Low temperature ferromagnetism in perovskite SrIrO 3 films
aa r X i v : . [ c ond - m a t . s t r- e l ] F e b Low temperature ferromagnetism in perovskite SrIrO films Rachna Chaurasia, K. Asokan, Kranti Kumar, and A. K. Pramanik ∗ School of Physical Sciences, Jawaharlal Nehru University, New Delhi - 110067, India. Materials Science Division, Inter University Accelerator Centre, New Delhi- 110 067, India. UGC-DAE Consortium for Scientific Research, Indore - 452001, India.
The 5 d based SrIrO represents prototype example of nonmagnetic correlated metal which mainlyoriginates from a combined effect of spin-orbit coupling, lattice dimensionality and crystal structure.Therefore, tuning of these parameters results in diverse physical properties in this material. Here, westudy the structural, magnetic and electrical transport behavior in epitaxial SrIrO film ( ∼
40 nm)grown on SrTiO substrate. Opposed to bulk material, the SrIrO film exhibits a ferromagneticordering at low temperature below ∼
20 K. The electrical transport data indicate an insulatingbehavior where the nature of charge transport follows Mott’s variable-range-hopping model. Apositive magnetoresistance is recorded at 2 K which has correlation with magnetic moment. Wefurther observe a nonlinear Hall effect at low temperature ( <
20 K) which arises due to an anomalouscomponent of Hall effect. An anisotropic behavior of both magnetoresistance and Hall effect has beenevidenced at low temperature which coupled with anomalous Hall effect indicate the development offerromagnetic ordering. We believe that an enhanced (local) structural distortion caused by latticestrain at low temperatures induces ferromagnetic ordering, thus showing structural instability playsvital role to tune the physical properties in SrIrO . PACS numbers: 75.47.Lx, 73.50.-h, 75.70.-i, 71.70.Ej
I. INTRODUCTION
In recent times, lots of scientific interest have beenplaced on Ir-based transition metal oxides.
Due to its5 d character, iridates exhibit strong spin-orbit coupling(SOC) and relatively weak electron correlation ( U ) effect.The delicate balance among competing energies such as,SOC, U and crystal field effect (CFE) gives exotic elec-tronic and magnetic properties in these materials wheremany of the phases are topologically relevant. In pres-ence of strong SOC, the t g d -orbitals split into low lying J eff = 3/2 and top lying J eff = 1/2 states. In case ofIr with 5 d electronic configuration, the J eff = 3/2state is fully filled while J eff = 1/2 remains half filled.The later J eff state being narrow, even a small U opensup a gap, thus giving a realization of J eff = 1/2 Mott-like insulating state. The in-built structural dimensionality has, however, adominant role on the magnetic and electronic propertiesof iridium oxide materials. For instance, in Ruddlesden-Popper (RP) series Sr n +1 Ir n O n +1 (which can be consid-ered as SrO.(SrIrO ) n where n layers of perovskite SrIrO is separated by magnetically and electronically inactiveSrO layer), a transition from paramagnetic (PM) andmetallic state in perovskite SrIrO ( n = ∞ ) to magneticand insulating state has been observed in layered Sr IrO ( n = 1) and Sr Ir O ( n = 2). Among the iridiumoxides, the perovskite SrIrO draws particular interest asit is shown to lie on the verge of ferromagnetic (FM) in-stability and metal-insulator transition (MIT). In addi-tion, the low temperature electronic state in SrIrO hasbeen characterized with a non-Fermi-liquid behavior. Therefore, tuning of parameters, such as SOC, U , struc-tural distortion, etc. by means of doping or lattice strainlikely to modify its ground state electronic and magnetic properties significantly. In fact, a recent band struc-ture calculation shows a line node near the Fermi surfacewhich is inherent to crystal structure of this material,and predicts SrIrO can host topological phases upontuning the SOC strength with suitable doping. Further,an emergence of antiferromagnetic (AFM) and insulatingstate with the substitution of a nonmagnetic and isova-lent element in SrIr − x Sn x O highlights the role of struc-tural distortion driven tuning of its physical properties. In present work, we have studied the magnetic andelectrical transport behavior in epitaxial SrIrO film ( ∼
40 nm) grown on SrTiO substrate. Bulk SrIrO adoptsa hexagonal structure grown in an ambient condition buta perovskite orthorhombic crystal structure can be real-ized if synthesized under high pressure. However, anepitaxial film grown on suitable substrate has an advan-tage that can host an orthorhombic SrIrO . Moreover,substrate strain can be used as a tuning parameter to getmodified magnetic and electronic behavior. There havebeen several studies for SrIrO film with different sub-strates. An angle-resolved photoemission spectroscopy(ARPES) study on perovskite SrIrO film reveals a nar-row semi-metallic band across Fermi level which mainlyoriginates due to combined effect of SOC, dimensional-ity and IrO octahedral rotations. The effects of sub-strate strain, film thickness and substrate temperatureon metal-insulator transition in perovskite SrIrO filmhave been studied. For SrIrO multilayers, many ex-otic phenomena such as, topological Hall effect, anoma-lous Hall effect, tuning of magnetic anisotropy, exchangebias, etc. have also been studied in heterostructures con-sisting of SrIrO and FM oxides. While most of thestudies have focused on exotic transport and electronicproperties, a less attention has been paid to the evolu-tion of magnetic behavior in SrIrO films. A dimension-ality controlled magnetic and electronic properties haverecently been studied in artificial [(SrIrO ) m ,(SrTiO ) n ]( m , n = 1, 2, 3,.....) superlatices. These studies showa low temperature magnetic ordering in SrIrO layer withthe transition temperature around 140 K for m , n = 1which systematically decreases with increasing m , andfurther show a close relation of resistivity with the mag-netic transition. A recent theoretical calculation showsthat the magnetic state in bulk perovskite SrIrO is sig-nificantly modified with the tuning of SOC. Therefore,the lattice strain or the distortion of local IrO octahedrain films likely to play vital role in determining its mag-netic behavior. The prominent example of strain inducedferromagnetism is CaRuO film where its bulk compo-nent has similar orthorhombic structure showing similarnon-magnetic and non-Fermi-liquid behavior. The present SrIrO film is found to be epitaxial withgood crystal quality. While the magnetic data indicatea development of (weak) FM ordering at low tempera-ture ( < ∼
20 K), the film remains insulating all overthe temperature. A positive, though small magnetoresis-tance (MR) is observed at 2 K which has correlation withits magnetic behavior. The nonlinear Hall effect coupledwith anisotropic-MR and -Hall effect further supports anevolution of FM state at low temperature. An increasingstrain or local structural distortion at low temperature isbelieved to induce the FM behavior.
II. EXPERIMENTAL METHODS
Epitaxial thin film of SrIrO with thickness ∼
40 nmhas been grown on (100) oriented SrTiO single-crystalsubstrate using pulsed laser deposition (PLD) techniqueequipped with KrF ( λ = 248 nm) laser. A phase-pure sto-ichiometric SrIrO polycrystalline pellet is used as a tar-get for film deposition. Before deposition, the substratehas been properly cleaned in ultrasonic cleaner alterna-tively with acetone and isopropyl alcohol for about 10min. The laser frequency and energy are used as 5 Hz and1.5 J/cm , respectively. The deposition has been donewith parameters such as, substrate to target distance5 cm, substrate temperature 750 ◦ C and oxygen pres-sure during deposition 0.1 mbar. In order to maintainthe oxygen stoichiometry, the deposited films have beenpost annealed at same deposition temperature 750 ◦ Cfor about 15 min at partial oxygen pressure around 500mbar. The thickness of the films has been estimated us-ing calibrated laser shot counts which closely matcheswith the thickness checked with FESEM. The structuralcharacterization of the films are performed with x-raydiffraction (XRD) where the data have been collectedusing a Panlytical diffractometer equipped with Cu- K α source. X-ray absorption spectroscopy (XAS) data havebeen collected from ‘ National synchrotron radiation re-search center’, Taiwan for O-K edge and Ir- L edge intotal electron yield and fluorescence modes, respectivelyby following standard procedure. Before XAS measure- FIG. 1: (color online) (a) the XRD pattern of SrIrO filmgrown on SrTiO (100) substrate are shown in semi-log scale.Left inset shows the expanded (100) Bragg peak with thick-ness fringes while the right inset presents an atomic force mi-croscope image of the film showing surface topography. (b)shows the φ -scan at (220) reflection. (c) shows the ω -scan(rocking curve) of SrIrO film around (200) reflection. ments, the x-ray photon energy has been calibrated us-ing a metallic gold foil for L edge absorption which isthe standard procedure followed in beamline. Further,XAS data are collected for various iridate samples hav-ing different Ir charge states in same beamtime and samebeamline which provides a better comparison. The elec-tronic transport and its angle dependent measurementsare done using a four-probe method in an insert attachedwith 9 Tesla magnet (Nanomagnetic). Both temperatureand magnetic field dependent magnetic properties of thefilm are measured with a superconducting quantum in-terference device (Quantum Design). The magnetic con-tribution due to film has been extracted after subtractingthe related moment of substrate. III. RESULTS AND DISCUSSIONA. Structural Analysis with x-ray diffraction
Fig. 1a shows θ -2 θ x-ray diffraction (XRD) plotof SrIrO film deposited on SrTiO (100) substrate.
530 540 550
DCB (b)
Photon Energy (eV)
O-K edge A Energy (keV) I n s t e n s it y ( a r b . un it ) Photon Energy (keV) I n s t e n s it y ( a r b . un it ) Ir-L edge(a) FIG. 2: (color online) (a) XAS spectra at Ir- L edge and (b)XAS spectra at O- K edge of the SrIrO film are shown atroom temperature. Inset of (a) shows the double derivativeof XAS spectra of Ir- L edge. A, B, C and D in (b) mark theenergy position for for the peak/hump. Bulk SrIrO is realized from Ruddlesden-Popper seriesSr n +1 Ir n O n +1 with n = ∞ where an infinite layersof perovskite SrIrO are stacked together forming a 3-dimensional structural network. Bulk SrIrO generallyadopts two different crystal structures based on synthe-sis protocol. At ambient pressure, SrIrO crystallizesin 6 H -hexagonal (monoclinic) structure while using highpressure synthesis method this material stabilizes in per-ovskite orthorhombic ( P bnm ) structure. As a target ma-terial for deposition of present film, single phase SrIrO is used which has been synthesized at ambient pressurehaving monoclinic- C2/c structure with lattice parame-ters a = 5.5982 ˚A, b = 9.6293 ˚A, c = 14.1949 ˚A and β = 93.228 o . The epitaxial growth of films has, however,advantage that a meta-stable orthorhombic phase can bestabilized with perovskite substrate. The used substrateSrTiO has cubic structure with lattice parameter a sub =3.90 ˚A. The pseudo-cubic ( pc ) lattice parameter a pc ( ≈ √ a + b ) of target SrIrO has been calculated fromits bulk orthorhombic lattice parameters ( a = 5.56 ˚A, b = 5.59 ˚A and c = 7.88 ˚A) giving its value 3.942 ˚Awhich corresponds to ∼ +1% compressive lattice strainfor the films deposited on SrTiO . Given that a pc and a sub has slight mismatch while the lattice parameter c of orthorhombic SrIrO matches closely with 2 a sub , theSrIrO films on SrTiO (100) substrate are likely to growalong (110) direction rather than having (100) orienta-tion.As shown in main panel of Fig. 1a, the XRD patternof deposited film exhibit only crystalline peaks withouttrace of any impurity or additional phase(s). A satel-lite peak near the substrate peak signifies the epitaxialgrowth of the SrIrO film. A magnified view of low-angle(100) reflection has been shown in left inset of Fig. 1awith thickness fringes. While the peaks due to substrateand film are not superimposed but they are very closewhich implies the film is under strain. The Bragg peaks related to SrTiO and SrIrO are observed at 2 θ = 22.74 ◦ and θ = 22.44 ◦ which gives the lattice parameter 3.90 ˚Aand 3.96 ˚A, respectively. The calculated lattice param-eter of SrIrO film is very close to its bulk a pc (3.942˚A). The SrTiO substrate in reality gives a compressivestrain ∼ +1.5% to SrIrO film.Further, an estimation regarding the thickness ( D ) ofdeposited film has been done using thickness fringes in(100) reflection with the following formula, D = ( m − n ) λ Sinθ m − Sinθ n ) (1)where θ m and θ n are the positions of m -th and n -thorder peaks and λ is the wavelength of x-ray used. Fol-lowing Eq. 1, we have calculated the thickness of SrIrO film ∼ ∼ ∼ φ -scan has been takenat (220) reflection for both film as well as SrTiO sub-strate, as shown in Fig. 1b. The SrTiO has four-foldsymmetric cubic structure with in-plane and out-of-planepseudo-cubic lattice parameter 3.90 ˚A. As seen in figure,the SrTiO and SrIrO φ -scan peaks are nearly equidis-tant with ∼ ◦ apart, and the peaks are close to eachother having a very small difference with ∆ φ ∼ ◦ . Thissuggests a four-fold structural symmetry of SrIrO filmand the film has single domain with cube on cube growth.Moreover, an unsplit nature of peaks in φ -scan data (Fig.1b) is in favor orthorhombic structure rather than mon-oclinic one. The θ -2 θ XRD pattern and the φ -scan un-derlines the fact that the present SrIrO film has taken anorthorhombic structure on SrTiO substrate. Our resultis in line with previous reports which has shown that thefilm thinner than 40 nm would take orthorhombic struc-ture while a thicker film is susceptible to mixed phaseof monoclinic and orthorhombic structures. We havefurther characterized the crystalline quality of depositedSrIrO film with a ω -scan (rocking curve) in XRD mea-surements. Fig. 1c shows the ω -scan of present SrIrO film, obtained around (200) reflection. The full widthhalf maxima (FWHM) of ω -scan has been calculated tobe around 0.5 ◦ which compares well with other reportof similarly thick films. The ω -scan usually signifiesabout the perfection in lattice planes and mosaic spread,therefore reasonably small FWHM suggests the presentfilm has parallel planes and less mosaic spread. All theseresults conclusively show that the present SrIrO film ishigh quality epitaxial film with an orthorhombic struc-ture, obtained on SrTiO substrate. B. X-ray absorption spectroscopy
To understand the Ir oxidation state as well as Ir-O hy-bridization in present film, x-ray absorption spectroscopy(XAS) measurements have been done at room tempera-ture. Fig. 2a shows normalized L (2 p / → d ) ab-sorption edge spectra for the SrIrO film which involvestransition to both 5 d / and 5 d / states. It is evident inFig. 2a that L edge occurs at 11219.5 eV. Usually, inXAS spectra the position of absorption edge largely de-pends on the ionic state of transition metal because thetransition metal - oxygen bond length mostly determinesthe energy shifting of absorption edge. With an increasein ionic state the bond-length becomes shorted, there-fore the energy edge occurs at higher energy. However, asmall difference in edge position has been observed evenfor same ionic state with different lattice structure. Incase of thin films where the lattice strain has significantrole on the bond length, the position of absorption edgemay vary with the bulk material. In case of iridium, theIr- L absorption edge has usually been seen to occur at11218.0, 11219.6, 11220.0, 11222.0 and 11222.5 eV for Ir,Ir , Ir , Ir and Ir charge state, respectively. Given that the absorption edge is very sensitive to localenvironment of anions, a small difference in energy po-sition may occur from system to system. The L edgeat 11219.95(4) eV in present film suggests iridium is inIr state. However, a little lower value of edge posi-tion compared to 11220 eV may be due to strain effectin film giving a modified Ir-O bond length or due topresence of small fraction of Ir related to oxygen va-cancy during film deposition. While there is difference inpeak position even for same element in different materi-als which is mainly due to different chemical environmentand composition, the fact is that absorption edgeincreases with increasing charge state. In this sense, the L XAS data imply a Ir charge state in present film. Tocheck whether there is any mixing of Ir oxidation states,we have plotted double derivative in inset of Fig. 2a.A distinct shoulder in double derivative of L spectra isgenerally considered to be an indicative of mixed chargestate, but no clearly visible shoulder in double deriva-tive of our L spectra is observed, as shown in inset ofFig. 2a. Thus, based on this results we conclude thatiridium mostly is in Ir oxidation state.We have additionally measured XAS spectra at O- K edge on present film to understand about hybridizationbetween Ir-5 d and O-2 p states. Following crystal fieldchemistry, the Ir- d orbitals in environment of IrO octa-hedra are split into low-lying t g ( d xy , d yz and d zx ) andhigh-lying e g ( d x − y and d z ) states. In octahedral en-vironment, the oxygen orbitals ( p x , p y and p z ) of six lig-ands (four basal and two apical) hybridize with transitionmetal d orbitals. Among t g orbitals, d xy hybridizes onlywith basal p x / p y while d yz and d zx hybridize with bothbasal p z and apical p x / p y orbitals. In case of e g orbitals, d x − y and d z engage in interaction with p x / p y and p z , respectively. The normalized O- K edge spectra for (b) - ( O e . Ir / ) T (K)
H = 1 kOe
ZFC FC
T (K) M ( / Ir) (a)
FIG. 3: (color online) (a) Temperature dependent magneti-zation data measured in 1 kOe applied field following ZFCand FC protocol are shown for SrIrO film. (b) shows theinverse magnetic susceptibility χ − (= ( M/H ) − ) as a func-tion of temperature, deduced from ZFC magnetization data.The red straight line is due to fitting of straight line followingCurie-Weiss law (discussed in text) while the arrow indicatesthe deviation from linearity. present film in Fig. 2b shows two distinct peaks (markedby A and B in plot) in lower energy regime at binding en-ergy ( E b ) around 529 and 532.5 eV, respectively while abroad hump is observed between 534 and 550 eV. The on-set energy of present O- K spectral edge agree with otherreport for SrIrO film. The peak at 529 eV (A marking)is due to hybridization between Ir- d xz / d yz and apical O- p x / p y while that at 532.5 eV (B marking) arises due tointeraction between Ir- t g and and basal O- p orbitals. Onthe other hand, the hybridization between Ir- e g and O- p orbitals causes broad hump in higher energy side. It canbe further noticed in Fig. 2 that the broad hump in highenergy side exhibits two prominent shoulders (marked byC and D in figure). While the above hybridization picturehas been discussed considering an isolated model of e g and t g orbitals, a recent theoretical study has, however,shown that in 5d based oxides, the crystal field effect,SOC and U has prominent role on mixing of e g and t g orbitals, hence influences the absorption edge spectra. Further, note that we observe a slight difference in peakpositions compared to bulk materials, which is probablydue to substrate induced strain in films which alters theextent of hybridization through local structural modifica-tion. Such modification in strength of hybridization vis-` a -vis peak position in XAS O- K edge spectra has beenobserved with chemical pressure in (Y − x Pr x ) Ir O . C. Magnetization Study
Fig. 3a shows the temperature dependent magnetiza-tion ( M ) data for SrIrO film, collected in applied field of1 kOe following zero field cooling (ZFC) and field cool-ing (FC) protocol. The moment of the film has beenextracted after subtracting the substrate contribution.On cooling, while ZFC and FC branches of magnetiza-tion exhibit a finite difference starting from high temper-ature but the moment in both measurements increasesmonotonically till about 20 K. Below 20 K, both M F C and M ZF C show a sharp increase till lowest measure-ment temperature of 2 K. This sharp increase in mo-ment below ∼
20 K implies a development of weak fer-romagnetism in SrIrO film at low temperature. How-ever, the moment of the film is quite low which is in linewith the fact that iridates are generally low moment sys-tems. Note, that both the low temperature FM state aswell as the obtained moment are in agreement with ar-tificial [(SrIrO ) m ,(SrTiO ) n ] superlatices. Further,the observed magnetic behavior of SrIrO film is con-sistent with our recent report of exchange bias behaviorin La . Sr . MnO /SrIrO multilayers at low temper-ature below ∼
20 K. Note, that similar sharp rise insusceptibility has also been observed in bulk SrIrO be-low ∼
15 K which has been shown due to proximity to FMinstability. At low temperatures, there is significant ex-change enhancement, though a FM ordering is not devel-oped which probably requires a triggering. In fact, simi-lar ferromagnetic instability has been predicted in theo-retical calculations for bulk SrIrO , hence any suitableperturbation likely to trigger the magnetism in this ma-terial. Along with chemical doping, the lattice strainarising from underlying substrate acts as driving force toinduce magnetic state in films. Further, the inverse mag-netic susceptibility χ − , calculated as ( M ZF C /H ) − , areshown in inset of Fig. 3b as a function of temperature.As evident in figure, above 20 K the χ − ( T ) shows a lin-ear increase where the behavior is similar to Curie-Weiss(CW) behavior, χ = C/ ( T − θ P ) where C and θ P arethe Curie constant and Curie temperature, respectively.However, the fitting parameters such as, C and θ P cannot be determined precisely. Nonetheless, the deviationfrom linear behavior of χ − ( T ) below around 20 K sug-gests an onset of FM at low temperature.To understand the low temperature magnetic state fur-ther, we have recorded magnetic hysteresis loop M ( H )at three different temperature 2, 10 and 20 K with fieldrange of ±
20 kOe. The magnetic contribution dueto SrTiO substrate has been subtracted from original M ( H ) data at each temperature. We have adopted theprotocol where a slope in M ( H ) data has been takenat high field regime. Assuming this slope represents thesusceptibility of substrate, moment of the substrate hasbeen calculated which has been used for subtraction toobtain the moment of film. Fig. 4 shows the representa-tive corrected M ( H ) plot for present SrIrO film at 2 K.Unlike bulk SrIrO , the M ( H ) plot shows an open hys-teresis where we find left and right coercive field H Lc =1293 Oe, H Rc = 1109 Oe, and upper and lower remnantmagnetization M Ur = 2.69 × − µ B /Ir, M Lr = 2.63 × − µ B /Ir, respectively. This asymmetry in M ( H ) plot( H Lc = H Rc ), which is clearly shown in lower inset of Fig.4, generally arises due to an exchange bias (EB) effect.For this present film, we calculate an exchange bias field -25 -20 -15 -10 -5 0 5 10 15 20 25-0.08-0.040.000.040.08 -2 -1 0 1-4-20240 5 10 15 2002468 M ( - / Ir) M ( / Ir)
H (kOe)
T (K)
H (kOe) M S ( - / Ir)
FIG. 4: (color online) Magnetic field dependent magnetiza-tion loop is shown for SrIrO thin film at 2 K. The upper insetshows the variation of saturated magnetization M s with tem-perature. The lower inset depicts an expanded view of M ( H )close to origin showing an asymmetry in magnetic hysteresisloop. H EB (= ( | H Lc | + | H Rc | )/2) around 92 Oe at 2 K whichreduces to about 81 Oe at 10 K. Usually, EB effect is re-alized when a system with FM/AFM interface is cooledin presence of magnetic field from high temperature and M ( H ) is measured at low temperature. The appliedfield biases the exchange interaction at interface which re-sults in a shifting of M ( H ) plot or EB phenomena. However, the interfaces in films or multilayers experi-ence various other factors, such as structural distortion,electronic reconstruction, strain, etc. which modify themagnetic characters at interface accordingly.
Thesein turn induces EB effect which has shown many manyinteresting properties. For instance, EB effect has beenobserved even in zero-field-cooled M ( H ) plot and insingle-layer magnetic films, which are rather unusual.The observed EB effect in present single layer SrIrO ,deposited on diamagnetic SrTiO , is likely due to straineffect. We speculate that due to an enhanced strain atinterface, some SrIrO layers adjacent to interface mayachieve an antiferromagnetic ordering which in contactwith rest ferromagnetic SrIrO layers results in an ex-change bias effect.Fig. 4 further shows a saturation in moment withinapplied field of 20 kOe. The temperature variation ofsaturation moment M s is shown in upper inset of Fig.4. We have obtained a very low M s which, however,agrees well with other studies of [(SrIrO ) m ,(SrTiO ) n ]superlatices. While iridates generally exhibit a lowmoment, the obtained M s is much lower than the ex-pected saturation moment 0.33 µ B /Ir, which can be cal-culated using M s = g J J eff µ B with g J = 2/3 and J eff = 1/2 for strong SOC dominated systems. For instance,at 2 K we obtain saturation moment around 7.5 × − µ B /Ir (upper inset in Fig. 4) which is roughly one orderlower than the expected value following the J eff model.A detail investigation involving the microscopic tools isrequired to understand the nature of low temperaturemagnetism in present film. However, at this stage we canspeculate that the nature of this low temperature mag-netic state may be either of FM type without the J eff state or a canted-AFM type with a J eff state. It can benoted that a weak FM has been observed in layered iri-date Sr IrO due to canted-AFM ordering which is drivenby Dzyaloshinskii-Moriya (DM) type antisymmetric in-teraction in this SOC dominated system. While restof the results and analysis suggest a FM ordering at lowtemperature in present film (discussed later), with theprogress of time the J eff model has been shown to devi-ate considerably in non-ideal (or distorted) octahedral ar-rangement in both theoretical and experimental studies.A recent theoretical study has even discussed the effectof CFE, SOC and U on the mixing of e g and t g statesin 5 d oxides. Very recently, an evolution of both spinand orbital moment has been shown with the lattice dis-tortion in 3 d -5 d double perovskite (Sr − x Ca x ) FeIrO . Therefore, the magnetism in iridates continues to be aninteresting subject of research.Nonetheless, the combination of EB effect, open hys-teresis loop in M ( H ) plot and the magnetic saturationin M ( H ) above 10 kOe imply the ferromagnetic natureof present SrIrO film at low temperature. The thermaldemagnetization of the saturation moment, as shown inupper inset of Fig. 4, further indicates low temperatureferromagnetic in SrIrO film. Opposed to paramagneticbulk SrIrO , this development of ferromagnetism in itsfilms is quite noteworthy. We believe that epitaxial lat-tice strain in films plays important role in stabilizing FMstate which has been similarly observed in other nonmag-netic perovskite material CaRuO . D. Temperature dependent electronic transportmeasurements
To understand the electronic transport behavior inpresent SrIrO film, we have measured temperature de-pendent resistivity ρ ( T ), as shown in Fig. 5. The re-sistivity value at room temperature is found to be ∼ With decreasing temperature, the ρ ( T ) increases monotonically indicating a semi-metallicor insulating behavior. The electronic transport behav-ior in SrIrO films are shown to be extremely sensitiveto the deposition temperature, film thickness and latticestrain. Previous studies with varying substrate tem-perature (500 - 800 o C) have shown that with increasingsubstrate temperature, the film resistivity increases lead-ing to an insulating behavior which is mainly due to aninhomogeneous Ir distribution. Further, epitaxial strainrealized either from reducing film thickness or using lat-tice mismatch substrates, has prominent role to increasethe resistivity as well as to induce the insulating state.Given that our substrate deposition temperature is 750
260 280 300 l n T -1/3 (K -1/3 ) (a) ( m - c m ) T (K) (b) ( m - c m ) T (K)
T (K) d / d T FIG. 5: (color online) (a) The electrical resistivity as a func-tion of temperature are shown for SrIrO film deposited onSrTiO substrate. The up arrow marks an anomaly in resis-tivity around its magnetic transition. Upper inset shows tem-perature derivative of resistivity data showing the anomalyat low temperature as indicated by down arrows. Lower insetshows a magnified view of metal to insulator transition around272 K. (b) shows the fitting of data with Mott variable-range-hopping model (Eq. 2) in two different temperature ranges.The solid lines are due to straight line fittings with Eq. 2.The arrow indicates the deviation from linearity in fitting inlow temperature around 20 K. o C and the substrate SrTiO has some lattice mismatchwith SrIrO , this semi-metallic or insulating behavior isa likely behavior. However, an overall low resistivity anda reasonably low ρ (5 K)/ ρ (300 K) ratio ( ∼ ρ ( T ) dataaround 272 K which has been shown in lower inset of Fig.5. This dip in ρ ( T ) appears to be a metal-insulator tran-sition (MIT) which has been similarly observed at differ-ent temperatures in SrIrO film with different substratesas well as with different film thickness. Here, it canbe noted that below this dip, the ρ ( T ) does not followany logarithmic temperature dependance which is typ-ical to the Kondo phenomenon. On cooling, a changein slope in ρ ( T ) is further evident at low temperature R / R ( ) ( % ) H (kOe) R / R ( ) ( - ) M (10 -8 emu ) FIG. 6: (color online) Magnetoresitance (defined in text) asa function of magnetic field are shown for SrIrO film at 2K with only positive field direction. The arrows indicate thedirection of field application. The solid line is due to fittingof MR data with quadratic field dependance. Inset shows thequadratic dependance of magnetoresitance on magnetization. around 30 K which is marked by an up arrow in Fig. 5.This change of slope in ρ ( T ) occurs around its magnetictransition which is prominently observed in its tempera-ture derivative, dρ / dT , where a maximum slope changeis observed around 50 and 10 K (marked by down ar-row), as shown in upper inset of Fig. 5. The dip andpeak in dρ / dT around 50 and 10 K, respectively suggestthe resistivity changes with relatively faster rate betweenthese temperatures. However, there is a disagreement be-tween the dip temperature in dρ / dT around 50 K and themagnetic ordering temperature in Fig. 3 which may bedue to an early onset of magnetic ordering. Nonetheless,the anomaly in ρ ( T ) at low temperature is connectedwith the magnetic ordering which has been similarly ob-served in [(SrIrO ) m , (SrTiO ) n ] ( m , n = 1, 2, 3,.....)superlatices. The ρ ( T ) data can be fitted using Mott’s variable-range-hopping (VRH) model, ρ = ρ exp[( T /T ) α ] (2) T = 21 . k B N ( ǫ F ) ξ (3)where α is related to the dimensionality ( d ) of systemwith α = 1/( d +1), T is the characteristic temperaturerelated to the leakage rate of localized states at Fermilevel, k B is the Boltzmann constant, N ( ǫ F ) is the densityof states (DOS) at Fermi level and ξ is the localizationlength. As shown in Fig. 5b, Eq. 2 can be fitted with ρ ( T ) in two distinct temperature regimes (5 - 15 K and67 - 205 K) with d = 2, giving T values 3.7 × − K and 22.56 K, respectively. The T exhibits distinctly low value in low temperature magnetic state. The T is inversely proportional to both N ( ǫ F ) and ξ (Eq. 3).Given that this is an insulating system, so an increaseof N ( ǫ F ) at low temperature is very unlikely. Therefore,this change in T can be explained with an increase of ξ ,suggesting localization length increases drastically withan onset of magnetism at low temperature. E. Magnetoresistance
To further understand electron transport behavior, wehave measured isothermal sheet resistance as a functionof magnetic field (up to 85 kOe) at 2 K where the fieldhas been applied parallel to the plane of film ( H || ab -plane). The resistance ( R ) has been measured withfield applied in both positive and negative directions.The percentage magnetoresistance (MR), ∆ R/R (0) =[ R ( H ) − R (0)] /R (0) × film at 2 K andshown in Fig. 6 with positive field direction. The filmshows positive MR i.e., resistance increases with appliedmagnetic field. The calculated value of MR is ∼ We, however, do not observe any cusp/peak inMR close to zero field which is usually seen due to weakantilocalization effect in strong SOC systems. Interest-ingly, a hysteresis in MR data between increasing anddecreasing field has been observed below around 40 kOefield. This hysteresis in MR appears to be connectedwith the magnetic state of film at low temperature, assimilarly a remnant magnetization has been observed in M ( H ) plot (Fig. 4). With application of high magneticfield, there is an induced FM moment which is retainedeven after applied field returns to zero. Given that FMspin ordering has significant effect on charge transportbehavior (Fig. 5), the difference in moment between in-creasing and decreasing field causes hysteresis in MR. Inhigh field regime ( >
40 kOe), hysteresis is not evidentdue to magnetic saturation. To check further, we haveplotted MR as a function of M in inset of Fig. 6. TheMR follows a linear behavior with square of magnetiza-tion in field range up to ∼ ∼ ∼
40 kOe), theMR follows a quadratic field dependance, MR ∝ B , asshown in Fig. 6 with solid line. F. Hall measurements
In an aim to investigate the nature of charge carri-ers and magnetic state, Hall voltage has been measuredas a function of magnetic field in low temperature mag-netic state at 2, 10 and 20 K with H || ab -plane. Fig. 7shows Hall voltage is linear with field (up to 20 kOe) K K H a ll vo lt a g e ( m V ) H (kOe) K FIG. 7: (color online) Hall voltage vs magnetic field are shownat three different temperatures i.e., 2, 10 and 20 K. The dataat 10 and 20 K are vertically shifted by 0.1 and 0.2 mV, respec-tively for clarity. The vertical arrows indicate the deviationfrom linearity at respective fields. at 20 K but with lowering in temperature a nonlinear-ity is introduced where the onset field for nonlinearitydecreases with decreasing temperature. Here, it can benoted that FM state in present film dominates or M ( T )shows steep rise below around 20 K (Fig. 3). A nonlin-ear Hall effect is an intrinsic phenomenon which can beexplained with various theoretical descriptions where thetwo-carrier transport model and anomalous Hall effect(AHE) are commonly noted ones. Considering that thefilm develops a FM ordering at low temperature (Fig. 3)and MR follows a quadratic field dependance signifying asingle carrier charge transport (Fig. 6), we have focusedon AHE to understand the present Hall effect behavior.In general, Hall resistivity ρ xy can be expressed as, ρ xy = R H + 4 πR s M (4)where H is the magnetic field and M is magnetization.While the first term is due to Lorentz force driven ordi-nary hall resistivity, the second term represents the con-tribution from anomalous Hall effect. The AHE typicallyarises in FM materials with broken time-reversal symme-try. While the ordinary Hall effect is due to Lorentz forceeffect, the origin of anomalous part is debated. Thoughthe mechanisms based on intrinsic or extrinsic (skew-scattering and side jump) scattering are usually thoughtto cause anomalous Hall effect, the SOC effect plays cru-cial role in all these mechanisms. As seen in Eq. 4, AHEis proportional to magnetization, and can effectively beused to investigate the magnetic state of a material. Fol-lowing Eq. 4, in Fig. 8a we have plotted Hall coefficient R H (= ρ xy /H ) as a function of magnetic susceptibility(= M/H ) at 2 K. A good linear fit to the experimen-tal data is obtained in high field regime, as shown inFig. 8a. From fitting, we obtain R = -2.5881 cm /C xy / H ( c m / C ) M/H (10 -3 emu/Oe) | R | ( c m C - ) (a) R S ( c m C - ) T (K) (b) n ( c m - ) (c) ( - m s - V - ) T (K)
T (K)
FIG. 8: (color online) (a) Hall coefficient R H (= ρ xy /H ) vsmagnetic susceptibility (= M/H ) are shown at 2 K followingEq. 4 for SrIrO film grown on SrTiO . Inset shows thecalculated values of | R | and R s with temperature. (b) and(c) show the variation of carrier concentration n and carriermobility µ , respectively at low temperatures. and R s = 4.35 × cm /C at 2 K. The R s , which isrelated to magnetic part, is roughly three orders higherthan R as observed in ordinary magnetic materials. Thenegative sign of R implies an electron like charge carri-ers in the system. The variation of both R and R s atlow temperatures are shown in inset of Fig. 8a, showingboth the parameters increases with lowering the temper-ature. From the relation R = -1/( ne ), we obtain car-rier concentration n = 2.4 × /cm at 2 K, whichis in good agreement of earlier reports and indicates lowcarrier concentration in the system. Mobility of chargecarriers has also been evaluated as µ = R / ρ xx , where ρ xx is the resistivity parallel to the direction of currentin presence of zero magnetic field. Calculated values of n and µ for different temperatures are shown in Fig. 8b and8c, receptively. Both charge concentration and mobilitydecreases with decreasing temperature. Nonetheless, thepresence of nonlinear Hall effect or anomalous Hall effectconfirms a development of FM ordering at low temper-ature below ∼
20 K in present film. This is in contrastwith bulk SrIrO which exhibits PM behavior at leastdown to 1.7 K, though a sharp rise in susceptibility be-low 15 K indicates this material is in proximity to FMinstability. R xx () R xy () (Degree) R xx () (Degree)
20 K
FIG. 9: (color online) Angular dependance of (a) longitudinalresistance R xx and (b) transverse resistance (planar Hall) areshown for present SrIrO film at 2 K with an applied field of20 kOe. Inset of (a) shows the angular dependence of R xx at20 K with 20 kOe field. G. Angle dependent Hall and resistivitymeasurements
The low temperature FM state has further been probedby measuring both longitudinal ( R xx ) and transverse( R xy ) resistance after rotating the film with respect tothe direction of applied magnetic field. In case of FM ma-terials with long-range spin ordering, the magnetizationhas influence on the scattering of carriers and the resis-tivity depends on angle between magnetization (or mag-netic field) and current directions which is usually termedas anisotropic magnetoresistance (AMR) and defined asdifference in MR when the current ( I ) is applied eitherparallel or perpendicular to the magnetization. Figs. 9aand 9b show the measured R xx and R xy as a function ofangle β between the magnetization and the current di-rection, respectively. The measurements have been doneat 2 K in presence of 20 kOe field where the applied fieldis in the range of saturation magnetization (see Fig. 4).Though the variation of R xx over angel β is not signifi-cant but it shows a sinusoidal variation where the R xx ( β )data are best fitted with the following equation, R xx = A + B cos( Cβ + D ) (5)where A is an offset parameter, B is the amplitudeof the angular dependance of MR, C is the multiplyingfactor to angle and D is the phase factor. Usually, the pa-rameter C takes value 2 but is present case, C = 1.75(3) gives better fitting with phase angle D = 126(5) degree.The variation of R xx with angle β implies the film devel-ops FM ordering at low temperature. In inset of Fig. 9a,we have shown the same R xx ( β ) collected at 20 K. Asseen in figure, R xx do not exhibit any noticeable angulardependance at 20 K which is in agreement with linearHall effect evidenced at same temperature (Fig. 7). Thisis primarily because at 20 K or above the FM ordering isnot strong enough to induce an angular dependance or itrequires higher magnetic field. We have further recordedplanner Hall resistance R xy as a function of angle β inhall geometry where current is applied in xx directionand voltage is measured in xy direction. As shown inFig. 9b, the variation of R xy is also sinusoidal with β ,and can be best fitted using Eq. 4 with C = 1 and neg-ligible phase factor D . Nonetheless, the dependance ofboth R xx and R xy with the angle ( β ) between the ap-plied current and magnetic field direction supports a lowtemperature FM ordering in present film. H. Temperature dependent structural investigation
Our previous results indicate an unusual FM state inSrIrO film at low temperature which we believe is causedby an enhanced strain or structural distortion. To exam-ine the structural evolution with temperature in presentfilm, temperature dependent XRD measurements havebeen done down to 20 K. The lattice parameter for bothfilm and substrate have been calculated from Bragg peak.The lattice parameters related to (100) and (200) peaksof SrIrO film and (200) peak of SrTiO substrate areshown in Figs. 10a, 10b and 10c, respectively.Here, it can be mentioned that the lattice parame-ters determined from (100) and (200) peaks of SrIrO exhibit a minor difference ( ∼ θ po-sition is different. Nonetheless, this difference in latticeparameter remains almost similar across the temperature(Fig. 10a and 10b). The Fig. 10 shows that the lat-tice parameter of film and substrate does not follow thesame behavior, particularly at low temperatures. Boththe SrIrO lattice parameters related to (100) and (200)Bragg peaks, initially decrease with decreasing tempera-ture till around 40 K and then show a sudden increase.On the other hand, lattice parameter of SrTiO sub-strate shows a continuous decrease indicating an anomalyaround 100 K (Fig. 10c). This anomaly arises becauseSrTiO has structural phase transition from room tem-perature cubic to tetragonal phase at 105 K where thelattice parameters show a minor different at low tempera-ture. However, this structural phase transition of SrTiO substrate will unlikely influence the magnetic ordering ofSrIrO film as the former happens at relatively high tem-perature. As evident in Fig. 10, the difference betweenlattice parameters of film and substrate increases at lowtemperature which generates strain resulting in more dis-tortion in local structure. The IrO octahedra become0distorted in terms of Ir-O bond-length and bond-anglewhich will have large ramification on the magnetic andelectronic properties. IV. SUMMARY AND CONCLUSION
The SrIrO lies at the boundary of magnetic instabil-ity where a close interplay between SOC and U has beenshown to give rise many exotic magnetic and electronicphases in this material. Even, realization of topolog-ical phases through tuning of SOC in SrIrO has beendiscussed using band structure calculations. The semi-metallic electronic structure in SrIrO , that is character-ized by line nodes and small density of states, makes thismaterial quite interesting compared to layered Sr IrO and Sr Ir O which are both magnetic and insulating.In this Ruddlesden-Popper series based oxide, the cornershared IrO octahedra plays a crucial role in stabilizingits physical properties. An increasing lattice strain real-ized from underlying substrate would introduce a struc-tural distortion at low temperatures in terms of modi-fication of bond-angle and bond-length between Ir andoxygen. Therefore, the IrO octahedra will be distortedwith a modified local environment which will influencethe 3-dimensional network of Ir-O-Ir chain. Usually, bothSOC and U are considered to be intrinsic atomic prop-erties which would largely remain unaltered in presentcase with no change in transition metal or its electronicconfiguration. The particular combination of SOC and U is not energetically favorable for the onset of mag-netic ordering in bulk SrIrO , even presence of magneticIr ( J eff = 1/2). However, the IrO octahedral (struc-tural) distortion with modified bandwidth and electronicstructure would compete with these energies. Therefore,a strong competition among SOC, U , structural distor-tion and bandwidth will eventually weaken the effectivestrength of SOC and promote the magnetic exchange.These would drive the system into different magnetic andelectronic states.The structural distortion driven magnetism has al-ready been evidenced in doped bulk SrIr − x Sn x O wherethe substitution of nonmagnetic, isovalent Sn for Ir induces a metal-to-insulator transition and an antifer-romagnetic transition ( T N ≥
225 K) which has beenexplained as combined effect of increased spin-spin ex-change interaction, decreased SOC and enhanced IrO octahedral distortion. Further, a dimensionality in-duced magnetic ordering and its relation with resistiv-ity anomaly has been shown in [(SrIrO ) m ,(SrTiO ) n ]superlatice where the properties are shown to largely de-pends on both m and n . Very recently, an evolutionof magnetic moment as well as electrical properties withstructural distortion has been shown in 3 d -5 d double per-ovskite (Sr − x Ca x ) FeIrO using both experimental dataand theoretical calculations. A complex orbital order-ing with AFM spin state has been shown in other doubleperovskite Sr CeIrO as a competition between SOC, U T (K) O u t - o f- P l a n e l a tti ce p a r a m e t e r ( ¯ ) (a) (b) SIO (200) (c) SIO (100) STO (200)
FIG. 10: (color online) (a) and (b) show the lattice parameterof SrIrO film calculated from (100) and (200) Bragg reflec-tion, respectively. (c) shows the lattice parameter of SrTiO substrate due to (200) Bragg reflection. and structural distortion. Here, it can be noted thatin our previous La . Sr . MnO /SrIrO multilayer, wehave observed an interface magnetic exchange interactionand related exchange bias effect below 40 K. Although adifferent magnetic state (weak FM ordering) is observedin present film compared to bulk doped SrIr − x Sn x O but an increasing structural distortion at low tempera-ture likely triggers the FM ordering in present SrIrO film. Nonetheless, iridates in general have delicate bal-ance among different competing energies such as, electroncorrelation, SOC, crystal field effect, therefore tuning ofany parameter leads to modification of electric and mag-netic properties.In summary, we have prepared epitaxial thin film ofSrIrO ( ∼
40 nm) on SrTiO (100) substrate. Structuralanalysis shows the film is of good quality. In contrast tobulk material, film shows an insulating behavior wherethe charge transport mechanisn follows follows Mott’svariable-range-hopping model. Further, magnetic mea-surements suggest a development of weak FM orderingat low temperature below ∼
20 K. The magnetoresistanceat 2 K is found to be positive showing a quadratic fielddependance in low field regime ( <
40 kOe). A nonlin-ear Hall effect is observed at low temperature below 20K which is believed to be caused by an anomalous Hallbehavior. The present film further shows an anisotropicmagnetoresistance and Hall voltage at low temperatures.These experimental observations are in favor of FM stateat low temperature. A sudden increase of lattice param-eter below ∼
40 K implies an increase of lattice strain1which causes (local) structural distortion that is believedto induce the low temperature FM ordering SrIrO film.Given that SrIrO has delicate energy balance SOC, di-mensionality and structural distortion which places thismaterial in close proximity to FM instability and metal-insulator transition, our study shows lattice strain plays avital role in tuning the physical properties in this simple,though unusual oxide. V. ACKNOWLEDGMENT
We acknowledge SERB-DST for funding the ‘Excimerlaser’, PURSE-DST for funding the ‘Helium liquefier’, FIST-DST for funding the ‘Low temperature High mag-netic field AFM/STM’ and UPE II-UGC for funding the‘Film deposition chamber’. We are thankful to Dr. AlokBanerjee and Dr. Rajeev Rawat, UGC-DAE CSR, Indoreand Dr. Ajay Kr. Shukla, NPL, Delhi for the magnetiza-tion, electrical resistivity, thin film XRD measurementsand discussions. RC is thankful to UGC, India for thefinancial support. ∗ Electronic address: [email protected] B. J. Kim, H. Jin, S. J. Moon, J.-Y. Kim, B.-G. Park, C.S. Leem, J. Yu, T. W. Noh, C. Kim, S.-J. Oh, J.-H. Park,V. Durairaj, G. Cao, and E. Rotenberg, Phys. Rev. Lett , 076402 (2008). B. J. Kim, H. Ohsumi, T. Komesu, S. Sakai, T. Morita, H.Takagi, T. Arima, Science , 1329 (2009). W. Witczak-Krempa, G. Chen, Y. B. Kim, and L. Balents,Annu. Rev. Condens. Matter Phys. , 57 (2014). G. Cao, A. Subedi, S. Calder, J.-Q. Yan, J. Yi, Z. Gai, L.Poudel, D. J. Singh, M. D. 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