Polaronic transport induced by competing interfacial magnetic order in a La 0.7 Ca 0.3 MnO 3 /BiFeO 3 heterostructure
Y. M. Sheu, S. A. Trugman, L. Yan, J. Qi, Q. X. Jia, A. J. Taylor, R. P. Prasankumar
aa r X i v : . [ c ond - m a t . s t r- e l ] F e b Polaronic transport induced by competing interfacial magnetic order in aLa . Ca . MnO /BiFeO heterostructure Y. M. Sheu, ∗ S. A. Trugman, L. Yan, J. Qi, Q. X. Jia, A. J. Taylor, and R. P. Prasankumar † Center for Integrated Nanotechnologies, MS K771,Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA The Peac Institute of Multiscale Sciences, Chengdu, Sichuan 610225, People’s Republic of China
Using ultrafast optical spectroscopy, we show that polaronic behavior associated with interfacialantiferromagnetic order is likely the origin of tunable magnetotransport upon switching the fer-roelectric polarity in a La . Ca . MnO /BiFeO (LCMO/BFO) heterostructure. This is revealedthrough the difference in dynamic spectral weight transfer between LCMO and LCMO/BFO at lowtemperatures, which indicates that transport in LCMO/BFO is polaronic in nature. This pola-ronic feature in LCMO/BFO decreases in relatively high magnetic fields due to the increased spinalignment, while no discernible change is found in the LCMO film at low temperatures. Theseresults thus shed new light on the intrinsic mechanisms governing magnetoelectric coupling in thisheterostructure, potentially offering a new route to enhancing multiferroic functionality. PACS numbers: 78.47.jg,73.20.Mf,75.70.Cn,75.47.Gk
The quest to achieve strong magnetoelectric (ME) cou-pling has driven the surge in research on multiferroic ma-terials over the past decade. However, this has been quitedifficult to accomplish using bulk materials, motivatingresearchers to explore other approaches, most notably theuse of transition metal oxide heterostructures [1–3]. Inthese novel systems, different degrees of freedom (DOFs)(e.g., charge, spin, and orbital) are coupled at a single in-terface between two different oxide layers to form a newstate that displays properties dramatically different fromthose of the individual layers [4–8]. Particular attentionhas been given to the coupling between ferromagnetic(FM), antiferromagnetic (AFM) and ferroelectric (FE)orders, as this could reveal new routes to realizing strongME coupling [1–3, 9–11].Heterostructures consisting of manganite and multi-ferroic layers are particularly promising in this regard.The most extensively studied combination [2, 3, 9, 10, 12]consists of the colossal magnetoresistive (CMR) mangan-ite La . Sr . MnO (LSMO) (or the similar compoundLa . Ca . MnO (LCMO)), which is ferromagnetic belowa critical temperature T c , and the canonical multiferroicBiFeO (BFO), which has coexisting coupled AFM andFE phases in which the magnetization can be switchedby an applied electric (E) field [13]. The combinationof these materials thus has great potential for exhibit-ing novel phenomena by coupling different FM, AFM,and FE phases across the interface. Indeed, a new in-terfacial state between LSMO and BFO has been discov-ered experimentally and discussed theoretically [2, 14–16]. This state displays an exchange-bias (EB) field andmagnetotransport that can be tuned by switching theFE polarization, increasing the potential for device con-trol through interfacial coupling. However, a completepicture of the interplay between FM and AFM orders inthis heterostructure has yet to be reached.Current understanding of the EB field, which arises from the interaction between FM and G-type AFM(AFM(G))orders at the interface, is based on spin cant-ing or pinning in BFO, with the assumption of negligiblecanting in the manganite layer [14–16]. However, giventhat the spin interaction is mutual, it is not clear whyFM spins in LSMO can induce AFM spins in BFO tocant, but the reverse has not been discussed. In fact,the interfacial spin-spin interaction J FM-AFM(G) could en-able the use of effective ’AFM staggered fields’ to controlFM spins in an alternating arrangement, which cannotbe done using an applied magnetic (B) field. Further-more, relatively little effort has been made to explainthe E-field switchable magnetotransport in the mangan-ite layer, which is arguably of equal importance.One can shed light on these issues using ultrafast op-tical spectroscopy (UOS), which has been demonstratedto be a sensitive probe of the charge, spin and orbitalorder in CMR manganites [17–23]. In particular, muchinsight into the physics of these systems has been ob-tained from probing large photoinduced changes in theoptical conductivity between low and high frequencies(known as ’dynamical spectral weight transfer’ (DSWT))[17–20, 24], which are strongly coupled to the magneto-transport properties [25].Here, we used UOS to uncover the polaronic natureof carrier transport in a LCMO/BFO heterostructure,originating from spin canting in LCMO near the inter-face with AFM ordered BFO. This was revealed throughthe difference in DSWT between a LCMO film and aLCMO/BFO heterostructure at low temperatures, andfurther supported by the observed increase in conductiv-ity when a B field was used to increase the spin alignment.These results can thus provide a new avenue to engineer-ing multiferroic functionality [2], since the AFM orderedspins in BFO interact with FM spins in LCMO across theinterface and are coupled to the FE polarization [13, 16].The samples studied here are a thin film of optimally E σ (a) (b) Drude-like Polaron-like (c) E σ ~1.6 eV
0 200 400 600 800 -1-0.8-0.6-0.4-0.200.2
Delay Time (ps) N o r m a li z e d D R / R LCMO/STO
10 K 50 K100 K150 K200 K300 Ksimulationsimulation 0 200 400 600 800 -1-0.8-0.6-0.4-0.20
Delay Time (ps) N o r m a li z e d D R / R LCMO/BFO/STO
10 K 75 K165 K300 K0 2 4-1-0.50
FIG. 1: Photoinduced change in optical reflectivity as a function of time delay at various temperatures, measured at 1.59eV in (a) a LCMO film and (c) a LCMO/BFO heterostructure. In LCMO, T c ∼
160 K, consistent with that obtained throughmagnetotransport measurements. The dashed and dashed-dotted lines are numerical simulations of 1D thermal diffusion acrossthe LCMO/STO interface at 10 K and 300 K, calculated as in Ref. 26. Insets show the dynamics at early times. The spike-likefeature in the data of (c) at 300 K at t =0 is due to the coherent artifact [27]; its superposition with the transient reflectivitysignal leads to the observed sub-ps oscillatory signal. (b) Schematic diagram of the optical conductivity versus energy ( < ∼ doped LCMO (thickness t =10 nm), which is an FM metal(FMM) below T c and a paramagnetic insulator (PMI)above T c ( ∼
260 K in the bulk, ∼
160 K in our film),and a LCMO(10 nm)/BFO(50 nm) bilayer heterostruc-ture, both grown on (001) SrTiO (STO) substratesby pulsed laser (KrF) deposition. The heterostructure(LCMO/BFO) is deposited by switching the target with-out breaking the vacuum. The substrate temperatureduring film growth is initially optimized and maintained.The oxygen pressure during deposition is 100 mTorr. Thesamples are cooled to room temperature in pure oxy-gen (350 Torr) by turning off the power supply to theheater without further thermal treatment [29]. The de-generate optical pump-probe measurements use an am-plified Ti:sapphire laser system producing pulses at a 250kHz repetition rate with ∼
150 fs duration and energy ∼ µ J/pulse at a center wavelength of 780 nm (1.59 eV).The incident pump fluence is ∼ µ J/cm , creating aphotoexcited carrier density of ∼ × cm − ( ∼ × − per unit cell). The photoinduced initial increase in thelattice temperature is 2-4 K, calculated from the heatcapacity, which should decrease with increasing equilib-rium temperature. The B-field-dependent measurementsare performed in a ∼ t >
75 nm), the first time constant ( τ < τ ∼ T c due to an increase in the spin specific heat, and the thirdtime constant, τ , is due to heat diffusion ( τ > τ and τ are much faster than in thicker films. How-ever, thickness-dependent measurements on a series ofLCMO films (not shown) revealed that their origin is thesame [31]. This was confirmed by a numerical simula-tion for 1D thermal diffusion across an interface, whichdemonstrated that the faster decay times in thin filmsresult from the strong influence of the substrate thermaldiffusion on thermal transport, rather than diffusion inthe films themselves (dashed and dashed-dotted lines inFig. 1(a)) [26]. Therefore, as in thicker films, the ∆R/Rsignals measured on our thin LCMO film originate fromlaser heating-induced DSWT [17, 18]. Importantly, thesesignals, obtained in a non-contact manner, can indicatewhether the equilibrium state of LCMO is metallic orinsulating, as follows.In optimally doped manganites, the time-integratedoptical conductivity displays a Drude-like feature in thelow temperature FMM state [32, 33] (blue solid line inupper panel of Fig. 1(b)). This feature evolves into ahigher energy ( ∼ T increases [32, 33](blue solid line in lower panel of Fig. 1(b)). This SWTdominates the low energy physics ( < T approaches T c from below, whichtrap electrons hopping from Mn to Mn sites, reduc-ing the conductivity [25, 32–36]. The formation of thispolaron peak is considered to be a signature of competi-tion between the electron kinetic energy and the J-T lat-tice distortion [25]. However, this temperature-inducedSWT can be reversed by an applied B field. The appliedfield enhances the spin alignment, increasing the conduc-tivity, which reduces the polaronic peak and recovers theDrude-like feature (i.e. following the blue solid line fromthe lower to the upper panel in Fig. 1(b)) [25, 36–41].DSWT then occurs after femtosecond photoexcitation,which transfers energy from the electronic subsystem tothe lattice within a few ps, causing ∆R/R at ∼ T c ,∆R/R > ρ of the metallicstate ( dρ/dT >
0; the DSWT transiently redistributes thespectrum to higher energies (upper panel of Fig. 1(b)), so d R/ dT (1.59 eV) > T c , ∆R/R < dρ/dT <
0; the DSWTtransiently redistributes the spectrum to lower energies(lower panel of Fig. 1(b)), so d R/ dT (1.59 eV) < T < T c ) and ’conductive’ ( T > T c )transients are given by the 10 K and 300 K traces in Fig.1(a), respectively. For clarity, we emphasize that a ’re-sistive’ ∆R/R transient indicates that the equilibriumstate is metallic, while a ’conductive’ transient indicatesthat the equilibrium state is insulating (due to polaronictransport). Finally, since an applied B field increasesthe conductivity, the ’conductive’ transient measured inthe insulating state evolves towards a ’resistive’ transientas the field increases [18–20]. In this manner, we can useUOS to reveal whether LCMO is insulating or metallic inequilibrium. (See appendix A for more details on DSWTin manganites.)We can now use this detailed characterization of ourthin LCMO films as a basis for understanding carriertransport in the LCMO/BFO heterostructure. First,we note that the ∆R/R signal in LCMO/BFO must bedue to photoinduced changes in the LCMO layer, sincethe band gap of BFO is greater than 2.6 eV, prevent-ing it from being directly photoexcited with our 1.59eV pump photons [43, 44]. In addition, we expect thatthe temperature-dependent DSWT in optimally dopedLCMO should stay the same when changing the materialunderneath, as long as no novel interface state is formed.This is observed in LCMO/BFO above ∼
165 K(through comparing Fig. 1(a) and (c)), indicating thatthe relaxation mechanisms remain the same at high tem-peratures. However, at low temperatures (
T <
100 K)the resistive transient observed in LCMO alone is re-placed by a conductive transient in LCMO/BFO, imply-
Delay Time (ps) N o r m a li z e d ∆ R / R LCMO/BFO, 10 K
LCMO, 10 K
FIG. 2: Photoinduced time-dependent reflectivity change inLCMO/BFO as a function of B field at 10 K. The inset showsthe ∆R/R signal from a single LCMO film, taken under thesame experimental conditions. ing a change in the magnetic and/or metallic properties ofLCMO when grown on BFO. In particular, the observedconductive transient at low T , similar to that observedat high T in LCMO alone, suggests that LCMO is moreinsulating when grown on BFO.This is likely most significant near the LCMO/BFOinterface, where the two materials can directly inter-act [2, 3, 14, 16]. Our UOS measurements can sensi-tively probe this interface. Since our 10 nm LCMO filmis much thinner than the laser absorption depth ( ∼ (a) (b) LCMO BFO LCMO BFMOLCMOLCMOMO MO
P P (c) Rs V G a b θ ij Mn Fe
FIG. 3: Schematic diagrams showing spin tuning through fer-roelectric switching, where we only show the spin componentperpendicular to the interface in BFO [47]. The spin align-ment is depicted in LCMO(FM)/BFO(G-type AFM) withFE polarization (a) towards the interface (darker gray withstronger interaction) and (b) away from the interface (lightergray with weaker interaction). (c) The corresponding mag-netoresistance R s in states (a) and (b) as a function of gatevoltage V g , sketched from Ref. 2. origin of this behavior in LCMO/BFO. Another possi-bility, charge transfer between LCMO and BFO, is pro-hibited in equilibrium, as Ref. 14 measures no changein the valence state of Fe . Photostriction has previ-ously been observed in BFO [46], but is unlikely to affectour results, since both our previous experiments [43] andtime-resolved x-ray diffraction [44] on BFO measured noresponse upon ∼ ∼
5) is also unlikely to influence our results,since the high temperature data ( ≥
165 K) on both sam-ples is very similar (Fig. 1), and thermal diffusion wouldnot be affected by a B field. A more detailed exclusionof these effects is presented in Appendix B. Instead, theconductive transient observed at low temperatures andhigh B fields in LCMO/BFO suggests that polarons playa significant role in the observed phenomena (similar tothe PMI phase of LCMO, Fig. 1(a)), likely by decreasingthe charge kinetic energy near the LCMO/BFO interface.Such polaronic behavior could arise from AFM order, or-bital order or a lattice distortion that creates a trappingpotential, all of which are likely coupled.The most likely explanation for the observed pola-ronic behavior when LCMO was grown on BFO is aninduced AFM order in LCMO (reducing the FM or-der). This can result from interfacial coupling to G-typeBFO, with its alternating spin alignment of Fe ions[14] at the interface, through the interfacial spin-spininteraction J FM-AFM(G) (Fig. 3(a)). For more insight,we consider that in optimally doped manganites, trans-port is governed by the double exchange interaction, inwhich the charge kinetic energy H k is determined by the hopping amplitude t ij (1+ < S i S j > /S ) / = t ij cos( θ ij / S i , S j are nearest neighbor Mn t g spins and θ ij is the angle between them (Fig. 3(a)). In the gen-eral Hamiltonian for Mn e g electrons, hopping is linked tophonons (H ph ) and the electron-phonon coupling (H e − p ),both of which are related to the formation of polarons[25]. As H k surpasses H ph and H e − p , the spins becomemore ordered and the state becomes more metallic, i.e.with increased conductivity. In LCMO/BFO, the in-duced AFM order at the interface will increase θ ij inLCMO (Fig 3(a)), suppressing in-plane electron hopping.This results in a discernible difference between LCMOand LCMO/BFO in a high B field, which increases thealignment of the induced AFM spins [38, 39] while keep-ing the ordered FM spins saturated. We note that neu-tron diffraction measurements on LSMO/BFO superlat-tices grown in the same system at LANL have revealeda reduced magnetization in the LSMO layers, supportingthis hypothesis [48].This picture, based on an induced interfacial AFM or-der in LCMO, can now be used to shed light on previ-ous work in which the magnetotransport was tuned byswitching the FE polarization [2, 16]. In LCMO/BFO,the separation l between Fe and Mn / ions acrossthe interface affects the strength of J FM-AFM(G) andtherefore θ ij (i.e. J FM-AFM(G) exponentially decays as ∼ e − l/d [49], where d is the atomic distance, ∼ ions towards the interface, Fig. 3(a)), the reductionin l enhances the effective ’AFM field’ (the magnitudeof J FM-AFM(G) and θ ij increase), decreasing the hop-ping amplitude. When the FE polarity is switched,the increase in l (Fig. 3(b)) causes the magnitude of J FM-AFM(G) to decrease, increasing the hopping ampli-tude. The change in l between the two FE polarity statesis > ∼ d . Therefore, tunable magnetotrans-port can be observed when switching the FE polarizationin BFO [2, 3] (Fig. 3(c)).Our results thus indicate that polarons originate fromcompeting interfacial FM/AFM order in LCMO/BFO.This mutual spin interaction results in a measurable mag-netic moment in the Fe ion of G-type BFO [14] and aninduced AFM order in the Mn ion of FM LCMO. Theionic distance between these two transition metals can bethen used to tune magnetotransport in these systems.In summary, our UOS studies reveal a difference in theDSWT between single layer LCMO and a LCMO/BFOheterostructure. This indicates that interfacial transportin LCMO/BFO is polaronic in nature and is modified ina magnetic field, while no change is observed in LCMO.In the framework of the double exchange interaction, wesuggest that an interfacial AFM order in LCMO is in-duced by an effective ’AFM field’ arising from G-typeantiferromagnetic ordered BFO, which reduces in-planeelectron hopping. Furthermore, by switching the FE po-larity in BFO, one can change the coupling strength bytuning the separation between Fe-Mn, providing a newavenue for achieving tunable magnetotransport [2, 3, 16]. ACKNOWLEDGMENT
This work was performed at the Center for IntegratedNanotechnologies, a U.S. Department of Energy, Officeof Basic Energy Sciences user facility and under the aus-pices of the Department of Energy, Office of Basic En-ergy Sciences, Division of Material Sciences. It was alsopartially supported by the NNSA’s Laboratory DirectedResearch and Development Program. Los Alamos Na-tional Laboratory, an affirmative action equal opportu-nity employer, is operated by Los Alamos National Secu-rity, LLC, for the National Nuclear Security Administra-tion of the U. S. Department of Energy under contractDE-AC52-06NA25396.
APPENDIX A: DYNAMIC SPECTRAL WEIGHTTRANSFER REVEALED BY DEGENERATEOPTICAL PUMP-PROBE SPECTROSCOPY
Colossal magnetoresistance in manganites originatesfrom a giant change in the conductivity that is controlledby the spin alignment (either through applying an exter-nal magnetic field or going to low temperatures). Thisin turn dominates the optical properties from terahertzto visible frequencies through spectral weight transfer(SWT), as schematically depicted in Fig. 1(b) of themanuscript. SWT is a well established concept in man-ganites (described in more detail in Refs. [25, 28, 32–35, 51]). At low temperatures, the optical conductivityin optimally doped manganites is dominated by a lowfrequency Drude peak, which is a signature of the metal-lic state. As the temperature increases, the amplitude ofthe Drude peak diminishes as the conductivity decreases(due to decreasing spin alignment), and a peak in thenear-to-mid-infrared (IR) concurrently increases, whichhas been widely attributed to the trapping of free car-riers into polaronic states [25, 32–35]. In the high tem-perature insulating phase (above T C ), a strong externalmagnetic (B) field can induce SWT from the IR pola-ronic peak to the low frequency Drude peak by orientingspins (and thus increasing the conductivity). In the lowtemperature metallic phase, the B field will not increasethe conductivity, since the spins are already well aligned.Optical measurements can therefore be significantly in-fluenced by the spin alignment through SWT. This canthen be extended to non-equilibrium optical measure-ments, in which the femtosecond pump pulse introducesheat when the sample is at low temperatures (result- ing in SWT to higher frequencies) and liberates carrierstrapped in polaronic states when the sample is at hightemperatures (initiating SWT to lower frequencies); thisis the idea of dynamic spectral weight transfer (DSWT)described in the main manuscript (and schematically de-picted in Fig. 1(b)). DSWT is a well-known concept thathas been widely shown to dominate the response in ul-trafast optical measurements on manganites [17–20, 24].Further support for this concept comes from previouswork on similar samples: in Figure 14 of Ref. [42], it wasclearly shown that for an LCMO film, the time constantassociated with DSWT was nearly identical when probingat both 1.5 eV and THz frequencies. This is strong evi-dence that a 1.5 eV probe is sensitive to DSWT, even inthe absence of complementary optical-pump, THz-probemeasurements. Therefore, the degenerate 1.5 eV pump-probe measurements described here can clearly revealthe transition from Drude-like to polaronic behavior inLCMO through DSWT, since the origin of the photoin-duced change in optical properties is the same.Finally, we emphasize that the strongest evidence forthe sensitivity of our ultrafast optical reflectivity mea-surements to the spin alignment is given by the field tun-ing experiment displayed in Fig. 3 of our manuscript:while we saw significant field tuning in LCMO/BFO,we observed no change in LCMO, strongly support-ing the field-induced increase in the conductivity ofLCMO/BFO, while the conductivity of LCMO/STO wassaturated as all the spins were already aligned at 10 K. APPENDIX B: EXCLUSION OF OTHERPOSSIBLE CONTRIBUTIONS TO OUR DATA
We have considered the influence of lattice mismatchand/or modifications of the lattice properties due to thechange in the layer underneath LCMO. Therefore, wehave performed temperature-dependent ultrafast opticalmeasurements of LC(S)MO on MgO (on which mangan-ites are relaxed, as measured with x-ray diffraction) andon STO (on which manganites are strained, also mea-sured with x-ray diffraction). We found that the resultswere very similar for different substrates, indicating thatsimple lattice mismatch or random interfacial disordershould not influence the observed data. We also con-sidered the effect of changes in the thermal diffusivityon our data. However, we can exclude this possibility,since our data on LCMO/BFO/STO and LCMO/STOis very similar at higher temperatures (Fig. 1(a) and1(c)), despite the fact that the thermal conductivitiesof BFO ( ∼ ∼ /K) and STO ( ∼ /K) at 300 K lead to a factor of ∼ ∼ /s) and STO ( ∼ /s). This should lead to a corresponding difference inthe time constant associated with thermal decay in ourdata, but we observed no significant difference (traces at300 K in Fig. 1(a) and 1(c)). Furthermore, the differencein thermal diffusivity increases with decreasing tempera-ture [52] while the specific heat of BFO varies little andthat of STO decreases 30% between 165 and 300 K, butour LCMO/BFO/STO and LCMO/STO data remainsquite similar down to ∼
165 K, making it very unlikelythat this would significantly influence our results (partic-ularly since the 50 nm BFO layer is much thinner thanthe STO substrate, which thus serves as a heat sink).Finally, because an AFM material has no response toan external B field of the magnitude used in our experi-ments, if the thermal diffusivity of BFO is the origin ofthe observed difference, then we would not expect to seethe B field dependence shown in Fig. 2.It is also worth considering the possible influence ofphotostrictive effects on our data, as they have been pre-viously observed in BFO [46]. In this context, it is im-portant to emphasize that our optical pump pulses onlyphotoexcite the LCMO layer, since the pump photon en-ergy (1.59 eV) is much lower than the bandgap ( ∼ ∗ Electronic address: [email protected] † Electronic address: [email protected][1] W. Eerenstein, M. Wiora, J. L. Prieto, J. F. Scott, andN. D. Mathur, Nat. Mater. , 348 (2007).[2] S. M. Wu, S. A. Cybart, P. Yu, M. D. 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