Quantitative disentanglement of spin Seebeck, proximity-induced and intrinsic anomalous Nernst effect in NM/FM bilayers
Panagiota Bougiatioti, Christoph Klewe, Daniel Meier, Orestis Manos, Olga Kuschel, Joachim Wollschläger, Laurence Bouchenoire, Simon D. Brown, Jan-Michael Schmalhorst, Günter Reiss, Timo Kuschel
QQuantitative disentanglement of spin Seebeck, proximity-induced and intrinsicanomalous Nernst effect in NM/FM bilayers
Panagiota Bougiatioti , Christoph Klewe , , Daniel Meier , Orestis Manos , Olga Kuschel , Joachim Wollschl¨ager ,Laurence Bouchenoire , , Simon D. Brown , , Jan-Michael Schmalhorst , G¨unter Reiss , and Timo Kuschel , Center for Spinelectronic Materials and Devices, Department of Physics,Bielefeld University, Universit¨atsstraße 25, 33615 Bielefeld, Germany Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA Department of Physics and Center of Physics and Chemistry of New Materials,Osnabr¨uck University, Barbarastrasse 7, 49076 Osnabr¨uck, Germany XMaS, European Synchrotron Radiation Facility, Grenoble, 38043, France Department of Physics, University of Liverpool, Liverpool L69 7ZE, UK Physics of Nanodevices, Zernike Institue for Advanced Materials,University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands (Dated: April 27, 2018)We identify and investigate thermal spin transport phenomena in sputter-deposited Pt/NiFe O (4 ≥ x ≥
0) bilayers. We separate the voltage generated by the spin Seebeck effect from theanomalous Nernst effect contributions and even disentangle the intrinsic anomalous Nernst effect(ANE) in the ferromagnet (FM) from the ANE produced by the Pt that is spin polarized due to itsproximity to the FM. Further, we probe the dependence of these effects on the electrical conductivityand the band gap energy of the FM film varying from nearly insulating NiFe O to metallic Ni Fe .A proximity-induced ANE could only be identified in the metallic Pt/Ni Fe bilayer in contrastto Pt/NiFe O x ( x >
0) samples. This is verified by the investigation of static magnetic proximityeffects via x-ray resonant magnetic reflectivity.
In the emerging fields of spintronics [1] and spincaloritronics [2] phenomena such as the spin Hall effect(SHE) [3] and the spin Seebeck effect (SSE) [4, 5] en-able the generation, manipulation and detection of spincurrents in ferro(i)magnetic insulators (FMI). The mostcommon path to detect a spin current is to use a nor-mal metal (NM) with a large spin Hall angle, such as Pt[6], Ta [7], Pd [8] and W [9] on top of an FM material.The inverse spin Hall effect (ISHE) [10] then leads to theconversion of the spin current into a transverse chargevoltage in the NM.Pt is employed frequently for generating and detect-ing pure spin currents, if adjacent to an FMI, althoughthe possibility of magnetic proximity effects (MPEs) hasto be taken into account. Due to its close vicinity tothe Stoner criterion [11] the FM can potentially gener-ate a Pt spin polarization at the interface. Consequently,this might induce additional parasitic effects preventingthe correct interpretation of the measured ISHE voltage.Therefore, a comprehensive investigation regarding themagnetic properties of the NM/FM interface is requiredto distinguish the contributions of such parasitic voltagesfrom the ISHE voltage generated by a pure spin current.In the case of SSE, the driving force for the spin currentin the FM or FMI is a temperature gradient. When a spincurrent is generated parallel to a temperature gradient,it is generally attributed to the longitudinal spin Seebeckeffect (LSSE) [4, 5]. However, when using the ISHE in anadjacent NM for the spin current detection, not only aproximity-induced ANE [12] can contaminate the LSSEsignal, but also an additional intrinsic ANE contribution could be present in case of studying ferromagnetic met-als (FMMs) or semiconducting ferro(i)magnets [13, 14].Mainly NM/FMI bilayers have been investigated, whileLSSE studies on NM/FMM are quite rare.However, Ramos et al . [14–17] and Wu et al . [18] in-dividually investigated the LSSE in magnetite, which isconducting at room temperature (RT) and, thus, has anintrinsic ANE contribution. They identified the LSSEin Pt/Fe O [14] and CoFeB/Fe O bilayers [18] by us-ing temperatures below the conductor-insulator transi-tion of magnetite (Verwey transition at 120 K) in orderto exclude any intrinsic ANE contribution. Ramos et al .further investigated the ANE in bulk magnetite withoutany Pt [15] and concluded that the ANE contributionsfor Pt/Fe O bilayers and multilayers should be quitesmall [16, 17]. In addition, Lee et al . [19] and Uchida etal . [20, 21] discussed that in Pt/FMM multilayers bothLSSE and ANE contribute, but did not disentangle theeffects quantitatively. Hence, a clear quantitative disen-tanglement of the LSSE in the FMM [22], the intrinsicANE in the FMM, and the proximity-induced ANE inthe NM is still pending.Some groups used Cu or Au interlayers to suppressthe MPE in NM/FMM bilayers [23–25]. However, apromising technique to distinguish between LSSE andproximity-induced ANE was first proposed by Kikkawa et al . [23, 26]. In their study, the voltage measuredtransverse to the thermal gradient in in-plane magnetized(IPM) and out-of-plane magnetized (OPM) configura-tions, leads to the sufficient separation of the aforestatedcontributions. So far, this technique was only used to a r X i v : . [ c ond - m a t . m t r l - s c i ] F e b study the proximity-induced ANE in NM/FMI bilay-ers. It has not yet been applied to fully conductingNM/FMM bilayers for the separation of the LSSE andANE contributions in the FMM. In our work, we extendthis technique to identify all three contributions quanti-tatively: LSSE, intrinsic ANE in the FM, and proximity-induced ANE. We will use this separation for investigat-ing these effects in Pt on different FM materials such asnearly-insulating NiFe O , semiconducting-like NiFe O x (4 > x > Fe .To confirm or exclude any possible static MPE at theinterface of a Pt/FM hybrid structure, element-selectivex-ray resonant magnetic reflectivity (XRMR) has beenused due to its sensitivity to magnetic moments at inter-faces [27, 28]. XRMR measurements were performed atthe XMaS beamline BM28 at ESRF (Grenoble, France)[29], at RT. Details for the XRMR technique, experimentand data processing can be found in the SupplementalMaterials II [30] (including Ref. [31]).We fabricated the films on MgAl O (MAO) substratesby reactive sputter deposition [32] starting from purehigh-resistive NiFe O (NFO) ( ∼
160 nm) up to themetallic Ni Fe (10.4 nm) with intermediate NiFe O x (60 nm) and NiFe O x (35 nm), with 4 > x > x > ∇ T inthe presence of an in-plane magnetic field along the x-axisinduces a transverse voltage along the y-axis. While mea-suring in this IPM configuration with Pt on top (IPM -Pt, Fig. 1(a)) we detect the LSSE voltage togetherwith both ANE contributions, intrinsic and proximity-induced. However, in the IPM geometry without Pt(IPM - no Pt, Fig. 1(c)) we are only sensitive to the in-trinsic ANE contribution.The LSSE voltage is determined according to the rela-tion E ISHE = S SSE J s × s (1)where E ISHE , S SSE , J s , and s denote the electric fieldinduced by ISHE, the SSE coefficient, the spin currentwhich enters the spin detector material and the spin po-larization vector, respectively. Moreover, the ANE con-tribution is described by the relation E ANE = D ANE ∇ T × M (2)where E ANE , D ANE , and M denote the electric field in-duced by ANE, the ANE coefficient, and the magnetiza-tion vector of the FM, respectively. LSSE +ANE redintr +ANE redprox
ANE intr - 𝐵 𝐴 ∙ 𝐴 ∙ ANE redprox (d) y z x 𝛻T L V M PtFM
MAO L T + V - L V M PtFM MAO 𝛻T + - 𝛻T V L V M FM MAO
IPM - no Pt(b) OPM - Pt (a) IPM - Pt (c) - 𝐴 ∙ (ANE redintr + ANE redprox ) V + - Normalization to the distance 𝐋 𝐕 and to the heat flux ANE redintr + ANE redprox
LSSE
𝐀𝐍𝐄 𝐢𝐧𝐭𝐫
𝐀𝐍𝐄 𝐩𝐫𝐨𝐱 φ q FIG. 1. Schematic illustration of (a),(c) in-plane magnetizedand (b) out-of-plane magnetized geometries, introducing thetemperature gradient ∇ T , the magnetization vector M , thedistance between the contacts L V and the total length of thesample L T , respectively. (d) Flow chart for the quantitativeseparation of both ANE contributions from the LSSE voltage.The light green and grey areas correspond to the intermediatesteps determining the correction factors A and B respectively,taking into account the reduction of the ANE signal due tothe additional Pt layer (spin polarized and/or non-magnetic). In the OPM geometry with Pt on top (OPM - Pt, Fig.1(b)), the application of an in-plane temperature gradi-ent ∇ T together with an out-of-plane magnetic field gen-erates a transverse voltage attributed to the intrinsic andproximity-induced ANE. In this configuration, the LSSEcan not be detected, since no out-of-plane spin currentwith the proper spin polarization direction is generated[23]. One major issue is to consider the reduction of theANE signal upon a placement of a Pt layer [14]. All ANEcontributions measured with Pt on top have in general re-duced contributions and this is indicated by the subscript“red” in Fig. 1 and throughout the whole manuscript.Figure 1(d) explains the flow chart for the quantitativedisentanglement of the three effects. As a first step, theelectric field is calculated from the measured voltages bynormalizing to the distance of the electric contacts L V .Then, this electric field is divided by the heat flux φ q that runs through the sample. The normalization to theheat flux as suggested by Sola et al [33, 34], allows elim-inating the systematic errors due to the thermal surfaceresistances and thermal contacts resulting in the effec-tive comparison between IPM and OPM configurationsas well as in the comparability of our results. Further de-tails on the heat flux normalization can be found in theSupplemental Materials III [30] (including Refs. [35, 36]).To estimate the ANE reduction due to the additional Ptlayer we used the ratio of conductances G of the NiFe O x and the Pt in a parallel arrangement [14] r = G NiFe O x G Pt = ρ Pt ρ NiFe O x t NiFe O x t Pt (3)with ρ : RT resistivity and t : thickness of the cor-responding layer. The reduced intrinsic ANE signal(ANE intrred ) from the OPM - Pt configuration is then cor-rected by the factor A = r +1 r [14] resulting in the pureANE intr = A · ANE intrred . This correction step in our cal-culations is highlighted by the light green area in Fig.1(d). Combined with the information on the ANE intr from the IPM - no Pt configuration (cf. Fig. 1(c)), i.e.,by subtracting the ANE intr from the corrected term, thismethod already yields a qualitative criterion for the exis-tence or absence of proximity-induced ANE in the sam-ple.For a quantitative evaluation, an additional correctionhas to be applied to the reduced proximity-induced ANEsignal (ANE proxred ) due to the additional non-magnetic Ptlayer, while the correction A on the term has to be re-versed (see light grey area in Fig. 1(d)). - 4 - 2 0 2 4- 0 . 8- 0 . 40 . 00 . 40 . 8 - 0 . 1 0 . 0 0 . 1- 0 . 4- 0 . 20 . 00 . 20 . 4 I P M - n o P t ( a )
Vnorm(=VLV-1 f q-1) (10-4mV W-1 m) L S S E + A N E i n t rr e d + A N E p r o xr e d
A N E i n t r - 1 0 - 5 0 5 1 0- 0 . 2- 0 . 10 . 00 . 10 . 2 ( c o r r e c t e d ) O P M - P tO P M - P t ( c ) ( b )
A N E i n t rr e d + A N E p r o xr e d
A N E i n t r + A N E p r o x - 0 . 0 2- 0 . 0 10 . 0 00 . 0 10 . 0 2
Vnorm(=VLV-1 f q-1) (10-4mV W-1 m) I P M - P t I P M - n o P t
H ( k O e )
L S S E + A N E i n t rr e d + A N E p r o xr e d
A N E i n t r
I P M - P t - 4 - 2 0 2 4- 0 . 6- 0 . 4- 0 . 20 . 00 . 20 . 4 ( c o r r e c t e d ) ( d )
H ( k O e ) - 0 . 6- 0 . 4- 0 . 20 . 00 . 20 . 4
A N E i n t rr e d + A N E p r o xr e d
A N E i n t r + A N E p r o x
FIG. 2. Normalized voltage plotted against the magnetic fieldstrength for (a), (b) Pt/NiFe O x and (c), (d) Pt/Ni Fe bilayers measured in (a), (c) IPM and (b), (d) OPM geome-tries with the corresponding separation of the ANE contribu-tion (intrinsic and proximity-induced) from the LSSE voltage.ANE intr + ANE prox (purple) regards the calculated ANE sig-nal after the implementation of the correction factors A and B , which correct the reduction of the measured ANE from theOPM - Pt configuration due to the additional Pt layer (spinpolarized and/or non-magnetic). The correction factor for the ANE proxred is given by B = d I + d II d I [14], where d I and d II are the thicknessesof the spin polarized Pt layer and the non-magnetic frac-tion, respectively, estimated by XRMR. Then, the cor-rected proximity-induced ANE contribution is denoted asANE prox = ( B/A ) · A · ANE proxred . For the polarized and unpolarized fraction of the Pt layers, the same resistivity ρ Pt was used.Exemplarily, for the Pt/Ni Fe (Pt/NiFe O x ) sam-ple the reduction of the ANE intr is estimated to be 47%(95%) by using the measured values for the RT resistiv-ity of Pt equal to ρ Pt = 1 . · − Ωm (1 . · − Ωm)for a Pt film with thickness t Pt = 3 . ρ Ni Fe (NiFe O x2 ) = 4 . · − Ωm(4 . · − Ωm) for a FM thickness of t Ni Fe (NiFe O x2 ) =10.4 nm (35 nm). Moreover, for the metallic Pt/Ni Fe bilayer the reduction of the ANE prox is estimated to be71% by considering d I = 1 . d II = 2 . O x and Pt/Ni Fe bilayers. ForPt/NiFe O x (Fig. 2(a)), the LSSE is the most promi-nent contribution to the total voltage signal, while for Pton metallic Ni Fe , the intrinsic ANE and the LSSEare of comparable magnitude (Fig. 2(c)). By compar-ing the difference between the ANE intr from the IPM -no Pt configuration and the ANE intr + ANE prox sig-nals (corrected ANE intrred + ANE proxred by A and B , as ex-plained above) we are able to quantitatively determinethe contribution from the proximity-induced ANE. Forthe non-metallic NiFe O x bilayer (Fig. 2(a),(b)) no dif-ference can be determined between the saturation val-ues of the ANE intr data from IPM - no Pt configuration(orange line in Fig. 2(a)) and the saturation valuesof the ANE intr + ANE prox signal (corrected OPM - Ptdata, purple line in Fig. 2(b)), which are extracted tobe V satnorm = (0 . ± .
02) 10 − mVW − m in both cases.Thus, the ANE prox is zero and can be neglected for thissample. On the contrary, for the Pt/Ni Fe bilayer(Fig. 2(c),(d)) the ANE intr + ANE prox is (46 ± intr signal unveiling the existence of MPE.Furthermore, for the Pt/NFO bilayer both ANE intr andANE intr + ANE prox signals are zero confirming the ab-sence of any ANE contribution in the pure Pt/NFO bi-layer [28, 37].Figure 3 illustrates the linear dependence of the volt-age in saturation on φ q , normalized to L V for all samples.The dashed lines are the calculated contributions of thepure LSSE and ANE prox extracted as described in thediagram of Fig. 1(d) after correcting the reduced ANEsignal arising from both the FM and the spin polarizedPt layer. In Fig. 3(a), the zero line contribution of bothtypes of ANE indicates the absence of MPE in Pt/NFObilayers [28, 37]. The low amount of mobile charge car-riers in the nearly-insulating NFO leads to a vanishingANE intr contribution [13].As shown in Figs. 3(a)-(c), the LSSE contribution is LSSE + ANE redintr + ANE redprox
Measured:
ANE intr
Calculated:
LSSEANE prox
ANE intr + ANE prox
ANE red intr + ANE red prox (c) (b) q (10 Wm -2 ) (a) Pt/NiFe O x Pt/NiFe O q (10 Wm -2 ) Pt/NiFe O x (d) V s a t no r m ( = V / L V )( m V / m ) Pt/NFO
FIG. 3. Normalized voltage in saturation against the heat fluxfor (a) Pt/NFO, (b),(c) Pt/NiFe O x / x , and (d) Pt/Ni Fe samples with the corresponding separation of the ANE con-tribution (intrinsic and proximity-induced) from the LSSEvoltage. dominant for all Pt/NiFe O x ( x >
0) bilayers that consistof oxides. Furthermore, the absence of any proximity-induced ANE is verified, since no difference between theANE intr and the ANE intr + ANE prox can be identified.Additionally, for the Pt/NiFe O x bilayer the ANE intr contribution is 14% larger than for the Pt/NiFe O x bi-layer pointing towards its more conducting character.For the Pt/Ni Fe bilayer (Fig. 3(d)), the enhancementof ANE intr + ANE prox due to the metallic character ofNi Fe and the MPE contribution is clearly displayed.Figure 4(a) shows the SSE ( S SSE = V sat norm φ q ) and ANE( D ANE = V sat norm φ q ) coefficients extracted from the corre-sponding slopes of the curves in Fig. 3, plotted againstthe RT value for the measured electrical conductivity.There is a pronounced increase of the D ANE when theconductivity increases, whereas the S SSE decreases.Figure 4(b) depicts the dependence of the SSE andANE coefficients on the optical band gap for the NFOand NiFe O x / x bilayers. A short description of theband gab determination can be found in the Supplemen-tal Materials V [30] (including Refs. [13, 32, 38–40]).It is clearly observed that the more conducting samplesare characterized by lower band gap energies, reflectingthe existence of additional electronic states in the bandgap. Additionally, the ANE intr coefficient increases fordecreasing band gap energy verifying the previous as-sumption of more mobile charge carriers at a reducedoxygen concentration. On the contrary, the SSE coeffi-cient increases for larger band gap energies. (b) SSE coefficient ANE coefficient
Band gap energy (eV) S SS E ( - m V W - m ) D AN E ( - m V W - m ) -2 -1 ( -1 m -1 ) S SS E ( - m V W - m ) D AN E ( - m V W - m ) (a) FIG. 4. SSE and ANE coefficients as a function of (a) theelectrical conductivity σ for NiFe O x / x (blue area), NFO(orange area) and Ni Fe (green area) samples and (b) theoptical band gap for NiFe O x / x and NFO samples. -1 ) a s y mm e t r y r a ti o ( a r b . un it s ) -202 a s y mm e t r y r a ti o ( a r b . un it s ) -1 )4x10 -2 -7 ∆β and ∆δ Pt (3.5nm)Ni Fe (10.4nm)0.8 (b) exp. datasimulation (c) Pt/NFO a s y mm e t r y r a ti o ( a r b . un it s ) -101-2 0.50.40.30.20.1scattering vector q (Å -1 )2x10 -2 (d) Pt/NFO
5% of Pt/Ni Fe spin polarization (a) Pt/Ni Fe -4 1.0 nm FIG. 5. (a) XRMR asymmetry ratio for Pt/Ni Fe andsimulation with the corresponding magnetooptic depth profile(b). (c) XRMR asymmetry ratio for Pt/NFO after using themagnetooptic depth profile of (b), (d) assuming 5% of thePt/Ni Fe spin polarization. The absence of MPE in Pt/NFO, Pt/NiFe O x / x sam-ples and the presence of MPE in the metallic Pt/Ni Fe bilayer is now confirmed by XRMR (Fig. 5). InFig. 5(a) the measured XRMR asymmetry ratio for thePt/Ni Fe bilayer is displayed. From the correspond-ing fitting and by comparing the experimental fit val-ues of ∆ δ and ∆ β derived from the magnetooptic depthprofile in Fig. 5(b) to ab initio calculations [28], we ob-tain a maximum Pt magnetic moment of (0 . ± . µ B per spin polarized Pt atom, consistent with earlier re-sults [41]. The effective spin polarized Pt thickness iscalculated to be (1 . ± .
1) nm similar to our previousinvestigations [41].In Fig. 5(c) the measured XRMR asymmetry ratio forthe Pt/NFO bilayer is presented along with a simulationusing a magnetooptic depth profile identical to the onederived for the Pt/Ni Fe bilayer. Obviously, the simu-lated asymmetry ratio of the Pt/NFO sample (Fig. 5(c))deviates strongly from the one of the Pt/Ni Fe sam-ple (Fig. 5(a)), although the same magnetooptic depthprofile (Fig. 5(b)) was used. This is due to the differentoptical constants of Ni Fe and NFO. Since no asym-metry was detected for the Pt/NFO sample, a potentialMPE present in this film must be significantly smallerthan in the all-metallic system.By decreasing the magnitude of the magnetooptic pa-rameters down to 5% of the Pt/Ni Fe spin polarization(Fig. 5(d)), we can estimate a detection limit leading toan upper limit for the maximum magnetic moment inPt of 0.04 µ B per spin polarized Pt atom. Moreover, forthe Pt/NiFe O x and Pt/NiFe O x samples a detectionlimit of 0.1 µ B and 0.01 µ B per spin polarized Pt atomis extracted in the same way, see Supplemental Mate-rials II [30]. Finally, possible MPEs can be neglecteddown to these limits for all samples except for the metal-lic Pt/Ni Fe bilayer, where a distinct spin polarizationin the Pt layer can be observed.In conclusion, we investigated thermal spin transportphenomena in Pt/FM bilayers and separated the intrinsicANE in the FM and proximity-induced ANE contribu-tions quantitatively from the LSSE for sputter-depositedNiFe O x bilayers. This new compact procedure is basedon the preparation of twin samples (with and withoutPt), different measurement geometries, the normalizationto the heat flux instead of the thermal gradient, and thedetermination of important correction factors to obtainquantitative LSSE and ANE values. In our work, we ex-tracted the dependence of the LSSE and intrinsic ANEcoefficients on the band gap energy and on the electri-cal conductivity of the samples. Furthermore, possiblestatic MPE in Pt were studied via XRMR. We found nomagnetic response down to our detection limits of 0.04 µ B , 0.1 µ B and 0.01 µ B per spin polarized Pt atom forPt/NFO, Pt/NiFe O x and Pt/NiFe O x , respectively.For the Pt/Ni Fe we calculated a maximum magneticmoment of 0.48 µ B per spin polarized Pt atom. AllXRMR results are well in line with the absence/presenceof proximity-induced ANE contributions. In a next stepthis technique of thermal transport effect separation al-lows to study the individual transport effects dependingon other properties of the samples, e.g., thicknesses androughnesses. 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