The longitudinal spin Seebeck effect is detected in sputter-deposited NiFe2O4
films using Pt as a spin detector and compared to previously investigated
NiFe2O4 films prepared by chemical vapor deposition. Anomalous Nernst effects
induced by the magnetic proximity effect in Pt can be excluded for the
sputter-deposited NiFe2O4 films down to a certain limit, since x-ray resonant
magnetic reflectivity measurements show no magnetic response down to a limit of
0.04 {\mu}B per Pt atom comparable to the case of the chemicallydeposited
NiFe2O4 films. These differently prepared films have various thicknesses.
Therefore, we further studied Pt/Fe reference samples with various Fe
thicknesses and could confirm that the magnetic proximity effect is only
induced by the interface properties of the magnetic material.Comment: 4 pages, 4 figure

Bouchenoire, Laurence

Bougiatioti, Panagiota

Brown, Simon D.

Klewe, Christoph

Kuschel, Olga

Kuschel, Timo

Meier, Daniel

Reiss, Günter

Schmalhorst, Jan-Michael

Wollschläger, Joachim

IEEE Transactions on Magnetics

English

arXiv

Timo Kuschel

Christoph Klewe

Panagiota Bougiatioti

Olga Kuschel

Joachim Wollschlager

Laurence Bouchenoire

Simon D. Brown

Jan-Michael Schmalhorst

Daniel Meier

Gunter Reiss

Crossref

Static Magnetic Proximity Effect in Pt Layers on Sputter-Deposited NiFe2O4and on Fe of Various Thicknesses Investigated by XRMR

The longitudinal spin Seebeck effect is detected in sputter-deposited NiFe2O4
films using Pt as a spin detector and compared to previously investigated
NiFe2O4 films prepared by chemical vapor deposition. Anomalous Nernst effects
induced by the magnetic proximity effect in Pt can be excluded for the
sputter-deposited NiFe2O4 films down to a certain limit, since x-ray resonant
magnetic reflectivity measurements show no magnetic response down to a limit of
0.04 {\mu}B per Pt atom comparable to the case of the chemicallydeposited
NiFe2O4 films. These differently prepared films have various thicknesses.
Therefore, we further studied Pt/Fe reference samples with various Fe
thicknesses and could confirm that the magnetic proximity effect is only
induced by the interface properties of the magnetic material

Wollschlaeger, Joachim

Brown, Simon D

Reiss, Guenter

University of Liverpool Repository

arXiv:1602.07895v1  [cond-mat.mes-hall]  25 Feb 2016IEEE TRANSACTIONS ON MAGNETICS 1Static Magnetic Proximity Effect in Pt Layers on Sputter-DepositedNiFe2O4 and on Fe of Various Thicknesses Investigated by XRMRTimo Kuschel1, Christoph Klewe1, Panagiota Bougiatioti1, Olga Kuschel2,3, Joachim Wollschla¨ger2,3,Laurence Bouchenoire4,5, Simon D. Brown4,5, Jan-Michael Schmalhorst1, Daniel Meier1, and Gu¨nter Reiss11Center for Spinelectronic Materials and Devices, Department of Physics, Bielefeld University, 33615 Bielefeld, Germany2Fachbereich Physik, Universita¨t Osnabru¨ck, 49069 Osnabru¨ck, Germany3Center of Physics and Chemistry of New Materials, Universita¨t Osnabru¨ck, 49069 Osnabru¨ck, Germany4XMaS, European Synchrotron Radiation Facility, Grenoble, 38043, France5Department of Physics, University of Liverpool, Liverpool, L69 7ZE, United KingdomThe longitudinal spin Seebeck effect is detected in sputter-deposited NiFe2O4 films using Pt as a spin detector and compared topreviously investigated NiFe2O4 films prepared by chemical vapor deposition. Anomalous Nernst effects induced by the magneticproximity effect in Pt can be excluded for the sputter-deposited NiFe2O4 films down to a certain limit, since x-ray resonant magneticreflectivity measurements show no magnetic response down to a limit of 0.04µB per Pt atom comparable to the case of the chemicallydeposited NiFe2O4 films. These differently prepared films have various thicknesses. Therefore, we further studied Pt/Fe referencesamples with various Fe thicknesses and could confirm that the magnetic proximity effect is only induced by the interface propertiesof the magnetic material.Index Terms—magnetic proximity effect, spin Seebeck effect, magnetic insulators, x-ray resonant magnetic reflectivity.I. INTRODUCTIONTHE SPIN SEEBECK EFFECT [1] has become an im-portant tool in spintronics [2] and spin caloritronics[3] to generate thermally driven spin currents. However, theexistence of the transverse spin Seebeck effect using in-planethermal gradients could neither be confirmed for magneticmetals ([4], [5], [6], [7], [8]), semiconductors [9], nor in-sulators [10]. All measured response could be related to theanomalous Nernst effect (ANE) [4] or to the longitudinal spinSeebeck effect [10], if there are unintended out-of-plane tem-perature gradients involved ([6], [7]). Recently, an additionalunintended contribution of the anisotropic magnetoresistancebased on inhomogeneous magnetic fields has been added [8]to the list of uncertainties in transverse spin Seebeck effectexperiments.The longitudinal spin Seebeck effect (LSSE) is more reliableand could be confirmed by a large number of groups ([11],[12], [13], [14], [15]). Here, the thermal gradient is typicallyaligned out-of-plane. So far, the LSSE was investigated inseveral iron garnets ([14], [15], [16]) and ferrites ([11], [12],[17], [18], [19], [20]). In most cases the thermally inducedspin current is detected by the inverse spin Hall effect (ISHE)[21] in an adjacent material such as Pt. Since Pt is close to theStoner criterion, it can easily be spin polarized at the interfacewhich could lead to a magnetic proximity effect (MPE) and,therefore, to an MPE induced ANE in LSSE experiments [22].The MPE in Pt adjacent to magnetic metals has beeninvestigated intensely by x-ray magnetic circular dichroism(XMCD), while such studies for Pt on magnetic insulators arestill rare. So far, unclear results are reported for Pt on yttriumManuscript received November 6, 2015. Corresponding author: T. Kuschel(email: tkuschel@physik.uni-bielefeld.de.)iron garnet ([23], [24]), while the MPE for Pt on CoFe2O4could recently be excluded using XMCD [25].In case of NiFe2O4 (NFO) deposited by chemical vapordeposition (CVD) we excluded the MPE in Pt using x-ray resonant magnetic reflectivity (XRMR) [26]. We furtherdemonstrated that XRMR gives a magnetic response indepen-dent of the Pt thickness by investigating a Pt(x)/Fe series withx from 1.8 nm to 20 nm [26]. This interface-sensitivity basedon the interference of reflected light at the interfaces of thebilayers is more advantageous compared to standard XMCDfluorescence measurements. This is, because the magneticcircular dichroism in reflection generates slightly different x-ray reflectivity (XRR) for opposite magnetization directions(±) due to a change of the optical constants ±∆δ and±∆β of the spin polarized material with the refractive indexn = 1− δ + iβ (δ: dispersion, β: absorption).In this paper, we investigate the MPE in Pt on sputter-deposited NFO [27] and use the results for the interpretationof LSSE measurements in this system. The sputter-depositedNFO films are typically thinner compared to the CVD pre-pared samples and have larger coercive fields. Therefore, weadditionally study the thickness dependence of the MPE inPt/Fe(x) reference samples with x from 1.1 nm to 18.2 nm.II. EXPERIMENTAL BACKGROUND AND DATA PROCESSINGFe was deposited on MgAl2O4(001) (MAO) substrates bydc magnetron sputtering in Ar+ atmosphere in the range of2 × 10−3 mbar. We prepared the NFO films on MAO byreactive co-sputter deposition in a pure oxygen atmosphereof the same pressure [27] prior the in-situ sputter depositionof Pt. The magnetic moment of the sputter-deposited NFOfilms determined by alternating gradient magnetometer is244 emu/ccm in the same range as for our other sputter-deposited NFO films using the same deposition parametersIEEE TRANSACTIONS ON MAGNETICS 2[27]. This value also agrees with the magnetic moment of theCVD prepared NFO samples as deduced from the vibratingsample magnetometer measurements presented in Ref. [26](using memu instead of µemu in Fig. 1(d) of Ref. [26], whichwas falsly labeled).XRMR was measured at room temperature at the XMaSbeamline BM28 [28] at ESRF (Grenoble, France) in θ − 2θscattering geometry using circularly polarized x-rays withthe off-resonant (11465 eV) and resonant (11565 eV) photonenergy regarding the Pt L3 absorption edge. For each angleof incidence θ an external magnetic field of ±200mT wasapplied in the scattering plane parallel to the sample surface,while the reflected intensity I± was detected. The degree ofcircular polarization of the x-rays was (88± 1)% as deducedfrom a model to describe the performance of phase-plates [29].The dependence of the non-magnetic XRR intensity I(zero magnetic field) and the magnetic XRMR asymmetryratio ∆I = I+−I−I++I−on the scattering vector q = 4 piλsin θ(λ: wavelength) was simulated with ReMagX [30]. The off-resonant XRR curves are fitted using literature values forthe optical constants. Thus, the obtained structural parameterssuch as thicknesses and roughnesses are used to fit the resonantXRR curves and determine the resonant optical parameters.As a last step, the resonant XRMR asymmetry ratios arefitted using the previouly obtained parameters, while varyingmagnetooptic profiles for the change of optical constants ∆βand ∆δ [31]. In order to obtain the magnetic moment of thePt, we compared the change of ∆β and ∆δ to theoreticalcalculations which have been done before [26].III. RESULTS AND DISCUSSIONIn Fig. 1(a) the off-resonant non-magnetic XRR intensityI(q) of Pt/Fe/MAO shows Kiessig oscillations for Pt(3.2 nm)with roughness of 0.4 nm and Fe(18.2 nm) with roughnessof 0.5 nm. The roughness of the substrate is 0.2 nm. Theresonant XRMR asymmetry ratio ∆I(q) in Fig. 1(b) detectedat 11565 eV is about 2% and changes sign when the helicity isreversed, which confirms the magnetic origin of the effect. Thechosen energy is slightly below the maximum of the absorptionedge (cf. Fig. 1(c)), since here the magnetic dichroism of thespin polarized Pt is maximal [26]. Compared to Ref. [26]the energy position of the absorption maximum is slightlyshifted due to small differences in the energy calibration. Thewhiteline intensity (ratio of absorption maximum and edgejump) in Fig. 1(c) is 1.32, which indicates a mainly metallicstate for Pt [26]. Furthermore, we varied the energy for a fixedscattering vector q = 0.22 A˚−1 at a maximum position of theasymmetry ratio (cf. Fig. 1(b)) as presented in Fig. 1(d) toillustrate the XRMR energy dependence. The effect is maximalnear the absorption maximum and vanishes at energies of morethan 20 eV below and above as also confirmed by a series ofXRMR scans for different energies [31].In total, we investigated Pt(3.3 ± 0.1) nm/Fe(x) for x =1.1 nm, 5.7 nm and 18.2 nm. The non-magnetic resonant XRRresults are presented in Fig. 2(a), (c) and (e) with similarroughnesses as mentioned before. The whiteline intensities ofthe absorption edges of the 1.1 nm and the 5.7 nm sample areeven lower (1.31 and 1.27) compared to the 18.2 nm sample.The resonant XRMR asymmetry ratios in Fig. 2(b), (d) and(f) are all in the range of about 2%, independent of the Fethickness. This result confirms that the Pt spin polarization isonly affected by the interface properties. The XRR model inFig. 2(g) and the magnetooptic profiles in Fig. 2(h) were usedas input parameters in the fits. There is excellent agreementbetween the models and both XRR and XRMR data. Themaximum in ∆β is slightly smaller for the thickest Fe film butall results are still comparable to previously found asymmetryratios ([26], [31]), if the degree of circular polarization is takeninto account (here: (88± 1)%, Ref. ([26], [31]): (99± 1)%).Since the energy for the XRMR measurements was chosenslightly below the absorption maximum, we used a fixed ratioof ∆β/∆δ = −3.5 for the fitting (cf. Ref. [26]). Using thecalibration of Ref. [26], we obtain a maximum Pt magneticmoment of (0.5± 0.1)µB for all samples.The LSSE curves of the sputter-deposited NFO sample arepresented in Fig. 3(a) for various temperature gradients. Thesemagnetic field loops have larger coercive fields [27] comparedto CVD prepared NFO [26]. However, we still reach 70%of magnetic saturation in the XRMR experiments using themagnetic field of ±200mT which is sufficient to observedichroic effects. The linear dependence of the LSSE on thetemperature difference (cf. Fig. 3(b)) and the typical cosineangular dependence in magnetic saturation (cf. Fig. 3(c))confirm the LSSE being of the same order as for CVDprepared NFO samples [11]. Since the pure sputter-depositedNFO film (without Pt layer) is insulating within the accuracyof our resistance measurement, we can exclude any ANE(a)fixed q atq=0.2211565 eVFig. 1. (a) Off-resonant (11465 eV) non-magnetic XRR intensity I and fitfor Pt/Fe/MAO. (b) Resonant (11565 eV) asymmetry ratio ∆I(q) for bothcircular polarizations of the x-rays. (c) Fluorescence spectrum at the Pt L3edge normalized to the edge jump. (d) Magnetic reflectivity for variation ofthe photon energy for a fixed value of q = 0.22 A˚−1.IEEE TRANSACTIONS ON MAGNETICS 3coming from the NFO material itself. Therefore, we checkthe Pt/NFO bilayer for a MPE and, thus, for a MPE inducedANE.The resonant XRR curve of Pt/NFO in Fig. 4(a) does onlyshow oscillations of the Pt layer due to the large thicknessof the NFO of about (160 ± 10) nm (determined using thecalibrated deposition rate). Therefore, the NFO acts quasi asa substrate and the NFO/MAO interface is not accessible asdiscussed before [26]. We still can determine a 3 nm thick Ptfilm with a roughness of 0.4 nm and a Pt/NFO interface with aroughness of 0.3 nm. The whiteline intensity of the absorptionedge is 1.35 which is slightly above the whiteline intensi-ties obtained for the Pt/Fe samples. The measured XRMRasymmetry ratio in Fig. 4(b) is compared to a simulation forthe Pt/NFO sample with the same magnetooptic profile of thespin polarization of the Pt/Fe(6 nm) interface. The simulatedasymmetry ratio is different compared to Pt/Fe, since theoptical constants of Fe and NFO vary. However, this curveclearly cannot be identified in the measured data. The largernoise at q = 0.2 A˚−1 and q = 0.4 A˚−1 is due to the reducedintensity in the XRR curve at these positions. Compared to anFig. 2. Resonant non-magnetic XRR (11565 eV) intensity I and correspond-ing asymmetry ratios ∆I for Pt(3.3±0.1 nm)/Fe(x) with (a),(b) x = 1.1 nm(average of 2 curves), (c),(d) x = 5.7 nm (average of 2 curves) and(e),(f) x = 18.2 nm (average of 4 curves). For the XRR fits the model in(g) and for the asymmetry fits the magnetooptic profiles in (h) are used.Fig. 3. LSSE measurements for Pt/NFO/MAO. (a) ISHE voltage for varioustemperature differences ∆T . Inset: Measurement geometry. (b) Saturationvalue of the ISHE voltage depending on ∆T . (c) Angular dependence ofthe ISHE voltage in magnetic saturation (H = 1000mT, ∆T = 23K).Fig. 4. (a) Resonant non-magnetic XRR (11565 eV) intensity I and fit forPt/NFO. (b) Corresponding asymmetry ratio ∆I (average of 8 curves). Thesame magnetooptic profile as for Pt/Fe(6 nm) from Fig. 2(h) was used for thesimulation. (d) Close-up of the ∆I(q) and simulation assuming 5% of thePt/Fe(6 nm) spin polarization as sketsched in the magnetooptic profile in (c).asymmetry ratio using a magnetooptic profile with 5% of thePt/Fe(6 nm) spin polarization (cf. Fig. 4(c),(d)) we can identifya lower detection limit which leads to a maximum magneticmoment of 0.04µB taking into account the degree of circularpolarization of the x-rays and the 70% magnetized state of theNFO for ±200mT. Any MPE in this Pt/NFO bilayer can beneglected down to that limit.IV. CONCLUSIONIn conclusion, we used XRMR to investigate the interfacespin polarization in Pt/Fe(x) and Pt/NFO bilayers. We canIEEE TRANSACTIONS ON MAGNETICS 4confirm that the Pt spin polarization is only induced by theinterface properties of the magnetic material, since the effectis independent from the thickness of the magnetic layer. Wefurther observed no MPE in Pt on sputter-deposited NFOdown to a limit of 0.04µB per Pt atom and, therefore,excluded the MPE induced ANE down to that limit for ourLSSE measurements in this bilayer system. These results arecomparable to previously investigated Pt/NFO bilayers withchemically prepared NFO [26].ACKNOWLEDGMENTThe authors acknowledge financial support by the DFGwithin the priority program SpinCaT (SPP 1538) viaKU 3271/1-1 and RE 1052/24-2.REFERENCES[1] K. Uchida, H. Adachi, T. Ota, H. Nakayama, S. Maekawa, E. Saitoh,Appl. Phys. Lett. 97, 172505 (2010).[2] A. Hoffmann, S. D. Bader, Phys. Rev. Applied 4, 047001 (2015).[3] G. E. W. Bauer, E. 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Static Magnetic Proximity Effect in Pt Layers on Sputter-Deposited NiFe2O4 and on Fe of Various Thicknesses Investigated by XRMR

Static Magnetic Proximity Effect in Pt Layers on Sputter-Deposited
  NiFe2O4 and on Fe of Various Thicknesses Investigated by XRMR

Static Magnetic Proximity Effect in Pt Layers on Sputter-Deposited NiFe2O4 and on Fe of Various Thicknesses Investigated by XRMR

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