Longitudinal magnetothermopower in permalloy spin valves

Accepted Manuscript Research articles Longitudinal Magnetothermopower in Permalloy spin valves Priyanga B. Jayathilaka, Casey W. Miller PII: DOI: Reference:

S0304-8853(17)31099-5 https://doi.org/10.1016/j.jmmm.2017.10.055 MAGMA 63269

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Journal of Magnetism and Magnetic Materials

Please cite this article as: P.B. Jayathilaka, C.W. Miller, Longitudinal Magnetothermopower in Permalloy spin valves, Journal of Magnetism and Magnetic Materials (2017), doi: https://doi.org/10.1016/j.jmmm.2017.10.055

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Longitudinal Magnetothermopower in Permalloy spin valves Priyanga B. Jayathilaka and Casey W. Miller Department of Physical Sciences, Faculty of Applied Sciences, Rajarata University of Sri Lanka, Mihintale, Sri Lanka School of Chemistry and Material Science, Rochester Institute of Technology, 85 Lomb memorial drive, Rochester, NY 14623, USA

Abstract We report measuring the longitudinal magnetothermopower (LMTP) and giant magnetoresistance (GMR) of Py/Cu/Py/FeMn spin valves grown on top of a thermally oxidized silicon substrate. These measurements showed that the LMTP behaves similarly to GMR, indicating a similar behavior of charge current and thermal current inside a spin valve. Since spin dependent interface scattering is the mechanism that governs GMR, these observations suggests that LMTP also originates from the spin dependent interface scattering of thermally induced current. However, there are differences between the LMTP and GMR signals, including a factor of two larger signal for the LMTP, that may indicate the LMTP is additionally sensitive to another scattering phenomenon in these heterostructures.

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1. Introduction Interest in magnetothermal phenomenon grew rapidly in recent times with the discovery of the spin Seebeck effect[1, 2], which offers potential to engineer novel energy harvesting devices [3]. In these magnetothermal effects, a temperature gradient applied along or across a sample leads to a measurable voltage distinct from the regular Seebeck effect [1, 4]. Previous Studies on magnetothermopower (MTP) [5, 6, 7, 8, 9, 10] on Cu/Co and Fe/Cr multilayers have indicated a correlation between MTP and magnetoresistance (MR). While magnetic field dependence studies have exhibited direct correlation between MR and MTP[11, 12], temperature dependence Preprint submitted to Journal Name

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studies have revealed differences, such as MR decreasing with temperatures while the MTP increases [13, 14], that are presumed to be related to the differing importance of electronic versus phononic contributions to the effects. In this article, we directly compare the longitudinal magnetothermopower (LMTP) effect to the giant magnetoresistance (GMR) in the same samples. The LMTP signal is quite similar to GMR, but has greater magnitude, indicating more effective spin dependent scattering of thermally driven current in spin valves.

Figure 1: (a) Schematic of Py/Cu/Py/FeMn spin valves used in the study. (b) Sample was placed on two thermoelectric modules and the resultant voltage was measured using a nanovoltmeters. 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37

All the samples were grown on (10 mm × 10 mm) thermally oxidized silicon substrates in an Argon ion sputtering system with a base pressure of 2 × 10−8 Torr. As illustrated in Fig. 1(a) Ta(5 nm)/Py(5 nm)/Cu(5 nm)/ Py(t nm)/FeMn(15 nm) spin valves were deposited at room temperature in 3 mTorr of ultra high purity Ar. The top Py layer was exchanged biased to ensure that the two Py layers switched independently. The Ta seed layer helps provide the (111) texture throughout the structure to give the maximum exchange bias [15]. A 100 Oe external magnetic field was applied during the deposition to induce uniaxial anisotropy in the sample[16]. Then the sample was placed in a set-up that was designed to apply a longitudinal temperature gradient , as illustrated in the Fig. 1(b).The samples were attached to two electrically isolated Aluminum blocks mounted on independently controlled thermoelectric cooling (TEC) modules (Custom Thermoelectrics part 03111-5L31-06CG) using Ag paint. Copper voltage leads were connected to the sample using pressed indium contacts. In order to minimize the effect of Anomalous Nernst effect (ANE) due to perpendicular temperature gradient samples were attached to the Al blocks from their edges, and the voltage leads were placed 2 mm from thermal baths. Previous studies have shown that this butt-mounting configuration reduces the temperature gradient along the sur2

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face normal to a negligible level. In contrast to the charge current used in current in-plane GMR measurements, here a thermal current was generated by applying a temperature gradient along the sample. Then the resultant voltage difference (V2 −V1 ) was measured using nanovoltmeters.

Figure 2: Field dependence of the longitudinal magnetothermopower signal from Py(5 nm)/Cu(5 nm)/Py(10 nm)/FeMn(15 nm) at room temperature using a temperature gradient of 30 K/cm. Magnetic field was swept parallel to the temperature gradient; black (red) data indicate decreasing (increasing) field. 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 2 shows the field dependence of the longitudinal magnetothermopower (LMTP) at room temperature with 30 K temperature difference (∆T = 30 K). Here, “longitudinal” implies we are measuring the electric field parallel to the heat flow. The in-plane magnetic field was swept linearly and parallel to the temperature gradient, bounded between ±400 Oe. Similar to the resistance change in magnetoresistance measurements, the LMTP voltage signal (∆Vmax = Vparallel −Vantiparallel ) showed a significant jump upon field sweeping as the magnetizations of the two Py layers switch between parallel and antiparallel configurations. The thermally generated current appears to undergo a spin dependent scattering process similar to what is observed in GMR. The LMTP signal was measured for temperature differences (∆T) varied from +30 K to -30 K in 5 K steps. Figure 3 shows a linear relationship between ∆V and ∆T, with the sign of ∆V reversing with the polarity of ∆T, as expected. This tells us that the higher temperature gradient generates a larger thermally generated current , which leads to larger ∆V values, and that the direction of the thermal current depends on the direction of the ∆T. Notably, the percentage voltage change (∆V /V )% of the LMTP signal was effectively independent of the temperature difference for all samples. 3

Figure 3: Dependence of the LMTP signal on temperature difference (∆T) for a Py(5 nm)/Cu(5 nm)/Py(10 nm)/FeMn(15 nm) spin valve. The magnetic field was swept between -400 Oe and +400 Oe to obtain the LMTP signal, as illustrated in Fig. 2.

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The effect of Py layer thickness was studied by varying the thickness of the top Py layer. A set of spin valves with top Py layer thickness varied from 7.5 nm to 35 nm was grown using an in situ mask exchanging system, which allowed all the samples to be grown in a single deposition run. Therefore, it is possible to conclude that any change in the signal can be exclusively due to the change in Py layer thickness, not run-to-run variations. A 30 K temperature difference was applied along the sample, and the field dependence of the LMTP signal was measured while linearly sweeping the magnetic field between ±400 Oe. A decrease in LMTP signal was observed as the Py layer thickness was increased (Fig 4). When the layer thickness increases, bulk properties start to dominate over interfacial properties. Thus, spindependent interface scattering, the dominant mechanism in GMR, becomes less important [17, 18]. The decrease in the LMTP signal with the increasing Py layer thickness indicates that the thermally generated current also behaves similarly to the voltage driven charge current inside the spin valve giving similar effects. The data were fitted to A + Bexp(−t/λ), with the decay constant λ found to be 16 nm. This behavior is similar to the variation of GMR with the ferromagnetic layer thickness in previously reported studies (λ = 15 nm at 4 K in [19]) [18]. For direct comparison between GMR and LMTP, both effects were measured on a single Py(5 nm)/Cu(5 nm)/Py(10 nm)/FeMn(15 nm) spin valve. The GMR measurement was measured using a four point resistivity technique with a 93 µA DC current applied along the sample and the resultant volt4

Figure 4: The thickness dependence of the voltage signal on Py(5 nm)/Cu(5 nm)/Py(t)/FeMn(15 nm) indicates an interface effect is present in the LMTP signal.

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age measured using nanovoltmeters. The external magnetic field was swept between ±900 Oe. LMTP measurements were made with a 30 K temperature difference (∇T = 30 K/cm) applied along the sample and the resultant voltage signal as before. The exact same contacts on the sample were used for both measurements. Both GMR and LMTP had similar field dependence as shown in Fig. 5 (a) and (c). There is a slight difference in exchange bias can be seen between LMTP and GMR. This change is likely due to the heating of the sample affecting the antiferromagnetic FeMn: when applying 30 K/cm temperature gradient, one side of the sample was held at 293 K and the other side was held at 323 K. This can cause the exchange bias to change because a portion of the sample was at a temperature closer to the N´eel temperature of the FeMn (TN ≈ 400 K [20]), which in turn can lead to a difference in switching fields. This is corroborated by the 3 K/cm gradient not having a significant shift in switching field relative to the GMR signal. This is also likely the source of the ∇T = 30 K/cm LMTP signal having quite different switching field distribution compared to the GMR signals and the ∇T = 3 K/cm LMTP signal. In addition to the slight switching field difference, a significant difference in magnitudes of GMR and LMTP signals was observed. The

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Figure 5: Current In-plane GMR and LMTP signals from a single Py(5 nm)/Cu(5 nm)/Py(10 nm)/FeMn(15 nm) spin valve with (a) 93µA dc current, (b) 13µA dc current, (c) ∇T = 30 K/cm, (d) ∇T = 3 K/cm. In all the measurements magnetic field was swept parallel to the current and the temperature gradient.

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magnitude of the LMTP signal is roughly twice that of the GMR signal. To check the consistency of these values, the current used in the GMR measurement was reduced to 13 µA and the temperature difference in LMTP measurement was reduced to 3 K, and the respective measurements were carried out again. Reducing the current and the thermal gradient did not significantly change the GMR and LMTP percentages (5 (b) and (d)). The noise level of the measurements went up due to the reduction of base voltages at lower current and temperature gradient. This observation further indicates that the LMTP values are intrinsically larger than the GMR values in spin valves. It is the interface scattering which governs the GMR [17, 19, 21]. The conduction electrons participating in the signal are the same whether they are driven by an externally applied potential difference or a thermal gradient. According to our observations, it is possible to suggest that the origin of the LMTP also related to the interface scattering of the thermally driven current. Also it is clear from the Fig. 5, that the LMTP signal from the spin valve is larger than the GMR value and has a wider switching field distribution. It is possible to suggest that the thermally driven current might scatter both from the bulk and the interface to give higher LMTP values and wider switching fields [22].Recent studies have suggested that thermal spin injection is 6

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less sensitive to the interface contrast to the electrical spin injection and also benefited by presence of oxide layer at the interface [23]. It is also possible that some other type of scattering or magnon drag related effect[24] cause the higher LMTP signal. Previous studies have also reported higher LMTP signals in different spin valve systems [25].Therefore further studying is necessary to understand this change. The wider switching field distribution has alsobeen reported in previous studies associated with thermally driven currents in magnetic thin films[1][26]. Thermally driven spin valves of the sort described here may be used as sensors in applications where heat is generated as a byproduct. This could enable a wide variety of applications in electrical and mechanical devices. Using thermal current to run sensing devices not only provide an easy way to remove the excess heat but also reduces electrical power requirements to run such sensors. In summary, we measured the longitudinal magnetothermopower and inplane GMR of Py/Cu/Py/FeMn spin valves. While both LMTP and GMR signals showed similar behavior with respect to the magnetic field and layer thickness, we observe differences in both magnitudes and the switching fields of the signals. Therefore it is possible to suggest that the interface scattering plays a major role in the origin of LMTP similar to the GMR, but bulk scattering or some other form of scattering may also take place to give the LMTP significantly higher magnitudes, which may enable spin caloritronic energy harvesting sensor devices.

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2. Acknowledgments

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This work was supported by NSF Grant 1522927. [1] K. Uchida, S. Takahashi, K. Harii, J. Ieda, W. Koshibae, K. Ando, S. Maekawa, E. Soitoh, Observation of spin seebeck effect, Nature 455 (2008) 778–781. [2] J. Xiao, G. E. W. Bauer, K.-c. Uchida, E. Saitoh, S. Maekawa, Theory of magnon-driven spin seebeck effect, Phys. Rev. B 81 (2010) 214418. [3] F. J. DiSalvo, Thermoelectric cooling and power generation, Science 285 (1999) 703–706.

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Here we Report measuring longitudinal magnetothermopower (LMTP) and giant magnetoreisitance (GMR) on py/Cy/PyFeMn spin valves. LMTP showed similar behavior to GMR indicating similar behavior of charge current and thermal current inside a spin valve. These observations suggest that LMTP also originate from spin dependent interface scattering. But magnitude of LMTP signal is twice the magnitude of GMR. This indicates that LMTP may additionally sensitive to another scattering phenomenon which needs to be studied