Session P41: Thermal Spintronics

Controlling heat by spin caloritronics

Interaction between spin and heat is actively studied in the field of spin caloritronics from the viewpoints of both fundamental physics and applications. Early studies on spin caloritronics mainly focused on phenomena that generate a spin current from a heat current, such as the spin Seebeck effect, toward the development of versatile thermoelectric generators. In contrast, there are many heat-generation phenomena that use spin and charge currents as input, e.g., the spin Peltier effect, which is the reciprocal of the spin Seebeck effect; the anisotropic magneto-Peltier effect, in which the charge-to-heat current conversion efficiency depends on the angle between the charge current and magnetization in a ferromagnet; and the anomalous Ettingshausen effect, in which a heat current is generated in the direction perpendicular to both the applied charge current and magnetization. Recently, we successfully observed the thermal response from these phenomena by means of an active infrared emission microscopy called the lock-in thermography, and demonstrated thermal control functions that cannot be actualized without using spins [1-4]. In this talk, we review our recent experimental results by focusing particularly on the thermal imaging measurements of the spin-caloritronic phenomena.

[1] S. Daimon, R. Iguchi, T. Hioki, E. Saitoh, and K. Uchida, Nature Commun. 7, 13754 (2016).
[2] T. Seki, R. Iguchi, K. Takanashi, and K. Uchida, Appl. Phys. Lett. 112, 152403 (2018).
[3] K. Uchida, S. Daimon, R. Iguchi, and E. Saitoh, Nature 558, 95-99 (2018).
[4] K. Uchida et al., Scientific Reports 8, 16067 (2018).   

Spin-Seebeck Effects in Pyrochlore Iridates

The spin-Seebeck effect (SSE) refers to voltage signals induced by the Inverse Spin Hall Effect (ISHE), with thermally driven spin currents through adjacent magnetic systems. It has been studied before mostly in collinear magnetic systems. Here, we present a general theory of the SSE in the case of a non-collinear antiferromagnetic insulator (AFI)/non-magnetic metal heterostructure. In our model, we apply a periodic boundary condition in the transverse plane and an open boundary condition at the interface. If there is a non-equilibrium temperature difference within the two materials, the exchange coupling between the electrons in the metal and magnons of interfacial spins in the AFI can give rise to a longitudinal spin current across the system. We also numerically compute the spin current using a pyrochlore iridate as the AFI where the spin configuration is the all-in-all-out (AIAO) ground state. We compare the results of different crystal interfaces arising from different crystalline orientations to make a connection with experiments and practical devices.   

Spin Seebeck effect induced by antiferromagnetic magnons and spin fluctuations in epitaxial FeF2 films

Recently, the spin Seebeck effect (SSE) in antiferromagnetic (AFM) Cr₂O₃ and MnF₂ has been reported and the signals were found to rise significantly above the spin-flop (SF) field. However, it is still unclear whether the SSE requires an induced magnetic moment or just the presence of AFM magnons. Here we report a study of the SSE of FeF₂/Pt heterostructures in which FeF₂ is an insulating AFM epitaxial film. The high SF field (42 T) of FeF₂ makes it a perfect candidate for studying the SSE below the SF field. The AFM properties of FeF₂ film are characterized by exchange bias. The SSE signal of FeF₂/Pt under different magnetic fields shows very similar temperature-dependent behaviors. First, there is a peak at around 11.6 K due to AFM-magnons’ contribution. Second, we observe a “bump” at the AFM ordering temperature (70 K), beyond which the SSE signal decays and persists up to 250 K under high magnetic fields, indicating that short-range spin fluctuations also contribute to the SSE signal.   

Spin Seebeck imaging of spin-torque switching in antiferromagnetic Pt/NiO/Pt heterostructures

We demonstrate spin Seebeck microscopy as a sensitive table-top method for imaging in-plane antiferromagnetic order in thin films. In Pt/NiO(111)/Pt samples, we resolve antiferromagnetic spin domains within crystalline twin domains and image the effects of DC current-induced spin-orbit torque switching. We find a linear correlation between spin-torque-induced changes in the integrated spin Seebeck signal and the spin Hall magnetoresistance, confirming that we image the Néel order. The measurements show that changes driven by spin-orbit torque can occur both by antiferromagnetic domain wall motion and domain flopping, and only a small fraction of the NiO sample is altered.   

Characterization of topological band structures by the anomalous Nernst effect

Resolving the structure of energy bands in transport experiments is a major challenge in condensed matter physics and material science. Sometimes, traditional electrical conductance or resistance measurements only provide very small signals, and thus limit the ability to obtain direct band structure information. In this case, it has been proven beneficial to employ thermoelectric measurements which are sensitive to the first derivative of the electrical charactoristics with respect to energy, rather than to its value itself. The common way to access the Berry curvature in topological materials directly via measurements is the anomalous Hall effect, but the corresponding signal can be too small to be detected when the topological band structures lie far off the Fermi level. In this work we propose to investigate topological band structure features utilizing the anomalous Nernst effect , that are elusive in anomalous hall measurements. This work demonstrates that anomalous Nernst measurements can be an effective tool for characterization of topological band structures away from Fremi level.   

A comparative study of spin Seebeck effect and anomalous Nernst effect in a ferromagneticmetal/normal-metal bilayer system.

In the emerging field of Spin Caloritronics, the central interest is the study of the interactions of spins with heat current. Thus a pure spin current got an enormous importance for such kind of studies. On the other hand, spin seebeck effect (SSE) pertains to the generation of pure spin current under application of a thermal gradient across a magnetic material. Although for magnetic-metals, the SSE signal is contaminated by the anomalous Nernst signals (ANE). In this work, we identify and investigate longitudinal spin seebeck effect (LSSE) phenomenon in La_0.7Ca_0.3MnO₃ /Pt bilayer system over a wide temperature range 20K ≤ T ≤ 295K. We separate out LSSE from the ANE. Further a detailed-analysis on LSSE shows a very good agreement with the current theoretical models.   

Enhancement of Thermal Spin Injection Effects in Nonlocal Spin Valves on Silicon Nitride Membranes

Nonlocal spin valves (NLSVs) have the unique capability to separate charge and spin currents, and thus are an important tool for both applied and fundamental research in nanomagnetism.[1,2] Previous research shows that changes to the composition of the NLSV substrate can have a marked effect on the background nonlocal resistance by changing the substrate thermal conductance.[3] Our research demonstrates enhancement of non-local spin resistance by the anomalous Nernst effect (ANE) and dramatically lowered thermal conductance in NLSVs fabricated on SiN membranes.[4,5]

We present refined values as low as 0.27 for the Nernst coefficient of permalloy as determined by 2D finite-element analysis of Py/Al NLSVs, where the thermal gradient is confined to two dimensions by the membrane-supported construction. We also present new insight into thermal spin injection effects other than ANE, including thermally-assisted electrical spin injection.

[1] Jedema, et al. Nature 410, 345 (2001)
[2] Ji, et al, APL 88, 052509 (2006)
[3] Kasai, et al. APL 104, 162410 (2014)
[4] Hojem, et al. (in preparation)
[5] Bennet, et al. (in preparation)   

Spin-orbit torque and Nernst effects in Bi-Sb/ferromagnet heterostructures

Topological insulators have gained considerable interested recently due to their potential as spin current generators for room temperature memory applications. In this work we show harmonic Hall measurements of spin-orbit torque (SOT) in MBE grown Bi-Sb/Co bilayers [1]. We find that signals in our samples are dominated by the ordinary Nernst effect (ONE). This thermal effect can be a spurious signal in harmonic Hall measurements and has not been taken into account in previous experiments.

We are proposing two control experiments to separate the ONE in Bi-Sb from SOT. First, we compare harmonic measurements in Bi-Sb/Co and Bi-Sb/Al. While SOT is absent in the sample without magnet, we still see large signals in the second harmonic Hall voltage due to the ONE. Second, we investigate the scaling of V_2ω with external magnetic field B_ext. While it is expected that SOT effects are suppressed at large magnetic fields, we find that the second harmonic voltage in Bi-Sb/Co scales linearly with the external field, as expected from the ONE.
These results provide an important contribution to the understanding of the ultra-large SHA that was recently reported in TI/ferromagnets.

[1] N. Roschewsky et al., arXiv:1810.05674   

Spin-Seebeck and anomalous Nernst effects in MnBi and Bi/MnBi composites

The spin-Seebeck effect (SSE) is an advective transport process in a bilayer composed of a ferromagnet (FM) and a nonmagnetic (NM) material with strong spin-orbit coupling. In a temperature gradient, the flux of magnons in the FM transfer spin angular momentum to the electrons in the NM, which, by the ISHE contributes an SSE voltage. In contrast, the intrinsic anomalous Nernst (ANE) conductivity in homogeneous FMs is understood as a non-advective process due to the effect of the temperature gradient on electrons: in MnBi, one can calculate the ANE conductivity from the Berry curvatures using a 32 band tight-binding Hamiltonian with the spin-orbit interaction and magnetization. We show that there can be an additional advective magnonic contribution to the ANE similar to the SSE effect but in uniform metallic FMs. Further, as was done in Ni/Pt ¹, we synthesized composites of aligned MnBi needles in a Bi matrix. In this geometry, we expect an additional SSE contribution from the magnons in MnBi to the Nernst effect in the Bi. In composites with Mn concentration far below the percolation threshold, we observe a large boost in the thermopower and Nernst over the Bi host, which we attribute to advective spin transport.

[1] S. Boona et al., Nat. Comm. 7 (2016)   

Magneto-thermoelectric measurements on epitaxial Heusler alloy thin films

Full Heusler compounds with the composition Co₂Mn(Al,Si) are of interest due to their nearly half-metallic behavior. We have measured the Seebeck and anomalous Nernst coefficients in epitaxial MgO(001)/Co₂MnAl_xS_1-x thin films using a novel all-electrical device fabricated through straightforward top-down photolithography techniques. Because the scattering rate cancels out in expressions for the thermoelectric coefficients (to first approximation), they can provide more direct information about the electronic structure than is accessible using only ordinary magneto-transport. Among our samples, we found Co₂MnAl to have the largest anomalous Nernst coefficient of -4 Am^-1K^-1 at room temperature, which is larger than the value of -2 Am^-1K^-1measured in bulk polycrystalline Fe [S. J. Watzman et al., Phys. Rev. B 94, 144407 (2016)] . We discuss how these measurements can be applied as a probe of a material’s spin polarization, so that they can be used as a tool for optimizing magneto-electronic properties of quaternary Heusler alloys.
  

Thermal Hall effect from Magnon-Phonon Couplings

In this letter, we propose a new mechanism to realize the thermal Hall effect from magnon-phonon couplings in exchange spin wave systems. We study a collinear ferromagnetic system on the 2D square lattice with an in-plane Dzyaloshinskii-Moriya interaction, where neither the magnon system nor the phonon system alone shows the thermal Hall effect. However, the magnon-phonon interaction can lead to a thermal Hall effect of a hybrid magnon-phonon system. Such an interaction can be directly tuned by an out-plane gate voltage.   

Pure Spin Current Driven by a Thermally Induced Magnon Chemical Potential

A major goal of spintronics is to develop devices that rely on spin current, rather than charge current. Of the few methods for generating pure spin current in magnetic insulators (MI), the spin Seebeck effect (SSE) is an attractive method, due to its simplicity. The SSE relies on a thermal gradient to generate a nonequilibrium distribution of magnons, collective spin quasiparticles, which carry spin current. Describing these magnons requires a thermally driven magnon chemical potential, never before measured in such a nonequilibrium system. Here we report the use of Brillouin light scattering for measuring a magnon chemical potential generated by a thermal gradient in the MI yttrium iron garnet (YIG): Y3Fe5O12. Boltzmann transport analysis allows for the quantification of spin currents due to the magnon temperature and chemical potential gradients. Finally, the range of energies and wavevectors of the magnons that contribute to the spin current are identified. Experimental determination of these items will facilitate advancing the theories describing coupled heat and spin transport. Furthermore, this technique allows for the determination intrinsic spin current generating ability of an MI, not possible using previous techniques.   

Effects of thermal spin disorder on the half-metallicity of Co2MnSi with antisite defects and Fe substitution

Heusler alloys based on Co₂MnSi have a high Curie temperature and high spin polarization at the Fermi level, which is promising for applications in spintronics. Band structure calculations show Co₂MnSi to be half-metallic at zero temperature. Alloying with Fe on the Mn sublattice is believed to shift the Fermi level closer to the middle of the half-metallic gap. Here, we study the effects of thermal spin fluctuations on the electronic spectrum near the Fermi level, both in pure Co₂MnSi and in the presence of crystallographic defects or Fe substitution for Mn.We find that all magnetic antisite defects are strongly exchange-coupled to the host magnetization, and thermal spin fluctuations do not easily destroy the half-metallic gap. In this respect, Co₂MnSi differs from NiMnSb, where Mn antisites on the Sb sublattice strongly reduce the spin polarization already at rather low temperatures [1]. We also find that partial substitution of Mn by Fe results in considerable changes in the Bloch spectral function near the Fermi level, which are very different from the rigid-band picture that is usually assumed.
[1] K. D. Belashchenko, J. Weerasinghe, S. Mu, and B. S. Pujari, Phys. Rev. B 91, 180408(R) (2015).