Thermal Spin-transfer Torque Research Articles

Linear-response analysis of thermal spin-transfer torque and Seebeck coefficients in diverse superlattice magnetic tunnel junctions

The thermal spin-transfer torque (TSTT) and Seebeck coefficients (SECs) are investigated for diverse superlattice magnetic tunnel junctions (DS-MTJs) including linear, Gaussian, Lorentzian, and Pöscl-Teller profiles. The present study is done in the linear response theory. The higher TSTT and SECs are found for the DS-MTJs in comparison with those for the regular types. The results can be utilized to improve in the fields of spintronics and spincaloritronics.

Domain-wall dynamics driven by thermal and electrical spin-transfer torque

According to spin Seebeck effect, a thermal spin current will be produced when a temperature gradient is applied to the metallic ferromagnet. Similar to the electrical spin transfer torque (ESTT) induced by spin-polarized current, there is also a thermal spin transfer torque (TSTT) caused by the thermal spin current, which can be attributed to the s-d interaction between the conduction electrons and the local magnetization. In this paper, the generalized analytical expressions of thermal spin current and TSTT are derived based on the spinor Boltzmann equation (SBE) under the local equilibrium approximation. This generalized TSTT has different terms compared to the previous phenomenological form, and the phenomenological coefficients can be determined theoretically within our framework. Combined with the SBE under the applied electric field and temperature gradient, we solve the Landau-Lifshitz-Gilbert-Levy equation to study the domain wall (DW) motion driven by the ESTT and TSTT. We investigated the temperature-dependent DW motion, the total spin torque, and TSTT in permalloys. We found that the velocity of the DW can be promoted by increasing temperature and temperature gradient, which provide a path to effectively utilize the Joule heating in spintronics devices.

Important Heat Contribution by Tunneling Spin Scattering in Magnetic Tunnel Junction

We investigated the heat contribution by tunneling spin scattering of magnetic tunnel junction in parallel (P) and antiparallel (AP) states using finite element simulation. It showed a maximum temperature increase of 23.2% from P to AP, which is even more significant than the heating asymmetry caused by different current directions. A remarkable enhancement of the temperature gradient was confirmed for contributing to the extra thermal spin-transfer torque to the free layer under a specific current direction. Both the contributions of enhanced temperature and temperature gradient should be the possible reasons for the asymmetry of the critical switching current density from P to AP and AP to P. We also extended the research to a double-barrier structure, clarifying its high performance from the aspect of heat generation. Our demonstration may offer efficient tunneling magnon excitation by using the tunneling spin scattering heat.

The unification of electric and thermal spin transfer torque in spintronics

There exist two kinds of spin transfer torque in spintronics: the electric spin transfer torque (STT) caused by spin-polarized current and the thermal spin torque (TSTT) caused by thermal spin current. In this manuscript, we will derive a unified torque which contain both of them. Based on spinor Boltzmann equation, we investigate the torques in a magnetoelectric device applied by voltage bias and temperature gradient, and find that the unified STT are contributed by two sources: the spin-polarized current induced by bias and the thermal spin current induced by the temperature gradient, moreover the TSTT can be further divided as local equilibrium TSTT and nonequilibrium TSTT. In addition to the usual electric STT and TSTT, a new term also appear in our expression, which can be interpreted by the inhomogeneity of magnetization. We illustrate them by an example of electric and thermal transport through a ferromagnet subject to some device, in which those torques originate from different sources are manifested quantitatively.

Improved thermal spin-transfer torque by anti-reflective Fabry-Pérot magnetic tunnel junctions

Abstract Improvement of thermal spin-transfer torque (TSTT) in magnetic tunnel junctions (MTJs) is predicted by an anti-reflective Fabry-Perot MTJ (AR-FPMTJ). It is investigated using non-equilibrium Green's function formalism. The presence of anti-reflective regions leads to boxcar (band-pass) transmission function in the AR-FPMTJ device compared to that in a superlattice MTJ as the magnitude of the TSTT in the former is almost six times greater than that in the latter. The present study suggests a novel application in the field of spintronics.

Enhancement of Thermal Spin Transfer Torque via Bandpass Energy Filtering

We propose employing the energy bandpass filtering approach in a magnetic tunnel junction device as a route to enhance the thermal spin transfer torque. Using the spin-resolved non-equilibrium Green's function formalism, we propose to harness the optical analog of anti-reflective coatings in a heterostructure MTJ device, and report a large boost in spin torque in the linear regime of temperature bias. In particular, we discuss the position of transmission function with respect to the Fermi energy that caters the maximum thermal effect. A “boxcar” transmission feature resulting from the anti-reflective configuration enhances charge and spin transport through the structure in comparison to a normal superlattice configuration. The thermally excited spin transfer torque is enhanced by almost five times with our device designs. Although, the thermally driven spin torque is much smaller than the voltage driven torque, this technique could potentially be an energy efficient way to switch the magnetization. This opens up a new viable area for spintronics applications and with the existing advanced thin-film growth technology, the optimized superlattice configurations can be achieved.

Magnetization switching in superlattice via thermal spin transfer torque

We theoretically investigate the switching characteristics of ferromagnets in the superlattice-like magnetic tunnel junction devices due to temperature bias. The critical switching spin current obstructs the anomaly flipping of magnetization in the stable condition of operation. We exploit the optical analog of anti-reflection in MTJ device design to harness its boxcar transmission feature. Using the non-equilibrium Green’s function spin transport formalism, we analyze the thermal spin-transfer torque switching in normal and anti-reflected superlattice configurations. It is observed that the operating temperature for complete flipping of magnetization in the case of anti-reflected superlattice is quite smaller than the normal superlattice. We also evaluate the thermal tunnel magnetoresistance (TMR) of both the device configurations and conclude that the switching and TMR ratio are the two different phenomena when a device operates on the temperature gradient alone. The discussed temperature variation is practically achievable with localized short laser pulses.

Strong current actions on ferrimagnetic domain walls in the creep regime

We observe domain-wall (DW) motion in ferrimagnetic TbFe wires with perpendicular anisotropy under combined field and current in the creep regime. The current action on the DW is double: Joule heating and spin-transfer torque. We propose a genuinely robust analysis of velocity, separating thermal effects and spin-transfer torque, quantifying the latter as an equivalent field in the so-called one-dimensional (1D) model. Its efficiency is much larger than in transition-metal ferromagnets above room temperature. The equivalent field reveals the large polarization-to-magnetization ratio in ferrimagnets despite a vanishing ${M}_{s}$. The usual 1D DW model is extended to mimic creep and predicts that, in low net magnetization systems, the internal DW structure precesses with currents above a field-independent threshold, leading to two propagation regimes with different mobilities. This is another example of how the internal DW magnetization is relevant in creep. We could not detect experimentally these two regimes, possibly because of the dispersion of the data.

Field- and temperature-modulated spin diode effect in a GMR nanowire with dipolar coupling

The magnetization dynamics of single Co/Cu/Co spin valves, embedded in electrodeposited nanowires of 30 nm avarage diameter, was observed using the spin-diode effect. The electrically-detected magnetic resonances were compared when using modulation of either the magnetic field or a laser irradiation. The effect of temperature modulation was accounted for by introducing the temperature dependence of the saturation magnetization and anisotropy, as well as thermal spin-transfer torque (TSTT). The predictions of the model are compared with experimental data. Two forms of modulation give rise to qualitative differences in the spectra that are accounted for by the model only if both temperature-modulated magnetization and TSTT are introduced in the model. On the contrary, the temperature modulation of the magnetic anisotropy has a smaller contribution.

Resonant enhancement of thermal spin-transfer torque in asymmetric double-barrier tunnel junctions

Charge current and thermal spin-transfer torque (TSTT) in asymmetric double-barrier magnetic tunnel junctions (DBMTJs) with a non-magnetic metal (NM) spacer as CoFeB/MgO/NM/MgO/LSMO are studied by non-equilibrium Green’s function (NEGF) formalism. It is found that the results show oscillatory behavior with an increase of the NM spacer thickness due to the resonant tunneling effect through the DBMTJs structures. About the dependence of the temperature difference and angle between the magnetizations, the charge current and TSTT are larger at the specific thicknesses (resonant positions) of the NM spacer than the others. The results also illustrate a magnitude enhancement for asymmetric DBMTJs compared with asymmetric single-barrier MTJs (SBMTJs). Therefore, it is possible to achieve a large TSTT for magnetization dynamics by new asymmetric DBMTJs.

Enhancement of thermal spin transfer torque by double-barrier magnetic tunnel junctions with a nonmagnetic metal spacer

Enhancement of thermal spin transfer torque in a double-barrier magnetic tunnel junction with a nonmagnetic-metal spacer is proposed in this study. The results indicate that, given the same temperature difference, thermal spin transfer torque and charge current density for the proposed double barrier magnetic tunnel junction configuration can be approximately twice as much as that of the traditional single-barrier magnetic tunnel junctions. This enhancement can be attributed to the resonant tunneling mechanism in the double-barrier structure.

Observation of thermal spin-transfer torque via ferromagnetic resonance in magnetic tunnel junctions

The thermal spin-transfer torque (TSTT) in magnetic tunneling junctions (MTJs) was systematically studied using electrical detection of ferromagnetic resonance (FMR). Evidence for the existence of TSTT in MTJs is observed. A temperature difference was applied across an MTJ acting as a TSTT on the free layer of the MTJ. The FMR of the free layer was then excited by a microwave current and electrically detected as a dc voltage. We found that the FMR line shape was changed by the TSTT, indicated by the ratio of dispersive and Lorentz components of the FMR spectra (D/L). D/L increases by increasing the temperature difference. In addition, we analyze the magnetization orientation dependence of TSTT and provide solid evidence that this dependence differs from the magnetization orientation dependence of spin-transfer torque driven by a dc bias.

Thermally driven transverse transports and magnetic dynamics on a topological surface capped with a ferromagnet strip

We theoretically study thermally driven transport of the Dirac fermions on the surface of a topological insulator capped with a ferromagnet strip. The generation and manipulation of anomalous Hall and Nernst effects are analyzed, in which the in-plane magnetization of the ferromagnet film is found to take a decisive role. This scenario is distinct from that modulated by Berry phase where the in-plane magnetization is independent. We further discuss the thermal spin-transfer torque as a backaction of the thermoelectric transports on the magnetization and calculate the dynamics of the anomalous Hall and Nernst effects self-consistently. It is found that the magnitude of the long-time steady Hall and Nernst conductance is determined by competition between the magnetic anisotropy and current-induced effective anisotropy. These results open up a possibility of magnetically controlling the transverse thermoelectric transports or thermally manipulating the magnet switching.

Thermal spin-transfer torque driven by the spin-dependent Seebeck effect in metallic spin-valves

The spin-dependent Seebeck effect converts thermal gradients into spin currents. It is now shown that this effect can be used to drive spin-transfer torques on picosecond timescales using the heat currents created by ultrafast pulses of laser light.

Comparison between thermal and current driven spin-transfer torque in nanopillar metallic spin valves

We investigate the relation between thermal spin-transfer torque (TSTT) and the spin-dependent Seebeck effect (SDSE), which produces a spin current when a temperature gradient is applied across a metallic ferromagnet, in nanopillar metallic spin valves. Comparing its angular dependence (aSDSE) with the angle dependent magnetoresistance (aMR) measurements on the same device, we are able to verify that a small spin heat accumulation builds up in our devices. From the SDSE measurement and the observed current driven STT switching current of 0.8 mA in our spin valve devices, it was estimated that a temperature difference of 230 K is needed to produce an equal amount of TSTT. Experiments specifically focused on investigating TSTT show a response that is dominated by overall heating of the magnetic layer. Comparing it to the current driven STT experiments we estimate that only ~10% of the response is due to TSTT. This leads us to conclude that switching dominated by TSTT requires a direct coupling to a perfect heat sink to minimize the effect of overall heating. Nevertheless the combined effect of heating, STT and TSTT could prove useful for inducing magnetization switching when further investigated and optimized.

Thermal spin-transfer torque in magnetic tunnel junctions (invited)

The thermal spin-transfer torque (TSTT) is an effect to switch the magnetic free layer in a magnetic tunnel junction by a temperature gradient only. We present ab initio calculations of the TSTT. In particular, we discuss the influence of magnetic layer composition by considering FexCo1–x alloys. Further, we compare the TSTT to the bias voltage driven STT and discuss the requirements for a possible thermal switching. For example, only for very thin barriers of 3 monolayers MgO, a thermal switching is imaginable. However, even for such a thin barrier, the TSTT is still too small for switching at the moment and further optimization is needed. In particular, the TSTT strongly depends on the composition of the ferromagnetic layer. In our current study, it turns out that at the chosen thickness of the ferromagnetic layer, pure Fe gives the highest thermal spin-transfer torque.

PARAMETER SPACE FOR THERMAL SPIN-TRANSFER TORQUE

Thermal spin-transfer torque describes the manipulation of the magnetization by the application of a heat flow. The effect has been calculated theoretically by Jia et al. in 2011. It is found to require large temperature gradients in the order of Kelvins across an ultra thin MgO barrier. In this paper, we present results on the fabrication and the characterization of magnetic tunnel junctions with three monolayer thin MgO barriers. The quality of the interfaces at different growth conditions is studied quantitatively via high-resolution transmission electron microscopy imaging. We demonstrate tunneling magnetoresistance ratios of up to 55% to 64% for 3 to 4 monolayer barrier thickness. Magnetic tunnel junctions with perpendicular magnetization anisotropy show spin-transfer torque switching with a critical current of 0.2 MA/cm2. The thermally generated torque is calculated ab initio using the Korringa–Kohn–Rostoker and nonequilibrium Green's function method. Temperature gradients generated from femtosecond laser pulses were simulated using COMSOL, revealing gradients of 20 K enabling thermal spin-transfer-torque switching.

Thermal spin pumping mediated by magnons in the semiclassical regime

We microscopically analyze thermal spin pumping mediated by magnons, at the interfacebetween a ferromagnetic insulator and a non-magnetic metal, in the semiclassical regime.The generation of a spin current is discussed by calculating the thermal spintransfer torque, which breaks the spin conservation law for conduction electrons andoperates the coherent magnon state. Inhomogeneous thermal fluctuations betweenconduction electrons and magnons induce a net spin current, which is pumped into theadjacent non-magnetic metal. The pumped spin current is proportional to thetemperature difference. When the effective temperature of magnons is lower than that ofconduction electrons, localized spins lose spin angular momentum by emittingmagnons and conduction electrons flip from down to up by absorbing all theemitted momentum, and vice versa. Magnons at the zero mode cannot contribute tothermal spin pumping because they are eliminated by the spin-flip condition.Consequently thermal spin pumping does not cost any kind of applied magneticfields.

Amplification of Spin Waves by Thermal Spin-Transfer Torque

We observe amplification of spin-wave packets propagating along a film of single-crystal yttrium iron garnet subject to a transverse temperature gradient. The spin waves are excited and detected with standard techniques used in magnetostatic microwave delay lines in the 1-2 GHz frequency range. The amplification is attributed to the action of a thermal spin-transfer torque acting on the magnetization that opposes the relaxation and which is created by spin currents generated through the spin-Seebeck effect. The experimental data are interpreted with a spin-wave model that gives an amplification gain in very good agreement with the data.

Manipulating the spin of single-molecule magnet by thermal spin-transfer torque

We investigate thermally driven electronic transport through a single-molecule magnet (SMM) coupled to a ferromagnetic (FM) lead and a normal-metallic lead. Due to the thermal spin-transfer torque effect, the orientation of the SMM’s spin can be manipulated by temperature bias or gate voltage to be parallel or antiparallel to the FM lead’s magnetization. The present device can be realized by current technologies and has potential application in molecular spintronics or quantum information processing.