Giant thermal spin-torque–assisted magnetic tunnel junction switching

Abstract

Spin-polarized charge currents induce magnetic tunnel junction (MTJ) switching by virtue of spin-transfer torque (STT). Recently, by taking advantage of the spin-dependent thermoelectric properties of magnetic materials, novel means of generating spin currents from temperature gradients, and their associated thermal-spin torques (TSTs), have been proposed, but so far these TSTs have not been large enough to influence MTJ switching. Here we demonstrate significant TSTs in MTJs by generating large temperature gradients across ultrathin MgO tunnel barriers that considerably affect the switching fields of the MTJ. We attribute the origin of the TST to an asymmetry of the tunneling conductance across the zero-bias voltage of the MTJ. Remarkably, we estimate through magneto-Seebeck voltage measurements that the charge currents that would be generated due to the temperature gradient would give rise to STT that is a thousand times too small to account for the changes in switching fields that we observe.MRAM | spintronics | spin caloritronics U sing heat to create potential gradients and charge currents has been a very active area of research in thermoelectrics (1). Spin caloritronics (2, 3) adds a new dimension to this concept by considering the use of heat to create spin-dependent chemical potential gradients in ferromagnetic materials (4). Traditionally, electric-current-driven spin currents have been used to transport spin angular momentum to change the magnetization of a magnetic material--a phenomenon known as spin-transfer torque (STT) (5-7). Heat currents can also create spin currents in magnetic materials; the transfer of spin angular momentum through this process has been named thermal-spin torque (TST) (8,9). A panoply of recent experiments that use spin currents generated by heat has been reported which includes the spin-Seebeck effect observed in ferromagnetic metals (10, 11), semiconductors (12) and insulators (13), thermal-spin injection from a ferromagnet into a nonmagnetic metal (14), the magnetoSeebeck effect observed in magnetic tunnel junctions (MTJ) (15-17), Seebeck spin tunneling in ferromagnet-oxide-silicon tunnel junctions (18), and several others (19,20). On the other hand, whereas there have been several theoretical predictions (8,9,21,22) of the TST, there have been few experiments to date. In one experiment, evidence of TST was established in Co-Cu-Co spinvalve nanowires (23). However, in this work the same current was used for both heating and probing the device, thus making it difficult to unravel the individual contribution of TST from the simultaneously generated STT.In our device, the heating current is distinct from the probing current, which helps to decouple pure temperature gradient effects from charge-current-driven STT effects. We find that a temperature gradient of ∼1 K/nm across a 0.9-nm-thick MgO tunnel barrier in an MTJ induces modest charge currents on the order of 1 × 10 3

Perspective: Ultrafast magnetism and THz spintronics

“…55 Spin-transfer torque driven by extreme temperature gradients seems feasible, 56,57 as indicated by researches published so far. 58,59 New concepts in spintronic devices focus on a more compact version of the magnetic device structures. This is possible by "bending" electrons and exploiting the third dimension.…”

Thermal spin torques in magnetic insulators

“…It was later shown that heat couples to magnetization dynamics [16][17][18]. The effect of heat on magnetization was also found in magnetic tunnel junctions [19] and metallic spin valves [20]. Slonczewski predicted that a spin-transfer torque induced by thermal magnons could be more efficient than the usual electrically induced spin torques [8].…”

Microscopic calculation of thermally induced spin-transfer torques

“…(8) the summation is carried out over normal and magnetic impurities with scattering strengths u i and u s , respectively. We assume a uniform and isotropic distribution of impurity positions (R j and R ) and impurityspin direction (S ), respectively, and take a quenched average as (9) where n i (n s ) is the concentration of normal (magnetic) impurities and S imp is the magnitude of the impurity spin. Equation (9) is a tensor in spinor space.…”

“…Such a large temperature gradient can be realized in magnetic nanostructures by focused pulsed laser heating. Evidence for thermal torques affecting the magnetization dynamics has been obtained in spin valves [8] and magnetic tunnel junctions [9]. Domain wall motion under a temperature gradient has been observed in magnetic insulators in which spin currents are carried by magnons [10].…”