Nanoscale magnetic heat pumps and engines

Abstract

We present the linear-response matrix for a sliding domain wall in a rotatable magnetic nanowire, which is driven out of equilibrium by temperature and voltage bias, mechanical torque, and magnetic field. An expression for heat-current-induced domain-wall motion is derived. Application of Onsager's reciprocity relation leads to a unified description of the Barnett and Einstein-de Haas effects as well as spin-dependent thermoelectric properties. We envisage various heat pumps and engines, such as coolers driven by magnetic fields or mechanical rotation as well as nanoscale motors that convert temperature gradients into useful work. All parameters ͑with the exception of mechanical friction͒ can be computed microscopically by the scattering theory of transport.

Spin caloritronics

“…In analogy to the Gilbert damping factor α the dissipation under an applied current is governed by a material parameter β c that for itinerant magnetic materials is of the same order as α [48]. In the case of a heat-current induced domain wall motion, the adiabatic thermal spin transfer torque [11] is also associated with a dissipative β T -factor that is independent of the charge-current β c [49,50]. β T has been explicitly calculated by Hals for GaMnAs [52].…”

“…The elements can be computed by scattering theory [50]. The matrix relation between generalized forces and currents implies a large functionality of magnetic materials.…”

Thermoelectric energy harvesting with quantum dots

“…Alternative spintronic heat engines based on nanowires with domain walls have been proposed in Ref. [154,155].…”

Enhancing the spin transfer torque in magnetic tunnel junctions by ac modulation

“…For a given area of around 46Å 2 [the diameter of a (5,5) CNT is about 6.8Å], the current density is about 1×10 8 A/cm 2 . To meet the requirement of switching current densities in permalloy [34] and MgO-based MTJs devices, which range from 1 × 10 6 to 1 × 10 9 A/cm 2 ,[3, 9, 35, 36] the area of the ferromagnetic leads can be at most a dozen times larger than that of the CNT's, which shows a promising potential for this kind of MTJ device.…”