Photon-assisted thermoelectric properties of noncollinear spin valves

Abstract

We report theoretical analysis of thermal-spin and thermoelectric properties of noncollinear spin-valves driven by a high frequency AC voltage bias. The spin-valve consists of two ferromagnetic contacts sandwiching a single-level or multi-level quantum dot (QD). A general formulation for the time-averaged thermal-spin and thermoelectric properties of spin-valves is derived within the nonequilibrium Green's function theory, which provides a starting point for further numerical calculations of these properties. Numerical results of a spinvalve having a spin-degenerate single-level QD are given as an example. The AC bias induces various photonassisted transmission peaks which can greatly enhance the Seebeck coefficients and the figures of merit, and offer a new possibility to tune both the spin-dependent and normal thermoelectric properties of the spin-valve. Details of these properties and how they depend on the non-collinearity of the spin-valve, magnetic polarization, temperature, AC bias, and other control parameters are reported. A particularly interesting result is the opposite dependency of the thermoelectric properties on the magnetic polarization and non-collinearity for contacts with or without spin accumulation.2

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

“…Recently, it was revealed that spin-orbit coupling may act as another mechanism to efficiently manipulate currentinduced torques. [6,18,19] An interesting possibility that has not been investigated so far, is if STT can be enhanced by applying an external ac modulation (without increasing the total bias) to the MTJ -although it has been known that such ac modulation can increase charge current flow [20].…”

Transient spin current under a thermal switch

“…where J spin = (J spin x , J spin y , J spin z ), σ = (σ x , σ y , σ z ), σ x/y/z are Pauli matrices, and the lesser Green's function is defined as [20] G < ns,kas (t, t ) = i c † kαs (t ) d ns (t) . Note that the integrals of time goes from 0 to +∞, different to the traditional form that goes from −∞ to +∞ [23]. Due to this difference, its inverse transformation is F (t, t ) only when both times variables are later than 0:…”

“…We consider a system which consists of two ferromagnetic (FM) leads and a single-level quantum dot (QD) between the leads. Similar FM/QD/FM systems have been widely studied previously [23,[30][31][32]. When t < 0, three parts are disconnected and in individual thermal equilibrium.…”

New horizons for microwave applications using spin caloritronics

“…Differing from these static transport measurements, Zhang et al performed a dynamic experiment and demonstrated magneto-Seebeck rectification in MTJs, which converts microwaves to a dc voltage through the thermal dynamics. This observation not only motivates the theoretical investigation of thermal-spin and thermoelectric properties (such as figures of merit) of MTJs driven by microwave fields [13], but also paves the way for utilizing spin caloritronics in microwave imaging applications [15,16].…”

“…Recently, the renaissance of thermoelectricity in spintronic devices and magnetic structures is trigged by the enhancement of thermoelectric efficiency in low-dimensional material systems [2,3]. Among various thin films and nanostructures, where spin caloritronics has been experimentally investigated, magnetic tunnel junctions (MTJs) consisting of two ferromagnetic layers separated by a thin insulator are one of the most attractive systems for spin caloritronics [4][5][6][7][8][9][10][11][12][13]. Similar to the tunnel magnetoresistance (TMR), which carries the application for data storage and data processing [14], recently a number of experiments performed on MTJs found a magneto-Seebeck effect with a large Seebeck coefficient (S) up to 1.0 mV/K at room temperature [8][9][10].…”