Seeking large thermoelectric effects in MgO-based tunnel junctions

Abstract

There is much controversy concerning thermoelectric effects in the MgO-based magnetic tunnel junctions (MTJs) as reported in some recent publications. To clarify the problem, we give calculations from atomic first-principles systemically. Large Seebeck coefficient (S) and up-limit of figure of merit (ZT) are predicted in double-and multi-barrier MTJs, with those in single-barrier MTJs being relatively small. By restraining the phonon thermal conductance through the introduction of one vacuum barrier or numbers of MgO barriers, the up-limit of ZT can be obtained. Room temperature m » --S 220 VK 1 and ZT » 3.5 are predicted in an asymmetric Fe | MgO | Fe | Vaccum | Fe MTJs. The resonant quantum-well states are suggested to be responsible for the enhanced thermoelectric effects in the MTJs with double-and multi-barrier.

Strain-induced enhancement of the Seebeck effect in magnetic tunneling junctions via interface resonant tunneling: Ab initio study

“…In the definition, since there is usually no periodicity along the z axis in MTJs, we use G/A, the conductance divided by the cross-sectional area of the MTJ, instead of the electrical conductivity, which is the analog of the resistance area product RA and used in Ref. [25]. The power factor of the Fe(7ML)/MgO(nML)/Fe(7ML)(001) MTJ (from n = 3 to n = 12) for the parallel magnetization configuration for the compressive and the extensive tetragonal distortions of the in-plane lattice parameter is shown in Fig.…”

“…For example, the interfacial termination has been found to affect the Seebeck effect in MTJs [18,23,24]. Furthermore, Jia et al [25] have reported that the interface resonant tunneling significantly changes the energy dependence of the tunneling conductance. They have calculated the Seebeck coefficient of Fe/MgO/Fe(001) MTJs, which are known to exhibit interface resonant tunneling due to the large DOS of minority-spin states at the interface [31][32][33].…”

“…The heat-to-charge current conversion, namely, the Seebeck effect, in MTJs has been recently investigated both experimentally [6][7][8][9][10][11][12][13][14][15][16] and theoretically [17][18][19][20][21][22][23][24][25][26] , in which most studies have considered the tunnel magneto-Seebeck (TMS) ratio 27 , the thermoelectric analog of the tunnel magnetoresistance ratio. MTJs with a large Seebeck effect have potential not only to produce a large TMS ratio, but also to realize various applications such as thermal energy harvesting, thermoelectric cooling in nanoscale spintronic devices 27 , and scanning Seebeck tunneling microscopy 28,29 .…”

“…They have calculated the Seebeck coefficient of Fe/MgO/Fe(001) MTJs, which are known to exhibit interface resonant tunneling due to the large DOS of minority-spin states at the interface [31][32][33] . While the enhancement of the Seebeck coefficient due to the interface resonant tunneling has been pointed out, detailed properties of the interface resonant tunneling have not been discussed in their work 25 . In particular, it is expected that the interface resonant tunneling strongly depends on the structural parameters of MTJs, such as the barrier thickness and lattice distortion.…”

Tunnel magneto-Seebeck effect

“…The TMS in double-barriers [108,109] and the role of magnon transport in the TMS [110] have been discussed recently from the theoretical side. However, experiments following these ideas have not yet been conducted.…”