Tunneling Anisotropic Magnetoresistance in Multilayer-(Co/Pt)/AlOx/PtStructures

Abstract

We report observations of tunneling anisotropic magnetoresitance (TAMR) in vertical tunnel devices with a ferromagnetic multilayer-(Co/Pt) electrode and a non-magnetic Pt counter-electrode separated by an AlOx barrier. In stacks with the ferromagnetic electrode terminated by a Co film the TAMR magnitude saturates at 0.15% beyond which it shows only weak dependence on the magnetic field strength, bias voltage, and temperature. For ferromagnetic electrodes terminated by two monolayers of Pt we observe order(s) of magnitude enhancement of the TAMR and a strong dependence on field, temperature and bias. Discussion of experiments is based on relativistic ab initio calculations of magnetization orientation dependent densities of states of Co and Co/Pt model systems.PACS numbers: 85.75. Mm,75.45.+j,75.50.Cc Anisotropic magnetoresistance (AMR) sensors replaced in the early 1990s classical magneto-inductive coils in hard-drive readheads launching the era of spintronics. Their utility has, however, remained limited partly because the response of these ferromagnetic resistors to changes in magnetization orientation originates from generically subtle spin-orbit (SO) interaction effects [1]. Currently widely used giant magnetoresistance [2] and tunneling magnetoresistance (TMR) [3] elements comprising (at least) two magnetically decoupled ferromagnetic layers provided a remarkably elegant way of tying the magnetoresistance response directly to the ferromagnetic exchange splitting of the carrier bands without involving SO-coupling. Large magnetoresistances in these devices are, nevertheless, obtained at the expense of a significantly increased structure complexity, necessary to guarantee independent and different magnetization switching characteristics and spin-coherence of transport between the ferromagnetic layers.Studies of AMR effects [4,5,6,7] in ferromagnetic semiconductor tunneling devices showed that AMR response can in principle be huge and richer than TMR, with the magnitude and sign of the magnetoresistance dependent on the magnetic field orientation and electric fields. Subsequent theoretical work predicted [8] that the tunneling AMR (TAMR) effect is generic in ferromagnets with SO-coupling, including the high Curie temperature transition metal systems. A detailed investigation of the TAMR is therefore motivated both by its intricate relativistic quantum transport nature and by its potential in more versatile alternatives to current TMR devices which will not require two independently controlled ferromagnetic electrodes and spin-coherent tunneling.Experimental demonstration of the TAMR in a tunnel junction with a ferromagnetic metal electrode has recently been reported [9] in an epitaxial Fe/GaAs/Au stack. The observed TAMR in this structure is relatively small, bellow 0.5%, consistent with the weak SO-coupling in Fe. In this paper we present a study of vertical tunnel devices in which the ferromagnetic electrode comprises alternating Co and Pt films. We build upon the extensive literature [10,11,12,13,14,15] o...

Anisotropy of spin polarization and spin accumulation in Si/Al2O3/ferromagnet tunnel devices

“…For CoPt film, the anisotropy in tunneling DOS was predicted 11 to be >12%. This was confirmed 8 by exploiting the enhanced SOC at the interface with CoPt based electrodes giving rise to large TAMR of 15%. Although the TAMR effect may be small in magnitude in tunnel contacts with transition metal FMs, it may influence the spin injection from a FM into a semiconductor (SC).…”

“…A feature that provides more insight is the anisotropy of the tunnel conductance, which may have various sources, including the TAMR. Previous reports on TAMR concluded that the change in tunnel resistance is due to the anisotropy in DOS [3][4][5]8,9,11,21 at the tunnel interface between a FM and an insulating barrier. As the magnetization direction is rotated, it faces different DOS, thereby changing the transport across the tunnel contact.…”

“…The in-plane [3][4][5][6][7] TAMR refers to the change in tunnel resistance when the magnetization is rotated in the plane of the magnetic layer. On the other hand, out-of-plane 2,5,[8][9][10] TAMR refers to the change in tunneling resistance when the magnetization is rotated from in-plane to out-of-plane.…”

Current-induced spin–orbit torques

“…Indeed, a metal/oxide bilayer is already half of an MTJ, which could be used as a readout element of a magnetic bit where the write functions are performed by the SO torque [15]. Moreover, several studies have pointed out the interplay between SO coupling and tunnelling anisotropic magnetoresistance in MTJs [99][100][101] and explicitly considered the dependence of the tunnelling conductance on interfacial Rashba spin splitting in metal systems [102,103] as well as the coexistence of Dresselhaus and Rashba fields in metal/semiconductor heterojunctions [31,65,104], providing additional functionalities to layers displaying strong SO effects. Further, as noted by Obata & Tatara [59], DW manipulation can be achieved by means of an SO torque, depending on the type of DW involved and easy axis magnetization direction, which may be useful in, for example, shift register or random access memory applications based on DW motion.…”

Tetragonal phase of epitaxial room-temperature antiferromagnet CuMnAs

“…Low Curie temperatures have prevented the direct integration of (III,Mn)V FM semiconductors into spintronic technologies. Nevertheless, new phenomena discovered in (Ga,Mn)As have been subsequently reported in the room-temperature metal FMs, relevant not only for basic science but also for practical spintronic applications [14][15][16] .…”