Tunneling Anisotropic Magnetoresistance: A Spin-Valve-Like Tunnel Magnetoresistance Using a Single Magnetic Layer

We introduce a new class of spintronic devices in which a spin-valve-like effect results from strong spin-orbit coupling in a single ferromagnetic layer rather than from injection and detection of a spin-polarized current by two coupled ferromagnets. The effect is observed in a normal-metal–insulator–ferromagnetic-semiconductor tunneling device. This behavior is caused by the interplay of the anisotropic density of states in (Ga,Mn)As with respect to the magnetization direction and the two-step magnetization reversal process in this material.

Figure 1

(a) Device schematic showing the contact geometry and the crystallographic directions. (b) Hysteretic magnetoresistance curves acquired at 4.2 K with 1 mV bias by sweeping the magnetic field along the 0°, 50°, and 55° directions. Spin-valve-like features of varying widths and signs are clearly visible, delimited by two switching events labeled $H_{c1}$ and $H_{c2}$. The magnetoresistance is independent of the bias direction or amplitudes up to 1 meV. (c) TAMR along 30° for temperatures from 1.6 to 20 K, showing a change of sign of the signal. The curves are vertically offset for clarity.

Figure 2

Polar plot compiled from individual magnetoresistance curves. The circles indicate the switching events $H_{c1}$ and $H_{c2}$ from the individual curves. The shaded areas are regions where the sample is in a high-resistance state. The solid lines are a fit to the model described in the text.

Figure 3

The relative difference between partial DOS at the Fermi energy for $M$ along $[010]$ and$[100]$ directions is plotted separately for each of the four occupied valence bands. Note that in the ferromagnetic state, even the near $k=0$ states, cannot properly be called light and heavy holes due to the $pd$ exchange interaction. Dashed lines correspond to $\mathrm{M}{\mathrm{n}}_{\mathrm{G}\mathrm{a}}$ concentration of 4%; solid lines corresponding to 6% Mn doping are shown for comparison.

Figure 4

The relative integrated DOS anisotropy is plotted for different Mn (left panel) and hole (right panel) concentrations. The $x$ axis represents the DOS at the Fermi energy that is assumed to contribute to tunneling, relative to the total DOS at the Fermi energy. Moving from left to right corresponds to gradually relaxing the momentum conservation condition.