Tunneling Anisotropic Magnetoresistance in Multilayer-$(\mathrm{Co}/\mathrm{Pt})/{\mathrm{AlO}}_x/\mathrm{Pt}$ Structures

We report observations of tunneling anisotropic magnetoresitance (TAMR) in vertical tunnel devices with a ferromagnetic multilayer-($\mathrm{Co}/\mathrm{Pt}$) electrode and a nonmagnetic Pt counterelectrode separated by an ${\mathrm{AlO}}_x$ 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. The discussion of experiments is based on relativistic ab initio calculations of magnetization orientation dependent densities of states of Co and $\mathrm{Co}/\mathrm{Pt}$ model systems.

Figure 1

Upper panels: schematic layer structures of device $A$ (left) and device $B$ (right). Lower panels: corresponding TAMR traces defined as $\mathbf{(}R(\theta )-R(0)\mathbf{)}/R(0)$ at $-5\mathrm{mV}$ bias and 4 K. Insets: SQUID magnetization measurements in out-of-plane magnetic fields.

Figure 2

(a) Resistance and (b) TAMR defined as $\mathbf{(}R(90°)-R(0)\mathbf{)}/R(0)$ of sample A measured at 10, $-5$, and $-10\mathrm{mV}$ bias and at 4 K. Inset: temperature dependence of the TAMR for the $-5\mathrm{mV}$ bias and 10 T field.

Figure 3

(a) Resistance and TAMR (inset) defined as $\mathbf{(}R(90°)-R(0)\mathbf{)}/R(0)$ of sample $B$ at 10 mV bias and 4 K. (b) Temperature and bias dependence of the TAMR at 10 T field.

Figure 4

(a) Theoretical DOSs at $\theta =0$ and 90° and (b) the relative DOS anisotropy defined as $\mathbf{(}\mathrm{DOS}(90°)-\mathrm{DOS}(0)\mathbf{)}/\mathrm{DOS}(0)$ as a function of energy measured from the Fermi level for the $\mathrm{Pt}/\mathrm{Co}$ and Co model systems. (c) Corresponding experimental differential conductances and (d) relative differential conductance anisotropies as a function of bias.

Figure 5

(a) Theoretical DOSs at $\theta =0$ and 90° for an unrelaxed $\mathrm{Pt}/\mathrm{Co}$ model systems. (b) Experimental TAMR defined as $\mathbf{(}R(\theta =90°,\phi )-R(\theta =0)\mathbf{)}/R(\theta =0)$ for samples $A$ and $B$. (c) Resistance measurements (inset) in two independent bias sweeps showing a discrete step in resistance for the upper curve. Corresponding differential conductances (main panel) showing a relative shift in the oscillatory bias dependence for the two measurements.