Changing Exchange Bias in Spin Valves with an Electric Current

We show that a high-density electric current, injected from a point contact into an exchange-biased spin valve, systematically changes the exchange bias. The bias can either increase or decrease depending upon the current direction. This observation is not readily explained by the well-known spin-transfer torque effect in ferromagnetic metal circuits, but could be evidence for the recently predicted current-induced torques in antiferromagnetic metals.

Figure 1

Point-contact magnetoresistance at different bias currents. Solid traces show point-contact resistance $R=V/I$ as a function of the applied magnetic field $B$ for a series of bias currents $I$ ($-35$, $-30$, $-25$, $-20$, $-15$, $-10$, $-5$, 5, 10, 15, 20, 25, 30, 35 mA). Black (gray) traces are for $B$ sweeps down (up). The MR sweeps at different currents are shifted along the vertical axis for clarity. The point-contact resistance at high fields is $0.92\Omega$.

Figure 2

Variation of exchange bias in standard and inverted spin-valve structures. 2D gray-scale plots show the point-contact magnetoresistance (down sweeps) as a function of the bias current in (a) standard, (b) inverted, and (c) symmetric spin-valve structures. Lighter color indicates higher resistance with black/white corresponding to (a) $0.919\Omega /0.926\Omega$, (b) $1.590\Omega /1.596\Omega$, (c) $2.720\Omega /2.733\Omega$. In (b) and (c) the current was stepped up from high negative values while in (a) it was stepped down from high positive ones. See text for details.

Figure 3

Schematic illustration of the influence of transport currents on exchange bias. (a) The surface layer of an antiferromagnet contains uncompensated magnetic moments (gray). A fraction of the uncompensated moments are pinned. They do not reverse when the adjacent ferromagnet reverses and are responsible for the existence of exchange bias. The dotted line indicates an antiferromagnetic domain. (b)-(c) Exchange bias will increase (decrease) if the configuration of pinned moments is altered to increase (decrease) the total component along the exchange-bias direction. The pinned moments are exchange coupled to the bulk antiferromagnet and will be reoriented by torques that act in the bulk of the antitferromagnet.