15 mars 2009
SPIN COUPLING IN DOMAIN WALLS
Contact: Gilles Gaudin

Fig. 1: The force being exerted by the mouse is equal to the one exerted by the mass on the plate.

In Hard disk drives and magnetic memories, information is stored in a magnetic domain, associating magnetization direction to logic 1 and 0, up or down direction, which is written using a magnetic field. To locally manipulate the border of these domains, the domain wall, with an electrical current would enable the creation of new memory architectures and magnetic logic elements that would be faster, denser and less power consuming. All that is required is to move this wall in a controlled fashion! Spintec has performed the first direct measurement of the term controlling the wall displacement efficiency within a magnetic nanowire submitted to a current flow. We have found a parameter that enables an increase in this efficiency by two orders of magnitude.

Why was the domain wall displacement efficiency not previously determined? Ideally, one would have wished to measure the wall velocity and compare it to theoretical models so as to understand the physical origins of the displacement, and then to optimise it. In practice, the wall does not move as freely as one might want:  its motion is hindered by the roughness at the nanowire edges and defaults, causing a sudden intrinsic slowdown, which often remains unknown.

Fig. 2a: In the patterned nanowire, the geometric constriction pins the wall in the area between the two red and orange domains of opposite orientation.

Fig. 2b: The current I tends to push the wall, creating a voltage change in the arms of the Hall cross. The same effect is obtained by applying a magnetic field in the same direction as the red domain. In this way, we can find the magnitude of the torque created by the current.

The spring in the wall

The problem can be looked at from a different angle, as shown in Fig. 1. What is the force exerted by the mouse on the spring? It is the same as the mass extending the spring by the same length.  Applying this principle to the domain wall: what is the torque exerted on the wall by the current?  It is the same as that of a known magnetic field, which would have the same effect. A wall that is held back by a constriction in the magnetic material (Fig. 2a) acts as a spring: every displacement of this wall, under the effect of the current, generates a voltage change in the Hall effect cross (Fig. 2b), that is proportional to the displacement itself. Since, at equilibrium, there are no problems due to friction in material defaults and damping, this could be done.

And Spintec has done it! The sample was a cobalt-patterned nanowire, sandwiched between two non-magnetic layers, creating an appropriate domain wall configuration. We then clearly showed that the current action was equivalent to the action of the magnetic field applied perpendicularly to the sandwich, and this equivalence gave us the first direct measurement of spin transfer torque efficiency.

Asymmetric sandwiches taste better

Comparing this efficiency in different materials, we have found that it was almost 100 times larger in Pt/Co/AlOx stacks as opposed to Pt/Co/Pt identical stacks. This difference is due to the asymmetry in the stack: the magnetic field felt by the electrons at one interface is not compensated at the other interface. Mastering this parameter to increase the displacement efficiency under a current flow will allow the walls to travel faster with less current, and perhaps the use of this effect to produce new memory and magnetic logic devices!

Further reading: I. M. Miron, et al., Physical Review Letters 102 (2009) 137202

The current pushes the magnet

In magnetic materials, one area where all the magnetic moments are aligned in the same direction is called a magnetic domain, and this alignment defines the direction of the local magnetization M. When two neighbouring domains have different orientations, the transition area separating them is called a “domain wall”. Within the wall, magnetization is continuously turning from one direction to another (orange to red arrows on the picture).

The electrons of the current also have an elementary magnetic moment, the spin. When a current flows through a structure having different domains and walls, these spins (blue arrows) align themselves to the local magnetization. The alignment poses no problem in a domain where all spins have the same orientation, but in the wall, local magnetization changes faster than the electron spin can adapt. The resulting misalignment creates a torque force on the magnetic moments of the wall, the spin transfer torque, thus rotating their direction. The consequence is a displacement of the domain wall, pushed by the current.

Maj : 20/02/2014 (971)