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OnderzoekZernike (ZIAM)FYSNDResearchMetallic Spintronics

Spin-Torque

Non-local spin transfer torque

In the conventional MRAM or local spin valve geometry the current is sent through the complete stack of layers (pillar) to apply a torque to or read out the magnetization state. Therefore, heating effects and magnetic field induced phenomena are often dominant and obscure the exact influence of spin transfer torque on the magnetization state. By creating a lateral (ie. in plane) structure it is possible to slightly alter this geometry to exclude these disadvantages.

We use a four-terminal non-local geometry where we physically separate the current and voltage paths. The nonlocal spin valve experiment is schematically depicted in Fig. 1a. Two permalloy (Py) electrodes are overlapped with a Cu strip, creating two ferromagnetic/nonmagnetic metal (F/N) interfaces. Spin injection across a F/N interface is well-described in terms of a two-current model with different conductivities for spin-up en spin-down. When a spin-polarized current flows across a F/N interface, the sudden change in spin-dependent conductivity causes a spin splitting of the electrochemical potential. As a result, a spin accumulation builds up in the region close to the interface and decays with the spin relaxation length. Electrical spin injection and detection in a nonmagnetic metal was demonstrated f rst by Johnson and Silsbee [1] and succeeded later by Jedema et al. [2] in a lateral structure, at room temperature, by performing nonlocal spin valve measurements.

The voltage across the Cu/FM2 interface depends on the direction of the spin accumulation and the magnetization and varies stepwise as a function of the magnetic field, due to the switching between parallel and anti-parallel state of both FMs (Fig. 2b). Because the injected spins get very effectively absorbed in the second FM, angular momentum is transferred and results in a torque on the magnetization. Our goal is to realize an efficient transfer of angular momentum from FM1 to FM2 such that the magnetization of the second ferromagnet can be manipulated purely by a spin current.

Fig. 1a) Non Local Spin Valve Geometry. Current is injected into the copper via FM1. The injected spins diffuse towards the second ferromagnet where it produces a torque on the magnetization. This torque can, if sufficiently large, switch the magnetization direction.
Fig. 1a) Non Local Spin Valve Geometry. Current is injected into the copper via FM1. The injected spins diffuse towards the second ferromagnet where it produces a torque on the magnetization. This torque can, if sufficiently large, switch the magnetization direction.
Fig .1b) NLSV signal as function of magnetic field. When the magnetization is swept from a negative field to a positive field one FM switches its magnetization before the other. Hence the stepwise changes in the generated spin voltage.
Fig .1b) NLSV signal as function of magnetic field. When the magnetization is swept from a negative field to a positive field one FM switches its magnetization before the other. Hence the stepwise changes in the generated spin voltage.

Modeling

We make use of  3D finite element modeling using Comsol Multiphysics to simulate the devices we fabricate. So far, we are in the process of optimizing the lateral geometry for efficient spin torque injection.

Fig. 2: 3D simulation of spin transport. This picture shows how a spin accumulation diffuses towards a 2nd FM when a current is sent non locally through the 1st FM.
Fig. 2: 3D simulation of spin transport. This picture shows how a spin accumulation diffuses towards a 2nd FM when a current is sent non locally through the 1st FM.
Experiments

To be updated soon.
References

  1. Phys. Rev. Lett. 55, 1790 (1985)
  2. Nature 410, 345 (2001)
Bachelor/Master projects

There are a couple of projects available concerning this topic. Please contact one of the people involved in case you are interested.

People involved in this project:

Joost Flipse - Ph.D.
Fasil Dejene - PhD
Bart van Wees - Group Leader

Last modified:09 July 2015 10.59 a.m.