Spin@RT : Domain Wall Spintronics

NiFe and CoFeB shape anisotropy devices

Figure 3. OOMMF2 simulation of the H structure with a 400nm constriction.

This type of structure is part of a magneto-electronic device can be used to extract the diffusive current spin polarization³ of this technologically important material via the domain wall resistance. Further, affects associated with reduction of dimensionality of the structure and the domain wall structure under non-equilibrium conditions (i.e. with an applied current density) can be explored at a length scale within the critical device dimensions (50-400nm).

Figure 4. Domain wall resistance in a 'tank' structure at 300 K

 In these multilayer devices, we study how to nucleate domain walls in the presence of   perpendicular magnetic anisotropy . The coercivity can then be altered by Ga+ ion implantation. The local mangetization couples to the anomalous Hall conductivity, and thuse, the position of a domain wall is inferred through electrical measurements. The propagation of a wall with applied magnetic field can be studied in Hall bar type devices. Real-time MOKE imaging can reveal the magnetic contrast and wall motion can be observed.

Figure 5. Magnetic field induced domain propagation observed by MOKE microscopy in Co/Pt multilayer Hall bars.

FePt and FePd L₁0 ordered materials

Figure 6 . Magnetic force micrograph of the labyrinth stripe domain structure in an L10 FePt film.

The highly anisotropic crystal structure of these materials leads to an extremely strong magnetosrystalline anisotropy, with K~3MJ/m³. The domain structures in these materials are of particular interest for studies of domain wall resistance, as they are so narrow. We have previously published4 on the magnetoresistive properties of FePd grown by MBE. The domain wall resistance is large enough to be easily measured since the very high value of K leads to narrow walls (typically <10 nm thick) and a dense stripe domain structure, so that volume fraction of the film occupied by the walls is high. The significance of this is that one can use the theory domain wall resistance of Levy and Zhang5 to determine the spin-polarisation of an ordinary diffusive current flowing in a metal, as the magnetoresistance ratio is determined only by the ratio of the spin-  to spin-  resistivity,  , whilst the overall scattering rate is irrelevant.

 Lorentz TEM of magnetic layers and devices

Lorentz TEM is a powerful tool for performing high resolution imaging of magnetic texture of structured materials. The University of Glasgow have been developing this teachnique for investigating domain nucleation and field driven motion in nanostructures created by focused ion beam milling.

Figure 7. Domain wall pinning in nanostructures

Micron-scale MOKE measurement at the University of Druham is an essential tool for detecting domain wall nucleation and propagation in devices with nanoscale critical dimensions. Quantitative information such as coercive field can be extracted and compared with micromagnetic simulation.

 

Further Reading