Nano scale thermo-electrical detection of magnetic domain wall propagation

A group of European researchers from the Institute of Physics and the Charles University in Prague, Czech Republic, the Physikalisch-Technische Bundesanstalt in Braunschweig, Germany, the Hitachi Cambridge laboratory in Cambridge and the National Physical Laboratory in Teddington, in the United Kingdom have developed an unprecedented thermo-electric method to detect the propagation of a magnetic domain wall with nanometer precision.

In magnetic nanowires with perpendicular magnetic anisotropy (PMA) magnetic domain walls (DW) are narrow and can move rapidly driven by current induced torques. This enables important applications like high-density memories for which the precise detection of the position and motion of a propagating DW is of utmost interest. Today’s DW detection tools are often limited in resolution, or acquisition speed, or can only be applied on specific materials. The researchers show that the anomalous Nernst effect provides a simple and powerful tool to precisely track the position and motion of a single DW propagating in a PMA nanowire. They detect field and current driven DW propagation in both metallic heterostructures and dilute magnetic semiconductors over a broad temperature range. The demonstrated spatial resolution below 20 nm is comparable to the DW width in typical metallic PMA systems. The work is currently under peer review at Nature Nanotechnology.

Legend:

• ANE DW detection in a (GaMn)(AsP) microwire: (a) Principle of ANE based DW detection: A transverse temperature gradient ∇Ty generates an ANE voltage V_ANE along the wire depending on x_DW.
• False colour electron micrograph of the device. Yellow: wire, red: Pt heater line; blue: Au nucleation strip line.
• Magnetisation reversal observed by ANE at various heater currents.
• ANE measurement during field induced DW propagation. Normalized V_ANE vs. B_app. P_heat = 0.5 mW. Sweep rate 0.25 mT/s. At B_app ≅ 0.05 mT a DW is nucleated (blue arrow). Plateaus result from pinning at intrinsic unintentional pinning sites. Inset: MOKE microscope images corresponding to the three pinned DW states marked (i)-(iii).
• ANE measurement of spin torque induced DW propagation. B_app = 0. ANE data is taken after application of 1μs pulses of current density j as indicated. For high current density high DW velocities up to 30 m/s are found. For lower current density DW propagation is hindered by pinning at various nonintentional pinning sites.