Spin Caloritronics

This is a brief overview of the state of the art of spin caloritronics, the science and technology of controlling heat currents by the electron spin degree of freedom (and vice versa).Comment: To be published in "Spin Current", edited by S. Maekawa, E. Saitoh, S. Valenzuela and Y. Kimura, Oxford University Pres

Single shot Kerr magnetometer for observing real-time domain wall motion in permalloy nanowires

Details are presented of a single shot focused magneto-optic Kerr effect (MOKE) magnetometer which is used to capture the movement of single domain walls (DWs) in permalloy (Ni80 Fe20) nanowires in real time. By probing the DW motion within the 1μm diameter laser spot of the instrument, DW velocity and pinning field distributions were obtained. An external field was ramped up linearly, and depinning of a DW from the same start position was observed at three different fields, indicating the stochastic nature of the DW motion

Effects of combined current injection and laser irradiation on Permalloy microwire switching

Combined field- and current-induced domain wall (DW) motion in Permalloy microwires is studied using fast magneto-optical Kerr-microscopy. On increasing the current density, we find a decrease of Kerr signal contrast, corresponding to a reduction in the magnetization, which is attributed to Joule heating of the sample. Resistance measurements on samples with varying substrates confirm that the Curie temperature is reached when the magneto-optical contrast vanishes and reveal the importance of the heat flow into the substrate. By tuning the laser power, DWs can be pinned in the laser spot, which can thus act as a flexible pinning site for DW devices

Direct imaging of current-induced domain wall motion in CoFeB structures

By direct x-ray photoemission electron microscopy imaging, we probe current-induced domain wall motion in 20 nm thick CoFeB wires. We observe transverse walls for all wire widths up to 1500 nm as a consequence of the small saturation magnetization of the material. High critical current densities above 1x10 12 A/m2 for wall displacement due to the spin transfer torque effect are found. The critical current densities jc increase further with decreasing wire width indicating that jc is governed by extrinsic pinning due to edge defects. In addition to wall displacements, we observe wall transformations to energetically favorable wall types due to heating. Owing to the high Curie temperature though, the sample temperature stays below the Curie temperature even for the highest current densities where structural damage sets in

Scaling of spin relaxation and angular momentum dissipation in permalloy nanowires

We study the relationship between the damping (α) and the nonadiabaticity of the spin transport (β) in permalloy nanowires. α is engineered by Ho doping, and from the characteristics of the current-induced domain-wall velocity, determined by high-resolution x-ray magnetic circular-dichroism photoemission electron microscopy, β due to spin relaxation is measured. We find that β scales with α and conclude that the spin relaxation that leads to nonadiabatic spin torque originates from the same underlying mechanism as the angular momentum dissipation that causes viscous damping

Magnetic-field-induced domain-wall motion in permalloy nanowires with modified Gilbert damping

Domain wall (DW) depinning and motion in the viscous regime induced by magnetic fields, are investigated in planar permalloy nanowires in which the Gilbert damping α is tuned in the range 0.008–0.26 by doping with Ho. Real time, spatially resolved magneto-optic Kerr effect measurements yield depinning field distributions and DW mobilities. Depinning occurs at discrete values of the field which are correlated with different metastable DW states and changed by the doping. For α<0.033, the DW mobilities are smaller than expected while for α≥0.033, there is agreement between the measured DW mobilities and those predicted by the standard one-dimensional model of field-induced DW motion. Micromagnetic simulations indicate that this is because as α increases, the DW spin structure becomes increasingly rigid. Only when the damping is large can the DW be approximated as a pointlike quasiparticle that exhibits the simple translational motion predicted in the viscous regime. When the damping is small, the DW spin structure undergoes periodic distortions that lead to a velocity reduction. We therefore show that Ho doping of permalloy nanowires enables engineering of the DW depinning and mobility, as well as the extent of the viscous regime