-Magnetization relaxation in the single molecule magnet Ni4 under continuous microwave irradiation, G. de Loubens, D. A. Garanin, C. C. Beedle, D. N. Hendrickson and A. D. Kent, Europhysics Letters 83, 37006 (2008).
Spin relaxation between the two lowest-lying spin-states has been studied in the $S=4$ single molecule magnet Ni$_4$ under steady state conditions of low amplitude and continuous microwave irradiation. The relaxation rate was determined as a function of temperature at two frequencies, 10 and 27.8 GHz, by simultaneously measuring the magnetization and the absorbed microwave power. A strong temperature dependence is observed below 1.5 K, which is notconsistent with a direct single-spin-phonon relaxation process. The data instead suggest that the spin relaxation is dominated by a phonon bottleneck at low temperatures and occurs by an Orbach mechanism involving excited spin-levels at higher temperatures. Experimental results are compared with detailed calculations of the relaxation rate using the universal density matrix equation.
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-Spin-torque driven ferromagnetic resonance of Co/Ni synthetic layers in spin valves, W. Chen, J.-M. L. Beaujour, G. de Loubens, A. D. Kent and J. Z. Sun, Applied Physics Letters 92, 012507 (2008).
Spin-torque driven ferromagnetic resonance (ST-FMR) is used to study thin Co/Ni synthetic layer with perpendicular anisotropy confined in spin valve based nanojunctions. Field swept ST-FMR measurements were conducted with a magnetic field applied perpendicular to the layer surface. The resonance lines were measured under low amplitude rf excitation, from 1 to 20 GHz. These results are compared with those obtained using conventional rf field driven FMR on extended films with the same Co / Ni layer structure. The layers confined in spin valves have a lower resonance field, a narrower resonance linewidth, and approximately the same linewidth vs frequency slope, implying the same damping parameter. The critical current for magnetic excitations is determined from measurements of the resonance linewidth vs dc current and is in accord with the one determined from I-V measurements.
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-Spin-transfer in High Magnetic Fields and Single Magnetic Layer Nanopillars, A. D. Kent, Chapter in Handbook of Magnetism and Advanced Magnetic Materials, Kronmuller, H. and Parkin S. (eds). John Wiley & Sons Ltd, Chichester, UK pp 2611-2629, 2007.
Wiley: Handbook of Magnetism and Advanced Magnetic Materials
This article reviews studies of the physics of spin-transfer in high magnetic fields in bilayer and single magnetic layer nanopillars. The focus is on phenomena associated with the action of spin-currents on the background magnetization of thin nanometer scale ferromagnetic layers, know as nanomagnets. Spin-currents are shown to lead to magnetic hysteresis in large fields perpendicular to the layers in bilayer nanopillars. This is consistent with the standard model of spin-transfer within a macrospin approximation. Spin-transfer induced excitations also occur in single magnetic layer nanopillars. This shows that the most elementary samples that exhibit spin-transfer consist of just a single thin magnetic layer, not two magnetic layers. Physics beyond the macrospin and one dimensional transport models are necessary to understand such excitaitons. Non-uniform spin-wave excitations are also seen in the traditional bilayer structures, which experiments reveal to have a rich phase diagram of current-induced magnetic excitations.
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