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Thermal Relaxation Rates of Magnetic Nanoparticles in the Presence of Fields and Spin-Transfer Effects



William H. Rippard, Ranko R. Heindl, Matthew R. Pufall, Stephen E. Russek, Anthony B. Kos


We have measured the relaxation time of a thermally unstable ferromagnetic nanoparticle incorporated into a magnetic tunnel junction (MTJ) as a function of applied magnetic field and voltage V (-0.38 V {less than or equal to} V {less than or equal to} +0.26 V) over a wide range of temperatures (283 K{less than or equal to} T{less than or equal to} 363 K). By analyzing the results within the framework of a modified Néel-Brown formalism we are able to determine the effective attempt time of the nanoparticle and also the bias dependences of the in-plane and out-of-plane spin transfer torques. We find that there is a significant linear modification of the effective energy barrier with voltage and a significant contribution of a “field-like” torque that is quadratic with voltage, in agreement with some previous studies but not others. The methods presented here do not require complicated models for device heating or calibration procedures, but instead directly measure how temperature, field, and voltage influence the energy landscape and thermal fluctuations of a two-state system. These results should have significant implications for designs of future nanometer-scale magnetic random access memory (MRAM) elements and provide a straightforward methodology to determine these parameters in other MTJ device structures.
Physical Review B


spin transfer, thermal relaxation, superparamagnetic, relaxation rates


Rippard, W. , Heindl, R. , Pufall, M. , Russek, S. and Kos, A. (2011), Thermal Relaxation Rates of Magnetic Nanoparticles in the Presence of Fields and Spin-Transfer Effects, Physical Review B, [online], (Accessed June 13, 2024)


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Created August 29, 2011, Updated November 10, 2018