Thermal and magnetoelastic effects in magnetic tunnel junction

Abstract

Heat dissipation in spintronic devices is currently a limiting factor for their further miniaturization, spurring scientists on seeking for more energy friendly mechanisms to manipulate spin. In this work, two new approaches for stimulating local spin or magnetization in a magnetic tunnel junction (MTJ), being a very important and industrially relevant device, are presented. The studies are based on thermal and magnetoelastic effects on the local spin of MTJs with the stimulus of heat pulses and acoustic pulses generated by femtosecond laser pulses. The information of the local spin, being included in the spin dependent electric conductance or resistance of the MTJ, is studied by time resolved and static resistance detection methods. Thereby, optically generated temperature and temperature gradient effects on the spin are considered separately. For the study of temperature effects, the temperature dependence of the resistance of the MTJ is employed. The measured resistance and temperature traces reveal that the resultant resistance change after laser excitation mainly results from temperature effects, which is supported by finite element simulations. Still, the local spin is also influenced by a temperature gradient effect. Based on the intrinsic magnetic properties of the MTJ, the temperature and temperature gradient effects can even be separated. The thermal effect on the magnetization can only be observed when the laser beam is focused directly on the MTJ, while the magnetization dynamics can be excited by acoustic pulses (surface acoustic waves) when the laser pulse hits the surface several μm away from the MTJ. The excited magnetization dynamics due to the magnetoelastic effect strongly depends on the laser heating position and applied magnetic field. Comparing the acoustic wave induced precession frequencies with precession induced by charge currents and with micromagnetic simulations, we identify spatially non-uniform magnetization modes localized close the edge regions as being responsible for the optically induced magnetization dynamics. Two acoustic pulses created by the laser even allow us to coherently control the magnetization precession. The study presented in the thesis shows that the manipulation of spins can be achieved with femtosecond laser pulse heating in a straightforward way. Additionally,the techniques employed in the study enable the use of MTJs for novel applications such as temperature and strain sensors with a large dynamic range.