Thermal Spin Transfer Torque
An electrical current flowing through a metallic spin-valve or a magnetic tunnel junction (MTJ) consisting of a first, ‘fixed’ magnetic layer separated by a non-magnetic spacer from a second, ‘free’ magnetic layer will gain a spin polarization under the influence of the fixed layer. The spin polarized current will then exert a torque on the local magnetization of free layer, allowing for its magnetization to be manipulated via the so-called “spin transfer torque” (STT), which can be exploited for consumer applications such as STT-RAM or tunable radio-frequency emitters or receivers. Theory predicts that similar effects can be observed in the absence of an electrical current, if a large enough thermal gradient can be applied across the spacer layer, as pure spin currents can be induced by thermal gradients . Indeed, it has been theoretically predicted that a temperature gradients of the order of 10K over three monolayers of an MgO barrier in an defect-free, ideal Fe / MgO / Fe tunnel junction can induce thermal spin-transfer torques large enough to cause magnetization dynamics and/or switching. The thermal torques in tunnel junctions with ultrathin MgO barriers are mediated by multiple scattering between interface states in the spacer .
The main objective of this project is to provide experimental evidence confirming the existence of thermal spin-transfer torques in MgO-based magnetic tunnel junctions and to investigate their angular dependence. We propose to achieve this by combining two previously demonstrated phenomena, namely that (1) spin-transfer torques alter FMR signals (in terms of damping and frequency)  and (2) 2D microresonators can be used to investigate magnetic resonance in single magnetic nanostructures . Specifically, we intend to use suitably designed microresonators in order to analyze how the FMR signal corresponding to the free layer of an in-plane MgO-based tunnel junction device is modified in the presence of a temperature gradient across the barrier.
Fig. 1 shows an SEM image of a resonator designed to allow for the characterization of the FMR response of a tunnel junction with a lateral size of the order of 1 µm. Microresonator fabricated by UV lthography has 20 µm loop diameter, with 3µm wide sample.
The experiments are carried by placing the device, which is fabricated in the microresonator, is loaded on the rotatable sample holder. Sample is measured in reflection geometry. DC magnetic field is swept with particular range while microwave power produces RF magnetic field. At resonance condition, reflected signal is measured by using lock-in technique (Fig2).
This project is funded under the Schwerpunktprogramm SPP 1538 Spin Caloric Transport (SpinCaT). (http://www.spincat.info/)
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· Prof. Günter Reiss, University of Bielefeld
Fig. 1. SEM image of magnetic tunnel junction integrated in microresonator.
Fig. 2. The sketch of experimental setup to allow ferromagnetic resonance measurement on single micro/nano sized sample..