Pumping laser excited spins through MgO barriers

Abstract

We present a study of the tunnel magneto-Seebeck (TMS) 1 effect in MgO based magnetic tunnel junctions (MTJs). The electrodes consist of CoFeB with inplane magnetic anisotropy. The temperature gradients which generate a voltage across the MTJs layer stack are created using laser heating. Using this method, the temperature can be controlled on the micrometer length scale: here, we investigate, how both, the TMS voltage and the TMS effect, depend on the size, position and intensity of the applied laser spot. For this study, a large variety of different temperature distributions was created across the junction. We recorded twodimensional maps of voltages generated by heating in dependence of the laser spot position and the corresponding calculated TMS values. The voltages change in value and sign, from large positive values when heating the MTJ directly in the centre to small values when heating the junction on the edges and even small negative values when heating the sample away from the junction. Those zero crossings lead to very high calculated TMS ratios. Our systematic analysis shows, that the distribution of the temperature gradient is essential, to achieve high voltage signals and reasonable resulting TMS ratios. Furthermore, artefacts on the edges produce misleading results, but also open up further possibilities of more complex heating scenarios for spincaloritronics in spintronic devices.

Anomalous Nernst effect and three-dimensional temperature gradients in magnetic tunnel junctions

“…From this measurement curve, the TMS ratio is calculated, which results in a ratio of approximately 50%. This is consistent with the findings reported in reference [11]. The measurement confirms the two possibilities for parallel alignment configuration of opposite direction, p1 for negative 0 and p2 for positive 0 .…”

“…TMR junctions provide a rich variety of possibilities to create anisotropic temperature profiles on nm to μm length-scales using positiondependent laser heating. Extending the scanning technique originally developed for the extraction of the preferably pure TMS signal, as introduced in reference [11], allows a systematic temperature gradient variation. The schematic in Figure 1a) depicts this experimental procedure.…”

“…The microscopic origin together with theoretical predictions of the TMS for multiple CoFe compositions with MgO barriers is given in reference [5,6] and is calculated by: TMS = ap − p min (| ap |, | p |) . (1) The TMS effect has been observed and analyzed for various combinations of barrier and electrode materials, showing thermovoltages in the μV range for MgO [7][8][9][10][11][12][13] andMgAl 2 O 4 [14,15], and reaching themV range for Heusler based MTJs [16]. All examined material configurations result in specific TMS ratios.…”

“…One must apply a large temperature gradient that across the junction and the whole junction area needs to be heated homogeneously. As a consequence, using all-optical laser heating, the spot size needs to be adapted to the junction size and positioned centrally in order to create a well-defined temperature gradient across both electrodes and generate reliable voltages [11]. Temperature gradients deviating from the out-ofplane direction, for example temperature inhomogeneities in the sample plane, lead to additional thermoelectric effects that influence the total Seebeck voltages.…”

Magnonics: spin waves connecting charges, spins and photons

“…This approach is of high interest in magnonics and is known as magnon spintronics. A study of the tunnel magneto-Seebeck effect in MgO based magnetic tunnel junctions is presented in [7]. The electrodes consist of CoFeB with in-plane magnetic anisotropy.…”