The tunnel magneto-Seebeck effect in magnetic tunnel junctions

by Marvin Walter
Doctoral thesis
Date of Examination:2013-11-14
Date of issue:2014-01-30
Advisor:Prof. Dr. Markus Münzenberg
Referee:Prof. Dr. Markus Münzenberg
Referee:Prof. Dr. Christian Jooß
Persistent Address: http://dx.doi.org/10.53846/goediss-4335

 

 

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Abstract

English

The present (cumulative) thesis focusses on thermomagnetoelectric effects in magnetic tunnel junctions. In particular, it deals with the measurement of the tunnel magneto-Seebeck (TMS) effect in tunnel junctions incorporating either ferromagnetic cobalt-iron-boron (CoFeB) alloys or ferromagnetic Heusler compounds as electrodes, and magnesium-oxide (MgO) as tunnel barrier. Within the last decade, the investigation of thermoelectric effects combined with spintronic effects has attracted considerable attention. The research field that has emerged is labeled “spin caloritronics”. Since the focus of this thesis is embedded in this field, it starts with a short introduction to thermoelectric effects and recent achievements in “spin caloritronics”. In the main part of the thesis, it is experimentally shown that the thermovoltage of a magnetic tunnel junction - generated by the Seebeck effect - can be switched between two states: one for a parallel (P) and one for an antiparallel (AP) magnetization configuration of the two ferromagnetic electrodes. The required temperature gradients are generated by focussing a modulated laser beam onto the microscopic tunnel junction, and the voltages are then recorded with a lock-in technique. The observed, time-dependent voltage signals can be understood by means of an electric model circuit. Thermal spin-transfer torque allows for the switching between P and AP magnetization configurations caused by a thermally generated, spin-polarized electron current. The parameter space for the aforementioned effect, as given by theoretical predictions, is investigated experimentally. In an outlook, possible control tactics for the TMS effect, such as electrode materials (e.g. Heusler compounds) and the application of bias voltages are discussed, and the thermoelectric efficiency of magnetic tunnel junctions is estimated.
Keywords: magnetic tunnel junctions; thermoelectric effects; Seebeck effect; thermopower; tunnel magneto-Seebeck effect; tunnel magnetoresistance; laser heating; spin-transfer torque; thermal spin-transfer torque
 
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