Alexander Boehnke

Physical characteristics and cation distribution of NiFe2O4 thin films with high resistivity prepared by reactive co-sputtering

We fabricated NiFe2O4 thin films on MgAl2O4 (001) substrates by reactive dc magnetron co-sputtering in a pure oxygen atmosphere at different substrate temperatures. The film properties were investigated by various techniques with a focus on their structure, surface topography, magnetic characteristics, and transport properties. Structural analysis revealed a good crystallization with epitaxial growth and low roughness and a similar quality as in films grown by pulsed laser deposition. Electrical conductivity measurements showed high room temperature resistivity (12 Ω m), but low activation energy, indicating an extrinsic transport mechanism. A band gap of about 1.55 eV was found by optical spectroscopy. Detailed x-ray spectroscopy studies confirmed the samples to be ferrimagnetic with fully compensated Fe moments. By comparison with multiplet calculations of the spectra, we found that the cation valencies are to a large extent Ni2+ and Fe3+.

Time-resolved measurement of the tunnel magneto-Seebeck effect in a single magnetic tunnel junction

Recently, several groups have reported spin-dependent thermoelectric effects in magnetic tunnel junctions. In this paper, we present a setup for time-resolved measurements of thermovoltages and thermocurrents of a single micro- to nanometer-scaled tunnel junction. An electrically modulated diode laser is used to create a temperature gradient across the tunnel junction layer stack. This laser modulation technique enables the recording of time-dependent thermovoltage signals with a temporal resolution only limited by the preamplifier for the thermovoltage. So far, time-dependent thermovoltage could not be interpreted. Now, with the setup presented in this paper, it is possible to distinguish different Seebeck voltage contributions to the overall measured voltage signal in the μs time regime. A model circuit is developed that explains those voltage contributions on different sample types. Further, it will be shown that a voltage signal arising from the magnetic tunnel junction can only be observed when the laser spot is directly centered on top of the magnetic tunnel junction, which allows a lateral separation of the effects.

Exchange bias effect in martensitic epitaxial Ni-Mn-Sn thin films applied to pin CoFeB/MgO/CoFeB magnetic tunnel junctions

The exchange bias effect is commonly used to shift the coercive field of a ferromagnet. This technique is crucial for the use of magnetic tunnel junctions as logic or memory devices. Therefore, an independent switching of the two ferromagnetic electrodes is necessary to guarantee a reliable readout. Here, we demonstrate that the intrinsic exchange bias effect of Ni-Mn-Sn can be used to apply a unidirectional anisotropy to magnetic tunnel junctions. For this, we use epitaxial Ni-Mn-Sn films as pinning layers for microfabricated CoFeB/MgO/CoFeB magnetic tunnel junctions. We compare the exchange bias field (

Structure and giant inverse magnetocaloric effect of epitaxial Ni-Co-Mn-Al films

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Comparison of laser-induced and intrinsic tunnel magneto-Seebeck effect inCoFeB/MgAl2O4and CoFeB/MgO magnetic tunnel junctions

) measured after field cooling in

On/off switching of bit readout in bias-enhanced tunnel magneto-Seebeck effect

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Enhancement of thermovoltage and tunnel magneto-Seebeck effect in CoFeB-based magnetic tunnel junctions by variation of the MgAl2O4 and MgO barrier thickness

\,kOe external field by magnetization measurements with

Large magneto-Seebeck effect in magnetic tunnel junctions with half-metallic Heusler electrodes

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Pumping laser excited spins through MgO barriers

obtained from tunnel magnetoresistance measurements. Consistent for both methods we find an exchange bias of about

Heusler compounds for spin caloritronics: Large magneto-Seebeck effects in magnetic tunnel junctions

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