Christian Sterwerf

High TMR Ratio in

Magnetic tunnel junctions with Fe1+zCo2-xSi (0 ≤ x ≤ 1)electrodes and MgO barrier were prepared on MgO substrates by magnetron co-sputtering. Maximum tunnel magnetoresistance (TMR) ratios of 262% at 15 K and 159% at room temperature were observed for x = 0.75. Correlations of the annealing temperature dependent atomic ordering and TMR amplitude are discussed. The high TMR for an intermediate stoichiometry is ascribed to the adjustment of the Fermi energy within a minority spin pseudo gap.

{\rm Co}_{2}{\rm FeSi}

Magnetic tunnel junctions with Fe1+zCo2-xSi (0 ≤ x ≤ 1)electrodes and MgO barrier were prepared on MgO substrates by magnetron co-sputtering. Maximum tunnel magnetoresistance (TMR) ratios of 262% at 15 K and 159% at room temperature were observed for x = 0.75. Correlations of the annealing temperature dependent atomic ordering and TMR amplitude are discussed. The high TMR for an intermediate stoichiometry is ascribed to the adjustment of the Fermi energy within a minority spin pseudo gap.

and

Thin highly textured Fe1+xCo2–xSi (0 ≤ x ≤ 1) films were prepared on MgO (001) substrates by magnetron co-sputtering. Magneto-optic Kerr effect (MOKE) and ferromagnetic resonance (FMR) measurements were used to investigate the composition dependence of the magnetization, the magnetic anisotropy, the gyromagnetic ratio, and the relaxation of the films. Both MOKE and FMR measurements reveal a pronounced fourfold anisotropy for all films. In addition, we found a strong influence of the stoichiometry on the anisotropy as the cubic anisotropy strongly increases with increasing Fe concentration. The gyromagnetic ratio is only weakly dependent on the composition. We find low Gilbert damping parameters for all films with values down to 0.0012±0.00010.0007 for Fe1.75Co1.25Si. The effective damping parameter for Co2FeSi is found to be 0.0018±0.00040.0034. We also find a pronounced anisotropic relaxation, which indicates significant contributions of two-magnon scattering processes that is strongest along the easy ax...

{\rm Fe}_{2}{\rm CoSi}

We investigated the structural and magnetic properties of sputter-deposited Mn-Fe-Ga compounds. The crystallinity of the Mn-Fe-Ga thin films was confirmed using X-ray diffraction. X-ray reflection and atomic force microscopy measurements were utilized to investigate the surface properties, roughness, thickness, and density of the deposited Mn-Fe-Ga. Depending on the stoichiometry, as well as the used substrates [SrTiO3 (001) and MgO (001)] or buffer layer (TiN), Mn-Fe-Ga crystallized in the cubic or the tetragonally distorted phase. Anomalous Hall effect and alternating gradient magnetometry measurements confirmed strong perpendicular magnetocrystalline anisotropy. Hard magnetic behavior was reached by tuning the composition. TiN-buffered Mn2.7Fe0.3Ga revealed sharper switching of the magnetization compared with the unbuffered layers.

Based Magnetic Tunnel Junctions

Magneto-optical and electronic properties of the Co2FeSi Heusler compound were studied by polar Kerr magneto-optical spectroscopy and ab-initio calculations. The thin-film samples were grown by dc/rf magnetron co-sputtering on MgO(100) substrates. A Cr seed layer was deposited prior to the Co2FeSi layer to achieve its epitaxial growth. The magneto-optical spectroscopy was carried out using generalized magneto-optical ellipsometry with rotating analyzer in the photon energy range from 1.4 to 5.5 eV with an applied magnetic field of up to 1.2 T. The polar Kerr spectra showed a smooth spectral behavior up to 5.5 eV indicating nearly free charge carriers. Experimental data were compared with ab-initio calculations based on density functional theory employing the full-potential linearized augmented plane wave method.

High TMR Ratio in Co2FeSi and Fe2CoSi Based Magnetic Tunnel Junctions

BaO has been introduced as a novel tunnel barrier material in magnetic tunnel junctions (MTJs). Due to the good agreement regarding lattice constants and crystal structure, we believe BaO to be particularly suitable as barrier in MTJs containing Heusler compound electrodes. Co₂FeSi/BaO/Fe MTJs have been fabricated by molecular beam epitaxy (MBE) and investigated in terms of microstructure, transport properties and tunnel magneto resistance (TMR). A TMR amplitude as high as 104% at room temperature (RT) has been achieved for very small bias voltages (<i>V</i>_Bias) and a strong dependence on <i>V</i>_Bias could be observed as the TMR ratio decreases with increasing <i>V</i>_Bias to about 14% at <i>V</i>_Bias = 10 mV.

Low Gilbert damping in Co2FeSi and Fe2CoSi films

Spin caloritronics studies the interplay between charge-, heat- and spin-currents, which are initiated by temperature gradients in magnetic nanostructures. A plethora of new phenomena has been discovered that promises, e.g., to make wasted heat in electronic devices useable or to provide new read-out mechanisms for information. However, only few materials have been studied so far with Seebeck voltages of only some microvolt, which hampers applications. Here, we demonstrate that half-metallic Heusler compounds are hot candidates for enhancing spin-dependent thermoelectric effects. This becomes evident when considering the asymmetry of the spin-split density of electronic states around the Fermi level that determines the spin-dependent thermoelectric transport in magnetic tunnel junctions. We identify Co2FeAl and Co2FeSi Heusler compounds as ideal due to their energy gaps in the minority density of states, and demonstrate devices with substantially larger Seebeck voltages and tunnel magneto-Seebeck effect ratios than the commonly used Co-Fe-B-based junctions.Expanding the scope of materials for spin caloritronics enhances the opportunity to achieve more energy efficient memory and sensor devices. Here the authors report the tunnel magneto-Seebeck effects in magnetic tunnel junctions with Co2FeAl and Co2FeSi Heusler compounds.

Structural and Magnetic Properties of Sputter-Deposited Mn–Fe–Ga Thin Films

Tetragonally distorted Mn3−xGax thin films with 0.1<x<2 show a strong perpendicular magnetic anisotropy and low magnetization and thus have the potential to serve as electrodes in spin transfer torque magnetic random access memory. Because a direct capping of these films with MgO is problematic due to oxide formation, we examined the influence of a CoFeB interlayer and of two different deposition methods for the MgO barrier on the formation of interfacial Mn-O for Mn62Ga38 by element specific X-ray absorption spectroscopy (XAS) and magnetic circular dichroism (XMCD). A highly textured L10 crystal structure of the Mn-Ga films was verified by X-ray diffraction measurements. For samples with e-beam evaporated MgO barrier no evidence for Mn-O was found whereas in samples with magnetron sputtered MgO, Mn-O was detected, even for the thickest interlayer thickness. Both XAS and XMCD measurements showed an increasing interfacial Mn-O amount with decreasing CoFeB interlayer thickness. Additional element specific f...

Magneto-optical spectroscopy of Co2FeSi Heusler compound

Thin highly epitaxial BiFeO3 films were prepared on SrTiO3 (100) substrates by reactive magnetron cosputtering. Detailed magneto-optic Kerr effect (MOKE) measurements on BiFeO3/Co-Fe bilayers were performed to investigate the exchange bias as a function of film thicknesses and Co-Fe stoichiometries. We found a maximum exchange bias of Heb = 92 Oe and a coercive field of Hc = 89 Oe for a 12.5 nm-thick BiFeO3 film with a 2 nm-thick Co layer. The unidirectional anisotropy is clearly visible in in-plane rotational MOKE measurements. Anisotropic magnetoresistance measurements reveal a strongly increasing coercivity with decreasing temperature, but no significant change in the exchange bias field.

Co

Tetragonally distorted Mn3−xGax thin films with 0.1<x<2 show a strong perpendicular magnetic anisotropy and low magnetization and thus have the potential to serve as electrodes in spin transfer torque magnetic random access memory. Because a direct capping of these films with MgO is problematic due to oxide formation, we examined the influence of a CoFeB interlayer and of two different deposition methods for the MgO barrier on the formation of interfacial Mn-O for Mn62Ga38 by element specific X-ray absorption spectroscopy (XAS) and magnetic circular dichroism (XMCD). A highly textured L10 crystal structure of the Mn-Ga films was verified by X-ray diffraction measurements. For samples with e-beam evaporated MgO barrier no evidence for Mn-O was found whereas in samples with magnetron sputtered MgO, Mn-O was detected, even for the thickest interlayer thickness. Both XAS and XMCD measurements showed an increasing interfacial Mn-O amount with decreasing CoFeB interlayer thickness. Additional element specific f...