Peter Klaer

Disentangling the Mn moments on different sublattices in the half-metallic ferrimagnet Mn3?xCoxGa

Ferrimagnetic Mn3−xCoxGa compounds have been investigated by magnetic circular dichroism in x-ray absorption (XMCD). Compounds with x>0.5 crystallize in the CuHg2Ti structure. A tetragonal distortion of the cubic structure occurs for x≤0.5. For the cubic phase, magnetometry reveals a linearly increasing magnetization of 2x Bohr magnetons per formula unit obeying the generalized Slater–Pauling rule. XMCD confirms the ferrimagnetic character with Mn atoms occupying two different sublattices with antiparallel spin orientation and different degrees of spin localization and identifies the region 0.6<x≤0.8 as most promising for a high spin polarization at the Fermi level. Individual Mn moments on inequivalent sites are compared to theoretical predictions.

Full tunability of strain along the fcc-bcc bain path in epitaxial films and consequences for magnetic properties.

Huge deformations of the crystal lattice can be achieved in materials with inherent structural instability by epitaxial straining. By coherent growth on seven different substrates the in-plane lattice constants of 50 nm thick Fe70Pd30 films are continuously varied. The maximum epitaxial strain reaches 8,3 % relative to the fcc lattice. The in-plane lattice strain results in a remarkable tetragonal distortion ranging from c/abct = 1.09 to 1.39, covering most of the Bain transformation path from fcc to bcc crystal structure. This has dramatic consequences for the magnetic key properties. Magnetometry and X-ray circular dichroism (XMCD) measurements show that Curie temperature, orbital magnetic moment, and magnetocrystalline anisotropy are tuned over broad ranges.

Anomalous transport properties of the half-metallic ferromagnets Co2TiSi, Co2TiGe and Co2TiSn

In this work, the theoretical and experimental investigations of Co2TiZ (Z=Si, Ge or Sn) compounds are reported. Half-metallic ferromagnetism is predicted for all three compounds with only two bands crossing the Fermi energy in the majority channel. The magnetic moments fulfil the Slater–Pauling rule and the Curie temperatures are well above room temperature. All compounds show a metallic-like resistivity for low temperatures up to their Curie temperature, above the resistivity changes to semiconducting-like behaviour. A large negative magnetoresistance (MR) of 55 per cent is observed for Co2TiSn at room temperature in an applied magnetic field of μ0H=4T, which is comparable to the large negative MRs of the manganites. The Seebeck coefficients are negative for all three compounds and reach their maximum values at their respective Curie temperatures and stay almost constant up to 950 K. The highest value achieved is −52 μVK−1 for Co2TiSn, which is large for a metal. The combination of half-metallicity and the constant large Seebeck coefficient over a wide temperature range makes these compounds interesting materials for thermoelectric applications and further spincaloric investigations.

Localized magnetic moments in the Heusler alloy Rh2MnGe

X-ray magnetic circular dichroism (XMCD) of core-level absorption (x-ray absorption spectroscopy, XAS) spectra in the soft x-ray region has been measured for the ferromagnetic Heusler alloy Rh2MnGe at the Rh M3,2 and Mn L3,2 edges. The ratio of Rh and Mn spin moments amounts to 0.05 which is smaller than the ratio of 0.1 determined by a local density approximation electronic band structure calculation. We have found that the orbital moments of the Rh 4d and Mn 3d states are very small. The observed Rh 2p XAS spectrum can be understood on the basis of the Rh 3d partial density of unoccupied states as is typical for metals. The observed features of the Mn 2p XAS and XMCD spectra are dominated by final state multiplets as is typical for oxides. The comparison of experimental and ab initio calculated XAS/XMCD spectra reveals a strong narrowing of the Mn 3d bands, indicating strongly localized Mn moments. The magnetic moments are considerably more localized for Rh2MnGe in comparison with the isoelectronic compound Co2MnGe. In spite of the strong localization of the Mn moment, the temperature dependences of sublattice magnetization are equal for the Mn and Rh sublattices in contrast to the prediction by a Heisenberg model. This might be attributed to the remaining itinerant character of the Rh moment.

Robustness of plasmonic angular momentum confinement in cross resonant optical antennas

Using a combination of photoemission electron microscopy and numerical simulations, we investigated the angular moment transfer in strongly enhanced optical near-fields of artificially fabricated optical antennas. The polarization dependence of the optical near-field enhancement has been measured in a maximum symmetric geometry, i.e., excitation by a normal incident planar wave. Finite-difference time-domain simulations for the realistic antenna geometries as determined by high-resolution electron microscopy reveal a very good agreement with experimental data. The agreement confirms that the geometrical asymmetries and inhomogeneities due to the nanoscale fabrication process preserve the circular polarization in the gap regions with strong near-field enhancement.

Element-specific ferromagnetic resonance in epitaxial Heusler spin valve systems

Time-resolved x-ray magnetic circular dichroism was used to investigate epitaxial MgO(100)/Co2Cr0.6Fe0.4Al and MgO(100)/Co2Cr0.6Fe0.4Al/Cr/CoFe films. The precessional motion of the individual sublattice magnetization, excited by continuous microwave excitation in the range 2?10?GHz, was detected by tuning the x-ray photon energy to the L3 absorption edges of Cr, Fe and Co. The relative phase angle of the sublattice magnetizations response is smaller than the detection limit of 2?. A weakly antiferromagnetically coupled CoFe layer causes an increase in the ferromagnetic resonance linewidth consisting of a constant offset and a component linearly increasing with frequency that we partly attribute to non-local damping due to spin pumping.

Interface and bulk magnetism of Co2Cr0.6Fe0.4Al and Co2CrAl thin films

The interface and bulk magnetic properties of epitaxial thin films of the Heusler compounds Co2Cr0.6Fe0.4Al/Mg/AlOx and CoCr2Al/Mg/AlOx are investigated. We compare the magnetization measured by x-ray magnetic circular dichroism experiments in surface sensitive total electron yield mode (information depth of 2–3 nm) and in bulk sensitive transmission mode. The pronounced temperature dependence of the magnetoresistance of tunneling junctions with Heusler electrodes, which is often related to weakened interface magnetism, is discussed. Evidence is given that this explanation does not apply to the compounds investigated here.

Magnetic and structural properties of Co2FeAl1?xSix thin films

We investigate different types of disorder which are predicted to influence the spin polarization by x-ray and electron diffraction (RHEED). We observe that the compound Co2FeAl grows in the B2 ordered structure. However, for Co2FeAl0.3Si0.7 the analysis revealed with increasing annealing temperature a crossover from B2 to L21 order. The magnetization of the samples is investigated by SQUID magnetometry and x-ray magnetic circular dichroism (XMCD) experiments in bulk sensitive transmission and surface sensitive total electron yield (TEY) mode. The results reveal the same surface and bulk moments of Co2FeAl. However, with increasing annealing temperature the Co2FeAl0.3Si0.7 thin films display a small decrease of the surface moment in contrast to a not changing bulk moment. This observation is relevant concerning potential device applications of Co2FeAl0.3Si0.7.

New Materials with High Spin Polarization Investigated by X-Ray Magnetic Circular Dichroism

We investigate element-specific spin and orbital magnetic moments of polycrystalline bulk Heusler alloys that are predicted to be half-metallic with composition Co2YZ (Y = Ti, Cr, Mn, Fe and Z = Al, Ga, Si, Ge, Sn, Sb) using magnetic circular dichroism in X-ray absorption spectroscopy (XAS/XMCD). In addition to stoichiometric compounds we also investigate composition series with partly replaced elements on the Y-site (Co2Fe x Cr1−x Si, Co2Mn x Ti1−x Si and Co2Mn x Ti1−x Ge) and on the Z-site (Co2MnGa1−x Ge x ) promising a tailoring of the Fermi level with respect to the minority band gap. We compare experimental results with theoretical predictions elucidating the influence of local disorder in the experimental samples. Moreover, we demonstrate that a consideration of electron correlation in local density approximation theories is necessary for reproducing experimental results. Increased orbital magnetic moments in respect to theoretical predictions put forward the role of spin–orbit coupling for half-metallic properties. For the case of single crystalline thin films we developed a method of simultaneous measurement of bulk and surface sensitive magnetic properties including those at the crucial interface to a tunneling barrier. Exploiting the comparison of bulk and interface information, film growth can be improved for specific applications. In order to directly determine the spin-resolved density-of-states function we present a calculation scheme for extracting this information from the XMCD spectra considering final-state electron correlations. We investigate the electronic properties of epitaxial Co2(Fe x Mn1−x )Si, Co2Fe(Al1−x Si x ), and Co2(Cr0.6Fe0.4)Al films on MgO(100) substrates and for the case of several bulk samples including Co2TiSb as a reference sample for normal metallic ferromagnetism. The experimental results, revealing the distribution of magnetic moments and the relative position of the Fermi energy as a function of the number of valence electrons, confirm the predicted possibility of tailoring the minority band gap using substitutional quaternary Heusler compounds. These findings may be of general importance for the understanding of the electronic structures in complex intermetallic compounds.