Markus Meinert

Exchange interactions and Curie temperatures of Mn2CoZ compounds.

The generalized Heusler compounds Mn(2)CoZ (Z = Al, Ga, In, Si, Ge, Sn, Sb) with the Hg(2)CuTi structure are of great interest due to their half-metallic ferrimagnetism. The complex magnetic interactions between the constituents are studied by means of first principles calculations of the Heisenberg exchange coupling parameters, and Curie temperatures are calculated from those. Due to the direct Mn-Mn exchange interaction in Mn(2)CoZ, the Curie temperature decreases, although the total moment increases when the valence electron number Z is increased. The exchange interactions are dominated by a strong direct exchange between Co and its nearest neighbor Mn on the B site, which is nearly constant. The coupling between the nearest neighbor Mn atoms scales with the magnetic moment of the Mn atom on the C site. Calculations with different lattice parameters suggest a negative pressure dependence of the Curie temperature, which follows from the decreasing magnetic moments. Curie temperatures of more than 800 K are predicted for Mn(2)CoAl (890 K), Mn(2)CoGa (886 K), and Mn(2)CoIn (845 K).

Ab initio prediction of ferrimagnetism, exchange interactions and Curie temperatures in Mn2TiZ Heusler compounds

The Heusler compounds Mn(2)TiZ (Z = Al, Ga, In, Si, Ge, Sn, P, As, Sb) are of great interest due to their potential ferrimagnetic properties and high spin polarization. Here, we present calculations of the structural and magnetic properties of these materials. Their magnetic moment follows the Slater-Pauling rule m = N(V) - 24. None of them is actually a perfect half-metallic ferrimagnet, but some exhibit more than 90% spin polarization and Curie temperatures well above room temperature. The exchange interactions are complex; direct and indirect exchange contributions are identified. The Curie temperature scales with the total magnetic moment, and it has a positive pressure dependence. The role of the Z element is investigated: it influences the properties of the compounds mainly via its valence electron number and its atomic radius, which determines the lattice parameter. Based on these results, Mn(2)TiSi, Mn(2)TiGe, and Mn(2)TiSn are proposed as candidates for spintronic applications.

Ferrimagnetism and disorder of epitaxial Mn2−xCoxVAl Heusler compound thin films

The quaternary full Heusler compound Mn{sub 2-x}Co{sub x}VAl with x = 1 is predicted to be a half-metallic antiferromagnet. Thin films of the quaternary compounds with x = 0-2 were prepared by dc and RF magnetron co-sputtering on heated MgO (0 0 1) substrates. The magnetic structure was examined by x-ray magnetic circular dichroism and the chemical disorder was characterized by x-ray diffraction. Ferrimagnetic coupling of V to Mn was observed for Mn{sub 2}VAl (x = 0). For x = 0.5, we also found ferrimagnetic order with V and Co antiparallel to Mn. The observed reduced magnetic moments are interpreted with the help of band structure calculations in the coherent potential approximation. Mn{sub 2}VAl is very sensitive to disorder involving Mn, because nearest-neighbour Mn atoms couple antiferromagnetically. Co{sub 2}VAl has B2 order and has reduced magnetization. In the cases with x {ge} 0.9 conventional ferromagnetism was observed, closely related to the atomic disorder in these compounds.

Itinerant and localized magnetic moments in ferrimagnetic Mn(2)CoGa thin films probed by x-ray magnetic linear dichroism: Experiment and ab initio theory

Epitaxial thin films of the half-metallic Xa compound Mn(2)CoGa (Hg(2)CuTi prototype) were prepared by dc magnetron co-sputtering with different heat treatments on MgO (001) substrates. High-quality films with a bulk magnetization of 1.95(5) mu(B) per unit cell were obtained. The L(3,2) x-ray magnetic circular dichroism spectra agree with calculations based on density functional theory (DFT) and reveal the antiparallel alignment of the two inequivalent Mn moments. X-ray magnetic linear dichroism, in good agreement with theory as well, allows us to distinguish between itinerant and local Mn moments. Based on noncollinear spin DFT, it is shown that one of the two Mn moments has local character, whereas the other Mn moment and the Co moment are itinerant.

Exchange interactions and Curie temperatures in Mn2CoZ compounds

The generalized Heusler compounds Mn(2)CoZ (Z = Al, Ga, In, Si, Ge, Sn, Sb) with the Hg(2)CuTi structure are of great interest due to their half-metallic ferrimagnetism. The complex magnetic interactions between the constituents are studied by means of first principles calculations of the Heisenberg exchange coupling parameters, and Curie temperatures are calculated from those. Due to the direct Mn-Mn exchange interaction in Mn(2)CoZ, the Curie temperature decreases, although the total moment increases when the valence electron number Z is increased. The exchange interactions are dominated by a strong direct exchange between Co and its nearest neighbor Mn on the B site, which is nearly constant. The coupling between the nearest neighbor Mn atoms scales with the magnetic moment of the Mn atom on the C site. Calculations with different lattice parameters suggest a negative pressure dependence of the Curie temperature, which follows from the decreasing magnetic moments. Curie temperatures of more than 800 K are predicted for Mn(2)CoAl (890 K), Mn(2)CoGa (886 K), and Mn(2)CoIn (845 K).

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+.

Unconventional Superconductivity in YPtBi and Related Topological Semimetals.

YPtBi, a topological semimetal with a very low carrier density, was recently found to be superconducting below T_{c}=0.77  K. In conventional theory, the nearly vanishing density of states around the Fermi level would imply a vanishing electron-phonon coupling and would, therefore, not allow for superconductivity. Based on relativistic density-functional theory calculations of the electron-phonon coupling in YPtBi, it is found that carrier concentrations of more than 10^{21}  cm^{-3} are required to explain the observed critical temperature with the conventional pairing mechanism, which is several orders of magnitude larger than experimentally observed. It is very likely that an unconventional pairing mechanism is responsible for the superconductivity in YPtBi and related topological semimetals with half-Heusler structure.

Static Magnetic Proximity Effect in Pt/NiFe2O4 and Pt/Fe Bilayers Investigated by X-Ray Resonant Magnetic Reflectivity.

The spin polarization of Pt in Pt/NiFe2O4 and Pt/Fe bilayers is studied by interface-sensitive x-ray resonant magnetic reflectivity to investigate static magnetic proximity effects. The asymmetry ratio of the reflectivity is measured at the Pt L3 absorption edge using circular polarized x-rays for opposite directions of the magnetization at room temperature. The results of the 2% asymmetry ratio for Pt/Fe bilayers are independent of the Pt thickness between 1.8 and 20 nm. By comparison with ab initio calculations, the maximum magnetic moment per spin polarized Pt atom at the interface is determined to be (0.6±0.1)  μB for Pt/Fe. For Pt/NiFe2O4 the asymmetry ratio drops below the sensitivity limit of 0.02  μB per Pt atom. Therefore, we conclude, that the longitudinal spin Seebeck effect recently observed in Pt/NiFe2O4 is not influenced by a proximity induced anomalous Nernst effect.

Modified Becke-Johnson potential investigation of half-metallic Heusler compounds

We have investigated the electronic structures of various potentially half-metallic Heusler compounds with the Tran-Blaha modified Becke-Johnson + local density approximation (TB-mBJLDA) potential within the density functional theory. The half-metallic gaps are considerably enhanced with respect to values from the Perdew-Burke-Ernzerhof (PBE) functional. In particular the unoccupied densities of states are modified by the mBJLDA potential, and agreement with experiment is considerably worse than for PBE results. The agreement of the densities of states can be improved by reducing the Tran-Blaha parameter c. However, ground-state properties such as the hyperfine fields are more accurately described by the PBE functional than by the mBJLDA. Despite its success for ionic and covalent semiconductors and insulators, we conclude that the mBJLDA is not a suitable approximation for half-metallic Heusler compounds. DOI: 10.1103/PhysRevB.87.045103

Phase stability of chromium based compensated ferrimagnets with inverse Heusler structure

Abstract Chromium based inverse Heusler compounds of the type Cr 2 YZ (Y=Co, Fe; Z=Al, Ga, In, Si, Ge, Sn) have been proposed as fully compensated half-metallic ferrimagnets. Such materials are of large interest for spintronics because they combine small magnetic moment with high spin polarization over a wide temperature range. We assess their thermodynamic stability by their formation enthalpies obtained from density functional theory calculations. All compounds under investigation are unstable. Cr 2 FeSi and Cr 2 CoAl are stable with respect to the elemental constituents, but decompose into binary phases. Cr 2 FeGe, Cr 2 CoGa, Cr 2 FeSn and Cr 2 CoIn are found to be unstable with respect to their elemental constituents. We identify possible binary decompositions.