M. Kallmayer

Improvement of structural, electronic, and magnetic properties of Co2MnSi thin films by He+ irradiation

The influence of 30 keV He+ ion irradiation on structural, electronic, and magnetic properties of Co2MnSi thin films with a partial B2 order was investigated. It was found that room temperature irradiation with light ions can improve the local chemical order. This provokes changes of the electronic structure and element-specific magnetization toward the bulk properties of a well-ordered Co2MnSi Heusler compound.

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.

Epitaxial film growth and magnetic properties of Co2FeSi

We have grown thin films of the Heusler compound Co_2FeSi by RF magnetron sputtering. On (100)-oriented MgO substrates we find fully epitaxial (100)-oriented and L2_1 ordered growth. On Al_2O_3 (11-20) substrates, the film growth is (110)-oriented, and several in-plane epitaxial domains are observed. The temperature dependence of the electrical resistivity shows a power law with an exponent of 7/2 at low temperatures. Investigation of the bulk magnetic properties reveals an extrapolated saturation magnetization of 5.0 mu_B/fu at 0 K. The films on Al_2O_3 show an in-plane uniaxial anisotropy, while the epitaxial films are magnetically isotropic in the plane. Measurements of the X-ray magnetic circular dichroism of the films allowed us to determine element specific magnetic moments. Finally we have measured the spin polarization at the surface region by spin-resolved near-threshold photoemission and found it strongly reduced in contrast to the expected bulk value of 100%. Possible reasons for the reduced magnetization are discussed.

Reduction of surface magnetism of Co2Cr0.6Fe0.4Al Heusler alloy films

Element specific magnetization has been determined at the surface and in the bulk of Co2Cr0.6Fe0.4Al Heusler alloy films grown on α-Al2O3 and capped by Al, using x-ray magnetic circular dichroism both in transmission and total electron yield. The magnetic moments for Co and Fe are considerably reduced at the upper surface in comparison to their values in the bulk of the film. The large reduction at room temperature of 17% for thick films averaged along the electron escape depth implies an even larger reduction at the topmost layer which is crucial for spin-dependent transport. The surface magnetization decreases additionally with respect to the bulk value with decreasing film thickness below 20nm.

Magnetic properties of Co2Mn1−xFexSi Heusler alloys

Co2Mn1−xFexSi Heusler alloys with Fe concentration x = 0–0.4 as prepared by arc melting show a L21 long range order for all Fe concentrations. Magnetic properties of Co2Mn1−xFexSi Heusler alloys were investigated by magnetometry and circular magnetic dichroism. The magnetization of the Fe doped Heusler alloys is in agreement with the Slater–Pauling values expected for half-metallic ferromagnets. Element specific magnetic moments as determined by x-ray absorption using the total electron yield method are in disagreement with theoretical predictions for x = 0 but approach the predicted values as the Fe concentration increases. Surprisingly small Fe concentration increases the magnetic moments of all constituents considerably.

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.

Compositional dependence of element-specific magnetic moments in Ni2MnGa films

Element-specific magnetic moments were investigated for epitaxial Ni2Mn1+xGa1−x and (Ni2MnGa)1−x(Co2FeSi)x Heusler films using x-ray absorption spectroscopy and x-ray circular magnetic dichroism in transmission. The epitaxial films of the Ni2MnGa-derived compositions were prepared by dc-sputtering on Al2O3 substrates at 773 K. X-ray diffraction confirms a (1 1 0) oriented growth. An increase in the Mn concentration reduces the magnetic spin moment of both Mn and Ni. An increase in the content of Co2FeSi in the Ni2MnGa compound leads to an increase in the Mn and Ni spin moments and to a decrease in Tm for 5% Co2FeSi and finally to a suppression of the phase transition for 20% Co2FeSi. The orbital moments of the stoichiometric Ni2MnGa films are larger compared with values obtained for films of a Mn-rich compound and smaller compared with Co2FeSi containing films. The results are discussed in the context of theoretical models.

Basic Properties of Magnetic Shape-Memory Materials from First-Principles Calculations

The mutual influence of phase transformations, magnetism, and electronic properties of magnetic-shape memory Heusler materials is a basic issue of electronic structure calculations based on density functional theory. In this article, we show that these calculations can be pursued to finite temperatures, which allows to derive on a first-principles basis the temperature versus composition phase diagram of the pseudo-binary Ni-Mn-(Ga, In, Sn, Sb) system. The free energy calculations show that the phonon contribution stabilizes the body-centered-cubic (bcc)-like austenite structure at elevated temperatures, whereas magnetism favors the low-temperature martensite phase with body-centered-tetragonal (bct) or rather face-centered-tetragonal (fct) structure. The calculations also allow to make predictions of magnetostructural and magnetic field induced properties of other (new) magnetic Heusler alloys not based on NiMn such as Co-Ni-(Ga-Zn) and Fe-Co-Ni-(Ga-Zn) intermetallic compounds.

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.

Suppression of martensitic phase transition at the Ni2MnGa film surface

We investigated magnetic and structural properties at the surface of epitaxial Ni2MnGa(110) Heusler films using x-ray absorption spectroscopy and x-ray magnetic circular dichroism both in transmission and total electron yield mode. The magnetic shape memory films were prepared by dc sputtering from a stoichiometric target onto sapphire substrates at an optimized substrate temperature of 773K. X-ray diffraction confirms a (110) oriented growth on Al2O3(112¯0) and an austenite to martensite transition at 270–280K. At the surface the martensitic phase transition and the magnetization are strongly suppressed. The deviation in the surface properties is caused by a Mn deficiency near the surface.