Thermomagnetic Materials: Thermomagnetic Properties Improved by Self-Organized Flower-Like Phase Separation of Ferromagnetic Co2Dy0.5Mn0.5Sn (Adv. Funct. Mater. 9/2012)

Itinerant half-metallic ferromagnetsCo2TiZ(Z=Si, Ge, Sn):Ab initiocalculations and measurement of the electronic structure and transport properties

This work reports on ab initio calculations and experiments on the half-metallic ferromagnetic Heusler compounds ${\text{Co}}_{2}\text{Ti}Z$ $(Z=\text{Si},\text{ }\text{Ge},\text{ }\text{Sn})$. Aim is a comprehensive study of the electronic-structure and thermoelectric properties. The impact of the variation in the main group element $Z$ on those properties is discussed. X-ray diffraction was performed on the compounds and the lattice parameters are compared to other ${\text{Co}}_{2}$-based compounds. Hard x-ray photoemission measurements were carried out and the results are compared to the calculated electronic structure. The experimentally determined electronic structure, magnetic propert…

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 disorde…

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…

Electronic structure of the austenitic and martensitic state of magnetocaloric Ni-Mn-In Heusler alloy films

Changes of the electronic and magnetic structure near the martensitic phase transition of Ni-Mn-In Heusler alloys doped with Co are investigated by experiment and theory. The nonstoichiometric Ni${}_{48}$Co${}_{5}$Mn${}_{35}$In${}_{12}$ epitaxial film undergoes a transition from a weakly magnetic martensitic phase below ${T}_{m}=350$ K to a ferromagnetic austenitic phase above ${T}_{m}$. Element-specific magnetic moments and the unoccupied density of states function is investigated using x-ray magnetic circular dichroism. We find an antiparallel alignment of Mn and Ni/Co magnetic moments in both phases. The electronic structure is calculated using the SPR-KKR Green's function approach consi…

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…

Effect of annealing on Co2FeAl0.5Si0.5thin films: A magneto-optical and x-ray absorption study

A series of Al and MgO-capped Co${}_{2}$FeAl${}_{0.5}$Si${}_{0.5}$ epitaxial thin films grown on MgO with various levels of L2${}_{1}$ ordering was obtained by in situ annealing. The films were studied by means of x-ray absorption spectroscopy, x-ray magnetic circular dichroism (XMCD), magneto-optical Kerr effect magnetometry, and Brillouin light scattering. We find the anisotropy constants decrease, while the spin wave stiffness increases as the samples are annealed to higher temperatures. The magnetization as determined by Brillouin light scattering reveals a maximum value at intermediate annealing temperatures. Surprisingly, the orbital-to-spin-moment ratio (as seen from XMCD) is essenti…

Charge transfer and tunable minority band gap at the Fermi energy of a quaternaryCo2(MnxTi1−x)GeHeusler alloy

We investigate the distribution of element-specific magnetic moments and changes in the spin-resolved unoccupied density of states in a series of half-metallic ${\text{Co}}_{2}({\text{Mn}}_{x}{\text{Ti}}_{1\ensuremath{-}x})\text{Ge}$ Heusler alloys using x-ray magnetic circular dichroism. The Co and Mn magnetic moments are oriented parallel while a small Ti moment shows antiparallel to the mean magnetization. The element-specific magnetic moments remain almost independent on the composition. Therefore, a replacement of Ti by Mn results in an increase in magnetization. The increase in magnetization with increasing $x$ follows the Slater-Pauling rule. The Fermi level decreases with respect to…

Element-specific magnetic properties of Co2(Mn1−xFex)Si films probed by x-ray magnetic circular/linear dichroism

Element-specific magnetic properties of epitaxial Co${}_{2}$Fe${}_{x}$Mn${}_{1\ensuremath{-}x}$Si/MgO(100) Heusler films prepared by laser ablation are measured by circular dichroism in x-ray absorption spectroscopy (XMCD). Surface and bulk magnetization of the 100-nm-thick films are equal as tested by comparing the total electron yield and transmission measurements and follow the generalized Slater-Pauling rule. For Co${}_{2}$FeSi, the large magnetization of 6 ${\ensuremath{\mu}}_{B}$ per formula unit indicates half-metallic behavior. Calculations using the local spin density approximation (LDA) result in a half-metallic band structure for this compound when an additional electron-electron…

Temperature dependence of x-ray absorption spectra in the ferromagnetic Heusler alloysMn2VAlandCo2FeAl

We investigate the temperature dependence of the spin-resolved unoccupied density of states (DOS) in ferromagnetic ${\text{Co}}_{2}\text{FeAl}$ and ferrimagnetic ${\text{Mn}}_{2}\text{VAl}$ epitaxial films on MgO(100) using x-ray magnetic circular dichroism. We observe an unexpected strong temperature dependence of the DOS beyond the change expected from the Fermi distribution function. An increase in spectral weight is observed for majority states below the Fermi energy in the case of ${\text{Mn}}_{2}\text{VAl}$ and for minority states above the Fermi energy in the case of ${\text{Co}}_{2}\text{FeAl}$. Reduced atomic order near the interface suppresses the unexpected temperature dependence…

Thermomagnetic Properties Improved by Self-Organized Flower-Like Phase Separation of Ferromagnetic Co2Dy0.5Mn0.5Sn

A thermodynamically stable phase separation of Co2Dy0.5Mn0.5Sn into the Heusler compound Co2MnSn and Co8Dy3Sn4 is induced by rapid cooling from the liquid phase. The phase separation forms an ordered flower-like structure on the microscale. The increased scattering of phonons at the phase boundaries reduces the thermal conductivity and thus improves thermoelectric and spincaloric properties.

Iron-based Heusler compounds Fe2YZ: Comparison with theoretical predictions of the crystal structure and magnetic properties

The present work reports on the new soft ferromagnetic Heusler phases Fe${}_{2}$NiGe, Fe${}_{2}$CuGa, and Fe${}_{2}$CuAl, which in previous theoretical studies have been predicted to exist in a tetragonal Heusler structure. Together with the known phases Fe${}_{2}$CoGe and Fe${}_{2}$NiGa these materials have been synthesized and characterized by powder x-ray diffraction, ${}^{57}$Fe M\"ossbauer spectroscopy, superconducting quantum interference device, and energy-dispersive x-ray measurements. In particular M\"ossbauer spectroscopy was used to monitor the degree of local atomic order/disorder and to estimate magnetic moments at the Fe sites from the hyperfine fields. It is shown that in con…

Tailoring the electronic structure of half-metallic Heusler alloys

We investigated element-specific magnetic moments and the spin-resolved unoccupied density of states (DOS) of polycrystalline ${\text{Co}}_{2}\text{Ti}Z$ $(Z=\text{Si},\text{ }\text{Ge},\text{ }\text{Sn},\text{ }\text{Sb})$, ${\text{Co}}_{2}{\text{Mn}}_{x}{\text{Ti}}_{1\ensuremath{-}x}\text{Si}$ and ${\text{Co}}_{2}{\text{MnGa}}_{1\ensuremath{-}x}{\text{Ge}}_{x}$ Heusler alloys using circular dichroism in x-ray absorption spectroscopy (XMCD). We find a small $(l0.03{\ensuremath{\mu}}_{B})$ Ti moment oriented antiparallel and a large $(g3{\ensuremath{\mu}}_{B})$ Mn moment oriented parallel to the Co moment of approximately $1{\ensuremath{\mu}}_{B}$ per atom in the investigated compounds. Orb…

Microscopic origin of magnetic anisotropy in martensitic Ni2MnGa

The microscopic origin of magnetic anisotropy in the shape memory alloy Ni${}_{2}$MnGa is investigated by means of x-ray magnetic circular dichroism in transmission mode. Field- and angle-dependent dichroism spectra of epitaxial Ni${}_{2}$MnGa(101)/MgO(001) films reveal pronounced differences for magnetization aligned parallel and perpendicular to the film plane. These differences are related to an anisotropy of the orbital magnetic moment in agreement with the observed out-of-plane magnetocrystalline anisotropy. The spectral variation of the x-ray absorption originates from changes in the spin-projected density of states when the magnetization vector is rotated from the easy to the hard ma…

Structure and Microscopic Magnetism of Epitaxial Ni-Mn-Ga Films

We report on the structural and magnetic properties of epitaxial thin films of the ferromagnetic shape memory material Ni–Mn–Ga prepared by DC magnetron sputter deposition. Different substrate materials, i.e., MgO(100) and Al2O3(11−20) allow for a tailored epitaxial growth. Using a sacrificial chromium buffer layer freestanding epitaxial films are obtained. In combination with photolithography partially freestanding structures such as microbridges are fabricated. The complex martensite crystal structure in substrate-constrained and freestanding films is studied by means of X-ray diffraction. The identified asymmetric twin variant configuration is associated with a macroscopic surface patter…

Element-specific magnetic moments and spin-resolved density of states in CoFeMnZ(Z=Al, Ga; Si, Ge)

Using circular dichroism in x-ray-absorption spectroscopy (XAS/XMCD), we determined element-specific magnetic moments and spin-resolved unoccupied densities of states (DOS) for Co, Fe, and Mn in the quaternary Heusler compounds CoFeMn$Z$ ($Z=\text{Al}$, Ga; Si, Ge). These compounds belong to a class of highly spin-polarized materials with cubic LiMgPdSn-type structure. Different structure models for the sublattice occupation leading to similar average magnetization values can be distinguished by comparison of element-specific moments with theory. We find that the compounds form similar structures, where Co, Fe, Mn, and $Z$ occupy the $X$, ${X}^{\ensuremath{'}}$, $Y$, and $Z$ sublattice of t…

Phase separation in the quaternary Heusler compound CoTi(1−x)MnxSb – A reduction in the thermal conductivity for thermoelectric applications

We investigate the phase separation of the solid solution CoTi(1−x)MnxSb into the two Heusler compounds CoTiSb and CoMnSb. Energy-dispersive X-ray spectroscopy measurements on the two-phase material reveal the presence of size- and shape-tunable CoTiSb regions in a CoMnSb matrix. We demonstrate that the formed phase and grain boundaries have a considerable influence on the phonon scattering processes, which leads to a reduction in the thermal conductivity by a factor of three compared to single-phase CoTiSb.

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 ci…

Spin-polarized photoelectrons resonantly excited by circularly polarized light from a fractional Ag film on GaAs(100)

We demonstrate a finite spin polarization of photoelectrons emitted from GaAs(100) covered by a fractional Ag film. The photoemission yield shows a sharp maximum for intermediate coverage and the spin polarization increases with increasing laser intensity. Photoelectrons are excited by circularly polarized 100 fs laser pulses of 800 nm wavelength. We recorded the photoemitted electrons using a photoemission electron microscope combined with a Mott spin polarimeter. The spin polarization is analyzed in dependence on the excitation frequency and intensity and on the average thickness of the silver film. The results are explained by a model combining multiphoton photoemission and optical field…

Full Tunability of Strain along the fcc-bcc Bain Path in Epitaxial Films and Consequences for Magnetic Properties

Strained coherent film growth is commonly either limited to ultrathin films or low strains. Here, we present an approach to achieve high strains in thicker films, by using materials with inherent structural instabilities. As an example, 50 nm thick epitaxial films of the ${\mathrm{Fe}}_{70}{\mathrm{Pd}}_{30}$ magnetic shape memory alloy are examined. Strained coherent growth on various substrates allows us to adjust the tetragonal distortion from $c/{a}_{\mathrm{bct}}=1.09$ to 1.39, covering most of the Bain transformation path from fcc to bcc crystal structure. Magnetometry and x-ray circular dichroism measurements show that the Curie temperature, orbital magnetic moment, and magnetocrysta…

Phase-separation-induced changes in the magnetic and transport properties of the quaternary Heusler alloyCo2Mn1−xTixSn

The quaternary Heusler compound ${\text{Co}}_{2}{\text{Mn}}_{1\ensuremath{-}x}{\text{Ti}}_{x}\text{Sn}$ with $x=0$, 0.2, 0.4, 0.5, 0.6, 0.8, and 1 shows a phase separation into the two Heusler compounds, ${\text{Co}}_{2}\text{MnSn}$ and ${\text{Co}}_{2}\text{TiSn}$. Only at the edges of the composition range a slight admixture of Mn and Ti to the respective other phase is observed. This phase separation leads to a distinct microstructure which can be altered by the composition of the material. Pronounced changes in the magnetic and electronic properties take place with varying composition. Two magnetic transitions occur which indicate different Curie temperatures for both phases. The reduct…

Spin-resolved unoccupied density of states in epitaxial Heusler-alloy films

We investigate the electronic properties of epitaxial ${\text{Co}}_{2}({\text{Fe}}_{x}{\text{Mn}}_{1\ensuremath{-}x})\text{Si}$, ${\text{Co}}_{2}\text{Fe}({\text{Al}}_{1\ensuremath{-}x}{\text{Si}}_{x})$, and ${\text{Co}}_{2}({\text{Cr}}_{0.6}{\text{Fe}}_{0.4})\text{Al}$ films on MgO(100) substrates using circular dichroism in x-ray absorption spectroscopy (XMCD). Considering final-state electron correlations, the spin-resolved partial density of states at the Co atom can be extracted from XMCD data. The experimental results corroborate the predicted half-metallic ferromagnetic properties of these alloys and reveal a compositional dependence of the Fermi energy position within the minority b…

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 L 3 absorption edges of Cr, Fe and Co. The relative phase angle of the sublattice magnetization's 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 at…