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Register a new userStructural, magnetic, and transport properties of Co$_2$FeSi Heusler films
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We report the deposition of thin Co$_2$FeSi films by RF magnetron sputtering. Epitaxial (100)-oriented and L2$_1$ ordered growth is observed for films grown on MgO(100) substrates. (110)-oriented films on Al$_2$O$_3$(110) show several epitaxial domains in the film plane. Investigation of the magnetic properties reveals a saturation magnetization of 5.0 $mu_B/f.u.$ at low temperatures. The temperature dependence of the resistivity $rho_{xx}(T)$ exhibits a crossover from a T^3.5 law at T<50K to a T^1.65 behaviour at elevated temperatures. $rho_{xx}(H)$ shows a small anisotropic magnetoresistive effect. A weak dependence of the normal Hall effect on the external magnetic field indicates the compensation of electron and hole like contributions at the Fermi surface.
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We report the structural, transport, electronic, and magnetic properties of Co$_2$FeGa Heusler alloy nanoparticles. The Rietveld refinements of x-ray diffraction (XRD) data with the space group Fm$bar {3}$m clearly demonstrates that the nanoparticles are of single phase. The particle size (D) decreases with increasing the SiO$_2$ concentration. The Bragg peak positions and the inter-planer spacing extracted from high-resolution transmission electron microscopy image and selected area electron diffraction are in well agreement with data obtained from XRD. The coercivity initially increases from 127~Oe to 208~Oe between D = 8.5~nm and 12.5~nm, following the D$^{-3/2}$ dependence and then decreases with further increasing D up to 21.5~nm with a D$^{-1}$ dependence, indicating the transition from single domain to multidomain regime. The effective magnetic anisotropic constant behaves similarly as coercivity, which confirms this transition. A complex scattering mechanisms have been fitted to explain the electronic transport properties of these nanoparticles. In addition we have studied core-level and valence band spectra using photoemission spectroscopy, which confirm the hybridization between $d$ states of Co/Fe. Further nanoparticle samples synthesized by co-precipitation method show higher saturation magnetization. The presence of Raman active modes can be associated with the high chemical ordering, which motivates for detailed temperature dependent structural investigation using synchrotron radiation and neutron sources.
We studied the structural and magnetic properties of FeC~thin films deposited by co-sputtering of Fe and C targets in a direct current magnetron sputtering (dcMS) process at a substrate temperature (Ts) of 300, 523 and 773,K. The structure and morphology was measured using x-ray diffraction (XRD), x-ray absorption near edge spectroscopy (XANES) at Fe $L$ and C $K$-edges and atomic/magnetic force microscopy (AFM, MFM), respectively. An ultrathin (3,nm) $^{57}$FeC~layer, placed between relatively thick FeC~layers was used to estimate Fe self-diffusion taking place during growth at different Ts~using depth profiling measurements. Such $^{57}$FeC~layer was also used for $^{57}$Fe conversion electron M{o}ssbauer spectroscopy (CEMS) and nuclear resonance scattering (NRS) measurements, yielding the magnetic structure of this ultrathin layer. We found from XRD measurements that the structure formed at low Ts~(300,K) is analogous to Fe-based amorphous alloy and at high Ts~(773,K), pre-dominantly a tifc~phase has been formed. Interestingly, at an intermediate Ts~(523,K), a clear presence of tefc~(along with tifc~and Fe) can be seen from the NRS spectra. The microstructure obtained from AFM images was found to be in agreement with XRD results. MFM images also agrees well with NRS results as the presence of multi-magnetic components can be clearly seen in the sample grown at Ts~= 523,K. The information about the hybridization between Fe and C, obtained from Fe $L$ and C $K$-edges XANES also supports the results obtained from other measurements. In essence, from this work, experimental realization of tefc~has been demonstrated. It can be anticipated that by further fine-tuning the deposition conditions, even single phase tefc~phase can be realized which hitherto remains an experimental challenge.
The composition dependence of the structural, magnetic, and transport properties of epitaxially grown Mn-Co-Ga films were investigated. The crystal structure was observed to change from tetragonal to cubic as the Co content was increased. In terms of the dependence of saturation magnetization on the Co content, relatively small value was obtained for the Mn$_{2.3}$Co$_{0.4}$Ga$_{1.3}$ film at a large {it K}$_textrm u$ value of 9.2 Merg/cm$^3$. Electrical resistivity of Mn-Co-Ga films was larger than that of pure Mn-Ga film. The maximum value of the resistivity was 490 $muOmega$cm for Mn$_{2.2}$Co$_{0.6}$Ga$_{1.2}$ film. The high resistivity of Mn-Co-Ga might be due to the presence of localized electron states in the films due to chemical disordering caused by the Co substitution.
Ferromagnetic resonance (FMR) technique has been used to study the magnetization relaxation processes and magnetic anisotropy in two different series of the Co2FeSi (CFS) Heusler alloy thin films, deposited on the Si(111) substrate by UHV sputtering. While the CFS films of fixed (50 nm) thickness, deposited at different substrate temperatures (TS) ranging from room temperature (RT) to 600^C, constitute the series-I, the CFS films with thickness t varying from 12 nm to 100 nm and deposited at 550^C make up the series-II. In series-I, the CFS films deposited at TS = RT and 200^C are completely amorphous, the one at TS = 300^C is partially crystalline, and those at TS equal 450^C, 550^C and 600^C are completely crystalline with B2 order. By contrast, all the CFS films in series-II are in the fully-developed B2 crystalline state. Irrespective of the strength of disorder and film thickness, angular variation of the resonance field in the film plane unambiguously establishes the presence of global in-plane uniaxial anisotropy. Angular variation of the linewidth in the film plane reveals that, in the CFS thin films of varying thickness, a crossover from the in-plane local four-fold symmetry (cubic anisotropy) to local two-fold symmetry (uniaxial anisotropy) occurs as t exceeds 50 nm. Gilbert damping parameter {alpha} decreases monotonously from 0.047 to 0.0078 with decreasing disorder strength (increasing TS) and jumps from 0.008 for the CFS film with t = 50 nm to 0.024 for the film with t equal 75 nm. Such variations of {alpha} with TS and t are understood in terms of the changes in the total (spin-up and spin-down) density of states at the Fermi level caused by the disorder and film thickness.
We present a comprehensive first principles electronic structure study of the magnetoelastic and magnetostrictive properties in the Co-based Co$_2$XAl (X = V, Ti, Cr, Mn, Fe) full Heusler compounds. In addition to the commonly used total energy approach, we employ torque method to calculate the magnetoelastic tensor elements. We show that the torque based methods are in general computationally more efficient, and allow to unveil the atomic- and orbital-contributions to the magnetoelastic constants in an exact manner, as opposed to the conventional approaches based on second order perturbation with respect to the spin-orbit coupling. The magnetostriction constants are in good agreement with available experimental data. The results reveal that the main contribution to the magnetostriction constants, $lambda_{100}$ and $lambda_{111}$, arises primarily from the strained-induced modulation of the $langle d_{x^2-y^2}|hat{L}_z|d_{xy}rangle$ and $langle d_{z^2}|hat{L}_x|d_{yz}rangle$ spin orbit coupling matrix elements, respectively, of the Co atoms.
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