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Register a new userTunable magnetization relaxation of Fe_{2}Cr_{1-x}Co_{x}Si half-metallic Heusler alloys by band structure engineering
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We report a systematic investigation on the magnetization relaxation properties of iron-based half-metallic Heusler alloy Fe$_{2}$Cr$_{1-x}$Co_${x}$Si (FCCS) thin films using broadband angular-resolved ferromagnetic resonance. Band structure engineering through Co doping (x) demonstrated by first-principles calculations is shown to tune the intrinsic magnetic damping over an order of magnitude, namely 0.01-0.0008. Notably, the intrinsic damping constants for samples with high Co concentration are among the lowest reported for Heusler alloys and even comparable to magnetic insulator yttrium iron garnet. Furthermore, a significant reduction of both isotropic and anisotropic contributions of extrinsic damping of the FCCS alloys was found in the FCCS films with x=0.5-0.75, which is of particular importance for applications. These results demonstrate a practical recipe to tailor functional magnetization for Heusler alloy-based spintronics at room temperature
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Though Weyl fermions have recently been observed in several materials with broken inversion symmetry, there are very few examples of such systems with broken time reversal symmetry. Various Co$_{2}$-based half-metallic ferromagnetic Heusler compounds are lately predicted to host Weyl type excitations in their band structure. These magnetic Heusler compounds with broken time reversal symmetry are expected to show a large momentum space Berry curvature, which introduces several exotic magneto-transport properties. In this report, we present systematic analysis of experimental results on anomalous Hall effect (AHE) in Co$_2$Ti$X$ ($X$=Si and Ge). This study is an attempt to understand the role of Berry curvature on AHE in Co$_2$Ti$X$ family of materials. The anomalous Hall resistivity is observed to scale quadratically with the longitudinal resistivity for both the compounds. The detailed analysis indicates that in anomalous Hall conductivity, the intrinsic Karplus-Luttinger Berry phase mechanism dominates over the extrinsic skew scattering and side-jump mechanism.
The low temperature heat capacity C_{p} of Fe_{1-x}Ga_{x} alloys with large magnetostriction has been investigated. The data were analyzed in the standard way using electron ($gamma T$) and phonon ($beta T^{3}$) contributions. The Debye temperature $Theta_{D}$ decreases approximately linearly with increasing Ga concentration, consistent with previous resonant ultrasound measurements and measured phonon dispersion curves. Calculations of $Theta_{D}$ from lattice dynamical models and from measured elastic constants C_{11}, C_{12} and C_{44} are in agreement with the measured data. The linear coefficient of electronic specific heat $gamma$ remains relatively constant as the Ga concentration increases, despite the fact that the magnetoelastic coupling increases. Band structure calculations show that this is due to the compensation of majority and minority spin states at the Fermi level.
The local atomic environments and magnetic properties were investigated for a series of Co(1+x)Fe(2-x)Si (0<x<1) Heusler compounds. While the total magnetic moment in these compounds increases with the number of valance electrons, the highest Curie temperature (Tc) in this series was found for Co1.5Fe1.5Si, with a Tc of 1069 K (24 K higher than the well known Co2FeSi). 57Fe Mossbauer spectroscopy was used to characterize the local atomic order and to estimate the Co and Fe magnetic moments. Consideration of the local magnetic moments and the exchange integrals is necessary to understand the trend in Tc.
We use scanning SQUID microscopy to investigate the behavior of vortices in the presence of twin boundaries in the pnictide superconductor Ba(Fe1-xCox)2As2. We show that the vortices avoid pinning on twin boundaries. Individual vortices move in a preferential way when manipulated with the SQUID: they tend to not cross a twin boundary, but rather to move parallel to it. This behavior can be explained by the observation of enhanced superfluid density on twin boundaries in Ba(Fe1-xCox)2As2. The observed repulsion from twin boundaries may be a mechanism for enhanced critical currents observed in twinned samples in pnictides and other superconductors.
Doping dependence of the superconducting state structure and spin-fluctuation pairing mechanism in the $Ba(Fe_{1-x}Co_{x})_{2}As_{2}$ family is studied. BCS-like analysis of experimental data shows that in the overdoped regime, away from the AFM transition, the spin-fluctuation interaction between the electron and hole gaps is weak, and $Ba(Fe_{1-x}Co_{x})_{2}As_{2}$ is characterized by three essentially different gaps. In the three-gap state an anisotropic (nodeless) electron gap $Delta_e (x, phi)$ has an intermediate value between the dominant inner $Delta_{2h}(x)$ and outer $Delta_{1h}(x)$ hole gaps. Close to the AFM transition the electron gap $Delta_e (x, phi)$ increases sharply and becomes closer in magnitude to the dominant inner hole gap $Delta_{2h}(x)$. The same two-gap state with close electron and inner hole gaps $Delta_{2h}(x) approx Delta_e (x, phi)$ is also preserved in the phase of coexisting antiferromagnetism and superconductivity. The doping dependence of the electron gap $Delta_e (x, phi)$ is associated with the strong doping dependence of the spin-fluctuation interaction in the AFM transition region. In contrast to the electron gap $Delta_e (x, phi)$, the doping dependence of the hole gaps $Delta_{1,2h}(x)$ and the critical temperature $T_{c}(x)$, both before and after the AFM transition, are associated with a change of the density of states $gamma_{nh}(x)$ and the intraband electron-phonon interaction in the hole bands. The non-phonon spin-fluctuation interaction in the hole bands in the entire Co concentration range is small compared with the intraband electron-phonon interaction and is not dominant in the $Ba(Fe_{1-x}Co_{x})_{2}As_{2}$ family.
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