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Register a new userMagnetic couplings and magnetocaloric effect in the GdTX (T=Sc, Ti, Co, Fe; X=Si, Ge) compounds
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We compute the magnetocaloric effect (MCE) in the GdTX (T=Sc, Ti, Co, Fe; X=Si, Ge) compounds as a function of the temperature and the external magnetic field. To this end we use a density functional theory approach to calculate the exchange-coupling interactions between Gd$^{3+}$ ions on each compound. We consider a simplified magnetic Hamiltonian and analyze the dependence of the exchange couplings on the transition metal T, the p-block element X, and the crystal structure (CeFeSi-type or CeScSi-type). The most significant effects are observed for the replacements Ti $to$ Sc or Fe $to$ Co which have an associated change in the parity of the electron number in the 3d level. These replacements lead to an antiferromagnetic contribution to the magnetic couplings that reduces the Curie temperature and can even lead to an antiferromagnetic ground state. We solve the magnetic models through mean field and Monte Carlo calculations and find large variations among compounds in the magnetic transition temperature and in the magnetocaloric effect, in agreement with the available experimental data. The magnetocaloric effect shows a universal behavior as a function of temperature and magnetic field in the ferromagnetic compounds after a scaling of the relevant energy scales by the Curie temperature $T_C$.
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We present a theoretical analysis of the magnetic phase diagram of CeTi$_{1-x}$Sc$_{x}$Ge and GdFe$_{1-x}$Co$_{x}$Si as a function of the temperature and the Sc and Co concentration $x$, respectively. CeScGe and GdCoSi, as many other RTX (R=rare earth, T=transition metal, X=p-block element) compounds, present a tetragonal crystal structure where bilayers of R are separated by layers of T and X. While GdFeSi and CeTi$_{0.75}$Sc$_{0.25}$Ge are ferromagnetic, CeScGe and GdCoSi order antiferromagnetically with the R 4f magnetic moments on the same bilayer aligned ferromagnetically and magnetic moments in nearest neighbouring bilayers aligned antiferromagnetically. The antiferromagnetic transition temperature $T_N$ decreases with decreasing concentration $x$ in both compounds and for low enough values of $x$ the compounds show a ferromagnetic behavior. Based on these observations we construct a simplified model Hamiltonian that we solve numerically for the specific heat and the magnetization. We find a good qualitative agreement between the model and the experimental data. Our results show that the main magnetic effect of the Sc $to$ Ti and Co $to$ Fe substitution in these compounds is consistent with a change in the sign of the exchange coupling between magnetic moments in neighbouring bilayers. We expect a similar phenomenology for other magnetic RTX compounds with the same type of crystal structure.
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 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.
A comprehensive ellipsometric study was performed on Fe$_{1-x}$Co$_{x}$Si single crystals in the spectral range from 0.01 eV to 6.2 eV. Direct and indirect band gaps of 73 meV and 10 meV, respectively, were observed in FeSi at 7 K. One of four infrared-active phonons that is energetically close to the direct absorption edge is coupled both to the electrons and to the low-energy phonon. This is evident from asymmetry in the phonon line shape and a reduction of its frequency when the absorption edge shifts across the phonon energy due to the temperature dependence of the direct band gap. As the temperature increases, the indirect gap changes sign, which manifests as a transition from a semiconductor to a semimetal. The corresponding gain of the spectral weight at low energies was recovered within an energy range of several eV. The present findings strongly support the model indicating that Fe$_{1-x}$Co$_{x}$Si can be well described in an itinerant picture, taking into account self-energy corrections.
Magnetic entropy and adiabatic temperature changes in and above the room-temperature region has been measured for La0.7Sr0.3Mn1-xMxO3 (M = Al, Ti) by means of magnetization and heat capacity measurements in magnetic fields up to 6 T. The magnetocaloric effect becomes largest at the ferromagnetic ordering temperature Tc that is tuned to ~300 K by the substitution of Al or Ti for Mn. While the substitution of Al for Mn drastically reduces the entropy change, it extends considerably the working temperature span and improves the relative cooling power. The magnetocaloric effect seems to be only lightly affected by Ti substitution. Although manganites have been considered potential for magnetic refrigerants, the magnetocaloric effect in these materials is limited due to the existence of short-range ferromagnetic correlations above Tc.
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