Search

Dirac semimetals show nontrivial physical properties and can host exotic quantum states like Weyl semimetals and topological insulators under suitable external conditions. Here, by combining angle-resolved photoemission spectroscopy measurements (ARPES) and first-principle calculations, we demonstrate that Zintl-phase compound SmMg$_2$Bi$_2$ belongs to the close proximity to a topological Dirac semimetallic state. ARPES results show a Dirac-like band crossing at the zone-center near the Fermi level ($E_\mathrm {F}$) which is further confirmed by first-principle calculations. Theoretical studies also reveal that SmMg$_2$Bi$_2$ belongs to a $Z_2$ topological class and hosts spin-polarized states around the $E_\mathrm {F}$. Zintl's theory predicts that the valence state of Sm in this material should be Sm$^{2+}$, however we detect many Sm-4$f$ multiplet states (flat-bands) whose energy positions suggest the presence of both Sm$^{2+}$ and Sm$^{3+}$. It is also evident that these flat-bands and other dispersive states are strongly hybridized when they cross each other. Due to the presence of Sm$^{3+}$ ions, the temperature dependence of magnetic susceptibility $\chi(T)$ shows Curie-Weiss-like contribution in the low temperature region, in addition to the Van Vleck-like behaviour expected for the Sm$^{2+}$ ions. The present study will help in better understanding of the electronic structure, magnetism and transport properties of related materials., Comment: 11 pages, 7 figures

We investigated the effect of off-stoichiometry on the Gilbert magnetic damping constant (\ensuremath{\alpha}) of Heusler alloy $\mathrm{C}{\mathrm{o}}_{2}\mathrm{MnSi}$ (CMS) thin films by employing time-resolved magneto-optical Kerr effect (TR-MOKE) measurements along with first-principles calculations based on the linear-response theory for magnetic damping. Because of the contribution of extrinsic damping arising from two-magnon scattering, the effective \ensuremath{\alpha} (${\ensuremath{\alpha}}_{\mathrm{eff}}$) extracted from the TR-MOKE responses showed dependences on the in-plane magnetic-field angle (${\ensuremath{\theta}}_{\mathrm{H}}$) and the field strength ($H$). Then, we obtained the smallest ${\ensuremath{\alpha}}_{\mathrm{eff}}$ (${\ensuremath{\alpha}}_{0}$) for each sample under the most reduced contribution from two-magnon scattering realized through varying ${\ensuremath{\theta}}_{\mathrm{H}}$ and $H$ values, which is the closest value to \ensuremath{\alpha}. The thus obtained ${\ensuremath{\alpha}}_{0}$ values of epitaxially grown off-stoichiometric $\mathrm{C}{\mathrm{o}}_{2}\mathrm{M}{\mathrm{n}}_{\ensuremath{\beta}}\mathrm{S}{\mathrm{i}}_{\ensuremath{\gamma}}$ ($\ensuremath{\gamma}=0.82$) films with various (Mn + Si) compositions, $(\ensuremath{\beta}+\ensuremath{\gamma})$, decreased with increasing $(\ensuremath{\beta}+\ensuremath{\gamma})$ from ${\ensuremath{\alpha}}_{0}=0.0057$ for (Mn + Si)-deficient $(\ensuremath{\beta}+\ensuremath{\gamma})=1.44$ to ${\ensuremath{\alpha}}_{0}=0.0036$ for $(\ensuremath{\beta}+\ensuremath{\gamma})=1.90$ being close to the stoichiometric one of $(\ensuremath{\beta}+\ensuremath{\gamma})=2.0$ at 300 K. It was also demonstrated that a half-metallic (Mn+Si)-rich CMS film with $\ensuremath{\beta}=1.30$ and $\ensuremath{\gamma}=0.90$ showed a low ${\ensuremath{\alpha}}_{0}$ of 0.0035. The dependence of ${\ensuremath{\alpha}}_{0}$ on $(\ensuremath{\beta}+\ensuremath{\gamma})$ in $\mathrm{C}{\mathrm{o}}_{2}\mathrm{M}{\mathrm{n}}_{\ensuremath{\beta}}\mathrm{S}{\mathrm{i}}_{\ensuremath{\gamma}}$ ($\ensuremath{\gamma}=0.82$) was well explained by the first-principles calculations. Through the systematic investigations of off-stoichiometric CMS with various values of $(\ensuremath{\beta}+\ensuremath{\gamma})$, it was clarified that the total density of states (DOS) at the Fermi level, $D({E}_{\mathrm{F}})$, plays the key role for determining the damping constant of CMS. Furthermore, it was revealed that the reduced minority-spin DOS at ${E}_{\mathrm{F}}$, caused by decreasing harmful $\mathrm{C}{\mathrm{o}}_{\mathrm{Mn}}$ antisites, is essential for reducing the damping constant of CMS. These findings demonstrate that appropriately controlling off-stoichiometry and film composition is thus promising for achieving half-metallicity and a low \ensuremath{\alpha} simultaneously for CMS thin films.

The FeIr/MgO multilayer system was found to exhibit large interfacial perpendicular magnetic anisotropy (PMA) and voltage-controlled magnetic anisotropy effects. We herein report the x-ray magnetic circular dichroism spectroscopy of both the Fe and Ir absorption edges in the FeIr/MgO system. Although Ir insertion at the Fe/MgO interface increased the PMA energy in the system, the orbital magnetic-moment anisotropy of Fe remained approximately constant. We found significant anisotropy in the orbital magnetic moment but not in the magnetic-dipole ${T}_{\mathrm{z}}$ term in the Ir atoms. Our experimental and theoretical results demonstrated that the large PMA of FeIr/MgO originates from second-order perturbation of the spin-orbit interaction energy of Ir. Moreover, the energy mainly originates from the spin-conserved virtual excitation process terms from the majority-spin bands of the Ir atoms located in the second atomic layer from MgO.

Precise magnetization measurements of a half-metallic ferromagnet ${\mathrm{Co}}_{2}\mathrm{TiSn}$ with the Heusler type structure have been carried out under pressure up to 1.27 GPa. At ambient pressure, the spontaneous magnetic moment ${M}_{\mathrm{s}}^{\mathrm{exp}}$ and the Curie temperature ${T}_{\mathrm{C}}^{\mathrm{exp}}$ are experimentally obtained to be $1.993(3)\phantom{\rule{4pt}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}$/f.u. and $376.76(7)$ K, respectively. The ${M}_{\mathrm{s}}^{\mathrm{exp}}$ is independent of applying pressure. The ${T}_{\mathrm{C}}^{\mathrm{exp}}$ decreases with the increase of pressure and the pressure derivative of the Curie temperature $d{T}_{\mathrm{C}}^{\mathrm{exp}}/dp$ is estimated to be $\ensuremath{-}3.15(9)$ K/GPa. Furthermore, the pressure effect on the electronic and magnetic properties of ${\mathrm{Co}}_{2}\mathrm{TiSn}$ has been theoretically investigated on the basis of first-principles density functional calculations by using the projector augmented wave method in order to evaluate the half metallicity of ${\mathrm{Co}}_{2}\mathrm{TiSn}$. Both the experimental and the theoretical results under pressure reveal that ${\mathrm{Co}}_{2}\mathrm{TiSn}$ is half metal. The experimental results of pressure-dependent magnetic properties are finally discussed on the basis of the spin fluctuation theory for itinerant electron magnetism.

We investigated the magnetic anisotropy of an iron layer on a Pt(001) surface and some related systems by employing the local spin density approximation in a theoretical ab initio approach. We found that the surface system $\mathrm{Pt}∕\mathrm{Fe}∕\mathrm{Pt}(001)$ showed a perpendicular magnetic anisotropy and its anisotropy energy per iron atom amounted to a value which is 2 times larger than the value of bulk FePt. The surface relaxation much enhances the anisotropy energies, related to a large attractive force between the iron and platinum layers. A remarkable cap effect---that the covering platinum layer changes anisotropy energy---was also found to exist. We investigated the microscopic origin of the perpendicular anisotropy in relation to the local densities of states of the Fe atom. These quantities were discussed as a fingerprint of magnetic anisotropy in comparison with the results of the Fe chain at the step edge on a vicinal surface Pt(664). The atomic orbital magnetic moments were enhanced at the respective surface atoms.