Magnetic anisotropy of Kondo semiconductor CeT2Al10(T=Ru,Os) in the ordered state

Abstract

We studied the applied magnetic field direction ($\ensuremath{\theta}$) dependence of the magnetic and transport properties of Ce$T$${}_{2}$Al${}_{10}$ ($T=$ Ru, Os) in the $ab$, $bc$, and $ca$ planes focusing on the magnetic anisotropy in the ordered state. The magnetization of NdOs${}_{2}$Al${}_{10}$ was also studied as a reference of Ce$T$${}_{2}$Al${}_{10}$. The results of NdOs${}_{2}$Al${}_{10}$ in the antiferromagnetic (AFM) state could be understood as a normal localized AFM compound. In Ce$T$${}_{2}$Al${}_{10}$, the magnetic susceptibility in the $ab$, $bc$, and $ca$ planes exhibits a smooth $\mathrm{cos}\ensuremath{\theta}$-like $\ensuremath{\theta}$ dependence both in the ordered state and the paramagnetic region. This suggests that the antiferromagnetic moment (${m}_{\mathrm{AFM}}$) is not so strongly fixed along the $c$ axis in the ordered phase, which indicates that the anisotropic AFM exchange interaction along the $c$ axis is not strong and a coupling between the orbital moment of the Ce ion and the lattice is also small. The $\ensuremath{\theta}$ dependence of ${H}^{*}$ in the $bc$ and $ca$ planes strongly supports the spin-flop transition from ${m}_{\mathrm{AFM}}\ensuremath{\parallel}c$ to ${m}_{\mathrm{AFM}}\ensuremath{\parallel}b$ at ${H}^{*}$ for $H\ensuremath{\parallel}c$ unexpected from the smallest magnitude of the magnetic susceptibility along the $b$ axis, which was recently proposed by our ${}^{27}$Al NMR studies. From the $\ensuremath{\theta}$ dependence of ${H}^{*}$ in the $ca$ plane, we propose that the phase II observed in a high-field magnetization curve a little below the critical field to the paramagnetic region for $H\ensuremath{\parallel}a$ is the spin-flop phase B with ${m}_{\mathrm{AFM}}\ensuremath{\parallel}b$. ${H}^{*}$ in CeOs${}_{2}$Al${}_{10}$ is about twice larger than that in CeRu${}_{2}$Al${}_{10}$, and when the temperature approaches to the transition temperature (${T}_{0}$) from the low temperature, ${H}^{*}$ of CeOs${}_{2}$Al${}_{10}$ exhibits the unusual increase, but that of CeRu${}_{2}$Al${}_{10}$ is reduced. As the origin of the different behavior of ${H}^{*}$ in these two compounds, we propose that the relative weight of the singlet component against the AFM component in the ordered state close to ${T}_{0}$ and also the $c$-$f$ hybridization along the $b$ axis are larger in CeOs${}_{2}$Al${}_{10}$ than in CeRu${}_{2}$Al${}_{10}$. We conclude that the anisotropic $c$-$f$ hybridization is the origin of the unusual magnetic anisotropy of Ce$T$${}_{2}$Al${}_{10}$ in the ordered state.