Ferromagnetic ordering along the hard axis in the Kondo lattice ${\mathrm{YbIr}}_3{\mathrm{Ge}}_7$

Ferromagnetic Kondo lattice compounds are far less common than their antiferromagnetic analogs. In this Rapid Communication, we report the discovery of a ferromagnetic Kondo lattice compound, ${\mathrm{YbIr}}_3{\mathrm{Ge}}_7$. As in almost all ferromagnetic Kondo lattice systems, ${\mathrm{YbIr}}_3{\mathrm{Ge}}_7$ shows magnetic order with moments aligned orthogonal to the crystal electric field (CEF) easy axis. ${\mathrm{YbIr}}_3{\mathrm{Ge}}_7$ is unique in that it is the only member of this class of compounds that crystallizes in a rhombohedral structure with a trigonal point symmetry of the magnetic site, and it lacks broken inversion symmetry at the local moment site. The ac magnetic susceptibility, magnetization, and specific heat measurements show that ${\mathrm{YbIr}}_3{\mathrm{Ge}}_7$ has a Kondo temperature $T_{\mathrm{K}}\approx 14$ K and a Curie temperature $T_{\mathrm{C}}=2.4\mathrm{K}$. Ferromagnetic order occurs along the crystallographic [100] hard CEF axis despite the large CEF anisotropy of the ground-state Kramers doublet with a saturation moment along [001] almost four times larger than the one along [100]. This implies that a mechanism which considers the anisotropy in the exchange interaction to explain the hard-axis ordering is unlikely. On the other hand, the broad second-order phase transition at $T_{\mathrm{C}}$ favors a fluctuation-induced mechanism.

Figure 1

(a) Inverse magnetic susceptibility $H/(M-M_0)$ vs $T$ with the polycrystalline average $M_{\text{avg}}=(M_{001}+2M_{100})/3$ (purple line) and Curie-Weiss fits between $T=400$ and 600 K (solid black lines). (b) Temperature-dependent ac magnetic susceptibility ${\chi }^{\prime }$ with $H_{\mathrm{ac}}$ along [001] (blue open symbols) and along [100] (red solid symbols) with different applied static fields $H$ applied also along [100].

Figure 2

Scaled temperature-dependent electrical resistivity $\rho /{\rho }_{300}$ of ${\mathrm{YbIr}}_3{\mathrm{Ge}}_7$ (red circles) with ${\mu }_{0}H=0$ (open symbols) and 9 T (solid symbols) for $H\left|\right|i\left|\right|\left[100\right]$. The nonmagnetic polycrystalline analog ${\mathrm{LuIr}}_3{\mathrm{Ge}}_7$ is shown with black symbols. The dashed line in the main panel shows a $-lnT$ increase in $\rho \left(T\right)$. The inset shows the absolute resistivity of ${\mathrm{YbIr}}_3{\mathrm{Ge}}_7$ with ${\mu }_{0}H=0$ and 9 T.

Figure 3

Magnetization isotherm $M\left(H\right)$ at $T=1.8$ K for $H\parallel \left[100\right]$ (solid red circles) and $H\parallel \left[001\right]$ (open blue circles). Left inset: Low field $M\left(H\right)$. Right inset: Ferromagnetic hysteresis in the ac susceptibility at 20 mK along $H\parallel \left[100\right]$.

Figure 4

(a) $H=0$ specific heat of ${\mathrm{YbIr}}_3{\mathrm{Ge}}_7$ (red symbols and line) and ${\mathrm{LuIr}}_3{\mathrm{Ge}}_7$ (black line). (b) Magnetic entropy of ${\mathrm{YbIr}}_3{\mathrm{Ge}}_7$ with $S_{\text{mag}}={\int }_0^T\frac{C_{\text{mag}}}{T}dT$, where $C_{\text{mag}}=C_p\left({\mathrm{YbIr}}_3{\mathrm{Ge}}_7\right)-C_p\left({\mathrm{LuIr}}_3{\mathrm{Ge}}_7\right)$.