Observation of two distinct energy scales in the magnetization measurements of the anisotropic antiferromagnet TmAgGe

TmAgGe is an antiferromagnet ($T_{N}$ = 4.2 K) that crystallizes in a variant of the hexagonal Fe$_{2}$P structure (three Tm atoms per unit cell). We have studied the magnetization of the TmAgGe single crystals in fields $\mu _{0}H$ of up to 65 T as a function of the field orientation and the temperature $T$. With \textbf{H} in the basal \textbf{ab}-plane, a number of metamagnetic transitions are observed for $\mu _{0}H \quad <$5 T and $T \quad < \quad T_{N. }$However, when \textbf{H}$\vert \vert $\textbf{c}, three steps in the magnetization occur between 30 and 35 T, persisting to $T \quad \cong $ 60 K. On tilting \textbf{H} away from \textbf{c, }both sets of features (high-field steps and low-field metamagnetism transitions) are seen, showing that they arise from two distinct mechanisms. The dependence of the high-field steps on $T$ and field orientation suggests that they are associated with crystalline electric field (CEF) level crossing; the CEF confines the moments to the \textbf{ab}-plane. By contrast, it is the rearrangement of the moments within the basal plane that gives to the low-field metamagnetic transitions. To the best of our knowledge, TmAgGe is the first intermetallic system in which these two energy scales (CEF and in-plane exchange) can be unambiguously distinguished in this way.