Magnetic frustration and weak Mn magnetic ordering in EuMn${}_2$P${}_2$

Abstract

We report on the electron spin resonance (ESR), heat capacity, magnetization, nuclear magnetic resonance (NMR), magnetic circular and linear dichroism (XMCD, XMLD), as well as the electrical resistivity of EuMn${}_2$P${}_2$ single crystals. Antiferromagnetic order of Eu was observed in several quantities at $T_{\mathrm{N}}^{\mathrm{Eu}}=18\mathrm{K}$. The temperature dependencies of ESR linewidth and resonance shift show, when approaching the Eu-ordered state, a divergence towards $T_{\mathrm{N}}^{\mathrm{Eu}}$, indicating the growing importance of magnetic correlations and the build-up of internal magnetic fields. An additional temperature scale of $\approx 47\mathrm{K}$ has considerable impact on linewidth, resonance field and intensity. This points to the presence of weak Mn-based ordering. The observed ESR line is interpreted as an Eu${}^{2+}$ resonance, which probes the weak magnetic background of the Mn subsystem. Such picture is suggested by the lineshape which keeps to be Lorentzian across the $47\mathrm{K}$ scale and by the ESR intensity which can be described by the same Curie-Weiss temperature above and below $47\mathrm{K}$. In the same temperature range anomalies were observed at $48.5\mathrm{K}$ and $51\mathrm{K}$ in the heat capacity data as well as a pronounced broadening of the NMR signal of the EuMn${}_2$P${}_2$ samples. In XMCD and XMLD measurements, this weak magnetic order could not be detected in the same temperature range which might be due to the small magnetic moment, with a potential $c$-component or frustration.