${\mathrm{Mn}}_4{\mathrm{Ta}}_2{\mathrm{O}}_9$ (MTO) is a magnetoelectric material with Ising-like antiferromagnetism (AFM) in which an applied magnetic field ($H$) induces electric polarization and an applied electric field produces magnetization. Here, we report results from our detailed investigations of the temperature ($T$) and $H$ dependence of magnetization ($M$) and heat capacity of MTO. These results show AFM ordering in MTO below $T_N \sim$ 102 K followed by additional transitions near $T_{S1} \sim$ 50 K and $T_{S2} \sim$ 20 K. The $M$ vs $H$ variations at 3 K show the presence of weak ferromagnetism (FM) below $T_{S1}$ superposed on AFM, with coercivity becoming zero for $T\ge T_{S1}$. This weak FM for $T\le T_{S1}$ is likely due to two inequivalent ${\mathrm{Mn}}^{2+}$ ions present in MTO. The magnetic susceptibility $\chi$ vs $T$ data for $T> T_N$ is used to determine two dominant AFM exchange constants $J_2/k_B$ = –11.3 K and $J_3/k_B$ = –22.9 K, which are in good agreement with those determined from the first-principles density-functional theory ($\mathrm{DFT}+U$) calculations. It is argued that the $H$-induced transition observed at 3 K at the critical field $H_c \sim$ 20 kOe is a spin flop transition whose surprising shift to higher $H$ values with increasing $T$ is explained in terms of $T$ dependence of anisotropic $\chi$, magnetic anisotropy, and exchange fields. The temperature dependence of magnetic specific heat and entropy shows the presence of short-range spin correlations ordering up to 140 K, a $\lambda$-type anomaly at $T_N$, and a broad anomalous peak near 40 K covering the region of $T_{S1}$ and $T_{S2}$. To further explore the nature of these transitions at $T_N, T_{S1}$, and $T_{S2}$, the $T$ dependence of various Raman modes of MTO was measured and compared with the lattice dynamics calculations from $\mathrm{DFT}+U$. For $T> T_N, T$ dependencies of the frequency and linewidth of the Raman modes are described by the three-phonon anharmonic approximation with considerable departures in these quantities observed at $T_N, T_{S1}$, and $T_{S2}$, which are then related to the strong spin-phonon coupling evident in MTO.

Close

PrevNext

View figure in article

• Ferromagnetism
• Metamagnetism
• Phonons
• Spin state transition
• Spin-phonon coupling
• DFT+U