Speaker
Description
Multiferroic NdMnO$_{3+\delta}$ belongs to the family of RMnO$_3$ (R= rare earth) perovskite and exhibits several fascinating properties, including negative magnetization, magnetocaloric effect (MCE), magnetic anisotropy, and spin reorientation (SR) within the Mn sublattice. Materials with a large MCE are highly desirable for magnetic refrigeration technologies, which provide an energy-efficient and environmentally friendly alternative to conventional gas-compression refrigeration. This study aims to investigate the impact of oxygen content on the MCE, critical behaviour, negative magnetization and magnetic anisotropy of NdMnO$_{3+\delta}$ crystals grown by the vertical optical floating zone technique in air, O$_2$ and Ar atmosphere.
The crystals were characterized by X-ray powder diffraction, electron scanning microscopy and thermogravimetric measurements. The significant increase of magnetization below 80 K is observed in $M$-$T$ curves showing the onset of magnetic ordering and the temperature dependent magnetization starts decreasing below $20$ K. Both magnetic transitions are also evident in the AC susceptibility ($\chi$$_{ac}$) curve, i.e., around $60$ K and $15$ K. A negative magnetization is observed in zero field cooled (ZFC) and field cooled (FC) regimes. As the temperature decreases from $50$ K, a reduction in magnetization is observed due to the long-range ordering of Nd$^{3+}$, influenced by competing exchange-coupled FM and weak FM interactions at the Nd$^{3+}$ site. When the magnetic moment reaches zero around $15$ – $50$ K, a negative magnetization emerges in the ZFC and FC regimes, attributed to the SR of Mn$^{3+}$. The M-H curves at different temperatures (i.e., $2$ - $150$ K) showing the presence of ferromagnetism at low temperatures while at high temperature a paramagnetic phase has appeared. The relationship between magnetic entropy $-\Delta S_{M}$ and temperature is used to study the MCE of NdMnO$_{3+\delta}$. A sharp increase of $-\Delta S_{M}$ absolute value is obtained around the ordering temperature ($10$ - $20$ K) of Nd sublattice, due to the rapid change in magnetization of NdMnO$_{3+\delta}$ as a result of the disruption Mn moments orientation resulting from the increased applied field and Nd sublattice ordering.
Acknowledgements
This publication is the result of the project implementation: VEGA 2/0004/25.
