Exploring the Electronic and Magnetic Behavior of NdPO₄: A First-Principles Study

With growing demand for cooling and rising interest in eco-friendly solutions, researchers are focusing on the development of advanced cooling technologies, including those utilizing the magnetocaloric effect by means of magnetocaloric materials. Such development necessitates a thorough understanding of these types of materials. In this context, a density functional theory (DFT) study of neodymium orthophosphate (NdPO₄) was conducted by employing the GGA + U + SOC approximation. The compound's electronic and magnetic properties, investigated in the monoclinic monazite structure, revealed an antiferromagnetic configuration, which can be described as two antiparallel sublattices, while electronic structure analysis showed a direct band gap energy of 3.39 eV, indicating an insulating behavior. To probe its magnetic properties, sum rules applied to X-ray magnetic circular dichroism (XMCD) and X-ray absorption spectra (XAS) near the M_5,4 edges coupled to pure DFT calculations yielded a total magnetic moment of 1.4 μ_B, where 2.92, -1.64, and 0.105 μ_B are the spin, orbital, and dipolar term contributions, respectively. Additionally, NdPO₄ shows a modest magneto-crystalline anisotropy with the b-axis identified as the hard and the ac plane as the easy magnetization directions. Finally, the superexchange was found to be very weak; however, it remains the driving mechanism over dipole-dipole interaction, especially between nearest neighbors, while driving the transition temperature around 0.32 K.