Interplay between crystal structure and magnetic susceptibility of tetragonal ROBr
Abstract
The average magnetic susceptibilities for trivalent rare earth (R3+) ions in polycrystalline tetragonal rare earth oxybromides (ROBr, R = Ce–Nd, Sm, Eu, and Tb–Yb) were measured between 2 and 298 K. The susceptibilities of all ROBr follow the paramagnetic Curie–Weiss behaviour down to low temperatures except for SmOBr and EuOBr where the effect of crystal field was observed already above 200 K. The observed effective magnetic moments μeff(RT) were generally higher than the free ion values. For the heavier R3+ ions (Ho3+ and Er3+) μeff was found to be less than the free ion value due to crystal field mixing in the opposite way. The Weiss constant θ was found to be negative for all ROBr, suggesting antiferromagnetic (AFM) ordering at lower temperatures but only SmOBr (SmOCl) and DyOBr (DyOCl) show such ordering at 5(8) and 8(11) K, respectively. The increasing interlayer distance between adjacent (RO)n+n layers from ROCl to ROBr and within the ROBr series lowers the Néel temperature. AFM ordering thus depends on the 3D interactions while 2D interactions have negligible effect. The temperature dependence of the experimental paramagnetic susceptibility for each ROBr was simulated with the aid of the van Vleck formalism based on the wave functions and energy level values obtained from spectroscopic data. Lattice expansion and consequent modifications in crystal field caused discrepancies at higher temperatures.
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