Magnetization steps in Pb1-xEuxSe: Determination and identification of the dominant antiferromagnetic exchange constant.

Abstract

The magnetization of three ${\mathrm{Pb}}_{1\ensuremath{-}x}{\mathrm{Eu}}_{x}\mathrm{Se}$ samples, with $x=1.3, 3.0, \mathrm{and} 4.1%$, was measured at 30 and 50 mK in magnetic fields $H$ up to 50 kOe, and at 0.6 K in fields up to 180 kOe. For $x=1.3%$ and with H\ensuremath{\parallel}[100], a magnetization step (MST) due to an energy-level crossing for isolated ${\mathrm{Eu}}^{2+}$ ions was observed at 30 and 50 mK. The magnetic field at this MST, 1.76\ifmmode\pm\else\textpm\fi{}0.2 kOe, was close to the predicted value ${H}_{c}=1.98$ kOe. At the same low temperatures (30 and 50 mK) but at higher fields, a magnetization "ramp" due to pairs was observed in all samples. For $x=3.0 \mathrm{and} 4.1%$ this ramp consisted of well-resolved MST's arising from pairs. A ramp due to open triplets was also observed in these two samples. The MST's due to pairs were used to obtain the value $\frac{J}{{k}_{B}}=\ensuremath{-}0.24\ifmmode\pm\else\textpm\fi{}0.03$ K for the dominant antiferromagnetic exchange constant. Comparisons between the measured magnetization curves at 30 or 50 mK and theoretical simulations indicates that this $J$ is the nearest-neighbor (NN) exchange constant ${J}_{1}$. At 0.62 K the magnetization of each of the three samples rose rapidly with $H$ in fields below several kOe. At higher fields a rounded ramp due to pairs and triplets was present. This ramp ended near 40 kOe, and complete saturation was achieved near 50 kOe. A model which includes only the NN exchange constant ${J}_{1}$ gave a reasonably good account for all the data at 0.62 K. Calculated magnetization curves for pairs, and for open and closed triplets, at various values of $\frac{{k}_{B}T}{|J|}$ are presented. The effects of the single-ion and dipole-dipole anisotropies on the MST's due to pairs are also discussed.