Variable temperatureX-band EPR ofGd3+inLaNbO4andPrNbO4crystals: Low-symmetry effect, influence of host and impurity paramagnetic ions on linewidth, and onset of antiferromagnetism

Abstract

EPR studies on ${\mathrm{Gd}}^{3+}$-doped diamagnetic ${\mathrm{LaNbO}}_{4}$ and paramagnetic ${\mathrm{PrNbO}}_{4}$ crystals containing ${\mathrm{Nd}}^{3+}$ and ${\mathrm{Ce}}^{3+}$ impurity Kramers ions were carried out at the X band (9.61 GHz) in the 4--295 K temperature range. The observed asymmetry in the extrema of ${\mathrm{Gd}}^{3+}$ EPR line positions in the angular variations of the EPR lines in the two hosts for the various transitions observed for the orientations of the external magnetic field about the Z and Y magnetic axes in the ac crystallographic plane was ascribed to the existence of the low, monoclinic site symmetry at the ${\mathrm{Gd}}^{3+}$ ion. The ${\mathrm{Gd}}^{3+}$ spin-Hamiltonian parameters were estimated from EPR line positions in the two crystals at 8, 73, and 295 K. The variation of the ${\mathrm{Gd}}^{3+}$ EPR linewidth as a function of temperature in the paramagnetic ${\mathrm{PrNbO}}_{4}$ host between 100 and 295 K is explained to be due to the dynamical dipolar and exchange interactions between the impurity ${\mathrm{Gd}}^{3+}$ ions and the host paramagnetic ${\mathrm{Pr}}^{3+}$ ions. The increase in the linewidth with decreasing temperature below 30 K in the two crystals implies the onset of antiferromagnetic ordering, consistent with ${T}_{c}=1.6$ and 2.4 K in ${\mathrm{LaNbO}}_{4}$ and ${\mathrm{PrNbO}}_{4}$ crystals. The observed decrease in the integrated intensity of ${\mathrm{Gd}}^{3+}$ EPR signal with lowering temperature in the two crystals is ascribed to the coupling of ${\mathrm{Gd}}^{3+}$ ions to the impurity ${\mathrm{Nd}}^{3+}$ and ${\mathrm{Ce}}^{3+}$ Kramers ions present in the samples.