ESR of Singlet-Ground-State Magnets at Very Low Temperatures

Abstract

The ESR spectra of NiSnCl₆·6H₂O(NCS) and NiPtCl₆·6H₂O(NCP) are studied at temperatures between 83 mK and 4.2 K. Below 1 K, marked shifts from the resonance positions expected from a single-ion spin-Hamiltonian occur and can be quantitatively described by a calculation based on the temperature-dependent moment method as well as by an approach of collective excitations, taking into account a small antiferromagnetic exchange interaction among Ni²⁺ ions, 2J=−(0.55±0.15)×10⁻² cm⁻¹ for NCS and −(1.00±0.08)×10⁻² cm⁻¹ for NCP. Combined with the direct determination of an axial crystal field parameter D with positive sign, 0.447±0.004 cm⁻¹ for NCS and 0.493±0.004 cm⁻¹ for NCP, it is confirmed that the substances are such the singlet-ground-state magnets that the non-existence of long-range order can be expected under zero magnetic field even at 0 K, The linewidth and line shape are also examined at these temperatures.