Incommensurate magnetic structure and crystal field in ErNi₂B₂C studied by ¹⁶⁶Er Mossbauer spectroscopy and specific heat measurements

Abstract

¹⁶⁶Er Mossbauer absorption spectra have been recorded in the superconductor ErNi₂B₂C (T_c=10rK), in zero field and in the temperature range 1.4rK-40rK. The spectra in the magnetically ordered phase (TN 5.5rK) are characteristic of an incommensurate magnetic structure with a maximum Er³⁺ moment of 8.2μB. We show that the magnetic transition is first order, with a small temperature range where magnetically ordered and paramagnetic domains coexist. The analysis of the magnetic and quadrupolar hyperfine interactions below and above T_N suggests that the Er³⁺ ground doublet is close to |J=15/2; J_z=±½> and shows that there is a low lying (10rK) excited doublet. The measured Er³⁺ electronic relaxation rate 1/T₁ shows an anomaly at 10K (T_c) suggesting that the conduction electrons that are exchange coupled to the 4f spin take part in the formation of the superconducting state. We also present specific heat data for Er_xY_1-xNi₂B₂C and Er_0.2Lu_0.8Ni₂B₂C which also evidence a low lying doublet and from which we propose a modified (improved) Er³⁺ crystal field level scheme (0, 9, 62, 71, 73, 181, 212 and 216 K) where the energies of the higher levels are consistent with published neutron scattering data.