Incommensurate magnetic structure and crystal field in ErNi2B2C Studied by 166Er Mössbauer spectroscopy and specific heat measurements

Abstract

166Er Mossbauer absorption spectra have been recorded in the superconductor ErNi2B2C (Tc = 10 K), in zero field and in the temperature range 1.4 K–40 K. The spectra in the magnetically ordered phase (TN ≃ 5.5 K) are characteristic of an incommensurate magnetic structure with a maximum Er3+ moment of 8.2 μB. 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 TN suggests that the Er3+ ground doublet is close to |J = 15/2; Jz = ± 1/2 > and shows that there is a low lying (≃ 10 K) excited doublet. The measured Er3+ electronic relaxation rate 1/T1 shows an anomaly at 10 K (Tc 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 ErxY1 − xNi2B2C and Er0.2Lu0.8Ni2B2C 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.