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

Abstract

¹⁶⁶Er Mössbauer absorption spectra have been recorded in the superconductor ErNi₂B₂C (T _c = 10 K), in zero field and in the temperature range 1.4 K–40 K. The spectra in the magnetically ordered phase (T _N ≃ 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 T _N suggests that the Er³⁺ ground doublet is close to |J = 15/2; J _z = ± 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 (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 _x Y_{1 − x} Ni₂B₂C 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.