The double nature of the ErNiSi2 magnetic structure neutron diffraction and magnetic measurements

The magnetic phase diagram of the compound ErNiSi2 of CeNiSi2 structure type (Cmcm, space group) has been studied by neutron diffraction and magnetic measurements. The paramagnetic neutron data confirmed the sample homogeneity. The analysis of the data collected in the magnetically ordered regime (TN=3.4 K) showed the existence of two distinct types of magnetic ordering associated with the wave vectors q1=(1/2, 0, 0) and q2=(qx, qy, qz) which are not related by symmetry. The q1 phase corresponds to a uniaxial antiferromagnetic arrangement of the Er moments along b associated with a 2a cell enlargement. The refined moment value at 1.5 K is μEr=7.6(1) μB. This ordering is stable only below T1=1.8 K. Above T1, a first-order transition accompanied by hysteresis effects, leads to the appearance of a second set of magnetic reflections. These reflections are associated with a wave vector in a general reciprocal lattice position q2=(qx, qy, qz) that has a temperature dependent length and orientation. At T1 the two phases coexist and are present in equal amounts. The wave vector components of the q2 phase at T1 are qx=0.1262(3), qy=0.022(2) and qz=0.2273(3) r.l.u. The structure corresponds to a sine wave modulated structure with an amplitude of 5.31 μB polarized along b. This structure dominates the high-temperature region, T>T1. Below T1 it is still present as a metastable phase but its relative amount decreases with decreasing temperature. The same holds for the q1 phase which exists as a metastable phase above T1. The coexistence region on heating spans the interval T1±0.45 K.