Magnetic phase transitions in the ternary carbides Ln 2Cr 2C 3 (Ln=Tb, Ho, Er)

Abstract

The magnetic structures of the monoclinic carbides Tb 2Cr 2C 3, Ho 2Cr 2C 3 and Er 2Cr 2C 3 (space group C2/m) have been investigated by neutron powder diffraction. Below the Néel temperatures T N=49(1), 14.0(5) and 6.8(5) K the magnetic moments of the terbium, holmium and erbium atoms order antiferromagnetically within the monoclinic ac-plane, respectively. In the case of Tb 2Cr 2C 3 the magnetic structure can be described with the propagation vector k=(0 1 0) and it is stable up to the Néel temperature. The magnetic ordering in Ho 2Cr 2C 3 and Er 2Cr 2C 3 is more complex requiring different sets of coexisting wave vectors pertaining to distinct domains and comprising incommensurate regions and lock-in phase transitions. For the erbium compound the low-temperature ordering is described by the coexisting wave vectors k 1=(0 0 1 2 ) and k 2=(0 1 0) and both are stable up to T t=5.5(3) K. Between 4.5(3) K and T N=6.8(5) K a sine-wave modulated structure with k 3=(τ x,0, 1 2 −τ z) appears, where the vector components vary with the temperature. At 5.3 K one obtains τ x =0.055(1) and τ z =0.070(1). For Ho 2Cr 2C 3 the ordering associated with k 1=(0 1 1 2 ) is stable between 1.6 K and T t=12.7(3) K. A second type of ordering appears below T N=14.0(5) K and corresponds to a sine-wave modulated structure with k 2=(τ x,0,–τ z) . The length of k 2 varies from (0.8856, 0, −0.1395) at 13 K to (0.9935, 0, −0.1075) at T t=10.2(2), where the modulated structure undergoes a transition to a square-wave one manifested by the presence of higher harmonics.