Re-entrant magneto-elastic transition in HoFe4Ge2 a neutron diffraction study

Abstract The re-entrant magneto-elastic transition of the antiferromagnetic HoFe 4 Ge 2 compound has been studied by neutron powder diffraction as a function of temperature. The magnetic phase diagram comprises the wave vectors: ( q 1o , q 2o , q 1t ) and three magnetic transitions, two of them occurring simultaneously with the structural changes at T c , T N =52 and T c′ , T i c1 =15 K, the third being purely magnetic at T i c2 =40 K. The first transition is of second order while the latter two of first order . The sequence of phases follows the path: P4 2 /mnm (HT), T c , T N =52 K→ Cmmm (IT): ( q 1o =(0,1/2,0) , T i c 2 =40 K ⇒ q 2o =(0,q y ,0)) , T c′ , T i c 1 =15 K ⇒ P4 2 /mnm (LT): q 1t =(0,1/2,0) . The magnetic structures described by the wave vectors ( q 1o , q 2o and q 1t ), where the components are referred to the reciprocal basis of the conventional Cmmm cell, correspond to canted multi-axial arrangements. The q 2o wave vector length of the amplitude modulated phase varies non-monotonously, decreasing fast just below T i c2 ,—slowly between 36 K— T c′ , T i c1 and jumping to the q 1t =(0,1/2,0) lock-in value at T c′ , T i c1 simultaneously with the first order re-entrant transition to the (LT) tetragonal phase. In the coexisting meta-stable orthorhombic phase from T c′ , T i c1 down to 1.5 K the length of the wave vector q 2o continues to decrease. To solve the magnetic structures of all the phases appearing in this complex situation, arising from competing ordering mechanisms and anisotropies of the underlying sublattices, we have used the simulated annealing method of global optimisation on high-resolution neutron powder diffraction data.