A study of cluster spin-glass behaviour at the critical composition [formula omitted]NiGe

Abstract

The solid solution Mn1-xFexNiGe with x~0.27 has been investigated via powder ray diffraction, dc magnetization, ac susceptibility, heat capacity, and magnetic relaxation measurements. The alloy Mn0.73Fe0.27NiGe crystallizes in a Ni2In-type hexagonal structure at room temperature. A coexistence of hexagonal and orthorhombic phases with a phase fraction of nearly 88:12 was found below 80 K. The dc susceptibility measurements reveal a paramagnetic to ferromagnetic transition at around Tt≃80 K and a thermal hysteresis observed during field-cooled-cooling and field-cooled-warming suggests the first order nature of the magnetic transition. The splitting between zero-field-cooled and field-cooled dc susceptibilities and the appearance of a frequency dependent peak in ac susceptibility indicate the onset of a spin-glass transition at Tg≃58 K. The relative shift in freezing temperature (δTf) is calculated to be ~0.03 and ~0.09, respectively from the real and imaginary parts of ac susceptibility indicating the formation of cluster spin-glass state. The analysis of frequency dependent Tf using power law yields a characteristic time for a single spin flip τ*≃1.5×10-10 s and critical exponent zν′≃5.5. Similarly, the analysis using the Vogel-Fulcher law results a characteristic time for a single spin flip τ0≃1.2×10-8 s, Vogel-Fulcher temperature T0≃54.8 K, and an activation energy Ea/kB≃72.8 K. The magnitude of τ* and τ0 together with a non-zero value of T0 add further evidence for the formation of cluster spin-glass. The magnetic relaxation and memory effect measurements also demonstrate the low temperature cluster spin-glass behaviour. The reason for the cluster spin-glass behaviour could be the difference in local environment of Mn atoms in the coexisting Mn-rich antiferromagnetic and Fe-rich ferromagnetic phases. Furthermore, Mn0.73Fe0.27NiGe shows the asymmetric response of magnetic relaxation with a change in temperature, belowTf, which can be explained by the hierarchical model. The low temperature heat capacity gives a large electronic coefficient γ≃25 mJ/mol K2 and the Debye temperature θD≃300 K. This value of θD is in close agreement with that calculated from the temperature variation of unit cell volume.