Abstract
The structural and magnetic properties of seven CeMn2Ge2-xSix compounds with x = 0.0–2.0 have been investigated in detail. Substitution of Ge with Si leads to a monotonic decrease of both a and c along with concomitant contraction of the unit cell volume and significant modifications of the magnetic states - a crossover from ferromagnetism at room temperature for Ge-rich compounds to antiferromagnetism for Si-rich compounds. The magnetic phase diagram has been constructed over the full range of CeMn2Ge2-xSix compositions and co-existence of ferromagnetism and antiferromagnetism has been observed in CeMn2Ge1.2Si0.8, CeMn2Ge1.0Si1.0 and CeMn2Ge0.8Si1.2 with novel insight provided by high resolution neutron and X-ray synchrotron radiation studies. CeMn2Ge2-xSix compounds (x = 0, 0.4 and 0.8) exhibit moderate isothermal magnetic entropy accompanied with a second-order phase transition around room temperature. Analysis of critical behaviour in the vicinity of TCinter for CeMn2Ge2 compound indicates behaviour consistent with three-dimensional Heisenberg model predictions.
Highlights
Due to the wide and interesting range of structural and magnetic phenomena - including magnetism, superconductivity, mixed valence, Kondo behaviour and heavy fermion - exhibited by layered RT2X2 rare earth compounds, (R is a rare earth, T is transition metal and X is Si or Ge) this series has attracted significant attention over the years (e.g.1–4)
Compared with the model reported by Fernandez-Baca et al.[18] using the (1 0 1-qz) vector, we found that the vector (0 0 qz; qz ~ 0.31) generates the (101)−/(101)+ and (103)−/(103)+ satellite peaks in their correct positions, and results in an excellent fit of the magnetic satellites overall
The total magnetic moment of CeMn2Ge2-xSix compounds at base temperature ~5 K is found to decrease with increasing Si content. This behaviour indicates that contraction of the unit cell leads to a reduction in the Mn moment value (e.g. μ Total ~ 3.16 μ ab (μ B) for x = 0.0 and μ Total ~ 2.02 μ B for x = 2.0) and agrees well with the tendency detected for both the LaMn2Ge2 and LaMn2Si2 systems where μ Total is found to decrease with decrease in the lattice parameter a51
Summary
Due to the wide and interesting range of structural and magnetic phenomena - including magnetism, superconductivity, mixed valence, Kondo behaviour and heavy fermion - exhibited by layered RT2X2 rare earth compounds, (R is a rare earth, T is transition metal and X is Si or Ge) this series has attracted significant attention over the years (e.g.1–4). The relatively simple body centred tetragonal ThCr2Si2-type structure offers scope for selection of the magnetic state via the strong dependence of Mn–Mn intraplanar and interplanar exchange interactions on daMn-Mn the Mn–Mn separation distance in the ab-plane. This information is expected to provide enhanced understanding of the correlation between magnetic properties and atomic distances.
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