Structural chemistry and magnetic properties of Ln MnFeGe 4 O 12 ( Ln = Y, Eu, Lu)

Abstract

Polycrystalline samples of LnMnFeGe 4 O 12 (Ln = Y, Eu, Lu) have been prepared using the ceramic method and characterised by a combination of magnetometry, Mössbauer spectroscopy, X-ray diffraction and neutron diffraction. They all adopt the space group P4/nbm with a ~ 9.670, c ~ 4.81 Å and show long-range antiferromagnetic order with transition temperatures 15 ≤ T N /K ≤ 17. The magnetic structure is the same in each case and consists of an A-type ordering of (001) planes; the ordered spins lie in the (001) plane.Comparison with isostructural compounds leads to the conclusion that subtle structural changes play a greater role than the electronic configuration of the cation in determining the magnetic structure.

Magnetic properties of GdBB′Ge4O12; BB′ = FeZn or GdCa

“…The large QS can be attributed to the low, 2/m, point symmetry at the 4f site and the resulting irregularity of the FeO6 coordination polyhedron. We encountered a similar situation in our study of YFeMnGe4O12 [5]. We note that a minor (5%) ferric impurity is evident near the centre of the spectrum.…”

“…The adoption of an A-type structure suggests that the interactions between next-nearest-neighbour cations are stronger than those between nearest neighbours. We have previously proposed [5] that the relative size of the cations on the 2b and 4f sites determines the type of ordering adopted because it determines the tilt angle of the octahedra of oxide ions that surrounds the 4f site, and hence the bond angles along the superexchange pathway. In the case of GdFeZnGe4O12 the low value, 0.658, of the ratio <r4f>/r2b would be expected to lead to the observed k = [0, 0, ½] magnetic structure.…”

“…The distribution of the cations over the two sites is determined by their relative size and charge. Our own research has focussed on compositions in which one or more of the cations is magnetic and we have previously reported the occurrence of paramagnetism, spin-glass behaviour, antiferromagnetism and weak ferromagnetism within the structure type [3][4][5][6][7][8]. Paramagnetism is likely to result when only one site is occupied by a magnetic cation, as in Y2CoGe4O12 [3], but the behaviour of the more magnetically-concentrated compositions is difficult to predict because several intercation interactions are in competition with each other.…”