The peculiar intrinsic magnetism of non-magnetic glasses: interpretation in terms of a Cellular Model for the glassy MR-Structure

The nature of the medium-range (MR) structure of glasses and other amorphous solids is still a ma􀀼er of debate. On the other hand, the magnetism of ordinary insulating glasses devoid of magnetic species other than trace impurities has always been considered as made up of Larmor diamagnetism of the inner-shell electrons and the small Langevin paramagnetism of the outer-shell electrons of the atomic impurities. In this paper, evidence will be given that a novel type of paramagnetism is found in insulating glasses (notably in the multi-silicates) that is intrinsic to the glassy nature of the structure of these solids. This is revealed by the temperature, T, and magnetic field, H, dependence of the magnetisation M=M(T,H), once the Larmor and Langevin contributions are carefully subtracted out. The remaining, intrinsic part of the magnetisation displays a peculiar behaviour in all the samples that we have studied using a SQUID magnetometer. While the non-Larmor magnetisation of most solids made up of atomic-scale magnets typically saturates to a constant value at large H/T values, the glassy intrinsic part of M presents a broad peak at high H and small T that does not have a known theoretical explanation. Moreover, at moderate H, the intrinsic M presents some small unexplained oscillations in T that are to be found in all of the glassy materials studied. The explanation offered here is that the MR-structure of glasses should be thought of as made up of jam-packed solid-like cells, without crystallinity of the bulk, and liquid-like atomic species in the voids between the cells. A quantum-mechanical model that deals with effective quasi-particles tunnelling in a local three-welled potential describes rather well the atomic species adsorbed on the cell walls. The model can successfully reproduce the observed behaviour of the measured M(T,H) while the oscillations are thought to be associated with thermal rearrangements of elongated cells in the MR-structure. The size of the cells is typically in the tens of nm range for these glasses. The MR-structure advocated here has also been discovered in a glass specimen produced from melting the sol-gel paste of a barium aluminium silicate in a special graphite crucible. The resulting specimen shows, even to the naked eye, a distinct cellular MR-structure where the cells are indeed elongated, of mm size, and are separated by fluid-like regions.