Dipole–dipole interactions as the source of spin-glass behaviour in exchangewise two-dimensional ferromagnetic layer compounds

We have studied magnetic layered materials that have in-plane ferromagnetic exchange interactions, and dipole–dipole only interplane interactions. These are biotite mica (including a nearly ideal annite end member) and new graphite bi-intercalation compounds that contain regularly stacked arrays of ferromagnetic and diamagnetic intercalates. All these strictly (exchangewise) two-dimensional materials exhibit hysteretic spin-glass magnetization cusps at temperatures up to 42 K. This proves that, in the presence of ferromagnetic correlations, classical dipole–dipole forces can play an important role in causing spin-glass behaviour at elevated temperatures. The layered materials described are ideal model systems for the study of a class of spin glasses that require only ferromagnetic exchange interactions. Below some characteristic temperature TFC, the latter interactions induce ferromagnetic correlations that give rise to significant dipole–dipole coupling leading to "domain configuration trapping" and an onset of temperature hysteresis at T ~ TFC.