Pressure-induced onset of long-range magnetic order in two-dimensional spin-frustrated CuFeO₂

Magnetic properties of two-dimensional spin-frustrated CuFeO₂ have been studied up to 19 GPa by means of 57Fe Mössbauer spectroscopy. The partially disordered spin arrangement at ambient pressure in the 11-16 K range, transforms with pressure to a long-range ordered “5-sublattice” phase with a distinct T_N, a similar role played by external magnetic field in neutron studies. This phase gradually substitutes for the “4-sublattice” magnetic ground state present at ambient pressure, reaching 100% at 19 GPa. Despite the presence of long-range order, this high pressure phase exhibits magnetic relaxation above 20 K attributed to the notwithstanding weak interplanar superexchange interaction. The dramatic twofold increase of T_N at 19 GPa is explained in terms of the unusual increase of the intraplanar direct exchange J_∥ caused by the anomalous anisotropic compression of CuFeO₂ in which c/a increases with pressure.