The zero-field glassy ground state and field-induced ferromagnetic transition in (La0.4Pr0.6)1.2Sr1.8Mn2O7

We have investigated glassy magnetic freezing in (La0.4Pr0.6)1.2Sr1.8Mn2O7 single crystals together with the field-induced transition to a metastable ferromagnetic phase using ac magnetic susceptibility and heat capacity measurements. The magnetization measurements show evidence for the development of a zero-field glassy ground state below 45 K along with a hysteretic, field-induced change in susceptibility associated with the transition to the ferromagnetic phase above 5 T. The heat capacity develops a clear peak at higher temperatures with the application of large magnetic fields, consistent with the development of a ferromagnetic order, while at low temperatures the Sommerfeld coefficient is monotonically reduced by an applied field, suggesting suppression of spin fluctuations. The heat capacity shows hysteretic behaviour, accompanied by a sharp decrease at a critical field, when held at fixed temperature, which does not recover on reducing the field back to zero. These measurements suggest that the zero-field ground state for (La0.4Pr0.6)1.2Sr1.8Mn2O7 consists of frozen disordered spin clusters, which develop into a metastable ferromagnetic state in modest magnetic fields.