Random magnetic anisotropy driven transitions in the layered perovskite LaSrCoO4

Attempts to unravel the nature of magnetic ordering in LaSrCoO4 (Co3+), a compound intermediate between antiferromagnetic (AFM) La2CoO4 (Co2+) and ferromagnetic (FM) Sr2CoO4 (Co4+), have met with limited success so far. In this paper, the results of a thorough investigation of dc magnetization and ac susceptibility in single-phase LaSrCoO4 provide clinching evidence for a thermodynamic paramagnetic (PM)-ferromagnetic (FM) phase transition at T-c = 220.5 K, followed at lower temperature (T-g = 7.7 K) by a transition to the cluster spin glass state (CSG). Analysis of the low-field Arrott plot isotherms, in the critical region near Tc , in terms of the Aharony-Pytte scaling equation of state clearly establishes that the PM-FM transition is basically driven by random magnetic anisotropy (RMA). For temperatures below similar to 30 K, large enough RMA destroys long-range FM order by breaking up the infinite FM network into FM clusters of finite size and leads to the formation of a CSG state at temperatures T <= 8 K by promoting freezing of finite FM clusters in random orientations. Increasing strength of the single-ion magnetocrystalline anisotropy (and hence RMA) with decreasing temperature is taken to reflect an increase in the number of low-spin Co3+ ions at the expense of that of high-spin Co3+ ions. At intermediate temperatures (30 K <= T <= 180 K), spin dynamics has contributions from the infinite FM network (fast relaxation governed by a single anisotropy energy barrier) and finite FM clusters (extremely slow stretched exponential relaxation due to hierarchical energy barriers).

Automated summary

The compound LaSrCoO4 displays a paramagnetic-ferromagnetic phase transition at 220.5 K, followed by a transition to a cluster spin glass state at 7.7 K. The transition is driven by random magnetic anisotropy, which leads to the formation of finite ferromagnetic clusters at low temperatures. The strength of the single-ion magnetocrystalline anisotropy increases with decreasing temperature.