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 Tc=220.5 K, followed at lower temperature (Tg=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 ≈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 KT180 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).