Unconventional critical magnetic behavior in the Griffiths ferromagnet La0.4Ca0.6MnO2.8□0.2 oxide

The effects of oxygen vacancy on the critical magnetic behavior in La0.4Ca0.6MnO2.8□0.2 around the paramagnetic-ferromagnetic (PM-FM) phase transition were investigated through various techniques such as modified Arrott plot, Kouvel-Fisher method and critical isotherm analysis via dc magnetization measurements recorded around the Curie temperature TC. The obtained critical exponents values are β∼0.8, γ∼0.7 and δ∼1.882 with TC∼164.5 K. Thus the scaling law γ+β=δβ   is fulfilled. The critical exponents obey the single scaling-equation of state M(H,ε)|ε|−β=f±(H|ε|−(β+γ))M(H,ε)|ε|−β=f±(H|ε|−(β+γ)) where, f+ for T>TC and f- for T>TC. The found exponents are inconsistent with any known universality class. These results attributed to the existence of Griffiths Phase (Triki et al. (2012) [1]) seem to actually reflect the unconventional critical scaling of the magnetic susceptibility.

Modified Arrott plots: M1/β vs. (μ0H)1/γ with (a) mean-field model (β=0.5, γ=1), (b) 3D-Heisenberg model (β=0.365, γ=1.336), (c) 3D-Ising model (β=0.325, γ=1.24), (d) tricritical mean-field model (β=0.25, γ=1) and (e) (β=0.79, γ=0.71).Figure optionsDownload as PowerPoint slideHighlights
► Study of the critical behavior for La0.4Ca0.6MnO2.8□0.2 compound.
► A typical second-order magnetic transition near TC.
► Unconventional critical exponents were found.