Critical phenomena near structural phase transitions studied by EPR

Abstract

Critical phenomena observed by electron paramagnetic resonance (EPR) near the phase transitions of SrTiO₃ and LaA1O₃ are reviewed. A short description of the second-order antiferrodistortive transitions in these compounds is first given. Then high accuracy EPR Fe³⁺ measurements of the temperature dependence of the (rotational) order parameter φ are described. These show the existence of a critical region near Tc in both compounds which scale and yield the static critical exponents β and γ. A survey of the EPR linewidfh broadening of the Fe^{3 +}-V_O, centre in SrTiO₃ due to the critical slowing down of local order parameter fluctuations follows. The frequency width Δ ω(T) of the local spectral density narrows strongly on approaching T_c . Close to Tc a quasi-static regime is created. This allows a determination of the probability distribution P(φl) from the EPR lineshapes. P(φl) deviates from a Gaussian closely above and below Tc.