Competing ground states in transition metal oxides: Behavior of itinerant Sr_1−xCa_xRuO₃ close to the classical and quantum critical ferromagnetic phase transition

The ferromagnetic (FM) phase transition of the itinerant electron-system Sr_1−xCa_xRuO₃ can be tuned by chemical composition resulting in a quantum critical point (QCP) at the critical concentration x_c≈ 0.7. Applying epitaxial pressure at constant x leads to a reduction of the Curie temperature T_c which is found to be proportional to the shrinkage of the unit-cell volume V_uc, shifting x_c to higher values for tensile strained films. Surprisingly, the tetragonal distortion seems to play here only a minor role. With increasing x the critical scaling of the order parameter shows unusual behavior. The magnetic critical exponents β, γ, and δ change systematically from typical mean-field values at x = 0 with increasing x towards β = 1, γ = 0.9 and δ = 1.6 at x = 0.7. The results are discussed with respect to a crossover from mean-field-like behavior at x = 0 to a line of fixed points that might emerge in the strong-disorder limit as the system approaches the QCP at or near x_c. Magnetic inhomogeneities are indeed suggested by a nonvanishing magnetic moment at x_c and the evidence of a Griffiths phase as well as glass-like behavior close to x_c. Although spin fluctuations certainly play an important role around x_c as proposed previously, our highly accurate data of the magnetization M(T,B) and specific heat C(T,B) for x = 0.7 suggest dynamic scaling with an unusual dynamic exponent z = 1.8, incompatible with standard spin-fluctuation theories at a ferromagnetic QCP.