Composition dependence of the anomalous Hall effect in Cax Sr1-x Ru O3 films

A series of 12 epitaxial films was grown across the entire range of the solid solution Cax Sr1-x Ru O3 in order to systematically examine the behavior of the anomalous Hall effect in this system. While all samples in this series are known from bulk studies to behave as Curie-Weiss paramagnets at high temperatures, samples with less than 70% Ca also order ferromagnetically with a maximal Curie temperature (TC) of ∼160 K for pure SrRu O3. Temperature (T) and magnetic field (H) dependent transport measurements were used in tandem with mean field simulations of sample magnetization (M) to track the composition and temperature dependence of the ordinary (Ro) and anomalous (Rs) parts of the Hall resistivity (ρH) in this system using the standard relation ρH = Ro B+ Rs 4πM. In the high-temperature Curie-Weiss paramagnetic regime, the only temperature dependence of the Hall resistivity comes from the anomalous portion, allowing the ordinary and anomalous contributions to ρH to be estimated via Curie-Weiss type fits. Rs was observed to be positive and nearly T independent at high temperatures (>200 K) and smoothly increased with increasing Ca content in this regime. In all ferromagnetic samples, Rs decreased significantly on cooling below TC in response to magnetic ordering, actually changing sign for samples with ≤20% Ca. This behavior is consistent with a two-component behavior of Rs, with the two different regimes (above TC and below TC) resulting from substantial changes in the band structure of this itinerant ferromagnet on crossing TC. The symmetric behavior of the anomalous Hall effect around the ferromagnetic→paramagnetic quantum phase transition is perhaps an indicator of hidden magnetic order in CaRu O3.