Magnetic and transport properties of single-crystal Ca2RuO4: Relationship to superconducting Sr2RuO4

Ca2RuO4, which has the single-layer tetragonal K2NiF4 structure, shows nonmetallic behavior for T<300 K unlike its isostructural counterpart, Sr2RuO4, which is metallic for all T<1300 K and which undergoes a superconducting transition temperature below T = 1.35 K, possibly with p-wave spin pairing. Magnetization, electrical-resistivity, and heat-capacity data for single-crystal Ca2RuO4 are presented. An antiferromagnetic transition is identified at T-N=110 K in all samples studied. The easy axis for magnetization is parallel to the a or b axis (in the Ru-O plane). Isothermal magnetization studies in fields to 30 T show a spin reorientation transition for B=3.5 T at T=105 K and a metamagnetic transition for T much greater than T-N at about 9 T. The saturation magnetization even at 30 T, M-sat approximate to 0.4 mu B/Ru, is less than expected for the Ru S=1 moment (M(sat)approximate to 2.mu/Ru). The electrical resistivity, p(T), increases with decreasing temperature by eight orders of magnitude for 70<T<300 K and fits a variable-range hopping model including correlations. The magnetoresistivity below T-N shows some dependence on spin orientation. In contrast to all the other Sr and Ca-based ruthenates, Ca2RuO4 has a relatively small low-temperature electronic specific-heat coefficient (gamma=4 mJ/mole K-2), and unlike them, Ca2RuO4 shows no anomaly in either p(T) or dp(T)/dT at T-N. Some magnetization results are presented for lightly Sr-doped Ca2RuO4. The data are contrasted with those of metallic Sr2RuO4 and discussed in terms of the weak spin disorder scattering in Ca2RuO4 compared to the very strong coupling in the other Sr-and Ca-based ruthenates.