Phase Transition from Antiferromagnetic Insulator to Ferromagnetic Metal in La_<2-x>Sr_xCoO_4 : Magnetization and NMR Studies

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Magnetic and zero-field nuclear magnetic resonance investigations have been made on the La_<2-x>Sr_xCoO_4 system (x=0-1.4), one of the isomorphous compounds of the superconducting cuprates. In the La-rich region (x≦0.5), the antiferromagnetic (AF)ordering persists and the internal magnetic field at 1.4 K remains almost constant (H_<int>(Co)≃200 kOe), though the Neel temperature shows an initial strong decrease with a small amount of Sr substitution. When x increases further (x≧0.6), the AF state suddenly transforms to a ferromagnetic state and the Curie temperature shows a maximum (T_c=220 K) at x=0.9. Spontaneous magnetizations at T=4.2 K are, however, non-zero (2.2-1.6μ_B) only in the range of x=0.7-0.9, and H_<int>(Co)strongly decreases from 190 kOe (x=0.6) to below 40 kOe (x=1. 1). ^<59>Co spin-lattice relaxation time T_1 for x=0.7 and 0.9 follows the relation T_1 T=const. , suggesting it is in a metallic state.
社団法人日本物理学会の論文
1993-04-15