Superparamagnetic properties of La1−xBaxCoO3 (0.0<x<0.18)

The static and dynamic magnetic properties of La_1−xBa_xCoO₃ (0<x<0.18) (LBCO) have been investigated by the dc and ac susceptibility measurements systematically. The spin-glass-like characteristics of magnetic behavior, which are always considered as an evidence of spin glasses for La_1−xSr_xCoO₃ (0<x<0.18) (LSCO), have been observed. However, just like that of LSCO, some magnetic behaviors, which are different from the canonical spin-glass characteristics, for example, the difference far above peak temperature T_p of zero-field-cooled curve between field-cooled magnetization (FCM) and zero-FCM (ZFCM) and the very broad peaks in the samples for x>0.10, are also observed simultaneously. Based on the comparisons with canonical spin-glass Au₉₆Fe₄ alloy and conventional superparamagnetic (SPM) Cu₉₇Co₃ alloy, the nature of the magnetic behaviors in LBCO is ascribed to the superparamagnetic cluster rather than spin glass. Though the magnetic properties behave as SPM, LBCO has one critical point different from that of the conventional SPM Cu₉₇Co₃ alloy. The segregated Co nanoparticles in the Cu₉₇Co₃ alloy are isolated by nonmagnetic background while the clusters in LBCO are embedded in an antiferromagnetic matrix. The SPM behavior of LBCO can only exist at low temperature because the antiferromagnetic matrix transfers to paramagnetic phase at Neél temperature T_N and the SPM cluster turns into ferromagnetic (FM) above T_N.