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Polycrystalline ceramics of Co(Cr 1- x Fe x ) 2 O 4 (0 ≤  x  ≤ 0.12) were experimentally studied based on a series of temperature and time-dependent dc magnetic measurements using different magnetic field histories. Magnetization in field cooling process was continuously decreased for doping content x in the range of 0 ≤  x  ≤ 0.04. Remarkable negative magnetization is observed when x reaches to 0.06 and persists up to x  = 0.1. Two-sublattice model is established and competition of the two magnetic sublattices is responsible for the phenomenon. The magnetic switching effect is realized just by changing the magnitude of the applied magnetic field and double magnetocaloric effects are obtained. These unique features under low magnetic fields show attractive for application in spintronic devices due to that the magnetic state can effectively be tuned through magnetic field or temperature. Besides, the system exhibits both positive and negative exchange bias fields which are considered to be originating from the unidirectional anisotropy of exchange coupling of antiferromagnetic/ferromagnetic phases and spin reorientation of the two sublattices magnetic moments, respectively. [ABSTRACT FROM AUTHOR]

Abstract: We have systematically measured susceptibility, angle dependent magnetization and specific heat on single crystal. When magnetic field (H) is rotated within ab plane, a twofold symmetry in magnetization is observed at 50 and 100K under , while a jump in magnetization is observed at certain angle with the twofold symmetry at 10K. Specific heat shows two successive transitions at and , respectively. The first one at shows antiferromagnetic (AFM) nature, while the second one at shows ferromagnetic (FM) behavior. These behaviors can be interpreted by the recently reported magnetic structure, and confirm the magnetic structure through another way. [Copyright &y& Elsevier]

Abstract: We have systematically measured resistivity, susceptibility and specific heat under different magnetic fields (H) in single crystals. It is found that a metamagnetic transition from A-type antiferromagnetism to ferromagnetism occurs at a critical field for magnetic sublattice of ions. The jump of specific heat is suppressed and shifts to low temperature with increasing H up to the critical value, then shifts to high temperature with further increasing H. Such behavior supports the metamagnetic transition. Further, a twofold in-plane magnetization is found in antiferromagnetic state which is similar to spin density wave (SDW) order. In contrast, isotropic in-plane magnetization is presented in both ferromagnetic and paramagnetic states. Moreover, ferromagnetic state becomes more easily established under external field with suppressing SDW order. An anisotropic exchange model is proposed to understand our findings. Our results suggest that metamagnetism of ions is somewhat related to SDW order. Finally, detailed H–T phase diagrams for and 0.15 crystals are given, and possible magnetic structure is proposed. [Copyright &y& Elsevier]

Abstract: The AC susceptibility and magnetoresistance (MR) of (, Pr, –0.2) polycrystals were studied. The substitution of Nd or Pr for La results in a slight decrease of the Curie temperature . For all samples, the cluster-freezing behavior is observed in AC susceptibility measurement, and the freezing temperature shifts to higher temperature with increasing x. The resistivity and negative MR are significantly enhanced by both Pr and Nd doping and the effect of Nd substitution on them is stronger than that of Pr substitution. These results suggest that the spin-disorder scattering is enhanced by substitution of Nd and Pr for La. A rapid decrease in MR is observed at about 90K under 1T, while at 70K under 7T. This behavior could arise from the transition induced by magnetic field from high-spin or intermediate-spin state of trivalent Co ions to low-spin state. The transition temperature is nearly the same for both of Nd and Pr cases and independent on the dopants. [Copyright &y& Elsevier]

Abstract: La0.5Sr0.5Co1-xRuxO3 system has been studied by the measurements of XRD, magnetic, electrical transport, and magnetoresistance properties. Magnetic measurements indicate that mostCo3+ andCo4+ ions are stabilized in intermediate spin IS state, while partial Co ions are converted into HS state. The FM state resistivity behavior forx=0, 0.025, 0.05 can be fitted using the formulaρ(T)=ρ0+ρ1T9/2+ρ2T2, while the insulating behavior forx=0.1, 0.2, 0.3 obeys the VRH model. Electronic phase separation ferromagnetic-cluster model and the quantum interference effects are used to explain the MR behavior. Meanwhile, in this system, we find that the magnetic field has two competing contributions to the MR. One is the suppression of spin-disorder scattering, which result in a negative magnetoresistance. The others are both the spin-state transition ofCo3+ in clusters and quantum interference effects, which are enhanced by an applied field and produce a positive magnetoresistance. This result is supported by the low-temperature positive MR forx=0.025 and 0.05. The ferromagnetic-cluster model is also supported by the shift of the low-temperature peak inχ″(T), which is corresponding to the freezing of the ferromagnetic clusters, indicating the decrease of the size of clusters with increasing x. [Copyright &y& Elsevier]