Magnetic coupling in Mn3O4-coated γ-MnOOH nanowires

Mn3O4-coated g-MnOOH nanowires were synthesized by using the hydrothermal method. X-ray diffraction and transmission electron microscopy studies reveal that the nanowires have a core (g-MnOOH)-shell (Mn3O4) structure. The magnetic transition temperature of Mn3O4 is slightly lower than the previously reported value because of the increased thermal disturbance for nanomaterials and the influence of the helical magnetism of g-MnOOH. The hysteresis loop at 50 K keeps the same shape as was measured below the Néel temperature of Mn3O4 because of the short-range order of g-MnOOH above the transition temperature. The short-range ordering is responsible for the deviation of the hysteresis loop at 300 K from the linear behavior of a normal paramagnetic phase. The magnetic coupling behavior between Mn3O4 and g-MnOOH also induces an exchange bias effect in the system. The hysteresis loop shifts to the positive direction with increasing measurement temperature. A lower barrier energy is requested for the reversal of magnetic moments on the interface to generate the exchange bias behavior because of the asymmetric magnetic natures of Mn3O4 and g-MnOOH, which should possess shorter responsive time compared with the traditional antiferromagnetic-ferromagnetic coupling induced exchange bias systems