The magnetic properties of Co (<D>=15 nm, <L>=130nm) nanowires are reported. In oxidized wires, we measure large exchange bias fields of the order of 0.1 T below T ~ 100 K. The onset of the exchange bias, between the ferromagnetic core and the anti-ferromagnetic CoO shell, is accompanied by a coercivity drop of 0.2 T which leads to a minimum in coercivity at $\sim100$ K. Magnetization relaxation measurements show a temperature dependence of the magnetic viscosity S which is consistent with a volume distribution of the CoO grains at the surface. We propose that the superparamagnetic fluctuations of the anti-ferromagnetic CoO shell play a key role in the flipping of the nanowire magnetization and explain the coercivity drop. This is supported by micromagnetic simulations. This behavior is specific to the geometry of a 1D system which possesses a large shape anisotropy and was not previously observed in 0D (spheres) or 2D (thin films) systems which have a high degree of symmetry and low coercivities. This study underlines the importance of the AFM super-paramagnetic fluctuations in the exchange bias mechanism.
Synthesis, Optical, and Magnetic Properties of Ba$_2$Ni$_3$F$_{10}$ Nanowires
采用低温水热法，在优化的工艺条件下成功制备了纯相Ba $_2$ Ni $_3$ F $_{10}$纳米线。在325 nm激发下，Ba $_2$ Ni $_3$ F $_{10}$纳米线呈现出三个发射带，峰值分别位于360 nm、530 nm和700 nm。结合第一性原理计算，可以用电子在t2 g和eg轨道之间的跃迁来解释其光致发光性质。在低于60 K的温度下观察到的清晰的磁滞回线表明Ba $_2$ Ni $_3$ F $_{10}$纳米线具有弱的铁磁性，这归因于纳米线的表面应变。观察到阻挡温度为55 K的交换偏置，这是由于Ba 2Ni 3F 10纳米线的反铁磁核/弱铁磁壳结构在冷场下的磁化钉扎所致。
