Exchange Anisotropy in Nanocasted Co₃O₄ Nanowires

Abstract

Antiferromagnetic Co3O4 nanowires with an average diameter of about 8 nm have been synthesized by using the nanocasting route. The nanowires were characterized by powder X-ray diffraction, transmission electron microscopy, and a superconducting quantum interference device magnetometer. The magnetic measurements show a unidirectional exchange anisotropy accompanied by an enhancement of the field-cooled coercivity at low temperature. These effects suggest the presence of an exchange interaction between the antiferromagnetic core and the surface spins. A strong dependence of the exchange field on the magnitude of the cooling field and a training effect have been observed. The dependence of exchange bias field on temperature reveals the important role played by the antiferromagnetic core in the exchange coupling.

The effects of size and orientation on magnetic properties and exchange bias in Co3O4 mesoporous nanowires

“…Figure 3 presents the Néel temperature (T N ), the coercive field (H C ), the exchange bias (H EB ) under 1 kOe and 50 kOe FC, and the average grain size (for easy comparison) versus the calcining temperature. From these results, we can conclude: (1) T N is consistent with previous reports [13][14][15] of an increase with increasing calcining Sample W350 presents obvious room temperature ferromagnetism, and the ferromagnetic transition is even obvious at high temperature [as shown in the inset of Fig. 2(d)].…”

Exchange bias in single-crystalline CuO nanowires

“…This behavior is similar to that found in some granular systems containing AFM and FM nanoparticles, 20,21 although it has not been reported for CuO nanoparticles 7 and differs from that observed in Co 3 O 4 nanowires, where H eb raised for increasing H cool . 9 A strong influence of H cool on the EB phenomenon may be expected due to the multivalley energy structure and multiple equivalent spin configurations of the SG-like phase. In the FC process, a preferred configuration is imposed upon the SG-like surface spins.…”

“…7,8 Only very recently, EB has been reported in certain 1D nanostructures. 9,10 Cupric oxide ͑CuO͒ is not only an AFM transition metal oxide ͑T N = 230 K͒ but also a narrow band gap semiconductor ͑E g = 1.3 eV at room temperature͒. Due to such dual semiconducting-magnetic properties, it is considered a relevant material for applications including gas sensors, catalysis, field emitters, electrochemical cells, and magnetic storage media.…”

“…The ZFC curve shows a clear maximum at T B ϳ19 K and an evident separation from the FC curve below T B , suggesting a SG-like behavior at low temperatures. 9,10,16 Such behavior can be attributed to uncompensated surface spins in the nanowires, which result frozen due to the applied field. In our case, T B is much lower than T N .…”

“…Similar models have been used in order to explain the magnetic properties of other oxide nanowires. 9,10 The exchange coupling present at the interface between the AFM core and the magnetic shell induces a unidirectional anisotropy in the latter layer. The strength of this anisotropy is measured by the exchange bias field ͑H eb ͒, defined as H eb = ͑H c1 +H c2 ͒ / 2, where H c1 and H c2 , respectively, represent the left and right coercive fields.…”