Unusual zero field cooled exchange bias and related mechanism in YBaCuFeO5-Ni0.3Zn0.7Fe2O4 composites

We report an unusual room temperature giant zero-field-cooled exchange bias (∼1 kOe) in an antiferromagnetic (100 - x)YBaCuFeO5-ferrimagnetic (x) Ni0.3Zn0.7Fe2O4 composite. The solid state route is adopted to prepare these dilute weight% (x = 1, 3, and 5) ferrite based composites. The incorporation of ferrite phase improves the interfacial tensile strain and grain boundary volume fraction in the said composite. The incommensurate to commensurate magnetic transition (TN2) of YBaCuFeO5 is shifted to a high temperature by 10 K with the lowest concentration and indistinguishable in higher concentrations due to the magnetic dominance of ferrites. The irreversibility of magnetization due to the field history mostly stems from the uncompensated spins and a competitive interaction among the magnetic phases at the interface. The magnetic isotherms show unusual negative exchange bias phenomena in the said system, and a large room temperature spontaneous exchange bias (∼1 kOe) is achieved with a dilute incorporation x = 5. The exchange bias field and coercive fields (HC) are, however, contrary to each other with temperature (and concentration) explained with a schematic model on the basis of dominating irreversible spins at high temperatures. In a field of ±50 kOe, the exchange bias field is dropped, while HC is increased, which might be due to the dominance of Zeeman energy over the uniaxial anisotropy. The non-collinear magnetic phase transition of YBaCuFeO5 at T ≤ 175 K plays a pivotal role in reducing the exchange bias compared to its collinear phase (300 K). Moreover, the extent of this bias field (∼1 kOe) can be considered a useful component in efficient device fabrication.