Exploring the Origin of Exchange Bias in Fe3O4 Films and Its Correlation with Film Thickness and Cooling Field

The study of exchange bias in various types of systems received great attention due to widespread applications in numerous magnetoelectronic devices and, in particular, understanding and controlling exchange bias in single layer films becomes crucial in advancing the spintronics field. With this connection, we here present a study on the origin of the exchange bias in the Fe₃O₄ (t nm) films with a polycrystalline structure fabricated via magnetron reactive sputtering directly onto a Si(100) substrate at ambient conditions and its correlation with thickness of the films and cooling fields. Structural and morphological studies reveal single phase polycrystalline Fe₃O₄ films and the average roughness increases with increasing thickness of the films. Magnetic properties show typical ferromagnetic behavior in Fe₃O₄ film, but interestingly, a considerable exchange bias (EB) of 550 Oe is noticed in 30 nm thick Fe₃O₄ film at 10 K when subjected to field cool. In addition, Fe₃O₄ (30 nm) film exhibits a spontaneous exchange bias without any external applied field. These observations can be understood from the magnetic exchange coupling at the interfaces between the ferromagnetic Fe₃O₄ and antiferromagnetic FeO, and the existence of a FeO can be validated from the magnetic anomalies in thermomagnetization data. Furthermore, the magnetic anomalies fade out at higher film thicknesses due to reduced (increased) volume of FeO (Fe₃O₄), causing suppression in the EB effect. The variation of EB with cooling field and film thickness demonstrates the tunability of EB in single layer Fe₃O₄ film and its suitability for possible applications in spintronics.