Mechanisms of exchange bias with multiferroic BiFeO3 epitaxial thin films (0710.2025v1)

Abstract: We have combined neutron scattering and piezoresponse force microscopy to study the relation between the exchange bias observed in CoFeB/BiFeO3 heterostructures and the multiferroic domain structure of the BiFeO3 films. We show that the exchange field scales with the inverse of the ferroelectric and antiferromagnetic domain size, as expected from Malozemoff's model of exchange bias extended to multiferroics. Accordingly, polarized neutron reflectometry reveals the presence of uncompensated spins in the BiFeO3 film at the interface with the CoFeB. In view of these results we discuss possible strategies to switch the magnetization of a ferromagnet by an electric field using BiFeO3.

• The paper demonstrates that the exchange field scales inversely with the ferroelectric and antiferromagnetic domain sizes in BiFeO₃ films.
• The paper employs neutron scattering and PFM to confirm G-type antiferromagnetic order and structural integrity of BiFeO₃ thin films.
• The paper reveals that uncompensated interfacial spins contribute to enhanced coercivity, offering potential for room-temperature spintronic applications.

Analysis of Exchange Bias Mechanisms in Multiferroic BiFeO$_3$ Epitaxial Thin Films

The paper investigates the mechanisms underlying exchange bias (EB) within CoFeB/BiFeO$_3$ heterostructures, focusing on the relationship between exchange bias and the multiferroic domain structure of BiFeO$_3$ (BFO) films. Utilizing neutron scattering and piezoresponse force microscopy (PFM), the paper identifies a dependency of the exchange field on the inverse of ferroelectric and antiferromagnetic domain sizes, drawing from Malozemoff's extension of exchange bias models to multiferroics. The examination is buttressed by polarized neutron reflectometry, which detects uncompensated spins at the BFO/CoFeB interface.

The researchers employ neutron diffraction to confirm that BFO maintains a G-type antiferromagnetic order in thin film form, despite the absence of the bulk cycloidal spin modulation. This finding correlates with a small antiferromagnetic domain size, which coupled with Malozemoff's adaptation, suggests that exchange bias scales inversely with domain size. Moreover, the EB effect and increased coercivity are attributed to the presence of uncompensated interfacial spins, the magnitude of which appears to influence the manipulation of magnetization.

Key Results and Observations:

1. Domain Size and Exchange Field Dependency: Empirical evidence supports a linear relationship between the exchange field and the inverse of the domain size, consistent with theoretical predictions derived from modified exchange bias models. This inversely proportional relationship posits a potential method for controlling magnetization in ferromagnetic films through domain engineering.
2. Structural Analysis with Neutron Diffraction: Neutron diffraction measurements delineate the presence of G-type antiferromagnetic order, while eliminating other possible antiferromagnetic configurations in the BFO films. This confirms the structural integrity and magnetic identity of BFO films akin to bulk materials, despite changes in strain and symmetry.
3. Uncompensated Spins and Spin Root Disorder: With polarized neutron reflectometry, the work elaborates on the presence of a magnetic moment within a thin BFO layer at the interface with CoFeB, indicative of uncompensated spins. These spins are implicated in the observed EB and coercivity enhancement, where the core reason could be random field effects and/or the structural disorder at the interface.
4. Implications for Spintronics and Device Applications: The paper suggests that electrical field manipulations of magnetic states in such heterostructures could facilitate innovative spintronic devices operating at room temperature, broadening practical applications in memory and logic devices.

The implications of these findings underscore a step towards the practical utilization of multiferroic materials, particularly BFO, in advanced electronic components. The potential for electrical control over magnetic properties at room temperature opens pathways for novel device architectures and positions BFO-based heterostructures as significant candidates in the field of spintronics.

Future Directions:

Future research may focus on refining techniques for precise domain size manipulation, exploring alternative multiferroic materials with similar or enhanced properties, and overcoming limitations related to interface quality and stability. Investigations into the intrinsic properties of these interfaces may render insights into optimizing performance and reducing energy costs in practical applications. Such advancements would significantly contribute to the field of materials science, particularly in the context of designing future-generation electronic devices.