Magnetic and structural properties of CoFe2O4 and NiFe2O4 thin films and heterostructures

Thin film heterostructures, combining layers of materials with different magnetic properties, have received high levels of attention as they often exhibit new behaviour not seen in the individual layers. In particular, heterostructures utilizing ferrites have shown modified magnetic properties, and have found applications in spin-filtering magnetic tunnel junctions and insulator spintronics. In this work, we investigate pulsed laser deposition grown CoFe2O4 (CFO) and NiFe2O4 (NFO) single films and heterostructures. NFO is of interest due to its large exchange splitting, its ability to form insulating or conducting phases depending on its growth conditions, and the possibility to control its magnetic anisotropy through strain. CFO, on the other hand, exhibits a large positive cubic magnetocrystalline anisotropy constant compared to other ferrites, high magnetostriction, and low conductivity. The difference in the magnetic anisotropy and electronic properties between NFO and CFO, but similar crystal structure, provides the motivation to combine them into a heterostructure. For this purpose, we have studied single films of CFO and NFO as well as bilayers of NFO/CFO and CFO/NFO grown on Al2O3 [001] with different thicknesses. Here we present how interfacial effects, such as exchange coupling and strain, affect the magnetic properties and structural properties of single films and heterostructures. Polarised neutron reflectivity (PNR), X-ray reflectivity (XRR), X-ray diffraction (XRD) and vibrational sample magnetometry (VSM) have been used to characterise samples’ magnetic and structural properties. Of particular importance, PNR was utilized to measure magnetic profiles as a function of depth across the chemically different layers. We first study [111] orientated CFO and NFO single films to investigate their growth on the strained ALO substrate and to have reference magnetic properties for later comparison to heterostructures. Both films showed a lower than bulk magnetisation, indicating anti-phase boundaries (APBs) defects play a large role in their magnetisation. A difference in chemical and magnetic profiles between NFO and CFO indicated a differing strength of effect from the ALO [001] interface on the density of APBs. We have then investigated bilayer samples of [111] orientated bilayers of CFO and NFO. Modified hysteresis loops and coercive field (Hc) values of the bilayers show evidence of exchange coupling. Certain magnetic properties of the bilayers depend on the layer thickness and growth order, i.e. CFO/NFO or NFO/CFO, on the ALO substrate. The measurements show an overall relative increased magnetization of NFO when grown on top of CFO, and depending on the layer order of the bilayer CFO and NFO show different Hc values. The changes in moment imply strain-related effects from the highly lattice mismatched substrate. The changes in Hc can be explained by a combination of strain-related effects, exchange coupling and layer thickness.