(111)-oriented CoFe2O4 and NiCo2O4 ultrathin films with tailored electronic and magnetic properties

• Applicant: Dr. Karsten Küpper
• Subject Area: Experimental Condensed Matter Physics ; Physical Chemistry of Solids and Surfaces, Material Characterisation
• Term: from 2017 to 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 392675618

During the first funding period of the project "Ferrimagnetic spinels with tunable electronic and magnetic properties", ultrathin, (001)-oriented ferrite and NiCo2O4 films were characterized in detail with respect to i) structural parameters (degree of inversion of the spinel, interface, morphology, antiphase boundaries), ii) valence states of the transition metals (cationic distribution), and iii) spin state and magnetic anisotropy. As already indicated, the anticipated second period will deal with ultrathin films of the same material which, however, have an orientation different from (001). Here, the (111)-orientation is highly interesting for both CoxFe3-xO4 (x<2) and NixCo3-xO4 (x<2) ultrathin films. For example, (111)-oriented CoFe2O4 ultrathin films can exhibit higher spin polarization and coercivity than (001)-oriented CoFe2O4 ultrathin films. High magnetoresistance and semiconducting behavior have been reported for (111)-oriented NiCo2O4 ultrathin films. These properties and their dedicated characterization are indispensable to use these materials for applications in, e.g., spin polarized tunnel currents, magnetic storage media or applications in the field of electrocatalysis or battery anodes. In order to study the complex interplay between double- and super-exchange mechanisms in dependence of the exact stoichiometry, the cation distribution and above mentioned structural parameters, we will produce (111)-oriented ultrathin films by means of reactive and oxygen plasma assisted MBE to maintain control of the growth down to the unit cell scale, resulting in films of excellent structural order. Characterization of the thin films will include structural analysis by means of LEED, XRR and GI(XRD), microstructural probes of the surface (SEM, AFM) and interfaces (TEM). Electronic structure and stoichiometry will be investigated by XPS and HAXPES experiments. Complementary (temperature dependent) measurements of the electrical resistivity measurements will be also carried out. The magnetic structure will be studied globally by means of MOKE, VSM/SQUID magnetometry, and magneto resistance measurements. For selected samples, synchrotron based approaches will by employed to gain element specific information of the chemical properties (HAXPES) and the magnetic properties (XMCD). The latter will be accompanied by XRMR for a few interesting samples. The experimental results will be further analyzed and compared with suitable theoretical approaches. This way we will develop a rather complete picture of the overall properties of the above mentioned highly interesting (stoichiometric and non-stoichiometric) systems with normal, mixed, or inverse spinel structure.

• DFG Programme: Research Grants