JRP CALL information
Supported By

European Commission

Short description of the work
Exchange interaction through interface between a ferromagnet and an antiferromagnet may result in the magnetic exchange bias (EB) effect [1]. The EB manifests itself as a shift of the magnetic hysteresis loops along the field direction when the sample is field cooled below the Néel temperature (TN) of the antiferromagnet. This property has become of great technological value for applications in magnetic sensors based on spin-valves or tunnel junctions [2]. In this project, we prepared several types of samples.

The first type of samples included bilayers consisting of an antiferromagnetic UO2 (bulk TN is 30.8 K) with a ferrimagnetic Fe3O4. The thickness of the UO2 layer was varied from 40 to 300 A and the thickness of magnetite has been kept constant (~150 A). The samples were prepared on an Omicron machine at ITU by dc sputtering from metallic targets using CaF2 (CaF2 has an identical crystal structure and a lattice parameter within 0.5% of UO2 and UO2 is known to grow epitaxially on CaF2 substrates [3]). The UO2 layer has been deposited on the substrate at 300 C and a partial oxygen pressure 1.2x10-6 mbar while Fe3O4 was deposited on the top of UO2 at room temperature (and the same partial O2 pressure) to avoid interdiffusion. All layers have been checked by XPS to ensure the desired stoichiometry of magnetic layers. The samples have been capped with a protective Mg layer to prevent oxidation. The magnetization study of one of the representative samples performed using MPMS3 VSM magnetometer (Quantum Design, USA) showed that the sample UO2(approximate thickness ~130A from the deposition rate)/Fe3O4(approximate thickness ~150A from the deposition rate) field cooled in a magnetic field of 1 T shows exchange bias ~850 Oe. Complete analysis of the EB effect in UO2/Fe3O4 thin films with a varied thickness of the UO2 layer will allow us to deduce the magnetic anisotropy constant in UO2 films.
Further, several UO2/Fe3O4 bilayers have been grown on fused silica substrates. In this case, both UO2 and magnetite are polycrystalline. The comparison of the EB effect in epitaxial and polycrystalline films will be carried out in order to reveal the influence of microstructure on EB in UO2/magnetite thin films.
In addition to the main part of the project - the use of hybrid structures based on UO2, we succeeded in preparing a set of bilayers based on another antifferomagnet, uranium nitride. This part of work was carried out simultaneously with the preparation of UO2/Fe3O4 bilayers using another sputtering machine implemented in 2015 at ITU.
The second set of samples included metallic layers.
Here we used an antifferomagnetic UN (bulk TN is 54 K) and Ni as the ferromagnetic counterpart of the bilayers. The UN layers have been deposited on various substrates and had different thicknesses (200-800 A for UN and 100-300 A for Ni). The use of Al2O3 (2 different cuts), CaF2, MgO commercial substrates provides various microstructure, which may influence the formation of magnetic moments and magnetic ordering in UN. The XPS study confirmed the formation of UN at the use of partial N2 pressure 2x10-5 mbar. Exchange bias effect in these materials will be studied and compared with the effects in oxide layers.

[1] W. H. Meiklejohn and C. P. Bean, Phys. Rev. B, 102 1413, (1956).
[2] B. Dieny et al., Phys. Rev. B, 43 1297 (1991).
[3] E. A. Tereshina et al., Thin Solid Films, 591 271 (2015).

Main visitor contact data
Name: Dr. Evgeniya Tereshina
Organisation: Institute of Physics Academy of Sciences of the Czech Republic

JRP Identification
JRP nr: TALI-C06-12
JRP title: Magnetic anisotropy of hybrid nanostructures based on uranium dioxide
JRP scope: Scope 3: Actinide materials

Visited Associated Pooled Facility
Visited APF during the stay: JRC-ITU - Labs & Hot-cells
Name of the APF Contact Person: Thomas Gouder

Other APF and organisation involved in the JRP
Other organisations involved:
Other APF involved in the project:

Description of the work done at the associated pooled facility
Start date of the stay: 11/9/2015
End date of the stay: 12/11/2015
Quantity of access: 25
Access Unit: Days