Mes: noviembre 2020

Novel Aurivillius Bi4Ti32xNbxFexO12 phases with increasing magnetic-cation fraction until percolation: a novel approach for room temperature multiferroism

Miguel Alguero,  Miguel Perez-Cerdan, Rafael P. del Real, Jesus Ricote and Alicia Castro

J. Mater. Chem. C, 2020, 8, 12457

DOI: 10.1039/d0tc03210grsc.li/materials-c

 

Aurivillius oxides with general formula (Bi2O2 (Am1BmO3m+1) are being extensively investigated for room-temperature multiferroism and magnetoelectric coupling. The chemical design strategy behind current investigations is the incorporation of magnetically active BiMO3 units (M: Fe3+, Mn3+, Co3+. . .) to the pseudoperovskite layer of known ferroelectrics like Bi4Ti3O12, increasing m. The percolation of magnetic cations at the B-site sublattice is required for magnetic ordering and thus, phases with m Z 5 are searched. Alternatively, one can try to directly substitute magnetic species for Ti4+ in the perovskite slab, without introducing additional oxygen octahedra. We report here the mechanosynthesis of Aurivillius Bi4Ti32xNbxFexO12 phases with increasing x values up to 1. A maximum magnetic fraction of 1/3, surpassing the threshold for percolation, was reached. Preliminary structural analysis indicated a continuous solid solution, though hints of structural changes between x = 0.25 and 0.5 were found.
Ceramic processing was accomplished by spark plasma sintering of the mechanosynthesized phases, including those with high-x ones with reduced thermal stability. This has enabled us to carry out full electrical characterization and to demonstrate ferroelectricity for all phases up to x = 1. Magnetic measurements were also carried out, and weak ferromagnetism was found for x = 1. Therefore, Bi4TiNbFeO12 is proposed to be a novel room-temperature multiferroic.

“Continuous and nanopatterned TbCo based heterostructures with in-plane and perpendicular anisotropy”

Nikita A. Kulesh, PhD by the Autonomous University of Madrid (Sobresaliente cum laude), supervised by Manuel Vázquez and Vladimir O. Vaskovskii

This thesis focuses on investigation of magnetic properties and magnetization reversal processes in continuous and antidot patterned TbCo ferrimagnetic amorphous films with perpendicular magnetic anisotropy (PMA). The search for new ways of tailoring magnetic anisotropy and hysteresis properties of magnetic films with PMA by adjusting their shape at the nanoscale level is the main objective.

In the first part, magnetic properties of amorphous TbCo films are investigated using auxiliary systems with simpler magnetic structures LaCo and GdCo. A possibility to produce TbCo layers having the same composition but different character of magnetic anisotropy is demonstrated. TbCo layers with PMA or in-plane magnetic anisotropy were used to induce unidirectional anisotropy in adjoining FeNi layers.

In the last part, micromagnetic simulation is employed to analyze and reproduce experimental results obtained for TbCo and GdCo antidot patterned films. An approach of step-by-step complication of micromagnetic model is used so separate and analyze sources of experimentally observed variations in magnetization processes.

This PhD research study derives from the collaboration with the Department of Magnetism and Magnetic Nanomaterials, Urals Federal University in Ekaterinburg.

“Enhanced in-plane magnetic anisotropy in thermally treated arrays of Co-Pt nanowires”

Fernando Meneses, Cristina Bran, Manuel Vázquez and Paula G. Bercoff

Materials Science and Engineering B 261 (2020) 114669

DOI: doi.org/10.1016/j.mseb.2020.114669

Ordered arrays of Co_xPt_100-x cylindrical nanowires (NWs) with x = 90, 80, 70 were synthesized by template-assisted electrodeposition using nanoporous alumina membranes of 55 nm pore diameter. The obtained NWs alloys crystallize in hcp and/or fcc structures, depending on the composition and thermal treatment, resulting in different magnetic behaviors. The magnetic anisotropy was studied as a function of composition and crystalline structure in as-deposited and annealed samples. An enhanced coercivity was obtained for the Co₇₀Pt₃₀ NWs array, in which in-plane anisotropy (e.g., perpendicular to the NWs axis) was found. These characteristics have not been reported for NWs of this kind. A simplified model of effective anisotropy including shape, magnetostatic interaction and magnetocrystalline contributions is presented, which appropriately describes the magnetic behavior of the Co_xPt_100-x NWs arrays before and after annealing.

This work has been performed in collaboration with the National University of Cordoba, Argentina within the framework of the i-COOP B 20037 project supported by CSIC.