- Natalia Rinaldi-Montes: Breakdown of the antiferromagnetic order in transition metal oxide nanoparticles: a matter of size
- 20. 4. 2016, 14:50
- lecture room F2, first floor Ke Karlovu 5
- more information
Abstract:
The possibility of tuning the magnetic behaviour of nanostructured 3d transition metal oxides has opened new avenues for their prospective application in fields ranging from energy storage and high-density magnetic recording to biomedicine. In this work we report on how the antiferromagnetism of a bulk material can be broken when reducing its size below a given threshold [1]. As antiferromagnetic (AF) systems need a high degree of symmetry in order to maintain a balanced magnetic structure, the magnetic frustration arising from size reduction is especially pronounced. We have combined the use of neutron diffraction, x-ray absorption spectroscopy and magnetic measurements with the aim of studying the influence of nanoparticle (NP) diameter reduction on the microstructure and magnetism of NPs composed of materials that are AF in their bulk forms (NiO, Cr2O3, CuO, MnO).
Our findings reveal that size effects such as average undercoordination, bond relaxation and static disorder, induce surface spin frustration, which competes with the expected antiferromagnetism, giving rise to a threshold diameter for the nucleation of the AF phase [1]. Hence, the magnetic microstructure of the NPs changes from a core(AF)/shell(spin-glass) morphology to a completely disordered magnetic state below a critical NP diameter. We have also explored the link between the NP morphology and the emergence of the exchange bias effect, which arises as a consequence of the exchange interaction at the interface between two magnetic phases with different anisotropy energies [2]. Finally, the knowledge acquired for low dimensional antiferromagnets has been used in order to shed light on the undermined exchange bias performance of core(transition metal)/shell(native transition metal oxide) NPs [3-4]. This type of interfaces plays a crucial role as stabilizing elements in spin valves and tunnelling devices.
References:
[1] N. Rinaldi-Montes et al., Nanoscale 6, 457 (2014)
[2] N. Rinaldi-Montes et al., Nanotechnology 26, 305705 (2015)
[3] N. Rinaldi-Montes et al., J. Mater. Chem C 3, 5674 (2015)
[4] N. Rinaldi-Montes et al., J. Magn. Magn. Mater. 400, 236 (2016)