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Spin-lattice coupling and structural transformations in frustrated magnets with spinel and MnWO4 structure


Author: Felea Viorel
Degree:doctor of physics and mathematics
Speciality: 01.04.10 - Semiconductors physics and engineering
Year:2012
Scientific adviser: Vladimir Ţurcan
doctor habilitat, associate professor (docent), Institute of Applied Physics of the ASM
Institution: Institute of Applied Physics of the ASM
Scientific council: DH 02-01.04.10-27.03.08
Institute of Applied Physics of the ASM

Status

The thesis was presented on the 10 February, 2012
Approved by NCAA on the 5 April, 2012

Abstract

Adobe PDF document1.55 Mb / in romanian

Keywords

magnetic semiconductors, spinel structure, spin-lattice interaction, ultrasound propagation, structural transitions, strong magnetic fields, bond frustrations, competing ferro-and antiferromagnetic exchange interactions, orbital ordering, multiferroic behavior

Summary

Structure of the thesis. The thesis consists of the introduction, five chapters, conclusions, 109 references, 113 pages of text, and 62 drawings. The obtained results were published in 11 scientific publications.

Field of investigation: physics of magnetic materials with strong electronic correlations.

Scope: Investigation of the spin-lattice interaction and elucidation of the mechanisms of the structural transitions induced by magnetic field and by the orbital ordering in AB2X4 compounds with spinel structure (A = Fe, Zn, Cd, B = Cr, X = S, Se) and MnWO4 utilizing ultrasound propagation in high magnetic fields.

Novelty and originality of the results. A scientific problem related to structural transitions in magnetic materials with strong frustration of exchange interactions was solved.

In FeCr2S4 crystals the anomalies in the ultrasound propagation were evidenced at the orbital-ordering tranzition induced by the static cooperativ Jahn-Teller effect, which takes place at 9 K, as well as at the spin-reorientation transition at 60 K.

In single crystals of ZnCr2Se4 an anomaly in the sound propagation is found at the transition into the antiferromagnetic phase with incommensurate spin structure.

In ZnCr2S4 antiferomagnet a sequence of magneto-structural transitions were revealed in high magnetic fields which indicates the appearance of new crystallographic structures with constant rigidity.

In MnWO4 multiferoic the anomalies in the ultrasound propagation were revealed at the three consecutive antiferromagnetic phase transitions at low temperatures, and one new phase was evidenced in high magnetic fields.

Theoretical importance: New experimental data related to spin-lattice and spin-orbital coupling in magnetic compounds with strong electronic correlations are important for deepening of the understanding of the physical phenomena in solids with magnetic ordering and for clarification of the mechanisms of the appearance of spontaneous dielectrical polarization.

Practical importance: The experimental results obtained for magnetic compounds with colossal magnetocapacitance and multiferroic behavior can be used for elaboration of the magnetoelectric tranducers based on the multiferroic phenomena, as well as for application in registration systems which use magnetic memory.