Date of Award

2021-08-01

Degree Name

Master of Science

Department

Physics

Advisor(s)

Harikrishnan H. Nair

Abstract

The coexistence of two order parameters is a particular occurrence in bulk single-phase materials. Such materials possessing (anti)ferromagnetism, ferroelectricity, and ferroelasticity are known as multiferroics. In this work we revisit BeCr2O4, which is one of the oldest material to be studied in this context. Cr3+ occupies octahedral 4a site and Be2+ occupies tetrahedral 4c site in this compound, forming a close packing structure with a 90° and 138° Cr-O-Cr bonds allowing magnetic superexchange interactions. In the present work, BeCr2O4 powder was prepared using solid-state reaction method and the crystal structure was studied in detail using laboratory and synchrotron X-ray diffraction. Rietveld refinement of the powder X-ray data was carried out using Pnma orthorhombic space group yielding lattice parameters, a = 9.4019(9), b = 5.4746(3), c = 4.4259(2). We discover new magnetic phase transitions in this compound at low temperature. The temperature dependent magnetic susceptibility revealed two close-by anomalies at TN1 = 7.56 K and TN2 = 25.07 K, along with a third transition around TN3 = 26 K. These magnetic transitions are observed in specifc heat also. Using the Curie-Weiss fit in the paramagnetic region, an efective magnetic moment is estimated to be 3.48(7) μB/Cr3+ and the Curie-Weiss temperature is estimated to be -40.21(16) K indicating antiferromagnetic interactions. We present microscopic details of multiple magnetic orders which are previously unreported, using magnetometry, specific heat, synchrotron X-ray diffraction, and neutron diffraction data. Lattice anomalies below TN were observed in neutron diffraction data collected in the 1.8 K - 200 K temperature range suggesting magnetostructural coupling. Combining the observed magnetic transitions, an H - T phase diagram outlining the paramagnetic and antiferromagnetic regions was constructed. Preliminary analysis of the magnetic structure of BeCr2O4 using the 10 K-data suggests an incommensurate k-vector (0,0.087,0).

Language

en

Provenance

Received from ProQuest

File Size

84 p.

File Format

application/pdf

Rights Holder

Hector Cein Mandujano

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