Crystal and spin structure and their relation to physical properties in some geometrical and spin spiral multiferroics

The aim of the present work has been to synthesize and to investigate crystal and spin structure in some geometrical and spin spiral multiferroics. Multiferroic materials exhibit two or more ferroic properties such as, ferroelectricity, ferromagnetism and ferroelasticity. These materials are considered as prime candidates for future computer data storage and spintronics. There are several classes of magnetoelectric multiferroics classified based on the origin of multiferroicity. The two types of multiferroic compounds investigated in the present dissertation are, geometrically frustrated systems including; hexagonal DyMnO3 (hDMO) and orthorhombic HoCrO3 (HCO) and spin spiral systems including wolframite type Mn0.9Co0.1WO4 (MCoW) and Mn0.9Cu0.1WO4 (MCuW). The samples were characterized by macroscopic techniques; specific heat and magnetization as well as microscopic techniques; x-ray diffraction and neutron scattering. Polycrystalline samples of HCO were prepared by solid-state reaction method and the phase purity is confirmed by x-ray diffraction measurements. From magnetization measurements we determined magnetic ordering temperature and also an indication of antiferromagnetic exchange interactions is found. The thermal properties of HCO can be very well described in terms of lattice, hyperfine and crystal field interaction contributions. From the specific heat data the contributions from lattice, hyperfine and crystal field interactions was determined. The hyperfine field at Ho site and the hyperfine splitting energy are deduced. Using a Schottky formula for multiple crystal field levels, five crystal field energies were computed. Magnetic ordering of both Cr and and Ho moments in HCO was determined by neutron powder diffraction (NPD) measurements. The thermal variation of Cr and Ho ordered moments was understood based on molecular field (MF) theory. NPD data also indicated the presence of spin fluctuations, which was confirmed by INS measurements. From INS data the correlation length of short range ordering was calculated using Selyakov-Scherrer formula. Thermal variation of lattice parameters shows anomalous changes around magnetic ordering temperature indicating the presence of magnetoelastic effect (ME) in HCO. Excessive change in unit cell volume (dV) due to ME was determined using Grüneisen approximation. The linear dependence of dV and Cr magnetic moments confirm the origin of ME effect. Single crystals of metastable hDMO were grown by optical floating zone technique. Magnetization and specific heat measurements indicated two high temperature and two low temperature phase transitions, associated with high and low temperature magnetic phases of Mn and Dy sublattices. From isothermal magnetization at low temperatures a magnetic phase diagram was constructed associated with induced antiferromagnetic to ferrimagnetic transition. From single crystal neutron diffraction measurements the high temperature ordering temperature of Mn sub lattice and low temperature magnetic ordering of Dy was determined. From our magnetization, specific heat and single crystal neutron diffraction experiments combined with the reported results we constructed a magnetic phase diagram of hDMO. It was found that at high temperature Mn and Dy sublattices order with different magnetic symmetries, which is attributed to weak 3d-4f interactions. This result is in striking contrast to other hexagonal rare earth manganites. Further thermal variation of lattice parameters confirmed the presence of ME effect. Polycrystalline samples of MCoW and MCuW were prepared by solid-state reaction method. From magnetization measurements the magnetic exchange interactions were found to be strongly antiferromagnetic. The magnetic structure at several temperatures below ordering temperature was determined by NPD measurements. An incommensurate magnetic phase is found in both the compounds. The magnetic structure of MCoW is in good agreement with the reported results while the magnetic ordering in MCuW is very different from the parent MnWO4 and other doped compounds in this family. In the whole temperature range MCuW exhibit sinusoidal spin arrangement while other compounds exhibit spiral spin ordering in certain temperature range. This provides a further opportunity to study the implications of magnetic ordering on the dielectric properties in this class of multiferroics.

Grantor dc:publisher

Forschungszentrum Jülich, Zentralbibliothek

Year dc:date

2012

Author dc:creator

• Chogondahalli Muniraju, Naveen Kumar

Contributors dc:contributor

• Brückel, Thomas

• info:eu-repo/classification/ddc/530
• Multiferroikum
• Magnetische Neutronenstreuung
• Neutronenstreuung
• Hyperfeinstruktur
• Röntgenbeugung
• Physik
• Kristallfeldwechselwirkung
• multiferroics
• magnetic neutron scattering
• crystal field interactions
• hyperfine levels
• x-ray diffraction

Statement dc:rights

• info:eu-repo/semantics/openAccess

Language dc:language

eng

Identifier

https://publications.rwth-aachen.de/search?p=id:%22RWTH-CONV-124401%22

OAI identifier oai:identifier

oai:publications.rwth-aachen.de:62916