In this chapter, we will restrict our attention to the ferrites and a few other closely related materials. The great interest in ferrites stems from their unique combination of a spontaneous magnetization and a high electrical resistivity. The observed magnetization results from the difference in the magnetizations of two non-equivalent sub-lattices of the magnetic ions in the crystal structure. Materials of this type should strictly be designated as “ferrimagnetic” and in some respects are more closely related to anti-ferromagnetic substances than they are to ferromagnetics in which the magnetization results from the parallel alignment of all the magnetic moments present. We shall not adhere to this special nomenclature except to emphasize effects, which are due to the existence of the sub-lattices.

Ferromagnetic materials

Negative thermal expansion (NTE) is an intriguing physical property of solids, which is a consequence of a complex interplay among the lattice, phonons, and electrons. Interestingly, a large number of NTE materials have been found in various types of functional materials. In the last two decades good progress has been achieved to discover new phenomena and mechanisms of NTE. In the present review article, NTE is reviewed in functional materials of ferroelectrics, magnetics, multiferroics, superconductors, temperature-induced electron configuration change and so on. Zero thermal expansion (ZTE) of functional materials is emphasized due to the importance for practical applications. The NTE functional materials present a general physical picture to reveal a strong coupling role between physical properties and NTE. There is a general nature of NTE for both ferroelectrics and magnetics, in which NTE is determined by either ferroelectric order or magnetic one. In NTE functional materials, a multi-way to control thermal expansion can be established through the coupling roles of ferroelectricity-NTE, magnetism-NTE, change of electron configuration-NTE, open-framework-NTE, and so on. Chemical modification has been proved to be an effective method to control thermal expansion. Finally, challenges and questions are discussed for the development of NTE materials. There remains a challenge to discover a “perfect” NTE material for each specific application for chemists. The future studies on NTE functional materials will definitely promote the development of NTE materials.

Negative thermal expansion in functional materials: controllable thermal expansion by chemical modifications

Structural characterization of R2BaCuO5 (R = Y, Lu, Yb, Tm, Er, Ho, Dy, Gd, Eu and Sm) oxides by X-ray and neutron diffraction

A phenomenological theory of magnetic states in noncentrosymmetric tetragonal antiferromagnets is developed, which has to include homogeneous and inhomogeneous terms (Lifshitz invariants) derived from Dzyaloshinskii-Moriya couplings. Magnetic properties of this class of antiferromagnets with low crystal symmetry are discussed in relation to the recently detected compounds ${\mathrm{Ba}}_{2}{\mathrm{CuGe}}_{2}{\mathrm{O}}_{7}$ and ${\mathrm{K}}_{2}{\mathrm{V}}_{3}{\mathrm{O}}_{8}.$ Crystallographic symmetry and magnetic ordering in these systems allow the simultaneous occurrence of chiral inhomogeneous magnetic structures and weak ferromagnetism. Incommensurate magnetic structures, chiral helices, with a rotation of the staggered magnetization accompanied by oscillations of the total magnetization, are possible. Field-induced reorientation transitions into modulated states have been studied and corresponding phase diagrams are constructed. Structures of magnetic defects (domain-walls and vortices) are discussed. In particular, vortices, i.e., localized nonsingular line defects, are stabilized by inhomogeneous Dzyaloshinskii-Moriya interactions in uniaxial noncentrosymmetric antiferromagnets.

https://arxiv.org/pdf/cond-mat/0206291.pdf

Magnetic structures and reorientation transitions in noncentrosymmetric uniaxial antiferromagnets

The large variety of magnetic phenomena observed in the Co based Laves phases are reviewed. Following the band structure calculations it is argued that the outstanding magnetic features of the RCo2 intermetallics are intimately related to the position of the Fermi level, which is near to a local peak in N(e). This is why the Co 3d-electron system reacts sensitively either to the molecular field of the R partner element or to the changes of external parameters such as a magnetic field or pressure. Magnetic, magnetoelastic and transport measurements of RCo2 compounds and related pseudobinaries such as R(Co1-xAlx)2 with R either magnetic or nonmagnetic rare earth element are shown and discussed. The conditions for the appearance of itinerant electron metamagnetism and spin fluctuations are outlined. In particular, the influences of spin fluctuations on physical properties, e.g. the susceptibility, thermal expansion and transport phenomena, are demonstrated.

https://iopscience.iop.org/article/10.1088/0953-8984/13/23/202/pdf

Physical properties of RCo2 Laves phases

This review treats the little-studied magnetic phase transitions of the order-order, spin-reorientation type, in which a rotation of the magnetization vector (or of die antiferromagnetism vector) with respect to the crystallographic axes occurs upon change of temperature, magnetic field, or external pressure. It presents theoretical ideas about magnetic phase transitions of the spin-reorientation type; it considers the effect of domain structure on spin-reorientation phase transitions, aad critical fluctuations during such transitions. Taking as an example rare-earth magnets (orthoferrites, iron-garnets, and rare-earth metals and intermetallic compounds), it considers the experimental data on various types of spin-reorientation transitions and on the anomalies of physical properties at such transitions.

http://iopscience.iop.org/article/10.1070/PU1976v019n07ABEH005274/pdf

Spin-reorientation transitions in rare-earth magnets

Magnetoelastic properties of RE-3d intermetallics

Metamagnetism of cuprates R2Cu2O5

Magnetic and spectral studies of Er2BaNiO5 and other rare earth nickelates

CONTENTS 1. Magnetic Properties of Rare-earth Ferromagnets 179 2. Neutron-Diffraction Studies of the Magnetic Structure of Rare-earth Ferromagnets 183 3. Theory of the Helicoidal Magnetic Structure 184 4. Nature of Magnetic Phase Transitions in Rare-earth Ferromagnets 190 5. Antiferromagnetism of Metals of the Cerium Subgroup 191 6. Magnetoelastic Properties of Rare-earth Ferromagnets 193 7. Electrical and Galvanomagnetic Properties of Rare-earth Ferromagnets 198 8. Magnetic Properties of Rare-earth Alloys 202 Literature 206

https://iopscience.iop.org/article/10.1070/PU1964v007n02ABEH003660/pdf

R. Z. Levitin

Papers

Spin-reorientation transitions in rare-earth magnets

Magnetoelastic properties of RE-3d intermetallics

Metamagnetism of cuprates R2Cu2O5

Magnetic and spectral studies of Er2BaNiO5 and other rare earth nickelates

Ferromagnetism and antiferromagnetism of rare-earth metals