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

A survey is given of the physical properties, composition and crystal structure of intermetallic compounds formed between rare-earth elements and 3d transition elements. Apart from binary compounds the results of pseudobinary series are also considered. The magnetic properties determined by the exchange interactions involving 4f as well as 3d electrons, are discussed together with experimental results available on magnetovolume effects and various resonance techniques such as NMR and the Mossbauer effect.

https://iopscience.iop.org/article/10.1088/0034-4885/40/10/002/pdf

Intermetallic compounds of rare-earth and 3d transition metals

A magnetically, electrically or mechanically responsive material can undergo significant thermal changes near a ferroic phase transition when its order parameter is modified by the conjugate applied field. The resulting magnetocaloric, electrocaloric and mechanocaloric (elastocaloric or barocaloric) effects are compared here in terms of history, experimental method, performance and prospective cooling applications.

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Caloric materials near ferroic phase transitions

Magnetocaloric effect: From materials research to refrigeration devices

The magnetic refrigeration technique based on the magnetocaloric effect (MCE) has attracted increasing interest because of its high efficiency and environment friendliness. in this article, our recent progress in exploring effective MCE materials is reviewed with emphasis on the MCE in the LaFe13-xSix-based alloys discovered by us. These alloys show large entropy changes over a wide temperature range near room temperature. The effects of magnetic rare-earth doping, interstitial atoms and high pressure on the MCE have been systematically studied. Special issues, such as appropriate approaches to determining the MCE associated with the first-order magnetic transition, the depression of magnetic and thermal hysteresis, and the key factors determining the magnetic exchange in alloys of this kind, are discussed. The applicability of giant MCE materials to magnetic refrigeration near ambient temperature is evaluated. A brief review of other materials with significant MCE is also presented.

Recent Progress in Exploring Magnetocaloric Materials

A detailed thermodynamical analysis of commensurate and incommensurate magnetic phases in localized magnetic systems within a mean-field theory is presented. Interesting results are obtained concerning the specific heat of amplitude-modulated magnetic structures: In particular, the specific-heat discontinuity at the ordering temperature ${\mathit{T}}_{\mathit{N}}$ is reduced by a factor of 2/3 relative to that expected in ferromagnetic or helimagnetic structures. In addition, the exact shape of the specific-heat curves strongly depends on the relative magnitude of the successive exchange couplings J(nQ). Under certain conditions, the maximum of specific heat at ${\mathit{T}}_{\mathit{N}}$ is shifted to a temperature lower than ${\mathit{T}}_{\mathit{N}}$. Recent experimental data on ${\mathrm{GdCu}}_{2}$${\mathrm{Si}}_{2}$, ${\mathrm{GdNi}}_{2}$${\mathrm{Si}}_{2}$, ${\mathrm{GdGa}}_{2}$, and ${\mathrm{GdCu}}_{5}$ compounds support very well these peculiarities.

Specific heat in some gadolinium compounds. II. Theoretical model

Magnetic measurements which have been performed on single crystals of the Gd${\mathrm{Co}}_{2}$, Ho${\mathrm{Ni}}_{2}$, and Ho${\mathrm{Co}}_{2}$ Laves phases lead to a quantitative analysis of the crystal-field effects on energy and magnetization anisotropies. The anisotropy due to cobalt is negligible. Crystal-field parameters have been determined from the magnetization variation in intense magnetic fields. The change of easy magnetization direction observed at 14 K on Ho${\mathrm{Co}}_{2}$ corresponds to a change, with temperature, of the level scheme of the $^{5}J_{8}$ ground-state multiplet. An entropy discontinuity is associated with this transition, leading to a specific-heat anomaly. For Ho${\mathrm{Co}}_{2}$, the values of the anisotropy constants at 0 K are ${K}_{1}=\ensuremath{-}{10}^{7}$ erg/${\mathrm{cm}}^{3}$ and ${K}_{2}={10}^{9}$ erg/${\mathrm{cm}}^{3}$. The interactions in Ho${\mathrm{Ni}}_{2}$ are weak compared to crystal-field effects; when a magnetic field is applied, the anisotropy increases so much that it is no longer possible to measure it.

Magnetic properties of single crystals of Gd Co 2 , Ho Ni 2 , and Ho Co 2

Lattice parameter analysis and studies of thermal expansion show that the orthorhombic CeNi compound is, like CeSn 3 , an intermediate valence compound in which the cerium valence state varies from 3.5 to 3.3 between 4 and 300 K. This compound behaves as an enhanced Pauli paramagnet in which the magnetic susceptibility along the c axis passes through a maximum at around 140 K. Magnetic susceptibility, resistivity and heat capacity measurements are characteristic of an almost magnetic Fermi liquid in which spin fluctuations are present. Unlike the other cerium intermediate valence compounds which are generally cubic, large anisotropic effects due to the local surroundings are observed in CeNi because of its orthorhombic symmetry. All these properties can be understood in the light of the band structure of this type of alloy in which a large hybridization occurs between the Ni 3d electrons and the Ce 5d electrons.

Intermediate valence state of cerium in CeNi

Some features common to many rare-earth intermetallic compounds with complex magnetic phase diagrams are explained using a realistic mean-field model, which takes into account the periodic-exchange-field and crystal-field effects which favor an easy magnetization axis, i.e.: (i) the change of an incommensurate or long-period commensurate structure near ${\mathit{T}}_{\mathit{N}}$ toward a simple commensurate one at low temperature, with and without intermediate structures of intermediate propagation vectors; (ii) the low-temperature metamagnetic process during which the high-temperature propagation vector(s) tend to be recovered; (iii) the trend for the propagation vectors to lock onto commensurate values, except near ${\mathit{T}}_{\mathit{N}}$. Exchange interactions alone are enough to account for these properties, the exact boundaries of the magnetic phase diagram being determined by the real variation of J(q) and crystal-field effects.

Competition between commensurate and incommensurate phases in rare-earth systems: Effects on H-T magnetic phase diagrams.

Magnetic and resistivity measurements performed on the $\mathrm{Ce}{\mathrm{Ni}}_{x}{\mathrm{Pt}}_{1\ensuremath{-}x}$ compounds show that this system is one of the best examples of a ferromagnetic dense Kondo system in which Ruderman-Kittel-Kasuya-Yosida exchange interactions compete with the Kondo effect. The variations of the Curie temperature ${T}_{C}$ and of the Kondo temperature ${T}_{K}$ as a function of the Ni content are in qualitative agreement with those calculated as a function of the product $|\ensuremath{\Gamma}n({E}_{F})|$ in the Kondo lattice model, where $\ensuremath{\Gamma}(l0)$ is the coupling constant between the $4f$ shell and the conduction band. The increase of $|\ensuremath{\Gamma}n({E}_{F})|$ as a function of the Ni content is suggested to be mainly due to the decrease of the separation between the $f$ level and the Fermi level.

D. Gignoux

Papers

Specific heat in some gadolinium compounds. II. Theoretical model

Magnetic properties of single crystals of Gd Co 2 , Ho Ni 2 , and Ho Co 2

Intermediate valence state of cerium in CeNi

Competition between commensurate and incommensurate phases in rare-earth systems: Effects on H-T magnetic phase diagrams.

Competition between the Kondo effect and exchange interactions in the Ce Ni x Pt 1 − x compounds