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

Magnetic refrigeration techniques based on the magnetocaloric effect (MCE) have recently been demonstrated as a promising alternative to conventional vapour-cycle refrigeration1. In a material displaying the MCE, the alignment of randomly oriented magnetic moments by an external magnetic field results in heating. This heat can then be removed from the MCE material to the ambient atmosphere by heat transfer. If the magnetic field is subsequently turned off, the magnetic moments randomize again, which leads to cooling of the material below the ambient temperature. Here we report the discovery of a large magnetic entropy change in MnFeP0.45As0.55, a material that has a Curie temperature of about 300 K and which allows magnetic refrigeration at room temperature. The magnetic entropy changes reach values of 14.5 J K-1 kg-1 and 18 J K-1 kg-1 for field changes of 2 T and 5 T, respectively. The so-called giant-MCE material Gd5Ge2Si2 (ref. 2) displays similar entropy changes, but can only be used below room temperature. The refrigerant capacity of our material is also significantly greater than that of Gd (ref. 3). The large entropy change is attributed to a field-induced first-order phase transition enhancing the effect of the applied magnetic field.

Transition-metal-based magnetic refrigerants for room-temperature applications

Magnetism and Magnetic Materials

A review is given of the formation, the crystal structure and the magnetic properties of several classes of rare earth based intermetallic compounds that lend themselves as starting materials of permanent magnets. These compounds include R2Fe14B, R2Fe14C and R2Co14B and the large class of ternary rare earth compounds having the tetragonal ThMn12 structure. Special emphasis is given to the changes in magnetic properties of R2Fe17 compounds observed after interstitial solution of C or N atoms. The magnetic properties of all these compounds are discussed in terms of current models based on intersublattice and intrasublattice exchange and the interplay between the rare earth sublattice anisotropy and 3D sublattice anisotropy. A substantial portion of the review is devoted to manufacturing routes of permanent magnets and a description of the coercivity mechanisms operative in the magnets. A comparison is made of the performance and economic costs of various types of magnets and novel applications are briefly discussed.

https://iopscience.iop.org/article/10.1088/0034-4885/54/9/001/pdf

New developments in hard magnetic materials

What we believe to be the first experimental results have been obtained on strong magnetic x-ray dichroism in the ${M}_{4,5}$ absorption spectra of magnetically ordered rare-earth materials, in accordance with recent predictions.

Experimental proof of magnetic-x-ray dichroism

A structural and magnetic characterization of the new ternary R2Fe17Nx compounds has been carried out by means of neutron powder diffraction and χac susceptibility measurements. Preliminary high field magnetization measurements are also presented. These new nitrides have been found to retain the 2:17 host metal symmetry. Nitrogen is accommodated in octahedral sites. The effects of nitrogenation on the magnetic properties are discussed. The high field measurements have allowed us to determine the anisotropy field of the Sm2Fe17N2.2 compound in the range from 4.2 to 300 K.

Structural and magnetic properties of ternary nitrides R2Fe17Nx (R ≡ Nd, Sm)

Electronic properties and critical current densities of superconducting (Y1Ba2Cu3O6.9)1−xAgx compounds

On three fluorides CeF3, NdF3, and PrF3 accurate susceptibilities measurements are reported from 2 up to 300 K under a magnetic field applied along the c axis. On the same samples, the optical Faraday rotation has been investigated in the temperature range 8–300 K at 0.6328 μm wavelength in a magnetic field up to 20 kOe. The reciprocal susceptibilities χ−1(T) follow a Curie Weiss law for T>100 K; the observed values of the paramagnetic Curie temperature are − 56 K (PrF3), − 114 K (CeF3), and − 26 K (NdF3). For T 77 K. (V and χ are expressed in deg cm−1 Oe−1 and in μB mol−1 Oe−1, respectively.) On one hand for NdF3 and PrF3, it ...

Magnetic susceptibility and Verdet constant in rare earth trifluorides

The influence of high magnetic fields on the first order magneto-elastic transition in MnFe(P1−yAsy) systems

An automatic device for measurements of the magnetic moment in high d.c. magnetic fields up to 20 T produced by a Bitter coil is presented. The extraction technique is used and a cryostat associated to a calorimeter permits us to cover the 1.4–500 K temperature range. The calibration, the accuracy and the sensitivity of the system are discussed.

M. Guillot

Papers

Structural and magnetic properties of ternary nitrides R2Fe17Nx (R ≡ Nd, Sm)

Electronic properties and critical current densities of superconducting (Y1Ba2Cu3O6.9)1−xAgx compounds

Magnetic susceptibility and Verdet constant in rare earth trifluorides

The influence of high magnetic fields on the first order magneto-elastic transition in MnFe(P1−yAsy) systems

Automatic device for precise magnetic measurements in high continuous magnetic fields