Etude des differentes interactions elementaires dans un cristal contenant des atomes magnetiques. On montre que certaines interactions correspondent a des champs magnetiques tres eleves. Description detaillee des methodes de mesure d'aimantation en champ pulse intense

High-field magnetism

Publisher Summary This chapter discusses the progress in spinel ferrite research. A large number of oxides with a metal–oxygen ratio of 0.75 as composition is known to crystallize into the spinel structure. Among these oxides, magnetite Fe 3 O 4 is an important compound, from which the spinel ferrites can be derived by partial substitution of the iron ions by other cations. Between most of the spinel oxides solid solutions can be formed, which means that a great variety of spinel oxides is possible. The simple spinel ferrites are those ferrites in which x = 0 and the bivalent metal ion is Mg or Cd or one of the bivalent transition metal elements Mn, Fe, Co, Ni, Cu, and Zn. Partial solid solutions of the simple ferrites are also possible with compounds that either do not exist or that do not have the spinel structure. A great variety in the chemical composition of spinel ferrites is possible. The diversity in composition results into a large range of physical properties that permits the tuning of the properties for specific applications and makes spinel ferrites of particular interest.

Chapter 3 Progress in spinel ferrite research

Detailed magnetization measurements for single crystals of Ni-rich Fe–Ni alloys have been carried out between 4.2 K and 300 K in external magnetic fields up to 16.9 kOe to a relative accuracy of 5×10 -5 . It is found by iterative least-squares analysis of these observed values that the temperature variation of magnetization consists of two parts: one is due to the spin-wave excitations and the other due to the Stoner-type excitations. The coefficient of the latter part has a considerable value at pure Ni, decreases with addition of Fe atoms to pure Ni, vanishing around 70 at%Ni, and reappears at about 65 at%Ni. A simple picture for metallic ferromagnetism at finite temperatures is proposed.

Magnetic Excitations in FCC Fe–Ni Alloys Revealed from Detailed Magnetization Measurements

Magnetization process of Hf0.8Ta0.2Fe2 in strong pulsed magnetic fields

Total angular momentum and atomic magnetic moments

The magnetic phenomena are ruled by two main energy terms: the exchange energy and the magnetocrystalline anisotropy energy. According to the amplitude of the exchange interaction, with respect to the thermal energy; acclrding to the relative importance of the terms of exchange energy, magnetocrystalline energy and of coupling energy of the moments with the applied field, the magnetic moments can adopt different relative orientations. High magnetic fields produce on some substances transitions from an antiferromagnetic to a ferromagnetic state, which we call metamagnetism [1]; numerous examples exist [2], particularly with the rare earth elements and compounds [3][4][5]. On substances with high anisltropy, high magnetic fields allow to precise some specific properties, such as the anisotropy of the magnetic moment [6], the quenching of the moment due to the crystalline field [7]. On the other hand, the analysis of the isotherms of the magnetizatiln with high magnetic fields alllw to interpret interesting properties of itinerant magnetism and enhanced magnetism [8] [9]. In the present article, I discuss two original results that we have obtained recently at the SNCI Grenlble: the first lne deals with the saturatiln of paramagnetism and the effect of the crystalline field, the second one concerns the properties of the spin waves.

R. Pauthenet

Papers

High Field Magnetism