Quantum phase transitions and thermodynamic properties in highly anisotropic magnets

The systems exhibiting quantum phase transitions (QPT) are investigated within the Ising model in the transverse field and Heisenberg model with easy-plane single-site anisotropy. Near QPT a correspondence between parameters of these models and of quantum phi^4 model is established. A scaling analysis is performed for the ground-state properties. The influence of the external longitudinal magnetic field on the ground-state properties is investigated, and the corresponding magnetic susceptibility is calculated. Finite-temperature properties are considered with the use of the scaling analysis for the effective classical model proposed by Sachdev. Analytical results for the ordering temperature and temperature dependences of the magnetization and energy gap are obtained in the case of a small ground-state moment. The forms of dependences of observable quantities on the bare splitting (or magnetic field) and renormalized splitting turn out to be different. A comparison with numerical calculations and experimental data on systems demonstrating magnetic and structural transitions (e.g., into singlet state) is performed.Comment: 46 pages, RevTeX, 6 figure

Magnetic domain patterns

La technique et l'interprétation des diagrammes de poudres magnétiques est brièvement passée en revue, d'un point de vue historique. Les diagrammes les plus simples observés sont ensuite décrits et expliqués dans la mesure du possible. Dans la troisième Partie, on décrit et discute de nouveaux diagrammes relatifs : a. à un monocristal dont la direction (111) est celle de facile aimantation (60 pour 100 Co, 40 pour 100 Ni); b. à un monocristal de cobalt; c. à un alliage polycristallin fer-silicium et d. à un alliage polycristallin pour aimants permanents (Alnico 5)

Quelques propriétés magnétiques, électriques et optiques des films obtenus par électrolyse et par évaporation thermique

Several experiments were carried out. Composites of thin layers of ferromagnetic materials, deposited by electrolysis, show an increase of the coercive force with decreasing thickness, as predicted by theory. The magnitude of the effect shows that both the cristal anisotropy and shape anisotropy contribute. Thin precipitates of some magnetic materials of high coercive force, studied by methods of measurements of the magnetic torque, are proved to be effective because of their shape anisotropy, magnetic crystal anisotropy being of secondary importance. Thin films of copper and others metals are produced by rapid evaporation (500 to 1000 Å /sec) and found to have a better mechanical structure than those produced slowly with the same residual gas pressure. The mean free path calculated from the resistivity has the expected magnitude. It is shown that a thin layer of the semiconductor GaSb is a photoelectric conductor. The electronic mobility (Hall effect) is of the order of 10-2 of that of the bulk material. Certain techniques are reported for preparing films of zinc 400 thick, on a free layer of cellulose acetate, the thickness of such composites being about 1 000 Å

Some ferrimagnetic and antiferromagnetic materials at low temperatures

The magnetization of single crystals and of polycrystals of a number of materials has been investigated at temperatures from 1. 3 to 300 °K, in fields up to 12,500 oersteds. Measurements were made on single crystals of LiMnPO4 (orthorhombic), Fe 3(PO4)2.4H2O (monoclinic), and HoFeO 3 (orthorhombic) ; polycrystalline materials were HoFeO3, Ho2O3, and Er2O3, PbO.Fe 12—xAlxO18, FeTiO3 and FeTiO 3-Fe2O3 solid solutions, CoMnO and MiMnO 3, CuF2.2H2O, MnF3, and CrF3 . The ferromagnetic single crystals show spontaneous magnetization along one crystallographic direction ; in Fe3(PO4) 2 .4H2O it was not possible to rotate the spontaneous magnetization out of the direction of the axis, in HoFe3O it is readily rotated away from the preferred orthorhombic axis at low temperatures. The spontaneous magnetization in HoFeO3 rises to about 3 Bohr magnetons as the temperature is lowered.Nous avons étudié l'aimantation de monocristaux et de polycristaux de quelques substances aux températures de 1,3 à 300 °K, dans les champs jusqu'à 12 500 œrsteds. Nous avons fait des mesures sur des monocristaux de LiMnPO4 (orthorhombique), Fe3(PO4)2.4H2O (monoclinique) et HoFeO3 (orthorhombique) ; les substances polycristallines étudiées sont HoFeO3, Ho 2O3 et Er2O3, PbO.Fe12—x AlxO18, FeTiO3 et FeTiO3-Fe 2O3 en solution solide, CoMnO3 et NiMnO3 , CuF2.2H2O, MnF3, et CrF3 . Pour les monocristaux ferromagnétiques, on observe une aimantation spontanée le long d'un axe cristallin spécifique ; dans le cas du Fe3(PO4)2.4H 2O il ne fut pas possible de faire dévier l'aimantation spontanée de la direction de l'axe monoclinique ; dans le cas de l'HoFeO3 il est possible aux basses températures de la faire dévier de l'axe privilégié orthorhombique. L'aimantation spontanée de l'HoFeO3 augmente jusqu'à 3 magnétons de Bohr environ si on baisse la température du cristal