The many theoretical studies of the various magnetic, thermal and magnetoelastic properties for paramagnetic and ferromagnetic transition metals and alloys in the simple itinerant electron model are reviewed. The important amendments to the simple itinerant electron model or Stoner model of magnetism due to the spin wave excitations and spin fluctuations are explained.

https://iopscience.iop.org/article/10.1088/0034-4885/44/4/001/pdf

Itinerant electron magnetism

Topologically close-packed structures of transition metal alloys

Measuring the magnetization of various alloys in the Ni3Al and Ni3Ga phases, we found that going to higher Ni concentrations within these phases, a transition from strongly exchange enhanced paramagnetism to weak ferromagnetism takes place at 74.5 and 76 at.% Ni, respectively. The shape of the reciprocal magnetic susceptibility vs temperature curves measured between 4.2° and 300°K is of the Stoner type and suggests for these alloys a density‐of‐states curve varying appreciably in the Fermi region leading to a low effective Fermi energy. This conclusion is confirmed by the strong curvature of the magnetization in high magnetic fields. These results and the fact that an increase of the magnetic field applied on ferro‐magnetic samples effects a large increase of the magnetization even at the lowest temperatures (4.2°K) and in the highest fields (200 kOe) led us to a description in terms of the itinerant electron model. The use of this model seems justified by the fact that in agreement with calculations by Edwards and Wohlfarth, the magnetic data of both paramagnetic and ferromagnetic compositions fit nicely on straight lines in σ2 vs H/σ plots, this being so for all temperatures (down to 4.2°K) and all magnetic fields between 4 and 200 kOe. Combining the magnetic data and low‐temperature specific‐heat data, we obtained some information on the bandshape at the Fermi level. We cannot conclude, however, to more than a nice qualitative agreement with the Stoner model at this moment. As would be expected in highly polarizable metals and alloys, small amounts of Fe dissolved both in paramagnetic and in ferromagnetic alloys in the Ni3Al and Ni3Ga phases lead to giant moment inducement.

Exchange‐Enhanced Paramagnetism and Weak Ferromagnetism in the Ni3Al and Ni3Ga Phases; Giant Moment Inducement in Fe‐Doped Ni3Ga

The itinerant electron model is considered in the limit of very low exchange splitting energies. Magnetic isotherms giving M(H, T) are derived to cover a wide range of fields and temperatures. The isotherms are transformed so as to show that plots of M 2 vs H/M at different temperatures give parallel straight lines. The differential susceptibility below and above the Curie temperature is derived as a function of T. The free energy corresponding to this model is calculated and several results concerning the specific heat of very weak itinerant ferromagnets are deduced. Some properties related to spin wave excitations are also considered. New experimental data on the material ZrZn2 are considered on the basis of the theory and shown to be in reasonable agreement with it. Several characteristic parameters of this material are deduced, for example a value of the exchange splitting energy 0.040 eV, and of the effective interation between the itinerant electrons 0.35 eV. The occurrence of very weak itinerant ferromagnetism in other substances is also discussed.

Very Weak Itinerant Ferromagnets; Application to ZrZn2

The origin of the Curie-Weiss law of the magnetic susceptibility universally observed in ferromagnetic materials is discussed from a point of view that the local spin fluctuation amplitude is almost conserved. A fully consistent treatment is presented which can deal with both the ordered phase and the paramagnetic phase on an equal footing. In the case of weakly ferromagnetic limit, Rhodes-Wohlfarth plot is revised from a new stand point. In the ordered phase, observed systematic trends of magnetization curves are explained in connection with the nature of dynamical spin fluctuation spectra. Some new interesting relations among various magnetic data are derived and compared with experiments. Temperature dependence of magnetic susceptibilities of Fe and Ni is also discussed.

On the Origin of the Curie-Weiss Law of the Magnetic Susceptibility in Itinerant Electron Ferromagnetism

The magnetization of ZrZn 2 has been measured as a function of magnetic field up to 48 kOe and of temperature from 2.0°K to 380°K. The intrinsic magnetization extrapolated to 0°K increases strongly with external field. The magnetization at low field is found to vary as T 2 at low temperatures. The Curie temperature for a typical sample is 17.8°K, whereas the hysteresis loop is observed up to 30°K. A comment on the Curie temperature quoted in literatures has been made. The difference in magnetic properties among different samples has been noticed. The itinerant electron model of ferromagnetism can account for qualitatively the observed magnetic properties, whereas the localized-spin model proves inappropriate for this material.

Shinji Ogawa

Papers

Magnetic Properties of ZrZn 2 —Itinerant Electron Ferromagnet