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

Ferromagnetism with high Curie temperature $T_c$, well above room temperature, and very small saturation moment has been reported in various carbon and boron systems. It is argued that the magnetization must be very inhomogeneous with only a small fraction of the sample ferromagnetically ordered. It is shown that a possible source of high $T_c$ within the ferromagnetic regions is itinerant electrons occupying a narrow impurity band. Correlation effects do not reduce the effective interaction which enters the Stoner criterion in the same way as in a bulk band. It is also shown how, in the impurity band case, spin wave excitations may not be effective in lowering $T_c$ below its value given by Stoner theory. These ideas are applied to CaB$_6$ and a thorough review of the experimental situation in this material is given. It is suggested that the intrinsic magnetism of the B$_2$ and O$_2$ dimers might be exploited in suitable structures containing these elements.

High-temperature ferromagnetism of $sp$ electrons in narrow impurity bands: Application to CaB$_6$

Ferromagnetism with high Curie temperature Tc, well above room temperature, and very small saturation moment has been reported in various carbon and boron systems. It is argued that the magnetization must be very inhomogeneous with only a small fraction of the sample ferromagnetically ordered. It is shown that a possible source of high Tc within the ferromagnetic regions is itinerant electrons occupying a narrow impurity band. Correlation effects do not reduce the effective interaction which enters the Stoner criterion in the same way as in a bulk band. It is also shown how, in the impurity band case, spin wave excitations may not be effective in lowering Tc below its value given by Stoner theory. These ideas are applied to CaB6 and a thorough review of the experimental situation in this material is given. It is suggested that the intrinsic magnetism of the B2 and O2 dimers might be exploited in suitable structures containing these elements.

https://iopscience.iop.org/article/10.1088/0953-8984/18/31/016/pdf

High-temperature ferromagnetism of sp electrons in narrow impurity bands: application to CaB6

The physics of ferromagnetic doped manganites, such as La 1-x Ca x MnO 3 with x ≈ 0.2-0.4, is reviewed. The concept of double exchange is discussed within the general framework of itinerant electron magnetism. The new feature in this context is the coupling of electrons to local phonon modes. Emphasis is placed on the quantum nature of the phonons and the link with polaron physics. However it is stressed that the manganites fall into an intermediate coupling regime where standard small-polaron theory does not apply. The recently-developed many-body coherent potential approximation is able to deal with this situation and Green's recent application to the Holstein double-exchange model is described. Issues addressed include the nature of the basic electronic structure, the metal-insulator transition, a unification of colossal magnetoresistance, pressure effects and the isotope effect, pseudogaps in spectroscopy and the effect of electron-phonon coupling on spin waves.

Ferromagnetism and electron-phonon coupling in the manganites

A new formula is derived for the spin-wave stiffness constant D and is compared with previous work. It is shown that in NiD is very sensitive to the 'ground-state' exchange splitting Delta in the d band and that hybridisation with the sp band has little effect on D. The observed value of D yields a value of 0.40 eV for Delta , which is larger than the observed quasiparticle exchange splitting of 0.33 eV, as expected theoretically. Calculations on D in Ni alloys, within the virtual crystal approximation, are compared with experimental results on NiCo and NiFe.

Spin waves in ferromagnetic transition metals. I. General formalism and application to nickel and its alloys

Previous numerical calculations for a model of nickel alloys, employing the coherent potential approximation (CPA) and the random phase approximation (RPA), are extended to iron alloys. It is shown that often the effective scattering potentials are sufficiently weak for a rigid band calculation of the spin wave stiffness constant D to be a good guide to the more elaborate CPA calculations. The case of NiFe alloys is re-examined in the light of a recent calculation by Riedinger and Nauciel-Bloch (ibid., vol.5, p.732 (1975)). It is shown that D is very sensitive to the assumed exchange splitting in pure nickel and that within the RPA reasonable agreement with experiment is obtained only by assuming an unusually large value for this parameter.

http://iopscience.iop.org/article/10.1088/0305-4608/6/4/019/pdf

D M Edwards

Papers

Theory of the spin wave stiffness constant in ferromagnetic transition metal alloys