Nonferromagnetic State Research Articles

XY model with weak random anisotropy in a symmetry-breaking magnetic field.

We present a numerical study of the two-dimensional classical XY model with weak random anisotropy at zero temperature. Zero-field configurations obtained by ultrafast cooling, first-magnetization curves, and hysteresis loops have been calculated for different random-anisotropy-to-exchange ratios. In zero field, a pinning of vortices by the random-anisotropy field occurs. It prevents the binding then collapsing of pairs of opposite charges and thus leads to a nonferromagnetic ground state. Applying a magnetic field causes a progressive depinning of vortices that disappear in pairs until saturation. Starting from saturation and decreasing the applied field leads, in zero-field, to a magnetic state of large remanent magnetization. However, an important aftereffect is observed. It should give smaller remanence after much longer computer time. Then, the reversal of magnetization in negative fields occurs through a peculiar process that involves the formation and collapse of new kinds of topological defects (infinite strings). These linear defects are in fact the ultimate stage in the shrinking of domains oriented in the initial direction of saturation. Their collapse occurs abruptly through the creation and propagation in opposite directions, along the defect, of an unbound vortex-antivortex pair.

Ferromagnetism in the one-band Hubbard model.

We consider the question of ferromagnetism in the one-band Hubbard model on the square lattice with infinite on-site repulsion using exact diagonalization techniques. Our results show a nonmonotonic behavior as a function of the number of holes ${N}_{h}$ away from half filling. We find that the spin per electron is generally finite but less than its maximum value and, for a fixed hole number, increases with increasing system size. We interpret this as indicating that the inifinite-U Hubbard model is ferromagnetic in the thermodynamic limit for a finite fraction of holes, but that there are nonferromagnetic states which are very close in energy.

An effect of pressure forced transition from ferro- to nonferromagnetic state in amorphous alloy Fe 60Mn 20B 20

The paper presents the experimental results on isothermal, pressure forced phase transition from ferro- to nonferromagnetic state in Fe 60Mn 20B 20 amorphous alloy. The experiments have been done for five temperatures within the range 50–65°C and within the pressure range up to 1 GPa. The total disappearence of ferromagnetism with the increase of pressure has been observed.

Soft heating for hyperthermia using temperature-sensitive magnetic powders and fine metal rings (abstract)

In local hyperthermia it is important to leave the temperature of surrounding normal tissues essentially unchanged while increasing the temperature of tumors to a desired level. Soft heating1 with temperature-sensitive magnetic materials is advantageous in heating deep-seated tumors because of the self-limiting ability. In order for the method to become available for large-scale tumors we have proposed to use temperature-sensitive magnetic powders, in which fine metal rings are dispersed. As the magnetic flux distribution varies with the temperature of the powders, the fine rings immersed in the powders with low temperatures generate heat more than rings located in the powders with high temperatures. Furthermore, the transition from the ferromagnetic to the nonferromagnetic state becomes sharp effectively. As a result it is expected to make the temperatures of powders in a broad region uniform. Figure 1 shows the temperature dependence of the magnetic powders used in experiments. Figure 2 shows the relation between the attained temperature of the powders and the ring resistances, comparing them in the case of iron powder. The figure reveals that it exists appropriate combination between the ring resistance, and the magnetic permeability of the powders.

Evidence for spin-glass behaviour in the random anisotropy axis model

The random anisotropy axis model (RAM) in two dimensions (d=2) is studied in the limit of infinite anisotropy strength using the large-cell renormalisation group method. It is found that the model exhibits a 'zero-temperature phase transition', to a nonferromagnetic ground state, with critical exponents consistent with those of the d=2 Ising spin glass model. It is concluded that the RAM belongs to the universality class of the Ising spin glass, at least for d=2 and probably for all d<or=4.

Longitudinal magnetoresistance and temperature dependence of the electrical resistance of NiCr hydrides

Abstract The transformation of ferromagnetic NiCr alloys with chromium contents between 0.5 and 7 at.% to a non-ferromagnetic hydride state leads to a minimum in the electrical resistance at low temperatures and to a negative contribution of the longitudinal magnetoresistance. This phenomenon can be understood, by analogy to the Kondo effect in dilute alloys, as the spin-flip scattering of conduction electrons at localized magnetic moments and its “freezing-out” by a magnetic field. Furthermore the results can be compared with those of other researchers on CuCr and non-dilute CuMn alloys.

Effect of Pressure on the Curie Temperature of Rare-Earth Metals. I. Gadolinium

An experimental investigation has been undertaken for the purpose of learning the effect of interatomic distance on the magnetic exchange interaction in ferromagnetic rare-earth metals. The first material which has been studied is gadolinium. The equipment used was the high-pressure apparatus of Professor G. C. Kennedy of the Institute of Geophysics (UCLA). A toroidal-shaped specimen of gadolinium was subjected to pressures up to about 40 kilobars (kbar). The gadolinium toroid served as a transformer core. A constant input voltage was applied and the output voltage was studied as a function of pressure and temperature. The Curie temperature, obtained from the output voltage, was found to be depressed linearly with pressure at a rate of -1.60\ifmmode^\circ\else\textdegree\fi{}C/(kbar) up to 21.5 kbar. Between 21.5 and 26.7 kbar a phase transition to a nonferromagnetic state occurred.

Band-Theoretic View of Ferromagnetic Nickel

Although the probable applicability of the itinerant electron concept to nickel has been recognized for some time, convincing support for this point of view has only emerged recently. Some of this information, notably the galvanomagnetic measurements of Fawcett and Reed1 and optical data extending to 11 eV,2 are discussed on the basis of a realistic model for the band structure of nickel which is closely related to the calculations of Hanus3 for the material in its nonferromagnetic state. The marked resemblance of many of these experimental results to those of copper is related to the corresponding similarity in the band structure of the two materials and thus supports the itinerant electron hypothesis in nickel. The assumption that the electron interaction giving rise to ferromagnetism is roughly k independent and splits the d bands in such a way that the lower spin-down bands are completely filled determines a Fermi level with respect to Hanus's calculations whose energetic position depends only weakly on the magnitude of the splitting. The resulting Fermi surfaces are shown to consist of three electron sheets and unimportant hole pockets. One electron surface, corresponding to spin-down carriers, is copper-like. The remaining surfaces are distorted spheres which do not contact any faces of the Brillouin zone. They are characterized by large anisotropies in electron velocities and large corresponding differences between optical and thermal masses. Besides being consistent with the galvanomagnetic measurement, these surfaces and their associated masses also lead to results for the magneton number, the electronic specific heat, the plasma frequency, and the dc conductivity of nickel which are in substantial agreement with experimental data. In particular, they show how the large difference between optical and thermal effective masses, reflected in the similarity of plasma frequencies in Cu and Ni on the one hand, and the order of magnitude difference between electronic specific heats on the other, may be accounted for. The structure in the optical properties at 0.3 and and 1.4 eV, and corresponding structure observed by Krinchik4 and collaborators at nearly the same energies by the ferromagnetic Kerr effect is associated with interband transitions which may be interpreted in a consistent, but not unique fashion in terms of the preceding band structure to lead to a ferromagnetic splitting of the d bands at the point L corresponding to about 1.7 eV. With the highly qualitative assumption that the splitting is rigid, the resulting band structure for the spin-down electrons would be very similar to the E vs k curves characteristic of copper as obtained by Segall, while that for the spin-up electrons would correspond to the results of Hanus, with the two sets matched at Γ1, the bottom of the s bands. The similarity in the optical properties of copper and nickel can be understood qualitatively on this basis. The fact that the position of the Fermi level, and hence the qualitative features of the Fermi surfaces, are relatively insensitive to the magnitude of the d band splitting is illustrated by the substantial agreement between the Fermi levels obtained from the present model and the one proposed independently by Phillips,5 also based on Hanus's calculations, which assumes the splitting to be between 0.5 and 0.6 eV.

Ferromagnetism of an Electron Gas

Magnetism, especially ferromagnetism, of an electron gas is studied on the basis of the Bohm-Pines collective description of electron interactions. Exchange interactions are derived from the screened Coulomb interaction, the interaction obtained from the second-order perturbation of the screened Coulomb interaction, and the weak interaction coming from the interaction between plasma and individual electrons. We assume effective mass for the electron mass. The total energy of the electron gas in the ferromagnetic state is compared with that in the non-ferromagnetic state, and the conditions of ferromagnetism for the electron gas are discussed. The one-electron approximation being adopted, the effective number of magnetic carriers in the electron gas is expressed as a function of temperature and magnetic field. Using the Weiss approximation, we express the spontaneous magnetization at an arbitrary temperature in terms of this effective number and the molecular field obtained from the proper average of the exchange interactions. At low temperatures, the spin-wave approximation is adopted. The Curie temperature, the paramagnetic susceptrbility above the Curie point, and the temperature variation of spontaneous magnetization at low temperatures are determined as functions of the effective mass and the electron mean distance. Using the effective masses determined so as to explain the experimental values of the electronic heats, we make a comparison with experimental values for Ni, Co and Fe. It shows satisfactory agreement in the case of positive hole gas. The strong paramagnetism of γFe, Pd and Pt are also satisfactorily explained, and the inverse of their susceptibilities is well expressed as the sum of a constant term and the term proportional to T 2 at comparatively lower temperatures.

Ferromagnetism and the Band Theory

Ferromagnetism and the Band Theory

Statistical Mechanics of Nearest Neighbor Systems II. General Theory and Application to Two-Dimensional Ferromagnets

This is a continuation of an effort to reduce to the solution of a characteristic value problem the rigorous calculation of thermodynamic properties of systems in which the intermolecular forces are sufficiently short ranged so that practically the entire potential energy of the system results from interactions between nearest neighbors. The partition function of such a system can be expressed in terms of the largest characteristic value of a linear operator equation and finally as the ratio of partition functions of systems with relatively few particles whose potential energy functions differ somewhat from those in the original system. A method of evaluating grand partition functions and one of introducing interactions between more distant neighbors are discussed. The general theory is applied to the calculation of magnetization, internal energy, and specific heat of two-dimensional ferromagnets on the basis of the Ising model. There seems to exist a λ-point phase transition in the change from ferromagnetic to nonferromagnetic states.