The quantum ground state of a Heisenberg ferromagnet with an “easy-plane” type anisotropy

For the Heisenberg ferromagnet with the easy-plane type anisotropy the ground state energy and the magnetization are found with the help of perturbation theory supposing that the anisotropy energy is less than the exchange one. The study is carried out exactly without using any spin operators representation. Therefore, it is valid for a spin of any magnitude.     

Peculiarities of the electronic transport in Co2CrAl and Co2CrGa half-metallic ferromagnets

We present results on the transport properties of the half-metallic ferromagnetic Heusler alloys Co₂CrAl and Co₂CrGa in the temperature range from 4 to 900 K. The peculiarities of the resistivity and the absolute differential thermoelectric power are considered within a two-current model of conductivity, taking into account the energy gap at the Fermi level in the electronic spectrum of alloys for electrons with spin opposite to the direction of the magnetization vector.     

Mean Field Dilute Ferromagnet: High Temperature and Zero Temperature Behavior

We study the mean field dilute model of a ferromagnet. We find and prove an expression for the free energy density at high temperature, and at temperature zero. We find the critical line of the model, separating the phase with zero magnetization from the phase with symmetry breaking. We also compute exactly the entropy at temperature zero, which is strictly positive. The physical behavior at temperature zero is very interesting and related to infinite dimensional percolation, and suggests possible behaviors at generic low temperatures. Lastly, we provide a complete solution for a (partially) annealed model. Our results hold both for the Poisson and the Bernoulli versions of the model.     

Block spins in the edge of an Ising ferromagnetic half-plane

The characteristic function of a block spin in the face of an Ising ferromagnetic half-plane is obtained in closed form. The distribution function for the block spin converges to a Gaussian at the critical temperature, but the normalization of the block is modified.Partially supported by NRC grant A9344.     

Magnetic Properties of Heisenberg Thin Films in an External Field

The magnetic properties of Heisenberg ferromagnetic films in an external magnetic field are investigated by means of the variational cumulant expansion (VCE). The magnetization can be in principle calculated analytically as the function of the temperature and the number of atomic layers in the film to an arbitrary order of accuracy in the VCE. We calculate the spontaneous magnetization and coercivity to the third order for spin-1/2 Heisenberg films with simple cubic lattices by using a graphic technique.     

Surface oxidation in a van der Waals ferromagnet Fe3-xGeTe2

We investigate the surface oxidation in a van der Waals ferromagnet Fe_3-xGeTe₂, a material widely utilized for spintronic applications. While known for its relative air-insensitivity, exposure to air during the handling process (e.g. device or heterostructure fabrication) can lead to reduction or disappearance of its magnetic signal. Comparison of x-ray absorption and x-ray magnetic circular dichroism spectra between pristine and air-exposed Fe_3-xGeTe₂ confirm a naturally oxidized surface layer on the material. The surface oxide layer has predominantly Fe³⁺ content. X-ray absorption spectroscopy done on micron-sized exfoliated Fe_3-xGeTe₂ flakes reveal that the change in the surface chemical properties can be quite significant for thin flakes. The surface modulation of Fe_3-xGeTe₂ can lead to inaccuracies in characterizing its interfacial magnetic and spin transport properties, and complicate device and heterostructure fabrication processes.     

Infinite divisibility of a bethe lattice ising model

It is shown that the probability distribution for the infinite-volume, free-boundary-condition Ising ferromagnet on the Bethe lattice under zero external field is infinitely divisible with respect to the group operation of pointwise multiplication of spin variables.     

Anisotropic ferromagnet with biquadratic exchange

A formulation, using the double time Green’s function method, is presented for the anisotropic spin-one ferromagnetic system in the presence of Ising part of the biquadratic exchange and an expression is given for the critical temperatureT _c in terms of two parameters. One is associated with the uniaxial anisotropy (treated exactly) and the other one corresponds to the biquadratic interaction (treated in RPA). The bilinear Heisenberg exchange term is treated in Callen’s approximation emphasizing the role of intersite correlation for small parameters. The prediction thatT _c increases with the biquadratic exchange is found to be consistent with the recent calculation based on high temperature series expansion.