We study a generalized isotropic XY model which includes both two- and four-spin mean-field interactions. This model can be solved in the microcanonical ensemble. It is shown that in certain parameter regions the model exhibits gaps in the magnetization at fixed energy, resulting in ergodicity breaking. This phenomenon has previously been reported in anisotropic and discrete spin models. The en...

We discuss the electronic quasiparticle spectrum of a ferromagnetic semiconductor, within the framework of the s-f model. Some exactly calculable limi ting cases (ferromagnetic saturation, atomic limit, centre of gravity of the conduction band, ....) as well as approximate theories for the general case are reviewed in order to understand the influence of temperature and finite band filling on t...

We compare different approximation schemes for investigating ferromagnetism in the periodic Anderson model. The use of several approximations allows for a detailed analysis of the implications of the respective methods, and also of the mechanisms driving the ferromagnetic transition. For the Kondo limit, our results confirm a previously proposed mechanism leading to ferromagnetic order, namely ...

We apply a modified mean-field density functional theory to determine the phase behavior of Stockmayer fluids in slit-like pores formed by two walls with identical substrate potentials. Based on the Carnahan-Starling equation of state, a fundamental-measure theory is employed to incorporate the effects of short-ranged hard-sphere-like correlations while the long-ranged contributions to the flui...

Dilute ferromagnetic oxides having Curie temperatures far in excess of 300 K and exceptionally large ordered moments per transition-metal cation challenge our understanding of magnetism in solids. These materials are high-k dielectrics with degenerate or thermally activated n-type semiconductivity. Conventional super-exchange or double-exchange interactions cannot produce long-range magnetic or...

In the electrically correlated metal URhGe ferromagnetic order (T C = 9.5 K) and superconductivity (T s = 0.25 K) coexist at ambient pressure. Here we report on chemical substitution of Rh by Ru, which enables one to tune the Curie temperature to T = 0. URuGe has a paramagnetic ground state and is isostructural to URhGe. We have prepared a series of polycrystalline URh 1−x Ru x Ge samples over ...

The square lattice spin-1/2 antiferromagnet containing a dilute concentration, δ, of randomly placed ferromagnetic nearest-neighbor bonds is studied at low-temperature via nonlinear σ-model techniques and by exact diagonalization. We generally find that long-range Néel order is destroyed above a critical strength in the defective ferromagnetic exchange coupling-constant given by |Kc|/J ∼ δ . We...

We present a solid-state synthesis of ferromagnetic lithium niobate nanoparticles (LiNbO₃) and their corresponding structural aspects. In order to investigate the effect of heat treatments, two batches of samples with a heat-treated (HT) and non-heat-treated (nHT) reduction at 650 °C in 5% of hydrogen/argon were considered to investigate the multiferroic properties and their corresponding struc...

We examine the ordering behavior of the ferromagnetic Ising lattice model defined on a surface with a constant negative curvature. Small-sized ferromagnetic domains are observed to exist at temperatures far greater than the critical temperature, at which the inner-core region of the lattice undergoes a mean-field phase transition. The survival of short-range order at such high temperatures can ...