Temperature behavior of bound magnetic polarons in antiferromagnetic chain

The behavior of a one-dimensional antiferromagnetic (AF) chain doped by non-magnetic donor impurities is analyzed, in the limit of low impurity density n  . The doping leads to the formation of ferromagnetically correlated regions localized near impurities (bound magnetic polarons or ferrons). The temperature evolution of the chain is calculated using an approximate variational method, and a Monte Carlo simulation. Both these methods give the similar results. The analysis of correlation functions for neighboring local spins demonstrates that the ferromagnetic correlations inside a ferron are significant even at high temperatures. The AF correlations in the rest part of the chain decay much faster with temperature. So, the ferron is a stable object that does not disappear even above the Néel temperature _TN$T_{\mathrm{N}}$. At rather small values of the electron–impurity coupling energy V$V$ (for V$V$ lower then the electron hopping integral t  ), the bound ferron depins from impurity retaining its magnetic structure. Such a depinning occurs at T∼V$T\sim V$.     

Measurements of magnetostriction of paramagnetic Fe–Mo–Cu–B metallic glasses

Magnetostriction of amorphous Fe₇₉Mo₈Cu₁B₁₂, (Fe₁₂Co₁)₇₉Mo₈Cu₁B₁₂ and (Fe₉Co₁)₇₉Mo₈Cu₁B₁₂ prepared by planar flow casting was measured using a direct method. The results indicate that magnetostriction in parallel (_λ∥)$({\lambda }_{\parallel })$ and perpendicular (_λ⊥)$({\lambda }_{\perp })$ directions of applied magnetic field is linearly dependent on magnetic field. In order to determine the influences of chemical composition and the conditions of sample preparation the magnetostriction of pure BCC-Fe, Cu and Mo were also measured. Samples containing Co with Curie temperatures slightly above room temperatures were shown to exhibit a hybrid magnetostriction behaviour with both ferromagnetic and paramagnetic features.     

Study of magnetoelectric coupling effect in the hexagonal ferroelectromagnet

Soft-mode theory based on Diffour model for ferroelectric subsystem, and mean-field theory as well as Heisenberg model for antiferromagnetic subsystem are utilized to investigate the magnetoelectric coupling effect in a hexagonal ferroelectromagnet, in which the ferroelectric and antiferromagnetic orders spontaneously coexist below a certain temperature. An anomaly of polarization at the magnetic transition temperature is ascribed to the effect of magnetoelectric coupling. The magnetic excitation has also been studied by spin-wave theory over the three-sublattice model. It is demonstrated that role of magnetoelectric coupling effect is not only related with the strength of magnetoelectric coupling but also special spin lattice structure. Our results show the magnetic specific heat induced by magnetic excitation experiences a suppression by the magnetoelectric coupling.     

The phenomenology of Dvali–Gabadadze–Porrati cosmologies

Cosmologists today are confronted with the perplexing reality that the universe is currently accelerating in its expansion. Nevertheless, the nature of the fuel that drives today's cosmic acceleration is an open and tantalizing mystery. There exists the intriguing possibility that the acceleration is not the manifestation of yet another mysterious ingredient in the cosmic gas tank (dark energy), but rather our first real lack of understanding of gravity itself, and even possibly a signal that there might exist dimensions beyond that which we can currently observe. The braneworld model of Dvali, Gabadadze and Porrati (DGP) is a theory where gravity is altered at immense distances by the excruciatingly slow leakage of gravity off our three-dimensional Universe and, as a modified-gravity theory, has pioneered this line of investigation. I review the underlying structure of DGP gravity and those phenomenological developments relevant to cosmologists interested in a pedagogical treatment of this intriguing model.     

Maximum entropy production principle in physics,chemistry and biology

The tendency of the entropy to a maximum as an isolated system is relaxed to the equilibrium (the second law of thermodynamics) has been known since the mid-19th century. However, independent theoretical and applied studies, which suggested the maximization of the entropy production during nonequilibrium processes (the so-called maximum entropy production principle, MEPP), appeared in the 20th century. Publications on this topic were fragmented and different research teams, which were concerned with this principle, were unaware of studies performed by other scientists. As a result, the recognition and the use of MEPP by a wider circle of researchers were considerably delayed. The objectives of the present review consist in summation and analysis of studies dealing with MEPP. The first part of the review is concerned with the thermodynamic and statistical basis of the principle (including the relationship of MEPP with the second law of thermodynamics and Prigogine's principle). Various existing applications of the principle to analysis of nonequilibrium systems will be discussed in the second part.     

The periodic Anderson model: Symmetry-based results and some exact solutions

The topic “magnetic impurities in metals” is certainly one of the most studied problems of the solid-state physics in the last years. The interest toward this argument relies on the fact that the interaction between the magnetic moment of the impurities and the conduction electrons of the host metal, is responsible for a large variety of physical phenomena.     

Magnetism of Fe,Co, and Ni nanowires encapsulated in carbon nanotubes

We have investigated the electronic and magnetic properties of Fe, Co, and Ni nanowires encapsulated in carbon nanotubes (CNTs) using spin polarized ab initio calculation. The incorporated systems with hollow region between the nanowire and the C shell have the enhanced magnetic moments compared to the ferromagnetic nanowires tightly wrapped by CNTs. The Co nanowire encapsulated in CNTs is a strong ferromagnet and has high spin polarization regardless of the distance between the nanowire and the C shell. The results show that the Co-filled CNTs are useful for spin polarized transport nanodevice.     

The method of increments—a wavefunction-based ab initio correlation method for solids

An overview of wavefunction-based correlation methods generalised for the application to solids is presented. Those methods based on a preceding Hartree–Fock treatment explicitly calculate the many-body wavefunction in contrast to the density-functional theory which relies on the ground-state density of the system. This review focus on the so-called method of increments where the correlation energy of the solid is expanded in terms of localised orbitals or of a group of localised orbitals. The method of increments is applied to a great variety of materials, from covalent semiconductors to ionic insulators, from large band-gap materials like diamond to the half-metal α$\alpha$-tin, from large molecules like fullerenes over polymers, graphite to three-dimensional solids. Rare-gas crystals where the binding is van der Waals like are treated as well as solid mercury, where the metallic binding is entirely due to correlation. Strongly correlated systems are examined and the correlation driven metal–insulator transition is described at an ab initio level.     

Anisotropy and magnetization process in soft magnets: Principles,experiments, applications

Understanding and controlling the anisotropy energy and its effects has proved vital to the development of soft magnetic materials and their applications. Indeed, acting on composition and structure and working out specific annealing treatments, a large variety of anisotropy-governed behaviors under DC and AC excitation can be obtained. These are discussed in the present paper, together with special problems arising in the characterization of anisotropic soft magnets and a few significant applications. It is stressed how features like J$J$–H$H$ loop shape, energy losses, and magnetoresistance effects can be controlled, in crystalline and amorphous materials, by the methods of induced anisotropy. The high-frequency behavior of these materials can be strongly affected by the anisotropy field via resonant absorption of energy. This calls for tradeoff between the values of permeability and resonance frequency.     

Room-temperature ferromagnetic Mn-alloyed ZnO films obtained by pulsed laser deposition

Ferromagnetic, semi-insulating Mn-alloyed ZnO films with a Curie temperature above 375 K have been grown by pulsed laser deposition on c-plane sapphire substrates. Antiferromagnetic coupling is revealed by temperature-dependent magnetization measurements. The antiferromagnetic coupling would be compatible with the observed weak ferromagnetism by assuming that the magnetic moments order antiferromagnetically but nonparallel (canted). We find a clear correlation between coercivity and mosaicity of the ferromagnetic Mn-alloyed ZnO films and explain it on the basis of a coercivity mechanism known from soft magnetic materials.     

Brownian rotational relaxation and power absorption in magnetite nanoparticles

We present a study of the power absorption efficiency in several magnetite-based colloids, to asses their potential as magnetic inductive hyperthermia (MIH) agents. Relaxation times τ$\tau$ were measured through the imaginary susceptibility component χ^″(T)${\chi }^{″}(T)$, and analyzed within Debye's theory of dipolar fluid. The results indicated Brownian rotational relaxation and allowed to calculate the hydrodynamic radius close to the values obtained from photon correlation. The study of the colloid performances as power absorbers showed no detectable increase of temperature for dextran-coated Fe₃O₄ nanoparticles, whereas a second Fe₃O₄-based dispersion of similar concentration could be heated up to 12 K after 30 min under similar experimental conditions. The different power absorption efficiencies are discussed in terms of the magnetic structure of the nanoparticles.     

Ferrimagnetic properties of Co/(Gd–Co) multilayers

Co/(Gd–Co) multilayers have been prepared by rf-sputtering and investigated by means of Transverse Magnetooptic Kerr Effect (TMOKE), SQUID and VSM magnetometry. The composition of amorphous _Gd0.36Co0.64${\mathrm{Gd}}_{0.36}{\mathrm{Co}}_{0.64}$ layers was chosen so that their saturation magnetization was dominated by Gd moments in all the temperature range. Co and Gd–Co layers formed a macroscopic ferrimagnetically coupled system displaying a compensation temperature. Complete magnetic moment compensation was found at such point. An inversion of TMOKE hysteresis loops and a divergent behaviour of coercivity were also observed. By changing the layers thickness it has been possible to control the magnetic characteristics of the Co/(Gd–Co) structures, in particular the compensation takes place at different temperatures.     

Monte Carlo investigation of the magnetic anisotropy in Fe/Dy multilayers

By Monte Carlo simulations in the canonical ensemble, we have studied the magnetic anisotropy in Fe/Dy amorphous multilayers. This work has been motivated by experimental results which show a clear correlation between the magnetic perpendicular anisotropy and the substrate temperature during elaboration of the samples. Our aim is to relate macroscopic magnetic properties of the multilayers to their structure, more precisely their concentration profile. Our model is based on concentration dependent exchange interactions and spin values, on random magnetic anisotropy and on the existence of locally ordered clusters that leads to a perpendicular magnetisation. Our results evidence that a compensation point occurs in the case of an abrupt concentration profile. Moreover, an increase of the non-collinearity of the atomic moments has been evidenced when the Dy anisotropy constant value grows. We have also shown the existence of inhomogeneous magnetisation profiles along the samples which are related to the concentration profiles.     

Needle-like domain structure in Co films deposited on Mo (1 1 0)

Magnetization reversal processes and domain structures have been studied in Mo(1 1 0)/Co(0 0 0 1)/Au(1 1 1) structures grown by molecular beam epitaxy on monocrystalline (11–20) sapphire substrates. Wedge-shaped samples with different Co thickness gradients relative to the Mo [0 0 1] direction were fabricated. Observation of the domain structure was performed at room temperature using Kerr microscopy in a Co thickness range varying from 5 to 50 nm, where the magnetization is oriented in the plane of the sample. A Co thickness-dependent coercivity field was determined through analysis of the domain wall position during the reversal process. A preferential orientation of magnetic domain walls was found, with the domains being needle-like. The orientation, as well as the size of the needles, depends on the Co thickness and the orientation of the magnetic field applied in the sample plane.