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.     

Coverage-dependent spin reorientation transition temperature of the Fe double-layer on W(1 1 0) observed by scanning tunneling microscopy

The magnetic domain structure of the Fe double-layer on W(1 1 0) is investigated using a variable-temperature scanning tunneling microscope. At low temperature the well-known periodic magnetic stripe domain structure is identified via the observation of domain walls. This is done with a non-spin-polarized tip by taking advantage of a spin–orbit coupling effect. At higher temperature a reorientation to an in-plane easy axis is observed. The spin reorientation temperature is found to be coverage-dependent and it is determined for samples with a coverage of 1.5–2.2 atomic layers of Fe on W(1 1 0).     

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.     

Monte Carlo study of half-magnetization plateau and magnetic phase diagram in pyrochlore antiferromagnetic Heisenberg model

The antiferromagnetic Heisenberg model on a pyrochlore lattice under external magnetic field is studied by classical Monte Carlo simulation. The model includes bilinear and biquadratic interactions; the latter effectively describes the coupling to lattice distortions. The magnetization process shows a half-magnetization plateau at low temperatures, accompanied with strong suppression of the magnetic susceptibility. Temperature dependence of the plateau behavior is clarified. Finite-temperature phase diagram under the magnetic field is determined. The results are compared with recent experimental results in chromium spinel oxides.     

Magnetization depth profile of (Fe/Dy) multilayers

An experimental and numerical study of the magnetization in (Fe 3 nm/Dy 2 nm) multilayers is presented. The samples were thermally evaporated under ultra-high vacuum at two different substrate temperatures, 320 and 570 K. In order to get the magnetization depth profile of these transition metal/rare earth (TM/RE) multilayers, a fine investigation of the structural, chemical, and magnetic properties was carried out. The samples were studied by X-ray reflectivity (XRR), high resolution transmission electron microscopy (HRTEM), conversion electron Mössbauer spectrometry (CEMS), SQUID magnetometry and polarized neutron reflectivity (PNR). Magnetization profiles were obtained by Monte Carlo simulations to support the PNR fits. The key role of the crystalline structure is emphasized by magnetic depth profile measurements performed using polarized neutron reflectometry. The antiparallel configuration of Fe and Dy layers’ magnetizations was evidenced, as well as the perpendicular magnetic anisotropy (PMA), especially in the case of the sample prepared at 570 K.     

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.     

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.     

Thermodynamics of the small clusters: The Gibbs free-energy derivation for the Honmura–Kaneyoshi method

A cluster density matrix is proposed for calculation of the thermodynamic averages in the effective field theory with correlations. On this basis, derivation of the Gibbs free-energy for the Honmura–Kaneyoshi method is made.     

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.     

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.     

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.     

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.     

Spin dynamics of layered triangular antiferromagnets with uniaxial anisotropy

The spin dynamics of the semiclassical Heisenberg model with uniaxial anisotropy, on the layered triangular lattice with antiferromagnetic coupling for both intralayer nearest neighbor interaction and interlayer interaction is studied both in the ordered phase and in the paramagnetic phase, using the Monte Carlo-molecular dynamics technique. The important quantities calculated are the full dynamic structure function S(q,ω)$S(\mathbf{q},\omega )$, the chiral dynamic structure function _Schi(ω)$S_{\mathrm{chi}}(\omega )$, the static order parameter and some thermodynamic quantities. Our results show the existence of propagating modes corresponding to both S(q,ω)$S(\mathbf{q},\omega )$ and _Schi(ω)$S_{\mathrm{chi}}(\omega )$ in the ordered phase, supporting the recent conjectures. Our results for the static properties show the magnetic ordering in each layer to be of coplanar 3-sublattice type deviating from 120^°${120}^{°}$ structure. In the presence of magnetic trimerization, however, we find the 3-sublattice structure to be weakened along with the tendency towards non-coplanarity of the spins, supporting the experimental conjecture. Our results for the spin dynamics are in qualitative agreement with those from the inelastic neutron scattering experiments performed recently.     

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.     

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.     

Single-domain versus two-domain configuration in thin ferromagnetic prisms

Thin ferromagnetic elements in the form of rectangular prisms are theoretically investigated in order to study the transition from single-domain to two-domain state, with changing the in-plane aspect ratio p  . We address two main questions: first, how general is the transition; second, how the critical value _pc$p_{\mathrm{c}}$ depends on the physical parameters. We use two complementary methods: discrete-lattice calculations and a micromagnetic continuum approach. Ultrathin films do not appear to split in two domains. Instead, thicker films may undergo the above transition. We have used the continuum approach to analyze recent magnetic force microscopy observations in 30 nm-thick patterned permalloy elements, finding a good agreement for _pc$p_{\mathrm{c}}$.     

Voltage-induced modification in magnetic coercivity of patterned Co50Fe50 thin film on piezoelectric substrate

A controlled modulation of magnetic properties through the inverse piezoelectric (PE) effect was investigated by means of the magneto-optical Kerr effect magnetometry in a periodic array of ferromagnetic (FM) Co₅₀Fe₅₀ stripes patterned on a commercial PE substrate. The coercive field (_Hc)$(H_{\mathrm{c}})$ rise (up to 80% in virgin cycle and 25% in subsequent cycles) when a DC electric field (up to 75 kV/cm) is applied on the PE substrate manifests a magnetoelectric coupling between the PE substrate and adjacent FM layer. The electric field dependence of _Hc$H_{\mathrm{c}}$ resembles the shear strain response to electric field of the PE constituent. The differences in the hysteresis loops shape when the magnetic field is oriented parallel and perpendicular to the stripes, reveal the interplay of the shape and stress anisotropy of the ferromagnet. Besides the inherent difficulties on constructing, an epitaxial hybrid system is a promising candidate of future random access memories designation.