Ab initio and Monte Carlo studies of phase transitions and magnetic properties of YCrO3: Heisenberg model

By using the density functional theory (DFT) and Monte Carlo simulations (MCS) with the Heisenberg model, we have studied magnetic properties of the bulk perovskite YCrO₃. The exchange couplings of the Heisenberg model and the magnetic anisotropy are investigated. The 110 direction in the crystalline structure of the compound has shown the minimum energy, it is the easy magnetic direction. Using Monte Carlo simulations, the magnetizations behavior, the effects of system parameters and the critical exponents of the compound YCrO₃ are implemented. It is shown that the bulk perovskite YCrO₃ belongs to the 3D Heisenberg universality class.     

Magnetic properties and structures of PrPt and NdPt

The magnetic properties of the CrB-type orthorhombic phase of PrPt and NdPt are presented. Below their ordering temperature which is 15 and 23 K respectively, these compounds exhibit a ferromagnetic behaviour. The magnetic structures determined by neutron diffraction are collinear. While in PrPt, the magnetic moments are parallel to c, in NdPt, the moments lie in the (a, c) plane and make an angle of 22° with a. These properties and especially the peculiar moment direction in NdPt are discussed in terms of crystalline electric field effects on the rare earth ion.     

Formation of phase domain structures in thin films under conditions of a first-order magnetic phase transition

This paper reports on the results of a theoretical investigation into the magnetic and resonance properties of thin films in the range of the transition from a paramagnetic state to a ferromagnetic state in the case where the magnetic transition is a first-order phase transformation. It is demonstrated that, in an external magnetic field directed perpendicular to the film plane, the formation of a specific domain structure consisting of domains of the coexisting paramagnetic and ferromagnetic phases can appear to be energetically favorable. The parameters of the equilibrium system of stripe phase domains and their dependences on the temperature, the magnetic field, and the characteristics of the material are calculated. The specific features of the magnetic resonance spectra under the conditions of formed stripe phase domains are considered. A relationship is derived for the dependence of the resonance field of the system of ferromagnetic domains on the magnetization and temperature. It is shown that the alternating external field can fulfill an orientation function in the formation of stripe phase domains.     

Magnetic domain structures of overquenched Nd–Fe–B permanent magnets studied by electron holography

Microstructures and magnetic domain structures of overquenched Nd–Fe–B permanent magnets have been investigated in detail by transmission electron microscopy. While magnetic domain boundaries are clarified by Lorentz microscopy, magnetization distribution in the domains is clearly observed by electron holography. In the as-quenched magnet, the size of the magnetic domains is in the range from 200 to 500 nm and the direction of the magnetic lines of force changes gradually in wide region, while in the annealed one having the crystalline phase of Nd₂Fe₁₄B, the direction of the magnetic lines of force changes drastically especially at the grain boundaries. Furthermore, the direction of the magnetic lines of force changes more drastically in the specimen annealed at 893 K than the specimen annealed at 843 K. This result clearly indicates that the magnetocrystalline anisotropy is enhanced with the increase of annealing temperature, resulting in strong domain wall pinning.     

Strain-induced modification of magnetic structure and new magnetic phases in rare-earth epitaxial films

Rare earths exhibit complex magnetic phase diagrams resulting from the competition between various contributions to the magnetic energy: exchange, anisotropy and magnetostriction. The epitaxy of a rare-earth film on a substrate induces (i) a clamping to the substrate and (ii) pseudomorphic strains. Both these effects are shown to lead to modifications of the magnetic properties in (0 0 1)Dy, (0 0 1)Tb and (1 1 0)Eu films. In Dy and Tb films, spectacular variations of the Curie temperature have been evidenced. Additionally, Tb films exhibit a new large wavelength magnetic modulation. In Eu films, one of the helical magnetic domains disappears at low temperature whereas the propagation vectors of the other helices are tilted. The link between structural and magnetic properties is underlined via magnetoelastic models. Moreover, molecular beam epitaxy permits the growth of Sm in a metastable dhcp phase. The magnetic structure of dhcp Sm has been elucidated for the first time. In this review, neutron scattering is shown to be a powerful technique to reveal the magnetic structures of rare-earth films.     

Growth of Mn5Ge3 ultrathin film on Ge（111）

The growth of Mn5Ge3 ultrathin films with different thicknesses, prepared by solid phase epitaxy, is studied. The results of scanning tunnelling microscopy and low energy electron diffraction studies show that the film can be formed and it is terminated with a （√3 × √3） R30° surface reconstruction when the thickness of Mn exceeds 3 monolayers. The magnetic properties show that the Curie temperature is about 300 K and the T^2-dependent behaviour is observed to remain up to 220 K.     

Order-disorder criticality, wetting, and morphological phase transitions in the irreversible growth of far-from-equilibrium magnetic films

An exhaustive numerical investigation of the growth of magnetic films in confined (d+1)-dimensional stripped geometries (d=1,2) is carried out by means of extensive Monte Carlo simulations. Films in contact with a thermal bath at temperature T, are grown by adding spins having two possible orientations and considering ferromagnetic (nearest-neighbor) interactions. At low temperatures, thin films of thickness L are constituted by a sequence of well-ordered domains of average length l_DL. These domains have opposite magnetization. So, the films exhibit “spontaneous magnetization reversal” during the growth process. Such reversal occurs within a short characteristic length l_R, such that l_Dl_RL. Furthermore, it is found that for d=1 the system is non-critical, while a continuous order-disorder phase transition at finite temperature takes place in the d=2 case. Using standard finite-size scaling procedures, the critical temperature and some relevant critical exponents are determined. Finally, the growth of magnetic films in (2+1) dimensions with competing short-range magnetic fields acting along the confinement walls is studied. Due to the antisymmetric condition considered, an interface between domains with spins having opposite orientation develops along the growing direction. Such an interface undergoes a localization–delocalization transition that is the precursor of a wetting transition in the thermodynamic limit. Furthermore, the growing interface also undergoes morphological transitions in the growth mode. A comparison between the well-studied equilibrium Ising model and the studied irreversible magnetic growth model is performed throughout. Although valuable analogies are encountered, it is found that the non-equilibrium nature of the latter introduces new and rich physical features of interest.     

Structure and frustration in liquid crystalline polyacrylates II. Thin-film properties

We report on the thin-film behaviour of a polyacrylate with phenyl benzoate mesogenic side groups and a narrow polydispersity. Depending on the degree of polymerisation, these polymers show a nematic, a smectic-A _d, a re-entrant nematic and a C phase with a two-dimensional monoclinic lattice. X-ray reflectivity and atomic-force microscopy have been used to characterize the structure and surface morphology. The system exhibits two stable side-chain packing configurations with incommensurate spacings that can be both stabilized at a free surface. Thin films in the nematic phase show a structural dewetting induced by the growth of surface domains of the C phase. Additionally, surface-induced ripples with a nanoscale lateral period form at the air-film interface. We attribute these patterns to a coupling between the local liquid crystalline ordering of the mesogenic side groups and the surface curvature energy. Received 28 February 2001 and Received in final form 6 August 2001     

Cylindrical domain structure in metamagnets and its stability conditions

The possibility of the existence of a cylindrical magnetic domain structure near the first-order phase transition line is investigated theoretically in metamagnets, compounds with anisotropy energy substantially exceeding the exchange interaction energy. The domain structure of such compounds is related to the kinetics of the transition from the paramagnetic to the antiferromagnetic state. The static properties of isolated cylindrical magnetic domains and their equilibrium lattices are studied using an energetic approach. It is shown that for a low-temperature metamagnetic phase transition domains of the existence of a cylindrical domain structure, lattices and isolated cylindrical magnetic domains, abut, on the domain of plane-parallel domain structure existence from both sides. The features of their behavior are determined as a function of the magnitude of the external magnetic fields.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 84–88, March, 1988.The author is grateful to Yu. I. Gorobets for supporting the research and for useful remarks, and also to D. A. Yablonskii and I. M. Vitebskii for fruitful discussions.     

Anisotropy of antiferromagnetic 180° domains in magnetoelectric LiMPO<Subscript>4</Subscript> (M = Fe,Co, Ni)

An unusual variety of magnetic, magnetoelectric and ferrotoroidic properties was observed in the lithium-orthophosphates LiMPO₄ with M = Fe, Co, Ni by optical second harmonic generation. In spite of a largely similar magnetic and crystallographic structure the compounds exhibit pronounced differences in the topology of antiferromagnetic 180° domains. In LiCoPO₄ the antiferromagnetic domains coexist with ferrotoroidic, i.e., magnetic vortex domains. For LiNiPO₄ it was shown that the weak ferrimagnetic moment of the LiMPO₄ compounds along the spin direction is rigidly coupled to the AFM order parameter so that the sign of the magnetoelectric effect is reversed by a magnetic field only. Further effects of a static magnetic field on the system are discussed.     

On the theory of impurities in a lattice of magnetic ions: Crystalline field effects

We present a formalism for treating the problem of impurities in a lattice of magnetic rare earth ions. Latter are subject to a crystalline field and special attention is paid to non-Kramers ions in a singlet ground state. Our calculations are restricted to the paramagnetic regime. We derive the conditions for magnetic localized modes to occur and discuss the appearance of local magnetic instabilities. It is shown that the impurity effects are especially large if the system is close to a magnetic phase transition. Furthermore we compute the influence of impurities on the magnetic transition temperature. For the case of vacancies or nonmagnetic impurities the dependence of the Curie temperature on impurity concentration is derived. It is demonstrated that small amounts of impurities can often completely suppress magnetic ordering.     

Quantum creep in layered antiferromagnetic superconductor

In the mixed state of layered superconductor, the antiferromagnetic order of magnetic ions can create the spin-flop domains along the phase cores of the Josephson vortices, and this property impact upon the creep rate in the antiferromagnetic superconductor. The activation of the creep at constant temperature can be either thermal or quantum, depending on the intensity, or direction of the applied magnetic field. It is also shown that the action, and hence the activation energy, is rendered temperature dependent, when the damping and inertial mass of the vortex are included, so that the quantum tunnelling rate becomes temperature dependent below the crossover temperature.     

Phase composition and magnetic structure in nanocrystalline ferromagnetic Fe–N–O films

The chemical and phase compositions and structure of the Fe–N–O films produced by reactive dc magnetron sputtering (in Ar or Ar + N₂ gas mixture atmospheres) under different conditions (energy parameters of magnetron, residual pressure in the magnetron chamber after preliminary pumping, operating pressure in gas mixture) have been investigated by energy-dispersive X-ray spectroscopy, X-ray diffraction analysis, and vibrating sample magnetometry. Impurity of nitrogen and oxygen, which are present in the sputtered films, participate in the formation of their phase composition and determine its features. Some phenomena inherent in the nanocrystalline films in the metastable state were found. These are the formation of supersaturated bcc interstitial αFe-based solid solution and precipitation of α’ nitrous martensite with bct crystal lattice. The magnetic structure of the Fe–N–O films, which is characterized by the existence of stochastic domains discovered by correlation magnetometry method, is discussed in terms of the random anisotropy model. It was found that two modes of the magnetic anisotropy field of stochastic domains are formed, which determine the existence of two modes of the coercive field found in the magnetic hysteresis loops.     

Phase domain structures in cylindrical magnets under conditions of a first-order magnetic phase transition

The magnetic and resonance properties of cylindrical magnets at first-order phase transition from paramagnetic to ferromagnetic state were theoretically studied. It has been shown that in the external magnetic field directed perpendicularly to the rotation axis, formation of a specific domain structure of paramagnetic and ferromagnetic layers can be energetically favorable. The parameters of cylindrical phase domains as well as their dependences on temperature, magnetic field and material characteristics have been calculated. Peculiarities of the magnetic resonance spectra appearing as a result of the phase domain formation have been considered. Dependence of the resonance field of the system of ferromagnetic domains on magnetization and temperature has been obtained.     

Magnetic hyperfine data of non-crystalline iron from Mössbauer spectroscopy

The magnetic hyperfine (hf) data of non-crystalline iron samples, which were prepared in various ways, indicate that the different magnetization behaviour of non-crystalline (nc) and crystalline (crys) material cannot always be related to a random molecular field throughout the nc solid. For heterogeneous samples containing impurity clusters or voids the influence of fluctuations in the local magnetic concentration on the low energy magnetic density of states appears to be predominant.Supported by the Bundesministerium für Forschung und Technologie.     

Non-linear behaviour of diamagnetic moments in Condon domain phase

A model for non-linear behaviour of a magnetization in strong de Haas van Alphen effect is proposed. It describes correctly measured values of the magnetic induction splitting due to Condon domains in silver, beryllium and aluminium. It is shown that the characteristics of magnetization curve in Condon domain phase depend on temperature, magnetic field and Dingle temperature.     

Effective temperature, thermodynamical functions, and equilibrium properties of excited domain structure

The theory of the equilibrium properties of stripe domain structure (DS) of nonhomogeneous magnets, induced by a variable magnetic field, is developed. It is shown that an induced DS is a thermodynamic system characterized by an effective temperature which may be many orders of magnitude higher than the room temperature. For a DS of this kind, the thermodynamic functions are derived and the equilibrium conditions are determined. It is found that the entropy term in the free energy function is responsible for DS fragmentation under the variable field action, for the hysteretic dependence of the DS period on the frequency and the amplitude of the variable field, and for phase transitions attended with a jump in the number of domains.     

An electron in a quantized plane wave and in a constant magnetic field

The Dirac equation is considered for an electron in a constant homogeneous magnetic field and in a quantized electromagnetic plane wave propagating along the direction of the former. This problem is shown to be reduced to the Schrödinger equation for a system of many interacting oscillators. The corresponding Hamiltonian is diagonalized. An equation is established relating the total energy with the moment of a system along the magnetic field. Approximate solutions of this equation are given. A behaviour of a system close to cyclotron resonance is discussed.     

Ground state of magnetic dipoles on a two-dimensional lattice: structural phases in complex plasmas

We study analytically and by molecular dynamics simulations the ground state configuration of a system of magnetic dipoles fixed on a two-dimensional lattice. We find different phases, in close agreement with previous results. Building on this result and on the minimum energy requirement we determine the equilibrium lattice configuration, the magnetic order (ferromagnetic versus antiferromagnetic), and the magnetic polarization direction of a system of charged mesoscopic particles with magnetic dipole moments, in the domain where the strong electrostatic coupling leads to a crystalline ground state. Orders of magnitudes of the parameters of the system relevant to possible future dusty plasma experiments are discussed.     

An optically stimulated luminescence study of flint related to radiation dosimetry

This article describes the essential features of photo-stimulated luminescence in flint. The emission is shown to be complex, arising from a number of different aspectsof the material. Under constant illumination, dose-dependent time-decaying anti-Stokes signals are superimposed on dose-independent, time-stable emission that is both Stokes and anti-Stokes shifted in energy. It is argued that the dose-dependent signals arise from the crystalline quartz phase of the material, whereas the dose independent signals originate from an amorphous-like phase of SiO₂. The microcrystallinity of the material leads to contrasting behaviour when compared with bulk crystalline quartz.