The magnetic phase diagram at zero external field of an ensemble of dipoles with uniaxial anisotropy on a FCC lattice is investigated from tempered Monte Carlo simulations. The uniaxial anisotropy is characterized by a random distribution of easy axes and its magnitude $λ_u$ is the driving force of disorder and consequently frustration. The phase diagram, separating the paramagnetic, ferromagnetic, quasi long range ordered ferromagnetic and spin-glass regions is thus considered in the temperature, $λ_u$ plane. This system is aimed at modeling the magnetic phase diagram of supracrystals of magnetic nanoparticles.
Dipolar dislocation loops, prevalent in fcc metals, are widely recognized as controlling many physical aspects of plastic deformation. We present results of 3D dislocation dynamics simulations that shed light on the mechanisms of their formation, motion, interactions, and large-scale patterning. We identify two main formation mechanisms, enabled by cross-slip, and show that arrays of dipoles can be easily formed as a result of the interaction between glide screw dislocations. We present a systematic analysis of the spectrum of possible junctions that can form as a result of mutual interaction …
Results are presented on analytic and computational analyses of the spin states associated with a 3D fcc lattice composed of ABC stacked kagome planes of magnetic ions with only long-range dipole-dipole interactions. Extending previous work on the 2D kagome system, where discrete six-fold discrete degeneracy of the ground state was revealed [Holden et al. Phys. Rev. B 91, 224425 (2015)], we show that the 3D lattice exhibits a continuous degeneracy characterized by just two spherical angles involving six sublattice spin vectors. Application of a Heat Bath Monte Carlo algorithm shows that therma…
The magnetic phase diagram at zero external field of an ensemble of dipoles with uniaxial anisotropy on a FCC lattice has been investigated from tempered Monte Carlo simulations. The uniaxial anisotropy is characterized by a random distribution of easy axes and its magnitude $λ_u$ is the driving force of disorder and consequently frustration. The phase diagram, separating the paramagnetic, ferromagnetic and spin-glass regions, was thus considered in the temperature, $λ_u$ plane. Here we interpret this phase diagram in terms of the more convenient variables namely the bare dipolar interaction a…
Based on a simple general relation for the Lorentz field, the precise values are obtained for the energies of ferromagnetic and basic antiferromagnetic states in sc, bcc, fcc, and diamond cubic lattices. Within both the 'nearest-neighbor' and group-theoretic approaches, a large variety of antiferromagnetic states revealed as corresponding to the lowest energies is addressed, with recognizing their spherical spin-orientation degeneracy, which turns out to be rather special in fcc lattices.
A diluted FCC magnet with modified long-range RKKY interaction and arbitrary Ising spin S is considered within two-sublattice model. In the molecular field approximation the Gibbs free-energy is derived, from which all magnetic thermodynamic properties can be self-consistently obtained. In particular, the phase diagrams are studied for different magnetic ion and free-charge concentration, the atomic short-range-order (Warren-Cowley) parameter being taken into account.
The relative stability among the multiple spin density wave (MSDW) states in fcc transition metals has been investigated on the basis of a Ginzburg-Landau type of free energy with terms up to the fourth order in magnetic moments. Obtained magnetic phase diagrams in the space of expansion coefficients indicate the possibility of various 3Q MSDW states in fcc transition metals: the commensurate 3Q state, the incommensurate linear 3Q state, and the incommensurate helical 3Q state. It is shown that these 3Q states are always stabilized, when they are compared with the corresponding 2Q and 1Q state…
The phase diagram of the Al-Li system was determined by means of first principles calculations in combination with the cluster expansion formalism and statistical mechanics. The ground state phases were determined from first principles calculations of fcc and bcc configurations in the whole compositional range while the phase transitions as a function of temperature were ascertained from the thermodynamic grand potential and the Gibbs free energies of the phases. Overall, the calculated phase diagram was in good agreement with the currently accepted experimental phase diagram but the simulatio…
A prototype Hamiltonian for the generic patterned magnetic structures, of dipolar interaction with perpendicular anisotropy, is investigated within the finite-size framework by Landau-Lifshift-Gilbert classical spin dynamics. Modifications on the ground state phase diagram are discussed with an emphasis on the disappearance of continuous degeneracy in the ground state of in-plane phase due to the finite lattice size effect. The symmetry-governed ground state evolution upon the lattice size increase provides a critical insight into the systematic transition to the infinite extreme.
The compositional ordering of Ag, Pb, Sb, Te ions in (AgSbTe$_{2}$)$_{x}$(PbTe)$_{2(1-x)}$ systems possessing a NaCl structure is studied using a Coulomb lattice gas (CLG) model on a face-centered cubic (fcc) lattice and Monte Carlo simulations. Our results show different possible microstructural orderings. Ordered superlattice structures formed out of AgSbTe$_{2}$ layers separated by Pb$_{2}$Te$_{2}$ layers are observed for a large range of $x$ values. For $x=0.5$, we see an array of tubular structures formed by AgSbTe$_{2}$ and Pb$_{2}$Te$_{2}$ blocks. For $x=1$, AgSbTe$_{2}$ has a body-cent…
We investigate freezing characteristics of a finite dipolar hexagonal plate by the Monte Carlo simulation. The hexagonal plate is cut out from a piled triangular lattice of three layers with FCC-like (ABCABC) stacking structure. In the present study an annealing simulation is performed for the dipolar plate uniaxially compressed in the direction of layer-piling. We find spin melting and refreezing driven by the uniaxial compression. Each of the melting and refreezing corresponds one-to-one with a change of the ground states induced by compression. The freezing temperatures of the ground-state …
We explore the equilibrium properties of a two-dimensional Ising spin model with short-range exchange and long-range dipolar interactions as a function of the applied magnetic field H. The model is studied through extensive Monte Carlo simulations that show the existence of many modulated phases with long range orientational order for a wide range of fields. These phases are characterized by different wave vectors that change discontinuously with the magnetic field. We provide numerical evidence supporting the existence of first order transitions between these phases. At higher fields our resu…
We show that a crystal of mesoscopic Fe8 single molecule magnets is an experimental realization of the Quantum Ising Phase Transition (QIPT) model in a transverse field, with dipolar interactions. Quantum annealing has enabled us to explore the QIPT at thermodynamical equilibrium. The phase diagram and critical exponents we obtain are compared to expectations for the mean-field QIPT Universality class.
We investigated numerically the high-temperature/high-pressure phase diagram of Xenon as modelled through the exp-6 interaction potential, which is thought to provide a reliable description of the thermal behaviour of rare gases under extreme conditions. We performed a series of extensive NVT Monte Carlo simulations which, in conjunction with exact computation of the solid free energy by the Frenkel-Ladd method, allowed us to precisely locate the freezing and the melting thresholds at each temperature. We find that, under isothermal compression, the exp-6 fluid freezes directly into a FCC soli…
We use first-principles techniques to re-examine the suggestion that transitions seen in high-P experiments on Mo are solid-solid transitions from the bcc structure to either the fcc or hcp structures. We confirm that in the harmonic approximation the free energies of fcc and hcp structures become lower than that of bcc at P > 325 GPa and T below the melting curve, as reported recently. However, we show that if anharmonic effects are fully included this is no longer true. We calculate fully anharmonic free energies of high-T crystal phases by integration of the thermal average stress with resp…
This thesis is a theoretical study of thermodynamic, point-defect formation and diffusion properties in Fe-Ni alloys with a focus on the magnetochemical effects. The results are derived from density functional theory (DFT) calculations and Monte Carlo (MC) simulations using a DFT-parametrized effective interaction model (EIM) with explicit atomic and spin variables. The first part of this work is focused on thermodynamics. We compute via DFT energetic, magnetic and vibrational properties and the bcc-fcc phase diagram, revealing the relative importance between magnetic and vibrational entropies…
A class of proposed coherent precipitate structures (Guinier-Preston zones) in the Al-Mg-Si alloy are investigated using first-principles density functional theory methods. The cluster expansion method is used to extract effective interaction parameters, providing the means for large scale energy calculations of alloy structures. The Mg$_1$Si$_1$ L$1_0$ structure and structures related to the Mg$_5$Si$_6$ $β''$ phase are studied in more detail, and e.g., precipitate/matrix interface energies are presented. Using direct first-principles calculations we show that the former phase is dynamically …
Iron, cobalt and nickel nanoparticles, grown in the gas phase, are known to arrange in chains and bracelet-like rings due to the long-range dipolar interaction between the ferromagnetic (or super-paramagnetic) particles. We investigate the dynamics and thermodynamics of such magnetic dipolar nanoparticles for low densities using molecular dynamics simulations and analyze the influence of temperature and external magnetic fields on two- and three-dimensional systems. The obtained phase diagrams can be understood by using simple energetic arguments.
The liquid-gas phase diagram for polydisperse dipolar hard-sphere fluid with polydispersity in the hard-sphere size and dipolar moment is calculated using extension of the recently proposed thermodynamic perturbation theory for central force (TPT-CF) associating potential. To establish the connection with the phase behavior of ferrocolloidal dispersions it is assumed that the dipole moment is proportional to the cube of the hard-sphere diameter. We present and discuss the full phase diagram, which includes cloud and shadow curves, binodals and distribution functions of the coexisting daughter …
Motivated by recent experimental results on GdRu$_2$Si$_2$ [Khanh, N.D., Nakajima, T., Yu, X. et al., \emph{Nat. Nanotechnol.} \textbf{15}, 444-449 (2020)], where nanometric square skyrmion lattice was observed, we propose simple analytical mean-field description of the high-temperature part of the phase diagram of centrosymmetric tetragonal frustrated antiferromagnets with dipolar interaction in the external magnetic field. In the reciprocal space dipolar forces provide momentum dependent biaxial anisotropy. It is shown that in tetragonal lattice in the large part of the Brillouin zone for mu…
The phase diagram of colloidal hard superballs, of which the shape interpolates between cubes and octahedra via spheres, is determined by free-energy calculations in Monte Carlo simulations. We discover not only a stable face-centered cubic (fcc) plastic crystal phase for near-spherical particles, but also a stable body-centered cubic (bcc) plastic crystal close to the octahedron shape, and in fact even coexistence of these two plastic crystals with a substantial density gap. The plastic fcc and bcc crystals are, however, unstable in the cube and octahedron limit, suggesting that the rounded c…
In this article mixed CuInP$_2$(S$_x$Se$_{1-x}$)$_6$ crystals were investigated by broadband dielectric spectroscopy (20 Hz - 3 GHz). The complete phase diagram has been obtained from these results. The phase diagram of investigated crystals is strongly asymmetric - the decreasing of ferroelectric phase transition temperatures in CuInP$_2$(S$_x$Se$_{1-x}$)$_6$ is much more flat with small admixture of sulphur then with small admixture of selenium. In the middle part of the phase diagram (x=0.4-0.9) the dipolar glass phase has been observed. In boundary region between ferroelectric order and di…
The phase diagram of adsorbate-induced row-type-alignments, such as missing-row reconstructions induced by adsorbate-atoms on the FCC(110) surface, is calculated by the Blume-Emmery-Griffiths (BEG) model. In the model, we introduce adatom-adatom and dipole-dipole interactions between nearest-neighbor (NN) and next-nearest-neighbor (NNN) rows. The calculation of the temperature versus adatom chemical potential phase diagram is performed using mean-field approximation. It is indicated that when NN and NNN interactions are competitive, there appear either dipole or coverage modulated (incommensur…
