Fcc Antiferromagnet Research Articles

Monte Carlo calculations of phase diagrams of Ising systems with competing interactions

Fcc antiferromagnets with nearest (Jnn) and next-nearest neighbor exchange (Jnnn) in a field H are studied by Monte Carlo methods. For Jnnn = 0, H = Hcl = 4¦Jnn¦ the “fully frustrated” system stays always disordered. In a centered rectangular lattice, layered structures (2×1), (3×1) and an incommensurate phase are found.

Spin glass in the site-diluted antiferromagnetic FCC

The diluted spin glass on the frustrated face-centered cubic lattice with the first and second neighbour interactions are considered by the square cactus approximation. Phase boundaries between the paramagnetic and ordered (ferromagnetic, superantiferromagnetic and spin glass) phases are obtained. The results qualitatively explain the experiments on EupSr1−pS and MnpCd1−pTe.

On the behavior of static susceptibilities in spin glasses

Linear and nonlinear susceptibilities of spin glasses in thermal equilibrium are qualitatively discussed in terms of magnetic short-range order. The apparent discrepancy between theoretical results, implying lack of Edwards-Anderson order and hence a divergence of the susceptibility at zero temperature, and experiments showing a susceptibility saturating at finite values, is considered. It is suggested that the susceptibility must indeed have a static maximum if the system exhibits a (“frustrated”) ferromagnetic phase with a reentrant phase boundary. At the reentrancy point, some exponents of the ferromagnet take twice their normal value, and hence a crossover near this point occurs similar to multicritical points. These observations are used to interpret a number of recent experiments, and it is shown that neither of them proves the existence of a static spin glass phase. As a quantitative example of gradual onset of order without a phase transition in three-dimensional systems, numerical results for susceptibility and specific heat of the fully frustrated Ising fcc antiferromagnet at its critical field are given.

Type I FCC antiferromagnets in a magnetic field: a realisation of the q=3- and q=4-state Potts models

It is suggested that phase transitions induced by a magnetic field H//(111) in certain type I FCC antiferromagnets provide a physical realisation of the q=3- and q=4-state Potts models. The (H, T) phase diagram associated with these antiferromagnets for a field lying in the (110) plane is studied using mean field approximation and renormalisation group calculations in d=- epsilon dimensions. The phase diagram exhibits a critical and two first-order surfaces connected by lines to tricritical and polycritical points.

Dynamics of the fcc Heisenberg antiferromagnet with nearest-neighbor interactions. Fully occupied lattice

Numerical techniques are used to investigate the spin dynamics of the nearest-neighbor fcc Heisenberg antiferromagnet on the fully occupied lattice. The dynamic structure factor and the distribution of modes are calculated in the harmonic approximation. Despite the absence of three-dimensional long-range order the dynamics of the disordered antiferromagnet resembles the dynamics of a fcc antiferromagnet with type-III order.

Sharp Transition from Ferromagnetic to Antiferromagnetic State in fcc Fe-Co Particles Precipitated in Cu Matrix

Mossbauer and magnetic measurements were made on Fe-Co particles precipitated in a Cu matrix in order to investigate magnetic properties of fcc Fe-Co alloys around the concentration for the ferromagnetic to antiferromagnetic transition, using the samples Cu 98 (Fe 1- x Co x ) 2 with x =0.3 ∼0.45. The antiferromagnetic fcc Fe-Co with a small moment was observed at x =0.3, whereas the ferromagnetic fcc Fe-Co with nearly the full moment appears and coexists with the antiferromagnetic fcc Fe-Co for x =0.35 ∼0.4. The antiferromagnetic Fe-Co disappears around x =0.45. This sharp magnetic transition associated with a small change in the composition is quite similar to the case of Fe-Ni Invar alloys.

The ground state of an impure FCC Ising antiferromagnet

It is shown that the almost pure FCC Ising antiferromagnet in its ground state has the same layered antiferromagnetic order as the pure FCC antiferromagnet. It is argued that this ground state persists until the concentration of magnetic sites falls to about 0.4. For concentrations between 0.18 and 0.4 the ground state is a spin glass.

Fluctuation-induced first-order transitions and symmetry-breaking fields. II. Systems with no stable fixed points

The crossover from first order to continuous transition induced by symmetry-breaking field, $g$, is analyzed for several models possessing no stable fixed point. Specifically, the ($g,T$) phase diagram associated with $n=6$ type-I fcc antiferromagnets (such as U${\mathrm{O}}_{2}$) and with $n=4$ type-II fcc antiferromagnets (such as TbP, TbAs, CeTe, and TbSe) is considered. The symmetry-breaking field $g$ corresponds to a magnetic field or to a uniaxial stress in certain symmetry directions. The phase diagrams are studied using large-$g$ expansions, mean-field calculations, and renormalization-group techniques in $d=4\ensuremath{-}\ensuremath{\epsilon}$ dimensions. It is found that in both cases the phase diagram is rather complex exhibiting tricritical points and critical end points.

Fluctuation induced first order transitions and symmetry breaking fields

Model Hamiltonians which possess no stable fixed point or which lie outside the domain of attraction of their stable fixed point, are known to yield first order transitions within the renormalization group approach. By applying a symmetry breaking field, g, a continuous transition may be restored. The crossover from first order to continuous transition induced by symmetry breaking fields is analyzed. Two Landau-Ginzburg-Wilson models are considered: (a) the n = 6-component model associated with type-I fcc antiferromagnets (such as UO2), and (b) the n = 4-component model associated with type-II fcc antiferromagnets (such as TbP, TbAs, CeTe and TbSe). The symmetry breaking field corresponds to a magnetic field or a uniaxial stress. The phase diagrams are studied using large g expansions, means field calculations, and renormalization group techniques in d = 4−ε dimensions. It is found that the (g,T) phase diagrams are rather complex exhibiting fourth order critical points, tricritical points and critical end points.

Magnetic phase transitions in the Pd2−xCuxMnIn system

The Heusler alloys have the L21 structure and occur at the compoistion X2MnZ. The majority, such as Cu2MnIn, are ferromagnetic with a moment of about 4 μB at the Mn sites but some, such as Pd2MnIn, are antiferromagnetic. The Pd2MnIn1−xSnx and Pd2MnIn1−ySby [1,2] series have magnetic structures that change from antiferromagnetic fcc type 2 to type 3A to ferromagnetism as the proportions of the Z site atoms In/Sn/Sb are varied. Recently two models have been proposed to explain the 3d exchange interactions in Heusler alloys. Stearns [3] suggests a model with three interactions and the X atoms playing a dominant role. Price [4] proposes a single interaction dependent only upon the average conduction electron concentration. Results are presented of magnetic and neutron diffraction measurements made on the new series Pd2−xCuxMnIn, in which the conduction electron concentration is varied via the X rather than the Z sites. The changes in magnetic structure observed are similar to those that occur in the series Pd2MnIn/Sn/Sb. The implications of the results are discussed.

Ordering of the Face-Centered-Cubic Lattice with Nearest-Neighbor Interaction

The phase diagram of an Ising fcc antiferromagnet in a field is investigated by a study of the ground state and Monte Carlo simulation of a lattice with 16 384 sites. At the critical field between the two "ordered" phases, the disordered phase is stable down to zero temperature due to "frustration" effects. The corresponding alloy phase diagram disagrees with all previous calculations. Order parameters and internal energy are briefly compared to experimental data on the Cu-Au system.

Phase transitions in n=4 type-II antiferromagnets

The Landau-Ginzburg-Wilson (LGW) Hamiltonian associated with n=4 type-II FCC antiferromagnets is discussed. It is shown that the model is expected to exhibit a first-order transition in d=3 dimensions. Recent experimental results on CeS, CeSe and CeTe are discussed.

Magnetic order in palladium-based heusler alloys: Part 2: Pd2MnIn1− ySby

Magnetization, susceptibility, X-ray and neutron-diffraction measurements have been on the Heusler alloy series Pd 2 MnIn 1− y Sb y , for 0 < y < 1. All the alloys were chemically and magnetically ordered with a moment of ≃4.2μ B located at the Mn sites. Their magnetic structures change from antiferromagnetic fcc type 2 at the In rich end, to antiferromagnetic fcc type 3A and then to ferromagnetism as the Sb content is increased. The results closely parallel those already reported on the isostructural series Pd 2MnIn 1- x Sn x and Pd 2MnSn 1- x Sb x with the changes in magnetic structures o ccurring at the same conduction electron concentrations. The results strongly support the postulate that the exchange interactions in Heusler alloys are essentially dependent upon the conduction electron concentration. The correlation between the data and an indirect double resonance exchange coupling mechanism is discussed.

Nuclear and atomic models

A theorem concerning fermion interaction is postulated and applied to the problems of atomic (electronic) and nuclear physics. Model building basedsolely upon the postulate thatadjacent like fermions must be singlet paired accounts for the closed shells ofboth nuclear and atomic structure. The implied antiferromagnetic FCC lattice of protons and neutrons in alternating layers has been found previously to be the lowest-energy solid configuration of nuclear matter (N=P) (Canuto and Chitre, 1974). The buildup of the FCC lattice from a central tetrahedron reproduces all of the shells and subshells of the isotropic harmonic oscillator, which of course is the basis for the shell model. In atomic structure, the singlet pairing of adjacent electrons implies closed-shell structures uniquely at the six noble gases and the three noble metals, Ni, Pd, and Pt. The basis for the postulate concerning fermions is found in terms of classical electrodynamics; it is a microscopic corollary of Biot-Savart's law that parallel currents attract whereas antiparallel currents repel.

Field-Dependent Magnetic Phase Transitions in Mixed-Valent TmSe

A neutron diffraction study of the field-dependent magnetic ordering in TmSe is reported. The magnetic strucutre in zero field is antiferromagnetic fcc type I with ${T}_{\mathrm{N}}=3.2$ K. The magnetic phase diagram may be understood as a successive domain reorientation and metamagnetic transitions for $T&lt;3$ K with increasing field. We can explain this quantitatively using a simple classical spin Hamiltonian. The mixed-valent character manifests itself mainly in a reduced moment and in a markedly altered crystal field.

Magnetic ordering of rare earth monochalcogenides. II. Neutron diffraction investigation of terbium sulphide, telluride and holmium telluride

For pt.I see ibid., vol.7, p.2023 (1974). Polycrystalline TbxS, TbxTe and HoxTe (0.85<or=x<or=1) with NaCl structure show antiferromagnetic FCC II order. The magnetic moments of TbxS and HoxTe are found to be oriented similarly to TbxSe, and equivalent monopnictides, and correspond to theoretical predictions based on exchange interactions and crystal-field effects. Corresponding to the orientation of the magnetic moments parallel to the direction (111), TbxS is trigonally distorted at 6K. The magnetic moments of HoxTe deviate 68 degrees from the (111) axis. In the case of TbxTe different angles of the magnetic moments (0 and 78 degrees ) with respect to the (111) direction were found for two investigated samples. Thus the magnetic order of rare earth monochalcogenides appears to be essentially modified by structure defects and non-stoichiometry.

Measurement of Spin-Wave Dispersion in NiO by Inelastic Neutron Scattering and Its Relation to Magnetic Properties

Inelastic neutron scattering techniques have been used to measure the spin-wave dispersion relations at 78\ifmmode^\circ\else\textdegree\fi{}K in the fcc antiferromagnet NiO. The energy dispersion has a steep initial slope (\ensuremath{\sim}250 meV \AA{}) and a high maximum energy (\ensuremath{\sim}117 meV) and is further characterized by a relatively low zone boundary energy in certain directions. The exchange parameters defined by ${\mathcal{H}}^{1,2}={J}_{j}{\stackrel{\ensuremath{\rightarrow}}{\mathrm{S}}}^{(1)}\ifmmode\cdot\else\textperiodcentered\fi{}{\stackrel{\ensuremath{\rightarrow}}{\mathrm{S}}}^{(2)}$ were determined by fitting the theoretical expression for the spin-wave energies to the experimental data corrected for instrumental resolution effects. The predominant interaction is a large antiferromagnetic exchange ${J}_{2}=221\ifmmode^\circ\else\textdegree\fi{}$K (19.01 meV) between next-nearest neighbors, which are linked by a 180\ifmmode^\circ\else\textdegree\fi{} superexchange path. The interaction between nearest neighbors, linked by a 90\ifmmode^\circ\else\textdegree\fi{} ${\mathrm{Ni}}^{2+}$---${\mathrm{O}}^{2\ensuremath{-}}$---${\mathrm{Ni}}^{2+}$ configuration, is much smaller and ferromagnetic in sign, ${J}_{1}=\ensuremath{-}15.9\ifmmode^\circ\else\textdegree\fi{}$K (-1.37 meV). A consequence of the relatively small value of ${J}_{1}$ is that the spin waves from the four domains present in the sample can only be resolved in a limited region of reciprocal space. These values of exchange interactions are in accord with simple ideas of covalency and overlap, and the results emphasize the behavior of NiO as a weakly covalent insulator. The density of magnon states, estimates of the transition temperature, and several thermomagnetic properties of NiO have been calculated from the measured exchange parameters using molecular field and random-phase-approximation Green's-function formulas.