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Single crystals of a honeycomb lattice antiferromagnet, ${\mathrm{Tb}}_{2}{\mathrm{Ir}}_{3}{\mathrm{Ga}}_{9}$, were synthesized, and the physical properties have been studied. From magnetometry, a long-range antiferromagnetic ordering at $\ensuremath{\approx}12.5$ K with highly anisotropic magnetic behavior was found. Neutron powder diffraction confirms that the Tb spins lie along the $\mathbf{a}$-axis, parallel to the shortest Tb-Tb contact. Two field-induced spin-flip transitions are observed when the field is applied parallel to this axis, separated by a plateau corresponding roughly to $M\ensuremath{\approx}{M}_{\mathrm{s}}/2$. Transport measurements show the resistivity to be metallic with a discontinuity at the onset of N\'eel order. Heat capacity shows a $\ensuremath{\lambda}$-like transition confirming the bulk nature of the magnetism. We propose a phenomenological spin Hamiltonian that describes the magnetization plateau as a result of strong Ising character arising from a quasidoublet ground state of the ${\mathrm{Tb}}^{3+}$ ion in a site of ${C}_{s}$ symmetry and expressing a significant bond-dependent anisotropy.

Thin flakes of Cr1/3NbS2 are fabricated successfully via microexfoliation techniques. Temperature-dependent and field-dependent magnetizations of thin flakes with various thicknesses are investigat...

Owing to their overall low energy scales, flexible molecular architectures, and ease of chemical substitution, molecule-based multiferroics are extraordinarily responsive to external stimuli and exhibit remarkably rich phase diagrams. Even so, the stability and microscopic properties of various magnetic states in close proximity to quantum critical points are highly under-explored in these materials. Inspired by these opportunities, we combined pulsed-field magnetization, first-principles calculations, and numerical simulations to reveal the magnetic field–temperature (B–T) phase diagram of multiferroic (NH4)2FeCl5⋅H2O. In this system, a network of intermolecular hydrogen and halogen bonds creates a competing set of exchange interactions that generates additional structure in the phase diagram—both in the vicinity of the spin flop and near the 30 T transition to the fully saturated state. Consequently, the phase diagrams of (NH4)2FeCl5⋅H2O and its deuterated analog are much more complex than those of other molecule-based multiferroics. The entire series of coupled electric and magnetic transitions can be accessed with a powered magnet, opening the door to exploration and control of properties in this and related materials.

International audience; The antiferromagnetic semiconductor MnTe has recently attracted attention for spintronics and high-performance thermoelectric applications. However, little is known about its vibrational and thermal transport properties and how these might relate to the electronic and magnetic structure, particularly as related to 3d Mn orbital correlations. Here, we calculate a physically justified Coulomb correlation parameter within the DFT+U framework. We couple this framework with the Heisenberg Hamiltonian and first-principles Boltzmann transport to understand the magnetic, vibrational, and phonon thermal transport properties of MnTe. We also perform inelastic neutron and nuclear inelastic x-ray scattering measurements of the total and partial phonon density of states, respectively. Very good agreement is obtained with the measured and calculated phonon density of states, and with available measurements for the band gap, local magnetic moments, Néel temperature, magnon dispersion, thermal conductivity, and phonon dispersion. This study demonstrates that the vibrational and magnetic degrees of freedom are not strongly coupled in MnTe, and provides a more comprehensive picture of this technologically promising material.

Electronic phase separation has been increasingly recognized as an important phenomenon in understanding many of the intriguing properties displayed in transition metal oxides. It is believed to produce fascinating functional properties in otherwise chemically homogenous electronic systems, e.g., colossal magnetoresistance manganites and high-${T}_{\mathrm{c}}$ cuprates. While many well-known electronically phase-separated systems are oxides, it has been argued that the same phenomenon should occur in other electronic systems with strong competing interactions. Here we report the observation of electronic phase separation in molecular $({\mathrm{ND}}_{4}{)}_{2}{\mathrm{FeCl}}_{5}\ifmmode\cdot\else\textperiodcentered\fi{}{\mathrm{D}}_{2}\mathrm{O}$, a type-II multiferroic. We show that two magnetic phases, one of which is commensurate and the other of which is incommensurate, coexist in this material. Their evolution under applied magnetic field produces emergent properties. In particular, our measurements reveal a field-induced exotic state linked to a direct transition from a paraelectric/paramagnetic phase to a ferroelectric/antiferromagnetic phase, a collective phenomenon that hasn't been seen in other magnetic multiferroics.

Due to the lack of suitable single crystals, the average g-factor of anisotropic polycrystalline samples are commonly estimated from either the Curie−Weiss susceptibility or the saturation magnetization. We show that the average g-factor obtained from the Curie constant is always greater than or equal to the average g-factor obtained from the saturation magnetization. The average g-factors are equal only for a single crystal or an isotropic polycrystal. We review experimental results for several compounds containing the anisotropic cation [Fe(C5Me5)2]+ and propose an experiment to test this inequality using a compound with a spinless anion.

Unlike the coercive field H c of a bulk ferrimagnet, which diverges at the compensation temperature T comp , the coercive field of a polycrystalline ferrimagnet with uni-axial anisotropy is shown to have a minimum at T comp . Despite this behavior, the field required for domain-wall motion still diverges at the compensation temperature. These ideas are used to treat a ferrimagnetic class of molecule-based magnets, the bimetallic oxalates, that exhibit a minimum coercivity at T comp .

Multiferroics permit the magnetic control of the electric polarization and the electric control of the magnetization. These static magnetoelectric (ME) effects are of enormous interest: The ability to read and write a magnetic state current-free by an electric voltage would provide a huge technological advantage. Dynamic or optical ME effects are equally interesting, because they give rise to unidirectional light propagation as recently observed in low-temperature multiferroics. This phenomenon, if realized at room temperature, would allow the development of optical diodes which transmit unpolarized light in one, but not in the opposite, direction. Here, we report strong unidirectional transmission in the room-temperature multiferroic BiFeO_{3} over the gigahertz-terahertz frequency range. The supporting theory attributes the observed unidirectional transmission to the spin-current-driven dynamic ME effect. These findings are an important step toward the realization of optical diodes, supplemented by the ability to switch the transmission direction with a magnetic or electric field.

The crossover from localized- to itinerant-electron behavior is associated with many intriguing phenomena in condensed-matter physics. In this paper, we investigate the crossover from localized to itinerant regimes in the spinel system Mn$_{1-x}$Co$_x$V$_2$O$_4$. At low Co doping, orbital order (OO) of the localized electrons on the V3+ ions suppresses magnetic frustration by triggering a tetragonal distortion. With Co doping, electronic itinerancy melts the OO and suppresses the structural phase transition while the reduced spin-lattice coupling produces magnetic frustration. Neutron scattering measurements and first-principles-guided spin models reveal that the non-collinear state at high Co doping is produced by weakened local anisotropy and enhanced Co-V spin interactions., 5 pages, 4 figures

Organic coatings often contain sufficient pigmentation that the pigment volume concentration (PVC) and the critical PVC (CPVC) must be carefully considered. Past work has shown that the CPVC can be identified with the random densest packing of pigment particles after considerations of the adsorbed polymer layer on the particles is taken into account, and that this packing can be predicted by empirical particle packing algorithms developed for mixtures of particle sizes. Recent studies have shown that local fluctuations of pigment and polymer concentrations in a film can lead to local film volumes that can exceed the CPVC even though the average PVC for the whole film (the global PVC) is below the CPVC. These effects are consistent with observations of void formation in films below the CPVC, and can be analyzed by local film statistics. We have also recently performed simulations of particle packing by the compression of multiple hard spheres that allows further insight into local concentration fluctuations in PVC within a film.

Many experiments have verified the presence of a spin-density wave (SDW) within the Cr spacer of Fe/Cr multilayers and wedges. We review the recently proposed interlayer magnetic coupling mediated by a SDW. Unlike previously proposed mechanisms, this magnetic coupling is strongly temperature-dependent. Depending on the temperature and the number N of Cr monolayers (ML), the SDW may be either commensurate (C) or incommensurate (I) with the BCC Cr lattice.

Several recent experiments have detected a spin-density wave (SDW) within the Cr spacer of Fe/Cr multilayers and wedges. We use two simple models to predict the behavior of a collinear SDW within an Fe/Cr/Fe trilayer. Both models combine assumed boundary conditions at the Fe-Cr interfaces with the free energy of the Cr spacer. Depending on the temperature and the number N of Cr monolayers, the SDW may be either commensurate (C) or incommensurate (I) with the bcc Cr lattice. Model I assumes that the Fe-Cr interface is perfect and that the Fe-Cr interaction is antiferromagnetic. Consequently, the I SDW antinodes lie near the Fe-Cr interfaces. With increasing temperature, the Cr spacer undergoes a series of transitions between I SDW phases with different numbers n of nodes. If the I SDW has $n=m$ nodes at $T=0$, then n increases by one at each phase transition from m to $m\ensuremath{-}1$ to $m\ensuremath{-}2$ up to the C phase with $n=0$ above ${T}_{\mathrm{IC}}(N).$ For a fixed temperature, the magnetic coupling across the Cr spacer undergoes a phase slip whenever n changes by one. In the limit $\stackrel{\ensuremath{\rightarrow}}{N}\ensuremath{\infty},$ ${T}_{\mathrm{IC}}(N)$ is independent of the Fe-Cr coupling strength. We find that ${T}_{\mathrm{IC}}(\ensuremath{\infty})$ is always larger than the bulk N\'eel transition temperature and increases with the strain on the Cr spacer. These results explain the very high $\mathrm{IC}$ transition temperature of about 600 K extrapolated from measurements on Fe/Cr/Fe wedges. Model II assumes that the I SDW nodes lie precisely at the Fe-Cr interfaces. This condition may be enforced by the interfacial roughness of sputtered Fe/Cr multilayers. As a result, the C phase is never stable and the transition temperature ${T}_{\mathrm{N}}(N)$ takes on a seesaw pattern as $ng~2$ increases with thickness. In agreement with measurements on both sputtered and epitaxially grown multilayers, model II predicts the I phase to be unstable above the bulk N\'eel temperature. Model II also predicts that the I SDW may undergo a single phase transition from $n=m$ to $m\ensuremath{-}1$ before disappearing above ${T}_{\mathrm{N}}(N).$ This behavior has recently been confirmed by neutron-scattering measurements on CrMn/Cr multilayers. While model I very successfully predicts the behavior of Fe/Cr/Fe wedges, a refined version of model II describes some properties of sputtered Fe/Cr multilayers.

Although absent in bulk transition metals, a noncollinear, helical (H) spin-density wave (SDW) is stabilized by steps at the interfaces in Fe/Cr multilayers. Using the random-phase approximation, we evaluate the phase boundary between the H SDW and the collinear, incommensurate (I) SDW found in bulk Cr. In agreement with neutron-scattering results, the I-to-H transition temperature T{sub IH} is always lower than the bulk N{acute e}el temperature T{sub N} and the nodes of the I SDW lie near the Fe-Cr interfaces. While a H SDW with a single {plus_minus}{pi}/2 twist has lower free energy than a I SDW above T{sub N} , H SDW{close_quote}s with larger twists are stable between T{sub IH} and T{sub N} . {copyright} {ital 1998} {ital The American Physical Society }

It is well known that Fe is the only impurity atom which exhibits Pauli paramagnetism below the Neel temperature of a dilutely doped Cr host. Magnetic measurements on two complementary Cr alloys now reveal that the Fe moment is about 6% larger in the incommensurate than in the commensurate spin-density wave state. A phenomenological model is presented which explains this difference by dividing the Fe moment into bound and unbound parts.

Based on a simple model with antiferromagnetic interactions at the Cr - Fe interfaces, we construct the magnetic phase diagram for the Cr spacer within Fe/Cr multilayers and wedges. The spin-density wave is found to undergo transitions between incommensurate (I) phases with a decreasing number of nodes n as the temperature increases for a fixed number N of Cr monolayers. In the limit , the incommensurate to commensurate (C) transition temperature from n = 1 to n = 0 nodes is independent of the Cr - Fe coupling strength but sensitively depends on the strain and doping. These results agree with recent neutron-scattering studies that the SDW inside Fe/Cr multilayers may be either C or I depending on the temperature and Cr thickness.

The high-temperature ferromagnet MnBi continues to receive attention as a candidate to replace rare-earth-containing permanent magnets in applications above room temperature. This is due to a high Curie temperature, large magnetic moments, and a coercivity that increases with temperature. The synthesis of MnBi also allows for crystals that are free of interstitial Mn, enabling more direct access to the key interactions underlying the physical properties of binary Mn-based ferromagnets. In this work, we use inelastic neutron scattering to measure the spin waves of MnBi in order to characterize the magnetic exchange at low temperature. Consistent with the spin reorientation that occurs below 140 K, we do not observe a spin gap in this system above our experimental resolution. A Heisenberg model was fit to the spin wave data in order to characterize the long-range nature of the exchange. It was found that interactions up to sixth nearest neighbor are required to fully parameterize the spin waves. Surprisingly, the nearest-neighbor term is antiferromagnetic, and the realization of a ferromagnetic ground state relies on the more numerous ferromagnetic terms beyond nearest neighbor, suggesting that the ferromagnetic ground state arises as a consequence of the long-ranged interactions in the system., Comment: 5 pages, 2 figures, 2 tables; Supplemental Information has 2 pages, 1 figure

The phase diagram of chromium alloys sensitively depends on the presence of an electron reservoir. With the formation of a spin-density wave (SDW), electrons are depleted from the nested electron-hole surfaces and for a finite reservoir the chemical potential will decrease. If the power \ensuremath{\rho} of the electron reservoir were infinite, then the chemical potential would remain constant and lightly doped CrMn alloys would experience a first-order transition from an incommensurate (I) to a commensurate (C) SDW state with decreasing temperature. With a power less than about 10, the reservoir will flip the phase boundary from one side of the triple point to the other, allowing a commensurate to incommensurate transition with decreasing temperature as observed experimentally. A finite reservoir also suppresses the first-order jumps in the SDW order parameter and wave vector. When \ensuremath{\rho}\ensuremath{\le}2, the CI transition is second order for all temperatures. When \ensuremath{\rho}g2, the transition is second order near the triple point but first order at lower temperatures.

The diruthenium compound [Ru2(O2CMe)4]3[Cr(CN)6] contains two interpenetrating sublattices that behave like giant antiferromagnetically-coupled moments with strong anisotropy. The preferred orientations of the total moment of each sublattice are determined from susceptibility measurements on a polycrystalline sample. In agreement with previous mean-field calculations for the magnetic ground state, the fits to the experimental magnetization imply that the sublattice moments are restricted to cubic diagonals rather than the cubic axis or the edge diagonals. The parameterization of the sublattice susceptibility indicates that the sublattice spin states are more distorted when they are aligned antiparallel.

Using mean field theory, we have constructed the phase diagram of chromium alloys. At zero temperature, we find a first-order phase transition from the incommensurate state with manganese doping. Within a small window of manganese concentrations, a first-order phase transition from the commensurate to the incommensurate states occurs with decreasing temperature, in agreement with experiments performed over 25 years ago on CrMn alloys.

The conventional theory of organic coatings assumes that the volume density of pigment particles is uniform throughout the sample. The coating is then described in terms of the pigment volume concentration Π, which equals the volume of pigment divided by the volume of pigment and polymer. Voids form in the coating when the pigment particles are randomly dense-packed, which implies that Π exceeds the critical pigment volume concentration (Φ c or that Λ=Π/Φ c > 1. Due to fluctuations in the local density of pigment however, some regions in the coating may become randomly dense-packed even below Φ c . Hence, voids may form in the densely-packed islands even when Λ N o is the smallest number of pigment particles in a densely-packed cluster that may contain a void; C q is the coarseness of the polymer space-filling in the volume of the sample not occupied by pigment. When C q =0 and Π > 1, the polymer completely fills the interstitial volume and the void concentration vanishes. But when C q > 0, voids may form in the densely-packed regions of the coating even below Φ c . The coarseness parameter C q , depends on the sample preparation, on the properties of the pigment and polymer, and on the pigment volume concentration Π. For any nonzero C q we conclude that optical measurements will systematically underestimate Φ c . On the other hand, since the density of polymer is less than half the density of pigment, the peak in the mass density p (Π) of the coating will overestimate Φ c . Unless C q is abnormally large, the void percolation threshold value Φ v is larger than Φ c and is a decreasing function of the coarseness C q . The predictions of this simple model are in good agreement with experiment, and are relevant to the general class of random concentrated composites which include organic coatings, ceramic polymer slips, composite solid polymer electrodes and some forms of battery separators.

We study the net chirality created by the Dzyaloshinskii-Moriya interaction (DMI) at the boundary between hexagonal layers of magnetic and nonmagnetic materials. It is shown that another mechanism besides elastic torsion is required to understand the change in chirality observed in Dy/Y multilayers during field cooling. This Rapid Communication demonstrates that, due to the overlap between magnetic and nonmagnetic atoms, interfacial steps may produce a DMI normal to the interface in magnetic heterostructures.

It is usually assumed that the pigment and polymer concentrations are homogeneously distributed in an organic coating. This would imply that voids only appear when the pigment volume concentration (PVC) p exceeds the critical pigment volume concentration (CPVC) pc. However, a great deal of experimental work suggests that voids actually appear even below the CPVC. In this article, we present a phenomenological theory for the effects of density fluctuations on the formation of voids both below and above the CPVC. If the distribution of the local PVC p(x) is Gaussian, then our theory contains two phenomenological parameters: a coarseness parameter Cp which is proportional to the width of the Gaussian distribution, and the diameter d0 of the smallest densely packed cluster of pigment particles. When Cp=0, the homogeneous model of void formation is recovered; but for any nonzero coarseness, voids will appear even below the CPVC. In agreement with earlier work, we find that optical measurements will underestima...

Several molecule-based magnets can be constructed from diruthenium tetracarboxylate building blocks. We study two such materials with antiferromagnetic interactions between Cr(III) ions and diruthenium paddle-wheel complexes. While collinear magnetic ordering in the threedimensional compound is frustrated by the easy-plane anisotropy on the diruthenium species, the two-dimensional compound has a collinear ferrimagnetic ground state because of the dominant coupling between the Cr ions and the in-plane diruthenium species.

Because of the competition between the spin-orbit coupling and the Jahn-Teller (JT) energies in Fe(II)Fe(III) bimetallic oxalates, we theoretically predict that an undistorted phase with ${C}_{3}$ symmetry about each Fe site may be recovered at low temperatures. Both lower and upper JT transitions bracketing the ferrimagnetic transition temperature ${T}_{c}$ are predicted for compounds that exhibit magnetic compensation. Comparisons with recent measurements and first-principles calculations provide strong evidence for the inverse JT transition below ${T}_{c}$.

Bimetallic oxalates are layered molecule-based magnets with transition metals M(II) and M'(III) coupled by the oxalate bridges ox=C2O4 in an open honeycomb structure. Fe(II)Fe(III) bimetallic compounds with spins S = 2 and S′ = 5/2 ferrimagnetically order below a transition temperature Tc that ranges from 30 to 48 K, depending on the cation between the layers. In small magnetic fields, several of these compounds exhibit "giant negative magnetization" below a compensation temperature of about 0.62 Tc. By studying the behavior of the orbital-doublet ground state produced by a C3-symmetric crystal field, we construct a reduced Hamiltonian that contains both exchange and spin-orbit interactions. This Hamiltonian is used to explain all of the important behavior of the Fe(II)Fe(III) bimetallic oxalates, including the stability of magnetic order in well-separated two-dimensional layers and the magnetic compensation in compounds with high transition temperatures. Fitting the parameters of this reduced Hamiltonian, we estimate that the z component of the orbital angular momentum of the ground-state doublet in compounds exhibiting magnetic compensation is about 0.27 and that the antiferromagnetic exchange coupling Jc between the Fe(II) and Fe(III) spins is about 0.45 meV. In a magnetic field perpendicular to the bimetallic layers, a spin-flop transition is predictedmore » at a field of about 3Jc/μB ≈ 25 T. A Holstein-Primakoff 1/S and 1/S′ expansion is used to evaluate the spin-wave spectrum and to estimate the spin-wave gap sw ≈ 1.65 meV in compounds that exhibit magnetic compensation. This significant spin-wave gap explains the stability of magnetic order in well-separated bimetallic layers. We predict that the negative magnetization can be optically reversed by near-infrared light. Breaking the C3 symmetry about each of the Fe(II) ions either through a cation-induced distortion or uni-axial strain in the plane of the bimetallic layer is expected to increase the magnetic compensation temperature.« less

Bimetallic oxalates are a class of layered organic magnets with transition metals M(II) and M'(III) coupled by oxalate molecules in an open honeycomb structure. Energy, structure, and symmetry considerations are used to construct a reduced Hamiltonian, including exchange and spin-orbit interactions, that explains the giant negative magnetization in some of the ferrimagnetic Fe(II)Fe(III) compounds. We also provide new predictions for the spin-wave gap, the effects of uniaxial strain, and the optical flipping of the negative magnetization in Fe(II)Fe(III) bimetallic oxalates.

The magnetic phase diagrams of all dilute Cr alloys can be explained by a simple theoretical model with coupled spin- and charge-density waves and a finite electron reservoir. If the charge-density wave and electron reservoir are sufficiently large, the paramagnetic to commensurate spin-density wave transition becomes strongly first order, as found in Cr{sub 1- x}Fe{sub x} and Cr{sub 1-x}Si{sub x} alloys. The observed discontinuity of the slope dT{sub N}/dx at the triple point and the bending of the CI phase boundary are also natural consequences of this model.

The magnetic phase diagram of chromium alloys sensitively depends on both electron damping and the presence of an electron reservoir. If the damping energy [Gamma] vanishes and the power [rho] of the reservoir is infinite, then lightly doped CrMn alloys would experience a first-order transition from an incommensurate ([ital I]) to a commensurate ([ital C]) spin-density wave (SDW) state with decreasing temperature. Either damping or a finite reservoir may flip the phase boundary from one side of the triple point to the other, allowing a commensurate-to-incommensurate transition with decreasing temperature as observed experimentally. Both damping and a finite reservoir suppress the first-order jumps in the SDW order parameter and wave vector. When [rho][le]2, the [ital C]-[ital I] transition is second order for all temperatures. When [rho][gt]2, the transition is second order near the tricritical point but first order at lower temperatures. Unlike electron damping, an electron reservoir does not shift the paramagnetic phase boundary and triple point.

${\mathrm{Cd}}_{2}{\mathrm{Os}}_{2}{\mathrm{O}}_{7}$ crystallizes in the pyrochlore structure and undergoes a metal-insulator transition (MIT) near 226 K. We have characterized the MIT in ${\mathrm{Cd}}_{2}{\mathrm{Os}}_{2}{\mathrm{O}}_{7}$ using x-ray diffraction, resistivity at ambient and high pressure, specific heat, magnetization, thermopower, Hall coefficient, and thermal conductivity. Both single crystals and polycrystalline material were examined. The MIT is accompanied by no change in crystal symmetry and a change in unit-cell volume of less than 0.05%. The resistivity shows little temperature dependence above 226 K, but increases by 3 orders of magnitude as the sample is cooled to 4 K. The specific heat anomaly resembles a mean-field transition and shows no hysteresis or latent heat. ${\mathrm{Cd}}_{2}{\mathrm{Os}}_{2}{\mathrm{O}}_{7}$ orders magnetically at the MIT. The magnetization data are consistent with antiferromagnetic order, with a small parasitic ferromagnetic component. The Hall and Seebeck coefficients are consistent with a semiconducting gap opening at the Fermi energy at the MIT. We have also performed electronic structure calculations on ${\mathrm{Cd}}_{2}{\mathrm{Os}}_{2}{\mathrm{O}}_{7}.$ These calculations indicate that ${\mathrm{Cd}}_{2}{\mathrm{Os}}_{2}{\mathrm{O}}_{7}$ is metallic, with a sharp peak in the density of states at the Fermi energy. We interpret the data in terms of a Slater transition. In this scenario, the MIT is produced by a doubling of the unit cell due to the establishment of antiferromagnetic order. A Slater transition---unlike a Mott transition---is predicted to be continuous, with a semiconducting energy gap opening much like a BCS gap as the material is cooled below ${T}_{\mathrm{MIT}}.$

Many experiments have verified the presence of a spin-density wave (SDW) within the Cr spacer of Fe/Cr multilayers and wedges. The authors review the recently-proposed interlayer magnetic coupling mediated by a SDW. Unlike previously proposed mechanisms, this magnetic coupling is strongly temperature-dependent. Depending on the temperature and the number N of Cr monolayers (ML), the SDW may be either commensurate (C) or incommensurate (I) with the bcc Cr lattice.

The phase diagram of chromium alloys is remarkably rich. At the Neel temperature of 310 K, pure chromium undergoes a weakly first-order phase transition into an incommensurate spin-density wave (SDW) state. When doped with more than 0.2% manganese, this transition becomes second order and the SDW becomes commensurate. Over 25 years ago, Koehler [ital et] [ital al]. and Komura, Hamaguchi, and Kunitomi observed a first-order commensurate-to-incommensurate (CI) transition in CrMn alloys. The temperature of this CI transition decreased to zero as the manganese concentration increases from about 0.2% to about 1.5%. Using mean-field theory, we have constructed the free energy and phase diagram of chromium alloys in the presence of electron scattering. In the absence of scattering, the phase diagram allows a first-order phase transition from the incommensurate to the commensurate states with decreasing temperature. But if the damping is sufficiently large, the phase-separation curve flips from the right side of the tricritical point to the left. So within a small window of manganese concentrations, the commensurate state undergoes a first-order transition into the incommensurate state with decreasing temperature, in agreement with the experiments of Koehler [ital et] [ital al]. At zero temperature, we find a first-order phase transition frommore » the incommensurate to the commensurate state with manganese doping, in agreement with the work of Komura, Hamaguchi, and Kunitomi. In the absence of damping, the zero-temperature energy gap [Delta](0) in the commensurate regime is independent of manganese concentration. But in the presence of damping [Delta](0) becomes an increasing function of the manganese concentration.« less

Bimetallic oxalates are molecule-based magnets with transition-metal ions M(II) and M(')(III) arranged on an open honeycomb lattice. Performing a Holstein-Primakoff expansion, we obtain the spin-wave spectrum of antiferromagnetically coupled bimetallic oxalates as a function of the crystal-field angular momentum L(2) and L(3) on the M(II) and M(')(III) sites. Our results are applied to the Fe(II)Mn(III), Ni(II)Mn(III) and V(II)V(III) bimetallic oxalates, where the spin-wave gap varies from 0 meV for quenched angular momentum to as high as 15 meV. The presence or absence of magnetic compensation appears to have no effect on the spin-wave gap.

Dynamical mean-field theory is used to study the magnetic instabilities and phase diagram of the double-exchange (DE) model with Hund's coupling J_H >0 in infinite dimensions. In addition to ferromagnetic (FM) and antiferromagnetic (AF) phases, the DE model supports a broad class of short-range ordered (SRO) states with extensive entropy and short-range magnetic order. For any site on the Bethe lattice, the correlation parameter q of a SRO state is given by the average q=, where theta_i is the angle between any spin and its neighbors. Unlike the FM (q=0) and AF (q=1) transitions, the transition temperature of a SRO state (T_{SRO}) with 00 but appears for J_H\neq 0. For p near 1, PS occurs between an AF with p=1 and either a SRO or a FM phase. The stability of a SRO state at T=0 can be understood by examining the interacting DOS,which is gapped for any nonzero J_H in an AF but only when J_H exceeds a critical value in a SRO state., Comment: 38 pages, 11 figures, submitted to New Journal of Physics

The coexistence of spin- and charge-density waves in pure Cr and its dilute alloys continues to fascinate both experimentalists and theorists. Using a three-band model, we show that the charge-density wave (CDW) in incommensurate (I) Cr alloys is governed by the same Coulomb interaction that produces a first-order N{acute e}el transition in commensurate (C) alloys. Therefore, we predict that the first-order paramagnetic-to-commensurate transition observed in CrFe and CrSi alloys is preceded by the growth of a large CDW in the I phase of each alloy. We also provide a justification for the three-band model in the I phase and show that it reduces to the correct form in the C limit. {copyright} {ital 1998} {ital The American Physical Society}

Despite the presence of only collinear, commensurate (C) and incommensurate (I) spin-density waves (SDWs) in bulk Cr, the interfacial steps in Fe/Cr multilayers are now believed to stabilize a helical (H) SDW within the Cr spacer. Yet HSDWs were first predicted in an Fe/Cr trilayer with perfect interfaces when the orientation of the Fe moments does not favor C ordering: if the number of Cr monolayers is even (odd) and the Fe moments are pointing in the same (opposite) direction, then a CSDW does not gain any coupling energy. Under these circumstances, a simple model verifies that H ordering is indeed favored over I ordering provided that the Fermi surface mismatch is sufficiently small or the temperature sufficiently high.

In order to understand the effects of density fluctuations in composite systems, we have studied the effects of compression rate on the final density and density variations in a system of hard spheres. Systems of 900 and 1800 mono‐sized spheres are placed in a box with periodic boundary conditions in the x and y directions and walls at z=0 and z=w(t). Using hard‐sphere interactions between the particles, the cell is compressed at a constant rate κ∝−dw/dt under isothermal conditions until the pressure diverges and an overlap occurs between the particles or with the walls. The final particle volume is then subdivided into smaller cells, each containing about 15 particles with a local density p(x). Macroscopic fluctuations in the density are measured by the coarseness parameter C p =σ p /p, where σ p is the standard deviation of the local densities and p is the average density. Surprisingly, the coarseness reaches a maximum at intermediate compression rates and, within the range of compression rates studied, the most homogeneous particle packings are generated by fast compressions. For compression rates above 1, the distribution function of the local densities is a Gaussian with a mean close to the random dense packing value p(x)≊0.64. For compression rates below 1, local regions vary from randomly dense‐packed [p(x)≊0.64] to close‐packed [ p(x)≊0.74] with a correspondingly high coarseness. So for κ

Neutron-scattering measurements have revealed that the spin-density wave (SDW) inside the Cr spacer of Fe/Cr magnetic multilayers may be either commensurate (C) or incommensurate (I) depending on the temperature and Cr thickness N. We theoretically evaluate the magnetic phase diagram of Fe/Cr multilayers under the assumption that the nodes of the SDW lie at the Cr-Fe interface, as found by recent neutron-scattering measurements. While the C phase is never stable under this assumption, the paramagnetic to I transition temperature T{sub N}(N) takes on a seesaw pattern as the SDW wave vector switches between different allowed values. {copyright} {ital 1998} {ital The American Physical Society}