Showing papers on "Exchange interaction published in 2004"
TL;DR: The discovered alloy has a saturation magnetization that is about 50% larger than that of FePt--a compound that has received considerable attention lately-with a uniaxial MAE that can easily be tailored reaching a maximum value that is 50% smaller than thatof FePT.
Abstract: In the first part of this work, the influence of hydrogen on the structural and magnetic properties of Fe/V(001) superlattices was studied. The local structure of the vanadium-hydride layers was determined by extended x-ray absorption fine structure (EXAFS) measurements. The magnetic ordering in a weakly coupled Fe/V(001) superlattice was investigated using the magneto-optical Kerr effect (MOKE). The interlayer exchange coupling is weakened upon alloying with hydrogen and a phase with short-range magnetic order was observed. The second part is concerned with first-principles calculations of magnetic materials, with a focus on magnetic recording applications. The uniaxial magnetic anisotropy energy (MAE) of Fe, Co, and Ni was calculated for tetragonal and trigonal structures. Based on an analysis of the electronic states of tetragonal Fe and Co at the center of the Brillouin zone, tetragonal Fe-Co alloys were proposed as a material that combines a large uniaxial MAE with a large saturation magnetization. This was confirmed by experimental studies on (Fe,Co)/Pt superlattices. The large uniaxial MAE of L10 FePt is caused by the large spin-orbit interaction on the Pt sites in connection with a strong hybridization between Fe and Pt. Furthermore, it was shown that the uniaxial MAE can be increased by alloying the Fe sublattice with Mn. The combination of the high-moment rare-earth (RE) metals with the high-TC 3d transition metals in RE/Cr/Fe multilayers (RE = Gd, Tb, Dy) gives rise to a strong ferromagnetic effective exchange interaction between the Fe layers and the RE layer. The MAE of hcp Gd was found to have two principal contributions, namely the dipole interaction of the large localized 4f spins and the band electron magnetic anisotropy due to the spin-orbit interaction. The peculiar temperature dependence of the easy axis of magnetization was reproduced on a qualitative level.
334 citations
TL;DR: In this paper, the fine structure of a confined exciton in the exchange field of a single Mn2+ ion (S=5/2) was analyzed in detail, and six emission lines were observed at zero magnetic field.
Abstract: The magnetic state of a single magnetic ion (Mn2+) embedded in an individual quantum dot is optically probed using microspectroscopy. The fine structure of a confined exciton in the exchange field of a single Mn2+ ion (S=5/2) is analyzed in detail. The exciton-Mn2+ exchange interaction shifts the energy of the exciton depending on the Mn2+ spin component and six emission lines are observed at zero magnetic field. Magneto-optic measurements reveal that the emission intensities in both circular polarizations are controlled by the Mn2+ spin distribution imposed by the exchange interaction with the exciton, the magnetic field, and an effective manganese temperature which depends on both the lattice temperature and the density of photocreated carriers. Under magnetic field, the electron-Mn interaction induces a mixing of the bright and dark exciton states.
279 citations
TL;DR: In this paper, the emission spectra of phenylene-based conjugated polymers were used to investigate the source of the constant exchange energy in the lowest singlet and triplet excited states.
Abstract: Electron–electron interactions in organic semiconductors split the lowest singlet and triplet states by the exchange energy, ΔEST. Measurement of singlet and triplet emission spectra in a large number of conjugated polymers yield an almost constant ΔEST value close to 0.7 eV. This is in contrast to the situation in molecules, where the exchange energy is found to depend on molecular size and to vary over a wide range. Quantum-chemical calculations are performed to address the origin of the constant exchange energy in phenylene-based conjugated polymers. The electron–hole separation in the lowest singlet and triplet excited states is found to be independent of the π-conjugated backbone, and saturates for chains longer than a few repeating units, resulting in a constant exchange energy. In shorter conjugated oligomers, confinement of the excitations destabilizes the singlet with respect to the triplet through exchange interactions and leads to a larger and size-dependent singlet–triplet energy separation.
230 citations
TL;DR: Saturation spectroscopy demonstrates that the neutral exciton behaves as a two-level system and the remaining problem for manipulating excitonic quantum states in this system is spectral fluctuation on a mueV energy scale.
Abstract: We show how the optical properties of a single semiconductor quantum dot can be controlled with a small dc voltage applied to a gate electrode. We find that the transmission spectrum of the neutral exciton exhibits two narrow lines with approximately 2 mueV linewidth. The splitting into two linearly polarized components arises through an exchange interaction within the exciton. The exchange interaction can be turned off by choosing a gate voltage where the dot is occupied with an additional electron. Saturation spectroscopy demonstrates that the neutral exciton behaves as a two-level system. Our experiments show that the remaining problem for manipulating excitonic quantum states in this system is spectral fluctuation on a mueV energy scale.
228 citations
TL;DR: In this article, the effect of exchange interaction between the hard-magnetic Y2Fe14B and softmagnetic Fe3B phases on the natural resonance phenomenon and microwave absorption properties was investigated.
191 citations
TL;DR: In this paper, the authors studied the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction in one-and two-dimensions in the presence of a Rashba spin-orbit (SO) coupling.
Abstract: We study theoretically the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction in one- and two-dimensions in presence of a Rashba spin-orbit (SO) coupling. We show that rotation of the spin of conduction electrons due to SO coupling causes a twisted RKKY interaction between localized spins which consists of three different terms: Heisenberg, Dzyaloshinsky-Moriya, and Ising interactions. We also show that the effective spin Hamiltonian reduces to the usual RKKY interaction Hamiltonian in the twisted spin space where the spin quantization axis of one localized spin is rotated.
151 citations
TL;DR: In this article, the generalized perturbation method with screened Coulomb interactions is used to obtain effective interactions that yield an accurate and physically transparent description of configurational energetics within the atomic sphere and coherent potential approximations.
Abstract: It is shown that, using the generalized perturbation method (GPM) with screened Coulomb interactions that ensures its consistency with the force theorem, one is able to obtain effective interactions that yield an accurate and physically transparent description of configurational energetics in the framework of the Korringa-Kohn-Rostoker method within the atomic sphere and coherent potential approximations. This is demonstrated with calculations of ordering energies, short-range order parameters, and transition temperatures in the CuZn, CuAu, CuPd, and PtCo systems. Furthermore, we show that the GPM can be used to obtain Heisenberg exchange interaction parameters, which, for instance, capture very well the magnetic configurational energy in bcc Fe.
113 citations
TL;DR: A lattice mean field theory for ferromagnetic ordering in diluted magnetic semiconductors is developed by taking into account the spatial fluctuations associated with random disorder in the magnetic impurity locations and the finite mean free path associated with low carrier mobilities.
Abstract: We develop a lattice mean field theory for ferromagnetic ordering in diluted magnetic semiconductors by taking into account the spatial fluctuations associated with random disorder in the magnetic impurity locations and the finite mean free path associated with low carrier mobilities. Assuming a carrier-mediated indirect RKKY exchange interaction among the magnetic impurities, we find substantial deviation from the extensively used continuum Zener model Weiss mean field predictions. Our theory allows accurate analytic predictions for Tc and provides simple explanations for a number of observed anomalies, including the non-Brillouin function magnetization curves, the suppressed low-temperature magnetization saturation, and the dependence of Tc on conductivity.
103 citations
TL;DR: The synthesis and structural characterisation of three small nickel(II) cages are reported, all stabilised by pivalate ligands, and are therefore expected to be new single molecule magnets.
Abstract: The synthesis and structural characterisation of three small nickel(II) cages are reported, all stabilised by pivalate ligands. The magnetic properties of the cages have been studied by a combination of susceptibility measurements and inelastic neutron scattering. For a dinuclear cage, [Ni2(μ-OH2)(O2CCMe3)4(HO2CCMe3)4]
1 the ground state is S
= 2, with a ferromagnetic exchange interaction between the Ni(II) centres of J
= 0.32 meV and DS=2
=
−0.09 meV in the ground state. For a tetranuclear heterocubane cage, [Ni4(μ3-OMe)4(O2CCMe3)4(MeOH)4]
2, two ferromagnetic exchange interactions are found and an S
= 4 ground state observed. While the zero-field splitting of this state cannot be determined unambigiously the most likely value is DS=4
=
−0.035 meV. For a tetranuclear nickel butterfly, [Ni4(μ3-OH)2(O2CCMe3)6(EtOH)6]
3, three exchange interactions are required, two anti-ferromagnetic and one weakly ferromagnetic; the resulting ground state is S
= 0. The data enable us to estimate the zero-field splitting of single Ni(II) ions involved in the cage as Di
=
+1.0 meV. Both 1 and 2 are therefore expected to be new single molecule magnets.
94 citations
TL;DR: In this article, the average concentration of Mn was determined to be 8.4mol% by x-ray fluorescence, which may suggest that ferrimagnetic (Zn,Mn)Mn2O4 exists at the surface.
Abstract: Wurtzite-type ZnO tetrapod nanostructures were prepared by evaporating Zn metal under humid argon flow. After the fabrication, Mn was doped into ZnO nanostructures by diffusion at 600°C. The average concentration of Mn was determined to be 8.4mol% by x-ray fluorescence. X-ray diffraction patterns obtained from the doped and undoped samples are almost the same. High-resolution transmission electron microscopy observations reveal the existence of surface layers. Magnetic measurements show that the sample has a very large coercivity HC=5500Oe at 5.5K and a Curie temperature TC=43K, which may suggest that ferrimagnetic (Zn,Mn)Mn2O4 exists at the surface. Exchange bias is clearly observed below 22K. Exchange bias is attributed to the exchange interaction between ferrimagnetic (Zn,Mn)Mn2O4 and spin-glass-like (or antiferromagnetic) phase in manganese oxides.
88 citations
TL;DR: Sun et al. as discussed by the authors showed that the short spin-spin correlation dominates the exchange interaction in the ferromagnetic Fe, Co, Ni, and Fe 3 O 2 2 nanosolids.
Abstract: Incorporating the recent bond order-length-strength correlation mechanism [Sun; et al. J. Phys. Chem. B 2002, 106, 10701] into the Ising premise has led to consistent insight, with an analytical expression, into the Curietemperature (Tc) suppression of ferromagnetic, ferroelectric, and superconductive nanosolids. The phase stability is related to the atomic cohesive/exchange energy that is lowered by the coordination number (CN) imperfection of the lower coordinated atoms near the surface edge. A numerical match between predictions and measurements for a number of specimens reveals that the short spin-spin correlation dominates the exchange interaction in the ferromagnetic Fe, Co, Ni, and Fe 3 O 2 nanosolids, whereas the long-range interaction dominates the exchange energy for the ferroelectric PbTiO 3 , PbZrO 3 , SrBi 2 Ta 2 O 9 , and BaTiO 3 and the superconductive MgB 2 nanosolids.
TL;DR: In this paper, the first and second-order Raman scattering and infrared reflection spectra of hexagonal HoMnO3 single crystals in the temperature range 10-300 K are reported.
Abstract: Polarized first- and second-order Raman scattering and infrared reflection spectra of hexagonal HoMnO3 single crystals in the temperature range 10–300 K are reported. Based on the symmetry analysis and comparison with the results of lattice dynamics calculations the observed lines are assigned to the lattice eigenmodes. The magnetic ordering of Mn ions, which occurs below TN = 76 K, is shown to affect Raman- and infrared-active phonons, which modulate Mn–O–Mn bonds and, consequently, the Mn–Mn exchange interaction.
TL;DR: The magneto-optical effect in dilute magnetic semiconductors (DMSs) is directly related to the interaction between the d electrons of the transition metal ions and the s, p electrons of host semiconductor as mentioned in this paper.
Abstract: The magneto-optical effect in dilute magnetic semiconductors (DMSs) is directly related to the interaction between the d electrons of the transition metal ions and the s, p electrons of the host semiconductor. We show the advantages of the magneto-optical effect of DMSs as regards fabricating magneto-optical waveguide devices that can be integrated with other semiconductor optical devices. We also discuss the advantages of magneto-optical spectroscopy for characterizing DMSs. Intrinsic ferromagnetism of In1−xMnxAs, Ga1−xMnxAs and Zn1−xCrxTe is confirmed by using magnetic circular dichroism (MCD) spectroscopy. The MCD analyses also show that Zn1−xTMxO (TM = Mn, Fe, Co, Ni or Cu), Ga1−xMnxN and Ga1−xCrxAs are paramagnetic DMSs with the s, p–d exchange interaction. Ferromagnetic behaviours observed in some transition metal doped ZnO, GaN and GaAs samples are attributed to unidentified precipitations not detectable by means of x-ray diffraction analysis.
TL;DR: In this paper, a theoretical analysis of the magnetization reversal process in a bi-layer structure with hard and soft (with metamagnetic transition) magnetic layers on an example of an $\mathrm{Fe}Mathrm{Pt}∕\mathrm {Fe}mathrm
Abstract: We present a theoretical analysis of the magnetization reversal process in a bi-layer structure with hard and soft (with metamagnetic transition) magnetic layers on an example of an $\mathrm{Fe}\mathrm{Pt}∕\mathrm{Fe}\mathrm{Rh}$ bi-layer. The latter leads to the formation of a new type of exchange spring which results in a significant reduction of the switching field in the temperature range of the metamagnetic (from antiferromagnetic to ferromagnetic state) transition in an FeRh layer. Analytic expressions for nucleation and switching fields are presented along with results of numerical micromagnetic simulations. The reduction of the switching field due to the metamagnetic transition is controlled by the following microscopic parameters: (i) the interfacial exchange coupling parameter ${J}_{12}$; (ii) saturation magnetization of the FeRh layer in a ferromagnetic phase; (iii) the metamagnetic transition temperature. The switching field dependence on the ${J}_{12}$ parameter is shown to saturate quickly as it approaches the bulk exchange interaction value which has been evaluated using first-principles method used also to verify the electronic nature of the metamagnetic transition. Theoretical results are discussed in the context of recent experimental observations.
TL;DR: In this paper, first principles calculations of Zn11X1S12 supercells in the zincblende structure, where X is a magnetic ion, show that when X=Cr, Fe, and Ni the materials are calculated to be half metallic and ferromagnetic.
Abstract: First principles calculations of Zn11X1S12 supercells in the zincblende structure, where X is a magnetic ion, show that when X=Cr, Fe, and Ni the materials are calculated to be half metallic and ferromagnetic, i.e., the densities of states of these bulk materials at the Fermi energy is calculated to be 100% spin polarized and the exchange interaction is ferromagnetic. This behavior persists even when on-site Coulomb “Hubbard U” potentials are applied.
TL;DR: In this paper, the symmetry of exchange interaction of charge carriers in semiconductor nanostructures (quantum wells and quantum dots) was analyzed, and it was shown that the exchange Hamiltonian of two particles belonging to the same energy band can be universally expressed via pseudospin operators of the particles.
Abstract: The symmetry of exchange interaction of charge carriers in semiconductor nanostructures (quantum wells and quantum dots) is analyzed. It is shown that the exchange Hamiltonian of two particles belonging to the same energy band can be universally expressed via pseudospin operators of the particles. The relative strength of the anisotropic exchange interaction is shown to be independent of the binding energy and the isotropic exchange constant.
TL;DR: In this article, it is shown that the original expressions for exchange integrals based on the magnetic force theorem (Liechtenstein et al 1984 J. Phys. F: Met. Rev. 14 L125) are optimal for calculations of the magnon spectrum.
Abstract: Starting from an exact expression for the dynamical spin susceptibility in the time-dependent density functional theory, a controversial issue regarding exchange interaction parameters and spin-wave excitation spectra of itinerant electron ferromagnets is reconsidered. It is shown that the original expressions for exchange integrals based on the magnetic force theorem (Liechtenstein et al 1984 J. Phys. F: Met. Phys. 14 L125) are optimal for calculations of the magnon spectrum, whereas the static response function is better described using the 'renormalized' magnetic force theorem given by Bruno (2003 Phys. Rev. Lett. 90 087205). This conclusion is confirmed by ab initio calculations for Fe and Ni.
TL;DR: In this paper, the cross-correlations of the phase diagrams of the n-layer ferroelectric thin film described by the transverse field Ising model have been studied in the framework of the mean field approach.
Abstract: By taking into account three different exchange interaction parameters (the surface, the inter-layer, and the intra-layer couplings) and two different transverse field parameters (on the surface and inside the film), the phase diagrams of the n-layer ferroelectric thin film described by the transverse field Ising model have been studied in the framework of the mean field approach. The results show that the features of the phase diagrams can be greatly modified by changing the exchange interactions or transverse field parameters. The crossover features of the parameters from the ferroelectric dominant phase diagram to the paraelectric dominant phase diagram are determined for the first time.
TL;DR: In this article, the high-field ESR spectra of the spin dimer material TlCuCl were described within the framework of an effective field theory and a good agreement between the theory and experiment was achieved, for all geometries and in a wide field range, under the assumption of a weak anisotropy of the interdimer as well as intradimer exchange interaction.
Abstract: Recently measured high-field ESR spectra of the spin dimer material ${\mathrm{TlCuCl}}_{3}$ are described within the framework of an effective field theory. A good agreement between the theory and experiment is achieved, for all geometries and in a wide field range, under the assumption of a weak anisotropy of the interdimer as well as intradimer exchange interaction.
TL;DR: In this article, it is shown that the original expressions for exchange integrals based on the magnetic force theorem (J. Phys. Rev. F14 L125 (1984)) are optimal for the calculations of the magnon spectrum whereas the normalized response function is better described by the ''renormalized'' magnetic forces theorem by P. Bruno (Phys. Lett. 90, 087205 (2003)).
Abstract: Starting from exact expression for the dynamical spin susceptibility in the time-dependent density functional theory a controversial issue about exchange interaction parameters and spin-wave excitation spectra of itinerant electron ferromagnets is reconsidered. It is shown that the original expressions for exchange integrals based on the magnetic force theorem (J. Phys. F14 L125 (1984)) are optimal for the calculations of the magnon spectrum whereas static response function is better described by the ``renormalized'' magnetic force theorem by P. Bruno (Phys. Rev. Lett. 90, 087205 (2003)). This conclusion is confirmed by the {\it ab initio} calculations for Fe and Ni.
TL;DR: In order to theoretically study on the magnetic coupling interaction in the Mn-Mn dimer, quantum chemical calculations on the equilibrium Mn−Mn distance R e, total energy E T, binding energies B e and exchange interaction energies E (S ) under different spin multiplets were performed at CASPT2 level of theory.
TL;DR: In this paper, the ion implantation technique has been used to fabricate a Co-rich layer in rutile: single-crystalline TiO2 substrates were heavily irradiated by Co+ ions with energy of 40?keV.
Abstract: The ion implantation technique has been used to fabricate a Co-rich layer in rutile: single-crystalline TiO2 substrates were heavily irradiated by Co+ ions with energy of 40?keV. The magnetic properties of as-prepared and post-annealed samples were studied by both inductive and Faraday magnetometry as well as ferromagnetic resonance (FMR). A ferromagnetic Curie temperature as high as 700?K was measured in our samples. The analysis of the magnetic hysteresis loop, the temperature dependence of the saturation magnetization, and strong out-of-plane anisotropy of the FMR spectra allow us to suppose that the origin of the macroscopic high-temperature ferromagnetism is the exchange interaction mediated by oxygen vacancies.
TL;DR: It is predicted that even typical ferromagnetic materials (e.g., Fe, Co, and Ni) should be possible to stabilize in a noncollinear magnetic order in, e.g, high pressure experiments.
Abstract: Two criteria have been identified here which determine whether a magnetic metal orders in a collinear (e.g., ferromagnet) or noncollinear (e.g., spin-spiral) arrangement. These criteria involve the ratio between the strength of the exchange interaction and the width of the electron bands, as well as Fermi-surface nesting between spin-up and spin-down sheets of the Fermi surface. Based on our analysis we predict that even typical ferromagnetic materials (e.g., Fe, Co, and Ni) should be possible to stabilize in a noncollinear magnetic order in, e.g., high pressure experiments.
TL;DR: In this paper, the influence of substitutional disorder on the magnetic properties of diluted Heisenberg spin systems is studied with regard to the magnetic stability of ferromagnetic diluted semiconductors (DMS).
Abstract: The influence of substitutional disorder on the magnetic properties of diluted Heisenberg spin systems is studied with regard to the magnetic stability of ferromagnetic diluted semiconductors (DMS). The equation of motion for the magnon Green's function within Tyablikov approximation is solved numerically for finite systems. The resulting spectral density is then used to estimate the magnetization and Curie temperature of an infinite system. This method is suitable for any form of a ferromagnetic exchange interaction. Besides different lattices and spin magnitude $S$, exchange interactions of different range are examined. The results show that, for short-range interaction, no magnetic order exists below the critical percolation concentration, whereas a linear dependence of the Curie temperature on the concentration of spins is found for ferromagnetic long-range interaction.
TL;DR: In this paper, the current state, achievements, problems and prospects of the intramolecular electron spin exchange interaction as a function of the nitroxide biradical composition and structure, type of the radical ring, temperature and the solvent nature are considered on the basis of the literature data and the results of their own experiments.
Abstract: The current state, achievements, problems and prospects of the intramolecular electron spin exchange interaction as a function of the nitroxide biradical composition and structure, type of the radical ring, temperature and the solvent nature are considered on the basis of the literature data and the results of our own experiments.
TL;DR: In this paper, the authors analyzed the strength and form of the exchange interaction under relevant conditions, and showed that when several spins are engaged in mutual interactions, the quantitative strengths or even qualitative forms of the interactions can change.
Abstract: Several prominent proposals have suggested that spins of localized electrons could serve as quantum computer qubits. The exchange interaction has been invoked as a means of implementing two qubit gates. In this paper, we analyze the strength and form of the exchange interaction under relevant conditions. We find that, when several spins are engaged in mutual interactions, the quantitative strengths or even qualitative forms of the interactions can change. It is shown that the changes can be dramatic within a Heitler-London model. Hund-Mulliken calculations are also presented, and support the qualititative conclusions from the Heitler-London model. The effects need to be considered in spin-based quantum computer designs, either as a source of gate error to be overcome or a new interaction to be exploited.
TL;DR: In this paper, the very low temperature properties of two pyrochlore compounds, Yb2Ti2O7 and Gd2Sn2O 7, were investigated using an ensemble of microscopic and bulk techniques.
Abstract: The very low temperature properties of two pyrochlore compounds, Yb2Ti2O7 and Gd2Sn2O7, were investigated using an ensemble of microscopic and bulk techniques. In both compounds, a first order transition is evidenced, as well as spin dynamics persisting down to the 20 mK range. The transition however has a quite different character in the two materials: whereas that in Gd2Sn2O7 (at 1 K) is a magnetic transition towards long range order, that in Yb2Ti2O7 (at 0.24 K) is reminiscent of the liquid-gas transition, in the sense that it involves a 4 orders of magnitude drop of the spin fluctuation frequency, with no long range order. We attribute these unusual features to the frustration of the antiferromagnetic exchange interaction in the pyrochlore lattice.
TL;DR: In this paper, the positions of a pair of spin-split SdH maxima are estimated at different Landau level (LL) indices to estimate the g-factor at different LL indices.
Abstract: Low-temperature magnetotransport measurements were performed on an Al0.25Ga0.75N∕GaN two-dimensional electron system. In this system, we observe Shubnikov-de Haas (SdH) oscillations in a perpendicular magnetic field B. By measuring the positions of a pair of spin-split SdH maxima, we are able to estimate the g‐factors at different Landau level (LL) indices. We find the g‐factor is enhanced over its bulk value in GaN (≅2) due to many-body exchange interactions. Moreover, the measured g‐factor increases with decreasing LL index, indicating that many-body electron–electron interactions become stronger as the number of occupied LLs decreases. Our results suggest that the exchange energy Eex shows an approximately linear B dependence.
TL;DR: In this paper, a spin-flop transition was observed at H sf ≃ 0.42±0.05 kbar with an S = 1/2 coupled spin dimer system TlCuCl 3 with a gapped ground state under magnetic field H parallel to the [201] direction.
Abstract: Magnetization measurements under hydrostatic pressure were performed on an S =1/2 coupled spin dimer system TlCuCl 3 with a gapped ground state under magnetic field H parallel to the [201] direction. With increasing applied pressure P , the gap decreases and closes completely at P c = 0.42±0.05 kbar. For P > P c , TlCuCl 3 undergoes antiferromagnetic ordering. A spin-flop transition was observed at H sf ≃0.7 T. The spin-flop field is approximately independent of pressure, although the sublattice magnetization increases with pressure. The gap and Neel temperature are presented as functions of pressure. The occurrence of the pressure-induced quantum phase transition is attributed to the relative enhancement of the interdimer exchange interactions compared with the intradimer exchange interaction.
TL;DR: A spin model that displays inverse melting and inverse glass transition is presented and analyzed, and the qualitative features of this model are shown to resemble a large class of inverse melting phenomena.
Abstract: A spin model that displays inverse melting and inverse glass transition is presented and analyzed. Strong degeneracy of the interacting states of an individual spin leads to entropic preference of the "ferromagnetic" phase, while lower energy associated with the noninteracting states yields a "paramagnetic" phase as temperature decreases. An infinite range model is solved analytically for constant paramagnetic exchange interaction, while for its random exchange analogous results based on the replica symmetric solution are presented. The qualitative features of this model are shown to resemble a large class of inverse melting phenomena. First and second order transition regimes are identified.