Showing papers on "Antiferromagnetism published in 2005"
TL;DR: It is proposed thatferromagnetic exchange here, and in dilute ferromagnetic nitrides, is mediated by shallow donor electrons that form bound magnetic polarons, which overlap to create a spin-split impurity band.
Abstract: Dilute ferromagnetic oxides having Curie temperatures far in excess of 300 K and exceptionally large ordered moments per transition-metal cation challenge our understanding of magnetism in solids. These materials are high-k dielectrics with degenerate or thermally activated n-type semiconductivity. Conventional super-exchange or double-exchange interactions cannot produce long-range magnetic order at concentrations of magnetic cations of a few percent. We propose that ferromagnetic exchange here, and in dilute ferromagnetic nitrides, is mediated by shallow donor electrons that form bound magnetic polarons, which overlap to create a spin-split impurity band. The Curie temperature in the mean-field approximation varies as (xdelta)(1/2) where x and delta are the concentrations of magnetic cations and donors, respectively. High Curie temperatures arise only when empty minority-spin or majority-spin d states lie at the Fermi level in the impurity band. The magnetic phase diagram includes regions of semiconducting and metallic ferromagnetism, cluster paramagnetism, spin glass and canted antiferromagnetism.
2,743 citations
TL;DR: The syntheses and magnetic susceptibilities of a pure series of rare copper minerals from the atacamite family with general formula ZnxCu4-x(OH)6Cl2 (0
Abstract: The syntheses and magnetic susceptibilities of a pure series of rare copper minerals from the atacamite family with general formula ZnxCu4-x(OH)6Cl2 (0 ≤ x ≤ 1) are reported. The structure of these compounds features a corner-sharing triangular kagome lattice of antiferromagnetically coupled Cu(II) ions. We correlate the onset of magnetic ordering with the mole fraction of paramagnetic Cu(II) ions located between kagome layers and demonstrate that the fully Zn-substituted compound shows no magnetic ordering down to 2 K, resulting in a highly spin-frustrated S = 1/2 lattice.
592 citations
TL;DR: In this paper, the authors demonstrate that geometrical frustration stabilizes a low-temperature spin-disordered state with coherence beyond the two-spin correlation length on a two-dimensional triangular lattice antiferromagnet.
Abstract: As liquids crystallize into solids on cooling, spins in magnets generally form periodic order. However, three decades ago, it was theoretically proposed that spins on a triangular lattice form a liquidlike disordered state at low temperatures. Whether or not a spin liquid is stabilized by geometrical frustration has remained an active point of inquiry ever since. Our thermodynamic and neutron measurements on NiGa2S4, a rare example of a two-dimensional triangular lattice antiferromagnet, demonstrate that geometrical frustration stabilizes a low-temperature spin-disordered state with coherence beyond the two-spin correlation length. Spin liquid formation may be an origin of such behavior.
399 citations
TL;DR: Second-harmonic generation (SHG) in magnetically ordered crystals is reviewed in this article, where the specific degrees of optical experiments -including spectral, spatial, and temporal resolution - lead to the observation of novel physical effects that cannot be revealed by other techniques of probing magnetism.
Abstract: Second-harmonic generation (SHG) in magnetically ordered crystals is reviewed. The symmetry of such crystals is determined by the arrangement of both the charges and the spins, so their contributions to the crystallographic and the magnetic structures, respectively, must be distinguished. Magnetic SHG is introduced as a probe for magnetic structures and sublattice interactions. The specific degrees of optical experiments - including spectral, spatial, and temporal resolution - lead to the observation of novel physical effects that cannot be revealed by other techniques of probing magnetism. These include local or hidden phase transitions, interacting magnetized and polarized sublattices and domain walls, and magnetic interfaces. SHG in various centrosymmetric and noncentrosymmetric crystal classes of antiferromagnetic oxides such as Cr2O3, hexagonal RMnO3(R=Sc,Y,In,Ho-Lu), magnetic garnet films, CuB2O4, CoO, and NiO, is discussed.
356 citations
TL;DR: In this article, it has been shown that epitaxial constraint results in the destruction of a space modulated spin structure in BiFeO3 films and that the induced magnetization is enhanced at low applied fields; the polarization is dramatically enhanced; whereas, the lattice structure for (111)c films and crystals is nearly identical.
Abstract: In BiFeO3 films, it has been found that epitaxial constraint results in the destruction of a space modulated spin structure. For (111)c films, relative to corresponding bulk crystals, it is shown (i) that the induced magnetization is enhanced at low applied fields; (ii) that the polarization is dramatically enhanced; whereas, (iii) the lattice structure for (111)c films and crystals is nearly identical. Our results evidence that eptiaxial constraint induces a transition between cycloidal and homogeneous antiferromagnetic spin states, releasing a latent antiferromagnetic component locked within the cycloid.
352 citations
TL;DR: In this article, a triangular lattice spin-1/2 system with antiferromagnetic Heisenberg and ring exchanges using variational approach focusing on possible realization of spin-liquid states was studied.
Abstract: We study triangular lattice spin-1/2 system with antiferromagnetic Heisenberg and ring exchanges using variational approach focusing on possible realization of spin-liquid states. Trial spin liquid wave functions are obtained by Gutzwiller projection of fermionic mean-field states and their energetics is compared against magnetically ordered trial states. We find that in a range of the ring exchange coupling upon destroying the antiferromagnetic order, the best such spin liquid state is essentially a Gutzwiller-projected Fermi sea state. We propose this spin liquid with a spinon Fermi surface as a candidate for the nonmagnetic insulating phase observed in the organic compound kappa-(ET)2Cu2(CN)3, and describe some experimental consequences of this proposal.
318 citations
TL;DR: In this article, the authors studied the charge order in LBCO with resonant soft X-ray scattering (RSXS) and observed significant modulation in both the doped-hole density and the Mottness, or the degree to which the system resembles a Mott insulator.
Abstract: Competition between magnetism and the kinetic energy of mobile carriers (typically holes) in doped antiferromagnets may lead to ‘stripe’ phases1,2,3,4, which are charged rivers separating regions of oppositely phased antiferromagnetism. In copper oxides the main experimental evidence for such coexisting static spin and charge order comes from neutron scattering in La1.48Nd0.4Sr0.12CuO4 (LNSCO; ref. 5) and La1.875Ba0.125CuO4 (LBCO; ref. 6). However, as a neutron is neutral, it does not detect charge but rather its associated lattice distortion7, so it is not known whether the stripes involve ordering of the doped holes. Here we present a study of the charge order in LBCO with resonant soft X-ray scattering (RSXS). We observe giant resonances near the Fermi level as well as near the correlated gap8,9, demonstrating significant modulation in both the doped-hole density and the ‘Mottness’, or the degree to which the system resembles a Mott insulator10. The peak-to-trough amplitude of the valence modulation is estimated to be 0.063 holes, which suggests11 an integrated area of 0.59 holes under a single stripe, close to the expected 0.5 for half-filled stripes.
310 citations
TL;DR: In this article, a phase diagram of the perovskite GdBaCo 2 O 5+x (GBCO) with Co ions in the 3+ state is presented, where the authors show that at low temperatures, the homogeneous distribution of doped carriers becomes unstable, and both the magnetic and transport properties point to an intriguing nanoscopic phase separation.
Abstract: Single crystals of the layered perovskite GdBaCo_{2}O_{5+x} (GBCO) have been grown by the floating-zone method, and their transport, magnetic, and structural properties have been studied in detail over a wide range of oxygen contents. The obtained data are used to establish a rich phase diagram centered at the "parent'' compound GdBaCo_{2}O_{5.5} -- an insulator with Co ions in the 3+ state. An attractive feature of GBCO is that it allows a precise and continuous doping of CoO_{2} planes with either electrons or holes, spanning a wide range from the charge-ordered insulator at 50% electron doping (x=0) to the undoped band insulator (x=0.5), and further towards the heavily hole-doped metallic state. This continuous doping is clearly manifested in the behavior of thermoelectric power which exhibits a spectacular divergence with approaching x=0.5, where it reaches large absolute values and abruptly changes its sign. At low temperatures, the homogeneous distribution of doped carriers in GBCO becomes unstable, and both the magnetic and transport properties point to an intriguing nanoscopic phase separation. We also find that throughout the composition range the magnetic behavior in GBCO is governed by a delicate balance between ferromagnetic (FM) and antiferromagnetic (AF) interactions, which can be easily affected by temperature, doping, or magnetic field, bringing about FM-AF transitions and a giant magnetoresistance (MR) phenomenon. An exceptionally strong uniaxial anisotropy of the Co spins, which dramatically simplifies the possible spin arrangements, together with the possibility of continuous ambipolar doping turn GBCO into a model system for studying the competing magnetic interactions, nanoscopic phase separation and accompanying magnetoresistance phenomena.
263 citations
TL;DR: In this paper, a paramagnetic behavior for the Co-doped ZnO samples with paramagnetic Co amount smaller than the nominal concentration was shown. But the remaining Co is antiferromagnetic coupled through oxygen.
Abstract: Polycrystalline Zn1−xCoxO diluted magnetic semiconductors have been prepared by coprecipitation technique in the concentration range 0⩽x⩽0.1. Structure, composition analysis, and optical absorption measurements revealed that cobalt is incorporated into the lattice, as Co2+ substituting Zn2+ ions, forming a solid solution with wurtzite structure instead of Co precipitates. Room- and low-temperature magnetization measurements reveal a paramagnetic behavior for the Co-doped ZnO samples with a paramagnetic Co amount smaller than the nominal concentration. χT versus T evidenced that the remaining Co is antiferromagnetically coupled through oxygen. This is further supported by a simple model that shows that as the Co concentration increases the amount of nearest neighbors Co atoms increases thus giving antiferromagnetic coupling and reducing the paramagnetic contribution.
242 citations
HGST1
TL;DR: In this paper, the temperature and field hysteresis of the magnetization in the first order antiferromagnetic to ferromagnetic phase transition in FeRh films grown onto $c$-axis sapphire and MgO (001) are investigated.
Abstract: The temperature and field hysteresis of the magnetization in the first order antiferromagnetic to ferromagnetic phase transition in FeRh films grown onto $c$-axis sapphire and MgO (001) are investigated. The transition to the ferromagnetic state upon heating and antiferromagnetic state upon cooling is generally broad indicating a heterogeneous transition due to defects so that antiferromagnetic and ferromagnetic domains coexist during the transition. However, the nucleation of antiferromagnetic domains upon cooling is abrupt for FeRh on $c$-axis sapphire which is indicative of homogeneous nucleation and growth of antiferromagnetic domains. The transition is further broadened when measuring with fields applied out of plane of the sample due to internal demagnetization fields. Temperature dependent remanent magnetization measurements reveal that field induced magnetization changes are irreversible during heating, but reversible during cooling. The field dependence of the shift in transition temperature is qualitatively modeled with an Ising spin type model utilizing a mean field approach. From this calculation a shift of $\ensuremath{-}10\phantom{\rule{0.3em}{0ex}}\mathrm{K}∕\mathrm{T}$ in transition temperature is determined in good agreement with the experimentally observed shift of $\ensuremath{-}8$ and $\ensuremath{-}9\phantom{\rule{0.3em}{0ex}}\mathrm{K}∕\mathrm{T}$ for FeRh films grown onto MgO (001) and c-axis sapphire, respectively.
228 citations
TL;DR: The new chemical synthesis methods allow us to produce large single-crystal samples of KFe3(OH)6(SO4)2, an ideal Kagomé lattice antiferromagnet, and reveal that the phase transition to the ordered ground-state is unusual.
Abstract: The collective behaviour of interacting magnetic moments can be strongly influenced by the topology of the underlying lattice. In geometrically frustrated spin systems, interesting chiral correlations may develop that are related to the spin arrangement on triangular plaquettes. We report a study of the spin chirality on a two-dimensional geometrically frustrated lattice. Our new chemical synthesis methods allow us to produce large single-crystal samples of KFe3(OH)6(SO4)2, an ideal Kagome lattice antiferromagnet. Combined thermodynamic and neutron scattering measurements reveal that the phase transition to the ordered ground-state is unusual. At low temperatures, application of a magnetic field induces a transition between states with different non-trivial spin-textures.
TL;DR: In this article, the crystal and magnetic structures of the magnetoelectric materials RMn2O5 (R=Tb,Ho,Dy) using neutron diffraction as a function of temperature were studied.
Abstract: We have studied the crystal and magnetic structures of the magnetoelectric materials RMn2O5 (R=Tb,Ho,Dy) using neutron diffraction as a function of temperature. All three materials display incommensurate antiferromagnetic ordering below 40 K, becoming commensurate on further cooling. For R=Tb,Ho, a commensurate-incommensurate transition takes place at low temperatures. The commensurate magnetic structures have been solved and are discussed in terms of competing exchange interactions. The spin configuration within the ab plane is essentially the same for each system, and the radius of R determines the sign of the magnetic exchange between adjacent planes. The inherent magnetic frustration in these materials is lifted by a small lattice distortion, primarily involving shifts of the Mn3+ cations and giving rise to a canted antiferroelectric phase.
Posted Content•
TL;DR: In this article, it was shown that the critical current for switching can be smaller in the antiferromagnetic case because of the absence of shape anisotropy and because spin torques act through the entire volume of an antiferron.
Abstract: Spintronics in ferromagnetic metals is built on a complementary set of phenomena in which magnetic configurations influence transport coefficients and transport currents alter magnetic configurations. In this Letter we propose that corresponding effects occur in circuits containing antiferromagnetic metals. The critical current for switching can be smaller in the antiferromagnetic case because of the absence of shape anisotropy and because spin torques act through the entire volume of an antiferromagnet. Our findings suggest that current-induced order parameter dynamics can be used to coarsen the microstructure of antiferromagnetic thin films.
TL;DR: In this article, a theory of magnetic relaxation is developed for geometrically frustrated three-dimensional magnets that can be described by an antiferromagnetic Ising model, where magnetic point defects treated as noninteracting quasiparticles characterized by well-defined energies, mobilities and effective magnetic charges play a fundamental role analogous to that of ion defects in the physics of ice or by electrons and holes in semiconductors.
Abstract: A theory of magnetic relaxation is developed for geometrically frustrated three-dimensional magnets that can be described by an antiferromagnetic Ising model. These magnetic materials are exemplified by some of the recently synthesized rare-earth oxide pyrochlores, such as Dy2Ti2O7, Ho2Ti2O7, or Yb2Ti2O7. A model based on an analogy between the spin ordering in Ising magnets and proton ordering in ice is proposed. In this model, magnetic point defects treated as noninteracting quasiparticles characterized by well-defined energies, mobilities, and effective magnetic charges play a fundamental role analogous to that of ion defects in the physics of ice or by electrons and holes in semiconductors. The proposed model is used to derive expressions for magnetic susceptibility as a function of frequency and temperature.
TL;DR: Reversible crystal-to-crystal transformation accompanied by change from ferromagnetic to antiferromagnetic ground states at 8 K upon dehydration-rehydration of the nanoporous coordination framework [CoII3(OH)2(C4O4)2].3H2O.
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TL;DR: In this paper, the structure and energy of YCrO3 with ferromagnetic and antiferromagnetic ordering were determined by means of first-principles density functional theory calculations, based on pseudopotentials and a plane wave basis.
Abstract: YCrO3 which has a monoclinic structure, shows weak ferromagnetism below 140 K (TN) and a ferroelectric transition at 473 K accompanied by hysteresis We have determined the structure and energetics of YCrO3 with ferromagnetic and antiferromagnetic ordering by means of first-principles density functional theory calculations, based on pseudopotentials and a plane wave basis The non-centrosymmetric monoclinic structure is found to be lower in energy than the orthorhombic structure, supporting the biferroic nature of YCrO3
TL;DR: In this paper, a novel molecule-based magnetic polymer Mn(N3)2(btr)2 1 (btr = 4,4‘-bi-1,2,4-triazole) was synthesized and characterized crystallographically and magnetically.
Abstract: A novel molecule-based magnetic polymer Mn(N3)2(btr)2 1 (btr = 4,4‘-bi-1,2,4-triazole) was synthesized and characterized crystallographically and magnetically. 1 crystallizes in the monoclinic system, space group P21/c, formula C8H8N18Mn, with a = 12.2831(4) A, b = 6.3680(1) A, c = 10.2245(3) A, β = 105.064(1)°, and Z = 2. Bridged by end-to-end azides, the Mn2+ ions form a two-dimensional layer with (4,4) topology; the layers are further connected to the three-dimensional network by the weak hydrogen bonds between ligands of btr. Magnetic studies on a polycrystalline sample show the existence of strong antiferromagnetic couplings between the adjacent Mn2+ ions, and the Neel temperature is TN = 23.7 K. In the ordered state below TN, detailed investigations on an oriented single-crystal sample of 1 reveal that the hidden spin-canting, metamagnetic transition, and spin-flop transition can appear in different circumstances. The ground state is of an antiferromagnet with hidden spin-canting. An external field ...
TL;DR: An interaction-induced cooling mechanism for two-component cold fermions in an optical lattice based on an increase of the spin entropy upon localization, an analogue of the Pomeranchuk effect in liquid helium 3, is proposed.
Abstract: We propose an interaction-induced cooling mechanism for two-component cold fermions in an optical lattice. It is based on an increase of the spin entropy upon localization, an analogue of the Pomeranchuk effect in liquid helium 3. We discuss its application to the experimental realization of the antiferromagnetic phase. We illustrate our arguments with dynamical mean-field theory calculations.
TL;DR: In this article, an unusually large magnetocaloric effect for the temperature region below 10 K was found for the Fe14 molecular nanomagnet, to large extent caused by its extremely large spin S ground state combined with an excess of entropy arising from the presence of low-lying excited S states.
Abstract: An unusually large magnetocaloric effect for the temperature region below 10 K is found for the Fe14 molecular nanomagnet. This is to large extent caused by its extremely large spin S ground state combined with an excess of entropy arising from the presence of low-lying excited S states. We also show that the highly symmetric Fe14 cluster core, resulting in small cluster magnetic anisotropy, enables the occurrence of long-range antiferromagnetic order below TN=1.87K.
Journal Article•
TL;DR: In this paper, an unusually large magnetocaloric effect for the temperature region below 10 K was found for the Fe14 molecular nanomagnet, to large extent caused by its extremely large spin S ground state combined with an excess of entropy arising from the presence of low-lying excited S states.
Abstract: An unusually large magnetocaloric effect for the temperature region below 10 K is found for the Fe14 molecular nanomagnet. This is to large extent caused by its extremely large spin S ground state combined with an excess of entropy arising from the presence of low-lying excited S states. We also show that the highly symmetric Fe14 cluster core, resulting in small cluster magnetic anisotropy, enables the occurrence of long-range antiferromagnetic order below TN=1.87K.
Posted Content•
TL;DR: Slow relaxation of the magnetization is observed at low temperature for the first time in an antiferromagnetic chain, following an activated behavior with Delta(tau)/k(B) = 47 K and tau(0) = 7 x 10(-)(11) s.
Abstract: Four discrete MnIII/MnII tetra-nuclear complexes with double-cuboidal core were synthesized. dc magnetic measurements show that both Mn2+ - Mn3+ and Mn3+ - Mn3+ magnetic interactions are ferromagnetic in three samples leading to an S = 9 ground state for the Mn4 unit. Furthermore, these complexes are Single-Molecule Magnets (SMMs) clearly showing both thermally activated and ground state tunneling regimes. Slight changes in the [Mn4] core geometry result in an S = 1 ground state in fourth sample. A one-dimensional assembly of [Mn4] units was obtained in the same synthetic conditions with the subsequent addition of NaN3. Double chair-like N3- bridges connect identical [Mn4] units into a chain arrangement. This material behaves as an Ising assembly of S = 9 tetramers weakly antiferromagnetically coupled. Slow relaxation of the magnetization is observed at low temperature for the first time in an antiferromagnetic chain, following an activated behavior with 47 K and tau_0 = 7x10^-11 s. The observation of this original thermally activated relaxation process is induced by finite-size effects and in particular by the non-compensation of spins in segments of odd-number units. Generalizing the known theories on the dynamic properties of poly-disperse finite segments of antiferromagnetically coupled Ising spins, the theoretical expression of the characteristic energy gaps were estimated and successfully compared to the experimental values.
TL;DR: In this paper, the synthesis and magnetic properties of (1−xFex)2O3−σ bulk ceramics with Cu co-doping were reported, and extensive structural and magnetic studies rule out the possibility that the observed magnetism is derived from magnetic impurity phases.
Abstract: The synthesis and magnetic properties of (In1−xFex)2O3−σ bulk ceramics with Cu co-doping are reported. Magnetic Fe ions are found to have high thermodynamic solubility (up to 20%) in the In2O3 host compound. The lattice constant decreases almost linearly as Fe doping concentration increases indicating the incorporation of Fe ions into the host lattice. The samples with high Fe concentration annealed under Ar reduced atmosphere were found to be ferromagnetic, and the Curie temperature is around 750K. The extensive structural and magnetic studies rule out the possibility that the observed magnetism is derived from magnetic impurity phases.
TL;DR: The hysteresis loop shift in sub-100 nm ferromagnetic- (FM-)antiferromagnetic (AFM) nanostructures can be either enhanced or reduced with respect to continuous films with the same composition, with varying the AFM layer thickness.
Abstract: The hysteresis loop shift in sub-100 nm ferromagnetic- (FM-)antiferromagnetic (AFM) nanostructures can be either enhanced or reduced with respect to continuous films with the same composition, with varying the AFM layer thickness. An enhancement of the coercivity and a reduction of the blocking temperature are also observed. These effects are mainly ascribed to the physical limitations that the dot sizes impose on the AFM domain size and the concomitant weakening of the pinning strength exerted by the AFM during magnetization reversal of the FM.
TL;DR: In this article, the lattice parameter of the Zn1−xMnxO was observed to increase with Mn concentration and transmission measurements showed systematic variations dominated by absorption from interatomic Mn2+ and Co2+ transitions, which was attributed to superparamagnetic Co clusters embedded in a diamagnetic ZnO matrix.
Abstract: Manganese and cobalt-doped ZnO have been produced using a modified melt-growth technique. X-ray diffraction measurements indicate that the samples are high-quality single crystals with ω−2θ full width at half maximum values of 78 arc sec for the undoped ZnO and 252 arc sec for Zn1−xMnxO (x=0.05). The lattice parameter of the Zn1−xMnxO was observed to increase with Mn concentration. Transmission measurements showed systematic variations dominated by absorption from interatomic Mn2+ and Co2+ transitions. No evidence of diluted magnetic semiconductor mean-field ferromagnetic behavior was observed in any of these nominally noncarrier-doped samples. The magnetic properties instead showed paramagnetic behavior for Zn1−xMnxO dominated by an antiferromagnetic Mn–Mn exchange interaction at low temperatures. Zn1−xCoxO showed hysteresis that was attributed to superparamagnetic Co clusters embedded in a diamagnetic ZnO matrix. It has been shown that in the bulk single-crystal form, intrinsic and noncarrier-doped Zn1−...
TL;DR: Using ab initio band structure calculations and symmetry arguments, this article showed that the magnetic property of the magnetic properties of Gd atoms in the alloy is antiferromagnetic, but the ferromagnetic phase can be stabilized by introducing electrons.
Abstract: Using ab initio band structure calculations and symmetry arguments, we show that the magnetic property of ${\mathrm{Ga}}_{1\ensuremath{-}x}{\mathrm{Gd}}_{x}\mathrm{N}$ is drastically different from TM-doped GaN. The coupling between Gd atoms in the alloy is antiferromagnetic, but the ferromagnetic phase can be stabilized by introducing electrons. Furthermore, we propose a model that may explain the recently observed colossal magnetic moments in this system, based on the polarization of donor electrons.
TL;DR: The magnetic structures and phase diagram have been determined, and reveal additional transitions below 8 K where the ferroelectrically displaced Ho3+ ions are ordered magnetically.
Abstract: Hexagonal HoMnO3 is a frustrated antiferromagnet (T(N)=72 K) ferroelectric (T(C)=875 K) in which these two order parameters are coupled. Our neutron measurements of the spin-wave dispersion for the S=2 Mn3+ on the layered triangular lattice are well described by a two-dimensional nearest-neighbor Heisenberg exchange J=2.44 meV, and an anisotropy D that is 0.28 meV above the spin-reorientation transition at 40 K and 0.38 meV below. For H parallel c the magnetic structures and phase diagram have been determined, and reveal additional transitions below 8 K where the ferroelectrically displaced Ho3+ ions are ordered magnetically.
TL;DR: The e-Fe2O3 has been obtained as nanoparticles by vacuum heat treatment of yttrium iron garnet in a silica matrix at 300 °C followed by annealing at 1000 °C for up to 10 h in air and employing formamide as a gel modifier as mentioned in this paper.
Abstract: The elusive e-Fe2O3 has been obtained as nanoparticles by vacuum heat treatment of yttrium iron garnet in a silica matrix at 300 °C followed by annealing at 1000 °C for up to 10 h in air and employing formamide as a gel modifier. Its nuclear structure is temperature independent as observed from the neutron powder diffraction patterns and has been modeled by the published structures on analogous MM‘O3 compounds. It displays complex magnetic properties that are characterized by two transitions: one at 480 K from a paramagnet (P) to canted antiferromagnet (CAF1) and the second at ca. 110 K from the canted antiferromagnet (CAF1) to another canted antiferromagnet (CAF2) that has a smaller resultant magnetic moment (i.e., smaller canting angle). The latter transition resembles that of Morin for α-Fe2O3 at 260 K. The magnetization shows unusual history dependence: it has a bifurcation below 100 K if the field is applied at low temperatures after zero-field-cooled, whereas the bifurcation is above 150 K if the ...
TL;DR: In this paper, the authors investigated the ground state and finite temperature properties of the half-filled Hubbard model on a honeycomb lattice using quantum Monte Carlo and series expansion techniques.
Abstract: We investigate ground state and finite temperature properties of the half-filled Hubbard model on a honeycomb lattice using quantum Monte Carlo and series expansion techniques. Unlike the square lattice, for which magnetic order exists at $T=0$ for any nonzero $U$, the honeycomb lattice is known to have a semimetal phase at small $U$ and an antiferromagnetic one at large $U$. We investigate the phase transition at $T=0$ by studying the magnetic structure factor and compressibility using quantum Monte Carlo simulations and by calculating the sublattice magnetization, uniform susceptibility, spinwave, and single hole dispersion using series expansions around the ordered phase. Our results are consistent with a single continuous transition at ${U}_{c}∕t$ in the range 4--5. Finite-temperature signatures of this phase transition are seen in the behavior of the specific heat, $C(T)$, which changes from a two-peaked structure for $Ug{U}_{c}$ to a one-peaked structure for $Ul{U}_{c}$. Furthermore, the $U$ dependence of the low temperature coefficient of $C(T)$ exhibits an anomaly at $U\ensuremath{\approx}{U}_{c}$.
TL;DR: In this article, it was shown that at the antiferromagnetic transition of YMnO{sub 3] there is a strong coupling between the spin and lattice degrees of freedom splitting two Mn-O(3) and MnO(4) bond distances on a basal plane.
Abstract: YMnO{sub 3} has an antiferromagnetic transition at 76 K with a ferroelectric transition at much higher temperature, making it a rare example of systems having both ferroelectric and magnetic transitions and thus a so-called multiferroic compound. Through high-resolution neutron diffraction studies, we have demonstrated here that at the antiferromagnetic transition of YMnO{sub 3} there is a strong coupling between the spin and lattice degrees of freedom splitting two Mn-O(3) and Mn-O(4) bond distances on a basal plane. This coupling then induces an unmistakable change in the electric dipole moments, i.e., a coupling between the magnetic and electric dipole moments. We discuss how this rare phenomenon can occur within a Ginzburg-Landau theory.
TL;DR: In this paper, a study of the magnetic properties of oxidized Co nanoparticles with an average grain size of 3nm, embedded in an amorphous Al2O3 matrix is presented.
Abstract: We present a study of the magnetic properties of oxidized Co nanoparticles with an average grain size of 3nm, embedded in an amorphous Al2O3 matrix. These nanoparticles can be considered as imperfect Co-core CoO-shell systems. Magnetization measurements after magnetic field cooling show a vertical shift of the hysteresis loop, while no exchange bias is observed. With a simple model, we show that there is a critical grain size for hybrid ferromagnetic-antiferromagnetic particles, below which exchange bias is absent for any ratio of ferromagnetic and antiferromagnetic constituents. The reason is that the interfacial exchange energy dominates over other energies in the system due to a large surface-to-volume ratio in the nanoparticles.