Conventional Antiferromagnets Research Articles

Classical antiferromagnetism inMnSc2S4: ASc45NMR study

$^{45}\mathrm{Sc}$ NMR has been measured for the spinel compound $\mathrm{Mn}{\mathrm{Sc}}_{2}{\mathrm{S}}_{4}$. The Knight shift and the spin-lattice relaxation rate show typical behaviors expected for conventional localized moment systems. The Sc site separates into two different magnetic sites in the antiferromagnetic state below ${T}_{N}\ensuremath{\simeq}2\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, pointing to a simple spin structure. These results suggest that $\mathrm{Mn}{\mathrm{Sc}}_{2}{\mathrm{S}}_{4}$ is a rather conventional antiferromagnet.

Heat capacity and critical behavior of hexagonalYMnO3

We report heat capacity $({C}_{p})$ measurements on high-quality single crystals of hexagonal $\mathrm{Y}\mathrm{Mn}{\mathrm{O}}_{3}$, a ferroelectric and frustrated triangular antiferromagnetic (AF) compound. The obtained values of the critical exponent $\ensuremath{\alpha}$ and amplitude ratio ${A}^{+}∕{A}^{\ensuremath{-}}$ for the AF transition are in good agreement with the standard three-dimensional Heisenberg universality class, and not with a chiral universality class associated with the 120\ifmmode^\circ\else\textdegree\fi{} spin structure. The low-temperature ${C}_{p}$ deviation from the Debye law can be adequately fitted with a single Einstein contribution, indicating that there is no evidence of the anomalous magnetic contributions reported earlier. We also suggest that there is no intrinsic linear term in the low-temperature ${C}_{p}$. These results are consistent with conventional long-range AF ordering in $\mathrm{Y}\mathrm{Mn}{\mathrm{O}}_{3}$.

Specific heat and materials analysis on U 1−xTh xPt 3 for [formula omitted

UPt 3 exhibits anomalous, possibly time-fluctuating antiferromagnetic (AFM) order below 6 K. Th substitution induces conventional AFM order with the same magnetic structure. Recent μSR studies on U 1− x Th x Pt 3 for 0 ⩽ x ⩽ 0.05 showed that the transition into the conventional AFM state was sharp for x = 0.05 , but broadened for x ⩽ 0.02 (Phys. Rev. B 84 (2003) 224421), indicative of a crossover behavior. We present X-ray diffraction and transmission electron microscopy (TEM) results that show no significant material inhomogeneity in those samples. However, specific heat measurements corroborate the μSR measurements and show signs of an increase near 7 K for x > 0.01 . This supports the conjecture that Th impurities slow down the fluctuating AFM, rendering them observable on the timescale of thermodynamic measurements.

Crossover from anomalous to conventional antiferromagnetism in Pd-doped UPt 3 studied via cantilever magnetometry

UPt 3 exhibits an anomalous form of antiferromagnetic (AF) order, which is thought to be fluctuating in time. Substitution of Pd for Pt induces conventional AF behavior. In an attempt to further probe the possible existence of an AF quantum critical point in U(Pt 1− x Pd x ) 3 at x c∼0.006, we have made cantilever magnetometer measurements on single crystalline U(Pt 1− x Pd x ) 3 for x=0.01, 0.02, and 0.05 in magnetic fields of 6 T. While readily observable for the samples with x=0.02 and 0.05, we find no evidence for the AF transition at x=0.01, in contrast to μSR measurements.

Structure and magnetism ofMnMgB2O5andMn2B2O5

We present an investigation of the magnetic properties of the pyroborates ${\mathrm{Mn}}_{2}{\mathrm{B}}_{2}{\mathrm{O}}_{5}$ and ${\mathrm{MnMgB}}_{2}{\mathrm{O}}_{5}$ using magnetization and electron-paramagnetic-resonance measurements. The pyroborates have in common with another well studied family of borates, the warwickites, substructures in the form of ribbons where the metals are located. The homometallic ${\mathrm{Mn}}_{2}{\mathrm{B}}_{2}{\mathrm{O}}_{5}$ system is shown to be a conventional three-dimensional antiferromagnet with Neel temperature ${\mathrm{T}}_{N}\ensuremath{\approx}24\mathrm{K}.$ Its magnetization isotherms present steps indicating spin-flop-like transitions involving distinct pairs of metal sites. The heterometallic compound ${\mathrm{MnMgB}}_{2}{\mathrm{O}}_{5}$ shows a susceptibility with a power-law temperature dependence $\ensuremath{\chi}(T)\ensuremath{\propto}{T}^{\ensuremath{-}\ensuremath{\alpha}},$ down to $T=1.8\mathrm{K}$ and a magnetization with a power-law field dependence $M(H)\ensuremath{\propto}{H}^{(1\ensuremath{-}\ensuremath{\alpha})}$ at low temperatures. This behavior is characteristic of a random singlet phase, in this case, in a system with spin $S=5/2.$ We discuss our magnetic results on the pyroborates in connection with their structure and compare them with those obtained in the warwickites.

Pressure effect of spin-Peierls material CuGeO 3

We studied the pressure effect of Cu 1− x Mg x GeO 3 by the magnetic susceptibility measurements. We observed the revival of spin-gap, which is supposed to be spin-Peierls gap, in the sample without the spin-Peierls phase at ambient pressure and the sudden decrease of antiferromagnetic (AF)-phase transition temperature at the same pressure. The observation suggests the existence of the phase transition between AF phase with spin-gap excitation and conventional AF phase, which is similar to that between dimerized-AF and uniform-AF phases in the temperature vs. concentration phase diagram previously studied (T. Masuda et al., Phys. Rev. Lett. 80 (1998) 4566).

Anelastic relaxation and around the critical Sr content x = 0.02

Anelastic relaxation and $^{139}$La NQR relaxation measurements in La$_{2-x}$Sr$_x$CuO$_4$ for Sr content x around 2 and 3 percent, are presented and discussed in terms of spin and lattice excitations and ordering processes. It is discussed how the phase diagram of La$_{2-x}$Sr$_x$CuO$_4$ at the boundary between the antiferromagnetic (AF) and the spin-glass phase (x = 0.02) could be more complicate than previous thought, with a transition to a quasi-long range ordered state at T = 150 K, as indicated by recent neutron scattering data. On the other hand, the $^{139}$La NQR spectra are compatible with a transition to a conventional AF phase around T = 50 K, in agreement with the phase diagram commonly accepted in the literature. In this case the relaxation data, with a peak of magnetic origin in the relaxation rate around 150 K at 12 MHz and the anelastic counterparts around 80 K in the kHz range, yield the first evidence in La$_{1.98}$Sr$_{0.02}$CuO$_4$ of freezing involving simultaneously lattice and spin excitations. This excitation could correspond to the motion of charged stripes.

ANELASTIC AND 139La NQR RELAXATION STUDY OF La2-xSrxCuO4 AROUND THE CRITICAL Sr CONTENT x = 0.02

Anelastic relaxation and 139 La NQR relaxation measurements in La 2-x Sr x CuO 4 with x ≃ 0.02 are discussed in terms of spin and lattice excitations, ordering processes and thermal depinning of charged stripes. Two possible scenarios are considered. On one hand, the phase diagram of La 2-x Sr x CuO 4 at the boundary xc ~ 0.02 between the antiferromagnetic (AF) and the spin-glass phase could be more complicate than previously thought, and a peak of magnetic origin in the NQR rate near 150 K could signal an unconventional transition to a quasi-long range ordered state. On the other hand, the 139 La NQR spectra favour a transition to a conventional AF phase around 50 K. In this case, the peak of the NQR rate around 150 K at 12 MHz would correspond to an anelastic peak around 80 K in the kHz range, and they would constitute the first direct evidence of the freezing of charged stripes involving simultaneously lattice and spin excitations. The barrier of thermal depinning and the frequencies of the stripes motions are thus estimated.

Numerical study of multisoliton configurations in a doped antiferromagnetic Mott insulator

We evaluate from first principles the self-consistent Hartree-Fock energies for multisoliton configurations in a doped, spin-$\frac{1}{2}$, antiferromagnetic Mott insulator on a two-dimensional square lattice. The microscopic Hamiltonian for this system involves a nearest-neighbor electron hopping matrix element t, an on-site Coulomb repulsion U, and a nearest-neighbor Coulomb repulsion V. We find that nearest-neighbor Coulomb repulsion on the energy scale of t stabilizes a regime of charged meron-antimeron vortex soliton pairs over a region of doping from $\ensuremath{\delta}=0.05$ to $0.4$ holes per site for intermediate coupling $3<~U/t<~8$. This stabilization is mediated through the generation of ``spin flux'' in the mean-field antiferromagnetic (AFM) background. Spin flux is a form of spontaneous symmetry breaking in a strongly correlated electron system in which the Hamiltonian acquires a term with the symmetry of spin-orbit coupling at the mean-field level. Spin flux modifies the single quasiparticle dispersion relations from that of a conventional AFM. The modified dispersion is consistent with angle-resolved photoemission studies and has a local minimum at wave vector $\stackrel{\ensuremath{\rightarrow}}{k}=\ensuremath{\pi}/2a(1,1)$, where a is the lattice constant. Holes cloaked by a meron vortex in the spin-flux AFM background are charged bosons. Our static Hartree-Fock calculations provide an upper bound on the energy of a finite density of charged vortices. This upper bound is lower than the energy of the corresponding charged spin-polaron configurations. A finite density of charge carrying vortices is shown to produce a large number of unoccupied electronic levels in the Mott-Hubbard charge transfer gap. These levels lead to significant band tailing and a broad midinfrared band in the optical absorption spectrum as observed experimentally. In the presence of a finite density of charged meron-antimeron pairs, the peak in the magnetic structure at $\stackrel{\ensuremath{\rightarrow}}{Q}=\ensuremath{\pi}/a(1,1)$, corresponding to the undoped AFM, splits into four satellite peaks that evolve with charge carrier concentration as observed experimentally. At very low doping $(\ensuremath{\delta}<0.05)$ the doping charges create extremely tightly bound meron-antimeron pairs or even isolated conventional spin polarons, whereas for very high doping $(\ensuremath{\delta}>0.4)$ the spin background itself becomes unstable to formation of a conventional Fermi liquid and the spin-flux mean field is energetically unfavorable. Our results point to the predominance of a quantum liquid of charged, bosonic, vortex solitons at intermediate coupling and intermediate doping concentrations.

Longitudinal spin fluctuations of coupled integer-spin chains: Haldane triplet dynamics in the ordered phase ofCsNiCl3

By means of high-resolution polarized neutron scattering from the spin-1 chain compound ${\mathrm{CsNiCl}}_{3},$ we have observed at least one additional mode of spin fluctuations, not contained in spin-wave theory, in the three-dimensional antiferromagnetically ordered phase of the material. The number of observed modes, their polarization, frequency, magnetic field dependence, and intensity show that, in contrast to the doublet-spin dynamics of conventional antiferromagnets, the ${\mathrm{Ni}}^{2+}$ spins of ${\mathrm{CsNiCl}}_{3}$ manifest triplet fluctuations where one component is parallel to the ordered moment. The results confirm that the Ginsburg-Landau model of Affleck and Wellman gives a realistic description of the dynamics of an ordered array of coupled integer-spin chains exhibiting the Haldane gap. The longitudinal fluctuations exist because the system lies not too far from a quantum critical point.

Dynamical properties of an antiferromagnet near the quantum critical point: Application toLaCuO2.5

For a system of two-chain spin ladders, the ground state for weak interladder coupling is the spin-liquid state of the isolated ladder, but is an ordered antiferromagnet (AF) for sufficiently large interactions. We generalize the bond-operator mean-field theory to describe both regimes, and to focus on the transition between them. In the AF phase near the quantum critical point (QCP) we find both spin waves and a low-lying but massive amplitude mode which is absent in a conventional AF. The static susceptibility has the form $\ensuremath{\chi}(T)={\ensuremath{\chi}}_{0}{+aT}^{2},$ with ${\ensuremath{\chi}}_{0}$ small for a system near criticality. We consider the dynamical properties to examine features arising from the presence of the amplitude mode, and compute the dynamic structure factor. ${\mathrm{LaCuO}}_{2.5}$ is thought to be such an unconventional AF, whose ordered phase is located very close to the QCP of the transition to the spin liquid. From the N\'eel temperature we deduce the interladder coupling, the small ordered moment, and the gap in the amplitude mode. The dynamical properties unique to near-critical AF's are expected to be observable in ${\mathrm{LaCuO}}_{2.5}.$

Magneto-optical effect in the weak ferromagnetsLaMO3(M= Cr, Mn, and Fe)

It is shown that antiferromagnetic spin order coupled through the spin-orbit interaction with the lattice distortion can lead to a nonvanishing ferromagnetic component of the orbital magnetization and strong optical nonreciprocity in transition-metal perovskite oxides with orthorhombic structure. This magneto-optical effect is expected to display very peculiar orientation dependence and disappears for particular directions of the antiferromagnetic spin magnetization. Based on the first principles band structure calculations, elements of the conductivity tensor relevant for this phenomenon are evaluated for the series of La MO 3 oxides with M5Cr, Mn, and Fe. @S0163-1829~97!06513-2# Magneto-optical properties of various compounds have been a subject of very intensive study for the past couple of decades due to their potential applicability in the technology of high density data storage. Traditionally, the range of materials considered in this field was essentially restricted to ferro- and ferrimagnets. For the conventional antiferromagnets, the magneto-optical effect is averaged out as a consequence of the time-reversal symmetry (T), though destroyed microscopically, but restored on the macroscopic scale. However, for several classes of the antiferromagnetic ~AFM! compounds T remains broken macroscopically, suggesting a nonreciprocity in optical phenomena. 1,2 Broad interest in this problem has been attracted recently in connection with spontaneous violation of T predicted by so-called anyon models of high-T c superconductivity and discovery of the nonreciprocal optical effects in magnetoelectric Cr 2O 3. 3 One of the modes of T violation in antiferromagnets gives rise to the phenomenon of the weak ferromagnetism. 1 Such breakdown is quite typical for many distorted perovskite transition-metal oxides. 4 Intuitively, the relationship between weak ferromagnetism and nonreciprocal optical behavior is straightforward: the net magnetic moment results in the famous Faraday and Kerr rotations, but the effect is believed to be small and proportional to the ‘‘weak’’ ferromagnetic magnetization. 5 However, such an intuitive picture is very incomplete. In the present work we show that local noncollinearity between spin and orbital magnetic moments proposed recently 6,7 for the ground state of compounds with relatively low symmetry is the crucial aspect of optical nonreciprocity in the weak ferromagnets. First, we propose a simple scenario of this noncollinearity stressing the local environment of magnetic sites and interatomic spin coupling. Generally, the orbital magnetic moment at the site i is related with the direction of the spin ,

Investigation of spin chirality by polarized neutrons.

The projection of the chiral spin fluctuations on the magnetization is discussed. It is shown that it determines the polarization dependent part of the neutron magnetic scattering. A strong enhancement of this scattering appears near the chiral phase transition in triangular lattice antiferromagnets as well as in all cases when the system is soft and its nonlinearity is important. The example of conventional antiferromagnets is considered. A possibility to observe chiral fluctuations in doped Cu${\mathrm{O}}_{2}$ planes as a precursor of a proposed chiral state is discussed.

Order from disorder in a kagome antiferromagnet

It will be discussed to what extent the large S approach to an antiferromagnet on a kagome lattice can explain the data of magnetothermal and neutron scattering experiments on a stacked kagome antiferromagnet Sr–Cr–Ga–O. In particular, it will be shown that quantum fluctuations lift the classical degeneracy related to a large amount of frustration in a kagome lattice and give rise to a finite velocity of spin-wave excitations which is by a factor S−1/3 smaller than for a conventional antiferromagnet.

Electron pairing in high-T c superconductors

For CuO systems, there may be considerable deviations from conventional antiferromagnetism when E d − E p ∼0. When holes are introduced, then these systems may become superconductors because of the attractions between electron pairs.

Evolution of the magnetic structure in TbxY1−xAg (abstract)

As nonmagnetic yttrium is substituted for terbium, TbAg changes from a conventional antiferromagnet into a complex magnetic system with components of both commensurate and incommensurate spin structures and bulk magnetic properties typical of spin-glass behavior for x≤0.5. We have carried out neutron diffraction experiments on polycrystalline samples with x=0.15, 0.5, and 1.0 to complement our earlier studies for x=0.3.1 For x=0.5, the system orders at T≊58 K into the mixed structure observed for x=0.3, but with a much sharper (1/2 1/2 0) peak. On cooling below 40 K the incommensurate peaks almost disappear, remaining only as low-intensity shoulders of the commensurate peak at the lowest temperature measured (T=4 K). Hysteresis is observed in the relative intensity of the two components of the structure between 45 and 55 K. For x=0.15, the system orders at T≊18 K into the two-component structure, but with an approximately constant intensity ratio and no evidence of hysteresis, similar to the behavior found for x=0.3. Thus, as the Tb concentration decreases, the relative stability of the commensurate and incommensurate structures appears to change. This balance between competing magnetic structures will be related to the spin-glass properties observed in this system.

Magnetic phase diagram of Rb2MnCl4, a quasi-two-dimensional uniaxial antiferromagnet

The phase diagram of Rb2MnCl4 in magnetic fields up to 8.3 T parallel to the tetragonal axis was determined by neutron scattering. Similar to K2MnF4, but unlike conventional three-dimensional antiferromagnets, the authors experimental data exhibit a bifurcation of the phase boundary uniaxial antiferromagnet/paramagnet into the phase boundaries uniaxial antiferromagnet/spin-flop phase and paramagnet/spin-flop phase at the multicritical point T*=50.5K, H*=5.55 T. The boundary lines between the uniaxial antiferromagnetic phase and the paramagnetic and spin-flop phases are well described by Tc(H)/Tc(0)= alpha (H)n, where alpha (H) is the field-dependent anisotropy and n approximately=0.04 is possibly a universal exponent.

Molecular-orbital basis for superconductivity in high- and low-dimensional metals

A real-space molecular-orbital description of electronic wave functions which are postulated to be the precursors of the superconducting state in high- and low-dimensional metals is presented, based on self-consistent X-alpha scattered-wave (SCF-Xα-SW) molecular-orbital calculations for clusters representing the local molecular environments in these materials. It is shown that there is a persistent correlation between the occurrence of superconductivity in a material and the existence of spatially delocalized molecular orbitals at the Fermi energy which are bonding within and anti-bonding between “layers” or “tubes” of overlapping atomic orbitals that span many atoms, forming a type of “electron network” at the Fermi energy, as exemplified by pπ “layered” molecular-orbital topologies in Al and (TMTSF) 2PF 6, and by dδ “tubular” molecular-orbital topologies in Nb and Nb 3Sn. This description of the precursor superconducting state is consistent with the original conjectures of London that the superconducting-state wave function is “molecular” in nature, “rigid” in character, and of wide spatial extent, from which observed physical properties ( e.g., diamagnetism and nondissipative electrical currents) of the superconducting state logically follow. The molecular-orbital model is further shown to be consistent with Cooper's concept of electron pairing in the superconducting state through a net attractive electron-electron interaction but differs from the Bardeen-Cooper-Schrieffer (BCS) theory of superconductivity in attributing the pairing to “valence-bond-like” electron occupation of the layered or tubular molecular orbitals at the Fermi energy, coupled with lattice ion displacements through a dynamic Jahn-Teller effect, rather than to the absorption and emission of virtual phonons. However, recent simple physical arguments presented by Weisskopf for the formation of Cooper pairs suggest that the two points of view are not incompatible. Approximate formulae for calculating the electron pair binding energy and superconducting transition temperature, analogous to the BCS formulae but expressed entirely in terms of two simple molecular-orbital parameters, are derived and applied to various superconductors. Moreover, through an elementary phase-space argument, the molecular-orbital description of Cooper pairs is shown to be qualitatively consistent with Schafroth's description of electron pairs as “quasi-molecules” undergoing Bose-Einstein-like condensation to the superconducting state. The above molecular-orbital criteria for superconductivity are diametrical to those for the occurrence of local magnetic moments and ferromagnetism, namely, the existence of spatially localized, exclusively antibonding ( e.g., dσ ∗, dπ ∗, or dδ ∗) molecuar orbitals at the Fermi energy. Furthermore, in contrast to the ordering of electron spins on atomic sublattices in conventional antiferromagnetism, the valence-bond-like correlation of Cooper-Schafroth electron pairs at the Fermi energy in composite bonding-antibonding layered or tubular molecular-orbital components, spatially delocalized between the atoms, corresponds to a type of conduction-electron “antiferromagnetism” or spin-density wave, thus offering an explanation for the occurrence of such antiferromagnetism in superconductors such as (TMTSF) 2PF 6 and Nb 3Sn. These molecular-orbital criteria therefore provide a conceptual basis for understanding the generally mutually exclusive incidence of superconductivity and magnetism among the elements of the Periodic Table, although they can also be used to explain the occasional coexistence of superconductivity and ferromagnetism or antiferromagnetism in some materials. The molecular-orbital model for superconductivity complements BCS theory in that it permits the prediction of which materials are likely to be superconductors and which are not, entirely on the basis of the molecular-orbital topology at the Fermi energy. It offers an explanation of the vanishing isotope effect in certain superconductors. It contributes to the clarification of the issue of non-phonon mechanisms of superconductivity. It provides insight into the correlations of superconductivity with other physical properties, such as lattice instabilities. It is readily applicable to superconductors lacking long-range crystalline order, such as amorphous alloys and small particles. Finally, the molecular-orbital approach can be used to explain in simple terms why some materials ( e.g., Cu, Ag, and Au) are neither superconducting nor magnetic, why certain quasi-one-dimensional organic solids, such as TTF-TCNQ, are not superconductors while others, such as (TMTSF) 2PF 6, are superconductors, and to assess critically the likelihood of superconductivity in certain other types of substances ( e.g., other types of organic solids; metallic hydrogen at attainable high pressures), and to suggest ways of systematically improving existing classes or synthesizing novel classes of superconducting materials.

On the influence of competing interactions on physical properties of spin systems: numerical exact results for finite Ising and Heisenberg systems

The ground-state properties and low-lying excitations of spin systems with competing interactions are studied for various finite two-dimensional S=1/2 Heisenberg systems of 8 spins and corresponding Ising systems of up to 60 spins. The response of the spin systems on the conflict in the interactions is studied using numerical exact results. In the Ising model the competition of the interactions influences the ground state and the excitations essentially. Contrary to conventional Ising systems in disordered strongly frustrated Ising systems the low-energy density of states is smooth, the gap between the ground state and the first excitations vanishes. The low-lying excitations are characterised by a large portion of multiple spin excitations. Thermodynamic consequences are indicated. In the Heisenberg case the influence of the conflict in the interactions is less obvious, the character of the ground state and of the first excitation has similarities with that of conventional antiferromagnets, the spins have an enlarged number of possibilities to evade the conflict in the interactions.

The development of magnetic order in superconducting systems

Two different classes of rare-earth (RE) ternary superconductors (RERh 4B 4 and REMo 6X 8, X= S, Se) have provided the first instances in which chemically ordered sublattices of magnetic ions exist in superconductors. Neutron scattering studies show that simple, conventional antiferromagnetism coexists with superconductivity in a number of systems, while destruction of superconductivity occurs with the onset of ferromagnetism. We summarize the magnetic structural details for the coexistent antiferromagnets, and review measurements on the superconducting → ferromagnetic transition in ErRh 4B 4.