Magnetic Order Parameter Research Articles

Influence of paramagnetic La2/3Sr1/3MnO3 layer on the multiferroic property of Bi0.8Ba0.2FeO3 film

Multiferroics simultaneously exhibit several order parameters such as ferroelectricity and antiferromagnetism, representing an appealing class of multifunctional material. As the only multiferroics above room temperature, BiFeO3 (BFO) becomes an attractive choice for a wide variety of applications in the areas of sensors and spintronic devices. The coexistence of several order parameters brings about novel physical phenomena, for example, the magnetoelectric coupling effect. It allows the reversal of ferroelectric polarization by a magnetic field or the control of magnetic order parameter by an electric field. Heterostructure interface plays an important role in enhancing the ferroelectric and magnetic properties of multiferroic materials. Furthermore, the magnetoelectric coupling at the interface between the antiferromagnetism BFO and a ferromagnetic film has the close relation with achieving a functional multiferroic-ferromagnetic heterostructure. In order to determine the relationship between the multiferroic property and the interface experimentally, we prepare the Bi0.8Ba0.2FeO3(BBFO)/La2/3Sr1/3MnO3(LSMO) heterostructure on an SrTiO3(STO) substrate by pulsed laser deposition, and the structure characteristics and ferroelectric and magnetic properties are investigated. X-ray diffraction analysis shows that BBFO and LSMO films are epitaxially grown as single-phase. The further study by high-resolution transmission electron microscopy determines that the BBFO film has a tetragonal structure. The ferroelectric and magnetic measurements show that the magnetic and the ferroelectric properties are simultaneously improved, and the maximum values of the remnant polarization (2Pr) and the saturation magnetization of the heterostructure at room temperature are about 3.25 C/cm2 and 112 emu/cm3, respectively. The reasons for enhancing the ferroelectric and ferromagnetic properties of heterostructure are demonstrated by X-ray photoelectron spectrum that shows being unrelated to the valence states of Fe element. On the contrary, interface effect plays a major role. In addition, the magnetic resistivities and dielectric properties of BBFO/LSMO heterostructure are investigated at temperatures in a range of 50 K to 300 K, finding that magnetoresistance (MR) and magnetodielectric (MD) are respectively about -42.2% and 21.9% at 70 K with a magnetic field of 0.8 T, and the transition of magnetic phase takes place near 180 K. Furthermore, the temperature dependences of magnetodielectric and magnetoloss (ML) present opposite tendencies, suggesting that magnetodielectric is caused by Maxwell-Wagner effect and the magnetoresistance. Experimental results reveal that heterogeneous interface effect shows the exceptional advantages in enhancing multiferroic property and magnetoelectric coupling effect of complex heterostructure material. It is an effective way to speed up the application of multiferroic materials.

Critical space-time networks and geometric phase transitions from frustrated edge antiferromagnetism.

Recently I proposed a simple dynamical network model for discrete space-time that self-organizes as a graph with Hausdorff dimension d(H)=4. The model has a geometric quantum phase transition with disorder parameter (d(H)-d(s)), where d(s) is the spectral dimension of the dynamical graph. Self-organization in this network model is based on a competition between a ferromagnetic Ising model for vertices and an antiferromagnetic Ising model for edges. In this paper I solve a toy version of this model defined on a bipartite graph in the mean-field approximation. I show that the geometric phase transition corresponds exactly to the antiferromagnetic transition for edges, the dimensional disorder parameter of the former being mapped to the staggered magnetization order parameter of the latter. The model has a critical point with long-range correlations between edges, where a continuum random geometry can be defined, exactly as in Kazakov's famed 2D random lattice Ising model but now in any number of dimensions.

Magnetically modulated critical current densities of Co/Nb hybrid.

By tuning morphology and size of magnetic subsystem, ferromagnet-superconductor (F/S) hybrid system provides an effective way to modulate superconductivity due to the interaction between superconducting and magnetic-order parameters at the mesoscopic length scale. In this work, we report on investigations of critical current density in a large-area Co/Nb hybrid via facile colloidal lithography. Here, Co hexagon shell array as a magnetic template build on Nb film to modulate the critical current density. A novel superconducting transition has been observed in I-V curve with two metastable transition states: double-transition and binary-oscillation-transition states. Importantly, such unusual behavior can be adjusted by temperature, magnetic field and contact area of F/S. Such hybrid film has important implications for understanding the role of magnetic subsystem modulating superconductivity, as well as applied to low-energy electronic devices such as superconducting current fault limiters.

Coexistence of Superconductivity and Ferromagnetism in Superconducting ErRh<sub>4</sub>B<sub>4</sub>

This research work focuses on the theoretical investigation of the possible coexistence of superconductivity and ferromagnetism in ErRh4B4. By developing a model Hamiltonian for the given system and by using the double time temperature-dependent Green’s function formalism, we obtained expressions for superconducting transition temperature (T C, ), magnetic ordering temperature (T m ), superconducting and magnetic ordering parameters (Δ) and (η) respectively. By using the experimental and theoretical values and by considering plausible approximations of the parameters in the obtained expressions, the phase diagrams of superconducting transition temperature versus magnetic ordering parameter and magnetic ordering temperature versus magnetic ordering parameter are plotted. Finally, by combining the two phase diagrams, we showed the possible coexistence of superconductivity and ferromagnetism in ErRh 4 B 4 . Keywords : Superconductivity, ferromagnetism, order parameter, conduction electrons, localized electrons, Green’s function.

Antisymmetric exchange in La-substituted BiFe0.5Sc0.5O3 system: symmetry adapted distortion modes approach

Abstract Neutron powder diffraction measurements on the 35 % La-substituted Bi1−x La x Fe0.5Sc0.5O3 composition revealed that the samples obtained under high-pressure (6 GPa) and high-temperature (1500 K) conditions crystalize into a distorted perovskite structure with the orthorhombic Pnma symmetry and the unit cell parameters: a o = 5.6745(2) Å, b o = 7.9834(3) Å and c o = 5.6310(2) Å. A long-range magnetic ordering takes place below 220 K and implies a G-type magnetic structure with the moments 4.10(4)μ B per Fe aligned predominately along the orthorhombic c-axis. The space group representation theory using the orthorhombic symmetry yields four bi-linear coupling schemes for the magnetic order parameters imposed by antisymmetric exchange interactions. The couplings are analysed based on symmetry adapted distortion modes defined in respect of the undistorted cubic perovskite structure. The approach allows a quantitative estimation of the coupling strength. It is shown that the experimentally found spin configuration combines the magnetic order parameters coupled by the atomic displacement modes with the largest amplitudes. The results indicate that the antisymmetric exchange is the dominant anisotropic term which fully controls the direction of the Fe3+ spins in the distorted perovskite lattice.

Coexistence of superconductivity and magnetism inCa1−xNaxFe2As2: Universal suppression of the magnetic order parameter in 122 iron pnictides

We present a detailed investigation of the magnetic and superconducting properties of ${\mathrm{Ca}}_{1\ensuremath{-}x}{\mathrm{Na}}_{x}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ single crystals with $x=0.00$, 0.35, 0.50, and 0.67 by means of the local probe techniques M\ossbauer spectroscopy and muon spin relaxation experiments. With increasing Na-substitution level, the magnetic order parameter is suppressed. For $x=0.50$ we find a microscopic coexistence of magnetic and superconducting phases accompanied by a reduction of the magnetic order parameter below the superconducting transition temperature ${T}_{c}$. A systematic comparison with other 122 pnictides reveals a square-root correlation between the reduction of the magnetic order parameter and the ratio of the transition temperatures ${T}_{c}/{T}_{N}$, which can be understood in the framework of a Landau theory. In the optimally doped sample with ${T}_{c}\ensuremath{\approx}34$ K, diluted magnetism is found and the temperature dependence of the penetration depth and superfluid density are obtained, proving the presence of two superconducting $s$-wave gaps.

Quantum phase transitions in two-dimensional strongly correlated fermion systems

In this article, we review our recent work on quantum phase transition in two-dimensional strongly correlated fermion systems. We discuss the metal–insulator transition properties of these systems by calculating the density of states, double occupancy, and Fermi surface evolution using a combination of the cellular dynamical mean-field theory (CDMFT) and the continuous-time quantum Monte Carlo algorithm. Furthermore, we explore the magnetic properties of each state by defining magnetic order parameters. Rich phase diagrams with many intriguing quantum states, including antiferromagnetic metal, paramagnetic metal, Kondo metal, and ferromagnetic insulator, were found for the two-dimensional lattices with strongly correlated fermions. We believe that our results would lead to a better understanding of the properties of real materials.

Self-consistent approach for magnetic ordering and excited site occupation processes in a two-level system

Ferromagnetic ordering in a two-level partially excited system is studied in detail. Magnitudes of magnetization (magnetic order parameter) and lattice ordering (excited level occupation number) are calculated self-consistently. The influence of an external magnetic field and excited level gap on the ferromagnetic phase transition is discussed.

Macroscopic degeneracy and order in the 3D plaquette Ising model

The purely plaquette 3D Ising Hamiltonian with the spins living at the vertices of a cubic lattice displays several interesting features. The symmetries of the model lead to a macroscopic degeneracy of the low-temperature phase and prevent the definition of a standard magnetic order parameter. Consideration of the strongly anisotropic limit of the model suggests that a layered, “fuki-nuke” order still exists and we confirm this with multi-canonical simulations. The macroscopic degeneracy of the low-temperature phase also changes the finite-size scaling corrections at the first-order transition in the model and we see this must be taken into account when analyzing our measurements.

Effect of Fe doping on the magnetic ordering temperature of ErMnO3

We studied the effect of Fe doping on structural, magnetic, and dielectric properties of hexagonal ErMnO3 system. For 50% doping of Fe on Mn site in ErMnO3 modulated its crystallographic structure from hexagonal to orthorhombic phase. Accompanied with the structural phase transition in ErMnO3, the magnetic properties are effectively modified. The Fe doped samples exhibit enhancement in antiferromagnetic ordering Néel temperature (TN) from 77 K (ErMnO3) to 280 K (ErFe0.5Mn0.5O3). The anomalies observed in the dielectric constant around TN in doped ErMnO3 samples indicate the coupling between electric and magnetic order parameters.

Bosons and Magnons in Ordered Magnets

In earlier experimental studies we have shown that in accordance with the principles of renormalization group theory the spin dynamics of ordered magnets is controlled by a boson guiding eld instead by exchange interactions between nearest magnetic neighbors. In particular, thermal decrease of the magnetic order parameter is given by the heat capacity of the boson eld. The typical signature of boson dynamics is that the critical power functions either at T = Tc or at T = 0 hold up to a considerable distance from critical temperature. The critical power functions of the atomistic models hold asymptotically at T = Tc or at T = 0 only. In contrast to the atomistic magnons eld bosons cannot directly be observed using inelastic neutron scattering. However, for some classes of magnets the eld bosons seem to have magnetic moment and thus are able to interact directly with magnons. This interaction, although weak in principle, leads to surprisingly strong functional modi cations in the magnon dispersions at small q-values. In particular, the magnon excitation gap seems to be due to the magnon boson interaction. In this communication we want to show that for small q-values the continuous part of the magnon dispersions can be tted over a nite q-range by a power function of wave vector. The power function can be identi ed with the dispersion of the eld bosons. It appears that for low q-values magnon dispersions get attracted by the boson dispersion and assume the dispersion of the bosons. This allows for an experimental evaluation of the boson dispersions from the known magnon dispersions. Exponent values of 1, 1.25, 1.5, and 2 have been identi ed. The boson dispersion relations and the associated power functions of temperature for the heat capacity of the boson elds are now empirically known for all dimensions of the eld and for magnets with integer and half-integer spin quantum number. These are two 2× 3 exponent schemes.

Ab initio investigation of the magnetic and ferroelectric properties of double perovskites LaPbMeSbO6 (Me = Mn, Fe, Co, Ni)

The results of ab initio calculations of the ferroelectric and magnetic properties of the recently synthesized double perovskites LaPbMeSbO6 (Me2+ = Mn2+, Fe2+, Co2+, Ni2+) have been presented. The ordered double perovskites with a layered ordering of cations A and A′ and a checkerboard ordering of cations B and B′ have been considered. The calculation of the lattice dynamics has revealed instabilities in the phonon spectra of the high-symmetry phase of these compounds. The condensation of unstable modes leads to a stable polar phase P21. The spontaneous polarization in this phase is found to be ~30 μC/cm2. The magnetic ground state is antiferromagnetic with an antiparallel direction of the magnetic moments in adjacent planes along the [001] direction. The Neel temperatures are estimated in the mean field approximation. The presence of the ferroelectric and magnetic order parameters in the structure suggests that the studied double perovskites with the simultaneous ordering of the cations are potential multiferroics.

Ferroelectric and Magnetodielectric Properties of Cobalt-Doped Sr x Ba1−x Nb2O6 Ceramics

Relaxor ferroelectrics Sr0.5Ba0.5Nb2O6 (SBN50) and Sr0.4Ba0.6Nb2O6 (SBN40), both pure and cobalt (Co) doped, have been synthesized and investigated to␣understand possible interaction between magnetic and electric order parameters. Synthesis was carried out via ceramic route. This paper reports the synthesis, crystal structure, dielectric properties, P–E hysteresis loops, M–H hysteresis loops, and magnetocapacitance (M c) of the Co-doped SBN compositions. All compositions were observed to exhibit useful values of M c, especially at frequencies less than 10 kHz.

Polarization control at spin-driven ferroelectric domain walls.

Unusual electronic states arise at ferroelectric domain walls due to the local symmetry reduction, strain gradients and electrostatics. This particularly applies to improper ferroelectrics, where the polarization is induced by a structural or magnetic order parameter. Because of the subordinate nature of the polarization, the rigid mechanical and electrostatic boundary conditions that constrain domain walls in proper ferroics are lifted. Here we show that spin-driven ferroelectricity promotes the emergence of charged domain walls. This provides new degrees of flexibility for controlling domain-wall charges in a deterministic and reversible process. We create and position a domain wall by an electric field in Mn0.95Co0.05WO4. With a magnetic field we then rotate the polarization and convert neutral into charged domain walls, while its magnetic properties peg the wall to its location. Using atomistic Landau-Lifshitz-Gilbert simulations we quantify the polarization changes across the two wall types and highlight their general occurrence.

Interfacial spin and heat transfer between metals and magnetic insulators

We study the role of thermal magnons in the spin and heat transport across a normal-metal/insulating-ferromagnet interface, which is beyond an elastic electronic spin transfer. Using an interfacial exchange Hamiltonian, which couples spins of itinerant and localized orbitals, we calculate spin and energy currents for an arbitrary interfacial temperature difference and misalignment of spin accumulation in the normal metal relative to the ferromagnetic order. The magnonic contribution to spin current leads to a temperature-dependent torque on the magnetic order parameter; reciprocally, the coherent precession of the magnetization pumps spin current into the normal metal, the magnitude of which is affected by the presence of thermal magnons.

Rhodopsin/Lipid Hydrophobic Matching—Rhodopsin Oligomerization and Function

Lipid composition of the membrane and rhodopsin packing density strongly modulate the early steps of the visual response of photoreceptor membranes. In this study, lipid-order and bovine rhodopsin function in proteoliposomes composed of the sn-1 chain perdeuterated lipids 14:0d27-14:1-PC, 16:0d31-16:1-PC, 18:0d35-18:1-PC, or 20:0d39-20:1-PC at rhodopsin/lipid molar ratios from 1:70 to 1:1000 (mol/mol) were investigated. Clear evidence for matching of hydrophobic regions on rhodopsin transmembrane helices and hydrophobic thickness of lipid bilayers was observed from 2H nuclear magnetic resonance order parameter measurements at low rhodopsin concentrations. Thin bilayers stretched to match the length of transmembrane helices observed as increase of sn-1 chain order, while thicker bilayers were compressed near the protein. A quantitative analysis of lipid-order parameter changes suggested that the protein adjusts its conformation to bilayer hydrophobic thickness as well, which confirmed our earlier circular-dichroism measurements. Changes in lipid order parameters upon rhodopsin incorporation vanished for bilayers with a hydrophobic thickness of 27 ± 1 Å, suggesting that this is the bilayer thickness at which rhodopsin packs in bilayers at the lowest membrane perturbation. The lipid-order parameter studies also indicated that a hydrophobic mismatch between rhodopsin and lipids triggers rhodopsin oligomerization with increasing rhodopsin concentrations. Both hydrophobic mismatch and rhodopsin oligomerization result in substantial shifts of the equilibrium between the photointermediates metarhodopsin I and metarhodopsin II; increasing bilayer thickness favors formation of metarhodopsin II while oligomerization favors metarhodopsin I. The results highlight the importance of hydrophobic matching for rhodopsin structure, oligomerization, and function.

Magnetoelastic relaxations in EuTiO3

The multiferroic properties of EuTiO3 are greatly enhanced when a sample is strained, signifying that coupling between strain and structural, magnetic or ferroelectric order parameters is extremely important. Here resonant ultrasound spectroscopy has been used to investigate strain coupling effects, as well as possible additional phase transitions, through their influence on elastic and anelastic relaxations that occur as a function of temperature between 2 and 300 K and with applied magnetic field up to 14 T. Antiferromagnetic ordering is accompanied by acoustic loss and softening, and a weak magnetoelastic effect is also associated with the change in magnetization direction below . Changes in loss due to the influence of magnetic field suggest the existence of magnetic defects which couple with strain and may play a role in pinning of ferroelastic twin walls.

Investigations on Fe-doped strontium barium niobate, single phase ferroelectric and magnetodielectric compounds

To understand possible interaction between magnetic and electric order parameters of Fe -doped Sr x Ba 1-x Nb 2 O 6 (SBN), Fe -doped Sr 0.5 Ba 0.5 Nb 2 O 6 (SBN50) and Sr 0.4 Ba 0.6 Nb 2 O 6 (SBN40) are synthesized and investigated. Synthesis has been carried out via ceramic route. The paper reports synthesis, crystal structure, dielectric properties, P–E hysteresis loops, M–H hysteresis loops and magneto-capacitance of the Fe -doped SBN compositions. All the compositions are observed to exhibit a useful value of magneto-capacitance, especially at frequencies less than 10 kHz.

Boson Fields in Ordered Magnets

Spin wave theory of magnetism reveals two severe shortcomings. First, it considers non quantized classical spins and, second, the predicted temperature power functions for the thermal decrease of the magnetic order parameter hold asymptotically at T = 0 only. As experiments unambiguously show the dynamics is di erent for magnets with integer and half-integer spin and the critical power functions at T = 0 of type ∆M ≈ T e or at T = Tc of type ≈ (Tc − T ) hold over a nite temperature range, independent of spin structure. The nite critical range unequivocally indicates that the dynamics of the spins is controlled by a eld of freely propagating bosons instead by exchange interactions. Consequently, eld theories are necessary for description of the thermodynamics of ordered magnets. The experimental indications will be discussed that the eld quanta are essentially magnetic dipole radiation emitted by the precessing magnetic moments. Since integer and half-integer spins precess di erently the generated eld quanta and the dynamics of the eld are correspondingly di erent.

Quasiclassical magnetic order and its loss in a spin-12Heisenberg antiferromagnet on a triangular lattice with competing bonds

We use the coupled cluster method (CCM) to study the zero-temperature ground-state (GS) properties of a spin-1/2 $J_{1}$--$J_{2}$ Heisenberg antiferromagnet on a triangular lattice with competing nearest-neighbor and next-nearest-neighbor exchange couplings $J_{1}>0$ and $J_{2} \equiv \kappa J_{1}>0$, respectively, in the window $0 \leq \kappa < 1$. The classical version of the model has a single GS phase transition at $\kappa^{cl}=$1/8 in this window from a phase with 3-sublattice antiferromagnetic (AFM) 120$^{\circ}$ N\'{e}el order for $\kappa < \kappa^{cl}$ to an infinitely degenerate family of 4-sublattice AFM N\'{e}el phases for $\kappa > \kappa^{cl}$. This classical accidental degeneracy is lifted by quantum fluctuations, which favor a 2-sublattice AFM striped phase. For the quantum model we work directly in the thermodynamic limit of an infinite number of spins, with no consequent need for any finite-size scaling analysis of our results. We perform high-order CCM calculations within a well-controlled hierarchy of approximations, which we show how to extrapolate to the exact limit. In this way we find results for the case $\kappa = 0$ of the spin-1/2 model for the GS energy per spin, $E/N=-0.5521(2)J_{1}$, and the GS magnetic order parameter, $M=0.198(5)$, which are among the best available. For the spin-1/2 $J_{1}$--$J_{2}$ model we find that the classical transition at $\kappa=\kappa^{cl}$ is split into two quantum phase transition at $\kappa^{c}_{1}=0.060(10)$ and $\kappa^{c}_{2}=0.165(5)$. The two quasiclassical AFM states (viz., the 120$^{\circ}$ N\'{e}el state and the striped state) are found to be the stable GS phases in the regime $\kappa < \kappa^{c}_{1}$ and $\kappa > \kappa^{c}_{2}$, respectively, while in the intermediate regimes $\kappa^{c}_{1} < \kappa < \kappa^{c}_{2}$ the stable GS phase has no evident long-range magnetic order.