Magnetic Order Parameter Research Articles

Pure electronic metal-insulator transition at the interface of complex oxides.

In complex materials observed electronic phases and transitions between them often involve coupling between many degrees of freedom whose entanglement convolutes understanding of the instigating mechanism. Metal-insulator transitions are one such problem where coupling to the structural, orbital, charge, and magnetic order parameters frequently obscures the underlying physics. Here, we demonstrate a way to unravel this conundrum by heterostructuring a prototypical multi-ordered complex oxide NdNiO3 in ultra thin geometry, which preserves the metal-to-insulator transition and bulk-like magnetic order parameter, but entirely suppresses the symmetry lowering and long-range charge order parameter. These findings illustrate the utility of heterointerfaces as a powerful method for removing competing order parameters to gain greater insight into the nature of the transition, here revealing that the magnetic order generates the transition independently, leading to an exceptionally rare purely electronic metal-insulator transition with no symmetry change.

Formation and structure-property correlation of new bulk Fe–B–Si–Hf metallic glasses

The present work is devoted to designing new Fe–B–Si–Hf multi-component bulk glassy alloys with good soft magnetic and mechanical properties, and also explored the structure-property correlation in the microalloyed Fe–B–Si amorphous alloy with Hf. Based on the cluster-plus-glue-atom model, a series of Fe–B–Si–Hf alloys with cluster formulas [Si–B2Fe8−xHfx]Fe (x=0–0.6) are produced. Our experiments show that a large glass-forming ability along with a critical glass formation size of 2.5mm is reached at [Si–B2Fe7.7Hf0.3]Fe (in at.%: Fe72.5B16.7Si8.3Hf2.5). This glassy alloy shows a high fracture strength ~4000MPa with a plastic strain of 0.5%, a high saturation magnetization ~1.45T and a low coercive force ~1.5A/m. The monotonic variations of the thermal glass stability, microhardness and the magnetic ordering parameters with the concentration of micro-alloying element Hf, the accessibility of calorimetric glass transition and the glass-forming ability of the Fe–B–Si–Hf glassy alloys are explained in terms of the intra- and inter-atomic cluster correlations in the amorphous structures.

Landau model for the multiferroic delafossite antiferromagnets

A symmetry based framework is used to describe the complex phase diagrams observed in the multiferroic delafossite compounds. A free energy Landau functional is derived from the analysis of the transformation properties of the most general incommensurate magnetic spin order parameter. A principle of maximal symmetry is invoked and the stability of each of the different higher symmetry phases considered. The competition between different potential ground states is analysed within the scope of a simplified model, which emphasizes the role of the symmetry allowed phase dependent biquadratic couplings. The cross-over between the different competing states is also discussed. The results show that the diverse set of phase diagrams that are experimentally observed in this class of triangular lattice antiferromagnets and, in particular, the stabilization of magnetically induced ferroelectric states, can be well interpreted and described within this integrated phenomenological approximation.

Modulated spin helicity stabilized by incommensurate orbital density waves in a quadruple perovskite manganite

Through a combination of neutron diffraction and Landau theory we describe the spin ordering in the ground state of the quadruple perovskite manganite ${\mathrm{CaMn}}_{7}{\mathrm{O}}_{12}$---a magnetic multiferroic supporting an incommensurate orbital density wave that onsets above the magnetic ordering temperature, ${T}_{\mathrm{N}1}=90$ K. The multi-$\mathbf{k}$ magnetic structure in the ground state was found to be a nearly-constant-moment helix with modulated spin helicity, which oscillates in phase with the orbital occupancies on the ${\mathrm{Mn}}^{3+}$ sites via trilinear magneto-orbital coupling. Our phenomenological model also shows that, above ${T}_{\mathrm{N}2}=48$ K, the primary magnetic order parameter is locked into the orbital wave by an admixture of helical and collinear spin density wave structures. Furthermore, our model naturally explains the lack of a sharp dielectric anomaly at ${T}_{\mathrm{N}1}$ and the unusual temperature dependence of the electrical polarization.

Coexistence of Superconductivity and Antiferromagnetism in High-T C Gd1+x Ba2−x Cu3O7−δ Superconductor

Following the experimental discovery of the coexistence of superconductivity and antiferromagnetism in high- TC superconductor of fully oxygenated superconducting Gd1.2Ba1.8Cu3O7.03 by Rogacki, a theoretical study has been made by considering a model Hamiltonian consisting of pairing interaction, intra- and inter-atomic Coulomb interactions, exchange interaction, and a new interaction involving scattering of Cooper pairs by the magnetic moments/spins of Gd ions. Employing Green’s function formalism and a suitable decoupling approximation technique, the coexistence of superconductivity and antiferromagnetism has been shown to be a very distinct possibility. The phase diagrams of superconducting gap parameter (Δ) versus temperature (T) for various values of magnetic order parameter (η), the superconducting transition temperature (TC) and antiferromagnetism order temperature (TN) versus magnetic order parameter (η) have been plotted. Finally, by combining the two phase diagrams, the coexistence of superconductivity and antiferromagnetism has been demonstrated. The obtained result is in broad agreement with the experimental observations.

Differentiation between ferroelectricity and thermally stimulated current in pyrocurrent measurements of multiferroicMMn7O12(M=Ca, Sr, Cd, Pb)

Multiferroics present a dynamic crossover between magnetic and electronic order parameters. Using both the conventional pyroelectric current (PC) and the bias electric field (BE) methods, the authors investigate the electric polarization of multiferroic $M$Mn${}_{7}$O${}_{12}$ ($M$= Ca, Sr, Cd, Pb). While the pyrocurrent measurements show intense peaks below the magnetic ordering temperature, the peaks are not observed in the biased electric field method, which indicates the peaks are produced by thermal stimulation, not ferroelectricity. In CaMn${}_{7}$O${}_{12}$, the electric polarizations polarization produced is quite small, which suggests that the large polarizations reported in previously are not associated with intrinsic ferroelectricity.

Synthesis, Characterization and Exploration of Multiferroic Properties in Nano-Crystalline Tb1−x YxMnO3 (0 ≤ x ≤ 0.4)

We report the synthesis and electric properties of nano-crystalline Tb1-xYxMnO3 (x = 0, 0.1, 0.2, 0.3 and 0.4) compounds prepared by gel-combustion method. These samples were characterized by a number of techniques including X-ray diffraction (XRD), Raman spectroscopy, specific-heat measurement, neutron diffraction, and magnetic field dependent pyrocurrent measurement. All the samples crystallize in the orthorhombic structure with space group Pnma at room temperature. Anomalies were observed in low temperature specific-heat measurement corresponding to magnetic and electric phase transitions. The magnetic phase transitions occurred at ~35, ~22-28 and ~7 K for all the samples. Signatures of coupling between magnetic and electric order parameters were revealed by pyrocurrent measurements carried out in presence of magnetic fields.

Topological Effects on Quantum Phase Slips in Superfluid Spin Transport

We theoretically investigate effects of quantum fluctuations on superfluid spin transport through easy-plane quantum antiferromagnetic spin chains in the large-spin limit. Quantum fluctuations result in the decaying spin supercurrent by unwinding the magnetic order parameter within the easy plane, which is referred to as phase slips. We show that the topological term in the nonlinear sigma model for the spin chains qualitatively differentiates the decaying rate of the spin supercurrent between the integer versus half-odd-integer spin chains. An experimental setup for a magnetoelectric circuit is proposed, in which the dependence of the decaying rate on constituent spins can be verified by measuring the nonlocal magnetoresistance.

Discontinuity in heat capacity of Fe0.5Co0.5(110) alloy thin films

In this work we calculate heat capacity of alloy thin films of FeCo on the surface of the plane (110), using three parameters, the concentration x(i), the lattice long range order parameter t(i) and the magnetic order parameter σ(i), being i the number of layers of the thin film. The formulations reported by Hill [1] in the context of small particles and Valenta's model [2] can be applied to the film structure when we treat a thin film as a system divided into subsystems equivalent to two-dimensional parallel layers. The FeCo bulk alloy is completely homogeneous while a thin film have spatial discontinuities in their surfaces. We consider three ferromagnetic thin films formed by 11, 15 and 19 layers in the Helmholtz's free energy, which is minimized applying their first partial derivatives with respect to chemical composition, long range order parameter and magnetic order parameter. We calculate internal energy and heat capacity as a function of temperature and we verify that have two jumps as are reported in literature for the bulk; there are many results of bulk or surface effects of FeCo, but no enough results about ferromagnetic FeCo thin films and this fact does this work interesting.

Frustrated Heisenberg antiferromagnet on the honeycomb lattice with spin quantum number s ≥ 1

The ground-state (GS) phase diagram of the frustrated spin-s J1-J2-J3 Heisenberg antiferromagnet on the honeycomb lattice is studied using the coupled cluster method implemented to high orders of approximation, for spin quantum numbers s = 1, 3/2, 2 , 5/2. The model has antiferromagnetic (AFM) nearest-neighbour, next-nearest-neighbour and next-next-nearest-neighbour exchange couplings (with strength J1 > 0, J2 > 0 and J3 > 0, respectively). We specifically study the case J3 = J2 = κJ1, in the range 0 < κ < 1 of the frustration parameter, which includes the point of maximum classical (s → ∞) frustration, viz., the classical critical point at κcl = 1/2, which separates the Neel phase for κ < κcl and the collinear striped AFM phase for κ > κcl. Results are presented for the GS energy, magnetic order parameter and plaquette valence-bond crystal (PVBC) susceptibility. For all spins s > 3/2 we find a quantum phase diagram very similar to the classical one, with a direct first-order transition between the two collinear AFM states at a value κc(s) which is slightly greater than κcl [e.g., κc(3/2) ≈ 0.53(1)] and which approaches it monotonically as s → ∞. By contrast, for the case s = 1 the transition is split into two such that the stable GS phases are one with Néel AFM order for κ < κc1 = 0.485(5) and one with striped AFM order for κ > κc2 = 0.528(5), just as in the case s = 1/2 (for which κc1 ≈ 0.47 and κc2 ≈ 0.60). For both the s = 1/2 and s = 1 models the transition at κc2 appears to be of first-order type, while that at κc1 appears to be continuous. However, whereas in the s = 1/2 case the intermediate phase appears to have PVBC order over the entire range κc1 < κ < κc2, in the s = 1 case PVBC ordering either exists only over a very small part of the region or, more likely, is absent everywhere.

Signs of high-temperature superconductivity in frustrated manganites La1 – y Sm y MnO3 + δ (y = 0.85, 1)

The characteristics signs of the coexistence of nanoscale superconductivity and fluctuating antiferromagnetic state of the spin-liquid type have been revealed for the first time in frustrated manganites La1‒ySmyMnO3 + δ (δ ~ 0.1, y = 0.85, 1.0) in the form of macroscopic quantization of magnetic properties in weak magnetic fields. A sharp decrease and oscillations of close-in-magnitude critical temperatures of transitions to fluctuating antiferromagnetic (TA) and superconducting (Tc0) states with an increase in the external magnetic field strength have been found. Quantum oscillations of the magnetization and magnetic susceptibility near the critical temperatures of fluctuating antiferromagnetic phase transitions of the A- and CE-types have been discovered and investigated in detail. It has been shown that the studied samples exhibit properties of a multicomponent composite in which at temperatures T < 60 K in weak magnetic fields, there coexist fluctuating charge and antiferromagnetic correlations of the A- and CE-types with properties of the spin-liquid state and a small fraction of the superconducting phase in the form of individual and Josephsontunnel- junction-coupled superconducting loops with low critical currents. It has been assumed that, in samples with samarium concentrations y ≥ 0.8 at temperatures below 60 K, there is a new inhomogeneous state of doped manganites of the magneto-electronic liquid crystal type with strong quantum fluctuations of the magnetic and electronic order parameters, which are similar to electronic liquid crystals in lightly doped high-temperature superconducting cuprates.

Magnetoelastic coupling forbidden by time-reversal symmetry: Spin-direction-dependent magnetoelastic coupling on MnO, CoO, and NiO

In the Landau free energy, which is a powerful tool for describing the physical properties and phase transitions in condensed-matter physics, it has been generally believed that time-reversal symmetry allows only even-power polynomials in the magnetic moment when magnetic transition happens. Although no experimental evidence supports it, this symmetry constraint is very strict in theory. On the other hand, MnO, CoO, and NiO have been extensively studied for several decades since these materials are used to test advance experimental and theoretical methods. However, their precise spin directions and phase-transition mechanism have remained as a long-standing problem until now. To resolve these issues, we used the cutting-edge time-of-flight neutron powder diffractometer (SuperHRPD) at the Japan Proton Accelerator Research Complex (J-PARC) to study the spin-direction-dependent magnetoelastic coupling in MnO, CoO, and NiO. We also constructed a group-subgroup structure relation from $Fm\ensuremath{-}3m$ to $C2/m$ with exchange striction and a type of magnetostriction (dipolar and roto magnetostriction). These unified viewpoints and the high-resolution neutron powder diffractometer enable us to construct an order-parameter vector diagram. The order-parameter vector diagram reveals distinguished order-parameter coupling and phase-transition characters by different ${\mathrm{\ensuremath{\Gamma}}}_{1}^{+}({A}_{g})$ and ${\mathrm{\ensuremath{\Gamma}}}_{2}^{+}({B}_{g})$ spin direction. Moreover, the experiment results show clearly that the ${\mathrm{CoO}}_{6}$ octahedral distortion and the Co magnetic moment couple through a magnetoelastic coupling ${\ensuremath{\epsilon}}_{\text{oct}}\ensuremath{\sim}{M}^{3}$, which is forbidden by time-reversal symmetry but allowed by the more general magnetic symmetry---the so-called rotation-time-reversal symmetry in the double antisymmetry space group. Rotation-time-reversal symmetry allows the coupling of odd-power polynomials in the spin vector and odd-power ones in the roto vector when both spin- and roto-axial vectors belong to the same irreducible representation in Landau free energy. Here, we show experimental evidence that the magnetic order parameter and static structural order parameter belong to the same irreducible representation in the materials. Experimental discovery of general magnetic symmetry opens a way to find the physics forbidden by time-reversal symmetry in condensed matter physics.

Large-s expansions for the low-energy parameters of the honeycomb-lattice Heisenberg antiferromagnet with spin quantum number s

The coupled cluster method (CCM) is employed to very high orders of approximation to study the ground-state (GS) properties of the spin-s Heisenberg antiferromagnet (with isotropic interactions, all of equal strength, between nearest-neighbour pairs only) on the honeycomb lattice. We calculate with high accuracy the complete set of GS parameters that fully describes the low-energy behaviour of the system, in terms of an effective magnon field theory, viz., the energy per spin, the magnetic order parameter (i.e., the sublattice magnetization), the spin stiffness and the zero-field (uniform, transverse) magnetic susceptibility, for all values of the spin quantum numbers in the range 12≤s≤92. The CCM data points are used to calculate the leading quantum corrections to the classical (s→∞) values of these low-energy parameters, considered as large-s asymptotic expansions.

Spin-nematic order in antiferromagnetic spinor condensates

Large spin systems can exhibit unconventional types of magnetic ordering different from the ferromagnetic or N\'eel-like antiferromagnetic order commonly found in spin 1/2 systems. Spin-nematic phases, for instance, do not break time-reversal invariance and their magnetic order parameter is characterized by a second rank tensor with the symmetry of an ellipsoid. Here we show direct experimental evidence for spin-nematic ordering in a spin-1 Bose-Einstein condensate of sodium atoms with antiferromagnetic interactions. In a mean field description this order is enforced by locking the relative phase between spin components. We reveal this mechanism by studying the spin noise after a spin rotation, which is shown to contain information hidden when looking only at averages. The method should be applicable to high spin systems in order to reveal complex magnetic phases.

Macrospin dynamics in antiferromagnets triggered by sub-20 femtosecond injection of nanomagnons.

The understanding of how the sub-nanoscale exchange interaction evolves in macroscale correlations and ordered phases of matter, such as magnetism and superconductivity, requires to bridging the quantum and classical worlds. This monumental challenge has so far only been achieved for systems close to their thermodynamical equilibrium. Here we follow in real time the ultrafast dynamics of the macroscale magnetic order parameter in the Heisenberg antiferromagnet KNiF3 triggered by the impulsive optical generation of spin excitations with the shortest possible nanometre wavelength and femtosecond period. Our magneto-optical pump–probe experiments also demonstrate the coherent manipulation of the phase and amplitude of these femtosecond nanomagnons, whose frequencies are defined by the exchange energy. These findings open up opportunities for fundamental research on the role of short-wavelength spin excitations in magnetism and strongly correlated materials; they also suggest that nanospintronics and nanomagnonics can employ coherently controllable spin waves with frequencies in the 20 THz domain.

Short- and long-range magnetic order in LaMnAsO

The magnetic properties of the layered oxypnictide LaMnAsO have been revisited using neutron scattering and magnetization measurements. The present measurements identify the N\'eel temperature ${T}_{N}=360(1)$ K. Below ${T}_{N}$ the critical exponent describing the magnetic order parameter is $\ensuremath{\beta}=0.33--0.35$, consistent with a three-dimensional Heisenberg model. Above this temperature, diffuse magnetic scattering indicative of short-range magnetic order is observed, and this scattering persists up to ${T}_{\text{SRO}}=650(10)$ K. The magnetic susceptibility shows a weak anomaly at ${T}_{\text{SRO}}$ and no anomaly at ${T}_{N}$. Analysis of the diffuse scattering data using a reverse Monte Carlo algorithm indicates that above ${T}_{N}$ nearly two-dimensional, short-range magnetic order is present with a correlation length of 9.3(3) \AA{} within the Mn layers at 400 K. The inelastic scattering data reveal a spin gap of 3.5 meV in the long-range ordered state, and strong, low-energy (quasielastic) magnetic excitations emerging in the short-range ordered state. Comparison with other related compounds correlates the distortion of the Mn coordination tetrahedra to the sign of the magnetic exchange along the layer-stacking direction, and suggests that short-range order above ${T}_{N}$ is a common feature in the magnetic behavior of layered Mn-based pnictides and oxypnictides.

Competition of Spin and Charge Orders in a Model Cuprate

We present results of the mean-field analysis of the competition between charge and spin orders in a model cuprate. Phase diagrams and temperature dependencies of the charge and magnetic order parameters are calculated for varying doping given different values of the on-site correlation parameter, inter-site density–density coupling, and spin exchange integral.

Spin phonon interactions and magnetodielectric effects in multiferroic BiFeO3–PbTiO3

Here, we report the observation of magneto-dielectric and magneto-structural coupling in (1 − x)BiFeO3–xPbTiO3 i.e.(1 − x)BF–xPT) solid solutions with compositions in the vicinity of morphotropic phase boundary, as manifested by a combination of temperature dependent magnetic, Raman and dielectric measurements. Whilst x-ray diffraction and Raman spectroscopy suggest absence of any structural phase transition between 90–300 K, temperature dependent magnetic studies reveal magnetic anomalies in the solid solutions. These results are complemented by identical observations in the dielectric measurements at similar temperatures indicating a coupling between magnetic and electric order parameters. Further, Raman studies on rhombohedral i.e. x = 0.20 samples reveal a coupling between the magnetic structure and the lattice, causing spin-phonon interactions that are possibly responsible for observed magneto-dielectric effects. Our results illustrate that the phase transitions in BiFeO3–PbTiO3 system are fewer than expected and are attributed to a spatial averaging in an inhomogeneous albeit single-phase material due to clustering of Fe- or Ti-ions on different length scales.

Competition of Spin and Charge Orders in a Model Cuprate

We present results of the mean-field analysis of the competition between charge and spin orders in a model cuprate. Phase diagrams and temperature dependencies of the charge and magnetic order parameters are calculated for different doping given different values of the on-site correlation parameter, inter-site density-density coupling, and Heisenberg spin exchange integral.

Magnetostructural coupling behavior at the ferromagnetic transition in double-perovskiteSr2FeMoO6

The ordered double-perovskite $\mathrm{S}{\mathrm{r}}_{2}\mathrm{FeMo}{\mathrm{O}}_{6}$ (SFMO) possesses remarkable room-temperature low-field colossal magnetoresistivity and transport properties which are related, at least in part, to combined structural and magnetic instabilities that are responsible for a cubic-tetragonal phase transition near 420 K. A formal strain analysis combined with measurements of elastic properties from resonant ultrasound spectroscopy reveal a system with weak biquadratic coupling between two order parameters belonging to ${\mathrm{\ensuremath{\Gamma}}}_{4}^{+}$ and $m{\mathrm{\ensuremath{\Gamma}}}_{4}^{+}$ of parent space group $Fm\overline{3}m$. The observed softening of the shear modulus by \ensuremath{\sim}50% is due to the classical effects of strain/order parameter coupling at an improper ferroelastic $({\mathrm{\ensuremath{\Gamma}}}_{4}^{+})$ transition which is second order in character, while the ferromagnetic order parameter $(m{\mathrm{\ensuremath{\Gamma}}}_{4}^{+})$ couples only with volume strain. The influence of a third order parameter, for ordering of Fe and Mo on crystallographic B sites, is to change the strength of coupling between the ${\mathrm{\ensuremath{\Gamma}}}_{4}^{+}$ order parameter and the tetragonal shear strain due to the influence of changes in local strain heterogeneity at a unit cell scale. High anelastic loss below the transition point reveals the presence of mobile ferroelastic twin walls which become pinned by oxygen vacancies in a temperature interval near 340 K. The twin walls must be both ferroelastic and ferromagnetic, but due to the weak coupling between the magnetic and structural order parameters it should be possible to pull them apart with a weak magnetic field. These insights into the role of strain coupling and relaxational effects in a system with only weak coupling between three order parameters allow rationalization and prediction of how static and dynamic properties of the material might be tuned in thin film form by choice of strain contrast with a substrate.