Magnetic Order Parameter Research Articles

Frustrated spin-12Heisenberg antiferromagnet on a chevron-square lattice

The coupled cluster method (CCM) is used to study the zero-temperature properties of a frustrated spin-half ($s={1}{2}$) $J_{1}$--$J_{2}$ Heisenberg antiferromagnet (HAF) on a 2D chevron-square lattice. Each site on an underlying square lattice has 4 nearest-neighbor exchange bonds of strength $J_{1}>0$ and 2 next-nearest-neighbor (diagonal) bonds of strength $J_{2} \equiv x J_{1}>0$, with each square plaquette having only one diagonal bond. The diagonal bonds form a chevron pattern, and the model thus interpolates smoothly between 2D HAFs on the square ($x=0$) and triangular ($x=1$) lattices, and also extrapolates to disconnected 1D HAF chains ($x \to \infty$). The classical ($s \to \infty$) version of the model has N\'{e}el order for $0 < x < x_{{\rm cl}}$ and a form of spiral order for $x_{{\rm cl}} < x < \infty$, where $x_{{\rm cl}} = {1}{2}$. For the $s={1}{2}$ model we use both these classical states, as well as other collinear states not realized as classical ground-state (GS) phases, as CCM reference states, on top of which the multispin-flip configurations resulting from quantum fluctuations are incorporated in a systematic truncation scheme, which we carry out to high orders and extrapolate to the physical limit. We calculate the GS energy, GS magnetic order parameter, and the susceptibilities of the states to various forms of valence-bond crystalline (VBC) order, including plaquette and two different dimer forms. We find that the $s={1}{2}$ model has two quantum critical points, at $x_{c_{1}} \approx 0.72(1)$ and $x_{c_{2}} \approx 1.5(1)$, with N\'{e}el order for $0 < x < x_{c_{1}}$, a form of spiral order for $x_{c_{1}} < x < x_{c_{2}}$ that includes the correct three-sublattice $120^{\circ}$ spin ordering for the triangular-lattice HAF at $x=1$, and parallel-dimer VBC order for $x_{c_{2}} < x < \infty$.

Ca-Doping of BiFeO3: The Role of Strain in Determining Coupling between Ferroelectric Displacements, Magnetic Moments, Octahedral Tilting, and Oxygen-Vacancy Ordering.

Elastic and anelastic properties of a member of the BiFeO3–CaFeO2.5 perovskite solid solution (BCFO), which is known to have multiple instabilities, have been investigated by resonant ultrasound spectroscopy. This phase, with 64% Bi and 36% Ca on the A site, is antiferromagnetic (TN ∼650 K) and has an ordered arrangement of oxygen vacancies with tetragonal lattice geometry. The inverse mechanical quality factor, Q–1, has a maximum near 100 K, correlating closely with a peak in dielectric loss, reported previously, consistent with a loss mechanism that involves the movement of oxygen vacancies accompanied by local lattice distortion. At higher temperature, there is a further acoustic loss peak that is correlated with complex impedance anomalies. There is no clear relationship to the magnetic transition, and the observations are interpreted as relating to ionic conductivity. A small stiffening, scaling with the square of the magnetic order parameter below TN, indicates that the main coupling with strain is biquadratic, confirming that conventional coupling of magnetic order with symmetry-breaking shear strains is weak in BCFO. Data from the literature for BCFO indicates that local strain fields are likely to be responsible for suppressing the spin cycloid present in BiFeO3.

Magnetic Behaviors of BaTiO&lt;sub&gt;3&lt;/sub&gt;-BaFe&lt;sub&gt;12&lt;/sub&gt;O&lt;sub&gt;19&lt;/sub&gt; Nanocomposite Prepared by Sol-Gel Process Based on Differences in Volume Fraction

Barium titanate BaTiO3 (BTO) - barium hexaferrite BaFe12O19 (BHF) nanocomposite could be as a raw material of multiferroic. Multiferroic is a class of materials with coupled electric, magnetic and structural order parameters that yield simultaneous effects of ferroelectric, ferromagnetism and ferroelasticity in the same material. This material has potential applications in such as spintronic devices and sensors. This work was an earlier research towards formation of multiferroic material. Knowing magnetic properties that will lead to a better understanding of magnetoelectric coupling in multiferroic material is the objective of this research.The samples were BTO and BHF prepared by sol-gel and then were mixed in bulk system by a conventional techniques in various of volume fraction between BTO : BHF = 1:1 ; 1:2 and 2:1, then samples were sintered at 925°C for 5, 10 and 15 hours. Composite phase study was carried out using X-Ray Diffraction (XRD). MPS Magnet Physik EP3 Permagraph L was used to characterize magnetic properties. XRD results confirm that composite with volume fraction of BTO : BHF = 1:1 with sintering at 925°C for 5 hours consists only of 2 phases BTO and BHF. There is impurity phase BaFe2O4 beside BTO and BHF phases at samples with volume fraction BTO:BHF = 1:2 and 2:1 for longer sintering. Composite with volume fraction of BTO:BHF = 1:1 for 5 hours sintering has a high value of remanent magnetization 0.081 T and the lowest value of intrinsic coersive 333.6 kA/m leading to good characteristics of multiferroic materials.

Magnetic Behaviors of BaTiO&lt;sub&gt;3&lt;/sub&gt;-BaFe&lt;sub&gt;12&lt;/sub&gt;O&lt;sub&gt;19&lt;/sub&gt; Nanocomposite Prepared by Sol-Gel Process Based on Differences in Volume Fraction

Barium titanate BaTiO3 (BTO) - barium hexaferrite BaFe12O19 (BHF) nanocomposite could be as a raw material of multiferroic. Multiferroic is a class of materials with coupled electric, magnetic and structural order parameters that yield simultaneous effects of ferroelectric, ferromagnetism and ferroelasticity in the same material. This material has potential applications in such as spintronic devices and sensors. This work was an earlier research towards formation of multiferroic material. Knowing magnetic properties that will lead to a better understanding of magnetoelectric coupling in multiferroic material is the objective of this research.The samples were BTO and BHF prepared by sol-gel and then were mixed in bulk system by a conventional techniques in various of volume fraction between BTO : BHF = 1:1 ; 1:2 and 2:1, then samples were sintered at 925°C for 5, 10 and 15 hours. Composite phase study was carried out using X-Ray Diffraction (XRD). MPS Magnet Physik EP3 Permagraph L was used to characterize magnetic properties. XRD results confirm that composite with volume fraction of BTO : BHF = 1:1 with sintering at 925°C for 5 hours consists only of 2 phases BTO and BHF. There is impurity phase BaFe2O4 beside BTO and BHF phases at samples with volume fraction BTO:BHF = 1:2 and 2:1 for longer sintering. Composite with volume fraction of BTO:BHF = 1:1 for 5 hours sintering has a high value of remanent magnetization 0.081 T and the lowest value of intrinsic coersive 333.6 kA/m leading to good characteristics of multiferroic materials.

Magnetism of powder samples and of single crystals

The temperature dependence of the magnetic order parameter of NiF2, MnCO3 and K2NiF4 measured on powder samples is compared with data obtained on single crystals. It is found that the dynamic dimensionality of the NiF2 and MnCO3 powder samples is one-dimensional (1D) for all temperatures T<TN. This shows that the powder grains are mono-domain particles. In the multi-domain single crystals isotropic 3D dynamic symmetry is observed. In order to explain isotropy in the bulk single crystals a dynamic averaging process over all domain orientations has to be postulated. For single crystal material of K2NiF4 a critical exponent of β=0.14±0.01 has been reported. Our neutron diffraction measurements on powder material yield to a very good approximation mean field critical behaviour indicating isotropic dynamics in the critical range. This unexpected result points to a dynamic averaging process over all differently oriented powder grains. As we have argued earlier, the observed universality in the dynamics of ordered magnets is due to a boson guiding field. Averaging over all powder grains, requests that the mean free path of the field bosons is larger than the size of the grains. Additionally, it must be assumed that the field quanta are able to tunnel across the interface between adjacent grains. In polycrystalline bulk samples of the 2D ferromagnet Rb2CrCl4 and of the 1D antiferromagnet KCuF3 also mean field critical behaviour is identified. As a consequence, a similar averaging process as in the K2NiF4 powder sample can be assumed to average over the mosaic structure of polycrystalline bulk material of Rb2CrCl4 and KCuF3. High quality single crystals of Rb2CrCl4 and of KCuF3 exhibit critical exponent of β~0.3. Dependence of the critical exponents on the mesoscopic morphology of the sample is considered as a typical indication of a boson controlled dynamics, and may partly explain the surprisingly broad distribution of critical exponent values reported in literature.

Two-dimensional incommensurate and three-dimensional commensurate magnetic order and fluctuations in La2−xBaxCuO4

We present neutron scattering measurements on single crystals of lightly doped $La_{2-x}Ba_{x}CuO_{4}$, with $0 \leq x \leq? 0.035$. These reveal the evolution of the magnetism in this prototypical doped Mott insulator from a three dimensional (3D) commensurate (C) antiferromagnetic ground state, which orders at a relatively high TN, to a two dimensional (2D) incommensurate (IC) ground state with finite ranged static correlations, which appear below a relatively low effective TN. At low temperatures, the 2D IC magnetism co-exists with the 3D C magnetism for doping concentrations as low as ? 0.0125. We find no signal of a 3D C magnetic ground state by x $\sim$? 0.025, consistent with the upper limit of x $\sim$? 0.02 observed in the sister family of doped Mott insulators, $La_{2-x}Sr_{x}CuO_{4}$. The 2D IC ground states observed for $0.0125 \leq x \leq 0.035$ are diagonal, and are rotated by 45 degrees within the orthorhombic basal plane compared with those previously reported for samples with superconducting ground states: $La_{2-x}Ba_{x}CuO_{4}$, with $0.05 \leq? x \leq? 0.095. We construct a phase diagram based solely on magnetic order parameter measurements, which displays much of the complexity of standard high temperature superconductivity phase diagrams discussed in the literature. Analysis of high energy-resolution inelastic neutron scattering at moderately low temperatures shows a progressive depletion of the very low energy dynamic magnetic susceptibility as x increases from 0.0125 to 0.035. This low energy, dynamic susceptibility falls off? with increasing temperature on a scale much higher than the effective 2D IC TN appropriate to these materials. Appreciable dynamic 2D IC magnetic fluctuations inhabit much of the "pseudogap" regime of the phase diagram.

Magnetic field induced polarization and magnetoelectric effect of Ba0.8Ca0.2TiO3-Ni0.2Cu0.3Zn0.5Fe2O4 nanomultiferroic

The xBa0.8Ca0.2TiO3-(1 − x)NiCuZn ferrite (x = 0.1, 0.3, 0.5, 0.7, and 0.9) nanocomposites were prepared by using sol-gel method. The densification of these composites was carried out using microwave sintering method. The magnetic field induced changes in the ferroelectric polarization loop may support the possible magnetoelectric coupling between Ba0.8Ca0.2TiO3 and NiCuZn ferrite phases. The observed change in ferroelectric polarization with applied magnetic field proves the coupling between magnetic and ferroelectric order parameters. The loop change is observed with the composition and with magnetic field. The magnetoelectric coefficient of the nanocomposite with x = 0.3 shows a value of 280 mV/cm Oe is obtained.

Modeling of ferroelectric control of magnetic domain pattern and domain wall properties

In the field of spintronics, various approaches via spin-polarized currents have been proven effective in controlling the direction of magnetization in ferromagnetic layers. However, overheating problems are unavoidable in view of the high current densities involved. Alternatively, electric field control of magnetization in multiferroics, which couple the electric and magnetic order parameters, could be a promising approach due to the possibility of “zero-current” operations. In the present study, we investigated transferring of domain patterns from a ferroelectric substrate onto a ferromagnetic thin film. Under an applied electric field, the magnetic domain patterns could be maintained or erased depending on the field direction. The thickness and motion of the magnetic domain walls could also be manipulated by the applied electric field. The possibility of ferroelectric control of local magnetism is a new approach for producing electric-field controlled spintronics devices.

Spin-split conduction band in EuB6and tuning of half-metallicity with external stimuli

We report an Eu ${L}_{3}$-edge resonant inelastic x-ray scattering (RIXS) investigation of the electronic structure of EuB${}_{6}$. We observe that the RIXS spectral weight around 1.1 eV increases dramatically when the system is cooled below the ferromagnetic ordering temperature and follows the magnetic order parameter. This spectral feature is attributed to the intersite excitation from the local 4$f$ orbital to the spin-split 5$d$ orbital on the neighboring site, illustrating the essential role of exchange splitting of the conducting electrons. Based on our density functional theory calculations and the RIXS data, EuB${}_{6}$ at low temperature can be consistently described with a semimetallic electronic structure with incomplete spin polarization. We propose that half-metallicity in EuB${}_{6}$ can be achieved utilizing the strong tunability of the electronic structure against gate voltage, strain, and magnetic field.

Raman spectroscopy of nanocrystalline Mn-doped BiFeO3 thin films

Enhancement of magnetic and electric properties in multiferroic Mn-substituted BiFeO3 thin films (BiFe1− x Mn x O3 with x = 0.00–0.05) grown via the low-temperature chemical solution deposition process on low-cost substrates like indium tin oxide (ITO)/glass and silicon is reported in this work. A detailed structural analysis by high-resolution X-ray diffraction and Raman spectroscopy indicates that the grown films exhibit rhombohedral distorted perovskite structure without any impure phase. Raman spectroscopic analysis shows all 13 Raman active (4A1 + 9E) phonon modes corresponding to distorted rhombohedral BFO R3c structure. BFO thin film with 5% Mn substitution showed maximum saturation and remnant magnetisation (M s = 16.70 emu/g and M r = 2.58 emu/g) and remarkably reduced leakage current due to distortion in the spin cycloid structure and suppression of oxygen vacancies due to partial replacement of Fe3+ ion by Mn3+ ion, respectively. These enhanced multifunctional properties highlight the potential applications of BFMO system in thin film devices which can couple magnetic and ferroelectric order parameters.

Realization of theπ-state in junctions formed by multiband superconductors with a spin density wave

Using a simple model of multi-band superconductors, which can be applied in particular to Fe\nobreakdash-based pnictides, we calculate the Josephson current $I_{\text{J}}$ in a tunnel junction composed by such superconductors. We employ the tunneling Hamiltonian method and quasiclassical Green's functions. We study both the case of coexistence of the superconducting ($\Delta $) and magnetic (SDW---spin density wave) order parameters and the case when only the superconducting order parameter exists. We show that the current $I_{\text{J}}$ depends on the mutual orientation of magnetization of the SDW in the case of non-ideal nesting when the coexistence of superconducting and magnetic order parameters is possible as it takes place in Fe\nobreakdash-based pnictides. It is found that the realization of the $\pi$\nobreakdash-junction is possible in both cases. We compare our results for multi-band superconductors without the SDW with those obtained earlier and find that they coincide if the tunneling matrix elements are real. If these elements are complex, a new term appears in the formula for the Josephson critical current.

Magnetic switching of ferroelectric domains at room temperature in multiferroic PZTFT

Single-phase magnetoelectric multiferroics are ferroelectric materials that display some form of magnetism. In addition, magnetic and ferroelectric order parameters are not independent of one another. Thus, the application of either an electric or magnetic field simultaneously alters both the electrical dipole configuration and the magnetic state of the material. The technological possibilities that could arise from magnetoelectric multiferroics are considerable and a range of functional devices has already been envisioned. Realising these devices, however, requires coupling effects to be significant and to occur at room temperature. Although such characteristics can be created in piezoelectric-magnetostrictive composites, to date they have only been weakly evident in single-phase multiferroics. Here in a newly discovered room temperature multiferroic, we demonstrate significant room temperature coupling by monitoring changes in ferroelectric domain patterns induced by magnetic fields. An order of magnitude estimate of the effective coupling coefficient suggests a value of ~1 × 10−7 sm−1.

Elastic and anelastic relaxations accompanying magnetic ordering and spin-flop transitions in hematite, Fe2O3

Hematite, Fe2O3, provides in principle a model system for multiferroic (ferromagnetic/ferroelastic) behavior at low levels of strain coupling. The elastic and anelastic behavior associated with magnetic phase transitions in a natural polycrystalline sample have therefore been studied by resonant ultrasound spectroscopy (RUS) in the temperature range from 11 to 1072 K. Small changes in softening and attenuation are interpreted in terms of weak but significant coupling of symmetry-breaking and non-symmetry-breaking strains with magnetic order parameters in the structural sequence . The transition at TN = 946 ± 1 K is an example of a multiferroic transition which has both ferromagnetic (from canting of antiferromagnetically ordered spin moments) and ferroelastic (rhombohedral → monoclinic) character. By analogy with the improper ferroelastic transition in Pb3(PO4)2, W and W′ ferroelastic twin walls which are also 60° and 120° magnetic domain walls should develop. These have been tentatively identified from microstructures reported in the literature. The very low attenuation in the stability field of the C2/c structure in the polycrystalline sample used in the present study, in comparison with the strong acoustic dissipation reported for single crystal samples, implies, however, that the individual grains each consist of a single ferroelastic domain or that the twin walls are strongly pinned by grain boundaries. This absence of attenuation allows an intrinsic loss mechanism associated with the transition point to be seen and interpreted in terms of local coupling of shear strains with fluctuations which have relaxation times in the vicinity of ∼10−8 s. The first order (Morin) transition occurs through a temperature interval of coexisting phases but the absence of an acoustic loss peak suggests that the relaxation time for interface motion is short in comparison with the time scale of the applied stress (at ∼0.1–1 MHz). Below the Morin transition a pattern of attenuation which resembles that seen below ferroelastic transitions has been found, even though the ideal low temperature structure cannot contain ferroelastic twins. This loss behavior is tentatively ascribed to the presence of local ferromagnetically ordered defect regions which are coupled locally to shear strains.

A unified model for the dynamics of driven ribbon with strain and magnetic order parameters

We develop a unified model to explain the dynamics of driven one dimensional ribbon for materials with strain and magnetic order parameters. We show that the model equations in their most general form explain several results on driven magnetostrictive metallic glass ribbons such as the period doubling route to chaos as a function of a dc magnetic field in the presence of a sinusoidal field, the quasiperiodic route to chaos as a function of the sinusoidal field for a fixed dc field, and induced and suppressed chaos in the presence of an additional low amplitude near resonant sinusoidal field. We also investigate the influence of a low amplitude near resonant field on the period doubling route. The model equations also exhibit symmetry restoring crisis with an exponent close to unity. The model can be adopted to explain certain results on magnetoelastic beam and martensitic ribbon under sinusoidal driving conditions. In the latter case, we find interesting dynamics of a periodic one orbit switching between two equivalent wells as a function of an ac magnetic field that eventually makes a direct transition to chaos under resonant driving condition. The model is also applicable to magnetomartensites and materials with two order parameters.

Photoinduced femtosecond formation of ferromagnetism in a strongly correlated antiferromagentic manganite

We report a pump threshold behavior in fs photoinduced magnetization in a strongly correlated manganite, which indicates the establishment of thermally-inaccessible ferromagnetic ground state and build-up of new magnetic order parameters at fs time scales

Large magnetoresistance and charge transfer between the conduction and magnetic electrons in layered oxyselenide BiOCu0.96Se

The electrical and magnetic properties of slightly Cu-deficient BiOCu(0.96)Se have been investigated using neutron and X-ray diffraction, ac magnetic susceptibility, magnetization and electric resistivity measurements. The layered BiOCu(0.96)Se crystallizes into a tetragonal lattice with a P4/nmm symmetry. Thermal profiles of the electrical resistivity reveal a semiconductor type behavior, but depart from its course at low temperatures when antiferromagnetic coupling becomes thermally loosened at 140 K. Positive magnetoresistances are obtained at all temperatures studied. With an applied magnetic field of 0.5 kOe, the magnetoresistance reaches 235% at 2 K. It decreases with increasing temperature, but stabilizes to 70% above 60 K. Both ferromagnetic and antiferromagnetic coupling are detected between the Cu spins in the SeCu4 pyramidal blocks, which results in a non-collinear spin arrangement at low temperatures. The antiferromagnetic component becomes disordered above T(N) = 140 K, whereas the ferromagnetic moment persists up to T(C) = 300 K. Interlayer charge transfer between the conduction and magnetic electrons gives rise to an anomaly in the magnetic order parameter.

Dynamic off-centering of Cr3+ions and short-range magneto-electric clusters in CdCr2S4

The cubic spinel CdCr2S4 gained recently a vivid interest, given the relevance of relaxor-like dielectric behavior in its paramagnetic phase. By a singular combination of local probe techniques, namely, pair distribution function and perturbed angular correlation, we firmly establish that the Cr ion plays the central key role on this exotic phenomenon, namely, through a dynamic off-centering displacement of its coordination sphere. We further show that this off-centering of the magnetic Cr ion gives rise to a peculiar entanglement between the polar and magnetic degrees of freedom, stabilizing, in the paramagnetic phase, short-range magnetic clusters, clearly seen in ultralow-field susceptibility measurements. Moreover, the Landau theory is here used to demonstrate that a linear coupling between the magnetic and polar order parameters is sufficient to justify the appearance of magnetic cluster in the paramagnetic phase of this compound. These results open insights on the hotly debated magnetic and polar interaction, setting a step forward in the reinterpretation of the coupling of different physical degrees of freedom.

Direct evidence of non-Zhang-Rice Cu3+centers in La2Li0.5Cu0.5O4

A well-isolated Zhang-Rice (ZR) singlet as a ground state of the Cu${}^{3+}$ center in hole-doped cuprates is a leading paradigm in modern theories of high-temperature superconductivity. However, a dramatic temperature evolution of the ${}^{6,7}$Li NMR signal in La${}_{2}$Li${}_{0.5}$Cu${}_{0.5}$O${}_{4}$, a system with a regular lattice of well-isolated Cu${}^{3+}$ centers, reveals significant magnetic fluctuations and suggests a quasidegeneracy to be a generic property of their ground state at variance with the simple ZR model. We argue for a competition of the ZR state with nearby states formed by a ``doped'' hole occupying purely oxygen nonbonding ${a}_{2g}(\ensuremath{\pi})$ and ${e}_{u}(\ensuremath{\pi})$ orbitals rather than a conventional ${b}_{1g}({d}_{{x}^{2}\ensuremath{-}{y}^{2}})$ Cu 3$d$-O 2$p$ hybrid. The temperature variation of the ${}^{6,7}$Li NMR line shape and spin-lattice relaxation rate point to a gradual slowing down of some magnetic order parameter's fluctuations without distinct signatures of a phase transition down to $T=2$ K. This behavior agrees with a stripelike ferrodistortive fluctuating $Ammm$ order in a two-dimensional structure of the (CuLi)O${}_{2}$ planes accompanied by unconventional oxygen orbital antiferromagnetic fluctuations.

Long range magnetic order in azurite from μSR

We report a Muon Spin Rotation (μSR) study on single crystalline azurite. The measurements revealed clear evidence for long range magnetic order below the Néel temperature at TN = 1.9 K. The spectra in the magnetically ordered regime were fitted with three different muon sites, which can be related to muons close to Cu2+ monomer and dimer ions, respectively. The spontaneous muon spin rotation frequencies revealed the development of a magnetic order parameter. Below 0.4 K, transverse damping data indicated a further change in the local magnetic environment of the Cu2+ monomer ions.

ChemInform Abstract: Electric Field Control of Magnetism in Multiferroic Heterostructures

AbstractReview: 632 refs.