Spiral Spin Structure Research Articles

Low temperature ferromagnetic ordering and dielectric properties of Bi1-xDyxFeO3 ceramics

Polycrystalline Bi1−xDyxFeO3 (x=0.03, 0.05, 0.07, 0.10, and 0.12) ceramics were synthesized by solid state reaction method. Rietveld refinement reveals that all the samples crystallize in the rhombohedral structure with non-centrosymmetric R3c space group. Substitution induced suppression of the spiral spin structure and Dy3+-Fe3+ interaction enhanced the room temperature magnetization. Temperature dependent magnetization curves suggest that the magnetic coupling between Dy3+-Fe3+ and Dy3+-Dy3+ ions enhanced at low temperatures range for x=0.07–0.12 samples. ESR study indicated enhanced magnetic properties due to change in magnetic behavior of Fe3+ ions. The change in the Fe-O-Fe bond angle corresponds to the increase in Neel transition temperature with increasing Dy concentration. The dielectric properties are improved with increasing Dy content in BiFeO3. The grain and grain boundary contributions decrease with increase in temperature for all samples, indicating a negative temperature coefficient of resistance.

Spontaneous Polarization Reversal in Multiferroic Mn<sub>0.85</sub>Co<sub>0.15</sub>WO<sub>4</sub> Induced by an External Magnetic Field

The multiferroic properties of Mn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.85</sub> Co <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.15</sub> WO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> are studied in magnetic fields oriented along the monoclinic c-axis. A spontaneous sign reversal of the ferroelectric polarization is observed upon decreasing temperature if cooled in a constant magnetic field. This unusual phenomenon is explained by the coexistence of different multiferroic phases with conical and spiral spin structures. These phases form domains which contribute to the electrical polarization in different temperature ranges. The sign reversal of the polarization is explained by an analysis of the contributions from the different domains to the net value of the polarization. It is concluded that the spiral and conical magnetic orders in neighboring domains are strongly coupled leading to the preservation of the spin chirality across the domain walls.

Magnetic structure and domain conversion of the quasi-2D frustrated antiferromagnet CuCrO2 probed by NMR

We have carried out 63,65Cu NMR spectra measurements in magnetic field up to about 15.5 T on single crystal of a multiferroic triangular-lattice antiferromagnet CuCrO2. The measurements were performed for perpendicular and parallel orientation of the magnetic field with respect to the c-axis of the crystal, and the detailed angle dependence of the spectra on the magnetic field direction within ab-plane was studied. The shape of the spectra can be well described in the model of spiral spin structure proposed by recent neutron diffraction experiments. When field is rotated perpendicular to crystal c-axis, we observed, directly for the first time, a remarkable reorientation of spin plane simultaneous with rotation of the incommensurate wavevector by quantitatively deducing the conversion of less energetically favorable domain to a more favorable one. At high enough fields parallel to c-axis, the data are consistent with either a field-induced commensurate spiral magnetic structure or an incommensurate spiral magnetic structure with a disorder in the c direction, suggesting that high fields may have influence on interplanar ordering.

Size dependence of structure and magnetic properties of CoCr2O4 nanoparticles synthesized by hydrothermal technique

The size dependence of structure and magnetic behavior has been investigated on spinel chromite CoCr2O4 nanoparticles (NPs) with different particles from ~2.8 to ~19.7 nm, synthesized by a hydrothermal technique. The microstructure analysis shows that the lattice parameter increases monotonically with reduced particle sizes. Compared with the bulk sample, all synthesized NPs exhibit lower Curie temperature (TC~87 K) from paramagnetic (PM) to collinear short-range ferrimagnetic (FiM) state, and this PM–FiM transition is gradually degraded with decreased particle sizes and nearly vanishes for ~2.8 nm NPs. No magnetic transition from short-range FiM order to long-range spiral spin structure is found at low temperatures contrast to the bulk ones. Instead, a cluster spin-glass (SG) transition appears for NPs with D≤5.4 nm. The dynamic behavior of the cluster SG can be described by a power law with the spin relaxation time in the range of 10−10–10−8 s and critical exponent zv′=9.9±0.6, in accordance with a three-dimensional Ising SG behavior. The glass transition temperature (Tg) decreases from 16.3 K for ~5.4 nm to 12.8 K for ~2.8 nm NPs.

Origin of enhanced multiferroic properties in Dy and Co co-doped BiFeO3 ceramics

Structural, magnetic and ferroelectric properties of polycrystalline BiFeO3, Bi0.9Dy0.1FeO3, BiFe0.97Co0.03O3 and Bi0.9Dy0.1Fe0.97Co0.03O3, which were prepared by solid-state method, have been investigated. The X-ray diffraction (XRD) patterns reveal that all the samples are in single phase and show rhombohedrally distorted perovskite structure with R3c space group. Both XRD and Raman-scattering studies show that Dy and Co co-doped sample has a compressive lattice distortion induced by co-substitution of Dy and Co ions at the A and the B sites, respectively. Dy and Co co-doping favors weak ferromagnetism ordering with evident magnetic hysteresis loop and enhances magnetization values at room temperature. Ferroelectric hysteresis loop for Dy and Co co-doped sample shows the nearly saturated polarization at 40 kV/cm and large remnant polarization. Dy dopant is prominent in the reduced leakage current density, while Co dopant is remarkable in the improved remnant magnetization in Bi0.9Dy0.1Fe0.97Co0.03O3 ceramic. By using a simple model, it was found that the anharmonicity of canted spiral cycloidal spin structure was responsible for the weak ferromagnetism of pure BiFeO3, and the enhanced magnetization in Co-doped sample was attributed to the transition from the incommensurate cycloidal spiral spin structure towards the G-type canted collinear antiferromagnetic structure.

Effect of Solvents on the Ferromagnetic Behavior of Undoped BiFeO&lt;sub&gt;3&lt;/sub&gt; Prepared by Sol-Gel

We here report the preparation of undoped BiFeO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> (BFO) thin films with sol-gel method using two solvents, i.e., acetic acid (AA) and ethylene glycol (EG). Films are annealed in the temperature range 100-300 °C for 60 mins. Films deposited using AA show the presence of bismuth rich phase Bi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">46</sub> Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">72</sub> along with BFO in the intermediary conditions. Pure phase BFO is obtained when films are deposited using EG as a solvent even under as-deposited conditions, and the pure phase persists after annealing in the temperature range 100 °C-300 °C. This is the lowest temperature reported for the preparation of phase pure BFO. Bandgap of AA-based films lies in the range 2.58-2.64 eV and that of EG-based films in the range 2.77-2.81 eV. Reduced value of dielectric constant, in AA-based films, is because of the presence of bismuth rich impurity phase as compared with the films prepared using EG. Bands in FTIR spectra at 487 and 552 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> are characteristic bands of FeO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> and BiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> groups, indicating the formation of BFO. Films deposited using AA show weak ferromagnetic behavior, while those deposited using EG show strong ferromagnetic behavior as opposed to antiferromagnetic nature of BFO, due to suppression of spiral spin structure.

Commensurate-incommensurate magnetic phase transition in the Fe-doped bilayer ruthenateCa3Ru2O7

Neutron diffraction studies have revealed an uncommon commensurate-incommensurate magnetic phase transition with decreasing temperature in the (\ensuremath{\sim}5%) Fe-doped bilayer ruthenate ${\mathrm{Ca}}_{3}(\mathrm{Ru},\mathrm{Fe}{)}_{2}{\mathrm{O}}_{7}$. An incommensurate phase formed of a cycloidal spiral spin structure coexists with a commensurate one below the phase transition at 42 K and persists down to the lowest temperature, accompanied by higher-order magnetic satellite peaks which indicate the formation of a magnetic soliton lattice. We ascribe these findings to the competing magnetic interactions in this system. This study demonstrates an effective approach to tune novel magnetic and electronic properties of ruthenates via $3d$ magnetic transition-metal substitution.

Structure–property relationship of GdCrO3-modified BiFeO3 ceramics

Solid solutions of perovskites (1−x)BiFeO3–xGdCrO3 (x=0, 0.1 and 0.2) has been synthesized under high-pressure conditions in an attempt to find magnetoelectric materials. Structural studies of these samples show a compositional-driven phase transition from rhombohedral for x=0 to orthorhombic symmetry for x=0.2, with a mixture of the two phases for x=0.1. Magnetic and ferroelectric properties of these samples are shown to correlate with evolution of their structural state. The addition of GdCrO3 in BiFeO3 leads to a ferroelectric–paraelectric phase transition. The magnetic properties of BiFeO3 are improved due to structural distortion and partial destruction of spiral spin structure caused by introducing GdCrO3. The best combination of multiferroic characteristics, with remanent polarization and magnetization of 10.1μC/cm2 and 0.118emu/g, respectively, was realized in the sample with x=0.1. The magnetodielectric and magnetoelectric effect of these samples were evaluated.

The phase transitions in CsFe(MoO4)2 triangular lattice antiferromagnet, neutron diffraction and high pressure studies

We report on the phase transitions in the triangular lattice antiferromagnet CsFe(MoO4)2 at low temperatures and high pressure using powder neutron and X-ray diffraction, specific heat, magnetic susceptibility and in situ high pressure Raman measurements. CsFe(MoO4)2 undergoes a structural phase transition induced by the rotation of (MoO4)2− tetrahedra at Tc=220K. The transformation is associated with a symmetry decrease from P-3m to P-3. Below 4.5K the antiferromagnetic long range order appears that is related to the nuclear unit cell by the modulation vector q=(1/31/30.5). The system adopts ‘120°’ spiral spin structure that has been found in several triangular lattice multiferroics. Under pressure CsFe(MoO4)2 undergoes a reconstructive phase transition to a much larger superstructure which preserves the super-exchange interactions at low temperatures and allows an additional magnetic long-range order at T∼20K.

Antiferromagnetic ground state in NpCoGe

NpCoGe, the neptunium analog of the ferromagnetic superconductor UCoGe, has been investigated by dc magnetization, ac susceptibility, specific heat, electrical resistivity, Hall effect, Np M\"ossbauer spectroscopy, and local spin-density approximation (LSDA) calculations. NpCoGe exhibits an antiferromagnetic ground state with a N\'eel temperature ${T}_{N}\ensuremath{\approx}13$ K and an average ordered magnetic moment $\ensuremath{\langle}{\ensuremath{\mu}}_{\mathrm{Np}}\ensuremath{\rangle}=0.80\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{B}$. The magnetic phase diagram has been determined and shows that the antiferromagnetic structure is destroyed by the application of a magnetic field (\ensuremath{\approx}3 T). The value of the isomer shift suggests a Np${}^{3+}$ charge state (configuration $5{f}^{4}$). A high Sommerfeld coefficient value for NpCoGe (170 mJ mol${}^{\ensuremath{-}1}$ K${}^{\ensuremath{-}2}$) is inferred from specific heat. LSDA calculations indicate strong magnetic anisotropy and easy magnetization along the $c$ axis. M\"ossbauer data and calculated exchange interactions support the possible occurrence of an elliptical spin-spiral structure in NpCoGe. The comparison with NpRhGe and uranium analogs suggests the leading role of 5f-d hybridization, the rather delocalized character of 5f electrons in NpCoGe, and the possible proximity of NpRuGe or NpFeGe to a magnetic quantum critical point.

Enhancement of magnetic and electrical properties in Sc substituted BiFeO3 multiferroic

Polycrystalline BiFe1−xScxO3 (x=0, 0.05, 0.1 and 0.15) compounds are prepared using solid state reaction. The XRD patterns show that all compounds are crystallized in rhombohedral structure with R3c space group. An induced weak ferromagnetism in Sc substituted BiFeO3 due to suppression of spiral modulated spin structure is revealed. In addition, a spin glass like behaviour is observed from the zero field cooled (ZFC) and field cooled (FC) magnetization curves in the low temperature region. Further, the coupling between the ferroelectric and (anti) ferromagnetic orders is evident from the appearance of anomaly in the dielectric data near the magnetic Néel temperature (373°C). The reduction of oxygen vacancies due to Sc substitution is evident from the ac conductivity data and the suppressed anomaly in dielectric data at 220°C. The temperature dependence of ac conductivity is consistent with correlated barrier hopping (CBH) model. The temperature dependent ac conductivity and activation energies indicate that electronic conduction, oxygen vacancies movement and creation of defects are the prime contributors to the ac conductivity in measured temperature regions. The improved magnetic and electrical properties due to the structural modification are prominent for novel device applications.

Nd doping effect on Bi1−xNdxFe0.97Co0.03O3 thin films: Microstructural, electrical, optical and enhanced multiferroic properties

Multiferroic Bi1−xNdxFe0.97Co0.03O3 (BNFCO) thin films with compositions x = 0.06, 0.09, 0.12, and 0.15 were deposited on fluorine-doped SnO2 substrates. The microstructure, chemical state, leakage mechanism, ferroelectric and magnetic properties were investigated. XRD analysis and Raman scattering spectra reveal that Nd doping gives rise to a triclinic structure transition for BNFCO films with respect to the original rhombohedral structure of BFO film. XPS analysis shows that the number of unoccupied states in the Bi 5d and Fe 3d orbits are increased with the increase of Nd content, which probably demonstrates a Fe/Bi-to-Nd electron transfer process. Fowler–Nordheim tunneling conduction mechanism associated with oxygen vacancies is responsible for the low breakdown voltage of BFO. The leakage mechanism of BNFCO is subject to trap-free Ohmic conduction due to the high bond strength among the Bi, O and Fe ions and the low concentration of oxygen vacancies, therefore, improves the resistivity in the high electric field. Highly enhanced ferroelectric properties with giant remanent polarization of 107.5 μC cm−2 is obtained via 15% Nd doping. The magnetic property is also increased by Nd doping, which is ascribed to the modified spiral spin structure and the small grain size effect. The band gap values of the BNFCO films are gradually decreased with the increase of Nd content.

Room temperature dielectric and magnetic properties of Gd and Ti co-doped BiFeO3 ceramics

Room temperature dielectric and magnetic properties of BiFeO3 samples, co-doped with magnetic Gd and non-magnetic Ti in place of Bi and Fe, respectively, were reported. The nominal compositions of Bi0.9Gd0.1Fe1–xTixO3 (x = 0.00-0.25) ceramics were synthesized by conventional solid state reaction technique. X-ray diffraction patterns revealed that the substitution of Fe by Ti induces a phase transition from rhombohedral to orthorhombic at x &amp;gt; 0.20. Morphological studies demonstrated that the average grain size was reduced from ∼1.5 μm to ∼200 nm with the increase in Ti content. Due to Ti substitution, the dielectric constant was stable over a wide range of high frequencies (30 kHz to 20 MHz) by suppressing the dispersion at low frequencies. The dielectric properties of the compounds are associated with their improved morphologies and reduced leakage current densities probably due to the lower concentration of oxygen vacancies in the compositions. Magnetic properties of Bi0.9Gd0.1Fe1–xTixO3 (x = 0.00-0.25) ceramics measured at room temperature were enhanced with Ti substitution up to 20% compared to that of pure BiFeO3 and Ti undoped Bi0.9Gd0.1FeO3 samples. The enhanced magnetic properties might be attributed to the substitution induced suppression of spiral spin structure of BiFeO3. An asymmetric shifts both in the field and magnetization axes of magnetization versus magnetic field curves was observed. This indicates the presence of exchange bias effect in these compounds notably at room temperature.

Effect of Sm-substitution on structural, electrical and magnetic properties of BiFeO3

Samarium-modified bismuth ferrites (i.e., Bi1−xSmxFeO3 (BSFO; x = 0, 0.05, 0.15, 0.25) were synthesized in a single-phase by a solid-state reaction method. Preliminary x-ray structural analysis suggested that crystal structure of the material is rhombohedral for x = 0.0, 0.05, 0.15 and orthorhombic for x = 0.25 composition. The surface morphology and textures of the pellet samples, recorded by scanning electron microscope at room temperature, show uniform distribution of the different size of plate- and rod-shaped grains on the surface of the samples. The grain size generally decreases with increase of Sm-content in BSFO. Detailed studies of permittivity and impedance parameters of the samples in a wide frequency range (1 kHz — 1 MHz) at different temperatures (30°C–450°C) provide many new and interesting results about relaxation processes. Room temperature magnetic hysteresis loops revealed that the spontaneous magnetization increases due to structural distortion and partial destruction of spiral spin structure caused by the substitution of Sm in BiFeO3. Enhancement in magneto-electric coupling coefficient of BFO was observed with increase of Sm in BSFO.

Complex and strongly anisotropic magnetism in the pure spin system EuRh2Si2

In divalent Eu systems, the 4f local moment has a pure spin state J = S = 7/2. Although the absence of orbital moment precludes crystal electric field effects, we report a sizable magnetic anisotropy in single crystals of EuRh2Si2. We observed a surprisingly complex magnetic behavior with three successive phase transitions. The Eu2+ moments order in a probably amplitude-modulated structure below 24.5 K, undergoing a further transition to a structure that is possibly of the equal-moment type, and a first order transition at lower temperatures, presumably into a spin spiral structure. The sharp metamagnetic transition observed at low fields applied perpendicular to the hard axis is consistent with a change from a spiral to a fan structure. These magnetic structures are presumably formed by ferromagnetic planes perpendicular to the c axis, stacked antiferromagnetically along c but not of type I, at least just below the ordering temperature. Since EuRh2Si2 is isoelectronic and isostructural to EuFe2As2 at room temperature, our results are also relevant for the complex Eu-magnetism observed there, especially for the transition from an antiferromagnetic to a ferromagnetic state observed in EuFe2P2 upon substituting As by P.

Interfacial magnetism and exchange coupling in BiFeO3–CuO nanocomposite

Ferromagnetic BiFeO3 nanocrystals of average size 9 nm were used to form a composite with antiferromagnetic CuO nanosheets, with the composition (x)BiFeO3/(100−x)CuO, x = 0, 20, 40, 50, 60, 80 and 100. The dispersion of BiFeO3 nanocrystals into the CuO matrix was confirmed by x-ray diffraction and transmission electron microscopy. The ferromagnetic ordering as observed in pure BiFeO3 occurs mainly due to the reduction in the particle size as compared to the wavelength (62 nm) of the spiral modulated spin structure of the bulk BiFeO3. Surface spin disorder of BiFeO3 nanocrystals gives rise to an exponential behavior of magnetization with temperature. Strong magnetic exchange coupling between the BiFeO3 nanocrystal and the CuO matrix induces an interfacial superparamagnetic phase with a blocking temperature of about 80 K. Zero field and field cooled magnetizations are analyzed by a ferromagnetic core and disordered spin shell model. The temperature dependence of the calculated saturation magnetization exhibits three magnetic contributions in three temperature regimes. The BiFeO3/CuO nanocomposites reveal an exchange bias effect below 170 K. The maximum exchange bias field HEB is 1841 Oe for x = 50 at 5 K under field cooling of 50 kOe. The exchange bias coupling results in an increase of coercivity of 1934 Oe at 5 K. Blocked spins within an interfacial region give rise to a remarkable exchange bias effect in the nanocomposite due to strong magnetic exchange coupling between the BiFeO3 nanocrystals and the CuO nanosheets.

Peculiar magnetism of BiFeO3 nanoparticles with size approaching the period of the spiral spin structure

Size effect of multiferroics is important for its potential applications in new type miniaturized multifunctional devices and thus has been widely studied. However, is there special size effect in the materials with spiral modulated spin structure (such as BiFeO3)? It is still an issue to be investigated. In this report, structural, magnetic and magnetoelectric coupling properties are investigated for sol-gel prepared BiFeO3 nanoparticles with various sizes. It is found that a structural anomaly arises for the particles with size close to the 62 nm period of the spiral modulated spin structure, which induces an obviously increased ferromagnetism. In addition, large magnetoelectric coupling effect is observed in 62 nm BiFeO3 nanoparticles. Our result provides another insight into the size effect of BiFeO3, and also a clue to the magnetic structure at nanoscale.

Effect of Quantum Confinement on Optical and Magnetic Properties of Pr–Cr-Codoped Bismuth Ferrite Nanowires

We report the effect of the variation of diameter on the optical, magnetic, and magnetodielectric properties of the Pr–Cr-codoped BiFeO3 (BFO) nanowires (NWs). Pr–Cr-codoped BFO NWs with different diameters (18, 35, 55, 100, 150, and 250 nm) have been fabricated by employing a simple wet chemical template-assisted route. The effect of quantum confinement has been found to have a significant influence on the room-temperature photoluminescence and Raman spectra of the NWs. An interesting blue shift in the band gap emission is observed in the photoluminescence spectra of the NWs as a result of quantum confinement. The position and the intensity of the Raman peaks are found to change significantly depending on the variation in the NW diameter. The room-temperature ferromagnetism of the codoped BFO NWs increases consistently with the decrease in the diameter of the NWs because of the suppression of the spiral spin structure and the increase in the number of uncompensated for spins at the NW surface (as the sur...

Effect of Dy substitution on structural, magnetic and optical properties of BiFeO3 ceramics

Bi1−xDyxFeO3 (x=0.0, 0.03, 0.05, 0.07, 0.10 and 0.12) ceramics were synthesized by solid state reaction method. Effects of Dy substitution on structural distortion, magnetic and optical properties of BiFeO3 were examined by X-ray diffraction, Raman and UV–Visible spectroscopy. The samples were found to crystallize in rhombohedral structure of BiFeO3 with R3c space group. The reduction in lattice parameters and unit cell volume indicate the distortion in FeO6 octahedra of the rhombohedral structure without any signature of phase transformation up to x=0.12. The predictable weak ferromagnetic hysteresis loops can be observed in the Dy doped samples with maximum remnant magnetization of 0.2103emu/g for x=0.12. The weak ferromagnetism is ascribed to the suppressed spiral spin structure and magnetically active characteristic of Dy3+ ions together with ferromagnetic coupling between Dy3+ and Fe3+ ions. With optical band gap in visible region, Dy doped BiFeO3 ceramics are potential material for optoelectronic device and solar cell applications.

First-order phase transition from an antiferromagnetic ferroelectric to a cycloidal multiferroic with weak ferromagnetism during the joint action of applied magnetic and electric fields

The thermodynamics of the phase transition in a perovskite-like multiferroic, in which an antiferromagnetic ferroelectric transforms into a new magnetic state where a spiral spin structure and weak ferromagnetism can coexist in applied magnetic field H, is described. This state forms as a result of a first-order phase transition at a certain temperature (below Neel temperature TN), where a helicoidal magnetic structure appears due to the Dzyaloshinskii-Moriya effect. In this case, the axes of electric polarization and the helicoid of magnetic moments are mutually perpendicular and lie in the ab plane, which is normal to principal axis c. Additional electric polarization p, which decreases the total polarization of the ferroelectric P, appears in the ab plane. The effect of applied magnetic and electric fields on the properties of a multiferroic with a helicoidal magnetic structure is described. An alternating electric field is shown to cause a field-linear change in magnetic moment m, whose sign is opposite to the sign of the change of electric field E. The detected hysteretic phenomena that determine the temperature ranges of overheating and supercooling of each phase are explained. A comparison with the experimental data is performed.