Spin Cycloid Research Articles

Enhancement in electrical and magnetic properties of (Li0.5Ga0.5)2+ and (Li0.5Er0.5)2+-Modified BiFeO3-BaTiO3 ceramics

Effects of (Li0.5Ga0.5)2+ and (Li0.5Er0.5)2+ doping on the phase structure, electrical, and magnetic properties of 0.75BiFeO3-0.25BaTiO3 (BFO-BT) ceramics were investigated and analyzed. X-ray diffraction measurements suggested a rhombohedral distorted perovskite structure and no structural transformation with the increasing doping content. Rietveld refinement results revealed that (Li0.5Ga0.5)2+ ions were more susceptible to replace the B-sites and (Li0.5Er0.5)2+ ions tended to substitute the A-sites. A significant improvement in the dielectric loss, ferroelectricity, and magnetization was observed for both (Li0.5Ga0.5)2+ and (Li0.5Er0.5)2+-modified BFO–BT ceramics without the addition of MnO2 compared to undoped ceramic samples. Remnant magnetization (Mr) of 0.35 emu/g was reached for LG6. The enhanced magnetic properties were related to the suppressed cycloidal spin structure, the presences of the local lattice disorder and the magnetic impurities induced by the (Li0.5Ga0.5)2+ and (Li0.5Er0.5)2+ substitution.

Spin-spiral state of a Mn monolayer on W(110) studied by soft x-ray absorption spectroscopy at variable temperature

The noncollinear magnetic state of epitaxial Mn monolayers on tungsten (110) crystal surfaces is investigated by means of soft x-ray absorption spectroscopy, to complement earlier spin-polarized STM experiments. X-ray absorption spectra (XAS), x-ray linear dichroism (XLD) and x-ray magnetic circular dichroism (XMCD) Mn L23-edge spectra were measured in the temperature range from 8 to 300 K and compared to results of fully-relativistic ab initio calculations. We show that antiferromagnetic (AFM) helical and cycloidal spirals give rise to significantly different Mn L23-edge XLD signals, enabling thus to distinguish between them. It follows from our results that the magnetic ground state of a Mn monolayer on W(110) is an AFM cycloidal spin spiral. Based on temperature-dependent XAS, XLD and field-induced XMCD spectra we deduce that magnetic properties of the Mn monolayer on W(110) vary with temperature, but this variation lacks a clear indication of a phase transition in the investigated temperature range up to 300 K - even though a crossover exists around 170 K in the temperature dependence of XAS branching ratios and in XLD profiles.

Coexistence of large negative and positive magnetodielectric response in Bi1−xCaxFe1−yTiyO3−δ nanoparticle ceramics

Magnetodielectric (MD) properties of as-prepared (AP) and air-annealed ${\mathrm{Bi}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{Fe}}_{1\ensuremath{-}y}{\mathrm{Ti}}_{y}{\mathrm{O}}_{3\ensuremath{-}\ensuremath{\delta}}$ nanoparticle ceramics made by spark plasma sintering process are investigated as a function of temperature. Aliovalent ${\mathrm{Ca}}^{2+}$ substitution at ${\mathrm{Bi}}^{3+}$ site creates oxygen vacancies (${\mathrm{V}}_{\mathrm{O}}$) in the lattice disrupting the intrinsic spin cycloid of ${\mathrm{BiFeO}}_{3}$, which are suppressed when the charge compensating ${\mathrm{Ti}}^{4+}$ is co-substituted. In addition, cation substitution reduces the grain size and increases surface oxygen vacancies. These lattice and surface ${\mathrm{V}}_{\mathrm{O}}$ defects play a significant role in enhancing the magnetic properties. Zero-field-cooled magnetization curves of all AP samples show a sharp Verwey-like transition at \ensuremath{\sim}120 K, which weakens on air-annealing. A coexistence of positive and negative MD [MD = $\frac{\mathrm{\ensuremath{\Delta}}\ensuremath{\varepsilon}(H)}{\ensuremath{\varepsilon}(H=0)}$; $\mathrm{\ensuremath{\Delta}}\ensuremath{\varepsilon}(H)=\ensuremath{\varepsilon}(H)\ensuremath{-}\ensuremath{\varepsilon}(H=0)$] response is observed, with the former dominating at 300 K and the latter at 10 K. As-prepared 5 at.% (10 at.%) Ca and Ca-Ti substituted ${\mathrm{BiFeO}}_{3}$ ceramics exhibit a maximum MD response of --10% (\ensuremath{\sim}+3%) at 10 K (300 K). Negative MD response diminishes for air-annealed ${\mathrm{Bi}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{Fe}}_{1\ensuremath{-}y}{\mathrm{Ti}}_{y}{\mathrm{O}}_{3\ensuremath{-}\ensuremath{\delta}}$ ceramics due to the reduction in ${\mathrm{V}}_{\mathrm{O}}$ concentration. Samples exhibiting dominant positive MD response show a similar trend for MD $vs$ H and ${M}^{2}$ vs H plots. This agreement between ${M}^{2}$ and $\mathrm{\ensuremath{\Delta}}\ensuremath{\varepsilon}(H)$ demonstrates a strong inherent MD coupling. On the contrary, negative MD does not follow this trend yet shows a linear relationship of MD vs ${M}^{2}$, suggesting a strong coupling between the magnetic and dielectric properties. Temperature-dependent MD studies carried out at 5 T show a gradual change from negative to positive values. Negative MD at low temperatures could be activated by the spin-lattice coupling, which dominates even at high frequency (1 MHz) under the applied field. Other contributions, including Verwey-like transition, magnetoresistance, and Maxwell-Wagner effects, do not influence the observed MD response. A prominent role of oxygen vacancies in altering the MD behavior of ${\mathrm{BiFeO}}_{3}$ is discussed in detail.

Synthesis of bismuth ferrite nanopowder doped with erbium ions

The potential for the practical application of bismuth ferrite (BFO) in information storage, microelectronic, and spintronic devices and in medical sensors of various purpose is limited by the presence of a spin cycloid. Its destruction, including destruction due to doping with rare earth elements and the transfer of BFO to a nanoscale state, contributes to the occurrence of ferromagnetism and the manifestation of the magnetoelectric effect. The study was aimed at the synthesis of bismuth ferrite nanopowder doped with erbium ions.By spray pyrolysis at a temperature of 760 °C, we synthesised BFO samples with a nominal degree of doping with erbium ions from 0.05 to 0.20. The data of X-ray diffraction analysis show that there is a small amount of Bi25FeO39 and Bi2Fe4O9 in the doped samples.The shift of the BFO reflections on diffraction patterns towards larger 2q angles is representative of the incorporation of erbium ions into the crystal lattice of BiFeO3. The morphological characteristics of the samples were determined using transmission electron microscopy. According to the data of electron probe X-Ray microanalysis, the real composition of the doped ErxBi1-xFeO3 samples is very close to the nominal.The particles of ErxBi1-xFeO3 powders synthesised by spray pyrolysis have a nearly spherical shape, the particle-size distribution is in the range of 5–300 nm, the predominant number of particles have a size in the range of 50-200 nm, and the agglomeration is weak. The decrease in the crystal lattice parameters and the unit cell volume of ErxBi1-xFeO3 and an increase in the degree of doping with erbium ions confirm the incorporation of Er3+ into the BFO crystal lattice to the bismuth position.

Surface engineered Tb and Co co-doped BiFeO3 nanoparticles for enhanced photocatalytic and magnetic properties

Controlled synthesis pure-phase Bi0.98Tb0.02Fe1−xCoxO3 (BTFCO, x = 0.01, 0.03, 0.05 and 0.07) nanoparticles with rhombohedral structure using a facile sol–gel method opens the doors to robust their magnetic and photocatalytic properties significantly. The microstructure analysis revealed improvement in the surface morphology with a decrease of average particle size from 64 to 38 nm. No mixed valance state of iron detected for any sample, which is confirmed by Mossbauer spectra. The saturation magnetization and remnant magnetization were increased dramatically from 0.802 and 0.118 emu/g for BFO to 2.261 and 0.737 emu/g, respectively by Tb, Co co-doping. The enhancement of the magnetic property is attributed to the breakdown of spin cycloidal structure, spin canting, and uncompensated spin on the surface of nanoparticles. Furthermore, the as-synthesized samples are highly effective for rhodamin B (RhB) degradation under visible light irradiation, and the possible photocatalytic mechanism discussed in detail. Bi0.98Tb0.02Fe0.93Co0.07O3 nanoparticles exhibit the best photocatalytic activity owing to their larger specific surface area, lower band gap, and reduced average particle size. Besides, 1.2% efficiency retention for the degradation of RhB dye using BTFCO7 after the 20th cycle suggests a potential candidate to address the current scenario in the environmental remediation technology.

Peculiar magnetism of Bi1−xDyxFeO3 ceramics at the morphotropic phase boundary

• Bi 1−x Dy x FeO 3 samples were synthesized by solid-state reaction method. • Dy substitution causes the formation of the R3c / Pnma morphotropic phase boundary. • The pinched hysteresis loop and maximum magnetization are observed. • The metamagnetic transition of antiferromagnetic clusters explains for the pinched hysteresis loop. We report systematic studies on the crystal structure, optical phonons, and magnetic properties of Bi 1−x Dy x FeO 3 ( x = 0.1–0.2) systems at a morphotropic phase boundary (MPB) of R3c rhombohedral and Pnma orthorhombic structures. The structural phase coexistence is verified by observing two distinct X-ray diffraction profiles throughout the composition range. Rietveld refinement confirms the structural evolution from R3c to Pnma as the Dy concentration increases and provides details on the phase percentage and lattice parameters. Raman studies clearly reveal two different types of phonon vibrations in the coexisting phase. Weak ferromagnetism is observed in a composition with a dominant Pnma phase, meaning that the substitution of Dy weakly suppresses the cycloidal spin structure of the R3c phase. A pinched hysteresis loop is observed across the MPB and the magnetization reaches a maximum value and then decreases continuously as the Dy concentration increases. These observations are explained in the framework of the metamagnetic transition of antiferromagnetic clusters and the contribution of phase boundary spins.

Direct Observation of Cycloidal Spin Modulation and Field-induced Transition in Néel-type Skyrmion-hosting VOSe2O5

We investigate the spin rotational structure of magnetic skyrmions in a tetragonal polar magnet VOSe2O5 via polarized small-angle neutron scattering (SANS). Spin polarization analysis of the scattered neutrons provides consistent evidence for the cycloidal spin modulation in all the incommensurate phases at zero and non-zero magnetic field along the c axis, including the triangular skyrmion-lattice phase. In the vicinity of the skyrmion phase, we performed extensive SANS measurements to unravel a field-induced incommensurate phase (IC-2 state). We discuss the possibility of anisotropic double-q state as an alternative spin structure to provisional square skyrmion-lattice state.

Theory and phase-field simulations of electrical control of spin cycloids in a multiferroic

Cycloidal spin orders are common in multiferroics. One of the prototypical examples is BiFeO3 (BFO) which shows a large polarization and a cycloidal antiferromagnetic order at room temperature. Here we employ Landau theory and phase-field simulations to analyze the coupled switching dynamics of polarization and cycloidal antiferromagnetic orders in BFO. We are able to identify 14 types of transitional spin structures between two cycloids and 9 electric-field-induced spin switching paths. We demonstrate the electric-field-induced rotation of wave vectors of the cycloidal spins and discover 2 types of cycloidal spin switching dynamics: fast local spin flips and slow rotation of wave vectors. Also, we construct road maps to achieve the switching between any two spin cycloids through multi-step applications of electric fields. The work provides a theoretical framework for the phenomenological description of spin cycloids and a fundamental understanding of the switching mechanisms to achieve electrical control of magnetic orders.

Enhanced high temperature ferromagnetism in Bi1−x R x FeO3 (R = Dy, Y) compounds

In this work we report experimental evidence for the weak high-temperature ferromagnetism in Bi1−x R x FeO3 (R = Dy, Y) compounds by systematic characterizations, excluding the possible side-effects from other iron-based impurities. Remarkable saturated magnetic moment was observed in the Y-substituted samples, Bi1−x Y x FeO3, which is larger than the moment obtained in Bi1−x Dy x FeO3, the Dy-substituted samples with antiferromagnetic background. The physical origin of the weak ferromagnetic transition is discussed and serious lattice distortions have been identified based on the x-ray diffraction and Raman scattering data, although the rhombohedral structure symmetry remains unchanged upon the substitutions. It is believed that the structural distortion suppressed cycloid spin structure is the main factor for the enhanced magnetization in Bi1−x R x FeO3 compounds. Additionally, the Dy3+–Fe3+ antiferromagnetic coupling, which strengthens the antiferromagnetic interaction in Bi1−x Dy x FeO3 compounds, acts as the driving force for the magnetic discrepancy between Bi1−x Y x FeO3 and Bi1−x Dy x FeO3 samples.

Enhanced magnetoelectric coupling characteristics of Mn2O3-modified BiFeO3-based ceramics

BiFeO3 is the most important room-temperature multiferroic material. However, the magnetoelectric coupling effect of BiFeO3 is weak, and an effective way to improve it is to suppress the spin cycloid structure. In this work, Mn ions were substituted into BiFeO3-based ceramics to enhance the magnetoelectric coupling effect. All samples were refined with the rhombohedral R3c structure. The grain size gradually decreased with increasing Mn substitution content. According to DSC analysis, the endothermic peak and exothermic peak exhibit an obvious thermal hysteresis behavior, indicating that it is the first-order ferroelectric transition. Meanwhile, a dielectric anomaly was also found at that transition temperature. The leakage current increased with increasing Mn content, which can be ascribed to the increased oxygen vacancies. The magnetic properties were enhanced for all samples, and the maximum Mr = 51.9 emu/mol was obtained for x = 0.03. In addition, the magnetoelectric properties were improved, and the value of linear magnetoelectric coefficient exhibits a maximum value αME = 0.38 mV/cm Oe at x = 0.03. Thus, the addition of Mn at Fe sites may destroy the spin cycloid structure and enhance the ME effect of BiFeO3.

Structural evolution and magnetic properties in Bi1-xNdxFeO3 ceramics

Ceramic Bi1-xNdxFeO3 solid solutions in the composition range of 0.10 ≤ x ≤ 0.20 were prepared to study the structural evolution and magnetic properties. The structural analysis of X-ray diffraction (XRD) and Rietveld refinement data revealed only a phase transition from the polar R3c rhombohedral to antipolar Pbam orthorhombic structures, while Raman scattering (RS) spectroscopy not only detected the phonon vibrations of the R3c and Pbam symmetries in the samples 0.10 ≤ x ≤ 0.16 but also found the phonon vibrations of the orthorhombic Pbam and Pbnm symmetries for x ≥ 0.16. The magnetic properties of these compounds revealed the weak ferromagnetism at high doping concentration. This observation indicated a crucial role of the destruction of the cycloidal spin structure for revealing the weak ferromagnetism. The magnetic aging behavior observed at room temperature confirmed the contribution of phase boundary spins to the magnetic properties.

Effect of transition metal element substitution on magnetoelectric properties of BiFeO3-BaTiO3 ceramics

BiFeO3 is the most typical multiferroic materials. However, the poor magnetoelectric coupling effect limits its practical application. In this work, the transition metal elements (Co, Mn) were introduced to suppress the spin cycloid structure of BiFeO3-based ceramics. The structure, ferroelectric, magnetic and magnetoelectric properties were investigated thoroughly. The XRD and SAED analysis confirmed the coexistence of tetragonal (T) and rhombohedral (R) phases of all samples. The piezoelectric properties were enhanced in Co-substituted samples. In addition, the magnetic properties were enhanced with the addition of Co/Mn ions. According to the analysis of the M-H loops and XRD patterns before and after poling, the electric-field induced magnetic change was observed in all samples, which can be attributed to the phase transition from R-phase to T-phase during poling. The room-temperature linear magnetoelectric coefficient αME was analyzed, and the maximum αME was obtained at BF-BT-005BC sample, where αME = 0.66 mV/cm Oe. The enhanced magnetic and magnetoelectric properties were ascribed to the distortion of the spiral spin structure by transition metal ions.

Bismuth orthoferrite nanocrystals: magnetic characteristics and size effects

BiFeO3 with 50–70 nm crystallites was obtained by glycine–nitrate combustion under varying conditions. These nanopowders demonstrated magnetic ordering at temperatures up to 400 K. The results were compared with the data known from other works, and it was shown that at the sizes lower than 70 nm the predominant contribution to the magnetization belongs to the crystallite surface. The effect of the increased magnetization, associated with the spin cycloid distortion, can also occur which, however, is less essential for crystallites of sizes ∼10 nm. For the first time, it was shown that the value of magnetization can be controlled technologically within several orders.

Magnetic Diagram of the High-Pressure Stabilized Multiferroic Perovskites of the BiFe1-yScyO3 Series

Magnetic properties of the high-pressure stabilized perovskite BiFe1-yScyO3 phases (0.1 ≤ y ≤ 0.9) have been studied by means of magnetization measurements and neutron diffraction. The metastable perovskites of this series undergo irreversible polymorphic transformations upon annealing, the phenomenon referred to as conversion polymorphism. It has been found that the solid solutions with y ≥ 0.70 exhibit no long-range magnetic ordering regardless of their polymorph modification, while those with y ≤ 0.60 are all antiferromagnets. Depending on the scandium content, temperature and structural distortions, three types of the antiferromagnetic orderings, involving collinear, canted and cycloidal spin arrangements, have been revealed in the phases obtained via conversion polymorphism and the corresponding magnetic phase diagram has been suggested.

The Experimentalist's Guide to the Cycloid, or Noncollinear Antiferromagnetism in Epitaxial BiFeO3

Bismuth ferrite (BiFeO3 ) is one of the most widely studied multiferroics. The coexistence of ferroelectricity and antiferromagnetism in this compound has driven an intense search for electric-field control of the magnetic order. Such efforts require a complete understanding of the various exchange interactions that underpin the magnetic behavior. An important characteristic of BiFeO3 is its noncollinear magnetic order; namely, a long-period incommensurate spin cycloid. Here, the progress in understanding this fascinating aspect of BiFeO3 is reviewed, with a focus on epitaxial films. The advances made in developing the theory used to capture the complexities of the cycloid are first chronicled, followed by a description of the various experimental techniques employed to probe the magnetic order. To help the reader fully grasp the nuances associated with thin films, a detailed description of the spin cycloid in the bulk is provided. The effects of various perturbations on the cycloid are then described: magnetic and electric fields, doping, epitaxial strain, finite size effects, and temperature. To conclude, an outlook on possible device applications exploiting noncollinear magnetism in BiFeO3 films is presented. It is hoped that this work will act as a comprehensive experimentalist's guide to the spin cycloid in BiFeO3 thin films.

Effect of Mn and Ba Codoping on a Magnetic Spin Cycloid of Multiferroic Bismuth Ferrite Nanoparticles

Bismuth ferrite (BFO) is the drosophila of research in multiferroic materials due to its simultaneous magnetic and electric ordering at room temperature. The unfortunate detail is its antiferromagn...

Nonreciprocity of spin waves in noncollinear magnets due to the Dzyaloshinskii-Moriya interaction

Broken inversion symmetry in combination with the spin-orbit interaction generates a finite Dzyaloshinskii-Moriya interaction (DMI), which can induce noncollinear spin textures of chiral nature. The DMI is characterized by an interaction vector whose magnitude, direction and symmetries are crucial to determine the stability of various spin textures, such as skyrmions and spin spirals. The DMI can be measured from the nonreciprocity of spin waves in ferromagnets, which can be probed via inelastic scattering experiments. In a ferromagnet, the DMI can modify the spin-wave dispersion, moving its minimum away from the $\Gamma$ point. Spin waves propagating with opposite wavevectors are then characterized by different group velocities, energies and lifetimes, defining their nonreciprocity. Here, we address the case of complex spin textures, where the manifestation of DMI-induced chiral asymmetries remains to be explored. We discuss such nonreciprocal effects and propose ways of accessing the magnitude and direction of the DMI vectors in the context of spin-polarized or spin-resolved inelastic scattering experiments. We show that only when a periodic magnetic system has finite net magnetization, that is, when the vector sum of all magnetic moments is nonzero, can it present a total nonreciprocal spin-wave spectrum. However, even zero-net-magnetization systems, such as collinear antiferromagnets and cycloidal spin spirals, can have spin-wave modes that are individually nonreciprocal, while the total spectrum remains reciprocal.

Fingerprints of spin-current physics on magnetoelectric response in the spin- 12 magnet Ba2CuGe2O7

The single-site anisotropy vanishes for the spin-1/2 as a consequence of Kramers degeneracy. We argue that similar property holds for the magnetically induced electric polarization P, which should depend only on the relative orientation of spins in the bonds but not on the direction of each individual spin. Thus, for insulating multiferroic compounds, P can be decomposed in terms of pairwise isotropic, antisymmetric, and anisotropic contributions, which can be rigorously derived in the framework of the superexchange (SE) theory, in an analogy with the spin Hamiltonian. The SE theory also allows us to identify the microscopic mechanism, which stands behind each contribution. The most controversial and intriguing one is antisymmetric or spin-current mechanism. In this work, we propose that the disputed magnetoelectric (ME) properties of Ba2CuGe2O7 can be explained solely by the spin-current mechanism, while other contributions are either small or forbidden by symmetry. First we explicitly show how the cycloidal spin order induces the experimentally observed P in the direction perpendicular to the xy plane, which can be naturally explained by the spin-current mechanism operating in the out-of-plane bonds. Then, we unveil previously overlooked ME effect, where the application of the magnetic field perpendicular to the plane not only causes the incommensurate-commensurate transition, but also flips P into the plane due to the spin-current mechanism operating in the neighboring bonds within this plane. In both cases, the magnitude and direction of P can be controlled by rotating the spin pattern in the xy plane. Our analysis is based on a realistic spin model, which was rigorously derived from the first-principles calculations and supplemented with the new algorithm for the construction of localized Wannier functions obeying the crystallographic symmetry of Ba2CuGe2O7.

Influence of Pr3+ and Lu3+ co-doping on the magnetoelectric coupling response in BiFeO3 mutiferroic ceramics

Perovskite-type bismuth ferrite (BiFeO3) multiferroic material is being considered as a potential candidate for magnetoelectric coupling and field-controlled multi-state memory applications. We report the Pr3+ and Lu3+ co-doping effect on the magnetoelectric coupling in BiFeO3 ceramics. In this study, Bi0.94Pr0.06FeO3 (BPO), Bi0.94Lu0.06FeO3 (BLO), and Bi0.94Pr0.03Lu0.03FeO3 (BPLO) ceramics were synthesized by conventional solid-state ceramic synthesis technique. The BPO sample displayed higher saturation magnetization as compared to BLO and BPLO samples which may be due to canting of spin cycloid of BFO and also due to the discrepancy between the antiparallel sublattice of Fe3+/Fe2+, the value of ferroelectric polarization of the BPO sample was found to be better than BLO and BPLO samples. However, the magnetocapacitance (MC) of the BPLO sample is found to be MC ~ 2.3% which is higher than that of BLO (MC ~ 1.4%) and BPO (MC ~ 1.7%) samples. In addition, the enhanced value of magnetoelectric coupling coefficient (αE) has been obtained for the BPLO sample. Overall, our results indicate that BPLO sample displays interesting magnetoelectric response for device applications.

Electronic charge transfer driven by spin cycloidal structure

Muon spin rotation and resonant soft X-ray scattering experiments on prototype multiferroics RMn2O5 (R = Y, Sm) are used to demonstrate that the local electric displacements are driven by the spin-current (SC) mechanism. Small local electric displacements were evaluated by observing spin polarization at ligand O ions, for which implanted muons served as an extremely sensitive probe. Our results for YMn2O5 provide evidence that the spin polarization of O ions forming a spin cycloid chain with Mn spins increases in proportion to the vector spin chirality (Si x Sj ) of the Mn ions. This relationship strongly indicates that the charge transfer between O and Mn ions is driven by the SC mechanism, which leads to the ferroelectricity accompanying O spin polarization.