Cycloidal Modulation Research Articles

Field-induced spin cycloidal modulation to antiferromagnetic transition and possible flexomagnetic effect in BiFeO3 nanoparticles

Beyond its various properties, the model multiferroic BiFeO3 (BFO) displays a rich magnetic structure illustrated in the bulk by its long period (∼62 nm) spin cycloidal modulation. Here, BFO nanoparticles are produced by a facile hydrothermal method and show average size of 8 nm and a narrow size distribution, as determined using x-ray diffraction analysis and transmission electron microscopy images. Mössbauer spectrometry (MS) unambiguously reveals that a cycloidal modulation does still exists with particles about 5 times smaller than the bulk cycloid. Combining macroscopic magnetic measurements and in situ Mössbauer spectrometry, we demonstrate that a critical magnetic field of ∼0.2 T destabilizes the cycloidal modulation to lead to a homogenous antiferromagnetic state, as the result of magnetic anisotropy due to magnetoelastic and surface-confinement effects. More interestingly, further increasing of the external magnetic field up to 8 T does not change the average magnetic hyperfine field and results into multiple Mössbauer sextets we propose to explain by a flexomagnetic effect i.e. magnetic anisotropies resulting from strain gradients due to a continuous variation of the coupling between magnetization and the structural distortion from the surface to the particle core.

Field-Induced Spin Cycloidal Modulation to Antiferromagnetic Transition and Possible Flexomagnetic Effect in Bifeo3 Nanoparticles

Field-Induced Spin Cycloidal Modulation to Antiferromagnetic Transition and Possible Flexomagnetic Effect in Bifeo3 Nanoparticles

The Influence of Annealing Temperature on the Structure and Magnetic Properties of Nanocrystalline BiFeO3 Prepared by Sol\u2013Gel Method

In this work, BiFeO3 powders were synthesized by a sol–gel method. The influence of annealing temperature on the structure and magnetic properties of the samples has been discussed. X-ray diffraction studies showed that the purest phase was formed in the temperature range of 400 °C to 550 °C and the samples annealed at a temperature below 550 °C were of nanocrystalline character. Mössbauer spectroscopy and magnetization measurements were used as complementary methods to investigate the magnetic state of the samples. In particular, the appearance of weak ferromagnetic properties, significant growth of magnetization, and spin-glass-like behavior were observed along with the drop of average grain size. Mössbauer spectra were fitted by the model assuming cycloidal modulation of spins arrangement and properties of the spin cycloid were determined and analyzed. Most importantly, it was proved that the spin cycloid does not disappear even in the case of the samples with a particle size well below the cycloid modulation period λ = 62 nm. Furthermore, the cycloid becomes more anharmonic as the grain size decreases. The possible origination of weak ferromagnetism of the nanocrystalline samples has also been discussed.

Magnetic field induced antiferromagnetic cone structure in multiferroic BiFeO3

Neutron diffraction measurements were performed under high magnetic fields up to 17 T in a multiferroic ${\mathrm{BiFeO}}_{3}$ single crystal, in which an intermediate magnetic (IM) phase has been found between the cycloid and canted antiferromagnetic phases [S. Kawachi et al., Phys. Rev. Mater. 1, 024408 (2017)]. We clearly found that the incommensurate magnetic peaks, which split perpendicular to the magnetic field in the cycloid phase, rotate by 90 deg to align parallel to the field in the IM phase. The magnetic structure in the IM phase can be best described by an antiferromagnetic cone (AF cone) structure. The transition from the cycloid to AF cone is of first order and the direction of the magnetic wave vector and the easy plane of the cycloidal component are rotated by 90 deg without changing the cycloidal modulation period, whereas the transition from the AF cone to canted antiferromagnetic phase is gradual and the cone angle becomes smaller gradually without changing the modulation period. Interestingly, the cycloidal component as well as the cone angle in the IM phase shows a large hysteresis between the field increasing and decreasing processes. This result, combined with the magnetostriction with a large hysteresis previously reported in the IM phase, suggests a strong magnetoelastic coupling.

Commensurate to incommensurate magnetic phase transition in honeycomb-lattice pyrovanadateMn2V2O7

We have synthesized single crystalline sample of Mn$_2$V$_2$O$_7$ using floating zone technique and investigated the ground state using magnetic susceptibility, heat capacity and neutron diffraction. Our magnetic susceptibility and heat capacity reveal two successive magnetic transitions at $T_{N1} =$ 19 K and $T_{N2} =$ 11.8 K indicating two distinct magnetically ordered phases. The single crystal neutron diffraction study shows that in the temperature ($T$) range 11.8 K $\le T \le$ 19 K the magnetic structure is commensurate with propagation vector $k_1 = (0, 0.5, 0)$, while upon lowering temperature below $T_{N2} =$ 11.8 K an incommensurate magnetic order emerges with $k_2 = (0.38, 0.48, 0.5)$ and the magnetic structure can be represented by cycloidal modulation of the Mn spin in $ac-$plane. We are reporting this commensurate to incommensurate transition for the first time. We discuss the role of the magnetic exchange interactions and spin-orbital coupling on the stability of the observed magnetic phase transitions.

Influence of flexoelectricity on the spin cycloid in (110)-oriented BiFeO3 films

The influence of film orientation, strain relaxation, and flexoelectric fields on the stability of the spin cycloid in (110)-oriented $\mathrm{BiFe}{\mathrm{O}}_{3}$ epitaxial films grown on $\mathrm{LaAl}{\mathrm{O}}_{3}$ substrates is investigated. By means of advanced x-ray-diffraction techniques, we show that thinner films have very large strain gradients which give rise to high flexoelectric fields. Using low-energy Raman spectroscopy and conversion electron M\"ossbauer spectroscopy (CEMS) we show that films up to 53 nm thick possess collinear antiferromagnetic order, with no cycloidal modulation. This suppression of the cycloid is proposed to be from strain and strain-gradient-induced flexoelectric fields. On the other hand, films thicker than 90 nm show a complex spin texture consistent with two separate cycloids, likely with different propagation directions. Interestingly, CEMS analysis suggests that the two cycloids have the same spin rotation plane. The multiple cycloids are suggested to arise from different ferroelastic domains (in turn influenced by twinning in the substrate) with different strain relaxation behaviors. These results offer insight into the factors that influence cycloid stability in the less common (110) film orientation and have implications for future magnonic devices.

Weak exchange striction between the 4f and 3d ions in the multiferroic GdMn2O5

We used single-crystal neutron diffraction to investigate the magnetic structures of the multiferroic ${\mathrm{GdMn}}_{2}{\mathrm{O}}_{5}$. The system undergoes a first-order incommensurate to commensurate magnetic transition below 33 K, accompanied by the appearance of electric polarization $P$. Upon cooling, $P$ increases smoothly while the magnetic order shows an abrupt enhancement in intensity below 20 K owing to the large increase of the rare-earth Gd moment. The contrasting temperature evolution of the magnetic order and polarization indicates the polarization is mainly driven by the exchange striction between the magnetic Mn ions. The incommensurate phase in the intermediate temperature range has a cycloidal modulation along the $c$ axis, with spin configuration projected in the $ab$ plane similar to the commensurate phase. The lack of observable bulk polarization suggests the exchange striction is not the dominant mechanism for the polarization in the incommensurate phase.

Strain and Magnetic Field Induced Spin-Structure Transitions in Multiferroic BiFeO3.

The magnetic-field-dependent spin ordering of strained BiFeO3 films is determined using nuclear resonant scattering and Raman spectroscopy. The critical field required to destroy the cycloidal modulation of the Fe spins is found to be significantly lower than in the bulk, with appealing implications for field-controlled spintronic and magnonic devices.

Enhanced ferromagnetism in BiFeO3 powders by rapid combustion of graphite powders

The magnetoelectric applications of BiFeO3 are strongly limited by the very weak ferromagnetism, due to the cycloidal modulated antiferromagnetic structure. In this paper, a rapid combustion method was adopted to prepare BiFeO3 powders. The amorphous BiFeO3 precursors prepared by sol-gel method were mixed with graphite powders and calcined at 350 °C for 5 minutes. BiFeO3 powders were crystallized well with grain size of 30 nm due to the local high temperature during the rapid combustion of graphite powders in short time, and the high crystallinity was confirmed by the Raman spectra. Clear room temperature ferromagnetism has been observed, with significantly large coercivity of 0.19 kOe and magnetization of 0.48 emu/g at 10 kOe, which is much larger than that of BiFeO3 powders sintered at 600 °C (negligible small coercivity and 0.092 emu/g at 10 kOe). The enhancement of ferromagnetism has been attributed to the suppression of the cancellation of canted spins due to the much smaller crystallite size than the cycloidal modulation period.

Effect of Nb doping on the morphology and multiferroic behavior of Bi0.9La0.1FeO3 ceramics

Synthesis and study of the crystal structure, microstructure, local ferroelectric, and magnetic properties of the Bi0.9La0.1Fe1−xNbxO3+x (x≤0.02) perovskites have been carried out to shed light on the conditions underlying the appearance of doping-driven instability of the cycloidal antiferromagnetic order in the polar phase of bismuth ferrite. The light aliovalent substitution has not been found to change the rhombohedral symmetry specific to the parent Bi0.9La0.1FeO3. It has been proven that the doping removes the cycloidal modulation characteristic of the rhombohedral phase to stabilize a weak ferromagnetic ferroelectric state. The antiferromagnetic-weak ferromagnetic transition is accompanied by a dramatic decrease in the average size of crystal grains and ferroelectric domains. The magnetic and morphological transformations can be understood by taking into consideration the charge-compensating mechanism involving the formation of lattice defects in the donor-doped materials. The observation of a correlation between the magnetic and morphological evolution in the Bi0.9La0.1Fe1−xNbxO3+x series suggests that structural defects should be considered as an essential factor controlling the magnetic ground state in BiFeO3-based multiferroics.

Ti doping-driven magnetic and morphological changes in multiferroic ceramics of Bi0.9La0.1FeO3

Investigation of the crystal structure, microstructure, local ferroelectric, and magnetic properties of the Bi0.9La0.1Fe1−yTiyO3 (0 ⩽ y ⩽ 0.1) perovskites has been carried out at room temperature to shed light on the conditions that favor the appearance of spontaneous magnetization in the polar phase of BiFeO3-based multiferroics. The compounds have been shown to possess the noncentrosymmetric rhombohedral structure (space group R3c) specific to the antiferromagnetic (cycloidal) phase of the parent Bi0.9La0.1FeO3. A gradual suppression of the rhombohedral distortions and a decrease of the ferroelectric polarization have been found in the series with increasing Ti content. The substitution dramatically affects the morphology of the ceramic samples: a drastic (from ~10 μm for y = 0 to ~1 μm for 0.02 ⩽ y ⩽ 0.1) decrease of the average grain size has been revealed. The decrease is accompanied by the formation of a nanodimensional ferroelectric domain structure. The origin of the morphological changes has been explained by the charge-compensating mechanism that involves the formation of lattice defects in the donor-doped materials. It has been proposed that the same mechanism can be responsible for the substitution-induced removal of the cycloidal modulation resulting in the establishment of a weak ferromagnetic state in the Bi0.9La0.1Fe1−yTiyO3 series.

Recording polarization gratings with a standing spiral wave

A scalable and robust methodology for writing cycloidal modulation patterns of optical axis orientation in photosensitive surface alignment layers is demonstrated. Counterpropagating circularly polarized beams, generated by reflection of the input beam from a cholesteric liquid crystal, direct local surface orientation in a photosensitive surface. Purposely introducing a slight angle between the input beam and the photosensitive surface normal introduces a grating period/orientation that is readily controlled and templated. The resulting cycloidal diffractive waveplates offer utility in technologies requiring diffraction over a broad range of angles/wavelengths. This simple methodology of forming polarization gratings offers advantages over conventional fabrication techniques.

BiFeO3 thickness dependence of the exchange bias in polycrystalline BiFeO3/NiFe bilayers

A clear exchange bias has been observed at room temperature in a series of BiFeO3(t)/NiFe(3.6 nm) bilayers, in which the BiFeO3 layers are single-phase polycrystalline with t varied from 8 nm to 240 nm. With increasing t, both the exchange bias field (HE) and the coercivity (HC) increase sharply and approach maxima when t is about 40 nm, which is close to one half the spin cycloidal modulation period (64 nm) of the bulk BiFeO3 material. The oscillatory variations of HE and HC with the BiFeO3 layer thickness suggest that the cycloidal spin structure may exist in polycrystalline BiFeO3 thin films. The angular dependence of the exchange bias exhibits collinear unidirectional and uniaxial anisotropies in the present BiFeO3/NiFe bilayers.

Neutron diffraction investigation of NdNi 2Si 2 single crystal

Neutron diffraction experiment was carried out on the single-crystal NdNi 2Si 2. Magnetic satellites reflections indicated that the magnetic structure is incommensurate with Q=(0.03, 0, 0.885). Numerical analysis of the experimental data determined that the magnetic structure exhibits a cycloidal modulation with Nd moment of 3.1 μ B .