Iron Spins Research Articles

Spin-Phonon Coupling in Iron-Doped Ultrathin Bismuth Halide Perovskite Derivatives.

Spin in semiconductors facilitates magnetically controlled optoelectronic and spintronic devices. In metal halide perovskites (MHPs), doping magnetic ions is proven to be a simple and efficient approach to introducing a spin magnetic momentum. In this work, we present a facile metal ion doping protocol through the vapor-phase metal halide insertion reaction to the chemical vapor deposition (CVD)-grown ultrathin Cs3BiBr6 perovskites. The Fe-doped bismuth halide (Fe:CBBr) perovskites demonstrate that the iron spins are successfully incorporated into the lattice, as revealed by the spin-phonon coupling below the critical temperature Tc around 50 K observed through temperature-dependent Raman spectroscopy. Furthermore, the phonons exhibit significant softening under an applied magnetic field, possibly originating from magnetostriction and spin exchange interaction. The spin-phonon coupling in Fe:CBBr potentially provides an efficient way to tune the spin and lattice parameters for halide perovskite-based spintronics.

ESR and magnetization studies of double perovskite Sr[formula omitted]FeNbO6

By pyrolysis of nitrate-organic mixtures, we synthesized double perovskite Sr2Fe0.6NbO5.4. We performed the study of the temperature dependencies of the EPR spectra, DC and AC magnetization, and magnetization isotherms in a wide temperature range of 4–300K. The EPR spectrum is described by the sum of two contributions below 125 K and indicates phase separation of the sample. The paramagnetic part of the inverse magnetic susceptibility is well approximated by the Curie–Weiss law with ΘCW≈−60K. The exchange integrals between iron spins, calculated from the temperature dependence of the ESR integral intensity and the Curie–Weiss temperature, coincide and are equal to J/kB≈10K. The negative sign of the Curie–Weiss temperature and the hysteresis loops indicate the antiferromagnetic canted nature of exchange interactions, which is further confirmed by the presence of a peak in the real part of the AC magnetization and its absence in the imaginary part.

Synthesis of nanostructured iron oxides and study of the thermal crystallization process using DSC and in situ XRD experiments

Using a coprecipitation route in aqueous medium with different precipitating agents (NH4OH and NaOH), iron-oxide nanostructures were prepared with distinct structural and magnetic properties. X-ray diffraction (XRD), 57Fe Mössbauer spectroscopy and N2 sorption experiments indicated that the sample synthesized with NaOH had nanocrystalline goethite as the dominant phase, with a specific surface area (SSA) of 80 m2/g, whereas the sample synthesized with NH4OH was X-ray amorphous, with SSA = 270 m2/g. The crystallization process of the amorphous material was investigated in detail, using in situ XRD experiments combined with differential scanning calorimetry (DSC). The X-ray diffractograms recorded at high temperatures for this sample showed first the formation of maghemite, which was then converted into hematite as the heat treatment temperature increased. DSC curves recorded at different heating rates allowed the assessment of the kinetics of the crystallization process, with identification of two consecutive steps (formation of maghemite and conversion of maghemite into hematite) and the determination of the corresponding activation energies (166 and 273 kJ/mol, respectively). From the magnetic viewpoint, the nanostructured iron oxide samples showed non zero magnetization values, indicating a strong contribution form the uncompensated iron spins at the particle surfaces, leading to a superparamagnetic regime (i.e., magnetically non-blocked state) observed for temperatures above 50 K for both samples. These results evidenced how the chemical composition and the structural features of nanostructured iron oxides can be controlled by the conditions employed in the synthesis and the posterior thermal treatments performed on these materials.

Disentangling the phase sequence and correlated critical properties in Bi0.7La0.3FeO3 by structural studies

This work addresses the study of the high-temperature phase sequence of ${\mathrm{Bi}}_{0.7}{\mathrm{La}}_{0.3}{\mathrm{FeO}}_{3}$ by undertaking temperature-dependent high-resolution neutron powder diffraction (NPD) and Raman spectroscopy measurements. A determination of lattice parameters, phase fractions, and modulation wave vector was performed by Pawley refinement of the NPD data. The analysis revealed that ${\mathrm{Bi}}_{0.7}{\mathrm{La}}_{0.3}{\mathrm{FeO}}_{3}$ exhibits an incommensurate modulated orthorhombic $Pn{2}_{1}a(00\ensuremath{\gamma})000$ structure at room temperature, with a weak ferromagnetic behavior, likely arising from a canted antiferromagnetic ordering. Above ${T}_{1}=543\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, the low-temperature modulated $Pn{2}_{1}a(00\ensuremath{\gamma})000$ evolves monotonically into a fractionally growing Pnma structure up to ${T}_{\mathrm{N}}=663\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. At 663 K, the low-temperature canted antiferromagnetic phase is suppressed concurrently with the switching of the former into a nonmodulated $Pn{2}_{1}a$ structure that continues to coexist with the Pnma one, until the latter is expected to reach the 100% fraction of the sample volume at high temperatures above 733 K. The $Pn{2}_{1}a$ space group is obtained from the Pnma one through the ${\mathrm{\ensuremath{\Gamma}}}_{4}^{\ensuremath{-}}$ polar distortion. Neutron diffraction and Raman spectroscopy results provide evidence for the emergence of noteworthy linear spin-phonon coupling. In this regard, magnetostructural coupling is observed below ${T}_{\mathrm{N}}$, revealed by the relation between the weak ferromagnetism of the canted iron spins and the ${\mathrm{FeO}}_{6}$ octahedra symmetric stretching mode. The correlation between magnetization and structural results from NPD provides definite evidence for the magnetic origin of the structural modulation. The analysis of the temperature-dependent magnetization and the magnetic peak intensity as well yields a critical exponent (\ensuremath{\beta}) value of 0.38. The lower limit of the phase coexistence temperature ${T}_{1}=543\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, marking the emergence of the Pnma phase, is also associated with the temperature whereupon the modulation magnitude starts to decrease.

Sequence of superconducting states in field cooled FeCr2S4

In the present article we discuss theoretically the emergence of superconductivity in field cooled FeCr2S4. The chromium electrons form a triplet t 2g states and due to antiferromagnetic exchange with the iron spins have Zeeman splitting. Applied, during preparation, magnetic field along the moment of iron ions, successively compensates the Zeeman splittings. The chromium electrons with zero Zeeman energy form Cooper pairs induced by iron magnons. In that way, we predict theoretically the existence of sequence of superconducting states in field cooled FeCr2S4. Actually there are three different superconductors prepared applying, during preparation, different magnetic fields. In these compounds superconductivity coexist with the saturated magnetism of iron ions.

Structural and Magnetic Tuning of LaFeO3 Orthoferrite Substituted Different Rare Earth Elements to Optimize Their Technological Applications

Lanthanum partially substituted rare earth (RE) elements to form RE0.7La0.3FeO3; RE = Ce, Pr, Nd, Sm and Gd) nanoparticles. They were synthesized using the citrate–nitrate auto-combustion method. The crystal structure and microstructure were refined by applying Rietveld profile refinements using the Maud Program. The morphology, magnetic and optical properties of the inspected samples have been explored using HRTEM, VSM, and UV–Vis diffuse reflectance, respectively. The prepared-samples distortion increases as the substituted RE ionic radius decreases. Iron spins of RE orthoferrite samples are known to be ordered Anti-ferromagnetically (AFM). They also possess weak ferromagnetism due to the Dzyaloshinskii-Moriya interaction. This feeble ferromagnetism is reflected in their soft behavior, whereas Sm0.7La0.3FeO3 sample exhibits the butterfly shape hysteresis loop. The remnant magnetization, saturation magnetization, and coercivity are relatively small, but sometimes impressively, enhanced through Lanthanum-rare-earth substitution. Introducing rare earth ions into LaFeO3 decreases the Fe–O–Fe angle and the consequent reduction of the super exchange interaction. In particular, the coercivity of Sm0.7La0.3FeO3 is remarkably improved relative to the parent sample. It has the largest coercivity of all the prepared nanoparticles. Due to the aforementioned substitution, a slight reflectance edge red shift is observed in the spectra. The relation between the magnetic properties of the investigated samples and the ionic radii of RE was investigated and discussed. The experimental results of this work can provide fundamental support for the research and development of multiferroic materials.

Robustness in Coupling between Iron and Rare Earth Spins in Rare Earth Orthoferrites

We report on very accurate magnetic measurements on large rare earth orthoferrites single crystals of ErFeO3 and NdFeO3. Our results show that the interaction between rare earth and iron spin system does not change during the spin-flip process. This implies that the coupling between the iron and rare earth spin systems is robust enough to withstand the effects of spin flipping against the magnetic anisotropy energy. This is despite rare eath ions, polarized by the ordered iron ions, being in partly metastable state and their magnetic moment decays with time.

Helical spin structure in the langasite family multiferroic Ba3TaFe3Si2O14, determined by Mössbauer spectroscopy

The magnetic structure of the Ba3TaFe3Si2O14 multiferroic of the langasite family has been studied using the Mössbauer spectroscopy and theoretical analysis within the full Hamiltonian of the combined hyperfine magnetic dipole and electric quadrupole interaction in the ground and excited states of 57Fe nuclei. The model is based on an antiferromagnetic triangular magnetic lattice with a 120° orientation of the iron spins in the ()-plane, which occurs below the Néel temperature . It was found that during translation along the c-axis, the magnetic moments of iron ions in triangular clusters rotate at an angle of . This angle remains constant with further translation, and a helical magnetic structure with a magnetic cell period of about 7c is formed. Excited states of magnetic moments arising due to a finite temperature are also considered. It has been established that the helical structure is well described by the proposed model in the entire temperature range T < TN.

Structural, Spectroscopic, and Theoretical Investigation of a T-Shaped [Fe3(μ3-O)] Cluster.

The synthesis, X-ray crystal and electronic structures of [Fe3(μ3-O)(mpmae)2(OAc)2 Cl3], 1, where mpmae-H = 2-(N-methyl-N-((pyridine-2-yl)methyl)amino)ethanol, are described. This cluster comprises three high-spin ferric ions and exhibits a T-shaped site topology. Variable-frequency electron paramagnetic resonance measurements performed on single crystals of 1 demonstrate a total spin ST = 5/2 ground state, characterized by a small, negative, and nearly axial zero-field splitting tensor D = -0.49 cm-1, E/D ≈ 0.055. Analysis of magnetic susceptibility, magnetization, and magneto-structural correlations further corroborate the presence of a sextet ground-spin state. The observed ground state originates from the strong anti-ferromagnetic interaction of two iron(III) spins, with J = 115(5) cm-1, that, in turn, are only weakly coupled to the spin of the third site, with j = 7(1) cm-1. These exchange interactions lead to a ground state with magnetic properties that are essentially entirely determined by the weakly coupled site. The contributions of the individual spins to the total ground state of the cluster were monitored using variable-field 57Fe Mössbauer spectroscopy. Field-dependent spectra reveal that, while one of the iron sites exhibits a large negative internal field, typical of ferric ions, the other two sites exhibit small, but not null, negative and positive internal fields. A theoretical analysis reveals that these small internal fields originate from the mixing of the lowest ST = 5/2 excited state into the ground state which, in turn, is induced by a minute structural distortion.

Solid State Collapse of a High-Spin Square-Planar Fe(II) Complex, Solution Phase Dynamics, and Electronic Structure Characterization of an Fe(II)2 Dimer.

Square-planar high-spin Fe(II) molecular compounds are rare, and until recently, the only four examples of non-macrocyclic or sterically driven molecular compounds of this kind shared a common FeO4 core. The trianionic pincer-type ligand [CF3-ONO]H3 (1) supports the high-spin square-planar Fe(II) complex {[CF3-ONO]FeCl}{Li(Sv)2}2 (2). In the solid state, 2 forms the dimer complex {[CF3-ONO]Fe}2{(μ-Cl)2(μ-LiTHF)4} (3) in 96% yield by simply applying a vacuum or stirring it with pentane for 2 h. A detailed high-frequency electron paramagnetic resonance and field-dependent (57)Fe Mössbauer investigation of 3 revealed a weak antiferromagnetic exchange interaction between the local iron spins which exhibit a zero-field splitting tensor characterized by negative D parameter. In solution, 2 is in equilibrium with the solvento complex {[CF3-ONO]FeCl(THF)}{Li2(Sv)4} (2·Sv) and the dimer 3. A combination of frozen solution (57)Fe Mössbauer spectroscopy and single crystal X-ray crystallography helped elucidate the solvent dependent equilibrium between these three species. The oxidation chemistry of 2·Sv was investigated. Complex 2 reacts readily with the one-electron oxidizing agent CuCl2 to give the Fe(III) complex {[CF3-ONO]FeCl2}{Li(THF)2}2 (4). Also, 2·Sv reacts with 2 equiv of TlPF6 to form the Fe(III) complex [CF3-ONO]Fe(THF)3 (5).

Interplay of iron and rare-earth magnetic order in rare-earth iron pnictide superconductors under magnetic field**Project supported by the National Natural Science Foundation of China (Grant Nos. 11104274, 11274310, and 11474287) and the Fundamental Research Funds for the Central Universities, China (Grant No. 27R1310020A).

The magnetic properties of iron pnictide superconductors with magnetic rare-earth ions under strong magnetic field are investigated based on the cluster self-consistent field method. Starting from an effective Heisenberg model, we present the evolution of magnetic structures on magnetic field in R/FeAsO (R = Ce, Pr, Nd, Sm, Gd, and Tb) and R/Fe2As2 (R = Eu) compounds. It is found that spin-flop transition occurs in both rare-earth and iron layers under magnetic field, in good agreement with the experimental results. The interplay between rare-earth and iron spins plays a key role in the magnetic-field-driven magnetic phase transition, which suggests that the rare-earth layers can modulate the magnetic behaviors of iron layers. In addition, the factors that affect the critical magnetic field for spin-flop transition are also discussed.

Adding remnant magnetization and anisotropic exchange to propeller-like single-molecule magnets through chemical design.

The selective replacement of the central iron(III) ion with vanadium(III) in a tetrairon(III) propeller-shaped single-molecule magnet has allowed us to increase the ground spin state from S=5 to S=13/2. As a consequence of the pronounced anisotropy of vanadium(III), the blocking temperature for the magnetization has doubled. Moreover, a significant remnant magnetization, practically absent in the parent homometallic molecule, has been achieved owing to the suppression of zero-field tunneling of the magnetization for the half-integer molecular spin. Interestingly, the contribution of vanadium(III) to the magnetic anisotropy barrier occurs through the anisotropic exchange interaction with iron(III) spins and not through single ion anisotropy as in most single-molecule magnets.

Magnetoelectric responses from the respective magneticRand Fe subsystems in the noncentrosymmetric antiferromagnetsRFe3(BO3)4(R = Eu, Gd, and Tb)

In rare-earth ($\mathit{R}$) ferroborates, R${\mathrm{Fe}}_{3}$(${\mathrm{BO}}_{3}$)${}_{4}$ with R = Eu, Gd, and Tb, the magnetoelectric (ME) responses appear to stem from both the antiferromagnetic order of the iron (Fe) spins and the magnetic moments on the $\mathit{R}$ ions. We measured the electric polarization ($P$) along the $a$ axis while rotating a magnetic field ($H$) around the $a$ axis and found that the target compounds show mutually distinctive $H$-direction dependencies. ${\mathrm{EuFe}}_{3}$(${\mathrm{BO}}_{3}$)${}_{4}$ (R = Eu) shows an almost constant spontaneous $P$ with a slight modulation when $H$ is slanted from the $c$ axis. The $H$-angle (${\ensuremath{\theta}}_{H}$) dependence of the $P$ can be described by a formula $P={P}_{0}\ensuremath{-}\ensuremath{\Lambda}{sin}^{2}{\ensuremath{\theta}}_{H}$. As for ${\mathrm{GdFe}}_{3}$(${\mathrm{BO}}_{3}$)${}_{4}$ and ${\mathrm{TbFe}}_{3}$(${\mathrm{BO}}_{3}$)${}_{4}$, they show highly anisotropic ${\ensuremath{\theta}}_{H}$ dependence of $P$, which characterizes the respective ME responses from their $\mathit{R}$ magnetic moments. In certain regions of ${\ensuremath{\theta}}_{H}$, the $P$ can be described by $P={P}_{0}\ensuremath{-}Ksin2{\ensuremath{\theta}}_{H}$ and $P={P}_{0}\ensuremath{\mp}\ensuremath{\Gamma}sin{\ensuremath{\theta}}_{H}$ for R = Gd and Tb, respectively. We devised a theory for the ME response of the individual magnetic ions in a R${\mathrm{Fe}}_{3}$(${\mathrm{BO}}_{3}$)${}_{4}$ crystal and applied it to these compounds focusing on their local symmetry and their ground-state multiplet structures. The above formulas successfully reproduce the observed results as the summation of $P$ from each magnetic subsystem, which in turn enables us to assign the first and second terms to the spontaneous $P$ due to a collinear antiferromagnetic ordering of the Fe spins and the ME response of the $\mathit{R}$ ion under $H$, respectively. The thermal and $H$-induced evolutions of the magnetic-ion resolved $P$ quantitatively agree with the theoretical predictions, ensuring the relevant microscopic ME mechanism for each magnetic ion. The measurement of angular dependence of $P$ is particularly useful to decompose the overlapped ME responses into the respective origins in the system with multiple magnetic subsystems.

New Routes to Synthesizing an Ordered Perovskite CaCu3Fe2Sb2O12 and Its Magnetic Structure by Neutron Powder Diffraction

The search for new double-perovskite oxides has grown rapidly in recent years because of their interesting physical properties like ferroelectricity, magnetism, and multiferroics. The synthesis of double perovskites, especially the A-site-ordered perovskites, in most cases needs to be made under high pressure, which is a drawback for applying these materials. Here we have demonstrated synthetic routes at ambient pressure by which we have obtained a high-quality duo-sites-ordered double perovskite, CaCu3Fe2Sb2O12, which has been previously synthesized under high pressure. The availability of a large quantity of the powder sample allows us to determine the crystal and magnetic structures by neutron powder diffraction (NPD) at 300 and 1.3 K. Measurements of the magnetization and heat capacity showed a ferrimagnetic transition at 160 K. A ferrimagnetic structure consisting of the uncompensated antiferromagnetic coupling between neighboring collinear copper and iron spins has been resolved from the low-temperature NPD data.

Magnetism of ordered and disordered alloys of R2Fe14B (R = Nd, Er) type

Magnetic susceptibility, magnetization and neutron diffraction measurements have been performed to study structure and magnetic states of crystalline and amorphous Nd2Fe14B and Er2Fe14B alloys. In the crystalline state there exists a large (about 20%) anisotropy of Er-sublattice magnetization. Values of the magnetic anisotropy constant of Er ions and the Er–Fe exchange-coupling parameter were estimated using temperature dependence of the Er- and Fe-sublattice magnetizations. Amorphous state of the samples was obtained by irradiation of fast (Eeff⩾1МeV) neutrons with a fluence up to the 1.2×1020n/сm−2 at 340K. It is shown that antiferromagnetic coupling between the rare-earth and iron spins is kept in the amorphous state. Amorphization of the samples is found to result in reduction of the Curie temperature (TC) by about 200K and almost total absence of coercivity. We suggest that the strong decrease of TC is a consequence of enhancement of negative Fe–Fe interactions as a result of dispersion of interatomic distances, which is a characteristic feature of the amorphous state.

A magnetic glassy phase in Fe1+ySexTe1−x single crystals

The evolution of magnetic order in Fe1+ySexTe1−x crystals as a function of Se content was investigated by means of ac/dc magnetometry and muon-spin spectroscopy. Experimental results and self-consistent density functional theory calculations both indicate that muons are implanted in vacant iron-excess sites, where they probe a local field mainly of dipolar origin, resulting from an antiferromagnetic (AFM) bicollinear arrangement of iron spins. This long-range AFM phase becomes progressively disordered with increasing Se content. At the same time all the tested samples manifest a marked glassy character that vanishes for high Se contents. The presence of local electronic/compositional inhomogeneities most likely favours the growth of clusters whose magnetic moment ‘freezes’ at low temperature. This glassy magnetic phase justifies both the coherent muon precession seen at short times in the asymmetry data, as well as the glassy behaviour evidenced by both dc and ac magnetometry.

Magnetic properties and spin dynamics in Ba(Fe1−xMnx)2As2 compounds studied by 57Fe Mössbauer spectroscopy

Ba(Fe1−xMnx)2As2 compounds with x = 0.016 and 0.064 have been studied by 57Fe Mössbauer spectroscopy in the temperature range from 30 to 300 K. The unusual magnetic splitting spectra at lower temperatures have been analyzed using the distribution of hyperfine field. It is found that the influence of Mn dopant spreads beyond the nearest Fe magnetic moments, and the Fe 3d electrons behave more localized compared with those in the electron-doped compounds. This reduces the hyperfine interactions between iron nucleus and the sounding electrons. The shape of the spin density wave is near-rectangular at 6.4%-Mn doping, indicating quite different interband interactions compared with electron-doped compounds. A distinct broadening of the spectral linewidth around the spin density wave transition temperature has been observed and the spin correlation time is deduced according to the linewidth. The correlation time is further related to the spin–lattice relaxation rate by a simple model and the magnetic fluctuations can be explored effectively. It is found that the magnetic fluctuations of iron spins in Mn-doped compounds can be described well using a phenomenological two-component model and the resulting Curie–Weiss temperature is far from the quantum critical point at the present doping levels.

Electric-field-induced generation and reversal of ferromagnetic moment in ferrites

The prospect of controlling the magnetization (M) of a material solely with an external electric field (E) could enable the development of low-power spintronics. Although there has been some success towards this end, most approaches involve controlling interactions at the interface between two different materials rather than switching of a single bulk phase. Here we report the ability to exert complete control over the generation and reversal of the bulk spontaneous M of the single-component multiferroics RFeO3 (R = Dy0.70Tb0.30, Dy0.75Gd0.25) with an E alone. We achieve this by controlling the anisotropic character of rare-earth magnetism and exploiting the competition between different magnetoelectric phases. We also show that whether M is reversed or retained on the E-induced polarization reversal depends on the E modulation speed. This is ascribed to the different dynamical characteristics of ferroelectric and multiferroic domain walls governed by the reversal dynamics of rare-earth moments and iron spins, respectively. The ability to modify a material’s magnetization with an electric field could enable lower-power electronic devices. Such ‘magnetoelectric’ behaviour is usually only seen at the interface between magnetostrictive and electrostrictive materials, but has now been observed in the bulk of single-component rare-earth ferrites.

Magnetic structure of almandine

The magnetic structure of almandine has been investigated by electronic structure calculations in the local spin density approximation in order to arrive at a more detailed understanding of the magnetic structure and the exchange pathways. The calculations are based on experimentally determined geometrical data of the crystal structure at 100 K. The calculated quadrupole splittings, spin-allowed d–d transitions and magnetic moment for iron atoms are in reasonable agreement with the respective experimental values obtained by Mossbauer and absorption spectroscopy, and magnetization measurements demonstrating the reliability of the calculations. The spin structure is derived from the calculated coupling constants for the possible exchange pathways. The competing superexchange pathways exist via oxygen bridges between directly neighboured iron ions and via edges of silicon tetrahedra and aluminium octahedra connecting more distant iron dodecahedra. Careful consideration revealed that almandine structure contains two identical interpenetrative sublattices of Fe dodecahedra connected via Al octahedra and Si tetrahedra. The calculations provide the information about ferromagnetic interaction between the iron spins within each sublattice, whereas the coupling between two magnetic sublattices is weakly antiferromagnetic via Al octahedra and Si tetrahedra. This antiferromagnetic interaction of two identical magnetic sublattices is in good agreement with the experiments and explains the Mossbauer spectra of almandine below the Neel temperature. Since almandine belongs to most abundant crystallized silicates that are main constituents of the earth and main components of cosmic dust, these results have scientific importance of studying the universe.

Structural and magnetic properties of the iron-containing langasite familyA3MFe3X2O14(A= Ba, Sr;M= Sb, Nb, Ta;X= Si, Ge) observed by Mössbauer spectroscopy

The occurrence of the long-range magnetic order of high-spin Fe${}^{3+}$ ions is found in iron-containing langasite family crystals ${A}_{3}M$Fe${}_{3}$${X}_{2}$O${}_{14}$ ($A$ = Ba, Sr; $M$ = Sb, Nb, Ta; $X$ = Si, Ge) at $T$ \ensuremath{\le} 40 K. The Neel temperature (${T}_{\mathrm{N}}$) values (27.1--37.1 K) depend on the composition and significantly increase with the strengthening covalence of Fe-O bonds in the 3$f$ tetrahedra. Splitting of iron positions into two magnetic sublattices is established in Ba${}_{3}$TaFe${}_{3}$Si${}_{2}$O${}_{14}$ and Ba${}_{3}$NbFe${}_{3}$Si${}_{2}$O${}_{14}$ crystals at $T$ ${T}_{\mathrm{N}}$. Iron magnetic moments in two sublattices are directed at different angles to the local crystal axes with a mean value \ensuremath{\approx}45\ifmmode^\circ\else\textdegree\fi{}. At the temperatures between 4.2 and 20 K, the iron spins in two sublattices rotate in opposite directions. It can be related to magnetic chirality in the helicoidal magnetic structure. The appearance of nonequivalent Fe${}^{3+}$ sites is associated with the structural phase transition $P$321 \ensuremath{\rightarrow} $P$3 (or $P$321 \ensuremath{\rightarrow} $C$2) induced by the magnetic transition. The presence of the polar threefold axis at this transition provides the conditions for the occurrence of the ferroelectric state. Such crystals can be considered as magnetically induced multiferroics.