Collinear Spin Structure Research Articles

Stoichiometry driven tuning of physical properties in epitaxial Fe3-xCrxO4 thin films

Tuning magnetic and electronic transport properties in spinel oxides requires a faithful description between chemical composition and cation site-occupation. Here this challenge is addressed using Fe3-xCrxO4 thin films grown by oxygen-assisted molecular-beam epitaxy within a wide range of composition (0.0 ≤ x ≤ 1.2). Spectroscopic measurements (e.g., X-ray magnetic circular dichroism), refined by theoretical simulations (e.g., crystal field multiplet), are performed to establish a quantitative link between chromium content, Fe2+/Fe3+ site-occupation and macroscopic physical properties of the layers. It is found that Fe3-xCrxO4 thin films (i) delay the transition from inverse to normal spinel configuration with increasing chromium content and (ii) promote collinear spin structure, at odds with bulk material. As a result, strong antiferromagnetic interactions are preserved between spins in tetrahedral and octahedral spinel sublattices, so that chromium-rich thin films exhibit Curie temperatures above room temperature and higher magnetization. Electron hopping is also favored by this singular cation distribution and electronic band gap is smaller than expected for these thin films. The cation site-occupation is therefore a key feature to consider for applications of Fe3-xCrxO4 thin films in spintronics and photocatalysis, as it enables manipulation of magnetic properties (Curie temperature and magnetization) and band gap engineering.

Excitonic transverse and amplitude fluctuations in noncollinear and charge-ordered RbFe2+Fe3+F6

RbFe$^{2+}$Fe$^{3+}$F$_{6}$ is an example of an antiferromagnet with charge ordering of the octahedrally coordinated Fe$^{2+}$ and Fe$^{3+}$ ions. As well as different spin values, Fe$^{2+}$ ($S=2$) and Fe$^{3+}$ ($S={5\over2}$) possess differing orbital ground states with Fe$^{2+}$ having an orbital degeneracy with an effective orbital angular momentum of $l=1$. The resulting low temperature magnetic structure is non collinear with the spins aligned perpendicular to nearest neighbors (S. W. Kim \textit{et al.} Chem. Sci. {\bf{3}}, 741 (2012)). The combination of an orbital degeneracy and non collinear spin arrangements introduces the possibility for unusual types of excitations such as amplitude modes of the order parameter. In this paper we investigate this by applying a multi-level analysis to model neutron spectroscopy data (M. Songvilay \textit{et al.} Phys. Rev. Lett. {\bf{121}}, 087201 (2018)). In particular, we discuss the possible origins of the momentum and energy broadened continuum scattering observed in terms of amplitude fluctuations allowed through the presence of an orbital degree of freedom on the Fe$^{2+}$ site. We extend previous spin-orbit exciton models based on a collinear spin structure to understand the measured low-energy excitations and also to predict and discuss possible amplitude mode scattering in RbFe$^{2+}$Fe$^{3+}$F$_{6}$.

Magnetic phase diagram of [formula omitted][FeCl5(H2O)] ([formula omitted] = K, Rb, NH4)

Erythrosiderites with the formula A2FeX5⋅H2O, where A = Rb, K, and (NH4) and X = Cl and Br are intriguing systems that possess various magnetic and electric phases, as well as multiferroic phases in which magnetism and ferroelectricity are coupled. In this report, we study the magnetic phase diagram of erythrosiderites as a function of superexchange interactions and magnetic anisotropies. To this end, we perform classical Monte Carlo simulations on magnetic Hamiltonians that contain five different superexchange interactions with single-ion anisotropies. Our phase diagram contains all magnetic ground states that have been experimentally observed in these materials. We argue that the ground states can be explained by varying the ratio of J4J2. For J4J2>0.95 a cycloidal spins structure is stabilized as observed in (NH4)2FeCl5⋅H2O and otherwise a collinear spin structure is stabilized as observed in (K,Rb)2FeCl5⋅H2O. We also show that the difference in the single-ion anisotropy along a- and c- axes is essential to stabilize the intermediate state observed in (NH)2FeCl5⋅H2O.

Anomalous Hall effect in antiferromagnetic Cr thin films

Bulk chromium is the simplest antiferromagnet in our general material database, which has been firmly believed to exhibit a collinear spin structure and a consequent vanishing anomalous Hall effect (AHE). In this work, we report an unexpected weak AHE in polycrystalline thin films of chromium. We attribute this phenomenon to the noncollinear spin textures induced by the local spin frustration and rearrangement in certain areas with high residual strain and defect densities. Moreover, a dominant underlying mechanism of intrinsic Berry phase is speculated. This could be a general feature for all the collinear antiferromagnetic thin-film materials with moderately high defect concentrations, making them promising candidates for emergent antiferromagnetic spintronics.

Magnetoelectric Coupling Triggered by Noncollinear Magnetic Structure in M‐Type Hexaferrite

AbstractDirect magnetoelectric (ME) coupling between magnetic and ferroelectric orders, which can be realized in spiral magnets, is vital in technological applications of multifunctional materials. Multisusceptible BaFe12O19 is noteworthy for its excellent magnetic, dielectric properties and thus the application in high‐density information storage, while its collinear spin structure limits the emergence of ferroelectrics and ME coupling. In this work, nonzero Dzyaloshinskii–Moriya (DM) interaction is created by partial substitution of the spin‐down Fe3+ sites to induce conical spin structure in BaFe12O19. Larger In3+ ions are introduced and prefer to occupy the spin‐down 4f1 and 4f2 lattice sites. The evolution of magnetization versus temperature adduces evidence of conical magnetic structure. As a result, direct ME and magnetodielectric coupling are obtained in In‐doped BaFe12O19 ceramics. An interlayer DM interaction model is built to discuss the intrinsic relationship among crystal structure, noncollinear magnetic structure, and ionic substitution. Moreover, BaZn0.9Zr0.9Fe10.2O19 ceramics are also prepared, and exhibit noncollinear magnetic structure and direct ME coupling since Zn2+ and Zr4+ also possess larger radii than Fe3+ and prefer to enter the spin‐down sites. The present result provides a feasible avenue to develop multiferroic and magnetoelectric coupling in ubiquitous M‐type hexaferrite.

A-Site and B-Site Cation Ordering Induces Polar and Multiferroic Behavior in the Perovskite NaLnNiWO6 (Ln = Y, Dy, Ho, and Yb)

Octahedral distortion in ABO3 perovskite materials is ubiquitous because of the ionic size mismatch between A and B cations, leading to various kinds of crystal symmetry. However, such a distortion always results in centrosymmetric structures except for the covalent bond formation because of the second-order Jahn–Teller (SOJT) effect that occurs with d0 or lone-pair cations. Here, we report that an unusual combination of the layered A-site cation ordering and B-site rock salt ordering in NaYNiWO6 prepared under high-pressure and high-temperature conditions results not only in a polar (P21) structure, as revealed by the neutron diffraction analysis, but exhibits multiferroic properties below the magnetic ordering of Ni2+ ions (TN = 21 K). Analysis of the neutron diffraction data at 20 K reveals an incommensurate sinusoidal spin ordering with the propagation vector, ki = (0.471(2), 0, 0.491(4)), and a commensurate collinear spin structure with kc = (0.5, 0, 0.5) below 18 K. X-ray diffraction data confirm the polar structure in Dy, Ho, and Yb compounds. All four compounds exhibit a switchable change in electric polarization (ΔP) at the magnetic ordering temperatures, demonstrating coupling between ferroelectricity and magnetism.

Easy-cone magnetic structure in (Cr0.9B0.1)Te

Although stoichiometric CrTe is difficult to synthesize because of the appearance of Cr vacancies, ferromagnetic Cr1−x Te compounds have attracted increasing attention. This work investigates single crystalline (Cr0.9 B0.1)Te with the Cr vacancies filled by B to stabilize the hexagonal crystal structure and shift the Fermi energy. The structural and magnetic properties have been characterized by experimental measurements and ab initio calculations. A collinear spin structure with an easy axis along c is observed at high temperature, whereas the magnetic moments localized at the Cr atoms gradually tilt away from the c-axis below 140 K. A maximum tilt angle of 30° is observed at a temperature of 2 K.

Thermodynamics of the pyrochlore-lattice quantum Heisenberg antiferromagnet

We use the rotation-invariant Green's function method (RGM) and the high-temperature expansion to study the thermodynamic properties of the Heisenberg antiferromagnet on the pyrochlore lattice. We discuss the excitation spectra as well as various thermodynamic quantities, such as spin correlations, uniform susceptibility, specific heat, and static and dynamical structure factors. For the ground state we present RGM data for arbitrary spin quantum numbers $S$. At finite temperatures we focus on the extreme quantum cases $S=\frac{1}{2}$ and 1. We do not find indications for magnetic long-range order for any value of $S$. We discuss the width of the pinch point in the static structure factor in dependence on temperature and spin quantum number. We compare our data with experimental results for the pyrochlore compound ${\mathrm{NaCaNi}}_{2}{\mathrm{F}}_{7}$ ($S=1$). Thus, our results for the dynamical structure factor agree well with the experimentally observed features at 3 ...8 meV for ${\mathrm{NaCaNi}}_{2}{\mathrm{F}}_{7}$. We analyze the static structure factor ${S}_{\mathbf{q}}$ to find regions of maximal ${S}_{\mathbf{q}}$. The high-temperature series of the ${S}_{\mathbf{q}}$ provide a fingerprint of weak order by disorder selection of a collinear spin structure, where (classically) the total spin vanishes on each tetrahedron and neighboring tetrahedra are dephased by $\ensuremath{\pi}$.

Room temperature ferromagnetism in BiFe1−xMnxO3 thin film induced by spin-structure manipulation

The evolution of crystal structure, spin structure, and macroscopic magnetization of manganese-substituted BiFeO3 (BiFe1−xMnxO3), a candidate for multiferroic materials, were investigated on bulk and epitaxial thin-film. Mn substitution for Fe induced collinear antiferromagnetic spin structure around room temperature by destabilizing the cycloidal spin modulation which prohibited the appearance of net magnetization generated by Dzyaloshinskii-Moriya interaction. For the bulk samples, however, no significant signal of ferromagnetism was observed because the direction of the ordered spins was close to parallel to the electric polarization so that spin-canting did not occur. On the contrary, BiFe1−xMnxO3 thin film on SrTiO3 (001) had a collinear spin structure with the spin direction perpendicular to the electric polarization at room temperature, where the appearance of spontaneous magnetization was expected. Indeed, ferromagnetic hysteresis behavior was observed for BiFe0.9Mn0.1O3 thin film.

MAGNETORESISTANCE IN NANOSIZE STRONTIUM FERROMOLYBDATE WITH DIELECTRIC INTERLAYERS

Single phase strontium ferromolybdate nanopowder with a double perovskite structure has been synthesized using the citrate gel technique at pH = 4. A superstructural ordering degree of the iron and molybdenum cations of 88% has been obtained. X−ray diffraction of pressed Sr2FeMoO6−δ pellets subjected to annealing at T = 700 K and p(O2) = 10 Pa has revealed the formation of the SrMoO4 phase at grain boundaries. The temperature dependence of the electrical resistivity in the 4.2 to 300 K range changes from a metal type one in the single phase Sr2FeMoO6−δ to a semiconductor type one in the Sr2FeMoO6−δ – SrMoO4 – Sr2FeMoO6−δ structure containing dielectric interlayers, indicating variable charge hopping in the latter structure. In the applied magnetic fields the temperature dependence does not change qualitatively; however, the resistivity decreases with increasing field, i.e., a negative magnetoresistance of up to 41% at T = 10 K and B = 8 T is observed. The external field forms a collinear spin structure, thus increasing the spin−polarized current through the barriers in the granular Sr2FeMoO6−δ – SrMoO4 – Sr2FeMoO6−δ heterostructure.

Charge transfer mechanisms in strontium ferromolybdate with tunneling barriers

The single-phase strontium ferromolybdate powder with the double perovskite structure has been synthesized by the citrate-gel method with superstructural ordering of iron and molybdenum cations of 88%. The formation of the SrMoO4 phase on the intergrain boundaries has been determined by means of the XRD studies, the Electrokinetic Sonic Analysis technique, and FESEM investigations of pressed Sr2FeMoO6-δ powders subjected to the isothermic treatment at 700 K and p(O2) = 10 Pa. The temperature dependence of the electrical resistivity in the temperature range 4.2–300 K has a metallic type in the single-phase Sr2FeMoO6-δ compound and the semiconductor type in the Sr2FeMoO6-δ–SrMoO4–Sr2FeMoO6-δ structure with dielectric sheaths. In the latter case, a hopping mechanism of charge transfer is observed. In the applied magnetic field, the temperature dependence does not change qualitatively; however, the resistivity decreases with increasing field, i.e., a negative magnetoresistive effect appears, reaching 43.6% at 10 T and 10 K. The external field forms a collinear spin structure, thus increasing the spin-polarized current through the energy barriers in the granular Sr2FeMoO6-δ–SrMoO4–Sr2FeMoO6-δ heterostructure.

Magnetic behavior of metastable Fe films grown on Ir(1 1 1)

We investigated the growth of ultra-thin Fe films on Ir(1 1 1) by means of in situ low energy electron diffraction and spin-resolved photoemission techniques. We observe a (1 × 1) diffraction pattern, characteristic of the fcc substrate, below four monolayers (ML). Then, a complex superstructure starts to develop, compatible with the formation of bcc-like Fe domains aligned with the substrate according to the Kourdjumov–Sachs orientation relationships. The analysis of the diffraction patterns reveals a progressive evolution towards a fully relaxed bcc lattice, characteristic of bulk Fe. Both photoemission (filled states) and inverse photoemission (empty states) results show characteristic features related to the contribution of the Fe layer, evolving towards those observed on the Fe (1 1 0) bcc surface. Spin resolution allows to detect a spectral polarization above 4 ML, corresponding to the formation of bcc Fe, which gradually increases indicating the formation of an in-plane magnetized ferromagnetic layer in thick films. No in-plane net magnetization is detected in thinner films, independent of the sample temperature down to 30 K. Following recent investigations on the Fe/Ir(1 1 1) system with microscopy techniques, we link this observation to the stabilization of a non collinear spin structure yielding an overall nil magnetization.

Microscopic evidence of magnetic and structure phase transition in multiferroic spinel FeV2O4

We report the microscopic evidence for magnetic and structural phase transitions in multiferroic spinel FeV2O4 from the hyperfine magnetic interaction. FeV2O4 sample shows three different crystal structures with the phase transitions from tetragonal to orthorhombic structure around 70 K, from orthorhombic to tetragonal structure around 109 K, and from tetragonal to cubic structure around 140 K. Mössbauer spectra of FeV2O4, obtained at various temperatures, were analyzed with severely distorted 8-line below TC, and doublet at TC. Also, the Mössbauer spectra change from doublet to singlet around TJT ≅ 140 K due to the reduction of Jahn-Teller effect. The value of electric quadrupole splitting (ΔEQ) is 3.05 mm/s at 4.2 K, indicating the noncollinear spin structure with strong polarization from the gap energy of 5T2g band, Δ1 ≅ 0. Whereas, there is collinear spin structure between TS ≅ 70 K < T < TC ≅ 109 K, since Δ1 in this temperature range increases from the value when T < TS due to the non-degenerate energy state with commensuration in the collinear state. Also, we have found that large polar angle θ for T < TS suggests the spin of the Fe2+ cations aligns along c-axis with the distortion in the a-b plane, while the small azimuthal angle θ suggest the direction of the spin is within ab-plane for TS < T < TC.

Magnetoresistive effect in nanosized strontium ferromolybdate with dielectric interlayers

Monophasic iron ferromolybdate nanopowders with a double perovskite structure have been synthesized using the citrate-gel technique at pH=4. A superstructural ordering degree of the iron and molybdenum cations of 88% has been obtained. X-ray diffraction of pressed Sr2FeMoO6−δ pellets subjected to annealing at T=700K and p(O2)=10Pa has revealed the formation of the SrMoO4 phase at grain boundaries. The temperature dependence of the electrical resistivity in the range from 4.2 to 300K switches from a metal type one in the monophasic Sr2FeMoO6-δ to a semiconductor type one in the Sr2FeMoO6−δ–SrMoO4–Sr2FeMoO6−δ structure containing dielectric interlayers, indicating variable range hopping in the latter. In the applied magnetic fields the temperature dependence does not change qualitatively; however, the resistivity decreases with increasing field, i.e., a negative magnetoresistance of up to 41% at T=10K and B=8T is observed. The external field forms a collinear spin structure, thus increasing the spin-polarized current through the granular Sr2FeMoO6−δ–SrMoO4–Sr2FeMoO6−δ heterostructure.

Exploration of antiferromagnetic CoO and NiO using reverse Monte Carlo total neutron scattering refinements

The atomic and magnetic structures of CoO and NiO have been probed using reverse Monte Carlo (RMC) refinements of neutron total scattering data. The results obtained show that the known magnetic structure for NiO can be recovered by the RMC process starting from random spin configurations, but it is insensitive to the spin direction in the {111} ferromagnetic planes. Refinements of the magnetic structure of CoO starting from random spin configurations result in collinear or non-collinear magnetic structures, consistent with those reported by other techniques. Starting from an ordered collinear spin structure for CoO and NiO leads to different results than when starting from a random arrangement of spins, which is evidence for configurational bias that highlights the need to take care when selecting a starting model for RMC refinements of magnetic structures.

Magnetoelectric memory effect in the Y-type hexaferrite BaSrZnMgFe12O22**Project supported by the National Natural Science Foundation of China (Grant Nos. 11534015 and 51371193).

We report on the magnetic and magnetoelectric properties of the Y-type hexaferrite BaSrZnMgFe12O22, which undergoes transitions from a collinear ferrimagnetic phase to a proper screw phase at 310 K and to a longitudinal conical phase at 45 K. Magnetic and electric measurements revealed that the magnetic structure with spiral spin order can be modified by applying a magnetic field, resulting in magnetically controllable electric polarization.It was observed that BaSrZnMgFe12O22 exhibits an anomalous magnetoelectric memory effect: the ferroelectric state can be partially recovered from the paraelectric phase with collinear spin structure by reducing magnetic field at 20 K. We ascribe this memory effect to the pinning of multiferroic domain walls, where spin chirality and structure are preserved even in the nonpolar collinear spin state.

Anisotropic lattice dynamics and intermediate-phase magnetism in delafossiteCuFeO2

Hyperfine interactions and Fe-specific lattice dynamics in ${\mathrm{CuFeO}}_{2}$ were investigated by nuclear resonance scattering methods and compared to ab initio lattice dynamics calculations. Using nuclear forward scattering the collinear spin structure at temperatures below about 11 K could be confirmed, whereas the nuclear forward scattering results in the intermediate temperature range between about 11 K and 14 K are incompatible with the assumption of a sinusoidal distribution of spins parallel to the $c$ axis of ${\mathrm{CuFeO}}_{2}$. The critical behavior of the average hyperfine field at the phase transition at about 14 K further supports a three-dimensional model for the magnetism in this compound. Moreover, using nuclear inelastic scattering by the ${}^{57}\mathrm{Fe}$ M\ossbauer resonance, Fe-specific lattice dynamics are found to be strongly anisotropic with stiffer bonds in the $ab$ plane of the crystal. The powder averaged, Fe partial density of phonon states can be well modeled using ab initio calculations and low-energy phonons are found to deviate from classical Debye-like behavior, indicating spin-phonon coupling in this compound. Besides, the theoretical phonon spectrum exhibits typical characteristics for delafossite-type material.

Magnetic properties of the multiferroic compounds Eu1-xYxMnO3 (x = 0.2 and 0.3)

Magnetic properties of multiferroic Eu1-xYxMnO3 with x = 0.2, 0.3 were studied by μSR and Mossbauer spectroscopy. Both compounds are known to become antiferromagnetic with Mn3+ being the only magnetic ion. We find TN = 47±0.5K for x = 0.2 and 45±0.5K for x = 0.3. Below TN three different magnetic states (AFM-1, AFM-2, AFM-3) are formed. The μSR parameters of the uppermost magnetic state (AFM-1) are alike in both compounds, and compatible with a commensurate modulated collinear spin structure. The magnetic ground state in x = 0.2 (AFM-3) is shown to be single phase. Its spectral parameters support the proposal of a cone-like spin structure, yet with incommensurate modulation. The ground state for x = 0.3 is found to have an incommensurate spiral spin structure. 151Eu Mössbauer spectroscopy measured the Eu hyperfine field induced by the Mn3+ ions. Its low value (~4T) means that mixing with higher Eu electronic states is small and that the Mn-Eu exchange coupling is weak (~0.5K). The various magnetic transitions appear as small irregularities in the temperature dependence of the hyperfine fields. The present results are discussed in terms of different published magnetic phase diagrams of Eu1-xYxMnO3.

Relation between spin and orbital magnetism in excited states of ferromagnetic materials

We report the first-principles study of the orbital magnetism, the magnetic anisotropy energy, the ratio of the spin, and the orbital moments in nano-sized systems perturbed from their magnetic ground state. We investigate one monolayer thick films of Co, Fe, and FePt. Two types of the perturbation are studied. First, the collinear spin structure is rotated continuously between the easy and hard axes. Second, the non-collinear spin structures are considered varying in both the angles between spin moments and the direction of the net magnetization. In agreement with the experiment we obtain a variety of behaviours. We show that the magnetic anisotropy energy can both increase and decrease with increasing magnetic disorder. The type of behaviour depends on the variation of the electronic structure with increasing angles between atomic moments. We obtain the effect of band narrowing accompanying the spin disorder that correlates with the band narrowing obtained experimentally in a laser irradiated system. In agreement with this experiment we show that the ratio of the spin and orbital moments can both remain unchanged and vary strongly. We analyse the applicability of Bruno's picture, which suggests proportionality between magnetic-anisotropy energy and orbital moment anisotropy for non-collinear spin configurations. We study the non-collinearity of the atomic spin and orbital moments and demonstrate that the response of the orbital moments to the variation of the spin structure can be unexpected and spectacular.

Electromagnon by chiral spin dynamics in the triangular lattice antiferromagnet

From high field electron spin resonance measurements in illuminating polarized light, we have revealed the existence of electromagnon, i.e., magnon excitation by oscillatory electric fields of light, in the field-induced 1/5-plateau phase of the triangular lattice antiferromagnet ${\mathrm{CuFeO}}_{2}$. We indicate that peculiar magnon modes, which generate uniform fluctuation of the vector spin chirality at wave vector $k=0$, appear in the magnetic ordered phase with a collinear spin structure on triangular lattice. Our experimental results demonstrate that such magnon modes couple with an electric component of light, leading to the emergence of the electromagnon. Moreover, the measurements in circularly polarized light exhibit an anomalous behavior that circular dichroism, which is usually found in magnetic resonance, is absent in the resonance signal of the electromagnon. The microscopic mechanism of the electromagnon in ${\mathrm{CuFeO}}_{2}$ is also discussed.