Collinear Antiferromagnetic Structure Research Articles

Spontaneous Magnetization Induced by Antiferromagnetic Toroidal Ordering.

The magnetic toroidal dipole moment, which is induced by a vortex-type spin texture, manifests itself in parity-breaking physical phenomena, such as a linear magnetoelectric effect and nonreciprocal transport. We elucidate that a staggered alignment of the magnetic toroidal dipole can give rise to spontaneous magnetization even under antiferromagnetic structures. We demonstrate the emergence of uniform magnetization by considering the collinear antiferromagnetic structure with the staggered magnetic toroidal dipole moment on a bilayer zigzag chain. Based on the model calculations, we show that the interplay between the collinear antiferromagnetic mean field and relativistic spin-orbit coupling plays an important role in inducing the magnetization.

Multimodal Approach Reveals the Symmetry-Breaking Pathway to the Broken Helix in EuIn2As2

Understanding and manipulating emergent phases, which are themes at the forefront of quantum-materials research, rely on identifying their underlying symmetries. This general principle has been particularly prominent in materials with coupled electronic and magnetic degrees of freedom, in which magnetic order influences the electronic band structure and can lead to exotic topological effects. However, identifying symmetry of a magnetically ordered phase can pose a challenge, particularly in the presence of small domains. Here we introduce a multimodal approach for determining magnetic structures, which combines symmetry-sensitive optical probes, scattering, and group-theoretical analysis. We apply it to EuIn2As2, a material that has received attention as a candidate axion insulator. While first-principles calculations predict this state on the assumption of a simple collinear antiferromagnetic structure, subsequent neutron-scattering measurements reveal a much more intricate magnetic ground state characterized by two coexisting magnetic wave vectors reached by successive thermal phase transitions. The proposed high- and low-temperature phases are a spin helix and a state with interpenetrating helical and Néel antiferromagnetic order termed a “broken helix,” respectively. Employing a multimodal approach, we identify the magnetic structure associated with these two phases of EuIn2As2. We find that the higher-temperature phase is characterized by a variation of the magnetic moment amplitude from layer to layer, with the moment vanishing entirely in every third Eu layer. The lower-temperature structure is similar to the broken helix, with one important difference: Because of local strain, the relative orientation of the magnetic structure and the lattice is not fixed. Consequently, the symmetry required to protect the axion phase is not generically protected in EuIn2As2, but we show that it can be restored if the magnetic structure is tuned with uniaxial strain. Finally, we present a spin Hamiltonian that identifies the spin interactions that account for the complex magnetic order in EuIn2As2. Our work highlights the importance of a multimodal approach in determining the symmetry of complex order parameters. Published by the American Physical Society 2024

Temperature- and magnetic field-induced magnetic structural changes in the Fe3Si/FeSi2 superlattice

Artificial lattices with semiconductor spacers are expected to exhibit changes in their magnetic structure owing to the control of their electronic states. The temperature (T) and magnetic-field (H ext) dependence of the in-plane magnetic structure of an [Fe3Si/FeSi2]20 superlattice with a nonmagnetic and semiconducting FeSi2 spacer layer is investigated using magnetization and polarized neutron reflectivity measurements. When H ext = 5 mT, nearly collinear antiferromagnetic (AF) structures are observed from 4 to 298 K. When H ext = 1 T, field-induced fan-like, noncollinear AF structures showing ferromagnetic components along H ext and transverse AF components are observed at low T.

Magnetic structure and properties of the honeycomb antiferromagnet [Na(OH2)3]Mn(NCS)3.

We report the magnetic structure and properties of a thiocyanate-based honeycomb magnet [Na(OH2)3]Mn(NCS)3 which crystallises in the unusual low-symmetry trigonal space group P3̄. Magnetic measurements on powder samples show this material is an antiferromagnet (ordering temperature TN,mag = 18.1(6) K) and can be described by nearest neighbour antiferromagnetic interactions J = -11.07(4) K. A method for growing neutron-diffraction sized single crystals (>10 mm3) is demonstrated. Low temperature neutron single crystal diffraction shows that the compound adopts the collinear antiferromagnetic structure with TN,neut = 18.94(7) K, magnetic space group P3̄'. Low temperature second-harmonic generation (SHG) measurements provide no evidence of breaking of the centre of symmetry.

Opticheskiy lineynyy dikhroizm v ab-ploskosti ferroborata NdFe3(BO3)4

he high-resolution optical absorption spectra of NdFe3(BO3)4 single crystals have been recorded at temperatures from 4 to 40 K in the IR range of f–f transitions in a Nd3+ ion. Light linearly polarized at different angles to the C2 axes in the basal plane has been passed along the trigonal C3 axis. Below the temperature of magnetic moment ordering into a collinear antiferromagnetic structure (TN ≈ 30 K), dichroism, that is, the absorption versus polarization dependence, arises. The temperature and angular dependences of dichroism indicate that the magnetic moments of iron are directed along the C2 axes up to about 17 K, the number of domains with variously directed C2 axes being different. The mechanism of linear dichroism has been discussed. Below 17 K, a smooth transition to the helicoidal magnetic phase has been observed, with the collinear phase coexisting with the helicoidal one. Data presented in this article contradict the earlier concept of magnetic moments fluctuating in the low-temperature phase near the C2 axis within the ±10° interval.

Low-frequency spin dynamics of the quasi-two-dimensional S = 12 antiferromagnet BaCdVO(PO4)2

A quasi-two-dimensional quantum magnet with competing ferro- and antiferromagnetic exchange interactions, $\mathrm{BaCdVO}{({\mathrm{PO}}_{4})}_{2}$, is studied using the magnetic resonance technique in a wide frequency range. The magnetic resonance spectra of the ordered phase confirm the collinear antiferromagnetic spin structure oriented along the $a$ axis. The frequency-field diagram of antiferromagnetic resonance demonstrates two low-frequency excitation branches with different energy gaps, indicating a biaxial nature of anisotropy. We observe a critical softening of the longitudinal resonance mode in a field of 3.8 T, which is the lower boundary of a previously proposed presaturation phase positioned below the saturation field of 6.5 T. An interpretation of the presaturation phase as a state with variable magnetization of spin-vacancy defects is proposed. It may be considered an alternative to the spin-nematic phase initially supposed.

Anomalous Hall effect and magnetic structure of the topological semimetal ( Mn0.78Fe0.22)Ge3

Me$_{3+\delta}$Ge, being a Weyl semimetal, shows a large anomalous Hall effect (AHE), which decreases slowly with an increase in $\delta$ from 0.1 to 0.4. However, AHE in this compound remains significantly large in the whole range of $\delta$ because of the robust nature of the topology of bands. To explore the possibility of tuning the anomalous transport effects in Weyl semimetals, we have studied the single-crystal hexagonal-(Mn$_{0.78}$Fe$_{0.22}$)$_3$Ge compound. Magnetization of this compound shows two magnetic transitions at 242 K ($T_{\text{N1}}$) and 120 K ($T_{\text{N2}}$). We observed that the AHE persists between $T_{\text{N2}}$ - $T_{\text{N1}}$ and vanishes below $T_{\text{N2}}$. Further, we performed single-crystal neutron diffraction experiments (using spherical neutron polarimetry and unpolarized neutron diffraction) to determine the magnetic structures of (Mn$_{0.78}$Fe$_{0.22}$)$_3$Ge at different temperatures. Our neutron diffraction results show that the sample possesses a collinear antiferromagnetic structure below $T_{\text{N2}}$. However, the magnetic structure of the sample remains noncollinear antiferromagnetic, the same as Mn$_3$Ge, between $T_{\text{N1}}$ to $T_{\text{N2}}$. The presence of AHE, and noncollinear magnetic structure in (Mn$_{0.78}$Fe$_{0.22}$)$_3$Ge, between $T_{\text{N1}}$ and $T_{\text{N2}}$, suggest the existence of Weyl points in this temperature regime. Below $T_{\text{N2}}$, AHE is absent, and the magnetic structure also changes to a collinear antiferromagnetic structure. These observations signify a strong link between the magnetic structure of the sample and AHE.

Cycloidal spiral magnetic structures in the spin-chain compounds BaRFeO4 ( R=Yb and Tm): Ordered Yb versus partly ordered Tm

$\mathrm{Ba}R\mathrm{Fe}{\mathrm{O}}_{4}$ compounds containing magnetic rare-earth ions ($R=\mathrm{Yb}$, Tm) were prepared by a conventional solid-state method in air. We present detailed measurements of magnetic properties (specific heat, magnetic susceptibility) to demonstrate the presence of three successive magnetic phase transitions (at ${T}_{\mathrm{N}1}$, ${T}_{\mathrm{N}2}$, and ${T}_{3}$) in both compounds and employed neutron diffraction to determine the magnetic structures. Both compounds are isostructural (orthorhombic space group $Pnma$) and magnetic ions form rings and chains along the $b$ direction. All magnetic structures are incommensurate with a propagation vector $\mathbf{k}=(0,\phantom{\rule{0.28em}{0ex}}0,\phantom{\rule{0.28em}{0ex}}{k}_{z})$. Magnetic ordering of ${\mathrm{Fe}}^{3+}$ ions occurs at ${T}_{\mathrm{N}1}$ and ${T}_{\mathrm{N}2}$, where two different irreducible representations (irreps) order. Below ${T}_{\mathrm{N}1}$, there is a collinear spin-density wave with ordered ${\mathrm{Fe}}^{3+}$ moments along the chain direction ($b$ axis). Below ${T}_{\mathrm{N}2}$, this component remains stable and an additional component inside the $ac$ plane appears. Each Fe chain adopts a collinear antiferromagnetic structure with a constant magnetic phase. The two Fe rings in the unit cell have a different chirality and a noncollinear coupling. The mixing of two irreps leads to a cycloidal spiral magnetic structure that allows spin-induced ferroelectric polarization at ${T}_{\mathrm{N}2}$. With the presence of modulated components both perpendicular and along the propagation vector $\mathbf{k}$, the magnetic structure can be viewed as a sum of a helix and a cycloid structure. For ${\mathrm{BaTmFeO}}_{4}$, the magnetic structure has a larger cycloidal contribution and the dielectric constant \ensuremath{\epsilon} exhibits a small peak at ${T}_{\mathrm{N}2}$. ${\mathrm{Yb}}^{3+}$ moments order at ${T}_{3}$ with each Yb chain having a constant magnetic phase and a collinear antiferromagnetic structure stabilized by $4f\ensuremath{-}4f$ electron-exchange interactions. In contrast, no constant magnetic phase is observed at the Tm chains. Below ${T}_{3}$, magnetic order of Tm2 ions is induced by $3d\ensuremath{-}4f$ electron-exchange interactions and Tm1 ions remain disordered down to the lowest measured temperature $T=1.6\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ due to frustration of magnetic exchange interactions. The obtained magnetic structures are compared with those of ${\mathrm{BaYFeO}}_{4}$.

Features of the magnetization in ceramics Bi0.86Sm0.14Fe1-xMnxO3 in the rhombohedral - orthorhombic morphotropic phase boundary

Crystal structure of the ceramic compounds Bi0.86Sm0.14Fe1-xMnxO3 prepared by solid state reaction method was studied using X-ray diffraction, scanning electron microscopy and Raman spectroscopy; magnetometry measurements were performed to study magnetic properties and to investigate a correlation between the magnetic and crystal structures of the compounds. The compounds with 0 = x ≤ 0.1 are characterized by the coexistence of the polar rhombohedral and the antipolar orthorhombic phases, an increase of Mn doping causes a stabilization of the orthorhombic phase. Room temperature magnetization data show a gradual reduction of the remanent magnetization with an increase of Mn concentration, while a decrease of temperature causes a nontrivial reduction of remanent magnetization which is determined by a transformation of the non-collinear antiferromagnetic structure towards the collinear antiferromagnetic structure. The magnetic state of the compounds notably depends on the phase ratio of the coexisting structural phases and consists of the antiferromagnetic phases associated with the respective rhombohedral and the orthorhombic phases as well as the ferromagnetic component associated with the phase boundary spins. An evolution of the mentioned magnetic subsystems is discussed as a function of the chemical composition and temperature.

Competition of magnetocrystalline anisotropy of uranium layers and zigzag chains inUNi0.34Ge2single crystals

Structural and thermodynamic properties of single-crystalline ${\mathrm{UNi}}_{1\ensuremath{-}x}{\mathrm{Ge}}_{2}$ with $x=0.66$ have been investigated by measuring magnetization, specific heat, and thermal expansion over a wide range of temperatures and magnetic fields. The measurements revealed the emergence of a long-range antiferromagnetic ordering of uranium magnetic moments below the N\'eel temperature ${T}_{\mathrm{N}}=45.5(1)\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ and the existence of two easy axes in the studied compound, namely, $b$ and $c$, which correspond to the plane of the uranium zigzag chains. Magnetic field applied along these two crystallographic directions induces in the system a first-order metamagnetic phase transition (from antiferromagnetism to field-polarized paramagnetism), and the width of the magnetic hysteresis associated with that transition reaches as much as about 40 kOe at the lowest temperatures. A magnetic phase diagram developed from the experimental data showed that the metastable region associated with that magnetic hysteresis forms a funnel that narrows toward the N\'eel point in a zero magnetic field. The four-layer Ising model has successfully predicted the collinear antiferromagnetic structure in ${\mathrm{UNi}}_{0.34}{\mathrm{Ge}}_{2}$ (known from earlier reports), its magnetic phase diagram, and temperature and field variations of its magnetization. Moreover, it suggests that the first-order phase transition extends down to a zero magnetic field, although it is barely detectable in the experiments performed in low magnetic fields. According to this model, the second-order phase transition occurs in the compound only in a zero field.

Magnetic properties of Gd[formula omitted]Cu[formula omitted]Sb[formula omitted

Here, we investigated Gd3Cu3Sb4 compound, with the focus on its magnetic structure. We performed magnetization and susceptibility measurements and determined X-ray photoelectron spectroscopy valence band spectra. We also performed a detailed density functional theory calculations. We found that collinear antiferromagnetic structure is preferred over the non-collinear one observed earlier in the analogue Ce3Cu3Sb4 compound. The calculated energy of the unit cell is lower, when assuming a collinear scenario, and magnetic measurements seem to also support this result.

Magnetic structure study of the sawtooth chain antiferromagnet hbox {Fe}_2hbox {Se}_2hbox {O}_7

A magnetic structure of the sawtooth-chain antiferromagnet hbox {Fe}_2hbox {Se}_2hbox {O}_7 was investigated by magnetization measurements, single crystalline and powder neutron diffraction experiments, and a further analysis on the Mössbauer spectra. These experiments revealed a nearly collinear antiferromagnetic structure with magnetic moments aligned along the b-axis, indicating dominant antiferromagnetic exchanges between Fe(1)–Fe(2) and Fe(2)–Fe(3) sites. The magnon dispersion relation derived from the linear spin wave approximation suggests the possible flat band nature of magnons.

Observation of novel charge ordering and spin reorientation in perovskite oxide PbFeO3

PbMO3 (M = 3d transition metals) family shows systematic variations in charge distribution and intriguing physical properties due to its delicate energy balance between Pb 6s and transition metal 3d orbitals. However, the detailed structure and physical properties of PbFeO3 remain unclear. Herein, we reveal that PbFeO3 crystallizes into an unusual 2ap × 6ap × 2ap orthorhombic perovskite super unit cell with space group Cmcm. The distinctive crystal construction and valence distribution of Pb2+0.5Pb4+0.5FeO3 lead to a long range charge ordering of the -A-B-B- type of the layers with two different oxidation states of Pb (Pb2+ and Pb4+) in them. A weak ferromagnetic transition with canted antiferromagnetic spins along the a-axis is found to occur at 600 K. In addition, decreasing the temperature causes a spin reorientation transition towards a collinear antiferromagnetic structure with spin moments along the b-axis near 418 K. Our theoretical investigations reveal that the peculiar charge ordering of Pb generates two Fe3+ magnetic sublattices with competing anisotropic energies, giving rise to the spin reorientation at such a high critical temperature.

Charge transfer enhanced magnetic correlations in type-II multiferroic Co3TeO6.

Magnetic structure of the Co ions in monoclinic Co3TeO6 in the antiferroelectric state at 16 K has been determined by neutron powder together with single-crystal diffractions. The indices of the magnetic reflections that appear at the incommensurate positions were determined by diffractions from a single crystal, which allow to uniquely identify the magnetic modulation vector. There are two crystallographically distinct Co layers. Magnetic incommensurability appears in the Co spins in the layers comprising zig-zag chains, with a magnetic modulation vector of (0.357, 0.103, 0.121) at 3 K but changes to (0.4439, 0, 0.137) at 16 K, while the Co ions in the honeycomb webs form a collinear antiferromagnetic structure. Thermal reduction rate of the Co moments in the honeycomb webs was found to be much smaller than those in the zigzag chains. Shifting of large amounts of electronic charge into the Co─O bonds in the honeycomb webs on warming is used to understand the behavior.

Two-dimensional magnetism in α−CuV2O6

Several previous studies reported that a one-dimensional Heisenberg chain model is inadequate in describing the magnetic properties of the low-dimensional quantum antiferromagnet $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{CuV}}_{2}{\mathrm{O}}_{6}$, but the origin for this observation has remained unclear. We have reinvestigated the magnetic properties of $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{CuV}}_{2}{\mathrm{O}}_{6}$ and found that our anisotropic magnetic susceptibility, neutron-powder diffraction, and electron paramagnetic spin-resonance measurements are in good agreement with extensive density-functional theory ($\mathrm{DFT}+U$) total energy calculations which indicate that the correct spin lattice model for $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{CuV}}_{2}{\mathrm{O}}_{6}$ is rather a $S=1/2$ 2D-Heisenberg antiferromagnetic lattice. The magnetic susceptibility data are well described by a rectangular Heisenberg antiferromagnet with anisotropy ratio $\ensuremath{\alpha}\ensuremath{\sim}$ 0.7 consistent with the DFT results. Quantum Monte Carlo simulations of the magnetic susceptibilities for a rectangular lattice Heisenberg antiferromagnet were performed in the anisotropy range 0.5 $\ensuremath{\le}\ensuremath{\alpha}\ensuremath{\le}$ 1.0. The results of the Quantum Monte Carlo calculations were cast into a Pad\'e approximant which was used to fit the temperature-dependent magnetic susceptibility data. Neutron-powder-diffraction measurements were used to conclusively solve the collinear antiferromagnetic structure of $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{CuV}}_{2}{\mathrm{O}}_{6}$ below the N\'eel temperature of $\ensuremath{\sim}22.4$ K.

Crystal and magnetic structure of polar oxide HoCrWO6

Polar magnetic oxide HoCrWO6 is synthesized and its crystal structure, magnetic structure, and thermodynamic properties are investigated. HoCrWO6 forms the polar crystal structure (space group Pna21 (#33)) due to the cation ordering of W6+ and Cr3+. There is an antiferromagnetic transition at TN = 24.5 K along with the magnetic entropy change (~5 J.Kg.−1K−1 at 70 kOe). Neutron diffraction measurement indicates that both Cr and Ho sublattices are ordered with the moment of 2.32(5)μB and 8.7(4)μB at 2 K, respectively. While Cr forms A-type collinear antiferromagnetic (AFM) structure with magnetic moment along the b axis, Ho sublattice orders in a non-coplanar AFM arrangement. A comparison with isostructural DyFeWO6 and DyCrWO6 indicates that the magnetic structure of this family of compounds is controlled by the presence or absence of eg electrons in the transition metal sublattice.

Fe57 Mössbauer spectroscopy study of cycloidal spin arrangements and magnetic transitions in BiFe1−xCoxO3

We report results of a $^{57}\mathrm{Fe}$ M\"ossbauer study focused on Co-substituted $\mathrm{BiF}{\mathrm{e}}_{1\ensuremath{-}x}\mathrm{C}{\mathrm{o}}_{x}{\mathrm{O}}_{3}$ ($x=0.05$). The experimental spectra were analyzed assuming the space-modulated cycloidal magnetic structure with high values of the anharmonicity parameter $m=0.47\ensuremath{-}0.92$ (at 4.7 K). The $m$ parameter increases with increasing temperature to reach its maximum value ($m\ensuremath{\approx}1$) for $x=0.10$, 0.15 corresponding to the transition to a collinear antiferromagnetic structure of easy-plane type (${\ensuremath{\mu}}_{\mathrm{Fe}}\ensuremath{\perp}{c}_{h}$). Such an unusual trend of the anharmonicity parameter $m$(T) increasing with temperature was well reproduced by the molecular field model in which the low-spin to intermediate-spin transition of $\mathrm{C}{\mathrm{o}}^{3+}$ is assumed. At low cobalt content, $\mathrm{BiF}{\mathrm{e}}_{0.95}\mathrm{C}{\mathrm{o}}_{0.05}{\mathrm{O}}_{3}$, the cycloidal structure is retained but with a very high degree of anharmonicity, corresponding to the nearly collinear spin order.

Magnetic interplay of Mn and Yb sites in YbMn2Si2 – Crystal field splitting

The crystal field splitting of YbMn2Si2 has been investigated over the temperature range 5–65 K using inelastic neutron scattering at a wavelength of 2.345 Å (resolution 800 μeV; dynamic range ∼10 meV). The excitation spectra have been analysed using a crystal field model above and below TN2, the temperature at which the collinear antiferromagnetic structure AFil transforms to the low-temperature structure in which the magnetic cell is doubled along the c-axis (TN1 = 526(4) K > T > TN2 = 32(2) K). The calculated excitation spectra show good agreement with the observed spectra for the unique environment of Yb3+ ions in the collinear antiferromagnetic structure AFil above TN2 and for the inequivalent sites of Yb3+ ions below TN2. This agreement has been obtained with a model for the low-temperature region in which a molecular field with optimal components in the x, y and z directions of Bx = 13.5 T, By = 65 T, Bz = 21.3 T is included. The pronounced components in the x and y directions are discussed in relation to the significant contraction of ∼0.1% of the c lattice parameter below the TN2 magnetic transition.

Field-induced magnetic incommensurability in multiferroicNi3TeO6

Using single crystal neutron scattering we show that the magnetic structure Ni$_3$TeO$_6$ at fields above 8.6 T along the $c$ axis changes from a commensurate collinear antiferromagnetic structure with spins along c and ordering vector $Q_C$= (0 0 1.5), to a conical spiral with propagation vector $Q_{IC}$= (0 0 1.5$\pm\delta$),$\delta\sim$0.18, having a significant spin component in the ($a$,$b$) plane. We determine the phase diagram of this material in magnetic fields up to 10.5 T along $c$ and show the phase transition between the low field and conical spiral phases is of first order by observing a discontinuous jump of the ordering vector. $Q_{IC}$ is found to drift both as function of magnetic field and temperature. Preliminary inelastic neutron scattering reveals that the spin wave gap in zero field has minima exactly at $Q_{IC}$ and a gap of about 1.1 meV consisting with a cross-over around 8.6 T. Our findings excludes the possibility of the inverse Dzyaloshinskii-Moriya interaction as a cause for the giant magneto-electric coupling earlier observed in this material and advocates for the symmetric exchangestriction as the origin of this effect.

Effect of Mn substitution on the crystal and magnetic structure of Bi1−xCaxFeO3−x/2 multiferroics

The room- and low-temperature neutron diffraction measurements of the Bi0.9Ca0.1Fe0.6Mn0.4O3+δ compound have been carried out to disclose the influence of Mn substitution on the multiferroic properties of the low-doped Bi1−xCaxFeO3−x/2 perovskites combining ferroelectric and weak ferromagnetic behavior. It has been proven that the material under study retains a polar R3c structure specific to the parent Bi0.9Ca0.1FeO2.95. The Mn doping results in the elimination of oxygen vacancies giving rise to the increase in spontaneous electric polarization. The chemical modification stabilizes the collinear antiferromagnetic structure at room temperature. The reorientation of the antiferromagnetic vector from the c to a axis takes place with decreasing temperature. Reflecting the competitive character of the superexchange interactions between Fe3+, Mn3+ and Mn4+, the coexistence of ferromagnetic glassy and antiferromagnetic long-range-ordered phases is observed at low temperatures.