Type Of Antiferromagnetic Order Research Articles

Understanding unconventional magnetic order in a candidate axion insulator by resonant elastic x-ray scattering

Magnetic topological insulators and semimetals are a class of crystalline solids whose properties are strongly influenced by the coupling between non-trivial electronic topology and magnetic spin configurations. Such materials can host exotic electromagnetic responses. Among these are topological insulators with certain types of antiferromagnetic order which are predicted to realize axion electrodynamics. Here we investigate the highly unusual helimagnetic phases recently reported in EuIn2As2, which has been identified as a candidate for an axion insulator. Using resonant elastic x-ray scattering we show that the two types of magnetic order observed in EuIn2As2 are spatially uniform phases with commensurate chiral magnetic structures, ruling out a possible phase-separation scenario, and we propose that entropy associated with low energy spin fluctuations plays a significant role in driving the phase transition between them. Our results establish that the magnetic order in EuIn2As2 satisfies the symmetry requirements for an axion insulator.

Stripe-yz magnetic order in the triangular-lattice antiferromagnet KCeS2

Yb- and Ce-based delafossites were recently identified as effective spin-1/2 antiferromagnets on the triangular lattice. Several Yb-based systems, such as NaYbO2, NaYbS2, and NaYbSe2, exhibit no long-range order down to the lowest measured temperatures and therefore serve as putative candidates for the realization of a quantum spin liquid. However, their isostructural Ce-based counterpart KCeS2 exhibits magnetic order below T N = 400 mK, which was so far identified only in thermodynamic measurements. Here we reveal the magnetic structure of this long-range ordered phase using magnetic neutron diffraction. We show that it represents the so-called ‘stripe-yz’ type of antiferromagnetic order with spins lying approximately in the triangular-lattice planes orthogonal to the nearest-neighbor Ce–Ce bonds. No structural lattice distortions are revealed below T N, indicating that the triangular lattice of Ce3+ ions remains geometrically perfect down to the lowest temperatures. We propose an effective Hamiltonian for KCeS2, based on a fit to the results of ab initio calculations, and demonstrate that its magnetic ground state matches the experimental spin structure.

Electronic and magnetic properties of VOCl/FeOCl antiferromagnetic heterobilayers

We study the electronic properties of the heterobilayer of vanadium and iron oxychlorides, VOCl and FeOCl, two layered air stable van der Waals insulating oxides with different types of antiferromagnetic order in bulk: VOCl monolayers are ferromagnetic (FM) whereas the FeOCl monolayers are antiferromagnetic (AF). We use density functional theory calculations, with Hubbard correction that is found to be needed to describe correctly the insulating nature of these compounds. We compute the magnetic anisotropy and propose a spin model Hamiltonian. Our calculations show that interlayer coupling is weak and hence the magnetic order of each monolayers is preserved in the heterobilayer. Thus, the heterobilayer combines antiferromagnetic and ferromagnetic orders. Interlayer exchange should lead both to exchange bias and to the emergence of hybrid collective modes that combine FM and AF magnons. The energy band of the heterobilayer show a type II band alignment, and feature spin-splitting of the states of the AF layer due to the breaking of the inversion symmetry.

Role of spin-glass behavior in the formation of exotic magnetic states in GdB6

Randomness and frustration are believed to be two crucial criteria for the formation of spin glass state. However, the spin freezing occurs in some well-ordered crystals below the related temperature Tf due to the instability of each spin state, which induces the variation of either magnetic moment value or exchange energy. Here we explore the new mechanism of the in-site originated disorder in antiferromagnets Gd0.73La0.27B6 and GdB6, which is caused by the random mutual shifts of Gd3+ spins from the centrally symmetrical positions in the regular cubic lattice. The universal scaling of ESR linewidth temperature dependencies to the power law ΔH(T) ~ ((T − TD)/TD)α with α = − 1.1 ± 0.05 in the paramagnetic phase of both compounds demonstrates the identity of the origin of magnetic randomness. In Gd0.73La0.27B6 the resulting random spin configurations freeze at Tf ≈ 10.5 K where the maximum of magnetization is observed. Below Tf the splitting of ZFC and FC magnetization curves takes place as well as the magnetic state depends on the antecedent sample history. In the case of GdB6 the coherent displacement of Gd ions compete with these random shifts forming an antiferromagnetic (AFM) phase at TN = 15.5 K, which prevails over the spin freezing at Tf ≈ 13 K, expected from the ESR data. The observation of the hysteresis of the ESR spectrum in the AFM phase suggests that its properties may be determined by the competition of two types of AFM orders, which results in formation of stable magnetic domains with nonequivalent positions of AFM Gd pairs at T < 10 K.

Antiferromagnetic structure of exchange-coupled La0.7Sr0.3FeO3 thin films studied using angle-dependent x-ray absorption spectroscopy

The magnetic structure of exchange-coupled antiferromagnetic (AF) layers in epitaxial $\mathrm{L}{\mathrm{a}}_{0.7}\mathrm{S}{\mathrm{r}}_{0.3}\mathrm{Mn}{\mathrm{O}}_{3}$ (LSMO)/$\mathrm{L}{\mathrm{a}}_{0.7}\mathrm{S}{\mathrm{r}}_{0.3}\mathrm{Fe}{\mathrm{O}}_{3}$ (LSFO) superlattices grown on (111)-oriented $\mathrm{SrTi}{\mathrm{O}}_{3}$ substrates was studied using angle-dependent x-ray absorption spectroscopy utilizing linearly polarized x rays. We demonstrate the development of the measurement protocols needed to determine the orientation of the LSFO antiferromagnetic spin axis and how it responds to an applied magnetic field due to exchange interactions with an adjacent ferromagnetic layer. A small energy difference exists between two types of AF order: the majority of the AF moments cant out-of-the-plane of the film along the 110 or 100 directions depending on the LSFO layer thickness. In response to an applied magnetic field, these canted moments are aligned with a single 110 or 100 direction that maintains a nearly perpendicular orientation relative to the LSMO sublayer magnetization. The remaining AF moments lie within the $(111)$ plane and these in-plane moments can be reoriented to an arbitrary in-plane direction to lie parallel to the LSMO sublayer magnetization. These results demonstrate that the magnetic order of AF thin films and heterostructures is far more complex than in bulk LSFO and can be tuned with orientation, thickness, and applied magnetic field.

Modeling of the structural and magnetic properties of Fe-Rh-(Z) (Z = Mn, Pt) alloys by first principles methods

The structural and magnetic properties of Mn- and Pt-doped Fe-Rh alloys are investigated by first principles calculations. The 16-atom supercell (Fe8Rh8-xZx) with different initial spin configurations including ferromagnetic and two types of antiferromagnetic orders are considered. It is shown that in a case of Fe8Rh8-xMnx, the ferromagnetic spin configuration in a cubic cell is energetically favorable compared to other magnetic configurations. The addition of Mn content stimulates the martensitic transformation from ferromagnetic cubic phase to antiferromagnetic tetragonal one. Moreover, an increase in Mn content leads to increase in the energy difference between cubic and tetragonal phases which is equivalent of increase in a temperature of structural transformation. Contrary, for Fe8Rh8-xPtx, an increase in Pt content results to appearance of stable tetragonal state with the checkerboard-like antiferromagnetic configuration.

Effect of the ionic radius of A-cations on the magnetic and magnetocaloric properties of charge-ordered manganite La0.5Ca0.5–x Sr x MnO3 (0 &lt; x &lt; 0.5)

The effect of substitution of Sr2+ ions for Ca2+ ions on the magnetic and magnetocaloric properties of a charge-ordered La0.5Ca0.5MnO3 manganite has been studied. The direct and inverse magnetocaloric effects corresponding to the paramagnetic–ferromagnetic and ferromagnetic–antiferromagnetic transitions have been detected. It has been shown that the substitution Ca2+ → Sr2+ leads to the suppression of the charge-ordered phase and to an increase in the Curie temperature TC. The evolution of the destruction of the charge-ordered phase is nonmonotonic, which is associated with the change in the type of antiferromagnetic ordering.

The Effect of Phase Decomposition on Magnetic Structure of Cu0.4Mn0.3Ni0.3Alloy

The purpose of present investigation was to study the e ects of phase decomposition of the Cu0.4Mn0.3Ni0.3 alloy on its magnetic ordering. The single-crystal sample was examined by elastic neutron scattering before and after ve subsequent annealing runs. The results indicate that in small fraction of the quenched sample volume there are two types of antiferromagnetic order: one of them AF1 of the long range, the other one of the short range. Ageing induces phase decomposition which yields disappearance of AF1 long range order and increase of the volume of new ordered phase. The results of the investigations of the aged sample indicate that regions of new tetragonal phase exhibit antiferromagnetic ordering with magnetic moments of Mn atoms arranged as in the pure compound MnNi but with unequal domain distribution. The intensity of magnetic component of super-structure re ections increases with the duration of ageing at lower rate than component due to atomic order.

Magnetism-dependent phonon anomaly in LaFeAsO observed via inelastic x-ray scattering

The phonon dispersion was measured at room temperature along ($0,0,L$) in the tetragonal phase of LaFeAsO using inelastic x-ray scattering. Spin-polarized first-principles calculations imposing various types of antiferromagnetic order are in better agreement with the experimental results than nonmagnetic calculations, although the measurements were made well above the magnetic ordering temperature, ${T}_{N}$. Splitting observed between two ${A}_{1g}$ phonon modes at 22 and 26 meV is only reproduced in spin-polarized calculations. Magnetostructural effects similar to those observed in the $A$Fe${}_{2}$As${}_{2}$ ($A=\text{Ca}$,Sr,Ba,Eu) materials are present in LaFeAsO. The inclusion of Fe spin is necessary to find reasonable agreement of the calculations with the measured spectrum well above ${T}_{N}$. On-site Fe and As force constants show significant softening compared to nonmagnetic calculations, however an investigation of the real-space force constants associates the magnetoelastic coupling with a complex renormalization instead of softening of a specific pairwise force.

Synthesis, structure, and magnetic properties of anion-substituted manganese chalcogenides

The anion-substituted solid solutions of the MnSe1 t xTex system have been synthesized. The crystal structure and magnetic properties of the synthesized solid solutions have been investigated. It has been shown that, in the concentration range 0 ≤ x ≤ 0.4, the solid solutions have a face-centered cubic structure. It has been revealed that an increase in the concentration of the substituting element in the MnSe1 − xTex system leads to an increase in the coefficient of thermal expansion of the sample. The investigation of the magnetic properties has been carried out at temperatures in the range 80 K < T < 1000 K in a magnetic field up to 8.6 kOe. It has been experimentally found that the type of antiferromagnetic order (the second type of ordering) remains unchanged over the entire concentration range up to x = 0.4 and that the paramagnetic Curie temperature and the Neel temperature decrease within the limits of 20%. Theoretical calculations have been performed using the Monte Carlo method, and the model of nanoclusters with an uncompensated antiferromagnetic moment has been proposed.

Quantum critical behavior driven by Hund’s rule coupling in quantum antiferromagnets

When localized spins on different $d$ orbitals prefer different types of antiferromagnetic ordering, the Hund's rule coupling creates frustration. Using spin-wave theory we study the case of two such orbitals on a square lattice coupled through Hund's rule such that the first one couples antiferromagnetically more strongly to its nearest neighbors, while the second couples more strongly to its next nearest neighbors. We find that the zero temperature phase diagram has four regions: one characterized by the familiar $(\ensuremath{\pi},\ensuremath{\pi})$ antiferromagnetic order, a second by the columnar $(\ensuremath{\pi},0)$ order, a third by a canted order, and a fourth region where a quantum-disordered state emerges. We comment on the possible relevance of these findings for the case of Fe-pnictide-based antiferromagnets.

Exotic antiferromagnetic transition in deformed pyrochlore lattice Ni2(OH)3Cl of atacamite-structure

Abstract As a part of an extensive study on the new geometric frustration series M2(OH)3X on deformed pyrochlore lattice, we have found an exotic antiferromagnetic order in atacamite-structure Ni2(OH)3Cl. Strong frustration exists in Ni2(OH)3Cl with antiferromagnetic transition below TN=4 K. While neutron diffraction witnessed unambiguously a long-range order, there is no signature of an order as detected by μSR. Instead, the detected local field is dynamically fluctuating at zero field. The present material may exemplify a novel type of antiferromagnetic order recently predicted for Heisenberg spins on pyrochlore lattice, wherein the squared Fourier amplitude orders but the amplitude itself fluctuates rapidly.

Quantum non-demolition detection of strongly correlated systems

Preparation, manipulation and detection of strongly correlated states of quantum many-body systems are among the most important goals and challenges of modern physics. Ultracold atoms offer an unprecedented playground for the realization of these goals. Here, we propose a method for detecting strongly correlated states of ultracold atoms in a quantum non-demolition scheme, that is, in the fundamentally least destructive way permitted by quantum mechanics. In our method, spatially resolved components of atomic spins couple to quantum polarization degrees of freedom of light. In this way, quantum correlations of matter are faithfully mapped on those of light; the latter can then be efficiently measured using homodyne detection. We illustrate the power of such spatially resolved quantum-noise-limited polarization measurement by applying this method to the detection of various standard and ‘exotic’ types of antiferromagnetic order in lattice systems, and by indicating the feasibility of detection of superfluid order in Fermi liquids.

Ising Spin Orders Conforming to Local Matching Rule and Long-Range Regularity of Interactions in Penrose Lattice

A simple arithmetic rule is proposed to correlate the matching rule in Penrose lattice with long-range Ising spin orders with the aim of clarifying the energetic mechanism of the orders under Ruderman–Kittel–Kasuya–Yoshida-like (RKKY) alternating interactions. Three different types of antiferromagnetic orders were found to be compatible with the rule. The orders showed sharp magnetic diffraction spots with the same peak widths as the ordinary lattice diffraction, which were consistent with the long-range orders enforced by the matching rule. The three types of the orders were most adequately interpreted as the orders in the occupation domain in the phason space. The RKKY interactions resulted in two of the orders in simulated annealing calculations. The results indicate the importance in the energetic mechanism of the consistency of the connectivity of the interaction signs inherent in the quasiperiodic structure.

Magnetic order of s = [formula omitted] spins on the FCC lattice at zero temperature calculated by exact diagonalization

The magnetic order of s = 1 2 spins with Heisenberg couplings on the FCC lattice at T = 0 has been probed by examining the ground state of finite clusters with periodic boundary conditions, for different ratios of the nearest and next-nearest-neighbour couplings, J 1 and J 2. Results for 32 spins show that the type of antiferromagnetic order is independent of the external field, in agreement with recent experimental results on nuclear magnetic order in silver. Results of calculations on 64 spins in a high field implies that the zero-field phase diagram in the ( J 1, J 2) plane is close to the classical predictions.

Doping effects on UPd2Al3

Abstract We present first results of a doping study on the U site on UPd2Al3. These results give further support for a tetravalent uranium configuration and an induced type of antiferromagnetic ordering. They confirm the importance of an unperturbed electronic configuration for both antiferromagnetic long-range ordering and heavy-fermion superconductivity. Implications for the interaction between both phenomena are discussed.

Antiferromagnetic nuclear ordering in copper: Comparison of theoretical spin structures against neutron-diffraction data

Neutron-diffraction measurements on nuclear spins in copper have shown two different types of antiferromagnetic ordering, characterized by the twelve cubic-symmetry-related →k=(π/a)(2/3,2/3,0) propagation vectors and the three →k=(π/a)(1,0,0) vectors. The particular propagation vectors that are stable in an external field depend on the alignment of →B with respect to the crystalline axes. In this paper, the observed selection rules are compared with theory and found to be in good agreement.

Magnetic Characteristics of Europium–Aluminum Intermetallics

Bulk magnetic characteristics of EuAl2 and EuAl4 are reported for temperatures ranging from 4°–300°K and for field strengths up to 20 kOe. To evaluate the influence of electron concentration on the magnetic interactions a series of EuxLa1−xAl2 alloys, with x ranging from 0.1 to 0.8, was studied over the same interval of temperature and field strength. Between 50° and 300°K all samples exhibited Curie–Weiss behavior with an effective moment characteristic of dipositive europium. EuAl4, EuAl2, and Eu0.8La0.2Al2 become antiferromagnetic below 20°K, two types of antiferromagnetic order being indicated for the latter two intermetallics. Increasing the electron concentration by replacing Eu with tripositive La alters the interaction from predominantly antiferromagnetic to predominantly ferromagnetic as the electron concentration approaches 9. The change in sign of the interaction occurs at about 8.4 electrons per magnetic atom. Eu0.1La0,9Al2 and Eu0,2La0.8Al2 are ferromagnetic at 4°K with Eu moments of about 5 μB per formula unit. Eigenvalues of the spin–Hamiltonian are obtained for Eu++ in a site of tetragonal symmetry and used to interpret the saturation moments observed for the ferromagnetic ternaries and the deviations from Curie–Weiss behavior for EuAl2 and the (Eu, La)Al2 alloys. The bulk magnetic properties can be quite successfully accounted for on the basis of an over-all (Zeeman-like) splitting of 480°K.

Magnetic properties of CeAs and CeSb deduced from neutron scattering data

Neutron inelastic scattering and powder diffraction techniques have been used to study the crystal field splitting and magnetic ordering properties of CeAs and CeSb. Powder diffraction data indicate a type 1 face-centred cubic magnetic ordering structure for CeAs with a moment of 068 μB±10% at 42 °K. This value demonstrates that the ground state of Ce3+ in CeAs is a doublet. Inelastic scattering measurements give directly a value of the crystal field splitting in this compound of 140±10 °K (12 meV). The powder diffraction data for CeSb could not be interpreted using any of the simple face-centred cubic types of antiferromagnetic ordering. The inelastic data show that the crystal field splitting is smaller than 25 °K (21 meV).