Ground-state Magnetic Structure Research Articles

151Eu Mössbauer study of magnetic ordering in flux-grown ferromagnetic and antiferromagnetic forms of EuCd2As2

EuCd2As2 is a remarkably complex magnetic semimetal that may behave as a topological insulator or host two pairs of Weyl points, depending on the growth conditions and the final magnetic state. Both antiferromagnetic (AFM) and ferromagnetic (FM) forms have been grown, and we show here, using 151Eu Mössbauer spectroscopy, that the differences between the AFM and FM forms extend well beyond their ground state magnetic structures. Whereas the AFM form undergoes a conventional AFM → paramagnetic transition on warming, the FM form passes through a complex incommensurate modulated state before becoming paramagnetic.

Study of ground state magnetic structure of L10-type MnAl via spin-spiral density wave

We investigated the ground state magnetic structure of L10-type MnAl with the density functional theory via the spin-spiral density wave state. Our results indicate that stoichiometric perfectly ordered MnAl prefers an antiferromagnetic state, while the ferromagnetic state is metastable. When a chemical disorder is introduced with the degree of order S = 0.9, the spiral period becomes longer than that of the antiferromagnetic structure, and below S = 0.8, the system adopts the ferromagnetic state. Furthermore, doping of Mn atoms into the Al sites above 5 at% led to the ferromagnetic state. We confirmed that chemical or compositional disorder induces ferromagnetic transitions in MnAl alloys.

Stacking and layer dependence of magnetic properties in Ti2C and Fe2C

Magnetic MXenes are becoming an important family of materials for exploring 2D magnetism. However, investigations into the interdependence of layer thickness, stacking patterns and magnetism in these materials, from a microscopic point of view, are still lacking. In this work, we have used density functional theory based calculations to understand the effects of layer thickness and stacking on the magnetic properties in two magnetic MXenes, Ti2C and Fe2C, in their monolayer and bilayer forms. The ground-state magnetic structures, magnetic moments, magnetic exchange interactions, magnetic transition temperatures and magnetic anisotropy energies are calculated and analysed using their electronic structures and standardised models. We find that in both systems an increase in layer thickness (monolayer to bilayer) affects the ground-state magnetic configuration, which is driven by the changes in the magnetic exchange interactions. While the effects of the stacking pattern are rather weak in Ti2C, they are substantial, both qualitatively and quantitatively, in Fe2C. The computed results are analysed from their electronic structures. The results suggest that fascinating physical effects can be obtained in Fe2C by tuning the layer thickness and stacking patterns, making it more suitable for device applications.

Neutron scattering sum rules, symmetric exchanges, and helicoidal magnetism in MnSb2O6

MnSb$_{2}$O$_{6}$ is based on the noncentrosymmetric $P321$ space group with magnetic Mn$^{2+}$ ($S={5/2}$, $L\approx 0$) spins ordering below $T_{\mathrm{N}}=12$ K in a helicoidal structure. The ground state magnetic structure, expected to be built and originate from 7 Heisenberg exchange constants, has been shown to be coupled to the underlying crystallographic chirality with polar domain switching being reported. We apply neutron spectroscopy to extract these symmetric exchange constants. Given the high complexity of the magnetic exchange network, crystallographic structure and complications fitting linear spin-wave models, we take advantage of multiplexed neutron instrumentation to use the first moment sum rule of neutron scattering to estimate the 7 exchange constants. We then use these parameters to calculate the low-energy spin-waves in the N\'eel state to reproduce the neutron response without strong antisymmetric coupling. Using Green's response functions, the stability of long-wavelength excitations in the context of proposed magnetic structures is then discussed. The results show the presence of strong exchange constants for the chiral exchange pathways and illustrate an underlying coupling between crystallographic and magnetic ``chirality" through predominantely symmetric exchange.

First-Principles Study of the Magnetic and Electronic Structure of NdB4

Due to their magnetic and physical properties, rare earth magnetic borides have been applied to a variety of critical technologies. In particular, rare earth tetraborides are more abundant as frustrated antiferromagnets. Here, the atomic structures, magnetic structures, and electronic structures of NdB4 have been studied by first-principle calculations. The ground state magnetic structure of NdB4 is determined. Moreover, the small energy difference between different magnetic structures means that there may be more than one magnetic structure that coexist. One can glean from the electronic structure of the magnetic ground state that the d orbital of Nd is strongly hybridized with the p orbital of B, and the f electron of Nd is highly localized. The computational results reveal the complexity of the magnetic structure and provide a theoretical basis for studying the magnetic ground state of NdB4.

Photoferroic prospective of multiferroic HoMnO3 compound, evaluated on the base of the DFT study of its magnetic, electronic, and optical properties

In this work, non-collinear spin density functional theory based all-electron full-potential linearized augmented plane wave electronic structure method have been employed to study magnetic, electronic, and optical properties of multiferroic HoMnO3 compound in order to evaluate its potential to be used in photovoltaic applications. Exchange and correlation effects of the Ho 4f- and Mn 3d-electrons were treated on the base of the local spin density approximation including effective Hubbard (Ueff) correction. We concluded that the ground state magnetic structure should be described by Γ3 irreducible representation of the P63cm space group, for both the Mn and the Ho magnetic sub-lattices. The calculated indirect and direct band gaps, corresponding to the transition from the O 2p-to the Mn 3d-states, are found to be 1.25 eV and 1.4 eV, respectively, in very good agreement with the experimental findings. Calculated dielectric tensor spectra are found to be in satisfactory agreement with the experiment as well. On the base of these results, we investigated photoferroic properties of the HoMnO3 by calculating its absorption coefficient spectrum and the effective mass tensor of the photogenerated electrons and holes. By comparing results with those of isostructural LuMnO3 and the YMnO3 multiferroics, we concluded that the HoMnO3 exhibits similar photoferroic properties, a fact that classifies it as a material suitable for photovoltaic applications.

Frustrated magnetic cycloidal structure and emergent Potts nematicity in CaMn2P2

We report neutron-diffraction results on single-crystal CaMn$_2$P$_2$ containing corrugated Mn honeycomb layers and determine its ground-state magnetic structure. The diffraction patterns consist of prominent (1/6, 1/6, $L$) reciprocal lattice unit (r.l.u.; $L$ = integer) magnetic Bragg reflections, whose temperature-dependent intensities are consistent with a first-order antiferromagnetic phase transition at the N\'eel temperature $T_{\rm N} = 70(1)$ K. Our analysis of the diffraction patterns reveals an in-plane $6\times6$ magnetic unit cell with ordered spins that in the principal-axis directions rotate by 60-degree steps between nearest neighbors on each sublattice that forms the honeycomb structure, consistent with the $P_Ac$ magnetic space group. We find that a few other magnetic subgroup symmetries ($P_A2/c$, $P_C2/m$, $P_S\bar{1}, P_C2, P_Cm, P_S1$) of the paramagnetic $P\bar{3}m11^\prime$ crystal symmetry are consistent with the observed diffraction pattern. We relate our findings to frustrated $J_1$-$J_2$-$J_3$ Heisenberg honeycomb antiferromagnets with single-ion anisotropy and the emergence of Potts nematicity

Green's function method for the two-dimensional frustrated spin- 12 Heisenberg magnetic lattice

The magnon Hedin's equations are derived via the Schwinger functional derivative technique, and the resulting self-consistent Green's function method is used to calculate ground state spin patterns and magnetic structure factors for 2-dimensional magnetic systems with frustrated spin-1/2 Heisenberg exchange coupling. Compared to random-phase approximation treatments, the inclusion of a self-energy correction improves the accuracy in the case of scalar product interactions, as shown by comparisons between our method and exact benchmarks in homogeneous and inhomogeneous finite systems. We also find that for cross-product interactions (e.g. antisymmetric exchange), the method does not perform equally well, and an inclusion of higher corrections is in order. Aside from indications for future work, our results clearly indicate that the Green's function method in the form proposed here already shows potential advantages in the description of systems with a large number of atoms as well as long-range interactions.

Unraveling the origin of the peculiar transition in the magnetically ordered phase of the Weyl semimetal Co3Sn2S2

Recent discovery of topologically non-trivial behavior in Co3Sn2S2 stimulated a notable interest in this itinerant ferromagnet (Tc = 174 K). The exact magnetic state remains ambiguous, with several reports indicating the existence of a second transition in the range 125 -- 130 K, with antiferromagnetic and glassy phases proposed to coexist with the ferromagnetic phase. Using detailed angle-dependent DC and AC magnetization measurements on large, high-quality single crystals we reveal a highly anisotropic behavior of both static and dynamic response of Co3Sn2S2. It is established that many observations related to sharp magnetization changes when B || c are influenced by the demagnetization factor of a sample. On the other hand, a genuine transition has been found at Tp = 128 K, with the magnetic response being strictly perpendicular to the c-axis and several orders of magnitude smaller than for B || c. Calculations using density-functional theory indicate that the ground state magnetic structure consist of magnetic moments canted away from the c-axis by a small angle (~ 1.5deg). We argue that the second transition originates from a small additional canting of moments within the kagome plane, with two equivalent orientations for each spin.

Electronic structure, magnetic order and Lifshitz transition in electron doped new structure 12442 type Fe-based superconductors

We investigate in detail the electronic structure of very recently discovered electron doped 12442-type iron based BaTh2Fe4As4(N1−xOx)2 compounds (x = 0.1–0.6), using Density Functional Theory (DFT) based first principles calculations. Unlike in many iron-based superconducting compounds, absence of As-4pZ orbital character, and dominant contributions from Fe-3d orbitals close to the Fermi level are noted. Formation energy calculations indicate x = 0.3 structure is the most stable which is consistent with experimental observation. Incorporating magnetic order at Fe sites in compound x = 0.3, our calculations predict stripe AFM as ground state magnetic structure. Calculated Arsenic height from the Fe square plane (hAs) for striped AFM configuration agrees well with the experiment. An important outcome of this calculation is the prediction of occurrence of Lifshitz transition at the same doping level (x = 0.6) for which the highest critical temperature is reported in literature. Detailed analysis of electronic structure results like estimation of radii of various Fermi pockets, orbital characters of Fermi surfaces lead to prediction of mixed d and s pairing symmetry.

Zigzag magnetic order in a novel tellurate compound Na4−δNiTeO6 with S = 1 chains

Na4−δNiTeO6 is a rare example in the transition-metal tellurate family of realizing an S = 1 spin-chain structure. By performing neutron powder diffraction measurements, the ground-state magnetic structure of Na4−δNiTeO6 is determined. These measurements reveal that below TN ∼ 6.8(2) K, the Ni2+ moments form a screwed ferromagnetic (FM) spin-chain structure running along the crystallographic a axis but these FM spin chains are coupled antiferromagnetically along the b and c directions, giving rise to a magnetic propagation vector of k = (0, 1/2, 1/2). This zigzag magnetic order is well supported by first-principles calculations. The moment size of Ni2+ spins is determined to be 2.1(1)μB at 3 K, suggesting a significant quenching of the orbital moment due to the crystalline electric field (CEF) effect. The previously reported metamagnetic transition near HC ∼ 0.1 T can be understood as a field-induced spin-flip transition. The relatively easy tunability of the dimensionality of its magnetism by external parameters makes Na4−δNiTeO6 a promising candidate for further exploring various types of novel spin-chain physics.

Order parameters for antiferromagnetic structures: A first-principles study of iridium manganese

We use density-functional theory to study the ordered states of iridium manganese, demonstrating a simple but powerful method for describing magnetic structures and the transitions among them. As an illustrative example, the coupling of magnetism and crystal structure in IrMn is examined through a rigorous exploration of strain space. We then generate order parameters for the ground-state magnetic structures, providing a comprehensive framework for understanding antiferromagnetic variants and their magnetic anisotropy. In particular, we show how the most direct path between two variant structures can be analytically determined; this technique is used to equate two seemingly contradictory prior studies.

Magnetic structure and exchange interactions in the Heisenberg pyrochlore antiferromagnet Gd2Pt2O7

The Heisenberg pyrochlore antiferromagnet ${\mathrm{Gd}}_{2}{\mathrm{Pt}}_{2}{\mathrm{O}}_{7}$ is one of a series of gadolinium pyrochlore compounds with a variety of B-site cations. Despite the expected simplicity of a spin-only ${\mathrm{Gd}}^{3+}$ Heisenberg interaction model, the gadolinium pyrochlore series exhibits various complex magnetic ground states at low temperature. ${\mathrm{Gd}}_{2}{\mathrm{Pt}}_{2}{\mathrm{O}}_{7}$ displays the highest temperature magnetic order of the series with ${T}_{N}=1.6$ K, which has been attributed to enhanced superexchange pathways facilitated by empty $5d\phantom{\rule{0.16em}{0ex}}{e}_{g}$ Pt orbitals. In this study, we use various neutron scattering techniques on an isotopically enriched polycrystalline $^{160}\mathrm{Gd}{}_{2}{\mathrm{Pt}}_{2}{\mathrm{O}}_{7}$ sample to examine the magnetic structure and spin-wave excitation spectrum below ${T}_{N}$ in order to extract the dominant exchange interactions. We find that the ground-state magnetic structure is the Palmer-Chalker state previously seen in ${\mathrm{Gd}}_{2}{\mathrm{Sn}}_{2}{\mathrm{O}}_{7}$ with an associated gapped excitation spectrum consistent with enhanced exchange interactions between further near-neighbor ${\mathrm{Gd}}^{3+}$ ions. We confirm this exchange model with analysis of the magnetic diffuse scattering in the paramagnetic regime using polarized neutrons.

Evolution from helical to collinear ferromagnetic order of theEu2+spins inRbEu(Fe1−xNix)4As4

The ground-state magnetic structures of the ${\mathrm{Eu}}^{2+}$ spins in recently discovered $\mathrm{RbEu}{({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Ni}}_{x})}_{4}{\mathrm{As}}_{4}$ superconductors have been investigated by neutron powder diffraction measurements. It is found that as the superconductivity gets suppressed with the increase in Ni doping, the magnetic propagation vector of the Eu sublattice diminishes, corresponding to the decrease in the rotation angle between the moments in neighboring Eu layers. The ferromagnetic Eu layers are helically modulated along the $\mathit{c}$ axis with an incommensurate magnetic propagation vector in both the ferromagnetic superconductor $\mathrm{RbEu}{({\mathrm{Fe}}_{0.95}{\mathrm{Ni}}_{0.05})}_{4}{\mathrm{As}}_{4}$ and the superconducting ferromagnet $\mathrm{RbEu}{({\mathrm{Fe}}_{0.93}{\mathrm{Ni}}_{0.07})}_{4}{\mathrm{As}}_{4}$. Such a helical structure transforms into a purely collinear ferromagnetic structure for nonsuperconducting $\mathrm{RbEu}{({\mathrm{Fe}}_{0.91}{\mathrm{Ni}}_{0.09})}_{4}{\mathrm{As}}_{4}$, with all the ${\mathrm{Eu}}^{2+}$ spins lying along the tetragonal (1 1 0) direction. The evolution from helical to collinear ferromagnetic order of the ${\mathrm{Eu}}^{2+}$ spins with increasing Ni doping is supported by first-principles calculations. The variation of the rotation angle between adjacent ${\mathrm{Eu}}^{2+}$ layers can be well explained by considering the change of magnetic exchange couplings mediated by the indirect Ruderman-Kittel-Kasuya-Yosida interaction.

Magnetic-field-induced ferroelectric states in centrosymmetric R2BaCuO5 ( R=Dy and Ho)

The linear magnetoelectric effect and multiferroicity phenomena occur independently due to breaking inversion symmetry below the magnetically ordered state of either transition metal or rare-earth ions. Here, we report the occurrence of a linear magnetoelectric effect and magnetic field-induced ferromagnetism and ferroelectricity below ${T}_{\mathrm{N}}^{R}$ in the orthorhombic green phases ${R}_{2}\mathrm{BaCu}{\mathrm{O}}_{5}$ ($R=\mathrm{Dy}$ and Ho). They undergo a long-range antiferromagnetic ordering of ${\mathrm{Cu}}^{2+}$ (${T}_{\mathrm{N}}^{\mathrm{Cu}}=18.5$ and 17.5 K) and ${R}^{3+}$ ions (${T}_{\mathrm{N}}^{\mathrm{Dy}}=10.7\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ and ${T}_{\mathrm{N}}^{\mathrm{Ho}}=8\phantom{\rule{0.16em}{0ex}}\mathrm{K}$). The neutron diffraction study reveals that these compounds undergo a first-order magnetic transition from the high-temperature centrosymmetric antiferromagnetic phase (${P}_{\mathrm{b}}{112}_{1}/n$) to the low-temperature noncentrosymmetric phases $Pn{m}^{\ensuremath{'}}a$ (Dy) and $P{112}_{1}^{\ensuremath{'}}/a$ (Ho), which allow linear magnetoelectric coupling. This is consistent with field-induced electric polarization, below ${T}_{\mathrm{N}}^{R}$, which varies linearly up to \ensuremath{\sim}1.2 T. Above a critical field $({H}_{\mathrm{c}}>1.2\phantom{\rule{0.16em}{0ex}}\mathrm{T})$, both compounds exhibit metamagnetic transitions with magnetization close to the saturation value ${M}_{s} \ensuremath{\sim}10.1\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}/\mathrm{f}.\mathrm{u}.$ (Dy) and $\ensuremath{\sim}11.8\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}/\mathrm{f}.\mathrm{u}.$ (Ho) at 7 T. Above the metamagnetic transition, a new polar state appears with large electric polarization, indicating field-induced ferroelectricity. We discuss the important role of $4f\text{\ensuremath{-}}3d$ coupling in determining the ground state magnetic structure responsible for the magnetoelectric coupling in both compounds.

Determination of the magnetic structures in orthoferrite CeFeO3 by neutron powder diffraction: first order spin reorientation and appearance of an ordered Ce-moment

High resolution and high intensity neutron powder diffraction are used to determine the temperature dependence of the crystallographic and magnetic structure of the orthoferrite CeFeO3. The high temperature G x -type magnetic coupling of the Fe-sublattice described by the Γ4 (G x A y F z ) irreducible representation changes at the spin reorientation temperature T SR = 228 K to a G y -type coupling of Γ1 (A x G y C z ). The spin reorientation is of first order and sees a hysteresis of about 2.5 K at T SR. Below 35 K faint magnetic peaks reflecting C z type magnetic coupling appear and are argued to be related to the Ce-sublattice. Magnetic moments at 2 K amount to μ Fe = 4.15 μ B and μ Ce = 0.11 μ B. CeFeO3 is only the second RFeO3 compound after DyFeO3 showing this ground state magnetic structure of the Fe-sublattice. The orthorhombic structure Pbnm is kept over the whole temperature range.

Magnetostructural coupling in ilmenite-type NiTiO3

We report the ground state magnetic structure and in-field magnetostrictive effects of $\mathrm{Ni}\mathrm{Ti}{\mathrm{O}}_{3}$ studied by means of zero-field and in-field single-crystal neutron diffraction, magnetization, and high-resolution dilatometry experiments. Zero-field neutron diffraction on $\mathrm{Ni}\mathrm{Ti}{\mathrm{O}}_{3}$ single crystals confirms an easy-plane antiferromagnet with a multidomain ground state. Upon application of external magnetic fields, neutron diffraction shows the evolution of domains with spins perpendicular to the applied field. The rotation of spins in the multidomain state exhibits pronounced lattice changes in the magnetostriction measurements. We see magnetization and magnetostriction measurements scale with each other in the multidomain state in accordance with phenomenological theories, revealing the strong coupling of spins to the lattice.

Фазовая диаграмма модели Поттса с числом состояний спина q=4 на гексагональной решетке

We have carried out Monte Carlo investigations of the phase transitions, thermodynamic properties and ground-state magnetic structures in two-dimensional 4-state Potts model on a hexagonal lattice with the competing exchange interactions. Researches are carried out for the value of interaction of next-nearest neighbor in the range of 0.0≤r≤1.0. Taking into account of antiferromagnetic interaction of next-nearest neighbor is shown to lead to the violation of the magnetic ordering. The phase diagram of dependence of critical temperature on the value of interaction of next-nearest neighbor is plotted. Of the orders of phase transitions has been carried out. It is established that the phase transition of the first order is observed in the ranges of 0.0≤r≤0.2 and 0.7≤r≤1.0. In the range of 0.3≤r≤0.6 frustration are observed.

Complex interplay of magnetic interactions in 5f -electron systems: The case of U2Ni2Sn

Recent experiments on a single crystal of ${\mathrm{U}}_{2}{\mathrm{Ni}}_{2}\mathrm{Sn}$ established a strong uniaxial magnetic anisotropy and an unusual magnetic structure that is antiferromagnetic both inside the U layers and between the layers. We apply fully relativistic first-principles calculations to investigate various magnetic configurations of the system. The calculations confirmed both the character of the magnetic anisotropy and the type of the ground-state magnetic structure. The study of the electron density of states shows that the lowest energy of the ground-state structure is related to the doubling of the magnetic unit cell and corresponding reduction of the Brillouin zone, leading to the formation of the energy gaps in the electronic structure. The mapping of the energies of the magnetic structures on the Heisenberg Hamiltonian of interacting atomic moments leads to a surprising result that only one of the interatomic exchange interactions is antiferromagnetic. We explain how this one antiferromagnetic interaction competing with several ferromagnetic interactions leads to the simultaneous presence of two types of antiferromagnetic behavior. We provide a simple mean-field estimate of the N\'eel temperature and conclude that short-range magnetic order, neglected on the mean-field level, may play an important role at temperatures around the critical temperature. We demonstrate the existence of self-consistent noncollinear magnetic states that limits the accuracy of treating ${\mathrm{U}}_{2}{\mathrm{Ni}}_{2}\mathrm{Sn}$ as a pure Ising magnet. We apply $\mathrm{GGA}+U$ method to examine the influence of the Hubbard parameter $U$ on the magnetic moments and energies of the magnetic configurations. An estimation of the influence of the contraction of the lattice on the magnetic properties is reported.

Role of magnetism in the stability of the high-entropy alloy CoCrFeMnNi and its derivatives

Multiprincipal element alloys, called high-entropy alloys represent a promising group of materials. They possess unique mechanical or electrical properties, which provide a wide range of potential applications. In general, mechanical or electrical properties of a material are influenced by the magnetic behavior. Therefore, in our work, we are exploring magnetic behavior of so-called ``Cantor alloy'' CoCrFeMnNi and its molybdenum based derivatives. Mo alloys were studied not only to carefully describe their properties but also to probe the magnetic behavior of the parent alloy by adding a nonmagnetic element. Based on ab initio calculations using the TB-LMTO-ASA (tight-binding linear-muffin-tin-orbital atomic-sphere approximation) method within CPA (coherent-potential approximation), we have found the ground-state magnetic structures of a particular alloy. We deal with various magnetic structures including complex structures beyond the simple FM or DLM phases. We show the influence of the presence of a particular element on the magnetic properties. It includes, e.g., magnitudes of magnetic moments or preferred magnetic phases. The calculations were extended by studying the magnetic exchange interaction employing the Liechtenstein formula. We clearly show the contribution of each element to the magnetism as a function of the composition or crystal structure. We provide a thorough description of magnetic behavior in the mentioned compounds.