Collinear Antiferromagnetic Phase Research Articles

Evidence of exchange striction and charge disproportionation in the magnetoelectric material Ni3TeO6

The chiral magneto-electric compound Ni3TeO6 is investigated through temperature-dependent synchrotron-based powder x-ray diffraction and x-ray absorption spectroscopy between 15 to 300 K. Our work provides direct evidence for the exchange-striction in the material around the concomitant onset point of collinear antiferromagnetic and magneto-electric phases. The x-ray absorption near edge spectra and x-ray photoelectron spectra show that the sample consists of both Ni2+ and Ni3+ ions in the lattice. The ionic state of Ni is found to be quite robust, and it is largely independent of the preparation route. Additionally, the minority Te4+ state is found to coexist with the majority Te6+ state, which may arise from the charge disproportionation between Ni and Te ions (Ni2+ + Te6+ --> Ni3+ + Te4+). The observed mixed valency of Ni is also confirmed by the total paramagnetic moment per Ni atom in the system. This mixed valency in the metal ions and the exchange-striction may be attributed to the observed magneto-electric effect in the system.

Diagonal and off-diagonal thermal conduction with resonant phonon scattering in Ni3TeO6

The coupling between phonon and magnon is ubiquitous in magnetic materials and plays a crucial role in many aspects of magnetic properties, most notably in spintronics. Yet, this academically and technologically interesting problem still poses a severe challenge to a general understanding of the issue in certain materials. We report that Ni3TeO6 exhibits clear evidence of significant magnon-phonon coupling in both longitudinal thermal conductivity ($\kappa_{xx}$) and thermal Hall coefficient ($\kappa_{xy}$). The Debye-Callaway model, a phenomenological description for phonon heat conduction, can explain the measured magnetic field dependence of $\kappa_{xx}(H)$: phonon scattering from spin fluctuation in the paramagnetic phase and additional scattering due to magnon-phonon coupling in the collinear antiferromagnetic phase. We further suggest that a similar approach could be applied to understand the finite $\kappa_{xy}$ values in $Ni_3TeO_6$.

Magnetic domains in two distinct antiferromagnetic phases of CuO

Multiferroic CuO showing two distinct antiferromagnetic (AFM) phases: incommensurate (ICM) spiral and commensurate (CM) collinear AFM phases, is investigated by resonant x-ray diffraction (RXD) at the Cu ${L}_{3}$ edge. In the ICM phase where spin-spiral-induced ferroelectricity develops, circular dichroism ascribed to the handedness of the spiral magnetic order is observed at the magnetic Bragg (0.506, 0, \ensuremath{-}0.483) reflection. By measuring two-dimensional maps of the reflection intensity, the spatial distribution of the ferroelectric domains is clarified and is found to be strongly influenced by crystallographic twin structures. In the CM phase, weaker but substantial circular dichroic signals are observed at the magnetic Bragg (0.5, 0, \ensuremath{-}0.5) reflection. It is found that the spatial distributions of the circular dichroic signals resemble those in the ICM phase, suggesting some relation between the ICM and CM phases. The origin of the circular dichroic RXD observed in CuO is discussed based on the experimental observations.

Frustrated spin-[formula omitted] Heisenberg magnet on an [formula omitted]-stacked honeycomb bilayer: High-order study of the collinear magnetic phases of the [formula omitted]–[formula omitted]–[formula omitted] model

The zero-temperature phase diagram of the frustrated spin-12J1–J2–J1⊥ Heisenberg magnet on an AA-stacked honeycomb bilayer lattice is studied using the coupled cluster method implemented to very high orders. On each monolayer the spins interact via nearest-neighbor (NN) and frustrating next-nearest-neighbor isotropic antiferromagnetic Heisenberg interactions with respective strength parameters J1>0 and J2≡κJ1>0. The two layers are coupled such that NN interlayer pairs of spins also interact via a similar isotropic Heisenberg interaction of strength J1⊥≡δJ1, which may be of either sign. In particular, we locate with high accuracy the complete phase boundaries in the κ–δ half-plane with κ>0 of the two quasiclassical collinear antiferromagnetic phases with Néel or Néel-II magnetic order in each monolayer, and the interlayer NN pairs of spins either aligned (for δ<0) or anti-aligned (for δ>0) to one another. Compared to the two-sublattice Néel order, in which all NN intralayer pairs of spins are antiparallel to one another, the four-sublattice Néel-II order is characterized by NN intralayer pairs of spins on the honeycomb lattice being antiparallel to one another along zigzag (or sawtooth) chains in a specified direction from among the three equivalent honeycomb-lattice directions, and parallel to one another for the corresponding interchain pairs.

Gapless quantum spin liquid and global phase diagram of the spin-1/2 [formula omitted] square antiferromagnetic Heisenberg model

The nature of the zero-temperature phase diagram of the spin-1/2J1-J2 Heisenberg model on a square lattice has been debated in the past three decades, and it remains one of the fundamental problems unsettled in the study of quantum many-body theory. By using the state-of-the-art tensor network method, specifically, the finite projected entangled pair state (PEPS) algorithm, to simulate the global phase diagram of the J1-J2 Heisenberg model up to 24×24 sites, we provide very solid evidences to show that the nature of the intermediate nonmagnetic phase is a gapless quantum spin liquid (QSL), whose spin-spin and dimer-dimer correlations both decay with a power law behavior. There also exists a valence-bond solid (VBS) phase in a very narrow region 0.56≲J2/J1≤0.61 before the system enters the well known collinear antiferromagnetic phase. We stress that we make the first detailed comparison between the results of PEPS and the well-established density matrix renormalization group (DMRG) method through one-to-one direct benchmark for small system sizes, and thus give rise to a very solid PEPS calculation beyond DMRG. Our numerical evidences explicitly demonstrate the huge power of PEPS for highly frustrated spin systems. Finally, an effective field theory is also proposed to understand the physical nature of the discovered gapless QSL and its relation to deconfined quantum critical point (DQCP).

Structural and magnetic phase transitions in Ca-substituted bismuth ferromanganites

Single-phase samples of the Bi0.85Ca0.15Fe1−xMnxO3 (0.1 ≤ x ≤ 0.5) perovskites have been prepared by a solid state reaction method and investigated using X-ray and neutron diffraction, scanning electron microscopy, piezoresponse force microscopy, magnetization, and ac susceptibility measurements. It has been found that the increase in Mn concentration induces a modification of crystal and magnetic structures giving rise to the canted antiferromagnetic polar phase → collinear antiferromagnetic polar phase → paramagnetic nonpolar phase transitions at room temperature. The appearance of spontaneous magnetization suggesting a transformation of the room-temperature antiferromagnetic structure characteristic of the ferroelectric Bi0.85Ca0.15Fe1−xMnxO3 compounds near the polar / nonpolar (R3c / Pnma) phase boundary is observed with decreasing temperature. The transformation of the magnetic arrangement can also be driven by a magnetic field application. Such metamagnetic transition can be accompanied by a large (up to ~4 emu/g for the sample with x = 0.4, when measured at T = 2 K) magnetization jump far exceeding that specific to a field-driven cycloidal order → canted order transformation in the pure BiFeO3. Nonpolar samples (x ≥ 0.45) acquire a large (a few emu/g) spontaneous magnetization at low temperature, a value that cannot be explained by a spin canting. It is shown that the low-temperature magnetic behavior of the polar and nonpolar Mn-enriched Bi0.85Ca0.15Fe1−xMnxO3 compounds can be successfully described in terms of an antiferromagnetic phase + glassy magnetic phase coexistence.

Terahertz spectroscopy of antiferromagnetic resonances in YFe1−xMnxO3 ≤x≤0.4 across a spin reorientation transition

We have conducted a terahertz spectroscopic study of antiferromagnetic resonances in bulk orthoferrite YFe1−xMnxO3 0≤x≤0.4. Both the quasi-ferromagnetic resonance mode and the quasi-antiferromagnetic resonance mode in the weak ferromagnetic Γ4 phase disappear near the spin reorientation temperature, TSR, for the onset of the collinear antiferromagnetic Γ1 phase (x ≥ 0.1). Below TSR, an antiferromagnetic resonance mode emerges and exhibits a large blueshift with decreasing temperature. However, below 50 K, this mode softens considerably, and this tendency becomes stronger with Mn doping. We provide a deeper understanding of such behaviors of the antiferromagnetic resonance modes in terms of the influence of the Mn3+ ions on the magnetocrystalline anisotropy. Our results show that terahertz time-domain spectroscopy is a useful, complementary tool in tracking magnetic transitions and probing the interaction between disparate magnetic subsystems in antiferromagnetic materials with multiple ionic species.

Coexistence of Magnetoelectric and Antiferroelectric-like Orders in Mn3Ta2O8

In materials showing a linear magnetoelectric (ME) effect, unconventional functionalities can be anticipated such as electric control of magnetism and nonreciprocal optical responses. Thus, the search for new linear ME materials is of interest in materials science. Here, using a recently proposed design principle of linear ME materials, which is based on the combination of local structural asymmetry and collinear antiferromagnetism, we demonstrate that an anion-deficient fluorite derivative, Mn3Ta2O8, is a new linear ME material. This is evidenced by the onset of magnetic-field-induced electric polarization in its collinear antiferromagnetic phase below TN = 24 K. Furthermore, we also find an antiferroelectric-like phase transition at TS = 55 K, which is attributable to an off-center displacement of magnetic Mn2+ ions. The present study shows that Mn3Ta2O8 is a rare material that exhibits both ME and antiferroelectric-like transitions. Thus, Mn3Ta2O8 may provide an opportunity to investigate the physics associated with complicated interactions between magnetic (spin) and electric dipole degrees of freedom.

Spin waves in the collinear antiferromagnetic phase of Mn5Si3

By combining two independent approaches, inelastic neutron-scattering measurements and density-functional-theory calculations, we study the spin waves in the collinear antiferromagnetic phase (AFM2) of ${\mathrm{Mn}}_{5}{\mathrm{Si}}_{3}$. We obtain its magnetic ground-state properties and electronic structure. This study allowed us to determine the dominant magnetic exchange interactions and magnetocrystalline anisotropy in the AFM2 phase of ${\mathrm{Mn}}_{5}{\mathrm{Si}}_{3}$. Moreover, the evolution of the spin excitation spectrum is investigated under the influence of an external magnetic field perpendicular to the anisotropy easy axis. The low-energy magnon modes show a different magnetic field dependence, which is a direct consequence of their different precessional nature. Finally, possible effects related to the Dzyaloshinskii-Moriya interaction are also considered.

Ultrafast spin-nematic and ferroelectric phase transitions induced by femtosecond light pulses

Optically induced phase transitions of the manganite ${\mathrm{Pr}}_{1/3}{\mathrm{Ca}}_{2/3}\mathrm{Mn}{\mathrm{O}}_{3}$ have been simulated by using a model Hamiltonian that captures the dynamics of strongly correlated charge, orbital, lattice, and spin degrees of freedom. Its parameters have been extracted from first-principles calculations. Beyond a critical intensity of a femtosecond light pulse, the material undergoes an ultrafast and nonthermal magnetic phase transition from a noncollinear to collinear antiferromagnetic phase. The light-pulse excites selectively either a spin-nematic or a ferroelectric phase, depending on the light polarization. The behavior can be traced to an optically induced ferromagnetic coupling between Mn trimers, i.e., polarons which are delocalized over three Mn sites. The polarization guides the polymerization of the polaronic crystal into distinct patterns of ferromagnetic chains determining the target phase.

Multiple phase transitions and high-field quadrupolar order in a model for β−TeVO4

Motivated by the complex behavior of the frustrated magnet $\beta$-TeVO$_4$, we study an anisotropic Heisenberg model for coupled spin-$1/2$ zigzag chains. Using cluster mean field approach to capture quantum correlations we find, upon reducing temperature in the absence of applied field, (i) a partially ordered state, (ii) a collinear antiferromagnetic phase, and (iii) an elliptical spiral state characterized by finite vector chirality. For finite fields, we find metamagnetic response close to saturation magnetization. We show via explicit calculations that the quadrupolar order parameter is finite in the metamagnetic regime. The exchange parameters reported in the ab-initio study of $\beta$-TeVO$_4$ are used in our study. We compare our results with those reported in recent experiments on $\beta$-TeVO$_4$ and highlight similarities as well as differences between experimental results and our cluster mean field calculations.

Real-space Berry curvature of itinerant electron systems with spin-orbit interaction

By considering an extended double-exchange model with spin-orbit coupling (SOC), we derive a general form of the Berry phase $\gamma$ that electrons pick up when moving around a closed loop. This form generalizes the well-known result valid for SU(2) invariant systems, $\gamma=\Omega/2$, where $\Omega$ is the solid angle subtended by the local magnetic moments enclosed by the loop. The general form of $\gamma$ demonstrates that collinear and coplanar magnetic textures can also induce a Berry phase different from 0 or $\pi$, smoothly connecting the result for SU(2) invariant systems with the well-known result of Karplus and Luttinger for collinear ferromagnets with finite SOC. By taking the continuum limit of the theory, we also derive the corresponding generalized form of the real space Berry curvature. The new expression is a generalization of the scalar spin chirality, which is presented in an explicitly covariant form. We finally show how these simple concepts can be used to understand the origin of the spontaneous topological Hall effect that has been recently reported in collinear and coplanar antiferromagnetic phases of correlated materials.

The Ir4+ substitution dependence of electric polarization as a probe of magnetic phase stability in multiferroic MnWO4

Hubnerite MnWO4 is a highly frustrated magnetic compound that has been known for its multiferroic properties. The intrinsic connection of ferroelectric polarization and magnetically frustrated structure allows an opportunity to probe the stability of magnetic structures against perturbations by means of measuring ferroelectric polarization. In this work, we investigate the ferroelectric polarization of Mn1 − 2xIrxWO4 to probe the stability of the low-temperature (T) collinear antiferromagnetic (AF1) phase against the Ir substitution, considering the strong spin-orbital coupling of Ir4+ that would enhance the single-ion anisotropy, on the one hand, and would favor the noncollinear spin alignment, on the other hand. Different from Mn1 − xRux/2WO4, it is suggested that the AF1 phase is only partially suppressed by the Ir substitution, allowing the emergence of the noncollinear antiferromagnetic (AF2) phase in coexistence with the collinear AF1 phase. Proper Ir substitution may promote both the magnetocrystalline anisotropy and the Dzyaloshinskii-Moriya interaction, thus making the modulation of the magnetic structure more complicated.

Observation of inverted hysteresis loop and thermomagnetic irreversibility in the antiferromagnetic Mn5Si3 alloy

${\mathrm{Mn}}_{5}{\mathrm{Si}}_{3}$ crystallizes in hexagonal ${\mathrm{D}8}_{8}$ type structure (space group $P{6}_{3}/mcm$) with two distinct crystallographic sites for Mn atoms. It undergoes two first-order phase transitions during cooling from paramagnetic to collinear antiferromagnetic phase around 100 K and subsequently to noncollinear antiferromagnetic phase around 66 K. Detailed investigation by dc magnetic measurements probe the presence of inverted hysteresis loop below 66 K. In addition, thermomagnetic and field-induced arrest are the two key features of this alloy. Such arrest of high temperature phase plays the pivotal role toward the observation of inverted hysteresis loop behavior. The robustness of the arrested states was checked by magnetic relaxation measurements.

Magnetic States and Metal—Insulator in Strongly Correlated Systems (Scientific Summary)

Various slave particle approaches to the consideration of electron correlations in many-electron models, in particular, in application to layered copper oxide systems, have been analyzed. The magnetic phase diagrams of the ground state have been considered within the t—t’ Hubbard and s—d exchange models for the square and cubic lattices depending on the degree of band filling and the interaction parameter with allowance for collinear ferromagnetic, antiferromagnetic, and incommensurate (helical) magnetic phases, as well as magnetic phase separation. The generalized Hartree—Fock and slave boson approaches including correlations have been used. The latter approach makes it possible to correctly include the contribution of doubly occupied sites to the energy and to adequately describe the paramagnetic phase. A criterion of a metal—insulator transition has been obtained by numerical calculations and the analytical expansion in the transfer integral between next-nearest neighbors t’ and in the direct antiferromagnetic gap Δ. The order of the transition in the square lattice changes from the second to the first with increasing t’ at a small value of t’ ∼ 0.05t, which is due to the presence of a van Hove singularity near the band center. For the simple and body-centered cubic lattices, the transition from the antiferromagnetic insulator state occurs to an antiferromagnetic metal phase, is a second-order transition, and is followed by a transition to a paramagnetic metal. The inclusion of the intersite Heisenberg interaction changes these results and the transition can become a first-order transition.

Magnetic phase diagram, magnetotransport and inverse magnetocaloric effect in the noncollinear antiferromagnet Mn5Si3

The antiferromagnet Mn$_5$Si$_3$ has recently attracted attention because a noncollinear spin arrangement has been shown to produce a topological anomalous Hall effect and an inverse magnetocaloric effect. Here we synthesize single crystals of Mn$_5$Si$_3$ using flux growth. We determine the phase diagram through magnetization and measure the magnetoresistance and the Hall effect. We find the collinear and noncollinear antiferromagnetic phases at low temperatures and, in addition, a third magnetic phase, in between the two antiferromagnetic phases which has ferromagnetic character. The latter magnetic phase might be caused by strain produced by Cu inclusions that lead to quenched fluctuations of the mixed character magnetic ordering in this compound.

Collinear antiferromagnetic phases of a frustrated spin-12 J1–J2–J1⊥Heisenberg model on an AA-stacked bilayer honeycomb lattice

The zero-temperature quantum phase diagram of the spin-$\frac{1}{2}$ $J_{1}$--$J_{2}$--$J_{1}^{\perp}$ model on an $AA$-stacked bilayer honeycomb lattice is investigated using the coupled cluster method (CCM). The model comprises two monolayers in each of which the spins, residing on honeycomb-lattice sites, interact via both nearest-neighbor (NN) and frustrating next-nearest-neighbor isotropic antiferromagnetic (AFM) Heisenberg exchange iteractions, with respective strengths $J_{1} > 0$ and $J_{2} \equiv \kappa J_{1}>0$. The two layers are coupled via a comparable Heisenberg exchange interaction between NN interlayer pairs, with a strength $J_{1}^{\perp} \equiv \delta J_{1}$. The complete phase boundaries of two quasiclassical collinear AFM phases, namely the N\'{e}el and N\'{e}el-II phases, are calculated in the $\kappa \delta$ half-plane with $\kappa > 0$. Whereas on each monolayer in the N\'{e}el state all NN pairs of spins are antiparallel, in the N\'{e}el-II state NN pairs of spins on zigzag chains along one of the three equivalent honeycomb-lattice directions are antiparallel, while NN interchain spins are parallel. We calculate directly in the thermodynamic (infinite-lattice) limit both the magnetic order parameter $M$ and the excitation energy $\Delta$ from the $s^{z}_{T}=0$ ground state to the lowest-lying $|s^{z}_{T}|=1$ excited state (where $s^{z}_{T}$ is the total $z$ component of spin for the system as a whole, and where the collinear ordering lies along the $z$ direction) for both quasiclassical states used (separately) as the CCM model state, on top of which the multispin quantum correlations are then calculated to high orders ($n \leq 10$) in a systematic series of approximations involving $n$-spin clusters. The sole approximation made is then to extrapolate the sequences of $n$th-order results for $M$ and $\Delta$ to the exact limit, $n \to \infty$.

Magnetic phase diagram of the strongly frustrated quantum spin chain system PbCuSO4(OH)2 in tilted magnetic fields

We report the $\mathbf{H}-T$ phase diagram of $S=1/2$ strongly frustrated anisotropic spin chain material linarite PbCuSO$_4$(OH)$_2$ in tilted magnetic fields up to 10 T and temperatures down to 0.2 K. By means of torque magnetometry we investigate the phase diagram evolution as the magnetic field undergoes rotation in $\mathbf{ba}^{\ast}$ and $\mathbf{bc}$ planes. The key finding is the robustness of the high field spin density wave-like phase, which may persist even as the external field goes orthogonal to the chain direction $\mathbf{b}$. In contrast, the intermediate collinear antiferromagnetic phase collapses at moderate deflection angles with respect to $\mathbf{b}$ axis.

Magnetic phase coexistence in DyNiAl4

The magnetic structure and properties of the rare earth intermetallic DyNiAl4 have been determined. Two magnetic phase transitions have been observed at TN = 20.2(1) K and TN’ = 14.6(1) K. Analysis via neutron diffraction has revealed that these correspond to the formation of two distinct magnetic phases, a low temperature collinear antiferromagnetic phase with kC = (0, 1, 0) and a higher temperature incommensurate phase with kI = (0.1745(6), 1, 0.0313(6)). The incommensurate phase consists of a sinusoidal modulation of the magnetic moment along the a- and c-axis directions. In addition, both of these phases have been found to coexist between 14.5 K and 16.1 K.

A comprehensive study of the magnetic properties of the pyroxenes series CaMgSi2O6–Co2Si2O6 as a function of Co content

We report a detailed study on the magnetic properties of the pyroxene series M2M1Si2O6, with M2 = Ca and M1 = Mg, where magnesium and then calcium are progressively substituted by cobalt. For cobalt site occupancy larger than 0.7 at the M1 site, a collinear antiferromagnetic phase is detected for T < TN1 = 12 K with a monodimensional character (i.e. M1 site intra-chain order parallel to c axis). Moreover the magnetization easy axis has been estimated to lie roughly along the [1 0 1] direction. Cobalt content ⩾0.5 at the M2 site (overall content 1.5) determines the formation of a new independent antiferromagnetic order with higher Néel temperature, involving only the M2 site intra-chain interactions. The incoming M2 site order is accompanied by a lowering of the space symmetry which yields to a weakly ferromagnetic resultant due to spin canted distribution of the magnetic moments either along the M1 or M2 chains. Furthermore, metamagnetic transitions are observed for both M1 and M2 site intra-chain orders at relatively low critical magnetic fields, around 2 T, suggesting that this series of pyroxenes can be used as a model system for investigating the fundamental aspects of magnetism in the matter.