Magnetization Plateau State Research Articles

Magnetocaloric effect and magnetic phase diagram of EuRhAl4Si2

We report magnetocaloric effect (MCE) landscape and rich magnetic phase diagram of the quaternary compound EuRhAl4Si2 for magnetic field applied along the long axis (H || [001]) of the tetragonal unit cell. The MCE turns out to be a non-monotonic function of magnetic fields and temperatures containing regions of normal as well as inverse MCE. Our high-resolution magnetization data reveal the appearance of first order minor magnetization jumps with emergent phase separation below 5.5 K, within the magnetization plateau state. The magnetic phase diagram of EuRhAl4Si2 provides a fertile ground for statistical models.

Emergent many-body composite excitations of interacting spin-1/2 trimers

Understanding exotic forms of magnetism in quantum spin systems is an emergent topic of modern condensed matter physics. Quantum dynamics can be described by particle-like carriers of information, known-as quasiparticles that appear from the collective behaviour of the underlying system. Spinon excitations, governing the excitations of quantum spin-systems, have been accurately calculated and precisely verified experimentally for the antiferromagnetic chain model. However, identification and characterization of novel quasiparticles emerging from the topological excitations of the spin system having periodic exchange interactions are yet to be obtained. Here, we report the identification of emergent composite excitations of the novel quasiparticles doublons and quartons in spin-1/2 trimer-chain antiferromagnet Na2Cu3Ge4O12 (having periodic intrachain exchange interactions J1-J1-J2) and its topologically protected quantum 1/3 magnetization-plateau state. The characteristic energies, dispersion relations, and dynamical structure factor of neutron scattering as well as macroscopic quantum 1/3 magnetization-plateau state are in good agreement with the state-of-the-art dynamical density matrix renormalization group calculations.

Defect-driven ferrimagnetism and hidden magnetization in MnBi2Te4

MnBi$_2$Te$_4$ (MBT) materials are promising antiferromagnetic topological insulators where field driven ferromagnetism is predicted to cause a transition between axion insulator and Weyl semimetallic states. However, the presence of antiferromagnetic coupling between Mn/Bi antisite defects and the main Mn layer can reduce the low-field magnetization, and it has been shown that such defects are more prevalent in the structurally identical trivial magnetic insulator MnSb$_2$Te$_4$ (MST). We use high-field magnetization measurements to show that the magnetization of MBT and MST occur in stages and full saturation requires fields of~$\sim$~60 Tesla. As a consequence, the low-field magnetization plateau state in MBT, where many determinations of quantum anomalous Hall state are studied, actually consists of ferrimagnetic septuple blocks containing both a uniform and staggered magnetization component.

Symmetry-protected topological order in magnetization plateau states of quantum spin chains

A symmetry-protected topologically ordered phase is a short-range entangled state, for which some imposed symmetry prohibits the adiabatic deformation into a trivial state which lacks entanglement. In this paper we argue that magnetization plateau states of one-dimensional antiferromagnets which satisfy the conditions $S\ensuremath{-}m\ensuremath{\in}$ odd integer, where $S$ is the spin quantum number and $m$ the magnetization per site, can be identified as symmetry-protected topological states if an inversion symmetry about the link center is present. This assertion is reached by mapping the antiferromagnet into a nonlinear sigma model type effective field theory containing a novel Berry phase term (a total derivative term) with a coefficient proportional to the quantity $S\ensuremath{-}m$, and then analyzing the topological structure of the ground state wave functional which is inherited from the latter term. A boson-vortex duality transformation is employed to examine the topological stability of the ground state in the absence/presence of a perturbation violating link-center inversion symmetry. Our prediction based on field theories is verified by means of a numerical study of the entanglement spectra of actual spin chains, which we find to exhibit twofold degeneracies when the aforementioned condition is met. We complete this study with a rigorous analysis using matrix product states.

Quasi-Spin Correlations in a Frustrated Quantum Spin Ladder

The quasi-spin correlations in a frustrated quantum spin ladder with one-half magnetization are theoretically studied by using the density-matrix renormalization-group method and the quasi- spin transformation. In this model, the frustration induces a gapless-to-gapful phase transition with a strong rung coupling. The gapful state is observed as the one-half magnetization plateau in the magnetization curve. In the magnetization-plateau state, we find that the quasi-spin dimer has a large expectation value with long-ranged correlations. This result does not only comes in useful to clarify the magnetization-plateau state, but gives a crucial information to understand the magnetization curve of the real compound BiCu2PO6, whose effective spin model corresponds to ours.

Almost Perfect Frustration in the Dimer Magnet Ba2CoSi2O6Cl2

We determined the crystal structure of Ba$_2$CoSi$_2$O$_6$Cl$_2$, which was synthesized in this work, and investigated its quantum magnetic properties using single crystals. This compound should be described as a two-dimensionally coupled spin-1/2 XY-like spin dimer system. Ba$_2$CoSi$_2$O$_6$Cl$_2$ exhibits a stepwise magnetization process with a plateau at half of the saturation magnetization, irrespective of the field direction, although all the Co$^{2+}$ sites are equivalent. This indicates that spin triplets are localized owing to the almost perfect frustration of interdimer exchange interactions. Thus, the spin states for the zero and 1/2 magnetization-plateau states are almost exactly given by the simple product of singlet dimers and the alternate product of singlet and triplet dimers, respectively.

Topological nature of magnetization plateaus in periodically modulated quantum spin chains

We unveil nontrivial topological properties of zero-temperature magnetization plateau states in periodically modulated quantum spin chains under a uniform magnetic field. As positions of plateaus are uniquely determined by the modulation period of exchange couplings, we find that the topologically nontrivial plateau state can be characterized by a nonzero integer Chern number and has nontrivial edge excitations. Our study clarifies the topological origin of the well-known phenomena of quantized magnetization plateaus in one-dimensional quantum spin systems and relates the plateau state to the correlated topological insulator.

Spin-singlet recombination responsible for the magnetization plateau in a tetrameric ladder-like system

The magnetic properties and quantum phase transitions (QPTs) of an anisotropic tetrameric ladder-like model compound Cu2CdB2O6, which is constructed by dimers embedded into two-leg ladder with weak coupling inside the leg, are investigated using Green's function theory. It is shown that for weak dimer-chain (J2) coupling, the ground state lies in a gapless phase with a 1/2 magnetization plateau appearing in a magnetic field, which was observed in experiment. However, with J2 coupling ascending, the ground state goes into a gapped phase with an additional zero plateau emerging. It is found that the magnetization plateau states originate from the intra-dimer and dimer-chain singlet recombination, which are explicitly evidenced by the sublattice magnetization and two-site entanglement entropy, whose behaviors signal the QPTs clearly.

Quantum anomalous Hall effect in kagome ice

An anomalous Hall insulator without magnetic long-range ordering is theoretically reported in the absence of the relativistic spin-orbit coupling. It is realized in itinerant electrons coupled with the Ising spins on a <111> kagome plane of pyrochlore spin ice in applied magnetic field. We find that the kagome-ice type local spin correlation in the magnetization plateau state opens a charge gap without magnetic ordering, which results in quantization of the Hall conductivity. By Monte Carlo simulation, we identify the anomalous Hall insulating region in the magnetic phase diagram, in addition to another anomalous Hall insulator in a fully-saturated state.

ChemInform Abstract: Highly Frustrated Magnetism in Spinels

AbstractReview: 97 refs.

Spin-chirality separation andS3symmetry breaking in the magnetization plateau of the quantum spin tube

We study the magnetization plateau state of the three-leg spin-$\frac{1}{2}$ tube in the strong rung coupling region, where ${S}_{3}$ symmetry breaking and the low-energy chirality degree of freedom play crucial roles. On the basis of the effective chirality model and density matrix renormalization group, we clarify that, as the leg coupling increases, the chirality liquid with gapless nonmagnetic excitations, the spin-imbalance phase, and the vector-spin-chirality ordered phase emerge without closing the plateau spin gap. The relevance of these results to experiments is also discussed.

Field-controlled magnetic order with insulator–metal transitions in a periodic Anderson-like organic polymer

The zero- and low-temperature behaviors of a quasi-one-dimensional organic polymer proposed as a symmetrical periodic Anderson-like chain model, in which the localized f orbitals hybridize with the conduction orbitals at even sites, are investigated by means of many-body Green's function theory. In the absence of magnetic field, the ground state of the system turns out to be ferrimagnetic. The temperature-induced phase diagrams have been explored, where the competition between the Hubbard repulsion U on the localized f orbital and the hybridization strength V makes an important impact on the transition temperature. In a magnetic field, it is found that a 1/3 magnetization plateau appears and two critical fields indicating the insulator-metal transitions at zero temperature emerge, which are closely related to the energy bands. Furthermore, the single-site entanglement entropy is a good indicator of quantum phase transitions. The temperature-field-induced phase diagram has also been attained, wherein the magnetization plateau state, the gapless phase and the spin polarized state are revealed. The temperature dependence of thermodynamic quantities such as the magnetization, susceptibility and specific heat are calculated to characterize the corresponding phases. It is also found that the up-spin and down-spin hole excitations are responsible for the thermodynamic properties.

First‐order phase transition between one‐third magnetization plateau states in an anisotropic (S, S′) = (1, 2) spin‐alternating chain

AbstractIn recent studies we find that three kinds of magnetization plateaux appear at one‐third of the saturation magnetization in the ground‐state magnetization curve of an anisotropic (S, S′) = (1, 2) spin‐alternating chain, in which S = 1 and S′ = 2 spins with on‐site anisotropies are alternatingly arranged and coupled antiferromagnetically. One of these plateaux is a quantum plateau, while the other two are classical plateaux. We here show that the first‐order phase transition between the two classical one‐third plateau states occurs when the on‐site anisotropy for the S = 1 spin is of easy‐axis type and its magnitude is sufficiently large. We numerically determine the phase transition line for this first‐order transition and briefly discuss the origin of its occurrence.

Formula omitted] spin systems on frustrated pyrochlore lattice

The effect of magnetic field on S = 3 2 Heisenberg antiferromagnets on pyrochlore lattice was investigated. Chromium spinels CdCr 2 O 4 and HgCr 2 O 4 exhibit first order field-induced transitions to magnetization plateau states with one-half moment of Cr 3 + . One-half moment is reasonably understood as formation of collinear three-up and one-down spin configuration for each tetrahedron, which is a building block of pyrochlore lattice. The effect of lattice distortions to stabilize this spin ordering state makes these magnetization plateau phases appear within wide range of magnetic field. It is quite likely that field-induced collinear three-up and one-down state is universal for antiferromagnetic Heisenberg spin systems on pyrochlore lattice, however, such pure spin system seems to be limited to chromium spinel oxides at present.

Spin Nematics and Magnetization Plateau Transition in Anisotropic Kagome Magnets

We study S=1 kagome antiferromagnets with an isotropic Heisenberg exchange J and strong easy-axis single-ion anisotropy D. For D>>J, the low-energy physics can be described by an effective S=1/2 XXZ model with antiferromagnetic Jz approximately J and ferromagnetic J perpendicular approximately J2/D. Exploiting this connection, we argue that nontrivial ordering into a "spin-nematic" occurs whenever D dominates over J, and discuss its experimental signatures. We also study a magnetic field induced transition to a magnetization plateau state at magnetization 1/3 which breaks lattice translation symmetry due to ordering of the Sz and occupies a lobe in the B/Jz-Jz/J perpendicular phase diagram.

Effective Hamiltonian for the pyrochlore antiferromagnet: Semiclassical derivation and degeneracy

In the classical pyrochlore lattice Heisenberg antiferromagnet, there is a macroscopic continuous ground-state degeneracy. We study the semiclassical limit of large spin length $S$, keeping only the lowest-order (in $1∕S$) correction to the classical Hamiltonian. We perform a detailed analysis of the spin-wave modes, and using a real-space loop expansion, we produce an effective Hamiltonian, in which the degrees of freedom are Ising variables representing fluxes through loops in the lattice. We find a family of degenerate collinear ground states, related by gaugelike ${Z}_{2}$ transformations, and provide bounds for the order of the degeneracy. We further show that the theory can readily be applied to determine the ground states of the Heisenberg Hamiltonian on related lattices and to field-induced collinear magnetization plateau states.

Magnetic-Field Induced Transition to the1/2Magnetization Plateau State in the Geometrically Frustrated MagnetCdCr2O4

The magnetization of the geometrically frustrated spinel CdCr2O4 was measured in pulsed fields of up to 47 T. We found a metamagnetic transition to a very wide magnetization plateau state with one half of the full moment of S=3/2 Cr3+ at 28 T, independent of the field direction. This is the first observation of magnetization plateau state realized in Heisenberg pyrochlore magnet. The plateau state can be ascribed to a collinear spin configuration with three-up and one-down spins out of four spins of each Cr tetrahedron. A large magnetostriction is observed at the transition in spite of the negligible spin-orbit couplings. We argue that spin frustration plays a vital role in this large spin-lattice coupling.

The [formula omitted]-magnetization plateau state in the 2D quantum antiferromagnet SrCu 2(BO 3) 2: spin superstructure, phase transition, and spin dynamics studied by high-field NMR

We report the NMR observation of spin superstructure in the 1 8 magnetization plateau phase of SrCu 2(BO 3) 2, a quasi-2D dimer singlet spin system, at high magnetic fields near 28 T . The 1 8 phase is characterized by crystalization of triplets in a rhomboid unit cell with oscillating magnetization and separated from uniform phase by a first-order transition. Comparison between the Cu and B NMR spectra and theoretical calculations on the Shastry–Sutherland model enabled us to determine the three-dimensional spin superstructure. We also found that subtle structural feature of the buckling of magnetic layers produces anisotropic interactions, which lead to field-induced staggered magnetization.

Spin superstructure in the [formula omitted]-magnetization plateau phase of the 2D orthogonal dimer spin system SrCu 2(BO 3) 2

Nuclear magnetic resonance (NMR) experiments have been performed on the quasi-two-dimensional quantum spin system SrCu 2(BO 3) 2. The Cu NMR spectrum in the 1 8 -magnetization plateau state at 27.6 T and 35 mK shows fine structure, providing clear evidence for a magnetic superlattice in the plateau state. We conclude that the superlattice has a rhomboid unit cell with oscillating spin polarization, which is in good agreement with the numerical calculation on the Shastry–Sutherland spin model.

Magnetic Superstructure in the Two-Dimensional Quantum Antiferromagnet SrCu 2 (BO 3 ) 2

We report the observation of magnetic superstructure in a magnetization plateau state of SrCu2(BO3)2, a frustrated quasi-two-dimensional quantum spin system. The Cu and B nuclear magnetic resonance (NMR) spectra at 35 millikelvin indicate an apparently discontinuous phase transition from uniform magnetization to a modulated superstructure near 27 tesla, above which a magnetization plateau at 1/8 of the full saturation has been observed. Comparison of the Cu NMR spectrum and the theoretical analysis of a Heisenberg spin model demonstrates the crystallization of itinerant triplets in the plateau phase within a large rhomboid unit cell (16 spins per layer) showing oscillations of the spin polarization. Thus, we are now in possession of an interesting model system to study a localization transition of strongly interacting quantum particles.