arXiv subjects

M. Kenzelmann

Publications and source records attributed to M. Kenzelmann.

At least 55 records · Page 3Linked to original sources

arXiv2008

Mesoscopic Phase Coherence in a Quantum Spin Fluid

Mesoscopic quantum phase coherence is important because it improves the prospects for handling quantum degrees of freedom in technology. Here we show that the development of such coherence can be monitored using magnetic neutron scattering from a one-dimensional spin chain Y2BaNiO5, a quantum spin fluid where no classical, static magnetic order is present. In the cleanest samples, the quantum coherence length is 20 nm, almost an order of magnitude larger than the classical antiferromagnetic correlation length of 3 nm. We also demonstrate that the coherence length can be modified by static and thermally activated defects in a quantitatively predictable manner.

cond-mat.str-el↗

arXiv2008

Complex Spin Structures and Origin of the Magneto-Electric Coupling in YMn2O5

By combining neutron four-circle diffraction and polarized neutron diffraction techniques we have determined the complex spin structures of a multiferroic, YMn2O5, that exhibits two ferroelectric phases at low temperatures. The obtained magnetic structure has spiral components in both the low temperature ferroelectric phases that are magnetically commensurate and incommensurate, respectively. Among proposed microscopic theories for the magneto-electric coupling, our results are consistent with both the spin-current model that requires a magnetic transverse (cycloidal) spiral structure in order to induce a spontaneous electric polarization and the magneto-restriction model. Our results also explain why the electric polarization changes at the commensurate-to-incommensurate phase transition.

cond-mat.str-el↗

arXiv2008

Effect of Inversion Symmetry on Incommensurate Order in Multiferroic RMn2O5, R=rare earth

Starting from the irreducible representations of the group of the wave vector we construct the spin wave functions consistent with inversion symmetry, neglected in the usual representation analysis. We obtain the relation between the basis functions of different members of the star of the wave vector. We introduce order parameters and determine their transformation properties under the operations of the space group of the paramagnetic crystal. The results are applied to construct terms in the magnetoelectric interaction which are quadratic and quartic in the magnetic order parameters. The higher order magnetoelectric interactions can in principle induce components of the spontaneous polarization which are not allowed by the lowest order magnetoelectric interaction. We also obtain the relation between the spin wave functions of the incommensurate phase and those of the commensurate phase which lead to analogous relations between the order parameters of these two phases.

cond-mat.mtrl-sci↗

arXiv2007

Direct transition from a disordered to a multiferroic phase on a triangular lattice

Competing interactions and geometric frustration provide favourable conditions for exotic states of matter. Such competition often causes multiple phase transitions as a function of temperature and can lead to magnetic structures that break inversion symmetry, thereby inducing ferroelectricity [1-4]. Although this phenomenon is understood phenomenologically [3-4], it is of great interest to have a conceptually simpler system in which ferroelectricity appears coincident with a single magnetic phase transition. Here we report the first such direct transition from a paramagnetic and paraelectric phase to an incommensurate multiferroic in the triangular lattice antiferromagnet RbFe(MoO4)2 (RFMO). A magnetic field extinguishes the electric polarization when the symmetry of the magnetic order changes and ferroelectricity is only observed when the magnetic structure has chirality and breaks inversion symmetry. Multiferroic behaviour in RFMO provides a theoretically tractable example of ferroelectricity from competing spin interactions. A Landau expansion of symmetry-allowed terms in the free energy demonstrates that the chiral magnetic order of the triangular lattice antiferromagnet gives rise to a pseudoelectric field, whose temperature dependence agrees with that observed experimentally.

cond-mat.mtrl-sci↗

arXiv2007

Phase diagram and spin Hamiltonian of weakly-coupled anisotropic S=1/2 chains in CuCl2*2((CD3)2SO)

Field-dependent specific heat and neutron scattering measurements were used to explore the antiferromagnetic S=1/2 chain compound CuCl2 * 2((CD3)2SO). At zero field the system acquires magnetic long-range order below TN=0.93K with an ordered moment of 0.44muB. An external field along the b-axis strengthens the zero-field magnetic order, while fields along the a- and c-axes lead to a collapse of the exchange stabilized order at mu0 Hc=6T and mu0 Hc=3.5T, respectively (for T=0.65K) and the formation of an energy gap in the excitation spectrum. We relate the field-induced gap to the presence of a staggered g-tensor and Dzyaloshinskii-Moriya interactions, which lead to effective staggered fields for magnetic fields applied along the a- and c-axes. Competition between anisotropy, inter-chain interactions and staggered fields leads to a succession of three phases as a function of field applied along the c-axis. For fields greater than mu0 Hc, we find a magnetic structure that reflects the symmetry of the staggered fields. The critical exponent, beta, of the temperature driven phase transitions are indistinguishable from those of the three-dimensional Heisenberg magnet, while measurements for transitions driven by quantum fluctuations produce larger values of beta.

cond-mat.str-el↗

arXiv2006

Comment on: "Ferroelectricity in spiral magnets"

There is much interest in the physics of materials that show a strong coupling between magnetic and electric degrees of freedom. In a recent paper by Mostovoy a theory is presented that is based on symmetry arguments and leads to quite general claims which we feel merit some further analysis. In particular, Mostovoy concludes that "spiral magnets are, in general, ferroelectric". We argue that this conclusion is not generally valid, and that the symmetry of the unit cell has to be taken into account by any symmetry-based magneto-electric coupling theory. In an attempt to avoid further confusion in the search of new multiferroic materials, we identify in this Comment some of the necessary symmetry properties of spiral magnets that can lead to ferroelectricity.

cond-mat.mtrl-sci↗

arXiv2006

Neutron scattering from a coordination polymer quantum paramagnet

Inelastic neutron scattering measurements are reported for a powder sample of the spin-1/2 quantum paramagnet $\rm Cu(Quinoxaline)Br_2$. Magnetic neutron scattering is identified above an energy gap of 1.9 meV. Analysis of the sharp spectral maximum at the onset indicates that the material is magnetically quasi-one-dimensional. Consideration of the wave vector dependence of the scattering and polymeric structure further identifies the material as a two-legged spin-1/2 ladder. Detailed comparison of the data to various models of magnetism in this material based on the single mode approximation and the continuous unitary transformation are presented. The latter theory provides an excellent account of the data with leg exchange $J_{\parallel}=2.0$ meV and rung exchange $J_{\perp}=3.3$ meV.

cond-mat.str-el↗

arXiv2005

Field dependence of magnetic ordering in Kagome-staircase compound Ni3V2O8

We present powder and single-crystal neutron diffraction and bulk measurements of the Kagome-staircase compound Ni3V2O8 (NVO) in fields up to 8.5T applied along the c-direction. (The Kagome plane is the a-c plane.) This system contains two types of Ni ions, which we call "spine" and "cross-tie". Our neutron measurements can be described with the paramagnetic space group Cmca for T < 15K and each observed magnetically ordered phase is characterized by the appropriate irreducible representation(s). Our zero-field measurements show that at T_PH=9.1K NVO undergoes a transition to an incommensurate order which is dominated by a longitudinally-modulated structure with the spine spins mainly parallel to the a-axis. Upon further cooling, a transition is induced at T_HL=6.3K to an elliptically polarized incommensurate structure with both spine and cross-tie moments in the a-b plane. At T_LC=4K the system undergoes a first-order phase transition, below which the magnetic structure is a commensurate antiferromagnet with the staggered magnetization primarily along the a-axis and a weak ferromagnetic moment along the c-axis. A specific heat peak at T_CC'=2.3K indicates an additional transition, which we were however not able to relate to a change of the magnetic structure. Neutron, specific heat, and magnetization measurements produce a comprehensive temperature-field phase diagram. The symmetries of the two incommensurate magnetic phases are consistent with the observation that only one phase has a spontaneous ferroelectric polarization. All the observed magnetic structures are explained theoretically using a simplified model Hamiltonian, involving competing nearest- and next-nearest-neighbor exchange interactions, spin anisotropy, Dzyaloshinskii-Moriya and pseudo-dipolar interactions.

cond-mat.str-el↗

arXiv2005

Magnetic inversion symmetry breaking and ferroelectricity in TbMnO3

TbMnO3 is an orthorhombic insulator where incommensurate spin order for temperature T_N < 41K is accompanied by ferroelectric order for T < 28K. To understand this, we establish the magnetic structure above and below the ferroelectric transition using neutron diffraction. In the paraelectric phase, the spin structure is incommensurate and longitudinally-modulated. In the ferroelectric phase, however, there is a transverse incommensurate spiral. We show that the spiral breaks spatial inversion symmetry and can account for magnetoelectricity in TbMnO3.

cond-mat.str-el↗

arXiv2005

Magnetically driven ferroelectric order in Ni$_3$V$_2$O$_8$

We show that for Ni$_3$V$_2$O$_8$ long-range ferroelectric and incommensurate magnetic order appear simultaneously in a single phase transition. The temperature and magnetic field dependence of the spontaneous polarization show a strong coupling between magnetic and ferroelectric orders. We determine the magnetic symmetry of this system by constraining the data to be consistent with Landau theory for continuous phase transitions. This phenomenological theory explains our observation the spontaneous polarization is restricted to lie along the crystal b axis and predicts that the magnitude should be proportional to a magnetic order parameter.

cond-mat.mtrl-sci↗

arXiv2005

The S=1/2 chain in a staggered field: High-energy bound-spinon state and the effects of a discrete lattice

We report an experimental and theoretical study of the antiferromagnetic S=1/2 chain subject to uniform and staggered fields. Using inelastic neutron scattering, we observe a novel bound-spinon state at high energies in the linear chain compound CuCl2 * 2((CD3)2SO). The excitation is explained with a mean-field theory of interacting S=1/2 fermions and arises from the opening of a gap at the Fermi surface due to confining spinon interactions. The mean-field model also describes the wave-vector dependence of the bound-spinon states, particularly in regions where effects of the discrete lattice are important. We calculate the dynamic structure factor using exact diagonalization of finite length chains, obtaining excellent agreement with the experiments.

cond-mat.str-el↗

arXiv2005

Magnetic Order and Spin Dynamics in Ferroelectric HoMnO$_{3}$

Hexagonal HoMnO$_{3}$ is a frustrated antiferromagnet (T$_{N}$=72 K) ferroelectric (T$_{C}$=875 K) in which these two order parameters are coupled. Our neutron measurements of the spin wave dispersion for the S=2 Mn$^{3+}$ on the layered triangular lattice are well described by a two-dimensional nearest-neighbor Heisenberg exchange J=2.44 meV, and an anisotropy $D$ that is 0.093 meV above the spin reorientation transition at 40 K, and 0.126 meV below. For $H\parallel c$ the magnetic structures and phase diagram have been determined, and reveal additional transitions below 8 K where the ferroelectrically displaced Ho$^{3+}$ ions are ordered magnetically.

cond-mat.str-el↗

arXiv2004

Competing Magnetic Phases on a "Kagome Staircase"

We present thermodynamic and neutron data on Ni_3V_2O_8, a spin-1 system on a kagome staircase. The extreme degeneracy of the kagome antiferromagnet is lifted to produce two incommensurate phases at finite T - one amplitude modulated, the other helical - plus a commensurate canted antiferromagnet for T ->0. The H-T phase diagram is described by a model of competing first and second neighbor interactions with smaller anisotropic terms. Ni_3V_2O_8 thus provides an elegant example of order from sub leading interactions in a highly frustrated system

cond-mat.str-el↗

arXiv2004

Bound spinons in an antiferromagnetic S=1/2 chain with a staggered field

Inelastic neutron scattering was used to measure the magnetic field dependence of spin excitations in the antiferromagnetic S=1/2 chain CuCl_2 2(dimethylsulfoxide) (CDC) in the presence of uniform and staggered fields. Dispersive bound states emerge from a zero-field two-spinon continuum with different finite energy minima at wave numbers q=pi and q_i approx pi (1-2 ). The ratios of the field dependent excitation energies are in excellent agreement with predictions for breather and soliton solutions to the quantum sine-Gordon model, the proposed low-energy theory for S=1/2 chains in a staggered field. The data are also consistent with the predicted soliton and n=1,2 breather polarizations and scattering cross sections.

cond-mat.str-el↗

arXiv2002

Structure of end states for a Haldane Spin Chain

Inelastic neutron scattering was used to probe edge states in a quantum spin liquid. The experiment was performed on finite length antiferromagnetic spin-1 chains in Y_2BaNi_{1-x}Mg_xO_5. At finite fields, there is a Zeeman resonance below the Haldane gap. The wave vector dependence of its intensity provides direct evidence for staggered magnetization at chain ends, which decays exponentially towards the bulk (xi = 8(1) at T=0.1K). Continuum contributions to the chain end spectrum indicate inter-chain-segment interactions. We also observe a finite size blue shift of the Haldane gap.

cond-mat.str-el↗

arXiv2002

Order to disorder transition in the XY-like quantum magnet Cs2CoCl4 induced by noncommuting applied fields

We explore the effects of noncommuting applied fields on the ground-state ordering of the quasi-one-dimensional spin-1/2 XY-like antiferromagnet Cs2CoCl4 using single-crystal neutron diffraction. In zero field interchain couplings cause long-range order below T_N=217(5) mK with chains ordered antiferromagnetically along their length and moments confined to the (b,c) plane. Magnetic fields applied at an angle to the XY planes are found to initially stabilize the order by promoting a spin-flop phase with an increased perpendicular antiferromagnetic moment. In higher fields the antiferromagnetic order becomes unstable and a transition occurs to a phase with no long-range order in the (b,c) plane, proposed to be a spin liquid phase that arises when the quantum fluctuations induced by the noncommuting field become strong enough to overcome ordering tendencies. Magnetization measurements confirm that saturation occurs at much higher fields and that the proposed spin-liquid state exists in the region 2.10 < H_SL < 2.52 T || a. The observed phase diagram is discussed in terms of known results on XY-like chains in coexisting longitudinal and transverse fields.

cond-mat.str-el↗

arXiv2001

Temperature dependence of single particle excitations in a S=1 chain: exact diagonalization calculations compared to neutron scattering experiments

Exact diagonalization calculations of finite antiferromagnetic spin-1 Heisenberg chains at finite temperatures are presented and compared to a recent inelastic neutron scattering experiment for temperatures T up to 7.5 times the intrachain exchange constant J. The calculations show that the excitations at the antiferromagnetic point q=1 and at q=0.5 remain resonant up to at least T=2J, confirming the recent experimental observation of resonant high-temperature domain wall excitations. The predicted first and second moments are in good agreement with experiment, except at temperatures where three-dimensional spin correlations are most important. The ratio of the structure factors at q=1 and at q=0.5 is well predicted for the paramagnetic infinite-temperature limit. For T=2J, however, we found that the experimentally observed intensity is considerably less than predicted. This suggests that domain wall excitations on different chains interact up to temperatures of the order of the spin band width.

cond-mat.str-el↗

arXiv2001

Evolution of spin excitations in a gapped antiferromagnet from the quantum to the high-temperature limit

We have mapped from the quantum to the classical limit the spin excitation spectrum of the antiferromagnetic spin-1 Heisenberg chain system CsNiCl3 in its paramagnetic phase from T=5 to 200K. Neutron scattering shows that the excitations are resonant and dispersive up to at least T=70K, but broaden considerably with increasing temperature. The dispersion flattens out with increasing temperature as the resonance energy Delta at the antiferromagnetic wave-vector increases and the maximum in the dispersion decreases. The correlation length xi between T=12 and 50K is in agreement with quantum Monte Carlo calculations. xi is also consistent with the single mode approximation, suggesting that the excitations are short-lived single particle excitations. Below T=12K where three-dimensional spin correlations are important, xi is shorter than predicted and the experiment is not consistent with the random phase approximation for coupled quantum chains. At T=200K, the structure factor and second energy moment of the excitation spectrum are in excellent agreement with the high-temperature series expansion.

cond-mat.str-el↗