Field-induced Magnetic Ordering Research Articles

Spinon Heat Transport in the Three-Dimensional Quantum Magnet PbCuTe_{2}O_{6}.

Quantum spin liquids (QSLs) are novel phases of matter which remain quantum disordered even at the lowest temperature. They are characterized by emergent gauge fields and fractionalized quasiparticles. Here we show that the sub-kelvin thermal transport of the three-dimensional S=1/2 hyperhyperkagome quantum magnet PbCuTe_{2}O_{6} is governed by a sizeable charge-neutral fermionic contribution which is compatible with the itinerant fractionalized excitations of a spinon Fermi surface. We demonstrate that this hallmark feature of the QSL state is remarkably robust against sample crystallinity, large magnetic field, and field-induced magnetic order, ruling out the imitation of QSL features by extrinsic effects. Our findings thus reveal the characteristic low-energy features of PbCuTe_{2}O_{6} which qualify this compound as a true QSL material.

Field-induced effects in the spin liquid candidate PbCuTe2O6

${\mathrm{PbCuTe}}_{2}{\mathrm{O}}_{6}$ is considered to be one of the rare candidate materials for a three-dimensional quantum spin liquid (QSL). This assessment was based on the results of various magnetic experiments, performed mainly on polycrystalline material. More recent measurements on single crystals revealed an even more exotic behavior, yielding ferroelectric order below ${T}_{\text{FE}}\ensuremath{\approx}1\phantom{\rule{0.28em}{0ex}}\text{K}$, accompanied by distinct lattice distortions, and a somewhat modified magnetic response which is still consistent with a QSL. Here we report on low-temperature measurements of various thermodynamic, magnetic, and dielectric properties of single-crystalline ${\mathrm{PbCuTe}}_{2}{\mathrm{O}}_{6}$ in magnetic fields $B\ensuremath{\le}14.5\phantom{\rule{0.28em}{0ex}}\text{T}$. The combination of these various probes allows us to construct a detailed $B\text{\ensuremath{-}}T$ phase diagram including a ferroelectric phase for $B\ensuremath{\le}8$ T and a $B$-induced magnetic phase at $B\ensuremath{\ge}11$ T. These phases are preceded by or coincide with a structural transition from a cubic high-temperature phase into a distorted noncubic low-temperature state. The phase diagram discloses a ferroelectric quantum critical point at ${B}_{c1}=7.9\phantom{\rule{0.28em}{0ex}}\text{T}$, where the second-order phase transition line associated with ferroelectric order is suppressed to zero. In addition, a magnetic quantum phase transition is revealed at ${B}_{c2}=11\phantom{\rule{0.28em}{0ex}}\text{T}$. The corresponding phase transition to a field-induced magnetic order at $B>{B}_{c2}$ is likely to be of first order. Field-induced lattice distortions, observed in the state at $T>1$ K and which are assigned to the effect of spin-orbit interaction of the ${\mathrm{Cu}}^{2+}$ ions, are considered as the key mechanism by which the magnetic field couples to the dielectric degrees of freedom in this material.

Investigating field-induced magnetic order in Han purple by neutron scattering up to 25.9 T

BaCuSi$_2$O$_6$ is a quasi-two-dimensional (2D) quantum antiferromagnet containing three different types of stacked, square-lattice bilayer hosting spin-1/2 dimers. Although this compound has been studied extensively over the last two decades, the critical applied magnetic field required to close the dimer spin gap and induce magnetic order, which exceeds 23 T, has to date precluded any kind of neutron scattering investigation. However, the HFM/EXED instrument at the Helmholtz-Zentrum Berlin made this possible at magnetic fields up to 25.9 T. Thus we have used HFM/EXED to investigate the field-induced ordered phase, in particular to look for quasi-2D physics arising from the layered structure and from the different bilayer types. From neutron diffraction data, we determined the global dependence of the magnetic order parameter on both magnetic field and temperature, finding a form consistent with 3D quantum critical scaling; from this we deduce that the quasi-2D interactions and nonuniform layering of BaCuSi$_2$O$_6$ are not anisotropic enough to induce hallmarks of 2D physics. From neutron spectroscopy data, we measured the dispersion of the strongly Zeeman-split magnetic excitations, finding good agreement with the zero-field interaction parameters of BaCuSi$_2$O$_6$. We conclude that HFM/EXED allowed a significant extension in the application of neutron scattering techniques to the field range above 20 T and in particular opened new horizons in the study of field-induced magnetic quantum phase transitions.

Revealing three-dimensional quantum criticality by Sr substitution in Han purple

Classical and quantum phase transitions (QPTs), with their accompanying concepts of criticality and universality, are a cornerstone of statistical thermodynamics. An exemplary controlled QPT is the field-induced magnetic ordering of a gapped quantum magnet. Although numerous "quasi-one-dimensional" coupled spin-chain and -ladder materials are known whose ordering transition is three-dimensional (3D), quasi-2D systems are special for several physical reasons. Motivated by the ancient pigment Han Purple (BaCuSi$_{2}$O$_{6}$), a quasi-2D material displaying anomalous critical properties, we present a complete analysis of Ba$_{0.9}$Sr$_{0.1}$CuSi$_{2}$O$_{6}$. We measure the zero-field magnetic excitations by neutron spectroscopy and deduce the magnetic Hamiltonian. We probe the field-induced transition by combining magnetization, specific-heat, torque and magnetocalorimetric measurements with low-temperature nuclear magnetic resonance studies near the QPT. By a Bayesian statistical analysis and large-scale Quantum Monte Carlo simulations, we demonstrate unambiguously that observable 3D quantum critical scaling is restored by the structural simplification arising from light Sr-substitution in Han Purple.

Field-induced magnetic transition and spin fluctuations in the quantum spin-liquid candidate CsYbSe2

Two-dimensional triangular-lattice materials with spin-1/2 are perfect platforms for investigating quantum frustrated physics with spin fluctuations. Here we report the structure, magnetization, heat capacity and inelastic neutron scattering (INS) results on cesium ytterbium diselenide, CsYbSe$_2$. There is no long-range magnetic order down to 0.4 K at zero field. The temperature dependent magnetization, $M(T)$, reveals an easy-plane magnetic anisotropy. A maximum is found in $M(T)$ around \emph{T}$\sim$1.5 K when magnetic field $H$ is applied in the $ab$ plane, indicating the short-range interaction. The low-temperature isothermal magnetization $M(H)$ shows a one-third plateau of the estimated saturation moment, that is characteristic of a two-dimensional frustrated triangular lattice. Heat capacity shows field-induced long-range magnetic order for both $H||c$ and $H||ab$ directions. The broad peak in heat capacity and highly damped INS magnetic excitation at $T$=2 K suggests strong spin fluctuations. The dispersive in-plane INS, centered at the (1/3 1/3 0) point, and the absence of dispersion along $c$ direction suggests 120$^{\circ}$ non-collinear 2D-like spin correlations. All these results indicate that the two-dimensional frustrated material CsYbSe$_2$ can be in proximity to the triangular-lattice quantum spin liquid. We propose an experimental low-temperature $H$-$T$ phase diagram for CsYbSe$_2$.

Field-induced double dome and Bose-Einstein condensation in the crossing quantum spin chain system AgVOAsO4

We present inelastic neutron scattering data on the quantum paramagnet AgVOAsO4 that establish the system is a S=1/2 alternating spin chain compound and provide a direct measurement of the spin gap. We also present experimental evidence for two different types of field-induced magnetic order between mu_0H_c1 = 8.4T and mu_0H_c2 = 48.9T, which may be related to Bose-Einstein condensation (BEC) of triplons. Thermodynamic measurements in magnetic fields up to 60T and temperatures down to 0.1K reveal a H-T phase diagram consisting of a dome encapsulating two ordered phases with maximum ordering temperatures of 3.8K and 5.3K respectively. This complex phase diagram is not expected for a single-Q BEC system and therefore establishes AgVOAsO4 as a promising multi-Q BEC candidate capable of hosting exotic vortex phases.

Crystal Growth of Quantum Magnets in the Rare-Earth Pyrosilicate Family R2Si2O7 (R = Yb, Er) Using the Optical Floating Zone Method

We report on the crystal growth of rare-earth pyrosilicates, R 2Si 2O 7 for R = Yb and Er using the optical floating zone method. The grown crystals comprise members from the family of pyrosilicates where the rare-earth atoms form a distorted honeycomb lattice. C-Yb 2Si 2O 7 is a quantum dimer magnet with field-induced long range magnetic order, while D-Er 2Si 2O 7 is an Ising-type antiferromagnet. Both growths resulted in multi-crystal boules, with cracks forming between the different crystal orientations. The Yb 2Si 2O 7 crystals form the C-type rare-earth pyrosilicate structure with space group C 2 / m and are colorless and transparent or milky white, whereas the Er-variant is D-type, P 2 1 / b , and has a pink hue originating from Er 3 +. The crystal structures of the grown single crystals were confirmed through a Rietveld analysis of the powder X-ray diffraction patterns from pulverized crystals. The specific heat of both C-Yb 2Si 2O 7 and D-Er 2Si 2O 7 measured down to 50 mK indicated a phase transition at T N ≈ 1.8 K for D-Er 2Si 2O 7 and a broad Schottky-type feature with a sharp anomaly at 250 mK in an applied magnetic field of 0.8T along the c-axis in the case of C-Yb 2Si 2O 7 .

Probing short-range magnetic order in a geometrically frustrated magnet by means of the spin Seebeck effect

Competing magnetic interactions in geometrically frustrated magnets give rise to new forms of correlated matter, such as spin liquids and spin ices. Characterizing the magnetic structure of these states has been difficult due to the absence of long-range order. Here, we demonstrate that the spin Seebeck effect (SSE) is a sensitive probe of magnetic short-range order (SRO) in geometrically frustrated magnets. In low temperature (2 - 5 K) SSE measurements on a model frustrated magnet \mathrm{Gd_{3}Ga_{5}O_{12}}, we observe modulations in the spin current on top of a smooth background. By comparing to existing neutron diffraction data, we find that these modulations arise from field-induced magnetic ordering that is short-range in nature. The observed SRO is anisotropic with the direction of applied field, which is verified by theoretical calculation.

Neutron diffraction investigation of theH−Tphase diagram above the longitudinal incommensurate phase ofBaCo2V2O8

The quasi-one-dimensional antiferromagnetic Ising-like compound BaCo2V2O8 has been shown to be describable by the Tomonaga-Luttinger liquid theory in its gapless phase induced by a magnetic field applied along the Ising axis. Above 3.9 T, this leads to an exotic field-induced low-temperature magnetic order, made of a longitudinal incommensurate spin-density wave, stabilized by weak interchain interactions. By single-crystal neutron diffraction we explore the destabilization of this phase at a higher magnetic field. We evidence a transition at around 8.5 T towards a more conventional magnetic structure with antiferromagnetic components in the plane perpendicular to the magnetic field. The phase diagram boundaries and the nature of this second field-induced phase are discussed with respect to previous results obtained by means of nuclear magnetic resonance and electron spin resonance, and in the framework of the simple model based on the Tomonaga-Luttinger liquid theory, which obviously has to be refined in this complex system.

Dispersionless Spin Waves and Underlying Field-Induced Magnetic Order in Gadolinium Gallium Garnet.

We report the results of neutron diffraction and inelastic neutron scattering on a powder sample of Gd_{3}Ga_{5}O_{12} at high magnetic fields. Analysis of the diffraction data shows that in high fields (B≳1.8 T) the spins are not fully aligned, but are canted slightly as a result of the dipolar interaction. The magnetic phase for fields ≲1.8 T is characterized by antiferromagnetic peaks at (210) and an incommensurate wave vector. The dominant contribution to inelastic scattering at large momentum transfers is from a band of almost dispersionless excitations. We show that these correspond to the spin waves localized on ten site rings, expected on the basis of nearest neighbor exchange interaction, and that the spectrum at high fields B≳1.8 T is well described by a spin wave theory.

Effects of magnetic field intensity on carbon diffusion coefficient in pure iron in γ-Fe temperature region

Effects of magnetic field intensity on carbon diffusion coefficient in pure iron in the γ- Fe temperature region were investigated using carburizing technology. The carbon penetration profiles from the iron surface to interior were measured by field emission electron probe microanalyzer. The carbon diffusion coefficient in pure iron carburized with different magnetic field intensities was calculated according to the Fick's second law. It was found that the magnetic field intensity could obviously affect the carbon diffusion coefficient in pure iron in the γ- Fe temperature region, and the carbon diffusion coefficient decreased obviously with the enhancement of magnetic field intensity, when the magnetic field intensity was higher than 1 T, the carbon diffusion coefficient in field annealed specimen was less than half of that of the nonfield annealed specimen, further enhancing the magnetic field intensity, the carbon diffusion coefficient basically remains unchanged. The stiffening of lattice due to field-induced magnetic ordering was responsible for an increase in activation barrier for jumping carbon atoms. The greater the magnetic field intensity, the stronger the inhibiting effect of magnetic field on carbon diffusion.

Unusual Magnetic Ordering Observed in Nanosized S = 1/2 Quantum Spin System (CH3)2NH2CuCl3

Specific heat measurements were performed on nanometer-sized particles of an S = 1/2 quantum spin system, DMACuCl3 [DMA = (CH3)2NH2]. A tiny peak was observed at a low temperature of approximately 1.5 K at zero magnetic field, in addition to a well-defined and commonly observed peak with the bulk system at approximately 0.8 K. Under magnetic fields, it increased in intensity and coincided with the phase boundary responsible for the field-induced magnetic ordering phase above about 6 T. 1H-NMR studies confirmed that such extra peaks also indicated magnetic ordering. The resultant phase diagram is considerably different from that observed previously in the bulk system. Such a difference may be caused by the presence of extra spins found in a nanosized system by ESR measurements.

Criticality in a disordered quantum antiferromagnet studied by neutron diffraction

Field-induced magnetic ordering in the structurally disordered quantum magnets Ni(Cl$_{1-x}$Br$_x$)$_2$$\cdot$4SC(NH$_2$)$_2$, $x=8$% and 13%, is studied by means of neutron diffraction. The order parameter critical exponent is found to be very close to its value $\beta=0.5$ expected for magnetic Bose-Einstein condensation in the absence of disorder. This result applies to temperatures down to 40 mK, and a 1 T range in magnetic field. The crossover exponent is found to be $\phi \sim 0.4$, for temperature ranges as small as 300 mK. Possible reasons for a discrepancy with recent numerical simulations and bulk measurements are discussed.

Field-induced magnetic ordering and single-ion anisotropy in the quasi-one-dimensional Haldane chain compound SrNi2V2O8: A single-crystal investigation

Field-induced magnetic ordering in the Haldane chain compound SrNi$_{2}$V$_{2}$O$_{8}$ and effect of anisotropy have been investigated using single crystals. Static susceptibility, inelastic neutron scattering, high-field magnetization, and low temperature heat-capacity studies confirm a non-magnetic spin-singlet ground state and a gap between the singlet ground state and triplet excited states. The intra-chain exchange interaction is estimated to be $J \sim 8.9{\pm}$0.1 meV. Splitting of the dispersions into two modes with minimum energies 1.57 and 2.58 meV confirms the existence of single-ion anisotropy $D(S^z){^2}$. The value of {\it D} is estimated to be $-0.51{\pm}0.01$ meV and the easy axis is found to be along the crystallographic {\it c}-axis. Field-induced magnetic ordering has been found with two critical fields [$\mu_0H_c^{\perp c} = 12.0{\pm}$0.2 T and $\mu_0H_c^{\parallel c} = 20.8{\pm}$0.5 T at 4.2 K]. Field-induced three-dimensional magnetic ordering above the critical fields is evident from the heat-capacity, susceptibility, and high-field magnetization study. The Phase diagram in the {\it H-T} plane has been obtained from the high-field magnetization. The observed results are discussed in the light of theoretical predictions as well as earlier experimental reports on Haldane chain compounds.

Field-induced long-range magnetic order in the spin-singlet ground-state system YbAl3C3: Neutron diffraction study

The $4f$-electron system YbAl$_3$C$_3$ with a non-magnetic spin-dimer ground state has been studied by neutron diffraction in an applied magnetic field. A long-range magnetic order involving both ferromagnetic and antiferromagnetic components has been revealed above the critical field H$_C\sim $ 6T at temperature T=0.05K. The magnetic structure indicates that the geometrical frustration of the prototype hexagonal lattice is not fully relieved in the low-temperature orthorhombic phase. The suppression of magnetic ordering by the remanent frustration is the key factor stabilizing the non-magnetic singlet ground state in zero field. Temperature dependent measurements in the applied field H=12T revealed that the long-range ordering persists up to temperatures significantly higher than the spin gap indicating that this phase is not directly related to the singlet-triplet excitation. Combining our neutron diffraction results with the previously published phase diagram, we support the existence of an intermediate disordered phase as the first excitation from the non-magnetic singlet ground state. Based on our results, we propose YbAl$_3$C$_3$ as a new material for studying the quantum phase transitions of heavy-fermion metals under the influence of geometrical frustration.

A search for field-induced ordering in the optimally doped Ba(Fe,Co)2As2 superconductor

A 57Fe Mössbauer search for field-induced magnetic order in optimally doped Ba(Fe1−xCox)2As2 in fields of up to 6 T showed no changes that could be attributed to field-induced order. We also observed no difference between the normal (30 K) and superconducting states (5 K). Any field-induced order is certainly less than 1% of the order present in the parent BaFe2As2.

Dzyaloshinsky-Moriya interaction and field-induced magnetic order in an antiferromagnetic honeycomb lattice compound Bi3Mn4O12(NO3) studied by high-field electron spin resonance

High-field electron spin resonance (ESR) measurements of ${\mathrm{Bi}}_{3}{\mathrm{Mn}}_{4}{\mathrm{O}}_{12}({\mathrm{NO}}_{3})$, which has been suggested recently as a new honeycomb lattice antiferromagnet with spin frustration, have been performed in a field region up to 20 T. An ESR measurement at 265 K shows an isotropic broad resonance, and the obtained $g$ value is $1.974\ifmmode\pm\else\textpm\fi{}0.016$, which is consistent with the ${\mathrm{Mn}}^{4+}$ ion in a crystal field with low symmetry. At 1.8 K, resonances, whose frequency-field relation corresponds to antiferromagnetic resonance (AFMR) with an easy-plane type anisotropy and a Dzyaloshinsky-Moriya (DM) interaction, appear above 6 T. This result is consistent with the field-induced magnetic order (FIMO) phase suggested by a previous neutron measurement. From the analysis of our AFMR result, the $D$ term of the DM interaction is estimated as 1.3 K, and the direction of the $D$ term is suggested to be along the $c$ axis, which is consistent with the crystal symmetry. Using this $D$ and the exchange interaction, the canted angle is estimated to be 2.42${}^{\ensuremath{\circ}}$, which can interpret the magnetization jump at the metamagnetic transition qualitatively. The possible origin of the FIMO phase is discussed with recent theoretical results within the ${J}_{1}\ensuremath{-}{J}_{2}$ model.

Uniaxial-stress enhancement of spin-driven ferroelectric polarization in a multiferroic CuFe1−xGaxO2

Delafossite compound CuFeO2 is a spin-driven magneto-electric multiferroic, in which magnetic field-induced or nonmagnetic impurity-induced proper helical magnetic ordering generates a spontaneous electric polarization through the spin-orbit-interaction mediated modulation of Fe 3d-O 2p hybridization. As is expected from spin-lattice tightly coupled character of this system, we found an enhancement of spin-driven ferroelectric polarization under [11̄0] uniaxial stress as spin-mediated piezoelectric effect. Preparing almost mono-domain state for multiferroic CuFe1−xGaxO2 sample with x = 0.035, we have performed polarized neutron diffraction and in-situ ferroelectric polarization measurements to investigate how the helical magnetic structure is modified and how much ferroelectric polarization increases under application of [11̄0] uniaxial stress up to 100 MPa. The uniaxial-stress variation of magnetic structural parameters characterizing helical magnetic ordering suggests decreasing of the ferroelectric polarization according to the calculation on the basis of d-p hybridization mechanism. Therefore, it is suggested that the enhancement of ferroelectric polarization originates not in variation of the magnetic structure but in the enhancement of the d-p hybridization coupling constants.

Effect of Jahn-Teller distortion on magnetic ordering in Dy(Fe,Mn)O3perovskites

The structural, magnetic properties of the rare-earth perovskites, DyFe${}_{1\ensuremath{-}x}$Mn${}_{x}$O${}_{3}$ (space group, orthorhombic Pbnm), were studied, and the magnetic phase diagram of the solid solution was investigated, which unveiled an unexplored hidden field-induced weak ferromagnetism in the material system with $x$ $=$ 0.5--0.9 (antiferromagnetically ordered along the $b$ axis and canted along the $c$ axis). In the solid solution, the Jahn-Teller distortion contributed by Mn${}^{3+}$ gives rise to the predominance of the bc plane sublattice with increasing Mn and also the effective onset of orbital ordering in the ab plane for $x$ $=$ 0.5 and above. These distinct features in the respective lattice and orbital degrees of freedom induce the Dy-(Fe,Mn) electronic interaction anisotropy primarily in the $\mathit{bc}$ plane and the growing in-($\mathit{ab}$)plane electronic anisotropy, effectively competing with each other above $x$ $=$ 0.5. The close entanglement of these anisotropies introduced by the various lattice, orbital, and correlated electronic characters results in the favoring of the Dzyaloshinskii-Moriya interaction dictating along the $a$ axis, in contrast to the conventionally observed $b$ axis and accounting for the hidden field-induced canted magnetic ordering in the $\mathit{bc}$ plane. The complex interplays of these multiple factors are discussed in this work and satisfactorily explain the rich magnetic phase diagram of the solid solution.

Magnetism and Disorder Effects on Muon Spin Rotation Measurements of the Magnetic Penetration Depth in Iron-Arsenic Superconductors

It is shown that attempts to accurately deduce the magnetic penetration depth λ of overdoped BaFe(1.82)Co(0.18)As₂ single crystals by transverse-field muon spin rotation (TF μSR) are thwarted by field-induced magnetic order and strong vortex-lattice disorder. We explain how substantial deviations from the magnetic field distribution of a nearly perfect vortex lattice by one or both of these factors is also significant for other iron-arsenic superconductors, and this introduces considerable uncertainty in the values of λ obtained by TF μSR.