Chiral Glass Research Articles

Magnetic irreversibility and magnetization processes in Ni5Al3/NiO core/shell nanoparticle system

This work brings out many interesting facets of magnetism in the Ni5Al3/NiO core/shell nanoparticle system. Theweakandstrongmagnetic irreversibility lines (TWI(H)andTSI(H)) reproduce the previously reportedH - Tphase diagram at fieldsH⩽30 Oe, but strong departures occur forH > 30 Oe. Comparison with the theoretically predictedH - Tphase diagram allows us to identifyTWIwithTCG+SG, where the paramagnetic (PM)-chiral glass (CG) and PM-spin glass (SG) phase transitions occursimultaneously, andTSIwithTSG, the temperature at which transition to the replica symmetry breakingSGstate takes place. TheTSI(H)transition line abruptly ends at the point (H≃30 Oe,T≃90K). AsHexceeds 30 Oe, a new transition appears which gets completely suppressed at fieldsH>1 kOewhere the magnetic irreversibility ceases to exist. Nointrinsiclong-range ferromagnetic ordering exists but fields as low as 3 kOe suffice to induce long-range ferromagnetic order. At fixed temperatures, the magnetocrystalline anisotropy fluctuations essentially govern the 'approach-to-saturation' in magnetization for fields in the range 3 - 70 kOe. The present nanocrystalline system behaves as an isotropic system with random easy axis in which the magnetization reversal occurs through the coherent rotation of the magnetizations of weakly-interacting single-domain Ni5Al3particles. Saturation magnetization, likeM(T) atH⩾2 kOe, exhibits an anomalous upturn at temperatures below ≈ 30 K. This upturn is associated with the anomalous softening of spin-wave modes which results in the thermal excitation of a large number of non-equilibrium (finite lifetime) magnons. At sub-Kelvin temperatures, these magnons undergo Bose-Einstein condensation.

Glassy Dynamics in Chiral Fluids.

Chiral active matter is enjoying a rapid increase of interest, spurred by the rich variety of asymmetries that can be attained in, e.g., the shape or self-propulsion mechanism of active particles. Though this has already led to the observance of so-called chiral crystals, active chiral glasses remain largely unexplored. Apossible reason for this could be the naive expectation that interactions dominate the glassy dynamics and the details of the active motion become increasingly less relevant. Here, we show that quite the opposite is true by studying the glassy dynamics of interacting chiral active Brownian particles. We demonstrate that when our chiral fluid is pushed to glassy conditions, it exhibits highly nontrivial dynamics, especially compared to a standard linear active fluid such as common active Brownian particles. Despite the added complexity, we are still able to present a full rationalization for all identified dynamical regimes. Most notably, we introduce a new "hammering" mechanism, unique to rapidly spinning particles in high-density conditions, that can fluidize a chiral active solid.

Monte Carlo study of site disordered [formula omitted] Heisenberg models in the presence of a magnetic field and a weak random anisotropy

We study the 3d Heisenberg site disordered model with and without weak random anisotropy D in the presence of a uniform external longitudinal magnetic field using Monte Carlo methods. The spin and the chirality correlation lengths are measured. We provide evidence that the ”mixed chiral” phase is a chiral glass at a finite temperature transition TCG>0, whereas the spin glass temperature transition is at TSG=0 for D=0. In the anisotropic case we find that the model exhibited a strong evidence the occurrence of finite temperature transitions in the both sectors the spin and the chirality. Our estimate of the spin and chirality correlation-length exponents are compatible with our previous values obtained for the same model without anisotropy and the 3d Heisenberg spin glass.

Chiral glass of charged DNA rods, cavity loops

Recently, the collective phase behaviors of charged DNA-viruses (rods) are explored in various low ionic strengths, where much slowing down relaxations are observed in equilibrium. These phases are distinguished by crossing the critical ionic strength, where the salt-dependent effective microscopic dynamics are important with non-monotonic multi-phase boundaries. This is also interested in terms of phase transition and replica symmetry breaking (RSB), realized by the divergence of relaxation time and the exponent power of correlation lengths in dynamic disorderorder transitions. RSB occurs as the result of many-body interactions of randomized orientations. Here, in this paper, one of the chiral mesophases, the X-pattern is highlighted for its long-time metastable state consisting of cavity loops bounded within the planar nematic, as a new type of a glass, noted as a chiral glass. The chiral glass is discussed in terms of the divergences for relaxations in the characteristic time as well the inverse of an optical pitch length, which are analyzed in the long-time-lapsed orientation textures via image-time correlations. The outcome highlights interesting aspects of charge effect in the ‘twist’ elasticity energy for optimizing the long-ranged repulsive electrostatic interactions among charged DNA rods.

In-field critical behavior and magnetocaloric effect in Ni5Al3/NiO nanoparticle compacts

Experimental evidence is presented for the transformation of the previously reported ‘zero-field’ paramagnetic (PM)-chiral glass and PM-spin glass transitions into a single PM-ferromagnetic (FM) phase transition at applied magnetic fields H ≥ 3 kOe in Ni5Al3/NiO nanoparticle compacts. Accurate values of the asymptotic critical exponents β, γ and δ for spontaneous magnetization (order parameter), ‘zero-field’ magnetic susceptibility and the critical M − H isotherm, respectively, have been determined from the magnetization, M(T, H), data using the generalized scaling equation of state (SES) analysis. The critical exponents β, γ and δ not only satisfy the Widom scaling relation (β + γ) = βδ but also unambiguously establish that the nanoparticle system in question behaves as a three-dimensional (3D) isotropic nearest-neighbor (INN) Heisenberg ferromagnet in the critical region. As a function of temperature, the isothermal magnetic entropy change, −ΔSM, computed from the M(T, H) data, exhibits two peaks: a sharp peak at T†(H) ≃ 10 K and a broad one at Tp(H) ≃ 140 K. The characteristic temperatures, T† and Tp, shift to higher temperatures with increasing magnetic field, H, in accordance with the power laws H1∕2 and H1∕3, respectively. While the peak at Tp(H) signifies the PM-FM phase transition, the peak at T†(H) is reminiscent of a transition to a reentrant spin glass state. Using the rescaled temperature axis, the reduced -ΔSM(T) curves for different applied magnetic fields collapse onto a single universal curve characteristic of magnetic materials that exhibit a second-order phase transition at Tc. The presently determined 3D INN Heisenberg values for the exponents β and γ are shown to correctly describe the observed variation of -ΔSM with H at the Curie temperature, T = Tc.

Chiral lanthanide lumino-glass for a circularly polarized light security device

Artificial light plays an essential role in information technologies such as optical telecommunications, data storage, security features, and the display of information. Here, we show a chiral lanthanide lumino-glass with extra-large circularly polarized luminescence (CPL) for advanced photonic security device applications. The chiral lanthanide glass is composed of a europium complex with the chiral (+)-3-(trifluoroacetyl)camphor ligand and the achiral glass promoter tris(2,6-dimethoxyphenyl)phosphine oxide ligand. The glass phase transition behavior of the Eu(III) complex is characterized using differential scanning calorimetry. The transparent amorphous glass shows CPL with extra-large dissymmetry factor of gCPL = 1.2. The brightness of the lumino-glass is one thousand times larger than that of Eu(III) luminophores embedded in polymer films of the same thickness at a Eu(III) concentration of 1 mM. The application of the chiral lanthanide lumino-glass in an advanced security paint is demonstrated.

Electron Beam Lithography of Magnetic Skyrmions.

The emergence of magnetic skyrmions, topological spin textures, has aroused tremendous interest in studying the rich physics related to their topology. While skyrmions promise high-density and energy-efficient magnetic memory devices for information technology, the manifestation of their nontrivial topology through single skyrmions and ordered and disordered skyrmion lattices could also give rise to many fascinating physical phenomena, such as chiral magnon and skyrmion glass states. Therefore, generating skyrmions at designated locations on a large scale, while controlling the skyrmion patterns, is the key to advancing topological magnetism. Here, a new, yet general, approach to the "printing" of skyrmions with zero-field stability in arbitrary patterns on a massive scale in exchange-biased magnetic multilayers is presented. By exploiting the fact that the antiferromagnetic order can be reconfigured by local thermal excitations, a focused electron beam with a graphic pattern generator to "print" skyrmions is used, which is referred to as skyrmion lithography. This work provides a route to design arbitrary skyrmion patterns, thereby establishing the foundation for further exploration of topological magnetism.

Monte Carlo studies of the spin-chirality decoupling in the three-dimensional Heisenberg spin glass

An extensive equilibrium Monte Carlo simulation is performed on the 3D isotropic Heisenberg SG model with the random nearest-neighbor Gaussian coupling, with particular interest in its chiral-glass (CG) and spin-glass (SG) orderings. For this model, the possibility of the spin-chirality decoupling, {\it i.e.\}, the CG order setting in at a higher temperature than that of the SG order was suggested earlier, but still remains controversial. We simulate the model up to the maximum size (linear dimension) $L=48$ under both periodic and open boundary conditions (BC). In locating the CG and SG transition temperatures $T_{{\rm CG}}$ and $T_{{\rm SG}}$ by the $L\rightarrow \infty$ extrapolation, a variety of independent physical quantities under the both BC are computed and utilized to get larger number of degrees of freedom (NDF). Thanks to the large NDF up to NDF=43, we succeed in obtaining stable and accurate estimates of the CG and SG transition temperatures, $T_{{\rm CG}}=0.142\pm 0.001$ and $T_{{\rm SG}}=0.131^{+0.001}_{-0.006}$. No sign of the size crossover is observed. For larger $L$, the CG correlation length progressively outgrows the SG correlation length at low temperatures. These results provide strong numerical support for the spin-chirality decoupling. The critical exponents associated with the CG and SG transitions are evaluated by use of the finite-size scaling with the scaling correction. For the CG transition, we get the CG exponents, $\nu_{{\rm CG}}=1.36\pm 0.10$ and $\eta_{{\rm CG}}=0.49\pm 0.10$, consistently with the corresponding experimental exponents of canonical SG. Implications to the chirality scenario of experimental SG ordering is discussed.

Chiral metallic glass nanolattices with combined lower density and improved auxeticity.

Auxetic materials are promising structural and functional candidates due to their unique lateral expansion when stretched, however, bulk metallic glasses (MGs) could not show any auxeticity because of their intrinsic isotropic nature. Here we construct chiral Cu50Zr50 metallic glass nanolattices with cavities, and investigate their auxeticity and underlying mechanism with molecular dynamics simulations. It is found that, compared to monolithic MGs, all the chiral metallic glass nanolattices (CMGNs) exhibit improved auxeticity and lower density. For CMGNs with cavities, the negative Poisson's ratio and ultimate tensile strength (UTS) increase first and then decrease with increasing cavity radius, with the cavity radius of 2.5 nm being the most favorable for auxeticity and enhanced UTS. The auxetic mechanism is attributed to the competition between rotation behavior and non-affine deformation under tension. Our study not only reveals the mechanism of auxeticity in CMGNs having cavities but also provides a feasible method to optimize their auxetic performance and density by structure designing of MGs.

Impact of chirality on the Glass Forming Ability and the crystallization from the amorphous state of 5-ethyl-5-methylhydantoin, a chiral poor glass former

The investigation of the glassy state of 5-ethyl-5-methylhydantoin (i.e. 12H, a chiral Active Pharmaceutical Ingredient) was attempted by Differential Scanning Calorimetry (DSC) and Fast Scanning Calorimetry (FSC). This compound exhibits a high crystallization propensity for every enantiomeric composition. Nevertheless, glassy states of pure enantiomer or mixtures between enantiomers were successfully reached by FSC at cooling rates of: 1000 °C/s and 300 °C/s respectively, even though limitations on the sampling reproducibility were evidenced due to FSC sample size. The Glass Forming Ability (GFA) was proven to increase with the counter-enantiomer content. From the glassy state, pure enantiomer displayed a more pronounced crystallogenic character (with a crystallization occurring 36 °C below Tg during ageing) than that of the mixture between enantiomers. Ageing of amorphous 12H promotes a strong nucleation behavior in both samples but enantiopure 12H crystallizes upon ageing while scalemic 12H evolves towards the metastable equilibrium. Finally, potential new phase equilibria (previously not reported) in the enantiomeric phase diagram could have been highlighted by FSC by recrystallization from the amorphous state.

Devil's staircase continuum in the chiral clock spin glass with competing ferromagnetic-antiferromagnetic and left-right chiral interactions.

The chiral clock spin-glass model with q=5 states, with both competing ferromagnetic-antiferromagnetic and left-right chiral frustrations, is studied in d=3 spatial dimensions by renormalization-group theory. The global phase diagram is calculated in temperature, antiferromagnetic bond concentration p, random chirality strength, and right-chirality concentration c. The system has a ferromagnetic phase, a multitude of different chiral phases, a chiral spin-glass phase, and a critical (algebraically) ordered phase. The ferromagnetic and chiral phases accumulate at the disordered phase boundary and form a spectrum of devil's staircases, where different ordered phases characteristically intercede at all scales of phase-diagram space. Shallow and deep reentrances of the disordered phase, bordered by fragments of regular and temperature-inverted devil's staircases, are seen. The extremely rich phase diagrams are presented as continuously and qualitatively changing videos.

Tunable exchange bias in dilute magnetic alloys – chiral spin glasses

A unidirectional anisotropy appears in field cooled samples of dilute magnetic alloys at temperatures well below the cusp temperature of the zero field cooled magnetization curve. Magnetization measurements on a Cu(13.5 at% Mn) sample show that this anisotropy is essentially temperature independent and acts on a temperature dependent excess magnetization, ΔM. The anisotropy can be partially or fully transferred from being locked to the direction of the cooling field at lower fields to becoming locked to the direction of ΔM at larger fields, thus instead appearing as a uniaxial anisotropy. This introduces a deceiving division of the anisotropy into a superposition of a unidirectional and a uniaxial part. This two faced nature of the anisotropy has been empirically scrutinized and concluded to originate from one and the same exchange mechanism: the Dzyaloshinsky-Moriya interaction.

Chirality-driven intrinsic spin-glass ordering and field-induced ferromagnetism in Ni3Al nanoparticle aggregates

Weak itinerant-electron ferromagnet Ni3Al is driven to magnetic instability (quantum critical point, QCP, where the long-range ferromagnetic order of the bulk ceases to exist) by reducing the average crystallite size to d=50nm. ‘Zero-field’ (H=0) linear and nonlinear ac-susceptibilities, measured on Ni3Al nanoparticle aggregates, with d=50nm (S1) and d=5nm (S2), provide strong evidence for two spin glass (SG)-like thermodynamic phase transitions: one at Ti(H=0)≃30K (Ti†(H=0)≃230K) and the other at a lower temperature Tp(H=0)≃8K (Th(H=0)≃52K) in S1 (S2). ‘In-field’ (H≠0) linear ac-susceptibility and dc magnetization demonstrate that the thermodynamic nature of these transitions is preserved in finite fields. The presently determined H–T phase diagrams for the samples S1 and S2 are compared with those predicted by the Kotliar–Sompolinsky and Gabay–Toulouse mean-field models and Monte Carlo simulations, based on the chirality-driven spin glass (SG) ordering scenario, for a three-dimensional nearest-neighbor Heisenberg SG system with or without weak random anisotropy. Such a detailed comparison permits us to unambiguously identify various ‘zero-field’ and ‘in-field’ SG phase transitions as: (i) the simultaneous paramagnetic (PM)-chiral glass (CG) and PM-SG phase transitions at Ti(H), (ii) the PM-CG transition at Ti†(H), (iii) the replica symmetry-breaking SG transition at Tp(H), and (iv) the continuous spin-rotation symmetry-breaking SG transition at Th(H). In the presence of random anisotropy, magnetization fails to saturate even at 90kOe in S1 whereas negligibly small anisotropy allows even fields as weak as 1kOe to saturate magnetization and induce ferromagnetism in S2. Due to the proximity to CG/SG-QCP, magnetization and susceptibility both exhibit non-Fermi liquid behavior over a wide range at low temperatures.

Fluctuation conductivity and the chiral glass state in disordered [formula omitted] thin films

In this communication we report on fluctuation conductivity experiments in several superconducting YBa2Cu3O6+d thin films grown by chemical solution deposition. These films are c-axis oriented and show granularity at a submicron scale. Two of the samples were submitted to ion implantation in order to study effects of local disorder. When the temperature approaches the superconducting transition from above we first observe the expected three-dimensional Gaussian fluctuation region. Then, a crossover occurs closely above the transition to a regime dominated by genuine critical fluctuations characterized by a large exponent. Based on previous theoretical predictions, we interpret these critical regimes as precursors of the superconducting chiral glass state.

NMR of guest-host systems: 8CB in chiral nematic porous glasses.

Liquid crystals confined to porous materials often have different critical phenomena and ordering than in the bulk. Through the selection of pore size, structure and guest liquid crystal, these systems could enable a variety of functional materials for applications such as sensors and displays. A recent example of such a system is chiral nematic mesoporous films infiltrated with liquid crystal 4-cyano-4'-n-octylbiphenyl (8CB), which has reversible thermal switching of its optical bandgap. The optical bandgap is lost when the ordered 8CB guests are heated above ∼50 °C, where the 8CB becomes isotropic. In this study, we have used NMR cryoporometry and pulsed-field gradient diffusion measurements to determine the pore sizes and structures of various chiral nematic mesoporous silica and organosilica films. Temperature and orientation-dependent wideline (15)N NMR spectra of films infiltrated with (15)N-labelled 8CB guests show that the ordering of the 8CB mesogens is consistent with an average orientation parallel to the chiral nematic pore axes. Inclusion of a large, orientation-dependent shift was necessary to fit the spectra, probably due to susceptibility differences between the 8CB guests and the organosilica host.

Phase transition in three-dimensional Heisenberg spin glasses with strong random anisotropies through a multi-GPU parallelization

We characterize the phase diagram of anisotropic Heisenberg spin glasses, finding both the spin and the chiral glass transition. We remark the presence of strong finite-size effects on the chiral sector. We find a unique phase transition for the chiral and spin glass sector, in the Universality class of Ising spin glasses. We focus on keeping finite-size effects under control, and we stress that they are important to understand experiments. Thanks to large GPU clusters we have been able to thermalize cubic lattices with up to 64x64x64 spins, over a vast range of temperatures.

Monte Carlo simulations of the three-dimensionalXYspin glass focusing on chiral and spin order

The ordering of the three-dimensional isotropic {\it XY} spin glass with the nearest-neighbor random Gaussian coupling is studied by extensive Monte Carlo simulations. To investigate the ordering of the spin and the chirality, we compute several independent physical quantities including the glass order parameter, the Binder parameter, the correlation-length ratio, the overlap distribution and the non-self-averageness parameter, {\it etc}, for both the spin-glass (SG) and the chiral-glass (CG) degrees of freedom. Evidence of the spin-chirality decoupling, {\it i.e.}, the CG and the SG order occurring at two separated temperatures, $0<T_{SG}<T_{CG}$, is obtained from the glass order parameter, which is fully corroborated by the Binder parameter. By contrast, the CG correlation-length ratio yields a rather pathological and inconsistent result in the range of sizes we studied, which may originate from the finite-size effect associated with a significant short-length drop-off of the spatial CG correlations. Finite-size-scaling analysis yields the CG exponents $\nu_{CG}=1.36^{+0.15}_{-0.37}$ and $\eta_{CG}=0.26^{+0.29}_{-0.26}$, and the SG exponents $\nu_{SG}=1.22^{+0.26}_{-0.06}$ and $\eta_{SG}=-0.54^{+0.24}_{-0.52}$. The obtained exponents are close to those of the Heisenberg SG, but are largely different from those of the Ising SG. The chiral overlap distribution and the chiral Binder parameter exhibit the feature of a continuous one-step replica-symmetry breaking (1RSB), consistently with the previous reports. Such a 1RSB feature is again in common with that of the Heisenberg SG, but is different from the Ising one, which may be the cause of the difference in the CG critical properties from the Ising SG ones despite of a common $Z_2$ symmetry.

K0.9982Li0.0018TaO3 Crystal Under External DC Electric Field

The influence of external dc electric field on dielectric properties of K1-xLixTaO3 x = 0.18% (henceforth KLT-x%) is presented. Electric permittivity dependence on dc electric field confirms the existence of short-range electric order. According to the suggestion made by us [1] the substitution of K+ ions by off-center Li+ in KTaO3 is considered as the source of induced polar clusters of short range order at low temperatures for this crystal. The low-temperature state of KLT-0.18 may be considered as a quantum superparaelectric one that may be toggled into an improper ferroelectric while applying an electric field [2] or a chiral glass state [3].

Polar states of the impurity system KTaO3:Li

Linear and nonlinear dielectric and related properties of the polar phases of K 1– x Li x TaO 3 are reviewed for Li + concentrations x = 0.005, 0.011 and 0.063. They range from single particle relaxation with a possible chiral glass transition at T cg ≈ 6 K to the generic glass state below T g ≈ 33 K and to the ferroelectric transition at T C + ≈ 104 K with field controllable internal random field-perturbation.

Dynamics of Li+ dipoles at very low concentration in quantum paraelectric potassium tantalate

The dynamics of Li+ dipoles distributed in the quantum paraelectric matrix of potassium tantalate is investigated via linear and nonlinear dielectric response. It is found that the highly diluted solid solution K0.995Li0.005TaO3, KLT(005), reveals nearly ideal Debye-type dipolar relaxation with conventional frequency dispersion at intermediate temperatures, 30&amp;lt;T&amp;lt;60 K. At lower temperatures a drastic increase in the linear dielectric susceptibility is measured as predicted by computer simulations Su et al. [J. Appl. Phys. 90, 6345 (2001)]. Being accompanied by a negative divergence of the third-order nonlinear susceptibility, a transition into a chiral glass state as T→Tg≈6 K is conjectured.