Glass Freezing Research Articles

Cooperative freezing in spin glasses and magnetic nanostructures

Relaxation processes in disordered nanostructures and spin glasses are investigated by examining the role of the magnetic anisotropy. To abstract from freezing processes associated with phase transitions, the consideration is restricted to a one-dimensional bond-disordered Ising spin glass. Using a Mattis-type gauge transformation, the spin glass is mapped onto a disordered ferromagnet. Based on the behavior of the spin chain we argue that spin-glass freezing is not necessarily related to a phase transition. The magnetic anisotropy, which is largely ignored in the Glauber approach to Ising dynamics, gives rise to cooperative deviations from the Arrhenius behavior and mimics Vogel–Fulcher relaxation.

Influence of Chemical and Phase Equilibria on the Medium-Range Order and Physicochemical Properties of Chalcogenide Glasses1

The problem associated with the influence of chemical and phase equilibria in melts on the properties of formed glasses is analyzed experimentally and theoretically by the example of chalcogenide glasses prone to glass formation. It is proved that inorganic glasses should be treated as microinhomogeneous systems formed by a set of nanometer-sized pseudophases that differ in character. The equivalentometry method has revealed that, in inorganic glasses, there exist at least two types of the pseudophases which have different chemical compositions and are responsible for microinhomogeneous structure of vitreous materials. It is established that the pseudophases in glasses are fragments composed of elementary substances and are also stable and metastable compounds. The concept of a variable number of components in vitrifying melts is introduced for the first time. This concept and the eutectoidal model of the vitreous state of matter allow one to reveal the types of chemical and phase equilibria whose freezing in glasses results in the formation of a set of the pseudophases responsible for glass microinhomogeneity. The regularities revealed make it possible to put forward the principles of analysis of composition–property diagrams for vitreous systems.

Phase coexistence of hyderogen-bonded mixed crystals

The complex dielectric constants and polarization of the hydrogen-bonded mixed crystals, K1−x(NH4)xH2PO4 and Rb1−x(NH4)xH2AsO4, near the ferroelectric-proton glass phase boundary have been investigated as a function of temperature. Coexistence of both phases was discovered from a glassy dispersion at low temperature and a ferroelectric anomaly. The dielectric dispersion due to the coexisting proton glass state occurred at lower temperatures than for pure proton glass composition. The detection of polarization after field-cooling or zero field-cooling the sample showed that the ferroelectric domain and polarization were retained in the glass freezing process. These results are qualitatively consistent with the expectation of the infinite ranged random-bond random-field model in the presence of macroscopic polarization.

Study of the anisotropic dielectric behavior ofRb1−x(NH4)xH2PO4(x=0.15and 0.17) mixed crystals

The complex dielectric constants and polarization of the mixed crystals of ${\mathrm{Rb}}_{1\ensuremath{-}x}({\mathrm{NH}}_{4}{)}_{x}{\mathrm{H}}_{2}{\mathrm{PO}}_{4}$ (RADP) with $x=0.15$ and 0.17 have been measured along the tetragonal a and c axes as a function of temperature. Phase coexistence of ferroelectric order and proton glass in RADP can be confirmed from a dielectric dispersion distinguished from the ferroelectric domain wall freezing at low temperature. The dielectric dispersion, which was ascribed to the proton glass state, occurred at much lower temperatures compared to pure proton glass composition. The detection of depolarization after field-cooling or zero-field cooling the sample, showed that the ferroelectric domain and polarization were retained in the glass freezing process. The result is qualitatively consistent with the expectation of the mean-field model in the presence of macroscopic polarization. The phase coexistence may be a common feature of ${\mathrm{KH}}_{2}{\mathrm{PO}}_{4}$ family mixed crystals.

Magnetic field–time-scaling relations and exotic spin correlations: a μSR study of spin glasses

In this work we analyze a longitudinal field [H] μSR experiment in the spin glass AgMn(0.5 at%) using a cut-off power-law dynamical field auto-correlation function q(t)=2ct −x exp [−(νt) y] . This function allows us to account for the data for more than 2.5 orders of magnitude in magnetic field and two orders in time. Our results suggest that spin glass freezing is associated with a dynamical transition from a cut-off power law to a pure cut-off.

Coexistence of the critical slowing down and glassy freezing in relaxor ferroelectrics

We have developed a dynamical model for the dielectric response in relaxor ferroelectrics that explicitly takes into account the coexistence of the critical slowing down and glassy freezing. The application of the model to the experiment in ${\mathrm{PbMg}}_{1/3}{\mathrm{Nb}}_{2/3}{\mathrm{O}}_{3}$ (PMN) allowed for the reconstruction of the nonequilibrium spin-glass state order parameter and its comparison with the results of recent NMR experiment [R. Blinc et al., Phys. Rev. Lett 83, 424 (1999)]. It is shown that the degree of the local freezing is rather small even at temperatures where the field-cooled permittivity exceeds the frequency-dependent permittivity by an order of magnitude. This observation indicates the significant role of the critical slowing down (accompanying the glass freezing) in the system dynamics. Also, the theory predicts an important interrelationship between the frequency-dependent permittivity and the zero-field-cooled permittivity, which proved to be consistent with the experiment in PMN [A. Levstik et al., Phys. Rev. B 57, 11 204 (1998)].

Possible sign change of Mn–Mn exchange interaction between ThMnAl and ThMnIn

ThMnAl and ThMnIn have the C-15, cubic Laves phase crystal structure. Pronounced thermomagnetic irreversibility is seen in the magnetisation of both compounds. A cusp in the AC susceptibility of ThMnAl near 43K which shows frequency dependence establishes the spin glass freezing of the Mn moments in this compound. This is further corroborated by the heat capacity data which do not exhibit any anomaly near the cusp temperature. On the other hand, the zero-field-cooled magnetisation of ThMnIn shows a sharp ferromagnetic-like increase around 270K, a broad peak around 180K and then decreases monotonically with temperature. Various scenarios based on the sign change of Mn–Mn exchange interaction between ThMnAl and ThMnIn due to the increased Mn–Mn bond length in the latter are discussed. Electrical resistivity is large in both compounds and shows a negative temperature coefficient indicating the disordered nature of both alloys.

Field cooling studies on anisotropic glass freezing in mixed crystal dipole glass

Glass freezing temperatures of a mixed crystal dipole glass, deuterated rubidium-ammonium dihydrogen phosphate (DRADP-x) mixed crystal, are measured by both dielectric constant measurements and field cooling remanent polarization studies along the a- and c-axis of the mixed crystals DRADP-x. It was first observed that the anisotropy in glass freezing of DRADP-x dipole glass was strongly dependent on the mixing concentration x across x=0.5, that is, a large anisotropy in DRADP-x for x < 0.5 (x=0.40) but very small anisotropy in DRADP-x for x > 0.5 (x=0.59).

Spin glass behavior probed by various methods with different time- and space-scale of observation: Ising spin glass Fe 0.50 Mn 0.50 TiO 3

Under the expectation that the behavior of a spin glass (SG) system shows various aspects depending on the time- and space-scale of observations, we have studied the typical Ising spin glass Fe0.50Mn0.50TiO3 by measuring dc-magnetization, ac-susceptibility, Mossbauer spectrum, neutron scattering intensity and μ+SR time spectrum. We have demonstrated that the application of various methods with different time- and space-scale of observations to the same system is useful to clarify the property of the spin glass freezing, the Ising character and the de Almeida and Thouless-line.

Proton glass freezing in hydrated lysozyme powders.

At room temperature, the dielectric relaxation of hydrated powder of the protein lysozyme is known to be due to protons migrating between ionized side chains. A recent study of this relaxation at lower temperatures suggested a behavior typical of proton glasses. An analysis of the complex dielectric susceptibility by a temperature-frequency plot presented here has revealed that ergodicity is broken due to the divergence of the longest relaxation time at 266 K, indicating specifically that this hydrated protein is a proton glass. A change in the temperature behavior of the static dielectric constant and the average relaxation frequency at 273 K indicates a further transition occurring at this temperature, whose nature remains to be investigated.

Reconstruction of the nonequilibrium spin glass order parameter in relaxor ferroelectric lead magnesium niobate

We performed a reconstruction for the order parameter of nonequilibrium spin glass state of PbMg 1/3Nb 2/3O 3 based on the dielectric response theory in relaxor ferroelectrics, which takes into account the competition between local glass like freezing and the critical slowing down of the dynamics in highly polarizable materials. The resultant values for the degree of local freezing rather small even at temperatures where the field-cooled permittivity exceeds the frequency dependent permittivity by an order of magnitude. This observation indicates the significant role of the critical slowing down (accompanied the glass freezing) in the system dynamics.

Glass transition, freezing and melting of liquids confined in the mesoporous silicate MCM-41

Glass transition, freezing and melting of liquids confined in the mesoporous silicate MCM-41

Glass transition, freezing and melting of liquids confined in the mesoporous silicate MCM-41

Abstract MCM-41 is a recently discovered mesoporous silicate that consists of a hexagonal arrangement of cylindrical pores. Because the pores are very well defined in size and shape, MCM-41 can be considered as a model material in comparison with previous types of porous glass and has therefore been used in the present study. In particular, we have focused on the properties of confined liquids and related phase transitions using an MCM-41 with a pore diameter of 4 nm. We discuss here the first stage of a study where the thermodynamics of a series of molecular liquids (water, cyclohexane, benzene, toluene, o-terphenyl and m-toluidine) are analysed by differential scanning calorimetry. Only a moderate decrease in the melting point of organie liquids has been observed. Moreover, analysing the freezing process, we have established thermal treatments that provide temperature ranges where confined liquids can be studied below the melting temperatures of both the bulk and the confined phases. Surprisingly, no si...

Non-exponential relaxation in macroscopic susceptibilities

The study of dynamical properties of crystals near structural phase transitions is a very challenging subject. The dynamics may have various origins of intrinsic (moving phase transition clusters, domain walls, molecular reorientations, glass freezing, …) or extrinsic (impurities, dislocations, …) nature. Some of these processes lead to a poly-dispersive behaviour of macroscopic susceptibilities. Frequently the obtained data are fitted by empirical functions, e.g. Cole-Cole-, Cole-Davidson-, Havriliak-Negami-, etc. functions, thus leaving the origin for the observed relaxation unclear. We present two examples of non-exponential relaxations whose origins can be described by simple models: The influence of order parameter fluctuations to the elastic susceptibility yielding a dispersion which resembles a Cole-Davidson-type behaviour and the influence of heat diffusion to macroscopic susceptibilities near phase transitions, respectively.

Deuteron NMR investigations of glass and phase transitions in(KI)1−x(ND4I)xmixed crystals

Powdered potassium ammonium iodide mixed crystals with ammonium concentrations of 20% and 70% have been studied using deuteron NMR techniques. For the less doped sample the deuteron line starts to broaden and the spin-lattice relaxation time ${T}_{1}$ becomes nonexponential below about 20 K. These observations indicate the breakdown of ergodicity and the onset of orientational glass freezing. At the lowest temperatures ${T}_{1}$ reaches a plateau value. For the 70% sample a relatively abrupt onset of two-phase behavior at $T&lt;35\mathrm{K}$ is inferred from measurements of the spin-spin and spin-lattice relaxation times. These observations are taken to indicate an order-disorder transition to a crystalline phase which exhibits dynamically distinguishable lattice sites.

Application of tikhonov regularization to dipole glass relaxation function

Relaxation distribution function g(τ) was derived from the experimental data of dielectric measurements by use of the Tikhonov regularization method of solving for the inverse solution of the integral transform defining the relaxation time distribution g(τ) in the dipole glass system undergoing the glass freezing. The distribution function g(τ) allows to calculate the average or effective relaxation time (τ). The temperature dependence of (τ) was studied in DRADP-x (x = 0.4) dipole glass and found to follow the Vogel-Fulcher law of glass freezing with the static freezing temperature T 0≃16.5K.

Dynamic mechanical analysis—a powerful tool for the study of phase transitions

Dynamic Mechanical Analysis (DMA-7, PERKIN ELMER) is a very useful device for determining the thermal expansion and the low-frequency elastic compliances of crystals. The complex compliances can be measured in a frequency range from 10mHz to 50Hz and for temperatures between 6 and 1300K. A great advantage is that also very small samples (a 0.1 mm thick) can be measured. Here we give an overview of our recent DMA-measurements in different systems and discuss the main results: – In KSCN Entropy fluctuations were found for the first time at low frequencies. – In Cm a glass transition was detected and the freezing process was studied in detail. Moreover, a very unusual elastic hardening was observed. – In ALHS ferroelastic domain freezing was found for the first time. From these examples one can see, that a variety of phenomena, i.e. low-frequency dynamics, due to entropy fluctuations, glass freezing, domain freezing, etc. can be well studied by Dynamic Mechanical Analysis.

Anisotropic glass freezing in rubidium/ammonium dihydrogen phosphate mixed crystal and its deuterated analogue

Anisotropic glass freezing has been studied via dielectric measurements in the dipole glass systems of rubidium/ammonium dihydrogen phosphate RADP(x) and its perdeuterated analogue D-RADP(x), where x denotes the ammonium concentration. The glass freezing temperatures for a-cut and c-cut crystals are measured, and found to reveal very small anisotropy in the protonated sample of RADP(0.43) but a considerable anisotropy at low frequency in the deuterated sample of D-RADP(0.40). A higher glass freezing temperature was observed along the c-axis as compared with the a-axis freezing temperature for D-RADP(0.40) crystal. This conforms with the prediction of the cluster model theory for . An Arrhenius plot of the logarithmic relaxation frequency versus the reciprocal temperature for D-RADP(0.40) gave the deuteron activation energies along the a-axis and along the c-axis.

μSR on the novel heavy‐fermion system Nd 2-y Ce y CuO 4

The magnetic correlations in Nd2-yCeyCuO4 have been studied by zero and longitudinal field μSR. Nd1.8Ce0.2CuO4 reveals a saturation of the muon relaxation rate below 0.5 K. Even down to 70 mK LF spectra evidence spin fluctuations in the dynamic regime with a rate of \sim 109 s-1 excluding long range magnetic order or spin glass freezing. The average Nd moment is estimated to be \approx 0.2\muB, i.e., strongly reduced from the value determined for the ground state Kramers doublet of Nd2CuO4. Extending former μSR measurements on Nd2CuO4, a gradual enhancement of the internal field has been detected below 10 K which is accompanied by an increase of the relaxation rate. These results are attributed to the development of ordered Nd moments.

Non-Fermi-liquid and magnetic behaviour of the system

The system exhibits an unconventional Kondo effect with non-Fermi-liquid characteristics at low temperatures for . Measurements of the low-temperature electrical resistivity and magnetization of high-purity samples with have been made as part of an effort to determine whether the non-Fermi-liquid behaviour persists to low U concentrations, i.e., whether it is a single-ion effect. Irreversible behaviour in the magnetization, reminiscent of spin glass freezing, is observed for samples with , while long-range antiferromagnetic ordering has previously been established by neutron diffraction measurements for a sample with x = 0.45. Magnetic relaxation, ac magnetic susceptibility, and specific heat measurements were performed to investigate this unusual magnetic ordering.