Glass Freezing Research Articles

Evaluation of magnetic inhomogeneities in non-stoichiometric Mg0.5Ca0.5Fe2O4 nanoferrite

Present work reports a systematic study on the evaluation of magnetic inhomogeneities in non-stoichiometric Mg0·5Ca0·5Fe2O4 nanoferrite (MCNF) by conducting exhaustive dc-magnetization, ac-susceptibility and 57Fe Mössbauer spectroscopic measurements and exchange bias investigations using training protocol down to 6 K. Rietveld fitting to PXRD established the formation of anticipated spinel fcc phase of MCNF (non-stoichiometric) along with a minute impurity phase of calcite. Scherrer method and HRTEM micrographs illustrated broad size distribution of MCNF nanoparticles with an average nanocrystallite size of ∼15 nm. Combined 57Fe Mössbauer spectroscopic and dc-magnetization analysis establishes coexistence of ferrimagnetic (67 %) & superparamagnetic (33 %) states at 300 K with notable Ms = 22 emu/g, Mr = 4 emu/g & Hc = 130 Oe and blocking of most of the nanoparticles of MCNF below 300 K. The coercivity followed the size-modified Kneller law for ferrimagnetic nanoparticles and the saturation magnetization abides the Bloch law. Moreover the frequency-dependent ac-susceptibility investigations revealed two magnetic transitions: (i) A transition at ∼ 330 K in the low frequency data attributed to the relaxation of blocked particles of bigger sizes under the superparamagnetic (SPM) regime and (ii) an irregularity at low temperatures is assigned to surface spin glass freezing. Surface spin glass freezing was further affirmed by the ageing experiments and dynamic scaling law. Furthermore, even the best fit to the dynamic scaling couldn't assert the existence of conventional spin glass phase due to slower spin-flip time of surface spins. A soft ferrimagnetic core of MCNF is enveloped with disordered surface spins, which manifest spin glass state. Concurrently, the findings of exchange bias at 30 K and training effect at 6 K affirmed that MCNF nanoparticles are presenting themselves as FM core- SG shell system. Our experimental findings suggested magnetic inhomogeneities comprised of superparamagnetism, ferrimagnetism and disordered surface spin glass state in the non-stoichiometric MCNF.

Efficient Synthesis of Highly Crystalline One-Dimensional CrCl3 Atomic Chains with a Spin Glass State.

One-dimensional (1D) magnetic material systems have attracted widespread interest from researchers because of their peculiar physical properties and potential applications in spintronics devices. However, the synthesis of 1D magnetic atomic chains has seldom been investigated. Here, we developed an iodine-assisted vacuum chemical vapor-phase transport (I-VCVT) method, utilizing single-walled carbon nanotubes (SWCNTs) with 1D cavities as templates, and high-quality and high-efficiency fabrication of 1D atomic chains of CrCl3 was achieved. Furthermore, the structure of CrCl3 atomic chains in the confined space of SWCNTs was analyzed in detail, and the charge transfer between the 1D atomic chains and SWCNTs was investigated through spectroscopic characterization. A comprehensive study of the dynamic magnetic properties revealed the existence of spin glass states and freezing of the 1D CrCl3 atomic chains at around 3 K, which has never been seen in bulk CrCl3. Our work established an effective strategy for the control synthesis of 1D magnetic atomic chains with promising potential applications in further magnetic-based spintronics devices.

The influence of polydextrose on freeze-dried unripe acerola (Malpighia emarginata DC.) by the concept of a state diagram

Unripe acerola powders obtained from freeze-drying without (AV) and with polydextrose (AP) were characterized by using differential scanning calorimetry (DSC) and vapor sorption isotherms at (25, 35, and 45) °C in water activity ( a w ) range of 0.10–0.90. State diagrams built from DSC results, such as the glass transition ( T g ) and freezing ( T m ) points, were compared with those of the sorption isotherms. The Gordon-Taylor equation presented a good fit for T g , in which the solids glass transition increased and the k parameter decreased in the presence of polydextrose. The isotherms obtained were Brunauer-Emmet-Teller (BET) type III, and the Guggenheim-Anderson-de Boer (GAB) equation fitted to the experimental data showed slightly higher monolayer water content for AP, and not affected by temperature. Although there was a divergence in the stability when comparing the criteria of a w and T g , polydextrose increased the a w and T g of freeze-dried unripe acerola and conferred a stabler powder. • The stability criterion between water activity and glass transition was studied. • State diagrams of unripe acerola with polydextrose were built. • Polydextrose used as a drying aid increased the glass transition temperature. • Sorption isotherms of unripe acerola powders were BET Type III.

Ferroic superglasses: Relaxor ferroelectrics PMN and SBN vs. CoFe superspin glass

Random electric fields (RF) due to charge disorder are at the origin of polar nanoregions (PNR) in the cubic relaxor PbMg1/3Nb2/3O3. They initiate Lacroix-Béné-type Cole-Cole semicircles, which achieve Cole-Davidson skew and dynamic power law criticality upon approaching superdipolar glass freezing. Below Tg ≈ 240 K percolation of PNR into microdomains is evidenced by interfacial creep and relaxation via Cole-Cole diagrams. Similar behavior occurs in uniaxial Sr0.8Ba0.2Nb2O6 (Tg ≈ 301 K). In contrast, matrix isolated ferromagnetic nanoparticles in CoFe/Al2O3 multilayers without RF interaction lack all of these effects except superglassy critical dynamics at Tg ≈ 46 K.

Dielectric relaxation in pure and doped with Cu lead germanate single crystal

Dielectric permittivity of pure and doped with copper lead germanate Pb5Ge3O11 (PGO) crystal was studied in wide frequency (20 Hz to 3 GHz) and temperature (120 K to 450 K) range. The ferroelectric phase transition in PGO crystals is mixed “order-disoder” and displacive. Second order phase transition temperature shifts to lower temperature with increase the concentration of Cu2+ dopant. At low temperatures the dielectric relaxation caused by off center Cu2+ ion dynamics was detected. Dielectric dispersion in PGO:Cu crystals is attributed to domain wall dynamics and dipole glass freezing. It is shown that the domain relaxation requires lower energy then for the random freezing dipoles.

Microbial cell disruption methods for efficient release of enzyme L-asparaginase

The efficacy of a simple laboratory method for cell disruption based on the glass bead stirring, sonication, osmotic shock, freezing and grinding, or use of solvents and detergents was assessed in this study, via measurements of the release of total protein and L-asparaginase activity. Three different microbial sources of L-asparaginase were used: Escherichia coli BL21 (DE3), Leucosporidium muscorum, and Aspergillus terreus (CCT 7693). This study adjusted and identified the best procedure for each kind of microorganism. Sonication and glass bead stirring led to obtaining filamentous fungus cell-free extracts containing high concentrations of soluble proteins and specific activity; however, sonication was the best since it obtained 4.61 ± 0.12 IU mg−1 after 3 min of operation time. Mechanical methods were also the most effective for yeast cell disruption, but sonication was the technique which yielded a higher efficiency releasing 7.3 IUtotal compared to glass bead stirring releasing 2.7 IUtotal at the same operation time. For bacterium, sonication proved to be the best procedure due to getting the highest specific activity (9.01 IU mg−1) and total enzyme activity (61.7 IU). The data presented lead to conclude that the mechanical methods appeared to be the most effective for the disintegration of the all microbial cells studies. This is the first report related to the experimental comparison of L-ASNase extraction procedures from different microorganisms, which can also be used for extracting periplasm located enzymes from other organisms.

DMA study of water's glass transition in nanoscale confinement.

Dynamic mechanical analysis (DMA) measurements of water confined in nanoporous silica have been performed as a function of temperature and frequency for different pore sizes (2.5-10 nm) at heating and cooling. Most of the data show three processes, P1, P2 and P3, where P1 and P2 depend on measurement frequency and P3 does not. The characteristic shift of P3 with pore size shows that this process corresponds to freezing/melting of "internal water", i.e. in the core of the pores. Thermal expansion data indicate - in agreement with e.g. [A. Taschin, P. Bartolini and R. Torre, Meas. Sci. Technol., 2017, 28, 014009] - that in all our nanoporous systems about 2 layers of water remain liquid much below the freezing point. Dynamic elastic measurements show clear signatures of glass freezing of this supercooled water in the vicinity of P1. Extrapolating the DMA data to the timescale (103 s) of adiabatic calorimetry unveils a systematic behaviour: P1(T) shows a clear size dependence for a broad range of pore diameters, i.e. 2.5 nm ≤ d ≤ 52 nm, implying (together with the corresponding activation energy 0.5 eV) that P1 corresponds to the glass-liquid transition of a few layers of supercooled water at Tg(d). An extrapolation of Tg(d) to d → ∞ yields Tg(∞) ≈ 136 K, the traditional value for bulk water. The small (liquid like) value of Young's modulus in a temperature region above P1 is most naturally explained assuming that the supercooled water in this range is still liquid, implying that Tg values of 160 K or even 210 K - as suggested by various authors - are unlikely.

Freezing in relaxor ferroelectrics and dipolar glasses

A recently proposed semi-phenomenological model of freezing in relaxor ferroelectrics, based on the concept of polar nanoregions (PNRs) embedded in a polarizable medium, is reviewed. A generalized Landau-type free energy for the medium is discussed, where the medium polarization couples linearly to the PNR polarization. When the fourth-order Landau coefficient is negative (b < 0), the correlation radius rc, which measures the PNR size, depends on the temperature T and the applied field E. As T is lowered or E increased, rc increases and the volume of a cluster of PNRs grows until the percolation limit is reached. This leads to a generalized expression for the Vogel-Fulcher (VF) relaxation time with a field-dependent VF freezing temperature T0(E). The case b > 0, in which the percolation mechanism cannot be realized, is considered to be appropriate for dipolar glasses.

Antiferromagnetic spin glass-like behavior in sintered multiferroic Aurivillius Bim+1Ti3Fem−3O3m+3 compounds

The structure, hyperfine interactions and magnetic properties of the series of multiferroic Bim+1Ti3Fem−3O3m+3 Aurivillius compounds with m=4–8 were studied using X-ray diffraction, 57Fe Mössbauer spectroscopy and vibrating sample magnetometry. Samples were prepared by the conventional solid-state sintering method. Bulk magnetic measurements showed that for m=4 the compound is paramagnetic down to 2K while in the compound with m=5 the antiferromagnetic type transition was observed at 11K. In the case of compounds with m=6–8 much more complex magnetic behavior was found. For these compounds a gradual spin freezing and antiferromagnetic spin glass-like ordering were observed on decreasing temperature. The temperature of spin glass freezing was determined as 260, 280 and 350K for m=6, 7 and 8, respectively. Room-temperature Mössbauer spectra of all the compounds studied confirm their paramagnetic state. However, liquid nitrogen and liquid helium temperature measurements reveal magnetic ordering with a residual paramagnetic phase contribution for the compounds with m=5–8.

Thermodynamic Properties of κ-(BEDT-TTF)2X Salts: Electron Correlations and Superconductivity

Heat capacity measurements of κ-(BEDT-TTF)2X (BEDT-TTF: Bis(ethylendithio) tetrathiafulvalene, X: counteranions) which are classified as two-dimensional (2D) dimer-Mott system are reported. At first, we explain structural and electronic features originated from rigid dimerization in donor arrangement in 2D layers. The antiferromagnetic Mott insulating phase located at low-pressure region in the phase diagram shows vanishing γ electronic heat capacity coefficient in the heat capacity, which claims opening of a charge-gap in this insulating state. Then, a systematic change of the γ around the Mott boundary region is reported in relation to the glass freezing of ethylene dynamics. The thermodynamic parameters determined by ∆Cp/γTc of 10 K class superconductors, κ-(BEDT-TTF)2Cu(NCS)2 and κ-(BEDT-TTF)2Cu[N(CN)2]Br demonstrate that a rather large gap with a strong coupling character appears around the Fermi-surface. On the other hand, the low temperature heat capacity clearly shows a picture of nodal-gap structure due to an anisotropic pairing. The comparison with lower Tc compounds in the κ-type structure is also performed so as to discuss overall features of the κ-type superconductors. The heat capacity measurements of hole-doped systems containing mercury in the counteranions show an anomalous enhancement of γ, which is consistent with the T1−1 of NMR experiments etc. The results of heat capacity measurements under high pressures are also reported.

Protein Structure in Amorphous Solid Phase

Protein-based medicines often require freezing or stabilization in carbohydrate glasses for storage prior to use. The structural stability of these proteins is of the great importance in the conditions required for pharmaceutical purposes. Problems involving aggregation and stability of the protein in freeze-dried formulations are of the challenges for the pharmaceutical industry.Small-angle neutron scattering (SANS) is uniquely qualified to study the structure of proteins in the liquid and solid phases that are biotechnologically relevant for proteins. The structural and conformational changes of a model protein, lysozyme, during the destabilizations in water- ice and carbohydrates systems were studied using SANS and MD simulations. X-ray diffraction measurements were performed to verify existence of cubic and hexagonal ice structures in protein-ice system. Measurements and modelling efforts to understand protein structural changes will be discussed and the interaction distances measured by SANS and proposed model protein structures in different carbohydrate glasses will be compared.

Finite size versus surface effects on magnetic properties of antiferromagnetic particles

The observation of finite magnetic moment in antiferromagnetic materials is quite unusual and has been immensely investigated in nanoparticle systems. Here, the structural and magnetic properties of NiO particles are explored by x-ray diffraction, extended x-ray absorption fine structure, and magnetization measurements. Using similar-sized particles with different surface defect structure, we show that the observed magnetic enhancement, which is present even beyond finite-size limit, is due to the surface effects. However, the well known spin glass freezing is found to occur only in nano-regime.

Magnetic properties of Mn-oxide nanoparticles dispersed in an amorphous SiO2 matrix

Samples of Mn-oxide nanoparticles dispersed in an amorphous SiO2 matrix with manganese concentration 0.7 and 3at% have been synthesized by a sol–gel method. Transmission electron microscopy analysis has shown that the samples contain agglomerates of amorphous silica particles 10–20nm in size. In silica matrix two types of Mn-rich particles are dispersed, smaller nanoparticles with dimensions between 3 and 10nm, and larger crystalline areas consisting of aggregates of the smaller nanoparticles. High-temperature magnetic susceptibility study reveals that dominant magnetic phase at higher temperatures is λ-MnO2. At temperatures below TC=43K strong ferrimagnetism originating from the minor Mn3O4 phase masks the relatively weak magnetism of λ-MnO2 with antiferromagnetic interactions. Magnetic field dependence of the maximum in the zero-field-cooled magnetization for both the samples in the vicinity of 40K, and a frequency shift of the real component of the ac magnetic susceptibility in the sample with 3at% Mn suggest that the magnetic moments of the smaller Mn3O4 nanoparticles with dimensions below 10nm are exposed to thermally activated blocking process just below the Curie temperature TC. Appearance of a maximum in the zero-field-cooled magnetization for both the samples below 10K indicates possible spin glass freezing of the magnetic moments at low temperatures which might occur in the geometrically frustrated Mn sublattice of the λ-MnO2 crystal structure.

Dynamic mechanical analysis of confined glass-forming liquids

Acoustic spectroscopy is a valuable tool, when the degrees of freedom of a material couple to strain. Since this is the case for molecular glass-forming liquids the dynamic mechanical response is a quantity of primary interest here. Dynamic mechanical analysis measurements (0.1–100 Hz) have been performed for salol, toluene and ortho-terphenyl confined in mesoporous (2.5–10 nm) silica matrices. Important parameters (relaxation times τ of the slow α-relaxation, change of T g with pore size, number and size of dynamically correlated molecules, etc.) characterizing the glass freezing in the three glass-forming liquids have been extracted from the measurements for various confining pore sizes. A comparison with literature data shows excellent agreement and demonstrates that mechanical measurements provide a very sensitive tool to probe the dynamics of molecular glass forming liquids in confined systems.

Magnetic ordering in ternary germanide Nd 2CuGe 6

We have performed a systematic investigation on a well-annealed polycrystalline Nd 2CuGe 6 sample by measuring its AC and DC susceptibility χ( T), magnetization M( H), magnetic relaxation M( t), electrical resistivity ρ( T) and specific heat C( T). The sharp peak observed at about T N = 9.4 K in χ AC( T), χ DC( T) and C( T) curves, respectively, indicates the occurrence of an antiferromagnetic order, which is further confirmed by the sudden decrease in ρ( T) at the same temperature. In the magnetically ordered state, M( H) shows an evident metamagnetic transition. Moreover, the χ DC( T) and M( t) measurements reveal evident irreversible magnetism and long-time magnetic relaxation effect below T N indicating the metastable characters of the magnetically ordered state. The formation of the metastable magnetic state cannot be explained as glass freezing of the neodymium magnetic moments in the Ce 2CuGe 6-type orthorhombic Nd 2CuGe 6 sample.

Nonlinear Screening Theory of the Coulomb Glass

A nonlinear screening theory is formulated to study the problem of gap formation and its relation to glassy freezing in classical Coulomb glasses. We find that a pseudogap ("plasma dip") in a single-particle density of states begins to open already at temperatures comparable to the Coulomb energy. This phenomenon is shown to reflect the emergence of short-range correlations in a liquid (plasma) phase, a process which occurs even in the absence of disorder. Glassy ordering emerges when disorder is present, but this occurs only at temperatures roughly an order of magnitude lower. Our result demonstrate that the formation of the plasma dip at high temperatures is a process distinct from the formation of the Efros-Shklovskii pseudogap, which in our model emerges only within the glassy phase.

μSRStudy of the Quantum Dynamics in the FrustratedS=32Kagomé Bilayers

We present muSR experiments in the S=3/2 kagomé bilayer compound Ba(2)Sn(2)ZnGa(10-7p)Cr(7p)O22 [BSZCGO(p)] and compare it to the isostructural SrCr(9p)Ga(12-9p)O19 [SCGO(p)], including for the latter new results for p > or =0.89. Quantum-dynamical low energy magnetic excitations are evidenced in this novel compound. We study the evolution of the muon relaxation rate with p, T, and field. A phenomenological model for the muon relaxation based on sporadic dynamics due to spin excitations in a singlet sea proposed by Uemura et al. is extended to all fields and T range. Its connection to the RVB picture is discussed, and we argue that such coherent states might mediate the interactions between "impurities" which induce the spin glass freezing.

Structure and physical properties of RE2Ge 2In ( RE = La, Ce, Pr, Nd)

The rare earth compounds RE 2Ge 2In ( RE = La , Ce, Pr, Nd) were synthesized by arc-melting the elements under argon and investigated by X-ray diffraction on both powders and single crystals. The structures of the praseodymium and neodymium compounds have been refined on the basis of the single crystal data: Mo 2FeB 2 type, P 4 / m b m , a = 753.2 ( 1 ) pm , c = 434.69 ( 6 ) pm , w R 2 = 0.0701 , 272 F 2 values for Pr 2Ge 2In and a = 749.6 ( 1 ) pm , c = 432.49 ( 7 ) pm , w R 2 = 0.0457 , 317 F 2 values for Nd 2Ge 2In, with 12 variable parameters for each refinement. These structures can be considered as intergrowths of distorted AlB 2 and CsCl related slabs of compositions REGe 2 and REIn, respectively. The Ge Ge distances within the AlB 2 slabs are 253 pm for both Pr 2Ge 2In and Nd 2Ge 2In. Magnetic measurements have indicated that La 2Ge 2In is a diamagnet, whereas Ce 2Ge 2In, Pr 2Ge 2In and Nd 2Ge 2In exhibit localized magnetism of RE 3+ ions with complex behavior at the lowest temperatures hinting at canted antiferromagnetism or spin glass freezing. All four compounds studied show metallic electrical conductivity.

Ferromagnetic cluster glass behavior inU2IrSi3

Measurements of the ac susceptibility, dc magnetization, magnetic relaxation, specific heat, and electrical resistivity of ${\mathrm{U}}_{2}{\mathrm{IrSi}}_{3}$ are reported. The investigated compound crystallizes in an ${\mathrm{AlB}}_{2}$-derived hexagonal structure and undergoes a ferromagnetic cluster glass transition at ${T}_{f}=10.5\mathrm{K}.$ Typical features of the ferromagnetic cluster glass state include (i) a frequency-dependent cusp in the ac susceptibility, (ii) an irreversibility in the temperature dependence of the dc magnetization, (iii) a very slow decay of the remanence, (iv) a small jump in the initial magnetization curve, and (v) a small specific heat anomaly, which contains a very small amount of magnetic entropy. The specific heat data also demonstrate a large value of the linear specific heat coefficient and the absence of true long-range magnetic order. Furthermore, a Kondo-effect-like behavior is observed in the temperature dependence of the electrical resistivity. It seems that both cluster glass freezing and Kondo-lattice effects make an impact on the low-temperature physical properties of ${\mathrm{U}}_{2}{\mathrm{IrSi}}_{3}.$

Acoustic Spectroscopy Near Phase Transitions

Here we give an overview about out recent elastic susceptibility measurements in different single crystals and discuss the main results - In KSCN entropy fluctuations were found at low frequencies - In C60 a glass transition was detected and the freezing process was studied in detail - In SrTiO3 and KMnF3 we have detected ferroelastic domain wall motion below Tc . - In SrTiO3 nonlinear elastic anomalies were found in the quantum paraelectric region. From these examples it becomes clear, that a variety of phenomena, i.e. low frequency dynamics due to entropy fluctuations, glass freezing, domain wall motion, soliton motion etc. can be well studied by acoustic spectroscopy, i.e. measurements of the complex dynamic elastic susceptibility in a broad temperature and frequency range.