Strain glass behaviour of Ni–Co–Mn–Sn ferromagnetic shape memory alloys

We report the direct experimental observations of the glassy behaviour in Ni–Co–Mn–Sn ferromagnetic shape memory alloys by doping sufficient substitutional point defect Co into the Ni sites (9 at%). The results showed that high level of Co doping had caused the complete suppression of the martensitic transformation and introduction of a strain glass transition in Ni–Co–Mn–Sn alloys. The strain glass transition was definitively characterized by the dynamic mechanical anomalies following the Vogel–Fulcher relationship and the signature nonergodicity of the frozen glass using a zero‐field‐cooled/field‐cooled heating measurement of static strain. The findings clarified the cause of vanishing of the martensitic transformation in Ni–Co–Mn–Sn alloy with high Co doping levels and the generality of glassy state in Ni–Mn based ferromagnetic shape memory alloys with high level of foreign elements doping. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Transverse magnetic ordering

There is an increasing number of ferromagnets and antiferromagnets which are observed to undergo either a further long range magnetic order or spin glass transition in components of the moment transverse to either the ferromagnetic or antiferromagnetic ordering direction. Necessary conditions include exchange frustration and some atomic disorder. We discuss the observation of transverse antiferromagnetic order in the ferromagnet (Fe, Mn)₃Si and the transverse spin glass phase observed in the ferromagnetic glassy metal a-(Fe, Zr) and the antiferromagnet γ-Mn–Cu.     

Tailoring the flow of soft glasses by soft additives

We examine the vitrification and melting of asymmetric star polymer mixtures by combining rheological measurements with mode coupling theory. We identify two types of glassy states, a single glass, in which the small component is fluid in the glassy matrix of the big one, and a double glass, in which both components are vitrified. Addition of small-star polymers leads to melting of both glasses, and the melting curve has a nonmonotonic dependence on the star-star size ratio. The phenomenon opens new ways for externally steering the rheological behavior of soft matter systems.     

Small activation entropy bestows high-stability of nanoconfined D-mannitol

It has been a long-standing puzzling problem that some glasses exhibit higher glass transition temperatures(denoting high stability) but lower activation energy for relaxations(denoting low stability). In this paper, the relaxation kinetics of the nanoconfined D-mannitol(DM) glass was studied systematically using a high-precision and high-rate nanocalorimeter.The nanoconfined DM exhibits enhanced thermal stability compared to the free DM. For example, the critical cooling rate for glass formation decreases from 200 K/s to below 1 K/s; the Tg increases by about 20 K–50 K. The relaxation kinetics is analyzed based on the absolute reaction rate theory. It is found that, even though the activation energy E~*decreases,the activation entropy S~*decreases much more for the nanoconfined glass that yields a large activation free energy G~*and higher thermal stability. These results suggest that the activation entropy may provide new insights in understanding the abnormal kinetics of nanoconfined glassy systems.     

Magnetic and electrical properties of the ZnGeAs<Subscript>2</Subscript>: Mn chalcopyrite

Doping of the ZnGeAs₂ semiconductor with manganese has produced compositions with spontaneous magnetization and high Curie temperatures of up to 367 K for the composition 3.5 wt% Mn. Their magnetic properties are characteristic of spin glasses at temperatures T < T _S and magnetic fields H < 11 kOe. In stronger fields, the spin glass state transforms into a phase with a spontaneous magnetization 4–5 times weaker than that to be expected under ferromagnetic ordering of all Mn ions. This is obviously a singly-connected ferromagnetic phase containing regions with frustrated bonds. The frustrated regions and the spin glass phase have inclusions of noninteracting ferromagnetic clusters, because these regions and the spin glass phase at low temperatures exhibit a strong increase in the magnetization M, with the dependence M(T) being described by the Langevin function. Measurements of the electrical resistivity ρ and the Hall effect have revealed that, for T < 30 K, the resistivity ρ of compositions with 1.5 and 3.5 wt % Mn is higher that at 30 K, which makes superexchange dominant and gives rise to the onset of the spin glass state. The nonuniform distribution of Mn ions in the spin glass phase accounts for the existence of isolated ferromagnetic clusters, their ferromagnetism being generated by carrier-mediated exchange. As the temperature increases still more, the increase in the mobility occurs faster than the decrease in the concentration, thus promoting an enhancement of the carrier-mediated exchange and growth of the ferromagnetic clusters in size, which at T = T _S come in contact. This signifies a transition from a multiply-to a singly-connected ferromagnetic phase, which contains microregions with frustrated bonds.     

Gels and glasses in a single system: evidence for an intricate free-energy landscape of glassy materials

In the free-energy landscape picture of glassy systems, their slow dynamics is due to a complicated free-energy landscape with many local minima. We show that for a colloidal glassy material multiple paths can be taken through the free-energy landscape. The evolution of the nonergodicity parameter shows two distinct master curves that we identify as gels and glasses. We show that for a range of colloid concentrations, the transition to nonergodicity can occur in either direction (gel or glass), accompanied by "hesitations" between the two. Thus, colloidal gels and glasses are merely global free-energy minima in the same free-energy landscape, and the paths leading to these minima can be complicated.     

Glassy dynamics of polymers confined to nanoporous glasses revealed by relaxational and scattering experiments

The glassy dynamics of poly(propylene glycol) (PPG) and poly(dimethyl siloxane) (PDMS) confined to a nanoporous host system revealed by dielectric spectroscopy, temperature-modulated DSC and neutron scattering is compared. For both systems the relaxation rates estimated from dielectric spectroscopy and temperature-modulated DSC agree quantitatively indicating that both experiments sense the glass transition. For PPG the segmental dynamics is determined by a counterbalance of adsorption and confinement effect. The former results form an interaction of the confined macromolecules with the internal surfaces. A confinement effect originates from an inherent length scale on which the underlying molecular motions take place. The increment of the specific-heat capacity at the glass transition vanishes at a finite length scale of 1.8 nm. Both results support the conception that a characteristic length scale is relevant for glassy dynamics. For PDMS only a confinement effect is observed which is much stronger than that for PPG. Down to a pore size of 7.5 nm, the temperature dependence of the relaxation times follows the Vogel-Fulcher-Tammann dependence. At a pore size of 5 nm this changes to an Arrhenius-like behaviour with a low activation energy. At the same pore size vanishes for PDMS. Quasielastic neutron scattering experiments reveal that also the diffusive character of the relevant molecular motions --found to be characteristic above the glass transition-- seems to disappear at this length scale. These results gives further strong support that the glass transition has to be characterised by an inherent length scale of the relevant molecular motions.Received: 1 January 2003, Published online: 14 October 2003PACS: 64.70.Pf Glass transitions - 77.22.Gm Dielectric loss and relaxation - 61.25.Hq Macromolecular and polymer solutions; polymer melts; swelling     

Long-lived ultrafast spin precession in manganese alloys films with a large perpendicular magnetic anisotropy

Spin precession with frequencies up to 280 GHz is observed in Mn(3-δ)Ga alloy films with a perpendicular magnetic anisotropy constant K(u)～15 M?erg/cm(3). The damping constant α, characterizing macroscopic spin relaxation and being a key factor in spin-transfer-torque systems, is not larger than 0.008 (0.015) for the δ=1.46 (0.88) film. Those are about one-tenth of α values for known materials with large K(u). First-principles calculations well describe both low α and large K(u) for these alloys.     

Muons as local probes of three-body correlations in the mixed state of type-II superconductors

The vortex glass state formed by magnetic flux lines in a type-II superconductor is shown to possess nontrivial three-body correlations. While such correlations are usually difficult to measure in glassy systems, the magnetic fields associated with the flux vortices allow us to probe these via muon-spin rotation measurements of the local field distribution. We show via numerical simulations and analytic calculations that these observations provide detailed microscopic insight into the local order of the vortex glass and more generally validate a theoretical framework for correlations in glassy systems.     

Mössbauer studies of polymers in the glassy state and near their glass transition

Mössbauer absorption spectra of glassy block and copolymers with lowT _G containing ferrocene have been recorded in a wide temperature range (4–330 K) to investigate the dynamical processes of the system, in the glassy state and around the glass transition. An inhomogeneous broadening due to conformational substates with different hyperfine parameters found at lower temperatures becomes motionally narrowed above 40 K. Whereas in the glassy state both the block and the copolymers reveal the same vibrational behaviour, we found differences in the vicinity of their respective glass transitions. For the block polymer an anomalous decrease of the recoilfree fractionf _A was found above the static glass transition. In agreement with neutron data on other polymers we attribute this behaviour to an onset of motions of the ferrocene units of higher than 10¹⁰ Hz. For the copolymers, however, a departure of lnf _A from linearity was detected already far below the staticT _G. We propose that this is caused by residual solvents which allow an easier cooperative rearrangement of the segments even belowT _G. First attempts to interpret the Mössbauer data by means of the predictions of the mode coupling theory are reported.     

The lithium ion conductor β-spodumene: an orientational glass

Polycrystals of β-spodumene were investigated using impadence spectroscopy for frequencies 0.1 Hz < ν < 235 GHz and for temperatures 5 K < T < 500 K. At high temperatures the Li ions are mobile and lead to dc and ac conductivity phenomena. These are analyzed in terms of the universal dielectric response and using the modulus formalism. The results obtained by both procedures are critically compared. At low temperatures or very high frequencies dielectric loss due to a dipolar freezing process is observed which bears a close resemblance to an orientational glass transition. To further confirm the glassy character of β--spodumene, we report measurements of the low temperature specific heat (3 K < T < 50 K) which provide evidence for the existence of excess contributions that are usually observed in amorphous materials.     

Magnetoelectric effect in an XY-like spin glass system Ni{x}Mn{1-x}TiO{3}

Magnetoelectric (ME) properties were investigated for an XY-like spin glass (SG) system, Ni{x}Mn{1-x}TiO{3} with an ilmenite structure. The ME effect is usually observed in systems with peculiar couplings between a crystal lattice and a magnetic order. Nonetheless, we found an antisymmetric ME effect with nonzero ME tensor elements below T{ME}=8-10 K in samples showing SG transitions. At T{ME}, no specific heat anomaly was observed, suggesting the absence of long-range magnetic order. We discuss the origin of the ME effect in the XY-like SG system in terms of an alignment of toroidal moments.     

Semiclassical approximation for the specific heat of non-crystalline solids at intermediate temperatures

The origin of the broad maximum in the specific heat (c _P) of glassy systems in the c _P/T ³ representation, in correspondence with the boson peak, has been described from a semiclassical perspective. A new model, based in the Debye approximation, taking into account the anharmonic vibrations of correlated atoms, is proposed. The theoretical predictions are shown to be in good agreement with the experimental data obtained in several glass samples in the temperature range 2–80 K.     

Low temperature elastic properties of a superconducting disordered metal

We have measured the elastic properties of a glass at audiofrequencies (0.2–1.5 kHz) in the temperature interval 0.01 to 10 K. In the superducting glassy metal Cu₆₀Zr₄₀ (T_c=0.31 K) both the sound velocity and attenuation are similar to that of an insulating glass below 0.05 K. Above that temperature (T_c>T>0.05 K) the relaxation process is mainly governed by electrons which are thermally excited above the BCS gap. Above the superconducting transition we observed new features in the sound velocity which were not observed in high frequency measurements.     

Zur Spinordnung von paramagnetischen Zusätzen in abschreckend kondensierten Schichten aus Supraleitern

The susceptibility of the dilute alloys Pb-Mn, Cu-Mn, Sn-Mn, and La-Gd is measured in the temperature range from 2 to 300 °K. The dilute alloys are produced by simultaneous evaporating of both components on a substrate at 14 °K. In this way one gets a statistic distribution of the paramagnetic impurities in the solvent. The Mn atoms, dissolved in Pb, obeyCurie's law. The paramagnetic Curie temperature equals zero at every chosen concentration for the mentioned systems. Mn in Pb shows an effective magnetic moment of 3.9 Bohr magnetons in the concentration range from 2 to 10 at% Mn. At low temperature there exists a quasi-antiferromagnetic ordering among the spins of the Mn atoms. The temperature at which this ordering appears, linearly increases with the concentration of Mn. With regard to the systems of Cu-Mn, Sn-Mn, and La-Gd we also find an antiferromagnetic spin ordering. The transition temperature of superconductivity is depressed by adding Mn to Pb. The decrease disappears when the Pb-Mn films are annealed to 290 °K. This agrees with the fact that Mn loses its magnetic moment after annealing the films to room temperature.     

A comparison of glass forming ability in Ag-Si and Au-Si alloys

Ag-Si alloys of various compositions were produced by laser-induced melting and quenching of vapour-deposited thin films. Despite cooling rates approaching 10¹⁰K/s the procedure fails to yield glassy phases for all compositions except those around 80 at.-% Si. This behaviour contrasts with that of the Au-Si system which under similar experimental conditions yields glasses at almost arbitrary concentrations. The difference in glass forming ability of these two systems, which have similar phase diagrams, is explained in terms of the difference in the heat of mixing of the liquid, and its consequent effect on both the thermodynamics and kinetics of these alloys.     

Precursor phenomena in frustrated systems

To understand the origin of the dynamical transition, between high-temperature exponential relaxation and low-temperature nonexponential relaxation, that occurs well above the static transition in glassy systems, a frustrated spin model, with and without disorder, is considered. The model has two phase transitions, the lower being a standard spin glass transition (in the presence of disorder) or fully frustrated Ising (in the absence of disorder), and the higher being a Potts transition. Monte Carlo results clarify that in the model with (or without) disorder the precursor phenomena are related to the Griffiths (or Potts) transition. The Griffiths transition is a vanishing transition which occurs above the Potts transition and is present only when disorder is present, while the Potts transition which signals the effect due to frustration is always present. These results suggest that precursor phenomena in frustrated systems are due either to disorder and/or to frustration, giving a consistent interpretation also for the limiting cases of Ising spin glass and of Ising fully frustrated model, where also the Potts transition is vanishing. This interpretation could play a relevant role in glassy systems beyond the spin systems case.     

Magnetic properties of a new BaO.Fe2O3.Na2O glass

Homogeneous BaO.Fe₂O₃.Na₂O glass containing 60% Fe₂O₃ is prepared by splat cooling technique. X-ray diffraction reveals the presence of a few small βNaFeO₂ crystallites in a glassy matrix. DTA studies show that βNaFeO₂ cristallizes first and BaFe₁₂O₁₉ next. In the temperature range 130K < T < 400K this glass shows Curie-Weiss behaviour with large negative Weiss constant. The magnetic susceptibility measurements below 130K exhibit a broad maximum near 90–110K. Mössbauer study reveals that the glass mainly consists of tetrahedral network of Fe³⁺O₄ and a hyperfine structure at low temperature; magnetic ordering temperature estimated is about (125 ± 5)K.     

LaGa-based bulk metallic glasses

We report the formation of La Ga-based bulk metallic glasses. Ternary La–Ga–Cu glassy rods of 2–3 mm in diameter can be easily formed in a wide composition range by the conventional copper mold casting method. With minor addition of extra elements such as Co, Ni, Fe, Nb, Y, and Zr, the critical diameter of the full glassy rods of the La–Ga–Cu matrix can be markedly enhanced to at least 5 mm. The characteristics and properties of these new La Ga-based bulk metallic glasses with excellent glass formation ability and low glass transition temperature are model systems for fundamental issues investigation and could have some potential applications in micromachining field.