Temperature-dependent Hysteresis Research Articles

Magnetization tunneling in an enneanuclear manganese cage

The enneanuclear mixed-valent manganese cage [Mn9O7(OAc)11(thme)(py)3(H2O)2] 1 {Mn9} possesses an S=17/2 ground state as a result of an antiferromagnetic interaction between three ferromagnetically coupled MnIV ions and a wheel of four MnIII and two MnII ions. AC magnetization measurements show a frequency-dependent out-of-phase signal at 3.4 K and 997 Hz, indicative of single-molecule magnetism behaviour. Data obtained from varying the frequency of oscillation of the AC field gives an effective energy barrier (Ueff) for the reversal of magnetization of 27 K. Magnetic measurements of single crystals of 1 at low temperature show time- and temperature-dependent hysteresis loops which contain steps at regular intervals of field. DC and AC relaxation measurements show both the temperature-dependent and temperature-independent regions, the latter being definitive evidence of magnetization tunneling in the lowest energy zero-field split component of the ground state.

Single-molecule magnets. A Mn12 complex with mixed carboxylate-sulfonate ligation: [Mn12O12(O2CMe)8(O3SPh)8(H2O)4

The synthesis and magnetic properties of a new member of the Mn12 single-molecule magnet family, [Mn12O12(O2CMe)8(O3SPh)8(H2O)4] (2), are reported. The compound was prepared by treatment of [Mn12O12(O2CMe)16(H2O)4] (1) with eight equivalents of PhSO3H in MeCN. Complex 2·4CH2Cl2 crystallizes in the triclinic space group P. The complex consists of a central [MnIV4O4] cubane held within a nonplanar ring of eight MnIII ions by eight μ3-O2− ions. Eight bridging acetate groups occupying equatorial sites, eight bridging benzenesulfonate groups in axial sites, and four terminal water molecules complete the peripheral ligation of the molecule. The spin of the ground state was established by magnetization measurements in the 2.00–7.00 T field range and 1.80–4.00 K temperature range. Fitting of the reduced magnetization data by full matrix diagonalization, incorporating only axial anisotropy, gave S = 10, g = 1.96 and D = −0.34 cm−1. The cluster exhibits out-of-phase ac susceptibility signals and temperature-dependent hysteresis loops at temperatures below 4.00 K, establishing 2 as a new single-molecule magnet.

Novel ferromagnetism in digital GaAs/Mn and GaSb/Mn alloys

Ferromagnetic III-Mn-V semiconductors are under intensive investigation for spintronic applications. In the present work digital alloys of GaAs/Mn and GaSb/Mn were fabricated by a combination of molecular beam epitaxy and atomic layer epitaxy. The Mn fraction in the layers was varied at constant III–V spacer thickness, and the III–V spacer-layer thickness was varied at constant Mn fraction (0.5 monolayer). Transmission electron micrographs showed good crystal quality with no evidence of three-dimensional (3D) Mn-V precipitates. The GaAs/Mn samples have Curie temperatures in the vicinity of 40 K and exhibit an anomalous Hall effect (AHE) similar to that seen in GaMnAs random alloys. These samples all show thermally activated resistance at zero field, characteristic of hopping conduction with evidence of Coulomb-gap-like behavior. The GaSb/Mn samples exhibit ferromagnetism (with temperature dependent hysteresis loops) and a strong AHE up to 400 K . At low temperatures the remanent magnetization initially drops rapidly with increasing temperature, indicative of Curie temperatures between 10 and 50 K depending on the Mn concentration. However, a substantial remanent magnetization persists to high temperature, suggesting a second phase with a Curie temperature above 400 K . These samples all showed essentially metallic behavior and weak negative magnetoresistance at low temperatures. These results are discussed in the context of a model of quasi-2D MnSb islands (for which there is direct evidence) embedded in a matrix of GaMnSb in the Mn-containing layers.

Three-dimensional Landau theory for multivariant stress-induced martensitic phase transformations. II. Multivariant phase transformations and stress space analysis

In this paper, the three-dimensional Landau model of austenite-martensite transformations constructed in Part I is generalized to include transformations between an arbitrary number of martensitic variants. The model can incorporate all temperature-dependent thermomechanical properties of both phases for arbitrary crystal symmetries, including higher-order elastic constants, and it correctly describes the characteristic features of stress-strain curves for shape-memory alloys and steels, namely, constant transformation strain tensors, constant or weakly temperature dependent stress hysteresis, and transformation at nonzero tangent moduli. Geometric representations of the conditions for phase equilibrium and phase transformations in six-dimensional stress space are developed. For the cubic-tetragonal phase transformation, equilibrium and transformation surfaces in three-dimensional stress space and the corresponding lines in the deviatoric-stress plane are found at various temperatures, and transformation processes are analyzed. All model parameters are obtained for the NiAl cubic-tetragonal phase transformation using the results of molecular dynamics simulations available in the literature.

Transition to superparamagnetism in a Cr-containing FINEMET-type alloy

The transition from a ferromagnetic to a superparamagnetic regime has been studied in a Cr-containing FINEMET-type alloy. Cr enhances the thermal stability of the alloy against nanocrystallization and reduces the Curie temperature of the remaining amorphous phase. Above room temperature, the temperature dependent hysteresis loop of a nanocrystalline sample can be described by a model of interacting superparamagnetic particles. The temperature dependence of the apparent magnetic moment of the particles obtained from the fittings is interpreted in an extended version of the model.

Preisach 모델을 이용한 ER유체의 온도별 히스테리시스 특성 고찰

This paper presents the identification of temperature-dependent hysteresis of an electro-rheological (ER) fluid under various operating temperatures using the Preisach model. As a first step, polymethylaniline (PMA) particles are prepared and mixed with silicone oil to make an ER fluid. A couette type electroviscometer is then employed to obtain the field-dependent shear stress. In order to show the suitability of the Preisach model to predict a physical hysteresis phenomenon of the ER fluid, two significant properties; the minor loop property and the wiping-out property are experimentally examined under three dominant temperature conditions. Subsequently, the Preisach model for the PMA-based ER fluid is identified using experimental multiple first order descending (FOD) curves. The effectiveness of the identified hysteresis model is verified in the time domain by comparing the predicted field-dependent shear stress with the measured one under the both specified and unspecified temperatures. In addition, the hysteresis model proposed in this work is compared to Bingham model.

Synthesis of CoCrFeO4 Nanoparticles Using Microemulsion Methods and Size-Dependent Studies of Their Magnetic Properties

Normal and reverse micelle microemulsion methods were used to synthesis single-phase CoCrFeO4 nanoparticles with a controlled size range of 6−16 nm and a size distribution of 14%. The size-dependent magnetic properties were characterized and found to agree well with Stoner−Wohlfarth theory. The results from nanoparticles prepared by the normal micelle and reverse micelle microemulsion methods were consistent with each other. The CoCrFeO4 nanoparticles did not display the types of anomalous magnetic properties reported for their bulk-phase material. Using CoCrFeO4 nanoparticles as precursors for the formation of a bulk sample resulted in a material that displayed an unusual susceptibility behavior but not the anomalous temperature-dependent hysteresis trend usually observed in conventional CoCrFeO4 bulk materials.

Above-room-temperature ferromagnetism in GaSb/Mn digital alloys

Digital alloys of GaSb/Mn have been fabricated by molecular-beam epitaxy. Transmission electron micrographs showed good crystal quality with individual Mn-containing layers well resolved, no evidence of three-dimensional MnSb precipitates was seen in as-grown samples. All samples studied exhibited ferromagnetism with temperature-dependent hysteresis loops in the magnetization accompanied by metallic p-type conductivity with a strong anomalous Hall effect (AHE) up to 400 K (limited by the experimental setup). The anomalous Hall effect shows hysteresis loops at low temperatures and above room temperature very similar to those seen in the magnetization. The strong AHE with hysteresis indicates that the holes interact with the Mn spins above room temperature. All samples are metallic, which is important for spintronics applications.

Modeling of temperature-dependent hysteresis within the framework of dynamical systems

It is demonstrated that temperature sensitive rate-independent deterministic hysteresis can be modeled within the framework of randomly perturbed fast dynamical systems with multiple attractors. By using the theory of large deviations, the mathematical machinery of identifying the attractors, through which such systems evolve in time, is presented.

Temperature dependence of AC-losses in Bi-2223 superconducting tapes

The losses of twisted multifilamentary Bi-2223/Ag tapes were measured in the temperature range between 4.2 K and the critical temperature T c∼110 K. The sample is placed in liquid helium (or in liquid nitrogen for measurements above 77 K), but is thermally insulated from the cooling bath. An ohmic heater is connected with the sample and its power is adjusted such that the desired temperature is reached. The loss measurement is performed by the standard magnetization technique. For a fixed AC-field amplitude the energy loss per cycle is determined, at a given temperature, as a function of frequency f. The extrapolation to f→0 gives the hysteresis loss. The measured total losses as a function of temperature show, at small field amplitudes, a minimum around 50 K. This is explained by the different temperature dependence of hysteresis and coupling losses. While coupling losses decrease with temperature, hysteresis losses increase for field amplitudes below the penetration field. A quantitative description based on the Bean model is included in the paper.

Temperature dependence of hysteresis and relaxation effects in hard magnetic materials

Hysteresis properties and viscosity effects have been studied at several temperatures (in the range from 140 to 500 K) on hard Sm(Co,Cu) 5 alloys. The application of a recently proposed theoretical approach permits one to identify the fluctuation field H f, connected to a typical activation volume, and the Preisach distribution, connected to microstructure. We found that: (i) at a single temperature, the model is able to reproduce the complex magnetization dynamics measured under pulsed field history excitation in a detailed fashion; (ii) the Preisach distribution for different temperatures can be rescaled onto a temperature-independent single curve through a temperature-dependent scale factor H A( T) equivalent to the anisotropy field.

Ferroelectricity in systems of induced and permanent dipoles: A Monte Carlo study

Dielectric and ferroelectric properties in systems of permanent and induced dipoles are calculated with a Monte Carlo method, the importance-sampling method. The systems are dipole chains and dipole planes, which can be looked at as subsystems of a perovskite structure. Depending on the system and the parameters we find spontaneous polarizations, ferroelectric domains, temperature-dependent hysteresis loops of the polarization, and a Curie–Weiss law for the susceptibility. These properties are mainly due to the cooperation of induced and permanent dipoles. The most difficult problem is the calculation of the spontaneous polarization, i.e., the calculation of the polarization without an applied field. In this case, a high iteration number is required.

Hysteresis of temperature dependences of thermopower and electrical resistivity in solid solutions on the basis of Eu ternary compounds

Hysteresis of temperature dependences of thermopower and electrical resistivity in solid solutions on the basis of Eu ternary compounds

Experimental and Theoretical Study of High Temperature-Stability and Low-Chirp 1.55 µm Semiconductor Laser with an External Fiber Grating

An experimental and theoretical study of the effect of temperature on the static and dynamic characteristics of packaged external fiber grating semiconductor lasers (FGL) is reported on. Operating in single frequency mode, the laser exhibits high output power (> 8 mW), high temperature stability of operating frequency (-3.4 GHz/K), and low static chirp (-60 MHz/mA). The observed hysteresis in wavelength versus temperature dependence is explained in the frame of a time-domain FGL model accounting for asymmetric nonlinear gain. The laser has low dynamic chirp (~16 MHz/mA) under 2.5 GB/s direct modulation, which is the key factor determining low penalty transmission over 312 km of SSM fiber. Dense WDM transmission performed at 2.6 Gbit/s over 117 km of SSM fiber shows that an FGL-based transmitter is a factor of 7 more tolerant to temperature variations than externally modulated DFB lasers.

Spontaneous twisting of an incommensurable improper ferroelastic Ba2NaNb5O15

The spontaneous twisting of a y-orientation single-crystal sample of barium-sodium niobate Ba2NaNb5O15 at a phase transition and into an incommensurable ferroelastic phase is investigated. Anomalies are found in the temperature dependence of the torsional deformation at temperatures 453, 533, and 573 K and anomalous hysteresis of the temperature dependences of the torsional deformation, measured on heating and cooling of the sample. The results obtained are discussed on the basis of an analysis of the temperature dependence of incommensurable 1q and 2q structures and ferroelastic twins.

Magnetization of the Layer Compounds AFeIIFeIII(C2O4)3 (A = Organic Cation), in Low and High Magnetic Fields: Manifestation of Néel N and Q Type Ferrimagnetism in a Molecular Lattice

Detailed bulk magnetization and magnetic susceptibility measurements are reported for representative examples of the series of ferrimagnetic tris-oxalato-ferrate(II,III) salts with general formula AFeIIFeIII(C2O4)3 (A = quaternary ammonium, phosphonium, or arsonium). The compounds all crystallize in two-dimensional hexagonal honeycomb lattices, but while some show conventional low field positive magnetization at low temperature (Néel type Q), others show a large negative magnetization below a compensation temperature Tcomp (Néel type N) under the same measurement protocol. Isothermal and temperature-dependent magnetizations are measured after zero field and field cooling. Temperature-dependent hysteresis measurements are also presented. In compounds that exhibit negative magnetization a discontinuity in the magnetization is observed at Tt (Tcomp < Tt < Tc) which correlates with the onset of a giant anisotropy and is indicative of a magnetostrictive transition. In compounds that exhibit positive magnetization, hysteresis behavior and frequency-dependent ac susceptibility data indicate a glassy magnetic order at low temperatures, possibly arising from the frustration of random anisotropy domains below a characteristic blocking temperature or imbalance and disorder of Fe(II) and Fe(III).

Temperature-dependent hysteresis in one-dimensional thermovisco-elastoplasticity

In this paper, we develop a thermodynamically consistent description of the uniaxial behavior of thermovisco-elastoplastic materials for which the total stress σ contains, in addition to elastic, viscous and thermic contributions, a plastic component σp of the form σp(x,t)=Ρ[e, θ(x,t)](x,t). Here ∈ and θ are the fields of strain and absolute temperature, respectively, and {Ρ[·, θ]}θ>0 denotes a family of (rate-independent) hysteresis operators of Prandtl-Ishlinskii type, parametrized by the absolute temperature. The system of momentum and energy balance equations governing the space-time evolution of the material forms a system of two highly nonlinearly coupled partial differential equations involving partial derivatives of hysteretic nonlinearities at different places. It is shown that an initial-boundary value problem for this system admits a unique global strong solution which depends continuously on the data.

On a System of Nonlinear PDEs with Temperature-Dependent Hysteresis in One-Dimensional Thermoplasticity

In this paper, we develop a thermodynamically consistent description of the uniaxial behaviour of thermoelastoplastic materials that are characterized by a constitutive law of the form σ(x,t)=P[ε,θ(x,t)](x,t), where ε,σ,θ denote the fields of strain, stress, and absolute temperature, respectively, and where {P[·,θ]}θ>0denotes a family of (rate-independent) hysteresis operators of Prandtl–Ishlinskii type, parametrized by the absolute temperature. The system of state equations governing the space-time evolution of the material are derived. It turns out that the resulting system of two nonlinearly coupled partial differential equations involves partial derivatives of hysteretic nonlinearities at different places. It is shown that an initial-boundary value problem for this system admits a global weak solution. The paper can be regarded as a first step towards a thermodynamic theory of rate-independent hysteresis operators depending on temperature.

A Hammerstein-based dynamic model for hysteresis phenomenon

The Hammerstein configuration, which includes a nonlinear static block followed by a linear dynamic block, is applied to model the rate-dependent and temperature-dependent hysteresis phenomenon. The nonlinear static block is realized by a modified Preisach model, which includes both the irreversible and reversible components of magnetization. The linear dynamic block is realized by a low-pass filter, which takes into account the rate-dependent effects of hysteresis. Temperature dependencies are incorporated into the model by fitting the model parameters as piecewise-linear functions of temperature. A procedure is described for the extraction of a single set of model parameters over the frequency, amplitude, and temperature ranges of interest. The theory is verified experimentally.

Electrical and optical properties of phenylene vinylene oligomers obtained by vacuum evaporation

Optical absorption, photoluminescence and electrical impedance of phenylene vinylene (PV) oligomer films deposited onto indium—tin oxide (ITO) substrates by evaporation in vacuum were studied in a wide range of temperatures (10–450 K) and a.c. frequencies (0.3–10 5 Hz). The material possesses an optical band of 2.8 eV. Photoluminescence spectra are Stokes shifted for 0.2 eV with respect to absorption and excitement luminescence. A.c. electrical properties of the material show some temperature-dependent hysteresis of the real part of dielectric permittivity, as well as several relaxation/conduction modes at low (− 100 to − 50 °C) and high (above 60 °C) temperatures.