Thermal Hysteresis Loop Research Articles

Generation of radially polarized beams based on thermal analysis of a working cavity

The laser oscillation and polarization behavior of a side-pumped Nd:YAG laser are studied theoretically and experimentally by a thermal model for a working cavity. We use this model along with the Magni method, which gives a new stability diagram, to show important characteristics of the resonator. High-power radially and azimuthally polarized laser beams are obtained with a Nd:YAG module in a plano-plano cavity. Special regions and thermal hysteresis loops are observed in the experiments, which are concordant with the theoretical predictions.

Time-domain terahertz spectroscopy of spin state transition in [Fe(NH2 − trz)3]2+ spin crossover compounds

We have measured the evolution of the THz spectrum of iron(II) spin crossover compounds within the low-spin/high-spin thermal hysteresis loop in the 0.6–6 THz frequency range. This study enabled to follow both the variations of the refractive optical index and absorption during the spin state transition. Marked absorptions centered ∼2 – 3 THz and ∼5 THz shifting with the spin state are revealed. Our work provides a means to store optically information and to read it out in the THz domain and also offers indications about the structural evolution occurring during the spin state transition.

Synthesis and characterizations of microwave sintered ferrite powders and their composite films for practical applications

Phase pure single phase ferrite powders of (NixR1−x)0.5Zn0.5Fe2O4 (R=Mn, Co, Cu; x=0, 0.5) were manufactured using microwave sintering at 930°C for 10min in air atmosphere. The powders were characterized for their structure, microstructure, thermal, and magnetic properties. Selected powders were used as fillers to prepare their composite films using polymethyl methacrylate polymers as matrix. The composite films were prepared using the melt blending approach and were tested for their microstructure, thermal, and magnetic hysteresis loop as well as 3D magnetic field space mappings using an electromagnetic compatibility scanner. Among the studied ferrites, cobalt doped ferrites and their composites showed the best electromagnetic interference (EMI) shielding effectiveness value and have potential for practical EMI shielding applications.

Graphical Abstract: Eur. J. Inorg. Chem. 18/2011

AbstractThe cover picture shows the influence of the bridging ligand in iron(II) 1D coordination polymers on the spin‐transition properties of those complexes. Rigid linkers like 4,4′‐bipyridine increase the cooperative interactions between the spin‐crossover centres, leading to thermal hysteresis loops. In contrast to this, flexible linkers like 1,3‐bis(4‐pyridyl)propane support the formation of zigzag chains with restraining interactions between the 1D chains that lead to stepwise spin transitions. Details are discussed in the article by B. Weber et al. on p. 2803 ff.

Two‐Step versus One‐Step Spin Transitions in Iron(II) 1D Chain Compounds (Eur. J. Inorg. Chem. 18/2011)

AbstractThe cover picture shows the influence of the bridging ligand in iron(II) 1D coordination polymers on the spin‐transition properties of those complexes. Rigid linkers like 4,4′‐bipyridine increase the cooperative interactions between the spin‐crossover centres, leading to thermal hysteresis loops. In contrast to this, flexible linkers like 1,3‐bis(4‐pyridyl)propane support the formation of zigzag chains with restraining interactions between the 1D chains that lead to stepwise spin transitions. Details are discussed in the article by B. Weber et al. on p. 2803 ff.

Photo-switchable spin-crossover iron(III) compound based on intermolecular interactions

Iron(III) spin-crossover compounds, [Fe(qnal)2]CF3SO3·MeOH (1·MeOH) and [Fe(qnal)2]CF3SO3·acetone (1·acetone) were prepared and their spin transition properties were characterized by magnetic susceptibility measurement, Mossbauer spectroscopy and single crystal analysis. Two iron(III) compounds exhibited abrupt spin transition with thermal hysteresis loop (T 1/2↑ = 115 K and T 1/2↓ = 104 K for 1·MeOH, and T 1/2↑ = 133 K and T 1/2↓ = 130 K for 1·acetone). Single crystal analysis revealed both of the structures in high-spin (HS) and low-spin (LS) states for 1·acetone. The difference of bond length between the HS and LS states for 1·acetone was ~0.10 A, which was corresponding to that of typical iron(III) SCO compounds. Specially, it showed strong intermolecular interactions by π–π stacking formed between the neighbor complexes leading to 2-D sheet. Both 1·MeOH and 1·acetone exhibited LIESST effect when it was illuminated at 1000 nm. We also confirmed that the introduction of strong intermolecular interactions, such as π–π stacking, can play an important role in LIESST effect.

Microstructures and thermochromic properties of tungsten doped vanadium oxide film prepared by using VOX–W–VOX sandwich structure

Abstract Tungsten doped vanadium oxide (VO X ) thin films were prepared by oxygen annealing VO X –W–VO X sandwich layers. X-ray photoelectron spectroscopy, X-ray diffraction and field emission scanning electron microscope were employed to characterize the compositions, crystal structures and surface morphologies, respectively. It was demonstrated that sandwich structure suppressed the crystallization of VO X , and that V 5+ was reduced by diffused W atom to V 4+ . The results of surface morphologies indicated that the grain arrangement of W doped vanadium dioxide film exhibited some regular patterns compared with the random grain distribution of undoped film. Electrical measurements showed that the square resistance of V 2 O 5 film and semiconductor-metal transition temperature of VO 2 –V 2 O 5 film decreased obviously after W doping. In addition, thermal hysteresis loop was observed in W doped V 2 O 5 film with thick W middle layer. The investigation of optical properties indicated that the optical band gap of W doped V 2 O 5 film decreased with the increase of thickness of W middle layer, and the optical switching performance in the near-infrared range of VO 2 –V 2 O 5 slightly weakened after W doping.

Iron(II) spin crossover complexes with branched long alkyl chain

The bzimpy iron(II) complexes, 1– 3, containing branched long alkyl chains were synthesized and characterized in detail. The temperature-dependant magnetic susceptibility of 1 showed gradual spin crossover behavior from low spin to high spin state, while 2 retained only low spin state in the same condition. Interestingly, 3 displayed an abrupt spin transition in temperature range from T 1/2↑ = 236 K to T 1/2↓ = 230 K with the thermal hysteresis loop about 6 K. The differential scanning calorimetric analysis of 3 revealed two species of liquid crystal phase transitions at 236 K and 351 K, respectively.

Aging in the electrical properties of nanostructured vanadium oxide thin film exposed to air

Radio frequency magnetron sputtering method is used to grow nanostructured VOx thin film on silicon nitride layer. X-ray diffraction and atomic force microscope are used to characterize the crystal structure and surface morphology, respectively. The variations of square resistance and thermal hysteresis loop are studied when the film is exposed to air for a long period of time, and the effects of these variations on the performance of device are analyzed. X-ray photoelectron spectrometer and Fourier transform infrared spectroscopy are employed to investigate the differences in composition and molecular structure between the fresh and aged films. The results indicate that the increase of square resistance is due to the oxidation of vanadium ions with low oxidation states. The reason to cause the change of thermal hysteresis loop is that the molecular structure of nanostructured VOx thin film is affected by the adsorbed atoms and functional groups.

Unique spin transition and wide thermal hysteresis loop for a cobalt(ii) compound with long alkyl chain

The cobalt(II) compounds with long alkyl chains, [Co(C12-terpy)(2)](BF(4))(2)·EtOH·0.5H(2)O(1·EtOH·0.5H(2)O) and [Co(C12-terpy)(2)](BF(4))(2) (1) was synthesized and characterized. The compound 1·EtOH·0.5H(2)O exhibits a "re-entrant spin crossover". The compound 1 exhibits the reentrant spin crossover and multi phase transitions with a wide thermal hysteresis loop.

Cooperative spin transition in a lipid layer like system

A novel iron(II) mononuclear spin transition complex [FeL(py)(2)] displays an abrupt spin transition around 225 K accompanied by a very wide thermal hysteresis loop (∼50 K) that spreads out over 100 K. Crystal structure analysis in both low-spin and high-spin states reveals a lipid layer-like arrangement of the complex molecules and provides insights into the spin switching mechanism.

Size effects upon phase transitions in vanadium oxide nanocomposites

A phenomenological model has been proposed for the formation of the major thermal hysteresis loop of optical parameters upon the semiconductor-metal phase transition in vanadium oxide nanocomposites. It has been demonstrated that the effects associated with the influence of nanocrystallite sizes on the phase transition temperature in an individual nanocrystallite, which is determined by the width of the elementary hysteresis loop and the position of the phase equilibrium temperature on the temperature scale, manifest themselves in the form of pronounced features in the loop shape. In particular, the size effects for VO2 polycrystalline films are observed in the formation of a trapezoidal shape of the optical thermal hysteresis loop with a wide lower base, whereas these effects for nanocomposites based on porous glasses with VO2 nanocrystals are revealed in the form of the optical hysteresis loop with a narrow lower base (wide upper base). The proposed model also explains the symmetric shape of the major hysteresis loop for vanadium oxide nanocomposites based on opals. The size effects in opal nanocomposites with strictly fixed sizes of pores and, therefore, nanocrystallite sizes manifest themselves in the form of well-defined steps in the heating and cooling branches of the major hysteresis loop.

Nanosecond study of spin state transition induced by a single nanosecond laser shot on [Fe(NH 2trz) 3] compounds inside and outside their thermal hysteresis loops

We show that the combination of a free running 80 MHz femtosecond Ti–sapphire oscillator with a nanosecond Nd:YAG laser makes it possible to record in real time, with a nanosecond time resolution, reversible or irreversible phase transitions photo-induced by a single laser pulse. This set-up is used to study the spin state transition photo-induced inside and outside the thermal hysteresis loops of a series of [Fe(NH 2trz) 3]X 2· nH 2O compounds (with X = NO 3 and Br). Original dynamics are presented and discussed. The results indicate that at room temperature, it is the laser induced heating that drives the recorded photo-induced phase transition.

Hysteresis effects in the magnetic-field-induced reverse martensitic transition in magnetic shape-memory alloys

We study the mechanism of magnetic-field-induced reverse martensitic transformations by considering the field-induced temperature-shifts in the thermal hysteresis loop of the magnetization and critical fields determined from magnetization isotherms. We consider first the temperature-shifts in a symmetric thermal hysteresis loop and extend the discussion to the case of the Heusler-based Ni50Mn34In16 magnetically superelastic alloy having a nonsymmetric thermal hysteresis loop. We obtain two different temperature-behaviors for the critical field depending on whether the initial measurement temperature lies on the forward or on the reverse transformation path of the thermal hysteresis.

Anisotropic properties and crystal structure of ferroelectric Na 0.5Bi 4.5Ti 4O 15

Bismuth layer-structured ferroelectric sodium bismuth titanate Na 0.5Bi 4.5Ti 4O 15 (NBT4) was prepared by conventional solid-state reaction method, and its dielectric and ferroelectric properties are investigated. The dielectric spectra of NBT4 ceramics obviously display anisotropic behaviors, in which the large thermal hysteresis loops with anisotropy are found for first-order phase transitions of NBT4. Based on the analyses of obtained XRD and SEM results, the special anisotropic responses maybe contributed to pseudo-perovskite blocks and bismuth layers (Bi 2O 2) 2+ for their stages of lattice transitions in NBT4 compound.

Microstructure and soft magnetic properties of Finemet-type ribbons obtained by twin-roller melt-spinning

Abstract Soft magnetic ribbons of Finemet-type (Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 ) alloys are synthesized by the twin-roller melt-spinning technique directly from the melt, at tangential wheel speeds of 15, 18, 19 and 20 m/s. The microstructure and the magnetic properties are characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), differential thermal analysis (DTA), thermo-gravimetric analysis (TGA) and hysteresis loops measurements. Samples cooled at 20 m/s are amorphous, while those quenched at lower wheel speeds are partially crystalline. All samples studied present saturation magnetization values (150–160 A m 2 /kg) higher than the commercial Finemet alloys (∼135 A m 2 /kg), obtained by controlled crystallization of amorphous single-roller melt-spun alloys. Optimal soft magnetic properties – σ S =(154±8) A m 2 /kg and H C =(6.9±0.9) A/m – are found in samples quenched at 19 m/s, consisting of size-distributed bcc Fe–Si nanograins (∼18 nm in average) embedded in an amorphous residual matrix. A minority nanocrystalline magnetic phase (≤10 nm) is also detected.

Monte Carlo simulations of phase transitions and lattice dynamics in an atom–phonon model for spin transition compounds

We apply here the Monte Carlo Metropolis method to a known atom–phonon coupling model for 1D spin transition compounds (STC). These inorganic molecular systems can switch under thermal or optical excitation, between two states in thermodynamical competition, i.e. high spin (HS) and low spin (LS). In the model, the ST units (molecules) are linked by springs, whose elastic constants depend on the spin states of the neighboring atoms, and can only have three possible values. Several previous analytical papers considered a unique average value for the elastic constants (mean-field approximation) and obtained phase diagrams and thermal hysteresis loops. Recently, Monte Carlo simulation papers, taking into account all three values of the elastic constants, obtained thermal hysteresis loops, but no phase diagrams. Employing Monte Carlo simulation, in this work we obtain the phase diagram at T=0 K, which is fully consistent with earlier analytical work; however it is more complex. The main difference is the existence of two supplementary critical curves that mark a hysteresis zone in the phase diagram. This explains the pressure hysteresis curves at low temperature observed experimentally and predicts a “chemical” hysteresis in STC at very low temperatures. The formation and the dynamics of the domains are also discussed.

Magnetic phase of the perovskiteCaCrO3studied withμ+SR

We investigated the magnetic phase of the perovskite ${\text{CaCrO}}_{3}$ by using the muon-spin relaxation technique accompanied by susceptibility measurements. A thermal hysteresis loop is identified with a width of about 1 K at the transition temperature. Within the time scale of the muon lifetime, a static antiferromagnetic order is revealed with distinct multiple internal fields which are experienced in the muon interstitial sites below the phase-transition temperature, ${T}_{N}=90\text{ }\text{K}$. Above ${T}_{N}$, lattice deformations are indicated by transverse-field muon-spin rotation and relaxation suggesting a magnetoelastic mechanism.

Coupling of blocking and melting in cobalt ferrofluids

Zero-field-cooling and field-cooling (FC) measurements were performed on ferrofluids of cobalt magnetic nanoparticles (MNPs) in various organic solvent. Two peaks, one broad peak corresponding to the blocking transition (TB), and one sharp peak corresponding to the melting of the solvent (TM), were observed. Furthermore, for a given MNP size, when the blocking and melting transitions were superposed by choosing an appropriate solvent, the strongest intensity of the sharp peak at the melting point of the organic solvent was obtained. This observation is explained by applying the M spectrum theory. Additionally, a first order, melting-induced magnetic phase transformation was observed at the melting point of the solvent. Associated with the first order phase transition and the supercooling effect, a thermal hysteresis loop in the FC curve was observed.

Goldilocks Effect in Magnetic Bistability: Remote Substituent Modulation and Lattice Control of Photoinduced Valence Tautomerism and Light-Induced Thermal Hysteresis

The thermal-induced and photoinduced valence tautomerism of a series of Co(dioxolene)(2)(4-X-py)(2) complexes (dioxolene = 3,5-di-tert-butylcatecholate or 3,5-di-tert-butylsemiquinonate; 4-X-py = 4-(X)pyridine, X = H (1), OMe (2), Me (3), CN (4), Br (5), NO(2) (6)) is described. The thermal valence tautomerism (ls-Co(III)(SQ)(Cat)(4-X-py)(2) <--> hs-Co(II)(SQ)(SQ)(4-X-py)(2)) is only observed for complexes 4, 5, and 6 where each is accompanied by a hysteresis loop of ca. 5 K. When a crystalline sample of 4-6 is held at 10 K in a SQUID magnetometer and irradiated with white light (lambda = 400-850 nm), the hs-Co(II) tautomer is formed. When the light source is removed, and the sample is slowly heated, the hs-Co(II) tautomer persists until ca. 90 K, approximately 40 K higher than the thermal stability of previously reported complexes. Heating and cooling the sample while maintaining irradiation results in the appearance of a new light-induced thermal hysteresis loop below 90 K (DeltaT = ca. 25 K). Below 50 K, the hs-Co(II) tautomer displays temperature-independent relaxation to the ls-Co(III) form, and above 50 K, the relaxation is thermally activated with an activation energy E(a) > ca. 1500 cm(-1). The coordination geometry (trans-pyridines), pyridine substitution, and crystal packing forces conspire to create the comparatively thermally stable photogenerated hs-Co(II) tautomer, thus providing an excellent handle for molecular and crystal engineering studies.