The influence of a static magnetic field up to 15 T on the manifestation of the Portevin-Le Chatelier effect in NaCl: Eu crystals

The influence of a static magnetic field on the instability of plastic flow (the Portevin-Le Chatelier effect) is revealed in NaCl: Eu quenched crystals. It is found that, in an external magnetic field, the yield stress of the crystals is reduced, the probability of plastic strain jumps and their amplitude decrease, and the amplitude distribution of the plastic strain jumps becomes random. The number of shear bands formed on the surface of crystals strained in the magnetic field is halved as compared to that observed without a magnetic field.     

Effect of magnetic field in the microhardness studies on calcium hydrogen phosphate crystals

Calcium hydrogen phosphate dihydrate (CaHPO₄·2H₂O, brushite) crystals are grown in gel medium at physiological pH by single diffusion method in the presence of different magnetic field strengths, namely 0, 0.1 and 0.2 Tesla. The results indicate that in the presence of magnetic field, the number of brushite crystals formed at the gel-solution interface reduces and the average size of the crystals increases. This shows that the growth of brushite crystal was favored by magnetism. Vickers and Knoop microhardness studies were carried out on the crystals grown at various magnetic fields. The Vickers (H_V) and Knoop (H_K) microhardness numbers were found to increase with increasing load. The values of Meyer’s index number (n) decrease while the magnetic field strength increases. Nevertheless the n values are greater than 1.6, which shows that the brushite crystals grown under magnetic fields still fall in the soft material category. The brittle index (Bi), yield strength (σ_y) and elastic stiffness constant (c₁₁) were calculated. The Young’s Modulus was calculated using the Knoop hardness value and found to decrease in higher magnetic field strengths.     

Domain structures in orthorhombic KNbO3 and characterisation of single domain crystals

Domain configurations in large KNbO₃ single crystals grown by a top seeded flux growth technique are studied by optical methods and etching techniques. The orientation of observed domain walls is consistent with the principles of mechanical compatibility and electrical neutrality. Experimental evidence for the existence of a new type of ferroelectric domain wall (S-wall) is presented. Miller indices of such walls are irrational and depend on spontaneous strain tensor coefficients. The preparation of large single domain crystals (up to 1/3 cm³) is described. Assessment of crystal perfection by x-ray topographic and interference methods reveals a very small mosaic spread (one minute of arc) and a good optical homogeneity.     

Effect of a weak magnetic field on stepwise deformation of lead in the large strain region

Precision measurements of the strain rate of lead at constant stresses in a magnetic field and without field and changes in the strain rate resulting from field turn-on and -off have been performed by the interferometric method. It has been shown that the entire stress-strain curve in the field and without field consists of steps of different amplitudes and lengths: from several tens of nanometers to several hundreds of micrometers. The magnetic field causes a certain strain enhancement and the redistribution of contributions of steps of different values. The magnetic field turn-on during creep results in a sharp increase in the strain rate, followed by its drop to the values larger than or close to those before the field turn-on. The field turn-off is accompanied by the reverse effect. The characteristics of strain steps at various scale levels and the magneto-plastic effect depend on the strain rate and the strain value. The observed features in the behavior of lead are related to its possible multiple recrystallization during creep.     

Effect of magnetic field on the microplastic strain rate for C60 single crystals

Microplastic deformation in a magnetic field and in a zero field, as well as after preliminary action of a magnetic field on C₆₀ crystals, is studied with the help of a laser interferometer, which makes it possible to measure the strain rate on the basis of linear displacements of 0.15 µm. It is shown that the introduction of a sample into the field and its removal from a field of 0.2 T directly during sample deformation lead to a change in the strain rate, the decrease in the rate being accompanied by a brief interruption of deformation. The sign of the effect depends on temperature: the magnetic field accelerates deformation at room temperature and slows it down at 100 K. Preliminary holding of a sample in a field of 0.2 or 2 T produces a similar effect on the strain rate. Possible reasons for the observed manifestations of the magnetoplastic effect in C₆₀ and the relation between the sign of the effect and the phase transition at 260 K are considered.     

Raman Spectroscopy and Magnetic Properties of Mn-Doped ZnO Bulk Single Crystal

Mn doped ZnO bulk single crystals are grown by the modified Bridgman method. The as-grown crystals are red in eolour. The additional Raman mode observed at 524cm^-1 is attributed to the Mn ions incorporating into ZnO crystal. The crystal exhibited paramagnetie under lower applied field below 2280 Oe. Then diamagnetism is observed in the crystal when the magnetic field rises up and becomes dominant under applied field above 5270 Oe. The magnetic susceptibility dependence on the temperature follows a Curie law indicating a typical paramagnetie characteristic under an applied field of 2kOe. No ferromagnetic ordering is observed in the as-grown Mn-doped ZnO crystal.     

High Pressure Optical Absorption in Cs 2 TCNQ 3 Complexes Grown Under the Influence of Magnetic Field

Crystals of the organic semiconductor Cs 2 TCNQ 3 have been grown under the influence of magnetic field of 5 T and their optical properties have been compared with the crystals grown without magnetic field. The magnetic field effect manifests itself as the enhancement of the intradimer charge transfer band S 1 , which appears at around 1.3 eV in the E//a polarization. The high-pressure optical absorption measurements have shown that the S 1 band of the crystal grown at 5 T contains a new component, which is significantly intensified with increasing pressure up to 1 GPa, but diminishes as pressure increases further up to 4 GPa, and remains constant at higher pressures. This new component is additional to the component of S 1 band which has similar properties to that of the crystals grown without magnetic filed, which continuously grows up to 3 GPa and abruptly broadens out at higher pressures. The new component of S 1 is strongly linked to the ferromagnetism in this substance.     

INVESTIGATION OF DEFECTS IN Ge-DOPED GaAs CRYSTAL GROWN IN A MAGNETIC FIELD

Ge-doped GaAs single crystals have been grown by liquid-encapsulated Czochralski method in absence and presence of a magnetic field of 4000Gauss. By means of high op-tical efficiency photoluminescence, spectra of the grown crystals at room temperature were obtained, which consist of two emission bands A and B at 1.39-1.42eV and 0.97-1.05eV, respectively. Comparing the photoluminescence mappings with microphotographs of etched wafers, Hall effect results and electron probe microanalyses of the n- and p-type crystals with different Ge concentrations, we considered that the emission bands A and B originate from Ge-related acceptor and donor complexes, respectively. The complexes were formed during crystal growth, mainly due to temperature fluctuations in molten Ge-doped GaAs. The concentrations and homogeneities of the defects can be improved by the application of a magnetic field during crystal growth to suppress the temperature fluctuation.     

Spin precession of neutrons in nonmagnetic single crystals

Diffraction of slow neutrons in nonmagnetic single crystals in the presence of an external steady homogeneous magnetic field is considered. It is shown that in this case precession of neutron spin takes place at a few frequencies.     

The contribution of Neutron and synchrotron radiation topography to the investigation of first-order magnetic phase transitions

Neutron and white beam synchrotron topography have proved to be valuable and complementary tools for the investigation of the coexistence of magnetic phases in single crystals. Neutron topographs indicate unambiguously which phase occupies a given region of the sample, whereas synchrotron radiation topographs give information about the coexistence-related lattice distortion and the dynamics of the phase transition. A few examples illustrate these capabilities. 1) Helimagnetic-ferromagnetic coexistence was followed in Tb and MnP by varying either the magnetic field or the temperature. The observed interfaces appear to result from competition between the elastic and magnetostatic terms of the total energy, the last term being dominant in the field-driven case. 2) The reorientational transition of hematite was shown to correspond, when spread over a wide temperature range, to a succession of abrupt transitions, probably related to an inhomogeneous incorporation of small amounts of impurities during growth. A similar feature also explains the occurrence of small ferromagnetic islands which remain in the helimagnetic phase of Tb.     

籽晶尺寸对宝石级金刚石单晶生长的影响

本文在国产六面顶压机上,在5.6 GPa, 1250—1450℃的高压高温条件下,分别选用边长0.8, 1.5和2.2 mm三种尺寸的籽晶,系统开展了Ib型宝石级金刚石单晶的生长研究.文中系统考察了籽晶尺寸对宝石级金刚石单晶生长的影响.首先,考察了籽晶尺寸变化对宝石级金刚石单晶裂晶问题带来的影响.研究得到了籽晶尺寸变大,裂晶出现概率增加的晶体生长规律.其次,在25 h的生长时间内,考察了上述三种尺寸籽晶生长金刚石单晶时,生长时间与单晶极限生长速度的关系.得到了选用大尺寸籽晶,可以提高优质单晶合成效率、降低合成成本的研究结论.借助扫描电子显微镜和光学显微镜,对三种尺寸籽晶生长金刚石单晶的表面形貌进行了标定.最后,傅里叶微区红外测试,对三种尺寸籽晶生长宝石级金刚石单晶的N杂质含量进行了表征.研究得到了选用大尺寸籽晶实现快速生长金刚石的同时,晶体的N杂质含量会随之升高的晶体生长规律.     

First-order phase transition in potassium dysprosium tungstate induced by the field renormalization and softening of the elastic moduli in the vicinity of a structural Jahn-Teller-type transition

The magnetostriction of KDy(WO₄)₂ single crystals is measured in an external magnetic field at temperatures below the temperature of a structural phase transition of the Jahn-Teller type. A steplike irreversible variation in the elastic strain is observed to occur with an increase in the magnetic field applied along the a or b axis of the monoclinic cell of the crystal. The residual change in the strain is retained after changing the sign of the magnetic field. The return to the initial state characterized by field-induced jumps in the strain is possible only after thermal cycling well above the structural phase transition temperature. The theory of this phenomenon is developed using a phenomenologically derived thermodynamic potential of the elastic sub-system that takes into account the crystal symmetry and the field renormalization of the elastic moduli. The jumplike transitions are interpreted as being due to the magnetic softening of the elastic moduli in the vicinity of the structural phase transition temperature.     

Serrated creep of zinc single crystals under compression in a magnetic field

The compressive creep rate of zinc single crystals was measured for sample deformation increments of 150 nm, which permits the measurement of deformation jumps larger than 300 nm. A weak magnetic field B = 0.2 T is shown to increase the average creep rate and decrease the height and sharpness of submicron-sized deformation jumps. Preliminary holding of a sample in a magnetic field also influences the creep rate and the characteristics of deformation jumps. The data are explained in terms of a model relating the effect of a magnetic field to the destruction of barriers to dislocation motion.     

Enhanced electron trapping by a static longitudinal magnetic field in laser wakefield acceleration

An externally applied longitudinal magnetic field was found to enhance the particle trapping in the laser wakefield acceleration. When a static magnetic field of a few tens of tesla is applied in parallel with the propagation direction of a driving laser pulse, it is shown from two-dimensional particle-in-cell simulations that total charge of the trapped beam and its maximum energy increase. The analysis of electron trajectories strongly suggests that the enhanced trapping originates from the suppression of the transverse motion by the magnetic field. The enhanced trapping by the magnetic field was observed consistently for various values of the plasma density, the amplitude of the laser pulse and pulse spot size.     

Strain characteristics and superelastic response of single crystals

The intermartensitic transformation, in a two-step complete thermoelastic martensitic transformation in Ni_53.2Mn_22.6Ga_24.2 single crystals, provides a much larger strain than that of the martensitic transformation. With a biasing magnetic field, the intermartensitic transformation strain is inhibited and the martensitic transformation strain is enhanced. Compressive stress–strain characteristics can be affected greatly by a static magnetic field. At low deformation temperature, the irreversible transformation strain induced by the stress becomes reversible, when a static magnetic field is applied. Further, the magnitude of the stress necessary for rearrangement of martensitic variants is dependent on the direction of the biasing magnetic field. Moreover, a well-defined character of the twin-boundary motion, similar to the soliton motion, has been observed upon loading or unloading.     

Growth rate of potash alum crystals: comparison of silent and ultrasonic conditions

The influence of power ultrasound on the growth rate of potash alum was investigated. The experiments on growth of potash alum crystals were carried out in a stirred double jacket tank in silent conditions as well as in the presence of power ultrasound (20 kHz) at 32 degrees C, with different initial crystal sizes. It was observed that the mass growth rate of potash alum was faster under ultrasound compared to that under silent conditions. The shape was not modified by ultrasound but the size of crystals, which are grown under ultrasound, are smaller and with higher density compared to those grown under silent conditions.     

Morphology,Crystallization, and Melting of Single Crystals and Thin Films of Star‐branched Polyesters with Poly(ϵ‐caprolactone) Arms as Revealed by Atomic Force Microscopy

The morphology and thermal stability of different sectors in solution‐ and melt‐grown crystals of star‐branched polyesters with poly(?‐caprolactone) (PCL) arms, and of a reference linear PCL, have been studied by tapping‐mode atomic‐force microscopy (AFM). Real‐time monitoring of melt‐crystallization in thin films of star‐branched and linear PCL has been performed using hot‐stage AFM. A striated fold surface was observed in both solution‐ and melt‐grown crystals of both star‐branched and linear PCL. The presence of striations in the melt‐grown crystals proved that this structure was genuine and not due to the collapse of tent‐shaped crystals. The crystals of the star‐branched polymers had smoother fold surfaces, which can be explained by the presence of dendritic cores close to the fold surfaces. The single crystals of linear PCL grown from solution showed earlier melting in the {100} sectors than in the {110} sectors, whereas no such sectorial dependence of the melting was found in the solution‐grown crystals of the star‐branched polymers. The proximity of the dendritic cores to the fold surface yields at least one amorphous PCL repeating unit next to the dendritic core and more nonadjacent and less sharp chain folding than in linear PCL single crystals; this evidently erased the difference in thermal stability between the {110} and {100} sectors. Melt‐crystallization in thin polymer films at 53–55°C showed 4 times faster crystal growth along b than along a, and more irregular crystals with niches on the lateral faces in star‐branched PCL than in linear PCL. Crystal growth rate was strictly constant with time. Multilayer crystals with central screw dislocation (growing with or without reorientation of the b–axis) and twisting were observed in both classes of polymers.     

Magnetodielectric effect in Co3O4 nanoparticles grown within a silica glass

Co₃O₄ nanocrystals of sizes in the range 17.5-33.1 nm were grown within a sol-gel-derived silica glass matrix. Rietveld analysis of the X-ray diffraction data showed that the crystals had tetragonal symmetry. This was explained as arising due to a tensile strain induced at the glass-crystal interface as a result of thermal expansion mismatch between the phases. The crystals showed ferromagnetic behaviour with superparamagnetism setting-in at temperatures above 15 K. They also exhibited ferroelectric characteristics with large remanent polarization. Change in dielectric constant as a function of magnetic field was observed in these nanocomposites. This was discussed on the basis of magnetostriction effect.     

Experimental and theoretical studies of the effects of collisions and magnetic fields on quantum beat

The effects of collisions and magnetic fields on quantum beat are treated by the density matrix method. Some experimental quantum beat results of biacetyl in the presence of collisions are presented and theoretically analysed. It is shown that in this case not only pure dephasing but also pressure dependent inherent decay rate constants can be observed; this implies that the Schrödinger method cannot be employed for this case. The magnetic field effect on quantum beat is examined also and it is shown that for the biacetyl molecule magnetic splitting can be observed even in fields of a few gauss.     

Inverted EPR signal from nitrogen defects in a synthetic diamond single crystal at room temperature

Electron paramagnetic resonance (EPR) in diamond single crystals was studied. The crystals were grown using apparatuses of the “split-sphere” type in a Ni-Fe-C system using the temperature gradient method with a subsequent high-temperature high-pressure treatment. It was found that, after the high-temperature high-pressure treatment of a diamond sample, the EPR signal from the lattice defects containing nitrogen atoms became inverted with the growth of the microwave power in an H₁₀₂ resonator. In a constant polarizing magnetic field, when the microwave power applied to the diamond was low, a resonance absorption by the nitrogen defects took place, whereas, when the microwave power was high, an emission was observed. The inversion of the EPR lines of a single nitrogen atom substituting for a carbon atom at a diamond lattice site could be caused by the presence of a nickel atom with an uncompensated magnetic moment at the adjacent tetrahedral interstitial site. In synthetic diamond crystals that were not subjected to high-temperature high-pressure treatment, the inversion of the EPR signal from nitrogen atoms (P1 centers, nitrogen in the C form) was absent.