Melt stirring effect of a weak magnetic field on crystal growth

Melt stirring effect of a weak magnetic field for the natural convection of liquid metal in an electrically adiabatic cubic enclosure heated from one vertical wall and cooled from an opposing wall was studied by a fully transient three-dimensional numerical analyses and the reasoning for melt stirring effect was clarified from the numerical results. Similar techniques were applied for the melt convection in a cylindrical Czochralski crystal growing crucible with an application of a vertical magnetic field. In a static crucible, central fluid column rotated in a magnetic field and in a rotating crucible, central fluid column did not rotate in a magnetic field. These peculiar characteristics could have been explained due to the Lorentz force.     

The influence of laser annealing in the presence of longitudinal weak magnetic field on asymmetrical magnetoimpedance response of CoFeSiB amorphous ribbons

Asymmetrical magnetoimpedance (AMI) is very important to further improve micromagnetic sensor performance in terms of linearity and sensitivity. This behavior was investigated for Co_68.15Fe_4.35Si_12.5B₁₅ amorphous ribbons irradiated by a 1064 nm Nd:YAG pulsed laser in air and in the presence of 3 Oe longitudinal magnetic field with changing pulse repetition rate. Results indicate that for different pulse repetition rates, various types of AMI profiles appear. For the samples annealed in the presence of longitudinal field, because of induced anisotropy, rising in asymmetry factor takes place and another peak in the magnetoimpedance (MI) profile transpires.     

Effect of magnetic field on anelasticity of KBr crystals

The effect of a weak (0.2 T) magnetic field on the amplitude dependences of the internal friction and the defect of Young’s modulus in KBr crystals has been studied. It is shown that, under the action of the magnetic field, the internal friction increases and the effective elastic modulus decreases, thus indicating an increase of sample plasticity. The preliminary treatment of the samples in a magnetic field results in the inverse effect—hardening.     

Influence of magnetic field on the nucleation rate control of mono paracetamol

Crystallization of paracetamol was carried out under magnetic field in pure aqueous solution at ambient condition for the first time. Solutions at different supersaturation levels σ = 0.28–1.6 were exposed to various magnetic flux densities in the range 160–510 Gauss and the grown crystals were compared with that grown in the absence of magnetic field. Both in the absence and presence of magnetic field, solution yields only stable mono paracetamol and does not favors the metastable ortho polymorph at all supersaturation levels. The presence of magnetic field reduces the rate of crystal nucleations as well as modifies the habit of the nucleated mono paracetamol. Crystals obtained in the presence of magnetic field were of good quality and highly transparent as compared to the crystals grown in the absence of magnetic field. The induction period of the nucleation in solution decreases significantly with increase in magnetic flux density at all supersaturation ranges. The internal structure and thermal stability of the grown paracetamol crystal was confirmed by PXRD and DSC analysis.     

Effect of a magnetic field on the orientation of hen egg-white lysozyme crystals

Recent experiments have shown that lysozyme crystallization in a magnetic field of the order of 1 T can result in a significant degree of orientation of the crystals. We present more extensive experimental results and propose a model to account for this phenomenon. Because of the small susceptibility anisotropy of most protein molecules, the orienting effect is unimportant for smaller aggregates, even those much larger than a critical nucleus. However, during sedimentation crystals grow larger and are more likely to become aligned. The degree of orientation thus depends on crystal growth rate and container geometry, in addition to magnetic field strength, as we have confirmed experimentally.     

Effect of magnetic field–current annealing on the magnetoimpedance of Co-based ribbons

The roles of applied magnetic field during the current annealing of Co_68.15Fe_4.35Si_12.5B₁₅ soft magnetic amorphous ribbons have been studied. The Joule heating process causes a complex domain structure which is related to the circular field generated by the current. We have shown that magnetic field–current annealing modifies the transverse permeability and under suitable conditions, enhances the GMI response.     

Impact of magnetic field on the nucleation and morphology of calcium carbonate crystals

The influence of static magnetic field of strength 0.75 T on the nucleation of calcium carbonate crystals has been investigated. Particle size analysis shows that magnetic field can cause marked difference in distribution. One of the major impacts of magnetic exposure is the increase in number of the critical nuclei formed. Also, magnetic field promotes the formation of parallelepipedic calcite crystals and the dissolution of the smaller crystals by Ostwald ripening mechanism. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

On the combined action of magnetic field and electric current on silicon plasticity

The specific features of the deformation behavior of low-resistivity p-type silicon single crystals have been studied using nontraditional methods of plastic deformation. Under the conditions of the combined action of a weak magnetic field and a direct electric current, a significant decrease in the plasticity of silicon is found during its active deformation when compared to the deformation in the absence of a magnetic field. An increase in the electrical conductivity of silicon samples subjected to active plastic deformation is found. The data that are obtained are compared with the previous data on single-crystal germanium. A possible explanation for the observed effects is given.     

Interplay between the magnetic field and the dipolar interaction on a magnetic nanoparticle system: A Monte Carlo study

We have studied the influence of the applied magnetic field on the blocking temperature (T_B) of a fine magnetic particle system. By means of a Monte Carlo technique we have simulated zero field cooling (ZFC) curves under different applied fields, obtaining the respective T_B as a function of H. We have focused our study on the limit H → H_K (where H_K is the anisotropy field), since the results found in the literature usually lack a detailed study of this range. The simulations were done at different sample concentration of the nanoparticles, with the purpose of observing how the magnetic dipolar interaction affects the field dependence of T_B. The classical expression predicts T_B to disappear for H ? H_K, independently of the dipolar interaction strength. Our simulations show that at strong interacting conditions T_B exists even for fields H > H_K.     

Influence of transverse magnetic field on thermocapillary flow in liquid bridge

For exploring the optimizing convection control technique by external magnetic field in floating zone crystal growth of semiconductor under microgravity, thermocapillary flow in a floating half‐zone model is simulated numerically, and the influences of both the transversal uniform magnetic field and the magnetic field generated by transversal four coils on thermocapillary flow are investigated. The results indicate that the transversal uniform magnetic field is likely to break the axisymmetrical structure of thermocapillary flow, which is unfavorable to the growth of high‐quality crystal; under the magnetic field generated by transversal four coils, both the mean and the maximum velocities increase with the increment of the distance between coils or the decrement of coil radius; and the convection tends to be more axisymmetrical with increasing coil radius. Compared to the transversal uniform magnetic field, the magnetic field generated by transversal four coils of appropriate radius and relative distance may not only suppress convection, but also enhance the axisymmetry of convection at the same time, and finally, the better convection control can be achieved. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Influence of magnetic field on the director profile in nematic layer with curved surface

A nematic liquid crystal layer with a curved upper boundary exposed to a magnetic field directed along the layer plane has been studied. Strong nematic anchoring at the lower boundary surface is assumed. A phenomenological expression for the director profile is proposed to solve the problem on the director profile distribution in this system and its stability with respect to the rotational deformations in relation to the degree of the upper-surface roughness and the magnetic field value. The distribution parameters are found by optimizing the expression for the free energy of the liquid crystal system.     

Polariton dynamics of a bounded weak ferromagnet in an external dc electric field tangential to the interface

Using the example of a semibounded easy-plane weakly ferromagnetic crystal, magnetized tangentially to its surface, the conditions have been determined under which, with the application of an external dc electric field along the easy-magnetization axis, a number of anomalies arise at the transmission and localization of OA and TM electromagnetic waves through the interface between a nonmagnetic dielectric and a weak ferromagnet due to quadratic magneto-optical interaction.     

Orientation of protein crystals grown in a magnetic field

Three crystal forms of hen egg-white lysozyme, and ribonuclease A and met-myoglobin crystals exhibited orientation in a magnetic field of <1 T. Magnetic field application is thus shown to be a possible means to orient protein crystals. The conditions for the orientation to occur and some of its applications are discussed.     

Effect of magnetic field on melt flow and crystal growth of oxide crystals

The flow in an oxide melt such as LiNbO, and TiO₂ in a high magnetic field was observed by using magnetic-field-applied Czochralski equipment for oxide crystals. It was found that the flows in oxides melts were very much different from these in a semiconductor melt. The single crystals of TiO₂ were grown in a magnetic field by using this equipment.     

Refracting properties of lyotropic nematic calamitic mesophase under magnetic field

The present study found the time-dependent extraordinary refractive index and birefringence of lyotropic nematic calamitic mesophase for different temperatures during its orientation under magnetic field. Based on the experimental results, the average angle between the mesophase directors and magnetic field was calculated to estimate the orientation dynamics of the mesophase during the director rotation from planar to homeotropic orientations. On the other hand, the textural properties were investigated during the orientation. The ordinary and extraordinary refractive indices were also measured depending on temperature to obtain the orientational order parameter of the mesophase. Using the macroscopic refractive index results, the microscopic polarizability was studied as well.     

Use of an inhomogenous magnetic field for silicon crystal growth

The flow of liquid silicon and oxygen transfer during crystal growth under three different types of cusp-shaped magnetic field were clarified using numerical simulation, flow visualization, and infrared measurement of oxygen concentration in grown crystals. Velocity vectors obtained from numerical simulation are almost parallel to cusp-shaped magnetic fields since flow parallel to a magnetic field does not produce a Lorentz force. This parallel flow enhances homogenization of oxygen concentration along the radial direction in grown crystals. Cusp-shaped magnetic fields can control the flow velocity at the top of the melt. Since melt with a low concentration of oxygen at the top of the melt transfers directly from the free surface to the solid-liquid interface, a low concentration of oxygen in crystals can be achieved. Separation of fluid flow between the near surface and bulk can produce a spatial distribution of the concentration in the melt, and therefore a low oxygen concentration can be obtained in grown crystals.     

Instability of the melt flow in VGF growth with a traveling magnetic field

The linear instability of a thermally stratified melt flow in the VGF configuration driven by a traveling magnetic field (TMF) is considered numerically and experimentally. The dependency of the instability threshold on the governing parameters is found for several cuts through the parameter space covering a wide range of possible applications. In a first approximation the linear instability occurs when the dimensionless TMF forcing parameter reaches the magnitude of the Grashof number. This is particularly true in a medium-sized crucible where the first instability is axisymmetric and sub-critical. As the Grashof number increases the flow develops self-similar boundary layers and the instability becomes three-dimensional. The instability originates in the bottom boundary layer where the convection tends to suppress the imposed temperature gradient in the central part of the melt zone. It is shown that the TMF may serve as a tool to control the phase interface shape without causing flow instationarity when the crucible diameter exceeds a certain value. This value is estimated to be around 6 cm for GaAs. The flow stays stable if the TMF is used for a reversal of the meridional flow with the aim to remove a possible dopant concentration peak on the axis.     

Modification of the malus law for the torsional deformation of lyotropic nematics in magnetic field on the basis of statistical approach

A method based on the statistical approach is proposed to calculate the light intensity for the torsional deformation of lyotropic nematic liquid crystals at violated Mauguin adiabatic approximation. Theoretical dependences of the light intensity on the magnetic field strength are obtained for two limiting cases of lyotropic nematic anchoring with bearing surfaces: infinite and low anchoring energies.     

Texture transformations and orientational properties of lyotropic nematics in magnetic field

Dynamics of changes of the morphological properties and processes of orientation in lyotropic nematic mesophases by application of external magnetic field have been investigated in detail. The dynamics of changes of the optical birefringence values in nematic‐calamitic mesophase vs. time has been determined. The optical mappings of nonequilibrium magnetically induced textures, which have been observed for this mesophase during orientational process, and changes of birefringence vs. time are presented. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

硒化镉晶体定向切割方法研究

根据硒化镉(CdSe)晶体结构的特点,发展了一种简便、快捷、准确的定向方法.此法只需观察CdSe晶体解理面上台阶的走向,便可确定CdSe晶体的C轴方向,即解理面上台阶延伸的方向为CdSe晶体的C轴方向,并采用X射线衍射法对结果进行了验证.已知C轴和某一晶面,借助激光正反射技术,可在CdSe单晶体上定向切割出任意所需的晶面.