晶粒尺寸对气相沉积薄膜磁取向生长的影响研究

为了研究强磁场下薄膜取向生长规律,采用真空蒸发气相沉积法分别制备了不同磁场方向生长的Zn和Bi薄膜.XRD结果发现磁化率差异较小的Zn薄膜在4T时产生了明显的取向生长,而磁化率差异较大的Bi薄膜在5T磁场强度还没有发生取向生长.SEM结果显示Zn薄膜和Bi薄膜晶粒尺寸上有明显的差别,利用Zn薄膜在4T磁场下的取向建立晶粒尺寸和取向生长的对应关系,提出薄膜发生取向时晶粒的磁化能须大于热能kT的420倍.薄膜是否发生取向生长取决于三个因素:薄膜单个晶粒的大小V,材料不同晶向的磁化率差异Δ关键词： 强磁场 磁取向 薄膜生长 材料电磁加工     

Entanglement and Optimal Dense Coding in Spin Chain with an Arbitrary Magnetic Field

The entanglement and optimal dense coding at entangled states of a 1D Ising chain in the presence of an external magnetic field with an arbitrary direction, are investigated. The entanglement concurrence and the optimal dense coding capacity are calculated for different orientations of the magnetic field. It has been found that the direction of external magnetic field has effects on the entanglement concurrence and optimal dense coding capacity. In the case of antiferromagnet, the quantum phase transition occurs when an external magnetic field is parallel to Ising orientation. The concurrence increases when the angle between the direction of magnetic field and Ising orientation become smaller at ground state in certain parameter regimes, so does the optimal dense coding. The maximum moves toward the direction perpendicular to the Ising orientation in higher temperature. In contrast, the more concurrence and optimal dense coding can be produced only in the case of an external magnetic field perpendicular to Ising orientation at zero and low temperature for ferromagnetic case.     

磁场方向调制的InAs量子点分子量子比特

在有效质量近似条件下研究了由两个垂直耦合自组织InAs量子点组成的双电子量子点分子的电子结构，在此基础上利用系统的总自旋提出了一种磁场方向调制的量子比特方案.电子的相关效应可以导致系统的总自旋在0和1之间转换，值得注意的是，通过调节外部磁场的方向来实现这种转换，而不是像以往那样通过改变外部磁场的大小.结果支持利用系统的总自旋作为磁场方向调制的量子比特的可能性，而且因为高质量的垂直耦合量子点分子的制作工艺已经成熟，所以这是一个非常现实的量子比特设计方案. 关键词： 量子点分子 磁场方向调制 量子比特     

Angular dependence of the photothermally modulated magnetic resonance in Gd thin films

In this paper we use electron spin resonance and photothermally modulated magnetic resonance techniques to investigate gadolinium thin films as a function of the orientation of the film surface with respect to the external magnetic field and of the temperature, around the magnetic phase transition temperature. We observe that, in the ferromagnetic phase, the resonance line is shifted up to higher external magnetic fields when the angle between the film surface and the field increases, revealing the magnetic anisotropy of the sample. At the same time, when the temperature is augmented to values higher than the phase transition temperature, the external field of the resonance collapses back to the expected value in the paramagnetic phase for all orientations. We also demonstrated that, even for the perpendicular orientation (magnetic field perpendicular to the sample surface), the photothermally modulated magnetic resonance signal is maximized near the magnetic phase transition temperature. Furthermore, in the ferromagnetic phase the photothermally modulated magnetic resonance intensity is very sensitive to the orientation, showing a significant enhancement in the perpendicular direction.     

Effect of electric and magnetic fields on the orientation structure of a ferronematic liquid crystal

We analyze uniform orientation phases in soft ferronematics (suspensions of magnetic nanoparticles in nematic liquid crystals) induced by electric and magnetic fields. It is shown that the competition between the electric and magnetic fields can lead to various sequences of orientation transitions in a ferronematic depending on the energy of coupling between the director and magnetization. We obtain and analyze phase diagrams of these transitions. A sequence of re-entrant transitions in the orientation structure (angular phase-homeotropic phase-angular phase-planar phase) is predicted for a certain range of the coupling energies and electric field strengths.     

Photonic anisotropic magnetoresistance in dense co particle ensembles

In this Letter, we experimentally show that the amplitude and arrival time of the terahertz optical transmission through dense ensembles of subwavelength-size ferromagnetic particles is strongly dependent on the orientation of an externally applied magnetic field. The attenuation and delay have the same magnetic field orientation dependence as the electrical anisotropic magnetoresistance inherent to bulk ferromagnetic metals. We envision the application of this magnetic effect in terahertz photonic devices.     

Correlations between the orientation of magnetic recording media determined by Mössbauer Spectroscopy and magnetic methods

The orientation of the easy magnetization axis of magnetic particles is a key parameter of the recording performance of magnetic recording media. Usually the orientation is measured by magnetic methods, but the applicability of the Mössbauer Spectroscopy has also been shown in the past. We show up and discuss the correlations between the results obtained by magnetic and Mössbauer measurements for the example of several magnetic tapes. We demonstrate that by a combination of both methods we are even able to estimate the mean canting angles distribution width of the easy axis of magnetization.     

Effect of magnetic fields on out-of-plane orientations of nematic liquid crystalline polymers under simple shear flow

The effect of magnetic fields on out-of-plane orientations of liquid crystalline polymers (LCPs) under simple shear flows is numerically analyzed using the Doi–Hess equation. The evolution equation for the probability distribution function of the LCP molecules is directly solved without any approximation closure. The initial director is parallel to the vorticity direction. Two cases of the magnetic fields are considered (1) the magnetic field parallel to the flow direction, and (2) the magnetic field parallel to the velocity gradient direction. For both cases a log-rolling orientation state is detected at low shear rates. However, the director is quickly aligned along the direction of magnetic fields because of the deformation of molecules. The field affects on the scalar order parameter rather than the major orientation direction for the magnetic field parallel to the flow direction. On the other hand regarding the magnetic field along the vorticity gradient direction, the effect of the magnetic field is more remarkable on the major orientation in comparison with the effect on the scalar order parameter. Also it is be found that the order parameter is increased obviously with increasing the magnetic fields. It is an efficient way to improve the performance of LCP materials.     

The influence of the orientation and strength of a magnetic field on the coherent population trapping in the Λ system

The influence of the orientation and strength of a magnetic field on the dynamics and dispersion of the populations of the multilevel Λ system upon spontaneous decay into thermostat levels is considered. The radiation field consists of two components and is specified by the vector-potential in the electric dipole approximation. From the solution of the Schödinger equation for a system consisting of an atom in a magnetic field + radiation field, the probability of populating a common level for the generalized Λ system is determined in the resonance approximation. The calculation of the dynamics and dispersion of the populations demonstrates their dependence on the orientation of the magnetic field vector with respect to the light field polarization vector and on the relationship between the magnetic field strength and radiation field intensities. The coherent population trapping occurs only in the case when Rabi frequencies either exceed or are comparable to the Zeeman splitting of magnetic sublevels. By varying the orientation of the magnetic field, it is possible to change the dynamics and dispersion of the populations, thus affecting the coherent population trapping.     

Spin and orbital magnetic moments of free nanoparticles

The determination of spin and orbital magnetic moments from the free atom to the bulk phase is an intriguing challenge for nanoscience, in particular, since most magnetic recording materials are based on nanostructures. We present temperature-dependent x-ray magnetic circular dichroism measurements of free Co clusters (N=8-22) from which the intrinsic spin and orbital magnetic moments of noninteracting magnetic nanoparticles have been deduced. An exceptionally strong enhancement of the orbital moment is verified for free magnetic clusters which is 4-6 times larger than the bulk value. Our temperature-dependent measurements reveal that the spin orientation along the external magnetic field is nearly saturated at ~20 K and 7 T, while the orbital orientation is clearly not.     

Surface analysis of Fe-Si-Al sputtered alloy films by conversion electron Mössbauer spectrometry

Fe-Si-Al alloy films were deposited on silicon wafers heated to various temperatures by DC Ar sputtering and the microstructure of the films was analyzed by CEMS. As-prepared films on cooled substrate contained superparamagnetic components in addition to magnetic components. The fine grains included yielded a random orientation of magnetic spins in the films. The spin orientation became perpendicular to the surface by annealing the sputtered films at more than 773 K. The magnetic fields in sputtered films on a heated substrate were parallel to the surface.     

Anisotropy of magnetoresistance in Be Co-doped GaMnAs

Magnetotransport properties of GaMnAs co-doped with Be have been studied as a function of applied magnetic field orientations and temperature. It was shown that magnetoresistivity phenomena in GaMnAs:Be depend not only on the relative orientation of the current and applied magnetic field, but also on the respective orientation of these two vectors to the crystalline axis.     

单晶Ni52Mn24Ga24中马氏体变体择优取向的物理表征

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Quantum Nucleation of Antiferromagnetic Bubbles with Tetragonal and Hexagonal Symmetries

We study the quantum nucleation in a nanometer-scale antiferromagnet placed in a magnetic field at an arbitrary angle. We consider the magnetocrystalline anisotropy with tetragonal symmetry and that with hexagonal symmetry, respectively. Different structures of the tunneling barriers can be generated by the magnitude and the orientation of the magnetic field. We use the instanton method in the spin-coherent-state path-integral representation to calculate the dependence of the rate of quantum nucleation and the crossover temperature on the orientation and strength of the field for bulk solids and two-dimensional films of antiferromagnets, respectively. We find that the rate of quantum nucleation and the crossover temperature from thermal-to-quantum transitions depend on the orientation and strength of the external magnetic field distinctly, which can be tested by use of existing experimental techniques.     

Probing of pair interaction of magnetic microparticles with optical tweezers

Magnetic interaction of paramagnetic Brownian submicron-sized particles is studied by optical tweezers technique. Correlation analysis allows one to extract magnetic interaction of two particles 0.4 μm in size, which are optically trapped at the distance of 3 μm one from each other and placed in a static magnetic field of 30 Oe, from the background of their Brownian motion. The magnetic interaction force is estimated to be of approximately 100 fN. Two configurations of the mutual orientation of the magnetic field vector and the line connecting two centers of optical traps are used in the experiment. For field vector orientation parallel/perpendicular to this line, the magnetic interaction is detected by the cross-correlation function increase/decrease in comparison with the absence of magnetic field on the time scales of 1 ms.     

Structural anisotropy and internal magnetic fields in trabecular bone: coupling solution and solid dipolar interactions

We investigate the use of intermolecular multiple-quantum coherence to probe structural anisotropy in trabecular bone. Despite the low volume fraction of bone, the bone-water interface produces internal magnetic field gradients which modulate the dipolar field, depending on sample orientation, choice of dipolar correlation length, correlation gradient direction, and evolution time. For this system, the probing of internal magnetic field gradients in the liquid phase permits indirect measurements of the solid phase dipolar field. Our results suggest that measurements of volume-averaged signal intensity as a function of gradient strength and three orthogonal directions could be used to non-invasively measure the orientation of structures inside a sample or their degree of anisotropy. The system is modeled as having two phases, solid and liquid (bone and water), which differ in their magnetization density and magnetic susceptibility. A simple calculation using a priori knowledge of the material geometry and distribution of internal magnetic fields verifies the experimental measurements as a function of gradient strength, direction, and sample orientation.     

Orientation of hyperfine magnetic fields of α-iron films produced by laser deposition

Iron films were produced by pulsed laser deposition (PLD) of iron in Ar gas and M?ssbauer spectra of these films were obtained at room temperature. The orientation of the hyperfine magnetic field was found to vary depending on the pressure of the Ar gas. Iron films produced at low Ar pressures exhibited magnetic fields parallel to the substrate surface. The magnetic field became increasingly perpendicular to the substrate with increasing Ar pressure. Collisions with Ar gas molecules reduced the translational energies of laser-evaporated iron atoms and thus the orientation of crystals formed on the substrate varied depending on the Ar pressure.     

The sheath criterion for a collisional plasma sheath at the presence of external magnetic field

The Bohm sheath criterion is modified for collisional plasma containing Boltzmann electrons and cold fluid ions at the presence of external magnetic field. Based on fluid model, the effects of the strength and the orientation of an external magnetic field on the upper and lower limits of Bohm sheath criterion have been studied by considering the collision frequency between ions and neutrals. The results show that the sheath criterion depends on the orientation and magnitude of magnetic field and the ion flow velocity at the sheath boundary.     

Magnetic field anisotropy based MR tractography

Non-invasive measurements of structural orientation provide unique information regarding the connectivity and functionality of fiber materials. In the present study, we use a capillary model to demonstrate that the direction of fiber structure can be obtained from susceptibility-induced magnetic field anisotropy. The interference pattern between internal and external magnetic field gradients carries the signature of the underlying anisotropic structure and can be measured by MRI-based water diffusion measurements. Through both numerical simulation and experiments, we found that this technique can determine the capillary orientation within 3°. Therefore, susceptibility-induced magnetic field anisotropy may be useful for an alternative tractography method when diffusion anisotropy is small at higher magnetic field strength without the need to rotate the subject inside the scanner.