MOCVD growth, structure and magnetic properties of Fe films grown on GaAs (001) substrates

Thin iron films have been grown on (001) GaAs substrates by low pressure metal organic chemical vapor deposition (LP-MOCVD) at different temperatures with the pressure of 150 Torr. X-ray diffraction (XRD) analysis showed that all films have only one strong diffraction peak (110). The surface of Fe film became smooth with increasing the growth temperature. Magnetization measurements showed that the Fe films grown at different temperatures were ferromagnetic with easy axis parallel to the film surface and hard axis perpendicular to the substrates. The field dependence of magnetization along two axes showed a remarkable difference, implying that the samples have strong magnetic anisotropy. Furthermore, when the applied magnetic field is perpendicular to the Fe surface, a sharp jump in the hysteresis loop could be observed, followed by a broad shoulder, which is related to the interface effect, the existence of carbon and the formation of 180^°/90^° magnetic domains.     

Neutron diffraction study of single crystal europium in an applied magnetic field

A single crystal of europium has been studied in applied magnetic fields up to 41.7 koe. At T_N = 90.5 ± 0.5°K a first order magnetic transition is observed. At 4.2°K the effect of an applied magnetic field in either a 〈100〉 or 〈110〉 direction is to stabilize a helix structure having τ along the field direction. This field stabilized structure remains even if the field is reduced to zero. The hysteresis associated with these transformations has been investigated and is discussed. No ferromagnetic components have been detected in fields up to 41.7 koe.     

Metamagnetic states in single crystal HoNi<Subscript>2</Subscript>B<Subscript>2</Subscript>C at <Emphasis Type="Italic">T</Emphasis> ≈ 1.9 K: Torque magnetometry study

Metamagnetic transitions in single-crystal rare-earth nickel borocarbide HoNi₂B₂C have been studied at T ≈ 1.9 K with a Quantum Design torque magnetometer. With increasing field, transitions to antiferromagnetic, ferrimagnetic, non-collinear, and saturated paramagnetic states take place. The critical fields of the transitions depend crucially on the angle θ between applied field and the easy axis [110]. Measurements of torque along the c axis have been made while changing the angular direction of the magnetic field (parallel to basal tetragonal ab planes) and with changing field at fixed angle over a wide angular range. Two new phase boundaries in the region of the non-collinear phase have been observed, and the direction of the magnetization in this phase has been precisely determined. At low field the antiferromagnetic phase is observed to be multidomain. In the angular range around the hard axis (?6° ≤ ? ≤ 6°, where ? is the angle between the field and hard axis [100]) the magnetic behavior is found to be “frustrated” with a mixture of phases with different directions of the magnetization.     

Magnetic field effects in the cooperative Jahn-Teller system TbVO4

Effects of a magnetic field on the behaviors of TbVO₄ which shows a cooperative Jahn-Teller phase transition have been investigated. The possibility that the system will return to the undistorted phase at low temperatures in a magnetic field is considered. We show that because of the weak coupling of the system to the magnetic field applied along the c-axis, the magnetic field required to see the transition of the crystal back to the undistorted phase is impractically high. Diluting the crystal with nonmagnetic ions is then a plausible approach in the study of this novel behavior of the system. We have also calculated the effects of a magnetic field applied in the basal plane and shown that the crystal would remain distorted for all temperatures except the cases when the field is applied along the crystallographic axes. Finally, we consider the effects of a magnetic field on the temperature behavior of the specific heat and show that the Schottky anomaly which occurs at a very low temperature (～1°K) will be shifted to a higher temperature by applying a magnetic field.     

Propagation of high power helicons in n-indium antimonide crystal

The propagation of high power helicon in n-InSb has been analysed taking into account the heating of the carriers by the electric field of the wave. The momentum and energy transfer of the electrons have been taken to be due to acoustic phonons and polar optical phonons scattering at 77°K respectively. The sample is assumed to be finite and the wave is incident on the semiconductor-free space interface. Calculations have been made for phase constant, attenuation constant, reflection coefficient and the electron temperature as function of the magnetic field and the wave amplitude. The theoretical results are found to be in qualitative agreement with the experimental observations of Laurinavichyus and Pozhela[14].     

Anomalous shear ultrasonic attenuation peak at 95.5°K in holmium2

An anomalous attenuation peak at 95.5°K has been observed for shear ultrasonic waves transmitted along the c-axis of two holmium single crystals. The velocity change across this temperature is on the order of 10^?4. In the presence of an external magnetic field, the peak moves toward high temperatures for both H∥a and H∥c. A rotation of the field in the basal plane produces a two-fold symmetry pattern in the attenuation. A double peak structure centered at the c-plane has also been observed when the field is rotated away from the a-axis. A considerable fraction of the attenuation peak appears to go as sin² θ where θ is the angle between the shear wave polarization and the external field.     

Double-walled carbon nanocones: stability and electronic structure

We have applied first-principles calculations, based on the density functional theory, toinvestigate the stability and electronic properties of double-walled carbon nanocones,60°60°, 120°120° and 60°120° with different rotation anglesbetween the walls. We have shown that the most favorable double-walled nanocone studiedhere is that of angles of 60°60°, with rotation angle of 36° and distance between apexes of 4.22 Å.We have found that, the interaction between the walls of rotated double-walled nanoconesintroduce geometric distortion in gap states, such as in Fermi level. These results shouldhave consequences on the field emission properties of double-walled carbon nanocones.Additionally, we also investigated the spin polarization of such structures, and we havefound unpaired electrons, which induces a total spin from 1 and 1/2 for 60°60° and 60°120° double cones, respectively.     

Magnetic structures of the rare earth orthotitanites RTiO3; R = Tb,Dy, Tm and Yb

The magnetic structures of the rare earth orthotitanites, RTiO₃, R = Tb, Dy, Tm and Yb, have been solved using neutron powder diffraction techniques.Two different types of magnetic structure have been found. One has the titanium and rare earth moments antiparallel along the c axis. The other structure has the rare earth moments in the ab plane with both ferromagnetic and antiferromagnetic components. In TbTiO₃, the terbium moment of (8.1 ± 0.4)μ_β has ferromagnetic and antiferromagnetic components along the [100] and [010] directions, respectively, with the moments lying at an angle of (36 ± 3)° to the [100] direction. In DyTiO₃, the dysprosium moment of (9.7 ± 0.7)μ_β has ferromagnetic and antiferromagnetic components along the [010] and [100] directions, respectively, with the moments making an angle of (31 ± 5)° to the [010] direction. TmTiO₃ has a thulium moment of (6.0 ± 0.4)μ_β in a ferromagnetic array along the [001] direction. The average titanium moment in the orthotitanites is (0.7 ± 0.3)μ_β in a direction antiparallel to the ferromagnetic component of the rare earth moment. The ytterbium moment in YbTiO₃ is quenched. It is found to be (1.7 ± 0.2)μ_β assuming a moment direction along [001]. The rare earth moment directions are found to be remarkably consistent in the series RMO₃, M = Ti, Cr, Fe and Al.     

Study of domain structures in FeCo/TiZr multilayers by means of 3D neutron depolarisation

Multilayers of composition 60(Co₆₈Fe₃₁V₁/Ti₇₅Zr₂₅) sputtered on Si wafers are studied by 3D neutron depolarisation analysis at glancing angles between −6.5° and 6.5°, both in zero and non-zero magnetic field. As the sample is rotated over a full turn in its plane, a 360° rather than 180° period is observed. This suggests that the magnetisation inside the CoFe sublayers is inclined by a few degrees out of the sample plane. Neighbouring FeCo sublayers cooperate by dipolar forces to form a structure of “inclined slab domains” with mutually antiparallel overall magnetisation. The thickness of these slabs includes 10–20 bilayers. In the hysteresis loop with the field parallel to the FeCo magnetisation a high remanence is found; when the field is (in-plane but) perpendicular to the FeCo magnetisation, the dipolar coupling is suppressed and a magnetisation loop with a lower remanence is observed.     

Critical current of YBa(2)Cu(3)O(7-delta) low-angle grain boundaries

Transport critical current measurements have been performed on 5 degrees [001]-tilt thin film YBa(2)Cu(3)O(7-delta) single grain boundaries with the magnetic field rotated in the plane of the film, phi. The variation of the critical current has been determined as a function of the angle between the magnetic field and the grain boundary plane. In applied fields above 1 T the critical current j(c) is found to be strongly suppressed only when the magnetic field is within an angle phi(k) of the grain boundary. Outside this angular range the behavior of the artificial grain boundary is dominated by the critical current of the grains. We show that the phi dependence of j(c) in the suppressed region is well described by a flux cutting model.     

理想磁流体中激波与矩形密度界面相互作用的数值研究

发展一套采用三阶WENO格式和混合GLM方法的理想磁流体数值方法,并对激波与矩形密度界面相互作用进行数值研究.通过圆极化阿尔芬波和旋转激波管问题对数值方法的稳定性和可靠性进行验证.在入射激波马赫数为10,界面内外气体密度比为10的情况,对比不同磁场中矩形密度界面的演变过程.结果表明,磁场能够减少界面上涡量的生成从而抑制界面不稳定性,并且磁场对界面的加速过程以及界面内外气体混合率有影响;而界面的存在将会使波后部分区域磁场增强;由于尖角的存在,矩形界面的发展与圆形界面不同.     

Magnetostriction relative to pretransition in Ni50.5Mn24.5Ga25 single crystal

The strain behaviors as well as the structural and magnetic changes relative to the pretransition in the Ni_50.5Mn_24.5Ga₂₅ single crystals have been characterized by various methods, such as pretransition strain, magnetostriction, magnetization measurements, and TEM observations. A large magnetostriction up to 505 ppm measured in the [001] direction of the sample is obtained at the pretransition temperature with only a low magnetic field of about 1 kOe applied along the [010] direction. We found that not only the pretransition strain pronounces a more large change, but also the magnetostriction at a certain temperature exhibits a more large magnitude for field applied along the [010] direction than with field along the [001] direction. It is concluded that the magnetoelastic interaction is responsible for the premartensitic transition, and the magnetoelastic interaction in the [010] direction is stronger than that in the [001] direction.     

Anisotropic magnetic properties and superzone gap formation in CeGe single crystal

Single crystals of CeGe and its non-magnetic analog LaGe have been grown by the Czochralski method. The CeGe compound crystallizes in the orthorhombic FeB-type crystal structure with the space group Pnma (#62). The anisotropic magnetic properties have been investigated for well oriented single crystals by measuring the magnetic susceptibility, electrical resistivity and heat capacity. It has been found that CeGe orders antiferromagnetically at 10.5?K. Both transport and magnetic studies have revealed large anisotropy, reflecting the orthorhombic crystal structure. The magnetization data at 1.8?K reveal metamagnetic transitions along the [010] direction at 4.8 and 6.4?T and along the [100] direction at a critical field of 10.7?T, while the magnetization along the [001] direction increases linearly without any anomaly up to a field of 16?T. From the magnetic susceptibility and the magnetization measurements it has been found that the [010] direction is the easy axis of magnetization. The electrical resistivity along the three crystallographic directions exhibits an upturn at T(N), indicating superzone gap formation below T(N) in this compound. We have performed crystal electric field analysis on the magnetic susceptibility and the heat capacity data and found that the ground state is a doublet, and the energies of splitting from the ground state to the first and second excited doublet states were estimated to be 39 and 111?K, respectively.     

导电突扩管中磁流体管流的数值模拟

采用基于OpenFOAM环境自主开发的低磁雷诺数磁流体求解器,对45°和90°突扩矩形管中液态金属流体在受到垂直流向的外加磁场作用时的速度、感应电流、压力的分布及突扩位置处的MHD三维现象进行数值模拟.结果表明：磁场沿突扩方向时,由于无回流涡,45°比90°突扩管在肩部位置速度分布更优.哈特曼数增大,强射流和突扩结构,在突扩肩部位置引发流动的不稳定性.伴随感应电流的不稳定,流动不稳定发展到突扩位置上游.磁场沿垂直突扩方向时感应电流的三维效应显著.哈特曼数增大,MHD压降显著增大.同方向磁场和相同哈特曼数,不同突扩角度的三维无量纲压力梯度无明显差异.     

Magnetization anisotropy of Ni dots with several tens of nanometer diameter

We have studied the magnetization of Ni dot with 50 to 70 nanometer diameter and 12 nanometer thickness using a magnetic force microscopy with an in-plane magnetic field. The Ni dots were prepared using self-assembled dot patterns with poly (styrene-b-methyl mathacrylate) diblock copolymers on Ni film and ion etching. It was found that the remanent magnetization direction of the dot was perpendicular to the plane as prepared. From the vibrating sample magnetometer measurement, a hysteresis loop was found in the perpendicular magnetization. When an in-plane external magnetic field was applied, the magnetization was rotated into a horizontal direction with low coercivity along the field direction.     

On the theory of electromagnetic surface waves in magnetized semiconductor plasma

The paper demonstrates existence of electromagnetic surface waves at the boundary separating a magnetized semiconductor plasma and a dielectric or metal. The external magnetic field is along the interface. As has been shown, slow surface waves of the helicon or Alfven type can exist only with their propagation vector directed obliquely with respect to the magnetic field. The ω-k relations have been found and ranges of existence established both in the frequency domain and that of angles between the propagation vector and the magnetic field.     

Observation of ferro- and antiferromagnetic domains in the Europium-Chalcogenides

Single crystals of the Eu-Chalcogenides show a scattering of light and a depolarization of polarized light at temperatures below the Curie- or Néel point, respectively, due to the existence of magnetic domains. These domains could be observed with a polarizing microscope. In antiferromagnetic EuSe (between 3.5° and 4.6°K), domains exist with optically active axes along the 6 [100] directions. The domain size in ferromagnetic EuS was at the limit of the resolution of the microscope. However, by applying a magnetic field larger domains were obtained. The direction of the magnetization within the domains is stabilized by internal stress. Due to the strong dichroism of the Eu-Chalcogenides, domains could be observed with only one polarizing unit.Busch, Junod andWachter [1, 2] have found a large red-shift of the absorption edge with the magnetization. By means of this effect domains with different magnetizations could be observed without any polarizer due to their different absorption.     

Interface induced exchange bias effect in La0.67Sr0.33MnO3/SrIrO3 multilayer

Epitaxial superlattices of ferromagnetic/paramagnetic La_0.67Sr_0.33MnO₃/SrIrO₃ materials have been prepared on SrTiO₃ (100) substrate using pulse laser deposition technique. An unexpected onset of interface magnetic interaction has been observed around 40 K. Interestingly, magnetic exchange bias effect has been observed in both field cooled and zero field cooled magnetization loops, however, the shifting of loop is opposite in both measurements. Exchange bias field vanishes as temperature increases to interface magnetic ordering temperature. Moreover, exchange bias field is found to decrease with increasing cooling field. We believe that tuning of magnetic exchange at interface during field cooling induces this evolution in nature of exchange bias field.     

Diurnal variations in cosmic ray intensity caused by high latitude magnetospheric-ionospheric current systems

Cosmic ray intensity measurements obtained with the neutron monitors at Thule (geomagnetic latitude, 84.8°) and McMurdo (geomagnetic latitude, −79.9°) in 2007–2009 were used to test the effect of the magnetic field generated by magnetospheric currents flowing along geomagnetic field lines in the high latitude region on cosmic ray intensity. The existence of such a relation in a region where the geomagnetic field lines are virtually radial should revise our concept of the unimpeded access that cosmic ray particles have to the Earth’s atmosphere.     

Creation of an antiferromagnetic exchange spring

We present evidence for the creation of an exchange spring in an antiferromagnet due to exchange coupling to a ferromagnet. X-ray magnetic linear dichroism spectroscopy on single crystal Co/NiO(001) shows that a partial domain wall is wound up at the surface of the antiferromagnet when the adjacent ferromagnet is rotated by a magnetic field. We determine the interface exchange stiffness and the antiferromagnetic domain wall energy from the field dependence of the direction of the antiferromagnetic axis, the antiferromagnetic pendant to a ferromagnetic hysteresis loop. The existence of a planar antiferromagnetic domain wall, proven by our measurement, is a key assumption of most exchange bias models.