Magnetic Impurity Band and Photoinduced Magnetic Phase Transition in Diluted Magnetic Semiconductors

Applying the dynamical coherent potential approximation (dynamical CPA) to a model of diluted magnetic semiconductors (DMSs), in which both random impurity distribution and thermal fluctuation of localized spins are taken into account, the spin-polarized band and the carrier spin polarization are calculated for various magnetizations. In order to clarify the role of impurity depth on the occurrence of ferromagnetism, three typical cases are investigated: (a) II-VI DMS, (b) deep impurity level, and (c) strong exchange interaction. The present study reveals that the impurity depth of magnetic ions strongly enhances the carrier spin polarization (CSP) and accordingly, leads to a high Curie temperature. This means that photoinduced ferromagnetism with high Curie temperature can be expected in a DMS with a deep impurity depth and strong exchange interaction.     

磁传感器与磁场的测量

在中学物理教学中，研究磁场传统的实验方法是用小磁针显示磁场的方向，用小磁针或铁粉显示磁感线分布．磁针和铁粉用来形象描述磁场功不可没，可对磁感强度的定量研究却显得无能为力．学生也只好在题海中领会感受磁感强度B的大小；1特斯拉有多大?在脑子里只有一个模糊抽象的印象．物理学对电磁现象的研究成果为各行各业提供了先进的检测设备，可是在自身的物理教学中对磁场的实验检测仍停留在4大发明的“司南”时代．近来上海的DIS实验(数字化实验系统)，     

Magnetic Textures and Dynamics in Magnetic Weyl Semimetals

Recent theoretical and experimental attempts have been successful in finding magnetic Weyl semimetal phases, which show nodal-point structure in the electronic bands as well as magnetic orders. Beyond uniform ferromagnetic or antiferromagnetic orders, nonuniform magnetic textures, such as domain walls and skyrmions, enrich the properties of the Weyl electrons even more in such materials. Here, a topical review on interplay between Weyl electrons and magnetic textures in those magnetic Weyl semimetals is given. The basics of magnetic textures in nontopological magnetic metals are reviewed first, and then the effect of magnetic textures in Weyl semimetals is discussed, regarding the recent theoretical and experimental progress therein. The idea of the fictitious “axial gauge fields” is pointed out, which effectively describes the effect of magnetic textures on the Weyl electrons and can well account for the properties of the electrons localized around magnetic domain walls.     

Exploitable Magnetic Anisotropy of Magnetic CrBr3 Monolayer

JETP Letters - We investigated the influence of Li and F adsorption on the ferromagnetism and magnetic anisotropy energy of CrBr3 monolayer based on the first-principles calculations. It is...     

Magnetic geodynamics

Several decades since the work E. P. Velikhov, Zh. Éksp. Teor. Fiz. 36, 1398 (1959) [Sov. Phys. JETP 9, 995 (1959)] [1] concerning magnetorotational instability was published, great astrophysical interest has been manifested in the mechanism of generating a magnetic field in a rotating well-conducting medium in view of difficulties in the development of the theory of anomalous matter transfer in accretion discs both upon the formation of stars and planets from gaseous conglomerations and upon the formation of a galactic core with a black hole at the center { S. A. Balbus and J. F. Hawley, Astrophys. J. 376, 214 (1991) [2] and G. Ruediger and R. Hollerbach, The Magnetic Universe (Wiley-VCH, Weinheim, 2004) [3]}. Attempts to experimentally observe the magnetorotational instability were successful only for spherical geometry in experiments initially devoted to the verification of geomagnetic dynamo theory {D. R. Sisan et al., Phys. Rev. Lett. 93, 114502 (2004) [4]}. In experiments with liquid sodium in the complete absence of temperature gradients and, therefore, convection, which is very important for the conventional theory of the geomagnetic dynamo, the generation of the magnetic field was obtained due to the development of the magnetorotational instability, which is usually ignored when developing the theory of the origin of the Earth’s magnetic field. The results obtained in this work enable one to develop a theory of geomagnetic dynamo that is primarily based on the magnetorotational instability, which provides a new insight into not only the origin of the Earth’s magnetic field but its evolution in time.     

Magnetic superconductors

Simultaneous occurence of magnetism and superconductivity in Mo₅SnGa_0.5S₆ has been reported by Alekseevskii, Dobrovol'skii and Tsebro. X-ray studies show that this results from two phase mixtures of Mo₃Sn_0.5S₄ and Mo₂Ga_0.5S₄ which have previously been reported to be superconducting and ferromagnetic respectively.     

Magnetic semiconductors

As far as the electrical conductivity is concerned, solids are usually classified as metals, semiconductors, or insulators. In metals the concentration of the charge carriers responsible for the electrical conductivity is large, whereas in semiconductors and insulators the carrier concentration is much smaller. The distinction between semiconductors and insulators is based on a difference in the nature of the conductivity. For semiconductors the charge carriers (electrons or holes) occupy the states of energy bands; these states are not Iocalized on particular atoms, but spread throughout the entire crystal. In such a situation the mobility of the carriers can be quite high and would, in fact, be infinite in a rigid periodic lattice; in this model the thermal motion of the atoms induces a scattering of the carriers and thus limits the conductivity to finite values. The classical examples of semiconductors are the elements Si and Ge and compounds such as GaAs, InSb, CdTe, ZnS, etc.     

Magnetic nanostructures

Magnetic materials have become controllable on the nanometre scale. Such fine structures exhibit a wide range of fascinating phenomena, such as lowdimensional magnetism, induced magnetization in noble metals, electron interference patterns, oscillatory magnetic coupling and 'giant' magnetoresistance. Magnetic multilayers with nanometre spacings are among the first metallic quantum structures to become incorporated into electronic devices, such as reading heads for hard discs. This article is intended to familiarize the reader with the physics and technology of magnetic nanostructures. It starts out with recent progress in nanofabrication, gives a tutorial on the connection between electronic states and magnetic properties, surveys the state of the art in characterization techniques, explains unique phenomena in two-, one- and zero-dimensional structures, points out applications in magnetic storage technology and considers fundamental limits to storage density. Particular emphasis is placed on the connection between magnetism and the underlying electronic states, such as the spin-split energy bands, s, p versus d states, surface states, and quantum well states.     

Magnetic monopoles

Summary Theoretical foundations for the existence of magnetic monopoles are given. Consequences extracted from cosmological and astrophysical considerations on the abundance of magnetic monopoles today are reviewed. The latest experimental results in the search for classical monopoles as well as for grand unified monopoles are summarized.

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Riassunto Si danno i fondamenti teorici per l'esistenza di monopoli magnétici. Si esaminano le conseguenze tratte da considerazioni cosmologiche e astrofisiche sull'abbondanza di monopoli magnetici attuale. Si riassumono i piú recenti risultati sperimentali sulla ricerca di monopoli classici e di quelli della grande unificazione.

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Резюме Приводятся теоретические обоснования сушствования магнитных монополей. Анализируются следствия из космологических и астрофизических рассмотрений, касающиеся распространенноси магнитных мононолей в настоящее время. Приводятся последние экспериментальные результаты по поискы классических монополей и монополей Великого Объединения.

     

Magnetic surfaces

The magnetic ground state, magnetic anisotropies and spin excitations of surfaces, interfaces and ultra-thin films of ferromagnetic 3d-metals are discussed. Enhanced magnetic ground state moments and altered hyperfine fields as predicted by ab initio band calculations have not been conclusively verified by experiments up to now. Future calculations should take into account dipolar fields and the role of interface roughness. Very large magnetic anisotropies are observed at magnetic surfaces and interfaces. In Ni/Cu multilayered films, the superposition of surface and stress-induced anisotropies was used to switch the easy axis of magnetization from the film plane to a perpendicular orientation by a proper choice of the Ni layer thickness. This could be an attractive possibility to develop new magnetic materials for technical applications. The temperature dependence of the spontaneous magnetization at surfaces and in ultra-thin films deviates from the behaviour of bulk material. Size effects as well as surface effects of spin wave excitations are discussed, comparing theoretical and experimental results. The need for more complete theories including surface exchange, surface anisotropy and realistic surface structures is emphasized.     

磁性石墨

德国来比锡大学的EsquinaziP教授及其同事们利用质子对石墨样品进行辐射,可使石墨在室温下转变为一种纯碳的、不含金属的、轻型的磁性材料.众所周知,碳元素具有许多值得注意的固体形态:一种是粉末状的石墨,由于它在二维层间只有松散的键约束,因此可用来作为润滑油或铅笔芯;第二种是钻石,由于它在三维空间内都有紧密的约束而成为坚硬的宝石;第三种就是著名的富勒球;其他还有碳纳米管等.所有这些固态碳都具有重要的电性质,但都没有磁性.迄今为止还没有任何一种由纯碳组成的样品具有磁性,当然除了保持温度在绝对零度附近的掺杂的石墨样品.来比…     

Magnetic nitrides

Nitrogen enters 3d metals and 3d–4f intermetallic compounds as an interstitial provoking a dilation of the lattice. Iron-based alloys are most susceptible to modification by nitrogen, which can turn them from weak to strong ferromagnets. In the 3d–4f compounds it is a major contributor to the crystal field. The influence of nitrogen in BCC, FCC and HCP iron, and then in iron-based intermetallics with 2 : 17, 1 : 12 and 3 : 29 structure types is reviewed. Important issues are the role of nitrogen in enhancing the iron moment and raising the Curie temperature. Progress in processing nitrogen-containing compounds is outlined, and applications of iron nitrides as permanent magnets and as materials for use in magnetic recording are discussed.     

Magnetic superconductors

The old problem of the coexistence of superconductivity and magnetism has obtained new interest since the discovery of the unusual magnetic and superconducting properties of some ternary compounds. In this report I want to discuss the influence of antiferromagnetic order on the superconducting state and, in particular, will compare different pairing approximations which have been proposed in the literature. Furthermore I will investigate the influence of fluctuations on the stability of the periodic spin structure and other superconducting phases at the onset of ferromagnetic order.     

Magnetic nanowires

We review recent developments in the research on magnetic nanowires electrodeposited into pores of membranes. Typical nanowires fabricated by this method have a diameter in the range 30–500 nm for a length of the order of 10 μm, and can be composed of a stack of layers of different metals with thicknesses in the nanometer range (multilayered nanowires). We describe the preparation methods and present typical examples of structural characterization. We review the magnetic properties with examples of results on both arrays of nanowires and isolated nanowires. We then describe the magnetoresistance properties of multilayered nanowires, and their interest for their understanding of the CPP–GMR and the determination of spin diffusion lengths. The last section is an overview on the perspectives of future research.     

Magnetic monopoles

These are notes of the first part of the lectures given at the JINR-ISU Baikal Summer School on Physics of Elementary Particles and Astrophysics (July 2010). I review classical monopole solutions of the SU(2) Yang-Mills-Higgs theory providing a pedagogical introduction into to the theory of non-Abelian monopoles both in the BPS limit and beyond of it. I briefly discuss monopole dynamics, the idea of the moduli space and some of the basic properties which are connected with the field theoretical aspects of these classical solutions.     

Oscillations of Magnetic Fluid Column in Strong Magnetic Field

Russian Physics Journal - The paper considers the results of measuring the elastic parameters (ponderomotive elasticity coefficient, oscillation frequency, attenuation coefficient) of the...     

Interaction of a Magnetic Vortex with Magnetic Anisotropy Nonuniformity

Journal of Experimental and Theoretical Physics - The problem of propagation of a magnetic inhomogeneity in the form of a magnetic vortex near a defect simulated by a crystallite with uniaxial...     

Diffusion-Weighted Magnetic Resonance Imaging in an Ultra-Low Magnetic Field

Magnetic resonance imaging is well known as a highly effective technique of medical visualization. One of its relatively new approaches is diffusion imaging. As a rule, the majority of magnetic resonance investigations in biology and medicine tends to be carried out in high magnetic fields (1.5 T and higher), but there are some advantages of the same experiments in low magnetic fields. It can be strongly useful, for example, for designing and testing new pulse sequences, training operators of magnetic resonance imagers, making new phantoms (model objects). In this study, diffusion-weighted imaging experiments in a low magnetic field 7 mT are performed in the first time. Nevertheless, this field is about two orders of magnitude bigger than an extremely low Earth field, and so concomitant gradients and polarization problems do not arise. In particular, diffusion weighted images of combined model samples (phantoms) are presented.