Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

Fast parallel algorithms for short-range molecular dynamics

The interaction of subpicosecond laser pulses with magnetically ordered materials has developed into a fascinating research topic in modern magnetism. From the discovery of subpicosecond demagnetization over a decade ago to the recent demonstration of magnetization reversal by a single 40 fs laser pulse, the manipulation of magnetic order by ultrashort laser pulses has become a fundamentally challenging topic with a potentially high impact for future spintronics, data storage and manipulation, and quantum computation. Understanding the underlying mechanisms implies understanding the interaction of photons with charges, spins, and lattice, and the angular momentum transfer between them. This paper will review the progress in this field of laser manipulation of magnetic order in a systematic way. Starting with a historical introduction, the interaction of light with magnetically ordered matter is discussed. By investigating metals, semiconductors, and dielectrics, the roles of nearly free electrons, charge redistributions, and spin-orbit and spin-lattice interactions can partly be separated, and effects due to heating can be distinguished from those that are not. It will be shown that there is a fundamental distinction between processes that involve the actual absorption of photons and those that do not. It turns out that for the latter, the polarization of light plays an essential role in the manipulation of the magnetic moments at the femtosecond time scale. Thus, circularly and linearly polarized pulses are shown to act as strong transient magnetic field pulses originating from the nonabsorptive inverse Faraday and inverse Cotton-Mouton effects, respectively. The recent progress in the understanding of magneto-optical effects on the femtosecond time scale together with the mentioned inverse, optomagnetic effects promises a bright future for this field of ultrafast optical manipulation of magnetic order or femtomagnetism.

/pdf/ultrafast-optical-manipulation-of-magnetic-order-2ms2t5k1zo.pdf

Ultrafast optical manipulation of magnetic order

Ferromagnetic materials exhibit intrinsic `easy' and `hard' directions of the magnetization. This magnetic anisotropy is, from both a technological and fundamental viewpoint one of the most important properties of magnetic materials. The magnetic anisotropy in metallic magnetic multilayers forms the subject of this review article. As individual layers in a multilayer stack become thinner, the role of interfaces and surfaces may dominate that of the bulk: this is the case in many magnetic multilayers, where a perpendicular interface contribution to the magnetic anisotropy is capable of rotating the easy magnetization direction from in the film plane to perpendicular to the film plane. In this review, we show that the (in-plane) volume and (perpendicular) interface contribution to the magnetic anisotropy have been separated into terms related to mechanical stresses, crystallographic structure and the planar shape of the films. In addition, the effect of roughness, often inherent to the deposition techniques used, has been addressed theoretically. Several techniques to prepare multilayers and to characterize their growth as well as methods to determine the magnetic anisotropy are discussed. A comprehensive survey of experimental studies on the perpendicular magnetic anisotropy in metallic multilayers containing Fe, Co or Ni is presented and commented on. Two major subjects of this review are the extrinsic effects of strain, roughness and interdiffusion and the intrinsic effect of the crystallographic orientation on the magnetic anisotropy. Both effects are investigated with the help of some dedicated experimental studies. The results of the orientational dependence studies are compared with ab initio calculations. Finally, the perpendicular surface anisotropy and the in-plane step anisotropy are discussed.

https://iopscience.iop.org/article/10.1088/0034-4885/59/11/002/pdf

Magnetic anisotropy in metallic multilayers

Research efforts addressing spin waves (magnons) in micro- and nanostructured ferromagnetic materials have increased tremendously in recent years. Corresponding experimental and theoretical work in magnonics faces significant challenges in that spin-wave dispersion relations are highly anisotropic and different magnetic states might be realized via, for example, the magnetic field history. At the same time, these features offer novel opportunities for wave control in solids going beyond photonics and plasmonics. In this topical review we address materials with a periodic modulation of magnetic parameters that give rise to artificially tailored band structures and allow unprecedented control of spin waves. In particular, we discuss recent achievements and perspectives of reconfigurable magnonic devices for which band structures can be reprogrammed during operation. Such characteristics might be useful for multifunctional microwave and logic devices operating over a broad frequency regime on either the macro- or nanoscale.

https://iopscience.iop.org/article/10.1088/0953-8984/26/12/123202/pdf

Review and prospects of magnonic crystals and devices with reprogrammable band structure

Magnons—the quanta of spin waves—propagating in magnetic materials with wavelengths at the nanometer-scale and carrying information in the form of an angular momentum, can be used as data carriers in next-generation, nano-sized low-loss information processing systems. In this respect, artificial magnetic materials with properties periodically varied in space, known as magnonic crystals, are especially promising for controlling and manipulating the magnon currents. In this article, different approaches for the realization of static, reconfigurable, and dynamic magnonic crystals are presented along with a variety of novel wave phenomena discovered in these crystals. Special attention is devoted to the utilization of magnonic crystals for processing of analog and digital information. Magnonic crystals for data processing 2

https://iopscience.iop.org/article/10.1088/1361-6463/aa6a65/pdf

Magnonic crystals for data processing

Laser-induced irreversible modifications of magnetic properties of molecular beam epitaxy grown X/Co/Y trilayers on Al2O3 substrates were investigated, with combinations of nonmagnetic metals Pt and Au as buffer X and capping layer Y. It is shown that the formation of regions with irreversibly rotated magnetization from in-plane to out-of-plane orientation occurred for Pt/Co/Pt, Pt/Co/Au, and Au/Co/Pt trilayers. In the Au/Co/Au system, the modification of magnetic properties was negligible. Pt/Co/Pt trilayers were also deposited by sputtering on Si/SiO2 substrates. In this case, a significant suppression of laser-induced magnetic modifications was observed and correlated with the lower thermal conductivity of the Si/SiO2 substrate.

Femtosecond Laser Pulse-Induced Perpendicular Magnetization in Co Ultrathin Films With Diverse Surroundings

Ultrathin ferromagnetic films are of great interest for study magnetic properties due to their potential application as a high density storage media. In cobalt films (2-8 monolayers) epitaxially grown on Au(111) one can observe the spin reorientation transition (SRT) from out-of-plane magnetization for thin films to in-plane for thick films [1]. According to the theoretical predictions [2], the domain size drastically decreases approaching SRT. The domains became too small to be observed with magneto-optical Kerr effects. Magnetic force microscopy (MFM) is a well-established technique for imaging of small magnetic structures. In cooperation with external magnetic fields, MFM can be used to study the magnetization process with nanometer spatial resolution, e.g. it allows to measure a hysteresis curve even for an individual particle, which can be compared to ensemble measurements by magneto-optical techniques. The advantage of MFM-constructed hysteresis loops is an opportunity of understanding the origin of dispersion in switching fields. It is not easy to obtain good quality MFM images for ultrathin magnetic film due to the low magnetic signal and possible sample magnetization reversal during scanning [1]. In present work we study the magnetic domain structure of cobalt film near SRT and the influence of an external magnetic field on it by MFM.

MFM STUDY OF DOMAIN STRUCTURE IN ULTRATHIN Co FILMS UNDER EXTERNAL MAGNETIC FIELD

Domain formation in polycrystalline garnet films

Algorithm of magnetic structure extraction and analysis in ultra thin magnetic film images is proposed. This algorithm allows to gain successful results and calculates a large number of characteristics of structures on magneto-optical images. The algorithm consists of three parts: background correction, structure extraction and calculation of dendrite characteristics. For correction of magnetic image we use methods for magneto-optic contrast. One of the basic features of structure extraction part is a preservation of border properties. Therefore the basic processing of extraction is applied for binary image. Proposed characteristics allow to describe and to classify dendrites. For analysis of complex magnetics structures topological properties are calculated for three basic objects types: body, holes and gulf. The software system ldquoZubrrdquo has been realized for magnetic structure image analysis.

The structure analysis of ultra thin magnetic film images

Masako SAKAMAKI, Takehisa KONISHI, Takashi FUJIKAWA, Shougo KOUZAI, Hitoshi ABE, Lech Tomasz BACZEWSKI, Andrzej WAWRO, Alexei PETROUTCHIK, Zbigniew KURANT, Andrzej MAZIEWSKI, Rohei SUMII, Kenta AMEMIYA* Graduate School of Advanced Integration Science, Chiba University, Chiba, Japan Faculty of Science and Technology, Keio University, Yokohama, Japan Institute of Physics, Polish Academy of Science, Warsaw, Poland Institute of Experimental Physics, University of Bialystok, Poland 5 KEK-PF, Tsukuba, Japan

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Andrzej Maziewski

Papers

Femtosecond Laser Pulse-Induced Perpendicular Magnetization in Co Ultrathin Films With Diverse Surroundings

MFM STUDY OF DOMAIN STRUCTURE IN ULTRATHIN Co FILMS UNDER EXTERNAL MAGNETIC FIELD

Domain formation in polycrystalline garnet films

The structure analysis of ultra thin magnetic film images

Magnetization Depth Profile in Ultrathin Mo/Co/Au and Au/Co/Au Trilayers