Magnetic Information Storage Research Articles

Future science at next generation neutron sources

We review some of the scientific opportunities which will become available with the advent of increased fluxes of polarized neutrons and corresponding advances in neutron instrumentation. These range from the study of magnetism in thin films and novel nanostructures with application in magnetic information storage and retrieval, to the study of spin-dependent inelastic scattering from highly correlated magnetic systems, frustrated magnetic systems, etc. Extensions of neutron spin echo techniques offer further opportunities for advances in our knowledge of the dynamics of soft condensed matter systems.

Magnetic modelling at nanoscale level

With the development of the information technology in all areas of present life, high-density data storage devices are in great demand. Currently, the bit dimension is below a micron and in the future will approach the nanoscale level. At present, not many theoretical approaches are available to explain the magnetic information storage properties at nanoscale level, as there is a lack of experimental data for example. This is due to difficulty in implementing measurement techniques for magnetic properties at this low scale. In this article, we have presented a short review of the most used methods to represent the physical behaviour at microscale level. Our goal was to apply some of these methods at a nanoscale range, where the physical properties change dramatically and various physical effects cannot be neglected that are not considered at micron scale. Some of the results regarding the simulation of magnetic properties of the nanostructured patterned media and comparison with experimental data are also presented.

Preface: phys. stat. sol. (a) 201/8

The KMS/SOMMA Meeting 2003 was held 3–6 December 2003 at Spapia Hotel, Daejeon, Korea. It was the 5th SOMMA (International Symposium on Magnetic Materials and Applications) organized by ReCAMM (Research Center for Advanced Magnetic Materials) of Chungnam National University. Since 2002, the Korean Magnetics Society (KMS) winter conference has been jointly held with SOMMA. This was the second time to have a KMS/SOMMA joint meeting. The main objective of the meeting was to provide an international forum to discuss up-to-date results on magnetism and magnetic materials. The conference brought together 360 participants from 12 countries. Sessions of the meeting were: Theory and Fundamentals, Magnetic Random Access Memory, Spintronics, Information Storage, Nanostructured Materials, Sensors, and Interdisciplinary. In these seven sessions, 325 papers were presented including 66 oral and 259 poster presentations. Since the symposium was held in Korea, this enabled a large number of Asian scientists to attend: 239 from Korea, 41 from Japan, 7 from Taiwan, and 5 from China. The conference program had 25 invited and plenary speakers. They were Y. Ando (Tohoku U.), M. Inoue (Toyohashi U. Tech), H. Kubota (Tohoku U.), K. Mohri (Nagoya U.), M. Sahashi, M. Takahashi, K. Takanashi, M. Tsunoda (Tohoku U.), and H. Yoda (Toshiba) from Japan; A. J. Freeman (Northwestern U.), A. T. Hanbicki (NRL), F. B. Humphrey (Boston U.), and S. Sun (IBM) from the USA; J. D. Boeck (IMEC, Belgium), B. Dieny (CEA, France), N. Garcia (CSIS, Spain), G. Reiss (Bielefeld U., Germany), T. Stobiecki (U. M. & M. Krakow, Poland), and M. Wolfram (Singulus Tech, Germany) from Europe; C. G. Kim, D. J. Kim (CNU), T. W. Kim (SAIT), S. H. Lim (KIST), Sung-Chul Shin (KAIST), and Yoon Hee Chung (POSTEC) from Korea. For the first time, the SOMMA Proceedings appear in physica status solidi. The Editors hope that the Proceedings could provide chances for deeper and wider understanding of the presentations as well as for enhanced relationship between all participants. We deeply appreciate the help of the editorial staff of physica status solidi for their efficient and kind help during the paper preparations and publications. Finally, we would like to take this opportunity to thank all members of the Advisory Committee, Organizing Committee, referees, and KMS staff for their effort before, during, and after the meeting.

Preface: phys. stat. sol. (b) 241/7

AbstractThe KMS/SOMMA Meeting 2003 was held 3–6 December 2003 at Spapia Hotel, Daejeon, Korea. It was the 5th SOMMA (International Symposium on Magnetic Materials and Applications) organized by ReCAMM (Research Center for Advanced Magnetic Materials) of Chungnam National University. Since 2002, the Korean Magnetics Society (KMS) winter conference has been jointly held with SOMMA. This was the second time to have a KMS/SOMMA joint meeting.The main objective of the meeting was to provide an international forum to discuss up‐to‐date results on magnetism and magnetic materials. The conference brought together 360 participants from 12 countries. Sessions of the meeting were: Theory and Fundamentals, Magnetic Random Access Memory, Spintronics, Information Storage, Nanostructured Materials, Sensors, and Interdisciplinary. In these seven sessions, 325 papers were presented including 66 oral and 259 poster presentations. Since the symposium was held in Korea, this enabled a large number of Asian scientists to attend: 239 from Korea, 41 from Japan, 7 from Taiwan, and 5 from China.The conference program had 25 invited and plenary speakers. They were Y. Ando (Tohoku U.), M. Inoue (Toyohashi U. Tech), H. Kubota (Tohoku U.), K. Mohri (Nagoya U.), M. Sahashi, M. Takahashi, K. Takanashi, M. Tsunoda (Tohoku U.), and H. Yoda (Toshiba) from Japan; A. J. Freeman (Northwestern U.), A. T. Hanbicki (NRL), F. B. Humphrey (Boston U.), and S. Sun (IBM) from the USA; J. D. Boeck (IMEC, Belgium), B. Dieny (CEA, France), N. Garcia (CSIS, Spain), G. Reiss (Bielefeld U., Germany), T. Stobiecki (U. M. & M. Krakow, Poland), and M. Wolfram (Singulus Tech, Germany) from Europe; C. G. Kim, D. J. Kim (CNU), T. W. Kim (SAIT), S. H. Lim (KIST), Sung‐Chul Shin (KAIST), and Yoon Hee Chung (POSTEC) from Korea.For the first time, the SOMMA Proceedings appear in physica status solidi. The Editors hope that the Proceedings could provide chances for deeper and wider understanding of the presentations as well as for enhanced relationship between all participants. We deeply appreciate the help of the editorial staff of physica status solidi for their efficient and kind help during the paper preparations and publications.Finally, we would like to take this opportunity to thank all members of the Advisory Committee, Organizing Committee, referees, and KMS staff for their effort before, during, and after the meeting.

Current-induced domain-wall switching in a ferromagnetic semiconductor structure.

Magnetic information storage relies on external magnetic fields to encode logical bits through magnetization reversal. But because the magnetic fields needed to operate ultradense storage devices are too high to generate, magnetization reversal by electrical currents is attracting much interest as a promising alternative encoding method. Indeed, spin-polarized currents can reverse the magnetization direction of nanometre-sized metallic structures through torque; however, the high current densities of 10(7)-10(8) A cm(-2) that are at present required exceed the threshold values tolerated by the metal interconnects of integrated circuits. Encoding magnetic information in metallic systems has also been achieved by manipulating the domain walls at the boundary between regions with different magnetization directions, but the approach again requires high current densities of about 10(7) A cm(-2). Here we demonstrate that, in a ferromagnetic semiconductor structure, magnetization reversal through domain-wall switching can be induced in the absence of a magnetic field using current pulses with densities below 10(5) A cm(-2). The slow switching speed and low ferromagnetic transition temperature of our current system are impractical. But provided these problems can be addressed, magnetic reversal through electric pulses with reduced current densities could provide a route to magnetic information storage applications.

Organized single-molecule magnets: direct observation of new Mn12 derivatives on gold

Gold adsorbates of the dodecamanganese(III,IV) single-molecule magnet (SMM) [Mn 12 O 12 ( L ) 16 (H 2 O) 4 ] where L =16-(acetylthio)hexadecanoate have been prepared and investigated by X-ray photoelectron spectroscopy and scanning tunneling microscopy (STM). The successful imaging of Mn 12 molecules by STM represents a first step toward the magnetic addressing of individual SMMs and the development of molecule-based devices for magnetic information storage.

Cover Picture: Cyanide‐Bridged Iron(III)–Cobalt(II) Double Zigzag Ferromagnetic Chains: Two New Molecular Magnetic Nanowires (Angew. Chem. Int. Ed. 13/2003)

Magnetic nanowires are among new objects that chemists can design with a view to magnetic information storage. The cover picture shows views of an ordered bimetallic double chain of iron(III) and cobalt(II) ions, bridged by cyanide groups. Each chain has remarkable anisotropic magnetic properties (axes of easy magnetization are shown) and shows slow relaxation of the magnetization. Find out more about these nanowires in the Communication by M. Julve, M. Verdaguer et al. on page 1483 ff.

TEM of epitaxial thin films controlled by planes extending (near) normal to interface; with application to two methods to reduce crystal orientations in polycrystalline magnetic media

Abstract This work is a study of heteroepitaxial interfaces as applied to multilayer-thin films for magnetic information storage media. With a goal to develop a film crystallography that optimizes the alignment of magnetic dipoles to coincide with the write/read signal of the recording head, two TEM observations have elucidated a better understanding of what controls heteroepitaxial interfaces. The classical approach to establishing “lattice matching” of interfaces is to model the top plane of atoms of the substrate and then align the next plane of atoms in the subsequently deposited film, i.e. plane A and plane B should “match” (with minimal misfit), with both planes being parallel to the interface. Such mechanism is valid for idealized slow MBE growth where the planes remain atomically flat. However, most film deposition conditions quickly violate this atomically flat configuration. Here growth on a roughened interface is shown to be controlled by the matching of planes that extend (normal or near-normal) across the interface. A second classical observation is the nucleation of bi-crystals, which naturally increases the number of crystal orientations in subsequent films. However, this work exhibits two cases of reducing orientations! One case has a 3-D isotropically oriented cubic film followed by a hexagonal film with 2-&-1/4-D isotropy, and a second case where a 2-D random cubic film is followed by a hexagonal film with 1-&-1/2-D isotropy. The understanding and control of these heteroepitaxial interfaces enables reduction of film orientations to enhance properties, such as 100 Gigabit per-square-inch magnetic recording.

The Little Maghemite Story: A Classic Functional Material

It is presumably one of the curiosities of our modern “information society” that the public at large is virtually unaware of the role of magnetism in everyday life. Ask your next door neighbors and you will find that some may perhaps associate magnetism with ship navigation using a compass; the majority probably consider the topic a long-forgotten elementary school experience. Nonetheless, magnetic information storage, either analog or digital, be it on tapes or on disks, is a truly universal technology and of tremendous importance both in the consumer and professional markets. The possibility of storing and retrieving essential information has become so natural that only a sudden data loss makes us realize how indispensable it really is—especially when your own computer hard disk suddenly crashes. While I personally believe that most digital information is better dumped than preserved, we all would certainly love to keep the audio and video memories of our beloved ones accessible for eternity. Here’s a little story reminding us of one of the materials that made it all possible, and of the science and scientists involved. Because of the opulent scientific literature, the following historical account can only be considered a rough

Soft x-rays for deep sub-100 nm lithography, with and without masks

The development of micro- and nanofabrication, their applications, and their dependent industries has progressed to a point where a bifurcation of technology development will likely occur. On the one hand, the semiconductor industry (at least in the USA) has decided to develop EUV and SCALPEL to meet its future needs. Even if the semiconductor industry is successful in this (which is by no means certain) such tools will not be useful in most other segments of industry and research that will employ nanolithography. As examples, MEMS, integrated optics, biological research, magnetic information storage, quantum-effect research, and multiple applications not yet envisioned will not employ the lithography tools of the semiconductor industry, either because they are too expensive, insufficiently flexible, or lacking in accuracy and spatial-phase coherence. Of course, direct-write electron-beam lithography can meet many of these non-semiconductor-industry needs, but in other cases a technique of higher throughput or broader process-latitude is necessary. Our experience at MIT in applying low-cost proximity x-ray nanolithography to a wide variety of applications leads us to conculde that this technology can provide the alternative path of a bifurcation. A new version of x-ray nanolithography, zone-plate-array lithography (ZPAL), does not require a mask, and has the potential to reach the limits of the lithographic process with a projection process.

Fabrication of 200 nm period nanomagnet arrays using interference lithography and a negative resist

Magnetic information storage density has increased at the rate of 60% per year for the past seven years. There is wide agreement that continuation of this trend beyond the physical limits of the continuous thin-film media currently used will likely require a transition to discrete, lithographically defined magnetic pillars. Interference lithography (IL) appears to be the most cost-effective means of producing two-dimensional arrays of such pillars. IL can rapidly expose large areas with relatively simple equipment, without the need for a mask, and with fine control of the ratio of pillar diameter to period. We show that negative-tone imaging yields three times the contrast of positive-tone imaging for the generation of holes in photoresist, suitable for subsequent deposition or electroplating of magnetic material. We use a negative i-line, chemically-amplified resist (OHKA THMR-iN PS1) to form 200 nm period arrays of magnetic dots in Co and Ni. Such arrays, with a variety of well controlled diameters, are used to study the effect of particle size on magnetic behavior.

Writing and reading of single magnetic domain per bit perpendicular patterned media

By fabricating patterned media with a large number of nanoscale single domain magnetic particles embedded in a nonmagnetic substrate, and by writing the magnetization for each of these particles in a desired direction, nonvolatile magnetic storage of information could reach densities much higher than what is currently thought possible for longitudinal continuous media. We have fabricated high aspect ratio perpendicular nickel columnar nanoparticles embedded in a hard Al2O3/GaAs substrate. We show that the magnetization states of the individual magnets can be controlled by demonstrating that prototype patterned “single magnetic domain per bit” data tracks can be written and read back using current magnetic information storage technology.

Soft X-Rays for Deep Sub-100 Nm Lithography, with and Without Masks

AbstractThe development of micro- and nanofabrication, their applications, and their dependent industries has progressed to a point where a bifurcation of technology development will likely occur. On the one hand, the semiconductor industry (at least in the USA) has decided to develop EUV and SCALPEL to meet its future needs. Even if the semiconductor industry is successful in this (which is by no means certain) such tools will not be useful in most other segments of industry and research that will employ nanolithography. As examples, MEMS, integrated optics, biological research, magnetic information storage, quantum-effect research, and multiple applications not yet envisioned will not employ the lithography tools of the semiconductor industry, either because they are too expensive, insufficiently flexible, or lacking in accuracy and spatial-phase coherence. Of course, direct-write electron-beam lithography can meet many of these non-semiconductor-industry needs, but in other cases a technique of higher throughput or broader process-latitude is necessary. Our experience at MIT in applying low-cost proximity x-ray nanolithography to a wide variety of applications leads us to conclude that this technology can provide an alternative path of a bifurcation. A new projection lithography technique, zone-plate-array lithography (ZPAL), does not require a mask, can operate from UV to EUV to x-rays, and has the potential to reach the limits of the lithographic process.

Measurement and interpretation of magnetic time effects in recording media

Magnetic time effects are highly relevant to magnetic information storage because of the large difference (up to 16 or 17 orders of magnitude) between the time scales of the recording process and the required storage stability. Magnetic time effects become more pronounced as the volume of the switching unit becomes smaller, and thus become of more practical importance as the microstructure of recording media is made finer in the pursuit of greater information storage density. Magnetic time effects taking place on time scales longer than about 10/sup -9/ s can be explained by a model of thermally assisted crossing of an energy barrier (Arrhenius-Neel formalism). Many recording phenomena can be explained within this regime. Observation of magnetization changes during exposure to a constant field (magnetic viscosity) can be interpreted to yield an estimate of the switching volume; in most cases, this volume is larger than the frequently cited activation volume. The dependence of coercivity on the time scale of the magnetic reversal precess (e.g., on the field sweep rate of a hysteresis loop) can also be used to deduce the volume of the switching unit and to estimate time effects relevant to information storage. A model and procedure for analysis of time-scale dependence of coercivity are described here and applied to typical advanced tape media, of both metal particulate (MP) and metal-evaporated (ME) composition. The resulting switching volume for the MP tape (where the microstructure is well defined) is in approximate agreement with the particle size seen by electron microscopy. The magnetic time effects are significantly stronger in the ME tape than in the MP tape. Measurement of coercivity ion at least two very different time scales (e.g., by vibrating-sample magnetometer and 60-Hz hysteresis loops) provides a convenient means of estimating the effective switching-unit volume, and hence the magnitude of the time effects to be expected in a recording application. The model also allows the estimation of the minimum particle or grain sizes that could be used with adequate stability of magnetic transitions in high-density information storage. The practical lower limit for metal-particle volume is found to be about one-fifth to one-fourth of the volume of current advanced particles.

Improved magnetic information storage using return-point memory

The traditional magnetic storage mechanisms (both analog and digital) apply an external field signal H(t) to a hysteretic magnetic material, and read the remanent magnetization M(t), which is (roughly) proportional to H(t). We propose a new analog method of recovering the signal from the magnetic material, making use of the shape of the hysteresis loop M(H). The field H, “stored’’ in a region with N domains or particles, can be recovered with fluctuations of order 1/N using the new method—much superior to the 1/N fluctuations in traditional analog storage.

SEMPA Studies of Exchange Coupling in Magnetic Multilayers

In the late 1980s, a number of exciting yet puzzling observations resulted from experiments investigating the coupling between two ferromagnetic layers separated by a nonferromagnetic spacer layer. A pioneering experiment by Grünberg et al. showed that Fe layers separated by a thin Cr spacer aligned with antiparallel magnetization, but with Au as the spacer layer, a parallel alignment occurred. The long-range magnetic dipole from each layer would tend to explain antiparallel alignment; small pinholes in the spacer layer would produce parallel alignment. Alternatively, the layers might be coupled through the spacer-layer conduction electrons by the Ruder man-Kittel-Kasuya-Yosida (RKKY) effect. This was expected to produce an oscillation in coupling as the spacer thickness increased, that is, an oscillation between parallel and antiparallel alignment. Oscillatory coupling was first observed by Parkin et al. Researchers had also found that, at spacer thicknesses where antiparallel alignment occurred, the Fe/Cr/Fe system can exhibit a giant magnetoresistance (GMR) effect, that is, an anomalously large change in resistance when a magnetic field is applied. The potential technological importance of the GMR effect to magnetic sensing and magnetic information storage added further impetus to the already rapidly growing area of research in magnetic multilayers.

Development and Preparation Processes of Barium Ferrite Particles for Magnetic Information Storage.

The preparation processes, morphlogical, structural and magnetic properties of BaFe12-2X (CoTi) XO19 and BaFe12-2X (CoSn) XO19 fine particles, prepared by chemical coprecipitation, synthesis from salt melts and glass crystallization method were investigated. The correlation between the magnetic properties, the particle morphology and the degrees of Co Ti and Co Sn substitutions of Ba ferrite particles were studied. These substituted Ba ferrite particulate media are of great interest for high density magnetic information storage.

Collapse of Apo- and Magnetoferritins at the Air-Water Interface

Synthesis of nanometer-size magnetic particles within the confined environment of apoferritin might have several uses in biomedical applications and in magnetic information storage devices. Successful use of these protein-encapsulated mineral particles in industrial and biomedical applications depends on the structural stability of these particles at various interfaces. We studied adsorption of apoferritin and magnetoferritin at the air-water interface. Transmission electron microscopy analyses of the protein films at the air-water interface indicate that the spherical protein shell of both apoferritin and magnetoferritin collapses into a two-dimensional disk shape at the interface. In the collapsed state, the magnetite core is released from the protein shell. The results suggest that the oligomeric state of magnetoferritin is not stable at hydrophobic surfaces.

Emerging instrumentation, measurements, and standards for magnetics (invited) (abstract)

Magnetics is second only to semiconductor technology in its importance to electronic and electrical products. The affected markets include magnetic materials, electronic components, magnetic information storage equipment, and electrical equipment. Our measurement capability strongly determines the ease with which commercial goals can be achieved. The existence of internationally recognized standards of measurement and standard reference materials assists in ensuring product compatibility. Magnetic measurement technology has made significant strides in the last few years. Usually this was driven by the demands of ever-increasing storage densities in magnetic recording, although superconductivity and high frequency electronics contributed as well. In this presentation, I will discuss an array of new instruments and measurement techniques with emphasis on the small scale measurement of magnetic parameters. In addition, I will present results of a recent NIST assessment of the present status and future development of measurements and standards in the field of magnetics.

The use of high temperature superconducting films for the magnetic storage of information

The authors discuss the first use of high-temperature superconducting films as an information recording medium in a format that could be modified for use in a large disc drive system. The system uses Abrikosov vortices to store integer units of magnetic flux in a hysteresis loop. A proof-of-principle system has been assembled using a 0.3 mu m film of YBCO on LaAlO/sub 3/ with T/sub c/=81 K; bits approximately 1.9 mm in diameter were recorded and read. It is noted that there are a great many improvements needed to make the system design a practical recording medium. However, this material has the potential advantage of being able to store a single bit of information with a single flux quantum, and thus the possibility for the storage of information with very high densities.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>