This review describes a new paradigm of electronics based on the spin degree of freedom of the electron. Either adding the spin degree of freedom to conventional charge-based electronic devices or using the spin alone has the potential advantages of nonvolatility, increased data processing speed, decreased electric power consumption, and increased integration densities compared with conventional semiconductor devices. To successfully incorporate spins into existing semiconductor technology, one has to resolve technical issues such as efficient injection, transport, control and manipulation, and detection of spin polarization as well as spin-polarized currents. Recent advances in new materials engineering hold the promise of realizing spintronic devices in the near future. We review the current state of the spin-based devices, efforts in new materials fabrication, issues in spin transport, and optical spin manipulation.

http://science.sciencemag.org/content/sci/294/5546/1488.full.pdf

Spintronics: a spin-based electronics vision for the future.

Spintronics, or spin electronics, involves the study of active control and manipulation of spin degrees of freedom in solid-state systems. This article reviews the current status of this subject, including both recent advances and well-established results. The primary focus is on the basic physical principles underlying the generation of carrier spin polarization, spin dynamics, and spin-polarized transport in semiconductors and metals. Spin transport differs from charge transport in that spin is a nonconserved quantity in solids due to spin-orbit and hyperfine coupling. The authors discuss in detail spin decoherence mechanisms in metals and semiconductors. Various theories of spin injection and spin-polarized transport are applied to hybrid structures relevant to spin-based devices and fundamental studies of materials properties. Experimental work is reviewed with the emphasis on projected applications, in which external electric and magnetic fields and illumination by light will be used to control spin and charge dynamics to create new functionalities not feasible or ineffective with conventional electronics.

/pdf/spintronics-fundamentals-and-applications-2dx38n6phu.pdf

Spintronics: Fundamentals and applications

Synthesis of monodisperse iron-platinum (FePt) nanoparticles by reduction of platinum acetylacetonate and decomposition of iron pentacarbonyl in the presence of oleic acid and oleyl amine stabilizers is reported. The FePt particle composition is readily controlled, and the size is tunable from 3- to 10-nanometer diameter with a standard deviation of less than 5%. These nanoparticles self-assemble into three-dimensional superlattices. Thermal annealing converts the internal particle structure from a chemically disordered face-centered cubic phase to the chemically ordered face-centered tetragonal phase and transforms the nanoparticle superlattices into ferromagnetic nanocrystal assemblies. These assemblies are chemically and mechanically robust and can support high-density magnetization reversal transitions.

/pdf/monodisperse-fept-nanoparticles-and-ferromagnetic-fept-12j5xql32h.pdf

Monodisperse FePt Nanoparticles and Ferromagnetic FePt Nanocrystal Superlattices

In this chapter, we will restrict our attention to the ferrites and a few other closely related materials. The great interest in ferrites stems from their unique combination of a spontaneous magnetization and a high electrical resistivity. The observed magnetization results from the difference in the magnetizations of two non-equivalent sub-lattices of the magnetic ions in the crystal structure. Materials of this type should strictly be designated as “ferrimagnetic” and in some respects are more closely related to anti-ferromagnetic substances than they are to ferromagnetics in which the magnetization results from the parallel alignment of all the magnetic moments present. We shall not adhere to this special nomenclature except to emphasize effects, which are due to the existence of the sub-lattices.

Ferromagnetic materials

Modern computing technology is based on writing, storing and retrieving information encoded as magnetic bits. Although the giant magnetoresistance effect has improved the electrical read out of memory elements, magnetic writing remains the object of major research efforts. Despite several reports of methods to reverse the polarity of nanosized magnets by means of local electric fields and currents, the simple reversal of a high-coercivity, single-layer ferromagnet remains a challenge. Materials with large coercivity and perpendicular magnetic anisotropy represent the mainstay of data storage media, owing to their ability to retain a stable magnetization state over long periods of time and their amenability to miniaturization. However, the same anisotropy properties that make a material attractive for storage also make it hard to write to. Here we demonstrate switching of a perpendicularly magnetized cobalt dot driven by in-plane current injection at room temperature. Our device is composed of a thin cobalt layer with strong perpendicular anisotropy and Rashba interaction induced by asymmetric platinum and AlOx interface layers. The effective switching field is orthogonal to the direction of the magnetization and to the Rashba field. The symmetry of the switching field is consistent with the spin accumulation induced by the Rashba interaction and the spin-dependent mobility observed in non-magnetic semiconductors, as well as with the torque induced by the spin Hall effect in the platinum layer. Our measurements indicate that the switching efficiency increases with the magnetic anisotropy of the cobalt layer and the oxidation of the aluminium layer, which is uppermost, suggesting that the Rashba interaction has a key role in the reversal mechanism. To prove the potential of in-plane current switching for spintronic applications, we construct a reprogrammable magnetic switch that can be integrated into non-volatile memory and logic architectures. This device is simple, scalable and compatible with present-day magnetic recording technology.

/pdf/perpendicular-switching-of-a-single-ferromagnetic-layer-1bprgrfvol.pdf

Perpendicular switching of a single ferromagnetic layer induced by in-plane current injection

The exchange field Hex transferred from a thick antiferromagnetic substrate to a thin exchange coupled ferromagnetic film is shown to reach a limiting value no matter how large the exchange coupling is. The limit is due to domain‐wall formation in the antiferromagnet. Numerical results based on a simple model for the interface are presented and compared to experimental results.

Simple model for thin ferromagnetic films exchange coupled to an antiferromagnetic substrate

With the present book we intend to give an account of the historical development,
the physical foundations and the continuing research underlying
the field of magnetism, one of the oldest and still vital field of physics. Our
book is written as a text book for students on the late undergraduate and
the graduate levels. It should also be of interest to scientists in academia and
research laboratories.

Throughout history, magnetism has played an important role in the development
of civilization, starting with the loadstone compass. Our modern
society would be unthinkable without the generation and utilization of electricity,
wireless communication at the speed of light and the modern hightech
magnetic devices used in information technology. Despite the existence
of many books on the topic, we felt the need for a text book that reviews the
fundamental physical concepts and uses them in a coherent fashion to explain
some of the forefront problems and applications today. Besides covering the
classical concepts of magnetism we give a thorough review of the quantum
aspects of magnetism, starting with the discovery of the spin in the 1920s.
We discuss the exciting developments in magnetism research and technology
spawned by the computer revolution in the late 1950s and the more recent
paradigm shift starting around 1990 associated with spin-based electronics or
“spintronics”. The field of spintronics was largely triggered by the discovery
of the giant magnetoresistance or GMR effect around 1988. It utilizes the
electron spin to sense, carry or manipulate information and has thus moved
the quantum mechanical concept of the electron spin from its discovery in the
1920s to a cornerstone of modern technology.

These historical and modern developments in magnetism are discussed
against the background of the development and utilization of spin-polarized
electron techniques and polarized photon techniques, the specialties of the
authors. It is believed that the technological application of magnetism will
continue with a growth rate close to Moore’s law for years to come. Interestingly,
the magnetic technology goals of “smaller and faster” are matched by
“brighter and faster” X-ray sources, which are increasingly used in contemporary
magnetism research. Novel ultra-bright X-ray sources with femtosecond
pulse lengths will provide us with snapshots of the invisible ultrafast magnetic
nanoworld. These exciting developments are another reason for the present book.

Last not least, this book is born out of our passion for the subjects discussed
in it. In the process we had to get to the bottom of many things and
understand them better or for the first time. This process took a deep commitment
and much time, with “the book” often preoccupying our minds. The
process was greatly aided by discussions with our colleagues and students and
we would like to thank them at this place. In particular, we need to thank
Ioan Tudosa for his critical comments and for helping us with numerous illustrations.
In this book we give an account of the field of magnetism that is
colored by personal taste and our way of looking at things. We hope that you
will enjoy the result.

Magnetism : From Fundamentals to Nanoscale Dynamics

In magnetic memory devices, logical bits are recorded by selectively setting the magnetization vector of individual magnetic domains either ‘up’ or ‘down’. In such devices, the fastest and most efficient recording method involves precessional switching1,2,3,4: when a magnetic field Bp is applied as a write pulse over a period τ, the magnetization vector precesses about the field until Bpτ reaches the threshold value at which switching occurs. Increasing the amplitude of the write pulse Bp might therefore substantially shorten the required switching time τ and allow for faster magnetic recording. Here we use very short pulses of a very high magnetic field5 to show that under these extreme conditions, precessional switching in magnetic media supporting high bit densities no longer takes place at well-defined field strengths; instead, switching occurs randomly within a wide range of magnetic fields. We attribute this behaviour to a momentary collapse of the ferromagnetic order of the spins under the load of the short and high-field pulse, thus establishing an ultimate limit to the speed of deterministic switching and magnetic recording.

The ultimate speed of magnetic switching in granular recording media.

Hydrogen is stored atomically in metal hydrides. Dissociative chemisorption and associative desorption are therefore important steps in the hydrogen absorption and desorption processes. We summarize the theoretical and experimental knowledge of hydrogen chemisorption on clean and precovered metal surfaces and correlate it with the techniques for preparing metal hydrides. At the surface of hydride-forming intermetallics, precipitates of d metals and a metallic subsurface are produced by surface segregation and decomposition. The subsurface and the precipitates are able to dissociate H2. Our recent work on the surface analysis of LaNi5, FeTi, Mg2Ni and ErFe2 is reviewed.

Surface effects and the formation of metal hydrides

Ultrafast magnetic field pulses as short as 2 picoseconds are able to reverse the magnetization in thin, in-plane, magnetized cobalt films. The field pulses are applied in the plane of the film, and their direction encompasses all angles with the magnetization. At a right angle to the magnetization, maximum torque is exerted on the spins. In this geometry, a precessional magnetization reversal can be triggered by fields as small as 184 kiloamperes per meter. Applications in future ultrafast magnetic recording schemes can be foreseen.

https://science.sciencemag.org/content/sci/285/5429/864.full.pdf

H. C. Siegmann

Papers

Simple model for thin ferromagnetic films exchange coupled to an antiferromagnetic substrate

Magnetism : From Fundamentals to Nanoscale Dynamics

The ultimate speed of magnetic switching in granular recording media.

Surface effects and the formation of metal hydrides

Minimum field strength in precessional magnetization reversal