The advent of semiconductor technology has impacted the lives of many of us since the 1970s. Silicon CMOS (complementary metal-oxide-semiconductor) devices are practically ubiquitous, and by the year 2000, the value of the semiconductor industry exceeded that of the automobile industry. The magnetic industry, on the other hand, is much smaller than the semiconductor industry. Engineering schools of universities rarely cover any courses in this discipline. Nonetheless, a tiny magnetic recording device is in the hard disk of every computer. Like CMOS devices, magnetic recording technology is being scaled down from generation to generation. At the time of writing, the physical size of the magnetic bit remains smaller than a DRAM bit on silicon chips.

Researchers working in these two communities had little in common until the development of the modern magnetic random access memory, or MRAM. A MRAM chip is built by integrating magnetic tunneling junction (MTJ) devices onto the silicon CMOS circuits. The research activity of MTJs in academia and industry, both hard disk and semiconductor, has been very active since it first showed signs of technology implication in the mid 1990s. That effort led to the mass production of the MTJ recording head in hard disk in 2006. In the same year, the semiconductor industry announced the first successful introduction of an MTJ memory product. The viability of MTJ technology is proven. It is expected that research activities will develop further, which will increase cooperation between these two research communities.

Introduction

Two thousand years ago, the Chinese invented the compass, a special metallic needle with one end always pointing to the North Pole. That was the first recorded human application of magnetism. Important understandings and developments were achieved in the mid 19th century and continue into the present day. Indeed, today, magnetic devices are ubiquitous. For example, to name just two: energy conversion devices provide electricity to our homes and magnetic recording devices store data in our computers. This chapter provides an introduction to basic magnetism. Starting from the simple attractive (or repelling) force between magnets, we define magnetic field, dipole moment, torque, energy and its equivalence to current. Then we will state the Maxwell equations, which describe electromagnetism, or the relationship between electricity and magnetism.

A great tutorial is provided by Kittel, which may be used to support students studyingChapters 1–4.

Magnetic forces, poles and fields

In the early days, magnetic phenomena were described as analogous to electrical phenomena: like an electric charge, a magnetic pole was considered to be the source of magnetic field and force. The magnetic field was defined through the concept of force exerted on one pole by another.

Introduction

The best description of spin-torque transfer can be found in the patent issued to John Slonczewski of IBM:

The subject of spin-torque transfer was not widely known until 1996. Due to its enormous technology potential, both academic and industrial research activities had been very active, and very rapid progresses have been made in recent years: from the first experimental verification of spin-torque transfer in giant magnetoresistance (GMR) film, to the implementation of this mechanism to magnetic tunneling junction devices. A large portion of this effort was directed towards the development of practical magnetic RAM chips based on the spin-torque-transfer mechanism.

Nanoscale magnets have at least one dimension in the nanometre range. They exhibit size-specific magnetic properties such as superparamagnetism, remanence enhancement, exchange averaging of anisotropy and giant magnetoresistance when the small dimensions become comparable to a characteristic magnetic or electrical length scale. Thin films are the most versatile magnetic nanostructures, and interface effects such as spin-dependent scattering and exchange bias influence their magnetic properties. Thin-film stacks form the basis of modern magnetic sensors and memory elements.

Matter behaves differently down in the nanoworld, where the length scales of interest range from about 1 nm up to about 100 nm. The atomic-scale structure of matter can usually be ignored, but the mesoscopic dimensions of the magnetic nano-objects are comparable to some characteristic length scale, below which the physical properties change. We have already encountered one important nanoscale object in bulk magnetic material – the domain wall. It is extended in two directions, but not in the third; the domain wall width δw is one of the characteristic lengths that concern us here.

The number of small dimensions in a nanoscale magnet may be one, two or three. Some examples of each are illustrated in Fig. 8.1. The one-small-dimension class includes magnetic thin films, which are at the heart of many modern magnetic devices. Magnetic and nonmagnetic layers can be stacked to make thin-film heterostructures, such as spin valves and tunnel junctions. The films are usually grown on a macroscopic substrate.

Resonance arises when the energy levels of a quantized system of electronic or nuclear moments are Zeeman split by a uniform magnetic field and the system absorbs energy from an oscillating magnetic field at sharply defined frequencies, which correspond to transitions between the levels. Classically, resonance occurs when a transverse AC field is applied at the Larmor frequency. Resonance methods are valuable for investigating the structure and magnetic properties of solids, and they are used for imaging and other applications. The resonant moment may be an isolated ionic spin or free radical, as in electron paramagnetic resonance (EPR), or a nuclear spin as in nuclear magnetic resonance (NMR). Otherwise it can be the ordered magnetization as in ferromagnetic resonance (FMR). Resonant effects are also associated with spin waves, and domain walls. The related techniques of Mössbauer spectroscopy and muon spin resonance provide further information on hyperfine interactions in solids.

A magnetic system placed in a uniform magnetic field B0 may absorb electromagnetic radiation at a precisely defined frequency ν0 = ω0/2π which falls in the radio-frequency or microwave range. The phenomenon is related to the Larmor precession of the magnetic moment, introduced in §3.2.2. In order to observe the resonance, an experimental geometry with crossed magnetic fields is needed. The steady uniform field defines the z-direction, while a high-frequency AC field bx = 2b1 cos ωt is applied in the perpendicular plane.

This book offers a broad introduction to magnetism and its applications, designed for graduate students and advanced undergraduates as well as practising scientists and engineers. The approach is descriptive and quantitative, treating concepts, phenomena, materials and devices in a way that emphasises numerical magnitudes, and provides a wealth of useful data.

Magnetism is a venerable subject, which underwent four revolutionary changes in the course of the twentieth century – understanding of the physics, extension to high frequencies, the avalanche of consumer applications and, most recently, the emergence of spin electronics. The reader probably owns one or two hundred magnets, or some billions if you have a computer where each bit on the hard disc counts as an individually addressable magnet. Sixty years ago, the number would have been at best two or three. Magnetics, in partnership with semiconductors, has created the information revolution, which in turn has given birth to new ways to research the subject – numerical simulation of physical theory, automatic data acquisition and web-based literature searches.

The text is structured in five parts. First, there is a short overview of the field. Then come eight chapters devoted to concepts and principles. Two parts follow which treat experimental methods and materials, respectively. Finally there are four chapters on applications. An elementary knowledge of electromagnetism and quantum mechanics is needed for the second part. Each chapter ends with a short bibliography of secondary literature, and some exercises.

Central to most magnetic measurements is the generation and detection of magnetic fields. Atomic-scale magnetic structure is best probed by neutron diffraction, while other atomic-scale element-specific information is provided by spectroscopic methods. Domain-scale magnetization measurements are made by magneto-optic methods or magnetic force microscopy, whereas macroscopic measurements of magnetization are made in open or closed circuit by a variety of methods. Spin-wave and other excitations are best explored by inelastic neutron scattering. Numerical methods of investigation are of growing importance for understanding the static and dynamic behaviour of real magnetic materials and magnetic systems.

Magnetism is an experimental science. Experiments serve to inspire and refine physical theory, besides providing all the quantitative information on which the applications depend. The traditional image of apparatus on a laboratory bench does not tell the whole story; some magnetic measurements are now conducted at national or multinational institutes built around large-scale facilities for generating high magnetic fields, neutron beams or intense streams of synchrotron radiation. Computers have evolved in the opposite sense, from central facilities to benchtop instruments for data acquisition, display and modelling. Numerical computations and simulations may be regarded as an experimental tool for investigating a model reality, where complex magnetic behaviour at the atomic, micromagnetic or system level can be investigated with the aid of a computer workstation.