bluuebunny/crossref_metadata_2025 huggingface dataset:
Magnetism in Alloys
Authors: Pinski F.J. , Staunton J.B. , Razee S.S.A. , Johnson D.D.
Published: October 2003

Magnetism in transition metal materials can be described in quantitative detail by spin‐density functional theory (SDFT). At low temperatures, the magnetic properties of a material are characterized in terms of its spin‐polarized electronic structure. It is on this aspect of magnetic alloys that is focused on here. From this basis, the early Stoner‐Wohlfarth picture of rigidly exchange‐split, spin‐polarized bands is shown to be peculiar to the elemental ferromagnets only. Shown are the origins of two commonly occurring features of ferromagnetic alloy electronic structures, and the simple structure of the Slater‐Pauling curve for these materials (average magnetic moment versus electron per atom ratio), can be traced back to the spin‐polarized electronic structure.

The details of the electronic basis of the theory can, with care, be compared to results from modern spectroscopic experiment.

Although SDFT is “first‐principled,” most applications resort to the local approximation (LSDA) for the many electron exchange and correlation effects. This approximation is widely used and delivers good results in many calculations.

The LDA in magnetic materials fails when it is straightforwardly adapted to high temperatures. This failure can be redressed by a theory that includes the effects of thermally induced magnetic excitations, but which still maintains the spin‐polarized electronic structure basis of standard SDFT.

Although not spin‐polarized “globally”—i.e., when averaged over all orientational configurations—the electronic structure is modified by the local‐moment fluctuations, so that “local spin‐polarization” is evident. A mean field theory of this approach and its successes for the elemental ferromagnetic metals and for some iron alloys are discussed. The dynamical effects of these spin fluctuations in a first‐principles theory remain to be included.

Also emphasized is how the state of magnetic order of an alloy can have a major effect on various other properties of the system, and effect upon atomic short‐range order by describing case studies of NiFe,FeV, andAuFe alloys is dealt with at length. The results of our calculations with details of “globally” and “locally” spin‐polarized electronic structure are linked. The relativistic generalization of SDFT and covered its implication for the magnetocrystalline anisotropy of disordered alloys, with specific illustrations for CoPt alloys are discussed.

In summary, the magnetic properties of transition metal alloys are fundamentally tied up with the behavior of their electronic “glues.” As factors like composition and temperature are varied, the underlying electronic structure can change and thus modify an alloy's magnetic properties. Likewise, as the magnetic order transforms, the electronic structure is affected and this, in turn, leads to changes in other properties. Here the focussed is the effect on ASRO, but much could also have been written about the fascinating link between magnetism and elastic properties—“Invar” phenomena being a particularly dramatic example.
Kinematic Diffraction of X Rays
Authors: Chen Haydn
Published: October 2003

Diffraction by x rays, electrons, or neutrons has enjoyed great success in crystal structure determination. For a perfectly ordered crystal, diffraction results in arrays of sharp Bragg reflection spots periodically arranged in reciprocal space. Analysis of the Bragg peak locations and their intensities leads to the identification of crystal lattice type, symmetry group, unit cell dimensions, and atomic configuration within a unit cell. On the other hand, for crystals containing lattice defects such as dislocations, precipitates, local ordered domains, surface, and interfaces, diffuse intensities are produced in addition to Bragg peaks. The distribution and magnitude of diffuse intensities are dependent on the type of imperfection present and the x‐ray energy used in a diffraction experiment.

Diffuse scattering is usually weak, and thus more difficult to measure, but it is rich in structure information that often cannot be obtained by other experimental means. Since real crystals are generally far from perfect, many properties exhibited by them are therefore determined by the lattice imperfections present. Consequently, understanding of the atomic structures of these lattice imperfections and of the roles these imperfections play is of paramount importance if these materials properties are to be exploited for optimal use.

This article addresses the fundamental principles of diffraction based upon the kinematic diffraction theory for x rays. (Nevertheless, the diffraction principles described here unit may be extended to kinematic diffraction events involving thermal neutrons or electrons.). In practice, most x‐ray diffraction experiments are carried out on crystals containing a sufficiently large number of defects that kinematic theory is generally applicable.

This article is divided into two major sections. In the first section, the fundamental principles of kinematic diffraction of x rays are discussed and a systematic treatment of theory is given. In the second section, the practical aspects of the method are discussed; specific expressions for kinematically diffracted x‐ray intensities are described and used to interpret diffraction behavior from real crystals containing lattice defects. Neither specific diffraction techniques and analysis nor sample preparation methods are described in this article.
Mechanical Testing
Authors: Abbaschian Reza
Published: October 2003

Basic mechanical property and materials strength measurements are obtained by standardized mechanical tests, many of which are described in this category. Each article in this category not only covers details, principles, and practical aspects of the various tests, but also provides comprehensive reference to the standard procedures and sample geometries dictated by the ASTM or other standards agencies. Also covered are additional mechanical testing techniques such as high‐strain‐rate testing, measurements under pressure, and tribological and wear testing. As such, the group of articles covers a broad spectrum of techniques used to assess materials behavior in a variety of engineering applications.
High‐Strain‐Rate Testing of Materials
Authors: Gray George T. (Rusty)
Published: October 2003

Experimental methods of probing material behavior at high rates of strain are concerned with measuring the variation of mechanical properties, such as strength and ductility, that can vary with strain rate. Interest in the high‐rate mechanical behavior of materials has continued to expand during the last four decades, driven by demands for increased understanding of material response in impact events. High‐rate tests also provide the data critically needed to create predictive constitutive model descriptions of materials. These descriptions strive to capture the fundamental relationships between the ways that each of the independent variables—stress, strain rate, strain, and temperature—independently affect the constitutive behavior of materials.

Measurements of the mechanical properties of materials are normally conducted via loading test samples in compression, tension, or torsion. Mechanical testing frames can be used to achieve nominally constant loading rates for limited plastic strains and thereby a constant engineering strain rate. Typical screw‐driven or servohydraulic testing machines are routinely used to measure the stress‐strain response of materials up to strain rates as high as 1 s
−1
. Specially designed testing machines, typically equipped with high‐capacity servohydraulic valves and high‐speed control and data acquisition instrumentation, can be used during compression testing to achieve strain rates as high as 200 s
−1
. Above this strain rate regime,

> 200 s
−1
, alternative techniques using projectile‐driven impacts to induce stress‐wave propagation have been developed.

Chief among these techniques is the split‐Hopkinson pressure bar, which is capable of achieving the highest uniform uniaxial stress loading of a specimen in compression at nominally constant strain rates of the order of 10
3
s
−1
. Stress is measured by using an elastic element in series with the specimen of interest. Stress waves are generated via an impact event and the elastic elements utilized are long bars such that the duration of the loading pulse is less than the wave transit time in the bar. Utilizing this technique, the dynamic stress‐strain response of materials at strain rates up to 2 × 10
4
s
−1
and true strains of 0.3 can be readily achieved in a single test.
Hardness Testing
Authors: Klansky Janice
Published: October 2003

Static indentation hardness tests such as Brinell, Rockwell, Vickers, and Knoop are frequently used methods for determining hardness. The basic concept utilized in all of these tests is that a set force is applied to an indenter in order to determine the resistance of the material to penetration. If the material is hard, a relatively small or shallow indentation will result, whereas if the material is soft, a fairly large or deep indentation will result.

These tests are often classified in one of two ways: either by the extent of the test force applied or the measurement method used. A “macro” test refers to a test where a load >1 kg is applied; similarly “micro” refers to a test where a load of ≤1 kg of force is applied. Additionally, some instruments are capable of conducting tests with loads as light as 0.01 g and are commonly referred to as ultralight or nanoindentation testers. Rockwell and Brinell testers fall into the macro category, whereas Knoop testers are used for microindentation tests. Vickers testers are employed for both macro and microindentation tests. The measurement methods available include a visual observation of the indentation or a depth measurement of the indentation. Rockwell and some nanoindentation testers are capable of determining the depth of the indentation, whereas Brinell, Knoop, and Vickers testers require an indentation diameter measurement. These visual measurements can be automated, and are discussed.

Hardness is not a fundamental property of a material, yet hardness testing is considered a useful quality‐control tool. Many properties are predicted from hardness values when combined with additional information such as alloy composition. The following is a list of such properties: resistance to abrasives or wear, resistance to plastic deformation, modulus of elasticity, yield strength, ductility, and fracture toughness. Some of these properties, such as yield strength, have numerical relationships with hardness values, whereas others such as fracture toughness are based on observations of cracks surrounding the indentations. Data analysis and conversions are discussed in detail.
Thermal Analysis
Authors: Dollimore David
Published: October 2003

Thermal analysis is the name given to the measurement of a sample as the sample is programmed through a predetermined temperature regime in a specified atmosphere.

This category of article presents a collection of thermal analysis techniques available for the characterization of the thermal properties of materials. The general nature of the available equipment along with the formal definitions, codes of practice, and nomenclature for each of the principal methods are described early. The most obvious property that can be measured against an imposed temperature regime is the sample mass; this is termed thermogravimetric analysis.

In addition to providing the materials scientist with information on the thermal stability and degradation products of a material under study, thermal analysis methods can reveal the phase properties, mechanical stabilities, thermal expansion coefficients, and electrical or magnetic properties as a function of temperature. While the temperature dependence of the various methods contained throughout this work is presented this category of articles compile the methods whose primary concern is the thermal properties and behavior of the material.
Thermal Analysis‐Definitions, Codes of Practice, and Nomenclature
Authors: Dollimore David
Published: October 2003

Those who practice the techniques of thermal analysis are well advised to notice and use the definitions, conventions, codes of practice, and methods of reporting data advocated by the International Confederation of Thermal Analysis and Calorimetry (ICTAC). These recommendations have been widely circulated and are to be found in the booklet
For Better Thermal Analysis
.

The recommended names and abbreviations of the various techniques in thermal analysis are listed. The classification of thermal analysis techniques is given. The definitions follow. The reporting of thermal analysis data is discussed. Publications on thermal analysis are listed.
Thermogravimetric Analysis
Authors: Dunn J.G.
Published: October 2003

The formal definition of thermogravimetry (TG is the preferred abbreviation, although TGA is also used) has been given by the Nomenclature Committee of the International Confederation for Thermal Analysis and Calorimetry (ICTAC) as “a technique in which the mass of a substance is measured as a function of temperature whilst the substance is subjected to a controlled temperature programme”.

In the above definition, a controlled temperature program means heating or cooling the sample at some predetermined and defined rate. Although it is common to have just one constant heating or cooling rate, it is also advantageous in some cases to have different rates over different temperature ranges and in some cases even a varying rate over a specific temperature range.

An apparatus called a thermobalance is used to obtain a thermogravimetric curve. By means of a thermobalance the temperature range over which a reaction involving mass change occurs may be determined.

The sample is usually a solid, or more rarely a liquid.

Thermogravimetric data can be presented in two ways. The TG curve is a plot of the mass against time or temperature, with the mass loss on the ordinate plotted downward and mass gains plotted upward relative to a baseline. Alternatively, data can be presented as a derivative thermogravimetric (DTG) curve, which is a plot of the rate of change of masswith respect to timeor temperatureagainst time or temperature. The DTG mass losses should also be plotted downward and the gains upward.

Conventionally, thermal analysis experiments are carried out at a constant heating rate, and a property change is measured as a function of time. An alternative approach is to keep the change in property constant by varying the heating rate. For TG, the rate of mass loss is kept constant by variation in heating rate. To achieve this, the mass change is monitored and the heating rate decreased as the mass loss increases, and vice versa. At the maximum rate of mass loss, the heating rate is a minimum. This gives mass losses over very narrow temperature ranges and sometimes enables two close reactions to be resolved. This method has the advantage of using fast heating rates when no thermal event is taking place and then slowing down the heating rate when a mass change is in progress.

Thermogravimetry does not give information about reactions that do not involve mass change, such as polymorphic transformations and double‐decomposition reactions. Also, it is not useful for identification of a substance or mixture of substances unless the temperature range of the reaction has already been established and there are no interfering reactions. However, when a positive identification has been made, TG by its very nature is a quantitative technique and can frequently be used to estimate the amount of a particular substance present in a mixture or the purity of a single substance. The types of processes that can be investigated by TG are given.
Combustion Calorimetry
Authors: O'Hare P.A.G.
Published: October 2003

In the absence of direct experimental information, one often must rely on a knowledge of thermodynamic properties to predict the chemical behavior of a material under operating conditions. For such applications, the standard molar Gibbs energy of formation

is particularly powerful; it is frequently derived by combining the standard molar entropy of formation

and the standard molar enthalpy of formation

often as functions of temperature and pressure. It follows, therefore, that the standard molar enthalpy of formation,

, is among the most valuable and fundamental thermodynamic properties of a material. This quantity is defined as the enthalpy change that occurs upon the formation of the compound in its standard state from the component elements in their standard states, at a designated reference temperature, usually (but not necessarily) 298.15 K, and at a standard pressure, currently taken to be 100 kPa or 101.325 kPa.

Many methods have been devised to obtain

experimentally. Those include the so‐called second‐ and third‐law treatments of Knudsen effusion and mass‐spectrometric results from high‐temperature vaporization observations, as well as EMF results from high‐temperature galvanic cell studies. Each of those techniques yields

for the studied process at a given temperature. To derive values of

, the high‐temperature results for

must be combined with auxiliary thermodynamic information (heat capacities and enthalpy increments at elevated temperatures).

Alternative procedures involve measurements of the enthalpies (or enthalpy changes) of chemical reactions,

, of a substance and the elements of which it is composed. Experimentally, the strategy adopted in such determinations is dictated largely by the chemical properties of the substance. The most obvious approach involves direct combination of the elements, an example of which is the combustion of gaseous H
2
in O
2
to form H
2
O. In the laboratory, however, it is often impossible under experimental conditions to combine the elements to form a particular compound in significant yield. An alternative, albeit less direct route has been used extensively for the past century or more, and involves measurements of the enthalpy change associated with a suitable chemical reaction of the material of interest. Here, the compound is “destroyed” rather than formed. Such chemical reactions have involved,
inter alia
, dissolution in a mineral acid or molten salt, thermal decomposition to discrete products, or combustion in a gas such as oxygen or a halogen. This last method is the the subject matter of this article.
Simultaneous Techniques Including Analysis of Gaseous Products
Authors: Dollimore David , Lerdkanchanaporn Supaporn
Published: October 2003

The concept of concurrently using two or more techniques of thermal analysis on a single substance has been promoted by advances in microprocessor control. This has made it possible to program and collect the data and process the information in many ways. Strictly speaking, the nomenclature “simultaneous” applied in such circumstance is not always correct. The mass change in thermogravimetry (TG) is generally the first signal received. The signal from the differential thermal analysis (DTA) probe will be received a little later as the heat has to diffuse through the sample and the crucible before reaching the sensing thermocouple. The sample is defined as the material (usually a solid) under investigation. Likewise, in evolved gas analysis (EGA), the gases are sampled but it then takes a finite time for the gas to be analyzed. In the mass spectrometer, there is an interface to allow the gas to be analyzed. In gas chromatography (GC), the signal of gas analysis takes considerable time to process, and consequently the gas analysis signal is intermittent and not received in “real time.”

In common with all other thermal analysis techniques, the following parameters must be considered: the sample characteristics, its mass, the sample holder, the temperature range for study, the rate of heating, and the atmosphere.

It must be remembered that a single thermal analysis method may not give the investigator sufficient information. Thus DTA will indicate if a transformation is exothermic or endothermic. It needs thermogravimetric analysis to indicate if there is mass change as well as gas analysis to identify any gaseous products involved.

Although “simultaneous techniques” is the approved term used by the International Union of Pure and Applied Chemistry (IUPAC), the recommended nomenclature for such simultaneous techniques is to use a hyphen to separate the two techniques, e.g., TG‐DSC. This has led to such techniques being called “hyphenated” techniques.
Electrical and Electronic Measurements
Authors: Barnes Peter A.
Published: October 2003

Electrical and electronic measurements of materials are among the most powerful techniques available for materials characterization. These measurements can reveal information that completely characterizes the electrical transport properties of a material. Furthermore, modern instrumentation makes precision measurements and computer interfacing straightforward at reasonable cost.
Conductivity Measurement
Authors: Wentworth Stuart M.
Published: October 2003

A material's conductivity, (or the inverse property, resistivity) relates to its ability to conduct electricity. In metals, conduction of electricity is tantamount to conduction of electrons, which depends on charge density and on scattering of the electrons by the crystal lattice (phonons) or by lattice imperfections. In semiconductors, conductivity is determined by the number of available charge carriers (electrons or holes) and the carrier mobilities. Because of the different mechanisms for conductivity, its dependence on temperature also differs. Conductivity increases with increasing temperature for semiconductors (more carriers are generated) and it decreases with increasing temperature for metals (more scattering by the lattice). Conductivity also depends on physical structure. In crystals, the crystal type and orientation affect conductivity because the electronic structure is intimately tied to the crystal structure. The size of the crystallites (grains) in polycrystalline materials is also important as it affects the scattering of carriers, and, at very small sizes may also affect electronic structure.

The accurate determination of a material's conductivity can be critical for understanding material composition or device performance. The method used to determine conductivity depends on whether the material is a bulk sample or a thin film. This article begins with a section on bulk measurements.

In the second section, two thin‐film approaches are presented which are most frequently used in semiconductor characterization: four‐point probing and the Van der Pauw method.

In the third section measurement approaches are presented that offer the possibility of measurements without damaging the material. These approaches are not as well developed as the contact approaches described in the first two sections. Specialized equipment is required, and accuracy can be rather limited.

The fourth section briefly describes the most common microwave techniques that can be employed to determine conductivity information at high frequencies. Expensive test equipment is required for these approaches, along with specialized test apparatus to probe or hold the samples. The conductivity is not directly measured, but is typically extracted from loss tangent determination.

Some general characteristics for the methods described in these four sections are listed.
Deep‐Level Transient Spectroscopy
Authors: Tin Chin‐Che
Published: October 2003

Defects are responsible for many different characteristic properties of a semiconductor. They play a critical role in determining the viability of a given material for device applications. The identification and control of defects have always been among the most important and crucial tasks in materials and electronic device development. The performance and reliability of devices can be significantly affected by only minute concentrations of undesirable defects. Since the determination of the type and quality of defects in a material depends on the sensitivity of a characterization technique, the challenge in materials characterization has been to develop detection methods with improved sensitivity.

Deep‐level transient spectroscopy (DLTS) probes the temperature dependence of the charge carriers escaping from trapping centers formed by point defects in the material. This technique is able to characterize each type of trapping center by providing the activation energy of the defect level relative to one of the energy band edges and the capture cross‐section of the traps. It can also be used to compute the concentration and depth profile of the trapping centers.

The spectroscopic nature of DLTS is a key feature that provides both convenience and sensitivity.

Deep‐level transient spectroscopy has been widely used for many different semiconductors. This technique has distinguished itself in contributing to the resolution of many defect‐related problems in several technologically important semiconductors such as silicon, the Group III to V and II to VI compounds, and alloys. Many variations of basic DLTS have also been developed for improved sensitivity and for more specialized applications in device structures different from the normalp‐nor Schottky barrier diodes.

Deep‐level transient spectroscopy is not able to determine the chemistry or the origin of a defect. Deep‐level transient spectroscopy data should therefore be used in conjunction with other techniques. A successful study of defects requires the concerted efforts of many researchers using various characterization techniques in order to derive a more accurate and consistent picture of the defect structure of a given material.
Carrier Lifetime: Free Carrier Absorption, Photoconductivity, and Photoluminescence
Authors: Linnros Jan , Grivickas Vytautas
Published: October 2003

The charge carrier lifetime is one of the most important parameters characterizing a semiconductor. This is mainly for two reasons: (1) the carrier lifetime determines the performance of devices while, at the same time, (2) it is a sensitive measure of material quality and cleanliness. Carrier lifetime may impact device operation directly, such as by affecting the turn‐off speed of a diode or a thyristor operating at high injections or the leakage current of a pn‐junction. Carrier lifetime may also affect operation indirectly. Thus, a high carrier lifetime is desirable in many cases, whereas a low lifetime favors specific applications. Furthermore, for optically active devices, one is interested in radiative recombination, and other recombination mechanisms must be suppressed to enhance quantum efficiency.

This article reviews the theory and common concepts in carrier‐lifetime measurement methodology and focuses on a few important and general techniques used to characterize lifetime in semiconductor materials. Thus, device lifetime characterization methods have been left out. The our aim here is to present basic principles of the methods, methodology, and associated difficulties, such that the reader will be able to critically analyze data provided by the methods and to perform simple measurements using the techniques. Also, a selection guide is provided to direct users to the most appropriate method for their specific needs.

The three methods described in this article are free carrier absorption (FCA), photoconductivity (PC), and photoluminescence (PL).

Free carrier absorptionis not of widespread use but is a very general tool applicable to most semiconductor materials and also to devices. It provides an exceptional advantage, as different geometries and very different injection regimes can be realized, and therefore many different materials and recombination mechanisms may be characterized. It is used here to illustrate many common concepts in lifetime measurement methods.

Photoconductivityis one of the few standard methods used both for wafer fabrication quality monitoring and for clean‐room process monitoring.

Photoluminescenceis a particularly useful method for characterizing direct‐band‐gap semiconductors, but has also been applied to indirect band‐gap materials. The PL method has also become the workhorse when small volumes must be analyzed, such as in thin‐film studies, for quantum‐well characterization, or for the rapidly expanding field of semiconductor nanostructures.

The article commences with a review of the carrier‐lifetime concept and various recombination mechanisms, also illustrating some major difficulties affecting lifetime characterization, such as trapping and surface recombination. Brief outlines of the different techniques and their fundamental principles of operation are given. The three different techniques are described in detail. A selection guide is provided for the different lifetime applications.
Capacitance–Voltage (C‐V) Characterization Of Semiconductors
Authors: Barnes Peter
Published: October 2003

Hillibrand and Gold first described the use of capacitance‐voltage (C‐V) methods to determine the majority carrier concentration in semiconductors. C‐V measurements are capable of yielding quantitative information about the diffusion potential and doping concentration in semiconductor materials. The technique employs
pn
junctions, metal‐semiconductor (MS) junctions (Schottky barriers), electrolyte‐semiconductor junctions, metal‐insulator‐semiconductor (MIS) capacitors, and MIS field effect transistors (MISFETSs). The discussions here emphasize
pn
junctions and Schottky barrier techniques.

C‐V measurements yield accurate information about doping concentrations of majority carriers as a function of distance (depth) from the junction. The major competing technique to C‐V measurements is the Hall effect which, while yielding added information about the carrier mobility, requires difficult, time‐consuming procedures to determine carrier‐depth profiles. In fact, C‐V profiling and Hall measurements can be considered complementary techniques.

In concert with deep‐level transient spectroscopy, C‐V measurements can quantitatively describe the free carrier concentrations together with information about traps. Defects appearing as traps at energies deep within the forbidden gap of a semiconductor can add or remove free carriers. The same Schottky diode can be used for both C‐V measurements and DLTS measurements. The C‐V apparatus and the DLTS apparatus are often a “bundled” setup.

C‐V profiling to determine the doping profile of the mobile majority carriers in a semiconductor is a powerful quantitative technique. Some precautions must be taken, especially in dealing with multiple layered structures such as high‐to‐low doping levels, quantum wells, and the presence of traps. Major considerations and precautions to be taken in dealing with C‐V profiling data acquisition and reduction are also covered in the ASTM standard. The cost of assembling a C‐V profiling apparatus will range between six thousand dollars for a simple, single temperature, manual apparatus to sixty thousand dollars for a fully automated apparatus. C‐V profiling used in concert with deep‐level transient spectroscopy to identify traps, as well as with Hall effect, to address transport mechanisms, provides a powerful set of tools at reasonable cost.
Characterization of

pn

Junctions
Authors: Baca Albert G.
Published: October 2003

The pn junction of a semiconductor material forms part of many commercially important devices such as bipolar junction transistors (BJTs), heterojunction bipolar transistors (HBTs), junction field‐effect transistors (JFETs), and various kinds of diodes (e.g., Zener, rectifying, laser, light emitting). Semiconductor engineers routinely use the current‐voltage (I–V) measurement technique both for the design of semiconductor devices and for yield or process checks during manufacture. The I–V measurement technique is also considered to be a method in materials research because it is used to characterize semiconductor materials and processes used to create these materials. The pn junction I–V technique is useful in materials research because complementary techniques often do not definitively answer whether the material quality is sufficiently good to operate as required in a semiconductor device. The technique can definitely be used on pn junctions present in state‐of‐the‐art semiconductor devices, but in most materials research laboratories, diodes with large feature sizes and considerably simplified sample preparation are preferred for rapid turnaround and lower equipment cost. The technique is most often used when the purpose of characterizing the material is for fabrication of pn junction devices such as BJTs, HBTs, JFETs, and diode structures for a number of applications, including semiconductor lasers and light‐emitting diodes. In that case, the method can be used primarily as a measure of material quality in a device application or as routine intermediate process characterization during device fabrication.
Magnetism and Magnetic Measurements
Authors: Chien C.L. , Xiao G.
Published: October 2003

This article introduces the sections on magnetism and magnetic measurements. Magnetism and magnetic materials have a long and illustrious history. Lodestone (a variety of magnetite) was the first magnetic material of technological importance—which often means military importance as well—due to its seemingly mystical power of seeking direction, a feat which never fails to fascinate a first‐time observer. Lodestones were known to the ancient Egyptian, Greek, and Chinese civilizations, and possibly others. About 2600 B.C., lodestone was instrumental in winning a decisive battle for the first emperor of China, fought in a dense fog against the invading barbarians.

The subject that we generally call magnetism began with the observation that some minerals, such as magnetite, could attract each other and certain other minerals. However, a fundamental understanding of magnetism had to await the advent of quantum mechanics and special relativity in the 20th century.

Today, magnetism enters many aspects of our lives, far beyond just compasses and refrigerator magnets. Magnetic materials and magnetic phenomena inconspicuously appear in many complex devices, for example, theft‐prevention systems in department stores, magnetic recording disks and read heads in the ubiquitous computers and cellular phones, and in magnetic resonance imaging apparatus. Some of the more exciting new developments are the spintronic devices, which are on the verge of becoming realities. These new devices are based on the spin properties of electrons, as opposed to charge alone, as in conventional electronics.

The subject of magnetism is inseparable from electrical charges, their translational motion and their angular momenta. The articles in this category cover magnetic fields and properties, measurement of magnetic fields and their effect on materials behavior.
Generation and Measurement of Magnetic Fields
Authors: Schneider‐Muntau Hans J. , Coey J.M.D. , Kuhns Phil , Rubin Larry , Schott Christian
Published: October 2003

In basic science areas, such as semiconductor physics, superconductivity, structural biology, and chemistry, magnetic fields are used as a powerful tool for analysis of matter because they can change the physical state of a system. In the presence of a magnetic field, the angular momenta (spins), the orbital motion of charged particles, and therefore the energy of the investigated physical system, are altered. One must realize, however, that the size of the induced effects are very modest compared to other physical quantities.

The use of magnetic fields and their effects on matter are based on a few micro‐ and macroscopic principles. These principles are utilized not only in basic science areas but also for significant industrial applications, and most of them are also employed to measure magnetic fields. These include the following: Magnetization of nuclei and its decay; induced currents; force on moving charged particles(“Lorentz force”); magnetization; and energy content.

Magnetic fields can be generated by magnets created from hard magnetic materials and by electromagnets. Permanent magnets offer a simple, fast, and cost‐effective way to provide small‐volume fields.Permanent magnetsare being employed for MRI applications, accelerator storage rings, and wiggler magnets. Dimagnetism, antiferromagnetism, and ferromagnetism are explained.
Magnetic Moment and Magnetization
Authors: McHenry Michael E.
Published: October 2003

The intrinsic magnetic properties of materials refer to

the origin, magnitude, and directions of atomic magnetic dipole moments;

the nature of the interaction between atomic magnetic dipole moments on neighboring atoms in the solid;

whether these result in collective magnetic phenomena; and

the resulting temperature dependence of the magnetization.

Materials exhibiting collective magnetic response possess additional intrinsic magnetic properties, including the strength of the coupling of magnetic moments to one another and to crystallographic directions, magnetoelastic coupling coefficients, and the temperature(s) at which magnetic phase transformations occur. The intrinsic magnetic properties of species at surfaces and interfaces are known to be distinctly different from those of the bulk in many cases. In this articles the theory of intrinsic magnetic properties, specifically of dipole moment and magnetization, as well as theory and examples of collective magnetic response are achieved.
Theory of Magnetic Phase Transitions
Authors: McHenry Michael E. , Laughlin David E.
Published: October 2003

The existence of magnetic order (collective magnetism) appearing in materials below a particular ordering temperature (e.g., the Curie temperature,
T
C
, or the Neel temperature,
T
N
) points to a class of physical phenomena which are described as
magnetic phase transition
s. The thermodynamics of these phase transitions can be described by energy functions (expressed in terms of intensive or extensive variables), in terms of magnetization − temperature (
M
−
T
) phase diagrams, or in terms of critical exponents that describe the variation of thermodynamic properties (as a function of the order parameter) as the ordering temperature is approached. In this article, the thermodynamics of magnetic ordering transitions, the order of the transitions, critical exponents, and thermodynamic variables are described. Magnetic phase transitions within the framework of the mean field or
Landau theory of phase transitions
with discussion of several magnetic equations of state are presented.
Thermomagnetic Analysis
Authors: McHenry Michael E.
Published: October 2003

The most common method of measuring magnetic ordering temperatures is through the use of
thermomagnetic analysis
techniques. These techniques rely on methods previously described. In essence, thermomagnetic analysis involves sensitive measurement of a magnetic dipole moment in a zero or an applied field as a function of temperature. The components of the system therefore include an appropriate magnetometer. Superconducting quantum interference device (SQUID), vibrating‐sample, and Faraday‐balance magnetometers are commonly used in the field. Large axial fields can be applied using a superconducting solenoid; smaller axial fields can be applied using wound‐metal solenoids. Transverse fields can be applied using split‐coil superconducting solenoids or electromagnets. Measurement of the temperature dependence of magnetic properties requires careful design of the sample chamber and methods for heating and cooling, as well as accurate thermometry for monitoring temperature. Measurement of magnetic transitions of the types described in the other articles may entail the measurement of magnetic properties at temperatures ranging from cryogenic to ∼1100°C (the
T

c

of elemental Co).

The data illustrated in the figures in this article, probing magnetic phase transitions, were taken using either a SQUID magnetometer or a vibrating sample magnetometer. An abbreviated discussion of these systems is given.

The principle of operation of a vibrating sample magnetometer (VSM) is based on placing a magnetic sample in a uniform magnetic field. The sample dipole moment is made to undergo a periodic sinusoidal motion at a fixed frequency using a transducer drive head to vibrate a sample rod.

The term SQUID is an acronym for superconducting quantum interference device. The principles of operation of a typical SQUID magnetometer system for measuring a sample are briefly summarized.
Techniques to Measure Magnetic Domain Structures
Authors: Celotta R.J. , Unguris J. , Kelley M.H. , Pierce D.T.
Published: October 2003

Critical to understanding the magnetic properties and the technological application of magnetic materials is the ability to observe and measure magnetic domain structure. The measurement challenge is quite varied. Magnetic fluids can be used to decorate and make simple domain structures visible while highly sophisticated electron microscopy‐based methods may be needed to explore and image nanometer‐scale magnetic phenomena. No one method will solve all of the important domain imaging problems. For that reason, the bases of all the important methods and the practical implications of each are discussed.

For purposes of comparison, several important characteristics of each domain imaging technique are briefly summarized.

There are several general comments that apply to every domain imaging method discussed in this article. First, each method is related either to a sample's magnetization or to the magnetic field generated by that magnetization. The former observation is of great usefulness in understanding or developing new materials, and the latter is of practical importance in devices. Second, all of the methods are limited to the topmost few micrometers of the sample, and are therefore dominated by magnetic structure related to the surface. None provide the domain structure of the interior of a thick sample where bulk magnetic microstructure is fully developed. Third, all of the methods require smooth, clean, damage‐free surfaces. In general, the surfaces need to have at least the equivalent of a mirror polish, otherwise, topography and surface stress will affect the measurement to varying degrees. Fourth, correlations exist between several of the parameters in each technique we describe. For example, it may not be possible to achieve the ultimate sensitivity specified at the highest resolution possible. Finally, almost all of the techniques use digital signal acquisition and can benefit from the use of modern image processing tools.

Expanded discussions of the basis of each technique, its practical aspects, sample preparation required, possible sample modifications, and problems that are specific to the technique described are given. Each section contains references to more complete treatments of the technique and recent examples of its use.
Magnetotransport in Metals and Alloys
Authors: Bass Jack
Published: October 2003

Electronic transport properties are fundamental to the classification of materials. The behaviors of the electrical resistivity ρ, the thermal conductivity κ, and the thermopowerSare used to define whether a material is a metal, a semi‐conductor, or an insulator. Studies of how ρ, κ, andSvary with impurity content (alloying), magnetic fieldB, sample size, deformation, etc., provide insight into the nature of current carriers and how they are scattered. Studies of magnetoresistance, the variation of ρ withB, can yield additional information about electronic structure, the current carriers, and their scattering. In systems involving magnetic metals,has technical applications to magnetic sensing and memory. The Hall effect and the thermopowerScan often be used to infer the sign of the charge of the majority current carriers, and the thermal conductivity κ, while closely related to ρ, can manifest differences that contain significant information. The Hall effect in semiconductors also finds use in sensors, but in metals it is smaller and usually of more interest for the physical insight it can provide into both nonmagnetic and magnetic metals. Changes with magnetic field in the thermal conductivity and thermoelectric coefficients of metals are usually small and difficult to measure. They have provided useful information about physical phenomena such as many‐body contributions to thermoelectricity and giant magnetoresistance in granular alloys and magnetic multilayers but are much less studied than the resistivity and Hall effect. Transport and magnetotransport measurements of metals and alloys are made over a wide range of temperatures extending from the lowest achievable temperature to the liquid state.

This article focuses on four measured quantities in solid metals and alloys in the presence of a temporally constant and spatially uniform magnetic fieldB—the electrical resistanceR, the Hall resistance, the thermal conductanceK, and the thermopowerS. The basic theory underlying these quantities applies also to liquid metals and alloys. The discussion begins with definitions and general information, including how to relate the measured quantities to the fundamental properties ρ, the Hall coefficient, κ, andS. These are followed by a brief description of the behaviors of ρ, κ, andSin zero magnetic field. Many more quantities than these four can be defined and measured, some of which have provided important information about metals. A few are briefly discussed.
Electrochemical Techniques
Authors: Bocarsly Andrew B.
Published: October 2003

Electrochemistry, in contrast to most materials characterization disciplines, contains a bifurcated methodology having applications both in materials synthesis and as a physical characterization technique. On the one hand, a variety of functional surfaces are prepared electrochemically, for example, the anodization of aluminum, electroetching of semiconductors, and electroplating of metallic films. Similarly, the electrosynthesis of bulk materials such as conducting organic polymers has taken on importance in recent years. On the other hand, electrochemistry has been demonstrated to be of use as a physical characterization technique for the quantification of conducting and semiconducting surfaces. The corrosion properties of metallic surfaces and semiconductor electronic properties are two key areas where electrochemical characterization has had a high impact. In keeping with the central analytical theme of this work, this category limits its focus to the class of electrochemical techniques specifically aimed at characterizing materials and their interfaces (as opposed to synthetic electrochemistry). In some cases, the line between materials synthesis and measurement can become hazy. The field of sensors based on chemically modified electrodes (CMEs) is a good example of this confusion. Often sensors of this type are electrosynthesized and the synthesis parameters represent an important portion of the systems characterization. Cases such as this are included in the topical coverage of these articles.