How to Select

How to Select a Magnet System

In principle, precision laboratory electromagnet systems are very similar. A basic system consists of two component parts: 1) a carefully designed and manufactured electromagnet assembly and 2) a precisely controlled power supply for excitation of the magnet coils.

Within these broad definitions are a wide variety of optional features which optimize a particular system for use in a particular application. The WSI standard electromagnet assemblies are available with pole piece diameters ranging from 4 inches to 15 inches. The air gap may be either fixed or variable (by one means or another) and the orientation of the yoke, or frame, may be either vertical, horizontal, or in some cases, be convertible from one to the other.

The excitation power supply may also vary in sophistication, performance and cost. Its real purpose, of course, is simply to supply direct current to energize the magnet coils, and therefore, the supply must be able to deliver sufficient current for the particular magnet size chosen. Furthermore, the stability (and accuracy) of the supply affects the stability, repeatability and "quality" of the magnetic field generated between the magnet poles. In many cases the regulation feedback is supplied by direct measurement of the resultant field. Some supplies have built in capability for "sweeping" the output current, and hence the magnetic field, over a pre-determined range.

In short, the primary factors to consider in the choice of a magnet system are:

The size of air gap required to accommodate the desired sample or experiment. This, in addition to consideration of the level of magnetic flux required and homo geneity, will suggest the basic pole piece size of the electromagnet assembly. WSI offers standard pole piece diameters of 4", 5", 7", 9.5", 10", 12" and 15". Other sizes can be manufactured on special order.
The requirement for fixed or variable air gap and vertical or horizontal field orientation. Actually, the air gap of all WSI magnets may be varied because even the "fixed gap" ver sions may be adjusted by means of interchangeable pole caps. Most of the continuously variable gap models are equipped with precision screw drives for precise gap width control.
The size and complexity of the power supply. The magnet size, required field strength, and volume of field really dictate the required current range while application consider- ations suggest the accuracy and stability requirements as well as other desirable features of the supply. WSI offers several standard supplies to cover all requirements.

Defining The Field

The magnetic field generated by a precision electromagnet system has several characteristics which should be considered in the final selection. First is the size of the air gap, but other important parameters include flux intensity (in gauss or kilogauss), stability, and homogeneity of the field within the gap. Stability of the field means it is independent of changes in line voltage, load impedance, noise, temperature drift or the introduction of extraneous magnetic fields.

Homogeneity is usually expressed as a fraction of the total field. The magnetic intensity nearly always decreases slightly off of the center point of the poles, but plots such as the ones shown indicate the homogeneity of the field. Homogeneity may be improved in many experiments by the use of optional ring-shim pole caps and/or µ-shims. Homogeneity is a function of air gap, pole face diameter and field intensity. Actually it is a function of electromagnet design and may be considered (all things being equal) as a "figure of merit" for any given electromagnet.

Compensating for Inhomogeneity

To a large extent, the air gap performance of a given electromagnet may be significantly improved by use of appropriate pole caps and/or other accessories. For example, homogeneity of the field may be improved dramatically by use of ring-shimmed pole caps. Another effective method at high field is the use of our patented "µ-shim" pole cap accessory which features continuous, repeatable precision adjustment and does not protrude into the working gap of the magnet.

The µ-shim compensators employ a one inch diameter ferromagnetic alloy rod inserted to a precisely set depth through the pole caps and pole core of the magnet. The µ-shim is keyed and precision ground with the pole cap thereby retaining parallelism with the pole face even during adjustment. The µ-shim essentially controls the air gap over a preselected diameter at the center of the pole face, there by compensating for radial field fall off and axial field increases. A micrometer head on each shim rod positioner provides quick and calibrated optimization of the shim position at any field strength.

Since the µ-shim is continuously adjustable, homogeneity optimization is not restricted to any one field strength. The µ-shim can be set while the magnet is operating, greatly facilitating rapid and precise adjustment with no auxiliary power supply required.

Improvements in magnetic field uniformities are particularly dramatic at higher field strengths where a two-to four-fold increase in field uniformity and volume occurs for most pole cap geometries. At certain high field strength geometries the use of the µ-shim accessory is necessary for conducting resonance studies.

For air gaps greater than 2 inches, ring shims are often employed to increase field strength at the outer diameter of the pole caps. This compensates for the natural fall off in field strength and thereby improves homogeneity across the pole face. Properly designed ring shims can improve homogeneity at the field for which they were designed by about a factor of five.

Applications

The applications of precision laboratory electromagnets are truly limited only by the researcher's imagination. Some of the most common uses are:

Magnetic Susceptibility Measurements
Hall Effect Studies
Magneto-Optics Experiments
NMR (Nuclear Magnetic Resonance)
EPR (Electron Paramagnetic Resonance)
Quantum Mechanics Analysis
Biological Studies

A brief description of some of these techniques is given here which, although by no means rigorous, is intended to show the considerations involved in system selection.

Susceptibility Measurements
The magnetic susceptibility (Xv) of a material is the ratio of the magnetization (M) of a unit volume of a sample to the magnetic field intensity (H) in the material. Measurement of magnetic susceptibility provides useful information about a material, particularly the ease with which it may be magnetized. The field required is generally small and homogeneity is not particularly critical. Since the measurement is a comparative one (with a sample of known susceptibility in the same field) field stability and repeatability are important. For most of these applications a 4" to 7" magnet with fixed gap is adequate when powered by a stable power supply.

Hall Effect Studies
The Hall Effect, while by no means a new discovery, has found widespread application for a variety of research purposes. Briefly stated, the Hall Effect phenomenon is this: if an electrical current is passed along the x axis of a block of conductive or semi-conductive material while the material is simultaneously subjected to a magnetic field which is perpendicular to the current flow (y axis), a voltage will be generated across the sample material in the mutually perpendicular axis (z). In this configuration, the voltage so generated is directly proportional to the intensity of the magnetic field and hence is a valuable measurement tool for determining magnetic flux. Furthermore, these techniques yield significant data on the structural properties of the sample. For most investigations, relatively low levels of field intensity are required and the 4", 5" or 7" inch magnets are usually adequate. Care should be taken in the selection of a power supply, however, as stability of the magnetic field is important for most applications.

Magneto-Optics Experiments
Several optical phenomena are dependent upon the generation of precise magnetic fields. These include the Zeeman Effects, Faraday Effect, Kerr Effect, Voight Effect, etc. These effects are most useful research tools relative to spectroscopy, as well as laser switching and other practical applications. Many of these experiments require axial access holes longitudinally through the pole pieces (and hence the field) and this should be considered in the choice of a magnet system for these applications.

NMR, EPR
NMR (Nuclear Magnetic Resonance) and EPR (Electron Paramagnetic Resonance) are sophisticated analysis techniques in which a sample material is subjected to irradiation by an RF energy source while simultaneously in the presence of a strong DC magnetic field. While the frequency of the RF source is held constant the magnetic field intensity is swept over a predetermined range. At some particular level, the RF absorption of the material will increase sharply. In the case of NMR, the molecules of the sample contain an isotope known to demonstrate a magnetic moment. The absorption mentioned occurs at the magnetic field unique to the resonance of the nucleus of the isotope thus a great deal of information about the crystal structure, reaction rates, relaxation times, etc. can be gained by an analysis of the resultant spectral lines. In NMR, RF frequencies range from a few MHz to about 100 MHz and the required magnetic field on the order of 10 kG.

In EPR experiments, the techniques employed are similar, although the results are substantially different. The sample materials are electrons rather than the isotope nucleus as in NMR. EPR is often used in analysis of impurities in semiconductors and other spectroscopic applications. The resonant frequencies are much higher, usually at about 9.5 GHz (X band) or 35 GHz (Ka band) with magnetic field intensities up to about 15 kG.

Cryogenics
One method often employed to reach extremely low temperatures (below 1° Kelvin) is called adiabatic demagnetization. The technique involves reducing the temperature of a paramagnetic salt as low as possible by conventional means. This is done while the salt is in the presence of a high intensity magnetic field. The field partially orders the atomic structure of the salt and when the field is removed, the rearrangement of the atomic spin systems consumes energy (heat) thereby reducing the temperature of the paramagnetic salt specimen. Any of the previously mentioned experiments are occasionally performed at cryogenic temperatures.

Other Applications
Extensive research is being done in many other fields which is aided by the use of precise magnetic fields. Many biological processes are altered by the application of magnetic fields which provide researchers with useful data on the precise nature of these phenomena. Similarly, semiconductor physics and quantum mechanics studies frequently require precise generation and control of magnetic fields. WSI has developed a number of specially adapted systems to aid in many of these requirements. We welcome your inquiry.

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