Magnetic property characterization system employing a single sensing coil arrangement to measure AC susceptibility and DC moment of a sample

A sensing coil arrangement including a pair of sensing coils connected in opposition is used to measure AC susceptibility and, using sample movement and a DC magnetization field source, also to sense signals for absolute DC moment measurements. Since the same sensing coil arrangement is used for both AC and DC measurements, the measurements can be made successively in situ without removing the sample from a sample space (e.g., within a cryogenic chamber). This is a big advantage, because the changed conditions associated with removing and replacing a sample between measurements can cause confusing, uncorrelated results. A high speed voltmeter is used to perform the signal analysis for the moment measurement. The system can be configured to yield high resolution DC moment measurements to 25 ppm and sensitivities to better than 5.times.10.sup.-5 emu.

FIELD OF THE INVENTION 
The present invention relates to measuring the magnetic characteristics of 
a sample, and more particularly relates to measuring magnetic parameters 
using both AC and DC measuring techniques. 
BACKGROUND AND SUMMARY OF THE INVENTION 
AC susceptometry and DC magnetometry are two different widely used 
techniques for obtaining information about the magnetic characteristics of 
a sample. The following provides a brief background discussion of each of 
these well known techniques. 
Briefly, in a typical dc magnetization measurement, a value for the 
magnetic moment m of the sample is measured for some applied dc field 
H.sub.dc. The magnetic moment m is a bulk sample property and is a measure 
of the magnetic field generated by the sample itself. The dc or static 
"susceptibility" is determined by dividing the magnetization by the 
applied field ( .chi..sub.dc =M/H.sub.dc ). When comparing different 
materials (or samples of the same material having different sizes), the 
macroscopic quantity of interest is magnetization per unit volume (or per 
unit mass) of the sample. 
Techniques for measuring the dc moment have long been commercialized in a 
variety of system products such as vibrating sample magnetometers (VSMs), 
force magnetometers, and SQUID magnetometers. The most commonly used dc 
magnetometers, such as the vibrating sample magnetometer (VSM) or SQUID 
magnetometer, generally use a detection coil to measure the change in 
magnetic flux due to the presence of a magnetized sample. If the sample 
does not have a permanent magnetic moment, an applied field is required. 
FIG. 1 schematically illustrates a typical prior art commercial DC 
magnetometer. In many commercial DC magnetometers, a magnet 50 (e.g., an 
electromagnet or superconducting solenoid) is provided to apply a constant 
magnetic field H.sub.dc to the sample 52 in order to magnetize the sample 
to a magnetic moment (m). A detection coil 54 and associated detection 
circuit 56 are also provided. There is no output from the detection 
circuit 56 until there is a magnetic flux change within the detection coil 
54 (Faraday's Law). The sample flux coupled to the detection coil 54 is 
commonly varied by moving the sample. In a VSM, the sample 52 is vibrated 
near detection coil 54. As the sample 52 moves, an ac signal is generated 
at a frequency determined by the sample oscillation. In a SQUID system, 
the sample 52 is simply passed through the detection coil 54. Typically, 
detection circuit 56 comprises a fluxmeter, lock-in amplifier, or SQUID 
electronics that is coupled to the detection coil 54 in order to measure 
the change in flux (i.e., the magnetic moment). A so-called "extraction 
technique" is known wherein the fluxmeter (detection circuit) 56 deflects 
in an amount proportional to the flux change (moment) as the sample is 
removed from the sensing coil. See Cullity, Introduction to Magnetic 
Materials, pages 61-81, and particularly, 65-66 (Addison-Wesley Publishing 
Co. 1972). 
Traditionally, fluxmeters, which are essentially digital voltmeters, are 
used for measuring secondary coil output signals over time. However, most 
fluxmeters have a relatively low input impedance (which can present a 
problem when the sensing coils to be used have a large variable 
resistance). In addition, fluxmeters may present potential measuring 
difficulties in dealing with drifts and thermal emfs. 
It is generally known to use an integrating digital voltmeter as a 
fluxmeter. It is apparently also known to use a digital voltmeter to 
measure voltages to indicate DC magnetization. For example, the Rebouillat 
and Sasaki references cited below each appear to teach using an 
integrating digital voltmeter as a fluxmeter for measuring magnetization. 
In addition, the Werle '142 patent (cited below) teaches a magnetic 
fluxmeter for measuring macro flux disturbances (e.g., caused by a ship 
passing by) which includes a digital voltmeter type indicator for 
indicating the level of a signal integrated in the analog domain. The 
Coley '990 patent (also cited below) appears to teach a digital voltmeter 
arrangement in his FIG. 4 tunneling susceptometer (see components 52-66). 
When properly calibrated, the output from the VSM or SQUID magnetometer 
yields the value of the magnetic moment for the sample. With knowledge of 
the sample volume (V), the magnetization (M) can be determined. 
Magnetization and the dc susceptibility are thus derived quantities. 
Usually, the moment is measured as a function of field, and the materials' 
magnetization curve (i.e., m or M versus H.sub.dc) is determined (see FIG. 
2) by repeating the measurement for different values of H.sub.dc. That is, 
in a dc magnetization measurement discrete points along some 
characteristic magnetization curve are measured. This permits measurement 
of several discrete points along the magnetization curve of the sample. If 
the magnetic field direction can be reversed, hysteresis curves can also 
be generated. 
DC magnetization/susceptibility measurements are extremely useful (e.g., 
for high field studies and for measuring hysteresis curves). However, 
sometimes additional (or different) information about the magnetic 
properties of a sample is required that is not available from a DC type 
measurement. For example, there are instances in which "complex" (real and 
imaginary) magnetic susceptibility must be measured in order to provide 
more complete information about the magnetic characteristics (e.g., 
relaxation characteristics) of a sample. AC susceptometery provides 
certain information (e.g., information about such "complex" parameters) 
that is not available from DC magnetometry. Moreover, in the AC 
measurement, copper wound coils can be used to generate very small 
amplitude AC fields (e.g,&gt;&gt;1 Oe) without complications arising from the 
remanent fields associated with iron-core or superconducting magnets. This 
means that the AC technique is very valuable in the study of low-field 
magnetic characteristics of a sample. 
In addition, with the capability to vary the frequency of the drive field 
in an AC technique, the magnetodynamics of the magnetic system can also be 
studied. Further, since in the AC measurement the slope of the 
magnetization curve is being measured, non-linear magnetization and 
magnetic transitions are often best studied using an AC measurement. 
AC susceptometry has thus been widely used for the characterization of 
magnetic materials for many years. However, prior to 1988, there was no 
serious commercial product available and ac susceptometer usage was almost 
universally "build your own" (so that, for example, many ac susceptometers 
were "laboratory assembled" using available components). Unlike the dc 
technique (where actual values for the magnetic moment m are measured), 
changes in m (i.e., .DELTA.m) are measured in the ac technique. Thus, the 
ac susceptibility gives an indication of the slope (dm/dH) of the 
magnetization curve. This is a fundamental difference between the ac and 
dc measurement techniques. 
The discovery of high T.sub.c superconductors led to a rapid increase in 
interest in magnetic measurements. High T.sub.c materials are 
characterized by relatively small first critical fields, H.sub.c1, and a 
small full penetration field, H.sub.p. Therefore, a reliable low-field 
magnetization measurement technique is necessary for the full magnetic 
characterization of these materials. 
In addition, the ac measurement can be used to differentiate between inter- 
and intragranular current coupling, and in determining the overall quality 
of a superconductor. An analysis of .chi." can provide information about 
the critical current density J.sub.c of these materials via the invocation 
of a suitable critical state model, and this sort of analysis has 
contributed to a better understanding of the mechanisms of 
superconductivity in these compounds. 
An analysis of the complex susceptibility can also provide information 
about relaxational processes that may be occurring in the system under 
study. For example, it can be used to study spin-lattice relaxation 
phenomena in paramagnetic compounds, domain wall movement in metamagnetic 
systems, and has contributed to a better understanding of spin-glass 
systems. 
Using the ac technique, the sample is generally centered within a detection 
coil and exposed to an applied AC magnetic field. The magnetic moment of 
the sample follows the applied field. The detection circuitry is generally 
balanced, with matched detection coils being provided in order to null out 
the changing flux due to the AC excitation. As a result, the detected 
change in flux is related only to the change in moment of the sample as it 
responds to the AC field. 
Using well established principles of ac susceptometry, Lake Shore 
Cryotronics (the assignee of the present invention) introduced its 7000 
line of ac susceptometers in the fall of 1988. FIG. 2A is a schematic 
block diagram showing this prior art ac susceptometer developed by Lake 
Shore. The basic principles of operation are described in Lake Shore's 
application note entitled "AC Susceptibility Measurement: Its Purpose and 
Process" (the disclosure of which is incorporated by reference herein for 
the purpose of providing discussion as to the state of the art). While no 
sample movement is required to perform AC susceptometery measurements, a 
motor and associated sample movement arrangement are provided in Lake 
Shore's ac susceptometer to move the sample in order to increase 
measurement accuracy and resolution. 
Needless to say, much work has been done in the past in regard to magnetic 
characteristic measuring techniques. The following documents relate to 
techniques for measuring the magnetic characteristics of a sample: 
Rillo et al, "Multipurpose a.c. and d.c. Equipment for Low Temperature 
Magnetic and Electric Measurements of Solids" (Abstract of paper presented 
at S ONR Workshop in May 1991); 
U.S. Pat. No. 3,528,001--Yntema 
U.S. Pat. No. 3,454,875--Bol et al 
U.S. Pat. No. 4,861,990--Coley 
U.S. Pat. No. 2,975,360--Bell 
U.S. Pat. No. 4,037,149--Foner 
U.S. Pat. No. 4,005,358--Foner 
U.S. Pat. No. 4,238,734--Steingroever et al 
U.S. Pat. No. 4,849,695--Muller et al 
U S. Pat. No. 5,008,621--Jiles 
U.S. Pat. No. 3,863,142--Werle 
Goldfarb et al, "Alternating-Field Susceptometry and Magnetic 
Susceptibility of Superconductors" (Office of Naval Research Workshop on 
Magnetic Susceptibility of Superconductors and Other Spin Systems, 
Berkeley Springs, W. Va. May 1991); 
Rebouillat, "High Resolution Automatic Magnetometer Using a Superconducting 
Magnet: Application to High Field Susceptibility Measurements", IEEE 
Trans. on Magnetics, v. MAG-8, n. 3 pp. 630-33 (Sept. 3, 1972); 
Sasaki, "Simple Precision Fluxmeter", 76 Nuclear Instruments and Methods n. 
1 pp. 100-2 (Dec. 1, 1969); 
Edwards et al, "Magnetometer for Surface Flux Density Measurement in MPI", 
29 British Journal of Non-Destructive Testing n. 5, pp. 304-306 (Sept. 
1987); 
Beckley et al, "Simplified Electronic Permeameter Suitable for Routine and 
Standards Use", 9 Measurement and Control n.10, p. T65-T70 (Oct. 1976); 
Marinaccio, "Op Amp Converts DVM To Fluxmeter", 48 Electronics n.10, pp. 
112-113 (May 15, 1975); 
De Mott, "Integrating Fluxmeter with Digital Readout", IEEE Journal on 
Magnetics v. MAG-6 n.2 pp. 269-71 (Jun. 2, 1970); and 
Press Release, "DOWTY RFL Offers The Only Digital Fluxmeter With IEEE-488 
Bus Operation", Dowty RFL Industries, Boonton N.J. (Mar. 24, 1986). 
Jiles teaches a measuring device capable of measuring various magnetic 
parameters (e.g., fluxmeter, gaussmeter, strain indicator), and uses the 
same overall transducer assembly for various measurements. The Dowty RFL 
press release describes a digital fluxmeter which measures "both flux 
density and total flux". 
In addition, AC susceptometers, DC magnetometers and Vibrating Sample 
Magnetometers are commercially available from several companies. For 
example: 
Quantum Design Inc. of San Diego, Calif. has been marketing a DC 
magnetometer since 1985, and as early as 1990 announced an option for 
making complex ac susceptibility measurements using elements of the SQUID 
detection system in its dc magnetometer. The means for making the ac 
measurements is not clear, nor has this group been able to qualify 
performance characteristics. In fact, it appears that no units have 
actually been delivered as of the filing date of the subject application. 
Cryogenic Consultants Ltd., London England has been marketing, for 
approximately two years, a DC magnetometer employing a SQUID detection 
system. This device has no ac measurement capabilities. Cryogenic 
Consultants also markets a vibrating sample magnetometer. 
Metronique Ingenierie of Le Bourget, France (no longer a going concern) 
introduced a SQUID (dc) Magnetometer in December 1989. They offered an ac 
measurement option which consisted of a separate set of sensing coils. The 
customer had to place a separate ac measurement insert into the dewar 
(refrigerator) system. 
EG&G Princeton Applied Research (C) Princeton N.J., long ago introduced 
a vibrating sample magnetometer (VSM)that makes dc magnetization 
measurements. This instrument in found in many installations around the 
United States. 
Princeton Measurements Corporation of Princeton N.J. introduced an 
Alternating Gradient Force Magnetometer (AGFM) about two years 
ago--possibly to compete against VSMs and room temperature applications 
for SQUID magnetometers. 
Phasetrack Instruments of Santa Clara, Calif. markets an ac susceptometer 
on a small scale. This instrument is not capable of making dc 
magnetization measurements. 
See also generally the following papers relating to magnetic characteristic 
measurement: John K. Krause and Jeffrey R. Bergen, "Understanding magnetic 
measurement techniques," Superconductor Industry, vol. 3, no. 4, pp. 
23-26, 1990; 
Jiles, Magnetization and Magnetic Materials, pages 47-68 (Chapman & Hall 
1991); 
Goldfarb, "Thermoremannt magnetization and superparamagnetism in 
nickel-manganese alloys", Ph.D. dissertation (Colorado State University 
1979); 
Khoder et al, "Calibration constant calculations for Magnetic 
Susceptibility"; 
Couach et al, "Study of Superconductors by AC Susceptibility", Cryogenics 
Vol. 25, pp. 695-99 (1985); 
Goldfarb et al, "Calibration of ac susceptometers for cylindrical 
specimens", Rev. Sci. Instrum. vol. 55, pp. 761-64 (1984): 
Zieba et al, "Superconducting Magnet Image Effects Observed With A 
Vibrating Sample Magnetometer", Rev. Sci. Instrum. Vol. 54, pp. 137-45 
(1983); and 
Rillo et al, "On the Sensitivity of High-TC Superconducting Ceramics as 
Magnetic-Field Sensors", Sensors and Actuators A-Physics, Vol. 27, N1-3 
pp. 775-80 (1991). 
For some time there has been a desire in the field to develop a practical, 
cost-effective commercial instrument capable of providing both ac and dc 
measurement techniques. As mentioned above, for a complete study of the 
magnetic properties of at least certain samples it is desirable to perform 
both ac and dc measurements. However, despite such desire, no one in the 
past has developed a practical, cost-effective instrument that is capable 
of accurately measuring the characteristics of a sample using both ac and 
dc techniques. 
The present applicants have developed a preferred embodiment magnetic 
measurement system that is versatile, highly accurate, and can measure 
both AC susceptibility and DC moment. The instrument provided in 
accordance with a presently preferred exemplary embodiment of the present 
invention includes various components (i.e., two oppositely wound 
secondary coils, a source of AC excitation current coupled to a primary 
winding, and a stepping motor for moving the sample between the coils) to 
perform AC susceptibility measurements. The instrument also includes a 
source of DC current that can be coupled to the primary winding. The motor 
and associated sampling positioning arrangement is also used to provide 
the motion needed for "extraction type" DC magnetization measurement. A 
high speed digital voltmeter monitors and records the output of the 
secondary coil(s). The recorded output of the digital voltmeter is 
numerically processed (using a computer) to yield the voltage integral 
indicative of magnetic moment. 
Thus, a presently preferred exemplary embodiment of the present invention 
provides a combination of the extraction technique for DC moment 
measurement with the AC susceptibility measurement using a common sensing 
structure--all within an instrument providing state-of-the-art electronics 
and computer control. By way of non-limiting example, applicants' 
presently preferred embodiment system provides the following advantages: 
A single instrument, requiring no hardware reconfiguration between 
measurement modes, that is capable of measuring both AC and DC magnetic 
response of materials; 
AC and DC measurements can be made without removing sample from 
dewar--thereby eliminating possible errors due to changes in set-up, 
changes in sample condition, etc.; 
Use of common components (e.g., coil assembly) permits common calibration 
factors to be used for both AC and DC measurements; 
Calibration constants for both AC and DC measurements can be calculated 
from considerations of coil geometry alone (unnecessary to calibrate with 
standard magnetic materials--since the coil assembly is precision wound 
and the system has been designed to avoid extraneous effects); 
Stepper motors and associated sample suspension/mounting structure used for 
DC magnetization extraction technique also used for moving sample during 
AC measurements in order to cancel out differences between the two coils; 
Dual opposed secondary coils cancel magnetic field noise; 
Non-zero voltage offsets within the digital voltmeter during DC 
measurements are cancelled during subsequent noise reduction 
analysis--permitting higher speed digital voltmeter data acquisition in 
order to minimize "dead time" between measurements and thereby increase 
measurement sensitivity; 
AC susceptibility measurement sensitivities comparable to or greater than 
those achievable using SQUID magnetometers (e.g., 2.times.10.sup.-8 emu); 
High dc measurement sensitivity levels (e.g., 5.times.10.sup.-5 emu) can be 
achieved with an effective dynamic range extending to&gt;10.sup.3 emu, 
comparable to Vibrating Sample Magnetometers; 
Broad dynamic range (10.sup.-8 to&gt;10.sup.3) to permit a wide array of 
material properties to be studied; 
AC susceptibility measurement over a wide range of temperatures (e.g.,&lt;4.2 
K to 325 K), amplitudes (e.g., 0.1 A m.sup.-1 () .00125 Oe) to 1600 
Am.sup.-1 (20 Oe) RMS), and frequencies (e.g., 1 Hz to 10 kHz); 
DC moment measurement over a wide range of temperatures and DC fields 
(e.g., 1.0 tesla or 5.0 tesla, plus or minus); 
Capability of measuring harmonic susceptibilities, and AC and DC resistance 
(e.g., Hall effect, Transport J.sub.c, magnetoresistance); 
For AC measurements, primary coil is driven with an AC current source so 
that the resultant AC field depends only on the "constancy" of the current 
source output--thereby eliminating complicated phase relationships 
dependent on measurement frequency or temperature; 
Virtual elimination of effects of eddy current generation in conductive 
materials or generation of persistent currents in superconductive 
solenoids inductively coupled to the secondary coils; 
Fully automated for unattended operation with data acquisition and control 
software that is quickly and easily tailored to address specific research 
requirements; 
Possible to input sample parameters (e.g., volume, mass) and 
demagnetization factors to assure that the resultant measurement is as 
accurate as possible; 
Wide range of materials and applications. The system is well suited for 
studying paramagnetic and ferromagnetic materials, amorphous alloys and 
diluted magnetic semiconductors, organic ferromagnets and organic 
superconductors (C.sub.60 compounds), conductive polymers, thin film 
recording media, etc. 
Ability to flexibly configure a common instrument platform for AC measuring 
capabilities, DC measuring capabilities, or both--without sacrificing 
accuracy of either measurement mode. 
Expandable configuration to permit purchasers to upgrade measuring system 
by adding additional measurement mode capabilities subsequent to purchase. 
Applicants have thus developed a single instrument that incorporates both a 
dc moment measurement scheme and an ac susceptometer. The instrument 
provides a sensitive dc measurement capability which can be simply 
implemented, without sacrificing any of the performance characteristics of 
the ac measurement. 
After considering a number of standard dc moment measurement schemes, 
applicants chose an extraction technique. Extraction refers to many 
variations of a basic method, but generally involves moving (extracting) a 
magnetized sample from within a sensing coil. The voltage induced in the 
coil is detected and integrated over time to yield the total flux change 
in the coil. The flux change is directly relatedto the magnetic moment of 
the sample. An extraction method is attractive for this application since 
all required experimental hardware is already in place for the ac 
susceptibility measurement. The only missing component is the means to 
detect the voltage induced in the sensing coils. After reviewing 
experimental requirements and instrument specifications, applicants 
decided to use a high speed digital voltmeter (DVM). The DVM 
specifications indicated that performance comparable to, or better than, a 
commercial fluxmeter could be achieved. 
One aspect of the present invention thus provides a relatively simple 
arrangement to provide a dc moment measurement capability in an ac 
susceptometer requiring minimal effort and hardware changes. Performance 
is comparable to many systems constructed solely as a dc magnetometer. An 
important element to the measurement in the preferred embodiment is the 
use of a high speed digital voltmeter (DVM) for the signal analysis. In 
addition to providing the required resolution and sensitivity needed for 
the moment measurement, the features of the DVM can be used to add further 
capabilities to the system at minimal expense. For example, with the 
addition of a sample probe, a dc resistance measurement can now be 
performed.

DETAILED DESCRIPTION OF A PRESENTLY PREFERRED EXEMPLARY EMBODIMENT 
FIG. 3 is a block schematic diagram of a presently preferred exemplary 
embodiment of a magnetic measuring system 100 in accordance with the 
present invention. FIG. 3 uses common reference numerals to designate 
elements that are similar or identical to those shown in prior art FIG. 
2A. However, the fact that common reference numerals are used between the 
two Figures does not necessarily mean that elements designated with common 
reference numerals are identical in all respects. For example, even though 
computer 112 appears in both prior art FIG. 2A and in FIG. 3, the FIG. 3 
computer executes different software to provide additional/different 
functionality (e.g., capability to measure dc magnetization) such that it 
is not identical to the FIG. 2A computer. 
Referring to FIG. 3, system 100 includes a cryostat 102 housing a coil 
assembly 104; a stepping motor 106 mechanically coupled to a sample probe 
108; a control unit 110; a computer 112; a superconducting magnet 114 and 
associated magnet power supply 116; a temperature controller 118 and 
associated temperature probe 120; an AC lock-in amplifier 122; and a 
digital voltmeter (DVM) 124. 
Referring now to FIG. 3 and to FIG. 4 (a simplified cutaway view of coil 
assembly 104), two sensing (secondary) coils 126a, 126b are identical but 
oppositely wound on a sapphire tube 128 in the preferred embodiment. Each 
coil 126a, 126b is perfectly wound and contains approximately 1600 turns 
with a mean diameter of 1.1 cm and a length of 1.9 cm in the preferred 
embodiment. Secondary coils 126a, 126b are positioned coaxially with a 
center-to-center distance of 3.8 cm in the preferred embodiment. A 1300 
turn primary coil 130 is wound directly over the top of the two secondary 
coils 126a, 126b in the preferred embodiment. Thus, the three coils 126a, 
126b and 130 are all coaxial with one another, with primary coil 130 being 
wound over and covering both of secondary coils 126a, 126b. As will be 
appreciated, the spacing between secondary coils 126a, 126b is such that 
when sample 140 is within the space defined within the interior of 
secondary coil 126a, it is outside of the interior space within (and is 
also magnetically isolated from) secondary coil 126b. However, so long as 
sample 140 is at least partially positioned within the interior space 
within any of secondary coils 126a, 126b, the sample is fully within the 
interior space within primary coil 13. 
The primary coil 130 is used in the preferred embodiment to generate the ac 
field for ac susceptibility measurements, but it can also be used to 
generate low level dc fields for the extraction type DC magnetization 
measurement. The overall design of the coil assembly 104 in terms of its 
physical size and number of turns is critical for the ac measurement (as 
those skilled in the art will readily recognize), as it determines the 
frequency response and ac field range. 
Temperature sensors 120 and control heaters 132 are also mounted with the 
coils 126, 130 on the outside of the sapphire tube 128. The entire coil 
assembly 104 is wrapped with coil foil and superinsulation and sealed at 
one (i.e., the lower) end 134 in the preferred embodiment. The sapphire 
tube 134 includes an open end 136 through which the sample probe may 
descend into an interior space 138 within the center of coil assembly 104. 
As will be explained in detail below, the sample probe 108 is positioned 
and moved by stepping motor 106 so as to precisely position and move a 
sample 140 within sample space 138 with respect to secondary coils 126a, 
126b. 
FIG. 5 is an exemplary cut-away view of cryostat 102 shown in FIG. 3 
mounted within a helium dewar (refrigerator) 142. As shown in FIG. 5, the 
open end 136 of the sapphire tube 128 is attached to the lower end 144 of 
a stainless steel tube 146 inside a vacuum jacket 147. This permits sample 
space 13B to be isothermal and thermally isolated from the cryogenic 
(e.g., liquid helium) bath 148. The sample space 138 is normally operated 
with helium exchange gas inside to provide the necessary thermal coupling 
between the sample 140 and the thermometers 120. Controlled temperatures 
from below 4.2 K to 330 K can be achieved. A vacuum line/wire feed 150 is 
coupled to a vacuum isolation space 152 within vacuum jacket 147 to permit 
vacuum to be drawn from the isolation space and to provide a path for 
electrical conductors coupling external equipment to the superconducting 
magnet 114, the coil assembly 104, and the other electrical components 
within cryostat 102. 
FIG. 6 is an elevated side view of an exemplary preferred embodiment sample 
probe 108. Sample probe 108 descends, in the preferred embodiment, through 
stainless steel tube 146 into sample space 138. The preferred embodiment 
provides access to the secondary coils 126a, 126b and sample space 138 
from above through a room temperature load seal assembly 156 and 
associated sample probe load seal (not shown) using a sample rod 154. 
Sample probe load seal assembly 156 in the preferred embodiment includes 
an O-ring seal 158 located on a lower portion thereof that makes a vacuum 
tight seal between the probe seal and the load seal. Side-mounted opposed 
thumb screws (not shown) in the load seal pull the probe seal 156 downward 
to compress the O-ring, and also hold the seal snugly in place to prevent 
any relative movement between the load seal and the probe seal during 
sample movement. 
Sample rod 154 in the preferred embodiment is a 0.64 cm diameter polished 
stainless steel rod with a nylon extension 160 and sample mount 162. The 
sample rod 154 slides through vacuum-tight Teflon O-ring probe seal 
assembly 156, permitting the sample 140 (which is disposed within sample 
mount 162) to be moved up and down while maintaining a vacuum seal in the 
sample space 138. This arrangement allows samples 140 to be exchanged 
while the system is at cyrogenic temperatures. In the preferred 
embodiment, sample mount 162 includes a nylon (e.g., Delrin) bushing 164, 
a Delrin sample holder 166, and a holder lid (not shown, may comprise a 
#10-24 threaded Delrin rod for mechanically coupling bushing 164 to sample 
holder 166). 
Once the sample 140 has been inserted into the sample probe 108 and the 
sample probe has been inserted into the sample space 138 within the 
secondary coils 126, an upper, threaded end 168 of the sample rod 154 is 
attached to stepping motor 106 via a finder nut 170. Stepping motor 106 
positions and moves the sample 140 between the two secondary coils 126a, 
126b under control of control unit 110. In the preferred embodiment, the 
ac measurement technique uses a two-position measurement scheme to 
eliminate uncertainties which arise from slight imbalances between the two 
secondary coils 126a, 126b. In the preferred embodiment, this same 
movement mechanism (i.e., stepping motor 106 and associated mechanical 
coupling) is used in the dc moment measurement. 
Referring once again to FIGS. 3 and 5 a 7.6 cm diameter 1 tesla 
superconducting magnet 114 is mounted on the outside of the vacuum space 
152 in direct contact with the liquid helium bath 148. In the preferred 
embodiment, the magnet 114 is designed to have a uniformity of better than 
.+-.0.1% over the full axial length of the secondary coils 126a, 126b. The 
large diameter is used to minimize inductive coupling to the primary and 
secondary coils 126, 130 which adversely affects the ac measurement. The 
magnet 114 is used in the preferred embodiment both for applying, to the 
sample, the magnetic field required for the dc moment measurement; and 
also for applying a dc bias field to the sample when making ac 
measurements. In order to vary fields easily and rapidly, a persistent 
switch is not used in the preferred embodiment. 
Referring once again to FIG. 3, system 100 is designed for automatic 
operation and all control is done through computer 112. The sample 
movement, temperature control, field control, and data acquisition are all 
executed under software control by computer 112. Magnet power supply 116 
selectively provides power to superconducting magnet 114 under control of 
computer 112 (e.g., via an RS-232 serial link 171). In the preferred 
embodiment, magnet power supply 116 comprises a convention Lake Shore 
Model 610 or 612 magnet power supply. 
In addition, computer 112 is connected, in the preferred embodiment, via an 
IEEE 488 interface bus 172 to permit it to control the operations of 
temperature controller 118, control unit 110, AC lock-in amplifier 122, 
and DVM 124. In the preferred embodiment computer 112 comprises a general 
purpose off the shelf personal computer (e.g., the Hewlett-Packard VECTRA 
(TM) computer and associated monitor and keyboard) that includes an 
IEEE-488 (GPIB) board and internal hard disk (not shown). Software stored 
on the internal hard disk permits computer 112 to control the operations 
of system 100. 
In the preferred embodiment, AC lock-in amplifier 122 comprises a 
commercially available EG&G Model 5209 Lock-in amplifier that is 
controlled by computer 112 via bus 172. Briefly, lock-in amplifier 
synchronizes with an AC reference (excitation) signal provided by control 
unit 110 in order to "lock in" to and amplify a version of the ac 
excitation signal received by secondary coils 126a, 126b. Lock-in 
amplifier 122 provides a highly accurate measurement of the parameters of 
the AC signal it is "locked in" to (synchronous detection being used to 
eliminate noise effects and to make the instrument phase sensitive so as 
to permit imaginary susceptibility components to be detected, as is well 
known). 
Control unit 110 preferably comprises a Lake Shore model 140 or 710 ACS 
Control Unit that includes an internal DC current source 110a, AC current 
source 110b, and motor control 110c. In the preferred embodiment, the 
outputs of the AC and DC current sources 110a, 110b are coupled to primary 
coil 130, and these current sources are operable under direct, 
programmable control by computer 112. Thus, computer 112 may cause an 
adjustable amount of AC and/or DC current to primary coil 130 by 
controlling DC current source 110a and AC current source 110b via bus 172. 
In the preferred embodiment, AC current source 110b provides a reference 
current output that is applied to a "REF AC In" terminal of Lock-In 
Amplifier 122 (as described above). 
Computer 112 also controls the operation of stepping motor 106 by writing 
appropriate information to control unit motor control block 110c. Stepper 
motor 106 in the preferred embodiment includes a movement motor head (not 
shown) providing a conventional stepping motor coupled to axially displace 
a threaded shaft, the threaded shaft being mechanically coupled to sample 
probe finger nut 170. The movement motor head preferably includes travel 
limit switches (not shown) of conventional design to limit the vertical 
travel of sample probe 108. 
In the preferred embodiment, the secondary coils 126a, 126b are connected, 
in series, alternately and selectively to either lock-in detector 
amplifier 122 for the ac measurement; or to digital voltmeter 124 for dc 
measurement. Such alternate connections are accomplished in the preferred 
embodiment via connector network 123 (a switching network is avoided in 
the preferred embodiment in order to reduce noise; instead, the user 
manipulates cables to manually disconnect coils 126a, 126b to either the 
lock-in amplifier or to the digital voltmeter depending upon which 
measurement, ac or dc, is desired). In the preferred embodiment, DVM 124 
comprises a modified Keithly Model 182 Sensitive Integrating Digital 
Voltmeter. This DVM 124 provides an output which is numerically processed 
by computer 112 to provide an integrated voltage value. 
The Keithly Model 182 in unmodified form has excellent sensitivity, but 
does not sample rapidly enough to provide desired sensitivity in the 
preferred embodiment because it provides a substantial "dead time" between 
samples during which auto-zeroing compensation routines are performed 
(i.e., in order to compensate for non-zero voltage offsets generated 
within the meter itself so as to provide absolute voltage readings). The 
preferred embodiment employs a Keithley meter that has been modified to 
reduce "dead time" between samples by eliminating zero offset compensation 
functions (such that the DVM does not provide absolute voltage readings 
but instead provides only relative voltage readings). Thus, the preferred 
embodiment modified DVM 124 spends a higher percentage of the time making 
measurements (thereby increasing the amount of time during a given 
measuring sequence during which the output voltages provided by sensing 
coils 126a, 126b are being sensed). As a consequence of this modification, 
the modified DVM may, when connected to a zero input voltage, tend to 
drift between slight positive and slight negative voltage readings due to 
drifting within the input amplifiers and other associated stages of the 
meter itself. The preferred embodiment compensates for such meter offset 
voltage drifting--and at the same time compensates for background DC 
voltage picked up by secondary coils 126a, 126b--such that the absolute 
voltage readings of the Keithly DVM can be sacrificed for reduced "dead 
time" realizable by decreasing the amount of overhead processing performed 
in the "dead time" between successive samples. Although the modified meter 
drifts slightly over time, it provides sufficient stability over the time 
a measurement is actually being made so that errors in the relative 
voltage measurements it provides during a particularly magnetic 
characteristic measurement are relatively negligible. While not presently 
preferred due to its decreased sensitivity relative to the modified 
Keithley DVM, it is also possible to use a Hewlett Packard Model HP 3458A 
integrating DVM since it may give satisfactory performance and may have 
its own specific advantages depending upon what is desired from the 
measurement. In the preferred embodiment, "integrating" DVM 124 integrates 
the output of sensing coils 126a, 126b over relatively short time periods 
during measurement to provide many successive samples to computer 112. 
Computer 112 numerically processes these successive samples (as described 
below) to provide a value indicating dc moment. 
Principles of Operation 
A. DC Magnetization 
Superconducting magnet 114 is activated during DC measurements in order to 
apply a constant, uniform magnetic field to sample 140. Primary coil 130 
is typically deactivated during DC measurements, although it is sometimes 
desirable to apply a controlled amount of DC current to primary coil 130 
in order to provide "fine" adjustment of the applied magnetic field. Thus, 
in the preferred embodiment magnet power supply 116 has an output that is 
adjustable in coarse increments; DC current source 110a permits "fine" 
magnetic field bias adjustments to permit the operator to, for example, 
measure more (or specific user-selected) points on the sample's 
magnetization curve. 
The "extraction method" is used during the dc measurement--such that 
stepping motor 106 moves the sample from the axial center of secondary 
coil 126b to the axial center of secondary coil 126a (and, in the 
preferred embodiment, back to the axial center of secondary coil 126b) 
during the time DVM 124 is acquiring a measurement. Sample movement 
results in a change in magnetic flux that is detected by secondary coils 
126a, 126b and induced voltage measured by DVM 124 to provide a measure of 
the net magnetic moment of the sample. Secondary coils 126a, 126b are 
wound identically (albeit in opposite directions) and are intended to have 
identical characteristics so that background voltage (e.g., due to stray 
magnetic fields) induced in one coil is cancelled by the same background 
voltage induced in the other coil. 
In the extraction measurement, the voltage as a function of time induced in 
the secondary coil by the moving sample [.nu.(t)] is not as important as 
determination of the integral of the voltage over time. This integral 
gives the total magnetic flux (.PHI.) change through the coil due to the 
sample movement: 
EQU .PHI.=-.intg..nu.(t)dt. (1) 
Note that the flux change will be independent of how the movement is 
executed; the result only depends on the starting and stopping points of 
the sample 140. Variations in the mechanics of the movement may change the 
shape and appearance of .nu.(t), but not the value of the integral. The 
use of two oppositely wound secondary sensing coils 126a, 126b as in the 
preferred embodiment will double the integral in (1), but more importantly 
will minimize induced noise from the environment and the applied field. 
The magnetic flux can be related to the magnetic moment (m) of the sample 
through a calibration coefficient, .alpha.: 
EQU m=.alpha..PHI.. (2) 
The calibration coefficient can be determined experimentally with known 
magnetic standard samples, or, if the coil and sample geometry are known, 
a value for .alpha. can be calculated. 
As mentionedabove, during the DC measurement the sample 140 is moved from 
the center of one of the secondary coils 126a, 126b to the center of the 
other secondary coil. Thus, the sample 140 remains within the region of 
the field uniformity created by superconducting magnet 114 and/or primary 
coil 130 at all times (since the secondary coils 126 each lie entirely 
within the interior space defined by superconducting magnet 114 in the 
preferred embodiment, and also each lie entirely within the interior space 
defined by primary coil 130). This movement also minimizes uncertainties 
related to sample positioning since the voltage induced by a sample at a 
coil center is effectivly zero. Note this will not be generally true for 
every two-coil system but depends on the coil/sample geometry and the flux 
coupling between the sample and each of the two secondary coils 126a, 
126b. 
Initially, it is necessary to position the sample 140 to be in the center 
of one or the other of secondary coils 126a, 126b--and for the operator to 
inform computer 112 which secondary coil the sample is positioned within. 
To initially position the sample 140 prior to a DC measurement, the 
operator sets a DC field and selects a "position" routine in the control 
software. The operator then uses his or her "best guess" to select which 
secondary coil 126 (upper or lower) the sample is believed to be within. A 
test scan is then performed to automatically move the sample by a 
displacement corresponding to the distance between secondary coils 126a, 
126b. This will yield voltage peak outputs of the type shown in FIG. 8. 
For a properly positioned sample, the voltage peaks will be perfectly 
symmetrical. for in incorrectly positioned sample, on the other hand, 
non-symmetrical peaks will result. In the preferred embodiment, an 
improperly positioned sample 140 should be repositioned by moving the 
sample up or down as needed. This process is repeated until a symmetrical 
voltage peak curve is obtained. 
This positioning technique may be difficult to perform for samples with 
very low signals. When the baseline "scatter" is comparable to the signal 
due to small sample size, it may be possible to increase the field 
strength in order to increase signal level. As a least resort, manual 
positioning of sample rod 154 (e.g., through a process of physically 
measuring the length of the sample, relative to the distance between 
secondary coils 126a, 126b, and marking the rod accordingly) may be used 
to properly position the sample. 
In order to obtain a measurable voltage signal (i.e., a voltage signal of a 
sufficiently high level to be measurable), a rapid sample movement is 
required. However, there is a limit as to how fast the stepping motor 106 
can move the sample rod 154 through the relatively tight O-ring seal 156. 
Changing to a different movement mechanism was considered, but the 
benefits of a stepping motor 106 outweighed the constraints it placed on 
the measurement. A sample velocity of several cm/sec was determined 
sufficient to meet the desired moment specifications. Except as noted, all 
measurements were performed with a sample velocity of 2.4 cm/sec. 
FIG. 7 is a high level flowchart of measuring steps performed by system 100 
to provide a DC magnetization measurement. Initially, computer 112 
controls magnetic power supply 116 (and possibly also DC current source 
110a) to provide generate a constant magnetic field of specified intensity 
(block 200). Secondary coils are coupled to DVM 124 (via connection 
network 123), and the DVM is programmed to integrate with desired sampling 
times and for a desired overall integration time (block 202). The voltage 
induced in the secondary coils 126a, 126b is logged with high speed 
integrating voltmeter 124. Voltmeter 124 allows computer 112 to specify 
the time period through which the voltmeter integrates the signal (.tau.) 
and the time interval between the start of each measurement (.DELTA.t). 
Note, by necessity, .DELTA.t&gt;.tau. with the difference representing the 
integral processing time required for the DVM instrument 124 to complete a 
single measurement. For best noise rejection, .tau. is set at an integral 
multiple of the power line cycle (PLC). For the maximum reading rate, 
.tau.t is set to the minimum permitted by DVM 124. All filtering, 
autoranging, and other features (e.g., auto zeroing) which may slow the 
reading rate are disabled. 
In the preferred embodiment, computer 112 controls DVM to begin monitoring 
voltage prior to controlling stepping motor to begin moving the sample 140 
(blocks 204, 206). In a single "scan" the sample is typically moved from 
the center of bottom coil 126b to the center of top coil 126a and then 
back to the center of bottom coil 126b. During a sample movement in the 
preferred embodiment, a series of approximately one hundred readings 
characterizing .nu.(t) are recorded and stored to the internal memory of 
DVM 124. Computer 112 then controls stepping motor 106 to cease moving the 
sample 140 (block 208), and finally, controls DVM to cease monitoring 
voltage (block 210). The voltage readings are then read back into the 
computer 112 (block 212) for processing. 
The "scan" of blocks 204-212 may be repeated several (e.g., up to 10) times 
to provide a single averaged value (e.g., for measuring samples with low 
level signals or when extra precision is required). The preferred 
embodiment system 100 permits a "half scan" mode (e.g., each movement of 
the sample 140, either up or down, defines one moment measurement). Such 
"half scans" are useful when rapid data acquisition is required (e.g., 
during a rapid field sweep). Since the accuracy and repeatability of the 
measurement is degraded somewhat when "half scan" is used, however, this 
mode is recommended for large samples or when non-critical measurements 
are being made. 
The first step of the processing by computer 112 involves the elimination 
of uncertainties related t thermal emf's. The thermal emf's and any zero 
offsets are easily removed by monitoring the voltage in the sensing coils 
for a short period of time both before the movement starts (the time 
between performing blocks 204 and 206 shown in FIG. 7 is timed in the 
preferred embodiment) and after the movement stops (the time between 
performing blocks 208 and 210 of FIG. 7 is thus also timed by computer 112 
in the preferred embodiment). The readings are linearly fit with respect 
to time in the preferred embodiment (FIG. 7 block 214) to create a 
baseline voltage which is subtracted from the measured voltages. This 
yields the true voltage data due to only the sample movement. Typical 
results are shown in FIG. 8. The voltage referred to in the description 
below is the voltage after correction for thermal emf's. 
Each voltage measurement made by the DVM(.nu..sub.i) is actually a mean 
integral value over the integration period: 
##EQU1## 
where t.sub.i denotes the start of the measurement process. What is 
required from (1) is the voltage integral over the complete pulse 
consisting of n voltage readings. This can be determined as follows: 
##EQU2## 
The second term in the summation, .epsilon..sub.i contains the 
contribution to the integral corresponding to the period .DELTA.t-.tau. 
the DVM is not integrating the voltage. If this time period is short, 
.epsilon..sub.i, can be evaluated by approximating the voltage V(t) as the 
means value of the two voltage readings which span the time period. 
EQU .epsilon..sub.i =[(V.sub.i +V.sub.i+1)/2](.DELTA.-.tau.) (5) 
Substituting into (4), rearranging, and taking note of the fact that the 
initial and final voltage readings are zero yields the final result: 
EQU .intg.V(t)dt=.SIGMA.V.sub.i .DELTA.t (6) 
This final expression appears exactly like a numerical integration using a 
trapezoidal approximation. However, what is physically occurring is not a 
numerical integration. This is a subtle but important distinction to 
understand. Since each reading of DVM 124 is actually proportional to the 
voltage integral over the time period .tau., the analysis using (6) will 
yield better results than what a strict numerical integration would imply. 
In a numerical integration, the error is a function of the data point 
spacing and curvature. The numerical error in (6) is due to the voltage 
approximation made in (5) and the magnitude of .DELTA.t-.tau., but not 
necessarily the number of data points. 
The numerical error is important to understand because it affects both the 
absolute accuracy of the measurement and the repeatability from one 
movement to the next. If the voltage measurements were perfectly 
synchronized with the sample movement, each .nu..sub.i would always occur 
at exactly the same point in the voltage pulse. The evaluation of (6) 
would always give exactly the same numerical result but would be in error 
by the approximations made in (5). However, perfect synchronization would 
require timing to the millisecond level between the mechanical and 
electronic components. The effort to guarantee this is not justified, so 
in practice the .nu..sub.i fall at different points along the pulse for 
each sample movement. Slightly different numerical errors then arise in 
the evaluation of (6) which creates a non-repeatability associated with 
the numerical processing. 
The time required to complete a moment measurement is dependent on how the 
measurement sequence is defined. A single scan measurement requires about 
15 seconds to execute. Multiple scans may increase the measurement time 
(e.g., up to 150 seconds if 10 scans is selected). Computer 112 generates 
a moment value at the conclusion of the moment measurement sequence (block 
216 FIG. 7). 
The experimental feasibility of this approach and analysis was initially 
tested with the HP 3458A set to a 1 PLC (16.666 msec) integration time as 
DVM 124. The time between measurements could be set to as low as 16.69 
msec, yielding an active DVM integration time over the total pulse of 
99.86%. A permanent magnet sample was used to supply an output signal well 
above the noise floor of the voltmeter and the sample was moved at 1 
cm/sec. The voltage integral was approximately 0.0104 volt-sec. Repeated 
movements of the sample showed a standard deviation of less than 25 ppm in 
the measurement and evaluation of (6). Also, as predicted in (5), with 
.DELTA.t set to 20 msec, the standard deviation for repeated measurements 
increased to 150 ppm. 
The effect of varying the integration time on the DVM was also tested using 
this same experimental set-up. The integration time was varied from 1 PLC 
to 40 PLC while the time between readings was fixed at 0.023 milliseconds 
greater than the integration time. The results, consistent within .+-.20 
ppm, verified the predicted independence on the number of voltage 
readings. Over two hundred points were recorded at 1 PLC while only 5 or 6 
were recorded at 40 PLC. In all situations encountered to date, evaluating 
the integral by summing over a large number of readings yields the same or 
slightly better repeatability than using longer integration times with a 
fewer number of voltage readings. For this reason, a 1 PLC integration 
time has been adopted in the preferred embodiment. 
The voltage noise floor and sensitivity limit can be determined by making 
repeated measurements with no sample present and comparing the standard 
deviation in the integral evaluations. Ideally, the results should be 
zero. The HP 3458A yields a lower limit of .+-.2.5.times.10.sup.- 
.perspectiveto. volt-seconds in the evaluation of the integral. This 
corresponds to a moment sensitivity of approximately .+-.10.sup.-3 emu 
(10.sup.-6 Am.sup.2). In the discussion of the preceding paragraphs, the 
standard deviation of 25 ppm reflects this noise floor and is not 
necessarily the limit to the reproducibility which might be achieved for 
large samples. 
Most demands are for higher sensitivity as opposed to high resolution. For 
this reason, the final system configuration (as mentioned above) actually 
uses a modified Keithley 182 Sensitive Voltmeter instead of the HP 3458A. 
The Keithley instrument offers approximately a factor of ten improvement 
in voltage sensitivity and resolution, yielding a lower limit of 
.+-.2.5.times.10.sup.-8 volt-seconds in the integral evaluation. This 
equates to about .+-.10.sup.-4 emu (10.sup.-7 Am.sup.2) in moment 
sensitivity. Voltages from the sensing coils of less than a microvolt can 
be readily detected and measured. 
The increased sensitivity of the Keithley is at the expense of some speed 
and versatility. With the Keithley, voltage readings are limited to 
approximately 30 millisecond intervals. At 1 PLC integration times, the 
voltage is only integrated over 55% of the total time. Numerical errors 
associated with the approximation in (5) then limit the accuracy and 
repeatability of the integral determination to about .+-.0.1%. However, 
this is more than sufficient for most applications. 
It will be understood that in some applications it may be desirable to 
change the magnetic field intensity between (or during) dc moment 
measurements. For example, it may be desirable to provide a magnetic field 
table to permit system 100 to automatically step through a series of 
predefined magnetic fields (so as to measure moment at various points 
along the magnetization curve of the sample). The preferred embodiment 
typically provides "wait" periods between field changes to permit the 
field to stabilize prior to making the next successive measurement, and 
also preferably directly monitors the current output of magnet power 
supply 116 to ensure accuracy. It is also possible to control magnet power 
supply to "ramp" at a user defined rate in order to "sweep" the magnetic 
field during a measurement (this may be useful to generate hysteresis 
curves for example). 
Calibration 
In order to complete the moment measurement, the value of the calibration 
coefficient in equation (2) must be determined. The value of .alpha. is a 
measure of the flux coupling between the sample and the secondary coils 
and will vary with the sample size and geometry. A proper calibration 
therefore requires a separate calibration coefficient for each sample 
geometry measured. 
However, samples are often small with respect to the sensing coils 126a, 
126b and in these situations they can be approximated by magnetic dipoles. 
With this assumption and the simple secondary coil 126 geometry employed 
in this system, an expression for .alpha. can easily be derived in closed 
form. The calculations are similar to those used to derive an ac 
calibration coefficient. A magnetic dipole is assumed centered in one of 
the secondary coils and the field is integrated over the geometry of both 
the secondary coils 126a, 126b to give the total magnetic flux. The flux 
contained in the empty coil represents only 0.6% of this total, indicating 
that the two secondary coils 126a, 126b are nearly independent of each 
other. The total flux change when the sample is moved to the second coil 
is twice the calculated value. This gives a value for .alpha. of 
approximately 5300 emu/volt-sec (5.3 Am.sup.2 /volt-sec). 
Larger samples with simple geometries, such as cylinders, can also be 
handled numerically. These types of calculations indicate that the dipole 
approximation is actually a very good approximation for most applications. 
For the present coil geometry and for samples up to 5 mm diameter and less 
than 10 mm long, the potential error due to the dipole approximation is 
less than 3. 
Since numerical values for .alpha. can be effectively calculated, the 
measurements made with preferred embodiment system 100 are absolute 
determinations of the moment. The accuracy is limited by the accuracy of 
the calibration coefficient which is estimated at 1 to 2 percent. The 
dimensions of the coil arrangement 104 are known to better than 1% and the 
effects of thermal expansion and contraction are only a few tenths of a 
percent. The two sensing coils 126a, 126b are matched to within 0.2% in 
the preferred embodiment as determined from their ac response or by 
measuring the induced voltage as the magnetic field is ramped. This level 
of calibration accuracy is comparable to what is often accomplished with 
standard samples. 
For completeness, the calculated value for the calibration coefficient has 
been confirmed experimentally using NIST standard Ni and MnF.sub.2 
samples. The agreement is within the experimental uncertainty. 
In a properly designed system which avoids eddy currents and other ac 
specific problems, the ac calibration follows directly from the dc 
calibration and vice versa. Both calibration coefficients relate the flux 
coupling between the coils and the sample. In the preferred embodiment, 
the DC calibration coefficient used is equal to .pi. times the ac 
calibration coefficient. Thus, a single calibration coefficient and 
associated analysis can be used for both ac and dc measurements. This has 
been tested experimentally by verifying that ac measurements are 
consistent with dc measurements for samples which show no ac related 
effects. 
Preferred embodiment system 100 also provides a capability to measure the 
background signal attributable to the sample mount 162, sample rod 154, 
and anything else that may generate a background signal (e.g., substrate 
material etc.) during a moment measurement. The capability for subtracting 
the background signal form the data being recorded with the sample present 
is provided by software executing on computer 112. Such "addenda data" 
should be recorded using fixed temperature points over an applicable 
temperature range, and at least two applied fields which span the intended 
range of use (where one field can be zero field). A special option in the 
calibration analysis provided by the preferred embodiment processes the 
"addenda" data and stored the temperature/field/moment data to a file for 
later use. System 100 will automatically subtract the addenda data from a 
measured sample moments upon demand. The operator need only check the 
temperature T and field H at which the sample data was recorded. System 
100 then uses the temperature/field/moment data in the addenda file to 
determine the addenda moment at temperature T and field H (i.e., a simple 
two-dimensional linear interpolation is performed); and subtracts the 
addenda moment from the actual sample moment. 
FIGS. 9-12 show exemplary software menu structures provided by software 
executing on computer 112 to permit preferred embodiment system 100 to 
make dc magnetization measurements. Appendix A provides a listing of menu 
breakdown and functional description of FIGS. 9-12. 
Experimental Results 
The dc moment measurement is made by making one complete movement cycle in 
the preferred embodiment; the sample is moved from the bottom to the top 
coil and the returned to the bottom. Two voltage integral evaluations are 
made and averaged to give the final moment output for the sample. This 
process requires approximately 15 seconds to complete. Averaging over 
multiple measurements improves the sensitivity and noise with a lower 
limit of &lt;5.times.10.sup.-5 emu (5.times.10.sub.8.sup.-, Am.sup.2). With 
the voltage ranges available in the DVM, there is no practical upper limit 
to the measured moment. 
FIGS. 13 and 14 illustrate with a NIST MnF.sub.2 standard sample what level 
of performance can be achieved. FIG. 13 shows the measured moment versus 
the applied field. The data should be linear over the range of fields 
used, and FIG. 14 shows the data plotted as a deviation from a straight 
line fit. The standard deviation about the line is 6.times.10.sup.-5 
emu--illustrating the excellent linearity in the measurement. 
Repeated measurements over the course of an hour on a sample at fixed field 
gives reproducibilities of better than .+-.0.1%. When all experimental 
aspects are considered, including data processing, sample positioning, 
field setting, etc., the overall measurement reproducibility is a few 
tenths of a percent. 
Image effects have been reported in superconducting magnets, and 
measurements were made to see if these effects could be detected. No 
effect was observed (&lt;0.1%) which is probably due to the relatively large 
bore of the magnet 114 with respect to the sensing coils 126a, 126b. 
FIGS. 15 and 16 show typical dc moment results to further illustrate the 
capability and flexibility of the system. FIG. 15 shows moment as a 
function of temperature for a single crystal YBCO sample at an applied 
field of 100e (the sample was zero field cooled and had a mass of 15.2 
mg). FIG. 16 shows experimentally measured hysteresis curves for a thin 
film YBCO sample at 4.2 K (open circles), 25 K (solid circles), and 50K 
(triangles). Note the low dc field used in the data of FIG. 15 and the 
smoothness of the hysteresis loops of FIG. 16 for the samples with 
relatively low moments. 
B AC Susceptibility 
System 100 performs ac susceptibility measurements in a manner that is 
substantially identical in principle to the ac measurement techniques used 
since 1988 in conjunction with Lake Shore's Model 7000 AC Susceptometer 
product (FIG. 2A is a block diagram of this prior art ac susceptometer 
arrangement showing similar or identical components to those used in the 
FIG. 3 preferred embodiment of the present invention). 
Briefly, to perform an AC susceptibility measurement using the FIG. 3 
preferred embodiment, the sample 140 is placed within sample space 140 and 
AC current source 110b is activated to provide a desired AC field to the 
sample via primary coil 130. Since the applied AC field is changing, the 
magnetization of the sample 140 changes in response. Secondary coils 126a, 
126b measure changes in the magnetic field due to the susceptibility of 
sample 140 (the magnetization of which "follows" changes in the applied 
field may lag behind in an amount dependent on the magnetic response of 
the sample). 
Superconducting magnet 114 does not need to be activated for AC 
susceptibility measurement. This is because most ac susceptibility 
measurements are desirably performed under low field conditions (although 
the superconducting magnet may be activated if high field ac measurements 
are desired, such as for "irreversibility line" type measurements). In 
addition, sample 140 is stationary during the time ac susceptibility 
parameters are being measured by lock-in amplifier 122. 
In the preferred embodiment, stepping motor 106 is used to position sample 
140 within the interior space of first one secondary coil 126b (at which 
time a first measurement is taken with sample 140 remaining stationary 
during the measurement); and is then used to position the sample within 
the interior space of the other secondary coil 126a (at which time a 
second measurement is made with the sample again remaining stationary 
during the second measurement). The two measurements are used by computer 
112 to ascertain the magnitude and direction of the error between the two 
coils (such that the error can be cancelled out from one of the two 
measurements in order to yield a highly accurate result). Hence, stepping 
motor 106 is used in the preferred embodiment during the ac susceptibility 
measurement process even though sample 140 is stationary during the time 
lock-in amplifier 122 is actually acquiring an ac measurement. 
While the invention has been described in connection with what is presently 
considered to be the most practical and preferred embodiment, it is to be 
understood that the invention is not to be limited to the disclosed 
embodiment, but on the contrary, is intended to cover various 
modifications and equivalent arrangements included within the spirit and 
scope of the appended claims. 
##SPC1##