ASTM A894/A894M-00(2005)
(Test Method)Standard Test Method for Saturation Magnetization or Induction of Nonmetallic Magnetic Materials
Standard Test Method for Saturation Magnetization or Induction of Nonmetallic Magnetic Materials
ABSTRACT
This test method deals with the standard practice of measuring saturation magnetization or induction of nonmetallic magnetic materials using a vibrating sample magnetometer, which could be commercially purchased from a manufacturer or constructed with normal machine shop facilities. The test specimen shall preferably be in the form of an isotropic sphere, the size of which shall depend on the measuring apparatus. Details concerning equipment setup and calibration, the measurement procedure, and the governing equations for numerical calculations are all discussed thouroughly.
SCOPE
1.1 This test method covers the measurement of saturation magnetization of magnetic materials using a vibrating sample magnetometer.
1.2 Explanation of symbols and abbreviated definitions appear in the text of this test method. The official symbols and definitions are listed in Terminology A 340.
1.3 The values stated in either customary (absolute (or practical) cgs-emu) units or SI units are to be regarded separately as standard. Within the text, the SI units are shown in brackets. The values stated in each system are not exact equivalents; therefore, each system shall be used independently of the other. Combining values from the two systems may result in nonconformance with this method.
This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
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Designation: A894/A894M – 00 (Reapproved 2005)
Standard Test Method for
Saturation Magnetization or Induction of Nonmetallic
Magnetic Materials
This standard is issued under the fixed designationA894/A894M; the number immediately following the designation indicates the year
of original adoption or, in the case of revision, the year of last revision.Anumber in parentheses indicates the year of last reapproval.
A superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
B 5 H 14pM ~cgsunits!
1. Scope
1.1 This test method covers the measurement of saturation
B 5 µ ~H 1 M! [SIunits#
magnetization of magnetic materials using a vibrating sample o
magnetometer.
1.2 Explanation of symbols and abbreviated definitions 3.1.1 In this test method, cgs units are given in parentheses
appear in the text of this test method. The official symbols and ( ) and SI units in square brackets [ ].
definitions are listed in Terminology A340. 3.2 The magnetization M is the magnetic moment per unit
1.3 The values stated in either customary (absolute (or volume of material. In a ferromagnetic or ferrimagnetic mate-
rial, M increases with the applied magnetic field H, but at
practical) cgs-emu) units or SI units are to be regarded
separately as standard. Within the text, the SI units are shown sufficiently high values of H, M approaches a constant maxi-
mum value called the saturation magnetization M (emu/cm )
in brackets. The values stated in each system are not exact
s
equivalents;therefore,eachsystemshallbeusedindependently or [A/m].The corresponding value of B−H=4pM (gauss) or
s
B−µ H=µ M [tesla] is called the saturation induction.Itis
of the other. Combining values from the two systems may
o o s
result in nonconformance with this method. sometimes given the label B .
s
3.3 If a sphere of isotropic magnetic material is placed in a
1.4 This standard does not purport to address all of the
safety concerns, if any, associated with its use. It is the uniform magnetic field, the sphere becomes uniformly magne-
tized in a direction parallel to the applied field. The magnetic
responsibility of the user of this standard to establish appro-
priate safety and health practices and determine the applica- field in the space outside the sphere is exactly that of a
magneticdipolelocatedatthecenterofthesphereandoriented
bility of regulatory limitations prior to use.
parallel to the magnetization of the sphere.The strength of this
2. Referenced Documents
magnetic dipole is equal to the total magnetic moment of the
2.1 ASTM Standards:
sphere, which is given by
A340 Terminology of Symbols and Definitions Relating to
m 5 Mv ~emu!or[A·m #
Magnetic Testing
3. Summary of Test Method
where:
3 3
3.1 The magnetic induction B, magnetic field strength H, v = is the volume of the sphere, (cm)or[m ].
Section 4 describes an apparatus that provides an indication
and magnetization M in a material are related by the following
equation (1): or reading proportional to the strength of this dipole field and
therefore proportional to the magnetization M of the sample. If
the proportionality constant between this reading and the
magnetic moment can be established, and if the volume of the
This test method is under the jurisdiction of ASTM Committee A06 on
sample is known, the magnetization of the sample is deter-
MagneticPropertiesandisthedirectresponsibilityofSubcommitteeA06.01onTest
Methods.
mined. Then if the sample can be shown to be magnetically
Current edition approved Nov. 1, 2005. Published November 2005. Originally
saturated, the saturation magnetization is determined.
approved in 1970 as F133. Last previous edition approved in 2000 as A894/
A894M–00. DOI: 10.1520/A0894_A0894M-00R05.
4. Apparatus
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM
4.1 The equipment used for the measurement is called a
Standards volume information, refer to the standard’s Document Summary page on
vibrating sample magnetometer(2) and is illustrated schemati-
the ASTM website.
cally in Fig. 1. The sample is attached to the end of a
The boldface numbers in parentheses refer to a list of references at the back of
this standard. nonmagnetic, nonconducting rod, and placed in a uniform
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
A894/A894M – 00 (2005)
practical advantages. The output of the tuned amplifier will be
an ac voltage, while the output of the lock-in amplifier will be
a dc voltage.
4.1.3 If a superconducting solenoid is used to provide the
magnetic field, it is usually most convenient to have the
direction of sample vibration parallel rather than perpendicular
to the field. The operation of the instrument is basically
unchanged,andalltheprovisionsofthisstandardapplytoboth
cases.
4.2 One version of the vibrating sample magnetometer uses
a second set of coils placed outside the magnetizing field and
a standard sample comprising a small permanent magnet
attached to the sample rod (see Fig. 2). In this case, the signal
from the permanent magnet can be balanced against the signal
from the sample, so that the apparatus is operated in a null
mode. Alternatively, the output from the second set of coils
may simply be used to monitor or control the amplitude of the
samplevibration.Avariablegapcapacitor,withoneplatefixed
and one attached to the sample rod, can be used to control the
amplitude of vibration in place of a second set of coils plus a
magnet.
4.3 An advantage of the vibrating sample magnetometer is
that the sample temperature may be easily raised or lowered
FIG. 1 S, Sample; R, Mounting Rod; D, Oscillating Drive
with simple heaters or refrigerators. Some precautions are
Mechanism; P, Magnet Pole Pieces; C, Measuring Coils
necessary in this case, but they are not a part of this test
method.
transverse magnetic field generated by an electromagnet or
solenoid. The sample and rod are oscillated or vibrated in a
direction perpendicular to the field. This oscillating drive may
be produced by attaching the end of the sample rod to a
loudspeaker cone or a similar electromagnetic oscillator and
driving the loudspeaker coil with an appropriate ac current.
Alternatively, the rod may be oscillated by a mechanical crank
or cam driven by a small motor. The frequency and amplitude
of the oscillation must be held constant, either by the mechani-
cal design of the apparatus or by an appropriate feedback
system.The operating frequency is usually chosen in the range
30to100Hz,andtheamplitudeisusuallychosentobe0.01to
0.1 cm [0.1 to 1 mm]. The operating frequency should not be
an integer multiple of the power frequency to avoid pickup of
spurious signals.
4.1.1 One or more coils are placed symmetrically with
respect to the sample, oriented so that the moving dipole field
of the sample produces a changing magnetic flux in the coils.
The resulting ac voltage in the coils is amplified and measured
and is proportional to the dipole moment of the sample and
therefore to the magnetization of the sample.
4.1.2 Various coil orientations are possible. In general, the
coil positions and coil connections are chosen to cancel the
effects of any time-varying fields other than those caused by
theoscillationofthesample.Foradiscussionofthedesignand
placement of these coils, see Refs 3 and 4. The coils typically
contain hundreds or thousands of turns to increase the ampli-
tude of the induced voltage. The signal may be amplified by a
tuned amplifier whose gain is maximum at the frequency of
oscillation, or preferably by a lock-in amplifier operated at the
FIG. 2 Ref, Reference Standard (Permanent Magnet); C , C ,
1 2
oscillation frequency. The coils may be connected in series or
Measuring Coils; M, Null-Indicating Meter; Res, Calibrated
as parallel inputs to a differential amplifier; the latter has some Variable Resistor. Other Parts as in Fig. 1.
A894/A894M – 00 (2005)
4.4 Vibrating sample magnetometers are commercially of the standard. However, the size of the standard and of the
available from several manufacturers in various countries, or unknown sample should be similar, especially if neither is
can be constructed with normal machine shop facilities. spherical.
6.1.3 Nickel is the most commonly used standard sample. It
5. Test Specimen can be obtained in high purity, resists oxidation and corrosion,
and has a saturation magnetization lower than that of iron and
5.1 The test specimen shall preferably be in the form of an
cobalt but higher than that of ferrites. The saturation magneti-
isotropic sphere. The size of the sphere will depend on the
zation of nickel at 20°C and 10-kOe [800-kA/m] applied field
measuring apparatus to be used, but for the usual instrument
3 3
may be taken (12) as 492 6 2 emu/cm [(492 6 2) 310
thesizewillbe0.5cm[5mm]orlessindiameter.Methodsfor
A/m]. The temperature coefficient of magnetization
producing small spherical samples are given in Refs (5-8).
is−0.05%per°C,andthefieldcoefficientisabout+0.2%per
5.1.1 Forthesampletobeisotropic,thecrystalsizeorgrain
kOe from 5 to 15 kOe [+2.5% per MA/m from 0.4 to 1.2
size of the sample material must be small compared to the
MA/m].
sample size. Furthermore, the crystals should be of random
6.2 Moment from Coil—The standard sample may be re-
orientation. If the sample is not isotropic, it is still possible to
placed by a coil of known dimensions and number of turns
measure the saturation magnetization, but the field required to
carrying a known dc current. Such a coil produces a dipole
reach saturation will depend on the direction in which the field
field the same as that produced by a spherical sample. The
is applied to the sample, and there will in general be a torque
magnitude of the equivalent moment is given by
acting on the sample which may be large enough to interfere
2 2 2
with the measurement.
m5pr ni/10 ~emu!or m5pr ni[A·m #
5.1.2 The same measuring technique can be applied to
highly anisotropic samples such as single crystals. In this case,
where:
the saturation magnetization is best measured by applying the
r = the radius of the coil (cm) or [m],
fieldparalleltothecrystallographicaxisofeasymagnetization;
n = the number of turns, and
thatis,paralleltotheaxisforwhichsaturationisattainedatthe
i ([A]) = the current.
lowest field.
Amultiple-layer coil may also be used, with the moments of
5.2
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