ASTM A894/A894M-00
(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
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 A340.
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.
1.4 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.
General Information
Relations
Standards Content (Sample)
NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
Contact ASTM International (www.astm.org) for the latest information
Designation: A 894/A 894M – 00
Standard Test Method for
Saturation Magnetization or Induction of Nonmetallic
Magnetic Materials
ThisstandardisissuedunderthefixeddesignationA894/A894M;thenumberimmediatelyfollowingthedesignationindicatestheyear
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 (e) indicates an editorial change since the last revision or reapproval.
1. Scope 3.1.1 In this test method, cgs units are given in parentheses
( ) and SI units in square brackets [ ].
1.1 This test method covers the measurement of saturation
3.2 The magnetization M is the magnetic moment per unit
magnetization of magnetic materials using a vibrating sample
volume of material. In a ferromagnetic or ferrimagnetic mate-
magnetometer.
rial, M increases with the applied magnetic field H, but at
1.2 Explanation of symbols and abbreviated definitions
sufficiently high values of H, M approaches a constant maxi-
appear in the text of this test method. The official symbols and
mum value called the saturation magnetization M (emu/cm )
s
definitions are listed in Terminology A340.
or [A/m].The corresponding value of B−H=4pM (gauss) or
1.3 The values stated in either customary (absolute (or s
B−µ H=µ M [tesla] is called the saturation induction.Itis
o o s
practical) cgs-emu) units or SI units are to be regarded
sometimes given the label B .
separately as standard. Within the text, the SI units are shown s
3.3 If a sphere of isotropic magnetic material is placed in a
in brackets. The values stated in each system are not exact
uniform magnetic field, the sphere becomes uniformly magne-
equivalents;therefore,eachsystemshallbeusedindependently
tized in a direction parallel to the applied field. The magnetic
of the other. Combining values from the two systems may
field in the space outside the sphere is exactly that of a
result in nonconformance with this method.
magneticdipolelocatedatthecenterofthesphereandoriented
1.4 This standard does not purport to address all of the
parallel to the magnetization of the sphere.The strength of this
safety concerns, if any, associated with its use. It is the
magnetic dipole is equal to the total magnetic moment of the
responsibility of the user of this standard to establish appro-
sphere, which is given by
priate safety and health practices and determine the applica-
bility of regulatory limitations prior to use.
m 5 Mv ~emu!or A·m
@ #
2. Referenced Documents
where:
2.1 ASTM Standards:
3 3
v = is the volume of the sphere, (cm)or[m ].
A340 Terminology of Symbols and Defniitions Relating to
2 Section 4 describes an apparatus that provides an indication
Magnetic Testing
or reading proportional to the strength of this dipole field and
3. Summary of Test Method therefore proportional to the magnetization M of the sample. If
the proportionality constant between this reading and the
3.1 The magnetic induction B, magnetic field strength H,
magnetic moment can be established, and if the volume of the
and magnetization M in a material are related by the following
3 sample is known, the magnetization of the sample is deter-
equation (1):
mined. Then if the sample can be shown to be magnetically
B 5 H 14pM ~cgsunits!
saturated, the saturation magnetization is determined.
B 5 µ H 1 M SIunits
~ ! @ # 4. Apparatus
o
4.1 The equipment used for the measurement is called a
vibrating sample magnetometer(2) and is illustrated schemati-
cally in Fig. 1. The sample is attached to the end of a
1 nonmagnetic, nonconducting rod, and placed in a uniform
This test method is under the jurisdiction of ASTM Committee A-6 on
transverse magnetic field generated by an electromagnet or
Magnetic Properties and is the direct responsibility of A06.01 on Test Methods.
Current edition approved Oct. 10, 2000. Published December 2000. Last
solenoid. The sample and rod are oscillated or vibrated in a
previous edition A894–95. (Formerly F133–70(1981)).
direction perpendicular to the field. This oscillating drive may
Annual Book of ASTM Standards, Vol 03.04.
3 be produced by attaching the end of the sample rod to a
The boldface numbers in parentheses refer to a list of references at the back of
this standard. loudspeaker cone or a similar electromagnetic oscillator and
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
A 894/A 894M
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
with simple heaters or refrigerators. Some precautions are
necessary in this case, but they are not a part of this standard.
4.4 Vibrating sample magnetometers are commercially
available from several manufacturers in various countries, or
can be constructed with normal machine shop facilities.
FIG. 1 S, Sample; R, Mounting Rod; D, Oscillating Drive
Mechanism; P, Magnet Pole Pieces; C, Measuring Coils 5. Test Specimen
5.1 The test specimen shall preferably be in the form of an
isotropic sphere. The size of the sphere will depend on the
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
oscillation frequency. The coils may be connected in series or
as parallel inputs to a differential amplifier; the latter has some
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.
FIG. 2 Ref, Reference Standard (Permanent Magnet); C , C ,
1 2
4.1.3 If a superconducting solenoid is used to provide the
Measuring Coils; M, Null-Indicating Meter; Res, Calibrated
magnetic field, it is usually most convenient to have the Variable Resistor. Other Parts as in Fig. 1.
A 894/A 894M
measuring apparatus to be used, but for the usual instrument zation of nickel at 20°C and 10-kOe [800-kA/m] applied field
3 3
thesizewillbe0.5cm[5mm]orlessindiameter.Methodsfor may be taken (12) as 492 6 2 emu/cm [(492 6 2) 310
producing small spherical samples are given in Refs (5-8). A/m]. The temperature coefficient of magnetization
5.1.1 Forthesampletobeisotropic,thecrystalsizeorgrain is−0.05%per°C,andthefieldcoefficientisabout+0.2%per
size of the sample material must be small compared to the
kOe from 5 to 15 kOe [+2.5% per MA/m from 0.4 to 1.2
sample size. Furthermore, the crystals should be of random MA/m].
orientation. If the sample is not isotropic, it is still possible to
6.2 Moment from Coil—The standard sample may be re-
measure the saturation magnetization, but the field required to
placed by a coil of known dimensions and number of turns
reach saturation will depend on the direction in which the field
carrying a known dc current. Such a coil produces a dipole
is applied to the sample, and there will in general be a torque
field the same as that produced by a spherical sample. The
acting on the sample which may be large enough to interfere
magnitude of the equivalent moment is given by
with the measurement.
2 2 2
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,
the saturation magnetization is best measured by applying the
where:
fieldparalleltothecrystallographicaxisofeasymagnetization;
r = the radius of the coil (cm) or [m],
thatis,paralleltotheaxisforwhichsaturationisattainedatthe
n = the number of turns, and
lowest field. i ([A]) = the current.
5.2 Nonspherical samples can be used if they are such that
Amultiple-layer coil may also be used, with the moments of
the demagnetizing factor is calculable and the field is applied
each layer computed separately and added together. The
parallel to an axis of symmetry. (The magnetic field of such
dimensions of the coil should be similar to the size of the
samplesisdipolar.)Thiswouldincludespheroidal(ellipsoidal)
sample to be measured. A difficulty of this method is that the
samples with the field applied parallel to the principal axis,
moment produced by a coil carrying a reasonable current
...
Questions, Comments and Discussion
Ask us and Technical Secretary will try to provide an answer. You can facilitate discussion about the standard in here.