1、Designation: A 894/A 894M 00 (Reapproved 2005)Standard Test Method forSaturation Magnetization or Induction of NonmetallicMagnetic Materials1This standard is issued under the fixed designationA894/A894M; the number immediately following the designation indicates the yearof original adoption or, in t
2、he case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval.A superscript epsilon (e) indicates an editorial change since the last revision or reapproval.1. Scope1.1 This test method covers the measurement of saturationmagnetization of magnetic mater
3、ials using a vibrating samplemagnetometer.1.2 Explanation of symbols and abbreviated definitionsappear in the text of this test method. The official symbols anddefinitions are listed in Terminology A 340.1.3 The values stated in either customary (absolute (orpractical) cgs-emu) units or SI units are
4、 to be regardedseparately as standard. Within the text, the SI units are shownin brackets. The values stated in each system are not exactequivalents; therefore, each system shall be used independentlyof the other. Combining values from the two systems mayresult in nonconformance with this method.1.4
5、 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2. Referenced Docu
6、ments2.1 ASTM Standards:2A 340 Terminology of Symbols and Defniitions Relating toMagnetic Testing3. Summary of Test Method3.1 The magnetic induction B, magnetic field strength H,and magnetization M in a material are related by the followingequation (1):3B 5 H 1 4pM cgs units!B 5 oH 1 M! SI units#3.1
7、.1 In this test method, cgs units are given in parentheses( ) and SI units in square brackets .3.2 The magnetization M is the magnetic moment per unitvolume of material. In a ferromagnetic or ferrimagnetic mate-rial, M increases with the applied magnetic field H, but atsufficiently high values of H,
8、 M approaches a constant maxi-mum value called the saturation magnetization Ms(emu/cm3)or A/m. The corresponding value of BH=4pMs(gauss) orBoH =oMstesla is called the saturation induction.Itissometimes given the label Bs.3.3 If a sphere of isotropic magnetic material is placed in auniform magnetic f
9、ield, the sphere becomes uniformly magne-tized in a direction parallel to the applied field. The magneticfield in the space outside the sphere is exactly that of amagnetic dipole located at the center of the sphere and orientedparallel to the magnetization of the sphere. The strength of thismagnetic
10、 dipole is equal to the total magnetic moment of thesphere, which is given bym 5 Mv emu! or Am2#where:v = is the volume of the sphere, (cm3)orm3.Section 4 describes an apparatus that provides an indicationor reading proportional to the strength of this dipole field andtherefore proportional to the m
11、agnetization M of the sample. Ifthe proportionality constant between this reading and themagnetic moment can be established, and if the volume of thesample is known, the magnetization of the sample is deter-mined. Then if the sample can be shown to be magneticallysaturated, the saturation magnetizat
12、ion is determined.4. Apparatus4.1 The equipment used for the measurement is called avibrating sample magnetometer (2) and is illustrated schemati-cally in Fig. 1. The sample is attached to the end of anonmagnetic, nonconducting rod, and placed in a uniform1This test method is under the jurisdiction
13、of ASTM Committee A06 onMagnetic Properties and is the direct responsibility of SubcommitteeA06.01 on TestMethods.Current edition approved Nov. 1, 2005. Published November 2005. Originallyapproved in 1970 as F 133. Last previous edition approved in 2000 as A 894/A 894M 00.2For referenced ASTM standa
14、rds, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.3The boldface numbers in parentheses refer to a list of references at the back ofthis st
15、andard.1Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.transverse magnetic field generated by an electromagnet orsolenoid. The sample and rod are oscillated or vibrated in adirection perpendicular to the field. This oscillating drive
16、 maybe produced by attaching the end of the sample rod to aloudspeaker cone or a similar electromagnetic oscillator anddriving the loudspeaker coil with an appropriate ac current.Alternatively, the rod may be oscillated by a mechanical crankor cam driven by a small motor. The frequency and amplitude
17、of the oscillation must be held constant, either by the mechani-cal design of the apparatus or by an appropriate feedbacksystem. The operating frequency is usually chosen in the range30 to 100 Hz, and the amplitude is usually chosen to be 0.01 to0.1 cm 0.1 to 1 mm. The operating frequency should not
18、 bean integer multiple of the power frequency to avoid pickup ofspurious signals.4.1.1 One or more coils are placed symmetrically withrespect to the sample, oriented so that the moving dipole fieldof the sample produces a changing magnetic flux in the coils.The resulting ac voltage in the coils is a
19、mplified and measuredand is proportional to the dipole moment of the sample andtherefore to the magnetization of the sample.4.1.2 Various coil orientations are possible. In general, thecoil positions and coil connections are chosen to cancel theeffects of any time-varying fields other than those cau
20、sed bythe oscillation of the sample. For a discussion of the design andplacement of these coils, see Refs 3 and 4. The coils typicallycontain hundreds or thousands of turns to increase the ampli-tude of the induced voltage. The signal may be amplified by atuned amplifier whose gain is maximum at the
21、 frequency ofoscillation, or preferably by a lock-in amplifier operated at theoscillation frequency. The coils may be connected in series oras parallel inputs to a differential amplifier; the latter has somepractical advantages. The output of the tuned amplifier will bean ac voltage, while the outpu
22、t of the lock-in amplifier will bea dc voltage.4.1.3 If a superconducting solenoid is used to provide themagnetic field, it is usually most convenient to have thedirection of sample vibration parallel rather than perpendicularto the field. The operation of the instrument is basicallyunchanged, and a
23、ll the provisions of this standard apply to bothcases.4.2 One version of the vibrating sample magnetometer usesa second set of coils placed outside the magnetizing field anda standard sample comprising a small permanent magnetattached to the sample rod (see Fig. 2). In this case, the signalfrom the
24、permanent magnet can be balanced against the signalfrom the sample, so that the apparatus is operated in a nullmode. Alternatively, the output from the second set of coilsmay simply be used to monitor or control the amplitude of thesample vibration.Avariable gap capacitor, with one plate fixedand on
25、e attached to the sample rod, can be used to control theamplitude of vibration in place of a second set of coils plus amagnet.4.3 An advantage of the vibrating sample magnetometer isthat the sample temperature may be easily raised or loweredwith simple heaters or refrigerators. Some precautions aren
26、ecessary in this case, but they are not a part of this testmethod.FIG. 1 S, Sample; R, Mounting Rod; D, Oscillating DriveMechanism; P, Magnet Pole Pieces; C, Measuring CoilsFIG. 2 Ref, Reference Standard (Permanent Magnet); C1, C2,Measuring Coils; M, Null-Indicating Meter; Res, CalibratedVariable Re
27、sistor. Other Parts as in Fig. 1.A 894/A 894M 00 (2005)24.4 Vibrating sample magnetometers are commerciallyavailable from several manufacturers in various countries, orcan be constructed with normal machine shop facilities.5. Test Specimen5.1 The test specimen shall preferably be in the form of anis
28、otropic sphere. The size of the sphere will depend on themeasuring apparatus to be used, but for the usual instrumentthe size will be 0.5 cm 5 mm or less in diameter. Methods forproducing small spherical samples are given in Refs (5-8).5.1.1 For the sample to be isotropic, the crystal size or grains
29、ize of the sample material must be small compared to thesample size. Furthermore, the crystals should be of randomorientation. If the sample is not isotropic, it is still possible tomeasure the saturation magnetization, but the field required toreach saturation will depend on the direction in which
30、the fieldis applied to the sample, and there will in general be a torqueacting on the sample which may be large enough to interferewith the measurement.5.1.2 The same measuring technique can be applied tohighly anisotropic samples such as single crystals. In this case,the saturation magnetization is
31、 best measured by applying thefield parallel to the crystallographic axis of easy magnetization;that is, parallel to the axis for which saturation is attained at thelowest field.5.2 Nonspherical samples can be used if they are such thatthe demagnetizing factor is calculable and the field is appliedp
32、arallel to an axis of symmetry. (The magnetic field of suchsamples is dipolar.) This would include spheroidal (ellipsoidal)samples with the field applied parallel to the principal axis,approximate spheroids such as thin sheets or thin films with thefield in the plane of the film, and long thin wires
33、 with the fieldapplied parallel to the wire axis.5.3 Nonspheroidal shapes can also be measured generallywith reduced accuracy, if the largest dimension of the sampleis small compared with the distance from the sample to themeasuring coils (see Section 4). For greatest accuracy, acalibration sample o
34、f the same size and shape as the unknownsample is required.6. Calibration and Calculation6.1 Three methods can be used to calibrate the instrument.See Ref (11) for a discussion of calibration methods andaccuracy.6.1.1 Standard SampleA sample of known saturationmagnetization Mrefand known volume vref
35、is measured. If thesignal (V) from this sample in the saturated state is Sref, thecalibration constant of the apparatus is given byk 5 Mrefvref/Srefemu/V! or Am2/V#An unknown sample of volume v is measured with allexperimental conditions held constant, giving signal S. Thenthe magnetization of the u
36、nknown sample is given byM 5 kS/v emu/cm3! or A/m6.1.2 If the image effect is significant, k must be determinedas a function of the applied field H. Any variation in k will bea function only of H, not of the magnetization of the sample orof the standard. However, the size of the standard and of theu
37、nknown sample should be similar, especially if neither isspherical.6.1.3 Nickel is the most commonly used standard sample. Itcan be obtained in high purity, resists oxidation and corrosion,and has a saturation magnetization lower than that of iron andcobalt but higher than that of ferrites. The satu
38、ration magneti-zation of nickel at 20C and 10-kOe 800-kA/m applied fieldmay be taken (12) as 492 6 2 emu/cm3(492 6 2) 3 103A/m. The temperature coefficient of magnetizationis 0.05 % per C, and the field coefficient is about +0.2 % perkOe from 5 to 15 kOe +2.5 % per MA/m from 0.4 to 1.2MA/m.6.2 Momen
39、t from CoilThe standard sample may be re-placed by a coil of known dimensions and number of turnscarrying a known dc current. Such a coil produces a dipolefield the same as that produced by a spherical sample. Themagnitude of the equivalent moment is given bym 5pr2ni/10 emu! or m 5pr2ni Am2#where:r
40、= the radius of the coil (cm) or m,n = the number of turns, andi (A) = the current.Amultiple-layer coil may also be used, with the moments ofeach layer computed separately and added together. Thedimensions of the coil should be similar to the size of thesample to be measured. A difficulty of this me
41、thod is that themoment produced by a coil carrying a reasonable current issmall compared with the moment of a strongly magneticsample of similar size.6.3 Operational Method (13, 14) (Also Called the SlopeMethod or Susceptibility Method)A material with highmagnetic permeability has a linear magnetiza
42、tion curve inrelatively small applied fields. The slope of the curve isgoverned by the demagnetizing factor of the sample. For asphere,M 5 n|Pp!H cgs! or M 5 3H SI6.3.1 A plot of the signal S versus applied field H gives aslope K given byK 5 S/H V/Oe! or Vm/A#6.3.2 Then the magnetization is related
43、to the measuredvoltage signal byM 5 S n|Pp!1/K! emu/cm3! or M 5 S 3/K! A/m#6.3.3 This calibration method has the disadvantage that itmust be carried out in relatively low fields, where the high-permeability sample is not near saturation. If the image effectis significant, the calibration will be dif
44、ferent at the high fieldswhere Msmust usually be measured. This method also requiresa sample of high permeability and low coercive field, so thatthe magnetization curve is linear and nonhysteretic in lowfields. Nickel is often a satisfactory material. A calibrationmade with a satisfactory high-perme
45、ability standard can beA 894/A 894M 00 (2005)3used for any sample of similar size, so long as the geometry ofthe instrument remains the same.6.4 It is sometimes desirable to determine the saturationmagnetization per unit mass s (emu/g) or Am2/kg. Thesample mass can always be measured with less error
46、 than thesample volume, and the mass is independent of temperature.The calibration methods of 6.1 and 6.2, but not of 6.3, may beused, with the obvious substitutions.7. Procedure7.1 The sample is prepared in a suitable shape, and itsvolume is determined by direct measurement, by Archimedesmethod, or
47、 by weighing and dividing the mass by the knowndensity. The sample is attached to the end of the rod andpositioned symmetrically along the x, y, and z axes with respectto the measuring coils. This positioning is best accomplishedby observing the voltage signal from the sample when asubstantial dc fi
48、eld is applied and adjusting the sample positionalong each axis in turn until the signal shows a maximum orminimum. A magnetic field sufficient to saturate the sample isthen applied, and the output signal corresponding to thissaturated state is recorded. The background signal originatingin the rod,
49、the adhesive, and the sample holder or substrate (ifany) is determined separately and subtracted from the totalsignal.7.2 Most samples, especially at room temperature, do notreach a state of precisely constant saturation magnetization inhigh fields. There is a small but nonzero high-field suscepti-bility, so that the magnetization continues to increase slightlywith increasing field. For the purposes of this standard, thesample is considered to be saturated if the measured magneti-zation decreases less than 1 %
