1、Designation: A894/A894M 00 (Reapproved 2011)1Standard Test Method forSaturation Magnetization or Induction of NonmetallicMagnetic Materials1This standard is issued under the fixed designation A894/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 () indicates an editorial change since the last revision or reapproval.1NOTEUpdated 6.3 and 6.3.2 editorially in October 2011.1. Scope1.1 This test method covers the me
3、asurement of saturationmagnetization of magnetic materials 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 A340.1.3 The values stated in either customary (
4、absolute (orpractical) cgs-emu) units or SI units are 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 sys
5、tems mayresult in nonconformance with this method.1.4 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 r
6、egulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:2A340 Terminology of Symbols and Definitions 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
7、 (1):3B 5 H14M cgs units!B 5 oH1M! SI units#3.1.1 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 ferrimagneticmaterial, M increases with the applied magnetic
8、field H, but atsufficiently high values of H, M approaches a constant maxi-mum value called the saturation magnetization Ms(emu/cm3)or A/m.The corresponding value of BH =4Ms(gauss) orBoH =oMstesla is called the saturation induction.Itissometimes given the label Bs.3.3 If a sphere of isotropic magnet
9、ic material is placed in auniform magnetic field, 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
10、 of the sphere. The strength of thismagnetic dipole is equal to the total magnetic moment of thesphere, which is given by:m 5 Mvemu! 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 dip
11、ole field andtherefore proportional to the magnetization M of the sample. Ifthe proportionality constant between this reading and themagnetic moment can be established, and if the volume of the1This test method is under the jurisdiction of ASTM Committee A06 onMagnetic Properties and is the direct r
12、esponsibility of Subcommittee A06.01 on TestMethods.Current edition approved Oct. 1, 2011. Published December 2011. Originallyapproved in 1970 as F133. Redesignated as A894. Last previous edition approvedin 2005 as A894/A894M00(2005). DOI: 10.1520/A0894_A0894M-00R11E01.2For referenced ASTM standards
13、, 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 stand
14、ard.Copyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United StatesNOTICE: This standard has either been superseded and replaced by a new version or withdrawn.Contact ASTM International (www.astm.org) for the latest information1sample is known, the m
15、agnetization of the sample is deter-mined. Then if the sample can be shown to be magneticallysaturated, the saturation magnetization 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 samp
16、le is attached to the end of anonmagnetic, nonconducting rod, and placed in a uniformtransverse magnetic field generated by an electromagnet orsolenoid. The sample and rod are oscillated or vibrated in adirection perpendicular to the field. This oscillating drive maybe produced by attaching the end
17、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 amplitudeof the oscillation must be held const
18、ant, 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 bean integer multiple of the power f
19、requency 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 amplified and measuredand is proportio
20、nal 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 caused bythe oscillation of the sample.
21、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 frequency ofoscillation, or preferab
22、ly 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 output of the lock-in amplifier will bea d
23、c 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 all the provisions of this standard ap
24、ply 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 permanent magnet can be balanced agai
25、nst 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 theFIG. 1 S, Sample; R, Mounting Rod; D, Oscillating Drive Mecha-nism; P, Magnet Pole Pieces; C, Measuring C
26、oilsFIG. 2 Ref, Reference Standard (Permanent Magnet); C1, C2, Mea-suring Coils; M, Null-Indicating Meter; Res, Calibrated VariableResistor. Other Parts as in Fig. 1A894/A894M 00 (2011)12sample vibration.Avariable gap capacitor, with one plate fixedand one attached to the sample rod, can be used to
27、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 arenecessary in this case, but they are not a par
28、t of this testmethod.4.4 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 anisotropic sphere. The size of the sph
29、ere 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 grainsize of the sample material must be
30、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 the fieldis applied to the sample,
31、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 best measured by applying thefield
32、 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 appliedparallel to an axis of symmetry. (Th
33、e 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 with the fieldapplied parallel to
34、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 of the same size and shape as the un
35、knownsample is required.6. Calibration and Calculation6.1 Three methods can be used to calibrate the instrument.See Ref (9) for a discussion of calibration methods andaccuracy.6.1.1 Standard SampleA sample of known saturationmagnetization Mrefand known volume vrefis measured. If thesignal (V) from t
36、his sample in the saturated state is Sref, thecalibration constant of the apparatus is given by:k 5 Mrefvref/Srefemu/V! orAm2/V#An unknown sample of volume v is measured with allexperimental conditions held constant, giving signal S. Thenthe magnetization of the unknown sample is given by:M 5 kS/v e
37、mu/cm3! or A/m#6.1.2 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 theunknown sample should be similar, e
38、specially 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 saturation magneti-zation of nickel at
39、 20C and 10-kOe 800-kA/m applied fieldmay be taken (10) as 492 6 2 emu/cm3(492 6 2)103A/m.The temperature coefficient of magnetization is 0.05 % perC, and the field coefficient is about +0.2 % per kOe from 5 to15 kOe +2.5 % per MA/m from 0.4 to 1.2 MA/m.6.2 Moment from CoilThe standard sample may be
40、 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 by:m 5 r2ni/10emu! or m 5 r2ni Am2#where:r = the radius of the coil (cm) or m,n
41、= 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 method is that themoment produced by a
42、coil carrying a reasonable current issmall compared with the moment of a strongly magneticsample of similar size.6.3 Operational Method (11, 12) (Also Called the SlopeMethod or Susceptibility Method)A material with highmagnetic permeability has a linear magnetization curve inrelatively small applied
43、 fields. The slope of the curve isgoverned by the demagnetizing factor of the sample. For asphere,M 5S34DH cgs! or M 5 3H SI#6.3.1 A plot of the signal S versus applied field H gives aslope K given by:K 5 S/H V/Oe! or Vm/A#6.3.2 Then the magnetization is related to the measuredvoltage signal by:M 5
44、SS34D1/K!emu/ cm3! or M 5 S 3/K! A/m#A894/A894M 00 (2011)136.3.3 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 different at the hi
45、gh 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-permeability standard
46、 can beused 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 (emu/g) or Am2/kg. Thesample mass can always be measured with less error than thesample volume, and the mass is i
47、ndependent 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 by weighing and dividing the mass by the
48、 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 field is applied and adjusting the sample p
49、ositionalong 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, 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 i
