1963/64, No. 9 227

FAST AND ACCURATE MAGNETIC MEASUREMENTSON SAMPLES WEIGHING A FEW MILLIGRAMMES

by G. W. van OOSTERHOUT *) and L. J. NOORDERMEER **).

In research and development work on magnetic materials it is important to be able to measurequickly and accurately the magnetic moments of samples weighing only a few milligrammes.Such measurements provide the basis for determining important magnetic properties, such assaturation magnetization, coerciveforce and remanence. An apparatus designedfor this pur-pose is described in this article.

In the development of magnetic materials it isimportant to knowat an early stage whether aparticular material is suited to the purpose for whichit is intended. This applies especially to materials- usually in powder form - for magnetic record-ing 1). At present it is not yet possible to decide frommagnetic measurements on a powder whether thematerial is suitable for making e.g. magnetic tape.For this purpose it is still necessary to produce anexperimental tape. It is, however, possible, bymeasuring the coercive force, saturation magneti-za tion and remanence of the material, to seewhetherit is definitely unsuitable. The making of experi-mental tapes can thus he limited to materials thathave at least some chance of success.

F or experimental purposes it is useful if the meas-urements can be carried out on small samples. Oneof the advantages is that only small quantities needbe made of the substances to be investigated, so thatfull benefit can be derived from the advantages ofpreparative chemistry on a small scale. Some tenyears ago it was still a very difficult matter to per-form magnetic measurements on samples weighingas little as a fewmilligrammes. One of the few prop-erties that could be measured on small samples wasthe saturation magnetization 2). In 1954 a methodwas developed in Philips Research Laboratories forrapidly and accurately measuring the magneticmoment of very small samples 3). The method wasintended at the time for measurements on galvani-cally deposited magnetic material. Since the proper-

*) Philips Research Laboratories, Eindhoven.**) Formerly with Philips Research Laboratories, Eindhoven.1) For a general introduetion to magnetic recording, see:

W. K.Westmijze,The principle of themagneticrecording andreproduetion of sound, Philips tech. Rev.I5, 84-96, 1953/54.

2) G. W. Rathenau and J. L. Snoek, Apparatus for measuringmagnetic moments, Philips Res. Repts, 1, 239, 1946.

3) G. W. van Oosterhout, A rapid method for measuringcoercive force and other ferromagnetic properties of verysmall samples, Appl. sci. Res. B6, 101-104, 1956/57. Amethod based on the same principle is described by:S. Foner, Vibrating sample magnetometer, Rev. sci. Instr.27, 548, 1956. For an improved version of this instrument,see: S. Foner, Versatile and sensitive vibrating-samplemagnetometer, Rev. sci. Instr. 30, 548-557, 1959.

621.317.42

ties of such materials depend on the thickness of thedeposited layer, it was necessary to measure samplesweighing a few milligrammes. The set-up designed tothis end has gradually grown to be a useful tool forinvestigations concerning magnetic tape: firstly, forthe above-mentioned selection of materials (pow-ders), and secondly for measurements on finishedtape. The apparatus is capable of measuring amagnetic moment as small as about 1.5X 10-12 Vsm,which is the saturation moment of about6 X 10-3mgof iron.After this apparatus had been used with good

results for several years, the question arose as towhether its sensitivity could he increased sufficientlyto permit the study of phenomena connected withthe print-through effect. This effect occurs in a tapewound on a spool, and is due to the magnetizationpattern in a winding being taken over by adjacentwindings. Bound up with this effect are the weakmagnetizations caused by the stray field of themagnetic tape itself, and which are therefore of theorder of one thousandth of the remanence. In orderto measure such weak magnetizations on a lengthof magnetic tape of e.g. 1.5 cm, a considerably highersensitivity was needed.

In the following we shall first briefly describe theprinciple of the method of measurement and explainhow the coercive force, the remanence and thesaturation magnetization can he derived frommeasurements of a magnetic moment. We shall thenconsider the steps taken to increase the sensitivitysufficiently to permit the detection of a magneticmoment of about 7.5X 10-14 Vsm (the saturationmoment of 3X10-4 mg of iron). The article concludeswith a few examples, including measurements con-cerning print-through.

Principle of the method

The principle of the method of measurement isillustrated in fig .1. The sample under study - which,as mentioned, usually consists of a little powder or apiece of magnetic tape - is applied over a length of

228 • PHILIPS TECHNICAL REVIEW VOLUME 25

about 1.5cm around a thin bar (diameter e.g. 2mm)of non-magnetic material, preferably glass. Thesample is magnetized in the direction of the bar axis.One end of the bar is connected to the speechcoil ofa loudspeaker. When the loudspeaker is connectedto ~malternating-current source, the sample movesto and fro axially with the bar. Identical search coilsare placed around the ends of the sample, coaxiallywith the bar. When the sample is in vibration, ane.m.f. is induced in these two coils. The coils areconnected in series opposition (astatic coils), so thatinterfering e.m.f.'s induced in the coils by fields from

SM000 000 0 000

u

o 0

Fig. I.The magnetic moment of a small sample M of magneticmaterial is measured by causing the sample to vibrate inside anastatic pair of coilsSl' The sample (usually a powder or a pieceof magnetic tape) is applied over a length of about 1.5 cm tothe surface of a bar Uwhich, driven by a loudspeaker system T,moves to and fro in the axial direction. VM electronic volt-meter, preceded by an amplifier. SM solenoid which cangenerate a homogeneous external magnetic field at M.

extraneous sources cancel each other. On the otherhand the e.m.f.'s produced by the sample reinforceone another. The resultant alternating e.m.f. is ampli-fied and measured with an electronic millivoltmeter,the deflection of which is a measure of the magneticmoment of the sample.

Determination of saturation magnetization, coerciveforce and remanence

The saturation magnetization of a material isdetermined by measuring the magnetic moment ofa sample of known weight in the magneticallysaturated state. The measurement must of coursebe done in a strong external field, which has thedirection of the vibrating bar and keeps the samplein the saturated state. This field is supplied by asolenoid, fitted coaxially with the bar and the searchcoils, or by an electromagnet, with holes in the polepieces for the bar to pass through. As a rule the directcurrent through the solenoid or through the electro-magnet shows a slight ripple, so that the externalfield contains an alternating component. This doesnot induce an e.m.f. in the astatic coils and thereforecauses no trouble.

When the external field is reduced to zero and themagnetic moment measured again, one can determinefrom this the remanence of the material.The coercive force is measured by letting the

external field grow in the reverse direction until thee.m.f. induced in the search coils reaches zero. Themagnetization of the sample is then zero, and thestrength of the external field is' equal to the coerciveforce.

When interpreting the measurements, account should betaken of the demagnetizing field of the sample itself. Thisapplies in particular to remanence measurements. These corn-plications of interpretation are not, however, characteristic ofthe method of measurement described here, and therefore willnot be dealt with in this article.

During the measurement the sample is shifted inrelation to the search coils until the induced e.m.f. ismaximum. Since the signal is in the form of analternating voltage that can readily be amplified,the sensitivity of this method is substantially higherthan that of the ballistic methods formerly used.

For the purpose of absolute measurements theapparatus is calibrated, using, for instance, a nickelsample of known weight which is magnetized tosaturation (for nickel 0.7 X 10-4Vsmfkg). The nickelsample is fitted over the same length of the bar inorder to eliminate the influence of the shape of thesamples.

Measuring arrangement with compensating device

In the measurement under fig. I the e.m.f. produceddepends on the amplitude and the frequency ofvibration of the sample. These two quantities are notentirely constant, which limits the accuracy of themeasurement. We have eliminated this limitation byadopting a compensating method. Mounted at an-other point on the bar carrying the sample is a smallpermanent magnet of platinum-cobalt+), surroundedby a second astatic pair of coils 82 (fig. 2). Themagnet and the sample are rigidly connected to eachother, and therefore vibrate identically. The voltagesinduced in the two pairs of coils consequently dependin the same way on amplitude and frequency, so thatchanges in these quantities do not affect the ratiobetween these voltages.

This ratio is measured with a compensating circuit.The electronic millivoltmeter, again preceded by anamplifier, now serves as a null instrument. Thesetting q of the voltage divider R2 at which the nullinstrument shows no deflection is a measure of the

4) Because of its lowtemperature coefficientand lowsensitivityto extraneous fields,platinum-cobalt is eminently suited foruse as a standard magnet. See: R. A. Mintern, Platinumalloy permanent magnets, Platinum Metals Rev. 5, 82-88,1961.

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1963/64, No. 9 MAGNETIC MEASUREMENTS ON SMALL SAMPLES 229

SM000 000 0 0 0 0

Fig.2. Refinement of the method in :fig.I: the e.m.f. inducedby the sample M in the pair of coils Sl is compensated by ane.m.f, induced by a platinum-cobalt magnet PMl in the pairof coils S2. This makes the measurement insensitive to changesin the amplitude and the frequency of vibration. The millivolt-meter VM now acts as a null instrument. The setting q ofvoltage divider R2 is a measure of the magnctic moment.Meaning of the other symbols as in fig, I.

required magnetic moment. Calibration can again becarried out using a sample of known magneticmoment.The essentials of the circuit actually employed are

shown in fig. 3. The coils SI' containing the sampleunder measurement, are connected to the calibrated,

Fig. 3. Essentials of the compensating circuit. -The pair ofcoils Sl' inside which the samplevibrates, is connected to a cali-brated voltage divider Rl. Theplatinmn-cobalt magnet vibratesinside the pair of coils S2. A fraction E2 of the e.m.f, induced inS2 appears across the voltage divider R2 via the fixed voltagedivider Ra-R4. Rs is a resistor with which the phase differencebetween the compared voltages Ex and Erer can be reduced tozero. When the circuit is earthed as shown here, no trouble isexperieneed from the four stray capacitances Cl>C2, Caand C4,

which are produced by the electrical screeningofthe cables inter-connecting the pairs of coils and the compensating circuit. Now,however, points Pand Q, between which the voltage must bereduced to zero, both have a certain potential with respect toearth. For this reason it is necessary to use a difference ampli-fier 5).

5) 'See e.g. G. Klein and J. J. Zaalberg van ZeIst, Generalconsiderations on difference amplifiers, Philips tech. Rev.22, 345-351, 1960/61, and by the same authors: Circuits fordifference amplifiers, I and Il, Philips tech. Rev. 23,142-150 and 173-180, 1961/62.

stepwise variable voltage divider RI' with which apart Ex = pEl of the voltage El across RI can betaken off. In the successive steps of this voltagedivider, p has the values 1, lj3, ljl0, lj30, ljl00,lj300 and ljl000. In this way Ex can be madesmaller than the voltage E2 produced by the per-manent magnet vibrating in S2. The setting q of R2is then adjusted until the null instrument shows nodeflection, after which q can be read off to threedecimal places. In this connection E2 is chosensuch that it is about 1000 times greater than thesmallest voltage detectable with the null instrument.For this purpose R2 is connected to the fixed voltagedivider R3-R4.

Owing to the inductances of the pairs of coils SIand S2' the currents through RI and R2 will showphase differences with respect to the vibration of thebar. Only when these differences are identical will thecurrents be in phase, which is necessary for exactcompensation. To make the phase shifts equal, aresistance R5 is put parallel with the coils 82• Forgiven pairs of coils SI and S2' the resistance R5 onlyhas to be adjusted once.

The combination of amplifier (about 1000X) andelectronic millivoltmeter as null· instrument makesit possible to observe voltages of 20 nanovolts.Special measures have been taken to turn this highsensitivity to good advantage. The compensatingcircuit and the amplifier are magnetically screened.The amplifier is fitted with low-noise valves at theinput, and is made selective by means of a bandpassfilter for the vibration frequency of the sample(125 cjs) with a pass band of 30 cjs. Moreover, themains harmonics at 100 and 150 cjs are additionallysuppressed. A photograph of the measuring arrange-ment is shown in fig. 4.

Stray capacitances

Care was taken in the design of the circuitry to avoid inter-ference from stray capacitances. The pairs of coils SI and S2are connected to the compensating circuit by cables with earthedscreening. These cables therefore have a capacitance to earthof a few hundred pF. At a frequency of 125cis a capacitanceof 100pF represents an impedance of more than 107 ohms. Thisis very high compared with the resistances, of the order of1000 ohms, used in the measuring circuits. The stray capaci-tances therefore have no significant influence on the amplitudesof the voltages that have to compensate each other. On theother hand the slight phase shifts which they give rise to inthese voltages result in a perceptihle difference voltage (fig.5).This causes no trouble provided the phase shifts are independentof the settings of the voltage dividers Rl and R2 (:fig.3), for thephase correction by means of Rs relates to a phase shift of thiskind. The phase shifts will be independent of the settings of thevoltage dividers when no currents flow through the two controlcontacts Pand Q. If the null instrument has an infinitely highinput impedance - or if we imagine it to be removed for a

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230 PHILlPS TECHNICAL REVIEW VOLUME 25

Fig. Lj.• The apparatus for measuring magnetic moments by the compensation method. S,and S2 are the astatic pairs of coils, inside which, respectively, the sample and a small plati-num-cobalt magnet (both fixed to the bar U) are caused to vibrate. Left, the loudspeakersystem T for the drive. ln the foreground, from left to right: the compensator (includingthe difference amplifier), a standard amplifier (1000 x) and a bandpass filter. In the back-ground: the amplifier that drives the loudspeaker system, and the electronic millivoltmeter(the null instrument). The solenoid SM is mounted on a carriage and can be slid aroundthe coils S"

moment - then the control contacts in a circuit as in fig. 3 willcertainly carry no current. No trouble is then experienced fromthe stray capacitances. The same then applies when the inputimpedance of the null instrument is not infinitely high, for oncecompensation has been effected it makes no difference whetherthe null instrument is connect.ed or disconnected.

In fig. 3 none of the terminals of the null instrument areearthed, in other words the first amplifying stage must bedesigned as a difference amplifier 5). It is in fact impossible

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Fig.5. A small phase difference spbetween the voltages Ex and Erer,which are to compensate each other,gives rise to a relatively large voltagedifference LIE.

to earth the circuit at one of the terminals of the null instru-ment, for in that case a current could flow via this earth con-nection and the stray capacitances through the control contact

connected to the earthed terminal. Movement of this contactwould then not be entirely without influence on the currentdistribution in the circuit, and would in general cause ;,Iightphase shifts, making exact compensation impossible.

The vibration frequency

In the original measuring arrangement (with nocompensating device) the sample was set in motionby a loudspeaker system operating on 50c/s. Sincethe sensitivity was relatively low, little inconveniencewas experienced from interfering fields of 50c/s andmultiples of it. When, however, the sensitivity wasincreased by introducing the compensation method,these fields became troublesome. Little could be donewith filters because of the fact that the frequency ofthe desired signal was also 50 c/s_ It was thereforenecessary to choose a different vibration frequency.A second source of interference that influenced the

choice of frequency was the noise of the first ampli-fier 'tuhe. This noise contains a component which is

1963/64, No. 9 MAGNETIC MEASUREMENTS ON SMALL SAMPLES 231

inversely proportional to the frequency 6). Fromthese considerations it is evident that the frequencyshould be chosen as high as practicable, avoiding asfar as possible multiples of 50 cjs. A limit is set to theheight of the frequency, however, by considerationsof noise nuisance and by the acceleration forces,·which are proportional to the square of the frequencyand threaten the connection between sample andbar. The frequency finally decided upon was 125 cjs.

The vibrator

The mechanical part of the vibrator is sketched infig.6. The bar, which carries the sample and theplatinum-cobalt magnet, is fixed to. a spring of a typethat combines high lateral stiffness with a long life 7).On the left in fig. 6 can he. seen the loudspeaker coiland the loudspeaker 'magnet. The current throughthe loudspeaker coil is supplied by an amplifier whichreceives its input signal from a second platinum-

Fig. 6. The vibrator. 1 loudspeaker magnet, 2 speech coil,3 mass of the vibrating system with which the frequency isadjusted, 4 spring (the vibrator contains two of these springs),5 coupling piece for the bar carrying the sample and the magnetfor the compensation voltage. PM2 platinum-cobalt magnetfixed to the same bar and contained inside a third astatic pairof coils Sa, connected to the input of the amplifier which drivesthe current through the speech coil (see fig.7).

cobalt magnet fixed to the vibrating bar, and con-tained in a third pair of coils S3' Because of thisfeedback the system is able to oscillate at its natural(mechanical) frequency (fig.7). The amplitude ofthe vibration is limited to about 1 mm by shuntingacross the amplifier output a small incandescentlamp, mounted opposite to a photo-resistorê) shunt-ing the input of the amplifier.

G) See K. S. Knol, Noise; a general survey, Philips tech. .Rev,20, 50-57, 1958/59.

7) Another application of a spring of the same type is describedin: B. Bollée and F. Krienen, The CERN 600MeV synchro-cyclotron at Geneva, Ill. The tuning-fork modulator,Philips tech. Rev. 22, 162-180, 1960/61, especially p. J69.

8) N. A. de Gier, W. van GooI and J. G. van Santen, Photo-resistors made of compressed and sintered cadmium sul-phide, Philips tech. Rev. 20, 277-287, 1958/59, esp. p. 286.

9) These measurements were made by B. J. G. Hamer,

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Fig.7. The pair of coils Sa' in whieh the magnet PM2 vibrates,is connected to the input of an amplifier AT, which feeds thespeech coil of the vibrator T. Due to this feedback the systemvibrates at its natural (mechanical) frequency. The photo-resistor LDR, connected to the input terminals of AT, and theelectric lamp G connected to the output terminals of AT ensurethat the vibration amplitude is limited to about 1mm. Av isthe difference amplifier, F the bandpass filter. .

Examples of applications 9)

Fig. 8 gives an example of coercive-force measure-ments. The coercive force here is determined as afunction of the thickness of a galvanically depositedlayer of a Co-Ni-P alloy. The shape of the curve is

2000xT04Vsm2

,

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I( ~

~x

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T500

TOOO

500

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Fig. 8. Coercive force of a galvanically deposited layer of Co-Ni-P as a function oflayer thickness d.The quantity ofmaterialvaries from 0.4 to 50 mg.

;

232 PHILIPS TECHNICAL REVIEW VOLUME 25

most remarkable, the coercive' force showing apronounced maximum at a layer thickness of about5 [Lm.With increasing layer thickness the quantityof material increased from 0.4 to 50 mg. Thesemeasurements were done with the relatively insensi-tive arrangement, without compensating device.As a second example fig. 9 shows two remanence

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300I

t 200

100

./00 600xKJ4Vs/mz

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-300

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Fig. 9.Remanent magnetization producedin a piece ofmagnetictape which - starting from a non-magnetized state - wasmagnetized by a fieldof the magnitude indicated on the abscissa(remanence curve).The two curves A and B (with branches BIand B2) relate to different non-magnetic initial states: thesymmetric curve A relates to conventional anhysteretic de-magnetization, curve B to a demagnetizing process followingthe path indicated by S in the inset. The difference betweenbranches BI and B2 is due to the fact that the magnetizing fieldfor these branches was respectively equal and opposite indirection to the fieldin which the samplewas saturated (pathsSI and S2 in the inset). Quantity of material approx. 1.5mg.Vertical scale not calibrated.

curves A and B measured on one sample weighing1.5 mg, using the set-up with compensating device.The procedure for measuring a remanence curve isas follows. First, the sample is demagnetized. Next,it is exposed for a short time to a magnetizing field.The field is then switched off and the residual mag-netization measured. This being done, the sample isagain demagnetized, and the measurement is repeat-ed for a different value of the magnetizing field, andso on. The measured values of remanence plottedagainst the magnetizing :fieldproducing them resultin a remanence curve. As the magnetization fieldincreases in strength the remanence curve of courseapproaches saturation remanence. Of the tworemanence curves in fig. 9 curve A was obtained afterconventional demagnetizing in an alternating field,the initially high amplitude of which is slowlyreduced to zero (anhysteretic demagnetization).Curve B, with branches BI and B2' relates to thesame sample, but in this case the non-magneti? state

was reached along the path marked by s in theinset. The marked difference between curves A andB demonstrates the fact that the state of a magneticmaterial is in general by no means unambiguouslydefined by the point corresponding to that state inthe I-H plane, but depends on the manner in whichthis point is reached. Only in the saturation statethis is not the case. In order to obtain a mate-rial in a well-defined magnetic state, it is there-fore necessary to start from saturation. This evidentlyalso applies to the non-magnetic state.

The third example is the remanence curve of apiece of magnetic tape, measured with the sensitiveversion of the apparatus, where the accurate meas-urements ~ere made even in the region of very lowremanence (fig. 10). These low remanence values areimportant in connection with the print-througheffect mentioned at the beginning of the article.

The fourth example relates to a study of the effectof grinding upon the magnetic properties of a' pow-der 10), the magnetic properties being investigated asa function of the grinding time. Since small samplescould be used, it was possible to take ten samplesfrom a charge of only 109 in the ball mill without

1000

L 5r 1-. --./

/

I/

//

/

//

2

100

5

5

.2

5

2

0,1o 200-J.l.oH

Fig. 10. Remanence curve for a piece of magnetic tape over awide range of remanence values I (log scale for I). Ir is thesaturation remanence. The processes responsible for print-through take place at values of }toIl smaller than roughly4.0X10-4 Vs/m2• Quantity of material approx. 1.5mg. Rerna-nence values of one thousandth of the saturation value canstill be measured on samples as small as this. Vertical scale notcalibrated.

10) G. W. van Oosterhout and C.J. Klomp, On the effect ofgrinding upon the magnetic properties of magnetite andzinc ferrite, Appl. sci. Res. B9, 288-296, 1962 (No. 4./5).

1963/64, No. 9 MAGNETIC MEASUREMENTS ON SMALL SAMPLES 233

appreciably changing the size of the charge. This sizehas a considerable influence on the grinding condi-tions.

Finally, it may be mentioned that the method ofmeasurement described here is also used in geological

Summary. When a sample of a magnetic material ---; whichpossesses a magnetic moment - is caused to vibrate inside acoil, an alternaring e.m.f. is induced in the coil.Since this e.m.f.can easily be amplified, a basis is thus provided for magneticmeasurements which are much more sensitive than ballisticmeasurements. In this way magnetic measurements can bedone on samples weighing only a few milligrammes. The useof an astatic pair of coilseliminatesinterference from extraneousalternating fields. The accuracy is increased by compensating

investigations for studyingthe "frozen -in" magnetismof rocks. From these measurements conclusions canbe drawn about the earth's magnetic field at thetime when the magnetism was frozen in (palaeo-magnetism).

the induced e.m.f. with an e.m.f. generated in a second astaticpair of coils, in which a-small permanent magnet, rigidly fixedto the sample, vibrates. Small variations in the amplitude andthe frequency of vibration then do not influence the results.With a set-up based on this principle, developed and used inPhilips Research Laboratories for the investigation of mag-netic-recording materials, it is possible to measure a magneticmoment of 7.5X 10-14Vsm, i.e. the saturationmoment of3X 10-4mg of iron. Methods of suppressing interference are discussed.

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