High sensitivity vibrating reed magnetometerW. Roos, K. A. Hempel, C. Voigt, H. Dederichs, and R. Schippan Citation: Review of Scientific Instruments 51, 612 (1980); doi: 10.1063/1.1136264 View online: http://dx.doi.org/10.1063/1.1136264 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/51/5?ver=pdfcov Published by the AIP Publishing Articles you may be interested in A high-sensitive static vector magnetometer based on two vibrating coils Rev. Sci. Instrum. 82, 124702 (2011); 10.1063/1.3664615 High sensitivity 2 T vibrating sample magnetometer Rev. Sci. Instrum. 70, 3035 (1999); 10.1063/1.1149865 Compensating vibrating reed magnetometer (invited) J. Appl. Phys. 64, 6002 (1988); 10.1063/1.342132 Improvement of sensitivity of the vibrating reed magnetometer Rev. Sci. Instrum. 59, 1388 (1988); 10.1063/1.1139674 A Vibrating Reed Magnetometer for Microscopic Particles Rev. Sci. Instrum. 41, 1241 (1970); 10.1063/1.1684777

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High sensitivity vibrating reed magnetometer w. Roos, K. A. Hempel, C. Voigt, H. Dederichs, and R. Schippan

Institut fiir Werkstoffe der Elektrotechnik. R WTH Aachen. Templergraben 55. D-5JOO Aachen. Federal Republic of Germany

(Received 20 November 1979; accepted for publication 23 January 1980)

A high sensitivity vibrating reed magnetometer useful for measurements on very small ferromagnetic particles is described. The reed is made of gold wire (18 ,...,m in diameter, 10 mm long). The mechanical vibrations of the reed are converted directly to an ac voltage by a piezoelectric ceramic. Using a lock-in amplifier magnetic moments as small as 10-9 Acm2 can be detected.

PACS numbers: 07.55. + x, 06.30.Lz

INTRODUCTION

A sensitive magnetometer for measurements on small ferromagnetic particles has been described by Zijlstra. 1

This so-called vibrating reed magnetometer makes use of a resonance step-up of a vibrating reed. The particle is fixed at one end of a thin gold wire. A force is exerted on the sample by a nonhomogeneous ac-magnetic field. The deflection of the wire which is proportional to the magnetic moment of the sample is observed using a stroboscope lamp. If the ac-frequency corresponds to the mechanical resonance frequency of the gold wire the deflection increases according to the quality factor of the resonance system. By this method magnetic moments as small as 10- 7 Acm2 can be resolved. I

We have developed a modified version of the vibrating reed magnetometer with a much more sensitive signal pick-Up with which magnetic moments of about 10-9

Acm2 (corresponding to barium ferrite crystals of 1 fLm size at room temperature) can be detected.

I. APPARATUS

Figure 1 shows a schematic diagram of the apparatus. The sample is fixed in wax at one end of the reed and positioned between two coils producing the inhomo­geneous magnetic field. The coils have an inner diameter of 3 mm. The distance between the two coils is 2 mm.

.--- amplifier f-- audiO - lock -In- r- * OSCillator amplifier

I magnet J '" /

COIl .. - gloss fiber

sample -\"reed ~ piezoleromic

/ / "" I Hall probe

FIG. I. Schematic diagram of the modified vibrating reed mag­netometer.

Each consists of 265 turns made of copper wire (300 fLm in diameter). The coils are fed by an audio oscillator (current 1.2 A). To avoid heating, each coil is fixed in a piece of water-cooled rectangular waveguide. The reed consists of a gold wire (I8 fLm in diameter. 10 mm long) which has a mechanical resonance frequency of about 62 Hz and a quality factor of nearly 70. The mechanical vibrations of the gold wire are transferred by a glass fiber (150 fLm in diameter) to a piece of bimorph piezo­electric ceramic with rectangular cross section (1.5 mm x 0.5 mm and 20 mm long) which converts the mechani­cal vibrations directly to an ac voltage. The glass fiber is necessary to avoid inductive disturbances due to the ac magnetic field. The length of the glass fiber (70 mm) is chosen so that its first and second mechanical reso­nance frequency (20 Hz and 130 Hz) are far away from that of the gold wire. The piezoelement is cemented at one end of a heavy copper rod. The voltage, which is proportional to the magnetic moment of the sample, is amplified by a lock-in detector which gets its reference frequency from the audio oscillator. The output signal is recorded as a function of the applied magnetic field strength which is measured with a Hall effect gauss­meter. The whole apparatus is carefully screened against

-600 -400 -200 zoo 400 kA/m 600 H-- ---

FIG. 2. Primary hysteresis loop together with minor loops of a single crystal of BaFe'20'9 with a size of about 5 J.Lm.

612 Rev. Sci. Instrum. 51(5), May 1980 0034-6748/80/050612-02$00.60 © 1980 American Institute of Physics 612

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electrical and acoustical noise. The resolving power of the instrument is about 10-9 Acm2

• With this high sensitivity we can obtain direct information about the mechanism of magnetization reversal of very small ferro­magnetic particles. 2

II. MEASUREMENTS AND DISCUSSION

Figure 2 shows the primary hysteresis loop together with minor loops of a single crystal of BaFel2019 of about 5 /Lm size. The magnetic moment of this sample is approximately 10-7 Acm2. The particle has been fixed at one end of the reed using a microscope. The sample has been oriented by heating up the wax with the sample in a field of 1000 kA/m. During the measurement the field has been applied parallel to the easy axis. Magne­tization jumps and domain wall motion can be clearly detected. Starting from positive saturation a wall is nu­cleated if the field strength reaches - 20 kA/m. The wall moves through the crystal to a point where the mag­netic moment of approximately one half of the crystal is reversed. If the field is subsequently varied the wall is displaced in a nearly reversible way as can be seen from the smaIl opening of the inner hysteresis loop. A high field strength is needed to remove the wall from the crystal. The diamagnetic moment of the gold wire

/L = VPXH

has considerable influence on the shape of the hysteresis curve. V is the volume. p the density, X the magnetic susceptibility per mass and H the applied field. If we assume that according to the experimental arrangement approximately VIO of the gold wire is magneticaIly effec­tive we have a diamagnetic moment of nearly -IO-H

Acm2 with p = 19,28 g/cm3 , X = -0,142 10-6 cm3/g and H = 500 kA/m. This closely corresponds to the nega­tive slope superimposed on the hysteresis loop in Fig. 2.

613 Rev. Sci. Instrum., Vol. 51, No.5, May 1980

~ ~V U 11500

1000

500

61.5

0.4 mbar ( 0 -190 )

62 Hz 62.5 f---

FIG. 3. Output signal of the lock-in detector versus oszillator fre­quency for various air pressures.

The sensitivity of the magnetometer is essentiaIly limited by the quality factor Q of the resonance system which depends on the viscosity of the surrounding gas. The resolving power of the instrument can therefore be further improved by evacuating the space around the reed. The effect of evacuation on sensitivity is presented in Fig. 3 which shows the output signal of the lock-in detector versus oscillator frequency for various air pres­sures. As can be recognized the quality factor is im­proved from Q = 70 at 1000 mbar to Q = 550 at 10-2

mbar. This fact shows that by evacuation magnetic moments even smaller than 10-9 Acm2 can be measured.

1 H. Zijlstra, Rev. Sci. Instrum. 41, 1241 (1970). 2 W. Roos, C. Voigt, H. Dederichs, and K. A. Hempel, to be

published in the Proc. Int. Conf. Mag., Munich Sept. 1979.

Vibrating reed magnetometer 613

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