A low cost modulation technique for magneto-optical measurementsGiovanni Isella and Ezio Puppin Citation: Review of Scientific Instruments 70, 2095 (1999); doi: 10.1063/1.1149719 View online: http://dx.doi.org/10.1063/1.1149719 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/70/4?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Simultaneous magnetoresistance and magneto-optical measurements of domain wall properties in nanodevices J. Appl. Phys. 115, 17C718 (2014); 10.1063/1.4862377 Electrochemical cell for in situ magneto-optic Kerr effect measurements Rev. Sci. Instrum. 74, 4723 (2003); 10.1063/1.1619583 A new tool for measuring polar magneto-optical Kerr hysteresis curves at high fields and low temperatures Rev. Sci. Instrum. 73, 2086 (2002); 10.1063/1.1468683 A focused magneto-optical Kerr magnetometer for Barkhausen jump observations Rev. Sci. Instrum. 71, 1752 (2000); 10.1063/1.1150532 Probing submicron nanomagnets by magneto-optics Appl. Phys. Lett. 73, 3947 (1998); 10.1063/1.122945

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A low cost modulation technique for magneto-optical measurementsGiovanni Isella and Ezio Puppina)

Dipartimento di Fisica, Istituto Nazionale per la Fisica della Materia, Politecnico di Milano,P.za L. da Vinci 32, 20132 Milano Italy

~Received 9 October 1998; accepted for publication 13 January 1999!

A modulation–demodulation apparatus for magneto-optical Kerr effect measurements is presented.This system is based on an intensity modulated laser and on a specially designed demodulationcircuit. This approach allows us to exploit the advantages offered by modulation at a much lowercost compared to the standard design based on expensive photoelastic modulators and generalpurpose lock-in amplifiers. ©1999 American Institute of Physics.@S0034-6748~99!04904-7#

Linearly polarized light undergoes a rotation of the po-larization plane after reflection from a magnetized metal sur-face. This Magneto-optical Kerr effect~MOKE!, constitutesthe physical basis of several techniques which allow us tomeasure the hysteresis loop in various geometries on thinand ultrathin film samples. Measurements are carried out bysending a beam of polarized light onto the sample surfaceand by measuring the polarization rotation of the scatteredbeam. The angle of rotation turns out to be proportional tothe magnetization of the sample.1 Difficulties are connectedwith the small value of the Kerr rotation which is in the orderof 1023 rad for a complete reversal of magnetization. In spiteof this fact a direct measurements of the Kerr signal has beenproposed by several authors with sensitivities down to thesubmonolayer regime.2 With this method the polarization ro-tation of a stabilized He–Ne laser beam is directly measuredby placing a polarizer~analyzer! close to extinction with re-spect to the polarization of the incident beam and by mea-suring the intensity of the transmitted light.

Another approach is based on compensation techniques.3

Briefly, the small magneto-optical signal is normally super-imposed to a constant background whose intensity is100–1000 times larger compared to the useful signal. In or-der to subtract this background various optical or electricaltechniques have been proposed. A common feature ofthese techniques is their cheapness. The drawback isthe higher sensitivity to noise sources such as ambient lightfluctuations, mechanical vibrations, and low frequencynoise.

Another standard practice is to modulate the polarizationof the light impinging onto the sample with a photoelasticmodulator and to recover the magneto-optical signal with alock-in amplifier.4 Modulation techniques present an excel-lent capability to recover small signals buried in an over-whelming noise. In optical apparatuses such as those forMOKE measurements, another significant advantage is thepossibility to work in the presence of ambient light. On theother hand, modulation–demodulation techniques have thedrawback of being relatively expensive~more than 10 000US dollars for a photoelastic modulator coupled with alock-in amplifier!.

The optimal solution would be a cheap modulation–demodulation technique. In this article we describe an ex-perimental arrangement which joins the advantages of modu-lation with a low cost. Even through the final signal to noiseratio attainable with this technique is worse if compared tothe above methods, its very low cost makes it appealing forapplications where an extreme sensitivity is not mandatory.

Two major improvements with respect to the availabletechniques have ben introduced. First, instead of modulatingthe polarization state of light a more conventional intensitymodulation has been used. Second, the usual general purposelock-in amplifier has been replaced with a specially designeddemodulation circuit based on standard commercial compo-nents. We estimate that the cost of the modulated lightsource along with the demodulation circuitry is in the orderof 500 US dollars.

The experimental setup is outlined in Fig. 1: An intensitymodulated laser beam is emitted by a laser diode~L! andthen polarized by a polarizer~P!. The data presented herehave been collected by using standard low cost polarizersnormally made for photographic cameras. The linearly polar-ized light is subsequently scattered by the sample~S!, whichis magnetized by means of two coils, and then revealed by aphotodiode~D! after being analyzed by a second polarizer~A!.

The working principle of the intensity modulation–demodulation technique is now briefly presented. IfI 0

1I sinvt is the intensity of the light emitted by the modu-lated laser, the intensity collected at the photodiode will be(I 01I sinvt)cos2(u1fK), whereu is the angle between the

a!Electronic mail: puppin@axp7000.cdc.polimi.it FIG. 1. Experimental setup.

REVIEW OF SCIENTIFIC INSTRUMENTS VOLUME 70, NUMBER 4 APRIL 1999

20950034-6748/99/70(4)/2095/2/$15.00 © 1999 American Institute of Physics

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polarizer and analyzer extinction axes, whilefK is the Kerrrotation. After demodulation the total signal is:I cos2(u1fK). In the hypothesis offK!1 this can be written asI cos2 u2I sin 2ufK . In order to maximize the ratio betweenthe magnetization dependent part of the signal,I sin 2ufK ,and the constant backgroundI cos2 u, the polarizer–analyzercouple should be set in a crossed configuration~u590°!. Alimit to the suitability of settingu590° is however fixed bythe non-null extinction ratio of the polarizer–analyzercouple,5 and by the need of optimizing the signal to noiseratio.2

In our apparatus we use a 3 mW TXlaser module byVector Technology with a wavelength of 670 nm. Themodulation carrier, which also works as a reference wave forthe demodulation circuit, is supplied by a function generator.The polarizers are two Polaroid sheets with an extinctionratio 1023. The optimal value for the modulation frequencyhas been observed to be in the 50–100 kHz range.

In order to recover the modulated signal a three stagescircuit has been realized, as shown in Fig. 2. In the first stagethe direct current~dc! component of the photocurrent fromthe light detector~D in Fig. 1! is compensated with a con-stant current generated with a variable resistor. The differ-ence current signal is then amplified with a two stage trans-impedance amplifier. The gain of this amplification stage is 3

MV for the components shown in Fig. 2. The transfer func-tion of the amplifier has one pole at 630 kHz and a secondpole at 8 MHz and is therefore adequate for minimizing thephase shift at the modulation frequency. Both the zeroing ofthe dc component of the photocurrent and the absence ofphase shifts are essential to ensure a correct working of thesubsequent demodulation stage. This stage is based on astandard integrated demodulator~AD 630! which performs amultiplication of the voltage signal with the reference signalused for driving the laser modulation. The product signal isthen integrated with a simple RC filter. The time constant hasbeen set to 100 ms. The data have been collected with ananalog input computer board. The PC also controls the mag-netic field generation. Good quality hysteresis loops can beattained with the use of this simple circuit, with no need forfurther signal conditioning~filtering or phase matching!.

Figure 3 shows a hysteresis loop taken from an Fe film90 nm thick grown on a MgO substrate. The acquisition timefor a loop like this is in the order of 1 min. In order toevaluate the signal to noise ratio of the acquisition chain it isfirst necessary to define what the useful signal and the mea-suring procedure of the noise are. The useful signal is repre-sented by the height of the hysteresis loop. In order to deter-mine the noise level a second measurement has been carriedout without applying the magnetic field, i.e., without chang-ing the sample magnetization. In this way the straight lineshown in Fig. 3 has been obtained. The standard deviation ofthe values obtained in this case represents the noise level.For the case shown in Fig. 3 the signal to noise ratio is equalto 30.

This work has been supported by the Italian Ministry ofUniversity and Scientific and Technological Researchthrough the MURST-COFFIN97 Project.

1M. J. Freiser, IEEE Trans. Magn.4, 152 ~1968!.2E. R. Moog and S. D. Bader, Superlattices Microstruct.1, 543 ~1985!; Z.Q. Qiu, J. Pearson, and S. D. Bader, Phys. Rev. Lett.67, 1646~1991!.

3J. A. C. Bland, M. J. Pagget, R. J. Butcher, and N. Bett, J. Phys. E22, 308~1989!.

4P. Q. J. Nederpl and J. W. Martens, Rev. Sci. Instrum.56, 687 ~1985!.5C. A. Ballentine, R. L. Fink, J. Araya-Pochet, and J. L. Erskine, Appl.Phys. A: Solids Surf.49, 459 ~1989!.

FIG. 3. Hysteresis loop taken from a 90-nm-thick Fe/MgO sample~dots!and a blank measure from the same sample~rhombs!.

FIG. 2. Demodulation circuit.

2096 Rev. Sci. Instrum., Vol. 70, No. 4, April 1999 G. Isella and E. Puppin

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