Automation of a torque magnetometer for measuring magnetic thin filmsM. Tejedor, A. Fernandez, and M. A. Cerdeira Citation: Review of Scientific Instruments 67, 851 (1996); doi: 10.1063/1.1146824 View online: http://dx.doi.org/10.1063/1.1146824 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/67/3?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Inplane periodic bicrystallinity in magnetic thin films Appl. Phys. Lett. 69, 857 (1996); 10.1063/1.117915 Magnetic relaxation at high linear densities in thin films with perpendicular magnetic anisotropy J. Appl. Phys. 79, 7920 (1996); 10.1063/1.362405 Variable temperature ultralow compliance torque magnetometer Rev. Sci. Instrum. 64, 802 (1993); 10.1063/1.1144162 Magnetic anisotropy in TbFe thin films prepared at oblique incidence J. Appl. Phys. 63, 3645 (1988); 10.1063/1.340670 Effect of oxidation on the magnetic properties of unprotected TbFe thin films J. Appl. Phys. 59, 1291 (1986); 10.1063/1.336519

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Automation of a torque magnetometer for measuring magnetic thin filmsM. Tejedor, A. Fernandez, and M. A. CerdeiraDepartamento de Fı´sica, Facultad de Ciencias, Universidad de Oviedo, 33007 Oviedo, Spain

~Received 30 March 1995; accepted for publication 11 December 1995!

An automated torque magnetometer for thin film measurements using the ‘‘45° method’’ has beendeveloped and built. Very low sampling rates~2 Hz! in the control process are allowed by fitting onthe sample holder a circular metallic piece that acts as an electromagnetic brake. ©1996 AmericanInstitute of Physics.@S0034-6748~96!01803-9#

The measurement of anisotropy in magnetic thin filmsby torque magnetometry is a direct and useful method.1,2 Asuitable procedure for attaining the magnetization saturationand the uniaxial perpendicular anisotropy is to measure thetorque on the sample as a function of the magnetic appliedfield when it is held at 45° relative to the plane of the film.3

It has been employed for this purpose in recent experiments.4

The main advantage of this procedure is related to the factthat the measurements are made in a fixed orientation of themagnetic applied fieldH. By measuring the acting torqueL,on the magnetic thin film for several values ofH, a linearrelation between (L/H)2 andL results, supposing thatH isgreater than the anisotropy field of the sample,Hk . Plotting(L/H)2 vs L, the values of the saturation magnetizationMs ,and the effective anisotropy constantKeff of the magneticthin films, can be obtained using

Ms5~2B!1/2/V, Keff5B/VA. ~1!

HereB is the intercept point with the (L/H)2 axis of the plot,A is its slope, andV is the volume of the sample. Based onthis method, a very simple torque magnetometer was previ-ously developed by us.5

Due to the fact that the parameters of the plot are deter-mined by least-squares fitting, a large number of experimen-tal points are suitable in order to perform statistical compu-tations. In the improved magnetometer described in thispaper, a data acquisition and control system provides us withan almost continuous flow of data, providing a large numberof experimental points. This can be done by changing thevalue ofH in a continuous mode, using the automatic sweepof the electromagnet power supply, and calculating the val-ues ofH and L through a data acquisition system pluggedinto a computer. A low sweep speed~0.5–1.35 T in 3 min! isadvisable in order to allow the automatic control task to usea very low sampling rate~2 Hz!. The dc current that pro-duces the counteracting torque is provided by a dc powersupply controlled by the computer. To cancel undesirable tor-sional and lateral vibrations, a suitable electromagnetic brakemust be located in the sample holder. A scheme of the newmagnetometer is presented in Fig. 1. It basically consists of aphosphorous bronze torsion fiber that supports the sampleholder, forming a semiturn around it. The ends of the torsionwire are attached to two jaws placed in the support system.These jaws also provide the electrical contact for the conduc-tor torsion fiber. The sample holder is a 34.5310.8f mmcylindrical piece of Teflon™ with two flat surfaces, placedon diametrical planes forming a 45° between them. Samples

are stuck on the upper surface and a nonmagnetic mirror isattached to the lower surface. An aluminum ring~9.6 mmhigh, 10.8 mm external diameter, 6.2 mm inner diameter! isattached on the top of the sample holder, producing an elec-tromagnetic damping. A laser provides a narrow light beamemployed for detecting torsions on the fiber by the conven-tional way of reflecting the light beam on the mirror of thesample holder and detecting the changes in the angle of re-flection by means of a dual photoresistance. The torque ex-erted on the sample by the applied magnetic field is mea-sured by means of another opposite torque produced by acurrent in the semiturn made by the conductor torsion fiberin the holder located between the electromagnet poles. Thiscurrent is provided by a dc power supply controlled by thecomputer. In the null position of the torquemeter, bothtorques are equal and are directly proportional to the currentthrough the torsion fiber and to the applied magnetic field.The constant of the magnetometer computed from the di-mensions of the semiturn is

FIG. 1. Scheme of the torque magnetometer.

851Rev. Sci. Instrum. 67 (3), March 1996 0034-6748/96/67(3)/851/2/$10.00 © 1996 American Institute of Physics This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP:

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T50.055 dyn cm/Oe A. ~2!

To test the automated torque magnetometer, we are go-ing to compare the results of a Co thin film when the mag-netometer is manually operated and when it is under auto-matic control. The basic features of the new magnetometerwhen it works in the manual mode are very close to those ofthe magnetometer previously developed by us.5 Somechanges, such as the electromagnetic brake and the new de-sign of the sample holder which improves its stability againstvibrations and allows for easier automation, have been in-cluded.

In Fig. 2 we show the plots of (L/H)2 vs L per unit ofvolume for a 1000-Å-thick Co film, measured using themanual and the automatic modes. The dashed line is thelinear regression of the ten experimental points, obtained bythe manual mode. This linear regression was calculated byleast-squares fitting, which determined that the coefficient ofthe regression was 0.998. The values ofMs andKeff obtainedusing Eq.~1! are shown in Table I. We can see that the resultfor Ms is in very good agreement with the value of 1422emu/cm3 given for Co in the literature.6 The continuous line

is the linear regression obtained from 358 experimentalpoints measured in automatic mode being the determinationcoefficient 0.997. The results ofMs andKeff are shown inTable I. The dispersion observed forL in Fig. 2 is due to asmall oscillation of the sample around the position of 45° asa consequence of the digital feedback employed. An analogangle-to-current feedback would improve the behavior of thesystem.

From the statistical analysis of the curve data, the ex-pected error for theA andB parameters, can be estimated as0.8% and 0.7%, respectively, for the values obtained in theautomatic mode, and 1.1% and 0.4%, respectively, for thevalues obtained in the manual mode. From these results thestatistical error forMs andKeff can be evaluated to be 0.4%and 1.4%, respectively, when these parameters are calculatedfrom the data obtained by automatic measurement and 0.2%and 1.3%, respectively, when the data are obtained bymanual measurement. Therefore there are not any significantdifferences detected in the statistical errors when the magne-tometer is working in the automatic or in manual mode.These errors are lower than those expected due to the experi-mental errors in the measurements of the sample volume,V,magnetic field,H, and magnetometer constant,T, which af-fects the computation ofMs andKeff .

1A. K. Agarwala, Rev. Sci. Instrum.59, 2265~1988!.2M. J. Pechan, A. P. Runge, and M. E. Bait, Rev. Sci. Instrum.64, 802~1993!.

3H. Miyajima, K. Sato, and T. Mizoguchi, J. Appl. Phys.47, 4669~1976!.4F. Hellman and E. M. Gyorgy, Phys. Rev. Lett.68, 1391~1992!.5M. Tejedor, A. Fernandez, B. Hernando, and J. Carrizo, Rev. Sci. Instrum.56, 2160~1985!.

6 Introduction to Magnetic Materials, edited by B. D. Cullity~Addison-Wesley, Reading, MA, 1972!, p. 617.

FIG. 2. (L/H)2 vs L plots for a Co film:~h! experimental points and linearregression~continuous line! obtained in automatic mode;~s! experimentalpoints and linear regression~dashed line! obtained in manual mode.

TABLE I. Cobalt film results obtained with the magnetometer working inmanual and automatic mode.

Manual mode Automatic mode

Ms 1.433103 emu/cm3 1.433103 emu/cm3

Keff 1.123107 ergs/cm3 1.133107 ergs/cm3

852 Rev. Sci. Instrum., Vol. 67, No. 3, March 1996 Notes This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitationnew.aip.org/termsconditions. Downloaded to IP:

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