NOISE POWER SECTRAL DENSITY IN SINGLE STRIP NiFeCo/Cu GMR SENSORS

A.F. Md Nor, E.W. Hill Manchester School of Engineering, University of Manchester, Dover Street,

Manchester MI3 9PL, UK

Introduction

Noise power spectral density measurements were performed in single strip NiFeCo/Cu GMR sensors for various width ranging from 2 pm to 20 pm. Two frequency ranges were used for noise investigations which are from 0.1 Hz to 100 Hz and from 10 Hz to IO kHz. The power spectrum of the noise was obtained at various bias fields in the absence and presence of an alternating external signal. In order to obtain accurate and repeatable data a program was developed using LabviewTM to automate the power spectrum measurements.

I/f Noise Analysis

The I/f noise slope for samples measured in the absence of the alternating external signal, showed no field dependence in both frequency ranges. The variation of l/f noise slope and the power spectral density S, with bias field along the hard axis for the 5 pm wide sample measured in the absence of the alternating external field is shown in figure 1. The slope of the l/f noise is about 1 at close to zero bias field in all samples investigated. This result is in rough agreement with Smith [ 11 that reported a noise power dependence on frequency of about 0.8 at zero bias in similar samples. On the other hand the l/f noise slope measured in the presence of the alternating drive field shows field dependent noise behaviour as shown in figure 2 for the 10 pm wide sample. The slope of the l/f noise is about 1.1 in the region close to zero bias but falls off at large bias field. This field dependent behaviour indicates that the signal-induced noise is dominant around zero bias-field- It has been reported that this signal-induced noise is responsible for the existence of the sharp peak near zero bias field [2]. The I/f noise is analysed using the Hooge parameter a.

References

[ I ] Smith N., Zeltser A.M.,Parker M.R. IEEE Trans. Magn. 32 p135 (1 996)

[2] Md Nor A.F., Hill E.W. IEEE Trans. Magn. 34 p1327 ( I 998)

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COMPARISON OF THE MAGNETOREFRATIVE EFFECT IN TRANSMISSION AND REFLECTION FOR GIANT MAGNETORESISTIVE C O - A ~ THIN FILMS.

D. Bozec, F. Canet, V.G. Kravets, S.M. Thompson and J.A.D. Matthew Department of Physics, University of York, York, YO10 5DD, UK

The magnetorefractive effect (MRE) measures a change of the dielectric function in the infrared region due to the change in conductivity when a magnetic field is applied to a giant magnetoresistive material [ 11. An experimental and theoretical study of the MRE has been performed in reflection and transmission for Co-Ag granular films. Samples of type Co,Ag,., with x ranging from 0.15 to 0.5 were grown on Si (11 1). For reflection measurements, samples 100 nm thick were grown onto standard Si (111) substrates whereas for the transmission study, films of 30 nm were deposited onto Si (1 11) substrates thinned down to a thickness of 90pm. The maximum GMR of 7% was obtained for a composition x = 0.25. Fourier Transform Infra Red spectroscopy was used to measure the change of reflection and transmission for s- and p - polarisation over a wavelength range from 2 to 25 p n in an applied magnetic field up to 15 kOe. Experimental results show that a maximum MRE effect of 11.3% is measured in transmission for Co35Ag65 whereas in reflection it only reaches 1.4% for s- polarisation and 0.25% for p - polarisation. Calculations of the spin dependent optical conductivity were performed for transmission and reflection 121. Spin dependence was introduced by calculating the optical conductivity using the Zhang and Levy model for GMR in granular materials 131. In agreement with experimental results, simulations show that the MRE effect is significantly larger in transmission than in reflection and good agreement is achieved between experiment and theory especially at high wavelength.

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Figure 1: Measurement of the MRE in reflection s- polarisation and in transmission for different concentration of Co.

[l] J.C. Valet and T. Valet in Magnetic ultrathin films, multilayers and surfaces, edited by E. Marinero, Materials Research Society, Pittsburgh, p477 (1995) [2] V.G. Kravets, D. Bozec, J.A.D. Matthew, S.M. Thompson, H. Menard, A.B. Horn and A.F. Kravets, Accepted for publication in Phys. Rev. B. (2001) [3] S. Zhang and P.M. Levy, J. Appl. Phys. 73 5315 (1993).

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