Electrical Noise in Ultra Thin Giant Magnetoresistors
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ABSTRACT We report measurements of low frequency 1/f excess noise for the series of La0. 75Sr 0 25MnO 3 (LSMO) giant magnetoresistive thin epitaxial films with thickness of 42, 50, 100, and 600 A. Fabricated manganite films experience semiconductor-normal metal (paramagnetic-ferromagnetic) phase transition with the temperature change. The transition manifests itself by the sharp change of resistivity and characteristic peak of magnetoresistivity. Thickness decrease results in lowering the transition temperature and increasing of resistivity. The noise spectra has 1/fa behavior with ct 1± 0.2. The voltage fluctuations spectral density shows quadratic dependence on current indicating that observed noise is caused by the resistance fluctuations. Noise level, temperature coefficient of resisistvity and magnetoresistance increase for thin films. Therefore, the operation point (transition temperature) can be tailored from 330 K to 220 K by changing only the film thickness while the performance of temperature and magnetic field LSMO sensors can be maintained almost constant in thickness range down to 100 A. INTRODUCTION In recent years, manganite films exhibiting giant magnetoresistance (GMR) have gained much attention. High magnetic sensitivity and large temperature coefficient of resistivity (TCR) attract the interest both in understanding mechanisms of GMR phenomenon and the
great potential of these materials for various applications (bolometers, magnetic field sensors, etc.). The belonging of GMR materials to perovskite family is an additional advantage. It allows to make smart devices fabricating epitaxial heterostructures with alternating layers which possess identical crystalline and different functional properties. For all CMR sensor applications, an important factor to be considered is the nature of resistance fluctuations which could limit the signal-to-noise ratio. Some studies, devoted to 1/f noise in giant magnetoresistive multilayers like Ni 80 Fe 20 [1], evidence that resistance fluctuations in magnetic materials are caused by the fluctuations of magnetic domain structure. For the l/f noise in manganite perovskite oxides the majority of researchers consider, that the origin of resistance fluctuations also could be related with magnetic properties of the material [2, 3]. For both bolometric and magnetoresistive sensors it is important to produce film with the phase transition and maximum of TCR and CMR at desired temperature range. In this paper we report the functional properties of epitaxial Lao 75Sr 0 25MnO 3 films have been successfully tailored within the temperature range of 400 around room temperature only by film thinning without drastically decreasing of film performance.
365 Mat. Res. Soc. Symp. Proc. Vol. 574 © 1999 Materials Research Society
EXPERIMENT Thin LSMO film series with the thickness of 42, 50, 100, and 600 A was produced by a 248nm KrF pulsed laser ablation of stoichiometric La 0 .75Sr 0. 25MnO 3 ceramic target. Films were grown on (001) SrTiO 3 (STO) single crystal substrates (5x5
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