Random Telegraph-like Signals in Ultrathin CMR Films
- PDF / 1,310,784 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 6 Downloads / 194 Views
Random Telegraph-like Signals in Ultrathin CMR Films A. Lisauskas, S. I. Khartsev, and A. M. Grishin Department of Condensed Matter Physics, Royal Institute of Technology, S-100 44 Stockholm, SWEDEN. ABSTRACT The aim of the present paper is to study low frequency (10 Hz - 10 kHz) noise in low dimensional colossal magnetoresistors (epitaxial CMR films). Two contributions in electrical fluctuations were observed: thermal noise, which depends on resistance and temperature, and excess part, which is determined by resistance fluctuations and are mostly related to film microstructure. For films, thicker than the critical thickness, excess noise spectra has 1/f α dependence with α = 1 ± 0.2. In films thinner than the critical one there are random telegraph like signals (TLS) with Lorentzian spectra appear on the background of 1/f noise. Noise spectroscopy reveals the relaxation process in 4.2 nm thick film has thermally activated character with an energy gap of 20 meV at temperatures below 156 K and 78 meV at T > 156 K. INTRODUCTION Rear earth manganites, exhibiting colossal magnetoresistance (CMR) effect, have been proved to be promising candidates for uncooled infrared radiation (IR) bolometers [1] and week magnetic field sensor [2,3] applications. The further exploration of CMR materials is believed to rely upon development of micromachining technique that will enable the use of materials as sensors for high-resolution thermovision cameras or magnetic reading heads for high-density information storage. However, the reduction of sensor size involves new physical effects that usually are not observed in the bulk form and should be studied before the industrial manufacturing will take place. In this paper we present results on low frequency (from 10 Hz to 10 kHz) noise spectroscopy in pulsed laser deposited thin CMR films. FILMS PREPARATION The series of ultra-thin LSMO films with the thickness of 4.2, 5, 10, 30, 60, and 90 nm was fabricated by a 248-nm KrF pulsed laser ablation of stoichiometric La0.75Sr0.25MnO3 ceramic target. Films were grown on (001) SrTiO3 (STO) single crystal substrates (5×5 mm2), laser radiation energy density was 3-4 J/cm2, pulse repetition rate was 30 Hz, and target-to-substrate distance was 55 mm. Depositions were carried out in an oxygen pressure of 200 mTorr at substrate temperature of 750 °C and finalized by in situ annealing in an oxygen pressure of 500 Torr for 10 minutes. The background pressure did not exceed 10-7 Torr. Thickness of films was determined by atomic force microscope (AFM). Comprehensive x-ray diffraction study [4] showed these films are highly c-axis oriented with the full width at half maximum (FWHM) of rocking curve of (001) Bragg reflection FWHM = 0.2°-0.3°. The film-substrate lattice mismatch for films grown on STO is (aLSMO – aSTO) / aSTO = - 0.6 % (calculated from bulk LSMO x-ray diffraction data). Such a small mismatch enables F7.11.1
b)
10
2
5
0
0
20
TCR, %K
15
-1
4.2 nm 5 nm 10 nm 30 nm 60 nm 90 nm
-4
10
-6
10
-8
10
-10
10
-12
d)
c)
H = 10 kOe
Data Loading...
