Evidence of Anisotropic Thermoelectric Properties in La 2/3 Ca 1/3 MnO 3 Thin Films Studied by Laser-Induced Transient V
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Mat. Res. Soc. Symp. Proc. Vol. 602 © 2000 Materials Research Society
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Fig. 1 Schematic sketch of an YBCO film deposited on a vicinal cut STO substrate
the low-indexed crystallographic axes [12]. In this case, the gradient of the electrical potential VO is related to the gradient of the lattice temperature VT by V(j = Sij VjT
((1)
In the thermopile experiment, the non-crystallographic system orientation is realised by depositing YBCO thin films onto STO (001) substrates intentionally miscut by an angle a towards the [010] direction as shown schematically in Fig. 1. This causes the CuO 2 planes of the
YBCO to grow with a tilt angle a with respect to the substrate surface plane. An illumination of the film with a short laser pulse generates a transient tem-perature gradient V±T, which gives rise to a voltage, U. Quantitatively, assuming VjT being, the integration of Equ. ( 1 ) yields Uj0o0] = l/2tA SAT sin2a
(2)
U[ 10]= 0
[I
=
length of illuminated film strip, t
=
film thickness, AT
=
temperature difference of top and
bottom of the film, AS = S•b - S, , the difference of the Seebeck coefficients in the ab-plane and
along the c-axis ]. In YBCO the signals detected at room temperature even for moderate laser fluence of 50 mJ/cm2 can be as large as 18 V for a sample with t = 250 nm and an illuminated length of 2 mm [11]. The experiments reported here, demonstrate the existence of an unexpected laser-induced voltage [ LIV ] in LanCa113MnO 3 thin films in the order of 1/10 of those measured in YBCO. The existence of these laser-induced signals suggests a microscopic mechanism comparable to that in YBCO implying a substantial anisotropy of the thermoelectric properties of La3Caj/MnO3. 126
I. EXPERIMENTAL TECHNIQUES Expitaxial LanCal/MnO 3 films were prepared on STO ( 001 )-oriented substrates with intentional miscut angles ox using the standard pulsed laser deposition technique[13], subsequently patterned and contacted by the usual lithographic and deposition procedures. The films, characterised by x-ray diffraction, magnetotransport and AFM, consist of single - phase material with properties typical of epitaxial CMR thin films [14,15]. The thermoelectric experiment is performed using an excimer laser [ KrF, I = 248 nm, pulse duration 28 ns ] as energy source in conjunction with an Oxford optical cryostat allowing measurements in the range 10 K < T < 300 K. For the measurement of the transient voltage signal, a HP 500 MHz oscilloscope was used allowing a time resolution of 2 nsec. The digitised data are collected by a computer using a standard IEEE 488 interface. Figure 2a shows the original raw data as recorded for a specimen deposited on a perfectly oriented (001) substrate together with the signals for a specimen deposited on a substrate with 10' tilt vs. the [010] direction.
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