Influence of Post-Deposition Annealing on Lattice Strain, Electrical Transport and Magnetic Properties in Epitaxial La 0
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The bulk-doped LCMO is a distorted perovskite with a pseudocubic lattice parameter (aop) of 3.881 A. The tilting of the MnO 6 octahedra results in a tetragonal structure with lattice parameters ao = b=,2ap and c 2 ao. In this paper, Miller indices are based on this tetragonal unit cell, indicated by a superscript "T". The tetragonal distortion enables one to distinguish between the (1 IO)T and (00I)T oriented films, using off-axis azimuthal (0) x-ray scans [11,12]. The ability to distinguish the two tetragonal orientations is crucial for the studying magnetism in these films. Detailed thickness-dependent lattice distortions and 3-dimensional strain dependent magnetic anisotropy of these films were reported elsewhere [12,13]. EXPERIMENTAL DETAILS The LCMO films (both 250 A and 4000 A) were grown on (001) LaA10 3 (a0 = 3.792 A) substrate from a stoichiometric target using a pulsed laser deposition technique at 700 TC with an oxygen partial pressure of 400 mTorr. The optimized laser energy density was kept at 0.75 J/cm2/pulse and the samples were cooled down at 300 Torr oxygen pressure. In order to study the influence of post-deposition annealing on the physical properties, we have subjected our samples to 950 °Cin flowing oxygen of 1 atm. pressure with the duration of 2, 5 and 10 hours. As strontium is known to diffuse from the SrTiO 3 substrate into the LCMO film in the process of annealing [14], we did not explore this aspect in the case of a LCMO film grown on (001) SrTiO3 substrate. To determine the associated 3D strain states, the in-plane and out-of-plane lattice parameters of as-grown and annealed films were detennined by normal and grazing incidence diffraction (GID) 0-20 scans using a four-circle x-ray diffractometer. Crystallographic domain structure and texture were determined by off-axis azimuthal 0 scans of nondegenerate reflections such as (I 1 I)T, (113 )T, and (22 ])T [12,13]. The temperature-dependent electrical transport measurements were carried out using a 4-point DC technique. Magnetic measurements (M(H,T)) were carried out by the DC extraction method using Oxford Instruments' MagLab System 2000M. RESULTS In order to explore the influence of annealing on electrical transport, the resistivity of 250
A and 4000 A thick LCMO films was measured as a function of temperature for both as-grown and annealed samples. Figure 1 shows the typical variation of resistivity (p) with temperature for 106
o
5 1lO
4000
A
320
As-grown
300 280
LCMO/LAO
4000
260
hrs.
10410
aý240 103
2 hrs.
220
200 1021 I11 ii.l l ii I 0 50 100 150
m
200
l iii
250
180 1800
1
300
T (K)
25oA
LCMOILAO 2
. .l
4 6 8 Annealing time (hrs)
10
Fig. 1.Temperature dependence of resistivity of Fig.2. Variation of Tp with annealing time for 4000 A LCMO/LAO film in as-grown and both 250 A and 4000 A films. various annealed states.
46
4000 A as-grown and annealed films after different annealing periods. Surprisingly, the resistivity peak temperature (Tp ) is found to be enhanced above room temperature (308 K) for a sho
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