Physical Measurements

The standard characterization of CMR thin films consists of resistance measurements versus temperature in zero field and under an applied magnetic field, by using the four probe technique and also in magnetization measurements. Results pertaining to strain effects will be discussed in the next section.

Several groups focused their studies on the surface [84— 86]. Extensive thin film surface studies were performed using two complementary techniques: atomic force microscopy and magnetic force microscopy [87]. Work was mostly on La1-x Srx MnO3 since this material exhibits the highest Curie temperature. It was also found that the properties of the surface are different from those of the bulk in both the electronic and the composition point of view [86]. For example, the surface termination and the Ca surface concentration depend on the overall Ca concentration in La1-X CaxMnO3 films [84] (the La/Ca ratio differs between the surface and in the film). The surfaces of La0 5Ca0 5MnO3 and La0 66Ca033MnO3 show a highly ordered grain pattern induced by strains [85], and the La0 65Sr035MnO3 surface exhibits a surface phase transition at 240 K (to be compared to 370 K for the bulk) [86].

Grain boundaries (GBS) strongly affect the properties of CMR materials. Low field magnetoresistance (LFMR) has been reported and attributed to the spin-dependent scattering of polarized electrons at the GB [88]. Researchers tried to enhance this property by artificially creating an interface between two elements. We will describe here only the intrinsic effect across natural GB (in polycrystalline thin films) and across artificial GB (in films deposited on bicrys-tal substrates). The precise influence of the substrate will be discussed in a separate part. Note that another method has been utilized to create artificial GB by scratching the LaAlO3 substrate before the deposition of the LCMO film [89]. The MR is subsequent in a field of 2 kOe and varies with the field orientation with respect to the GB.

The simplest way of creating a natural GB is to grow the film on polycrystalline substrates [90]. Most of this work was done by IBM [90,91] on La-Ca-Mn-O (LCMO) and LSMO films. The p — T curve of such films depends on the grain size, as shown on Figure 9: resistivity in zero field decreases when the grain size increases, but the peak temperature of approximately 230 K is almost independent of the grain size [90]. Gu et al. show that the LFMR at low temperature has a dramatic dependence on the nature of the inplane GB [92]. The reduction of zero-field low-temperature resistivity might be explained by the spin-polarized tunneling across half-metallic grains. Another possibility to obtain polycrystalline samples is to decrease the deposition temperature. The resulting GB results from a lower crystalline quality of the film [93].

Bicrystal substrates having a single GB have also been used to study the transport across a GB. LCMO and LSMO thin films were deposited on bicrystalline SrTiO3 substrates having a specific misorientation angle [94,95]. To measure the properties of the GB only, the film was patterned into a Wheatstone bridge. The GB resistance and its magnetic field dependence are strongly dependent on the misorienta-tion angle [95]. The MR increases with an increase of the misorientation angle of the bicrystal [95]. The change of resistance is 3% under 2 mT magnetic field at 300 K for La07Sr03MnO3. At 77 K, a large bridge resistance (27%) is observed during magnetic field sweeps between ±200 mT

Figure 9. p(T) under different magnetic fields for polycrystalline LCMO films with different grain sizes and an epitaxial film. The inset shows the zero-field resistivity at 10 K as a function of the average grain size for La075MnO3 (LXMO). Reprinted with permission from [88], A. Gupta and J. Z. Sun, J. Magn. Magn. Mater. 200, 24 (1999). © 1999, Elsevier Science.

Figure 9. p(T) under different magnetic fields for polycrystalline LCMO films with different grain sizes and an epitaxial film. The inset shows the zero-field resistivity at 10 K as a function of the average grain size for La075MnO3 (LXMO). Reprinted with permission from [88], A. Gupta and J. Z. Sun, J. Magn. Magn. Mater. 200, 24 (1999). © 1999, Elsevier Science.

over a temperature range down to 77 K. Steenbeck et al. utilized La0 8Sr0 2MnO3 films grown on SrTiO3 bicrystals with a misorientation angle of 36.8° [96] or 24° [97]. They found that the GB magnetoresistance occurs at low temperature, separated from the intrinsic MR near TC, and that the sign of the MR at the GB depends on the domain structure and H [96]. Moreover, current-voltage measurements show that the field dependence might not be related to tunneling [98].

Phase separation was suspected in La0 4Ba01Ca0 5MnO3 films [99] and confirmed by the noise probe method [100] in La2/3Ca1/3MnO3 films; in this work the authors attribute the origin of the random telegraph noise to a dynamic mixed-phase percolative process, where manganese clusters switched back and forth between two phases that differ in their conductivity and magnetization. This spatial inho-mogeneity in doped manganite thin films was also investigated in La1—xCaxMnO3 [101] using scanning tunneling microscopy. The phase separation is observed below the Curie temperature where different structures of metallic and more insulating areas coexist and are field dependent. This suggests that the insulator to metal transition at TC should be viewed as a percolation of metallic ferromagnetic domains.

0 0

Post a comment