Physical Properties

The physical properties of manganites [111] such as insulator-to-metal transition [145], magnetoresistance [103], coercive field [146], or microstructure [72, 147] are strongly dependent on the thickness. As an example, the MR value calculated as AR/R(H) for H = 6 T exhibits a strong dependence on film thickness as shown in Figure 15 for La07Ca03MnO3. The curves show a maximum MR for a thickness near 110 nm with a value of 106% and, on either side of the peak, the MR ratio is drastically lower. Transport properties are mostly affected and the magnetization is only moderately changed [148] with thickness as seen for La0.6Sr0.4MnO3 deposited on MgO or SrTiO3. At an intermediate thickness around 100 nm, the films usually recover the properties of the bulk compounds. Even when the film

films grown on (100)-LaAl03. Reprinted with permission from [103], S. Jin et al., Appl. Phys. Lett. 67, 557 (1995) © 1995, American Institute of Physics.

Figure 15. Thickness dependence of the MR for La067Ca033MnO3 thin

Figure 16. (a) Temperature dependence of the resistivity for La0 67Sr0 33MnO3 films with varying thickness on NdGaO3 and LaAlO3. (b) Thickness dependence of the conductance of films at 14 K. Reprinted with permission from [111], J. Z. Sun et al., Appl. Phys. Lett. 74, 3017 (1999) © 1999, American Institute of Physics.

films grown on (100)-LaAl03. Reprinted with permission from [103], S. Jin et al., Appl. Phys. Lett. 67, 557 (1995) © 1995, American Institute of Physics.

Figure 16. (a) Temperature dependence of the resistivity for La0 67Sr0 33MnO3 films with varying thickness on NdGaO3 and LaAlO3. (b) Thickness dependence of the conductance of films at 14 K. Reprinted with permission from [111], J. Z. Sun et al., Appl. Phys. Lett. 74, 3017 (1999) © 1999, American Institute of Physics.

is under low epitaxial stress (case of NGO), TIM varies greatly from 182 (3.5 nm) to 264 K (165 nm), as seen for La07Ca03MnO3 films [145].

Films thinner than 100 nm have properties different from the bulk and are most of the time unusual. For example thin La1-xBaxMnO3 films (t < 100 nm) exhibit a TC higher than in the bulk due to an anomalous tensile strain effect when deposited on SrTiO3. Consequently, the resulting film shows room temperature ferromagnetism and an enhancement of the magnetoresistance [19]. In (La,Ca)MnO3 films, the thinnest films which present full magnetization grow with the fo-axis of the structure perpendicular to the substrate, whereas the thicker films grow with the fo-axis in the plane of the substrate and do not present full magnetization [72]. Another example of the thickness dependence is seen in La0.67Ca033MnO3 on SrTiO3, which is ferromagnetic around 150 K but remains insulating [71]. Biswas et al. [139] have explained this behavior by the coexistence of two different phases, a metallic ferromagnet (in the highly strained region) and an insulating antiferromagnet (in the low strain one). This nonuniformity induces, under a magnetic field, an insulator-to-metal transition resulting in a large CMR effect. But the metallic behavior of the bulk La0 7Ca0 3MnO3 can be retained for a thickness down to 6 nm when the SrTiO3 substrate is treated to obtain an atomically flat TiO2 terminated surface [149]. For La09Sr01MnO3 (t < 50 nm) on (100)-SrTiO3, Razavi et al. [150] reported an unexpected insulator-to-metal transition, most probably due to La deficiency. Nevertheless, Sun et al. [111] have estimated the "dead layer" for La0 67Sr0 33MnO3 to be around 3 nm for NdGaO3 and 5 nm for LaAlO3 (Fig. 16). The magnetic, transport, and structural properties of La07Sr03MnO3 deposited were interpreted recently in terms of a magnetic (1 nm) and an

Figure 15. Thickness dependence of the MR for La067Ca033MnO3 thin

Figure 17. p(T) under varying magnetic fields for different thicknesses of Pr05Ca05MnO3 thin films grown on SrTiO3. Arrows indicate the direction of the temperature dependence.

electrical (insulating) dead layer (respectively 1 and 0.4 nm thick) [151].

More recently, the robustness of the CO state was studied by Prellier et al. [121]. In Pra5Caa5MnO3, the thicker film induced the less stable state; that is, a small magnetic field as compared to the bulk is required to destroy the CO state and induce a metallic behavior (Fig. 17). In Nd05Sr05MnO3, the (110)-films show a strained and a quasi-relaxed layer. The latter increases with film thickness whereas the strained one has a constant thickness [143]. The coexistence of two strain regimes inside the same film was also seen in La0 66Ca033MnO3 films on SrTiO3 [14] and LaMnO3 deposited on NdGaO3 [152]: at the interface a cubiclike dense layer (5 nm thick) is observed while the upper layer shows a columnar growth. These two distinct thickness ranges behave differently with respect to the thickness dependence of the magnetotransport properties [102]; the upper range (t > 20 nm) is weakly thickness-dependent whereas the lower one not. These results [102,107] show the evidence for the effect of Jahn-Teller type distortion and confirm theoretical explanations [104]. In Nd2/3Sr1/3MnO3 films, the release of the strain as the thickness increases [141] results in a first-order phase transition.

Thus, these results show the thickness dependence of the physical properties of the film, but it seems difficult to estimate these changes precisely. For example, considering a Pr07Sr03MnO3 film deposited on NdGaO3, is it possible to evaluate the TIM and the TC for 200 nm thick film? There is no report of such calculations, and one of the reasons is that the properties of the film depend not only of the substrate but also on the growth conditions. It will be necessary to answer this question in the future.

The improvement in controlled heterostructures and multilayers is a necessary stage for the realization of many devices and circuits. Structures with new properties such as superlattices were also widely studied.

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