Superlattices

Thin film methods offer a powerful and versatile technique for growing new structures, as previously seen for example in cuprates. This is due to strain effects that can stabilize structures which do not exist under classical conditions of pressure and temperature. For example, various metastable perovskites, which cannot be formed in bulk or can only be prepared under high pressure, such as BiMnO3 [110], YMnO3 [119], and atomically ordered LaFe0.5Mn0.5O3 [153], are synthesized via a pulsed laser method or by using injection MOCVD-like NdMn7O12 [154].

Also interesting is the construction of new compounds, such as artificial superlattices, that show unique physical properties since different types of magnetism can be combined by building the desirable structure at the atomic layer level [155]. Growth conditions such as the oxygen pressure or the deposition temperature are easy to control in the thin film process, allowing the synthesis of metastable phases.

Another approach to obtain exotic properties via new phases is the method of artificial superlattices. Preliminary films were grown by stacking a magnetic layer (La07AQ3MnO3 with A = Sr, Ba,...) and another per-ovskite, usually an insulator (such as SrTiO3) [156]. This allows a continuous variation of the in-plane coherency strain in the films [157-159]. High quality films showing a clear chemical modulation by the presence of satellite peaks around the main diffraction peak were obtained [157,160]. In the case of LCMO, the metallic transition is suppressed and the MR enhanced at low temperature when the thickness of the LCMO layer is decreased to 2.5 nm [158]. The MR (MR = 100 * [R(0) - R(H)]/R(0)) is calculated to be 85% at H = 5 T over a wide temperature range (10-150 K). A systematic study of La07Ba03MnO3/SrTiO3 superlattices shows that the decrease of the LBMO layer thickness results in the broadening of the MR peak vs temperature [156]. Such studies also confirm the importance of strains and the relevance of Jahn-Teller electron-phonon coupling in doped manganites, as pointed out by Lu et al. [157]. Following the same idea, La2/3BaX/3MnO3/LaNiO3 multilayers were synthesized. Magnetization measurements show evidence of antiferromagnetic coupling between LBMO layers when the thickness of the LaNiO3 spacer is 1.5 nm or less [161].

The magnetic exchange interactions have been extensively studied in La(Sr)MnO3/LaMO3 (M = Fe, Cr, Co, Ni) [162,163]. The authors showed that the ferromagnetism is systematically affected by the adjacent magnetic layers via the interface, and they propose an expression of TC on the basis of the molecular field image. The magnetotransport properties of superlattices such as La0 6Pb0 4MnO3/ La0.85MnO3 [164] or La0.7MnO3/Pb0L65Ba0.0SCa0JMnO3 [165]

were also investigated. An enhancement of the magnetoresistance is obtained in these materials. Also grown were c-axis YBa2Cu3O7/La0 67Ba033MnO3 superlattices [166]. Above TC, the CMR persists up to room temperature, and below TC the superlattices exhibit a quasi-two-dimensional superconductivity of the YBa2Cu3O7 layers coexisting with magnetism in the La0 67Ba033MnO3 [166]. An increase in the thickness of the antiferromagnetic La0 6Sr0 4FeO3 layer in between La0 6Sr0 4MnO3 layers induces a strong magnetic frustration around the superlattice interfaces, leading to a reduction of the magnetic temperature transition and of the ferromagnetic volume [159].

Salvador et al. used the PLD technique to create A-site ordering in films of (LaMnO3)/(SrMnO3) superlattices [167]. An increase of the superlattice period leads to a decrease in the TC and in TIM or to a low magnetization value.

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