Double and Triple Perovskites A3Mn2O7 and A4Mn3O10

Although the majority of studies have been done on simple perovskites AMnO3, colossal magnetoresistance also occurs in (La, A)3Mn2O7 (A = Ca, Sr). These compounds belong to the Ruddlesden-Popper phases whose general formula is A„+1B„O3b+1 (see Fig. 4). Two parents of this family were synthesized in thin film form with n = 2 and n = 3.

Films of La2-2xCa1+2xMn2O7 (x = 0.3) were deposited on (001)-MgO by single-target magnetron sputtering [56,57]. The c-axis oriented films of La2-2xSr1+2xMn2O7 (x = 0.4) can be grown on (001)-SrTiO3 under limited conditions (above 900 ° C for the deposition temperature and below 100 mTorr for the oxygen partial pressure) [58] which are different from the typical conditions used for (La, Sr)MnO3 films. On SrTiO3 substrates, the resistivity curves shows a transition at 100 K which coincides with a magnetic transition for La2-2xSr1+2xMn2O7 films on SrTiO3 [58]. Films on MgO are a-axis oriented, which means that the long parameter is in the plane of the substrate. These films evidence

Figure 4. Idealized crystal structure of the layered manganites Lan_nxSr1+nxMnnO3n+1. Note the intergrowth between insulating rocksalt layers and the perovskite layers. The infinite term (n = to) corresponds to the La0 7Sr03MnO3 perovskite.

Figure 2. M(T) for as-deposited and postannealed La08MnO3_s films. Annealing 1—600 ° C, 3 h. Annealing 2—600 "C, 24 h. Annealing 3— 700 "C, 3 h. Annealing 4—800 "C, 3 h. Reprinted with permission from [50], S. Pignard et al., J. Appl. Phys. 82, 4445 (1997). © 1997, American Institute of Physics.

Figure 4. Idealized crystal structure of the layered manganites Lan_nxSr1+nxMnnO3n+1. Note the intergrowth between insulating rocksalt layers and the perovskite layers. The infinite term (n = to) corresponds to the La0 7Sr03MnO3 perovskite.

two types of ferromagnetic ordering that possibly result from anisotropic exchange interactions for 0.22 < x < 0.55 [59]. Magnetoresistance is observed in a wide temperature range below the ferromagnetic transitions on MgO and is accompanied by an hysteresis on SrTiO3 [58]. Epitaxial films of (La,Sr)3Mn2O7 can also be grown artificially by atomic-layer stacking of SrO and (La,Sr)MnO3 [60].

Contrary to the previous one, the La3-3x Cax+3xMn3O9 (x = 0.3) compound can be stabilized, but only in the form of thin films and not in the form of bulk [61]. Features similar to those reported for the double perovskite (n = 2) were also observed for the n = 3 compound, indicating a correlation between the dimensionality (or the c-axis bond configuration) and the magnetotransport properties [61].

Thus, it appears that the increase of the c axis reduces the magnitude of the CMR at low temperature and this may be attributed to the increased magnitude of the doubleexchange transfer matrix and a better ferromagnetic spin alignment.

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