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Design Rules for Manganites with Novel Magnetic and Electronic Properties

100%

Dabrowski B., Chmaissem O., Mais J., Kolesnik S.

Acta Physica Polonica A

2004

vol. 105

issue 1-2

45-56

We have been systematically developing strategies for making new perovskite manganites with novel magnetic and electronic properties. This effort requires a two-fold approach: understanding the dependence of properties on chemical and structural factors and development of the ability to synthesize desired compounds. We show that, similar to other single valent 3d systems, the magnetic superexchange interactions in AMnO_3 manganites (A = rare or alkaline earth's) are dependent on the Mn-O-Mn bond angle which is a function of interatomic distances A-O and Mn-O. The local structural disorder on the A-site suppresses magnetic interactions. Recently, by comparing disordered (randomly mixed La/Ba) and ordered (forming Ba/La/Ba layers along the c-axes) perovskites we have demonstrated much more conspicuous effects of structural and charge disorder for mixed-valent manganites. We show that by stabilizing the ordered structure; i.e., by suppressing local structural and charge disorder on the A-site, a substantial increase in T_C from 340 to 365~K can be achieved. A similar control of order on the Mn-site was achieved for SrMn_{1-y}Ga_y O_{3-y/2} compounds near y=0.5, while SrMn_{1-y}Fe_yO_3 compounds are always randomly mixed. To achieve these compounds, we have been systematically developing special synthesis techniques for extending chemical composition ranges far beyond those previously achieved, developing rules for predicting which compositions should be possible to produce by these special techniques, and establishing methods for selectively ordering or disordering mixtures of metal atoms on the A- or Mn-sites.

Novel Properties of Atomically Arranged Perovskites

88%

Dabrowski B., Kolesnik S., Chmaissem O., Suescun L., Mais J.

Acta Physica Polonica A

2007

vol. 111

issue 1

15-25

Perovskites AMO_{3-δ} attain unique electronic, magnetic, ferroelectric, thermoelectric, mixed-conducting, and other functional properties through selection of the A- and M-site ions, their fractions, ionic sizes and valences, spin states, and orbital orderings, as well as the oxygen content and vacancy ordering. We illustrate here our systematic exploration of the effects of composition, temperature, pressure, and oxygen content on the thermodynamic stability and magnetic properties for La_{1-x-y}Sr_xBa_yMnO_3 manganites. Parameters controlling magnetic transitions are identified through examples of single-valent compounds of RMnO_3 (the Mn-O-Mn bond angles that can be equivalently described in terms of the tolerance factor) and Sr_{1-x}Ca_xMnO_3 (the tolerance factor and disorder described in terms of the variance of A-site ion sizes), and the mixed-valent La_{0.5}Sr_{0.5-y}Ba_yMnO_3 (the tolerance factor, variance of sizes, and the local strains described in terms of the elongated Mn-O bonds). By using an example of kinetically stable, atomically-ordered layered-perovskites we show that improved useful properties, such as the increase in the Curie temperature T_C and enhancement of colossal magneto-resistive effect at room temperature, can be achieved through decreases in structural distortions such as variance of sizes and local strains.

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2 Acta Physica Polonica A

1 2007

1 2004

2 Chmaissem O.

2 Dabrowski B.

2 Kolesnik S.

2 Mais J.

1 Suescun L.

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Design Rules for Manganites with Novel Magnetic and Electronic Properties

Novel Properties of Atomically Arranged Perovskites