Giant oxygen isotope shift in the magnetoresistive perovskite La_1–xCa_xMnO_3+y

Abstract

FERROMAGNETIC perovskites of the form La_1–XMe_xMnO_3–Y (where Me is Ca or Sr) have been known¹ since 1950, but there has been a recent resurgence of interest following the discovery of giant magnetoresistance in this class of compounds^2,3. The compounds contain both Mn³⁺ and Mn⁴⁺ ions; as the electronic ground state of the Mn³⁺ ions is degenerate, their energy is lowered by a spontaneous distortion of the surrounding lattice—the Jahn–Teller effect⁴. The charge carriers in these materials are strongly coupled to (and mediate the ferromagnetic interaction between) the manganese ions⁵, suggesting that localized lattice distortions could also play an important role in determining the electronic and magnetic properties of these compounds. Here we investigate this possibility by examining the effect on the ferromagnetic transition temperature of varying the oxygen isotope mass (replacing ¹⁶O with ¹⁸O). For La_0.8Ca_0.2MnO_3+y, we measure an isotope shift of >20 K, significantly larger than that found for any magnetic or electronic phase transition in other oxides. In contrast, we observe no significant isotope shift for the structurally related ferromagnet SrRuO₃, in which the Jahn–Teller effect is negligible. These results imply that the large isotope shift arises from coupling of the charge carriers to Jahn–Teller lattice distortions, and we suggest that such Jahn–Teller 'polarons' may also be responsible for the magnetoresistive properties of these materials.