Oxygen-isotope effect on the charge-ordering transition ofLa1−xCax

Abstract

The colossal-magnetoresistance compound La 1Ϫx Ca x MnO 3 shows a transition to commensurate charge ordering (Mn 3ϩ -Mn 4ϩ ) for xϭ50%. In the range xϭ42-50 %, the high-temperature paramagnetic insulating phase transforms first into a conducting ferromagnetic phase at the Curie temperature T C , and at somewhat lower temperature T CO into the charge-ordered phase. The oxygen isotope effect on T C is small in this range, ␣ O ϭϪdlnT C /dlnm O Ϸ0.14. In contrast to this, the charge-ordering transition shows a large and negative isotope effect, i.e., the heavier mass ( 18 O) favors charge ordering. At the border of the existence of the charge-ordered phase, 18 O substitution can induce this transition in an 16 O sample that does not show this transition. The effect can be reversed by back exchange. These results show the extreme mass dependence of the charge-ordering transition in this system. ͓S0163-1829͑98͒51510-X͔

Mechanisms for giant isotope effects in manganites and other oxide semiconductors

“…8 But in the same compound with xϾ0.4 the isotope shift of T C amounts only to 2%. 9 A giant isotope effect manifests itself quite differently. At xϭ0.41 and 0.42 in the initial 16 O sample the magnetization decreases with increasing temperature monotonically, but after the replacement 16 O for 18 O a peak of the magnetization M appears slightly below the Curie point.…”

Oxygen-isotope effect of the paramagnetic-insulating to ferromagnetic-metallic transition inLa1−xCaxFranck1, Isaac2, Chen3 et al. 1998Phys. Rev. BSelf Cite668601View full textAdd to dashboardCite

“…Large isotope shifts are seen in the charge-ordering transition, they will be reported in a separate paper. 23 In contrast to this, the isotope shift of the ferromagnetic T c is quite small, with ␣ O near 0.1 in this range. At the low Ca concentration of 20% we find a very large isotope exponent of ␣ O ϭ0.83, in excellent agreement with the data of Zhao et al 17 For a slightly larger x of 22% Ca, ␣ O drops to about 0.38 and then continues to fall with increasing Ca concentration.…”

Oxygen isotope effect on the vibrational modes ofLa1−xCaxMnOIrwin1, Chrzanowski2, Franck3 1999Phys. Rev. BSelf Cite284210View full textAdd to dashboardCite

“…The observation [23][24][25] of large oxygen isotope shifts in the Curie temperature also emphasized 26 the importance of electron-phonon interactions in CMR compounds. The precise nature of this coupling, its relative importance, and the origin of the large isotope effect itself are not well understood.…”