The oxygen-isotope effect of the ferromagnetic transition in La 1Ϫx Ca x MnO 3 was investigated from x ϭ20% to xϭ43%. This is the range of the conducting ferromagnetic phase. We find that ␣ O ϭ Ϫ⌬ ln T c /⌬ ln m decreases from 0.36 to 0.14 with increasing Ca concentration. A large value of ␣ O ϭ0.83 was found for xϭ20%, it is possibly connected with excess oxygen content. The isotope effect decreases with increasing tolerance factor, pointing to the importance of double exchange. The isotope effect at 35% Ca is independent of magnetic field.

No abstract

It is generally believed that the electrical transport in magnetoresistive manganites above T C is governed by small polarons associated with large lattice distortions. We present evidence which confirms the existence of magnetic clusters ͑polarons͒ associated with minimal lattice distortions above the Curie temperature. We take advantage of the fact that the Mössbauer spectra of the relatively large and slowly fluctuating magnetic polarons consist of incompletely collapsed magnetic component with extensive wings.

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

Muon spin rotation measurements of Sr 2 Y(Ru 1Ϫu Cu u )O 6 ͑for uϭ0.1) reveal two distinct muon sites: one located in a SrO layer ͑which is superconducting at low temperatures͒ and the other in a Y(Ru 1Ϫu Cu u )O 4 layer ͑which is magnetically ordered at low temperatures͒. A precursor spin-glass state due to the Ru moments is detected in high fields ͑Ϸ3.3 kOe͒ in Y(Ru 1Ϫu Cu u )O 4 layers, with a spin-glass temperature of T G ϭ29.25 K. The Y(Ru 1Ϫu Cu u )O 4 layers order ferromagnetically in the a-b planes at the Néel temperature, T N Ϸ23 K. This in-plane ferromagnetism alternates direction between adjacent Y(Ru 1Ϫu Cu u )O 4 planes, resulting in a net antiferromagnetic structure. Although the onset of superconductivity is observed both by electron spin resonance and by dc susceptibility to occur for temperatures up to about T c,onset Ϸ49 K, this superconductivity is adversely affected by the Ru moments that fluctuate for TϾT N producing magnetic fields that break pairs in the SrO layers. The muons, as well as other probes, sense the more-robust static superconductivity for TϽT G . In fact, resistance measurements only show zero resistance below T N , at which temperatures the Ru moments that fluctuated for TϾT N are frozen in-plane. Hence strictly speaking, the superconducting transition temperature is the same as T N , which is far below T c,onset . Below T N there are no pair breaking fluctuating magnetic fields in the SrO layers where the hole condensate resides.