ELSEVIER Physica C 282-287 (1997) 190--193 PHYSICA G EPR evidence for Jahn-Teller polarons in Lal_xCaxMnO3+y A. Shengelaya, Guo-meng Zhao, H. Keller and K. A. Mfiller Physik-Institut der Universitiit Ziirich, CH-8057 Zfirich, Switzerland We present the results of the electron paramagnetic resonance (EPR) study in the mixed valence perovskite Lal-.CaffiMnOs+v for xeo and 1SO isotope substituted compounds. The characteristic differences observed in EPR intensity and linewidth for the two isotope samples can be explained by a model in which a bottlenecked spin relaxation takes place from the exchange-coupled constituent Mn 4+ ions via the Mns+ Jahn-Teller ions to the lattice. For x----0.2 the ferromagnetic exchange energy J exhibits a lSo/lSo oxygen isotope effect of -,~-10~. The observed isotope effects provide the microscopic evidence for the formation of Jalm-Teller polarons in doped manganese perovskites. I. Introduction Recently, the ferromagnetic sys- tems Lal_zMezMnO3+~ (where Me=Ca,Sr,Ba) have become the focus of scientific and technolog- ical interest because of the colossal magnetoresis- tance (CMR) effects found in these materials [1]. Doped manganese perovskites are mixed-valent systems containing Mn s+ and Mn 4+ ions. The magnetic and electronic properties in these com- pounds have traditionally been examined with the double exchange (DE) model, which consid- ers the transfer of an electron between neighbor- ing Mn 3+ and Mn 4+ ions through the Mn-O-Mn path [2]. However, recent theoretical considera- tions indicated that DE alone does not explain the CMR, and that polaronic effects due to a very strong electron-phonon coupling should be included [3,4]. The strong electron-phonon cou- pling is expected because the electronic ground state of the Mn 3÷ ions is degenerate and this de- genera~:y is removed by a spontaneous distortion of the surrounding lattice, known as the Jahn- Teller (JT) effect [5]. A recent demonstration of a giant oxygen isotope shift of > 20K on the fer- romagnetic transition temperature T~ by Zhao et al. [6] provided direct experimental evidence of the strong coupling of the charge carriers to JT lattice distortions and of JT polaron formation [7] in Lai_=Ca~MnOs+y. 0921-4534/97/$17.00 © Elsevier Science B.V. All fights reserved. PII S0921-4534(97)00 ! 92-5 50 ! I I I i I I o.o 11 .,.o 7/-- -50 1 , , i i t 2000 3OOO 4000 5O00 H(G) Figure 1. EPR signal of 160 and 180 samples of La0.sCa0.2MnO3+y measured at T:300 K under identical experimental conditions. The fits with Lorentzian line shape are indicated by solid lines. 2. Experiments We measured the temperature dependence (To < T < 3To) of the Electron Paramegnetic Resonance (EPR) for ceramic powder samples of La1_zCa~MnOs+y with x-0.1, 0.2 which were substituted by different oxygen isotopes (160 and IsO). The samples used here are the same as those studied by Zhao et al. [6]. A strong sym- metric EPR signal with a lineshape very close to Lorentzian and g-value of 2.0 was observed