D.L. HUBER; G. ALEJANDRO; A. CANEIRO; M.T. CAUSA; F. PRADO; M. TOVAR; S. OSEROFF

PHYSICAL REVIEW B

AMER PHYSICAL SOC

Lugar: New York; Año: 1999 vol. 60 p. 12155 - 12161

1098-0121

We have analyzed recent results for the electron paramagnetic resonance linewidths in La12xCaxMnO3 andrelated materials over the range 0<x<1. In all cases studied, the linewidth away from magnetic and structuraltransitions can be fitted to the expression @x0(T)/x(T)#DHp.p.(`), where the first factor is the ratio of theCurie susceptibility x0(T) to the measured susceptibility x(T) and the second factor is a temperatureindependentconstant. Formally exact and approximate expressions for DHp.p.(`) are derived. In the case ofLaMnO3, the linewidth in the distorted O8 phase arises from the interplay of crystal field ~single ion! anisotropyand antisymmetric ~Dzialozhinsky-Moriya! exchange together with isotropic superexchange. Above theJahn-Teller transition to the pseudocubic O phase, the single-ion anisotropy is much weaker and the linewidtharises primarily from the antisymmetric interactions. Similar behavior is also found in both phases of theless-distorted, oxygenated material LaMnO3.04. In the case of CaMnO3, the distortions of the oxygen octahedraare also small and the antisymmetric interactions are again dominant. In the intermediate region 0,x,1,DHp.p.(`) decreases with increasing Ca ~i.e., Mn41) concentration in contrast to the Curie-Weiss temperature,which peaks near x51/3, reflecting the importance of the double-exchange mechanism. It is proposed that theabsence of a signature of the double-exchange interaction in the infinite-temperature linewidth is a consequenceof the fact that the time scale associated with the changes in Mn valence arising from double exchangeis too long to affect the decay of the correlation function in DHp.p.(`) so that the linewidth is determinedprimarily by the superexchange interactions between the Mn ions.