Static and dynamic double-exchange effects in doped manganites
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C PROPERTIES OF SOLID
Static and Dynamic DoubleExchange Effects in Doped Manganites K. O. Khutsishvili and N. P. Fokina Dzhavakhishvili State University, Tbilisi, 0128 Georgia email: nath_fokina@daadalumni.de Received February 5, 2009
Abstract—It is shown that antiferromagnetic ordering in doped manganites with strong doubleexchange interaction is transformed into ferromagnetic canted ordering with residual antiferromagnetic behavior in the basal plane as a result of hopping of mobile electron. The canting angle between the core magnetiztions is controlled by the competition of the Heisenberg antiferromagnetic exchange and double exchange. The tem peratures of the paramagnet–antiferromagnet and paramagnet–canted ferromagnetic phase transitions are calculated. The results on the dependence of the magnetization in the canted phase and critical temperatures on the doping degree are in qualitative agreement with experiment. The form of uniform oscillations of core magnetiztions in the canted ferromagnetic phase of a doped manganite sample with hopping conduction is analyzed with and without allowance for relaxation of mobile electrons to the lattice. We propose a mecha nism for the ferromagnetic resonance broadening and its resonance frequency shift in a ferromagnetic con ducting sample (hopping conduction) of doped manganite due to double exchange. The resonance frequency shift and the ferromagnetic resonance damping constant (linewidth) are calculated in this model. In contrast to other relaxation mechanisms, the model is based on the fact that mobile electrons rapidly relax to the lat tice (over a time on the order of the precession period). PACS numbers: 85.75.d, 75.47.Lx, 76.50.+g DOI: 10.1134/S1063776109110090
1. INTRODUCTION Doped manganites with the general formula R1 ⎯ xMxMnO3 (R is a rareearth ion and M is an alkali earth ion) exhibit interesting magnetic and transport properties, among which the colossal magnetoresis tance (CMR) effect is the most promising for practical applications. Since the transport and magnetic prop erties of doped manganites strongly correlate, consid erable efforts are being made to analyze their static [1– 3] and dynamic (electron paramagnetic and ferromag netic resonance) properties [4–9]. Transport proper ties of doped manganites, including CMR, are ana lyzed in [2] based on the calculation of the magneto electric constant in the spinwave model of the magnetic system. The double exchange mechanism was discovered by Zener [10] and subsequently studied by Anderson and Hasegawa [11], as well as De Gennes [12] and other researchers [1–3]. It is associated with hopping of egelectrons (which will henceforth be referred to as mobile electrons) between Mn3+ and Mn4+ ions and plays the major role in the peculiar magnetic properties of these compounds. Undoped manganites (RMnO3) are always antifer romagnetic (AFM). However, as was observed long ago in [13], the substitution of bivalent alkaliearth ions for trivalent rareearth ions in manganites leads to the emergence
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