High temperature susceptibility in electron doped Ca_1 xY_xMnO₃: double exchange versus superexchange

Abstract

Abstract. We present a study of the magnetic properties of the electron doped manganites Ca 1-x Y x MnO 3 (for 0 ≤ x ≤ 0.25) in the paramagnetic regime. For the less doped samples (x ≤ 0.1) the magnetic susceptibility, χ(T), follows a Curie-Weiss (CW) law only for T > 450 K and, below this temperature, χ -1 (T) shows a ferrimagnetic-like curvature. We approached the discussion of these results in terms of a simple mean-field model where double exchange, approximated by a ferromagnetic Heisenberg-like interaction between Mn 3+ and Mn 4+ ions, competes with classical superexchange. For higher levels of doping (x ≥ 0.15), the CW behaviour is observed down to the magnetic ordering temperature (T mo ) and a better description of χ(T) was obtained by assuming full delocalization of the e g electrons. In order to explore the degree of delocalization as a function of T and x, we analyzed the problem through Montecarlo simulations. Within this picture we found that at high T the electrons doped are completely delocalized but, when T mo is approached, they form magnetic polarons of large spin that cause the observed curvature in χ -1 (T) for x ≤ 0. IntroductionManganites, responding to the formula B 1-x A x MnO 3 (with B = divalent alkaline earth and A = trivalent rare earth), have prompted a burst of research activity in the last decade, as they enclosed not only very rich physics but also interest in technological applications. The dynamics of these systems is mainly governed by the Mn ions, whose average valence changes with x between 4+ and 3+. The Mn 4+ ions have noncompensated spins in t 2g 3 configuration that give rise to localized S = 3/2 spins. On the other hand, the Mn 3+ ions have an extra e g electron (with s = ½), that couples ferromagnetically (FM) to the t 2g spins. The e g electrons tend to be itinerant and lower their kinetic energy by polarizing with FM character the localized t 2g spins. This process is known [1] as double exchange (DE) and in these materials competes with the classical superexchange (SE) interaction between Mn ions.In Ca 1-x La x MnO 3 were the whole series, from x = 0 to x = 1, can be obtained with the perovskite structure, the behaviour in the electron and hole doping regions were found different. The parent compounds are indeed quite different: while LaMnO 3 , a Jahn-Teller system, displays A-type [2] antiferromagnetism, where ferromagnetic (FM) planes order antiferromagnetically (AFM), the non Jahn-Teller CaMnO 3 is a G-type antiferromagnet, where each magnetic moment orders AFM with his nearest neighbours [2]. Most studies have been performed on hole doped compounds where ferromagnetism and colossal magnetoresistance (CMR) were found. In the range of electron doping, transport and magnetization studies on Ca 1-x La x MnO 3 showed that a complete FM state is never reached and different models were discussed in order to explain the magnetic properties at low temperatures [3,4]. These studies showed that in this region of doping new and very interesting phenomena appear as a conseq...

Temperature evolution of crystal field interactions across the Jahn-Teller transition in aLa7/8Sr1/8Alejandro1, Passeggi2, Vega3 et al. 2003Phys. Rev. B30270View full textAdd to dashboardCite

“…At high temperatures the apparent moment decreases down to 5.02 B , which is still larger than the average expected for an appropriate mixture of Mn 3ϩ and Mn 4ϩ free ions, e f f av ϭ4.78 B . A Curie-Weiss law with a C constant different from the free-ion theoretical value has been observed in several manganites 15,19 where localized moments strongly interact with itinerant electrons. This behavior has been modeled, 20 in a mean-field approximation, for the case of the double-perovskite Sr 2 FeMoO 6 , where well-localized Fe 3ϩ ions interact antiferromagnetically with delocalized electrons.…”

Magnetic properties of nanosized mixed valent manganites CaMnO3 and Ca0.7La0.3Mn1−xCexO3 (x=0; 0.2)

“…Nanosized CaMnO 3 exhibits phase transition to paramagnetic state at T N = 120 K (Fig. 1a) which is very close to a Néel temperature for both bulk [12] and nanosized (100-200 nm) [9] Ca-manganite. Large difference between ZFC and FC magnetization branches indicates the presence of a strong magnetic anisotropy.…”

“…DC magnetization measurements of nanosized Ca-manganite with the average particle size of 35 nm does not show significant change of T N ≈ 120 K in respect to its bulk counterpart [12] as well as to nanosized system with 100-200 nm particle size [9]. Partial substitution of calcium and manganese by lanthanum and cerium strongly affect magnetic behavior of the parent nanosized Ca-manganite.…”

“…The position of the inflection points are found to be at 230 K (S1) and 255 K (S2). According to phase diagrams established for bulk La 0.3 Ca 0.7 MnO 3 compound [5,12] these points can be ascribed to charge ordering phase transitions and thus above determined temperatures are denoted in Fig. 1b by T CO .…”

Specific effects of nanometer scale size on magnetic ordering in La1−xCaxMnO3 (x=0.1, 0.3 and 0.6) manganites

“…derived [10] for ferrimagnet and applied recently to manganites [11] was used for fitting. The applicability of Eq.…”