Polaron Activation Energy as Evidenced by EMR in Colossal Magnetoresistive Nanowires

Appl. Magn. Reson. (2008) 34, 1–XXX DOI 10.1007/s00723-007-0000-0 Printed in The Netherlands

Applied Magnetic Resonance

Polaron Activation Energy as Evidenced by EMR in CMR Nanowires A. Popa1, D. Toloman1, M. N. Grecu1,2, G. Mihailescu1, Al. Darabont1, C. V. L. Pop1, O. Raita1, M. Fardis1,3, S. Idziak1,4, S. K. Hoffmann1,4, and L. M. Giurgiu1 1 

National Institute for Research and Development of Isotopic and Molecular Technologies, Cluj-Napoca, Romania 2  National Institute for Materials Physics, Bucharest, Romania 3  Institute of Materials Science, National Center of Scientific Research Demokritos, Athens, Greece 4  Institute of Molecular Physics, Polish Academy of Sciences, Poznañ, Poland

Received 25 May 20007; revised 2 July 2007 © Springer-Verlag 2008

Abstract. In the present work an X-band electron magnetic resonance (EMR) investigation in the paramagnetic regime of colossal magnetoresistive manganite La2/3Ca1/3MnO3 (LCMO) nanowires was carried out. The temperature dependence of the EMR line width has been analyzed in terms of the small polaron hopping scenario. From this analysis, the polaron activation energy Ea in LCMO nanowires was evaluated. A discussion is given concerning the factors which could explain the smaller value of Ea as compared with the corresponding ones for LCMO nanoparticles.

1 Introduction The perovskite manganites La1#xCaxMnO3 (LCMO) are the compounds well-known for their colossal magnetoresistive (CMR) properties. Coexistence of ferromagnetism and metallic conduction in CMR manganites has been explained in terms of the double-exchange (DE) interaction. However, the DE interaction alone is not sufficient to explain the observed CMR and an additional mechanism on the basis of the polaronic effects was included [1]. It results in a polaronic contribution to the conduction in the paramagnetic regime (PM) of CMR manganites. Recently, CMR manganites with nanoscale dimensions have become of great interest for their potential technological applications. These nanostructured materials could exhibit enhanced electronic and magnetic properties as compared with their conventional microscale counterparts [2]. Regarding the template for the growth of nanoscale CMR manganites, the porous anodized alumina oxide (AAO) template has proven to be very flexible [3].

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A. Popa et al.

The study of the internal dynamics in the nanometric CMR is very important because the DE mechanism is basically a dynamical process. Since electron magnetic resonance (EMR) spectroscopy ideally suits for probing the dynamics of spins over a wide temperature range, a number of EMR studies has been performed in nanostructured CMR [4–6]. EMR could also provide experimental evidence of small polarons in both microsized and nanosized CMR [7–10]. In our previous publications, we presented systematic EMR investigations on the effects of the grain size reduction on the exchange coupling integral between manganese spins and the polaron activation energy [10, 11]. In the present work we have investigated the temperature dependence of the EMR line width in the PM regime in order to get information about the polaron activation energy of LCMO nanowires. Some preliminary results have been recently reported [12].

2 Experimental The perovskite-like La2/3Ca1/3MnO3 nanowires were synthesized using an AAO template by the procedure described in ref. 12. The X-ray powder diffraction analysis confirmed that the samples are composed of a single phase. As revealed by scanning electron microscopy (SEM), the mean diameter of LCMO nanowires is about 40 nm [12]. X-band EMR investigations on LCMO nanowires ground into powders were carried out in the 80–400 K temperature range. In the PM regime the EMR spectrum of LCMO nanowires consists of a single line with g ; 2 similar to that observed for the case of LCMO nanoparticles [10]. The line shape was found to be Lorentzian over the investigated temperature range (Fig. 1). It confirms the presence of the exchange-narrowed manganese dipolar fields [13]. In order to evaluate the EMR parameters, the derivative spectra were fitted with a Lorentzian

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Fig. 1. X-band EMR spectra at T " 100 K for La2/3Ca1/3MnO3 nanowires with a mean diameter of about 40 nm. The solid line represents the fit with a Lorentzian line shape.

EMR Investigation of CMR Nanowires

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line shape with fitting parameters being the half-width at the half-height @B1/2 of the corresponding absorption line and the resonance field B0.

3 Results and Discussion In what follows we present and discuss the temperature dependence of the EMR line width @B1/2 for LCMO nanowires. This dependence is shown in Fig. 2 in comparison with the characteristic one for LCMO nanoparticles which was previously reported [10]. The line width goes through a minimum at Tmin ; (1.1–1.2)Tc, where Tc is the magnetic transition temperature from the ferromagnetic metallic state to the paramagnetic insulator state. This behavior is a general feature already observed for bulk and nanosized CMR manganites [10, 14]. In accordance with the above empirical relation between Tmin and Tc, we have estimated a value of Tc ; 110–120 K for LCMO nanowires with d ; 40 nm. As can be seen from Fig. 2, the behavior of @B1/2(T) around Tmin indicates that the magnetic transition occurs in a larger temperature range for LCMO nanowires as compared with that corresponding to LCMO nanoparticles having Tc " 245 K and the mean diameter of 150 nm [10]. The observed variation of Tc as a function of the size reduction suggests that the Mn4!-to-Mn3! ratio is different for LCMO nanowires and nanoparticles. The decrease of Tc also indicates that the magnetic disorder on the surface increases as the size decreases. It could be the reason for the observed broader magnetic transition in LCMO nanowires (Fig. 2). In order to get more information about the magnetic behavior of nanowires, the magnetization measurements and the determination of Mn4! content by redox titration are planned.

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Fig. 2. Temperature dependences of the line width @B1/2 for LCMO: nanowires with d ; 40 nm (+), nanoparticles with d ; 150 nm (J) [10]. The solid lines are the best fit to Eq. (1) in the PM regime (T