Manganese Perovskite Nanoparticles and the Downturn of Inverse Susceptibility above the Curie Temperature

Abstract

In the La 0.75 Sr 0.25 MnO 3 nanoparticle system for hyperthermia a downturn in the inverse susceptibility above the Curie temperature T c was observed and interpreted in terms of a finite width of the T c distribution.PACS numbers: 75.75.Fk General informationThe magnetic study of the nanoparticles usually comprises the determination of the ZFC and FC susceptibilities which corresponds to the measurement after zero--field and field-cooled procedures, respectively. In the present work however we focus our attention on the region above T c , where in many bulk manganites a downturn of the inverse susceptibility was found and explained by the presence of a Griffith phase [1]. In studying the nanoparticles for hyperthermia we observed the same effect, for which the interpretation was suggested using the distribution of T c . Results and discussionThe SQUID magnetometer measurements were performed on the manganite La 0.75 Sr 0.25 MnO 3 in the form of the bulk sample (LSMOB) and two 20 nm nanoparticle samples LSMO and LSMO@SiO 2 , where the second one was encapsulated in SiO 2 [2]. The temperature dependence of the magnetization M under the applied field 7.95 kA/m was measured when cooling the sample from 400 to 300 K. Well above T c , M can be separated to a Curie-Weiss (C-W) part and a small contribution M imp arising from ferromagnetic impurities. The FC susceptibility χ g per unit of the weight and the main experimental result -the quantity d(1/χ g )/dT (Fig. 1a) was then evaluated from the corrected values M − M imp . In order to explain the observed downturn of 1/χ g , which manifests itself in a maximum of d(1/χ g )/ dT we assume a Gauss distribution function f (T c ) characterized by the average value T ca = 335 K and a given dispersion σ, which is approximately equal to the halfwidth of f (T c ). This assumption is realistic if we realize that there is a random distribution of the nanoparticle sizes and that T c depends on the nanoparticle diameter [3]. For a given T , T c , g-factor and spin S the reduced magnetization M r = M/M 0 (M 0 is the saturated value of M for T approaching zero) and susceptibility χ r = χ/M 0 can be calculated using the molecular field method. This procedure consists in solving the transcendental equationwhere λ = 3k B T c /(gµ B (S +1)) and B S (x) is the Brillouin function for spin S. Taking g = 2, S = 5/2 (this value approximately corresponds to the effective spin deduced from the experiments on the bulk samples) and σ as a parameter we determined M r and finally d(1/χ r )/ dT by integrating the product M r (T, T c ) over T c (Fig. 1b). Letting aside absolute values we shall compare the form of the curves displayed in Fig. 1a and b. For σ = 0.25 K the (792)

Griffiths-Like Phase in Nanocrystalline Manganite La_{0.85}Ca_{0.15}MnO_3

“…It consists in the simultaneous application of the magnetic susceptibility and electron spin resonance studies. The applied method follows that one proposed by Marysko et al [3,4].…”