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MAGNETIC SYSTEMS 2009Ferromagnetic domains in nanosized erbium perovskitesCooperative phenomena constitute an important researcharea because of many applications at the frontiers of chemistry,physics and electronics. Perovskites manganiteshave received growing attention because of mixed-valenceMn 4+ /Mn 3+ and resulting magnetoresistance effect. Wepreviously reported interesting results in Er(Co,Mn)O 3 dueto an antiferromagnetic interaction between Er and |Co,Mn|sublattices, producing a reversal of the magnetic momentand formation of ferromagnetic domains. An avalanchemechanism occurs under moderate magnetic fields (∼35 kOe), leading to the rotation of the domains in one ormore steps, depending on the rate of variation of the appliedfield and on the grains formation [Peña et al., J. Magn.Magn. Mater. 312, 78 (2007)].In order to investigate the influence of the micro-structurewe have elaborated nanosized materials and correlated themagnetic response to the grains size and to boundaries percolation.ErCo 0.50 Mn 0.50 O 3 compound was prepared at700 ◦ C by a citrate method and further calcined at increasingtemperatures. Samples were characterized by X-raydiffraction both before and after the sintering conditions,confirming the presence of a pure perovskite orthorhombicPbnm structure and phase purity was checked by energydispersive analysis. The micro-structure, characterizedby scanning electron microscopy, consists of homogeneousspherical grains of 20 − 30 nm diameter for as-preparedsample synthesized at 700 ◦ C, which progressively growwith increasing sintering temperature, attaining 100 − 200nm and a very good percolation at 950 ◦ C, that is, whengrains fuse together and grain barriers almost disappear.Magnetic measurements were performed on the startingmaterial and sintered pellets.temperatures of 950 ◦ C and above, that is, when grain barrierstend to disappear. We correlate this sudden jump withthe reorientation of ferromagnetic domains. At the sametime, the critical field H c decreases with increasing annealingtemperature suggesting that ferromagnetic domains rotatemore easily inside a large homogeneous grain since nobarriers are present.To confirm the reorientation phenomenon, we have performedsubsequent runs at 2 K, measuring the full M(H)loop. The starting bulk corresponds to a piece of the pelletsintered at 950 ◦ C. This piece was then crushed into finepowder (sieved at 80 µm) and let free to rotate under theaction of the applied field. In a third run, this same powderwas homogeneously dispersed inside a drop of liquefiedvacuum grease, then frozen at 2 K in order to blockany rotation of the fine particles, and performed a M(H)loop. Finally, the gel-type solution was heated at 350 K insidethe cryostat, and a 50 kOe field was applied. The gelwas then cooled under the same static field down to 2 K,with all domains oriented parallel to the applied field. Figure138 shows the full M(H) loop under these 4 differentexperimental set-ups.Figure 138: M(H)-loops measured under 4 different conditionsfor ErCo 0.50 Mn 0.50 O 3 sample calcined at 950 ◦ C.Figure 137: Positive part of magnetization loops at T = 2 K forgiven calcination temperatures (in ◦ C).Figure 137 presents part of the M(H)-loops for selectedpellets. Below an annealing temperature of about 950 ◦ C,the magnetization increases smoothly with increasing field,until an inflexion point occurs at about 40 kOe. This inflexionpoint transforms into a sudden jump at H c , for sinteringIt can immediately be noticed that the most significantchange in the magnetization loop concerns the height of thejump, which increases by about 60% in the oriented powderwith respect to the bulk ceramics. It is also evident that,when the particles are homogeneously dispersed and not allowedto rotate (run 3), the magnetization jump is smoothedout and becomes just an inflexion point. In conclusion, wehave unambiguously shown that grains size and percolationare the most important mechanisms in the rotation of magneticdomains in this material.A. B. AntunesC. M. Campos, O. Peña (Sciences Chimiques de Rennes,Université de Rennes 1, Rennes, France), G. Pecchi (Facultadde Ciencias Químicas, Universidad de Concepción, Concepción, Chile)94