Effects of crystallite size on magnetic properties of thermal-hydro decomposition prepared La1−xSrxMnO3 nanocrystalline powders

The effects of crystallite size (D) on the two magnetic parameters, saturation magnetization (MS) and coercivity (HC), of nanocrystalline powders of La1−xSrxMnO3 (LSMO) are investigated. LSMO samples with 0.0 ≤ x ≤ 0.5 are synthesized by a thermal-hydro decomposition method at different temperatures (TD) between 600 and 1000 °C, and are then characterized with XRD and VSM techniques at room temperature. The approximate crystallite sizes are in the range 11–45 nm, which increase with increasing TD but decrease with increasing x. Soft ferromagnetic behavior is found in all Sr-doped samples. The MS values for each Sr concentration (0.2 ≤ x ≤ 0.5) strongly depends on D, suggesting the presence of magnetically dead layers at the surface. The thickness of this magnetically dead layer (tm) is reduced by an increase in TD and x (only x ≤ 0.3), and shows the lowest value in the LS3M sample (x = 0.3) for all TD. The MS increase with increasing TD or D corresponds to a decrease in tm. From analysis of the case for a TD of 900 °C, the variation in MS is not only controlled by tm but also by the average ionic radius size in A-site (〈rA〉〈rA〉), and the size mismatch factor of the A-site cations (σ2). The enhancement in the MS value at 〈rA〉〈rA〉 ≤ 1.244 Å (x ≤ 0.3) can be explained by the continuous decrease in tm and the increase in x   and 〈rA〉〈rA〉, including the variations in dMn–O and θMn–O–Mn in a direction to encourage FM coupling via double exchange interaction. Whereas the reduction in MS values for 〈rA〉〈rA〉 > 1.224 Å (x ≥ 0.4) is probably due to the higher tm and σ2. The HC values of all LSMO samples strongly vary with D. The single magnetic domain is present for all samples with D lower than 33 nm, while some LSMO samples with D higher than 40 nm contain multi domains. The range of D of 33–40 nm is the possible range in which domain wall formation begins.

The plot of the thickness (tm) and the ratio of magnetically dead layer (%tm) varying in term of D−1.653 versus crystallite size of the La0.7Sr0.3MnO3 (LS3M) nanocrystalline in the range of 10–45 nm. The tm decreases with increasing of D but slightly increases for the sample prepared at 1200 °C. It is also seen that the ratio of tm (%tm) existed in a particle for each sample decreases as D increases in term of D−1.653 as illustrated with good curve fitting in the figure. This result confirms that the magnetically dead layer for LS3M tends to diminish when D is large or in bulk sample form.Figure optionsDownload as PowerPoint slide