Size Dependence of Charge Order and Magnetism in Sm0.35Ca0.65MnO3 – Magnetization and Electron Paramagnetic Resonance Studies

Abstract

We report a systematic tracking of the consequence of size reduction to nanoscale on charge order (CO) and magnetic properties of electron doped Sm0.35Ca0.65MnO3 by magnetization and electron paramagnetic resonance (EPR) measurements. The bulk form of this system is charge ordered below 275 K and antiferromagnetic (AFM) below 130 K. The bulk sample and nanoparticles of various sizes (mean diameter ~ 15, 30, 60, 90 nm) were prepared by sol-gel method. Magnetic and X-band EPR measurements were carried out at temperatures from 10-300 K. Our studies show that the highly robust CO in the bulk gets weakened by size reduction and the nanoparticles exhibit ferromagnetic (FM) ordering[1]. Magnetization in the paramagnetic region arising due to FM fluctuations caused by double exchange interaction is found to decrease as particle size decreases. However, at low temperature the trend of FM magnetization as a function of the size is found to be reversed as shown in Fig 1. Analysis of magnetic susceptibilities shows that magnetization in the FM phase is not due to the strengthening of double-exchange FM interactions but it is attributed to the dominance of surface effects. A detailed study on the EPR intensity, line width and g-factor reveals that in 15 nm particles though long range CO disappears, the signature of short range CO persists[2]. Line width variation indicating weakening of the AFM interactions and the development of the short-range FM phase are discussed based on Fig 2. As the particle size decreases the relative contribution of the surface ions increases. The prevalence of translational symmetry breaking, defects and surface imperfections lead to short range FM ordering in the system. We discuss the origin of these effects in terms of the presently existing models.