Predicting a major role of surface spins in the magnetic properties of ferrite nanoparticles

Room‐temperature magnetization hysterisis measurements were conducted on Mn 0.5 Zn 0.5 Gd x Fe (2‐x) O 4 ferrite nanoparticles, with x = 0, 0.5, 1.0, 1.5. The structure of this ferrite is normal spinel where the added of Gd 3+ ions occupied the octahedral sites and replaces Fe 3+ ions. The saturation magnetization was found to increase with the initial addition of the Gd 3+ ions followed by a sharp decrease with further addition of Gd 3+ ions. The Curie temperature was found to increase up to Gd 3+ concentration of x = 1.0, and then decreases at x = 1.5. These results were attributed to the surface spins. Because the size of Gd 3+ ions is larger than that of Fe 3+ ions, the substitution of Fe 3+ ions with the Gd 3+ ions results in surface disorder which results in surface spins. A core‐shell magnetization model was introduced where several factors were combined to explain our results. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)