Crystal Structure and Magnetic properties of Fe-substituted nanoscale Hydroxyapatite

Magnetic nanoscale hydroxyapatite (HAp) of chemical formula Ca$_{(5-x)}$Fe$_{x}$(PO$_{4})_{3}$OH has been prepared by a chemical precipitation method where $x$ varies from 0 to 1.26. Single phase HAp is identified in XRD patterns of samples with $x \quad \le $ 0.30 while maghemite (Fe$_{2}$O$_{3})_{ }$is formed as a secondary phase for $x \quad \ge $ 0.60. The average crystallite size as calculated by the Scherrer equation varies from 16 nm to 28 nm. Rietveld refinement reveals a decrease of the unit cell for $x \quad \le $ 0.15. Magnetic moment measurements as a function of temperature at applied field \textit{$\mu $}$_{0}H$ = 1.5 T shows a two component system: a temperature-dependent paramagnet (PM) or superparamagnet (SPM) and a roughly temperature-independent ferromagnetic (FM) component. No FM activity is shown for low $x$, followed by increased activity for higher $x$. Increasing SPM activity is observed for $x\le $0.60. Hysteresis measurements show irreversible loops for $x \quad \ge $ 0.22.