Structural and magnetic characterization of Fe-Mo Oxide nanoparticles

Fe-Mo oxides with different Fe:Mo ratio x (0.5 to 1.5) were synthesized by aerogel process. As microns-diameter spherical shells comprising of nanoscaled needle-like grains, XRD patterns correspond to a beta-FeMoO$_{4}$ structure independent of x. SQUID measurements on two selected samples with x = 1.0 and 1.5, respectively, reveal a broad transition to antiferromagnetism at low temperatures. In addition, a slowly decreasing magnetic susceptibility with decreasing temperature below approximately 100 K for x = 1.0, but not for x = 1.5, presumably reflects a beta- to alpha-FeMoO$_{4}$ phase transition. After being annealed at 500$^{\circ}$C for 2 hr, both samples transform to a Fe$_{2}$(MoO$_{4})_{3}$ structure. The antiferromagnetic ordering becomes well defined at T$_{N}$ = 13 K. A secondary anomaly near 5 K likely arises from spin reorientation in the magnetic frustrated systems. These observations are corroborated by calorimetric and M\"{o}ssbauer measurements. Specific heat data also yield lattice softening and the associated entropy change. \newline *On leave from Barton County Community College, Great Bend, Kansas. \newline **Present address: Indraprastha University, Delhi, India