Silica-Coated and Bare Akaganeite Nanorods: Structural and Magnetic Properties

We report on structural and magnetic properties of uniform silica-coated akaganeite nanorods with length of L similar to 80 +/- 15 nm and diameter D similar to 15 +/- 5 nm as well as silica shell thickness of about 5 nm. Unexpected negative difference between field-cooled (FC) and zero-field-cooled (ZFC) magnetization Delta M = M-FC - M-ZFC < 0, room temperature ferromagnetism, and exchange bias effect have been found. The nanorods are investigated by X-ray powder diffraction (XRPD), transmission electron microscopy (TEM), and vibrating sample magnetometer (VSM) measurements. The magnetic measurements were also performed on bare akaganeite nanorods in order to discriminate the effects of silica coating on the magnetic properties. The measured coercivity and exchange bias effect of bare beta-FeOOH nanorods are much lower compared with same properties of SiO2@beta-FeOOH nanorods, emphasizing the effect of silica coating on the magnetic properties. These results are discussed considering the core shell structure of akaganeite nanorods; i.e., the inner part of the akaganeite nanorod has antiferromagnetic ordering, whereas the nanorod surface exhibits some disorder spin state.