Influence of rare earth (RE = Y, La, Gd, Ho) substitution on the structural, magnetic and rheological properties of magnetite based ferrofluids

In the present study, magnetite based ferrofluids having the composition Fe _3-xRE _xO ₄ (x = 0.0, 0.06) (RE = Y, La, Gd, Ho) have been prepared using chemical co-precipitation method to study the role of different rare-earth (RE) substitution on the various structural, microstructural, magnetic and rheological properties. X-ray diffraction (XRD) study confirm the spinel structure of the ferrite, which is further supported by fourier transform infrared spectroscopy (FTIR) study. The crystallite sizes of the samples are calculated from the XRD data using Scherrer formula, and found to be in the range 8 nm to 14 nm which is in agreement with the particle size obtained from transmission electron microscope (TEM) study. Dynamic light scattering (DLS) study shows the hydrodynamic particle size in the range 42 nm to 68 nm which is comparatively higher than those obtained from XRD, indicating the presence of aggregates in the ferrofluid samples. Vibrating sample magnetometer (VSM) measurement shows superparamagnetic behaviour for the investigated samples which is desirable for biomedical applications. Various parameters like resonant field (H _rf), g-value, peak-to peak linewidth (ΔH _pp) and spin–spin relaxation time (T ₂) are calculated from the electron spin resonance spectroscopy (ESR) studies. It is observed that ΔH _pp decreased with RE substitution indicating improvement in magnetic homogeneity, while the spin–spin relaxation time (T ₂) is found to be of the order of picoseconds which points towards their suitability for magnetic resonance applications. Rheological measurements have been performed under a shear rate flow at varying magnetic field to investigate the flow properties of the ferrofluid samples. Shear thinning behaviour with large yield stress is observed in the non-Newtonian fluids.