Gadolinium substitution effect on the thermomagnetic properties of Ni ferrite ferrofluids

• NiFe2O4, and NiFe1.88Gd0.12O4 were prepared by the self-combustion method.
• Two organic ferrofluids were prepared with the smallest nanoparticles of both samples (~50 nm).
• Increments in thermal conductivity are observed under low-intensity applied magnetic fields.
• Gd induces magnetocaloric effects in NiGd-ferrite ferrofluids.
• Thermal properties of NiGd ferrofluids can by optimized the by applying low magnetic fields.

This work is focused on the structural and magnetic characterization of Gd-doped Ni ferrite nanoparticles and the preparation of a ferrofluid for applications in heat-transfer devices. For this purpose, spinel ferrites NiFe2O4, and NiFe1.88Gd0.12O4 were prepared by the self-combustion method. The substituted sample was obtained with a small amount of Gd inclusion and the excess appeared as GdFeO3. The smallest nanoparticles of both samples were properly coated and dispersed in kerosene. Thermal conductivities of the produced ferrofluids were measured at 25 °C under an applied magnetic field. There is a significant enhancement in the thermal conductivity of the ferrofluid prepared with NiGd ferrite with respect to the one with Ni ferrite, in presence of a magnetic field. This effect is directly related to the well-known magnetocaloric effect of Gd.