Role of Superparamagnetic Nanoparticles in Humidity Sensing Behavior of Holmium doped Manganese-Bismuth ferrites for Relative Humidity Sensor applications

Rare earth-doped ferrites are exhibiting remarkable surface phenomena. Wherein the process of adsorption is most significant. Humidity sensors are the realization of an adsorption mechanism. Therefore, rare earth-doped ferrites are suitable for humidity sensor applications. Considering this fact, in the present work, Holmium(Ho3+) Mn-Bi ferrites are prepared by the solution combustion method with the general formula Mn0.95Bi0.05Fe2-xHoxO4 (x = 0 to 0.03). The effect of holmium doping on the structural, microstructural, magnetic, and humidity properties was analyzed. The X-ray diffraction revealed the formation of a cubic spinel structure. The average crystallite size was obtained by using the Scherrer method, with a range of 7 to 13 nm. The presence of nanoparticles in samples increases the surface area required to favor adsorption mechanisms in the samples. The Scanning Electron Microscopy micrographs showed the surface morphology with the presence of pores, holes, and on their surfaces. Vibrating sample magnetometry confirmed superparamagnetic behavior. The saturation magnetization (M-s) was found to decrease with an increase of Ho+3 content in the samples. We observed the decrements of resistance with the increment of relative humidity from 11 to 97% relative humidity and the increase of sensing response with an increase in relative humidity. At 54% relative humidity, maximum humidity hysteresis is found to be 2%. The sensing response time and recovery time were recorded to be 79 s and 91 s, respectively. The difference between sensing response and recovery time is small. These kinds of samples may be used in applications of sensing material.