Advancing humidity sensing and magnetocaloric properties of spinel structural CoCr2O4 nanoparticles achieved via innovative bismuth doping by combustion synthesis

Abstract

This study explores the influence of Bi-doping on Co_1-xBi_xCr₂O₄ (x = 0–0.2) nanoparticles synthesized via the solution combustion method, focusing on humidity sensing and magnetocaloric effects. The investigation reveals two magnetic transitions: the Curie temperature (T_C) marks the paramagnetic to ferrimagnetic shift, while the spiral transition temperature (T_S) indicates a spiral spin order transition. Magnetization measurements demonstrate that −ΔS_M and relative cooling power (RCP) values vary with Bi concentration, making these nanoparticles viable for magnetic refrigeration above liquid nitrogen temperatures. Analyzing magnetic entropy variation, the modified Arrott plots and Kouvel-Fisher approach affirm second-order phase transitions. The sensing response exhibits growth alongside relative humidity (RH) and Bi concentration, culminating in an impressive ∼97.56 % sensing response for the 20 % Bi-doped sample. This heightened humidity sensing performance with increased Bi content can be attributed to synergistic effects. These results highlight the potential of 20 % Bi-doped Co_1-xBi_xCr₂O₄ nanoparticles as promising contenders for enduring and practical humidity sensing applications.