The structural, magnetic and magnetic entropy changes on CoFe₂O₄/CoFe₂ composites for magnetic refrigeration application

Highlights: •CoFe₂O₄/CoFe₂ composites synthesized through thermal reduction method. •Different composition of CoFe₂ and CoFe₂O₄ phases were obtained. •The composite with high fraction of CoFe₂ show magnetic entropy change near RT. •An appreciable magnetic entropy change and relative cooling power were achieved. - Abstract: In this article, the magnetocaloric effect on thermally reduced CoFe₂O₄/CoFe₂ composite was reported. Co-precipitated CoFe₂O₄ nanoparticles were thermally reduced at 500 and 800 °C for 2 h under H₂ atmosphere. The reduction process resulted in different fractions of soft and hard (CoFe₂O₄/CoFe₂) magnetic composites and the products were characterized through various procedures. The structure of CoFe₂O₄/CoFe₂ composites was found to be cubic with an average crystallite size of 31 and 30 nm for the samples reduced at 500 and 800 °C, respectively. The stoichiometry of the composites confirmed that the percentage of soft CoFe₂ magnetic phase was increased to 87% with temperature up to 800 °C for 2 h, besides showed accumulation of nanoparticles. The room temperature saturation magnetization of the CoFe₂O₄/CoFe₂ composites increases from 184 to 190 emu/g as a consequence of high concentration of CoFe₂ phase. Conversely, the sample reduced at 800 °C attained the maximum magnetization of 200 and 221 emu/g at 300 and 5 K, respectively. Field cooled (FC) and zero field cooled magnetization (ZFC) and the isothermal magnetic entropy change $\left(\Delta S_m\right)$ on the CoFe₂O₄/CoFe₂ composite (with 87% of soft CoFe₂ magnetic phase) were estimated. The composite showed the interesting positive and negative magnetic entropy change $\left(\Delta S_m\right)$ of 0.923 and −0.923 J/kg.K⁻¹ near room temperature at around 310 and 290 K, respectively.