Giant Magnetocaloric Effect in Cr- and Fe-Substituted Frustrated Magnet Gadolinium Gallium Garnets, Gd ₃ Ga ₅ O ₁₂ , for Magnetic Cooling at Cryogenic Temperatures

The beneficial effects of Cr and Fe substitutions in enhancing the magnetocaloric properties of the frustrated magnetic system gadolinium gallium garnets Gd₃Ga₅O₁₂, i.e., Gd₃CrGa₄O₁₂ and Gd₃FeGa₄O₁₂, are discussed for their potential use as magnetic refrigerant materials. Evaluations of their structural, electronic, magnetic, and thermal properties were carried out to investigate the effect of magnetic ion substitution in the frustrated magnetic lattice and its magnetocaloric properties. For both Cr and Fe substitutions, the cubic <i>Ia</i>3̅<i>d</i> structure remains preserved, and antiferromagnetic correlations are observed with very negligible magnetic hysteresis at low temperatures. A maximum isothermal magnetic entropy change, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mo>-</mml:mo><mml:mi>Δ</mml:mi><mml:msubsup><mml:mrow><mml:mi>S</mml:mi></mml:mrow><mml:mrow><mml:mi>m</mml:mi></mml:mrow><mml:mrow><mml:mrow><mml:mi>max</mml:mi></mml:mrow></mml:mrow></mml:msubsup></mml:mrow><mml:mo>∼</mml:mo><mml:mn>49.60</mml:mn><mml:mspace></mml:mspace><mml:mi>J</mml:mi><mml:mo>/</mml:mo><mml:mi>kg</mml:mi><mml:mspace></mml:mspace><mml:mi>K</mml:mi></mml:math>, adiabatic temperature change, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>Δ</mml:mi><mml:msubsup><mml:mrow><mml:mi>T</mml:mi></mml:mrow><mml:mrow><mml:mrow><mml:mi>ad</mml:mi></mml:mrow></mml:mrow><mml:mrow><mml:mrow><mml:mi>max</mml:mi></mml:mrow></mml:mrow></mml:msubsup></mml:mrow><mml:mo>∼</mml:mo><mml:mn>14.1</mml:mn><mml:mspace></mml:mspace><mml:mi>K</mml:mi></mml:math>, and relative cooling power, RCP^max ∼ 645.53 J/kg for Gd₃CrGa₄O₁₂ and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mo>-</mml:mo><mml:mi>Δ</mml:mi><mml:msubsup><mml:mrow><mml:mi>S</mml:mi></mml:mrow><mml:mrow><mml:mi>m</mml:mi></mml:mrow><mml:mrow><mml:mrow><mml:mi>max</mml:mi></mml:mrow></mml:mrow></mml:msubsup></mml:mrow><mml:mo>∼</mml:mo><mml:mn>35.3</mml:mn><mml:mspace></mml:mspace><mml:mi>J</mml:mi><mml:mo>/</mml:mo><mml:mi>kg</mml:mi><mml:mspace></mml:mspace><mml:mi>K</mml:mi></mml:math>, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>Δ</mml:mi><mml:msubsup><mml:mrow><mml:mi>T</mml:mi></mml:mrow><mml:mrow><mml:mrow><mml:mi>ad</mml:mi></mml:mrow></mml:mrow><mml:mrow><mml:mrow><mml:mi>max</mml:mi></mml:mrow></mml:mrow></mml:msubsup></mml:mrow><mml:mo>∼</mml:mo><mml:mn>12.1</mml:mn><mml:mspace></mml:mspace><mml:mi>K</mml:mi></mml:math>, and RCP^max ∼ 549.72 J/kg for Gd₃CrGa₄O₁₂ systems are observed, respectively, under a magnetic field change of 9 T. This significant enhancement in the magnetic entropy of the Cr substituted system could be attributed to the changing nature of the magnetic ground state and the considerable change in magnetic frustration that enhances the Gd spin loop. The presence of giant magnetocaloric parameters makes both systems promising competitors in the field of magnetic refrigeration technology for cooling applications at cryogenic temperatures.