Inverse and conventional dual magnetocaloric effects in Ni substituted Y-type Sr2Zn2-xNixFe12O22 hexaferrites

The effects of Ni substitution on magnetic and magnetocaloric effect (MCE) are investigated in polycrystalline Y-type hexaferrites of Sr₂Zn_2-xNi_xFe₁₂O₂₂ (x = 0.0, 0.8, and 2.0). With the increasing of temperature, the M-T curves indicate successive magnetic structure transitions exist in Sr₂Zn_2-xNi_xFe₁₂O₂₂ (x = 0.0, 0.8, and 2.0), which play significant roles in MCE behaviors. As the Ni²⁺ doping ratio increases, the shape of $-\Delta S_M-T$ curves near room temperature evolves gradually from a table-like to a peak-like form. Particularly, a significant contrast between CMCE and IMCE is observed below 100 K, whose behavior can be inherently exploited for heat sink in magnetic refrigeration applications. Among the samples in this series, Sr₂Zn_1.2Ni_0.8Fe₁₂O₂₂ plays the best CMCE and IMCE performances, with the maximum magnetic entropy change ($-\Delta S_M^{\max }$) values of 0.78 J/kg K at 354 K and −0.56 J/kg K at 16 K for $\Delta H=50kOe$ , respectively. Our work demonstrates that Sr₂Zn_2-xNi_xFe₁₂O₂₂ hexaferrites have great potential to be tailored for magnetic refrigeration with special needs such as different operating temperature zones, Ericsson cycle or heat sink at refrigeration.