Study On Phase Transition Regulation And Enhancement Of Magnetocaloric Effect In Gd₂TMMnO₆ （TM=Co,Ni） Double Perovskite

The design and development of refrigeration materials is one of the key research directions of magnetic refrigeration technology.Gd base double perovskite Gd2TMMnO6 exhibits rich physical properties due to the coexistence and competition of various internal magnetic interactions.Morever,because it contains a large number of unpaired 3d and 4f electrons,Gd2TMMnO6 has a large total angular momemtum quantum number,which is expected to be one of the candidated for magnetic refrigeration materials.In this paper,relevant experimental studies on phase transition regulation of Gd2TMMnO6(TM=Co,Ni)and enhancement of magnetocaloric effect are carried out.The main research contents and specific research results are summarized as follows:(1)Gd2TMMnO6(TM=Co,Ni)samples were successfully synthesized by solid phase reaction method.Magnetic measurement showed that Gd2CoMnO6 and Gd2NiMnO6 exhibit magnetic phase transition at 111 K and 128 K,respectively,which were attributed to ferromagnetic superexchange interactions of Co-Mn and Ni-Mn,respectively.Due to the interaction of Gd with Co/Mn,an obvious antiferromagnetic transition can be observed in Gd2CoMnO6 near 42 K,while Gd2NiMnO6 exhibits magnetic order at low temperature(20 K),which may be related to lattice distortion caused by the difference of Co and Ni ion radii and the antipotential disorder caused by ion mixing valence.In addition,Gd2CoMnO6 and Gd2NiMnO6 at 4 K and 6 T were 16.6 J·kg-1·K-1(23.3 J·kg-1·K-1).And by comparative studies,the Gd2NiMnO6 in negligible hysteresis loss and the relatively low temperature of Gd magnetic ordering induce and drive better magnetocaloric effect.(2)The doping of Gd2CoMnO6 was investigated using non-magnetic Ti4+.Magnetic measurements showed that the temperature of the ferromagnetic phase transition due to Co-Mn interaction in Gd2CoMn1-xTixO6(0 ≤x ≤1)gradually decreases with increasing Ti content,indicating that Ti doping had significant inhibitory effect on this phase transition.Besides,the-ΔSMmax increases monotonically from 16.6 J·kg-1·K-1 at x=0 to 27.2 J·kg-1·K-1 at x=1,indicated that the Ti4+ doping significantly enhances the magnetocaloric effect of this system.It is concluded from the analysis that the dilution of nonmagnetic Ti4+ leads to weaker couping between magnetic ions,which in turn is more easily controlled by the external magnetic field,and the doping also leads to the onset of low temperature phase transition to the paramagnetic phase instead of the ferromagnetic phase,thus causing more significant magnetic order changes,and thus magnetocaloric becomes stronger with increasing doping amount.(3)Gd2NiMn1-xTixO6(0 ≤x ≤1)series monoclinic double perovskite samples were successfully synthesized by solid-phase reaction method.The magnetic measurement showed that the ferromagnetic phase transition temperature of Gd2NiMn1-xTixO6(0≤x ≤1)exhibited trend consistent with that of Gd2CoMn1-xTixO6,it restated indicate that Ti4+plays an effective role in suppressing the ferromagnetic phase transition in this system.The-ΔSMmax of the samples first decreasing monotonically from 23.3 J·kg-1·K-1 at x=0 to 13.70 J·kg-1·K-1 at x=0.4,and then monotonically increasing to 27.6 J·kg-1·K-1 at x=1.Combining the M-T curves and comparing the results of Gd2CoMn1-xTixO6,this non-monotonic variation of the magnetocaloric effect in Gd2NiMn1-xTixO6 was correlated with the M-T hump(antiferromagnetic interaction of Gd with Ni/Mn)presented by the amount of intermediate doping(x=0.4-0.6).