Structure, Magnetic Properties And Magnetocaloric Effect Of Gd-Mn-M Melt-spun Alloys And Gd-Si-Ge-Ni Alloys

Compared with crystalline materials, amorphous alloys exhibit many superior properties,such as high electric resistance and low eddy current loss, good corrosion resistance andmechanical properties. As magnetic refrigeration materials, especially amorphous Gd-basedalloys, amorphous alloys possess interesting features: tuneable magnetic ordering temperature,large transition temperature range and no obvious thermal and/or magnetic hysteresis, whichare suitable for magnetic Ericsson cycle. Therefore, amorphous materials have been receivedgreat attention in the field of magnetic refrigeration.In the work, firstly, Gd-Mn based alloy ribbons with dopant Ge and Si elements wereprepared by using melt-spinning technique, and their structures, magnetic properties andmagnetocaloric effect were investigated. The results show that the crystalline phases（α-Gd+GdMn₂） are observed in Gd-Mn alloy without the third element, which indicatesthat the glass-forming ability of binary eutectic Gd-Mn alloy is not good. With theaddition of Ge and Si, the typical amorphous structures are obtained in the ternaryGd-Mn-M （M=Ge, Si） alloys, the glass-forming ability is improved obviously. Withincreasing the temperature, Gd₆₅Mn₃₀Ge₅, Gd₆₅Mn₂₅Ge₁₀, Gd₆₅Mn₃₀Ge₅and Gd₆₅Mn₂₅Si₁₀glassy ribbons undergo a second-order transition from ferromagnetic to paramagnetic around212,229,221and218K under a magnetic filed of0.05T, respectively. Under a magnetic fieldchange of0-5T, the maximum of magnetic entropy change values of those alloys are not large,but the refrigeration capacity （RC） values are625,615,625and660J/kg, respectively. That isvery suitable for the application of room temperature magnetic refrigeration.The critical behavior of Gd₆₅Mn₃₅Ge_x（x=5,10） amorphous ribbons in the vicinity oftransition temperature were analysed by Kouvel-Fisher method. The fitted critical exponentsare checked to be effective and accurate by means of the scaling analysis. The critical valuesmeans that a short-range magnetic coupling plays dominant roles in Gd₆₅Mn₃₅Ge_x（x=5,10）amorphous ribbons, and the change of value of β presents change of magnetic orderprimely.The crystallization of Gd₆₅Mn₂₅Si₁₀amorphous ribbons was performed. With theincrease of crystallization temperature, α-Gd, GdMn₂and Gd₅Si₃phases are precipitated in turn. Similarly, the Curie temperature and magnetic entropy change of the alloys are lowerthan those of pure Gd, owing to these impurity phases. However, the magnetic transitions ofGdMn₂and Gd₅Si₃are not detectable.Furthermore, the influences of Ni on structures, magnetocaloric effect and hysteresiseffect of Gd₅Si_2.05Ge_1.95alloy by vacuum arc-melting method were investigated. The dopingof Ni could suppress the formation of the orthorhombic Gd₅Si₂Ge₂phase to some extent. TheCurie temperature of Gd₅Si_2.01Ge_1.91Ni_0.08alloy is300K, and no obvious thermal or magnetichysteresis is observed. However, Gd₅Si₂Ge_1.9Ni_0.1alloy undergoes a field-induced first-ordercrystallographic phase change around295K, and a second-order transition occurres around301K. The thermal hysteresis is about6¹3K, and the maximum magnetic hysteretic loss isabout14J/kg. The peak of magnetic entropy variation of Gd₅Si_2.05-xGe_1.95-xNi_2x（2x=0.08,0.1）are8.0and9.1Jkg-1K-1for applied-field changes from0to5T, respectively. Low hystereticloss and large magnetocaloric effect make the alloys as a potential magnetic refrigerant atroom temperature.