The Regulation Of Magnetic Transition And Investigation Of Magnetic Properties In Ni-Co-Mn-Ti Alloys

Magnetic refrigeration technology,which is efficient,energy-saving and environmentally friendly,is considered as the most promising refrigeration technique to replace the traditional gas compression refrigeration.With the swift and violent development of electronics technology,the components of devices trend to be miniaturization.Miniature refrigeration has been a research hotspot in magnetic refrigeration.The key question that miniature refrigeration must solve is to find solid refrigerants that can be used in its miniaturization.Present studies mainly focus on the ribbons and films.This is because ribbons and films have bigger ratio for surface to volume which is beneficial to the heat exchange of samples.Moreover,ribbons and films usually need shorter time in heat treatment for forming stable structure,sometime even completely avoiding the heat treatment,which can greatly reduce the energy losses and cost.NiMn-based alloys have attracted more and more interests for its multifunctional properties near the temperature of martensitic transformation.This paper studied the preparations,transformations and related physical properties of Ni-Co-Mn-Ti alloy ribbons and films which completely consist of transition metals by substituting Co for Ni in this system.The effects of heat treatment on ribbons' magnetic properties and depositional temperature on films have been studied.The films' exchange bias is also investigated.The main results are as follows: 1.The microstructure and magnetic properties in Ni₃₇Co₁₃Mn₃₅Ti₁₅ ribbons.The introduction of Ti in the binary NiMn alloys can acquire Ni₅₀Mn_50-xTi_x and Mn₅₀Ni_50-yTi_y.In the two systems,the increase in the content of Ti not only stabilizes the austenite but also can decrease the temperature of martensite transition obviously.Ni₅₀Mn₃₅Ti₁₅ alloys are 5M structure at room temperature?RT?.Here,we introduce Co into Ni₅₀Mn₃₅Ti₁₅ for obtaining Ni₃₇Co₁₃Mn₃₅Ti₁₅ alloys and studying its regulation of transition and magnetocaloric properties.Ni₃₇Co₁₃Mn₃₅Ti₁₅ alloy ribbons are prepared by melt-spun after electric arc melting.Some of ribbons were taken out to engage in the high temperature annealing.The X-ray diffraction?XRD?shows that melt-spun ribbons crystallize predominately the B2 structure and annealed ribbons reveal principally 5M structure at RT.The field emission scanning electron microscope?FESEM?observations depict that the crystals of melt-spun and annealed ribbons are columnar and grow perpendicular to the surface.The annealed ribbons have bigger grains.And it is can be seen that big grains are synthesised by the aggregation of little grains.The thermodynamic and magnetic measuring demonstrate that heat treatment makes the transformation temperature increase from 200 K to 325 K.Their enthalpy changes are 7.5 Jg-1 and 7.9 Jg-1,maximum magnetic entropy changes are 10.7 Jkg-1K-1? 23.8 Jkg-1K-1 under 0-5 T field change.They also have high effective cooling power with 101.1 Jkg-1 and 70.8 Jkg-1 under 0-5 T field change,respectively.2.The preparation and magnetic properties of Ni-Co-Mn-Ti films.In this work,four Ni-Co-Mn-Ti alloy films under different deposition temperature?DT?were prepared by DC magnetron sputtering technique.Their structures,microstructures,compositions and magnetic properties are characterized by XRD,FESEM and vibrating sample magnetometer?VSM?,respectively.We find the DT has no effect on the compositions and thicknesses of films.But it can change films' microstructures and magnetic properties.The higher DT will lead to bigger grain size.And it can be proved that high DT can reduce the limitation of substrate to film in XRD results.The coercivity and remanence increase with the improvement of DT too.In addition,the resistance measurement suggests the transformation can be achieved on low field which is more forceful than the magnetic measuring.All these predict this kind of alloy film has great potential on the practical application of magnetic film materials.3.The exchange bias in Ni-Co-Mn-Ti alloy filmsIn this part,two Ni-Co-Mn-Ti alloy films were fabricated in different depositive parameter.The exchange bias was achieved in this system at the first time that we explored it as follows.All the grain sizes of films are very uniform which can be observed in the FESEM data.The enhancement of deposition time can not only increase the thickness of films but change the shape of particles to be flake.The grain size is enlarged too.Atomic force microscope?AFM?observations demonstrated that the films grew in the pattern of island.And the improvement of DT will decrease the roughness of films.Hysteresis loops show that both films have positive exchange bias.It means that the ferromagnetice?FM?and antiferromagnetice phase coexist in the films.