Research On Some Related Magnetic Effects In Magnetic-Phase-transition Alloys

Magnetic alloys undergo a phase transition under the magnetic field which accompanies with magnetocaloric and magnetostriction/magnetostrain effects and so on.As multifunctional materials,they have wide applications in many fields,such as magnetic refrigeration,sensor and transducer.However,some problems such as narrow phase-transition temperature region and poor mechanical properties have hinded their applications.In this thesis,we investigate the magnetostriction/magnetostrain and magnetocaloric effects in three kinds of magnetic-phase-transition alloys.Their performances are remarkably improved by bonding and orientation,high-magnetic-field solidification and electric field manipulation.1.The magnetostrain effects in Mn0.96sCoGe alloy by bonding and orientationThe magnetic-phase-transition materials based on transition elements,MnM,X(M'=Co,Ni,X=Si,Ge)alloys,exhibit many interesting physics phenomena including magnetocaloric,magnetoresistance and negative thermal expansion effects around phase transition temperature.At the same time,these materials have the advantages of low cost and simple preparation process,which make them become the hot spots in the research field of magnetic-phase-transition alloys.As a member of MnM,X family,stoichiometric MnCoGe alloy undergoes a first-order martensitic transformation from a hexagonal Ni2In-type structure to an orthorhombic TiNiSi-type structure at 650 K,accompanied with a giant negative thermal expansion effect.However,there are few reports on magnetostrain effects in this material due to the decoupling between magnetic phase transition and structure transformation,which makes it difficult to be driven by a magnetic field.In addition,this alloy naturally collapses into powders owing to its drastic martensitic transformation,and is difficult to be oriented,which hinder their applications for magnetostrain.To slove these problems,firstly,some vacancies of Mn element are introduced in this alloy and as a result,a magnetic-field driven martensitic transformation is observed in Mn0.965CoGe alloy,which is ascribed to the change of Mn-Mn distance and structural stability between the martensitic phase and the austenitic phase.Secondly,the alloy powders are bonded with epoxy resin and aligned in a magnetic field,which result in a textureded Mno.965CoGe alloy.The measurement results demonstrate that the field-aligned Mn0.965CoGe alloy shows a large magnetostrain effect near room temperature.The maximal value of magnetostrain reachs up to 925 ppm at 270 K.2.The magnetostrictive effects of MnCoSi-based alloys prepared by high-magnetic-field solidificationIn recent years,the development of large and reversible magnetostrictive materials at room temperature becomes one of the research hotspots in the field of magnetic materials.MnCoSi alloy is synthesized by relatively low-cost transition elements and main group elements.With the application of magnetic field,this alloy would undergo a metamagnetic transition from an antiferromagnetic phase to a ferromagnetic phase,accompanying with a large lattice distortion,which suggests that it is a promising magnetostriction material.However,from the practical application point of view,there are server problems in this material,such as large critical magnetic field,poor orientation and poor mechanical property,which seriously hinder their applications for magnetostrition.By elements substitution or introducing vacancies of Mn element,the tricritical points of MnCoSi-based alloys are adjusted below room temperature.Meanwhile,the critical magnetic fields which driving metamagnetic transition are tuned below 1 T.By applying high-magnetic-field solidification,the textured and dense samples are obtained.At room temperature,large low-field reversible magnetostriction effects are observed.Especially for Mno.ssCoSi alloy,its magnetostrictive value reaches up to 1160 ppm under the magnetic field of 1 T at 300 K,which is comparable to some RFe2-based alloys.We hope that this study can provide the experimental evidence for widening the applications of magnetic-phase-transition alloys and high magnetic field.3.Electric field tuning of magnetocaloric effect in FeRh0.96Pd0.04/PMN-PT heterostructureFeRh alloy has been extensively studied due to its excellent performance in magnetic response during antiferromagnetic-ferromagnetic transition.As a giant magnetocaloric material,stoichiometric FeRh alloy has some problems as follows:(1)the metamagnetic transition temperature is far away from room temperature,(2)the refrigerant temperature region is narrow.These drawbacks largely hinder their application as magnetic refrigeration.In this study,by doping a small amount of Pd element,the phase transition temperature of FeRh is tuned to room temperature.Then a FeRh0.96Pd0.04(FRP)/PMN-PT heterostructure is prepared by magnetron sputtering.An electric field of 8 kV/cm applying on the PMN-PT substrate would generate strain which then transfers to the FeRh0.96Pd0.04 film,leading to the shift of transition temperature from 300 K to 325 K.As a result,the refrigeration temperature region of FRP film is broadened from 35 K to 47 K.These results show that the working temperature region of FRP film can be effectively broadened by external stimuli.