The Regulation Of Magnetic Transition And Magnetocaloric Properties In Magnetic Alloys

Magnetic transition alloys possess many interesting physical phenomena, including magnetocaloric effect, magnetoresistance, and magnetostriction owning to the abrupt changes of magnetization and resistivity around the transition temperature. As multifunctional materials, these alloys have important application in many fields. In the present paper, we studied the regulation of magnetic transition and related magnetocaloric properties in three kinds of magnetic alloys. The main results are as follows:1. The regulation of magnetic transition and related magnetocaloric properties in Ni-Mn-Co-Sn ferromagnetic shape memory alloy.Ni-Mn-based ferromagnetic shape memory alloys (FSMAs) are a kind of new multifunctional materials, which have been developing in recent years. They show thermoelastic and shape memory effects simultaneously, which can be controlled by temperature and magnetic field respectively. In the Ni-Mn-X (X=In, Sn, Sb) FSMAs, they undergo a ferromagnetic transition from ferromagnetic austenite to paramagnetic one and a martensitic transformation from ferromagnetic austenite to weak magnetic martensite on cooling. Around their transition temperatures, abundant physical properties are achived due to the abrupt change of magnetization and resistivity. In order to tune the transitions of FSMAs and investigate their physical properties around the transition temperatures, we prepare the Ni-Mn-Co-Sn ribbons and deal with them by pre-deformation, annealing, and high pressure annealing. The main results of the effect of these external factors on the ribbons are listed as below:(â…°) In the pre-deformed Ni-Mn-Co-Sn ribbons, the martensitic transformation temperature increase while the Curie temperature of austenite keeps almost unchanged. Though the magnetic entropy change of the pre-deformed ribbons around martensitic transformation decreases slightly, the refrigerant capacity (RC) increases due to the broad temperature span; (â…±) Annealing makes the martensitic transformation temperature and Curie temperature of austenite increase obviously in Ni-Mn-Co-Sn ribbons. The magnetic entropy change around the martensitic transformation of these ribbons increases largely, but the effective RCeff is almost unchanged. Around the Curie temperature of austenite, both the magnetic entropy change and RCeff increase remarkably after annealing. (â…²) Both the martensitic transformation and austenitic Curie temperatures increase evidently in the high-pressure-annealed Ni-Mn-Co-Sn alloy. More interestingly, an intermediate phase is also observed prior to the martensitic transformation in this alloy. Due to the existence of intermediate phase, two sequent positive magnetic entropy change peaks are obtained around the martensitic transformation, which broadens the working temperature interval.2. The regulation of magnetic transition and related magnetocaloric properties in MnCoGe based alloys.MnCoGe alloy is a collinear ferromagnet with the Curie temperature of TC=345 K. This compound has the orthorhombic TiNiSi-type crystal structure at room temperature and transforms diffusionlessly to the hexagonal Ni2In-type structure at Tt=650 K. Because this structural transformation occurs between two paramagnetic states, the change of the magnetization around the structural transformation is very small, which can not be distinguished from the thermomagnetic curves. In order to realize the magnetoestructural transition in these alloys, we prepared two series of MnCoGe based alloys, Mn1-xVxCoGe and Mn1+xCo1-xGe. The experimental results indicate that Tt can be largely reduced by the V-substitution or tuning the Mn/Co ratio, which makes the structural transformation and magnetic transition couple intimately. In these two series of alloys, the first order magnetostructural transformations from the ferromagnetism to paramagnetism are achieved around room temperature. The magnetization appears an abrupt change, which induces a large magnetic entropy change in the relatively low field. It is worth pointing out that the magnetic hysteresis losses of these two kinds of alloys are almost negligible, which is important for the practical applications.3. The spin reorientation transition and related magnetocaloric effects in RCo4Al-based alloys.RCo5 are ferromagnetic alloys, which have the CaCu5 structure and P6/mmm space group. In these alloys, two spin reorientation transitions (SRTs) are induced due to the coexistence of two competing magnetocrystalline anisotropys. One is the planar anisotropy of the rare earth sublattice R (Pr,Nd,Tb,Dy,Ho et al.), the other is axial anisotropy of Co sublattice. The magnetization shows an abrupt change around these SRTs, which would lead to some interesting physical phenomena. The substitution for the R ions and the substitution of Al and Ga for Co element can change the magnetocrystalline anisotropy energy of R and Co sublattices, and then, the SRTs can be regulated. Therefore, we prepared the Nd1-xDyxCo4Al and Pr1-xNdxCo4Al alloys and investigated their SRTs and related magnetocaloric effect in this paper. The experimental results indicat that the transition temperatures are tuned to around room temperature and SRT occurs in a very broad temperature interval for these two alloys. Mealwhile, the moderate magnetic entropy change values are observed in these alloys. In addition, we also prepared the ferrimagnetic DyCo4Al alloy and investigated the magnetic and magnetocaloric properties around the compensate temperature. The aforementioned investigation should be a beneficial attempt to explorate new room temperature magnetic refrigeration materials.