| Magnetic alloys, which experience the magnetic-field-induced phase transition, show large magnetostrictive and magnetocaloric effects. These multi-functional materials have attracted considerable attention due to their potential applications in actuator, sensor, magnetomechanical device and magnetic cooling refrigerator. Recently, large magnetostrictive and magnetocaloric effects havd been found in some compounds with first-order transition, such as La-Fe-Si alloys, Gd-Si-Ge alloys, Ni-Mn based magnetic shape memory alloys and MM’X (M=Mn, M’=Co or Ni, X=Si or Ge) alloys. However, the reported effects couple with some drawbacks, such as irreversibility, large hysteresis, poor mechanical property, high driving field and low transition temperature, which greatly limited their further applications. Thus, it is still a challenge to overcome these drawbacks and fulfill magnetic-phase-transit ion alloys for their applications. In this study, we investigate the magnetic-field-induced phase transition in three groups of Mn-based alloys and improve their magnetostrictive or magnetocaloric performance by tuning content, altering sintering method and controlled with electric field.1. Large reversible magnetostrictive effect in the Gd1-xSmxMn2Ge2 (x= 0.37, 0.34) alloys at room temperatureLarge lattice distortion and magnetic-field-induced strain appear in alloys possessing magnetoelastic or magneto structural transition. Compared with magneto structural transformation, the magnetic-field-induced strain generated from magnetoelastic transition is highly reversible, which is important for application. However, the reported magnetoelastic transitions usually need high driving magnetic field and occur far from room temperature. Therefore, exploring a material showing magnetoelastic transition and large low-field magnetostriction at room temperature is meaningful.We find that by tuning the ratio of Gd and Sm, the Mn-Mn distance in Gd1-xSmxMn2Ge2 system can be adjusted to be close to the critical Mn-Mn distance, which leads to the thermal-and magnetic-field-induced antiferromagnetic-ferromagnetic transition around room temperature in the Gd0.66Sm0.34Mn2Ge2 and Gdo.63Smo.37Mn2Ge2 alloys. Experimental results prove that this transition is a magnetoelastic one, which is accompanied by lattice distortion. During the magnetic-field-induced magnetoelastic transition, large reversible magnetostriction with minimal hysteresis and high stability is obtained under low magnetic field.2. Textured, dense and giant magnetostrictive alloy from fissile polycrystalTextured material usually promises an enhanced physical effect along the preferred orientation. However, how to orientate the material, especially from a fissile alloy, is still a great challenge. The magnetic-field-induced metamagnetic transition in transition-metal-based alloy, MnCoSi, is accompanied with considerable changes in lattice parameters, suggesting that this alloy would be a promising magnetostrictive material. But limited magnetostriction in polycrystalline and poor mechanical property limit its potential application.We prepare some textured and dense MnCoSi1-x (x=0; 0.01; 0.02) alloys by an approach called high-magnetic-field solidification with a slow cooling rate. Through this special method, the crystal grains can be orientated and the cracks can be eliminated, leading to the textured and dense bulks. Furthermore, in order to get rid of the magnetic hysteresis and decrease the driving field for metamagnetic transition, tricritical points of MnCoSi-based alloys are carefully adjusted. By introducing the vacancies of main group element, which is first reported in this system, MnCoSi1-x alloys show reduced driving field and room-temperature tricritical points. As a result, giant reversible magnetostriction, which is comparable to RFe2-based alloy, can be observed in MnCoSi0.98 at room temperature with a reduced driving magnetic field. Compared with RFe2-based alloy, these alloys are made up of transition metals and Si, which largely decrease the cost of raw materials. Our results suggest that Si-vacant MnCoSi1-x alloys have promising application for acoustic transducer, actuator, stress sensor and so on.3. Electric field control of the magnetocaloric effectMagnetoelectric coupling has attracted ever-increasing interest due to its novel physical mechanism and potential application. Magnetic cooling refrigeration, which is based on magnetocaloric effect, is regarded as an environment friendliness and high-efficiency refrigerating technology for substituting conventional vapor compression refrigeration in the future. It is well known that Heusler-type Ni-Mn-Z (Z= In, Sn, Sb) alloys can show large inverse magnetocaloric effect owing to the magnetic-field-induced magneto structural transformation. However, due to the first-order nature of the transformation, two important drawbacks, limited operating temperature region and large thermal/magnetic irreversibility, largely hinder their application as refrigerant.In this study, we demonstrate that magnetocaloric effect can be significantly improved by an electric-field-controlled method. We investigate the manipulation of electric field on magnetocaloric effect in a Ni-Co-Mn-In/PMN-PT laminate. The thermal and magnetic hysteresis of Ni-Co-Mn-In can be obviously reduced with the application of electric field. Moreover, the operating temperature window of the sample can be remarkably extended. Taking advantage of the electric field control of magnetocaloric effect, we can overcome two important hurdles mentioned above, which is helpful to enhance the efficiency of active magnetic cooling refrigerator. Since the optimal working temperature region of the refrigerant can be tuned by the electric field, a model for active magnetic refrigerator with high efficiency is thereby proposed in principle. |
