Wenxiu Zhu

Stable magnetostructural coupling with tunable magnetoresponsive effects in hexagonal ferromagnets

The magnetostructural coupling between the structural and the magnetic transition has a crucial role in magnetoresponsive effects in a martensitic-transition system. A combination of various magnetoresponsive effects based on this coupling may facilitate the multifunctional applications of a host material. Here we demonstrate the feasibility of obtaining a stable magnetostructural coupling over a broad temperature window from 350 to 70 K, in combination with tunable magnetoresponsive effects, in MnNiGe:Fe alloys. The alloy exhibits a magnetic-field-induced martensitic transition from paramagnetic austenite to ferromagnetic martensite. The results indicate that stable magnetostructural coupling is accessible in hexagonal phase-transition systems to attain the magnetoresponsive effects with broad tunability.

Vacancy-tuned paramagnetic/ferromagnetic martensitic transformation in Mn-poor Mn1-xCoGe alloys

It is shown that a temperature window between the Curie temperatures of martensite and austenite phases around the room temperature can be obtained by a vacancy-tuning strategy in Mn-poor Mn1-xCoGe alloys (0≤x≤0.050). Based on this, a martensitic transformation from paramagnetic austenite to ferromagnetic martensite with a large magnetization difference can be realized in this window. This gives rise to a magnetic-field–induced martensitic transformation and a giant magnetocaloric effect in the Mn1−xCoGe system. The decrease of the transformation temperature and of the thermal hysteresis of the transformation, as well as the stable Curie temperatures of martensite and austenite, are discussed on the basis of the Mn-poor Co-vacancy structure and the corresponding valence-electron concentration.

Magnetic-field-induced transformation in FeMnGa alloys

A kind of ferromagnetic shape memory alloy with off-stoichiometric composition of Heusler alloy Fe2MnGa has been synthesized. By optimizing composition, the martensitic transformation has been modified to occur at about 163 K accompanying spontaneous magnetization, which enables a magnetic field-induced structural transition from a paramagnetic parent phase to a ferromagnetic martensite with high magnetization of 93.8 emu/g. The material performs a quite large lattice distortion through the transformation, (c−a)/c=33.5%, causing a shape memory strain upto 3.6%. Such large lattice distortions strongly influence the electron structures, and thus some special physical behavior related to the transport and conductive properties is investigated.

Giant exchange bias based on magnetic transition in gamma-Fe2MnGa melt-spun ribbons

Giant exchange bias with a shift up to 3.86 kOe has been observed in gamma-Fe2MnGa alloy in which a ferromagnetic to antiferromagnetic transition takes place at 250 K. The transition can be suppressed to lower temperatures by higher magnetic fields at a shift rate of 10.3 K/kOe. Exchange bias and enhanced coercivity occur simultaneously, revealing an exchange coupling between the coexisting antiferromagnetic and ferromagnetic phases. Meanwhile, the internal exchange coupling inside the antiferromagnetic clusters dynamically ensures their unidirectional anisotropy during their size changing following the external magnetic field

Development of a single-stage pulse tube refrigerator capable of reaching 49 K

Abstract The orifice pulse tube refrigerator is a new type of cryocooler which was reported to be capable of reaching 60 K. The present authors have achieved a temperature of 49 K for the first time with a single-stage orifice pulse tube refrigerator by improving the regenerator, the hot end heat exchanger and the insulation of the low temperature sections. A refrigeration power of 12 W could be obtained at 77 K at the cold end. The relation between the ratio of regenerator volume to pulse tube volume and the minimum temperature of the orifice pulse tube refrigerator was experimentally investigated. Methods for further improving the performance of the orifice pulse tube refrigerator are discussed.

Ferromagnetic exchange interaction between Co and Mn in the Heusler alloy CuCoMnAl

The ferromagnetic exchange interaction between Co and Mn in Heusler alloys has been phenomenologically investigated by analyzing the composition dependence of the magnetic moment and the Curie temperature in a series of quaternary CuCoMnAl alloys. The curves of the composition dependence of the magnetic moment show an interesting valleylike profile and their minima are positioned at different Co contents for different Mn concentrations. The ferromagnetic Co–Mn exchange interaction is a short-range effect which is only effective at the nearest-neighbor distance. At this distance, the exchange interaction can be further enhanced by a Mn-rich composition, but it might be destroyed by the lattice distortion due to the martensitic transformation.

Significant disorder-induced enhancement of the magnetization of Fe2CrGa by ball milling

It is reported that ball milling gives rise to a different atomic configuration in Fe2CrGa than the order obtained upon preparation by arc melting. After ball milling, the magnetic moment has values of 3.2 to 3.9u2009μB/f.u., which is significantly higher than in arc-melted samples, and the Curie temperature increases by about 200u2009K. Combination of first-principles calculations and experimental results indicates that Fe2CrGa crystallizes in the Hg2CuTi-based structures with either Fe-Ga or Cr-Ga disorder, depending on the preparation method. It is shown that magnetic interactions play a crucial role in adopting atomic configurations which disobey the empirical rule.A new disordered atom configuration in Fe2CrGa alloy has been created by ball-milling method. This leads to a significant enhancement of the magnetic moment up to 3.2~3.9 {mu}B and an increase of Curie temperature by about 200 K, compared with the arc-melt samples. Combination of first-principles calculations and experimental results reveals that Fe2CrGa alloy should crystallize in Hg2CuTi based structure with different atomic disorders for the samples prepared by different methods. It is addressed that magnetic interactions play a crucial role for the system to adopt such an atomic configuration which disobeys the empirical rule.

Study on miniature pulse tube cryocooler for space application

Abstract Pulse tube refrigerator (PTR) is a new type of mechanical cryocooler with the potential of long-term operation in space. Theoretical and experimental studies are currently on the way in the Cryogenic Laboratory of Chinese Academy of Sciences (CL/CAS) in order to develop a 85 K/250 mW class pulse tube cryocooler to be used to cool space-borne infrared devices. A theoretical model is established based on the analyses of the thermodynamic behavior of gas parcels in oscillating flow regenerators. It helps us to understand the cooler and can be used to study the influence of DC flow on the refrigeration performance. The flow resistance of the regenerator is an important factor for the cooler performance. A test bench, including a hot-wire anemometer has been set up to investigate the flow resistance characteristics of regenerators with oscillating flow. The results of measurement are correlated and served for practical design. The Oxford type linear pressure wave generator with flexure bearings is also under development in CL/CAS. The prototype miniature pulse tube cryocooler, driven by a linear pressure wave generator of 1.06 cm 3 maximum swept volume, provides at present 200 mW net cooling power at 81 K with 28.4 W input power. Improvements are being made to further increase the cooling power and reduce the input power.

Unusual magnetic anisotropy in the ferromagnetic shape-memory alloy Ni50Fe23Ga27

Unusual magnetic anisotropy of the ferromagnetic shape-memory alloy Ni50Fe23Ga27 has been observed. The anisotropy of the austenite becomes very large, even larger than that of the martensite in ribbon samples. Lowering the temperature from 300K to 80K, the saturation field of the austenite is dramatically increased from 200 Oe up to 6 kOe. This high-anisotropy behavior clearly highlights the demagnetization effect of the martensitic transformation. The physical mechanism is attributed to a collective effect coming from the atomic disorder, the premartensitic transformation, and the off-stoichiometric Ga-rich composition of the alloys

Liquid metal actuated ejector vacuum system

An ejector vacuum system using nontoxic room-temperature liquid metal as actuating fluid was fabricated and experimentally demonstrated. With physical merits of high density, fluidity, and ultralow vapor pressure, the liquid metal could serve as a high momentum density carrier fluid. The main performance parameters of the liquid metal actuated ejector were thus found to be significantly improved by orders superior as compared with that of water actuating system. Under oxidation protection, an ultimate vacuum pressure of 170u2009Pa was achieved and capacity of evacuating a 500u2009ml nitrogen reservoir from atmospheric pressure to 480u2009Pa within 96 s was obtained.