H. Z. Luo

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

A coupling of martensitic and metamagnetic transitions with collective magneto-volume and table-like magnetocaloric effects

A coupling of the first-order paramagnetic-to-induced-ferromagnetic martensitic and the second-order antiferromagnetic-to-ferromagnetic metamagnetic transitions was found in MnNi0.8Fe0.2Ge alloy. Based on the coupling, a magneto-volume effect driven by the martensitic transition and a table-like magnetocaloric effect generated by the successive magnetic phase transitions arise collectively. By using the magneto-volume effect, the internal stress in the volume-expansion martensitic transition was determined at 350 MPa. The magnetocaloric effect, with a wide working temperature range of 26 K around room temperature, shows a small hysteresis loss (5 J kg−1) and a large net refrigerant capacity (157 J kg−1).

Realization of multifunctional shape-memory ferromagnets in all-d-metal Heusler phases

Heusler ferromagnetic shape-memory alloys (FSMAs) normally consist of transition-group d-metals and main-group p-elements. Here, we report the realization of FSMAs in Heusler phases that completely consist of d metals. By introducing the d-metal Ti into NiMn alloys, cubic B2-type Heusler phase is obtained and the martensitic transformation temperature is decreased efficiently. Strong ferromagnetism is established by further doping Co atoms into the B2-type antiferromagnetic Ni-Mn-Ti austenite. Based on the magnetic-field-induced martensitic transformations, collective multifunctional properties are observed in Ni(Co)-Mn-Ti alloys. The d metals not only facilitate the formation of B2-type Heusler phases but also establish strong ferromagnetic coupling and offer the possibility to tune the martensitic transformation.

Role of covalent hybridization in the martensitic structure and magnetic properties of shape-memory alloys: The case of Ni50Mn5+xGa35-xCu10

The influence of covalent hybridization on the martensitic structure and magnetic properties of Ni50Mn5+xGa35−xCu10 shape-memory alloys has been investigated. It is found that the lattice distortion (c − a)/a of L10 martensite linearly increases upon substitution of Mn for Ga, showing a change of slope at Ga = 25 at. %, which is ascribed to a weakened covalent hybridization between main-group and transition-metal atoms. Moreover, due to the competition between the covalent hybridization and the magnetic ordering of the substituted Mn atoms, the magnetic moment per formula unit and the Curie temperature show maxima at Ga = 25 at. % as well. This behavior is closely associated with the corresponding changes of the strength of the covalent hybridization. The results, therefore, suggest that a careful control of the concentration of main-group atoms in Heusler alloys may serve as a tuning parameter for finding multifunctional materials.

Tuning exchange bias by thermal fluctuation in Fe52Mn23Ga25 melt-spun ribbons

In Fe52Mn23Ga25 ribbons, the exchange bias becomes very sensitive to the proportions and sizes of the ferromagnetic (FM) and antiferromagnetic (AFM) phases. With high cooling fields, the AFM clusters with small sizes are embedded in the FM matrix which is kinetically arrested at low temperatures, resulting in a small exchange bias. The arrested FM phase can be dearrested by thermal fluctuation, and simultaneously, the proportion and sizes of AFM clusters increase with a stable unidirectional anisotropy. With different thermal fluctuations, a continuous tuning of exchange bias field from 0.202 kOe to 2.11 kOe is realized.

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 3.9 μB/f.u., which is significantly higher than in arc-melted samples, and the Curie temperature increases by about 200 K. 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.

New spin injection scheme based on spin gapless semiconductors: A first-principles study

Spin injection efficiency based on a conventional and/or half-metallic ferromagnet/semiconductor is greatly limited by the Schmidt obstacle due to conductivity mismatch; here we proposed that by replacing the metallic injectors with spin gapless semiconductors can significantly reduce the conductive mismatch to enhance spin injection efficiency. By performing first-principles calculations based on superlattice structure, we have studied a representative system of Mn2CoAl/semiconductor spin injector scheme. The results showed that a high spin polarization was maintained at the interface in systems of Mn2CoAl/Fe2VAl constructed with (100) interface and Mn2CoAl/GaAs with (110) interface, and the latter is expected to possess long spin diffusion length. Inherited from the spin gapless feature of Mn2CoAl, a pronounced dip was observed around the Fermi level in the majority spin density of states in both systems, suggesting fast transport of the low-density carriers.

DISORDER ENHANCED MAGNETIC MOMENT IN Fe2CrAl RIBBONS

The electronic structure and magnetic properties of Fe2CrAl have been investigated theoretically and experimentally. The theoretical calculations show that there is an energy gap in the minority-spin band in the electronic structure of ordered Fe2CrAl with the L21 structure and that the spin-polarization ratio is as high as 93%. The calculated total spin moment is 0.97 μB/f.u. However, in melt-spun Fe2CrAl ribbons, the B2 type structure, an atomic-site disordered L21 structure, is found and this atomic site disorder gives rise to a magnetic moment of 2.02 μB in the ribbons which is much higher than the theoretical value. The calculations prove that the enhanced magnetic moment is mainly due to the Fe–Cr and Fe–Al types of disorder. In contrast, the Cr–Al type disorder has little influence on the total spin moment and the spin-polarization ratio.

Windows open for highly tunable magnetostructural phase transitions

An attempt was made to tailor the magnetostructural transitions over a wide temperature range under the principle of isostructural alloying. A series of wide Curie-temperature windows (CTWs) with a maximal width of 377 K between 69 and 446 K were established in the Mn1−yCoyNiGe1−xSix system. Throughout the CTWs, the magnetic-field-induced metamagnetic behavior and giant magnetocaloric effects are obtained. The (Mn,Co)Ni(Ge,Si) system shows great potential as multifunctional phase-transition materials that work in a wide range covering liquid-nitrogen and above water-boiling temperatures. Moreover, general understanding of isostructural alloying and CTWs constructed in (Mn,Co)Ni(Ge,Si) as well as (Mn,Fe)Ni(Ge,Si) is provided.

Magnetostructural martensitic transformations with large volume changes and magneto-strains in all-d-metal Heusler alloys

The all-d-metal Mn2-based Heusler ferromagnetic shape memory alloys Mn50Ni40-xCoxTi10 (x = 8 and 9.5) are realized. With a generic comparison between d-metal Ti and main-group elements in lowering the transformation temperature, the magnetostructural martensitic transformations are established by further introducing Co to produce local ferromagnetic Mn-Co-Mn configurations. A 5-fold modulation and (3, -2) stacking of [00 10] of martensite are determined by XRD and HRTEM analysis. Based on the transformation, a large magneto-strain of 6900 ppm and a large volume change of -2.54% are observed in polycrystalline samples, which makes the all-d-metal magnetic martensitic alloys of interest for magnetic/pressure multi-field driven applications.