E. K. Liu

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.

Giant magnetocaloric effect in isostructural MnNiGe-CoNiGe system by establishing a Curie-temperature window

An effective scheme of isostructural alloying was applied to establish a Curie-temperature window in isostructural MnNiGe-CoNiGe system. With the simultaneous accomplishment of decreasing structural-transition temperature and converting antiferromagnetic martensite to ferromagnetic state, a 200 K Curie-temperature window was established between Curie temperatures of austenite and martensite phases. In the window, a first-order magnetostructural transition between paramagnetic austenite and ferromagnetic martensite occurs with a sharp jump in magnetization, showing a magnetic entropy change as large as −40 J kg−1 K−1 in a 50 kOe field change. This giant magnetocaloric effect enables Mn1−xCoxNiGe to become a potential magnetic refrigerant.An effective scheme of isostructural alloying was applied to establish a Curie-temperature window in isostructural MnNiGe-CoNiGe system. With the simultaneous accomplishment of decreasing structural-transition temperature and converting antiferromagnetic martensite to ferromagnetic state, a 200 K Curie-temperature window was established between Curie temperatures of austenite and martensite phases. In the window, a first-order magnetostructural transition between paramagnetic austenite and ferromagnetic martensite occurs with a sharp jump in magnetization, showing a magnetic entropy change as large as −40 J kg−1 K−1 in a 50 kOe field change. This giant magnetocaloric effect enables Mn1−xCoxNiGe to become a potential magnetic refrigerant.

Facile fabrication and growth mechanism of 3D flower-like Fe3O4 nanostructures and their application as SERS substrates

A template-free solvothermal combined with precursor thermal transformation method has been developed for the preparation of flower-like Fe3O4 nanostructured hollow microspheres. The reaction mechanism and the self-assembly evolution process were studied, and it was found that the synthetic conditions for the precursor such as reaction time, urea concentration and non-aqueous media are all crucial for the formation of the flower-like hierarchical precursors. The flower-like Fe3O4 microspheres obtained by calcining the precursor in Ar gas exhibit superparamagnetic behavior and show relative high saturation magnetization at room temperature. To endow them with SERS activity, silver coating was conducted by magnetron sputtering. The obtained Fe3O4/Ag hybrid microflowers make a positive influence on the high sensitivity of SERS to 4-pyridinethiol (4-Mpy) and Rhodamine 6G (R6G) molecules when compared with the silver film substrates. More importantly, the detection limit of Fe3O4/Ag hybrid microflowers for R6G dye can reach up to 10−15 M, which meets the requirements of ultratrace detection of analytes using SERS. Thus, the SERS-active magnetic hybrids prepared in this work may possibly be used as an optical probe with magnetic function for application in high-sensitivity bioassays.

Magneto-transport properties of oriented Mn2CoAl films sputtered on thermally oxidized Si substrates

Spin gapless semiconductors are interesting novel class of materials by embracing both magnetism and semiconducting. Its potential application in future spintronics requires realization in thin film form. In this letter, we report a successful growth of spin gapless Mn2CoAl films on thermally oxidized Si substrates by magnetron sputtering deposition. The films deposited at 673K are well oriented to (001) direction and display a uniform-crystalline surface. Magnetotransport measurements on the oriented films reveal a semiconducting-like resistivity, small anomalous Hall conductivity and linear magnetoresistance (MR) representative of the transport signatures of spin gapless semiconductors. The magnetic properties of the films have also been investigated and compared to that of bulk Mn2CoAl, with small discrepancy induced by the composition deviation.

Ferromagnetic structures in Mn2CoGa and Mn2CoAl doped by Co, Cu, V, and Ti

The structure and magnetic properties in doped Heusler alloys of Mn2CoGa and Mn2CoAl have been investigated by experiments and calculations. The main group elements of Ga or Al in the systems are substituted by the magnetic or non-magnetic transition metals, Co, Cu, V, and Ti. Three kinds of local ferromagnetic structures, Co-Mn-Co, Mn-Co-Mn, and Mn-Co-V, have been found. They embed in the native ferrimagnetic matrix and increase the magnetization with different increments. The Co-Mn-Co ferromagnetic structure shows the largest increment of 6.18 mu(B)/atom. In addition, interesting results for non-magnetic Cu increasing the magnetization and the V atom having a large ferromagnetic moment of about 1.0 mu(B) have been obtained. The exchange interaction energy can be increased by the newly added Co and depleted by supporting a ferromagnetic coupling in other substitution cases and showing the variation of the T-C. Our calculation of electronic structure verifies the strong d-d hybridization when the three ferromagnetic structures are achieved. It has also been found that the covalent bonding from the Ga and Al determines the generation of the local ferromagnetic structure and the tolerance for dopant content

Unusual lattice constant changes and tunable magnetic moment compensation in Mn50−xCo25Ga25+x alloys

We report on unusual lattice parameter changes and tunable magnetic moment compensation in Mn50−xCo25Ga25+x (x = 0-25) Heusler alloys by substituting Ga for Mn. The observed lattice parameter first increases with increasing Ga content x, showing a maximum at x = 12.5, and then abnormally decreases due to the enhanced covalence effect between transition-metal and main-group atoms. Moreover, a tunable magnetic moment compensation was also observed due to the diversification in role of the main magnetic contributor when the structure varies from Hg2CuTi-type Mn2CoGa to Cu2MnAl-type CoMnGa2. These results provide an alternative way to simultaneously tune both the structural and magnetic properties of Heusler alloys, which is particularly important for developing flexible spintronics devices.

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.

Structural transitions, magnetic properties, and electronic structures of Co(Fe)-doped MnNiSi compounds

The structural transitions, magnetic properties, and electronic structures of Co(Fe)-doped MnNiSi compounds are investigated by x-ray powder diffraction, differential scanning calorimetry (DSC), magnetic measurements, and first-principles calculations. Results indicate that all samples undergo a martensitic transition from the Ni2In-type parent phase to TiNiSi-type orthorhombic phase at high temperatures. The substitution of Co(Fe) for Mn in Mn1−xCoxNiSi (x = 0.2, 0.3, and 0.4) and Mn1−yFeyNiSi (y = 0.26, 0.30, 0.36, 0.46, and 0.55) samples decreases the structural transition temperature and Curie temperature of martensite. The martensite phases show a typical ferromagnetic behavior with saturation field being basically unchanged with increasing Co(Fe) content, while the saturation magnetization shows a decreasing tendency. The theoretically calculated moments are in good agreement with the experimentally measured results. The orbital hybridizations between different 3d elements are analyzed from the dist...

Towards fully compensated ferrimagnetic spin gapless semiconductors for spintronic applications

Extensive first-principles calculations suggest that inverse Heusler compounds , , , and are the candidates to achieve fully compensated ferrimagnetic spin gapless semiconductors. It is shown that only the holes can be 100% spin polarized in , while both the excited electrons and the holes around the Fermi level 100% spin polarized in the others. A simple rule for searching potential fully compensated ferrimagnetic spin gapless semiconductors in Heusler compounds is proposed. Due to the spin gapless semiconducting and the fully compensated ferrimagnetic properties, these compounds offer distinct advantage towards the development of the practical spintronic devices.

Structure, magnetism, and magnetic compensation behavior of Co50-xMn25Ga25+x and Co50-xMn25+xGa25 Heusler alloys

The structure, magnetism, magnetic compensation behavior, exchange interaction and electronic structures of Co50-xMn25Ga25+x and Co50-xMn25+xGa25 (x=0-25) alloys have been systematically investigated by both experiments and first-principles calculations. We found that all the samples exhibited body centered cubic structures with a high degree of atomic ordering. With increasing Ga content, the composition dependence of lattice parameters shows a kink point at the middle composition in Co50-xMn25Ga25+x alloys, which can be attributed to the enhanced covalent effect between the Ga and the transition metals. Furthermore, a complicated magnetic competition has been revealed in Co50-xMn25Ga25+x alloys, which causes the Curie temperature dramatically decrease and results in a magnetic moment compensation behavior. In Co50-xMn25+xGa25 alloys, however, with increasing Mn content, an additional ferrimagnetic configuration was established in the native ferromagnetic matrix, which causes the molecular moment monotonously decreases and the exchange interaction enhances gradually. The electronic structure calculations indicate that the Co50-xMn25+xGa25 alloys are likely to be in a coexistence state of the itinerant and localized magnetism. Our study will be helpful to understand the nature of magnetic ordering as well as to tune magnetic compensation and electronic properties of Heusler alloys.