H. G. Zhang

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.

Phase diagram, ferromagnetic martensitic transformation and magnetoresponsive properties of Fe-doped MnCoGe alloys

The crystal structure and magnetoresponsive properties of Fe-doped MnCoGe alloys have been investigated using x-ray diffraction (XRD) and magnetic measurements. By alloying the Fecontaining isostructure compounds into MnCoGe, a magnetostructural transition from paramagnetic austenite to ferromagnetic martensite with large magnetization difference can be realized in a temperature window between the Curie temperatures of the austenite and martensite, resulting in a magnetic-field-induced martensitic transformation and large magnetic-entropy changes. A structural and magnetic phase diagram of Fe-doped MnCoGe alloys is presented.

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.

Structure and magnetic properties of Fe2NiZ (Z=Al, Ga, Si and Ge) Heusler alloys

The Heusler alloys Fe2NiZ (Z=Al, Ga, Si and Ge) have been synthesized and investigated focusing on the phase stability and the magnetic properties. The experimental and theoretical results reveal the covalent bonding originated from p-d hybridization takes an important role in these alloys, which dominates the stability of ordered structure but leads to the decline of the band splitting. The electronic structure shows the IV group main group element (Si and Ge) provides stronger covalent effect than that of the III group element (Al and Ga). It has been found that the variations of the physical parameters, lattice constants, critical ordering temperature, magnetic moments and Curie temperature, precisely follow these covalent characters.

Half-metallicity and anisotropy magnetoresistance properties of Heusler alloys Fe2Co1-xCrxSi

In this paper, we investigate the half-metallicity of Heusler alloys Fe2Co1-x CrxSi by first principles calculations and anisotropy magnetoresistance measurements. It is found that, with the increase of Cr content x, the Fermi level of Fe2Co1-xCrxSi moves from the top of valence band to the bottom of conduction band, and a large half-metallic band gap of 0.75 eV is obtained for x=0.75. We then successfully synthesized a series of Heusler Fe2Co1-x CrxSi polycrystalline ribbon samples. The results of X-ray diffraction indicate that the Fe2Co1-x CrxSi series of samples are pure phase with a high degree of order and the saturation magnetic moment follows half-metallic Slater-Pauling rule. Except for the two end members, Fe2CoSi and Fe2CrSi, the anisotropic magnetoresistance of Fe2Co1-x CrxSi (x=0.25, 0.5, and 0.75) shows a negative value suggesting they are stable half-metallic ferromagnets

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.

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.

Site preference and compensation behavior in Co(Cr, Mn)2O4 system

Site preference of doped Mn ions in CoCr2−xMnxO4 (x = 0–2) series has been derived separately from structure and magnetic measurement. It shows that parts of the doped Mn ions occupy the A (Co) sites when x < 0.5. And then, it takes the two B (Cr) sites in turn before and after x = 1.3. This site preference behavior results in a role conversion of the magnetic contributors and, thus, leads to the composition dependent magnetic compensation. Temperature induced compensation and negative magnetization have also been found in several samples, which is attributed to the large energy barrier between the ferromagnetic and antiferromagnetic spin arrangement. A structure transition from cubic to tetragonal symmetry has been detected.

Wide temperature window of magnetostructural transition achieved in Mn0.4Fe0.6NiSi1−xGax by a two-step isostructural alloying process

A new approach has been proposed in this work, which provides an effective way for tuning the structural transition in MM’X systems with relatively high transition temperature. With this method, a temperature window as wide as 275 K for the magnetostructural transition has been achieved in the MnNiSi alloy system. The maximum magnetic entropy change of the system is as high as 13.3 J/kgK, which, together with the large temperature window, enables the Mn0.4Fe0.6NiSi1−xGax system to be a promising candidate for magnetic refrigerant applications.