Jl Chen

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.

Structure and negative thermal expansion in the PbTiO3–BiFeO3 system

The structures of (1−x)PbTiO3–xBiFeO3 (x=0.3 and 0.6) were investigated by means of the neutron powder diffraction. A splitting shift between Fe and Ti atoms was found along the c axis in 0.7PbTiO3–0.3BiFeO3; however, this splitting does not appear in 0.4PbTiO3–0.6BiFeO3. The tetragonal phase of PbTiO3–BiFeO3 exhibits a large spontaneous polarization. The negative thermal expansion of PbTiO3 is significantly enhanced in a wide temperature range by the BiFeO3 substitution. The average bulk thermal expansion coefficient of 0.4PbTiO3–0.6BiFeO3 is a¯v=−3.92×10−5°C−1, which is much strong in the known negative thermal expansion oxides.

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.

Magnetic properties and magnetocaloric effects in R3Ni2 (R = Ho and Er) compounds

Magnetic and magnetocaloric properties of R3Ni2 (R = Ho and Er) compounds have been investigated. Both Ho3Ni2 and Er3Ni2 compounds undergo two successive phase transitions: spin reorientation transition and second-order ferromagnetic-paramagnetic transition. The maximal values of magnetic entropy change are achieved to be 21.7 J kg−1 K−1 for Ho3Ni2 and 19.5 J kg−1 K−1 for Er3Ni2 for a field change of 0-5 T. A large refrigerant capacity (RC) of 496 J kg−1 in the composite material is also obtained. Large reversible magnetocaloric effect and RC indicate the potentiality of R3Ni2 (R = Ho and Er) compounds as candidates for low-temperature magnetic refrigerant.

Dissolving, trapping and detrapping mechanisms of hydrogen in bcc and fcc transition metals

First-principles calculations are performed to investigate the dissolving, trapping and detrapping of H in six bcc (V, Nb, Ta, Cr, Mo, W) and six fcc (Ni, Pd, Pt, Cu, Ag, Au) metals. We find that the zero-point vibrations do not change the site-preference order of H at interstitial sites in these metals except Pt. One vacancy could trap a maximum of 4 H atoms in Au and Pt, 6 H atoms in V, Nb, Ta, Cr, Ni, Pd, Cu and Ag, and 12 H atoms in Mo and W. The zero-point vibrations never change the maximum number of H atoms trapped in a single vacancy in these metals. By calculating the formation energy of vacancy-H (Vac-Hn) complex, the superabundant vacancy in V, Nb, Ta, Pd and Ni is demonstrated to be much more easily formed than in the other metals, which has been found in many metals including Pd, Ni and Nb experimentally. Besides, we find that it is most energetically favorable to form Vac-H1 complex in Pt, Cu, Ag and Au, Vac-H4 in Cr, Mo and W, and Vac-H6 in V, Nb, Ta, Pd and Ni. At last, we examine the detr...

Martensitic and magnetic transformation in Mn50Ni50−xSnx ferromagnetic shape memory alloys

A martensitic transformation (MT) from a body-centered-cubic austenitic phase to a tetragonal martensitic phase has been found in Mn50Ni50−xSnx (0 ≤ x ≤ 11) alloys. The martensitic transformation temperature can be decreased by about 71.6 K by increasing the Sn concentration by 1 at. %. For 9 ≤ x ≤ 11, Mn50Ni50−xSnx ferromagnetic shape memory alloys are obtained. Due to the large magnetization difference (ΔM = 60 emu/g) and small thermal hysteresis (ΔT = 6 K) in the Mn50Ni40Sn10 alloy, a two-way magnetic-field-induced martensitic transformation is observed with dT/dH = 2 K/T.

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

Superelasticity of CoNiGa : Fe single crystals

We have fabricated CoNiFeGa single crystals with excellent superelasticity. The superelastic strains of 4% and 6.7% in compression have been obtained along the [001] and [110] directions, respectively. These single crystals show strong anisotropy in strains, superelastic parameters, and even transformation path related to the different crystalline directions. A large superelastic strain up to 11% has been obtained in tension test. The perfect superelasticities have also displayed in bending and torsion tests.

Martensitic transformation and magnetic properties of Co-Ni-Al shape memory alloy ribbons

We report on the magnetic and martensitic transformation properties of Co39Ni33Al28 ferromagnetic shape memory alloy ribbons. We found that the phase formation of Co39Ni33Al28 is strongly dependent on the method of preparation. The conventional as-cast ingot sample contains a large amount of the γ phase embedded in the primary β phase, while melt-spun ribbons contain the pure β phase. Co39Ni33Al28 ribbons exhibit a perfect thermoelastic martensitic transformation from a cubic to a tetragonal structure at 240 K during cooling. The martensite structure can be well described by the L10 lattice, similar to that of Ni–Al alloys. The martensitic phase at 5 K exhibits a saturation magnetization of 48.67 emu g −1 and saturates at about 8000 Oe. A large increase in coercive force after ageing at 500uC for 1 h has been found due to the change in atomic chemical ordering. The temperature dependence of the saturation magnetization indicates that magnetization can be well interpreted by spin-wave theory at temperatures lower than 200 K. The material shows a recoverable strain of 500 ppm upon the martensitic transformation.

Giant magnetocaloric effect in Ho12Co7 compound

Magnetic properties and magnetocaloric effects of Ho12Co7 compound are investigated by magnetization and heat capacity measurement. The Ho12Co7 compound undergoes antiferromagnetic (AFM)-AFM transition at T1 = 9 K, AFM-ferromagnetic (FM) transition at T2 = 17 K, and FM-paramagnetic transition at TC = 30 K, with temperature increasing. There are two peaks on the magnetic entropy change (ΔSM) versus temperature curves and the maximal value of –ΔSM is found to be 19.2 J/kg K with the refrigerant capacity value of 554.4 J/kg under a field change from 0 to 5 T. The shape of the ΔSM-T curves obtained from heat capacity measurement is in accordance with that from magnetization measurement. The excellent magnetocaloric performance indicates the applicability of Ho12Co7 as an appropriate candidate for magnetic refrigerant in low temperature ranges.Magnetic properties and magnetocaloric effects of Ho12Co7 compound are investigated by magnetization and heat capacity measurement. The Ho12Co7 compound undergoes antiferromagnetic (AFM)-AFM transition at T1 = 9 K, AFM-ferromagnetic (FM) transition at T2 = 17 K, and FM-paramagnetic transition at TC = 30 K, with temperature increasing. There are two peaks on the magnetic entropy change (ΔSM) versus temperature curves and the maximal value of –ΔSM is found to be 19.2 J/kg K with the refrigerant capacity value of 554.4 J/kg under a field change from 0 to 5 T. The shape of the ΔSM-T curves obtained from heat capacity measurement is in accordance with that from magnetization measurement. The excellent magnetocaloric performance indicates the applicability of Ho12Co7 as an appropriate candidate for magnetic refrigerant in low temperature ranges.