arXiv subjects

E. K. Liu

Publications and source records attributed to E. K. Liu.

At least 37 records · Page 2Linked to original sources

arXiv2014

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 pressure 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) and a large net refrigerant capacity (157 J/kg).

cond-mat.mtrl-sci↗

arXiv2014

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.

cond-mat.mtrl-sci↗

arXiv2014

First-principles investigation of half-metallic ferromagnetism of half-Heusler compounds XYZ

We investigate the electronic structure and magnetism of half-Heusler compounds XYZ (X, Y=V, Cr, Mn, Fe, Co and Ni; Z=Al, Ga, In, Si, Ge, Sn, P, As, and Sb) using the ab initio density functional theory calculations. Nine half-metals with half-Heusler structure have been predicted with the half-metallic gap of 0.07-0.67 eV. The calculations show that the formation energies for these nine half-Heusler compounds range from -1.32 to -0.12 eV/f.u., and for CoCrSi, CoCrGe, CoFeGe, CoMnSi, CoMnGe, FeMnGe and FeMnAs, the total energy differences between the half-Heusler structure and the corresponding ground-state structure are small (0.07-0.76 eV/f.u.), thus it is expected that they would be realized in the form of thin films under metastable conditions for spintronic applications. The stability of the half-metallicity of CoCrGe and FeMnAs to the lattice distortion is also investigated in detail.

cond-mat.mtrl-sci↗

arXiv2013

A new class of topological insulators from I-III-IV half-Heusler compounds with strong band inversion strength

In this paper, by first principle calculations, we investigate systematically the band topology of a new half-Heusler family with composition of I(A)-III(A)-IV(A). The results clearly show that many of the I-III-IV half-Heusler compounds are in fact promising to be topological insulator candidates. The characteristic feature of these new topological insulators is the naturally strong band inversion strength (up to -2eV) without containing heavy elements. Moreover, we found that both the band inversion strength and the topological insulating gap can be tailored through strain engineering, and therefore would be grown epitaxially in the form of films, and useful in spintronics and other applications.

cond-mat.mtrl-sci↗

arXiv2013

Crossover of magnetoresistance in the zerogap half-metallic Heusler alloy Fe2CoSi

This work reports on the band structure and magneto-transport investigations of the inverse Heusler compound Fe2CoSi. The first-principles calculations reveal that Fe2CoSi has a very peculiar band structure with a conducting property in the majority spin channel and a nearly zero bandgap in the minority spin channel. The synthesized Fe2CoSi sample shows a high-ordered inverse Heusler structure with a magnetic moment of 4.88 μB at 5 K and a high Curie temperature of 1038 K. With increasing temperature, a crossover from positive to negative magnetoresistance (MR) is observed. Complemented with the Hall effect measurements, we suggest the intriguing crossover of MR can be ascribed to the dominant spin carriers changing from the gapless minority spin channel to the majority spin channel at Fermi level.

cond-mat.mtrl-sci↗

arXiv2013

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

In this paper, we investigate the half-metallicity of Heusler alloys Fe2Co1-xCrxSi 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 Heusler Fe2Co1-xCrxSi polycrystalline ribbon samples. The results of X-ray diffraction indicate that the Fe2Co1-xCrxSi 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-xCrxSi (x=0.25, 0.5, 0.75) show a negative value suggesting they are stable half-metallic ferromagnets.

cond-mat.mtrl-sci↗

arXiv2013

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.

cond-mat.mtrl-sci↗

arXiv2013

Topological Insulators in Hexagonal Wurtzite-type Binary Compounds

We propose new topological insulators in hexagonal wurtzite-type binary compounds based on the first principles calculations. It is found that two compounds AgI and AuI are three-dimensional topological insulators with a naturally opened band-gap at Fermi level. From band inversion mechanism point view, this new family of topological insulators is similar with HgTe, which has s (Gamma 6) - p (Gamma 8) band inversion. Our results strongly support that the spin-orbit coupling is not an essential factor to the band inversion mechanism; on the contrary, it is mainly responsible to the formation of a global band gap for the studied topological insulators. We further theoretically explore the feasibility of tuning the topological order of the studied compounds with two types of strains. The results show that the uniaxial strain can contribute extremely drastic impacts to the band inversion behavior, which provide an effective approach to induce topological phase transition.

cond-mat.mtrl-sci↗

arXiv2013

Disorder-induced significant enhancement in magnetization of ball-milled Fe2CrGa alloy

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 μB 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.

cond-mat.mtrl-sci↗

arXiv2013

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

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.

cond-mat.mtrl-sci↗

arXiv2013

A New Spin Gapless Semiconductors Family: Quaternary Heusler Compounds

Using first-principles calculations, we investigate the band structures of a series of quaternary LiMgPdSn-type Heusler compounds. Our calculation results show that five compounds CoFeMnSi, CoFeCrAl, CoMnCrSi, CoFeVSi and FeMnCrSb possess unique electronic structures characterized by a half-metallic gap in one spin direction while a zero-width gap in the other spin direction showing spin gapless semiconducting behavior. We further analysis the electronic and magnetic properties of all quaternary Heusler alloys involved, and reveal a semi-empirical general rule (total valence electrons number being 26 or 28) for indentifying spin gapless semiconductors in Heusler compounds. The influences of lattice distortion and main-group element change have also been discussed.

cond-mat.mtrl-sci↗

arXiv2012

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

We have investigated the impact of covalent hybridization on martensitic structure and magnetic properties of Ni50Mn5+xGa35-xCu10 shape memory alloys. We found that the lattice distortion ((c-a)/a) of L10 martensite monotonously changes with the substitution of Mn for Ga atoms and shows a kink behavior at Ga(at.%)= 25 due to the weakened covalent effect between main-group and transition-metal atoms. Moreover, owing to the competition between covalence hybridization and magnetic ordering of introduced Mn atoms, the molecular magnetic moment and Curie temperature coincidently show maximums at Ga(at.%)=25 as well. These behaviors are closely associated with corresponding changes of the strength of covalent hybridization. The results therefore suggest that careful control of the concentration of main-group atoms in Heusler alloys can serve as an additional general tuning parameter for searching new multifunctional materials.

cond-mat.mtrl-sci↗

arXiv2012

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 Fe-containing 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 magnetic-field-induced martensitic transformations and large magnetic-entropy changes. A structural and magnetic phase diagram of Fe-doped MnCoGe alloys has been proposed.

cond-mat.mtrl-sci↗

arXiv2012

Compensation effect and magnetostriction in CoCr2-xFexO4

The magnetic compensation and magnetostriction properties in Fe doped CoCr2O4 samples have been investigated. Structural and magnetic measurements imply that the doped Fe3+ ions initially occupy the B1(Cr) sites when x<0.1, and then mainly take the A(Co) sites. This behaviour results in a role conversion of magnetic contributors and a composition compensation between two competitively magnetic sublattices at x=0.1. Temperature dependence compensation has also been found in the samples with x=0.1~0.22, with the compensation temperature in the range of 40~104 K. The Fe3+ doping also modulates the exchange interaction of the system and prevents the formation of long range conical order of spins. The magnetoelectric transition temperature at 23K in CoCr2O4 is shifted to lower temperature by increasing the dopants. The magnetostriction effect in this system has been observed for the first time. The strain has a maximum value of about 280ppm at x=0.4. The magnetostriction is in consistent with the behaviour of the two magnetic compensations.

cond-mat.mtrl-sci↗

arXiv2012

Generalized Slater-Pauling rule for the inverse Heusler compounds

We present extensive first-principles calculations on the inverse full-Heusler compounds having the chemical formula X$_2$YZ where (X = Sc, Ti, V, Cr or Mn), (Z = Al, Si or As) and the Y ranges from Ti to Zn. Several of these alloys are identified to be half-metallic magnets. We show that the appearance of half-metallicity is associated in all cases to a Slater-Pauling behavior of the total spin-magnetic moment. There are three different variants of this rule for the inverse Heusler alloys depending on the chemical type of the constituent transition-metal atoms. Simple arguments regarding the hybridization of the d-orbitals of neighboring atoms can explain these rules. We expect our results to trigger further experimental interest on this type of half-metallic Heusler compounds.

cond-mat.mtrl-sci↗

arXiv2012

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.

cond-mat.mtrl-sci↗

arXiv2012

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 and Al are substituted by the magnetic or non-magnetic transition metals, Co, Cu, V, and Ti in the alloy systems. 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 6.18μB /atom. In addition, interesting results for non-magnetic Cu increasing the magnetization and the V atom having a large ferromagnetic moment of about 1.0μB 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 TC. 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 effect from the Ga and Al determines the generation of the local ferromagnetic structure and the tolerance for dopant content.

cond-mat.mtrl-sci↗

arXiv2010

Field-induced multiple metamagnetization in phase transition from paramagnetic austenite to ferromagnetic martensite in MnNi1-xFexGe

Tailoring the phase stability and magnetic structure by combining the antiferromagnetic MnNiGe martensite with ferromagnetic MnFeGe austenite, a magnetic-field-induced martensitic transformation has been achieved in MnNi1-xFexGe system. The doped Fe enables the isolated structural and magnetic transitions of MnNiGe to coincide at room temperature and also establishes ferromagnetic couplings in six-Mn-centering-one-Fe-atom clusters. The ferromagnetic clusters transit the antiferromagnetic state in martensite to a ferromagnetic state and the alloys are further induced to a stronger ferromagnetic state by external field, resulting in a large magnetization difference up to 50 emu/g across the transformation. The field-induced martensitic transformation thus experiences a multiple metamagnetization process.

cond-mat.mtrl-sci↗