Yunlong Jin

Structural disorder and magnetism in the spin-gapless semiconductor CoFeCrAl

Disordered CoFeCrAl and CoFeCrSi0.5Al0.5 alloys have been investigated experimentally and by first-principle calculations. The melt-spun and annealed samples all exhibit Heusler-type superlattice peaks, but the peak intensities indicate a substantial degree of B2-type chemical disorder. Si substitution reduces the degree of this disorder. Our theoretical analysis also considers several types of antisite disorder (Fe-Co, Fe-Cr, Co-Cr) in Y-ordered CoFeCrAl and partial substitution of Si for Al. The substitution transforms the spin-gapless semiconductor CoFeCrAl into a half-metallic ferrimagnet and increases the half-metallic band gap by 0.12 eV. Compared CoFeCrAl, the moment of CoFeCrSi0.5Al0.5 is predicted to increase from 2.01 μB to 2.50 μB per formula unit, in good agreement with experiment.

Magnetism and electronic structure of CoFeCrX (X = Si, Ge) Heusler alloys

The structural, electronic, and magnetic properties of CoFeCrX (X = Si, Ge) Heusler alloys have been investigated. Experimentally, the alloys were synthesized in the cubic L21 structure with small disorder. The cubic phase of CoFeCrSi was found to be highly stable against heat treatment, but CoFeCrGe disintegrated into other new compounds when the temperature reached 402 °C (675 K). Although the first-principle calculation predicted the possibility of tetragonal phase in CoFeCrGe, the tetragonal phase could not be stabilized experimentally. Both CoFeCrSi and CoFeCrGe compounds showed ferrimagnetic spin order at room temperature and have Curie temperatures (TC) significantly above room temperature. The measured TC for CoFeCrSi is 790 K but that of CoFeCrGe could not be measured due to its dissociation into new compounds at 675 K. The saturation magnetizations of CoFeCrSi and CoFeCrGe are 2.82 μB/f.u. and 2.78 μB/f.u., respectively, which are close to the theoretically predicted value of 3 μB/f.u. for their...

Phase composition and nanostructure of Zr2Co11-based alloys

The effect of Mo addition on phase composition and nanostructure of nanocrystalline Zr16Co84−xMox (x = 0–2.0) melt spun at 55 m/s has been investigated. All the ribbons consist mainly of a hard magnetic Zr2Co11 phase with rhombohedral crystal structure but also contain minor amounts of soft-magnetic phases. The increase in cell volume on alloying suggests that Mo mainly enters the rhombohedral Zr2Co11 structure and occupies the Co site. Mo addition promotes the formation of the hard magnetic phase and increases its volume fraction. The mean grain size of the hard magnetic phase remains almost unchanged with the increase of Mo content. But the average grain size of the soft magnetic phase decreases from about 200 nm to 50 nm. This promotes the exchange coupling of the hard and soft magnetic phases and thus leads to a significant increase in coercivity and isotropic energy product, from 0.6 kOe and 0.5 MGOe for x = 0 to 2.9 kOe and 4.2 MGOe for x = 1.5.

Half-metallicity in highly L21-ordered CoFeCrAl thin films

The structural, magnetic, and electron-transport properties of Heusler-ordered CoFeCrAl thin films are investigated experimentally and theoretically. The films, sputtered onto MgO and having thicknesses of about 100 nm, exhibit virtually perfect single-crystalline epitaxy and a high degree of L21 chemical order. X-ray diffraction and transmission-electron microscopy show that the structure of the films is essentially of the L21 Heusler type. The films are ferrimagnetic, with a Curie temperature of about 390 K, and a net moment of 2 μB per formula unit. The room temperature resistivity is 175 μΩ cm; the carrier concentration and mobility determined from the low temperature (5 K) measurement are 1.2 × 1018 cm−3 and 33 cm2/V s, respectively. In contrast to the well-investigated Heusler alloys such as Co2(Cr1−xFex)Al, the CoFeCrAl system exhibits two main types of weak residual A2 disorder, namely, Co-Cr disorder and Fe-Cr disorder, the latter conserving half-metallicity. Point-contact Andreev reflection yiel...

Magnetic force microscopy study of Zr 2 Co 11 -based nanocrystalline materials: effect of Mo addition

The addition of Molybdenum was used to modify the nanostructure and enhance coercivity of rare-earth-free Zr2Co11-based nanocrystalline permanent magnets. The effect of Mo addition on magnetic domain structures of melt spun nanocrystalline Zr16Co84-xMox (x = 0, 0.5, 1, 1.5, and 2.0) ribbons has been investigated. It was found that magnetic properties and local domain structures are strongly influenced by Mo doping. The coercivity of the samples increases with the increase in Mo content (x ≤ 1.5). The maximumenergy product (BH)max increases with increasing x from 0.5 MGOe for x = 0 to a maximum value of 4.2 MGOe for x = 1.5. The smallest domain size with a relatively short magnetic correlation length of 128 nm and largest root-mean-square phase shift Φrms value of 0.66° are observed for the x = 1.5. The optimal Mo addition promotes magnetic domain structure refinement and thus leads to a significant increase in coercivity and energy product in this sample.

Effect of boron doping on nanostructure and magnetism of rapidly quenched Zr2Co11-based alloys

The role of B on the microstructure and magnetism of Zr16Co82.5-x Mo1.5B x ribbons prepared by arc melting and melt spinning is investigated. Microstructure analysis show that the ribbons consist of a hard-magnetic rhombohedral Zr2Co11 phase and a minor amount of soft-magnetic Co. We show that the addition of B increases the amount of hard-magnetic phase, reduces the amount of soft-magnetic Co and coarsens the grain size from about 35 nm to 110 nm. There is a monotonic increase in the volume of the rhombohedral Zr2Co11 unit cell with increasing B concentration. This is consistent with a previous theoretical prediction that B may occupy a special type of large interstitial sites, called interruption sites. The optimum magnetic properties, obtained for x = 1, are a saturation magnetization of 7.8 kG, a coercivity of 5.4 kOe, and a maximum energy product of 4.1 MGOe.

Effect of disorder on the resistivity of CoFeCrAl films

Structural and electron-transport properties of thin films of the ferrimagnetic Heusler compound CoFeCrAl have been investigated to elucidate structure-property relationships. The alloy is, ideally, a spin-gapless semiconductor, but structural disorder destroys the spin-gapless character and drastically alters the transport behavior. Two types of CoFeCrAl films were grown by magnetron sputtering deposition at 973 K, namely polycrystalline films on Si substrates and epitaxial films on MgO (001) substrates. The resistivity decreases with increasing temperature, with relatively small temperature coefficients of –0.19 μΩcm/K for the polycrystalline films and –0.12 μΩcm/K for the epitaxial films. The residual resistivity of the polycrystalline films deposited on Si is higher than that of the epitaxial film deposited on MgO, indicating that the polycrystalline films behave as so-called dirty metals.

Effect of disorder on the magnetic and electronic structure of a prospective spin-gapless semiconductor MnCrVAl

Recent discovery of a new class of materials, spin-gapless semiconductors (SGS), has attracted considerable attention in the last few years, primarily due to potential applications in the emerging field of spin-based electronics (spintronics). Here, we investigate structural, electronic, and magnetic properties of one potential SGS compound, MnCrVAl, using various experimental and theoretical techniques. Our calculations show that this material exhibits ≈ 0.5 eV band gap for the majority-spin states, while for the minority-spin it is nearly gapless. The calculated magnetic moment for the completely ordered structure is 2.9 μB/f.u., which is different from our experimentally measured value of almost zero. This discrepancy is explained by the structural disorder. In particular, A2 type disorder, where Mn or Cr atoms exchange their positions with Al atoms, results in induced antiferromagnetic exchange coupling, which, at a certain level of disorder, effectively reduces the total magnetic moment to zero. This...

Structural, magnetic, and electron-transport properties of epitaxial Mn2PtSn films

The growth of new magnetic materials on suitable insulating substrates is an important part of the development of spin-electronics devices for memory or information processing. Epitaxial thin films of Mn2PtSn were grown on a MgO [001] substrate by magnetron co-sputtering of the constituents. Structural, magnetic, and electron-transport properties were investigated. The epitaxial Mn2PtSn film has an inverse tetragonal structure with the c-axis aligned in the plane of the MgO substrate. The lattice constants determined using XRD and TEM analysis are c = 6.124 A and a = b = 4.505 A. The orientation of Mn2PtSn c-axis which is 45° away from the a-axis of MgO has resulted in a small lattice mismatch of about 2.8%. The measured saturation magnetization is 5.3 μB/f.u., which is smaller than the first-principles calculated value of 6.4 μB/f.u. for ferromagnetic spin arrangement. Magnetization measurements determined the bulk magnetocrystalline anisotropy constant Kv of about 11.3 Merg/cm3 (1.13 MJ/m3). The electron-transport behavior is similar to that of normal magnetic metals. These results indicate that Mn2PtSn may have promising applications in spintronic devices.The growth of new magnetic materials on suitable insulating substrates is an important part of the development of spin-electronics devices for memory or information processing. Epitaxial thin films of Mn2PtSn were grown on a MgO [001] substrate by magnetron co-sputtering of the constituents. Structural, magnetic, and electron-transport properties were investigated. The epitaxial Mn2PtSn film has an inverse tetragonal structure with the c-axis aligned in the plane of the MgO substrate. The lattice constants determined using XRD and TEM analysis are c = 6.124 A and a = b = 4.505 A. The orientation of Mn2PtSn c-axis which is 45° away from the a-axis of MgO has resulted in a small lattice mismatch of about 2.8%. The measured saturation magnetization is 5.3 μB/f.u., which is smaller than the first-principles calculated value of 6.4 μB/f.u. for ferromagnetic spin arrangement. Magnetization measurements determined the bulk magnetocrystalline anisotropy constant Kv of about 11.3 Merg/cm3 (1.13 MJ/m3). The electro...

Spin localized magnetism and electron transport in Fe 2 Ti 1−x Co x Si

Bulk alloys of Co-substituted Fe<inf>2</inf>TiSi, forming the series Fe<inf>2</inf>Ti<inf>1−x</inf>Co<inf>x</inf>Si (x = 0, 0.25, 0.75 and 1), have been investigated by first-principle density functional calculations.