Yung Huh

Magnetism and electron transport of MnyGa (1 < y < 2) nanostructures

Nanostructured MnyGa ribbons with varying Mn concentrations including Mn1.2Ga, Mn1.4Ga, Mn1.6Ga, and Mn1.9Ga were prepared using arc-melting and melt-spinning followed by a heat treatment. Our experimental investigation of the nanostructured ribbons shows that the material with y = 1.2, 1.4, and 1.6 prefers the tetragonal L10 structure and that with y = 1.9 prefers the D022 structure. We have found a maximum saturation magnetization of 621 emu/cm3 in Mn1.2Ga which decreases monotonically to 300 emu/cm3 as y reaches 1.9. Although both the L10- and D022-MnyGa samples show a high Curie temperature (Tc) well above room temperature, the value of Tc decreases almost linearly from 702 K for Mn1.9Ga to 551 K for Mn1.2Ga. All the ribbons are metallic between 2 K and 300 K but the Mn1.2Ga also shows a resistance minimum near 15 K. The observed magnetic properties of the MnyGa ribbons are consistent with the competing ferromagnetic coupling between Mn moments in the regular L10-MnGa lattice sites and antiferromagnet...

Magnetic properties of NiO and (Ni, Zn)O nanoclusters

Rock-salt NiO and Ni0.7Zn0.3O nanoparticles were investigated by x-ray diffraction, atomic-force microscopy, and magnetic measurements. Nanoparticle diameters varied from 8 to 30 nm depending on reaction conditions. There are two main magnetization contributions, the field-induced spin canting of the antiferromagnetic sublattices and the magnetization rotation caused by uncompensated spins interacting with the magnetic field. The former is a bulk effect, modified by the presence of Zn, whereas the latter is a nanoscale effect that increases with decreasing particle size. The relative contributions of the two effects depend on particle size with a critical size of about 18 nm resulting in bulklike behavior.

Magnetic and Structural Properties of Rapidly Quenched Tetragonal Mn

Nanostructured Mn_3-x Ga ribbons with x = 0, 0.4, 0.9 and 1.1 were prepared using arc-melting, melt-spinning and annealing. As-spun samples crystallized into hexagonal D0₁₉ and cubic L2₁ Heusler crystal structures based on the concentration of Mn in Mn_3-x Ga. Upon vacuum-annealing the samples at 450 °C for about 50 hours, both the hexagonal and cubic structures transformed into a tetragonal D0₂₂ structure. High-temperature x-ray diffraction and high-temperature magnetometry showed that the samples with low Mn content (Mn_1.9 Ga and Mn_2.1 Ga) retain their tetragonal structure up to 850 K but the samples with high Mn concentrations (Mn_2.6 Ga and Mn_3.0 Ga) undergo a structural phase transition from tetragonal to hexagonal phases around 800 K. The magnetic properties of Mn_3-x Ga ribbons were very sensitive to Mn concentration, where the magnetization and anisotropy energy increased and the coercivity decreased as x increased from 0 to 1.1. Although the Curie temperatures of Mn_2.6 Ga and Mn_3.0 Ga samples could not be determined because of the structural phase transition, the Curie temperature decreased with increasing x in Mn_3-x Ga. The maximum magnetization of 57 emu/g (300 emu/cm³) and the coercivity of 6.5 kOe were measured in the Mn_1.9 Ga and Mn_3.0 Ga ribbons, respectively.

_{3 - {\rm x}}

The structural, magnetic and electron-transport properties of cubic Mn3Ga have been investigated. The alloys prepared by arc melting and melt-spinning show an antiferromagnetic spin order at room temperature but undergo coupled structural and magnetic phase transitions at 600 and 800 K. First-principles calculations show that the observed magnetic properties are consistent with that of a cubic Mn3Ga crystallizing in the disordered Cu3Au-type structure. The samples exhibit metallic electron transport with a resistance minimum near 30 K, followed by a logarithmic upturn below the minimum. The observed anomaly in the low-temperature resistivity has been discussed as a consequence of electron scattering at the low-lying excitations of the structurally disordered Mn3Ga lattice.

Ga Nanostructures

The increasing interest in spin-based electronics has led to a vigorous search for new materials that can provide a high degree of spin polarization in electron transport. An ideal candidate would act as an insulator for one spin channel and a conductor or semiconductor for the opposite spin channel, corresponding to the respective cases of half-metallicity and spin-gapless semiconductivity. Our first-principle electronic-structure calculations indicate that the metallic Heusler compound Ti2MnAl becomes half-metallic and spin-gapless semiconducting if half of the Al atoms are replaced by Sn and In, respectively. These electronic structures are associated with structural transitions from the regular cubic Heusler structure to the inverted cubic Heusler structure.

Structural and magnetic transitions in cubic Mn3Ga

Structural, electronic, and magnetic properties of a Heusler-type CoFeCrAl alloy have been investigated experimentally and by model calculations, with a focus on the alloys spin-gapless semiconductivity. The as-quenched samples are ferrimagnetic at room temperature with a Curie temperature of about 456 K, which increases to 540 K after vacuum annealing at 600 °C for 2 h. The saturation magnetizations of the as-quenched and 600 °C-annealed samples are 1.9 µB/f.u. and 2.1 µB/f.u., respectively, which are very close to the value predicted by the Slater–Pauling curve. The resistivity shows a nearly linear decrease with increasing temperature, from about 930 µΩ cm at 5 K to about 820 µΩ cm at 250 K, with dρ/dT of about −5 × 10−7 Ω cm K−1. We explain this high resistivity and its temperature dependence as imperfect spin-gapless semiconducting behavior, with a negative band-gap parameter of 0.2 eV.

Investigation of spin-gapless semiconductivity and half-metallicity in Ti2MnAl-based compounds

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, electron transport and effect of disorder in CoFeCrAl

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...

Structural disorder and magnetism in the spin-gapless semiconductor CoFeCrAl

Since industrial revolution by the end of nineteenth century, the consumption of fossil fuels to drive the economy has grown exponentially causing three primary global problems: depletion of fossil fuels, environmental pollution, and climate change (Andreev and Grilikhes, 1997). The population has quadrupled and our energy demand went up by 16 times in the 20th century exhausting the fossil fuel supply at an alarming rate (Bartlett, 1986; Wesiz, 2004). By the end of 2035, about 739 quadrillion Btu of energy (1 Btu = 0.2930711 Whr) of energy would be required to sustain current lifestyle of 6.5 billion people worldwide (US energy information administration, 2010). The increasing oil and gas prices, gives us enough region to shift from burning fossil fuels to using clean, safe and environmentally friendly technologies to produce electricity from renewable energy sources such as solar, wind, geothermal, tidal waves etc (Kamat, 2007). Photovoltaic (PV) technologies, which convert solar energy directly into electricity, are playing an ever increasing role in electricity production worldwide. Solar radiation strikes the earth with 1.366 KWm-2 of solar irradiance, which amounts to about 120,000 TW of power (Kamat 2007). Total global energy needs could thus be met, if we cover 0.1% of the earth’s surface with solar cell module with an area 1 m2 producing 1KWh per day (Messenger and Ventre, 2004). There are several primary competing PV technologies, which includes: (a) crystalline (c-Si), (b) thin film (a-Si, CdTe, CIGS), (c) organic and (d) concentrators in the market. Conventional crystalline silicon solar cells, also called first generation solar cells, with efficiency in the range of 15 21 %, holds about 85 % of share of the PV market (Carabe and Gandia, 2004). The cost of the electricity generation estimates to about

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

4/W which is much higher in comparison to