Bishnu Dahal

Electrical transport and magnetic properties of cobalt telluride nanostructures

Transition metal telluride CoTe nanostructures have been synthesized using a wet-chemical method. These nanostructures exhibit NiAs-type hexagonal crystal structure with a space group of P63/mmc. The magnetic measurements show spontaneous magnetization and hysteresis, and the coercivity increases with a decrease in temperature. The saturation magnetization was calculated to be around 0.2 μB/Co atom. The magnetic transition temperature was found to be above room temperature similar to that of the bulk CoTe. The measured bandgap of the nanostructures is 2.15 eV and they exhibit p-type semiconducting behavior.

Ferrimagnetic Co1+δ Te nanostructures

Transition metal telluride, Co1+δ Te nanostructures are synthesized using the hydrothermal method. These nanostructures exhibit filled NiAs-type hexagonal crystal structure also known as Ni2In structure with the space group p63/mmc. The Co1+ δ Te nanostructures exhibit hard ferrimagnetic behavior below 40 K. The coercivity increases with the decrease in temperature, which is around 4.90 kOe at 3 K. The saturation magnetization is 0.6 μ B/Co atom. Electrical transport measurements show that the Co1+ δ Te nanostructures are nonmetallic in nature with the resistance increases with the decrease in temperature. It does not follow the thermal excitation law for semiconductors, but can be explained by the Motts three-dimensional variable range hopping model.

Ultrahigh vacuum deposition of higher manganese silicide Mn4Si7 thin films

The authors have successfully grown one of the higher manganese silicides, Mn4Si7 thin films on silicon (100) substrates using an ultrahigh vacuum deposition with a base pressure of 1 × 10−9 Torr. The thickness of the film was varied from 65 to 100 nm. These films exhibit a tetragonal crystal structure and display paramagnetic behavior as predicted for the stoichiometric Mn4Si7 system. They have a resistivity of 3.32 × 10−5 Ω m at room temperature and show a semimetallic nature.

Ferromagnetism in Fe2CrAl nanowires

The authors have successfully grown Fe2CrAl nanowires on polished Si/SiO2 substrates using the electrospinning method. The diameter of nanowires varies from 50 to 300 nm. These nanowires exhibit a cubic crystal structure with lattice disorder. The nanowires are ferromagnetic with a Curie temperature greater than 400 K, much higher than that the bulk reported value. This higher Curie temperature is attributable to disordering in lattice sites, which causes an enhanced exchange interaction between pairs of iron atoms.

Magnetic nanoparticle-loaded alginate beads for local micro-actuation of in vitro tissue constructs

Magnetic nanoparticles (MNPs) self-align and transduce magnetic force, two properties which lead to promising applications in cell and tissue engineering. However, the toxicity of MNPs to cells which uptake them is a major impediment to applications in engineered tissue constructs. To address this problem, MNPs were embedded in millimeter-scale alginate beads, coated with glutaraldehyde cross-linked chitosan, and loaded in acellular and MDA-MB-231 cancer cell-seeded collagen hydrogels, providing local micro-actuation under an external magnetic field. Brightfield microscopy was used to assess nanoparticle diffusion from the bead. Phase contrast microscopy and digital image correlation were used to track collagen matrix displacement and estimate intratissue strain under magnetic actuation. Coating the magnetic alginate beads with glutaraldehyde-chitosan prevents bulk diffusion of nanoparticles into the surrounding microenvironment. Further, the beads exert force on the surrounding collagen gel and cells, resulting in intratissue strains of 0-10% tunable with bead dimensions, collagen density, and distance from the bead. Cells seeded adjacent to the embedded beads are subjected to strain gradients without loss of cell viability over two days culture. This study describes a simple way to fabricate crosslinked magnetic alginate beads to load in a collagen tissue construct without direct exposure of the construct to nanoparticles. The findings are significant to in vitro studies of mechanobiology in enabling precise control over dynamic mechanical loading of tissue constructs.

Ferromagnetic, spin glass, and antiferromagnetic behaviors in Cd1−xMnxTe nanowires

The Cd1−xMnxTe (x = 0–0.18) nanowires were grown using a wet-chemical synthesis. The synthesized Cd1−xMnxTe nanowires have diameters in the range of 50–100 nm and display zinc-blend crystal structure. In bulk, Cd1−xMnxTe is extensively studied, but the difficulty in doping Mn in CdTe nanowires delayed the understanding of the properties at the nanoscale. The authors have used a cation exchange method to incorporate Mn in CdTe nanowires. Their magnetic behavior can be tuned by varying the concentration of Mn ions. The CdTe nanowires were paramagnetic while doping small amount of Mn ions introduces ferromagnetic behavior at low temperatures. As the manganese concentration is increased in CdTe, both spin glass and antiferromagnetic behaviors are observed. This is in contrast to what is observed in bulk, where only paramagnetic behavior is observed for x < 0.17.The Cd1−xMnxTe (x = 0–0.18) nanowires were grown using a wet-chemical synthesis. The synthesized Cd1−xMnxTe nanowires have diameters in the range of 50–100 nm and display zinc-blend crystal structure. In bulk, Cd1−xMnxTe is extensively studied, but the difficulty in doping Mn in CdTe nanowires delayed the understanding of the properties at the nanoscale. The authors have used a cation exchange method to incorporate Mn in CdTe nanowires. Their magnetic behavior can be tuned by varying the concentration of Mn ions. The CdTe nanowires were paramagnetic while doping small amount of Mn ions introduces ferromagnetic behavior at low temperatures. As the manganese concentration is increased in CdTe, both spin glass and antiferromagnetic behaviors are observed. This is in contrast to what is observed in bulk, where only paramagnetic behavior is observed for x < 0.17.

Core–shell FeNi–NixFe3−xO4 nanowires

Core–shell Fe0.7Ni0.3–NixFe3−xO4 nanowires were fabricated using a three step process. Initially, NiFe2O4 nanowires were fabricated using the electrospinning method; these nanowires were reduced to form Fe0.7Ni0.3 nanowires. The Fe0.7Ni0.3 nanowires were then naturally oxidized to form a shell of NixFe3−xO4 on the surface, obtaining Fe0.7Ni0.3–NixFe3−xO4 core–shell nanowires. The core Fe0.7Ni0.3 and the shell NixFe3−xO4 are crystalline in nature. The core–shell structure is very stable, and even after prolonged exposure to dry air, it maintains the core–shell structure and the magnetic hysteresis character of the bimagnetic system.