A. K. Das

Temperature dependence of solubility limits of transition metals (Co, Mn, Fe, and Ni) in ZnO nanoparticles

X-ray diffraction studies on bulk amount of chemically prepared nanocrystalline powder of Zn1−xTMxO (TM=Co, Mn, Fe, and Ni) show that the evolution of secondary phases (Co3O4, Mn3O4, Fe3O4, or NiO) along with the single phase Zn1−xTMxO strongly depend on growth temperature and doping concentration. The highest solubility limits of Co, Mn, Fe, and Ni in ZnO are 30%, 30%, 20%, and 3% (atomic weight), respectively. The magnetization measurement shows that the secondary phase formation reduces the magnetization of single phase Zn1−xTMxO, which may be the important clue that the secondary phase is not responsible for magnetism in Zn1−xTMxO.

Microstructural and magnetic properties of ZnO:TM (TM=Co,Mn) diluted magnetic semiconducting nanoparticles

We have investigated the structural and the magnetic properties of 3d transition metal (TM) doped Zn1−xTMxO (TM=Co,Mn) diluted magnetic semiconducting nanoparticles for different doping concentrations (0⩽x⩽0.4) synthesized by chemical “pyrophoric reaction process.” From x-ray diffraction measurements the solubility limits of Co and Mn in ZnO nanoparticles are found to be strongly dependent on growth (calcinations) temperature (Tg). The highest solubility limit of both Co2+ and Mn2+ in ZnO at Tg∼300°C is found to be ∼30%. High resolution transmission electron microscopy studies show that Zn1−xTMxO particles are single crystalline of high quality with a wide particle size distribution in nanometric regime. The non-mean-field-like very strong concave nature of temperature dependent magnetization curves is observed at very low temperature in both the systems without showing any distinct magnetic transition. The magnetic behaviors of those Mn2+ and Co2+ doped ZnO semiconducting nanoparticles are observed to be...

Spin glass behaviour and extrinsic origin of magnetodielectric effect in non-multiferroic La2NiMnO6 nanoparticles

We report magnetization, dielectric and dc transport properties of La(2)NiMnO(6) nanoparticles. Both dc and ac magnetization measurements indicated a metastable magnetic behaviour with random ferromagnetic and antiferromagnetic interactions below 110 K; critical slow-down, memory and rejuvenation properties signify the spin glass nature. The dc resistivity shows a semiconducting nature but the temperature dependent magnetoresistance (MR) shows a peak at the spin glass transition. The colossal dielectric property and its frequency dependence were interpreted using the Maxwell-Wagner (MW) interfacial polarization model. Impedance analysis along with magnetodielectric (MD) and magnetoresistance (MR) indicates that the observed MD originates from the combined effect of MR and MW interfacial polarization.

Self-assembled Ge nanostructures on polymer-coated silicon: Growth and characterization

Self-assembled Ge nanoparticles have been grown on polymer-coated Si substrates by thermal evaporation under high vacuum utilizing the nonwetting condition given by the surface free-energy relation σGe≫σpolymer. The nanostructures have been characterized by Raman spectroscopy, atomic-force microscopy (AFM), and optical microscopy. Raman spectrum shows a prominent Ge–Ge vibration peak at 302 cm−1. AFM and optical microscopy show the formation of isolated Ge islands (≲100 nm base, ≲25 nm height), nanowires (160 nm base, 25 nm height), and islands in linear chains. The possibility of embedding such nanostructures in waveguide structures are discussed.

Magnetic out-of-plane component in MnAs/GaAs(001)

From highly sensitive superconducting quantum interference device magnetometry and magnetic force microscopy, we deduce a small out-of-plane magnetization component of MnAs/GaAs(001) films. Its temperature dependence is substantially different from the dominating in-plane magnetization, particularly as it is still detectable above the phase transition temperature of MnAs films. Our measurements indicate that the out-of-plane component is due to small isolated magnetic “grains” within the film.

Epitaxial growth of silver on Br-passivated Si(111) substrates under high vacuum

Abstract Ag thin films (∼125 nm) were deposited on Br-passivated vicinal (4° miscut) Si(111) surfaces at room temperature under high vacuum conditions. The films have been characterized by Rutherford backscattering spectrometry (RBS) and channeling, X-ray diffraction and transmission electron microscopy and diffraction measurements. The [111] axis of the Ag epilayer is tilted from the substrate [111] orientation by 0.4° towards the substrate surface normal. The films are grainy with a mosaic spread of 0.74°. The crystal quality of the Ag layer improves and the mosaic spread decreases to 0.37° upon annealing in high vacuum at higher temperatures (400 and 500°C) as observed from RBS/channeling and high resolution X-ray diffraction measurements. The tilt angle of the Ag[111] axis and the layer strain also decrease to some extent upon annealing at 500°C.

Spontaneous nanostructural island formation and layer-to-island mass transport in ge layers on si(111) surfaces

We have deposited Ge on Br-passivated Si(111) surfaces under high vacuum (HV) conditions at room temperature (RT). Ge has grown in a layer-plus-island growth mode. Atomic force microscopy (AFM) measurements on the as-deposited samples show the formation of nanostructural islands. On a 500°C-annealed sample, the size and the density of islands increase. High resolution X-ray diffraction (HRXRD) and ion channeling experiments show the lack of epitaxial growth. However, Raman spectroscopy measurements show the polycrystallinity of the Ge layer. X-ray reflectivity (XRR) and Raman spectroscopy results show that the Ge/Si interface is sharp for the as-deposited layer and there is no significant intermixing even in the annealed samples. AFM, XRR and Raman spectroscopy results, taken together, indicate mass transport from the Ge layer to Ge islands. The temperature dependence of this mass transfer provides effective activation energy of 0.45±0.04 eV.

Microstructural, magnetic, and optical properties of Zn1- x(Mnx/2Cox/2)O (x=0.1 and 0.2) semiconducting nanoparticles

We have investigated structural, magnetic, and optical properties of Zn1−x(Mnx∕2Cox∕2)O (x=0.1 and 0.2) diluted magnetic semiconducting nanoparticles synthesized by chemical “pyrophoric reaction process.” X-ray diffraction analysis clearly shows that the samples are single phase in ZnO wurtzite structure, where the average crystallite size of samples is found to be in the nanometric regime (∼10nm). From the Curie-Weiss fit, as well as from the calculated value of effective exchange constant (Jex), which is found to be negative, we can assert that the nature of magnetic ground state of both of these samples are antiferromagnetic (AFM). This is further established by the concave nature of isothermal Arrott-Belov-Kouvel plots at the ground state (5K) without having any spontaneous magnetization in both of the samples. When both Mn and Co dopant concentrations (x) are increased in the ZnO matrix, the magnitude of AFM interaction (∣Jex∣) is found to enhance. This observed magnetic behavior has been best explai...

Large magnetodielectric response in Pr0.6Ca0.4MnO3/polyvinylidene fluoride nanocomposites

We have studied the magnetic field effect on low frequency dielectric properties of Pr0.6Ca0.4MnO3/polyvinylidene fluoride nanocomposite with 22.5% volume fraction of Pr0.6Ca0.4MnO3 nanoparticles. A strong magnetodielectric response was observed below 120 K where Pr0.6Ca0.4MnO3 nanoparticles show the magnetic phase transition indicating a direct correlation between magnetism and dielectric properties. A large change in the dielectric permittivity ∼30% has been observed in a magnetic field of 4.6 T with loss as low as 0.17 at 70 K. The observed magnetodielectric response has been attributed to the decrement of polaron activation barrier of Pr0.6Ca0.4MnO3 nanoparticles with the increase in magnetic field.

Magnetic semiconducting diode of p-Ge1−xMnx/n-Ge layers on silicon substrate

We have synthesized Ge-based magnetic diode composed of a Mn-doped Ge film grown on lightly As-doped Ge on silicon substrate. p-Ge1−xMnx/n-Ge heterostructure behaves like a conventional diode under forward and reverse biases and works like a spin valve below Curie temperature (∼50 K) under zero (B=0) and nonzero (B=300 mT) magnetic fields at forward bias (+2 V). A hysteretic behavior of p-n junction current with small coercive magnetic field implies the nonvolatility of the diode. Thus, a single element of p-Ge1−xMnx/n-Ge on silicon substrate deserves nonvolatility, rectification, and spin-valve-like functionality.