Ravi Kumar

Magnetocaloric effect in Heusler alloys Ni50Mn34In16 and Ni50Mn34Sn16

We present results of detailed ac susceptibility, magnetization and specific heat measurements in Heusler alloys Ni50Mn34In16 and Ni50Mn34Sn16. These alloys undergo a paramagnetic to ferromagnetic transition around 305 K, which is followed by a martensitic transition in the temperature regime around 220 K. Inside the martensite phase both the alloys show signatures of field-induced transition from martensite to austenite phase. Both field- and temperature-induced martensite–austenite transitions are relatively sharp in Ni50Mn34In16. We estimate the isothermal magnetic entropy change and adiabatic temperature change across the various phase transitions in these alloys and investigate the possible influence of these transitions on the estimated magnetocaloric effect. The sharp martensitic transition in Ni50Mn34In16 gives rise to a comparatively large inverse magnetocaloric effect across this transition. On the other hand the magnitudes of the conventional magnetocaloric effect associated with the paramagnetic to ferromagnetic transition are quite comparable in these alloys.

Evaluation of manganite films on silicon for uncooled bolometric applications

Pulsed-laser-deposited polycrystalline/amorphous films of mixed-valent manganites [La0.7Ca0.3MnO3 (LCMO), La0.5Sr0.5MnO3 (LSMO), La0.5Ba0.5MnO3 (LBMO), and (La0.6Pr0.4)0.67Ca0.33MnO3 (LPCMO)] grown at low temperature (450u200a°C) on single crystal (001) silicon substrate are evaluated for uncooled bolometric applications. It is shown that the temperature coefficient of resistance (TCR) and electrical noise (Sv) depend on the chemical composition. The optimum performance is found for LCMO with TCR of ∼7% K−1 and spectral noise ∼8.9×10−13u2002V2/Hz. In LBMO and LSMO the noise is much lower, but so is the TCR (1.5%–2%u2002K−1). In LPCMO the TCR is high (∼10%u2002K−1) and the noise is as well.

The effect of substitution of Mn by Fe and Cr on the martensitic transition in the Ni50Mn34In16 alloy

The potential shape memory alloy Ni(50)Mn(34)In(16) is studied with partial substitution of Mn with Fe and Cr to investigate the effect of such substitution on the martensitic transition in the Ni-Mn-In alloy system. The results of ac susceptibility, magnetization and electrical resistivity measurements show that while the substitution with Cr increases the martensitic transition temperature, the substitution with Fe decreases it. Possible reasons for this shift in martensitic transition are discussed. Evidence of kinetic arrest of the austenite to martensite phase transition in the Fe substituted alloys is also presented. Unlike the kinetic arrest of the austenite to martensite phase transition in the parent Ni(50)Mn(34)In(16) alloy which takes place in the presence of high external magnetic field, the kinetic arrest of the austenite to martensite phase transition in the Fe doped alloy occurs even in zero magnetic field. The Cr substituted alloys, on the other hand, show no signature of kinetic arrest of this phase transition.

Exposition of semiconducting and ferromagnetic properties of pulsed-laser-deposited thin films of LaFe1-xNixO3 (x=0.3, 0.4, and 0.5)

We have explored the possibility of ferromagnetic semiconducting property in the epitaxial thin films of LaFe1−xNixO3 (x=0.3, 0.4, and 0.5) grown on (001) oriented LaAlO3 substrate. We observe that substitution of Ni in the series leads to the increase in conductivity of the samples with conduction being controlled by the disorder-induced localization of charge carriers. All these samples show ferromagnetic behavior at room temperature while their magnetization decreases with increase in Ni concentration in the composition. The results have been explained on the basis of the close interplay between the electrical and magnetic properties.

Magnetoelectric properties of BixCo2−xMnO4 (0⩽x⩽0.3)

We present the structural, dielectric and magnetization study of single phase polycrystalline BixCo2−xMnO4 (0⩽x⩽0.3), synthesized by a conventional solid state route. All the samples have the cubic spinel structure with Fd3m space group. Bi-substitution in Co2MnO4 stabilizes the ferroelectric transition at a temperature of ∼350K and enhances the dielectric constant with a relaxor behavior. The capacitance-voltage (C-V) measurements confirm the ferroelectric nature at room temperature. Ferrimagnetic nature of the Co2MnO4 is preserved in the Bi-substituted samples. Magnetocapacitive coupling proves candidature of these materials from an application point of view.

Observation of electron confinement in InP/GaAs type-II ultrathin quantum wells

The issue of type-II band alignment for InP/GaAs heterostructure is addressed by means of simple layer architecture of ultrathin quantum wells (QWs). From specific signatures of the radiative recombination in type-II QWs especially the cube root dependence of blueshift in the lowest excitonic transition energy on excitation power in photoluminescence measurements indicates that the observed luminescence is originating from spatially separated electrons and holes. Such a blueshift is seen to increase with the QW thickness again confirming a type-II band alignment. A direct evidence of electron confinement in the conduction band of InP is provided by the capacitance voltage measurements.

Magnetic and calorimetric investigations of ferromagnetic shape memory alloy Ni54Fe19Ga27

We report results of magnetization and differential scanning calorimetry measurements in the ferromagnetic shape memory alloy Ni54Fe19Ga27. This alloy undergoes an austenite–martensite phase transition in its ferromagnetic state. The nature of the ferromagnetic state, both in the austenite and the martensite phase, is studied in detail. The ferromagnetic state in the martensite phase is found to have higher anisotropy energy as compared with the austenite phase. The estimated anisotropy constant is comparable to that of a well-studied ferromagnetic shape memory alloy system NiMnGa. Further, the present study highlights various interesting features accompanying the martensitic transition (MT). These features suggest the possibility of either a premartensitic transition and/or an inter-MT in this system.

Determination of band offsets in strained InAsxP1−x/InP quantum well by capacitance voltage profile and photoluminescence spectroscopy

InAsxP1−x/InP quantum wells (QWs) with excellent crystalline and interfacial quality are grown by metal organic vapor phase epitaxy as confirmed from the cross-sectional transmission electron microscopy, high resolution x-ray diffraction and photoluminescence measurements. The electron confinement in InAsxP1−x/InP QW states is determined by capacitance voltage measurements, where we find that the electron accumulation increases with increasing QWs thickness and arsenic composition. This is explained by the variation of the band offset and hence the effective change in the position of the electronic energy level from Fermi level with QWs composition and thickness. The conduction band offset (ΔEc) for InAsxP1−x/InP QWs has been obtained by solving the self consistent set of Schrodinger and Poisson equations and fitting the theoretical carrier density profile with the apparent carrier density measured from experiments. The ΔEc values in strained InAsxP1−x/InP QWs have been obtained which fits to the expressi...

Low- and high-density InAs nanowires on Si(0 0 1) and their Raman imaging

Micro-Raman imaging along with other techniques are applied to study the morphology, structure and crystalline quality of various types of InAs nanowires (NWs). The NWs of low and high densities are formed using metal organic vapor phase epitaxy. Raman mapping is effectively used as a local probe to gain information about the structure and crystalline quality of low-density NWs where the conventional characterization techniques are not very useful. However, for high-density NWs, the image and crystalline quality obtained from the LO phonon strongly corroborate with scanning electron microscopy and x-ray diffraction (XRD) results, respectively. These low-density (104xa0cm−2) and high-density (108xa0cm−2) NWs are grown on Si(0u20090u20091) under various growth conditions such as catalyst-assisted and catalyst-free growth, growth on native oxide-covered and oxide-cleaned Si, grooved Si surfaces and also varying the V/III ratio and growth temperature. NWs (1xa0µm long and 50–100xa0nm wide) with high density and tapered NWs (50–80xa0µm long and 200–500xa0nm wide at the tip) with low density are formed under different growth conditions. The growth of hillock- and wire-like structures is observed under the same growth condition. Raman, XRD, scanning electron microscopy and atomic force microscopy analyses confirm that the hillocks are grown along the 〈0u20090u20091〉 direction, whereas the wires are grown along [1u20091u20090] directions in the plane of Si(0u20090u20091). Furthermore, the Raman analysis of these NWs confirms that the smaller NWs have much better crystalline quality (half-width of LO phonon frequency ∼6xa0cm−1) compared to the larger NWs (half-width of LO phonon frequency ∼15xa0cm−1) although both NWs are oriented with the Si(0u20090u20091) surface.

Conduction band offset and quantum states probed by capacitance–voltage measurements for InP/GaAs type-II ultrathin quantum wells

Quantum states in InP/GaAs type-II ultrathin quantum wells (QWs) are investigated through temperature dependent capacitance–voltage (C–V) measurements. We observe a well-defined peak in the apparent carrier density (ACD) profile for the ultrathin QWs at low temperatures in the vicinity of QWs. ACD peak value is found to decrease with the reduction in QW thickness, indicating quantum confinement effect. Decrease in the ACD peak value and increase in its width with increasing temperature confirms that the observed peak in the ACD profiles is related to the two dimensional electrons occupying the quantum states formed in the ultrathin QWs. We do not observe appreciable peak shift in ACD profiles with temperature, which is attributed to the less temperature dependence of the Debye length because of the high doping density used in the barrier region of InP/GaAs ultrathin QWs. We determine a strained value of 180u2009±u200930 meV for the conduction band discontinuity by simulating the C–V profile through the self-consi...