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  • 1. Chastellain, M.
    et al.
    Petri, A.
    Gupta, Amita
    KTH, Superseded Departments, Materials Science and Engineering.
    Rao, K. Venkat
    KTH, Superseded Departments, Materials Science and Engineering.
    Hofmann, H.
    Superparamagnetic silica-iron oxide nanocomposites for application in hyperthermia2004In: Advanced Engineering Materials, ISSN 1438-1656, E-ISSN 1527-2648, Vol. 6, no 4, p. 235-241Article in journal (Refereed)
    Abstract [en]

    Superparamagnetic iron oxide particles embedded in silica are studied for application in hyperthermia. The temperature increase is studied when submitting the samples to a weak alternating magnetic field. The influence of the iron oxide size distribution, saturation magnetization, out of phase susceptibility and anisotropy constant is discussed. A theoretical calculation of power loss is carried out and agrees with experimental data.

  • 2.
    Gu, Ziyan
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Engineering Material Physics.
    Gupta, Amita
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Engineering Material Physics.
    Lopez Ponce, Enrique
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Engineering Material Physics.
    Xu, Zhihui
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Engineering Material Physics.
    Rao, K. V.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Engineering Material Physics.
    Transport and Soft Magnetic Properties of Ultra-high Strength Co43Fe20Ta5.5B31.5 Bulk Glassy alloy2005In: Journal of Materials Research, ISSN 0884-2914, E-ISSN 2044-5326Article in journal (Other academic)
  • 3.
    Gupta, Amita
    KTH, Superseded Departments, Materials Science and Engineering.
    Novel room temperature ferromagnetic semiconductors2004Doctoral thesis, comprehensive summary (Other scientific)
    Abstract [en]

    In today's information world, bits of data are processed bysemiconductor chips, and stored in the magnetic disk drives.But tomorrow's information technology may see magnetism (spin)and semiconductivity (charge) combined in one 'spintronic'device that exploits both charge and 'spin' to carry data (thebest of two worlds). Spintronic devices such as spin valvetransistors, spin light emitting diodes, non-volatile memory,logic devices, optical isolators and ultra-fast opticalswitches are some of the areas of interest for introducing theferromagnetic properties at room temperature in a semiconductorto make it multifunctional. The potential advantages of suchspintronic devices will be higher speed, greater efficiency,and better stability at a reduced power consumption.

    This Thesis contains two main topics: In-depth understandingof magnetism in Mn doped ZnO,and our search and identificationof at least six new above room temperature ferromagneticsemiconductors. Both complex doped ZnO based new materials, aswell as a number of nonoxides like phosphides, and sulfidessuitably doped with Mn or Cu are shown to give rise toferromagnetism above room temperature. Some of the highlightsof this work arediscovery of room temperature ferromagnetismin

        ZnO:Mn

        ZnO doped with Cu (containing no magnetic elements init!)

        GaP doped with Cu (again containing no magnetic elementsin it!)

        Enhancement of Magnetization by Cu co-doping inZnO:Mn

        CdS doped with Mn, and a few others not reported in thisthesis.

    We discuss in detail the first observation of ferromagnetismabove room temperature in the form of powder, bulk pellets, in2-3 μm thick transparent pulsed laser deposited films ofthe Mn (<4 at.% ) doped ZnO. High-resolution transmissionelectron microscopy (HRTEM) and electron energy lossspectroscopy (EELS) spectra recorded from 2 to 200nm areasshowed homogeneous distribution of Mn substituting for Zn a 2+state in the ZnO lattice. Ferromagnetic Resonance(FMR) technique is used to confirm the existence offerromagnetic ordering at temperatures as high as 425K. The abinitio calculations were found to be consistent with theobservation of ferromagnetism arising from fully polarized Mn 2+state. The key to observed room temperatureferromagnetism in this system is the low temperatureprocessing, which prevents formation of clusters, secondaryphases and the host ZnO from becoming n-type. The electronicstructure of the same Mn doped ZnO thin films studied usingXAS, XES and RIXS. revealed a strong hybridization between Mn3d and O 2p states, which is an important characterstic of aDilute magnetic Semiconductor (DMS).

    It is shown that the various processing conditions likesintering temperature, dopant concentration and the propertiesof precursors used for making of DMS have a great influence onthe final properties. Use of various experimental techniques toverify the physical properties, and to understand the mechanisminvolved to give rise to ferromagnetism is presented. Methodsto improve the magnetic moment in Mn doped ZnO are alsodescribed. New promising DMS materials (such as Cu doped ZnOare explored).

    The demonstrated new capability to fabricate powder,pellets, and thin films of room temperature ferromagneticsemiconductors thus makes possible the realization of a widerange of complex elements for a variety of new multifunctionalphenomena related to Spintronic devices as well asmagneto-optic components.

  • 4.
    Gupta, Amita
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Engineering Material Physics.
    Cao, Hongtao
    Parekh, Kinnari
    Rao, K. Venkat
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Engineering Material Physics.
    Raju, A. R.
    Waghmare, Umesh V.
    Room temperature ferromagnetism in transition metal (V, Cr, Ti) doped In(2)O(3)2007In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 101, no 9, p. 09N513-Article in journal (Refereed)
    Abstract [en]

    Indium oxide is chosen as the host material for doping Ti, V, and Cr transition metal ions. Theoretical calculations based on density functional theory within a local spin density approximation show that V-V separation of 5.6 A is more stable with a strong ferromagnetic coupling. Our calculations clearly predict that substitution of vanadium for indium should yield ferromagnetism in In(2)O(3). Experimentally, (In(0.95)TM(0.05))O(3) (TM=Ti,V,Cr) were prepared using sol-gel as well as solid state reaction methods. Superconducting quantum interference device magnetization measurements as a function of field and temperature clearly showed that the V and Cr doped samples are ferromagnetic with Curie temperature well above room temperature. Thin films deposited by pulsed laser ablation using these materials on sapphire substrates exhibit a preferred 222 orientation normal to the plane of the film. The magnetic moment for (In(0.95)V(0.05))O(3) film deposited in 0.1 mbar oxygen pressure was estimated to be 1.7 mu(B)/V and is comparable to the theoretical value of 2 mu(B)/V.

  • 5. Kobayashi, M.
    et al.
    Ishida, Y.
    Hwang, J. I.
    Song, G. S.
    Takizawa, M.
    Fujimori, A.
    Takeda, Y.
    Ohkochi, T.
    Okane, T.
    Saitoh, Y.
    Yamagami, H.
    Gupta, Amita
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Engineering Material Physics.
    Cao, Hongtao
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Engineering Material Physics.
    Rao, K. Venkat
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Engineering Material Physics.
    Hybridization between the conduction band and 3d orbitals in the oxide-based diluted magnetic semiconductor In2-xVxO32009In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 79, no 20, p. 205203-Article in journal (Refereed)
    Abstract [en]

    The electronic structure of In2-xVxO3 (x=0.08) has been investigated by photoemission spectroscopy and x-ray absorption spectroscopy (XAS). The V 2p core-level photoemission and XAS spectra revealed that the V ion is in the trivalent state, which is the same valence state as that of In in In2O3. The V 3d partial density of states obtained by the resonant photoemission technique showed a sharp peak above the O 2p band. While the O 1s XAS spectrum of In2-xVxO3 was similar to that of In2O3, there were differences in the In 3p and 3d XAS spectra between the V-doped and pure In2O3. The observations give clear evidence for hybridization between the In-derived conduction band and the V 3d orbitals in In2-xVxO3.

  • 6. Moeck, P.
    et al.
    Noice, L.
    Li, C.
    Gupta, Amita
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Erni, R.
    Browning, N. D.
    Rao, K. Venkat
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Structural effects of transition metal oxide calcinations on wurtzite type semiconductors that are ferromagnetic at room temperature2007In: Novel semiconductor materials for room-temperature ferromagnetism, Materials Research Society, 2007, p. 31-36Conference paper (Refereed)
    Abstract [en]

    Gallium nitride powders and zinc oxide powders were each calcined with a few weight percent of copper oxide and/or magnesium oxide either in air or N2. Powder X-ray diffractometry, transmission electron microscopy, energy dispersive X-ray spectroscopy, and electron energy loss spectroscopy were performed in order to observe calcination induced structural effects on these wurtzite type semiconductors. We note that our earlier magnetic results on Cu doped GaN are qualitatively consistent with recent first principle calculations [Wu et al., Appl. Phys. Lett. 89 (2006) 62505].

  • 7. Noice, L.
    et al.
    Seipel, B.
    Grathoff, G.
    Gupta, Amita K.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Engineering Material Physics.
    Moeck, P.
    Rao, Venkat
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Engineering Material Physics.
    Structural studies of ZnO calcined with transition metal oxides2006In: Mater Res Soc Symp Proc, 2006, p. 455-460Conference paper (Refereed)
    Abstract [en]

    Powder X-ray diffraction analyses of Mn-and Cu-doped ZnO powders calcined at 500°C, show shifts in the wurtzite type semiconductor's lattice constants and unit cell volume which correspond to the nominal concentrations of both transition metal dopants. Marked reductions in the a-lattice constant and unit cell volume for a small concentration of Cu dopants, which is not maintained upon increased Cu concentration, suggest a change in the copper ion hybridization state due to the dopant concentration. In all the samples, only ZnO and CuO phases were detected, aiding the ascertainment of any ferromagnetic response from the samples as arising from the formation of a true dilute magnetic semiconductor.

  • 8. Owens, Frank J.
    et al.
    Gupta, Amita
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Engineering Material Physics.
    Rao, K. Venkat
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Engineering Material Physics.
    Iqbal, Zafar
    Guillen, J. M. Osorio
    Ahuja, R.
    Guo, J. -H
    Unusual room temperature ferromagnetism in bulk sintered GaP doped with copper2007In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 43, no 6, p. 3043-3045Article in journal (Refereed)
    Abstract [en]

    Robust room temperature ferromagnetism is obtained in single phase Gallium Phosphide doped with Cu2+ prepared by simple solid state reaction route. The saturation magnetization at 300 K is 1.5 x 10(-2) emu/g and the coercivity was found to be 125 Oe. A strong ferromagnetic resonance signal confirms the long range magnetic order which persists to temperatures as high as 739 K. X-ray absorption spectroscopy (XAS) indicate that Cu is in a +2 state. Ab initio calculations also show that the ferromagnetic ordering is energetically favorable in Cu doped GaP. When the spin-orbit coupling is included we get an enhanced total magnetic moment of 0.31 mu(B) with a local moment on Cu 0.082 and on P 0.204 mu(B). per atom.

  • 9. Sharma, P
    et al.
    Gupta, Amita
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Owens, F J
    Inoue, A
    Rao, K Venkat
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Room temperature spintronic material - Mn-doped ZnO revisited2004In: Journal of Magnetism and Magnetic Materials, ISSN 0304-8853, E-ISSN 1873-4766, Vol. 282, p. 115-121Article in journal (Refereed)
    Abstract [en]

    We report the first observations of ferromagnetism above room temperature for small atomic percentages of Mn doped in ZnO. In a homogeneous bulk pellet sample sintered at low temperatures (similar to 500degrees C Mn is found to carry an average moment of 0.16 mu(B) per Mn ion. We have obtained room temperature ferromagnetic ordering in samples containing around 2% Mn in the form of powder, bulk pellets and 2-3 mum, thick transparent laser-ablated films. The new capability to fabricate ferromagnetic Mn-doped ZnO semiconductors promises new devices related to spintronic devices as well as magneto-optic components.

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