Browsing by Author "Lauter, V"

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    Ferromagnetism of Co, Eu Co-doped ZnO and 5%-Co doped TiO2 magnetic semiconductors
    (Australian Institute of Physics, 2014-02-04) Lee, OJ; Lou, X; Lee, WT; Lauter, V; Triani, G; Li, S; Yi, JB
    Diluted magnetic semiconductor has attracted wide interest due to its potential applications in spintronics devices. Oxide semiconductor based diluted magnetic semiconductors has been investigated in detail for possible ferromagnetism above room temperature. However, most of the diluted magnetic semiconductors show very weak ferromagnetism. The magnetic moment is originated from the doped magnetic element, such as Fe, Co, Ni. Rare-earth element, which shows strong spin-orbit coupling, may enhance the magnetic anisotropy of the diluted magnetic semiconductors, thus enhances the ferromagnetism. In this work, we used both Co and Eu to co-dope ZnO and deposited Co doped TiO2 thin films in order to achieve a diluted room-temperature magnetic semiconductor with strong ferromagnetism. 4%Co and 4%Eu or 6% Eu were used for the doping by implantation in ZnO and 5%Co-TiO2 film were deposited on LaAlO3 substrate under different oxygen partial pressures from 10-4 to 10-6 torr. For the ZnO-based thin films, XRD analysis indicates there is no secondary or impurity phase. Magnetic measurement by SQUID shows room temperature ferromagnetism. Polarized neutron reflectometry (PNR) analysis illustrates that ZnO film is 100 nm in thickness and the magnetic layers is around 30 nm, which is in consistent with the penetration depth of Co and Eu implantation, indicating the magnetic moment is due to the Co and Eu co doping. 4%Co, 4%Eu codoped ZnO film has a saturation magnetization of 3.57 emu/cm3, while 4%Co, 6%Eu co doped ZnO film has a saturation magnetization of 9.62 emu/cm3, indicating the significant enhancement of saturation magnetization by more rare earth element doping. For the TiO based thin films, XRD analysis show epitaxial growth and that the films have anatase phases. TEM confirms the single crystal like microstructure. EDX mapping indicates that Co is uniformly distributed in the TiO2 matrix, suggesting effective doping of Co dopant. Magnetic measurement shows that film deposited under lower oxygen partial pressure has a larger saturation magnetization. PNR shows that the magnetization is uniformly distributed along the film thickness. The magnetization for the film deposited under an oxygen partial pressure of 10-6 torr is about 4.2 emu/cm3, which is much smaller than that measured by SQUID (30 emu/cm3). This suggests a magnetic dead layer on the film surface.
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    In-situ polarized 3 He-based neutron polarization analyzer for SNS magnetism reflectometer
    (Insitute of Physics, 2010-12-16) Lee, WT; Tong, X; Pierce, J; Fleenor, M; Ismaili, A; Robertson, JL; Chen, WC; Gentile, TR; Hailemariam, A; Goyette, R; Parizzi, A; Lauter, V; Klose, F; Kaiser, H; Lavelle, C; Baxter, DV; Jones, GL; Wexler, J; McCollum, L
    We report here the construction and neutron transmission test results of an in-situ polarized 3 He-based neutron polarization analyzer system for the Magnetism Reflectometer at the Spallation Neutron Source, Oak Ridge National Laboratory. The analyzer uses the Spin-Exchange Optical Pumping method to polarize the 3 He nuclei of a cell of 3 He gas. Polarized neutrons scattered from the sample are intercepted by the polarized 3 He gas which strongly absorbs neutrons in one spin-state while allowing most neutrons in the other spin-state to pass through. To maintain a stable analyzing efficiency during an experiment, the 3 He gas is continuously polarized in - situ on the instrument. Neutron transmission measurements showed that 73% 3 He polarization was reached in this setup. © 2010, Insitute of Physics.
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    Structural and magnetic properties of epitaxial Fe25Pt75
    (American Vacuum Society, 2009-07) Lu, Z; Walock, MJ; LeClair, PR; Mankey, GJ; Mani, P; Lott, D; Klose, F; Ambaye, H; Lauter, V; Wolff, M; Schreyer, A; Christen, HM; Sales, BC
    Epitaxial films of Fe25Pt75 have a number of different magnetic phases as a function of temperature and chemical order. For example, chemically ordered epitaxial films have two distinct antiferromagnetic phases at temperatures below similar to 160 K and exhibit paramagnetism above that temperature. In sharp contrast, chemically disordered epitaxial films are ferromagnetic with a Curie temperature that is greater than 400 K. It is demonstrated that by varying the substrate temperature during growth, epitaxial films with varying degrees of chemical order can be produced and it is possible to produce an alloy with the same composition throughout the film with a modified magnetic structure. The authors used polarized neutron reflectivity to gauge the magnetism of a Fe25Pt75 sample produced with a periodic variation in the growth temperature and showed that the sample exhibits a reduced Curie temperature of approximately 300 K as compared to bulk Fe25Pt75. © 2009, American Vacuum Society