Erik Helgren

Gate-controlled magnetic properties of the magnetic semiconductor (Zn,Co)O

Electric field-controlled ferromagnetism of (Zn,Co)O is demonstrated via anomalous Hall effect measurements. The electron carrier concentration in this material is 1.65x10(20) cm(-3) as measured via ordinary Hall effect at 4 K, and an anomalous Hall effect is observed up to 6 K, but with no hysteresis at any temperature. With positive electric gate field, the carrier concentration is increased by approximately 2%, resulting in a clear magnetic hysteresis at 4 K. The ability to reversibly induceeliminate ferromagnetism by applied gate field alone, measured via the effect on the carriers, is a clear sign of carrier-induced ferromagnetism in this system.

Beneficial effects of annealing on amorphous Nb–Si thin-film thermometers

Amorphous Nb–Si alloys have a temperature-dependent resistivity which can be tuned over many decades by controlling composition and are used for thin-film thermometers. Annealing at temperatures from 100 to 500 °C produces dramatic but easily controlled increases in resistivity, both magnitude and temperature dependence, for insulating and metallic samples with compositions ranging from 8–15 at. %Nb. A transition from metal to insulator is induced by annealing an initially metallic sample. Annealing produces thermal stability against subsequent heat treatment, allowing annealed films to be used as low-temperature thermometers even when they are cycled to temperatures as high as 500 °C. Cross-section transmission electron microscopy and energy-dispersive x-ray analysis show that the initially amorphous films develop Nb-rich clusters within an amorphous Nb-depleted matrix, explaining the observed resistivity increase.

Effects of annealing on amorphous GdxSi1−x near the metal-insulator transition

Annealing of amorphous Gd–Si films produces large changes in magnetic and magnetotransport properties. The materials have spin-glass freezing and enormous negative magnetoresistance (MR) in the unannealed state but show drastically reduced MR and magnetization on annealing. These changes can be explained by high resolution transmission electron micrographs and energy-dispersive x-ray analysis which show the appearance of nanocrystalline clusters of GdSi and GdSi2 in an amorphous background. A comparison with the nonmagnetic analog Y–Si shows similar modification of electrical properties.

Concentration dependent microstructure and transport properties of the magnetic semiconductor Gd-Si

The transport properties and microstructure of amorphous GdxSi1−x alloys are presented. The conductivity increases from x=0 through the metal-insulator transition (x=14 at.%), up to a dopant concentration of 25 at.%. A sharp cusp in the magnitude of the conductivity is then observed and the flattening of the conductivity versus temperature curve occurs at higher concentrations. These transport results are explained in terms of high-resolution electron micrographs which demonstrate the formation of nano-crystallites at x≥25 at.%. The flattening of the conductivity versus the temperature curve is identical to the results for annealing of a-GdxSi1−x alloys with a low Gd concentration.

Effect of magnetic Gd adatoms on the transport properties of ultrathin gold films

Ultrathin two-dimensional gold films have been grown on an amorphous Ge underlayer by quench condensation at low temperature, followed by adsorption of magnetic Gd atoms and nonmagnetic Y atoms. The resulting electrical transport as a function of temperature and composition has been investigated in situ. Gold films of different sheet resistances R have been used for the Gd and Y adsorption platform. The temperature and thickness dependence of the conductance G G=1 /R indicates that the Au films cross from a strongly localized regime, where conductivity is through hopping and where electron correlation effects are expected to be strong, to a weakly localized regime. The system is shown to be sensitive to different added electronic states, in that adding Gd or Y increases G, but much less than adding the same amount of Au for all initial G values. No difference is observed down to 5 K between added Gd and Y, showing that there is no effect of the Gd magnetic moments on electrical transport. The absence of magnetic localization and dominance of adding electronic states over added electronic potential disorder in this quench-condensed ultrathin system is discussed and attributed to the intrinsically high electronic concentration of Au.

Magnetic Rare Earth (Gd) Implanted Tetrahedral Amorphous Carbon ( ta- C)

Tetrahedrally bonded amorphous carbon (ta-C) thin films were prepared by mass selected ion beam deposition (MSIBD) using 100 eV carbon ions at room temperature. Gadolinium, a magnetic rare earth element, was implanted as a dopant into ta-C with two different fluences. The peak doping level of the ta-C:Gdx layers was 4 or 7 at.%, respectively. The Gd is believed to be electrically activated in as-implanted films, although contributions from damage-induced sp 2 sites cannot be ruled out. A characteristic crossover temperature (T’) was found, below which there is a large negative magnetoresistance (MR) with strong temperature dependence. Thermal annealing greatly increases the sample conductivity due to the increase of sp 2 sites. However, the MR persists at least up to an annealing temperature of 500°C. Magnetically, the Gd dopants behave like non-interacting local moments. Similarities and differences in physical properties between the ta-C:Gdx films and other Gd doped amorphous semiconductors are compared.