Shireen Adenwalla

A class of boron-rich solid-state neutron detectors

Real-time solid-state neutron detectors have been fabricated from semiconducting boron–carbon alloys, deposited by plasma-enhanced chemical vapor deposition. Single neutrons were detected and signals induced by gamma rays were determined to be insignificant. The source gas closo-1,2-dicarbadodecaborane (ortho-carborane) was used to fabricate the boron–carbon alloys with only the natural isotopic abundance of 10B. Devices made of thicker boron–carbon alloy layers enriched in 10B could lead to increased detection efficiency and active diodes could use the inherent micron scale spatial resolution, increasing the range of possible applications.

Ferroelectric Control of Magnetic Anisotropy

We demonstrate unambiguous evidence of the electric field control of magnetic anisotropy in a wedge-shaped Co film of varying thickness. A copolymer ferroelectric of 70% vinylidene fluoride with 30% trifluoroethylene, P(VDF-TrFE) overlays the Co wedge, providing a large switchable electric field. As the ferroelectric polarization is switched from up to down, the magnetic anisotropy of the Co films changes by as much as 50%. At the lowest Co thickness the magnetic anisotropy switches from out-of-plane to in-plane as the ferroelectric polarization changes from up to down, enabling us to rotate the magnetization through a large angle at constant magnetic field merely by switching the ferroelectric polarization. The large mismatch in the stiffness coefficients between the polymer ferroelectric and metallic ferromagnet excludes typical magnetoelectric strain coupling; rather, the magnetic changes arise from the large electric field at the ferroelectric/ferromagnet interface.

Boron carbide/n-silicon carbide heterojunction diodes

The fabrication, initial structural characterization, and diode measurements are reported for a boron carbide/silicon carbide heterojunction diode. Current–voltage curves are obtained for operation at temperatures from 24 to 351 °C. Plasma-enhanced chemical-vapor deposition (PECVD) -deposited undoped boron carbide material is highly crystalline and consists of a variety of polytypes of boron carbide (BC) with crystal sizes as large as 110 nm. Crystal phases are similar to those for PECVD BC on Si but only partially match known boron and boron-rich BC phases.

Effect of bias on neutron detection in thin semiconducting boron carbide films

Neutron detection signatures in thin films of semiconducting boron carbide were measured as a function of three applied reverse bias voltages, 0, 1.57 V and 3.15 V. Thermal neutrons were detected at zero bias. As the bias was increased, an increase in efficiency and in peak channel positions was observed. These two significant effects have been attributed to an increase in charge collection at higher bias voltages.

Closely linear temperature dependence of exchange bias and coercivity in out-of-plane exchange-biased [Pt/Co]3/NiO (11 Å) multilayer

Strong out-of-plane exchange biasing has been observed in out-of-plane exchange biased [Pt (5 A)/Co (4 A)]3/NiO (11 A) multilayer with perpendicular easy axis. Both the exchange field HE and coercivity HC display a closely linear temperature dependence except at very low temperatures. A thin NiO layer coated on the top of a Pt/Co multilayer has a great effect on the domain pattern of the Pt/Co multilayer, which is in a more irregular configuration with much smaller domain sizes than the uncoupled Pt/Co multilayer. A simulation according to Malozemoff’s random field model gives a good agreement to the experimental temperature dependence of HE and HC, suggesting that the closely linear temperature dependences of HE and HC are strongly related to the behavior of the temperature of anisotropies and of the spin rotation inside the domain walls of a thin NiO layer. The blocking temperature of TB=220 K is much higher than that observed in ferromagnetic/thin NiO systems with in-plane anisotropy.

Boron carbide based solid state neutron detectors: the effects of bias and time constant on detection efficiency

Neutron detection in thick boron carbide(BC)/n-type Si heterojunction diodes shows a threefold increase in efficiency with applied bias and longer time constants. The improved efficiencies resulting from long time constants have been conclusively linked to the much longer charge collection times in the BC layer. Neutron detection signals from both the p-type BC layer and the n-type Si side of the heterojunction diode are observed, with comparable efficiencies. Collectively, these provide strong evidence that the semiconducting BC layer plays an active role in neutron detection, both in neutron capture and in charge generation and collection.

Ferroelectric control of magnetocrystalline anisotropy at cobalt/poly(vinylidene fluoride) interfaces.

Electric field control of magnetization is one of the promising avenues for achieving high-density energy-efficient magnetic data storage. Ferroelectric materials can be especially useful for that purpose as a source of very large switchable electric fields when interfaced with a ferromagnet. Organic ferroelectrics, such as poly(vinylidene fluoride) (PVDF), have an additional advantage of being weakly bonded to the ferromagnet, thus minimizing undesirable effects such as interface chemical modification and/or strain coupling. In this work we use first-principles density functional calculations of Co/PVDF heterostructures to demonstrate the effect of ferroelectric polarization of PVDF on the interface magnetocrystalline anisotropy that controls the magnetization orientation. We show that switching of the polarization direction alters the magnetocrystalline anisotropy energy of the adjacent Co layer by about 50%, driven by the modification of the screening charge induced by ferroelectric polarization. The effect is reduced with Co oxidation at the interface due to quenching the interface magnetization. Our results provide a new insight into the mechanism of the magnetoelectric coupling at organic ferroelectric/ferromagnet interfaces and suggest ways to achieve the desired functionality in practice.

Magnetization dynamics triggered by surface acoustic waves

Investigations into fast magnetization switching are of both fundamental and technological interest. Here we present a low-power, remote method for strain driven magnetization switching. A surface acoustic wave propagates across an array of ferromagnetic elements, and the resultant strain switches the magnetization from the easy axis into the hard axis direction. Investigations as a function of applied magnetic field as well as unidirectional anisotropy (the exchange bias) reveal excellent agreement with prediction, confirming the viability of this method.

Semiconducting boron-rich neutron detectors

Semiconducting boron-rich boron-carbon alloys have been deposited by plasma-enhanced chemical vapor deposition. Heterojunction diodes made with 276nm thick nanocrystalline layers of these alloys have been used as real-time solid-state neutron detectors. Individual neutrons were detected and signals induced by gamma rays were determined to be insignificant. Linearity of detection was demonstrated over more than two orders of magnitude in flux. The neutron detection performance was unaffected by > 1 x 1015 neutrons / cm2. The source gas closo-1,2-dicarbadodecaborane (ortho-carborane) was used to fabricate the boron carbon alloys with only the natural isotopic abundance of 10B. Devices made of thicker boron carbon alloy layers enriched in 10B could lead to increased detection efficiency.

Influence of dynamical scattering in crystalline poly(vinylidene fluoride-trifluoroethylene) copolymers

The effective Debye temperature of poly(vinylidene fluoride-trifluoroethylene) copolymers was measured using photoemission and neutron diffraction techniques. An effective Debye temperature of 53±11 K is obtained from the photoemission data and 69±3.5 K from neutron diffraction measurements. This effective Debye temperature is a consequence of the temperature-dependent dynamic motions perpendicular to the surface of these crystalline polymer films.