Jason N. Armstrong

Role of magnetostatic interactions in micromagnetic structure of multiferroics

While it is well known that magnetoelastic coupling governs the magnitude of field-induced strain in magnetic shape memory alloys, the present study shows that the zero-field micromagnetic structure and the pathway leading to the field-induced strain is governed by magnetostatic coupling across martensite twins. The micromagnetic investigations reveal a new energy barrier to the motion of domain walls arising from magnetostatic coupling between walls across the twin planes.

Channel saturation and conductance quantization in single-atom gold constrictions

It is shown that the finite elasticity of atomic-sized gold constrictions allows for a continuous and reversible change in conductance. This is achieved by superposition of atomic-scale or subatomic-scale mechanical oscillations on a steadily retracting/approaching gold tip against a gold substrate. Through these perturbation studies, we report the direct observation of channel saturation and conductance quantization in stable, single-atom gold constrictions. In addition, the origin of peaks in conductance histograms is explained, and the peaks alone are shown to be insufficient in evaluating the stability of atomic configurations.

Anisotropic Curie temperature materials

Existence of anisotropic Curie temperature materials [E. R. Callen, Phys. Rev. 124, 1373 (1961)] is a longstanding prediction - materials that become paramagnetic along certain crystal directions at a lower temperature while remaining magnetically ordered in other directions up to a higher temperature. Validating Callens theory, we show that all directions within the basal plane of monoclinic Fe7S8 single crystals remain ordered up to 603 K while the hard c-axis becomes paramagnetic at 225 K. Materials with such a large directional dependence of Curie temperature opens the possibility of uniquely new devices and phenomena.

An integration approach to microfluidic flow sensing and actuation using electrolytic bubbles

In this paper we demonstrate an integration approach for making high-density microfluidic systems. A complex microfluidic system including both sensors and actuators was constructed on silicon chip. Electrically addressable bubble-based valves were used to regulate the fluid flow. A number of electrolytic bubble sensors were placed in parallel channels (sensing limb) connected with the main flow channel for measurements of open channel pressure in real-time. All the fluidic components were made using a single microfabrication process. The pressure dependence of the bubble-based sensor was systematically investigated by applying an inlet pressure ranging from 101 kPa to 133 kPa, while keeping the outlet pressure at atmosphere. The results show that open flow pressure can be accurately measured using the bubble-based sensor located in an adjacent sensing limb. The bubble-shrinking rate can also serve as a measurable parameter for the pressure in main fluidic channel. The experimental data validated with 3D numerical simulation results. The electrolytic bubble-based approach provides an ability to integrate a large number of microfluidic components on a monolithic lab-chip.

Physical properties of a two-component system at the Fermi and Sharvin length scales

Previously, we have reported the measurement of various physical properties at the Fermi and Sharvin length scales in pure elements (1-component systems). In the present study, the evolution of physical properties is mapped in a 2-component system at these length scales, using Au-Ag alloys. These alloys are well known to have complete solubility in each other at all compositions in the bulk and an ideal system to vary the surface energy of the alloy simply by changing the alloy composition. At sample sizes where surface effects dominate (less than ∼2–3 nm), varying the alloy composition is found to cause dramatic changes in force required to rupture the bonds (strength) as well as atomic cohesion (modulus) and can be directly attributed to segregation of higher surface energy Au from the lower surface energy Ag. In other words, the Au-Ag system with complete solubility in the bulk exhibits segregation at these length scales. This breakdown of bulk solubility rules for alloying (the so-called Hume-Rothery ...

Single-atom spintronics

Recent work on magnetic quantum point contacts (QPCs) was discussed. Complete magnetoresistance loops across Co QPCs as small as a single atom was measured. The remarkable feature of these QPCs is the rapid oscillatory decay in magnetoresistance with the increase of contact size. In addition, stepwise or quantum magnetoresistance loops are observed, resulting from varying transmission probability of the available discrete conductance channels because the sample is cycled between the ferromagnetic (F) and antiferromagnetic (AF) aligned states. Quantized conductance combined with spin dependent transmission of electron waves gives rise to a multi-channel system with a quantum domain wall acting as a valve, i.e., a quantum spin-valve. Behavior of a few-atom QPC is built on the behavior of a single-atom QPC and hence the summarization of results as ‘single-atom spintronics’. An evolutionary trace of spin-dependent electron transmission from a single atom to bulk is provided, the requisite hallmarks of artefact-free magnetoresistance is established across a QPC – stepwise or quantum magnetoresistance loops and size dependent oscillatory magnetoresistance.