Matthew R. Sullivan

Ballistic magnetoresistance in nickel single-atom conductors without magnetostriction

Large ballistic magnetoresistance (BMR) has been measured in Ni single-atom conductors electrodeposited between microfabricated thin films. These measurements eliminate magnetostriction related artifacts. By making measurements on single atom conductors, the benchmark for the incontrovertible evidence against magnetostriction is set at the unyielding condition of the known quantum mechanical principles, namely,

Role of magnetostatic interactions in micromagnetic structure of multiferroics

1{G}_{o}=2{e}^{2}}{h=1}∕12\phantom{\rule{0.2em}{0ex}}900{\mathrm{\ensuremath{\Omega}}}^{\ensuremath{-}1}

In situ study of temperature dependent magnetothermoelastic correlated behavior in ferromagnetic shape memory alloys

(for ferromagnetic contacts the unit of conductance being

Method to study temperature and stress induced magnetic transitions

\frac{1}{2}{G}_{o}=1∕25\phantom{\rule{0.2em}{0ex}}800{\mathrm{\ensuremath{\Omega}}}^{\ensuremath{-}1}

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

) is the universal threshold ballistic conductance of an unbroken single atom contact below which even an angstrom separation of the contact due to magnetostriction is immediately signaled by an abrupt and large increase in tunneling resistance of several hundred thousand ohms across the gap. The present approach to electrodeposited point contacts between microfabricated thin films also provides an independent confirmation of Garcias original BMR experiments on atomic point contacts that were made by a mechanical method [N. Garcia, M. Mu\~noz, and Y.-W. Zhao, Phys. Rev. Lett. 82, 2923 (1999)]. There are many intricacies and subtleties to be resolved and understood in the highly interesting BMR phenomenon. Conclusive elimination of magnetostriction related artifacts, which is most easily invoked as a primary alternative explanation to the electronic origin of BMR, is one step towards a better understanding of these atomic scale entities. In addition, several explanations of null effects in some of the reported literature are given.

Fundamental Investigation of Ferromagnetic Shape Memory Alloys: A New Perspective

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.

Single-atom spintronics

This study reports the first in situ observation of temperature-dependent micromagnetic and twin structure in oriented single crystals of Ni–Mn–Ga Heusler alloys. Micromagnetic measurements were made over a temperature interval of 50 to −35 °C covering both forward and reverse martensitic transformation. Magnetic domains in the martensite phase were found to be uniformly spaced (25–30 μm); the direction of the domain walls conforms to the changing direction of the magnetic easy axis as they traverse from one twin to another. The martensite twins could be reoriented in applied fields as low as 1300 Oe.

Temperature- and field-dependent evolution of micromagnetic structure in ferromagnetic shape-memory alloys

A new method called magnetic transition spectrum (MTS) is described for studying magnetic phase transitions. The MTS method is an electronic method that monitors the dynamics of the micromagnetic structure as a function of temperature, stress, or any other perturbation that can cause a sudden variation in flux inside the magnetic material. It is based on the same principle upon which the well-known and established Barkhausen method is based, namely, Faraday’s law. However, instead of applying a magnetic field as in the Barkhausen method, temperature or stress is the external “force.” The efficacy of the MTS method is illustrated by studying magnetic transitions in magnetic shape memory alloys. The MTS method is simple to implement and is equally applicable for studying magnetic transitions in other systems, such as, for example, dynamics of exchange anisotropy, using the Co–CoO system, by cooling the sample across the Neel temperature. In general, it can be used to study magnetic phase transitions driven ...

Magnetoelastic and magnetostatic interactions in exchange-spring multilayers

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.

Pathways of structural and magnetic transition in ferromagnetic shape-memory alloys

In the present study, the evolution of micromagnetic structure and microstructure is studied in-situ both as a function of temperature and applied magnetic field, using single crystal Fe-Pd and Ni-Mn-Ga Heusler alloys. Through the development of a novel technique called ‘Magnetic Transition Spectrum’ to study temperature dependent domain dynamics, the relative sequence of micromagnetic reconfiguration with respect to the martensitic transformation has been determined for the first time. Results show that the FSMAs may be viewed as magnetic mosaics , a new perspective, which is also more amenable to modeling the physical properties of these alloys. Finally, the concept of magnetic mosaics has been used to synthesize a novel class of materials with engineered magnetic anisotropies , and is briefly discussed.