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Current Research Interests

Most of what we do relates to the relatively new field of "spin electronics." In recent years, there has been an enormous interest in so-called spin electronics devices, driven by a rising demand for ultra-high density magnetic storage and non-volatile magnetic memories. These devices utilize the fundamental principle that electrons carry not only charge, the property exploited in traditional semiconductor devices, but also spin. In semiconductor devices, an electric field is used to control the transport of conduction electrons, and hence, electric currents. In magnetic materials, where spin up and spin down electron populations are unequal, an electric current will similarly be spin polarized. In this case, a magnetic field can be used to manipulate spin polarized electrical currents, providing an additional channel of information as well as an additional degree of freedom for designing novel devices. Spin polarized transport, however, relies crucially on electrons retaining their spin information within the device structure. This is where we come in: how to generate highly spin polarized currents, maintain them, and manipulate them.

You can see the slides from a few fairly general talks I've given in the past here.

Facilities & Techniques in the LeClair Laboratory

The most basic characterization on a finished device structure, and arguably the most technologically important, is the measurement of the electrical transport characteristics. We perform a wide variety of electrical transport measurements (current, voltage, and their derivatives) as a function of temperature, magnetic field and other variables. Our main system is a Quantum Design Physical Property Measurement System (PPMS). This system allows measurements in a controlled temperature and magnetic field environment, with temperatures ranging from 0.3-400K (with a 3He insert) and magnetic fields from 0-7T. The entire system is computer controlled, allowing programmed temperature, current, or field sweeps during a measurement.

[photo]
The PPMS system and external electronics.

Coupled with external electronics and custom software - which Quantum Design makes very easy to do, by the way - we can perform a number of detailed transport investigations, such as Hall effect, magnetoresistance, temperature-dependent conductivity, and tunneling spectroscopy. A couple of these techniques are described in more detail below. Usually, our studies involve measuring derivatives of current with respect to voltage, such as dI/dV, with a basic setup something like this:

[photo]
A block diagram of what we usually do. Click to enlarge; a schematic of the summing amp is here.

You can find a few more details here. Below are some of the less common (but very powerful) techniques we employ regularly.

Magnetotransport

Using the PPMS system, a variety of temperature- and field-dependent transport phenomena can be measured, such as:
  • Magnetoresistance (GMR, TMR) as a function of temperature, frequency, and dc bias
  • Hall Effect
  • Resistance versus temperature
  • Impedance spectroscopy as a function of temperature, field, and dc bias

    • Inelastic tunneling spectroscopy

    (IETS) is a powerful technique utilized to characterize tunneling structures. IETS involves measuring derivatives of the tunneling current-voltage characteristics (dI/dV and d2I/dV2) in order to probe more sensitively the changes in conductance properties when excitations occur as a part of the tunneling process. IETS has been used extensively in the past to probe vibrational modes in tunnel barrier materials (it may be considered complementary to Raman or IR spectroscopy), and more recently, to probe magnetic excitations in MTJs. The unique advantage of IETS is that it provides information only on those processes that influence transport.

    [photo]
    Example IETS measurements, made on Al/Al2O3/4-X-benzoic acid/Pb junctions, with X={iodo,chloro}.

    For more information: P.K. Hansma, "Tunneling Spectroscopy", Plenum Press, New York (1982); E.L. Wolf , "Principles of Electron Tunneling Spectroscopy", Oxford University Press, London (1985)

    Spin-polarized tunneling:

    one of our main and most powerful tools for assessing prototype structures will be the Meservey-Tedrow spin polarized tunneling (SPT) technique. SPT is a direct and unambiguous manner measure of the spin polarization (P) of tunneling electrons - the only direct method with direct relevance for the TMR effect. It utilizes a superconducting electrode as a spin detector, with the ability to probe within about 1meV of the Fermi energy (i.e., the order of the superconducting energy gap).

    [photo]
    Conductance (dI/dV) of a S/I/N tunnel junction with Zeeman splitting of the density of states in the superconductor due to a magnetic field B. In (a) the normal metal has zero polarization, P=+50% in (b), and P=-75% in (c). The thin solid and dashed lines in (a) show the spin-up and spin-down partial densities of states. The conductance curves are calculated for T=0.1Tc and &muB=0.6&Delta .

    In superconductor-insulator-normal metal tunneling, dI/dV(V) is proportional to the BCS density of states. The SPT technique achieves spin sensitivity by applying a magnetic field (2-3T) parallel to the film plane. If the superconductor is sufficiently thin (4-5nm), such that the orbital screening currents are negligible, the effects of the electron spin interacting with the magnetic field may be observed. The result is Zeeman splitting of the states in the superconductor, which can be used to probe the spin polariation of tunneling electrons.

    [photo]
    Example SPT measurements, made on Al/Al2O3/Co junctions.

    The SPT technique has crucial advantages over, e.g., TMR or point contact Andreev reflection. One is the ability to measure the sign (majority or minority) of the spin polarization. From our point of view, another crucial advantage is that a robust, well-tested and microscopic theory of SPT has been developed over the years. This, coupled with the simple, fundamental nature of the experiment makes SPT much more agreeable for detailed comparison with theory. Though restricted to low temperatures and bias voltages, SPT is an ideal technique for our synergistic experimental and theoretical approach to these novel spintronic systems, which require at this stage more fundamental characterization. Complimentary to the SPT experiments, we still utilize the TMR effect to judge the efficiency of spin transport in structures more closely resembling real devices.

    For more information: R. Meservey and P.M. Tedrow, "Spin-polarized electron tunneling", Phys. Rep. 238, 173-234(1994); here [pdf], or here [pdf]

    Noise characterization

    In electronics and communication systems, noise is a random fluctuation or variation of an electromagnetic analog signal such as a voltage or a current. Electronic noise is a characteristic of all electronic circuits. Depending on the circuit, the noise generated by electronic devices can vary greatly and arise through several different mechanisms. Contributions such as thermal noise and shot noise are inherent to all devices, while other types depend mostly on manufacturing quality and defects. Though noise usually has a negative connotation, it does have its uses -- for instance, noise power is used in low-temperature thermometry, and the study of noise can be a powerful technique for elucidating the microscopic mechanisms of conduction. During the spring and summer of 2010, we are busy building up our very own noise spectroscopy setup, which we hope to have fully operational by fall 2010.

    Optical characterization

    During spring/summer 2007, we started a small effort to investigate optically active spin electronic devices. This started as a summer research project for an undergraduate intern (REU), and the initial successes led to the initiation of a new research focus. Currently, two graduate students are involved at some level in this project.

    As a requirement for this project, I purchased a simple UV-Visible-Near Infrared spectrophotometer. The system we have (Ocean Optics USB4000 + tungsten-deuterium source) allows one to shine light of a particular wavelength (from 200nm-1000nm) onto a sample and measure the amount of light reflected or transmitted from that sample at any wavelength within the instrument's range. The incident wavelength of light as well as the detected wavelength can be varied over the whole 200-1000nm range, which allows one to measure the wavelength and energy-dependence of light absorption and reflection from a sample. The ability to have different incident and measured wavelengths allows one to measure, e.g., Raman modes or fluorescence behavior.

    The system consists of a tungsten-deuterium light source coupled with a wavelength selector (monochromator), which allows one to illuminate a sample with only one wavelength of light, fiber optic cables to guide the light to a desired point, and a spectrometer to measure the absolute light intensity at a particular wavelength. The whole system is controlled through an existing PC using a USB interface. Also included are optical accessories, such as a reference 'leg' fiber (to calibrate the output light intensity), various fiber and spectrometer holders to aid in positioning, control software, and the necessary cabling.

    This equipment will allow us to carefully measure the optical properties of materials as a function of light energy, which helps us determine (for example) how efficiently prototype photodetectors and solar cells can convert light energy into electrical energy. The Ocean Optics design we have chosen in particular is perfectly suited to our needs and substantially more inexpensive than competing systems.

    National Labs

    We also make use of facilities at a number of national laboratories for more exotic measurements. For example:
  • X-ray magnetic circular dichroism, Synchrotron Radiation Center
  • X-ray magnetic circular dichroism, Brookhaven National Laboratory
  • Neutron reflectometry, Spallation Neutron Source

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