Session T5: Measuring Magnetism at the Nanoscale

Prospects for Imaging Magnetic Nanoparticles Using a Scanning Squid Microscope

Magnetic nanoparticles have a number of present and proposed uses: in the fields of nanobiotechnology for magnetic separation, magnetic manipulation, magnetic sensing, and in situ heating; for high density storage in both conventional and patterned media; and for spintronic devices. Although there are well established techniques for measuring the magnetic properties of large numbers of particles, it is desirable to magnetically image individual nanoparticles and clusters with small numbers of nanoparticles to determine such properties as their coercive fields, magnetic moments, and anisotropy energies. Wernsdorfer and co-workers [1] have shown that the magnetic reversal fields of small magnetic particles can be determined using a nanoSQUID. However, in these experiments nanoparticles were deposited directly on the SQUID. Such a technique would be difficult to use for the determination of, for example, the distribution in particle properties of a collection of particles. Woods and coworkers [2] determined the anisotropy energy of a film of magnetic particles from SQUID microscope measurements of the magnetic noise. In these experiments a large number of particles were included in the region sensed by the SQUID pickup loop, so that only average properties were determined. Measurement of the magnetic properties of individual nanoparticles is a challenge using any scanning probe microscopy, but is possible with the scanning SQUID microscope. In this talk I will describe different modes for imaging magnetic nanoparticles, present simple calculations of the size of signal expected for these modes as a function of such parameters as the size and saturation magnetization of the particles, the size of the pickup loop, and the spacing between the SQUID pickup loop and the nanoparticle, and compare these signals with the noise currently and ultimately available in scanning SQUID sensors [3]. I conclude that such measurements should be possible with the very small pickup loop (0.6 $\mu$m diameter) nanoSQUIDs that have now been demonstrated [4]. We have built and operated a high spatial resolution, variable sample temperature scanning SQUID microscope for imaging magnetic nanoparticles. I will describe this microscope and present results on imaging magnetic nanoparticles. * Work done in collaboration with Beena Kalisky, Lisa Qian, and Kathryn Moler. \\[4pt] [1] W. Wernsdorfer {\it et al.} {\it Phys. Rev. Lett.} {\bf 78}, 1791 (1997). \\[0pt] [2] S.I. Woods, J.R. Kirtley, S. Sun, and R.H. Koch, {\it Phys. Rev. Lett.} {\bf 87}, 137205 (2001). \\[0pt] [3] J.R. Kirtley, {\it Supercond. Sci. Technol}. {\bf 22}, 064008 (2009). \\[0pt] [4] N.C. Koshnick, M.E. Huber, J.A. Bert, C.W. Hicks, J. Large, H. Edwards, and K.A. Moler, {\it Appl. Phys. Lett.} {\bf 93}, 243101 (2007).   

Soft X-Ray Microscopy: Imaging Magnetism at Small Sizes

The manipulation of spins on the nanoscale is of both fundamental and technological interest. In spin based electronics the observation that spin currents can exert a torque onto local spin configurations which can e.g. push a domain wall has stimulated significant research activities in order to provide a fundamental understanding of the physical processes involved. Magnetic soft X-ray microscopy is a unique analytical technique combining X-ray magnetic circular dichroism (X-MCD) as element specific magnetic contrast mechanism with high spatial and temporal resolution. Fresnel zone plates used as X-ray optical elements provide a spatial resolution down to currently $<$12nm [1] thus approaching fundamental magnetic length scales such as the grain size [2] and magnetic exchange lengths. Images can be recorded in external magnetic fields giving access to study magnetization reversal phenomena on the nanoscale and its stochastic character [3] with elemental sensitivity [4]. Utilizing the inherent time structure of current synchrotron sources fast magnetization dynamics with 70ps time resolution, limited by the lengths of the electron bunches, can be performed with a stroboscopic pump-probe scheme. In this talk I will review recent achievements with magnetic soft X-ray microscopy with focus on current induced wall [5] and vortex dynamics in ferromagnetic elements [6]. Future magnetic microscopies are faced with the challenge to provide both spatial resolution in the nanometer regime, a time resolution on a ps to fs scale and elemental specificity to be able to study novel multicomponent and multifunctional magnetic nanostructures and their ultrafast spin dynamics.\\[4pt] References\\[0pt] [1] W. Chao, et al., Optics Express 17(20) 17669 (2009) \\[0pt] [2] M.-Y. Im, et al, Advanced Materials 20 1750 (2008) \\[0pt] [3] M.-Y. Im, et al., Phys Rev Lett 102 147204 (2009) \\[0pt] [4] M.-Y. Im, et al., Appl Phys Lett 95 182504 (2009) \\[0pt] [5] L. Bocklage, et al., Phys Rev B 78 180405(R) (2008) \\[0pt] [6] S. Kasai, et al., Phys Rev Lett 101, 237203 (2008)   

Detecting Biomolecular Interactions with Semiconductor Hall Sensors

Semiconductor Hall magnetometry is a magnetic measurement technique with ultrahigh magnetic moment sensitivity and broad temperature and magnetic field operation ranges. These attributes make such devices, especially those based on high-mobility semiconductor heterostructures, ideal candidates as sensors for detecting biomolecular interactions using superparamagnetic labels. Magnetic moment sensitivity better than 10$^{4 }\mu _{B}$/Hz$^{1/2}$ is demonstrated at low-temperature on GaAs/AlGaAs devices,\footnote{Y. Li et al., \textit{PRL} \textbf{93}, 246602 (2004).} while room-temperature detection of a \textit{single} superparamagnetic bead is realized with micro-Hall sensors based on InAs quantum wells.\footnote{G. Mihajlovic et al., \textit{APL} \textbf{87}, 112502 (2005).} The implementation of magnetic detection of protein binding\footnote{P. Manandhar, K.-S. Chen, et al., \textit{Nanotechnology} \textbf{20}, 355501 (2009).} and DNA hybridization with InAs micro-Hall devices will be presented in this talk. Details of the sensing scheme, including fabrication and passivation of the devices, selective biomolecular functionalization of the Hall crosses, measurements of the Hall signals in response to specific biomolecular binding, and verification of the specificity of the Hall sensing via extensive fluorescence microscopy, will be described. The results demonstrate significant potential of the semiconductor Hall sensors for high-speed biomolecular sensing.   

Nanoscale Magnetic Field Sensors For Magnetic Recording

I present two possible paths to achieving magnetic sensitivity to mT magnetic fields on the tens of nm length scale at GHz frequencies, required by future magnetic recording systems. Extraordinary Magneto Resistance (EMR) is based on the synergistic combination of two effects derived from the Lorentz force acting on charge carriers in hybrid structures comprising a high mobility semiconductor and a metal shunt: Hall and Corbino (where current is directed away from a metal inclusion in a semiconductor when magnetic field is applied). Our earlier devices (resolution 150nm) were built from InAs 2DEGs and provided sensitivities S = 5 microV/Oe, comparable to GMR sensors. Recently we have employed graphene as the high mobility channel, greatly improving the potential spatial resolution since it is only one atom thick and easily located near the sensor surface. We obtain S =10 microV/Oe when both electrons and holes are present near the Dirac point. The Spin Torque Oscillator Sensor, also sensitive to mT fields on the nanoscale, has two ferromagnetic layers (reference and sense) a few nm thick separated by a non-magnetic conductor in a pillar a few tens of nm in diameter. The current from the reference layer is spin polarized, and is used to excite persistent oscillations of the magnetization in the sense layer, whose frequency of oscillation changes with magnetic field. Our modeling shows that dispersions of 150 GHz/T are possible, or a few GHz shift in response to the field above recorded bits. The change in frequency is measured by a phase detector sensitive to the oscillating voltage signal generated by the GMR that arises as the sense layer precesses with respect to the reference layer. Model results show that high SNR may be possible.   

Magnetoresistive Sensors in Biological Assays

Magnetic beads or nanoparticles can be used as ``labels'' in biochemical assays by attaching the beads to the biospecies of interest using a bio-specific attachment. Once the labels are attached, they can be used to manipulate, capture, and detect the species to be analyzed. Magnetoresistive (MR) sensors may be used to detect and count these labels, and thus make an inference about the concentration of the species of interest. MR technology is especially promising for biosensor applications where making the detector small and integrated with related sample handling tools to form a ``lab-on-a-chip'' miniaturized system. The function of the MR sensors is to detect stray magnetic fields from the beads while they are exposed to a magnetic excitation field. Generally, the stray fields from beads and clusters of beads are complicated functions of geometry, so some care is required to relate the detected magnetic signal to the number and location of the bead labels. This presentation will begin with a broad overview of results from many groups working in this area. For convenience, the applications are divided into three categories, detection of: flowing magnetic beads, immobilized beads, and scanned samples. Next will be some discussion of how the choice of spintronic sensor technology might affect detection capabilities (AMR, GMR, TMR, Hall effect, etc). Then, challenges relating to integration of MR sensors into microfluidic products will be discussed. This is the focus of the presenter's current day-to-day work on developing and producing MR-based biosensors. And finally, a description of possible future avenues of study and development will be presented.