Philosophy & Scope

Research Archive

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Prof. Kannan M. Krishnan (Principal Investigator)

Magnetic Actuation of MEMS Devices

Dr. Alec Pakhomov, Visiting Scientist
Vasilios Tsirimiagos, Graduate Student
Chris Petz, Undergraduate Student


This project was a collaborative effort between Microvision and the UW. Micro-electro-mechanical system (MEMS)-based scanners are a natural choice for the scanning mirror requirements of scanned beam displays. By developing a MEMS-compatible magnetic film deposition material and process, actuation forces can be directly applied to the scanner.

The project team is developed materials and processes for fabrication of hard micromagnets for magnetic actuation of MEMS devices. The first phase of the project is focused on:
- Selection of magnetic materials with parameters consistent with the device requirements.
- Fabrication of hard magnetic films by appropriate means.
- Investigation of heat treatments leading to the desired properties yet compatible with silicon processing.
- Recommendations of optimum film preparation techniques and patterning methods for Phase II.

Hard magnetic films often suffer from instability over time and temperature, or incompatibility with certain MEMS fabrication techniques. The goal of this first phase of the project was to identify those magnetic films that overcome these limitations. It is anticipated that there will be a phase II to this project whereby the University of Washington and Microvision team will perform the deposition of the material optimized in Phase I in thin film form and patterned onto actual MEMS. Both deposition and patterning will be optimized to achieve best torque performance.

Coercivity Limits and Mechanism in Nanocomposite Fe-Nd-B Alloys

Dr. Er. Girt (postdoctoral fellow)
Dr. W. Grogger (postdoctoral fellow)

Dr. D. Crew, U.W. Australia (Collaborator)
Dr. T. Schrefl, U. Vienna(Collaborator)
Prof. Z. Althunian, McGill University(Collaborator)
Prof. G. Thomas, UC Berkeley (collaborator)


We have studied the magnetic behavior of a range of Fe13.1NdxB (2 < x < 147) alloys as a function of the Nd content and annealing temperature (400-700ƒC) and characterized them by a wide range of measurements including x-ray diffraction, differential scanning calorimetry, thermogravimetric analysis and electron microscopy. These annealed samples show a systematic increase in room-temperature coercivity with Nd content, culminating in the largest value (~2.8T) ever reported in the Fe-Nd-B system when x ~ 147. Energy-filtered images, using spatially resolved measurements of inner-shell ionization edges, was critical in evaluating the particle shapes (platelets with the crystallographic c-axis normal to the plate), size (~100 x 40 x 25 nm) and distribution. For such randomly oriented, non-interacting particles, the largest observed coercivity, Hc ~ 2.8 T is ~90% of the theoretical limit expected for Stoner-Wohlfarth coherent rotation behavior including demagnetization effects. Initial magnetization curves and a systematic series of minor loop measurements show that the coercivity mechanism changes from the pinning-type at low Nd content, to the nucleation-type at higher Nd concentrations. In addition, the samples show a low-temperature spin reorientation and a peak in coercivity as a function of temperature. To understand these results we have carried out a systematic evaluation of the physical microstructure and the distribution of magnetic spins at the appropriate length-scale.


D. C. Crew, Er. Girt, M. Guilot, D. Suess, T. Schrefl and Kannan M. Krishnan, " The effect of magnetic interactions between grains on reversal behavior in diluted Nd2Fe14B", PRB (in press)

Er. Girt, Kannan M. Krishnan, G. Thomas and E. Girt, "Coercivity limits and mechanism in nanocomposite Fe-Nd-B alloys", J. Mag. Mag. Mat., 231, 219 (2001)

R. C. Woodward, N.T. Gorham, R. Street, D.C. Crew, E. Girt and Kannan M. Krishnan, "Coevrcivity, time dependence and reversible magnetisation in Nd-rich Nd-Fe-B alloys", IEEE Trans. Mag. 37, 2493 (2001)

Er. Girt, Kannan M. Krishnan, M. Guillot, I. P. Swainson, Z. Altounian, G. Thomas, "Structural and magnetic properties of Nd2Fe17-xGax (x = 0 - 2)", J. Appl. Phys. 87, 5323 (2000)

Er. Girt, Kannan M. Krishnan, G. Thomas, Z. Altounian, "Approaching the theoretical coercivity of Nd2Fe14B : microstructural evaluation and interparticle interactions", J. Appl. Phys. (in press)

Er. Girt, Kannan M. Krishnan, G. Thomas, Z. Altounian, "Nanocomposite Nd-rich Nd-Fe-B alloys: approaching ideal Stoner-Wohlfarth type behavior", Appl. Phys. Lett. 76, 1746 (2000)

Y. Zhang, W. Tang, G.C. Hadjipanyis, C. Chen, C. Nelson and Kannan M. Krishnan, " Evolution of microstructure, microchemistry and coercivity in 2:17 type sm-Co magnets with heat treatment ",IEEE Trans Mag. 37, 2525 (2001)
Y. Zhang, W. Tang, G. C. Hadjipanayis, J. Liu, M. S. Walmer, E. C. Nelson and Kannan M. Krishnan, "Effect of cell size on magnetic properties and domain structures of sintered Sm(Co,Cu,Fe,Zr) 2:17 magnets", J. Appl. Phys. (in press)
. Y. Zhang, M. Corte-Real, G.C. Hadjipanayis, J. Liu, M. S. Walmer and Kannan Krishnan, "Magnetic hardening studies in sintered Sm(Co, Cux, Fe, Zr)z 2:17 high temperature magnets" J. Appl. Phys. 87, 6722 (2000)

M. Benaissa, Kannan M. Krishnan and E.E.Fullerton, "Magnetic anisotropy and its microstructural origin in epitaxially grown bicrystal Sm-Co thin films", IEEE Trans. Mag., 34, 1204 (1998)

Tai Nguyen, Kannan M. Krishnan, L. H. Lewis, Y. Zhu and D.O. Welch, "Microstructure and composition in rapidly quenched FeNdB-based hard magnet alloys", J. Appl. Phys.,79, 4848 (1996)


Patterned Media by Ion-Beam Irradiation

G.J. Kusinski, (Graduate Student)
J. D. Wright (Graduate Student)

D. Weller, IBM/Almaden (Collaborator)
B. Terris, IBM/Almaden (Collaborator)
G. Thomas, UC Berkeley (Collaborator)
R. Ranjan, Seagate (Collaborator)


(111) textured (Co0.3nm/Pt1nm)10 multilayers with perpendicular anisotropy were grown using electron beam evaporation and were patterned into magnetic arrays by a Ga Focussed Ion Beam (FIB). The reversal processes of these arrays were studied by a magnetic soft x-ray transmission microscope (X-TXM) at a spatial resolution of 25nm. Element specificity (Co L3 absorption edge) and magnetic contrast due to magnetic circular dichroism (MCD) were used for the first time to image such nanometer scale elements. Samples imaged at remanance, after applying a field of +10kOe, show significant decrease in intensity for irradiated regions as a result of the change in magnetization from out of plane to in plane (Figure 1a). The matrix and the squares have the same intensity, and the irradiated lines have lower intensity, demonstrating upward-perpendicular magnetization and in-plane magnetization, respectively. Reversing the applied field direction resulted in partial switching of the 240nm square patterns at fields below the sample coercivity (HC^ = 6.3kOe). At remanance, after applying a vertical field of -3.4kOe, the matrix is still magnetized perpendicular (upward), the FIB lines are magnetized in-plane but the squares are magnetized downward at the edges and upward in the centers, as shown by dark and bright contrast, respectively (Figure 1b). The reversal process appears to originate at the edges of the patterns and propagate into both the squares and the surrounding region. The same area imaged at 750eV, (off the Co edge), shows no contrast indicating no topographical changes due to the patterning. Details of the magnetizing experiment including the reversal mechanism for the samples exposed to different irradiation dose are discussed in the paper. These results are correlated with the microstructure, which was investigated using conventional, high resolution and energy filtered transmission electron microscopy (TEM).


G.J. Kusinski, K.M. Krishnan, G. Denbeaux, G. Thomas, B.D. Terris and D. Weller "Magnetic imaging of ion- irradiation-patterned Co/Pt multilayers using complementary electron and photon probes', Appl. Phys. Lett. 79, 2211 (2001)

D. Weller. L. Folks, M. Best, E.E. Fullerton, B.D. Terris, G. J. Kusisnski, Kannan M. Krishnan and G. Thomas, 'Growth, structural and magnetic properties of high-coercivity Co/Pt multilayers", Jour. Appl. Phys., 89, 7525 (2001)

G.J. Kusinski, K.M. Krishnan, G. Denbeaux, G. Thomas, B.D. Terris and D. Weller "Magnetic imaging of ion- irradiation-patterned Co/Pt multilayers using complementary electron and photon probes', Appl. Phys. Lett. 79, 2211 (2001)

Role of Stoichiometry and Structure in Colossal Magneto-Resistive Oxide Thin Films

Dr. H. L. Ju (Postdoctoral Fellow)
Dr. A. R. Modak (Postdoctoral Fellow)
Prof. S. Sundar Manoharan (Visiting Scientist)
Dr. H. C. Sohn (Postdoctoral Fellow)

Dr. O.I. Lebedev (Collaborator)
Prof. D. Lederman (Collaborator)

Synthesis by pulsed laser ablation and novel sol-gel chemical routes. Fundamental investigation of the role of elemental substitutions, crystallinity, epitaxy, strain, oxidation state and grain boundaries in determining their galvanomagnetic properties. Characterization by electron microscopy and x-ray scattering/dichroism. Tailored structures to reduce field dependence and submicrometer magnetoresistive devices are being developed. Current work is in CMR manganites and new work is being initiated in half-metallic ferromagnets.

We prepared a range of well characterized samples by the sol-gel method developed in our laboratory. By careful doping we controlled the divalent doping (in one set of samples) and the oxygen content (in the second set).

We measured the temperature dependent magnetotransport behaviour of these complex oxides. The electronic structure of these well characterized samples was measured by electron energy-loss spectroscopy using the newly installed CM200/FEG microscopes. No discernible change in the Mn L3,2 edges were observed either as a function of doping or oxygen content. However, the O-K edge(1s->2p transitions) intensity, sensitive to the 2p hole density on oxygen sites, varied systematically with the resistivity for both sets of samples.

By these measurements, we showed that these manganites are charge-transfer type insulators with significant conductivity due to holes on oxygen sites. This provides new insight into th transport mechanism in these oxide and suggests that the double exchange model, which implicitly assumes a Mott-Hubbard type insulator, has to be revised to include the role of oxygen hole mobility in the transport.


H. L. Ju, H.C. Sohn and Kannan M. Krishnan, " Evidence for oxygen 2p hole driven conductivity in La1-xSrxMnO3 and La0.8Sr0.2MnOz CMR thin films", Phys. Rev. Lett., 79, 3230 (1997)

Kannan M. Krishnan and H. L. Ju, "Role of stoichiometry and structure in colossal magneto-resistive La1-xSrxMn1-yRuyO3+? ", Phys. Rev (B), 60, 14793 (1999)

H. L. Ju, Kannan M. Krishnan and D. Lederman, "Evolution of strain dependent transport properties in ultra-thin LSMO films", Jour. Appl. Phys., 83, 7073 (1998).

S. Sundar Manoharan, H. L. Ju and Kannan M. Krishnan, " Unusual substitution effect of Ru in bulk and thin films of La0.7Sr0.3Mn1-xRuxO3" , Jour. Appl. Phys., 83,7183 (1998)

H. L. Ju and Kannan M. Krishnan, "Effect of vacuum annealing on oxygen stoichiometry and resistivity in sol-gel derived films", Sol. Stat. Comm. , 104,419 (1997)

O.I. Lebedev, C. Van Tandeloo, S. Amelinckx, H. L. Ju and Kannan M. Krishnan, "HREM study of strained epitaxial La0.7Sr0.3MnO3 thin films, Phil Mag A, 90, 673-691 (2000)

Kannan M. Krishnan, A. R. Modak, C. A. Lucas, H. Baumann and R. Michel, "Role of epitaxy and polycrystallinity in the magneto- resistance and magnetization of La0.8Sr0.2MnO3 thin films", J. Appl. Phys., 79, 5169 (1996