Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session X32: Focus Session: Magnetic Sensors |
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Sponsoring Units: GMAG Chair: Thomas Crawford, University of South Carolina Room: Morial Convention Center 225 |
Friday, March 14, 2008 8:00AM - 8:12AM |
X32.00001: Challenges for picoTesla Magnetic-Tunnel-Junction Sensors William Egelhoff, Philip Pong, Robert McMichael, Edmund Nowak, Alan Edelstein, James Burnett, Greg Fisher The extension of small, inexpensive, low-power, low frequency, ultra-sensitive magnetic sensors to fields between 1 nanoTesla and 1 picoTesla, an area currently dominated by fluxgates, optically-pumped magnetometers, and SQUIDS, would be a paradigm shift for the field of magnetic sensors. The necessary elements for picoTesla MTJ sensors have been identified by modeling the noise characteristics. The results help identify the experimental challenges that exist in the integration of the necessary components of the sensor and illustrate the trade-offs that must be considered. For example, values of the TMR above 100{\%} contribute very little, while lowering the saturation field of the free layer below 10 Oe is essential. These and other insights identify the critical issues than need attention and can guide research into productive directions. [Preview Abstract] |
Friday, March 14, 2008 8:12AM - 8:24AM |
X32.00002: X-Ray Diffraction Studies of CoFeB/MgO/CoFeB Magnetic Tunnel Junctions Pinshane Huang, John Read, Robert Buhrman MgO-based magnetic tunnel junctions (MTJs) show high levels of tunneling magnetoresistance (TMR),\footnote{ Y. Lee, et al., Appl. Phys. Lett 90, 212507 (2007).} a very desirable trait for magnetic random access memory and hard drive read heads. Because theory links tunnel junction crystallinity with high TMR, studies of MTJ crystal structure have implications for the development of high-performance MTJs\footnote{ J. Mathon and A. Umerski, Phys. Rev. B. 63, 220403 (2001).}$^{,}$\footnote{ W. H. Butler, et al., Phys. Rev. B. 63, 054416 (2001).}. Varying the anneal temperature, MgO growth methods, and seed layers, we have examined texturing in CoFeB/MgO/CoFeB tunnel junctions using x-ray diffraction. We investigated MTJs with rf-sputtered or electron-beam evaporated MgO barrier layers, and we used TaN, Ta/Ru, and Ta/CuN as alternative seed layers for the MTJ growth. We report on changes in both tunnel barrier and electrode crystallization as a function of anneal temperature, which we find to be dependent on both the MgO deposition method and the seed layer composition. Our complete MTJ structures using rf-sputtered MgO barriers achieve TMR in excess of 200{\%}, and correlation with these XRD results sheds light on the complex dependence of MTJ performance on growth and processing conditions. [Preview Abstract] |
Friday, March 14, 2008 8:24AM - 8:36AM |
X32.00003: Chemical and electronic studies of CoFeB / MgO / CoFeB magnetic tunnel junctions J. Read, J. Cha, P. Huang, W. Egelhoff, D. Muller, R. Buhrman MgO based magnetic tunnel junctions (MTJs), particularly the CoFeB/MgO/CoFeB system, exhibit large tunneling magnetoresistance (TMR) which makes them viable for MRAM [1] and sensor applications. Careful engineering of the MgO tunnel barriers, CoFeB electrodes, and their interfaces is essential for optimizing device performance [2,3], which motivates investigation of the chemical and electronic properties of high quality MTJs. We correlate scanning tunneling (STS), x-ray photoelectron (XPS) [4], and electron energy loss (EELS) [5] spectroscopies with current-in-plane tunneling (CIPT) measurements to gain insight on the electronic structure and chemistry of MgO MTJ structures. The measurements reveal that quite high TMR ($>$200{\%}) can be obtained when there is substantial boron in the tunnel barrier, showing that proper doping of the MgO layer plays a significant role in the performance of such MTJs. We will discuss the impact of materials properties upon transport measurements and provide suggestions for greater control over MTJ device characteristics. [1] Parkin, Nat. Mater. 3, 862 (2004). [2] Nagamine, APL 89, 162507 (2006). [3] Lee, APL 90, 212507 (2007). [4] Read, APL 90, 132503 (2007). [5] Cha, APL 91, 062516 (2007). [Preview Abstract] |
Friday, March 14, 2008 8:36AM - 8:48AM |
X32.00004: FMR spectroscopy with very large precession cone angle in magnetic tunnel junctions Takahiro Moriyama, Xin Fan, John Q. Xiao Unlike the small angle precession with low power microwave excitation, which has been extensively studied using conventional ferromagnetic resonance (FMR) techniques, the large angle precession involves nonlinear response of the magnetizations. Large angle ferromagnetic resonance (LA-FMR) measurements can help to understand the magnetization dynamics with high power excitation generally found in current induced spin switching and microwave assisted switching, etc. Recently we have performed magnetoresistance measurements in IrMn/FeCo/AlOx/Permalloy (Py) magnetic tunnel junctions excited by a large power microwave [1]. Using the same structure, we electrically detected the LA-FMR of the Py by measuring the tunneling resistance in a sweeping external dc magnetic field. We found that the resonance frequency depends on the microwave power, i.e. precession cone angle, as well as the dc magnetic field, which can be well explained by using the Landau-Lifshitz-Gilbert equation in non-linear regime. The results also suggest that this new experimental technique to detect FMR is very useful for characterizing the LA-FMR. \newline [1] T. Moriyama et al., Appl. Phys. Lett. 90 (15), 152503 (2007). [Preview Abstract] |
Friday, March 14, 2008 8:48AM - 9:00AM |
X32.00005: Technique for Minimizing the Effect of 1/$f$ Noise in Magnetic Sensors Alan Edelstein, Greg Fischer, James Burnette, Shu-Fan Cheng, Edmund Nowak, William Egelhoff, Cathy Nordman Sensors such as magnetic tunnel junctions with MgO barriers offer the possibility of increased sensitivity. The magnetoresistance of these junctions can be as large a 400{\%}. Unfortunately, these magnetoresistance devices suffer from having considerable 1/$f$ noise. We have a device, the MEMS flux concentrator, that modulates the field at the position of the sensor and thus increase the frequency of the field to be detected to kHz frequencies where the 1/$f$ noise is much smaller. It does this by having flux concentrators on MEMS flaps that are driven to move electrostatic comb drives. The flaps on each side of the sensor are connected by springs so that the desired motion is a normal mode. The signal appears as sidebands that can be demodulated using a lock-in amplifier. The device will increase the sensitivity at 1 Hz of many sensors by a factor of 100. Tests indicate that the device does not increase the noise and that it will function at frequencies lower than 1 Hz. Results of initial tests will be reported. [Preview Abstract] |
Friday, March 14, 2008 9:00AM - 9:12AM |
X32.00006: Study of magnetization switching in synthetic antiferromagnets Cosmin Radu, Dorin Cimpoesu, Leonard Spinu, Alexandru Stancu Synthetic Antiferromagnet (SAF) structures are very important in designing modern spintronic devices. The theoretical studies of the toggle writing mode in MRAM$^{1}$ use the concepts of SAF critical curve, which is a generalization of the astroid from the coherent rotation model in the case of uniaxial anisotropy. Although extensively studied theoretically$^{2,3}$ there are no methods proposed to experimentally determine the critical curve of a SAF structure. We propose a way for determining the critical curve of the switching fields using reversible susceptibility experiments (RS) and we prove this to be more sensitive to the switching characteristics of SAF structures than a regular hysteresis loop. For certain coupling strengths the entire critical curve can't be determined using standard RS experiments and a strategy for revealing these hidden parts of the critical curve is proposed. 1. L. Savtchenko, B. N. Engel, N. D. Rizzo, M. F. Deherrera, and J. A. Janesky, US Patent 6,545,906 B1, (2003). 2. S. Y. Wang and H. Fujiwara, J. Magn. Magn. Mater. 286, 27-30 (2005). 3. H. Fujiwara, S. Y. Wang, and M. Sun, J. Appl. Phys. 97, 10P507-10P507-5 (2005). Work supported by DARPA grant HR0011-07-1-0031. [Preview Abstract] |
Friday, March 14, 2008 9:12AM - 9:24AM |
X32.00007: ABSTRACT WITHDRAWN |
Friday, March 14, 2008 9:24AM - 9:36AM |
X32.00008: Exchange bias in a ferrimagnetic/antiferromagnetic system M.R. Hossu, S. Demirtas, M.B. Salamon, A.R. Koymen The effect of antiferromagnetic FeMn on ferrimagnetic (Co4nm/Gd4nm)$_{ 4}$ multilayer was investigated by measuring the exchange bias and coercivity fields. It was observed that magnetic properties depend on whether the multilayers are Co or Gd terminated. The exchange bias increased at low temperatures from 50Oe to 350Oe when FeMn layers are on both surfaces of Co terminated [FeMn10nm/Co4nm/ (Gd4nm/Co4nm)$_{ 4}$/FeMn10nm] multilayer, compared to single FeMn layer. However for the Gd terminated [FeMn10nm/Gd4nm/ (Co4nm/Gd4nm)$_{ 4}$/FeMn10nm] multilayer the Gd/FeMn interface does not induce exchange bias. Moreover exchange spring behavior is observed around compensation temperature due to the uncompensated moment in FeMn layers. [Preview Abstract] |
Friday, March 14, 2008 9:36AM - 9:48AM |
X32.00009: Optical Manipulation of Paramagnetic Particles with On-Chip Detection Using Spin Valve Sensors Lena Wai-Yi Lui, Ke Bin Li, Sean O'Shea, Chorng-Haur Sow In this work, we present a new combinatory approach where an optical tweezers was used to trap and position a single super-paramagnetic particle over a Spin Valve sensor, with the particle then detected by the sensor. This approach is demonstrated using super-paramagnetic particles of 2 $\mu $m (Micromer$^{\mbox{{\textregistered}}}$-M, Micromod) together with a Spin Valve sensor with dimensions of 2 x 4 $\mu $m$^{2}$ whereby a single magnetic particle was positioned over the sensor and a corresponding drop in the voltage across the sensor was detected. The results are explained using a simple model where the particle is treated as a pure dipole. [Preview Abstract] |
Friday, March 14, 2008 9:48AM - 10:00AM |
X32.00010: One dimensional Brownian rotation of magnetic microspheres Miri Shlomi, Brandon McNaughton, Raoul Kopelman, Panos Argyrakis Many experimental observations of 3D random rotations of particles have been interpreted using Einstein's 1D Gaussian solution. However, in contrast to 1D, 3D rotations are non-commutative, and cannot rigorously be described by Einstein's model. To test the potential discrepancy between theory and experiment, we monitor a particle rotating freely around a single fixed axis, subsequently comparing it with the particle's 3D Brownian rotation. To carry out these observations, we use a fluorescent ferromagnetic microsphere that has one hemisphere coated with an opaque metal (aluminum), while the other is left intact. In rotating, the particle undergoes intensity fluctuations which are observed in a fluorescent microscope. Restraining the rotation by aligning the ferromagnetic particles with an external magnetic field, allows us to see, for the first time, Brownian rotation of a sphere around a fixed axis. [Preview Abstract] |
Friday, March 14, 2008 10:00AM - 10:12AM |
X32.00011: Single bacterial cell detection with nonlinear rotational frequency shifts of driven magnetic microspheres Brandon McNaughton, Rodney Agayan, Ron Smith, Raoul Kopelman, Roy Clarke Shifts in the nonlinear rotational frequency of magnetic microspheres, driven by an external magnetic field, offer a dynamic approach for the dynamic detection of single bacterial cells. We demonstrate this capability by measuring such frequency shifts when an \textit{Escherichia coli }attaches to the surface of a 2.0 micron magnetic microsphere, thereby affecting the drag of the system. From this change in drag, the nonlinear rotation rate was reduced, on average, by a factor of 3.8. Sequential bacterial attachments were also monitored using this approach. [Preview Abstract] |
Friday, March 14, 2008 10:12AM - 10:24AM |
X32.00012: Length dependence of the magnetorheological properties of cobalt microwires dispersed in silicone oil. Richard Bell, Joshua Karli, Jeffrey Krug, Darin Zimmerman The rheological responses and dispersion stability of magnetorheological (MR) fluids were experimentally investigated. In order to improve the stability of the MR fluids, spherical particles were replaced with cobalt microwires (260 nm diameter) with varying aspect ratios ranging from 10 to 65 and suspended in silicone oil (0.17 Pa$\cdot $s). Under an external magnetic field ($H_{0})$ and a steady shear flow, the yield stress of the fluids display a $H_{0}^{\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} }$ dependence. The apparent yield stress at magnetic saturation increased linearly with the median length of the particles until a maximum at a wire length of 7 $\mu $m was reached. Further increasing the length of the microwires resulted in a linear decrease in the yield stress of the fluids. Furthermore, the MR fluids containing microwires with lengths greater than 6 $\mu $m exhibited a larger yield stress than fluids containing only 1.6 $\mu $m diameter spherical particles. The microwire-based fluids also display improved stability against rapid sedimentation as compared to the spherical cobalt particles. [Preview Abstract] |
Friday, March 14, 2008 10:24AM - 10:36AM |
X32.00013: Half-metallic ferromagnetism in iron-antimony based Skutterudites with monovalent filler atoms Andreas Leithe-Jasper, Walter Schnelle, Helge Rosner, John Mydosh, Yuri Grin We report the thermodynamic, magnetic and electronic properties of of the filled Skutterudites AFe$_4$Sb$_{12}$ (A=Na,K,Tl) in a joined experimental and theoretical study. Unexpectedly, these compounds show a ferromagnetic transition at $T_c$ $\sim$ 85\,K.[1] According to electronic structure calculations and point-contact Andreev reflection [2] these systems show a rather large spin polarization. On the other hand, these itinerant magnets exhibt strong spin fluctuations. A brief comparison with compounds based on A=Ca,Sr,Ba,Yb,La where spin fluctuations impede long magnetic order will be presented.\newline \newline [1] A. Leithe-Jasper, W. Schnelle, H. Rosner, N. Senthilkumaran, A. Rabis, M. Baenitz, A. Gippius, E. Morozova, J.A. Mydosh, and Yu. Grin, Phys. Rev. Lett. 91, 037208, (2003). \newline [2] G. Sheet, H. Rosner, S. Wirth, A. Leithe-Jasper, W. Schnelle, U. Burkhardt, J. A. Mydosh, P. Raychaudhuri, and Yu. Grin, Phys. Rev. B 72, 180407, (2005). [Preview Abstract] |
Friday, March 14, 2008 10:36AM - 10:48AM |
X32.00014: Interplay of Magnetic and Structural Anisotropy in Co$\vert$Ni Multilayer Thin Films Joseph Dvorak, Kathryn Krycka, Jean-Marc Beaujour, Wenyu Chen, Andrew Kent, Chi-Chang Kao Interfacial perpendicular magnetic anisotropy (PMA) is important for spin transfer devices and has been predicted to overcome dipolar shape anisotropy for [t Co$\vert$2t Ni] multilayers with thicknesses, t, of 4 $\mathring{A}$ or less [1]. layered between Cu, however, thickness dependent PMA (t=1, 2, 3, 4 and 6 $\mathring{A}$) is not sufficiently strong to produce perpendicular magnetization. Anomalous diffraction reveals that the Co and Ni are expansively strained by the Cu within the sample plane. As calculated in reference 2 this trigonal strain would be sufficient to overcome the PMA. Ferromagnetic resonance measurements [3] indicate that the net Lande g-factors are enhanced above bulk for all thicknesses, and increase further with decreasing layer thickness. By applying element-specific x-ray magnetic circular dichroism (XMCD) we have been able to study the Co and Ni individually. Both elements show increasing spin to orbit ratios with decreasing thickness magnetized either in-plane or along sample normal. In all cases the spin to orbit ratio is enhanced along the sample normal compared with the in-plane direction. \newline [1] Phys. Rev. Lett. 68, 682 (1992) \newline [2] Phys. Rev. B 69, 104426 (2004) \newline [3] Eur. Phys. J. B 59, 475 (2007) [Preview Abstract] |
Friday, March 14, 2008 10:48AM - 11:00AM |
X32.00015: Classical and Quantum Routes to Linear Magnetoresistance Jingshi Hu, T.F. Rosenbaum We show that either adding a few parts per million of the proper chemical impurities to indium antimonide, a well-known semiconductor, or redesigning the material's structure on the micrometer scale, can transform its response to an applied magnetic field. The former approach is a spectacular manifestation of magnetotransport in the extreme quantum limit, when only one Landau band is partially filled; the latter a classical outgrowth of disorder, turned to advantage. In both cases the magnetoresistive response – at the heart of magnetic sensor technology – can be converted to a simple, large and linear function of field that does not saturate. Harnessing the effects of disorder has the further advantage of extending the useful applications range of such a magnetic sensor to very high temperatures by circumventing the usual limitations imposed by phonon scattering. [Preview Abstract] |
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