Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session U14: Focus Session: Magnetization Dynamics and Half Metals |
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Sponsoring Units: GMAG DMP FIAP Chair: John Freeland, Argonne National Laboratory Room: Colorado Convention Center Korbel 4D |
Thursday, March 8, 2007 8:00AM - 8:12AM |
U14.00001: Theoretical limit of the minimal magnetization switching field and the shape of a field pulse for minimal reversal time of Stoner particles X.R. Wang, Z.Z. Sun The theoretical limit of the minimal magnetization switching field and the optimal field pulse design for uniaxial Stoner particles are investigated. Two results are obtained. One is the existence of a theoretical limit of the smallest magnetic field out of all possible designs. It is shown that the limit is proportional to the damping constant in the weak damping regime and approaches the Stoner-Wohlfarth (SW) limit at large damping. For a realistic damping constant, this limit is more than ten times smaller than that of so-called precessional magnetization reversal under a non-collinear static field, showing a big room for possible improvement in current available strategies. The other is on the optimal field pulse design: If the magnitude of a magnetic field does not change, but its direction can vary during a reversal process, there is an optimal design that gives the shortest switching time. The switching time depends on the field magnitude, damping constant and magnetic anisotropy. The two results can be used to evaluate various magnetization reversal strategies. Reference: Z.Z. Sun, and X.R. Wang, Phys. Rev. B 73, 092416 (2006); 74, 132401 (2006); Phys. Rev. Lett. 97, 077205 (2006). [Preview Abstract] |
Thursday, March 8, 2007 8:12AM - 8:24AM |
U14.00002: Coupled Precession Modes in Indirect Exchanged-Coupled Thin Films Steven Michalski, Jian Zhou, Bob Buckley, Ralph Skomski, Roger Kirby Static and dynamic magnetic properties of exchange-coupled magnetic layers have been investigated by magneto-optical measurements. Our samples are [Pt/Co] multilayers with perpendicular magnetic anisotropy (PMA) exchange-coupled to a Co (Ni) layer with in-plane magnetic anisotropy by a variable thickness intervening Pt layer. The magnetic properties of such systems are controllable by tuning the exchange strength and PMA. To measure the magnetization precession, we use a femtosecond laser in a pump-probe experiment with direct optical excitation and preliminary measurements using a magnetic field pulse excitation. Both the strength and the angle of an external applied magnetic field were varied and for many samples, two modes with two distinct precession frequencies were observed, with frequencies that depend on the strength and the angle of the applied field. Our results are interpreted by a LLG-based model which predicts two modes whose behaviors depend on the strength and sign of the exchange coupling. This model is in a good qualitative agreement with our data and allows us to estimate the magnitude of the exchange coupling between the two layers. This work is supported by NSF-MRSEC, NCMN, and the W. M. Keck Foundation. [Preview Abstract] |
Thursday, March 8, 2007 8:24AM - 8:36AM |
U14.00003: Frequency-dependent magnetization response of CoFe thin film Mingqiang Bao, Alexander Khitun, Jooyoung Lee, Kang L. Wang, Ajey P. Jacob Frequency-dependent magnetization dynamics in CoFe thin films is investigated. Magnetization has been generated and detected by a pair of micrometer-wide antennas. The experimental data have been mapped showing the frequency-dependence of both the amplitude and the phase under external magnetic field. The response on a continuous single frequency excitation shows complex behaviors including those of harmonic resonances and half-frequency resonances. Furthermore, the device time-domain response of any input voltage/current pulse under any external magnetic field (below 350 Oe) can be reconstructed from our measured frequency-domain maps through the Fourier transform, and is validated with our time-domain measurement data. \newline \newline *The authors would like to thank Dr. S. Wang and Dr DW Lee for growing CoFe thin-films. The work was supported in part by Western Institute of Nanoelectronics (WIN) and the Microelectronics Advanced Research Corporation (MARCO) Focus Center on Functional Engineered Nano Architectonics (FENA). [Preview Abstract] |
Thursday, March 8, 2007 8:36AM - 9:12AM |
U14.00004: Vortex Dynamics Imaged by Time-Resolved Scanning Transmission X-Ray Microscopy. Invited Speaker: Substantial progress in the understanding of magnetic vortex dynamics has been achieved by implementing 100 ps time-resolved magnetic imaging techniques into a Scanning Transmission X-Ray Microscope (ALS, Berkeley, BL 11.0.2) with a lateral resolution of 20-40 nm. Gyrotropic vortex motion excited by in-plane alternating magnetic fields was studied in micron-sized ferromagnetic vortex structures. \newline The vortex core, extending over a range of about 20 nanometers in the center of a vortex structure, plays a key role in vortex dynamics. We have been able to perform time-resolved imaging of the out-of-plane magnetization distribution of the excited vortex core. In addition, we have discovered new switching schemes to change the direction of the vortex core polarization in micron-sized ferromagnetic vortex structures, either (i) by altering the amplitude of an alternating magnetic field at a frequency close to the eigenfrequency of the gyrotropic vortex motion or (ii) by applying a short burst (e.g., one single period) of ac magnetic fields [1]. \newline Magnetic vortex cores have already been discussed as candidates for magnetic data storage, but for the switching of their polarization large magnetic fields in the order of half a Tesla were required so far. The vortex core reversal schemes presented here need significantly lower magnetic fields (down to a few mT). The switching mechanism as reproduced by micromagnetic simulations [1] involves: (i) creation of a vortex--antivortex pair, both with opposite polarisation and (ii) annihilation of the antivortex with the original vortex. At the end a vortex with opposite polarization remains. \newline \newline [1] B.Van Waeyenberge, A.Puzic, H.Stoll et al., \textit{Magnetic vortex core reversal by excitation with short bursts of an alternating field}, Nature, Nov.23 (2006) [Preview Abstract] |
Thursday, March 8, 2007 9:12AM - 9:24AM |
U14.00005: Highly-coherent magnetic vortex oscillations driven by a dc spin-polarized current V.S. Pribiag, G.D. Fuchs, P.M. Braganca, O. Ozatay, J.C. Sankey, D.C. Ralph, R.A. Buhrman, I.N. Krivorotov While it has been demonstrated that dc spin-polarized currents can drive microwave spin-wave oscillations in magnetic multilayers via the spin-transfer torque (STT) effect [1], little is known about persistent STT-driven oscillations in strongly non-uniform systems. We report the use of STT to excite steady-state gigahertz-frequency oscillations of a magnetic vortex. We use an elliptical Py-Cu-Py nanopillar spin-valve structure in which one of the Py layers is sufficiently thick that its magnetization assumes a vortex configuration. The oscillations, which can be obtained in essentially zero applied field, are highly coherent, with full-widths at half maximum of less than 300 kHz at room temperature being obtained under certain bias conditions. We will discuss the observed sensitivity of the oscillation line-width to magnetic defects. We will also present measurements of the temperature-dependence of the oscillations, which we are pursuing to obtain a more complete understanding of how magnetic imperfections and thermal fluctuations determine the performance of this new type of nanomagnetic STT oscillator. We will also discuss the use of STT-driven ferromagnetic resonance to examine the various magnetic modes that can be present in these nanoscale vortex structures. [1] S. I. Kiselev \textit{et al.}, \textit{Nature} (London) \textbf{425}, 380 (2003). [Preview Abstract] |
Thursday, March 8, 2007 9:24AM - 9:36AM |
U14.00006: Driven dynamic mode-splitting of magnetic vortices Kristen Buchanan, Marcos Grimsditch, Frank Fradin, Sam Bader, Val Novosad It has been established theoretically and experimentally that a magnetic vortex in restricted geometry possesses a translational excitation that corresponds to circular motion of the vortex core at a characteristic frequency. Here we explore the effect of increased driving-field amplitude on this dynamic mode using a microwave reflection technique. We find a new effect - the vortex translational eigenmode splits into two peaks. The splitting in frequency is $>$25\% for driving magnetic fields $<$25 Oe for micron-sized permalloy ellipses that are 40-nm thick. Splitting effects were detected for driving fields as low as 3 Oe in circular dots. Micromagnetic modeling suggests this effect could be indicative of nonlinear fold-over but further theoretical work will be required to develop a full understanding of these observations. [Preview Abstract] |
Thursday, March 8, 2007 9:36AM - 9:48AM |
U14.00007: Electronic and magnetic properties of the La$_{0.7}$Sr$_{0.3}$MnO$_{3}$/SrTiO$_{3}$ interface from first principles calculations Hand Zenia, Gillian Gehring, Walter Temmerman We present results of first principles calculations of the electronic and magnetic properties of the La$_{0.7}$Sr$_{0.3}$MnO$_3$/SrTiO$_{3}$ interface. We are interested in the changes with respect to the bulk as concerns ferromagnetism and half-metallicity. The bulk calculations give a nearly half-metallic ground state for the manganite. In transport the system is predicted to be totally half-metallic. This latter property is preserved at the interface only if the magnetic coupling between the interface region and the bulk is ferromagnetic. We have looked at the two possible interface terminations between La$_{0.7}$Sr$_{0.3}$MnO$_3$ and SrTiO$_{3}$ and found that one of the interfaces preserves the bulk properties of La$_{0.7}$Sr$_{0.3}$MnO$_3$ whereas the other type of termination suppresses them. Reducing the number of holes at the interface by introducing LaMnO$_{3}$ restores ferromagnetism. Hence the possibility of engineering interfaces to improve the Tunneling Magneto-Resistance yield in the La$_{0.7}$Sr$_{0.3}$MnO$_3$/SrTiO$_{3}$/La$_{0.7}$Sr$_{0.3}$MnO$_3$ tunnel junctions. [Preview Abstract] |
Thursday, March 8, 2007 9:48AM - 10:00AM |
U14.00008: Heusler Alloys for CPP-GMR C. Culbert, M. Williams, M. Chshiev, P. LeClair, W. H. Butler Half-Metallic full Heusler alloys of composition X$_{2}$YZ and structure type L2$_{1}$ have aroused interest because of their potential application in CPP-GMR spin valves for readers in hard disk drives. The X and Y are typically transition metals and the Z is a non-transition metal element. The structure of these alloys can be viewed as a variant of bcc in which (100) atomic layers of X alternate with layers of YZ. The alloys Co$_{2}$MnSi and Co$_{2}$MnGe have received particular attention because of their high T$_{C}$ which exceeds 900K. We have performed first-principles calculations using the VASP code in GGA to investigate the properties of these materials. We have found them to be half-metals in bulk in agreement with previous work. We obtained minority gaps at the Fermi energy of 0.36 and 0.51eV for Co$_{2}$MnGe and Co$_{2}$MnSi, respectively. We also investigated multilayers consisting of Heusler and various possible spacer materials. Interestingly, we found that for one or two atomic layers of Cr alternating with Co2MnGe along (100), the system remained half-metallic. Cr can actually be used in this way to increase the minority gap. We found that Co$_{2}$MnGe slabs were typically not half metallic, but slabs terminated in a pure Mn layer retained a minority gap. [Preview Abstract] |
Thursday, March 8, 2007 10:00AM - 10:12AM |
U14.00009: Spin Polarization and Spin Transport in Co-based Heusler alloys Muhammad M. Faiz, Raghava P. Panguluri, Sabine Wurmehl, Claudia Felser, Boris Nadgorny The Co-based Heusler alloys are of special interest for possible spintronic applications due to their high Curie temperatures and high magnetic moment per unit cell. Co$_{2}$FeSi is especially promising as a candidate half-metal as it has a Curie temperature of approximately 1100K and the integer magnetic moment of 6$\mu _{B}$ per unit cell [Ref. S. Wurmehl et al]$^{1}$. The samples have been prepared by arc melting of stochiometric quantities of pure metals in argon atmosphere followed by annealing in sealed quartz tube\textbf{s} at 1300K. Here, we report comparative spin polarization, $P$, in Co$_{2}$FeSi and Co$_{2}$MnFeSi using Point Contact Andreev Reflection Spectroscopy (PCAR). We have also studied spin transport in Heusler/Au bilayers. Variable thickness Au films were deposited on top of the Heusler samples and the PCAR technique was then used to probe $P $on the Au side. We will give the estimates of the spin diffusion length in Au based on these measurements and compare the results with conventional ferromagnetic spin injectors. 1. S. Wurmehl et al., J. Appl. Phys. 99, 08J103 (2006). [Preview Abstract] |
Thursday, March 8, 2007 10:12AM - 10:24AM |
U14.00010: A magneto-optical study of Heusler alloy Co$_{2}$MnGe films grown on Ge(111) substrates Pranaba Muduli, Wil Rice, Brian Collins, Liang He, Frank Tsui We report an extensive study of magneto-optic properties and magnetic anisotropy of ternary alloys of Co, Mn and Ge over a wide range of composition near the Heusler alloy compound Co$_{2}$MnGe grown on a single Ge(111) substrate by combinatorial molecular-beam epitaxy. The scanning magneto-optic Kerr effect (MOKE) study reveals a range of room temperature ferromagnetic phases near the Heusler alloy composition. A ridge of higher MOKE intensity near the Co/Mn atomic ratio of two is found, which appears to correlate with the structural perfection of the epitaxial film. The MOKE hysteresis loops in this composition range exhibit asymmetries that are associated with the presence of a large second-order magneto-optic effect. The magnetic anisotropy in the same composition range can be described by a combination of a six-fold and a uniaxial term. A single domain model in conjunction with the second-order MOKE is used to explain the magnetic anisotropy in these alloys. The model reveals a systematic change of magnetic anisotropy and the second-order MOKE coefficients as a function of composition and structural and chemical ordering of the films. [Preview Abstract] |
Thursday, March 8, 2007 10:24AM - 10:36AM |
U14.00011: The electronic band structure of CoS$_{2}$ Ning Wu, Yaroslav Losovyj, David Wisbey, Kirill Belashchenko, Wai-Ning Mei, Michael Manno, Lan Wang, Chris Leighton, Peter Dowben We have identified a strongly dispersing band of CoS$_{2}$(100), with both sulfur and cobalt weight, along the $\Gamma $-X direction of the bulk Brillouin zone, from photon energy dependent angle resolved photoemission studies. From the critical points of the experimental band structure, the inner potential is estimated at about 4 to 5 eV, consistent with LEED I(V) analysis. The small inner potential indicates that CoS$_{2}$ has a narrow band width, consistent with the theoretical expectations. The clearly favored structural model from the LEED I(V) analysis is sulfur with cobalt terminated surface. [Preview Abstract] |
Thursday, March 8, 2007 10:36AM - 10:48AM |
U14.00012: Spin polarization of the ferromagnetic semimetal EuB$_{6}$ Xiaohang Zhang, Stephan von Molnar, Peng Xiong, Zach Fisk Much progress has been made recently in the understanding of the electronic properties of EuB$_{6}$. However, the details of the electronic structure remain controversial. Several band structure calculations have produced different degrees of conduction-valence band overlap for the two spin subbands. In particular, some calculations predict a half-metallic band structure, ie, 100{\%} spin polarization at the Fermi level$^{1}$. We have performed direct measurements of the spin polarization of EuB$_{6}$ crystals using Andreev reflection spectroscopy. Planar junctions of EuB$_{6}$/Pb were fabricated on crystals grown with an Al flux method. The conductance spectra were measured using phase-sensitive detection at several temperatures below T$_{C}$ of Pb. The spectra are well-described by the spin-polarized BTK model. More than seven junctions were measured and a spin polarization of 55 $\pm $ 10{\%} is obtained. Our results indicate that in ferromagnetic EuB$_{6}$ the electrons and holes at the Fermi level are not fully spin-polarized. This work was supported by a FSU Research Foundation PEG grant and NSF grant under DMR-0503360. $^{1}$J. Kunes and W.E. Pickett, PRB 69, 165111 (2004); M. Kreissle and W. Nolting, PRB 72, 245117 (2005). [Preview Abstract] |
Thursday, March 8, 2007 10:48AM - 11:00AM |
U14.00013: Sharp switching of the magnetization in Fe$_{1/4}$TaS$_{2}$ Emilia Morosan, Henny Zandbergen, Lu Li, Minhyea Lee, Joseph Checkelsky, Michael Heinrich, Theo Siegrist, N. Phuan Ong, Robert Cava Anisotropic magneto-transport measurements are reported on Fe$_{1/4}$TaS$_{2}$ single crystals grown by vapor transport. Both the magnetization and resistivity are extremely anisotropic, with the magnetic moments aligned parallel to the c crystallographic direction. Fe$_{1/4}$TaS$_{2}$ orders ferromagnetically below T$_{C}$ = 160 K and displays very sharp hysteresis loops in the ordered state for H$\vert \vert $c. The corresponding magnetoresistance is negative, and it qualitatively reproduces the features observed in the M(H) data, by showing a sharp drop around the critical field H$_{s}$ for the moment reversal. For field applied within the ab plane, the magnetization remains small and linear in field up to 5 T, and the magnetoresistance is positive and quadratic in field, with no visible hysteresis. The squareness of the H$\vert \vert $c M(H) loops and the high critical field for the magnetization switch (H$_{s }$= 3.7 T at T = 2 K) allow us to classify Fe$_{1/4}$TaS$_{2}$ as a strong ferromagnet. [Preview Abstract] |
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