Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session J16: Focus Session: Magnetic Nanostructures III |
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Sponsoring Units: DMP GMAG Chair: Valentyn Novosad, Argonne National Laboratory Room: D173 |
Tuesday, March 22, 2011 11:15AM - 11:27AM |
J16.00001: Studies of isolated and interacting ferromagnetic gapped nanorings Jie Li, Sheng Zhang, Jason Bartell, Chris Grigas, Cristiano Nisoli, Paul Lammert, Vincent Crespi, Peter Schiffer We have used micromagnetic simulation and magnetic force microscopy (MFM) to study isolated and interacting permalloy nanorings that are lithographically fabricated with gaps that prevent a rotationally symmetric magnetic state. The gapped nanorings have inner and outer radii of 200 and 300 nm respectively, and the gap has a subtended width of $\sim $20 degrees. The nanorings generate a strong magnetic field only in the gap, and thus the magnetization states of gapped nanorings are much more accessible to MFM imaging than complete rings. We have investigated the properties of these gapped nanorings, including the anisotropy in their coercive field and the relative alignment of the magnetic polarization in coupled pairs. [Preview Abstract] |
Tuesday, March 22, 2011 11:27AM - 11:39AM |
J16.00002: Broad-Band FMR of Patterned Square Arrays of Square Permalloy Antidots Vinayak Bhat, Joseph Sklenar, John Ketterson, Lance DeLong We have used electron beam lithography to pattern 25-nm-thick Permalloy films with square arrays of square antidots of size D = 300, 400, 500 and 700 nm and same lattice constant d = 1000 nm, using a lift-off technique. Broadband FMR\footnote{C. C. Tsai et al., Rev. Sci. Instrum \textbf{80}, 023904 (2009)} was used to observe localized modes\footnote{C. T. Yu, M. J. Pechan, and G. J. Mankey, Appl. Phys. Lett. \textbf{83}, 3948 (2003)}$^,$\footnote{Minghui Yu et al., J. Appl. Phys \textbf{101}, 09F501 (2007)} showing four-fold rotational symmetry for in-plane DC magnetic field. We have studied FMR spectra spanning the ferromagnetic hysteresis regime around 250 MHz, up to the saturation regime ending near 14 GHz, and observe the appearance and disappearance of various FMR modes, especially at frequencies below 7 GHz. We have observed history-dependent modes below 3 GHz that may be associated with domain walls. [Preview Abstract] |
Tuesday, March 22, 2011 11:39AM - 11:51AM |
J16.00003: Broadband Magnetic Resonance Measurements on Periodic Patterned Disc and Hole Arrays J. Sklenar, V.S. Bhat, L. Delong, V. Metlushko, C.C. Tsai, O. Chernyashevskyy, K. Rivkin, J.B. Ketterson We have made broadband ferromagnetic resonance measurements on patterned permalloy arrays consisting of circular dots (discs) and both square and circular anti-dots (holes). We employ a transmission meander line approach as opposed to a resonant cavity technique, and cover the frequency range 10MHz to 20GHz. Experiments are performed at a fixed frequency by sweeping the field (through positive and negative values) and at a fixed field while varying the frequency; both magnetic field and frequency modulation are employed to suppress noise and background effects. Experiments on hole arrays show two dominant resonances which from their symmetry appear to be standing spin waves centered at the X- points of the square Brillouin zone. Low field measurements on disc arrays where the field is swept over varying ranges in the region where the sample is hysteretic while tracking the history dependent disappearance and reappearance of the uniform FMR mode, allows a determination of the phase boundaries separating the single and double vortex states, and are in agreement with simulations by Rivkin. [Preview Abstract] |
Tuesday, March 22, 2011 11:51AM - 12:03PM |
J16.00004: Exciton-Mn ion interaction in CdTe quantum dots Anna Trojnar, Marek Korkusinski, Eugene Kadantsev, Pawel Hawrylak We develop a microscopic theory of optical properties of quantum dots containing a single magnetic ion [1,2] which includes electron-hole correlations, short range exchange of Mn ion with electron and with heavy hole, long range electron-hole exchange, quantum dot anisotropy and external strong magnetic field. A new quantum interference (QInt) effect between electron-hole Coulomb scattering and scattering by Mn ion is obtained. Special role is played here by configurations with electron and hole on the p-shell and degenerate with it configurations with electron (hole) on s-shell and hole(electron) on a d-shell. QInt is shown to significantly reduce exciton-Mn coupling. The signature of this QInt effect in emission and absorption spectrum is discussed. The effect of strong magnetic field on the characteristic emission spectrum is discussed and the limitations of the spin model are established.\\[4pt] [1] S.-J.Cheng et al, Eur. Phys. Lett. 81, 37005 (2008)\\[0pt] [2] L.Besombes, Phys. Rev. Lett. 93, 207403 (2004). [Preview Abstract] |
Tuesday, March 22, 2011 12:03PM - 12:15PM |
J16.00005: Antiferromagnetic coupling in cobalt atomic clusters on (110) surface of tungsten Renat F. Sabirianov, Pavel Lukashev, Axel Enders We report results of the first principles calculations on the structural and magnetic properties of cobalt atomic clusters on (110) surface of tungsten. We found that for certain geometry these clusters can exhibit antiferromagnetic order. The result is unexpected, as in the bulk as well as in the thin films and free standing clusters Co always exhibits ferromagnetic structure. We compare results for Co with the ones for the analogues Fe atomic clusters. We found that Fe clusters deposited on (110) surface of tungsten tend to couple ferromagnetically similar to bcc Fe in considered geometries. In our calculations we analyzed different configurations of atomic islands, in particular N=3, 4, 5, 6, 8, 12, where N is the number of atoms in the cluster. We perform full structural and magnetic relaxation, and we show that depending on the geometry and number of cobalt atoms in the cluster, the system can be non-magnetic (N=4, 6, 8), ferromagnetic (N=3, 5) and antiferromagnetic or ferrimagnetic (N=4, 12). We present phenomenological model to explain this intriguing magnetic properties of Co atomic islands on (110) surface of tungsten. [Preview Abstract] |
Tuesday, March 22, 2011 12:15PM - 12:27PM |
J16.00006: Non-liftoff block copolymer nanolithography of magnetic nanodot arrays A. Baruth, M.D. Rodwogin, A. Shankar, M.A. Torija, M.J. Erickson, M.A. Hillmyer, C. Leighton Nanolithographic techniques based on self-assembled block copolymer templates offer exceptional potential for fabrication of large-area nanostructure arrays from a wide variety of functional materials. Despite significant progress with control of the template ordering, and development of pattern transfer schemes, significant issues exist with common techniques such as lift-off and etching. Here, we demonstrate successful execution of a nanolithographic process based on climate-controlled solvent annealing of easily degradable cylinder-forming poly(styrene-$b$-lactide) block copolymer films that avoids both lift-off and the most challenging aspects of etching. Essentially, we use an overfill/planarize/etch-back ``Damascene-type'' process, exploiting the large Ar ion beam etch rate contrast between polystyrene and typical metals. The process is demonstrated via formation of a large-area array of 12 nm thick, 25 $\pm $ 3 nm diameter Ni$_{80}$Fe$_{20}$ nanodots ($\sim $0.4 x 10$^{12}$ dots/in$^{2})$ with hexagonally-close-packed local order. Extensive microscopy, magnetometry, and electrical measurements provide detailed characterization of the pattern formation and fidelity. We argue that this generic approach can be applied to a wide variety of materials and is scalable to even smaller feature sizes. Funded by NSF MRSEC. [Preview Abstract] |
Tuesday, March 22, 2011 12:27PM - 12:39PM |
J16.00007: Investigating the origin of the magnetic switching field distribution in bit patterned media Olav Hellwig, Bastian Pfau, Christian Guenther, Stefan Eisebitt, Thomas Hauet, Elizabeth Dobisz, Xiaoyu Xu, Yaney Deborah Bit patterned media (BPM) is a promising approach for extending densities in magnetic data storage. One critical challenge for BPM is a tight magnetic switching field distribution (SFD), i.e. the bit-to-bit variations in reversal field. The SFD has three components: the dipolar interactions between neighboring islands within the array, pattern non-uniformities, such as variations in island size, position or shape and the so-called intrinsic SFD of each individual island, which is due to variations in the intrinsic magnetic material properties. We use soft X-ray spectro-holography and high resolution transmission electron microscopy (TEM) to study the origin of the magnetic SFD in BPM. For this we fabricated pattern arrays with 80 nm islands by e-beam lithography and integrated these into a SiN membrane design suitable for x-ray transmission studies. After identifying individual easy and hard to switch islands in the tails of the SFD we performed plane-view TEM analysis of these islands and correlate their magnetic with their structural properties, such as misaligned grains or irregular island shapes. [Preview Abstract] |
Tuesday, March 22, 2011 12:39PM - 12:51PM |
J16.00008: Control of Magnetic States of Cobalt Nanorings by an External Azimuthal Field Nihar Pradhan, Tianyu Yang, Abbey Licht, Yihan Li, Mark Tuominen, Katherine Aidala Ferromagnetic nanorings attract interest due to their potential application in high density data storage and Magnetoresistive Random Access Memory (MRAM) devices. These nanorings show multidomain stable states that need to be well controlled by external in-plane or circular magnetic fields. This talk presents a new method to generate circular magnetic fields to control the magnetic states in different geometries of Cobalt nanoring structures, of varying diameter, width and thickness. A solid platinum AFM tip was used to pass current through a single nanoring, generating a circular magnetic field. In applying this field we were able to change the state of the individual ring without affecting the states of other neighboring rings. The evolution of the magnetic states of individual symmetric and asymmetric Cobalt nanorings with applied azimuthal field will be presented. [Preview Abstract] |
Tuesday, March 22, 2011 12:51PM - 1:03PM |
J16.00009: Rotationally induced magnetic chirality in clusters of single-domain permalloy islands and gapped nanorings Sheng Zhang, Jie Li, Jason Bartell, Paul Lammert, Vincent Crespi, Peter Schiffer We have studied magnetic moment configurations of clusters of single-domain ferromagnetic islands in different geometries.\footnote{J. Li , S. Zhang, J. Bartell, C. Nisoli, X. Ke, Paul E. Lammert, Vincent H. Crespi, and P. Schiffer, Physical Review B \textbf{82}, 134407 (2010).} The magnetic moments of these clusters are imaged by MFM after rotational demagnetization, following our previous protocols.\footnote{R. F. Wang, C. Nisoli, R. S. Freitas, J. Li, W. McConville, B. J. Cooley, M. S. Lund, N. Samarth, C. Leighton, V. H. Crespi, P. Schiffer, Nature \textbf{439}, 303 (2006).} We observed that two types of the clusters showed a significant imbalance of their two-fold degenerate ground states after demagnetization, and this inequality is correlated to the rotational direction of the demagnetization. A similar imbalance was also found in nano-scale rings with a small gap: the chirality of their magnetic state can be precisely controlled by the rotational direction during demagnetization. [Preview Abstract] |
Tuesday, March 22, 2011 1:03PM - 1:39PM |
J16.00010: Chirality control and vortex manipulation in asymmetric Co dots Invited Speaker: Magnetic vortices in sub-micron sized dots have gained considerable interests in recent years due to their fascinating physics and potential applications in information storage, spin-torque oscillators, and magnetic memory and logic devices. Reproducible control of the vortex chirality is of critical importance for these studies. Here we report on two distinctly different chirality control mechanisms in asymmetric Co dots. Arrays of Co dots were fabricated using electron beam lithography and the circular symmetry was broken by introducing a flat edge. Below a critical diameter and/or thickness, chirality control is achieved by the nucleation of a single vortex within each dot, as conventionally observed. The vortex can be manipulated to annihilate at particular sites under different field orientations and cycle sequences. Interestingly, above these critical dimensions a new chirality control mechanism is realized by the nucleation and subsequent coalescence of double vortices, resulting in a single vortex at remanence with the \textit{opposite} chirality as found in smaller dots. Magneto-optical Kerr effect and magnetic force microscopy measurements confirm this new process. Micromagnetic simulations not only reproduce the experimentally observed behavior, but also elucidate the delicate interplay between exchange, demagnetization, and Zeeman energies and the role of fractional vortices bound to the dot edge. [Preview Abstract] |
Tuesday, March 22, 2011 1:39PM - 1:51PM |
J16.00011: Manipulation of Magnetization States of Permalloy Nanorings by an External Azimuthal Field Tianyu Yang, Nihar Pradhan, Abby Goldman, Moureen Kemei, Abbey Licht, Yihan Li, Mark Tuominen, Katherine Aidala This experimental research investigates a new method of manipulating the magnetic states of ferromagnetic nanorings using a circular magnetic field directed along the ring circumference. This type of azimuthal field can naturally select a vortex magnetization of desired chirality. The understanding of the magnetization switching behavior in an azimuthal field could lead to new designs of practical magnetic data storage devices. Symmetric and asymmetric nanorings made of permalloy are fabricated by a standard technique using electron-beam lithography and e-beam evaporation. Azimuthal fields are generated by passing current through an atomic force microscope tip, which is positioned at the center of the ring. The magnetic field direction and magnitude are controlled by the current. We demonstrate control over switching from an onion state to a vortex state, and also between two vortex states, using magnetic force microscopy to image the resulting magnetic states. [Preview Abstract] |
Tuesday, March 22, 2011 1:51PM - 2:03PM |
J16.00012: Magnetization reversal of surface and subsurface Co/Pt multilayers in a porous matrix B.J. Kirby, M.T. Rahman, R.K. Dumas, J.E. Davies, Kai Liu, C. Lai Deposition of magnetic multilayers onto porous host matrices has been studied as a simple and cost-effective method for fabrication of nano-patterned magnetic arrays [1]. For such structures, the magnetic reversal properties of the surface multilayer are twofold dependent on the size and depth of the host pores. First, the pore size determines the lateral size of the surface multilayer with respect to that of a single domain. Second, the pore size determines the amount and location of magnetic material within the pore - material that can exchange couple to the surface multilayer. To study these effects, we have used polarized neutron reflectometry to measure the structural and field-dependent magnetic depth profiles of a series of Co/Pt multilayers deposited on nanoporous alumina (diameter: 13, 20, or 28 nm). Despite the film porosity, we observe robust spin-dependent reflectivities, providing strong sensitivity to interfaces throughout the structure. The determined nuclear profiles show impressive agreement with cross-sectional transmission electron microscopy, and the magnetic profiles feature clearly distinct surface and subsurface magnetizations. The surface magnetization reversal and the role of exchange coupling will be discussed. [1] M. T. Rahman, et al., APL. 94, 042507 (2009). [Preview Abstract] |
Tuesday, March 22, 2011 2:03PM - 2:15PM |
J16.00013: Hysteresis-Loop Overskewing Ralph Skomski, T.A. George, D.J. Sellmyer The performance of permanent magnets is largely determined by the magnetostatic energy stored in free space (energy product), which depends on both materials properties and magnet geometry. The latter usually differs from laboratory shapes such as spherical samples, and demagnetizing-field corrections must be applied to compare different geometries. However, in nanostructures, especially in thin films, the macroscopic demagnetizing factors \emph{D} predicted from Maxwell's equations [1] yield unphysical overskewed hysteresis loops [2]. The overskewing is probably a nanoscale effect, but its origin has remained controversial. Our explanation is that nanoscale magnetization processes violate a main condition for the applicability of macroscopy demagnetizing factors, namely the uniform character of the magnetization. In bulk magnets, the magnetization inhomogeneities effectively average to zero, but this is no longer the case if any of the dimension of the magnet becomes small. We explicitly consider granular thin films, where we find a real-structure dependent reduction \emph{D}, as contrasted to the sometimes assumed infinite slope \emph{M}(\emph {H}) at coercivity. --- This research is supported by BREM (RS), ARPA-E, DOE (DJS), and NCMN. --- \textbf{References:} [1] J. A. Osborn, Phys. Rev. \textbf {67}, 351 (1945); [2] R. Skomski, J.- P. Liu, and D. J. Sellmyer, Phys. Rev. B \textbf{60}, 7359 (1999). [Preview Abstract] |
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