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7 May 2013

Volume 113, Issue 17, Articles (17xxxx)

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J. Appl. Phys. 113, 174302 (2013); http://dx.doi.org/10.1063/1.4798262 (4 pages)

Yuichiro Kurokawa, Takehiko Hihara, and Ikuo Ichinose
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back to top Patterned Films

Highly stable signal propagation in a consecutively tuned nanomagnet array

Zheng Li and Kannan M. Krishnan

J. Appl. Phys. 113, 17B901 (2013); http://dx.doi.org/10.1063/1.4794137 (3 pages)

Online Publication Date: 28 February 2013

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A key function of magnetic quantum-dot cellular automata (MQCA) is signal propagation in the nanomagnet array, for which a clocking field is required. However, the misalignment of the clocking field and the resultant low stability for signal propagation is one of the main challenges for its application. Here, we modeled and fabricated a progressively shape-tuned nanomagnet array combined with a reversal clocking field with progressively reduced amplitude. Based on micromagnetic simulations, Fe nanomagnet arrays were fabricated by electron beam lithography and their magnetization states characterized by magnetic force microscopy demonstrated correct signal propagation against clocking field misalignment up to ±5°. Furthermore, cascade-like signal propagation was observed. This novel design provides high stability and directional control in signal propagation within the nanomagnet array and potentially paves the way for addressing the misalignment issue in MQCA structures.
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75.50.Tt Fine-particle systems; nanocrystalline materials
81.07.Ta Quantum dots
85.40.Hp Lithography, masks and pattern transfer
75.75.-c Magnetic properties of nanostructures
68.37.Rt Magnetic force microscopy (MFM)

Information transport in field-coupled nanomagnetic logic devices

J. Kiermaier, S. Breitkreutz, I. Eichwald, M. Engelstädter, X. Ju, G. Csaba, D. Schmitt-Landsiedel, and M. Becherer

J. Appl. Phys. 113, 17B902 (2013); http://dx.doi.org/10.1063/1.4794184 (3 pages)

Online Publication Date: 1 March 2013

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The information transport in field-coupled nanomagnetic logic (NML) systems is demonstrated by investigating signal propagation in a circular chain of magnets. Design criteria for the magnet layout, signal injection timing, and the required clocking field are presented. The strong interaction between the magnets is estimated by simulations and verified by hysteresis curve measurements. Signal transmission in the magnetic wire is confirmed by magnetic force microscopy measurements, especially the propagation of a metastable pair of magnets with parallel magnetization. For the first time, a field-coupled magnetic logic device is successfully operated for hundreds of clocking cycles. Extensive studies verify the reliability and robustness of information transport in field-coupled NML systems from perpendicular magnetic media.
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84.30.Sk Pulse and digital circuits
84.32.Hh Inductors and coils; wiring
85.70.-w Magnetic devices

Lateral Interaction of transverse magnetic domain walls

Chunghee Nam, M. D. Mascaro, and C. A. Ross

J. Appl. Phys. 113, 17B903 (2013); http://dx.doi.org/10.1063/1.4794187 (3 pages) | Cited 1 time

Online Publication Date: 6 March 2013

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Magnetic domain walls (DWs) that are spatially close are formed with 360° (360DW) or 540° (540DW) configurations in Co magnetic rings. The 360DW is formed in a continuous magnetic structure whereas the 540DW configuration comprises the 360DW and 180DW with a narrow nonmagnetic gap. Individual transverse DWs play a key role in forming 360DWs or 540DWs because their magnetic polarity and chirality reduce the magnetostatic energy depending on the DW configurations. The magnetostatic interaction between individual DWs results in the variation of the local stray field, which is consistent with micromagnetic simulations.
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75.60.Ch Domain walls and domain structure
75.25.-j Spin arrangements in magnetically ordered materials (including neutron and spin-polarized electron studies, synchrotron-source x-ray scattering, etc.)
75.78.Cd Micromagnetic simulations
75.78.Fg Dynamics of domain structures
68.37.Rt Magnetic force microscopy (MFM)

Closely spaced nanomagnets by dual e-beam exposure for low-energy nanomagnet logic

Faisal A. Shah, Gyorgy Csaba, Katherine Butler, and Gary H. Bernstein

J. Appl. Phys. 113, 17B904 (2013); http://dx.doi.org/10.1063/1.4794362 (3 pages)

Online Publication Date: 6 March 2013

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The effect of nanomagnet spacing on required clock field has been studied by micromagnetic simulation for supermalloy (Ni79Fe16Mo5) dots with dimensions 90 × 60 × 20 nm3 and 120 × 60 × 20 nm3. Reduction of the inter-magnet spacing for both dimensions has resulted in reduction of the required clock field in the simulation. A dual e-beam exposure technique has been developed to allow fabrication of ultra dense features using conventional poly(methylmethacrylate) e-beam resist. Nanomagnet logic (NML) datalines of supermalloy dots with ∼10 nm and ∼15 nm spacing have been fabricated using dual e-beam exposure with a 3σ overlay accuracy of ∼4 nm. Fabricated NML datalines have been characterized using magnetic force microscopy for various clock fields. Datalines of both spacing have shown proper NML functionality with a clock field as low as 60 mT.
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75.78.Cd Micromagnetic simulations
61.80.Fe Electron and positron radiation effects
75.50.Tt Fine-particle systems; nanocrystalline materials
75.75.Cd Fabrication of magnetic nanostructures

Vortex state formation and stability in single and double layer nanorings and nanodisks

Meng Zhu, Christoph Mathieu, Werner Scholz, Sridhar Dubbaka, and Michael Kautzky

J. Appl. Phys. 113, 17B905 (2013); http://dx.doi.org/10.1063/1.4798247 (3 pages)

Online Publication Date: 2 April 2013

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Quasi-static magnetic properties of microscopic Ni80Fe20 disks, rings, and double layered disks and rings were studied via longitudinal magneto-optical Kerr effect (MOKE) and magnetic force microscopy (MFM), concomitant with micromagnetic simulations to elucidate the systems' vortex properties and remagnetization behavior. The features were fabricated lithographically, with diameters between 0.6 and 2 micrometers, and thicknesses of 50 and 100 nm. Key results are: (i) Dual-vortex to single-vortex transitions were observed at nucleation in 100 nm thick disks. (ii) Vortex nucleation and annihilation fields increase with single layer disk thickness whereas they decrease with thickness in single layer ring features. (iii) Double-layer disks separated by a Ru spacer indicate interactions of vortex cores at nucleation and avoidance of vortex core movements, whereas double-layer rings show successive switching in- and out- of vortex states with strong interactions only at vortex-to-onion transition.
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75.75.-c Magnetic properties of nanostructures
75.50.Tt Fine-particle systems; nanocrystalline materials
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
78.20.Ls Magneto-optical effects
81.30.Hd Constant-composition solid-solid phase transformations: polymorphic, massive, and order-disorder

Power reduction in nanomagnet logic using high-permeability dielectrics

Peng Li, Gyorgy Csaba, Michael Niemier, X. Sharon Hu, Joe Nahas, Wolfgang Porod, and Gary H. Bernstein

J. Appl. Phys. 113, 17B906 (2013); http://dx.doi.org/10.1063/1.4799520 (3 pages)

Online Publication Date: 5 April 2013

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In nanomagnet logic (NML), tens-of-mT fields are required to initiate the ordering of nanomagnets, and clock them into their computational ground state. This work investigates material-based approaches that are being used to reduce the nulling clock field. As test cases, arrays of both standalone and coupled magnets are covered with enhanced-permeability dielectric (EPD) films. Experimental results demonstrate that EPDs can reduce the fields required to saturate standalone magnets along the hard-axis by 60%, and coupled magnets by 75%. Experiments show that the remanent magnetization of a nanomagnet array is unchanged after being coated with EPDs, demonstrating that no undesired remanence is added to the NML system. The 75% reduction in switching field translates to a 94% reduction in energy dissipation due to reduced Joule-heating of field-generating wires.
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75.75.-c Magnetic properties of nanostructures
75.50.Tt Fine-particle systems; nanocrystalline materials
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
77.55.-g Dielectric thin films

Probing the magnetization reversal process of permalloy nano-rings with high wall height-to-thickness ratios

C. T. Chao, Y. C. Huang, C. Y. Kuo, Lance Horng, and J. C. Wu

J. Appl. Phys. 113, 17B907 (2013); http://dx.doi.org/10.1063/1.4799777 (3 pages)

Online Publication Date: 8 April 2013

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We report a study of magnetization reversal process on a tall magnetic nano-ring with a wall height-to-thickness ratio of up to 21. Samples in a hexagonal lattice pattern of ring array with an outer/inner diameter of 300/260 nm and a ring height of 420 nm are fabricated using electron beam lithography in conjunction with an ion beam etching technique. A longitudinal magneto-optical Kerr effect (MOKE) measurement reveals that meta-stable states exist during the magnetization reversal process. In particular, the exact magnetization configurations at the top end of the nano-ring are illustrated using magnetic force microscopy (MFM) that is facilitated with a photoresist etch-back technique. Most interestingly, MFM images of two sets of head-to-head (H2H)/tail-to-tail (T2T) domain walls are captured during the magnetization reversal process. It is believed that the domain walls formation and evolution processes at the top end of the nano-ring are mediated by the anti-symmetrical distribution of the vortex domain wall on the tubular sidewalls. Simulation results using Object Oriented Micromagnetic Framework confirm the MOKE hysteresis loop and MFM imaging.
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75.60.Jk Magnetization reversal mechanisms
81.65.Cf Surface cleaning, etching, patterning
78.20.Ls Magneto-optical effects
81.16.Nd Micro- and nanolithography

Enhanced magnetic performance of metal-organic nanowire arrays by FeCo/polypyrrole co-electrodeposition

X. J. Luo, W. B. Xia, J. L. Gao, S. Y. Zhang, Y. L. Li, S. L. Tang, and Y. W. Du

J. Appl. Phys. 113, 17B908 (2013); http://dx.doi.org/10.1063/1.4800835 (3 pages)

Online Publication Date: 10 April 2013

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FeCo/polypyrrole (PPy) composite nanowire array, which shows enhanced magnetic remanence and coercivity along the nanowires, was fabricated by AC electrodeposition using anodic aluminum oxide templates. High resolution transmission electron microscopy shows that PPy grows on the surface of FeCo nanowires forming a coaxial nanowire structure, with a coating layer of about 4 nm. It suggests that the decreased dipolar interaction due to the reduced nanowire diameters is responsible for the enhancement of magnetic performance. The possible mechanism of this coating may be that PPy is inclined to nucleate along the pore wall of the templates.
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75.75.Cd Fabrication of magnetic nanostructures
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
81.05.Qk Reinforced polymers and polymer-based composites
81.16.-c Methods of micro- and nanofabrication and processing
82.45.Qr Electrodeposition and electrodissolution

Perpendicular magnetic tunnel junctions with synthetic antiferromagnetic pinned layers based on [Co/Pd] multilayers

Yao-Jen Chang, A. Canizo-Cabrera, Valentin Garcia-Vazquez, Yang-Hua Chang, and Te-ho Wu

J. Appl. Phys. 113, 17B909 (2013); http://dx.doi.org/10.1063/1.4799974 (3 pages)

Online Publication Date: 12 April 2013

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We fabricated MgO-based perpendicular magnetic tunnel junctions (p-MTJ) with Ta/CoFeB magnetic electrodes. Synthetic antiferromagnetic (SAF) pinned layers with perpendicular magnetic anisotropy (PMA) were included into the p-MTJs by using two Co/Pd multilayers (MLs) separated by a thin Ru spacer layer. The MTJs stack has the structure bottom contact/free layer CoFeB (1.0)/MgO (1)/pinned layer CoFeB (1.0)/Ta spacer layer/SAF/Ru cap layer/top contact (the units in parenthesis are in nanometers). The SAF was optimized by changing the repetition period n in one of the Co/Pd multilayers and the Ru thickness in order to obtain PMA with antiferromagnetic (AFM) coupling. The Ru spacer values were 0.7, 0.75, 0.8, 0.85, and 0.9 nm. The magnetic studies show that all magnetic films, including the Ta/CoFeB layers, are perpendicularly magnetized. The two Co/Pd MLs are AFM coupled for n > 2. Controlling the Ru thickness, the interlayer exchange coupling strength Jiec can be tailored. Jiec vs. Ru thickness exhibits a simple exponential decay. The electrical properties of the full p-MTJ with SAF show a low resistance-area (RA) product of 44.7 Ω μm2 and a tunnel magnetoresistance (TMR) ratio of 10.2%.
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75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.30.Et Exchange and superexchange interactions
75.30.Gw Magnetic anisotropy
75.50.Ee Antiferromagnetics
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects

Energy barrier versus switching field for patterned Co80Pt20 alloy and Co/Pt multilayer films

Jeroen de Vries, Thijs Bolhuis, and Leon Abelmann

J. Appl. Phys. 113, 17B910 (2013); http://dx.doi.org/10.1063/1.4801399 (3 pages)

Online Publication Date: 18 April 2013

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Two Co/Pt multilayer samples have been fabricated with a difference in the number of bilayers, leading to a total magnetic layer thickness of 3 nm and 20 nm. From these films, large arrays of magnetic islands have been patterned using laser interference lithography and ion beam etching. We have investigated the switching field distribution (SFD) of approximately 80 islands and thermal switching field distribution SFDT of individual islands of both samples using the anomalous Hall effect. We compare the results of these measurements with the (SFDT) of a previously investigated alloy with a magnetic layer thickness of 20 nm by comparing the results of over 1000 hysteresis loops of a single weak island and a single strong island. We found that that the energy barrier for the multilayer islands increases with increasing switching field, whereas it was previously found that the energy barrier for the alloy stays constant with varying switching fields. When comparing the two multilayer samples, we observe that the grain size, anisotropy, and switching field distribution are more or less independent on thickness, whereas the switching field at both 0 K and 300 K decreases with film thickness.
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75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
68.55.at Other materials
72.15.Gd Galvanomagnetic and other magnetotransport effects
73.50.Jt Galvanomagnetic and other magnetotransport effects (including thermomagnetic effects)
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.70.Ak Magnetic properties of monolayers and thin films
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