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Magnetics Letters, IEEE

Date 2011

About the Cover: For more information on Louis Néel, please see Cover 3 (back, inside cover).

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  • [Front cover]

    Article#: 0000101
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  • IEEE Magnetics Society Information

    Article#: 0000201
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  • Table of contents

    Article#: 0000302
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  • Numeric Simulations of a Novel Wideband Electromagnetic Band Gap Metamaterial Utilizing Oriented Cobalt-Substituted Z-Type Barium Hexaferrites

    Article#: 0500104
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (399 KB) |  | HTML iconHTML  

    The material parameters of an electromagnetic bandgap (EBG) metamaterial consisting of metallic Sievenpiper structures prepared upon a Co2Z hexaferrite substrate having μr = 12 to 14 and εr = 10 to 12 are reported. A bandwidth of 50%-75% of L-band was realized in simulation. The synthesis of the Co2Z hexaferrite material with optimized properties was achieved via a modified aqueous coprecipitation method with yields of up to 25 g/L. Magnetically oriented Co2Z substrates were demonstrated to exhibit a zero-field ferromagnetic resonance frequency of ~1.1 GHz that enabled device application from ultrahigh frequency (UHF) through L-band frequencies. EBG metamaterials designed and simulated on an oriented Co2Z hexaferrite substrate were compared to standard dielectric EBG designs. Results indicate that standard dielectric EBG designs, which operate over a 50% bandwidth and profile height of ~λo/10, are replaceable with an equivalent bandwidth Co2Z EBG metamaterial designed to a height of ~λo/95. The applications of the proposed metamaterial toward ultralow profile antenna designs at UHF through L-band are discussed. View full abstract»

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  • Improved Sensitivity and Noise in Magneto-Electric Magnetic Field Sensors by Use of Modulated AC Magnetostriction

    Article#: 2500104
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (456 KB) |  | HTML iconHTML  

    A magnetic field sensor based on the nonlinear nature of the magnetostrictive response of a magneto-electric (ME) heterostructure has two orders of magnitude improvement in sensitivity and signal-to-noise ratio compared with a conventional dc-biased configuration. The sensor consists of a longitudinally magnetized and transversely poled lamination of iron-cobalt-boron (Metglas) and lead zirconate titanate (PZT). The ac-modulated sensor has enhanced environmental noise immunity and does not require a dc magnetic bias field. Combined, these advantages hold promise for the development of miniature ME sensor elements for applications with size and weight limitations. View full abstract»

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  • Spin-Torque Nano-Oscillator as a Microwave Signal Source

    Article#: 3000104
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (238 KB) |  | HTML iconHTML  

    We evaluate a possibility to use spin-torque nano-oscillators (STNOs) and oscillator arrays as new sources of microwave signals for telecommunication devices. The microwave signal generated by STNO can be received as oscillation of the device magnetoresistance (MR) or as direct electromagnetic emission of two effective magnetic dipoles. We calculate the dipolar microwave power emitted by STNO in free space and in several types of transmission lines and resonators. We demonstrate that although the power of a single STNO received through the MR effect is, typically, larger than the power of direct microwave emission of effective dipoles, the latter mechanism might have an advantage for sufficiently large arrays of coupled STNOs. View full abstract»

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  • Demonstration of Ultralow Bit Error Rates for Spin-Torque Magnetic Random-Access Memory With Perpendicular Magnetic Anisotropy

    Article#: 3000204
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    Bit error rates below 10-11 are reported for a 4-kb magnetic random access memory chip, which utilizes spin transfer torque writing on magnetic tunnel junctions with perpendicular magnetic anisotropy. Tests were performed at wafer level, and error-free operation was achieved with 10 ns write pulses for all nondefective bits during a 66-h test. Yield in the 4-kb array was limited to 99% by the presence of defective cells. Test results for both a 4-kb array and individual devices are consistent and predict practically error-free operation at room temperature. View full abstract»

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  • Domain Structure in CoFeB Thin Films With Perpendicular Magnetic Anisotropy

    Article#: 3000304
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (447 KB) |  | HTML iconHTML  

    Domain structures in CoFeB-MgO thin films with a perpendicular easy magnetization axis were observed by magneto-optic Kerr-effect microscopy at various temperatures. The domain-wall surface energy was obtained by analyzing the spatial period of the stripe domains and fitting established domain models to the period. In combination with superconducting quantum interference device measurements of magnetization and anisotropy energy, this leads to an estimate of the exchange stiffness and domain-wall width in these films. These parameters are essential for determining whether domain walls will form in patterned structures and devices made of such materials. View full abstract»

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  • Analysis and Design of Hanle-Effect Spin Transistors at 300 K

    Article#: 3000404
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (524 KB) |  | HTML iconHTML  

    We propose a new Hanle-effect device based on a MOSFET type of spin transistor to reveal the dynamics of spin-polarized electron transport in the Si MOS inversion channel. The proposed device has the ability to detect spin signals with high sensitivity and to distinguish spin transport signals from other, spurious signals. Spin transport behavior induced by the Hanle effect in the device were theoretically analyzed and were well correlated with the universality of electron mobility in the MOS inversion channel. The Hanle-effect spin device can elucidate the true nature of spin transport in Si MOS inversion channels. View full abstract»

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  • Asymmetric Interfacial Perpendicular Magnetic Anisotropy in Pt/Co/Pt Trilayers

    Article#: 3000504
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    An asymmetry in the interfacial anisotropies of Pt/Co and Co/Pt interfaces was observed in Pt/Co/Pt sputtered trilayers, the interfacial anisotropy arising from the bottom Pt/Co interface being significantly higher than that from the top Co/Pt one. Interdiffusion at the top interface is believed to be the main factor for this asymmetry. It dramatically decreases the anisotropy of the stack when the cobalt layer is thinner than 1 nm. By introducing ultrathin layers of materials immiscible with Co and acting as a diffusion barrier at the Co/Pt interface, the effective anisotropy can be doubled in this low Co thickness range. This is of great interest for spintronic devices, particularly for out-of-plane magnetized magnetoresistive random access memory structures that require high perpendicular magnetic anisotropy when their lateral dimensions are reduced below 45 nm. View full abstract»

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  • Influence of Periodic Patterning on the Magnetization Response of Micromagnetic Structures

    Article#: 3500104
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (487 KB) |  | HTML iconHTML  

    The magnetization response of single, patterned, thin-film Permalloy (Ni_80Fe_20, Py) elements embedded in a coplanar waveguide (CPW) is investigated. The anisotropic magnetoresistance (AMR) effect serves as the detection mechanism in current-modulated differential resistance (dV/ dI) and rectification effect measurements, which reveal magnetization dynamics over a broad frequency range. Differently shaped Py elements had distinctly different magnetization responses. In rectangular Py elements, the dynamic low frequency response shows two resonant excitations, whereas multimode ferromagnetic resonance (FMR) occurs in the high frequency range. Disk-shaped patterning significantly increases the magnetic loss in comparison to simple rectangular elements. View full abstract»

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  • Two-Bit-Per-Dot Patterned Media for Magnetic Storage

    Article#: 4500104
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (356 KB) |  | HTML iconHTML  

    Bit-patterned recording media are made of arrays of physically separated magnetic nanodots, each nanodot carrying one bit of information. We demonstrate that by stacking perpendicular-to-plane and in-plane magnetized thin films within each magnetic dot, it is possible to obtain a 2-bit-per-dot multilevel magnetic recording system without penalty to the readout signal-to-noise ratio. The stray field above the dots was measured and analyzed by electron holography for different magnetic configurations and compared with numerical simulations. The perpendicular-to-plane magnetized layer mainly influences the stray field above the center of the dots, whereas the in-plane magnetized layer determines the stray field above the gap between dots. The areal density of information can thus be doubled with this media by making an optimum use of the whole area of the medium. View full abstract»

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  • Ferrimagnetic Sr _{1.5} Ba _{0.5} Zn _2 Fe _{12} O _{22} (Zn-Y) Single Crystal With Planar Anisotropy

    Article#: 5000104
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (443 KB) |  | HTML iconHTML  

    Single crystals of Sr1.5Ba0.5Zn2Fe12O22 were grown from the flux systems of BaCO3, SrCO3, and Na2CO3 by a high temperature, slow cooling, flux method. X-ray diffraction confirmed that the crystal structure is an extended single crystalline array of Zn-Y composition with an in-plane easy axis crystallographic orientation. The obtained single crystals were characterized for magnetic and microwave properties. The anisotropy field (HA) and the in-plane magnetization (σs) were 10 kOe and 22.5 emu/g, respectively. Ferrimagnetic resonance (FMR) measurements showed 41 Oe of linewidth (ΔH ) at 10 GHz. View full abstract»

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  • M-type Hexaferrite for Gigahertz Chip Antenna Applications

    Article#: 5000204
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (331 KB) |  | HTML iconHTML  

    We demonstrate that Sn/Zn-substituted strontium M-type hexaferrite (SrSn2.5Zn2.5Fe7O19 = Sn/Zn-SrM) can provide high performance and a uniform radiation pattern over a wide frequency band for gigahertz chip-antenna applications. Saturation magnetization and coercivity were 68.5 emu/g and 46 Oe. Real parts of permeability and permittivity were 1.37 (loss tan δμ = 0.13) and 22.2 (loss tan δε = 0.09) at 2.44 GHz, respectively. A Bluetooth Sn/Zn-SrM chip antenna has a bandwidth of 780 MHz (2.15-2.93 GHz) at S11 <; -10 dB, maximum three-dimensional (3-D) peak gain of 1.25 dBi, and radiation efficiency of 60% at 2.4 GHz. Our antenna performance simulation suggests that radiation efficiency can be increased up to 98% with a low magnetic loss tan δμ of 0.01. 3-D radiation patterns were omnidirectional in the Bluetooth frequency range. Therefore, Sn/Zn-substituted strontium M-type hexaferrite is a promising magnetic material for future gigahertz antenna applications. View full abstract»

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  • An In Situ Anneal Study of Graded Anisotropy FePtCu Films

    Article#: 5500104
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (470 KB) |  | HTML iconHTML  

    We provide a detailed study of how the anisotropy (Ku) gradient in a compositionally graded FePtCu film gradually develops as a function of the postannealing temperature (TA). By utilizing the in situ annealing and magnetic characterization capabilities of a physical property measurement system, the evolution of the induced Ku gradient is elucidated. For low TA, the sample primarily remains in the low-Ku A1 phase. At intermediate TA, the gradual development of an A1 to L10 anisotropy gradient occurs. As TA is further increased, a well-developed L10 gradient is realized. Finally, annealing temperatures greater than 475°C reduce the gradient until the film is effectively uniform for TA ≥ 525°C and higher, presumably due to interdiffusion of the Cu through the film thickness. The resulting coercivity shows a nonmonotonic dependence on TA with an initial steep increase as the L10 fraction of the sample increases, a local minimum at TA = 525°C where the gradient vanishes, and a final increase as the uniform L10 film orders completely. View full abstract»

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  • Electrical and Mechanical Manipulation of Ferromagnetic Properties in Polycrystalline Nickel Thin Film

    Article#: 6000104
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (441 KB) |  | HTML iconHTML  

    The ferromagnetic properties of a 35 nm polycrystalline nickel thin film deposited on a single-crystal, optical, ferroelectric lithium niobate substrate are magnetoelectrically tunable. The coercive field of the nickel film, which has low magnetocrystalline anisotropy and appropriate magnetoelasticity, changes by about 80% owing to the anisotropic strain produced by the substrate. In addition, mechanical strain changes the coercive field by about 260% and increases the normalized remanence bm Mbmr/bm Mbms from 0.3 to 1.0. This giant tunability would be achievable by combining polycrystalline Ni thin film with a ferroelectric substrate having large anisotropic piezoelectric coefficients. View full abstract»

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  • Resonance-Based Wireless Energizing of Piezoelectric Components

    Article#: 6000204
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (404 KB) |  | HTML iconHTML  

    A new technique of wirelessly transmitting electric energy to piezoelectric components is explored. The proposed method improves energy transfer efficiency by using the strongly coupled magnetic resonance as well as the mechanical resonance of the piezoelectric component. In the design, a printed spiral receiving coil connected across the piezoelectric component is used to capture energy wirelessly from the magnetic fields generated by a source resonator. Experimentally, the transmission of electric energy to the piezoelectric components is enhanced when the operating frequency of the generated magnetic field is close to the mechanical resonance frequency of the piezoelectric component. The power delivered to the piezoelectric component depends on the driving frequency, vibration modes of the piezoelectric plate, design, and separation distance of the source and receiving coils. At the resonant frequency 772 kHz of the piezoelectric component operating in the thickness vibration mode, a maximum power of 1.26 W and energy conversion efficiency of 48% have been achieved wirelessly by the piezoelectric component with an input ac source power of 2.6 W across the source coil and 4 cm separation distance of the source and receiver coils. View full abstract»

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  • Novel Silicon-Embedded Coreless Transformer for On-Chip Isolated Signal Transfer

    Article#: 6500103
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (449 KB) |  | HTML iconHTML  

    In this letter, a novel silicon-embedded coreless transformer is proposed and demonstrated. The transformer is fabricated in the thick bottom layer of a silicon substrate and connected to the frontside through vias opened in the thin top layer where all other components of the system can be fabricated. A 5-turn coreless transformer fabricated using this monolithic transformer technology achieves a small area of 2 mm2 and a good voltage gain of larger than -0.8 dB (load = 50 Ω, best reported so far) from 12 to 100 MHz. This technology shows great potential for on-chip isolated signal transfer. View full abstract»

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  • Discrete Cosine Transform Technique for Measuring Domain Widths in Magneto-Optic Garnet Films

    Article#: 6500204
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (356 KB) |  | HTML iconHTML  

    One use of engineered magneto-optic garnet thin films is to visualize magnetic patterns with high spatial resolution. Of these engineered films, films with out-of-plane anisotropy exhibiting a stripe domain pattern are found to work well for resolving patterns in digital storage magnetic media. The width of the stripe domains in garnet films has become a performance metric for the films' ability to visualize high spatial-frequency magnetic patterns. Current manual image-based methods for measuring stripe domain width are tedious, complicated, and susceptible to bias and human error. In this paper, an improved measurement method with repeatable precision utilizing the discrete cosine transform is presented and compared with the manual measurement of the same sample image. The measurements were in agreement. View full abstract»

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  • 2011 Index IEEE Magnetics Letters Vol. 2

    Article#: 9500105
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    Article#: 9500201
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  • Why we joined [Advertisement]

    Article#: 9500301
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  • IEEE Magnetics Letters Publication Information

    Article#: 0000401
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  • Blank page [back cover]

    Article#: 0000501
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Aims & Scope

IEEE Magnetics Letters is a peer-reviewed, archival journal covering the physics and engineering of magnetism, magnetic materials, applied magnetics, design and application of magnetic devices, bio-magnetics, magneto-electronics, and spin electronics.

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Ron B. Goldfarb, Ph.D.
National Institute of Standards and Technology