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Magnetics Letters, IEEE

Volume 3: 2012

About the Cover: For more information on Richard M. Bozorth, please see Cover 3 (back, inside cover).

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  • Front Cover

    Publication Year: 2012 , Article#: 0000101
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  • IEEE Magnetics Society Information

    Publication Year: 2012 , Article#: 0000201
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  • IEEE Magnetics Letters Publication Information

    Publication Year: 2012 , Article#: 0000301
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  • Blank page

    Publication Year: 2012 , Article#: 0000401
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  • Table of Contents

    Publication Year: 2012 , Article#: 0000502
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  • Exact Analytical Demagnetizing Factors for Long Hollow Cylinders in Transverse Field

    Publication Year: 2012 , Article#: 0500104
    Cited by:  Papers (2)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (224 KB) |  | HTML iconHTML  

    Analytical expressions are derived for the magnetometric Nm and fluxmetric Nf demagnetizing factors of infinitely long, hollow cylinders with uniform susceptibility in transverse uniform applied field. Whereas Nm depends on both permeability μ and the ratio of the internal and external radii of the cylinder, Nf does not depend on μ. The magnetic induction and magnetic field of materials with conjugate relative permeabilities μ and 1/μ are related. View full abstract»

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  • Forces between a permanent magnet and a soft magnetic plate

    Publication Year: 2012 , Article#: 0500204
    Cited by:  Papers (2)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (1744 KB) |  | HTML iconHTML  

    Forces between a hard/permanent magnet of arbitrary shape and an ideally soft magnetic plate in close proximity are derived analytically from the image method applied to magnetostatics. We found that the contact force, defined as the force required to detach the hard magnet from the plate, coincides with that between two identical touching permanent magnets. Furthermore, if the hard and the soft magnets are displaced by some amount, their attraction equals that between two identical permanent magnets displaced by twice that amount. Experimental results are presented that validate the theoretical framework and highlight its limits of applicability. View full abstract»

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  • Waveguide Tubes Coated With Inhomogeneous Lossy Materials for Superior Shielding Above and Below Cutoff Frequency

    Publication Year: 2012 , Article#: 0500304
    Cited by:  Papers (1)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (636 KB) |  | HTML iconHTML  

    A new approach for enhanced electromagnetic interference shielding of metallic waveguide (WG) tubes coated with inhomogeneous lossy materials is proposed based on numerical simulation. Shielding enhancement is achieved by using stepped variations of complex permittivity and complex permeability coating materials along the WG transverse direction. Such variation in the coating layer provides multiple wave reflections below and above the WG cutoff frequency. The underlying principle is applied to a rectangular WG operating in the dominant TE10 mode. Simulation examples of rectangular WGs coated with uniform and stepped lossy materials are demonstrated. The proposed approach shows remarkably better performance when compared with WGs coated with uniform lossy materials. View full abstract»

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  • Universal Behavior of Biomolecule-Concentration-Dependent Reduction in AC Magnetic Susceptibility of Bioreagents

    Publication Year: 2012 , Article#: 1500104
    Cited by:  Papers (1)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (292 KB) |  | HTML iconHTML  

    Magnetic nanoparticles biofunctionalized with antibodies are useful for specifically labeling target biomolecules. By measuring magnetic signals after the association between biofunctionalized magnetic nanoparticles and target biomolecules, the concentration of target biomolecules can be quantitatively detected. One of measuring methodologies is so-called immunomagnetic reduction (IMR), in which the reduction in ac magnetic susceptibility of magnetic reagent is a function of the concentration of target biomolecules. In this letter, the relationship between the magnetic reduction signal of reagent and the concentration of target biomolecules is explored. According to the experimental results on various kinds of target biomolecules, such as proteins and chemicals, the magnetic reduction signal as a function of concentration of various target biomolecules can be scaled to a universal curve. This universal curve is a logistic function. This implies that there exists a unique mechanism for the association between the target biomolecules and biofunctionalized magnetic nanoparticles in an IMR assay. View full abstract»

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  • Low Energy Magnetic Domain Wall Logic in Short, Narrow, Ferromagnetic Wires

    Publication Year: 2012 , Article#: 3000104
    Cited by:  Papers (11)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (330 KB) |  | HTML iconHTML  

    We present circuit simulation results of an implementation of universal logic that operates at low switching energy. Information is stored in the position of a single domain wall in a thin, short ferromagnetic wire. The gate is switched by current-driven domain wall motion, and information is read out using a magnetic tunnel junction. The inputs and outputs of the device are currents controlled by voltage clocks, making it compatible with CMOS. Using devices that operate at 100-1 mV, we simulate a shift register circuit and a full-adder circuit. The simulations show that the magnetic logic gates can operate at lower switching energy than CMOS electronics. View full abstract»

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  • CoFeB Thickness Dependence of Thermal Stability Factor in CoFeB/MgO Perpendicular Magnetic Tunnel Junctions

    Publication Year: 2012 , Article#: 3000204
    Cited by:  Papers (4)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (253 KB) |  | HTML iconHTML  

    Thermal stability factor Δ of the recording layer was studied in perpendicular anisotropy CoFeB/MgO magnetic tunnel junctions (p-MTJs) with various CoFeB recording layer thicknesses and junction sizes. In all series of p-MTJs with different thicknesses, Δ is virtually independent of the junction sizes of 48-81 nm in diameter. The values of Δ increase linearly as the recording layer thickness increases. The slope of the linear fit is explained well by a model based on nucleation-type magnetization reversal. View full abstract»

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  • Spin-Transfer Torque Switching Above Ambient Temperature

    Publication Year: 2012 , Article#: 3000304
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (364 KB) |  | HTML iconHTML  

    We report the temperature dependences of tunneling magnetoresistance ratio, coercivity, thermal stability, and switching current distribution of magnetic tunnel junctions (MTJs) in the temperature range 25-80°C, the most probable working environment for spin-transfer torque random access memory (STT-RAM). Two distinct temperature dependence of the switching current density are apparent due to two switching mechanisms: a switching current density decrease with increasing temperature in the long-pulse ( >; 1 μs) regime, a result of thermally activated switching, but no decrease in the short-pulse (>;10 ns) regime, as a result of precessional switching. In the temperature range studied, the switching current density variation is less sensitive to environmental temperature than it is to switching time. Thus, switching time is the more important factor to consider in STT-RAM design. View full abstract»

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  • Appearance of Anisotropic Magnetoresistance and Electric Potential Distribution in Si-Based Multiterminal Devices With Fe Electrodes

    Publication Year: 2012 , Article#: 3000404
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (349 KB) |  | HTML iconHTML  

    We have investigated the influence of anisotropic magnetoresistance (AMR) on nonlocal signals in Si-based multiterminal devices with ferromagnetic Fe electrodes. The AMR of the Fe electrodes has a significant influence on nonlocal signals when the in-plane device structure is not optimized. Our conformal mapping calculations show that it is virtually impossible to realize a pure spin current by spin diffusion because of the electric potential distribution in the depth direction in the Si channel when the ferromagnetic electrodes are directly fabricated on the Si channel. We discuss the influence of structure on the electric potential distribution, which is indispensable for an analysis of spin-dependent transport. View full abstract»

    Open Access
  • Experimental Study of the Variation in Oscillation Characteristics of Point-Contact-Based Spin-Torque Oscillators

    Publication Year: 2012 , Article#: 3000504
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (374 KB) |  | HTML iconHTML  

    This paper presents an experimental study of the variation in oscillation characteristics of circular point-contact-based spin-torque oscillators (PC-STOs). We first measured the oscillation behavior of multiple, identically fabricated PC-STOs, then made the same measurements on a PC-STO and rotated the azimuth angle of the bias field \varphi . Our results show considerable device-to-device variations among identical PC-STOs. More striking is large changes within a single PC-STO for a small \varphi rotation, which is totally inconsistent with the geometrical symmetry of the device. These results suggest that the giant magnetoresistance stack has some form of microstructural inhomogeneity, which causes a strong \varphi dependence of the oscillation behavior, eventually resulting in the device-to-device variation. Some possible sources of the inhomogeneity are discussed based on these observation results. View full abstract»

    Open Access
  • Write error rate slopes of in-plane magnetic tunnel junctions

    Publication Year: 2012 , Article#: 3000604
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (325 KB) |  | HTML iconHTML  

    Understanding bit error rates of magnetic tunnel junctions (MTJs) is critical for designing reliable spin-transfer-torque magnetoresistive random access memory. In this letter, we study the write error rate (WER) of two types of in-plane MTJs with the same film stacks except for the free layer and the capping layer. By comparative analysis of these two MTJ splits that show significantly different WER characteristics, we find that reducing average switching voltages does not necessarily improve WER slopes, resulting in decreased write margins for a sufficiently low WER requirement. Various magnetic measurements suggest that WER slopes and slope asymmetries are more strongly correlated to spin torque efficiencies rather than thermal stability factors. View full abstract»

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  • Microwave-Assisted Magnetic Reversal in Perpendicular Media

    Publication Year: 2012 , Article#: 3500104
    Cited by:  Papers (1)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (318 KB) |  | HTML iconHTML  

    Microwave-assisted magnetic reversal (MAMR) has not yet been characterized in hard perpendicular media. We investigate MAMR in such media experimentally for monolayer and bilayer magnetic thin films. The coercive field can be reduced by 8% in our geometry. The response occurs over a broad frequency range which is due to the excitation of long-wavelength spin-wave modes which can help drive reversal. Our results qualitatively match simple micromagnetic modeling. View full abstract»

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  • Magnetization Dynamics in Vortex-Imprinted Ni _{80} Fe _{20} /Ir _{20} Mn _{80} Square Elements

    Publication Year: 2012 , Article#: 3500204
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (305 KB) |  | HTML iconHTML  

    The dynamic behavior of the vortex magnetization on micrometer-sized, ferromagnetic/antiferromagnetic (FM/AFM) bilayer exchange-coupled square elements is investigated using time-resolved Kerr effect microscopy and micromagnetic modeling. 10 \mu m \times 10 \mu m square elements consisting of Ni _{80} Fe _{20} /Ir _{20} Mn _{80} bilayers were prepared and subsequently field cooled at various field strengths H_{\rm cf} , resulting in different exchange-bias field H_{\rm ex} profiles at the FM/AFM interface. With the presence of the H_{\rm ex} , the vortex core is displaced from the center of the square and consequently the fourfold symmetry of the Landau vortex state breaks. The spin-wave eigenmode excited upon application of a magnetic field pulse is strongly dependent on the level of the displacement of the vortex configuration, in which the mode frequency decreases for the growing domain, whereas it increases for the shrinking domain. View full abstract»

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  • Magnetic Frustration in Circular Arrays of Dipoles

    Publication Year: 2012 , Article#: 4000104
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (360 KB) |  | HTML iconHTML  

    Magnetostatically interacting structures consisting of 1.5 μm diameter rings of radially oriented nanomagnets were investigated using magnetic force microscopy. An even number of nanomagnets, 16 in this case, formed a ground state in which the magnetizations of neighboring nanomagnets were oriented antiparallel. However, an odd number of nanomagnets, 15, led to a frustrated state in which at least one pair of neighboring nanomagnets was magnetized parallel, giving a net radial magnetic charge. The “defect” in the structure was moved around the ring by a modest external field. View full abstract»

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  • Large Exchange Bias Field in (Pt/Co) _{\bf 3} /IrMn/Co Trilayers With Ultrathin IrMn Layers

    Publication Year: 2012 , Article#: 4000204
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (304 KB) |  | HTML iconHTML  

    The field shift of the hysteresis loop, called exchange bias field, in ferromagnetic/antiferromagnetic (F/AF) Co/IrMn bilayers, or (Pt/Co),,/IrMn structures, appears only above a critical thickness of IrMn, which is related to the IrMn magnetocrystalline anisotropy and to the interfacial coupling between the F and AF layers. In this letter, we show that this critical IrMn thickness can be considerably reduced by sandwiching the IrMn layer between two F layers or multilayers having parallel or orthogonal anisotropy, i.e., the first being magnetized in plane and the second out of plane. An in-plane exchange bias field of 20 mT could be measured in structures of the form (Pt/Co)3/IrMn/Co (5 nm) having an IrMn layer as thin as 3 nm at room temperature. Several explanations are proposed and discussed. The first is associated with structural variations of texture resulting in changes in the antiferromagnet grains' anisotropy energy. The second is based on an enhancement of the stability of the antiferromagnet spin lattice resulting from an indirect intergrain coupling. The third relies on a decrease of the Co/IrMn interfacial coupling due to an out-of-plane canting of the interfacial spin. View full abstract»

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  • Novel Approach to Control Diameter of Self-Rolled Magnetic Microtubes by Anodizing Ti Layer

    Publication Year: 2012 , Article#: 4000304
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (944 KB) |  | HTML iconHTML  

    Stress engineering in multilayered thin-film patterns is the key to control the diameter of self-rolled magnetic structures. Arrays of magnetic microtubes were fabricated via a deterministic release of rectangular thin-film patterns of Ti(20 nm)/Ni(30 nm)/Au(2 nm) and Ti(20 nm)/GaFe(40 nm)/Au(15 nm) from a sacrificial underlayer of Cu. It is demonstrated that the expansion of the anodized Ti layer leads to a reduction of the diameter of the tubes by almost 50%. The changes of shape of the magnetic patterns from flat to tubular and further reduction of the tube diameter due to anodizing were accompanied by changes of their magnetic characteristics. View full abstract»

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  • Spontaneously Formed FePt Graded Granular Media With a Large Gain Factor

    Publication Year: 2012 , Article#: 4500104
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (428 KB) |  | HTML iconHTML  

    In conventional magnetic recording media fabrication processes, magnetically soft regions of composite or graded material are prepared by varying either the substrate temperature or the film composition during deposition. In this work, ultrathin FePt granular media with graded composition was fabricated directly by spontaneous layer diffusion between FePt and Pt layers during the film deposition. Electron energy loss spectroscopy analysis on individual FePt columnar grains identified the composition distribution of Fe and confirmed the Fe composition gradient. A large “gain factor” of 3.74, indicative of a relatively high-thermal energy barrier, was obtained in this spontaneously formed FePt graded granular media. View full abstract»

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  • Role of Dipolar Interactions on the Thermal Stability of High-Density Bit-Patterned Media

    Publication Year: 2012 , Article#: 4500204
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (336 KB) |  | HTML iconHTML  

    We have characterized the magnetic reversal and thermal stability of bit-patterned media with a composite structure of [Co (0.25 nm)/Pd (0.7 nm)]5 /Fe(X)/[Pd (0.7 nm)/Co (0.25 nm)] 5, where X = 1, 1.5 , and 2 nm. For 25 nm diameter islands separated by 35 nm, the average thermal stability of the islands is confirmed by analyzing the time-dependent coercive fields. However, by further analyzing the time-dependent hysteresis loop shape, we find a broad distribution of the effective energy barriers. We quantitatively show that this energy barrier distribution arises primarily from the dipolar interactions in these densely packed arrays and not from intrinsic distributions. View full abstract»

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  • Novel system design for readback at 10 terabits per square inch user areal density

    Publication Year: 2012 , Article#: 4500304
    Cited by:  Papers (2)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (1480 KB) |  | HTML iconHTML  

    A novel system design for sensing very high density magnetic recording data, such as that envisioned for two-dimensional magnetic recording, is proposed. The key idea is a rotated sense head, so that the shields are aligned down-track, combined with oversampled signal processing to regain the lost down-track resolution. Based on a random Voronoi grain model, simulation indicates that for bits with dimension of 8 nm × 6 nm, 5.5 nm grains, and a reader with 4 nm × 18 nm × 18 nm free layer and 11 nm shield-shield spacing, the bit error rate can drop from 20.9% for a normally positioned head array to 4.2% for a single-rotated single head with sampling period of 2 nm, a minimum mean squared error equalizer, and pattern-dependent noise prediction detector. The user density computed using the Shannon capacity limit is greatly increased to 10.1 Tb/in2. View full abstract»

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  • Magnetic switching properties of Fe-Ni-Pt thin films with graded magnetocrystalline anisotropy

    Publication Year: 2012 , Article#: 4500404
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (408 KB) |  | HTML iconHTML  

    Exchange-coupled composites and graded magnetocrystalline anisotropy Ku films allow a lower switching field while maintaining a given thermal stability relative to a conventional single Ku thin film. In this letter, [001] oriented single composition, multilayered single compositions, and a linearly graded composition of L10FexNi0.48-xPt0.52 thin films were sputter-deposited. The films were patterned into an array of ~70-80 nm diameter nanodots by electron beam lithography and Ar ion etching. A relative improvement in the switching field-to-thermal stability of ~41% was achieved for the continuous graded film as compared to the single composition Fe0.23Ni0.25Pt0.52 film. However, the patterned diameter size was still too large to prevent lateral magnetic domain wall motion; consequently, the absolute switching field-to-thermal stability value was low. View full abstract»

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  • Efficient Magnetic Gradient Field Generation With Arbitrary Axial Displacement for Magnetic Particle Imaging

    Publication Year: 2012 , Article#: 6500104
    Cited by:  Papers (3)
    Save to Project icon | Request Permissions | Click to expandQuick Abstract | PDF file iconPDF (1460 KB) |  | HTML iconHTML  

    The magnetic particle imaging method applies a magnetic gradient field featuring a field-free point (FFP) to determine the spatial distribution of superparamagnetic nanoparticles. Such a field is generated by two opposing electromagnetic coils, carrying currents in opposite directions. To achieve a large field of view, the FFP needs to be shifted in space. This can be realized by superimposing currents flowing in the same direction in both coils. For large displacements, the power loss increases drastically. In this letter, a new coil geometry is proposed, which is considerably more efficient for generating an FFP at off-center positions. It consists of four coils in which the currents are optimized to generate the required field at minimal power loss. View full abstract»

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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