bluuebunny/crossref_metadata_2025 huggingface dataset:
Determination of the intrinsic high-frequency magnetoimpedance spectra of multilayer systems
Authors: García-Arribas Alfredo , de Cos David , Barandiarán José Manuel
Published: June 2006

Magnetoimpedance results up to 3 GHz are presented in a Ni–Fe∕Au∕Ni–Fe multilayer film. The impedance of the sample is measured using microwave techniques in a microstrip-based test fixture. We describe in detail the procedure to obtain the intrinsic impedance of the sample free from contributions of the measurement setup. The process follows a method proposed for cylindrical samples in coaxial lines, but the procedure is here adapted to planar geometry. The impedance spectra so obtained clearly reveal the existence of ferromagnetic resonance, which is acceptably reproduced by simple theoretical models.
Inelastic tunneling spectroscopy of magnetic tunnel junctions based on CoFeB∕MgO∕CoFeB with Mg insertion layer
Authors: Miao Guo-Xing , Chetry Krishna B. , Gupta Arunava , Butler William H. , Tsunekawa Koji , Djayaprawira David , Xiao Gang
Published: June 2006

Magnetic tunnel junctions (MTJs) based on textured MgO barriers have thus far shown the highest tunneling magnetoresistance (TMR) at room temperature. In contrast to traditional magnetic tunnel junctions, it appears that the large TMR observed in these systems arises from a type of coherent tunneling in which the symmetry of the Bloch state wave functions plays a critical role. We have fabricated MTJs with artificial asymmetric barriers by depositing a thin layer of Mg of varying thickness (0–10 Å) prior to the growth of the MgO barrier into otherwise identical CoFeB∕MgO∕CoFeB MTJs. The inelastic tunnel spectrum shows magnon and phonon excitation peaks similar to traditional Al2O3 barriers, and an additional peak at about 300 meV. The conventional interpretation that this peak corresponds to density of states of the s electrons in the ferromagnetic electrodes, however, does not apply in the MgO system.
La
2
∕
3
Sr
1
∕
3
MnO
3
–
La
0.1
Bi
0.9
MnO
3
heterostructures for spin filtering
Authors: Gajek M. , Bibes M. , Varela M. , Fontcuberta J. , Herranz G. , Fusil S. , Bouzehouane K. , Barthélémy A. , Fert A.
Published: June 2006

We have grown heterostructures associating half-metallic La2∕3Sr1∕3MnO3 (LSMO) bottom electrodes and ferromagnetic La0.1Bi0.9MnO3 (LBMO) tunnel barriers. The layers in the heterostructures have good structural properties and top LBMO films (4 nm thick) have a very low roughness when deposited onto LSMO∕SrTiO3(1.6nm) templates. The LBMO films show an insulating behavior and a ferromagnetic character that are both preserved down to very low thicknesses. They are thus suitable for being used as tunnel barriers. Spin-dependent transport measurements performed on tunnel junctions defined from LSMO∕SrTiO3∕LBMO∕Au samples show a magnetoresistance of up to ∼90% at low temperature and bias. This evidences a spin-filtering effect by the LBMO layer, with a spin-filtering efficiency of ∼35%.
Explorations of the magnetization of Ga1−xMnxS over a wide range of concentrations, 0.008<x<0.18
Authors: Tracy J. L. , Mourad R. , Garner J. , Pekarek T. M. , Miotkowski I. , Ramdas A. K.
Published: June 2006

Calculations and measurements of the magnetization of Ga1−xMnxS, a III-VI diluted magnetic semiconductor crystal, are reported. Results extend over a wide range of concentrations: x=0.18, 0.13, 0.099, 0.079, 0.062, 0.032, and 0.008. The magnetization was measured at temperatures from 50 to 400 K in magnetic fields up to 7 T. The experimental data are compared with a model of the magnetization that is derived using the energy levels of a singlet Hamiltonian which posits the manganese atoms are not interacting with each other. The Hamiltonian consists of crystal-field, spin-orbit, spin-spin, and Zeeman interactions of the 3d electrons of the Mn+3 substitutional ions. The spin-orbit parameter used in the model was λ=23cm−1, independent of concentration. At smaller values of x the singlet model agreement with the experiment is excellent. For larger values of x and low temperatures the agreement deteriorates somewhat as expected due to the antiferromagnetic coupling of the Mn ions.
Influence of different substrates on phase separation in La1−x−yPryCaxMnO3 thin films
Authors: Gillaspie Dane , Ma J. X. , Zhai Hong-Ying , Ward T. Z. , Christen Hans M. , Plummer E. W. , Shen J.
Published: June 2006

Large scale phase separation between ferromagnetic metallic and charge-ordered insulating states in La1−x−yPryCaxMnO3 (LPCMO) crystals and thin films is very sensitive to structural and magnetic changes and is responsible for the enhanced magnetoresistance in LPCMO compared to its parent compounds. By epitaxially growing LPCMO thin films on different substrates, the strain on the LPCMO thin films can be changed, thereby controlling the energy balance between the two phases. LPCMO films of several different thicknesses have been grown on NdGaO3 (NGO), SrTiO3 (STO), SrLaGaO4 (SLGO), and LaAlO3 (LAO) substrates. The compressive strain from the LAO and SLGO substrates suppresses the long-range charge ordering in these samples and enhances magnetoresistance and magnetic hystereses. Conversely, the tensile strain from the STO and NGO substrates enhances the long-range charge ordering and reduces the magnetoresistance and magnetic hystereses.
Remote two-dimensional imaging of giant magnetoresistance in a synthetic spin valve with spatial resolution
Authors: Stirk S. M. , Vick A. J. , Thompson S. M. , Matthew J. A. D. , Lee A. F.
Published: June 2006

Spatially resolved measurements of giant magnetoresistance (GMR) in a synthetic spin valve multilayer are demonstrated using an infrared charge-coupled device camera to remotely sense images of the sample temperature. The field-dependent emissivity of the spin valve is shown to be proportional to the GMR, and a sudden change in the apparent temperature of the material associated with the free layer rotation in an applied magnetic field is observed. The optical nature of this technique gives excellent potential for in situ monitoring and whole-wafer characterization of the magnetoresistance in spin valves on a spatial scale during growth.
Shape-induced ultrahigh magnetic anisotropy and ferromagnetic resonance frequency of micropatterned thin Permalloy films
Authors: Zhuang Y. , Vroubel M. , Rejaei B. , Burghartz J. N. , Attenborough K.
Published: June 2006

Magnetic anisotropy Hk>200Oe was observed from bar-shaped Permalloy strips. The film was deposited on sputtered Cr seed layer by electroplating under ∼800Oe external magnetic field. Tiny degradation of Hk was observed after 30 min postannealing at 400 °C in the absence of external magnetic field. The Permalloy strip with in-plane aspect ratio of 40:1 showed the ferromagnetic resonance at ∼5.3GHz. The ferromagnetic resonance frequency and magnetic anisotropy decreased to ∼1GHz and ∼50Oe, respectively, as the in-plane aspect ratio reduced from 40:1 to 10:1. In comparison, Permalloy strips plated on combined Ti∕TiN seed layers did not show clear anisotropy features.
Thickness-dependent properties of (110)-oriented La1.2Sr1.8Mn2O7 thin films
Authors: Takamura Yayoi , Chopdekar Rajesh V. , Grepstad Jostein K. , Suzuki Yuri , Marshall Ann F. , Vailionis Arturas , Zheng Hong , Mitchell John F.
Published: June 2006

We have grown (110)-oriented thin films of the bilayer manganite La1.2Sr1.8Mn2O7 [n=2 phase of the Ruddlesden-Popper (RP) family] by pulsed laser deposition. The structural, magnetic, and magnetotransport properties of these films depend strongly on the growth conditions as well as the film thickness. Optimal growth conditions of 900°C and 100mTorr O2 balance the oxygenation of the n=2 phase without causing its decomposition into the n=1 and ∞ phases of the RP family. With increasing film thickness, the film evolves from a mixture of the n=1, 2, and ∞ phases to a film composed primarily of the n=2 phase. The thicker films possess bulklike properties with a ferromagnetic/paramagnetic transition that coincides with a resistivity maximum at a reduced Tc∼90K (vs 120K in bulk) and an anisotropy ratio, ρc∕ρab∼20–200 over the temperature range 5–380K.
Self-assembled nanocrystalline epitaxial manganite films on SrTiO3∕Si heterostructures
Authors: Hunter D. , Dadson J. B. , Zhang K. , Lasley B. , Lord K. , Williams T. M. , Rakhimov R. R. , Pradhan A. K. , Zhang J. , Sellmyer D. J.
Published: June 2006

We report the growth and magnetic characterizations of La0.7Ba0.3MnO3 and La0.7Sr0.3MnO3 films on SrTiO3-buffered Si (100) and Si (111) substrates by pulsed-laser deposition. The structural studies show the epitaxial nature of the films. The microscopic studies show that the films consist of nanocrystalline particles. All films display sharp magnetic and electrical transitions associated with the colossal magnetoresistance behavior at and above room temperature, illustrating the superior quality of the films.
Neutron-scattering studies of magnetism in multiferroic HoMnO3 (invited)
Authors: Vajk O. P. , Kenzelmann M. , Lynn J. W. , Kim S. B. , Cheong S.-W.
Published: June 2006

Hexagonal HoMnO3 is a frustrated antiferromagnet (TN=72K) ferroelectric (TC=875K) with a rich magnetic phase diagram consisting of multiple temperature- and field-dependent phases. Previously observed anomalies in the dielectric constant at magnetic phase transitions indicate strong coupling between the ferroelectricity and magnetism. Neutron-diffraction measurements in a magnetic field reveal new intermediate-field phases at low temperatures. Inelastic neutron-scattering measurements are used to establish the primary magnetic interactions and demonstrate that the spin dynamics in HoMnO3 are well described by a simple two-dimensional nearest-neighbor Heisenberg antiferromagnetic exchange J=2.44meV and a temperature-dependent anisotropy D.
Magnetostructural transitions and adiabatic temperature variation in polycrystal and single-crystal Ni2MnGa alloys
Authors: Sasso C. P. , Pasquale M. , Giudici L. , Besseghini S. , Villa E. , Lewis L. H. , Lograsso T. A. , Schlagel D L.
Published: June 2006

The magnetocaloric response of off-stoichiometric Ni2MnGa polycrystal and single-crystal samples with a Curie temperature coincident with the martensitic transition temperature was investigated. The direct measurement of temperature change (ΔTad) during an adiabatic transformation under a maximum field variation ΔH=5.6×106A∕m (7T) was performed. The polycrystalline sample shows a maximum ΔTad≈1.5K at a temperature close to the structural transition. The value of ΔTad and the 20K span of the ΔTad peak encourage further studies of the effect of microstructure on the magnetocaloric response of Ni2MnGa.
Model for strain-induced metal-insulator phase coexistence in colossal magnetoresistive perovskite manganites (invited)
Authors: Ahn K. H. , Lookman T. , Bishop A. R.
Published: June 2006

There is considerable evidence from new generations of high resolution microscopies and scattering techniques for intrinsically multiscale structures and dynamics in complex transition-metal oxides. In particular, the coexistence of submicrometer-size insulating and metallic domains in the same sample of perovskite manganites is believed to be crucial to the understanding of colossal magnetoresistance in these materials, and has been a puzzle to both theorists and experimentalists. In this work, we demonstrate, using an atomic-scale description of lattice distortions and long-range strains, that the presence of multiple local energy minimum states with different distortions provides a natural mechanism for such multiphase coexistence within the same material. The framework provides a basis for engineering nanoscale patterns of metallic and insulating phases and understanding other novel features observed in manganites, such as precursor short-range ordering and quasielastic scattering near the phase-transition temperature, hysteretic and glassy dynamics, metastability, and photoinduced insulator-metal transition.
Magnetoresistive DNA chips based on ac field focusing of magnetic labels
Authors: Ferreira H. A. , Cardoso F. A. , Ferreira R. , Cardoso S. , Freitas P. P.
Published: June 2006

A study was made on the sensitivity of a magnetoresistive DNA-chip platform being developed for cystic fibrosis diagnostics. The chip, comprised of an array of 2.5×80μm2 U-shaped spin-valve sensors integrated within current line structures for magnetic label manipulation, enabled the detection at 30Hz of 250nm magnetic nanoparticles from 100pM down to the pM range (or a target DNA concentration of 500pM). It was observed that the sensor response increased linearly with label concentration. Noise spectra obtained for these sensors showed a thermal noise of 10−17V2∕Hz with a 1∕f knee at 50kHz at a 1mA sense current, showing that lower detection limits are possible.
Theoretical study on the stability of ferromagnetism and resistivity of dilute magnetic semiconductors at finite temperature
Authors: Sakuma Akimasa
Published: June 2006

Based on the s-d model, where the concentration of the d sites is low enough, magnetism and transport properties are investigated for the dilute magnetic semiconductors. In case in which the Fermi level lies near the upper edge of the s band, ferromagnetism is realized when the site energy of the d state (Ed) is located around the edge of the s band. In this case, down spin states of the d band are expelled out of the s band, resulting in a formation of the gap in the down-spin band. This leads to a half-metallic character accompanied by the resonance enhancement of the ferromagnetic interaction. The resistivity has a residual value at T=0 as the effects both of random distribution of the localized spins and energy difference between the s and d states.
Effect of annealing conditions on the Fe incorporation and ferromagnetism of Sn1−xFexO2: A Raman spectroscopic investigation
Authors: Mathew Xavier , Hays J. , Mejía-García C. , Contreras-Puente G. , Punnoose A.
Published: June 2006

In order to investigate the effect of annealing conditions on the structural and magnetic properties, we have employed detailed Raman spectroscopic investigations on 5% Fe-doped SnO2 samples annealed at different temperatures in the range 200–900°C. Systematic changes observed in the Raman spectra with increasing annealing temperature are correlated with similar changes in the structural and magnetic properties of these samples. The presence of incorporated Fe ions was confirmed from the appearance of vibrational local modes of Fe in the Raman spectra. It is concluded that the ferromagnetism in Sn0.95Fe0.05O2 annealed in the 350–600°C temperature range is due to the incorporated Fe ions and is destroyed completely when TA>600°C due to the expulsion of Fe from the SnO2 lattice, which is in agreement with the disappearance of the Fe local modes in the Raman spectra.
Composition-dependent structural and magnetic properties of Co1−xPtx (0.09⩽x⩽0.86) nanowire arrays
Authors: Zhang Jun , Li Wuxia , Jones Grenville A. , Shen Tiehan H.
Published: June 2006

Highly ordered 14nm diam cobalt platinum alloy nanowire arrays with Pt content ranging from 9to86at.% have been electrodeposited into nanoporous anodic aluminum oxide templates. The crystal structure of the as-deposited Co1−xPtx nanowire arrays changes progressively from hcp to a mixture of hcp and fcc phases and finally to pure fcc with increasing Pt content. The coercivity, squareness, and saturation magnetization of the as-deposited nanowire arrays have been investigated as a function of nanowire composition.
Analysis and optimization of a Maglev system based on the Halbach magnet arrays
Authors: Ham C. , Ko W. , Han Q.
Published: June 2006

A systematic analysis and an optimization are presented for an electrodynamic suspension Maglev system based on the Halbach magnet arrays that promises to develop a feasible Maglev system. The magnetic field was modeled using scalar potential and Fourier series methods and then confirmed with the corresponding finite element method (FEM) result. The optimization of the array was derived based on the performance index defined as the ratio of the square of flux to the magnet weight of a unit volume. The study results were applied to the Maglev system at the University of Central Florida for measuring the current capability and optimizing the performance while gaining some insight into the system stability.
Magnetic memories: Memory hierarchy and processing perspectives (invited)
Authors: Fontana R. E. , Hetzler S. R.
Published: June 2006

A variety of magnetic memory devices, including MRAM, spin-torque RAM, plated wire, and bubbles, either are or have been under investigation as candidate “universal memory” devices. Such devices would combine the performance attributes of DRAM with the nonvolatility of a disk drive. This paper examines the past and present magnetic memories from two perspectives: (1) processing and extendability to smaller dimensions, and (2) the memory hierarchy of the applications to which they are directed. From a processing requirement, magnetic memories must converge to the cell sizes of competitive memory technologies, which range from 8F2 for DRAM to 0.5F2 for disk drives, where F is the minimum processing feature. From a memory hierarchy perspective, magnetic memories must balance the advantages of nonvolatility with the market demands for performance and low cost, and must address the application shifts away from classical enterprise storage hierarchy to the now rapidly developing mobile storage needs. The success of magnetic memories will depend on their ability to fulfill the evolving memory hierarchy requirements of a large set of new and nontraditional applications and on their ability to approach the device sizes of competing DRAM and flash devices.
The effects of oxygen on intergranular exchange and anisotropy dispersion in Co∕Pd multilayers for perpendicular magnetic recording media
Authors: Speetzen Neal J. , Huang Xiaobo , Stadler Bethanie J. H. , Victora R. H.
Published: June 2006

A typical way to analyze anisotropy dispersion (σHk) in perpendicular magnetic recording media is to simply calculate the slope parameter α(dM∕dH) from a hysteresis loop. However, α is also dependent on intergranular exchange, and is therefore not an accurate measure of σHk. In this work, σHk and intergranular exchange were varied in high coercivity Co∕Pd multilayers on InSn seed layers in order to determine their relationship with coercivity, saturation magnetization, and α. These parameters were altered using various concentrations of oxygen as measured by Auger sputter depth profiles, and a minor loop coercivity method was used to find σHk independent of exchange. A general cross-referencing analysis was developed to characterize the important parameters of media using hysteresis loops. And in this study, the analysis showed that oxygen in Co-Pd multilayers reduced intergranular exchange coupling as expected, but it also increased σHk at higher concentrations.
Exchange coupling between ferromagnetic and antiferromagnetic layers via Ru and application for a linear magnetic field sensor
Authors: Wang Dexin , Daughton Jim , Nordman Cathy , Eames Pete , Fink Jon
Published: June 2006

Exchange coupling is observed in a trilayer structure of ferromagnet-Ru-antiferromagnet and the coupling strength is found to be a function of the thickness of the Ru spacer layer. This is the first observation for such a trilayer structure and may help to shed light on the illusive mechanism of exchange coupling in these systems. This unique coupling is used to bias the sense layer in a magnetic tunnel junction structure so that the magnetization orientations of the sense layer and the pinned layer can be independently controlled. Sensor devices are fabricated with a bipolar output, a medium sensitivity, and a wide field range. The results show that this biasing scheme is well suited for magnetic tunnel junctions used in magnetic field sensors.
Fabrication and characterization of granular-type (FePt∕Fe3Pt)- and (FePt∕fcc FePt)-tilted magnetic recording media
Authors: Singh Amarendra K. , Yin Jinhua , Ko Hnin Yu Yu , Suzuki Takao
Published: June 2006

The magnetic properties of tilted media with a film stack structure of [(fccFePtxnm) or (Fe3Ptynm)]∕FePt∕MgO∕FePt∕MgO4nm∕SiO24nm∕glass were investigated. The angular dependence of coercivity from the film normal to the in plane exhibits a broad peak in the case of Fe3Pt, implying the tilt of the easy axis from the film normal. The coercivity of the media with a hard layer decreases by about 50% from its initial value with an increase in Fe3Pt layer thickness. However, the media with fcc FePt exhibit about a 70% decrease in coercivity for layer thickness up to 3 nm. There is not much change in the α[=4π(dM∕dH)H=Hc] value (α∼2–3) with an increase in the Fe3Pt layer thickness.
Spin valves with synthetic exchange bias Co70Ni10Pt20∕Ru∕CoFe
Authors: Qiu J. J. , Li K. B. , Zheng Y. K. , Luo P. , Wu Y. H.
Published: June 2006

Co 70 Ni 10 Pt 20 ∕ Ru ∕ Co 90 Fe 10 trilayer was used as pinning/pinned layer in spin valves after deposition condition of CoNiPt was studied. The influence of hard-layer, soft-layer, and Cu-spacer-layer thicknesses on exchange bias, interlayer coupling, free-layer coercivity, and magnetoresistance (MR) ratio were systematically studied. Oscillation of interlayer coupling and exchange bias with respect to the thickness of spacer or hard layer are found. MR of a spin valve with structure Cr(3)∕CoNiPt(4)∕Ru(0.8)∕CoFe(2.3)∕Cu(2.05)∕CoFe(2.6)∕Cu(1.1)∕Ta(1) (in nanometer) reached 10.48%, whereas its coercivity of free layer was increased to 190 Oe and is attributed to two reasons: one is the poor (111) texture and small grain size of free layer with Cr as the seed layer, and the other is magnetostatic coupling between the free layer and hard layer.
Multiscale micromagnetic simulation of giant magnetoresistance read heads
Authors: Ertl O. , Hrkac G. , Suess D. , Kirschner M. , Dorfbauer F. , Fidler J. , Schrefl T.
Published: June 2006

The Landau-Lifshitz-Gilbert equation and quasistatic Maxwell equations were solved simultaneously to calculate the read back signal of giant magnetoresistance read heads with a hybrid finite-element/boundary element method. The finite-element simulations show the influence of the sense current on the linearity of the reader, the effect of the exchange bias field on the sensor performance, and the influence of the Gilbert damping constant on the decay time of the read back voltage. All parts of the system, the layers of the giant magnetoresistance sensor, the hard bias magnets, the shields, and the recording layer are treated micromagnetically. In addition, the influence of the sense current onto the magnetization is taken into account self-consistently. The current distribution in the giant magnetoresistance stack is calculated from local resistivity which depends on the magnetization of the free and of the pinned layer.
Effect of top Ru deposition pressure on magnetic and microstructural properties of CoCrPt–SiO2 media in two-step Ru layer
Authors: Park S. H. , Kim S. O. , Lee T. D. , Oh H. S. , Kim Y. S. , Park N. Y. , Hong D. H.
Published: June 2006

Two-step-deposited Ru layers of 20 nm have been employed as an underlayer for CoCrPt–SiO2 perpendicular magnetic recording media. The bottom Ru layer of 10 nm is deposited at 3 mTorr for good texture of hcp (0002) and the top Ru layer of 10 nm is deposited at various conditions. Sputtering power and Ar pressure are variables for the top Ru layer deposition. As the sputtering pressure of the top Ru layer increases, coercivity (Hc) of the CoCrPt–SiO2 layer increases and then saturates or slightly decreases with a further increase of sputter pressure. The Hc increase is mainly due to exchange decoupling among grains of CoCrPt. The c-axis alignment of the Ru layer by the two-step deposition method is much improved compared to that of a single-step-deposited Ru layer when the sputtering pressure is higher than 3 mTorr. The grain size of Ru layer is smaller and the grains are well separated by void boundaries when the top Ru sputtering pressure is high. There is a one-to-one relationship between small Ru grains and CoCrPt grains. Although the grain size of CoCrPt layer is small with a good c-axis alignment in the case of lower-pressure-deposited top Ru underlayer, Hc is small. The reason will be discussed in terms of grain interconnection.
Spin-torque-driven magnetization dynamics in nanomagnets subject to magnetic fields perpendicular to the sample plane
Authors: Bonin Roberto , Bertotti Giorgio , Mayergoyz Isaak D. , Serpico Claudio
Published: June 2006

We analytically solve the Landau-Lifshitz-Gilbert equation with the addition of the spin-torque term proposed by Slonczewski [J. Magn. Magn. Mater. 159, L1 (1996)] for the case of external magnetic field perpendicular to the sample plane. By using methods of nonlinear dynamical system theory we construct the complete stability diagram of the spin-torque-driven dynamics in the field-current control plane. Our predictions are compared with recent experimental results.
Study of synthetic ferrimagnet-synthetic antiferromagnet structures for magnetic sensor application
Authors: Guedes A. , Mendes M. J. , Freitas P. P. , Martins J. L.
Published: June 2006

There has been a growing interest in using both synthetic ferrimagnet (SF) free and synthetic antiferromagnet (SAF) pinned layers for head and memory applications. In particular, for linear sensor applications, these structures lower the magnetostatic fields present at the free layer through the reduction of its effective thickness (teffSF). This allows higher sensitivity but at the expense of an increased offset field H0(Néel coupling fieldHf+interlayer demagnetizing fieldHdSAF). In this work, results on a series of patterned 3×1 and 6×2μm2 top-pinned SF-SAF spin valves are analyzed and compared with a three-dimensional micromagnetic simulation in order to clarify the role of the different ferromagnetic layers in the overall offset field and sensitivity. H0 varies as 1∕teffSF[teffSF=(Mata−Mbtb)∕MeffSF]. The magnetostatic field acting on the SF coming from the SAF (HdSAF) can act as a biasing field, partially counterbalancing the Néel coupling field (Hf) leading to a reduction of H0. In this work the offset field was reduced from an initial value of 25 Oe in a quasicompensated SAF to a value of −6Oe, by unbalancing the SAF and consequently increasing its effective moment (teffSF=15Å).
Planar Hall-effect magnetic random access memory
Authors: Bason Y. , Klein L. , Yau J.-B. , Hong X. , Hoffman J. , Ahn C. H.
Published: June 2006

We suggest a type of magnetic random access memory (MRAM) that is based on the phenomenon of the planar Hall effect (PHE) in magnetic films, and we demonstrate this idea with manganite films. The PHE-MRAM is structurally simpler than the currently developed MRAM that is based on magnetoresistance tunnel junctions, with the tunnel junction structure being replaced by a single-layer film.
Complex magnetic ordering in Tb3Ag4Sn4
Authors: Perry Laura K. , Ryan D. H. , Canepa F. , Napoletano M. , Mazzone D. , Riani P. , Cadogan J. M.
Published: June 2006

Tb 3 Ag 4 Sn 4 has been studied using both bulk magnetization and Sn119 Mössbauer spectroscopy. We find evidence for a first-order magnetostructural transition at 28K, followed by a spin reorientation at 13K. Three tin sites with hyperfine fields of 8.0, 4.2, and 3.5T are observed at 2K with area ratios of 2:1:1. Magnetization at 5K is linear up to 9T, indicating antiferromagnetic ordering.
Analysis of Barkhausen effect signals in surface-modified magnetic materials using a hysteretic-stochastic model
Authors: Lo C. C. H. , Kinser E. R. , Jiles D. C.
Published: June 2006

The effects of microstructural variations with depth on Barkhausen effect (BE) signals in surface-modified ferrous materials have been studied through measurements and simulations based on a hysteretic-stochastic model. The BE signals measured from an unhardened sample show a peak near zero field. In contrast, the BE signals in surface-hardened samples with different case depths exhibit low-amplitude pulses near zero field and a peak at a high reverse field, which are attributed to irreversible magnetization processes in the soft core and the hardened surface layer, respectively. Theoretical analysis showed that the model parameters k and ξ, which describe the domain-wall pinning strength and the range of interaction of a domain wall with pinning sites, respectively, are related to each other as ξ=a ρ−1∕2=b∕k via the pinning site density ρ, where a and b are constants. The relationship was used to simulate BE signals of the surface-hardened samples as a sum of signals generated at different depths by taking into account signal attenuation due to eddy current shielding. The simulated results were found to exhibit the general features observed in the experimental results.
Transport, thermal, and magnetic properties of RuSr2(Gd1.5Ce0.5)Cu2O10−δ, a magnetic superconductor
Authors: Naugle D. G. , Rathnayaka K. D. D. , Krasovitsky V. B. , Belevtsev B. I. , Anatska M. P. , Agnolet G. , Felner I.
Published: June 2006

Resistivity, thermoelectric power, heat capacity, and magnetization for samples of RuSr2(Gd1.5Ce0.5)Cu2O10−δ were investigated in the temperature range 1.8–300K with a magnetic field up to 8T. The resistive transitions to the superconducting state are found to be determined by the inhomogeneous (granular) structure, characterized by the intragranular, Tc0, and intergranular, Tcg, transition temperatures. Heat capacity, C(T), shows a jump at the superconducting transition temperature Tc0≈37.5K. A Schottky-type anomaly is found in C(T) below 20K. This low-temperature anomaly can be attributed to splitting of the ground term S7∕28 of paramagnetic Gd3+ ions by internal and external magnetic fields.
Thermal effects of heated magnetic disk on the slider in heat-assisted magnetic recording
Authors: Xu B. X. , Hu S. B. , Yuan H. X. , Zhang J. , Chen Y. J. , Ji R. , Miao X. S. , Chen J. S. , Chong T. C.
Published: June 2006

In this paper, the real time thermal effect of the heat-assisted magnetic recording (HAMR) on the slider is investigated with the HAMR platform. The net temperature rises of the slider caused by laser absorption and heat radiation of the locally heated recording magnetic disk are studied by monitoring the calibrated read sensor resistances in different rotation speeds. The maximum slider temperature increase of about 3.5°C caused by the locally heated recording disk is obtained when the media net temperature increase is 85°C at 2000rpm rotation speed. A higher temperature case is also discussed.
Co
72
Fe
20
B
8
: Structure, magnetism, and tunneling spin polarization
Authors: Paluskar P. V. , Kohlhepp J. T. , Swagten H. J. M. , Koopmans B.
Published: June 2006

We present measurements of tunneling spin polarization of Co72Fe20B8 in Al∕AlOx∕CoFeB tunnel junctions using superconducting tunneling spectroscopy. We consistently observe high spin polarization (above 53%), higher than that measured for comparable CoFe alloys, indicating a crucial role played by the 8% atomic boron. X-ray diffraction measurements on 700Å thick films reveal an amorphous structure for as-deposited films, while anneals above 250°C initiate a gradual crystallization. Superconducting quantum interference device magnetometery shows that the magnetic moment exhibits minor changes after anneals, but there is a dramatic change in the coercivity after anneals above 400°C. However, these structural and magnetic changes induce only a slight change in the tunneling spin polarization of CoFeB.
Bias field effects on the toggle mode magnetoresistive random access memory
Authors: Wang S.-Y. , Fujiwara H. , Sun M.
Published: June 2006

Our previous study revealed that setting the induced intrinsic anisotropy orthogonal to the shape anisotropy to the toggle-MRAM (toggle mode magnetoresistive random access memory) element, thus giving zero total anisotropy, can reduce the operating field substantially with a greater margin. It has been confirmed that further reduction of the operating field is possible by applying a bias field keeping a substantial operating field margin, and that by choosing a proper bias field and the coupling field between the two magnetic layers composing a synthetic antiferromagnet memory element, the “orthogonal anisotropy” toggle-MRAM can be endowed with a greater margin than the conventional Stoner-Wohlfarth-type MRAM for the same strength of operating field, memory life, and half-select disturb robustness.
Thermodynamic aspects of magnetic-field-driven phase transformations in Gd-Si-Ge alloys
Authors: Basso Vittorio , LoBue Martino , Sasso Carlo P. , Bertotti Giorgio
Published: June 2006

A temperature-dependent Preisach model is proposed to describe hysteresis in materials in which a structural phase transformation driven by the magnetic field takes place. The model is applied to the interpretation of the temperature dependence of hysteretic magnetization curves measured for Gd-Si-Ge alloys. Predictions about the entropy exchanged during transformations under increasing or decreasing field reveal the role played by internal dissipative processes.
Magnetic reversal in three-dimensional exchange-spring permanent magnets
Authors: Shield J. E. , Zhou J. , Aich S. , Ravindran V. K. , Skomski R. , Sellmyer D. J.
Published: June 2006

In this paper, we investigate the magnetization reversal in single-phase RE2Fe14B and two-phase α-Fe∕RE2Fe14B with varying nanoscale grain structures and intergranular exchange interactions produced via controlled segregation during crystallization. We show that the loss of coercivity arises because domain-wall processes dominate the magnetic reversal as the exchange interactions increase. Micromagnetic modeling corroborates a transition to strongly cooperative magnetic reversal as the exchange interactions increase. The magnetic reversal is controlled by the growth of interaction domains via discrete domain-wall motion, and the coercivity is intrinsically limited by the presence of interaction domains. To alleviate this problem, we have built an additional length scale into the structure that is below the interaction domain size but above the limit for intergranular exchange interactions to be significant. These “single-interaction domain” structures retain nucleation-type magnetic reversal and high coercivity. We show experimentally that nanocomposite Sm-Co/Co with this additional length scale has excellent coercivity and nucleation-controlled reversal.
Magnetic properties of Co3O4 nanoparticles mineralized in Listeria innocua Dps
Authors: Resnick D. A. , Gilmore K. , Idzerda Y. U. , Klem M. T. , Allen M. , Douglas T. , Arenholz E. , Young M.
Published: June 2006

Temperature-dependent magnetic measurements are reported for 4.34 nm antiferromagnetic Co3O4 nanoparticles mineralized in the Listeria innocua Dps protein cage. ac measurements show a superparamagnetic blocking temperature of roughly 5.4 K and give an extracted anisotropy energy density of (7.6±0.4)×104J∕m3. The Néel temperature for the Co3O4 nanoparticles, determined with dc magnetometry, was determined to be roughly 15±2K.
A class of micromachined magnetic resonator for high-frequency magnetic sensor applications
Authors: Kim Yong-Seok , Yu Seong-Cho , Lu Hong , Lee Jeong-Bong , Lee Heebok
Published: June 2006

A class of LC resonators for micromagnetic sensor devices is reported, which is fabricated by means of the microelectromechanical system (MEMS) technique. The micro-LC resonator consists of a solenoidal microinductor with a bundle of soft magnetic microwire cores and a capacitor connected in parallel to the microinductor. The core magnetic material is a tiny glass-coated Co83.2B3.3Si5.9Mn7.6 microwire fabricated by a glass-coated melt-spinning technique. The solenoidal microinductors fabricated by the MEMS technique were 500–1000μm in length with 10–20 turns. The changes of inductance as a function of external magnetic field in microinductors with appropriately annealed microwire cores were varied as much as 370%. Because the permeability of ultrasoft magnetic microwire is changing rapidly as a function of external magnetic field, the inductance ratio as well as magnetoimpedance ratio (MIR) in a LC resonator is varied accordingly as a function of external magnetic field. The MIR curves can be tuned very precisely to obtain maximum sensitivity.
Surface spin freezing of ferrite nanoparticles evidenced by magnetization measurements
Authors: Alves C. R. , Aquino R. , Depeyrot J. , Cotta T. A. P , Sousa M. H. , Tourinho F. A. , Rechenberg H. R. , Goya G. F.
Published: June 2006

Magnetization and in-field Mössbauer measurements were performed on copper ferrite nanoparticles with average sizes ranging from 3.5 to 10.4nm. Our results show that the nanoparticles are well-crystallized single domains with a magnetically disordered surface shell. A sharp increase in the saturation magnetization at low temperatures, in addition to the usual modified Bloch behavior, was observed for the smallest particles. This jump in magnetization curves seems to be related to the freezing of the surface spins below a temperature of about 45K.
Spin relaxation of excitons in nonmagnetic quantum dots: Effect of spin coupling to magnetic semiconductor quantum dots
Authors: Lee S. , Dobrowolska M. , Furdyna J. K.
Published: June 2006

The effect of spin coupling on the spin polarization of exciton was investigated in double quantum dot (QD) structures, one layer consisting of self-assembled nonmagnetic CdZnSe quantum QDs, and the other of CdMnSe diluted magnetic semiconductor QDs, both layers embedded in ZnSe. The magneto-photoluminescence (PL) studies on this double-layer structure were carried by detecting a single circular polarization in order to investigate the polarization dependence of the PL intensity. We have observed two well separated PL peaks in double QDs, corresponding to emissions from the CdZnSe and CdMnSe QD layers, respectively. It appears that the degree of the PL polarization emitted by the CdZnSe QDs in this double-layer structure is much larger than that observed in a single layer of nonmagnetic CdZnSe QDs. We discussed this enhancement of polarization in terms of antiparallel spin interaction between excitons localized in the nonmagnetic QDs and in the magnetic QDs.
Effect of intragranular exchange on exchange-coupled composite media
Authors: Kapoor Manish , Shen Xiao , Victora R. H.
Published: June 2006

A micromagnetic approach was adopted to study the effect of the intragranular exchange parameter, A(ergs∕cm), on the switching of exchange-coupled composite (ECC) media. A single grain of ECC media was simulated by subdividing it into small cubes and introducing appropriate exchange within different regions of the grain. We find full coupling between the hard and soft regions to be necessary to facilitate switching of a hard region with very high anisotropy, Ku(∼2×107ergs∕cm3). This method also allows for rotation mechanism analysis within the ECC grain. We report nonuniform rotation that is observed as a domain nucleating in the bottom region of the soft part and moving its way up to and through the hard/soft layer interface, if the intragranular exchange coupling is appropriate. Field sweep-time analysis revealed that ECC media was capable of following the rapidly changing field more efficiently than regular perpendicular media.
Magnetic ordering in Pd∕Mn oxide nanocomposites
Authors: Lewis L. H. , Baumberger E. , Gambino R. J.
Published: June 2006

An increase in the magnitude of resistivity and magnetic susceptibility (10×) with increase of oxygen in the assist beam during reactive ion-beam assisted deposition of Pd94Mn6 nanocomposite films is reported. These results imply that a portion of the oxygen is implanted in the as-deposited films within the Pd matrix, forming a metastable (PdMnO) solid solution that creates an increase in the effective electronic density of states at the Pd Fermi level and fosters an electronic state that approaches the cooperative magnetism. Vacuum annealing of films is hypothesized to foster the formation of Mn oxides that exhibit antiferromagnetism in the temperature range 50<T<300K, with a ferromagnetic component that is confirmed at low temperature, with 0.2μB∕Mn and coercivity of 57.5Oe at 5K in the 10% oxygen sample. The data are consistent with annealing-induced precipitation of a Mn oxide ferrimagnetic or uncompensated antiferromagnetic nanoscopic phase with a number density that increases with increasing amount of implanted oxygen in the film. The ferromagnetic contribution that appears at low temperatures may be associated with ferromagnetic polarization of Pd surrounding the precipitates.
Methodology of pole piece design in permanent magnets
Authors: Abele Manlio G. , Tsui Winnie , Rusinek Henry
Published: June 2006

Magnet shimming methodology is important for achieving the stringent field uniformity required for magnetic-resonance imaging. Shimming is necessary to correct for design limitations and to compensate for magnet magnetization and fabrication tolerances. An exact theory of the shimming procedure is presented, based on the cylindrical harmonic expansion of the magnetostatic potential within the imaging region. Cancellation of the harmonics is achieved by modulating the magnetostatic potential at the parallel interfaces between the pole pieces and the cavity. This spatial filtering is implemented in two complementary ways. An active filter is generated by distributing magnetized material on the pole piece surfaces. An independent, passive filter solution is achieved by deriving the geometric profile of the pole piece from an image dipole distribution inside a ferromagnetic body. Both active and passive methodologies are demonstrated on an example of a recently introduced conical permanent magnet.
Investigation of the phase separations and the local electronic structures of Zn1−xTxO (T=Mn, Fe, Co) magnetic semiconductors using synchrotron radiation
Authors: Lee S. S. , Kim G. , Wi S. C. , Kang J.-S. , Han S. W. , Lee Y. K. , An K.-S. , Kwon S. J. , Jung M. H. , Shin H. J.
Published: June 2006

The spatial chemical distributions of doped T ions and their local electronic structures in Zn1−xTxO (T=Mn, Fe, Co; x⩽0.1) have been investigated simultaneously by using scanning photoelectron microscopy (SPEM), photoemission spectroscopy (PES), and soft-x-ray absorption spectroscopy (XAS). The measured SPEM images for Zn1−xTxO single crystals reveal the uniform chemical distributions of T ions, indicating the homogeneous substitution of T ions for Zn sites. The T2p XAS spectra of Zn1−xTxO show that Mn and Co ions are in the divalent states, while Fe ions are in the Fe2+–Fe3+ mixed-valent states. The valence-band PES study of Zn1−xTxO (T=Mn, Fe, Co) shows that T3d states lie near the top of the O 2p valence bands. Only ferromagnetic Zn1−xTxO shows a significantly large Fe 3d photoemission intensity near EF, in contrast to the negligible Mn and Co 3d photoemission intensities near EF in nonferromagnetic Zn1−xTxO (T=Mn, Co), suggesting that the high carrier density is important in determining the ferromagnetic behavior of Zn1−xTxO.
Effects of Tb∕Pt∕Ru underlayer on microstructure and magnetic properties of CoPtCr–SiO2 perpendicular media
Authors: Vokoun David , Lai Chih-Huang , Liao Yu-Yi , Lin Meng-Shian , Jiang Ruo-Fan , Huang Rong-Tan , Chu Yen-Chang , Goryczka Tomasz
Published: June 2006

The effects of a composite Tb∕Pt∕Ru underlayer on the microstructure and magnetic properties of the CoPtCr–SiO2 perpendicular recording layer were studied. Among various underlayers of Pt∕Ru, Ta∕Pt∕Ru, and Tb∕Pt∕Ru, we demonstrated that the media with the Tb∕Pt∕Ru underlayer exhibited an enhancement of SiO2 segregation at grain boundaries, a refinement of the grains, the narrow c-axis distribution, and consequently, improved magnetic properties.
Magnetocaloric properties of the Ni2Mn1−x(Cu,Co)xGa Heusler alloys
Authors: Gomes A. M. , Khan M. , Stadler S. , Ali N. , Dubenko I. , Takeuchi A. Y. , Guimarães A. P.
Published: June 2006

We have investigated the magnetocaloric properties on the Ni2Mn1−xAxGa Heusler alloys with partial substitution of Mn by A=Co (x=0.10, 0.20, and 0.30) and Cu (x=0.15 and 0.20) in the vicinity of the martensitic transition by measuring magnetization curves at magnetic field up to 20kOe and in the temperature range of 250–300K. The changes of the magnetic part of entropy dependence on magnetic field and temperature have been evaluated.
Determination of intrinsic switching field distributions in perpendicular recording media (invited)
Authors: Berger A. , Lengsfield B. , Ikeda Y.
Published: June 2006

We describe a method for the experimental characterization of intrinsic switching field distributions in perpendicular media. The method is based upon a comparison between the major loop and a set of recoil loops, which start at a certain distance ΔM away from saturation. In particular, we measure the applied field difference ΔH between the recoil loops and the major loop at identical M values, for which the average demagnetizing fields are the same. By analyzing complete ΔH(M,ΔM) data sets taken from multiple recoil loops, we gain a detailed measure of the intrinsic switching field distribution D(HS). We have also studied the reliability and precision of this method by means of micromagnetic modeling and find it to be robust and overall very precise in reproducing the input parameters. In particular, the method is sufficiently insensitive to intergranular exchange coupling as long as it is only moderate in strength. Using polar-Kerr effect measurements of recording disk samples, we demonstrate the capabilities of our approach and its suitability for being a practical characterization tool for perpendicular recording media.
Effects of dipolar interactions on the magnetic properties of γ-Fe2O3 nanoparticles in the blocked state
Authors: Nunes W. C. , Cebollada F. , Knobel M. , Zanchet D.
Published: June 2006

The role played by dipolar interactions in the coercive field and remanence of nanoparticle systems is studied using different concentrations of nearly monodisperse nanoparticles of γ-Fe2O3. The 9.0nm particles are isolated by organic encapsulation and diluted in paraffin. Magnetic properties were investigated by means of hysteresis obtained at different temperatures, zero-field-cooled (ZFC) and field-cooled (FC) magnetization curves, and the so-called ΔM technique. The analysis of the experimental data points to the existence of a demagnetizing role played by the dipolar interaction in the coercive field.
Spin-wave spectrum of a magnonic crystal with an isolated defect
Authors: Kruglyak V. V. , Sokolovskii M. L. , Tkachenko V. S. , Kuchko A. N.
Published: June 2006

Real magnonic crystals—periodic magnetic media for spin-wave (magnon) propagation—may contain some defects. We report theoretical spin-wave spectra of a one-dimensional magnonic crystal with an isolated defect. The latter is modeled by insertion of an additional layer with thickness and magnetic anisotropy values different from those of the magnonic crystal constituent layers. The defect layer leads to appearance of several localized defect modes within the magnonic band gaps. The frequency and the number of the defect modes may be controlled by varying parameters of the constituent layers of the magnonic crystal.
Compensation of the thermal drift in the sensitivity of fundamental-mode orthogonal fluxgates
Authors: Plotkin Anton , Paperno Eugene , Samohin Alexander , Sasada Ichiro
Published: June 2006

A method is suggested that reduces the temperature coefficient of the sensitivity of fundamental-mode orthogonal fluxgates by an order of magnitude. For the background magnetic fields greater than 20μT, the method provides even better reduction, down to 100ppm∕°C, which is comparable with the temperature coefficient of conventional parallel-type fluxgates operated in closed-loop configuration. The fluxgate prototype has demonstrated a 20-fold reduction of the thermal drift in its sensitivity. The suggested method is solely based on the processing of signals generated by fundamental-mode orthogonal fluxgates in open-loop configuration and does not require any additional hardware. This makes the open-loop fundamental-mode orthogonal fluxgates competitive with closed-loop parallel-type ones in terms of both resolution and accuracy, while still keeping them simpler than the parallel fluxgates.