bluuebunny/crossref_metadata_2025 huggingface dataset:
Superparamagnetic NiFeCo layers as free layers in magnetic tunnel junctions
Authors: Schad R. , Alouach H. , Harrell J. W. , Shamsuzzoha M. , Wang D.
Published: May 2003

In order to study the possibility of using superparamagnetic films as a hysteresis-free switching layer in magnetic tunnel junctions or spin valves we prepared thin, discontinuous NiFeCo layers on different buffer layers on Si3N4/Si wafers. The magnetic properties were measured as a function of temperature for various layer thicknesses. We observe a transition from ferromagnetic behavior via superparamagnetic to paramagnetic behavior as either the film thickness is reduced or the temperature is increased. For a 0.8 nm thick (nominal thickness) NiFeCo layer grown on 10 nm Cu the coercivity reaches essentially zero at around 260 K, indicating a transition from ferromagnetic to superparamagnetic behavior. At that point, the slope of the steepest part of the hysteresis loop diverges. A further increase in temperature causes the saturation magnetization Ms (estimated at sufficiently large fields to saturate the sample) to sharply decrease. The drop of Ms with increasing temperature points to a Curie temperature (transition towards paramagnetism) at about T=350–400 K. Thinner films show reduced temperatures for those transitions, whereas thicker films are still ferromagnetic at room temperature. We found that these transition temperatures are not only a function of the film thickness but also vary with the material on which the NiFeCo layers are prepared. This points toward the importance of the microstructure of such films. Transmission electron microscope images do reveal a pronounced granularity of the NiFeCo layer.
Interplay of lattice strain and spin-polarization in ferromagnetic–insulator–ferromagnetic thin films: La0.7Ca0.3MnO3/LaAlO3/La0.7Sr0.3MnO3
Authors: Khatua Sanghamitra , Kundaliya Darshan C. , John J. , Rayaprol S. , Shinde S. R. , Kuberkar D. G. , Pinto R. , Malik S. K.
Published: May 2003

Epitaxial thin films of ferromagnet/insulator/ferromagnet layers (La0.7Ca0.3MnO3/LaAlO3/La0.7Sr0.3MnO3) of different thickness were fabricated by pulsed laser deposition. Microstructural analysis shows island growth on the thicker film surface. dc magnetization study demonstrates the strong signature of domain wall freezing for the composite film. An interesting feature is a cusp/upturn in field-cooled curve in the composite film-1, much below TB, the blocking temperature. The low temperature cusp disappears in the composite film-2. We suggest that a quasistable charge ordering via spin localization at low temperature, especially for film-1, is the origin of this feature.
Quasiparticle-injection effects in YBa2Cu3O7-δ/La1/3Ca2/3MnO3/La2/3Ca1/3MnO3 heterostructures
Authors: Baca E. , Saldarriaga W. , Osorio J. , Campillo G. , Gómez M. E. , Prieto P.
Published: May 2003

We have grown YBa2Cu3O7-δ (40 nm)/La1/3Ca2/3MnO3 (2 nm)/La2/3Ca1/3MnO3 (30 nm) junctions for quasiparticle-injection measurements. Ferromagnetic (F), antiferromagnetic (AF), and superconducting (S) thin films were sequentially grown “in situ” using a dc-sputtering technique at 850 °C in pure oxygen at pressures around 3.5 mbar. The temperature dependence of the resistivity of the junction, without any current injection, presented coexistence between magnetism and superconductivity with a metallic insulating transition at 186 K, and zero resistance at 52 K. When current was injected from the F layer into the S layer, superconductivity suppression was obtained at temperatures below 52 K; furthermore, small values of current injection were necessary to change the electrical behavior up to the critical temperature. A negative current gain (G) has been observed and described by the relation G∝ατeff, where τeff is the relaxation time of the quasiparticle. The use of an AF layer as an insulating barrier is also discussed.
Tuning structure and roughness in exchange biased NiO/permalloy bilayers
Authors: Thomas Luc , Negulescu Béatrice , Dumont Yves , Tessier Michel , Keller Niels , Wack André , Guyot Marcel
Published: May 2003

Polycrystalline NiO thin films have been grown by pulsed laser deposition on quartz substrates. These films exhibit a strong texture, which can be tuned by changing deposition parameters such as substrate temperature or oxygen partial pressure. By varying the deposition temperature from room temperature up to 900 °C, (220), (111), and (200) textured films are prepared. In the temperature zones separating these orientations, competition between different growth directions leads to smaller crystallites, characterized by broader diffraction lines. Surface roughness measured by atomic force microscopy is strongly correlated with these structural features. Roughness is minimum for highly textured samples (about 7Å for 500 Å thick films), and it exhibits two peaks in the intermediate zones, with maximum values of about 40 Å. In order to correlate exchange bias with these structural features, 100 Å thick FeNi layers were deposited by rf sputtering on top of the 500 Å thick NiO films. Hysteresis loops were measured at 10 K by superconducting quantum interference device magnetometry after the samples were cooled in a 100 Oe magnetic field. Exchange bias is maximum for (111) oriented samples. No clear correlation between exchange bias and surface roughness is observed at low temperature. Exchange bias temperature dependence strongly depends upon NiO films deposition temperature. The blocking temperature, for which the exchange bias vanishes, varies between 150 K for (220) oriented samples and 250 K for (111) textured samples, and it exceeds room temperature for (200) films.
Crossovers from in-plane to perpendicular anisotropy in magnetic tunnel junctions as a function of the barrier degree of oxidation
Authors: Rodmacq B. , Auffret S. , Dieny B. , Monso S. , Boyer P.
Published: May 2003

Pt/Co/M sandwiches (M=Al, Ta, Cr) were prepared by sputtering. The M capping layer was oxidized either by natural oxidation in air or by exposure to an oxygen plasma. By measuring the extraordinary Hall effect in these systems, we observed that the magnetic anisotropy of the Co electrode is extremely sensitive to the degree of oxidation of the capping layer. In particular, when M=Al, the magnetic anisotropy is found in plane when the AlOx layer is under or overoxidized whereas it is out of plane when this oxide layer is stochiometric alumina. The conditions of oxidation which give the perpendicular anisotropy in Pt/Co/AlOx are found to be the same than those which lead to a maximum of specular reflection at Co/AlOx interface in Co/Ru/Co/Cu/Co/AlOx synthetic spin-valves. This crossover of anisotropy therefore provides a very useful way for controlling the oxidation of tunnel barriers. Furthermore, this phenomenon can be used to study the aging of naturally oxidized tunnel junctions exposed to air, and it is also observed with other capping materials such as Ta or Cr.
Gold-coated iron nanoparticles for biomedical applications
Authors: Chen Min , Yamamuro Saeki , Farrell Dorothy , Majetich Sara A.
Published: May 2003

We describe the magnetic properties of gold-coated iron nanoparticles, and the effects of low pH and heat treatment. Acicular iron and spherical iron based nanoparticles were coated with a thin layer of gold. The morphology and magnetic properties of magnetic particles were examined using transmission electron microscopy and alternative gradient magnetometry. While the small spherical particles had relatively uniform layers coatings, the larger acicular particles had many gold clusters decorating the surface. The original acicular iron nanoparticles had a specific magnetic moment of 145 emu/g and a coercivity of 1664 Oe. Corrosion tests showed good corrosion resistance for gold-coated commercial iron particles even in a 1.0×10−3 M HCl solution at 80 °C for 12 h, compared with uncoated particles.
Nonuniform demagnetizing field and magnetization in element of patterned NiFe films
Authors: Zhai Y. , Li J. , Yan Y. , Zhang X. Y. , Lu M. , Zhai H. R. , Shi J.
Published: May 2003

Patterned arrays of micron and submicron rectangular Ni81Fe19 elements studied by ferromagnetic resonance (FMR) previously showed that an in-plane shape anisotropy existed in the element, with two origins. One is contributed from a quasiuniform magnetization described by conventional demagnetizing factors, another is an effect of the non-uniform magnetization in the nonellipsoidal element. The results of micromagnetic numerical calculation provide further evidence of the nonuniform demagnetizing field and nonuniform magnetization in the rectangular elements. The calculated profile of demagnetizing field shows that as the aspect ratio increases the non-uniform demagnetizing field decreases, as the data of in-plane FMR require. When the static field is not along either of the edges the demagnetizing field become more nonuniform. Not only its magnitude but also its direction is nonuniform, which causes nonuniform static and microwave field and thus meets the condition for magnetostatic mode excitation as observed by FMR. When the field is along the film normal, the demagnetizing fields at the film edges differ much from the uniform field in the central portion and the magnetization changes gradually across the side. These meet the requirement of dynamic pinning and the excitation of the spin waves with k vector along the narrow side of the elements that was observed by FMR.
Domain nucleation and growth of La0.7Ca0.3MnO3−δ/LaAlO3 films studied by low temperature magnetic force microscopy
Authors: Liebmann M. , Kaiser U. , Schwarz A. , Wiesendanger R. , Pi U. H. , Noh T. W. , Khim Z. G. , Kim D.-W.
Published: May 2003

The field-dependent domain structure of epitaxial La0.7Ca0.3MnO3−δ thin films grown on a LaAlO3(001) substrate has been studied as a function of film thickness (50 and 100 nm) and oxygen content (optimum and deficient) by means of magnetic force microscopy at 5.2 K. The epitaxially grown films show a stress induced out-of-plane anisotropy. All samples exhibit a maze type domain structure at zero field. Domain size and contrast depend on film thickness. The effect of oxygen content could not clearly been determined. Field-dependent measurements were performed by ramping a perpendicular magnetic field of up to 800 mT continuously during imaging. Domain nucleation and growth takes place by discrete reorientation of regions, which have diameters similar to the final domain width.
Magnetic clusters in Nd1−xSrxMnO3 (0.3⩽x⩽0.5): An electron-spin resonance study
Authors: Angappane S. , Rangarajan G. , Sethupathi K.
Published: May 2003

The paramagnetic electron-spin resonance (ESR) linewidth of polycrystalline Nd1−xSrxMnO3 (x=0.3, 0.33, 0.4, and 0.5) increases in a quasilinear manner up to 400 K due to the formation of paramagnetic clusters, which is also confirmed by the observation of an activated behavior of the ESR intensity above TC. Upon cooling, a pair of ferromagnetic resonance lines appear well above the magnetic transition temperature (TC, TN) in all cases. This suggests the presence of ferromagnetic clusters at a temperature T*>TC. The two-line structure below T* is discussed in terms of phase separation and formation of magnetic clusters and their coexistence with a charge-ordered phase for x>0.4.
Oxidation process of AlOx-based magnetic tunnel junctions studied by photoconductance
Authors: Koller P. H. P. , Vanhelmont F. W. M. , Boeve H. , Coehoorn R. , de Jonge W. J. M.
Published: May 2003

The oxidation process of Co/AlOx/Co magnetic tunnel junctions has been investigated by photoconductance, in addition to traditional transport measurements. The shape of the photoconductance curves is explained within the framework of a simple qualitative model, assuming an oxidation time dependent imbalance of the incident forward and reverse hot electron fluxes, as well as inelastic scattering processes in the oxide. Due to the large sensitivity of the technique, the presence of unoxidized Al beneath the barrier layer can be monitored very accurately. The disappearance of a negative contribution to the photocurrent indicates the complete oxidation of the barrier layer, which coincides with the maximum magnetoresistance. From a Fowler analysis, the barrier height is determined as a function of oxidation time. The observed disagreement of the effective barrier heights determined by this technique and those found by Simmons fits demonstrates the added value of photoconductance studies.
Soft x-ray longitudinal magneto-optical Kerr effect measured from Co/Cu multilayers at the Co L2,3 edges
Authors: Hecker M. , Mertins H.-C. , Abramsohn D. , Gudat W. , Schneider C. M.
Published: May 2003

We report on x-ray magneto-optical reflectivity investigations of Co/Cu multilayers with circularly polarized synchrotron light. The individual layer thicknesses of the samples were tuned to the second maximum of the giant magnetoresistance. An external magnetic field was applied parallel to the sample surface and the scattering plane utilizing the longitudinal magneto-optical Kerr effect. The reflectometry curves measured at the Co L3 absorption edge clearly show antiferromagnetic (AFM) coupling of the Co layers by half-integer-order Bragg peaks of magnetic origin, which disappear for nonresonant excitation. Whereas the first-order magnetic peak overlaps with strong charge scattering contributions, the higher-order magnetic peaks are more pronounced and sensitive to magnetic effects. In an applied field, the magnetic peaks are suppressed due to a reduction of the AFM layer coupling, and an additional intensity modulation occurs due to asymmetry effects. Measurements of the photon energy dependence of the Bragg peak positions were employed close to the Co L2,3 edges to determine the dispersive part of the magneto-optical constants of the Co/Cu multilayer.
Properties of thin IrMn in exchange biased multilayers
Authors: Eckert J. C. , Stern N. P. , Snowden D. S. , Sparks P. D. , Carey M. J.
Published: May 2003

We report striking behavior in thin IrMn exchange-biased heterostructures. We have studied exchange-biased multilayers with IrMn thicknesses from 2 to 80 Å with 50-Å Ti over- and underlayers. The resistance of spin valves with Si/50-Å Ti/40-Å NiFe/8-Å Co/30-Å Cu/30-Å Co/tIrMn/50-Å Ti as a function of temperature shows anomalous behavior for 12 Å<tIrMn<26 Å. These features are not seen for spin valves in which the ferromagnetic layers are replaced with CoFe. To isolate the effects of the IrMn, resistance and magnetization versus temperature for structures of 50 Å Ti/ tIrMn /50 Å Ti were measured. Our measurements are suggestive of a model involving a mixed hcp/fcc phase of IrMn, in which the hcp phase undergoes a magnetic phase transition, and the resistance is influenced by a combination of the magnetic transition and the structural properties of the IrMn layer.
Co layer thickness dependence of exchange biasing for IrMn/Co and FeMn/Co
Authors: Seu K. A. , Huang H. , Lesoine J. F. , Showman H. D. , Egelhoff W. F. , Gan L. , Reilly A. C.
Published: May 2003

We present a high resolution study of the ferromagnetic layer thickness dependence of exchange bias field (HEB) and coercivity (HC) in IrMn/Co and FeMn/Co bilayers using the magneto-optical Kerr effect. Samples are sputtered wedges on silicon with Co thicknesses ranging from 1 to 17 nm. The IrMn/Co (with exchange bias interface energy of ∼0.14 erg/cm2) shows square loops, a smooth increase in HEB with inverse thickness, and a complicated behavior for coercivity, perhaps due to competition with thickness dependent coercive mechanisms. The FeMn/Co (with exchange bias interface energy of ∼0.059 erg/cm2) shows more rounded loops, a plateau of HEB with decreasing thickness, and a smooth increase in coercivity with inverse thickness.
Finite temperature micromagnetics and magnetic measurements of submicron patterned permalloy thin films
Authors: Deak James G.
Published: May 2003

Micromagnetic simulation using the deterministic Landau–Lifshitz–Gilbert (LLG) equation is inadequate for predicting the coercivity of submicron patterned thin films. The discrepancy results because the deterministic LLG equation only provides a zero-temperature description of the magnetization processes of a ferromagnetic material. In order to properly simulate the coercivity, the stochastic LLG equation, which includes thermal effects through a fluctuating magnetic field, must be used. Direct comparison of measurements of the coercivity of arrays of submicron patterned permalloy thin films with simulation has been used to show that the second-order Heun scheme is adequate for this purpose. In addition, the simulated temperature dependence of the magnetization of the patterned bits tracks the measured dependence.
X-ray study of the interdiffusion and interfacial structure in ferrimagnetic Gd1−xCox/Co multilayers
Authors: González J. A. , Andrés J. P. , López de la Torre M. A. , Riveiro J. M. , Hase T. P. A. , Tanner B. K.
Published: May 2003

The strong interdiffusion that takes place in the Gd/Co multilayered system greatly influences the structure, composition, and thus the magnetic properties of the layers. We have conducted grazing incidence x-ray scattering experiments using synchrotron radiation on Gd1−xCox/Co multilayers with x=0, 0.37 and x=0.60 to study the interdiffusion process that takes place and the interfacial structure that results. Probing the scattering vector normal to the surface (specular scattering) showed excellent multilayer growth for all samples. For alloyed Gd samples, good multilayer growth was observed with Co thickness down to below 6 Å. Fits to the reflectivity data show a reduction in the thickness of the Co layer, significantly away from its nominal value. This effect was more exaggerated in the pure Gd sample (x=0). We have successfully modeled this by assuming a significant portion of the Co has interdiffused into the pure Gd layer. Off-specular scattering experiments show that the interface roughness was conformal in all samples and was not related to the interdiffusion processes at the interfaces.
Obliquely sputtered Co/Cr thin film tape for bidirectional recording
Authors: Nguyen L. T. , Lisfi A. , Lodder J. C.
Published: May 2003

We report the growth and properties of a thin film tape, which can be equivalently recorded in both directions. The experimental tape consists of a 20 nm thick Co layer grown on top of a Cr underlayer (120 nm). The two layers were consecutively sputtered at incident angle of 70° at room temperature onto a rotating drum covered with a polymer substrate. In such growth geometry the running direction of drum is perpendicular to the incidence plane of arriving atoms. As a result of this configuration, a medium with a good orientation of the easy axis along the recording direction as well as a high coercivity of 180 kA/m has been prepared. Recording measurements have been carried out and illustrate the bidirectional recording behavior of this experimental tape, which seems to be very promising for high-density tape recording.
Two-band model of spin-polarized tunneling incorporating discrete charging energy
Authors: Wang X. , Jalil M. B. A.
Published: May 2003

Tunneling transport across a double-junction system, consisting of a small magnetic metallic island, coupled to ferromagnetic contacts by tunnel barriers, is studied by incorporating the effects of source–drain Va and gate Vg voltages, and the island charging energy into the model Hamiltonian. The transmission coefficients and current across the double barrier are evaluated using quantum mechanical transfer matrix method. The tunneling J–Va characteristic exhibits a staircase pattern, while the tunneling current oscillates with the gate voltage. The device also exhibits a bias-dependent tunneling magnetoresistance with a peak value exceeding 35%. We attribute these behaviors to the combined effect of spin-polarized tunneling and discrete charging of the island.
Semiclassical theory of spin transport in magnetic multilayers
Authors: Urban R. , Heinrich B. , Woltersdorf G.
Published: May 2003

A semiclassical model of the spin momentum transfer in ferromagnetic film (FM)/normal metal (NM) structures is presented. It is based on the Landau–Lifshitz equation of motion and the exchange interaction in FM, and on the spin diffusion equation in NM. The internal magnetic field is treated by employing Maxwell’s equations. A precessing magnetization in FM creates a spin current which is described by spin pumping proposed by Tserkovnyak et al. The back flow of spins from NM into FM is assumed to be proportional to the spin accumulation in NM as proposed by Silsbee et al. These theoretical calculations are tested against the experimental results obtained by different groups. A good agreement was found for Py/Cu samples, but spin pumping is significantly enhanced in Py/Pt systems.
Terahertz dynamics of photogenerated carriers in ferromagnetic InGaMnAs
Authors: Khodaparast G. A. , Larrabee D. C. , Kono J. , King D. S. , Kato J. , Slupinski T. , Oiwa A. , Munekata H. , Sanders G. D. , Stanton C. J.
Published: May 2003

We have measured the picosecond transient carrier response of InGaMnAs/InP by two-color pump–probe spectroscopy, using an intense near-infrared beam as the pump and a 52.5 μm (5.7 THz) beam as the probe. We observed strongly nonexponential decays, especially at high pump fluences and low temperatures, where a pronounced dip developed in the terahertz differential transmission. This dip disappeared at the highest fluence where a transmission plateau versus time delay was observed. Our band structure calculations suggest that this intriguing behavior may be due to carrier dynamics associated with Γ–L intervalley scattering that becomes efficient when ferromagnetism modifies the band structure.
Effect of pole tip anisotropy on the recording performance of a high density perpendicular head
Authors: Patwari Mohammed S. , Batra Sharat , Victora R. H.
Published: May 2003

A perpendicular recording head for an areal density of 1 Tbit per square inches has been developed using a self-consistent three-dimensional (3D) micromagnetic model. The soft underlayer and the recording layer have been included in the model. The head consists of a probe type tip protruding from a collar. The tip has saturation magnetization (Ms) of 24 kG while the collar has lower Ms of 10 kG. The magnitude and orientation of anisotropy field (Hk) in the tip is varied to obtain the best recording performance. A perpendicularly oriented Hk in the tip reduces flux spreading, thereby enhancing the recording field in the writing track while reducing the offtrack field. However, simulations show that the head’s performance suffers in terms of high remanent field and slower frequency response. Simulations show that a lower remanent field can be achieved by applying ac demag pulse to the tip. A detailed comparison has been made between two cases of perpendicular Hk of 1 kOe and longitudinal Hk of 10 Oe in the tip of the head. Results show a clear tradeoff in terms of recording field and frequency response. Finally, simulations show that the designed head is capable of recording areal density of 1 TBPSI on a recording layer of Ms of 700 emu/cc, thickness of 20 nm, Hk of 18 kOe and average grain diameter of 6 nm.
Energy surface model of single particle reversal in sub-Stoner–Wohlfarth switching fields
Authors: Gao Kai-Zhong , Boerner Eric D. , Bertram H. Neal
Published: May 2003

A simple energy surface model is introduced to explain single domain particle switching under sub-Stoner–Wohlfarth fields. The criterion is that if the initial energy exceeds the subsequent energy barrier maximum, then the particle can reverse. Reversal will occur only for small damping constants and short field rise times, so the magnetization dynamics explores a sufficiently large portion of the energy surface. The results are confirmed by micromagnetic analyses and are consistent with both large scale micromagnetic simulation and experimental data. This effect may be utilized for ultrahigh density (1 Tbit/in2) and ultrahigh data rate magnetic recording (>2 Gbit/s).
Longitudinal write-process modeling including measured angular-dependent coercivity
Authors: Olson Terry , Hsiao Wenchien , Lu Danzhu
Published: May 2003

Recent angular VSM measurements indicate that the remanent coercivity of various types of longitudinal media has an applied field angular dependence that approximates the behavior of Stoner–Wohlfarth particles with easy axes oriented isotropically in-plane. This is in contrast to the Kondorsky style switching (in-plane field, domain wall switching) inherently assumed in most models of the write process. In this article, we incorporate the measured angular dependent switching into a Williams–Comstock style write model. We find that the vertical spacing dependence of the transition width or “a” parameter can be much lower than that expected when only the in-plane component of the head field participates in switching the media. This phenomena is verified experimentally by writing at different spacings and reading at a constant spacing. We find that the experimental data can only be matched if the measured angular dependent switching is taken into account.
Ultrafine NiFe2O4 powder fabricated from reverse microemulsion process
Authors: Fang Jiye , Shama Narayan , Tung Le Duc , Shin Eun Young , O’Connor Charles J. , Stokes Kevin L. , Caruntu Gabriel , Wiley John B. , Spinu Leonard , Tang Jinke
Published: May 2003

NiFe 2 O 4 ultrafine powder with high crystallinity has been prepared through a reverse microemulsion route. The composition in starting solution was optimized, and the resulting NiFe2O4 was formed at temperature of around 550–600 °C, which is much lower than that observed from the solid-state reaction. Magnetic investigation indicates that samples are soft-magnetic materials with low coercivity and with the saturation magnetization close to the bulk value of Ni ferrite.
Properties of epitaxial yttrium iron garnet films grown from BaO flux
Authors: Kučera M. , Nitsch K. , Maryško M. , Štěpánková H.
Published: May 2003

Yttrium iron garnet, Y3Fe5O12, films of excellent purity were grown onto Gd3Ga5O12 substrates by liquid phase epitaxy from a lead-free BaO–B2O3–BaF2 flux. The aim of using the BaO flux was to reduce the content of impurities and intrinsic defects in the garnet films and to obtain samples with minimum microwave losses. The films were characterized by the nuclear magnetic resonance (NMR), ferromagnetic resonance, and parallel pumping method. The NMR spectra demonstrated a considerable reduction of impurity concentration compared to garnet films produced from a conventional PbO–B2O3 flux. Likewise, the NMR and spin wave linewidths of BaO-grown films were very narrow comparable with results obtained on the best (111) films and bulk samples.
Magnetic scattering in Fe–Cr multilayers in the ferromagnetic state at low temperatures
Authors: Patel R. S. , Majumdar A. K. , Hebard A. F. , Temple D.
Published: May 2003

We report here an interpretation of the electrical resistivity (ρ) below 15 K of two Fe–Cr multilayers (30 layers of 20 Å Fe/10 and 12 Å Cr) produced by Xe ion-beam sputter deposition. These multilayers have a negative giant magnetoresistance (GMR) of ∼21% at 4.2 K and Hsat=13 kOe. Very high-resolution resistance measurements (better than 3 ppm) were made down to 2.3 K at every (100±10) mK. Excellent results were obtained when the ρ(T) data in the ferromagnetic state (H=Hsat) were fitted to an equation containing the residual resistivity [ρ(0)], electron–phonon interband s–d scattering (Bloch–Wilson integral) and a magnetic BT2 term, where B is proportional to s–d interaction strength responsible for the electron–magnon s–s scattering. The value of B is found to be typically (9±2)×10−5 μΩ cm K−2 compared to the much smaller value of 1.5×10−5 μΩ cm K−2 for bulk crystalline Fe. The fits without the magnetic term are distinctly inferior as seen from their residuals plotted against T. However, the data in the antiferromagnetic state could be fitted very well with only the lattice term and ρ(0). We have restricted our analysis to T⩽15 K in order to avoid electron–magnon s–d scattering which dominates above 20 K and is very difficult to estimate. Thus magnetic scattering in 3d metals are best interpreted at low temperatures even for these magnetic multilayers.
Magnetoresistance in magnetic multilayers with the nano-oxide layer
Authors: Chen Sui-Pin , Chang Ching-Ray , Lai Chih-Huang
Published: May 2003

From the linear response Boltzmann transport equation in the relaxation-time approximation, we investigate the structure consisting of magnet–metal–magnet multilayers inserted by the nano-oxide layer. Moreover, we propose a modified Boltzmann transport equation with the appropriate boundary conditions in multilayers instead of solving the complicated coupled equations. We find that the nano-oxide layer causes the decrement of the diffusive scattering effect and the increment of the reflective effect, and the magnetoresistance ratios vary with different positions of the nano-oxide layer. The theoretical results are reasonable in agreement with the experimental data.
Effect of Sm–Co layer thickness on magnetic properties of crystallized Sm–Co/Cr films
Authors: Takei Shigeto , Morisako Akimitsu , Matsumoto Mitsunori
Published: May 2003

Sm–Co/Cr film was prepared with substrate heating and the dependence of crystal structure and magnetic properties on the Sm–Co layer thickness was studied. The x-ray diffraction diagram indicated that the Sm–Co layer was crystallized by the substrate heating. The coercivity was higher than 3 kOe even though the thickness of the Sm–Co layer was as thin as 2.5 nm. It was found that the substrate heating during deposition was effective to prepare the films with higher coercivity at ultrathin Sm–Co layer.
Application of magnetic printing method to hard-disk media with double recording layers
Authors: Ono Takuya , Kuboki Yoshiyuki , Ajishi Yoshifumi , Saito Akira
Published: May 2003

The magnetic printing method, which can duplicate soft magnetic patterns containing digital information such as servosignals formed on a master disk onto recording media, enables signals to be written to hard-disk media having high coercivities above 6000 Oe. We propose the application of the magnetic printing method to a hard-disk medium having double recording layers, one layer of which has high coercivity and is to be printed with digital information. This double recording layer medium is a hard-disk medium that has a magnetic read-only-memory (MROM) layer. In this study, we demonstrated a method for printing to this medium, which has MROM, and discussed the magnetic properties and recording performances of this medium.
Effect of microstructure on the oscillating interlayer coupling in spin-valve structures
Authors: Kools J. C. S. , Devasahayam A. J. , Rook K. , Lee Chih-Ling , Mao M.
Published: May 2003

It has been well established that the interlayer coupling in a spin valve is well described as a sum of terms associated with pinholes, magnetostatic (Néel), and oscillating exchange interlayer coupling [(OXC) or Ruderman–Kittel–Kasuya–Yosida]. We experimentally studied the effect of interface roughness on the OXC term. We systematically varied the microstructure by variation of the sputtering geometry, the sputtering pressure, the seed layer and by application of low-energy ion bombardment at the interfaces (“beam treatment”). It is found that smoothening the stacks leads to a stronger OXC, both when suppressing long-range (∼200 Å) and short-range (∼20Å) roughness. Néel coupling on the other hand, is found to be more sensitive to long-range waviness.
Resistance of multilayers with long length scale interfacial roughness
Authors: Alicea Jason , Hershfield Selman
Published: May 2003

The resistance of multilayers with interfacial roughness on a length scale large compared to the layer spacing is obtained using the Boltzmann equation. Both the current-perpendicular-to-plane (CPP) and current-in-plane (CIP) geometries are considered in the limits where the mean-free paths are short and long compared to the atomic spacing. In the short mean-free path limit, the resistance decreases in the CPP geometry and increases in the CIP geometry. In the long mean-free path limit, the resistance increases in both configurations due to enhanced surface scattering. The giant magnetoresistance can either be enhanced or reduced by roughness depending on the sample parameters. Estimates of the short and long mean-free path effects in Fe/Cr multilayers are obtained using experimentally determined parameters.
Tunneling magnetoresistance in ultrathin Co–SiO2 granular films
Authors: Honda S. , Yamamoto Y.
Published: May 2003

Tunneling giant magnetoresistance (MR) properties have been examined for ultrathin Co–SiO2 granular films, forming the pseudo-two-dimensional alignment of Co granules. The resistivity follows well the relation for the tunneling transport between Co granules. The values of resistivity and the activation energy for tunneling depend on both the Co composition and the film thickness, owing to the change in morphological structure. In the 35 vol % Co films of 5 and 10 nm thickness, the MR ratio increased with reducing temperature similar to thick films. On the other hand, in the 28 and 32 vol % Co films thinner than 5 nm, the MR ratio decreased drastically below 100 K with reducing temperature reflecting the two-dimensional alignment of Co granules.
Low temperature deposition of c-axis oriented Sr-ferrite thin films prepared by facing targets sputtering with mixture gas of Ar and Kr
Authors: Nakagawa Shigeki , Mizuno Keisuke , Naoe Masahiko
Published: May 2003

We have succeeded in depositing Sr-ferrite (SrM) thin films with only several nanometer thick with apparent c-axis orientation. Pt underlayer was effective to prepare SrM films with c-axis orientation. Although perpendicular coercivity, Hc⊥, of the Ba-ferrite (BaM) and SrM films prepared by sputtering with Ar decreased in the thickness region below 40 nm, Hc⊥ of SrM films prepared by sputtering with Ar and Kr mixture gas revealed high value around 3 kOe even though the thickness of them were below 40 nm. SrM films prepared by Ar and Kr mixture gas revealed small c-axis dispersion angle, Δθ50, as small as 2°. Remanent magnetization, Mr, of SrM/Pt films was more stable than that of BaM/Pt bilayered films. Sputtering with Ar and Kr mixture gas was effective to decrease the preparation temperature of SrM films as low as 450 °C in as-deposited state.
Active hydrostatic bearing with magnetorheological fluid
Authors: Hesselbach J. , Abel-Keilhack C.
Published: May 2003

Special bearings based on magnetic fluids are well known in literature. These bearings use the magnetic pressure inside a ferrofluid that is exposed to a magnetic field. The biggest disadvantage of this principle is the small load that can be supported. In one reference [B. M. Berkovsky, V. F. Medvedev, and M. S. Krakov, Magnetic Fluids, Engineering Applications (Oxford University Press, Oxford, 1993)], the specific load is specified as 1 N cm−2. To support heavy loads very large support areas are needed. We will present a completely different concept for bearings with magnetorheological fluids. Hydrostatic bearings get their load bearing capacity from the hydrostatic pressure produced by an external pump and should not be confused with hydrodynamic bearings presented in another reference [R. Patzwald, M. S. thesis, Institute für Werkzeugmaschinen und Fabrikbetrieb, Technische Universität, Berlin (2001)]. The main disadvantage of hydrostatic bearings is that the bearing gap varies with the payload. Conventional systems compensate for these variations with a change of the oil flow rate, that is done, for example, by external valves. Our contribution will present a hydrostatic bearing that uses magnetorheological fluids. Due to the fact that magnetorheological fluids change their rheological properties with the change of an external magnetic field, it is possible to achieve a constant bearing gap even if the payload changes. The great advantage of this system compared to valve based systems is the short response time to payload changes, because the active element (i.e., the fluid) acts directly inside the bearing gap, and not outside like in the case of valves.
Precision in situ flying height measurement during track seeking process with intersymbol interference compensation
Authors: Wan Lei , Yuan Zhimin , Liu Bo
Published: May 2003

In order to use the harmonic ratio to measure the flying height with wide usable bands and high signal to noise ratio for ultrahigh density magnetic recording systems, the harmonic selection channel should be made narrow. But intersymbol interference (ISI) could be, therefore, introduced when this setup is used in the track seeking process. As a result, the measurement accuracy deteriorates significantly. Due to the band-limited nature of the harmonic selection channel, a linear equalizer based on the minimum mean-square error algorithm can be adopted to compensate for ISI and the accuracy achievable is 99%.
Magnetic and structural characterization and ferromagnetic resonance study of thin film HITPERM soft magnetic materials for data storage applications
Authors: Okumura H. , Twisselmann D. J. , McMichael R. D. , Huang M. Q. , Hsu Y. N. , Laughlin D. E. , McHenry M. E.
Published: May 2003

HITPERM/SiO 2 single and multilayer thin films have been produced using a target composition of (Fe70Co30)88Zr7B4Cu1. The as-deposited HITPERM film contains small bcc (or B2) nanocrystals of volume fraction less than 10% surrounded by an amorphous matrix. The lattice parameter of the nanocrystal is about 5% larger than an equilibrium FeCo phase. The saturation induction determined from FMR measurements (1.53±0.08 T) is consistent with VSM and SQUID measurements (1.45–1.5 T) and also with as-spun amorphous ribbons (1.55–1.62 T). The Landé g-factors (2.15±0.05) are typical of transition metals, particularly, of Fe. The Landau–Lifshitz–Gilbert damping parameters of the single and multilayered films are small (α = 0.0055±0.0004) with each layer acting almost independently. Neither thickness variation of each layer nor the number of stacking significantly affects the damping process in a range of film thicknesses of 50–150 nm, while the coercivities are strongly dependent on those parameters. This supports a notion that the damping parameter is an intrinsic property.
Accurate analysis of the magneto-optical permittivity tensor of Y3Fe5O12
Authors: Allen Gary A. , Dionne Gerald F.
Published: May 2003

The permittivity tensor elements ε0 (diagonal) and ε1 (off-diagonal) elements of Y3Fe5O12 (YIG) were analyzed as a function of frequency to gain insight into the cause of electric dipole transitions that produce Faraday or Kerr effects in a magnetized state. A continuous set of spectral measurements of the reflection Kerr ellipticity and rotation were carried out with high density ceramic specimens utilizing a photoelastic modulated ellipsometer across the visible band from 2.0 to 4.0 eV. The spectra of ε0 and ε1 deduced from the measurements were then fitted to select individual transitions. From the line shapes, it was determined that the ε1 transitions responsible for the Kerr effects are of the orbitally split ground state (paramagnetic) type, despite the fact that the pure ground state of the Fe3+ ion has no orbital magnetic moment. From the results of the analysis, quantitative estimates from a molecular-orbital model are offered for the magnitude of spin-orbit splitting of the Fe3+ S6 ground states hybridized covalently with Fe3+ T1g4 excited states at neighboring sites of the opposing magnetic sublattice.
Evolution of stress with L1 ordering in FePt and FeCuPt thin films
Authors: Wierman K. W. , Platt C. L. , Howard J. K. , Spada F. E.
Published: May 2003

The evolution of stress in 50 nm FexPt1−x (x=0.41, 0.43, 0.49, 0.52, and 0.56) sputtered films was monitored in situ as a function of the temperature. In the as-deposited state, films have a disordered face-centered-cubic (fcc) structure with a strong (111) fiber texture and 685–978 MPa compressive stress. Below 200 °C, thermal expansion of the FexPt1−x fcc lattice produced an initial increase in residual in-plane compressive stress. Above 250 °C, a transition to a low stress state was observed, with the relaxation in the Fe0.52Pt0.48 film showing stronger temperature dependence. Comparisons of stress to a sputtered film of Fe0.35Cu0.15Pt0.5 indicate a faster transition rate to the low stress state can be achieved with the addition of Cu. In situ high temperature x-ray diffraction measurements show a decrease in (111) d spacing with an increase in temperature, and are consistent with the changes observed in the residual stress measurements. The large stress transition appears to arise from changes in the fcc phase prior to L10 phase transformation, because a substantial increase in film coercivity appears only after the transition to the low stress state has been completed.
Effect of stress applied on the magnetization profile of Fe–Si–B amorphous wire
Authors: Zhukova V. , Zhukov A. , Blanco J. M. , Gonzalez J. , Gómez-Polo C. , Vázquez M.
Published: May 2003

The effect of tensile stress, σ, on hysteresis loops and magnetization profiles of Fe77.5B15Si7.5 amorphous wires has been measured. Measurements performed by a short movable coil permit one to obtain the dependence of remanent magnetization, μ0Mr, on the position of this coil, L. Increasing applied stress in tensile stress is applied, the initially rectangular local hysteresis loop measured for L=4 cm (at ℓ=8 cm) becomes inclined, but then recovers its rectangular shape for larger stress. The μ0Mr(σ) dependence shows an increase with σ for all L. The stress dependence of both μ0Mr and H* can be understood in terms of a competition between magnetostatic and magnetoelastic energy.
Magnetic properties of FeCuNbSiB nanocrystalline alloy powder cores using ball-milled powder
Authors: Kim G. H. , Noh T. H. , Choi G. B. , Kim K. Y.
Published: May 2003

Cold-pressed nanocrystalline powder cores were fabricated using powders of nanocrystalline ribbons which were ball milled for short time. Their magnetic properties at high frequency were measured. The powder size ranges from 20 to 850 μm and the contents of the glass binder are between 1 and 8 wt %. For cores composed of large particles of 300–850 μm with 5 wt % glass binder, we obtained a stable permeability of 100 up to 800 kHz, a maximum level 31 of quality factor at frequency of 50 kHz, and 320 mW/cm3 core loss at f=50 kHz and Bm=0.1 T. This is mainly due to the good soft magnetic properties of the powders and the higher insulation of powder cores which cause low eddy current losses.
Magnetic behavior of iron-oxoclusters prepared in an organosilica sol–gel matrix
Authors: Dubenko I. S. , Rao M. S. , Roy S. , Dave B. C. , Ali N.
Published: May 2003

The crystal structure and magnetization of nanoscale enTMOS–Fe2O3 sol–gel composites with weight iron concentration x, varying from 0.003 to 0.065, have been studied by the transmission electron microscopy technique and a superconducting quantum interference device magnetometer. The clusters are crystallized in a hexagonal crystal structure. All the samples demonstrate a superparamagnetic behavior with antiferomagnetic cluster–cluster coupling at low temperature. The effective paramagnetic moment, μeff, has been found to vary in the range from 5.9 (S=5/2) to 2.5 μB per iron ion. The concentration dependence of the μeff shows a minimum for x∼0.01. At a low iron concentration x<0.01, μeff is practically independent of x and equals about 6 μB per Fe ion. The concentration interval 0.01<x<0.07 is characterized by a monotonical increase of μeff from 2.5 to about 3 μB per Fe ion. Thus, an abrupt variation of μeff (about two times) is observed at x≈0.01. It has been shown that such behavior can be caused by competition between the uncoupled “surface” and antiferromagnetically coupled “bulk” Fe magnetic moments.
Continuous thin barriers for low-resistance spin-dependent tunnel junctions
Authors: Wang Jianguo , Liu Yaowen , Freitas P. P. , Snoeck E. , Martins J. L.
Published: May 2003

The occurrence of pinholes in thin barrier low-resistance junctions degrades the TMR signal and increases the coupling field between pinned and free layers. The tunnel junction coupling field (Hf), junction resistance and TMR signal dependence on the barrier thickness was studied for various types of barriers (HfOx,HfAlOx,ZrAlOx,AlOx). Micromagnetic simulation was employed to simulate the coupling field versus pinhole density. From the coupling field results, HfOx makes the thinnest continuous barriers, followed by doped HfAlOx and ZrAlOx, and then AlOx. HfAlOx and ZrAlOx offer the best compromise between low resistance (1–5 Ω μm2) and reasonable TMR (12%–14%). Pure HfOx can be made with R×A products of 0.4 Ω μm2 but the TMR does not exceed 5.5%.
On the shape optimization of magnetic random access memory element design
Authors: Zhu Xiaochun , Zhu Jian-Gang
Published: May 2003

In designing the shape of magnetic random access memory elements, a sufficient difference in switching current threshold between a full-select element and a half-select memory element is critical. In this article, we present a systematic micromagnetic study of the margin of switching threshold for two specific shapes: eye shaped and ellipse. It is found that at small magnetic thickness, the eye-shaped element exhibits a higher switching threshold margin than the ellipse. However, for relatively thick storage layers, the opposite becomes true. The switching threshold is also a strong function of the initial magnetization state for the ellipse.
Structure and thermomagnetic properties of polycrystalline Ni–Mn–Ga thin films
Authors: Castaño F. J. , Nelson-Cheeseman B. , O’Handley R. C. , Ross C. A. , Redondo C. , Castaño F.
Published: May 2003

The structure and thermomagnetic properties of pulsed laser deposited polycrystalline Ni–Mn–Ga alloy films, grown onto thermally oxidized Si(100) wafers, have been investigated. The 300-nm-thick films were deposited at substrate temperatures between 500 and 600 °C from targets made from slices of a Ni2MnGa single crystal. The trends in the average composition suggest that samples deposited around 500 °C are composed of the Ni2MnGa (L21) phase as well as a nonmagnetic B2 phase. On increasing the temperature of the substrate, the Mn content as well as the saturation magnetization increase suggesting the appearance of a different magnetic phase that is tentatively ascribed to the Ni3(MnGa) (L11) magnetic phase.
Magnetic and structural properties of nonstoichiometric Ni2MnGa alloys with Ni and Ga excess
Authors: Golub Vladimir O. , Vovk Andriy Ya. , O’Connor Charles J. , Kotov Vitaliy V. , Yakovenko Peter G. , Ullakko Kari
Published: May 2003

The influence of nickel and gallium excess on local structure and magnetic properties of Ni–Mn–Ga alloys has been studied. Nuclear magnetic resonance experiments showed that excessive Ni occupies Mn and Ga positions. This leads to the appearance of low-frequency line from the nearest Mn55 nuclei and generation of fractional nuclear echo signals due to the increase of electrical field gradients on these nuclei. Magnetic measurements revealed the difference of Curie temperature determined from ac susceptibility and extrapolated from high-temperature magnetization behavior. The most probable explanation of this fact is the reduction of manganese–manganese indirect exchange via the faults in Mn–Ga layers interchange caused by excessive Ga.