bluuebunny/crossref_metadata_2025 huggingface dataset:
Thermal spin excitations in epitaxial Fe nanostructures on GaAs(001)
Authors: Kipferl W. , Dumm M. , Rahm M. , Bayreuther G.
Published: May 2003

Thermal spin excitations in confined ferromagnetic structures become increasingly important, e.g., because they reduce tunnel magnetoresistance in highly integrated magnetic memories and the stability of stored information. Here, the effect of lateral confinement on the temperature dependence of magnetization in ultrathin films was studied. Epitaxial Fe films were grown on GaAs(001) by molecular beam epitaxy. Patterning into dot arrays with several million dots of well defined circular shape was accomplished by electron beam lithography, lift-off, and ion beam etching. The magnetic properties of the samples were investigated by superconducting quantum interference device magnetometry between 10 and 350 K. All films—in addition to the fourfold magnetocrystalline anisotropy—have an in-plane uniaxial magnetic anisotropy with the easy axis along [110], which is fully conserved during patterning. The temperature dependence of the spontaneous magnetization for T<0.5TC can be well described by Bloch’s law, MS(T)=M(0)(1−BT3/2), for all samples. For a dot diameter of 500 nm the spin wave parameter B is significantly increased compared to the extended 14 ML film, which in turn shows about twice the bulk value of BFe=5×10−6 K−3/2. The enhancement of spin wave excitations with decreasing film thickness and lateral dimension is discussed in comparison to existing theories and model simulations.
Flux measurements and modeling of the magnetic hysteresis for a zero-voltage switching dc-to-dc converter
Authors: Hauser H. , Himmelstoss F. , Wurm P.
Published: May 2003

A half-bridge bidirectional step-up dc/dc converter with zero voltage switching is introduced. The losses in the semiconductors are reduced in this concept but the higher current ripple leads to increased losses in the MnZn ferrite inductor. The resulting unsymmetric hysteresis including minor loops due to switching oscillations is predicted using reversible and irreversible energy contributions.
Proximity effects at epitaxial Co/FeMn thin film systems (invited)
Authors: Matthes F. , Rzhevskii A. , Tong L.-N. , Malkinski L. , Celinski Z. , Schneider C. M.
Published: May 2003

We prepared exchange-biased Co/FeMn structures to examine the interaction between single crystalline ferromagnetic and antiferromagnetic films with well-defined interfaces. Using x-ray magnetic circular dichroism (XMCD) and magneto-optical Kerr effect we studied in situ layered systems of fcc Co and FeMn epitaxially grown onto Cu(001) templates. XMCD studies reveal that in the FeMn layer a certain fraction of Fe is polarized at the Co/FeMn interface. The magnetic moment on the Fe atoms is parallel to the Co magnetization direction. Compared to Fe the measured dichroic signal for Mn was considerably smaller. Experiments with epitaxially grown trilayer systems of Co/FeMn/Co showed that the proximity of the ferromagnetic material influences the properties of the antiferromagnetic material leading, e.g., to a reduction of the temperature at which a marked increase of the coercive field occurs. Strong ferromagnetic and 90° coupling between the Co films observed at different thicknesses of the FeMn spacer do not affect the proximity effect at the antiferromagnet/ferromagnet interface.
Dependence of exchange coupling on NiO grain size in NiO/NiFe bilayers
Authors: Nam Chung-Hee , Cho B. K. , Lee Seonghoon
Published: May 2003

The texture and grain-size effects on the exchange bias in sputtered polycrystalline NiO/NiFe bilayers were studied. Two oriented antiferromagnetic NiO layers along (111) and (200) planes were fabricated on SiO2/Si(100) substrates by varying the Ar/O2 ratio. An exchange anisotropy field Hex was detected in both NiO/NiFe bilayers with a NiO(111) plane where Ni moments are in ferromagnetic (FM) order, and with a NiO(200) plane where Ni moments are in fully compensated antiferromagnetic (AF) order. In order to clarify the presence of the Hex in a NiO(200)/NiFe bilayer, we prepared NiO(200) layers with different grain sizes by controlling a total pressure at a constant Ar/O2 ratio in a sputter chamber. We observed that the Hex of the bilayer films with small grains of NiO(200) is larger than the Hex with large grains. This observation is consistent with a model that the exchange interaction is caused by the reorientation of the moments with AF layer spins rotating, rather than FM layer spins rotating at the interface of the bilayer.
Abnormal temperature dependence of exchange bias in the NiFe5/Ta0.2/IrMn8 system
Authors: Li Kebin , Guo Zaibing , Han Guchang , Qiu Jinjun , Wu Yihong
Published: May 2003

Exchange bias (Hex) between NiFe and IrMn across a spacer layer such as Ru, Cr, Cu, and Ta has been studied. Hex is almost destroyed when a 0.2-nm-thick Ta layer is inserted between NiFe and IrMn. Abnormal temperature (T) dependence of Hex (there is two magnetization hysteresis loops with different blocking temperatures; the T dependence is neither linear nor nature of Fermi-Dirac statistics, it is an exponential decay function) has been observed in the NiFe/Ta/IrMn system. The abnormal hysteresis loops are associated with two antiferromagnetic (AFM) phases formed due to interface diffusion after magnetic annealing. The exponential decay Hex with temperature is due to complicated coupling mechanisms involved in the system. It is suggested that the strength of the exchange bias should be the nature of long-range interaction while the blocking temperature of the system should be mainly determined by the structure of the FM/AFM interface.
Fe–Co–V alloy with improved magnetic properties and high-temperature creep resistance
Authors: Liu S. , Bauser S. , Turgut Z. , Coate J. , Fingers R. T.
Published: May 2003

Advanced power systems require soft magnetic materials with a combination of high saturation magnetization and high creep resistance. When the Fe–Co–V alloy laminate is used in a rotor at high temperatures (500–600 °C) coupled with very high rpm, significant creep occurs, which destroys the device integrity. Since grain boundary slide is predominantly responsible for creep deformation in the Fe–Co–V alloy at temperature higher than ∼430 °C, the approach in this study was to reduce the volume fraction of the grain boundaries by making a Fe–Co–V alloy with very large grains. Very large grains, up to mm range, were readily obtained after small cold deformation of ∼3% followed by a normal recrystallization anneal. Fe–Co–V alloy with large grains displays lower coercivity and higher permeability than the commercial Fe–Co–V alloy. Even though its yield strength at 600 °C is lower than the commercial Fe–Co–V, the creep strains of the Fe–Co–V alloy with large grains are only 1/10–1/2 of that for the commercial alloy in the initial and middle periods of the creep test performed at 600 °C under 150 MPa. At the end of the test before the specimens were broken, the Fe–Co–V alloy with large grains showed an accelerated creep strain rate. Better balancing the high-temperature yield strength and creep resistance by creating the optimum grain size would further improve the high-temperature creep resistance of the Fe–Co–V alloy.
Conducted noise suppression effect up to 3 GHz by NiZn ferrite film plated at 90 °C directly onto printed circuit board
Authors: Kondo K. , Chiba T. , Ono H. , Yoshida S. , Shimada Y. , Matsushita N. , Abe M.
Published: May 2003

A NiZn ferrite film (3 μm thick) was deposited at 90 °C by the spin-spray ferrite plating from an aqueous solution onto a 50 Ω microstrip line formed on an epoxy printed circuit board (PCB). A strong magnetic loss was caused by the ferrite film in a GHz range, ΔPloss reaching 67% attenuation at 3 GHz, the upper limit of our measurement. Furthermore, the reflection loss was very weak, S11 being smaller than 7%. Thus plated NiZn ferrite films hold strong promise to be actually applied to a type of thin film electromagnetic noise suppressors; the films can be directly deposited onto noise sources (semiconductor elements or electronic circuits) to attenuate conducted-electromagnetic noises in the GHz range. Because the plated NiZn ferrite film was magnetically isotropic in film plane, the noise suppressors will be isotropic, attenuating noise electromagnetic waves radiated from any directions. The NiZn ferrite film was also plated on a flat glass substrate as a standard, which exhibited natural resonance frequency of fr=500 MHz and initial real permeability of μ′=50. Compared to these values, the film on the PCB had higher fr of 850 MHz, though reduced in μ′ to 40. But the spectrum of the imaginary permeability μ″ shifted to a higher frequency range. This facilitated the strong magnetic loss at the high frequencies. The higher fr for the film on the PCB may be attributed to the undulated columnar structure of the film which was observed by scanning electron microscopy.
Fabrication of L1 ordered FePt alloy films by monatomic layer sputter deposition
Authors: Shima T. , Moriguchi T. , Seki T. , Mitani S. , Takanashi K.
Published: May 2003

L 1 0 ordered FePt alloy films with perpendicular magnetic anisotropy have been prepared by alternating Fe (001) and Pt (001) monatomic layers on MgO (001) substrates at low temperatures below 300 °C. In addition to molecular beam epitaxy (MBE) technique reported so far, ultra high vacuum compatible dc magnetron sputtering was used for the sample preparation, and the properties for sputtered films are compared with those for MBE films. When the Ar gas pressure during sputtering is equal to or higher than 5 mTorr, sharp (001) and (003) superlattice peaks have been observed in the x-ray diffraction (XRD) patterns, and the magnetization measurements show that the films are perpendicularly magnetized. However, the uniaxial anisotropy energy Ku for sputtered films is smaller than that for MBE films even when the long range order parameter estimated from the intensity ratio of the superlattice to fundamental XRD peaks is almost the same in both cases.
Evolution of in-plane magnetic anisotropy in sputtered FeTaN/TaN/FeTaN sandwich films
Authors: Nie H. B. , Ong C. K. , Wang J. P. , Li Z. W.
Published: May 2003

FeTaN/TaN/FeTaN sandwich films, FeTaN/TaN, and TaN/FeTaN bilayers were synthesized by using rf magnetron sputtering. The magnetic properties, crystalline structures, microstructures, and surface morphologies of the as-deposited samples were characterized using angle-resolved M–H loop tracer, VSM, XRD, TEM, AES and AFM. An evolution of the in-plane anisotropy was observed with the changing thickness of the nonmagnetic TaN interlayer in the FeTaN/TaN/FeTaN sandwiches, such as the easy-hard axis switching and the appearing of biaxial anisotropy. It is ascribed to three possible mechanisms, which are interlayer magnetic coupling, stress, and interface roughness, respectively. Interlayer coupling and stress anisotropies may be the major reasons to cause the easy–hard axis switching in the sandwiches whereas, magnetostatic and interface anisotropies may be the major reasons to cause biaxial anisotropy in the sandwiches, in which magnetostatic anisotropy is the dominant one.
Orbital order and anisotropic transport properties in doped manganites induced by epitaxial coherency strain
Authors: Klein J. , Philipp J. B. , Reisinger D. , Opel M. , Marx A. , Erb A. , Alff L. , Gross R.
Published: May 2003

Epitaxial coherency strain is a crucial parameter strongly influencing the complex interplay between structure as well as spin, charge, and orbital degrees of freedom in the doped manganites. We have grown epitaxial manganite thin films on various substrates using ultrahigh vacuum laser molecular beam epitaxy with RHEED control. In order to study transport perpendicular to the film plane, mesa structures have been fabricated into the epitaxial films. For coherently strained La2/3Ca1/3MnO3 and La2/3Ba1/3MnO3 thin films grown on SrTiO3 and NdGaO3 substrates, an unusual transport anisotropy is observed. Whereas metallic behavior is found within the plane of biaxial coherency strain, for transport perpendicular to this plane an insulating behavior and nonlinear current–voltage characteristics are observed. This behavior can be interpreted in terms of a strain induced orbital order effect at fixed doping. Our results demonstrate the effect of epitaxial coherency strain in heterostructures required for magnetoelectronic devices.
Mapping spatial variations of the coercivity on patterned magneto-optical materials
Authors: Wu Te-ho , Ye L. X. , Lee J. M. , Wu J. C. , Shieh Han-Ping D.
Published: May 2003

The spatial variation of coercivity on patterned magneto-optical (MO) materials has been studied with a Kerr microscope. Our results show that the coercivity values on arrays made by holes are larger than those observed on the land regions. In contrast with nonpatterned MO media, which present a sharp Gaussian-like distribution, the coercivity distribution in the patterned material is much wider and shows an exponential-type distribution. This article maps the spatial variation of the coercivity on patterned magnetic materials with a spatial resolution of 0.1×0.1 μm2.
Enhanced magnetoresistance in nanocrystalline magnetite
Authors: Venkatesan M. , Nawka S. , Pillai S. C. , Coey J. M. D.
Published: May 2003

The magnetic and magnetotransport properties of nanoparticulate magnetite with different grain sizes are investigated using x-ray diffraction, microscopy, magnetometry, and magnetoresistance measurements. The magnetization varies significantly with grain size and is sensitive to preparation conditions. The reduction in saturation magnetization in coprecipitated particles is probably due to the surface spin disorder. Magnetoresistance of pressed powder compacts is significantly enhanced in material composed of small grain size magnetite particles prepared by coprecipitation. Useful magnetoresistance persists well above room temperature in sintered ceramic material.
SrRuO
3
/
SrTiO
3
/
SrRuO
3
heterostructures for magnetic tunnel junctions
Authors: Herranz G. , Martı́nez B. , Fontcuberta J. , Sánchez F. , Garcı́a-Cuenca M. V. , Ferrater C. , Varela M.
Published: May 2003

We report on the growth and characterization of SrRuO3 single layers and SrRuO3/SrTiO3/SrRuO3 heterostructures grown on SrTiO3(100) substrates. The thickness dependence of the coercivity was determined for these single layers. Heterostructures with barrier thickness tb=1, 2.5, and 4 nm were fabricated, with electrodes having thickness ranging from 10 to 100 nm. The hysteresis loops of heterostructures with tb=2.5 nm, 4 nm reveal uncoupled magnetic switching of the electrodes. Therefore, these heterostructures can be used for the fabrication of magnetic tunneling junctions.
Media background effect on the performance in perpendicular recording
Authors: Zhu Wenzhong , Kaiser David , Judy Jack , Palmer Dean
Published: May 2003

The worst case bit error rate performance degradation due to the effect of stray fields from the recording layer on the reading and writing processes has been measured for a perpendicular recording system. The system consists of a single pole writer with a conventional spin-value reader and a double layer recording medium. A new method is developed to distinguish the reading and writing effects separately by adjusting the write current time asymmetry. Both effects have also been measured by using the dipulse extraction method. The data show that the measured writing effect is correlated to the value which one gets from adjusting write current time asymmetry.
Finite element study of the effect of permeable media on longitudinal writer field calculations
Authors: Zhou Yuming , Giusti Jim , Fernandez-de-Castro Juan
Published: May 2003

The effect of permeable media on writer field calculations was examined in this study via a finite element analysis in which the nonlinear characteristics of both the writer and the permeable media were represented. The applied field of the writer was calculated for the two cases: one without a permeable media and the other with a permeable media adjacent to the gap of the writer. No significant difference was found between the applied fields of the two cases, indicating that the demagnetizing fields present in typical longitudinal media are not large enough to perturb the writer pole magnetization, and thus the writer field during writing. The results validated the common practice of using applied fields directly from writer simulations for write process analysis in longitudinal recording.
High-sensitivity in-plane vector magnetometry using the alternating gradient force method
Authors: Thomas Luc , Rahmani Anas , Renaudin Patrice , Wack André
Published: May 2003

The alternating gradient force magnetometer is a highly sensitive tool particularly suited for thin films magnetometry. The measurement technique is based upon the alternating force generated on a magnetized sample by a set of field-gradient coils. The so-induced sample oscillation is directly proportional to the sample’s magnetization. High sensitivity measurements are achieved by mounting the sample at the end of a cantilever attached to a piezoelectric bimorph element, and by tuning the excitation frequency close to the mechanical resonance of the sample-cantilever assembly. Here we describe a new design that allows to measure both in-plane components of the magnetization of a thin film sample, for any direction of the external magnetic field within the sample’s plane. By rotating the sample-probe assembly, we find the output signal to be proportional to the projection of the alternating force along the sense axis of the piezoelectric bimorph. Besides, the resonance frequency of the system remains unchanged. Thus, hysteresis loops can be measured accurately for various angles between the applied field and an in-plane anisotropy axis. The signal only vanishes when the alternating force is orthogonal to the bimorph axis. Moreover, we have designed a set of two pairs of gradient coils, whose axis are orthogonal to one another. By varying the excitation current within these two pairs of coils, it is possible to rotate the alternating gradient direction, to detect magnetization components along or perpendicular to the external field.
Permeability tensor of magnetized ferrites at microwave frequencies: A comparison between theory and experiment
Authors: Mallégol Stéphane , Quéffélec Patrick , Le Floc’h Marcel
Published: May 2003

The permeability tensor of partially magnetized ferrites or composite materials, given by a recent mathematical model developed in our laboratory, is compared with measured one over the 50 MHz–6 GHz frequency range. The characterization method used to measure the tensor components μ, κ is based on the broadband determination of the S parameters of a nonreciprocal strip transmission line. The quasistatic electromagnetic analysis of the measurement cell permits the expression of μ and κ analytically from the S parameters. The theoretical approach used to calculate μ and κ is based on the extension of the effective medium approximation (EMA) to the case of anisotropic magnetic materials. The interactions between magnetic domains and the hysteresis phenomenon are taken into account in our calculations. The good agreement observed between simulated and measured μ and κ data for polycrystalline ferrites and ferrimagnetic or ferromagnetic loaded composites provides an experimental validation of the model.
Preparation and characterization of MnZn–ferrite nanoparticles using reverse micelles
Authors: Morrison S. A. , Cahill C. L. , Carpenter E. E. , Calvin S. , Harris V. G.
Published: May 2003

Research on manganese zinc ferrites (MZFO) has undergone a renewal in recent years as advances in synthetic techniques promise smaller grain sizes and corresponding changes in material properties. Current techniques for nanoscale synthesis of ferrites, however, produce a broad distribution of particle sizes, thus limiting the density of compacted materials, and consequently altering coercivity [C. Rath et al., J. Appl. Phys. 91, 2211 (2002)]. To minimize porosity, bulk materials need to be pressed from uniform particles. Wet chemical synthesis performed in reverse micelles, in which pools of water are encased by surfactant molecules in an excess volume of oil, provides the greatest control over size and morphology. During synthesis, surfactant molecules keep particles separated and restrict particle growth. This affords greater control over the size and shape of the particles grown in the micelles and commonly results in highly uniform morphologies [J. P. Chen et al., J. Appl. Phys. 76, 6316 (1994); C. Liu et al., J. Phys. Chem. B. 104, 1141 (2000)]. As a first step, it is necessary to produce pure phase, nanosized ferrite particles, therefore in this study, analysis of the powder of a sample prepared by a reverse micelle technique is compared to a sample prepared by a traditional ceramic method. Future studies will focus on the porosity and subsequent material properties of compacted forms of the pure phase samples.
Multilayer configuration for experiments of spin precession induced by a dc current
Authors: Berger L.
Published: May 2003

Until now, most predictions about spin precession induced by a CPP dc current have concerned a simple “asymmetric” configuration consisting of a free magnetic layer and of a thicker pinned magnetic layer. In the present work, we propose a different configuration where the precessing free magnetic layer is sandwiched between two thicker pinned magnetic layers having opposite magnetizations. In this “antisymmetric” configuration, the spin current and accumulation arising from expansion/contraction are nearly three times as large as in the asymmetric configuration, for given current and layer thickness. Moreover, both interfaces of the free magnetic layer are now active to generate drive torques. This should result in a reduction of the critical current density needed for spin oscillations, by a factor of about six.
Role of oxygen exposure in different positions in the synthetic spin valves
Authors: Li Kebin , Han Guchang , Qiu Jinjun , Luo Ping , Guo Zaibing , Zheng Yuankai , Wu Yihong
Published: May 2003

The magnetoresistance (MR) properties of a basic Ta/NiFe/IrMn/CoFe/Ru/CoFe/Cu/CoFe/Cu/Ta synthetic spin valve (SV) system have been systematically studied by using O2-soaked layers in five different positions. It is found that about 10% of MR ratio enhancement can be achieved without sacrificing other merits of SVs when the O2-soaking dose is controlled in the vicinity of 10−3 Pa s. It is probably due to improvement of the spin-dependent scattering (increasing the spin-dependent transmission coefficient and reducing the diffusion scattering coefficient) after O2 soaking either within the pinned layers or within the spacer layer.
Numerical analysis of narrow-track single-pole-type head with side shields for 1 Tb/in.2
Authors: Kanai Yasushi , Mohammed Osama A. , Matsubara Ryo , Muraoka Hiroaki , Nakamura Yoshisha
Published: May 2003

To improve the performance of perpendicular magnetic recording using a single-pole-type (SPT) head and double-layered medium, a finite-element method simulation has been carried out with a calculation model for 1 Tb/in.2. Additional side shield poles have been introduced to reduce the side-writing field while maintaining the recording field strength. The side-writing ratio, assumed to be (field strength at adjacent track center)/(field strength at home track center), was 0.31 without the side shields. The optimized side shield allows this to be reduced to 0.11. A much narrower off-track field distribution than with conventional heads was obtained with the SPT head.
Stray field sensitivity of perpendicular recording heads
Authors: Shukh Alexander
Published: May 2003

An effect of stray field on perpendicular head performance was studied by three-dimensional finite-element method modeling. Perpendicular writers in a drive are exposed to the stray field up to 20 Oe, which generates an erase field in double layer media. The coefficient of a stray field magnification by writer can be of several dozens. Main and return poles of the writer generate the same polarity erase field in the media but the field is stronger under the main pole due to considerably smaller pole size at an air-bearing surface (ABS). The erase field depends on parameters of the media such as keeper permeability μK and head-keeper spacing (HKS) but is not sensitive to keeper thickness. The erase field exhibits a maximum at μK≈20 and decreases with μK or HKS increase. An incident angle of the stray field relatively to a medium plane substantially affects the field strength exhibiting a maximum at about 30° from normal for conventional single pole writer. The recess of the return pole from the ABS causes a strong increase of the erase field at the main pole. The field more than doubles when the return pole has 0.5 μm recess. The erase field can be controlled by writer design optimization. Simulation of stray field sensitivity and efficiency for a conventional single pole and cusp-field writers and their modifications has shown that the cusp-field writer has the lowest sensitivity to the stray field but slightly lower efficiency than conventional writer.
Coercivity enhancement by Ru pinning layer in FePt thin films
Authors: Zhao Z. L. , Ding J. , Chen J. S. , Wang J. P.
Published: May 2003

The face-centered-tetragonal-(001) FePt thin films were prepared by dc magnetron sputtering Fe50Pt50 layer onto Cr91Ru9 underlayer at 400 °C. In order to enhance the coercivity and to obtain a relative lower exchange coupling medium with nucleation reversal mechanism, a nonmagnetic Ru pinning layer inserted in the magnetic layer is described to introduce controllable defects to impede domain-wall motion. A 60% increasing of the out-of-plane coercivity was observed after 1 nm Ru nonmagnetic pinning layer added between the FePt magnetic layers. From angular dependence of the coercivity measurement, the magnetization reversal mechanism was observed to shift from the typical domain-wall motion behavior to the nucleation mode by increasing the thickness of the pinning layer.
Studies of magnetic properties of the stabilizing layer for synthetic antiferromagnetically coupled media
Authors: Shan Z. S. , Bitman L. , Malhotra S. S. , Stafford D. , Bian B. , Bertero G. , Xu Y. F. , Yan M. L. , Sellmyer D. J.
Published: May 2003

The effects of the stabilizing layer thickness and its temperature dependence on the magnetic properties were investigated experimentally. These results were used to analyze the magnetic structure of the thin stabilizing layer and its effect on the coupling strength, which is valuable for improving the design of synthetic antiferromagnetically coupled media.
Effects of Pt content on the microstructure and magnetic properties of CoTbPt thin films
Authors: Kuo P. C. , Lie C. T. , Chen S. C. , Chou C. Y. , Wu T. H. , Chang L. Y.
Published: May 2003

This work presents a single-layer CoTbPt film for use as a hybrid-recording medium. Co77.5−xTb22.5Ptx films with x=0–14 at. % are fabricated on glass and naturally oxidized silicon wafer substrates by dc magnetron sputtering. The magnetic film is sandwiched between SiNx protective layers to prevent oxidization. The effects of Pt content on the magnetic properties and microstructure of the film are investigated. X-ray diffraction and transmission electron microscope diffraction patterns revealed that all the films are amorphous. It shows that substituting a small amount of Co for the nonmagnetic element Pt will increase the saturation magnetization, the perpendicular remanence, and the perpendicular coercivity of the Co77.5Tb22.5 film. It is found that the saturation magnetization of the Co75.3Tb22.5Pt2.2 film is about 245 emu/cm3, the perpendicular squareness is about 0.8, perpendicular coercivity is about 2100 Oe at room temperature and the coercivity decreases rapidly with increasing temperature. The coercivity is decreased to about 50 Oe when temperature is increased to 200 °C.
Mn–Zn spinel ferrite thin films prepared by high rate reactive facing targets sputtering
Authors: Nakagawa Shigeki , Saito Shunsuke , Kamiki Taro , Kong Sok-Hyun
Published: May 2003

Mn–Zn ferrite thin films were deposited by sputtering a Mn0.6Zn0.4Fe2 alloy target, which was fabricated by powder metallurgy, in a mixture gas of Ar and O2. A 12 nm thick Pt underlayer was deposited at a substrate temperature of 300 °C. All specimen films were prepared in a reactive facing targets sputtering system. The degree of oxidization of the film is strongly related to the oxidation condition at the surface of the target. The surface condition of the target can be estimated by monitoring the discharge voltage. The discharge current–voltage characteristics clarify that there are three regions for the surface condition of the target, i.e. “oxidized,” “metallic,” and “transition.” Mn–Zn ferrite films with (111) crystallite orientation were deposited on Pt(111) underlayer under the transition region condition. The film prepared at O2 partial pressure ratio of 25% revealed 4.8 kG of saturation magnetization 4πMs. The deposition rate of the reactive sputtering method is 16 times as high as that of the conventional method.
Exchange-biased CoTaZr soft underlayer for perpendicular recording
Authors: Tanahashi Kiwamu , Kikukawa Atsushi , Hosoe Yuzuru
Published: May 2003

As a way of controlling the domain structure of a soft magnetic underlayer, we introduced a Pd/antiferromagnetic-MnIr/CoFe trilayer below a CoTaZr soft magnetic underlayer in perpendicular recording media, and we examined the effect of exchange biasing on the spike noise. Films consisting of layer structure—Pd (5 nm thick)/MnIr (7.5 nm)/CoFe (1–10 nm)/CoTaZr (50–200 nm)—were sputter deposited on precoated glass disks. The Fe content in the CoFe layer was varied from 5 to 70 at. %. After deposition, the films were heated to 250 °C and cooled in a magnetic field. Both uniaxial and unidirectional anisotropies were induced along the radial direction of the disk. It was found that the Pd layer promoted a face-centered-cubic-MnIr (111) crystalline texture, while the CoFe layer enhanced the exchange bias field Hex by 1.5–3.2 times, compared with that in the case of a Pd/MnIr/CoTaZr film. The value of Hex was strongly dependent on the CoFe alloy composition. By using the Pd/MnIr/Co60Fe40 trilayer, spike noise was suppressed when the thickness of the CoTaZr layer ranged from 50 to 200 nm. This suppression is probably due to the fact that the exchange biasing restrained the formation of domain walls in the CoTaZr layers.
Effect of N2 addition in sputter gas on giant magnetoresistance response of PtMn bottom spin-valve films
Authors: Mao M. , Devasahayam A. J. , Kools J. C. S. , Wang J. , Su C.
Published: May 2003

Interlayer coupling in spin-valve films is usually dominated by the ferromagnetic Néel coupling of a magnetostatic nature due to interfacial roughness. The addition of N2 as a second sputter gas species during deposition of thin layers in PtMn bottom spin-valve films has shown effectiveness in the reduction of interfacial roughness and, therefore, control of ferromagnetic Néel coupling. The interlayer coupling field has been more effectively reduced from an original 38.8 Oe down to 6.1 Oe at a Cu spacer layer thickness of 24 Å with the addition of 4 sccm N2 gas during the Cu spacer layer deposition. In addition to higher giant magnetoresistance values over 10% at thinner Cu spacer layer thicknesses, a pronounced oscillatory dependence of the interlayer coupling field on Cu spacer layer thickness has been observed. Apparently, nitrogen serves as surfactant and helps layer-by-layer growth of Cu on a CoFe pinned layer, resulting in smoother CoFe/Cu and Cu/CoFe interfaces.
Particle–particle interaction in magnetic fluids: A static magnetic birefringence investigation
Authors: Figueiredo L. C. , Gravina P. P. , Skeff Neto K. , Morais P. C. , Silva L. P. , Azevedo R. B. , Wagener M. , Buske N.
Published: May 2003

Static magnetic birefringence (SMB) is used to investigate particle–particle interaction in iron-nitride-based MF samples. Particularly interesting are the SMB data at fields below 1 kG, where particle interaction plays a key role. The SMB data were curve fitted using the contribution of particle agglomerates plus the field dependence of the magnetic susceptibility associated with agglomerates. In addition to the peak susceptibility of the chainlike structures, the fitting procedure allows determination of the fraction of both isolated (8%) and chainlike (92%) particles. The mean particle diameter (12.3 nm) and the diameter dispersity (0.14) obtained from the transmission electron microscopy data are used to support the fitting of the SMB data.
Magnetic properties of Fe3C nanograins embedded in carbon matrix
Authors: Lee Y. H. , Han T. C. , Huang J. C. A.
Published: May 2003

Magnetron dc cosputtering of a composite target of graphite disk plus iron rods was used in manufacturing carbon films with Fe3C nanograin inclusions. Both temperature- and field-dependent magnetizations, M(T) and M(H), were measured for samples of various carbon concentrations (from 37% to 85%). M(T) were measured in both conditions of zero-field cooling and a field cooling at H=100 Oe. Experimental results of χ(T), obtained from M(T), of zero-field cooling, were theoretically fitted by using Wolhfarth’s model of noninteracting particles with log–normal distribution function of particle size.
Thermal stability of coupled ferromagnetic and superparamagnetic particles
Authors: Safonov Vladimir L. , Bertram H. Neal
Published: May 2003

We consider a single-domain ferromagnetic particle with uniaxial anisotropy coupled to a single-domain soft ferromagnetic particle (superparamagnetic particle). The problem of thermally agitated magnetization reversal in this case can be reduced to the random magnetization dynamics of the first particle with an effectively larger anisotropy field. The magnetic external field is also altered in a manner that depends on the sign of the coupling and can be either enhanced or suppressed.
Magnetic properties and spin dynamics in magnetic molecule {Mn3}
Authors: Suh B. J. , Procissi D. , Jung J. K. , Bud’ko S. , Jeon W. S. , Kim Y. J. , Jung D.-Y.
Published: May 2003

We report dc magnetization and nuclear magnetic resonance (NMR) measurements in a trinuclear manganese compound, {Mn3}, with formula [Mn3O(O2CCH3)6(C5H5N)3]⋅C5H5N. The magnetic properties are characterized by two antiferromagnetic (AF) coupling constants, J=−7.9 K and J′=−27.6 K. H1 NMR linewidth strongly depends on both the magnetic field and temperature, and is quantitatively explained by the dipolar interaction between proton nuclei and Mn ion spins. The strong enhancement of T1−1 at low temperatures is ascribed to the slowing down of magnetic fluctuations resulting from building up of AF correlations. From the T dependence of T1−1 at low T, we obtained the gap ΔNMR=19 K, in qualitative agreement with the gap Δ≅12 K obtained from susceptibility χ.
Ferromagnetic CuII–CuII exchange interactions in a polyoxomolybdate-based nanocluster
Authors: Kögerler Paul , Müller Achim
Published: May 2003

We present initial magnetic properties and a simple Heisenberg model for a recently isolated new member of the {Mo57M6}-type family, a nanometer-sized polyoxomolybdate cluster anion comprising a triangular Cu6II prism. The exchange interactions between the six s=1/2 spin centers are weak compared to the exchange found for other members of the {Mo57M6} cluster family with M=FeII, FeIII, and VIV but display ferromagnetic exchange interactions of CuII–O–MoVI–O–CuII pairs, i.e., within two equilateral Cu3II triangles. The exchange interaction between the two triangles is antiferromagnetic. The observed temperature dependence of χT is described well by a Heisenberg model that employs a ferromagnetic intratriangle exchange constant J1=1.9 K and antiferromagnetic intertriangle coupling of J2=−1.4 K that eventually results in an S=0 ground state.
Size effect on the ordering of FePt granular films
Authors: Takahashi Y. K. , Ohkubo T. , Ohnuma M. , Hono K.
Published: May 2003

We have investigated the A1→L10 ordering process of FePt granular films with different particle sizes. Although FePt particles larger than ∼7 nm ordered by annealing at 600 °C, the ordering did not proceed when the particle size was smaller than ∼4 nm in diameter. This suggests that there is a size dependence on the ordering of FePt particles. Calculation of the Helmholtz free energy using the Lennard-Jones potential suggested that Tc decreases below the annealing temperature when the particle size is decreased to a few nanometers suggesting that experimental observation is physically feasible.
High density magnetic recording on protein-derived nanoparticles
Authors: Hoinville Jay , Bewick Angus , Gleeson David , Jones Richard , Kasyutich Oksana , Mayes Eric , Nartowski Artur , Warne Barnaby , Wiggins Jason , Wong Kim
Published: May 2003

Self-organized Co/Pt nanoparticulate arrays offer a novel approach to fabricating magnetic recording media with the potential for supporting Terabit/in.2 recording densities. Protein-derived Co/Pt nanoparticles are prepared within apoferritin from aqueous reactants, with synthesis conditions controlling grain size, structure, and composition. Smooth films on glass disk substrates are produced by either spin coating or dip coating from aqueous dispersions of the precursor material. Films are typically annealed at 590 °C for 60 min with a 19 kPa (190 mBar) partial pressure of H2 to form the L10 phase. By varying the annealing conditions we are able to produce coercivities in the range of 500–8000 Oe. Electrical testing of Co/Pt nanoparticulate media using a contact test recorder shows that these nanoparticle films are capable of sustaining recording densities of more than 12.6 Gbits/in.2 (143.6 kfci, kilo flux changes per inch). In this article we will present results from finished films with regard to film structure, magnetic properties, and recording capabilities.
Fabrication of ordered FePt nanoparticles with a cluster gun
Authors: Stoyanov S. , Huang Y. , Zhang Y. , Skumryev V. , Hadjipanayis G. C. , Weller D.
Published: May 2003

In this study we present a technique to obtain ordered fct FePt particles embedded in a C matrix. FePt nanoparticles are formed inside a high-pressure sputtering cell, called a particle gun (PG), and subsequently deposited on the substrate through a small orifice. These particles have a uniform size distribution with an average particle size that can be controlled in the range of 3–10 nm by adjusting the sputtering cell pressure, power, distance between the magnetron and the orifice, and by using a liquid nitrogen cooling jacket. The particles are converted to the L10 phase as they pass through a specially designed heating stage, attached to the top of the PG, heated by halogen lamps, thus avoiding alloying and oxidation effects. A strong dependence of coercivity on both the particle size and temperature was observed.
Magnetic characteristics of ultrathin Fe films with Ar bombardment during and after deposition
Authors: Iwatsubo S. , Naoe M.
Published: May 2003

The three kinds of Fe films, without Ar bombardment (BNON), with Ar bombardment after deposition (BAFT), and with Ar bombardment during deposition (BDUR) were prepared by dual ion beam sputtering. The relationship between the morphology and the magnetic characteristics of the Fe films has been investigated as a function of the film thickness tF in the wide range between 0.5 and 600 nm. The sputtering voltage and current were fixed at 1200 V and 50 mA, respectively. The Ar bombardments for BAFT and BDUR were carried out at the ion acceleration voltage of 200 V for the period. The features of the film growth were very different among them. The films of BNON changed from island structure to layer structure at tF between 8 and 16 nm. The films of BAFT and BDUR revealed smoother surfaces than that of BNON and were in layer structure at tF above 1 nm. However, the saturation magnetization 4πMS of BDUR abruptly decreased at tF below 8 nm, since the Ar bombardment knocked Fe atoms into the surface region of glass substrate at the initial growth stage. On the other hand, 4πMS of BNON and BAFT took almost constant value of 21.5 kG with a fluctuation of about 5%. The coercivity HC of BAFT took the minimum value among them at tF below 4 nm. These results indicate Ar bombardment after the deposition is useful for preparing the ultrathin Fe films with excellent soft magnetic properties.
Structural effects on the magnetic character of yttrium–iron–garnet nanoparticles dispersed in glass composites
Authors: Taketomi Susamu , Shapiro Alexander J. , Shull Robert D.
Published: May 2003

By absorbing amorphous yttrium–iron–garnet nanoparticles into nanometer pores of the sponge-like structure of porous silica glass [controlled pore glass (CPG)] followed by heat treatment, we obtained dispersed-nanocrystal/glass composites. We prepared samples using different extremal heat treatments: low temperature long calcination (700 °C, 2 h) and high temperature short calcination (1000 °C, 0.1 h). From the difference between the secondary electron image in a field emission scanning electron microscope (FESEM) and the backscattered electron image of the same sample surface area, it is concluded that the nanoparticles were imbedded on and just beneath the surface of the CPG granules. This was confirmed by a cross-sectional backscattered electron image of the sample in the FESEM. In this image nanoparticles of 20–40 nm were monodispersed in a 2 μm thick shell of fused glass and, inside this shell, the inner core of CPG granules preserved their sponge-like structure and contained no nanoparticles. Powder x-ray diffraction revealed that the synthesized nanoparticles were dominantly ε-Fe2O3, however, many small diffraction peaks consistent with those of Fe5Y3O12 and FeYO3, were also observed. The magnetization hysteresis loop curves revealed that the high temperature heat-treated samples were paramagnetic, whereas the low-temperature heat-treated sample was a mixture of a small amount of ferro- or ferrimagnetic material with a majority being paramagnetic material.
Microwave permeability spectra of ferromagnetic thin films over a wide range of temperatures
Authors: Ledieu M. , Schoenstein F. , Le Gallou J.-H. , Valls O. , Queste S. , Duverger F. , Acher O.
Published: May 2003

Ferromagnetic materials are of great interest today for high frequency applications in microelectronics (M-RAMs, planar inductors, etc.) or magnetic recording systems, taking advantage of the high levels of saturation magnetization of ferromagnetics. We are presenting a setup permitting permeability measurements from 10 MHz up to 6 GHz in the 77–400 K temperature range. An existing single-coil perturbation technique is modified to allow precise and accurate microwave measurements. An experimental validation of the technique is performed. Measurements on a CoFeSiB thin film are presented. The clear effect of the temperature on the whole permeability spectra is shown for soft ferromagnetic thin films, including CoNbZr.
Microstructure and magnetic properties of the FeTaCN nanocrystalline thin films
Authors: Chou C. Y. , Kuo P. C. , Yao Y. D. , Chen S. C. , Sun A. C. , Lie C. T.
Published: May 2003

FeTaCN films were deposited on quartz substrates by cosputtering of Fe and TaC targets at room temperature with different N2 flow rate ratios in the sputtering gas. The as-deposited films were postannealed in vacuum for 30 min at various temperatures. The effects of annealing temperature on the N2 flow rate ratio and film thickness on the magnetic properties and microstructure of the film were investigated. X-ray diffraction and transmission electron microscopy analyses show that the as-deposited FeTaCN film has a nanocrystalline structure or mixing phases of nanocrystalline and amorphous. Nanocrystalline as-deposited film with good soft magnetic properties (in-plane coercivity Hc∥=1∼2 Oe and 4πMs=12–14 kG) can be obtained by controlling the N2 flow rate ratio and film thickness. The soft magnetic properties can be improved by postannealing the as-deposited film at 200–300 °C as the N2 flow rate ratio is higher than 5 vol %. For the Fe71.03Ta6.1C7.2N15.67 film, the Hc∥ value decreases as the film thickness is increased when the annealing temperature is lower than 400 °C. After annealing at 300 °C, its Hc∥ is about 3.57 Oe as the film thickness is 50 nm and Hc∥ will decrease to 0.18 Oe as the film thickness is increased to 1000 nm.
Printing superparamagnetic colloidal particle arrays on patterned magnetic film
Authors: Yellen Benjamin , Friedman Gary , Feinerman A.
Published: May 2003

Thin ferromagnetic film patterned into isolated islands is used to direct the assembly of superparamagnetic colloidal particles into two-dimensional (2D) arrays. It is confirmed that only one particle per island is retained when particles are slightly larger than the island, whereas particles that are smaller can form various patterns on the islands. Ferromagnetic islands positioned under a template of microwells are shown to promote highly regular 2D arrays, and external uniform magnetic field can be used to bias the particles to attract or to repel from the exposed end of the ferromagnetic islands.
Antiferromagnetically coupled media performance
Authors: Tang Kai , Doerner Mary , Xiao Qi-Fan , Tang Li , Mercado Mark , He Jizhong , Prichard Roeshan , Rice Philip
Published: May 2003

Magnetic and recording properties of antiferromagnetically coupled (AFC) media are studied in comparison to conventional media. AFC media have superior magnetic recording properties compared to conventional media in terms of media signal to noise ratio (S0NR) increase and transition pulse width (PW50) reduction. Such improvements are most significant for media with (11.0) texture, small underlayer grain size, and low dispersion of the crystallographic texture. AFC media with (10.0) texture do not perform as well. In particular, AFC media have wider PW50 than conventional media in this orientation. For (11.0) textured media, the large underlayer grain size reduces media SNR improvement of AFC media while exchange field reduction is the major drawback of the deteriorated crystallographic texture.
Improvement of exchange bias field by introduction of oxygen during deposition of Ru layer in antiferromagnetic coupled media
Authors: Djayaprawira D. D. , Domon H. , Yoshimura S. , Takahashi M.
Published: May 2003

It is shown that the antiferromagnetic exchange coupling J in antiferromagnetic coupling (AFC) media using a CoCr-based magnetic layer can be increased up to 65% by introducing oxygen with pressure of 2×10−5 Torr during deposition of a Ru spacer layer. The thermal stability factor KV/kT is also increased from 91 to 106 in the same manner. The increase of J by introducing oxygen during deposition of the Ru spacer layer does not originate from improvement of the flatness of Ru interfaces, but is suggested to be due to the formation of a layer with high Co concentration at the Ru interfaces. It is concluded that the proposed method is effective in increasing the thermal stability of AFC media with a conventional structure through the improvement of J.
Chopping techniques for low-frequency nanotesla spin-dependent tunneling field sensors
Authors: Jander A. , Nordman C. A. , Pohm A. V. , Anderson J. M.
Published: May 2003

Three chopping techniques to address 1/f noise in spin-dependent tunneling magnetoresistive sensors are investigated. These include modulation of the sensitivity using orthogonal fields, modulation, and second-harmonic generation using the nonlinear response of the magnetoresistive element and modulation of the flux concentrator permeability. Of these, only the second technique resulted in a slight reduction in low-frequency noise. In order to achieve significant noise reduction by chopping, domain noise will have to be reduced.