bluuebunny/crossref_metadata_2025 huggingface dataset:
Thermal stability of pinned synthetic ferrimagnets with perpendicular magnetic anisotropy
Authors: Sort J. , Rodmacq B. , Garcia F. , Auffret S. , Dieny B.
Published: May 2004

The thermal stability of pinned synthetic ferrimagnets with perpendicular anisotropy is investigated. These structures consist of two [Pt/Co] mutilayers (MLs) antiferromagnetically coupled through a Ru spacer, in which one of them is, in turn, exchange biased with FeMn. The switching fields of both MLs can be tuned, at a given temperature, by varying the relative number of (Pt/Co) repeats comprised in each ML. These fields are determined by the values of the exchange constant through the Ru, ARu, and the exchange constant with FeMn, Apinning. Interestingly, for some compositions, it is found that the order in which the two MLs switch during magnetization reversal changes depending on temperature. This effect is due to the different thermal evolution of ARu and Apinning.
Influence of magnetic field on the tunneling current in magnetic 10-nm-scale point contact junctions using tunneling atomic force microscopy
Authors: Miyamoto Yasuyoshi , Kuga Kiyoshi , Hayashi Naoto , Machida Kenji , Aoshima Ken-ichi
Published: May 2004

Point-contact junctions were fabricated by contacting an atomic force microscope (AFM) cantilever with a soft magnetic material coating on to [Al-O barrier/Fe3O4 half-metal] films using a tunneling AFM. The observed distribution of tunneling current through a thin barrier of Al-O in specimen point-contact junctions was influenced by the strength of the applied magnetic field and by the quality of the barrier at a measured position. Magnetization of the magnetically soft coating on the AFM cantilever was easily affected with alignment along the applied H direction, even when the contact size was as small as several tens of nanometers.
Optical and magneto-optical properties of magnetic nanocomposites FePt-SiO2
Authors: Gan’shina E. , Aimuta K. , Granovsky A. , Kochneva M. , Sherbak P. , Vashuk M. , Nishimura K. , Inoue M.
Published: May 2004

Structural, magnetic, magnetotransport, optical, and magneto-optical properties of (FePt)1−X(SiO2)X thin films in as-deposited state and after annealing were investigated. Magneto-optical properties were measured in the transversal Kerr effect (TKE) geometry. Magneto-optical spectra in nanocomposites (FePt)1−X(SiO2)X depend greatly on volume fraction of metal and annealing conditions. The TKE reaches the maximum value for compositions close to the percolation threshold X≈57% in as-deposited samples, as well as a maximum of the magnetoresistance. After annealing at 700 °C the TKE value significantly increases and the maximum of the magneto-optical response is observed at X≈70%. The magneto-optical spectra transformation after annealing is mainly due to the disorder-order phase transitions in FePt granules, changes in the mean granular size, and alloy microstructure. The TKE spectra calculated in the framework of the effective medium approximation for as-deposited samples are in good agreement with the experiment.
Investigation of ultrafast demagnetization and cubic optical nonlinearity of Ni in the polar geometry
Authors: Wilks R. , Hicken R. J. , Ali M. , Hickey B. J. , Buchanan J. D. R , Pym A. T. G. , Tanner B. K.
Published: May 2004

Femtosecond optical pump-probe experiments were performed upon a Ni(720 Å)/Si(100) sample in the polar geometry with the pump beam close to normal incidence. A signal due to the ultrafast demagnetization effect was observed when the pump pulse was linearly polarized. When the pump was elliptically polarized, additional peaks were observed at zero time delay, resulting from the specular inverse Faraday effect (SIFE) and the specular optical Kerr effect (SOKE). By comparing measurements made with different pump helicities, the SIFE and SOKE peaks and the demagnetization signal were found to superpose in a linear fashion. From the dependence of the peak height upon the pump polarization, values of χxxyy=(1−3i)×10−10 rad cm3 erg−1 and χxyyx=(−9+2i)×10−12 rad cm3 erg−1 were deduced for the nonvanishing components of the local cubic susceptibility tensor. For applied fields less than the saturation value, the sudden reduction of the thin film demagnetizing field leads to an imbalance of the torques acting upon the magnetization, causing it to precess.
Magnetic properties of densely packed arrays of Ni nanowires as a function of their diameter and lattice parameter
Authors: Vázquez M. , Pirota K. , Hernández-Vélez M. , Prida V. M. , Navas D. , Sanz R. , Batallán F. , Velázquez J.
Published: May 2004

High-quality densely packed hexagonal arrays of Ni nanowires have been prepared by filling self-ordered nanopores in alumina membranes. Nanowires with different diameter d (18–83 nm) and lattice parameter D (65 and 105 nm) have been studied by atomic force, high resolution scanning electron microscopies, Rutherford backscattering, and vibrating sample magnetometer techniques. Axial loops coercivity and remanence decrease with increasing ratio diameter to lattice parameter, r, until nanowires start to interconnect locally. Additionally, hysteresis of in-plane loops increases with packing factor. In order to interpret the experimental results, multipolar magnetostatic interactions among nanowires with increasing ratio r are considered.
Mössbauer studies of dynamic Jahn-Teller relaxation on the Cu-substituted sulfur spinel
Authors: Kim Sam Jin , Son Bae Soon , Lee Bo Wha , Kim Chul Sung
Published: May 2004

Samples of Fe1−xCuxCr2S4 (x=0.0, 0.1, 0.3, and 0.5) have been studied with Mössbauer spectroscopy, x-ray diffraction, magnetization, and magnetoresistance. A cusp-like anomaly is observed for the sample x=0.1 in the both field-cooled and zero-field-cooled magnetization curves near 130 K under an applied field H=100 Oe. The charge state of iron ions are ferrous for the samples x=0.0 and x=0.1, whereas they are ferric for the samples x=0.3 and x=0.5. The Mössbauer spectra for the sample x=0.1 show asymmetric line broadening, and it is considered to be dynamic Jahn-Teller relaxation. The unusual reduction of magnetic hyperfine field below 110 K can be interpreted in terms of cancellation effect between the mutually opposite orbital current field HL and Fermi contact field HC.
Exchange bias system of Fe/KFeF3
Authors: Pang W. , Stamps R. L. , Malkinski L. , Celinski Z. , Skrzypek D.
Published: May 2004

Epitaxial Fe/KFeF3 bilayers were grown using molecular beam epitaxy on GaAs (100). FMR measurements revealed two peaks for samples with an Fe seed layer. A peak with fourfold symmetry was identified as the main layer Fe whereas a peak with twofold symmetry appeared to be due to the seed layer Fe. The magnetocrystalline anisotropy for the fourfold peak is ∼400 Oe at 300 K whereas a large anisotropy of 1.5 kOe is associated with the seed layer. A large rotatable anisotropy of 280±20 Oe was found for a sample with Fe thickness 2.1 nm. All Fe/KFeF3 bilayers checked by superconducting quantum interference device exhibit exchange bias and enhanced coercivity. The blocking temperature of the system is close to the Néel temperature 112 K of KFeF3. One sample has polycrystalline KFeF3 and shows a much larger exchange bias and coercivity than the samples having single crystal KFeF3.
Lateral standing spin waves in permalloy antidot arrays
Authors: Yu Chengtao , Pechan Michael J. , Burgei Wesley A. , Mankey Gary J.
Published: May 2004

Spin wave modes in permalloy antidot arrays have been investigated with ferromagnetic resonance at 9.7 GHz. In contrast to a quadratic dispersion expected for exchange standing spin waves, nearly linear relationship exists between the resonance field and mode index, and it can be approximately described by the dipole-dipole and exchange theory. Time-dependent micromagnetic simulations show that the spin wave modes are a result of lateral confinement from the vacant holes. Furthermore, the simulations visually reveal the existence of localized spin waves due to localized boundary conditions in antidots arrays.
Giant negative photoconductivity in La0.7Ca0.3MnO3 thin films
Authors: Moshnyaga V. , Giske A. , Samwer K. , Mishina E. , Tamura T. , Nakabayashi S. , Belenchuk A. , Shapoval O. , Kulyuk L.
Published: May 2004

The increase of the resistance up to two orders of magnitude under laser illumination (λ=760 nm) was observed in La0.7Ca0.3MnO3 (LCMO) epitaxial thin films in ferromagnetic state. Optical absorption also increases by 10–15 % and the magnetic second-harmonic generation signal decreases down to zero under the irradiation. The light induced changes are reversible with characteristic relaxation times τ∼1–30 s. Magnetic field, B=4 T, suppresses the photoconductivity and decreases its relaxation time. Photoinduced effects are caused by the injection of a large number of extra carriers, which change the (antiferromagnetic) AFM/FM phase balance in LCMO, favoring the insulating AFM state.
Dynamics of the magnetic field-induced first order magnetic-structural phase transformation of Gd5(Si0.5Ge0.5)4
Authors: Leib J. , Snyder J. E. , Lograsso T. A. , Schlagel D. , Jiles D. C.
Published: May 2004

The system Gd5(SixGe1−x)4 for 0.4⩽x⩽0.5 has been shown to have an unusual first order, coupled magnetic-structural phase transformation at the Curie temperature. Above the transformation temperature Tc, the material is paramagnetic with a monoclinic structure; below Tc, it is ferromagnetic with an orthorhombic structure. Another unusual feature of this phase transformation is that an applied magnetic field can increase Tc by 5 K per tesla. In this study, the magnetic-structural transformation in single crystal Gd5Si2Ge2 was triggered by holding the sample at a temperature just above Tc, then using an applied field to increase Tc beyond the sample temperature, thereby inducing the magnetic-structural transformation. The dynamics of this field-induced phase transformation at various temperatures just above Tc were observed by measuring the magnetization as a function of time. This magnetization change is caused by the first order phase transformation which is distinctly different from the magnetization reversal which one observes in conventional magnetic relaxation experiments. The transformation could be modeled as a thermal activation process with a single energy barrier of height 4.2±0.2 eV.
Growth-induced uniaxial magnetic anisotropy in Co/Cu(100)
Authors: Zhao H. W. , Wu Y. Z. , Won C. , Qiu Z. Q.
Published: May 2004

Growth-induced uniaxial magnetic anisotropy of Co/Cu(100) films were investigated using surface magneto-optic Kerr effect and scanning tunneling microscopy (STM). We found that the Co films off-grown at 230 K show an in-plane uniaxial magnetic anisotropy with the easy magnetization axis perpendicular to the growth incident plane. STM measurements show that the low temperature grown Co film consists of smaller islands without obvious anisotropic roughness or elongated islands. This result implies that the dipolar interaction does not dominate the uniaxial magnetic anisotropy. The CO absorption experiment further suggests that the uniaxial magnetic anisotropy originates from the magnetocrystalline step anisotropy of the Co film surface.
Magnetism of L1 compounds with the composition MT (M=Rh, Pd, Pt, Ir and T=Mn, Fe, Co, Ni)
Authors: Kashyap A. , Skomski R. , Solanki A. K. , Xu Y. F. , Sellmyer D. J.
Published: May 2004

The electronic band structure of ordered equiatomic compounds of 3d transition elements (Mn, Fe, Co, Ni) with nonmagnetic 4d and 5d elements (Rh, Pd, Pt, Ir) are investigated by linear muffin-tin orbital calculations. The systematic study considers 3d and 4d/5d spin moments and interatomic exchange interactions, with emphasis on the comparison between ferromagnetic and antiferromagnetic order. Total and site-resolved exchange interactions are calculated from first principles, and the obtained exchange constants are used to estimate ordering temperatures on a mean-field level.
Enhancement of the magnetization saturation in magnetite (100) epitaxial films by thermo-chemical treatment
Authors: Zhou Y. , Jin Xuesong , Shvets I. V.
Published: May 2004

The effect of thermo-chemical treatment on the ease of saturation in a magnetic field of epitaxial magnetite (100) thin films grown on MgO (100) substrates was investigated. It was found that the films maintained a fully strained state with the MgO substrate during the treatment in air. No other iron oxide phase apart from the magnetite was observed in the treated film. Yet, remarkably the treated films showed a higher magnetization compared to that of original ones. A model involving a change in antiferromagnetic coupling at the antiphase boundaries during the treatment is proposed.
Ferromagnetic nanospheres of perovskite manganite La0.7Ca0.3MnO3 prepared by template replication in porous carbon framework
Authors: Kim Y. N. , Kim J. C. , Lee E. K. , Hwang Y. S. , Hur N. H. , Yun Y. J. , Park G. S. , Kim J. T.
Published: May 2004

Nanospheres of perovskite manganite La0.7Ca0.3MnO3 (LCMO) were prepared using porous carbon replica as a template. Close-packed arrays of silica spheres were infiltrated with a sucrose solution, which was then carbonized. After removal of the silica template, pores in the replica were filled with metal acetate precursors. The filled replica was sintered at 800 °C. The resulting hollow LCMO nanospheres were characterized by magnetization, electrical transport, scanning electron microscope, and transmission electron microscope measurements. The magnetization data of LCMO show a ferromagnetic transition near 250 K.
Isomer-selective detection of microsolvated oxonium and carbenium ions of protonated phenol: Infrared spectra of C6H7O+–Ln clusters (L=Ar/N2, n⩽6)
Authors: Solcà Nicola , Dopfer Otto
Published: May 2004

Infrared photodissociation (IRPD) spectra of clusters composed of protonated phenol (C6H7O+) and several ligands L are recorded in the O–H and C–H stretch ranges using a tandem mass spectrometer coupled to a cluster ion source. The C6H7O+–Ln complexes (L=Ar/N2, n=1–6) are generated by chemical ionization of a supersonic expansion. The IRPD spectra of mass selected C6H7O+–Ln clusters obtained in various C6H7O+–Lm fragment channels (m<n) display the unambiguous fingerprints of at least two different C6H7O+ nucleation centers: the oxonium ion (5) and the carbenium ion(s) corresponding to protonation of phenol in ortho and/or para position (1/3). These two classes of C6H7O+–Ln isomers show very different fragmentation behavior upon IR excitation, facilitating the assignment of the observed vibrational transitions. The vibrational frequency shifts as a function of cluster size reveal that the microsolvation of 1/3 and 5 in Ar and N2 begins with the formation of intermolecular hydrogen bond(s) to the acidic OH group(s) and proceeds by the formation of intermolecular π-bonds to the respective six-membered rings. The analysis of photofragmentation branching ratios yields estimated ligand binding energies of the intermolecular OH- and π-bonds for solvation of the different C6H7O+ isomers. The effects of microsolvation on the properties of 1/3 as reactive intermediates in electrophilic aromatic substitution reactions are discussed. Comparison of clusters of protonated phenol with those of neutral phenol reveals the drastic protonation-induced changes in the topology of the intermolecular potential of aromatic molecules interacting with a nonpolar solvent. Moreover, the results show that the IRPD process can be used to selectively generate a spectroscopically clean ion beam of either 1/3 or 5 with some control over their internal energies.
Effect of Cu inserted layer between the IrMn and CoFeB interface on magnetic properties of CoFeB
Authors: Joo S. J. , Hong D. H. , Lee T. D.
Published: May 2004

In order to reduce the coercivity of an amorphous CoFeB layer of 10 nm thickness in a CoFeB/IrMn antiferromagnetically coupled layer, a very thin interlayer was inserted. In the present study, effects of the nonmagnetic interlayer at the interface of a ferromagnetic/antiferromagnetic on the exchange field, the coercivity and the saturation magnetization were studied. As the nonmagnetic interlayer, Cu, Ag, and Ta were used. When Cu and Ag were inserted, there was a long range bias field effect but Ta did not show any bias effect at the 0.6 nm thickness. These results are consistent with the previous articles [Gkemeijer et al. Phy. Rev. Lett. 79, 4270 (1997); J. Appl. Phys. 81, 4999 (1997)]. Thermal stability of the 0.6 nm Cu interlayer was investigated by the annealing experiment. Up to 200 °C annealing the Cu layer was stable. The dynamic magnetic reversal of the CoFeB layer with and without Cu inserted layer was observed.
Reversible and irreversible magnetoresistance of quasisingle domain permalloy microstructures
Authors: Steiner M. , Pels C. , Meier G.
Published: May 2004

Permalloy microstructures are investigated by magnetoresistance measurements at 2.0 K and by magnetic-force microscopy at room temperature. While the reversible anisotropic magnetoresistance is determined to be 2.4% at saturation fields of Bsat=1020 mT, the irreversible switching yields a resistance change of the order of 0.05% at 13 mT. By tilting the external magnetic field relative to the easy axis of the quasi single-domain microstructures insight in the reversal process is gained. Comparison with an analytical model provides evidence for magnetization reversal by curling.
Anomalous magnetic properties in Co3O4 nanoparticles covered with polymer decomposition residues
Authors: Li Shandong , Bi Hong , Cui Baozhi , Zhang Fengming , Du Youwei , Jiang Xiqun , Yang Changzheng , Yu Qing , Zhu Yuping
Published: May 2004

Co 3 O 4 nanoparticles with and without covering of residues from the decomposed polymer at the surface have been fabricated, respectively. It has been revealed that anomalous magnetic properties of the nanoparticles covered with the residues deviate greatly from those of the ones without residues. In comparison with the “naked” Co3O4 nanoparticles, for the “covered” sample, the saturation field, coercivity, loop shift and average permanent magnetic moments per Co3O4 particle are enhanced remarkably. It is believed that the anomalous magnetic properties and their significant difference are caused by the presence of residues at the surface, consistent to the reported results in literature.
Revised magnetic phase diagram for FexMn5−xSi3 intermetallics
Authors: Candini A. , Moze O. , Kockelmann W. , Cadogan J. M. , Brück E. , Tegus O.
Published: May 2004

The magnetic and crystal structures of the series FexMn5−xSi3 (x=4,3.5,3,2,1) have been reinvestigated by neutron powder diffraction between 315 and 8 K. Fe atoms are found to preferentially occupy the 4d site of the hexagonal D88 structure (P63/mcm). The compound Fe4MnSi3 orders ferromagnetically below 300 K. Refinement of the neutron data for this compound indicates that the moments are aligned at ∼40° off the c axis. A high moment at the 6g site is due to a large Mn moment which is forced into a ferromagnetic alignment with the Fe moments. Associated with this moment is a moderate magnetic entropy change, with a maximum value of 4 J kg−1 K−1 for a magnetic field change of 5 T. The magnetic structure of Fe3.5Mn1.5Si3 is similar to that of Fe4MnSi3, but the Curie point is lower, below 240 K. A feature for this composition is the appearance of a spiral structure below 60 K. The compounds with x=1, 2, and 3 are all antiferromagnets with a spiral structure observed for Fe3Mn2Si3 at all temperatures up to the Néel point of 150 K. Two separate antiferromagnetic structures exist for Fe2Mn3Si3, one between 120 and 45 K and another structure for temperatures below 45 K. Similar features in the magnetic ordering are exhibited by FeMn4Si3. At temperatures between 70 and 95 K, the magnetic structure is similar to that of the low temperature phase of Fe2Mn3Si3. Accompanying the magnetic transition at 70 K is a structural distortion to space group Cmcm, a feature also observed for Mn5Si3.
Dependence of magnetic tunnel junction’s reliability on oxidation condition
Authors: Kim Kwang-Seok , Jang Y. M. , Park J. M. , Cho B. K.
Published: May 2004

Time-dependent dielectric breakdown measurements under constant voltage stress were carried out for magnetic tunnel junctions (MTJs), prepared by different oxidation techniques. Insulating barriers in MTJs were fabricated by oxidation of a predeposited Al layer with different oxidation techniques, such as conventional O2 plasma, off-axis O2 plasma, Ar-mixed O2 plasma, and off-axis Ar-mixed O2 plasma oxidations. The time to breakdown (TBD) of the MTJs was estimated from Weibull failure distribution plot. The estimated TBD when 63% of the junctions formed by conventional O2 plasma oxidation failed was estimated to be about 55 s. The TBD increased up to about 250, 750, and 2,500 s for the junctions of off-axis, Ar-mixed, and off-axis Ar-mixed O2 plasma oxidation, respectively. We believe that the enhanced reliability of MTJs is due to the reduction of plasma damage during oxidation process, resulting in low electron trap density in the Al2O3 tunneling barrier. Low-frequency 1/f noise power density was measured to estimate the relative electron trap density as a function of the oxidation method, which supports our conjecture about the enhanced reliability of MTJs.
Hysteresis modeling of tunneling magnetoresistance strain sensor elements
Authors: Hauser Hans , Rührig Manfred , Wecker Joachim
Published: May 2004

Utilizing the inverse magnetostriction effect, magnetic tunneling junction (MTJ) elements have been demonstrated to also be useful as highly sensitive stress or strain transducers. The prediction of the stress dependent hysteresis of the tunneling magnetoresistance R is done by the energetic model (EM), minimizing the total magnetization work which consists of the Zeeman energy density, of magnetocrystalline and stress anisotropy of the magnetization rotation processes, and of reversible and irreversible contributions of domain wall displacements. Furthermore, the law of approach to saturation and the initial susceptibility relate the parameters of the EM to the effective anisotropy energy which has to be overcome for a magnetization reversal. The calculated R(H) curves are in good aggreement with the measurements, allowing several predictions of the MTJ parameters as magnetostriction constant, stress (strain) sensitivity and hysteresis, and misalignment of the easy axis.
Magnetoresistance and Hall effect in Co implanted GaAs hybrid magnetic semiconductors
Authors: Honda S. , Sakamoto I. , Shibayama B. , Sugiki T. , Kawai K. , Nawate M.
Published: May 2004

The hybrid Co/GaAs ferromagnetic semiconductor has been prepared by the ion-implantation method, and the sample surface has been sputter-etched for examining the magnetic and transport properties as a function of the depth. Extraordinary Hall effect increases with depth until 120 nm etching, and the Hall resistance is proportional to the square of resistance, indicating the side-jump mechanism, in the depth region of 10–60 nm. The depth profiles of the saturation magnetization and the conductivity are estimated analytically, and then the Hall coefficient is evaluated as a function of etching depth.
Effect of postgrowth annealing on the morphology and magnetic properties of MnAs thin films grown on GaAs(001) substrates
Authors: Nakane R. , Sugahara S. , Tanaka M.
Published: May 2004

We have investigated the effect of postgrowth annealing on the surface morphology and magnetic properties of monocrystalline MnAs thin films epitaxially grown on GaAs(001) substrates by molecular beam epitaxy. After the growth of MnAs, the postgrowth annealing was carried out in the temperature range between 330 and 400 °C, with keeping the MnAs surface exposed to the As flux. The surface morphology and crystallinity of MnAs thin films were drastically improved by optimizing the annealing temperature. The magnetic properties, such as saturation magnetization, coercivity, and the magnetoresistance characteristics of the MnAs films were sensitive to their crystallinity, which was determined by the annealing temperature.
Placement of epitaxial magnetic Co dots on Ru(0001) via substrate modifications
Authors: Li Dongqi , Cheng Ruihua , Yu Chengtao , Pearson J. , Fradin F. Y. , Bader S. D.
Published: May 2004

Co has been epitaxially grown onto flat and modified Ru(0001) substrates via molecular beam epitaxy at elevated temperature and characterized ex situ with atomic force and magnetic force microscopy. On flat Ru(0001), Co grows into three-dimensional Co islands (dots) of ∼20–900 nm or a flat Co film network with deep holes (antidots) in truncated pyramidal shapes, which randomly distribute on the surface. When the Ru surface morphology is modified via either scratches or e-beam lithography, the dots align along the edges of the surface features (∼4–100 nm deep), and form dot chains, wires, arrays, and dots around lithographic patterns. The dots exhibit in-plane single domains with magnetic easy axis along the edges of these surface features.
Mesoscopic quantized properties of magnetic-dipolar-mode oscillations in disk ferromagnetic particles
Authors: Kamenetskii E. O. , Shavit R. , Sigalov M.
Published: May 2004

Magnetic-dipolar-mode or magnetostatic (MS) oscillations in ferrite samples have the wavelength much smaller than the electromagnetic wavelength at the same frequency and, at the same time, much larger than the exchange-interaction spin wavelength. This intermediate position between the “pure” electromagnetic and spin-wave (exchange-interaction) processes reveals very special behaviors of the geometrical effects. It was shown recently that magnetic-dipolar-mode oscillations in a normally magnetized ferromagnetic disk are characterized by discrete energy levels resulting from the structural confinement. In this article we give results of the energy spectra in MS-wave ferrite disks taking into account nonhomogeneity of the internal dc magnetic field.
Effect of thermal stability on magnetoresistance in NiO spin valve
Authors: Zhang A. M. , Wu X. S. , Sun L. , Sheng W. T. , You B. , Du J. , Lu M. , Hu A. , Jiang S. S.
Published: May 2004

The interface structure and magnetoresistance (MR) for the single spin valve (SV) of Co/Cu/Co structure with a NiO layer at the top or under the bottom were investigated. Glancing incident x-ray reflectivity and x-ray diffuse scatter studies show that the interface roughness of NiO on Co is much larger than that of Co on NiO. The large roughness may be one of the main factors of suppressing spin reflectivity. The interface between NiO and Co becomes more flat after annealing. Different temperature dependence of MR was observed for the top and the bottom SV. We attribute the above phenomena to the competition between the roughness and the exchange effects of the NiO/Co interface.
Exchange bias anisotropy on the dynamic permeability of thin NiFe layers
Authors: Queste S. , Dubourg S. , Acher O. , Barholz K. U. , Mattheis R.
Published: May 2004

30 nm permalloy thin films have been grown by sputter deposition with and without an 8 nm IrMn exchange bias layer. The microwave permeability of these layers has been investigated in the 100 MHz–3 GHz range with a permeameter based on a microstrip cell, applying an external field along the easy axis. A comparison between the bias field deduced from magneto-optical Kerr effect measurements and a fit of the dynamic permeability spectra is made. Significant hysteresis effect on the microwave permeability of the biased sample is observed and modeled.
Magnetic anisotropy and transport properties of epitaxially grown MnAs/GaAs digital alloys
Authors: Song J. H. , Cui Y. , Lee J. J. , Kim Yunki , Ketterson J. B. , Cho Sunglae
Published: May 2004

An enhancement of both Tc and the magnetic anisotropy, as well as unusual electrical transport properties, are observed in superlattice thin films consisting of metallic ferromagnet MnAs and semiconducting GaAs. Structural analysis indicates that the MnAs layer is hexagonal rather than zinc blende. All films studied display room temperature ferromagnetism with a Tc higher than that of a single-layer MnAs film. A MnAs(5 nm)/GaAs(5 nm) superlattice showed the highest Tc (345 K) and the largest magnetic anisotropy. Furthermore, the temperature-dependent resistance of this sample shows semiconductor-like behavior and exhibited negative magnetoresistance, suggesting an interplay between the charge carriers and localized magnetic ions.
Artifacts in ballistic magnetoresistance measurements (invited)
Authors: Egelhoff W. F. , Gan L. , Ettedgui H. , Kadmon Y. , Powell C. J. , Chen P. J. , Shapiro A. J. , McMichael R. D. , Mallett J. J. , Moffat T. P. , Stiles M. D. , Svedberg E. B.
Published: May 2004

We have carried out an extensive search for credible evidence to support the existence of a ballistic magnetoresistance (BMR) effect in magnetic nanocontacts. We have investigated both thin-film and thin-wire geometries for both mechanically formed and electrodeposited nanocontacts. We find no systematic differences between mechanically formed and electrodeposited nanocontacts. The samples we have investigated include mechanical contacts between ferromagnetic wires, electrodeposited nanocontacts between ferromagnetic wires, ferromagnetic nanocontacts electrodeposited on Cu wires, nanocontacts electrodeposited between ferromagnetic films anchored on wafers, ferromagnetic nanocontacts electrodeposited on Cu films anchored on wafers, nanocontacts between two ferromagnetic films connected by a pinhole through an insulating film, and nanocontacts formed by focused ion-beam etching. In none of these samples did we find credible evidence for a BMR effect. However, we did find a number of artifacts due to magnetostrictive, magnetostatic, and magnetomechanical effects that can mimic BMR.
Size and structure control of magnetic nanoparticles by using a modified polyol process
Authors: Hinotsu T. , Jeyadevan B. , Chinnasamy C. N. , Shinoda K. , Tohji K.
Published: May 2004

Synthesis strategies for the size and structure controlled Ni, Co, and FePt nanoparticles by manipulating the reaction kinetics of the polyol process are reported. In the case of Ni, particle size varied from a few micron to a few tens of nanometer, and the structure from fcc to a mixture of fcc and hcp was realized. The Co particle structure has been changed from a mixture of fcc and hcp at micron size range, to the fcc dominant mixture of fcc and hcp in the submicron size range, then to ε- and hcp-Co and finally to hcp-Co in the nanosize range. In the case of FePt, particles had fcc and fct phases with 5–10 nm in diameter. The magnetic properties of these particles are also reported.
Characterization of interfacial reactions in magnetite tunnel junctions with transmission electron microscopy
Authors: Peng Yingguo , Park Chando , Zhu Jian-Gang , White Robert M. , Laughlin David E.
Published: May 2004

To make a uniform AlOx barrier layer in tunnel junctions, a thin layer of Al is often sputtered first and then oxidized. In this study, we sputtered a thick layer of Al onto Fe3O4 and then employed high resolution transmission electron microscopy and x-ray energy dispersive spectroscopy to investigate the interfacial microstructures. Two new layers have been found and investigated at the Al/Fe3O4 interface. The interfacial reaction has been determined to be: Fe3O4+Al⇒Fe+(a)AlOx, where a denotes the amorphous state. The consequence of the interfacial reaction to transport properties is also discussed.
Electrical and magnetic properties of spinel-type magnetic semiconductor ZnCo2O4 grown by reactive magnetron sputtering
Authors: Kim Hyun Jung , Song In Chang , Sim Jae Ho , Kim Hyojin , Kim Dojin , Ihm Young Eon , Choo Woong Kil
Published: May 2004

We report on the growth of spinel ZnCo2O4 films using reactive magnetron sputtering and their electrical and magnetic properties, with particular emphasis on the relation of Curie–Weiss temperature (TCW) and conduction type. The conduction type and carrier concentration in these films were found to be dependent on the oxygen partial pressure ratio in the sputtering gas mixture. The highest electron and hole concentration at 300 K were 1.37×1020 and 2.81×1020 cm−3, respectively. A ferromagnetic coupling (TCW>0) was observable in p-type ZnCo2O4, whereas an antiferromagnetic interaction (TCW<0) was found for n-type and insulating ZnCo2O4, revealing hole-induced ferromagnetic transition in ZnCo2O4.
Manipulation of the metamagnetic transition and entropy change in Gd5(Si,Ge)4
Authors: Lewis L. H. , Yu Ming-hui , Welch D. O. , Gambino Richard J.
Published: May 2004

100 nm coatings of Fe and of Al were found to alter the magnetic attributes, such as the initial susceptibility and metamagnetic transition field, of the giant magnetocaloric material Gd5(Ge,Si)4 and thereby increase the entropy change for a given applied magnetic field, relative to uncoated samples. The experimental data strongly suggest that these coatings confer a strain that propagates through the material and enhances the effect of the magnetic field in the metamagnetic phase transition range. A simplified Landau theory of coupled magnetic and structural transitions was developed for Gd5(Si1−xGex)4 and used to analyze the thermodynamics and phase transitions of this material under the influence of magnetic fields and elastic strain fields.
Magnetic properties of MnGeAsP films grown on GaAs (100) by molecular beam epitaxy
Authors: Cui Y. , Lee J. J. , Song J. H. , Luan L. , Kim Y. , Ketterson J. B. , Cho S.
Published: May 2004

Magnetic MnGeAsP films with the nominal composition MnGe(As1−xPx)2 were grown on GaAs(100) by solid source molecular beam epitaxy. The films were grown keeping the Mn, Ge, and As fluxes constant while varying the P flux via its cell temperature. A streaky reflection high energy electron diffraction pattern with intensity variations along each reflection line was observed, suggesting small domain sizes in the films. The incorporation of P in the films was examined by x-ray photoelectron spectroscopy, and an increase of the P/As peak ratio was observed with increasing phosphorous cell temperature. Superconducting quantum interference device magnetic measurements revealed a clear change in the magnetic properties as phosphorous was introduced into the films. The coercive field decreases with increasing P/As ratio. The measured Curie temperatures were over 380 K for P/As ratios larger than 2, which is higher than that of a nominal MnGeAs2 and MnGeP2 film.
Intermittency of Barkhausen avalanche in Co nanothin films
Authors: Kim Dong-Hyun , Shin Sung-Chul
Published: May 2004

We investigate the statistical distribution of separation time ΔT between the adjacent two Barkhausen jumps in Co films having the thickness ranging from 5 to 50 nm. By means of a magneto-optical microscope magnetometer, we determine the time-dependent magnetization curve and the separation time ΔT between two jump events during avalanche process from the directly observed time-resolved domain evolution patterns. Through a statistical analysis of ΔT, we find that the distribution P(ΔT) seems to follow a power-law behavior with the same form within the error range, irrespective of the film thickness.
Micromagnetic simulation of ferromagnetic part-spherical particles
Authors: Boardman Richard P. , Fangohr Hans , Cox Simon J. , Goncharov Alexander V. , Zhukov Alexander A. , de Groot Peter A. J.
Published: May 2004

The paramagnetic size limit for current magnetic storage media, particularly in sputtered grain storage, is being approached rapidly. To further increase media storage density, patterned media can be used which only need a single grain to store one bit of data. Chemical self-assembly techniques offer cost-effective methods to create templates, from which periodic arrays of magnetic structures can be formed. In contrast to systems of dots prepared by standard lithography, which have a cylindrical shape, dots prepared by chemical self-assembly template techniques are often spherical or part spherical in shape. In this article, we investigate the properties of such magnetic shapes using micromagnetic simulations. To represent accurately the geometry produced through chemical self-assembly methods, we attach a partial sphere (lower part) to a small ellipsoidal dome. We compute the hysteresis loops for various dot sizes and compare them with experimental results. In those below a critical diameter (140 nm in nickel), the hysteresis loop is square-like, resembling the uniform rotation of magnetization once the critical field is exceeded. For larger sizes, the hysteresis loop reverses reversibly around zero applied field but shows minor loops, placed symmetrically at the onset of magnetization reversal. These correspond to vortices penetrating and exiting the structure. In summary, we find that the coercive field of the droplets becomes zero above a critical diameter where the magnetization reversal behavior changes from single domain-like to vortex-like. Our results agree with experimental measurements performed on such structures.
Ion deposited nanoscale permalloy and Co films generated from laser excitation
Authors: Cadieu F. J. , Chen Li
Published: May 2004

A modified pulsed laser deposition system has been used to deposit permalloy and Co films from ions generated due to the impact of 248 nm excimer laser pulses. A synchronously pulsed magnetic field coil with a wide entrance throat and a tapered bore in conjunction with a set of permanent magnets has been used to capture and concentrate the ion and electron beam flux under vacuum conditions onto the substrates. Co and other films such as diamond like carbon (DLC) are strongly adherent when deposited onto even room temperature substrates. Permalloy films deposited onto Si (111) substrates with thicknesses from 10 to 50 nm exhibit anisotropic magnetoresistance of approximately 2% for magnetic fields applied in-plane parallel versus perpendicular to the current direction. Over 1 order of magnitude intensity modulation has been observed in x-ray reflectivity measurements for Co films, 7–15 nm thickness, deposited onto R-plane sapphire substrates and onto Si (111) substrates. In contrast to this Co films simultaneously deposited onto A-plane sapphire substrates hardly exhibited x-ray reflectivity fringes indicating a large surface roughness. Similarly DLC films deposited onto (111) Si substrates exhibited order of magnitude intensity modulations indicating a very small surface roughness. Scanning electron microscope measurements indicated smooth films were obtained with no discernible particulates.
Oscillatory magnetic anisotropy in Fe/Cr/Fe trilayers
Authors: Kim Ki-Yeon , Kim Dong-Hyun , Shin Sung-Chul , You Chun-Yeol
Published: May 2004

We report our experimental finding that there exists an oscillation of magnetic anisotropy with varying Cr sublayer thickness along the direction normal to the easy axis in Fe/Cr/Fe trilayers, where the easy axis is induced along a certain direction in the film plane by applying an external field during film deposition. Interestingly enough, the squareness of the hysteresis loop measured along the direction transverse to the easy axis exhibits an oscillatory behavior with a period of 5–6 Å Cr sublayer thickness. We also witness that the magnetic anisotropy measured by a torque magnetometer shows an oscillating behavior with the same periodicity. Domain evolution pattern investigated using a time-resolved magneto-optical microscope reveals that domain configuration correspondingly oscillates between simple 180° type and complex non-180° type with varying Cr thickness.
Magnetic properties of laser-induced ferromagnetic cobalt quantum dots passivated by ultrathin Ta film
Authors: Yang Jung Yup , Yoon Kap Soo , Do Young Ho , Koo Ja Hyun , Kim Chae Ok , Hong Jin Pyo , Son Si Ok , Kim Hyun Jung
Published: May 2004

The ferromagnetic quantum dots (FMQDs) are fabricated by exposing a pulsed Nd:YAG laser (wavelength=355 nm) on ultrathin cobalt (Co) films under an external magnetic field of 1000 Oe. Various dot diameters and densities of more than 3.5×1010/cm2 are ultimately realized by changing laser power, scan condition and initial film thickness. In addition, both AlN and Ta thin films as capping layers are used to protect the FMQDs from natural oxidation effect. The observed coercivity values of uncapped and capped FMQDs are changed from 600 up to 200 Oe with the initial Co thickness of 2 nm, respectively. The observation of magnetic force microscopy and hysteresis loops clearly confirms in-plane single domain state for the Ta-capped FMQDs.
Spin-polarized tunneling effects observed on the oxygen-terminated Fe3O4 (111) surface
Authors: Berdunov N. , Murphy S. , Mariotto G. , Shvets I. V. , Mykovskiy Y. M.
Published: May 2004

Under oxidizing preparation conditions the magnetite (111) surface reconstructs to a highly ordered superlattice. This surface reconstruction represents an oxygen termination of the magnetite bulk. We employ spin-polarized (SP) scanning tunneling magnetization (STM) to study the spin-dependent tunneling between a magnetite (111) sample and an antiferromagnetic tip through a vacuum barrier. Atomic scale STM images show significant magnetic contrast corresponding to variations in the local surface states induced by oxygen vacancies. The local variations of the tunneling magnetoresistance around these vacancies correspond to 150%. By employing SP–STM measurements and first principles calculations we could conclude that an oxygen top layer considerably changes the SP properties of the magnetite surface.
Structural ordering effects in Fe nanoparticle two- and three-dimensional arrays
Authors: Farrell D. , Ding Y. , Majetich S. A. , Sanchez-Hanke Cecilia , Kao Chi-Chang
Published: May 2004

Two- and three-dimensional arrays were prepared by self-assembly of iron nanoparticles with similar magnetic moments and interparticle separations, and characterized both magnetically and structurally. The rapid magnetization decay in the three-dimensional (3D) arrays suggests a relaxation mechanism than has been previously reported, perhaps associated with the existence of domain walls within large structurally ordered regions. Small angle x-ray scattering indicates the presence of such regions in the 3D arrays.
Multidomain and incoherent effects in magnetic nanodots
Authors: Skomski R. , Kashyap A. , Sorge K. D. , Sellmyer D. J.
Published: May 2004

The magnetic ground state and the magnetization reversal of aspherical nanoparticles are investigated by model calculations and micromagnetic simulations. Essential deviations from Kittel’s domain theory occur for very flat and very elongated particles, for particles whose size is comparable to the domain-wall width, and when the particle shape is strongly nonellipsoidal. For example, the single-domain state of square rod is much less stable than that of comparable elongated ellipsoids, because most of the surface charge of long ellipsoids is located on the sides. Another specific feature of flat particles is suppression of the curling mode.
Magnetic nanotubes produced by hydrogen reduction
Authors: Sui Y. C. , Skomski R. , Sorge K. D. , Sellmyer D. J.
Published: May 2004

FePt and Fe3O4 nanotubes are produced by hydrogen reduction in nanochannels of porous alumina templates and investigated by electron microscopy, x-ray diffraction, and superconducting quantum interference device magnetometry. Loading the templates with an Fe chloride and Pt chloride mixture, followed by hydrogen reduction at 560 °C, leads to the formation of ferromagnetic FePt nanotubes in the alumina pores. An Fe nitrate solution, thermally decomposed at 250 °C and reduced in hydrogen for 2.5 h at the same temperature, yields Fe3O4 tubes. The versatility of the method indicates that materials with a wide range of parameters can be produced.
Evidence of exchange-induced spin polarization in the magnetic semiconductor EuS
Authors: Müller C. , Lippitz H. , Paggel J. J. , Fumagalli P.
Published: May 2004

The semiconducting character and the large spin moment of 7/2 μB of europium sulfite are important preconditions for applications in the field of spintronics. However, the low Curie temperature (TC=16.5 K) excludes EuS from commercial application. We have studied a variety of Co/EuS multilayer samples deposited in a molecular beam epitaxy system with varying numbers of layers and different single-layer thickness. Magneto-optic Kerr effect measurements at low temperatures (10 K) indicate an antiferromagnetic coupling between EuS and Co moments.
Properties of thin film europium oxide by x-ray magnetic circular dichroism
Authors: Holroyd Johnathon , Idzerda Y. U. , Stadler Shane
Published: May 2004

Magnetic circular dichroism was measured as function of temperature for the Eu M5 and M4 absorption edges in single crystal EuO thin films. The M5 edge is resolved into two major features, one with the same helicity dependence as the M4 edge, and one with opposite helicity dependence. While the ratio of the absorption peaks with similar helicity dependence remains constant as a function of temperature, the ratio of those with opposite dependence is found to change linearly with temperature below 75 K. The physical mechanism for this linear behavior in the absorption peak intensity is not well understood.
Growth and magnetism of FePt:C composites in nanoscale channels
Authors: Sui Y. C. , Zhou J. , Li X. Z. , Skomski R. , Sellmyer D. J.
Published: May 2004

Nanochannels of porous alumina films were used as nanoreactors for the reaction of hydrogen gas with a mixture of Fe nitrate and Pt chloride. This results in the formation of of FePt clusters within the nanochannels. Both the sizes of the clusters and the coercivity the cluster assembly increase with the increase of annealing temperature. In order to reduce excessive agglomeration at high temperature, carbon was introduced by chemical vapor deposition and FePt:C composites in nanoscale channels were created. When FePt clusters were synthesized with carbon and heat treated at high temperatures, cluster sizes were much smaller than those without carbon, suggesting that the introduced carbon serves effectively to block the agglomeration of clusters. The coercivity of the FePt:C composite containing the smaller clusters is as high as 29.0 kOe.
Comparative x-ray absorption spectroscopy study of Co-doped SnO2 and TiO2
Authors: Lussier A. , Dvorak J. , Idzerda Y. U. , Ogale S. B. , Shinde S. R. , Choudary R. J. , Venkatesan T.
Published: May 2004

We performed x-ray absorption spectroscopy measurements at the cobalt L2,3 edge and the oxygen K edge of Co-doped SnO2 and Co-doped TiO2. Our measurements confirm that doped cobalt atoms are in the same local environment in both compounds. Furthermore, the results support the idea that cobalt atoms occupy substitutional cation sites. Additionally, the oxygen spectral shapes offer insight into a possible cause for the observed giant magnetic moment of cobalt atoms present in SnO2, but not in TiO2.
Synthesis and magnetic behavior of self-assembled Co nanorods and nanoballs
Authors: Ma Wei-Wen , Yang Yan , Chong Chong-Tow , Eggeman Alexander , Piramanayagam S. N. , Zhou Tie-Jun , Song Tao , Wang Jian-Ping
Published: May 2004

ε-cobalt nanoparticles with different shapes and sizes, e.g., rod-shaped Co (2×25 nm) and ball-shaped Co (diameter 4–9 nm), were synthesized by the rapid pyrolysis of octacarbonyldicobalt in o-dichlorobenzene solution in the presence of the surfactant, oleic acid. The size, size distribution, and the shape of the nanoparticles were influenced by changing the preparation conditions, such as the molar ratio of the surfactant (oleic acid) to reactant (cobalt carbonyl). The effects of the preparation conditions and annealing temperature on the shape and magnetic properties of cobalt nanoparticles have been investigated. It was found that as-synthesized rod-shaped cobalt nanoparticles (2×25 nm) show paramagnetic properties, but as-synthesized ball-shaped cobalt nanoparticles exhibit superparamagnetic behavior. After postannealing at 773 K for 30 min, the rod-shaped Co nanoparticles formed hcp structure and show ferromagnetic properties.