bluuebunny/crossref_metadata_2025 huggingface dataset:
Transition state in magnetization reversal
Authors: Brown G. , Novotny M. A. , Rikvold Per Arne
Published: May 2003

We consider a magnet with uniaxial anisotropy in an external magnetic field along the anisotropy direction, but with a field magnitude smaller than the coercive field. There are three representative magnetization configurations corresponding to three extrema of the free energy. The equilibrium and metastable configurations, which are magnetized approximately parallel and antiparallel to the applied field, respectively, both correspond to local free-energy minima. The third extremum configuration is the saddle point separating these minima. It is also called the transition state for magnetization reversal. The free-energy difference between the metastable and transition-state configurations determines the thermal stability of the magnet. However, it is difficult to determine the location of the transition state in both experiments and numerical simulations. Here it is shown that the computational Projective Dynamics method, applied to the time dependence of the total magnetization, can be used to determine the transition state. From large-scale micromagnetic simulations of a simple model of magnetic nanowires with no crystalline anisotropy, the magnetization associated with the transition state is found to be linearly dependent on temperature, and the free-energy barrier is found to be dominated by the entropic contribution at reasonable temperatures and external fields. The effect of including crystalline anisotropy is also discussed. Finally, the influence of the spin precession on the transition state is determined by comparison of the micromagnetic simulations to kinetic Monte Carlo simulations of precession-free (overdamped) dynamics.
Analysis of spectral noise density of hysteretic systems driven by stochastic processes
Authors: Mayergoyz I. , Dimian M.
Published: May 2003

A technique for computing spectral noise density of hysteretic systems driven by stochastic inputs is discussed. This technique is based on the mathematical machinery of stochastic processes on graphs. Computational results illustrating this technique are presented.
Intrinsic damping and intentional ferromagnetic resonance broadening in thin Permalloy films
Authors: Twisselmann D. J. , McMichael R. D.
Published: May 2003

Detailed ferromagnetic resonance measurements on thin Ni80Fe20 films are described that determine characteristics of intrinsic damping and the effects of intentionally created defects on linewidth. Measurements are made as a function of frequency with magnetization oriented in-plane and normal to the plane. For nominally uniform films, the ferromagnetic resonance linewidth is linear in frequency for both in-plane and normal magnetization and is well described by Landau–Lifshitz damping with a constant damping parameter. The effect of inhomogeneity was investigated by depositing the Ni80Fe20 films on specially prepared substrates, one with parallel, random grooves and one with polycrystalline, antiferromagnetic NiO. The linewidth in these samples is generally larger with magnetization in-plane than it is with magnetization normal. In the NiO/Ni80Fe20 film, the in-plane linewidth is linear in frequency with a significant zero-frequency linewidth. In the Ni80Fe20 sample on the grooved substrate, the linewidth has a more complicated frequency dependence.
Fully controlled precessional switching of a macrospin in a cross-wire geometry
Authors: Hiebert W. K. , Lagae L. , Das J. , Bekaert J. , Wirix-Speetjens R. , De Boeck J.
Published: May 2003

Ultrafast scanning Kerr microscopy measurements of half-precession-cycle, quasiballistic magnetization switching in a Permalloy™ macrospin are presented. The 20×7 μm2, 15-nm ellipse-shaped element is embedded in a continuous film of thinner Py, which effectively eliminates the remanent state closure domains and strongly increases coherence of dynamic motions in the large element. A perpendicular crossing-wire geometry allows pulse excitations to both the easy and hard axes. Data for well-terminated, ringing-suppressed ballistic switching and poorly-terminated switching with ringing are presented with hard axis pulses as small as 12 Oe and as short as ∼400-ps driving precessional switching. With addition of an easy-axis overlapping pulse, half-select precessional switching into a metastable state is observed; however, large spatial nonuniformities are introduced into the element dynamics, which ultimately defeat the half-select switching process.
Nonlinear microscopic relaxation of uniform magnetization precession
Authors: Safonov Vladimir L. , Bertram H. Neal
Published: May 2003

Dynamic relaxation for nonlinear magnetization excitation is analyzed. For direct processes, such as magnon–electron scattering and two-magnon scattering, the relaxation rate is determined from the linear case simply by utilizing the magnetization oscillation frequency for nonlinear excitation. For an indirect process, such as slow-relaxing impurities, the analysis gives an additional relaxation term proportional to the excitation level. In all cases the effective magnetization damping is increased compared to Landau–Lifshitz–Gilbert damping.
90° coupling induced by exchange biasing in PtMn/CoFe10/CoFe2O4/CoFe10 films
Authors: Maat S. , Gurney B. A.
Published: May 2003

We observe 90° coupling of the two CoFe10 layers in sputter-deposited PtMn/CoFe10/CoFe2O4/CoFe10 films, in which the first CoFe10 layer is exchange biased to PtMn. The 90° coupling of the two CoFe10 layers is induced by the unidirectional anisotropy of the CoFe10 pinned layer, since it is absent in the as-deposited sample, exhibiting no exchange biasing. We observe that the coupling changes from 90° to collinear for increasing CoFe2O4 thickness or by inserting a thin Al layer between the pinned layer and CoFe2O4 layer. A model, which can qualitatively explain the observed behavior by assuming magnetic roughness due to spin-frustration at the CoFe2O4/CoFe10 interfaces is proposed. The observed effect could potentially be used to set the easy axes of two ferromagnetic layers in device structure perpendicular to each other in a single annealing step, for example, the flux closure layer and the in-stack biasing layer in spin or tunnel valves with in-stack biasing.
Correlation between structure and magnetic properties of manganite-based multilayers
Authors: Sirena M. , Haberkorn N. , Granada M. , Steren L. B. , Guimpel J.
Published: May 2003

We have studied the structure, magnetic, and transport properties of manganite-based multilayers A/Bi, with A=La0.55Sr0.45MnO3 (metallic ferromagnet), and spacers (Bi) of different nature: B1=SrTiO3 (nonmagnetic insulator), B2=La0.9Sr0.1MnO3 (insulator ferromagnet), and B3=La0.67Ca0.33MnO3 (paramagnetic-insulator/ferromagnet-metal). The samples are strongly textured in the direction perpendicular to the sample surface, and present a good interface quality with small roughness and interdiffusion. We have found that in the A/B1/A trilayers, the ferromagnetic electrodes are ferromagnetically coupled for thin spacer layers and becomes decoupled for spacer thickness larger than 3 nm. Instead, the others multilayers are ferromagnetic for all spacer thicknesses and temperature range. This result was expected for temperatures well below the spacer Curie temperature. We attributed the ferromagnetic behavior of the system, found for temperatures above the ordering temperature of the spacer, to direct exchange coupling through short-range ordered zones in spacer layer. As expected from magnetization results, in fact, no extrinsic magnetoresistance was measured in these systems in the temperature range between 4.2 and 300 K.
Magnetic interactions in Ni–Cu/Cu superlattice nanowire arrays
Authors: Robinson A. , Schwarzacher W.
Published: May 2003

The remanent magnetization of Ni–Cu/Cu superlattice nanowires was investigated. Arrays of superlattice nanowires were prepared by template deposition through polycarbonate nanoporous membranes using a single electrolyte bath. The thicknesses of the nickel-rich layers (tNi) and copper layers (tCu) were independently controlled by monitoring the current during deposition. The wire diameter was determined by transmission electron microscopy imaging to be 80 nm. A study of the remanent magnetization at 20 K for a range of values of tNi and tCu reveals the existence of magnetic interactions within each array. It is noted that for an array with tNi of 200 Å, the strength of the interactions decreases with increasing tCu, indicating that the interactions are taking place between nickel layers. However the interaction strength appears to reach a minimum level, beyond which an increase in tCu does not correspond to a decrease in the interaction strength observed. A minimum interaction level is also observed for an array with tNi of 50 Å, for all values of tCu investigated.
Evolution of magnetic anisotropy in epitaxial Fe films by ferromagnetic resonance
Authors: Zhai Y. , Shi L. , Zhang W. , Xu Y. X. , Lu M. , Zhai H. R. , Tang W. X. , Jin X. F. , Xu Y. B. , Bland J. A. C.
Published: May 2003

Single-crystal Fe films of 4 to 33 monolayers (ML) on GaAs (100) prepared by molecular-beam epitaxy were studied by ferromagnetic resonance (FMR). The evolution of both in-plane and out-of-plane anisotropies was observed. The FMR data show a predominant in-plane uniaxial magnetic anisotropy in the films with the thickness t less than 12.7 ML, with an easy axis along the 〈11_0〉 direction. An in-plane fourfold anisotropy due to cubic magnetocrystalline anisotropy starts to appear and coexists with uniaxial magnetic anisotropy when t=8.4 ML, and increases with increasing film thickness. For t=33 ML, the cubic anisotropy constant K1 reaches 28.2×104 erg/cm3, which is 60% of the value for bulk bcc Fe. A strong perpendicular anisotropy in the ultrathin Fe films was observed. For t=4 ML, the perpendicular anisotropy constant is as high as 14.6×106 erg/cm3. It decreases with increasing the thickness t and reaches about 2.1×106 erg/cm3 for 33 ML, while a small fourth-order perpendicular anisotropy appears and increases with the same trend as the cubic anisotropy.
Spin-reorientation transition of epitaxial Cu/Ni/Cu (001) structure
Authors: Hwang H. M. , Park J. C. , You D. G. , Park H. S. , Jeong K. , Lee J. , Kim T. G. , Song J. H.
Published: May 2003

We have studied the spin-reorientation transition of Cu/Ni/Cu (001) system by measuring the strain, coercive field, and magnetic anisotropy of epitaxial Cu/Ni/Cu (001) films. We found that the critical thickness (tc) of Ni for the coherent growth of Ni on Cu is below 25 Å and does not occur at the peak in the effective magnetic anisotropy constant (Keff) versus 1/t curve, where t is the Ni thickness. The 60-Å Ni film, which is highly strained and shows out-of-plane magnetization, has been irradiated by 1-MeV C ions. After irradiation, the magnetization lies in the plane and the strain in the Ni film is almost released. This shows the close relationship between strain and out-of-plane magnetization in this system.
Structural order and magnetic anisotropy transition in Co/Fe multilayers
Authors: Carbucicchio M. , Bennett S. , Berry F. J. , Prezioso M. , Rateo M. , Turilli G.
Published: May 2003

Co/Fe multilayers were electron-beam evaporated in ultrahigh vacuum onto quartz substrates keeping the Co layer thickness (10 nm) constant and changing that of Fe (10–30 nm). For Fe layer thicknesses up to 24 nm, the magnetization substantially lies in the film plane and shows a uniaxial magnetic anisotropy. The coercive field measured along the easy axis is ∼10 Oe, and the x-ray reflectivity patterns show a superlattice behavior. For a Fe layer thickness equal to 30 nm, the in-plane texture strongly decreases, the coercive field increases up to ∼100 Oe, the magnetization direction forms an out-of-plane angle of ∼36° and a stripe magnetic domain structure takes place. The observed in-plane anisotropy and the changing in the magnetic order as a function of the iron layer thickness is discussed and justified, assuming that the growth of the first Co layer occurs by the nucleation of ordered zones, influencing the subsequent layer order via exchange interaction.
Ferromagnetic resonance study on single and multiple layers of rare-earth, transition-metal alloys
Authors: Wang Yixing , Zhao Yanling , Wang Shiqi , Lu Mu , Zhai Hongru , Zhai Ya , Shono Keiji , Yu Xiangyou
Published: May 2003

Single layers of GdFeCo (40 nm), GdFeCoSi (40 nm), TbFeCo (40 nm), and their bilayers and trilayers with the compositions near those in the medium of magnetic super-resolution magneto-optical (MO) storage made by sputtering on glass substrate with buffer and capping layers of SiN (5 nm) were studied by measuring the hysteresis loop and by ferromagnetic resonance (FMR). TbFeCo layer shows strong perpendicular anisotropy. GdFeCoSi layer shows easy-plane anisotropy. However, in some films an inverted loop was observed in the direction of film normal. FMR shows the existence of two phases with, respectively, easy-plane and easy-normal anisotropy, which explains the inverted loop. FMR on a GdFeCo layer shows a cone anisotropy with easy direction at 30° with respect to film normal. TbFeCo/GdFeCoSi shows a double Hc hysteresis loop and an easy-plane anisotropy for the GdFeCoSi sublayer. In GdFeCo/GdFeCoSi bilayers, both sublayers show easy-cone anisotropy with directions at 70° and 60° from the normal. GdFeCo/GdFeCoSi/TbFeCo trilayer film shows predominantly the resonance peak of the GdFeCoSi sublayer with easy-cone anisotropy of about 60° and a unidirectional anisotropy due to the interfacial exchange effect.
Experimental study of magnetic anisotropy in multilayered, metal-evaporated tapes
Authors: Bottoni Giancarlo
Published: May 2003

The magnetic anisotropy in metal-evaporated thin films for flexible media is studied, mainly to determine the distribution of anisotropy fields. The analyzed films have different compositions (Co and CoNi) and number of superimposed layers. The presence of a multilayered structure affects the average anisotropy field and the anisotropy distribution width in the analyzed metal-evaporated films. Such evolution is compared with the change of the macroscopic magnetic properties of the tapes. In addition, the effect of the temperature decrease on the magnetic anisotropy distribution is studied in the metal-evaporated films, in which the presence of exchange anisotropy at low temperature is observed. The results are discussed with respect to the relationship between the evolution of magnetization switching with the number of layers and the decrease in temperature.
Fluctuating field calculations for a bimodal medium
Authors: Rao Sandeep , Bennett Lawrence H. , Torre Edward Della , Chen Andrew Peijie , Fry Richard A.
Published: May 2003

Bimodal magnetic behavior has been observed in a (0.3-nm Co/1.2-nm Pt)15 multilayer film. In that film, the Kerr rotation and ellipticity exhibit magneto-optic hysteresis loops with two distinct field-dependent steps, so that there are four stable magnetization states. The magnetic aftereffects of the two distinct types of decay at the two sets of holding fields have been analyzed. Thus, it is possible to measure and calculate the two types of decay individually. We show that the shift in log time of the aftereffect curves for both the sets are linear. The fluctuation fields for the two states are calculated using these methods. We also confirm experimentally the conjecture of the Preisach–Arrhenius model that the shape of the aftereffect curve is the same as the major hysteresis curve. This helps in finding the chemical potential.
Magnetic properties of the U(Ge1−xNix)2 system
Authors: Rojas D. P. , Medina A. N. , Gandra F. G. , Cardoso L. P.
Published: May 2003

Magnetic results for the U(Ge1−xNix)2 series of compounds with x=0.1, 0.2, 0.3, and 0.4 are reported. For x⩽0.2 the system is ferromagnetic with Tc close to 52 K. For the remaining samples we found a spin-glass ground state with Tf around 30 K. The specific heat for the x=0.3 sample present an anomaly at 50 K, which does not seems to be of magnetic origin. We discuss the results in terms of the possible formation of the compound U2NiGe3.
Electric, thermal, and magnetic properties of hexagonal and cubic UPtSn
Authors: Lin C. L. , Zan J. A. , Yuen Tan , Myer G. H.
Published: May 2003

Resistivity, specific heat, and magnetization measurements have been carried out for polycrystalline UPtSn. Samples as cast and those annealed at temperatures below 300 °C crystallize in a hexagonal structure. The structure of samples annealed between 800 and 935 °C transforms into the cubic MgAgAs phase. The hexagonal UPtSn sample undergoes a ferromagnetic transition at TC=27 K. Below TC both the zero-field-cooled (ZFC) and field-cooled (FC) magnetization data initially increase as the temperature is decreased. However, when the temperature is lowered further, the ZFC magnetization drops significantly whereas the FC magnetization saturates. This irreversible behavior in the ZFC and FC curves could be attributed to intrinsic domain-wall pinning. On the other hand, the cubic UPtSn sample is paramagnetic down to the lowest measured temperature 0.5 K. The resistivity of cubic UPtSn is unusually large and exhibits a semiconductor-like behavior.
Magnetoresistance of magnetite films prepared by reactive evaporation
Authors: Furubayashi Takao
Published: May 2003

It was attempted to fabricate magnetite films by evaporating metallic iron in an oxygen atmosphere onto substrates at room temperature. From the characterization by x-ray diffraction and Mössbauer spectroscopy, the obtained films were found to consist of α-Fe, FeO, and Fe3O4 depending on the oxygen pressure. Pure magnetite was obtained when prepared in 5×10−6 Torr oxygen. The magnetite film exhibited negative magnetoresistance resulting from spin-dependent scattering at the grain boundaries. The result indicates that magnetite films with spin-polarized electrons can be prepared on substrates at room temperature.
Tunneling in epitaxial Fe/Si/Fe structures with strong antiferromagnetic interlayer coupling
Authors: Gareev R. R. , Pohlmann L. L. , Stein S. , Bürgler D. E. , Grünberg P. A. , Siegel M.
Published: May 2003

Fe(5 nm)/Si(0.8–2 nm)/Fe(5 nm) structures are grown by molecular-beam epitaxy on Ag(001) buffered GaAs substrates. Ferromagnetic tunneling junctions with crossed electrodes and junction areas ranging from 22 to 225 μm2 are patterned using photolithography. Antiparallel alignment of the magnetizations due to antiferromagnetic interlayer coupling, which is confirmed by longitudinal magneto-optical Kerr effect hysteresis loops, exists for the whole range of spacer thicknesses. Transport properties in current perpendicular to the sample plane geometry are examined by the four-point method in the temperature range from 4 K to room temperature. As a function of spacer thickness, the junctions show a strong increase of the resistance times area product from ≈1 Ω μm2 to more than 10 kΩ μm2. The dI/dV−V curves are parabolic and asymmetric and thus characteristic for trapezoidal tunneling barriers. The mean barrier heights derived from Brinkman fits range from 0.3 to 0.8 eV. The zero-bias resistance of the tunneling junctions moderately decreases with temperature by less than 10% over the whole measured temperature range. All these transport properties fulfill the necessary and sufficient criteria for elastic tunneling.
Intermixing of aluminum-magnetic transition-metal bilayers
Authors: Buchanan J. D. R. , Hase T. P. A. , Tanner B. K. , Chen P. J. , Gan L. , Powell C. J. , Egelhoff W. F.
Published: May 2003

Grazing incidence x-ray scattering has been used to study interfacial intermixing in thin films of aluminum/transition metal bilayers grown by dc magnetron sputter deposition at room temperature. As with all transition metals, the ferromagnets Fe, Co and Ni have dramatically different interface widths between X/Al and Al/X (X=Fe,Co,Ni). Intermixing lengths are larger for X on Al than for Al on X.
Thermal dependence of magnetotransport in nanogranular magnetic media
Authors: Jalil M. B. A.
Published: May 2003

Magnetotransport simulation is performed on granular nanomagnets (Co) in insulator, of average radius of 2.5 nm, over a temperature range (5<T<1000 K) which straddles the single-domain and superparamagnetic regimes. The M-H hysteresis is calculated based on a two-state model, which is solved analytically using the Master Equation. The two-state model is then refined to account for fluctuations into states in the vicinity of the two minima. The occupation probability of these states is determined by a birth–death chain analysis. The resulting M-H hystereses show decreasing coercivity with T. At higher T>200 K, the M-H curve approaches the Langevin function, but with a small discrepancy, due to the intrinsic anisotropy of Co. The magnetization results are then combined with a stochastic Monte Carlo transport model which combines the effects of stochastic spin-polarized tunneling, Coulomb blockade, and the magneto- and electrostatic influence of the contacts. The tunneling magnetoresistance shows a complex thermal dependence, with distinct behavior for different types of contact electrodes used.
Conductance anomaly and density of state effects in magnetic tunnel junctions
Authors: Xiang X. H. , Zhu T. , Du J. , Xiao John Q.
Published: May 2003

We have investigated the bias dependence of conductance in magnetic tunnel junctions and found that the conductance minima can have a large shift from zero bias and the shifts are spin dependent. These anomalous behaviors have not been observed in tunnel junctions based on nonmagnetic electrodes. With a modified Brinkman’s model, by incorporating the voltage dependent density of states of the ferromagnetic electrodes, these conductance anomalous behaviors can be explained. The proposed model can also explain the bias dependence of magnetoresistance and resistance over a large bias range. All these results demonstrate that the density of states of the ferromagnetic electrodes play an important role in defining the bias dependence behaviors in magnetic tunnel junctions.
Microstructure characteristics of hot-pressing Pr–Fe–B–Cu
Authors: Pan Jing , Liu Xincai , Cai Changchun , Xu Zhifeng
Published: May 2003

In order to research the formation mechanism of [006] texture of cast/hot-pressed magnets, effects of compositions, hot deformation parameters, and thermal annealing on [006] texture and microstructures of hot-pressed magnets of PrxFe93.5−xB5Cu1.5 (x=15–19) have been studied under the conditions of hot-pressing temperature T=973–1273 K, strain rate V=10−6–100/S, and strain E=0%–80%. Higher praseodymium content is detrimental to the formation of the ideal texture [006]. Only at a hot-pressing temperature of 973–1123 K and without fractures during the hot deformation, can [006] texture of PrxFe93.5−xB5Cu1.5 be obtained. Below the critical temperature of the equal strength among Pr2Fe14B and the residual grain-boundary phases, 1173 K, the boundary phases have a higher strength than that of Pr2Fe14B, and most of deformation can be focused within Pr2Fe14B grains and after thermal annealing at 1123 K for 8 h, I006 become more distinguished. The Br of these magnets reach 1320 mT and their magnetic domains, on the longitudinal section, are platelike, with domain walls parallel to the hot deformation direction. The domains on the transverse section are mazelike.
Longitudinal and perpendicular exchange bias in FeMn/(FeNi/FeMn)n multilayers
Authors: Sun L. , Zhou S. M. , Searson P. C. , Chien C. L.
Published: May 2003

Exchange bias in ferromagnetic (FM)/antiferromagnetic (AF) bilayers is usually investigated in the longitudinal configuration with the exchange coupling established in the film plane. In this work, we report on the perpendicular exchange bias in FeMn(8 nm)/[FeNi(2 nm)/FeMn(8 nm)]n multilayers induced by perpendicular field cooling. The thin FeNi layers give rise to large values of the exchange field and coercivity, and n=15 allows a sufficiently large magnetization for the measurements. Even though the soft FeNi layers have an intrinsic in-plane anisotropy, perpendicular exchange bias has been observed after cooling in a perpendicular external field. The exchange field in the perpendicular configuration is about 0.85 that of the longitudinal case. In both the longitudinal and perpendicular configurations, the exchange field decreases quasilinearly with temperature. The squareness of perpendicular hysteresis loops decreases with increasing temperature.
Biodetection using magnetically labeled biomolecules and arrays of spin valve sensors (invited)
Authors: Ferreira H. A. , Graham D. L. , Freitas P. P. , Cabral J. M. S.
Published: May 2003

On-chip spin-valve sensors (2×6 μm2) were used to detect the binding of streptavidin-functionalized superparamagnetic labels to sensor surface-bound biotin. Both micron-sized and nanometer-sized labels were studied. The detection of biomolecular recognition was demonstrated using 2 μm Micromer®-M and 250 nm Nanomag®-D labels, with signals ranging from ∼300 μV to ∼2 mV (8 mA sense current; ∼15 Oe in-plane magnetizing fields). For smaller labels, detection of biomolecular recognition was not achieved. The capability of detecting single labels was demonstrated for label moments down to 2×10−12 emu (2 μm labels), corresponding to signals of 100–400 μV. Although, theoretical calculations suggest that the minimum measurable moment is in the order of 6×10−15 emu, due to noise limitations of the present setup, this limit is in the order of 2×10−13 emu, corresponding to a single 250 nm label. On-chip tapered aluminum current line structures were used for placement of magnetic labels at sensor sites.
Switching behavior of cross contacting current perpendicular to plane giant magnetoresistance structure fabricated by two-step ion milling technique
Authors: Oishi H. , Nozaki Y. , Matsuyama K.
Published: May 2003

A current perpendicular to plane giant magnetoresistance structure with a cross contact geometry was fabricated by a two-step ion-milling technique. Multilayer pillars with various layer structures and a lateral dimension down to 0.4 μm were embedded between orthogonal top and bottom electrodes. The microfabricated cross point structure minimizes the influence of electrode resistance in measurements of magnetoresistance in current perpendicular to plane geometry. Individual switching of Co layer was observed in a pillar with layer structure of [Co(10 nm)/Cu(4 nm)/Co(2 nm)/Cu(10 nm)]3 and lateral dimension of 0.4×2.0 μm2. Distinct three-step resistance change was observed for the rising part of the magnetoresistance (MR) curve, corresponding to the switching of Co(2 nm) layers. Somewhat complicated MR behavior in the falling part suggests the existence of a local minimum configuration during the switching of Co(10 nm) layers. The switching behavior of magnetic layers with interlayer dipole coupling was studied by micromagnetic simulations. Markedly different switching field of Co layers can be attributable to the variation of the dipole fields at each layer.
Combustion synthesis of the magnetoresistive double perovskite (Ba1.6Sr0.4)FeMoO6
Authors: Douvalis A. P. , Venkatesan M. , Velasco P. , Fitzgerald C. B. , Coey J. M. D.
Published: May 2003

Structural, magnetic, and magnetoresistive properties of the double perovskite (Ba1.6Sr0.4)FeMoO6, produced by two different methods are compared. The low field magnetoresistance effect at room temperature reaches a value of 10% in a field of 425 mT in samples prepared by combustion synthesis, whereas a field of 1110 mT is required for the same magnetoresistance in samples prepared by solid state reaction. The proportion of Fe/Mo antisite defects, determined by Mössbauer spectroscopy, is twice as great for the former as for the latter, and influences both the magnetic and magnetoresistive properties. The valence state of iron ions in different cation neighbor environments in (Ba1.6Sr0.4)FeMoO6, evaluated from the Mössbauer hyperfine parameters, is consistent with that found for a series of other isostructural iron-based double perovskites.
Slow relaxation of grain boundary resistance in a ferromagnetic manganite
Authors: Kozlova N. , Dörr K. , Eckert D. , Handstein A. , Skourski Y. , Walter T. , Müller K.-H. , Schultz L.
Published: May 2003

The resistance relaxation of a polycrystalline ferromagnetic La0.7Sr0.3MnO3 thin film has been studied. Time-dependent resistance data R(t), recorded after field pulses of 7 and 47 T, respectively, show a pronounced relaxation of approximately logarithmic type for 10 ms<t<20 s and temperatures T<100 K. The resistance relaxation has also been measured in a superconducting quantum interference device magnetometer, yielding similar relaxation rates. An unusual increase of the relaxation rate with decreasing temperature down to 4.2 K is observed. While polycrystalline samples show this type of relaxation, it is absent in an epitaxial film, indicating the origin in the grain boundary regions between misaligned grains. Slow relaxation might be caused by spin glass-like magnetic order at grain boundaries; however, no freezing temperature has been found down to 4.2 K.
Exchange coupling between Cr and ferromagnetic thin films
Authors: Yang F. Y. , Chien C. L.
Published: May 2003

Exchange bias has been observed in Ni81Fe19/Cr bilayers. The exchange bias field for Cr thickness tCr larger than 62 Å increases with tCr and levels off at tCr=150 Å, whereas coercivity increases without saturation. The blocking temperature increases with the Cr thickness. Due to the presence of commensurate spin density waves in the thicker but strained Cr layers, blocking temperature as much as 425 K has been observed. Exchange bias has also been observed in Co/Cr bilayers.
Bias and temperature dependence of hot electron transport in a spin-valve structure
Authors: Hong Jisang , Wu R. Q.
Published: May 2003

We have explored spin-dependent hot electron transport in a magnetic tunneling junction at finite temperatures. The inelastic mean-free path has been extracted from either theoretical calulations or experimental measurements. The temperature dependence of hot electron spin polarization has been modeled according to P0[1−(T/Tc)3/2]. The bias dependence of magnetocurrent agrees in trend with experiments, which indicates that the model adopted here indeed displays the key factors in these phenomena.
Spin-wave frequency discretization in submicron rectangular prisms
Authors: Gubbiotti G. , Candeloro P. , Businaro L. , Di Fabrizio E. , Gerardino A. , Zivieri R. , Conti M. , Carlotti G.
Published: May 2003

Spin-wave frequency discretization has been observed by Brillouin light scattering in an array of tangentially magnetized 800 nm×550 nm Ni81Fe19 rectangular prisms with thicknesses of 30 nm and dot separations of 200 nm. For a large wave vector interval, several discrete dispersionless modes were observed. The experimental frequencies were compared to those calculated using a recently developed analytical model for flat uniformly magnetized rectangular prisms. The agreement between the experimental and calculated frequencies is very good. In addition to the above mentioned modes, a low frequency mode was experimentally detected over a wide range of transferred wave vectors. Finally, the decrease in frequency of the first perpendicular standing spin-wave mode, observed in the patterned sample with respect to in the continuous film, was successfully reproduced.
Structure and magnetic properties of (Nd1−xHox)3Fe23−yCo6Vy compounds
Authors: Liu B. D. , Li W. X. , Wang J. L. , Wu G. H. , Yang F. M. , Li Y. X.
Published: May 2003

The Ho-substituted (Nd1−xHox)3Fe23−yCo6Vy compounds with x=0–0.9 have been synthesized and their structure and intrinsic magnetic properties have been investigated by x-ray diffraction and magnetic measurements. It is found that all the investigated compounds crystallize in the Nd3(Fe,Ti)29-type structure with monoclinic symmetry and A2/m space group. The unit-cell volume V shows a decreasing tendency with increasing Ho content, reflecting the lanthanum contraction. All the compounds show easy-plan type anisotropy at room temperature. The Curie temperatures Tc are almost independent of the Ho content and the spin reorientation temperature, Tsr, decreases with increasing Ho content from 266 K for x=0 to 127 K for x=0.7. No spin reorientation occurs for x=0.9. The saturation magnetization Ms decreases linearly with increasing Ho content due to the antiferromagnetic coupling between Ho and Nd moments. The anisotropy field Ba increases first, going through a maximum at x=0.5, y=2.0, and then decreases with increasing Ho content.
Nanoparticle surface charge density in ionic magnetic fluids: The effect of particle–particle interaction
Authors: Fanyao Qu , Morais P. C.
Published: May 2003

In this study, particle–particle interaction in ionic magnetic fluids is systematically investigated by self-consistently solving the coupled Schrödinger and Poisson equations. It was found that particle–particle interaction changes the colloid characteristics by changing the energy level position E and the nanoparticle surface charge density σ. Formation of bonding and antibonding states due to particle–particle interaction in a two-particle system (dimer) was studied as a function of particle diameter, particle–particle distance, and band offset. The coupling strength within the dimer is characterized by a systematic dependence of both the bonding and antibonding levels and surface charge density upon particle diameter, particle–particle distance, and band offset values. The quantum-model picture discussed in the article opens up new insights into the physics of coupled colloidal particles.
Observation of ion beam induced magnetic patterning using off-specular polarized neutron reflectometry
Authors: Telling N. D. , Langridge S. , Dalgliesh R. M. , Grundy P. J. , Vishnyakov V. M.
Published: May 2003

The long-range magnetic structure in Co/Pt multilayers magnetically patterned by ion irradiation is observed by off-specular polarized neutron reflectivity. While both specular and off-specular measurements indicate the formation of an artificial domain structure when the sample is in its remanent state, resonant peaks seen in the diffuse scatter reveal long-range magnetic ordering with periodicity in agreement with the design value. These peaks are completely suppressed when the sample is saturated in plane, confirming their origin in the magnetic patterning of the multilayer.
Domain-wall trapping in a ferromagnetic nanowire network
Authors: Saitoh E. , Tanaka M. , Miyajima H. , Yamaoka T.
Published: May 2003

The magnetic domain configuration in a submicron Ni81Fe19 wire network has been investigated by magnetic force microscopy. To improve the responsivity of the magnetic force microscope, an active quality factor autocontrol method was adopted. In the remanent state, domain walls were observed trapped firmly at the vertexes of the network. The magnetic domain configurations appear to minimize the exchange energy at the vertexes. These results indicate that the magnetic property of the ferromagnetic network can be described in terms of the uniform magnetic moments of the wires and interwire magnetic interactions at the vertexes. The observed structure of the domain walls is well reproduced by micromagnetic simulations.
CoPt hard magnetic nanoparticle films synthesized by high temperature chemical reduction
Authors: Sui Y. , Yue L. , Skomski R. , Li X. Z. , Zhou J. , Sellmyer D. J.
Published: May 2003

The synthesis of hard-magnetic CoPt nanoparticle films by hydrogen reduction of Co nitride and Pt chloride mixture is reported. Thin porous alumina films are adopted as the carrier of the initial aqueous solution and of the final reducing products of CoPt nanoparticles. It is found that chemical ordering of L10–CoPt occurs above 400 °C. Partial phase transformation occurs in the alumina substrate when the treatment temperature is higher than 600 °C. The film coercivity increases with increasing annealing temperature and reaches a maximum value of 24.2 kOe when the reduction is carried out at 700 °C for 2 h.
High anisotropy Sm–Co nanoparticles: Preparation by cluster gun technique and their magnetic properties
Authors: Stoyanov S. , Skumryev V. , Zhang Y. , Huang Y. , Hadjipanayis G. , Nogués J.
Published: May 2003

Sm–Co nanoparticles with 7 nm mean diameter have been obtained by gas condensation in a cluster gun. By absorbing light from quartz halogen bulbs, the nanoparticles are annealed during their flight from the cluster gun to the substrate and are embedded in a carbon matrix by cosputtering. This in situ annealing results in nanoparticles with moderate coercivity. The moderate coercivity and its strong temperature dependence imply a strong effect of the thermal fluctuations, which can be partly overcome by embedding the nanoparticles in an antiferromagnetic matrix (CoO) rather than in a paramagnetic C.
Study of the enhancement of the magnetic properties of Fe70Al30 in the order-disorder transition
Authors: Apiñaniz E. , Plazaola F. , Garitaonandia J. S. , Martı́n D. , Jimenez J. A.
Published: May 2003

The order-disorder transition produced by ball milling in the intermetallic Fe70Al30 has been systematically studied by x-ray diffraction, calorimetry, and Mössbauer experiments. These techniques show a monotonous transformation that ends after 6 h of milling. In the transition, the lattice parameter increase amounts to 0.7% and there is a large enhancement of the alloy’s magnetism.
On the first-order phase transition in MnFeP0.5As0.4Si0.1
Authors: Tegus O. , Brück E. , Dagula W. , Li X. W. , Zhang L. , Buschow K. H. J. , de Boer F. R.
Published: May 2003

We have studied the first-order magnetic phase transition and related physical properties of MnFeP0.5As0.4Si0.1 by means of x-ray diffraction, magnetization, thermal expansion, and electrical resistance measurements. The compound crystallizes in the hexagonal structure with Fe2P-type. The temperature dependence of magnetization shows a first-order magnetic phase transition around 299 K. There is no crystallographic structural transition associated with the magnetic phase transition. The linear thermal expansion shows that this magnetic phase transition is accompanied by a negative volume change. The electrical resistance shows an anomalous behavior at the transition, which is probably caused by the change in c/a ratio.
Nanocrystalline NdFeB magnets fabricated by a modified hot-working process
Authors: Kim Hyoungtae , Cho Sungho , Kim Yoonbae , Kim Hakshin , Kapustin G. A.
Published: May 2003

Nanocrystalline and textured NdFeB magnets were prepared from isotropic melt-spun Nd12Dy2Fe73.2Co6.6Ga0.6B5.6 powder by a modified hot-working process using direct joule heating. The isotropic magnet obtained by current applied (CA) press was composed of randomly oriented Nd2Fe14B grains of 50–100 nm in diameter, and the optimized magnetic properties were Br=0.81 T (8.1 kG), Hci=2022 kA/m (25.4 kOe), and (BH)max=117 kJ/m3 (14.7 MG Oe). After CA deformation, the grains grew to platelets with 50–100 nm in thickness and 200–400 nm in length, and the magnet showed excellent properties of Br=1.31 T (13.1 kG), Hci=1457 kA/m (18.3 kOe), and (BH)max=333 kJ/m3 (41.8 MG Oe).
Spin-rotation phenomena and magnetic phase diagrams of hexagonal RMnO3
Authors: Fiebig M. , Lottermoser Th. , Pisarev R. V.
Published: May 2003

Magnetic phase diagrams of the hexagonal RMnO3 (with R=Sc, Y, Ho, In, Er, Tm, Yb, Lu) compounds in the magnetic-field/temperature plane are established by linear and nonlinear magneto-optical techniques. Eight different magnetic phases of the Mn3+ sublattice and at least three different phases of the R3+ sublattices are distinguished. Phase coexistence and photoinduced modification of the magnetic structure are observed. Magnetic reorientations of the Mn3+ lattice can occur via in-phase or antiphase rotation of the triangularly frustrated spins in the adjacent crystallographic planes.
Dynamic modeling of tubular moving-magnet linear actuators
Authors: Clark R. E. , Jewell G. W. , Howe D.
Published: May 2003

This article examines the influence of eddy currents on the dynamic performance and efficiency of a small reciprocating permanent magnet actuator for use in air compressors. Transient finite element simulations are compared with results from magnetostatic analyses and measurements. It is shown that induced eddy currents reduce the efficiency by ∼5%. The influence of the excitation current wave form on the eddy current loss and the actuator efficiency is also examined.
Comparative study of the electronic structure of ternary superconductors MoRuP and ZrRuP in the orthorhombic and hexagonal phases
Authors: Ching W. Y. , Xu Yong-Nian , Ouyang Lizhi , Wong-Ng W.
Published: May 2003

The electronic structures of the ternary superconductors MoRuP and ZrRuP in the orthorhombic (o-) and hexagonal (h-) phases were calculated by a first-principles method and then compared. For h-MoRuP, the theoretically predicted structure was used. Based on the comparative studies of the electronic structures of the known superconductors of o-MoRuP (Tc=15.5 K), o-ZrRuP (Tc=4 K), and h-ZrRuP (Tc=13 K), it is argued that h-MoRuP may have a transition temperature Tc exceeding 20 K.
Cation intermixing and ordering phenomenon in M–O layer of MSr2YCu2Oz (M-1212) compounds with M=Fe, Co, Al, and Ga: A neutron powder diffraction study
Authors: Awana V. P. S. , Takayama-Muromachi E. , Malik S. K. , Yelon W. B. , Karppinen M. , Yamauchi H. , Krishnamurthy V. V.
Published: May 2003

Various MSr2YCu2Oz (M-1212 with M=Fe, Co, Al, and Ga) compounds have been synthesized through a solid-state reaction route. All these compounds are derived from the basic CuBa2YCu2O7−δ structure. Rietveld structural refinement of the room-temperature neutron diffraction data for Fe-1212 reveals that nearly half of Fe remains at the M site, while the other half goes to the Cu site in the CuO2 plane. For the M=Co, Al, and Ga compounds, the basic unit cell gets doubled due to two distinct M sites in the M–O layer and the space group Ima2, without any cation intermixing is found to be more appropriate. For the M=Fe sample, magnetic ordering of Fe spins is seen from magnetization data below 70 K. In the case of the M=Co compound, a shallow down-turn in magnetic susceptibility is seen below 150 K, indicating low-dimensional magnetic characteristics of the Co–O layer. The compounds with M=Al and Ga show paramagnetic behavior in their magnetization data down to 5 K. The oxygen content, established for each compound based on oxygen occupancies, refined from the neutron diffraction data comes close to 7.0 for each.
Improving sintered NdFeB permanent magnets by powder compaction in a 9 T superconducting solenoid
Authors: Mulcahy T. M. , Hull J. R. , Rozendaal E. , Wise J. H. , Turner L. R.
Published: May 2003

Commercial-grade magnet powder (Magnequench UG) was axial die pressed in the 76.2 mm warm bore of a 9 T superconducting solenoid. Otherwise, processing was performed as part of normal factory operations. This pressing was done to improve the alignment of the anisotropic single-crystal particles of the compact and, thus, the remanent magnetization of the sintered cylindrical permanent magnets (12.7 mm diameter). Although the press was operated in batch mode for this proof-of-concept study, its design enables automated production. Improvements of up to 8% in magnetization and 16% in energy products were obtained, as the alignment field H was increased above the 2 T maximum field of electromagnets used in industry. The greatest improvements were obtained for magnets with the smallest length-to-diameter ratios, L/D<0.5. The production of quality magnets in this near-final-shape size range is currently being pursued by industry to eliminate expensive machining steps. To understand the potential for 2–8 T alignment fields to overcome the distortions created in the otherwise uniform field by the self-field of short compacts, electromagnetic code (Opera) calculations were made. A simple material model was used to predict the distortions. The trends in the predicted field-line inclinations, with L/D and H, compare to trends in the improvement of the magnetic properties.
Magnetic properties of Mn–Al system alloys produced by mechanical alloying
Authors: Saito Tetsuji
Published: May 2003

Mechanical alloying of Mn-Al and Mn-Al-C powder mixtures resulted in the formation of α-Mn solid solution. Heat treatment of these mechanically alloyed powders resulted in the formation of the metastable τ phase. The mechanically alloyed powders annealed at relatively low temperatures (500–700 °C) showed small saturation magnetization of less than 1 emu/g in both Mn–Al and Mn–Al–C powders. On the other hand, the Mn–Al powders showed saturation magnetization of 8 emu/g and their Mn–Al–C counterparts exhibited large saturation magnetization of more than 20 emu/g when annealed at relatively high temperature (800–1100 °C). Although the annealed Mn–Al powders showed small coercivity, the Mn–Al–C powders exhibited large coercivity, up to 3.3 kOe, after selected heat treatment.
Determination of binding constant Kb of biocompatible, ferrite-based magnetic fluids to serum albumin
Authors: Tedesco A. C. , Oliveira D. M. , Lacava Z. G. M. , Azevedo R. B. , Lima E. C. D. , Gansau C. , Buske N. , Morais P. C.
Published: May 2003

In this work, we investigated the interaction between molecular-coated magnetic nanoparticles (MC-MNPs) and serum albumin proteins (BSA) through the fluorescence quenching of the tryptophan residue present in BSA after the binding of MC-MNPs to specific sites. Three different biocompatible magnetic fluid (BMF) samples based on magnetite or cobalt–ferrite MNPs coated with citric acid or dextran were used. The binding constant and the stoichiometry of the investigated MNPs indicate that the BMF based on cobalt–ferrite is more site specific and more strongly bound to the BSA than the BMFs based on magnetite. The results may direct the design of new magnetic drug-carriers based on BMFs.
Kinetics of pinhole nucleation and growth in magnetic tunnel junctions
Authors: Shang Changhe , Chen Yingjian , Moon Ki-seok
Published: May 2003

Tunnel junctions with different pinhole probabilities were obtained by varying natural oxidation condition. In pinhole-free junctions, the observed abrupt changes in magnetoresistance and junction resistance with increasing bias current are the signature of dielectric breakdown, and are attributed to the process of pinhole nucleation. On the contrary, low RA junctions show gradual changes in magnetoresistance and junction resistance with voltage, indicating the absence of pinhole nucleation, but only the process of pinhole growth starting from the inherent pinholes. Experimentally, the activation energy of pinhole growth (EG∼0.3 eV) is found to be much lower than that of pinhole nucleation (EN∼0.6eV), which explains the easy breakdown behavior observed in low RA junctions.
Effect of atomic disorder on transport through magnetic tunnel junctions
Authors: Humphry K. J. , Stamps R. L. , Folks L.
Published: May 2003

Spin-dependent tunneling through magnetic junctions is sensitive to material properties near the interface and in the barrier. Results of calculations are presented showing how electron transmission through a point contact tunnel junction is affected by atomic disorder in the barrier. Giant variations in the transmission probability are found with limited disorder. Sharp peaks appear in the tunnel current when defects exist at positions in the barrier that facilitate electron hopping across the contact. Consequences for thin film tunnel junction devices are also discussed, with reference to experiments showing strong spatial variations in tunnel current.