bluuebunny/crossref_metadata_2025 huggingface dataset:
Exchange coupling in FePt permanent magnets
Authors: Simizu S. , Obermyer R. T. , Zande B. , Chandhok V. K. , Margolin A. , Sankar S. G.
Published: May 2003

FePt (for 40–60 at. % Fe) exhibits an order–disorder transformation. The disordered phase is face centered cubic and magnetically soft while the ordered phase is tetragonal and shows high magnetic anisotropy. Since the changes in volume between the two phases are small, it is easy for the soft and hard phases to coexist in a uniform manner. Thus, we have an ideal system with which to investigate the basic features of exchange coupled magnets. Bulk Fe0.6Pt0.4 exhibits reasonably large permanent magnetic properties with a maximum energy product of ∼15 MG Oe (120 kJ/m3) without the need for special processes to promote grain alignment. The high energy product is partially a result of the high ratio of remanence to the saturation induction which amounts to 0.68 as opposed to the ratio of 0.5 for an assembly of randomly oriented uniaxial magnets. This enhanced remanence is predicted by the exchange-spring magnet model for a mixture of cubic and uniaxial phases. In order to verify that the high remanence of the FePt magnets is indeed caused by this exchange mechanism and not by a fortuitous formation of magnetic texture during sample preparation, we have prepared cubic samples of Fe0.6Pt0.4 and measured the B–H loops along the three principal directions of the cube using a vibrating sample magnetometer. The hysteresis loops are identical in all three directions, indicating the absence of strong texture in this material. After correction of the demagnetizing factor, the ratio of remanence to saturation induction is 0.66–0.68. This is consistent with that in the exchange coupling model. The highest energy product in our study was found for the as-quenched samples. Further annealing, with the intention of promoting the formation of hard magnetic phases, reduced the coercivity and remanence. This suggests that improved results may be achieved by initial suppression of the formation of the hard phases by modification of the quenching process and alloying. An ideal exchange coupled system could be designed based on such a starting material.
Ordering in the site frustrated Heisenberg ferromagnet revisited
Authors: Beath A. D. , Ryan D. H.
Published: May 2003

Monte Carlo simulations of frustrated and nonfrustrated versions of the three-dimensional site disordered Heisenberg model have been used to determine the sequence and nature of magnetic ordering. At sufficiently high concentrations, x, of antiferromagnetic sites for an infinite percolating network to form (0.31<x<0.69), two ordering events occur: ferromagnetic and antiferromagnetic order that coexist. Coupling the two networks introduces frustration but does not destroy either transition. However, it appears to change the universality class of the antiferromagnetic transition.
Magnetic properties and specific heat of new spin glass Mn0.5Fe0.5PS3
Authors: Takano Yoshiki , Arai Atsuko , Takahashi Yumiko , Takase Kouichi , Sekizawa Kazuko
Published: May 2003

We have found spin glass phenomena in a layered Mn0.5Fe0.5PS3 compound and investigated its magnetic properties and specific heat. The temperature dependence of the zero field cooled (ZFC) magnetization along the easy axis, M∥ under dc applied field Ha below 1 kOe shows a cusp at Tg=32.6 K. The FC and ZFC curves measured at Ha below 1 kOe separate at almost the same temperature as Tg. For Ha larger than 1 kOe, the temperature at which FC and ZFC curves separate deviates from Tg and shifts toward lower temperature and the peak becomes broader. Magnetic hysteresis is still observed above 70 kOe at 2 K. The remarkable feature of this spin glass phase is that the spin freezing is not suppressed up to a rather high magnetic field compared with similar spin glass (Mn,Fe)TiO3. The cusp temperature of χ′(ω) does not show magnetic field amplitude dependence from 1 to 10 Oe and increases with an increase in frequency, obeying the Vogel–Fulcher rule. The temperature dependence of χ″(ω) shows two extra peaks at Tm1 and Tm2 below Tg. The time dependence of thermoremanent magnetization MTRM below Tg shows power law dependence. The value of the molar specific heat is about an order of magnitude larger than that of MnPS3 and FePS3 at 2 K. The T-linear term characteristic of spin glass is observed below about 10 K. The temperature dependence of the magnetic specific heat shows no anomaly at Tg, Tm1=9 and Tm2=16 K, and shows two peaks at Tc1=13 and Tc2=67 K.
Curie–Weiss analysis of ferromagnetic and glassy transitions in nanostructured GdAl2
Authors: Williams D. , Shand P. M. , Stark C. , Pekarek T. , Brown R. , Yue Lanping , Leslie-Pelecky D. L.
Published: May 2003

Structural inhomogeneity on length scales comparable to magnetic interaction lengths can produce complex magnetic behavior. Crystalline GdAl2 is a ferromagnet with a Curie temperature of 170 K, while amorphous GdAl2 thin films exhibit classic spin-glass behavior with a freezing temperature of 16 K. Nanostructured GdAl2, made by mechanically milling initially crystalline GdAl2, exhibits ferromagnetic and spin-glass-like transitions; however, the spin-glass-like transition occurs at a higher temperature than the freezing temperature of amorphous GdAl2 thin films. Curie–Weiss analysis suggests that the paramagnetic-to-ferromagnetic transition is due to the ferromagnetic ordering of small GdAl2 clusters and that the glassy transition is most likely due to spin-glass-like ordering of a surface/interface phase.
Magnetic properties of Mn- and Eu-doped ZnS nanocrystals
Authors: Tsujii Naohito , Kitazawa Hideaki , Kido Giyuu
Published: May 2003

The magnetic properties of Mn- and Eu-doped ZnS nanocrystals have been investigated. The results suggest that most of the Mn ions are coordinated on the surface of the nanocrystal, and the amount of Mn doped inside the nanocrystal does not exceed one per nanocrystal. Low-temperature magnetization shows the absence of antiferromagnetic interaction between Mn ions, in contrast to the bulk materials. For Eu-doped nanocrystal, the magnetic susceptibility data reveal the coexistence of Eu2+ and Eu3+ ions, though the starting material contains only Eu3+ ions.
Electronic structure and exchange interactions in V15 magnetic molecules: LDA+U results
Authors: Boukhvalov D. W. , Dobrovitski V. V. , Katsnelson M. I. , Lichtenstein A. I. , Harmon B. N. , Kögerler P.
Published: May 2003

Single-molecule magnets V15 (K6[V15As6O42(H2O)]⋅8H2O) have attracted a great deal of attention recently. These magnets are promising systems for studying such fundamental problems as spin tunneling, spin coherence, and low-temperature spin relaxation. To understand in detail the internal magnetic and electronic structure, and the intramolecular interactions responsible for the formation and low-energy excitations in V15 molecules, we have performed electronic structure calculations using the local spin density approximation with on-site repulsion paramter U (so-called LSDA+U) approach. The calculated values of magnetic moments and charge states of vanadium ions agree well with experiments confirming the V4+ state of vanadium ions. We found that for U∼4–5 eV, good description of known properties of V15 molecule can be achieved. Comparing the results of the band-structure calculations with the experiments, we have found that the LDA+U description of the V15 magnet is in agreement with experimental data.
An Fe4 cluster with a slowly relaxing paramagnetic excited state: [Fe4O(OH)5(tacn)4]I7⋅2.5 H2O
Authors: Zipse D. , Abboud K. A. , Dalal N. S.
Published: May 2003

We report the synthesis, x-ray structure, and magnetic susceptibility of a magnetic compound [Fe4O(OH)5(tacn)4]I7⋅2.5 H2O (Fe4), where tacn=1, 4, 7 triazacyclononane. Fe4 crystallizes in the monoclinic space group, P2(1)/c, with cell parameters a=18.7604(9) Å, b=12.5840(6) Å, c=24.064(1) Å, α=90°, β=108.783(1)°, and γ=90°. Dc susceptibility, χdc, experiments show the magnetic moment of Fe4 saturates at approximately 0.2 μB, giving a spin ground state of S=0. However, on application of dc fields, a peak develops in χdc, which is ascribed to the presence of a low lying excited magnetic state. Interestingly, ac susceptibility experiments show frequency dependence of χ″ and thermally activated magnetization switching dynamics, with a potential energy barrier of 26 K. The origin of these observations still remains unexplained.
Magnetic and magneto-optical properties of ultrathin Fe50Pt50 films with Ag layers inserted
Authors: Zhang Zhengang , Kang Kyongha , Suzuki Takao
Published: May 2003

Ultrathin Fe50Pt50 (2 nm)/Ag (x nm)/Fe50Pt50 (2 nm)/Ag (x nm)/Fe50Pt50 (2 nm) multilayer films are deposited onto a MgO (100) substrate by laser ablation and ex situ annealed at 630 °C for 15 min, with the Ag inserted layer changing in thickness from x=0 to 4 nm. (001) textured, highly ordered (ordering parameter S∼0.9) FePt films are obtained when x⩾1 nm. Both the perpendicular coercivity and anisotropy field (Hk) show a dependence on the inserted Ag layer thickness, with a peak appearing at x=1 nm. In the sample with the highest Hk, the rotational mode contributes to magnetization reversal, while in lower Hk samples domain wall motion plays a dominant role. The polar Kerr spectroscopy and optical constants of these samples are measured with photon energy in the range of 1.4–6.8 eV. The feature of diagonal dielectric elements at around 3.8 eV reveals the Ag plasma resonance effect. However, no obvious Kerr effect enhancement at the Ag plasma edge is observed.
Depth profiles of magnetostatic and dynamic characteristics in annealed Co66Fe4B15Si15 amorphous ribbons
Authors: Rheem Y. W. , Kim C. G. , Lee B. S. , Jin L. , Kim C. O. , Shalyguina E. E. , Gan’shina E. A.
Published: May 2003

We investigated the magnetostatic and dynamic properties of annealed and etched Co66Fe4B15Si15 amorphous ribbons. The near-surface magnetic characteristics of the annealed samples showed the gradual growth of a hard magnetic layer with increasing annealing time; that is, coercivity Hc and saturation field Hs increase from 15 to 600 Oe and from 45 to 1500 Oe, respectively. On the etching effect on the magnetic property in an 8 h annealed sample, the hard magnetic layer near the sample surface was retained for the sample with etching time tetch<40 s, but the influence of the ribbon core on the near-surface magnetic properties of the sample was discovered for tetch⩾50 s. When the thickness of hard magnetic layer is about 1 μm, the asymmetric giant magnetoimpedance (GMI) profiles, the so-called GMI valve, do not show hysteresis. As the thickness is reduced, a hysteretic GMI is observed due to the cyclic change of magnetization in the hard magnetic layer under a cyclic field.
Applied voltage dependence of nano-oxidation of ferromagnetic thin films using atomic force microscope
Authors: Takemura Yasushi , Kidaka Seiichi , Watanabe Keizo , Nasu Yasuaki , Yamada Tsutomu , Shirakashi Jun-ichi
Published: May 2003

Nanodots of Ni, CoFe, and Cr oxide were fabricated by the nano-oxidation technique using atomic force microscope. The dot size was controlled from 40 to 200 nm by changing the pulse voltage applied to the cantilever from 2 to 10 V. In order to evaluate the size of the nanostructures quantitatively, the electric field emitted from the cantilever was calculated. The threshold electric field strength was defined as the minimum strength to promote the oxidation. The threshold field strength of the order of 107 V/m was derived by fitting the experimental results. The voltage dependence of the size of fabricated Cr-oxide dots was fitted well by the calculation. The dot size of the ferromagnet-based oxide was fluctuating and did not agree with the calculation. From the theoretical analysis, it was suggested that the size of the nanostructures did not depend on the distance between the cantilever and film surface, but significantly depended on the curvature radius of the cantilever.
Fabrication and anisotropy investigations of patterned epitaxial magnetic films using a lift-off process
Authors: Heyderman L. J. , Kläui M. , Rothman J. , Vaz C. A. F. , Bland J. A. C.
Published: May 2003

We demonstrate a technique to fabricate patterned epitaxial magnetic films using a lift-off process and investigate the magnetic properties using the magneto-optical Kerr effect (MOKE). 150 μm epitaxial face-centred-cubic squares are patterned and a method to extract the anisotropy constant from the hard axis switching field is presented. The maximum switching field on the hard axis can be read easily from the transverse magnetization loop and a second-order magneto-optical effect present in these thin-film systems is used to extract transverse and longitudinal magnetization hysteresis loops from single-longitudinal MOKE measurements. The anisotropy values obtained for the lift-off elements are consistent with those of nonpatterned continuous films of the same material so indicating that the lift-off process presented here has no adverse effects on the epitaxial growth.
Analysis of thermal effects in thin-film media
Authors: Peng Qingzhi , Richter H. J.
Published: May 2003

A simple solution for dynamic coercivity was derived to study the effect of the second-order anisotropy K2 on thermally assisted magnetization reversals for both perpendicular and longitudinal media. For longitudinal media, the energy barrier is calculated by adopting the approach of approximate expansion of total energy. In the balance between the thermal stability and writability, only the media with easy axis well oriented along the field direction will benefit from the effect of K2 due to a relatively larger increase in the energy barrier than in the intrinsic switching field.
High-frequency electron paramagnetic resonance investigations of tetranuclear nickel-based single-molecule magnets
Authors: Edwards R. S. , Maccagnano S. , Yang E.-C. , Hill S. , Wernsdorfer W. , Hendrickson D. , Christou G.
Published: May 2003

We report preliminary high-frequency electron paramagnetic resonance (EPR) investigations for several tetranuclear nickel complexes which exhibit single-molecule magnetism, including low-temperature (below ∼1 K) hysteresis loops and resonant magnetic quantum tunneling. The combination of a cavity perturbation technique and a split-coil magnet facilitates high-sensitivity, multifrequency (40 to 200+ GHz), angle dependent single-crystal EPR measurements. The data confirm the expected S=4 ground states, and a negative magnetocrystalline anisotropy for each member in the series. An unusual splitting of the easy-axis EPR peaks is observed, which may be interpreted in terms of distinct Ni4 species within the crystals. Overall, however, the trends associated with the splitting, as well as the EPR linewidths and shapes, suggest that intermolecular exchange interactions are important. Indeed, differences between the EPR spectra obtained for different complexes correlate nicely with the expected strength of exchange interactions, as determined both from intermolecular contact distances and from independent hysteresis measurements.
Unconventional Heisenberg spin triangle in magnetic molecule {V15}: A proton nuclear magnetic resonance study
Authors: Procissi D. , Suh B. J. , Jung J. K. , Kögerler P. , Vincent R. , Borsa F.
Published: May 2003

We present results of H1 nuclear magnetic resonance (NMR) and relaxation in polyoxovanadate cluster {V15} with formula K6[V15As6O42]⋅9(H2O). The data in {V15} are compared with the published data in {V6} with the formula [CN3H6]4Na2[H4{V3L}2P4O4]⋅14H2O, which contains two independent spin triangles. Temperature T and field H dependence of the NMR linewidth are well explained by the magnetic dipolar interaction between the proton nuclei and vanadium ion spins in both compounds. T and H dependence of proton relaxation rate T1−1 show great differences in the two compounds. The difference indicates a completely different spin dynamics for the two Heisenberg spin-triangle systems whereby in {V15} the fluctuations of the VIV (s=1/2) in the central triangle are dominated by the interlayer coupling with the VIV spins in the two upper and lower hexagons.
Tunneling splitting of magnetic levels in Fe8 detected by H1 NMR cross relaxation
Authors: Furukawa Y. , Aizawa K. , Kumagai K. , Ullu R. , Lascialfari A. , Borsa F.
Published: May 2003

Measurements of proton NMR and the spin lattice relaxation rate 1/T1 in the octanuclear iron (III) cluster [Fe8(N3C6H15)6O2(OH)12]⋅[Br8⋅9H2O], in short Fe8, have been performed at 1.5 K in a powder sample aligned along the main anisotropy z axis, as a function of a transverse magnetic field (i.e., perpendicular to the main easy axis z). A big enhancement of 1/T1 is observed over a wide range of fields (2.5–5 T), which can be attributed to the tunneling dynamics; in fact, when the tunneling splitting of the pairwise degenerate m=±10 states of the Fe8 molecule becomes equal to the proton Larmor frequency a very effective spin lattice relaxation channel for the nuclei is opened. The experimental results are explained satisfactorily by considering the distribution of tunneling splitting resulting from the distribution of the angles in the hard xy plane for the aligned powder, and the results of the direct diagonalization of the model Hamiltonian.
Effective spin Hamiltonian for magnetic clusters in presence of S mixing
Authors: Carretta S. , Liviotti E. , Magnani N. , Amoretti G.
Published: May 2003

A quite general and straightforward procedure to study the contributions to the high-order anisotropy terms due to the mixing between states with different spin S in magnetic clusters has been presented. This procedure is based on a perturbational approach and consists of adding to the effective spin Hamiltonian some terms depending on the anisotropic interactions causing the mixing. These terms are written as functions of some new operators which depends on the total spin S. The present approach has been applied to study the behavior of the cluster Cr8.
Magnetoresistance magnetometry of (Ni80Fe20)1−xIrx wires with varying anisotropic magnetoresistance ratio
Authors: Vaz C. A. F. , Blackburn E. , Kläui M. , Bland J. A. C. , Gan Li , Egelhoff W. F. , Cambril E. , Faini G. , Wernsdorfer W.
Published: May 2003

We report room and low temperature magnetoresistance measurements on submicrometer scale elements (wires and crosses, 1.0, 0.5, and 0.1 μm) of (Ni80Fe20)1−xIrx (x≈2%) with varying Ir concentrations. For the particular Ir concentrations studied, the anisotropic magnetoresistance (AMR) ratio varied between −0.043% and −0.035% at room temperature. For the single wires (0.5–1 μm) the MR versus field characteristic is similar to that reported for Ni80Fe20 wires of similar size, but an opposite sign of the AMR is observed for x>2%. For H parallel to the wire axis, the reversal is determined by the sweeping of a few domains which give rise to “ears” in the MR–H characteristic. For H perpendicular to the wire, a parabolic variation is seen corresponding to rotation of the magnetization, but a negative slope in the MR-H characteristic is obtained at high fields as in permalloy wires. This suggests that the high field behavior originates from a distinctly different mechanism (e.g., surface scattering) from that of AMR.
Junction area dependence of breakdown characteristics in magnetic tunnel junctions
Authors: Kim Kwang-Seok , Cho B. K. , Kim T. W. , Park W. J.
Published: May 2003

Breakdown characteristics of the magnetic tunnel junctions (MTJ) with different junction areas of S=200 μm2 and S=0.5 μm2 are investigated under constant voltage stress. The breakdown process is found to be quite different for the two junction areas. For the large junctions with S=200 μm2, magnetoresistance (MR) ratio decreases gradually with increasing time of constant voltage stress and lasts for 10–30 min. The low frequency 1/f noise power also increases with increasing stress time due to the Johnson and shot noises, caused by current flowing through ohmic shorts or pinholes across tunnel barrier. Meanwhile, the junctions with S=0.5 μm2 show abrupt junction breakdown after stress time of 1–7 h. There is no significant change in both MR ratio and its bias dependence during the voltage stress. In particular, soft-breakdown events are observed before total breakdown occurs. The junction breakdown of small junction area is of intrinsic nature of the junction while the one of large junction area is due to extrinsic factors.
Modeling of the magnetomechanical effect: Application of the Rayleigh law to the stress domain
Authors: Li L. , Jiles D. C.
Published: May 2003

Stress is one of the principal external factors affecting the magnetization of materials. The magnetomechanical effect, that is, the change of magnetization of a magnetic material resulting from the application of stress, has attracted attention because of its scientific complexity. An improved model equation for interpreting the magnetomechanical effect has been developed based on extension of the previous equation to include the Rayleigh law. According to the previous theory of the magnetomechanical effect, which is based on the “law of approach,” application of stress induces changes in magnetization toward anhysteretic magnetization which itself is stress dependent, and the rate of change of magnetization with the input elastic energy is dependent on the displacement of the prevailing magnetization from the anhysteretic magnetization. The theory has been refined by including a linear term in the model equation in addition to the well-known quadratic term. It was found that the modified theory provides a much better description of the magnetization changes under stress, particularly at small applied stress amplitudes and when the stress changes sign.
Thermal expansion studies on the unusual first order transition of Gd5Si2.09Ge1.91: effects of purity of Gd
Authors: Han M. , Jiles D. C. , Snyder J. E. , Lo C. C. H. , Leib J. S. , Paulsen J. A. , Pecharsky A. O.
Published: May 2003

Two polycrystalline samples were made by using high purity Gd and commercial Gd, respectively, but with Si and Ge starting materials of the same purity in both cases. Thermal expansion results showed that both samples exhibited a first order phase transformation, with a discontinuity in thermally-induced strain and with hysteresis in the Curie temperature. Magnetic force microscopy has been used to demonstrate the magnetic phase transformation process from paramagnetic to ferromagnetic upon cooling. It was found that the Curie temperature was lower and the thermally-induced strain higher, in the sample made from lower purity level Gd starting materials compared with the sample made from high purity Gd metal. These results indicate that the impurities (mainly C, O, N, and F) in the Gd starting material can significantly alter the strain and Curie temperature of Gd5(SixGe1−x)4 alloys.
Construction of hysteresis loops of single domain elements and coupled permalloy ring arrays by magnetic force microscopy
Authors: Zhu Xiaobin , Grütter P. , Metlushko V. , Hao Y. , Castaño F. J. , Ross C. A. , Ilic B. , Smith H. I.
Published: May 2003

Magnetic structure and magnetization reversal of permalloy ring arrays and elongated permalloy particle array were studied by magnetic force microscopy (MFM). For single domain permalloy particles, the hysteresis loop is constructed by counting the percentage of switched elements imaged at remanence. For permalloy ring elements, two different states are energetically stable: a vortex state and an onion state. Their hysteresis loop is obtained by MFM imaging at a field between the switching fields of these two states. The magnetostatic coupling among these ring elements is directly revealed.
Effects of Ag buffer layer on the magnetic properties of ultrathin Co/Ge(111) films
Authors: Tsay J. S. , Nieh H. Y. , Yang C. S. , Yao Y. D. , Chin T. S.
Published: May 2003

The influences of an Ag buffer layer on the magnetic properties of ultrathin Co/Ge(111) films grown at ambient temperature were studied using the surface magneto-optic Kerr effect technique. Due to the intermixing of Co adatoms and Ge(111) substrate, a nonferromagnetic layer forms at the initial stage of the deposition for Co/Ge(111) films. We demonstrate experimentally that the ferromagnetic inactive layer can be reduced by introducing an Ag buffer layer and the orientation of the easy axis of magnetization remains unchanged. Three magnetic phases have been resolved for Co/Ag/Ge(111) films with a Ag thickness less than 6 monolayers. In the phase diagram, the boundary between ferromagnetic and nonmagnetic regions shifts to a lower Co thickness from 10 to 5 monolayers as the thickness of the Ag layer increases from 0 to 6 monolayers.
Exchange bias in Co/Fe/FexMn1−x/Cu(100) ultrathin films
Authors: Kuo C. C. , Pan W. , Chen Y. C. , Lin Minn-Tsong
Published: May 2003

Stable and well-grown face-centered-cubic Fe films were prepared on buffer layers with varying lattice constants by depositing FexMn1−x alloy film on Cu(100) single crystal. No ferromagnetic ordering was observed at the stage of 30 ML Fe on the FexMn1−x/Cu(100) systems in the temperature range from 100 to 350 K. Furthermore, capping of Co on Fe/FexMn1−x/Cu(100) was employed as the probe of antiferromagnetic ordering by study of exchange bias coupling in these films. The exchange bias of the hysteresis loops can be observed after field cooling of the films. Further analyses by varying the measurement temperature and Fe coverage of the films were also carried out to clarify the origin of the exchange bias coupling observed. The exchange bias field found here is attributed to the interlayer coupling between the Co and Fe–Mn films through the spacing layer Fe.
Simple transition parameter expression including grain size and intergranular exchange
Authors: Wang Xiaobin , Bertram H. Neal
Published: May 2003

A microtrack model approach is utilized to extend the simple William–Comstock (WC) formula to include finite grain size and intergranular exchange. The model assumes microtracks with finite size grains in each microtrack and random phase of grain centers between microtracks. Intergranular exchange is modeled as a mean field. For WC parameters much less than grain size, the model gives a finite transition parameter limit . The effect of exchange is to reduce the transition parameter.
Anisotropy characterization of garnet films by using vibrating sample magnetometer measurements
Authors: Andrei Petru , Dimian Mihai , Krafft Charles , Mayergoyz Isaak , Mircea Dragos Iulian , Rojas Rafael
Published: May 2003

A technique for the anisotropy characterization of garnet thin films is presented. A vectorial vibrating sample magnetometer is used to measure the magnetization from which the components parallel and orthogonal to the direction of the applied field are deduced. It is shown that this measurement can be used to retrieve the anisotropy function g(θ), the critical field curve, the anisotropy constants, as well as the anisotropy field.
Analysis of magnetic aftereffects in strontium hexagonal ferrites with W-type stoichiometry
Authors: Hernández-Gómez P. , Torres C. , de Francisco C. , Muñoz J. M. , Hisatake K.
Published: May 2003

The relaxation of the initial permeability has been measured in polycrystalline Sr hexaferrites with the initial composition of W phase (SrO⋅9Fe2O3). The samples have been prepared by means of standard ceramic techniques at different temperatures in the 1250 °C<T<1420 °C range in air as well as CO2 sintering atmospheres and then rapidly quenched to provide the presence of crystal vacancies. X-ray diffraction spectra reveal the presence of hexaferrite as the main phase. In the temperature range between 80 and 500 K, the magnetic disaccommodation is represented by means of isochronal curves. The isochronal spectra show the presence of different after-effect processes whose maxima lie at 365 K, 300 K, and 165 K (respectively, A, B, and D peaks). They correspond to thermally activated relaxation processes with activation energies of 1 eV, 0.8 eV, and 0.5 eV, respectively. Processes A and B are connected to ionic reorientations of ferrous cations and lattice vacancies within the octahedral sites in spinel-like S blocks whereas D peak is associated with a similar process in face-sharing octahedral sites located in the hexagonal R blocks of the lattice. Regarding the corresponding isochronal spectra of barium W-type ferrites, we can observe that the C process centered at 240 K does not appear in Sr hexaferrites. This suggests that the relaxation processes which take place in the hexagonal R blocks are not favorable in Sr hexaferrites, probably due to the smaller size of the Sr ion.
Anomalous magnetoresistance behavior of CoFe nano-oxide spin valves at low temperatures
Authors: Ventura J. O. , Sousa J. B. , Salgueiro da Silva M. A. , Freitas P. P. , Veloso A.
Published: May 2003

We report magnetoresistance curves of CoFe nano-oxide specular spin valves of MnIr/CoFe/nano-oxidized CoFe/CoFe/Cu/CoFe/nano-oxidized CoFe/Ta at different temperatures from 300 to 20 K. We extend the Stoner–Wolfarth model of a common spin valve to a specular spin valve, introducing the separation of the pinned layer into two sublayers and their magnetic coupling across the nano-oxide. We study the effect of different coupling/exchange (between the antiferromagnetic layer and the bottom sublayer) field ratios on the magnetization and magnetoresistance, corresponding with the experimentally observed anomalous bumps in low temperature magnetoresistance curves.
Magnetoresistance effect in Ag–Fe3O4 and Al–Fe3O4 composite films
Authors: Hsu Jen-Hwa , Chen Shang-Yi , Chang Wen-Ming , Jian T. S. , Chang Ching-Ray , Lee Shan-Fan
Published: May 2003

The Agx–(Fe3O4)1−x and Agx–(Fe3O4)1−x composite films were prepared by dc sputtering on Si(100) substrates. The x-ray diffraction results show that the films contain essentially only the cubic inverse spinal phase from Fe3O4 and face-centered cubic phase from Ag or Al. The transmission electron microscopy images indicate that the metal granules are randomly distributed with Fe3O4 grains. The resistivity determined from the four-probe method decreases rapidly with increasing metal content. At x≒0.5, a percolation occurs. The conducting path is formed from metal granules in series with Fe3O4 grains. The magnetoresistance (MR) is defined to be {R(H=0.8 T)−R(H=0)}/R(H=0). It has been found that MR is isotropic and the appearance of Ag granules has significant impact on the MR effect. Furthermore, a positive MR region appears with 0.011<x<0.1 in Agx–(Fe3O4)1−x. On the contrary, the incorporation of Al granules does not have the same effect on MR as in Agx–(Fe3O4)1−x. A slow increase of MR with Al content might be due to Coulomb blockade. The extra contribution to MR in Agx–(Fe3O4)1−x can be attributed to spin injection from Fe3O4 into Ag granules so that spin accumulation in Ag granules impedes the current causing a larger resistance under a field.
Role of Ag seed layer for CoCrPt/Ti perpendicular recording media
Authors: Jang Pyungwoo , Hong Sooyoul , Kim Jongryul
Published: May 2003

2 nm metal seed layers (M=Al, Cu, Ni, Cr, Ag, Mg, Fe, Co, Pd, Au, Pt, Mo, and Hf) were sputtered to increase coercivity (Hc) and anisotropy (Ku) of CoCrPt/Ti perpendicular recording media. Among them 2 nm Ag seed layer was very effective to increase Hc of (Co78Cr22)100−xPtx/Ti (x=14,20). However, the effect was more pronounced when (Co78Cr22)100−xPtx films became thinner. In addition α [=4π(dM/dH)Hc] decreased when the Ag layer was used. The film thickness below which the seed Ag layer was effective was reduced with decreasing Pt content. However, x-ray diffraction data showed that the Ag seed layer did not promote (002) texture of Ti and CoCrPt layers. In magnetic force microscope observations, domain size was reduced when the Ag seed layer was used. The reason for the higher coercivity of CoCrPt films with the Ag seed layer is thought to be due to change of exchange constant of the grains, for which the grain boundary area plays an important role. Effects of film thickness and Pt content can also be explained successfully by the variation of exchange constant due to grain boundary.
Spin-memory loss and current-perpendicular-to-plane-magnetoresistance in sputtered multilayers with Au
Authors: Kurt H. , Chiang Wen-C. , Ritz C. , Eid K. , Pratt W. P. , Bass J.
Published: May 2003

We derive a spin-diffusion length at 4.2 K in sputtered Au, lsfAu=35−5+65 nm, spin-memory-loss at Au/Cu interfaces, δAu/Cu=0.13−0.02+0.08, and Au/Cu interface specific resistance, 2ARAu/Cu=0.35−0.05+0.10 fΩ m2. We also show that exchange biased spin valves with Au sandwiched between Co layers produce changes in specific resistance, AΔR, comparable to those for Cu and Ag.
Synthesis and characterization of Zn2U (Ba4Zn2Fe36O60) hexaferrite powder
Authors: Lisjak D. , Drofenik M.
Published: May 2003

Polycrystalline Zn2U (Ba4Zn2Fe36O60) was synthesized. The high-energy milling of the starting powders activated the synthesis of the Zn2U at 1100 °C. Energy dispersive spectroscopy supported by x-ray diffraction analysis confirmed that pure Zn2U was synthesized in the temperature range 1200–1300 °C. The magnetic properties of the Zn2U were measured. The coercivity varied with the synthesis conditions, while the saturation magnetization was independent of the synthesis conditions. Nevertheless, both the coercivity and the saturation magnetization were different from those of the basic components of the Zn2U crystal structure: M (BaFe12O19) and Zn2Y (Ba2Zn2Fe12O22).
Effects of rapid thermal annealing on nanostructure, texture and magnetic properties of granular FePt:Ag films for perpendicular recording (invited)
Authors: Shao Y. , Yan M. L. , Sellmyer D. J.
Published: May 2003

We report effects of rapid thermal annealing on nanostructure, texture, and magnetic properties of granular FePt:Ag films. It was found that the orientations of FePt grains were dependent strongly on the as-deposited multilayer structure and the annealing processes. Through the control of the annealing processes, (001) textured L10 granular FePt:Ag films were obtained. Magnetic measurements revealed that saturation magnetization Ms, perpendicular coercivity Hc, and remanence ratio Mr/Ms, were also dependent on the annealing temperature, annealing time and Ag content. Perpendicular Hc values increased from 1.5 to 15 kOe when the annealing temperature changed from 400 to 600 °C for 600 s. Perpendicular Hc values remained about 11 kOe when the annealing time changed from 5 to 600 s. The effect of Ag content on magnetic properties is reported.
Postannealing effects on magnetic properties and microstructure of CoCrPt/Ti perpendicular recording media
Authors: Roy Anup G. , Nuhfer N. T. , Laughlin David E.
Published: May 2003

In this work, we investigate the postannealing effects on the magnetic and structural properties of CoCrPt perpendicular films. We observe a coercivity of 5000 Oe in the films with a 2 nm CrMn overlayer, which is about two times larger than the coercivity of similar films without a CrMn overlayer. This increment is attributed to the decoupling of grains by diffusion of CrMn from the top layer through the grain boundaries. An increase in the negative nucleation field and a decrease in intergranular exchange coupling with annealing temperatures was observed for the films with a CrMn overlayer. On the other hand, the films without a CrMn overlayer show the opposite trends except at high annealing temperature (450 °C). We observe a coercivity of ∼7600 Oe and a negative nucleation field of ∼2400 Oe for a film with a CrMn overlayer annealed at 450 °C for 5 min.
Microelectromagnetic ferrofluid-based actuator
Authors: Melikhov Y. , Lee S. J. , Jiles D. C. , Schmidt D. H. , Porter M. D. , Shinar R.
Published: May 2003

Computer simulations were used to investigate the performance of a microscale ferrofluid-based magnetic actuator developed for liquid dispensing in microfluidic channels. The actuation was based on the movement of a ferrofluid plug in a magnetic field gradient generated by on-chip effectively infinite parallel conductors. The movement, positioning, and retaining of ferrofluid plugs with different lengths at various locations along a microfluidic channel were investigated for two cases. In case (a), the magnetic field gradient was generated by a single conductor; when the ferrofluid reached its equilibrium position, the current was switched off and the nearest neighbor conductor was energized. A similar, consecutive on/off current switching was performed for case (b), where a set of conductors was energized simultaneously.
Angle dependent magnetization reversal of thin film magnetic recording media
Authors: Coffey Kevin R. , Thomson Thomas , Thiele Jan-Ulrich
Published: May 2003

The results of angular measurements of the remanent switching fields for granular longitudinal and perpendicular magnetic recording media are presented and compared to idealized models of magnetic switching. Co alloy longitudinal and perpendicular recording media are found primarily to have a Stoner–Wohlfarth switching character at vibrating sample magnetometer time scales. Since the Stoner–Wohlfarth model does not consider the effects of thermal activation, the angular dependence of the time independent switching field Hswro(θ) was determined from a Sharrock analysis. This approach shows a closer agreement between experimental data and model. For the case of a representative high density longitudinal recording medium, we additionally investigate the switching field distribution.
Low temperature thermomagnetic print for hard disk servo writing
Authors: Ikeda Hiroyuki , Seo Yuzo , Arita Yoji
Published: May 2003

Thermomagnetic print, proposed in the 1980’s, is expected to apply to hard disk drive servo writing. The print needs to be done after certification of media using magnetic head, so decomposition of lubricant on the media surface is the issue. Thermomagnetic print can be done at lower temperatures than Curie point, utilizing the small change of Hc by heating. But since the print should be done in short term to avoid thermal diffusion, the effective Hc is much higher than the static Hc. In the process, proposed in this article, the dynamic effect is suppressed. A part of the printing field is generated in a very short period, so the dynamic Hc at nonheated areas is also increased and the divergence of Hc is kept large enough. For the experiment, KrF excimer laser was used as the light source. Si mask was faced on the media with 0.5 μm spacing. For printing narrow pattern less than 1 μm in width on media of Hc 3.4 kOe, the optimum laser power was 90 mJ/cm2 and the optimum printing field was 3.1 kG including 1.0 kG of 0.1 ms pulse field. The signal-to-noise ratio was 16 dB, which was 3 dB better than the existing process. The maximum temperature is near 200 °C, estimated by numerical method.
Self-assembled monolayers on ta-C surfaces: Effect of sp3/sp2 ratio on adsorption rate and friction
Authors: Choi Junho , Kato Takahisa
Published: May 2003

Self-assembled monolayers of 1H,1H,2H,2H-perfluorodecyltrichlorosilane (FDTS), CF3(CF2)7(CH2)2SiCl3 were deposited on tetrahedral amorphous carbon (ta-C) surfaces by immersion process. The ta-C films were deposited on silicon substrates by using a filtered cathodic arc system and the sp3/sp2 ratios of the ta-C films were controlled by changing the sample bias voltage. In results of the x-ray photoelectron spectroscopy measurements, it was found that the adsorption rate of silane molecules on sample A (whose sp3 content is about 85%) was higher than that of on sample B (whose sp3 content is about 65%). This trend was qualitatively the same as that found for water contact angle measurement. The friction forces of FDTS/sample A and FDTS/sample B measured by lateral force microscope show FDTS/sample A has lower friction value than that of FDTS/sample B. We conclude the following: the density of dangling bonds, which are effective adsorption sites for silane molecules, increases with sp3 content. Consequently, the adsorption rate of silane molecules increases with increasing sp3 content. The friction force decreases with increasing sp3 content of the ta-C film since more of a close-packed self-assembled monolayer can be formed.
Effect of nitrogen plasma treatment at the Al2O3/Fe interface in magnetic tunnel junction
Authors: Shim Heejae , Cho B. K. , Kim Jin-Tae , Kim T. W. , Park W. J.
Published: May 2003

We investigated the effects of nitrogen plasma treatment on top surface of Fe pinned layer for short times (tex=0, 10, 30, and 60 s) in magnetic tunnel junctions and annealing of the junctions. The nitrogen-treated junctions show much reduced magnetoresistance (MR) ratio and significantly lower resistance-area (RA) products compared with the untreated junction, i.e., MR≈3%, RA≈30 kΩ μm2 for tex=10 s and MR≈10%, RA≈60 kΩ μm2 for tex=0 s. The untreated junction showed enhanced MR ratio up to about 17% and higher RA (≈70 kΩ μm2) upon thermal annealing at Ta=230 °C, as expected. For the nitrogen-treated junctions, while the MR ratio also increases up to about 16% upon annealing at Ta=230 °C, which is almost the same value as the one of the optimal reference junction, the RA values of the annealed junctions still keep as low as their initial values. We believe that the redistribution of nitrogen during the annealing process is responsible for the change of properties of nitrogen-treated junction. The bias dependence of MR and the estimation of effective barrier height and thickness are studied and found to be consistent with the observed changes in nitrogen-treated junctions.
Organic luminescent devices and magnetoelectronics
Authors: Davis A. H. , Bussmann K.
Published: May 2003

Developments in magnetoelectronics are advancing by combination of once disparate areas of research in magnetic materials, semiconductor electronics, and optoelectronics. We explore the integration of magnetic materials with organic semiconductors. Because small spin–orbit coupling in these materials minimizes spin relaxation, they may be useful in spintronic applications. Motivated by a theoretical investigation into spin-dependent exciton formation that predicts a magnetoluminescence valve effect, we attempt to manipulate spin-polarized holes and electrons in an effort to generate magnetic field dependent luminescence in organic light emitting diodes (OLEDs). We have fabricated various functional OLEDs consisting of ferromagnetic electrodes sandwiching a organic semiconducting bilayer, thus demonstrating that hole and electron injection from magnetic electrodes is possible. However, magnetic transition metal anodes produce higher turn-on voltages and significantly reduced lifetimes compared to indium–tin–oxide based OLED’s.
Detection of single micron-sized magnetic bead and magnetic nanoparticles using spin valve sensors for biological applications
Authors: Li Guanxiong , Joshi Vikram , White Robert L. , Wang Shan X. , Kemp Jennifer T. , Webb Chris , Davis Ronald W. , Sun Shouheng
Published: May 2003

We have fabricated a series of highly sensitive spin valve sensors on a micron scale that successfully detected the presence of a single superparamagnetic bead (Dynabeads M-280, 2.8 μm in diameter), and thus showed suitability for identifying biomolecules labeled by such magnetic beads. By polarizing the magnetic microbead on a spin valve sensor with a dc magnetic field and modulating its magnetization with an orthogonal ac magnetic field, we observed a magnetoresistance (MR) signal reduction caused by the magnetic dipole field from the bead that partially cancelled the applied fields to the spin valve. A lock-in technique was used to measure a voltage signal due to the MR reduction. A signal of 1.2 mV rms or 5.2 mΩ of resistance reduction was obtained from a 3 μm wide sensor and a signal of 3.8 mV rms or 11.9 mΩ from a 2.5 μm wide sensor. Micromagnetic simulations were also performed for the spin valve sensors with a single bead and gave results consistent with experiments. Further experiments and simulations suggested that these sensors or their variations can detect 1–10 Co nanoparticles with a diameter of ∼11 nm, and are suitable for DNA fragment detection.
Synthesis and aging effect of spherical magnetite (Fe3O4) nanoparticles for biosensor applications
Authors: Gee S. H. , Hong Y. K. , Erickson D. W. , Park M. H. , Sur J. C.
Published: May 2003

The chemical coprecipitation process was used to synthesize about 7 nm, spherical magnetite nanoparticles to study magnetic properties and the aging effect. As-produced spherical magnetite nanoparticles have been aged in the atmosphere for 19 months. Magnetic properties and aging effect were studied by Mössbauer spectroscopy at a temperature ranging from 77 to 300 K, vibrating sample magnetometer, and x-ray diffraction. Saturation magnetization and coercivity were found to be 49 emu/g and nearly 0 Oe at room temperature, respectively. A singlet Mössbauer spectrum was observed at room temperature, implying superparamagnetic behavior of the particles, while a two-sextet spectrum was observed at 77 K. The particle size in this study is about 7 nm, which is smaller than the superparamagnetic size of 26 nm as calculated from Neel’s theory of single domain particles. After having aged these particles for 19 months, all magnetic properties and their original shapes were retained. Superparmagnetic magnetite nanoparticles synthesized in this study can be applied to microbead applications of a biosensor.
Magnetic resonance and light microscopy investigation of a dextran coated magnetic fluid
Authors: Lacava L. M. , Garcia V. A. P. , Kückelhaus S. , Azevedo R. B. , Lacava Z. G. M. , Silva O. , Pelegrini F. , Gansau C. , Buske N. , Morais P. C.
Published: May 2003

A dextran-coated magnetite-based magnetic fluid (MF) sample (DexMF) was developed for cancer diagnostic and therapeutic purposes. In order to perform biological studies DexMF samples were endovenously injected into female Swiss mice. Magnetic resonance (MR) spectra showed a broad line around g=2, typical of magnetic nanoparticles (MNPs) suspended in a nonmagnetic matrix. The MR data showed that MNPs essentially spread in liver, spleen, and bone marrow. MNPs in blood stream were found up to 60 min after injection. Histological analysis also showed MNP agglomeration in liver, spleen, and bone marrow, from 1 h up to 28 days. No damaged cells or any other kind of alteration were observed in the investigated tissues. The data suggested that DexMF sample is biocompatible and adequate for biomedical applications.
Magnetotransport properties of lithographically defined lateral Co/Ni80Fe20 wires
Authors: Husain M. K. , Adeyeye A. O.
Published: May 2003

In this article we have investigated the magnetization reversal process of laterally defined coupled magnetic structures consisting of micron-sized sputtered Co and Ni80Fe20 wires lying side by side at temperatures ranging from 3 to 300 K. We have used a microfabrication technique to create an array of planar, laterally coupled magnetic wires made of two ferromagnetic materials. We observed two distinct peaks in the magnetoresistance (MR) curves corresponding to the magnetization reversals of Co and Ni80Fe20 wires. Below a critical temperature of 20 K we observed an asymmetric shift in the Ni80Fe20 peak position for both forward and reverse field sweeps due to the exchange coupling between the ferromagnetic (Ni80Fe20) and antiferromagnetic (Co–oxide at the interface of Co and Ni80Fe20 formed during fabrication) parts. The Co peaks gradually disappeared as the temperature was reduced. At low temperature we also observed that the Ni80Fe20 peaks in the MR loops are considerably shifted to larger fields corresponding to the increase in coercivity.