bluuebunny/crossref_metadata_2025 huggingface dataset:
Physical measurements in a permanent magnet field varying spatially
Authors: Arrott A. S. , Templeton T. L.
Published: June 2006

A high-field permanent magnet with the field varying from −2.6to+2.6T over a distance of 23cm has been constructed for Virginia State University by Neomax Inc. The field varies along a 52-cm-long access port with a square cross section of 12×12mm2. The purpose of this magnet is to facilitate measurements in an arbitrary sequence of fields in which the position of the sample determines the applied field. The maximum field is high enough that the demagnetizing fields for thin oblate spheroids can be overcome for any known ferromagnetic material. The use of this magnet in an induction magnetometer is demonstrated.
Transition between onion states and vortex states in exchange-coupled Ni–Fe∕Mn–Ir asymmetric ring dots
Authors: Sasaki Isao , Nakatani Ryoichi , Endo Yasushi , Kawamura Yoshio , Yamamoto Masahiko , Takenaga Takashi , Aya Sunao , Kuroiwa Takeharu , Beysen Sadeh , Kobayashi Hiroshi
Published: June 2006

The transition between onion states and vortex states in exchange-coupled Ni–Fe∕Mn–Ir asymmetric ring dots has been investigated. A direction of domain wall motion, during the transition from the single-domain state to the vortex state via the onion state, depends on a sweep direction of an external field. This dependence fixes the directions of vortical magnetizations in the vortex states. The derivative of the amount of the domain wall motion with respect to the external field depends on the sweep direction of external field, and thus the hysteresis loop becomes asymmetric.
Micromagnetic study of pinning behavior in percolated media
Authors: Suess Dieter , Fidler Josef , Porath Karina , Schrefl Thomas , Weller Dieter
Published: June 2006

A micromagnetic model has been developed describing the influence of the domain wall pinning in a magnetic thin film consisting of a granular microstructure with perpendicular magnetocrystalline anisotropy and a film thickness in the range of 5–16nm. The diameter of the prismatic domain wall pinning centers has been varied between 2 and 10nm. The stability of a single written bit is investigated. Thermal stability (energy barrier=80kBT300) can be guaranteed if the diameters of the pinning sites are equal or larger than the domain wall width and the film thickness is about 16nm. For defects on the order of 6nm magnetically very hard materials with values of Ku of 4×106J∕m3 are required.
Three-dimensional modeling for magnetic field shielding in a high electric power process
Authors: Zucca Mauro , Crotti Gabriella , Bottauscio Oriano , Li Xiangping , Di Pardo Massimo , Chiampi Mario
Published: June 2006

A modeling approach is used to investigate the shielding efficiency of ferromagnetic shields for the reduction of magnetic fields produced by industrial high electric power devices. Different ferromagnetic materials and their behavior in combination with pure conductive screens is analyzed and quantified. A comparison with experimental results performed in proximity to an actual device is finally reported to validate the modeling analysis.
Development of liquid phase epitaxy-grown (Bi, Gd, Lu)-substituted thin-film iron garnets
Authors: Nistor I. , Holthaus C. , Mayergoyz I. D. , Krafft C.
Published: June 2006

The liquid phase epitaxy growth of single-crystal iron garnet thin films of composition Gd3−x−yLuxBiyFe5−zGazO12 is discussed. The Bi, Lu, and Gd contents are identified for which the garnet films have in-plane magnetization as well as sufficiently large Faraday rotation and can be grown at relatively small melt undercooling temperatures. The results of extensive characterization of the magnetic and optical properties of these garnet films as a function of melt compositions and undercooling are reported.
Simulations of nanomagnet clusters with perpendicular uniaxial anisotropy for multilevel data storage
Authors: Xiao Q. , Krotkov R. V. , Tuominen M. T.
Published: June 2006

This work describes simulations used to investigate a multilevel data storage unit based on a hexagonal cluster of interacting uniaxial single domain nanomagnets. The accessibility and stability of the discrete magnetization states are studied using material parameters characteristic of Co3Pt. The switching properties of the clusters can be tuned by modifying the geometry, providing the ability to engineer desirable magnetic properties. These results provide guidance for increasing effective data storage density through the introduction of multilevel data units in perpendicular storage media.
Planar Hall effect in orthogonal submicrometer Co wires
Authors: Huang Y. S. , Wang C. C. , Adeyeye A. O. , Tripathy D.
Published: June 2006

We report on the planar Hall effect (PHE) at the junction of two orthogonal 200-nm-wide Co wires. Both the experimental and simulated PHE curves reveal that the magnetization switching at the junction is strongly associated with the reversal processes of the individual wire. As the orientation of applied field is varied with respect to the easy axes of the wires, the switching field and the direction of the spin rotation at the junction can be well controlled.
Effect of crystalline quality and substitution on magnetic anisotropy of SrRuO3 thin films
Authors: Kolesnik S. , Yoo Y. Z. , Chmaissem O. , Dabrowski B. , Maxwell T. , Kimball C. W. , Genis A. P.
Published: June 2006

Magnetic anisotropy of pure and substituted laser-ablated SrRuO3 and SrRu0.9Cr0.1O3 thin films has been studied by rotational magnetization experiments in a superconducting quantum interference device (SQUID) magnetometer. The magnetic easy axis for epitaxial (single domain) SrRuO3 and SrRu0.92O3 thin films lies in the (001) plane and is tilted away from the normal to the surface by 23°-26°. Its direction lies between the [110] and [010] axes. For the lower oxygen pressure grown SrRuO3 thin film, which consists of intertwined crystal nanodomains the magnetic easy axis is no longer locked-in to the (001) plane but is oriented closer to the normal to the surface that is the resultant orientation of easy axes for different domains. Similar effect is observed in single-domain SrRu0.9Cr0.1O3 thin film with random Cr substitution on the RuO6 octahedra. A role of ruthenium deficiency observed in all the studied thin films is also discussed.
Magnetization dynamics driven by the combined action of ac magnetic field and dc spin-polarized current
Authors: Finocchio G. , Krivorotov I. , Carpentieri M. , Consolo G. , Azzerboni B. , Torres L. , Martinez E. , Lopez-Diaz L.
Published: June 2006

The spin-polarized current flowing through a ferromagnet can apply a torque able to drive magnetization reversal or to excite persistent dynamical states of magnetization. In the present work we simulate the dynamic behavior of nanomagnets in nanopillar spin valve structures due to the combined action of ac magnetic fields and dc spin-polarized current. The simulations are performed using a micromagnetic model which includes the effect of the spin-polarized current. We also discuss possible experimental realizations of the ac-field-assisted current-induced switching. Two particular cases of the ac-field polarization are studied: circular and linear. We find that in both cases ac field accelerates the switching process.
A neutron-scattering investigation of the magnetic structure and magnetic excitations in nanocrystalline Tb
Authors: Vecchini C. , Moze O. , Hoser A. , Prager M. , Argyriou D. , Michels A. , Roessli B. , Weissmüller J. , Lemier C. , Viswanath R. N.
Published: June 2006

The magnetic structure and magnetic excitations in nanocrystalline Tb have been investigated by neutron diffraction and neutron spectroscopy. This is a report on the long-range magnetic order and the magnetic excitations in a nanocrystalline elemental rare earth. Refinement of the neutron-diffraction data reveals an “average” magnetic structure of each crystallite which contains a significant out-of-plane component to the magnetic moment as well as a suppression of the high-temperature antiferromagnetic phase observed for coarse-grained Tb. The inelastic-neutron-scattering measurements reveal the presence of a magnetic excitation of approximately 10meV at 2.5K. The excitation energy decreases with increasing temperature. The origins of this excitation are discussed with particular reference to the magnetic modes at the zone center observed for single-crystal Tb.
Ferromagnetism stabilization of ultrathin SrRuO3 films: Thickness-dependent physical properties
Authors: Toyota D. , Ohkubo I. , Kumigashira H. , Oshima M. , Ohnishi T. , Lippmaa M. , Kawasaki M. , Koinuma H.
Published: June 2006

Ultrathin films of SrRuO3 (SRO) have been grown on SrTiO3 (001) substrates to study thickness-dependent physical properties. We have revealed the existence of two critical film thicknesses where a metal-insulator transition occurs at a film thickness of 4–6 ML (monolayer) and the value of Curie temperature saturates at about 15 ML. The electrical and magnetic properties are well correlated with the thickness-dependent evolution of surface morphology of the ultrathin SRO films. These experimental results suggest the importance of the disorder associated with the unique growth mode transition in SRO films.
Annealing effects on structural and transport properties of rf-sputtered CoFeB∕MgO∕CoFeB magnetic tunnel junctions
Authors: Park Chando , Zhu Jian-Gang , Moneck Matthew T. , Peng Yingguo , Laughlin David E.
Published: June 2006

Annealing effects on the structural and transport properties of sputtered CoFeB∕MgO∕CoFeB magnetic tunnel junctions deposited on SiO2∕Si were investigated. At the as-deposited state, the CoFeB was amorphous at the CoFeB∕MgO interface. High-resolution transmission electron microscope image clearly shows that after annealing at 270°C for 1h, crystallization of amorphous CoFeB (three to four monolayers) with lattice matching to MgO (100) occurred locally at the interface between MgO and CoFeB, producing a magnetoresistance (MR) around 35%–40%. After annealing at 360°C for 40min, the MR increased to 102%. The increase in the MR with annealing is attributed to the complete formation of (100) crystalline structure of CoFeB well lattice matched with the (100)-oriented MgO barrier. The bias voltage dependence of the MR shows a consistent correlation with each CoFeB∕MgO interface.
Low-frequency noise in MgO magnetic tunnel junctions
Authors: Md Nor Anis Faridah , Kato Takeharu , Ahn Sung Jin , Daibou Tadaomi , Ono Kazunaga , Oogane Mikihiko , Ando Yasuo , Miyazaki Terunobu
Published: June 2006

Noise measurements have been performed in MgO-based tunnel junctions with normalized resistance in the range of 105–107Ωμm2 and various magnetoresistance ratios were investigated. Noise measurements in the frequency range of 1–1000Hz shows magnetically dependent pure 1∕f power spectra at low frequency. The 1∕f noise scales with bias voltage, indicating that the 1∕f noise can be attributed to magnetic tunnel junction resistance fluctuations. Bias voltage dependence of random telegraph noise (RTN) was observed, indicating electronic origin due to the charge-trapping mechanism. In the presence of the easy-axis bias field, our data exhibit a magnetic-field dependence of RTN that originates from magnetization fluctuations. A phenomenological noise parameter, defined for the comparison of noise levels in different junctions, was shown to be independent of the junction resistance-area product in the range investigated. This observation suggests that the decrease in tunnel resistance does not play an important role in reducing 1∕f noise.
Ba-hexaferrite films for next generation microwave devices (invited)
Authors: Harris V. G. , Chen Zhaohui , Chen Yajie , Yoon Soack , Sakai Tomokuza , Gieler Anton , Yang Aria , He Yongxue , Ziemer K. S. , Sun Nian X. , Vittoria Carmine
Published: June 2006

Next generation magnetic microwave devices require ferrite films to be thick (>300μm), self-biased (high remanent magnetization), and low loss in the microwave and millimeter wave bands. Here we examine recent advances in the processing of thick Ba-hexaferrite (M-type) films using pulsed laser deposition (PLD), liquid-phase epitaxy, and screen printing. These techniques are compared and contrasted as to their suitability for microwave materials processing and industrial production. Recent advances include the PLD growth of BaM on wide-band-gap semiconductor substrates and the development of thick, self-biased, low-loss BaM films by screen printing.
Temperature-induced spin reorientation in TbMn6Sn6−xGax
Authors: Perry Laura K. , Ryan D. H. , Venturini G. , Cadogan J. M.
Published: June 2006

The spin reorientation process in TbMn6Sn6−xGax has been investigated using Sn119 Mössbauer spectroscopy. The Sn site hosting the substituting Ga atoms is identified as Sn-2d. The reorientation is confirmed to be a 90° rotation of the Mn moments from ab plane to c axis upon cooling through Tsr. The reorientation temperature Tsr is observed to decrease with increasing Ga doping at 255±18K∕Ga. Finally, we evaluate the anisotropic contributions to the hyperfine field at the Sn sites as seen through the spin reorientation.
Spin-transfer switching in MgO-based magnetic tunnel junctions (invited)
Authors: Diao Zhitao , Pakala Mahendra , Panchula Alex , Ding Yunfei , Apalkov Dmytro , Wang Lien-Chang , Chen Eugene , Huai Yiming
Published: June 2006

We present spin-transfer switching results for MgO-based magnetic tunneling junctions (MTJs) with large tunneling magnetoresistance ratio of up to 150% and low intrinsic switching current density (Jc0) of (2–3)×106A∕cm2. The low intrinsic switching current density is attributed to high tunneling spin polarization (TSP) in MgO-based MTJs. The current switching data are discussed based on a qualitative study of TSP in MgO-based MTJs. Additional film stack modification needed to decrease the switching current to meet the requirement of advanced magnetoresistive random access memory application is also discussed.
Crossover in heating regimes of thermally assisted magnetic memories
Authors: Sousa R. C. , Kerekes M. , Prejbeanu I. L. , Redon O. , Dieny B. , Nozières J. P. , Freitas P. P.
Published: June 2006

This work investigates the tunnel junction heating process for micrometer and submicrometer size junctions to be used in a thermally assisted magnetic random access memories write scheme. The time evolution of the heating process was obtained from experimental measurements and numerical thermal simulations. Simulation results show an initial temperature regime at very short pulse widths associated with the intrinsic heating of the junction (adiabatic regime). In this regime, for the same power density, the temperature increase is independent of the junction area. In the studied geometry, for pulse widths around 1ns and higher an additional heating occurs in the electrical leads (diffusion regime). The write power density is in this case lower for large junction areas. The use of thermal barriers is an effective mean to decrease the power density required for writing and to eliminate its junction area dependence.
Memory properties in a Landau-Lifshitz hysteresis model for thin ferromagnetic sheets
Authors: Van de Wiele Ben , Dupré Luc , Olyslager Femke
Published: June 2006

The paper deals with a two-dimensional numerical model for the evaluation of the electromagnetic hysteretic behavior of thin magnetic sheets when applying a unidirectional magnetic field. The time variation of the magnetization vector m in each space point obeys the Landau-Lifshitz equation. The effective field is the result of several contributions: the applied field, the magnetostatic field, the anisotropy field, and the exchange field. Microstructural features, such as grain size and crystallographic texture, are introduced in the micromagnetic model by dividing the geometry in subregions, each with its own magnetic preferable directions. In the article, numerical experiments are presented aiming at low-frequency applications. The presented micromagnetic model is used to study magnetic memory material properties.
Evidence for strong spin-lattice coupling in multiferroic RMn2O5 (R=Tb,Dy,Ho) via thermal expansion anomalies
Authors: dela Cruz C. R. , Yen F. , Lorenz B. , Park S. , Cheong S.-W. , Gospodinov M. M. , Ratcliff W. , Lynn J. W. , Chu C. W.
Published: June 2006

Thermal expansion measurements were done on single-crystal RMn2O5 (R=Tb,Dy,Ho) along the principal crystallographic axes. Distinctive anomalies were observed in the linear thermal expansivities at critical temperatures marking the onset of long-range antiferromagnetic order (TN1), ferroelectricity (TC1), as well as at temperatures when anomalous changes in the polarization, dielectric constant, and spin wave incommensurability have been previously reported. These observations suggest that the coupling between the magnetic orders and the dielectric properties is mediated by the lattice distortion. In particular, a possible scenario as to how ferroelectricity arises and subsequently is altered as a result of the subtle changes in the highly frustrated magnetic order, is discussed in the light of the strong spin-lattice coupling observed in these materials. Neutron diffraction measurements on HoMn2O5 show a spin reorientation at 23K corresponding to a step-like anomaly in the dielectric constant.
Induced interfacial diffusive scatterings from different mean free paths between adjacent materials
Authors: Chen Sui-Pin , Chang Ching-Ray
Published: June 2006

To understand the influence of different mean free paths between adjacent materials on diffusive scatterings of a transport electron, we study an A-B bilayer with two infinite planes by use of the linear response Boltzmann transport equation in the relaxation time approximation with given boundary conditions. Our theoretical results show that the asymmetry in the mean free path induces a directional interfacial diffusive scattering when the electrons transport across the A-B interface.
Large room-temperature spin-dependent tunneling magnetoresistance in a Fe3O4-polymer composite system
Authors: Wang Wendong , Yu Minghui , Chen Yuxi , Tang Jinke
Published: June 2006

Powders of polymer and α-Fe2O3 nanoparticles were pressed into pellets. The samples were annealed at 250°C in pure hydrogen flow. X-ray diffraction spectra indicate that there is a complete phase transformation from α-Fe2O3 to Fe3O4. Giant negative magnetoresistance (MR) was observed at room temperature and the MR ratio is over 14% in an applied field of 5.5T. The maximum MR ratio is higher than 22% at 130K. The drastic enhancement of the MR ratio is attributed to the fact that the polymer is an excellent barrier material and, more importantly, prevents the oxidation of the surface of Fe3O4. Our results clearly suggest that there is a high degree of spin polarization at room temperature for half-metallic Fe3O4.
Magnetic properties of CaRu1−xFexO3
Authors: Mizusaki S. , Taniguchi T. , Okada N. , Nagata Y. , Hiraoka N. , Nagao T. , Itou M. , Sakurai Y. , Ozawa T. C. , Noro Y.
Published: June 2006

The occurrence of ferromagnetism was studied in the CaRu1−xFexO3 system by means of magnetic Compton scattering, Mössbauer effect, and magnetization measurements using polycrystalline specimens. Ferromagnetism is induced for extremely small Fe doping (x⩾0.02) to paramagnetic CaRuO3, and the TC increases as the Fe content increases. The temperature dependence of reciprocal magnetic susceptibility shows a characteristic of ferrimagnetism. A sextet with an isomer shift of the Fe3+ ion and the hyperfine field of about 450kOe were observed in the Mössbauer effect measurement below TC, suggesting the existence of magnetic order. The magnetic Compton scattering experiment revealed the occurrence of a large magnetic moment on Ru and an antiferromagnetic coupling between Fe and Ru spin moments. A possible model for the occurrence of ferrimagnetism has been proposed.
Structural characterization of Co100−xFex nano-oxide layer
Authors: Endo Hiroaki , Doi Masaaki , Hasegawa Naoya , Sahashi Masashi
Published: June 2006

For the structural characterization of a Co100−xFex nano-oxide layer (NOL), the exchange bias properties of the Co100−xFex-natural oxidized NOL in the specular spin-valve (SPSV) system were investigated. The exchange bias energy (Jex) increased monotonically with the increasing Fe content for the Co100−xFex-NOL. The enhancement of both the magnetoresistance ratio and the exchange bias field (Hex) was realized by increasing the Fe content in the Co100−xFex-NOL. It should be mentioned that Hex more than 800Oe is obtained by the insertion of Co30Fe70-NOL, even in NOL-SPSV, which is a remarkably higher pinning field than that ever reported on IrMn-SV. This high exchange bias field is considered to be realized by the formation of an Fe-rich fcc phase at the interface of IrMn.
Coercivity and magnetic viscosity in mechanical milled nanocrystalline YCo5
Authors: Elizalde Galindo J. T. , Matutes-Aquino J. A. , Costes M. , Broto J. M.
Published: June 2006

High coercivity and magnetic viscosity in nanocrystalline YCo5 alloy were observed. The alloy was obtained by arc-melting the raw materials, mechanical milling for 4h with subsequent heat treatment for short times at 1103K and a final quenching in water. The x-ray diffraction pattern showed a single hexagonal 1:5 phase. The average crystallite size determined by transmission electron microscopy was 13nm. Magnetic measurements were carried out at 85K in a pulsed field magnetometer varying the field in the range 50–250kOe and keeping the pulse duration equal to 0.3s. A maximum coercivity of 50kOe was measured when applying a maximum magnetic field of 250kOe. The viscosity parameter was measured varying the dH/dt rate in the range 511–1113kOe∕s. The calculated activation volumes from viscosity data were smaller than the crystallite volume from transmission electron microscopy, consistent with their different physical meanings.
Prediction and analysis of magnetic forces in permanent magnet brushless dc motor with rotor eccentricity
Authors: Liu Z. J. , Li J. T. , Jabbar M. A.
Published: June 2006

In design of permanent magnet motors for high-precision applications, it is sometimes necessary, early in the design stage, to have a detailed analysis of the effect of rotor eccentricity that may result from manufacturing imperfectness or use of fluid dynamic or aerodynamic bearings. This paper presents an analytical model for electromagnetic torque and forces in permanent magnet motors with rotor eccentricity. The model gives an insight to the relationship between the effect of the eccentricity and the other motor design parameters on the electromagnetic forces. It is shown that the calculated magnetic forces obtained from this model agree well with those obtained from numerical simulations that are very computationally demanding.
Effect of thermal fluctuation field on the noise performance of a perpendicular recording system
Authors: Batra Sharat , Scholz Werner , Roscamp Thomas
Published: June 2006

We have studied the effect of thermal fluctuations on the transition jitter of a perpendicular recording system using a micromagnetic recording model. A stochastic thermal field term is added to the total effective field in the Landau-Lifshitz-Gilbert equation of motion to simulate thermal fluctuations. The recording model uses a head field generated by a finite element model in the presence of a soft underlayer. A Monte Carlo simulation of 300 isolated transitions is done to calculate the variation in transition jitter. Transition jitter is significantly increased in the presence of a stochastic thermal field for different media parameters. Our results highlight that the thermal fluctuation fields must be included in a realistic assessment of the system performance.
Magnetic aftereffect and magnetic force microscopy studies of Fe–B∕FePt-type nanocomposite ribbons
Authors: Chang C. W. , Chang H. W. , Chiu C. H. , Chang W. C. , Fang Y. K. , Han B. S.
Published: June 2006

Magnetic aftereffect and surface magnetic domain structure of (Fe0.675Pt0.325)100−xBx(x=12–20) nanocomposites have been investigated for correlating with their corresponding phases and magnetic properties. Exchange-coupling effect between grains is present for all the studied ribbons as evidenced by Henkel plot. The volume fraction of magnetically soft phases decreases with increasing B concentration from x=12 to x=18. It leads to the reduction in the activation volume of the reverse domain. Among all samples, the (Fe0.675Pt0.325)82B18 ribbon having the highest intrinsic coercivity (Hci) exhibits minimum value in activation volume V=1.1×10−18cm3, because it possesses the least volume fraction of the magnetic soft phases, i.e., Fe2B or Fe3B phases. Magnetic force microscopy studies reveal the magnetic domain structures of the ribbons, confirming the existence of strong exchange coupling between magnetic grains.
Design and control of the phase current of a brushless dc motor to eliminate cogging torque
Authors: Jang G. H. , Lee C. J.
Published: June 2006

This paper presents a design and control method of the phase current to reduce the torque ripple of a brushless dc (BLDC) motor by eliminating cogging torque. The cogging torque is the main source of torque ripple and consequently of speed error, and it is also the excitation source to generate the vibration and noise of a motor. This research proposes a modified current wave form, which is composed of main and auxiliary currents. The former is the conventional current to generate the commutating torque. The latter generates the torque with the same magnitude and opposite sign of the corresponding cogging torque at the given position in order to eliminate the cogging torque. Time-stepping finite element method simulation considering pulse-width-modulation switching method has been performed to verify the effectiveness of the proposed method, and it shows that this proposed method reduces torque ripple by 36%. A digital-signal-processor-based controller is also developed to implement the proposed method, and it shows that this proposed method reduces the speed ripple significantly.
Fabrication of oriented L10-FeCuPd and composite bcc-Fe∕L10-FeCuPd nanoparticles: Alloy composition dependence of magnetic properties
Authors: Naganuma Hiroshi , Sato Kazuhisa , Hirotsu Yoshihiko
Published: June 2006

Oriented and well-isolated L10-FeCuPd ternary alloy nanoparticles have been fabricated by electron-beam evaporation followed by postdeposition annealing. A single L10 phase was formed in the FeCuPd nanoparticles with (Fe+Cu) content lower than 48 at. %. A strong preferential c-axis orientation along the film normal direction was achieved by Cu addition, which leads to a strong perpendicular magnetic anisotropy. Also, a lowering of the ordering temperature by 50 K compared to the binary L10-FePd nanoparticles was achieved by Cu addition. By contrast, composite particles composed of the bcc Fe and the L10-FeCuPd were formed when the (Fe+Cu) content was higher than 52 at. %. Coexistence of the bcc Fe and the L10-FeCuPd was confirmed by high-resolution transmission electron microscopy and nanobeam electron diffraction. It was found that perpendicular magnetic anisotropy of the L10-FeCuPd nanoparticles on the NaCl substrate is sensitive to the alloy composition.
Magnetic structure variations during giant magnetoresistance training in spin valves with picoscale antiferromagnetic layers
Authors: Moyerman S. , Eckert J. C. , Borchers J. A. , Perdue K. L. , Doucet M. , Sparks P. D. , Carey M. J.
Published: June 2006

Microscopic models of exchange bias focus on the formation of domains in the antiferromagnet or the ferromagnet, or on a small induced moment in the antiferromagnet. Previous giant magnetoresistance (GMR) measurements, however, reveal exchange bias and training effects in CoFe-based spin valves with antiferromagnetic IrMn layers as thin as 0.4 nm. Polarized neutron reflectometry studies of a related spin valve with a 1.6 nm IrMn layer were carried out for several points along the GMR hysteresis curve to probe separately the free and pinned CoFe layers. These measurements confirm that the two ferromagnetic CoFe layers are aligned in parallel in saturating fields. During the first field cyle, regions of high resistance correspond to an antiparallel alignment of the CoFe layers as expected. Significant changes in this antiparallel structure are observed during the second field sweep, and a magnetic spiral forms and persists in the pinned CoFe layer. High-field saturation seems to reduce the effectiveness of the pinning and thus gives rise to training. These results have implications for the origin of exchange bias in spin valves with thin antiferromagnetic layers.
Magnetic effects in an itinerant electron antiferromagnetic Cr+1.72at.%Fe alloy single crystal
Authors: Prinsloo A. R. E. , Derrett H. A. , Alberts H. L. , Venter A. M.
Published: June 2006

Measurements of the temperature and pressure dependences of the longitudinal-mode ultrasonic wave velocities (νL) as well as the temperature dependence of the electrical resistivity (ρ) of a Cr1−cFec alloy single crystal with c=0.0172 are reported. The νL−T curve displays two prominent anomalies of magnetic origin. One is a deep minimum at the Néel temperature (TN=274K) and the other a sharp change in slope at a lower temperature Tt=243K. The anomaly at Tt is absent on the ρ−T curve, but the Néel transition is accompanied by a very prominent continuous rise in ρ on cooling through TN. The observations at Tt cannot be directly explained in light of the present magnetic phase diagram of the Cr–Fe system. There is a large contribution of about 50% from magnetic effects to the residual resistivity of Cr+1.72at.%Fe at 0 K. This contribution is ascribed to spin-density-wave and local impurity effects.
Temperature dependence of induced Ni2+ moment fluctuations in the Eu2BaNiO5 Haldane system
Authors: Voyer C. J. , van Lierop J. , Ryan D. H. , Cadogan J. M.
Published: June 2006

Eu 151 and Eu153 Mössbauer spectroscopies have been used to study the magnetism of the Ni2+ moments in Eu2BaNiO5 by way of the exchange-induced Eu3+ moments. These independent and complementary probes indicate that at high temperatures all of the moments in the Haldane quantum spin chains are paramagnetic. On cooling below 25 K, fluctuations of ∼90% of the Ni2+ moments slow down, while the remaining 10% remain paramagnetic (indicating the presence of chain defects). Both Eu151 and Eu153 Mössbauer measurements yield the same Ni2+ moment. Furthermore, below 5 K, the 90% chain component becomes static and ordered, in excellent agreement with the ordering temperature expected from the rare-earth exchange energy.
Self-organization and magnetic domain microstructure of Fe nanowire arrays
Authors: Rougemaille N. , Schmid A. K.
Published: June 2006

Starting from essentially flat nanometer-thick Fe films, epitaxially grown at room temperature on W(110) surfaces, we used carefully tuned annealing schedules to produce periodic arrays of nanoscale ferromagnetic wires. The structural transition from continuous films to nanowire arrays is accompanied with an in-plane 90° rotation of the spontaneous magnetization. Using spin-polarized low-energy electron microscopy to map the local magnetization directions while annealing, we studied the role of the dewetting mechanism on the self-organization and magnetization reorientation processes.
Microfluidic high gradient magnetic cell separation
Authors: Inglis David W. , Riehn Robert , Sturm James C. , Austin Robert H.
Published: June 2006

Separation of blood cells by native susceptibility and by the selective attachment of magnetic beads has recently been demonstrated on microfluidic devices. We discuss the basic principles of how forces are generated via the magnetic susceptibility of an object and how microfluidics can be combined with micron-scale magnetic field gradients to greatly enhance in principle the fractionating power of magnetic fields. We discuss our efforts and those of others to build practical microfluidic devices for the magnetic separation of blood cells. We also discuss our attempts to integrate magnetic separation with other microfluidic features for developing handheld medical diagnostic tools.
Local environment of Mn atoms in IV-VI ferromagnetic semiconductor Ge1−xMnxTe
Authors: Fukuma Y. , Sato H. , Fujimoto K. , Tsuji K. , Kimura A. , Taniguchi M. , Senba S. , Tanaka A. , Asada H. , Koyanagi T.
Published: June 2006

Mn L 2 , 3 x-ray absorption and magnetic circular dichroism study has been performed on IV-VI ferromagnetic semiconductor Ge1−xMnxTe with different Mn compositions x=0.12, 0.32, 0.51, and 1. The absorption and dichroism line shapes are found to be almost independent of the Mn composition, suggesting that the local chemical environment of Mn atoms is the same for Ge1−xMnxTe with the wide range of x. On the basis of the configuration interaction analysis for a MnTe6 cluster, the electronic structure parameters such as the p-d hybridization and the p-d exchange constant have been estimated.
Non-Boltzmann energy distributions in spin-torque devices
Authors: Visscher P. B. , Apalkov D. M.
Published: June 2006

Most previous work on energy distributions in spin-torque devices has assumed a Boltzmann form exp(−E∕kBTeff), with an effective temperature that is modified by the spin torque. We consider in this article a regime in which this is not even qualitatively true (for example, the distribution is not even monotonic in E), and give a method for quantitatively calculating the distribution. This is a necessary ingredient of any calculation of the noise spectrum or the switching current.
On the quality of molecular-beam epitaxy grown Fe∕MgO and Co∕MgO(001) interfaces
Authors: Sicot M. , Andrieu S. , Tiusan C. , Montaigne F. , Bertran F.
Published: June 2006

We investigated in detail the structural, chemical, and magnetic properties of Fe at the interface with MgO. The samples were grown by molecular-beam epitaxy and analyzed by using reflection high-energy electron diffraction, scanning tunneling microscopy, Auger electron spectroscopy, x-ray photoemission spectroscopy (XPS), x-ray-absorption spectroscopy (XAS), and x-ray magnetic circular dichroism (XMCD) measurements. We discuss the presence or absence of C and O contaminants at the surface of the metallic film. We actually show that when growing the first Fe layer on the initial MgO substrate, a carbon contamination at the surface of the annealed Fe layer is observed leading to a C(2×2) surface structure. We propose a method to eliminate this carbon contamination and to get interfaces free of carbon. On the other hand, we observed that the reactivity of the Fe surface to oxygen contamination is greatly influenced by the presence of carbon or absence on the surface. The influence of oxygen adsorption at the Fe surface on the MgO growth will also be discussed. Concerning the hybridization between Fe or Co with MgO, XPS, and XAS-XMCD analyses clearly demonstrate that the charge transfer is weak, and that the magnetization at the interface is larger than in bulk (3μB/at for Fe). With such clean interfaces, reproducible magnetoresistance equal to 160% are obtained on Fe∕MgO(3nm)∕Fe(001) 100×100μm2 tunnel junctions at room temperature in our laboratory. Finally, we show that pollution at the bottom Fe∕MgO interface modify drastically spin-dependent tunneling properties.
Soft-x-ray spectroscopic investigation of ferromagnetic Co-doped ZnO
Authors: Krishnamurthy S. , McGuinness C. , Dorneles L. S. , Venkatesan M. , Coey J. M. D. , Lunney J. G. , Patterson C. H. , Smith K. E. , Learmonth T. , Glans P. -A. , Schmitt T. , Guo J. -H.
Published: June 2006

The electronic properties of cobalt-doped ZnO were investigated through site-selective and element-sensitive x-ray-absorption spectroscopy in the vicinity of the Co L2,3 edge, the oxygen K edge, and at the Zn L3 edge. The spectroscopic measurements of the ferromagnetic cobalt-doped ZnO films appear to have additional components in the O K edge x-ray-absorption spectrum not observed in the undoped films. The observed features may derive from both hybridization with unoccupied Co 3d states and also from lattice defects such as oxygen vacancies. Only minor changes in the Zn L3 edge spectra were observed. These observations are consistent with a polaron percolation model in which the ferromagnetic coupling is mediated by shallow donor electrons trapped in oxygen vacancies and couples the Co atoms substituted on Zn sites in the hexagonal wurtzite ZnO structure.
Magnetic irreversibility in the antiferromagnetic state of UPt2Si2
Authors: Süllow S. , Otop A. , Klenke J. , Feyerherm R. , Hendrikx R. W. A. , Mydosh J. A.
Published: June 2006

We present a detailed investigation of the magnetothermal hysteresis observed in the magnetically ordered phase of the moderately mass-enhanced U intermetallic UPt2Si2. Aside from the antiferromagnetically ordered state below TN=31K, in a susceptibility study we establish the presence and field dependence of two irreversibility temperatures Tirr and Tirr2. Comparing the susceptibility data to the result of a neutron-scattering study, we find the lower temperature Tirr2 to be accompanied by an anomaly in the temperature dependence of various magnetic Bragg peaks, viz., a reduction of the intensity. Tentatively, we attribute this behavior to a modification of the symmetry of the magnetically ordered phase. Altogether, our findings imply that the magnetic phase diagram of UPt2Si2 is much richer than previously thought, and similar to related uranium (122) intermetallic compounds.
Oxide-based dilute ferromagnetic semiconductors: ZnMnO and Co:TiO2
Authors: Pradhan A. K. , Hunter D. , Lasley-Hunter B. , Dadson J. B. , Zhang Kai , Rakhimov R. R. , Zhang Jun , Sellmyer D. J.
Published: June 2006

We report on ferromagnetic properties of ZnMnO and Co:TiO2 films grown by pulsed laser deposition with varying growth conditions. We have demonstrated that ZnMnO films show ferromagnetic properties at room temperature. However, oxygen plays a dominant role in the occurrence of ferromagnetism. Introducing carriers into ZnMnO films did not improve the ferromagnetic properties. Our experimental results indicate that the mechanism for ferromagnetism lies, probably, within the perspective of charge transfer between Mn ions through oxygen. On the other hand, our experimental results suggest that the ferromagnetism in Co:TiO2 films is controlled by the presence of small metal Co2+ clusters in the rutile TiO2 matrix, which are mainly present at the interface and on the surface of the films.
Exchange-coupling effects in perpendicular composite materials
Authors: Schuermann K. C. , Dutson J. D. , Wu S. Z. , Harkness S. D. , Valcu B. , Richter H.-J. , Chantrell R. W. , O’Grady K.
Published: June 2006

The effect of interlayer thickness on the magnetic properties of exchange-coupled composite media has been studied. Films consisting of a CoPtO hard magnetic layer and a CoCrPt soft layer separated by a Pt interlayer with a thickness varying from 0to40Å have been fabricated. At low Pt thicknesses (⩽16Å) the exchange coupling forces the layers to switch as a single unit with a reduction in the hard magnetic layer coercivity of >2.5kOe. Thicker Pt layers induce sufficient decoupling to permit each layer to reverse separately. The variation of the coercivity with Pt thickness is complex and suggests that the coupling between the layers is a combination of dipolar and exchange effects.
Effects of current on vortex and transverse domain walls
Authors: He J. , Li Z. , Zhang S.
Published: June 2006

By using the spin torque model in ferromagnets, we compare the response of vortex and transverse walls to the electrical current. For a defect-free sample and a small applied current, the steady-state wall mobility is independent of the wall structure. In the presence of defects, the minimum current required to overcome the wall pinning potential is much smaller for the vortex wall than for the transverse wall. During the wall motion, the vortex wall tends to transform to the transverse wall. We construct a phase diagram for the wall mobility and the wall transformation driven by the current.
Magnetic properties of cobalt-ferrite nanoparticles embedded in polystyrene resin
Authors: Vaishnava P. P. , Senaratne U. , Buc E. , Naik R. , Naik V. M. , Tsoi G. , Wenger L. E. , Boolchand P.
Published: June 2006

Samples of maghemite and cobalt-ferrite nanoparticles (sizes, 3–10nm) were prepared by cross-linking sulfonated polystyrene resin with aqueous solutions of (1) FeCl2, (2) 80%FeCl2+20%CoCl2, (3) FeCl3, and (4) 80%FeCl3+20%CoCl2 by volume. Chemical analysis, x-ray powder-diffraction, and Fe57 Mössbauer spectroscopic measurements show that samples 1 and 3 consist of γ-Fe2O3 nanoparticles (sizes, ∼10 and 3nm) and sample 2 and 4 consist of CoxFe3−xO4 nanoparticles (sizes, ∼10 and 4nm). The temperature dependence of the zero-field-cooled and field-cooled magnetizations at low temperatures, together with a magnetic hysteresis in the M versus H data below blocking temperatures, demonstrate superparamagnetic behavior. The introduction of Co in the iron oxide-resin matrix results in an increase in the blocking temperature of nanoparticles.
Thermal hysteresis in the magnetization of the layered III-VI diluted magnetic semiconductor In1−xMnxSe
Authors: Pekarek T. M. , Ranger L. H. , Miotkowski I. , Ramdas A. K.
Published: June 2006

Magnetic properties of single-crystalline In1−xMnxSe (x=0.10) have been measured. A prominent thermal hysteresis in the magnetization is observed between 90 and 290K. The magnetization is reversible (deviating by only ∼0.8%) from 400 down to ∼120K along the upper branch of the hysteresis. In contrast, the lower branch magnetization is irreversible from 5 up to 290K and deviates by 30% of the 0.010emu∕g hysteresis splitting at 140K. Magnetic-field hysteresis loops at 200K between −7 and +7T demonstrate that changing the magnetic field does not allow movement between the upper and lower branches of the thermal hysteresis. This magnetic behavior is consistent with a charge-density wave. However, the observed ΔT=200K hysteresis in In1−xMnxSe is roughly an order of magnitude larger than other previously reported values of ΔT.
Magnetic imaging with soft x-ray spectroholography
Authors: Hellwig Olav , Eisebitt Stefan , Eberhardt Wolfgang , Schlotter William F. , Lüning Jan , Stöhr Joachim
Published: June 2006

We present recent advances in imaging magnetic nanostructures via lensless holographic x-ray techniques. In addition to the tunable energy and polarization this method also uses the coherent photon flux that is now available at third generation synchrotron radiation sources. Our spectroscopic imaging technique is based on the direct Fourier inversion of a holographically formed interference pattern, thus we refer to it as spectroholography. By exploiting the magnetic dichroism in resonance at the L3 absorption edges of the magnetic transition metals (wavelength of ∼1–2nm and energy of ∼700–900eV), images of magnetic nanostructures have been obtained with a spatial resolution below 50nm. Using Si3N4-membrane substrates we combine in our experimental setup the sample itself rigidly with the holographic mask structure consisting of an 800nm gold layer with a micronsized object aperture and a nanosized reference hole on the backside of the membrane. Such a compact implementation leaves plenty of space for installing extreme sample environments such as high magnetic fields and low temperatures that are difficult to combine with conventional x-ray imaging techniques based on focusing optics.
Structural and magnetic properties of Laves compounds Dy1−xPrx(Fe0.35Co0.55B0.1)2 (⩽x⩽1)
Authors: Ren W. J. , Li D. , Sui Y. C. , Liu W. , Zhao X. G. , Liu J. J. , Li J. , Zhang Z. D.
Published: June 2006

Dy 1 − x Pr x ( Fe 0.35 Co 0.55 B 0.1 ) 2  (0⩽x⩽1) Laves compounds with a cubic MgCu2-type structure were synthesized by arc melting and subsequent annealing. The lattice parameter of the Laves compounds linearly increases, while the Curie temperature TC decreases with increasing Pr content. The saturation magnetization Ms at 5K or 295K for the Dy1−xPrx(Fe0.35Co0.55B0.1)2 alloys decreases to reach a minimum, then increases with increasing Pr content. The composition for magnetic moment compensation is about x=0.55 at 295K and x=0.65 at 5K, respectively. The magnetostriction λ‖ or λ⊥ at room temperature was investigated either parallel or perpendicular to the applied field using a standard strain gauge technique.
Dual current-perpendicular-to-plane giant magnetoresistive sensors for magnetic recording heads with reduced sensitivity to spin-torque-induced noise
Authors: Childress J. R. , Carey M. J. , Kiselev S. I. , Katine J. A. , Maat S. , Smith N.
Published: June 2006

Dual IrMn-pinned current-perpendicular-to-plane giant magnetoresistive sensor structures have been investigated, and their signal and noise characteristics are compared to similar standard single sensors, using test structures with sizes down to 50nm. The intrinsic signal ΔRA product is found to be increased by a factor of nearly 2 between single and dual spin valves, although the parasitic resistance due to the second 80-nm-thick IrMn-pinned layer stack limits the net increase in ΔR∕R to a factor of 1.5. For the single spin valve with antiparallel free and reference layers, the noise power spectral density shows a large asymmetry between positive and negative current flows due to strong spin-torque effects for positive (electrons flow from reference layer to free layer) current densities above 1×107A∕cm2. For the dual sensor, however, the critical current density is symmetric in polarity and is increased to above 5×107A∕cm2, resulting in a 5-fold increase in practical sensor output voltage.
CoPtCr
−
SiO
2
media with Ku2 magnetic anisotropy term fabricated with Pd seed layers
Authors: Sato H. , Shimatsu T. , Kondo T. , Watanabe S. , Aoi H. , Muraoka H. , Nakamura Y. , Okamoto S. , Kitakami O.
Published: June 2006

We tried to enhance the grain isolation of CoPtCr–SiO2 media with Pd seed layers while simultaneously inducing the second energy term of the uniaxial anisotropy, Ku2. The addition of 5%N2 to the Ar gas used during the deposition of the Pd seed layer was found to reduce the Pd grain size. This structural change in the seed layer enhances the grain isolation of CoPtCr, without degrading the c-axis orientation, while simultaneously inducing a Ku2 component. However, adding N2 gas to the Pd seed layer increases the CoPtCr grain size to about 10nm. Moreover, these media have magnetization curves with relatively low squareness, this is related to the appearance of a cone state of magnetization induced by the Ku2 term. Magnetic analysis revealed that, in addition to a reduction of the grain size, a reduction of the saturation magnetization is also necessary to realize practical CoPtCr–SiO2 media, which take advantage of the Ku2 term.
Wide temperature range study of multilayer Fe∕Au∕Tb films
Authors: Kalita V. M. , Lozenko A. F. , Ryabchenko S. M. , Trotsenko P. O. , Shypil O. V. , Pogorily A. M.
Published: June 2006

The studies of Fe∕Au∕Tb multilayers (mls) prepared by e-beam evaporation are presented. We found that for mls obtained by seven reiterations of the layer sequence: ⋯∕24ÅFe∕17.5ÅAu∕36ÅTb∕17.5ÅAu∕⋯ only the Fe layers are ferromagnetic (FM) at T>100K, while Tb layers are paramagnetic. At T<100K Tb layers become also FM and their magnetic moments are oriented opposite to the Fe ones. The Fe and Tb layer magnetic moments mFe and mTb compensate each other at Tcomp∼40K. At lower temperatures mTb exceeds mFe and total magnetic moment increases significantly. M(H) curves and ferromagnetic resonance data show that the films are magnetically nonuniform, with significant local anisotropy dispersion (from easy plane to easy axes). On average this value is equal to the easy-plane anisotropy of the pure Fe films, caused by the shape anisotropy of the sample.