bluuebunny/crossref_metadata_2025 huggingface dataset:
Switching rate of magnetoresistive random access memory element: Verifying transition state theory
Authors: Wang Shuxia , Visscher P. B.
Published: June 2006

In this paper we show that the “transition state theory” (TST) used in chemical rate theory works surprisingly well for magnetic systems. It does not work for the simplest case (uniaxial anisotropy), which may explain why it has not previously been applied to other cases such as thin film switching. To check the TST, we develop a method for defining and computing switching rates by Landau-Lifshitz simulation, and apply it to a specific system (thin-film magnetoresistive random access memory element) for which it gives results similar to the TST.
Probabilistic behavior in subnanosecond spin transfer torque switching
Authors: Ito K. , Devolder T. , Chappert C. , Carey M. J. , Katine J. A.
Published: June 2006

We studied the magnetization dynamics induced by spin transfer torque (STT) in CoFe∕Ru∕CoFe∕Cu∕NiFe nanopillars using Landau-Lifshitz-Gilbert micromagnetic simulations including the STT term given by Slonczewski [J. Magn. Magn. Mater. 159, L1 (1996)] and a Gaussian thermal random force. Our simulation results clearly show the probabilistic switching behavior. The antiparallel to parallel switching at 77K requires a smaller overdrive current than at 300K. The parallel to antiparallel switching is not dependent on temperature. Our results show that C-like bending induced by the Ampere field plays an important role rather than the misalignment which arises from the thermal magnetization fluctuation.
Deposition of perfluoropolyether lubricant films on Si-incorporated diamondlike carbon surfaces
Authors: Choi Junho , Kawaguchi Masahiro , Kato Takahisa , Ikeyama Masami
Published: June 2006

In this article, the bonding characteristics of perfluoropolyether (PFPE) molecules, having a hydroxyl end group, regarding Si-incorporated amorphous carbon surfaces were investigated. The Si contents in the carbon films were varied from 0to10.3at.%. The PFPE lubricant layers were deposited on the carbon surfaces using the traditional dip coating and vacuum vapor deposition. It was found that the Si incorporation enhances the bonding of the PFPE molecules on the carbon surfaces. Moreover, the bonding of the PFPE molecules increases with the increasing Si content in the carbon film. This is attributed to the structural and chemical modifications of the hydrogen-terminated carbon surfaces due to the Si incorporation into the carbon network, which increases the adsorption sites for the PFPE molecules. The bonding of the PFPE molecules is enhanced using a vapor deposition technique compared to the traditional dip coating. The Si-incorporated carbon films represent a reasonably high hardness and a good bonding characteristic for the PFPE molecules, which could be good candidates as a protective coating for magnetic storage disks.
Analysis of diffusive interface resistance for measurements with perpendicular current in Fe∕Nb multilayers
Authors: Huang S. Y. , Lee S. F. , Yu C. Y. , Hsu S. Y. , Yao Y. D.
Published: June 2006

In this study, we investigated the electron transport properties of interface between a ferromagnet and a superconductor with flowing current perpendicular to plan (CPP) at 4.2K in Fe∕Nb multilayers. The CPP resistance increased linearly with layer thickness and with bilayer number. We extracted the unit area resistance value of one pair of interface for superconducting and normal Fe∕Nb by using a series resistor model. Hence, we can quantitatively analyze the interface resistance between Fe and Nb in the diffusive regime.
Simulated domain wall dynamics in magnetic nanowires
Authors: Kunz Andrew
Published: June 2006

The simulated domain wall dynamics in rectangular 10nm thick, 2000nm long Permalloy wires of varying width is presented. In the absence of an applied field the static domain wall length is found to be linearly dependent to the width of the nanowire. As magnetic fields of increasing strength are applied along the wire’s long axis, the domain wall motion changes from a uniform reversal to a steplike reversal. The onset of the stepping motion leads to a decrease in the domain wall speed. By continuing to increase the field it is possible to decrease the time between steps increasing the domain wall speed. The critical field associated with the crossover from uniform to nonuniform reversal decreases as the wire width increases.
Finite-temperature anisotropy of magnetic alloys
Authors: Skomski R. , Mryasov O. N. , Zhou J. , Sellmyer D. J.
Published: June 2006

The temperature dependence of the magnetic anisotropy of ferromagnetic materials is analyzed. Simple ferromagnets, such as Fe and Co, obey the m=n(n+1)∕2 power laws predicted by the Callen and Callen [Phys. Rev. 129, 578 (1963)] theory, but in alloys, the applicability of the theory is an exception rather than the rule. Many alloys, such as the rare-earth transition-metal intermetallics and L10 magnets, violate a basic assumption of the theory, namely, that the single-ion anisotropy and the spontaneous magnetization have the same origin. This is the reason for significant deviations from the Callen and Callen behavior, such as the m=2 law we obtained for L10 alloys.
Magnetic and Mössbauer spectroscopy studies of nanocrystalline iron oxide aerogels
Authors: Carpenter E. E. , Long J. W. , Rolison D. R. , Logan M. S. , Pettigrew K. , Stroud R. M. , Kuhn L. Theil , Hansen B. Rosendahl , Mørup S.
Published: June 2006

A sol-gel synthesis was used to produce iron oxide aerogels. These nanocrystalline aerogels have a pore-solid structure similar to silica aerogels but are composed entirely of iron oxides. Mössbauer experiments and x-ray diffraction showed that the as-prepared aerogel is an amorphous or poorly crystalline iron oxide, which crystallized as a partially oxidized magnetite during heating in argon. After further heat treatment in air, the nanocrystallites are fully converted to maghemite. The particles are superparamagnetic at high temperatures, but the magnetic properties are strongly influenced by magnetic interactions between the particles at lower temperatures.
Pattern storage and recognition using ferrofluids
Authors: Ban Shuai , Korenivski V.
Published: June 2006

An implementation of an associative memory based on a ferromagnetic nanocolloid is proposed. The design contains inductive input and output units for training the ferrofluid as well as sensors incorporated into the output units for performing recall. Using Monte Carlo simulations of the system we demonstrate the possibility of creating nanoparticle configurations that can serve to associate input/output pattern pairs.
Ab initio study on stability of half-metallic Co-based full-Heusler alloys
Authors: Miura Yoshio , Shirai Masafumi , Nagao Kazutaka
Published: June 2006

We perform the first-principles density functional calculations of Co-based full-Heusler alloys, exploring 25 possible combinations of Co2YZ (Y=Ti, V, Cr, Mn, and Fe; Z=Al, Ga, Si, Ge, and Sn) in order to clarify the stability for the Co-related atomic disorder, because this type of disorder is considered to degrade the spin polarization of Co2YZ. We found that the disorder between Co and Y atoms correlates with the total valence electron charges around Y atom, because a difference in valence electron charges between Co and Y atoms leads to a different shape of the local potential at each site. This means that Ti-based alloys are better than Cr-, Mn-, and Fe-based alloys in preventing the atomic disorder between Co and Y atoms. From our results, we propose stable Co-based full-Heusler alloys with excellent prospects for half-metallic ferromagnets.
Structure and magnetism of nanocrystalline exchange-coupled (Ni0.67Co0.25Fe0.08)89−xZr7B4Cux (x=,1) films
Authors: Joshi Shashidhar , Yoon Soack Dae , Yang Aria , Sun Nian X. , Vittoria Carmine , Harris Vincent G. , Goswami Ramasis , Willard Matthew , Shi Ning
Published: June 2006

Structural and magnetic characterizations of nanocrystalline films of (Ni0.67Co0.25Fe0.08)89−xZr7B4Cux (x=0,1) alloys are reported. The films were grown on quartz substrates using pulsed laser deposition from homogeneous targets of the above compositions at substrate temperatures ranging from ambient to 600°C. Structural properties were measured by x-ray diffraction, atomic force microscopy, and transmission electron microscopy, whereas the magnetic properties were measured by vibrating sample magnetometry and ferromagnetic resonance. The resulting films exist as a two phase alloy with face-centered-cubic metallic grains suspended in an amorphous matrix. For both the x=1 and x=0 alloys, the softest magnetic properties (coercivity Hc<0.5Oe, 4πMs∼7000G) coincided to a deposition at 300°C in which the fcc grain size (D) was 6–8nm separated by an amorphous phase of ∼1nm. At higher substrate temperatures (Ts) grain size follow a Ts2 dependence, while at lower temperatures the grain size is comparable but the volume of the amorphous matrix is larger and hence the exchange coupling is comparatively weak. A power law relationship is observed between the coercivity and grain size with HcαD3. These results are consistent with the results of Suzuki et al. [J. Magn. Magn. Mater. 177–181, 949 (1998)] for the nanostructure being dominated by a uniaxial anisotropy.
Enhanced spin accumulation in a superconductor
Authors: Urech M. , Johansson J. , Poli N. , Korenivski V. , Haviland D. B.
Published: June 2006

A lateral array of ferromagnetic tunnel junctions is used to inject and detect nonequilibrium quasiparticle spin distribution in a superconducting strip made of Al. The strip width and thickness are kept below the quasiparticle spin diffusion length in Al. Nonlocal measurements in multiple parallel and antiparallel magnetic states of the detectors are used to in situ determine the quasiparticle spin diffusion length. A very large increase in the spin accumulation in the superconducting state compared to that in the normal state is observed and is attributed to a diminishing of the quasiparticle population by the opening of the gap below the transition temperature.
Stress polarity dependence of breakdown characteristics in magnetic tunnel junctions
Authors: Kim Kwang-Seok , Jang Y. M. , Nam C. H. , Lee Ki-Su , Cho B. K.
Published: June 2006

Time-dependent dielectric breakdown (TDDB) measurements under constant voltage stress with positive and negative bias polarities are carried out for magnetic tunnel junctions (MTJs) with different oxidation status (under-, optimal, and overoxidation). We found that there is significant polarity dependence in the TDDB and speculated that the polarity dependence is due to both intrinsic and extrinsic origins. Optimally oxidized MTJs with positive bias on the top electrode show shorter times to breakdown (tBD’s) and lower barrier height than with negative bias, indicating that asymmetric band structure, in part, causes the polarity dependence. On the other hand, under- and overoxidized MTJs show much shorter tBD’s than optimally oxidized one and show a higher 1∕f noise power density for positive bias than for negative bias, indicating that the polarity dependence is also, in part, due to the interface states, which acts like precursors for the dielectric breakdown.
Fabrication and testing of deep submicron annular vertical magnetoresistive random access memory elements
Authors: Moneck Matthew T. , Zhu Jian-Gang
Published: June 2006

Robust magnetic switching and nonvolatility make magnetic random access memory an attractive prospect for future memory designs. However, there has been some concern over whether or not standard fabrication processes can be used to produce annular shaped memory elements. In this paper we present the fabrication and test results of deep submicron annular memory elements defined by electron beam and optical lithographies. The annular memory cells consist of a current perpendicular to plane giant magnetoresistive (CPP GMR) stack containing two ferromagnetic layers with a nonmagnetic interlayer where thin Cu laminations were included in the ferromagnetic layers to enhance the CPP GMR effect.
Influence of in-plane field on the thermal stability of perpendicular media
Authors: Uesaka Y. , Endo H. , Kodama N. , Nakatani Y. , Hayashi N. , Fukushima H.
Published: June 2006

In order to investigate the effect of single pole type head scanning on output signal stability of perpendicular media, 2kOe static magnetic field was applied to the media. The results are as follows. Coupling of the in-plane component of the magnetic field and thermal agitation decrease output signal. On the other hand, the perpendicular component of the magnetic field scarcely affects the stability of the output signal. Exchange interaction between grains in the recording layer increases the stability of the output signal. Easy axis distribution of the recording layer decreases the stability. Exchange interaction between recording layer and soft magnetic underlayer (SUL) scarcely affects the stability. Anisotropy field of SUL does not affect the stability.
Micromagnetic simulations of nanosecond magnetization reversal processes in magnetic nanopillar
Authors: Finocchio G. , Carpentieri M. , Azzerboni B. , Torres L. , Martinez E. , Lopez-Diaz L.
Published: June 2006

In this paper we study by means of the spin torque model the fast switching behavior of the Co(20nm)∕Cu(5nm)∕Co(2.5nm) magnetic multilayers of two different cross sections: ellipse (130×70nm2) and ellipse (130×40nm2). Simulations have been performed at zero and room (300K) temperatures, these point out that the magnetization inversion occurs by nucleation processes in three main steps, for both structures. In particular, for zero temperature the third step of the switching depends on the value of the spin-polarized current. Furthermore, for all of the simulated currents the switching processes are thermally activated and smoother with respect to zero temperature.
Thermal stability and recording writability of hard/soft stacked perpendicular media
Authors: Inaba Y. , Shimatsu T. , Aoi H. , Muraoka H. , Nakamura Y. , Kitakami O.
Published: June 2006

Magnetic properties and recording performance were studied for hard/soft stacked media consisting of a magnetically hard CoPtCr–SiO2 layer (10nm thickness) underneath a soft granular layer. Various kinds of soft layers [NiFe(Cr)–SiO2,CoCr–SiO2,…] were used in the present study. Transmission electron microscope images revealed that the grains of the soft layer grew on CoPtCr grains in all kinds of stacked media with SiO2 segregated at the grain boundaries. However, it was revealed that the grain isolation becomes degraded at the film surface when the soft layer was thicker than a critical thickness, and this thickness was about 7nm for (NiFe∕CoPtCr)–SiO2 media. The stacked structure reduced the coercivity, with no significant change in the slope of the magnetization curve. The reduction in dynamic coercivity H0 reached about 2kOe for stacked media with 3nm soft layers, allowing a reduction of the saturation recording current. Furthermore, no significant changes in the signal to media noise ratio SNmR and thermal stability were observed on stacking the soft layer. We demonstrated that hard/soft stacked media with thin soft layers show better recording writability with no notable change in SNmR or thermal stability.
Concentration effect on high field pinning and transport behaviors in nano-Y2BaCuO5 doped Sm–Ba–Cu–O superconductor
Authors: Chen Shih-Yun , Liao Yang-Chung , Chen In-Gann , Wu Maw-Kuen
Published: June 2006

Recent studies indicate that the addition of nanoscale RE2BaCuO5 (nmRE211) (RE=Y, Sm, and Nd) in melt-texture-grown Sm–Ba–Cu–O bulk material can enhance the critical current density (Jc), especially in high magnetic fields. It is known that the δTc pinning is the reason for high field peak effect in Jc-H curves. The Jc-H curves exhibit enhancement compared with those of undoped samples. The analysis of first derivations of R-T curves found two peaks in dR∕dT curves representing two characteristic temperatures in high magnetic fields, where the lower temperature is attributed to the weak superconducting phase. The relative strength of the two peaks varies with the concentration of nmY211 additions. This argument is consistent with microstructural observations and pinning analysis results.
Intra-atomic aspects of magnon-plasmon interactions
Authors: Skomski R. , Dowben P. A.
Published: June 2006

Magnon-plasmon interactions are modeled by considering the spin-dependent dielectric response of atoms placed in crystalline environment. Hund’s exchange rules favor parallel spin alignment, but the strength of the exchange depends on the displacement of the centers of gravity of the atomic spin-up and spin-down electron charge clouds. The intra-atomic exchange is modeled by considering a Hubbard-type interaction, and interatomic interaction then yields a k-space dispersion. The eigenmodes of the plasma are a mixture of spin-up and spin-down degrees of freedom, described by a 2×2 interaction matrix. Minority and majority bands yield different plasmon frequencies. However, these modes are not orthogonal but coupled by intra-atomic exchange and obtained by explicit matrix diagonalization. The effect is largest for small wave vectors, in agreement with experiment.
Impedance behavior of spin-valve transistor
Authors: Peng T. Y. , Chen S. Y. , Hsieh L. C. , Lo C. K. , Huang Y. W. , Chien W. C. , Yao Y. D.
Published: June 2006

The magnetoimpedance (MZ) effect of the pseudo-spin-valve transistor (PSVT) was investigated at room temperature in the frequency ranged from 100Hzto15MHz. The PSVT can be regarded as a complex combination of resistors, inductors, and capacitors, while the impedance (Z) consists of a real part, the resistance (R), and an imaginary part, the reactance (X). Besides, all these components exhibit magnetic hysteresis. It is due to the frequency dependent behavior that R does not reach a minimum at the resonant frequency (fr). The frequency dependences of MZ and MX ratios cross zero at fx=6.5MHz and at fr=3.65MHz, respectively. The shape of magnetoreactance (MX) loop is reverse to the magnetoresistance (MR) loop; furthermore, MX ratio changes sign from negative at ffr. The MZ loop also reverses shape and sign after crossing fx. For instance, the MZ loop with a ratio of 0.077% at 6MHz switches to −0.086% and −0.125% at 7 and 8MHz, respectively.
Micromagnetic simulation of the complex transverse susceptibility in nanostructured particulate systems
Authors: Cimpoesu Dorin , Stancu Alexandru , Spinu Leonard
Published: June 2006

The paper is dedicated to the micromagnetic study of the real and imaginary parts of the transverse susceptibility (TS) temperature dependence for an interacting particulate system. Due to its fundamental and technological interest, we considered in our study two-dimensional arrays of particles as magnetic patterned media. It is shown that the interactions between particles strongly affect the TS signal. The effects of volume and anisotropy field dispersions are also analyzed.
Effect of acrylic acid vapor on the head∕disk interface
Authors: Ji Rong , Liew Thomas , Chong T. C.
Published: June 2006

The slider-disk interactions are sensitive to environmental and outgas factors. In this study, the impact of water vapor and acrylic acid vapor on the head∕disk interface is investigated. The related tribological performance and tribological failure mechanism is reported.
Stress dependence of coercivity in nanocrystalline Fe79Hf7B12Si2 glass-coated microwires
Authors: Garcia C. , Zhukov A. , Gonzalez J. , Zhukova V. , Varga R. , del Val J. J. , Larin V. , Chizhik A. , Blanco J. M.
Published: June 2006

The stress dependence of the coercivity of thin Fe79Hf7B12Si2 glass-coated microwires exhibiting a nanocrystalline microstructure is presented. Such nanocrystalline microstructure was developed by means of 1h annealing at temperatures between 450 and 600°C. The evolution of the grain size (15–35nm) with the annealing temperature was obtained from x-ray diffraction measurements. The applied tensile stress dependence of the coercive field of both as-cast and annealed samples has been measured. The coercivity monotonically decreases with the applied tensile stress for the as-cast and the low annealing temperature (Tann<500°C) samples. This fact should be ascribed to the positive magnetostrictive character of these samples. Nevertheless, the samples annealed above 500°C behave in the opposite sense, the coercivity increases with the applied tensile stress, as a consequence of the negative effective magnetostriction of the annealed samples with a massive presence of α-Fe(Si) and α-Fe nanograins.
Magnetic properties of indium-substituted BiCaVIG single crystals
Authors: Xu Zhicheng
Published: June 2006

Indium-substituted BiCaVIG ({Bi3−2xCa2x}[Fe2](Fe3−xVx)O12)single crystals were grown in PbO–Bi2O3 flux with indium content, y⩽0.144 per molecular formula. Desirable magnetic properties, i.e., low saturation magnetization, MS=2.38×105A∕m, and also low magnetocrystalline anisotropy field, K1∕MS=2.6×103A∕m, have been obtained. The results can be elucidated in terms of single-ion contributions to the magnetocrystalline anisotropy from the Fe3+ ions on octahedral (A) and tetrahedral (D) lattice sites of the iron garnet structure and the effect of indium substitution.
Modeling and measurement of spin torques in current-perpendicular-to-plane giant magnetoresistive (invited)
Authors: Smith Neil
Published: June 2006

This paper discusses some aspects of practical modeling of spin-torque effects in current-perpendicular-to-plane (CPP) giant magnetoresistive magnetic multilayers, with emphasis on considerations of primary importance for sensor and/or read head applications of CPP spin valves (SVs). These include an extension of the well-known Valet-Fert transport model [Phys. Rev. B 48, 7099 (1993)] to the general case of noncollinear magnetization vectors in CPP multilayer of arbitrary layer structure as well as the application of the model to realistic CPP-SV structures. Simple analytical models for the angular dependence of spin-torque critical currents in CPP-SV devices are reviewed and compared with numerical simulations which include thermal fluctuations. Finally, the paper also contains some recent high frequency spectral measurements of spin-torque-induced noise in dual spin-valve sensors.
Intrinsic and nonlocal Gilbert damping parameter in all optical pump-probe experiments
Authors: Djordjevic Marija , Eilers Gerrit , Parge Anne , Münzenberg Markus , Moodera J. S.
Published: June 2006

The study of magnetization dynamics on the femtosecond time scale is an important task for the implementation of future ultrafast spintronics. With the time resolution inherent using femtosecond laser pulses in all optical pump-probe experiments, the basic time constants of magnetic precessional modes as well as the energy dissipation processes, which determine the Gilbert damping, can be studied. The dominant magnetic relaxation modes for the thin Ni films have frequencies in the range of 1.5–13GHz. The corresponding Gilbert damping parameter is found to be dependent on the precession mode. The α values range from 0.05 to 0.8 for highly damped modes. The nonlocal Gilbert damping due to evanescent spin currents and two-magnon scattering is studied for double layers Ni∕Cr∕Si(100) with varied Ni thicknesses. A large increase of the damping parameter for films with a thinner Ni layer is observed.
Carrier induced ferromagnetism in Nb doped Co:TiO2 and Fe:TiO2 epitaxial thin film
Authors: Hitosugi T. , Kinoda G. , Yamamoto Y. , Furubayashi Y. , Inaba K. , Hirose Y. , Nakajima K. , Chikyow T. , Shimada T. , Hasegawa T.
Published: June 2006

We have investigated the carrier induced magnetic properties of anatase Ti1−x−yNbxMyO2 (M=Co,Fe) epitaxial films grown by the pulsed laser deposition technique. For Ti0.95−xNbxCo0.05O2, the n-type carrier density could be controlled in a wide range (4.9×1017cm−3to2.7×1021cm−3) by Nb doping (x=0–0.2). The temperature dependence of the resistivity showed metallic behavior, suggesting that Ti0.95−xNbxCo0.05O2 undergoes a semiconductor to metal transition along with a slight carrier doping less than x<0.03. In both Co-doped and Fe-doped films, we have confirmed hysteresis in M-H curves, and the anomalous Hall effect at room temperature. This strongly suggests that the charge carriers are spin polarized and mediate ferromagnetic interaction between local spins on transition metal ions. In the case of Ti0.94−xNbxFe0.06O2, ferromagnetism is sensitive to carrier concentration. That is, the x=0.002 film is nonmagnetic even at 3K, while room-temperature ferromagnetism appears at x=0.01.
Scaling behavior of thermally assisted magnetization reversal in nanomagnets
Authors: Braun Hans-Benjamin
Published: June 2006

Energy barriers and the corresponding magnetization configurations of a nanowire of arbitrary length are derived analytically. The results interpolate between the Néel-Brown theory and the soliton-antisoliton mechanism for long samples. It is shown that the reversal mechanism depends on the time scale being probed. For sufficient size, one and the same particle reverses uniformly for fields close to the anisotropy field at short time scales, but reverses nonuniformly at low fields and large time scales. Remarkably, the energy barrier for nonuniform reversal is considerably lower than that for uniform reversal at small time scales.
Magnetic properties of self-organized lateral arrays of (Fe,Ag)∕Mo(110) nanostripes
Authors: Borca B. , Fruchart O. , Meyer C.
Published: June 2006

We report the fabrication of self-organized arrays of Fe nanostripes with a lateral period of 3.5nm by sequential deposition of Fe and Ag on Mo(110). The wires display a strong in-plane uniaxial magnetic anisotropy along their length and are superparamagnetic above TB=185±15K. The large value of nucleation volumes, which is inferred from the analysis of the thermal dependence of coercivity below TB, suggests the existence of interactions between the wires.
NMR study of heavy fermion superconductor with no inversion symmetry CePt3Si
Authors: Ueda K. , Koyama T. , Hamamoto K. , Kohara T. , Motoyama G. , Oda Y.
Published: June 2006

We report Pt195 and Si29 NMR studies of an antiferromagnetic heavy fermion superconductor CePt3Si with no inversion symmetry in the crystal structure. The temperature dependence of spin-lattice relaxation rates (1∕T1) for Pt and Si nuclei measured between 2 and 300K might be explained by the contributions from a low-lying crystalline-electric-field level and a quasiparticle due to a hybridization between the ground state and conduction electrons. Just below the superconducting transition temperature (Tc) no remarkable enhancement in 1∕T1 was observed within an experimental error. No significant decrease in Si29 Knight shift was observed for parallel direction to the c axis below Tc. This indicates that spin triplet pairing is probably realized for the superconductivity.
Magnetic-field-induced switching between ferroelectric phases in orthorhombic-distortion-controlled RMnO3
Authors: Noda K. , Akaki M. , Kikuchi T. , Akahoshi D. , Kuwahara H.
Published: June 2006

We have investigated the dielectric and magnetic properties of Eu0.595Y0.405MnO3 without the presence of the 4f magnetic moments of the rare-earth ions and have found two ferroelectric phases with polarization along the a and c axes in a zero magnetic field. A magnetic-field-induced switching from one to the other ferroelectric phase took place in which the direction of ferroelectric polarization changed from the a axis to the c axis by the application of magnetic fields parallel to the a axis. In contrast to the case of TbMnO3, in which the 4f moments of Tb3+ ions play an important role in such a ferroelectric phase switching, the magnetic-field-induced switching between ferroelectric phases in Eu0.595Y0.405MnO3 does not originate from the magnetic transition of the rare-earth 4f moments but from that of the Mn 3d spins.
Time resolved studies of edge modes in magnetic nanoelements (invited)
Authors: Kruglyak V. V. , Keatley P. S. , Hicken R. J. , Childress J. R. , Katine J. A.
Published: June 2006

Micromagnetic simulations have been performed to investigate the frequencies and relative amplitudes of resonant magnetic modes within nanomagnetic elements of varying size that have been previously studied by time resolved Kerr magnetometry. The magnetic response of a nanoscale element generally consists of the edge and center localized modes. For 2.5nm thick elements, a crossover from center to edge mode excitation occurs as the element size is reduced to less than 220nm. Additional modes appear in the spin wave spectrum as the thickness of the element is increased. The frequency of the edge mode is particularly sensitive to the strength of the exchange interaction, dipolar interactions with nearest neighbor elements, and rounding of the corners of the element. Simulations with in-plane pulsed fields show that the edge mode becomes dominant in elements of somewhat larger size, emphasizing the importance of the edge mode in technological applications.
Magnetic transitions in lattice-matched, ordered FePt3 based antiferromagnetic/ferromagnetic films
Authors: Mani P. , Krishnamurthy V. V. , Robertson J. L. , Klose F. , Mankey G. J.
Published: June 2006

Measurements of lattice-matched antiferromagnetic/ferromagnetic films which are ideal layered systems to study exchange bias are reported. Epitaxial films of FePt3 have two kinds of antiferromagnetic ordering. The spin ordering phase with wave vector Q1=(12120) has a Néel temperature TN=160K and that with wave vector Q2=(1200) has TN=100K. Neutron diffraction confirmed the presence of Q2=(1200) antiferromagnetic ordering in 200nm Fe25Pt75 grown on MgO(100). The loop shift and coercivity of a trilayer film of CoPt3∕FePt3∕CoPt3 decrease with increasing temperature, consistent with the observed Néel temperature of FePt3. The x-ray diffraction rocking curve widths of films grown on MgO(100) and Al2O3(112̱0) are compared and related to the loop shifts that are observed in the films with lattice-matched antiferromagnetic/ferromagnetic interfaces.
Switching of composite media by wall propagation
Authors: Mukherjee Sonali , Berger Luc
Published: June 2006

Using simple scaling arguments, we calculate the external field required to propagate a domain wall from the soft to the hard phase in a composite media. In the absence of the external field the domain-wall energy is lower in the soft phase, resulting in an energy barrier between the hard and soft phases. It is shown that as the external field increases the domain-wall energy in the soft phase also increases, and there is a corresponding wall energy reduction in the hard phase, resulting in the lowering of the energy barrier. The switching field is obtained as the field, which equalizes the domain-wall energies in both phases, reduces the wall barrier to zero, and leads to wall propagation and reversal. The calculation allows identification of field dependent anisotropies and wall widths of the two phases, which become equal at the reversal field. The reversal field is found to linearly depend upon the intrinsic anisotropy difference and inversely on the sum of magnetization of the two phases. Moreover, the reversal field is found to be independent of the volume of the two phases. An attempt at exhaustive comparison with experiments is made.
Enhanced low field magnetoresistance in Sr2FeMoO6-glass composites
Authors: Wang Xianjie , Zhang Xingquan , Sui Yu , Cheng Jinguang , Liu Zhiguo , Miao Jipeng , Huang Xiqiang , Lu Zhe , Qian Zhengnan , Su Wenhui , Tang Jinke
Published: June 2006

In this paper, we report the enhancement of the low field magnetoresistance in Sr2FeMoO6-glass composites with different wt% percents of glass. The crystal structure of Sr2FeMoO6 does not change by adding glass, and the glass is most likely located at the grain boundaries. The low field magnetoresistance up to 1T of Sr2FeMoO6-glass composites at 10K all shows obvious butterfly-shaped field dependence other than the pure Sr2FeMoO6 bulk sample. The magnetoresistance of Sr2FeMoO6-glass composites at 10K is enhanced gradually with increasing the glass concentration and reaches 39% with a wt% of 50%. The enhancement of low field magnetoresistance in Sr2FeMoO6-glass composites can be well explained by the spin-dependent tunneling at the glass boundaries, and it also allowed us to conclude that the spin polarization of Sr2FeMoO6 is at least 80% at low temperature.
Strain and magnetic anisotropy of epitaxial Ni∕Cu(001) nanostructures
Authors: Lee S. G. , Shin S. W. , Jang J. W. , Hwang H. M. , Jang H. K. , Lee J. , Lee J. H. , Song J. H. , Choi J.-Y. , Lee H. S.
Published: June 2006

The magnetic anisotropy of isolated epitaxial Ni∕Cu(001) nanostructure on Si(001) has been studied together with structural characterization. The strain of the Ni nanostructure is released such that the perpendicular strain of nanostructure with tNi=10nm decreases to −0.48% from −1.17% of film with the same thickness. Because of this, strain becomes the main factor in determining magnetic anisotropy of Ni∕Cu nanostructure in the investigated Ni thickness range (tNi=4.5–10nm).
Ferroelectricity induced by incommensurate magnetism (invited)
Authors: Harris A. Brooks
Published: June 2006

Ferroelectricity has been found to occur in several insulating systems, such as TbMnO3 and Ni3V2O8 which have more than one phase with incommensurately modulated long-range magnetic order. Here we give a phenomenological model which relates the symmetries of the magnetic structure as obtained from neutron diffraction to the development and orientation of a spontaneous ferroelectric moment induced by the magnetic ordering. This model leads directly to the formulation of a microscopic spin-phonon interaction which explains the observed phenomena. The results are given in terms of gradients of the exchange tensor with respect to generalized displacements for the specific example of NVO. It is assumed that these gradients will now be the target of first-principles calculations using the LDA+U or related schemes.
Spectroscopic measurements of Zeeman splitting of the density of states in high temperature superconducting tunneling junctions
Authors: Alvarez G. A. , Iguchi I. , Wang X. L. , Dou S. X. , Yao Q. W.
Published: June 2006

We report c-axis tunneling spectroscopy investigations in high magnetic fields (I-V characteristics and tunneling conductance dI∕dV) of high quality planar junctions fabricated from c-axis oriented NdBa2Cu3O7−δ∕PrBa2Cu3O7−δ∕NdBa2Cu3O7−δ thin film multilayers. The tunneling conductance in a parallel magnetic field reveals Zeeman splitting of the quasiparticle density of states. In the presence of spin-orbit interactions, our measurements for the tunneling density of states and the tunneling conductance show a spin splitting of gμBH in energy space of the density of states peak. Here the quantity μB is the Bohr magneton, g is the g factor of the electron, and H is the applied magnetic field. The magnitude of the splitting is attributed to the magnetic moment of the quasiparticles. The spin splitting of the density of states in HTS multilayer tunneling junctions could be used as a very powerful technique to determine the relative spin polarization in magnetic materials.
Magnetic structures in UCuSn
Authors: El-Khatib Sami , Llobet Anna , Purwanto Agus , Robinson Robert A. , Lee Seung-Hun , Lynn Jeffrey W. , Chang Sung , Nakotte Heinz
Published: June 2006

We report neutron-diffraction results on UCuSn which crystallizes in the orthorhombic P21cn structure. This compound undergoes two magnetic transitions around TN=62K and TM=25K. At T=52K, UCuSn exhibits a noncollinear arrangement of uranium moments of ∼1.5μB∕U-atom with components in all three principal directions. Our diffraction data provide further evidence that the transition around 25K is indeed magnetic in origin, although our refinements did not lead to a conclusive answer on its exact nature.
Spin oscillation of a quantum dot embedded in a ferromagnetic ring with two interacting electrons
Authors: Feng Jin-Fu , Jiao Chun-Hong , Wu Xiao-Shan , Jiang Shu-Sheng
Published: June 2006

The spin oscillation of a quantum dot embedded in a ferromagnetic ring is investigated. We suppose that there are two electrons with an opposite spin in the system, which is a sort of excited state of the system that includes ferromagnetism. By numerically solving the time-dependent Schrödinger equations for the many-body states, the spin oscillations of a quantum dot are derived and studied in detail. It is found that the amplitude and frequency of the oscillations depend sensitively on the exchange energy of ferromagnetic lead, the coupling strength between the dot and ferromagnetic lead, the gate voltage, and Coulomb repulsion energy of quantum dot. We show also that the magnetic moment on the dot can be controlled by tuning the exchange field of ferromagnetic lead.
High frequency susceptibility of closure domain structures calculated using micromagnetic modeling
Authors: Kaya Ahmet , Bain James A.
Published: June 2006

In this work we use micromagnetic simulation to compute the complex magnetic susceptibility of several Permalloy elements having closure domain structures. The average imaginary susceptibility of the structures typically shows multiple maxima, indicating multiple ferromagnetic resonance modes. Two-dimensional maps of the susceptibility show that the lower frequency resonances (0.9–1.5GHz) are typically associated with 90° domain walls, while higher frequency resonances (2–4GHz) are associated with regions of more uniform magnetization oriented orthogonal to the driving field. This trend is understood by looking at the total local fields within each resonating regions. Quantitative agreement is found between the resonance frequencies predicted by a simple Kittel analysis and those observed in these different regions, when all components of the local field (demagnetization, exchange, anisotropy, and external fields) are considered. In the lower resonant frequency domain wall regions, the resonance condition is set by the small positive difference between a large demagnetizing field that stiffens the magnetic system and a large internal exchange fields that softens the magnetic system. In contrast, the relatively uniform magnetization regions are marked either by a large exchange field acting as the stiffening term and slightly overcoming a large demagnetizing term or a small demagnetizing term acting as the stiffening term and overcoming an even smaller exchange field.
Magnetic properties of Fe1−xCoxCr2S4
Authors: Noda R. , Kamihara Y. , Matoba M.
Published: June 2006

We designed a series of samples with Co substitution (Fe1−xCoxCr2S4, 0⩽x⩽1) and studied the Co doping effect in FeCr2S4. Spin glass transitions are observed in lower temperature regions and ferrimagnetic transitions are observed in higher temperature regions. The paramagnetic-ferrimagnetic transition temperature (TC) increases with the amount of Co substitution. In order to clarify the magnetic character at the transition point, a critical exponent was obtained at about TC. The exponent changes from Ising to Heisenberg as x increases.
Energy losses in disordered hysteretic systems with dipolar interactions
Authors: Hovorka Ondrej , Friedman Gary
Published: June 2006

Numerical simulations are used to study behavior of energy losses in the dipolar random coercivity interacting switch model. It is demonstrated that switching losses are not only due to inherent properties of individual switches, but also contain an excess contribution resulting from their interactions. Furthermore it is demonstrated that the losses are not necessarily the same along different branches of nonsymmetric minor hysteresis loops. This conclusion contradicts the results obtained on the basis of the Preisach hysteresis model.
Effects of Monte Carlo parameters on the simulated magnetic properties of Stoner-Wohlfarth particles
Authors: Russell C. M. P. , Unruh K. M.
Published: June 2006

Hysteresis loops for two-dimensional and three-dimensional systems of Stoner-Wohlfarth particles have been simulated by the Monte Carlo method as a function of the temperature and the magnitude of the angular aperture used to update the magnetization direction. A comparison of the reduced coercivity obtained from these hysteresis loops with those obtained from Néel’s relaxation model have allowed a consistent set of temperature dependent update angles to be determined for a given number of Monte Carlo steps.
Influence of spin polarization enhancement layer to rare earth–transition metal thin films for perpendicular tunneling magnetoresistance evaluated by ferromagnetic Hall effect
Authors: Hatori T. , Okuda M. , Nakagawa S.
Published: June 2006

Magnetic properties of ion beam sputter-deposited TbFeCo and GdFeCo thin films with their compensation point were studied using ferromagnetic Hall effect measurement. Gdx(Fe90Co10)1−x thin films sputter deposited using Kr exhibited high squareness ratio and sharp magnetization reversal compared with those prepared using Ar as sputtering gas. Lower kinetic energy of sputtering ions was effective to attain higher squareness ratio. Ferromagnetic Hall effect clarifies the magnetization reversal process of a FeCo(xnm) layer contacted to a TbFeCo(50nm) layer. FeCo layer with thickness less than 2nm is magnetized together with TbFeCo layer. Ferromagnetic Hall measurement is a powerful tool to determine the magnetization process of very thin films and magnetically coupled layers.
Velocity of vortex walls moved by current
Authors: Jubert P.-O. , Kläui M. , Bischof A. , Rüdiger U. , Allenspach R.
Published: June 2006

Current-induced domain-wall motion experiments in 27nm thick and 200–500nm wide Fe20Ni80 wires are reported. By imaging the domain-wall position after current injections, the mean wall velocities are determined. The initial velocity is found to be constant for pulse lengths between 2 and 25μs but decays after about ten injections. For samples with an increasing wire width the initial velocity is reduced.
Tunneling magnetoresistance of magnetic tunneling junction cell measured by conducting atomic force microscopy with ramping dc bias voltage rate
Authors: Shu Min-Fong , Canizo-Cabrera A. , Hsu Chih-Cheng , Chen C. C. , Wu J. C. , Li Simon C. , Yang Chao-Chen , Wu Te-ho
Published: June 2006

Magnetoresistance (MR) ratio in a magnetic tunneling junction cell with a structure of SiO2 (20nm)∕Ta (5nm)∕Cu (20nm)∕Ta (5nm)∕NiFe (2nm)∕Cu (5nm)∕MnIr (10nm)∕CoFe (4nm)∕Al–O (1.5nm)∕CoFe (4nm)∕NiFe (20nm)∕Ta (50nm) was measured by conducting atomic force microscopy to obtain I-V curves. Tunnel magnetoresistance was characterized from these nonlinear I-V curves. MR values of 33.9%, 30.5%, 30.3%, and 28% were obtained when applying magnetic fields of ±150Oe at various dc bias voltage ramping rates. Several ramping rate values were 0.498, 4.65, 9.3, and 27.9Hz, respectively.
Iron-based microstrip band-stop filters at higher microwave frequency range: Design optimization using shape anisotropy
Authors: Khivintsev Y. V. , Kuanr Bijoy K. , Harward Ian , Camley R. E. , Celinski Z.
Published: June 2006

Microwave filters that use thin films of ferromagnetic metals are now being established as an option compared to YIG based filters due to their higher frequency response at very small magnetic fields. The aim of the present investigation is to significantly boost the operating frequency of notch filters in very low applied magnetic fields. To do this, we fabricated a series of notch filters with Fe films of different thicknesses and shapes. The stop-band response of each of these filters cover four waveguide bands (X, KU, K and Ka) ranging from 8to40GHz with an applied magnetic field up to 4.5kOe. The frequency characteristics of these filters at zero field can be significantly changed by changing the geometry of the magnetic element. For example, an Fe film which is 7.5μm wide, 100nm thick, and 3mm long has a stop band centered at 9.5GHz, but this is increased to 22GHz for an 800nm thick Fe film. For narrower signal lines (4.5μm wide), we observed further increases in the frequencies: 11GHz for a 100nm Fe film increased to 25GHz for an 800nm thick Fe film. This upshift in frequency is due to the shape anisotropy of the Fe film. In addition, we investigated the temperature variations in the properties of the devices from room temperature to +85°C
Magnetization reversal and field annealing effects in perpendicular exchange-biased Co∕Pt multilayers and spin valves with perpendicular magnetization
Authors: van Dijken Sebastiaan , Crofton Matthew , Czapkiewicz M. , Zoladz M. , Stobiecki T.
Published: June 2006

A study on the Co layer thickness dependence of the magnetization reversal process and magnetoresistance of perpendicular exchange-biased Co∕Pt multilayers and spin valves with perpendicular magnetization is presented. The hysteresis of [20ÅPt∕tÅCo]3∕100Å IrMn∕20Å Pt multilayers with t⩾5Å are found to be asymmetric after deposition. This asymmetry reflects a lateral variation in the perpendicular exchange-bias direction due to the growth of IrMn onto multidomain Co∕Pt multilayers. Magnetic annealing in a perpendicular field removes the switching asymmetry and leads to a maximum exchange-bias field of 16.3mT for t=7Å. Perpendicular spin valves with an optimized structure of 50 Ta∕20Å Pt∕tÅ Co∕30Å Cu∕tÅ Co∕20Å Pt∕6Å Co∕3Å Pt∕100 IrMn∕20Å Pt are found to exhibit good switching behavior but limited magnetoresistance for small Co layer thickness. The magnetoresistance of these spin valves increases with t up to a maximum of 3.9% for t=10Å, beyond which it decreases due to simultaneous magnetization reversal in the ferromagnetic layers. Magnetic field annealing of these top-pinned structures reduces the exchange-bias field and the magnetoresistance.
Room-temperature deposition of nanocrystalline CaWO4 films by pulsed laser and their optical properties
Authors: Ryu Jeong Ho , Yoon Jong-Won , Shim Kwang Bo , Sasaki Takeshi , Koshizaki Naoto
Published: June 2006

Nanocrystalline CaWO4 films were synthesized by pulsed laser deposition at various Ar gas pressures without substrate heating or postannealing treatment. Crystallinity, chemical composition, absorbance, optical energy band gap, and photoluminescence (PL) of the CaWO4 films were found to be strongly dependent on the Ar pressure. The optical energy band gap was in a range of 4.9–4.5eV and PL peak position in the spectra was positioned in a blueshifted region around 3.29eV at room temperature. Compared with emission at around 2.95eV of bulk CaWO4 targets under the same excitation conditions, our results clearly demonstrate the blueshift of PL emission induced by the quantum-size effect.
Temperature induced differences in the nanostructure of hot-wire deposited silicon-germanium alloys analyzed by anomalous small-angle x-ray scattering
Authors: Goerigk G. , Williamson D. L.
Published: June 2006

The nanostructure of hydrogenated amorphous silicon-germanium alloys, a-Si1−xGex:H (x=0.62–0.70), prepared by the hot-wire deposition technique applying different substrate and filament temperatures was analyzed by anomalous small-angle x-ray scattering experiments. The pure-resonant scattering contribution, which is related to the structural distribution of the Ge component in the alloy, was separated from the total small-angle scattering for one sample series. For all alloys the Ge component was found to be inhomogeneously distributed. The shape of the pure-resonant and the mixed-resonant scattering curves reveal significant differences indicating the presence of a third phase, probably hydrogen clusters and/or voids. The thin films showed improved microstructure when lowering the filament temperature to 1800°C. Additional improvement was achieved by optimizing the substrate temperature (between 260 and 305°C) resulting in suggested mass fractal structures of Ge with the fractal dimension p<1.6 and a size of about 40nm. The nature of the microstructural changes induced by changes in filament temperature compared to those induced by the changes in substrate temperature were clearly different. The improved microstructure of the alloys could be correlated with improved optoelectronic properties of the material.