bluuebunny/crossref_metadata_2025 huggingface dataset:
Magnetic properties of Fe(2−x)RuxZr system (abstract)
Authors: Bhattacharya Sarbari , Deshmukh M. M. , Paulose P. L. , Nagarajan V.
Published: July 2002

Pseudobinary systems of the type Fe(2−x)MxZr (M=transition metal) have been studied extensively for their typical magnetic properties.12 The common features of such systems are a breakdown of magnetic order and change in crystal structure from cubic C15 (Cu2Mg) type to hexagonal C14 (MgZn2 type) with a variation in concentration (x). The origin of the structural change is not very clear. The Fe moment is thought to be influenced by the near neighbor environment and variation in electronic structure. The present investigation involving the substitution of Fe with isoelectronic Ru assumes importance in light of this and complements the previous reported work. We find that for x<0.5 the structure is C15. For x=0.6, a structural transition to C14 type takes place. The structural transition in the case of Ru occurs at x=0.5 as against values between 0.2 and 0.4 reported for similar studies with other transition metals like V, Cr, and Mn. There seems to be a clear indication of electronic influence on the structural change. In the case of V, which has five outer electrons, the structural change occurs at a relatively low substitution of x=0.2, whereas in the case of Ru, which has eight outer electrons, the structural change occurs at x=0.5. For x=0.1 to 0.3, the samples show a behavior similar to that of Fe2Zr, which is a ferromagnet with a saturation moment of 1.7 Bohr magnetron/Fe atom at 5 K. In the case of x=0.4, however, the saturation moment undergoes a sharp fall to 1.1 Bohr magnetron. Moreover, x=0.4 shows re-entrant behavior with a Tc of about 325 K and a spin freezing temperature of 130 K.
Spin dynamics of lightly doped La1−xSrxMnO3 (abstract)
Authors: Moudden A. H. , Vasiliu-Doloc L. , de Leon-Guevara A. M. , Pinsard L. , Revcolevschi A.
Published: July 2002

Neutron scattering has been used to study the spin dynamics of the lightly doped La1−xSrxMnO3, with x between 0 and 18. We show that as the dopant concentration increases from 0 to 0.09, the magnetic long-range order progressively changes from a slightly to a strongly canted antiferromagnetic order, whereas the transition temperature is continuously reduced from about 140 K to about 120 K. The spin waves show a strongly anisotropic dispersion relation that can be described by a strong ferromagnetic in-plane coupling Jab and a weak antiferromagnetic out-of-plane coupling Jc. The spin wave dispersion exhibits a finite gap that continuously decreases with the dopant concentration from about 2.5 meV for x=0 to about 1.5 meV for x=0.09. Furthermore, while the ferromagnetic in-plane coupling is slightly enhanced from SJab∼1.8 meV for the undoped sample to SJab∼2.0 meV for 9% doping, the out-of-plane antiferromagnetic coupling is strongly reduced from SJc∼−1.2 meV to SJc∼−0.4 meV. In the same concentration range, the system remains orthorhombic with a continuously decreasing lattice distortion, which could indicate that part of the reduction in the out-of-plane antiferromagnetic coupling can be ascribed to steric effects, whereas the enhancement of the ferromagnetic coupling could be solely associated with electronic effects. Preliminary studies of the spin-wave linewidths in the two systems x=0 and x=0.09 show a significant increase in the damping of the in-plane ferromagnetic spin waves. These results clearly show that La1−xSrxMnO3 evolves into a two-dimensional ferromagnetic system near the concentration x=0.1.
Exchange coupled domain walls: Resonance in multilayers
Authors: Stamps R. L. , Wigen P. E. , Carriço A. S.
Published: July 2002

Exchange coupling between magnetic films can lead to an attractive force between domain walls in the separate films. The coupling between the films allows for small amplitude oscillations around the equilibrium configuration of the walls, analogous to optic and acoustic type spin wave resonance modes. Since the restoring force acts only over the length of a domain wall, this suggests the possibility of sensitive measurements of the local coupling by studying domain wall resonances. The effects of applied fields are also examined. With antiferromagnetic coupling, small static fields push the walls apart and result in different behaviors of the acoustic and optic domain wall resonances.
180° walls structures in exchange-coupled multilayers with in-plane uniaxial anisotropy
Authors: Labrune M. , Miltat J.
Published: July 2002

First, the number of irreducible wall configurations is derived, both for an odd and an even number of magnetic layers. Second, the comparison of simulated wall structures in stacks made of three and four magnetic layers serves as an illustration of the interplay between flux–closure mechanisms and interlayer exchange interactions. Flux closure is shown either to compete or accommodate with ferromagnetic-type interlayer exchange interactions owing to its short- or long-range character. Last, an equivalent single layer behavior is shown to develop with an increasing number of magnetic layers, magnetic layer thickness, and interlayer (ferromagnetic-type) exchange parameter.
Magnetoelastic coupling in TbFe2 (110) thin films
Authors: Ciria M. , Arnaudas J. I. , Dufour C. , Oderno V. , Dumesnil K. , del Moral A.
Published: July 2002

We have determined the rhombohedral magnetoelastic stress of a Laves phase TbFe2 (110) single-crystal film, grown by molecular-beam epitaxy. The film thickness was 1300 Å. The magnetoelastic stress was directly measured by using a low-temperature cantilever capacitive method, between 300 and 10 K. The isotherms clearly display the coercive field but, unlike bulk alloy behavior, do not saturate even at the maximum field of 12 T. The determined rhombohedral magnetoelastic parameter of the film is Bε,2=−0.43 GPa, at 0 K and 12 T, which is 0.67 times the value for bulk TbFe2. Bε,2 follows a power m3 of the reduced magnetization m, indicating a single-ion volume origin for the rhombohedral magnetoelastic stress of this film. Measurements performed in a 300 Å TbFe2 (110) film deposited onto a YFe2 buffer show that the coercive field is drastically lowered and that the magnetoelastic distortion is negligible.
Magnetoresistance of Ce3Cu3Sb4: Ferromagnetic semiconductor (abstract)
Authors: Paulose P. L. , Patil Sujata
Published: July 2002

Ferromagnetic semiconductors are an interesting class of materials. The first one, CrBr3 was discovered only in 1960. Magnetic semiconductors usually show a prominent peak in the resistivity and a very large negative magnetoresistance (MR) in the vicinity of Tc, which is not well understood. Recently, we have reported Ce3Cu3Sb4 to be a ferromagnetic semiconductor with a Tc of 10 K.1 To our knowledge, this is the first Ce system of this type and is a further addition to the rich varieties of ground states exhibited by Ce systems. Here, we report the MR studies on Ce3Cu3Sb4 in magnetic fields up to 8 T in the temperature range 4.2–300 K. The resistivity of Ce3Cu3Sb4 shows a rise with decrease in temperature from 300 K exhibiting a peak at 19 K followed by a drastic fall at low temperatures. An activation type fit to the data gives a band-gap energy of 84 K. The resistivity peak is broadened considerably in a field of 8 T and the peak is found to shift to higher temperatures by about 10 K. The MR is small and negative (<0.5%) down to about 60 K and then gradually peaks to a large value of −30% at 12 K. The increased conduction in the vicinity of Tc shows that the conduction band is influenced by the magnetic spins presumably due to s–f interactions. The band-gap energy is found to remain constant even in a field as high as 8 T. This may rule out the magnetic polarons to be the cause of activation type of resistivity behavior.
Metamagnetism and electronic structure of UNiGa
Authors: Havela L. , Sechovský V. , Aoki Y. , Kobayashi Y. , Sato H. , Prokeš K. , Mihálik M. , Menovsky A. A.
Published: July 2002

Antiferromagnetic uranium intermetallic compound UNiGa shows a large reduction of electrical resistivity in conjunction with the metamagnetic transition in fields below 1 T. On the basis of specific heat, Hall effect and thermopower measurements in various fields we suggest that the reason for the giant magnetoresistance is a substantial Fermi surface reconstruction at the transition.
Study of mechanical alloying of Sm and Fe
Authors: Seifu D. , Oliver F. W. , Hoffman E. , Aning A. , Suresh Babu V. , Seehra M. S. , Catchings R. M.
Published: July 2002

Mechanical alloying of Sm and Fe with the composition of SmFe3 was studied using x-ray-diffraction (XRD), Mössbauer, and magnetization measurements. Data taken as a function of milling time for up to 20 h show significant changes occurring during ball milling. The XRD studies show that the initial crystalline Bragg reflections changed to a broad maximum, which is attributed to the formation of an amorphous phase. The initial six-line pattern in the Mössbauer spectrum, characteristic of magnetic ordering, changed to a broad singlet, characteristic of a nonmagnetic material. Magnetization measurements revealed that the coercive field was at its maximum after 5 h of milling and decreased sharply as the milling time increased. The remanent magnetization was at its maximum between 5 and 10 h of milling. The final product of the ball milling, which exhibited the characteristics of an amorphous paramagnetic material in its XRD and Mössbauer spectrum, was studied after heat treatment. The XRD and the Mössbauer spectra of the heat treated alloy show that substantial changes occurred during heat treatment in that sharp Bragg reflections, characteristic of crystalline materials, reappear and the alloy changed from a paramagnetic to a ferromagnetic state.
Micromagnetic behavior and recording performance in high density 5 Gb/in.2 CoCrPtTa medium
Authors: Kisker H. , Abarra E. N. , Yamada Y. , Glijer P. , Suzuki T.
Published: July 2002

Micromagnetic studies of the magnetization process in high density 5 Gb/in.2 CoCrPtTa and 0.8 Gb/in.2 CoCrTa media were carried out in conjunction with recording performance measurements. The maximum value of the δM curves measured at room temperature and at 373 K is smaller for CoCrPtTa compared to CoCrTa which indicates lesser intergranular exchange coupling in the quaternary medium. At room temperature, the Barkhausen volume V*, which is the smallest switching unit in magnetization reversal, corresponds approximately to one grain for CoCrPtTa but to three to four grains for CoCrTa. These results are consistent with the lower media noise found for CoCrPtTa especially at high bit densities. A decrease in V* to the size of one grain for CoCrTa is observed for temperatures below 200 K. The reduced intergranular exchange is confirmed by δM measurements at 77 K. Low coupling, however, leads to low values of Mr which leads to a reduction in the reading signal. The value of Mr for CoCrTa at 77 K is 60% greater than that of CoCrPtTa at room temperature despite the reduced exchange coupling indicating that larger values of Mr can still be achieved while confining the switching volume to no more than a grain.
Exchange coupling through composite nonmagnetic spacer layer in magnetic superlattices
Authors: Takahashi Y. , Inomata K.
Published: July 2002

We calculated the exchange coupling between ferromagnetic layers in magnetic sandwiches, in which a potential scattering layer is inserted in a nonmagnetic spacer layer. The Green’s function method is used for the calculation. The numerical results are obtained with varying the potential height and width. The Rudermann–Kittel–Kasuya-Yoshida-like exchange coupling is changed significantly in phase and amplitude with increasing the change of the potential barrier properties. However, the change of the oscillation period is not so remarkable. It is shown from the numerical results that the transition from ferromagnetic to antiferromagnetic coupling, or the reverse, can be induced by changing the potential parameters under a definite structure of magnetic sandwiches.
Biquadratic exchange coupling in sputtered Fe/Cr/Fe(100) sandwich structures
Authors: Chesman C. , Azevedo A. , Rezende S. M. , de Aguiar F. M. , Bian X. , Parkin S. S. P.
Published: July 2002

We have used magnetoresistance, magneto-optical Kerr effect, ferromagnetic resonance (FMR), and Brillouin light scattering to investigate interlayer exchange coupling in Fe(40 Å)/Cr(t)/Fe(40 Å) trilayers. At the right-hand side of the first antiferromagnetic peak (13 Å<t<16 Å), a common feature in both transport and magnetic measurements is the presence of sudden discontinuous jumps as the magnetic field is varied. By treating both transport and magnetic data on equal footing, we demonstrate that all measurements can be described by model calculations which elegantly take into account the same set of phenomenological parameters, characterizing the magnetic anisotropy, Zeeman, and bilinear and biquadratic exchange energies. Temperature-dependent FMR measurements reveal intriguing similarities between the bilinear and biquadratic coupling behaviors.
Local structure of as-prepared and partially reduced Co,Ti,Sn-substituted Ba-hexaferrite powder
Authors: Snyder J. E. , Harris V. G. , Harrell J. W. , Parker F. T. , Kitahata S.
Published: July 2002

In this study, the local structure around Fe and Co atoms in as-prepared and partially reduced Co,Ti,Sn-substituted Ba-hexaferrite powders are examined using extended x-ray absorption fine structure (EXAFS). It appears that the substitutional atoms in general and Co in particular show a preference for the Fe(IV) and Fe(III) sites. After partial reduction in H2 at 300 °C, both the Fe and Co EXAFS show a mixture of two phases: substituted Ba-hexaferrite and bcc metal. There does not appear to be any large preference in reduction among the five different types of Fe sites. However, a much larger fraction of the Co is reduced than is the Fe. Thus the partially reduced sample is not merely a collection of unreduced and totally reduced particles. The after-reduction spectra for both Fe and Co EXAFS were fit using a linear combination of spectra from as-prepared material and bcc metal, which enabled quantitative calculation of the relative fraction of each metal reduced.
Signal to noise ratio comparison of SAL versus dual-stripe magnetoresistive heads under comparable reliability conditions
Authors: George Peter K. , Zhu Jian-Gang
Published: July 2002

At high current densities the reliability of a magnetoresistive head is predominantly determined by the temperature rise of the sensor. Current dual-stripe magnetoresistive head (DSMR) designs operate two sensors, each at 2×107 A/cm2, resulting in twice the temperature rise of a typical SAL head. The proper design scaling for reliability would require a similar temperature rise for both heads with each sensor operating at 2×107 A/cm2. In this article the implications of this assumption are investigated with respect to the signal to noise ratio. The scaled DSMR design requires thinner sensors and lower bias current resulting in a higher output. The outputs of the SAL and DSMR heads are baselined via experimental data. A micromagnetic model is used to scale the results for comparison. The higher sensor output of the DSMR is compensated by the increase in preamp noise resulting from a differential versus single ended preamplifier, and from the increased Johnson noise due to higher resistance DSMR elements. Several limits are examined relative to the media noise and the bandwidth of the system. Practical resistances of the leads are included in the analysis as well as preamp noise figures. The results show a 0.2 dB gain of the DSMR head relative to the SAL head, not including linear density differences. This is at constant current density and constant joule heating. The comparison is expected to worsen at constant linear density. For a constant 25 μV noise floor the higher output of the DSMR design gives a 3 dB improvement over SAL. Thinning the SAL sensor to the same thickness as the DSMR sensor largely negates this advantage. The DSMR design does not show a definitive advantage over SAL despite claims often referred to in the literature. The role of DSMR heads in future areal density improvements will be discussed based upon these results.
The role of NiAl underlayers in longitudinal recording media (abstract)
Authors: Ross C. A. , Nolan T. P. , Ranjan R. , Lu D.
Published: July 2002

NiAl has been proposed as a replacement for the Cr underlayer in longitudinal recording media.1 NiAl has a B2 ordered structure with a lattice parameter close to that of Cr. Hence the same epitaxial relationships that exist in CoCrPt/Cr could also exist in CoCrPt/NiAl. Although high coercivity media have been made using a NiAl underlayer, a detailed microstructural study of the CoCrPt/NiAl is lacking. We have investigated the crystal orientation and epitaxial relationships in CoCrPt/NiAl bilayers deposited by DCM sputtering at a range of deposition temperatures up to 250 °C and sputter rates of 0.5–10 nm/s. Samples were examined by high resolution transmission electron microscopy in cross section and plan view. None of the samples showed the (002) NiAl/(11̈20)Co orientation that would be expected by analogy to CoCrPt/Cr films deposited at high temperatures. The CoCrPt crystals in the CoCrPt/NiAl are oriented with their c axes at a range of angles to the film plane, accounting for the lower squareness (Sq) of the magnetic hysteresis loop compared to samples on Cr underlayers. There is no evidence of the “bicrystal” structure in CoCrPt/NiAl as is seen on films grown on Cr.2 Grain size in NiAl is smaller than in Cr giving a smaller grain size in the CoCrPt overlayer and potentially lower medium noise. However, coercivity of CoCrPt/NiAl is lower than CoCrPt/Cr. The coercivity increases as the NiAl film thickness increases, and this is believed to be caused by topographic isolation of the magnetic grains.3 Recording measurements on CoCrPt/NiAl and CoCrPt/Cr media show that noise is lower in isotropic media made on the NiAl underlayer compared to oriented media made on the Cr underlayer. The microstructural results will be compared with previously published results on NiAl underlayers and the utility of NiAl as an underlayer in high density media will be assessed.
Anomalous heat capacity of antiferromagnet FeBr2 in a magnetic field
Authors: Katori H. Aruga , Katsumata K. , Katori M.
Published: July 2002

Heat capacity measurements in external magnetic fields (H) have been performed on the Ising antiferromagnet FeBr2. The temperature dependence of the magnetic heat capacity shows two anomalies for 1.4 T⩽H⩽2.9 T. The anomaly at a higher temperature indicates the phase transition from the paramagnetic to antiferromagnetic phase. The anomaly at a lower temperature is formed by a sharp peak superposed on a broad shoulder. The existence of two peaks shows the occurrence of a new phase transition in addition to the antiferromagnetic transition in magnetic fields. A Monte Carlo simulation for the heat capacity reproduces the peak indicating the antiferromagnetic transition and the broad shoulder. The nonuniform spin configurations in the layers with negative moment, which is caused by the competition between the nearest-neighbor ferromagnetic and the next-nearest-neighbor antiferromagnetic interactions in the triangular Fe-planes, is shown to produce the broad shoulder.
High density recording with keepered media and planar heads
Authors: Coughlin T. M. , Hoinville J. R. , Gooch B. R. , Haines W. G. , Payne A.
Published: July 2002

Modeling and experimental results are presented for keepered longitudinal recording media and planar (undershoot-reduced) thin film recording heads with low flying heights for areal densities >1 Gb/in.2. The keeper layer is magnetically coupled to the medium magnetic transitions, reducing the transition demagnetization and narrowing the transition length by about 10% in the media after recording. The reproduced bias field and the transition fields combine in the keeper to produce a partially saturated region, thereby modifying the fields from the medium transitions at the head during playback. We present experimental data on the write and read process for keepered media. Boundary element model results are presented which explain the amplitude gain and pulse asymmetries observed experimentally. Use of a keepered medium allows areal density improvements >20% through higher bits per inch.
Novel giant magneto-optical Kerr effect in cerium compounds (invited)
Authors: Schoenes J. , Pittini R.
Published: July 2002

The recently discovered novel giant polar magneto-optical Kerr rotation of 90° in CeSb is compared to previous record Kerr signals of the order of 10°, and to another newly discovered giant Kerr signal of 22° in CeS. An analysis of the temperature and field dependence of the off-diagonal conductivity spectra of CeSb is presented which allows an empirical assignment of several structures to electronic transitions. In particular, a folding of bands due to magnetic superstructures in a ferrimagnetic phase is shown to account for three additional peaks.
Propagation properties of microwave magnetic envelope solitons in thin yttrium–iron–garnet films at 5 GHz (abstract)
Authors: Xia Hua , Kabos Pavel , Patton Carl E. , Ensle Harold E.
Published: July 2002

Microwave magnetic wave packets propagating in thin yttrium–iron–garnet (YIG) films show potential for novel devices as well as improved understanding of the basic properties of linear and nonlinear waves. The propagation characteristics of these excitations that have been studied up to now, however, do not provide a clear separation between linear and soliton regimes or a clear separation of the different contributions to the decay during propagation. The objective of this work was to study such characteristics for 5 GHz, 13–40 ns wide backward volume wave (BVW) magnetostatic wave square pulses in both the low-power linear and in the high-power soliton regimes and address these issues. The measurements were made with a delay time structure with a long and narrow 7.2 μm thick YIG film and 50 μm wide transducers, and input powers from 5 mW to 2 W. The output peak power Pout versus input pulse power Pin exhibits the same nonlinear response reported previously,1 with a linear response region A followed by a region B response with a more rapid increase in Pout and a high power region C in which Pout goes through a maximum and decreases. However, the integrated output pulse power, or pulse energy, is a strictly linear function of input pulse power over both the A and the B regions. At the same time, one finds a small but measurable increase in the average propagation velocity for the pulses as power is increased. The measured decay in the total pulse energy with propagation time leads to an unambiguous separation of the decay contributions due to dispersion and loss. This allows, in turn, a clear separation between the linear pulse and soliton regimes. The results were modeled from the nonlinear Schrödinger (NLS) equation with propagation and damping terms included. Agreement is good for regions A and B but the NLS model fails completely for region C.
Instability analysis of SAL-biased magnetoresistive heads using rotational applied field
Authors: Mitsumata C. , Shimoe O.
Published: July 2002

Magnetoresistive (MR) head achieves high areal density in a magnetic recording system due to high sensitivity of the magnetic field. The high sensitivity of MR sensor, however, potentially causes an instability of readback signals. This instability appears as a variation of the output wave form during alternating reading and writing processes. In this study, MR heads were measured by a new technique that used rotational applied field. This technique is intended for checking the stability of magnetic domains against the arbitrary direction of the magnetic field. As a result, deviations of transfer curves for the MR sensor correlated to variations of the output voltage. This deviation seems to be caused by the Barkhausen jump in MR sensors. In order to analyze the Barkhausen jump, a simulation using the Landau–Lifshitz–Gilbert (LLG) equation was carried out. A calculated transfer curve of the MR sensor was similar to a measured curve, and it included two different origins of the Barkhausen jump. The first type of jump happened in the MR film. A rotational applied field generated domain walls in the active region of the MR film. The second type of jump occurred in the soft adjacent layer (SAL) film. The appearance of vortex domain walls changed the bias field from the SAL film, and then it forced a bias state variation in the MR film. The first type and second type of Barkhausen jump showed 0.05 and 0.01 Ω resistance change, respectively. But, in reducing the MR sensor height, the Barkhausen jump was suppressed, even in the case of a small domain stabilizing bias field.
One- and two-dimensional magnetization processes in amorphous alloys
Authors: Appino C. , Beatrice C. , Ferrara E. , Pelazza M. , Fiorillo F.
Published: July 2002

The hysteresis loop and loss properties of near-zero-magnetostrictive amorphous ribbons have been investigated as a function of the induced macroscopic magnetic anisotropy Ku from dc to 10 kHz. It is found that, following the evolution of Ku, imposed by means of sequential field annealings along the longitudinal (LD) and transverse (TD) directions, the hysteresis loop shape and area and the frequency dependence of energy losses suffer dramatic changes, dictated by the value of Ku and the local distributed anisotropies K. On passing from longitudinal to transverse macroscopic easy axis, coherent spin rotations come into play and, in association with the domain wall displacements, eventually provide a net magnetization reversal along both LD and TD. The hysteresis loss component Wh correspondingly increases and the energy loss at high frequencies decreases. It is concluded that: (1) optimal quasi-static magnetic behavior requires relatively high and positive Ku values (>50–100 J/m3), with longitudinal plane rigid walls and (2) the best high frequency properties are attained for Ku∼0, where flexible walls and complex domain patterns are created.
Spin structure in ferrimagnetic/antiferromagnetic Fe3O4/NiO superlattices (abstract)
Authors: Lind D. M. , Erwin R. W. , Borchers J. A. , Lee Seung Hung , Majkrzak C. F.
Published: July 2002

Neutron diffraction studies indicate that synthetic single crystalline [001] and [110] Fe3O4/NiO superlattices, prepared by oxygen-plasma-assisted molecular-beam epitaxy, exhibit long range antiferromagnetic ordering through the NiO layers despite the contrasting symmetries of the NiO rocksalt and Fe3O4 spinel unit cells, and that this ordering extends through multiple superlattice repeats. The structural and magnetic coherence in the Fe3O4 layers is confined to individual bilayers (<10 nm) due to interfacial stacking faults and this reduction gives rise to broadening of the selected neutron reflections. The coherence lengths for the antiferromagnetic and ferrimagnetic layers also differ in-plane, suggestive of magnetic frustration through the interfaces. Strong magnetic coupling at the Fe3O4/NiO interfaces, which leads to exchange biasing and to an enhancement of the NiO Neél temperature above the bulk value of 520 K (and approaching the TCurie=858 K for Fe3O4), has been observed. In contrast to results observed for a 1 μm NiO film, polarized neutron diffraction measurements of the [Fe3O4(6.8 nm)|NiO(3.4 nm)] superlattice found no preferred direction for NiO spins within a single 111 domain in a small field, but did show a tendency to reorient under application of a 1.5 T magnetic field. This seems consistent with the picture of an exchanged coupled but frustrated interface, since we have observed that the magnetic spins follow the field direction.
Magnetic excitations in the spin ladder system (VO)2P2O7
Authors: Garrett A. W. , Nagler S. E. , Barnes T. , Sales B. C.
Published: July 2002

We present the results of elastic and inelastic neutron scattering measurements made on a powder sample of the spin-ladder compound (VO)2P2O7. The elastic scattering measurements made at a temperature of 10 K showed no magnetic structure, consistent with a singlet ground state. The inelastic measurements, also at 10 K, revealed three magnetic modes at 3.5, 6.0, and 14 meV. Theoretical treatments of spin ladder models point to an identification of the 3.5 meV mode as the minimum in the one-magnon dispersion, and the 14 meV mode as the local minimum in the associated two-magnon continuum. However, we have been unable to explain the scattering at 6.0 meV using existing theories of spin ladders.
Observation of single ion transitions in the ordered states of FeCl2
Authors: Yamaguchi H. , Katsumata K.
Published: July 2002

Electron spin resonance (ESR) experiments using a far infrared laser and a superconducting magnet have been performed on a single crystal sample of the metamagnet FeCl2. A new ESR signal has been observed at a much higher frequency than the ordinary antiferromagnetic and ferromagnetic resonance frequencies. The new ESR signal originates from a transition within the lowest triplet state of single Fe2+ ions in the presence of exchange fields.
Calculation of the canting angle dependence of the resistivity in Cu|Co spin valves
Authors: Brown R. H. , Nicholson D. M. C. , Butler W. H. , Zhang X.-G. , Shelton W. A. , Schulthess T. C. , MacLaren J. M.
Published: July 2002

We consider the in-plane electrical conductivity of a spin valve, specifically, a system consisting of a copper slab between two cobalt slabs. We calculate the dependence of the resistivity on the canting angle, where the canting angle θ is defined as the angle between the magnetization vectors of two ferromagnetic slabs in a spin valve. To calculate the electronic structure, we utilize the layer-KKR formalism. Electron scattering by impurities, phonons, magnons, etc. is modeled using a layer and spin-dependent complex self-energy. Scattering rates are chosen to match Cu and Co resistivities. We assume a spin asymmetry scattering rate factor of 7 in Co, matching the Fermi-level minority-to-majority density-of-states ratio. No additional interfacial scattering is included. The nonlocal layer dependent conductivity is calculated using the Kubo–Greenwood formula for systems consisting of 3 and 7 monolayer fcc (111) Cu slabs in Co. We find electron channeling in Cu dominates the conductivity and at θ=π/2 the resistivity is increased by 4.9% and 2.3% from a linear 1−cos θ dependence. We find giant magnetoresistive GMR values of 64% and 36% for 3 and 7 monolayers of Cu, respectively.
Direct observation of the high frequency write response of recording heads using the magnetic force microscope (abstract)
Authors: Proksch Roger , Schmidt Jake , Austvold Shane , Skidmore George
Published: July 2002

A direct measure of the high frequency (HF) response of recording heads is becoming increasingly important as storage densities head toward 10 Gb/in.2. One of the quantities for characterizing head response is the value of the magnetic field at high write frequencies. Typical, HF testing of heads has been mostly indirect; the recording head is used to write data to a disk and the head performance is evaluated based on the readback signal. The results of the testing depend on the test media and the readback system in addition to the actual HF performance of the recording head. To our knowledge, the only direct testing of HF head response was done with a scanning magneto-optic photometer which images the dynamic domain structure of the recording head.1 Unlike these other techniques, the magnetic force microscope (MFM) responds directly to the magnetic field above the recording head. Normally, the MFM response time is limited by the resonant frequency of the cantilever, typically 60–80 kHz in our system. However, by either AM or FM modulation of the HF drive of the recording head, vibrations were excited in the MFM cantilever and the response of the head was characterized at head drive frequencies in excess of 50 MHz.2 In this work, we have measured the response of a metal-in-gap recording head from a commercially available disk drive (Seagate, Inc., Bloomington, MN). As expected, we found that the head had a single pole rolloff at roughly 1 MHz. The effects of dc and ac biasing were also studied.
Quantitative magneto-optic field imaging of recording heads (abstract)
Authors: Heidmann Juergen , Weller Dieter
Published: July 2002

With increasingly higher recording densities and narrowing track widths approaching 1 micron in particular it is getting more important to optimize the field distribution of the write head. The goal is to minimize side writing due to fringing fields at the pole edges as well as distortions of the write field contour along the track width emanating from gap saturation. The imaging of written transitions in a recording medium using a magnetic force microscope yields information only about the integral response of the head/medium magnetic system. We have developed a technique to measure the magnetic field distribution at the gap quantitatively using a transducer layer deposited directly on the air bearing surface. Other than an in-plane magnetized medium with hysteretic behavior the perpendicularly oriented Co/Pt multilayer shows reversible nonhysteretic rotation of magnetization when subjected to the in-plane component of the recording head. Consequently, a quantitative determination of the write field is possible that is not the 1,2 Y. The previously reported magneto-optic method’s perpendicular component of magnetization is detected using the polar Kerr effect and the calibrated Kerr contrast is then translated into an in-plane-field contour. Magnetic properties of the sensing layer were tailored to the range of write fields to be measured with anisotropy and coercivity fields up to 60 and 8 kOe, respectively.
Recording performance of the combination of magnetoresistive head and thin metal evaporated tape
Authors: Kagawa K. , Okabe A. , Yoshida S. , Yaoi T. , Sugawara N. , Takiguchi M. , Okazaki Y. , Hayashi K.
Published: July 2002

Thin metal evaporated (ME) tape with low remanence-thickness product (Mrδ), high coercivity (Hc) and low noise was designed to match a magnetoresistive (MR) head. This article focuses on the high density recording performance of a shielded MR head/thin ME tape media designed for use with an MR head. The PW50 measurement was adopted to certify that the depth of recording was restricted by magnetic layer thickness. Reduced second harmonic distortion at low density shows saturation-free operation of the MR head. Frequency domain measurements indicate drastically reduced ac erase noise level and higher output level at short wave length. High readback density (neglecting intertrack interference) of 1.07 Gbits/in2 with signal-to-noise ratio (SNR) after equalization of 18.7 dB was attained for a FeNi (AMR) head and 1.29 Gbits/in.2 with SNR of 20.0 dB for a spin-valve (GMR) head. The performance of the MR head/thin ME tape system was compared to commercial ME tape and thin metal particulate tape in terms of noise characteristics, depth of recording, and SNR after equalization.
Integrated dc/rf design theory for ferrite circulators
Authors: Schloemann Ernst
Published: July 2002

An integrated design approach is described, in which the dc and rf aspects of circulator design are both addressed for a realistic physical model of the microwave ferrite. The microwave ferrite disc is conceptionally subdivided into a center puck and a sequence of rings, each of which may have a different magnetization. The dipole interaction between the rings is taken into account by means of a dipolar interaction matrix, which is a generalization of the conventional demagnetizing factor. The rf behavior of the ferrite (including off-resonance loss and “low-field loss”) is characterized by appropriate theoretical expressions derived on the same physical basis as the equations that characterize the dc behavior.
Simple full micromagnetic model of exchange bias behavior in ferro/antiferromagnetic layered structures (abstract)
Authors: Koon Norman C.
Published: July 2002

It is shown using full micromagnetic relaxation calculations that exchange bias behavior is predicted for single-crystal ferro/antiferromagnetic layers with a fully compensated interface. The particular example most fully studied has a bcc/bct lattice structure with a fully compensated (110) interface plane. Only bilinear Heisenberg exchange was assumed, with anisotropy only in the antiferromagnet. In spite of the intuitive notion that exchange coupling between a ferromagnet and an antiferromagnet across a fully compensated plane of the antiferromagnet should be zero, we find strong coupling, comparable to the bilinear exchange, with a 90° angle between the ferromagnetic and antiferromagnetic axes of layers far from the interface in absence of an applied field. Even though the 90° coupling has characteristics resembling “biquadratic” exchange, it originates entirely from frustrated bilinear exchange. The development of exchange bias is found to originate from the formation of a domain wall in the antiferromagnet via the strong 90° exchange coupling and pinning of the wall by the magnetocrystalline anisotropy in the antiferromagnet. Because the large demagnetizing factor of the ferromagnet tends to confine its magnetization to the plane, the exchange bias is found to depend mainly on the strength and the symmetry of the in-plane component of anisotropy. Although little effort was made to analyze specific systems, the model reproduces many of the qualitative features observed in real exchange bias systems and gives reasonable semiquantitative estimates for the bias field when exchange and anisotropy values consistent with real systems are used.
Dependence of side-erasing effect and core efficiency on side taper angle of high Bs metal in gap head
Authors: Itoh Takahiko , Kobayashi Hiroshi , Ozeki Tatsuo
Published: July 2002

The side-erasing effect is a very serious problem, especially in guardband-less recording systems with a narrow track pitch (e.g., 10 μm pitch of consumer digital VCRs). We report the dependence of side-erasing effect and output voltage on the oblique side taper plane angle of high Bs (Fe–Zr–N) metal in gap (MIG) heads. We measured the side-erasing width written by several kinds of MIG heads (gap length=0.21 μm, track width=14 μm, gap depth=12 μm) with various side taper angles (45°, 60°, and 90°) between the gap plane and the side taper plane on metal evaporated tapes and on two types of particulate tapes. Measurement conditions were 10.2 m/s relative velocity, 2 μm wave length, and 720, 960, and 1750 Oe Hc tapes. The side-erasing width decreases with increasing side taper angle in the MIG head or with increasing tape Hc. The difference between the side-erasing width for a 60° and a 90° head is small. However, the output voltage decreases with increasing side taper angle. The normalized output voltage of 45° head at 20.4 MHz is superior 0.58 and 2.38 dB to the 60° and 90° head, respectively. Simulation results of the side-fringing field and the core efficiency using a three-dimensional finite element method, incorporating the decreasing of Fe–Zr–N permeability on tilted substrates, indicate a similar trend to the experimental results. The 60° side taper angle MIG head is more suitable than the 45° and 90° heads for high density and guardband-less recording, because it is a good compromise between output voltage and side-erasing width.
Spin dynamics of Er3+ in ErBa2Cu3O7−δ
Authors: Skanthakumar S. , Lynn J. W. , Dogan F.
Published: July 2002

Neutron scattering techniques are used to study the magnetic excitations of the Er ions in superconducting ErBa2Cu3O7, a system where the Er ions order antiferromagnetically at TN=0.62 K and the sublattice magnetization obeys the relation for a 2D S=1/2 Ising model. Our data, which were taken on a cold neutron triple axis spectrometer using a horizontally focusing analyzer, reveal that there is a large gap of about 0.20 meV in the excitation spectrum, about four times the thermal energy kTN, as expected for an Ising system. However, we also observe dispersion of the spin waves in the a-b plane. These excitations are found to renormalize with temperature and only diffuse paramagnetic scattering is observed well above the ordering temperature.
Neutron standing wave resonator to measure the flux line lattice in thin film superconducting samples (abstract)
Authors: Adenwalla S. , Fullerton E. E. , Felcher G. P.
Published: July 2002

Flux line lattices in superconductors have been extensively studied in bulk samples by a variety of techniques including neutron diffraction. We present a technique that is useful for studying the flux lattice in a thin film in the configuration where the field (and hence the flux lines) is parallel to the sample surface. This configuration provides information about the in-plane order and is not accessible by other means (e.g., scanning tunnel microscope, Bitter decoration). A 5000-Å-thick niobium film epitaxially grown on an MgO substrate is studied by polarized neutron reflectometry (PNR) in this parallel field geometry. In order to enhance the signal, a neutron standing wave resonator is created in the superconducting film by spin casting a thin layer of deuterated polystyrene on the Nb. The increased scattering caused by the presence of flux lines in the sample leads to a depletion of the reflected neutron beam at the resonant wave vector. Our results show the expected depletion in the superconducting state for fields at which flux lines are expected to be present. The field and temperature dependence of the data are consistent with scattering arising from flux lines. We discuss plans to prove that the source of the increased scattering is indeed the flux lines. The most conclusive evidence for a well-ordered flux lattice in the sample is the presence of off-specular diffraction, strongly enhanced by the standing wave in the resonator. Measurements to see this off-specular signal have proved unsuccessful, leading us to conclude that we may not have a well-ordered lattice in our superconducting sample.
Materials characteristics of a single grained Y–Ba–Cu–O superconductor
Authors: Chen In-Gann , Jamn Gwo , Hsu Jen-Chou , Wu M. K.
Published: July 2002

Single grained Y–Ba–Cu–O superconducting material has demonstrated its unique magnetic properties, such as strong flux trapping (up to 10 T at 40 K) and strong flux shielding (up to 18 T at 4 K) by eliminating the grain boundary weak link and improving the flux pinning effect. This improvement of magnetic properties in YBCO superconductors is due to the advance of materials processing techniques, i.e., top-seeding and melt-texturing method. With the TSMT method, single grained YBCO samples up to several centimeters in diameter can be produced with very high reproducibility. In this report, the TSMT method will be introduced. The results of the TSMT-YBCO sample’s superconducting properties, i.e., magnetic field intensity and distribution uniformity, levitation force, etc., as well as their correlation with various materials processing methods will be discussed.
Potential giant magnetoresistance perovskites: The manganites La1−xAx+MnO3 (A+=K+, Na+, Rb+, Ag+, Tl+, Cu+, and Li+) (abstract)
Authors: Diehl T. , Chaudouët P. , Joubert J. C. , Pierre J.
Published: July 2002

Rare-earth alkaline earth substitute manganites perovskites, Ln1−xAx2+MnO3 (A=Ca, Sr, Ba, Pb), have retained considerable attention in regard to their colossal magnetoresistance coefficient for compositions in the range 0.1⩽x⩽0.4. Compounds where A2+ cations replaced monovalent A+ cations such as K+ Ag+, etc., have been known for a long time1 and have even been tested as oxidation catalysts for exhaust gazes of internal combustion engines. Surprisingly enough, however, they have given rise to very little investigation as potential giant MR candidates. Preliminary structural features (cell distortions), magnetic and electric properties versus temperature are presented for the series of compounds of general composition La1−xAx+Mn1−2x3+Mn2x4+O3, where A+=Rb, K, Na, Ag, Tl, Cu, and Li. Lanthanum can be partly or totally replaced by Pr, Nd or other rare-earth cations or yttrium. Most of the phases of the lanthanum subseries are rhombohedral (R-3c) and ferromagnetic. Curie temperatures are strongly dependent on x, with a maximum around 330 K for Na, K, or Ag. A sharp decrease of the resistivity is observed just below the Curie temperature, accompanied by a very steep increase of the spontaneous magnetization, uncharacteristic of a conventional Brillouin-type curve. This may be related to a first-order crystallographic transition accompanying the isolant–metal transition. The average magnetic moment of Mn is about 3.55 μB at 20 K for the composition La0.8K0.2MnO3. Preliminary magnetoresistance measurements on bulk ceramic samples display a MR effect of the same amplitude as the case of the alkaline earth substituted La manganites.
The spring constant in the buckling of tightly jacketed double-coated optical fibers
Authors: Shiue Sham-Tsong
Published: July 2002

The primary coating in real optical fiber applications is a soft polymer with a low Young’s modulus that is used as a strain buffer to minimize microbending loss. The secondary coating and jacket are hard polymers with higher Young’s moduli and sufficient thicknesses to sustain the mechanical force. The buckling of optical fibers causes additional transmission loss, thereby necessitating that it be minimized. The optical fiber’s spring constant is a critical factor in the buckling phenomenon. Moreover, a higher value of the spring constant corresponds to a higher resistance to buckling. A closed form solution of the spring constant in the buckling of tightly jacketed double-coated optical fibers is presented. The effects of polymeric coatings on the spring constant are also considered. To obtain a higher value of the spring constant and adhere to optical fiber applications, the thickness of the primary coating was decreased, and the Poisson’s ratio of the primary coating and the Young’s moduli and Poisson’s ratios of the secondary coating and jacket were increased.
Positron annihilation investigations of vacancies in InP produced by electron irradiation at room temperature
Authors: Bretagnon T. , Dannefaer S. , Kerr D.
Published: July 2002

Positron lifetime investigations were done on a series of InP samples irradiated to various doses with 2.5 MeV electrons. In n-type materials, positron lifetimes of 265±5 and 338±15 ps are attributed to indium vacancy–interstitial complexes and divacancy–interstitial complexes, respectively. In p-type materials these defects were not observed. Thermal annealing took place up to 200 °C for both defect types. Introduction rates were estimated to be 0.1 cm−1 for VIn⋅InI and ∼0.05 cm−1 for the divacancies. The divacancies showed a temperature dependence of the trapping rate, which suggests a thermally activated process. No evidence for VP vacancies could be found in neither p-type nor n-type materials.
Magnetism and morphology of Fe/Cu(111) films below two-dimensional percolation
Authors: Shen J. , Skomski R. , Klaua M. , Jenniches H. , Sundar Manoharan S. , Kirschner J.
Published: July 2002

The magnetic properties of γ-iron films on Cu(111) below the two-dimensional percolation threshold of about 1.4 monolayer are investigated. Iron on flat Cu(111) surfaces forms triangular patches, while it forms stripes along step edges on vicinal surfaces. The films exhibit an easy magnetization axis perpendicular to the film plane and magnetic hysteresis at sufficiently low temperatures. The magnetization curves of the triangular iron patches are discussed in terms of Ising superparamagnetism. From Kerr hysteresis loops, we deduce that the first anisotropy constant K1 in the superparamagnetic films is of order 0.345±0.020 MJ/m3.