bluuebunny/crossref_metadata_2025_split huggingface dataset:
High field effective linewidth due to conductivity losses in Zn2Y hexagonal ferrite at 10, 20, and 35 GHz (abstract)
Authors: Truedson J. R. , McKinstry K. D. , Kabos P. , Patton C. E. 
Published: July 2002

A simple technique to analyze the effective linewidth at high field, corresponding to the far field tail of ferromagnetic resonance (FMR) absorption, was recently reported. This technique has now been applied at 10, 20, and 35 GHz to single crystal Zn2Y hexagonal ferrite with planar anisotropy. Far field effective linewidth (ΔHeffFF) measurements were made for a series of 2–3 mm-diam, 0.08–1.33-mm-thick c-plane disks. The ΔHeffFF is taken to represent the intrinsic linewidth, in the sense that there are no two magnon or inhomogeneous linebroadening contributions to the losses on the far-field FMR tail. ΔHeffFF at each frequency increases with the square of the disk thickness. This increase is consistent with an eddy current loss process. A fit of the data to a simple eddy current model yields resistivities of 10.3, 9.7, and 8.3 Ω cm at 10, 20, and 35 GHz, respectively. These values are consistent with a resistivity of 2 Ω cm from 10 GHz dielectric measurements and 20 Ω cm from the resistivity measurements.
These results are similar to previous results for barium ferrite. However, the microwave response in these Zn2Y materials also shows many anomalies. First, ΔHeffFF is larger, not smaller, than the corresponding FMR linewidth. Second, ΔHeffFF also depends on the orientation of the microwave field relative to the crystallographic c plane. The losses for out-of-plane microwave pumping are larger by a factor of 2–3 than for in-plane pumping. Research sponsored by the United States Office of Naval Research, Contract N00014-90-J-4078. The single crystal samples were kindly provided by M. A. Wittenauer, Purdue University.
Frequency dependence of the ferromagnetic resonance and effective linewidth in single crystal manganese doped barium ferrite (abstract)
Authors: Karim R. , Patton C. E. , Ball S. D. 
Published: July 2002

The frequency dependence of the ferromagnetic resonance (FMR) linewidth and effective linewidth has been measured in Mn doped single crystal barium hexaferrite (BaFe12−xMnxO19) with x=0.1. Mn doped samples were used to minimize conductivity losses. Measurements were made on c-plane thin disks magnetized to saturation along the disk axis and c direction. A shorted waveguide technique was used to measure the FMR profiles in the range of 55–90 GHz. The FMR field position increased linearly at 0.35 kOe/GHz, with a zero field extrapolation of 34.7 GHz as expected for uniform mode FMR in barium ferrite. The linewidth ΔH increased linearly with frequency at 0.29 Oe/GHz with a zero frequency extrapolation of 18 Oe. A high-Q cavity method was used to measure the high field effective linewidth, ΔHeffFF at 10, 20, 35, and 60 GHz. The effective linewidth increased linearly with frequency at 0.4 Oe/GHz, with a zero frequency extrapolation of 4 Oe. There were no dependencies of ΔH or ΔHeffFF on sample thickness. These results indicate that (1) eddy current contributions to ΔH and ΔHeffFF in these Mn doped barium ferrite materials are absent, (2) ΔHeffFF is significantly lower than ΔH and appears to be a good measure of the intrinsic losses, and (3) both linewidths increase linearly with frequency at about the same rate.
These intrinsic linewidths in barium ferrite are still significantly greater than linewidths in yttrium iron garnet. Research sponsored in part by the United States Office of Naval Research, Contract No. N00014-90-J-4078. The single crystal samples were kindly provided by M. A. Wittenauer, Purdue University.
Structural and magnetic characteristics of Co81Cr19/Al multilayers deposited by plasma-free sputtering with Kr gas (abstract)
Authors: Takahashi Takakazu , Naoe Masahiko 
Published: July 2002

In multilayers deposited by sputtering, interfaces between each layer are necessary to be extremely sharp and flat. Therefore, the kinetic energy and number of sputtering gas atoms and ions recoiled from the target plane must be satisfactorily decreased. So, in this study, Kr, which is heavier than both Co and Cr in atomic weight, was used as a sputtering gas, while Ar used in conventional sputtering is lighter than them. The Co81Cr19/Al multilayers have been prepared on plasma-free glass slide substrate at room temperature by alternately depositing Co81Cr19 and Al layers, using the facing targets sputtering (FTS) apparatus. The thickness of Co81Cr19 layer lCo-Cr was varied in the range of 45–140 Å, while the thicknesses of Al layer lAl were approximately 7 and 14 Å. The total layer thickness was in the range of 1450–1530 Å. It has been found from the small angle x-ray diffraction diagrams and the scanning electron microscope (SEM) images that the clear periodic structure could be formed in all specimen multilayers. In the multilayers with lal of 7 Å, the Co81Cr19 layers were intrinsically composed of hcp phase crystallites with c-axis orientation Δθ50 normal to the layer plane ranging from 5.4 to 8.4. In the multilayers with lAl of 14 Å, the Co81Cr19 layers seemed to be composed of hcp phase crystallites with very large Δθ50 of which value could not be estimated. However, for all specimen multilayers, the interplanar spacing between (002) planes d(002) were in the range of 2.033–2.038 Å. These values were nearly equal to those of Co81Cr19 single layer films (2.035 Å).
M s and Hc were significantly varied in the range of 500–1150 emu/cc and 35–260 Oe, respectively. In the multilayers with lAl of 7 Å, (Mr/Ms)1 was in the range of 0.28–0.36 and the perpendicular magnetic anisotropy was rather remarkable. On the other hand, in the multilayers with lAl of 14 Å, (Mr/Ms)1 was in the range of 0.61–0.7 and the in-plane magnetic anisotropy was significantly dominant. Co-Cr grains for lAl of 14 Å were finer than those for lAl of 7 Å. These results may indicate that the kinetic energy and number of atoms and ions recoiled from the target plane were significantly decreased by using Kr instead of Ar. So, the multilayers deposited by sputtering with Kr gas had excellently sharp and flat interfaces suitable for use of the apparent magnetoresistance effect.
Large-band-gap SiC, III-V nitride, and II-VI ZnSe-based semiconductor device technologies
Authors: Morkoç H. , Strite S. , Gao G. B. , Lin M. E. , Sverdlov B. , Burns M. 
Published: July 2002

In the past several years, research in each of the wide-band-gap semiconductors, SiC, GaN, and ZnSe, has led to major advances which now make them viable for device applications. The merits of each contender for high-temperature electronics and short-wavelength optical applications are compared. The outstanding thermal and chemical stability of SiC and GaN should enable them to operate at high temperatures and in hostile environments, and also make them attractive for high-power operation. The present advanced stage of development of SiC substrates and metal-oxide-semiconductor technology makes SiC the leading contender for high-temperature and high-power applications if ohmic contacts and interface-state densities can be further improved. GaN, despite fundamentally superior electronic properties and better ohmic contact resistances, must overcome the lack of an ideal substrate material and a relatively advanced SiC infrastructure in order to compete in electronics applications. Prototype transistors have been fabricated from both SiC and GaN, and the microwave characteristics and high-temperature performance of SiC transistors have been studied. For optical emitters and detectors, ZnSe, SiC, and GaN all have demonstrated operation in the green, blue, or ultraviolet (UV) spectra.
Blue SiC light-emitting diodes (LEDs) have been on the market for several years, joined recently by UV and blue GaN-based LEDs. These products should find wide use in full color display and other technologies. Promising prototype UV photodetectors have been fabricated from both SiC and GaN. In laser development, ZnSe leads the way with more sophisticated designs having further improved performance being rapidly demonstrated. If the low damage threshold of ZnSe continues to limit practical laser applications, GaN appears poised to become the semiconductor of choice for short-wavelength lasers in optical memory and other applications. For further development of these materials to be realized, doping densities (especially p type) and ohmic contact technologies have to be improved. Economies of scale need to be realized through the development of larger SiC substrates. Improved substrate materials, ideally GaN itself, need to be aggressively pursued to further develop the GaN-based material system and enable the fabrication of lasers. ZnSe material quality is already outstanding and now researchers must focus their attention on addressing the short lifetimes of ZnSe-based lasers to determine whether the material is sufficiently durable for practical laser applications. The problems related to these three wide-band-gap semiconductor systems have moved away from materials science toward the device arena, where their technological development can rapidly be brought to maturity.
Noise characteristics and detectivity of YBa2Cu3O7 superconducting bolometers: Bias current, frequency, and temperature dependence
Authors: Fardmanesh M. , Rothwarf A. , Scoles K. J. 
Published: July 2002

Meander line patterned infrared detectors, with values of resistance Ronset at the onset temperature Tc onset of 3–5 kΩ, were fabricated from YBa2Cu3O7−x superconducting material on MgO and SrTiO3 crystalline substrates. Noise voltages from the samples were measured versus bias current, radiation modulation frequency, and temperature, in both the normal and superconducting states. Four major types of voltage noise were identified according to where they occurred in temperature relative to Tconset and the zero resistance temperature Tc zero, and their dependence on frequency and bias current. They were also associated with the granularity of the superconducting film, which is related to the substrate material used. From these observations a specific cause for each type of noise is suggested. The results are as follows. (i) In the normal state with temperature T≳Tc onset, noise with a magnitude that is consistent with thermal (Johnson) noise is seen, but it depends linearly on bias current above a threshold value, at low frequencies.
The suggested noise source is conductivity fluctuations due to Cooper pairs. (ii) Noise was found to occur below Tc zero in granular films. With increasing bias current its magnitude increases, and it shifts to a lower temperature range; however, the noise magnitude becomes constant as the current goes to zero. It is weakly dependent on frequency above 400 Hz. Suggested cause is voltage fluctuations in superconductor–normal–superconductor junctions at grain boundaries. (iii) This noise also occurs below Tc zero with peaks at various temperatures. With increasing bias current the peaked noise spreads to lower temperatures, but the noise goes to zero as the bias current goes to zero. Its suggested cause is magnetic flux tube motion. (iv) This noise occurs between Tc onset and Tc zero and is present in all samples, but lowest on samples prepared on SrTiO3 substrates. Its suggested cause is fluctuations in the volume fraction of the superconducting phase along the current path. While the measured detectivity D* of our samples at a wavelength of 20 μm was only 106 cm Hz1/2/W, engineering changes can be expected to raise the value to above 1010 cm Hz1/2/W.
Barkhausen noise in FeCoB amorphous alloys (abstract)
Authors: Durin G. , Bertotti G. 
Published: July 2002

In recent years, the Barkhausen effect has been indicated as a promising tool to investigate and verify the ideas about the self-organization of physical complex systems displaying power law distributions and 1/f noise. When measured at low magnetization rates, the Barkhausen signal displays 1/fα-type spectra (with α=1.5÷2) and power law distributions of duration and size of the Barkhausen jumps. These experimental data are quite well described by the model of Alessandro et al. which is based on a stochastic description of the domain wall dynamics over a pinning field with brownian properties. Yet, this model always predicts a 1/f 2 spectrum, and, at the moment, it is not clear if it can take into account possible effects of self-organization of the magnetization process. In order to improve the power of the model and clarify this problem, we have performed a thorough investigation of the noise spectra and the amplitude distributions of a wide set of FeCoB amorphous alloys. The stationary amplitude distribution of the signal is very well fitted by the gamma distribution P(ν)=νc−1 exp(−ν)/Γ(c), where ν is proportional to the domain wall velocity, and c is a dimensionless parameter. As predicted in Ref. , this parameter is found to have a parabolic dependence on the magnetization rate. In particular, the linear coefficient is related to the amplitude of the fluctuations of the pinning field, a parameter which can be measured directly from the power spectra. In all measured cases, the power spectra show α exponents less than 2, and thus poorly fitted by the model.
Actually, the absolute value of the high frequency spectral density is not consistent with the c parameter determined from the amplitude distribution data. This discrepancy requires to introduce effects not taken into account in the model, as the propagation of the jumps along the domain wall. This highly enhances the fit of the data and indicates effects of propagation on the scale of a few millimeters. These results are analyzed in terms of new descriptions of the statistical properties of the pinning field based on fractional brownian processes.
Magnetic properties and crystal structure of FeTaAIN soft magnetic materials for MIG head (abstract)
Authors: Li Yiqun , Cai Bingchu , Zeng Xianglin , Xu Dong 
Published: July 2002

Fe-based nitride films have been expected as promising head core materials used for high-density magnetic recording MIG head due to their large saturation flux density combined with excellent soft magnetic properties. Recent studies found that the addition of 8–13 at. % Ta in Fe–N resulted in a remarkable improvement of thermal stability. It was also reported that small amount of Al appeared to inhibit the formation of γ′-Fe4N phase and to prevent the grain growth during annealing. In this paper, the magnetic properties and crystal structure of a new Fe–M–N soft magnetic film, FeTaAlN, has been reported. FeTaAlN films about 3 μm thick were prepared on glass substrate by dc magnetron sputtering method in Ar+N2 plasma and then annealed in vaccum. The films generally present high 4πMs (15–20 kG). X-ray diffraction results show that both as-deposited and annealed films have only two phases, an α-Fe phase with perfered (110) axis orientation and a Ta3N5 phase. Annealing is essential in lowering Hc without seriously diminishing 4πMs of the films; the film with the lowest Hc (about 0.5 Oe) has a 4πMs of above 15 kG. The peak broadenings of both the α-Fe phase and the Ta3N5 phase are observed as annealing temperature increases. Meanwhile, the (110) peaks of α-Fe shift slightly to higher 2θ value. This indicates that the interstitial nitrogen atoms come out of α-Fe lattice when the films are annealed and a part of b.c.t. α-Fe is transferred into b.c.c. α-Fe, which causes the broadening of α-Fe peak. On the other hand, the segregated nitrogen atoms may react with Ta atoms that dissolved substitutionally in α-Fe to form fine Ta3N5 particles and make the Ta3N5 peak broadened.
Thus, it is thought that the decrease of Hc is mainly due to the reduction of internal stress. The magnetostrictions of the films are on the order of 10−6. FeTaAIN film presents high saturation flux density and good soft magnetic properties even after annealing at 500 °C, and it is potentially a superior substitute for the core materials of high-density recording MIG head.
Complex anisotropies in sputtered Co90Fe10 alloy thin films (abstract)
Authors: Yun E. J. , Win W. , Walser R. M. 
Published: July 2002

Among all the known magnetic materials with vanishingly small magnetostriction, alloys with the composition Co90Fe10 have the highest magnetization at 300 K. Despite this combination of useful properties, thin films of these alloys have been little studied. This study investigated the magnetic characteristics of Co90Fe10 alloy thin films deposited by rf diode sputtering. The hysteresis loops of nominal 1000-Å-thick Co90Fe10 films deposited in ≊150 Oe planar field exhibited complex biaxiality. Two, orthogonal easy axis loops with different coercivities were observed. The direction corresponding to the hysteresis loops with the smallest (largest) coercivity are referred to as the soft (easy) axis. Dual hard axes were found at ±68° on either side of the easy axis. Similar biaxial hysteresis loops were observed in Co90Fe10 films deposited by sequential sputtering from pure Co and Fe targets on static or rotating substrates, and in films deposited from a Co90Fe10 alloy target on stationary substrates. The magnetization and resistivity of the sequentially sputtered films were 19.66 kG and 18.4 μΩ cm, while those deposited from the alloy target were 19.00 kG and 14.5 μΩ cm. The biaxiality was sensitive to a number of parameters which appeared to destabilize the soft axis state compared to the hard axis states. With increasing film thickness (to 3400 Å), the biaxiality disappeared, and films deposited under similar conditions exhibited rotatable anisotropy. The biaxiality could also be reduced, and the coercivity increased, by increasing the magnetic bias applied during deposition. Films annealed at 370 °C became uniaxial when a 1.3 kOe field was applied parallel to the easy axis, and became isotropic when the field was parallel to the hard axis.
The biaxiality was enhanced, and Hc reduced, by depositing 1000-Å-thick Co90Fe10 films onto a 500 Å 80Ni20Fe underlayer. In all cases, whenever the biaxiality was eliminated, the films exhibited a rotatable uniaxial anisotropy. The biaxiality of these films likely has its origin in the coupling of two uniaxial anisotropies as previously analyzed. The deposited films are highly polycrystalline. In addition, the symmetries of the planar magnetic state obtained were the same for crystalline and amorphous substrates. Thus, it is unlikely that either uniaxial anisotropy is due to magnetocrystallinity, but are probably induced. The experimental trends observed in the study can in principle be understood in terms of a two layer model in which a rotatable, stress induced uniaxial anisotropy in an underlayer, is exchange coupled to the field induced uniaxial anisotropy of an overlayer. In this model the complex biaxiality is modified by experimental parameters which modify the relative strength and coupling strength of these anisotropies.
Resolution of conflicts concerning Fe16N2 (abstract)
Authors: Arrott A. S. 
Published: July 2002

The 1994 3M-Intermag meeting featured an invited session on Fe16N2 in which many of the research workers in this exciting field put forth apparently conflicting views on the nature and possibilities for the use of, perhaps, the most promising transition metal based alloy for magnetic applications. The debate centers on the promise of 3μB per atom at transition metal densities if pure Fe16N2 could be made. A particularly compelling resolution of the conflicts was not mentioned at the meeting, even though it has been in the past. The conflict arises primarily from the original classification of Fe16N2 as a phase denoted by α″ to distinguish it from Fe8N classified as a phase denoted by α′ and the substitutional alloy α-Fe(N). The conflicts arise because data are interpreted as a mixture of two phases α′ and α″. For any measured property, it is then sufficient to know the ratio of the two component phases for two different compositions to deduce the apparent properties of the individual phases. But this is not the case. The distinction between α″ and α′ is not that of two separate phases but, rather, the end points of a continuum characterized by the degree of order. This makes it necessary to produce the fully ordered state in order to determine its properties, unless it is possible to have enough data about those properties for various degrees of order to justify extrapolation to the fully ordered state. The most ordered Fe16N2 is produced using molecular beam epitaxy. It also shows the highest magnetic moments. The high magnetic moments are supported by Mössbauer measurements of large hyperfine fields in bulk materials (not the molecular beam epitaxy material) and by some of the theoretical calculations for the ordered state.
One of the criticisms of the molecular beam epitaxy work is that those thin films do not decompose into Fe4N and Fe(N) on heating as do bulk alloys of the same composition. (The implication is that the thin films contain something else.) But this criticism is not valid, simply because one cannot predict the stability of a highly ordered state from observations of a weakly ordered state. This is particularly the case where the ordering is accompanied by large lattice distortions. The existing literature is analyzed to extract the dependence of magnetic moment on the degree of order. These considerations lend support to those who are pursuing Fe16N2 for its potential for producing a magnetic material with 3μB per atom at transition metal densities. How to increase the degree of order in bulk materials becomes a central concern.
Giant magnetoresistance as a probe of interfacial electronic character (invited) (abstract)
Authors: Parkin S. S. P. 
Published: July 2002

The giant magnetoresistance (GMR) effect exhibited by magnetic multilayers and related structures is very sensitive to the nature of the interfaces between the magnetic and nonmagnetic layers. We have explored the dependence of GMR on the electronic character of these interfaces by inserting additional thin magnetic layers at the interfaces. Insertion of thin Co layers in Ni–Fe/Cu/Ni–Fe or thin Ni–Fe layers in Co/Cu/Cu exchange biased sandwiches leads to an increase or decrease, respectively, in the magnitude of the GMR effect; the variation is monotonic with thickness of the inserted layer. In contrast, insertion of thin layers of Fe at the Ni–Fe/Cu interfaces in Ni–Fe/Cu/Ni–Fe exchange biased sandwich structure results in a nonmonotonic variation of GMR with Fe layer thickness; for intermediate thicknesses in particular the GMR is substantially reduced. The magnetism and structure of the Fe layers is explored in related sputter-deposited single-crystalline Ni/Fe/Cu thin-film structures prepared using thin Fe/Pt seed layers grown at high temperatures on single crystalline polished wafers of MgO(100), MgO(110), and (0001)Al2O3. The magnetism of the Ni, Fe, and Cu layers is examined using x-ray magnetic circular dichroism (XMCD) and the structure of the Fe is analyzed using extended x-ray absorption fine structure (EXAFS) studies. These experiments show that the Fe moment varies nonmonotonically with thickness resulting from a structural phase transition from fcc to bcc Fe with increasing thickness.
The Fe displays a very small magnetic moment for an intermediate range of Fe thickness for which the Fe structure is fcc. This range of thickness depends on the crystalline orientation of the Fe. It is within this same range of thickness that the GMR is suppressed in related exchange biased sandwich structures. Interestingly we find that such nonferromagnetic layers of fcc Fe can be used as spacer layers in magnetic multilayers of, for example, Ni/Fe. These multilayers display both antiferromagnetic interlayer coupling of the Ni layers and giant magnetoresistance, which each oscillate as a function of Fe thickness.