Narrow Field Range Research Articles

Magnetoelastic energy in domains separated by 90° walls

An iron whisker with a current along its axis has four domains in a frame about a central core. The domains in the frame are magnetized perpendicular to the axis. The central core is magnetized along the axis. The magnetoelastic energy and its corresponding effective field, found analytically using the calculus of variations, account for the experimental observation that for each current, below a critical current, there is a narrow range of fields for which there are two stable sizes for the central core.

Magnetic-field dependence of Hall resistance in thin films of pure bismuth.

Hall resistance has been measured in thin films of pure bismuth at 4.2 K in magnetic fields up to about 8 T. Fields were applied parallel to the trigonal axis, which is perpendicular to the plane of the film, and the thicknesses ranged from 30 to 300 nm. In thinner films, the Hall resistance remained in the negative region. In films of intermediate thickness, as the field increased the Hall resistance first decreased, reaching a minimum in the negative region, then increased to enter the positive region at higher fields. For thicker films, except in a narrow range of low fields, the Hall resistance was positive and increased in magnitude with the field. Superimposed on the general trend of the Hall resistance versus magnetic field, Shubnikov\char21{}de Haas\char21{}type oscillations have also been observed.

What makes basic research economically useful?

Economic analysis has helped us understand the strong economic dimension in the explosive growth of science, and (more recently) the reasons for continuing public subsidies. However, the growing domination of the “market failure” approach has led to the analytical neglect of two major questions for policy-makers. How does science contribute to technology? Are the technological benefits from science increasingly becoming international? On the former, too much attention has been devoted to the relatively narrow range of scientific fields producing knowledge with direct technological applications, and too little to the much broader range of fields, the skills of which contribute to most technologies. On the latter, national systems of science and of technology remain closely coupled in most major countries, in spite of the technological activities of large multinational firms. Empirical research is needed on concentration, scale and efficiency in the performance of basic research, where techniques and insights from the applied economics of industrial R&D are of considerable relevance. There is no convincing evidence so far of unexploited economies of scale in basic research. This evidence shows that many policies for greater “selectivity and concentration” in basic research have been misconceived. Economists and other social scientists could help by formulating more persuasive justifications for public subsidy for basic research, and by making more realistic assumptions about the nature of science and technology.

Observation of intermediate spin states in Er/Y superlattices

Low temperature hysteresis measurements, performed on a series of Er/Y superlattices with Er interlayer thickness ranging from 37 to 90 Å, suggest the existence of intermediate spin states that are stabilized in a field. While bulk Er becomes ferromagnetic at 20 K, even at 10 K these samples transform to a state carrying (1)/(7) the saturation moment. This state, which is observed only at high temperatures in bulk Er, is stable at 10 K to 15 kOe, beyond which the magnetization saturates. The widths of hysteresis loops associated with the low and high field transitions, respectively, are 0.5 and 2.5 kG, many times that of Er. Just below the critical field the spin structure passes through a narrow field range where the magnetization is approximately Ms/2. No such state is observed in bulk Er in zero field. This intermediate state is more apparent on reducing the field after saturation, and becomes more stable as the Er interlayer thickness is decreased. The magnetization is not found to decay appreciably when the field is held for 10 h on the Ms/2 plateau. Preliminary data on thin films also show the existence of the Ms/7 state at low temperatures, but with the range of stability decreasing with increasing Er film thickness. We discuss these phenomena in terms of modifications of the exchange interaction due to epitaxial strains.

Observation of auto-oscillations and chaos in subsidiary absorption in yttrium iron garnet

Auto-oscillations of the dynamic magnetization and routes to chaos for the first-order transverse pump spin-wave instability have been studied in single-crystal yttrium-iron-garnet (YIG) films. The measurements reported here were made on a 20.8-μm-thick YIG film at 9.4 GHz with the static and microwave fields in the plane of the film. Auto-oscillations at 100–400 kHz were observed in the power absorbed by the film over a relatively narrow static field range of 1100–1460 Oe, compared to the first-order instability (FOI) range of 0–1630 Oe. The auto-oscillation frequency and threshold microwave field amplitude were both strongly field dependent. The threshold amplitudes were about a factor of 2 larger than the FOI threshold amplitudes. At even higher power levels and for an even narrower field range of 1300–1380 Oe, the auto-oscillations showed frequency changes indicative of chaotic behavior. Several different subharmonic bifurcation routes to chaos were observed for different fields within the chaotic region.

Ion selectivity of temperature-induced and electric field induced pores in dipalmitoylphosphatidylcholine vesicles.

Temperature and electric field are known to alter the permeability of the bilayer membrane in phospholipid vesicles. A study of cation selectivity of these membrane pores is reported for multilamellar liposomes (MLV) and unilamellar large vesicles (ULV, 95 +/- 5 nm diameter) of dipalmitoylphosphatidylcholine (DPPC). The permeability of ULV to Rb+ was 1.0 X 10(-6) micrograms/s at 22 degrees C and increased to 1.1 X 10(-5) micrograms/s at the gel to liquid-crystalline transition temperature (Tm) of the bilayer, at 42 degrees C. The permeability of ULV to Rb+ continued to increase beyond the Tm and reached 1.0 X 10(-4) micrograms/s at 56 degrees C, a 100-fold increase over the permeability at 22 degrees C. In contrast, the permeability of ULV to Na+ showed a local maximum of 6.0 X 10(-6) micrograms/s at 42 degrees C and decreased at temperatures higher or lower than the Tm. For MLV, the permeability to both Rb+ and Na+ peaked dramatically at the phase transition temperature, 42 degrees C, and subsided at lower and higher temperatures. When ULV were exposed to an electric field, the permeability to Rb+, Na+, and sucrose surged at a field strength of 30 kV/cm; 30 kV/cm can induce a transmembrane potential of 210 mV. In ULV, the electrically perforated lipid bilayer exhibited selectivity for Rb+ over Na+ only at a narrow electric field range, between 31 and 33 kV/cm. For MLV, no well-defined breakdown voltage was recorded.(ABSTRACT TRUNCATED AT 250 WORDS)

Crystallization in electric fields of a semiflexible copolymer: [formula omitted] terephthalate

This work has studied crystallization in electric fields of a semiflexible polymer containing poly(ethylene terephthalate) (PET) segments. Electric (E) stimuli were found to either suppress or enhance PET crystallization depending on E-field strength. Over a relatively narrow field range the effect changes from suppression to enhancement of crystallinity. It is suggested that either inter-chain cooperative phenomena or electromechanical effects in the context of nucleation and/or growth may be affecting crystallization. For example, electrical alignment of rigid (mesogenic) segments in the copolymer structure leading to distension of the crystallizable and more flexible PET segments.

Irreversibility and metastability in spin-glasses. II. Heisenberg model

We numerically compute the various history-dependent magnetizations for Heisenberg spinglasses with and without anisotropy. The exchange interactions are of short range and have a Gaussian probability distribution. Our approach closely follows that of paper I. In the absence of anisotropy, a Heisenberg spin-glass is found to have no irreversibility. The field-cooled and zero-field-cooled magnetizations are macroscopically equivalent and magnetic hysteresis is absent. This macroscopic reversibility is a consequence of the accessibility of the rotationally degenerate field-cooled state and does not correspond to microscopic reversibility. Both Dzyaloshinsky-Moriya (DM) and uniaxial anisotropy introduce macroscopic irreversibility. In the latter case the hysteresis loops are like those which we found for Ising spins. In the former case, in some situations, we find displaced loops, which look similar to those seen in Mn-containing spin-glasses. To get qualitative agreement with experiment, we must also impose a tendency towards ferromagnetism. This ferromagnetic tendency (which corresponds to a displaced Gaussian exchange distribution with ${J}_{0}g0$) is essential in order to maintain rigid rotation of the spins in response to field rotations. This rigidity is a fundamental assumption in other approaches which explain analytically why DM anisotropy leads to displaced hysteresis loops. Finally we study the coexistent (longitudinal) ferromagnetic---(transverse) spin-glass phase proposed by Gabay and Toulouse. The behavior of the coexistent spin-glass is very similar to that of typical spin-glasses in very large applied fields. We see no indication for reentrant behavior, as is often observed experimentally, in our temperature-dependent moderate-field magnetizations. Furthermore, a calculation of the zero-field (${J}_{0},T$) phase diagram for isotropic systems shows no evidence for reentrant behavior. We cannot rule out the possibility that a reentrant transition exists only in some narrow range of low magnetic fields.

The Pulsar Magnetic Window

Triggering of the spark discharges in the Ruderman and Sutherland model by background gamma rays is shown to be effective only within a narrow range of neutron star magnetic fields centred on 2.5 × 1012 gauss. This calculated range of field strengths is in good agreement with ‘observed’ values, suggesting that (a) such triggering is operative, and (b) that neutron stars with much stronger fields do not function as pulsars.

Magnetic hysteresis of CuMn in the spin glass state

We present a series of measurements of the magnetic hysteresis properties of alloys of CuMn in the concentration range 0.06% at. to 4.7% at. when these are cooled well into the spin glass state i.e. T/Tg<0.3. Previous studies (1) made at higher concentrations have revealed the occurrence of a complete reversal of magnetisation in a narrow range of coercive fields and a shifted hysteresis loop. Quite unexpectedly we find that going to lower Mn concentration increases the cooperative nature of the magnetisation reversal to the point where the hysteresis becomes square and that complete, spontaneous (less than 0.1 second time constant) magnetisation jumps are observed in low magnetic field (typically-150 gauss) together with a shifted square center. We report on the effect on the hysteresis of a number of variables: the magnetisation, the concentration, the temperature, the metallurgy, the time and the application of a static transverse magnetic field. These observations lead to the existence of a well defined anisotropy field in the spin glass state.

Recent developments in the theory and materials for digital recording

There is ample evidence that further advancement in the state of the art of digital magnetic recording systems hinges to a large extent on the limitations imposed by the writing process, and in particular on the phenomena of recording demagnetization and asymmetric peak shift. To adequately handle these problems one must consider the dynamic interaction of the writing-demagnetization process self-consistently and integrally, instead of treating these processes as independent and unrelated. The evolution of our theoretical understanding of saturation recording is discussed, with particular emphasis on the iterative process which offers the greatest promise for the adequate treatment of the nonlinear writing-demagnetization phenomena. The results of the theoretical analysis are used to predict the desired properties of the recording materials for high bit packing densities, where recording demagnetizing is severely limiting system performance. Besides the well-known criteria of small thickness and relatively low remanence and high coercivity, a very narrow range of switching fields (steep-sided hysteresis loop) is also very desirable. In this regard the recent development of a superior gamma-ferric oxide coating is most exciting. It consists of nondendritic particles which can be well dispersed and aligned resulting in phenomenal orientation ratios, narrow range of switching fields, and superior recording performance.

Magnetic and magnetooptic effects under phase transitions in antiferromagnets

A complex study of magnetization change, optical Faraday effect, and light absorption spectrum has been carried out on uniaxial antiferromagnetic crystals of MnF 2 of FeCO 3 under the phase transitions induced by strong external magnetic field. The measurements were made at 4.2 and 20.4 K, which is lower than Neel temperature (T n =68 and 38 K for MnF 2 and FeCO 3 , respectively). The field used was pulsed up to 400 kOe. MnF 2 : The magnetic properties have been studied in detail employing a pickup coil method in a narrow range of magnetic fields near the spin-flopping field (H c =92 kOe) with a strict field orientation along the tetragonal axis of the crystal symmetry. The angular field deviation ψ from the c 4 axis is controlled to within 1.5'. In the narrow range of angles ψ c depends on the sample form and the magnetization process is of a jump-like character. The measurements of the longitudinal (along the magnetic field) and transversal susceptibility components made using three pickup coils of mutually perpendicular axes and three coaxial coils shifted along the sample axis show that at ψ c ± 200 Oe the sample magnetically cleaves into interchanging layers of 10-5cm 3 volume which stretch along the symmetry axis of the sample and differ from each other by a longitudinal component of the magnetization vector.

Experimental Study of the Host NMR Linewidth and Spin-Lattice Relaxation Rate in DiluteCuFeAlloys below the Kondo Temperature

In order to determine the properties of single magnetic impurities in the Kondo state and the effects of these single magnetic impurities on the host-conduction-electron spin system, the Fe-impurity contributions to the ${\mathrm{Cu}}^{63}$-host nuclear-magnetic-resonance (NMR) linewidth $\ensuremath{\Delta}{H}_{i}$, and spin-lattice relaxation time ${T}_{\mathrm{li}}$, have been studied over a wide Fe-concentration range ($0<c<1260$ ppm) in $\mathrm{Cu}\mathrm{Fe}$. The NMR-linewidth measurements made from 1.65 to 77\ifmmode^\circ\else\textdegree\fi{}K and in magnetic fields from 2 to 16 kOe and in some cases up to 60 kOe, show the anomalous behavior of the slope $S=\frac{d\ensuremath{\Delta}{H}_{i}}{\mathrm{dH}}$ originally observed by Heeger et al. and studied for a 480-ppm $\mathrm{Cu}\mathrm{Fe}$ alloy by Golibersuch and Heeger exists over a wide Fe-concentration range. This anomalous behavior, which consists of the transition from a constant slope at low fields, ${S}_{L}$, to a smaller magnitude slope at high fields, ${S}_{H}$, occurs in a relatively narrow range of fields about some critical field ${H}_{c}$. This behavior clearly results from the single-impurity contribution to the NMR linewidth as evidenced by the linear concentration dependence of both ${S}_{L}$ and ${S}_{H}$ and also by the concentration independence of $\frac{{S}_{L}}{{S}_{H}}$. ${S}_{H}$ has the same ${(T+29)}^{\ensuremath{-}1}$ temperature dependence as the bulk susceptibility, while ${S}_{L}$ is enhanced for $H<{H}_{c}$ and $T<{T}_{c}\ensuremath{\approx}6$ \ifmmode^\circ\else\textdegree\fi{}K. At 1. 65 \ifmmode^\circ\else\textdegree\fi{}K, ${S}_{H}=(1.50\ifmmode\pm\else\textpm\fi{}0.10)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}c$, ${S}_{L}=(2.83\ifmmode\pm\else\textpm\fi{}0.10)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}c$ ($c$ in ppm), and $\frac{{S}_{L}}{{S}_{H}}=1.9$. These results show that the Ruderman-Kittel-Kasuya-Yosida-like oscillatory conduction-electron spin polarization existing about an impurity for $T\ensuremath{\gg}{T}_{K}$ is either enhanced for $T<{T}_{c}$ and $H<{H}_{c}$, or else an additional long-range oscillatory spin polarization is formed in the Kondo state. From the inverse concentration dependence of ${H}_{c}$ we conclude that long-range interactions of sufficient strength exist between Fe spins via the $d\ensuremath{-}d$ double-resonance mechanism to effectively saturate the extra oscillatory spin polarization in successively smaller applied fields as the Fe concentration increases. The impurity-induced host relaxation rate is linear in Fe concentration up to at least 300 ppm, decreasing from ${T}_{\mathrm{li}}^{\ensuremath{-}1}=2.3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}c$ ($c$ in ppm) for 2.65 kOe to ${T}_{\mathrm{li}}^{\ensuremath{-}1}=2.5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}c$ for 15 kOe at 1. 65 \ifmmode^\circ\else\textdegree\fi{}K. The low-concentration data follow a single curve when plotted as $T{(c{T}_{\mathrm{li}})}^{\ensuremath{-}1}$ vs $\frac{T}{H} (0.1\frac{^{\ensuremath{\circ}}\mathrm{K}}{\mathrm{kOe}}<\frac{T}{H}<1.0\frac{^{\ensuremath{\circ}}\mathrm{K}}{\mathrm{kOe}})$. Comparison of this curve with the existing high-temperature ($T\ensuremath{\gg}{T}_{K}$) theories would imply that the spin-lattice relaxation in the liquid-helium temperature range is dominated by a dipolar coupling of the nuclei to longitudinal dipolar fluctuations of the impurity spin. These results are discussed in the light of the ${T}_{\mathrm{li}}$ data for $T>{T}_{K}$ which does not appear to be consistent with this mechanism suggesting that none of the $T\ensuremath{\gg}{T}_{K}$ relaxation mechanisms may be simply extended to the region $T<{T}_{K}$.

Anhysteretic Magnetization of an Assembly of Single-Domain Particles

A kinetic analysis of anhysteretic magnetization is presented that departs from previous analyses by including the time dependence of particle switching. Each particle is assigned a switching probability which depends upon the applied field and its period. The growth of anhysteretic remanence is shown to occur in a narrow range of fields immediately below the particle coercivity. In contrast to previous analyses, the kinetic analysis predicts a finite anhysteretic susceptibility for noninteracting particles that is large but not infinite as previously supposed.

Studies of the emission from indium antimonide

Experimental studies on the radio-frequency emission-- from InSb under the influence of electric and magnetic fields in the range of 10 MHz to 10 GHz, including the effect of temperature variations around 77° K, are presented. Indications of very weak emission for a narrow range of electric fields in the absence of a magnetic field are observed.