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We investigate the effects of high frequency strain on the depinning of magnetic domain walls in perpendicular anisotropy materials. Micron wide stripes of [Co(0.3 nm)/Pt(0.6 nm)]5 are patterned between a pair of identical inter-digital transducers that generate high frequency (114.8 MHz) standing surface acoustic waves. We use magneto-optical Kerr effect microscopy to characterize the thermally-assisted depinning of domain walls at defect sites within the strips. Our results show that the excitation of the domain walls with surface acoustic waves results in an increase in their depinning probabilities by approximately a factor of 10. Our data are consistent with a model in which the magnetoelastic anisotropies induced by the acoustic waves modulate the energy barriers that pin the domain walls. These results suggest an alternative route to domain wall depinning in thin films and nanostructures and are relevant to the development of racetrack memories, where domain wall pinning can result in reduced velocities and non-deterministic motion.

Domain walls in magnetic thin films are being explored for memory applications and the speed at which they move has acquired increasing importance. Magnetic fields and currents have been shown to drive domain walls with speeds exceeding 500 m/s. We investigate another approach to increase domain wall velocities, using high frequency surface acoustic waves to create standing strain waves in a 3 micron wide strip of magnetic film with perpendicular anisotropy. Our measurements, at a resonant frequency of 248.8 MHz, indicate that domain wall velocities increase substantially, even at relatively low applied voltages. Our findings suggest that the strain wave derived effective magnetic field acts as an additional driver for domain wall motion.

The writing of micrometer-scaled exchange bias domains by local, laser heating of a thin-film heterostructure consisting of a perpendicular anisotropic ferromagnetic Co/Pd multilayer and a (0001) oriented film of the magnetoelectric antiferromagnet Cr2O3 (chromia) is reported. Exchange coupling between chromia's boundary magnetization and the ferromagnet leads to perpendicular exchange bias. Focused scanning magneto-optical Kerr measurements are used to measure local hysteresis loops and create a map of the exchange bias distribution as a function of the local boundary magnetization imprinted in the antiferromagnetic pinning layer on field cooling. The robust boundary magnetization of the Cr2O3 fundamentally alters the exchange bias mechanism, enabling the writing of micrometer-scaled regions of oppositely directed exchange bias using a focused laser beam.

With the increasing interest in deploying ferroelectric polymer in flexible electronics and electro-mechanics, high-throughput and low-cost fabrication of 3D ferroelectric polymer nanostructures on flexible substrates can be a significant basis for future research and applications. Here, we report that large arrays of ferroelectric polymer nanopillars can be prepared directly on soft, flexible substrates by using low-cost polydimethylsiloxane (PDMS) soft-mold reverse nanoimprint lithography at 135 °C and at pressures as low as 3 bar. The nanopillar arrays were highly uniform over large areas of at least 200 × 200 μm and had good crystallinity with nearly optimum (110) orientation. Furthermore, the method leaves little or no residual polymer layer, fully isolating the nanopillars to avoid cross-talk and, obviating the need for additional etching processes that arises with conventional low-contrast nanoimprinting. The ferroelectric properties of individual nanopillars were probed by piezoresponse force microscopy, which showed that they exhibited switchable and bi-stable polarization. In addition, the polarization hysteresis loops probed by pyroelectric measurements of the entire array showed that the nanopillar capacitor arrays had good ferroelectric switching characteristics, over areas of at least 1 mm × 1 mm.

The ability to control magnetic domain patterns with high frequency strain waves could result in a novel, fast method for moving domain walls. We examine the motion of magnetic domain walls in Co/Pt multilayers when subject to a high frequency strain standing wave generated by a focused surface acoustic wave transducer. Our ability to map focused strain waves using optical methods allow us to make subsequent magnetic measurements without losing the location of strain nodes and antinodes. Our results indicate that domain walls move preferentially towards strain antinodes and away from the nodes.

The influence of substrate strain on the magnetic properties of Co–Pt multilayers is investigated by density functional theory (DFT) and model calculations. The lattice parameters for strained Co–Pt multilayers, which are needed as an input for first-principle calculations, are determined by a hybrid approach where DFT is used to determine the lattice constants and atomic positions of the free multilayer, and the effect of the strained substrate is treated continuum mechanically. The lowest order anisotropy constants have been calculated for unstrained and strained hcp Co2Pt4(111) and fcc Co1Pt5(111) multilayers on LiNbO3, and in the strained case, the anisotropy is nonunixial with nonzero anisotropy constant K1 and K1’. By involving both multilayer and substrate mechanical properties, our calculations go beyond the usually considered magnetoelastic anisotropy mechanisms.

The development of novel neutron detectors requires an understanding of the entire neutron detection process, a process which depends strongly on material properties. Here we present accurate measurements of the neutron detection efficiency of an unenriched 640 nm thick boron carbide solid state neutron detector grown by plasma enhanced chemical vapor deposition as a function of the neutron wavelength at a time-of-flight facility. The data were compared to that obtained simultaneously by a calibrated nitrogen detector over the same wavelength range. The measured spectra of both detectors fit a Maxwell–Boltzmann wavelength distribution, thereby indicating that the boron carbide detector can be used as a reliable beam monitor. Measurements of the material properties (density, thickness and elemental composition) of the semiconducting boron carbide enable a precise calculation of the ideal expected neutron detection efficiency. The calculated neutron detection efficiency for the effective moderator temperature (obtained from a fit to the Maxwell–Boltzmann distribution) showed excellent agreement with the experimentally determined neutron detection efficiency of 1.25%. Higher efficiencies may be obtained either by increased film thickness and/or 100% 10B enrichment of the boron carbide source molecule.

Electric field control of magnetization is one of the promising avenues for achieving high-density energy-efficient magnetic data storage. Ferroelectric materials can be especially useful for that purpose as a source of very large switchable electric fields when interfaced with a ferromagnet. Organic ferroelectrics, such as poly(vinylidene fluoride) (PVDF), have an additional advantage of being weakly bonded to the ferromagnet, thus minimizing undesirable effects such as interface chemical modification and/or strain coupling. In this work we use first-principles density functional calculations of Co/PVDF heterostructures to demonstrate the effect of ferroelectric polarization of PVDF on the interface magnetocrystalline anisotropy that controls the magnetization orientation. We show that switching of the polarization direction alters the magnetocrystalline anisotropy energy of the adjacent Co layer by about 50%, driven by the modification of the screening charge induced by ferroelectric polarization. The effect is reduced with Co oxidation at the interface due to quenching the interface magnetization. Our results provide a new insight into the mechanism of the magnetoelectric coupling at organic ferroelectric/ferromagnet interfaces and suggest ways to achieve the desired functionality in practice.

Training effects in a new class of exchange biased ferromagnet/antiferromagnet/ferromagnet trilayers (Co/NiO/[Co/Pt] 3 ) with mutually orthogonal easy axes have been measured and successfully modeled. Previous experiments have demonstrated an enhanced blocking temperature as well as the ability to isothermally field tune the magnitude of the room temperature in-plane exchange bias. These effects have been attributed to the presence of the [Co/Pt] multilayer with perpendicular magnetic anisotropy, which variably pins the backside NiO domains. Here we show that the tuning of the exchange bias and the blocking temperature enhancement are highly dependent on both the temperature and the in-plane remanence of the normally out-of-plane [Co/Pt] multilayer, achieved using modest in-plane set fields. Training effects and their dependence on temperature and in-plane remanence are modeled using a thermodynamic approach. The in-plane remanence of the [Co/Pt] acts only to set the equilibrium exchange bias value and sets the scale for the blocking temperature; it has no effect on the training. We conclude that training effects occur only at the Co/NiO interface and that the relaxation towards equilibrium is confined to this interface. The field enhanced blocking temperature and isothermal tuning of exchange bias in these magnetic heterostructures with mutually orthogonal easy axes could play a role in the enhancement of exchange bias effects in future spin-valve devices. A thorough knowledge of the training effects is essential to account for the fundamental relaxation mechanisms that occur with repeated field cycling.

The motion of domain walls in thin ferromagnetic films is of both fundamental and technological interest. In particular, the ability to use drivers other than magnetic fields to control the positions of domain walls could be exciting for memory applications. Here, we show that high frequency dynamic strain produced by surface acoustic waves is an efficient driver of magnetic domain walls in ferromagnetic films with perpendicular anisotropy. A standing surface acoustic wave of resonant frequency 96.6 MHz increases the domain wall velocities in thin films of [Co/Pt]n by an order of magnitude compared to magnetic fields alone. This effect is highly resonant, effectively ruling out thermal effects, and the velocity shows distinct variations in the domain wall velocity at the nodes and antinodes of the standing wave. The data indicate that standing strain waves can drive the domain wall motion from the creep to the flow regime as the amplitude increases. Hence, strain waves could provide an alternative route to rapid domain wall motion.The motion of domain walls in thin ferromagnetic films is of both fundamental and technological interest. In particular, the ability to use drivers other than magnetic fields to control the positions of domain walls could be exciting for memory applications. Here, we show that high frequency dynamic strain produced by surface acoustic waves is an efficient driver of magnetic domain walls in ferromagnetic films with perpendicular anisotropy. A standing surface acoustic wave of resonant frequency 96.6 MHz increases the domain wall velocities in thin films of [Co/Pt]n by an order of magnitude compared to magnetic fields alone. This effect is highly resonant, effectively ruling out thermal effects, and the velocity shows distinct variations in the domain wall velocity at the nodes and antinodes of the standing wave. The data indicate that standing strain waves can drive the domain wall motion from the creep to the flow regime as the amplitude increases. Hence, strain waves could provide an alternative route t...

Neutron detection signatures in thin films of semiconducting boron carbide were measured as a function of three applied reverse bias voltages, 0, 1.57 V and 3.15 V. Thermal neutrons were detected at zero bias. As the bias was increased, an increase in efficiency and in peak channel positions was observed. These two significant effects have been attributed to an increase in charge collection at higher bias voltages.

Two independent techniques for modeling boron-based solid-state neutron detectors are presented—one using the GEANT4 Monte Carlo toolkit and the other one an analytical approach using a simplified physical model. Results of these techniques are compared for three different types of solid-state boron carbide detector. These results provide the basis for distinguishing between conversion layer and other solid-state detectors.

The ferroelectric and piezoelectric properties of newly synthesized polymer systems have been studied. To date PVDF and its copolymers P(VDF-TrFE) have provided the bulk of the knowledge pertaining to ferroelectricity in polymers. Recently, ultrathin ferroelectric films of P(VDF-TrFE) 70:30 have been fabricated using the Langmuir-Blodgett technique [4]. In this study, various new polymers have been synthesized by chemically altering the PVDF structure. This alteration was performed in order to enhance the amphiphilic nature of the polymer and thus improve the LB film quality and control. Various chemical groups have been used to replace the electropositive hydrogen and electronegative fluorine found in the traditional PVDF chemical structure, including Nitrile, Ester, and Methyl groups. In all cases the resulting chemical structure provides for a net dipole moment directed from the electronegative side of the monomer to the electropositive side. However, to obtain ferroelectricity these microscopic dipole...

The paramagnetic phase of La 0.65 Ca 0.35 MnO 3 and La 0.65 Pb 0.35 MnO 3 powders was investigated by electron spin resonance. The resonance spectra, for a range of temperatures, was used to examine the presence of polarons and bipolarons. The resonance lines are compatible with the presence of polarons ( g ≈2), but the characteristic high-spin bipolaron lines ( g ≈4) are absent. The results are inconsistent with the presence of high-spin bipolarons. The evidence with respect to low-spin bipolarons is less conclusive, although the temperature dependence of the resonance line parameters speaks against the presence of low-spin bipolarons.

Strong out-of-plane exchange biasing has been observed in out-of-plane exchange biased [Pt (5 A)/Co (4 A)]3/NiO (11 A) multilayer with perpendicular easy axis. Both the exchange field HE and coercivity HC display a closely linear temperature dependence except at very low temperatures. A thin NiO layer coated on the top of a Pt/Co multilayer has a great effect on the domain pattern of the Pt/Co multilayer, which is in a more irregular configuration with much smaller domain sizes than the uncoupled Pt/Co multilayer. A simulation according to Malozemoff’s random field model gives a good agreement to the experimental temperature dependence of HE and HC, suggesting that the closely linear temperature dependences of HE and HC are strongly related to the behavior of the temperature of anisotropies and of the spin rotation inside the domain walls of a thin NiO layer. The blocking temperature of TB=220 K is much higher than that observed in ferromagnetic/thin NiO systems with in-plane anisotropy.

The angular dependence of the magnetization reversal process in the exchange biased ferromagnetic/antiferromagnetic bilayers has been investigated carefully using the Stoner–Wohlfarth rotation model. Depending strongly on the orientation of the applied field and the competition between the unidirectional and uniaxial anisotropic energies, the magnetization rotation can occur at either the same side or the different sides of the field direction for the decreasing and increasing field branches of the hysteresis loop. The calculated results and the magnetooptical Kerr effect have been used to understand the magnetization reversal process in the NiFe/NiO bilayers, which is caused mainly by the magnetization rotation.

We have investigated the exchange coupling between an epitaxial Ni (50 A) film with an out-of-plane magnetic easy axis and a NiO (600 A) film by polar magneto-optic Kerr-effect measurements. The temperature dependences of exchange field HE for both as-deposited and field-cooled states exhibit the same blocking temperature TB∼130 °C. The exchange field HE for the field-cooled state is lower than that for the as-deposited state. The hard-axis in-plane loop shows a much smaller value of HE. No coercivity enhancement is observed. The data are suggestive of linear coupling across the ferromagnet/antiferromagnet interface.

The structure of La0.65Pb0.35MnO3 has been studied by neutron powder diffraction. From the temperature dependence of the lattice parameters, we observe that La0.65Pb0.35MnO3 remains single phase in the temperature interval from 25 to 400 K, with a rhombohedral (R3c) structure. The collinear Mn spins suffer a thermally driven reorientation transition close to the Curie temperature, as obtained from powder samples (using neutron diffraction) and from thin films [using ferromagnetic resonance (FMR)]. The reorientation transition may indicate the presence of local Jahn–Teller (JT) lattice distortions, which are prevented from developing with long-range order in the R3c crystal structure.

CrO2 thin films fabricated on single crystal TiO2 (100) substrates by vapor phase transport grow with strongly textured growth characteristics and strong preferential orientation of the crystallites. We have used magneto-optical Kerr effect (MOKE) and alternating gradient force magnetometry (AGFM) to study the in-plane magnetic anisotropy. Both the coercive field, HC, and remnant magnetization Mr/Ms show strong twofold in-plane symmetry and the results, vs. angle, fit a coherent rotation model of domain reversal. We verify that the magnetic easy axis is along the c-axis of the crystal and the first order anisotropy constant, K1, is estimated to be about (2.3±0.2)×105 erg/cm3, consistent with the previous results. Near Tc, there are strong indications that the contributions to the magnetic anisotropy are strongly altered.

Organic ferroelectric materials, including the well-known poly(vinylidene fluoride) and its copolymers, have been extensively studied and used for a variety of applications. In contrast, the VDF oligomer has not been thoroughly investigated and is not widely used, if used at all. One key advantage the oligomer has over the polymer is that it can be thermally evaporated in vacuum, allowing for the growth of complex heterostructures while maintaining interfacial cleanliness. Here, we report on the ferroelectric properties of high-quality VDF oligomer thin films over relatively large areas on the order of mm2. The operating temperature is identified via differential scanning calorimetry and pyroelectric measurements. Pyroelectric measurements also reveal a stable remanent polarization for these films which persists over very long time scales, an important result for non-volatile data storage. Temperature dependent pyroelectric and capacitance measurements provide compelling evidence for the phase transition ...

We report on the design, operation, and performance of a thermal evaporation chamber capable of evaporating organic thin films. Organic thin films are employed in a diverse range of devices and can provide insight into fundamental physical phenomena. However, growing organic thin films is often challenging and requires very specific deposition parameters. The chamber presented here is capable of cooling sample substrates to temperatures below 130 K and allows for the detachment of the sample from the cooling stage and in situ transport. This permits the use of multiple deposition techniques in separate, but connected, deposition chambers without breaking vacuum and therefore provides clean, well characterized interfaces between the organic thin film and any adjoining layers. We also demonstrate a successful thin film deposition of an organic material with a demanding set of deposition parameters, showcasing the success of this design.

We present evidence for a change in electron–phonon coupling across a bulk phase transition. Below the lattice stiffening transition at around 160 K, there is a change in the diffracted peak width observed by neutron and X-ray scattering techniques. Also, the electronic band structure of the copolymer is shifting in binding energy below 160 K, decreasing the density of states near the Fermi level. The value of the effective Debye temperature above the transition temperature is approximately 50 K, while below 160 K, the value of the Debye parameter is 245 K. We postulate that the coupling between electrons and phonons results in a static distortion of the lattice (below 160 K), and this distortion “melts” above 160 K.

Unique insight into the nature of ferroelectricity is emerging from the study of the first two-dimensional ferroelectric films, which are made by Langmuir-Blodgett deposition of vinylidene fluoride copolymers. These films are the first truly two-dimensional ferroelectrics, with thickness-independent bulk ferroelectric properties and a separate surface phase transition. The films offer a unique system for the study the relationship between ferroelectric properties and mesoscopic structure. The structure at all length scales was probed using complementary techniques: X-ray and neutron diffraction, scanning tunneling microscopy, atomic force microscopy, electric field microscopy, low-energy electron diffraction, angle-resolved inverse photoemission spectroscopy, infrared ellipsometry, electron energy-loss spectroscopy, scanning electron microscopy, optical polarization microscopy. We also introduce new results from dynamic pyroelectric scanning microscopy.

Organic-based electronic devices are rapidly increasing in popularity, making it essential to understand and characterize the interface between organic materials and metallic electrodes. This work reports on the characterization of the interface between thin films of an emerging organic ferroelectric, vinylidene fluoride (VDF) oligomer, and Co, an important high Curie temperature ferromagnet. Using a wide battery of experimental techniques, it is shown that VDF oligomer thin films as thin as 15 nm can halt, or prevent, Co oxidization in atmospheric conditions, a necessary condition for device applications. Selectivity of magnetic properties, such as remanent magnetization, is enabled by the clarification of the time scale of Co oxidation, a topic on which there are many conflicting reports. Furthermore, this work shows evidence of chemical bonding at the interface between VDF oligomer and Co, a result with important implications for organic spintronic devices. These results establish the suitability of VDF oligomer for organic-based electronic devices.

Surface structures and a surface structure phase transition are identified that are distinct from the known bulk ferroelectric-paraelectric phase transition of crystalline copolymer films of vinylidene fluoride (70%) with trifluoroethylene (30%). The temperature-dependent changes in the surface structure are accompanied by the physical rotation of the polar group (CH{sub 2}-CF{sub 2}). These changes in the surface structure are compared to the bulk phase transition. We show that the bulk structural transition, while distinct from the surface, is qualitatively similar in both thick and thin Langmuir-Blodgett-grown films. (c) 2000 The American Physical Society.

The surface electronic structure in crystalline copolymer films of vinylidene fluoride (70%) with trifluoroethylene (30%) has been studied by photoemission and inverse photoemission as a function of alkali metal (sodium) doping. Sodium doping introduces at least two new states into the band (HOMO-LUMO) gap well away from the Fermi level. While the sodium-doped copolymer is observed to resemble an n-type semiconductor, the change in electronic structure with temperature suggests that dynamic distortions lead to a photoemission initial state splitting of the lower Hubbard-like bands. There is a decrease in the effective Debye temperature with sodium doping which may, in turn, enhance the dynamic Jahn-Teller distortion.

Materials possessing perpendicular anisotropy have applications in high density magnetic data storage technology. In this paper, we present initial attempts to measure the fast magnetization switching from out-of-plane to in-plane in perpendicular anisotropy Co/Pt multilayers. Interaction with surface acoustic waves (SAW) results in strain induced anisotropy changes in Co/Pt multilayers arising from the magnetostrictive properties of ferromagnetic Co, and changing the easy axis of magnetization. The strain amplitude required to overcome perpendicular magnetic anisotropy is close to 1%. Such large strains are achievable using annular interdigital transducers (AIDT), in which the electrodes follow the velocity curve of the piezoelectric substrate. When excited at the fundamental resonance frequency, SAW propagating towards the center undergo constructive interference and create an intense strain at the focal center. We successfully fabricated AIDT using photolithography with a fundamental resonance of 87.56 MHz. An elliptic structure of Co/Pt multilayers was deposited at the focal center using electron beam lithography (EBL) and magnetron sputtering. The magnetization of the elliptic patterned Co/Pt was measured using focused MOKE and magnetic force microscopy (MFM), confirming that the ellipse is in a single domain state with out of plane anisotropy. Preliminary attempts to measure the magnetization rotation arising from the strain waves did not show the expected magnetization rotation signature. Possible reasons for this are discussed.

Magnetization measurements of a series of Au/Fe/Ni trilayers were carried out by spin-polarized neutron reflectometry and SQUID magnetometry. The bottom layer of Ni had the same thickness for all samples (140 A) while the top layer of Fe varied from sample to sample (16–90 A). The magnetic moment of iron was found to be dependent on the layer's thickness. For Fe layers of 32 A or less, the Fe magnetization was an order of magnitude smaller than that of bulk BCC Fe, indicating that Fe is either in a non-magnetic state or that its ordering is antiferromagnetic. For Fe layers of 60 A or more, the Fe moment was found to be in the plane of the film, constant throughout its entire layer, and with a value (2.0 μ B /atom) approaching that of bulk iron. This magnetic transition probably can be attributed to a structural phase transition from the FCC to the BCC structure.

The Zeeman splitting was measured for neutrons specularly reflected from a magnetic layer in a particular geometry. The neutron spins were quantized along an external magnetic field H, and the reflecting layer was magnetized in a different direction, causing some of the reflected neutrons to flip spin. The laws of conservation of energy and momentum for the spin-flipped neutrons impose the condition θf2 = θi2 ± 1.47 × 10−7 Hλ2, where θi, θf are, respectively, the incident and reflected angles in radians, H is expressed in kOe and λ (the neutron wavelength) in A. Spin-flipped neutrons are reflected at an angle significantly different from the angle of incidence even in fields of a few kOe, although the Zeeman splitting energy amounts to less than 10−7 eV.

We show that the chemical depth profile of a film of unknown structure can be retrieved unambiguously from neutron reflection data by adding to the system a known magnetic layer. Three independent reflectivities are obtained by taking measurements with the sample magnetized in a magnetic field perpendicular to the surface and subsequently parallel to it, and using in the latter geometry neutrons polarized either in the direction of the field or opposite to it. The procedure consists of two steps. First, from the three reflectivities both the real and imaginary parts of the reflection coefficient of the unknown film are extracted within the framework of the rigorous dynamical theory. Second, the neutron scatteringlength density (and consequently the chemical depth profile) is obtained by a suitable numerical technique for the conventional Schrodinger inverse scattering problem. Computer experiments were conducted for selected cases: starting from the profiles the reflectivities were calculated in a limited range of q and then the original profiles were successfully recovered. The influence on the accuracy of the recovered depth profile of the counting statistics and the cutoffs at low and high q are discussed.

The effects on the interlayer coupling of the Cr Neel transition is studied in Fe/Cr(001) superlattices. The Neel transition is suppressed for Cr layer thickness 42 A of Cr, the Neel temperature TN initially increases rapidly and then asymptotically approaches its bulk value with a three-dimensional transition-temperature shift exponent value of λ = 1.4 ± 0.3. Neutron diffraction confirms both the Cr antiferromagnetic order and the existence of the incommensurate, transverse spin density wave whose nesting wave vector is the same as that of bulk Cr. The ordering of the Cr dramatically alters the coupling of the Fe layers. The biquadratic Fe interlayer coupling observed for T > TN vanishes below TN as confirmed by polarized neutron reflectivity. The behavior can be understood in terms of finite-size and spin frustration effects at rough Fe-Cr interfaces.

Polarized-neutron reflectivity together with magnetization and magnetotransport measurements on a (001)-oriented [Fe(14 \AA{})/Cr(74 \AA{})${]}_{20}$ superlattice confirms the existence of 90\ifmmode^\circ\else\textdegree\fi{} alignment of adjacent Fe layers due to biquadratic interlayer coupling. Each Fe layer is in a single domain state and the magnetic structure is coherent throughout the layered stack. The biquadratic coupling, however, is suppressed below the Cr N\'eel temperature (${\mathit{T}}_{\mathit{N}}$=187 K) as the Fe layers uncouple. But by field cooling through ${\mathit{T}}_{\mathit{N}}$ it is possible to retain a metastable biquadratic arrangement. \textcopyright{} 1996 The American Physical Society.

Domain wall (DW) contribution to the magnetoresistance (MR) is illustrated by direct observation of field-dependent magnetic force microscope images that are correlated with MR curves for micron sized zigzag wires. The Permalloy zigzag wires were formed using focused ion beam milling, a simple one-step microfabrication technique. Our results show domain walls cause a decrease in the resistance that is more closely associated with the creation of a large amount of DWs in each segment of the zigzag wire than with the DWs pinned at the corners.

We have developed a large amplitude piezoceramic scanner which should have numerous applications. Scanning tunneling microscopy (STM) and other scanning probe microscopies predominantly use piezoceramics for the scanning elements. Similarly adaptive optics, high resolution lithography, and micromanipulators are other examples of research which regularly utilize piezoceramic scanners. We present a new geometry for a piezoceramic scanner which allows for both high resolution (∼nanometers) and large amplitude (∼400 μm) displacements. The cross‐shaped geometry makes it possible to produce extremely long pieces with very high tolerances. We have shown its effectiveness by using it as the major component of a low temperature STM (LTSTM). This LTSTM is unique in two distinct ways: the scan range at low temperature is a factor of 10 larger than those reported and the coarse, approach mechanism is a single component piezoceramic—making coarse approach in situ much quieter and easier than in other designs.

The flexoelectric effect, which is a linear coupling between a strain gradient and electrical polarization, is a fundamental electromechanical property of all materials with potential for use in nanoscale devices, where strain gradients can be quite large. We report a study of the dependence of the flexoelectric response on thickness in ultrathin films of polar and non-polar polymers. The measurements of the flexoelectric response in non-polar polyethylene and the polar relaxor polymer polyvinylidene-co-trifluoroethylene-co-chlorofluoroethylene were made using a bent cantilever method and corrected for the contribution from the electrode oxide. The results show that the value of the flexoelectric coefficient increases with decreasing thickness, by up to a factor of 70 compared to the bulk value, reaching such enhanced values in films of only 10 nm thickness. These results are consistent with a model accounting for interfacial contributions, and underline how large electromechanical coupling can be produce...

The correlation between ferromagnetic domain formation and exchange bias in a series of NiFe/NiO samples with varying NiO thicknesses has been investigated using the magneto-optic Kerr effect and magnetic force microscopy. Below a critical thickness (15 nm) of NiO, the exchange bias HE is zero and ripple domains exist in the NiFe layer. Above this critical thickness, cross-tie type domain walls appear concurrently with the appearance of exchange bias. Both the number of cross-tie domain walls and the exchange bias increase with an increase in NiO thickness, reaching a maximum at 35 nm NiO, after which both show a gradual decrease. This variation of domain wall formation in the NiFe layer with the NiO thickness possibly reflects the variation of the domain structure in the NiO layer through interfacial exchange coupling.

Organic thin films have numerous advantages over inorganics in device processing and price. The large polarization of the organic ferroelectric oligomer vinylidene fluoride (VDF) could prove useful for both device applications and the investigation of fundamental physical phenomena. A VDF oligomer thin film vacuum deposition process, such as thermal evaporation, preserves film and interface cleanliness, but is challenging, with successful deposition occurring only within a narrow parameter space. We report on the optimal deposition parameters for VDF oligomer thin films, refining the parameter space for successful deposition, resulting in a high yield of robust ferroelectric films. In particular, we investigate the influence of deposition parameters on surface roughness, and the role that roughness plays in sample yield. The reliable production of ferroelectric films allowed us to perform detailed measurements of previously unreported properties, including the Curie temperature, the temperature and thickness dependence of the coercive field, the melting temperature, and the index of refraction. The ability to successfully grow robust, switchable, well-characterized films makes VDF oligomer a viable candidate in the field of organic ferroelectrics.

Real-time solid-state neutron detectors have been fabricated from semiconducting boron–carbon alloys, deposited by plasma-enhanced chemical vapor deposition. Single neutrons were detected and signals induced by gamma rays were determined to be insignificant. The source gas closo-1,2-dicarbadodecaborane (ortho-carborane) was used to fabricate the boron–carbon alloys with only the natural isotopic abundance of 10B. Devices made of thicker boron–carbon alloy layers enriched in 10B could lead to increased detection efficiency and active diodes could use the inherent micron scale spatial resolution, increasing the range of possible applications.

We demonstrate unambiguous evidence of the electric field control of magnetic anisotropy in a wedge-shaped Co film of varying thickness. A copolymer ferroelectric of 70% vinylidene fluoride with 30% trifluoroethylene, P(VDF-TrFE) overlays the Co wedge, providing a large switchable electric field. As the ferroelectric polarization is switched from up to down, the magnetic anisotropy of the Co films changes by as much as 50%. At the lowest Co thickness the magnetic anisotropy switches from out-of-plane to in-plane as the ferroelectric polarization changes from up to down, enabling us to rotate the magnetization through a large angle at constant magnetic field merely by switching the ferroelectric polarization. The large mismatch in the stiffness coefficients between the polymer ferroelectric and metallic ferromagnet excludes typical magnetoelectric strain coupling; rather, the magnetic changes arise from the large electric field at the ferroelectric/ferromagnet interface.

The fabrication, initial structural characterization, and diode measurements are reported for a boron carbide/silicon carbide heterojunction diode. Current–voltage curves are obtained for operation at temperatures from 24 to 351 °C. Plasma-enhanced chemical-vapor deposition (PECVD) -deposited undoped boron carbide material is highly crystalline and consists of a variety of polytypes of boron carbide (BC) with crystal sizes as large as 110 nm. Crystal phases are similar to those for PECVD BC on Si but only partially match known boron and boron-rich BC phases.

A review of ultrasonic measurements on the three heavy fermion compounds UPt3, URu2Si2 and CeCu6 is given. These compounds show one or more of the three types of co-operative behavior; Kondo-like effect, antiferromagnetism and superconductivity; all these features are observed as ultrasonic anomalies. Both UPt3 and CeCu6 show a strong coherence effect which is akin to a Kondo-like transition in the electronic system. Velocity anomalies are found at the antiferromagnetic transition for URu2Si2. UPt3 also has an antiferromagnetic transition which is believed to influence the subsequent superconducting ordering. For UPt3, the superconductivity is of an unconventional nature; multiple superconducting phases have been observed and a complete phase diagram has been obtained from high resolution velocity measurements.

In this paper we present a thermodynamic analysis of the ${\mathrm{UPt}}_{3}$ superconducting phase diagram in the H-T plane. The analysis relates the specific-heat jumps to the slopes of the four phase-transition lines. The existing data are found to be in good mutual agreement, and consistent with the assumption that the inner phase-transition line at low field, ${\mathit{T}}_{\mathit{c}}^{\mathrm{*}}$, is second order for both H^\ensuremath{\perp}c^ and H^?c^. For H^\ensuremath{\perp}c^, the inner phase-transition line at higher field, ${\mathit{H}}_{\mathrm{FL}}$, is first order albeit with a small latent heat. For H^?c^, it can be second order.

This review describes sound propagation in the normal state and the superfluid B phase of 3He. The l=1 nature of Cooper pairing in superfluid 3He leads to a complex collective mode spectrum. A number of the collective modes couple to sound. The expressions for the collective mode frequencies are derived using a phenomenological theory. Cryogenic and ultrasonic measurement techniques are briefly discussed, followed by a review of the results of sound propagation experiments, in particular those that show coupling to the collective modes. The effects of fields, textures and dispersion are also reviewed.

We present ultrasonic velocity data for the heavy fermion superconductor UPt 3 . UPt 3 is a strong candidate for unconventional superconductivity. Conclusive evidence for this comes from the observation of multiple superconducting phases. Earlier measurements of the phase diagram relied on a variety of different measurements on different samples. The phase boundaries obtained agreed qualitatively, but the important region near the H c 2 curve remained an open question. Our velocity measurements present the first complete phase diagram of UPt 3 based on a single measurement on a single sample. The sharp velocity signatures allow us to trace these signatures to within the width of the normal-superconducting transition. Our results indicate that there is a tetracritical point on the H c 2 curve.

The composition in the surface region of the colossal magnetoresistance perovskite manganite materials, La0.65Pb0.35MnO3 has been studied using angle resolved x-ray photoemission spectroscopy. The results are consistent with a Mn–O terminal layer and Pb segregation to the near-surface region. This is similar to the behavior exhibited by La0.65Ca0.35MnO3 and La0.65Sr0.35MnO3. The O–Mn–O terminal layer appears to be close to the Mn3+ valence state.

Crystalline films of La0.65Pb0.35MnO3 grown on Si (100) substrates by rf sputtering have been investigated by ferromagnetic resonance and MOKE. The dependence of the ferromagnetic resonance line parameters on the sample orientation and temperature is investigated. An expression for the angular dependence of the resonance field on the orientation of the sample is proposed, based on the assumption that close to the Curie temperature the Zeeman and demagnetizing terms are dominant. MOKE data support this hypothesis. The temperature dependence of the magnetization at saturation, in relative units, estimated from ferromagnetic resonance agrees with SQUID data.

Measurements of both the lower and upper critical fields along all three principal axes of the heavy-fermion superconductor UPt{sub 3} have been performed. A kink in the temperature dependence of the lower critical field, {ital H}{sub {ital c}1}, is observed for all three orientations, which is in agreement with a recent theory by Hess, Tokuyasu, and Sauls. Also, an anisotropy of the temperature dependence of the lower critical field is observed at low temperatures ({ital T}{lt}290 mK). The Landau-Ginzburg parameters are estimated from the experimental results.

The effective Debye temperature of poly(vinylidene fluoride-trifluoroethylene) copolymers was measured using photoemission and neutron diffraction techniques. An effective Debye temperature of 53±11 K is obtained from the photoemission data and 69±3.5 K from neutron diffraction measurements. This effective Debye temperature is a consequence of the temperature-dependent dynamic motions perpendicular to the surface of these crystalline polymer films.

Heterostructures of Co 4 nm/NiO 1.1 nm/[Co 0.4 nm/Pt 0.6 nm] with mutually orthogonal easy axes allow for isothermal tuning of the hysteresis loop shifts along the applied field axis at room temperature, as well as displaying a greatly enhanced blocking temperature. The loop shifts can be varied up to 35 Oe through the application of moderate dc magnetic fields of 3 kOe. The presence of the [Co/Pt] multilayer with perpendicular anisotropy is responsible for a significant enhancement of the blocking temperature. For this thickness of NiO, the blocking temperature is expected to be well below 50 K, in contrast to the observed blocking temperature of 225 K. These effects may be tailored by a judicious choice of materials. The dependence of the loop shift on applied field will vary depending on the coercivity and remanance of the ferromagnetic layers. The enhancement of the blocking temperature is highly dependent on the in-plane and out-of-plane anisotropy constants of the intervening antiferromagnet. Both effects are directly relevant to a variety of modern spintronic applications.