R. C. C. Ward

Mechanism of the structural phase transformations in epitaxial YHx switchable mirrors

The detailed mechanisms of the structural phase transformations that occur in epitaxial Y–hydride switchable mirrors are revealed with high resolution transmission electron microscopy (both cross sectional and plan view). The triangular ridge network that develops in Y prior to the α–β transformation is a result of {1012} deformation twinning. The basal plane that is originally parallel to the film/substrate interface is rotated by twinning over 5.6° and transformed into a prismatic plane and similarly the prismatic plane is transformed into a basal plane giving a final crystal reorientation for the ridge of 95.6°. After transformation to β, nearly vertical Σ3{111} twin boundaries arise in the ridges. In contrast, horizontal twin boundaries develop in the β domains to prevent macroscopic shape changes. Inbetween the two twin variants within the domains, Shockley partial dislocations are persistently present, which enable efficient reversible β–γ switching of the mirror.

Rb-doped potassium titanyl phosphate for periodic ferroelectric domain inversion

This article reports the dielectric, conductive, and ferroelectric properties of Rb-doped potassium titanyl phosphate (RKTP) crystals, of formula type RbxK1-xTiOPO4, where x=0.0065 or 0.013, both of which are shown to undergo ferroelectric switching at room temperature with coercive fields varying from 2.3 to 2.7 kV/mm. The measured spontaneous polarizations are 0.18(±0.01) and 0.19(±0.01) Cm-2 for RbxK1−xTiOPO4 with x=0.0065 and x=0.013, respectively. The dependence of the conductivity of Rb:KTP on electric-field amplitude at high fields leads to distorted ferroelectric hysteresis loops. As a result of thermal effects and the redistribution of vacancies, the switching times and switching current peaks vary throughout a series of switching processes at a frequency of 20 Hz. The measured switching times range from 2 to 6 ms and are inversely proportional to the peak current densities. The dielectric relaxation of Rb:KTP is found to be further from the Debye response than that of KTP. A correlation effect i...

The structural and magnetic characterization of molecular-beam-epitaxy-grown FeMn-NiFe exchange-biased bilayers

Structural and magnetic characterizations have been performed on NiFe-FeMn exchange-biased bilayers grown by molecular beam epitaxy. The growth of the films showed high-quality epitaxy with well-defined flat interface between the layers. Magnetic measurements confirmed that the films showed not only a comparable exchange anisotropy field (H/sub ex/) to that of sputtered films but also a lowered threshold thickness of 2 nm at which H/sub ex/ vanishes for the FeMn.

Magnetic structures of Laves phase superlattices

We present a determination of the magnetic structures of three Laves phase superlattice samples of structure [70/30]60, [150/100]50, and [50/70]60, where the structure is given as [t1xa0Axa0DyFe2/t2xa0Axa0YFe2]N, grown by molecular beam epitaxy. The experiments were performed using magnetization measurements and neutron scattering measurements at the NRU reactor at Chalk River in Canada. For the [70/30]60 sample, a magnetic field parallel to the scattering vector, Q, was applied to determine the nuclear component of the scattered intensity. The magnetic structure with the field aligned along the [001] had all the moments aligned along the field direction and the magnitude of the moment on the iron site was temperature independent with a value of 2.3(0.3)xa0μB. The moment on the dysprosium site was found to decrease with temperature from about 10xa0μB to a value of 6(0.5)xa0μB at 300 K. When the field was applied in the [10] direction the magnetic moments were found to rotate out of the (110) epitaxial plane towards the [110] direction by an angle ψ = 10° at 300 K, which increased at 4 K to an angle of ψ = 40° close to the [100] out-of-plane direction. For the [150/100]50 sample, when a 6 T magnetic field was applied along the [10] direction, the main peak of the magnetically sensitive (11) reflection was found to decrease in intensity while the scattering at the satellite peaks increased. This change in intensity is due to the formation of magnetic exchange springs in the soft YFe2 layers of the superlattice. Detailed measurements around the (11) reflection and calculations for an exchange spring model give excellent agreement between the model and the experiment. Finally, the [50/70]60 sample showed unexpected behaviour because the moments aligned largely perpendicular to an applied field. This is similar to a spin-flop phase of an antiferromagnet and it is argued that this occurs because the net moments on the DyFe2 and YFe2 layers are nearly equal.

Structure of DyFe2/YFe2 Laves phase superlattices grown by molecular beam epitaxy

The crystal structure of a series of six high quality single crystal Laves phase superlattice samples, of structure [t1 A DyFe2/t2 A YFe2]N, grown by molecular beam epitaxy have been probed using a high resolution triple crystal x-ray diffractometer. A study of the scattering near the 2 2 0, 4 4 0 and reflections has revealed the presence of several superlattice peaks, showing that the samples exhibit a high degree of superlattice modulation. The typical mosaic spread is less than 0.9°, while the superlattice coherence lengths are typically 2000 A. Fitting of a model to the data using a differential evolution algorithm with a Rietveld refinement has confirmed the high quality of these samples and shows that the interface widths are typically A. Measurements of the in-plane and out-of-plane lattice parameters show that the samples are subjected to a shear that is slightly anisotropic in the plane. Transverse scans through the Bragg and superlattice peaks show that the width is mainly dominated by a mosaic crystal effect, with a small contribution arising from the correlated roughness at the bilayer interfaces. Finally, high resolution electron microscopy images show directly that these superlattices exhibit a high degree of modulation.

High potassium KTiOPO4 crystals for the fabrication of quasi-phase matched devices

This paper reports a suppressed ionic conductivity in a KTiOPO4 (KTP) crystal grown from a potassium-rich K4P2O7 flux. Potassium-rich KTP is shown to have an ionic conductivity, which is reduced by one order of magnitude relative to normal flux-grown KTP and is explained by an improved potassium–oxygen stoichiometry. However, it is found that the frequency dispersion of the conductivity is almost identical in form to that of conventional KTP, showing that the fundamental Debye-like origin of the conductivity is unchanged as the number of potassium vacancies is decreased. This behaviour is contrasted with that of Rb-doped KTP, in which the conductivity is reduced by a factor of more than 100 and the dispersion is drastically changed to resemble a disordered system rather than a Debye-like relaxor. Potassium-rich KTP is also found to have a reduced low-frequency dielectric constant, a contracted Cole–Cole plot and a lowered ferroelectric coercive electric field of only 3.0(±0.1) kV mm−1 at room temperature. Successful periodic poling of a potassium-rich KTP crystal to produce a grating of 60 μm period is demonstrated at room temperature for the first time. X-ray topographs proving the existence of periodic domains throughout the bulk of potassium-rich KTP in both the usual [100] grating orientation and the unconventional [010] orientation are presented.

Modeling the magnetic properties of DyFe2/YFe2 superlattices

The Stoner–Wohlfarth model has proved reasonably successful in describing the coercivities of antiferromagnetically coupled DyFe2/YFe2 hard/soft superlattices in the absence of magnetic exchange springs. In particular, the coercivity rises sharply as the net magnetic moment of the superlattice approaches zero. However the situation becomes more complicated as the thickness of the YFe2 layers is increased. Two distinct “instability fields” can be identified: the bending field BB, signifying the onset of a magnetic exchange spring, and the irreversible switching field BIS associated with magnetic reversal. We have developed a computational model to address this problem. In particular, it is shown that the two instability fields in question are characterized by vanishing eigenvalues in the matrix formed by the double energy derivatives ∂2E/∂θi∂θj, where E is the total energy and θi the angle of each individual monolayer. It is shown that the model provides a very good description of the M–Bapp loops of DyFe2...

The structural characterisation of molecular beam epitaxy-grown exchange-biased bilayers

AbstractNiFe yFeMn exchange-biased thin films were grown in an applied magnetic field of approximately 200 Oe using molecularbeam epitaxy. In order to obtain the optimum conditions for the epitaxial growth of the thin films, various chemical treatments ofthe Si (111) substrates and insertion of a Cu buffer layer were used. The 7 =7 surface reconstruction of Si (111) was found outto be the crucial factor for the epitaxial growth. This surface promotes the reaction of Si with the Cu buffer layer to form epitaxialh0-Cu Si which then allows good epitaxy of NiFe and FeMn. In situ reflection high-energy electron diffraction and high resolution 3 electron microscopy observation confirmed the epitaxial growth of the films to be of high quality with well defined flat interfacebetween the layers. 2002 Elsevier Science B.V. All rights reserved. Keywords: Exchange-biased films; Reflection high-energy electron diffraction; High resolution electron microscopy; Molecular beam epitaxy 1. IntroductionResults obtained since 1988 by many researchers ongiant magnetoresistance (GMR ) materials w1,2x haveshown an increase in magnetoresistance of about a factorof 10 over the anisotropic magnetoresistance materialsthat have dominated the field of read heads for diskdrives. The production of read heads for disk drivesbased on the GMR effect was driven by the necessityto obtain larger signals from ever-smaller storage bitsand thus keep pace with improvements in hard diskdrives. In particular, the exchange biased spin valve hasbeen verified as one of the leading devices for high-density magnetic read heads. Spin valve type read headsconsist of a magnetically hard layer whose magnetisationis pinned by an antiferromagnet, separated by a spacerfrom a magnetically soft film whose magnetisation canreverse freely in applied magnetic fields. High-, low-,and intermediate-resistance states depend on the relative

The structure of rare earth thin films: holmium and gadolinium on yttrium

Single-crystal holmium and gadolinium layers have been grown on yttrium substrates using the molecular beam epitaxy technique and their structures investigated using high resolution x-ray scattering. The experiments were performed using a Philips MRD diffractometer in Oxford, and with the XMaS facility at the ESRF. Holmium layers with a thickness below give scattering that is characteristic of a pseudomorphic film structure with the same in-plane lattice parameter as the yttrium substrate to within 0.05%. For layers thicker than , there is a sharp reduction in misfit strain due to the creation of edge dislocations. The transverse lineshape of the holmium peaks exhibits a two-component lineshape for thicknesses above , but below about 500 A. Above 500 A the lineshape of the transverse scans becomes Gaussian and is characteristic of a mosaic crystal. The gadolinium layers show no sharp change of strain for layers as thick as 2920 A and the transverse peak shape remained similar for all films. This is characteristic of pseudomorphic film growth and a failure to nucleate dislocations.

Room temperature coercivities of Tb1-xDyxFe2 (110) molecular beam epitaxy grown films

It has been known for many years that the rare-earth intermetallic compound terfenol (Tb0.3Dy0.7)Fe2 is characterized by “giant magnetostriction,” which has found practical applications. In this article we report the magnetic properties of a series of molecular beam epitaxially (MBE) grown (110) Tb1−xDyxFe2 alloy films, on sapphire substrates. All the measurements were performed at room temperature with the field applied along an in-plane [110] axis. However, unlike bulk Tb1−xDyxFe2, it is shown that the coercivity does not fall to zero at the magic ratio of x∼0.7. Instead, the coercivities fall almost on a straight line, with the maximum coercivity for pure TbFe2 (0.64 T) and the smallest for DyFe2 (0.22 T). The difference between the bulk and MBE films is attributed to the presence of a magnetoelastic strain term, induced during crystal growth by the sapphire substrate. In practice, the measured coercivities can be described, approximately, with a modified Stoner–Wohlfarth model.