Magnetic phase diagram of epitaxial dysprosium

We have determined the magnetic phase diagram of Dy as a function of epitaxial strain , applied field H, and temperature T. $roman Y sub x roman Lu sub 1-x- alloys were employed as templates to clamp the films at selected strains. The separate roles of epitaxial clamping and strain are identified for the first time. There is a clearly defined transition as the strain is changed at low temperature from the clamped helical phase to the ferromagnetic phase. The transition is modeled by a linear coupling treatment of the magnetoelastic strains

New configurations of rare-earth superlattices

We have successfully grown high-quality Dy/Y rare-earth superlattices with their a or b axes perpendicular to the growth plane, at choice. Earlier efforts by molecular-beam-epitaxy methods produced only growth along the c axis. In other research, we have grown almost freestanding superlattices of Dy/Y and other hexagonal rare-earth superlattices. These new configurations make a variety of significant new experiments accessible

Positive giant magnetoresistance in Dy/Sc superlattices

We have discovered large positive magnetoresistance in Dy/Sc superlattices at low temperatures. These and other magnetotransport phenomena lack the hysteresis of the observed Dy magnetization. We offer a speculative interpretation in terms of interfacial reflectivity

Growth of rare-earth monolayers on synthetic fluorine mica

We have grown single-crystal rare-earth films on cleaved faces of synthetic fluorine mica fluorophlogopite by molecular-beam-epitaxy techniques. This has made it possible to measure material properties such as magnetism in monolayer structures

Brillouin-light-scattering study of long-wavelength spin waves in a single-crystal 300- gadolinium film

The temperature dependence of the energy of ferromagnetic spin waves in an epitaxially grown 300- [0001] Gd film is shown to depend on the bulk values of the c-axis magnetic-stiffness constant Dc, defined by (q)=tsumiDiqi2, where qa1, and the axial-anisotropy constant P2, defined by scrHaniso =P2 (Sz)2+.... Two bulk spin waves and one Damon-Eshbach surface magnetostatic wave were probed with Brillouin light scattering. The bulk spin waves were found to be sensitive to the exchange interaction. In contrast, the Damon-Eshbach surface magnetostatic wave, although insensitive to the exchange interaction, is influenced noticeably by the axial magnetic anisotropy P2(T) present in Gd. Ignoring surface anisotropy, we extracted values of Dc(T) and P2(T) from the Brillouin data and from the magnetization of the Gd film determined by a superconducting-quantum-interference-device magnetometer. Within the experimental errors, these values are reasonably consistent with the bulk values from the literature

Brillouin-light-scattering study of long-wavelength acoustic phonons in single-crystal dysprosium films

We have employed Brillouin scattering to investigate elastic-wave velocities as a function of temperature in Dy thin films and in bulk Dy. The single-crystal films were synthesized by molecular-beam epitaxy to ensure high crystalline and interfacial quality. Surface (Rayleigh) and guided-wave (Lamb) modes were probed by the Brillouin technique through the surface ripple-coupling mechanism. Results taken near room temperature in the paramagnetic phase agree very well with a layered-elastic model in which the materials of the samples are represented by elastic behavior using bulk single-crystal elastic data from the literature. At lower temperatures anomalies were found for samples with the thinner Dy layers: Measured wave velocities differed by up to 2% from the model predictions, and the ratio of Lamb-to-Rayleigh intensities was twice the predicted value. The anomalies coincide with the ferromagnetic transitions of the films, and they are believed to originate from coupling between the phonons and the magnetic system in the near-surface region probed by the Brillouin scattering

Anisotropic magnetic behavior in Dy/Y films and superlattices

By neutron diffraction we show that superlattices of Dy and Y grown by molecular-beam epitaxy along the hcp b axis exhibit little magnetic coupling between successive Dy layers, even for Y spacers as thin as 9 atomic planes (26). Previous studies of Dy/Y superlattices grown along the hcp c axis established that long-range three-dimensional helimagnetic ordering takes place even through Y spacer layers as thick as 120. This highly anisotropic coupling behavior is shown to have its origin in nearly-two-dimensional nesting features of the Y and Dy Fermi surfaces. Nesting along the c axis gives rise to sharp peaks along c in the wave-vector-dependent magnetic susceptibility, and causes the exchange coupling to exhibit long-range oscillations in real space. The lack of nesting features along the b axis leaves a rapid exponential decay of the exchange interaction with spin separation. From magnetic measurements by superconducting-quantum-interference-device magnetometry on b-axis superlattices and films, we deduce that the first-order ferromagnetic transition of Dy is suppressed, and that the critical field required to produce the ferromagnetic alignment is much higher than the c-axis counterpart. This difference arises from anisotropy of the energy balance of the system. The magnetic coherence in b-axis superlattices and films is anisotropic and exhibits an unusual temperature dependence

Magnetic structure in Dy/Sc superlattices

We have investigated magnetic order in superlattices of Dy and Sc grown along the hcp c axis by molecular beam epitaxy (MBE) techniques. Our neutron diffraction experiments reveal that individual Dy layers order ferromagnetically below Tc∼150 K. The magnetic coherence length along the growth direction is less than the Dy-layer thickness. Previous studies of rare-earth superlattices with Y or Lu as spacer layers have shown that magnetic coherence propagates through sufficiently thin nonmagnetic interlayers. This arises from the long-range exchange interaction that originates from nesting features in the Fermi surface of the spacer material. The lack of coupling in Dy/Sc superlattices reflects the very different Fermi surface of Sc, with much weaker nesting than Y and Lu

SiL: An Approach for Adjusting Applications to Heterogeneous Systems Under Perturbations

Scientific applications consist of large and computationally-intensive loops. Dynamic loop scheduling (DLS) techniques are used to load balance the execution of such applications. Load imbalance can be caused by variations in loop iteration execution times due to problem, algorithmic, or systemic characteristics (also, perturbations). The following question motivates this work: "Given an application, a high-performance computing (HPC) system, and both their characteristics and interplay, which DLS technique will achieve improved performance under unpredictable perturbations?" Existing work only considers perturbations caused by variations in the HPC system delivered computational speeds. However, perturbations in available network bandwidth or latency are inevitable on production HPC systems. Simulator in the loop (SiL) is introduced, herein, as a new control-theoretic inspired approach to dynamically select DLS techniques that improve the performance of applications on heterogeneous HPC systems under perturbations. The present work examines the performance of six applications on a heterogeneous system under all above system perturbations. The SiL proof of concept is evaluated using simulation. The performance results confirm the initial hypothesis that no single DLS technique can deliver best performance in all scenarios, while the SiL-based DLS selection delivered improved application performance in most experiments