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Simple modeling of readback response based on granular micro-structure

Magnetization transitions, readback waveforms, and transition jitter are formulated for Voronoi modeling using error functions. Suitable coefficients for the error functions are provided by the parameters of the granular media, i.e., average grain diameter and grain size distribution. TEM images of real media are used to confirm the fitted results. Random grain positions and uniform switching field distribution are assumed. Readback waveforms for isolated transitions are calculated by reciprocity. The pulse width, T50, is formulated from the medium parameters and readback conditions. An error function with the same T50 and slope at the transition center is a good fit. An analytical expression for jitter noise is presented comprising grain size and track width parameters.

Magnetism and electronic structure of (001)- and (111)-oriented LaTiO3 bilayers sandwiched in LaScO3 barriers

In this study, the magnetism and electronic structure of LaTiO3 bilayers along both the (001) and (111) orientations are calculated using the density functional theory. The band insulator LaScO3 is chosen as the barrier layer and substrate to obtain the isolating LaTiO3 bilayer. For both the (001)- and (111)-oriented cases, LaTiO3 demonstrates the G-type antiferromagnetism as the ground state, similar to the bulk material. However, the electronic structure is significantly changed. The occupied bands of Ti are much narrower in the (111) case, giving a nearly flat band. As a result, the exchange coupling between nearest-neighbor Ti ions is reformed in these superlattices, which will affect the Néel temperature significantly.

Design of grid-type microwave absorbers with high-permittivity composites of Ag-coated Ni-Zn ferrite particles

We investigate the microwave absorbing characteristics in grid-shaped rubber composite sheets containing Ag-coated Ni-Zn ferrite particles. The improvements in the microwave absorbance are obtained through the insertion of a periodic air cavity in the high-permittivity composites. In the bulk specimens containing conductive and magnetic Ag-coated ferrite microspheres, the impedance matching is not satisfied due to the high dielectric permittivity of the composite. The insertion of an air cavity in those absorbers reduces the permittivity and permeability, and thereby leading to impedance matching at a higher frequency. In the grid-type absorber with an optimum air cavity volume rate, the reflection loss can be decreased to as low as −30 dB at 10.5 GHz with a small layer thickness of 2 mm. The proposed grid-type microwave absorber has advantages of a small matching thickness and a considerable reduction in weight in comparison with conventional ferrite composite absorbers.

Current-induced spin torque resonance of magnetic insulators affected by field-like spin-orbit torques and out-of-plane magnetizations

The spin-torque ferromagnetic resonance (ST-FMR) in a bilayer system consisting of a magnetic insulator such as Y3Fe5O12 and a normal metal with spin-orbit interaction such as Pt is addressed theoretically. We model the ST-FMR for all magnetization directions and in the presence of field-like spin-orbit torques based on the drift-diffusion spin model and quantum mechanical boundary conditions. ST-FMR experiments may expose crucial information about the spin-orbit coupling between currents and magnetization in the bilayers.

Heavy fermion Ce3Co4Sn13 compound under pressure

The non-magnetic heavy fermion compound Ce3Co4Sn13 was studied under pressure. We report single crystalline measurements of electrical resistivity as a function of temperature ρ(T) under pressure. Some characteristic features related to a structural transition (TS ), crystalline field effects (TCEF ), and a low temperature maximum (Tmax ), possibly connected simultaneously to the onset of Kondo lattice coherence and short range magnetic correlations, were identified in the ρ(T) data. A pressure-temperature phase diagram with TS and Tmax was constructed by mapping these features. Like for most Ce-based heavy fermion compounds, Tmax moves to higher temperatures with pressure, indicating that it is related to the Kondo energy scale, due to the increase of hybridization induced by pressure. On the other hand, TS , associated to a superlattice distortion and probably combined with a charge density wave transition, decreases as a function of pressure. However, differently from the Sr3− x Ca x Ir4Sn13 system, where a superlattice quantum phase transition is observed [L. E. Klintberg et al., Phys. Rev. Lett. 109, 237 008 (2012)], in Ce3Co4Sn13 TS  ∼ 154 K, at ambient pressure (P = 0), seems to stabilize at around 143 K for P ≥ 19 kilobars. We also investigated ρ(T) in external magnetic fields, at P = 0. Negative magnetoresistance and increase of Tmax are observed, suggesting suppression of low temperature short range magnetic correlations.

Enhanced magnetic hysteresis in Ni-Mn-Ga single crystal and its influence on magnetic shape memory effect

Enhanced magnetic hysteresis due to boron doping in combination with magnetic shape memory effect in Ni-Mn-Ga single crystal results in new interesting functionality of magnetic shape memory (MSM) alloys such as mechanical demagnetization. In Ni50.0Mn28.5Ga21.5 single crystal, the boron doping increased magnetic coercivity from few Oe to 270 Oe while not affecting the transformation behavior and 10 M martensite structure. However, the magnetic field needed for MSM effect also increased in doped sample. The magnetic behavior is compared to undoped single crystal of similar composition. The evidence from the X-ray diffraction, magnetic domain structure, magnetization loops, and temperature evolution of the magnetic coercivity points out that the enhanced hysteresis is caused by stress-induced anisotropy.

Computer learns to play classic video games

Physics Today Daily Edition - 26 February 2015

BBC: A computer program developed by Google DeepMind has learned how to play 49 classic Atari video games. In about half the games, it was able to match the abilities of a professional human player. What makes this achievement significant is that the program was not specifically designed to play the games. Instead, it was given only the basic information needed to play them: the raw pixels on the screen and the goal of getting a high score. From that information the program could be presented with any of the games and, in the course of a few hours, learn to play the game with varying levels of success.

Surprisingly large quasar dates from very early universe

Physics Today Daily Edition - 26 February 2015

Los Angeles Times: An object spotted by the Sloan Digital Sky Survey appears to be a black hole 12 billion solar masses in size. Its redshift suggests it formed when the universe was only 875 million years old. Of the known black holes formed in the universe's first billion years, it is by far the most massive and luminous. This black hole is also a quasar and is pulling in so much of the surrounding material that massive amounts of radiative energy are being released. And that's what makes it unusual. Normally, the pressure from that radiation is expected to gradually slow the rate at which material falls into a black hole. The finding that the black hole reached such a great size in such a short period of time challenges current understanding. Further observations of this black hole could provide more clues into black hole formation and evolution. As the light it emits passes through the material in the intergalactic medium,  which was much denser 13 billion years ago, that light could also provide information about the growth of the universe itself.

Magnetic phase transitions and monopole excitations in spin ice under uniaxial pressure: A Monte Carlo simulation

In this work, we explore the spin ice model under uniaxial pressure using the Monte Carlo simulation method. For the known spin ices, the interaction correction (δ) introduced by the uniaxial pressure varies in quite a wide range from positive to negative. When δ is positive, the ground state characterized by the ferromagnetic spin chains is quite unstable, and in real materials it serves as intermediate state connecting the ice state and the long range ordered dipolar spin ice ground state. In the case of negative δ, the system relaxes from highly degenerate ice state to ordered ferromagnetic state via a first order phase transition. Furthermore, the domain walls in such ferromagnetic state are the hotbed of the excitations of magnetic monopoles, thus indicating that the uniaxial pressure can greatly increase the monopole density.

Towards nonvolatile magnetic crossbar arrays: A three-dimensional-integrated field-coupled domain wall gate with perpendicular anisotropy

A novel three-dimensional (3D)-integrated domain wall gate (DWG) providing direct control of the DW propagation in a magnetic nanowire with perpendicular magnetic anisotropy is presented. The pinning of field-driven DWs in a notch is controlled by the fringing fields of a subjacent gate magnet. Depending on the state of the gate magnet, the propagating DW is either pinned in the notch or its movement is supported. Theoretical calculations of controlled DW pinning are shown. We experimentally demonstrate control of the DW motion in magnetic nanowires by other domains in a separated gate layer using magnetic force microscopy. Regarding potential applications, the 3D DWG enables storing and buffering of magnetic domains in order to control the signal flow of 3D-integrated perpendicular nanomagnetic logic. Furthermore, due to the uniformity of input, output, and gate contact, the 3D DWG can act as nonvolatile logic device in order to realize field-driven logic gates and magnetic crossbar arrays.

Metastable bcc phase formation in 3d ferromagnetic transition metal thin films sputter-deposited on GaAs(100) substrates

Co100− x Fe x and Ni 100− y Fe y (at. %, x = 0–30, y = 0–60) films of 10 nm thickness are prepared on GaAs(100) substrates at room temperature by using a radio-frequency magnetron sputtering system. The detailed growth behavior is investigated by in-situ reflection high-energy electron diffraction. (100)-oriented Co and Ni single-crystals with metastable bcc structure are formed in the early stage of film growth, where the metastable structure is stabilized through hetero-epitaxial growth. With increasing the thickness up to 2 nm, the Co and the Ni films start to transform into more stable hcp and fcc structures through atomic displacements parallel to bcc{110} slide planes, respectively. The stability of bcc phase is improved by adding a small volume of Fe atoms into a Co film. The critical thickness of bcc phase formation is thicker than 10 nm for Co100− x Fe x films with x ≥ 10. On the contrary, the stability of bcc phase for Ni-Fe system is less than that for Co-Fe system. The critical thicknesses for Ni 100− y Fe y films with y = 20, 40, and 60 are 1, 3, and 5 nm, respectively. The Co100− x Fe x single-crystal films with metastable bcc structure formed on GaAs(100) substrates show in-plane uniaxial magnetic anisotropies with the easy direction along GaAs[011], similar to the case of Fe film epitaxially grown on GaAs(100) substrate. A Co100− x Fe x film with higher Fe content shows a higher saturation magnetization and a lower coercivity.

Modifying exchange-spring behavior of CoPt/NiFe bilayer by inserting a Pt or Ru spacer

We herein explore the possibility of obtaining tunable tilted magnetic anisotropy in ordered-CoPt (5 nm)/NiFe(tNiFe) bilayers through modifying their exchange spring behavior by inserting Pt and Ru-spacers. The tuning process of tilt angle magnetization of NiFe-layer was systematically investigated by varying the Pt or Ru thickness (tPt or tRu) from 0 to 8 nm at different thicknesses of NiFe (tNiFe = 1.5, 4.0, and 6.0 nm). Polar magneto-optic Kerr effect (p-MOKE) studies reveal that the bilayers grown in absence of spacers exhibit almost a rectangular hysteresis loop. With the insertion of Pt-spacer, the loop becomes more and more tilted as tPt increases; whereas, in the case of Ru-spacer, the nature of the loops is not simply changing in one direction. The estimated SQR⊥ (= θ r/θ s) values from the p-MOKE loops are found to monotonically decrease with increasing tPt when tPt ≦ 4 nm. In contrast, in the case of Ru-spacer, an oscillatory behavior for the SQR⊥ values is apparent when tRu ≦ 4 nm. As a result, an oscillatory tilted angle of NiFe spin configuration was obtained in the case of Ru-spacer; while a decoupling effect was prominent for the Pt-spacer. The results of present study reveal that the insertion of Pt and Ru-spacers as an appropriate means for realizing tunable tilted magnetic anisotropy in the CoPt/NiFe exchange springs.

Complete vertical M-H loop shift in La0.7Sr0.3MnO3/SrRuO3 thin film heterostructures

In the current work, we have epitaxially integrated La0.7Sr0.3MnO3/SrRuO3 (LSMO/SRO) BLs with the technologically important substrate Si (100) using pulsed laser deposition. Interestingly, at 4 K, under the magnetic field sweep of ±1500 Oe, a complete vertical M-H loop shift is observed in the sample prepared with 180 nm SRO thickness, which is unusual. This vertical shift persists even up to a field sweep range of ±6000 Oe, at which point the shift disappears and a symmetrical hysteresis loop centered at the origin is observed. In contrast, at the same temperature, under the same field sweep range, we observe a normal M-H loop (no or little vertical shift) from the sample with 45 nm SRO thickness. In both the cases, the LSMO thickness was held constant at ∼100 nm. It appears that SRO moment is frozen in place in the latter case, providing a clear demonstration of the effect that biasing layer (SRO) thickness can have on the magnetic characteristics of bilayer films. We attribute this vertical shift to the strong interplay between the uniaxial magnetocrystalline anisotropy and microscopic interface domain structure.

Effect of Ru thickness on spin pumping in Ru/Py bilayer

We report the effect of Ru thickness (t Ru) on ferromagnetic resonance (FMR) line-width of Ru(t Ru)/Py(23 nm) bilayer samples grown on Si(100)/SiO2 substrates at room temperature by magnetron sputtering. The FMR line-width is found to vary linearly with frequency for all thicknesses of Ru, indicating intrinsic origin of damping. For Ru thicknesses below 15 nm, Gilbert-damping parameter, α is almost constant. We ascribe this behavior to spin back flow that is operative for Ru thicknesses lower than the spin diffusion length in Ru, λsd. For thicknesses >15 nm (>λsd), the damping constant increases with Ru thickness, indicating spin pumping from Py into Ru.

Building An Earthquake Resistant Home

Inside Science - 26 February 2015

Stanford team develops design modification for a more structurally sound home.

General integrable n-level, many-mode Janes-Cummings-Dicke models and classical r-matrices with spectral parameters

Using the technique of classical r-matrices and quantum Lax operators, we construct the most general form of the quantum integrable “n-level, many-mode” spin-boson Jaynes-Cummings-Dicke-type hamiltonians describing an interaction of a molecule of N n-level atoms with many modes of electromagnetic field and containing, in general, additional non-linear interaction terms. We explicitly obtain the corresponding quantum Lax operators and spin-boson analogs of the generalized Gaudin hamiltonians and prove their quantum commutativity. We investigate symmetries of the obtained models that are associated with the geometric symmetries of the classical r-matrices and construct the corresponding algebra of quantum integrals. We consider in detail three classes of non-skew-symmetric classical r-matrices with spectral parameters and explicitly obtain the corresponding quantum Lax operators and Jaynes-Cummings-Dicke-type hamiltonians depending on the considered r-matrix.

Coupling capacitance between double quantum dots tunable by the number of electrons in Si quantum dots

Tunability of capacitive coupling in the Si double-quantum-dot system is discussed by changing the number of electrons in quantum dots (QDs), in which the QDs are fabricated using pattern-dependent oxidation (PADOX) of a Si nanowire and multi-fine-gate structure. A single QD formed by PADOX is divided into multiple QDs by additional oxidation through the gap between the fine gates. When the number of electrons occupying the QDs is large, the coupling capacitance increases gradually and almost monotonically with the number of electrons. This phenomenon is attributed to the gradual growth in the effective QD size due to the increase in the number of electrons in the QDs. On the other hand, when the number of electrons changes in the few-electron regime, the coupling capacitance irregularly changes. This irregularity can be observed even up to 40 electrons. This behavior is attributable the rough structure of Si nano-dots made by PADOX. This roughness is thought to induce complicated change in the electron wave function when an electron is added to or subtracted from a QD.

Growth of very large InN microcrystals by molecular beam epitaxy using epitaxial lateral overgrowth

Very thick InN (∼40 μm) was grown by molecular beam epitaxy using the epitaxial lateral overgrowth (ELO) technique. In some regions, the ELO of InN was observed as expected, indicating an important step toward fabricating quasi-bulk InN substrates. Interestingly, most parts of the sample consist of large flat-topped microcrystals and well-faceted microstructures. This is likely due to local growth condition variations during ELO, which is supported by an experiment where ELO of InN was performed on a substrate with various stripe mask patterns. TEM characterization of a flat top InN microcrystal revealed few stacking faults and only related threading dislocations. Defect-free small faceted microcrystals were also observed. The thick InN crystals show a narrow photoluminescence spectrum with a peak at 0.679 eV and linewidth of 16.8 meV at 4 K.

Effects of lithium doping on microstructure, electrical properties, and chemical bonds of sol-gel derived NKN thin films

Highly (100/110) oriented lead-free Li x (Na 0.5K0.5) 1 − x NbO3 (LNKN, x = 0, 0.02, 0.04, and 0.06) thin films are fabricated on Pt/Ti/SiO2/Si substrates via a sol-gel processing method. The lithium (Li) dopants modify the microstructure and chemical bonds of the LNKN films, and therefore improve their electrical properties. The optimal values of the remnant polarization (P r = 14.3 μC/cm2), piezoelectric coefficient (d 33 = 48.1 pm/V), and leakage current (<10−5 A/cm2) are obtained for a lithium addition of x = 0.04 (i.e., 4 at. %). The observation results suggest that the superior electrical properties are the result of an improved crystallization, a larger grain size, and a smoother surface morphology. It is shown that the ion transport mechanism is dominated by an Ohmic behavior under low electric fields and the Poole-Frenkel emission effect under high electric fields.

Enhancement in the structure quality of ZnO nanorods by diluted Co dopants: Analyses via optical second harmonic generation

We report a systematic study about the effect of cobalt concentration in the growth solution over the crystallization, growth, and optical properties of hydrothermally synthesized Zn 1−xCoxO [0 ≤ x ≤ 0.40, x is the weight (wt.) % of Co in the growth solution] nanorods. Dilute Co concentration of 1 wt. % in the growth solution enhances the bulk crystal quality of ZnO nanorods, and high wt. % leads to distortion in the ZnO lattice that depresses the crystallization, growth as well as the surface structure quality of ZnO. Although, Co concentration in the growth solution varies from 1 to 40 wt. %, the real doping concentration is limited to 0.28 at. % that is due to the low growth temperature of 80 °C. The enhancement in the crystal quality of ZnO nanorods at dilute Co concentration in the solution is due to the strain relaxation that is significantly higher for ZnO nanorods prepared without, and with high wt. % of Co in the growth solution. Second harmonic generation is used to investigate the net dipole distribution from these coatings, which provides detailed information about bulk and surface structure quality of ZnO nanorods at the same time. High quality ZnO nanorods are fabricated by a low-temperature (80 °C) hydrothermal synthesis method, and no post synthesis treatment is needed for further crystallization. Therefore, this method is advantageous for the growth of high quality ZnO coatings on plastic substrates that may lead toward its application in flexible electronics.


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