The spin transfer torque is a phenomenon in which angular momentum of a spin polarized electrical current entering a ferromagnet is transferred to the magnetization. The effect has opened a new ...research field of electrically driven magnetization dynamics in magnetic nanostructures and plays an important role in the development of a new generation of memory devices and tunable oscillators. Optical excitations of magnetic systems by laser pulses have been a separate research field whose aim is to explore magnetization dynamics at short time scales and enable ultrafast spintronic devices. We report the experimental observation of the optical spin transfer torque, predicted theoretically several years ago building the bridge between these two fields of spintronics research. In a pump-and-probe optical experiment we measure coherent spin precession in a (Ga,Mn)As ferromagnetic semiconductor excited by circularly polarized laser pulses. During the pump pulse, the spin angular momentum of photo-carriers generated by the absorbed light is transferred to the collective magnetization of the ferromagnet. We interpret the observed optical spin transfer torque and the magnetization precession it triggers on a quantitative microscopic level. Bringing the spin transfer physics into optics introduces a fundamentally distinct mechanism from the previously reported thermal and non-thermal laser excitations of magnets. Bringing optics into the field of spin transfer torques decreases by several orders of magnitude the timescales at which these phenomena are explored and utilized.
We show that effective electrical control of the magnetic properties in the ferromagnetic semiconductor (Ga,Mn)As is possible using the strain induced by a piezoelectric actuator even in the limit of ...high doping levels and high Curie temperatures, where direct electric gating is not possible. We demonstrate very large and reversible rotations of the magnetic easy axis. We compare the results obtained from magneto-transport and SQUID magnetometry measurements, extracting the dependence of the piezo-induced uniaxial magnetic anisotropy constant upon strain in both cases and detailing the limitations encountered in the latter approach.
We present an experimental and theoretical study of magnetocrystalline anisotropies in arrays of bars patterned lithographically into (Ga,Mn)As epilayers grown under compressive lattice strain. ...Structural properties of the (Ga,Mn)As microbars are investigated by high-resolution X-ray diffraction measurements. The experimental data, showing strong strain relaxation effects, are in good agreement with finite element simulations. SQUID magnetization measurements are performed to study the control of magnetic anisotropy in (Ga,Mn)As by the lithographically induced strain relaxation of the microbars. Microscopic theoretical modeling of the anisotropy is performed based on the mean-field kinetic-exchange model of the ferromagnetic spin-orbit coupled band structure of (Ga,Mn)As. Based on the overall agreement between experimental data and theoretical modeling we conclude that the micropatterning induced anisotropies are of the magnetocrystalline, spin-orbit coupling origin.
We report experimental and theoretical studies of magnetic domain walls in an in-plane magnetized (Ga,Mn)As dilute moment ferromagnetic semiconductor. Our high-resolution electron holography ...technique provides direct images of domain wall magnetization profiles. The experiments are interpreted based on microscopic calculations of the micromagnetic parameters and Landau-Lifshitz-Gilbert simulations. We find that the competition of uniaxial and biaxial magnetocrystalline anisotropies in the film is directly reflected in orientation dependent wall widths, ranging from approximately 40 nm to 120 nm. The domain walls are of the Néel type and evolve from near-\(90^{\circ}\) walls at low-temperatures to large angle 1\(\bar{1}\)0-oriented walls and small angle 110-oriented walls at higher temperatures.
Vehicle-to-vehicle (VTV) wireless communications have many envisioned applications in traffic safety and congestion avoidance, but the development of suitable communications systems and standards ...requires accurate models for the VTV propagation channel. In this paper, we present a new wideband multiple-input-multiple-output (MIMO) model for VTV channels based on extensive MIMO channel measurements performed at 5.2 GHz in highway and rural environments in Lund, Sweden. The measured channel characteristics, in particular the nonstationarity of the channel statistics, motivate the use of a geometry-based stochastic channel model (GSCM) instead of the classical tapped-delay line model. We introduce generalizations of the generic GSCM approach and techniques for parameterizing it from measurements and find it suitable to distinguish between diffuse and discrete scattering contributions. The time-variant contribution from discrete scatterers is tracked over time and delay using a high resolution algorithm, and our observations motivate their power being modeled as a combination of a (deterministic) distance decay and a slowly varying stochastic process. The paper gives a full parameterization of the channel model and supplies an implementation recipe for simulations. The model is verified by comparison of MIMO antenna correlations derived from the channel model to those obtained directly from the measurements.
We demonstrate dynamic voltage control of the magnetic anisotropy of a (Ga,Mn)As device bonded to a piezoelectric transducer. The application of a uniaxial strain leads to a large reorientation of ...the magnetic easy axis which is detected by measuring longitudinal and transverse anisotropic magnetoresistance coefficients. Calculations based on the mean-field kinetic-exchange model of (Ga,Mn)As provide microscopic understanding of the measured effect. Electrically induced magnetization switching and detection of unconventional crystalline components of the anisotropic magnetoresistance are presented, illustrating the generic utility of the piezo voltage control to provide new device functionalities and in the research of micromagnetic and magnetotransport phenomena in diluted magnetic semiconductors.
The large saturation magnetization in conventional dense moment ferromagnets offers flexible means of manipulating the ordered state through demagnetizing shape anisotropy fields but these dipolar ...fields, in turn, limit the integrability of magnetic elements in information storage devices. We show that in a (Ga,Mn)As dilute moment ferromagnet, with comparatively weaker magnetic dipole interactions, locally tunable magnetocrystalline anisotropy can take the role of the internal field which determines the magnetic configuration. Experiments and theoretical modeling are presented for lithographically patterned microchannels and the phenomenon is attributed to lattice relaxations across the channels. The utility of locally controlled magnetic anisotropies is demonstrated in current induced switching experiments. We report structure sensitive, current induced in-plane magnetization switchings well below the Curie temperature at critical current densities 10^5 Acm^-2. The observed phenomenology shows signatures of a contribution from domain-wall spin-transfer-torque effects.
Joint time-variant channel estimation and multi-user detection are key building-blocks for wireless broadband communication for mobile users at vehicular speed. We propose an iterative receiver for a ...multi-carrier (MC) code division multiple access (CDMA) system in the uplink. Multi-user detection is implemented through iterative parallel interference cancellation and conditional linear minimum mean square error (MMSE) filtering. MC-CDMA is based on orthogonal frequency division multiplexing (OFDM), thus time-variant channel estimation can be performed for every subcarrier individually. The variation of a subcarrier over the duration of a data block is upper bounded by the maximum Doppler bandwidth which is determined by the maximum velocity of the users. We exploit results from the theory of time-concentrated and bandlimited sequences and apply a Slepian basis expansion for time-variant subcarrier estimation. This approach enables time-variant channel estimation without complete knowledge of the second-order statistics of the fading process. The square bias of the Slepian basis expansion is one order of magnitude smaller compared to the Fourier basis expansion. The square bias of the basis expansion is the determining factor for the performance of the iterative joint channel estimation and data detection. We present an iterative linear MMSE estimation algorithm for the basis expansion coefficients in a multi-user system. The consistent performance of the iterative receiver using the Slepian basis expansion is validated by simulations for a wide range of velocities
The use of London atomic orbitals (LAOs) in a nonperturbative manner enables the determination of gauge-origin invariant energies and properties for molecular species in arbitrarily strong magnetic ...fields. Central to the efficient implementation of such calculations for molecular systems is the evaluation of molecular integrals, particularly the electron repulsion integrals (ERIs). We present an implementation of several different algorithms for the evaluation of ERIs over Gaussian-type LAOs at arbitrary magnetic field strengths. The efficiencies of generalized McMurchie–Davidson (MD), Head-Gordon–Pople (HGP), and Rys quadrature schemes are compared. For the Rys quadrature implementation, we avoid the use of high precision arithmetic and interpolation schemes in the computation of the quadrature roots and weights, enabling the application of this algorithm seamlessly to a wide range of magnetic fields. The efficiency of each generalized algorithm is compared by numerical application, classifying the ERIs according to their total angular momenta and evaluating their performance for primitive and contracted basis sets. In common with zero-field integral evaluation, no single algorithm is optimal for all angular momenta; thus, a simple mixed scheme is put forward that selects the most efficient approach to calculate the ERIs for each shell quartet. The mixed approach is significantly more efficient than the exclusive use of any individual algorithm.
Background
More than 50% of colorectal cancer (CRC)–related deaths could be prevented with early detection and treatment. Federally Qualified Healthcare Centers (FQHCs)—safety-net facilities that ...provide primary care services to uninsured, underinsured and minority patients—report some of the lowest CRC screening rates. Effective CRC screening interventions within FQHCs may help to promote CRC screening utilization among patients from diverse backgrounds.
Focus of Article
This study was designed to explore average-risk, patient perceptions of facilitators and impediments of using the Fecal Immunochemical Test (FIT) kit for CRC screening within the social marketing framework of the 4 “P’s”—product, price, place, and promotion.
Research Aim
To conduct formative research with priority populations (i.e., diverse, FQHC patients), assess acceptability of the FIT kit, and identify ways to better meet preferences and needs.
Program Design/Approach
Qualitative, Focus Groups.
Importance to the Social Marketing Field
Findings from this study would aid in the future development of a community-based prevention marketing intervention to promote CRC screening within FQHCs that serve diverse patient populations.
Methods
Focus groups were conducted in English and Spanish with CRC screening age-eligible patients from a South Florida FQHC System. Interview guides were designed to better understand FQHC patient perceptions of CRC Screening via the FIT kit (product) by up-to-date and not up-to-date participants. Additional topics included likes/dislikes (price), preferred location for completing CRC screening and receiving CRC prevention education (place), and strategies for persuasive communication to increase CRC screening among patients who visit FQHCs (promotion). Data were analyzed to identify FIT screening facilitators and impediments using thematic coding in MaxQDA.
Results
Five focus groups, involving 36 participants aged 50 and older, were conducted. Over half of participants self-identified as Hispanic (36%) or non-Hispanic Black (28%). Key facilitators were the convenience and simplicity of at-home testing (product), presumed affordability of it compared to colonoscopy (price), and ability to return by mail (place). Participants also shared numerous strategies for promotion, including direct referral from Primary Care Providers, mass media campaigns, mailed or electronic reminders from the clinic, and peer advocacy events in churches.
Recommendations for Research or Practice
Social marketing provides an adequate approach for developing a patient-centered campaign to promote CRC screening uptake at FQHCs. Future CRC screening promotional campaigns at FQHCs should build on the motivation that patients must prioritize their health by emphasizing the convenience of completing CRC screening at home with the capability of mailing it back. Clinics should also work to reduce patient responsibility by developing automated clinic system reminders to openly communicate with patients about CRC screening.
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