Spin-dependent electron transport through the ferromagnet/semiconductor interface induced by photon excitation

Circularly polarized light was used to excite electrons with a spin polarization perpendicular to the film plane in 3 nm Au/5 nm Co/GaAs (110) structures. At perpendicular saturation, the bias dependence of the photocurrent was observed to change in the range around 0.7 eV, corresponding to the Schottky barrier height. The photocurrent is observed to change significantly as a function of the magnetization direction with respect to the photon helicity, indicating spin-dependent transport between the semiconductor and the ferromagnetic layer at room temperature

Spin-polarized electron transport processes at the ferromagnet/semiconductor interface

Circularly polarized light was used to excite electrons with a spin polarization perpendicular to the film plane in ferromagnet/semiconductor hybrid structures. The Schottky characteristics at the interface were varied by using NiFe, Co and Fe as the ferromagnet. The Schottky characteristics were clearly observed with NiFe and Co/GaAs, while almost ohmic I-V characteristics were seen with Fe/GaAs. At negative bias a helicity-dependent photocurrent, dependent upon the magnetization configuration of the film and the Schottky barrier height, was detected upon modulating the polarization from right to left circular, For the magnetization along or perpendicular to the surface normal, the helicity-dependent photocurrent In or I 0, respectively, was measured. The asymmetry P=(In-I0)/(In+I0) of the helicity-dependent photocurrent decreases upon increasing the doping density of the GaAs substrates. P also decreases with photon energy h¿ as found for the polarization of photoexcited electrons in GaAs. In NiFe/GaAs samples for h¿=1.59 eV, P=16% for n+=1023 m-3 and P=-23% for p-=1025 m-3 doped substrates, i.e. P is comparable in magnitude to the theoretically predicted spin polarization of 50% for the optically pumped conduction band in GaAs. The results provide unambiguous evidence of spin-polarized electron transport through the ferromagnet/semiconductor interface and show that the Schottky barrier height controls the spin-polarized electron current passing from the semiconductor to the ferromagnet. The asymmetry data indicates that spin-polarized electrons are transmitted from the semiconductor to the ferromagnet with a high efficiency

Pseudo-Hall effect and anisotropic magnetoresistance in a micronscale Ni80Fe20 device

The pseudo-Hall effect (PHE) and anisotropic magnetoresistance (AMR) in a micronscale Ni80Fe20, six-terminal device, fabricated by optical lithography and wet chemical etching from a high quality UHV grown 30 Angstrom Au/300 Angstrom Ni80Fe20 film, have been studied. The magnetisation reversal in different parts of the device has been measured using magneto-optical Kerr effect (MOKE), The device gives a 50% change in PHE voltage with an ultrahigh sensitivity of 7.3%Oe(-1) at room temperature. The correlation between the magnetisation, magneto-transport properties, lateral shape of the device and directions of the external applied field is discussed based on extensive MOKE, AMR and PHE results

Influence of lateral geometry on magnetoresistance and magnetisation reversal in Ni80Fe20 wires

The magnetisation reversal processes and magnetoresistance behaviour in micron-sized Ni80Fe20 wires with triangular and rectangular modulated width have been studied. The wires were fabricated by electron beam lithography and a lift-off process. A combination of magnetic force microscopy (MFM), magneto-optical Kerr effect (MOKE) and magnetoresistance (MR) measurements shows that the lateral geometry of the wires greatly influences the magnetic and transport properties. The width modulations modify not only the shape-dependent demagnetising fields, but also the current density. The correlation between the lateral geometry, the magnetic and the transport properties is discussed based on MFM, MOKE and MR results

Magnetic domain studies of permalloy wire-based structures with junctions

Permalloy (Ni-80 Fe-20) wire-based structures (30 nm thick and 1 . w . 10 mum wide) with junctions (crosses, networks, H-shapes, rectangular chains and ring chains) prepared on a GaAs (100) substrate were observed in both their demagnetized and remanent states by magnetic force microscopy (MFM) in order to investigate the role of junction geometry in domain formation, Except in ring chains, two classes of domain configuration are found at the junction: (i) a domain wall-like feature due to abrupt spin rotation and (ii) a triangle-shape domain consistent with a flux closure configuration, Ring chains, on the other hand, form vortex domains at every other junction. The MFM observations are compared with micromagnetic calculations which qualitatively support the magnetic domain configurations

Human resources needs for universal access to antiretroviral therapy in South Africa: a time and motion study

<p>Background - Although access to life-saving treatment for patients infected with HIV in South Africa has improved substantially since 2004, treating all eligible patients (universal access) remains elusive. As the prices of antiretroviral drugs have dropped over the past years, availability of human resources may now be the most important barrier to achieving universal access to HIV treatment in Africa. We quantify the number of HIV health workers (HHWs) required to be added to the current HIV workforce to achieve universal access to HIV treatment in South Africa, under different eligibility criteria.</p> <p>Methods - We performed a time and motion study in three HIV clinics in a rural, primary care-based HIV treatment program in KwaZulu-Natal, South Africa, to estimate the average time per patient visit for doctors, nurses, and counselors. We estimated the additional number of HHWs needed to achieve universal access to HIV treatment within one year.</p> <p>Results - For universal access to HIV treatment for all patients with a CD4 cell count of ≤350 cells/μl, an additional 2,200 nurses, 3,800 counselors, and 300 doctors would be required, at additional annual salary cost of 929 million South African rand (ZAR), equivalent to US$141 million. For universal treatment (‘treatment as prevention’), an additional 6,000 nurses, 11,000 counselors, and 800 doctors would be required, at an additional annual salary cost of ZAR 2.6 billion (US$ 400 million).</p> <p>Conclusions - Universal access to HIV treatment for patients with a CD4 cell count of ≤350 cells/μl in South Africa may be affordable, but the number of HHWs available for HIV treatment will need to be substantially increased. Treatment as prevention strategies will require considerable additional financial and human resources commitments.</p&gt

Magnetic domain evolution in permalloy mesoscopic dots

Permalloy (Ni80Fe20) squares (30 nm thick and w mu m wide; 1 less than or equal to w less than or equal to 200 mu m) and circular disks (30 nm thick and r mu m diameter; 1 less than or equal to r less than or equal to 200 mu m) prepared on a GaAs (100) substrate were observed in both their demagnetized and remanent states by magnetic force microscopy (MFM) associated with non-contact atomic force microscopy (NC-AFM). The squares (2 less than or equal to w mu m) exhibited conventional closure domains and the corner plays a very important role in creating new walls. The circular disks, on the other hand, formed either vortex domain (5 less than or equal to r less than or equal to 20 mu m) or multi-domain (50 less than or equal to r mu m) states, The magnetization rotation is observed by MFM to change according to the size and shape of the elements, The MFM observations are supported by micromagnetic calculations which confirm the effect of the corner on the domain wall formation

Magnetization reversal in mesoscopic Ni80Fe20 wires: A magnetic domain launching device

The magnetization reversal process in mesoscopic permalloy (Ni80Fe20) wire structures has been investigated using scanning Kerr microscopy, magnetic force microscopy (MFM) and micromagnetic calculations. We find that the junction offers a site for reversed domain wall nucleation in the narrow part of the wires. As a consequence, the switching field is dominated by the domain nucleation field and the junction region initiates reversal by the wall motion following the nucleation of domains. Our results suggest the possibility of designing structures that can be used to “launch” reverse domains in narrow wires within a controlled field rang

Ferromagnetic/III-V semiconductor heterostructures and magneto-electronic devices

The interface magnetic and electronic properties of two Fe/III-V semiconductor systems, namely Fe/GaAs and Fe/InAs, grown at room temperature have been studied. A "magnetic interface", which is essential for the fabrication of magneto-electronic (ME) devices, was realized in both Fe/GaAs and Fe/InAs systems with suitable substrate processing and growth conditions. Furthermore, Fe/InAs was shown to have favorable interface electronic properties as Fe forms a low resistance ohmic contact on InAs. Two prototypes of ME device based on Fe/InAs are also discussed

Magnetization reversal and magnetic anisotropy in Co network nanostructures

The magnetization reversal and magnetic anisotropy in Co network structures have been studied using magneto-optic Kerr effect (MOKE). An enhancement of the coercivity is observed in the network structures and is attributed to the pinning of domain walls by the hole edges in the vicinity of which the demagnetizing field spatially varies. We find that the magnetization reversal process is dominated by the intrinsic unaxial anisotropy (2K(u)/M(s)approximate to 200 Oe) in spite of the shape anisotropy induced by the hole edges. The influence of the cross-junction on the competition between the intrinsic uniaxial anisotropy and the induced shape anisotropy is discussed using micromagnetic simulations