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To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.icarus.2007.04.025 Byline: Lon L. Hood (a), Nicola C. Richmond (a)(b), Keith P. Harrison (c), Robert J. Lillis (d) Keywords: Mars; Magnetic fields; Mars; interior; Mars; surface Abstract: Maps of the vector components of the martian crustal magnetic field over the strongly magnetized Terra Cimmeria/Sirenum region are constructed using Mars Global Surveyor magnetometer data. Although pronounced east-west trending anomalies are present on the radial and north field component maps at the mapping altitude ([approximately equal to]360-380 km), these trends are much less prominent at the lower aerobraking altitude ([approximately equal to]90-150 km). Comparisons with similar maps produced using artificial data at the aerobraking altitude indicate that elongated sources in this region may have maximum lengths along the martian surface of [approximately equal to]500 km and maximum aspect ratios of [approximately equal to]2. Iterative forward modeling of several relatively isolated anomalies in the mapped region yields paleomagnetic pole positions consistent with those estimated in previous studies of other anomalies using mapping phase and science phasing orbit data. On this basis, it is inferred that sources in the studied region are most probably magnetized primarily in northward or southward directions. Using this additional constraint, iterative forward modeling is then applied to determine a magnetization distribution that is consistent with data at both the aerobraking altitude and the mapping altitude. The model magnetization distribution, which includes 41 discrete sources, again indicates no highly elongated sources. An examination of surface geology in the region as well as a consideration of the global distribution of anomalies suggests that magmatic intrusions (e.g., subsurface dike swarms), cooling in the presence of water, are the most likely sources of the magnetic anomalies. Author Affiliation: (a) Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721, USA (b) Planetary Science Institute, 1700 East Fort Lowell, Suite 106, Tucson, AZ 85719, USA (c) Southwest Research Institute, 1050 Walnut Street, Suite 400, Boulder, CO 80309, USA (d) Space Sciences Laboratory, University of California, Berkeley, CA 94720, USA Article History: Received 30 November 2006; Revised 11 April 2007

A multilayer magnetic quantum dot sample containing 5 layers of Fe quantum dots (QDs) embedded in six layers of ZnS spacer was grown by molecular beam epitaxy (MBE). High-resolution transmission electron microscopy (HRTEM) observations reveal that the Fe QDs are single crystalline with spherical shape of diameters around 3 to 4 nm and area density of 1.5 x [10.sup.12] [cm.sup.-2]. Its zero-field cooled (ZFC) and field cooled (FC) curves measured at low field (100 Oe) show the magnetic relaxation effect with a blocking temperature around 26 K. The hysteresis loop measured at 5 K shows a coercivity of 83 Oe, confirming the slow relaxation process and coercivity enhancement attributed to the nanoparticle nature of the sample. Index Terms--Magnetic anisotropy, quantum dots.

This paper deals with micromagnetic model of magnetization reversal in nano scale-point contact devices driven by nonuniform injection of a spin-polarized current. A computational study of the magnetization reversal in the nanosecond regime will be presented considering the influence of the current density distribution below the aperture region on the reversal time. For high current values, a strong dependence of the reversal time on the current distribution has been observed. Finally, results of micromagnetic simulations show that the reversal time versus current behaviour (at T = OK) is monotonic, very different from the switching processes observed in standard spin valves and magnetic tunnel junctions with uniform current injection. Index Terms--Nanosecond regime, nano-point contacts, spin polarized current.

This paper interprets and reproduces, by means of full micromagnetic simulations, the pioneering experimental data on magnetization dynamics driven by spin polarized current of the experiment by Kiselev et al. The effect of the spatial dependence of the polarization function together with either nonuniform magnetostatic coupling from the fixed layer and classical Ampere field are shown to play a fundamental role in the magnetization dynamics. A detailed study of the stable magnetization self-oscillations shows that for high field and high current regimes, the dynamics is localized in the sides of the structure, where the energy dissipated by damping and the energy provided by the spin flow compensate exactly. Index Terms--Magnetization dynamics, micromagnetic modeling, nanopillar, spin torque.

Using micromagnetics, we demonstrate that the RF field produced by a spin valve can be used to reverse the magnetization in a magnetic nanoparticle. The RF field is generated using a current that specifically excites a uniform spin wave in the spin valve. This current is swept such that the chirped-frequency generated by the valve matches the angular-dependent resonant frequency of the anisotropy-dominated magnetic nanoparticle, as a result of which the magnetization reversal occurs. The switching is fast, requires currents similar to those used in recent experiments with spin valves, and is stable with respect to small perturbations. This phenomenon can potentially be employed in magnetic information storage devices or recently discussed magnetic computing schemes. Index Terms--Ferromagnetic resonance, micromagnetics, spintronics.

Aiming at a micromagnetic model that describes the macroscopic material behavior starting from microstructural features, there is a need for efficient time stepping schemes for the integration of Landau-Lifshitz-equation in material samples with dimensions of order [micro]m. This paper presents two related semianalytical time schemes. The time stepping algorithms are compared with other explicit time stepping schemes. Here, memory efficiency, time efficiency, convergence, and precision are checked. Further, special attention goes to 1) preservation of the magnetization magnitude in each FD cell; 2) a nonincrease of the free energy when applying a constant field; and 3) conservation of the systems free energy in the case of zero damping. Index Terms--Hysteresis, Landau-Lifshitz equation, micromagnetics, semianalytical integration.

We present the results of Monte Carlo simulations of the 3-D Heisenberg model with twisted boundary conditions. The boundaries are chosen to have a saturated magnetization in equal and opposite directions which imposes a twist on the order parameter profile centered at the midpoint of the sample. The magnetization profiles are distinct from mean field theory. We present a detailed examination of the magnetization profile and suggest experiments which can verify the finite size scaling forms we observe. Index Terms--Critical phenomenon, domain walls, ferromagnetism, Monte Carlo.

We study the anisotropic magnetoresistance (AMR) of a 2-D periodic square array of connected permalloy rings with periodicity of 1 [micro]m combining experimental and computational techniques. The computational model consists of two parts: 1) the computation of the magnetization and 2) the computation of the current density. For 1), we use standard micromagnetic methods. For 2), we start from a potential difference applied across the sample, compute the resulting electric potential, and subsequently the corresponding current density based on a uniform conductivity. We take into account the backreaction of the magnetoresistive effects onto the current density by self-consistently computing the current density and conductivity until they converge. We compare the experimentally measured AMR curve (as a function of the applied field) with the numerically computed results and find good agreement. The numerical data provides insight into the characteristics of the AMR data. Finally, we demonstrate the importance of taking into account the spatial variation of the current density when computing the AMR. Index Terms--Current density, finite element analysis, magnetoresistance, modeling, Nmag, numerical analysis.

The effect of surface anisotropy on the magnetization reversal in small magnetic particles is investigated. The model considers particles of cubic shape cut from a tetragonal crystal with cube faces in the (001) and equivalent planes. In particles having diameters of less than about 10 nm, the coercivity approaches the Stoner-Wohlfarth limit, but the anisotropy field differs from that of the bulk of the particles. With increasing particle size, the nucleation modes acquire the character of magnetic surface or bulk modes that reduce the coercivity. Index Terms--Coercivity, magnetic particles, nucleation, surface anisotropy.

Controllable remanent states have been studied on the microstructured magnetic tunnel junction (MTJ) rings through magnetoresistance measurements. These rings were designed accordingly with an outer/inner diameter of 2/1 and 1/0.5 [micro]m to reveal two and one metastable states, respectively, during the magnetization reversal process on the free layer. The distinct magnetoresistance levels based on the tunneling magnetoresistance effect are associated with the relative alignment of magnetization of free layer and pinned layer. As a result, four and three controllable remanent magnetic states on the free layer were manipulated by ramping external magnetic fields, applied in the biasing direction, with various field ranges, giving rise to four and three stable mangetoresistance values at zero field. These results may provide a great potential in magnetic multibit memory applications using ring-shaped cells. Index Terms--Magnetic tunneling junction (MTJ), magnetoresistance, multibit application, ring-shaped cells.

The magnetization distribution and the formation of magnetic domain wall (DW) in ferromagnetic metal wires with a nanoscale constriction have been investigated in details using the micromagnetic simulation. It is found that the angle of the nanoconstriction plays an important role in controlling the formation of the magnetic DW. For different ferromagnetic metals, NiFe, Ni, Fe and Co, the domain structures and formation of the DW are also found to be distinctly different. In the NiFe wires, the optimum constriction angle for a well defined head-to-head/tail-to-tail DW is around 10[degrees] while in Ni, it is around 14[degrees]. For large constriction angles in Fe and Co wires, the magnetizations across the nanocontact tend to align along the same direction without a DW. However, Fe and Co wires tend to form complex vortex magnetic domains or single domains in the wires and across the nanocontacts in sharp contrast with the NiFe and Ni wires of the same shape and size. Index Terms--Magnetic film, magnetization reversal process, simulation, spin configuration.