Surveyor launch opportunities for mid-1965 through 1968

Lunar launch data - ascent characteristics of Atlas-Centaur AC-15 configuration and mission ground rules applied to models for Surveyor flight

Validity of the scattering length approximation in strongly interacting Fermi systems

We investigate the energy spectrum of systems of two, three and four spin-1/2 fermions with short range attractive interactions both exactly, and within the scattering length approximation. The formation of molecular bound states and the ferromagnetic transition of the excited scattering state are examined systematically as a function of the 2-body scattering length. Identification of the upper branch (scattering states) is discussed and a general approach valid for systems with many particles is given. We show that an adiabatic ferromagnetic transition occurs, but at a critical transition point kF a much higher than predicted from previous calculations, almost all of which use the scattering length approximation. In the 4-particle system the discrepancy is a factor of 2. The exact critical interaction strength calculated in the 4-particle system is consistent with that reported by experiment. To make comparisons with the adiabatic transition, we study the quench dynamics of the pairing instability using the eigenstate wavefunctions.Comment: 7 pages, 7 figure

Landau-Zener-Stueckelberg effect in a model of interacting tunneling systems

The Landau-Zener-Stueckelberg (LZS) effect in a model system of interacting tunneling particles is studied numerically and analytically. Each of N tunneling particles interacts with each of the others with the same coupling J. This problem maps onto that of the LZS effect for a large spin S=N/2. The mean-field limit N=>\infty corresponds to the classical limit S=>\infty for the effective spin. It is shown that the ferromagnetic coupling J>0 tends to suppress the LZS transitions. For N=>\infty there is a critical value of J above which the staying probability P does not go to zero in the slow sweep limit, unlike the standard LZS effect. In the same limit for J>0 LZS transitions are boosted and P=0 for a set of finite values of the sweep rate. Various limiting cases such as strong and weak interaction, slow and fast sweep are considered analytically. It is shown that the mean-field approach works well for arbitrary N if the interaction J is weak.Comment: 13 PR pages, 15 Fig

Major loop reconstruction from switching of individual particles

Major hysteresis loops of groups of isolated 60 mm square garnet particles of a regular two-dimensional array, have been measured magnetooptically. Individual loops for each particle were measured, and the statistics of the distribution of coercivities and interaction fields was determined. It is shown that from the measured coercivity distribution and calculated magnetostatic interaction fields the major hysteresis loop can be reconstructed. The switching sequence, and the major loop of an assembly of 535 particles were calculated numerically for two cases: first, when calculating the magnetostatic interaction, the 25 particles were assumed to be isolated; second, the major loop of the same 25 particles, embedded into a 939 square, was reconstructed taking into account the interactions among all 81 particles. The numerically simulated major hysteresis loops agree very well with the measured loops, demonstrating the reliability of numerical modeling

Distribution of magnetic domain pinning fields in GaMnAs ferromagnetic films

Using the angular dependence of the planar Hall effect in GaMnAs ferromagnetic films, we were able to determine the distribution of magnetic domain pinning fields in this material. Interestingly, there is a major difference between the pinning field distribution in as-grown and in annealed films, the former showing a strikingly narrower distribution than the latter. This conspicuous difference can be attributed to the degree of non-uniformity of magnetic anisotropy in both types of films. This finding provides a better understanding of the magnetic domain landscape in GaMnAs that has been the subject of intense debate

Tricritical behavior in itinerant quantum ferromagnets

It is shown that the peculiar features observed in the low-temperature phase diagrams of ZrZn_2, UGe_2, and MnSi can be understood in terms of a simple mean-field theory. The nature of the ferromagnetic transition changes from second order to first order at a tricritical point, and in a small external magnetic field surfaces of first-order transitions emerge which terminate in quantum critical points. This field dependence of the phase diagram follows directly from the existence of the tricritical point. The quantum critical behavior in a nonzero field is calculated exactly.Comment: 4pp., 4 eps figure

Hysteretic properties of a magnetic particle with strong surface anisotropy

We study the influence of surface anisotropy on the zero-temperature hysteretic properties of a small single-domain magnetic particle, and give an estimation of the anisotropy constant for which deviations from the Stoner-Wohlfarth model are observed due to non-uniform reversal of the particle's magnetisation. For this purpose, we consider a spherical particle with simple cubic crystalline structure, a uniaxial anisotropy for core spins and radial anisotropy on the surface. The hysteresis loop is obtained by solving the local (coupled) Landau-Lifschitz equations for classical spin vectors. We find that when the surface anisotropy constant is at least of the order of the exchange coupling, large deviations are observed with respect to the Stoner-Wohlfarth model in the hysteresis loop and thereby the limit-of-metastability curve, since in this case the magnetisation reverses its direction in a non-uniform manner via a progressive switching of spin clusters. In this case the critical field, as a function of the particle's size, behaves as observed in experiments.Comment: 12 pages, 15 eps figure