The Eighth Workshop on
SPIN POLARIZATION AND MAGNETIC
EFFECTS IN NANO-SYSTEMS

ABSTRACTS

Controlling the Curie Temperature and Coercive Field of Ferromagnetic
III-Mn-V Semiconductors by Annealing, Extrinsic Doping, and Multi-Layer Proximity Effects
J. K. Furdyna, Department of Physics, University of Notre Dame, Notre Dame, Indiana


Very significant strides have recently been made in developing ferromagnetic III-V-based semiconductors (e.g., GaMnAs, InMnAs, and GaMnSb) grown in thin layer form by low-temperature molecular beam epitaxy (MBE). These new materials have already opened a number of fundamental issues in magnetism and magneto-transport, and their development holds promise of integrating ferromagnetic (FM) and non-magnetic semiconductors, in new devices that depend on electron charge and on its spin [1]. For such future devices, it is essential to have to be able to control and manipulate the magnetic properties of the III-V-based FM semiconductor. I will briefly discuss the techniques used for fabricating these new materials, and the mechanisms that underlie their ferromagnetism. I will then focus on several methods for optimizing and controlling their magnetic properties -- especially their Curie temperature and coercive field. Specifically, I will discuss the effects achieved by interfacing these materials with other magnetic systems, such as II-Mn-VI magnetic semiconductors; by extrinsic doping, aimed at increasing the number of free holes in these alloys; by post-growth annealing; and by novel materials design, such as the formation of "digital alloys", where Mn-containing atomic layers are inserted periodically into the III-V crystal matrix during epitaxial growth.

Current Induced Magnetization Rotation via Spin Momentum Transfer
M. W. Covington. Seagate Research, 1251 Waterfront Place, Pittsburgh, PA

A large current density (~1x108 A/cm2) of spin polarized conduction electrons can exert a substantial torque on the localized spins in a nanoscale ferromagnet. This effect, referred to as spin momentum transfer, has been observed to induce switching or precession of the magnetization in pillars made from ferromagnetic/non-magnetic multilayers. However, a better quantitative measure of the magnitude and angular dependence of this torque will come from measurements of magnetization rotation by spin transfer. To that end, we present experimental results from current perpendicular to the plane (CPP) devices consisting of a multilayer of CoFe/Cu/CoFe/Ru/CoFe. For this system the device magnetoresistance is a measure of the relative angle between the two CoFe layers adjacent to the Cu interlayer. For ~100 nm diameter pillars, and for many bias fields, the resistance versus current displays a smooth, non-hysteretic change between parallel and antiparallel states that is indicative of magnetization rotation. Experimental data will be compared to Slonczewski's numerical calculations of the Landau-Lifshitz equation with the spin transfer term, and to a model that minimizes the free energy of layers having uniform magnetization. Finally, if time permits, experimental high frequency dynamical results will also be presented.

Direct measurements of Supermagnetism in Cobalt Nanoparticle Films
Roger Koch, IBM Research. Yorktown Heights, New York

This talk introduces the concept of using of magnetic noise to understand superparamagnetism in magnetic thin films. We have used a direct probe of superparamagnetism to determine the complete anisotropy energy distribution of a Co nanoparticle film. The films were composed of self-assembled lattices of uniform Co nanoparticles that were 3 or 5 nm in diameter. A variable temperature scanning-SQUID microscope was used to measure temperature-induced spontaneous magnetic noise in the samples. Accurate measurements of the anisotropy energy distribution of small volume samples were made. Knowledge of these distributions is critical in the magnetic design of nanoparticle devices and media.

The Physical Origin of Some of the Terms in the Landau-Lifshitz-Gilbert (LLG) Equation
Roger Koch, IBM Research, Yorktown Heights, New York

I will discuss our (1) experimental program to understand the physical origin of the damping term in the LLG equation in thin magnetic thin films and (2) our attempts to verify the proper term to add to the LLG equation in the presence of a non-equilibrium spin current. We find that in thin films of permalloy that the measured loss term (alpha) in the LLG equation correlates extremely well the electrical resistivity of the film as the thickness or microstructure is varied. It is generally accepted that the coupling of the d electrons to the s electrons in a conducting magnetic film is the source of magnetic loss, so one would expect a correlation with the electrical resistance. Our data is a striking demonstration of this correlation. Secondly, I will report on our experiments to observe the time response of a submicron thin film magnet when magnetic reversal occurs via non-equilibrium spin current. By measuring the reversal time, a qualitative determination of the proper term in the LLG equation to add to account for a non-equilibrium spin current can be unambiguously determined. )

Studies Of Magnetic Nanowire Arrays In Hexagonally Ordered Porous Alumina
L. E. Wenger, Department of Physics and Astronomy, Wayne State University, Detroit, Michigan

Interest in the fabrication and characterization of nanostructures has grown during the last decade due to a multitude of applications envisioned for these structures, ranging from high density magnetic recording media to magnetic sensors. From a more fundamental point of view, interest has focused on understanding their unusual magnetic properties, such as higher coercivities as compared to those of thin films or bulk materials, and of the magnetic reversal mechanism in these nanostructures. This talk will focus on the synthesis, structural characterization, and magnetic properties of Ni, Co, Fe, and Fe-Co nanowires electrodeposited in hexagonally ordered alumina with pore sizes ranging from 12 to 52 nm. Although the magnetic properties of these nanowires arrays are primarily determined by shape anisotropy, magnetic hysteresis and relaxation data support the hypothesis that localized nucleation volumes, not single domain nanowires, dominate the reversal of the magnetization and that the large magnetostatic coupling of the nanowires in the arrays has a serious impact on their utility for many applications.

Spin-Memory-Loss (Spin-Flipping) in Metals and at Metallic Interfaces
J. Bass, Department of Physics and Astronomy, Michigan State University, East Lansing , Michigan.

The original model by Fert in 1988 for Giant Magnetoresistance (GMR) in a ferromagnetic/non-magnetic (F/N) multilayer assumed that the spins of electrons never flip as the electrons traverse the multilayer. For the first several years of GMR studies, the spin-flip (spin-diffusion) lengths in both F- and N-metals were assumed to be long enough that any spin-flipping could be neglected. Since 1994, evidence has accumulated that spin-flipping occurs more frequently than initially suspected, both within metals and at metallic interfaces. In addition to its scientific interest, such spin-flipping has potential technological importance because it can reduce GMR. I'll review how spin-flipping lengths in metals and spin-flipping probabilities at interfaces are measured, and what we have learned so far about spin-flipping in F- and N-metals, at N1/N2 and F/N metal interfaces, and in superconducting metals.