B. A. Jones

Large magnetic anisotropy of a single atomic spin embedded in a surface molecular network.

Magnetic anisotropy allows magnets to maintain their direction of magnetization over time. Using a scanning tunneling microscope to observe spin excitations, we determined the orientation and strength of the anisotropies of individual iron and manganese atoms on a thin layer of copper nitride. The relative intensities of the inelastic tunneling processes are consistent with dipolar interactions, as seen for inelastic neutron scattering. First-principles calculations indicate that the magnetic atoms become incorporated into a polar covalent surface molecular network in the copper nitride. These structures, which provide atom-by-atom accessibility via local probes, have the potential for engineering anisotropies large enough to produce stable magnetization at low temperatures for a single atomic spin.

Modification of the Landau-Lifshitz equation in the presence of a spin-polarized current in colossal- and giant-magnetoresistive materials

We derive a continuum equation for the magnetization of a conducting ferromagnet in the presence of a spin-polarized current. Current effects enter in the form of a topological term in the Landau-Lifshitz equation . In the stationary situation the problem maps onto the motion of a classical charged particle in the field of a magnetic monopole. The spatial dependence of the magnetization is calculated for a one-dimensional geometry and suggestions for experimental observation are made. We also consider time-dependent solutions and predict a spin-wave instability for large currents.

Non-Fermi Liquid Behavior and Griffiths Phase in {ital f} -Electron Compounds

We study the interplay among disorder, RKKY, and Kondo interactions in f -electron alloys. We argue that the non-Fermi liquid behavior observed in these systems is due to the existence of a Griffiths phase close to a quantum critical point. The existence of this phase provides a unified picture of a large class of materials. We also propose new experiments that can test these ideas. {copyright} {ital 1998} {ital The American Physical Society}

Reaching the magnetic anisotropy limit of a 3d metal atom

Maximizing atomic magnetic memory A study of the magnetic response of cobalt atoms adsorbed on oxide surfaces may lead to much denser storage of data. In hard drives, data are stored as magnetic bits; the magnetic field pointing up or down corresponds to storing a zero or a one. The smallest bit possible would be a single atom, but the magnetism of a single atom —its spin—has to be stabilized by interactions with heavy elements or surfaces through an effect called spin-orbit coupling. Rau et al. (see the Perspective by Khajetoorians and Wiebe) built a model system in pursuit of single-atom bits—cobalt atoms adsorbed on magnesium oxide. At temperatures approaching absolute zero, the stabilization of the spins magnetic direction reached the maximum that is theoretically possible. Science, this issue p. 988; see also p. 976 A cobalt atom bound to a single oxygen site on magnesia has the maximum magnetic anisotropy allowed for a transition metal [Also see Perspective by Khajetoorians and Wiebe] Designing systems with large magnetic anisotropy is critical to realize nanoscopic magnets. Thus far, the magnetic anisotropy energy per atom in single-molecule magnets and ferromagnetic films remains typically one to two orders of magnitude below the theoretical limit imposed by the atomic spin-orbit interaction. We realized the maximum magnetic anisotropy for a 3d transition metal atom by coordinating a single Co atom to the O site of an MgO(100) surface. Scanning tunneling spectroscopy reveals a record-high zero-field splitting of 58 millielectron volts as well as slow relaxation of the Co atom’s magnetization. This striking behavior originates from the dominating axial ligand field at the O adsorption site, which leads to out-of-plane uniaxial anisotropy while preserving the gas-phase orbital moment of Co, as observed with x-ray magnetic circular dichroism.

Non-Fermi-liquid behavior in U and Ce alloys: Criticality, disorder, dissipation, and Griffiths-McCoy singularities

In this paper we provide a theoretical basis for the problem of Griffiths-McCoy singularities close to the quantum critical point for magnetic ordering in U and Ce intermetallics. We show that the competition between the Kondo effect and Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction can be expressed in Hamiltonian form, and that the dilution effect due to alloying leads to a quantum percolation problem driven by the number of magnetically compensated moments. We argue that the exhaustion paradox proposed by Nozi\`eres is explained when the RKKY interaction is taken into account. We revisit the one- and two-impurity Kondo problem, and show that in the presence of particle-hole symmetry-breaking operators the system flows to a line of fixed points characterized by coherent (clusterlike) motion of the spins. Moreover, close to the quantum critical point, clusters of magnetic atoms can quantum mechanically tunnel between different states either via the anisotropy of the RKKY interaction or by what we call the cluster Kondo effect. We calculate explicitly from the microscopic Hamiltonian the parameters which appear in all the response functions. We show that there is a maximum number

CONFORMAL-FIELD-THEORY APPROACH TO THE TWO-IMPURITY KONDO PROBLEM : COMPARISON WITH NUMERICAL RENORMALIZATION-GROUP RESULTS

{N}_{c}

Evidence for a Common Physical Description of Non-Fermi-Liquid Behavior in Chemically Substituted f -Electron Systems

of spins in the clusters such that, above this number, tunneling ceases to occur. These effects lead to a distribution of cluster Kondo temperatures which vanishes for finite clusters, and therefore leads to strong magnetic response. From these results we propose a dissipative quantum droplet model which describes the critical behavior of metallic magnetic systems. This model predicts that in the paramagnetic phase there is a crossover temperature

Origin of Perpendicular Magnetic Anisotropy and Large Orbital Moment in Fe Atoms on MgO

{T}^{*},

Theory of exchange coupling in magnetic multilayers

above which Griffiths-McCoy-like singularities with magnetic susceptibility

First-principles calculation of the single impurity surface Kondo resonance.

\ensuremath{\chi}(T)\ensuremath{\propto}{T}^{\ensuremath{-}1+\ensuremath{\lambda}}