A. M. Portis

Bio: A. M. Portis is an academic researcher from University of California, Berkeley. The author has contributed to research in topics: Resonance & Magnetic resonance force microscopy. The author has an hindex of 9, co-authored 9 publications receiving 848 citations.

Electronic Structure of F Centers: Saturation of the Electron Spin Resonance

Abstract: It is shown that the unusual observed saturation behavior of the microwave electron spin resonance associated with $F$ centers in KCl, NaCl, and KBr crystals can be accounted for if the overall width is ascribed to interaction between the $F$-center electrons and the nuclear magnetic moments of the ions adjacent to the $F$ centers. The measured saturation factor gives for $F$ centers in KCl a spin-lattice relaxation time of 2.5\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}5}$ sec at room temperature. The observed saturation behavior in which only the absorption saturates is in marked disagreement with the Kramers-Kronig relations. However it is shown, that the Kramers-Kronig relations are not applicable to saturated systems. Expressions which avoid the use of these relations are presented for saturable systems.

Electronic structure of f centers: hyperfine interactions in electron spin resonance

Abstract: It is shown that the observed width of the microwave electron spin resonance absorption lines associated with $F$ centers in KCl, NaCl, and KBr crystals can be attributed to hyperfine interactions between the $F$-center electron and the nuclear magnetic moments of the ions adjacent to the $F$ center. The width arises from the distribution of nuclear moment components. Theoretical calculations of the width are in good agreement with observation provided that the $F$-center wave function is treated as a linear combination of atomic orbitals; wave functions calculated on continuum models are shown to be unsatisfactory. The theory is confirmed by a comparison of observations on $F$ centers in crystals of ${\mathrm{K}}^{39}$Cl and ${\mathrm{K}}^{41}$Cl. If the width is attributed to interactions with the nearest sets of K and Cl ions, the experiments lead directly to quantitative values of the electronic charge density at the K and Cl nuclei: one finds ${|\ensuremath{\Psi}(\mathrm{K})|}^{2}=0.70\ifmmode\times\else\texttimes\fi{}{10}^{24}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ and ${|\ensuremath{\Psi}(\mathrm{Cl})|}^{2}=0.12\ifmmode\times\else\texttimes\fi{}{10}^{24}$ ${\mathrm{cm}}^{\ensuremath{-}3}$.

MAGNETIC PROPERTIES OF KMnF$sub 3$. II. WEAK FERROMAGNETISM

Abstract: The static magnetic properties of single-crystal KMn${\mathrm{F}}_{3}$ have been studied by magnetic torsion measurements. These measurements are consistent with a transition to uniaxial antiferromagnetism below 88.3\ifmmode^\circ\else\textdegree\fi{}K. Below 81.5\ifmmode^\circ\else\textdegree\fi{}K the magnetic behavior is complex with the development of hysteresis and discontinuities in the torsion. Further, the torsion increases linearly with magnetic field in this range. These observations suggest the development of weak ferromagnetism in this crystal below 81.5\ifmmode^\circ\else\textdegree\fi{}K. From a comparison of the direction of the weak moment and the known distortions in the crystal structure it is concluded that the weak moment results from a canting of the magnetic sublattices because of differences in the sublattice anisotropy. Between 81.5\ifmmode^\circ\else\textdegree\fi{} and 88.3\ifmmode^\circ\else\textdegree\fi{} a moment appears only in strong magnetic fields. It is shown that the moment is developed in a field because of the increased parallel susceptibility of a canted antiferromagnet. The canting transition is interpreted as a first order transition of the Jahn-Teller type. The antiferromagnetic transition itself is associated with a change in lattice parameter and is interpreted as an exchange-controlled first-order transition.

Theory of Ferromagnetic Resonance in Rare Earth Garnets. II. Line Widths

Abstract: The spins of rare earth ions in the garnets are coupled strongly both to the lattice phonons and, by an exchange interaction, to the ferric spin lattice. The rare earth spins thus provide a powerful relaxation channel for the ferric lattice. Two contributions to the line width may be distinguished: a coherent process (in which the total magnetic moment of the ferric lattice relaxes without changing the magnitude of the moment) is dominant at temperatures from 0\ifmmode^\circ\else\textdegree\fi{}K up to just below the Curie temperature. Near and above ${T}_{c}$ a fluctuation process (in which ferric spins flip locally) is dominant. The theoretical results describe the order of magnitude and the temperature dependence of the observed line widths in the rare earth garnets and in impure yttrium iron garnet, if one assumes in the absence of direct experimental knowledge that the relaxation frequency $\frac{1}{\ensuremath{\tau}}$ of the relevant rare earth ions is \ensuremath{\sim}${10}^{\ensuremath{-}12}$ sec at 400\ifmmode^\circ\else\textdegree\fi{}K. As the temperature is increased from 0\ifmmode^\circ\else\textdegree\fi{}K, the width increases until a maximum is reached when $\frac{1}{\ensuremath{\tau}}$ becomes comparable with the ferric-rare earth exchange frequency. Above this temperature the width decreases until near ${T}_{c}$, where there is a sharp rise.

Relations between the Concentrations of Imperfections in Crystalline Solids

Abstract: Publisher Summary The chapter presents a study on relations between the concentrations of imperfections in crystalline solids. Many properties of crystalline solids, such as the electronic or ionic conductivity, the color, the luminescence, and the magnetic susceptibility are determined by the presence of imperfections. Generally, six types of primary imperfections are distinguished; namely phonons, electrons and holes, excitons, vacant lattice sites and interstitial atoms or ions, foreign atoms or ions in either interstitial or substitutional positions, and dislocations. In addition atoms of the base crystal may be present at lattice sites normally occupied by other atoms. Five types of primary imperfections—namely, electrons and holes, vacant lattice sites, interstitials, misplaced lattice atoms, and foreign atoms—are discussed in this chapter. The chapter presents a new treatment of these problems by making use of a graphical representation. This treatment, together with the use of a band scheme for the electronic energy levels, greatly facilitates the application of the theory and the deduction of conclusions from it. Apart from a few exceptions, binary nonmetallic compounds of the formula M X will be considered almost exclusively. Here M indicates an element of a more electropositive character (metal) and X an element of a more electronegative character.

NMR in rotating solids

Abstract: The NMR free induction decay from a spinning sample having inhomogeneous anisotropic interactions (chemical shifts, first order quadrupole couplings) takes the form of a train of rotational spin echoes. The Fourier transform of the echo envelope is a sharp spectrum from which the effects of anisotropy have been removed. The Fourier transform of the echo shape contains information concerning the anisotropies: This information can be extracted by a moment analysis. The effects of localized homonuclear spin–spin interactions are to convert the ’’isotropic’’ spectrum into a characteristic powder pattern. Second order quadrupole coupling produces a similar effect. It is shown in this case that the usual second‐order level shifts cannot be used to calculated this pattern, which must be described by a proper average Hamiltonian theory. Finally it is shown that rotational spin echoes provide a convenient means of studying very slow random molecular rotations (τc≲1 sec).

Electronic measurement and control of spin transport in Silicon

Abstract: The spin lifetime and diffusion length of electrons are transport parameters that define the scale of coherence in spintronic devices and circuits. As these parameters are many orders of magnitude larger in semiconductors than in metals, semiconductors could be the most suitable for spintronics. So far, spin transport has only been measured in direct-bandgap semiconductors or in combination with magnetic semiconductors, excluding a wide range of non-magnetic semiconductors with indirect bandgaps. Most notable in this group is silicon, Si, which (in addition to its market entrenchment in electronics) has long been predicted a superior semiconductor for spintronics with enhanced lifetime and transport length due to low spin-orbit scattering and lattice inversion symmetry. Despite this promise, a demonstration of coherent spin transport in Si has remained elusive, because most experiments focused on magnetoresistive devices; these methods fail because of a fundamental impedance mismatch between ferromagnetic metal and semiconductor, and measurements are obscured by other magnetoelectronic effects. Here we demonstrate conduction-band spin transport across 10 mum undoped Si in a device that operates by spin-dependent ballistic hot-electron filtering through ferromagnetic thin films for both spin injection and spin detection. As it is not based on magnetoresistance, the hot-electron spin injection and spin detection avoids impedance mismatch issues and prevents interference from parasitic effects. The clean collector current shows independent magnetic and electrical control of spin precession, and thus confirms spin coherent drift in the conduction band of silicon.

g Factors and Spin-Lattice Relaxation of Conduction Electrons

Abstract: Publisher Summary This chapter is concerned with spin-resonance absorption—that is, power absorption from an oscillating magnetic field—by conduction electrons. The e place of spin resonance in investigations of the band structure contrasts the cases of paramagnetic salts and conductors. In the former, the levels of the ions are discrete; when they have an odd number of electrons each level is at least twofold degenerate. An external magnetic field splits this degeneracy. The microwave spectroscopy of the energy bands is not confined to spin resonance, but includes cyclotron resonance. Of the two, undoubtedly the latter provides the most direct information on the energy bands. Spin resonance does not play the central role that it does in paramagnetic salts. However, it usually plays a useful supplementary role when something about the band structure is known, serving either as a check on the band model or determining the values of additional parameters. The use of spin resonance in semiconductors and semimetals grow as better materials are made and detailed knowledge about their band structure becomes available.