L. E. De Long

Dynamic magnetic response of infinite arrays of ferromagnetic particles

Recently developed techniques to find the eigenmodes of a ferromagnetic particle of arbitrary shape, as well as the absorption in the presence of an inhomogeneous radio-frequency field, are extended to treat infinite lattices of such particles. The method is applied to analyze the results of recent ferromagnetic resonance experiments and yields substantially good agreement between theory and experiment.

Direct imaging of coexisting ordered and frustrated sublattices in artificial ferromagnetic quasicrystals.

We have used scanning electron microscopy with polarization analysis and photoemission electron microscopy to image the two-dimensional magnetization of permalloy films patterned into Penrose P2 tilings (P2T). The interplay of exchange interactions in asymmetrically coordinated vertices and short-range dipole interactions among connected film segments stabilize magnetically ordered, spatially distinct sublattices that coexist with frustrated sublattices at room temperature. Numerical simulations that include long-range dipole interactions between sublattices agree with images of as-grown P2T samples and predict a magnetically ordered ground state for a two-dimensional quasicrystal lattice of classical Ising spins.

Criteria for the occurence of ferromagnetism and weak magnetic order in narrow-band metals

Abstract Phenomenological correlations between the occurence of different types of magnetic order, the value of the electronic specific heat divided by temperature γ∗, and the shortest distance d between Ce and U atoms in a number of metallic compounds are presented. Ferromagnetism is found to occur over restricted ranges of d and γ∗, and the magnetic ordering temperature Tm and the ordered moment μor are maximized for d≈4.0−4.2 A. “Weak magnetic order”, for which μor⪡0.5μB, is found to occur in two distinct regimes: Large values of Tm occur for d less than the “Hill limits” dH≈3.4 A (Ce) and 3.5 A (U), with γ∗≈3.5×104erg cm-3K-2, while very low Tm are found in several heavy fermion compounds near d≈4.15 A and γ∗≈1×105erg cm-3K-2. A plot of Tm vs. volume-scaled μor implies that metallic 4f, 5f and 3d transition metal materials can be systematically described within a unified picture. The small exchange enhancement of the magnetic susceptibility of heavy fermion compounds and the surprising trend that ferromagnetism occurs in relatively strongly hybridized systems may be explained by the wavevector dependence of the electronic self-energy that is responsible for large effective mass.

Ferromagnetic resonance study of eightfold artificial ferromagnetic quasicrystals

We have performed broadband (10 MHz–18 GHz) and narrowband (9.7 GHz) ferromagnetic resonance (FMR) measurements on permalloy thin films patterned with quasiperiodic Ammann tilings having eightfold rotational symmetry. We observed highly reproducible mode structures in the low-frequency, hysteretic regime in which domain walls and unsaturated magnetization textures exist. A minimum of 10 robust modes were observed in patterned samples, compared to the single uniform mode observed in unpatterned permalloy films. The field dependence and approximate eightfold rotational symmetry of the FMR spectra are in good agreement with micromagnetic simulations that confirm the importance of patterning for controlling static and dynamic magnetic response.

Structural, magnetic, and transport properties of a novel class of ferromagnetic semiconductors: SrM2±xRu4±xO11 (M=Fe,Co)

A major obstacle to the implementation of spin-polarized semiconductor devices is the current absence of suitable room-temperature, soft ferromagnetic semiconductors (FSs). Dilute magnetic semiconductors are under intense study for applications in spintronics. However, the weak solubility of randomly placed magnetic ions in the semiconductor host makes these materials unsuitable for devices. It is, therefore, crucial to develop a room-temperature FS based on a periodic array of magnetic ions. We have found that ternary ruthenium ferrites exhibit long-range ferromagnetic order well above room temperature, accompanied by narrow-gap semiconducting properties that include a large anomalous Hall conductance, low resistivity, and high carrier concentration. The physical properties can be tuned by simple chemical substitution of two elements, Fe and Co, or by varying the relative concentration of 3d and 4d elements within a homogeneity range that we have established. These promising properties—manifest within a ...

Magnetic response of aperiodic wire networks based on Fibonacci distortions of square antidot lattices

The static and dynamic magnetic responses of patterned ferromagnetic thin films are uniquely altered in the case of aperiodic patterns that retain long-range order (e.g., quasicrystals). We have fabricated permalloy wire networks based on periodic square antidot lattices (ADLs) distorted according to an aperiodic Fibonacci sequence applied to two lattice translations, d1 = 1618 nm and d2 = 1000 nm. The wire segment thickness is fixed at t = 25 nm, and the width W varies from 80 to 510 nm. We measured the DC magnetization between room temperature and 5 K. Room-temperature, narrow-band (9.7 GHz) ferromagnetic resonance (FMR) spectra were acquired for various directions of applied magnetic field. The DC magnetization curves exhibited pronounced step anomalies and plateaus that signal flux closure states. Although the Fibonacci distortion breaks the fourfold symmetry of a finite periodic square ADL, the FMR data exhibit fourfold rotational symmetry with respect to the applied DC magnetic field direction.

Whatever happened to Mott

Abstract In the case of randomly placed ion damage tracks in a bulk superconductor, theories suggest that increasing the applied field beyond the “saturation field” H s should result in a single, sharp depinning transition (analogous to the Mott–Hubbard metal-insulating transition) accompanied by an abrupt decrease in magnetic hysteresis and critical current density J c , and increased dissipation by an interstitial flux line (IFL) liquid phase. However, in the case of thin film patterned with an ordered lattice of artificial pinning centers (APC), anomalies in hysteresis and dissipation are observed at multiple “matching fields” nH n =1 H s , where n ⩽ s is a positive integer. Unexpected matching anomalies observed at “supermatching fields” ( n > s ), signal the existence of several ordered IFL lattice phases, rather than a simple IFL liquid phase. The relationship of such multiple anomalies to a possible Mott transition is an open question. We discuss the effects of temperature, applied DC field and field/film-plane angle on the mobility and non-linear dynamics of flux lines in a patterned thin film. We show how sensitive AC susceptibility measurements performed at variable electromagnetic drive and frequency address fundamental questions concerning the existence of the Mott transition and the conditions for melting of the IFL lattice and depinning of multiquantum fluxoids contained at the APC; and we review the serious experimental difficulties in distinguishing equilibrium versus dynamic depinning phenomena.

Nonstochastic magnetic reversal in artificial quasicrystalline spin ice

We have measured the isothermal DC magnetization of Penrose P2 tilings (P2T) composed of wire segments of permalloy thin film. Micromagnetic simulations reproduce the coercive fields and “knee anomalies” observed in experimental data and show magnetic shape anisotropy constrains segments to be single-domain (Ising spins) at low fields, similar to artificial spin ice (ASI). Mirror symmetry controls the initial reversal of individual segments oriented parallel to the applied field, followed by complex switching of multiple adjacent segments (“avalanches”) of various orientations such that closed magnetization loops (“vortices”) are favored. Ferromagnetic P2T differ from previously studied ASI systems due to their aperiodic translational symmetry and numerous inequivalent pattern vertices, which drive nonstochastic switching of segment polarizations.

Complex magnetic order and spin chirality on the Kagomé lattices of BaMn2.49Ru3.51O11 and BaFe3.26Ti2.74O11

Anomalies in the magnetization of single-crystal BaMn2.49Ru3.51O11 at temperatures T1=183 K, T2=171 K, and T3=128 K, signal complex magnetic order induced by competing ferro- and antiferromagnetic correlations and magnetic frustration within the Kagome (hexagonal ab-) plane. The T2- and T3-anomalies and unconventional transverse magnetoresistance are observed only for applied field H directed in the Kagome plane, suggesting a topological Hall effect is generated by nonzero scalar chirality of spins canted out of the Kagome plane, but is suppressed in a collinear structure induced by only modest H⊥c. In contrast, the magnetic susceptibility of an isostructural BaFe3.26Ti2.74O11 single-crystal reveals magnetic transitions at T1=250 K and T2=85 K for H oriented both parallel and perpendicular to the c-axis. The rapid low-field increase of the magnetic moment at μoH<1 T, followed by nonsaturation with a near-linear increase at high fields, are typical of a canted antiferromagnet or ferrimagnet.

Flux line depinning in a magnet-superconductor levitation system

Abstract The AC loss characteristics of a magnet-superconductor system were studied with the magnet fixed to the free end of an oscillating cantilever located near a stationary melt-textured YBCO pellet. Below a threshold AC field amplitude ≈2 Oe, the dissipation of the oscillator is amplitude-independent, characteristic of a linear, non-hysteretic regime. Above threshold, dissipation increases with amplitude, reflecting the depinning and hysteretic motion of flux lines. The threshold AC field is an order of magnitude higher than that measured for the same YBCO material via AC susceptometry in a uniform DC magnetic field. A partial lock-in of flux lines between YBCO ab planes is proposed as the mechanism for the substantial increase of the depinning threshold.