R. Wolfe

Scanning Hall probe microscopy

We describe the implementation of a scanning Hall probe microscope of outstanding magnetic field sensitivity (∼0.1 G) and unprecedented spatial resolution (∼0.35 μm) to detect surface magnetic fields at close proximity to a sample. Our microscope combines the advantages of a submicron Hall probe fabricated on a GaAs/Al0.3Ga0.7As heterostructure chip and the scanning tunneling microscopy technique for precise positioning. We demonstrate its usefulness by imaging individual vortices in high Tc La1.85Sr0.15CuO4 films and superconducting networks, and magnetic bubble domains.

Room‐Temperature Ferromagnetic Materials Transparent in the Visible

The only known crystals which have a spontaneous ferromagnetic moment at room temperature and are transparent well into the visible spectrum are ferric borate, FeBO3, and ferric fluoride, FeF3. Each of these rhombohedral materials is a canted antiferromagnet with a small moment lying in the plane perpendicular to its unique axis. The Curie temperatures are 348° and 363°K and the room‐temperature moments are 115 and 40 G, respectively. Both materials contain ferric ions in a slightly distorted octahedral environment. Both are green with a series of absorption bands in the visible, a fundamental absorption edge in the violet (FeBO3) or ultraviolet (FeF3), and a large Faraday rotation per unit absorption at room temperature, as high as 14°/dB at 5250 A for FeBO3 and 16°/dB at 4050 A for FeF3. Recent measurements of the magnetic, optical, magneto‐optical, and microwave resonance properties of these materials are discussed. Potential applications of room temperature transparent ferromagnets include microwave m...

EFFECTS OF A MAGNETIC FIELD ON THE THERMOELECTRIC PROPERTIES OF A BISMUTH‐ANTIMONY ALLOY

The thermoeiectric properties of an alloy of 88 at.% bismuth and 12 at.% antimony in transverse magnetic fields up to 17,000 Ce are reported. Graphs of variation of resistivity with magnetic field strength and of variation of thermoelectric figure of merit with temperature show that resistivity rises with field strength. Magnetothermoelectric results with pure bismuth are similar to those for this alloy. (D.C.W.)

MAGNETO‐OPTICAL PROPERTIES OF A GREEN ROOM‐TEMPERATURE FERROMAGNET: FeBO3

Ferric fluoride (FeF3) is a transparent green uniaxial weak ferromagnet (canted antiferromagnet) with a Curie temperature of 365 °K. FeF3 is far more transparent in the visible and uv than any other known material which has a spontaneous magnetic moment at room temperature. FeF3 at room temperature has 6 absorption bands between 7000 A and the band edge which is at 2440 A. There is a local absorption minimum of α = 4.4 cm−1 in the green at 5225 A. The Faraday rotation is 180 °/cm at 5225 A and increases to 1100 °/cm at 3000 A. The birefringent phase retardation at 5200 A is about 80 000 °/cm. The two indices of refraction are about 1.54 in the visible spectrum.

Thin‐film waveguide magneto‐optic isolator

A thin‐film waveguide Faraday rotator which provides nonreciprocal polarization rotation of ±45° for light of wavelength 1.45 μm with an extinction ratio of 500:1 has been demonstrated. This isolator structure makes use of laser annealing to reverse the sublattice magnetization and hence the sign of the Faraday effect in localized regions of a (Bi, Ga) yttrium iron garnet film. A novel geometrical pattern was designed for these regions to overcome the effects of the inherent thin‐film birefringence.

MnAlGe Films for Magneto‐Optic Applications

MnAlGe is a uniaxial ferromagnetic compound with a saturation 4πMs of 3600 G, and Curie temperature of 245°C. We have studied polycrystalline thin films of this compound formed on various substrates by getter sputtering. The optimum substrate temperature to obtain films with uniform magnetic properties was 500°C. Films prepared at this temperature have a substantial component of magnetization normal to the plane of the substrate, and a coercive force of approximately 2000 Oe. Information was written into films approximately 700‐A thick by Curie point writing or with a fine permanent magnet wire, and read with good optical contrast by means of the polar Faraday effect. The relatively low Curie temperature is advantageous in reducing the power required for Curie point writing for magneto‐optic memory or holographic applications.

Radio‐Frequency Determination of New Growth‐Induced Anisotropy in Garnets for Bubble Devices

A new type of magnetocrystalline anisotropy has been observed under {110} faces in several fluxgrown mixed rare‐earth iron garnets. This anisotropy is different in direction and magnitude from that previously reported in similar garnets under {211} faces. In Eu2ErGa0.6Fe4.4O12 with 4πM near 350 G at 300°K, microwave resonance measurements have shown that {110} face material has a large effective uniaxial anisotropy. The [100] in the growth plane is the easy axis, [011] normal to this plane is medium, and [011] parallel to this plane is hard. Using a resonance equation appropriate to the orthorhombic magnetic symmetry and neglecting the relatively small cubic terms, the effective uniaxial anisotropy field is ∼1300 Oe. This anisotropy leads to a new cutting procedure (type 3 cut) for bubble devices: {100} platelets are cut normal to the natural {110} faces. Using 15‐μ‐thick platelets of this type, bubbles ∼8μ in diameter were propagated. The domain wall mobility is ∼170 cm/sec/Oe. These properties are comp...

Magnetization and Mössbauer effect in single crystal Fe3BO6

Abstract The iron borate Fe3BO6 is found to be a weak ferromagnet with a Curie temperature of 508°K and a spin reorientation at 415°K. Spin orientations for the two iron sites above and below the reorientation are deduced.

FERROMAGNETIC RESONANCE IN FeBO3, A GREEN ROOM‐TEMPERATURE FERROMAGNET

Iron borate (FeBO3) is a transparent, green, room‐temperature weak ferromagnet (a canted anti‐ferromagnet) with a Curie temperature of 348°K. Ferromagnetic resonance measurements, i.e., linewidth, anisotropy fields, and g factor vs temperature and frequency are reported for the first time on this material. A low‐temperature linewidth maximum of unknown origin is also observed. Its narrow linewidth, large magnetic anisotropy, large optical Faraday rotation, and transparency in the green (the latter two reported elsewhere) made FeBO3 of interest for devices such as electronically tunable cavities and filters up to ∼100 GHz and high‐speed magneto‐optic modulators.

Directionally-conductive, optically-transparent composites by magnetic alignment

Ferromagnetic metal spheres (silver-coated Ni, 0.5-2% in volume) dispersed in a thin layer of transparent polymer medium were subjected to a vertical magnetic field for alignment into the chain-of-spheres (or column) configuration. When the polymer was solidified, the resultant composite contained vertically aligned, but laterally isolated, columns of conductive particles in a sheet of transparent matrix. The sheet material transmitted in excess of 90% of incident light in the visible range, and was highly conductive only in the thickness direction. While optically and electrically anisotropic, the magnetic columns in the composite exhibited an essentially isotropic magnetic behavior for the given height and spacing of the columns. The material, if suitably modified, may be useful as a transparent position sensor for visual communication devices such as touch-sensitive screens or write pads. >