F.B. Humphrey

Large Barkhausen and Matteucci effects in FeCoSiB, FeCrSiB, and FeNiSiB amorphous wires

The mechanism of the sensitive and stable large Barkhausen effects of as-cast amorphous magnetostrictive wires having three composition systems was investigated using measured values of stress-induced anisotropy constant saturation magnetostriction, residual internal stress, and MH hysteresis loop squareness M/sub r//M/sub s/. The entire composition range of FeCo is compared to FeCr with up to 10 at.% and FeNi with Ni up to 12 at.%. Matteucci effects were also investigated in magnetostrictive and nonmagnetostrictive amorphous wires magnetized with a longitudinal AC field, AC wire current, or AC field perpendicular to the wire axis. >

Large Barkhausen effect and Matteucci effect in amorphous magnetostrictive wires for pulse generator elements

The mechanisms producing the sensitive large Barkhausen effect and the Matteucci effect in as-prepared amorphous magnetostrictive wires are investigated using colloid technique domain observations and Sixtus-Tonks domain propagation characteristics. Theoretical analysis of the pulse height of induced voltage at the pick-up windings and between both ends of the wire are made. This analysis is compared with experimental results. The Matteucci effect was remarkably improved by twisting or twisting then annealing the as-prepared wires. Jitter-less pulse generation is realized in the latter case.

Domain observations of Fe and Co based amorphous wires

Domain observations were made on Fe- and Co-based amorphous magnetic wires that exhibit a large Barkhausen discontinuity during flux reversal. Domain patterns observed on the wire surface were compared with those found on a polished section through the center of the wire. It was found that the Fe-based wire consists of a shell and core region with a third region between them. This fairly thick transition region made up of domains at an angle of about 45 degrees to the wire axis lacks the closure domains of the previous model. The Co-based wire does not have a clear core and shell domain structure. The center of the wire has a classic domain structure expected of uniaxial anisotropy with the easy axis normal to the wire axis. When a model for the residual stress quenched-in during cooling of large Fe bars is applied to the wire, the expected anisotropy is consistent with the domain patterns in the Fe-based wire. >

Micromagnetic behavior of conical ferromagnetic particles

Large area arrays of cobalt and nickel particles with truncated conical shapes and diameters of 80–120 nm have been prepared using interference lithography combined with an evaporation and lift-off process. The magnetic hysteresis has been measured and the remanent states of the particles have been compared with a three-dimensional micromagnetic model. The model shows a transition from “flower” to “vortex” magnetization states as the particle size increases. The distribution of switching fields and the magnetostatic interactions between particles have been characterized. Both lead to a slow approach to saturation in the hysteresis loops. The suitability of such arrays for data storage is discussed.

Jitter-less pulse generator elements using amorphous bistable wires

New jitter-less pulse generator elements are presented using amorphous magnetostrictive wires. These elements induce sharp voltage pulses with ∼ 6V/cm2.Oe.turn due to the large Barkhausen jumps and with ∼ 6V/cm3.Oe due to the Matteucci effect using external ac fields of more than 0.1 Oe at frequencies between 0.01 Hz - 10 k Hz. Jitter of the pulse inducing phase is less than 1/10 and the pulse height is about twice that of Wiegand wires, respectively. The critical field of the large Barkhausen jump is controlled by heat treatment, etching, or twisting the wires. These wires are expected to be useful for high-performance pulse generator elements for high resolution rotary encorders by combining them with magnet ring, torque sensors, proximity sensors and magnetometers.

Domain wall structure in Permalloy films with decreasing thickness at the Bloch to Néel transition

The Bloch to Neel wall transition is investigated in Permalloy films between 160 and 10 nm thickness using direct integration of the Landau–Lifshitz–Gilbert equation in a three-dimensional Cartesian lattice. At 80 nm, the wall is a symmetric Bloch wall characterized by two adjoining vortices with the magnetization at the wall center pointing perpendicular to the plane of the material throughout the thickness. The Bloch to Neel transition takes place between 35 and 30 nm, below which the wall becomes a symmetric Neel wall. For the Bloch walls, our wall energy per unit area calculations match reasonably well the results of A. Hubert’s Ritz method calculations [Magnetic Domains (Springer, New York, 1998), p. 251] and A. E. Labonte’s numerical calculations [J. Appl. Phys. 40, 2450 (1969)]. For the Neel walls, however, our results indicate an approximately 70% higher energy for thicknesses of 30 nm and below, since the Neel wall tails are included. For thicknesses below 160 nm, the anisotropy energy component ...

Bistable magnetization reversal in 50 µm diameter annealed cold-drawn amorphous wires

Amorphous magnetostrictive 50 μm diameter Fe 77.5 Si 7.5 B 15 wires exhibiting square and bistable hysteresis loops have been obtained by field-tension annealing and flash annealing treatments of cold-drawn as-quenched wires. Highest switching field, 1.3 Oe, and domain drive field, 0.5 Oe, for annealing temperature 330 C , time 30 min, axial tension 150 Kg/mm2and axial field 200 Oe are an order of magnitude improvement over the as-quenched (AQ) state. Corresponding induced peak voltage is 0.5 mV/turn with exciting sinusoidal field 2 Oe and 60 Hz. Shorter pulse generation elements (≈ 2 cm) than for AQ wires (≈ 6 cm) were realized with 0.5 mV/turn peak voltage. Maximum squareness ratio is close to unity, twice that of AQ (125 μm) wires.

Dynamics and relaxation of large Barkhausen discontinuity in amorphous wires

Domain wall processes (domain wall configuration, propagation, and collapse) in magnetostrictive amorphous wires of the composition Fe/sub 77.5/Si/sub 7.5/B/sub 15/ were investigated. The wires were held under tensile stress (up to 1700 MPa in the case of as-quenched). The domain wall length and normal mobility (or damping) as functions of applied stress were found experimentally and from an ellipsoidal domain model. This allows the losses to be separated into eddy current and spin relaxation contributions. It was demonstrated that the spin relaxation contribution to the total damping parameter becomes dominant with increase of tension and leads to a dramatic decrease of the wall mobility. This is the reason cold-drawn and then tension-heated wires with high residual stress exhibit a much lower mobility in spite of the smaller diameter. The process of domain collapse at a collision of two domain walls is accompanied by a very sharp voltage pulse. It is shown that during the collapse the domain is affected by a growing internal magnetic field connected with an excess of domain surface energy in comparison with magnetostatic energy. >

Applications of amorphous wire

Abstract In-water-quenched amorphous magnetic metal wire has been available for a decade. In this time, the unique properties of the wire have created many new uses, especially in the field of electronic devices and sensors. Because of the near-ideal mechanical properties of amorphous metal, the wire can be die drawn to various wire diameters which can be much smaller than is possible with polycrystalline magnetic materials. Diameters of any value from the as-cast value of about 135 μm down to 10 μm are available. Also the wire can be used in applications where mechanical strength as well as unique magnetic characteristics are needed. Since magnetic metals are typically magnetostrictive, changing the chemical composition of the amorphous wire can be used to change the magnetostriction which, in combination with residual or applied stress, provides yet another opportunity to tailor the magnetic character to the device needs. Flux reversal lies at the heart of all electronic devices since the ultimate output is always a voltage to interact with the other electronic elements. Wire can have a longitudinal flux change or a circumferential flux change or a combination of two. The most curious, although not necessarily the most useful, is the re-entrant longitudinal reversal found in magnetostrictive as-cast and die-drawn tension-annealed wire. Re-entrant reversal takes the same time and hence generates the same voltage essentially independent of the drive field amplitude or frequency within the range of electromechanical devices. It is useful for tachometers where the frequency changes or security sensors where the drive field changes over a wide range. Since the reversal mechanism involves a magnetic domain wall propagating along the wire, distance sensors are also possible. By twisting the magnetostrictive wire, the longitudinal flux can be coupled to the circumferential flux which links the wire itself. This Matteucci effect allows the sensor to be driven or the output voltage observed between each end of the wire, providing a great convenience and flexibility for some types of devices. The ability to drive the reversal using a current through the wire is also popular with non-magnetostrictive wires. Here, the sharp and predictable demagnetizing effects resulting from the wire geometry make the wire attractive for electronic device applications such as the data tablet, miniature chokes and transformers.

Thickness dependent wall mobility in thin Permalloy films

Domain wall mobility in Permalloy films has been calculated as a function of thickness at 10, 80, and 160 nm which reflects the structure change of Neel, symmetric Bloch and C-shaped (asymmetric Bloch) domain walls. The mobility has been derived from the dynamics of a single nonperiodic domain wall using direct integration of the Landau–Lifshitz–Gilbert equation in a Cartesian lattice. This investigation allows for a detailed examination of spin precession, wall motion and overall magnetization distortion as the wall is moved in the presence of fields ranging from 0.5 to 5 Oe applied in the easy axis direction. At 10-nm-thick films, the mobility of a Neel wall is 30 m/s Oe. Wall motion takes place without noticeable distortion in the magnetization distribution in the vicinity of the Neel wall. For 80 nm-thick films, the mobility of a symmetric Bloch wall is 5 m/s Oe, or 20% less than the theoretical prediction for the mobility of a 180° domain wall model. At dynamic equilibrium, the symmetric Bloch wall h...