External Bias Magnetic Field Research Articles

Direct overwrite using through-thickness temperature gradients

A type of direct overwrite using exchange-coupled bilayer magnetooptical media, without the need for an initializing magnetic field, was experimentally demonstrated. This was accomplished at a constant bias magnetic field by manipulating the through-thickness temperature gradient using a combination of short and long laser pulses. One layer of this bilayer medium is rare-earth poor, and the other layer is rare-earth rich without a compensation temperature. With long pulses the magnetization direction of the recording layer is aligned to the external bias magnetic field, while short pulses cause the subnetwork magnetization of the recording layer to be aligned to that of the reference layer by exchange-coupling. The magnetization direction of the reference layer always remains unchanged, while the magnetization of the recording layer is switched by the change between short and long laser pulses.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

The dipole-exchange spin wave spectrum for anisotropic ferromagnetic films with mixed exchange boundary conditions

A theory is developed for dispersion characteristics of spin waves in ferromagnetic films taking into account both dipole-dipole and exchange interactions, crystallographic anisotropy and mixed exchange boundary conditions on the film surfaces. An arbitrary orientation of the external bias magnetic field with respect to the orientation of crystallographic axes and the film normal is assumed. The influence of crystallographic anisotropy on the spin wave spectrum of a ferromagnetic film is discussed. The theoretical results obtained are compared with the results of experiments performed in yttrium-iron garnet (YIG) and Mn-ferrospinel films.

Microwave oscillator using arrays of long Josephson junctions

The authors report on measurements performed on integrated superconducting devices based on arrays of long Josephson tunnel junctions operating in the resonant fluxon oscillation regime (i.e. biased on the zero field steps). The electromagnetic coupling among the junctions causes a mutual phase-locking of the fluxon oscillations with a corresponding increase of the emitted power and a decrease of the signal linewidth. This phase-locked state can be controlled by means of an external DC bias current and magnetic field. The effect of the generated microwave signal has been observed on a small Josephson tunnel junction coupled to the array via a microstrip transmission line. The feasibility of the reported devices as local oscillators in an integrated microwave Josephson receiver is discussed. >

Propagation of an extraordinary wave in a thin ferrite film in the absence of an external magnetic field

The propagation of an extraordinary wave in a unbiased ferrite film of (Bi,Lu) yttrium iron garnet is discussed. This, to our knowledge, is the first reported observation of this type of magnetostatic wave propagation. Two separate modes were observed to propagate in the material, with the mode selection resulting from the initial preparation of the film prior to measurement. These modes propagate in different ranges, 4.2–5.1 GHz and 2.63–3.13 GHz, and exhibit different propagation characteristics. The effect of an external magnetic field on the two modes is also reported for three mutually perpendicular direction-of-bias fields. Two of these directions are in-plane and the third is normal to the plane of the ferrite. This material has potential applications for microwave signal processing devices in that they do not require an external magnetic bias field for proper operation. This can greatly reduce the size of present magnetostatic wave devices, as well as allow these devices to acquire a more planar geometry.

Effect of Selective Tilt for GGG (111) Substrate Surface on the Magnetic Domain in Bubble Garnet LPE Layers

GGG (111) substrate surfaces are selectively tilted in an attempt to locally stabilize stripe domains. This method utilizes the magnetization easy axis inclination away from [111] in the tilted section. When an in-plane magnetic field (H r ) is applied to the substrate surfaces in a direction traversing the tilted section, a magnetic field is simultaneously produced in the magnetization easy axis direction of the tilted section. When an external bias magnetic field (H z ) is applied in the [111] direction, the effective magnetic field parallel to the magnetization in the tilted section can be modulated by H r . The results of our experiments demonstrate that this method can be used to simultaneously stabilize both the stripe domains of the tilted sections and the bubbles of the untilted regions at given H z .

Magnetostatic Wave Propagation in a Finite YIG-Loaded Rectangular Waveguide

The propagation of magnetostatic waves (MSW) in a waveguide partially loaded with a low-loss ferrite slab is investigated theoretically. The most common low-loss ferrite material used for MSW propagation is epitaxial yttrium iron garnet (YIG). A YIG slab is placed inside and along the guide and not in contact with the sidewalls of the wavegnide. The dc magnetic field is assumed to be parallel to the YIG slab and perpendicular to the direction of propagation. Using the integral equation method, the dispersion relation is found to be an infinitely large determinant equal to zero. Proper truncation of this determinant and numerical analysis to find its roots are carried out in this work. It is seen that in order to obtain high values of group time delay, the YIG slab must be narrow and placed at the bottom of the guide. On the other hand, to maximize the device bandwidth, a narrow YIG slab positioned at the top inside surface of the waveguide is preferred. It is also noticed that there exists a tradeoff between the time delay and the device bandwidth and that maximization of one property leads to a poor value in the other. Thus, some design compromises should be made. It is also observed that the frequency range of operation of the device can be adjusted by an external magnetic bias field. This property of tuning the device to operate in any frequency range adds an extra dimension of flexibility to the operation and also to the design of these devices.

Increased particle confinement observed with the use of an external dc bias field in a spheromak experiment

Spheromaks are formed in a mesh flux conserver in the presence of an external dc bias magnetic field. The particle confinement is improved when the spheromak separatrix is put inside the metal mesh by the application of positive bias flux. The spheromaks remain stable to tilt instabilities with ratios of bias-to-spheromak flux of up to 47±7%.

Internal bias effect of double layer epitaxial garnet films

A method of eliminating the external magnetic bias field necessary to sustain stable cylindrical magnetic domains (magnetic bubbles) is presented. This method uses double-layer garnet films. The films were grown on the

Thin-Film Surface Bias on Magnetic Bubble Materials

A method of reducing the external magnetic bias field to maintain stable cylindrical domains (magnetic bubbles) in uniaxial orthoferrites or garnets is presented. A thin film of a permanent magnetic material is sputtered onto a platelet inducing an exchange coupling interaction across the interface. This interaction manifests itself as an effective bias field which reduces the external applied field required to stabilize bubbles. So far the largest effective bias obtained has been about 25 Oe, using Co3.2Cu1.3Fe0.5Ce0.25Sm0.75 sputtered onto a 15-μ-thick platelet of Sm0.55Tb0.45FeO3. Dynamic properties do not seem to be appreciably altered for low-speed operation in a T-bar rotating field device.

A Coupled Magnetic Film Device for Associative Memories

It is well known that a pair of magnetostatically coupled films with easy axes at 90° to each other exhibit biaxial anisotropy. However, when an external bias magnetic field is applied along the easy direction of one of the films, only two states of the coupled system have minimum energy. It can be shown that these two stable states are not collinear in magnetization orientations and the resulting switching characteristic is quite different from that of a uniaxial coupled film device. A new associative memory device based on the above-mentioned switching characteristics is presented in this paper. Nondestructive content interrogation is feasible on this device by applying a current on the strip line intervening between two magnetic layers. When the polarity of the interrogation current mismatches the state of the device, a signal is induced in a detection strip line orthogonal to the interrogation line and located on top of both magnetic films. On the other hand, when the current polarity matches the device state, none or very little flux change is experienced by the detection line. The mismatch signal is always of one polarity independent of the device state. A device sample with electroplated magnetic and conductive layers and photoetched strip lines has been fabricated. The scheme of two storage cells per bit is adopted to cancel capacitive noise. Two 2-bit (4 cells) words have been successfully operated in a parallel by bit content search mode. The measured mismatch to match ratio for a single bit is better than eight.

The satellite ferrimagnetic power limiter : L. J. Varnerin, R. L. Comstock, W. A. Dean and R. W. Kordon, Bell Syst. Tech. J.XLII, 1817 (July 1963)

To limit the 4080-mc local oscillator signal power input to the beat oscillator modulator of the Telstar satellite communications repeater, a subsidiary absorption limiter was used which consisted of an optically polished sphere of single-crystal yttrium iron garnet (YIG), placed in a resonant transmission cavity between the amplified 4080-mc output of the traveling-wave tube and the BO modulator input. The limiter holds the output power nearly constant above a given input threshold; below this threshold the YIG is linear and introduces only a small loss. The threshold is determined, for a given sample at a given frequency, by the external magnetic bias field. Temperature compensation over the desired range was obtained by orienting the crystal with the dc magnetic field along a [100] or “hard” axis. The total weight of the limiter package, including the bias magnet and cavity, is 13 ounces

The Satellite Ferrimagnetic Power Limiter

To limit the 4080-mc local oscillator signal power input to the beat oscillator modulator of the Telstar satellite communications repeater, a subsidiary absorption limiter was used which consisted of an optically polished sphere of single-crystal yttrium iron garnet (YIG), placed in a resonant transmission cavity between the amplified 4080-mc output of the traveling-wave tube and the BO modulator input. The limiter holds the output power nearly constant above a given input threshold; below this threshold the YIG is linear and introduces only a small loss. The threshold is determined, for a given sample at a given frequency, by the external magnetic bias field. Temperature compensation over the desired range was obtained by orienting the crystal with the dc magnetic field along a [100] or “hard” axis. The total weight of the limiter package, including the bias magnet and cavity, is 13 ounces