Permalloy Stripes Research Articles

Observations of magnetisation distribution in narrow permalloy strips

We have used a scanning laser microscope which employs a novel detection tochnique to allow information on magnetisation distributions in the plane of the sample to be obtained. Results are presented which show that the magnetisation distributions found across the strips differ from those computed from the theory.

Magnetics of nonlaminated, bilaminated, and multilaminated permalloy stripes

Using the magnetoresistive response technique and the micro-Kerr domain imaging technique, we have studied the magnetics of 2000-Å-thick nonlaminated, bilaminated, and multilaminated permalloy stripes with a length of 2400 μm, widths varying from 100 to 2 μm, and with the uniaxial anisotropy easy axis transverse to the length of the stripes. Results show that stripes from all three types of films exhibit non-single-domain remanent states that are hysteretic in their longitudinal magnetic responses. For the nonlaminated structures, closure domains are formed for the minimization of transverse demagnetization energy. For the laminated structures, longitudinally magnetized regions along the edges are formed as a basic feature of the flux-closure configuration in the laminated films. Furthermore, for the multilaminated structures, the large number of degrees of freedom and the low wall energies have created an abundance of low-energy remanent states that might cause the small-field response of the structure to creep with time. Finally, the use of transverse biasing is found to be ineffective in eliminating hysteresis in the bilaminated structures because this hysteresis is not a consequence of transverse symmetry in the structure.

Magnetoresistive measurement of the exchange constant in varied thickness Permalloy films (abstract)

A recently developed method1 for the magnetoresistive measurement of the exchange constant in Permalloy films is applied to multiple films of varied thickness. Passing a dc current along the easy axis of a thin-film single-domain Permalloy stripe generates an inhomogeneous internal magnetic field. This field opposes exchange forces and causes the film magnetization to fan in opposite directions through the film thickness, forming a variable angle Bloch wall parallel to, and extending throughout, the film plane. The magnetization fanning is readily observable via dc magnetoresistive detection. Values for the exchange constant A are obtained by comparing experimental measurements with a micromagnetic model1 that employs A as the single free parameter. Present measurements on several films of thickness between 2000 and 4000 Å confirm the predicted strong film thickness dependence of magnetization fanning amplitude, and are all shown to be consistent with a single value for the Permalloy exchange constant of A=1.1±0.1×10−6 ergs/cm. This value is equal to the previous1 independently obtained value. The present technique for measurement of A is demonstrated to yield consistent values for the exchange constant in different films of varied thickness, and does not require additional postulates (as do spin-wave resonance techniques) concerning surface anisotropy or saturation magnetization inhomogeneities.

Conduction of flux at high frequencies by a charge-free magnetization distribution

A charge-free, analytically tractable model for flux conduction by pure rotation in Permalloy strips at high frequencies is proposed. An expression for the magnetization rotation versus applied field is presented. The model is extended to give important qualitative results for shapes other than simple strips. In particular, the region where the narrow pole tip joins an abruptly widened yoke in a thin film recording head is considered. It is shown that the desired flux spreading in this region can only be achieved with accompanying wall motion. >

Characteristics of bubble memory devices using multilayer permalloy films

We have investigated the bias field and detector's signal characteristics of a bubble memory device using multilayer Permalloy interleaved with Si spacers. Coercivity of two-layer Permalloy stripe pattern becomes minimum in the range from 3 to 5 nm of the Si spacer thickness, and is reduced to approximately half that with a single-layer Permalloy. In the three-layer Permalloy device, the bias field margin increases 2.7 Oe on the average, and the amplitude of signal increases by 30%, the maximum of noise amplitude is only about one-tenth, and the drive field causing maximum noise level is reduced to half, compared with a single-layer Permalloy device. The calculated decrease in potential well of three-layer Permalloy pattern is in good agreement with increase in bias field margin.

Magnetoresistive measurement of the exchange constant in permalloy

A current passed through a thin ferromagnetic stripe, with magnetization along the stripe (easy) axis, will generate an internal field that tries to fan the magnetization in opposite directions through the film thickness, but which is opposed by exchange forces. Methfessel et al. observed this effect experimentally in Permalloy using inductive methods and determined a value for A, the exchange constant, of 0.55 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-6</sup> ergs/cm. This is about half the value normally measured using spin wave resonance. We have extended the theoretical analysis and repeated the experiment on small, 3000 Å thick Permalloy stripes using dc magnetoresistive detection instead of induction. By comparing theoretical and experimental results, we have determined a value for A of 1.1 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-6</sup> erg/cm ± 10%. No assumptions about surface anisotropy or magnetization inhomogenieties, required to explain spin wave modes, are needed. We believe this is the most reliable value reported for A in Permalloy.

Conduction of flux at high frequencies in Permalloy strips by small-angle rotations

Data presented here on the permeability of Permalloy strips at high frequency indicates that flux is conducted by small-angle rotations without significant inhibition due to coupled domain wall motion. A new theory is proposed which postulates the existence of small angular variations of magnetization within transversely oriented domains in order to allow for flux conduction by rotation without the need for domain wall motion. Results from a computer simulation of this mechanism are presented.

Measurement system for two-dimensional magnetic field distributions, applied to the investigation of recording head fields

The system described is built around a very accurate positioner into which a sensitive transducer and the object of analysis is mounted. The properties of the applied magnetoresistive transducer are described. This transducer, a very narrow permalloy strip placed at the edge of a glass substratum, can be used to measure both components of the field distribution. The analysis of the measured results can be accomplished with the help of a computer simulation of the transducer response curves. The performance of the system is demonstrated by measurements on a number of ferrite heads and conclusions about the so called 'dead layer'-structures on these heads are given.

Magnetostatic surface modes in a thin film with nonuniform in-plane fields

The experimental observation of magnetostatic surface waves (MSSW's) in a rectangular YIG film placed between strips of permalloy and in the plane of the strips is reported. In such a configuration, the DC magnetic field between the permalloy strips is primarily in-plane with a minimum halfway between and increasing toward each strip. In the presence of such a gradient, a series of discrete modes appears near the low frequency end of the normally continuous MSSW band. The frequencies of these modes lie below the bottom of the conventional MSSW band as calculated from the field at the center of the sample where the field is minimum. However, reversing the bias field reduces the coupling to these modes indicating a nonreciprocal surface localization. Estimates of the group velocities of these modes based on the slope of phase vs. frequency data indicate that the discrete modes travel several times faster than conventional MSSW's.

Stretching and sensing of 1-µm bubbles in ion-implanted contiguous disk devices

A sensor-stretcher designed for use in ion-implanted contiguous disk devices is described. The current activated stretcher forms part of a self-aligned circuit and is defined simultaneously with the propagation structure. The sensor is a thin Permalloy stripe, 100 μm in length, adjacent to the stretch conductor. Detection of 1 μm diameter bubbles has been achieved at frequencies up to 146 kHz, over a 30-Oe (10 percent) bias field margin. The figure of merit, ΔV/V, for the detector was relatively independent of sensor width in the range 2.5- 10 μm and was ∼0.30%; maximum output was ∼10 mV. Noise resulting from imperfect balance of active and dummy detector was found to be a problem in narrow detectors (< 10 μm) due to high coercivity. However, the coercivity may be reduced substantially by in-situ passivation with a 70 Å Si layer. Stroboscopic observation of bubble stretching indicates expansion velocities of 20-90 m/s, (depending on drive) in representative EuTmGaIG/GdYTmGaIG composites. The stretch current pulse for reliable expansion is a function of the magnitude and phase of the in-plane rotating field and is typically 200 mA, 2.5 μs.

Theoretical considerations on the design of magnetic bubble detectors

The resistance change experienced in a Permalloy stripe when magnetized by an expanded bubble domain is considered from a theoretical standpoint. A model is developed after consideration of demagnetizing field strengths and thermal and electrical constraints. From this model the output signal level to be expected for any given expansion of the domain may be computed and used as a design aid.

Permeability of narrow permalloy stripes

The effective permeability between 1 MHz and 100 MHz was measured for 1-3 μm thick permalloy(80Ni- 20Fe) films in 2-3 cm diameter sheets and compared with the effective permeability for the same films after etching them into 51μm wide stripes on 127μm centers. Solid permalloy films show a 15%-40% loss of effective permeability at all frequencies after having been etched into stripes. Permalloy films made with nonmagnetic metal laminations show no significant permeability change after etching. Films made with dielectric laminations show no significant permeability change at low frequencies and a 20%-100% increase in the high frequency permeability after etching.

The crosstie memory

The crosstie memory stores information in magnetic domain walls in permalloy films about 350 A thick. The domain walls are also used as shift register tracks. Serrated edges on narrow thin film permalloy strips are used to center a domain wall in each strip and to provide stable positions for crossties and Bloch lines. A magnetoresistance detector uses the same information bearing permalloy film as the magnetoresistive element. The entire crosstie memory can be fabricated on a silicon wafer or chip using conventional photolithographic procedures as are used in fabricating integrated circuits. Thus, a magnetic memory can be combined with semiconductor drivers, decoders, and sense amplifiers on the same chip. The memory is intended to serve as a block oriented random access memory (BORAM). Important background information such as static and dynamic stability conditions, Bloch line mobility, propagation and observation techniques are reviewed. Also, design goals and the role such device is expected to play in memory technology are presented. Anticipated performance includes a shift rate of 20 × 106bits/sec, a bit density greater than 1.5 × 105bits/cm2, an operating temperature range from -50 °C to 100 °C, nonvolatility, low cost, and low power consumption.

377. Magnetic properties of thin permalloy strip films with stripe domain structure

377. Magnetic properties of thin permalloy strip films with stripe domain structure

Wall Motion Reversal in Easy-Axis-Coupled Film Strips

Wall motion reversal of the magnetization in coupled film strips is examined theoretically and experimentally. The structure is a conductor strip sandwiched between two Permalloy strips having their easy directions across the strips. If a train of current pulses of sufficient amplitude passes through the conductor strip, the Permalloy layers may be switched around the conductor in either of two sense depending on the polarity of the current. Criteria are presented for coupling (that is, that the two layers remain essentially oppositely magnetized) as a function of width of the strip, thickness of the Permalloy, coercivities of the films, and thickness of the intermediate conductor. It is found that coupling may break down either if the strip is too narrow or too wide. For strips which are coupled and have relatively thin enclosed conductor layers, the multiple-pulse field necessary to switch the film strip is about the average of the coercivities of the two Permalloy layers. For thicker conductor layers, this field is reduced by a geometric factor accurately predicted by a model using only magnetostatic interactions between the two Permalloy layers. Data are presented for 2000-Å layers of Permalloy in 0.125- to 4-mm strips with separations between the Permalloy strips between 0.6 and 8.6 μ. The coercivities of the films range between 0.6 and 6 Oe.

Demonstration of Magnetic Domain-Wall Storage and Logic

The operations of read-in, shifting, conditional erase, and fan-out in a system of domain-wall storage and logic proposed by Smith have been experimentally demonstrated. The several operations were performed by controlling the current flowing through a configuration of wires placed closely under a 5 to 40 mil wide, 50 to 300 A thick Permalloy strip, and the sequence of events was observed on the top side of the film using the Bitter technique. The basic wiring geometry was a narrow-spaced grid of series-connected parallel wires that made an angle of 50° to 75° with the strip of film, the smaller angles being necessary for the thinner films. Conditional erase was most successful in films below 100 A thick, as in this thickness range walls of the same sense form double walls which require relatively large fields for erasure.

Resistivity measurements on permalloy films

The electrical resistance of permalloy films, made by evaporation in vacuum from permalloy strips, is found to show only a small temperature dependence. Measurements of the coercive force of these permalloy films have also been carried out, using the magnetoresistance method. Just as in the case of the bulk material the coercive force of these permalloy films is very small.

The Electrical Resistance of Iron Wires and Permalloy Strips at Radiofrequencies

The ratio of a.c. to d.c. resistances of iron wires and permalloy strips has been measured in the frequency range 1.5–6.0 megacycles per sec. Empirical equations obtained are compared with existing theoretical equations derived on the assumption of constant permeability.