Small Permalloy Research Articles

Probing the oxidation kinetics of small permalloy particles

The oxidation of permalloys is important to apply in a wide range. The oxidation and diffusion mechanisms of small permalloy particles with different Fe content are studied by using thermal gravimetric analysis (TGA) and microstructure characterizations. Fe2O3/(Ni, Fe)3O4 plays a key role in the morphology evolution and diffusion mechanisms of small NiFe particles upon oxidation. The activation energies of grain boundary diffusion for the NiFe alloys increase from 141kJ/mol to 208kJ/mol as the Fe content increases from 0 to ~50wt%. We have developed a diffusion process resolved temperature programed oxidation (PR-TPO) analysis method. Three diffusion mechanisms have been recognized by using this method: In addition to the grain boundary diffusion and lattice diffusion, our TGA analysis suggests that the phase conversion from Fe2O3 to (Ni, Fe)3O4 induces diffusion change and affects the diffusion process at the intermediate temperature. Relevant oxidation kinetics and diffusion mechanisms are discussed.

Thin-Film Microsusceptometer With Integrated Nanoloop

We report the design and performance of thin-film microsusceptometers intended for magnetic measurements on samples at variable temperature down to the low milliKelvin range and excitation frequencies of up to about 1 MHz. The devices are realized as first-order gradiometers with two circular loops of 60- or 30- $\mu\hbox{m}$ average diameter resulting in a total inductance of 360 or 250 pH, respectively. An integrated excitation coil generates a magnetic field with a transfer coefficient of 0.1 T/A at the sample position, whereas the Josephson junctions are located in a field-reduced area. The susceptometers are fabricated by a conventional Nb/AlOx/Nb trilayer process. In order to enhance the sensitivity to the level required for the measurement of submicrometer samples, an extra detection loop of 450-nm inner diameter was integrated into one of the pickup loops by using a focused ion beam. We show that this device is able of detecting signals from very small Permalloy samples. An optimized susceptometer design with a predicted superconducting quantum interference device inductance of 12 pH is also presented, which can achieve an equivalent spin noise of 8 $\mu_{\rm B}/\!\surd\hbox{Hz}$ at 4.2 K ( $\mu_{\rm B}$ is the Bohr magneton) when being equipped with a nanoloop of 100-nm line width and separation.

Dynamical measurements with a nuclear magnetic resonance force microscope

We report imaging and dynamical measurements using a H3e nuclear magnetic resonance force microscopy probe. Relaxation-time measurements and a one-dimensional image were obtained for H1 nuclei in a micron-scale crystal of (NH4)2SO4. The force detection was made possible by a small Permalloy magnet, which supplied a field gradient of 500 T/m. These experiments were performed in the sample-on-oscillator configuration at room temperature, where the oscillator had a resonance frequency of 1.5 kHz and a spring constant of 0.03 N/m. The proton magnetic moments underwent cyclic adiabatic inversions (CAIs) under the influence of a frequency-modulated rf field. Scanning the position of the magnet with respect to the sample provided a micron-scale image with a signal-to-noise ratio of 4.3. A spin nutation signal was also obtained; those data imply a rotating rf field of 14 G. Using a 90°-τ-180°-t-90°-CAI sequence, a spin echo was mapped out, with a full width at half maximum of 8 μs. We also discuss future applications of this instrument toward relaxation measurements of single-crystal MgB2 at low temperatures.

Bi-stable and tri-stable Magnetic domain reconfigurations of a small permalloy pattern using current pulses

Realization of pulsed-current selectable domain configurations of small permalloy islands is reported. In the islands with 65–100nm thickness and micron-sized lateral dimensions, the magnetic domain pattern can be uniquely set into either a four or seven closure domain configuration by applying either a positive or negative 10ns current pulse at the density of 107 A/cm2. The transition is completely reversible. This phenomenon appears to belong to a family of current-induced magnetization reordering, whose main mechanism appears to be domain wall motion due to s-d exchange force directed along the electron flow, and partially influenced by the Amperian field produced at the contact regions. This paper presents two cases, namely, bi-stable and tri-stable configurations, and discusses the mechanism as well as power requirements in using this approach for memory device applications.

The evolution of dynamic switching diagrams of small Permalloy ellipses with magnetic field pulse duration

Switching diagrams of a thin-film 3 × 1 micrometer elliptical permalloy element in a crossed-wire geometry are measured for different pulsed field durations using time-resolved scanning Kerr microscopy. “Astroid”-like diagrams were observed for pulse durations as short as 900 ps full-width half-maximum. Our study indicates an asymmetry of the switching scenarios and resulting diagrams for positive and negative hard-axis field values, stemming from misalignment of the ellipse major axis with respect to the easy axis of the deposited film. Spatially resolved scans reveal the reluctance to switching of sample edges. The results are compared to Landau–Lifshitz–Gilbert micromagnetic simulations.

Intrinsic perpendicular anisotropy and exchange biasing in CoO/permalloy multilayers

For permalloy/CoO multilayers, the exchange field and the coercivity of the perpendicular and longitudinal exchange biasing have been found to decrease with increasing temperature. After zero field cooling, perpendicular magnetic anisotropy is observed. For small permalloy layer thickness, perpendicular anisotropy film can be established at low temperatures.

Magnetization configurations and hysteresis loops of small permalloy ellipses

We investigated systematically the easy axis magnetization reversal of 20 nm thick permalloy ellipses with a fixed major axis of 1.47 µm and minor axes of 0.22–1.47 µm. Lorentz transmission electron microscopy was used to image the micromagnetic configurations during magnetization reversal. Hysteresis loops of single ellipses were recorded by means of micro-Hall magnetometry and could be traced back to certain reversal mechanisms observed by Lorentz microscopy. In most cases, the magnetization reversal is initiated by the evolution of a magnetization buckling, followed by the formation of a single, a double, or a trapped vortex configuration. For ellipses with high aspect ratio (length-to-width ratio), the magnetization switches in the reversed magnetic field without creation of a stable vortex configuration. Our experiments show that the characteristic field values for vortex creation, single vortex annihilation, and switching strongly depend on the shape anisotropy of the elements.

Investigation of the magnetic interaction of small Permalloy particles

Permalloy island arrays and undulated lines were systematically studied by MFM to clarify the effect of neighbor magnetic interaction. The islands are 860 nm /spl times/320 nm and 35 nm thick, single domains and patterned to have preferential interaction along the easy and hard axes. The switching field for easy axis interaction is relatively high, i.e., 370 Oe with a 32 Oe spread; while the hard axis interaction produces a switching field of 130 Oe and a slightly wider distribution of over 40 Oe. A third array is comprised of undulated lines with connected segments. Magnetic charges accumulate at the ends of each segment, and yield the strongest possible interaction along the easy axis. In this case all segments on one line simultaneously switch at a specific field. The switching field increases linearly from 150 Oe with the number of segments up to a maximum number of 50, and approaches a constant field of 290 Oe. The results favorably compare with the prediction for parallel rotation of the Jacobs-Bean chain of spheres model.

Slow magnetization dynamics of small permalloy islands

The conditions that lead to specific domain configurations and the associated switching characteristics of small permalloy islands were studied by using magnetic force microscopy. By measuring a large number of particles, it was established that islands that have nonzero remanent moments (nonsolenoidal) exist in one of three distinct configurations, namely: (a) true single domain, (b) quasisingle domain with edge closure patterns, and (c) multidomain with nonuniform internal magnetization. The configuration depended upon the island width as well as the aspect ratio. Islands that are 310 nm wide or less are true single domain particles at low aspect ratios (∼1.87) and higher, while islands wider than 500 nm always exhibited edge closure domains even for very large aspect ratios. In the range between 310 and 500 nm, the onset of single domain behavior was a function of the aspect ratio and thickness. Our studies involving in situ applied field similarly revealed the mechanisms of the reversal processes for each of the configurations, which correlated quite well with the values of the switching fields.

Direct Observation of Magnetic Relaxation in a Small Permalloy Disk by Time-Resolved Scanning Kerr Microscopy

Magnetic oscillations were studied in a single, lithographically patterned disk ( $8\ensuremath{\mu}\mathrm{m}\mathrm{diam}\ifmmode\times\else\texttimes\fi{}0.1\ensuremath{\mu}\mathrm{m}\mathrm{thick}$) of permalloy placed in an in-plane biasing magnetic field, excited by subnanosecond pulses of perpendicular field. Local changes of the perpendicular magnetization component were detected by time-resolved scanning magneto-optical Kerr microscopy, allowing direct observation of the nonuniform spatial evolution of the magnetization. It was found that the modal frequency of the oscillations is very close to the ferromagnetic resonance frequency. The Landau-Lifshitz-Gilbert equation with damping constant equal to 0.008 describes broad features of the data, although departures due to the nonuniform response are also evident.

The role of vortices in the magnetization process of small dimension permalloy films

A micromagnetic model has been developed for small dimensional permalloy films, such as those being considered for high density recording. The results show that variations in the magnetization occur through the formation of vortices which give rise to Barkhausen noise in the transfer curve.

Memory effects and nucleation of domain structures in small Permalloy shapes

The processes of nucleation and stabilization of domain structures were studied by means of the reflection Kerr effect in small Permalloy discs subjected to alternating demagnetization. Patterns with vortex and composite topologies were observed with probabilities depending on the disc radius. Demagnetizations below a critical amplitude were unable to erase the memory of the previous state; on the contrary, demagnetizing above that threshold yielded statistically independent results, that is, complete erasure. Image-processing techniques and dynamical observations during demagnetization allowed analysis of the random nucleation of a domain structure and its subsequent stabilization inside an energy basin of given topology.

Simulation of magnetoresistive response in a small Permalloy strip

A micromagnetic simulation program has been used to calculate the magnetoresistance of a free-standing 4 μm×1 μm×200 Å Permalloy strip for a variety of numerical hysteresis loops in which a constant bias field is applied along the easy axis and a variable field is applied along the hard, in-plane axis. The main purpose of this study is to correlate noise in the magnetoresistive response with micromagnetic behavior and to show that the noise is considerably reduced by the application of the bias field. Two different methods for evaluating the magnetoresistive response are discussed.

Scanning electron microscopy with polarization analysis: An update

Over the past ten years the technique of scanning electron microscopy with polarization analysis (SEMPA) has rapidly evolved from a scientific curiosity to a useful analytical tool for looking at a material's magnetic microstructure. Several reviews of the technique have been published elsewhere. SEMPA has been successfully used to analyze various technological problems such as: noise in magnetic and magneto-optical recording media, domain wall motion in thin film recording heads, and domain structures in small Permalloy shapes. Basic science applications of SEMPA include quantitative studies of the influence of the surface on the structure of magnetic domains and domain walls, and studies of magnetic microstructures in ultra-thin (0.1 - 1 nm) ferromagnetic films. Many current applications of SEMPA make use of the technique's surface sensitivity to probe the magnetism of thin films and multilayers.

Magnetoresistive switching of small permalloy sandwich structures

The magnetoresistive (MR) switching behavior of small permalloy/silicon monoxide/permalloy sandwich elements is investigated. A comparison with single layer elements shows that the best MR-switching results are found for the sandwich structures with the smallest dimensions (10×100 μm). A theoretical calculation based on the magnetostatic coupling between the two permalloy layers of the sandwich and on inhomogeneous demagnetizing fields shows good agreement with the experimental results. The formation of magnetic domain configurations other than those found in small single layer elements is illustrated using a ferrofluid observation technique.

Study of domain formation in small permalloy magnetoresistive elements

Our past work has shown that the Barkhausen noise in the transverse response of small (10–100 μm) permalloy MR sensors originates from domain wall activities; namely, the systematic creation, intensification, and wall-state transition of buckling domain structures. The cause and details of domain formation, however, remain unresolved. In this work, through direct domain observations and wall population studies, we will show that magnetostatic energy effects constitute the most basic cause of domain formation. Easy-axis dispersion (δK) effects can also cause domains, but primarily for the shorter elements. Also, we will show how the magnetic configurations along the top and bottom edges of the element affect the geometry of the resultant buckling domains, and how fluctuations in these edge configurations produce fluctuations in the domain geometry and, consequently, in the MR response. Finally, we will show that domain activities are not suppressed by different orientations of the easy axis, although interesting changes in the domain behavior may be produced.

Efficiency of High-Frequency Mixing in a Permalloy Toroid

A theoretical harmonic analysis is made of the output voltage from a small, etched Permalloy sheet core which is energized by the sum of two low-amplitude currents of different frequencies. The magnitudes of the two fundamentals, the sum and the difference frequencies, are predicted by the analysis. The two-frequency analysis of a hysteretic device is complicated by the multivaluedness of the input—output relation. The small signal behavior of a Permalloy core is further complicated by the lack of a model for high-frequency drive (∼1.0 Mc/sec). The analysis of mixing begins, therefore, with an experimentally measured high-frequency minor loop of a saturated core. The amplitude and frequency of the applied current is selected to insure nonswitching behavior. Once the flux—current relation is established, multiple Fourier analysis is used to calculate the output voltage waveform. The theoretical amplitudes of the frequency components are compared with measured values from a small Permalloy core (o.d.=23 mil, i.d.=17 mil, thickness=0.25 mil).

Nonlinear Coil Generators of Short Pulses

Small permalloy coils and circuits have been developed which produce pulses well below a tenth of a microsecond in duration with repetition rates up to a few megacycles. The construction of these coils is described. Low-power circuits suitable for different types of drive and different frequency ranges are discussed.

An Alternating Current Probe for Measurement of Magnetic Fields

A method of measuring magnetic field strengths is described which utilizes the incremental permeability of a small Permalloy core. Alternating-current excitation is used which in turn permits amplification and hence a relatively high sensitivity and accuracy.