Rate coefficients of the Arrhenius-Neel form are calculated for thermally activated magnetic moment reversal for dual layer exchange-coupled composite (ECC) media based on the Langer formalism and are applied to study the sweep rate dependence of MH hysteresis loops as a function of the exchange coupling I between the layers. The individual grains are modelled as two exchange coupled Stoner-Wohlfarth particles from which the minimum energy paths connecting the minimum energy states are calculated using a variant of the string method and the energy barriers and attempt frequencies calculated as a function of the applied field. The resultant rate equations describing the evolution of an ensemble of non-interacting ECC grains are then integrated numerically in an applied field with constant sweep rate and the magnetization calculated as a function of the applied field H. MH hysteresis loops are presented for a range of values I and a figure of merit (FOM) that quantifies the advantages of ECC media is proposed. The results are also used to examine the accuracy of certain approximate models that reduce the complexity associated with the Langer based formalism and which provide some useful insight into the reversal. Of particular interest is the clustering of minimum energy states that are separated by relatively low energy barriers into "metastates." It is shown that while approximating the reversal process in terms of "metastates" results in little loss of accuracy, it can reduce the run time of a Kinetic Monte Carlo (KMC) simulation of the magnetic decay of an ensemble of dual layer ECC media by 2~3 orders of magnitude. The essentially exact results presented in this work for two coupled grains are analogous to the Stoner-Wohlfarth model of a single grain and serve as an important precursor to KMC based simulation studies on systems of interacting dual layer ECC media.

The kinetic Monte-Carlo algorithm as well as standard micromagnetics are used to simulate MH loops of high anisotropy magnetic recording media at both short and long time scales over a wide range of temperatures relevant to heat-assisted magnetic recording. Microscopic parameters, common to both methods, were determined by fitting to experimental data on single-layer FePt-based media that uses the Magneto-Optic Kerr effect with a slow sweep rate of 700 Oe/s. Saturation moment, uniaxial anisotropy, and exchange constants are given an intrinsic temperature dependence based on published atomistic simulations of FePt grains with an effective Curie temperature of 680 K. Our results show good agreement between micromagnetics and kinetic Monte Carlo results over a wide range of sweep rates. Loops at the slow experimental sweep rates are found to become more square-shaped, with an increasing slope, as temperature increases from 300 K. These effects also occur at higher sweep rates, typical of recording speeds, but are much less pronounced. These results demonstrate the need for accurate determination of intrinsic thermal properties of future recording media as input to micromagnetic models as well as the sensitivity of the switching behavior of thin magnetic films to applied field sweep rates at higher temperatures.

A procedure is developed to study the evolution of high anisotropy magnetic recording media due to thermally activated grain reversal. It is assumed that the system is composed of single domain grains that evolves by passing through a sequence of relatively long-lived metastable states punctuated by abrupt reversals of individual grains. Solutions to the rate equations describing the sequence of metastable states are calculated using kinetic Monte Carlo. Transition rates are formulated from the Arrhenius-Neel expression in terms of the material parameters, temperature, and applied field. Results obtained from this method are shown to be in good agreement with those calculated from finite-temperature micromagnetics. The method is applied to study the rate dependence of finite-temperature MH loops and the thermal degradation of a recorded bit pattern in perpendicular recording media. A significant advantage of the procedure is its ability to extend simulations over time intervals many orders of magnitude greater than is feasible using standard finite-temperature micromagnetics with relatively modest computational effort. DOI: 10.1103/PhysRevB.87.064405

Our previously developed kinetic Monte Carlo algorithm is used to simulate M-H loops of high anisotropy magnetic recording media at long time scales relevant to the experimental measurements using the magneto-optic Kerr effect. Micromagnetic parameters are fit to loop data taken at 300 K and at a sweep rate of 700 Oe/s on a single-layer media developed for heat-assisted magnetic recording. Significantly different fitted parameters result from standard micromagnetic simulations that can access only sweep rates many orders of magnitude faster. Sensitivities of the loops to anisotropy, saturation magnetization, and various distributions are reported.

A Kinetic Monte-Carlo algorithm is applied to examine MH loops of dual-layer magnetic recording media at finite temperature and long time scales associated with typical experimental measurements. In contrast with standard micromagnetic simulations, which are limited to the ns-μs time regime, our approach allows for the direct calculation of magnetic configurations over periods from minutes to years. The model is used to fit anisotropy and coupling parameters to experimental data on exchange-coupled composite media which are shown to deviate significantly from standard micromagnetic results. Sensitivities of the loops to anisotropy, inter-layer exchange coupling, temperature, and sweep rate are examined.

A microwave phase shifter is a device used to introduce phase change in a propagating electromagnetic wave in a waveguide. A series of microstrip transmission lines, with an iron film of 300 nm thickness placed at various places inside a SiO2 dielectric layer, was fabricated and tested as phase shifters based on ferromagnetic resonance principle. It is observed that the differential phase shift obeys a Sin2 – law (derived from perturbation theory) given by; Δβ ∝ Sin2(πx/h). Here, x is the distance of Fe film inside SiO2 dielectric from the Cu conductors, h is the height of SiO2 dielectric. This give Δβ as minimum when Fe film is at the two edges of the dielectric and maximum when Fe is at the center of the dielectric. The differential phase shift varies as high as 350° at resonance (20 GHz), when Fe is at the middle of the dielectric. For high (at 25 GHz) and low (at 8 GHz) frequency operation, far above and far below resonance, the differential phase shift is ∼125° and 250° when Fe at middle of the dielectric. This trend is same for absorption – it is a maximum when Fe at the middle.

There is need for non-reciprocal devices such as circulators and isolators. Although such devices are common at frequencies below 10 GHz, there is a lack of compact, low-weight, devices at higher microwave frequencies. This paper examines the non-reciprocal behavior associated with attenuated total reflection (ATR) for multi-layered dielectric and magnetic structures. Non-reciprocal behaviors produced by ATR have been explored for semi-infinite magnetic materials. This paper focuses on ATR behavior with magnetic films of finite thickness, from thick layers of around 3 cm to thin layers of about 1 µm. The results show significant non-reciprocity even for magnetic layers less than 0.1 cm thick, with reflection loss differences of more than 30 dB between positive and negative signal propagation. Results are presented for yttrium iron garnet and M type barium hexagonal ferrites. The two materials allow nonreciprocal behavior at different frequencies, 5–20 GHz for the garnet and 45–80 GHz for the hexagonal ferrite.

An iterative method is developed to obtain a semi-analytic description of field-induced switching for interacting Stoner-Wohlfarth-like particles. We investigate systems of particles with distributions in anisotropy orientations, anisotropy strength, and saturation magnetization as well as magnetostatic and exchange interactions between particles. As an illustration of the technique, M-H hysteresis loops are calculated and compared with dynamic micromagnetic simulations. The results for the iterative method show good agreement with the simulations and, in some cases, are on the order of 100 times faster for parameter values typical of magnetic recording media.

We explore the response of a magnetic bilayer to a driving microwave field using micromagnetic simulations. The bilayer consists of 8 nm of a material with a high uniaxial anisotropy and 56 nm of a material with a lower uniaxial anisotropy. The width and length of the structure is 100 × 100 square microns. A small applied field, opposite to the magnetization, switches most of the lower anisotropy material but not the higher anisotropy material, forming a domain wall between the two materials. We evaluate the frequencies of the magnetic eigenmodes for the entire system using Fourier analysis and then drive the structure with an oscillating magnetic field at each of the eigenfrequencies. When the oscillating microwave field is added, the static switching field required to align both layers is decreased compared to the undriven case. With a driving field strength of 120 Oe the switching field is reduced by about 40%, from 1.12 kOe for the undriven case to 0.55 Oe for the driven case.

Using a micromagnetics calculation, we explore the properties of a submicron magnetic square with microwave assisted switching. For a 10×160×160 nm3 structure of Fe–Ti–N, there are three particular stable magnetic states for reversal fields up to −320 Oe. One can switch between these different states by adding a microwave field. The strength and the frequency of the microwave field determine the final state. A microwave field of up to 30 Oe does not change the magnetization. Fields of 50 to 75 Oe result in an intermediate state, while larger microwave fields produce a reversed ground state.

A hexagonal ferrite thin film-based planar millimeter-wave phase shifter was demonstrated. The device made use of an M-type barium ferrite (BaM) thin film prepared by pulsed laser deposition and a coplanar waveguide geometry. The phase tuning relied on ferromagnetic resonance in the BaM film. The device showed a phase tuning rate of 43°/(mm kOe) and an insertion loss of 3.1 dB/mm in the on-resonance regime. In off-resonance regimes, the device showed smaller loss and smaller tuning rates. The experimental results were confirmed by theoretical calculations.

We present experimental data on the power-dependent shift in the ferromagnetic resonance frequency of permalloy ribbons magnetized in plane. We find the sign of the shift depends on the strength of the applied static magnetic field. For low applied fields, the power-dependent shift is upwards in frequency, while for high fields the mode softens with increasing power. We summarize two theoretical studies that are qualitatively compatible with this result. One is an analytic treatment of very thin films and the second a numerical simulation of a thin rectangular prism.

Recently there has been interest in small, planar, high frequency (10–50 GHz) signal processing devices which are based on metallic ferromagnets. This paper, in contrast, presents theoretical results for devices utilizing a hexagonal ferrite. The notch filter results show attenuation of frequencies in the 40–60 GHz range with applied fields in the 1–5 kOe range. For geometries similar to the ultrasmall metallic devices, the transmission loss at the notch can be from 20 to 140 dB/cm depending on the thickness of the hexagonal ferrite film and the inclusion of dielectric spacers. The phase shifter results show phase shifts of up to 360°, with losses below 2 dB/cm. These results are obtained for devices using thin films around 1 μm in thickness for the hexagonal ferrite, and for reasonable linewidths below 200 Oe.

Microwave filters that use thin films of ferromagnetic metals are now being established as a valuable option compared to yttrium iron garnet based filters due to their higher frequency response. In these filters the signal propagation is inhibited over a wide frequency band, depending on the applied dc magnetic field. However, the continuous application of an applied field to achieve an operating frequency in the higher gigahertz range increases the power consumption of the device. The main contribution of this article is to provide techniques which significantly boost the operating frequency of notch filters in zero or very low applied magnetic fields. To do this, the authors fabricated high quality epitaxial Fe films which are interlayer exchange coupled through nonmagnetic Si layer of different thicknesses. The films were used in flip-chip geometry on top of a Cu-coplanar waveguide to create band-stop filters. In contrast to filters based on Fe alone, the multilayer filters can operate above 25GHz with a very small applied magnetic field. The observed upshift in frequency is attributed to the induced interlayer exchange coupling energy mediated through the nonmagnetic Si layer between the two Fe layers. These frequency shifts are in good agreement with theoretical calculations of the ferromagnetic resonance modes taking into account anisotropy, exchange, and Zeeman energies.

We study small thin-layer magnetic notch filters which operate in the 5–40 GHz range. Past theoretical work has concentrated on a structure where the magnetic film was right next to one of the conductive films in a waveguide. Here we present a theoretical model, which investigates the properties of a waveguide with two dielectric films and one magnetic film placed between two outer conductive layers. The results show this more general structure produces a deeper attenuation and a narrower peak compared to the earlier structure. The additional attenuation varies from 0 to 30 dB/cm, depending on the thickness and position of the magnetic film. This article also examines the reflection of the guided waves as they enter the notch filter. The results from an effective medium calculation show that a signal experiences the largest return losses near the ferromagnetic resonance frequency of the magnetic film, with typical losses below −4 dB. The return loss can be reduced significantly if the linewidth in the ferromagnetic film is increased. The effective medium results are compared to an experiment that measured return losses in a microstrip device. The experiment had maximum return losses between −4 and −8 dB.

We present results for tunable microwave band-stop and bandpass filters on a microstrip geometry. These structures, prepared by sputtering on GaAs substrates, are compatible in size and growth process with on-chip high-frequency electronics. For the notch filters, we observed power attenuation up to /spl sim/100 dB/cm and an insertion loss on the order of /spl sim/2-3 dB for both Permalloy- and Fe-based structures. The operational frequency ranges from 5 to 35 GHz for external fields below 5 kOe. We discuss methods to increase operational frequency and reduce device linewidth. Using these techniques we are able, for example, to obtain an operational frequency of 11GHz at zero applied field and to narrow the device linewidth from 3 GHz to 330 MHz. The operational frequency, which can be obtained from the ferromagnetic resonance condition, is set by material properties such as saturation magnetization M/sub s/, anisotropy fields, the gyromagnetic ratio, and the magnitude of an applied field H. Thus, by using different materials and external fields one can create devices which function over a wide range of frequencies.