Microwave-assisted Magnetization Reversal Research Articles

Microwave-Assisted Magnetization Reversal in Dispersed Nanosized Barium Ferrite Particles for High-Density Magnetic Recording Tape

This paper investigates microwave (MW)-assisted magnetization reversal (MAMR) in dispersed nanosized barium ferrite (BaFe) particles. MAMR in dispersed nanosized BaFe particles was observed using vector network analyzer ferromagnetic resonance spectroscopy with coplanar waveguides, in this investigation. The average size of BaFe particles was 1750 nm3. A frequency-dependent reduction in the switching field was observed in response to an MW impulse of duration 50 ns and with a frequency of approximately 22 GHz. This result suggests that MAMR was observed in dispersed nanosized BaFe particles.

Out-of-Plane Auto-Oscillation in Spin Hall Oscillator With Additional Polarizer

The theoretical investigation on magnetization dynamics excited by the spin Hall effect in metallic multilayers having two ferromagnets is discussed. The relaxation of the transverse spin in one ferromagnet enables us to manipulate the direction of the spin-transfer torque excited in another ferromagnet, although the spin-polarization originally generated by the spin Hall effect is geometrically fixed. Solving the Landau-Lifshitz-Gilbert-Slonczewski equation, the possibility to excite an out-of-plane auto-oscillation of an in-plane magnetized ferromagnet is presented. An application to magnetic recording using microwave-assisted magnetization reversal is also discussed.

Spin torque oscillator for microwave assisted magnetization reversal

A theoretical study is given for the self-oscillation excited in a spin torque oscillator (STO) consisting of an in-plane magnetized free layer and a perpendicularly magnetized pinned layer in the presence of a perpendicular magnetic field. This type of STO is a potential candidate for a microwave source of microwave assisted magnetization reversal (MAMR). It is, however, found that the self-oscillation applicable to MAMR disappears when the perpendicular field is larger than a critical value, which is much smaller than a demagnetization field. This result provides a condition that the reversal field of a magnetic recording bit by MAMR in nanopillar structure should be smaller than the critical value. The analytical formulas of currents determining the critical field are obtained, which indicate that a material with a small damping is not preferable to acheive a wide range of the self-oscillation applicable to MAMR, although such a material is preferable from the viewpoint of the reduction of the power consumption.

An analytical computation of magnetic field generated from a cylinder ferromagnet

An analytical formulation to compute a magnetic field generated from an uniformly magnetized cylinder ferromagnet is developed. Exact solutions of the magnetic field generated from the magnetization pointing in an arbitrary direction are derived, which are applicable both inside and outside the ferromagnet. The validities of the present formulas are confirmed by comparing them with demagnetization coefficients estimated in earlier works. The results will be useful for designing practical applications, such as high-density magnetic recording and microwave generators, where nanostructured ferromagnets are coupled to each other through the dipole interactions and show cooperative phenomena such as synchronization. As an example, the magnetic field generated from a spin torque oscillator for magnetic recording based on microwave assisted magnetization reversal is studied.

Stability Analysis of Microwave-Assisted Magnetization Reversal in Exchange Coupled Nano Magnets

The stability analysis was applied to the study of magnetization dynamics under action of microwave magnetic field in exchange coupled nano magnets. It was found that the stability criteria represe...

Microwave-assisted magnetization reversal using transient precession of magnetization in permalloy hexagons

Microwave-assisted magnetization reversal utilizing a transient precession of magnetization was demonstrated in a permalloy hexagon by applying a 25-ns-wide microwave field and a 500-ps-wide pulsed field with a tunable delay to the microwave field. The switching field in a combination of these two fields becomes smaller than that in only the microwave field, and this additional reduction in switching field oscillates relative to the delay time. From the comparison with the results of micromagnetic simulations, we found that the oscillatory behavior is attributed to the beats in transient precession that occurs in the early stage of microwave-field-induced magnetization excitation.

Microwave-Assisted Magnetization Reversal in Exchange-Coupled Composite Media Using Linearly Polarized Microwave Fields

We have experimentally demonstrated a microwave-assisted magnetization reversal (MAMR) in exchange-coupled composite (ECC) media using the linearly polarized microwave fields generated by an electrically shorted coplanar waveguide. The MAMR frequency exhibiting a maximum reduction of coercive field can be decreased using 16 nm-thick ECC media without the suppression of the thermal stability of magnetization. The threshold amplitude of microwave field required for the successful MAMR is only 3% of the anisotropy field of the medium. A splitting of ferromagnetic resonance spectrum, which suggests an appearance of stochastic MAMR, can be observed after applying microwave fields. The stochastic MAMR gives rise to the threshold behavior in the condition of microwave field amplitude needed for MAMR.

Magnetization switching by microwaves initially rotating in opposite direction to precession

A common understanding of magnetization switching in microwave-assisted magnetization reversal is that the rotation direction of the microwaves should be the same as the precession direction of the magnetization. In this letter, however, we show that microwaves initially rotating opposite to the magnetization precession destabilize the magnetization at an equilibrium and induce switching more efficiently when the microwave frequency depends on time. This argument is analytically deduced from an energy balance equation. We also establish a model satisfying this condition and confirm magnetization switching solely by microwaves by using numerical simulation.

Cooperative microwave-assisted magnetization reversal with pulsed fields in 2-µm-wide nickel–iron wires at nanosecond time scales

The switching dynamics of nickel–iron wires (length, 100 µm; width, 2 µm) were investigated by magnetization reversal under a combined 20-ns-wide microwave impulse and a 2-ns-wide pulsed field. The switching field was clearly reduced by cooperative switching and was independent of the pulsed field width (for widths between 2 and 10 ns). This result suggests that magnetization assisted by the combined microwave field occurs within 2 ns.

Magnetization Reversal in Exchange Spring Bilayer System Under Circularly Polarized Microwave Field

Microwave assisted magnetization reversal are studied in the bulk bilayer exchange coupled system. We investigate the nonlinear magnetization reversal dynamics in a perpendicular exchange spring media using Landau-Lifshitz equation. In the limit of the infinite thickness of the system, the propagation field leads the reversal of the system. The reduction of the switching field and the magnetization profile in the extended system are studied numerically. The possibility to study the dynamics analytically is discussed and an approximation where two P-modes are coupled by an interaction field is presented. The ansatz used for the interaction field is validated by comparison with the numerical results. This approach is shown to be equivalent to two exchange coupled macrospins.

Microwave-Assisted Magnetization Reversal for Granular Media by a Resonant Cavity Method

Microwave-assisted magnetization reversal was verified by a cavity method with intention to apply homogenous microwave to a millimeter-size CoPtCr-SiO2 medium. By applying 34 GHz microwave with various power, magnetization reversal of the medium was assisted. The amount of the reversal magnetization was estimated using an internal magnetic field evaluated with ferromagnetic resonance (FMR) field. As a result, it is found that: 1) when the incident angle of external field is set at 90° (perpendicular), assisting effect depends on microwave power and magnetization of the media; 2) mode value of switching field distribution of the media was reduced 300 Oe by 160-mW microwave assisting; and 3) when the incident angle of external field is set at 45° during the microwave assisting, dispersion of the SFD decreased with increasing microwave power.

Physical principles of microwave assisted magnetic recording

While the basic physics of Microwave Assisted Magnetization Reversal (MAMR) phenomenon is well established both theoretically and experimentally, its application in a practical magnetic recording environment was so far studied primarily with the help of micromagnetic recording models. In this work, we instead attempt to use analytical formulation and simple numerical models to understand the main challenges as well as benefits that are associated with such a system. It appears that the main difference between the previously introduced theory [G. Bertotti et al., Phys. Rev. Lett. 86, 724 (2001); K. Rivkin et al., Appl. Phys. Lett. 92, 153104 (2008); S. Okamoto et al., J. Appl. Phys. 107, 123914 (2010).] and recording environment is that both the RF and DC magnetic fields are applied at a substantial angle to the anisotropy axis. While the associated symmetry breaking prevents one from describing the reversal process explicitly, it is possible to approximate the solutions well enough to satisfactorily match numerical models both in the case of wire and Spin Torque Oscillator generated RF fields. This approach allows for physical explanation of various effects associated with MAMR such as high gradient of writeable anisotropy and reduction of track width, and offers a clear guidance regarding future optimization of MAMR recording.

Microwave-Assisted Magnetization Reversal of Exchange-Coupled Composite Nanopillar With Large Gilbert Damping Constant

Microwave-assisted magnetization reversal (MAMR) characteristics of a single-exchange-coupled composite nanopillar, which consists of ferromagnetically coupled soft and hard magnetic sections, have been discussed in terms of Gilbert damping constant using micromagnetic simulation. The results indicate the occurrence of significant MAMR even when the damping constant of the hard magnetic section is large. The results obtained in this paper greatly contrast those obtained with single-phase hard magnetic pillars. We also found that the magnetization switching time tends to be shorter when the damping constant of the hard magnetic section is large.

Electrical detection of microwave assisted magnetization reversal by spin pumping

Microwave assisted magnetization reversal has been investigated in a bilayer system of Pt/ferromagnet by detecting a change in the polarity of the spin pumping signal. The reversal process is studied in two material systems, Pt/CoFeB and Pt/NiFe, for different aspect ratios. The onset of the switching behavior is indicated by a sharp transition in the spin pumping voltage. At a threshold value of the external field, the switching process changes from partial to full reversal with increasing microwave power. The proposed method provides a simple way to detect microwave assisted magnetization reversal.

Frequency dependence of critical switching asteroid of CoCrPt–SiO2 granular film under 50-ns microwave impulse

Understanding the dynamic process of microwave-assisted magnetization reversal (MAMR) in a dipole–dipole coupled granular perpendicular medium is essential for its application to future magnetic storage devices. We experimentally investigated the magnetization reversal process in CoCrPt–SiO2 granular films in terms of the dependence of the switching field on the strength of an applied in-plane radio-frequency magnetic field. The reversal process changed from non-uniform rotation to uniform rotation when the frequency increased toward the ferromagnetic resonance frequency. In the resonant condition, the switching field agreed well with a model assuming uniform rotation of the magnetization in each grain. The agreement suggests that the decoherence of ferromagnetic resonance due to a dynamic dipolar field is weak in adequately exchange-coupled granular films with a saturation magnetization of 415 emu/cm3. This finding is significant for the development of the practical medium necessary for MAMR writing.

Observation of microwave-assisted magnetization reversal in perpendicular recording media

This letter reports microwave-assisted magnetization reversal (MAMR) in a 700-Gbit/in2 perpendicular media sample. The microwave fields were applied by placing a coplanar waveguide on the media sample and feeding it with narrow microwave pulses. The switching states of the media grains were measured by magnetic force microscopy. For microwaves with a frequency close to the ferromagnetic resonance (FMR) frequency of the media, MAMR was observed for microwave power higher than a certain threshold. For microwaves with certain high power, MAMR was observed for a broad microwave frequency range which covers the FMR frequency and is centered below the FMR frequency.

Damping dependence in microwave assisted magnetization reversal

This letter reports a demonstration of microwave assisted magnetization reversal (MAMR) in a CoFeB film and the damping dependence in MAMR through the measurement of ferromagnetic resonance (FMR). Spin-pumping in non-ferromagnetic/ferromagnetic films provides a large range variation of Gilbert damping constants in magnetic samples when changing the thickness of non-ferromagnetic layers without changing the ferromagnetic film. An evident dependence of switching fields on the damping constant is observed in the presence of microwaves. The trend of the experimental data is well reproduced by a numerical simulation based on the Landau–Lifshitz–Gilbert equation. The result indicates that the large damping decreases the efficiency of microwaves in reducing the magnetization switching field.

Micromagnetic study on microwave-assisted magnetic recording in perpendicular medium with intergrain exchange coupling

The influence of intergrain exchange coupling on the properties of microwave-assisted magnetization reversal has been investigated. The results of micromagnetic simulation suggest that the microwave frequency realizing the magnetization reversal is increased as the exchange stiffness constant among the magnetic grains becomes larger than 1 × 10−7 erg/cm if an inhomogeneous magnetization reversal occurred. The numerically expected increase in the microwave frequency was observed experimentally in perpendicularly magnetized Co/Pd multilayer with an exchange stiffness constant of ∼1 × 10−6 erg/cm. The simulations of signal recording and reproducing processes in a granular medium with assistance of a microwave field also suggest that an adequate intergrain exchange coupling is required to inhibit the nucleation of small island domains that will suppress the amplitude of the signal-noise ratio of the readback signal.

Competition between pumping and damping in microwave-assisted magnetization reversal in magnetic films

This paper reports the demonstration of microwave-assisted magnetization reversal (MAMR) in large-damping materials and the observation of a saturation effect for the enhancement in the MAMR process. The experiments were carried out on ${\text{Fe}}_{70}{\text{Co}}_{30}$ thin films. A reduction in the switching field was observed in the presence of microwaves. The level of such a reduction depends on the frequency and power of the microwaves. With increasing the microwave duration, the switching field decreases first but then approaches a lower limit. This saturation of the switching field reduction was interpreted in terms of the pumping-damping competition. The interpretation was supported by the measurements of the switching field as a function of the microwave duration for different conditions.

Microwave-assisted magnetization reversal in a Co/Pd multilayer with perpendicular magnetic anisotropy

Microwave-assisted magnetization reversal in a rectangle of a Co/Pd multilayer with a perpendicular magnetic anisotropy is examined using vector network analyzer ferromagnetic resonance (FMR) spectroscopy. A microwave field is applied along the in-plane direction of the rectangle together with a negative dc easy-axis field smaller than the coercive field. Broadening or splitting of the peak profile in the FMR spectrum suggesting the formation of multidomain structure appears after the microwave field is applied. The dominance of microwave-assisted nucleation of magnetization is supported by the frequency dependence of the probability with which the multidomain structure appears.