Microwave Assisted Magnetization Reversal on Exchange Coupled Composite Media

Microwave-assisted magnetic recording (MAMR) on single layer and exchange coupled composite (ECC) media was simulated. The high-frequency field was modeled with a macrospin model and also micromagnetically using a spin-torque oscillator integrated into the write head. ECC media with a range of hard/soft layer thicknesses were optimized for MAMR, and the gain over a non-MAMR system was established. The results show a bigger gain from using MAMR when the hard/soft layer thickness ratio was small. MAMR systems were also less susceptible to changes in magnetostatic interactions between medium grains.

With the requirement of increasing data areal density in hard disk drive (HDD) recording, one of the main applied approaches that effectively reduce recording bit size is to incorporate a spin transfer torque (STT) device in the magnetic writer, either to reverse a magnetic layer in the device in order to guide the magnetic flux of the writer pole more concentrated towards the into and out-of air bearing surface (ABS) directions [1] , or to drive a magnetic layer into large angle oscillations for a sufficiently large radiofrequency (rf) field in the media to realize the assist [2] . Micromagnetic simulations of the magnetic device inside the write gap (WG) between the main writer pole and its magnetic shield with an alternating write gap field, were conducted incorporating STT interactions between the magnetic layers. Modeling results for understanding assist performance optimized by STT device design from these two main approaches are discussed in this talk.

The potential of microwave-assisted magnetic recording on exchange-coupled composite (ECC) media is investigated by numerically simulating read/write processes with respect to the physical and magnetic properties of the soft section of ECC media. To obtain desirable recording performance, the thickness of the soft section must be comparable to the exchange length. Under such condition, the optimal microwave frequency decreases owing to the non-uniformity of the reversal mode. A relatively high anisotropy field in the soft section produces high signal-to-noise ratios. Moreover, large saturation magnetization of the soft section enhances the magnetization reversal of ECC grains but excessive enhancement is likely to induce wide writing, which decreases the high track density.

Microwave assisted magnetic recording (MAMR) has been proposed as one of the prospective recording technologies for forthcoming tera-bit-class magnetic recording. In this paper, the technological and related physical issues of MAMR are reviewed. MAMR is the combined technology of microwave assisted switching (MAS) and spin torque oscillator (STO). MAS is a magnetization switching technology in which magnetization switches through excitation of its precessional motion by a radio frequency (rf) field. The MAS behavior in which nonlinear large angle precession is excited has been a challenging issue from scientific and technological viewpoints. Recently developed theories successfully predict the MAS behavior of a single macrospin in which all spins in a magnet are assumed to behave coherently. For a magnetic element with a size larger than the exchange length, spatially non-uniform magnetization precession modes become dominant instead of the coherent precession mentioned above, resulting in significant enhancement of the MAS effect. This size dependent MAS effect is experimentally very well verified for nanosized dots with perpendicular magnetization. Meanwhile, the STO is used as an rf field generator in MAMR technology. The aforementioned large angle magnetization precession dynamics are also a crucial problem for the stable operation of STO. While various types of STOs have been proposed so far, the optimum structure of STO for MAMR is still under investigation. The combination of the MAS and STO may provide frequency domain recording architecture which enables us to realize a new recording scheme, such as multilevel recording.

For increasing areal density in hard disk drives (HDDs), the physical clearance between the read/write element and the surface of the disk has been continuously decreasing to 1 nm or below [1]. At such a low clearance, the contact between the head and the disk is inevitable to occur, so head wear is becoming a critical issue in the development of HDD. The contact between the head and the disk induces a frictional heating, which may generate an additional thermal protrusion in the contact area of the head, and causes more wear. On the other hand, the target clearance in a HDD is generally determined by pulling back a setting TFC power from the touchdown point, accurately identifying the touchdown point is very significant for the clearance control in hard disk drive. A thermal protrusion is caused by friction-heating in the status of touchdown. Therefore, it is very necessary to quantitatively understand on friction induced thermal protrusion and clearance loss.

As continued reducing the track width (TW) of magnetic read sensor for gaining areal density in hard disk drive (HDD) industry, stripe height (SH) is needed to be reduced as keeping aspect ratio(TW/SH) for maintain resistance and signal constant. This paper is proposed to characterize the free layer behavior of tunneling magneto resistive (TMR) head with multiple SH by using ferro magnetic resonance analyzer (FMRA) that is able to analyze by layer-level state without applying external field. The results are found that, at shorter SH, effective stiffness field of free layer is increased which is based on decreasing FMR amplitude and increasing FMR frequency. As a result, magnetic read sensor is shown less sensitivity and signal to noise ratio (SNR) with shorter SH. The SH is a correlated function of the effective stiffness field in free layer that is referred to changed shape anisotropy field which has an effect to read ability.

A micromagnetic study on the magnetization reversal in bit patterned media (BPM) with each bit consisting of an exchange coupled composite (ECC) layer stack is presented. The simulations reveal superior magnetic properties of the combined ECC/BPM scheme, in particular for graded media, using uncorrelated distributions of magnetic anisotropy values in order to lower the switching field while keeping a high thermal stability of the media. In this study, a route for narrowing the switching field distribution of the bit array is provided as well, which is vital for the applicability of the BPM concept in magnetic data storage.

The signal-to-noise ratio (SNR) gain of microwave-assisted magnetic recording (MAMR) with a spin-transfer oscillator is analyzed by a micromagnetic model simulating the write process on a CoX/Pt media, by including the Voronoi polycrystalline microstructure built upon a regular mesh of micromagnetic cells. With a fixed head of 40 nm-width main pole and a 10.5 kOe maximum perpendicular field, a 2 dB SNR gain is found for low-coercivity media (0 K $H_{c} kOe) and a 4 dB gain is obtained for high-coercivity media (0 K $H_{c}>14$ kOe). Besides, the MAMR performance considering the finite rise time of single-pole-type head field in a bit is also calculated. It is found that the MAMR SNR gain is obtained in the media with lower anisotropy because of the weak writing capacity during the rise time.

In this study, an exchange coupled composite (ECC) film with an antiferromagnetic sandwich structure L10-FePt/[Co/Ni]5/Ru/[Co/Ni]10 (FePt-CN-Ru) is prepared via dc magnetron sputtering and measured by vibrating sample magnetometer. The results show that FePt-CN-Ru film exhibits ECC characteristics, with excellent angular tolerance, low remanance magnetization, and a narrow switching field. Its thermal stability is higher than that of conventional ECC film. Finally, magnetization reversal is demonstrated through micromagnetic simulation.

In this article, the recording physics of a ring head with a single layered perpendicular medium (Ring/SL) is studied and the results are compared with a single pole head with a double layered perpendicular medium (SPT/DL) by using various simulations and experiments. The Ring/SL has much lower effective medium coercivity than the SPT/DL due to the substantial longitudinal field component of ring head and the incoherent rotation mode of medium magnetizations. Furthermore, switching time of the Ring/SL is estimated as only 10%∼30% of the SPT/DL. In the Ring/SL, the head field gradient of 40 Oe/nm is enough for maximizing SNR. The Ring/SL shows very low noise characteristics especially at high linear density. The signal output of the Ring/SL is smaller than the SPT/DL, but it is large enough to be detected. Therefore, it can be concluded that the combination of single layered perpendicular medium and ring head is highly promising for ultrahigh density recording.

For most of the past 50 years, the areal density of hard disks-a measure of how many bits of data that engineers can squeeze into a given area-increased by an average of nearly 40 percent each year. Lately, though, that rate has slowed to around 10 percent. Everyone who works on magnetic storage is well aware of this problem, but only in the past year or so have executives from Seagate Technology and Western Digital, the leading manufacturers of hard drives, very publicly split on how to solve it. In back-to-back announcements in October 2017, Western Digital pledged to begin shipping drives based on what is known as microwave-assisted magnetic recording (MAMR) in 2019, and Seagate said it would have drives that incorporate heat-assisted magnetic recording (HAMR) on the market by 2020.

To improve writing capability of high magnetic anisotropy medium - L10-FePt, graded medium multilayer is one key candidate. Therefore, investigation of magnetic material properties of graded media multilayer is essential. In this work, we focus on magnetic properties of L10-FePt/Fe graded media multilayer such as hysteresis loop, magnetic energy and magnetic domain by the micromagnetic simulation the object oriented micromagnetic framework software based on the Landau - Lifshitz - Gilbert equation. The graded media multilayer can achieve reducing the switching field, Hsw, below available writing head field with high thermal stability. The anisotropy energy in graded media is higher than single layer and bilayer media that results in unstable magnetization in preferred direction. The proposed graded (f) and (g) structures are high grading magnetocrystalline anisotropy constant, Ku, that can reduce Hsw below single layer, bilayer and available writing head field. The hysteresis loop of low grading Ku structures has different steps; however, it disappear with high grading Ku structures of graded (e) - (g) structures. They have narrow loop compared with single layer. The results from this work lead to improve magnetic trilemma issue and increase data density.

Microwave-assisted magnetic recording (MAMR) is one of the promising candidate technologies for next-generation hard disk drives. In MAMR, the high-frequency field from a spin-torque oscillator (STO) in the recording head is utilized to reduce the magnetization switching field of the medium. Micromagnetic modeling has shown the potential of MAMR; however, in terms of experimental results with actual heads and media, only little evidence of media magnetization reversal and improvement on overwrite has been reported. In this paper, we focus upon the experimental results of MAMR gain defined as a difference in the read-write performance when the STO is activated compared with when not activated. In conclusion, we show clear bit error rate improvement by activating the STO, giving an areal density increase of around 5–10% after considering the penalty associated with the magnetic core width increase. We also confirm that the magnetization switching in the STO is fast enough up to 2.3 GHz, and that there is no negative impact on the adjacent track interference nor on the far track interference.

Microwave assisted magnetic recording (MAMR) has recently emerged as a candidate to solve the trilemma in magnetic recording. In MAMR the magnetizations of recording medium can be switched with much lower magnetic field by magnetic resonance, which however requires a higher frequency oscillator than 20 GHz. In this paper, it is described that spin torque oscillator (STO) using a magnetic material with a large negative uniaxial anisotropy has a possibility to generate higher frequency and stable magnetic field and it might be preferable to a conventional oscillator with soft magnetic materials.

Understanding the heat transfer behavior at the nanoscale head-disk interface (HDI) in hard disk drives is crucial for head design, media design, and failure analysis of the current hard disk drive (HDD) industry, especially for the emerging technologies including heat-assisted magnetic recording (HAMR), microwave-assisted magnetic recording (MAMR), and 2-D magnetic recording (TDMR). Previous experimental studies of both static touchdown technique and theoretical developments of the wave-based phonon conduction have shown enhanced heat transfer at the HDI. To better understand the heat transfer behavior across the HDI, a series of the simulation is necessary to connect the theory and the detailed geometric model of the HDI. In this article, we developed a finite-element model to explain the temperature change of the head during a static touchdown experiment. The wave-based phonon conduction theory is integrated into the simulation through iteration. The simulation results trend in agreement with the theoretical development and the experimental results. This simulation strategy can also be implemented for flying heads to predict the heat transfer behavior under HAMR conditions.