Spin Torque Oscillator With Negative Magnetic Anisotropy Materials for MAMR

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.

Hard disk drives (HDDs) have been investigated to determine how to increase their areal recording density. Recently, however, HDDs have been having problems with noise, thermal fluctuations, and writability simultaneously, commonly known as a trilemma. Microwave-assisted magnetic recording (MAMR) is a promising recording method to solve this trilemma [1-4]. In MAMR, a spin torque oscillator (STO) is placed between a conventional write head and a trailing shield. Because the AC field from the STO decreases the switching magnetic fields of the medium, the writability can be improved. As a result, the trilemma can be solved. Therefore, a high frequency AC field with strong amplitude is needed. In this study, we examined methods to increase the AC-field frequency by using a micro-magnetic simulator that takes into account magnetic interactions between the write head, the medium, and the STO.

We investigated spin torque induced magnetization dynamics in the mag-flip spin torque oscillators (STOs) of diameters D from 29 to 96 nm comprising of an in-plane magnetized field generation layer (FGL) Fe67Co33 (7 nm) with high saturation magnetization, μ0Ms ∼ 2.3 T, and perpendicular FePt(10 nm)/Co2FeGa0.5Ge0.5(3 nm) highly spin polarized spin injection layers. Out-of-plane high frequency, f ∼ 21–26 GHz, spin torque induced oscillation with a large cone angle in FGL was observed under nearly perpendicular external magnetic field μ0Hext of 1.1 T for the pillar D of 29 and 42 nm. Our micromagnetic simulation results indicated that ac magnetic fields of about 0.15 to 0.2 T are obtainable from the STOs having the same stacking structure and size as the experiment, which is large enough for the applications to microwave assisted magnetic recording technology.

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.

Chemical surface modification for the preparation of MgO cladding to enhance magnetic properties of Fe-Si-Nb-B-Cu nanocrystalline soft magnetic composites

ISAMMA 2007 was the first consolidated symposium of the three previous symposia held in the Asian region: ISPMM (International Symposium on Physics of Magnetic Materials) of Japan which started in 1987, ISAMT (International Symposium on Spintronics and Advanced Magnetic Technologies) of Taiwan and SOMMA (International Symposium on Magnetic Materials and Applications) of Korea, both of which started in 1999. Considering that there are too many international and domestic meetings around in recent years, I am sure that this kind of consolidation will benefit the people working in magnetics research in the region. ISAMMA 2007 was held on Jeju island, Korea, during the period 28 May to 1 June 2007. The number of participants was 533 from 26 countries. These statistics clearly indicate that the meeting was truly international! The number of presentations was even higher, there being a total of 613 presentations (174 oral presentations including invited talks and 439 poster presentations). A wide range of subjects were covered in the symposium. The technical sessions included:– Biomagnetics– Magnetotransport– Thin films and multilayers– Magnetoresistive Random Access Memory (MRAM)– Nanostructured materials– Spintronics– Magnetic oxides and multiferroics– Magnetic recording and magneto‐optical recording– Magnetic theory and calculation– Hard magnetic materials– Soft magnetic materials– Magnetic sensors and devices– Spin dynamics– Magnetic oxidesThe number of manuscripts submitted was approximately 320, among which ∼200 manuscripts were accepted and included in the proceedings. It was a challenging task to deal with such a large number of manuscripts in a short period of time. The formidable task has been completed successfully mainly thanks to the great efforts by the Publication Editors: Dr. Naoki Honda (Akita Research Institute of Advanced Techology, Japan), Prof. Minn‐Tsong Lin (National Taiwan University, Taiwan), Prof. Masahiro Yamaguchi (Tohoku University, Japan), and Prof. Kyung‐Jin Lee (Korea University, Korea). The assignment of reviewers in some sessions was problematic, causing extra burden to the Editors. Due to this and other unexpected problems, the job the editors have done is much beyond the originally anticipated one. My sincere thanks go to you all for your professional service and time, without which the proceedings would have not been issued in the present form. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Advanced electro-magnetic machines and systems require new materials with improved properties. Heterogeneous 3D nanomodified soft magnetic materials could be efficiently applied. Multistage technology of iron particle surface nanomodification by sequential oxidation and Si-organic coatings will be reported. The thickness of layers is 0.5-5 nm. Compaction and annealing are the final steps of magnetic parts and components shaping. The soft magnetic composite material shows the features: resistivity is controlled by insulating coating thickness and equals up to ρ =10-4 Ω⋅m for metallic state and ρ =104 Ω⋅m for insulator state, maximum magnetic permeability is μm = 2500 and μm = 300 respectively, induction is up to Bm=2.1 T. These properties of composite soft magnetic material allow applying for transformers, throttles, stator-rotor of high-efficient and powerful electric machines in 10 kHz–1MGz frequency range. For microsystems and microcomponents application, good opportunity to improve their reliability is the use of nanocomposite materials. Electroplating technology of nanocomposite magnetic materials into the ultra-thick micromolds will be presented. Co-deposition of the soft magnetic alloys with inert hard nanoparticles allows obtaining materials with magnetic permeability up to μm=104, magnetic induction of Bs=(0.62–1.3) T. Such LIGA-like technology will be applied in MEMS to produce high reliable devices with advanced physical properties.

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.

Electron holography, which visualizes magnetic and/or electric fields in materials on the nanometre scale, is a powerful tool for the study of fundamental issues in physics as well as the characterization of advanced materials. This paper presents an overview of recent electron holography studies on advanced magnetic materials, which include hard magnetic materials (both nucleation-type and pinning-type magnets), soft magnetic materials (both classical alloys and recently developed nanostructured materials), magnetic recording materials (Co–CoO tape and other related topics) and magnetic functional materials (ferromagnetic shape memory alloys and colossal magnetoresistive manganites).

A potential technology for use in future hard disk drives is microwave-assisted magnetic recording (MAMR). In a MAMR drive a spin-torque oscillator (STO) is integrated into the write head and applies a high frequency (HF) magnetic field to the recording medium along with the field from the write head. The HF field reduces the switching field of the medium, allowing media with higher uniaxial anisotropy to be used. As the write head moves over the recording medium the HF field seen by the medium grains changes from elliptical to circular to linear and back again. In this work the influence of circular, elliptical and linear HF fields on recording media was examined by calculating hysteresis loops for single and multiple grains.

Soft-hard magnetic composites have attracted much attention as magnetic materials for permanent magnets due to high coercivity and saturation magnetization in various fields such as motor, storage device and magnetic actuators. Hard magnetic materials with low saturation magnetization improve magnetic properties through exchange coupling effect with soft magnetic materials. Rare earth metals, which are used as magnetic materials in permanent magnetic, have issued such as high cost due to resource scarcity. Transition metals have been used for fabricating magnetic material with high magnetic properties due to low cost and abundant in the resource. Spinel magnetic materials with transition metals have excellent chemical, thermal stability, large magnetocrystalline anisotropy and magnetic properties. Composite magnetic materials in one dimensional (1D) morphology are effective in improving coercivity. In this work, soft-hard composite fiber has been fabricated by electrospinning method using iron oxide as soft magnetic material and cobalt ferrite as hard magnetic material. The change in the oxidation state of iron oxide can be controlled with sintering atmosphere and influences on the magnetic properties. In comparing with simple CoFe2O4, the fabricated Fe3O4–CoFe2O4 exhibits excellent saturation magnetization of 133.63 emu g−1 and enhanced coercivity of 2746.9 Oe.

Replacing Ir with Rh in a CoIr system possessing negative uniaxial magnetocrystalline anisotropy (Ku) substantially reduces its magnetic damping and coercivity by more than half while retaining its high negative Ku. Moreover, a higher saturation magnetization (Ms) and more isotropic coercivity are achieved. Such material development makes it particularly suitable for use as the soft underlayer (SUL) of magnetic recording media for reducing noise, and as the oscillation layer of a spin-torque oscillator (STO) for achieving higher oscillation frequency, larger AC magnetic field and lower driving current, which can be readily integrated with the current recording head for microwave-assisted magnetic recording. Finally, we recommend a composite free layer by coupling CoIr with a spin polarizer (Co or Co/Cu/Co) for the enhancement of the spin-polarization rate and, therefore, the improvement of STO efficiency. These could pave the way for CoIr-based materials to be implemented in devices requiring a negative Ku with low damping and high ‘softness’, such as oscillators.

A micromagnetic model analysis was carried out considering two types of write heads and three types of spin-torque oscillators (STOs). For write heads, we considered a small-sized head to reduce calculation time and a large-sized head with dimensions close to commercial heads. For STOs, we used a double-layered STO with reflection spin torque, a double-layered STO with transmission spin torque, and a tri-layered STO with two spin injection layers. We also carried out micromagnetic simulations of the STOs without the write heads (isolated STO) to compare the oscillation states. It was found that, compared with isolated STOs in a uniform applied field, a larger applied field was necessary for STOs integrated into a write head. This result shows that the write head should be designed to generate a large, well-shaped gap field to ensure stable STO oscillation.

To overcome superparamagnetic limit, microwave assisted magnetic recording (MAMR) is one interesting magnetic recording technology. Therefore, the effect of microwave on magnetization reversal in media should be analyzed. In this work, we propose the MAMR to decrease switching field (coercivity, Hsw) in exchange coupled composite (ECC) media by using the micromagnetic simulation based on the Landau - Lifshitz - Gilbert equation. The Hsw of single layer and ECC media without microwave field is 110.90 and 7.7 kOe, respectively. When the oscillating microwave field is added, Hsw of single layer media with microwave frequency of 2.5 - 40 GHz is lower than 110.90 kOe. Likewise, Hsw of ECC media with microwave frequency of 5 - 16 GHz is lower than 7.7 kOe and has the lowest value of 4.9 kOe at frequency of 10 GHz. The results from this work lead to solve superparamagnetic limit and increase areal density in hard disk drive.

In the last few years there has been rapid progress in the application of nuclear magnetic resonance clinical imaging as a diagnostic aid (Partain et al, 1983; Steiner, 1983; Worthington, 1983). Records kept by the operators of NMR imaging equipment indicate that about 1800 individuals have now been examined without incident within the United Kingdom. In view of this progress, an advisory group (Appendix) appointed by the National Radiological Protection Board recommends some modifications to the original advice given in 1981 (NRPB, 1981). The principal modifications are that: (1) It is no longer considered necessary to exclude volunteers with a history of epilepsy or cardiac disease. (2) It is not obligatory that a registered medical practitioner trained in resuscitation techniques be present during an exposure. (3) Higher limits are recommended for rates of change of magnetic flux density when the period of change is less than 10 ms. (4) The radiofrequency limit of 15 MHz has been removed. Theoretical considerations indicate that the interaction of an applied magnetic field with the intrinsic magnetic moments of protons and electrons within a molecule could alter its rate of chemical reaction (McLauchlan, 1981). Chemical reactions involving radical pair intermediates are expected to be sensitive to magnetic fields for this reason and a number of enzyme reactions that might be expected to be affected have been identified (A. Cass, personal communication). However, there is no experimental confirmation of these theoretical possibilities at present.