Heat assisted magnetic recording is the most promising technology for high density recording . Due to thermal diffusion of the media, its density potential is limited for continuous wave laser heating . Short pulse laser heating can greatly increase the density potential [1][2][3] . This heating strategy also leads to a low transducer temperature increase [4] . Therefore, short pulse laser heating is a must approach to reach the full potential of HAMR technology . In this talk, the recording performance of short pulse laser heating at density of 4-5 Tb/in 2 is analyzed . The results showed that the media magnetic switching time becomes a key factor to determine recording performance for pulse width of less than 200 ps . A relatively large damping constant a is desired to speed up its switching time . For a higher density recording, a relatively high bit density is preferable because a lower track density could be recorded by applying a higher magnetic field H which can relax the demand for a . Fig . 1 shows the dependences of the required minimal a and magnetic field H on desired recording density for bit length of 6 .5nm . Considered the measured FePt damping constant [5], a recording density of 4 .5 Tb/in 2 is achievable for FePt media with pulse width of 100ps . Density of 5 T/in 2 is also possible to be achieved by using the media with a larger saturation magnetization M s , which will lead to a fast switching speed . In real implementation of short pulse laser heating for HAMR, there are four factors for short pulse laser series to affect the recording performance: i) . Phase shift between laser pulse and magnetic field for the uniform pulse laser series [3, 6]; ii) . Frequency jitter of the pulse laser series; iii) . Laser pulse width variation; and iv) . Laser pulse power variation . The effects of these factors on the recording performance were analyzed . Fig . 2 shows the dependence of SNR on phase shift at laser pulse width of 100ps . Unlike the wide pulse heating, short pulse heating gives a narrower deterioration gap because of the short heating period . Fig . 3 shows the effects of the media peak temperature variation on SNR at pulse width of 100ps . SNR decrease with the increase of peak temperature deviation . At sigma of 6%, SNR decrease about 2dB . Pulse laser generation with pulse width of ≤ 100ps is a key issue for short pulse laser heating . In this talk, the different technologies for semiconductor laser to generate short pulse laser output are discussed . It is indicated that gain-switching technology is a better way to generate short pulse output (≤100ps) for HAMR application . Fig . 4 shows the pulse laser waveforms with pulse frequency of 2 GHz and pulse width of 97ps generated by gain-switched semiconductor laser . The frequency standard deviation (s) is 9 .23MHz and the pulse width standard deviation is 5 .4ps . Finally, some of other concerns for short pulse laser heating are also discussed .
Active magnetic bearing is a promising candidate for next generation spindle motors of hard disk drives. In this paper, an Iterative Learning Control scheme is proposed to suppress the rotor poison runout induced by the unbalanced forces in AMB. The ILC controller can produce the synchronous compensation force by iteratively "learning" rotor runout information. This compensation force makes the rotor rotate about its geometric axis.Index terms-active magnetic bearings, spindle motors.
There is a growing need to reduce the size and cost of power converter, which was widely used in portable/wireless devices like mobile phones, palmtop terminals, organizers, and other versatile communication tools. [1,2] For this purpose, the use of high frequencies (e.g. > 100 MHz) combined with thin ultra-soft high magnetic moment (Bs) films is desirable. High moment soft magnetic films are also widely used in modem electromagnetic devices, such as a high-frequency field-amplifying component, read-rite heads for magnetic disk memories in computers, and magnetic shielding material in tuners. [3] Although there are a lot of soft magnetic films, such as Fe based Co, Si, N, B, Al, Ti, Cr, Zr, Hf, Nb, and Ta alloys that have been achieved currently, an ultra-soft (Hc < 0.3 Oester (Oe)) magnetic film with high magnetic moment (Bs > 1.0 Tesla (T)) is yet to be achieved.[3] Herein, we report a novel approach to fabricate ultra-soft magnetic NiFe films from a sulfate salt based solution containing a little quantity of Cu2+ additive via electrodeposition, which is well known as a cheap and simple way to prepare metal films. The measurements from atomic force microscopy (AFM) and alpha-stepper showed that the thickness of prepared films ranged from tens of nanometers to micrometers. The magnetic characterization performed using vibrating sample magnetometer (VSM) and quantum design superconducting quantum interfaces device magnetometer (SQID) showed that the film possessed very soft magnetic property with an easy axis coercivity (Hce) and hard axis coercivity (Hch) no bigger than 0.15, 0.04 Oe, respectively. The film also had a high saturation magnetization of almost 1.65 T and good anisotropy. The NiFe films were electrodeposited on either Si (100) or SiO2 (amorphous) wafers. A seed layer such as Au, Cu or NiFe with thickness of 20 -30 nm was sputtered on each wafer as electrical conducting layer for the electrodeposition. The permalloy NixFe100-x (x = 45 -82) films were fabricated at the temperature of 40 ± 2 o C from a solution of 0.2 mol/l NiSO4.6H2O, 0.025 -0.035 mol/l FeSO4.7H2O, 0.28 mol/l NH4Cl, 0.4 mol/l H3BO3, and low concentration of Cu2+ additive. The electroplating system used was 55 l Paddle-Cell with a DC power. The atom ratio of Ni to Fe in the electrodeposited permalloy films was determined by energy dispersive spectra. The electroplating current density was controlled in the range of 15 -25 mA/cm2. Table 1 shows that the little amount (< 0.001 mol/l) of Cu2+ additive (added as CuSO4.5H2O or CuCl2.2H2O), greatly decreased the coercivity of Ni55Fe45 permalloy films without decreasing their Bs. It is because the Cu and O components (which decrease the Bs value) were of very low content in the deposited film and thus could not be detected with EDS. The Hce decreased from 0.5 to 0.15 Oe while the Hch decreased from 0.3 to 0.03 Oe when Cu2+ concentration increased from 0.0 to to 0.0006 g/l in electroplating solution. Figure 1 shows that the film changed from not having to having anisotropy when Cu2+ concentrat...
Self-Organized Magnetic Array (SOMA) is a promising candidate for future ultrahigh density recording media with an areal density of Tb/in2 and beyond [1]. Such type of recording media is composed of grains with a uniform size distribution and a non-magnetic coating to reduce exchange coupling. In this paper, a 3D finite element micromagnetic model is used to simulate the switch dynamics of SOMA recording system. The influence of media grain properties such as anisotropy field distribution is investigated. Our studies also include the signal to noise ratio (SNR) with respect to different bit-array alignments. The fluctuations of signal and noise are investigated under the condition that the transition location is shifted in a square assembled array. In conventional finite element micromagnetic simulations, the hybrid FEM/BEM is commonly used approach to solve the demagnetization field [2]. However, the computational cost associated with BEM, i.e., is very expensive for large-scale problems. In this study, we employed the FFTM algorithm [3] to overcome this computational bottleneck. Figure 1 shows the complexity plots for this part of the calculation using: (i) direct evaluation of the dense matrix-vector product, and (ii) accelerated with FFTM. It is obvious that FFTM can be significantly more efficient that the direct approach. A more detailed discussion on this fast algorithm will be given in the full paper. The recording layer is modeled by 3D spherical particles with a distribution of out-of-plane easy axis anisotropy. The nanoparticles are FePt-based, with diameters of 4 nm and inter-particle distance of 2 nm. An analytical solution [4] is applied to describe the write field for perpendicular recording system. Figure 2 shows the field distribution for horizontal (Hy) and perpendicular (Hz) components along the recording direction at the medium center. Transitions are recorded in a 32*32 array. The angle between the head trailing edge and x-dimension of the array denotes the bit-array alignment status (Fig. 3). The magnetic recording media is simulated using the combination of the micromagnetic modeling and the analytical write field. Figure 4 shows signal and noise fluctuations inside the transition position shifting range-one grain pitch in x direction. For 0 degree alignment, the coincidence of signal minimum and noise maximum corresponds to the situation that transition center is located at the grain site. In the full paper, the effect of anisotropy distribution and damping effect on recording performance will be also discussed. Fig.1 Complexity plots of direct (dashedlines) and FFTM methods (solid-lines). Fig.2 Schematic diagram of head field distribution calculated from analytical head model: Horizontal (Hy) and perpendicular (Hz) field component at media center. Fig 4 SNR fluctuations of different bit array alignments in perpendicular oriented SOMA media Fig.3 Transition position in the modeling array with head-media alignments at (a) 0 degree; (b) 10 degree
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