Perpendicular Media Research Articles

The influence of exchange-bias on the resonance frequencies and damping in IrMn3/[Co/Pt] multilayers

In order to realise single on-chip three axis magnetic field sensing technology without the need for convoluted die stacking, perpendicular media must be exploited. This poses additional challenges in both static-bias and the dynamics of the stack. Presented here are ferromagnetic resonance measurements (FMR) on exchange-biased multilayers of IrMn3/[Co/Pt]n. We chart the evolution of the different anisotropies caused by both magnetic field annealing and increasing repetition number n. The damping of these stacks does not obey the usual symmetry seen in single and bilayer samples with in-plane anisotropy, but instead follows a sin2(θ) dependence. This, we propose, is entirely dominated by spin diffusion from the Co layers into the adjacent heavy metal layers. This is modelled using a simple effective circuit model, providing a guide for the optimisation of complex multilayers.

Simulation Analysis of Thermal Decay and Read-Write Characteristics Depending on the Structure of Sputtered Tape Media

We developed a magnetic recording simulator that takes into account thermal agitation by employing a probabilistic switching model based on the Arrhenius-Neel equation. Using this simulator, we investigated the impact of Capped and <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">H_k</i> -Graded media on sputtered perpendicular media for magnetic tape recording. Our simulation results were in good agreement with the experimental data, as evidenced by the consistency of the M-H curves and read-write characteristics. Specifically, we found that the use of Capped media enhances writability, but it also increases the transition noise, while Graded media has no significant effect on writability. Furthermore, our thermal decay simulation showed a correlation between the reproduction output decay and the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">M_r/M_s</i> decay, and no clear relationship between the media SNR decay over time and the media <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">K_uV/k_BT</i> . These findings illustrate the potential of our simulator as a valuable tool for designing magnetic media.

Pattern constraints limiting nonlinear transition shift in high density magnetic recording

Nonlinear transition shift (NLTS) produced by the interaction between transitions is a major source of distortions in magnetic recording. NLTS must be reduced to realize higher density and maintain signal quality. However, there is some confusion about the production and behavior of NLTS in previous reports. In this paper, combined with micromagnetic simulation, we have developed a complete theoretical model to predict NLTS on perpendicular magnetic media. The results show that one-level NLTS (caused by one previous transition) will monotonically decrease as the present transition becomes further away from the previous one, and multilevel NLTS (caused by consecutive previous transitions) will firstly increase and quickly approach a constant value as there are more consecutive transitions. Particularly, it is demonstrated that NLTS and its impact on the media signal-to-noise ratio can be significantly limited by pattern constraints.

Zero-State Insert Dual-Layer HAMR Media Recording

Heat-assisted magnetic recording (HAMR) can support extremely high areal densities based on the thermal stability argument. However, significant challenges remain to reduce switching probability distribution (SPD) to improve media SNR. Exchange coupled composite (ECC)-type architectures, such as the designs employed in perpendicular media, are being explored for HAMR media and show potential promise at reducing SPD. In this article, we propose a new approach for the media design that consisting of an exchange decoupled dual-layer HAMR media architecture. In this design, a few net zero magnetization state grains form near the transition resulting in a transition noise reduction when a small difference in a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$T_{\text {c}}$ </tex-math></inline-formula> of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\approx 5$ </tex-math></inline-formula> % was applied between the two layers. A geometric 2-D recording model was used to explore this dual-layer media design space and transition noise response. The significant transition noise reduction under the grain size limit condition is attributed to a negative correlation between noise from the top and bottom layers. This simple model was supplemented with micromagnetic recording simulations that verify stable zero-state grains are established near the transition during the recording process. The simulations confirm the significant transition noise reduction, but an increase in the dc noise was observed. Once the dc noise was addressed by increasing the applied field magnitude, a significant SNR gain of 1.4 dB was achieved compared with the equivalent single-layer media.

Synthesis and Characterization of Composites with Y-Hexaferrites for Electromagnetic Interference Shielding Applications

The current research is focused on the chemical process and characterization of Co-based Y-type hexaferrite, electrochemically active polypyrrole doped with dodecylbenzene sulphonicacid (PPy-DBSA) and their composites. The microemulsion technique was used to produce hexaferrite with the formula Sr2Co2Fe12O22. The resistivity of pure ferrite specimens was 103 ohm-cm, which was lower than the 106 ohm-cm resistivity of the monomer utilized in the polymerization operation. As the temperature increases, the DC resistance decreases, revealing the specimens’ semiconductor nature. The cole-cole plots have been used to assess whether significant grain boundaries were involved in the dielectric relaxation process. By increasing the frequency, the electrochemical performance of all specimens was enhanced. Using the rate equation, ionic conductivity demonstrates that polarons are responsible for conduction. Because of the characteristics of the polymer PPY-conducting DBSA, the composites PPY/DBSA + Sr2Co2Fe12O22 exhibit a higher dielectric loss of 35 at 1 MHz. This specimen is perfect for electrical radiation shielding (EMI).These ferrites are widely used as permanent magnets, in microwave devices, high-density perpendicular media, and rigid disk media without lubricant and protective layers.

Switching Field Distribution in BN/FePtCAg/MgTiON and FePtCAg/MgTiOBN Films.

BN is the currently required segregant for perpendicular FePt media. We found that BN can be diffused from the MgTiOBN intermediate layer during a high temperature process. The FePtCAg film sputtered on MgTiOBN layers illustrates higher perpendicular magnetocrystalline anisotropy (Ku) (1.43 × 107 erg/cm3) and coercivity (normal to film surface) (17 kOe) at 350 K compared to BN/FePtCAg/MgTiON film. From the microstructure, the FePtCAg film shows the granular structure on the MgTiOBN intermediate layer, but parts of the irregular FePt grains are agglomerated and partially separated in the matrix, with grains size being, on average, 26.7 nm. Cross-sectional imaging showed that the FePt grains have a truncated pyramid shape with a lower wetting angle, which is influenced by the surface energy of MgTiOBN. BN segregation at FePt grains or boundaries is still not clear. Using the electron energy loss spectrum (EELS), we found that part of the BN atoms were clearly observed in the FePt lattice and iron-boride oxide was indexed in the x-ray photoelectron spectroscopy (XPS) spectra. To determine the effects of BN segregant (from capping layer or intermediate layer) on the magnetic switching behavior of FePtCAg film, the intrinsic-(ΔHint = 6.17 kOe, 6.54 kOe) and extrinsic- (ΔHext = 0.80 kOe, 0.39 kOe) switching field distribution (SFD) were measured by plotting saturated major- and unsaturated minor- hysteresis loops to evaluate the crystal orientation and microstructure (grains volume and distribution) for BN/FePtCAg/MgTiON and FePtCAg/MgTiOBN films, respectively. The main contribution of intrinsic SFD is the c-axis misalignment for the BN/FePt/MgTiON sample; however, the dispersed magnetic anisotropy has a higher input to intrinsic SFD for FePtCAg/MgTiOBN/CrRu film.

High-resolution alternating magnetic force microscopy using an amorphous FeB-based tip driven by an inverse magnetostrictive effect: Imaging of the high-density magnetic recording media

The improved spatial resolution of the alternating magnetic force microscopy (A-MFM) technique developed in this work is demonstrated using a tip coated with a very thin magnetostrictive Fe-B-based amorphous film. Its magnetization can be periodically reversed using an applied alternating current magnetic field, which leads to alternating mechanical stress at the tip end, thus affecting localized magnetization reversal via an inverse magnetostrictive effect. Spatial resolution improvement is achieved via effective reversible magnetization volume at the sharpened tip end, which promotes a decrease in the total magnetic volume and maintains sufficient sensitivity at a lower coating thickness. A-MFM uses frequency modulation induced by an off-resonant alternating magnetic force between the tip and the sample. Therefore, a magnetic force can be selectively detected near the sample surface, independent of non-magnetic interactions. Due to their smooth surfaces without crystalline grain boundaries, large magnetization, and large magnetostriction compared with those of other magnetic materials, Fe-based amorphous films are suitable for tip coating applications. A 500-kfci perpendicular magnetic recording medium (PMR) was imaged using a 6-nm thick amorphous Fe-B coated tip to produce highly resolved images with minimal detection of the wavelength of the magnetic field of less than 10 nm. Also, 1600-kfci PMR imaging was successfully achieved, with the images showing individual ∼ 15-nm bits.

Analyzer-free, intensity-based, wide-field magneto-optical microscopy

In conventional Kerr and Faraday microscopy, the sample is illuminated with plane-polarized light, and a magnetic domain contrast is generated by an analyzer making use of the Kerr or Faraday rotation. Here, we demonstrate possibilities of analyzer-free magneto-optical microscopy based on magnetization-dependent intensity modulations of the light. (i) The transverse Kerr effect can be applied for in-plane magnetized material, as demonstrated for an FeSi sheet. (ii) Illuminating that sample with circularly polarized light leads to a domain contrast with a different symmetry from the conventional Kerr contrast. (iii) Circular polarization can also be used for perpendicularly magnetized material, as demonstrated for garnet and ultrathin CoFeB films. (iv) Plane-polarized light at a specific angle can be employed for both in-plane and perpendicular media. (v) Perpendicular light incidence leads to a domain contrast on in-plane materials that is quadratic in the magnetization and to a domain boundary contrast. (vi) Domain contrast can even be obtained without a polarizer. In cases (ii) and (iii), the contrast is generated by magnetic circular dichroism (i.e., differential absorption of left- and right-circularly polarized light induced by magnetization components along the direction of light propagation), while magnetic linear dichroism (differential absorption of linearly polarized light induced by magnetization components transverse to propagation) is responsible for the contrast in case (v). The domain–boundary contrast is due to the magneto-optical gradient effect. A domain–boundary contrast can also arise by interference of phase-shifted magneto-optical amplitudes. An explanation of these contrast phenomena is provided in terms of Maxwell–Fresnel theory.

Large and Local Magnetoresistance in a State-of-the-Art Perpendicular Magnetic Medium.

PurposeMagnetotransport properties of granular oxide-segregated CoPtCr films were studied on both macroscopic and microscopic length scales by performing bulk and point-contact magnetoresistance measurements, respectively. Such a perpendicular magnetic medium is used in state-of-the-art hard disc drives and, when combined with magnetotransport phenomena for read/write operations, may lead to a novel concept for magnetic recording with high areal density.Materials and MethodsThe CoPtCr films were deposited by an epitaxy-like sputtering and contained several perpendicularly magnetized granular-media layers with different coercivities; they are very much like the state-of-the-art perpendicular magnetic medium, which can be found in today’s hard disc drives. Magnetoresistive properties of bulk films were assessed by measuring the film resistance in the standard Van der Pauw geometry, while the local transport was probed by the point-contact technique.ResultsThe bulk measurements showed only a negligible magnetoresistance of less than 0.02%. In contrast, the local point-contact measurements revealed giant-magnetoresistance-like changes ΔR in local resistance of the contact R with more than 10,000% ratio ΔR/R.ConclusionThe observed large and local magnetoresistive effect could be tentatively attributed to a tunnel magnetoresistance between oxide-segregated CoPtCr grains with different coercivities. The tunneling picture of electronic transport in our granular medium was confirmed by the observation of tunneling-like current–voltage characteristics of the contacts and bias dependence of the contact magnetoresistance – both the local point-contact resistance and magnetoresistance were found to decrease with the applied dc bias.

Universal thermal decay produced by time scaling in magnetic films and recording media

The logarithmic relaxation of the magnetisation in recording media under different applied negative fields can collapse onto an universal decay curve, by applying a scaling law based on the fluctuation field Hf, which describes the time-dependent magnetic behaviour in terms of thermal effects. In this work the scaling approach is investigated, in which multiple measurements of thermal decay curves in varying holding fields are scaled via the fluctuation field in order to reproduce the long-time decay of magnetisation. The scaling technique is tested using a numerical model of a granular perpendicular media and then applied to experimental data. Scaling is shown to be successful for systems of multiple layers of soft (Ni) and hard (Co) magnetic materials, coupled by a Pt interlayer and for CoCrPt granular recording media systems. Based on a dataset of decay curves in negative fields, the data lifetime of magnetic recording media can be quantified, this representing an important factor for media design and production.

Alternating Magnetic Force Microscopy: Effect of Si doping on the temporal performance degradation of amorphous FeCoB magnetic tips

In order to increase the resolution of traditional MFM and extend its applications in the past, we created an Alternating Magnetic Force Microscopy (A-MFM) with a magnetic force modulation by applying an off-resonance AC magnetic field. A new magnetic tip has been developed for A-MFM in this work. Perpendicular magnetic recording medium with 500 kfci recording density is successfully imaged and analyzed by using the new tips coated by 6–15 nm amorphous FeCoSiB-based soft magnetic film. The effect of Si doping to the magnetic film on the A-MFM imaging was studied. It was found that Si additive had no effect on the imaging properties, such as signal-to-noise ratio and spatial resolution (kept at ∼5 nm level). But on other side, Si additive significantly degraded the chemical stability of FeCoSiB film due to oxidation in ambient atmosphere. Surface passivation by metal and boron oxides is considered to have a main effect on improvement of chemical stability of Si-free magnetic coating. The results have shown that Si-free FeCoB-based magnetic tips are appropriate candidates for A-MFM imaging in ambient conditions.

Frequency dependence of microwave-assisted switching in CoCrPt granular perpendicular media

The microwave-assisted switching properties of a CoCrPt–TiO2 granular perpendicular medium are investigated by detecting the anomalous Hall effect. We improved the device structure to be able to generate a linearly polarized microwave field of 950 Oe without a significant increase in temperature. Coercivity decreases linearly with increasing microwave frequency and the microwave assistance effect is most enhanced by the application of microwaves of 18 GHz frequency with a coercivity reduction ratio of 55%.

Relaxation dynamics in magnetic antidot lattice arrays of Co/Pt with perpendicular anisotropy

The topic of magnetic antidot lattice (MAL) arrays has drawn attention from both fundamental research as well as from application point of view. MAL arrays are promising candidates for making domain engineering in thin films. For various applications it is necessary to understand the magnetization reversal mechanism as well as the relaxation dynamics. In this context we have studied magnetic antidot lattice (MAL) arrays of Co/Pt with perpendicular anisotropy fabricated by combination of photolithography and sputtering deposition. Kerr microscopy domain imaging for the continuous thin film reveals the formation of typical bubble domains of perpendicular media with high anisotropy. However, presence of periodic holes in the MAL arrays lead to nucleation of localised smaller bubbles. We have performed simulations using object oriented micromagnetic framework (OOMMF) which reproduced the experimental results even considering antidot arrays in nano dimension. In literature it has been reported that in MAL arrays with in-plane anisotropy the domain propagation gets significantly hindered by the presence of the holes. However here we show that in perpendicularly magnetized Co/Pt the propagation of the domain walls is not restricted by the presence of the antidots. Further we have performed magnetic relaxation study and found that the global relaxation time for the MAL arrays of Co/Pt is faster as compared to it’s parent thin film. This behavior is opposite to what has been observed in literature for in-plane magnetized MAL arrays.

Effect of heat-treatment on the magnetic and optical properties of Sr0.7Al0.3Fe11.4Mn0.6O19

Sr0.7Al0.3Fe11.4Mn0.6O19 was synthesized by sol-gel technique and sintered at varying temperatures 750 °C, 850 °C, 950 °C and 1050 °C for 5 h. We investigate the role of heat-treatment on the structural, magnetic and optical properties of Al–Mn doped strontium hexaferrite. XRD analysis indicates the formation of M-type hexaferrites with some traces of α-Fe2O3. The presence of two prominent peaks between 400 cm−1 and 600 cm−1 in the spectra of FTIR gives the possibility of the emergence of M-type hexagonal ferrites. FESEM micrograph shows that the grains of the prepared sample possess Plate-like hexagonal structure. EDX spectra confirmed the stoichiometry of the prepared sample. Recrystallization of the synthesized sample and formation temperature of 650 °C is observed in TGA analysis. Variation of the sintering temperature results in decrease of the band gap. Magnetic parameters were calculated from the M–H loop obtained from vibrating sample magnetometer (VSM). High value of coercivity (6750 Oe) is observed at 850 °C. Remanent and saturation magnetization decreases with increase heat-treatment temperature. The synthesized sample may be useful in perpendicular recording medium (PRM) and high frequency application.

Exchange-coupling of hard and soft magnetic sublattices and magnetic anomalies in mixed spinel NiFe0.75Cr1.25O4 nanoparticles

A set of single-crystalline nanoparticles (NPs) of nickel-chromium ferrite NiFe0.75Cr1.25O4 with a cubic spinel structure were synthesized and investigated. The NPs size can be varied from about 5 to 50 nm by the final annealing of the precursor at different temperatures. The distribution of cations over the tetrahedral (A) and the octahedral [B] sites (Fe0.75 Ni0.25) [Ni0.75 Cr1.25] O4 was established from the magnetic and Mössbauer measurements. In large NPs, the magnetic structure at low temperatures is close to the collinear antiferromagnetic (AFM) structure of the Neel type; and the total magnetic moment Mtot of the ferrite coincides with the direction of the B-sublattice moment. Several size-dependent magnetic anomalies were revealed. Three types of magnetic ions present in the A- and B- sublattices cause the competition of AFM and FM exchange interactions resulting in the highly frustrated magnetic ordering and the occurrence of canted magnetic structure in the octahedral B-sublattice. The frustrated structure is very flexible and significantly subjected to temperature and applied field. It results in several magnetic anomalies observed, including the occurrence of magnetic compensation, abnormal behavior of ZFC and FC magnetization curves and hysteresis loops. It was shown that magnetic anomalies can be explained in terms of exchange coupling of “soft” and “hard” magnetic B- and A-sublattices. This effect in the (Fe0.75 Ni0.25) [Ni0.75Cr1.25] O4 NPs can be considered as an atomic-scale analog of a similar effect observed in two-phase exchange-coupled alloys developed for permanent magnets and for the perpendicular recoding media.

Thickness Dependence of Magnetic Switching Dynamics of Barium-Ferrite as A High-Density Perpendicular Magnetic Storage Media

Micromagnetic study of material thickness dependence of Barium-ferrite nano-dot magnetization dynamics has been performed. The used materials characteristics in this research represent the properties of Barium-ferrite. Barium-ferrite was modeled as a nano-dot with a surface area of 50 50 nm2 and its thickness varies from 5 nm to 100 nm. This nano-dot was simulated using micromagnetic simulator software by solving Landau-Lifshitz-Gilbert equation. According to this study, obtained that the Barium-ferrite nano-dot has excellent thermal stability. Magnetization rate of this nano-dot decreases exponentially with the increase of thickness. The fastest magnetization rate observed in 5 nm of nano-dot thickness, meanwhile 45 nm for the slowest rate. Magnetization reversal mode of this Barium-ferrite nano-dot is dominated by domain wall nucleation and propagation. During the propagation of the domain wall, the exchange interaction becomes the main aspect compared to the other contributed energies.

1-6 二層垂直記録媒体のためのPRML方式の一検討

1-6 二層垂直記録媒体のためのPRML方式の一検討

Influence of exchange on signal-to-noise ratio in [CoX/Pt]4 media

In longitudinal hard disk drives, the medium Signal-to-Noise Ratio (SNR) is higher with better grain segregation or lower inter-grain exchange. In current energy assisted magnetic recording system, multilayer perpendicular media are utilized; thus, it is significant to study the influence of grain segregation on SNR, as well as the relevant percolation phenomenon, to give suggestions on the recording media design. In this study, micromagnetic recording models of Microwave Assisted Magnetic Recording (MAMR) is built up to calculate SNR to find optimized [CoX/Pt]4 media parameters such as the inter-grain exchange Agb and the anisotropy orientation distribution αθ, with different field generation layer (FGL) saturation in the spin torque oscillator (STO). The constrained relationship between Agb and αθ in MAMR have been estimated, and the medium SNR will be optimized in the perpendicular [CoX/Pt]4 with a proper but not lowest exchange.

Measurement and Simulation of Nanoscale Gap Heat Transfer Using a Read/Write Head With a Contact Sensor

Heat-assisted magnetic recording (HAMR), microwave-assisted magnetic recording, and other new technologies are being developed to boost the storage capacity of current hard disk drives (HDDs). Understanding the mechanisms of heat transfer across the head-disk interface is of major importance, because it is closely related to the design of HDDs, including lubricant flow and contact issues, especially for the reliability design of HAMR drives. In this paper, we report on a series of experiments and simulations with a perpendicular media recording (PMR) head to better understand the head temperature change and to validate the wave-based phonon conduction theory. From the static touchdown experiments, it is found that even without the air bearing, the head temperature of the thermal flight-height control (TFC) slider reduces when the clearance between the head and disk is less than 1 or 2 nm. The effect of lubricant on the disk surface is also studied. When the head temperature is relatively low, repeated touchdowns show that the starting point gets closer to the disk, suggesting that meniscus forces from the lubricant on the thermal fly-height control protrusion gradually reduce the head-disk clearance. But when the head temperature is relatively higher, the head can “melt through” the lubricant layer. The wave-based phonon conduction theory is integrated into an ANSYS simulation to study the steady state temperature distribution and protrusion profile of the head in the static touchdown experiments. The simulation result shows the same trend as the experiments of cooling before the head is in contact with the disk.

Use of an embedded contact sensor to study nanoscale heat transfer in heat assisted magnetic recording

A near field transducer is employed in the heat assisted magnetic recording technology in order to focus the light energy into a nanoscale spot on the disk. This is necessary to heat the high coercivity magnetic media to their Curie temperature, so the write transducer can record the data. However, the heat transfer mechanism across the head disk interface (HDI) is still not well understood. The current perpendicular media recording systems have a thermal fly-height control means in the air bearing slider near the read/write transducers for placing the transducers within 1 to 2 nm of the rotating disk. In order to monitor this near contact spacing, this system also uses an embedded contact sensor (ECS). Here, we investigate how this ECS can be used to study the heat transfer across the nanoscale gap between the read/write transducer and the disk. This study shows that the self heating effect of the ECS is strong when its current bias is too high. But this self heating effect can be isolated from other heat sources, which allows us to use the ECS for the desired heat transfer measurements. The experiments show that the heat transfer across the HDI is a strong function of the head-disk spacing.