Comparison of media properties between hard/soft stacked composite and capping layer perpendicular recording media

The effect of soft layer thickness (t_Soft) of CoTaZr–SiO₂ and low Pt-containing CoCrPtO layers on media properties in hard/soft (H/S) stacked media is compared to media properties in conventional capping layer (CL) media. Coercivity and coercivity squareness in H/S stacked media continuously decrease with increasing t_Soft, while they increase in CL media. H/S stacked media with CoTaZr–SiO₂ layers having higher saturation magnetization and in-plane magnetic anisotropy constant exhibit stronger demagnetization effect. Compared to CL media, H/S stacked media with CoCrPtO soft layers improve signal-to-noise ratio and magnetic write width. However, the use of a relatively soft layer deteriorates adjacent track erasure and does not improve media writeability due to compensation effect between softer and harder layers to be used. These phenomena can be understood as undesirable side effects of a soft layer: higher demagnetization field and larger lattice mismatch.     

The role of short exchange length in the magnetization processes of L10-ordered FePt perpendicular media

A three-dimensional micromagnetic model with non-uniform grain size distribution has been built up to study the magnetization process in FePt L1₀ perpendicular media. A 3D model of a single FePt magnetic grain is also set up for comparison. The high magneto-crystalline anisotropy K_u results in a short exchange length l_ex in FePt nanograins. Therefore a magnetic grain is divided into smaller grids on the order of l_ex. The simulated perpendicular and longitudinal loops are consistent with experiments, and it is explained why the measured perpendicular H_c is relatively smaller compared with the saturation field of the longitudinal loop in the FePt perpendicular medium.     

3-D FEM analysis of thermal degradation in writing and reading characteristics of a perpendicular magnetic head

In order to achieve high-density recording, the detailed behavior of thermal degradation should be investigated. In this paper, the degradation of magnetization of high-density recording medium is examined using the 3-D finite element method (FEM) combined with the modeling of Stoner–Wohlfarth (SW) particles and Neel–Arrhenius switching probability. It is shown that the anisotropy field H_k suppressed the thermal degradation and the saturation magnetization M_s enhances it. The thermal degradation is also changed by the amplitude of magnetization.     

Thermal stability and switching field of hard/soft-stacked perpendicular media

The time dependence of remanence coercivity and thermal stability were investigated for hard/soft-stacked media consisting of a magnetically hard granular layer underneath a very thin soft layer with a large saturation magnetization, M_s. The values of remanence coercivity at measurement times t′=10³ and 10⁻⁵ s (pulse field) were measured, and defined as H_r and H_r^P. The remanence coercivity on the recording time scale, H_r (1 ns), and the energy barrier, ΔE/kT, were evaluated by fitting H_r and H_r^P to Sharrock's equation taking into account the power law variation of the energy barrier, n. The value of H_r (1 ns) for a (Co–Pt)–SiO₂ (9 nm)/Co–SiO₂ (2 nm) stacked medium with an interfacial coupling control layer was about 9 kOe, which was less than half of that of a (Co–Pt)–SiO₂ (9 nm) conventional medium (=21.3 kOe). The value of ΔE/kT for the stacked medium was about 111 (n=0.7), and was not significantly different from the conventional medium. Moreover, no significant difference in the rate of decrease of H_r with increasing temperature was observed between media with and without interlayers. These results indicate that the use of a thin soft layer with high M_s was effective at significantly reducing H_r with no notable change in thermal stability.     

Characteristics of the magnetic microstructure of amorphous and nanocrystalline ferromagnets with a random anisotropy: Theoretical estimates and experiment

An experimental determination (both direct and indirect) of the characteristics of the magnetic microstructure, namely, the ferromagnetic correlation radius R _f and the rms fluctuation of the mean anisotropy D ^1/2〈H _a〉, is performed for amorphous and nanocrystalline ferromagnets with a random anisotropy characterized by the quantities R _c and D ^1/2 H _a, respectively. The magnetization curves of amorphous and nanocrystalline ferromagnets are found to exhibit a dependence on H that is caused by the alignment of the magnetizations of individual magnetic blocks with the field.     

Recording simulation beyond 2 Tbit/in on bit-patterned media with shielded planar head

A simple technique for bit-patterned media was proposed to increase achievable areal recording densities beyond 2 Tbit/in². Introduction of longitudinal magnetic anisotropy to the media indicated reduced effect of magnetostatic interaction between the dots. Recording simulation with a shielded planar pole head exhibited increased write shift margins in both down and cross track directions compared with that of the perpendicular anisotropy media. It was suggested that recording of an areal density of 2.5 Tbit/in² would be realized with a down and cross track margins of 3.5 and 4.0 nm, respectively. Better recording performance at high areal densities is expected if suitable head could be designed.     

Micromagnetic studies for bit patterned media above 2 Tbit/in.

Bit patterned media (BPM) recording is a candidate for extremely high density magnetic recording. A micromagnetic model is built up to analyze the phase diagram of the correct-write-in condition in BPM above 2 Tb/in.² fabricated by lithography or ion irradiation methods. The target of the study is to acquire the relationship between the recording performance and the magnetic properties of the media. The medium includes the polycrystalline grains and grain boundary. In BPM fabricated by lithography with FCT structure, two phase diagrams of the correct-write-in condition are found for the anisotropy angular distribution Δθ, the ratio of tetragonal anisotropy K₂₂ to uniaxial anisotropy K₁ and the uniaxial anisotropy distribution ΔK₁. In BPM fabricated by ion irradiation methods, two phase diagrams of the correct-write-in condition are analyzed for the ratio of saturation magnetization M′_s/M_s, anisotropy field H′_k/H_k and the exchange field H′_ex/H_ex in the ion irradiated region and the bit islands.     

Anisotropy at nano-grain boundary and effective anisotropy between magnetically soft and hard nano-grains

An expression of anisotropy at grain boundary suitable for different coupling conditions has been given, based on which the effective anisotropy between soft and hard grains 〈K_sh〉 has been calculated in nanoscaled magnetic materials. The results showed that 〈K_sh〉 increases rapidly first, then the increase becomes slow, and 〈K_sh〉 decreases with increasing D_s for the same value of D_h. 〈K_sh〉 decreases with increasing D_s for all given D_h, and the decrease rate becomes slow with increasing D_h. In order to obtain higher effective anisotropy between magnetically soft and hard grains, D_h should not be less than 25 nm, and D_s should be about 10 nm.     

Preparation and recording characteristics of granular-type perpendicular magnetic recording media with thin intermediate layer

Granular-type media with thin Ru intermediate layer were prepared on a highly oriented high-B_s FeCo soft underlayer (SUL). A CoPt–TiO₂ recording layer on a Ru intermediate layer of only 2 nm had high-crystal orientation, high H_c of 6.5 kOe, and a high squareness ratio (SQ) of 0.99. The magnetic property of the SUL was also good. The recording performance was measured for the media with different Ru intermediate thicknesses by using a single-pole-type (SPT) head. The media had large reproduced output even for the Ru intermediate layer thickness of 2 nm.     

Fluctuation description of the phase transition from a homogeneous magnetization state to domain structure in thin ferromagnetic films induced by film thickness change

It is revealed that fluctuations of the vector of magnetization in a homogeneous magnetization state have domain-like character. It manifests as well in formulae describing the energy of fluctuation as in the shape of correlation functions. The condition of fluctuation energy positive determination allows to obtain analytic formulae describing critical the thickness L_c of thin films and period λ of the domain structure appearing when L = L_c. The difficulties connected with interpretation of the obtained results in the formalism of the scaling theory are discussed.     

Anomalous fluctuations of 2D-electron recombination radiation intensity in the quantum Hall regime

Anomalous intensity fluctuations are observed in the spectrum of radiative ecombination of quasi-two-dimensional (2D) electrons with photoexcited holes in a single quantum well. The fluctuations are observed exclusively under the conditions of the quantum Hall effect (QHE). It is shown that, if the QHE conditions are not fulfilled, the radiation intensity fluctuates strictly following the Poisson distribution 〈δN ²〉/〈N〉= 1), whereas in the QHE regime the fluctuation amplitude increases by several orders of magnitude (〈δ N ²〉/〈N〉～10²). It is demonstrated that the maxima of the emission noise amplitude coincide with the maxima of inverse magnetoresistance of 2D electrons in the QHE regime and correspond to establishing an anomalously high uniformity of the system.     

Resin-bonded permanent magnetic films with out-of-plane magnetization for MEMS applications

Resin-bonded permanent magnets with out-of-plain direction of magnetization and improved magnetic properties for magnetic MEMS actuator have been created. The material investigated consists of magnetically anisotropic strontium ferrite particles embedded into epoxy resin matrix upto a volume loading of 80%. Intrinsic coercivity H_ci of 6000 Oe (480 kA/m), residual magnetic flux density B_r up to 4000 G (0.4 T) and maximum energy product (BH)_max of 3.0 MG Oe (23.6 kJ/m³) have been attained due to magnetic-field-induced alignment of the ferrite particles during curing process.     

Figures of merit of magnetostrictive single crystal iron-gallium alloys for actuator and sensor applications

Energy density, magnetomechanical coupling factor and a dimensionless sensing gage factor of 〈1 0 0〉 oriented single crystal iron-gallium alloys with 16, 17.5, 19, 24.7 and 29 at% gallium were studied as functions of stress and magnetic field. To estimate these quantities, the samples were characterized under different quasi-static stress and magnetic field conditions. The experimental behavior was modeled using an energy-based non-linear approach. Both the experimental data and the model simulations were used to calculate material parameters such as magnetic permeability, piezo-magnetic strain coefficient (d₃₃), inverse piezo-magnetic coefficient (stress sensitivity, d*₃₃) and Young's modulus in the material. These quantities were used to obtain energy density, magnetomechanical coupling factor and sensing gage factor as functions of magnetomechanical conditions. Maximum energy density of around 3 kJ/m³, magnetomechanical coupling factor higher than 0.75 and sensing gage factor on the order of 10³ were calculated.     

A study on the order–disorder phase transformations of rapidly solidified Sm–Co-based permanent magnets

The structural transformation from the metastable, disordered TbCu₇-type SmCo₇ structure to the equilibrium, ordered Th₂Zn₁₇-type Sm₂Co₁₇ structure was revealed by X-ray diffraction analysis using Rietveld refinement. The magnetic properties depended strongly on the stage of the transformation, as the coercivity depended on the annealing temperature. The as-solidified alloys in the TbCu₇-type structure exhibited coercivity as high as 7.85 kOe, which increased to greater than 9 kOe after heat treatment. The magnetization processes were also strongly influenced by the structural state. Initially it was dominated by nucleation processes, which gave way to domain wall pinning-controlled magnetization processes with the development of the Sm₂Co₁₇ structure. Transmission electron microscopy revealed the development of antiphase domains during heat treatment, which apparently served as the domain-wall pinning sites during magnetization reversal.     

Anisotropy dispersion in (CoCrPt)1−x(SiO2)x perpendicular recording media

The distribution of easy axis orientation in perpendicular media is of technological importance because it affects the value of S^* (see Fig. 1), which quantifies the switching field distribution (SFD) and hence partially determines the data density achievable on a given medium. The distribution is controlled by the crystallographic orientation of grains and factors such as intergranular exchange and dipolar coupling. Due to strong demagnetising fields in the perpendicular orientation, traditional measurements of remanence as a function of angle are difficult to interpret and have required the use of large-scale computational models. In this work we have utilised the variation of coercivity H_C with angle, which has the advantage that at H_C the global demagnetising field is zero. Additionally, since such materials follow essentially the Stoner–Wohlfarth mode of reversal, the variation of H_C with angle, H_C(θ), is much greater than that for the remanence. We find that for (CoCrPt)_1−x(SiO₂)_x, where the level of exchange coupling is controlled, the distribution of magnetic easy axes is narrower when the exchange coupling is reduced, but dipolar coupling between the grains is strong and affects the magnetisation reversal significantly.     

Electrical and magnetic properties of CoFeAlO thin films

Influences of oxygen-partial pressure and annealing on the electrical and magnetic properties of CoFeAlO thin films were systematically investigated by means of resistivity, permeability, magnetization and ferromagnetic resonance (FMR) measurements. It was found that, with increasing oxygen-partial pressure or under annealing, the electrical resistivity of the film increased and the magnetic softness decreased, which is attributed to the microstructural change of the film. Interestingly, an as-deposited Co_45.30Fe_20.65Al_19.34O_14.71 film was found to exhibit an inverted hysteresis loop with negative coercivity, and this peculiar phenomenon disappeared upon effects of oxygen-partial pressure and annealing. It was also found that the as-deposited films owned a narrow FMR line width that increased with increasing oxygen-partial pressure or under annealing.     

Optimisation of bit patterned media for 1 Tb/in

Micromagnetic simulations were used to investigate the influence of patterned media geometry on the signal to noise ratio (SNR), adjacent track erasure and write margin for a target recording density of 1 Tb/in². For an ideal patterned medium the readback noise was a maximum when the read head was directly over the dots and a minimum at the transitions. The SNR improved for smaller dots due to the larger dot separation. However, the ideal media with the highest SNR were also the most susceptible to dispersions of dot size and position. Low temperature simulations suggest that large write margins are available; however, at room temperature the write margin can be much reduced. Increasing the rise time of the write head had a deleterious effect on the write margin and the write margin was zero for rise times of more than 0.45 ns. Nevertheless, error-free writing at 1 Tb/in² could be achieved using appropriate head geometries and material parameters.     

Magnetization and Static Magnetic Susceptibility of Fine-Crystalline High-Temperature YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript> Superconductors Synthesized by the Sol–Gel Method

A comparative study of the magnetization and static magnetic susceptibility of high-temperature superconductors (HTSC) YBa₂Cu₃O_y synthesized by two variants of the sol–gel method with different average sizes of crystallites 〈 D〉 ranging 0.4–2 μm has been performed in constant magnetic fields (Н ≤ 6 kOe). It has been shown that the different annealing temperatures and times, at which their crystal structure is formed, change both the average sizes of crystallites 〈D〉 and the sizes of the structural homogeneity regions 〈l〉 and, at the same time, the magnetic field penetration depth (λ) and the coherence length (ξ). As a result, such parameters as 〈D〉 ~ λ and 〈l〉 ~ ξ become comparable, leading to a change in the physical characteristics of HTSCs. It has also been shown that the superconducting transition temperature T_c determined from the measurements of magnetic characteristics in constant magnetic fields remains within values optimal for superconductivity (T_c ≈ 92 K) in the case of an optimal number (y) of oxygen atoms, which determine the levels of charge doping for a given compound.     

Damage processes in Fe 3 O 4 magnetic insulator irradiated by swift heavy ions. Experimental results and modelisation

The behavior of the magnetic properties of magnetite Fe₃O₄ irradiated by swift heavy ions is investigated by magnetization measurements. Although there is no induced structural phase transformation, both coercive field and saturation magnetization are sensitive to ion irradiation and exhibit different behaviors depending on the ion fluence range. In the low fluence regime, the coercive field increases, which is evidence for a strong pinning of magnetic domain boundaries by the induced defects. The magnetization shows a decrease in the saturation value and tends to reorient perpendicularly to the ion track axis. At high fluence, the initial magnetic properties of the sample are nearly restored. The changes in the magnitude and the direction of magnetization are interpreted by magnetostrictive effects related to the stress induced by irradiation. A phenomenological model is applied to reproduce the fluence evolution of the saturation magnetization, assuming relaxation of the stress induced around the core of defects of the tracks by overlapping effects at high fluence. The results are compared to those obtained in the case of yttrium iron garnet Y₃Fe₅O₁₂. Received 18 April 2001 and Received in final form 24 July 2001     

The coupling effect between ferroelectric and frustrated antiferromagnetic ordering in hexagonal ferroelectromagnet

Effects of the magnetoelectric coupling between the frustrated antiferromagnetic and ferroelectric ordering in hexagonal ferroelectromagnet are investigated by the soft-mode theory and molecular-field approximation. Applying the Heisenberg model for frustrated triangular antiferromagnets with exchange anisotropy and Diffour model for ferroelectric interaction, we discuss thermodynamic properties of the hexagonal ferroelectromagnetic system, including mean magnetization 〈s_i〉, polarization p, magnetization susceptibility χ_m, and polarization susceptibility χ_p, in a possible coupling form related to a combination of electric polarization and spin correlation. It is found that polarization induced by magnetic coupling leads to an anomaly in χ_p and a cusp in χ_m at low-temperature, which is consistent qualitatively with experimental results in hexagonal ferroelectromagnet YMnO₃.