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.     

Study of magnetization reversal of Co/Pd bit-patterned media by micro-magnetic simulation

Bit-patterned media based on a single-bit-per-island may be a promising candidate for perpendicular magnetic recording at the Tb/in² level because they could provide a lower noise and higher density. The understanding of magnetization reversal processes in such patterned media is important. In this work, the range of single domain island size based on Co/Pd bit-patterned media was determined. Demagnetization effect, dipolar interactions and switching field distribution (SFD) for bit-patterned media were quantitatively studied by the simulation based on Landau-Lifshitz-Gilbert equation. The total hysteresis loops and SFD were comparable with the experiment ones. The SFD increased from 2σ=1.2 kOe (as the calculated intrinsic SFD) to the experimental value of 1.9 kOe due to dipolar interactions which is in a good agreement with the experimental results (2.0 kOe). Optimized patterned structure with a minimized SFD and maximized data storage densities was found to have an island size of 10 nm and islands separation of 20 nm. The calculated ratio of SFD/H_c (H_c: the coercivity) is 9.2%, which is below the threshold of 10% for 1 Tb/in² pattern media.     

Synthesis and magnetic properties of Au-coated amorphous Fe20Ni80 nanoparticles

Gold-coated nanoparticles of Fe₂₀Ni₈₀ (permalloy) have been synthesized by a microemulsion process. The as-prepared samples consist of ∼5 nm diameter particles of amorphous Fe₂₀Ni₈₀ that are likely encapsulated in B₂O₃. One or more Fe₂₀Ni₈₀@B₂O₃ particles are subsequently encapsulated in 8-20 nm gold nanospheres, as determined by TEM and X-ray powder diffraction (XRD) line broadening. The gold shells were found to be under expansive strain. Magnetic data confirm the existence of a superparamagnetic phase with a blocking temperature, T_B, of ∼33 K. The saturation magnetization, M_S, of the as-prepared, Au-coated sample is ∼65 emu g⁻¹ at 5 K and ∼16 emu g⁻¹ at 300 K. The coercivity, H_C, is ∼280 Oe at 5 K.     

Effects of the Hf content on the microstructure and magnetic properties of Co-Hf-Ta thin films

Effects of the Hf content in Co-Hf-Ta thin films on the microstructure and magnetic properties were investigated in this study. It was found that appropriate Hf addition can effectively refine the Co grain size. Co grain sizes sharply decreased from 50 nm down to 2.3 nm with increasing the Hf content from 1.02 at.% to 2.81 at.%, leading to the reduced magneto-crystalline anisotropy. The Co-Hf-Ta thin films with small Co grains reveal low anisotropy field, low coercivity, and high resistivity. By optimizing the Hf content, the film with Hf concentration of 2.81 at.% exhibits excellent soft magnetic properties: high saturation magnetization (4πM_S ∼ 13.6 kG), and low coercivity (H_C ∼ 0.6 Oe). The effective permeability of the film reaches 800 and remains constant up to 1 GHz.     

Switching field distribution of magnetic fine particles

In the magnetic recording media the switching field distribution (SFD) is an important micromagnetic characteristic curve. However, in general case, methods of assessing SFD have not presented a reasonable relation between the macroscopic measurements and the microscopic properties of the system. The degree of alignment of particles easy axis in a texture and the angular dependence of switching field are two fundamental factors for the distribution function. A technique for determining the switching field distribution curve due to these factors is reported. This technique is performed for textured iron fine particles and samples of CrO₂ and γFe₂O₃ commercial tapes. The distribution differs from a gaussian function. However, the width and the mean switching field equal to twice of the width of dM_r/dH curve and the remanent coercivity, respectively. Furthermore, it is shown that the width of the switching field distribution decreases with increasing the degree of easy axis orientation.     

Intermediate layer thickness dependence on switching field distribution in perpendicular recording media

The effect of intermediate layer (IL) thickness on crystallographic texture and magnetic properties of CoCrPtSiO₂ granular perpendicular recording media was investigated with switching field distribution (SFD) as the focus. Even though the c-axis orientation of the Co-based recording layer (RL) broadens with the reduction of IL thickness, the SFD becomes narrower. This result demonstrates that the intrinsic SFD is not directly dependent on c-axis orientation of the recording layer but instead dependent on the magnitude of exchange coupling. It is thus possible to have a medium with thin IL and narrow SFD. This is desirable for bit-patterned media (BPM), where highly exchange-coupled grains are required.     

Influence of Nb substitution for Fe on the magnetic and magneto-impedance properties of amorphous and annealed Fe76.5−xSi13.5B9Cu1Nbx (x=0–7) ribbons

We report a systematic study of the influence of Nb substitution for Fe on the magnetic properties and magneto-impedance (MI) effect in amorphous and annealed Fe_76.5−xSi_13.5B₉Cu₁Nb_x (x=0, 1, 2, 3, 4, 5, 6, and 7) ribbons. The amorphous ribbons were annealed at different temperatures ranging from 530 to 560 °C in vacuum for different annealing times between 5 and 20 min. We have found that for the as-quenched amorphous ribbons, the substitution of Nb for Fe first increases the saturation magnetization (M_s) and decreases the coercivity (H_c) until x=3, for which the largest M_s∼152 emu/g and the smallest H_c∼1.3 Oe are obtained, then an opposite trend is found for x>3. The largest MI ratio (ΔZ/Z∼38% at f=6 MHz) is achieved in the amorphous ribbon with x=3. A similar trend has been observed for the annealed ribbons. The most desirable magnetic properties (M_s∼156 emu/g and H_c∼1.8 Oe) and the largest MI ratio (ΔZ/Z∼221% at f=6 MHz) are achieved for the x=3 sample annealed at 540 °C for 15 min. A correlation between the microstructure, magnetic properties, and MI effect in the annealed ribbons has been established.     

Prospects for bit patterned media for high-density magnetic recording

Prospects for bit patterned media (BPM) of more than 1 Tb/in² are discussed. Improvement in the pattern drawing for small feature size and high precision is necessary for fabrication process. Deviation in the magnetic properties should be estimated and reduced. The etching damage seems not to be large. Design of the substructure of the magnetic dot is necessary for reducing the deviation. BPM is also a good template for technologies to increase the recording density. Combination of BPM with heat-assisted recording or exchange-coupled layers is advantageous for high-density recording.     

The effect of multi-step milling and annealing treatments on microstructure and magnetic properties of nanostructured Fe-Si powders

Nanostructured Fe_1−xSi_x (x=0.05, 0.1, 0.15 and 0.2) powders are prepared by different multi-step milling and annealing treatments. The microstructure and magnetic properties are investigated for all alloys. The minimum crystallite size of as-annealed powders (∼40 nm) is found to be larger than in as-milled ones (∼15 nm). It is found that microstrains of 2- and 4-step processes are close to those of the as-received powders. The lattice parameter decreased ∼0.5% and 0.9% for the powders that experienced milling and annealing at the last step, respectively. The Fe₈₀Si₂₀ powders prepared by 1- and 4-step treatments show the maximum (40-125 Oe) and minimum (20-26 Oe) coercivity, respectively. With increase in milling time, mass magnetization increased for all processes. This can be ascribed to diminution in magneto-crystalline anisotropy due to grain refinement. The maximum mass magnetization (160-199 Am²/kg) is achieved for the 4-step process.     

MFM studies of magnetic domain patterns in bulk barium ferrite (BaFe12O19) single crystals

Magnetic domain patterns in bulk barium ferrite (BaFe₁₂O₁₉; BaM) single crystals on the basal plane and the prism plane were measured and studied by magnetic force microscopy (MFM). The surface domain pattern is in the form of flowers or star on the basal plane and long elongated spikes or stripe domains on the prism plane. The change in domain structure with applied field (H_app) and the thickness (T) dependence on domain width (δ) was observed. The domain width decreased from 32 to 9 μm for the crystals of 800-100 μm thicknesses, respectively.     

Pulsed-field-remanence measurements on individual magnetotactic bacteria

We measured pulsed-magnetic-field remanences of individual cells of two types of magnetotactic bacteria. Wild-type magnetic vibrios displayed square remanence curves with the reversal field, H_rev reaching values up to 825 Oe (65.7 kA/m). We attribute the generally high values of H_rev to the elongated shape of the magnetosomes (length-to-width ratio, e≈1.2) and to the comparatively short distances between the magnetosomes (8 nm). Cells of the rod-shaped Magnetobacterium bavaricum, on the other hand, which frequently contain more than 600 magnetosomes per cell, could gradually be demagnetised; the coercivity of remanence, H_cr of individual cells always ranged between 600 and 700 Oe (47.7–55.7 kA/m). The non-square remanence curves of M. bavaricum reflect a distribution of elongations and interactions between adjacent strands of magnetosomes within the braid-like chains.     

Synthesis and magnetic characterization of Zn0.6Ni0.4Fe2O4 nanoparticles via a polyethylene glycol-assisted hydrothermal route

Zn-doped nickel ferrite nanoparticles (Zn_0.6Ni_0.4Fe₂O₄) have been prepared via a surfactant, polyethylene glycol assisted hydrothermal route. X-ray powder diffractometry (XRD), Fourier transform infrared spectroscopy, transmission electron microscopy (TEM), and vibrating scanning magnetometry (VSM) were used for the structural, morphological, and magnetic characterizations of the product, respectively. TEM analysis revealed that the nanoparticles have a narrow size distribution, with average particle size of 15±1 nm, which agrees well with the XRD based estimate of 14±2 nm. The absence of saturation and remanent magnetization, and coercivity in the high temperature region of the M-H curve and non-zero magnetic moments indicate superparamagnetism of the nanoparticles with a canted spin structure. The appearance of a peak on the temperature-dependent zero-field cooling magnetization curve at ∼190 K indicates the blocking temperature of the sample.     

Quantitative analysis of magnetization reversal in patterned strip wire by magnetic force microscopy

Magnetic force microscopy (MFM) was used to investigate the magnetization reversal process in a patterned strip wire of permalloy thin film. The magnitude of the phase-shift of tapping mode MFM changed with the varying interactive magnetic force between the magnetic tip and the sample. By analyzing the change in values of the phase-shift, the behaviors of magnetization reversal of different local regions in a patterned strip wire can be quantitatively analyzed. The intensity of the phase-shift in the wider end is stronger than that in the narrower one. In contrast, due to a strong anisotropic effect, the coercive force in the narrower end (9 Oe) is larger than that in the wider one (8 Oe). Therefore, the H_c in the neck section could become strongly affected by the competition of the head-to-tail magnetic configurations in the two parts of the strip wire, and this results in a small H_c in the neck section. In addition, in a simple neck shape connection in a strip NiFe wire, a single domain configuration can be easily changed to a two single domain magnetic configuration.     

Influence of patterning fluctuation on read/write characteristics in discrete track and bit patterned media

The read/write characteristics of non-patterned media (NPM), discrete track media (DTM), and bit patterned media (BPM) are examined by modeling the magnetization distribution of NPM and patterning fluctuation of DTM. By comparing spin-stand measurement with calculation, the magnetization distribution of NPM was well characterized with a new Voronoi cell magnetic cluster model, in which the cluster size at the track edge, 〈D_edge〉, was larger than that at the track center, 〈D_center〉 by a factor of two. Based on an analysis of patterning fluctuations seen in SEM images of DTM, line-edge roughness (LER) was modeled as a long-wavelength center-line roughness (CLR) plus a short-wavelength line-width roughness (LWR). It was confirmed that the standard deviation of the patterning fluctuation was much smaller than that of the magnetic fluctuation for NPM. This allowed DTM to achieve higher off-track performance than NPM. By examining the 747 curves, it was revealed that DTM could have an advantage in track-density of up to approximately 25% assuming patterning fluctuations can be well controlled at high track density. In BPM, fabricating accurate dots is essential. The relationship between dot defect rate and patterning fluctuation was examined, and the maximum allowable standard deviation of LER was derived as 2 nm for achieving 1 Tbspi.     

Magnetic microstructures of a high coercivity Nd-Fe-B sintered magnet in remanent and incomplete thermal demagnetization states

Magnetic microstructures of a high coercivity Nd-Fe-B sintered magnet in remanent and incomplete thermal demagnetization states have been revealed by using magnetic force microscopy (MFM) with high coercivity tips. MFM results indicate that specimens in a remanent state are single domain and their magnetizations align with the direction of the magnetizing field. The evolution of the magnetic domains with annealing temperatures shows that the thermal demagnetization process consists of four stages. Nd-Fe-B should be heat-treated at about 120-170 °C to make its magnetic state stable before practical applications.     

Magnetic properties of Nd–Fe–B film magnets prepared by RF sputtering

Highly oriented films of ∼6 μm in thickness consisting of the Nd₂Fe₁₄B compound phase were obtained by a three-dimensional sputtering method at room temperature and the subsequent crystallization by annealing. The c-axis orientation and coercivity of film samples were sensitive to the sputtering parameters and annealing conditions. The optimum temperature and time for annealing were 650 °C and 30 min to show the highest coercivity without any deterioration for the orientation of Nd₂Fe₁₄B grains, and furthermore the degree of c-axis orientation was increased by decreasing the Ar gas pressure or input power for sputtering. The resultant film magnets with good magnetic properties of B_r=∼1.06 T, H_C=∼371 kA/m, and (BH)_max=∼160 kJ/m³ were obtained under the optimized parameters for sputtering.     

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.     

Effect of Co underlayer on soft magnetic properties and microstructure of FeCo thin films

High saturation magnetization soft magnetic FeCo (=Fe₆₅Co₃₅) films were prepared using a thin Co underlayer. The FeCo/Co films exhibited a well-defined in-plane uniaxial anisotropy with easy axis coercivity (H_ce) of 10 Oe and hard axis coercivity (H_ch) of 3 Oe, and a half reduction of H_c with H_ce=4.8 Oe and H_ch=1.0 Oe was obtained when the composition was adjusted to 25 at% Co. The effective permeability of the films remains flat around 250 to 800 MHz. The saturation magnetostriction was 5.2×10⁻⁵ and the intrinsic stress was 0.8 GPa in FeCo single layer, both were slightly reduced by Co underlayer. The Co underlayer changed the preferred orientation of the FeCo films from (2 0 0) to (1 1 0) but more significantly, reduced the average grain size from ∼74 to ∼8.2 nm. It also reduced the surface roughness from 2.351 to 0.751 nm. The initial stage and interface diffusion properties were examined by TEM and XPS.     

Improving permanent magnetic properties of rapidly solidified nanophase RE-TM-B alloys by compositional modification

Rapid solidification is one of the most important techniques to produce nanocrystalline rare-earth-transition metal-boron (RE-TM-B) hard magnetic materials. To achieve high performance on these NdFeB-based alloys, compositional modification and microstructure optimization have been frequently employed. In this short review, various substitutions and doping elements have been discussed regarding to their behaviors in adjusting the individual or combined hard magnetic properties as well as the microstructure based on our recent results. It has been demonstrated that Pr and Dy enhance coercivity _jH_C, whereas Sm reduces _jH_C due to their effects on intrinsic properties. Co improves the thermal stability as well as the microstructure. Introducing Fe₆₅Co₃₅ is a possible approach to enhance the magnetization and maximum energy product (BH)_max. As a doping element, Ta was found to play an important role to obtain an appropriate combination of magnetic properties for this type of alloys.     

Exchange bias and magnetothermal properties in Fe@Mn nanocomposites

We have studied the Exchange Bias (EB) effect in nanocomposite films consisting of Fe nanoparticles (mean size ∼1.9 nm) embedded in an antiferromagnetic Mn matrix. They were produced by co-deposition through a gas aggregation cluster source and molecular beam epitaxy and have different Fe volume filling fractions (2.2% and 24.8%). The exchange field, higher in the sample with higher Fe concentration (at T=5 K, H_ex∼460 Oe for 24.8% and ∼310 Oe for 2.2% ), in both the samples decreases with increasing T, finally disappearing at T∼40 K. The EB properties have been studied in conjunction with results on the thermal dependence of the magnetic coercivity, zero-field-cooled and field-cooled magnetization and thermoremanence. The different Fe content strongly affects the magnetothermal properties, featuring superparamagnetic relaxation in the diluted sample and a reentrant ferromagnet-type transition in the concentrated one. Hence, the EB properties of the two samples have been discussed in consideration of such peculiarities of the magnetic behavior and highlighting the role of the Mn matrix.