A survey of recent advances in magnetic recording materials

Magnetic recording coatings are still made predominantly of iron oxide particles but the newer particles are significantly better in magnetic properties, dispersibility and orientability than the particles used, say, ten years ago. Chromium dioxide particles show excellent recording performance (particularly at densities above 1000 flux changes per millimeter) but they are presently being challenged by the new cobalt-modified iron oxides. These are formed by diffusing cobalt into the surface of acicular iron oxide particles and it is claimed that the particles prepared in this way are much more stable with respect to temperature and stress than the older cobalt-substituted iron oxides. Metal particles, by virtue of their high moment density and high coercivity, would be ideal for high density recording if they could be passivated permanently. The paper reviews improvements which have been made within the last nine years in the properties of particles for magnetic recording applications and discusses how the improvements were effected.     

Chromium dioxide particles for magnetic recording

Chromium dioxide was introduced in the later 1960s as the first magnetic material capable of high density recording. Since that time a continuous improvement of material properties has been made possible. The most recent improvement concerns particles with coercivities well above 70 kA/m which can be prepared by a modified hydrothermal process without using expensive Ir doping. Although CrO₂ has a somewhat higher specific magnetization, the static magnetic properties of CrO₂ and cobalt-modified iron oxides are comparable. There are, however, some significant differences in recording performance. CrO ₂ pigments show better print-through to noise ratios than Co-modified iron oxides of the same mean magnetic volume. There are no problems with erasability, which becomes increasingly difficult to control for high-coercivity cobalt-modified iron oxides. Finally, CrO ₂ is the only pigment material which allows thermomagnetic duplication     

A new inexpensive method for the preparation of acicular precursors for magnetic recording media

Magnetic materials meant for audio/video recording applications necessitate that polycrystalline materials be in acicular shape. So preparation of acicular precursors for magnetic storage materials assumes significance. The employment of aqueous solutions do not produce needle shape crystallites. Glycerol is one of the complexing media used for the precipitation of ferrous oxalate dihydrate. An inexpensive method using starch for preparation of acicular particles is described. The influence of an additive namely Gd on acicularity is also investigated.     

Recommendation of a simple and universally applicable method for measuring the switching field distribution of magneting recording media

The most common methods for assessing the switching field distribution (SFD) of magnetic recording media are compared for samples of acicular magnetic particles made of different types of magnetic materials and having various degrees of particle orientation. It is shown that the method of using the maximum slopedM/dH = M_{R}/H_{c}(1-S*)atM = 0of the magnetic hysteresis loopM(H)can be used to characterize SFD of the irreversible magnetization processes which correlate to the recording performance. Thus, (1-S*) is recommended as the most easily accessible and universally applicable figure of merit for SFD.     

Current trends in applications of magnetic ceramic materials

Magnetic oxides, a major constituent of magnetic ceramic materials, are most extensively used in a variety of applications as soft, moderate and hard ferrites. I review here its applications with special emphasis on some recent developments in magnetic recording materials. This work was partially supported by CSIR, New Delhi.     

Hydrothermal synthesis of chromium dioxide powders and their characterisation

Chromium dioxide (CrO₂) powders have been synthesised by decomposing CrO₃ and Cr₂O₅ powders under hydrothermal conditions in the temperature range of 300–500°C and pressure range of 250–1200 bars. Oxides of antimony and iron have been used as modifiers to induce acicular morphology. A novel method of using alkali metal salts such as chlorides and carbonates as mineralisers produces CrO₂ with superior magnetic characteristics. The particle size distributions have been correlated with the magnetic properties of the materials. The products obtained have properties rendering them useful for magnetic recording applications.     

Magnetic particles derived from iron nitride

Due to its high saturation magnetization (σ_s(0K)≈208 emu/g) and to the ease of obtaining it as a particulate material, Fe₄N could be a suitable magnetic material for replacing highly reactive metallic iron pigments (σ _s(0K)≈218 emu/g) in high-density recording. Throughout a nitriding process of properly selected precursors Fe₄N can be obtained in either of two morphologies: acicular or isotropic. Further protection of the magnetic particles can be ensured through coating of the precursor particles prior to thermal decomposition in NH₃/H₂ atmosphere or through substitutions with selected elements. The obtained passivated nitrides are characterized by high magnetic properties (H_c≈600 Oe; σ_s≈150 emu/g) and chemical stability well adapted for use as magnetic pigments for high-density recording     

Soft-magnetic composites from insulated iron powder and their potential technological applications

We have studied soft-magnetic composite materials fabricated from iron powder encapsulated in magnetic oxides. The results demonstrate that the use of such materials in power supplies, electric motors, and other devices ensures their stable operation under various conditions.     

Polymer-coated metal and alloy particles for magnetic recording

Fine particles of cobalt and an iron-cobalt alloy were prepared and encapsulated in a plastic film to protect them from oxidation. The magnetic properties of the coated powders were determined and compared with the properties of coated iron powders studied previously. The thermal degradation of magnetic properties was also investigated. Polymer-coated iron and cobalt powders were incorporated in a Hypalon rubber matrix to obtain additional magnetic property data and to study recording characteristics. The experimental materials exhibited better high-frequency response in magnetic recording than a standard γ-Fe2O3material, and one material had a higher signal output as well.     

An investigation of magnetic oxides' defects and conflict of anisotropies for diluted powders

Magnetostatic measurements and rotational hysteresis were carried out on extremely diluted samples of pure, surface modified, bulk doped iron oxides and CrO2. It is found that in all these materials but pure γ-Fe2O3a conflict of anisotropy is active. As a result, the values of remanence ratio jrcan be either lower or higher than the theoretical value of ½ for strictly uniaxial particles depending on the angle between the two anisotropy axis and the relative value of the anisotropy constants. It is therefore found that whilej_{r} < 1/2for acicular magnetite and CrO2, the introduction of cobalt always produces an increase of jr: the extreme effect of such conflict can be found in high remanence isotropic particles. Such a conflict is minimized in Surface Modified Materials. From rotational hysteresis, Hcvs. angle with applied field and, over all, from CF and IFF parameters, it is suggested that the postreatment of the iron oxides with the Co-containing solutions, beside the increase of Hc, has a beneficial effect on the morphology of the particles.     

Preparation of coated iron powders by chemical plating

Small iron particles can be produced by reduction of γ-Fe2O3and α-FeOOH with hydrogen; they show outstanding magnetic properties and can be used for recording media. The main difficulty concerns the tendency of these particles to oxidize. We have studied a preparation process in two steps: a) reduction of iron oxides or oxy-hydroxides; b) chemical plating in the same furnace. The magnetic properties of the iron powders chiefly depend on the reduction step. The water content in the gas leaving the furnace has been shown to be the main parameter determining the obtainment of powders with high magnetic properties. The iron powders, which were pyrophoric, have been quenched in chemical plating bath of suitable formulation and coated with cobalt or copper. Typical magnetic properties are: saturation magnetization σsat= 130-165 emu/g, intrinsic coercivityjHc= 350-450 Oe; best magnetic properties: σsat=155 emu/g,jHc=700 Oe. The coated powders have been submitted to heat treatment in air saturated with water, showing a good resistance to oxidation.     

Strain induced anomalous red shift in mesoscopic iron oxide prepared by a novel technique

Nano magnetic oxides are promising candidates for high density magnetic storage and other applications. Nonspherical mesoscopic iron oxide particles are also candidate materials for studying the shape, size and strain induced modifications of various physical properties viz. optical, magnetic and structural. Spherical and nonspherical iron oxides having an aspect ratio, ∼2, are synthesized by employing starch and ethylene glycol and starch and water, respectively by a novel technique. Their optical, structural, thermal and magnetic properties are evaluated. A red shift of 0·24 eV is observed in the case of nonspherical particles when compared to spherical ones. The red shift is attributed to strain induced changes in internal pressure inside the elongated iron oxide particles. Pressure induced effects are due to the increased overlap of wave functions. Magnetic measurements reveal that particles are superparamagnetic. The marked increase in coercivity in the case of elongated particles is a clear evidence for shape induced anisotropy. The decreased specific saturation magnetization of the samples is explained on the basis of weight percentage of starch, a nonmagnetic component and is verified by TGA and FTIR studies. This technique can be modified for tailoring the aspect ratio and these particles are promising candidates for drug delivery and contrast enhancement agents in magnetic resonance imaging.     

Effects of acicular ferrite on charpy impact properties in heat affected zones of oxide-containing API X80 linepipe steels

This study was concerned with effects of acicular ferrite on Charpy impact properties in heat affected zones (HAZs) of two API X80 linepipe steels containing oxides. In the one steel, Mg and O₂ were additionally added to form a larger amount of oxides than the other steel, which was a conventional X80 steel containing a considerable amount of Al and Ti. Various HAZ microstructures were obtained by conducting HAZ simulation tests under different heat inputs of 35 kJ cm⁻¹ and 60 kJ cm⁻¹. Oxides present in the API X80 linepipe steels were complex oxides whose average size was 1-2 μm, and the number of oxides increased with increasing amount of Mg and O₂. The volume fraction of acicular ferrite present in the steel HAZs increased with increasing number of oxides, and decreased with increasing heat input. When the volume fraction of acicular in the HAZ was higher than 20%, Charpy impact energy at −20 °C was higher than 100 J as the ductile fracture mode was dominant. Particularly in the steel HAZs having a larger amount of oxides, Charpy impact properties were excellent because oxides worked as nucleation sites of acicular ferrite during welding. Charpy impact properties of the HAZs could be well correlated with the volume fraction of acicular ferrite and number of oxides under different heat input conditions.     

Defects-Driven Ferromagnetism in Undoped Dilute Magnetic Oxides:A Review

In the past several decades, dilute magnetic semiconductors, particularly the dilute magnetic oxides have evolved into an important branch of materials science due to their potential application in spintronic devices combining of properties of semiconductors and ferromagnets. In spite of a major effort devoted to the mechanism of ferromagnetism with a high Curie temperature in these materials, it still remains the most controversial research topic, especially given the unexpected d0 ferromagnetism in a series of undoped wide-band-gap oxides films or nanostructures. Recently, an abundance of research has shown the critical role of various defects in the origin and control of spontaneous magnetic ordering, but contradicting views from intertwined theoretical calculations and experiments require more in-depth systematic research. In our previous work, considerable efforts have been focused on two major oxides, i.e. ZnO and ZrO 2. This review will present a summary of current experimental status of this defect-driven ferromagnetism in dilute magnetic oxides(DMOs).     

包覆型氧化铁黄颜料微结构与耐热性能研究

氧化铁黄颜料因耐热性差而限制了在塑料加工和卷材涂料中的应用。本研究以氧化铁黄为前驱体, 采用沉淀法合成氢氧化铝包覆氧化铁黄颜料, 采用XRD、FT-IR、TG-DTA、SEM&EDS和TEM等方法表征包覆型铁黄颜料的结构, 探讨了反应pH对复合材料微结构以及耐热性能的影响。结果表明, pH为4时, 铁黄表面包覆层为无定型氢氧化铝; pH提高至6、8和10时, 表面包覆层为晶态薄水铝石相。包覆后氧化铁黄颜料耐热性有了较大提升。特别在pH为8、10时, 铁黄颜料 240℃下耐热处理30 min后色差值较小, 与表面包覆层形成的薄水铝石相密切相关。铁黄包覆前后, 保持了原先的针状结构, 未出现团聚; 当pH为10时, 包覆后铁黄颜料除针状结构外, 还出现了较粗的晶态棒状物, 可能与羟基氧化铝在反应过程中自身成核有关, 解释了DTA图谱上246℃处出现的特殊吸热峰。本研究为耐温铁黄颜料开发提供了理论与实践指导。     

Remanence properties of substituted Ba-ferrite particles for highdensity magnetic recording

In this paper, the remanence properties of Co-Sn, Co-Ti and Co-Ti-Sn substituted Ba-ferrite (BaF) oriented particulate samples are compared with those of some oriented acicular particulate samples. A new parameter, the minor remanence distribution (MRD), is proposed to review the remanence properties of magnetic particles and the capabilities for resisting the recording demagnetization of magnetic recording media. It is shown that the MRD values of the oriented BaF particulate samples were smaller compared to oriented Co-γ-Fe₂O₃ samples, even though the squareness ratios (SR) of some of the BaF samples were smaller than those of the Co-γ-Fe₂O₃ samples. It Is the small MRD, SFD_r, IRS and large DH _r of a medium that can result in a large resistance to the effects of recording demagnetization and therefore in superior characteristics for high density magnetic recording. Since Co-Sn substituted BaF platelet-shaped particles exhibit these characteristics and have a very low temperature coefficient of coercivity, these particles can be expected to be a promising candidate for high density magnetic recording     

Magnetic-Pole Flip by Millimeter Wave

In the era of Big Data and the Internet of Things, data archiving is a key technology. From this viewpoint, magnetic recordings are drawing attention because they guarantee long-term data storage. To archive an enormous amount of data, further increase of the recording density is necessary. Herein a new magnetic recording methodology, “focused-millimeter-wave-assisted magnetic recording (F-MIMR),” is proposed. To test this methodology, magnetic films based on epsilon iron oxide nanoparticles are prepared and a focused-millimeter-wave generator is constructed using terahertz (THz) light. Irradiating the focused millimeter wave to epsilon iron oxide instantly switches its magnetic pole direction. The spin dynamics of F-MIMR are also calculated using the stochastic Landau–Lifshitz–Gilbert model considering all of the spins in an epsilon iron oxide nanoparticle. In F-MIMR, the heat-up effect of the recording media is expected to be suppressed. Thus, F-MIMR can be applied to high-density magnetic recordings.     

A magnetic recording tape based on iron particles

Tapes based on acicular iron particles give an outstanding recording performance compared to γ-Fe2O3and CrO2tapes. Especially in the short wavelength region the signal to noise ratio is, respectively, 12 and 7 dB better, whereas the bias noise level is somewhat lower. The iron tapes have a high coercivity, between 80 and90 times 10^{3}A/m, and hence the required bias is about 9 dB higher than for γ-Fe2O3. The print through properties are excellent.     

铁氧化物纳米晶自组装空心微球的制备、表征及其应用

铁氧化物纳米材料和纳米结构空心微球分别代表了材料研究中组分和结构的研究热点. 而由铁氧化物纳米晶自组装形成的空心微球的研究则是二者相结合, 具有重要的科学意义和良好的应用前景. 虽然已发展了多种方法制备各种单质及化合物的空心微球, 但铁氧化物纳米晶自组装空心微球的制备方法报道较少. 本文简要介绍了近几年发展起来的多种铁氧化物纳米晶自组装空心微球的一些制备方法, 利用上述方法, 制备出了多种不同组成单元、不同尺寸、不同空心程度的铁氧化物纳米晶自组装空心微球, 对所制备的铁氧化物纳米晶自组装空心微球进行了表征, 并初步介绍了所制备的铁氧化物纳米晶自组装空心微球在药物缓释和环境领域中的应用.     

Thermomagnetic recording: Physics and materials

Thermomagnetic recording is the production of a remanent magnetization in a material by cooling it from a critical temperature in a small magnetic field. The most prominent applications for thermomagnetic recording presently under consideration are high density information storage and tape copying; other applications, e.g., display and magnetic printing, can also be envisioned. In this review of the field, we discuss the proposed applications of thermomagnetic recording, the physical principles of the various magnetic phenomena that can be used for this purpose, and the degree to which specific materials meet the requirements for thermomagnetic recording.