Multiferroic oxide composites: synthesis,characterisation and applications

Abstract

The nature of mechanical strain mediated electromagnetic coupling in multiferroic composites has been studied extensively in recent years. This review is on composites with ferromagnetic or ferrimagnetic oxides and ferroelectrics. Systems studied so far include samples with spinel ferrites, hexagonal ferrites or lanthanum manganites for the ferromagnetic phase and barium titanate, lead zirconate titanate (PZT), lead magnesium niobate–lead titanate (PMN-PT) or lead zinc niobate–lead titanate (PZN-PT) for the ferroelectric phase. Bilayer and multilayer heterostructures, bulk composites, core shell nanoparticles and core shell nanotubes and nanowires were investigated for their response to magnetic fields, termed direct magnetoelectric effect (DME). Several systems show a giant low frequency DME and resonance enhancement at bending and electromechanical resonance. The response of the composites to an electric field, called converse ME effect, is found to be strong in several ferrite–ferroelectric composites. The potential for use of the composites for pico-Tesla magnetic sensors and high frequency electric field tunable ferrite signal processing devices are also addressed in this review.     

Effect of Barium on Morphological Transition and Magnetic and Dielectric Properties in Ferrite Nanoparticles

Barium ferrite (BaFe₂O₄) nanoparticles were synthesized by auto combustion method under different weight percentages of barium. The role of barium in the behavior of spinel ferrite property is identified from this study. XRD exhibits prominent orientation of (212) for BaFe₂O₄ has confirmed especially in 20 and 30 wt% of barium. The addition of barium metal induced the specific vibration in FTIR spectra and such changes coincide well with the particle size. Further, the EDX spectrum reflects the atomic percentage of elemental presence in the samples of barium ferrite. Addition of barium on ferrite nanoparticles reduces the intensity of fluorescence. The morphological changes occurred due to increasing doping concentration of barium and is visualized from the FESEM and TEM images. The formation of different morphologies such as spherical, hexagonal platelets and small rectangular bar shape are observed only due to inclusion of barium at surfactant medium. The magnetic properties of the barium ferrite samples are studied by VSM. It reveals that 35.11 emu/g saturation magnetization (M _s ) with 3775.08 Oe coercivity. The change in values of coercivity (H _c ) from 3775.08 to 1572.95 Oe due to the variation of barium levels confirmed that the role of barium induced the hard magnetic behavior. The dielectric study also indicates the significance of barium ferrite in the variation of dielectric constant.     

Statistical analysis of signal and noise in magnetic recording

A general theory is developed for the power spectrum of the induced voltage at the read head in magnetic recording, originating from the particulate nature of the tape. The theory is general in that it yields the signal power as well as the noise spectrum, including the effects of clustered particulate media. The statistics of all relevant parameters such as particle length, particle magnetic moment, and partial penetration of the signal into the layer have been taken into account. Experiments on nonoriented Fe2O3showed a discrepancy at short wavelengths of the order of 10 dB. This is qualitatively explained by particle interaction mechanisms.     

Intrinsic coercive force variation with packing in fine particle assemblies

A formula is given to describe the variation of intrinsic coercive force with packing in elongated single domain (ESD) iron particle assemblies. A similar formula, which describes this variation in barium ferrite particle assemblies, is formally identical to the other one by simply changingp(packing) into1 - p(porosity). Magnetic interactions are considered, and an overall interaction field is defined that may account for intrinsic coercive force variations in the case of ESD iron particles. Experimental results on barium ferrite particle assemblies are presented that suggest the existence of a critical packing factor (p sim 0.15) separating two different, but not specified, modes of interaction.     

球磨工艺对钡铁氧体浆料粒径和磁性能的影响

研究钡铁氧体永磁材料制备过程中,球磨时间和分级球磨等球磨工艺对浆料粒度的影响,以及浆料粒度对钡铁氧体磁性能的影响。结果表明:在高能球磨过程中,浆料粒径随球磨时间增加呈减小趋势,当球磨时间超过其极限值后基本保持不变。采用分级球磨后浆料粒径明显减小,且颗粒粒径分布均匀,但只有采用合适的球磨工艺才能达到最好的分散效果。钡铁氧体永磁材料的取向度、剩余磁化强度随颗粒粒径的减小呈增大趋势。     

Properties, preparation, advances and limitations of materials andmedia for perpendicular recording

The trends in perpendicular magnetic recording research are discussed, with emphasis on clarifying the trend of research on recording media, their materials, and head materials. Among numerous proposed perpendicular recording media materials, barium ferrite powder and evaporated Co-Cr films seem to lead in practical applications, since their mass production seems to be very successful. There still exist crucial points in their development. However, improvements are continuously taking place. Other candidate perpendicular recording media for a device which has a ring head are also discussed. It is concluded that, for future high-density magnetic recording, utilizing fully the perpendicular component of media magnetization will furnish the key to success     

Crucial points in perpendicular recording

Since the introduction of perpendicular recording on a floppy disc by IWASAKI in 1977 and its equivalent design on a rigid disc (SPH-like sensor + double-layer medium) in 1981, many tests have been carried out on different R/W sensors. For each test the main goal was the fci record or the improvement of the magnetic layer. Seen from the recording system point of view, the head and the medium are looked at as a unit through a specification, unchanging with increasing area density. For example, a minimum of 26 dB and 70 % must be achieved for the S/N ratio and the resolution respectively. By considering the noise of the best electronic channel (with a thin film head), and ignoring mechanical and medium noises, the output signal must be at least 250 μv pop. For a 50 Kfci application, however, a sensor does not yet exist. Using a ferrite head with a 1.2 μm gap length to write on FeTbGd, the level of the signal will not be high enough to be used. It is improved with a 0.6 μm gap head but then, the field doesn't allow us to write ! Such problems exist also with thin film heads or SPH like sensors on rigid discs. To improve the R/W process, the trend is to use a double layer medium e.g. CrCo/FeNi. The results show that this direction is not necessarily the best. For example, when erasing or over-writing with the head, some domains appear in the FeNi film which create noise from the track or its edge. Another example is the fact that the optimum parameters for a medium such as CrCo are not always compatible with the characteristics of the head (i.e. Hc, the thickness, the crystallographic orientation, the bit stability compared to the write field, the signal, the noise...).     

吸附钡铁氧体的多孔陶粒吸波材料制备及其性能

以硝酸铁、硝酸钡、柠檬酸为原料,采用溶胶-凝胶法合成钡铁氧体溶胶,将其吸附于多孔陶粒表面和微孔中,经焙烧制备得到电磁吸波功能陶粒;再将该功能陶粒用作集料制备新型水泥基复合吸波材料。研究了合成的钡铁氧体的物相、形貌、复介电常数和复磁导率;结果表明合成的钡铁氧体纯度高,合成温度对其结晶度和晶体形貌有影响,经1000℃煅烧制得的钡铁氧体具有良好的吸波能力;对制得的功能陶粒进行了测试,结果表明钡铁氧体能包覆至陶粒表面和孔壁上;采用弓形法测得的功能陶粒水泥基吸波材料在8~18GHz频率范围内的电磁波反射率明显优于采用碎石和普通陶粒制备的试样。     

Particulate magnetic recording media: a review

After describing briefly the principles of magnetic recording and the manufacture of media, the author reviews the history, characteristics, and development trends of the most important particulate magnetic recording materials. These are acicular iron oxides, chromium dioxide, cobalt-modified iron oxides, acicular metal particles, and barium ferrite. A trend common to most of the materials is particle size reduction     

Bicontinuous microemulsions as media for electrochemical studies

Biocontinuous microemulsions of didodecyldimethylammonium bromide (DDAB)/dodecane/water were explored as media for voltammetric reductions and oxidations of ruthenium(III) hexaammine, ferrocyanide, ferrocene, cob(II)alamin, and several polycyclic aromatic hydrocarbons (PAHs). These clear microemulsions are conductive and are believed to contain a dynamic extended network of interconnected water tubules. Agreement of simulated and experimental voltammetric data shows that electrochemical theory for homogeneous media is followed to a good approximation in DDAB microemulsions. Diffusion of electroactive solutes did not reflect the high bulk viscosities of the microemulsions. Non-polar molecules and ions diffuse as if they were in neat oil or aqueous media, respectively, and voltammograms with good signal to noise ratio are obtained. Reductions of PAHs in the microemulsions occurred by an ECE-type pathway, with nearly reversible one-electron reductions achieved at high scan rates.     

Media limitations on achievement of maximum detection window

The recording performance of a digital recording system is primarily dependent upon the isolated half pulse width (T50), signal amplitude, intersymbol interference (ISI), signal to noise ratio (SNR) and the horizontal detection window margin (Tm). These characteristics are related to the thickness and magnetic properties of the media, as well as the head/media interface. This paper addresses the relationship of the media characteristics in longitudinal digital magnetic recording for both particulate and thin film media to the overall recording system performance. The results are shown to proceed from further development of the quasi-optimum time containment filter analysis for media noise dominant and electronic (white) noise dominant recording channels [1], [2].     

Preparation and magnetic properties of barium ferrite fineparticles by the coprecipitation salt-catalysis method

The authors describe catalytic action by NaCl in the reaction between α-Fe₂O₃ and BaCO₃ and the development of a new process for manufacturing barium ferrite fine particles. Pure α-FeOOH and reagent-grade BaCO₃ are mixed and heated at temperatures from 300 to 800°C with and without NaCl. The BaCO₃ and α-Fe₂O₃ powders which are formed during heating react and change to barium ferrite at temperatures 100°C lower than NaCl than without it. A hematite defect structure is maintained up to a relatively high temperature due to the presence of the salt, which promotes the formation of barium ferrite. The manufacturing process involves mixing an aqueous solution of metallic chlorides with a mixture of NaOH and Na ₂CO₃. This alkaline coprecipitate slurry is then neutralized by HCl solution, filtered, and dried. The dried powder, containing salts produced by the reaction, is heated at 840°C. After the salts are dissolved in water, barium ferrite fine particles are obtained     

Barium ferrite magnetic recording media

Barium ferrite particulate media have generated a lot of interest for advanced magnetic recording applications because they offer the potential to combine high recording densities with relatively low manufacturing cost. They consist of small (sub-tenth micron) plateletshaped particles with competing orthogonal anisotropies (crystalline and shape) of comparable magnitude. These anisotropies, along with the quasi-perpendicular characteristics of the barium ferrite coatings impart to them many subtle and surprising properties, requiring a careful and judicious choice of parameters for each application. The choices include the aspect ratio of the particles, their coercivity, the particle-to-binder loading, and the degree and direction of magnetic orientation. The problem areas include dispersion and orientation of the particles, overwrite characteristics of the coatings, thermal coefficients of the magnetic parameters and maintaining media coercivities at moderate levels. I this paper, we discuss the effect of the particle and coating parameters on the ensuing magnetic and recording properties of the media, and the types of choices that should be made to minimize the impact of some of the potential problems mentioned above.     

Characterisation of the deagglomeration behaviour of nanoscaled barium hexaferrite powders by measurement of electrokinetic effects

Single crystalline barium hexaferrite crystals form very firm and relatively large particle agglomerates. The fabrication of composites and coatings require isolated crystals. For it is necessary to know their deagglomeration behaviour. In this paper we present the zeta potential in dependence on the pH ‐ value of three several modified barium hexaferrite powders which dispersed in aqueous media. The zeta potential describes the extension of the electrical layer and the surface barrier of suspended particles and gives information to prepare singularized barium hexaferrite particles in aqueous media.     

钡铁氧体磁粉在液相中的分散性能

为了提高分散作用效果,选用聚乙二醇-20000(PEG-20000)、十二烷基硫酸钠(SDS)及二者组成的复配分散剂,利用机械球磨与添加分散剂相结合的分散方法制备了钡铁氧体浆料,通过考察分散剂的用量、浆料的pH值、zeta电位对分散性的影响,根据红外光谱分析,研究了分散剂的分散效果及作用机理,并测定了分散前后磁场成型各向异性钡铁氧体材料的磁性能。结果表明:加入分散剂可以改善钡铁氧体的分散效果,且SDS和PEG-20000的用量为钡铁氧体质量的1%和2%时,分散效果最好。SDS和PEG-20000均为静电和空间位阻稳定作用。分散后磁场成型各向异性钡铁氧体材料的磁性能得到提高。     

A new theory of recording media noise

Previous investigations of the noise in particulate or grainy recording media have considered statistical variations in the processes by which the particles become magnetized. The theory of noise presented includes also statistical variations in the packing density of the particles. An extremely simple analysis shows that, when both of these phenomena are included properly, the noise power of recording media may always be expected to depend upon the magnetization, or signal level, and the particle packing factor. It is found that the recording media should always provide higher signal-to-noise ratios (SNRs) than was previously supposed. It is pointed out that the signal recovery or detection techniques employed today in magnetic storage devices cannot yield optimum SNRs or bit error rates. Some algebraic and/or conceptual errors in the published literature on noise are discussed.<>     

Erase profiles of floppy disk heads

An experimental method is described to measure the amplitude profiles across written tracks used in rigid and floppy disk drives. This method allows determination not only of read, write, and side-erasure widths associated with the heads, but also of the detailed shape of the written profiles. These profiles may be obtained for tracks in the as-written condition, or after modification by the write or erase functions of the head. This method was applied to floppy disk heads in order to determine the trim erase characteristics of straddle erase, tunnel erase, and the implicit side erase characteristic of the read/write core itself. A number of typical track profiles are shown, demonstrating the usefulness of this technique. It was found that the straddle erase elements exhibit erasure both under the air gap and under the poles straddling the read/write core; the tunnel erase elements exhibit uniform erasure across their erase gaps but are subject to azimuthal misalignment effects; and the read/write cores themselves exhibit an implicit erase function during normal writing which, in the case studied, extended about 360 μm (9 μm) to each side of the core. Applications of this method to evaluate heads for use at high track densities are also outlined.     

"Real time" noise measurements with sensitivity exceeding the standard thermal noise limit

A possibility of "real-time" noise measurements with spectral resolution better than a standard thermal noise limit has been experimentally demonstrated at microwave frequencies. The enhancement in the sensitivity of spectral measurements was achieved due to more efficient use of the signal power via the power recycling technique. By utilizing such a technique, the noise floor of a 9 GHz "real time" measurement system was reduced by 3 dB below the standard thermal noise limit. This makes possible the characterization of intrinsic fluctuations in individual low-noise microwave components, such as ferrite circulators, without the need for cascading them or using the cross-correlation signal processing.     

High bit density (100 Mb/cm2) with single-layer Co-Crmedia and ring heads in perpendicular magnetic recording

The signal and noise of single-layer Co₇₉Cr₂₁ media are measured with ring heads to estimate the area density that can be achieved. Densities as high as 100 Mb/cm² (1 μm²/bit) are expected when a signal-to-noise ratio sufficient for an error probability less than 10 ^-5 is required. As a comparison, densities estimated from data from metal-evaporated tape and CrO₂ tape are given. In the frequency response of the single-layer media, an additional minimum was observed for a wavelength slightly larger than the gap length. This is probably caused by the bipolar nature of the perpendicular recording field of a ring head     

热处理温度对M-Ba铁氧体微米纤维结构和磁性能的影响

以柠檬酸和金属盐为原料,采用有机凝胶先驱体转化法成功制备了直径为500～600nm的钡铁氧体(BaFe12O19)微米纤维。通过XRD、SEM和VSM等技术对所制备的目标纤维进行了表征。结果表明,经750℃焙烧2h后,可获得M-Ba铁氧体纯相。随着焙烧温度的升高,晶粒逐渐长大,经850℃焙烧2h后,纤维主要由比较规则的片状晶粒组成。钡铁氧体纤维的磁性能主要受晶粒尺寸和测试温度的影响。经950℃焙烧2h后,组成纤维的晶粒尺寸约为62nm,室温下测得的饱和磁化强度和矫顽力均取得最大值,分别约为67A.m2/kg和328kA/m。在液氮(77K)条件下,纤维的饱和磁化强度有显著提高,最大值约为87A.m2/kg,这主要是由于纳米晶的表面自旋有序程度提高造成的。