Hexagonal Ferrite Fibres and Nanofibres

Abstract

Hexagonal ferrites, or hexaferrites, are hugely important materials commercially and technologically, with common applications as permanent magnets, magnetic recording and data storage media, components in electrical devices operating at wireless frequencies, and as GHz electromagnetic wave absorbers for EMC, RAM and stealth technologies. Hexaferrites are all ferrimagnetic materials, and their magnetic properties are intrinsically linked to their crystalline structures, all having a strong magnetocrystalline anisotropy; that is the induced magnetisation has a preferred orientation within the crystal structure. They can be divided into two main groups: those with an easy axis of magnetisation (known as uniaxial), the hard hexaferrites, and those with an easy plane (or cone) of magnetisation (known as ferroxplana or hexaplana), soft ferrites. The common hexaferrite members are:M-type ferrites, such as BaFe12O19and SrFe12O19Z-type ferrites (Ba3Me2Fe24O41)Y-type ferrites (Ba2Me2Fe12O22)W-type ferrites (BaMe2Fe16O27)X-type ferrites (Ba2Me2Fe28O46)U-type ferrites (Ba4Me2Fe36O60)where Me = a small 2+ion such as cobalt, nickel or zinc, and Ba can be fully substituted by Sr. Generally, the M ferrites are hard, the Y, Z and U ferrites are soft, and the W and X ferrites can very between these two extremes, but all have large magnetisation (M) values.There is currently increasing interest in composite materials containing hexaferrite fibres. It had been predicted that properties such as thermal and electrical conductivity, and magnetic, electrical and optical behaviour will be enhanced in material in fibrous form. This is because a continuous fine fibre can be considered as effectively one-dimensional, and it does not behave as a homogeneously distributed solid. Although the intrinsic magnetisation of the material is unaffected, the effective magnetisation of an aligned fibre sample should be greater when a field is applied parallel with fibre alignment compared to when applied perpendicularly to fibre alignment. This feature was first demonstrated by the author for aligned hexaferrite fibres in 2006. This chapter will deal with progress in the manufacture and properties of hexaferrite fibres, from the first syntheses of BaM, SrM,Co2Y,Co2Z, Co2W, Co2X and Co2U micron-scale fibres by the author 12-15 years ago, to recent developments in M ferrite hollow fibres and nanofibres, and hexaferrite-coated CNTs (carbon nanotubes).The relative properties of all reported hexaferrite fibres are compared and summarised at the end of this chapter.

Strain-mediated magneto-electric interactions in hexagonal ferrite and ferroelectric coaxial nanofibers

“…Electrospinning techniques were used for the synthesis of ferroic and multiferroic nanofibers and involved the following steps: (i) preparation of sol of ferrite and ferroelectrics, (ii) electrospinning by dispensing the sol in a syringe through a needle and by applying an electric field of 1-2 kV/cm between the needle and a rotating aluminum drum, and (iii) fibers are collected from the aluminum drum, dried at 50°C for 24 h, and then annealed at high temperatures to form the ferroic phases. [42,43,[46][47][48][49][50][51][52][53][54][55][56] The fibers are formed from the sol droplets when the electrostatic force overcomes the force due to surface tension.…”

“…Such fibers can be fabricated by electrospinning or blow spinning. [24,42,43] The synthesis of coaxial fibers of spinel ferrites and ferroelectrics by electrospinning was reported by several groups. [25][26][27][28][29] They have synthesized ferrite-ferroelectric core-shell nanowires and tubes on porous anodized aluminum oxide or membrane templates by a combined sol-gel and electrochemical deposition techniques.…”

Recent Progress in Hexagonal Ferrites Based Composites for Microwave Absorption

“…[110][111][112][113] The basic units of M-type ferrite and cubic spinal ferrites that retain a hexagonal crystal structure with magnetization parallel to the c-axis make up the crystal structure of Y-type. [70,114] The structure of y-type is said to be an alternate stacking of tetrahedral (T), and spinel (S) blocks parallel to the caxis with space group R-3m [115][116][117][118][119] and a = 5.9 Å and c = 43.5 Å as lattice parameters. [120,121] The formula for Y-type is A 2 M 2 Fe 12 O 22 , where A can be Ba or Sr and M is any divalent metal ion [122,123] and are attractive prospects for microwave device applications at high-frequency.…”

Enhanced Structural, Dielectric, and Magnetic Properties of Mn-Yb-doped Y-type Hexaferrites

“…Hexaferrites are classi ed into different types based on their chemical composition, such as M-, Y-, Z-, X-, U-, and W-type hexagonal ferrites [10][11][12][13]. The Y-type hexagonal ferrites' crystalline structure was described as an alternating stacking of the S and T blocks along the caxis with space group R-3m [14][15][16]. Furthermore, the Y-type hexagonal ferrite belongs to the R-3m space group, with a magnetic structure consisting of the S-and T-spin blocks along the c-axis, with the S-block containing AFe4O8, and the T-block including A2Fe8O14, where A is a divalent metal ion.…”