Nanocomposite Permanent Magnets Based on SrFe12O19-Fe3O4 Hard-Soft Ferrites

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.

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Recent Trends of Magnetic Materials. Soft-ferrites and Permanent Magnets.

Journal of the Japan Society of Powder and Powder Metallurgy ◽

10.2497/jjspm.41.857 ◽

1994 ◽

Vol 41 (7) ◽

pp. 857-862

Author(s):

Hisae Minatono

Keyword(s):

Magnetic Materials ◽

Permanent Magnets ◽

Recent Trends ◽

Soft Ferrites

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Neutron Depolarization in Submicron Ferromagnetic Materials

Textures and Microstructures ◽

10.1155/tsm.11.127 ◽

1989 ◽

Vol 11 (2-4) ◽

pp. 127-142 ◽

Cited By ~ 39

Author(s):

M. Th. Rekveldt

Keyword(s):

Permanent Magnets ◽

Dynamic Range ◽

Domain Size ◽

Larmor Precession ◽

Metal Foils ◽

Domain Structures ◽

Direction Cosines ◽

The Mean ◽

Neutron Depolarization ◽

Soft Ferrites

The neutron depolarization technique is based on the loss of polarization of a polarized neutron beam after transmission through ferromagnetic substances. This loss, caused by Larmor precession in individual domains, determines the mean domain size, the mean square direction cosines of the domains and the mean magnetization. The method is complementary to the neutron scattering technique with respect to the size of the inhomogeneities to be determined and the dynamic range accessible. Only the static applications of the method in studying domain structures are considered. As examples will be treated metal foils under stress, oriented ferroxdur permanent magnets, soft ferrites, recording tapes and thin films. In most of these examples magnetic correlations between neighbouring domains are subject of study.

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EM of rapidly solidified permanent magnets

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100121399 ◽

1992 ◽

Vol 50 (1) ◽

pp. 198-199

Author(s):

Raja K. Mishra

Keyword(s):

Melt Spinning ◽

Permanent Magnets ◽

Dark Field Image ◽

Dark Field ◽

Maximum Energy ◽

Quench Rate ◽

Maximum Energy Product ◽

Energy Product ◽

Annular Dark Field ◽

Rapidly Solidified

The discovery of a new class of permanent magnets based on Nd2Fe14B phase in the last decade has led to intense research and development efforts aimed at commercial exploitation of the new alloy. The material can be prepared either by rapid solidification or by powder metallurgy techniques and the resulting microstructures are very different. This paper details the microstructure of Nd-Fe-B magnets produced by melt-spinning.In melt spinning, quench rate can be varied easily by changing the rate of rotation of the quench wheel. There is an optimum quench rate when the material shows maximum magnetic hardening. For faster or slower quench rates, both coercivity and maximum energy product of the material fall off. These results can be directly related to the changes in the microstructure of the melt-spun ribbon as a function of quench rate. Figure 1 shows the microstructure of (a) an overquenched and (b) an optimally quenched ribbon. In Fig. 1(a), the material is nearly amorphous, with small nuclei of Nd2Fe14B grains visible and in Fig. 1(b) the microstructure consists of equiaxed Nd2Fe14B grains surrounded by a thin noncrystalline Nd-rich phase. Fig. 1(c) shows an annular dark field image of the intergranular phase. Nd enrichment in this phase is shown in the EDX spectra in Fig. 2.

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Microstructure and microanalysis of sintered Fe-Nd-B permanent magnets

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100178082 ◽

1990 ◽

Vol 48 (4) ◽

pp. 990-991

Author(s):

Mahesh Chandramouli

Keyword(s):

Electron Microscope ◽

Liquid Phase ◽

Permanent Magnets ◽

Phase Distribution ◽

Energy Dispersive Spectrometer ◽

Nucleation And Growth ◽

Liquid Phase Sintering ◽

Domain Wall Energy ◽

Oxide Phases ◽

Scanning Electron

Magnetization reversal in sintered Fe-Nd-B, a complex, multiphase material, occurs by nucleation and growth of reverse domains making the isolation of the ferromagnetic Fe14Nd2B grains by other nonmagnetic phases crucial. The magnets used in this study were slightly rich in Nd (in comparison to Fe14Nd2B) to promote the formation of Nd-oxides at multigrain junctions and incorporated Dy80Al20 as a liquid phase sintering addition. Dy has been shown to increase the domain wall energy thus making nucleation more difficult while Al is thought to improve the wettability of the Nd-oxide phases.Bulk polished samples were examined in a JEOL 35CF scanning electron microscope (SEM) operated at 30keV equipped with a Be window energy dispersive spectrometer (EDS) detector in order to determine the phase distribution.

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Characterisation of planar crystal defects in Y2Fe17Cx (0.6 ≤ x ≤ 1.6) magnetic material by High Resolution Electron Microscopy

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100177003 ◽

1990 ◽

Vol 48 (4) ◽

pp. 774-775

Author(s):

W. Coene ◽

F. Hakkens ◽

T.H. Jacobs ◽

K.H.J. Buschow

Keyword(s):

Electron Microscopy ◽

High Resolution ◽

Permanent Magnets ◽

Annealing Treatment ◽

High Resolution Electron Microscopy ◽

Crystal Defects ◽

Ordered Structure ◽

X Ray ◽

Planar Crystal ◽

Resolution Electron

Intermetallic compounds of the type RE2Fe17Cx (RE= rare earth element) are promising candidates for permanent magnets. In case of Y2Fe17Cx, the Curie temperature increases from 325 K for x =0 to 550 K for x = 1.6 . X ray and electron diffraction reveal a carbon - induced structural transformation in Y2Fe17Cx from the hexagonal Th2Ni17 - type (x < 0.6 ) to the rhombohedral Th2Zn17 - type ( x ≥ 0.6). Planar crystal defects introduce local sheets of different magnetic anisotropy as compared with the ordered structure, and therefore may have an important impact on the coercivivity mechanism .High resolution electron microscopy ( HREM ) on a Philips CM30 / Super Twin has been used to characterize planar crystal defects in rhombohedral Y2Fe17Cx ( x ≥ 0.6 ). The basal plane stacking sequences are imaged in the [100] - orientation, showing an ABC or ACB sequence of Y - atoms and Fe2 - dumbbells, for both coaxial twin variants, respectively . Compounds resulting from a 3 - week annealing treatment at high temperature ( Ta = 1000 - 1100°C ) contain a high density of planar defects.

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