Hexagonal ferrites 1

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       The hexagonal ferrites, also known as hexaferrites, have attracted an exponentially growing amount of interest since their discovery in the 1950s. These materials, which make up the majority of all magnetic materials produced globally and are now of enormous commercial and technological significance, have a wide range of functions and applications. Common uses for permanent magnets include data storage, magnetic recording, and components for electrical devices, especially those that operate at microwave/GHz frequencies. The significant hexaferrite family members are depicted below, where Me = a tiny 2+ ion like cobalt, nickel, or zinc, and Ba can be changed to Sr:                   

  • M-type ferrites, such as BaFe12O19 (BaM or barium ferrite), SrFe12O19 (SrM or strontium ferrite), and cobalt–titanium substituted M ferrite, Sr- or BaFe12-2xCoxTixO19 (CoTiM).
  • Z-type ferrites (Ba3Me2Fe24O41) such as Ba3Co2Fe24O41, or Co2Z.
  • Y-type ferrites (Ba2Me2Fe12O22), such as Ba2Co2Fe12O22, or Co2Y.
  • W-type ferrites (BaMe2Fe16O27), such as BaCo2Fe16O27, or Co2W.
  • X-type ferrites (Ba2Me2Fe28O46), such as Ba2Co2Fe28O46, or Co2X.
  • U-type ferrites (Ba4Me2Fe36O60), such as Ba4Co2Fe36O60, or Co2U.

     Although hexaferrite having various cations (substituted or doped) will also be covered, M, W, Z, and Y ferrites containing strontium, zinc, nickel, and magnesium will be highlighted as being the most well-known hexagonal ferrites. The magnetic properties of the hexagonal ferrites are inextricably tied to their crystalline forms. They are all ferrimagnetic materials. The induced magnetization in each of them has a preferred orientation inside the crystal structure, which is known as magneto crystalline anisotropy, or MCA. They can be split into two primary categories: uniaxial hexaferrites, which have an easy axis of magnetization, and ferroxplana or hexa-plana ferrites, which have an easy plane (or cone) of magnetization. Here, we'll talk about the ferrites' structure, production, solid state chemistry, and magnetic characteristics. The synthesis and characteristics of bulk ceramic ferrites will be the main topics of this review. This is due to the substantial body of research on thin film hexaferrites, which justifies a separate evaluation.

     In the past ten years, interest in hexaferrites for increasingly unusual uses has skyrocketed. This is notably true for composite materials, electromagnetic wave absorbers for EMC, RAM, and stealth technologies (especially the X and U ferrites), as well as electronic components for mobile and wireless communications at microwave/GHz frequencies. Additionally, there has been a noticeable recent interest in nanotechnology, the creation of nanofibers, the impact of fibre alignment and orientation in hexaferrite fibres, and composites made of carbon nanotubes (CNT). The discovery of single-phase magnetoelectric/multiferroic hexaferrites—first, Ba2Mg2Fe12O22 Y ferrite at cryogenic temperatures, and recently Sr3Co2Fe24O41 Z ferrite at ambient temperature—has been one of the most intriguing developments. We cover a number of M, Y, Z, and U ferrites that have recently been identified as room-temperature multiferroics.


Introduction

        Man has employed magnetic materials in various forms since Neolithic man used a piece of hung lodestone as a compass. But it wasn't until electricity came along that the magnetic processes started to be understood. Currently, it is understood that lodestone is an iron mineral termed magnetite, one of the diverse groups of magnetic ceramics based on iron (III)oxide known as ferrites. The structural class of compounds known as spinel’s includes magnetite, Fe3O4, which has the chemical formula MeFe2O4, where Me is a divalent cation in the case of magnetite, Fe2+. These substances have a cubic structure, but there is also a class of ferrites known as hexaferrites that have a hexagonal crystal structure. With BaM hexaferrite alone accounting for 50% of the total magnetic materials generated globally, at over 300,000 tonnes per year, these materials have grown significantly in importance both commercially and technologically. They have a wide range of uses and applications. 

The discovery, composition and characteristics of the hexagonal ferrites

     The magnetic mineral magnetoplumbite was initially described in 1925, and in 1938 it was shown that its crystal structure, PbFe7.5Mn3.5Al0.5Ti0.5O19, was hexagonal. PbFe12O19, also known as pure PbM, was discovered to be the synthetic form of magnetoplumbite. Several isomorphous compounds, including BaFe12O19, were also suggested. However, this material was not structurally investigated until after World War II, when Philips Laboratories took the lead in developing ferrites under the direction of Snoek. It has been established that BaFe12O19, also known as barium ferrite, hexaferrite, barium hexaferrite, ferroxdure, M ferrite, and BaM, has a hexagonal structure. Wijn and Braun's research into the BaO-Fe2O3 system led to the creation of more intricate hexagonal compounds (BaFe18O27), which contained both divalent and trivalent iron species. Jonker, Wijn, and Braun also discovered additional compounds when the ternary BaO-Fe2O3-MeO system was heated at 1200–1400 C, where Me is a small divalent cation. Philips Laboratories released thorough reports on all the major hexaferrite phases in the 1950s, which culminated in the publication of Smit and Wijn's outstanding book "Ferrites" in 1959. The end members of this system are the cubic MeFe2O4 spinel and BaM, with zero populations of Me and Ba, respectively. Table lists the physical traits of M ferrites and cobalt hexagonal ferrites according to how their discoverers categorised them.

Ferrite

Formula

Molecular mass (g)

Density

(g cm-3)

c (Å)

Magnetisation at room temp

BaM

BaFe12O19

1112

5.28

23.18

uniaxial

SrM

SrFe12O19

1062

5.11

23.03

uniaxial

Co2Y

Ba2Co2Fe12O22

1410

5.40

43.56

In plane

Co2Z

Ba3Co2Fe24O41

2522

5.35

52.30

In plane

Co2W

BaCo2Fe16O27

1577

5.31

32.84

In cone

Co2X

Ba2Co2Fe28O46

2688

5.29

84.11

In cone

Co2U

Ba4Co2Fe36O60

3624

5.31

38.16

In Plane

    The hexagonal crystal structures of all of these compounds were discovered to contain two crystalline lattice parameters: a, the width of the hexagonal plane, and c, the height of the crystal. In a magnetic field, each had a preferential direction for magnetization, resulting in an MCA that often ran parallel to the c-axis and emerged from the hexagonal crystal's basal plane. The magnetization is effectively fixed in the direction of the c-axis by this uniaxial anisotropy, and it can only be changed at the expense of large anisotropic energy.

 



     However, it was discovered that some compounds with a divalent cation, particularly those containing cobalt, had a plane of spontaneous magnetization in the basal plane that was parallel to the c-axis. These substances, known as ferroxplana ferrites, now include contain substances with a magnetic cone oriented at an angle of 0 to 90 degrees to the c-axis. The magnetization is nevertheless confined in this plane or cone by a high magnetic anisotropy energy even though the direction of the magnetization can readily rotate within the plane or cone through an angle of 360.




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