Spinel structure ferrites (ferrimagnetic) and ferromagnetic materials

‘Ferrites’ has become a generic term for all magnetoceramic materials widely used in magnetoelectronic applications.

1. ADAMA SCIENCE AND TECHNOLOGY UNIVERSITY SCHOOL OF APPLIED NATURAL SCIENCES DEPARTMENT OF APPLIED CHEMISTRY Material Chemistry PhD Program Chem 7147 Course Seminar on: “Ferrite and Ferromagnetic Materials” By : Ararso Nagari Submitted to: Dr. Enyew January, 2019 Adama , Ethiopia 1

2. Outline Introduction Ferrites Crystal Structure , Properties and uses, Classification, Applications, Synthesis Ferromagnetic Materials Properties, Types, Applications Summary 2 Ferrites and Ferromagnetic Materials

3. One classification of magnetic materials—into diamagnetic , paramagnetic , and ferromagnetic — is based on how the material reacts to a magnetic field. In recent years, other types of magnetic ordering of substances into antiferromagnets and Ferrimagnetics is discovered 3 Introduction

4. ‘ Ferrites ’ has become a generic term for all magnetoceramic materials widely used in magnetoelectronic applications. These are generally chemically inert materials, which lose oxygen before melting at high temperatures. Ferrites are usually ferrimagnetic ceramic compounds derived from iron oxides. Ferrites 4

5. Magnetite (Fe3O4) is a famous example. Like most of the other ceramics, ferrites are hard, brittle, and poor conductors of electricity . Many ferrites adopt the spinel structure with the formula AB2O4 , where A and B represent various metal cations , usually including iron (Fe). …. 5

6. True ferrites contain iron(III) and are classified into two main groups: ‘ cubic ferrites ’ ( MFe2O4 ) with a spinel structure , which are sometimes called ferrospinels , and ‘ hexagonal ferrites ’ ( MFe12O19 ) with the magnetoplumbite structure (M = divalent cation, e.g. Mn, Fe, Co, Ni, Cu, Mg). Spinel ferrites usually adopt a crystal motif consisting of cubic close packed ( fcc ) oxides ( O 2 − ) with A cations occupying one eighth of the tetrahedral holes and B cations occupying half of the octahedral holes , i.e., A 2+ B 3+ 2 O 2− 4 . Crystal Structure of Ferrites 6

7. Ferrite crystals do not adopt the ordinary spinel structure, but rather the inverse spinel structure : One eighth of the tetrahedral holes are occupied by B cations, one fourth of the octahedral sites are occupied by A cations, and the other one fourth by B cation. Some ferrites adopt hexagonal crystal structure, like barium and strontium ferrites BaFe12O19 (BaO:6Fe2O3) and SrFe12O19 (SrO:6Fe2O3 ). 7 ….

8. It is also possible to have mixed structure spinel ferrites with formula [M 2+ 1−δ Fe 3+ δ ][M 2+ δ Fe 3+ 2− δ ]O 4 where δ is the degree of inversion. The magnetic material known as " ZnFe " has the formula ZnFe2O4 , with Fe 3 + occupying the octahedral sites and Zn 2+ occupy the tetrahedral sites, it is an example of normal structure spinel ferrite. …. 8

9. This is a structure for small to medium sized cations whose ideal composition takes the form AB2O4 . The general chemical formula of a ferrospinel molecule is M 2+ Fe 2 3+ O 4 2- , where M 2+ represents a divalent metal ion such as Zn 2+ , Fe 2+ , Mg 2+ , Mn 2+ , Cd 2 + , etc ., The basic structure is derived from a cubic closest packing of spheres with a doubled face-centered cubic unit cell . Cubic Ferrites: The Spinel Structure 9

10. Such a cell contains 32 octahedral holes and 64 tetrahedral holes , which can be broken down into a number of crystallographically different sublattices. Thus, in the spinel structure 50% of the octahedral holes are designated as B positions and one eighth of the tetrahedral holes as A sites, yielding the composition A 8 B 16 O 32 for the unit cell. 10 ....

11. The structure is difficult to view in three dimensions The A and B cations can have valences between 1 and 6, the only requirement being that their sum adds up to 8. Although typically thought of as a structure for transition metal ions, spinels form with a number of the smaller alkali and alkaline-earth cations as well as post-transition metals such as Zn +2 , Cd +2 , Ga +3 , In +3 , Ge +4 , and Sn +4 . …. 11

12. Some examples include TiCo 2 O 4 , CdCr 2 O 4 , FeCr 2 O 4 , Fe 3 O 4 , ZnAl 2 O 4 , FeGa 2 O 4 , NiV 2 O 4 . Assignment of A or B sites to cations in spinel type oxides based upon their chemical formulae can often be misleading. Thus, for the mixed-valence oxide NiFe2O4, we might conclude that the Ni +2 ions are on the tetrahedral A sites while the Fe +3 ions were on the B sites as implied by the formula. …. 12

13. 13 .... Ferrites crystallize in the form of a cubic structure. Each corner of a ferrite unit cell consists of a ferrite molecule

14. 14 …. The small filled circles represent metal ions, the large open or shaded circles represent oxygen ions: (a) tetrahedral or A sites ; (b) octahedral or B sites; and (c) one-fourth of the unit cell of a cubic ferrite. A tetrahedron and an octahedron are marked .

15. …. Normally there are two types of structures in ferrites- ferrospinels : Regular spinel and Inverse spinel 15

16. In this type, each divalent metal ion occupies 1 tetrahedral site and each trivalent metal ion occupies 1 octahedral site. Totally in an unit cell, there will be 8 tetrahedral (8 A) sites and 16 octahedral (16B) sites. Hence, the sites A and B combined to form a regular spinel ferrite structures as shown in Fig below. 16 Regular spinel structure

17. On the other hand, the mineral spinel, MgAl2O4 and Ni +2 Cr 2 +3 O 4 , have the site distributions predicted by the chemical formula and are referred to as normal spinels. The Spinels, ‘ zinc ferrites ’ (Zn +2 x M +2 1−x Fe +3 2 O 4 ), having the second metal, M, in oxidation state two (M is frequently Mn or Ni); when x = 1 it is that of a normal spinel in which all the Fe +3 ions occupy octahedral sites because Zn +2 displaces Fe +3 at the tetrahedral sites. 17 ....

18. In a ferrite unit cell there are 8 molecules. Therefore in a ferrite unit cell, there are 8 divalent metal ions, 16 ferric ions and 32 Oxygen ions. If only the oxygen ions in ferrite crystal are considered, it is found that they constitute a close packed face centered cubic structure. 18 ….

19. The schematic representation of zinc ferrite molecule as shown in Fig below Fig. Regular spinel structure 19 ....

20. In these arrangement it is found that for every four O2- ions there are 2 octahedral sites (surrounded by 6 O2- ions) and one tetrahedral site (surrounded by 4 O2- ions). The metal ions are distributed over these tetrahedral sites (A sites) and octahedral sites (B sites). Thus in ferrites the number of octahedral sites is twice the number of tetrahedral sites. 20 ….

21. In this type half of the B sites (8 sites) are occupied by divalent metal ions and the remaining half of the B sites (8 sites) and all the A sites are occupied by the trivalent metal ions For example, In fact, the actual distribution places all the Ni +2 on octahedral sites and half of the iron in tetrahedral sites giving a site preference formula of Fe +3 [Ni +2 . 5 Fe +3 . 5 ] 2 O 4 . 21 Inverse spinel structure

22. Magnetite, Fe 2+ Fe 2 3+ O 4 2 − is inverse spinel, with O 2− ions forming a fcc lattice and iron cations occupying interstitial sites. Half of the Fe 3+ cations occupy tetrahedral sites while the other half, along with Fe 2+ cations, occupy octahedral sites. The schematic representation of a ferrous ferrite molecule is shown in Fig. 22 ....

23. The Spinels ‘zinc ferrites’, Zn +2 x M +2 1 −x Fe +3 2 O 4 , when x = 0 the structure is that of an inverse spinel (i.e. half the Fe +3 ions occupy tetrahedral sites in the close-packed array of oxide ions, the other half of the Fe +3 ions occupying octahedral sites ) 23 ....

24. It is an interesting example of intergrowth structures Are a series of magnetic oxides based upon the structure of PbFe12O19, magnetoplumbite . The structure can be described in terms of five HCP layers of 4 oxygens each, in which one of the layers has one-quarter of its oxygen replaced by the large Pb +2 ion to yield a PbO19 framework. Hexagonal Ferrites: Magnetoplumbite structure 24

25. The iron then fill octahedral and tetrahedral holes in a spinel-like fashion. The layer sequencing is BABAB. …. 25

26. Now a day the term ferrite is used for magnetic oxides whose main constituent is iron oxide. In terms of their magnetic properties, the different ferrites are often classified as "soft", "semi-hard" or "hard ", which refers to their low or high magnetic coercivity , as follows . 26 Classification of Ferrites

27. Ferrites that are used in transformer or electromagnetic cores contain nickel, zinc, and/or manganese compounds. They have a low coercivity and are called soft ferrites. The low coercivity means the material's magnetization can easily reverse direction without dissipating much energy (hysteresis losses), while the material's high resistivity prevents eddy currents in the core, another source of energy loss. 27 Soft Ferrites

28. Because of their comparatively low losses at high frequencies, they are extensively used in the cores of RF transformers and inductors in applications such as switched mode power supplies. 28 ....

29. The most common soft ferrites are: a ) Manganese-zinc ferrite (Mn, Zn), with the formula ( Mn a Zn (1-a) Fe 2 O 4 ). MnZn have higher permeability and saturation induction than NiZn . b ) Nickel-zinc ferrite ( NiZn ), with the formula ( Ni a Zn (1-a) Fe 2 O 4 ). NiZn ferrites exhibit higher resistivity than MnZn , and are therefore more suitable for frequencies above 1 MHz. 29 ….

30. Cobalt ferrite , CoFe2O4 (CoO·Fe2O3), is in between soft and hard magnetic material and is usually classified as a semi-hard material . It is mainly used for its magnetostrictive applications like sensors and actuators thanks to its high saturation magnetostriction (~ 200 ppm). CoFe2O4 has also the benefits to be rare-earth free, which makes it a good substitute for Terfenol -D . 30 Semi-hard Ferrites

31. In contrast, permanent ferrite magnets are made of hard ferrites, which have a high coercivity and high remanence after magnetization. These are composed of iron and barium or strontium oxides. The high coercivity means the materials are very resistant to becoming demagnetized, an essential characteristic for a permanent magnet. They also conduct magnetic flux well and have a high magnetic permeability. 31 Hard Ferrites

32. This enables these so-called ceramic magnets to store stronger magnetic fields than iron itself. They are cheap, and are widely used in household products such as refrigerator magnets. The maximum magnetic field B is about 0.35 tesla and the magnetic field strength H is about 30 to 160 kiloampere turns per meter (400 to 2000 oersteds ). The density of ferrite magnets is about 5g/cm3 . 32 ….

33. The most common hard ferrites are: Strontium ferrite , SrFe12O19 (SrO·6Fe2O3), used in small electric motors, micro-wave devices , recording media, magneto-optic media, telecommunication and electronic industry. Strontium hexaferrite (SrFe12O19) is well known for its high coercivity due to its magnetocrystalline anisotropy. 33 ....

34. It has been widely used in industrial applications as permanent magnets and, because they can be powdered and formed easily, they are finding their applications into micro and nano -types systems such as biomarkers, biodiagnostics and biosensors. Barium ferrite , BaFe12O19 (BaO·6Fe2O3), a common material for permanent magnet applications. 34 ....

35. Barium ferrites are robust ceramics that are generally stable to moisture and corrosion-resistant. They are used in e.g. loudspeaker magnets and as a medium for magnetic recording, e.g. on magnetic stripe cards . 35 ....

36. Properties and Uses of Ferrites (M 2+ Fe 2 3+ O 4 2- ) The susceptibility (  ) is very large and positive. It is represented by,  = C / ( T  ), when T > TN When T<TN, they behave as ferrimagnetic materials. Mechanically, they have pure iron character . They have low tensile strength and are brittle and soft . In these, all valence electrons are tied up by ironic bonding and they are bad conductors with high resistivity of 1011  m. 36

37. They are soft magnetic materials and so they have low eddy current losses and hysteresis losses. Most spinel ferrites used in device applications are not simple ternary oxides but rather contain proprietary mixtures of several different magnetic ions tailor made for specific applications. 37 ….

38. So-called (Mn, Zn) ferrites of general formula Zn +2 x Fe +3 1−x [Mn +2 1−xF ]O 4 and related (Ni, Zn) ferrites are used as cores for inductors or transformers and in TV deflector yokes . (Mg, Mn) ferrites and yttrium iron garnets including those partially substituted by Al or Gd find numerous applications in microwave circuitry One of the biggest applications of magnetic oxides is for information storage . 38 ….

39. Computer memories typically utilize (Mn, Mg) based spinels. Hexagonal barium ferrites are used in floppy discs . Fe3O4 is used as a magnetic toner in photocopiers and as a magnetic printing ink while the defect spinel γ-Fe2O3 and γ-Fe2O3 /Fe3O4 mixtures compete with ferromagnetic CrO2 for the audio and video magnetic tape market. 39 ....

40. In addition to composition, the shape and size of the particles plays a crucial role in the quality and performance of the tape. In this respect CrO2 is superior because of its normal, needle-like crystal habit. 40 ….

41. Ferrimagnetics materials, ferrites, have wide application potential in different fields such as telecommunication, electronic industries, due to their interesting electrical and magnetic properties. The simultaneous requirement of optical, electric and magnetic properties in the advanced electronics, Microwave and computer technologies have focused the attention of research works on ferrites. 41 Applications

42. …. It was also observed that these properties are significantly be modified by substitution of divalent or trivalent cations, to suit the material for particular application. Therefore , investigations of structural, electrical and magnetic properties of the ferrites become a field of interest for many researchers. 42

43. Synthesis techniques play an important role in controlling the size and surface area of materials. Generally, Ferrites are manufactured by powder metallurgical process by mixing, compacting and then sintering at high temperatures followed by age hardening in magnetic fields. The synthesis of magnetic materials (ferrites) has been reported using different chemical methods; that is, sol-gel, precipitation, solid-state reaction, micro-emulsion. 43 Synthesis of Ferrites

44. In this method, the formation of a gel provides a high degree of homogeneity and reduces the need for atomic diffusion during the solid state calcinations. A solution of the appropriate precursors is formed first, followed by conversion into a homogeneous oxide (gel) after hydrolysis and condensation. Drying and sub- sequent calcination of the gel yields an oxide product. 44 Sol-Gel Method

45. In the precipitation method the precipitation of substances normally soluble under the employed conditions. An inclusion occurs when the impurity occupies a lattice site in the crystal structure of the carrier, resulting in a crystallographic defect, which can occur when the ionic radius and charge of the impurity are similar to those of the carrier. An occlusion occurs when an adsorbed impurity is physically trapped inside the crystal as it grows. 45 Precipitation Method

46. Solid-state synthesis methods are the most widely used. This method involves mixing of raw materials and can take place with both wet and dry processes. Assisted by grinding or ball milling heating at high temperatures in refractory containers for several hours or days to produce the desired product Often, the reactants are pressed into pellets to promote internal reaction and to minimize contact with the container 46 Solid-State Reaction Method

47. Ferromagnetic Material Some materials become spontaneously magnetized at low temperatures because of the interaction between the magnetic atoms called ferromagnetic . A type of material that is highly attracted to magnets and can become permanently magnetized The relative permeability is much greater than unity and are dependent on the field strength. These have high susceptibility . 47

48. E.g., Fe, Co, Ni, Cr, Mn 48 ....

49. Ferromagnetic materials, such as iron, contain unpaired electrons ( domains ), each with a small magnetic field of its own , that align readily with each other in response to an external magnetic field. This alignment tends to persists even after the magnetic field is removed . 49 ....

50. CrO2 is a metallic ferromagnet which can be used as a magnetic recording medium Thermal decomposition of CrO3 is the most common way to obtain this interesting material. The decomposition proceeds sequentially, while at ambient atmosphere it is almost impossible to freeze the rapid process at CrO2 before Cr2O3 forms. 50 CrO2

51. Ferromagnetic materials , including iron and nickel, lose their normal strong residual magnetism at a characteristic high temperature, called the Curie temperature. Electrical resistance usually decreases with decreasing temperature These and many other phenomena observed in solids depend on energy quantization and can best be described in terms of effective “particles” such as phonons, polarons , and magnons . 51 Ferromagnetic Materials Properties

52. in nuclear magnetic resonance imaging, an important diagnostic tool used by doctors in today's most powerful particle accelerators to keep the accelerated particles focused and moving in a curved path Scientists are developing magnetic levitation trains that use strong magnets to enable trains to float above the tracks, reducing friction 52 ....

53. Types of Ferromagnet Actinide ferromagnets A number of actinide compounds are ferromagnets at room temperature or exhibit ferromagnetism upon cooling. PuP is a paramagnet with cubic symmetry at room temperature, but which undergoes a structural transition into a tetragonal state with ferromagnetic order when cooled below its Tc =125 K. In its ferromagnetic state, PuP's easy axis is in the < 100> direction. 53

54. In NpFe2 the easy axis is <111 >. Above Tc ≈ 500 K NpFe2 is also paramagnetic and cubic. Cooling below the Curie temperature produces a rhombohedral distortion wherein the rhombohedral angle changes from 60° (cubic phase) to 60.53°. An alternate description of this distortion is to consider the length c along the unique trigonal axis (after the distortion has begun) and a as the distance in the plane perpendicular to c . 54 ....

55. In the cubic phase this reduces to c/a = 1.00. Below the Curie temperature which is the largest strain in any actinide compound . NpNi2 undergoes a similar lattice distortion below Tc = 32 K, with a strain of (43 ± 5) × 10−4 . NpCo2 is a ferrimagnet below 15 K. 55 ....

56. ….. Lithium gas A lithium gas cooled to less than one kelvin can exhibit ferromagnetism . Fermionic lithium-6 was cooled to less than 150 nK (150 billionths of one kelvin) using infrared laser cooling. This demonstration is the first time that ferromagnetism has been demonstrated in a gas. 56

57. Tetragonal ruthenium Body-centered tetragonal ruthenium exhibits ferromagnetism at room temperature. 57 ....

58. is the basis of the electric motor and the transformer. important in the computer revolution. Computer memories can be fabricated using bubble domains, thus serving as the units of the binary number system used in computers. Magnetic materials are also important constituents of tapes and disks on which data are stored. 58 Applications

59. Magnetic Materials are those materials in which a state of magnetization can be induced. Such materials when magnetized create a magnetic field in the surrounding space . They have many application in electronics specially for data storage and in medical for drug delivery 59 Summary