Hello everyone. In this lecture, we are going to talk about magnetic ceramics. So firstly, let's think about the origin of magnetic moment in materials. The magnetic moments of each electron has two sources. The one is spin magnetic moments and the other one is permanent orbital. So as shown here, spin of the electron produces a magnetic field with a direction. So we can find that one of two directions up or down. Also, electrons orbiting around the nuclei create a magnetic field. So this like moving electrons result in small current loop. So let's think about the magnetic moment in transition metals. The inner energy level that is not completely filled. So one of the transition metals, copper, has completely filled 3d shells. So copper is no net moment metal whereas manganese, the first five electrons have the same spin. So after half of the 3d-level is filled, the pairs with opposing spins form. So each atom in a transition metal provide the permanent magnetic moments and this is related with the number of unpaired electrons. Each atom behaves as a magnetic dipole. So you should know some important terminologies which is related with the magnetic moments. The Bohr magneton is strength of a magnetic moments of an electron, U_B. Magnetization, the total magnetic moment per unit volume. Magnetic permeability, it is a ratio between magnetization and magnetic field. Magnetic susceptibility, it is the ratio between magnetization and applied field. Domain, it is the region in which all of the magnetization directions are aligned. Curie temperature, Tc. Above Tc, the ferromagnetic properties should be transferred into paramagnetic properties. Generally, there are three different types of magnetization. The first, diamagnetic, the second, paramagnetic, and third ferromagnetic. In diamagnetics, the dipoles oppose the magnetic field, H and susceptibility of diamagnetics is lower than one. But sometimes it shows the magnet minus susceptibility. So for example, the copper, silver, gold, aluminum at room temperature can show the diamagnetic characteristics. Superconductors is a perfect diamagnetic material at below Curie Temperature. In paramagnetics, the magnetic field H is removed, the effect is lost, and the susceptibility value is just radial than 1.0. So diamagnetic and paramagnetic are non-magnetic. Materials with unpaired electron, such as aluminum, titanium, copper alloys are paramagnetic materials. In ferromagnetics, due to the spontaneous magnetization, the susceptibility value is very high, so about 10 to the six. This is related with the unfilled 3d and 4f shells. So this like ferromagnetic can be found in iron, cobalt, nickel, and several rare elements such as Gd and Nd. So ferromagnetics has uncancelled electron spins. So this results in permanent magnetic moment. Small magnetic field can make large magnetization in ferromagnetic material. In ferromagnetic material, you can find things like magnetic moment aligned with magnetic field and also find things like hysteresis curve. The loop traced out by magnetization in a ferromagnetic and ferrimagnetic material as the magnetic field is cycled. So you can find saturation magnetization value, the MS, to which it means that all of the dipoles have been aligned. Ferromagnetic material can be divided into two categories. The first is hard magnet with permanent magnetic moment and high coercivity. Another type of ferromagnetic material is soft magnetic material, which can be used as cores for electrical equipment such as motors. Small coercivity of soft magnetic material can show the small hysteresis and can minimize energy losses. So it shows the rapid response to high frequency magnetic field. The ferrite is one of the most important ferromagnetic material. So this is the ceramic material made by mixing and firing the large proportions of iron oxide blended with small proportions of one or more additional metallic elements, such as barium, mangan, nickel, zinc. They are insulators and ferrimagnetic materials. The harder ferrite with higher coercivity, they can be used for permanent magnets, such as the refrigerators, the loudspeakers, and small electric motors. Softer ferrite material with low coercivity, so this material can easily change their magnetization and can act as conductors of magnetic field and can be used in the electronics industry to make efficient magnetic cores. The permanent ferrite magnets are hard ferrite with high coercivity and high remanence. So the composition is iron oxide and barium or strontium elements. So they have high magnetic permeability and can be widely used in household products such as refrigerator magnets. The one important type of hard ferrite is strontium ferrite. The composition is SrO and six iron oxide. This material has high coercivity due to its magnetocrystalline anisotropy and can be used in small electric motors, the micro-wave devices, recording media, the magneto-optic media, the telecommunications and electronic industry and also can be used as biomarkers, bio diagnostics and biosensors. So another type of hard ferrite is barium ferrite. The competition is barium oxide and six iron oxide. It is stable to moisture and corrosion-resistant, so it can be used for louder speaker magnets, the medium for magnetic recording. Ferrite that are used in transformer and electromagnetic cores are soft ferrite. So they contains nickel, zinc, mangan and have the low coercivity. So magnetization can be easily reverse direction without dissipating too much energy. So they have high resistivity, so effectively prevent the eddy current in the core. So this means that low loss at high frequency, so they can be used as cores of RF transformers and inductors. The one type of software ferrite is manganese-zinc ferrite. So it has higher permeability and saturation induction than nickel-zinc ferrite. Another type of software ferrite is nickel-zinc ferrite. It has higher resistivity than manganese-zinc ferrite, so more suitable for frequencies above one megahertz. Then finally, this like the ferrite, the hard magnetic material, have so many advantages, such as low cost and the good oxidation registers. But as shown here, their magnetic performance is much lower than other alloy and intermetallic based magnetic material. So we should develop, we should find an effective approaches to enhance the magnetic properties of this magnetic ceramics based on the nanostructuring and defect engineering technologies. Thank you.
