Magnetic Ceramics EBB443-Technical Ceramics Dr. Sabar D. Hutagalung School of Materials & Min. Res. Eng., Universiti Sains Malaysia
Introduction • Materials may be classified by their response to externally applied magnetic fields as • diamagnetic, • paramagnetic, or • ferromagnetic. • These magnetic responses differ greatly in strength.
Introduction • Diamagnetism is a property of all materials and opposes applied magnetic fields, but is very weak. • Most materials are diamagnetic and have very small negative susceptibilities (about 10-6). • Example: Inert gases, hydrogen, many metals (Bi, Ag, Cu, Pb), most non-metals and many organic compounds. • A superconductor will be a perfect diamagnet since there is no resistance to the forming of the current loops.
Introduction • Paramagnetism is stronger than diamagnetism and produces magnetization in the direction of the applied field, and proportional to the applied field. • Paramagnetics are those materials in which the atoms have a permanent magnetic moment arising from spinning and orbiting electrons. • The susceptibilities are therefore positive but again small (in range of 10-3 – 10-6). • The most strongly paramagnetic substances are compound containing transition metal or rare earth ions and ferromagnetics above Tc.
Introduction • Ferromagneticeffects are very large, producing magnetizations sometimes orders of magnitude greater than the applied field and as such are much larger than either diamagnetic or paramagnetic effects.
Relative Permeability • The magnetic constant, m0 = 4p x 10-7 T m/A is called the permeability of space. • The permeabilities of most materials are very close to m0 since most materials will be classified as either paramagnetic or diamagnetic. • But in ferromagnetic materials the permeability may be very large and it is convenient to characterize the materials by a relative permeability.
Relative Permeability Some representative relative permeabilities: • Magnetic iron: 200 • Nickel: 100 • Permalloy (78.5% Ni, 21.5% Fe): 8,000 • Mumetal (75% Ni, 2% Cr, 5% Cu, 18% Fe): 20,000
Magnetic Field • The magnetization of a material is expressed in terms of density of net magnetic dipole moments,min the material. • We define a vector quantity called the magnetization M by M = mtotal/V • Then the total magnetic fields B in the material is given by B = B0 + m0M • where m0 is the magnetic permeability of space and B0 is the externally applied magnetic field.
Magnetic Field • When magnetic fields inside of materials are calculated using Ampere’s law or the Biot-Savart law, then the m0 in those equations is typically replaced by just m with the definition m = Kmm0 where Km is called the relative permeability. • If the material does not respond to the external magnetic field, then Km = 1. • Another commonly used magnetic quantity is the magnetic susceptibility which specifies how much the relative permeability differs from one. • Magnetic susceptibility, cm = Km - 1
Magnetic Field • For paramagnetic and diamagnetic materials the relative permeability is very close to 1 and the magnetic susceptibility very close to zero. • For ferromagnetic materials, these quantities may be very large. • Another way to deal with the magnetic fields which arise from magnetization of materials is to introduce a quantity called magnetic field strength, H. • It can be defined by the relationship H = B0/m0 = B/m0 - M
Magnetic Field • The relationship for B above can be written in the equivalent form B = m0(H + M) • H and M will have the same units, amperes/meter. • Ferromagnetic materials will undergo a small mechanical change when magnetic fields are applied, either expanding or contracting slightly. • This effect is called magnetostriction.
Flux Magnet • By definition, magnetic energy is the product of the flux density in the magnetic circuit and the magnetizing force it took to excite the material to that flux level. • Energy = B x H • The unit of energy in the SI system is the Joule, in the CGS system it is the ERG. • In permanent magnet design a special energy density, or energy product, is also used to indicate energy and storage properties per unit volume. • The CGS unit of energy product is the Gauss-Oersted, the SI unit is the Joule Per Meter3. • 1 joule = 107 ergs • 1 joule per meter3 = 125.63 gauss-oersted
Flux Magnet Tesla in SI units: • 1 Tesla = 10,000 Gauss • 1 Tesla = 1 Weber/m2 • 1 Gauss = 1 Maxwell/cm2 • Flux density is one of the components used to determine the amount of magnetic energy stored in a given geometry.
Ferromagnetism • Iron, nickel, cobalt and some of the rare earths (gadolinium, dysprosium) exhibit a unique magnetic behavior which is called ferromagnetism because iron (ferric) is the most common and most dramatic example. • Ferromagnetic materials exhibit a long-range ordering phenomenon at the atomic level which causes the unpaired electron spins to line up parallel with each other in a region called a domain. • Ferromagnetism manifests itself in the fact that a small externally imposed magnetic field can cause the magnetic domains to line up with each other and the material is said to be magnetized.
Ferromagnetism • Ferromagnets will tend to stay magnetized to some extent after being subjected to an external magnetic field. • This tendency to "remember their magnetic history" is called hysteresis. • The fraction of the saturation magnetization which is retained when the driving field is removed is called the remanence of the material, and is an important factor in permanent magnets. • All ferromagnets have a maximum temperature where the ferromagnetic property disappears as a result of thermal agitation. • This temperature is called the Curie temperature (Tc). • Ferromagnetic materials are spontaneously magnetized below a temperature term the Curie temperature.
Soft & Hard Magnetic • Soft magnetic, or core products, do have the ability to store magnetic energy that has been converted from electrical energy; but it is normally short-term in nature because of the ease to demagnetize. • This is desirable in electronic and electrical circuits where cores are normally used because it allows magnetic energy to be converted easily back into electrical energy and reintroduced to the electrical circuit. • Hard magnetic materials (PMs) are comparatively difficult to demagnetize, so the energy storage time frame should be quite long.
Soft & Hard Magnetic • Hard magnetic: high remanent magnetization (Br), high coercivities (Hc), difficult to demagnetize, broad B-H hysterisis loop.
Magnetic Domains • The microscopic ordering of electron spins characteristic of ferromagnetic materials leads to the formation of regions of magnetic alignment called domains. • The main implication of the domains is that there is already a high degree of magnetization in ferromagnetic materials within individual domains, but that in the absence of external magnetic fields those domains are randomly oriented. • A modest applied magnetic field can cause a larger degree of alignment of the magnetic moments with the external field, giving a large multiplication of the applied field.
Magnetic Ceramics • All ferro- and ferrimagnetic materials exhibit the “hysteresis effect” (a nonlinear realtionship between applied magnetic field, H and magnetic induction, B). • Many materials have important magnetic properties, including elemental metals, transition metal alloys, rare earth alloys and ceramics. • Among the magnetic ceramics, ferrites are the prodominant class.
Ferrites • Ferrites using Fe2O3 as the major raw material. • Ferrites crystallize in a large variety of structures: • Spinel, • Garnet, and • Magnetoplumbite. • Spinel: 1 Fe2O3 : 1 MeO, (MeO=transition metal oxide). • Garnet: 5 Fe2O3 : 3 Me2O3 (Me2O3=rare earth metal oxide) • Magnetoplumbite: 6 Fe2O3 : 1 MeO (MeO=divalent metal oxide from group II, BaO, CaO, SrO).
Ferrites • The spinel ferrite are isostructural with the naturally occuring spinel MgAl2O4 and conform to general formula AB2O4. • The realatively large oxygen anions are arranged in cubic close packing, with octahedral and tetrahedral interstitial site occupied by transistion metal cations. • The rare earth yittrium iron garnet, Y3Fe5O12 (YIG) is prototypical of the rare earth ferromagnetic insulators.
MAGNETORESISTIVE EFFECT • In magnetoresistive effect, the resistance of a material changes in the presence of magnetic field. • Similarly as the Hall effect, the magnetoresistive effect is caused by the Lorentz force which rotates the current lines by an angleqH. • The deflection of the current paths leads to an increase in the resistance of the semiconductor. • For small angles ofqHthe resistance R is: R R0(1 + tan qH2 ) • The applicationsare in magnetic sensors.
Giant Magnetoresistance (GMR) • The giant magnetoresistance (GMR) is the change in electrical resistance of some materials in response to an applied magnetic field. • GMR effect was discovered in 1988 by two European scientists working independently: Peter Gruenberg of the KFA research institute in Julich, Germany, and Albert Fert of the University of Paris-Sud . • They saw very large resistance changes - 6 percent and 50 percent, respectively - in materials comprised of alternating very thin layers of various metallic elements. • These experiments were performed at low temperatures and in the presence of very high magnetic fields.
Giant Magnetoresistance (GMR) • It was discovered that the application of a magnetic field to magnetic metallic multilayers such as Fe/Cr and Co/Cu, in which ferromagnetic layers are separated by nonmagnetic spacer layers of a few nm thick, results in a significant reduction of the electrical resistance of the multilayer.
In the absence of the magnetic field the magnetizations of the ferromagnetic layers are antiparallel. • Applying the magnetic field, which aligns the magnetic moments and saturates the magnetization of the multilayer, leads to a drop in the electrical resistance of the multilayer.
Intrinsic Magnetoresistance • SrRuO3 • Tl2Mn2O7 • CrO2 • La0.7(Ca1-ySry)0.3MnO3 • Fe3O4 • CaCu3Mn4O12 (CCMO)