08 Magnetic Properties Flashcards
- Describe a magnetic dipole.
The macroscopic magnetic prop. of a material are a consequence of interactions between an external magnetic field and the magnetic dipole moments of the constituent atoms.
magnetic dipoles in some respect are analagous to electric dipoles. Magn. dipoles may be thought of as small bar magnets composed of north and south poles instead of positive and negative electric charges. Magnetic dipole moments are represented by arrows. The force of magnetic fields exerts a torque that orients the dipole with the field like a compass needle lines up with the Earth´s magnetic field.
- Calculate the magnetic field strength (H) within a coil of wire given the number of wire turns, the length of the coil, and the magnitude of the current.
- Determine the magnetic flux density for a specified field strength
(a) in a vacuum given the permeability of a vacuum, and
(b) within some solid material given its permeability.
- Compute the relative permeability for some material given its permeability, and the permeability of a vacuum.
µR = µ/µ0
- Calculate the magnetic susceptibility of some material given the value of its relative permeability.
- Determine the magnetization of some material given its magnetic susceptibility and the applied magnetic field strength.
- From an electronic perspective note and briefly explain the two sources for magnetic moments in materials.
Magnetic moments arise from electron orbital motions and the spins on electrons.
an e- orbital magn. moment is equal to the product of the Bohr magneton and the e- magnetic quantum number
an e- spin magnetic moment is plus or minus the value of the Bohr magneton (plus for spin up, minus for spin down).
net atomic magnetic moment is the sum of moments of all e-, if the net magnetization cancels out, the atom possesses no magn. moment
- (a) Briefly explain the nature and source of diamagnetism.
(b) Note the order-of-magnitude value for the volume susceptibility of diamagnetic materials.
a) results from changes in e- orbital motion that are induced by an external field H.
b) The effect is extremely small (with susceptibilities on the order of -10^(-5)) All materials are diamagnetic.
- (a) Briefly explain the nature and source of paramagnetism.
(b) Note the order-of-magnitude value range for the volume susceptibility of paramagnetic materials.
a) Paramagn. materials are those having permanent atomic dipoles, which are acted on individually and aligned in the direction of an external field.
b) it´s a relatively small but positive susceptibility ranging from about E-5 to E-2
- (a) Briefly explain the nature and source of ferromagnetism.
(b) For a ferromagnetic material, compute the maximum saturation magnetization, given the number of Bohr magnetons per atom, the value of the Bohr magneton, Avogadro’s number, and the density and atomic weight of the material.
a) large and permanent magnetiztionsmay be established within the ferromagnetic metasl (Fe, Ni, Co)
atomic magnetic dipol moments are of spin origin, which are coupled and mutually aligned with moments of adjacent atoms
large magnetic susceptibilities up to 10^6
b) - not important
- Briefly explain the nature and source of antiferromagnetism.
Antiparallel coupling of adjacent cation spin moments is found for some ionic materials. Those in which there is total cancellation of spin moments are termed antiferromagnetic.
- (a) In terms of the crystal structure of cubic ferrites, explain the source of ferrimagnetism.
(b) Calculate the saturation magnetism for a cubic ferrite given its composition, the number of Bohr magnetons associated with each cation type, the value of the Bohr magneton, and the unit cell edge length.
For cubic ferrites, the net magnetization results from the divalent ions (e.g. Fe2+) that reside on octahedral lattice sites, the spin moments of which are all mutually aligned.
b) not so important
- (a) Define Curie temperature.
(b) Briefly explain why saturation magnetization diminishes with increasing temperature for ferromagnetic and ferrimagnetic materials.
a) Curie temperature is the point at wich the material looses its permanent magnetic property, saturation magnetization = 0
b) with rising temp. , increased thermal vibrations tend to counteract the dipole coupling forces in ferromagnetic and ferrimagnetic materials. Consequently, the saturation magnetization gradually decreases with temperature, up to the Curie temperature, at which point it drops to near zero.
- Describe the natures of (a) a domain, and (b) a domain wall.
a) domains = small-volume regions in which all net dipole moments are mutually aligned an the magnetization is saturated, occurs in ferromagneic and ferrimagn. materials
b) adjacent domains are seperated by domain boundaries = domain walls where a gradual change in magnetic dipole orientation occurs
- (a) Describe magnetic hysteresis.
(b) Explain why ferromagnetic and ferrimagnetic materials experience magnetic hysteresis.
(c) In terms of magnetic hysteresis, explain why these materials may be permanent magnets.
a) mag. hysteresis is the lag of the B-field behind the applied H-field when the magnetic field H is reversed
b) Because a ferro- or ferrimagnetic material is composed of domains! When H is reversed there is a change in domain structure by the motion of the domain walls.
c) Due to the resistance to movement of these domain walls, both hysteresis and permanent magnetization (or remanence = residual magnetic flux density B) result as a consequence.
- Given the complete hysteresis loop for a ferromagnetic or ferrimagnetic material, determine:
(a) the initial permeability,
(b) the remanence, and
(c) the coercivity.
a) the initial permeability may be the slope of the B-versus-H curve at H=0 (see in prev. Figure)
b) remanence = residual magnetic flux density = residual magnetiziaton is the value of the B-field when H = 0
c) coercivity = coercive field is the value of the H-field when B = 0
- (a) Define hard magnetic material.
(b) Cite the characteristics that are required in order for a ferromagnetic or ferrimagnetic material to be magnetically hard.
a) For hard magn. materials domain wall motion is much more difficult, which results in larger hysteresis loops, because greater fields are required to demagnetize these materials. The magnetizaiont is more permanent.
b) see Fig.
- Briefly explain how information is stored on and retrieved from a magnetic medium using a recording head.
magnetic medium e.g. hard disk drive HDD and magnetic tapes
HDD is composed of nanometer size grains of an HCP (hexagonal close packed) cobalt-chromium alloy. these grains are oriented such that their direction of easy magnetization (i.e. [0001]) is perpendicular to the plan of the disk.
the transferrence of data/info to and from a magnetic medium is done by a read/write head (see Fig.)
- Define the superconductive (a) critical temperature, (b) critical magnetic field, and (c) critical current density.
the superconducting state ceases to exist if temperature, magnetic field OR current density exceeds a critical value
a), b), c) therefore they are known as critical values
- Briefly describe the Meissner effect.
valid for type I superconductors: while in the superconducting state, they are completely diamagnetic - that is, all of an applied magnetic field is excluded from the body of material (Meissner Effect)
