Magnetic properties Flashcards

Question

What are ferromagnetic materials?

Answer 1

Xm>>0 Ferromagnetism is like paramagnetism – that is the magnetic dipole moments align with the applied magnetic field, H, but the dipoles form domains that get stuck with a given alignment.

Answer 2

The ferromagnetic domains can be the same as the crystallites in polycrystalline material – or the crystallites can align to form bigger domains. So usually gases cannot be ferromagnetic because they don’t have crystallites but they can be paramagnetic. A material can only be ferromagnetic when the temperature (thermal energy) is below the Currie temperature so that the domain structure can form – Currie Temperature is different for different ferromagnetic materials

Answer 3

Even when the H field is removed there is a permanent magnetization left behind. Only if the material is heated to a point where the thermal energy disrupts the domain structure can the material lose its magnetisation.

Answer 4

The B left when the H field returns to zero is called the remanence, R, and means there’s a permanent magnetization left – thus ferroelectric materials are used in permanent magnets and data storage devices such hard drive disks in computers. Also from this curve a property called the coercivity, C, can be measured which is a measure of how large a field has to applied to cause magnetisation S is point of saturation

Answer 5

The curve increases with initial magnetisation. as the field is removed it doesn’t go back down the blue curve instead continues onto the red curve, as it remembers that a magnetic field has been applied in the past it does not return to 0 and so retains some of the magnetisation

Answer 6

To get back to zero you must apply magnetic field in the opposite direction

Answer 7

As the applied field (H) increases... --the magnetic moment aligns with H. “Domains” with aligned magnetic moment grow at expense of poorly aligned ones!

Answer 8

As the applied field (H) increases... --the magnetic moment aligns with H. “Domains” with aligned magnetic moment grow at expense of poorly aligned ones! Arrows are different lengths and begin to grow as applied magnetic field and magnetic induction increase All domains align to form one large domain

Answer 9

If the moments of the aligned and anti-aligned ions balance completely so as to have zero net magnetization, despite the magnetic ordering

Answer 10

If you have a hot material it can misalign the domains, if it gets above the curie temperature then the material will no longer be ferromagnetic. Because you then have enough thermal energy that domains can misalign randomly since they jiggle around due to the thermal energy

Answer 11

As your ferromagnetic material is heated it up and becomes non ferromagnetic, apply a field as it is cooling down and it will line up all the magnetic domains

Answer 12

1. initial (unmagnetized state) 2. apply H, cause alignment 3. remove H, alignment stays! => permanent magnet! 4. Coercivity, HC Negative H needed to demagnitize!

Answer 13

large coercivity --good for perm magnets --add particles/voids to make domain walls hard to move (e.g., tungsten steel: Hc = 5900 amp-turn/m

Answer 14

mall coercivity--good for elec. motors (e.g., commercial iron 99.95 Fe)

Answer 15

- 26metals + 100’sofalloys & compounds • Unfortunately,not this simple: Jc (critical density) = critical current density if J > Jc not superconducting Hc (magnetic field) = critical magnetic field if H > Hc not superconducting

Answer 16

It is higher in a super conductor, but it has to be at a temperature below which material is superconductivity

Answer 17

``` • 1987- new results published for Tc > 30 K – ceramics of form Ba1-x Kx BiO3-y – Started enormous race. -- Y Ba2Cu3O7-x Tc = 90 K --Tl2Ba2Ca2Cu3Ox Tc = 122 K ``` •Values now stabilized at ca. 120K–important to get above 77K the temperature of liquid nitrogen because liquid nitrogen can be used as a much less expensive coolant than liquid He (4K).

Answer 18

Super conductors expel magnetic fields normal superconductor This is why a super conductor will float above a magnet

Answer 19

current only in outer skin - so amount of current limited current flows within wire

Answer 20

When a magnetic materials with a magnetic domain structure – ferromagnets - have a magnetic field applied the domains re-order and change their dimension –this behaviour is called Magnetostriction and it causes the humming noise that comes from electrical transformers

Answer 21

When the temperature is increased in Invar materials the thermal expansion is opposed by the magnetostriction because the material is less ferromagnetic as it approached its Currie temperature.

Answer 22

Where the two lines of the graph meet in the postive x and y axis

Answer 23

The saturation flux density / 1.26 x 10^5 (permeability in a vacuum)

Answer 24

when the graph hits the positive y axis

Answer 25

when the graph hits the negative x axis