a

Magnetic Domains

so let's talk about magnetic domains all

right now we need to start off with a

very important word that tells us how

important magnetic fields are to a

certain substance and that word is

ferromagnetic if you have a material

that is Ferro magnetic then what that

means is that it's atoms have fairly

strong magnetic fields associated with

them all right now what that means is

that magnetic forces are going to tend

to try to push those magnetic moments

the little magnetic fields that are

associated with each of the atoms or

molecules of the substance it's going to

try to align them all together so what

we usually have in a ferromagnetic

material at an ordinary temperature is

something that looks like this the atoms

that are right next to each other will

tend to align in a certain direction and

we'll get these magnetic domains that

are associated with magnetic fields that

are essentially all in the same

direction now notice that we've got

neighboring domains that don't have

magnetic fields in the same direction

and that's because the temperature is

too high to allow everything to fully

fully align if I want a permanent magnet

though I need everybody fully aligned so

what I'll do is I'll apply an external

magnetic field like this and what that

will do is it will just kind of switch

the orientations so that they align more

with this external magnetic field and

that's the idea that's how we magnetize

a permanent magnet nice external

magnetic field switch it so that the

magnetic domains become larger and

larger and larger and then we get our

nice permanent magnet that's called a

ferromagnetic phase transition all right

now let's look a little bit more

carefully into the way that this takes

place so at high temperature

we've got lots of randomness going

around the atoms have too much energy to

really listen to what the magnetic

fields want them to do so we're gonna

get a random orientation and lots of

these very very tiny magnetic domains

all right at very low temperature on the

other hand the atoms and molecules of

the material don't have enough energy to

fight with the magnetic field so they're

just going to align and they'll even do

that on their own even if I don't um

impose an external magnetic field

they'll just align with themselves as

long as it's cold enough the temperature

that distinguishes between these two

things is called the the Curie point

temperature and this phase transition

that takes us from a permanent magnet to

a non permanent magnet with just kind of

random orientations of these magnetic

moments is called the curie transition

alright so let's just talk real quickly

about the two major types of

ferromagnetic materials we've got a hard

ferromagnetic material in which the

alignment will stay if a material is a

hard Ferro magnet then if I imposed this

magnetic field of this external magnetic

field that all of the magnetic domains

will coincide and then if I take that

magnetic field off I'll still have a

magnetic field and that's this permanent

magnet all right so these are like these

ceramic magnets refrigerator magnets all

those things are hard ferromagnetic

materials now what about a soft matte

ferromagnetic material it's still

ferromagnetic so that means that it's

still going to interact largely with the

magnetic field but it's not going to be

as loyal to any specific direction of

the magnetic field so this is like iron

you know like paperclips steel

refrigerator

doors right why does the magnet stick to

a refrigerator door I mean if I take two

magnets and put them next to each other

sometimes they'll stick and sometimes

they'll repel but if I take a permanent

magnet put it on a refrigerator door

then take it off and turn it around put

it back on the refrigerator door

there's no repulsion that's because the

refrigerator door will just do whatever

the external magnetic field tells it to

do it's a soft ferromagnetic material so

that means that it's still at this kind

of random orientation the magnetic

domains will align if I have an external

magnetic field but if I take that

external magnetic field away they don't

care anymore

maybe they'll stay aligned for a little

while and you've probably seen this on a

screwdriver right you've got those

little screwdrivers where it'll hold a

screw but it's not real strong it's a

really really really kind of weak

magnetic field and that's because that

screwdriver

base is made of iron which is a soft

ferromagnet you can make it a little bit

of a magnet all right and it'll stay

just kind of a tiny bit but if you try

to take another magnet and put it there

in opposition to what it's magnetic

field was it'll just drop what he was

doing and listen to the new magnetic

field so that's the difference soft

ferromagnet hard Ferro magnet permanent

magnets these are just magnetic

materials like refrigerator doors and

that's magnetic domains