# Curie point for ferromagnetic materials

1. Oct 19, 2011

### Low-Q

Hi,

I have searched the web for a graph that shows the ferromagnetic properties of a metal like iron or nickel as they warms up to the Curie temperature - when the metals becomes non-ferromagnetic. I cannot find any good images or graphs that shows this very clear. I have been told that the Curie point is very sharp, meaning that at temperatures slightly under or over the Curie temperature makes the metal ferromegnetic or not. Can someone please help to explain this to me?

Br.

Vidar

2. Oct 19, 2011

### nasu

There is a sketch of the magnetization versus temperature for a ferromagnetic material here:
http://www.irm.umn.edu/hg2m/hg2m_b/hg2m_b.html
Scroll down a little, to get to ferromagnetism.
An experimental image cuve for nickel is contained in this graph:
http://media.web.britannica.com/eb-media/68/268-004-A2511D3C.gif

The Curie point is "sharp" in the sense that is the point of a phase transition. It is somewhat similar to the melting point. Above it the material is liquid, below it it's solid.

Here it is the ferro- to para- magnetic phase transition. However this is a so called "second order" phase transition whereas the melting is "first order". It means (between other things) that the order parameter (magnetization) does not have a jump over the transition point. It goes to zero when temperature is increased towards Tc and remains zero on the high temperature side of the transition.

Last edited: Oct 19, 2011
3. Oct 20, 2011

### Low-Q

Is it possible to make a relatively high efficient motor of ferromagnetic materials if the material is close to its Curier temperature so just small changes in temperature (which require "small" energy input) will make a "Curier engine" to work?
Say the engine is inside an insolated box where the temperature is possible to be kept high.

I am not talking about over unity, but another way to make an engine.

br.

Vidar

4. Oct 20, 2011

### nasu

The "high" temperature is not really a problem. There are ferromagnetic materials with low Curie temperature. Not very common I guess as the interest is in having it high enough to avoid demagnetization. Gadolinium is an example (TC less than 300 K).
Regarding the motor, I don't know, there are many things to consider.

5. Oct 21, 2011

### DrDu

Get some Gadolinium metal at Ebay, it is relatively cheap (for it being a rare earth) and has its Curie point at about 20 deg. Celsius. Nice toy!