What are magnetic materials

Let's quickly look at the questions raised in the previous module:1. What kind of optimal hysteresis curve do I even need for the planned application? Let's take the two main areas of application: Transformer core and magnetic storage.The transformer core must be ferromagnetic in order to have the greatest possible magnetic flux B. to be able to transport from the primary coil to the secondary coil. What one would want is the induced flow B. the primary field H follows as perfectly as possible.In other words: we need a material with as little hysteresis as possible - B.(H) should be as straight a line as possible, as shown below. 
However, there is no ideal, hysteresis-free magnetization curve in technically relevant materials. What we have are at best Soft magnets with a very slim hysteresis and thus small values ​​for and Hysteresis losses are now small, but not entirely avoidable.The power generation industry would be happy if the existing (Fe-based) materials still 1 % or 2 % could be made "softer". 1 % less losses 100 MW is also something.In addition to hysteresis losses, there is a second problem: Every deviation of the magnetization curve from a perfect straight line means that the output signal is somewhat distorted compared to the input signal. Give one sin (t) in, there are also higher harmonics - sin (nt) with a correspondingly small amplitude with out.
A soft magnetic material is used for one Permanent magnet be unsuitable because its permanent magnetization - the Remanence - is small. But for von bits we need a permanent magnet. Because we store data by using a "Print head"Imprint a permanent magnetization at a certain coordinate in a magnetic material on tape or disk. This magnetization should remain unchanged for many years.The magnetic bit must therefore be strong enough - even if it only takes up a tiny area - so that it is not accidentally erased in the event of the slightest disturbance, and in order to "Read head"to generate a strong signal. It shouldn't be too strong either, because that makes erasing and writing too time-consuming. The material should therefore have the following hysteresis curve: 
The hysteresis curve should be as rectangular as possible, coercive force and remanence have precisely defined values. We need one Hard magnet.The magnetization hardly changes above a certain field.If the direction of the field is reversed, hardly anything changes at first, then the magnetization flips over quite suddenly.Hysteresis losses are inevitable. There is even a much deeper principle in this remark: information is subject to thermodynamics, energetically (and entropically) it is never "free". Writing and reading "cost" energy. Almost all applications of magnetic materials require either hard or soft magnets. This will provide answers to the 2. and 3.Question from the previous module is now relevant 
  First we have to select a basic material. We have a wide selection for this:We can choose between the three ferromagnetic elements Fe, Ni, Co choose or start alloying these metals.Then we can make connections with almost magnetic elements like Cr, Mn, or O make to end up with "exotic". The link shows what is going on around permanent magnets.To do this, we quickly do a small task:The field is too big and too complex to go deeper into it. So let's take a quick look at the question: Can I adjust the hysteresis curve for a given material? The answer is yes. Always within limits, of course. Why and how Well, hysteresis curves always describe how easy or difficult it is to move domain walls. Since domain walls can be pinned to defects, this is a function of the internal defect structure - and I can change that.Let's just look at a couple of examples:  
Between "cured" iron, i. H. Iron with few dislocations and large grains, and highly deformed iron with small grains and high dislocation densities, there is a large difference in the hysteresis curves.Obviously, the domain movement in the severely deformed iron is much more difficult, as was to be expected.It will have to be similar in all other materials. Instead of deforming, we can also introduce atomic flaws ("dirt"), make small precipitates, .... However, we always only make the material "harder". 
We can also just put the material under mechanical tension, i.e. change the distance between the atoms by pulling or pushing in one direction. Since the interaction between the magnetic moments of the atoms, which forces the alignment, depends very much on the distance, something should happen:The effects are indeed remarkable, especially in nickel:
In this case the expansion is towards the external field H
In this case the expansion is towards the external field H
We find a large drop in, but not a large difference in.Again, no big differences in the, but a big increase in the. We have ideal hard magnetic behavior.The last word on the subject:The future of "magnetism" looks good. With an increasing understanding of the fundamentals, coupled with micro and nanotechnologies, many new types of products will become possible.

© H. Föll (MaWi for ET&IT - Script)