Magnetic Semiconductors

In order to get deep understanding of this topic we need to first understand what is semiconductor and what are magnetic materials. So lets start the discussion about magnetic materials.

MAGNETIC MATERIALS
All the elements discovered by us are described in periodic table and the way each of this elements are distinguished in periodic table is dependent upon electronic structure of this elements. Electronic structure is the way total no of electron is distributed in the shell. For example Hydrogen as 1 electron in its shell. Now consider the example of Al which has atomic mass 13 and its electronic configuration is 2,8,3 here nucleus and first two completely filled shell are bind up together and they are called core because they have no significant contribution since they are completely filled orbitals. The outermost shell has three electrons and hence contribute to magnetic moment. An electron contribute to magnetic moment in two way
1)orbital motion - An orbiting electron can be considered as current carrying loop which contribute to magnetic moments.
2)Spin moments - An electron is associated with a intrinsic property ( like mass ) which is called spin magnetic moment.

Now each orbit in a shell can accommodate two electron of opposite spin. So if there is equal no of up spin and down spin electron the net magnetic moment is zero and hence the element will not show any magnetic behaviors which is not the case of Aluminum. Here outermost shell has 3 electrons and so it should have some magnetic moment. But as we observe Al is not magnetic. The reason is atomic Al is indeed magnetic but the solid Al is non-magnetic because in solid Al all outermost electron get detached from the core and now free to move throughout the crystal, thats why its metallic in nature as well as non-magnetic.

Silicon-
Now consider the case of Si it has atomic no of 14 and it has 4 electron in outermost shell but in this case the bonding is covalent bond and the electrons are not free to move around the crystal which makes it non-metallic and 4 electrons make it non-magnetic. Now if we dope Si with little bit of Al say 1:15000, Al will fit exactly at the same place of Si but one electron less. This vacancy of electron is called hole and it is free to move around the crystal and hence can contribute to current contribution similarly if it is doped with Br there will be extra electron which will be now free to move around and can contribute to current conduction. This kind of materials are called semiconductors.

Iron-
Iron has atomic no 26 and its electronic configuration is 2,8,14,2. The outermost 4th shell electrons are free to move around making it metal but the 3rd unfilled orbital electrons are bound to the core and hence can't move since it is partially filled it contribute to magnetic moment.

Gedollinium (Gd)-
It has electronic configuration of 2,8,18,25,9,2 the outermost electron are free to move around and hence make it metal second most outer shell is half filled and give net magnetic moments.

Now we have enough of knowledge about magnetic and semiconductor materials and we will try to come up with a material which is both magnetic as well as semiconductor.

Magnetic Semiconductors-
If we dope semiconductor with the atoms having magnetic behavior we can get magnetic semiconductor. If we consider doping Si with Gd the problem is that Gd has a very big core and can not fit in the crystalline structure of Si. Now from here the research starts. One way is to make the Si amorphous by some means and then dope it with Gd. During this research you need to meke various sample with different doping techniques and different doping concentration then take there characteristics etc in order to come up with a material which has good magnetic as well as semicondutor property.

If we can come up with this type of material it can have remarkable technological applications. One application is Giant Magneto Resistance. Consider the case of Doping some X atom which has magnetic property with Y semiconductor now this X atom will take some definite position in the Y crystalline structure with random spin polarization. Now if by some means ( actually applying electric filed across the material and at the semiconductor metal junction applying magnetic field at metal side to make all spin polarized to same direction ) we can inject same spin electron into this material due to randomly oriented X atoms spin this elections will not be so comfortable in flowing across the material and hence will show some resistance. Now if we apply the magnetic filed across this material all the doped atom X will now become polarized to same direction as of polarization of the incoming electron and hence the elections can now move more easily across the material giving us less resistance. This phenomenon is actually called magneto resistance.

One of the application of this kind of material is in hard disk we have in our computer. We know that the data is stored in this disk in form of magnetic poles. Now when this kind of material will be allowed to pass over this stored data disk surface, depending upon the magnetic field at a particular site the resistance of the material will change and hence the current in the circuit will change which can be detected to read the data stored.

This kind of thiking bring us to whole new idea of electronics which is called "SPINTRONCS".
where we use spin of the electron to store data as well us to transfer information. How spin can be used in making next generation memory devices and memories will be the topic of my other post. That is it for now.

Enjoy!