Ferromagnetic substances

Ferromagnetism is a basic mechanism by which certain elements are attracted to magnets. Ferromagnetism is the strongest type of magnetism and is responsible for the common phenomenon of magnetism in magnets found in our everyday life. Some elements respond weakly to external magnetic fields, and the forces are usually so weak that they can be detected only in a laboratory. A refrigerator magnet is an example of ferromagnetism used to hold notes on a refrigerator door in everyday life.

So, we can conclude that ferromagnetism is that phenomenon that makes certain elements (in the periodic table) attract magnets very strongly, and eventually, these elements behave like magnets themselves. The phenomenon has been named after the element Ferrum (commonly known as Iron).

Causes of Ferromagnetism

Atoms that have unpaired electrons behave like tiny magnets and have magnetic moment in them. Most materials with such atoms do not behave like magnets and show only slight attraction towards magnets. This phenomenon is called Paramagnetism.

But there are a few elements on the periodic table such as Fe, Co, Ni, and Gd (Iron, Cobalt, Nickel, and Gadolinium, respectively), which show strong attraction towards magnets, though the elements mentioned here too have atoms that have unpaired electrons. These elements are Ferromagnetic materials. To understand why only a few elements fall in the category of ferromagnet, we need to see the difference between the paramagnetic and ferromagnetic elements’ atomic orientation/alignment.

 Let us understand with the help of an example :

If we take a piece of Aluminium, it does not behave like a magnet meaning it is a paramagnet. If we try to understand why we are saying the element Al is a paramagnet, we need to see the alignment of atoms in a piece of paramagnet (here Aluminium). Atoms in the piece of Aluminium are randomly oriented( as shown in figure 1 below), meaning all the atoms are randomly moving in different directions. Thus, almost all the magnetic moments set off with each other.

 But in the same way, if we study the orientation of atoms in a piece of Ferromagnet, say iron, we will observe that there are groups of atoms that are aligned in a similar direction. See Figure 2 below to see the orientation of atoms in a piece of iron aligned in a similar direction in groups. These groups are called magnetic domains. So, the primary cause of ferromagnetism lies in the alignment of atoms in any element.                                                                                               

Ferromagnetic materials :

As we are through with the concept of ferromagnetism, we can define ferromagnetic materials as elements showing strong magnetization properties primarily due to their atomic orientation. Examples of ferromagnetic materials include Iron, Cobalt, Nickel, Gadolinium and metallic alloys, etc.

Impact of external magnetic field on ferromagnetic materials:

If we get a bar of magnet and take it close to the Paramagnets, it will get slightly attracted towards the magnet. But let’s take a bar magnet closer to any Ferromagnetic substance such as Cobalt. It tends to turn the atoms in the entire domain and align them in the external magnetic field direction, making the domains even stronger. Eventually, all the different domains may be aligned in the same direction as the external magnetic field is stronger. We might thus get a giant magnetic domain where all the atoms are oriented in the same direction. As a result, that particular piece of cobalt will become incredibly strong magnetised, and resultantly, it will show very strong attraction towards the external magnetic field. The alignment of atoms is strong in Ferromagnetic substances due to the quantum mechanical effect.

Now, when we remove that bar magnet or, say external magnetic field out of these Paramagnetic materials, they lose their slight magnetic property as well very quickly. But in the case of ferromagnetic materials, the same is not true. Even after removing the external magnetic field(bar magnet here), the orientation of atoms stays aligned in the Ferromagnetic Substances (Cobalt here). The entire magnetic domain stays as it is. 

Effect of temperature on ferromagnetic materials :

If we heat a ferromagnetic material at a very high temperature (say T >770oC for Iron), it will lose all of its magnetic domain, meaning heating at a very high temperature will make the orientation of atoms random in a Ferromagnet making it a Paramagnet. The temperature at which a Ferromagnet loses its ferromagnetism and becomes a paramagnet is called Curie temperature. The Curie temperature for iron is 770oC, for cobalt 1121oC, for nickel 354oC, and for gadolinium 20oC. So, we can see that even at room temperature, gadolinium might lose its ferromagnetic properties. However, if we cool them back below their respective curie temperature, the elements will spontaneously get back their Ferromagnetic property, restoring their magnetic domains.