Paramagnetic, Diamagnetic, Ferromagnetic, Antiferromagnetic and Ferrimagnetic behavior of substances

What is Paramagnetic, Diamagnetic, Ferromagnetic, Antiferromagnetic and Ferrimagnetic behavior of substances?

What is the effect of temperature on these properties?


Paramagnetic substances:

These substances are attracted by the magnetic field and have unpaired electrons. These lose magnetism in the absence of magnetic field. Some examples of paramagnetic substances are transition metals like Cr, Mn, Co, Fe and metal oxides like CuO,VO2.

Diamagnetic substances:

These substances are weakly repelled by magnetic field and have no unpaired electrons. Some examples of diamagnetic substances are H2O, benzene, NaCl, TiO2.

Ferromagnetic substances:

Such substances are strongly attracted by magnetic field. In the absence of magnetic field magnetic field these substances have magnetic domains randomly oriented in such a way that their overall magnetic moment gets cancelled. But in the presence of magnetic field they are oriented in such a way that they together produce a strong magnetic field. These show permanent magnetism even in the absence of magnetic field. Some examples are Fe, Ni, Co, CrO2.

Magnets present in refrigerator doors are ferromagnets.

Antiferromagnetic substances:

These substances are expected to possess paramagnetism or ferromagnetism on the basis of unpaired electrons but actually have zero mac moment. Magentic domains in such substances are aligned in opposite direction thus no overall magnetic effect is formed. Some examples are MnO, MnO2, Mn2O3, NiO.

Ferrimagnetic substances:

The ferrimagnetic substances are weakly attracted by external magnetic field and possess magnetic domains in opposite directions in unequal proportions. This occurs due to presence of different type of ions in a simgle compound. Magnetite Fe3O4 containing both ferrous and ferric ions is ferrimagnetic. Other ferrites like  MgFe2O4, BaFe12O19, also exhibit ferrimagnetism.

Effect of temperature on magnetism



 

Pramagnetic substances generally obey Curie’s law. It states that extent of magnetization in paramagnetic substances varies inversely with temperature.

         X = C/T : where C= curie temperature and T= temperature

In paramagnetic substances magnetic moments are aligned in orderly manner in the direction of magnetic field. As the temperature increases this ordered arrangement is difficult to maintain due to which extent of magnetization decreases.

The ferromagnetic,antiferromagnetic and ferrimagnetic solids change into paramagnetic substances at a particular temperature. For example ferrimagnetic Fe3O4 on heating to 850k becomes paramagnetic. This is due to greater alignment of spins in one direction on heating.

Curie temperature:

Each ferromagnetic substance has a particular temperature above which no ferromagnetic behavior is observed. This is called Curie temperature.

Neel temperature:

The temperature above which an antiferromagnetic substance shows paramagnetism is Neel temperature.

 

Some common problems:

Q: What characteristic of a ferromagnetic material distinguishes it from paramagnetic one? What type of interaction must occur in the solid to bring about ferromagnetic behavior?

Ans: In a ferromagnetic substance, magnetic moments on the sites interact with one another throughout the lattice. That is they are close enough physically, and overlap of orbitals is such that individual magnetic sites couple to form much larger magnetic moment throughout the solid. Because of these interactions continued existence of magnetic moment does not require continuous application of external magnetic field. That is ferromagnetic materials form permanent magnets.

Q: What on atomic level distinguishes paramagnetic substance from diamagnetic one? How does each behave in the magnetic field?

Ans: The unpaired electrons in a paramagnetic substance cause it to be weakly attracted to a magnetic field. A diamagnetic material, where all electrons are paired, is very weakly repelled by a magnetic field.

Q: Predict whether following substances are paramagnetic, diamagnetic, or ferromagnetic: (a)Ni, (b)Mg, (c)NaCl, (d)CoO.

Ans: Ni – ferromagnetic

Mg _ diamagnetic, NaCl _ diamagnetic, CoO _ paramgnetic

 

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