Magnetism: Properties and Types of Magnets

Methods by which a magnet can be demagnetized can be given as mentioned below: 
$(1)$ Heating a magnet strongly
$(2)$ Hammering the magnet several times or repeated hammering
$(3)$ By passing an alternating current through the magnet
Hence, the correct options $(A)$ and $(B)$.

As the attraction occurs only in between two unlike poles, thus north pole of magnet induces south pole in the nail and north at the other end.

Magnetic force

Materials that can be magnetized, which are also the ones that are strongly attracted to a magnet, are called ferromagnetic (or ferrimagnetic). These include iron, nickel, cobalt, some alloys of rare-earth metals, and some naturally occurring minerals such as lodestone.

A magnet can be easily turned into a permanent magnet by excessive heating.

Magnetite is a naturally occurring iron ore that exhibits strong magnetic properties, historically known as lodestone.

Casting of a magnetic material, pressing and sintering of magnetic material and extruding of magnetic material are the processes used for making permanent magnets.

Iron, cobalt and steel are ferromagnetic materials which can be used to make magnet but ebonite cannot be used to make magnets.

Among the following options, magnetite is the only natural magnet.

The phenomenon of converting a substance into a magnet by flowing an electric current through a coil around it is known as electromagnetism and the magnet so formed is known as electromagnet.

When a magnet is treated above 176℉ (80)℃ magnet will quickly loose their magnetic properties. The magnet will become permanently demagnetized if exposed to those temperatures for a certain length of time.

By passing an alternating current the magnetic property can be removed because the alternating current produces a reverse magnetizing field which opposes the object magnetism.

All the three statements are true that the ferromagnetic materials lose their magnetism on heating above the Curie temperature,

A mixture of iron and nickel is magnetized by applying an external magnetic field. This convert the mixture into a temporary magnet for the time when the field has been applied.

As the magnet can include the magnetism is femo magnetic materials.

Soft iron pieces are placed across the ends of bar magnets when stored; such pieces are called magnetic keepers. If the two magnets are placed side by side there will be mutual repulsion or attraction that weakens the magnetic strength. So magnets are placed side by side with opposite poles near to each other.