Magnetism

Step 1: Understanding the Concept:
Ferromagnetism is characterized by a strong alignment of atomic magnetic moments due to quantum mechanical exchange interactions. This alignment is opposed by thermal agitation. The magnetic susceptibility ( ) is a measure of how strongly a material becomes magnetized in an external magnetic field.
Step 2: Detailed Explanation:
As the temperature of a ferromagnetic material increases, the thermal energy causes increased random vibrations of the atoms. This thermal agitation works against the exchange forces, disrupting the long-range ordering of the magnetic domains. This leads to a decrease in the overall magnetization and hence a decrease in magnetic susceptibility.
This effect becomes very pronounced as the temperature approaches the Curie temperature ( ). At , the thermal energy becomes strong enough to completely overcome the exchange forces, and the material undergoes a phase transition from ferromagnetic to paramagnetic. Above the Curie temperature, the material is paramagnetic, and its susceptibility continues to decrease with increasing temperature according to the Curie-Weiss law, .
Step 3: Final Answer:
Therefore, for a ferromagnetic material, the magnetic susceptibility consistently decreases as the temperature increases.

Step 1: Understanding the Concept:
Antiferromagnetic materials are those in which the magnetic moments of adjacent atoms align in an antiparallel configuration. In the absence of an external magnetic field, the net magnetization is zero. We need to identify the typical range of their magnetic susceptibility ( ).
Step 2: Detailed Explanation:
When an external magnetic field is applied to an antiferromagnetic material, it causes a slight canting of the antiparallel moments, resulting in a small, net magnetization in the direction of the field. This means their magnetic susceptibility is small and positive. It is generally of the same order of magnitude as that of paramagnetic materials.
Let's analyze the given options:

(A) to : This range is small and negative, which is characteristic of diamagnetic materials.
(B) to : This indicates a range of small positive values (assuming it means the values between and ). This range is consistent with the small, positive susceptibility of both paramagnetic and antiferromagnetic materials.
(C) to : This is a large negative range and is not physically realistic for susceptibility.
(D) to : This is a large positive range, characteristic of ferromagnetic materials.

Step 3: Final Answer:
By elimination, the only plausible range for the small, positive susceptibility of antiferromagnetic materials is the one given in option (B).

Step 1: Understanding the Concept:
Domain theory is a model used in physics to explain the magnetic behavior of certain materials.
Step 2: Detailed Explanation:
The domain theory was proposed by Pierre Weiss in 1906 to explain the properties of ferromagnetic materials. It postulates that these materials are composed of small regions called magnetic domains. Within each domain, the atomic magnetic moments are aligned parallel to each other due to strong exchange forces, resulting in a strong local magnetization. However, in an unmagnetized piece of material, the domains are oriented randomly, so their magnetic fields cancel each other out, and the net magnetization is zero.
When an external magnetic field is applied, the domains aligned with the field grow at the expense of others, and the domains can also rotate to align with the field, leading to a strong overall magnetization. This theory successfully explains phenomena unique to ferromagnetism, such as magnetic saturation, hysteresis, and the existence of the Curie temperature.
Diamagnetism and paramagnetism are explained by the behavior of individual atoms and do not involve domains. Superconductivity involves the Meissner effect and is explained by BCS theory.
Step 3: Final Answer:
Domain theory is the cornerstone for explaining the behavior of ferromagnetic materials.