Para-, dia- and ferromagnetism and temperature effects
In this section, we will explore the fascinating world of magnetism at the atomic level. Materials respond differently to external magnetic fields due to the behavior of their constituent atoms and electrons. We will focus on three main types of magnetic behavior: paramagnetism, diamagnetism, and ferromagnetism. We will also examine how temperature influences these magnetic properties.
Diamagnetism
Diamagnetism is a fundamental property of all materials, though it is often masked by stronger magnetic effects like paramagnetism or ferromagnetism. When a material is placed in an external magnetic field, the orbital motion of electrons within the atoms is slightly altered. This alteration induces a magnetic dipole moment that opposes the applied field. Consequently, diamagnetic materials are weakly repelled by a magnet.
The induced magnetic dipole moment (m) in a diamagnetic material is proportional to the applied magnetic field (B). Mathematically, this can be expressed as:
m = -k * B
where 'k' is a small, positive constant representing the diamagnetic susceptibility. The negative sign indicates that the induced moment opposes the applied field.
Key characteristics of diamagnetic materials:
- They are weakly repelled by a magnet.
- They do not possess permanent magnetic dipoles.
- Their magnetic susceptibility is small and negative (typically around -10-6 to -10-5).
- Diamagnetism is independent of temperature.
Examples of diamagnetic materials include:
- Water (H2O)
- Copper (Cu)
- Gold (Au)
- Bismuth (Bi)
- Nitrogen (N2)
Consider a superconductor. When a magnetic field is applied below its critical temperature, it expels the magnetic field entirely from its interior. This phenomenon, known as the Meissner effect, is a perfect example of diamagnetism.
Paramagnetism
Paramagnetism arises from the presence of atoms or molecules that possess permanent magnetic dipole moments. These permanent dipoles are due to unpaired electrons in their atomic structure. In the absence of an external magnetic field, these dipoles are randomly oriented, resulting in no net magnetization. However, when an external magnetic field is applied, these dipoles tend to align themselves with the field, creating a net magnetic moment that enhances the applied field.
The alignment of these permanent dipoles is opposed by thermal agitation. As a result, the magnetization of a paramagnetic material increases with the applied field strength and decreases with increasing temperature. This behavior is described by the Curie's Law.
Curie's Law states that the magnetic susceptibility (χ) of a paramagnetic material is inversely proportional to its absolute temperature (T):
χ = C / T
where 'C' is the Curie constant, a material-specific property.
Key characteristics of paramagnetic materials:
- They are weakly attracted by a magnet.
- They possess permanent magnetic dipoles due to unpaired electrons.
- Their magnetic susceptibility is small and positive (typically around 10-5 to 10-3).
- The susceptibility decreases with increasing temperature as described by Curie's Law.
Examples of paramagnetic materials include:
- Aluminum (Al)
- Platinum (Pt)
- Magnesium (Mg)
- Liquid Oxygen (O2)
- Sodium (Na)
Imagine holding a piece of aluminum near a strong magnet. You would feel a slight pull, but it would be much weaker than the pull you'd feel from a ferromagnetic material. This weak attraction is characteristic of paramagnetism.
Ferromagnetism
Ferromagnetism is a much stronger form of magnetism exhibited by materials like iron, nickel, and cobalt. In these materials, the atoms possess permanent magnetic dipole moments, similar to paramagnetic materials. However, in ferromagnetic substances, there is a strong quantum mechanical interaction between adjacent atomic dipoles, called the exchange interaction. This interaction causes the dipoles to align parallel to each other over large regions called magnetic domains, even in the absence of an external magnetic field.
In an unmagnetized ferromagnetic material, these magnetic domains are randomly oriented, so the net magnetization is zero. When an external magnetic field is applied, the domains aligned with the field grow in size, and the magnetic moments within other domains tend to rotate and align with the field. This leads to a very strong magnetization.
Ferromagnetic materials exhibit hysteresis, meaning their magnetization depends not only on the current applied field but also on their past magnetic history. They can retain their magnetism even after the external field is removed, becoming permanent magnets.
Key characteristics of ferromagnetic materials:
- They are strongly attracted by a magnet.
- They can be permanently magnetized.
- They exhibit magnetic hysteresis.
- They have very high magnetic susceptibility, which is positive and can be very large (order of 103 to 106).
Examples of ferromagnetic materials:
- Iron (Fe)
- Nickel (Ni)
- Cobalt (Co)
- Gadolinium (Gd) (at low temperatures)
- Alloys like steel and Alnico
A simple example is a common refrigerator magnet. It's made of a ferromagnetic material that has been magnetized and retains its magnetic properties, allowing it to stick to the refrigerator door.
Temperature Effects on Magnetic Materials
Temperature plays a crucial role in determining the magnetic behavior of materials, particularly paramagnetic and ferromagnetic substances. Thermal energy tends to disrupt the alignment of magnetic dipoles, opposing the ordering effects of external magnetic fields or internal exchange interactions.
Effect on Diamagnetic Materials
As mentioned earlier, diamagnetism is an intrinsic property arising from the orbital motion of electrons. This motion is not significantly affected by thermal agitation. Therefore, diamagnetic susceptibility is essentially independent of temperature.
Effect on Paramagnetic Materials
Paramagnetism is directly influenced by temperature. The thermal motion of atoms tends to randomize the orientation of the permanent magnetic dipoles. A higher temperature means greater thermal agitation, which makes it harder for the external magnetic field to align the dipoles. This leads to a decrease in magnetization and magnetic susceptibility as temperature increases. This relationship is precisely described by Curie's Law:
χ = C / T
As T increases, χ decreases.
Memory Trick for Curie's Law:
Think of 'C' for Constant and 'T' for Temperature. Susceptibility (χ) is like a temperature gauge for magnetism. As temperature (T) goes UP, the magnetic alignment gets messier, so the susceptibility (χ) goes DOWN. It's an inverse relationship: χ ∝ 1/T.
Effect on Ferromagnetic Materials
Ferromagnetic materials also exhibit a strong temperature dependence. At low temperatures, the exchange interaction is strong enough to maintain the parallel alignment of magnetic dipoles within domains, leading to ferromagnetism. However, as the temperature increases, thermal energy starts to overcome the exchange interaction.
There is a critical temperature for each ferromagnetic material, known as the Curie temperature (TC). Above the Curie temperature, the thermal agitation becomes so significant that it completely disrupts the long-range order of the magnetic domains. The material loses its ferromagnetic properties and transitions into a paramagnetic state.
The magnetic susceptibility of a ferromagnetic material above its Curie temperature follows the Curie-Weiss Law:
χ = C / (T - TC)
where 'C' is the Curie constant and 'TC' is the Curie temperature. This formula shows that as T approaches TC from above, the susceptibility increases significantly, and becomes infinite at T = TC (in the ideal case). Beyond TC, the material behaves paramagnetically, and its susceptibility still decreases with increasing temperature, but according to the Curie-Weiss law, not the simple Curie law.
Let's look at some Curie temperatures for common ferromagnetic materials:
| Material | Curie Temperature (TC) in °C | Curie Temperature (TC) in Kelvin (K) |
|---|---|---|
| Iron (Fe) | 770 °C | 1043 K |
| Nickel (Ni) | 354 °C | 627 K |
| Cobalt (Co) | 1115 °C | 1388 K |
| Gadolinium (Gd) | 19 °C | 292 K |
The fact that Gadolinium's Curie temperature is close to room temperature means that it is ferromagnetic at room temperature but becomes paramagnetic if heated slightly above it. This is a crucial property for certain applications.
Key Takeaway:
Diamagnetic materials are repelled by magnets, paramagnetic materials are weakly attracted and their susceptibility decreases with temperature (Curie's Law), and ferromagnetic materials are strongly attracted and retain magnetism below a critical Curie temperature (TC), above which they become paramagnetic (Curie-Weiss Law).
Summary Table of Magnetic Properties
To consolidate our understanding, let's summarize the key differences between these magnetic behaviors.
| Property | Diamagnetism | Paramagnetism | Ferromagnetism |
|---|---|---|---|
| Origin | Induced dipole moment opposing field (orbital motion of electrons) | Permanent dipoles (unpaired electrons) aligning with field | Strong exchange interaction causing parallel alignment of permanent dipoles in domains |
| Response to External Field | Weak repulsion | Weak attraction | Strong attraction |
| Permanent Magnetism | No | No | Yes (below TC) |
| Magnetic Susceptibility (χ) | Small, negative (e.g., -10-6) | Small, positive (e.g., 10-5 to 10-3) | Large, positive (e.g., 103 to 106) |
| Temperature Dependence | Independent of temperature | Decreases with temperature (χ ∝ 1/T, Curie's Law) | Strongly temperature dependent. Loses ferromagnetism above Curie Temperature (TC) and becomes paramagnetic (χ ∝ 1/(T-TC), Curie-Weiss Law). |
| Examples | Water, Copper, Gold, Bismuth | Aluminum, Platinum, Oxygen, Sodium | Iron, Nickel, Cobalt, Steel |
Applications
Understanding these magnetic properties is crucial for numerous technological applications:
- Magnetic Resonance Imaging (MRI): Utilizes the magnetic properties of atomic nuclei, often involving paramagnetic contrast agents.
- Data Storage: Hard drives and magnetic tapes rely on the ferromagnetism of materials to store information.
- Electric Motors and Generators: Employ strong ferromagnetic materials for their cores to enhance magnetic fields.
- Magnetic Levitation (Maglev) Trains: Use powerful magnets (often superconducting, exhibiting perfect diamagnetism) for frictionless travel.
- Medical Devices: Magnetic nanoparticles, which can be ferromagnetic or superparamagnetic, are used for targeted drug delivery and hyperthermia treatment.
- Shielding: Diamagnetic materials can be used to shield sensitive equipment from magnetic fields.
The interplay between atomic structure, external fields, and temperature dictates how a material behaves magnetically, leading to a diverse range of phenomena and applications that are fundamental to modern science and technology.