Magnetism in matter is all about how different materials respond to magnetic fields. From weak attraction to strong repulsion, materials can be classified as paramagnetic, diamagnetic, or ferromagnetic based on their magnetic behavior.

Understanding these categories helps explain why some materials make great magnets while others don't. We'll explore how magnetic dipoles align, the concept of hysteresis, and how factors like temperature affect magnetic properties.

Magnetism in Matter

Categories of magnetic materials

• Paramagnetic materials exhibit weak attraction to external magnetic fields
  • Magnetic dipoles align parallel to the applied field resulting in a net magnetic moment (aluminum, platinum, oxygen gas)
• Diamagnetic materials display weak repulsion from external magnetic fields
  • Magnetic dipoles align antiparallel to the applied field leading to a small negative magnetic susceptibility (copper, silver, water, most organic compounds)
• Ferromagnetic materials show strong attraction to external magnetic fields
  • Magnetic dipoles align parallel to the applied field creating a large net magnetic moment
  • Retain magnetization even after the external field is removed allowing for permanent magnets (iron, nickel, cobalt, and their alloys)

Magnetic dipole alignment

• Paramagnetic materials have randomly oriented magnetic dipoles in the absence of an external field
  • When an external field is applied, dipoles partially align parallel to the field resulting in a weak net magnetic moment
  • Alignment is weak and disappears when the external field is removed causing the material to lose its magnetization
• Diamagnetic materials have no net magnetic dipole moment in the absence of an external field
  • When an external field is applied, induced magnetic dipoles align antiparallel to the field creating a small negative magnetic susceptibility
  • Alignment is weak and disappears when the external field is removed leaving the material unmagnetized
• Ferromagnetic materials have magnetic dipoles aligned in small regions called magnetic domains
  • Domains are randomly oriented in the absence of an external field resulting in no net magnetization
  • When an external field is applied, domains align parallel to the field creating a strong net magnetic moment
  • Strong alignment persists even after the external field is removed, resulting in permanent magnets that retain their magnetization
  • The spin magnetic moment of electrons in ferromagnetic materials contributes significantly to their strong magnetic properties

Hysteresis and magnetic susceptibility

• Magnetic susceptibility $(\chi)$ measures the degree to which a material becomes magnetized in response to an external magnetic field
  1. Paramagnetic materials have a small, positive $\chi$ indicating weak attraction to the field
  2. Diamagnetic materials have a small, negative $\chi$ indicating weak repulsion from the field
  3. Ferromagnetic materials have a large, positive $\chi$ indicating strong attraction to the field
• Hysteresis is a phenomenon observed in ferromagnetic materials where the magnetization $(M)$ of the material depends on its magnetic history
  • Hysteresis loop plots $M$ vs. applied magnetic field $(H)$ and shows key features
    1. Saturation magnetization is the maximum $M$ achieved under a strong external field
    2. Remanent magnetization is the $M$ retained after the external field is removed
    3. Coercivity is the magnitude of the external field required to reduce $M$ to zero
  • Hysteresis is not observed in paramagnetic and diamagnetic materials as they do not retain magnetization after the external field is removed

Magnetic properties and environmental factors

• Magnetic permeability describes how easily a material can be magnetized in response to an external magnetic field
• The Curie temperature is the point at which a ferromagnetic material loses its permanent magnetic properties and becomes paramagnetic
• Magnetic field strength (H) is a measure of the intensity of an external magnetic field and affects the degree of magnetization in materials
• The magnetization curve illustrates the relationship between the applied magnetic field and the resulting magnetization in a material