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All materials exhibit some magnetic response to an applied field. Diamagnetism (χ < 0) is the universal tendency of orbital electron motion to oppose an applied field, present in all materials but typically weak (~10-5). Paramagnetism (χ > 0) arises from alignment of permanent magnetic moments: in insulators with localized moments, the Curie law χ = C/T describes the competition between field alignment and thermal disorder. In metals, Pauli paramagnetism gives a temperature-independent χ = μ_B2 g(E_F), reflecting that only Fermi-surface electrons contribute to the spin response. The net susceptibility of a material is the sum of all contributions.
Magnetism in condensed matter begins with two universal but weak effects. Diamagnetism is present in every material: an applied magnetic field induces tiny orbital currents (Lenz's law at the atomic scale) that produce a moment opposing the field. The resulting susceptibility chi_dia = -e2 N <r2> / (6mc2) is negative, small (~10-5), and temperature-independent. It is the dominant magnetic response only in materials with no unpaired electrons — noble gases, many ionic crystals, and organic molecules.
Paramagnetism occurs when atoms or ions carry permanent magnetic moments (from unpaired electrons). In insulators with localized moments, each moment independently tries to align with the field while thermal agitation randomizes it. The Langevin/Brillouin theory gives the Curie law: chi = C/T, where the Curie constant C depends on the magnitude of the atomic moment. This 1/T dependence is the signature of thermal demagnetization: at high temperature, moments are randomly oriented and the susceptibility is small; at low temperature, alignment is easier and chi grows. The saturation magnetization is reached only when mu B >> k_BT.
In metals, paramagnetism takes a different form. Conduction electrons have spin-1/2 magnetic moments, but the Fermi-Dirac distribution restricts which electrons can respond to a field. Only those within ~k_BT of the Fermi level have access to empty states with opposite spin. This produces Pauli paramagnetism: chi_Pauli = mu_B2 g(E_F), which is temperature-independent and much smaller than Curie paramagnetism would predict for the same number of spins. The ratio chi_Pauli / chi_Curie is of order k_BT/E_F ~ 1/100 at room temperature. Additionally, the orbital motion of conduction electrons contributes Landau diamagnetism, which is exactly -1/3 of the Pauli susceptibility for free electrons (and modified by band structure effects in real metals).
The magnetic properties of a material are the sum of all contributions: core-electron diamagnetism, Pauli spin paramagnetism, Landau orbital diamagnetism, and (if present) Curie paramagnetism from localized moments. Van Vleck paramagnetism — a temperature-independent correction from virtual transitions to excited states — can also contribute. In most simple metals, the net susceptibility is weakly paramagnetic (Pauli > Landau + core diamagnetism). These weak magnetic responses are the "background" on which cooperative phenomena — ferromagnetism, antiferromagnetism, and spin glass behavior — are built.