Magnetic cores can be divided into two major categories:

• Structural (shape)
• Material

These major core categories will then be sub-divided into additional categories. Further below are a list of core structures and a list of magnetically "soft" core materials.

Core Structures
Toroids (rings) are the core type geometry of choice for optimizing performance. A toroid of round cross-section offers better performance than one of rectangular cross-section, but for practical and economic reasons toroids of rectangular cross-section are much more prevalent. The symmetry of their circular geometry minimizes the amount of external magnetic flux produced. Consequently they produce much lower amounts of unwanted electromagnetic interference. Unlike other core types, turns can be wound along the entire length of the core thereby allowing more turns per layer. The mean turn length will be shorter than that of other core types of equal power capability hence lower winding resistance and lower winding losses. Compared against other core types, a toroidal coil has a lot of surface area from which it can dissipate heat hence it cools much better than other core types.  Cooler windings result in higher efficiency and may allow more utilization of the core’s capability. For the circular nature, the magnetic path of a toroid is an unbroken continuous path unless intentionally broken. There is no air gap in the magnetic path (unless intentionally added) hence optimal use can be made of high permeability materials.

Ferrite toroids and stacks of stamped lamination rings are examples of this. A tape wound core is the next closest example.

Toroids are manufactured in practically all “soft” magnetic materials. Toroid Cores can be coated with insulation to provide electrical isolation between the core and the winding(s).  Some toroid cores are “boxed” to provide isolation. Some toroid cores are “boxed” because the core material is sensitive to stresses produced by the winding processes.

Bar, Slab, or Rod
“Soft” magnetic metal alloys are available in Bar, Rod, or Slab shapes. These core shapes find use in D.C. applications such as D.C. powered solenoids and D.C. relays. They can be used in very low frequency (below 50 Hz) A.C. applications. They do have some limited use at A.C. line frequencies. For a solid core, A.C. core losses per unit weight (or unit volume) become more pronounced as the cross sectional area increases. This is why silicon steel, nickel-iron, and cobalt alloy cores use a stack of laminations. The laminations divide the cross-section into a stack of much smaller cross-sections. D.C. applications are subject to far less core losses.  They only experience A.C. core losses (and the heat produced) during transitional events.

Powder Cores extend the useful A.C. frequency range of the materials listed in the previous paragraph.  A non-magnetic binding material is used to bind the small magnetic powder particles together.  The binding material also serves to insulate the particles from one another thereby reducing eddy current flow in the core.  This extends the useful frequency range, but there is a trade-off.  The binding material adds a distributed air gap to the core.  The distributed air gap reduces the permeability of the core.  The core requires more magnetizing VA.  Bars, slabs, and rods can be purchased in powder iron materials.  The selection of sizes is somewhat limited.  Larger sizes can be assembled from smaller sizes.

Ferrites are a magnetic form of ceramics.  Ferrite has very high electrical resistivity.  Even at high frequencies the eddy currents remain low.  With suitable gauss de-rating, some types of ferrite cores can use above 1 megahertz.  Bar, slabs, and rods can be purchased in ferrite materials, but the selection of sizes is limited.  Larger sizes can be assembled from smaller sizes.