AlNiCo Magnets in Depth

Following last year’s All About Pickup Magnets, here’s another excellent article on guitar pickup magnets, courtesy of Pete Biltoft at Vintage Vibe Guitars.   Thanks Pete for the permission to post this here!

In this article, Pete consolidates his own expert research on pickups, along with a bunch of information from the Wikipedia magnet entries and Magnet Kingdom, to give us an overview of magnets in general, as well an in-depth look at AlNiCo magnets for guitar pickups.

June 2011

Howdy!

A recent question from a customer made me think that it was time to go back and review the properties of AlNiCo magnets as used in guitar and bass pickups.

Historical documents suggest ancient Greeks living in the prefecture of Magnesia in Thessaly (modern Manisa, Turkey), first discovered and observed the properties of naturally occurring magnetic materials.

The earliest known surviving descriptions of magnets and their properties are from Greece, India, and China around 2500 years ago. The properties of lodestones and their affinity for iron were written of by Pliny the Elder in his encyclopedia, Naturalis Historia.

Figure 1. Naturally occurring lodestone attracting paperclips.

First a few magnet basics:

A magnet is defined as a material or object which produces a magnetic field. This invisible magnetic field exerts a force that attracts ferromagnetic materials, such as iron and nickel and attracts or repels other magnets.

The magnetic field (usually denoted B) is a vector field. The magnetic field vector at a given point in space is specified by two properties:

1. Its direction, which is along the orientation of a compass needle.
2. Its magnitude (also called strength), which is proportional to how strongly the compass needle orients along that direction.

In SI (metric) units, the strength of the magnetic field is given in Tesla (T). The Gauss (G) is a non-metric unit also used to quantify magnetic field strength.

The magnetic field extending from a bar and rod magnet can be depicted as shown in Figure 2.

Figure 2. Diagram of a bar magnet with magnetic flux lines.

Pole naming conventions:

Here at Vintage Vibe Guitars we use a compass as shown in Figure 3 to determine the magnetic field direction.

Figure 3. A pickup which attracts the red end of a compass needle (just like the Earth’s North Pole) is referred to by us as being north up. This is the opposite of the scientific convention for definition of the magnetic field orientation.

Alnico magnets are a class of ferromagnetic materials made by casting or sintering a combination of aluminum, nickel and cobalt with iron and small amounts of other elements added to enhance the properties of the magnet. Sintering offers superior mechanical characteristics, whereas casting delivers higher magnetic fields and allows for the design of intricate shapes. Alnico magnets resist corrosion and have physical properties more forgiving than ferrite, but not quite as desirable as a metal.

Magnetizing ferromagnets

Ferromagnetic materials can be magnetized in the following ways:

• Heating the object above its Curie temperature, allowing it to cool in a magnetic field and hammering it as it cools. This is the most effective method and is similar to the industrial processes used to create permanent magnets.
• Placing the item in an external magnetic field will result in the item retaining some of the magnetism on removal. Vibration has been shown to increase the effect. Ferrous materials aligned with the Earth’s magnetic field that are subject to vibration (e.g., frame of a conveyor) have been shown to acquire significant residual magnetism.
• Stroking: An existing magnet is moved from one end of the item to the other repeatedly in the same direction.

Demagnetizing ferromagnets

Magnetized ferromagnetic materials can be demagnetized (or degaussed) in the following ways:

• Heating a magnet past its Curie temperature (810 to 860 Celsius for AlNiCo alloys); the molecular motion destroys the alignment of the magnetic domains. This always removes all magnetization.
• Placing the magnet in an alternating magnetic field with an intensity above the material’s strength and then either slowly drawing the magnet out or slowly decreasing the magnetic field to zero. This is the principle used in commercial demagnetizers to demagnetize tools and erase credit cards and hard disks.
• Some demagnetization or reverse magnetization will occur if any part of the magnet is subjected to a reverse field above the magnetic material’s coercivity.
• Demagnetization progressively occurs if the magnet is subjected to strong alternating magnetic fields.
• Hammering or jarring: the mechanical disturbance tends to randomize the magnetic domains. This process will leave some residual magnetization.

Strength of AlNiCo magnet grades often used in pickups

Figure 4. Data on AlNiCo magnet grades from www.magnetkingdom.com

Note that the strength of AlNiCo-2 magnets is greater than that of AlNiCo-3 magnets.

High magnetic field strength while it can offer increased output can also have an undesirable effect: magnetic pull on the strings. Sustain in an instrument can be reduced by magnetic pull from the pickup(s) which can be caused by strong magnets or having the pickup(s) adjusted too close to the strings.

Here is a table which offers some descriptive terms which may be helpful in comparing the tone and output of different magnet types in a given pickup:

Vintage Vibe Guitars offers a range of single coil and humbucking pickups designed to allow the owner to change the magnets. Please visit our website or contact us via email:

Best wishes,

Pete Biltoft

Vintage Vibe Guitars

website: www.vintagevibeguitars.com

email: [email protected]