Magnet Materials - Overview

Precision Magnetics Inc (PMI) supplies all types, grades and common sizes of permanent magnet alloys. The following is an overview of the types of magnets available from PMI that can be customized and precision machined into shapes and sizes required by our customers.

Neodymium Iron Boron

Neodymium magnet (also known as NdFeB, NIB, or Neo magnet) is the most widely-used type of rare-earth magnet. Neodymium Iron Boron is a permanent magnet made from an alloy of neodymium, iron, and boron to form the Nd2Fe14Btetragonal crystalline structure and are the strongest type of permanent magnet available in the market today. Application includes motors, tools, hard disk drives, and magnetic fasteners.

Neodymium is available in both sintered and bonded forms. Major characteristics includes Residual Flux Density (Br), High Hc, High BHmax. It is lower in cost than Samarium Cobalt. Major disadvantage of Neodymium is its sensitivity to temperature variations and rusts or oxidizes easily. For practical use, Neodymium needs to be plated to prevent the oxidation.

It is possible to operate some Neodymium to a  maximum temperature of about 250°C. Energy product ranges from 10 MGOe to about 64 MGOe is also possible. The Relative cost index by weight is 30.

Neodymium tetragonal Nd2Fe14B crystal structure has exceptionally high uniaxial magnetocrystalline anisotropy (HA~7 teslas). This gives the compound the potential to have high coercivity (i.e., resistance to being demagnetized). The compound also has a high saturation magnetization (16 kG) and typically 1.3 tesla. Therefore, as the maximum energy density is proportional to Js2, this magnetic phase has the potential for storing large amounts of magnetic energy (BHmax = 512 kJ/m3 or 64 MGOe), considerably more than samarium cobalt (SmCo) magnets. In practice, the magnetic properties of neodymium magnets depend on the alloy composition, microstructure, and manufacturing technique employed.

Samarium Cobalt

A samarium–cobalt (SmCo) magnet is another type of strong permanent rare earth magnet made of an alloy of samarium and cobalt. They were developed in the early 1970s and are the second-strongest type of magnet. Samarium-cobalt has higher temperature ratings and higher coercivity. However, SmCo are brittle, and prone to cracking and chipping. Samarium–cobalt magnets have maximum energy products (BHmax) that range from 16 megagauss-oersteds (MGOe) to 32 MGOe up to a theoretical limit of 34 MGOe.

SmCo are available in two series which are the Series 1:5 and Series 2:17.

The Series 1:5 samarium–cobalt magnet alloys (generally written as SmCo5, or SmCo Series 1:5) have one atom of rare earth samarium and five atoms of cobalt. By weight this magnet alloy will typically contain 36% samarium with the balance cobalt. The energy products of these samarium–cobalt alloys range from 16 MGOe to 25 MGOe. These samarium–cobalt magnets generally have a reversible temperature coefficient of -0.05% per °C. Saturation magnetization can be achieved with a moderate magnetizing field. SmCo5 magnets have a very high coercivity (coercive force); that is, they are not easily demagnetized.This series of magnet is easier to calibrate to a specific magnetic field than the SmCo 2:17 series magnets.

In the presence of a moderately strong magnetic field, unmagnetized magnets of this series will try to align its orientation axis to the magnetic field. Unmagnetized magnets of this series when exposed to moderately strong fields will become slightly magnetized. This can be an issue if postprocessing requires that the magnet be plated or coated. The slight field that the magnet picks up can attract debris during the plating or coating process causing for a potential plating or coating failure or a mechanically out-tolerance condition.

The Series 2:17 samarium-cobalt (written as Sm2Co17, or SmCo Series 2:17) are age-hardened with a composition of two atoms of rare-earth samarium and 13–17 atoms of transition metals (TM). The TM content is rich in cobalt, but contains other elements such as iron and copper. Other elements like zirconium,hafnium, and such may be added in small quantities to achieve better heat treatment response. By weight, the alloy will generally contain 25% of samarium. The maximum energy products of these alloys range from 20 to 32 MGOe. These alloys have the best reversible temperature coefficient of all rare-earth alloys, typically being -0.03% per °C. The "second generation" materials can also be used at higher temperatures

SmCo is highly resistant to oxidation. Most grades function well up to about 300°C. Relative cost index (by weight): 60.

Ferrite (Ceramic)

Ferrites are non-conductive ferromagnetic ceramic compounds derived from iron oxides such as hematite (Fe2O3) or magnetite (Fe3O4) and oxides of other metals. Ferrites are ceramics and is hard and brittle. Ferrites are classified as "soft" or "hard", which refers to their low or high magnetic coercivity.

Permanent ferrite magnets are hard ferrites with high coercivity and high remanence after magnetization. Ferrites magnets are composed of iron and barium or strontium oxides. The high coercivity means the materials are very resistant to becoming demagnetized. They also conduct magnetic flux well and have a high magnetic permeability. This enables ferrites or ceramic magnets to store stronger magnetic fields than iron. Ferrites magnets are cheap, widely used in low cost household products such as refrigerator magnets. The maximum magnetic field B is about 0.35 tesla and the magnetic field strength H is about 30 to 160 kiloampere turns per meter (400 to 2000 oersteds). The density of ferrite magnets is about 5g/cm3.

The most common ferrites magnets are Barium ferrite, BaFe12O19 (BaO·6Fe2O3) and Strontium ferrite, SrFe12O19 (SrO·6Fe2O3).

Barium ferrites are robust ceramics that are generally stable to moisture and corrosion-resistant. They are used as subwoofer magnets and as a medium for magnetic recording such as magnetic stripe cards. Cobalt ferrite, CoFe2O4 (CoO·Fe2O3) are used in some media for magnetic recording.

Barium or Strontium Ferrite exhibit good resistance to demagnetization, and are the lowest cost materials available today.  Energy product ranges from 1.1 MGOe to about 4.5 MGOe. Relative cost index (by weight): 1.5.

Alnico

Alnico refers to iron alloys consisting of iron with aluminium (Al), nickel (Ni) and cobalt (Co), giving it the name al-ni-co. It also has the addition of copper, and sometimes titanium. Alnico alloys are ferromagnetic, with a high coercivity (resistance to loss of magnetism) and are used to make permanent magnets. Before the development of rare earth magnets in the 1970s, Alnico was the strongest type of magnet. Common trade names for Alnico are: Alni, Alcomax, Hycomax, Columax, and Ticonal.

The composition of alnico alloys is typically 8–12% Al, 15–26% Ni, 5–24% Co, up to 6% Cu, up to 1% Ti, and the balance is Fe.

Alnico magnets produce magnetic field strength at their poles as high as 1500 gauss (0.15 tesla), or about 3000 times the strength of Earth's magnetic field. Some brands of alnico are isotropic and can be efficiently magnetized in any direction. Other types, such as alnico 5 and alnico 8, are anisotropic, with each having a preferred direction of magnetization, or orientation. Anisotropic alloys generally have greater magnetic capacity in a preferred orientation than isotropic types. Alnico'sremanence (Br) may exceed 12,000 G (1.2 T), its coercivity (Hc) can be up to 1000 oersted (80 kA/m), its energy product ([BH]max) can be up to 5.5 MGOe (44 T·A/m). Alnico magnets can produce a strong magnetic flux in closed magnetic circuit and has relatively small resistance against demagnetization.

Alnico alloys have some of the highest Curie temperatures of any magnetic material, around 800 °C (1,470 °F), although the maximum working temperature is normally limited to around 538 °C (1,000 °F). They are the only magnets that have useful magnetism even when heated red-hot. This property, as well as its brittleness and high melting point, is due to intermetallic bonding between aluminium and its other constituents. They are also one of the most stable magnets if they are handled properly. Alnico magnets are electrically conductive, unlike ceramic magnets.

Alnico magnets are available in cast and sintered forms. They are the most temperature stable magnets of all the different types, and may be used with no significant losses up to about 500°C. They are relatively easily demagnetized, resisting oxidation well. Energy product ranges from 1.4 MGOe to about 7.5 MGOe. Relative cost index (by weight): 25.

Flexible Magnet

Flexible magnets are manufactured by binding ferrite or Rare Earth magnet powders in a variety of carriers, such as vinyl such that the magnet material that can be bent, twisted, coiled.

Available grades are from 0.6 MGOe, to about 1.6 MGOe in the Ferrite based materials and up to about 6 MGOe in the Rare Earth based materials. Relative cost index (by weight, for bonded ferrite): 1.

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