5.

0 Permanent Magnet Stability

The ability of a permanent magnet to support an external magnetic field results from
small magnetic domains "locked" in position by crystal anisotropy within the magnet
material. Once established by initial magnetization, these positions are held until acted
upon by forces exceeding those that lock the domains. The energy required to disturb the
magnetic field produced by a magnet varies for each type of material. Permanent
magnets can be produced with extremely high coercive forces (Hc) that will maintain
domain alignment in the presence of high external magnetic fields. Stability can be
described as the repeated magnetic performance of a material under specific conditions
over the life of the magnet.

Factors affecting magnet stability include time, temperature, reluctance changes, adverse
fields, radiation, shock, stress, and vibration.

5.1 Time

The effect of time on modern permanent magnets is minimal. Studies have shown that
permanent magnets will see changes immediately after magnetization. These changes,
known as "magnetic creep", occur as less stable domains are affected by fluctuations in
thermal or magnetic energy, even in a thermally stable environment. This variation is
reduced as the number of unstable domains decreases. Rare Earth magnets are not as
likely to experience this effect because of their extremely high coercivities. Long-term
time versus flux studies have shown that a newly magnetized magnet will lose a minor
percent of its flux as a function of age. Over 100,000 hours, these losses are in the range
of essentially zero for Samarium Cobalt materials to less than 3% for Alnico 5 materials
at low permeance coefficients.

5.2 Temperature

Temperature effects fall into three categories:

Reversible losses.

Irreversible but recoverable losses.

Irreversible and unrecoverable losses.

5.2.1. Reversible losses.

These are losses that are recovered when the magnet returns to its original temperature.
Reversible losses cannot be eliminated by magnet stabilization. Reversible losses are
described by the Reversible Temperature Coefficients (Tc), shown in table 5.1. Tc is
expressed as % per degree Centigrade. These figures vary for specific grades of each
material but are representative of the class of material as a whole. It is because the
temperature coefficients of Br and Hc are significantly different that the demagnetization
curve develops a "knee" at elevated temperatures.
Table 5.1 Reversible Temperature Coefficients of Br and Hc
Material Tc of Br Tc of Hc
NdFeB -0.12 -0.6
SmCo -0.04 -0.3
Alnico -0.02 0.01
Ceramic -0.2 0.3

5.2.2. Irreversible but recoverable losses.

These losses are defined as partial demagnetization of the magnet from

exposure to high or low temperatures. These losses are only recoverable by
remagnetization, and are not recovered when the temperature returns to its
original value. These losses occur when the operating point of the magnet
falls below the knee of the demagnetization curve. An efficient permanent
magnet design should have a magnetic circuit in which the magnet operates
at a permeance coefficient above the knee of the demagnetization curve at
expected elevated temperatures. This will prevent performance variations at
elevated temperatures.

5.2.3. Irreversible and unrecoverable losses.

Metallurgical changes occur in magnets exposed to very high temperatures

and are not recoverable by remagnetization. Table 5.2 shows critical
temperatures for the various materials, where

TCurie is the Curie temperature at which the elementary magnetic

moments are randomized and the material is demagnetized; and

Tmax is the maximum practical operating temperatures for general classes

of major materials. Different grades of each material exhibit values
differing slightly from the values shown here.

Table 5.2 Critical Temperatures for Various Materials

Material TCurie Tmax*
Neodymium Iron Boron 310 (590) 150 (302)
Samarium Cobalt 750 (1382) 300 (572)
Alnico 860 (1580) 540 (1004)
Ceramic 460 (860) 300 (572)
(Temperatures are shown in degrees Centigrade with the Fahrenheit equivalent in parentheses.)

*Note that the maximum practical operating temperature is dependent on

the operating point of the magnet in the circuit. The higher the operating
point on the Demagnetization Curve, the higher the temperature at which
the magnet may operate.

Flexible materials are not included in this table since the binders that are
used to render the magnet flexible break down before metallurgical changes
occur in the magnetic ferrite powder that provides flexible magnets with
their magnetic properties.

Partially demagnetizing a magnet by exposure to elevated temperatures in a

2.0 Modern Magnet Materials

There are four classes of modern commercialized magnets, each based on their material composition.
Within each class is a family of grades with their own magnetic properties. These general classes are:

Neodymium Iron Boron

Samarium Cobalt

Ceramic

Alnico

NdFeB and SmCo are collectively known as Rare Earth magnets because they are both composed of
materials from the Rare Earth group of elements. Neodymium Iron Boron (general composition
Nd2Fe14B, often abbreviated to NdFeB) is the most recent commercial addition to the family of
modern magnet materials. At room temperatures, NdFeB magnets exhibit the highest properties of
all magnet materials. Samarium Cobalt is manufactured in two compositions: Sm1Co5 and Sm2Co17 -
often referred to as the SmCo 1:5 or SmCo 2:17 types. 2:17 types, with higher Hci values, offer
greater inherent stability than the 1:5 types. Ceramic, also known as Ferrite, magnets (general
composition BaFe2O3 or SrFe2O3) have been commercialized since the 1950s and continue to be
extensively used today due to their low cost. A special form of Ceramic magnet is "Flexible" material,
made by bonding Ceramic powder in a flexible binder. Alnico magnets (general composition Al-Ni-Co)
were commercialized in the 1930s and are still extensively used today.

These materials span a range of properties that accommodate a wide variety of application
requirements. The following is intended to give a broad but practical overview of factors that must be
considered in selecting the proper material, grade, shape, and size of magnet for a specific
application. The chart below shows typical values of the key characteristics for selected grades of
various materials for comparison. These values will be discussed in detail in the following sections.

Table 2.1 Magnet Material Comparisons

Material Grade Br Hc Hci BHmax Tmax (Deg C)*
NdFeB 39H 12,800 12,300 21,000 40 150
SmCo 26 10,500 9,200 10,000 26 300
NdFeB B10N 6,800 5,780 10,300 10 150
Alnico 5 12,500 640 640 5.5 540
Ceramic 8 3,900 3,200 3,250 3.5 300
Flexible 1 1,600 1,370 1,380 0.6 100
* Tmax (maximum practical operating temperature) is for reference only. The maximum practical operating temperature of
any magnet is dependent on the circuit the magnet is operating in.