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MAGNETIC PROPERTIES OF ALNICO MAGNETS

There are two types of anisotropic alnico magnets commercially available, sintered alnico magnets and cast alnico magnets. They are manufactured in different ways and have different finished magnetic properties. Sintered anisotropic alnico magnets have a maximum energy product range of 4.45-5.5 MGOe while stronger cast anisotropic alnico magnets range from 4.25-10 MGOe. Anisotropic magnets have their direction of magnetism permanently set during their manufacturing process giving them a stronger magnetic performance. Isotropic alnico magnets are also available, but in a narrower range of cast and sintered grades. Isotropic alnico magnets are less commonly used as they have no preferred direction of magnetism and are therefore weaker, but they can be magnetised in different ways after manufacture giving greater flexibility in magnetic circuit design.

Alnico magnets are typically manufactured with a magnetic length greater than their diameter such as rod or bar shapes. Traditionally horseshoe-shaped magnets are made from alnico, as they are effectively a bar magnet cast in a horseshoe shape, giving the greatest length-to-diameter ratio for the size of the magnet. Alnico magnets are susceptible to demagnetisation and a greater length-to-diameter ratio helps counteract this. Below, you can see a list of all the commercial available alnico magnet grades and their magnetic properties.

Cast anisotropic

GradeRemanence (BR)
mT
(KGS)
Coercive Force (Hcb)
kA/m
(kOe)
Intrinisc Coercive Force (Hcj)
kA/m
(kOe)
Max. Energy Product (BH) max
kJ/m3
(MGOe)
Max. Operating Temp
TW
Alnico 5 1100-1250
(11.0-12.5)
50
(0.65)
54
0.68
34-44
(4.2-5.5)
525°C
Alnico 6 1150
(11.5)
58
(0.73)
60
(0.75)
28
(3.5)
525°C
Alnico 5DG 1300
(13)
56
(0.7)
58
(0.73)
52
(6.5)
525°C
Alnico 8 800-900
(8.0-9.0)
115
(1.4)
117
(1.46)
38-44
(4.7-5.5)
555°C
Alnico 8HC 720
(7.2)
150
(1.2)
152
(1.9)
36
(4.5)
555°C
Alnico 9 1000-1080
(10.0-10.8)
120
(1.5)
122
(1.53)
60-80
(7.5-10.0)
555°C

 

Cast isotropic

GradeRemanence (BR)
mT
(KGS)
Coercive Force (Hcb)
kA/m
(kOe)
Intrinisc Coercive Force (Hcj)
kA/m
(kOe)
Max. Energy Product (BH) max
kJ/m3
(MGOe)
Max. Operating Temp
TW
Alnico 3 650
(6.5)
38
(0.48)
40
0.50
10
(1.25)
450°C
Alnico 2 750
(7.5)
45
(0.56)
46
(0.58)
12
(1.5)
450°C
Alnico 8 550
(5.5)
90
(1.2)
97
(1.2)
18
(2.2)
555°C

 

Sintered anisotropic

GradeRemanence (BR)
mT
(KGS)
Coercive Force (Hcb)
kA/m
(kOe)
Intrinisc Coercive Force (Hcj)
kA/m
(kOe)
Max. Energy Product (BH) max
kJ/m3
(MGOe)
Max. Operating Temp
TW
Alnico 5 1150
(11.5)
48
(0.6)
50
0.63
34
(4.2)
525°C
Alnico 6 1100
(11.0)
58
(0.73)
60
(0.75)
28
(3.5)
525°C
Alnico 8 800-920
(8.0-9.2)
125
(1.56)
127
(1.59)
48
(5.5)
555°C
Alnico 8HC 720
(7.2)
150
(1.8)
152
(1.9)
36
(4.5)
555°C

 

Sintered isotropic

GradeRemanence (BR)
mT
(KGS)
Coercive Force (Hcb)
kA/m
(kOe)
Intrinisc Coercive Force (Hcj)
kA/m
(kOe)
Max. Energy Product (BH) max
kJ/m3
(MGOe)
Max. Operating Temp
TW
Alnico 3 650
(6.5)
40
(0.50)
42
0.53
10
(1.25)
450°C
Alnico 2 750
(7.5)
45
(0.56)
46
(0.58)
12
(1.5)
450°C
Alnico 8 600-620
(6.0-6.2)
100
(1.2)
105
(1.3)
20
(2.2)
555°C

 

The greatest strength of Alnico magnets, and one of the reasons they are still used today is that they have the highest temperature coefficient of all permanent magnetic materials, which means their performance deteriorates less than anyth other permanent mangetic material per degree Celsius rise in temperature. Alnico magnets can maintain their magnetic performance up to an incredible 450 degrees Celsius, while Alnico 5 and above will maintain their performance in temperatures up to 525 degrees Celsius. They also have excellent resistance to corrosion as their iron content is very low and only after prolonged exposure to moisture will they begin to corrode. To add further protection from corrosion Alnico magnets are often painted red.

The main limitation of Alnico material is that it is easily demagnetised by a stronger or opposing magnetic field, even forcing two repelling Alnico magnets together can weaken their magnetic performance. To counteract this it is normal for Alnico magnets to be manufactured so that their length is five times their diameter (width for block magnets) as a magnet with a greater length to diameter to ratio has more resistance to demagnetisation than a magnet with a lesser length to diameter ratio.

At Magnet Expert we offer a limited range of Alnico magnets in our stock range, however we specialise in manufacturing bespoke Alnico magnets on a made-to-order basis. If you have an application which requires Alnico magnets, then get in touch with our team today on 0845 5194701 who will be happy to provide advice.