Physics:Alnico

Short description: Family of iron alloys

A "horseshoe magnet" made of Alnico 5, about 1 inch high. The metal bar (bottom) is a keeper, which is placed across the poles when the magnet is not in use. This helps to preserve the magnetization.

Alnico is a family of iron alloys which in addition to iron are composed primarily of aluminium (Al), nickel (Ni), and cobalt (Co), hence the acronym al-ni-co. They also include copper, and sometimes titanium. Alnico alloys are ferromagnetic, and are used to make permanent magnets. Before the development of rare-earth magnets in the 1970s, they were the strongest type of permanent magnet. Other trade names for alloys in this family 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 rest is Fe. The development of alnico began in 1931, when T. Mishima in Japan discovered that an alloy of iron, nickel, and aluminium had a coercivity of 400 oersteds (32 kA/m), double that of the best magnet steels of the time.

Properties

Alnico alloys can be magnetised to produce strong magnetic fields and have a high coercivity (resistance to demagnetization), thus making strong permanent magnets. Of the more commonly available magnets, only rare-earth magnets such as neodymium and samarium-cobalt are stronger. 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's remanence (B_r) may exceed 12,000 G (1.2 T), its coercivity (H_c) can be up to 1000 oersteds (80 kA/m), its maximum energy product ((BH)_max) can be up to 5.5 MG·Oe (44 T·A/m). This means that alnico can produce a strong magnetic flux in closed magnetic circuits, but has relatively small resistance against demagnetization. The field strength at the poles of any permanent magnet depends very much on the shape and is usually well below the remanence strength of the material.

Alnico alloys have some of the highest Curie temperatures of any magnetic material, around 800 °C (1,470 °F), although the maximal 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 the result of the strong tendency toward order due to intermetallic bonding between aluminium and other constituents. They are also one of the most stable magnets if they are handled properly. Alnico magnets are electrically conductive, unlike ceramic magnets.

MMPA class	IEC code ref.	Composition by weight (Fe comprises remainder)					Magnetic properties								Physical properties									Thermal properties
							Max. energy product, (BH)max		Residual induction, Br		Coercive force, Hc		Intrinsic coercive force, Hci		Density		Tensile strength		Transverse modulus of rupture		HRC	Thermal expansion coefficient (10−6 per °C)	Electrical resistivity, at 20 °C (μΩ·cm)	Reversible temp. coefficient, (% per °C)			Curie temp.		Max. service temp.
		Al	Ni	Co	Cu	Ti	(MGOe)	(kJ/m3)	(gauss)	(mT)	(Oe)	(kA/m)	(Oe)	(kA/m)	(lb/in3)	(g/cm3)	(psi)	(MPa)	(psi)	(MPa)				Near Br	Near max. energy prod.	Near Hc	(°C)	(°F)	(°C)	(°F)
Isotropic cast AlNiCo
Alnico 1	R1-0-1	12	21	5	3	-	1.4	11.1	7200	720	470	37	480	38	0.249	6.9	4000	28	14000	97	45	12.6	75
Alnico 2	R1-0-4	10	19	13	3	-	1.7	13.5	7500	750	560	45	580	46	0.256	7.1	3000	21	7000	48	45	12.4	65	-0.03	-0.02	-0.02	810	1490	450	840
Alnico 3	R1-0-2	12	25	-	3	-	1.35	10.7	7000	700	480	38	500	40	0.249	6.9	12000	83	23000	158	45	13.0	60
Anisotropic cast AlNiCo
Alnico 5	R1-1-1	8	14	24	3	-	5.5	43.8	12800	1280	640	51	640	51	0.264	7.3	5400	37	10500	72	50	11.4	47	-0.02	-0.015	+0.01	860	1580	525	975
Alnico 5DG	R1-1-2	8	14	24	3	-	6.5	57.7	13300	1330	670	53	670	53	0.264	7.3	5200	36	9000	62	50	11.4	47
Alnico 5-7	R1-1-3	8	14	24	3	-	7.5	59.7	13500	1350	740	59	740	59	0.264	7.3	5000	34	8000	55	50	11.4	47
Alnico 6	R1-1-4	8	16	24	3	1	3.9	31.0	10500	1050	780	62	800	64	0.265	7.3	23000	158	45000	310	50	11.4	50	-0.02	-0.015	+0.03	860	1580	525	975
Alnico 8	R1-1-5	7	15	35	4	5	5.3	42.2	8200	820	1650	131	1860	148	0.262	7.3	10000	59	30000	207	55	11.0	53	-0.025	-0.01	+0.01	860	1580	550	1020
Alnico 8HC	R1-1-7	8	14	38	3	8	5.0	39.8	7200	720	1900	151	2170	173	0.262	7.3	10000	59	30000	207	55	11.0	54	-0.025	-0.01	+0.01	860	1580	550	1020
Alnico 9	R1-1-6	7	15	35	4	5	9.0	71.6	10600	1060	1500	119	1500	119	0.262	7.3	7000	48	8000	55	55	110.	53	-0.025	-0.01	+0.01	860	1580	550	1020
Isotropic sintered AlNiCo
Alnico 2	R1-0-4	10	19	13	3	-	1.5	11.9	7100	710	550	44	570	45	0.246	6.8	65000	448	70000	483	45	123.4	68
Anisotropic sintered AlNiCo
Alnico 5	R1-1-10	8	14	24	3	-	3.9	31.0	10900	1090	620	49	630	50	0.250	6.9	50000	345	55000	379	45	11.3	50
Alnico 6	R1-1-11	8	15	24	3	1	2.9	23.1	9400	940	790	63	820	65	0.250	6.9	55000	379	100000	689	45	11.4	54
Alnico 8	R1-1-12	7	15	35	4	5	4.0	31.8	7400	740	1500	119	1690	134	0.252	7.0	50000	345	55000	379	45	11.0	54
Alnico 8HC	R1-1-13	7	14	38	3	8	4.5	35.8	6700	670	1800	143	2020	161	0.252	7.0			55000	379	45	11.0	54

As of 2018, Alnico magnets cost about 44 USD/kg (US$20/lb) or US$4.30/BH_max.

Alnico 5 magnet used in a magnetron tube in an early microwave oven. About 3 in (8 cm) long.

Classification

Alnico magnets are traditionally classified using numbers assigned by the Magnetic Materials Producers Association (MMPA), for example, alnico 3 or alnico 5. These classifications indicate chemical composition and magnetic properties. (The classification numbers themselves do not have any direct relation to the properties of the magnet; for instance, a higher number does not necessarily indicate a stronger magnet.)

These classification numbers, while still in use, have been deprecated in favor of a new system by the MMPA, which designates Alnico magnets based on maximum energy product in megagauss-oersteds and intrinsic coercive force as kilooersteds, as well as an IEC classification system.

Manufacturing process

Advertisement by Jensen Radio Manufacturing Co. for Alnico 5 loudspeakers in 1945. As illustrated, Alnico 5 allowed a dramatic reduction in size and weight of magnet needed to produce a given flux, from 90 oz in 1930 to 4.6 oz.

Alnico magnets are produced by casting or sintering processes. Cast alnico is produced by conventional methods using resin bonded sand molds. Sintered alnico magnets are formed using powdered metal manufacturing methods. Sintering alnico is suitable for complex geometries.

Most alnico produced is anisotropic, meaning that the magnetic direction of the grains is oriented in one direction. Anisotropic alnico magnets are oriented by heating above a critical temperature and cooling in the presence of a magnetic field. Both isotropic and anisotropic alnico require proper heat treatment to develop optimal magnetic properties—without it alnico's coercivity is about 10 Oe, comparable to technical iron, which is a soft magnetic material. After the heat treatment alnico becomes a composite material, named "precipitation material"—it consists of iron- and cobalt-rich precipitates in rich-NiAl matrix.

Assortment of Alnico magnets in 1956. Alnico 5, developed during World War 2, led to a new generation of compact permanent magnet motors and loudspeakers.

Alnico's anisotropy is oriented along the desired magnetic axis by applying an external magnetic field to it during the precipitate particle nucleation, which occurs when cooling from 900 °C (1,650 °F) to 800 °C (1,470 °F), near the Curie point. Without an external field there are local anisotropies of different orientations due to spontaneous magnetization. The precipitate structure is a "barrier" against magnetization changes, as it prefers few magnetization states requiring much energy to get the material into any intermediate state. Also, a weak magnetic field shifts the magnetization of the matrix phase only and is reversible.

Uses

Alnico cow magnet, used to bind sharp metal wire and other iron objects that may be ingested by the animal and otherwise cause damage to the digestive tract

Alnico magnets are widely used in industrial and consumer applications where strong permanent magnets are needed. Examples are electric motors, electric guitar pickups, microphones, sensors, loudspeakers, magnetron tubes, and cow magnets. In many applications they are being superseded by rare-earth magnets, whose stronger fields (B_r) and larger energy products (B·H_max) allow smaller-size magnets to be used for a given application.

The high temperature resistance of alnico magnets leads to many uses that cannot be filled by less resistant magnets, such as in magnetic stirring hotplates.

Further reading

• MMPA 0100-00, Standard Specifications for Permanent Magnet Materials