Samarium–cobalt (SmCo) magnets belong to the category ofrare-earth magnets and are composed ofsamarium (Sm), arare-earth element, andcobalt (Co), atransition metal. They are among the strongest permanent magnets.
They were developed in the early 1960s based on work done by Karl Strnat atWright-Patterson Air Force Base and Alden Ray at theUniversity of Dayton. In particular, Strnat and Ray developed the first formulation of SmCo5.[1][2]
Samarium–cobalt magnets are generally ranked similarly in strength toneodymium magnets,[3] but have higher temperature ratings and highercoercivity.
Some attributes of samarium–cobalts are:
| Property (unit) | Neodymium | Sm-Co |
|---|---|---|
| Remanence (T) | 1–1.5 | 0.8–1.16 |
| Coercivity (MA/m) | 0.875–2.79 | 0.493–2.79 |
| Relative permeability (–) | 1.05 | 1.05–1.1 |
| Temperature coefficient of remanence (%/K) | –0.09..–0.12 | −0.03..–0.05 |
| Temperature coefficient of coercivity (%/K) | −0.40..–0.65 | −0.15..–0.30 |
| Curie temperature (°C) | 310–370 | 700–850 |
| Density (g/cm3) | 7.3–7.7 | 8.2–8.5 |
| CTE, magnetizing direction (1/K) | (3–4)×10−6 | (5–9)×10−6 |
| CTE, normal to magnetizing direction (1/K) | (1–3)×10−6 | (10–13)×10−6 |
| Flexural strength (N/mm2) | 200–400 | 150–180 |
| Compressive strength (N/mm2) | 1000–1100 | 800–1000 |
| Tensile strength (N/mm2) | 80–90 | 35–40 |
| Vickers hardness (HV) | 500–650 | 400–650 |
| Electricalresistivity (Ω·cm) | (110–170)×10−6 | (50–90)×10−6 |
Samarium–Cobalt magnets are available in two "series", namely SmCo5 magnets and Sm2Co17 magnets.[7][8]
These samarium–cobalt magnet alloys (generally written as SmCo5, or SmCo Series 1:5) have one atom of rare-earthsamarium per five atoms of cobalt. By weight, this magnet alloy will typically contain 36%samarium with the balancecobalt.[9] The energy products of these samarium–cobalt alloys range from 16 MG·Oe to 25 MG·Oe, that is, approx. 128–200 kJ/m3. These samarium–cobalt magnets generally have areversible temperature coefficient of -0.05%/°C. Saturation magnetization can be achieved with a moderate magnetizing field. 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 their orientation axis to the magnetic field, thus becoming 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 coating failure or a mechanically out-of-tolerance condition.
Br drifts with temperature and it is one of the important characteristics of magnet performance. Some applications, such as inertial gyroscopes and travelling wave tubes (TWTs), need to have constant field over a wide temperature range. The reversible temperature coefficient (RTC) ofBr is defined as
To address these requirements, temperature compensated magnets were developed in the late 1970s. For conventional SmCo magnets,Br decreases as temperature increases. Conversely, for GdCo magnets,Br increases as temperature increases within certain temperature ranges. By combiningsamarium andgadolinium in the alloy, the temperature coefficient can be reduced to nearly zero.
SmCo5 magnets have a very highcoercivity (coercive force); that is, they are not easily demagnetized. They are fabricated by packing wide-grain lone-domain magnetic powders. The crystal system is hexagonal with space group P6/mmm. All of themagnetic domains are aligned with the easy axis direction, which is the one perpendicular to the hexagonal base in the lattice of the crystal. In this case, all of the domain walls are at 180 degrees. When there are no impurities, the reversal process of the bulk magnet is equivalent to lone-domain motes, where coherent rotation is the dominant mechanism. However, due to the imperfection of fabricating, impurities may be introduced in the magnets, which form nuclei. In this case, because the impurities may have lower anisotropy or misaligned easy axes, their directions of magnetization are easier to spin, which breaks the 180° domain wall configuration. In such materials, the coercivity is controlled by nucleation. To obtain much coercivity, impurity control is critical in the fabrication process.
These alloys (written as Sm2Co17, or SmCo Series 2:17) are age-hardened with a composition of two atoms of rare-earth samarium per 13–17 atoms of transition metals (TM). The arrangement of the atoms is rhombohedral in the space group R-3m. The TM content is rich in cobalt, but contains other elements such as iron and copper. Other elements likezirconium,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, what is about 160-260 kJ/m3. These alloys have the best reversible temperature coefficient of all rare-earth alloys, typically being -0.03%/°C. The "second generation" materials can also be used at higher temperatures.[10]
In Sm2Co17 magnets, the coercivity mechanism is based ondomain wall pinning. Impurities inside the magnets impede the domain wall motion and thereby resist themagnetization reversal process. To increase the coercivity, impurities are intentionally added during the fabrication process.
Samarium–cobalt alloys are typically machined in the unmagnetized state. Samarium–cobalt should be ground using a wet grinding process (water-based coolants) and a diamond grinding wheel. The same type of process is required if drilling holes or other features that are confined. The grinding waste produced must not be allowed to completely dry as samarium–cobalt has a low ignition point. A small spark, such as that produced with static electricity, can easily initiate combustion.[11]
The reduction/melt method and reduction/diffusion method are used to manufacture samarium–cobalt magnets. The reduction/melt method will be described since it is used for both SmCo5 and Sm2Co17 production. The raw materials are melted in an induction furnace or arc furnace filled with argon gas. The mixture is cast into a mold and cooled with water to form an ingot. The production of the two phases is not the same, this can be understood by looking at the phase diagram. in fact the 1:5 phase is not stable at room temperature.[12] Typically it is possible to keep the 1:5 phase with a fast quenching after an annealing process.
The ingot is pulverized and the particles are further milled to further reduce the particle size. This process is important to because the control of the grain size is fundamental for the control of the coercive field.[13] The resulting powder is pressed in a die of desired shape, in a magnetic field to orient the magnetic field of the particles. Sintering is applied at a temperature of 1100˚C–1250˚C, followed by solution treatment at 1100˚C–1200˚C and tempering is finally performed on the magnet at about 700˚C–900˚C.[14][15] It then is ground and further magnetized to increase its magnetic properties. The finished product is tested, inspected and packed.[citation needed]
Samarium can be substituted by a portion of otherrare-earth elements includingpraseodymium,cerium, andgadolinium, the problem is the effects that this substitutions can have on the Curie temperature and on the coercive field. The cobalt can be substituted with a portion of othertransition metals includingiron,copper, andzirconium.[16][17]
Fender used one of designerBill Lawrence'sSamarium Cobalt Noiseless series of electric guitarpickups in Fender's Vintage Hot Rod '57Stratocaster.[18] These pickups were used inAmerican Deluxe Series Guitars andBasses from 2004 until early 2010.[19]
Samarium–cobalt magnets are used in aerospace and defense due to their exceptional magnetic properties.[20] They are utilized in high-performance motors andactuators, precision sensors andgyroscopes, and satellite systems where stability and reliability are essential.[21] They are also used in medical technologies, includingMRI machines,pacemakers, and medical pumps.[22]
In the mid-1980s some expensive headphones such as the Ross RE-278 used samarium–cobalt "Super Magnet" transducers.
Other uses include: