The strength of the reverse magnetic field needed to bring the magnetization (or flux density) of a magnetized material back to zero. Materials with small coercivity are soft magnetic, and those with large coercivity hard magnetic.
Japanese name: 保磁力
Coercivity appears in 6 practice questions across 2 topics, including Magnetic materials, Electronic, magnetic and optical materials.
Coercivity expresses how strong a reverse field is needed to reverse the magnetization of a magnetized material, and its unit is A/m (1 Oe ≈ 79.6 A/m). In soft magnetic materials it ranges from below 1 A/m for high-permeability alloys such as Supermalloy to a few to a few tens of A/m for electrical steel sheet. In permanent magnets, ferrite magnets reach a few hundred kA/m and the intrinsic coercivity of neodymium magnets about 1000–3000 kA/m. Such large differences among ferromagnets arise because coercivity depends strongly on microstructure, not just on composition.
The anisotropy field calculated from magnetocrystalline anisotropy is the theoretical upper limit of coercivity, but actual coercivities are much smaller. This is because magnetization reverses when reverse domains nucleate at defects and domain walls move to spread them. The more inclusions, precipitates, grain boundaries and internal stresses catch (pin) domain walls, the larger the coercivity. The goal is a microstructure in which reverse domains hardly nucleate for permanent magnets, and a clean microstructure in which domain walls move easily for soft magnetic materials.
In ordinary soft magnetic materials, grain boundaries hinder domain wall motion, so larger grains lower the coercivity. When the grain size falls below a few tens of nanometers, smaller than the magnetic exchange length, however, the anisotropies of neighboring grains are averaged out and cancel, and coercivity drops sharply in proportion to the sixth power of grain size (Herzer's random anisotropy model). Nanocrystalline soft magnetic alloys exploit this effect. It is a design approach opposite to that for permanent magnets, whose coercivity peaks at the single-domain particle size.
Coercivity is found by applying a field to a specimen, measuring its magnetization curve (hysteresis loop), and reading the field at which magnetization or flux density becomes zero. For permanent magnets, a closed-circuit BH tracer is used to eliminate the effect of the demagnetizing field, and a vibrating sample magnetometer (VSM) is used for powders and thin films. Magnetic recording media need a coercivity that meets two conditions at once: small enough to be written, yet large enough that thermal fluctuations do not erase the data. Coercivity changes greatly with temperature, so it is evaluated at the service temperature.
Answer: C. It peaks near the critical single-domain size, then falls toward zero as the particles become superparamagnetic
Large particles contain many domains, and their magnetization reverses by the motion of domain walls, which move even in weak fields, so the coercivity is small. Making the particles smaller reduces the number of walls, and below a critical size they become single-domain particles with no walls. Reversing the magnetization of a single-domain particle requires rotating the whole magnetization against the magnetic anisotropy, so the coercivity peaks around this size. Making them smaller still makes the energy barrier to reversal, KV, small compared with the thermal energy, so the magnetization reverses spontaneously by thermal activation (superparamagnetism) and the coercivity approaches zero. Coercivity depends strongly on particle size and microstructure, not only on the anisotropy, so it is not constant, and below the critical size it does not keep increasing. Particles for magnetic recording media are designed between these upper and lower limits.
Answer: A. HcJ ≥ HcB
When B reaches zero at a reverse field H = −HcB, B = µ₀(H + M) gives M = HcB > 0, so the magnetization is still positive. Bringing M to zero requires an even stronger reverse field, so HcJ ≥ HcB. The options HcJ ≤ HcB and always equal are therefore wrong. Also, in a reverse field M does not exceed the remanent magnetization, so HcB cannot exceed Br/µ₀. In materials with a very large HcJ, such as neodymium magnets, HcB levels off near this upper limit of Br/µ₀. Whether a magnet resists demagnetization at high temperature or in a strong reverse field is judged by HcJ, which shows whether the magnetization itself reverses. In soft magnetic materials with small coercivities, the difference between the two is negligibly small.
In SI units it is A/m, and kA/m is also common. In the older CGS unit oersted (Oe), 1 Oe = 1000/(4π) A/m ≈ 79.6 A/m. It is sometimes given as µ₀Hc in tesla (T), converted to flux density units.
HcB is the reverse field that brings the flux density B to zero, and HcJ the reverse field that brings the magnetization M to zero; HcJ ≥ HcB always. Whether a magnet resists demagnetization is judged by HcJ, which expresses reversal of the magnetization itself.
Reduce impurities and inclusions, remove internal stress and strain by annealing, and enlarge the grains. Other methods are choosing compositions with small magnetocrystalline anisotropy and magnetostriction, or nanocrystallizing to average out the anisotropy.
Large particles contain many domains, and their magnetization reverses by the motion of domain walls, which move even in weak fields, so the coercivity is small.
As particles of a ferromagnet are made smaller, from a few …A guide to the upper limit of coercivity is the anisotropy field HA = 2K/(µ₀Ms); the larger K is, the harder it is to turn the magnetization away from the easy axis.
What is the main material reason that neodymium magnets, wi…A soft magnetic material with small coercivity and a narrow loop is therefore needed.
Which correctly describes a soft magnetic material suitable…The more the domain walls are pinned by inclusions and grain boundaries, the larger the coercivity.
When an external magnetic field is applied to a demagnetize…Nd₂Fe₁₄B has a rather low Curie temperature of about 310°C, and as the temperature rises its magnetocrystalline anisotropy falls and its coercivity drops sharply.
What is the main purpose of adding Dy or Tb to neodymium ma…Magnetic domain Saturation magnetization Domain wall Magnetocrystalline anisotropy Superparamagnetism Soft magnetic material Eddy-current loss Curie temperature Maximum energy product Indirect-bandgap semiconductor