The transition layer at the boundary between neighboring domains, across which the direction of the magnetic moments rotates gradually. Its thickness is set by the balance between exchange and anisotropy energies.
Japanese name: 磁壁
Domain wall appears in 3 practice questions across 2 topics, including Magnetic materials, Electronic, magnetic and optical materials.
Field: Magnetic materials
A domain wall is the transition layer between two domains of different orientation, across which the magnetic moments rotate gradually to change direction. Besides 180° walls, with opposite magnetization on the two sides, 90° walls also form in cubic crystals such as iron, whose easy axes lie along three mutually perpendicular directions. Inside thick specimens the walls are Bloch walls, in which the moments rotate within the plane of the wall, so no magnetic poles appear on the wall. In magnetic thin films thinner than the wall width, Néel walls, in which the moments rotate within the plane of the film, have the lower energy.
For a 180° wall in a material with uniaxial anisotropy, the thickness can be estimated as about δ = π√(A/K) and the energy per unit area as γ = 4√(AK), using the exchange stiffness A and anisotropy constant K. Applying these to iron (A ≈ 2×10⁻¹¹ J/m, K ≈ 4.8×10⁴ J/m³) gives δ of about 64 nm and γ of about 4 mJ/m². In Nd₂Fe₁₄B, whose anisotropy is extremely large (A ≈ 8×10⁻¹² J/m, K ≈ 4.9×10⁶ J/m³), δ is as thin as about 4 nm. The thinner the wall, the more easily it is caught even by small inhomogeneities.
Much of the change in magnetization under an applied field occurs by domain-wall motion. Impurities, precipitates, strain and the like in a material make the wall energy vary from place to place, and walls get caught at places of low energy (pinning). A caught wall breaks free suddenly and jumps as the field increases, so the magnetization changes in fine steps that can be heard as noise when picked up by a coil. This is the Barkhausen effect. Permanent magnets are considered in two types: the pinning type, such as Sm₂Co₁₇-based magnets, whose coercivity comes from pinning of walls, and the nucleation type, such as Nd-Fe-B and SmCo₅ magnets, which suppress the formation of reversed domains.
Because how easily walls move reflects the internal state of a material, measuring Barkhausen noise in steels is used to examine residual stress and grinding burn nondestructively. The walls themselves can be observed by magnetic force microscopy and Lorentz microscopy. In devices, racetrack memory has been proposed, in which walls lined up in a nanowire are moved by current to store information, and research on moving walls by current-driven spin transfer is advancing. Microstructures are designed to make walls move easily in soft magnetic materials and with difficulty in pinning-type magnets, so domain walls are a central object of materials design.
Answer: A. At weak fields, mainly domain wall motion occurs; at strong fields, the magnetization within the domains rotates, approaching saturation
A demagnetized ferromagnet is divided internally into many domains with different directions of magnetization, which cancel overall. When a weak field is applied, domain walls, the boundaries between domains, move so that domains oriented close to the field direction grow, and the magnetization increases. As the field is increased further, the magnetization within the remaining domains itself rotates toward the field direction, approaching the saturation magnetization. Within a domain the atomic moments are already aligned, so they do not rotate independently one by one. Nor does the number of domains increase; domains with favorable orientations grow. The more the domain walls are pinned by inclusions and grain boundaries, the larger the coercivity.
Answer: B. To lower the magnetostatic (demagnetizing-field) energy of the poles that appear on the surface when the whole body is uniformly magnetized
If the whole crystal is magnetized in one direction, large magnetic poles appear on its surface, creating fields outside and inside (the demagnetizing field inside), with a large magnetostatic energy. Splitting into domains with different directions lets the poles cancel and lowers this energy greatly. On the other hand, inside a domain wall at the boundary between domains the moments turn gradually, so the energy rises by the exchange and anisotropy energies. The actual domain size is set by the balance between the two. The exchange interaction acts to align all the moments, so splitting into domains actually increases the exchange energy. Moments randomized by thermal vibration describe the paramagnetic state above the Curie temperature.
In a Bloch wall the magnetic moments rotate within the plane of the wall, so no magnetic poles appear on the wall. In a Néel wall the moments rotate within the plane of the film, and poles appear on the wall. Bloch walls are stable in thick specimens, and Néel walls in films thinner than the wall width.
Yes. The boundary between domains is not a plane of zero width but a layer of finite thickness across which the magnetic moments change direction gradually, and that layer is called a domain wall. Its thickness differs with the material and is estimated at tens of nanometers in iron and a few nanometers in permanent magnet materials of high anisotropy.
One with small magnetocrystalline anisotropy and magnetostriction and few impurities, precipitates, internal strains and fine grain boundaries to catch the walls. Soft magnetic materials such as permalloy and grain-oriented electrical steel are examples, and walls are made to move easily by raising purity and removing strain by annealing.
When a weak field is applied, domain walls, the boundaries between domains, move so that domains oriented close to the field direction grow, and the magnetization increases.
When an external magnetic field is applied to a demagnetize…On the other hand, inside a domain wall at the boundary between domains the moments turn gradually, so the energy rises by the exchange and anisotropy energies.
What is the main reason a ferromagnet splits into many magn…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 …Magnetic domain Coercivity Saturation magnetization Curie temperature Superparamagnetism Eddy-current loss Ferrimagnetism Magnetocrystalline anisotropy Maximum energy product