Magnetic domains and magnetic domain walls are core concepts for understanding the microscopic mechanisms of magnetic materials and for exploring new magnetic phenomena, which are the topics of this article.
Definition of Magnetic Domains
A magnetic domain is a “miniature magnetic cluster” within magnetic materials and is central to understanding hysteresis loss.
The Essence of Magnetic Domains
Each magnetic domain is a tiny region where the magnetic moments of all the atoms are aligned in the same direction, akin to compact groups of tiny magnets.
An unmagnetized iron core (such as that in a non-energized electric motor) has countless magnetic domains oriented randomly, resulting in the absence of any external magnetic effect.
Key Changes in Magnetic Domains
When an electric motor is energized, it generates an alternating magnetic field, instructing these disordered magnetic domains to rearrange and align uniformly along the direction of the external magnetic field.
However, the direction of the alternating magnetic field keeps changing, requiring the magnetic domains to repeatedly “turn” and adjust their orientations. During this process of “turning,” friction and resistance occur between the magnetic domains, which is the source of hysteresis loss.
Definition of Magnetic Domain Walls
A magnetic domain wall is the “transition boundary” between adjacent magnetic domains with different orientations within the magnetic material. Its movement and direction directly affect the magnetization process and the extent of losses in the material, serving as the “dividing line” between domains.
The magnetic domains within materials are not chaotically packed; rather, adjacent domains have differing magnetic field directions, leading to the formation of a very thin transitional region known as the domain wall.
The domain wall acts as a transition area where the atomic magnetic moments gradually change direction from one domain to that of an adjacent domain. This transition layer requires a certain number of atomic magnetic moments to participate, resulting in a domain wall of a specific thickness. Measured thickness ranges from 10^-6 to 10^-7 m. The domain wall thickness is approximately several hundred atomic layers, allowing for a “smooth transition” of the magnetic field direction between adjacent domains to prevent energy instability caused by sudden changes.
Classification of Magnetic Domain Walls
Based on the manner of atomic magnetic moment transition, magnetic domain walls can be classified into Bloch walls and Néel walls.
The characteristic of Bloch walls is that the magnetic moment direction within the domain wall remains parallel to the wall plane during changes. Bloch walls typically exist in larger magnetic materials. When the thickness of a ferromagnetic material decreases to the dimensions of two dimensions, specifically in thin films with thicknesses between 1 to 100 nm.
Néel walls are likely to form. These are types of domain walls where magnetic poles appear on the plane of the wall (for instance, when the magnetic moment is perpendicular to the wall). Néel walls generally only form in magnetic films thinner than a certain critical size. In thicker magnetic layers, energetically, it is more favorable to form Bloch walls.
Core Functions of Magnetic Domain Walls
Key to the Magnetization Process: When materials get magnetized, it is not just a single domain that directly aligns, but rather through the movement of domain walls (expanding aligned domains, shrinking anti-aligned domains) that overall magnetization is achieved. The smoother the movement of the domain walls, the easier the magnetization.
Direct Relation to Losses: When domain walls move, they encounter resistance from impurities and defects in the material, requiring energy consumption. This energy loss significantly contributes to hysteresis loss.
The more stable and less resistive the movement of the domain walls, the lower the material’s hysteresis loss, which translates to a higher magnetic permeability, ultimately influencing the efficiency of magnetization and losses.
Influence of Different Substances on Magnetic Domains and Domain Walls
Polycrystalline materials consist of many small single-crystal particles, resulting in a magnetic domain structure that is far more complex than that of single crystals. The distribution of the various magnetic domains is intermixed and, overall, nearly devoid of specific regularity. However, within each grain, the atomic magnetic moments still align in a certain direction, while small cavities formed by impurities, pores, or other physical defects are situated within the domain walls.
The shapes, orientations, and quantities of magnetic domains and walls in polycrystals can vary significantly, influenced by the inhomogeneity in grain growth and the irregular shape of grain boundaries.
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