View Single Post
 
Old 05-01-2018, 03:32 PM
bistander bistander is online now
Gold Member
 
Join Date: Apr 2015
Posts: 1,543
Magnetic domains

Over the course of research into literature on magnetism I came across this interesting geology. It's quite long, and dry, so I snipped and pasted a bit about domains and Bloch wall. The source is listed so you can find the context and bibliography. There are also some microphotographs, empirical data and analysis.

Enjoy.

Quote:
The atomic coordination in ferromagnetic grains gives rise to sub-grain magnetic domains; this feature is important in explaining ferromagnetic behaviour and the size dependence of ferromagnetic grains on their anisotropic behaviour. In each sub-grain, the electron spins are parallel. The coordinated electron spins give rise to an internal magnetic field, which is equal to an external demagnetizing field (in the opposite direction) (Fig. A3a). A large external field is, however, energetically inefficient and the size of a single domain of parallel electron spins is limited to around 01 μm. In grains larger than this, the coordinated electron spins divide themselves into domains divided by Bloch walls, arranged so as to minimize the external field (Fig. A3b).


Quote:
Fig. A3.
(a) SD and MD configurations serve to minimize the external demagnetizing field. (b) Movement of the Bloch wall in MD grains gives rise to a magnetization when an external field is applied. (c) Inverse susceptibility anisotropy of a SD grain.
View largeDownload slide
(a) SD and MD configurations serve to minimize the external demagnetizing field. (b) Movement of the Bloch wall in MD grains gives rise to a magnetization when an external field is applied. (c) Inverse susceptibility anisotropy of a SD grain.
Quote:
In grains that are around 01 μm, multi-domain (MD) behaviour may not be stable and the grain will exhibit some single domain (SD) behaviour (Fig. A3c); these are pseudo-single domains (PSD). Grains smaller than 003 μm are not large enough to retain coordinated magnetic moments and thus cannot retain magnetization and behave essentially paramagnetically. This is super-paramagnetism (SPM) and is important for studies involving nanoparticles.
From: https://academic.oup.com/petrology/a...6/1187/1490561.

For those interested in the real Bloch wall, the link below will reference a thesis that covers the subject very well. A free download is easy to find.
Roberts, H. G., 2008. Magnetism and transport in nanostructured domain wall systems. Thesis (Doctor of Philosophy (PhD)). University of Bath.
Magnetism and transport in nanostructured domain wall systems - Opus

Regards,

bi
Attached Images
File Type: jpeg m_egn022a3.jpeg (34.2 KB, 63 views)
__________________
 

Last edited by bistander; 05-01-2018 at 06:53 PM. Reason: Added link
Reply With Quote