Magnetic domains is a microareas ( with size normally about micrometers), with homogeneous magnetization (both in direction and amplitude) We consider magnetic layer, where magnetization vector is arranged only in perpendicular axes to the sample surface. Changebility of this case is frequently analizyse as a magnetic bar structure in infuence external magnetic fields H. Along with increase volume of this fields, domain where orientation magnetisation vector M is the same as magnetic fields H increase too. In our description we will use parameter known as reduced magnetization m, which can define based on domain structure size: m(H)=(d+(H)-d–(H))/(d+(H)+d–(H)).
Picture F1 Domain structure bar model at magnetic fields.
For investigate magnetization process and visualization domain structure we use Faraday effect (Pic. F2) – plane of linear polarizated light, after transmition by magnetic layer is rotated on angle +fi or -fi depending on magnetization vector turning in oposite directions. In accordance with the Malus law value of light after transmition by magnetic layer is proportionaly to:
Cos2(alpha +/- fi )
where alpha is a angle between main polarization plane. Turning one of polarizer can possible to turn out light transmitted by magnetic domain with defined magnetization vector, what is showing on the picture F2. Round insertion in the picture illustrate real image of magnetic structure recording by CCD camera in different polarizer position.
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Picture F2 Domain structure visualization with use Farraday effect. Parts A and B respoding with two analizer – polarizer positions. (Animation – visualization of magnetic domains )
Frequently light transmitting by sample space has many domain is focusing on the single sensor then signal S(H) recoring by its can bind with the sample magnetization by equation:
S(H)=a + b*m(H)
Cooficients a i b depending on value of angles kątów alpha i fi. Measuring signal S, in various value external magnetic fields H applied to magnetic sample allow to determine magnetization (histeresis) loop for the sample m(H)
Our avaiable by Internet, experimental setup is equiped in light sensor and CCD camera (having mesh of sensors). The setup make possible for remote user measure both histeresis loop m(H) and recording pictures of domain structure in various value of magnetic field. Picture 3 illustrate magnetization process for thin magnetic layer recorded with use CCD camera and single light sensors. From its analyse we can see that incrase the amlitude of magnetic fields H increase volume of domains with the same direction of magnetization vector as a field H, increase te value m too. The sample going to monodomain state where external fields is greter then saturation field |H|>Hs. When decrase value of magnetic fields from |H|>Hs we can observe in field Hn (nucleation field) appearing domains with opposite direction of magnetization vetor to H. Value Hn its less than Hs This bahaviour is named histeresis effect. For Internet experiment we choose the sample – garnet layer, with histeresis effect in small magnetic fields (small comparing with Hn and Hs) which we can omit.
Picture F3 Example of histeresis loop and images of domain structures recorded with use our experimental setup. (Animation magnetization process). For more detailed analysis we choose rectangular area with size 20 x10 mikrometers.
With magnetic domain we can also involve very popular application in form as magnetic tapes or another magnetic media storage which its frequently use in digital/computer technology or geomagnetic earth investigate (magnetic memory of earth)
Additionaly materials we can find:
- Encyklopedia of Compentatory Physics, PWN Warszawa, 1983, Article:
“Domain structure and magnetization process”, H. i R. Szymczakowie, p 585
“Magnetooptic”, W. Wardzyński, p 590
“Magnetic memory”, H. Lachowicz p 596 - “Physics – encyklopedical guide”, B. M. Jaworski, A. A. Dietłaf, PWN, Warszawa 1998
- “Physics”, handbook for second year of middle school, Jerzy Ginter,
- Theory decribed bar domain structure in magnetic fields.
- Magnetic hysteresis – Wikipedia
- Image processing basics