Magnetic Domain and Domain Walls. 1. Domain walls Bloch wall Neel wall Cross-Tie wall 2. Magnetic domains Uniaxial wall spacing Closure domain Stripe domains 3. Some methods for the domain observation SEMPA MFM Magneto-optical. Relevant Energy Densities. Exchange energy.
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1. Domain walls
2. Magnetic domains
3. Some methods for the domain observation
In 1907, Weiss Proposed that magnetic domains that are regions inside the material that are magnetized in different direction so that the net magnetization is nearly zero.
Domain walls separate one domain from another.
P. Weiss, J.Phys., 6(1907)401.
Schematic of ferromagnetic material containing a 180o domain wall (center).
Left, hypothetical wall structure if spins reverse direction over one atomic
distance. Right for over N atomic distance, a. In real materials, N: 40 to 104.
In the case of Bloch wall, there is significant cost
in exchange energy from site i to j across the domain
wall. For one pair of spins, the exchange energy is :
Surface energy density is
In the other hand, more spins are oriented in directions
of higher anisotropy energy. The anisotropy energy per unit area increases with N approximately as
minimizes the sum with respect to N
thus the wall thickness
The minimized value No
is of order
, where A is the exchange stiffness
constant. A=Js2/a ～10-11 J/m (10-6 erg/cm), thus the wall
thickness will be of order 0.2 micron-meter with small aniso-
tropy such as many soft magnetic materials
The wall energy density is obtained by substituting into
Comparision of Bloch wall, left, with charged surface on
the external surface of the sample and Neel wall, right,
with charged surface internal to the sample.
Energy per unit area and thickness of a Bloch wall and a
Neel wall as function of the film thickness. Parameters
used are A=10-11 J/m, Bs=1 T, and K=100 J/m3.
In the case of Neel wall, the free energy density can be approximated as
Minimization of this energy density with respect to δN
For t/δN ≤1, the limiting forms of the energy density
σN and wall thickness δN follow from above Eq.
Calculated spin distribution in a thin sample containing a
180o domain wall. The wall is a Bloch wall in the interior,
but it is a Neel wall near the surface.
The charge on a Neel wall can destabilize it and cause
it to degenerate into a more complex cross-tie wall
Once domains form, the orientation of magnetization in each domain and the domain size are determined by
Domain wall energy
Domain formation in a saturated magnetic material is driven by the magnetostatic (MS) energy of the single domain state (a). Introduction of 180o domain walls reduces the MS energy but raises the wall energy; 90o closure domains eliminate MS energy but increase anisotropy energy in uniaxial material
The number of domains is W/d
and the number of walls is (W/d)-1. The area of single wall
is tL The total wall energy is
The wall energy per unit volume
Domain Size d ? by the magnetostatic (MS) energy of the single domain state (a). Introduction of 180
The equilibrium wall spacing
may be written as
Variation of MS energy density
and domain wall energy density
with wall spacing d.
For a macroscopic magnetic ribbon by the magnetostatic (MS) energy of the single domain state (a). Introduction of 180;
L=0.01 m, σw= 1mJ/m2, ｕoMs= 1 T and t = 10ｕm, the wall spacing is a little over 0.1 mm.
The total energy density reduces to
According to the Eq.(for do) for thinner sample the
equilibriumwall spacing do increases and there are
(The magnetostatic energy of single domain)
Single domain size
Variation of the critical thickness with
the ratio L/W for two Ms (σdw=0.1mJ/m2)
If using the parameters:
L/W=5, σdw≈ 0.1 mJ/m2,ｕoMs= 0.625 T; tc ≈13.7 nm;
Domain walls would not be expected in such a film. It is for a typical thin film magnetoresistivity (MR) read head.
Consider σ90 =σdw /2, the wall energy fdw
increases by the factor 1+0.41d/L; namely
Hence the energies change to
Geometry for estimation of equilibrium
closure domain size in thin slab of ferro-
magnetic material. If Δftot < 0, closure
Energy density of △f by the magnetostatic (MS) energy of the single domain state (a). Introduction of 180tot versus sample length L
forｕo Ms=0.625 T, σ=0.1 mJ/m2, Kud=1mJ/m2,
and td=10-14 m2.
Single domain partcle
σdw πr2 =4πr2(AK)1/2
△EMS≈ (1/3)ｕo Ms2V=(4/9)ｕo Ms2πr3
The critical radius of the sphere would be that which makes these two energies equal(the creation of a domain wall spanning a spherical particle and the magnetostatic energy, respectively).
rc≈ 3nm for Fe
rc≈ 30nm for γFe2O3
If the anisotropy is not that strong, the magnetization will tend to follow the particle surface
The spin rotate by 2π
radians over that radius
(a) A domain wall similar to
that in bulk; (b) The magneti-
zation conforms to the surface.
sphere by breaking the sphere into cylinders of radius r, each of which
has spins with the same projection on the axis symmetry
Construction for calculating the exchange energy of a
particle demagnetized by curling.
If this volume of a exchange energy density cost is equated to the magnetostatic energy density for a uniformly magnetizes sphere, (1/3)ｕoMs2, the critical radius for single-domain spherical particles results:
Critical radius for single-domain behavior versus saturation magnetization.
For spherical particles for large Ku, 106 J/m3 and small one.
Spin configuration of stripe domains
Spin configuration in volume of a
The slant angle of the spins is given as, θ = θo sin ( 2πx/λ )
The total magnetic energy (unit wavelength);
When w >0 the stripe
Minimizing volume of a wrespect toλ
Using eq.(1) we can get the condition for w>0,
(b) After switch off a strong
H along the direction normal
to striple domain.
(a) After switch of H along
(c)As the same as (b), but
using a very strong field.
with 120 nm thick by Lolentz electron microscopy.
ProbabilityP per unit time for switching out of the metastable
state into the more stable demagnetzed state:
the first term in the right side is an attempt frequancy factor equal approxi- mately 109 s-1.
Δf is equal to ΔNµo Ms2 or Ku .
For a spherical particle with Ku = 105 J/m3 the superparamagnetic radii
for stability over 1 year and 1 second, respectively, are
Paramagnetism and Superparamagnetism volume of a
Paramagnetism describes the behavior of materials that
have a local magnetic moments but no strong magnetic
interaction between those moments, or. it is less than kBT.
Superparamagnetism:the small particle shows ferromagnetic
behavior, but it does not in paramagnete. Application of an
external H results in a much larger magnetic response than would
be the case for paramagnet.
The M-H curves of superparamagnts can resemble those of ferromagnets but with two distinguishing features;
(1)The approachto saturation follows a
(2) There is no coecivity. Superpara-
magnetic demagnetization occurs without
coercivity because it is not the result of
the action of an applied field but rather of
Langevin function versus s;
M = NµmL(s); s = µmB/kBT
Some important parameters volume of a
Principle: when an energetic primary electron or photon enters a ferromagnetic material, electron can be excited and escape from the material surface. The secondary electrons collected from the small area on the surface are analyzed to determine the direction of magnetization at the surface from which they were emitted.
The vertical p
(a) magnetic surface domain structure on Fe(100). The arrows indicate the measured
polarization orientation in the domains. The frame shows the area over which the polari-
zation sistribution of (b) is averaged.
Below, structure of (SEMPA)Fe film/ Cr wedge/ Fe whisker illustrating the
Cr thickness dependence of Fe-Fe exchange. Above, SEMPA
image of domain pattern generated from top Fe film. (J. Unguris et
al., PRL 67(1991)140.)
Geometry for description of MFM
technique. A tip scanned to the
surface and it is magnetic or is
coated with a thin film of a hard
or soft magnetic material.
Domain structure of epitaxial Cu/t (SEMPA)Ni /Cu(100) films imaged by
MFM over a 12 µm square: (a) 2nm Ni, (b) 8.5 nm Ni, (c) 10.0
Nm Ni; (d) 12.5nm Ni (Bochi et al., PRB 53(1996)R1792).
Magneto-optical Effect (SEMPA)
θ k is defined as the main polarization plans is tilted over
a small angle;
(a) (SEMPA)Assembly of apparatus
(b) Rotation of polarization
of reflecting light.
Domain on MnBi Alloys (SEMPA)
The magnetic domains on the thin plate MnBi alloys observed by
Magneto-optical effect; (a) thicker plate (b) medium (c) thinner.
(Roberts et al., Phys. Rev., 96(1954)1494.)
(a) Bitter Powder method;
(b) Lorentz Electron Microscopy;
(c) Scanning Electron Microscopy;
(d) X-ray topograhy;