Domain walls separate magnetic domains within thin films. There are two types of spin configurations within domain walls - Bloch walls and Neel walls. Bloch walls favor spin rotation around an axis normal to the wall plane, while Neel walls favor spin rotation around an axis parallel to the film plane. For thin films, Neel walls are typically favored over Bloch walls due to magnetostatic energy considerations. The Stoner-Wohlfarth model describes coherent magnetization rotation in single-domain thin films under an applied magnetic field, predicting rectangular hysteresis loops and an anisotropy field Hk of 2Ku/Ms for irreversible switching between magnetic easy axes.

1. Domain and Domain Walls
Switching in Thin Films
Santosh Kumar Manchal
S7IC
ICED

2. Course Outcome
To impart basics of deposition and
characteristics of thin films

3. Perquisite
• What Is a crystal.
– A solid can be differentiated as amorphous and crystal.
– A crystal has well structured and highly ordered constituents
• What is Uniaxial Anisotropy
– Uniaxial means single axis.
– anisotropy means certain properties like elasticity, hardness, electric and
thermal conductivity, polarizability, and magnetization etc. of a crystal are
directional
– Attained if during deposition a unidirectional magnetic field is applied
• Ek= Kusin2(A) is energy of magnetization vector making an angle ‘A’ with direction
of applied field.
– Easy axis ie EA -> minimum Ek
– Hard axis ie HA -> maximum Ek

4. DOMAIN

5. What is Magnetic Domain
• In ferromagnetic materials the atoms align with the
magnetic fields, where magnetic forces tends to push
all magnetic moments in same direction.
• In thin films, ideally all spins are perfectly aligned to
applied magnetic field direction.
• However there is a deviation.

6. • A magnet can be broken down into several small
magnets.
• Each of these individual segments of the magnets
can be termed as domains.
• The walls which exist between each of these domains
are termed as domain walls

9. FERRO FLUID

11. DOMAIN WALLS

12. • Domain wall is an interface which
separates magnetic domains.
• Domains in thin films are separated
by vertical walls perpendicular to
the plane of the film.
• Horizontal walls are not possible as
their thickness(~1x10-7 m) is smaller
than wall width (~1x10-6m)

14. Domain wall motion
• Application of a magnetic field in the plane of film,
for a film having a planar easy axis, magnetizes the
sample to saturation.
• It produces a single domain magnetic structure by
domain wall motion.
• This structure is subjected to ripples and noise

15. Barkhausen effect
• The Barkhausen effect is a
name given to the noise in
the magnetic output of a
ferromagnetic when the
magnetizing force applied
to it is changed. It is
caused by rapid changes
of size of magnetic
domains

16. Width of
Domain Walls
• Determined by condition of minimization of the sum
of the exchange energy.
• It includes the anisotropy energy, the domain wall
energy and the magneto-static energy.
• Increase in wall width will reduce the exchange
energy but will increase the domain wall, and
anisotropy energy.

17. Spin Configuration
• There are 2 types of spin configuration possible
within the domain wall.
– Bloch Wall Spin Config/:
– Neel Wall Spin Config/:

18. Spin Configuration
in Bloch wall
• Spin changes from one direction
to another by rotating about an
axis, normal to plane of wall.
• The component of magnetic field to the wall is normal and
there is no volume magnetic pole.
• Free magnetic poles are created at intersection of walls which
are negligible in bulk materials as these poles are situated far
apart.

20. • For an in-plane magnetized film, below a certain
thickness, the magnetostatic energy predominates
over other two.
• This makes it unfavorable foe bloch wall formation.
• In such cases the spin within the wall takes up a
second configuration known as Neel wall.

21. Spin configuration
in Neel Wall
• Spin changes from one direction
to another by rotating about an
axis, perpendicular to the plane of the wall.
• The poles at the surface are eliminated and volume
poles are created within the walls as the magnetic
field component of the wall is not continuous.

22. comparison
• Bloch wall is energetically more stable than Neel wall.
• For thin film application Neel wall is favored.
• When thickness decreases, the bloch wall do not
essentially changes into a neel wall, instead an
intermediate cross tie wall structure is absorbed.
• A cross tie wall structure is like a neel wall having
alternative intervals of oppositely directed senses of
rotation of magnetization within the walls.

23. SWITCHING IN THIN FILMS

24. • Due to uniaxial anisotropy, the hysteresis loop of thin
film unlike ordinary ferromagnetic material depends
on the direction of applied magnetic field
• This is illustrated by STONER-WOHLFARTH model of
coherent rotation.

25. Stoner–Wohlfarth Model
• In the Stoner−Wohlfarth model, the magnetization of thin film does
and it is represented by a vector M. This vector rotates as the
magnetic field H changes. The magnetic field is only varied along a
single axis; its scalar value h is positive in one direction and negative
in the opposite direction.
• This theory is applied for analyzing the switching behavior of an
ideal single domain film.
• The magnetization energy is given by;
E = -MS HIICOS(A)-MS HPSIN(A)+KUSIN2(A)

26. • On differentiating E with respect to (A), yields a rectangular
hysteresis loop. There are two stable states of magnetization for Hll
which corresponds to two remnant states +-Mr (ideally Mr = Ms).
• The critical field for an irreversible change from one state to
another i.e. Switching is Hk=2Ku/Ms is called the anisotropy field.

27. • For field along hard axis, magnetization varies linearly
with field due to gradual rotation of magnetization
away from the easy axis for field values upto Hk.
• For higher field values, the magnetization vector
remains along the hard axis.
• Switching from EA to HA is a reversible process
• Hk is the minimum value of field required to pull
magnetization from EA to HA.

28. • In thin film reversal along EA does not takes place by
coherent rotation but by domain wall motion
– Growth of reverse domain along EA
– Expansion of elongated reverse domains by
parallel shift in HA
Theoretically coercivitiy ie coercive domain wall
force must be less than Hk=2Ku/Ms.

29. ThAnK yOu : )