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Microwave Magnetics
Graduate Course
Electrical Engineering (Communications)
2nd Semester, 1389-1390
Sharif University of Technology
Magnetostatic waves and oscillations 2
General information
� Contents of lecture 10:
• Magnetostatic waves and oscillations
� Introduction
� Magnetostatic waves
� Magnetic potential
� Magnetostatic waves in metallized plates
� Normally magnetized plates
� Transversely magnetized plates
� Magnetostatic waves in tangentially magnetized free plates
� Surface waves
� Excitation of magnetostatic waves
� Magnetostatic wave devices
Magnetostatic waves and oscillations 3
(i) Introduction
� Let us start our discussion with an example: an
unbounded magnetic film placed on top of a ground plane
� The film is magnetized parallel to its plane in the z-
direction
0 0,M Hx
y
z
d
Magnetostatic waves and oscillations 4
(i) Introduction
� We look for solutions which represent waves
propagating in the y-direction, we assume them to be
uniform along the magnetization (z-direction)
x
y
z
( ) exp( ) ( ) exp( )x j y x j y� �� � � ���e e h h
Magnetostatic waves and oscillations 5
(i) Introduction
� To analyze the structure we resort to equations we had
used before (inside the magnetic material)
x
y
z
� �2
2 20 02 2
0akx z z
� ��� � �� �
� �� �� � �
� � � � �� �� � �� �
2z z z
z
h h eh�
�
� � ��
� �2
2 20 02 2
0akx z z
� ��� � ��
��
� �� � �� � � � �� �� � �� �
��� �
�2
z z zz
e e he
Magnetostatic waves and oscillations 6
(i) Introduction
� Two decoupled sets result.
� 1st set (inside magnetic material):
� �2
2 202
0d
kdx
�� ��� � ��
�zz
ee
2 20
1
( ) /
a
a a
j j
j j d dx
� � �
�� � � � �
� � � � �� �� � � � �� �
�� � � �� �� � � �� �
� �
� �
zx
zy
eh
eh
x
y
z
0� � ��� �x y ze e h
Magnetostatic waves and oscillations 7
(i) Introduction
� 2nd set (inside magnetic material): x
y
z
0� � �� � �x y zh h e
� �2
2 202
0d
kdx
�� �� � ��
��z
z
hh
0
1
/
j
j d dx
�
�� �
� �� �� �� � � �� �� �� � � �
x z
y z
e h
e h
��
��
� 2nd set not affected by magnetic properties since �|| = 1 (ac
magnetic field parallel to static magnetization).
Magnetostatic waves and oscillations 8
(i) Introduction
� 1st set: Region I: 0 < x < d
2xk
� �2
2 202
0d
kdx
�� ��� � ��
�zz
ee
� �( ) sin xx A k x��ze � � � �
0
cos sinx x x
Ak k x k x
j�
�� ��
� �� �� ��
yh
a�� ��
�
d
x
y
Magnetostatic waves and oscillations 9
(i) Introduction
� 1st set: Region II (air): x >d
2,0xk
� �2
2 202
0d
kdx
�� � ��
�zz
ee
� �,0( ) exp xx B jk x� ��ze � �,0
,00
expxx
k Bjk x
��� � ��
yh
d
x
y
Magnetostatic waves and oscillations 10
(i) Introduction
� Resulting propagation equation:
� � ,0cotx x xk k d jk� ��� �
� �2 2 2 2 2 20 0 0cot ak d k j k
��� � �� � � � �
�� � �� � � � �
� The left hand side is always real. Since
the right hand side should be real �0k� �
2 2 2 20 0k j k� �� � � � Why the minus sign?
Magnetostatic waves and oscillations 11
(i) Introduction
� There are two classes of solutions. The 1st class satisfies
2 2 20 0k k� ���� � Condition: �� > 0
� These solutions are surface waves (why?) Like ordinary
(TE) surface waves on grounded dielectrics, they have a
frequency cutoff which depends on the film thickness,
dielectric constant, etc.
� The magnetic material, however, makes � dependent on
the direction of propagation.
Magnetostatic waves and oscillations 12
(i) Introduction
� Numerical example:
2 GHz
6 GHz
4
1 mm
H
M
f
f
d
�
��
��
-1 (mm )�
(GHz)f
� In any case 0k� ����
0� �
� Note also that the phase velocities are comparable to
that of light (e.g. at 50 GHz: vp = 2�f / � ~ 2.85 x 108 m/s)
0� �
Magnetostatic waves and oscillations 13
(i) Introduction
� However, there is a second branch of solutions in which
2 20k� ����
� These are also surface waves (why?). They only exist for
particular frequencies, but the range mainly depends on
magnetic properties
� �2 2 2 2 2 20 0 0coth ak d k k
�� �� � �� � � �
�� � �� � � � �
Magnetostatic waves and oscillations 14
(i) Introduction
� Numerical example:
-1 (mm )�
2 GHz
6 GHz
4
1 mm
H
M
f
f
d
�
��
��
H Mf f�
2M
H
ff �
� These �’s are large compared to to k0 !
� Wavelengths (2�/�) are short compared to
electromagnetic wavelength 2�/k0.
f�0� �
0� �
Magnetostatic waves and oscillations 15
(i) Introduction
� These waves are called slow waves
� In thinner films (here the thickness was 1mm) the velocity
and wavelength can be even 2-3 orders of magnitude
smaller than electromagnetic velocities!
� Note also that the phase velocities can be much smaller
than that of light: for example
• left moving wave at 7 GHz: vp = 2�f / � ~ 0.665 x 108 m/s
• right moving wave at 4.7 GHz: vp = 2�f / � ~ 0.45 x 108 m/s
Magnetostatic waves and oscillations 16
(i) Introduction
� These slow waves can satisfy
� Dispersion equation may have been approximated by
2 20k� � ���
� �2 2 2 2 2 20 0 0coth ak d k k
�� �� � �� � � �
�� � �� � � � �
� �coth ad�
� � � � �� �� �
2 20k� �
Magnetostatic waves and oscillations 17
(i) Introduction
� Comparison with the exact result (lines: exact, points:
approximate) show the accuracy of this approximation,
in particular for short wavelength’s (large �’s)
-1 (mm )�
H Mf f�
2M
H
ff �
f�0� �
0� �
Magnetostatic waves and oscillations 18
(ii) Magnetostatic waves
� So, with magnetic materials, it is possible to have slow
waves with short wavelength’s
0pv c� 00
pp
v c
f f� �� ��
� What is more: to obtain these solutions we may
assume that the velocity of light is infinite
0 0pk v c� �� �
� This approximation means that we neglect all the
propagation effects!
Magnetostatic waves and oscillations 19
(ii) Magnetostatic waves
� This means that, when dealing with slow wave
solutions, in Maxwell equations we are allowed to
neglect the displacement current term everywhere:
� �
00
0 0
d
d
j
j
���
� �
�� �
�� � � � � � �
�� � � �� � �
e h e
h e j h
�
�
� This leads to the magnetostatic approximation:
� � 0�� � �� � �h j h�
Magnetostatic waves and oscillations 20
(ii) Magnetostatic waves
� In electromagnetic problems we are accustomed to the
idea that wave propagation requires the displacement
current.
� Clearly here we have waves which do not need the
electric field for propagation!
� These are called magnetostatic waves. They are, in fact,
waves of the magnetization propagating inside the
medium.
Magnetostatic waves and oscillations 21
(ii) Magnetostatic waves
� To see the physical origin of these waves recall the
linearized Landau-Lifshitz equation
0 0
( , )( , ) ( , )
d tt t
dt� �� � � � �
m rm r H M h r
( , )tM r0M
( , )tm r
x
y
� � � �0x
s y y
dmM h H m
dt� �� �
� � � �0y
s x x
dmM h H m
dt� �� � �
M� H�
Magnetostatic waves and oscillations 22
(ii) Magnetostatic waves
( , )tM r0M
( , )tm r
x
y
� Differentiation with respect to time:
22
2
yxH x M H M x
dhd mm h
dt dt� � � �� � �
22
2
y xH y M H M y
d m dhm h
dt dt� � � �� � � �
� The field h is the ac magnetic field which contains the
externally applied field, and the “demagnetization”
field generated by the magnetization itself.
Magnetostatic waves and oscillations 23
(ii) Magnetostatic waves
� Let us assume that no external ac field is applied.
� Besides, since we are adopting the magnetostatic
approximation anyway, we use the approximation
0( , ) ( , ) ( ) ( , )M
V
t t G t dV� � � �� � �� � � ��h r h r r r m r
� If we neglect the effect of the boundary of the volume V
(in reality this is wrong but this is just a qualitative
argument)
� �0( , ) ( ) ( , )V
t G t dV� � � � �� � � � ��h r r r m r
Magnetostatic waves and oscillations 24
(ii) Magnetostatic waves
� The demagnetization field is related to the change of the
magnetization in space (its second derivative)
� It can lead to wavelike behavior. For instance, imagine
for some reason we can neglect my and hy
22
2
yxH x M H M x H M x
dhd mm h h
dt dt� � � � � �� � � �
2
0 2
( , )( ) x
x
V
m th G dV
x
�� ��� �� � � ���� ��
rr r
Magnetostatic waves and oscillations 25
(ii) Magnetostatic waves
� This is a wavelike equation (not a wave equation
because of the integration involved)
� Hence: change of magnetization in space induces
demagnetization fields which interact with the motion
(rotation) of the magnetization
� This leads to wavelike phenomena
2 22
02 2
( , )( ) 0x x
H M H x
V
d m m tG dV m
dt x� � �
�� ��� �� � � �� ���� ��
rr r
Magnetostatic waves and oscillations 26
(ii) Magnetostatic waves
� Remarks:
• Whatever the mechanism, everything ‘is’ already covered by
the full Maxwell equations. The magnetostatic approach
does not bring about new phenomena such as slow waves:
these are already in the Maxwell equations.
• Magnetostatic approach only leads to a simplified formalism
which allows us to study a certain class of solutions.
• Yet, magnetostatic approach cannot cover all possible
solutions. Had we neglected the displacement current in our
example, we could not have found the conventional surface
waves with 2 2 20 0k k� ���� �
Magnetostatic waves and oscillations 27
(ii) Magnetostatic waves
• Short wavelength magnetostatic waves can be used to
design compact devices at microwave frequencies (MSW
devices)
• Magnetostatic approach allows us to perform an
approximate, but simple analysis of these components.
• Magnetostatic waves are sometimes called spin waves, but
that is not completely accurate. They are, actually, the long
wavelength limit of the spin waves. A more accurate terms
is: non-exchange spin waves.
Magnetostatic waves and oscillations 28
(iii) Magnetic potential
� The magnetostatic equations are often solved in
materials where the electric (conduction) currents are
negligible. Therefore
� �0 0�� � �� � �h h�
� Let us introduce
�� �h Magnetic potential
� Then the first equation is automatically satisfied
Magnetostatic waves and oscillations 29
(iii) Magnetic potential
� 2nd equation:
� � 0�� � �h�
2 2 2
2 2 20
x y z
� � �� �� �� � �
� � �� �� � �� ��
� �a does not appear in this equation! But it affects the
problem through boundary conditions.
0
0
0 0
a
a
j
j
� �� �
�
� �� �� �� �� �� ��
�
Walker equation
Magnetostatic waves and oscillations 30
(iv) Magnetostatic waves in metallized plates
� Consider an unbounded magnetic plate metallized on
both sides
� We consider two cases:
• Normally magnetized plate
• Tangentially magnetized plate
0 0,M H 0 0,M H
Magnetostatic waves and oscillations 31
(iv) Normally magnetized metallized plates
� 1st case: normally magnetized plate
� We had seen similar systems before (transversely
magnetized waveguides, microstrips on normally
magnetized magnetic substrates)
� But now we will not restrict ourselves to solutions
uniform along z-direction
xy
z
0 0,M H
Magnetostatic waves and oscillations 32
(iv) Normally magnetized metallized plates
� It suffices to consider waves along one direction only
(because of the rotational symmetry of the system)
� Solution written as
xy
z
yk
( , ) ( ) exp( )yy z f z jk y� � �
Magnetostatic waves and oscillations 33
Magnetostatic waves and oscillations 34
(iv) Normally magnetized metallized plates
� Boundary conditions on perfect metallized surfaces
xy
z
yk
( 0) ( ) 0z z d� � � �z zb b
d
�� � � ��b h� � z
���
�zb
Magnetostatic waves and oscillations 35
(iv) Normally magnetized metallized plates
� Solution:
( ) cosn z
f z Ad
�� �� � �� �
� Such solutions only exist when �< 0:
2 2
2 2H
� ��
� �� ���
yk
1n � 2n �H� � ��� �
ny
nk
d
��
� ��
Magnetostatic waves and oscillations 36
(iv) Normally magnetized metallized plates
� Dispersion curves
� �� �
2 22 2
221 /
n My H H
ny
nk
d n k d
��� � �
� �
� �� �� � � � �� ��� �
� Numerical example:
-1 (mm )yk
2 GHz
6 GHz
0.1 mm
H
M
f
f
d
���
Hf
f�
(GHz)f
1n �
2n �
3n �
1(3.5 GHz) 0.157mm!n� � �
Magnetostatic waves and oscillations 37
(iv) Normally magnetized metallized plates
� Overall solution:
� �( , ) cos exp nn y
n zy z A jk y
d
�� � �� �� �
� �
� �( , ) cos expn ny n y
n zy z jk A jk y
d
�� �� � �� �� �
yh
� �( , ) sin exp nn y
n n zy z A jk y
d d
� �� �� � �� �� �
zh
� These waves have sinusoidal behavior inside the
sample. They are called volume waves.
Magnetostatic waves and oscillations 38
(iv) Normally magnetized metallized plates
� Remarks about volume waves:
• Dispersion curves are independent of the propagation direction
because of the rotational symmetry of the problem. The same
result is found for waves propagating in the x-direction or along
any other direction.
• All modes can propagate between �H and �� . There is no
size-dependent cutoff frequency as in a classical
electromagnetic waveguide.
• The mode n=0 is not a solution. It results in a zero magnetic
field (constant �).
• The waves have a group velocity parallel to the phase velocity:
they are forward waves.
Magnetostatic waves and oscillations 39
(iv) Normally magnetized metallized plates
� The electric field does not enter the magnetostatic
equations in the first place, but can be perturbatively
found after the magnetic field h is solved
00
d
j�
��� �
�� � � � � � �e h e�
� For simplicity assume there are no charges
(conduction or external)
0 0j���� � � � � � �e h e�
Magnetostatic waves and oscillations 40
(iv) Normally magnetized metallized plates
� We expect
0ny a
djk
dz�� �� � �y
z y
ee h
xy
z
yk
d
0
dj
dz�� �� �x
y
eh
0nyk
��� �x ze h
0ny
djk
dz� � �z
y
ee
� �( ) exp nyz jk y� ��e e
Magnetostatic waves and oscillations 41
(iv) Normally magnetized metallized plates
� �( , ) cos expn ny n y
n zy z jk A jk y
d
�� �� � �� �� �
yh
� �( , ) sin exp nn y
n n zy z A jk y
d d
� �� �� � �� �� �
zh
� �0( , ) sin exp nn yn
y
n n zy z A jk y
k d d
� ��� � �� �� �
� �xe
� We shall not compute the other components here!
� Note: the electric field satisfies the boundary conditions.
Magnetostatic waves and oscillations 42
(v) Tangentially magnetized metallized plates
� Let us now turn our attention to a tangentially
magnetized, metallized plate
� Consider waves propagating with a wave vector
z
x
y0 0,M H
� � � �0, , 0, sin , cosy z k kk k k � �� �k
k�k
Magnetostatic waves and oscillations 43
(v) Tangentially magnetized metallized plates
� We look for wave solutions of the type:
0 0,M H
k�k
� �( , , ) ( ) exp y zx y z f x jk y jk z� � � �
� Walker’s equation leads to
� �2
2 22
( )( ) 0y z
d f xk k f x
dx� �� � �
z
x
y
Magnetostatic waves and oscillations 44
(v) Tangentially magnetized metallized plates
� Boundary conditions lead to:
0 0,M H
k�k
0 0 and aj x x dx y
� �� �� �
� � � �� �
z
x
y
d
0 0 and a y
dfk f x x d
dx� �� � � �
Magnetostatic waves and oscillations 45
(v) Tangentially magnetized metallized plates
� General solution:
� � � �( ) sin cosx xf x A k x B k x� �
22 z
x y
kk k
�� � �
� Boundary conditions �
0x y ak A k B� �� �
� � � �sin 0y a x xk A k B k d� �� � �
Magnetostatic waves and oscillations 46
(v) Tangentially magnetized metallized plates
� Setting determinant to zero
� 1st solution:
� � � �2 2 2 2 sin 0x y a xk k k d� �� �
� � 2 2 2 2sin 0 or 0x x y ak d k k� �� � �
222 z
x y
kn nk k
d d
� ��
� �� � � � � � �� �
Magnetostatic waves and oscillations 47
(v) Tangentially magnetized metallized plates
222 2 cos
sin kk
nk
d
� ��
�� � � �� � �� � � �
� �� �
� Result:
� Regardless of the value of n, in order to have
propagation: 2
2 cossin 0k
k
��
�� �
� We should have µ < 0 like in the previous case;
otherwise this condition cannot be satisfied
Magnetostatic waves and oscillations 48
(v) Tangentially magnetized metallized plates
� Even then, propagation only occurs for certain angles
2 1tan k� �
� �
Propagation region
� This limitation does not depend on thickness
Magnetostatic waves and oscillations 49
(v) Tangentially magnetized metallized plates
� The width of the propagation cone at each frequency:
2 22
2 2
1tan H
k
� ��
� � ��
�� � �
�
H� �� � ���
� Propagation cone becomes very narrow near �H , but
covers the whole plane near ��
H� � ��� �
Magnetostatic waves and oscillations 50
(v) Tangentially magnetized metallized plates
� Within the propagation cone the dispersion relation is:
222 2 cos
sin kn k
nk
d
� ��
�� � � �� � �� � � �
� �� �
� �� �
222
2
sin /
1 /k n
H H M
n
n k d
n k d
� �� � � �
�
� ��� �� �
�� �� �
� The dispersion curve depends on the propagation angle
Magnetostatic waves and oscillations 51
(v) Tangentially magnetized metallized plates
� For every angle, the limit k � 0 corresponds to the same
frequency ��
-1 (mm )k
f�
f
� But, with increasing k,
frequency decreases
finally reaching
� �2sin
k
H H M k
�
� � � �
�� �
�
1n �2n �
3n �
� The higher n, the later this limit is reached
Magnetostatic waves and oscillations 52
(v) Tangentially magnetized metallized plates
� Propagation along static magnetization (z)
-1 (mm )k
f�
1n �2n �
3n �
� �0k� �
Hf
0 0,M Hk
d
Magnetostatic waves and oscillations 53
(v) Tangentially magnetized metallized plates
-1 (mm )k
f�
Hf
� Propagation nearly perpendicular to z � �/ 2k� ��
0 0,M Hk
d
� The dispersion curves become nearly flat
Magnetostatic waves and oscillations 54
(v) Tangentially magnetized metallized plates
� Magnetic potential:
( ) cos sin
cos sin sin
y an
n ak
k dn x n xf x B
d n d
k dn x n xB
d n d
�� �� �
�� ��
� �
� �� � � �� �� � � �� �� � � �� �
� �� � � �� �� � � �� �� � � �� �
� �( , , ) ( ) expn y zx y z f x jk y jk z� � � �
( , , )x y z�� �h
Magnetostatic waves and oscillations 55
(v) Tangentially magnetized metallized plates
� Note that for all angles the dispersion curves have a
negative slope: the group velocity is opposite to phase
velocity
� These waves are called backward volume waves
� The are called volume waves since fields have a
sinusoidal behavior inside the magnetic material
� How does the wave front of these waves look like if we
have an isotropic source?
Magnetostatic waves and oscillations 56
(v) Tangentially magnetized metallized plates
� But, there also exist a second class of solutions:
2 2 2 2 2 20x y a z yk k k k� � ��� � � � �
� ky and kz are real numbers (related to propagation).
Therefore this solution exists only if
� �2 2
2 20 0M H
M H
� � �� � � � �
� �� ��
� �� � � � � � �
�
� Propagation
angle: � �
2 22
2 2
1tan k
M H
� ��
� � � ��
�
�� � �
� �
Magnetostatic waves and oscillations 57
(v) Tangentially magnetized metallized plates
� Propagation angle is a function of frequency:
k
f� H Mf f�
f
k�
2
�
k�
Magnetostatic waves and oscillations 58
(v) Tangentially magnetized metallized plates
� Note that this is a single mode (there is no mode
number n like in the previous case)
� Besides, apart from the propagation angle, nothing can
be said about the relation between the frequency and
the magnitude of the wave number k.
� This means the dispersion relation is flat: at any allowed
frequency, every value of k is allowed� flat dispersion
� Of course, this is an artifact of the magnetostatic
approximation. But it can be said that in the true case
the dispersion is “almost” flat.
Magnetostatic waves and oscillations 59
(v) Tangentially magnetized metallized plates
� Also, note that2
2 2 2 2 2 20 ax y a x yk k k k
�� �
�� �
� � � � � � �� �
� Since ky is real, kx should be imaginary
� The corresponding magnetic potential and field should
have an exponential behavior inside the magnetic
material
Magnetostatic waves and oscillations 60
(v) Tangentially magnetized metallized plates
� � � �( ) cos siny ax x
x
kf x B k x k x
k
��
� �� �� �
� �
a yx x x
kk jq q
��
� � � �( ) exp xf x B q x� �
� This type of solution is called a magnetostatic surface
wave because the field drops exponentially inside the
magnetic material
� It is concentrated near the top or bottom surface
depending on the sign of qx
Magnetostatic waves and oscillations 61
(v) Tangentially magnetized metallized plates
� Summarizing the results:
• Between �H and �� we have volume waves. There are different
modes. At each frequency, the modes can propagate within a
certain range of angles with respect to the static magnetization.
All modes represent backward waves: their group velocity is
opposite to their phase velocity.
• Between �� and �H + �M we have a surface wave. To each
frequency their corresponds a specific propagation angle. But
the dispersion curve is flat. The field is concentrated at the top or
bottom interface (with the metal) depending on the propagation
direction.
Magnetostatic waves and oscillations 62
(v) Tangentially magnetized metallized plates
� Remember, for normally
magnetized metallized
plates:
� Forward volume waves
between �H and ��
� Independent of the
propagation angle
0 0,M H
-1 (mm )yk
Hf
f�
(GHz)f
1n �
2n �
3n �
Magnetostatic waves and oscillations 63
(v) Tangentially magnetized metallized plates
� Tangentially magnetized
metallized plates:0 0,M H
-1 (mm )k
f�
1n �2n �
3n �
Hf
H Mf f�� Angle-dependent, back-
ward volume waves
between �H and �� .
� Flat-dispersion surface
wave between �� and �H
+ �M . Frequency
depends on angle.
Magnetostatic waves and oscillations 64
(vi) Tangentially magnetized free plate
� We now turn to the case of a tangentially magnetized
free plate (no metallization)
� This case is more difficult to solve, but it is important
due to its rich physics, and application in MSW devices
0 0,M Hk�
kz
x
y
Magnetostatic waves and oscillations 65
(vi) Tangentially magnetized free plate
� Walker’s equation
0 0,M H
k�k
z
x
y
2 2 2
2 2 20
x y z
� � ��� �� � �
� � �� �� � �� �
� �( , , ) ( ) exp y zx y z f x jk y jk z� � � �
Magnetostatic waves and oscillations 66
(vi) Tangentially magnetized free plate
� Inside the magnetic material (0<x<d)
d
x
y
� �2
2 22
( )( ) 0y z
d f xk k f x
dx� �� � �
� Outside the magnetic material (x<0, x>d) we have �=1:
� �2
2 22
( )( ) 0y z
d f xk k f x
dx� � �
Magnetostatic waves and oscillations 67
(vi) Tangentially magnetized free plate
� Inside the magnetic material (0<x<d)
d
x
y
� � � �( ) sin cosx xf x A k x B k x� �
22 z
x y
kk k
�� � �
Magnetostatic waves and oscillations 68
(vi) Tangentially magnetized free plate
� Outside the magnetic material (x<0, x>d):
d
x
y
� �exp ( ) ( )
exp( ) 0
C k x d x df x
D kx x
� � � ��� ����
� �2
2 22
( )( ) 0y z
d f xk k f x
dx� � �
2 2y zk k k� �
Magnetostatic waves and oscillations 69
(vi) Tangentially magnetized free plate
� Boundary conditions at magnetic material/air interface:
: continuousy z xh ,h ,b
0 0,M H
k�k
z
x
y
: continuousy
z
jk
jk
��
�
�� � � �� � � �� �� � � ��� � � �
y
z
h
h
Magnetostatic waves and oscillations 70
(vi) Tangentially magnetized free plate
0 ajx y
� �� � �
� �� �� �� �� �� �
xb
� Inside magnetic plate:
� Outside magnetic plate:
0 x
��
��
�xb
Magnetostatic waves and oscillations 71
(vi) Tangentially magnetized free plate
� System
to solve:
0 x
��
��
�xb
x
y
� � � �sin cos exp( )x x y zA k x B k x jk y jk z� � � � �� �� �
0 ajx y
� �� � �
� �� �� �� �� �� �
xb
� �exp ( ) exp( )y zC k x d jk y jk z� � � � � �
0 x
��
��
�xb
� �exp exp( )y zD kx jk y jk z� � � �
Magnetostatic waves and oscillations 72
(vi) Tangentially magnetized free plate
� Matching:
D B�
x y akD k A k B� �� �
� � � �sin cosx xC A k d B k d� �
� � � �� � � �
cos
sin
x y a x
x y a x
kC k A k B k d
k B k A k d
� �
� �
� � �
� �
Magnetostatic waves and oscillations 73
(vi) Tangentially magnetized free plate
� Dispersion equation:
� �2 2 2 2 2
cot2
x y ax x
k k kk k d
k
� �
�
� ��
� � 2 21 1cot
2x x z yk k d k kk
���
� �� � � �� �
� �
k�k
z
x
y
Magnetostatic waves and oscillations 74
(vi) Tangentially magnetized free plate
� Note that: 2
2 cossin k
x kk k�
��
� � �
� Hence, we distinguish two situations:
Volume waves: 2
: real
10, tan
x
k
k
� ��
� � �
Surface waves: 2
: imaginary
1tan
x
k
k
��
� �
22 cos
sin 0kk
��
�� � �
22 cos
sin 0kk
��
�� � �
Magnetostatic waves and oscillations 75
(vi) Tangentially magnetized free plate
� Again, volume waves are
allowed within certain angles
1tan k� �
��
� � � �2 21 1
cot cos sin2k k k
k
kdgg
� � � �� ��
� �� � � �� � � �� �
� �
� For any angle, the dispersion equation of volume waves is
� � 2 2sin cos /k k kg � � � �� � �
Magnetostatic waves and oscillations 76
(vi) Tangentially magnetized free plate
� Example: volume waves for
0k� �k
-1 (mm )k
f�
Hf
1n �
2n �
3n �� Again different modes
appear which are all
backward waves
0k� �
1 1cot
2
kd�
� �
� � � �� � � �� � � �� �� �� �� � � �
Magnetostatic waves and oscillations 77
(vi) Tangentially magnetized free plate
� For surface waves we have
� �2
2 cossin 0k
k kg�
� ��
� � �
� � � �2 21 1
coth cos sin2k k k
k
kdgg
� � � �� ��
� �� � � �� � � �� �
� �
� Left hand side positive� there are solutions only if
2 21 1cos sin 0
2 k k� � ���
� �� � � �� �
� �
We can also take the minus sign, it does not matter
Magnetostatic waves and oscillations 78
(vi) Tangentially magnetized free plate
� We should have2 2 2
22 2
cossin 0k
k
� ��
� � � �
� �� � �
�
2 22 2
2 2
1 1cos sin 0
2 k k
�� � �
� � ���
� � � �� � � � �� � �� �
� �� �2 2 21 1sin
2 2H M H M k H� � � � � �� � � � �
� �2 2sinH H M k� � � �� � �
Magnetostatic waves and oscillations 79
(vi) Tangentially magnetized free plate
� If � � ��
� � � �� �2 21 1sin
2 2H H M H M H M k H� � � � � � � � �� � � � �
�� � � � Impossible (Why?)
� Only possibility: � � �� � �� � � �
� But then: �� � �
2sin Hk
H M
��
� ��
�Necessary condition
Magnetostatic waves and oscillations 80
(vi) Tangentially magnetized free plate
� But these conditions are not enough, we should also have
� �2 21 1cos sin
2 k k kg� � � ���
� �� � � �� �
� �2 2 2 2
2 2 2 2
� �� � � �� �
� � ���
� �
� � � �� �22 2 2 2 2 2� � � ��� � � �� �
Magnetostatic waves and oscillations 81
(vi) Tangentially magnetized free plate
� These relations lead to an upper frequency limit dependent
of propagation angle
� �� �2 2sin
2 sinH M H M k H
M k
� � � � � ��
� �
� � ��
-1 (mm )k
f�
1
2H Mf f�
2k
�� �
3k
�� �
4k
�� �
Magnetostatic waves and oscillations 82
(vi) Tangentially magnetized free plate
� The opposite is also true: like volume waves, surface
waves on a free plate are only allowed within a certain
range of angles
Propagation region
2 2
sin kM H
� � ��
� ��� �
��
Magnetostatic waves and oscillations 83
(vi) Tangentially magnetized free plate
� Summarizing:
• Free magnetized plate allows
backward volume modes in
the frequency range between
�H and �� . Between �� and
�H + �M/2 a surface waves
mode is allowed.
• But the actual dispersion
relation and frequency range
of propagation depends on the
propagation angle in both
cases. -1 (mm )k
f�
Hf
1n �2n �
3n �
1
2H Mf f�
Volume modes
Surface mode
Magnetostatic waves and oscillations 84
(vii) Surface waves
� A particularly important case is that of surface waves
propagating perpendicular to the direction of static
magnetization
� We analyze this case in more detail
0 0,M H
2k
�� �
k
z
x
y
Magnetostatic waves and oscillations 85
(vii) Surface waves
� The dispersion equation becomes
� � 1 1coth
2kd �
��
� �� � �� �
� �
2y yk k k� �
� �� �
22
22
12 1exp(2 )
2 1 1a
a
kd� �� � �
� � � � ��
�
� �� �� � �
� � � �
� �� �
22
22
11ln
2 1a
a
kd
� �
� �
� �� �� � �
� �� �� �
Magnetostatic waves and oscillations 86
(vii) Surface waves
� These waves propagate in the range
k
x
y
2M
H
�� � �� � � �
� Magnetic potential inside the magnetic layer:
� � � �sin cos exp( )x x yA k x B k x jk y� � � �� �� �
2x y yk k jk� � �
2y yk k k� �
Magnetostatic waves and oscillations 87
(vii) Surface waves
� From the matching equation it follows that
y y ay a a
x y
k kk k sAj
B k jk
�� �� � �
�� �� � � �
1 0
1 0
yy
y y
kks
k k
���� � �� ���
� � � �sinh cosh exp( )ay y y
sB k x k x jk y
��
�� ��
� � �� �� �
Magnetostatic waves and oscillations 88
(vii) Surface waves
� We rewrite this as
� � � � � �1 sinh cosh exp( )a y
Bs kx kx jk y� � �
�� � � �� �� �
� � � �exp( ) exp ( ) expyjk y U k x d U kx� � �� �� � � � �� �
� �21aU s� �� � � � � �2
1aU s� �� � � �
Magnetostatic waves and oscillations 89
(vii) Surface waves
� Plotting the magnetic potential, it is found that waves
moving in the +y direction (s=1) concentrate near the
bottom surface of the magnetic plate
� Those moving in the –y direction (s=-1) concentrate
near the top surface of the plate
yk
0M
yk
S =1 S = -1
x
y
Magnetostatic waves and oscillations 90
(vii) Surface waves
� What about a half infinite magnetic plate?
� What is its dispersion equation?
� What about the field profile?
Magnetostatic waves and oscillations 91
(vii) Surface waves
� Free plate is not the only structure whose surface waves
have been studied (in view of possible application)
� Various structures have been considered containing
metallic ground planes
� Solid lines: waves
moving in –y direction
(near top surface)
� Dashed lines: waves
moving in +y direction
(near bottom surface)
Magnetostatic waves and oscillations 92
(viii) Excitation of MSW’s in magnetic films
� Open or half-open (grounded) magnetic plates form the
basis of MSW devices. But how are these waves excited?
� By conventional current carrying lines (transducers) on the
surface or close to the surface of the magnetic film
Microstriptransducer
Meandertransducer
Lattice transducer
Magnetic film
Magnetostatic waves and oscillations 93
(viii) Excitation of MSW’s in magnetic films
� Roughly speaking, the amplitude of the excited MSW with
a (tangential) wave vector k is proportional to Fourier
transform of current density at that wave vector:
Microstriptransducer
� �( ) expV
j dV�� J r k r
Magnetostatic waves and oscillations 94
(ix) MSW devices
� MSW devices: based on
excitation and reception
of MSW’s in a finite or
infinite magnetic film
� Mostly based on
excitation of surface
waves, but volume
waves used as well
Magnetostatic waves and oscillations 95
(ix) MSW devices
� These devices benefit from the following properties of the
MSW’s:
• Broad frequency range
( )H H H M� � � � � ��� � � � Volume waves
H M� � � �� � � � Surface waves (grounded plates)
By applying very high dc magnetic fields (up to 2 Tesla by using
permanent magnets) or using materials with a high saturation
magnetization the range of 1-50GHz may be covered
Magnetostatic waves and oscillations 96
(ix) MSW devices
• Properties of MSW devices can be tuned by changing the applied
magnetic field
• Wavelength’s are short, for instance for surface MSW’s propagating
perpendicular to the magnetization in a tangentially magnetized
free plate wavelength is proportional to the film thickness. Using
thin films leads to very short wavelengths
� �� �
22
22
11ln
2 1a
a
kd
� �
� �
� �� �� � �
� �� �� �
� �� �
22
22
124 / ln
1a
a
dk
� ��� �
� �
� �� �� � � �
� �� �� �
Magnetostatic waves and oscillations 97
(ix) MSW devices
• It is possible to change the dispersion properties of MSW’s by the
choice of the wave type and by changing the layer thickness,
adding ground planes, etc.
Magnetostatic waves and oscillations 98
(ix) MSW devices
• The losses are comparatively law (if single-crystalline high quality
films are used)
• Transducers (for exciting MSW’s) are easy to design
� For these reasons MSW devices were investigated in the
1970’s and early 1980’s:
• Delay lines (phase shifters)
• Filters
• Resonators
Magnetostatic waves and oscillations 99
(ix) MSW devices
� Delay lines: consist of a transmitting transducer and a
receiving transducer
� The resulting time delay (phase shift) can be large because
MSW’s are slow (propagation constants are large)
Magnetostatic waves and oscillations 100
(ix) MSW devices
� These devices can be tuned by changing the dc magnetic
bias. They can also be reciprocal or non-reciprocal
� The dispersion characteristics can be engineered to
realize true wideband delay lines (small dispersion over a
wide frequency band) or to have other properties
Magnetostatic waves and oscillations 101
(ix) MSW devices
� MSW filters:
• Wide band filters can be built by using the natural propagation
ranges of MSW’s between transducers
• Narrow-band filters built by engineering the transducers. For
instance note that the amplitude of the wave is proportional to
� �( ) expV
j dV�� J r k r
Magnetostatic waves and oscillations 102
(ix) MSW devices
� For a lattice transducer with N elements each carrying a
current density
J
xs
01
( ) ( , , )N
nn
J r J x x y z�
� � ���
0J
� � � � � �1
00
( ) exp ( ) exp expN
xnV V
j dV j dV jk ns�
�
� �� � �� �
� ��� �J r k r J r k r
Magnetostatic waves and oscillations 103
(ix) MSW devices
� � � �� �
� � � �� �
1
0
1 expexp
1 exp
sin / 2 exp 1 / 2
sin / 2
Nx
xn x
xx
x
jk sNjk ns
jk s
k sNjk s N
k s
�
�
�� �
�
�� �� �
�
� Therefore, one can select
just certain values of wave
number (thus certain
frequencies) for excitation
xk
� �� �
sin / 2
sin / 2x
x
k sN
k s
Magnetostatic waves and oscillations 104
(ix) MSW devices
� MSW resonators: utilize the formation of standing
MSW’s in a ‘finite’ magnetic sample excited by a
transducer
� The standing wave is formed by the reflection of the
MSW off the edges of the finite film
� Since wavelength is short, resonators are small
Standing wave
Magnetostatic waves and oscillations 105
(ix) MSW devices
� For a more detailed overview see
W.S. Ishak, “Magnetostatic wave technology: a review”, Proceedings of
the IEEE, Vol. 78, Issue 2, 1988.
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