moke

ObjectiveThe aim of this experiment is to characterize the magnetization process of the magnetic bulk and thin film samples and recording their hysteresis loop and magnetic domains structures by using kerr microscopy

Introduction

Magneto-optic Kerr effect (MOKE) Magneto-optic Kerr effect (MOKE) is one of the magneto-optic effects. It describes the changes of light reflected from magnetized media. This reflection can produce several effects, including,

1) rotation of the direction of polarization of the light

2) introduction of ellipticity in the reflected beam and

3) a change in the intensity of the reflected beam.

Small rotations of the polarization plane of the light are caused by magneto-optical interaction with the sample and become visible in the MOKE microscope.

Depending on the orientation of the magnetization vector relative to refractive surface and the plane of incident light beams, there are three different types of Magneto-optical Kerr effect.

1) Longitudinal Kerr effect

2) Polar Kerr effect.

3) Transverse (Equatorial) Kerr effect

Figure 1: Different types of Kerr effect

In the longitudinal Kerr effect the magnetization vector is parallel to both the reflection surface and the plane of incidence. If the magnetization vector is perpendicular to the reflection surface and parallel to the plane of incidence, the effect is called the polar Kerr effect. When the magnetization is perpendicular to the plane of incidence and parallel to the surface it the effect is called transverse Kerr effect.

Experimental WorkEquipment and Accessories:

Moke microscope Quadro pole electromagnet Sample

Table 1: Different samples for MOKE microscopy

Sample No.

Type of the sample Thickness Pattern shape Pattern size

1 FeCoBSi Thin film 160 nm - -

2 FeCoBSi Thin film (patterned) 160 nm square 40 µm

3 FeCoBSi Thin film (patterned) 160 nm circle 20 µm

4 FeCoBSi Thin film (patterned) 160 nm circle 40 µm

5 FeCoBSi Thin film (patterned) 160 nm square 20 µm

6 Bulk ( NiFeCoB ) 10 µm - -

7 Bulk ( NiFeCoB ) 10 µm - -

Experimental ProcedureFirst the Microscope was switched on and sensitivity of light was adjusted using the slit in order to get the Longitudinal Kerr effect. After that the sample was placed into the sample holder and desired magnetic field was applied. Samples were oriented in different direction and then magnetized to find out easy axis for magnetization (at easy axis we get maximum saturation value for minimum applied field).

By using Lab-view software, background image of the sample was obtained. Then this image was subtracted from the image containing domains information to get the images that show the pattern of magnetic domain wall. An AC field with decreasing amplitude was then applied to demagnetize the samples along the easy axis direction and then perpendicular to easy axis direction. The same procedure was then repeated for different samples, to get the hysteresis loops and domains images along easy axis direction and perpendicular to easy axis direction.

Results:Hysteresis loops for different orientations:

Sample 1

-5 -4 -3 -2 -1 0 1 2 3 4 5

-1.5

-1

-0.5

0

0.5

1

1.5

Field (mT)

Nor

mal

ized

Inte

nsity

(I/Im

ax)

Figure 1: Hysteresis loop of sample 1, green, black and blue showing the hysteresis loop in different direction. (Green & black: along easy axis, blue: perpendicular to easy axis)

Sample 2

-15 -10 -5 0 5 10 15

-1.5

-1

-0.5

0

0.5

1

1.5

Field (mT)

Nor

mal

ized

Inte

nsity

(I/Im

ax)

Figure 2: Hysteresis loop of sample 2, green, black, pink and blue showing the hysteresis loop in different direction.

Sample 3

-25 -20 -15 -10 -5 0 5 10 15 20 25

-1.5

-1

-0.5

0

0.5

1

1.5

Field (mT)

Nor

mal

ized

Inte

nsity

(I/Im

ax)

Figure 3: Hysteresis loop of sample 3, green, pink and blue showing the hysteresis loop in different direction.

Sample 4

-2 -1.5 -1 -0.5 0 0.5 1 1.5 2

-1.5

-1

-0.5

0

0.5

1

1.5

Field (mT)

Nor

mal

ized

Inte

nsity

(I/Im

ax)

Figure 4: Hysteresis loop of sample 4, green and blue showing the hysteresis loop in different direction (Blue along easy axis and green along perpendicular to easy axis)

Sample 5

-2 -1.5 -1 -0.5 0 0.5 1 1.5 2

-1.5

-1

-0.5

0

0.5

1

1.5

Field (mT)

Nor

mal

ized

Inte

nsity

(I/Im

ax)

Figure 5: Hysteresis loop of sample 5, green, blue and pink showing the hysteresis loop in different direction (Blue and green along easy axis, pink along perpendicular to easy axis)

Sample 6

-2 -1.5 -1 -0.5 0 0.5 1 1.5 2

-1.5

-1

-0.5

0

0.5

1

1.5

Field (mT)

Nor

mal

ized

Inte

nsity

(I/Im

ax)

Figure 6: Hysteresis loop of sample 6, green, and blue showing the hysteresis loop in different direction.

Sample 7

-2 -1.5 -1 -0.5 0 0.5 1 1.5 2

-1.5

-1

-0.5

0

0.5

1

1.5

Field (mT)

Nor

mal

ized

Inte

nsity

(I/Im

ax)

Figure 7: Hysteresis loop of sample 7, green, and blue showing the hysteresis loop in different direction (Green along the easy axis, blue along perpendicular to easy axis)

Domain Structure of different Samples along and perpendicular to Easy

axis:

Sample 1(1)

(2)

Figure 8: Domain alignment in sample 1, (1) along the easy axis and (2) along the hard axis.

Sample 2

(1) (2)

(3) (4)

Figure 9: Domain alignment in sample 2, (1) along the easy axis and (2) along the hard axis, (3) closure domains at 0° AC (sensitive horizontal one ) and (4) closure domains at 90° AC (sensitive to vertical one).

Sample 3

(1) (2)

(3) (4)


Sample 4

(1) (2)

(3) (4)


Sample 5

(1) (2)

(3) (4)


Sample 6

(1)

(2) (3)

Figure 13: Domains in the magneto-strictive material, (1), (2) and (3) domain in the different region of the sample due to the different stress pattern (2) At 0° AC direction (3) at 90° AC direction

Sample 7

(1)

(2) (4)

Figure 14: Domains in the magneto-strictive material, (1), (2) and (3) domain in the different region of the sample due to the different stress pattern (2) At 0° AC direction (3) at 90° AC direction

Saturation, Reminance and Coercive field of the sample:Sample 1 Loop 1 Loop 2 Loop 3

Saturation (I/Is) 0.98 2 3

Reminance (Ir) 0.98 0.7823 .056

Coercive field (mT) 0.1057 0.15241 0.197

Sample 2 Loop 1 Loop 2 Loop 3

Saturation (Is) 1.2833 1.2241 1.17822

Reminance (Ir) 0.0155 0 0.155



Saturation (Is) 1.47 1.37 1.37

Reminance (Ir) 0.08 0.0395 0.0181


Sample 4 Loop 1 Loop 2

Saturation (Is) 1.3064 1.282

Reminance (Ir) 0.03 0.0424

Coercive field (mT) 0.0185 0.0302


Saturation (Is)

Reminance (Ir)

Coercive field (mT)

Sample Loop 1 Loop 2 Loop 3

Saturation (Is)

Reminance (Ir)

Coercive field (mT)

Sample Loop 1 Loop 2 Loop 3

Saturation (Is)

Reminance (Ir)

Coercive field (mT)

Discussion:Figure 1 to 7 showing the hysteresis loop of samples which are mentioned in the table 1, Hysteresis loop is between normalized intensity and normalized field. In figure 1 to 7 different hysteresis loop of different color having different area. FeCoBSi is a soft ferromagnetic material therefore t small area under the curve in hysteresis loops in figure 1 to 5 observed.

In figure 1, due to magnetic anisotropy we have different hysteresis loop, black loop represents the easy axis where blue represents the hard axis. The hysteresis loops were obtained by rotating samples in different direction relative to the applied magnetic field in order to find the easy axis and hard axis (perpendicular to easy axis) for magnetization. Due magneto anisotropy in the material there is certain direction in the material where domains align easily means require less field to achieve saturation magnetization it is termed as an easy axis, where there are other plane (normally considered perpendicular to easy axis) which require high field to get saturation magnetization termed as hard axis. In FeCoBSi thin film an uniaxial anisotropy is found and it is found that it is due to the spin orbital interaction.

In figure 2, 3, 4 and 6 it is difficult to observe the easy and hard axis from hysteresis loop and also one can analyze easily that some loops are not symmetric, the reasons behind are sample surface was corroded, other reasons are due to the drift and applying magnetizing and demagnetizing several times to the samples.

In figure 5, easy and hard axis observed again due to the uni-axial anisotropy, green and black loop represent the easy axis whereas pink loop indicating the hard axis.

In figure 7, green loop represents the easy axis whereas blue loop represent the hard axis. Sample 6 and sample 7 are magneto-strictive materials, in this kind of material stress induce magnetic anisotropy is observed

Figures 8 t 13 indicate the domain images of samples. The better contrast of domain images in MOKE microscopy is due to oblique plain imaging, position of polarizer and analyzer and subtracting the images from background images. Figure 8 shows the domain structure of the sample 1 after the demagnetization along the easy axis and perpendicular to the easy axis respectively.

Figure 9 to 12 show the domain images of pattern sample where (1) and (2) indicate domain structure after demagnetization along the easy and hard axis whereas (3) and (4) shows the sensitivity of closure domain to horizontal and vertical. Furthermore high contrast in (1) and (2) as compared to the (3) and (4) because in (3) and (4) sample align in the hard axis whereas closure domain sensitive to the easy axis.

In figure 9 to 12 in (1) and (2), arrow indicating the alignment of magnetic domains this is due to the uniaxial anisotropy. Also in pattern sample beside uniaxial anisotropy there is also shape anisotropy, during the experiment it was found that high field value are required to completely align pattern sample as compare to the non-pattern sample (sample 1), moreover for small pattern more higher field required as compare to bigger pattern.

It is noted that in figure 9 to 12 in (1) and (2) that domains width are larger in (1) i.e. domain structure demagnetize along easy axis whereas domain widths are smaller in the (2) i.e. domain structure demagnetize along perpendicular to easy axis.

Calculated domain width can be seen in table 2 from figure 9 to 12 for pattern samples, sample 2 to 5.

Table 2: Calculated domain width

Number Domain width (µm) , Sample demagnetize along easy axis

Domain width (µm), Sample demagnetize along perpendicular to easy axis

Sample 2

9.51 6.72

Sample 3

6.21 4.72

Sample 4

12.53 6.92

Sample 5

4.52 2.27

Figure 6 and 7 represent the domain structure in magneto-strictive material in which domain due to the magnetic induced stress anisotropy. Sample 6 and 7 having the magnetostriction

constant of 38x10-6 and 11x10-6 . By observing the domain the in the material one can estimate the stress pattern in the material.

Conclusion:In Ferromagnetic material, due to magnetic anisotropy there are easy and hard axis, they have their characteristic hysteresis loop as well domain images. Beside magnetic anisotropy, there is also shape anisotropy in the pattern samples. In magnetostriction material, domain images observed due to the stress induce anisotropy.

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