Lab E

Moke & Kerr Microscopy

Group 9 members: AshKan Djaberi Dashtestani

Susan Tavakoli Farzaneh Vaghefi

Date Performed:

May, 11th, 2013

TA: Bhaskarjyoti Sarma

Advanced Materials

Lab Materials Science II

Introduction: Modern technical applications range from permanent magnets to high-frequency

transducers and magneto-optical thin films in hard disc drives. The material properties still

have to be improved to achieve higher storage density for example. Iron, cobalt and nickel

are the most often used elements, since they have a high itinerant magnetic moment. Rare

earth elements can improve themagnetic properties as well as non-ferromagnetic elements

such as aluminum or carbon. Crystal grain structure and grain boundaries determine the

magnetic properties. Therefore the microstructure of the solid is essential for controlling

wanted or unwanted magnetic domain boundary movement. For adequate material design

the important properties have to be determined in the lab. This can be done with a

“vibrating sample magnetometer” (VSM) or “Foner magnetometer”. The magnetic moment

of a sample against an external magnetic field or versus temperature can be measured with

this device.

Types of Magnetism:

Diamagnetism

Diamagnetism is the property of an object which causes it to create a magnetic field in

opposition to an externally applied magnetic field, thus causing a repulsive effect.

Specifically, an external magnetic field alters the orbital velocity of electrons around their

nuclei, thus changing the magnetic dipole moment. According to Lenz's law, this opposes

the external field. Negative susceptibility is the most important diamagnetic property which

can be measured. It is independent of the applied field and has the absolute value of:

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In all materials diamagnetism can be found, but sometimes other stronger types of

magnetism may superimpose it. So only if there are no other types of magnetism one

would call a material diamagnetic. Examples for such materials would be materials with

completely filled electron shells e.g. noble gases, noble metals, nonmetals.

Superconductors can even show a susceptibility of -1, therefore they are known as ideal

diamagnets.

Paramagnetism

Paramagnetism is a form of magnetism that occurs only in the presence of an externally applied magnetic field. Paramagnetic materials are attracted to magnetic fields and hence have a relative magnetic permeability of ≥1 (a positive magnetic susceptibility). The magnetic moment induced by the applied field is linear in the field strength and rather weak. It typically requires a sensitive analytical balance to detect the effect and modern measurements on paramagnetic materials are often conducted with a SQUID magnetometer.

If the spin is not equal zero and there are already atomic dipole moments existing which can be oriented along an external field, then this field is enhanced. The susceptibility is therefore larger than zero and the phenomenon is called paramagnetism. It depends on

temperature and the magnetic field strength. Usually is in the range of to .

The Curie-Weiß law holds for such magnets:

The paramagnetic Curie temperature defines the temperature below which is not possible anymore to align the dipole moments.

Collective Magnetism

It has three important classes and the function of field and temperature describes the susceptibility in the case of collective magnetism.

Ferromagnetism Ferromagnets will tend to stay magnetized to some extent after being subjected to an external magnetic field. This tendency to "remember their magnetic history" is called hysteresis. The fraction of the saturation magnetization which is retained when the driving field is removed is called the remanence of the material, and is an important factor in permanent magnets..

Ferrimagnetism Solid is made up of two different ferromagnetic sublattices with different magnetizations. Below

Tc a non-vanishing magnetization can be measured

Antiferromagnetism the magnetic moments of atoms or molecules, usually related to the spins of electrons, align in

a regular pattern with neighboring spins (on different sublattices) pointing in opposite

directions. This is, like ferromagnetism and ferrimagnetism, a manifestation of ordered

magnetism. Generally, antiferromagnetic order may exist at sufficiently low temperatures,

vanishing at and above a certain temperature, the Néel temperature (named after Louis Néel,

who had first identified this type of magnetic ordering). Above the Néel temperature, the

material is typically paramagnetic.

Hysteresis curves M describes the induced magnetic behavior without application of an external magnetic field.

For a macroscopic description of the magnetic properties, the M-H diagram (hysteresis loop) is

used. Important parameters are:

: Saturation magnetization. It is the maximum a material can contribute to the magnetic

induction B. For the temperature above Tc it vanishes, so the hysteresis loop becomes smaller.

The value is temperature independent.

: Coercive field. With this important parameter you can distinguish between hard and soft

magnetic materials. ⁄ means it is easy to change the direction of magnetization,

thus it is a soft magnetic material. A sample with ⁄ would be called a hard magnetic

material.

: Remanence. This is the amount of magnetization the sample keeps after switching off the

applied field.

Fig.1: Hysteresis loop

Experimental Setups

Vibrating Sample Magnetometer A vibrating sample magnetometer (VSM) operation is based on that a changing magnetic field

will produce an electric field. This electric field can be measured and can tell us information

about the changing magnetic field. A VSM is used to measure the magnetic behavior of

magnetic materials. This picture shows diagram of a Vibrating Sample Magnetometer (VSM).

The sample to be studied is kept in a constant magnetic field. If the sample is magnetic, this

constant magnetic field will magnetize the sample by aligning the magnetic domains, or the

individual magnetic spins, with the field. The stronger the constant field, the larger the

magnetization will be. The magnetic dipole moment of the sample will create a magnetic field

around the sample, sometimes called the magnetic stray field. As the sample is moved up and

down, this magnetic stray field is changing as a function of time and can be sensed by a set of

pick-up coils. The alternating magnetic field will cause an electric current in the pick-up coils.

This current will be proportional to the magnetization of the sample. The greater the

magnetization applied, the greater the current induced.

Using controlling and monitoring software, the system can tell us how much the sample is

magnetized and how its magnetization depends on the strength of the constant magnetic field.

Fig. 2: Scheme of a VSM

The magneto-optical effect and Kerr-microscopy The magneto-optical Kerr effect is a well-established technique to study magnetization

properties. The effect is based on the fact, that the plane of polarization of light is rotated when

the light is reflected from a magnetic material. This Fig.3 is a schematic representation of the

Kerr effect which occurs when polarized light is reflected from a magnetic material while a

magnetic field is acting. In this case, the plane of polarization of the incident light rotates

proportionally to the intensity of the magnetic field. The Kerr rotation is an experimental

evidence of the interaction between light and a magnetic field.

Fig. 3: Magneto-Optical Kerr Effect

Frequently a computer is connected to the microscope to do the imaging.

Results and Discussion:

Kerr-microscopy (Co-sample)

The sample was first demagnetized by applying an AC field. As can be seen from Fig.4, there are

two domains which can be diagnosed as brighter and darker regions which are probably

pointing in opposite directions.

Fig 4.Sample after demagnetization

Fig 5.Co Sample, easy axis in magnetic field

Fig 6.Co Sample, hard axis in magnetic field

The white domain contribution became bigger after changing the polarity. Also, the black domain contribution has direct relation by field strength. To investigate the properties of the hard axis, the sample was rotated by 90° and again demagnetized. For the domains it is not easy to grow along this axis and they are visible as thin black lines. Magnetic field strength had to be higher to be much higher to reach saturation magnetization.

Fig 7.Fe-Ni is a soft material. After applying DC field

Fig 8. Easy axis is shown after applying magnetic field

Fig 9. Hard axis is shown after applying magnetic field

It is clear that in both pictures, it is hard to create domain in hard and easy axis.

-0.0025

-0.002

-0.0015

-0.001

-0.0005

0

0.0005

0.001

0.0015

0.002

0.0025

-150 -100 -50 0 50 100 150

Mag

ne

tiza

tio

(em

u)

Magnetic field (H)

-0.0025

-0.002

-0.0015

-0.001

-0.0005

0

0.0005

0.001

0.0015

0.002

0.0025

-150 -100 -50 0 50 100 150

Mag

ne

tiza

tin

(em

u)

Magnetic field(H)

VSM

The experiment is done on A Fe-Ni film. The data for the three obtained hysteresis loops was

processed by a PC. The sample was cooled throughout the measurement, to avoid melting.

Fig 10. Hysteresis loop along the easy axis

Fig .10 shows an in-plane measurement of the easy axis of the thin film.

Then, we Rotate the plan measurement of the sample by 90° and plotted the hysteresis loop

which shown in Fig11:

Fig 11. Hysteresis loop along the hard axis

In order to calculate the thickness we use the below formula:

(1) , V=A .t (2)

Where is t is thickness, is magnetic saturation and A is area:

From (1) and (2)

⁄

We know that and A=25 and : t= 21.8 nm