The work reported here mainly emphasis on magnetostriction and magnetisation properties of NiFe/FeCo multilayer thin films. Ni81Fe19 exhibit low coercivity and near zero magnetostriction while Fe50Co50 exhibit high coercivity and high magnetostriction. In simple words this project aims to study the change in properties of FeCo by changing the thickness of NiFe underlayer. A thin film with low coercivity and high magnetostriction constant has potential applications in the field of strain sensors. There are three stages involved in this project where in the first stage a number of multilayer thin films are fabricated using different techniques like RF sputtering, DC sputtering and thermal evaporation. Second stage involves characterization of thin films by MOKE, Villari method, XRD and MFM. Third stage involves of analysis of data obtained and various thin films are grown. These three stages are successfully completed and expected results are obtained which is report is presented here. In simple words this project aims to study the change in properties of FeCo by changing the thickness of NiFe underlayer. Thin films studied during the project showed some good results with low coercivity and high magnetostriction which has applications in the field of strain sensors.
Section -I
Introduction.
The first accounts of Magnetism date back to the ancient Greeks who also gave magnetism its name. Magnetic compasses have been used from thousands of years; however the potential of magnetic materials is properly described in 20th century which laid foundation to modern technology. The applications of magnetic materials are emerging and became indispensable in our daily lives and industries.
Hard and soft magnetic materials are crucial for energy conversion, particularly for converting electric energy to mechanical energy, which is important to meet the challenges of the climate change, depletion of fossil fuels and global warming. The application of magnetic materials in information technology is increasing rapidly. Development in Nanoscience and technology brought revolutionary progress in processing and characterizing of materials. At the nano scale ferromagnetic thin films are often incorporated into multilayered structures, which have been found to have magnetic and transport properties different than realized in bulk magnetic materials.
They are mainly three categories of magnetic materials namely paramagnetic, diamagnetic and ferromagnetic based on orientation of magnetic moments. Diamagnetic materials have no magnetic moments and no magnetisation in zero applied field, however a small negative moment is induced when field is applied. Paramagnetic materials have magnetic moments randomly oriented throughout the sample giving zero magnetisation. When field is applied magnetic moments align in the direction of magnetic field and net magnetisation increases with increase in field as the moments become more ordered. The magnetic moments become disordered when the field is removed. Like paramagnetic materials, ferromagnetic materials also have randomly ordered magnetic moments. When field is applied magnetic moments align parallel to the field and also parallel to each other in the direction of magnetic field to maintain a lowered energy state. The moments are aligned parallel even when the field is removed.
The work represented here is based on ferromagnetic materials Nickel (Ni), Iron (Fe) and Cobalt (Co). Properties of ferromagnetic materials is determined from hysteresis loop, hysteresis loop is obtained when a ferromagnetic material is placed in a field which increases from zero to some peak value, decreases to equal and opposite value through zero and then return again to original peak value. A hysteresis loop is represented in the Fig 1.1 ,
Fig 1.1 Hysteresis loop [1]
where the saturation magnetization (MS) represents the maximum value in the direction of applied magnetic field that the magnetic dipole moment per unit volume can take. All the magnetic moments are aligned in the direction of field in this state. The remanent magnetization (Mr) represents the remaining magnetization in the sample after the field is reduced to zero from saturation. The coercivity (Hc) is the field required to reduce the magnetization to zero from saturation.[1]
There are two types of magnetic materials namely soft magnetic materials and hard magnetic materials. Soft magnetic materials are saturated in low fields, but hard magnetic materials need high field to fully magnetise.
Magnetic Interaction
Magnetic anisotropy
The property of being directional dependent is called anisotropy, when internal energy of the system is dependent on the direction of the spontaneous magnetisation then it is called magnetic anisotropy. Two types of magnetic anisotropy is found in ferromagnetic materials, magnetocrystalline anisotropy which is related to the crystal symmetry of the material and magnetostrictive anisotropy which is related to mechanical stress.
Magnetocrystalline anisotropy
Interface and volume anisotropy
In multilayers the magnetic anisotropy energy is split in to two components namely volume contribution and interface contribution, the relation between these is given as follows
Magnetocrystalline anisotropy in bulk systems in dominated by volume term in bulk systems, however in multilayer thin films surface term is becomes more significant.
Single ion anisotropy
The interaction between the orbital state of a magnetic ion and the surrounding crystalline field determines the single ion anisotropy, which is often refered as magnetocrystalline anisotropy. This is present throughout the layer volume of the magnetic layer, and contributes to the volume anisotropy term; however addition or subtraction of this term depends upon the crystal orientation. It also contributes to the interface anisotropy; this is due to the reduced symmetry at the interface, however addition or subtraction of this term depends upon the crystal properties.
Shape anisotropy
Dipolar interactions determine the shape anisotropy of magnetic dipolar anisotropy. This is depending on the shape of the sample. This is very important in thin films as it produces in plane magnetization
In a thin film the dipolar anisotropy energy per unit volume is given by
E=1/2
Where ms is the saturation magnetisation which is uniform throughout the layer, angle
Magnetic domains
Ferromagnetic material have regions called magnetic domains, within which the direction of magnetization is largely uniform . the domain size and the orientation of the magnetic moments in domain regions are determined by the magnetoelastic, magneto static, local anisotropy and domain wall energy. Domains are formed in order to reduce the magnetostatic energy, which in turn reduces the internal energy of the system. Exchange energy tends to keep adjacent magnetic moments parallel to each other.
Internal energy of the system is also minimized by the exchange energy and local anisotropy energy by aligning the magnetic moments either parallel to each other or in the local easy axis.
Domain walls
The formation of domain walls is influenced by the thickness of the film. in general the domains walls are aligned as shown in fig, which is called bloch wall, where the magnetisation within the system rotates 180 and can inplane or out of plane.
As the film thickness decreases say less than 100nm then different type of domain walls are formed called neel wall. As the thickness of the film decreases domain wall transition occurs from bloch to neel to decrease magnetostatic energy, however exchange energy is increased. Complications can arise due to defects such as crystallographic defects, inclusion of impurities with large internal stresses which can lead to change in orientation if magnetic moments.
Ferromagnetic-ferromagnetic coupling
Pinned domain walls exists near the interface at which the magnetization is pinned in a direction different from the easy axis of the material, this transistion thickness is know as exchange length.
Magnetostriction
In 1942 james joule found that there is change in physical dimensions of a ferromagnetic materials when a magnetic field is applied along thee direction of magnetisation. This phenomenon is known as either joule effect, magnetostriction or magnetoelastic. This occurs due to the internal strain produced in the material, which converts magnetic filed into kinetic response. The converse of this effect i called villari effect or stress induced naisortopy
Magneoelastic energy
The energy accumulated due to dipole-dipole interactionsin a ferromagnetic is defined as magnetoelastic energy.
Multilayers and thinfilms
Strain in thin films and multilayers can be produced by the growth conditions, such as lattice mismatch between layers or thermal stress caused by differences in thermal expansion coefficients of adjacen
Project road map
This work involves three stages where in the first stage a number of thin films are fabricated using different fabrication techniques like rf sputtering, dc sputtering and thermal evaporation. In the second stage the thin films grown are characterised by MOKE magnetometer, XRD and MFM. In the final stage data obtained from the characterization stage is analysed and more films are grown according the results obtained. This process is shown in the form of block diagram in the below diagram.
Project plan
Fabrication of Ni81Fe19 and Fe50Co50 multilayer thin films using different techniques
Transverse MOKE magnetometer is used to investigate properties of thin films
Magnetostriction constant (λs) of thin films is measured by using villari model.
By using XRD and MFM
Evaluating the results from the above steps
Adding NiFe layers or varying the thickness of the layer
Results and discussion
Applications
Thin films with low coercivitry and high magnetostriction have potential applications in the field of strain sensors. Which is mems applications, one of such application is discussed below.
consider are based on polycrystalline Fe-Co. The alloy
series has been studied in bulk form for many years [20] with
a bcc solid solution existing across the composition range
of 0%-60% Co. The saturation magnetostriction constant
peaks at the disordered equi-atomic composition, with
a value of 150 ppm. Taking published data for the saturation
magnetostriction constants of Fe Co [20], [21], an isotropic polycrystal should show a net magnetostriction constant of approximately
80 ppm. We have demonstrated [22] that this level
of magnetostriction can be achieved in polycrystalline 300 nm
thick films on glass substrates, but at the expense of coercivities
greater than 500 A/m even after annealing. The anisotropy field
was 3800 A/m. Work continues to explore routes
to produce highly textured FeCo films, using appropriate seed layers, in order to achieve higher net magnetostriction constants
for the films.
Write about shoms project..a brief discussion...i hope i had an paper
Literature review
Properties of NiFe
NiFe is known as permalloy. Permalloy exhibit high magnetic permeability, low coercivity near zero magnetostriction and significant anisotropic magentoresistance. There are three compositions of alloys. Of these 78% percent nickel and 22% iron has near zero magnetostriction and magnetocrystalline anisotropy and also high initial permeability. Nickel 65% exhibits a strong response to field annealing while maintaining k1~0, 50% nickel with remaining iron has higher flux density as well as their responsiveness. The crystal structure of permalloy is generally face centered cubic structure. For alloys with 30% Ni the structure is Body centered cubic structure. Permalloys with Curie temperature more than 400o respond very well to magnetic field. There are four anisotropies that can occur in NiFe, they are magnetocrystalline anisotropy, magnetoelastic anisotropy, thermomagnetic anisotropy and slip induced anisotropy. All these can be avoided by annealing and by seeking k1 and λ=0.[1]
Properties of FeCo
FeCo is a soft magnetic material, it is known as permendur which means 50% of Cobalt and 50% of Iron. These exhibit very high saturation induction, low magnetic anisotropy, large magnetic permeability and high Curie temperature. An equiatomic FeCo shows order-disorder transformations. These transformations occur at 730o. Properties of ordered and disordered FeCo are different. FeCo at above 950o shows face centered cubic structure; below 850o it shows body centered cubic structure. Below 730o it forms ordering, which exhibits high coercivity. [9]
Permalloys and permendurs are used as components of the sense layer spin-valve read heads. Coercivity and magnetic anisotropy of NiFe decreases with decrease in thickness regardless of the film structure when it is sandwiched with Ta. In FeCo films they exhibit an increase in coercivity and magnetic anisotropy with decrease in thickness below 100 Ao, here the magnetic anisotropy appears to be magnetoelastic in nature [2]. Soft magnetic properties of FeCo layer are best at the thickness of ~100nm. As FeCo has large magnetostriction it is difficult to achieve soft magnetic properties. When these films are sandwiched with NiFe, they show soft magnetic properties at 2.4T of saturation magnetisation. [3] FeCo films generally shows high coercivity values over 50 Oe with no distant uniaxial anisotropy. Coercivity can be reduced by proper nitridation or deposition on various under-layers. An appropriate under-layer can improve the soft magnetic softness of FeCo films. Increase in film thickness increases the coercivity due to increase of grain size. [4] FeCo alloy shows large linear magnetostriction constant. The coercivity of FeCo alloy along the hard axis decrease as the decrease in Ar pressure during the sputtering deposition, and also shows that it exhibits low coercivity at a thickness of 50-100nm [5] The high coercivity of FeCo layer can be decreased from 140 to 12 Oe when FeCo is grown on CoO, it is due to the decrease in grain size from 20-35 to 5-15nm. [6]
These FeCon and feco alloys shows a large saturation magnetostriction constant in the range of (40-65) ppm and a relatively high anisotropy constant k1 of ̃̃10kj/m-3, which make it difficult to achieve good magnetic softness in the feco and fecon alloys
A coercivity of 960 A/m s12 Oed was
achieved in a 30-nm-thick FeCo50 at. % alloy film seeded
with a thin CoO layer.3 The soft magnetism in the CoOseeded
FeCo film was believed to be due to the fine grain
size induced by the CoO seedlayer.3 A coercivity of
400 A/m s5 Oed was achieved in the 100-nm-thick FeCoN
swith ,30 at. % Cod films.4 A very thin ,2.5-nm-thick Permalloy
alloy film was used as a seedlayer for the FeCoN film
with a thickness of 100 nm, which resulted in a low coercivity
of less than 80 A/m s1 Oed.4,5 Similar magnetic softness
was achieved in 50-nm-thick FeCo35 at. % alloy films
seeded with a thin layer of NiFe, Cu, and Ru, which showed
a low coercivity of 80-240 A/m s1-3 Oed in the hard
axis;6,7
The mean grain sizes of the
NiFe-, Cu-, and Ru-seeded 30-nm-thick FeCo35 at. % films
were found to be in the range of 10 nm, which was in contrast
to the mean grain size of ,50 nm in the Ta-seeded
FeCo film that did not show good soft magnetic properties.
The magnetic softness in the FeCo films on NiFe, Cu, and
Ru was attributed to the fine grain sizes induced by the seedlayers.6,7 A low coercivity of 720 A/m s9 Oed was reported
in a single-layer FeCo film, which was also attributed
to a small mean grain size of 7.2 nm.11
The mean grain size usually increases with the film
thickness, which often causes deteriorated soft magnetic
properties, as shown in the FeCo50 at. % films.2 Stripe domain
forms when the film thickness reaches a certain limit in
a soft magnetic film which degrades the magnetic
softness.12,13 Most of the published data on soft magnetic
FeCo and FeCoN films showed a thickness of
30-100 nm.2-8
Ref code 1
These properties include low coercivity, low
crystalline anisotropy, large magnetization, small magnetostricti
on, and relatively large magnetoresistive coefficient.
The thickness of Permalloy film, in most applications,
varies from several nanometers to several micrometers.
However, the shear stress at the substrate and film interface
increases as the film thickness increases.4
•
5 When a critical
thickness is reached, the shear stress is greater than the yield
stress at the interface causing the film to lose adhesion. Films
having compressive stress buckle and films having tensile
stress shrink or crack, therefore, the in-plane stress of the
film has to be controlled very carefully to ensure the film's
integrity and reliability.
The variation of stress with film thickness has been observed
in man.y metallic films, and can be attributed to the
morphology difference at different film thickness. Several
different mechanisms for the origin of the thin film's intrinsic
stress have been reported.2
!,22 These mechanisms include
lattice expansion, surface tension, grain size coalescence, effect
of the annealing of the grain boundary, defects, and impurity
incorporation. During the film growth, one or several
of these mechanisms can become dominant at a given thickness.
Ref code 2
A slight lattice deformation in FeCo alloy films yields significantly
large magnetostrictive energy of grains because of large
linear magnetostriction constants, resulting in degraded magnetic
softness [3]. Therefore, the appearance of soft magnetic
properties in the FeCo/NiFe(Cr) films is probably related to the
reduction of lattice deformation besides the reduction of grain
size [3], [4]. Moreover, it is likely that a high degree of preferred
grain orientation, which is often achieved by the use of seed
layers, plays an important role to derive soft magnetic properties
in FeCo films [5], which may lead to the appearance of softness
in FeCo/NiFe(Cr) films.
It is likely that less lattice deformation, besides relatively
small grain size and highly preferred grain orientation, is a key
to derive soft magnetic properties in the FeCo alloy films, and
the hetero-epitaxial growth of FeCo(110) plane on NiFe(111)
plane results in a high stability and reproducibility of the soft
magnetic properties of FeCo films.
Ref code 3
The monotonous change of ls with decreasing tmag can
be attributed to the dominance of interface magnetostriction
at smaller film thicknesses.3 While Nee´l's two-component
description3 of magnetostriction, ls5lb1li /(tmag2t0), is
valid over a relatively large thickness range for NiFe films, it
is only effective over a limited thickness range for CoFe
films. As shown in Fig. 4, the linearity of ls vs 1/(tmag
2t0) curve exists over the thickness range of 35Å,tNiFe
<200Å for NiFe films and of 80Å,tCoFe<200Å for CoFe
films. The bulk components, lb , and the interface component,
li , have been estimated by fitting the data to the twocomponent
expression. The results are listed in Table I. lb
ranges from 21.7831026 to 21.9931026 for NiFe films
and from 23.5831026 to 25.56331026 for CoFe films.
The interfacial contribution, li , of CoFe films is almost ten
times larger than that of NiFe films. Although there exist
significant differences in the thickness dependence of ls
among the three series of NiFe or CoFe films, the values of
ls are typically in the 1026 range.
Ref code 4
It was found
that the Permalloy underlayer, rather than the cap layer, is
more important in achieving the low coercive fields in Fe-
Co-N films.3 The magnetic softness of polycrystalline thin films is
closely related to the structural and compositional characteristics
in the thin films, such as grain size,5 crystallographic
texture,6 and strain and stress state.7 These characteristics
can also affect the other properties such as anisotropy and
magnetorestiction, which are intimately correlated with the
magnetic softness
Another possible cause for the magnetic softness may be
related to the magnetoelastic anisotropy that is determined
by the stress state and saturation magnetostriction in the film.
The saturation magnetostriction constants of the Fe-Co-N
films with different Permalloy underlayer thicknesses drop
slightly with the increase of Permalloy underlayer thickness,
as shown in Fig. 4.
Ref code 5
metallic thin-film multilayered structures. This results both
from magnetotransport effects that the materials show @e.g.,
giant magnetoresistance ~GMR! ~Ref. 1!#, and from their
widespread applications in the data storage industry
This yields an interface width ~defined as
10%-90% of the nominal layer compositions! of ;1.2 nm.
By comparison, the interface produced when CoFe is grown
on NiFe, shown on the left side of the Fig. 2 profile, is found
to be more diffuse, with a measured width of ;1.8 nm. Averaging
the measurements over profiles taken from three different
areas across the interfaces yields widths of 1.1
60.2 nm for NiFe grown on CoFe and 1.760.2 nm for CoFe
grown on NiFe.
Ref code 6
Previous work has studied the effect of underlayers
[3,4], the substrate material [5] and the rate of deposition
[6] on the magnetic properties of Fe1-xCox films (where x
ranges from 35 to 65). Jung et al studied 50 nm Fe65Co35
films grown on glass substrates and on thin Cu underlayers
[4]. They found that the Cu underlayer induced a uniaxial
anisotropy into the films compared to the isotropic films
grown just on glass. The Cu underlayer also reduced the
coercive field and the stress in the films. The
magnetostriction remained around ~ 50 ppm, although the
structure changed from <200> texture with no Cu layer to
<110> texture with the Cu underlayer. Jung et al also
studied other metallic underlayers [3], and found that
NiFe, Ru and Ta/NiFe layers also induced uniaxial
anisotropy in the FeCo films, while reducing the film
stress. They determined this was due to the underlayer
changing the texture of the film from <200> on glass to
<110>.
with Cu radiation [12]. It is observed that for all the films
the <110> peak has its centroid above 45o. This may be
due to stresses in the films or the sample height
displacement. Assuming that the shift in the peak was due
to stresses in the films, the lattice constants were also
determined (Fig. 3b). For all the films the lattice constants
were smaller than the bulk value. Taking a Young's
modulus for a 2μm thick FeCo film on Si as Y = 165 GPa
[13], and the average strain given by the change in lattice
parameter depicted in Fig. 3b, gives a compressive stress
of σ = 1.4GPa on the film. This is not an unreasonable
value as taking,
3
2 net Kσ = λ σ (4)
with λnet = 30 ppm, yields a stress anisotropy constant of
Kσ ~ 6.3x103 J.m-3. This in turn yields an anisotropy field
0
2
K
s
H K
M
σ
μ
= (5)
of 53 kAm-1 using a saturation induction (μ0Ms) of 2.4 T,
which is of the order of the measured values (60 nm DC
non-rotated film). The RF non-rotated and 600K DC
rotated films had lattice constants closest to the bulk value
suggesting a lower film stress than in other films. It is
these films that most closely follow equation (1). This may
imply that the surface/interface magnetostriction does not
dominate in films having high stress. Further investigation
is required, starting with a simple post-deposition anneal
of these films to reduce the residual stress.
Ref code 7
FeCo films typically
show in-plane nearly isotropic square hysteresis loops with
coercivity Hc.80 Oe @Fig. 1~a!#.2-4 The saturation magnetostriction
is also very large1 with values of the constant ls
from 45- 6531026 which in polycrystalline films if stresses
are large, can result in large nonuniform local magnetostrictive
anisotropies and potentially high Hc . However, Platt
et al.3 reported that a significant reduction in Hc from 140 to
12 Oe was observed when Fe50Co50 was grown on CoO.
Based on microstructural and other data, they concluded that
the cause was a reduction in grain size from 20-35 to 5-15
nm. Wang et al.,5 also reported that similar results could be
obtained if (Fe70Co30)N films were grown on NiFe. Later,
they6 proposed that increased exchange coupling between the
(Fe70Co30)N grains mediated by the NiFe underlayer caused
the coercivity reduction
We demonstrate that the primary effect of the Cu, NiFe,
and Ru underlayers is to reduce grain size in FeCo, which
causes a reduction in Hc quantitatively consistent with ripple
theory.
Ref code 8
Fabrication techniques
Thin films are grown using different techniques like sputtering and evaporation
Substrate
Silicon substrate is used throughout the project. Substrate has substantial effects in the change in properties of the film. Exchange coupling acts at the interface
Silicon
Thickness
380±50 µm
Youngs modulus
170Gpa
Poisson ratio
0.27
orientation
<100>
RF Sputtering:
Nordiko NM2000 sputtering system is used to sputter deposit the NiFe and FeCo thin films by radio frequency (RF) magnetron sputtering. This process includes formation of plasma by bombarding the target with inert gas ions, which causes ejection of atoms from the target material. The atoms that are ejected from the target are made to sputter on the substrate, which in turn forms the thin film. Sputtering of atoms on to the substrate depends on the sputtering parameters i.e., pressure and power. Sputter-up mode is used i.e., substrate is placed above the target, which are separated by a distance of 6cm. Three target electrodes can be used at a time to sputter multi-layered films in Nordiko NM2000. Growth rate of thin films is high as magnetron source is used, and mainly depends on the sputtering parameters namely sputtering power and pressure. Growth rate can be increased by increasing the sputtering power, which in turn increases the temperature. Growth rate can also be increased by increasing the pressure at low pressures. [1]
The properties of the resulting thin films are sensitive to sputtering power, temperature and pressure. Two types of substrates are used namely silicon-based and glass-based substrates, which are cleaned by rinsing in acetone and isopropanol. Clean-room gloves and non-magnetic tweezers are used to handle the substrates. The schematic diagram of sputtering system is shown in fig [1].
The composition of iron (Fe) in NiFe target is 19% and 50% in FeCo target. In order to investigate the properties of NiFe and FeCo thin films a range of films are grown with different thicknesses. This project mainly aims at the properties of NiFe and FeCo multilayered thin films, but during the course of training a wide range of monolayer films are grown. Five thin films are grown of which four are monolayers and one multilayer, which are grown on silicon substrate. These are grown based on the previously calibrated values of NiFe and FeCo, as shown in Table 1.
Table 1 Calibrated values of NiFe and FeCo thin films
Sample
Ar Pressure (mTorr)
Base pressure(Torr)
Forward power(W)
Time(min)
Thickness(nm)
NiFe
4.5
6.5 X 10-6
50
20
75
FeCo
4.5
6.5 X 10-6
100
60
38
Fig [1] Schematic representation of Nordiko NM 2000 [1]
The table below shows the samples and the growing conditions at which the thin films are grown. Thickness of the sample depends on the sputtering time and sputtering rate. Table 2: Growth parameters of the thin films
Sample
Ar Pressure mTorr
Base Pressure (Torr)
Forward power (W)
Reverse power (W)
Time (min)
Thickness (nm)
NiFe
4.5
6.5 x 10-6
50
2
20
75 ± 4
FeCo
4.5
6.5 x 10-6
100
2
20
13 ±1
NiFe
4.5
6.5 x 10-6
25
0
3
~ 5
NiFe/FeCo
4.5
6.5 x 10-6
25/100
0
35
5/20 ±2
FeCo
4.5
6.5 x 10-6
100
0
30
20 ± 2
Dc sputtering
Base pressure 8e-8
Voltage 45W
Time 4 min
Growth pressure 7e-3 mbarr
Temperature room temperature
Thicknesss 15nm
Thermal evaporater
All of the structures studied here were patterned from thin films deposited using a Wordentec thermal evaporator, shown schematically in fig. 3.4. The deposition material was placed in an alumina covered tungsten-wound crucible and the evaporator was evacuated to a base pressure of ~ 3-10-7 mbar. An electric current was passed through the crucible to heat the material and initiate evaporation. Evaporated material migrated towards the sample substrates, fixed to an indexed substrate holder using a 4:21 PMMA/anisole mixture, and an Edwards FTM5 film thickness rate monitor. A mask in front of the substrate holder enabled up to five different depositions and also allowed the deposition rate to be monitored without depositing material onto a substrate. The rate monitor measured the deposition rate and the deposited thickness with a resolution of 1 Å/s and 1 Å, respectively. Once the desired thickness had been deposited, a shutter was used to block evaporated material reaching the substrates. After deposition, the sample thickness was confirmed using an atomic force microscope
Characterisation techniques
MOKE:
MOKE (Magneto Optic Kerr Effect) magnetometer is used to study the properties of thin films that were grown. Optical anisotropy is observed in magnetic materials when external magnetic field is applied, this is due to the magnetisation of surface domains. Magneto optic effects will arise due to the presence of optical anisotropy, which is also known as magneto-optic kerr effect. This effect is used to obtain hysteresis loops of the thin films.
He-Ne laser is used with wavelength (λ=633nm) which is made to incident on the sample at an angle of 450. Before the light is made to incident on the sample, light is p-polarized by a Glan Taylor polariser and passed through a lens of focal length 30cm. The reflected light from the thin film is made to pass through analyser onto a photo- detector. The intensity of the light was measured by the photo-detector and by using a computer based program a hysteresis loop is obtained. The sample holder can be rotated freely in the plane of the magnetic field through 360o. [1]
For each thin film data is obtained by rotating the sample holder from 0o to 180o at an interval of 30o. For all the graphs 256 data points are taken with a shape factor of 0.4 and analyser angle 30o degrees. To get the accurate data three averages are taken. As the experiment is conducted in open environment, noise is included in the data. Hence drift and symmetry of the loop should be corrected; this can be done by normalising the data. This includes a series of steps to be performed using Microsoft excel program. The data obtained is utilised to determine coercivity, remanence and saturation point of each thin film.
By externally straining the thin film we can determine the magnetostriction of the film. These films are strained under bending radii of 300mm, 400mm and 500mm, and are investigated by using MOKE. Eight average readings are taken with 256 data points and shape factor 0.4. Data is then normalized in order to reduce the noise. Magnetostriction can be calculated from the formula [2]
Hk is the anisotropy field, τ is thickness of the substrate, Y is young's modulus of the substrate, R is the bend radius, v is Poisson ratio of the substrate.
Fig 2 , MOKE magnetometer [2]
Magnetostriction constant
The magnetostriction constant is determined by Villari method. It involves straining of films i.e., bending the films over five known radii and measuring the magnetisation loop. It is given by
Hk is the anisotropy field, τ is thickness, Y is young's modulus, R is the bend radius, v is Poisson ratio. [12]
XRD
We use XRD (X-Ray Diffraction) to determine the grain size and change in lattice constant of the thin film. It characterises the texture of the films. The average grain size in the films is determined using Scherrer equation [12].
Siemens D5000 xray diffractrometer is used to determine the grain size and lattice constant in thin films. These are observed in the range of 30-80 degrees with an increment of 0.2 per minute.
MFM
A magnetic force microscope (MFM) is essentially the same as an atomic force
microscope (AFM). However, the scanning probe is coated with a layer of magnetic
material, which may be magnetically hard (e.g., CoCr) or magnetically soft (amorphous
ferromagnetic alloy; e.g., FeBSiC); a tip is shown in Figure 4.24. It is usual to take a line
scan in contact mode to give the topographic information, and then rescan the line at a
fixed flying height of a few tens of nanometres to obtain the magnetic contrast. In
contact mode van derWaals forces, which have a much shorter range than the magnetic
forces, dominate and only topography is seen. At the flying height, the longer-range
Figure 4.24
magnetic interactions between the tip dipole moment and the stray field from the sample
are detected (Figure 4.25).
An oscillating cantilever (typically 70 kHz) would typically be used for high sensitivity.
The tip is sensitive to the force gradient @Fz/@z, which modifies the effective
cantilever spring constant keff: this produces a change in angular frequency
!0 ¼
ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
keff _ @Fz=@z
me
s
°4:36Þ
where me is the mass of the cantilever. If, as is usual, @Fz/@z _ keff then we can write:
!
0 ¼ !0 1 _ @Fz=@z
2keff
_ _
so _! ¼ !
0 _ !0 _ _ !0
2keff
@Fz
@z
: °4:37Þ
For an attractive force there is a decrease in resonance frequency, and vice versa; this
change is used to generate the contrast. As an example, Figure 4.26 shows data from a
