Chapter 1
Introduction
1.1
A Brief History of Fe-N System
Nitridation of 3d transition metals is a well-known process for improving their tribological properties. Nitriding process was first developed in the early 1900s, with a
motivation that metal nitrides have high melting point that would provide excellent
temperature stability, hardness and improved corrosion resistance properties that
have tremendous technological applications. Preliminary, nitridiation of metal was
performed by passing ammonia gas on heated metal melts, in this process the gas
gets dissociated to form metal nitrides. Following this process nitridation of a number of metals such as Ti, Cr, V, Mo, etc. was done. Such metal nitrides have been
utilized for various industrial applications such as in gears, crankshafts, camshafts,
valve parts etc. In spite of tremendous applications, magnetic metal nitrides, especially iron nitride (Fe-N) was rarely studied until 1920s. Between 1920-1930 first
works on Fe-N system were reported [1; 2]. During this period different Fe-N phases
such as α-Fe(N), γ 0 -Fe4 N, ε − Fe3−z N, and, ζ − Fe2 N were identified. Around 1950s,
Jack has revived the Fe-N system and discovered a new phase of Fe-N compound
which is denoted as α00 −Fe16 N2 [3]. Moreover, using X-ray diffraction, a systematic
study on different Fe-N phases was performed and a first phase diagram of this
system has been proposed which is shown in figure 1.1(a) [3; 4].
Until 1970s, the main area of research interest in these compounds is to investigate their tribological and crystallographical properties. During this period personal
computers had arrived at our door. With the advent of computer technology, FeN compounds were investigated from the prospect of their possible applications in
magnetic memory devices. In 1972, Kim and Takahashi observed a giant magnetic moment in α00 −Fe16 N2 phase [5], after this discovery, research interest in this
1
1. Introduction
compound grew immensely. During 1980s, the phase diagram of Fe-N system is
supplemented with magnetic transitions observed in these compounds as shown in
figure 1.1(b) [6]. Between 1985-1998, a number of researchers have individually proposed the existence of a nitrogen rich phase with equi-atomic concentration of Fe
and N, which was later named as iron mononitride (FeN) [7; 8]. In year 2002, Du
et al. proposed a revised phase diagram of Fe-N system as shown in figure 1.1(c) [9].
Recently in 2011, Kardonina et al. compiled a data set available on Fe-N system
presenting the current state of the Fe-N phase diagram [10].
1.2
Preparation of Fe-N Compounds
Fe-N compounds can be prepared by various physical and chemical methods. A
general preparation route that has been employed to prepared Fe-N compounds in
bulk is to heat the iron powder/foil in presence of nitrogen or ammonia gas. However,
with this method the maximum solubility limit of nitrogen in iron is limited to about
33 at.%. Considering the potential application of Fe-N compounds in electronic
devices, growth of these compounds in the thin films form is quite necessary. In this
direction, Fe-N compounds were grown using different physical vapor deposition
techniques such as e-beam evaporation (eBE), pulsed laser deposition (PLD), and
reactive sputtering [11]. In eBE technique, iron balls are evaporated in the presence
of nitrogen gas. The maximum nitrogen that can be incorporated in iron with this
technique is about 33 at.%. However, instead of using pure nitrogen, if eBE was
performed in the presence of nitrogen plasma the maximum nitrogen incorporation
can be increased to 50 at.%.
In another approach Fe-N compounds can be prepared using PLD, in which laser
ablation of Fe was performed in the presence of nitrogen gas. With this technique
the maximum nitrogen incorporation that can be achieved is about 50 at.%. However, in this technique due to congruent evaporation of Fe there is always a high
probability of having un-reacted clusters of Fe in the deposited Fe-N thin films. The
simplest method to obtain a single phase thin films of Fe-N compound is to perform
reactive sputtering of Fe in the presence of nitrogen gas. In this method, Fe target
is sputtered using a mixture of Ar and N2 gases. By this process, the formation of
different Fe-N phases can be controlled by varying the partial pressure of nitrogen.
In this method the maximum nitrogen concentration that can be achieved is about
50 at.%.
2
1. Introduction
(a)
(b)
(c)
Figure 1.1:
and [9](c).
1.3
Phase diagram of Fe-N system obtained in reference [4](a), [6](b),
Various Phases of Fe-N System
It has been observed that various Fe-N phases can be formed by increasing the
nitrogen content which are shown in the phase diagram (figure 1.1(c)) of the system.
These phases have distinct crystal structure and magnetic properties. Interestingly,
different phases of Fe-N compound have unique technological applications. In this
section we will discuss the formation of different Fe-N phases with increasing nitrogen
concentration. When nitrogen concentration is varied between 0-10 at.%, N atoms
get incorporated into the interstitial sites of bcc-Fe lattice that produces strain in the
lattice, however the overall structure remains same and the obtained phase is denoted
as α-Fe(N). Strain developed in the Fe lattice due to interstitial N atoms have
multiple effects. First, it results in nanocrystallization of Fe, secondly, it generates
magnetic anisotropy in the sample. Nanocrystallization occurs due to the dominance
3
1. Introduction
of strain energy over grain boundary energy that leads to the formation of smaller
grains.
It is well known that when grain size in a magnetic thin films is below ferromagnetic exchange length then according to random anisotropy model film possess
excellent soft magnetic properties. These properties of α-Fe(N) phase makes it a potential candidate for various applications such as high frequency read-write heads,
magnetic memory devices, etc. At exactly 11 at.% of N, α00 −Fe16 N2 phase gets
formed having a bct structure in which interstitial N atoms are arranged in an ordered fashion. This compound is mostly studied due to the observance of a giant
magnetic moment with the reported value varies between 2.1 to 3.2 T. Since fabrication of this compound in a single phase form is very difficult, the α00 −Fe16 N2 phase
remains controversial from both theoretical and experimental points of view. The
next phase is γ 0 -Fe4 N having a CaTiO3 type (perovskite) structure, which is formed
at 20 at.% N. This compound has well defined magnetic properties and have a chemically inert surface. This compound was also explored for high magnetic moment
and magnetic moment as large as 2.7µB /Fe atom was observed [12]. It was proposed
that this compound can replace pure Fe for the usage in various spintronics devices,
magnetic read-write heads, etc. With further increasing N concentration between
25-33 at.% ε − Fe3−z N phase gets formed having hcp structure. This compound
displays interesting magnetic behavior with increasing N concentration.
It was found that at z=0 the compound is ferromagnetic at room temperature
and the Curie temperature decreases between 560 K to <4.2 K with increasing N
content [13]. From the application point of view, this compound is used as a precursor to get single phase magnetic nitrides and for various tribological coatings. At
exactly 33.33 at.% of N, ζ − Fe2 N phase gets formed, in this phase due to ordering of
N atoms it has rhombohedral crystal structure. The magnetic state of this phase is
different in the powder and the thin films form. A powder sample of ζ − Fe2 N phase
shows an antiferromagnet ordering with Neel temperature at 9 K. Whereas, in the
thin films state it is a weak itinerant ferromagnet and its Curie temperature depends on nitrogen stoichiometry [14]. With further increasing N content to about
50 at.% equi-atomic iron mononitride phase can be formed. This compound can
only be prepared in the thin films form and its exact crystal structure remains controversial for a long time, which will be discussed later. In addition, a Fe3 N4 phase
with even more than 50 at.% N was theoretically predicted by Ching et al. but has
not yet been evidenced experimentally [15]. It was proposed that it has a spinel
structure with a weak ferromagnetic ordering at room temperature.
4
1. Introduction
1.4
Thermal Stability of Magnetic Fe-N Compounds
Magnetic Fe-N compounds were mainly studied owing to their interesting magnetic
properties and possible usage in various technological applications. However, it was
found that the thermal stability of Fe-N compound is poor [16; 17]. The severity of
this problem can be understood from the observed facts that when Fe-N compounds
were subject to a heat treatment, even at a moderate temperature of 450 K, they
undergo structural and magnetic transformations that are generally undesirable [18;
19; 20; 21; 22]. The poor stability in this system arises due to poor affinity and high
heat of formation (∆Hf◦ ) of Fe-N. Table 1.1 compares the value of heat of formation
of magnetic Fe-N with other transition metal nitrides (X-N). It can be seen that
transition metal nitrides have a relatively smaller value of ∆Hf◦ as compared to
magnetic Fe-N. The smaller value of ∆Hf◦ for X-N is also related to their high
affinity for N as compared to Fe. In 1960, Evans and Phelke did a detailed study
to calculate the affinity of X-N as compared to Fe. On the basis of Sievert’s law for
Fe-N solutions the activity coefficient of nitrogen in X doped Fe-N sample can be
written as [23]:
at.% N in Fe
(1.1)
at.% N in Fe − X
Based on the above equation an interaction parameter between X-N can be
derived given by [24]:
fN =
eX
N =
δ log(fN )
δ log(at.% X)
(1.2)
Evans and Pehkle determined the values of eX
N for various elements and their
◦
values at a temperature of 1600 C are tabulated in table 1.1. A high value of
eX
N for X-N compound indicates a poor bonding between Fe and N as compared to
X-N. Further, a high value of ∆Hf◦ does not favor the formation of a stable iron
nitride compound as easily as some iron oxides (typical value of ∆Hf◦ for iron oxide
is -826 kJ mol−1 ) that results in observed poor thermal stability in Fe-N compounds.
1.5
Effect of Additives/Dopants on Thermal Stability
The poor thermal stability of iron nitrides is a prime impediment in succeeding
Fe-N compounds as tribological coatings or device applications. To improve the
5
1. Introduction
thermal stability of this compound an addition of element X (X=Al, Ti, Zr, Si etc.)
was proposed in literature. Initially, the choice of additives was based on its high
affinity and low heat of formation of X-N as compared to Fe-N. It was proposed that
in the presence of dopants thermal stability of Fe-N system gets enhanced in two
ways. First, by increasing the binding energy of nitrides that cause suppression of
N diffusion from nitrides. Secondly, by forming traps for the released nitrogen that
results in the formation of X-N compound. With this proposition various Fe-X-N
thin films (X= Al, Co, Si, Ti, Zr, Ta, Hf, Rh etc.) were investigated during the last
two decades [22; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33; 34; 35; 36; 37; 38; 39; 40; 41;
42; 43].
The addition of these dopants in Fe-X-N thin films was done in a very small
amount of the order of a few at.%. At low concentration, dopants can either get
dissolved substitutionally into bcc-Fe lattice or accommodated into the grain boundary region [33; 40]. Due to such behavior lattice volume gets affected that depends
on the size of dopants. Table 1.1 compares atomic size of different additives with
respect to Fe. It is expected that dopants with bigger atomic size as compared to
Fe may produce expansion in the lattice, whereas smaller size dopants may produce
compression. Since, diffusion of nitrogen may also get affected due to a variation
in lattice volume, it could be presumed that atomic size of dopants should play a
crucial role in affecting the thermal stability of Fe-X-N thin films. However, various
reports on Fe-X-N thin films using varieties of additives fails to explain clearly what
properties of an additive control the thermal stability of Fe-X-N thin films.
Most of the research done on Fe-X-N thin films have found that Al, Ti, Zr or Ta
are the most suitable dopants. It was observed that these dopants not only improves
the thermal stability, but also significantly affect the magnetic properties of Fe-X-N
thin films. Das et al. with Ti doping observed that due to bigger atomic size of
Ti as compared to Fe, substitutional incorporation of Ti produces expansion in Fe
lattice that has influenced the magnetic properties of system, which was explained
Table 1.1: Atomic radius (r), heat of formation (∆Hf◦ ) and affinity (eX
N ) of X-N
with respect to Fe-N.
Element
Zr
Ti
Ta
Al
Fe
r(pm)
206
176
200
118
146
∆Hf◦ (kJ mol−1 )
-360
-338
-237
-321
-10
6
eX
N
-0.63
-0.53
-0.032
-0.028
–
1. Introduction
in terms of change in ferromagnetic exchange length [40]. Wang et al. observed
that with Ti addition thermal stability of Fe-X-N thin films also gets improved
significantly [32]. With Ta addition it was found that soft magnetic property and
magnetic anisotropy of Fe-Ta-N thin films get significantly altered [24; 27; 29; 30; 33].
Liu et al. studied the uniaxial magnetic anisotropy and the thermal stability in Al,
Ta and Zr doped Fe-X-N thin films. It was found that Al addition resulted in
superior thermal stability as compared to Ta [35; 38].
To understand these results, it was claimed that larger expansion created by Ta
addition could have lowered the diffusion barrier of interstitial nitrogen that results
in poor thermal stability. However, in an another study, Chechenin et al. measured
nitrogen desorption in Fe-Zr-N thin films and found that it gets significantly reduced
with the Zr addition that leads to improvement in the thermal stability of Fe-Zr-N
thin films [22; 36]. Since the atomic size of Zr and Ta is almost similar, therefore it is
expected that both should have similar effects on suppressing N diffusion. However,
from the observed dissimilarity one cannot understand adequately the underlying
mechanism responsible for the enhancement in thermal stability of Fe-X-N thin films.
Most importantly the whole mechanism of improvement in thermal stability of Fe-XN thin film is based on one assumption that in the presence of X, nitrogen diffusion
gets suppressed. Surprisingly, no N self-diffusion measurements were available in
literature in these compounds that can support such assumptions. Apart from it,
any effect of Fe self-diffusion on the thermal stability of Fe-X-N thin film was also
not studied in the literature. Therefore, in a part of this thesis (Chapter 3, 4 and
5) we have systematically addressed these issues.
1.6
Puzzle about the Crystallographical Structure
of Iron Mononitride Thin Films
We have seen that as nitrogen content is increased to about 50%, iron mono-nitride
(FeN) phase is formed. This is a non-magnetic compound and has been prepared in
the form of thin films, only. FeN is known to exist mainly in the two phases with fcc
structures: γ 00 -FeN with ZnS-type structure (lattice constant a=0.433 nm) and γ 000 FeN with NaCl type structure (lattice constant a=0.450 nm). A third phase called
as γ4 with lattice constant a = 0.866 nm was also observed [44]. Recently nonmagnetic iron mononitrides have emerged as a promising material in spintronics
applications [16; 45; 46]. A controlled annealing of FeN produces the γ 0 -Fe4 N phase
and thus provides a source of spin injection for semiconductors or diluted magnetic
7
1. Introduction
semiconductors [47].
Since the first preparation of γ 000 -FeN by Oueldennaoua et al. [7] and γ 00 -FeN by
Nakagawa et al. [8], several attempts were made to prepare and characterize thin
films of γ 00 /γ 000 -FeN [17; 18; 48; 49; 50; 51; 52; 53; 54; 55; 56; 57; 58]. Most of
the studies concluded that both γ 00 and γ 000 phases of FeN coexist. However such
coexistence remained controversial. Theoretical studies in iron mononitride predict
that the lattice parameter of FeN in the NaCl structure should be between 0.39
and 0.42 nm, which is considerably lower than the experimentally obtained value of
0.45 nm [59; 60]. Some theoretical simulations performed on this compound proposed that its stable structure should have a NaCl-type structure [61], but such
observations could not be supported experimentally. In recent studies the existence
of the γ 000 -FeN phase in the NaCl-type structure was questioned [45; 46; 57], and
concluded that the γ 000 -FeN compound should have ZnS-type structure. To sort out
the controversy related to possible stable structure of FeN phase in chapter 5 we
have systematically studied this compound.
1.7
Atomic Self-Diffusion in Thin Films
Diffusion process in general can be described as transport of matter from one point
to another in the presence of a concentration gradient. In the absence of any concentration gradient, defects and vacancies present in a solid mediates the atomic
diffusion process. The interesting thing is that even in the absence of any defect
and vacancies atomic diffusion is an inevitable process and can occur by exchanging
atomic positions. In all these cases the diffusion process can be activated by external forces such as pressure, temperature, ion irradiation etc [62; 63]. On this basis
a self-diffusion process can be defined as the measure of atomic diffusivity of any
atomic species in a solid in the absence of any chemical potential gradient. Study of
atomic self-diffusion is very important as different phase transformation, solid state
reaction, and various microstructural changes occurring in a solid state matter are
generally controlled by atomic diffusion. Moreover, knowledge of atomic diffusion is
utmost important for various technological applications such as in the fabrication
of microelectronic and optoelectronic devices. These devices are generally grown
in a thin film form with an average thickness of the order of diffusion distance at
operating temperature. Therefore, for long term stability of these devices knowledge
of atomic diffusion is necessary.
In thin films, atomic diffusion is highly influenced by the microstructure and
8
1. Introduction
various imperfections arises during the film growth, such as solid-solid interfaces and
grain boundaries that act as a short-circuiting path for the fast diffusion. Although
the diffusion process in amorphous and crystalline systems is relatively well-studied,
relatively less data is available about nanocrystalline systems. While in large grained
poly-crystals, vacancy type diffusion mechanisms are well-established, in case of
amorphous alloys collective hopping of a large group of atoms similar to liquids
takes place [64]. However, in nanocrystalline materials the situation becomes even
more complicated, where the portion of atoms at or near grain boundaries can be as
high as 50% [65]. The grain boundaries are un-relaxed and especially loosely packed
and significant grain growth may occur at relatively low temperatures by grain
boundary migration, which is controlled by grain boundary diffusion. Therefore,
knowledge of atomic self-diffusion in nanocrystalline materials is very important.
One of the main difficulties in probing diffusion in nanocrystalline alloys is a very
narrow temperature regime where nano-crystalline alloys are stable.
Therefore, diffusion measurements should be carried out close to room temperatures. It is expected that diffusivities in this temperature range could be as low as
10−20 -10−25 m2 s−1 and diffusion lengths of a few nm and below need to be probed.
For this purpose a combination of techniques which can determine diffusivities in
this range are neutron reflectometry (NR) and secondary ion mass spectrometry
(SIMS). Neutron reflectivity is a technique which has emerged as a superior method
to measure ultra low diffusivities, as demonstrated by various studies [66; 67; 68; 69].
However, for the direct information about the diffusion mechanism SIMS is a well
suited technique [18; 19; 70]. In this work these techniques have been thoroughly
used to investigate any effect of dopants on atomic diffusion of Fe and N. Moreover,
as pointed out earlier non-magnetic FeN compound is known for its precursor nature
to get single phase magnetic nitrides. Such kind of phase transformation is indeed
controlled by atomic diffusion of Fe and N. Therefore, we have also studied Fe and
N self-diffusion in FeN compound that has rather provided some surprising results
that will be discussed in chapter 4 and 6.
1.8
High Power Impulse Magnetron Sputtering:
A Modern Tool for Thin Film Deposition
HiPIMS is a recently developed technique for the deposition of thin films. Unique
plasma conditions associated with it, makes it a preferred choice over conventional
deposition methods [71; 72; 73; 74]. As compared to dc-MS technique, the plasma
9
1. Introduction
density in HiPIMS discharge is of the order of 1019 m−3 , about 2 orders of magnitude
larger than that in dc-MS plasma [72]. In this technique a high power pulse is applied
to a magnetron target at low duty cycle (between 0.1-1%) that produces a highly
dense plasma. In such kind of situation it was observed that the number of sputtered
ionized species exceeds neutrals. The volume fraction of ionized species depends
on various process parameters such as pulse duration, gas pressure, peak current
etc [72; 75]. These characteristic properties of HiPIMS plasma results in improving
film qualities such as film density, hardness, surface roughness, better adhesion,
dense microstructure etc. Moreover, due to high metal ionization in HiPIMS process,
it is expected that thin films deposited via reactive sputtering would display superior
properties.
As such HiPIMS technique has been frequently utilized for the deposition of
metal nitrides such as Al-N [76], Cr-N [77; 78; 79], Ti-N [80], Nb-N [81], etc. and
metal oxide thin films such as TiO2 [82; 83], Al2 O3 [84; 85], ZnO [86], ZrO2 [87],
and Fe2 O3 [88; 89]. In these studies, it was observed that properties of films deposited using reactive HiPIMS process are superior. Konstantindis et al. found that
formation of rutile phase in TiO2 thin film is more favorable as compared to anatase
phase when sputtered using HiPIMS technique. Moreover, HiPIMS deposited films
show higher refractive index [83]. Ehiasarian et al. observed that pretreatment using
HiPIMS process has improved the adhesion and mechanical properties of CrN thin
films [90; 91]. Similarly, Reinhard et al. observed improvement in corrosion resistance properties of HiPIMS treated CrN/NbN superlattice structure [81]. Recently,
Zhao et al. observed that the optical transmittance of Zirconia thin films deposited
using HiPIMS process is more as compared to dc-MS process [87].
Looking at the vast capabilities of HiPIMS technique in depositing various kinds
of thin films, it is surprising to note that HiPIMS process has not yet been applied
for the deposition of magnetic thin films. Very recently, HiPIMS process has been
employed to deposit Fe2 O3 [88; 89] and FeCuNbSiB thin films [92]. Still magnetic
nitride films have not yet been studied with HiPIMS technique. It is well known
that transition metal magnetic nitrides are an important class of materials for their
usage in various technological applications [93; 94]. Therefore, it will be immensely
useful to study magnetic nitride films deposited using HiPIMS technique which will
be discussed in chapter 7.
10
1. Introduction
1.9
Aim of Present Work
In brief objectives of this thesis are:
1. To understand the role of additives on phase formation of Fe-N thin films.
2. Optimization of additives for achieving maximum thermal stability.
3. Effect of additives on structure, magnetic properties and thermal stability of Fe-N
thin films.
4. Influence of additive elements on iron and nitrogen self-diffusion.
5. To determine the exact crystal structure of iron mononitride thin films.
6. Correlation between atomic self-diffusion and phase formation in iron mononitride
thin films.
7. Effect of HiPIMS plasma on structural and magnetic properties of Fe-N thin films.
To realize above mentioned aim, Fe-N thin films were deposited using dc-MS
and IBS and HiPIMS techniques, these techniques are presented and discussed in
chapter 2. To investigate the structural properties, X-ray diffraction (XRD) and
reflectivity (XRR) technique is utilized. Short range structure and magnetic properties were investigated using conversion electron Mössbauer spectroscopy (CEMS),
while the local structure was obtained using both hard and soft x-ray absorption
spectroscopy (XAS). To understand the role of additive elements on phase transformation and thermal stability, Fe and N self-diffusion measurements were performed.
To measure self-diffusion of Fe neutron reflectivity (NR), nuclear resonance reflectivity (NRR), and secondary ion mass spectroscopy (SIMS) was used.
Whereas, NR and SIMS techniques are utilized to measure N self-diffusion.
Superconducting quantum interference device-vibrating sample magnetometer (SVSM) and magneto optic Kerr effect (MOKE) spectroscopy have been used to investigate magnetic properties. To investigate the thermal stability, Fe-N thin films
were vacuum annealed and subsequently structural, magnetic and diffusion measurements were performed. The surface morphology of samples was studied using atomic
force microscopy (AFM). The average concentration of additives was measured using
energy dispersive x-ray analysis and secondary neutral mass spectroscopy (SNMS).
NR measurements were performed at AMOR beamline at SINQ/PSI Switzerland.
And NRR measurements were performed at P01 beamline at PETRA-III DESY,
Hamburg Germany using resonant x-rays of 14.4 keV energy.
11
1. Introduction
In Chapter 3 a systematic investigation was performed to understand the effect
of additives on thermodynamics of Fe-N system. Different Fe-N phases were prepared using dc-MS by increasing the nitrogen partial pressure (defined as RN2 (in %) =
P N2
× 100, where PN2 and PAr is N2 and Ar gas flow) was varied between 0(PN2 +PAr )
100%. Additives used in this work are Al, Ti and Zr. First the doping concentration
was kept minimum at 2 at.% of Al, 3 at.% of Ti, and 3 at.% of Zr and phase formation was studied. Using XRD and polarized neutron reflectivity, it was observed
that with Al and Ti doping nitrogen rich phase gets formed at lower RN2 , whereas,
with Zr doping nitrogen rich phase get formed at slightly higher RN2 as compared
to un-doped sample. At RN2 =10% nanocrystalline Fe(N) with bcc Fe structure get
formed that is having good soft magnetic properties. The thermal stability of Fe-XN thin films deposited at this RN2 was investigated by performing vacuum annealing
and subsequent XRD measurements. In the un-doped sample it was observed that
different Fe-N phases get evolved with increasing annealing temperature. With Al
or Ti doping thermal stability improves marginally, whereas, with Zr doping thermal stability improves significantly even upto 673 K. Further than, concentration of
Al is increased to 3 at.%, 6 at.%, and 12 at.%. It was observed that the thermal
stability improves remarkably at these concentration. A similar process is carried
out to optimize Ti concentration and it was found that the optimum concentration
of dopants are 5 at.%of Al, 5 at.% of Ti, and 3 at.% of Zr. At this level of doping
it was observed that in addition to enhancement in thermal stability, soft magnetic
properties of films also get improved remarkably.
Chapter 4 To understand the observed improvement in the thermal stability,
Fe and N self-diffusion measurements was performed on isotope multilayer samples
of type [57 Fe-X-N/natural Fe-X-N]n and [Fe-X-15 N/Fe-X-natural N]n using PNR, NRR
and SIMS techniques. In the first section, we have discussed results based on Fe selfdiffusion measurements performed on Fe-X-N (X=0, 3 at.% of Al, and 1.6 at.% of
Zr) thin films with 11 at.% of N deposited using IBS. At this composition, nitrogen get incorporated into the interstitial site of bcc-Fe lattice that produce strain
in the Fe lattice and generates a perpendicular magnetic anisotropy (PMA). The
dopant concentration is chosen in such a manner so that the PMA is retained. It
was observed that the structural and the magnetic stability gets significantly enhanced with Al doping, whereas Zr doping has only a marginal effect. From PNR
measurements it was found that self-diffusion of Fe gets suppressed with both additives. A correlation between the thermal stability and the diffusion process gives a
direct evidence that the enhancement in the thermal stability is primarily diffusion
12
1. Introduction
controlled. In the second section, the question pertaining to role of N diffusion on
thermal stability is addressed. At optimum doping level, Fe-X-N (X= 0, 5 at.% of
Al, 5 at.% of Ti and 3 at.% of Zr) thin films were deposited using dc-MS and Fe and
N self-diffusion measurements were performed. It was observed that N self-diffusion
gets suppressed with Al doping whereas Ti or Zr doping results in somewhat faster
N diffusion. On the other hand, Fe self-diffusion seems to get suppressed by any
dopant of which heat of nitride formation is significantly smaller than that of iron
nitride. Importantly, it was observed that N self-diffusion plays only a trivial role,
as compared to Fe self-diffusion, in affecting the thermal stability of iron nitride thin
films. A combined picture of diffusion, structural and magnetic stability has been
drawn to understand the obtained results.
Chapter 5 Iron mononitride (FeN) is known to exist in different phases and there
is a debate about the exact crystal type and the coexistence of these phases. We
prepared single phase iron mononitride thin films by dc-MS with various deposition
rates (sputtering power) and investigated them with XRD, NR and low temperature
and high magnetic field Mössbauer spectroscopy. In addition XAS measurements at
Fe and N absorption edges were performed. It was observed that with an increase in
the sputtering power a more disordered structure is formed while the local chemical
environment of iron remains unaffected. The low temperature and high magnetic
field Mössbauer spectroscopy measurements confirmed that the FeN phase is paramagnetic even at 5 K and an applied magnetic field of 5 T reflects magnetic splitting
caused by the applied field which is a characteristic of ZnS-type FeN.
Chapter 6 In this chapter a detailed study of Fe and N self-diffusion was performed using NR and SIMS on non-magnetic γ 000 -FeN thin films. The isotopic multilayers structure of type: [FeN(10 nm)|57 FeN(5 nm)]×10 and [FeN(9 nm)|Fe15 N(9 nm)]×25
were prepared using dc-MS. To get mononitride phase nitrogen alone was used as
the sputtering gas. From NR measurements, it was observed that nitrogen diffuses
slower than iron, although the atomic size of iron is larger than that of nitrogen.
The obtained value of pre-exponential factor and activation energy of Fe and N selfdiffusion indicates that a significantly larger group of N atoms participates in the
diffusion process than of Fe. Since NR is an indirect technique to study atomic diffusion we have employed the SIMS technique which directly gives detail information
about the diffusion mechanism. SIMS measurements were performed on a trilayer
structure of type [FeN(100 nm)|57 Fe15 N(2 nm)|FeN(100 nm)]. It was observed that
with annealing, the concentration profile of 57 Fe takes a skew shape, in contrast
to 15 N profile that has a Gaussian shape throughout the annealing temperatures.
13
1. Introduction
The skewness in the concentration profile of Fe indicates that fast diffusion of Fe
prevails along grain boundaries, whereas for N, volume type diffusion is a dominant
process. Using Le Claire’s analysis, it was observed that Fe grain boundary diffusion
has three distinct kinetic regimes with increasing temperature. The observed selfdiffusion results were then applied to understand the structural and the magnetic
phase transformation in FeN thin films. These studies eventually help to update the
position of FeN phase in Fe-N phase diagram.
Chapter 7 In this chapter we studied phase formation, structural and magnetic
properties of iron-nitride thin films deposited using HiPIMS and dc-MS. The nitrogen partial pressure during deposition was systematically varied both in HiPIMS
and dc-MS. Resulting Fe-N films were characterized for their microstructure, magnetic properties and nitrogen concentration. We found that HiPIMS deposited Fe-N
films show a globular nanocrystalline microstructure and improved soft magnetic
properties. In addition, it was found that the nitrogen reactivity impedes in HiPIMS as compared to dc-MS. Obtained results can be understood in terms of distinct
plasma properties of HiPIMS.
In Chapter 8 a general conclusion of present thesis work is provided and future
scope is discussed.
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