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CHAPTER 2

TRANSISTION METAL DOPED GALLIM NITRIDE -

A DILUTE MAGNETIC SEMICONDUCTOR

2.1 INTRODUCTION

Diluted magnetic semiconductor (DMS) materials, which utilize

both the spin and the charge properties of carriers, have become attractive

because of the interest in investigating the fundamental physical properties of

such materials and in potential applications for many promising spintronic

devices (Ohno 1998, Fiederling et al 1999, Dietl et al 2000). Spintronics

refers commonly to a phenomenon in which the spin of electrons in a solid

state environment plays the determining role. These devices have wide

applications in spin-valve transistor, spin-light emitting diodes, non-volatile

memory, optical isolator, ultrafast optical switches. As GaN doped with

transition metal will retain ferromagnetism at practical operating temperature

as predicted by theoretical calculations and the other advantage is that there

are already an existing process technology for this material in other

applications such as photodedectors and light emitting diodes (LEDs).

There is a wide class of semiconducting materials which is

characterized by the random substitution of a fraction of the original atoms by

magnetic atoms. The materials are commonly known as semimagnetic

semiconductors (SMSC) or diluted magnetic semiconductors (DMS). The

most common DMS are II-VI compounds (like CdTe, ZnSe, CdSe, CdS, etc.),

with transition metal ions (e.g. Mn, Fe, Ni or Co) substituting the original

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cation site. There are also materials based on IV-VI (e.g. PbTe, SnTe) and

recently III-V compound semiconductors (e.g. GaAs, InSb, GaN) crystals

(Russell et al 2004, Ham et al 2006). These mixed crystals (semiconductor

alloys) may be considered as containing two interacting subsystems. The first

of these is the system of delocalized conduction and valence band electrons.

The second is the random, diluted system of localized magnetic moments

associated with the magnetic atoms. The fact that both the structure and the

electronic properties of the host crystals are well known means that they are

perfect for studying the basic mechanisms of the magnetic interactions

coupling the spins of the band carriers and the localized spins of magnetic

ions. The coupling between the localized moments results in the existence of

different magnetic phases (such as paramagnets, spin glasses and

antiferromagnets).

(A) (B) (C)

Figure 2.1 Schematic diagram of (A) Pure or nonmagnetic semiconductor

(B) Dilute magnetic semiconductor (C) dilute magnetic

semiconductor with ferromagnetic interactions

The exchange interaction between spin of s, p electrons of host with

the d-electron of the doped transition–metal ion is responsible for magnetic,

optical and conductive properties of DMS materials (Furdyna 1998).

Possibility of ferromagnetism in GaMnN above room temperature based on

first principle calculations has been proposed (Dietl et al 2000). Calculation of

27

energetic and magnetic properties of manganese doped GaN with higher

concentration shows that Mn related cluster is energically favorable and is

responsible for the observed magnetism in the system (van Schilfgaarde and

Mryasov 2001). Investigations have shown that ferromagnetic behavior is

dominated by a short range double exchange mechanism in III-V compound

semiconductors when transition metal forms a random alloy (Sato et al 2001).

Ferromagnetic phase in (GaFe)N below 100K has been reported (Akinaga et

al 2000). Prediction of weak ferromagnetism in copper doped GaN and no

effect on magnetization with the creation of gallium vacancy is observed by

Rosa et al (Rosa et al 2007). DFT theory has shown that chromium doping in

GaN results in the formation of chromium cluster which exhibit anti

ferromagnetism (Cui et al 2005). Eu doping in GaN results in a high magnetic

moment of 6 µB (Goumari et al 2008). DFT calculations based on full potential

linearized augmented plane wave method have explored the possibility of

defect-induced magnetism in wurtize GaN (Hong 2008). The nitrogen

vacancy defect structure has no sign of magnetic state. Nonetheless, very

interestingly it has been found that GaN with a gallium vacancy defect can

show induced local magnetic moment in nitrogen atoms. The four nitrogen

atoms in the tetrahedron sites with the gallium vacancy as neighbor have

magnetic moments of 0.23 and 0.29 µB. The contribution of larger magnetic

moment is from the nitrogen which is at a larger distance from the gallium

vacancy.

In the recent past, much work has been carried out on manganese

doped GaN. The theoretical observation shows that Mn-Mn interaction is

responsible for ferromagnetism when Mn is buried inside the bulk but is

antiferromagnetic when the coupling takes place at the surface of GaN matrix

(Wang et al 2004). P-type GaMnN quantum well is ferromagnetic at room

temperature with weak spin-exchange interaction is reported (Kim et al 2005).

Prediction of room temperature ferromagnetic behavior in MOCVD grown

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iron doped GaN is observed (Kuo et al 2008). We have studied nickel doped

GaN using TB-LMTO method and observed that the system having no defects

is ferromagnetic for lower percentage of dopant (Munawar et al 2010).

Theoretical and experimental result shows that copper doped GaN is

ferromagnetic with high spin polarization at Tc above room temperature

(Ganz et al 2010). Implantation of Co into p-type GaN grown by MOCVD

and reported ferromagnetism at 320 K, in most cases the damage induced due

to ion implantation is removed on samples annealing (Lee et al 2003). Weak

magnetic polarization has been predicted in defects or residual oxygen of

GaN samples (Rosa et al 2007). Investigations have been carried out on the

theoretical aspects and on the synthesis of cobalt doped GaN. Extensive

characterizations on the electrical, optical and magnetic properties have been

carried out and cobalt doped GaN is found to be a suitable candidate in the

field of spintronics.

2.2 TB-LMTO CALCULATION ON COBALT DOPED GaN

The calculations have been performed using TBLMTO method

based on the density functional theory (Anderson 1975). The kinetic energy

outside the muffin-tin spheres (where the potential is constant) is chosen to be

zero. The Wigner- Seitz cells located at the atomic or interstitial sites are

replaced by the atomic sphere and the spherically symmetric potential within

the sphere is extended to their boundaries. In energy calculations, we include

only s, p and d orbital’s and neglected all higher partial waves and integrate

the radial Schrödinger equation out to the boundary of the sphere. The local

density approximation (LDA) parameterized by Barth and Hedin (Barth and

Hedin 1972) was used for the exchange and correlation potential. Coulomb

potential energy caused by electron-ion interaction is described in which

orbital of Ga (3d4s), N (2p3s), Co(3d4s) and Ni(3d4s) were treated as valence

electrons. The accuracy of the total energy calculation within the density

functional theory, using LDA, is sufficient to predict which structure is

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thermodynamically stable i.e., have lowest free energy. Empty sphere

were introduced in all cases in order to keep the overlap of atomic sphere

within 16 %.

GaN crystallize in the wurtzite structure with lattice constants

a = 3.188 Å and c = 5.189 Å (Lagerstedt. and Monemar 1979). Periodic

2x2x2, 4x4x2, 3x3x2 and 2x2x3 supercells of GaN matrix was used as the

calculation units. Each supercell has equal number of real and empty atoms.

Defects such as Ga and N vacancies are considered in the supercell. By

replacing Ga atom by transition metal namely, nickel and cobalt in these

supercells, we get the corresponding dopant concentration, Necessary care has

been taken not to exceed the doping level as beyond 12.5% for Ni. The

Wigner-Seitz sphere radii are chosen in such a way that the sphere boundary

potential was minimum and the charge flow in accordance with the electro

negativity criteria. Spin polarized scalar relativistic calculations are performed

to obtain the total energy, density of states (DOS) and magnetic moment. 144

k-points in the irreducible part of the Brillouin Zone were used to calculate

the total and partial density of states by means of tetrahedron method (Stokes

et al 1987). The k and E convergence are checked by increasing the number of

K points and the energy convergence criteria. TB-LMTO’s have almost

universal decay and with ASA, The main advantage of this scheme is that it is

computationally effective.

2.3 EXPERIMENT

Synthesis of pure and doped GaN was carried out using a

experimental set-up as shown in Figure 2.2. Pure gallium metal (5N purity)

and transition metal namely cobalt was taken in the alumina boat and is kept

at the middle of the furnace (Senthil kumar and Kumar 2002) which consists

of a 80mm diameter quartz tube and of length 100 cm (with wall thickness of

3mm) .The assembly has an inlet and an outlet of diameter 10mm.

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Figure 2.2 Experimental setup for the synthesis of pure and doped

Gallium nitride

Nitrogen gas is continuously flowed from the beginning of the

experiment for a clean ambient in the reactor and to remove the native oxide.

The reactor is kept inside a resistively heated furnace. A ceramic muffle of

length 100mm has been used for winding the heating element (Al Kanthal

wire: SWG 18 type).

The windings are made in such a way that the space between the

winding gradually becomes narrower at the ends of the tube compared to the

centre of the tube to compensate the heat loss at the ends of the reactor. The

windings are insulated by using a paste of corundum cement. A thermocouple

of K type is used to measure the reactor temperature. A Eurothrum

temperature controller Model 2604 with accuracy of 0.1 °C is used. Upon

attaining the growth temperature (~ 1223 K) ammonia gas of 3 slm is allowed

to flow for 8 hours and then the system is allowed to cool to room

temperature at an interval of 100 K/h under the flow of nitrogen. The

synthesized sample is etched with HF and HNO3 in 1:1 ratio in order to

remove unreacted metals, the final product is dried and then pulverized to a

fine powder. Structural investigations have been carried out using X-ray

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diffraction (XRD) studies. The emission properties of the samples are studied

by Photoluminescence (PL) measurement with He-Cd (325nm) laser as the

excitation source at room temperature. Magnetization and temperature

dependent magnetic properties of the samples have been studied using

superconductor quantum interface device (SQUID) measurements.

2.4 RESULTS AND DISCUSSIONS

The schematic representations of the unit cell for wurtzite GaN

with vacancies used in our calculation are presented in Figure 2.3. The figure

represent Ga, N, Ga vacancy, N vacancy and Ni are represented by Violet,

Gray, Yellow, Blue and light Green balls.

(A) (B) (C)

(D) (E) (F)

Figure 2.3 Schematic diagram of (A) GaN Supercell with two Ni doped

to form dimer, (B) two Ni at different site (C) GaN Supercell

with Ga vacancy (D) Ni doped at Ga site with a Ga vacancy

(E) GaN Supercell with N vacancy, (F) Ni doped at Ga site

with N vacancy

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2.4.1 Electronic and Magnetic Properties of Nickel Doped GaN

The electronic structure of GaN in the wurtzite structure is obtained

by means of TB-LMTO method. It is a direct band gap material as reported in

literature (Strite and Morkoç 1992). One and two atoms of gallium were

replaced by Ni to obtain a dopant concentration of 6.25 and 12.5 % in 2x2x2

supercell respectively. Since the substitution of Ni impurity in the GaN lattice

was done by replacing the Ga atom, the effect of the structural relaxation has

been neglected (Das et al 2003). When the transition metal (Ni) defect is

introduced in GaN, the five fold degenerate d-states split into two degenerate

d-states by the crystal field splitting (vanSchilfgaarde and Mryasov 2001).

One of the degenerate d-state involves in the bonding and antibonding with N

– p state, while the other act as a non-bonding state (eg and t2g) as shown in the

Figure 2.4, 2.5. As the t2g band lies above eg band, the tetrahedral bonding of

Ni atom in the GaN structure is confirmed (Das et al 2006). Thus the spin of

the defect state may give rise to the magnetic moment.

Figure 2.4 Electronic Band Structure of 6.25 % Ni doped GaN

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Figure 2.5 Electronic Band Structure of 12.5 % Ni doped GaN

Figure 2.6 shows the total Density of State (DOS) of 6.25 % and

12.5 % Ni doped GaN. The half metallic nature is clearly seen in 6.25 %

when compared to 12.5 %. The partial density of states for Ni–3d and N-2p

which are given in Figure 2.7 shows the p-d hybridization of Ni-N in the

bonding state.

(A) (B)

Figure 2.6 (A) Density of states of 6.25 % Ni doped GaN (B) DOS of

12.5 % Ni doped GaN

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(A) (B)

Figure 2.7 Partial density of states of Ni-3d and N-p in (A) Spin up

(B) Spin down state

The magnetic moment per Ni atom in 6.25 and 12.5 % are 1.13 B

and 1.32 B respectively, but the total magnetic moment in each case are

different because nickel atom polarizes the four nitrogen atoms bonding with

it. In 6.25 % concentration, the three equidistant Ni which bonds with N

atoms have a magnetic moment of 0.1808 B and the other N atom has 0.2151

B magnetic moment. For 12.5 % concentration, i.e., when two Ga atoms are

replaced by Ni, calculations are optimized by substituting the Ni atoms at two

different sites. Figure 1A shows the formation of the two Ni atoms which

forms a dimer with N atom and the Ni – Ni distance is 3.253 Å (Assadi et al

2009). The magnetic moment per Ni atom is found to be 1.13 B and the total

moment was also found to decrease when compared to 6.25 % concentration.

This is due to the interaction of Ni spins via N atom bonded to them. When Ni

is substituted as shown in Figure 2.3(B), the magnetic moment is found to

increase per Ni atom (1.32 B) and the system is not energetically favorable

when compared to the configuration shown in Figure 2.3(A). From this we

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conclude that when Ni-Ni distance is 3.253 Å, the magnetic moment per Ni

atom is 1.13 B and the magnetic moment was less when compared to the

6.25 % concentration. For the other position, the magnetic moment is found to

increase at Ni site. For 12.5 % concentration the half metallic nature

disappears and becomes a ferromagnetic alloy. Hence, we conclude that only

for a small addition of Ni in GaN one can achieve spintronic in these systems.

In case of defect created by Ga vacancy as show in Figure 2.3(C).

The magnetic moment is due to the spin interaction of p-p electron of

surrounding nitrogen atom which is in the tetrahedral bonding state. The three

N atoms are at a distance of 3.689 Å from the Ga vacancy which give a

magnetic moment of 0.13 µB. The other N which is at a distance of 3.678 Å

from VGa contributes to a magnetic moment of 0.43 4µB. For Ni doped GaN

with Ga vacancy as shown in Figure 2.3(D). A strong Ni-N bond is formed by

charge transfer from d electron of Ni to acceptor level of VGa. Hence, the 3d

electron of Ni has no role in spin polarization and therefore, the observed

magnetic moment is due to the dangling bond electron of N which is 0.299

µB. The Nitrogen vacancy defect as shown in Figure (2.3E) has no role in

spin polarization. Nickel substituted in the gallium site with nitrogen vacancy

as shown in Figure 2.3(F) has a magnetic moment of 1 µB and is similar to the

6.25 % Ni dopant case but the moment is slightly less.

2.4.2 Electronic and Magnetic Properties of Cobalt Doped GaN

We calculated the magnetic moment for 1.56 % Co doped GaN

which corresponds to replacing one Ga atom by Co in a matrix of 32 Ga

atoms. The effects of structural relaxation due to impurities are neglected

because of small radial difference between Ga and Co atoms. Other forms of

defects such as interstitial or anti-site are not considered because they are

energetically strongly unfavorable when compared to Co occupying the

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substitution site. The Co atom is in tetrahedral bonding with four nitrogen

atom. The bonding length of one of the nitrogen atom with Co is 1.946 Å with

a magnetic moment of 0.20 µB. The remaining nitrogen atoms show a

magnetic moment of 0.21 µB. The magnetic moment of Co atom per cell is

2.56 µB. This highly localized moment arises due to spin polarization from the

3d electron of Co which does not contribute to the p-d hybridization from 2p

and 3d electrons of N and Co atom. For 2.77 % and 4.16 % of Co dopants in

GaN, this is obtained by replacing Ga by Co in a matrix of 18 and 12 Ga

atoms. The calculated magnetic moment per cobalt atoms is same as

compared to 1.56 % but decrease to 1.48 µB for 4.16 %.

(A) (B)

Figure 2.8 Total density of state for (A) 2.77 % Co doped GaN (B) 4.16 %

Co doped GaN

Figure 2.8 (A, B) shows the total density of state of 2.77 % and

4.16 % Co doped GaN. It is observed form the density of states figure that

2.77 % concentration of Co in GaN is perfectly half metallic with almost 100

% spin polarization. For 4.16 % of Co dopant the system becomes metallic

but is still ferromagnetic, because the 3d electron of cobalt has a band in the

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band gap of GaN. As a result, the contribution for spin polarization from 3d

electron is small, which in turn decreases the magnetic moment value of 1.08

µB for 4.16 % Co dopant.

2.5 CHARACTERISATION OF COBALT DOPED GaN

2.5.1 X-ray Diffraction Analysis

The most common use of powder (polycrystalline) X-ray

diffraction is chemical analysis. This can include phase identification.

Investigations on high/low temperature phases, solid solutions and

determination of unit cell parameters of a material. A typical diffraction

spectrum consists of a plot of reflected intensities versus the detector angle 2

or depending on the goniometer configuration. The 2-Theta values for the

peak depend on the wavelength of the anode material of the X-ray tube. It is

therefore customary to reduce a peak position to the interplanar spacing d that

corresponds to the h, k, l planes that caused the reflection. The value of the d-

spacing depends only on the shape of the unit cell. We get the d-spacing as a

function of 2-Theta from Bragg’s law d = n /2 sin . Once we know the d-

spacing and the corresponding indices h, k, l, we can calculate the lattice

parameter of the unit cell.

The synthesized GaN was characterized using D8 Brucker AXS X-

ray diffractometer with Cu K source. Figure 2.9 shows the XRD spectra of

synthesized GaN. The sample is in polycrystalline nature, and confirmed the

wurtzite structure of pure GaN. The lattice parameter calculated from the

XRD data is a =3.186Å and c =5.174Å, which are in good agreement with

the reported values (Senthil kumar and Kumar 2002).

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Figure 2.9 XRD of synthesized pure Gallium Nitride powder sample at

1223 K

2.5.2 Photoluminescence

Photolumiscence (PL) is a non-destructive technique to characterize

the intrinsic properties of materials like band to band edge recombination to

evaluate the band gap energy, shallow defect levels like donor or acceptor

levels. When light (laser beam) of sufficient energy is incident on a material,

photons are absorbed and electronic excitations are created. Eventually, these

excitations relax and the electrons return to the ground state. If radiative

relaxation occurs, the emitted light is called Photoluminescence (PL). This

light can be collected and analyzed to yield a wealth of information about the

photoexcited material. The PL spectrum provides the transition energies,

which can be used to determine electronic energy levels. The PL intensity

gives a measure of the relative rates of radiative and nonradiative

recombination. Variation of the PL intensity with external parameters like

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temperature and applied voltage can be used to characterize further the

underlying electronic states and bands. A monochromator is used to get a

single wavelength of light. The photo detector senses the radiative emission

and then it processes this weak signal for analysis. The PL setup used in our

experiment is shown in Figure 2.10.

Figure 2.10 Photoluminescence system

The emission properties of 1 % and 5 % Co doped GaN are

measured at room temperature using 325 nm He-Cd laser as excitation source

and PL spectra can be recorded for a wide range of 300-1500nm. It is

observed that there is a blue shift in the band edge emission as the cobalt

concentration increases. The broad band edge emission is due to the

polycrystalline nature of the synthesized samples. The emission at 3.24 eV is

mostly due to nitrogen vacancy defects (Keller et al 1995) followed by its first

phonon replica at 3.13 eV.

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Figure 2.11 PL spectrum of Co doped GaN samples obtained with

325 nm excitation at room temperature

2.5.3 Magnetic Studies

One of the most sensitive forms of magnetometry is

superconducting quantum interference device SQUID magnetometer. This

uses the properties of electron-pair wave coherence and Josephson junctions

to detect very small magnetic fields with resolutions up to ~10-14 kilo Gauss

or greater. The central element of a SQUID is a ring of superconducting

material with one or more weak links called Josepheson’s Junctions. With

weak-links at two points, the critical current ic is much less than the critical

current of the main ring. This produces a very low current density making the

momentum of the electron-pairs small. The wavelength of the electron-pairs

is thus very long leading to little difference in phase between any parts of the

ring. The Schematic representation of a SQUID is shown in Figure 2.12.

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Figure 2.12 Schematic diagram of a Superconducting Quantum

Interface Device (SQUID)

Most common use of SQUID is in magnetic property measurement

systems (MPMS). This is typically done over a temperature range from 4 K to

190 K. Figure 2.13 shows the magnetization curve of 1 % and 5 % Co doped

GaN as a function of magnetic field at 10 K. The observed magnetic moment

at 3000 Oe is 2.03 µB and 0.54 µB. From hysteresis loop it is clear that the

synthesized sample is a perfect ferromagnetic. A clear opening of hysteresis

cycle is observed only for 1 % Co doped GaN. But the trend is no longer

observed for higher percentage of dopant. Due to the solubility limit of

transition metal cobalt which may be less than 1% in GaN (Zaj c et al 2008),

so as the concentration of dopant increases, there is a good possibility for the

formation of other impurity phases such as CoGa which shows paramagnetic

behavior (Booth and Marshall 1970).

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Figure 2.13 Magnetization curve of 1% and 5% Co doped GaN as a

function of applied magnetic field at 10 K

The other less possible impurity is CoOx because the synthesis of

GaN is done in nitrogen ambient to prevent oxidation, which may have

antiferromagnetic behavior. This may be the possible reason for the observed

decrease in magnetic moment values as the concentration of Cobalt dopant

increases. Figure 2.14 shows the temperature dependent magnetization (M-T

curve) of synthesized 1 % and 5 % Co doped GaN with a magnetic field of

3000 Oe. It is clear that there is no drastic decrease in magnetization value as

temperature increases till 300 K for 1 % Co doped GaN. This indicates that

the ferromagnetic transition temperature is above room temperature, since as

temperature increases the ferromagnetic order remains unchanged due to

strong alignment of 3d electron of cobalt in the applied field direction. For

higher percentages of cobalt, the magnetization decreases rapidly at very low

temperature.

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Figure 2.14 Temperature dependent magnetization (M-T curve) of 1 %

and 5 % Co doped GaN under a magnetic field of 3000 Oe

The theoretical calculations have shown a large value of the

magnetic moment when compared with the experimental values. The reason

may be due to the structural defects, Co occupying the interstitial site and the

small strain produced due the ionic radii difference of Co which is not taken

into account while calculating the magnetic moment values and also due to

the variation in the percentage of dopant used while synthesizing the samples.

The origin of the observed ferromagnetism from SQUID measurement is not

clear and may be due to formation of isolated Co cluster in synthesized GaN.

2.6 CONCLUSIONS

Theoretical calculations have been carried out to investigate the

possibility of inducing ferromagnetism in GaN by the substitution of Ni and

Co. When the Ni concentration is 6.25% the system shows half metallic and

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ferromagnetic behavior. The dopant site preference shows that Ni dimer via N

is energetically more stable when compared to the other position as discussed.

When a vacancy is created the system becomes metallic. When Ni is

substituted in Ga site with either a Ga or N vacancy the system shows

magnetism. In all these cases half metallic nature is absent. The half metallic

behavior is observed upto 2.77 % Co dopant, but for higher percentage of

dopant the system become metallic.

The structural and optical properties of doped GaN were studied

using X- ray diffractometer and photoluminescence respectively. The

magnetic properties were studied at 10 K using superconducting quantum

interface device (SQUID). From temperature dependent magnetization

measurements it is observed that there is no drastic change in the magnetic

moment up to room temperature for lesser cobalt dopant. However, we

assertively conclude from both experimental and theoretical observations that

only lower percentage of dopant in GaN is a suitable candidate for spintronic

devices operating at and above room temperature. The difference in magnetic

moment value between the experimental and theoretical results may be due to

the influence of structural defects on the ferromagnetic order and also the

position of cobalt occupancy in GaN matrix. To induce half metallic

ferromagnetism in GaN it is suggested that a low percentage of Ni or Co

without any vacancy in the system may be suitable to function as the good

candidate for spintronic applications.