Magnetic Properties of Co-doped Fe2O3 Thin Films

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ScienceDirect Materials Today: Proceedings 2 (2015) 5674 – 5678

International Conference on Solid State Physics 2013 (ICSSP’13)

Magnetic Properties of Co-doped Fe2O3 Thin Films Aseya Akbar*, Sidra Bashir, Saira Riaz and Shahzad Naseem Centre of Excellence in Solid State Physics, University of the Punjab, Lahore-54590, Pakistan

Abstract Among the various phases of iron oxide, Fe2O3 is the most stable form of iron oxide that shows antiferromagnetic behaviour. Doping of different metal ions in α-Fe2O3 will lead to its new technological and industrial applications and enhancement of its performance in existing applications. We here report synthesis and characterization of Cobalt (Co) doped Fe2O3 thin films with dopant concentration varied as 1%-7%. XRD peaks shift to slightly higher angles as compared to undoped thin films due to the smaller ionic radii of cobalt (0.72Å) as compared to iron (0.74Å). Room temperature magnetic properties, studied using Vibrating Sample Magnetometer, show increase in saturation magnetization as well as coercivity in doped iron oxide films due to canting of spin structure that arises because of imbalance in magnetic moment of two sublattices created by incorporation of cobalt in Fe2O3. ©2015 2015Elsevier Elsevier Ltd. rights reserved. © Ltd. AllAll rights reserved. Selectionand andPeer-review Peer-review under responsibility the Committee Members of International Conference on Solid State Physics Selection under responsibility of theofCommittee Members of International Conference on Solid State Physics 2013 2013(ICSSP’13) (ICSSP’13). Keywords: Thin films; Iron oxide; Magnetic properties

* Corresponding author. Tel.: +92-423-5839387. E-mail address: [email protected]

2214-7853 © 2015 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of the Committee Members of International Conference on Solid State Physics 2013 (ICSSP’13) doi:10.1016/j.matpr.2015.11.108

Aseya Akbar et al. / Materials Today: Proceedings 2 (2015) 5674 – 5678

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1. Introduction Iron oxides are commonly occurring compounds which are extensively used in many technological applications. Their applications extend from industrial to scientific use. From the pre-historic era where oxides of iron were mainly used as pigments and dyes to the current century where their applications draw a wide circle including magnetic storage, photo electrochemistry, gas sensors, energy storage contrast agents for cancer treatment and electrochromism [1-6]. Hematite (α -Fe2O3), also known as ferric oxide, is blood red in color and is extremely stable at ambient conditions. It is often the end product of other iron oxide transformations. Hematite is a semiconductor material with optical band gap of 2.20 eV [1]. Hematite has a corundum structure having hexagonal unit cell with a = 0.5034nm, c = 1.375. There are six formula units per unit cell. Hematite can also be indexed in rhomobohedral system with two formula units per unit cell with a=0.5427, α =55.3o [1, 7-9]. The structure of hematite is composed of arrays of oxygen ion along the (001) plane with the iron cations on the octahedral or tetrahedral interstitial sites. α-Fe2O3 forms with a stoichiometric metal to oxygen ratio especially when it is finely divided. At 1400 K oxygen evaporation is substantial and hematite transforms into magnetite at 1550K [1, 10-13]. Hematite has canted antiferromagnetism below transition temperature also known as Morin temperature, and above this temperature it is weak parasitic ferromagnetism with 0.1 gauss.cm3g-1 saturation. By neutron diffraction, it was found that the spins lie in (111) planes with the orientation +--+ along the trigonal axis. Below 10oC, the spins are aligned along [111] axis by a strong exchange field. Above this temperature anisotropy field confines the spin to the basal plane; this anisotropy is variable and very small. Morin temperature is dependent on the particle size and it decreases with the decrease in particle size [8-12, 18]. Hematite is weak ferromagnetic at room temperature due to canted spins with a saturation magnetization of 0.4 Am2/kg [1]. It undergoes a phase transition at 260 K (the Morin temperature TM) to antiferromagnetic state. Above the Neel temperature of 956 K hematite is paramagnetic [1]. Below 960 K, the Fe3+ ions are anitferromagnetically aligned. In a basal plane the spins are parallel to each other but antiparallel to the spins in the neighbouring planes. The magnetic easy axis is along the c-axis below 260 K. Above 260K the easy axis is within the basal plane. So below Neel temperature, an electron travelling in the basal plane will experience a ferromagnetically aligned situation [1]. The magnetic properties of hematite are dependent on parameters like cation substitutions, crystal size and particle size. Decreasing the particle size decreases the Morin temperature and for particles of size less than 8-20 nm it completely disappears. Poor crystallinity and substitution of cation lowers Neel temperature and Morin temperature [1]. The electrical conductivity of the material between Neel and Morin temperatures is very poor without the presence of impurities. This is thought to be result of the very low drift mobilities of charge carriers in this range, which require thermal activation to hop between lattice sites. Conductivity in hematite is highly anisotropic, observed experimentally, confirmed theoretically, and explained by its antiferromagnetic structure [1, 10-15]. For enhancing the magnetic properties of hematite thin films we here report synthesis and characterization of cobalt doped hematite thin films using sol-gel method. Variation in structural and magnetic properties is correlated with variation in dopant concentration. 2. Experimental Details Synthesis of pure hematite phase films were carried out using Fe(NO3)3.9H2O as precursor dissolved in DI water. Ethylene glycol was then added to the above solution. The solution was stirred at 60˚C for several hours before a stable sol was obtained [2, 3]. For cobalt doped iron oxide synthesis, cobalt nitrate Co(NO3)3.9H2O was dissolved in DI water and was added to Fe2O3 in appropriate proportions. The dopant concentration was varied as 1-9%. The sol was aged at room temperature and then the films were deposited on copper substrate. The films were annealed at 300˚C in the presence of vacuum under applied magnetic field of 500Oe for 60min. Films were characterized structurally using Bruker D8 Advance X-ray Diffractometer with CuKα radiations (λ=1.5406Å). For studying the magnetic properties Lakeshore’s 7407 Vibrating Sample Magnetometer was used.

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3. Results and Discussion Fig. 1 shows XRD patterns for undoped and cobalt doped iron oxide thin films. Presence of diffraction peaks corresponding to (012), (104), (110), (006), (113), (202), (024), (214) and (217) indicate the formation of α-Fe2O3 phase of iron oxide. No peaks corresponding to cobalt oxide were observed even at dopant concentration of 7% (Fig. 1(c)). Slight shift of peak positions was observed with increase in dopant concentration due to slight difference in ionic radius of cobalt as compared to iron. The shift in peak positions indicates that the dopant has been successfully incorporated into the host lattice. Crystallite size (t), strain (Δd/d) and dislocation density (δ) of cobalt doped iron oxide thin films was calculated using Eqs. (1)-(3).

t=

0.9 λ B cos θ

Strain =

δ =

(1)

Δd d exp − d pdf = d d pdf

1 t2

(2)

(3)

Where, λ is the wavelength (1.5406Å), B is the full width at half maximum, dexp is the d-spacing calculated from XRD pattern and dpdf is the d-spacing taken from JCPDS card no. 87-1165, θ is the diffraction angle.

Fig. 1. XRD patterns for cobalt doped iron oxide thin films with dopant concentration (a) 1% (b) 5% (c) 7%

Fig. 2. M-H curves for Co-doped Fe2O3 thin films

Crystallite size, strain and dislocation density of cobalt doped iron oxide thin films are listed in table 1. Crystallite size increased from 30.3nm to 37.9nm as the dopant concentration was increased to 5%. With further increase in dopant concentration, crystallite size decreased to 27.8nm. It can be predicted that at low dopant concentration the atoms occupy the substitutional positions. At high dopant concentration, the probability of dopant atoms residing on grain boundaries increased. This leads to destruction in crystallite size. In addition, strain in thin films also affects the crystallite size. Decrease in strain with increase in dopant concentration to 5% (table 1) also resulted in increase in crystallite size. The increased crystallite size thus resulted in few number of grain boundaries and the reduced number of grain boundaries at low dopant concentration thus resulted in decreased dislocations in

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Aseya Akbar et al. / Materials Today: Proceedings 2 (2015) 5674 – 5678

thin films (Table 1). Table 1 Structural parameters of cobalt doped iron oxide thin films Dopant concentration Crystallite size (nm) Dislocation density (1015 (%) lines/m2) 1 30.3 1.08922 5 37.9 0.69618 7 27.8 1.29393

Strain (10-3) 6.53 5.587 6.014

Lattice parameters (a,c), x-ray density (ρ) and porosity of cobalt doped Fe2O3 thin films is calculated using Eqs. (4)(6).

sin 2 θ =

ρ=

λ2 ⎛ 2 λ2l 2 2 ⎜ h + k + hk ⎞⎟ + 2

3a 2 ⎝

⎠

1.66042ΣA V

⎡ ρ exp ⎤ Porosity(%) = ⎢1 − ⎥ × 100 ⎣ ρ std ⎦

4c

(4)

(5)

(6)

Where, (hkl) represent the miller indices, ΣA is the sum of atomic weights of the atoms in the unit cell, V is the volume of unit cell (V=0.866a2c), ρexp is the experimental density calculated using Eq. (5), ρstd is bulk density of iron oxide. Table 2 shows lattice parameters plotted as a function of dopant concentration. The lattice parameters decreased as the dopant concentration was increased to 5% thus leading to reduction in unit cell volume. This decrease in unit cell volume from 303Å to 286Å is due to smaller ionic radius of cobalt as compared to that of iron. This reduced unit cell volume leads to increased density of the films indicating more compact structure of cobalt doped iron oxide thin films. Table 2. Lattice parameters, unit cell volume, x-ray density and porosity of cobalt doped iron oxide thin films Dopant a (Å) c(Å) Unit cell Porosity X-ray concentrati volume (Å3) (%) density on (%) (g/cm3) 1 5.089 13.69 307.033 5.09 0.1159 5 5.015 13.154 286.4953 5.165 0.0956 7 5.0202 13.1653 287.3419 5.103 0.1132 Although no peaks corresponding to aluminum or aluminum oxide were observed in the XRD pattern (Fig. 1) even with high dopant concentration of 9% but variation in crystallite size (Table 1) and lattice parameters (Table 2) strongly indicated that the dopant has been successfully incorporated in the α-Fe2O3 lattice. Fig. 2 shows magnetic properties of cobalt doped hematite thin films. The films show ferromagnetic behavior and saturation magnetization increases as the dopant concentration increases. In undoped hematite thin films the spin orbit coupling between two adjacent planes gives rise to uncompensated magnetic moment [16-22]. In case of doped Fe2O3 thin films Co and Fe have electronic configuration of 3d74s2 and 3d64s2 respectively. When cobalt is substituted for iron the spin down d bands get completely filled up while the remaining one electron resides in spin up band, and as the result of which net magnetization arises in doped hematite thin films.

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Aseya Akbar et al. / Materials Today: Proceedings 2 (2015) 5674 – 5678

4. Summary Cobalt doped iron oxide thin films were prepared using sol-gel method. The dopant concentration was varied as 1%, 5% and 9%. XRD results indicated that cobalt successfully replaced iron in the host lattice. The peak positions shifted to higher angles due to lower ionic radius of cobalt as compared to that of iron. Cobalt doped Fe2O3 thin films showed ferromagnetic behavior because of the presence of uncompensated spins arising from cobalt doping. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]

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