氧化铁的磁性

IEEE TRANSACTIONS ON MAGNETICS, VOL. 50, NO. 8, AUGUST [1**********]

Magnetic and Magnetization Properties of Co-Doped

Fe 2O 3Thin Films

Aseya Akbar, Saira Riaz, Robina Ashraf, and Shahzad Naseem

Centre of Excellence in Solid State Physics, University of the Punjab, Lahore 54590, Pakistan

Amongst the various phases of iron oxide, hematite (Fe2O 3) is the most stable form, which shows antiferromagnetic behavior with ferromagnetic canting at room temperature. Doping of different metal ions in α-Fe 2O 3will not only lead to its new technological and industrial applications but also enhance its performance in existing applications. In this paper, we report synthesis and characterization of cobalt (Co)-dopedFe 2O 3thin ﬁlmswith dopant concentration in the range of 0%–10%.XRD peaks shift to slightly higher angles as compared with undoped thin ﬁlmsdue to smaller ionic radii of cobalt (72pm) as compared with iron (74pm). Room temperature magnetic properties, studied using vibrating sample magnetometer, show increase in saturation magnetization with increase in dopant concentration up to 8%.Further increase in dopant concentration to 10%degrades magnetic properties, which might be because of the presence of more atoms at the grain boundaries. Index Terms —Ferromagnetic, hematite, sol-gel, thin ﬁlms.

I. I NTRODUCTION

AGNETIC thin ﬁlms,ferromagnetic and antiferromag-netic, have attracted considerable attention because of their unique properties that make them important for techno-logical and industrial applications. Possible use of magnetic thin ﬁlmsin magnetic sensors, spintronic and high density magnetic storage devices has resulted in a great deal of interest. This stimulated interest in magnetic and transport properties of multilayered thin ﬁlmsthat stems from discovery of giant magnetoresistance (GMR)and tunneling magnetore-sistnace (TMR)for the advancements of spintronic materials and devices [1]–[4].

Among the various materials of interest iron oxide, espe-cially hematite (α-Fe 2O 3) phase, is an important candidate. Hematite (α-Fe 2O 3) , 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 2.2eV [5].

α-Fe 2O 3has a corundum structure with hexagonal unit cell composed of six formula units. Lattice parameters of hematite are a =5. 034Å,c =13. 75Å[6].This material can also be indexed in rhomobohedral system with two formula units per unit cell with a =5. 427Å,α=55. 3°.The structure of α-Fe 2O 3has close-packed arrays of oxygen along the (001)plane with the iron cations in the octahedral and tetrahedral interstitial sites. Hematite (α-Fe 2O 3) is formed with a stoichiometric metal-to-oxygen ratio especially when it is ﬁnelydivided [4].However, at 1400K, oxygen evapora-tion is substantial and hematite transforms into magnetite at 1550K [4]–[6].

Hematite is weakly ferromagnetic at room temperature because of canted spins with a saturation magnetization of 0.4Am 2/kg[6].It undergoes a phase transition at

Manuscript received December 17, 2013; revised February 20, 2014; accepted March 4, 2014. Date of current version August 15, 2014. Corresponding author:A. Akbar (e-mail:[email protected]).

Color versions of one or more of the ﬁguresin this paper are available online at http://ieeexplore.ieee.org.

Digital Object Identiﬁer10.1109/TMAG.2014.2311826

260K (theMorin temperature T M ) to antiferromagnetic state. Above the Neel temperature (∼956K), hematite is paramag-netic. Below 960K, the Fe 3+ions are anitferromagnetically aligned [6].In the basal plane, the spins are parallel to each other but antiparallel to the spins of the neighboring planes [6].The magnetic easy axis is along the c axis below 260K and above 260K the easy axis is within the basal plane. Therefore, below Neel temperature, an electron traveling in the basal plane will experience a ferromagnetically aligned situation [7]–[10].

To enhance the room temperature magnetic properties of hematite, we here report synthesis and characterization of undoped and cobalt-doped Fe 2O 3thin ﬁlmsusing sol-gel and spin coating method. Dopant concentration is varied in the range 0%–10%.Structural and magnetic properties are correlated with variation in dopant concentration.

II. E XPERIMENTAL D ETAILS

A. Materials

FeCl 3·6H 2O and Co(NO3) 2·4H 2O (Sigma–Aldrich,99.99%pure), were used without further puriﬁcation.Ethanol and n -hexane (Sigma–Aldrich,99.99%pure) were used as solvents.

B. Sol Synthesis and Film Preparation

Two different solutions were prepared prior to the ﬁnalsol synthesis. FeCl 3·6H 2O was dissolved in deionized (DI)water and n -hexane was added to the solution that was stirred vigorously at room temperature. Another solution was prepared by dissolving NaOH in ethanol. Two distinguishable layers appeared after mixing of both the solutions. Finally mixed solution was heated on a hot plate at 60°Cfor several hours to obtain single-layered clear sol. Sol was aged at room temperature for 48h before the ﬁlmdeposition. For cobalt-doped iron oxide thin ﬁlms,cobalt nitrate Co(NO3) 2·4H 2O was dissolved in DI water and added to the iron oxide sol with variation in cobalt concentration (2%,4%,6%,8%,and 10%).

Undoped and Co-doped thin ﬁlmswere deposited onto copper (Cu)substrates. Cu was ﬁrstlyetched by diluted

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2201204 Fig. 1. XRD pattern of undoped and cobalt-doped Fe θ°view of 42°–45°2O at 300°C.Inset:expanded 2showing 3thin ﬁlmsannealed shift of peak positions to higher angles.

HCl and then repeatedly rinsed with DI water. It was then ultrasonically agitated at room temperature for 10min in acetone and isopropyl alcohol to remove the residual organic impurities.

Sols were spin coated onto Cu substrates at 3000r/minfor 30s and then aged at room temperature for 24h. Films were annealed at 300°Cin the presence of vacuum under 500Oe applied magnetic ﬁeld(MF)for 60min. C. Characterization Tools

Bruker D8Advance X-ray diffractometer (XRD)was used to study the phase and crystalline structure of undoped and cobalt-doped iron oxide thin ﬁlms.X-ray diffractometer used copper target with λ=1. 5406Å(Niﬁltered).Lake Shore’svibrating sample magnetometer (VSM)was used to study the room temperature magnetic properties.

III. R ESULTS AND D ISCUSSION

Fig. 1shows XRD patterns for undoped and Co-doped Fe 2O 3thin ﬁlmsprepared using sol-gel method after MF annealing at 300°C.Appearance of (110),(012),(202),and (024)planes (Fig.1) indicates the formation of hematite (α-Fe 2O 3) pure phase (JCPDScard no. 87–1165)at a low temperature of 300°C.Hematite (α-Fe 2O 3) phase persisted even after doping of 10%and peaks corresponding to cobalt oxide or metal cobalt were not observed.

Crystallite size was calculated using (1)[11]and is plotted as a function of dopant concentration along with crystallinity in Fig. 2

t =

k λ

B cos θ

(1)

where k is the shape factor taken as 0.9, λis the wavelength, B is full width at half maximum (FWHM),and θis the diffraction angle.

Crystallite size increased with increase in the doping con-centration up to 4%(Fig.2). This low level of doping may result in the dopant atoms being dissolved in the lattice properly. However, beyond 4%it is possible that some of

the

IEEE TRANSACTIONS ON MAGNETICS, VOL. 50, NO. 8, AUGUST

2014

Fig. 2. Crystallite size and crystallinity as a function of dopant concentration.

dopant atoms go to the interstitial positions or sit on the grain boundaries; it is to be kept in mind that it is a polycrystalline ﬁlmand as such there are a large number of grain boundaries. These atoms will destroy the crystalline structure and will also result into a decreased crystallite size [12],as shown in Fig. 2. Further, stresses occur because of the difference in ionic radii of the host and dopant atoms, and this can also be another reason for the decrease in crystallite size [13].

Shift in peak positions, to slightly higher angles, are observed with the increase in dopant concentrations up to 10%.This shift of peak positions to higher angles is due to smaller ionic radii of cobalt (72pm) as compared with that of iron (74pm). The small ionic radius of cobalt leads to decrease in unit cell [Fig.3(b)]that causes decrease in d-spacing which according to Bragg’slaw shifts the peak positions to higher angles [11].

Previously, pure phase hematite thin ﬁlmswere prepared at relatively high temperatures. Lian et al. [14]prepared hematite thin ﬁlmsusing sol-gel method at temperature of 500°C.Kumar et al. [15],Glasscock et al. [16],and Souza et al. [17]also reported annealing at 500°Cto obtain hematite phase. Lattice parameters (a , c ) and unit cell volume (V ) were calculated using (2)and (3)[11]and are shown as a function of dopant concentration in Fig. 3

sin 2θ=λ2 22

λ2l 2

3a 2h +k +hk +4c

(2)where (hkl ) represent the miller indices, λis the wavelength (1.5406Å)

V =0. 866a 2c .

(3)

X-ray density (ρ)[11]was calculated using

ρ=

1. 66042 A

(4)

where A is the sum of atomic weights of the atoms in the unit cell, ρis in g/cm3, and V is the volume of unit cell in Å3. The porosity values were calculated

using

Porosity (%) =1−ρexp

ρstd

×100(5)

AKBAR et al. :MAGNETIC AND MAGNETIZATION PROPERTIES OF Co-DOPED Fe 2O 3THIN FILMS

2201204

Fig. 3. (a)Lattice parameters and (b)unit cell volume of Co-doped Fe thin

ﬁlms.

2O 3Fig. 4. X-ray density and porosity as a function of dopant concentration.

where ρexp is the calculated X-ray density and ρstd is the standard density taken from the JCPDS data.

X-ray density and porosity of the ﬁlmsis shown as a function of dopant concentration in Fig. 4. Increase in X-ray density, with increased dopant concentration, indicates formation of compact structure with cobalt incorporation. Fig. 5shows M –H curves for undoped and cobalt-doped Fe 2O 3thin ﬁlms.It can be seen from Fig. 5that even undoped iron oxide thin ﬁlmsshow ferromagnetic behavior as opposed to antiferromagnetic nature of hematite.

In the temperature range 260–950K (13°C–677°C),(111)planes arrange themselves to form layers of Fe 3+cations [6].Spins interact ferromagnetically within the same plane while antiferromagnetic coupling arises with the spins of the adjacent planes, that is, antiparallel arrangement of spins. Because of spin orbit coupling canting between two adjacent planes arises that produces uncompensated magnetic moment of Fe 3+cations which is the cause of ferromagnetic behavior

[6].

Fig. 5.

M–Hcurves for Co-doped Fe 2O 3thin

ﬁlms.

Fig. 6. Coercivity and saturation magnetization as a function of dopant concentration.

The uncompensated magnetic moments seem to have appeared during sol’ssynthesis [18]since in our case even undoped ﬁlmsshow ferromagnetic behavior.

Fig. 6shows variation in saturation magnetization (M ) as a function of varying dopant concentration. s ) and coercivity (H c M s , in Co-doped Fe 2O 3thin ﬁlms,increases up to dopant concentration of 8%as shown in Fig. 6. However, a sharp decrease in M s value was observed by further increasing dopant concentration to 10%,which might be because of the presence of more atoms at the grain boundaries as seen in the XRD results with a slight increase in crystallite size from 8%to 10%.Ziese and Thornton [19]reported that doping in iron oxide generated extra electrons in the host lattice. In nonmagnetic lattice, these electrons can propagate freely through the crystal. However, in a magnetic lattice localized spin order is present that hinders the motion of doped charge carries. According to Hund’srule, strong exchange interaction exists among the d electrons [19].This strong exchange inter-action will force the electrons to take the direction of spin of localized electrons. As a result extra electrons with spinup will be available but cannot hop into the neighboring spin down site [19].

In addition, if a canted structure with angle between two sublattices is present then the total energy can be estimated using [19]

E (θ)=J S 2cos θ−6tx cos

θ2

(6)

where E is energy of the system, J is exchange interaction, S is spin quantum number, θis angle between spins of the two sites, t is the hopping matrix, and x is the dopant concentration.

2201204 TABLE I

C OMPARISON OF S ATURATION M AGNETIZATION

C O -D OPED T HIN F ILMS

The energy will be minimized under the cos θ

condition given in

3t 2=

2J S 2

x . (7)Equation (7)represents that by increasing x, spin structure becomes canted resulting in the presence of both ferromagnetic and antiferromagnetic ordering [19].Moreover, order will be purely ferromagnetic for a condition given in

x >x c =

2J S 2

(8)where x c is the critical concentration below which the mag-netic structure is undistorted.

Cobalt with electronic conﬁgurationof [Ar]3d74s 2has one more electron than iron ([Ar]3d64s 2) with less energy of d state. Cobalt atom donates one d and two s electrons to oxygen that results in remaining six electrons on cobalt. When substituted for Fe with spin down electron, the spin down d band gets completely ﬁlledwith remaining one d-electron residing in spinup band. This results in net magnetization of 1μB. Thus, canting of spin structure results because of the imbalance created by incorporation of cobalt in Fe 2O 3lattice, which in turn results in increased magnetization values in Co-doped Fe 2O 3thin ﬁlms[20].Best magnetic properties are observed with dopant concentration of 4%–8%(Fig.5). With increase in dopant concentration above 8%,a large number of defects can result in inadequate alignment of spins. This leads to less prominent canting of spin structure resulting in a fewer number of uncompensated magnetic moments thus reducing the magnetic properties [21].In addition, at high dopant concentration (≥10%)the reduction in magnetic moment arises owing to the presence of adjacent cobalt ions with different oxidation states of 2+and 3+with antifer-romagnetic coupling in Fe 2O 3lattice [9].Comparison of saturation magnetization of Co-doped Fe 2O 3thin ﬁlmswith literature can be seen in Table I.

IV. C ONCLUSIONS

Undoped and cobalt-doped (2%–10%)hematite (α-Fe 2O 3) thin ﬁlmshave been prepared using sol-gel and spin coating method. The ﬁlmswere annealed at 300°Cin the presence of a magnetic ﬁeldof 500Oe. XRD results indicated the formation of phase pure hematite in undoped and doped thin ﬁlms.Ferromagnetic behavior has been observed even in undoped Fe 2O 3thin ﬁlms.Increase in saturation magneti-zation, ∼2.1emu/cm3, was observed in Co-doped iron oxide

IEEE TRANSACTIONS ON MAGNETICS, VOL. 50, NO. 8, AUGUST 2014

thin ﬁlmsup to a dopant concentration of 8%which is lost with further increase in the dopant concentration.

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