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Physica B 403 (2008) 224–230

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Effect of pressure on the resistivity of spinel ferrite

M.A. Ahmeda,�, S.F. Mansourb, S.I. El-Deka

aMaterials Science Laboratory (1), Physics Department, Faculty of Science, Cairo University, Giza, EgyptbPhysics Department, Faculty of Science, Zagazig University, Zagazig, Egypt

Received 3 December 2006; received in revised form 18 August 2007; accepted 27 August 2007

Abstract

The far-infrared spectra of six mixed ferrites (CdxCo1�x+tTitFe2�2tO4; x ¼ 0.2, 0.00ptp0.25) in the range 200–1000 cm�1 are

reported. Two strong high-frequency bands n1 and n2 are observed. The first one n1 is assigned to the tetrahedral and the second n2 isassigned to the octahedral site. A small kink n3 around near n2 is observed and its intensity increases with divalent octahedral metal ion

concentration. Seebeck coefficient measurements showed that the substitution of tetravalent ion Ti4+ does not change the polarity of the

Seebeck coefficient from p- to n-type. The activation energy and the carrier mobility m were also calculated. The effect of mechanical

pressure on the dc resistivity was investigated.

r 2007 Published by Elsevier B.V.

Keywords: CoCd ferrite; FTIR; Dc resistivity; Pressure effect

1. Introduction

Ferrites play a useful role in many magnetic applicationsbecause their electrical conductivity is relatively low incomparison with that of magnetic materials. Their physicalproperties are strongly dependent on cation distributionamong the tetrahedral (A) and octahedral (B) sites in thecrystals. The dielectric properties of Co1�xCdxFe2O4

(0pxp1) ferrites were studied by Hemeda and Barakat[1]. The electrical conductivity of ferrites with compositionFe3O4, CdFeO4, and CoxZn1�xFe2O4 (0pxp1) wasstudied by Mousa et al. [2] in N2 atmosphere as a functionof temperature. Fe3O4, ZnFe2O4, and CdFe2O4 showedn-type conduction, whereas CoFe2O4 showed p-typeconduction [2].

Inverse spinel cobalt ferrite (CoFe2O4), exhibitingferrimagnetism below 793K [3], has been paid a great dealof attention for its applications such as high-densitymagnetic and magneto-optic recording media. The octahe-dral Co2+(d7) ions in CoFe2O4 are in the high-spin stateand the tetrahedral and the octahedral Fe3+(d5) ions are inthe high-spin state with the spin directions antiparallel to

front matter r 2007 Published by Elsevier B.V.

ysb.2007.08.216

ng author. Tel.: +2025676742.

ss: moala47@hotmail.com (M.A. Ahmed).

each other. Co-ferrite is especially interesting because of itscubic magnetocrystalline anisotropy [4], high coercivity [5],moderate saturation magnetization, high chemical stabi-lity, wear resistance, and electrical insulation [6].The goal of our study is to prepare and investigate the

structural characterizations and the temperature depen-dence of the dc electrical conductivity and the effect of Tiaddition on the mobility of Co–Cd ferrite. Also, one aimedto probe the sensing properties of the prepared samples byapplying external mechanical pressure to open a new era inthe applications of substituted Co-ferrite.

2. Experimental techniques

Samples of the formula CdxCo1�x+tTitFe2�2tO4; x ¼

0.2, 0.00ptp0.25 were prepared by the conventional solid-state reaction from analar grade form oxides (BHD).Stoichiometric amounts were good mixed and grindedusing agate mortar for 3 h, transferred to agate ball mill foranother 3 h. The samples were pressed into pellets formusing uniaxial press of pressure 5� 108N/m2. Presinteringwas carried out in air at 850 1C for 6 h with a heating rateof 2 1C/min and then cooled to room temperature with thesame rate as that of heating, regrinded again, sievedand pressed into disks of diameter 1 cm and thickness

ARTICLE IN PRESS

20 30 40 50 60 70 80

*

x = 0.20

x = 0.05

*

x = 0.15

*

x

2θ

** x = 0.25

(620)(3

11)

(422)

(440)

(511)

(400)

x = 0.00

Inte

nsity

(220)

20 30 40 50 60 70 80

*

*

*

x = 0.10

**

Tra

nsm

itta

nce %

1000 800 600 400 200

Wavenumber (Cm-1)

x = 0

.25

x = 0.20

x = 0.15

x = 0.05

x =

0.10

x = 0.0

585 4

32

398

Fig. 1. IR absorption spectra of (CdxCo1�x+tTitFe2�2tO4; x ¼ 0.2,

0.00ptp0.25) in the range 200–1000 cm�1. (*CoTiO3).

M.A. Ahmed et al. / Physica B 403 (2008) 224–230 225

offfi1.5mm and fired finally at 1150 1C for another 10 h inair with a heating rate of 2 1C/min.The IR spectra were recorded at room temperature in the

range 200–1000 cm�1 using Perkin-Elmer infrared spectro-meter model 1430.For the electrical properties measurements, the two

surfaces of each pellet were good polished, coated withsilver paste and then checked for good conduction.Dc resistivity was measured using two probe techniquewith the potential drop method. Seebeck coefficient voltagemeasurements were carried out using the differentialmethod where a constant temperature difference of 10Kwas maintained across the two surfaces of the samples andthe resulting emf was recorded using a microvoltmetertaking into consideration its sign. The effect of mechanicalpressure on the dc resistivity was carried out using ahomemade apparatus where the pressure was calculatedfrom P ¼ mg/A, where A is the cross-sectional area and m

is the load.

Table 2

Values of the activation energies for CdxCo1�x+tTitFe2�2tO4; x ¼ 0.20,

0.00ptp0.25

t EI (eV) EII (eV)

0.0 0.22 0.22

0.05 0.14 0.13

0.10 0.33 0.27

0.15 0.20 0.44

0.25 0.20 0.75

0.00 0.05 0.10 0.15 0.20 0.25

0.35

0.40

0.45

0.50

(I2/I

1)

Co Ti content (t)

Fig. 2. Dependence of (IA/IB) on CoTi content (t).

Table 1

IR transmission bands of CdxCo1�x+tTitFe2�2tO4; x ¼ 0.20,

0.00ptp0.25

x n1 (cm�1) I1 n2 (cm

�1) I2 n3 (cm�1) I3 yD (K)

0.00 585 55.8 398 18.6 432 2.9 706.7

0.05 589 53.0 389 23.5 433 3.9 703.0

0.10 589 50.0 391 19.6 432 3.9 704.6

0.15 582 49.0 389 10.8 432 3.4 698.0

0.20 585 50.0 392 11.7 430 4.0 702.4

ARTICLE IN PRESSM.A. Ahmed et al. / Physica B 403 (2008) 224–230226

3. Results and discussion

3.1. Spectroscopic analysis

The X-ray diffractograms of CdxCo1�x+tTitFe2�2tO4;x ¼ 0.20, 0.00ptp0.25 reveal a single-phase spinel cubicstructure and homogeneity. Details of structural analysis,density, and porosity were published later [7]. Infraredstudies for the investigated samples indicated the presenceof two strong and high-frequency absorption bands n1 andn2 in the expected range of 582–589 cm�1 and389–398 cm�1, respectively, Fig. 1. The band positionsand intensities are given in Table 1. The change in the bandposition is due to the change in the Fe3+–O2� internucleardistances for the tetrahedral and octahedral sites, respec-tively [8–12]. A small kink n3 near n2 around 400 cm�1 withlow intensity appeared for all compositions. The intensityof this band n3 increases with the concentration of cobaltwhich is confirmed by the results of X-ray. Accordingly,

S (

mV

/K) 1

1.5 2.0

1.5 2.0

1.5 2.0

1.5 2.0

0

0.1

0.0

0.1

0.0

-0.1

6

3

0

100

Fig. 3. The variation of Seebeck co

this band can be assigned to the divalent metal ion–oxygencomplexes in octahedral sites. The IR spectra of the systemCdxCo1�x+tTitFe2�2tO4 (0.00ptp0.25) reveal that theabsorption band n1 does not show any splitting or shoulderwhich indicates no excess of Fe2+ ions. The suggestedcation distribution [7] for this system is given by

ðCd0:2Fe1þy�tÞA½Co0:8þtTitFe1�y�t�

BO4 for 0:00ptp0:1,

y ¼ 0:03

ðCd0:2CozFe1�y�tÞA½Co0:8þt�zTitFe1þy�t�

BO4

for 0:15ptp0:25; y ¼ 0:03; 0:03pzp0:05

It is well known [13] that Cd2+ ions have strongpreference for A-site while Co2+ ions have strong B-sitepreference. The site preference leads predominantly to aninverse spinel structure. When Ti4+ and Co2+ ions subs-titute Fe3+ with higher concentration, the migration ofsome Co2+ from B-site to A-site takes place. This can be

2.5 3.0 3.5

2.5 3.0 3.5

2.5 3.0 3.5

2.5 3.0 3.5

0/T (K-1)

t=0.25

t=0.15

t=0.05

t=0

efficient (S) with temperature.

ARTICLE IN PRESS

300 400 500 600 700 800

0

3

6

300 400 500 600

0

1

2

3

T(K)

μ (

m2/ V

.s)

x10

-5x10

-6μ (

m2

/ V

.s)

t=0.00

t=0.05

t=0.10

t=0.15

t=0.25

Fig. 4. The variation of mobility with temperature for (a) t ¼ 0.0–0.15

and (b) t ¼ 0.25.

0.00 0.05 0.10 0.15 0.20 0.25

0.20

0.40

1.95

2.00

2.05

μ (m

2/ V

.s)x

10

-5

CoTi content (t)

Fig. 5. Dependence of mobility at 630K on CoTi content (t).

M.A. Ahmed et al. / Physica B 403 (2008) 224–230 227

understood from the intensity ratio of I2 to I1 as shown inFig. 2. As a matter of fact, the intensity I1 is a functionof the change in dipole moment with internuclear distancedm/dr [14]. This value represents the contribution of theionic bond Fe–O in the lattice. So, one can conclude thatthe IR spectra give an idea about the change in themolecular structure of the ferrite as the result of theperturbation exerted on Fe–O bonds by introducingthe Ti4+ ions. The Debye temperature of the investigatedcomposition was calculated using the following relation [9]and reported in Table 1. yD ¼ lc=k � nav ¼ 1:438nav, and itwas found to be in the range of 700K, where nav representsthe average value of n1 and n2 frequencies. The obtainedcalculations are reported in Table 1.

3.2. Dc conductivity and mobility

The values of the activation energy were calculated fromthe dependence of the dc conductivity on the absolutetemperature for the investigated samples as a function ofCoTi content and are reported in Table 2.

The reported data of the activation energy indicate thesemi-conducting like behavior of the investigated samples.All the above-mentioned observations indicate an increasein the activation energy in the high-temperature regionwhile nearly stable values were observed in the low-temperature range. The activation energy (EII) in thehigh-temperature region is greater than that in the low-temperature one (EI), because the charge carrier needsmore energy to overcome the electron phonon scattering.

Seebeck coefficient measurements in the tempera-ture range 300–670K, Fig. 3, indicate that the samplesexhibit p-type conduction. This is mainly attributed tothe existence of Ti4+ ions in the B-sites which act aselectrostatic trapping centers for the electrons and thestarting of the initiation of small polarons. The mobilitywas calculated from the data of Seebeck coefficient (S) andplotted versus absolute temperature in Fig. 4a and b fromthe relation [15]

m ¼1

nerand S ¼

2:3k

elog

1� C

C

� �

where m is the drift mobility, r is the dc resistivity, e is theelectronic charge, k is the Boltzmann constant, and C

represents the concentration of Fe2+ ions in the octahedralsites. C can be calculated from n ¼ CN, where N is thenumber of iron ions at the octahedral sites ¼ 1.35� 1022

atom/cm3 for the cubic spinel lattice and n is the number ofcharge carriers. The general trend of the data gives a stableregion up to about 500K, after which an increase of m withtemperature reaching a peak at about 650K, and thenincreases again. This supports the hopping conductionmechanism and that the conductivity is due to thermallyactivation mobility and not due to generation of carriers.Many remarks are observed, namely, the stable regiondecreases with increasing t up to t ¼ 0.15, the peakdisappeared at t ¼ 0.15.

At t ¼ 0.25 (Fig. 4b), m is nearly constant up toffi470Kafter which it increases giving a small hump at 500K andincreases drastically from 550K. Some of the vacancies actas trapping centers for the hopping electrons between Fe2+

ARTICLE IN PRESSM.A. Ahmed et al. / Physica B 403 (2008) 224–230228

and Fe3+ ions, thereby decreasing the ratio Fe2+/Fe3+.This in turn increases the mean free path of the carrierswith the result of increasing the mobility with Ti content(Fig. 5). Though one can say that increasing Co2+ andTi4+ ions content at the expense of Fe3+ enhances thepredominance of p-type conduction mechanism.

The dependence of the dc resistivity on the appliedmechanical pressure at room temperature is illustrated inFig. 6a–c. The data show that the resistivity decreases withincreasing the external applied pressure, this can be due tothe relative decrease of the hopping length with thepressure thereby increasing Fe2+/Fe3+ ratio as well asthe hopping probability enhancing the well-known Verwaymechanism.

It is well known that [16] when a crystal is deformedelastically under influence of applied stresses it returns to

0.0 0.5 1.020

30

40

50

60

0.0 0.5 1.0315

320

325

330

335

340

345

0.0 0 .5 1 .0

40

45

92

93

94

ρ (

Ω.m

)ρ

(Ω

.m)

ρ (

Ω.m

)

P *10

P *1

P *10

Fig. 6. Dependence of dc resistivity at room temperature on pressure for (a

its original state upon removal of the stresses. Cubiccrystals (bcc, fcc) require 3 elastic constant. For smallvalues of strain Hook’s law is obeyed, i.e. the strain islinearly proportional to the applied stress. For pure crystal,the critical shear stress lies in the range of 106–107 dyn/cm2.However, if the stresses are sufficiently large certainamount of deformation takes place after removal of thestresses, the crystal has been plastically deformed.Fig. 6c is a typical curve clarifying the dependence of the

resistivity of the sample with t ¼ 0.05 on the pressure inboth increasing and decreasing runs. A clear hysteresis isobserved keeping the change in r reversible (no permanentchange in r when the pressure is removed gradually). SinceCo-ferrite is characterized by high anisotropy the observedhysteresis can be interpreted as follows: the appliedpressure, which is a mechanical force affects directly the

1.5 2.0

1.5 2.0

1 .5 2 .0

t = 0.10

5 (N/m2)

05 (N/m2)

5 (N/m2)

t=0.00

t=0.05

increasing

decreasing

, b) t ¼ 0.0 and 0.1 and (c) t ¼ 0.05 (when increasing and decreasing).

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0 5 10 15 20 25 30 35 40

100

200

300

0 5 10 15 20 25 30

20

25

30

0 5 10 15 20 25

10

20

30

t (min)

IIIII

It=0.00

t=0.05

ρ(Ωm)

t=0.10

Fig. 7. Dependence of dc resistivity at room temperature on the time at the maximum pressure 2� 105N/m2.

Table 3

Values of the Dr/Dt (Om/min) at the maximum pressure 2� 105N/m2

Co–Ti content (t) Dr/Dt (Om/min)

Region I Region II

0.00 21.17 (0-8.5min) 5.33 (8.5-16min)

0.05 1.75 (0-4min) 0.66 (4-10min)

0.10 4.33 (0-3min) 0.80 (3-5min)

M.A. Ahmed et al. / Physica B 403 (2008) 224–230 229

dipoles orientation and/or domain wall motion. These twoparameters together with the anisotropy control therelaxation that occurs in the domains when the appliedstress is removed.

Fig. 7a–c correlates the variation of the resistivity withtime at the maximum applied pressure (2� 105N/m) forthe samples with 0.0ptp0.10, respectively, at the max-imum load (pressure). The resistivity was found to decreasewith time, the data in the figure is divided into threeregions, the first one in which a drastic drop in r isobserved. In the second region r is decreased graduallyuntil reaching nearly stable values in region III. The valuesof Dr/Dt are calculated and reported in Table 3. In the firsttwo regions, the data clarified that maximum Dr/Dt isobtained at t ¼ 0.00. This indicates the fast response of thissample to the applied pressure which opens a new era in the

field of application. This pressure and time dependence of rsuggests the use of this ferrite with t ¼ 0.0 to be used insensors applications and memory devices at room tem-perature.

4. Conclusion

1.

The ferrimagnetic samples CdxCo1�x+tTitFe2�2tO4;x ¼ 0.20, 0.00ptp0.25 reveal a single-phase spinelcubic structure and homogeneity.

2.

The resistivity values increase with increasing t anddecrease with increasing applied pressure.

3.

At the maximum pressure, the resistivity valuesdecreased monotonically.

4.

A hysteresis was observed on measuring r in bothincreasing and decreasing runs with pressure.

5.

Recommendation of the use of such ferrite (t ¼ 0.0) insensors and memory devices.

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