DC magnetisation study of nCoO/(1-n)ZnO nanocomposites

(1)

DC magnetization study of

nCoO/(1-n)ZnO nanocomposites

1

2

2 N. Guskos , G. Zolnierkiewicz , J. Typek , A. Guskos , D. Sibera , and U. Narkiewicz

1

Institute of Physics, West Pomeranian University of Technology, Al.Piastow 48, 70-311 Szczecin, Poland.

2

Institute of Chemical and Environment Engineering, West Pomeranian University of Technology,

ul. Pulaskiego 10, 70-322 Szczecin, Poland

?Nanocomposites of the general formula nCoO/(1-n)ZnO (where the composition index n=0.4, 0.5, 0.6 and 0.7) were prepared by using a microwave hydrothermal synthesis at pressure 1.5 MPa applied

for 15 min. At first, a mixture of cobalt and zinc hydroxides was obtained by addition of 2 M solution of KOH to the 20% solution of a proper amount of Zn(NO )·6H O and Co(NO )·6H O in water and then treated in ₃ ₂ ₃ ₂ a solvothermal microwave reactor. Next, the obtained materials were washed with deionized water to remove salt residues. Finally, the materials were dried at 100°C for 24 h.

?The morphology of samples was investigated using scanning electron microscope (SEM, Hitachi) followed by the phase composition of the samples determined by the X-ray diffraction (XRD, CoKá

radiation, X'Pert Philips). The specific surface area of the nanopowders was determined using the Brunauer–Emmett–Teller (BET) method with the equipment Gemini 2360 of Micromeritics. The helium pycnometer AccuPyc 1330 of Micromeritics was applied to determine the density of powders. Magnetization studies were carried on Quantum Design Magnetic Property Measurements System MPMS XL-7 with a superconducting quantum interference device magnetometer in magnetic fields up to 70 kOe and in 2–300 K temperature range.

Experimental

Fig. 1. XRD patterns for nCoO/(1-n)ZnO nanocomposites with n=0.5, 0.6, and 0.7. Peaks attributed to ZnO are marked as Z,

peaks attributed to ZnCo O are marked as S. The not marked 2 4

peaks are attributed to Co(OH) .2

Fig. 3. Temperature dependence of dc susceptibility ÷(T) in ZFC mode for four different nanocomposite. Inserts show magnified low temperature range of ÷(T).

Fig. 2. SEM images of nCoO/(1-n)ZnO nanocomposites with different composition index: (a) n=0.5; (b) n=0.6; (c) n=0.7.

Fig. 9. Magnetisation as a function of reciprocal external magnetic field used to

obtain magnetization saturation M for four S

investigated nanocomposite at 2 K. 20 30 40 50 60 70 Z Z Z Z Z Z Z Z S S S S S S S n=0.7 n=0.6 In te n si ty [A rb . u n its ] 2Q [deg] n=0.5 S Z

According to the results of the XRD analysis, the XRD spectra revealed the presence of ZnO, Co(OH) and ZnCo O phases (Fig. 1). Spinel phase ZnCo O content increases with increasing CoO content 2 2 4 2 4

in the samples, while the ZnO content decreases simultaneously. The mean crystallite sizes of the detected phases were determined using Scherrer's formula. In particular, the mean crystallite size of ZnCo O ₂ ₄

varied from 8 to 12 nm. SEM images of nCoO/(1-n)ZnO nanocomposites with different composition index n=0.5, 0.6, and 0.7 are shown in Fig. 2. The investigations by SEM allowed to distinguish three different

3 2

types of morphology: small spheroidal forms and large plates or rods (Fig. 2). The values of helium density of the investigated samples were in 4.6-4.7 g/cm range, the specific surface area in 19-21 m /g range. The obtained results shows that the helium density and specific surface area of the samples is at a similar level in all studied samples. The low density of samples may be due to the presence of cobalt hydroxide, the presence of this phase was confirmed by XRD analysis.

Results

a)

b)

_c)

•

At least two magnetic components: paramagnetic system of

ZnCo O with antiferromagnetic interaction and ferromagnetic 2 4

component in the low temperature range

2+

•

High-temperature paramagnetic system consists mostly of Co

7

ions (3d , S=3/2) in tetrahedral sites in AB O structure₂ ₄

•

In the low temperature range (T<10 K) the paramagnetic

system of ZnCo O strongly agglomerated nanoparticles the ₂ ₄ spin freezing is observed and at 2.5 K ZFC branch of magnetic susceptibility reaches a maximum

•

The ferromagnetic component observed at low temperatures

3+

may be connected with Co in octahedral positions and oxygen vacancies

•

Ferromagnetic component play bigger role in nanocomposites

with larger Co content

Conclusions

0 2 4 6 8 10 12 n = 0.4 10 Oe 100 Oe 1000 Oe 1 / c [1 0 4 g × O e × e m u -1] 0 2 4 6 8 10 12 n = 0.5 10 Oe 100 Oe 1000 Oe 0 50 100 150 200 250 300 0 1 2 3 4 5 n = 0.6 Temperature [K] 10 Oe 100 Oe 1000 Oe 10000 Oe 70000 Oe 0 50 100 150 200 250 300 0 1 2 3 4 5 6 n = 0.7 10 Oe 100 Oe 1000 Oe 10000 Oe 70000 Oe 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 10 12 14 16 18 20 22 24 26 28 30 32 34 n=0.4 n=0.5 n=0.6 M [e m u × g -1] 1/H [10-5 Oe-1] n=0.7 T=2 K 10 Oe 100 O e 1000 Oe 10000 Oe 70000 O e ZFC FC ZFC FC ZFC FC ZFC FC ZFC FC n=0.4 -7.8 -7.6 -6.1 -6.5 -5.0 -5.3 n=0.5 10.9 -0.3 -3.8 -4.4 -4.3 -3.5 n=0.6 -4.7 -4.6 -3.6 -3.5 -2.6 -2.6 -1 .9 -1.9 -5.1 -4.8 n=0.7 -3.5 -5.1 -0.6 -2.2 2.2 -1.7 0.4 -0.1 -1.1 0.2 10 Oe 100 Oe 1000 Oe 10000 Oe 70000 Oe ZFC FC ZFC FC ZFC FC ZFC FC ZFC FC n=0.4 -2.5E-4 -2.7E-4 -5.9 E-4 -6.2E-4 -7.1E-4 -7.5 E-4

n=0.5 -0.0016 -0.0017 -3.3 E-4 -3.8E-4 -6.1E-4 -5.9 E-4

n=0.6 -7.2E-4 -7.5E-4 -8.7 E-4 -8.8E-4 -9.2E-4 -9.2 E-4 -8.8E-4 -8.7E-4 -1E-3 -9.9E-4

n=0.7 0.0018 0 .0016 1.1E-4 -2E-5 -3.7E-4 -4.8 E-4 -9.7E-4 -9.8E-4 -0.0011 -0.0010

10 Oe 100 Oe 1000 Oe 10000 Oe 70000 Oe ZFC FC ZFC FC ZFC FC ZFC FC ZFC FC n=0.4 2.40 2.42 2.42 2.43 2.40 2.42 n=0.5 2.38 2.66 2.77 2.80 2.80 2.79 n=0.6 2.82 2.82 2.88 2.88 2.88 2.87 2.83 2.83 2.89 2.88 n=0.7 2.65 2.73 2.65 2.73 2.62 2.72 2.64 2.65 2.65 2.63 10 Oe 100 Oe 1000 Oe 10000 Oe 70000 Oe ZFC FC ZFC FC ZFC FC ZFC FC ZFC FC n=0.4 0.72 0.73 0.73 0.74 0.72 0.73 n=0.5 0.71 0.89 0.96 0.98 0.98 0.97 n=0.6 0.99 1.00 1.03 1.04 1.04 1.03 1.00 1.00 1.04 1.04 n=0.7 0.88 0.93 0.88 0.93 0.86 0.93 0.87 0.88 0.88 0.86 0.4 0.5 0.6 0.7 -7.5 -7.0 -6.5 -6.0 -5.5 -5.0 -4.5 -4.0 -3.5 -3.0 -2.5 -2.0 -1.5 2.4 2.5 2.6 2.7 2.8 2.9 T c [K ] Concentration index n FC 1000 Oe m eff [ m B /C o ] -60 -40 -20 0 20 40 60 -30 -25 -20 -15 -10 -5 0 5 10 15 20 25 30 n=0.4 n=0.5 n=0.6 n=0.7 M [e m u × g -1] Field [kOe] 2 K

Table 1. Values of in Eq. (1) obtained from

least squares fitting.

-1 -1

C [emu*K*mol *Oe ]

Table 2. Values of meff [mB/ Co] obtained from Eq. (2)

Table 3. Values of T [K] in Eq. (1) obtained from least squares fitting.C

Table 4. Values of a -1 -1 in Eq. (1) obtained from least squares fitting.

[emu*Oe *mol ] Ms [emu/g] MR [emu/g] HC [Oe] n=0.4 15.67 0.022 27.3 n=0.5 26.30 0.047 34.9 n=0.6 33.04 0.100 73.7 n=0.7 34.21 0.124 101.5 0 50 100 150 200 250 300 0 50 100 150 200 250 300 350 400 450 500 550 10 Oe Fit 100 Oe Fit 1000 Oe Fit 10000 Oe Fit 70000 Oe Fit 1 / c [m o l Co × O e × e m u -1 ] Temperature [K] n=0.7 FC 2 4 6 8 10 12 0.0006 0.0008 0.0010 0.0012 0.0014 0.0016 c [e m u × O e -1 × g -1 ] Temperature [K] ZFC 10 Oe FC 10 Oe ZFC 100 Oe FC 100 Oe ZFC 1000 Oe FC 1000 Oe n = 0.7

a

T

C

+

-=

c

C eff =2.82787× m Table 5. Values of hysteresis loop

parameters at 2 K.

Equation 1.

Equation 2.

Fig. 7 Concentration

dependence of T C

(left axis) and ì /Co B (right axis) in FC modes in magnetic field 1000 Oe.

Fig. 6. Comparison of ZFC and FC susceptibility in low temperature range for n=0.7 nanocomposite.

-1

Fig. 5. Fitting of Eq. (1) to experimental points ÷ (T) in FC mode for n=0.7 nanocomposite.

Fig. 4. Temperature dependence of inverse dc

susceptibility in FC mode for different external magnetic fields in four investigated nanocomposites.

Fig. 8. Hysteresis loops at 2 K for four investigated nanocomposites.