Magnetic resonance study of nanocrystalline 0.10MnO/0.90ZnO

(1)

Magnetic resonance study of

nanocrystalline

0.10MnO/0.90ZnO

1

1 G. Zolnierkiewicz , J. Typek ,

N. Guskos , U. Narkiewicz , D. Sibera

1, 2

3

1

Institute of Physics, Faculty of Mechanical Engineering and Mechatronics,

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

2

Solid State Section, Department of Physics, University of Athens Panepistimiopolis, 15 784 Zografos, Greece

3

Institute of Chemical and Environmental Engineering,

West Pomeranian University of Technology, Al. Piastów 17, 70-310 Szczecin, Poland

Temperature study of the magnetic spectrum of 0.10MnO/0.90ZnO nanocomposite revealed the existence of three kinds of magnetic centers. Two components of the observed spectra, designated as S1 and S2, are connected with defects in magnetic structure of the

2+

ZnMnO nanoparticles: the S1 spectrum displaying hfs is formed by isolated Mn ions, while the S2 line is attributed to the clusters of the ₃

4+

exchange-coupled Mn . Both paramagnetic centers are probably located on the surfaces of ZnMnO nanoparticles. The FMR line S3 is ₃ produced by the ZnMnO nanoparticles in a superparamagnetic phase.₃

In Fig. 1 a selection of the registered magnetic resonance spectra of 0.10MnO/0.90ZnO are presented. In the upper panel in Fig. 1 three spectra obtained at T<30 K are shown, in the bottom panel a few spectra taken at higher temperatures are presented. At low temperatures the spectra of two components are easily to recognized. One component forms a collection of many narrow lines that are presumably the result of a hyperfine structure (hfs) of manganese ion and will be further designated as component S1. The other component constitutes a single, broader line that is superimposed on the S1 spectrum. That component will be designated as S2. Both components are visible in all spectra registered in 4-300 K range. At temperatures T>30 K yet another component is visible – a very broad line that spans the whole range of magnetic field. This component will be designated as S3. 0 1000 2000 3000 4000 5000 6000 7000 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 -15 -10 -5 0 5 10 15 d c "/ d H [a rb . u n it s ] M agnetic field [G] 29.7 K 60 K 100 K 290 K _{200 K} 4 K 10 K 29.7 K 0 1000 2000 3000 4000 5000 6000 7000 -1500 -1000 -500 0 500 1000 1500 d c "/ d H [a rb . u n it s ] M agnetic field [G ] E xperim ent Fitting T=290 K 3250 3300 3350 3400 0 50 100 150 200 250 300 360 400 440 480 520 560 600 R e so n a n ce fie ld [G ] T em perature [K ] P e a k-to -p e a k lin e w id th [G ] 0 5 0 1 0 0 1 5 0 2 0 0 2 5 0 3 0 0 0 .0 0 .5 1 .0 1 .5 2 .0 2 .5 3 .0 3 .5 0 5 1 0 1 5 2 0 2 5 3 0 3 5 0 .0 0 .5 1 .0 1 .5 2 .0 2 .5 3 .0 In te g ra te d in te n si ty I int [a rb .u n it s] T e m p e r a t u r e [ K ] 1/ I int [a rb .u n it s] T e m p e r a t u r e [ K ] 50 100 150 200 250 300 0 500 1000 1500 2000 2500 3000 3500 2000 2500 3000 3500 4000 4500 5000 5500 R e so n a n ce fie ld [G ] Temperature [K] P e a k-to -p e a k lin e w id th [G ] 4 0 6 0 8 0 1 0 0 1 2 0 1 4 0 0 .5 1 .0 1 .5 2 .0 2 .5 3 .0 5 0 1 0 0 1 5 0 2 0 0 2 5 0 3 0 0 3 4 5 6 7 8 9 1 0 1 1 1 2 1/ I int[ ar b. un its ] T e m p e r a tu r e [K ] In te gr at ed in te ns ity I int [a rb .u ni ts ] T e m p e r a tu r e [K ] 2600 2800 3000 3200 3400 3600 3800 4000 4200 4400 -22 -20 -18 -16 -14 -12 -10 -8 -6 -4 -2 0 2 4 6 8 10 d c "/ d H [A rb . u n it s ] M agnetic field [G ] T = 10 K E xperim ent S im ulation

Conclusions

In this work, we present study of magnetic properties of sample containing nanosize ZnO powders doped with MnO magnetic dopand, by using the magnetic resonance method. The magnetic resonance spectra registered at temperatures in the 4-300 K range will be analyzed in term of three different components and attributed to specific magnetic centers.

Aim of the work

The investigated sample was synthesized by the wet chemical method. Initially, from an aqueous solution of nitrides the mixture of manganese hydroxides and zinc hydroxides was obtained. Then the obtained hydroxides were filtered, dried at the temperature of 70°C and calcined 1 hour at 300°C. The final sample contained 10 wt.% MnO and 90 wt.% ZnO. The phase composition was determined by X-ray diffraction (CoK radiation, X'Pert Philips). It was found that the sample contains only _á two phases: ZnO and ZnMnO . The mean crystallite size of magnetic ZnMnO was calculated by using Scherrer's formula ₃ ₃ and was found to be 9 nm.

Magnetic resonance spectra were obtained on a conventional X–band (í = 9.4 GHz) Bruker E 500 spectrometer with the 100 kHz magnetic field modulation. The measurements were carried in the 4 – 290 K temperature range using an Oxford Instrument helium-flow cryostat. The registered spectra are the first derivative of the absorption curve with respect to the sweeping field H.

Experimental

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The S1 spectrum is ascribed to isolated Mn ions which appear as defects in the ZnMnO structure. They might appear on the ₃

4+

surface of the nanograins or inside the grains. Simulation of the hfs of S1 lines as produced by the Mn (S=3/2) ion was not

4+

successful. For the broad S2 spectrum clusters of the exchange-coupled Mn ions are responsible. The hfs structure is not visible in S2 spectrum because it was washed out by a stronger interaction between neighboring ions. The very broad line of the S3 spectrum is most likely attributed to the magnetic ZnMnO nanoparticles. Thus the S3 spectrum is the ferromagnetic ₃ resonance (FMR) line originating in a system of magnetic nanoparticles in the superparamagnetic state. FMR line vanishes from the spectrum before the magnetic structure of nanoparticles freezes at T . Because the S2 and S3 spectra display similar _f

4+

changes at the same temperature 75 K thus the magnetic clusters of Mn ions must be influenced by the magnetic field of a nanoparticle. A possible connection will be that the S2 clusters are formed on the surface of a nanoparticle thus ensuring a close interaction of both systems.

Attribution of the spin system to the S1, S2, and S3 spectra

()

[

S

]

E

( )

S

A

S

I

D

HS

g

H

=

b

+

_z2

-

1 /

3 +

1 +

_x2

-

_y2

+

Spectrum S1

Spectrum S2

_{Spectrum S3}

Fig. 1. Examples of registered spectra of nanocrystalline 0.10MnO/0.90ZnO at low (top panel) and high (bottom panel) temperatures.

Fig. 2. Experimental (black) and fitted (red) magnetic resonance specta of nanocrystalline 0.10MnO/0.90ZnO at 290 K. The fitting included only S2

and S3 spectra components. Fig. 3. Experimental (black) and simulated (red) magnetic resonance specta of

nanocrystalline 0.10MnO/0.90ZnO at 10 K. Only S1 and S2 spectra componenets were included in the simulation.

Fig. 4. Temperature dependence of the resonance field (open squares, right axis) and the peak-to-peak linewidth (filled squares, left axis) for the S2

spectrum component.

Fig. 5. Temperature dependence of the integrated intensity of the S2 spectrum component. The inset shows the temperature dependence of the inverse of the integrated intensity. The straight line in inset is the best fit to

the Curie-Weiss law.

Fig. 6. Temperature dependence of the resonance field (open squares, right axis) and the peak-to-peak linewidth (filled squares, left axis) for

the S3 spectrum component.

Fig. 7. Temperature dependence of the integrated intensity of the S3 spectrum component. The inset shows the temperature dependence of

the inverse of the integrated intensity. The straight line in inset is the best fit to the Curie-Weiss law.

According to XRD study the only magnetic phase is ZnMnO ₃ and it appears as nanoparticles with an average size of 9 nm. ZnMnO has a T higher than 300 K and exhibits a spin-glass ₃ _C behavior with the freezing temperature T =15 K. The other _f important information is that most of manganese ions in

4+

ZnMnO is in Mn oxidation state. ₃

g=2,00232, A=80 G, D=225 G, E=15 G,

Gaussian-shape line, peak-to-peak linewidth DB=8 G

2

I =A· (ÄB )int pp

I =const/(T-T )int CW