SYNTHESIS AND MAGNETIC PROPERTIES OF HEMATITE NANOPARTICLES

Conventional sol-gel procedure resulted in formation of hematite nanoparticles, embedded in porous silica matrix. TEM studies confirmed that the average particle size of hematite is around 15nm, with narrow size distribution. Selected area electron diffraction (SAED) showed the formation of the hematite phase.X-ray diffraction and Mössbauer measurements confirmed formation of alpha phase of iron oxide nanoparticles with the same average size .Magnetic measurements using SQUID magnetometer showed typical behavior for superparamagnetic nanoparticle systems such as blocking temperature, irreversibility of zero-field cooled (ZFC)and field cooled (FC) curves and emergence of magnetic hysteresis below blocking temperature. Magnetization behavior was measured in both regimes: blocked and superparamagnetic.Magnetic moments of nanoparticles were determinate by comparing obtained data with values predicted by Langevin model. Experimental results are in good agreement with numericalcalculations of ZFC and FC magnetization based on the superparamagnetic blocking model. Calculated values for anisotropy constants are in predicted extent for hematite nanoparticle samples.Superparamagnetic relaxation phenomena were investigated by Mossbauer spectroscopy in wide range of temperatures, from 10K to 295K. Although weak ferromagnetism was expected in thewhole temperature range due to absence of Morin transition, the coexistence of two sextet components appeared in the Mossbauer spectrum..

Marija Perović, Ana Mraković, Marin Tadić, Dragana Marković, Vojislav Spasojević, The Vinca Institute of Nuclear Science, Condensed Matter Physics Laboratory POB 522, 11001 Belgrade, Serbia

TB

(K)

Tirr

(K)K

(105erg/cm3)

M100Öe 62 290 1,21

M500Öe 60,5 270 1,18

M1000Öe 56 240 1,13

M10000Öe 20 50 0,39

0

2

4

6

T=5K

M(e

mu/

g)

0

2

4

6

T=5K T=300K

M(e

mu/g

) Magnetization as a function of field was measured forseveral temperatures: T=5K, 40K, 250K, 300K. Aboveblocking temperature, there is no hysteresis That ischaracteristic for superparamagnetic systems.Hysteresis, measured on 5K showed lack of saturation.Coercive field and remanent magnetization were

ethanol solution of TEOS

α-Fe2O3

sol formation

ε-Fe2O3

aqueous solutionof

Fe(NO3)3·9H2OHNO3 catalyst

hydrolysisstirring

aeration, drying gelation

heat treatment400-1050C

Preparation route:

α-Fe2O3 nanoparticles (15nm) in a silica matrix containing 45 wt.% of hematite

Sol gel method is very useful for obtainingparticles of well defined shape and size,without significant interparticle interactions.It allows control of size and interactions,while silica matrix ensures uniform orderingand geometry of vacancies. Preparationincluded several stages: mix of etanolsolution of tetraetoxysilan with water solutionof iron nitrate Fe(NO3)3�9H20 in ratio4,5:1:19. mixture homogenization 1h,gelation process on air, drying of gel, fewdays on 55°C and heat treatment 4h, 200 °C

TEM:

0 50 100 150 200 250 300 3500,00

0,02

0,04

0,06

0,08

0,10

0,12

0,14

0,16

0,18

H=100Oe

ZFC

FC

M[e

mu

/g]

T[K]

0 50 100 150 200 250 300 350 4000,0

0,5

1,0

1,5

2,0

2,5

3,0

3,5 H=10000Oe

D D

M[e

mu/g

]

T[K]

Magnetization as a function of temperature wasmeasured for the different fields: H=100Öe, 500Öe,1000Öe i 10000Öe.There is no observation of Morin transition in the wholerange of temperatures, which is expected fornanoparticles of ∼15nm mean diameter. Blockingtemperature is found to be on 62K. From the relation KV= 25kBTB effective anisotropy constant was derived.Comparison of value K=(1,2±0,1)⋅105 erg/cm3 to bulkvalue K=8�104 erg/cm3 suggested important contributionof surface anisotropy in the case of hematitenanoparticle sample.Temperature of irreducibility Tirr=290K for themeasurement in field of 100 Öe points on progressiveblocking of superparamagnetic nanoparticle moments.Measurements on α-Fe2O3 30% samples showed thatprobably presence of weak interactions in our samplecaused increase of blocking temperature.

SQUID measurements:

ICA

ME

20

09

Transmission electron micrograph of α–Fe2O3/SiO2 (a) silica grain with embedded α–Fe2O3 nanoparticles (b) high resolution image of selected grain region (c) the SAEDpattern of the same region

from the fitting parameters:µp =1509 µ B , d =13 nm

( )2

2

2

log

exp2 σ

µ

πµσµ

= cxAf

( ) ( ) µµµ

µµ

µ

dfTk

HLNTHM

B∫

=

max

min

,

-60000 -40000 -20000 0 20000 40000 60000-6

-4

-2

0

H(Oe)

-60000 -40000 -20000 0 20000 40000 60000-6

-4

-2

0

H(Oe)

Langevin fit

-50000 -30000 -10000 10000 30000 50000-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

-2

-1.5

-1

-0.5

0

0.5

1

1.5

2

0 1000 2000 3000 4000 50000,0000

0,0002

0,0004

0,0006

0,0008

lognormal distribution functionxc=1509.595σ=0.34

f(µµ µµ)

µµµµ

Coercive field and remanent magnetization wereHc=700Öe i Mr=0,61emu/g due to weak ferromagnetismof hematite nanoparticles.With the assumption that weak interparticle interactionsare negliable, curve M(H) on 300K was fitted byLangevin functions including log normal distribution ofparticle size.From the fitting parameters, magnetic moment andmean particle diameter were derived: µp =1465 µB id=14 nm. Results showed good agreement with particlesize derived by XRD and TEM techniques. Alsoparameters of distrubion functions were obtained.The occurrence of hematite particles embedded and well dispersed into the SiO2 is confirmed by

TEM observations. SAED pattern of the same area confirmed α-phase of iron oxide. Narow sizedistribution of particles with mean diameter d≈13nm was observed, with no particleagglomeration.

20 30 40 50 60

2Θ

X-ray diffraction spectra of iron oxideparticles embedded in a silica matrix

The XRD spectrum of the sampleshowed well-defined hematitediffraction pattern. No iron-containing impurity phases wereobserved. The average particle sizewas obtained from the linebroadening of the diffraction peaksusing the Scherrer formula , whereis the wavelength (CuKα=1,54181A)and θ is the Bragg angle. Assumingspherical shape the particle diameterd was obtained from the FWHM linebreadth β by setting =0,916.Amorphous pattern of silica is visible.Mean particle diameter is found tobe d=15nm.

XRD:

Mössbauer spectra:

0 20 40 60 80 100 120

46

47

48

49

50

51

52

53

Bh

f (T

)

Temperature (K)

S1

S2

S3

-12 -10 -8 -6 -4 -2 0 2 4 6 8 10 12

0,96

0,97

0,98

0,99

1,00

rela

tive r

ansm

issi

on

velocity (mm/s)

10K

-12 -10 -8 -6 -4 -2 0 2 4 6 8 10 12

0,96

0,98

1,00

velocity (mm/s)

10K

0,96

0,98

1,00

20K

0,96

0,98

1,00

80K

0,96

0,98

1,00

rela

tive tra

nsm

issi

on

120K

0,98

0,99

1,00

150K

0,98

0,99

1,00

220K

0,98

0,99

1,00

295K

Although weak ferromagnetism was expected in thewhole temperature range due to the absence of Morintransition, the coexistence of two sextet componentsappeares in the Mossbauer spectrum. One of thepossible explanations can be found in the fact thathematite particles certainly have a great number of ironatoms at their surface, which are likely to interact withthe silica network. Particle-matrix interactions mightinvolve either the presence of Fe-O-Si bridges at theinterface of the two oxides or the replacement oftetrahedral Fe(III) sites in the iron oxide phase by Si(IV).There is no evidence by XRD or TEM for formation ofsome other iron containing compound .There are suggestions that additional component can beattributed to grain boundaries and interfacial regions,while the first one accounts for the core of grains. Thefirst component in the low temperature spectrum,corresponding to core grains atoms, shows hyperfinefield smaller than those of the corresponding bulk phase,because of the nanocrystalline character of the particles.Additional decrease of hyperfine field was observed inthe second component. The decrease of the meanhyperfine field in this case must be connected with thosegrain boundaries and interfacial regions with higherdegree of disorder.At intermediate temperatures, the sextet and doubletcoexist. Collapse of the sextet is due tosuperparamagnetic relaxation of the particles. Sextetsare not fully collapsed till T=296K. It can be suggestedthat the apparent higher blocking temperature could becaused by an increase in the superparamagneticrelaxation time due to higher anisotropy energyconstant. Such an effect could be related to the influenceof adsorbed molecules on the surface anisotropy.Another explanation can be found in the fact thatinterparticle interactions can cause similar behavior, butin the case of samples prepared by sol gel method thatis very unlikely.The average hyperfine fields, Bhf(T) at lowtemperatures, where no doublet is present, were used todetermine an effective anisotropy constant, K.

Mössbauer absorption spectra were obtained in a standard transmission geometry,using a source of 57Co in rhodium. Calibration was performed using a 25 µm thicknatural iron foil; the isomer shift values are referred to α- Fe. The spectra were fittedwith a simple model with two or three sextets and one or two doublets. The fittedhyperfine parameters of the components clearly correspond to hematite.

0 12

hf hf

kTB B

KV

= −

1. P. Kaushik et al., Colloids and Surfaces A 293 (2007) 162-1662. V. Chhabra et al., Materials Letters 26 (1996) 21-263. J. M. Florez et al. Hyperfine Interactions (2005) 165: 253–2594. G. Ennas et al., Chem. Mater., (1998), 10 (2), 495-5025. S.Morup, C. Ostenfeld, Hyperfine Interactions 136 (2001), 125–131,6. F. Bødker, S. Mørup, Europhys. Lett., 52 (2) (2000), pp. 217–2237. M. F. Hansen, C. B. Koch, S. Morup, Phys. stat. sol. (a) 173, (1999),3058. G. Rixecker , R. Birringer , U. Gonser , and H. Gleiter9. J.-M. Greneche, Hyperfine Interactions 148/149: 79–89, 2003.10.J.M. Greneche, A. Slawska-Waniewska, J. Mag. Mag. Mat. 215/216 (2000) 264-267

literature:

Modified Langevin function fit of M(H) data, T=300K