Nanoparticles with “onion-like” core (α-Fe) / double shell ... · atypical “onion-like”...

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Contribution (Oral/Poster/Keynote) TNT2009 September 07-11, 2009 Barcelona-Spain Nanoparticles with “onion-like” core (α-Fe) / double shell (γ-Fe/Fe-oxide) structure M.P. Fernández a , P. Gorria a , R. Boada b , J. Chaboy b , M. Sevilla c , A.B. Fuertes c , J.-M. Greneche d , J.A. Blanco a . a Departamento de Física, Universidad de Oviedo, Calvo Sotelo s/n, 33007, Oviedo, SPAIN. b Instituto de Ciencia de Materiales de Aragón and Dpto. de Física de la Materia Condensada, CSIC-Universidad de Zaragoza, 50009, Zaragoza, SPAIN c Instituto Nacional del Carbón, CSIC, Ap. 73, 33080, Oviedo, SPAIN. d LPEC, UMR 6087, Université du Maine, 72085 Le Mans Cedex 9, FRANCE [email protected] The large variety of magnetic scenarios displayed by metal transition nanoparticle systems is greatly influenced by the reduced size and/or miscellaneous morphologies of the nanoparticles, because surface, interface or finite-size effects play an important role [1-3]. Nevertheless, the magnetic behaviour may depend on the surrounding media or matrix in which the nanoparticles are dispersed as well. The complete understanding of such behaviour is of major importance for any particular application from the technological point of view. One of the most studied systems are those containing iron nanoparticles, which very often present a core-shell morphology due to the strong and fast reactivity of iron with oxygen. In this situation, strong magnetic interactions between the iron particle core and the iron oxide shell give rise to modifications of the coercivity, the magnetic anisotropy and to the appearance of an exchange bias effect [4]. Recently, exchange bias has been reported for iron nanoparticles coated by a thin ferrimagnetic iron oxide layer at temperatures much lower than that corresponding to the Néel temperature (T N ) of the iron oxide. This effect is originated by the freezing of the iron oxide spins due to low temperature spin-glass like behaviour. We will show in this contribution how the magnetic behaviour of iron nanoparticles, randomly dispersed in activated carbon, is strongly correlated with the microstructure and the complex nanoparticle morphology, through the combination of several experimental techniques such as x-ray powder diffraction, scanning and transmission electron microscopy, Fe-K edge x-ray absorption and Mössbauer spectroscopies and magnetic measurements. The iron nanoparticles were randomly dispersed in a commercial porous carbon using a low cost and simple chemical synthesis route [5,6]. The process is based in a pyrolysis taken place at the intersections between the nanopores of the activated carbon (see Figure 1a) [7,8]. This method allows the fabrication of several grams of powder samples with iron weight percentages above 15 %. The NPs present a broad particle-size distribution (5-50 nm) with an atypical “onion-like” morphology: Core (α-Fe) – inner layer (γ-Fe) – outer layer (Fe-oxide) (see Figures 1b-1c). The estimated phase percentages (by combining XRD, Mössbauer and XANES spectroscopies) are around: 44% (α-Fe), 32% (γ-Fe) and 24% (Fe-oxides) (see Figures 2 and 3). The γ-Fe layer remains paramagnetic down to 77 K, while the whole system does not reach a fully superparamagnetic regime even at 750 K, probably due to the high blocking temperature of the largest NPs (see Figures 4a-4b). However, the nanoparticles exhibit exchange bias below 60 K (H ex 150 Oe at T = 5 K), a temperature much lower than T N of Fe oxides (ferrimagnetic magnetite, maghemite or a non-stoichiometric mixture of them) (Figure 4b). Hence, we suggest that the existence of the exchange bias could be due to the combination of two effects, (i) the oxide shell-core interaction, governed by a spin freezing at the surface of the particles due to low-temperature spin-glass behaviour of the oxide layer, and (ii) the possible antiferromagnetic coupling between α-Fe core and γ-Fe layer. The financial support for this research work provided from FEDER and the Spanish MICINN (MAT2008-06542-C04, MAT2008-00407) is acknowledged. One of us, M.P.F. thanks MICINN for the award of a FPI grant cofinanced by the European Social Fund. Oral Poster

Transcript of Nanoparticles with “onion-like” core (α-Fe) / double shell ... · atypical “onion-like”...

Page 1: Nanoparticles with “onion-like” core (α-Fe) / double shell ... · atypical “onion-like” morphology: Core (α-Fe) – inner layer (γ-Fe) – outer layer (Fe-oxide) (see Figures

Contribution (Oral/Poster/Keynote)

TNT2009 September 07-11, 2009 Barcelona-Spain

Nanoparticles with “onion-like” core (α-Fe) / double shell (γ-Fe/Fe-oxide) structure

M.P. Fernández a, P. Gorria a, R. Boada b, J. Chaboy b, M. Sevilla c, A.B. Fuertes c, J.-M. Greneche d, J.A. Blanco a.

a Departamento de Física, Universidad de Oviedo, Calvo Sotelo s/n, 33007, Oviedo, SPAIN. b Instituto de Ciencia de Materiales de Aragón and Dpto. de Física de la Materia Condensada,

CSIC-Universidad de Zaragoza, 50009, Zaragoza, SPAIN c Instituto Nacional del Carbón, CSIC, Ap. 73, 33080, Oviedo, SPAIN.

d LPEC, UMR 6087, Université du Maine, 72085 Le Mans Cedex 9, FRANCE

[email protected]

The large variety of magnetic scenarios displayed by metal transition nanoparticle systems is greatly influenced by the reduced size and/or miscellaneous morphologies of the nanoparticles, because surface, interface or finite-size effects play an important role [1-3]. Nevertheless, the magnetic behaviour may depend on the surrounding media or matrix in which the nanoparticles are dispersed as well. The complete understanding of such behaviour is of major importance for any particular application from the technological point of view. One of the most studied systems are those containing iron nanoparticles, which very often present a core-shell morphology due to the strong and fast reactivity of iron with oxygen. In this situation, strong magnetic interactions between the iron particle core and the iron oxide shell give rise to modifications of the coercivity, the magnetic anisotropy and to the appearance of an exchange bias effect [4]. Recently, exchange bias has been reported for iron nanoparticles coated by a thin ferrimagnetic iron oxide layer at temperatures much lower than that corresponding to the Néel temperature (TN) of the iron oxide. This effect is originated by the freezing of the iron oxide spins due to low temperature spin-glass like behaviour. We will show in this contribution how the magnetic behaviour of iron nanoparticles, randomly dispersed in activated carbon, is strongly correlated with the microstructure and the complex nanoparticle morphology, through the combination of several experimental techniques such as x-ray powder diffraction, scanning and transmission electron microscopy, Fe-K edge x-ray absorption and Mössbauer spectroscopies and magnetic measurements. The iron nanoparticles were randomly dispersed in a commercial porous carbon using a low cost and simple chemical synthesis route [5,6]. The process is based in a pyrolysis taken place at the intersections between the nanopores of the activated carbon (see Figure 1a) [7,8]. This method allows the fabrication of several grams of powder samples with iron weight percentages above 15 %. The NPs present a broad particle-size distribution (5-50 nm) with an atypical “onion-like” morphology: Core (α-Fe) – inner layer (γ-Fe) – outer layer (Fe-oxide) (see Figures 1b-1c). The estimated phase percentages (by combining XRD, Mössbauer and XANES spectroscopies) are around: 44% (α-Fe), 32% (γ-Fe) and 24% (Fe-oxides) (see Figures 2 and 3). The γ-Fe layer remains paramagnetic down to 77 K, while the whole system does not reach a fully superparamagnetic regime even at 750 K, probably due to the high blocking temperature of the largest NPs (see Figures 4a-4b). However, the nanoparticles exhibit exchange bias below 60 K (Hex ≈ 150 Oe at T = 5 K), a temperature much lower than TN of Fe oxides (ferrimagnetic magnetite, maghemite or a non-stoichiometric mixture of them) (Figure 4b). Hence, we suggest that the existence of the exchange bias could be due to the combination of two effects, (i) the oxide shell-core interaction, governed by a spin freezing at the surface of the particles due to low-temperature spin-glass behaviour of the oxide layer, and (ii) the possible antiferromagnetic coupling between α-Fe core and γ-Fe layer. The financial support for this research work provided from FEDER and the Spanish MICINN (MAT2008-06542-C04, MAT2008-00407) is acknowledged. One of us, M.P.F. thanks MICINN for the award of a FPI grant cofinanced by the European Social Fund.

Oral

Poster

Page 2: Nanoparticles with “onion-like” core (α-Fe) / double shell ... · atypical “onion-like” morphology: Core (α-Fe) – inner layer (γ-Fe) – outer layer (Fe-oxide) (see Figures

Contribution (Oral/Poster/Keynote)

TNT2009 September 07-11, 2009 Barcelona-Spain

References: [1] J.L. Dormann, D. Fioranni, E. Tronc, Magnetic relaxation in fine-particle systems, John Wiley & Sons, (1997). [2] P. Gorria, M. Sevilla, J.A. Blanco, A.B. Fuertes, Carbon 44 (2006) 1954. [3] P. Gorria, M.P. Fernández, M. Sevilla, J.A. Blanco, A.B. Fuertes, Phys. Status Solidi-RRL 3 (2009) 4. [4] J. Nogués, J. Sort, V. Langlais, V. Skumryev, S. Suriñach, J. S. Muñoz, Phys. Reports, 422 (2005) 65. [5] A.B. Fuertes, P. Tartaj, Chem. Mater. 18 (2006) 1675. [6] A.B. Fuertes, P. Tartaj, Small 3 (2006) 275. [7] M.P. Fernández, D.S. Schmool, A.S. Silva, M. Sevilla, A.B. Fuertes, P. Gorria, J.A. Blanco, J. Non-Cryst. Solids 354 (2008) 5219-5221. [8] M.P. Fernández, D.S. Schmool, A.S. Silva, M. Sevilla, A.B. Fuertes, P. Gorria, J.A. Blanco, J. Magn. Magn. Mater. DOI: 10.1016/j.jmmm.2009.04.058. Figures:

50 nm

Fe-oxideγ-Feα-Fe

Nanoparticlesporous activated carbon

a)

b)

c)

Figure 1. a) Schematic drawing of interconnected

nanopores where Fe-NPs are deposited. b) “Onion-like” core (α-Fe) / double shell (γ-Fe/Fe-oxide)

structure. c) TEM image of Fe-AC NPs. The arrow points an “onion-like” nanoparticle.

0

2

40 60 80 100 120 140 160

Fe-AC

Inte

nsity

( x

10 3 c

ount

s)

angle, 2θ (deg) Figure 2. Observed (red points) and calculated

(solid line) room temperature XRD pattern of Fe-AC sample. Positions of Bragg reflections are

represented by vertical bars (first series correspond to γ-Fe and the second to α-Fe crystalline phases).

The observed-calculated difference pattern is depicted at the bottom of the figure.

0 20 40 60 80 100 120

44% (α -Fe)24% (γ -Fe2O3)32% (γ -Fe)

Nor

m. A

bsor

ptio

n (a

rb. u

nits

)

E - Eo(eV)

Fe-AC simulation

Figure 3. Fe K-edge XANES spectrum for Fe-AC

composite (black line) together with the simulation (red line) considering a mixture of 44% (α-Fe) 24%

(Fe-oxide) and 32% (γ-Fe).

0

100

200

300

400

0 100 200 300

H (O

e)

T (K)

Hex

Hc biggest Fe-NPs

blocked

b)

0.4

0.6

0.8

1

M (e

mu/

g)

H appl

= 100 Oe

a)FC

ZFC

Figure 4. a) M(T) ZFC-FC magnetization

measurement under an applied magnetic field of 100 Oe. b) Temperature dependences of the coercive, Hc,

and exchange bias, Hex , fields.

Oral

Poster