The effect of hydrogen incorporation in the nanocrystalline iron particles on their magnetic properties

The effect of hydrogen incorporation in the nanocrystalline iron particles on their magnetic properties

International Journal of Hydrogen Energy 26 (2001) 503–505 www.elsevier.com/locate/ijhydene The e%ect of hydrogen incorporation in the nanocrystalli...

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International Journal of Hydrogen Energy 26 (2001) 503–505

www.elsevier.com/locate/ijhydene

The e%ect of hydrogen incorporation in the nanocrystalline iron particles on their magnetic properties A.A. Novakovaa; ∗ , T.Yu. Kiselevaa , O.V. Agladzea , N.S. Perova , B.P. Tarasovb a Department

of Physics, Moscow M.V.Lomonosov State University, 117234 Moscow, Russia for New Chemical Problems RAS, 142432, Chernogolovka, Russia

b Institute

Abstract The e%ect of hydrogen on the structure and magnetic properties of nanocrystalline iron powders was investigated by XRD, M8ossbauer spectroscopy and magnetic measurements. The nanocrystalline Fe powders were produced by two methods: iron hydroxide reduction in hot hydrogen
1. Introduction Nanocrystalline (n.c.) materials are known as polycrystals with the size of individual crystallites in the order of several (1–10) nm. They consist of the following two components [1–3]: a crystalline component formed by all atoms located in the lattice of the crystallites (grains) and an interfacial component comprising all atoms which are situated in the grain boundaries between the crystallites. The structure of grain boundaries depends on how the n.c. material was prepared and strongly a%ects its magnetic properties [4,5]. In this work we have studied the in
2. Experimental Samples of n.c. iron were produced with the same (∼13 nm) mean grain size by two methods: from iron



Corresponding author. E-mail address: [email protected] (A.A. Novakova).



hydroxide reduction in hot hydrogen
0360-3199/01/$ 20.00 ? 2001 International Association for Hydrogen Energy. Published by Elsevier Science Ltd. All rights reserved. PII: S 0 3 6 0 - 3 1 9 9 ( 0 0 ) 0 0 0 8 3 - 5

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A.A. Novakova et al. / International Journal of Hydrogen Energy 26 (2001) 503–505

Table 1 Structural and magnetic parameters of the Fe samples Structural parameters

Magnetic parameters

Sample

D (nm) ±2

 × 10−3 ±0:2

a (nm) ±0:002

Hc (Oe) ±0:2

(G cm3 =g) ±50

Carboxylic Fe Reduced in H2 Ball-milled in H2 during 8 h

42 14 13

0.7 0.8 4.3

2.868 2.867 2.863

1.8 1.9 13.2

240 146 270

Fig. 1. X-ray di%raction patterns of carboxylic Fe (a), Fe reduced in H2 (b), and Fe ball-milled in H2 (c).

As seen from the table the strains values  in ball-milled iron are three times higher than in the reduced one. XRD-patterns proIle analysis showed that line broadening comes from the small grain size for reduced n.c. iron and from a combination of grain reInement and high defect levels for ball-milled n.c. Fe. To separate grain and surface fractions parameters in our samples, we applied the M8ossbauer spectroscopy. The M8ossbauer spectra are shown in Fig. 2. The sample of initial carboxylic iron has the crystalline -Fe standard parameters of isomer shift and hyperIne Ield (see Table 2). The spectrum of reduced n.c. iron (Fig. 2(b)) consists of two components corresponding to -Fe particles and to their ultraIne surfaces (∼ 2% of the total atoms amount) having the M8ossbauer parameters typical for amorphous Fe(III) oxy-hydroxides. Increased M8ossbauer linewidth for -Fe component in comparison to the carboxylic Fe sample may be caused by the grain-interface disordering range. M8ossbauer spectrum of n.c. ball-milled Fe sample (Fig. 2(c)) includes two subspectra corresponding to -Fe grains (84%) and to grain-boundary region (16%). The hyperIne parameters of subspectra are given in Table 2. As a result of mechanical strains during ball milling, a lot of defects such as dislocations and inclinations appear in grain boundaries region of n.c. Fe [2]. M8ossbauer parameters of grain boundary component: reduced hyperIne

Fig. 2. M8ossbauer spectra of carboxylic Fe (a), Fe reduced in H2 (b), and Fe ball-milled in H2 (c).

A.A. Novakova et al. / International Journal of Hydrogen Energy 26 (2001) 503–505

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Table 2 The M8ossbauer parameters of samples under investigation Sample

Subspectra

(mm=s)

H (kOe)

M (mm=s)

G (mm=s)

S (%)

Carboxylic Fe

Grain Grain boundaries -Fe grain Fe(III)-oxy-hydroxide Grain Grain boundaries

0 — 0 0.41 0 0

330 — 330 — 331 314

— — — 0.99 — —

0.26 — 0.34 0.55 0.27 0.55

100 — 98 2 84 16

Reduced Fe Ball-milled Fe

Ield value and broadened linewidth — re
creates diPculties for the magnetic domain-wall rotation in practically monodomain grains and a%ects the coercieve Ield value.

4. Discussion

[1] Gleiter H. Nanocrystalline materials. Progr Mater Sci 1989;3:223–9. [2] Gleiter H. Nanostruct. Mater 1992;1:1. [3] Herr U, Jing J, Birringer R, Gonser U. Appl Phys Lett 1987;8:50. [4] LoSer J, Van Swygenhoven H, Wagner W, Meier J, Doudin B, Ansermet J-Ph. Nanostruct Mater 1997;9:523. [5] Del Bianko L, Hernando A, Bonetti E, Navarro E. Phys Rev B 1997;56(14):56. [6] Newman JF, Shreir LL. J Iron Steel Inst 1969;207:1369. [7] Oriani RA. Acta Metallur 1970;18:147. [8] Allen-Booth DM, Newitt J. Acta Metallur 1975;22:171. [9] Choo WY, Lee JY, Cho CG, Hwang SH. J Mater Sci 1981;16:1285.

We have received two n.c. iron samples with the same grain size but having di%erent structures of grain-boundary region. In spite of active H2 -gas-working atmosphere hydrogen dilution and interactions with core iron grains are negligible in both cases. Nevertheless, in reducing process hydrogen interactions with the surface leads to the oxy-hydroxide ultrathin amorphous surface layer chemical formation on iron particles. Magnetic interactions between ferromagnetic iron grains and an antiferromagnetic interfacial layer [4] resulted in observed drastical decrease of magnetization value. In ball-milled sample, hydrogen absorption by defective grain boundaries causes the increase of their density and change of magnetic interactions, correspondingly. This fact

Acknowledgements This work was supported by the Russian Foundation for Basic Research (Grant 99-02-16135). References