Structure and magnetic properties of Cr nanoparticles and Cr2O3 nanoparticles

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Physica B 358 (2005) 332–338 www.elsevier.com/locate/physb

Structure and magnetic properties of Cr nanoparticles and Cr2O3 nanoparticles W.S. Zhanga,b,, E. Bru¨ckb, Z.D. Zhanga, O. Tegusb, W.F. Lia, P.Z. Sia, D.Y. Genga, K.H.J. Buschowb a

Shenyang National Laboratory for Materials Science, Institute of Metal Research, and International Centre for Materials Physics, Chinese Academy of Sciences, Wenhua Road, Shenyang 110016, China b Van der Waals-Zeeman Instituut, Universiteit van Amsterdam, Valckenierstraat 65, 1018 XE Amsterdam, The Netherlands Received 24 January 2005; accepted 24 January 2005

Abstract We have synthesized Cr nanoparticles by arc-discharge and Cr2O3 nanoparticles by subsequent annealing the asprepared Cr nanoparticles. The structure of these nanoparticles is studied by means of X-ray diffraction, X-ray photoelectron spectroscopy, and high-resolution transmission electron microscope. Most of the particles show a good crystal habit of well-defined cubic or orthorhombic shape, while some small particles show spherical shape. The asprepared Cr nanoparticles have a BCC Cr core coated with a thin Cr2O3 layer. Cr in the core of the particles heated at 873 K for 4 h is changed to Cr2O3. The results of magnetic measurements show that the Cr nanoparticles exhibit mainly antiferromagnetic properties, in addition to a weak-ferromagnetic component at lower fields. The weak-ferromagnetic component may be ascribed to uncompensated surface spins. For the field-cooled Cr2O3 nanoparticles, an exchange bias is observed in the hysteresis loops, which can be interpreted as the exchange coupling between the uncompensated spins at the surface and the spins in the core of the Cr2O3 nanoparticles. r 2005 Elsevier B.V. All rights reserved. PACS: 75.50.Kj; 81.05.Ys Keywords: Arc-discharge; Cr nanoparticles; Weak ferromagnetism; Exchange bias

Corresponding author. Shenyang National Laboratory for

Materials Science, Institute of Metal Research, and International Centre for Materials Physics, Chinese Academy of Sciences, Wenhua Road, Shenyang 110016, China. Tel.:+86 24 2397 1853; fax: +86 24 2389 1320. E-mail addresses: [email protected] (W.S. Zhang), [email protected] (E. Bru¨ck).

1. Introduction The magnetism of magnetic nanoparticles is a challenging subject in physics because the role of the surface spins increases as the particle size is decreased, which differs substantially from that of

0921-4526/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2005.01.469

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bulk counterparts [1]. It is also very fascinating that, compared with bulk materials, the nanoscale magnetic materials find diverse technological applications in information storage and spintronics [2]. The magnetic properties of Cr nanoparticles of about 13 nm were studied by Tsunoda et al. [3], who reported that the Ne´el temperature TN varies in the temperature range from 300 to 350 K. Fitzsimmons et al. [4] indicated that TN of Cr particles with a size of 73 nm is about 120 K, while Cr particles of 16 nm become ordered antiferromagnetically only at 20 K [5]. Abdul-Razzaq and Seehra [6] pointed out that the conflicting results may originate from undetectable surface oxides whose magnetization is much larger than that of pure Cr and, therefore, the effect of the oxide layer on the magnetic properties of the Cr nanoparticles deserves it to be studied. Bulk Cr is an antiferromagnet with a Ne´el temperature TN of 311 K [7], while bulk Cr2O3 is also an antiferromagnet with T N ¼ 307 K [8]. For antiferromagnetic (AFM) nanoparticles, the effect of uncompensated spins on their surfaces is another factor that may affect the magnetic properties. Ne´el [9] suggested that very fine particles of an AFM material should exhibit particular magnetic properties such as superparamagnetism and weak ferromagnetism. Recently, many researchers investigated the effect of the uncompensated surface spins of different nanoparticles, like CuO [10], NiO [11,12], ferritin [13,14], and a-Fe2O3 [15,16]. In this paper, we report on the fabrication of Cr nanoparticles by arc-discharge and of Cr2O3 nanoparticles by subsequent annealing the asprepared Cr nanoparticles, and we study the structure and magnetic properties of these nanoparticles.

2. Experimental procedures Cr nanoparticles were prepared by means of the arc-discharge method in an atmosphere consisting of a mixture of hydrogen gas (H2) and argon gas (Ar). The anode was bulk Cr of 99.15% purity, while a tungsten needle of 2 mm in diameter served

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as the cathode. The chamber was evacuated to 3 mPa before the introduction of the gases. A mixture of H2 (25%) and Ar (75%) was introduced into the chamber, till the gas pressure in the chamber reached 240 hPa. The gas mixture serves as a reactant gas and as a source of hydrogen plasma. A voltage in the range of 20–30 V was applied between the cathode and the anode for generating a discharge current between 20 and 100 A. After 2–3 h of evaporation, the deposits on the water-cooled wall of the chamber were collected as the product in our experiments. To produce Cr2O3 nanoparticles, we heated the as-prepared Cr nanoparticles at 873 K for 4 h in air. The samples for transmission electron microscope (TEM) observation were prepared in two steps [17,18]: First, the deposit was dispersed in ethanol in an ultrasound bath and then a drop of the suspension was transferred onto a carbon-coated TEM mesh grid and the ethanol was allowed to evaporate. The samples were examined in a JEOL 2000EX HRTEM operating at 200 kV . X-ray diffraction (XRD) spectra were recorded at room temperature in a Riguku D/max-2500pc diffractrometer with Cu Ka radiation and a graphite monochromator. The as-prepared powder samples were compacted into round plates of 10 mm diameter and about 1 mm thickness for t he measurements in the RIBER LAS-3000 Mk-2 X-ray photoelectron spectroscopy (XPS) spectrometer, while the Mg Ka line was used as an X-ray source. Magnetic measurements were performed using a superconducting quantum interference device (SQUID) magnetometer (MPMS 5S, Quantum Design) in applied fields up to 5 T. For measurements of the zero-fieldcooled (ZFC) and field-cooled (FC) temperature dependence of magnetization, the samples were first cooled at zero field from room temperature to 5 K, then the magnetization was measured in a field of 10 mT from 5 to 350 K. After this, the samples were cooled from 350 to 5 K maintaining the same field. Then t he FC curves were measured with increasing temperature.

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3. Results and discussion The XRD pattern of the as-prepared Cr nanoparticles is shown in Fig. 1(a), which shows good agreement with single-phase Cr patterns from literature [19]. Fig. 1(b) shows the XRD pattern of the nanoparticles, after annealing in air at 873 K for 4 h, which can be indexed as Cr2O3 phase [20]. The typical morphology of the as-prepared Cr nanoparticles is represented in a HRTEM micrograph (Fig. 2). Most of the particles show a good crystal habit of well-defined cubic or orthorhombic shape, while some small particles show a spherical shape (Fig. 2(a)). The size of the particles varies from 10 nm to about 50 nm. Electron diffraction patterns of these particles are consistent with BCC Chromium (Fig. 2(b)). From the HRTEM image, one can find that there is a thin coating layer around the nanoparticles, which is expected to be chromium oxide because it is easy to form an oxide layer on the surface of the nanoparticles, when they are exposed to air [21,22]. Note, Cr nanoparticles with cubic shape were found earlier by Kimoto and Nishida [23]. The reason why we have

Intensity ( arb. units)

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10

Cr2O3

(a) Cr

20

30

40 50 60 70 2 Theta ( Degrees )

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Fig. 1. XRD patterns of (a) the as-prepared Cr and (b) the asannealed Cr2O3 nanoparticles.

not found any observable XRD peaks of chromium oxides in the XRD patterns of the Cr nanoparticles is that the oxide layer is too thin to give rise to a detectable XRD peak. To gain more information on the surface chemistry, we analyzed our samples with XPS. The species covering the surface of the nanoparticles can be detected by the kinetic-energy analysis of both valence-band and inner-shell photoelectrons. To obtain information on the surface properties of the Cr nanoparticles, the binding energies of Cr2p3/2 and O1s electrons were measured by analyzing the XPS spectra. In Fig. 3(a), we show the XPS spectrum obtained from the compacted powder. The spectrum can be fitted by a peak at 576.14 eV corresponding to Cr2O3 and a peak at 573 eV corresponding to metallic Cr. The solid lines are the experimental curves and the dot line is the fitted curve. This is indicating that the surface oxide layer is very thin because the penetration depth of the photons in the XPS experiment is 3 nm. The XPS spectrum of the O1s electrons, displayed in Fig. 3(b), also confirms the presence of only Cr2O3. From the XRD, HRTEM and XPS analyses above, we conclude that the as-prepared Cr nanoparticles have a BCC Cr core coated with a thin Cr2O3 layer, and that, for the particles heated at 873 K for 4 h, Cr in the core is transformed into Cr2O3. Plots of the magnetization M versus applied field B measured at 5 and 300 K are shown in Figs. 4 and 5 for the as-prepared Cr nanoparticles and Cr2O3 nanoparticles. For both types of nanoparticles, except for a weak-ferromagnetic (WFM) component at lower fields, the magnetization curves are essentially linear up to 5 T without tendency towards saturation. For the as-prepared Cr nanoparticles, the WFM component appears to be saturated around 0.8 T. We derive a spontaneous moment of 0.08 Am2/kg at 300 K when extrapolating to B ¼ 0: Since one of the chromium oxides, CrO2, is a ferromagnet with a Curie temperature TC of about 394 K, one might speculate that the WFM component may be due to the presence of some CrO2. However, we can exclude this. The absence of CrO2 in the present nanoparticles is evident from our XPS analysis.

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Fig. 2. (a) HRTEM images and (b) electron diffraction patterns of the as-prepared Cr nanoparticles.

575.9eV

Cr2O3

530.7eV

Cr

570 (a)

O1s Intensity ( arb. unit )

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Cr2p3/2

575 580 585 Binding energy (eV)

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Cr2O3

525 (b)

530 535 Binding energy (eV)

Fig. 3. (a) Cr2p3/2 (the solid line is the experimental curve and the dot line are the fitted curves) and (b) O1s survey patterns of the surface binding energy of the as-prepared Cr nanoparticles.

Additionally, the WFM component is observed also in the samples heat-treated in air for about 4 h at 873 K, under which condition it is known that CrO2 is not stable. Also, when measuring the temperature dependence of the magnetization in 50 mT above room temperature, we could hardly find any change around 394 K (see the inset of Fig. 7). We therefore think the existence of WFM in the Cr nanoparticles and the Cr2O3 nanoparticles should be ascribed to uncompensated surface

spins. It is known that short-range ferromagnetic ordering on the surface can extend to rather high temperatures [10]. This phenomenon is very similar to that found in CuO nanoparticles. Many researchers have studied surface spin disorder in nanoparticles, which is an important factor in the magnetic relaxation, and have reported experimental evidence for this phenomenon [10–16]. Bulk Cr is antiferromagnetically ordered below its Ne´el temperature TN311 K [7],

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with a very low susceptibility of about wE4  108 m3/kg. Bulk Cr2O3 is also an antiferromagnet with Ne´el temperature TNE307 K [8]. As suggested by Ne´el, a net magnetic moment can appear in AFM nanoparticles, due to an imbalance of spins ‘up’ and ‘down’ near the surface [9]. As the particle size is reduced, the ratio of surface to volume increases and the magnetic behavior of the surface become apparent. Richardson et al. [24] discussed the various cases of Ne´el’s model for the uncompensated spins p ¼ nA  nB ; where nA and nB are the numbers of atoms on a twosublattice antiferromagnet. The magnetic moment for B  0 depends on p, which in turn depends on the crystal structure, the particle morphology, and the particle size.

Cr2O3at 5 K Cr at 5 K Cr2O3 at 300 K Cr at 300 K

0.6

0.4

0.2

0.0

0.5

1.0 1.5 Magnetic Field ( T )

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Fig. 4. Magnetization of the as-prepared Cr nanoparticles and the as-annealed Cr2O3 nanoparticles measured as a function of applied field at 5 and 300 K. The dashed line shows the extrapolation to zero field.

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Cr 0.4 -4

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Fig. 5 shows the hysteresis loops of the Cr nanoparticles and the Cr2O3 nanoparticles at 300 K and at 5 K. At 300 K, the magnetization of the Cr nanoparticles (0.5 Am2/kg at 5 T) is more than three times larger than that of bulk Cr but much lower than that of the Cr2O3 nanoparticles (1.4 Am2/kg at 5 T). In addition to the fielddependent magnetic response it also shows a considerable spontaneous magnetization. The difference between the Cr and Cr2O3 nanoparticles is smaller at 5 K. It is evident that the surface layer of the particles strongly affects the magnetic properties of the Cr nanoparticles, and also of the Cr2O3 nanoparticles. This is because nanoparticles have a large surface/volume ratio, and the surface spins will make a major contribution to the magnetization. At 300 K, the hysteresis loops for both Cr and Cr2O3 show very small coercivity and remanence. When the temperature is decreased to 5 K, both coercivity and remanence increase. To check for possible exchange-bias effects, we measured the hysteresis loops at different temperatures, after cooling down from 350 K in a field of 50 mT (see Fig. 6 for the 5 K results). For the Cr nanoparticles, no shift in the FC hysteresis loops is observed down to 5 K, i.e., the bias field BE  0: However, for the Cr2O3 nanoparticles, exchange bias sets in around room temperature and a bias field of about 40 mT is observed at 5 K. Exchange bias is of great interest because of the reduction of the saturation field needed to observe giant magnetoresistance in an exchange-biased system [25]. Usually, exchange bias is found when materials with FM–AFM interfaces are cooled through the Ne´el temperature (TN) of the AFM

Cr2O3 Cr

1

-4

-2

0

0

2

4

-1 -2

-1.2

(a)

Magnetic Field (T)

(b)

Magnetic Field (T)

Fig. 5. Hysteresis loop at (a) 300 K and (b) 5 K for the as-prepared Cr and the as-annealed Cr2O3 nanoparticles.

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FC ZFC

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Magnetization (Am2/ kg)

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-0.2

Magnetic Field (T)

(b)

Magnetic Field (T)

Fig. 6. Comparison of the hysteresis loops for (a) Cr and (b) Cr2O3 nanoparticles in 5 T at 5 K under field-cooled and zero-field-cooled conditions.

0.030 0.04

0.03

2

M (Am / kg)

0.025

M (Am2/ kg)

0.020 0.015

0.02

0.01 320

0.010

340 360 380 Temperature (K)

400

0.005 Cr2O3 Cr

0.000 0

50

100 150 200 250 300 350 400 Temperature (K)

Fig. 7. Zero-field-cooled and field-cooled (in a field of 10 mT) temperature dependence of magnetization of the as-prepared Cr and as-annealed Cr2O3 nanoparticles. Inset: Magnetization M plotted against temperature (in a field of 50 mT) for Cr2O3 nanoparticles.

(with the Curie temperature, TC of the FM larger than TN) [26]. Here, in the Cr2O3 nanoparticles, the WFM component of the surface layer may serve as the FM part. The reason that we do not observe any loop shifts in the Cr nanoparticles is that the exchange coupling between the uncompensated surface spins and the spins in the Cr core may be too weak. When the core is oxidized into Cr2O3, the exchange coupling becomes strong enough to create the exchange bias. This kind of

loop shifts has also been found in pure ferrimagnetic or AFM nanoparticles by other researchers [11,27]. The temperature-dependent magnetization of the as-prepared Cr nanoparticles and the Cr2O3 nanoparticles, measured in ZFC and FC processes in a relatively low field of 10 mT, is plotted in Fig. 7. Both types of the particles do not exhibit typical AFM behavior, since there is no peak around 310 K that is representative of TN. This could also be explained as an effect of the imbalance of the surface spins. For nanoparticles, the magnetization on the surface can easily dominate the AFM contribution due to uncompensated surface spins, and thus we could not observe the characteristics of the AFM component.

4. Conclusion Cr nanoparticles coated with Cr2O3 and Cr2O3 nanoparticles have been prepared by means of arcdischarge in a mixture of hydrogen and argon gas. Magnetic measurements show that a WFM component and an antiferromagnetic component coexist in both the nanoparticles. Uncompensated surface spins are at the origin of the WFM component. For the Cr2O3 nanoparticles, when cooled in 5 T, exchange bias is observed in the hysteresis loops, which is interpreted as resulting from the exchange coupling between the uncompensated surface spins and the spins in the Cr2O3

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core. The spins on the surface make a distinct contribution to the magnetization of the nanoparticles.

Acknowledgements We gratefully acknowledge the financial supports from the National Natural Science Foundation of China under Grants No. 59725103 and No. 50171070. This work has been also supported by scientific exchange program between China and The Netherlands. References [1] Z.D. Zhang, ‘‘Nanocapsules’’, in: H.S. Nalwa (Ed.), Encyclopedia of Nanoscience and Nanotechnology, vol. 6, American Scientific Publishers, 2004, pp. 77–160. [2] R.H. Kodama, A.E. Berkowitz, Phys. Rev. B 59 (1999) 6321. [3] Y. Tsunoda, H. Nakano, S. Matsuo, J. Phys.: Condens. Matter 5 (1993) L29. [4] M.R. Fitzsimmons, J.A. Eastman, R.A. Robinson, R. Movshovich, J.W. Lynn, J. Appl. Phys. 78 (1995) 1364. [5] M.R. Fitzsimmons, J.A. Eastman, R.A. Robinson, A.C. Lawson, J.D. Thompson, R. Movshovich, J. Satti, Phys. Rev. B 48 (1993) 8245. [6] W. Abdul-Razzaq, M.S. Seehra, Phys. Stat. Sol. (a) 193 (2002) 94. [7] E. Fawcett, Rev. Mod. Phys. 60 (1988) 209. [8] S. Foner, Phys. Rev. 130 (1963) 183. [9] L. Ne´el, in: C. Dewitt, B. Dreyfus, P.G. de Gennes (Eds.), Low Temperature Physics, Gordon and Breach, New York, 1962, p. 413.

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