in (TiC N -Si-(C-N))/C
x
1-x
studied by ESR
Spin reorientation processes
1,2,*
4
N. Guskos
,
4
2
1
P. Figiel , A. Guskos , and K. A. Karkas
3
2
2
, E. A. Anagnostakis , G. Zolnierkiewicz , J. Typek , A. Biedunkiewicz
1
Solid State Section, Department of Physics, University of Athens, Panepistimiopolis, 15 784 Zografos, Athens, Greece;
2
Institute of Physics, Szczecin University of Technology, Al.Piastow 17, 70-310 Szczecin, Poland;
3
Department of Computer and Communication Engineering, University of Thessaly, Pedion Areos, 38 334 Volos, Greece;
4
Institute of Materials Science and Engineering, Szczecin University of Technology, Al. Piastow 17, 70-310 Szczecin, Poland
Conclusions
Two processes, one being the competition between magnetic trivalent titanium ions clusters (or agglomerates) and the other the reorientation of spin system by internal magnetic field, are deductible from interpretation of the temperature-evolution of the ESR spectra of (TiC +SiC)/C x
samples before and (TiC N +Si N +SiC) after thermal annealing x 1-x 3 4
process in a NH -atmosphere studied in this work. These processes we 3
discuss as liable to influence the nature of ferromagnetic interaction within such technological important multinanocomposite magnetic systems.
Acknowledgments
This paper and the work it concerns were partially generated in the context of the MULTIPROTECT project, funded by the European Community as contract N° NMP3-CT-2005-011783 under the 6th Framework Programme for Research and Technological Development.
Introduction
Recently, the ternary nanomaterial system Ti-Si-C is attracting intense interest, especially for exploring different binary (TiC, SiC) or ternary (Ti SiC , Ti Si C ) nanocomposite 3 2 5 3 x
phases [1-5]. Titanium carbide (TiC) is a metallic compound, in which the electrical charge is transported via conduction electrons, with outstanding physical properties like high hardness, high melting point, and low thermal conductivity [6-13]. TiC titanium carbide is nonstoichiometric; the carbon-to-metal ratio can be changed in a wide range (x x
~ 0.47-0.97) without changing its crystal structure [14]. Titanium- carbide-related composites exhibit physical properties allowing for application in metallurgy, especially in tribology or in manufacturing protective coatings for the cutting tools.
The spectroscopic properties of the above nanomaterial system studied by the electron spin resonance (ESR) method have shown the existence of two paramagnetic centres in titanium carbides covered by carbon. One is understood to be arising from trivalent titanium ions and the other from conducting electrons or magnetic localized centres [15-17]. An additional magnetic measurement has shown coexistence of ferromagnetic and superconducting state [18]. The existence of even a very small number of magnetic clusters or agglomerates embedded in the titanium carbide matrix could have significant influence on the physical properties of these materials. That could improve their mechanical properties which could indeed be favourable for metallurgical applications.
In this report, ESR method has been employed to study the carbon-hosted system (TiC N -Si-(C-N))/C, obtained by a sol-gel method, over an extended temperature range x 1-x
(4-130 K), in order to probe the presence of charge transfer interactions and monitor molecular dynamics, as well as for the possibility detecting ferromagnetism.
Results and discussion
The XRD measurements for sample 2 have shown transformation from (TiC +SiC)/C to (TiC N +Si N +SiC)/C (Fig. 1) and this process x 0.3 0.7 3 4
eliminates partially free carbon. This would increase corrosion resistance of this nanocomposite.
Figure 3 presents the temperature dependence of the ESR spectra in the range of 4-130 K for samples 1 and 2, at the same spectrometer conditions and normalised to unit sample mass. A narrow ESR line is dominating in both samples, centred at g=2.0029(2) with peak-to-peak linewidth H =0.27(1) mT and at g=2.0032 with H =0.15(1) mT at 130 K for samples 1 and 2, respectively. pp pp
Lineshape analysis shows slight asymmetry of the resonance line, thus deviating from a pure Lorentzian function and suggesting the presence of a weak anisotropy and/or a relatively broad resonance background of a very low amplitude. However, the linewidth of the ESR line, which would directly reflect any variation in the spin-lattice relaxation rate and most importantly the dynamic averaging of the g-anisotropy [19], hardly changes in the corresponding temperature interval, suggesting a different origin of the observed effect.
Sample 2 produced more intense ESR signal than sample 1. This could indicate that the thermal annealing process plays a certain role in the formation of additional paramagnetic centres coming from the increase of the concentration of surrounding carbon by about 9 % (as
2 2
calculated from the ratio A ÄH /A ÄH at high temperature, with A being peak-to-peak spectral amplitude and ÄH the peak-to-peak 1 1 2 2
linewidth. Indeed, after the thermal annealing process more carbon must have been bonded to nanoparticles within the second sample.
Both types of samples, on the other hand, showed similar temperature dependence of their ESR spectra. Titanium carbide or nitride has not, in general, produced such a kind of signal, but after sample carburization it must be forming a narrow ESR line by means of localised magnetic centres related to a titanium-carbide narrow, Dysonian-like, resonance due to conducting electrons [16].
The ESR line narrowing after thermal annealing processes is suggesting that the exchange interaction for sample 2 is more intense. A large number of localised spin- centres and a small number of conduction carriers in the polycrystalline graphite (or carbon nanotubes) could be coupled by a strong exchange-interaction, giving a single Lorentzian-like ESR line [19]. In many fullerides a very narrow ESR line arising after extensive aging in air [20] has been observed, in contrast to our case [21]. Titanium carbide encapsulated in carbon has already exhibited a narrow ESR line with Dysonian lineshape in previous investigations of our group [24,25], whereas the ternary nanomaterial system Ti-Si-C appears changing to Lorentzian lineshape, probably in connexion with lowering of the conduction carrier density. The edge state, furthermore, of small-size graphite fibres and the curvature of graphite sheets (like those, of thickness below 20 nm cloaking the sample titanium carbide nanoelements) have been interpreted as capable of inducing magnetic moments [22,23].
Figure 4 presents the obtained temperature dependence of the prime ESR parameters, g -factor, linewidth (ÄH ), and normalized eff pp
resonance intensity IESR(T), in the range of 4-130 K, for samples 1 and 2. The g -factor decreases in both samples, with some sharp change eff
observed at 60 K for sample 1 and 50 K for sample 2 (Fig. 4a). The approximate temperature-rate of change for the g-factor is
-6 -1 -5 -1 -6 -1 -5
Äg/ÄT~9.7x10 K (130 – 60 K) and Äg/ÄT~1.8x10 K (55 – 4 K) for sample 1, and Äg/ÄT~5.0x10 K (130 – 55 K) and Äg/ÄT~1.1x10
-1
K (50 – 4 K) for sample 2. At the higher temperature-region the above gradient is 1.95 times and at the lower temperature-region 1.65 times greater, for sample1. It is believed that competition between the magnetic interactions, reorientation of spins, and freezing processes could be responsible for such behaviour of the g-factor.
The linewidth of the narrow ESR line, which would directly reflect any variation in the spin-lattice relaxation, increases intensively with decreasing temperature in the corresponding temperature interval and some anomaly is observed at 80 K and 50 K (Fig. 4b). This process is essentially stronger for sample 2, suggesting that the magnetic interaction increases after thermal annealing. The sharp change of the linewidth at some temperatures is similar to frustration in magnetic systems, with the broadening of the resonance line is connected with an approach to magnetically ordered state [24,25].
Figures 4c and 4d show the temperature dependence of the ESR integrated intensity (IESR(T)) and its inverse for both samples. In both
cases, two different regimes of the Curie–Weiss temperature-evolution are singled out. The fitting of IESR(T) to a Curie-Weiss law, C/(T-Q), with C the Curie constant and Q the Curie-Weiss temperature, shows the presence in both regions of a positive Curie-Weiss temperature,
Q1=23(1) K and Q2=2(1) K for sample 1, and Q1=36(1) K and Q2=7(1) K for sample 2. Such a positive value of Q verifies the presence of an essential ferromagnetic interactions between the localized magnetic centres. The evolution of the ESR spectra can be further assessed within the whole temperature range through the g(T) and H (T) variations, as shown in Fig. 4a and 4b. pp
Abstract
Samples of (TiC N -Si-(C-N))/C encapsulated in carbon cages x 1-x
have been prepared by a sol-gel method, prior to and after subjection to thermal annealing. In both kinds of sample a very narrow electron spin resonance (ESR) line arising from electron magnetic localized centres can be observed. At low temperature, in particular, a spin reorientation process, plausibly influencing the character of magnetic interaction, is registered. The thermal annealing process in a NH -atmosphere, on the other hand, is 3
identified as a critical factor for determination of the critical points of magnetic transition within the nanocomposite, as well as influencing the interaction mechanism for each major temperature regime.
Corresponding author:e-mail: ngouskos@phys.uoa.gr
Figure 4 Temperature dependence of the ESR parameters of the (TiC N +Si N +SiC)/C
x 1-x 3 4system:
a) g factor, b) linewidth ÄH , c) ESR integrated intensity I
eff pp ESR(T) and d) inverse of the ESR integrated intensity 1/I
ESR(T).
Experimental procedure
Two nanocrystalline powder samples of TiC +SiC encapsulated in carbon cages were prepared x
using a non-hydrolytic sol-gel organotitanium precursor technique [19]. Sample 1 (TiC +SiC)/C, x
was obtained at first and this sample after NH -atmosphere thermal annealing 3
(TiC N +Si N +SiC) was designated as sample 2. During thermal annealing, the free carbon 0.3 0.7 3 4
content has been lowered from 18 to 0.6 wt.%.
X-ray diffraction (XRD) measurements, by means of a copper X-ray tube ( l = 1.5406 Å) operating at a high voltage of 40 kV and at a current-level of 40 mA, were performed for sample characterisation (Fig. 1). The average crystallite size was estimated according to the
Sherrer formula: t=0.9l/B cosqB (1)
where t is average grain size, B is the width of the XRD peak at half intensity, l is X–ray wavelength, and qB is peak angle. Scanning electron microscopy (SEM) image (Fig. 2) details the composition of the second sample as made up of aggregates of cubic SiC+TiC N nanoparticles x 1-x
dispersed in carbon and a small amount of Si N nanofibers3 4 .
The measurements of the temperature evolution of the ESR spectrum of samples, containing around 20 mg of sample powder and placed in 4 mm diameter quartz tubes, were obtained by a conventional X-band (í = 9.4 GHz) Bruker E 500 spectrometer, with a 100 kHz magnetic field modulation. The measurements were carried out in the range from 130 K down to liquid helium temperature, with a ÄT=±1.0 K stability and by means of an Oxford cryogenic system.
335 336 337 338 339 340 -40000 -30000 -20000 -10000 0 10000 20000 30000 40000 d H /d c " [A rb . u n it s ] Magnetic field H [mT] 4 K 130 K a) 335 336 337 338 339 340 -80000 -60000 -40000 -20000 0 20000 40000 60000 80000 d c "/ d H [A rb . u n it s ] Magnetic field H [mT] 3.8 K 130 K b)
Figure 3 Temperature dependence of the ESR spectra of the (TiC N +Si-(C-N))/C x 1-x a) sample 1, and b) sample 2.
system:
Figure 1 XRD spectra of the (TiC N +Si N +SiC)/C 0.3 0.7 3 4 binary system. Figure 2 SEM picture of the (TiC N +Si N +SiC)/C .
x 1-x 3 4 system 0 20 40 60 80 100 120 140 0.00000 0.00001 0.00002 0.00003 0.00004 0.00005 0.00006 0 5 10 15 20 25 30 0.000000 0.000001 0.000002 0.000003 0.000004 0.000005 0.000006 Sample 1 Sample 2 1 /I E S R (T ) Temperature T [K] d) Temperature T [K] 1 /IES R (T ) 0 20 40 60 80 100 120 140 0 50 100 150 200 250 300 350 400 Sample 1 Sample 2 I ES R (T )/ I ES R (1 3 0 ) Temperature T [K] c) 0 20 40 60 80 100 120 140 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Sample 1 Sample 2 L in e w id th D H p p [m T ] Temperature T [K] b) 0 20 40 60 80 100 120 140 2.0020 2.0024 2.0028 2.0032 Sample 1 Sample 2 g e ff Temperature T [K] a)
