FM AFM Crossover in Vanadium Oxide Nanomaterials

ISSN 0021-3640, JETP Letters, 2010, Vol. 91, No. 1, pp. 11 15. Pleiades Publishing, Inc., 2010. Original Russian Text S.V. Demishev, A.L. Chernobrovkin, V.V. Glushkov, A.V. Grigorieva, E.A. Goodilin, N.E. Sluchanko, N.A. Samarin, A.V. Semeno, 2010, published in Pis ma v Zhurnal Éksperimental noі i Teoreticheskoі Fiziki, 2010, Vol. 91, No. 1, pp. 12 16. FM AFM Crossover in Vanadium Oxide Nanomaterials S. V. Demishev a, *, A. L. Chernobrovkin a, V. V. Glushkov a, A. V. Grigorieva b, E. A. Goodilin b, N. E. Sluchanko a, N. A. Samarin a, and A. V. Semeno a a Prokhorov General Physics Institute, Russian Academy of Sciences, Russia, 119991 Moscow * e-mail: demis@lt.gpi.ru b Faculty of Materials Sciences, Moscow State University, Moscow, 119991 Russia Received November 5, 2009 The magnetic properties of nanomaterials based on vanadium oxide (multiwall nanotubes, nanorods, and nanolayers) have been investigated in the temperature range of 1.8 220 K by high-frequency (60-GHz) EPR. A transition from a ferromagnetic temperature dependence to an antiferromagnetic temperature dependence has been observed in nanorods and nanotubes with a decrease in the temperature. The FM AFM crossover observed near T C ~ 110 K is accompanied by a low-temperature increase in the Curie constant by a factor of 2.7 7. The comparison of the experimental data for various VO x nanoparticles indicates that the most probable cause of the change in the type of magnetic interaction is a change in the concentration of V 4+ magnetic ions. DOI: 10.1134/S0021364010010030 1. Nanomaterials based on vanadium oxide VO x attract the attention of researchers, because they have various applications and unusual magnetic properties. For example, multiwall VO x nanotubes doped with lithium and iodine exhibit the transition to the ferromagnetic (FM) state with high Curie temperatures T C ~ 400 450 K [1]. Undoped VO x nanotubes are considered as antiferromagnets whose magnetic subsystem is a mixture of V 4+ magnetic ions with spin S = 1/2 and V 5+ nonmagnetic ions (S = 0) [2 4]. The disorder characteristic of such a system is manifested at low temperatures in the form of an anomalous powerlaw dependence of the susceptibility, χ(t) ~ 1/T ξ, which indicates the appearance of the quantum critical regime and Griffiths phase [4]. Another interesting feature of the magnetism of VO x nanotubes is the presence of antiferromagnetic (AFM) dimers in the sample, which are responsible for both the deviation from the Curie Weiss law [2, 3] and the appearance of a specific absorption line in the EPR spectrum [4]. As a rule, VO x nanotubes are prepared by the method of the hydrothermal treatment of vanadium pentoxide V 2 O 5 in the presence of various organic compounds such as long-chain amines and alcohols [2, 3, 5, 6]. It is remarkable that this method can provide morphologically different nanomaterials with common fragments (motive) by varying the synthesis conditions (the time of hydrothermal treatment, the ph of the medium [3, 5, 6]). For example, an intermediate material, which is a stack of 4 to 8 layers of vanadium oxide (VO x nanolayers), is formed at the initial stage of the hydrothermal treatment in the process of the synthesis of nanotubes, and an increase in the synthesis time leads to the scrolling of such layers to a multiwall nanotube. It is interesting that a small change in the composition of a liquid medium (introduction of various organic components) in the same regime of hydrothermal treatment makes it possible to obtain VO x nanorods instead of nanotubes. Thus, it is of interest to comparatively analyze the magnetic properties of nanoparticles with close motives of the structure based on vanadium oxide and, as a result, to clarify the nature of the anomalous magnetism in VO x nanotubes. In this work, using the data obtained for the samples of VO x nanorods, nanolayers, and nanotubes, we demonstrate that the standard interpretation of the magnetism of nanoparticles under consideration (doped and undoped materials are ferromagnetic and antiferromagnetic, respectively) [1 3] requires a significant correction. Under certain conditions, the transition from the ferromagnetic temperature dependence of the magnetic susceptibility to the antiferromagnetic temperature dependence can be observed in vanadium oxide nanoparticles with a decrease in the temperature. We argue that the appearance of such an FM AFM crossover is attributed to the concentration of magnetic centers in the sample and is characteristic of diluted systems. 2. The samples of VO x nanorods, nanolayers, and nanotubes were obtained by the scheme described in [5, 6]. The resulting nanomaterials were characterized by X-ray analysis and electron microscopy. Figure 1 shows the microphotographs of VO x nanorods, nanolayers, and nanotubes. The oxidation degree of vanadium, which characterizes the ratio of the concentra- 11

12 DEMISHEV et al. Fig. 1. Electron microphotographs of VO x (1, 1a) nanorods, (2) nanolayers, and (3, 3a) nanotubes. tions of V 4+ and V 5+ ions, is determined from the X-ray photoemission spectra. Since a structure-forming organic template is present in the interlayer space of the samples of VO x nanolayers and nanotubes, the magnetic properties are analyzed by the high-frequency EPR method, which makes it possible to separate the contribution from the magnetic subsystem of paramagnetic centers of V 4+ [4]. The experiments were performed at a frequency of 60 GHz in the temperature range of 1.8 to 220 K with a temperature stabilization accuracy of no worse than 0.01 K. The EPR spectrometer is described in detail in [7]. 3. Figure 2a shows the EPR spectra of VO x nanotubes. In addition to the main line A, which corresponds to paramagnetic ions V 4+ and whose intensity increases with a decrease in the temperature, line B is also observed. The amplitude of this line decreases at low temperatures and it is not observed in the spectra at T < 100 K. Line B was attributed in [4] to the contribution from V 4+ V 4+ antiferromagnetic dimers. Note that the existence of antiferromagnetic dimers in the VO x nanotubes also follows from the data on the static magnetic susceptibility [2, 3]. Each of the EPR spectra of VO x nanorods and nanolayers consists of a single line similar to line A in Fig. 2a and does not contain line B. Thus, the formation of antiferromagnetic dimers is characteristic of vanadium oxide nanotubes. The g factor corresponding to EPR on V 4+ ions (line A) is almost the same, g 1.95 ± 0.02, for VO x nanorods, nanolayers, and nanotubes and is independent of the temperature (see Fig. 2b). The g factor corresponding to line B for VO x nanotubes is in the range of 2.4 to 2.6 (as seen in Fig. 2, the error in the determination of this parameter is much larger; for this reason, certain conclusions regarding its temperature dependence cannot be made). Let us consider the temperature dependences of the integral intensity of line A for VO x nanotubes and its analog for VO x nanorods and nanolayers. This parameter is proportional to the magnetic susceptibility χ(t) for the paramagnetic subsystem of V 4+ ; it is convenient to analyze the magnetic susceptibility using the Curie Weiss law χ(t) = C/(T Θ). The results are shown in Fig. 3 in the form of a χ 1 (T) dependence. It is seen that the ferromagnetic temperature dependence of χ(t) with Θ FM = 63 K is observed for nanorods (see Fig. 3a) at T > T C ~ 110 K; below T C, the dependence is antiferromagnetic with = 12 K. This FM AFM crossover is accompanied by an increase in the Curie constant by a factor of 2.7 for T < T C (straight lines 1 and 2 in Fig. 3a). Since the g factor and, therefore, the magnetic moment of the V 4+ ion are temperature independent (see Fig. 2b), the concentration of magnetic centers changes significantly at T ~ T C according to Fig. 3a. The region of ferromagnetic correlations (T > T C ) corresponds to a lower concentration of magnetic centers as compared to the low-temperature antiferromagnetic region (T < T C ).

FM AFM CROSSOVER IN VANADIUM OXIDE NANOMATERIALS 13 T C Θ FM T C Θ FM Fig. 2. (a) EPR spectra for VO x nanotubes and (b) g factors for VO x (1) nanorods, (2) nanolayers, and (3) nanotubes for (open points) A and (closed points) B resonances. It is interesting that the anomalies of the static magnetic susceptibility χ(t) of VO x nanorods near T ~ 120 K were previously observed in [8], where they were attributed to the transition from one antiferromagnetic Curie Weiss dependence to another one. However, in that work, the EPR spectrum included three rather than one line, which indicates the existence of a noticeable amount of technological paramagnetic impurities in the samples. Moreover, Park et al. [8] analyzed the total magnetic susceptibility of the sample that was measured by a SQUID magnetic spectrometer, which hampered the correct model-free separation of the magnetic contribution associated with V 4+ ions. In our opinion, these circumstances explain the difference between our results and the results obtained in [8] and emphasize the fundamental importance of the use of EPR data in the analysis of the magnetic properties of VO x nanomaterials. In contrast to VO x nanorods, VO x nanolayers exhibit the antiferromagnetic temperature dependence χ(t) with = 30 K throughout the temperature range (see Fig. 3b). In this case, the deviations from the Curie Weiss law at T < 50 K are associated with the transition to the power-law asymptotic dependence χ(t) ~ 1/T ξ with ξ = 0.87 ± 0.02, which is similar to the dependence observed in [4] for the VO x nanotube samples [4]. Using the cluster model of the Fig. 3. Temperature dependences of the magnetic susceptibility of VO x (a) nanorods, (b) nanolayers, and (c) nanotubes. The open and closed points are the data for lines A and B, respectively. The solid lines correspond to the (1) ferromagnetic and (2) antiferromagnetic Curie Weiss dependences. Dashed line 3 is the approximation by the model of antiferromagnetic dimers. disordered critical quantum system [9], which was previously successfully used to analyze the critical quantum phenomena in the doped CuGeO 3 compound [10], it can be shown that the formation of the Griffith phase at T < 50 K in VO x nanolayers makes it possible to quantitatively describe the temperature dependence χ(t) in the case under consideration. The detailed analysis of the quantum criticality in VO x nanomaterials is beyond the scope of this work and will be reported elsewhere. According to Fig. 3c, VO x nanotubes exhibit both the FM AFM crossover with Θ FM = 92 K and = 36 K, which is accompanied by an increase in the Curie constant at T < T C by a factor of 7, and a lowtemperature deviation from the Curie Weiss law; the latter effect can be attributed to the approach to the power-law asymptotic behavior with the exponent ξ = 0.78 ± 0.02. Thus, the magnetic behavior of nanotubes has the features characteristic of nanorods and nanolayers (see Fig. 3). 4. Let us analyze the possible causes that can be responsible for the FM AFM crossover observed in this work in VO x nanomaterials. The data shown in Figs. 1 and 3 indicate that the crossover appears in

14 DEMISHEV et al. elongated particles, nanorods and nanotubes. According to the X-ray diffraction data, the unit cell in the VO x plane in nanolayers that are half-finished for nanotubes is close to a square cell, contains seven vanadium atoms, and has the cell parameter ~6.13(1) Å; the distance between the VO x layers is 33 Å. The description of the structure of VO x nanotubes is a complex problem owing to the bending of VO x layers under scrolling [11]. However, in the zeroth approximation, both the size of the quasiunit cell in the scrolled VO x plane and the interlayer distance in VO x nanotubes almost coincide with the respective structure parameters for VO x nanolayers. The unit cell in the VO x plane of nanorods contains two vanadium atoms having the sizes 4.35(5) and 3.56(5) Å (the latter parameter corresponds to the rod growth direction) and the distance between the VO x planes in this material is 10.839(8) Å. Thus, the crystallographic structure of VO x nanorods is significantly different from VO x nanolayers and nanotubes close in structure. The FM AFM crossover is observed in VO x nanotubes and is not observed in VO x nanolayers (see Fig. 3). Thus, the direct explanation of the FM AFM crossover effect by the structure of the samples is problematic. According to the EPR data, g(t) = const (see Fig. 2) and the magnetic moment of the V 4+ ion does not change near the crossover; therefore, the concentration of magnetic centers should not change. The antiferromagnetic and ferromagnetic regions correspond to higher and lower concentrations of the V 4+ ions, respectively. Let us compare nanorods and nanolayers with this point of view. The oxidation degrees of vanadium are +4.2 and + 4.8 for VO x nanolayers and nanorods, respectively; therefore, nanolayers contain 80% of V 4+ (magnetic) ions and 20% of V 5+ (nonmagnetic) ions, whereas nanorods contain 20% of V 4+ (magnetic) ions and 80% of V 5+ (nonmagnetic) ions. Taking into account these data and X-ray diffraction data, the ratio of the volume concentrations of V 4+ ions in VO x nanolayers and nanorods is ~2; this parameter recalculated to the concentration of V 4+ ions in the plane is ~16. Thus, the magnetic subsystem of nanolayers is more concentrated than that for nanorods and the antiferromagnetic behavior is characteristic of VO x nanolayers. As a result, the concentration of V 4+ magnetic ions is the parameter determining the type of the magnetic interaction in vanadium oxide nanoparticles. At first glance, the data for VO x nanolayers and nanotubes (see Fig. 3) contradict this hypothesis. Indeed, the oxidation degree and concentration of vanadium atoms are almost the same for these materials, whereas their magnetic behaviors are significantly different. However, some magnetic ions V 4+ in VO x nanotubes are bounded into antiferromagnetic dimers [2 4]. In order to estimate the concentration of various spin fragments, we approximated the temperature dependence of the integral intensity of line B (see Fig. 2) by the standard expression for the magnetic susceptibility of antiferromagnetic dimers [2] (line 3 in Fig. 3c) with the known spin gap Δ = 720 K [2, 4]. From the data presented in Fig. 3c, the ratio of the Curie constants for dimers and isolated paramagnetic centers V 4+ is about 120 and 18 for the ranges T > T C and T < T C, respectively. This makes it possible to estimate the fraction of antiferromagnetic dimers as ~99 and 95% for the ranges T > T C and T < T C, respectively. Thus, the magnetic subsystem of isolated V 4+ ions in nanotubes is diluted as compared to nanolayers, where antiferromagnetic dimers are absent. As a result, VO x nanotubes and nanorods have a much smaller concentration of magnetic centers than VO x nanolayers and exhibit the FM AFM crossover (see Fig. 3). Thus, the data of this work indicate that the character of the magnetic interaction in VO x nanorods, nanolayers, and nanotubes is apparently determined by the concentration of V 4+ ions and the ferromagnetic and antiferromagnetic regions correspond to the diluted and concentrated cases, respectively. In the framework of such an approach, the FM AFM crossover can be explained by the localization of an electron on a V 5+ center, i.e., by the e + V 5+ V 4+ process occurring at T ~ T C, which increases the concentration of magnetic centers in the sample volume. This assumption can be independently checked by analyzing the transport properties of VO x nanostructures, which is the subject of future investigations. 5. In this work, a transition from a ferromagnetic temperature dependence to an antiferromagnetic temperature dependence has been observed in vanadium oxide nanorods and nanotubes with a decrease in the temperature. The FM AFM crossover occurs near T C ~ 110 K and is accompanied by the low-temperature increase in the Curie constant by a factor of 2.7 7. The comparison of the experimental data for various VO x nanoparticles indicates that the most probable cause of the change in the type of the magnetic interaction is a change in the concentration of V 4+ magnetic ions. This work was supported by the Russian Academy of Sciences (programs Strongly Correlated Electrons in Metals, Semiconductors, and Magnetic Materials, and Quantum Physics of Condensed Media ) and by the Russian Foundation for Basic Research (project nos. 07-03-00749-a, 07-03-12182-ofi, 09-03-01122-a, and 09-03-00602-a). REFERENCES 1. L. Krusin-Elbaum, D. M. Newns, H. Zeng, et al., Nature 431, 672 (2004). 2. E. Vavilova, I. Hellmann, V. Kataev, et al., Phys. Rev. B 73, 144417 (2006).

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