Electrical properties of ferromagnetic Ag:CrO 2 particles

Transcription

1 Indian Journal of Engineering & Materials Sciences Vol. 15, April 2008, pp Electrical properties of ferromagnetic Ag:CrO 2 G P Singh a, S Ram a *, A K Thakur b & R N P Choudhary b a Materials Science Centre, b Department of Physics and Meteorology, Indian Institute of Technology, Kharagpur , India Received 16 December 2006; accepted 28 February 2008 A stable silver modified CrO 2, i.e., Ag:CrO 2 of nanocomposite was synthesized with a novel chemical method involving Cr 6+, Ag + and NO 3 - species in an aqueous solution. A single phase Ag:CrO 2 of submicron size (~33 nm average crystallite size) of a Pnnm orthorhombic structure (lattice parameters a = nm, b = nm and c = nm) is obtained after annealing at 573 K in air. Microstructure reveals single domain Ag:CrO 2 of prismatic shapes (aspect ratio L/b ~ 1-2) of average b ~30 nm width and 40 to 50 nm length (L). The temperature dependence electrical resistance R (T) shows a phase transition from a metallic to a semiconductor like at a specific temperature T C, which is found to be 405 K at a frequency f = 1 khz. At 10 khz, the dielectric constant ε r has as large a value as 49, with power loss As a function of increasing the f-value, both ε r and power loss have been shown to decrease monotonically to the values 28 and 0.06 respectively. The ε r gets saturated at f 400 khz. Owing to unusual transport properties, the halfmetallic magnetic encapsulated in a metallic or nonmetallic host are renewing immense research interest nowadays for several applications such as giant magnetoresistance, magneto-optics devices, and microwave absorbers 1-4. Earlier investigations indicate that the silver additives has unique tunable properties for controlling the microstructure and in turn the transport properties effectively in several ferroelectric perovskites 5,6 and superconducting ceramics RBa 2 Cu 3 O 7 (R = Y, Pr, Nd). 7-9 Furthermore, CrO 2 presents a novel half-metallic ferromagnet with the Curie temperature T C = 392 K. It has been considered as a pertinent candidate for magneto-electronic devices such as spin valve field sensors, magneto random access memory systems, magneto-optical recording, or spin polarized scanning tunneling microscopy The spin split Cr 4+ (3d 2 ) electronic band conduction results in nearly 100% spin polarization in CrO The conduction process involves intergranular tunneling that is affected by the so-called Coulomb blockade, with a strong temperature dependence 16,17. As a consequence, it is highly influenced by the microstructure, and in turn the method of the sample synthesis, especially in a polycrystalline form of powders or films with small crystallites at a nanoscale. Unfortunately, at atmospheric pressure, CrO 2 degrades into antiferromagnetic Cr 2 O 3 at the surface at *For correspondence( sram@matsc.iitkgp.ernet.in) an early temperature as 473 K. 16,17 A complete CrO 3 1/2CrO 2 + 1/4O 2 conversion lies at temperatures above 673 K depending on the microstructure. Such structural changes in this example lead to deteriorate very sensitively, especially the saturation magnetization. 18 As a result, very special methods under controlled conditions are demanded to derive a substantially stable CrO 2 with stable properties. A thermal or hydrothermal decomposition of CrO 3, Cr(OH) 3, or Cr(CO) 6, or a sputtering from a CrO 3 target, 18 is often used to get a CrO 2 product or the devices. Besides expensive and cumbersome to a large-scale production, these methods as such hardly offer a control of the microstructure with a homogeneous size distribution of the sample. In this paper, a simple chemical method involving a silver nitrate precursor, with a reaction of dispersed Cr 6+ cations in water, is used to synthesize a silver modified stable CrO 2, i.e., Ag:CrO 2 as a nanocrystalline powder. It comprises of single domain useful for fabrication of CrO 2 electronic devices and components. Experimental Procedure A silver stabilized ferromagnetic single domain CrO 2, i.e., Ag:CrO 2 of nanocomposite is obtained by exploring a simple chemical reduction reaction of CrO 3 to CrO 2 in the presence of a model reductant Ag + in aqueous medium. The CrO 3 CrO 2 transformation is performed by adding drop-wise an aqueous AgNO 3 solution to an aqueous CrO 3 solution

2 172 INDIAN J. ENG. MATER. SCI., APRIL 2008 with magnetic stirring at K in air. At early stage of the reaction, a nanocolloid appears of Ag:CrO 2 (blackish colour) dispersed in the solution. At outcome of the reaction, a pure sample was recovered by filtering and washing in distilled water (hot) to remove if any unreacted reactants followed by drying at K in air. A further annealing of 2 h at 573 K in air ensured a stable Ag:CrO 2 sample. The results were analyzed with X-ray diffraction, scanning electron micrograph (SEM), and impedance analyzer. The impedance analyzer of HIOKI 3535 LCR HITESTER was used to study the electrical and dielectric behaviours of the Ag:CrO 2 nanocomposites. These measurements were performed on cold pressed thin Ag:CrO 2 billets with a diameter of 15 mm and a thickness of 2.5 mm. A silver paste was used for making a good ohmic contact between the electrodes. Annealing at 373 K in air for 30 min ensured a good contact with the sample. The data were collected with a computer interface to the impedance analyzer, with f-value varied in khz range. Results and Discussion Synthesis of surface stabilized Ag:CrO 2 nanocomposite As summarized in Fig. 1 with a flow chart, when adding an aqueous AgNO 3 to an aqueous CrO 3 under heating conditions, the CrO 3 transforms into CrO 2 in ambient air pressure. A drop-wise addition of AgNO 3 solution to the CrO 3 solution under magnetic stirring reflects in the initiation of the reaction by changing of the initial colour of the sample as a function of the reaction time. Here, the magnetic stirring produces the refreshed reactive surfaces of AgNO 3. Such AgNO 3 species, having plenty of active free Ag + - and NO 3 species, react with Cr 6+ (from the CrO 3 solution) and form a Cr 4+ -Ag complex of a kind of a polymeric layer. Successive changes observed in the initial Cr 6+ - Ag + colour infer a dynamic reaction in operation in divided reaction centers. The layer renders a selfcontrolled reaction Cr 6+ Cr 4+ with active Ag + and NO - 3 groups from the AgNO 3 used. A colloid solution occurs as a stable Cr 4+ - Ag product. However, the reaction is highly sensitive to the final temperature. Mobile Cr 6+ -Ag + - and NO 3 groups, under hot conditions in solution at K, serve as active reaction species to react one another in small groups. As a result, the Cr 4+ nucleates and grows as CrO 2 within a shell of the initial Ag layer (serves as a template), depending on the Cr 6+ -concentration, Fig. 1 A systematic diagram showing the procedure of synthesis of surface stabilized Ag:CrO 2 nanocomposite dispersion, and other experimental conditions. Finally, a pure sample was recovered by filtering and washing in distilled water (hot) to remove if any unreacted reactants followed by drying at K in air. A further annealing of 2 h at 573 K in air ensured formation of a stable Ag:CrO 2 of nanocomposite. Structural analysis of surface stabilized Ag:CrO 2 nanocomposite The X-ray diffraction analysis of the Ag:CrO 2 nano annealed at 573 K in air reveals the significant change in the CrO 2 crystal structure. All the diffraction peaks are indexed assuming the Pnnm type orthorhombic structure in comparison to the P4 2 /mnm tetragonal structure in bulk CrO 2 19, with the lattice parameters a = nm, b = nm, and c = nm. An average crystallite size d~33 nm is obtained from the full width half at maxima (fwhm) of the peaks by using the Debye Scherrer relation. The crystal unit cell has a density ρ = 4.93 g/cm 3 as determined by the lattice volume V o = abc =

3 SINGH et al.: ELECTRICAL PROPERTIES OF FERROMAGNETIC Ag:CrO 2 PARTICLES 173 nm 3, assuming z = 2 formula units. This is a reasonably larger value in comparison to ρ = 4.90 g/cm 3 in pure CrO 2 of a = nm and c = nm, 19 indicating a partial Ag-involvement in the lattice. Moreover, no any distinct peak of independent silver or its oxide in the crystalline form is visible within a detectable limit of the X-ray diffractogram. The Ag used in this experiment is virtually dissolved in part of an amorphous structure of presumably nanocomposite Ag:CrO 2, and that indicates a promptly good dispersion or solubility of small Agspecies in the sample. In this novel method, a CrO 2 lattice forms with in-situ CrO 3 to CrO 2 transformation in the presence of the Ag + species (a model reductant). Part of small Ag species of atomic level succeeds to dissolve in a composite structure in process to the reaction in a dispersed medium. This is not feasible so easily in reacting CrO 2 with independent silver in a classical thin deposition method. SEM images (Fig. 2) of Ag:CrO 2 nanocomposite annealed at 573 K in air at ambient pressure have prismatic shapes. Average width (b) in these lies at a scale of ~30 nm. It represents effective d-value derived from the fwhm values. Average length L in such lies in a narrow nm range, with L/b = 1-2. An average value of d = 33 nm, determined from the fwhm values, ascribes average width in the basic components (crystallites) in the clusters. In a simple case, it represents effective thickness in the clusters. Electrical properties of surface stabilized Ag:CrO 2 nanocomposite Figure 3 shows the temperature dependence of electrical resistance R (T) of the surface stabilized Ag:CrO 2 nanocomposites at zero magnetic fields. The curve shows that a metallic (dr/dt>0) to a semiconductor like (dr/dt<0) transition behaviour occurs at T C = 405 K when heating from 250 to 420 K. In Fig. 3, the peak position in the plot in the Ag:CrO 2 nanocomposites is found at the higher temperature, which corresponds to the ferromagnetic transition temperature T c, i.e., 405 K relative to 392 K in pure CrO 2 (ref.10). Further, the R (T) of the composite is ~ 12.4 kω, which is higher than ~ 9 kω for pure CrO 2 at 300 K. As the temperature increases, a maximum R (T) value of 17.5 kω is attained in Ag:CrO 2 nanocomposites against a value 15.6 kω for pure CrO 2. An increased R (T) value in Ag:CrO 2 nanocomposite constituting of an amorphous Ag surface is possible in an inhomogeneous scattering of valence electrons, with a shorter mean free path in the nanosized. Figure 4 shows ε r -values measured as a function of f-values in khz range for Ag:CrO 2 at room temperature. The initial ε r -value, which is 48.6 at 10 khz, decreases monotonically to a value as small as 28.3 at 400 khz. Above 400 khz, the ε r -value decreases steadily following an almost linear plot, with a slope of per Hz, with a ε r -value 27.8 at 1 MHz. Over low f-values, a highly f-sensitive ε r response arises due to blocking the charge transport in the surface modified Ag:CrO 2 nanocomposite, i.e., effectively weak interconnecting network in between the. Fig. 2 SEM micrograph of the Ag:CrO 2 nanocomposite Fig. 3 The temperature dependent electrical resistance of Ag:CrO 2

4 174 INDIAN J. ENG. MATER. SCI., APRIL 2008 strong interdependent behaviour of the transport properties. Other characteristic physical properties known in CrO 2 and derived ceramic composites of different origins are compared in Table 1. Fig. 4 The variation of ε r against f-value for the Ag:CrO 2 Table 1 A comparison in typical transport properties in CrO 2 and representative magnetoceramic composites at room temperature S.No. Sample Resistivity ρ dc (kω-cm) ε r ρ dc Power References loss* 1 CrO 2 bulk pellets Cr 2 O 3 -CrO 2 films TiO 2 -CrO 2 films CrO 2 films Ag.CrO 2 pellets Present work *The ε r and power loss values are reported for a typical 500 khz f-value Over higher f-values, the presumably insulating Ag-surface layer (diamagnetic) plays a crucial role in inhibiting the charge transport by controlling the spin dynamics in a rather steady state of the CrO 2 (3d 2 ) magnons. The ε r and power loss values are reported for a typical 500 khz f-value. A variation of the power loss (figure is not given here) demonstrates qualitatively the f-dependent behaviour very similar to the ε r -value as shown in Fig. 1. The power loss has a value of 0.42 at 10 khz and that is diminished further to as small a value as 0.06 upon raising the f-value to 1000 khz. The power loss becomes reasonably small over f-values above 400 khz. At a common f ~500 khz, a value of the power loss found to be 0.08 in this example is much lower in comparison to a value 7.4 reported in Si- CrO 2 composite thin films 20, or 0.48 in a TiO 2 -CrO 2 composite powder 21. Clearly, the Ag-additive is highly effective in controlling the power loss especially at high f-values. The results envisage a Conclusions The single domain Ag:CrO 2 (33 nm size), synthesized by a chemical method, have an orthorhombic crystal structure of lattice parameters a = nm, b = nm, and c = nm. The temperature dependent electrical resistance R(T) shows a phase transition from a metallic to a semiconductor like at 405 K. The dielectric constant ε r as well as the power loss presents a two-step f- dependency, namely (i) a nonlinear complex polynomial behaviour over low f-values and (ii) a linear behaviour over high f-values, i.e., above 400 khz. At 10 khz, the ε r -value is as large as 49 ((power loss 0.42). Upon increasing the f-value, both the ε r and power loss are decreasing monotonically to the values 28 and 0.06 respectively. The linear parts of the plots demonstrate almost steady responses of the respective properties to the f-values, with a strictly controlled power loss of ~ 0.06, i.e., an order of smaller value in comparison to those in other known magnetoceramics. Acknowledgments This work is supported in part by the Council of Scientific & Industrial Research (CSIR), New Delhi. References 1 Kuanr B J, Marvin D L, Christensen T M, Camley R E & Celinski Z, Appl Phys Lett, 87 (2005) Kim S S, Kim S T, Yoon Y C & Lee K S, J Appl Phys, 97 (2005) 10F905-10F Gong He Y, Nie R, He Y & Zhao Z, J Appl Phys, 98 (2005) Sohn J, Han S H, Yamaguchi M & Lim S H, Appl Phys Lett, 89 (2006) Kalyanaraman R, Oktyabrsky S & Narayan J, J Appl Phys, 85 (1999) Srivastava A, Kumar D, Singh R, Venkataraman H K & Eisenstadt W R, Phys Rev B, 61 (2000) Jia Q X, Yan F, Mombourquette C & Reagor D, Appl Phys Lett, 72 (1998) Jaydevan K P, Liu C Y & Seng T Y, Appl Phys Lett, 85 (2004) Kurian J, Sato H, Makimoto T & Naito M, Appl Phys Lett, 87 (2005) Dai J & Tang J, Phys Rev B, 63 (2001) Liang J J, Lee S F, Yao Y D, Wu C C, Shyu S G & Yu C, J Magn & Magn Mater, 239 (2002) Chen Y J, Zhang X Y & Li Z Y, Chem Phys Lett, 375 (2003)

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