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EFFECT OF HIGH PRESSURES AND HIGH TEMPERATURES ON STRUCTURAL AND MAGNETIC CHARACTERISTICS OF NANOSTRUCTURED SOLID SOLUTIONS
Zm_xFexO
T. V. Dyachkova1, V.N. Krasil'nikov1, O.I. Gyrdasova1, E. V. Shalaeva1, A. P. Tyutyunnik1, V. V. Marchenkov2, Yu. G. Zaynulin1, H. W. Weber3
1 Federal State government-financed research institution Institute of Solid State Chemistry of the Ural Branch of the Russian Academy of Sciences, Ekaterinburg , Russia
2Federal State government-financed research institution Institute of Metal Physics of the Ural Branch of the Russian Academy of Sciences, Ekaterinburg, Russia
3Atominstitut, Vienna University of Technology, Vienna, Austria
dyachkova@ihim.uran.ru, kras@ihim.uran.ru, gyrdasova@ihim.uran.ru, shalaeva@ihim.uran.ru, tyutyunnik@ihim.uran.ru, march@imp.uran.ru, zaynulin@ihim.uran.ru, weber@ati143.ac.at
PACS 71.20.Nr, 73.63.Bd, 75.50.Pp
Nanostructured solid solutions of the composition Zni_xFexO (0 ^ x ^ 0.075) with tubular aggregate morphology, synthesized by the precursor method, were subjected to thermobaric treatment at P = 5 GPa and T = 600-700oC. Using the samples with x = 0.05 as an example, it was shown that the application of pressure leads to morphology variation, reduction of structural parameters and to an increase in ferromagnetism.
Keywords: High pressure - high temperature, nanomaterials, glycolate, semiconductors.
Received: 16 June 2014 Revised: 30 June 2014
1. Introduction
Increased interest in zinc oxide, doped with cations of ferromagnetic metals, such as manganese, iron, cobalt and nickel, is due to the need to find new magnetic materials for spintronics that open up possibilities of developing magnetoresistive memory cells, spin light-emitting diodes and field-effect transistor, as well as magnetic field sensors and quantum computer components [1-3]. Although such materials as solid solutions Zn1-xMxO contain only several atomic percents of magnetic impurities with negligibly small exchange interaction between them, ferromagnetism can occur in them even at room temperature. The magnetically ordered state observed for low-dimensional forms of zinc oxide, for example, nanopowders and thin films, consists not only in a greater degree of imperfection of such structures as compared with bulk objects, but also in the appearance of quantum-dimensional effects that manifest themselves during the transition into the nanodimensional state [2, 4-11]. The unique combination of semiconducting and ferromagnetic properties of the solid solutions Zn1-xFexO makes them promising materials for practical application in the development of devices based on the spin-dependent transport effect and attracts the
attention of a wide range of specialists including those concerned in synthesis of nanomate-rials, whose efforts are directed at the elaboration of new methods for synthesis of samples with preassigned composition that provide the possibility to control the degree of dispersity and structure imperfection and to affect their magnetic properties [7-11].
Among the methods of synthesis of room-temperature ferromagnetics based on iron-doped zinc oxide described in the literature, the most widespread techniques are solid-phase synthesis [12, 13], self-propagating high-temperature synthesis [14, 15], solvothermal synthesis [11, 16], sol-gel synthesis [17, 18] and synthesis by deposition from aqueous solution in the form of hydroxides [19, 20]. A method for the production of polycrystalline ferromagnetic samples of the composition Zno.99Feo.010 from mixtures of ZnO and Fe304 oxides by the solid-phase reaction technique combined with high-temperature treatment (2 and 5 GPa) has been reported previously [21].
The problem of synthesizing nanostructured iron-doped zinc oxide with a high degree of dispersity and intrinsic imperfection can be solved via the precursor method, the most important advantage of which is the possibility of dosed replacement of zinc by a magnetic metal in the precursor matrix. Thermal treatment of precursor under selected conditions allows the synthesis of oxides with a preset composition and expected morphological and dimensional aggregate parameters. So, as a result of heating in air at temperatures above 400°C, the octahedral crystals of Zn1-xFex(0CH2CH20) glycolate undergo a pseudomor-phous transformation into octahedral aggregates of the oxide Zn1-xFexO, while extended crystals of formate glycolate Zn1-xFex(HC00)(0CH2CH20)1/2 - into oxide nanotubes with diameters of 150-300 nm consisting of 10-15 nm crystallites [22].
The aim of this work was to examine the effect of annealing temperature and pressure on the structural and magnetic properties of nanodispersed solid solutions Zn1-xFex0 with tubular aggregates. In the synthesis of the samples, the advantages of the precursor method were combined with the advantages of thermobaric treatment. Nanotubular samples of the composition Zn0 95Fe0.050, preliminarily synthesized by the precursor technique [22], were treated at quasi-hydrostatic pressure (5 GPa) at a temperature of 600-700°C; after that, their phase composition, microstructure and magnetic characteristics were studied.
2. Experiment technique
For the production of nanostructured iron-doped zinc oxide with tubular aggregate morphology, we employed the precursor method; the precursor was the formate glycolate of the composition Zn1-xFex(HC00)(0CH2CH20)1/2, which was synthesized by the following reaction [22-23]:
Zm-xFex(HC00)2 ■ 2H20 + 1 /2H0CH2CH20H = Zn1-xFex(HC00)(0CH2CH20)1/2 + HC00H f +2H2O ( )
The formate Zn1-xFex(HC00)2-2H20 necessary for reaction (1) was synthesized by reaction between dilute formic acid and mixtures of zinc oxide and iron with heating:
(1 - x)Zn0 + xFe + 2HC00H + H20 = Zm-xFex(HC00)2 ■ 2H20 + XH2 f (2)
Using microscopic analysis, it was found that formate glycolate Zn1-xFex(HC00) (0CH2CH20)1/2 is isolated from the solution in ethylene glycol in the form of fibrous or needle-shaped crystals (reaction 1) upon exposure at 120°C for 2 h. The degree of substitution of iron for zinc in Zn1-xFex(HC00)(0CH2CH20)1/2 does not exceed 10 at. %. In order to produce solid solution samples of the composition Zn1-xFex0 with tubular aggregate morphology, the precursor was heated in air with a rate of 10°C/min up to 500°C,
exposed at this temperature for 2 h and cooled to room temperature with the furnace. The synthesized samples in the form of finely-dispersed orange powders that served as precursors were examined using X-ray diffraction analysis and then were subjected to thermobaric treatment. The experiments were carried out on a hydraulic press in a standard toroid-type high-pressure chamber. The powder of the composition Zno.95Feo.05O was tamped tightly into a graphite cup that served simultaneously as a heater and then it was placed into a container made of lithographic stone - a natural mineral consisting mainly of calcium carbonate CaCO3. To prevent contamination of the sample with carbon, the inside walls of the heater were isolated with boron nitride. The sample was compressed between the press anvils until the necessary pressure was reached, and after that, the temperature was raised. Upon exposure at a preset temperature, the sample under pressure was quenched by abrupt decrease of temperature. Then the pressure was released and the container with the samples was removed from the press. The experimental conditions and the structural characteristics of the produced preparations are given in Table 1.
Table 1. The crystallographic characteristics of the Zn0 95Fe0.05O sample before treatment (1) and after treatment at P = 5 GPa, T = 600°C (2) and P = 5GPa, T = 700°C (3)
Sample a c P T (°C) Sp. gr. Content, CSR
No. (A) (A) (GPa) weight % (nm)
1 3.2502 5.2074 - - P6 3mc 100 m% 46.04
2 3.2509 5.2059 5 600 P6 3mc 100 m% 123.61
3 3.2489 5.2034 5 700 P6 3mc 84.0 m % 93.83
3.2690 5.2247 P6 3mc 9.4 m % 33.65
The phase analysis of the precursors and the products of their thermolysis was performed be means of a POLAM S-112 polarizing microscope in transmitted light (the refractive indices were determined by the immersion method) and a STADI-P X-ray powder automated diffractometer (STOE, Germany) in CuKa1 radiation using the X-ray diffraction database PDF-2 (Release 2009). Thermogravimetric analysis was carried out on a SETSYS EVOLUTION thermal analyzer (SETARAM, France) at a heating rate of 10°C/min in air. The size and shape of the particles of the thermolysis products were determined by scanning electron microscopy on a JSM JEOL 6390LA device. The structure of the thermolysis products was studied by transmission electron microscopy on a JEM-200 CX microscope. To determine the content of zinc and iron, elemental analysis was performed by the atomic absorption spectroscopy method in acetylene-air flame on a Perkin-Elmer device and by atomic emission method on a JY-48 spectrum analyzer with inductively coupled plasma. The magnetic properties of the synthesized Zn1-xFexO samples were measured in the Atom Institute of the Vienna University of Technology on a MPMS XL7 SQUID magnetometer produced by Quantum Design in magnetic fields to 10 kOe in the temperature range from 4.2 to 330 K, as well as in the Multiple Access Center at the IMP UB RAS on a 7407 VSM vibration magnetometer produced by Lake Shore Cryotronics in magnetic fields to 17 kOe at room temperature.
3. Results and discussion
According to the X-ray phase analysis data, the thermolysis products of the precursor in air are solid solutions of the composition Zn1-xFexO (0^ x ^0.075) with wurtzite structure
(Fig. 1). Thermal decomposition of formate glycolate Zn1-xFex(HCOO)(OCH2CH2O)i/2 is an exothermic process, occurring in two stages from ~280-500°C, which agrees with the presence of two anion types, HCOO- and OCH2CH2O2-, in the crystal structure of this compound. The mass loss of the Zn0.95Fe0.o5(HCOO)(OCH2CH2O)1/2 sample determined by thermogravimetric analysis differed insignificantly from that calculated with allowance for its transformation into Zn0.95Fe0.05O. The common feature of thermal decomposition of Zni-xMx(HCOO)(OCH2CH2O)i/2 (M = V, Cr, Mn, Fe, Co, Ni, Cu) - the solid solutions, in which d metals substitute for zinc in the Zn(HCOO)(OCH2CH2O)1/2 structure - is that the fibrous crystals of this substance transform during heating in air into aggregates of Zn1-xMxO oxide having a tubular structure [22-29]. The size and microstructure of Zn0.95Fe0.05(HCO) tubes depend on the formation conditions of precursor crystals and their heat treatment, as well as on the type and concentration of the dopant. The length of the tubular aggregates Zn1-xFexO can exceed 30 ^m and their diameter is 100-300 nm depending on the iron concentration (Fig. 2 a,b). As the concentration of iron increases, the tube walls become thinner and during rapid heating this can lead to their rupture in the direction parallel to their lengthening.
I
40000 i---i—-*-----'------h
30000
20000
20 40 60 80
29
Fig. 1. The X-ray diffraction patterns of Zn0.95Fe0.05O sample with tubular morphology: before treatment (1) and after treatment at P = 5 GPa at T = 600oC (2) and T = 700oC (3)
The bright-field electron-microscope images of Zn0.95Fe0.05O oxide aggregates formed in the thermal decomposition of Zn0.95Fe0.05(HCOO)(OCH2CH2O)i/2 during heating in air show that they are tubular quasi-one-dimensional structures built of crystallites with an average dimension of ~10 nm (Fig. 3 a,b). The Debye selected-area diffraction pattern (Fig. 3c) and the dark-field image (Fig. 3d) in (100)ZnO, (002)ZnO and (101)reflections correspond to the polycrystalline nanodispersed structure of quasi-one-dimensional compounds
Zn0.95Fe0.05O.
Fig. 2. The XEM image of the sample of the composition Zn0.95Fe0.05O produced by thermolysis of Zno.95Feo.o5(HCOO)(OCH2CH2O)1/2 at 500°C in air
The powders of the composition Zn0.95Fe0.05O, produced by the precursor method were treated at different pressures (P = 5 - 9 GPa) and temperatures (T = 500, 600, 700, 900°C). The experiments showed that the initial wurtzite ZnO structure (sp. gr. — P63mc) remains only in a very narrow interval of pressures P = 5 GPa and temperatures T = 600-700°C. Under softer treatment conditions, an impurity phase of the composition Zn5(OH)6(CO3)2 is found, whereas under more rigorous conditions, the high-temperature cubic phase ZnO (sp. gr. — Fm3m) and ZnFe2O4 emerge. As is seen from Fig. 1 and Table 1, the crystal structure typical of the initial sample Zn0.95Fe0.05O with tubular-shaped aggregates (1) remains after thermobaric treatment at 600°C (2). However, the morphology of the samples changes dramatically - the tubular form of the particles transforms into a round shape with lamellar inclusions (Fig. 4a). When the temperature is increased to 700°C (3), two phases with different lattice parameters are formed, one of which is iron-rich (84.0 mass %) and the other, on the contrary, is iron-depleted (9.4 mass %). The shape of the particles becomes more rounded (Fig. 4b). Besides the principal lines belonging to different forms of Zn1-xFexO, the X-ray diffraction pattern (Fig. 1) contains lines of boron nitride and graphite (heater material). According to our estimates, the coherent scattering region (CSR) increases with temperature.
Figure 5 presents the magnetization curves at T = 4.2 K (-268.95°C) for Zn0.95Fe0.05O samples 2 and 3 (see Table 1) treated at P = 5 GPa, T = 600°C and T = 700°C. It is seen that a hysteresis loop with coersive force of 165 and 400 Oe is observed for the both samples, respectively. The values of magnetization M estimated in 10 kOe field are M = 9.71-10-2 emu/g (or 4.1-10-2 /Fe) and M = 2.02-10-1 emu/g (or 8.5-10-2^B/Fe), respectively. The temperature dependences M(T) in 1000 Oe field displayed in Fig. 6 allow us to suppose that the ferromagnetic state can be observed up to room temperatures at least in sample 3.
Fig. 3. The bright-field electron-microscope images of quasi-one-dimensional Zn0.95Fe0.05O with tubular morphology (a, b). The Debye selected-area diffraction pattern (c) and the dark-field image (d) in (100)ZnO, (002)ZnO and (101)Zno reflections correspond to the polycrystalline nanodispersed structure of quasi-one-dimensional compounds. The average size of the crystallites is ~10 nm
Therefore the magnetization curves have been measured at T = 290 K (17°C) (Fig. 7). It was found that at room temperature, sample 2 is paramagnetic, i. e. magnetization increases linearly with a stronger magnetic field, while hysteresis is absent. By contrast, for sample 3, a hysteresis loop with coercive force of 100 Oe is observed, and magnetization estimated in 10 kOe field is M = 8.25-10-2 emu/g (or 3.5-10-2^B/Fe). Thus, it is shown that the increase in the treatment temperature from T = 600°C to T = 700°C results in more than twofold enhancement of magnetization in the region of liquid helium temperatures (transition from sample 2 to sample 3). At room temperature, sample 2 transforms into a paramagnetic state, whereas sample 3 still exhibits ferromagnetic properties. In work [21], investigations have been performed into magnetization of Zn0.99Fe0.01O samples synthesized by annealing of a mixture of ZnO and Fe3O4 at P = 0; 2 and 5 GPa and T = 600°C for 30 min, which exhibited ferromagnetism in the temperature range from 5 K (-268°C) to 300 K (27°C). Moreover, the authors [21] also observed that magnetization grows when the pressure is increased during synthesis up to P = 5 GPa at constant temperature T = 600°C. Like in
□
20kV X17.000 1pm * 09 47 BES 20kV X6.000 2pm 09 44 BES
Fig. 4. The XEM images of the particles of the sample with the initial composition Zn0.95Fe0.05O after heating under a pressure of 5 GPa at 600 (a) and 700°C (b)
Fig. 5. The magnetizations curves of samples 2 (S2) and 3 (S3) at T = 4.2 K. The hysteresis loops for the both samples are shown in the inset
0.02
Ol
E
CD
0.01
0.00
o
-o-S2 S3 H=1000 Oe
0 \
■ v "tJ-O-O-O-O „o^-o-o-o-o-o-o-o-o-o^-^
D \ □ \
\ • V
1 i I.I.
100
200
300
T(K)
Fig. 6. The temperature dependences of magnetization for samples 2 (S2) and 3 (S3) in 1000 Oe field
Fig. 7. The magnetizations curves of samples 2 (S2) and 3 (S3) at T = 290 K. The hysteresis loop is shown in the inset
work [21], we obtained comparable magnetization values for iron-doped zinc oxide samples, though their saturation fields are higher.
Thus, in the studies performed, it was established that thermobaric treatment of the oxide of the composition Zn0.95Fe0.05O with tubular-shaped aggregates at T = 600°C and P = 5 GPa leads to the appearance of ferromagnetism and to the transformation of nanotubes into agglomerates having a rounded shape, which is similar to the shape of the particles of room-temperature ferromagnetics produced by solid-phase annealing of ZnO and Fe3O4 oxide mixtures under identical conditions [21].
Acknowledgments
The work was supported by the RFBR (grant No. 12-03-00453-) and the Presidium of RAS (project No. 12--3-1009).
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