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ISSN 0031 918X, The Physics of Metals and Metallography, 2012, Vol. 113, No. 13, pp. 1244­1256. © Pleiades Publishing, Ltd., 2012.

Ferromagnetism of Nanostructured Zinc Oxide Films
B. B. Straumala, b, A. A. Mazilkina, c, S. G. Protasovaa, c, P. B. Straumald, A. A. Myatieve, G. SchÝtza, E. Goeringb, and B. Baretzkyb
a

Institute of Solid State Physics, Russian Academy of Sciences, ul. Akademika Osip'yana 2, Chernogolovka, 142432 Russia b Karlsruhe Institute of Technology, Institute of Nanotechnology, Hermann von Helmholtz Platz 1, Eggenstein Leopoldshafen, 76344 Germany c Institute of Intelligence Systems (former Institute of Materials Science) of Max Plank Society, Heisenbergstrasse 3, Stuttgart, 70569 Germany dBaikov Institute of Metallurgy and Materials Science, Leninskii pr. 49, Moscow, 117991 Russia e National University of Science and Technology "MISiS," Leninskii pr. 4, 119049 Moscow, Russia e mail: straumal@issp.ac.ru
Received May 5, 2012

Abstract--The paper presents a review of the causes of the occurrence of ferromagnetic properties in zinc oxide. It is shown that ferromagnetism only occurs in polycrystals at a fairly high density of grain boundaries. The critical grain size is about 20 nm for pure ZnO and over 1000 nm for zinc oxide doped with manganese. The solubility of manganese and cobalt in zinc oxide increases considerably with diminishing grain size. Even at the critical grain size, the ferromagnetic properties depend significantly on the film texture and the struc ture of intercrystalline amorphous layers. DOI: 10.1134/S0031918X12130030

INTRODUCTION Grain boundaries (GBs) in polycrystalline materials affect their basic physical and technological properties, such as strength, ductility, corrosion resistance, and dif fusion permeability. One of the factors which controls the influence of GBs on the properties of polycrystals is their high adsorption capacity. As early as in the 1950s, McLean predicted that the adsorption of the second component on GBs should increase its total solubility and, therefore, cause the shift of lines on phase diagrams [1]. However, for the materials whose grain size exceeds a few microns, this effect is vanishingly weak. The recent advent of nanocrystalline materials, especially those in which multilayer adsorption can occur on GBs, result ing in the formation of interlayers of grain boundary phases (for example, in bismuth doped zinc oxide used to produce varistors [2]), introduces possible variations into the phase diagrams due to the high adsorption capacity of GBs. Nanocrystalline oxides are very interesting objects, which can be used as semiconductor membranes, materials for gas sensors, and solid electrolytes of fuel elements, as well as protective, decorative, and func tional coatings (for example, energy saving or self cleaning windows); they are also applied in solid state electronics. The theoretical prediction of ferromag netic properties of zinc oxide doped with cobalt, man ganese, or iron at room temperature by Tomasz Dietl [3] gave rise to an avalanche of experimental works [4]. This is explained by the fact that the manifestation of high temperature ferromagnetism in a transparent

wide band semiconductor such as zinc oxide makes it an extremely promising material for spintronics, since it is fundamentally possible to control the electrical properties of the material by applying the magnetic field, as well as its magnetic properties by applying the electric field. However, as is noted in review [4], the presence or absence of high temperature ferromag netism in zinc oxide is related to crystalline defects and, to a large extent, such interelation remains inap prehensible. Therefore, the influence of the effect of GBs on ferromagnetism in zinc oxide is one of the most topical trends in the modern condensed matter physics and materials science. EXPERIMENTAL Thin zinc oxide films doped with cobalt or manga nese were deposited on aluminum foil substrates by the fluid ceramics method. A mixture of fluid organic acids and metal ions was applied to a substrate, then dried at 150°C. Zinc (II) butanoate dissolved in the organic sol vent (the zinc concentration was 1 to 4 kg/m3) was used as a precursor to the formation of pure zinc oxide films. To synthesize zinc oxide films doped with 0.1 and 10 at % of Mn, the zinc (II) butanoate solution was mixed with manganese (III) butanoate solution in the corresponding proportions. The butanoate precur sor was applied to polycrystalline aluminum foils and sapphire monocrystals with a (102) orientation. Then, the films were subjected to pyrolysis in air or argon at 500, 550, and 600°C. The oxidation of organic acid

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films occurred simultaneously with their pyrolysis. As a result, thin films of zinc oxide doped with cobalt or manganese were formed; they were pore free and con tained equiaxed zinc oxide grains (Fig. 1). The films were slightly green and transparent. The film thickness was determined by electron probe X ray microanaly sis (EPMA) and transmission electron microscopy (TEM) and varied whithin 50­200 nm. The cobalt concentration in the films was 0­52 at % and that of manganese was 0­47 at %. The concentration of cobalt, manganese, and zinc in the oxide films was measured by atomic absorption spectroscopy using a PerkinElmer spectrometer and EPMA using a Tescan Vega TS5130 MM electron microscope equipped with an Oxford Instruments energy dispersion spectrometer. TEM examination was carried out using a JEM 4000FX microscope at an accelerating voltage of 400 kV. TEM was applied to study the film crystalline structure, espe cially on interfaces, and to detect possible pure cobalt or manganese oxide particles, as well as to measure the grain size in the pure and doped zinc oxide films. X ray diffraction was studied in Fe K radiation using a Sie mens diffractometer with a graphite monochromator and a flow gas detector. The grain size was calculated from the angular dependence of the line broadening [5]. The grain size in the specimens under study was 10 ± 2 nm. The magnetic properties were measured by a SQUID (Quantum Design MPMS 7 and MPMS XL) superconductive quantum interferometer. The mag netic field was applied parallel to the specimen plane. The diamagnetic signal induced by the specimen holder and substrate was subtracted carefully from the magnetization curves. RESULTS AND DISCUSSION Increase of Total Solubility of Second Component in a Polycrystal Due to Grain Refinement With increasing the content of the doping compo nent x, the solubility limit of two component and mul ticomponent alloys is achieved at a certain concentra tion cs. At higher concentrations cs, the second phase appears in the grain bulk. As c continues to increase, only the amount of the second phase increases, while that of the first phase remains equal to cs. The solubility limit in the bulk increases with increasing temperature. The simplest way to measure cs is to observe variations in the lattice parameter of the solid solution by, e.g., X ray diffractometry. The lattice parameter changes continu ously, i.e., it can increase or diminish, as x grows to cs. At x > cs the lattice period does not change and the intensity maxima corresponding to the second phase appear in the diffraction pattern. If an alloy contains the surfaces or interfaces enriched in the second component, the total concen tration of this component xt exceeds its concentration xv in the grain bulk of the solid solution. The difference between xt and xv grows with increasing specific area of
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50 nm
Fig. 1. Dark field electron microscopy image of thin nanocrystalline zinc oxide film obtained by fluid ceramics method.

external surfaces and interfaces, e.g., when grain refinement occurs. If the grain size is fairly small, the difference between xt and xv can be measurable. This is explained by the fact that X ray diffractometry (XRD) only registers diffraction from bulk phases. The com ponent located in thin surface layers or in the inter faces remains invisible for the XRD. The intensity peaks appear on the diffraction pattern only when the coherently scattering domain is sufficiently large (the grain size is about 5 nm or greater). At the same time, XRD is capable of measuring the grain size using the angular dependence of the peak width. McLean assumed that, in fine grained materials, the total solubility limit csa is higher than the bulk sol ubility limit cs [1]. He calculated this difference for the Fe­C system with a grain size of 1­10 m for the case of simple Langmuir type grain boundary segregation [1]. There is a great deal of evidence that, in micro and nanograined materials, csa > cs [6­11]. However, XRD studies of the shift of the solubility csa­cs depending on the grain size d are highly labor inten sive and, to our knowledge, have never been carried out. ZnO presents an excellent possibility for these investigations and is widely used as a transparent con ductive oxide in thin film semiconductor technologies and as a material for varistors (ZnO doped with Bi2O3) and gas sensors. Moreover, when being applied as fer romagnetic semiconductor, it is a promising material for future spintronics. Ferromagnetic semiconductors enable electric control over magnetic states and the magnetic transformation of electric signals. In 2000, T. Dietl et al. theoretically predicted that ZnO doped with small amounts of magnetic impurities, such as manganese or cobalt, possesses ferromagnetic proper ties [3]. This work gave rise to numerous experimental studies and over 1200 papers have been published since then that deal with low doped semiconductors. The presence or absence of ferromagnetism in doped ZnO depends on the method of synthesis. These studies make it possible to calculate the dependence of csa­cs
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1246 0.5220 Lattice parameter c, nm

STRAUMAL et al.

0.5215

0.5210

0.5205

0.5200 0 10 20 30 40 Concentration Co, at % 50 60

Fig. 2. Dependence of lattice parameter c of zinc oxide films doped with cobalt on its concentration [12].

Lattice parameter increment c, nm

0.06 0.05 0.04 0.03 0.02 0.01 0 0

Monocrystal 1000 nm 100 nm 20 nm present work

10 20 30 Concentration Mn, at %

40

50

Fig. 3. Variations in zinc oxide lattice period with increasing manganese concentration for different grain sizes [13].

on the grain (particle) size d, since the presence or absence of the second phase (manganese or cobalt oxide) was checked experimentally in specimens in almost all works dealing with the ferromagnetism of zinc oxide. The solubility limit of these elements in nanocrys talline ZnO films with a grain size of 20 nm was deter mined from the variations in the lattice period of zinc oxide with increasing cobalt or manganese concentra tion. The lattice period of zinc oxide grows linearly to 33 at % of Co (Fig. 2) [12]. Above 33 at % of Co the second phase, Co2O3 with the cubic lattice appears and the period of the wurtzite zinc oxide lattice ceases to increase as the cobalt concentration rises. This means that the solubility limit csa of cobalt in thin zinc oxide films is 33 at % of Co at 550°C (the synthesis temperature of the films). The solubility limit of man

ganese is approximately 33 at % of Mn (Fig. 3) [13]. The comparison of these values with the literature data (Fig. 3) shows that the finer the grain size, the slower the growth in the lattice period with increasing man ganese concentration and the higher the concentra tion at which the solubility limit is reached, i.e., when the second phase Mn3O4 appears. We explain this phe nomenon by the adsorption of the second component in the grain boundaries. To discover ferromagnetism in doped ZnO, it is important to be sure that the oxide is free of any sec ond phase particles capable of affecting the magnetic behavior of the specimen. This meant that each pub lished work of the study of ferromagnetism in ZnO reports the data on the concentration of the doping element and notes the presence or absence of the sec ond phase. These data have allowed us to plot the lines
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FERROMAGNETISM OF NANOSTRUCTURED ZINC OXIDE FILMS (a)
Solid solution

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1200 1000 Temperature, °C 800

on basis of ZnO

Grain size over 1000 nm

Two phases

600 400 200 0 0 10 40 30 20 Concentration Co, at % (b)
Solid solution Two phases Solubility limit Solid solution Two phases Solubility limit

50

1200 1000 Temperature, °C 800
Solid solution on basis of ZnO

600 400 200 0 0 10

Grain size below 20 nm

Two phases

20 30 40 Concentration Co, at %

50

Fig. 4. Solubility limit of cobalt in zinc oxide polycrystals with grain size of (a) over 1000 nm and (b) below 20 nm [12]. Solid symbols correspond to the single phase specimens, open symbols correspond to the two phase specimens, and asterisks corre spond to the solubility limit.

of the solubility limit of cobalt and manganese in zinc oxide for various grain sizes (Fig. 4 and 5) [12, 13]. Figure 4 shows the lines of the solubility limit of cobalt for a grain size of more than 1000 nm and less than 20 nm, as well as the experimental points. Figure 5 shows the lines of the solubility limit of cobalt and manganese for various grain sizes without experimen tal points. It is clearly seen how rapidly the total solu bility of the cobalt and manganese increases with diminishing grain size. In the specimens without grain boundaries and only an extended external surface (nanopowders, nanowires, and tetrapods), the total solubility also increases with diminishing particle size, but much slower than in the case of polycrystals with grain
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boundaries (see Fig. 6). The estimations of the adsorp tion capacity of grain boundaries and the external sur face show that an adsorption layer 2­4 Co or Mn monolayers in capacity is formed on the external sur faces and over ten monolayers appear at the grain boundaries [12, 13]. Thus, the accumulation of cobalt and manganese on GBs and free surfaces sharply shifts the solubility limit of these elements in zinc oxide towards higher concentrations. For example, at 550°C, the total solu bility of cobalt does not exceed 2 at % of Co in zinc oxide bulk and amounts to 33 at % in the nanocrystal line specimen with a grain size of less than 20 nm. The solubility of manganese at 550°C increases from 12 to 40 at %. The shift of the solubility limit in the polycrys
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1248 (a) 1200 Temperature, ° 1000 800 600 400 200 0 0 10 20 30 40 Concentration Co, at % (b) One phase (ZnO)

STRAUMAL et al.

Grain size over 1000 nm Grain size over 100­1000 nm Grain size over 20­100 nm Grain size below 20 nm

50

1200 Temperature, ° 1000 800 600 400 200 0 0

Grain size over 1000 nm 500 nm 10­100 nm 100 nm

Two phases 10 20 30 40 Concentration Mn, at % 50

Fig. 5. Solubility limit of (a) cobalt [12] and (b) manganese [13] in zinc oxide polycrystals with different grain sizes.

tal with GBs is greater than in nonsintered powder free of GBs, while the grain size is the same. This means that GBs are capable of accumulating approximately two to four times greater amounts of cobalt or manga nese than free surfaces. Therefore, the phase diagrams of the substances with a grain size of less than 1000 nm should be examined again. Especially sharp changes in the phase diagrams can be expected at a grain size of less than 100 nm. Critical Grain Size for Occurrence of Ferromagnetic Properties in Zinc Oxide Although over 1000 works that deal with the discov ery of ferromagnetism in pure and doped zinc oxide have been published, the experimental results are very contradictory. An analysis of the literature data shows that pure and doped zinc oxide monocrystals and the specimens produced by the sintering common coarse grain powders (with a particle size of over 10 m) always remain diamagnetic or paramagnetic. The

specimens produced by pulsed laser deposition (PLD) almost always demonstrate ferromagnetism at room temperature. The zinc oxide specimens synthesized by methods of wet chemistry or chemical vapor deposi tion (CVD) possess intermediate properties; they can be both paramagnetic and ferromagnetic. We assumed that the presence of ferromagnetic properties in zinc oxide correlates with the specific area of grain boundaries in unit volume sGB. We deter mined that this characteristic is based on the data reported in works on ferromagnetism in both pure and manganese doped zinc oxide [14]. The results of these calculations for pure and manganese doped zinc oxide are shown in Fig. 7 in the temperature­specific densities of boundaries sGB coordinates. Here, "tem perature" refers to the temperature of the annealing or fabrication of specimens. The results clearly demonstrate the existence of the dependence of the ferromagnetic properties of pure and doped zinc oxide specimens on the specific area of boundaries. The specimens only possess ferromag netic properties if the specific area of the boundaries exceeds the critical value sth. Free surfaces do not result in ferromagnetism, even if their specific area is great and oxide particles are very small, but no grain boundaries are present in the specimen. If zinc oxide nanoparticles or nanowires are not completely sin tered and free of grain boundaries, they also do not possess the ferromagnetic properties. Similar results were obtained for the specimens of zinc oxide doped with cobalt. Thus, the presence of manganese or cobalt is not necessary for the ferromagnetic proper ties to occur in zinc oxide. Even pure zinc oxide can be ferromagnetic; the fine grain size or high specific area sGB play a crucial role, rather than doping with mag netic atoms, as was initially assumed [3]. Nevertheless, the presence of manganese or cobalt in the zinc oxide lattice facilitates transition to the ferromagnetic state and shifts the critical size towards greater grain sizes (Fig. 7). For example, in a number of works in which the specific grain size fell between the critical size for the pure zinc oxide and that doped with manganese, paramagnetic properties were observed in pure zinc oxide and ferromagnetic properties were observed in specimens doped with manganese [15, 16]. Figure 8 shows the magnetization curves for the pure zinc oxide films and films doped with 0.1 and 10 at % manganese (in units of 10­3 B/f.u. at room tem perature). These curves were obtained after the sub tracting the magnetic contribution of the substrate and specimen holder. The magnetization is 2 â 10­3 B/f.u. = 0.16 emu/g for the zinc oxide films doped with 0.1 at % manganese, 0.8 â 10 3 B/f.u. = 0.04 emu/g for the zinc oxide films doped with 10 at % manganese, and 1â 10­3 B/f.u. = 0.06 emu/g for pure zinc oxide deposited on an aluminum substrate. Thus, the extremely small grain size and, hence, high specific
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FERROMAGNETISM OF NANOSTRUCTURED ZINC OXIDE FILMS (a) 1200 1000 Temperature, °C 800 600 400 200 0 0 10 20 30 40 Concentration Co, at % (b) 50 Two phases Solid solution Two phases Solubility limit Solid solution on basis of ZnO Particle size over 1000 nm

1249

Particle size 20­100 nm

Particle size 6 nm

1200 1000 Temperature, °C

Solid solution on basis of ZnO

> 1000 nm 800 600 400 200 0 0 10 < 100 nm Solid solution Two phases Solubility limit 20 30 40 Concentration Mn, at % 50 Two phases

Fig. 6. Solubility limit of (a) cobalt [12] and (b) manganese [13] in zinc oxide powders with different particle sizes.

density of grains in the studied pore free films make it possible to observe the ferromagnetic behavior of both pure and doped zinc oxide. The first signs of the ferro magnetic behavior of pure zinc oxide films were obtained quite recently; our work was the second study in which ferromagnetism was observed in undoped zinc oxide. The saturation magnetization grows linearly with increasing thickness, i.e., the mass of a zinc oxide film. The magnetization curves of pure zinc oxide show hysteresis with a coercive force Hc of about 0.02 T for the zinc oxide film on the sapphire substrate (Fig. 9a) and about 0.01 T for the zinc oxide film on the alumi num substrate (Fig. 9b). Figure 9 presents only the magnified middle part of the magnetization curves to
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illustrate clearly the value of the coercive force. These values are close to the coercive force found in works of other authors in pure zinc oxide (Hc = 0.005­0.02) or exceed it. At 40 K, the saturation magnetization of the pure zinc oxide films deposited on a sapphire substrate is only by 40% higher than that at the room tempera ture. This means that the Curie temperature of the films under study greatly exceeds room temperature. Effect of Doping and Grain Boundary Structure on Ferromagnetic Behavior of Zinc Oxide Films Among doped magnetic semiconductors, zinc oxide doped with manganese is one of the most prom ising materials for applications in spintronics, since
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STRAUMAL et al. (a) Equivalent size of equiaxed grains, m 10­4 10­6 Pure ZnO

10­2 1400 1200 Temperature, °C 1000 800 600 400 200 0 10
2

10

­8

108 104 106 2/m3 Ratio of area of boundary to volume sGB, m (b) Equivalent size of equiaxed grains, m

10­2 2000 1800 1600 Temperature, °C 1400 1200 1000 800 600 400 200 0 102 ZnO doped with manganese

10­4

10

­6

10­8

Monocrystals and monocrystalline films Equiaxed grains, dense material Equiaxed grains, porous material Elongated and flat grains, dense material Elongated and flat grains, porous material

108 104 106 2/m3 Ratio of area of boundary to volume sGB, m

Fig. 7. Ferromagnetic (solid symbols) and paramagnetic or diamagnetic (open symbols) behavior of films of (a) pure zinc oxide and (b) zinc oxide doped with manganese depending on specific area of grain boundaries sGB (ratio of area of boundaries to vol ume) at various temperature of specimen synthesis T. Vertical dashed lines mark threshold value sth. Large symbols are experi mental data from our work [14].

manganese ions possess the maximum magnetic moment among 3d transition metals. Moreover, man ganese has an almost completed 3d band, which makes it possible to achieve the stable spin polarized state.

The saturation magnetization depends nonmono tonically on the manganese concentration (Fig. 10) [17]; it increases by almost three orders of magnitude when a very small amount of manganese is added
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FERROMAGNETISM OF NANOSTRUCTURED ZINC OXIDE FILMS 2 1 0 ­1 ­2 ­9 ­6 Substrate: Al foil Synthesis temperature: 550°C ­3 0 3 Magnetic field, T 6 9 ­0.4 ZnO + 10 at % Mn Pure ZnO ZnO + 0.1 at % Mn (a) Magnetization, 10­3 µB/f.u. 1.0 0.5 0 ­0.5 ­1.0 0 ­0.2 0.2 Magnetic field, T (b) 0.4 Pure ZnO/sapphire

1251

Magnetization, 10­3 µB/f.u.

Magnetization, 10­3 µB/f.u.

Fig. 8. Magnetization of films of pure zinc oxide and zinc oxide doped with 0.1 and 10 at % of manganese at room temperature (in [14]).

0.6 0.4 0.2 0 ­0.2 ­0.4 ­0.6

Pure ZnO/Al

(0.1 at %) to pure zinc oxide. As the manganese con centration continues to increase, the saturation mag netization drops rapidly and becomes nearly indistin guishable from the background concentration at 5 at % of manganese. At a greater amount than 5 at % of manganese, the magnetization increases again. The saturation magnetization drops again to the back ground values only in the vicinity of 30 at % of manga nese, when the second phase Mn3O4 appears in zinc oxide with the wurtzite lattice in addition to manga nese solid solution. We assume that, at small manga nese concentrations, the growth in the saturation magnetization results from the injection of bivalent manganese ions and charge carriers into pure zinc oxide. The decrease of the saturation magnetization in the range of 0.1­5 at % of manganese is caused by an increase in the share of tri and quadricvalent manga nese ions. The repeated growth of the saturation mag netization at greater concentrations than 5 at % of manganese is due to to the formation and growth of multilayer segregation films rich in manganese at the grain boundaries of zinc oxide. Similar nonmonotonic variations in the magneti zation with increasing manganese concentration are reported in the literature and are observed in nanoc rystalline specimens of manganese obtained by other methods. However, the minima and maxima of the magnetization are found at other manganese concen trations. This can be explained by different topologies of the grain boundary network in these materials. Figure 11 shows the bright field micrographs of the doped zinc oxide films with 10 at % of Mn (Fig. 11a) and 15 at % of Mn (Fig. 11b) obtained by high resolu tion transmission electron microscopy [18]. The direct resolution of the lattice makes crystalline zinc oxide grains with the wurtzite lattice visible. It can be clearly seen in Fig. 11a that the amorphous phase interlayers are located in the boundaries between these grains. With
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­0.4

­0.2 0 0.2 Magnetic field, T

0.4

Fig. 9. (a) Magnetic hysteresis at room temperature for pure zinc oxide films deposited on sapphire single crystal. (b) Magnetic hysteresis at room temperature for pure zinc oxide film deposited on aluminum foil. Only the magnified middle part of the magnetization curves is presented to demonstrate clearly coercive force values.

0.18 0.16 Magnetization, emu/g 0.14 0.12 0.10 0.08 0.06 0.04 0.02 0 0 5

Al3O3 substrate Al substrate
ZnO ZnO + Mn3O4

10 15 20 25 Mn concentration, at %

30

35

Fig. 10. Dependence of saturation magnetization of zinc oxide doped films on manganese concentration [17]. Vol. 113 No. 13 2012


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STRAUMAL et al.

()

5 nm

A

A B

B

C

C

(b)

5 nm

Fig. 11. Bright field micrographs of films of zinc oxide doped with manganese obtained by high resolution trans mission electron microscopy. (a) Zinc oxide with 10 at % of manganese. Amorphous interlayers between nanocrys talline ZnO grains are seen. (b) Zinc oxide with 15 at % of manganese. Crystalline ZnO grains are surrounded by amorphous interlayers. Inserts show Fourier transform patterns for amorphous and crystalline areas [18].

increasing manganese concentration from 10 to 15 at %, the amount of the amorphous phase grows. It can clearly be seen from the comparison of the structures presented in Figs. 11a and 11b. Thus, one of the grains of zinc oxide with the wurtzite lattice in Fig. 11a

(in the middle of the photo) is surrounded on all sides by an amorphous area rich in manganese. Since the amorphous interlayers in the specimen with 15 at % of Mn are already thick, we have managed to obtain Fou rier transform patterns for the amorphous and crystal line areas. The corresponding crystalline grains (to the left and right) and the amorphous interlayer (in the middle) are designated as A, B, and C in Fig. 11b. Figure 11 demonstrates clearly what happens when manganese is gradually added to nanocrystalline zinc oxide. Some of the manganese atoms fall into the grain lattice during doping and contribute to the shift of X ray diffraction peaks from the coherent scattering regions (the grains with the wurtzite lattice). The remaining manganese atoms (approximately two thirds) fall into the amorphous interlayers that sur round and separate the grains. These amorphous interlayers become thicker with increasing manganese concentration. Our quantitative estimation shows that, at 30 at % of Mn, when the solubility limit for a grain size of 20 nm is reached, the thickness of the grain boundary interlayers is six to ten monolayers of manganese oxide, while that of the layers on the exter nal surface is two monolayers [13]. This situation dif fers radically from the case of one layer McLean adsorption (on the surface of GBs). The morphology of the amorphous areas between zinc oxide grains enriched in manganese differs con siderably from that of very homogeneous and uni formly thin amorphous interlayers of the grain bound ary prewetting phases in the ZnO:Bi2O3 specimens obtained by fluid phase sintering [19]. Thus, the amorphous areas in the specimen with 15 at % of Mn (Fig. 11b) surround some zinc oxide grains on all sides and only partially penetrate between the remaining grains. This microstructure strongly resembles the morphology of two phase polycrystals, in which the second phase fully wets some boundaries and partially wets the remaining boundaries. It is well known from the experiments with metal alloys that the distribution over misorientation and orientation of grain boundaries significantly changes the behavior of polycrystals. We compared the mag netic properties of nanocrystalline zinc oxide films with the same grain size, which was considerably smaller than the threshold value, but with different misorientation and orientation distributions of boundaries. Figure 12 shows the X ray diffraction spectra for the thin zinc oxide films deposited on the sapphire monoc rystals with orientation (102) and annealed in air (the upper curve) or argon (the lower curve) [20]. The X ray spectrum for the film annealed in argon (the lower curve) contains three peaks, which correspond to the reflections 100, 002, and 101 of the hexagonal wurtzite lattice of zinc oxide. The X ray diffraction spectrum from the film annealed in air (the upper curve in Fig. 12) contains practically only one peak that corre
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FERROMAGNETISM OF NANOSTRUCTURED ZINC OXIDE FILMS 12 Intensity, arb. u. 8 4 0 ­4 ­8 44 ­12 ­6 ­4 ­2 0 Field, 2 4 D

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F

Air (ferromagnetic and textured)

Ar (diamagnetic and random)

002

100

36

38

40 2, deg

42

101

J, 10

­6

emu

Fig. 12. X ray diffraction curves for ferromagnetic (upper curve) and diamagnetic (lower curve) zinc oxide thin films deposited on sapphire substrate with orientation (102) and annealed in air (upper curve) and argon (lower curve) [20].

6

sponds to the 002 reflection from zinc oxide with a hex agonal wurtzite structure. This means that the film syn thesized in air has a pronounced texture. Figure 13 shows the magnetization curves for thin zinc oxide films annealed in air (black triangles) and argon (black circles) [20]. The black triangles corre spond to the magnetization curve obtained after the subtraction of the contribution of the uncovered sap phire substrate. The film annealed in air possesses pro nounced ferromagnetic properties and has a saturation magnetization of about 12 â 10­6 emu (at applied fields of over 6 T). The film annealed in argon does not dem onstrate the ferromagnetic behavio