Magnetic Properties of Zno:Co Layers Obtained by Pulsed Laser Deposition Method

Materials Science-Poland, 36(3), 2018, pp. 439-444 http://www.materialsscience.pwr.wroc.pl/ DOI: 10.1515/msp-2017-0114

Magnetic properties of ZnO:Co layers obtained by pulsed laser deposition method

IRENEUSZ STEFANIUK1,BOGUMIŁ CIENIEK1,∗,IWONA ROGALSKA1,IHOR S.VIRT2,1, AGNIESZKA KOSCIAK´ 1

1Faculty of Mathematics and Natural Sciences University of Rzeszow, Rzeszow, Poland 2The Ivan Franko State Pedagogical University in Drohobycz, Drohobycz, Ukraine

We have studied magnetic properties of zinc oxide (ZnO) composite doped with Co ions. The samples were obtained by pulsed laser deposition (PLD) method. Electron magnetic resonance (EMR) measurements were carried out and temperature dependence of EMR spectra was obtained. Analysis of temperature dependence of the integral intensity of EMR spectra was carried out using Curie-Weiss law. Reciprocal of susceptibility of an antiferromagnetic (AFM) material shows a discontinuity at the Néel temperature and extrapolation of the linear portion to negative Curie temperature. The results of temperature dependence of EMR spectra for the ZnO:Co sample and linear extrapolation to the Curie-Weiss law indicated the AFM interaction between Co ions characterized by the Néel temperatures TN = 50 K and TN = 160 K for various samples. The obtained g-factor is similar to g-factors of nanocrystals presented in literature, and the results conﬁrm that in the core of these nanocrystals Co was incorporated as Co2+, occupying Zn2+ sites in wurtzite structure of ZnO. Keywords: electron magnetic resonance; ZnO:Co; diluted magnetic semiconductor; Curie temperature

1. Introduction Antiferromagnetic (AFM) couplings are preferred between transition metal atoms in Co-doped ZnO, resulting in a spin-glass state. The FM ordering Nanosized zinc oxide (ZnO) doped with transi- is realized by electron doping, based on first- tion metal (TM) ions may form diluted magnetic principles spin density functional calculations [9]. semiconductors (DMSs). DMSs can find possible For the AFM properties, CoO phase: a well-known applications in the spin-dependent semiconductor AFM material with the Néel temperature T of electronics [1] due to the spin-spin exchange inter- N 291 K is responsible [10]. In ZnO:Co, only a few actions between localized magnetic moments and films showed FM features, while the others showed electrons of conduction band [2]. For practical ap- spin-glass-like behavior [10]. plications, it is a key issue to raise the Curie temperature TC of DMSs above room temperature. We have studied magnetic properties of zinc Room temperature ferromagnetism (FM) has been oxide composite doped with high concentrations observed for several magnetically doped materi- of Co. The pulsed laser deposition method (PLD) als such as ZnO [3–5]. At room temperature, FM was used to produce the samples on sapphire ZnO, especially Co-doped, has been a subject of substrates (Al2O3). special interest: this material is optically transpar- ent, with the band gap of 3.3 eV and the exci- In the present study, we report electron mag- ton binding energy of 60 meV [6]. The existence netic resonance (EMR) properties of ZnO:Co lay- of FM ordering in Co-doped ZnO was first pro- ers. The measurements of temperature depen- posed theoretically. It was attributed to the dou- dence of EMR spectra were performed. Analy- ble exchange interaction between Co ions [7,8]. sis of the temperature dependence of the integral intensity of EMR spectra was carried out using ∗E-mail: [email protected] the Curie-Weiss law.

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1.1. Crystal structure of ZnO ZnO has three crystal forms: hexagonal wurtzite, cubic zinc blende and cubic rock salt which is rarely observed. The wurtzite structure is most commonly used as it has the highest stability under normal working conditions. ZnO has stable lattice parameters of wurtzite structure a = 0.3296 nm and c = 0.52065 nm (hexagonal crystal system, P63mc space group) [11]. The structure of ZnO consists of alternating planes composed of O2− and Zn2+ ions which are tetrahedrally coordinated and stacked along the c-axis on an alternate basis (Fig.1). The Zn2+ and O2− ions create a normal dipole moment and instant polarization which results in diversification of surface energy. The tetrahedral coordination in ZnO results in noncentral symmet- ric structure, and consequently, in piezoelectricity and pyroelectricity in ZnO. Hexagonal and zinc blende structures do not show inversion symmetry which is a property like these that are responsible for the piezoelectricity and pyroelectricity of ZnO. Fig. 1. Crystal structure of zinc oxide (wurtzite). Because Co2+ ions have a smaller ionic radius (0.58 Å) than Zn2+ ions (0.60 Å), a substitution of 2+ 2+ Zn by Co in ZnO lattice matrix in the wurtzite Mn2+ d5 in ZnO [15], D = −0.0093 meV for Mn2+ structure is possible [12, 13]. d5 in GaN [16]), and even for Co2+ (d7,S = 3/2) In wurtzite materials like ZnO, substitutional where D is significantly larger (0.342 meV) [17]. impurities are subjected to a trigonal crystal field. Crystal-field energy level diagram for the effec- Via the spin-orbit interaction, this anisotropic crys- tive spin S = 3/2 of the spin-orbit coupling due + tal field induces a strong magnetic anisotropy to large ZFS for the four-coordinated Co2 ions which can be described by a zero-field split- is presented in Fig.2. ting (ZFS) of the fundamental state and by an Due to the very large zero-field splitting, in anisotropic effective g-factor in an effective spin practice only the transition between spin levels Hamiltonian Hs in the form: with Ms = ±1/2 are possible to observe. 2 Hs = µBgkHzSz + µBg⊥ (HxSx + HySy) + DSz (1) where S is the spin quantum number, D describes 2. Experimental zero-field splitting and gk and g⊥ are the ef- A widely applied in ceramic technology solid fective g-factors for directions of magnetic field state reaction method was used to obtain hard so- parallel and perpendicular to the wurtzite c-axis, lutions of ZnO:Co. Materials of special cleanness respectively. were used as initial components for preparation of The zero-field splitting constant for this partic- the load. The powders of CoCO3 compounds were ular impurity in ZnO is very small. This is typi- obtained by colloidal grinding to the size of par- cal of S = 5/2 moments (e.g. D = −0.0074 meV ticles 50 nm to 100 nm, and mixed up with small for Fe3+ d5 in ZnO [14], D = −0.0027 meV for amount of water and ZnO powder in the jasper jar Magnetic properties of ZnO:Co layers obtained by pulsed laser deposition method 441

of 0.3 Hz. Time of a layer growth was about 30 minutes. The layer thickness was 300 nm for sample N88 and 500 nm for sample N79. Concentration of cobalt in both samples was x = 4 % (Zn1 − xCoxO). EMR measurements in the X-band were performed on Bruker multifrequency and multireso- nance FT-EPR ELEXSYS E580 spectrometer. The temperature of the samples was controlled in the range of 95 K to 300 K using the Bruker liquid nitrogen gas-ﬂow cryostat with 41131 VT digital controller, and in the range of 4 K to 300 K using the Oxford Instruments ESR 900 liquid helium gas- ﬂow cryostat with Mercury iTC digital controller.

3. Results The EMR spectra of ZnO:Co layers, for sam- Fig. 2. Crystal-field energy level diagram for the effec- ples N79 and N88, are shown in Fig.3. EMR tive spin Seff = 3/2 of the spin-orbit coupling due to large ZFS for four-coordinated Co2+ spectra for both samples consist of a broad line 2+ ions. near 1500 Gs and 3300 Gs resulting from Co ions, and two groups of narrow lines assigned to the impurities in the sapphire substrate, near of a planetary mill SAND-1-1. Time of mixing and 2000 Gs (strong intensity), and near 3400 Gs grating was determined by the homogenization de- (small intensity). gree and was set to 16 h. The mixture was dried in 2+ a temperature of 120±5 °C. The EMR features for Co content smaller than that in our samples should be further split The mixture was annealed in the chamber fur- into eight transitions due to hyperfine interac- nace VTP-06M1 with periodic operation in two tion. However, for higher Co2+ concentrations, stages. The primary mixture for the activation (de- x > 0.1 % [18], only one broad feature related composition) of CoCO and doping of the ZnO ma- 3 to g⊥ and one broad feature related to gk were terial was annealed in air at 700±5 °C for 4 hours. observed. The reported values of g⊥ = 4.55 and Pellets with a diameters of 11.5 nm to 15 mm, 1 nm gk = 2.24 for bulk ZnO:Co and g⊥ = 4.53 and to 2.5 mm thick were formed by applying pressure gk = 2.22 for thin ZnO:Co film, with the concen- of 40 MPa to 60 MPa on a hydraulic press PG-10 tration of cobalt of 10 % [18, 19], are similar to g without use of plastificators. The pellets were an- values obtained by us g⊥ = 4.56N79; 4.79N88 and nealed in air for the next 3 hours to improve their gk = 2.167N79; 2.196N88. mechanical hardness. The accuracy of temperature control was ±5 °C. Maximal temperature of an- In our earlier work we observed EMR signal nealing, corresponding to the isothermal area of the with g ≈ 2 which corresponds to g-value typical curve of heating-cooling, was 1110 °C. of donors in ZnO [20, 21]. The temperature dependence of EMR spectra Samples of zinc-oxide doped with cobalt on for sample N79 is summarized in Fig.4. sapphire substrate were prepared by PLD method. The substrate temperature was 200 °C. Beam pa- The EMR spectra of impurities and various de- rameters were as follows: 0.2 nm, 0.5 J in 10 ns fects in ZnO show more than ten features in the pulse, Gaussian distribution, repetition frequency g-value range of 1.96 to 2.06. Shallow donors and 442 IRENEUSZ STEFANIUK et al.

Fig. 3. EMR spectra of ZnO:Co layers (samples N79 and N88) at room temperature.

Fig. 4. EMR spectra of the ZnO:Co layer (sample N79). deep N acceptors are described in the literature as a source of EMR lines with those g-values [22] (Fig.4, broad line near 3300 Gs, visible only in low temperatures).

4. Discussion From the EMR spectra, the effective spectro- scopic g-factor, the peak-to-peak linewidth of the resonance line Hpp, EMR intensity and the reso- Fig. 5. Temperature dependence of EMR spectra of the nance field Hr were determined. Based on these sample ZnO:Co (a) g⊥(T), (b) Hpp (T), (c) EMR data we obtained the temperature dependencies of intensity (T), and (d) inverse of integral intensity Hpp (T), EMR intensity (T), g⊥ (T), and integral (susceptibility). intensity as presented in Fig.5. Generally, Co2+ ions are characterized by very short spin-lattice relaxation time, therefore their data [23–25]. However, in our case we have stud- EMR spectra can be clearly observed only at ied Zn1 − xCoxO with high concentration of cobalt low temperatures (near liquid helium) practically (x = 4 %), therefore we had to take into ac- in all crystalline solids according to references count magnetic interactions. We observed EMR Magnetic properties of ZnO:Co layers obtained by pulsed laser deposition method 443 lines from helium to room temperature without the literature [9], where the calculations indicated visible hyperfine structures. This proves the cor- that the AFM coupling was more pronounced with rectness of the assumptions and is consistent with increasing Co density. the description of EMR spectra of cobalt ions reported in the literature [26]. 5. Conclusions We used the Curie-Weiss law to analyse the temperature dependence of the integral intensity, In conclusion, we have investigated the na- which is directly proportional to the magnetic sus- ture of magnetic interactions between Co ions in ceptibility χ. A linear increase of χ−1(T) at higher ZnO through EMR measurements. Due to AFM temperatures can be fitted to the Curie-Weiss law. superexchange interactions between Co ions, Co- Reciprocal of susceptibility of AFM samples show- doped ZnO prefers the spin-glass-like state, while ing the discontinuity at the Néel temperature and the FM state is stabilized by electron carriers. the extrapolation of the linear portion to the nega- Due to high concentration of cobalt ions in tive Curie temperature are shown in Fig.5d. Zn1 − xCoxO (x = 4 %) the spectra of cobalt ions 2+ The results of temperature dependence of Co in this material are characterized by magnetic EMR spectra for the sample ZnO:Co and lin- interactions over a wide temperature range. ear extrapolation to the Curie-Weiss law indicate The sample having a smaller thickness (N88) the AFM interaction between Co ions character- is characterized by more stable parameters which ized by the Néel temperatures TN(N88) = 50 K, may suggest that the layer has fewer defects (is of TN(N79) = 160 K. better quality). The effects of magnetic interactions can be in- g-factor obtained in this work is similar to g fac- ferred from the magnetization data, but the inter- tor in nanocrystals presented in literature [32]. The action is AFM and short-ranged. 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