III. RESULTS AND DISCUSSION

The structures of the particles were characterized using XRD patterns analysis and are shown in Fig. 1. The ZnGa₂O₄ diffractograms are observed as dominant in the patterns, which are in accordance with the Inorganic Crystal Structure Database (ICSD) card with PDF#00-038-1240. Furthermore, indexing and calculating the lattice parameters were performed using the CRYSFIRE subprogram. The adjacent intensive peaks, between 30° and 80°, were used in lattice parameter calculations. The structure was found coherent to the ICSD cards. Additionally, the crystal structures of (Zn_1–x Co _x )Ga₂O₄ (x = 0.5, 0.10, and 0.20) particles were found cubic with the lattice parameters of 8.295 ± 0.006 Å, 8.288 ± 0.007 Å, and 8.285 ± 0.008 Å, respectively. An impurity phase, such as Co compounds or contamination elements during the synthesis abrading from WC vial and balls, was not observed. The distribution particle size is almost the same size interval, changing from 20 to 100 nm, as in the previous study. ^{Reference Can, Gafferi, Aksoy, Shah and Fırat18} The optical analyses were used to understand the positions of Co atoms in lattice and the band gap calculation of each doping amount. The optical studies were done using UV-vis spectra, as shown in Fig. 2. The band gaps were calculated from the inflexion point between 228 and 335 nm of the UV-vis curve. The extremum values of differentiation of each curve were used in calculation of the band gap energy by Eq. (1).

FIG. 1.

The XRD patterns of synthesized particles (the Miller indices were specified on the patterns).

FIG. 2.

UV-vis absorption spectra of (Zn_1–x Co _x )Ga₂O₄ (x = 0.05, 0.10, and 0.20) particles.

E _g, h, λ, and c indicate band gap energy, Planck constant, wave length, and speed of light, respectively. The calculated band gaps were found as 4.97, 4.93, and 4.93 eV for 5%, 10%, and 20% Co doping amount, respectively. The calculated band gaps for Co-doped ZnGa₂O₄ particles were in accordance with the reported literature values, which changed from 4.4 to 5.0 eV. ^{Reference Zhang, Zhang, Lan, Chen and Wang34,Reference Binet and Gourier35} Furthermore, doping with transition metals caused narrow band gaps. ^{Reference Zhang, Zhang, Lan, Chen and Wang34} However, the narrowing in band gap disappeared for Co amount higher than 10%. No specific change in the band gap was observed despite increasing Co amount up to 20% in the lattice. The specific absorptions of 3d ⁷ symmetry, in addition to the electronic transition between band gaps, were detected between 510 and 730 nm as pointed out in Fig. 2. Substituting Co atoms in the tetrahedral site at the ⁴A₂(F) ground level, permitted transitions, Ʋ₁(Co⁺² _Td): ⁴T₂(F) → ⁴A₂(F), Ʋ₂(Co⁺² _Td): ⁴T₁(F) → ⁴A₂(F), and Ʋ₃(Co⁺² _Td): ⁴T₁(P) → ⁴A₂(F), which were in (infrared) IR, near-IR, and visible regions, ^{Reference Bouloudenine, Viart, Colis and Dinia36} respectively. The near-IR transitions in between ⁴T₁(F) → ⁴A₂(F) electronic transitions were found at 1312 and 1406 nm. In addition, the transition in visible region of UV-vis absorption spectra at 554, 595, and 640 nm should correspond to ⁴A₂ (F) → ²A₁(G), ⁴A₂(F) → ⁴T₁(P), and ⁴A₂(F) → ²E(²G) transitions, ^{Reference Koidl37–Reference Denisov, Volk, Malyarevich, Yumashev, Dymshits, Zhilin, Kang and Lee39} respectively. The intensities of transitions are sensitive to the Co amount in tetrahedral site. ^{Reference Mindru, Gingasu, Marinescu, Patron, Diamandescu, Feder, Calderon-Moreno and Stanica40,Reference Volk, Malyarevich, Yumashev, Alekseeva, Dymshits, Shashkin and Zhilin41} Thus, the increased doped atoms in tetrahedral site caused an increase at intensities. ^{Reference Mindru, Gingasu, Marinescu, Patron, Diamandescu, Feder, Calderon-Moreno and Stanica40} However, a systematic change in intensity from the amount of Co atoms in lattice could not be observed, thus indicating that the Co atoms in the lattice were not only in the tetrahedral site.

The replacement of Co atoms in the lattice was also analyzed using Raman spectra, shown in Fig. 3. The spinel structure related phonon modes in AB₂O₄ structures with space group Fd3m were classified as follows ^{Reference Kang, Kim, Lee, Bahng, Choi, Park, Kim, Kim, Hwang, Mho, Eom, Yu, Song and Kim42–Reference Bouchard and Gambardella44} :

FIG. 3.

Raman spectra of (Zn_1–x Co _x )Ga₂O₄ (x = 0.05, 0.10, and 0.20) particles.

The infrared-active mode is defined by 4T_1u and the Raman active modes by A_1g, E_g, and 3T_2g, first-order Raman modes. The first-order Raman active modes in relation with spinel ZnGa₂O₄ structure were observed at 712 cm⁻¹ (A_1g), 609 cm⁻¹ (T_2g), and 468 cm⁻¹ (3T_2g) which corresponds to Zn²⁺ ions in the tetrahedral sites and independent from the motions of Ga³⁺ ions in the octahedral sites. ^{Reference Lopez-Moreno, Rodriguez-Hernandez, Munoz, Romero, J.Manjon, Errandonea, Rusu and Ursaki45,Reference Van Gorkom, Haanstra and Boom46} On the other hand, the low frequency modes at around 131 cm⁻¹ (T_2u), 167 cm⁻¹ (T_1u: infrared active), and 183 cm⁻¹ (T_2g: Raman active) were Zn ions related modes as mentioned in the study of Lopez-Moreno et al. ^{Reference Lopez-Moreno, Rodriguez-Hernandez, Munoz, Romero, J.Manjon, Errandonea, Rusu and Ursaki45} Furthermore, as illustrated in Fig. 3, doping Co atoms raises the infrared active mode at around 540 cm⁻¹ (4T_1u); a finding reported in a previous study as well. ^{Reference Van Gorkom, Haanstra and Boom46,Reference Boppana and Lobo47} Increasing the Co amount in the lattice disorders the structure. Thus by deforming the structure's periodicity, the vibration modes of the bonds change. The second-order Raman line was obtained at 670 cm⁻¹, which originated from the disordered structure due to exceeding the stoichiometric composition to the ZnO rich site ^{Reference Iida and Okazaki48} or/and Co ions in octahedral site. ^{Reference Withnall, Lipman, Fern, Rose and Silver49} While at approximately 670 cm⁻¹, the intensity of Raman line became apparent with an increasing Co amount, a decrease of Raman lines occurred for A_1g, E_g, and 3T_2g modes. These lines assigned the disordered structure. Thus, these results can be correlated to the Co atoms in the octahedral site, as also seen in those findings of the previous study. ^{Reference Can, Gafferi, Aksoy, Shah and Fırat18}

The replacement of Co atoms in both tetrahedral and octahedral sites caused a specific change in magnetic behavior of ZnGa₂O₄ semiconductors. The magnetization versus magnetic field σ(H) curves was measured at 300 K, as shown in Fig. 4(a) for (Zn_1–x Co _x )Ga₂O₄ (x = 0.05, 0.10, and 0.20) particles. The decline at coercivity value was observed with an increasing Co amount in lattice. Enhancing at the Co concentration makes the Co atoms periodic in lattice as mentioned before. Therefore, the formation periodicity for Co atoms in the lattice leads to an increase in the amount of antiferromagnetic coupled Co atoms. The increase of Co concentration upto 20% leads to a decrease at the coercivity value down below the instrument sensitivity, which is between ±50 Oe. On the other hand, the coercivities for (Zn_1–x Co _x )Ga₂O₄ (x = 0.05 and 0.10) particles, are at around 139 ± 50 and 91 ± 50 Oe, respectively, which is indicating the long term magnetic ordering (the magnetization sensitivity of VSM apparatus of PPMS is ±50 Oe). The replacement of 20% Co atoms made the antiferromagnetic interaction dominant in all particles; a similar observation was reported earlier for oxide magnetic semiconductors, such as ZnO, SnO, TiO, InO etc. ^{Reference Gao, Li, Liu, Song, Liu and Jiang7,Reference Berardan and Guilmeau8,Reference Can, Firat and Ozcan50,Reference Punnoose, Hays, Thurber, Engelhard, Kukkadapu, Wang, Shutthanandan and Thevuthasan51} However, the ferromagnetic ordering did not totally disappear. Thus, the competition between antiferromagnetic and ferromagnetic interactions cannot be neglected for the oxide diluted magnetic semiconductors. The inverse DC magnetic susceptibility versus temperature χ⁻¹(T) curves are shown in Fig. 4(b). The χ⁻¹(T) curves were used to determine the effectiveness of competition between ferromagnetic and antiferromagnetic coupling on the magnetization. As seen in Fig. 4(b), the linearity at the high temperatures, observed in temperature intervals of 260–300 K, 180–300 K, and 180–300 K for 5%, 10%, and 20% Co-doped particles, disappeared with decrease in temperatures. The linear fits (signed using red lines on the figure) were done to the linear regions on χ⁻¹(T) curves to obtain Curie–Weiss temperatures, which were found from the intersection points on the axis. The Curie–Weiss temperatures for 5%, 10%, and 20% doping amounts were found as 32.5, 4, and −4.6 K, respectively, as shown in Fig. 4(b). The positive Curie–Weiss temperature for (Zn_1–x Co _x )Ga₂O₄ (x = 0.05 and 0.10) particles and negative Curie–Weiss temperature for (Zn_0.8Co_0.20)Ga₂O₄ are the parameters that prove the dominancy of ferromagnetic and of antiferromagnetic interactions, respectively. ^{Reference Lawes, Risbud, Ramirez and Seshadri6,Reference Berardan and Guilmeau8} In addition, the humpback, which observed at the χ⁻¹(T) curves between 45 and 120 K, also assigns the combination of two separate magnetic interactions, double exchange and super exchange, in the lattice.

FIG. 4.

(a) Room temperature magnetization curves of (Zn_1–x Co _x )Ga₂O₄ (x = 0.05, 0.10, and 0.20) particles and (b) inverse DC magnetic susceptibility curves of (Zn_1–x Co _x )Ga₂O₄ (x = 0.05, 0.10, and 0.20) particles (inset shows the DC magnetic susceptibility curves).

The DC magnetic susceptibility versus temperature χ(T) curves, in addition, are shown as the inset in Fig. 4(b). The characteristic of χ(T) curves is coherent with the Curie–Weiss law, which is also used for the wide band gap semiconductors. ^{Reference Wang, Herron, Moiler and Bein53} However, the studies in literature assign the temperature-independent components in the χ(T) curves ^{Reference Berardan and Guilmeau8,Reference Punnoose, Hays, Thurber, Engelhard, Kukkadapu, Wang, Shutthanandan and Thevuthasan51–Reference Wang, Herron, Moiler and Bein53,Reference Kobayashi, Ishida, Hwang, Mizokawa, Fujimori, Mamiya, Okamoto, Takeda, Okane, Saitoh, Muramatsu, Tanaka, Saeki, Tabata and Kawai55,Reference Timm, von Oppen and Hofling57} for DMSs. The temperature-independent component originates from both free electrons and the non-interacting impurity spins due to Co atoms in the (Zn_1–x Co _x )Ga₂O₄ lattice, which are known as the Pauli and Curie susceptibilities, respectively. ^{Reference Kobayashi, Ishida, Hwang, Mizokawa, Fujimori, Mamiya, Okamoto, Takeda, Okane, Saitoh, Muramatsu, Tanaka, Saeki, Tabata and Kawai55–Reference Timm, von Oppen and Hofling57} Thus, these ionic transition metals that do not interact with each other, especially for DMSs, do lead strong antiferromagnetic interactions that arise the temperature-independent paramagnetic component in susceptibility curves. ^{Reference Kobayashi, Ishida, Hwang, Mizokawa, Fujimori, Mamiya, Okamoto, Takeda, Okane, Saitoh, Muramatsu, Tanaka, Saeki, Tabata and Kawai55} Moreover, Kobayashi et al. point to the Co ions as having a Néel temperature above the room temperature for the origin of temperature-independent paramagnetic component. ^{Reference Kobayashi, Ishida, Hwang, Mizokawa, Fujimori, Mamiya, Okamoto, Takeda, Okane, Saitoh, Muramatsu, Tanaka, Saeki, Tabata and Kawai55} To add the effects of non-magnetic matrix, the Curie–Weiss equation was modified as shown in Eq. (2). The χ(T) curves [inset of Fig. 4(b)] were fit to Eq. (2) to separate and analyze the effectiveness of all magnetic behaviors. ^{Reference Berardan and Guilmeau8,Reference Punnoose, Hays, Thurber, Engelhard, Kukkadapu, Wang, Shutthanandan and Thevuthasan51–Reference Cao, McCall, Shepard, Crow and Guertin54}

χ₀, C, T, and θ were used to assign the temperature-independent part (consisting the Pauli susceptibility, Landau diamagnetism, and core diamagnetism ^{Reference Dutta, Seehra, Thota and Kumar32,Reference Cao, McCall, Shepard, Crow and Guertin54} ), the Curie constant, temperature, and the asymptotic Curie–Weiss temperature, respectively. The calculated values were shown in Table I; and also, used in Eq. (3) to calculate effective magnetic moment (µ_eff) values (Table I depicts the calculations). ^{Reference Mandal, Das, Nath, Karmakar and Satpati52}

TABLE I.

The calculated χ₀, C, and µ_eff values according to the Curie–Weiss law.

k _B, C, M _w, N, and x are the Boltzmann constant (1.38 × 10⁻¹⁶ erg/K), the Curie constant, the molecular weight of (Zn_1–x Co _x )Ga₂O₄ (268.48, 268.16, and 267.52 a.u. for the amount of x = 0.05, 0.10, and 0.20, respectively), Avogadro number (6.02 × 10²³), and Co ratio in lattice, respectively.

The increase of “χ₀” by the Co amount in the lattice proved the enhancement of the temperature-independent component of susceptibility, in addition to the diamagnetic component. ^{Reference Kobayashi, Ishida, Hwang, Mizokawa, Fujimori, Mamiya, Okamoto, Takeda, Okane, Saitoh, Muramatsu, Tanaka, Saeki, Tabata and Kawai55,Reference Tovar, Causa, Butera, Navarro, Martinez, Fontcuberta and Passeggi56} The increase of Pauli susceptibility in 20% Co-doped particles assigns the enhancement of antiferromagnetic interactions. The calculated “µ_eff” values, shown in Table I, are higher than the value for spin only μ (Bohr magnetron value) = 3.87 μ_B of Co⁺² ions ^{Reference Lawes, Risbud, Ramirez and Seshadri6,Reference Dutta, Seehra, Thota and Kumar32} ; however, the “µ_eff” values are in accordance with the mentioned values in the literature for Co⁺² ions. ^{Reference Lawes, Risbud, Ramirez and Seshadri6} The high µ_eff values per Co atoms indicated the effectiveness of both ferromagnetic and antiferromagnetic coupling upon magnetization. The competitions between ferromagnetic and antiferromagnetic interactions lead to the decrease of µ_eff values by increasing the Co contents in the lattice. Furthermore, the Co⁺³ ions on the octahedral site were the other reason of low µ_eff values at high Co contents. The Co³⁺ ions in octahedral sites had no magnetic moments because of completely occupied t _2g levels ^{Reference Rack, Peterson, Potter and Park25} and, consequently, these replacements caused the lowering of µ_eff values. In addition, the presence of Co²⁺ (tetrahedral site) and Co³⁺ (octahedral site) ions also caused a change in the magnetic properties due to the nonstoichiometry of oxygen; this finding requires further analyses.