Magnetic properties of Li and Fe co-doped NiO

Solid State Communications 149 (2009) 297–300

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Magnetic properties of Li and Fe co-doped NiO S. Manna, S.K. De ∗ Department of Materials Science, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700 032, India

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Article history: Received 25 July 2008 Received in revised form 10 November 2008 Accepted 29 November 2008 by E.V. Sampathkumaran Available online 6 December 2008

a b s t r a c t The substitution of Fe within a NiO lattice produces significant ferromagnetic properties with a coercive field of 614 Oe with a high Curie temperature. Simultaneous doping of Fe and Li reduces the magnetic transition temperature significantly. A large coercive field of 1716 Oe at 5 K is observed in Li doped systems. Random substitution of non-magnetic Li induces a spin glass phase in diluted NiO. Electrical conductivity increases with doping of Fe and Li in NiO. © 2008 Elsevier Ltd. All rights reserved.

PACS: 72.15 75.30 Keywords: A. Magnetic ordered materials A. Spin glasses B. Chemical synthesis D. Exchange and super exchange

1. Introduction Among transition metal monoxides (TMO, TM = Mn, Fe, Co, Ni), Nickel monoxide (NiO) is a highly interesting strongly correlated electron system [1–3]. Bulk NiO crystallizes in a cubic NaCl structure and reveals antiferromagnetic behavior below the Neel temperature, TN = 523 K. Magnetism in NiO appears due to superexchange interaction between Ni ions. Different kinds of magnetic phase mainly depend on sign and the relative strength of the nearest neighbor and next nearest neighbor exchange interactions [4]. The doping of aliovalent metal ions into a NiO lattice modifies electronic and magnetic properties. Dilute magnetic systems have attracted much attention because of complex magnetic spin order [5]. Magnetic properties of randomly diluted NiO and CoO by divalent alkaline Mg have been studied in details [6,7]. Experimentally it was found that the crystal structure of both parent compounds does not change and also the lattice parameter does not change considerably upon dilution by Mg. The Neel temperature decreases in solid solutions of Nix Mg1−x O and Cox Mg1−x O, 0 < x > 1 with increase of concentration of the non-magnetic element Mg. Magnetization data suggests spin glass like behavior below the Neel temperature. Electrical and optical properties of NiO are significantly tuned by doping with monovalent Li ion. Electrical con-

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ductivity and optical transparency can be controlled by incorporating Li into a NiO lattice [8,9]. Zhao et al. [10] showed that the current density is enhanced due to an increase of hole concentration and optical absorption spectra are red shifted due to defects introduced by Li. A very high dielectric constant is observed in Li and Ti co-doped NiO and remains independent of frequency for a wide range of frequency [11,12]. Li doped NiO can be used as thermoelectric hydrogen gas sensor [13]. Thus Li doped NiO exhibits a rich variety of intriguing physical properties. The antiferromagnetic ground state of NiO can be tuned by replacing Ni by other 3d transition metal ions. It has been found that Fe doped NiO nanoparticles reveal room temperature ferromagnetism [14]. Ferromagnetic properties of Fe and Li doped NiO thin films grown by pulsed laser deposition and sol–gel spin coating methods have been studied [15,16]. Size reduction in doped NiO nanoparticles and thin films plays an important role in the ferromagnetic phase due to an uncompensated spin sublattice. The main objective of the present work is to study magnetic properties of Li and Fe doped NiO bulk samples to rule out a finite size effect. We have investigated dc magnetization as a function of temperature (300 K–5 K) and magnetic field (0–5 T). 2. Experimental Polycrystalline Fe and Li doped NiO samples were synthesized by a sol–gel technique. Ni(NO3 )2 , 6H2 O(A.R.), Fe(NO3 )3 , 9H2 O (A.R.), LiNO3 (A.R.) and citric acid were employed as starting raw materials. Three different ceramic samples, i.e., Fe0.02 Ni0.98 O (L-0), Li0.03 Fe0.02 Ni0.95 O (L-1), Li0.05 Fe0.02 Ni0.93 O (L-2), were

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Fig. 2. Scanning electron micrograph of sample L-1. Fig. 1. XRD patterns of various Li and Fe doped NiO at room temperature.

designed and prepared by the following procedure. Stoichiometric amounts of Ni(NO3 )2 , LiNO3 and citric acid were mixed and dissolved into distilled water. Afterwards required amount of Fe(NO3 )3 , 9H2 O solution was added slowly into the above mixed solution, followed by heating and stirring to form a semitransparent gel. The dried gel was then calcined at 800 ◦ C for 1 h in air to obtain the powders with different compositions. X-ray diffraction patterns of the samples were recorded by high resolution X’Pert PRO Panalytical X-ray diffractometer in the range 35◦ –100◦ using Cu Kα radiation. The morphology of powder samples was characterized by scanning electron microscope (SEM), JEOL JSM6700F. Magnetization measurements were carried out by Vibrating Sample Magnetometer (VSM), Cryogenic Limited, UK. 3. Results and discussion X-ray diffraction (XRD) patterns shown in Fig. 1, exhibit six (111), (200), (220), (311), (222) and (400) characteristic peaks of cubic crystalline NiO. All these peaks appear in Fe and Li doped samples at the same angular positions as that of pure NiO. This clearly establishes the absence of any impurity phase in the substitutions of Fe and Li at Ni site. The ionic radii of Ni2+ , Fe3+ and Li+ are 0.69 Å, 0.64 Å and 0.68 Å respectively. The ionic radii of all constituent elements are very close. Hence, crystalline structure of doped samples are not modified in comparison with pure NiO. The scanning electron micrographs are displayed in Fig. 2. The microstructure reveals the granular structure of polycrystalline samples. The grains are dense and arranged regularly. Secondary phase segregation is not found on micrographs. This indicates a homogeneous distribution of doped Fe and Li elements in the matrix of NiO. Temperature dependence of field cool (FC) and zero field cool (ZFC) magnetization from room temperature to low temperatures for Fe doped NiO are presented in Fig. 3. The splitting between ZFC and FC data occurs above room temperature. FC and ZFC magnetization decrease with decrease of temperature. Such a behavior of magnetizations with temperature is consistent with the antiferromagnetic behavior of NiO [17]. The temperature variation of FC and ZFC magnetizations for Li doped samples are displayed in Fig. 4. Both FC and ZFC magnetization data reveal a quite different behavior in comparison with L0 samples as shown in Fig. 3. Paramagnetic to ferromagnetic transition occurs below room temperature at about 242 K. Magnetic transition temperature and variation of FC and ZFC with temperature do not change considerably with increase in Li concentration from 3% to 5% as evidenced from Fig. 4. FC magnetization

Fig. 3. Field cool (FC) and zero field cool (ZFC) magnetization of 2% Fe doped NiO at 0.05 T.

increases continuously with decrease of temperature while ZFC remains almost constant at low temperatures. Such a characteristic behavior of FC magnetization data is attributed to ferromagnetism in Li doped material. Similar temperature dependency of FC and ZFC magnetizations were also found in Mg doped NiO and CoO and also Sr doped EuTe diluted antiferromagnets [6,7,18]. Magnetic characterizations of as prepared samples were performed by magnetic hysteresis measurements at different temperatures. The magnetization (M) as a function of applied magnetic field (H) for Fe doped NiO at room temperature and 5 K are displayed in Fig. 5. The appearance of a hysteresis loop at room temperature indicates the existence of a weak ferromagnetic phase. The hysteresis loop closes at about 4000 Oe. The magnetization at higher magnetic fields attains saturation. The saturation magnetization is 2.8 emu/g at 300 K and 3.3 emu/g at 5 K. The coercive field increases from 245 to 527 Oe with a lowering of the temperature from 300 K to 5 K. Remanence magnetization changes from 0.606 emu/gm at 300 K to 1.11 emu/g at 5 K. A distinct ferromagnetic behavior is reflected from a well defined M–H hysteresis loop for a 2% doping concentration of Fe which is consistent with earlier results reported by Lin et al. [19]. Li doped NiO samples do not show a hysteresis loop at room temperature. Magnetization varies almost linearly with the applied magnetic field as shown in Fig. 6(a). Fig. 6(b) reveals magnetization vs. magnetic field for Li doped samples at 5 K. A small shaped hysteresis loop is found below the magnetic transition temperature. Magnetization increases continuously with the applied magnetic field in contrast to the saturation

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Fig. 4. Field cool (FC) and zero field cool (ZFC) magnetization vs. temperature of sample L-1 (a) and L-2 (b) at 0.05 T.

Fig. 5. Magnetization (M) versus magnetic field (H) of 2% Fe doped NiO (sample L-0) at 5 K (a) and 300 K (b).

behavior of samples without Li. The highest coercive filed for 5% Li doped is 1716 Oe which is almost three times higher than that of L0. Remanence magnetization at 5 K is 0.286 emu/g. The AFM structure of NiO consists of two spin sublattices. Each sublattice exhibits ferromagnetic order along the (111) direction. The moments of adjacent sublattice are aligned in the opposite direction to generate an AFM structure. Transition metal Fe ion has two stable valence states Fe2+ and Fe3+ . Very recently Douvalis et al. [20] from Mossbauer studies proposed the existence of Fe3+ ions in a NiO matrix. Trivalent state of Fe introduces Ni vacancy in a NiO lattice. As a result of it, the AFM spin order of pure NiO is partially destroyed and favors a ferromagnetic spin order. Moreover, ferromagnetism may originate from the mixed valence state of Fe due to double exchange interaction [21]. Substitution of Ni in Fe doped NiO by monovalent Li induces Ni3+ and Fe3+ states for charge neutrality. X-ray photoelectron emission and K-edge X-ray absorption measurements predicted that the charge compensating hole has oxygen character [22, 23]. The correlated spin dynamics of Ni2+ in paramagnetic and antiferromagnetic phases suggest that the Neel temperature decreases with an increase in Li concentration [24]. The present observation of a lowering of the Neel temperature is consistent with spin dynamics results. Large coercivity in Li doped samples may arise from disorder and defects. Random substitution of Li and Fe produces cationic disorder in a NiO lattice. Higher Hc clearly suggests that the

substitution of Ni by Li increases crystalline anisotropy. Exchange bias originating from the coexistence of ferromagnetic and antiferromagnetic regions induces anisotropy [25] which may play an important role in ferromagnetic order. In general transition metal oxides, smaller coercivity and larger Neel temperature are found. Replacement of Ni by Li reduces TN from 534 K (pure NiO) to 243 K for Li doped samples. Temperature dependence of ZFC in Li doped systems is a characteristic of a spin glass system [6,7,18]. Ferromagnetic exchange interaction between mixed valence states through intervening oxygen such as Ni3+ –O–Ni2+ and Fe3+ –O–Fe2+ exists. Exchange interaction between like ions such as Ni2+ –O–Ni2+ produces antiferromagnetic coupling. Randomness of Li and Fe and the competition between ferromagnetic and antiferromagnetic interaction are responsible for a spin glass state. Pure NiO with NaCl structure is an antiferromagnetic Hubbard–Mott insulator possessing very low electrical conductivity (<10−13 S cm−1 ) at room temperature. Electrical conductivity of a Fe doped sample is 7.21 × 10−7 S cm−1 much higher than undoped NiO. The creation of Ni vacancies by Fe enhances electrical conductivity. Substitution of monovalent Li causes an increase of electrical conductivity, 3.02 × 10−4 S cm−1 and 6.63 × 10−4 S cm−1 for L-1 and L-2 samples. Replacement of Ni2+ by Li+ increases charge carrier concentration due to the appearance of Ni3+ state for charge balance [26,27]. Thus Li doped samples exhibit higher electrical conductivity. Magnetic transition temperature decreases with Li substitution.

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room temperature for Fe doped samples. The incorporation of Li into Fe doped NiO modifies the magnetic transition temperature significantly. Cationic disorder and the coexistence of ferromagnetic and antiferromagnetic phases may lead to spin glass type behavior in diluted NiO system. References

Fig. 6. Magnetization (M) versus magnetic field (H) of sample L-2 at 5 K (a) and 300 K (b).

4. Conclusion The appearance of magnetic hysteresis at room temperature suggests that the ferromagnetic Curie temperature is above

[1] S. Hufner, Adv. Phys. 43 (1994) 183. [2] S.L. Dudarev, G.A. Botton, S.Y. Savrasov, C.J. Humphreys, A.P. Sutton, Phys. Rev. B 57 (1998) 1505. [3] A.B. Shick, A.I. Leichtenstein, W.E. Pickett, Phys. Rev. B 99 (1988) 10763. [4] M.S. Seehra, Phys. Rev. B 38 (1988) 11898. [5] M. Auslous, R.J. Elliot (Eds.), Magnetic PhaseTransitions, Springer-Verlag, Berlin, 1983. [6] Z. Feng, M.S. Seehra, Phys. Rev. B 45 (1992) 2184. [7] R. Kanan, M.S. Seehra, Phys. Rev. B 35 (1987) 6847. [8] A. Matsuda, S. Akiba, M. Kasahara, T. Watanabe, Y. Akita, M. Yoshimoto, Appl. Phys. Letts. 90 (2007) 182107. [9] U.S. Joshi, Y. Matsumoto, K. Itaka, M. Sumiya, H. Koinuma, Appl. Surf. Sci. 252 (2006) 2524. [10] R. Zhao, Y.H. Lin, X. Zhou, M. Li, C.W. Wan, J. Appl. Phys. 100 (2006) 046102. [11] J. Wu, C.W. Nan, Y. Lin, Y. Deng, Phy. Rev. Letts. 89 (2002) 217601. [12] J. Kim, Y. Lee, M.G. Kim, A. Souchkov, J.S. Lee, H.D. Drew, S.J. Oh, C.W. Nan, E.J. Choi, Phys. Rev. B 70 (2004) 172106. [13] M. Matsumiya, F. Qiu, W. Shim, N. Izu, N. Murayama, S. Kanzaki, Thin Solid Films 419 (2002) 213. [14] J. wang, J. Cai, Y.H. Lin, C.W. Nan, Appl. Phys. Letts. 87 (2005) 202501. [15] W. Yan, W. Weng, G. Zhang, Z. Sun, Q. Liu, Z. Pan, Y. Guo, P. Xu, S. Wei, Y. Zhang, S. Yan, Appl. Phys. Letts. 92 (2008) 052508. [16] Y.H. Lin, R. Zhao, C.W. Nan, M. Ying, M. Kobayashi, Y. Oki, A. Fujomri, Appl. Phys. Letts. 89 (2006) 202501. [17] G. Srinivasan, M.S. Seehra, Phys. Rev. B 29 (1984) 6295. [18] F.J. Borgermann, H. Maletta, W. Zinn, Phys. Rev. B 35 (1987) 8454. [19] Y. Li, J. Wang, J. Cai, M. Ying, R. Zhao, M. Li, C.W. Nan, Phys. Rev. B 73 (2006) 193308. [20] A.P. Douvalis, L. Jankovic, T. Bakas, J. Phys: Condens. Matter. 19 (2007) 436203. [21] C. Zener, Phys. Rev. 82 (1951) 403. [22] J. Elp, H. Eskes, P. Kuiper, G.A. Sawatzky, Phys. Rev. B 45 (1992) 1612. [23] P. Kuiper, G. Kruizinga, J. Ghijsen, G.A. Sawatzky, Phys. Rev. Letts. 62 (1989) 221. [24] M. Corti, S. Marini, A. Rigamonti, F. Tedoldi, Phys. Rev. B 56 (1997) 11056. [25] A.E. Berkowitz, K. Takano, J. Magn. Magn. Mater. 200 (1999) 552. [26] D. Adler, J. Feinleib, Phys. Rev. B 2 (1970) 3112. [27] E. Antolini, Mater. Chem. Phys. 82 (2003) 937.