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Structure and magnetic properties of Mn doped α-Fe2O3
Physica B: Condensed Matter 574 (2019) 411663
Contents lists available at ScienceDirect
Physica B: Condensed Matter journal homepage: www.elsevier.com/locate/physb
Structure and magnetic properties of Mn doped α-Fe2O3 a,∗
b
b
b
a
T b
c
R. Nikam , S. Rayaprol , S. Mukherjee , S.D. Kaushik , P.S. Goyal , P.D. Babu , S. Radha , V. Sirugurib a b c
Department of Physics, Pillai College of Engineering, New Panvel, Navi Mumbai 410206, India UGC- DAE Consortium for Scientific Research, Mumbai Centre, B. A. R. C Campus, Trombay, Mumbai 400085, India Department of Physics, University of Mumbai, Vidyanagari, Mumbai 400098, India
A R T I C LE I N FO
A B S T R A C T
Keywords: Magnetic materials Iron oxide Structure Neutron diffraction
Magnetic transition metal oxides are interesting candidates for both academic as well as industrial researches. In the present work, we study the influence of partial Mn doping on structural and magnetic and dielectric properties of α-Fe2O3 using x-ray and neutron diffraction, magnetization and dielectric measurements. In particular, this paper deals structural, magnetic and dielectric properties of Fe1.95Mn0.05O3. The compound is found to crystallize in corundum type (α-Al2O3) having rhombohedral lattice structure with hexagonal setting in the space group (R-3c). It is seen that partial substitution of Fe by Mn in α-Fe2O3 (known as hematite) drastically alters the Morin transition in the hematite compound by reducing the ordering temperature from a value of 260 K to about 220 K. Information about ferrimagnetic structure of Fe1.95Mn0.05O3 has been obtained from the magnetic part of the neutron diffraction analysis. The results of room temperature neutron diffraction, temperature dependent magnetization and dielectric studies are reported and discussed in the present work in order to understand the correlation between structure and magnetism in this compound and ways in which magnetism and other physical properties can be controlled.
1. Introduction Physics of transition metal oxides has always been exciting for understanding the basic magnetism and their potential use in diverse applications [1,2]. Among the magnetic transition metal oxides, the iron oxide based systems, are among widely studied compounds in recent literature owing to their fascinating structural and magnetic properties and their potential for technological applications such as pigments, catalysts, sensors, environmental pollutant clean up agents, electrode materials, biomedical materials, etc. [3–8]. In particular, there is renewed interest in hematite (α-Fe2O3) in recent years [9–13]. This compound has very high resistance to corrosion and is found to be very stable and is very economical for commercial application purposes also as it is widely available in nature. Its properties such as biocompatibility and non-toxicity make it even more attractive. The magnetic and other physical property studies have been widely reported in the literature [9–15]. As per the reports, hematite α-Fe2O3 is known to undergo magnetic transition from paramagnetic to ferrimagnetic/canted antiferromagnetic around the Néel temperature (TN) 960 K. There is another magnetic transition called the Morin transition (TM) exhibited by bulk α-Fe2O3 around 260 K. In addition to above two
∗
transitions, temperature dependence of magnetization of nanoparticles of α-Fe2O3 exhibit a ‘blocking temperature’ TB (~50 K). It is known that all the three ordering temperatures, TN, TM and TB for nanoparticles depend on the size of the nanoparticles [11–13]. It may be mentioned that, in general, magnetic properties of α-Fe2O3 depend on the synthesis conditions, particularly in the preparation of nanoparticles [13,16–19]. In the present work, we have prepared samples using the mechanochemical synthesis method, which is also one of the ways of producing single phase compounds in bulk and nanocrystalline form [20–24] in relatively less amount of time and effort. 2. Experimental details Polycrystalline Fe1.95Mn0.05O3 has been prepared by mechanochemical synthesis in a high energy planetary ball mill using similar approach as reported earlier [23,24]. Stoichiometric quantities of Fe2O3 (99.9% pure) and MnO2 (99.9% pure) were taken in a tungsten carbide (WC) jar along with 10 mm WC balls. The powder to ball ratio was kept at 1 g: 10 (10 mm dia) balls. The mixture was ball milled at a speed of 400 rpm for a total time of about 18 h. The final product was sintered at
Corresponding author. Pillai College of Engineering, New Panvel, Navi Mumbai 410206, India. E-mail addresses: [email protected], [email protected] (R. Nikam).
https://doi.org/10.1016/j.physb.2019.411663 Received 4 January 2019; Received in revised form 15 July 2019; Accepted 24 August 2019 Available online 26 August 2019 0921-4526/ © 2019 Elsevier B.V. All rights reserved.
Physica B: Condensed Matter 574 (2019) 411663
R. Nikam, et al.
Fig. 1. Rietveld refinement of the X-ray diffraction data taken at room temperature. The Miller indices of the Bragg peaks are also shown.
825 °C for 72 h to homogenize the powder. The resulting powder was cold pressed into a 10 mm diameter pellet and sintered again at 825 °C for 24 h. Room temperature X-ray powder diffraction (XRD) pattern was recorded using Bruker D8 Advanced X-ray diffractometer. The room temperature neutron diffraction measurement was carried out on diffractometer (PD-3) at Dhruva reactor, Trombay [25]. The magnetization measurements were carried out on a Quantum Design PPMS based Vibrating Sample Magnetometer (9T-QD-PPMS-VSM) in the temperature range 5 K–350 K. Temperature dependent dielectric measurements were done in a frequency range of 20 Hz–2 MHz using Keysight LCR meter (E4980A) coupled to a closed cycle refrigerator (CCR).
Fig. 2. Crystal structure of the Mn doped α-Fe2O3 from the refinement of the room temperature neutron diffraction data. The Fe/Mn is at the centre of the polygons surrounded by oxygen atoms.
3. Results and discussions In Fig. 1, RT-XRD pattern of Fe1.95 Mn0.05O3 is shown along with Rietveld refinement profile and the difference curve. The Miller indices for the peaks along with refined cell parameters are also shown and this confirms that the compound has corundum type (α-Al2O3) structure in the space group R-3c in hexagonal setting. There is slight decrease in the unit cell volume of Fe1.95Mn0.05O3 compared to the undoped αFe2O3 hematite compound [18]. No peaks from other phases are found, suggesting that single phase Fe1.95Mn0.05O3 compound is formed [22–24]. The structure of Fe1.95Mn0.05O3 (Fig. 2) is similar to pure αFe2O3 structure [18]. That is, Fe and Mn ions in Fe1.95Mn0.05O3 share the Wyckoff site 12c whereas oxygen resides on the Wyckoff site 18e of the R-3c space group. Room temperature neutron diffraction pattern has been analyzed by the Rietveld refinement method using the FullProf program suite [26,27]. The raw data along with the calculated profile of Fe1.95Mn0.05O3 as measured at room temperature are shown in Fig. 3. It is to be noted that the first vertical ticks (green lines) indicate the nuclear Bragg peak positions and the second row of the vertical ticks corresponds to the magnetic Bragg peak positions. The structural part of the diffraction pattern was refined using the structural model that was used for refining the XRD data. To determine the magnetic structure, the propagation vector, k = (0 0 0), was used to describe the orientation of the magnetic spins with respect to the crystallographic unit cell. The irreducible representations for the space group R-3c were
Fig. 3. Rietveld refinement of the neutron diffraction data taken at room temperature. The data was refined for both nuclear (crystallographic) and magnetic structures. The first row of vertical tick marks indicates the Bragg peaks corresponding to the nuclear structure whereas the second row of the vertical ticks indicates magnetic Bragg peaks. The Rietveld profile fitting parameters are: Bragg R-factor = 6.3, Rf-Factor = 4.44, Rp = 7.4, Rwp = 9.4, Rexp = 3.60, χ2 = 6.8, Magnetic R-factor = 3.7.
2
Physica B: Condensed Matter 574 (2019) 411663
R. Nikam, et al.
Fig. 6. Magnetic anisotropy (defined as the difference between the magnetic susceptibility in FC and ZFC states) is shown here as a function of temperature for magnetic susceptibilities measured in H = 100 Oe. Fig. 4. Magnetic structure of the Mn doped α-Fe2O3 from the refinement of the room temperature neutron diffraction data. The arrows indicate the spins are lying on the ab-basal plane and are stacked along the c-axis. The refinement shows that the sample is in ferrimagnetic state at room temperature.
determined using the propagation vector k = (0 0 0) and the program BasIreps, available in the FullProf suite [27,28]. This analysis shows that the magnetic structure of Fe1.95Mn0.05O3 is similar to that of αFe2O3 at room temperature. The magnetic moment obtained from the refinement is 3.36μB per formula unit. However, this value is less than that reported in the paper by Hill et al., [29]. This decrease in the value can be due to partial substitution of Mn for Fe, and secondly, due to the synthesis method used. There is finite possibility of formation of some volume fraction of nanoparticles of Mn doped α-Fe2O3 leading to the overall decrease in magnetism of the compound. The magnetic structure of Fe1.95Mn0.05O3, comprising spins of Fe3+ is shown in Fig. 4. The magnetic structure of Fe1.95Mn0.05O3, unlike α-Fe2O3, comprises two antiferromagnetically coupled unequalferromagnetic sub-lattices [29,30]. This is clearly seen by the two sets of magnetic moments (short and long arrows, pointed in opposite directions, Fig. 4). The moments lie on the ab or basal plane of the hexagonal unit cell. The antiferromagnetic coupling of two unequal magnetic sub-lattices gives rise to the overall ferrimagnetism in this Mn-substituted compound at room temperature. Magnetic susceptibility (χ = M/H) was measured as a function of temperature in zero field cooled (ZFC) and field cooled (FC) states of the sample. In ZFC case, the sample was cooled to the lowest temperature without applying any external magnetic field, whereas in the case of FC measurements, sample was cooled in the external field. In both ZFC and FC measurements, susceptibility was measured while warming after applying the external magnetic fields (100 Oe and 5 kOe, respectively). χ(T) corresponding to two applied fields, namely, low field of 100 Oe, and a relatively higher field of 5 kOe, is plotted in Fig. 5. Both measurements show that sample is in high magnetization state and as the sample is cooled value of the susceptibility decreases monotonically with decreasing temperature up to 220 K and then falls suddenly. The peak around 220 K, by analogy to α-Fe2O3, can be taken as the Morin transition (TM) for Fe1.95Mn0.05O3 [11–13]. The χ(T) in ZFC case remains flat, as if independent of temperature, till T~60 K. Below 60 K, there is another anomaly, observed more clearly in the 5 kOe measurement (bottom panel of Fig. 5), where χ(T) goes through a small peak around 50 K. The anomaly around 50 K is in close vicinity of the observation of blocking temperature, TB, in nanoparticles of αFe2O3 [11–13]. Since the sample studied here has been prepared by mechanochemical method, it is quite possible that some agglomerated nano-particles would form and get dispersed within the bulk sample. The presence of these agglomerated nano-particles might be responsible
Fig. 5. Magnetic susceptibility of the Mn doped α-Fe2O3 measured as a function of temperature under two different applied fields, (top) 100 Oe and (below) 5 kOe. The sample was measured in the zero field cooled (ZFC) and field cooled (FC) states of the sample.
3
Physica B: Condensed Matter 574 (2019) 411663
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Fig. 7. Magnetization (M) measured at various temperatures is plotted as a function of ramping field (H). (a) Overall, M vs. H profiles at all temperatures does not saturate till the highest field of measurement, indicating ferrimagnetic state of the sample. (b) A small hysteresis loop is observed around the origin for fields less than 10 kOe at all temperatures. Inset in figure (b) depicts the coercive field observed as a function of temperature.
noted that the magnetic anisotropy increases sharply below 100 K. Magnetization (M) as a function of ramping field was measured at several temperatures across TB and TM and the results are shown in Fig. 7. For temperatures below TB, M varies non-linearly for initial fields, between ± (0–5) kOe, and then varies linearly with the field in the ± (5–90) kOe) range. It is interesting to observe that for all temperatures, M(H) shows small hysteresis loop around origin. None of the M(H) curves show any signs of saturation till the highest measured field, indicating the ferrimagnetic nature of the sample at room temperature. In the inset of Fig. 7 (b), the coercive field (HC) observed from the M(H) loops is plotted as a function of temperature. HC decreases
for the system showing the blocking temperature. The FC curves for both field (i.e., H = 100 Oe and 5 kOe) measurements show identical features till 220 K, and below this temperature, χ(T) does not level off as in the case of ZFC, but goes through minima around 140 K, below which there is a rapid increase in χFC(T), which results in separation between ZFC and FC curves. To highlight this aspect, the difference between ZFC and FC measurements in case of χ(T) measured in a field of 100 Oe, which is a measure of the magnetic anisotropy, is plotted in Fig. 6. The plot shows that there are significant differences between ZFC and FC and the magnetic anisotropy exhibits a clear peak around 220 K. Further, it is 4
Physica B: Condensed Matter 574 (2019) 411663
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observed around 50 K. Magnetization measurements along with magnetic structure derived from the neutron diffraction analysis shows that the compound is ferrimagnetic at room temperature with signatures of weak ferromagnetism or canted antiferromagnetism at temperatures below TMand TB. Dielectric measurements show two anomalies at temperatures corresponding to TMand TB. It will therefore be interesting and rewarding to study the origin of magnetoelectric coupling in Fe1.95Mn0.05O3. Detailed work in this direction is currently under progress. Further systematic studies on other physical properties of this compound and extension of this work to other concentrations of Fe and Mn in the stoichiometric form, Fe2-xMnxO3 are currently in progress. Conflict of interest form Authors declare no conflict of interest. Acknowledgments Authors would like to thank M. Venugopal, UGC-DAE-CSR (Mumbai Centre) for his assistance during sample preparation and data collection. We acknowledge Mukul Gupta (UGC-DAE-CSR, Indore Centre) and S. Bhattacharya (TPD, BARC) for providing XRD facilities. References
Fig. 8. Capacitance as a function of temperature, measured at several frequencies is shown here for Fe1.95Mn0.05O3. The inset shows the conductance as a function of temperature measured in a frequency of 1 MHz.
[1] C.N.R. Rao, Transition metal oxides, Annu. Rev. Phys. Chem. 40 (1989) 291–326. [2] Polona Umek, Andrej Zorko, Denis Arčon, Magnetic properties of transition-metal oxides: from bulk to nano, in: Ralf Riedel, I-Wei Chen (Eds.), Ceramics Science and Technology: Volume 2: Materials and Properties - 1, 2010 (Chapter 19). [3] V. Panchal, U. Bhandarkar, M. Neergat, K.G. Suresh, Controlling magnetic properties of iron oxide nanoparticles using post-synthesis thermal treatment, Appl. Phys. A 114 (2014) 537–544. [4] S.I. Srikrishna Ramya, C.K. Mahadevan, Preparation and structural, optical, magnetic, and electrical characterization of Mn2+/Co2+/Cu2+doped hematite nanocrystals, J. Solid State Chem. 211 (2014) 37–50. [5] H. Liang, X. Xu, W. Chen, B. Xu, Z. Wang, Facile synthesis of hematite nanostructures with controlled hollowness and porosity and their comparative photocatalytic activities, CrystEngComm 16 (2014) 959–963. [6] S. Sivakumar, D. Anusuya, C.P. Khatiwada, J. Sivasubramanian, A. Venkatesan, P. Soundhirarajan, Characterizations of diverse mole of pure and Ni-doped α-Fe2O3 synthesized nanoparticles through chemical precipitation route, Spectrochim. Acta A Mol. Biomol. Spectrosc. 128 (2014) 69–75. [7] F. Sánchez-DeJesús, A.M. Bolarín-Miró, C.A. Cortés-Escobedo, R. Valenzuela, S. Ammar, Mechanosynthesis, crystal structure and magnetic characterization of Mtype SrFe12O19, Ceram. Int. l40 (2014) 4033–4038. [8] A. Zelenakova, V. Zelenak, S. Michalik, J. Kovac, M.W. Meisel, Structural and magnetic properties of CoO-Pt core-shell nanoparticles, Phys. Rev. B 89 (2014) 104417. [9] N.M. Deraz, A. Alarifi, Novel processing and magnetic properties of hematite/maghemite nano-particles, Ceram. Int. 38 (2012) 4049–4055. [10] T. Adinaveen, J.J. Vijaya, L.J. Kennedy, Studies on the structural, morphological, optical, and magnetic properties of α-Fe2O3 nanostructures by a simple one-step low temperature reflux condensing method, J. Supercond. Nov. Magnetism 27 (2014) 1721–1727. [11] H.M. Lu, X.K. Meng, Morin temperature and Néel temperature of hematite nanocrystals, J. Phys. Chem. C 114 (2010) 21291–21295. [12] Marin Tadic, Matjaz Panjan, Vesna Damnjanovic, Irena Milosevic, Magnetic properties of hematite (α-Fe2O3) nanoparticles prepared by hydrothermal synthesis method, Appl. Surf. Sci. 320 (2014) 183–187. [13] A.S. Teja, P.Y. Koh, Synthesis, properties, and applications of magnetic iron oxide nanoparticles, Prog. Cryst. Growth Charact. Mater. 55 (2009) 22–45. [14] V. Baron, J. Gutzmer, H. Rundolöf, R. Tellgren, Neutron powder diffraction study of Mn-bearing hematite, α-Fe2-xMnxO3, in the range 0 ≤ x ≤ 0.176, Solid State Sci. 7 (2005) 753–759. [15] Dinesh Varshney, Arvind Yogi, Structural and electrical conductivity of Mn doped hematite (α-Fe2O3) phase, J. Mol. Struct. 995 (2011) 157–162. [16] B. Balaraju, M. Kuppan, S. Harinath Babu, S. Kaleemulla, N. Madhusudhana Rao, C. Krishnamoorthi, Girish M. Joshi, G. Venugopal Rao, K. Subbaravamma, I. Omkaram, D. Sreekantha Reddy, Structural, optical and magnetic properties of αFe2O3 nanoparticles, Mech., Mater. Sci. Eng. J. (2017) 9. [17] M. Mohamed Rafi, K. Syed Zameer Ahmed, K. Prem Nazeer, D. Siva Kumar, M. Thamilselvan, Synthesis, characterization and magnetic properties of hematite (α-Fe2O3) nanoparticles on polysaccharide templates and their antibacterial activity, Appl. Nanosci. 5 (2015) 515–520. [18] Jianmin Ma, Jiabiao Lian, Xiaochuan Duan, Xiaodi Liu, Wenjun Zheng, α-Fe2O3: hydrothermal synthesis, magnetic and electrochemical properties, J. Phys. Chem. C 114 (2010) 10671–10676. [19] S. Chakrabarty, T.K. Jana, K. De, S. Das, K. Dey, K. Chatterjee, Morphology
from 5 K to 100 K and then increases for 220 K before coming down again at 350 K. The behavior of HC tends to mimic the anisotropy behavior shown in Fig. 6. This behavior indicates that around 220 K, there is a change in the magnetic structure in this compound as well [29]. At T ≤ TB, for low fields, the non-linear variation in magnetization may be connected with canted antiferromagnetic or weak ferromagnetic state of the sample, as observed in the case of undoped α-Fe2O3 [29,30]. As discussed in Introduction, there is tremendous interest in iron oxide-based compounds for practical applications owing to their possible applications in spintronic devices, etc. In this connection, we explored the possibility of Fe1.95Mn0.05O3 for magnetoelectric applications by measuring its dielectric response as a function of temperature. In Fig. 8, the capacitance (C) of Fe1.95Mn0.05O3 measured at various frequencies is plotted as a function of temperature. It is interesting that the plot of capacitance as a function of temperature, (C vs. T), shows a sharp peak between 220 K and 250 K. This is very much similar to the observation of the Morin-like transition around this temperature in the plot of χ(T). Another interesting observation made from the dielectric measurements was that, in the conductance plot (inset of Fig. 8), a small anomaly was observed around 50 K, which is in good agreement with the anomaly observed in magnetic susceptibility measurement which has been attributed to blocking temperature TB. These interesting observations are encouraging enough to explore the possibility of magneto-electric coupling in this compound. A detailed study involving temperature dependent neutron diffraction, magnetoelectric measurements and other complementary studies are currently underway for better understanding of possible multiferroic properties of Fe1.95Mn0.05O3.
4. Conclusions The preliminary studies on structural, magnetic and dielectric properties of Fe1.95Mn0.05O3 have been carried out using X-ray and neutron diffraction, magnetization and dielectric measurements. The compound Fe1.95Mn0.05O3crystallizes in corundum type (α-Al2O3) structure in the space group R-3c in hexagonal setting. The partial doping of Mn for Fe reduces the Morin transition (TM) from 260 K to 220 K, but does not affect the blocking temperature, TB, which is 5
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[20] [21]
[22] [23] [24]
[25]
dependent magnetic properties of α-Fe2O3 nanostructures, Mater. Res. Exp. 1 (2014) 046104. T. Tsuzuki, F. Schäffel, M. Muroi, P.G. McCormick, α-Fe2O3 nano-platelets prepared by mechanochemical/thermal processing, Powder Technol. 210 (2011) 198–202. J. André-Filho, L. León-Félix, J.A.H. Coaquira, V.K. Garg, A.C. Oliveira, Size dependence of the magnetic and hyperfine properties of nanostructured hematite (αFe2O3) powders prepared by the ball milling technique, Hyperfine Interact. 224 (2014) 189–196. S. Rayaprol, S.D. Kaushik, Magnetic and magnetocaloric properties of FeMnO3, Ceram. Int. 41 (2015) 9567–9571. S. Rayaprol, S.D. Kaushik, P.D. Babu, V. Siruguri, Structure and magnetism of FeMnO3, AIP Conf. Proc. 1512 (2013) 1132. R. Nikam, S. Rayaprol, P.S. Goyal, P.D. Babu, S. Radha, V. Siruguri, Structural and magnetic properties of Fe-doped Mn2O3 orthorhombic bixbyite, J. Supercond. Nov. Magnetism 31 (2018) 2179–2185. V. Siruguri, P.D. Babu, M. Gupta, A.V. Pimpale, P.S. Goyal, A high resolution
powder diffractometer using focusing optics, Pramana - J. Phys. 71 (2008) 1197. [26] H.M. Rietveld, A profile refinement method for nuclear and magnetic structures, J. Appl. Crystallogr. 2 (1969) 65. [27] T. Roisnel, J. Rodriguez-Carvajal, WINPLOTR, Laboratoire Leon Brillouin(CEA296CNR), Centred Etudes de Saclay,91191 Gifsur Yvette, Cedes(France) and 297 Laboratoire de Chimie du Solideet Inorganique Moleculaire (UMR6511), 298 Universite de Rennes 1, 35042 Rennex Cedex(France). [28] C. Ritter, Neutrons not entitled to retire at the age of 60: more than ever needed to reveal magnetic structures, Solid State Phenom. 170 (2011) 263–269. [29] A.H. Hill, F. Jiao, P.G. Bruce, A. Harrison, W. Kockelmann, C. Ritter, Chem. Mater. 20 (2008) 4891–4899. [30] Adrian H. Hill, Henrik Jacobsen, J. Ross Stewart, Feng Jiao, Niels P. Jensen, Sonja L. Holm, Hannu Mutka, Seydel Tilo, Andrew Harrison, Lefmann Kim, Magnetic properties of nano-scale hematite, α−Fe2O3, studied by time-of-flight inelastic neutron spectroscopy, J. Chem. Phys. 140 (2014) 044709.
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Contents lists available at ScienceDirect
Physica B: Condensed Matter journal homepage: www.elsevier.com/locate/physb
Structure and magnetic properties of Mn doped α-Fe2O3 a,∗
b
b
b
a
T b
c
R. Nikam , S. Rayaprol , S. Mukherjee , S.D. Kaushik , P.S. Goyal , P.D. Babu , S. Radha , V. Sirugurib a b c
Department of Physics, Pillai College of Engineering, New Panvel, Navi Mumbai 410206, India UGC- DAE Consortium for Scientific Research, Mumbai Centre, B. A. R. C Campus, Trombay, Mumbai 400085, India Department of Physics, University of Mumbai, Vidyanagari, Mumbai 400098, India
A R T I C LE I N FO
A B S T R A C T
Keywords: Magnetic materials Iron oxide Structure Neutron diffraction
Magnetic transition metal oxides are interesting candidates for both academic as well as industrial researches. In the present work, we study the influence of partial Mn doping on structural and magnetic and dielectric properties of α-Fe2O3 using x-ray and neutron diffraction, magnetization and dielectric measurements. In particular, this paper deals structural, magnetic and dielectric properties of Fe1.95Mn0.05O3. The compound is found to crystallize in corundum type (α-Al2O3) having rhombohedral lattice structure with hexagonal setting in the space group (R-3c). It is seen that partial substitution of Fe by Mn in α-Fe2O3 (known as hematite) drastically alters the Morin transition in the hematite compound by reducing the ordering temperature from a value of 260 K to about 220 K. Information about ferrimagnetic structure of Fe1.95Mn0.05O3 has been obtained from the magnetic part of the neutron diffraction analysis. The results of room temperature neutron diffraction, temperature dependent magnetization and dielectric studies are reported and discussed in the present work in order to understand the correlation between structure and magnetism in this compound and ways in which magnetism and other physical properties can be controlled.
1. Introduction Physics of transition metal oxides has always been exciting for understanding the basic magnetism and their potential use in diverse applications [1,2]. Among the magnetic transition metal oxides, the iron oxide based systems, are among widely studied compounds in recent literature owing to their fascinating structural and magnetic properties and their potential for technological applications such as pigments, catalysts, sensors, environmental pollutant clean up agents, electrode materials, biomedical materials, etc. [3–8]. In particular, there is renewed interest in hematite (α-Fe2O3) in recent years [9–13]. This compound has very high resistance to corrosion and is found to be very stable and is very economical for commercial application purposes also as it is widely available in nature. Its properties such as biocompatibility and non-toxicity make it even more attractive. The magnetic and other physical property studies have been widely reported in the literature [9–15]. As per the reports, hematite α-Fe2O3 is known to undergo magnetic transition from paramagnetic to ferrimagnetic/canted antiferromagnetic around the Néel temperature (TN) 960 K. There is another magnetic transition called the Morin transition (TM) exhibited by bulk α-Fe2O3 around 260 K. In addition to above two
∗
transitions, temperature dependence of magnetization of nanoparticles of α-Fe2O3 exhibit a ‘blocking temperature’ TB (~50 K). It is known that all the three ordering temperatures, TN, TM and TB for nanoparticles depend on the size of the nanoparticles [11–13]. It may be mentioned that, in general, magnetic properties of α-Fe2O3 depend on the synthesis conditions, particularly in the preparation of nanoparticles [13,16–19]. In the present work, we have prepared samples using the mechanochemical synthesis method, which is also one of the ways of producing single phase compounds in bulk and nanocrystalline form [20–24] in relatively less amount of time and effort. 2. Experimental details Polycrystalline Fe1.95Mn0.05O3 has been prepared by mechanochemical synthesis in a high energy planetary ball mill using similar approach as reported earlier [23,24]. Stoichiometric quantities of Fe2O3 (99.9% pure) and MnO2 (99.9% pure) were taken in a tungsten carbide (WC) jar along with 10 mm WC balls. The powder to ball ratio was kept at 1 g: 10 (10 mm dia) balls. The mixture was ball milled at a speed of 400 rpm for a total time of about 18 h. The final product was sintered at
Corresponding author. Pillai College of Engineering, New Panvel, Navi Mumbai 410206, India. E-mail addresses: [email protected], [email protected] (R. Nikam).
https://doi.org/10.1016/j.physb.2019.411663 Received 4 January 2019; Received in revised form 15 July 2019; Accepted 24 August 2019 Available online 26 August 2019 0921-4526/ © 2019 Elsevier B.V. All rights reserved.
Physica B: Condensed Matter 574 (2019) 411663
R. Nikam, et al.
Fig. 1. Rietveld refinement of the X-ray diffraction data taken at room temperature. The Miller indices of the Bragg peaks are also shown.
825 °C for 72 h to homogenize the powder. The resulting powder was cold pressed into a 10 mm diameter pellet and sintered again at 825 °C for 24 h. Room temperature X-ray powder diffraction (XRD) pattern was recorded using Bruker D8 Advanced X-ray diffractometer. The room temperature neutron diffraction measurement was carried out on diffractometer (PD-3) at Dhruva reactor, Trombay [25]. The magnetization measurements were carried out on a Quantum Design PPMS based Vibrating Sample Magnetometer (9T-QD-PPMS-VSM) in the temperature range 5 K–350 K. Temperature dependent dielectric measurements were done in a frequency range of 20 Hz–2 MHz using Keysight LCR meter (E4980A) coupled to a closed cycle refrigerator (CCR).
Fig. 2. Crystal structure of the Mn doped α-Fe2O3 from the refinement of the room temperature neutron diffraction data. The Fe/Mn is at the centre of the polygons surrounded by oxygen atoms.
3. Results and discussions In Fig. 1, RT-XRD pattern of Fe1.95 Mn0.05O3 is shown along with Rietveld refinement profile and the difference curve. The Miller indices for the peaks along with refined cell parameters are also shown and this confirms that the compound has corundum type (α-Al2O3) structure in the space group R-3c in hexagonal setting. There is slight decrease in the unit cell volume of Fe1.95Mn0.05O3 compared to the undoped αFe2O3 hematite compound [18]. No peaks from other phases are found, suggesting that single phase Fe1.95Mn0.05O3 compound is formed [22–24]. The structure of Fe1.95Mn0.05O3 (Fig. 2) is similar to pure αFe2O3 structure [18]. That is, Fe and Mn ions in Fe1.95Mn0.05O3 share the Wyckoff site 12c whereas oxygen resides on the Wyckoff site 18e of the R-3c space group. Room temperature neutron diffraction pattern has been analyzed by the Rietveld refinement method using the FullProf program suite [26,27]. The raw data along with the calculated profile of Fe1.95Mn0.05O3 as measured at room temperature are shown in Fig. 3. It is to be noted that the first vertical ticks (green lines) indicate the nuclear Bragg peak positions and the second row of the vertical ticks corresponds to the magnetic Bragg peak positions. The structural part of the diffraction pattern was refined using the structural model that was used for refining the XRD data. To determine the magnetic structure, the propagation vector, k = (0 0 0), was used to describe the orientation of the magnetic spins with respect to the crystallographic unit cell. The irreducible representations for the space group R-3c were
Fig. 3. Rietveld refinement of the neutron diffraction data taken at room temperature. The data was refined for both nuclear (crystallographic) and magnetic structures. The first row of vertical tick marks indicates the Bragg peaks corresponding to the nuclear structure whereas the second row of the vertical ticks indicates magnetic Bragg peaks. The Rietveld profile fitting parameters are: Bragg R-factor = 6.3, Rf-Factor = 4.44, Rp = 7.4, Rwp = 9.4, Rexp = 3.60, χ2 = 6.8, Magnetic R-factor = 3.7.
2
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Fig. 6. Magnetic anisotropy (defined as the difference between the magnetic susceptibility in FC and ZFC states) is shown here as a function of temperature for magnetic susceptibilities measured in H = 100 Oe. Fig. 4. Magnetic structure of the Mn doped α-Fe2O3 from the refinement of the room temperature neutron diffraction data. The arrows indicate the spins are lying on the ab-basal plane and are stacked along the c-axis. The refinement shows that the sample is in ferrimagnetic state at room temperature.
determined using the propagation vector k = (0 0 0) and the program BasIreps, available in the FullProf suite [27,28]. This analysis shows that the magnetic structure of Fe1.95Mn0.05O3 is similar to that of αFe2O3 at room temperature. The magnetic moment obtained from the refinement is 3.36μB per formula unit. However, this value is less than that reported in the paper by Hill et al., [29]. This decrease in the value can be due to partial substitution of Mn for Fe, and secondly, due to the synthesis method used. There is finite possibility of formation of some volume fraction of nanoparticles of Mn doped α-Fe2O3 leading to the overall decrease in magnetism of the compound. The magnetic structure of Fe1.95Mn0.05O3, comprising spins of Fe3+ is shown in Fig. 4. The magnetic structure of Fe1.95Mn0.05O3, unlike α-Fe2O3, comprises two antiferromagnetically coupled unequalferromagnetic sub-lattices [29,30]. This is clearly seen by the two sets of magnetic moments (short and long arrows, pointed in opposite directions, Fig. 4). The moments lie on the ab or basal plane of the hexagonal unit cell. The antiferromagnetic coupling of two unequal magnetic sub-lattices gives rise to the overall ferrimagnetism in this Mn-substituted compound at room temperature. Magnetic susceptibility (χ = M/H) was measured as a function of temperature in zero field cooled (ZFC) and field cooled (FC) states of the sample. In ZFC case, the sample was cooled to the lowest temperature without applying any external magnetic field, whereas in the case of FC measurements, sample was cooled in the external field. In both ZFC and FC measurements, susceptibility was measured while warming after applying the external magnetic fields (100 Oe and 5 kOe, respectively). χ(T) corresponding to two applied fields, namely, low field of 100 Oe, and a relatively higher field of 5 kOe, is plotted in Fig. 5. Both measurements show that sample is in high magnetization state and as the sample is cooled value of the susceptibility decreases monotonically with decreasing temperature up to 220 K and then falls suddenly. The peak around 220 K, by analogy to α-Fe2O3, can be taken as the Morin transition (TM) for Fe1.95Mn0.05O3 [11–13]. The χ(T) in ZFC case remains flat, as if independent of temperature, till T~60 K. Below 60 K, there is another anomaly, observed more clearly in the 5 kOe measurement (bottom panel of Fig. 5), where χ(T) goes through a small peak around 50 K. The anomaly around 50 K is in close vicinity of the observation of blocking temperature, TB, in nanoparticles of αFe2O3 [11–13]. Since the sample studied here has been prepared by mechanochemical method, it is quite possible that some agglomerated nano-particles would form and get dispersed within the bulk sample. The presence of these agglomerated nano-particles might be responsible
Fig. 5. Magnetic susceptibility of the Mn doped α-Fe2O3 measured as a function of temperature under two different applied fields, (top) 100 Oe and (below) 5 kOe. The sample was measured in the zero field cooled (ZFC) and field cooled (FC) states of the sample.
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Fig. 7. Magnetization (M) measured at various temperatures is plotted as a function of ramping field (H). (a) Overall, M vs. H profiles at all temperatures does not saturate till the highest field of measurement, indicating ferrimagnetic state of the sample. (b) A small hysteresis loop is observed around the origin for fields less than 10 kOe at all temperatures. Inset in figure (b) depicts the coercive field observed as a function of temperature.
noted that the magnetic anisotropy increases sharply below 100 K. Magnetization (M) as a function of ramping field was measured at several temperatures across TB and TM and the results are shown in Fig. 7. For temperatures below TB, M varies non-linearly for initial fields, between ± (0–5) kOe, and then varies linearly with the field in the ± (5–90) kOe) range. It is interesting to observe that for all temperatures, M(H) shows small hysteresis loop around origin. None of the M(H) curves show any signs of saturation till the highest measured field, indicating the ferrimagnetic nature of the sample at room temperature. In the inset of Fig. 7 (b), the coercive field (HC) observed from the M(H) loops is plotted as a function of temperature. HC decreases
for the system showing the blocking temperature. The FC curves for both field (i.e., H = 100 Oe and 5 kOe) measurements show identical features till 220 K, and below this temperature, χ(T) does not level off as in the case of ZFC, but goes through minima around 140 K, below which there is a rapid increase in χFC(T), which results in separation between ZFC and FC curves. To highlight this aspect, the difference between ZFC and FC measurements in case of χ(T) measured in a field of 100 Oe, which is a measure of the magnetic anisotropy, is plotted in Fig. 6. The plot shows that there are significant differences between ZFC and FC and the magnetic anisotropy exhibits a clear peak around 220 K. Further, it is 4
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observed around 50 K. Magnetization measurements along with magnetic structure derived from the neutron diffraction analysis shows that the compound is ferrimagnetic at room temperature with signatures of weak ferromagnetism or canted antiferromagnetism at temperatures below TMand TB. Dielectric measurements show two anomalies at temperatures corresponding to TMand TB. It will therefore be interesting and rewarding to study the origin of magnetoelectric coupling in Fe1.95Mn0.05O3. Detailed work in this direction is currently under progress. Further systematic studies on other physical properties of this compound and extension of this work to other concentrations of Fe and Mn in the stoichiometric form, Fe2-xMnxO3 are currently in progress. Conflict of interest form Authors declare no conflict of interest. Acknowledgments Authors would like to thank M. Venugopal, UGC-DAE-CSR (Mumbai Centre) for his assistance during sample preparation and data collection. We acknowledge Mukul Gupta (UGC-DAE-CSR, Indore Centre) and S. Bhattacharya (TPD, BARC) for providing XRD facilities. References
Fig. 8. Capacitance as a function of temperature, measured at several frequencies is shown here for Fe1.95Mn0.05O3. The inset shows the conductance as a function of temperature measured in a frequency of 1 MHz.
[1] C.N.R. Rao, Transition metal oxides, Annu. Rev. Phys. Chem. 40 (1989) 291–326. [2] Polona Umek, Andrej Zorko, Denis Arčon, Magnetic properties of transition-metal oxides: from bulk to nano, in: Ralf Riedel, I-Wei Chen (Eds.), Ceramics Science and Technology: Volume 2: Materials and Properties - 1, 2010 (Chapter 19). [3] V. Panchal, U. Bhandarkar, M. Neergat, K.G. Suresh, Controlling magnetic properties of iron oxide nanoparticles using post-synthesis thermal treatment, Appl. Phys. A 114 (2014) 537–544. [4] S.I. Srikrishna Ramya, C.K. Mahadevan, Preparation and structural, optical, magnetic, and electrical characterization of Mn2+/Co2+/Cu2+doped hematite nanocrystals, J. Solid State Chem. 211 (2014) 37–50. [5] H. Liang, X. Xu, W. Chen, B. Xu, Z. Wang, Facile synthesis of hematite nanostructures with controlled hollowness and porosity and their comparative photocatalytic activities, CrystEngComm 16 (2014) 959–963. [6] S. Sivakumar, D. Anusuya, C.P. Khatiwada, J. Sivasubramanian, A. Venkatesan, P. Soundhirarajan, Characterizations of diverse mole of pure and Ni-doped α-Fe2O3 synthesized nanoparticles through chemical precipitation route, Spectrochim. Acta A Mol. Biomol. Spectrosc. 128 (2014) 69–75. [7] F. Sánchez-DeJesús, A.M. Bolarín-Miró, C.A. Cortés-Escobedo, R. Valenzuela, S. Ammar, Mechanosynthesis, crystal structure and magnetic characterization of Mtype SrFe12O19, Ceram. Int. l40 (2014) 4033–4038. [8] A. Zelenakova, V. Zelenak, S. Michalik, J. Kovac, M.W. Meisel, Structural and magnetic properties of CoO-Pt core-shell nanoparticles, Phys. Rev. B 89 (2014) 104417. [9] N.M. Deraz, A. Alarifi, Novel processing and magnetic properties of hematite/maghemite nano-particles, Ceram. Int. 38 (2012) 4049–4055. [10] T. Adinaveen, J.J. Vijaya, L.J. Kennedy, Studies on the structural, morphological, optical, and magnetic properties of α-Fe2O3 nanostructures by a simple one-step low temperature reflux condensing method, J. Supercond. Nov. Magnetism 27 (2014) 1721–1727. [11] H.M. Lu, X.K. Meng, Morin temperature and Néel temperature of hematite nanocrystals, J. Phys. Chem. C 114 (2010) 21291–21295. [12] Marin Tadic, Matjaz Panjan, Vesna Damnjanovic, Irena Milosevic, Magnetic properties of hematite (α-Fe2O3) nanoparticles prepared by hydrothermal synthesis method, Appl. Surf. Sci. 320 (2014) 183–187. [13] A.S. Teja, P.Y. Koh, Synthesis, properties, and applications of magnetic iron oxide nanoparticles, Prog. Cryst. Growth Charact. Mater. 55 (2009) 22–45. [14] V. Baron, J. Gutzmer, H. Rundolöf, R. Tellgren, Neutron powder diffraction study of Mn-bearing hematite, α-Fe2-xMnxO3, in the range 0 ≤ x ≤ 0.176, Solid State Sci. 7 (2005) 753–759. [15] Dinesh Varshney, Arvind Yogi, Structural and electrical conductivity of Mn doped hematite (α-Fe2O3) phase, J. Mol. Struct. 995 (2011) 157–162. [16] B. Balaraju, M. Kuppan, S. Harinath Babu, S. Kaleemulla, N. Madhusudhana Rao, C. Krishnamoorthi, Girish M. Joshi, G. Venugopal Rao, K. Subbaravamma, I. Omkaram, D. Sreekantha Reddy, Structural, optical and magnetic properties of αFe2O3 nanoparticles, Mech., Mater. Sci. Eng. J. (2017) 9. [17] M. Mohamed Rafi, K. Syed Zameer Ahmed, K. Prem Nazeer, D. Siva Kumar, M. Thamilselvan, Synthesis, characterization and magnetic properties of hematite (α-Fe2O3) nanoparticles on polysaccharide templates and their antibacterial activity, Appl. Nanosci. 5 (2015) 515–520. [18] Jianmin Ma, Jiabiao Lian, Xiaochuan Duan, Xiaodi Liu, Wenjun Zheng, α-Fe2O3: hydrothermal synthesis, magnetic and electrochemical properties, J. Phys. Chem. C 114 (2010) 10671–10676. [19] S. Chakrabarty, T.K. Jana, K. De, S. Das, K. Dey, K. Chatterjee, Morphology
from 5 K to 100 K and then increases for 220 K before coming down again at 350 K. The behavior of HC tends to mimic the anisotropy behavior shown in Fig. 6. This behavior indicates that around 220 K, there is a change in the magnetic structure in this compound as well [29]. At T ≤ TB, for low fields, the non-linear variation in magnetization may be connected with canted antiferromagnetic or weak ferromagnetic state of the sample, as observed in the case of undoped α-Fe2O3 [29,30]. As discussed in Introduction, there is tremendous interest in iron oxide-based compounds for practical applications owing to their possible applications in spintronic devices, etc. In this connection, we explored the possibility of Fe1.95Mn0.05O3 for magnetoelectric applications by measuring its dielectric response as a function of temperature. In Fig. 8, the capacitance (C) of Fe1.95Mn0.05O3 measured at various frequencies is plotted as a function of temperature. It is interesting that the plot of capacitance as a function of temperature, (C vs. T), shows a sharp peak between 220 K and 250 K. This is very much similar to the observation of the Morin-like transition around this temperature in the plot of χ(T). Another interesting observation made from the dielectric measurements was that, in the conductance plot (inset of Fig. 8), a small anomaly was observed around 50 K, which is in good agreement with the anomaly observed in magnetic susceptibility measurement which has been attributed to blocking temperature TB. These interesting observations are encouraging enough to explore the possibility of magneto-electric coupling in this compound. A detailed study involving temperature dependent neutron diffraction, magnetoelectric measurements and other complementary studies are currently underway for better understanding of possible multiferroic properties of Fe1.95Mn0.05O3.
4. Conclusions The preliminary studies on structural, magnetic and dielectric properties of Fe1.95Mn0.05O3 have been carried out using X-ray and neutron diffraction, magnetization and dielectric measurements. The compound Fe1.95Mn0.05O3crystallizes in corundum type (α-Al2O3) structure in the space group R-3c in hexagonal setting. The partial doping of Mn for Fe reduces the Morin transition (TM) from 260 K to 220 K, but does not affect the blocking temperature, TB, which is 5
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