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Influencing magnetism of quasi 1D spin-chain compound Ca3CoMnO6 by Ni substitution at Co site
Journal of Magnetism and Magnetic Materials 486 (2019) 165264
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Research articles
Influencing magnetism of quasi 1D spin-chain compound Ca3CoMnO6 by Ni substitution at Co site
T
⁎
S. Rayaprola, , S.D. Kaushika, Kiran Singhb,1, V. Siruguria, Thomas Hansenc, E.V. Sampathkumarand UGC – DAE Consortium for Scientific Research, Mumbai Centre, BARC Campus, Mumbai 400085, India UGC – DAE Consortium for Scientific Research, Indore Centre, University Campus, Khandwa Road, Indore 4520017, India c Institut Laue-Langevin, Grenoble, 38042 Grenoble Cedex 9, France d Tata Institute of Fundamental Research, Homi Bhabha Road, Colaba, Mumbai 400005, India a
b
A R T I C LE I N FO
A B S T R A C T
Keywords: Neutron diffraction Spin-chain compounds Magnetic structure Magneto-dielectric effect
Earlier studies show that the multiferroicity in the spin-chain compound Ca3CoMnO6 critically depends upon the Co/Mn ratio. In order to establish the role of Co and Mn in controlling the multiferroic properties, we intend to disturb this ratio to find out its implications on the physical properties. The present study is aimed at studying modification of the properties of Ca3CoMnO6 by partial substitution of Ni for Co. The combined results of structural, magnetic and dielectric studies show interesting changes as a function of Ni substitution. There is a drastic difference in M(H) behavior at T < TN for Ca3Co1−xNixMnO6 (x = 0.0, 0.2) compounds. The magnetodielectric properties are also largely different for x = 0.0 and 0.2 samples. The MDE for x = 0.0 is negative with ∼7% variation, whereas for x = 0.2 it is positive with about 1% variation over the temperature range of measurement. Therefore, Ni substitution clearly alters the magnetic structure resulting in the change of magnetic space group, thus influencing the overall magnetic properties.
1. Introduction Understanding low-dimensional magnetism is at the forefront of advancing quantum communications. One dimensional spin-chain compounds are fascinating compounds for understanding the low-dimensional magnetism, as they are fertile grounds for experimentations and theoretical calculations. Magnetism of spin-chain compounds of the type A3MXO6 (where A = Ca, Sr; and M and X are transition metal ions, which can be magnetic or non-magnetic such as Co, Ni, Mn, Zn, Rh, Ir, etc.) is both enriching and exciting as many physical phenomenon are observed in this class of largely isostructural materials [1–4]. Depending upon the choice of M and X ions, many interesting situations in magnetism have been observed, such as long range magnetic ordering, partially disordered antiferromagnetism, highly frustrated magnetism, etc. [1–9]. The magnetism of spin-chain compounds has strong ramifications for spintronic applications also [10,11]. Ferroelectricity in certain compounds is driven by magnetism, that is, when the system undergoes magnetic ordering, inversion symmetry is lost and give rise to the ferroelectric polarization [11–14]. The lattice inversion symmetry can be removed either by spin-spiral ordering in
one-dimensional (1D) chains made up of similar ions, or by the up-updown-down spin order of alternatively-placed two dissimilar ions in a 1D chain. Compounds such as LiCu2O2, LiCuVO4, and TbMnO3 are known to exhibit spin-spiral ordering of the former kind, whereas so far only Ca3CoMnO6 is known to exhibit spin-spiral ordering of the latter type [12–15]. In our study on quasi-1D spin-chain compound, Ca3CoMnO6, we observed that the spin-configuration could be altered by applying an external magnetic field, leading to changes in dielectric constant [2]. The spin-chain compound, Ca3Co2O6, can be taken as the parent compound of Ca3CoMnO6, where half of the Co ions are replaced by Mn, thus realizing a system where the charges of magnetic ions are arranged in up-up-down-down fashion, enabling electric polarization to arise because of the breaking of inversion symmetry on magnetic sites due to magnetic ordering. Studies on the system Ca3Co2−xMnxO6 have shown that ferroelectricity is observed for a certain range of Co/Mn ratio only [3,11]. The stoichiometric Ca3CoMnO6 exhibits perfect ionic order, that is, both Co and Mn completely occupy trigonal prismatic and octahedral positions along the c-axis, respectively. Contrary to the expectations, higher value of dielectric constant is observed for off-stoichiometric samples, i.e., for the ratio of Co/Mn ≠ 1, however in the
⁎
Corresponding author at: UGC-DAE Consortium for Scientific Research, Mumbai Centre, BARC Campus – Trombay, Mumbai 400085, India. E-mail address: [email protected] (S. Rayaprol). 1 Presently at Dr. B. R. Ambedkar National Institute of Technology, Jalandhar 144011, India. https://doi.org/10.1016/j.jmmm.2019.165264 Received 15 January 2019; Received in revised form 16 April 2019; Accepted 1 May 2019 Available online 02 May 2019 0304-8853/ © 2019 Elsevier B.V. All rights reserved.
Journal of Magnetism and Magnetic Materials 486 (2019) 165264
S. Rayaprol, et al.
close vicinity of 1. As the Co/Mn ratio deviates from one, the local Co/ Mn disorder allows enhancement of bulk spin long-range ordering and thus the formation of large ferroelectric domains. It has been reported in the literature that multiferroicity in Ca3CoMnO6 compounds critically depends upon the Co/Mn ratio [16]. In the present work, we study the influence of Ni substitution for Co, in Ca3CoMnO6. Ca3CoMnO6 is known to exhibit a broad magnetic transition around 13 K with one more transition below 10 K [2,6]. As Ni2+ replaces Co2+, it can be expected that the lower magnetic moment of Ni, and one extra electron contributed by it, would affect both magnetic as well as dielectric properties. With this premise, we present and discuss here the results of neutron diffraction, magnetization and dielectric studies on Ca3Co0.8Ni0.2MnO6 compound. We prepared a series of samples with different × (Ni substitution) values, and found singlephase sample up to x = 0.2, therefore it is worthwhile to compare two samples with same structure but different Co/Mn ratio to test the magnetic properties. 2. Experimental details Polycrystalline samples of the series Ca3Co1−xNixMnO6 (x = 0.0 and 0.2) were prepared by solid-state reaction of high purity starting compounds CaCO3, Co3O4, NiO and MnO2 all with stated purity better than 99.9%. Stoichiometric quantities were thoroughly ground using agate mortar and pestle and calcined at 800 °C for 12 h. The mixtures were then repeatedly ground and heated at 1200 °C for total of 120 h followed by final heat treatments at 1275 °C for 72 h. Samples were characterized for single-phase formation by room temperature X-ray diffraction measurements using Cu-Kα radiation as well as room temperature neutron diffraction measurement on PD-3 at Dhruva reactor, Trombay using neutrons at wavelength 1.48 Å [17]. Low temperature neutron diffraction (ND) experiments were carried out on D20 at Institute Laue Langevin (ILL) using neutrons at wavelength 2.41 Å [18]. Magnetodielectric measurements were carried out at JNU, New Delhi (x = 0.0 sample) and TIFR, Mumbai (x = 0.2) on a homemade dielectric set up coupled to commercial cryogen free magnet and physical property measurement system, respectively. Magnetic measurements were carried out on a commercial SQUID magnetometer (x = 0.0) and vibrating sample magnetometer (x = 0.2), respectively.
Fig. 1. Room temperature neutron diffraction data for Ca3Co1−xNixMnO6 (x = 0.0 and 0.2) fitted using Rietveld refinement method. Rietveld fitted parameters are also included in the figures.
and the deviation from the linearityconfirms the persistence short-range magnetic correlations in both the compositions up to (∼130 K). The values of paramagnetic Curie temperature (θp) are found to be ∼−42 K and ∼−57 K for x = 0.0 and 0.2 respectively. The negative sign of θp indicates antiferromagnetic correlations in the compounds. The values of effective magnetic moment per formula unit for x = 0.0 and 0.2 are 6.11 µB and 6.05 µB respectively, which are in close agreement with the calculated moment values of 6.24 µB and 6.14 µB assuming free ion Co2+, Mn4+ and Ni2+ values. The frustration parameter (f), which is given by the ratio of θp and magnetic ordering temperature (TN) increases with increasing Ni content from 3.2 (x = 0.0) to 6.7 (x = 0.2). Magnetization measured as a function of magnetic field at 2 K is also shown in Fig. 4. Though for both the samples, the magnitude of magnetization at highest measure field (90 kOe) is almost same, there is noticeable difference in the features. For x = 0.0 sample, as reported earlier also, M varies linearly for lower values of H and around 25 kOe undergoes spin-flip kind of transition and then increases sharply with increasing H. M exhibits hysteresis like behavior between 20 and 90 kOe. On the other hand, for x = 0.2 sample, M actually varies nonlinearly between 0 kOe and 90 kOe, with no observable hysteresis for upward and downward magnetic field variation. The characteristic step like feature observed in x = 0.0 sample is missing in x = 0.2 sample. The dielectric constant as a function of temperature for x = 0.0 sample was reported by some of us earlier [9]. In Fig. 5(a) we show the dielectric constant (ε′) measured in a frequency of 100 kHz under H = 0 and 50 kOe applied fields. A clear peak is observed around 13 K and then ε′ falls rapidly until T ∼ 40 K and above this temperature ε′ increases as T increases. The features are almost similar for H = 0 and 50
3. Results and discussions Fig. 1 shows the room temperature neutron diffraction patterns for Ca3CoMnO6 and Ca3Co0.8Ni0.2MnO6, fitted to a K4CdCl6 type rhombohedral structure (hexagonal setting), space group R-3c, using Rietveld refinement method employed in FullProf program [19,20]. Both the compounds show excellent agreement with the calculated structural model, establishing single-phase nature of the samples. With Ni substitution for Co, there is a slight increase in unit cell parameters. However, the refinement clearly shows that there is no change in the crystallographic structure with Ni substitution. In Fig. 2, the low field (≤500 Oe) magnetic susceptibility is plotted as a function of temperature. For x = 0.0 sample (top panel of Fig. 2), there is a broad hump below 20 K, with a peak around (TN = ) 13 K, and reaches a minima around 5 K. Below 5 K there is a further increase in χ as T → 0. However, in the case of x = 0.2 sample (bottom panel of Fig. 2), there is a slight variation in the feature of χ(T) around (TN = ) 9 K as if there is magnetic ordering taking place around this temperature. In both the cases, it may be noted that the difference between the curves obtained in zero-field-cooled (ZFC) and field-cooled (FC) protocols is minimal that too below 5 K only. Magnetic susceptibility was measured in higher fields (H = 5 kOe for x = 0.0 and 50 kOe for x = 0.2, respectively) also and is shown in Fig. 3. The low-field features are seen in high fields also (Fig. 3, left panels). From the high temperature, linear region of the inverse susceptibility plots (right panels of Fig. 3), Curie-Weiss regime is inferred 2
Journal of Magnetism and Magnetic Materials 486 (2019) 165264
S. Rayaprol, et al.
Fig. 2. Magnetic susceptibility (χ = M/H) for Ca3Co1−xNixMnO6 (x = 0.0 and 0.2) measured at low fields in zero field cooled (ZFC) and field cooled (FC) state of the samples.
Fig. 3. Magnetic susceptibility (χ = M/H) for Ca3Co1−xNixMnO6 (x = 0.0 and 0.2) measured in 5 and 50 kOe. Lines through the points in left hand side curves serve as guides to the eyes, and the ones in the right hand side curves are obtained by least squares fit of the high temperature data.
kOe except for the magnitude of ε′. From the difference in ε′ for H = 0 and 50 kOe, we obtained the magnetodielectric (MDE) constant using the relation,
Δε ' =
' ' (ε50 kOe − ε0 )
ε0'
× 100
' where ε0' and ε50 kOe are dielectric constant values measured in 0 and 50 kOe fields respectively. In Fig. 5(b), the magnetodielectric constant is shown with respect to temperature for both the samples. It can be clearly seen here that there is a drastic reduction in the magnitude of MDE with Ni substitution, however the main point to observe here is that there is a change in the sign of MDE for x = 0.2 sample. In order to understand the influence of Ni substitution for Co, on the magnetic structure of Ca3CoMnO6, we carried out low temperature neutron diffraction measurements on D20 at ILL. The Fig. 6 shows the ND plots measured at T = 2 K for both x = 0.0 and 0.2 samples. Using the same structural model as used for room temperature refinement, the crystallographic structure for low temperature profiles were also refined. No structural change takes place as a function of temperature or Ni substitution. However, as clearly seen from Fig. 6, there is a marked difference in the magnetic structure (magnetic Bragg peak position). As shown in the Fig. 6, for x = 0.0 sample (top panel), a strong magnetic Bragg peak could be observed around Q value of 1.04 Å−1. However, the width of the peak indicates that short-range magnetic order is present along with the long-range magnetic order. The width of the broad diffuse peak underneath the strongest magnetic Bragg reflection (1 0 1) has been used for an estimation of the correlation length at 2 K. The estimation is based on the reciprocal relation between cluster size D (˚ A) and peak half-width broadening FWHM (degrees) according to the equation [5] D = λ × 57.3° . By taking the actual
Fig. 4. Magnetization as a function of field is plotted for Ca3Co1−xNixMnO6 (x = 0.0 and 0.2) measured at 2 K. The black arrows indicates the direction of the applied field.
instrumental resolution), the calculation results in a characteristic value of 65 Å for the antiferromagnetic chain segments that coexist with the long-range magnetic order. The magnetic structure of x = 0.0 sample could be modeled using the propagation vector k = (0, 0, 1/8), whereas x = 0.2 sample shows a different magnetic structure and the propagation vector required to model it is k = (0, 1/8, 1/8). Basis vectors and irreducible representations were obtained using these respective k vectors using Baslreps program [20]. On the basis of symmetry elements used, the magnetic space group, identified using the Bilbao Crystallographic Server in BNS
cosθ × FWHM
experimental parameters (neutron wavelength λ = 2.41 Å, central diffuse peak position 2θ = 22.42° and FWHM = 2.15° after correction for 3
Journal of Magnetism and Magnetic Materials 486 (2019) 165264
S. Rayaprol, et al.
Fig. 6. Low temperature neutron diffraction patterns for Ca3Co1−xNixMnO6 (x = 0.0 and 0.2) measured on D20 at ILL. The patterns have been fitted by Rietveld refinement method using both nuclear and magnetic structural model. The vertical arrows marks the strongest magnetic Bragg peak. In both the panels, the first row of vertical lines indicate nuclear Bragg peak and the second row indicates magnetic Bragg peak positions.
magnetization, magneto-dielectric properties can be understood in terms of the differences observed in the magnetic structure. Apparently, the magnetic chains are disrupted by Ni substitution, due to the change in the symmetry conditions and propagation vector.
Fig. 5. (a) Dielectric constant for Ca3Co0.8Ni0.2MnO6 measured in H = 0 and 50 kOe. (b) Magnetodielectric constant for Ca3Co1−xNixMnO6 (x = 0.0 and 0.2). The lines through the data points serve as guides to the eyes.
4. Conclusions setting [21–23] is P1 (1.1) for x = 0.0 and Cc (9.37) for x = 0.2, which clearly shows the difference in the magnetic symmetry conditions for both structures. The magnetic structure with respect to this refined profile is shown in Fig. 7(a). The magnetic structure for x = 0.0 sample is made up of up-up-down-down-up type of arrangement of spins [2,11]. In Fig. 6 (bottom panel), Rietveld refined neutron diffraction pattern for x = 0.2 sample is shown. In the case of x = 0.2 sample, the strong magnetic Bragg peak is observed at Q = 1.3 Å−1, which indicates the difference in the magnetic structure. In Fig. 7(b), the magnetic structure for this compound is shown. Compared to the x = 0.0 sample, the marked difference is that both Mn and Co spins change their orientation with respect to each other as one progresses from one unit cell to another. The idea of showing the structure in 1 × 3 × 1 form is to express the antiferromagnetic structure seen clearly along the b-direction of the crystallographic unit cell. The difference in the
Magnetization measurements show reduction in the magnetic ordering temperature (TN) from 13 K (x = 0.0) to ∼ 9 K (x = 0.20), whereas correspondingly the frustration parameter, given as the ratio of paramagnetic Curie temperature (θp) and Néel temperature (TN), increases, indicating strong influence of Ni substitution on the magnetic properties. The difference in M(H) behavior also at T < TN for both compounds is that there is absence of hysteresis and spin-flip transition. Ni substitution clearly alters the magnetic structure i.e., change in magnetic space group from P1 for x = 0.0 to Cc for x = 0.2, resulting in changes in the overall magnetic properties. Results of neutron diffraction, magnetization and magnetodielectric measurements show that Ni substitution for Co induces strong changes in magnetic and dielectric properties offering a tool in tailoring the physical properties using substitution. Detailed studies on temperature dependent neutron diffraction experiments for both the compounds 4
Journal of Magnetism and Magnetic Materials 486 (2019) 165264
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Fig. 7. (a) The schematic of the magnetic structure of Ca3CoMnO6 is shown in 1 × 3 × 1 format. Black and grey arrows show Mn and Co magnetic moments. (b) The schematic of the magnetic structure of Ca3Co0.8Ni0.2MnO6 is shown in 1 × 3 × 1 format. Black and grey arrows show Mn and Co/Ni magnetic moments.
with and without magnetic fields are currently under way, and is expected to throw more light on the mechanism of magnetism and other physical properties in these interesting compounds.
177 (2004) 3270. [5] M. Lowenhaupt, W. Schäfer, A. Niazi, E.V. Sampathkumaran, Eur. Phys. Lett. 63 (2003) 374. [6] S. Rayaprol, K. Sengupta, E.V. Sampathkumaran, Solid State Commun. 128 (2003) 79. [7] Asad Niazi, P.L. Paulose, E.V. Sampathkumaran, Phys. Rev. Lett. 88 (2002) 107202. [8] E.V. Sampathkumaran, Asad Niazi, Phys. Rev. B 65 (2002) 180401R. [9] S. Rayaprol, K. Sengupta, E.V. Sampathkumaran, Phys. Rev. B 67 (2003) 180404R. [10] H. Katsura, N. Nagaosa, A.V. Balatsky, Phys. Rev. Lett. 95 (2005) 057205. [11] Y.J. Choi, H.T. Yi, S. Lee, Q. Huang, V. Kiryukhin, S.-W. Cheong, Phys. Rev. Lett. 100 (2008) 047601. [12] R. Flint, H.-T. Yi, P. Chandra, S.-W. Cheong, V. Kriyukhin, Phys. Rev. B 81 (2010) 092402. [13] S.J. Gong, Q. Jiang, Phys. Lett. A 333 (2004) 124. [14] S.-W. Cheong, M. Mostovoy, Nat. Mater. 6 (2007) 13. [15] Y. Zhang, H.J. Xiang, M.-H. Whangbo, Phys. Rev. B 79 (2009) 054432. [16] V. Kiryukhin, S. Lee, W. Ratcliff II, Q. Huang, H.T. Yi, Y.J. Choi, S.-W. Cheong, Phys. Rev. Lett. 102 (2009) 187202. [17] V. Siruguri, P.D. Babu, M. Gupta, A.V. Pimpale, P.S. Goyal, Pramana, J. Phys. 71 (2008) 1197. [18] T.C. Hansen, P.F. Henry, H.E. Fischer, J. Torregrossa, P. Convert, Meas. Sci. Technol. 19 (2008) 034001. [19] H.M. Rietveld, J. Appl. Cryst. 2 (1969) 65. [20] J. Rodríguez-Carvajal, Commission on Powder Diffraction (IUCr), Newsletter 26 (2001) 12. [21] M.I. Aroyo, J.M. Perez-Mato, D. Orobengoa, E. Tasci, G. de la Flor, A. Kirov, Bulg. Chem. Commun. 43 (2011) 183. [22] M.I. Aroyo, J.M. Perez-Mato, C. Capillas, E. Kroumova, S. Ivantchev, G. Madariaga, A. Kirov, H. Wondratschek, Z. Krist. 221 (2006) 15. [23] M.I. Aroyo, A. Kirov, C. Capillas, J.M. Perez-Mato, H. Wondratschek, Acta Cryst. A62 (2006) 115.
Acknowledgements SR, SDK and VS acknowledges DST, India and ILL, France for providing financial support and hospitality to carry out neutron diffraction experiments at ILL through the experiment number 5-31-2106. Authors also thank Dr. A. K. Banerjee (CSR-Indore) and K. K. Iyer (TIFRMumbai) for their help in magnetization and magnetodielectric measurements, respectively. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.jmmm.2019.165264. References [1] K.E. Stitzer, J. Darriet, H.-C. zur Loye, Curr. Opn. Solid State Mat. Sci. 5 (2001) 535. [2] S.D. Kaushik, S. Rayaprol, J. Saha, N. Mohapatra, V. Siruguri, P.D. Babu, S. Patnaik, E.V. Sampathkumaran, J. Appl. Phys. 108 (2010) 084106. [3] T. Basu, K.K. Iyer, K. Singh, E.V. Sampathkumaran, Sci. Rep. 3 (2013) 3104. [4] S. Rayaprol, K. Sengupta, E.V. Sampathkumaran, Y. Matsushita, J. Solid State Sci.
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Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Research articles
Influencing magnetism of quasi 1D spin-chain compound Ca3CoMnO6 by Ni substitution at Co site
T
⁎
S. Rayaprola, , S.D. Kaushika, Kiran Singhb,1, V. Siruguria, Thomas Hansenc, E.V. Sampathkumarand UGC – DAE Consortium for Scientific Research, Mumbai Centre, BARC Campus, Mumbai 400085, India UGC – DAE Consortium for Scientific Research, Indore Centre, University Campus, Khandwa Road, Indore 4520017, India c Institut Laue-Langevin, Grenoble, 38042 Grenoble Cedex 9, France d Tata Institute of Fundamental Research, Homi Bhabha Road, Colaba, Mumbai 400005, India a
b
A R T I C LE I N FO
A B S T R A C T
Keywords: Neutron diffraction Spin-chain compounds Magnetic structure Magneto-dielectric effect
Earlier studies show that the multiferroicity in the spin-chain compound Ca3CoMnO6 critically depends upon the Co/Mn ratio. In order to establish the role of Co and Mn in controlling the multiferroic properties, we intend to disturb this ratio to find out its implications on the physical properties. The present study is aimed at studying modification of the properties of Ca3CoMnO6 by partial substitution of Ni for Co. The combined results of structural, magnetic and dielectric studies show interesting changes as a function of Ni substitution. There is a drastic difference in M(H) behavior at T < TN for Ca3Co1−xNixMnO6 (x = 0.0, 0.2) compounds. The magnetodielectric properties are also largely different for x = 0.0 and 0.2 samples. The MDE for x = 0.0 is negative with ∼7% variation, whereas for x = 0.2 it is positive with about 1% variation over the temperature range of measurement. Therefore, Ni substitution clearly alters the magnetic structure resulting in the change of magnetic space group, thus influencing the overall magnetic properties.
1. Introduction Understanding low-dimensional magnetism is at the forefront of advancing quantum communications. One dimensional spin-chain compounds are fascinating compounds for understanding the low-dimensional magnetism, as they are fertile grounds for experimentations and theoretical calculations. Magnetism of spin-chain compounds of the type A3MXO6 (where A = Ca, Sr; and M and X are transition metal ions, which can be magnetic or non-magnetic such as Co, Ni, Mn, Zn, Rh, Ir, etc.) is both enriching and exciting as many physical phenomenon are observed in this class of largely isostructural materials [1–4]. Depending upon the choice of M and X ions, many interesting situations in magnetism have been observed, such as long range magnetic ordering, partially disordered antiferromagnetism, highly frustrated magnetism, etc. [1–9]. The magnetism of spin-chain compounds has strong ramifications for spintronic applications also [10,11]. Ferroelectricity in certain compounds is driven by magnetism, that is, when the system undergoes magnetic ordering, inversion symmetry is lost and give rise to the ferroelectric polarization [11–14]. The lattice inversion symmetry can be removed either by spin-spiral ordering in
one-dimensional (1D) chains made up of similar ions, or by the up-updown-down spin order of alternatively-placed two dissimilar ions in a 1D chain. Compounds such as LiCu2O2, LiCuVO4, and TbMnO3 are known to exhibit spin-spiral ordering of the former kind, whereas so far only Ca3CoMnO6 is known to exhibit spin-spiral ordering of the latter type [12–15]. In our study on quasi-1D spin-chain compound, Ca3CoMnO6, we observed that the spin-configuration could be altered by applying an external magnetic field, leading to changes in dielectric constant [2]. The spin-chain compound, Ca3Co2O6, can be taken as the parent compound of Ca3CoMnO6, where half of the Co ions are replaced by Mn, thus realizing a system where the charges of magnetic ions are arranged in up-up-down-down fashion, enabling electric polarization to arise because of the breaking of inversion symmetry on magnetic sites due to magnetic ordering. Studies on the system Ca3Co2−xMnxO6 have shown that ferroelectricity is observed for a certain range of Co/Mn ratio only [3,11]. The stoichiometric Ca3CoMnO6 exhibits perfect ionic order, that is, both Co and Mn completely occupy trigonal prismatic and octahedral positions along the c-axis, respectively. Contrary to the expectations, higher value of dielectric constant is observed for off-stoichiometric samples, i.e., for the ratio of Co/Mn ≠ 1, however in the
⁎
Corresponding author at: UGC-DAE Consortium for Scientific Research, Mumbai Centre, BARC Campus – Trombay, Mumbai 400085, India. E-mail address: [email protected] (S. Rayaprol). 1 Presently at Dr. B. R. Ambedkar National Institute of Technology, Jalandhar 144011, India. https://doi.org/10.1016/j.jmmm.2019.165264 Received 15 January 2019; Received in revised form 16 April 2019; Accepted 1 May 2019 Available online 02 May 2019 0304-8853/ © 2019 Elsevier B.V. All rights reserved.
Journal of Magnetism and Magnetic Materials 486 (2019) 165264
S. Rayaprol, et al.
close vicinity of 1. As the Co/Mn ratio deviates from one, the local Co/ Mn disorder allows enhancement of bulk spin long-range ordering and thus the formation of large ferroelectric domains. It has been reported in the literature that multiferroicity in Ca3CoMnO6 compounds critically depends upon the Co/Mn ratio [16]. In the present work, we study the influence of Ni substitution for Co, in Ca3CoMnO6. Ca3CoMnO6 is known to exhibit a broad magnetic transition around 13 K with one more transition below 10 K [2,6]. As Ni2+ replaces Co2+, it can be expected that the lower magnetic moment of Ni, and one extra electron contributed by it, would affect both magnetic as well as dielectric properties. With this premise, we present and discuss here the results of neutron diffraction, magnetization and dielectric studies on Ca3Co0.8Ni0.2MnO6 compound. We prepared a series of samples with different × (Ni substitution) values, and found singlephase sample up to x = 0.2, therefore it is worthwhile to compare two samples with same structure but different Co/Mn ratio to test the magnetic properties. 2. Experimental details Polycrystalline samples of the series Ca3Co1−xNixMnO6 (x = 0.0 and 0.2) were prepared by solid-state reaction of high purity starting compounds CaCO3, Co3O4, NiO and MnO2 all with stated purity better than 99.9%. Stoichiometric quantities were thoroughly ground using agate mortar and pestle and calcined at 800 °C for 12 h. The mixtures were then repeatedly ground and heated at 1200 °C for total of 120 h followed by final heat treatments at 1275 °C for 72 h. Samples were characterized for single-phase formation by room temperature X-ray diffraction measurements using Cu-Kα radiation as well as room temperature neutron diffraction measurement on PD-3 at Dhruva reactor, Trombay using neutrons at wavelength 1.48 Å [17]. Low temperature neutron diffraction (ND) experiments were carried out on D20 at Institute Laue Langevin (ILL) using neutrons at wavelength 2.41 Å [18]. Magnetodielectric measurements were carried out at JNU, New Delhi (x = 0.0 sample) and TIFR, Mumbai (x = 0.2) on a homemade dielectric set up coupled to commercial cryogen free magnet and physical property measurement system, respectively. Magnetic measurements were carried out on a commercial SQUID magnetometer (x = 0.0) and vibrating sample magnetometer (x = 0.2), respectively.
Fig. 1. Room temperature neutron diffraction data for Ca3Co1−xNixMnO6 (x = 0.0 and 0.2) fitted using Rietveld refinement method. Rietveld fitted parameters are also included in the figures.
and the deviation from the linearityconfirms the persistence short-range magnetic correlations in both the compositions up to (∼130 K). The values of paramagnetic Curie temperature (θp) are found to be ∼−42 K and ∼−57 K for x = 0.0 and 0.2 respectively. The negative sign of θp indicates antiferromagnetic correlations in the compounds. The values of effective magnetic moment per formula unit for x = 0.0 and 0.2 are 6.11 µB and 6.05 µB respectively, which are in close agreement with the calculated moment values of 6.24 µB and 6.14 µB assuming free ion Co2+, Mn4+ and Ni2+ values. The frustration parameter (f), which is given by the ratio of θp and magnetic ordering temperature (TN) increases with increasing Ni content from 3.2 (x = 0.0) to 6.7 (x = 0.2). Magnetization measured as a function of magnetic field at 2 K is also shown in Fig. 4. Though for both the samples, the magnitude of magnetization at highest measure field (90 kOe) is almost same, there is noticeable difference in the features. For x = 0.0 sample, as reported earlier also, M varies linearly for lower values of H and around 25 kOe undergoes spin-flip kind of transition and then increases sharply with increasing H. M exhibits hysteresis like behavior between 20 and 90 kOe. On the other hand, for x = 0.2 sample, M actually varies nonlinearly between 0 kOe and 90 kOe, with no observable hysteresis for upward and downward magnetic field variation. The characteristic step like feature observed in x = 0.0 sample is missing in x = 0.2 sample. The dielectric constant as a function of temperature for x = 0.0 sample was reported by some of us earlier [9]. In Fig. 5(a) we show the dielectric constant (ε′) measured in a frequency of 100 kHz under H = 0 and 50 kOe applied fields. A clear peak is observed around 13 K and then ε′ falls rapidly until T ∼ 40 K and above this temperature ε′ increases as T increases. The features are almost similar for H = 0 and 50
3. Results and discussions Fig. 1 shows the room temperature neutron diffraction patterns for Ca3CoMnO6 and Ca3Co0.8Ni0.2MnO6, fitted to a K4CdCl6 type rhombohedral structure (hexagonal setting), space group R-3c, using Rietveld refinement method employed in FullProf program [19,20]. Both the compounds show excellent agreement with the calculated structural model, establishing single-phase nature of the samples. With Ni substitution for Co, there is a slight increase in unit cell parameters. However, the refinement clearly shows that there is no change in the crystallographic structure with Ni substitution. In Fig. 2, the low field (≤500 Oe) magnetic susceptibility is plotted as a function of temperature. For x = 0.0 sample (top panel of Fig. 2), there is a broad hump below 20 K, with a peak around (TN = ) 13 K, and reaches a minima around 5 K. Below 5 K there is a further increase in χ as T → 0. However, in the case of x = 0.2 sample (bottom panel of Fig. 2), there is a slight variation in the feature of χ(T) around (TN = ) 9 K as if there is magnetic ordering taking place around this temperature. In both the cases, it may be noted that the difference between the curves obtained in zero-field-cooled (ZFC) and field-cooled (FC) protocols is minimal that too below 5 K only. Magnetic susceptibility was measured in higher fields (H = 5 kOe for x = 0.0 and 50 kOe for x = 0.2, respectively) also and is shown in Fig. 3. The low-field features are seen in high fields also (Fig. 3, left panels). From the high temperature, linear region of the inverse susceptibility plots (right panels of Fig. 3), Curie-Weiss regime is inferred 2
Journal of Magnetism and Magnetic Materials 486 (2019) 165264
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Fig. 2. Magnetic susceptibility (χ = M/H) for Ca3Co1−xNixMnO6 (x = 0.0 and 0.2) measured at low fields in zero field cooled (ZFC) and field cooled (FC) state of the samples.
Fig. 3. Magnetic susceptibility (χ = M/H) for Ca3Co1−xNixMnO6 (x = 0.0 and 0.2) measured in 5 and 50 kOe. Lines through the points in left hand side curves serve as guides to the eyes, and the ones in the right hand side curves are obtained by least squares fit of the high temperature data.
kOe except for the magnitude of ε′. From the difference in ε′ for H = 0 and 50 kOe, we obtained the magnetodielectric (MDE) constant using the relation,
Δε ' =
' ' (ε50 kOe − ε0 )
ε0'
× 100
' where ε0' and ε50 kOe are dielectric constant values measured in 0 and 50 kOe fields respectively. In Fig. 5(b), the magnetodielectric constant is shown with respect to temperature for both the samples. It can be clearly seen here that there is a drastic reduction in the magnitude of MDE with Ni substitution, however the main point to observe here is that there is a change in the sign of MDE for x = 0.2 sample. In order to understand the influence of Ni substitution for Co, on the magnetic structure of Ca3CoMnO6, we carried out low temperature neutron diffraction measurements on D20 at ILL. The Fig. 6 shows the ND plots measured at T = 2 K for both x = 0.0 and 0.2 samples. Using the same structural model as used for room temperature refinement, the crystallographic structure for low temperature profiles were also refined. No structural change takes place as a function of temperature or Ni substitution. However, as clearly seen from Fig. 6, there is a marked difference in the magnetic structure (magnetic Bragg peak position). As shown in the Fig. 6, for x = 0.0 sample (top panel), a strong magnetic Bragg peak could be observed around Q value of 1.04 Å−1. However, the width of the peak indicates that short-range magnetic order is present along with the long-range magnetic order. The width of the broad diffuse peak underneath the strongest magnetic Bragg reflection (1 0 1) has been used for an estimation of the correlation length at 2 K. The estimation is based on the reciprocal relation between cluster size D (˚ A) and peak half-width broadening FWHM (degrees) according to the equation [5] D = λ × 57.3° . By taking the actual
Fig. 4. Magnetization as a function of field is plotted for Ca3Co1−xNixMnO6 (x = 0.0 and 0.2) measured at 2 K. The black arrows indicates the direction of the applied field.
instrumental resolution), the calculation results in a characteristic value of 65 Å for the antiferromagnetic chain segments that coexist with the long-range magnetic order. The magnetic structure of x = 0.0 sample could be modeled using the propagation vector k = (0, 0, 1/8), whereas x = 0.2 sample shows a different magnetic structure and the propagation vector required to model it is k = (0, 1/8, 1/8). Basis vectors and irreducible representations were obtained using these respective k vectors using Baslreps program [20]. On the basis of symmetry elements used, the magnetic space group, identified using the Bilbao Crystallographic Server in BNS
cosθ × FWHM
experimental parameters (neutron wavelength λ = 2.41 Å, central diffuse peak position 2θ = 22.42° and FWHM = 2.15° after correction for 3
Journal of Magnetism and Magnetic Materials 486 (2019) 165264
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Fig. 6. Low temperature neutron diffraction patterns for Ca3Co1−xNixMnO6 (x = 0.0 and 0.2) measured on D20 at ILL. The patterns have been fitted by Rietveld refinement method using both nuclear and magnetic structural model. The vertical arrows marks the strongest magnetic Bragg peak. In both the panels, the first row of vertical lines indicate nuclear Bragg peak and the second row indicates magnetic Bragg peak positions.
magnetization, magneto-dielectric properties can be understood in terms of the differences observed in the magnetic structure. Apparently, the magnetic chains are disrupted by Ni substitution, due to the change in the symmetry conditions and propagation vector.
Fig. 5. (a) Dielectric constant for Ca3Co0.8Ni0.2MnO6 measured in H = 0 and 50 kOe. (b) Magnetodielectric constant for Ca3Co1−xNixMnO6 (x = 0.0 and 0.2). The lines through the data points serve as guides to the eyes.
4. Conclusions setting [21–23] is P1 (1.1) for x = 0.0 and Cc (9.37) for x = 0.2, which clearly shows the difference in the magnetic symmetry conditions for both structures. The magnetic structure with respect to this refined profile is shown in Fig. 7(a). The magnetic structure for x = 0.0 sample is made up of up-up-down-down-up type of arrangement of spins [2,11]. In Fig. 6 (bottom panel), Rietveld refined neutron diffraction pattern for x = 0.2 sample is shown. In the case of x = 0.2 sample, the strong magnetic Bragg peak is observed at Q = 1.3 Å−1, which indicates the difference in the magnetic structure. In Fig. 7(b), the magnetic structure for this compound is shown. Compared to the x = 0.0 sample, the marked difference is that both Mn and Co spins change their orientation with respect to each other as one progresses from one unit cell to another. The idea of showing the structure in 1 × 3 × 1 form is to express the antiferromagnetic structure seen clearly along the b-direction of the crystallographic unit cell. The difference in the
Magnetization measurements show reduction in the magnetic ordering temperature (TN) from 13 K (x = 0.0) to ∼ 9 K (x = 0.20), whereas correspondingly the frustration parameter, given as the ratio of paramagnetic Curie temperature (θp) and Néel temperature (TN), increases, indicating strong influence of Ni substitution on the magnetic properties. The difference in M(H) behavior also at T < TN for both compounds is that there is absence of hysteresis and spin-flip transition. Ni substitution clearly alters the magnetic structure i.e., change in magnetic space group from P1 for x = 0.0 to Cc for x = 0.2, resulting in changes in the overall magnetic properties. Results of neutron diffraction, magnetization and magnetodielectric measurements show that Ni substitution for Co induces strong changes in magnetic and dielectric properties offering a tool in tailoring the physical properties using substitution. Detailed studies on temperature dependent neutron diffraction experiments for both the compounds 4
Journal of Magnetism and Magnetic Materials 486 (2019) 165264
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Fig. 7. (a) The schematic of the magnetic structure of Ca3CoMnO6 is shown in 1 × 3 × 1 format. Black and grey arrows show Mn and Co magnetic moments. (b) The schematic of the magnetic structure of Ca3Co0.8Ni0.2MnO6 is shown in 1 × 3 × 1 format. Black and grey arrows show Mn and Co/Ni magnetic moments.
with and without magnetic fields are currently under way, and is expected to throw more light on the mechanism of magnetism and other physical properties in these interesting compounds.
177 (2004) 3270. [5] M. Lowenhaupt, W. Schäfer, A. Niazi, E.V. Sampathkumaran, Eur. Phys. Lett. 63 (2003) 374. [6] S. Rayaprol, K. Sengupta, E.V. Sampathkumaran, Solid State Commun. 128 (2003) 79. [7] Asad Niazi, P.L. Paulose, E.V. Sampathkumaran, Phys. Rev. Lett. 88 (2002) 107202. [8] E.V. Sampathkumaran, Asad Niazi, Phys. Rev. B 65 (2002) 180401R. [9] S. Rayaprol, K. Sengupta, E.V. Sampathkumaran, Phys. Rev. B 67 (2003) 180404R. [10] H. Katsura, N. Nagaosa, A.V. Balatsky, Phys. Rev. Lett. 95 (2005) 057205. [11] Y.J. Choi, H.T. Yi, S. Lee, Q. Huang, V. Kiryukhin, S.-W. Cheong, Phys. Rev. Lett. 100 (2008) 047601. [12] R. Flint, H.-T. Yi, P. Chandra, S.-W. Cheong, V. Kriyukhin, Phys. Rev. B 81 (2010) 092402. [13] S.J. Gong, Q. Jiang, Phys. Lett. A 333 (2004) 124. [14] S.-W. Cheong, M. Mostovoy, Nat. Mater. 6 (2007) 13. [15] Y. Zhang, H.J. Xiang, M.-H. Whangbo, Phys. Rev. B 79 (2009) 054432. [16] V. Kiryukhin, S. Lee, W. Ratcliff II, Q. Huang, H.T. Yi, Y.J. Choi, S.-W. Cheong, Phys. Rev. Lett. 102 (2009) 187202. [17] V. Siruguri, P.D. Babu, M. Gupta, A.V. Pimpale, P.S. Goyal, Pramana, J. Phys. 71 (2008) 1197. [18] T.C. Hansen, P.F. Henry, H.E. Fischer, J. Torregrossa, P. Convert, Meas. Sci. Technol. 19 (2008) 034001. [19] H.M. Rietveld, J. Appl. Cryst. 2 (1969) 65. [20] J. Rodríguez-Carvajal, Commission on Powder Diffraction (IUCr), Newsletter 26 (2001) 12. [21] M.I. Aroyo, J.M. Perez-Mato, D. Orobengoa, E. Tasci, G. de la Flor, A. Kirov, Bulg. Chem. Commun. 43 (2011) 183. [22] M.I. Aroyo, J.M. Perez-Mato, C. Capillas, E. Kroumova, S. Ivantchev, G. Madariaga, A. Kirov, H. Wondratschek, Z. Krist. 221 (2006) 15. [23] M.I. Aroyo, A. Kirov, C. Capillas, J.M. Perez-Mato, H. Wondratschek, Acta Cryst. A62 (2006) 115.
Acknowledgements SR, SDK and VS acknowledges DST, India and ILL, France for providing financial support and hospitality to carry out neutron diffraction experiments at ILL through the experiment number 5-31-2106. Authors also thank Dr. A. K. Banerjee (CSR-Indore) and K. K. Iyer (TIFRMumbai) for their help in magnetization and magnetodielectric measurements, respectively. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.jmmm.2019.165264. References [1] K.E. Stitzer, J. Darriet, H.-C. zur Loye, Curr. Opn. Solid State Mat. Sci. 5 (2001) 535. [2] S.D. Kaushik, S. Rayaprol, J. Saha, N. Mohapatra, V. Siruguri, P.D. Babu, S. Patnaik, E.V. Sampathkumaran, J. Appl. Phys. 108 (2010) 084106. [3] T. Basu, K.K. Iyer, K. Singh, E.V. Sampathkumaran, Sci. Rep. 3 (2013) 3104. [4] S. Rayaprol, K. Sengupta, E.V. Sampathkumaran, Y. Matsushita, J. Solid State Sci.
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