- Email: [email protected]
Synthesis, structure and magnetic properties of layered manganate Sr4Mn3−xCrxO10 (x=0, 0.2)
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 313 (2007) 52–56 www.elsevier.com/locate/jmmm
Synthesis, structure and magnetic properties of layered manganate Sr4Mn3xCrxO10 (x ¼ 0, 0.2) M. Zaghrioui, F. Schoenstein, C. Autret-Lambert, M. Gervais Laboratoire d’Electrodynamique des Mate´riaux Avance´s, UMR 6157 CNRS – CEA, Universite´ Franc- ois Rabelais, UFR Sciences et Techniques, Parc Grandmont, 37200 Tours, France Received 23 June 2006; received in revised form 27 November 2006 Available online 27 December 2006
Abstract The manganates Sr4Mn3xCrxO10 (x ¼ 0 and 0.2) have been synthesized by solid state reaction. Powder X-ray diffraction analysis shows orthorhombic symmetry with space group Cmca for both compounds. The magnetic susceptibility measurements show an antiferromagnetic transition at 192 and 176 K for x ¼ 0 and 0.2, respectively. The magnetic susceptibility data were estimated using a model based on spin exchange antiferromagnetic interactions in isolated (Mn4+) trimer; a paramagnetic contribution due to the chromium ions was added in the case of Cr-doped materials. r 2007 Elsevier B.V. All rights reserved. Keywords: Magnetic susceptibility; Layered manganate; Crystal structure; Cr-doped manganate
1. Introduction Layered Ruddlesden–Popper (RP) manganates of general formula An+1BnO3n+1 have attracted much attention of solid state chemists because of their interesting properties. The hole-doped manganites R1xAxMnO3 (n ¼ N) (R ¼ rare earth and A ¼ divalent cation) and R3xAxMn2O7 (n ¼ 2) were largely studied due to their colossal magnetoresistance (CMR) and a variety of physical properties [1–6]. Moreover, these materials offer the possibility of applications in magnetic switches and data read-out devices. These studies lead the researchers naturally to be interested in the other members of the RP series, in particular A4Mn3O10 (n ¼ 3). This last phase, with A ¼ Ca, is relatively easy to elaborate by conventional methods under air. In contrast, the synthesis is more difficult for A ¼ Ba or Sr and even more difficult for A ¼ rare earth [7–9]. The structure of the RP (An+1MnnO3n+1) compounds consists of a regular intergrowth of single rock-salt with multiple perovskite layers. n ¼ N corresponds to an infinite three-dimensional network with an infinite number Corresponding author. Tel.: +33 254552105; fax: +33 254552138.
E-mail address: [email protected] (M. Zaghrioui). 0304-8853/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.12.014
of perovskite layers. n ¼ 2 and n ¼ 3 (for A ¼ Ca) can be described like a mixture of 2D and 3D networks with two and three perovskite slabs of corner sharing MnO6 octahedra separated of rock-salt layer, whereas for n ¼ 3 with A ¼ Sr and Ba, i.e. with a large cation on the A-site, the structure consists of Mn3O12 units formed with three faces sharing MnO6 octahedra and connected by two of their three final oxygen atoms [10–12]. Certain studies concerning n ¼ 3 materials are interested in doping on the Mn site by other transition metals. In the case of Ni and Cu with x ¼ 1, the compounds have Sr4Mn2MO9 (M ¼ Ni, Cu) chemical composition [13,14]. Their structure is hexagonal perovskite-type, which can be described as a composite structure with two subsystems having the same a parameter and two different c parameters. This structure contains transition metals in chains of oxide polyhedra (one trigonal prism and two faces sharing octahedra). Mn4+ cations occupy the octahedra whereas Ni2+/Cu2+ are in the trigonal prism. All these results exhibit that for Sr4Mn3xMxO10 (M ¼ Ni, Cu), the structure is orthorhombic for x ¼ 0 and hexagonal for x ¼ 1. Between these two compositions, the evolution of the structure is not detailed. We thus decided to study the evolution of structure and physical properties in the range
ARTICLE IN PRESS M. Zaghrioui et al. / Journal of Magnetism and Magnetic Materials 313 (2007) 52–56
101 Sr4Mn2.8Cr0.2O10+δ
100 Weight loss (%)
0oxo1 for different M cations (Cr, Ni and Co). This paper deals with monophasic Cr-doped manganates obtained for xp0.2. The synthesis of Sr4Mn3xCrxO10 (x ¼ 0 and 0.2) manganates, the powder X-ray diffraction (XRD) at room temperature, Rietveld refinements and the zero field cooling (ZFC) magnetic susceptibility were presented. The w(T) data were fitted using a simple model based on spin exchange interactions in isolated trimer; a paramagnetic contribution due to the chromium ions was added in the case of Cr-doped materials.
53
99 98 97 96 95 Sr2Mn0.9Cr0.1O3.5 +MnO 94 0
100
2. Experimental
300 400 500 600 Temperature (°C)
700
800
900
Fig. 1. TGA weight loss curve of Sr4Mn2.8Cr0.2O10+d powder in 1 atm H2/ Ar gas.
5600 4900 4200 3500 Intensity (a.u.)
Polycrystalline samples of nominal composition Sr4Mn3O10 and Sr4Mn2.8Cr0.2O10 were prepared by standard solid state reaction. Required amounts of SrCO3, Mn2O3 and Cr(NO3)2 were well-ground, pressed into pellets and fired in air at 950 1C for 30 h. The purpose of this first firing is to eliminate carbonates and nitrates. The samples obtained were ground, pelletized and sintered in air at 1200 1C for 1 day. This was done five times. After each sintering, the powders were monitored by XRD. Beyond 5 days of annealing, no change was observed in the XRD patterns. Thermogravimetric analysis (TGA) was used to determine oxygen content in a Perkin-Elmer TGA7 system under a reducing atmosphere (flowing 10% H2 in Ar). X-ray data were recorded at room temperature using a Brucker D 8 diffractometer with Cu Ka radiation over the range 151o2yo1001, with a step size of 0.021. Rietveld refinement of the data was done using FULLPROF program. Magnetic susceptibilities were measured with a Manics Faraday-based magnetosusceptometer over the temperature range 80–300 K. w(T) data were collected in ZFC in applied field of 10 kG.
200
Yobs Ycalc Yobs-Ycalc Bragg_position
2800 2100 1400 700 0 -700 -1400 15.0 22.5 30.0 37.5 45.0 52.5 60.0 67.5 75.0 82.5 90.0 2θ (°)
Fig. 2. Rietveld refinement profile of the X-ray powder diffraction for Sr4Mn2.8Cr0.2O10. The calculated and observed patterns are shown on the top by the solid line and the dots, respectively. The verticals marks in the middle show the Bragg reflections in the Cmca space group. The trace on the bottom is the plot of the difference between the calculated and the observed intensities.
3. Results and discussion 3.1. Thermogravimetric analysis
3.2. Structure
The oxygen content was determined by TGA in a PerkinElmer TGA7 system under a reducing atmosphere (flowing 10% H2 in Ar). The samples were heated up to 900 1C at a constant rate of 2 1C/min. The TGA curve obtained for Sr4Mn2.8Cr0.2O10+d is presented in Fig. 1. A complete reduction to SrO, manganese and chromium metal did not occur, but a partial reduction was evidenced by XRD. The final products of the TGA were Sr2(Mn,Cr)O3.5 and MnO for all samples. The oxygen contents were estimated from the total weight loss, which is equal to 4.85% and 4.9% for Sr4Mn3O10+d and Sr4Mn2.8Cr0.2O10+d, respectively. Assuming this result, the samples can be formulated as Sr4Mn3O10.05 and Sr4Mn2.8Cr0.2O10.07, i.e. d ¼ 0.05 and 0.07.
The powder XRD (PXRD) patterns of both samples can be indexed based on orthorhombic symmetry in space group Cmca (Fig. 2). Sr4Mn2.8Cr0.2O10 is isostructural with Sr4Mn3O10, Ba4Mn3O10 and with Ba4Ti2PtO10 [10,15]. This structure can be described as a stacking along b of octahedral [Mn3O10]N layers interleaved with strontium and folded along the c-axis as described before (Fig. 3). The cell parameters of Sr4Mn3O10 have been determined from Rietveld refinement of PXRD patterns, starting with parameters reported by Floros et al. [10] for Sr4xBax Mn3O10 compounds. The lattice parameters obtained are summarized in Table 1. They are very close to those reported in Ref. [10]. The atomic positions and cell parameters of Sr4Mn2.8Cr0.2O10 shown in Table 2 have
ARTICLE IN PRESS M. Zaghrioui et al. / Journal of Magnetism and Magnetic Materials 313 (2007) 52–56
54
been estimated from Rietveld refinement with the parameters obtained for Sr4Mn3O10 as an initial model. Mn and Cr atoms were localized on the same sites with a constant Cr/Mn occupancy ratio. All the oxygen sites were assumed to be complete. The cell parameters obtained for Sr4Mn2.8Cr0.2O10 are slightly larger than those of Sr4Mn3O10. The calculated interatomic distances show that the Mn1 octahedra are slightly distorted, whereas the Mn2 octahedra present a large distortion (Fig. 3).
Mn1/Cr octahedra Mn2/Cr octahedra Sr
b a
c
Fig. 3. Refined crystal structure of Sr4Mn2.8Cr0.2O10.
Table 1 Refined parameters of Sr4Mn3O10 at room temperature
Space group a (A˚) b (A˚) c (A˚) V (A˚3) Z RBragg (%) w2
This work
Ref. [10]
Cmca 5.4757(1) 12.4618(1) 12.5284(1) 854.904(8) 4 6.4 1.3
Cmca 5.4766(1) 12.4659(2) 12.5282(2) 4 5.02
3.3. Magnetic susceptibility The w(T) curves for Sr4Mn3O10 and Sr4Mn2.8Cr0.2O10 exhibit a bump at TN ¼ 192 and 176 K, respectively, which corresponds to the antiferromagnetic (AFM) transition (Fig. 4). The TN values correspond to a sign change of dw/dT. Floros et al. [10] have already observed this transition at 200 K for Sr4Mn3O10. The fit of the w(T) data above TN (paramagnetic regime) using the Curie–Weiss law (w ¼ w0 þ C=ðT yÞ) allows to determine the Curie temperature y and the effective magnetic moment meff/Mn. The values obtained for y are 511 (36) and 390 (11) K, and for meff are 4.5(2) and 4.04(4) mB, for Sr4Mn3O10 and Sr4Mn2.8Cr0.2O10, respectively. Results are in good agreement with data reported in literature [10]. The negative y values indicate the predominance of strong AFM interactions in these compounds, which explain the AFM transition. The increase of y in Cr-doped compound suggests a decrease in AFM interactions, what leads to the decrease of TN. Moreover, the effective magnetic moment values obtained are higher than the values expected for free Mn4+. This can be explained neither by an orbital contribution arising from the 4A2g ground state of Mn4+ nor by the presence of Mn3+ as reported by Floros et al. [10]. In addition meff decreases when the material is Cr-doped. This result suggests that chromium ions have effective moment lower than Mn4+ ions, what excludes the presence of Cr2+ in high spin configuration (meff ¼ 4.90 mB). The magnetic properties of Sr4Mn3O10 were analyzed by a theoretical model containing a linear Mn3 ¼ Mn2a– Mn1–Mn2b system, which corresponds to the trimeric [Mn3O10] described above. The Heisenberg–Hamiltonian for this system can be written as H ¼ 2J½SMn2a SMn1 þ SMn1 SMn2b , where J is the exchange coupling constant between neighboring Mn1 and Mn2 sites (negative for an AFM interaction, positive for a ferromagnetic interaction) and S is the spin operator. Then the molar magnetic susceptibility for an isolated trimeric Mn4+ (S ¼ 3/2) is obtained using a
Table 2 Refined structural parameters of Sr4Mn2.8Cr0.2O10 at room temperature Atom
Wyckoff position
x
y
z
Biso (A˚2)
Occupancy
Sr1 Sr2 Mn1 Cr1 Mn2 Cr2 O1 O2 O3 O4
8f 8f 4a 4a 8f 8f 8f 8f 16g 8f
0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.234(1) 0.2500
0.4685(2) 0.2616(2) 0.0000 0.0000 0.1306(4) 0.1306(4) 0.034(1) 0.280(1) 0.104(1) 0.125(2)
0.1416(2) 0.3888(2) 0.0000 0.0000 0.1513(4) 0.1513(4) 0.148(1) 0.142(1) 0.0381(8) 0.2500
0.375 0.468 0.86 0.86 1.000 1.000 0.700 0.700 0.700 0.700
0.5000 0.5000 0.2333 0.0167 0.4667 0.0333 0.5000 0.5000 1.0000 0.5000
Space group Cmca; a ¼ 5.48014(4) A˚, b ¼ 12.4655(2) A˚, c ¼ 12.5316(2) A˚, V ¼ 856.07(1)A˚3. RBragg ¼ 6.81%, w2 ¼ 1.53.
ARTICLE IN PRESS M. Zaghrioui et al. / Journal of Magnetism and Magnetic Materials 313 (2007) 52–56
55
6J 9J 9J þ 2 exp þ 10 exp þ 4 exp kB T kB T kB T 7J 12J þ 6 exp þ 4 exp . kB T kB T
0.00750
TN =196 K
χ (emu/mol)
0.00725
In the case of Sr4Mn2.8Cr0.2O10, a paramagnetic contribution of chromium ions was added to the magnetic susceptibility of the isolated trimer. The total susceptibility is then expressed as: C (2) w ¼ ð1 pÞwtrimer þ p , T
0.00700
0.00675
0.00650 50
100
150 200 Temperature (K)
250
300
250
300
0.00750
χ (emu/mol)
0.00725 TN =176 K 0.00700
0.00675
0.00650 50
100
150 200 Temperature (K)
Fig. 4. The molar magnetic susceptibility for Sr4Mn3O10 (a) and Sr4Mn2.8Cr0.2O10 (b) as a function of temperature, measured in a field of 10 kG. The solid lines correspond to the calculated magnetic susceptibility.
Heisenberg–Dirac–Van-Vleck model. The denominator was modified according to the Curie–Weiss law to give: wtrimer ¼
Ng2 b2 A , 3kB ðT yÞ B
(1)
where 3J 2J A ¼ 141 þ 52:5 exp þ 15 exp kB T kB T 5J 6J J þ 126 exp þ 52:5 exp þ 1:5 exp kB T kB T kB T 6J 9J 9J þ 15 exp þ 1:5 exp þ 247:5 exp kB T kB T kB T 7J 12J þ 15 exp þ 52:5 exp kB T kB T
3J 2J 5J B ¼ 12 þ 6 exp þ 4 exp þ 2 exp kB T kB T kB T 6J J þ 8 exp þ 6 exp kB T kB T
where p is the fractional value. For the nominal composition, p is equal to 1/15 [p ¼ 0.2/(2.8+0.2)]. The least-square fitting routine was used to fit the experimental data to Eqs. (1) and (2) corrected for temperature-independent paramagnetic contribution, w0, and with assumed g value of 2. The best fits give the parameters: J/kB ¼ P13.63 K and P y ¼ 570 and R ¼ 5.3 104 [R ¼ (wcal–wexp)2/ (wexp)2], and J/kB ¼ 18.2 K, y ¼ 425 K, C ¼ 1.04, p ¼ 0.07 and R ¼ 2.6 104 for Sr4Mn3O10 and Sr4Mn2.8Cr0.2O10, respectively. The magnetic susceptibilities calculated by using the fit parameters are shown as the solid lines in Fig. 4a and b. The magnetic exchange interactions J are AFM in both samples, and they are very close. The Curie temperatures are in agreement with those obtained for the paramagnetic regime using Curie–Weiss law. Moreover, the p value is close to the fractional value of the nominal composition, which suggests a weak magnetic interaction between Cr and Mn4+. Finally, for Cr-doped materials, the Curie constant value obtained corresponds to two unpaired spins for each chromium cation, which indicates that the chromium oxidation state is Cr2+ in low spin configuration or Cr4+. According to the TGA, we can then suggest that the chromium, in Sr4Mn2.8Cr0.2O10, is present as Cr4+. 4. Conclusion Here, we present the synthesis, structure and magnetic study of manganates Sr4Mn3xCrxO10 (x ¼ 0 and 0.2). Manganates were obtained by solid state reaction in air. The XRD patterns were refined with Cmca space group. This structure is characterized by trimers formed by facesharing octahedra. Then the magnetic properties can be interpreted in terms of isolated trimers. In the case of Crdoped compounds, a paramagnetic contribution was added which corresponds to the magnetic contribution of chromium ions. The modelization gives AFM interactions between Mn4+ ions, high Curie temperature suggesting a strong AFM interaction and an oxidation state of Cr as Cr4+. References [1] B. Raveau, A. Maignan, V. Caignaert, J. Solid State Chem. 117 (1995) 424.
ARTICLE IN PRESS 56
M. Zaghrioui et al. / Journal of Magnetism and Magnetic Materials 313 (2007) 52–56
[2] A. Urushibara, Y. Moritomo, T. Arima, A. Asamitsu, G. Kido, Y. Tokura, Phys. Rev. B 51 (1995) 14103. [3] P.G. Radaelli, M. Marezio, H.Y. Hwang, S. Cheong, B. Batlogg, Phys. Rev. B 54 (1996) 8992. [4] Y. Moritomo, A. Asamitsu, H. Kuwahara, Y. Tokura, Nature 380 (1996) 141. [5] T. Kimura, Y. Tomioka, A. Asamitsu, Y. Tokura, Phys. Rev. Lett. 81 (1998) 5920. [6] D.N. Argyriou, J.F. Mitchell, J.B. Goodenough, O. Chmaissen, S. Short, J.D. Jorgensen, Phys. Rev. Lett. 67 (1998) 3380. [7] N.S. Witte, P. Goodman, F.J. Lincoln, R.H. March, S.J. Kennedy, Appl. Phys. Lett. 72 (1998) 853. [8] H. Asano, J. Hayakawa, M. Matsui, Phys. Rev. B 57 (1998) 1052.
[9] M.M. Cruz, M.D. Carvalho, A. Casaca, G. Bonfait, F.M. Costa, M. Godinho, J. Magn. Magn. Mater. 226–230 (2001) 800. [10] N. Floros, M. Hervieu, G. Van Tendeloo, C. Michel, A. Maignan, B. Raveau, Solid State Sci. 2 (2000) 1. [11] K. Boulahya, M. Parras, U. Amador, J.M. Gonzalez-Calbet, Solid State Ionics 172 (2004) 543. [12] K. Boulahya, M. Parras, J.M. Gonzalez-Calbet, U. Amador, J.L. Martinez, M.T. Fernandez-Diaz, Phys. Rev. B 69 (2004) 24418. [13] A. El Abed, E. Gaudin, J. Darriet, M.H. Whangbo, J. Solid State Chem. 163 (2002) 513. [14] S. Aygu¨n, S. Shiley, D.P. Cann, Mater. Lett. 58 (2004) 3645. [15] V.G. Zubkov, A.P. Tyutyunnik, I.F. Berger, V.I. Voronin, G.V. Bazuev, C.A. Moore, P.D. Battle, J. Solid State Chem. 167 (2002) 453.
Journal of Magnetism and Magnetic Materials 313 (2007) 52–56 www.elsevier.com/locate/jmmm
Synthesis, structure and magnetic properties of layered manganate Sr4Mn3xCrxO10 (x ¼ 0, 0.2) M. Zaghrioui, F. Schoenstein, C. Autret-Lambert, M. Gervais Laboratoire d’Electrodynamique des Mate´riaux Avance´s, UMR 6157 CNRS – CEA, Universite´ Franc- ois Rabelais, UFR Sciences et Techniques, Parc Grandmont, 37200 Tours, France Received 23 June 2006; received in revised form 27 November 2006 Available online 27 December 2006
Abstract The manganates Sr4Mn3xCrxO10 (x ¼ 0 and 0.2) have been synthesized by solid state reaction. Powder X-ray diffraction analysis shows orthorhombic symmetry with space group Cmca for both compounds. The magnetic susceptibility measurements show an antiferromagnetic transition at 192 and 176 K for x ¼ 0 and 0.2, respectively. The magnetic susceptibility data were estimated using a model based on spin exchange antiferromagnetic interactions in isolated (Mn4+) trimer; a paramagnetic contribution due to the chromium ions was added in the case of Cr-doped materials. r 2007 Elsevier B.V. All rights reserved. Keywords: Magnetic susceptibility; Layered manganate; Crystal structure; Cr-doped manganate
1. Introduction Layered Ruddlesden–Popper (RP) manganates of general formula An+1BnO3n+1 have attracted much attention of solid state chemists because of their interesting properties. The hole-doped manganites R1xAxMnO3 (n ¼ N) (R ¼ rare earth and A ¼ divalent cation) and R3xAxMn2O7 (n ¼ 2) were largely studied due to their colossal magnetoresistance (CMR) and a variety of physical properties [1–6]. Moreover, these materials offer the possibility of applications in magnetic switches and data read-out devices. These studies lead the researchers naturally to be interested in the other members of the RP series, in particular A4Mn3O10 (n ¼ 3). This last phase, with A ¼ Ca, is relatively easy to elaborate by conventional methods under air. In contrast, the synthesis is more difficult for A ¼ Ba or Sr and even more difficult for A ¼ rare earth [7–9]. The structure of the RP (An+1MnnO3n+1) compounds consists of a regular intergrowth of single rock-salt with multiple perovskite layers. n ¼ N corresponds to an infinite three-dimensional network with an infinite number Corresponding author. Tel.: +33 254552105; fax: +33 254552138.
E-mail address: [email protected] (M. Zaghrioui). 0304-8853/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.12.014
of perovskite layers. n ¼ 2 and n ¼ 3 (for A ¼ Ca) can be described like a mixture of 2D and 3D networks with two and three perovskite slabs of corner sharing MnO6 octahedra separated of rock-salt layer, whereas for n ¼ 3 with A ¼ Sr and Ba, i.e. with a large cation on the A-site, the structure consists of Mn3O12 units formed with three faces sharing MnO6 octahedra and connected by two of their three final oxygen atoms [10–12]. Certain studies concerning n ¼ 3 materials are interested in doping on the Mn site by other transition metals. In the case of Ni and Cu with x ¼ 1, the compounds have Sr4Mn2MO9 (M ¼ Ni, Cu) chemical composition [13,14]. Their structure is hexagonal perovskite-type, which can be described as a composite structure with two subsystems having the same a parameter and two different c parameters. This structure contains transition metals in chains of oxide polyhedra (one trigonal prism and two faces sharing octahedra). Mn4+ cations occupy the octahedra whereas Ni2+/Cu2+ are in the trigonal prism. All these results exhibit that for Sr4Mn3xMxO10 (M ¼ Ni, Cu), the structure is orthorhombic for x ¼ 0 and hexagonal for x ¼ 1. Between these two compositions, the evolution of the structure is not detailed. We thus decided to study the evolution of structure and physical properties in the range
ARTICLE IN PRESS M. Zaghrioui et al. / Journal of Magnetism and Magnetic Materials 313 (2007) 52–56
101 Sr4Mn2.8Cr0.2O10+δ
100 Weight loss (%)
0oxo1 for different M cations (Cr, Ni and Co). This paper deals with monophasic Cr-doped manganates obtained for xp0.2. The synthesis of Sr4Mn3xCrxO10 (x ¼ 0 and 0.2) manganates, the powder X-ray diffraction (XRD) at room temperature, Rietveld refinements and the zero field cooling (ZFC) magnetic susceptibility were presented. The w(T) data were fitted using a simple model based on spin exchange interactions in isolated trimer; a paramagnetic contribution due to the chromium ions was added in the case of Cr-doped materials.
53
99 98 97 96 95 Sr2Mn0.9Cr0.1O3.5 +MnO 94 0
100
2. Experimental
300 400 500 600 Temperature (°C)
700
800
900
Fig. 1. TGA weight loss curve of Sr4Mn2.8Cr0.2O10+d powder in 1 atm H2/ Ar gas.
5600 4900 4200 3500 Intensity (a.u.)
Polycrystalline samples of nominal composition Sr4Mn3O10 and Sr4Mn2.8Cr0.2O10 were prepared by standard solid state reaction. Required amounts of SrCO3, Mn2O3 and Cr(NO3)2 were well-ground, pressed into pellets and fired in air at 950 1C for 30 h. The purpose of this first firing is to eliminate carbonates and nitrates. The samples obtained were ground, pelletized and sintered in air at 1200 1C for 1 day. This was done five times. After each sintering, the powders were monitored by XRD. Beyond 5 days of annealing, no change was observed in the XRD patterns. Thermogravimetric analysis (TGA) was used to determine oxygen content in a Perkin-Elmer TGA7 system under a reducing atmosphere (flowing 10% H2 in Ar). X-ray data were recorded at room temperature using a Brucker D 8 diffractometer with Cu Ka radiation over the range 151o2yo1001, with a step size of 0.021. Rietveld refinement of the data was done using FULLPROF program. Magnetic susceptibilities were measured with a Manics Faraday-based magnetosusceptometer over the temperature range 80–300 K. w(T) data were collected in ZFC in applied field of 10 kG.
200
Yobs Ycalc Yobs-Ycalc Bragg_position
2800 2100 1400 700 0 -700 -1400 15.0 22.5 30.0 37.5 45.0 52.5 60.0 67.5 75.0 82.5 90.0 2θ (°)
Fig. 2. Rietveld refinement profile of the X-ray powder diffraction for Sr4Mn2.8Cr0.2O10. The calculated and observed patterns are shown on the top by the solid line and the dots, respectively. The verticals marks in the middle show the Bragg reflections in the Cmca space group. The trace on the bottom is the plot of the difference between the calculated and the observed intensities.
3. Results and discussion 3.1. Thermogravimetric analysis
3.2. Structure
The oxygen content was determined by TGA in a PerkinElmer TGA7 system under a reducing atmosphere (flowing 10% H2 in Ar). The samples were heated up to 900 1C at a constant rate of 2 1C/min. The TGA curve obtained for Sr4Mn2.8Cr0.2O10+d is presented in Fig. 1. A complete reduction to SrO, manganese and chromium metal did not occur, but a partial reduction was evidenced by XRD. The final products of the TGA were Sr2(Mn,Cr)O3.5 and MnO for all samples. The oxygen contents were estimated from the total weight loss, which is equal to 4.85% and 4.9% for Sr4Mn3O10+d and Sr4Mn2.8Cr0.2O10+d, respectively. Assuming this result, the samples can be formulated as Sr4Mn3O10.05 and Sr4Mn2.8Cr0.2O10.07, i.e. d ¼ 0.05 and 0.07.
The powder XRD (PXRD) patterns of both samples can be indexed based on orthorhombic symmetry in space group Cmca (Fig. 2). Sr4Mn2.8Cr0.2O10 is isostructural with Sr4Mn3O10, Ba4Mn3O10 and with Ba4Ti2PtO10 [10,15]. This structure can be described as a stacking along b of octahedral [Mn3O10]N layers interleaved with strontium and folded along the c-axis as described before (Fig. 3). The cell parameters of Sr4Mn3O10 have been determined from Rietveld refinement of PXRD patterns, starting with parameters reported by Floros et al. [10] for Sr4xBax Mn3O10 compounds. The lattice parameters obtained are summarized in Table 1. They are very close to those reported in Ref. [10]. The atomic positions and cell parameters of Sr4Mn2.8Cr0.2O10 shown in Table 2 have
ARTICLE IN PRESS M. Zaghrioui et al. / Journal of Magnetism and Magnetic Materials 313 (2007) 52–56
54
been estimated from Rietveld refinement with the parameters obtained for Sr4Mn3O10 as an initial model. Mn and Cr atoms were localized on the same sites with a constant Cr/Mn occupancy ratio. All the oxygen sites were assumed to be complete. The cell parameters obtained for Sr4Mn2.8Cr0.2O10 are slightly larger than those of Sr4Mn3O10. The calculated interatomic distances show that the Mn1 octahedra are slightly distorted, whereas the Mn2 octahedra present a large distortion (Fig. 3).
Mn1/Cr octahedra Mn2/Cr octahedra Sr
b a
c
Fig. 3. Refined crystal structure of Sr4Mn2.8Cr0.2O10.
Table 1 Refined parameters of Sr4Mn3O10 at room temperature
Space group a (A˚) b (A˚) c (A˚) V (A˚3) Z RBragg (%) w2
This work
Ref. [10]
Cmca 5.4757(1) 12.4618(1) 12.5284(1) 854.904(8) 4 6.4 1.3
Cmca 5.4766(1) 12.4659(2) 12.5282(2) 4 5.02
3.3. Magnetic susceptibility The w(T) curves for Sr4Mn3O10 and Sr4Mn2.8Cr0.2O10 exhibit a bump at TN ¼ 192 and 176 K, respectively, which corresponds to the antiferromagnetic (AFM) transition (Fig. 4). The TN values correspond to a sign change of dw/dT. Floros et al. [10] have already observed this transition at 200 K for Sr4Mn3O10. The fit of the w(T) data above TN (paramagnetic regime) using the Curie–Weiss law (w ¼ w0 þ C=ðT yÞ) allows to determine the Curie temperature y and the effective magnetic moment meff/Mn. The values obtained for y are 511 (36) and 390 (11) K, and for meff are 4.5(2) and 4.04(4) mB, for Sr4Mn3O10 and Sr4Mn2.8Cr0.2O10, respectively. Results are in good agreement with data reported in literature [10]. The negative y values indicate the predominance of strong AFM interactions in these compounds, which explain the AFM transition. The increase of y in Cr-doped compound suggests a decrease in AFM interactions, what leads to the decrease of TN. Moreover, the effective magnetic moment values obtained are higher than the values expected for free Mn4+. This can be explained neither by an orbital contribution arising from the 4A2g ground state of Mn4+ nor by the presence of Mn3+ as reported by Floros et al. [10]. In addition meff decreases when the material is Cr-doped. This result suggests that chromium ions have effective moment lower than Mn4+ ions, what excludes the presence of Cr2+ in high spin configuration (meff ¼ 4.90 mB). The magnetic properties of Sr4Mn3O10 were analyzed by a theoretical model containing a linear Mn3 ¼ Mn2a– Mn1–Mn2b system, which corresponds to the trimeric [Mn3O10] described above. The Heisenberg–Hamiltonian for this system can be written as H ¼ 2J½SMn2a SMn1 þ SMn1 SMn2b , where J is the exchange coupling constant between neighboring Mn1 and Mn2 sites (negative for an AFM interaction, positive for a ferromagnetic interaction) and S is the spin operator. Then the molar magnetic susceptibility for an isolated trimeric Mn4+ (S ¼ 3/2) is obtained using a
Table 2 Refined structural parameters of Sr4Mn2.8Cr0.2O10 at room temperature Atom
Wyckoff position
x
y
z
Biso (A˚2)
Occupancy
Sr1 Sr2 Mn1 Cr1 Mn2 Cr2 O1 O2 O3 O4
8f 8f 4a 4a 8f 8f 8f 8f 16g 8f
0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.0000 0.234(1) 0.2500
0.4685(2) 0.2616(2) 0.0000 0.0000 0.1306(4) 0.1306(4) 0.034(1) 0.280(1) 0.104(1) 0.125(2)
0.1416(2) 0.3888(2) 0.0000 0.0000 0.1513(4) 0.1513(4) 0.148(1) 0.142(1) 0.0381(8) 0.2500
0.375 0.468 0.86 0.86 1.000 1.000 0.700 0.700 0.700 0.700
0.5000 0.5000 0.2333 0.0167 0.4667 0.0333 0.5000 0.5000 1.0000 0.5000
Space group Cmca; a ¼ 5.48014(4) A˚, b ¼ 12.4655(2) A˚, c ¼ 12.5316(2) A˚, V ¼ 856.07(1)A˚3. RBragg ¼ 6.81%, w2 ¼ 1.53.
ARTICLE IN PRESS M. Zaghrioui et al. / Journal of Magnetism and Magnetic Materials 313 (2007) 52–56
55
6J 9J 9J þ 2 exp þ 10 exp þ 4 exp kB T kB T kB T 7J 12J þ 6 exp þ 4 exp . kB T kB T
0.00750
TN =196 K
χ (emu/mol)
0.00725
In the case of Sr4Mn2.8Cr0.2O10, a paramagnetic contribution of chromium ions was added to the magnetic susceptibility of the isolated trimer. The total susceptibility is then expressed as: C (2) w ¼ ð1 pÞwtrimer þ p , T
0.00700
0.00675
0.00650 50
100
150 200 Temperature (K)
250
300
250
300
0.00750
χ (emu/mol)
0.00725 TN =176 K 0.00700
0.00675
0.00650 50
100
150 200 Temperature (K)
Fig. 4. The molar magnetic susceptibility for Sr4Mn3O10 (a) and Sr4Mn2.8Cr0.2O10 (b) as a function of temperature, measured in a field of 10 kG. The solid lines correspond to the calculated magnetic susceptibility.
Heisenberg–Dirac–Van-Vleck model. The denominator was modified according to the Curie–Weiss law to give: wtrimer ¼
Ng2 b2 A , 3kB ðT yÞ B
(1)
where 3J 2J A ¼ 141 þ 52:5 exp þ 15 exp kB T kB T 5J 6J J þ 126 exp þ 52:5 exp þ 1:5 exp kB T kB T kB T 6J 9J 9J þ 15 exp þ 1:5 exp þ 247:5 exp kB T kB T kB T 7J 12J þ 15 exp þ 52:5 exp kB T kB T
3J 2J 5J B ¼ 12 þ 6 exp þ 4 exp þ 2 exp kB T kB T kB T 6J J þ 8 exp þ 6 exp kB T kB T
where p is the fractional value. For the nominal composition, p is equal to 1/15 [p ¼ 0.2/(2.8+0.2)]. The least-square fitting routine was used to fit the experimental data to Eqs. (1) and (2) corrected for temperature-independent paramagnetic contribution, w0, and with assumed g value of 2. The best fits give the parameters: J/kB ¼ P13.63 K and P y ¼ 570 and R ¼ 5.3 104 [R ¼ (wcal–wexp)2/ (wexp)2], and J/kB ¼ 18.2 K, y ¼ 425 K, C ¼ 1.04, p ¼ 0.07 and R ¼ 2.6 104 for Sr4Mn3O10 and Sr4Mn2.8Cr0.2O10, respectively. The magnetic susceptibilities calculated by using the fit parameters are shown as the solid lines in Fig. 4a and b. The magnetic exchange interactions J are AFM in both samples, and they are very close. The Curie temperatures are in agreement with those obtained for the paramagnetic regime using Curie–Weiss law. Moreover, the p value is close to the fractional value of the nominal composition, which suggests a weak magnetic interaction between Cr and Mn4+. Finally, for Cr-doped materials, the Curie constant value obtained corresponds to two unpaired spins for each chromium cation, which indicates that the chromium oxidation state is Cr2+ in low spin configuration or Cr4+. According to the TGA, we can then suggest that the chromium, in Sr4Mn2.8Cr0.2O10, is present as Cr4+. 4. Conclusion Here, we present the synthesis, structure and magnetic study of manganates Sr4Mn3xCrxO10 (x ¼ 0 and 0.2). Manganates were obtained by solid state reaction in air. The XRD patterns were refined with Cmca space group. This structure is characterized by trimers formed by facesharing octahedra. Then the magnetic properties can be interpreted in terms of isolated trimers. In the case of Crdoped compounds, a paramagnetic contribution was added which corresponds to the magnetic contribution of chromium ions. The modelization gives AFM interactions between Mn4+ ions, high Curie temperature suggesting a strong AFM interaction and an oxidation state of Cr as Cr4+. References [1] B. Raveau, A. Maignan, V. Caignaert, J. Solid State Chem. 117 (1995) 424.
ARTICLE IN PRESS 56
M. Zaghrioui et al. / Journal of Magnetism and Magnetic Materials 313 (2007) 52–56
[2] A. Urushibara, Y. Moritomo, T. Arima, A. Asamitsu, G. Kido, Y. Tokura, Phys. Rev. B 51 (1995) 14103. [3] P.G. Radaelli, M. Marezio, H.Y. Hwang, S. Cheong, B. Batlogg, Phys. Rev. B 54 (1996) 8992. [4] Y. Moritomo, A. Asamitsu, H. Kuwahara, Y. Tokura, Nature 380 (1996) 141. [5] T. Kimura, Y. Tomioka, A. Asamitsu, Y. Tokura, Phys. Rev. Lett. 81 (1998) 5920. [6] D.N. Argyriou, J.F. Mitchell, J.B. Goodenough, O. Chmaissen, S. Short, J.D. Jorgensen, Phys. Rev. Lett. 67 (1998) 3380. [7] N.S. Witte, P. Goodman, F.J. Lincoln, R.H. March, S.J. Kennedy, Appl. Phys. Lett. 72 (1998) 853. [8] H. Asano, J. Hayakawa, M. Matsui, Phys. Rev. B 57 (1998) 1052.
[9] M.M. Cruz, M.D. Carvalho, A. Casaca, G. Bonfait, F.M. Costa, M. Godinho, J. Magn. Magn. Mater. 226–230 (2001) 800. [10] N. Floros, M. Hervieu, G. Van Tendeloo, C. Michel, A. Maignan, B. Raveau, Solid State Sci. 2 (2000) 1. [11] K. Boulahya, M. Parras, U. Amador, J.M. Gonzalez-Calbet, Solid State Ionics 172 (2004) 543. [12] K. Boulahya, M. Parras, J.M. Gonzalez-Calbet, U. Amador, J.L. Martinez, M.T. Fernandez-Diaz, Phys. Rev. B 69 (2004) 24418. [13] A. El Abed, E. Gaudin, J. Darriet, M.H. Whangbo, J. Solid State Chem. 163 (2002) 513. [14] S. Aygu¨n, S. Shiley, D.P. Cann, Mater. Lett. 58 (2004) 3645. [15] V.G. Zubkov, A.P. Tyutyunnik, I.F. Berger, V.I. Voronin, G.V. Bazuev, C.A. Moore, P.D. Battle, J. Solid State Chem. 167 (2002) 453.