Synthesis, structure and magnetic properties of (Eu_1−xMn_x)MnO_3−δ

Abstract

The solid solution (Eu_1−xMn_x)MnO_3−δ (0 ≤ x ≤ 0.126) has been synthesized using a conventional solid-state method. Powder X-ray diffraction and selected area electron diffraction data reveal that all samples crystallize in the space group Pnma. Magnetic measurements indicate that the temperature of the paramagnetic to antiferromagnetic phase transition increases with an increase of Mn in the Eu sites. The neutron diffraction data for ¹⁵³EuMnO₃ at 3 K confirmed that the magnetic structure of EuMnO₃ at low temperature is a canted A-type antiferromagnetic structure with the magnetic space group Pnma1′.

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Introduction

Studies of LnMnO₃ (Ln = Y, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) have been enhanced after the discovery of both magnetic and ferroelectric order (called multiferroics) in TbMnO₃,¹ YMnO₃,² and HoMnO₃.³ Lots of studies have focused on the doping of LnMnO₃ (ref. 4–10) at the Ln and/or Mn sites to find better multiferroics. Particularly, several reports indicate that Mn can be doped into the Ln site.^11–17 This may help us to understand that the properties of LnMnO₃ reported in the literature can be different because the composition may be different. Therefore, it should be very useful to have a systematic study on the relationship between the properties and the composition of Ln_1−xMn_xMnO₃. Although it is known that Mn can be doped into the Eu site of EuMnO₃,¹⁸ detailed studies are fewer. In addition, our previous work^15–17 has indicated that Tb_1−xMn_yMnO₃ can show very interesting magnetic properties. Therefore, careful studies into the synthesis, structure and magnetic properties of Mn doped EuMnO₃ are performed. The details are presented below.

Experimental

Samples with the nominal formula (Eu_1−xMn_x)MnO_3−δ (x = 0, 0.025, 0.05, 0.075, 0.1, 0.125 and 0.15) (named M1, M2, M3, M4, M5, M6 and M7, respectively) were synthesized using a traditional solid-state reaction from stoichiometric amounts of Eu₂O₃ (99.95%) or ¹⁵³Eu₂O₃ (99.995%), and MnCO₃ (A.R.). Here the formula is noted as (Eu_1−xMn_x)MnO_3−δ instead of (Eu_1−xMn_x)MnO₃ to highlight that the oxygen component is not exactly analyzed. Some oxygen vacancies may appear in the samples. The oven-dried reagents were mixed and homogenized through about thirty minutes of grinding with an agate mortar and a pestle. The mixtures were subjected to 6 h of calcination at 800 °C. They were then pressed into pellets to undergo four 12 h heat treatments at 1200 °C, followed by a furnace cooling every time with intermediate grinding. All the treatments were carried out under air. The weights of the samples were monitored before and after heat treatment. The maximum difference was about 4 mg for the 6 g samples. Therefore, the final compositions of the samples were considered the same as the initial ones. Powder X-ray diffraction (PXRD) data were collected on a PANalytical X'Pert³ Powder diffractometer with Cu Kα_α (λ₁ = 0.15405 nm and λ₂ = 0.15443 nm) radiation (2θ range: 5–120° for 2 h; step size: 0.0131°) at 40 kV and 40 mA at room temperature. Neutron powder diffraction (NPD) data for ¹⁵³EuMnO₃ were collected on the instrument Echidna at the OPAL reactor (Lucas Heights, Australia) at the Australian Nuclear Science and Technology Organization (ANSTO), at λ = 1.62150 Å. The X-ray diffraction data were analyzed using GSAS software.^19,20 The magnetic properties were investigated using a Cryogenic physical property measurement system (PPMS) from 2 to 300 K. Selected area electron diffractions (SAED) were carried out on a JEM2100F, with a 200 kV accelerating voltage. The X-ray photoelectron spectroscopy (XPS) patterns were obtained with a UK Kratos AxisUltra spectrometer with an Al Kα X-ray source operating at 15 kV and 15 mA. The chamber pressure was less than 5.0 × 10⁻⁹ torr. Electron binding energies were calibrated against C 1s emission at E_b = 284.8 eV.

Results and discussion

Solid solution of (Eu_1−xMn_x)MnO_3−δ

The X-ray diffraction patterns of the studied samples are shown in Fig. 1. The data for M1 to M6 are very similar, with the reflections moving systematically to a higher angle when more Mn is in the samples. These patterns agree well with that reported for EuMnO₃ (ref. 21) in the space group Pnma, which indicates that the space group Pnma may be fitted to describe the structure of these samples. After considering that the true structure for a Perovskite compound is very complex,^22–24 selected area electron diffraction (SAED) patterns of the above samples have been obtained and analyzed. The typical data are shown in Fig. 2, which confirm that the space group Pnma can be used to describe the structure of M1 to M6. Then the structure of EuMnO₃ is used to refine the corresponding data, with the supposition that some Mn occupies Eu sites, using the Rietveld method using GSAS. Good refinement is obtained for all of the data with R_wp ≤ 0.023 and R_p ≤ 0.013 (the corresponding data is listed in Tables S1 and S2 in the ESI†). Typical Rietveld plots are shown in Fig. 3. For M7, a small amount of Mn₃O₄ impurity was detected, which has been marked in Fig. 3d. Then a two phase model is used to refine the data for M7.

Fig. 1 Powder X-ray diffraction patterns of the samples with the nominal formula (Eu_1−xMn_x)MnO_3−δ. * reflections from the impurity.

Fig. 2 Selected area electron diffraction patterns of M5 along the [100], [011̄, and [11̄1] directions in the space group Pnma.

Fig. 3 Rietveld plots of the powder X-ray diffraction patterns for the samples M1 (a), M6 (b), and M7 (c and d). The symbol “+” represents the observed value, the solid line represents the calculated value, the marks below the diffraction patterns are the calculated reflection positions, and the difference curve is shown at the bottom of the figure.

As shown in Fig. 4, with an increase in Mn in (Eu_1−xMn_x)MnO_3−δ, the lattice parameters a and c decrease, and b increases, which agrees well with Vegard's law:^25,26

a_Ox = a_O0(1 − x) + a_O1x (1)

b_Ox = b_O0(1 − x) + b_O1x (2)

c_Ox = c_O0(1 − x) + c_O1x (3)

where a_Ox(b_Ox, c_Ox), a_O0(b_O0, c_O0), and a_O1(b_O1, c_O1) are the lattice parameters a (b, c) of orthorhombic (Eu_1−xMn_x)MnO_3−δ, pure orthorhombic EuMnO₃ and supposed orthorhombic MnMnO₃. x is the composition variable, given by Mn/(Eu + Mn). Using the data shown in Fig. 4 and eqn (1)–(3), the maximum value for x in the solid solution (Eu_1−xMn_x)MnO_3−δ is found to be 0.126. That is, the range for the solid solution (Eu_1−xMn_x)MnO_3−δ is 0 ≤ x ≤ 0.126. The lattice parameters of the solid solution (Eu_1−xMn_x)MnO_3−δ follow well the relationship , which indicates that the solid solution (Eu_1−xMn_x)MnO_3−δ belongs to an orthorhombic OI structure.²⁷

Fig. 4 Variation of the lattice parameters a (a), b (b), c (c) and the volume of the unit cell V (d) for the solid solution (Eu_1−xMn_x)MnO_3−δ.

XPS data for (Eu_1−xMn_x)MnO_3−δ

In order to assess the oxidation states of Eu and Mn in the samples, XPS analysis has been performed. The corresponding data are shown in Fig. 5. The XPS spectra for Eu 3d in the samples M1 to M6 are almost the same. The two peaks at around 1135.6 and 1165.2 eV are attributed to Eu 3d_5/2 and Eu 3d_3/2, which indicates that the valence state of Eu in the samples is +3.^28,29 The XPS spectra of Mn 2p are also almost the same for all six samples, as shown in Fig. 5b. The corresponding Mn 2p_3/2 peak is located at about 641.9 eV, which agrees well with the binding energy of Mn 2p_3/2 in α-Mn₂O₃.^30,31 Thus, the valence state of Mn is +3 in the solid solution (Eu_1−xMn_x)MnO_3−δ.

Fig. 5 XPS spectra of Eu 3d_5/2 (a) and Mn 2p_3/2 (b) for M1 to M6.

Magnetic properties

As reported previously,³² “negative magnetization” in the ZFC (zero field cooling) curve can be easily observed for M1 (EuMnO₃), which is attributed to the effect of a trapped negative field in the solenoid of a superconducting magnet. Two characteristic temperatures can be assigned to M1: the temperature where the ZFC and FC (field cooling) curves start to depart, which is attributed by us to the phase transition temperature from PM to AFM, noted as T_N1, which is 52 K; and the peak temperature in the ZFC curves, which is attributed to the phase transition temperature from AFM to cAFM, noted as T_N2, which is 45 K, as listed in Table 1. These values agree well with data reported by other researchers.^33–36

Table 1 The T_N1, T_N2, Curie constant (C), θ, μ_cal, and μ_exp for M1 to M6

Sample	M1	M2	M3	M4	M5	M6
TN1 (K)	52	53	55	58	64	70
TN2 (K)	45	42	32	40	42	42
C (emu mol^−1 K)	4.87	5.10	5.18	5.21	5.24	5.25
θ (K)	−84	−54.0	−48.6	−36.1	−19.6	−21.9
μcal (μB)	5.96	5.99	6.02	6.04	6.07	6.09
μexp (μB)	6.24	6.39	6.44	6.45	6.47	6.48

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With an increase of Mn in the samples, the “negative magnetization” in the ZFC curve disappears, as shown in Fig. 6. Similarly to M1 (EuMnO₃), two characteristic temperatures, T_N1 and T_N2, can be obtained from the temperature dependence of the magnetic susceptibility in the samples M2 to M6 in ZFC and FC modes, which are listed in Table 1. It is found that with an increase of Mn in the system, the PM to AFM phase transition temperature, T_N1, increases. This means that Mn is a good dopant to increase T_N1 in Eu_1−xMn_xMnO_3−δ. A similar phenomenon is also found for Mn doped TbMnO₃.¹⁷

Fig. 6 Typical temperature dependence of the magnetization of (Eu_1−xMn_x)MnO_3−δ: χ–T and χ⁻¹–T curves for M2 (a and d), M4 (b and e), and M6 (c and f), respectively.

The linearity of χ⁻¹(T) above the temperature T_N1 shown in Fig. 6d–f suggests that the reciprocal magnetic susceptibility follows the Curie–Weiss (CW) law, χ = C/(T − θ), where χ is the magnetic susceptibility of the paramagnetic phase, C is the Curie constant, T is the temperature, and θ is the Weiss constant. The corresponding Curie constants C, Weiss constants θ and effective paramagnetic moments are obtained and presented in Table 1. The effective magnetic moment calculated from the Curie constant C increases with an increase in Mn in the sample, and agrees well with the expected value under the supposition³⁷ that μ_eff² = (1 + x)μ_eff²(Mn³⁺) + (1 − x)μ_eff²(Eu³⁺), with μ(Mn³⁺) = 4.90 μ_B, and μ(Eu³⁺) = 3.40 μ_B. The obtained Weiss constant θ is negative, which indicates that the magnetic interaction is antiferromagnetic in nature.

The magnetic hysteresis loops at selected temperatures have been measured and typical data are shown in Fig. 7. The magnetic hysteresis loops show a clear butterfly shape at 10 K, indicating that the samples are metamagnetic at this temperature. The maximum magnetization value, at H = 7 T at 10 K, increases from 12.8 to 38.9 emu g⁻¹ with an increase in Mn content from 0.025 to 0.125, indicating that doping of Mn at the Eu site makes Eu_1−xMn_xMnO₃ easier to magnetize, which is similar to Eu_1−xY_xMnO₃ (ref. 4) and Eu_1−xSr_xMnO₃.³⁸

Fig. 7 The magnetic hysteresis loops for M2, M4 and M6 ((Eu_1−xMn_x)MnO_3−δ, x = 0.025 (a, d, g), 0.075 (b, e, h), and 0.125 (c, f, i)) at selected temperatures.

When the temperature increases, the butterfly shape of the M–H curve changes to a normal shape, as shown in Fig. 7d–f, which indicates that the samples become canted antiferromagnetic, rather than metamagnetic. At 60 K, a straight line is observed for M2 and M3, which agrees well with the finding that the PM to AFM phase transition temperature T_N1 for M2 and M3 is lower than 60 K, as listed in Table 1. But for M4, M5, and M6, the M–H curves are not straight lines at 60 K.

Neutron diffraction of ¹⁵³EuMnO₃

As mentioned previously,^32,39 it is hard to get neutron diffraction data for EuMnO₃ because the neutrons can be absorbed by Eu. Therefore, the magnetic structure of EuMnO₃ has not been solved until now, although T. Goto et al.³⁴ have predicted that it has a phase transition from PM to ICAFM (incommensurate antiferromagnetic) at ∼50 K, and a further transition from ICAFM to cAAFM (canted A-type antiferromagnetic) ordering at ∼44 K. It has been reported that neutron diffraction data can be obtained for ¹⁵³EuMnO₃,³⁹ where only data from around room temperature has been presented, which confirms that ¹⁵³EuMnO₃ crystallizes in the space group Pnma. Therefore, ¹⁵³EuMnO₃ has been synthesized in our lab and sent away for neutron diffraction data collection at 60 and 3 K. As shown in Fig. 8a, the neutron diffraction data for ¹⁵³EuMnO₃ at 60 K can be refined well using the space group Pnma with the parameters listed in Table 2, which may indicate that the structure of ¹⁵³EuMnO₃ does not change between 300 K and 60 K. However, new diffraction peaks are found in the neutron data from ¹⁵³EuMnO₃ at 3 K, which can be attributed to the A-type⁴⁰ magnetic ordering of Mn in ¹⁵³EuMnO₃, as shown in Fig. 8c with the refinement parameters listed in Table 2. In the first work on the magnetic structure of Mn in the doped perovskite LaMnO₃, E. O. Wollan et al.⁴⁰ established a note system of A, B, C, D, E, F, and G type to note the different possible magnetic structures of Mn. Specially, A-type magnetic ordering means that the magnetic moment arrangement of Mn is similar to that shown in Fig. 8c. That is, the magnetic moments of the four-close-neighboring Mn in the first and the second layer are inversed. As shown in Fig. 8c, the moment of Mn in the first layer directs to the positive c-axis, while the moment of Mn in the second layer directs to the negative c-axis. Our results confirm the prediction of a cAAFM phase for EuMnO₃ by T. Goto et al.³⁴ More careful studies are planned to confirm the ICAFM phase for ¹⁵³EuMnO₃.

Fig. 8 Rietveld plots of the neutron diffraction data from ¹⁵³EuMnO₃ at 60 K (a) and 3 K (b), and the magnetic structure of ¹⁵³EuMnO₃ at 3 K (c).

Table 2 Rietveld refinement details for the neutron diffraction data from ¹⁵³EuMnO₃ at 60 K and 3 K

	60 K	3 K
a Rp is sum(|I0 − IC|)/sum(I0), and Rwp is the weighted R factor for the neutron diffraction data.
Phase 1	Nuclear phase	Nuclear phase
Space group	Pnma	Pnma
Lattice parameters (Å)	a = 5.8217(2), b = 7.2138(3), c = 5.3242(8)	a = 5.8183(2), b = 7.2082(3), c = 5.3269(2)
Atom	x, y, z	x, y, z
Eu	0.0787(3), 0.2500, 0.9844(3)	0.0800(3), 0.2500, 0.9845(3)
Mn	0.0000, 0.0000, 0.5000	0.0000, 0.0000, 0.5000
O1	0.4722(3), 0.2500, 0.0957(3)	0.4678(3), 0.2500, 0.0974(3)
O2	0.3236(3), 0.0462(3), 0.7063(3)	0.3249(3), 0.0478(3), 0.7066(3)
Phase 2		Magnetic phase
Space group		Pnma1′
Lattice parameters (Å)		a = 5.8183(2), b = 7.2082(3), c = 5.3269(2)
Atom		(x, y, z)/(Mx, My, Mz)
Mn		(0.0000, 0.0000, 0.5000)/(−0.554, 0.190, 2.868)
R factor^a	Rwp = 0.033, Rp = 0.026	Rwp = 0.041, Rp = 0.031

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Conclusions

Solid solution Eu_1−xMn_xMnO_3−δ has been synthesized using a conventional solid state method at 1200 °C under air. The powder X-ray diffraction and selected area electron diffraction data confirm that all samples crystallize in the space group Pnma at room temperature. The range of x for the solid solution obtained in the present case is assessed to be 0 ≤ x ≤ 0.126. With the doping of Mn in the Eu site, the paramagnetic to antiferromagnetic phase transition moves to a higher temperature. In addition, the neutron diffraction data from ¹⁵³EuMnO₃ at 3 K confirmed that the magnetic structure of EuMnO₃ at low temperatures is canted A-type antiferromagnetic.

Acknowledgements

This work is supported by the National Key Basic Research Project of China (2010CB833103), and the National Natural Science Foundation of China (Grants 21271014, 11275012). We thank Dr M. Avdeev for assistance in collecting the neutron power diffraction data at the OPAL facility.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra25951k

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