Synthesis, Structure and Magnetic Properties of Layered Perovskite Sm1.5SrBa0.5Mn2O7

The layered perovskite oxide Sm1.5SrBa0.5Mn2O7 was synthesized by the conventional method of co-precipitation. Its structure has been solved by powder X-ray diffraction. The diffraction patterns are consistent with the I4/mmm symmetry, with tetragonal lattice parameters a=3.8398(7) Å and c=20.3814(5) Å. The structure of Sm1.5SrBa0.5Mn2O7 is similar to Sr3Ti2O7. Magnetic measurements were also performed in the temperature range 2–360 K. They showed a strong presence of antiferromagnetic interactions below Neel temperature TN=25 K. The variation of the magnetization with the magnetic field in a temperature of 5 K was also analyzed. No saturation was observed up to the high applied magnetic field of 10 T.

Numerous systematic studies were dedicated to investigate the structural, electronic and magnetic mechanisms in order to obtain a key tuning of observed magnetoresistance. However, we still believe that the origin of these transitions is related to the double-exchange interactions [13] which implies a ferromagnetic pairing between Mn 3+ (t 3 2g e 1 g ) and Mn 4+ (t 3 2g e 0 g ) ions and the Jahn-Teller effect [14]. One of the main obstacles preventing the use of these materials in magnetic industry is the requirement of a very strong magnetic field. Several experimental efforts were devoted to search for new materials with a high magnetoresistance in relatively weak fields [15,16].
Some previous works were about the manganites family of samarium-based double perovskite type [17,18]. The study of the Sm 1.4 Sr 1.6 Mn 2 O 7 showed that this compound presents simultaneously a metal-insulator (M-I) and a rough antiferromagnetic transition at a temperature close to 118 K [17]. Phases of the Sm 2−2x Sr 1+2x Mn 2 O 7 system (with x = 0.2, 0.4, 0.5) were also investigated; these studies revealed that the Curie temperature (T C ) decreases when increasing x, and the phase with x = 0.2 presents a metalinsulator transition (M-I) near T C , and a maximal value of magnetoresistance (MR) of 89.88 % at 105 K under an external field of 5 T [18]. Also, the substitution of Sr 2+ with Ba 2+ in La 1.4 (Sr 1−x Ba x ) 1.6 Mn 2 O 7 with (0.0 ≤ x ≤ 0.6) was found to lower the T C from 161 to 94 K, and to increase the values of magnetic entropy when increasing x [19].
In continuation of our previous work, we will present structural and magnetic studies of a new phase with general formula Sm 1.5 SrBa 0.5 Mn 2 O 7 . The sample was prepared by the co-precipitation method and characterized by X-ray diffraction (XRD). The magnetic measurements versus temperature and under applied magnetic field will be presented and discussed.

Synthesis
The co-precipitation method was employed to synthesize the Sm 1

Instrumental Analysis
The phase purity and homogeneity were determined from the powder diffraction pattern recorded at room temperature in the range of 2θ = 10-119 • with a step of 0.02. The crystalline structure was obtained from Rietveld refinement [20] using the FULLPROF code [21].
The magnetization measurements versus temperature and under applied magnetic field of 0.05 T were taken in the temperature range of 2-360 K, using a BS2 magnetometer developed at Néel Institute. Also, magnetizations at 5 K were measured under several magnetic fields up to 10 T.

Structural Characterization
The structure of the synthesized Sm 1.5 SrBa 0.5 Mn 2 O 7 sample was determined by X-ray powder diffraction. The structural refinement of this compound was done by the Rietveld method using Fullprof software. The results show that the sample is single phase and all diffraction peaks are indexed in the quadratic system isotypic to that of Sr 3 Ti 2 O 7 with the I4/mmm as space group.
We noticed that inside the structure of this compound, two separate sites exist that can host Sm 3+ , Ba 2+ , and Sr 2+ cations: the first one is the perovskite cage of dodecahedral coordination (site 2b) the other one is of a particular coordination as it is coordinated to nine oxygen atoms: 1 + 4 + 4 (site 4e). Also, we noticed that manganese ions Mn 3+ and Mn 4+ are located in an octahedral environment site 4e. A refinement of all parameters leads to the following reliability factors: R B = 7.26 % R F = 7.50 % χ 2 = 1.87. The refined parameters are: a = 3.8398(7) Å, and c = 20.3814(5) Å. The structure of Sm 1.5 SrBa 0.5 Mn 2 O 7 was generated by ATOMS software [22]. The XRD profile and the structure of Sm 1.5 SrBa 0.5 Mn 2 O 7 are respectively presented in Figs. 1 and 2. Table 1 summarizes the obtained crystallographic characteristics as well as the atomic positions, while the distances and interatomic angles are gathered in Table 3. To complete the structural study of this compound, we checked that the only existence rule on the planes hkl is h + k + l = 2n which implies the presence of a symmetry center in the lattice. The resulting X-ray diffraction lines are presented in Table 4.
A comparison of the unit cell parameters a, c and the ratio c/a as well as the volume of the structure of our compound with those of a similar structure Sm 1.6 Sr 1.4 Mn 2 O 7 is summarized in Table 2. It is concluded from this comparison that the parameters c, c/a and the volume of the lattice of our product are slightly larger than those of the similar structure Sm 1.6 Sr 1.4 Mn 2 O 7. This is partly due to a relatively large difference between the ionic radii of Ba 2+ (r = 1.61 Å) and Sr 2+ (r = 1.44 Å), and secondly, due to the t 3 2g d 1 z2 configuration of the Mn 3+ ion leading to the Jahn-Teller effect (elongation along z) which is a cooperative effect.

Magnetic Characterization
The magnetization was measured in a large temperature range between 2 and 300 K using a BS2 magnetometer at Néel institute. The related measurement is presented in Fig. 3. One can notice that the sample cooling under a weak magnetic field of 0.05 T provokes an increase of the magnetic moment and reaches a maximum at Néel temperature T N = 25 K, and then decreases to T = 2 K. This result suggests the presence of a strong antiferromagnetic interactions below Néel temperature T N = 25 K.
The variation of the inverse molar magnetic susceptibility 1/χ M versus temperature is reported in Fig. 4. The effective magnetic moment (μ eff ) was calculated from the linear zone at high temperatures using the Curie law. The value of μ eff is found to be equal to 6.70μ B , the magnetic moment of manganese μ Mn was calculated from the total effective moment and theoretical magnetic moment of samar-      ium ion Sm 3+ using the relation proposed by Subramanian et al. [23]: μ 2 eff = n 1 μ 2 (Sm 3+ ) + n 2 μ 2 (Mn) where n 1 = 1.5 and n 2 = 2.0 are the numbers of samarium ion Sm 3+ and manganese ion in mixed Mn 3+ and Mn 4+ , respectively. The μ(Sm 3+ ) is the theoretical magnetic moment (0.84μ B ) of samarium ion Sm 3+ and μ(Mn) is the magnetic moment of manganese. We obtained a value of 4.63μ B for μ(Mn). Using the latter value of μ(Mn) and the theoretical values of moments for both Mn 3+ and Mn 4+ , which are 4.9μ B and 3.9μ B , respectively, we found 78 % and 22 % of Mn 3+ and Mn 4+ ions, respectively. Such percentages are close to the theoretical percentages 75 % and 25 % of Mn 3+ and Mn 4+ , respectively.
The Curie-Weiss temperature T C = 110 K was determined by applying a linear extrapolation to the curve presenting the inverse of molar magnetic susceptibility   of saturation even with strong magnetic fields (the maximal magnetic field ≈10 T). This phenomenon can be explained by the fact that Mn 3+ , Mn 4+ and Sm 3+ ions are the elements carrying magnetic moments. This should lead to a competition of magnetic interactions between these ions at a low temperature. Similar behavior was observed in the Er 2 Mn 2 O 7 [24] compound in which magnetic interaction between erbium ions Er 3+ and manganese Mn 4+ takes place.

Conclusions
A new phase with the composition Sm 1.5 SrBa 0.5 Mn 2 O 7 has been synthesized by conventional co-precipitation method. Its structure has been determined by the Rietveld analysis of XRD data. The results point out to the tetragonal lattice with the I4/mmm as space group. The magnetic measurements revealed that the compound shows antiferromagnetic behavior with a Néel temperature of 25 K. No magnetization saturation was observed even when the magnetic field increased up to 10 T. The presence of Mn in different valence states was confirmed by magnetic susceptibility measurements. Results are in good agreement with structural data.