The effect of strontium non-stoichiometry on the physical properties of double perovskite Sr2FeMoO6

Journal of Solid State Chemistry 222 (2015) 115–122

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The effect of strontium non-stoichiometry on the physical properties of double perovskite Sr2FeMoO6 L. Harnagea n, P. Berthet LPCES–ICMMO, UMR 8182 CNRS, Université Paris-Sud XI, 91405 Orsay Cedex, France

art ic l e i nf o

a b s t r a c t

Article history: Received 13 August 2014 Received in revised form 9 November 2014 Accepted 18 November 2014 Available online 26 November 2014

We present a detailed study on the effect of strontium non-stoichiometry on the properties of double perovskite Sr2FeMoO6 (SFMO). A citrate route has been used to prepare high quality polycrystalline samples with (2
x)Sr:Fe:Mo (x ¼0, 0.02, 0.04) and (2 þ x)Sr:Fe:Mo (x ¼ 0.02, 0.04, 0.10) cationic ratios. The strontium deficient samples exhibit a significant decrease in their values of saturation magnetization (Ms), Curie temperature (Tc) and magnetoresistance (MR) compared to the stoichiometric SFMO. On the other hand, the samples prepared with an excess of strontium, respectively those with x ¼ 0.02 and 0.04, show remarkably superior magneto–transport characteristics, despite of their increased level of Fe/Mo disorder and somewhat diminished magnetic properties compared to the stoichiometric SFMO. We also show that these samples exhibit superior MR values under low magnetic field and persisting up to temperatures as high as 400 K. & 2014 Elsevier Inc. All rights reserved.

Keywords: Double perovskite Citrate route Magnetoresistance Magnetic properties

1. Introduction Since 1998, when I. Kobayashi et al. predicted the half–metallic nature of the ferromagnetic Sr2FeMoO6 (SFMO), there has been a considerable interest to understand and improve the roomtemperature magnetoresistance of this compound which has several potential applications including its use in data storage devices. The compound exhibits a Curie temperature (Tc) of about 400 K and presents the advantages of large room–temperature magnetoresistance due to intergrain tunneling of the spin–polarized electrons through the grain boundaries [1,2]. However, despite these remarkable properties several issues have remained poorly understood and largely unsolved. For example, the exact role of non-stoichiometry, anti-site disorder, nature of the grain boundaries (i.e., their chemical composition and thickness) and their connectivity, etcetera still deserve investigations. In a previous article, we tried to address some of these issues in nominally stoichiometric SFMO samples synthesized using a wet-chemical method [3]. In the present article we discuss the role of Sr off-stoichiometry on the low-field magnetoresistance (LFMR) above room temperature, which is an important issue concerning practical applications that we alluded to above. In particular, we investigated the properties of samples prepared with (2
x)Sr:Fe:Mo (x¼0.02, 0.04) and (2þ x)Sr:Fe:Mo (x¼0.02, 0.04 and 0.10) cationic ratios, in order to understand the influence of Sr off-stoichiometry on the magnetic and magneto-resistive properties

n

Corresponding author. E-mail address: [email protected] (L. Harnagea).

http://dx.doi.org/10.1016/j.jssc.2014.11.017 0022-4596/& 2014 Elsevier Inc. All rights reserved.

of SFMO. Here we aim to identify the changes in the magnetic and magnetoresistive properties of SFMO due to self-substitution at the Sr site. Prior to this work, most previous studies dealt with electronic modification induced by substitution of strontium with monovalent (for example, Na þ and K þ ) [4,5], iso-valent (for example, Ca2 þ and Ba2 þ ) [6] and tri-valent (for example, La3 þ ) [7] metal ions. We come across with only one study dealing with strontium non-stoichiometry (Sr2
xFeMoO6 (0 rx r0.4) [8]. In this study the authors reported a drastic decrease of Curie temperature and saturation magnetization when the holes are introduced in the system. We should mention that they studied samples with a nonstoichiometry in strontium higher than 10% and we are interested in identifying the changes in the physical properties when the strontium deficit is smaller than 10%. Similarly, very scarce studies are dealing with self-substitutions type Sr2FexMo2
xO6 (0.8rxr1.2, [9,10] or 0.8rxr1.5, [11]), Sr2Fe1þ xMo1
xO6 (
1rxr0.25, [12]) or Sr2FeMo1
xO6 (xr0.06, [13]). It has been found for Sr2FexMo2
xO6 compounds [9–11] that the values of the order parameter, saturation magnetization and magnetoresistance are the highest and are decreasing as x deviates from 1. Similarly, Sr2Fe1þ xMo1
xO6 compounds [12] exhibit lower values of the Curie temperature and saturation magnetization once x deviates from zero. On the other hand, a small amount of Mo vacancies seems to improve the magnetic properties as shown in SrFeMo1
xO6 by Lü et al. [13]. Analogously, only few studies are dealing with oxygen nonstoichiometry in Sr2FeMoO6. To the best of our knowledge, the only experimental study analyzing more than one sample of Sr2FeMoO6
δ (0.006rδr0.091) was reported by R. Kircheisen et al. [14]. They

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observed that the unit cell volume and the antisite concentration increases with increasing the oxygen non-stoichiometry in Sr2FeMoO6
δ samples. In addition to increased antisite disorder, the oxygen non-stoichiometry decreases further the magnetization at saturation.

resistance and magnetoresistance measurements, conventional fourprobe technique was used, both, in a home-built set-up and in the cryostat chamber of the SQUID magnetometer. Magnetoresistance was defined in percentage using the following expression: MR%¼ ((RH
R0)/R0)  100, where RH and R0 represent the resistance values in presence and in absence of the magnetic field, respectively.

2. Experimental details 3. Results and discussions 3.1. Crystal structure and microstructure The XRD powder diffraction patterns of each sample, recorded at room temperature, are shown in the Fig. 1 using a logarithmic scale in order to better evidence eventual low intensity peaks revealing a secondary phase. Fig. 1 show that the parent compound SFMO (notated as Sr2.00 in the figure) is prepared as a single phase with a tetragonal crystal structure (space group I4/mmm) in agreement with most previous reports (see, for example, ref [3]). The compositions with strontium deficit, mainly the 2% strontium deficient sample (notated Sr1.96 in the figure), contain an impurity phase which could be identified as Fe2Mo. The compositions prepared using a small excess of strontium (Sr2.02, Sr2.04 and Sr2.10)) contain SrMoO3 as the main impurity phase. The amount of this impurity phase in the samples increases with the increase of strontium excess (Sr2.02 to Sr2.10 in Fig. 1). Additionally, in the sample Sr2.10 we also observed the presence of other impurity phases including Fe2Mo and FeO (Fig. 1). It should be mention here that the parasitic phase SrMoO3 and also to some extent SrMoO4 and Sr2MoO6 were reported in some previous works for stoichiometric SFMO samples (for example, ref. [17]). The presence of these impurity phases in such cases indicates that the main phase SFMO had formed with a slight deficit in molybdenum. A careful inspection reveals that the XRD patterns of the main phase present in the samples prepared with a deficit or with an excess of strontium are similar to that of SFMO (Fig. 1). However, the intensity of the superstructure peak (101) is reduced with respect to 100 10 1

Relative XRD Intensity%

All the samples discussed in this manuscript were prepared using a citrate route (for details, see [3]). Precursors Sr(NO3)2, Fe(NO3)3n9H2O, (NH4)6Mo7O24n4H2O and C6H8O7 were dissolved in distilled water to prepare the initial solutions with concentrations fixed at 0.24 mol/L Sr2þ , 0.12 mol/L Fe3þ , 0.12 mol/L Mo6þ and 1.44 mol/L C6H8O7. These solutions were then mixed to obtain a master solution with the desired chemical composition which was used further to prepare a particular sample. In the master solution the number of moles of citric acid (nC6H8O7) with respect to the total number of moles of cations (i.e., 3þ 6þ nþ nnMþ ¼n2þ Sr þnFe þnMo (moles)) was kept at 6 (i.e., nC6H8O7/nM ¼ 6) and the pH of the solution was maintained at 1.25. These particular conditions (pH, nC6H8O7/nnMþ ) were chosen after a careful analysis of samples obtained using different other conditions (like for example, nC6H8O7/nnMþ ¼2, 4, 6 in an acid or basic solution). In our work we observed optimized values for the physical properties of SFMO samples prepared with nC6H8O7/nnMþ ¼6 and a pH of 1.25. The master solution was evaporated relatively slowly using three different heating rates: 70 1C/3 h, 100 1C/3 h, 150 1C/1 h on a laboratory hot plate equipped with a magnetic stirrer. At the end of the evaporation process, we observed the formation of a gel which was dried at 250 1C. At this temperature the gel starts swelling, emanating gases, and slowly transformed itself into a black solid foam containing organic residues. This solid foam was then decomposed in air at 800 1C for 2.5 h resulting in a powder containing SrMoO4 and SrFeO3
x. The resulting powder was finely ground, pelletized and submitted to heat treatments at 980 1C for 10 h followed by 1080 1C for 10 h under a reducing atmosphere of Ar with 10% H2. The flow of Ar with 10% H2 was fixed at 210 mL/h during the first heat treatment which was performed on a batch weighting typically 2.5 g. Under these conditions the samples, particularly those prepared with an excess of strontium, contained significant quantities of SrMoO4 as an impurity phase (this compound typically appears when the reaction medium is too oxidant). To eliminate the impurity phase SrMoO4, which typically appears when the sintering atmosphere is not sufficiently reducing; during the second heat treatment we increased the Ar-10% H2 gas flow to 1600 mL/h from 210 mL/h in the previous heat treatment. The powder X-ray diffraction pattern of the sample obtained after second heat treatment did not show the presence of SrMoO4 secondary peaks. For an easier and clearer presentation of the data we will refer to our samples as Sr1.98 and Sr1.96 for the strontium deficient compositions, as Sr2.00 for the parent compound and as Sr2.02, Sr2.04 and Sr2.10 for those with a strontium excess. Structural characterizations of the synthesized samples were carried out using powder X-ray diffraction (XRD) patterns recorded with a Panalytical X’Pert Pro MPD diffractometer equipped with a primary beam Johansson monochromator, providing Cu Kα1 radiation. Parameters refinements were carried out using Rietveld or Le Bail [15] methods with the GSAS (General Structure Analysis System) program [16]. The microstructure and the stoichiometry of the samples were accessed using a Scanning Electron Microscope (SEM) (Philips XL 30 (SEMFEG)) equipped with an energy-dispersive x-ray spectroscopy probe (EDX). The chemical composition was determined on wellpolished surfaces and the microstructure was observed on fractured surfaces of the samples. Magnetic properties were measured using a SQUID magnetometer (Quantum Design MPMS 5). For electrical

100 10 1 10

20

30

40

50

60

70

80

90 100

Θ Fig. 1. (Color online) XRD powder diffraction for the samples prepared with a deficit of strontium (Sr1.98, Sr1.96), the parent compound SFMO (Sr2.00) and the samples with a strontium excess (Sr2.02, Sr2.04, Sr2.10).

244.8 5.572

a = b ( ) 7.902 c( ) 7.900

5.570 5.568

1.96 1.98 2.00 2.02 2.04 2.06 2.08 2.10

7.898 7.896

Sr2±x

224 400 215 323 411 006 314 330 402 116 420 332 305 413 206 422 404 107 325 431 501 226 510 334 316 424 512 217 415 521 433 503

004 220 213 301 114 310 222 204 312 105 321 303

RWP(%) - 6.49

20

40

224 400 215 323 411 006 314 330 402 116 420 332 305 413 206 422 404 107 325 431 501 226 510 334 316 424 512 217 415 521 433 503

204 312 105 321 303

004 220 213 301 114 310 222

101 002 110

103 211 202

RP(%) - 4.77

60

80

100

2θ (deg)

112 200

exp simulation 2

χ - 1.96

RWP(%) - 9.01 RP(%) - 6.76

20

40

60

80

100

2θ (deg) Fig. 3. (Color online) Rietveld refinement of the diffraction pattern for Sr- deficient samples Sr1.98 and Sr1.96 and for the parent compound Sr2.00. In the figure are represented: the experimental data and the simulated curve (black points and red line), the difference between experimental and calculated profile (blue line), the baseline which has been subtracted (green line), the calculated Bragg positions (magenta sticks) and the diffraction peaks indexing. The quality factors RP(%), RWP(%) and χ2 are indicate for each sample.

0.045 0.040 0.035

I101/(I200+I112)

2

224 400 215 323 411 006 314 330 402 116 420 332 305 413 206 422 404 107 325 431 501 226 510 334 316 424 512 217 415 521 433 503

245.0

100

χ - 2.27

004 220 213 301 114 310 222 204 312 105 321 303

245.2

80

exp simulation

103 211 202

0.02

Sr2.10

Sr1.98 Sr1.96

60

112 200

Sr1.98

101 002 110

0.03

40

RP(%) - 3.94

2θ (deg)

Intensity (arb.units)

Sr2.02 Sr2.04

c( )

) I101/(I200+I112) a=b( )V(

Sr2.00

2

χ - 1.83

103 211 202

20

Sr2.00 0.04

exp simulation RWP(%) - 5.10

101 002 110

Intensity (arb.units)

Sr1.96

Intensity (arb.units)

the same peak in SFMO (Sr2.00 in the Fig. 1), which indicates a higher degree of Fe/Mo disorder in the structure. Since the degree of Fe/Mo order can be related to the increase in intensity of the superstructure peak (101), the degree of order in the structure can be estimated by comparing the integrated intensity of the superstructure peak (101) and the principal peak (110þ 200). Although, the integrated intensity of these peaks may be related also to factors such as the grain size and their crystallinity, to instrumental errors, to inappropriate preparation of the sample measured, such as the grinding of the sample, the preparation of the sample holder, etcetera, it is a safe qualitative method to estimate the degree of order in SFMO since: (i) we compare the rapport of the integrate intensities of the peaks and (ii) to a good extent we could exclude the influence of these factors by preparing the samples under identical conditions (duration of grinding, use of spinner type sample holder, quantity of sample used, data acquisition time, etc.) for powder x-ray diffraction experiments. Furthermore, the samples reported in this work exhibit similar microstructure. The working parameters of the X-ray diffractometer (for example, the alignment) was checked on a regular basis. Fig. 2a shows the variation of I101/(I110 þ I200) for all the samples. As expected, a systematic decrease in the relative intensity for both series of compounds prepared with deficit or excess of strontium is observed. Rietveld refinements of the X-ray diffraction data collected at room temperature were performed using the tetragonal crystal structure with the space group I4/mmm for Sr1.96, Sr1.98 and Sr2.00 samples (Fig. 3). For the samples Sr2.02, Sr2.04 and Sr2.10, we refined only the lattice parameters using the Le Bail method [15]. The Rietveld refinement of the X-ray diffraction data for these samples is complicated and may be inaccurate since the diffraction peaks due to secondary phase are small and are in close proximity of the most intense peak due to SFMO.

117

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L. Harnagea, P. Berthet / Journal of Solid State Chemistry 222 (2015) 115–122

0.030 0.025 0.020 0.015 0.010 0.005 0.04

0.08

0.12

0.16

0.20

0.24

ASD/f.u Fig. 2. (Color online) (a) Relative integrated intensity of the superstructure peak; (b) volume of the unit cell;(c) lattice parameters in function of the nominal stoichiometry in strontium and (d) correlation between the ASD obtained from Rietveld refinement for several SFMO samples and their relative integrated intensity of the superstructure peak.

The Rietveld refinement, which gives a reasonably good estimate of the antisite disorder (ASD) in the structure [3], confirmed that Sr1.98 and Sr1.96 samples exhibit a higher ASD than that of SFMO (Sr2.00). An increase of ASD from 5.8% for SFMO to 13.4%, 16.5% for Sr1.98, Sr1.96, respectively, is observed. The higher concentration of ASD in Sr1.98, Sr1.96 samples is likely due to strontium vacancies that favor preferential occupation of surrounding B/B’ sites by the cation having higher formal valence (i.e., Mo5 þ ). As shown in Figs. 2b and c, the lattice parameters decrease compared with those of stoichiometric SFMO for all samples prepared with strontium off-stoichiometry. The decrease of the unit cell volume for these compositions can have a steric origin due to

L. Harnagea, P. Berthet / Journal of Solid State Chemistry 222 (2015) 115–122

cations vacancies and an electronic origin due to valence change of the Fe-Mo couple as discussed in more details in the later part of the manuscript. However, it is very difficult to estimate the changes of the lattice parameters induced by steric effects in a perovskite structure due to additional contributions such as, for example, unscreened electrostatic repulsions between the anions that surround the vacancy, oxygen displacement, etcetera. The reduction of the lattice parameters of Sr1.98, Sr1.96 samples compared with SFMO (Sr2.00) is quite similar to that of samples prepared by substituting Sr2 þ with Naþ [4]. If we suppose that these compounds are stoichiometric in oxygen, in the presence of a vacancy, the valence of Fe–Mo couple should increase to insure the electrical neutrality of the formula unit. As a consequence the distance between these cations will decrease due to stronger electrostatic attraction with the intervening oxygen ion. In the case of the samples prepared with an excess of strontium, the XRD patterns (Fig. 1) show the presence of SrMoO3 as the dominant secondary phase. If one assumes that this phase results from the excess strontium used in the reaction, the main phase should form with a chemical composition Sr2FeMo1
xO6 where x is the concentration of strontium taken in excess during sample preparation. This assumption is appropriate to describe the Sr2.02 and Sr2.04 samples but in the case of the Sr2.10 sample which also exhibits small amounts of two other impurity phases a more complicated picture must be considered with an undefined Fe:Mo ratio. As in the case of the samples with strontium deficit, molybdenum non–stoichiometry can explain the observed decrease of the lattice parameters. Like previously, the existence of molybdenum vacancies in the structure implies that the Fe-Mo valence increases and the relative distance between these cations decrease and consequently the lattice parameters are reduced. We should mention that the reduction of the lattice parameters for the Sr2.02 and Sr2.04 samples (Figs. 2b, c) is similar with that observed for compounds prepared with molybdenum deficiency by Lü et al. [13]. We observed that the lattice parameters for the Sr2.10 sample are not very different from those of Sr2.04 and that the ASD disorder in these samples prepared with an excess of strontium looks to be quite similar, as can be seen in the Fig. 2a where I101/(I110 þ I200) doesn't show a significant variation from sample to sample. In order to quantify the ASD concentration in these samples, the quantity I101/(I110 þI200) may be expressed in the units of ASD as typically reported using the Rietveld refinement. To achieve this, we plotted the ASD concentration obtained by Rietveld refinements and also those from the ratio I101/ (I110 þ I200) extracted from the experimental diffractogram for several of our SFMO samples (Fig. 2d). A linear correlation is found between the two which allowed to estimate an ASD concentration of about 11% to 12% for Sr2.02, Sr2.04 and Sr2.10 samples. We should also mention here that our plot which correlates the ASD with I101/(I110 þI200) is in good agreement with the one reported by Balcells et al. [18].

The SEM images of samples Sr1.98, Sr2.00 and Sr2.02 are shown exemplarily in Fig. 4. For the samples prepared with a deficit in strontium (Sr1.98, Sr1.96), we observed, on the SEM images taken on the fractured surfaces, areas with grains well connected having an average size of about 1.4 mm and areas presenting a high level of porosity. These samples present microstructural characteristics similar to those of the reference sample Sr2.00. On the other hand, the samples prepared with an excess in strontium Sr2.02, Sr2.04 and Sr2.10 present slightly different microstructural characteristics compared (Figs. 4b, c) to Sr2.00. These samples displayed, as shown exemplary in Fig. 4c, grain size ranging between less than 0.5 mm and about 1.5 mm. The secondary phase SrMO3 could not be identified in any of the SEM images which is most likely due to the fact that its average atomic number is rather similar to that of SFMO. However an inhomogeneous distribution of this impurity cannot be discarded. The EDX analysis performed on the Sr non-stoichiometric samples remained inconclusive since the changes in the stoichiometry of our samples which we intend to detect are within the error limit (1 to 2 at. %) of the technique. 3.2. Magnetic properties The hysteresis curves recorded at 5 K for all the samples are shown in Fig. 5 and present a coercive field (Hc) of approximate 9576 Oe, with the exception of Sr2.10 which has a Hc of 12576 Oe, and a remnant magnetization ranging between 0.15 and 0.4 mB/f.u. Fig. 5 also shows that the magnetization of samples Sr1.98, Sr1.96

4

2

M (μB/f.u)

118

0

Sr1.96 Sr1.98 Sr2.00 Sr2.02 Sr2.04 Sr2.10

-2

-4

-50 -40 -30 -20 -10

0

10 20 30 40 50

H (kOe) Fig. 5. (Color online) Magnetization vs. magnetic field recorded at 5 K. Inset shows the values of magnetization in function of strontium concentration used to prepare the samples. The estimate error of the measurement it is around 0.02 mB/f.u. for each sample.

Fig. 4. SEM images of a) Sr1.98; b) Sr2.00 and c) Sr2.02 samples.

L. Harnagea, P. Berthet / Journal of Solid State Chemistry 222 (2015) 115–122

M ð1Þ S ¼ 4
8 U ASDðμB =f :u:Þ

ð1Þ

This equation may be applied to calculate the saturation magnetization of strontium deficient Sr1.98 and Sr1.96 samples if there is no modification of the valence of the Fe-Mo coupl (this is the case when oxygen vacancies compensate strontium ones). Conversely, a deficit of strontium without the formation of oxygen vacancies leads to an increase of the valence of the Fe-Mo couple which in the framework of the FIM model is fully borne by the Mo ions. In this case, Eq. (1) can be modified as followed in order to take into account this effect: M ð2Þ S ¼ 4 þ Δ
ð8 þ 2 U ΔÞ U ASDðμB =f :u:Þ;

ð2Þ

where Δ represents the increase of molybdenum valence (Δ ¼2x where x is the concentration of strontium vacancies, so Δ is 0.04 for Sr1.98 and 0.08 for Sr1.96). Eq. (2) gives values slightly higher than Eq. (1) (Table 1). Both are close to the experimental values for Sr1.98 but lower for Sr1.96. The higher experimental value of Ms obtained for Sr1.96 compared to Sr1.98 (Table 1) could be related to the presence of a small amount of ferromagnetic parasitic phase Fe2Mo in the sample as observed by XRD. Sánchez et al. [8] in their study of Sr-deficient samples also observed a drastic decrease of Ms values with the increase of Sr non-stoichiometry. As mentioned previously the samples prepared with an excess in strontium (Sr2.02, Sr2.04 and Sr2.10) are actually SFMO double perovskites with molybdenum vacancies. In this case the experimental saturation magnetization Ms should be compared to the calculated one using a model which has to take into account: (i) the effect of Fe/Mo ASD in the structure, (ii) the effect of molybdenum vacancies and (iii) the change in the valence of Fe–Mo couple induced by them. If we suppose that all the strontium excess is reacting with molybdenum to form SrMoO3 and that there is no oxygen vacancies, the chemical composition of the main phase for the samples Sr2.02 and Sr2.04 is Sr2FeMo1
xO6, where x represents the concentration of molybdenum vacancies, which is equal to that of the strontium excess, respectively 0.02 and 0.04. If only the molybdenum cations change their valence in order to assure the electrical neutrality of the phase Sr2FeMo1
xO6 and if we are taking in account the molybdenum vacancies and the ASD the FIM model Table 1 Experimental saturation magnetization (Ms) and calculated values using the ferrimagnetic model (FIM). Sample

ASD

Mexp S (μB/f.u)

M(1) S (μB/f.u)

M(2) S (μB/f.u)

Sr1.96 Sr1.98 Sr2.00 Sr2.02 Sr2.04 Sr2.10

0.165 0.134 0.058

3.00 2.96 3.65 3.55 3.47 3.14

2.68 2.93 3.54

2.73 2.96

 0.11  0.11  0.12

M(3) S (μB/f.u),

from Eq. (1) changes as: M ð3Þ S ¼ 4 þ6 Ux
ð8 þ 12U xÞ UASDðμB =f :uÞ:

ð3Þ

In the case of Sr2.10 sample, the lattices parameters, the estimated Fe/Mo order in the structure and the sample purity obtained from XRD diffractogram indicate that the main phase formed in this case has a chemical composition similar to that of Sr2.04. As expected, some differences are observed between the experimental and calculated Ms which certainly are related to the estimative evaluation of ASD and molybdenum vacancies, and to the fact that an unknown quantity of molybdenum vacancies may be found on iron sites. In order to estimate the accuracy of this model several Sr2FeMo1
xO6 compositions should be prepared and carefully studied. The Curie temperatures (Tc) of all the samples investigated are obtained from the magnetization versus temperature M(T) curves recorded under H ¼1000 Oe (Fig. 6), as the inflection point (minimum of the first derivative). All the samples are ferromagnetic with values of Tc higher than 385 K (Fig. 6). The value of Tc (inset Fig. 6) is reduced by approximately 6% for Sr1.98, Sr1.96 samples compared to Sr2.00 (Tc¼ 411 K), while for the samples prepared with an excess of strontium the reduction of Tc is only about 2% to 4%. These variations of Tc are related in a complex way with several factors linked to structural, electronic and chemical properties of the sample. It was proposed [20,21] that the delocalized electrons of molybdenum, coupled antiferromagnetically (AFM) with the 3d5 localized electrons of iron, mediate the magnetic coupling leading to a ferromagnetic interaction between the iron cations. Taking into account the mean field theory the intensity of the ferromagnetic interaction is governed by two parameters: (i) the exchange energy between the cores spins (Fe) and those of itinerants electrons (Mo–4d1) and (ii) the number of electrons per formula unit in the spin down band. The increase of the number of electrons in the conduction band (spin-down band), obtained by the substitution of the alkaline earth metal “A” by a trivalent rare earth metal leads to an augmentation of Tc [7,22–24]. As previously mentioned in the manuscript all the non-stoechiometric compositions discussed here exhibit cationic vacancies, therefore, in order to assure the electroneutrality of the formula unit, the molybdenum valence should be greater than 5 (5þΔ, Δ¼2x-for the samples prepared with a deficit of strontium, respectively Δ¼5x-for those prepared with excess strontium, where x represents the concentration of cationic vacancies) assuming the valence of iron unchanged. As a consequence the average magnetic interactions are weaker leading to a decrease in Tc value. Following this approach strontium deficit samples, Sr1.98 and Sr1.96 should 28 24

M (emu/g)

and Sr2.10 does not reach complete saturation even at 50 kOe. This behavior may arise due to a higher degree of ASD in the structure. As shown in Fig. 5 and its inset the values of saturation magnetization (Ms) decrease on both sides around the stoichiometric composition Sr2.00 and the most pronounce decrease of Ms is recorded for the samples prepared with strontium deficit (Sr1.96, Sr1.98). The theoretical value of Ms for a stoichiometric, perfectly ordered, sample of SFMO is 4 mB/f.u., as expected for antiferromagnetically coupled Fe and Mo sublattices. Using the ferrimagnetic model (FIM) (the cations of iron or molybdenum placed in antisite are coupled antiferromagneticaly with their neighbors [18,19]), we can estimate the effect of ASD on the values of Ms employing the equation

119

20 16 Sr1.96 Sr1.98 Sr2.00 Sr2.02 Sr2.04 Sr2.10

12 8 4 0 300

350

400

450

500

T(K)  3.21  3.31  3.22

Fig. 6. (Color online) Evolution of the magnetization vs. temperature under an applied magnetic field of 1000 Oe for all the samples. Inset shows the values of Tc in function of the nominal strontium stoichiometry. The estimate error of the measurement it is less than 1.5 K for each sample.

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exhibit Tc values superior than those of samples Sr2.02 and Sr2.04, which is contrary to the experimental observations. Therefore, it appears necessary to invoke other factors to account for the observed variation of Tc (Fig. 6) such as the effect of ASD. Since the lowest value of Tc occurs for the samples with the highest ASD concentration, it appears that these defects have a negative influence on Tc. It is worth noting that if the Tc values are determined by extrapolating the magnetization versus temperature to zero magnetization then all the values come out to be higher than 400 K. This criterion to determining Tc looks to be more adequate and in line with the magneto-resistive properties for Sr1.98, Sr1.96 samples.

3.3. Electric and magneto-resistive properties The samples resistivity normalized to its value at 300 K (ρ(T)/ρ (300 K)) as a function of temperature is shown in Fig. 7. The evolution of the resistivity as a function of temperature is roughly similar for all the samples with the exception of Sr2.10 sample which presents a more pronounced variation (Fig. 7). As Sr2FeMoO6 is a half-metal, the d electrons of the iron cations belonging to the spin-up band which is full do not participate significantly to the electrical conduction, a task carried out by the delocalized electrons of the molybdenum cations belonging to the spin-down band. As discussed before, the two series of samples under investigation exhibit cationic vacancies either in strontium (Sr1.98 and Sr1.96 samples) or molybdenum (Sr2.02, Sr2.04 and Sr2.10 samples). Based on the partial densities of states investigates by Kuepper et al. [25], when small amount of hole is introduced into the compound, both the density of state of molybdenum and iron decrease, but the density of state of molybdenum decreases faster than density of state of iron. On the other hand, it was argued and experimentally proved that the Mo-band states at the Fermi level become gradually filled with electrons upon electron doping [7,23]. Based on these observations we will consider, although further experiments are needed to verify this conjecture, that it is the average molybdenum valence that increases in order to maintain the electrical neutrality, thereby reducing the number of electrons in the conduction band. Hence, the intrinsic grain resistivity is expected to increase for both series of compounds. Moreover, the resistivity will increase with the increase of any other defects (i.e., ASD) in the structure. However, due to the extrinsic contributions (grain boundaries and their characteristics such as connectivity, chemical composition, etc.) it is impossible to evaluate the intrinsic contribution due to vacancies and ASD. ρ300 Κ (Ω∗cm)

Sr1.96,0.014 Sr1.98,0.041 Sr2.00,0.038 Sr2.02,0.131 Sr2.04,0.126 Sr2.10,0.035

1.6

1.4

a

0 Sr2.02 Sr2.04 Sr2.10

Sr1.96 Sr1.98 Sr2.00 5K

10 20 30 40 50

-50 -40 -30 -20 -10

0

10 20 30 40 50

H(kOe)

b

0

Sr1.96 Sr1.98 Sr2.00 300 K

2

Sr2.02 Sr2.04 Sr2.10

4

%MR

ρ(T)/ρ(300K)

1.8

Resistivity values at room temperature are ranging from 0.01 to 0.13 Ω ncm. More precisely, with the exception of Sr2.02 and Sr2.04, all the samples present a resistivity value smaller than 0.04 Ω ncm. These values are comparable to those previously reported for SFMO polycrystalline samples [1,3]. All the samples prepared with a deficit or an excess of strontium present negative magnetoresistance under high (
50 kOe to 50 kOe, Fig. 8a) or low magnetic field (
8.5 kOe to 8.5 kOe, Fig. 8b, Fig. 9a-d). Fig. 8a shows the MR of all the samples at 5 K under high magnetic fields. At this particular temperature the best MR values of 44.9% (H ¼50 kOe) and 40.7% (H¼ 10 kOe) are observed for the stoichiometric sample of Sr2FeMoO6 (Sr2.00). It may also be noticed (Fig. 8a) that the samples prepared with an excess of strontium exhibit a less pronounced decrease of MR values comparative to Sr2.00 Fig. 8b shows the MR at 300 K under low magnetic fields (
8.5 kOe and 8.5 kOe). In these conditions a significant decrease of MR values is still observed for the samples with strontium deficit (Sr1.98, Sr1.96) compared to Sr2.00. However, with the exception of Sr2.10, the samples prepared with an excess of strontium present the MR values slightly higher (Sr2.02, Sr2.04) than Sr2.00. We should mention that the MR values observed for Sr2.02, Sr2.04 samples are the highest obtained in these particular preparation conditions compared with various samples prepared during the course of this study. A particular feature of our samples is the presence of MR even at temperatures as high as 400 K, near their Curie temperature (Fig. 9a-d). To the best of our knowledge the persistence of MR at temperatures higher than 300 K was reported only very scarcely

%MR

120

6

1.2 8

1.0

10

0

50

100

150

200

250

300

T(K) Fig. 7. (Color online) Evolution of the normalized resistivity at 300 K in function of temperature for all the samples. The values quoted after the sample names are those of their resistivity at 300 K in Ω.cm (the errors are of the order of the last significant digit).

-8

-6

-4

-2

0

2

4

6

8

H(kOe) Fig.8. (Color online) Magnetoresistance vs. applied magnetic field at low temperature (a) 5 K and room temperature (b) 300 K for all the samples. The error bar of the measurement is MR%7 0.02*MR%.

L. Harnagea, P. Berthet / Journal of Solid State Chemistry 222 (2015) 115–122

0

0

Sr1.96

1

121

Sr2.00 400 K

2

400 K

380 K

%MR

360 K

3

340 K

%MR

380 K

2

360 K

4

340 K

6

320 K

4

320 K

8

300 K

300 K

5 -8

-6

-4

-2

0

2

4

6

10

8

-8

-6

-4

-2

0

2

0

6

8

12

Sr2.04

2

400 K

4

360 K

6

340 K

8

320 K

10

300 K

Sr1.96 Sr1.98 Sr2.00 Sr2.02 Sr2.04 Sr2.10

10

380 K

8

%MR

%MR

4

H(kOe)

H(kOe)

H = 8.5 kOe

6 4 2

-8

-6

-4

-2

0

2

4

6

8

300

320

H(kOe)

340

360

380

400

T(K)

Fig. 9. a-d (Color online) Magnetoresistance vs. magnetic field for (a) Sr1.96; (b) Sr2.00 and (c) Sr2.04 samples at different temperatures. (d) The evolution of MR values at H¼ 8.5 kOe in the temperature range 300 K to 400 K, for the investigated samples. The error bar of the measurement is MR%7 0.02*MR%.

(MR of about 1% at 360 K at 1.25 kOe [26], MR of about 1.4% at 400 K for 10 kOe [27] and MR higher than 1% at 400 K for only 2 kOe [3]). Returning to the analysis of the data, it should be mentioned that in the case of Sr1.98 and Sr1.96 samples the MR is observed at temperatures slightly higher than the Tc values (388 and 385 K) determined from the inflexion point of the magnetization curves. For these particular samples, this observation confirms that this method to determine the Tc values is underestimating the temperature where the ferrimagnetism disappears. The significant decrease of MR values at low temperature for Sr1.98, Sr1.96 samples compared to Sr2.00 arises most likely from the increase of the ASD concentration (Fig. 8a). The ASD decrease the conduction electron polarization which cannot remain that of a perfect half–metal. For samples prepared with an excess of strontium Sr2.02 and Sr2.04 the MR values at 5 K are comparable to that of the reference compound Sr2.00 (Fig. 8a). However, the MR for these samples seems to reach the saturation value with more difficultly (Fig. 7a) which may be related to the increase in ASD. Nevertheless, it may be noted that the ASD do have only a moderate effect on the MR of these samples compared to the samples with deficit in strontium which exhibit a higher ASD concentration. For comparable deviations in stoichiometry, it seems like the high electron polarization is maintained in spite of the pronounced reduction of the conduction electrons (Δ ¼ 5 U x compared to Δ ¼ 2 U x). The Sr2.10 sample present a smaller MR value compared to Sr2.04 although their perovskite phase is similar. This reduction of MR can be due to a higher content of impurities which can depolarize the conduction electrons. The MR values of Sr2.02 and Sr2.04 samples recorded at 300 K and under a magnetic field larger than 2 kOe are the highest (Fig. 8a, Fig. 9c,d). These results are unexpected if we take in account the

majority of their properties. This enhancement of the MR is most likely due to their particular microstructure (Fig. 4c) but it cannot be ignored that these enhanced MR values are observed for samples presenting a deficit in molybdenum. Previous studies [13,28] have suggested that a small deficit in molybdenum in the double perovskite structure is favorable for the increase of MR. Conforming to these previous studies the best MR value was recorded for a sample with about 3% molybdenum vacancies. Retuerto et al. [28], observed a MR value of 6.5% at 300 K and 3 kOe compared to 9.8% at 290 K for 10 kOe reported by Lu et al. [13] for a SFMO sample with molybdenum deficit. These values are smaller than those observed in the present study in similar conditions, respectively 9% at 3 kOe and about 11% at 8.5 kOe (Fig. 9c,d), which shows that the samples prepared using the citrate route with the same nominal composition exhibit superior properties.

4. Conclusions To conclude, we undertook a detailed study to quantify and understand the effect of small Sr off-stoichiometry on structural stability, magnetic and magnetotransport properties of Sr2FeMoO6 samples. In particular, how the low-field magnetoresistance (LFMR) at and above room temperature will be affected by small variations in the Sr contents of the sample. Six double perovskite samples with Sr content ranging from 1.96 to 2.10 were prepared using the citrate route. The unit cell volume decreases for both the deficient and the excess Sr containing samples. The microstructural characteristics of the off-stoichiometric samples are similar to those of the stoichiometric sample, as are their resistivity values. More significant changes are observed in the magnetic and magnetoresistive properties.

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We find that the Sr-deficient samples could be easily obtained in their phase pure form or with a minimal amount of secondary phase. In these samples the Fe/Mo antisite disorder is found to enhance with increasing off-stoichiometry. The Ms, Tc and MR values are found to decrease significantly in the Sr-deficient samples as compared to the stoichiometric compound. The increased degree of antisite disorder in addition to the presence of Sr-vacancies appears to be responsible for the observed deterioration of the magnetic and magnetoresistive properties. The samples prepared with excess Sr, presents a different situation. Here, the synthesis conditions used did not allow the formation of a solid solution Sr2 þ xFeMoO6 þ x (x¼0.02, 0.04 and 0.10). The minor extra peaks in the powder x-ray diffraction of these samples indicate the presence of a parasitic SrMoO3 phase. The excess Sr, therefore, appears to form the parasitic phase SrMoO3 rather than allowing the solid-solution formation. As a result the resulting double-perovskite phase forms with a slight Mo deficit. These samples also present a higher degree of Fe/Mo disorder in the structure than the stoichiometric SFMO. The Ms and Tc values are found to decrease upon increase of the excess strontium content (x). However, the samples with x¼0.02 and 0.04 present remarkable magnetoresistive characteristics. At low temperature the MR of these samples is similar to SFMO, despite their higher ASD in the structure. At room temperature and even as 400 K the MR values for these samples are superior to those observed for the reference sample, SFMO. To the best of our knowledge these MR values are the highest reported till now for this family of double perovskite for samples with micrometric grain size. From our study we can draw following conclusions: (i) the strontium deficiency is detrimental to the magnetoresistive properties of SFMO; (ii) on the other hand, samples prepared with Srexcess exhibit higher magnetoresistance values at low field and up to high temperatures despite their higher antisite disorder. Our study also raises an interesting question on the underlying mechanism responsible for superior values of LFMR in the Sr-excess samples. In this regard, further investigations should focus on the specific role played by the parasitic phase(s), molybdenum vacancies and oxygen off-stoichiometry. Acknowledgment We thank Dr. Jurca Ciprian Bogdan for his help and useful discussions regarding the electrical and magnetoresistive measurements. We are grateful to Prof. Dr. Nita Dragoe for his valuables suggestions regarding the Rietveld refinement and M. François Brisset for his help and cooperation during SEM observations.

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