Journal of Alloys and Compounds 586 (2014) 261–266

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Journal of Alloys and Compounds

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DC magnetization studies of nano- and micro-particles of bilayeredmanganite LaSr2Mn2O7

0925-8388/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.jallcom.2013.10.002

⇑ Corresponding author. Address: Department of Physics, Shahrood University ofTechnology, Shahrood 3619995161, Iran. Tel./fax: +98 273 3395270.

E-mail address: ebrahim_ghazi@yahoo.com (M.E. Ghazi).

M.H. Ehsani a,b, M.E. Ghazi a,⇑, P. Kameli c, F.S. Razavi d

a Department of Physics, Shahrood University, Shahrood 36155-316, Iranb Department of Physics, Semnan University, Semnan 35195-363, Iranc Department of Physics, Isfahan University of Technology, Isfahan 84156-8311, Irand Department of Physics, Brock University, St. Catharines L2S3A1, Canada

a r t i c l e i n f o a b s t r a c t

Article history:Received 4 April 2013Received in revised form 11 September2013Accepted 1 October 2013Available online 12 October 2013

Keywords:ManganitesSol–gelGrain sizeMagnetic properties

Systematic studies of the magnetic properties of LaSr2Mn2O7 as a function of crystalline grain size provideinformation on how the crystalline grain size affects the magnetic and charge ordering in this compound.The half-doped bilayered manganite LaSr2Mn2O7 (x = 0.5) in its bulk form has the CE-type anti-ferromagnetic (CE-AFM) charge ordering phase transition. In this work, we prepare LaSr2Mn2O7 ceramicsamples using the Pechini sol–gel method in order to produce different grain sizes, and the effects ofdifferent crystalline grain sizes between 200 and 1000 nm on the magnetic properties of the compound,obtained by a SQUID magnetometer, are investigated. The DC magnetization (DCM) measurements for allsamples indicate that the crystalline grain size has no considerable effect on the charge ordering transi-tion temperature, TCO (just a few degrees shift to lower temperatures as the grain sizes become bigger).The width of the peak observed in these measurements (transition to the charge ordering phase) becomesbroader, and susceptibility measurement at the zero field cooling (ZFC) and filed cooling (FC) increases asthe grain size becomes systematically smaller. The results obtained from the magnetic hysteresis curvesconfirm the ground state phase of the system is anti-ferromagnetic and the Arrott plots obtainedmanifest existence of the first and second order magnetic phase transition in all samples. In addition,in the sample with a grain size of 200 nm, enhancement of the magnetic properties, which is accompa-nied with the formation of FM phase on the surface of grain or particle, is observed.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction the surface-to-volume ratio increases, with a large number of

Layered mixed-valence manganese oxides with the generalformula (La,Sr)n+1MnnO3n+1 have attracted much interest due totheir particular physical properties. Among the family of thesecompounds, the bilayered manganites (n = 2) La2�2xSr1+2xMn2O7

have a 2D character, and exhibit an anisotropic reduction in theone-electron (eg) band width, and show a rich magnetic phase dia-gram which depends strongly upon the doping level x [1–3]. In thehalf-doped compound LaSr2Mn2O7 (x = 0.5), transition to the anti-ferromagnetic phase (AFM) with decrease in temperature isaccompanied with the charge ordering (CO) state [4–6], which isalso observed in the 3D half-doped manganites according to theGoodenough prediction [7].

Systematic studies of magnetic and electrical properties ofmanganites as a function of particle size manifest that the particlesize affects the magnetic properties and the CO phase nature. Infact, when the size of the magnetic particles reduces, the value of

atoms residing at the grain boundaries in which their propertiesare different from in the grains. For example, the chargeordered state observed in bulk manganites can be suppressed innano-particles. In addition, tendency for ferromagnetic (FM) orderat the surface of nano-sized manganites presenting AFM/CO bulkorder has been observed [8–11].

These studies suggest that the bilayered manganites La2�2x-

Sr1+2xMn2O7, similar to other manganites, offer unique opportuni-ties to investigate the influence of particle size on their magneticproperties. In our previous study, in order to investigate the effectof particle size, we prepared a number of LaSr2Mn2O7 ceramic sam-ples using the sol–gel method, and the effects of particle size on theelectrical properties of this compound were reported [12]. In thiswork, we report the DC magnetization (DCM) studies of thesamples with nano- and micro-sized particles.

2. Experimental and synthesis

LaSr2Mn2O7 fine powders were prepared by the Pechini sol–gel method. All thechemicals required (analytical grade reagents) were purchased from the MerckCompany and used as received without further purification. Highly pure powders

Fig. 1. Rietveld refinement of XRD pattern for sample SB-1450.

Table 1Lattice parameters (a and c), unit cell volume (V), average crystallite size (D), andgrain size. Numbers in the parentheses indicate errors on the last significant digit.

Sample a (Å) c (Å) V (Å3) D (nm) Grain size (nm)

SB-1280 3.863(5) 20.002(8) 298.585(7) 22.5(7) 200SB-1350 3.868(0) 19.958(4) 298.612(9) 26.6(8) 500SB-1450 3.869(4) 19.986(8) 299.259(4) 30.6(3) 1000

262 M.H. Ehsani et al. / Journal of Alloys and Compounds 586 (2014) 261–266

of the nitrate precursor reagents La(NO3)3�6H2O, Mn(NO3)2�4H2O, Co(NO3)2�4H2O,and Sr(NO3)2 were weighted in appropriate proportions. Details of the sol–gel pro-cess and powder preparation have been reported in Refs. [12,13]. The samples weresynthesized at pH = 7 and calcinations temperature 1000 �C. The black powdersobtained were cold-pressed into pellets of 10 mm diameter and thickness of about2–3 mm under the pressure of 20 ton/cm2. Three pellets were sintered for 6 h at1280, 1350 and 1450 �C separately and labeled as SB-1280, -1350, -1450 formagnetic studies.

Structural properties of the samples were studied by X-ray diffraction (XRD)(ADVANCE-D8 model). The microstructures of the samples were analyzed by fieldemission scanning electron microscope (FE-SEM). The ac susceptibility measure-ments were performed using a PPMS model 6000. The DC magnetization was mea-sured using a MPMS model 6500.

Fig. 2. FE-SEM photographs for the compounds (a) SB

3. Results and discussion

The XRD data was analyzed by the Rietveld refinement usingthe FULLPROF software and Pseudo-Voigt function. It was foundthat every X-ray reflection can be indexed by the Sr3Ti2O7-type(327-type) tetragonal structure with the I4/mmm space group. Atypical Rietveld refinement pattern for the sample SB-1450 isshown in Fig. 1. The obtained lattice parameters and unit cell vol-umes for the samples are given in Table 1. The average crystallitesizes of the samples were calculated using Scherrer’s formula,D = 0.9k/(bcosh), where D is the average crystallite size, k is theX-ray wavelength equal to 1.5406 Å, h is the Bragg angle and b isthe corrected full width at half maximum .

Morphology of the samples obtained by FE-SEM is shown inFig. 2. The average grain sizes were estimated to be 200, 500, and1000 nm for the SB-1280, -1350, -1450 samples, respectively. Asthe images show, the average grain sizes increase with increasein the sintering temperature. Also in the images of the SB-1280and SB-1350 samples, porosities between the grains is clear butwith increase in sintering temperature (in SB-1450 sample), thisporous structure gradually disappears. Fig. 2c shows the grainsare almost tightly connected without porosity.

In the half-doped bilayered manganite LaSr2Mn2O7, there is atransition to the CE-type charge-orbital ordering state at �225 K.This state starts melting at �170 K, where the A-type AFM spinordering begins to form. Below this temperature, spins are ferro-magnetically aligned in the a-b plane; however they couple anti-ferromagnetically along the c-axis. In addition, the minor CE-AFMstate has been reported to co-exist with the major A-AFM orderingstate below 145 K and suppressed below �100 K but not com-pletely [4,6,14-17]. Also, reports on single crystals samples ofbi-layered manganite LaSr2Mn2O7 revealed CE-AFM ground statewhich is very sensitive to the doping level [5,15].

We investigated both the zero-field cooling (ZFC) and fieldcooling (FC) temperature dependence of DC magnetization (DCM)of the samples at the magnetic fields H = 100, 500, and 1000 Oe,and the results obtained were shown in Fig. 3a–c. For all samples,in the ZFC curves obtained at the magnetic field 100 Oe, one canobserve a peak around 220 K, which is onset of the transitioninto the charge–orbital ordered state. By applying higher fields,e.g. 500 or 1000 Oe, this transition temperature shifts to lowertemperatures.

-1280, (b) SB-1350, (c) SB-1450 (at 1 lm scale).

Fig. 3. Temperature dependence of magnetization in both ZFC (filled symbols) and FC (open symbols) modes for samples (a) SB-1280, (b) SB-1350, and (c) SB-1450. Inset:field dependence of charge ordering temperature.

M.H. Ehsani et al. / Journal of Alloys and Compounds 586 (2014) 261–266 263

Variation in charge ordering transition temperature (TCO) versusapplied filed is shown in the inset of Fig. 3a–c. As it can be seen inthese figures, TCO in all samples decreases with increase in theapplied filed but it does not show considerable dependence onthe particle size (only a few degrees shift to lower temperaturesas the grain sizes become bigger), indicating stability of the COstate in this particle size range. Also DCM measurements for thesamples exhibit an irreversibility between the FC and ZFC curves,as indicated by the appearance of large bifurcation between thembelow a temperature, marked as Tirr in the figures. This irreversibil-ity occurred at a higher temperature for the SB-1280 sample,which has smaller particles. The magnetization behavior shownfor the samples can be considered as a superposition of the FMand AFM components, implying that the FM component comesfrom the in-plane (ab plane) FM correlation and AFM one comesfrom coupling along the c-axis. From the FC DCM measurement,it can be deduced that there is a competition between the AFMand FM phases in the CO state, and that with decrease in temper-ature, the FM component dominates with a small linear increase inmagnetization.

We can see in Fig. 3a that for the sample with 200 nm grain size,SB-1280, the FC magnetization data bifurcates from the ZFC dataaround room temperature, with a hint of a weak ferromagneticcomponent that could be attributed to the surface moments ofthe particles. Previous studies have manifested that the robustcharge ordering in bulk manganites is weakened in nano-scalemanganites accompanied with appearance of a weak ferromagne-tism [18–22]. Although, there is no complete explanation for this

behavior but Dong and coworkers have proposed a phenomenolog-ical model for explanation of this exotic phenomenon [23]. Basedon their argument, when the grain size is down to nano-scale,the surface relaxation becomes considerable due to promotion ofthe surface/volume ratio. The Mn cations settled on the surfacelayer of grains have lower neighbors compared to the bulk. There-fore, the AFM superexchange (SE) is relaxed on the surface layer,and it seems that grain surface may favor FM state rather thanCO state that their fractions depend on the particle size.

To investigate the magnetic ground state, hysteresis loops of thesamples were measured up to 50 kOe at different temperatures,and the typical data was presented in Fig. 4a–d. It is well-knownthat in the FM phase, magnetization should increase rapidly byan applied magnetic field and then it should saturate at highermagnetic fields, while in the AFM long-range CO phase, it shouldincrease linearly with an applied magnetic field. In the M–H curvesfor the samples, a rapid increase up to 4500 Oe and then a linearincrease above this filed were observed. However, the magnetiza-tion does not exhibit saturation behavior even at 50 kOe. Anotherpoint is the existence of hysteresis behavior at temperatures below220 K. This behavior is also reported for the LaSr2Mn2O7 compoundby Zhang and coworkers [15,24]. It has been suggested that there isa superposition of both FM and AFM components in this compound[25]. The FM one appears because of interalayer FM doubleexchange interactions, while the dominant phase between the lay-ers is the AFM coupling. The difference between the FC and ZFCpatterns for the samples at low temperatures confirms the coexis-tence of both phases.

Fig. 4. Field dependence of magnetization for samples (a) SB-1280, (b) SB-1350, (c) SB-1450 at selected temperatures and (d) hysteresis loops of the samples at 20 K.

264 M.H. Ehsani et al. / Journal of Alloys and Compounds 586 (2014) 261–266

Also as it can be seen in Fig. 4d, there are hysteresis loops for allsamples at 20 K, and the magnitude of magnetization for theSB-1280 sample is higher than that for others samples. Thus the ob-served increase in magnetization by reduction of grain size could berelated to the formation of the FM phase on grain boundaries.

For further DCM studies and to check the nature of the magnetictransitions, we used the Banerjee criterion to plot H/M versus M2

curves (Arrott plot) in the critical region [26–29]. According to thiscriterion, a positive or negative slope for the curves indicateswhether the magnetic phase transition is second order or firstorder. As it can be seen in Fig. 5a–c, around the charge orderingtemperature at 220 K, curves clearly exhibit a positive slope inthe entire of M2 range. However, as the figures indicate, there isa disorder in the patterns for the curves versus temperature attemperatures below 220 K when the particle size becomes smaller,especially in SB-1450. Thus we cannot exactly define the type oftransition. It is possible that there is a mixture of first order andsecond order phase transition in the SB-1280 and SB-1350samples.

In order to further investigate the nature of the phase transitioninvolved, we performed the susceptibility measurement at the ZFC(cooling and heating) modes. The thermal hysteresis (TH) can giveus information about the existence of a first order phase transition[30] or even TH phenomenon is observed to be more pronouncedaround the transition temperature in the phase-separated manga-nites [31].

Figs. 6 and 7 show the data for ac susceptibilities measured inthe ac field 10 Oe for the samples SB-1450 (with 1 lm grain size)

and SB-1280 (with 200 nm grain size). In both runs, the heatingand cooling rates were the same. The TH nature was observed inthe SB-1450 sample around the transition temperature of the COphase (see Fig. 6). This is a signature of the existence of a first ordermagnetic transition. This hysteresis behavior was also observed forthe SB-1280 sample but in the entire temperature range, as it canobviously be seen in Fig. 7, indicating the first order phase transi-tion. This behavior is related to the phase-separation and coexis-tence of the FM phase which was formed on the surface ofparticles and AFM phase causing a broad hysteresis in the entiretemperature range.

Thus according to the above discussion and the results obtainedfrom the Arrott plots (Figs. 5a–c), there is a mixture of first and sec-ond order of magnetic phase in the samples. In addition, the occur-rence of Tirr near room temperature as well as the obvious THbehavior in the entire temperature range indicates the tendencyfor the FM phase formation on the surface of samples with nano-scale particle sizes.

4. Conclusion

DC magnetization study of the half-doped layered manganitesprepared with different grain sizes indicate that the magnetizationamplitude for the samples increase with reduction in the grain sizein these samples. This is accompanied with the enhancement of FMnature evidences in hysteresis loops measurements for the sam-ples. This can be related to effects of the grain surface that in this

Fig. 5. Arrott plot (H/M versus M2) for the samples at different temperatures: (a) SB-1280, (b) SB-1350, and (c) SB-1450.

Fig. 6. Real and imaginary parts of susceptibility for SB-1450 sample (1 lm grainsize).

Fig. 7. Real and imaginary parts of susceptibility for SB-1280 sample (200 nm grainsize).

M.H. Ehsani et al. / Journal of Alloys and Compounds 586 (2014) 261–266 265

charge ordered-anti-ferromagnetic manganite, the surface prefersto have ferromagnetic phase instead of having anti-ferromagneticstate when the size of the grains decreases to nanoscale. Theresults obtained for this study indicate that there is a mixture of

first and second order magnetic phase transition in all samples.The result obtained from susceptibility measurements confirm,existence of the FM phase on the grain surface for sample withsmaller grain size.

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