J Supercond Nov Magn (2012) 25:1049–1054DOI 10.1007/s10948-011-1348-5

O R I G I NA L PA P E R

Magnetism and Resistances of Slightly Dy Doped LaMnO3 SolidSolutions

Hao Liu · Hongguang Zhang · Yongtao Li ·Yuanyuan Chen · Lingshan Chen · Xueguang Dong ·Kai Chen · Qi Li

Received: 16 September 2011 / Accepted: 17 October 2011 / Published online: 11 November 2011© Springer Science+Business Media, LLC 2011

Abstract The magnetism of Dy-doped LaMnO3 solid solu-tions synthesized using sol–gel method was measured andexplained by the analysis of local structure. The saturatedmagnetization of the samples decreased with increasing Dyconcentration, and a magnetic phase transition, whose criti-cal exponents differed from the traditional ferromagnetic toparamagnetic transition of most perovskite manganites, oc-curred at around 150 K. The resistances of the samples couldbe well fitted by polaron hopping model. These propertieswere attributed to the spin–lattice coupling in the samples.

Keywords EXAFS · Disorders · Magnetism

1 Introduction

The perovskite manganites have been extensively studiedfor their novel properties and potential applications [1]. Thestructure of perovskite manganites always deviates from theideal cubic structure by both the rotation of MnO6 octa-hedron and the stretch of the Mn–O bonds [2]. The Jahn–Teller effect of Mn3+ and GdFeO3-type disorder lowers thesymmetries of the crystals [3]. The chemical disorder aris-ing from doping different cationic ions leads to other kindsof lattice distortions [4]. For these materials synthesizedwith conventional way like the solid state or wet chemistry

H. Liu · H. Zhang · Y. Li · Y. Chen · L. Chen · X. Dong ·K. Chen · Q. Li (�)Department of Physics, Southeast University, Nanjing 211189,P.R. Chinae-mail: qli@seu.edu.cn

Y. LiCollege of Science, Nanjing University of Postsand Telecommunications, Nanjing, 210003, P.R. China

methods, A-site or B-site vacancies are easily produced [5].The complex structure leads to various of magnetic inter-actions, such as the double exchange interaction (DEI) [6]between nearby Mn3+–Mn4+, the super-exchange interac-tion of Mn3+–Mn3+, the Dzyaloshinskii–Moriya interaction(DMI) [7, 8] originating from anisotropy exchange interac-tion, single-ion spin effect and bi-quadratic interaction [9].As to LaMnO3, the ground state is A-type antiferromagnetic(AFM), i.e., the ferromagnetic (FM) order in ab plane andAFM order along c axis. While for the Mn ions in hexagonalDyMnO3, there is an transition from commensurate AFM toincommensurate AFM around 20 K and further transition toparamagnetic (PM) state at about 70 K [10]; Dy sublatticeare AFM ordered along c axis but FM ordered in ab planebetween 8 K and 68 K; when the temperature goes down to8 K, Dy sublattice is FM ordered along c axis with AFMcorrelation in ab plane [11]. However, what the magneticproperty is when Dy ions are doped at La sites in an or-thorhombic LaMnO3 is still not clear. Here, Dy ions weredoped into LaMnO3 lattices to offer clues for understandingthe evolution from the A-type magnetic structure to spiralmagnetic structure of DyMnO3 system.

2 Experimental Details

Powder samples of La1−xDyxMnO3 with nominal stoi-chiometry of x = 0.05, 0.10 and 0.15 have been synthe-sized by sol–gel method. Structural characterization is ob-tained using the X-ray diffraction (XRD) technique at theAnalysis Center of Southeast University. X-ray absorptionfine structure (XAFS) data of both Dy and Mn K-edgesare collected at the National Synchrotron Radiation Lab-oratory (NSRL), University of Science and Technology ofChina. Magnetic susceptibility measurements from 70 K to

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Fig. 1 The XRD data of samples La1−xDyxMnO3 and the Rietveld re-fined results of the x = 0.15 sample. The insert of the top panel showsthe details of the XRD data around 30 degrees (see the text)

room temperature are carried out using a vibrating samplemagnetometer (VSM). The measurements of direct current(DC) resistivity are carried out on a Quantum Design Physi-cal Property Measurement System by a standard four probemethod.

3 Results

Figure 1 shows the XRD patterns of the samples. The pat-terns of the x = 0.05 and 0.10 samples are in good agree-ment with the orthorhombic structure while the pattern ofthe x = 0.15 sample shows two tiny peaks that do not be-long to the space-group of Pnma (as shown by the aster-isks in the top panel of Fig. 1). The pattern of the samplehas been refined by the Rietveld method using the Rietan-FP program [12], and the Rwp (reliability index of weightedpattern) of the refinement for the x = 0.15 sample has beenoptimized by taking into account the contribution of very alittle amount of hexagonal structure (as shown in the bottompanel of Fig. 1). Although the structural transition from or-

Table 1 The structure and magnetic parameters of the samples ofLa1−xDyxMnO3 with x = 0.05, 0.10, and 0.15 obtained from the ex-perimental data (see the text)

x = 0.05 x = 0.10 x = 0.15

a (Å) 5.477 5.474 5.468

b (Å) 7.762 7.778 7.777

c (Å) 5.521 5.522 5.520

gLa 0.910 0.894 0.849

gDy 0.048 0.079 0.112

Θab (degree) 157.45 ± 1.79 156.08 ± 1.91 151.33 ± 1.60

Θc (degree) 174.79 ± 5.42 170.93 ± 3.98 173.98 ± 3.12

γ 0.23 0.68 0.52

Tc (K) 149 144 139

Trange (K) 174 167 162

Tc∗ (K) 193 236 226

Ms (µB/f.u.) 0.015 0.01 0.008

ξ (nm) 0.88 0.83 0.79

thorhombic to hexagonal structure can be achieved by an-nealing the samples in high pressure or high temperature[13] for DyMnO3, the ionic radius difference between Laand Dy ions can lead to local distortion, in addition to theJahn–Teller effect caused by Mn3+ ions. The distortion canintroduce internal pressure and promote the formation ofhexagonal cluster around some Dy ions. The lattice param-eter a of the samples decreases with the increase of Dy dop-ing concentration (as shown in Table 1), but the x = 0.10 hasthe largest values of b and c among the three samples. Thisis mainly because of the competition between the latticeshrinkage due to La site cationic deficiencies or the evap-oration of La site ions (see Table 1 of the refined occupationrate of La site gLa and gDy) and the lattice expansion causedby the doping of Dy ions.

The VSM data of the samples are shown in Fig. 2. Allthe three doped samples undergo FM to PM transition withrather broad transition temperature ranges, i.e., Tc = 149–174 K, 144–167 K, and 139–162 K for x = 0.05, 0.10,and 0.15, respectively. With the linear fitting of 1/χ ∼ T

curve the Tcs are obtained to be 188 K, 157 K for sam-ples of x = 0.05,0.10, respectively. There is no linear partin the 1/χ ∼ T curve of the x = 0.15 sample. The satura-tion magnetization of the samples decreases with increasingDy concentration, as shown in the middle panel of Fig. 2.The x = 0.05 sample undergoes an FM-PM transition and its1/χ ∼ T curve mainly consists of linear part above the Curietemperature. This indicates little correlation between spinsat temperature above PM-FM phase transition. The nonlin-ear part of the 1/χ ∼ T curve (at temperature below the lin-ear part, see Fig. 2 as shown by the nonlinear fit solid line)indicates the possible polaron correlations between nearbymagnetic ions as discussed in [14]. At a temperature above

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Fig. 2 The M–T , χ−1–T curves and the M–H curves ofLa1−xDyxMnO3 solid solutions. In the χ−1–T figures, the tempera-tures at the intersection points of linear fit lines and χ−1 = 0 lines are188 K and 157 K for x = 0.05, 0.10, respectively (see the text)

Tc, the x = 0.10 sample has a visible nonlinear part super-positioning with linear part. The nonexistence of linear partfor the x = 0.15 sample indicates the existence of correla-tion effects of spins in the sample for a large temperaturerange.

The nonlinear part of 1/χ ∼ T curves of all the threesamples can be well fitted by power law function χ−1 ∼(T − T ∗

c )1−γ (γ > 0) (see Table 1), where γ means crit-ical exponent. We calculated the mean Mn–O–Mn anglein ab plane Θab [2, 9], as shown in Table 1 by ORFFE[12] program. The angle decreases with the increase of dop-ing concentration; the mean Mn–O–Mn angle along c axis[2, 9], Θc , do not revolve monotonously with doping con-centration as shown in Table 1. The γ values change con-sistently with Θc proving that the interaction between ab

planes plays an important role in the magnetic properties ofLa1−xDyxMnO3 solid solution.

The M–H curves at 120 K (below transition tempera-ture) show two plateaus as shown in Fig. 2: one is at highapplied field (5000 Oe) and the other is at low applied field(300 Oe). Previous work [15] attributes the plateau at highapplied field to the rotation of polarons and the one at lowapplied field to the growth of polarons. The M–H curvesat 180 K (just above the phase transition temperature) arealso shown in Fig. 2. The plateau at low applied field canbe clearly seen while the one at high field vanishes. It seemsthat the growth of polaron dominates the phase transitionprocess.

Figure 3 (upper panel) shows the near-edge XAFS spec-tra of Mn K-edge of La1−xDyxMnO3 (0 ≤ x ≤ 0.15) sam-ples with LaMnO3 as a standard sample. The edge positionsof the Dy doped samples are the same indicating same va-lence of the Mn ions (the edge positions were defined as thezero point of the second-order derivation of the absorptioncurve), but a little higher than that of LaMnO3 which maybe due to the variation of local structure and La site cationicdeficiencies. Thus, the linear part of the 1/χ ∼ T curvesof the x = 0.05 sample can be attributed to the variationof Mn valence which leads to the DE interaction betweennearby Mn3+–Mn4+ ions. Figure 3 (bottom panel) showsthe XANES spectra of Dy L3-edge absorption edge. All theedges locate at 7792 eV indicating the existence of Dy3+ions but absence of Dy4+ [16].

The Fourier-transformed curves of the EXAFS data ofMn K-edge are shown in Fig. 4. The peak around 0.15 nm(denoted as peak B in Fig. 4) is due to the Mn-O singlescattering paths, and the peak around 0.32 nm (denoted aspeak D) is mainly due to the second shell of Mn–La (Dy)or Mn–Mn paths. The peak B of LaMnO3 can be well fittedby two Mn–O single scattering paths with different Mn–Olengths as shown in the middle panel of Fig. 4. It is mainlybecause of the Jahn–Teller effect which transits the structureof LaMnO3 from R − 3c space-group to Pnma space-group.There is very little contribution of the shorter Mn–O singlescattering paths to peak B as shown in the fitted small peakat low r . It indicates a small concentration of Pnma phaseor few Mn sites with shortened Mn–O bonds. When Dy ionsare doped, the intensity of peak A increases. This is mainly

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Fig. 3 The XANES curves of La1−xDyxMnO3 with x = 0.05, 0.10,and 0.15. The upper panel is Mn K-edges and the lower panel is thedata of Dy L3-edges

because the structure becomes distorted from R − 3c space-group structure and some Mn–O bonds have stretched. Theintensity of peak D decreases with the increase of Dy con-centration. For the x = 0.10 sample, the intensity of peakC is comparable with that of peak D. Peak C cannot bethe mathematic tails from the Fourier Transform process ofXAFS because the peak positions keep constant by usingdifferent k range [17, 18]. When the doping concentrationreaches 0.15, the whole curve is flatter and no peak can beclearly identified in high r range. This is mainly because ofthe high X-ray absorption ability of Dy ions.

The DC transport properties of the samples are shown inFig. 5. The resistance of all the three samples increases withthe decrease of temperature and no insulator to metallic tran-sition is found in the temperature range around the PM-FMtransition. The data can be best fitted by the polaron vari-able range hopping model (VRH, ρ(T ) ∝ exp[(T1/T )n])with n = 1/2 (see insert of Fig. 5; the solid line is the fittedcurve by VRH model). The correlation lengths (estimatedby ξ = 2.8e2(4πε0kBT1)

−1, see Table 1. The dielectric con-stant κ has been set to 1 as in [19]) of the samples decreasewith the increase of doping concentration.

Fig. 4 The Fourier transforms of the EXAFS spectra of Mn K-edge.The lower panel is the FEFF fit of the x = 0.00 and x = 0.10 data

4 Discussion

The differences between chemical pressure of La and Dyions make the doped samples more distorted and even tran-sit the symmetry of the lattices, especially for the x = 0.15sample, in which a little amount of hexagonal structure isfound. Although LaMnO3 and La1−xDyxMnO3 (0 < x ≤0.15) are synthesized with the same process, more La sitevacancies appear in La1−xDyxMnO3 (0 < x ≤ 0.15). This

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Fig. 5 The DC resistance of La1−xDyxMnO3 with x = 0.05, 0.10,0.15 from top to bottom, insert shows VRH model fitted curve

is mainly because the chemical disorder promotes the evap-oration of La ions.

As the resistances of the samples can be well fitted bythe VRH model, the polaron hopping process dominatesthe transport properties of the samples. Previous works onLa1−xCaxMnO3 system have reported that the stretchedMn–O bonds favors the formation of polarons [20]. InLa1−xDyxMnO3 (0 < x ≤ 0.15) solid solutions the Mn-Obonds are compressed as seen by the Fourier transform ofEXAFS data shown in the top panel of Fig. 4. As the Mn Kedge positions of the doped samples are same, the concen-tration of Mn4+ ions is also same for the samples, i.e., theaverage carriers concentration is same for the doped threesamples. The only variable is the concentration of Dy ions.Local structure disorder promotes the formation of polaronsbecause the redistribution of electrons between ions, i.e., thechange of Mn-O bonds lengths influences the hopping pro-cess between nearby Mn ions. For no insulator-metal tran-sition process or MR effect found in the samples, the Dyions may block the electrons from further hopping in largeareas, i.e., polarons can hop in smaller areas, and graduallyevolve from large ones to small ones. This is also the rea-son of the decrease of correlation length with increasing Dydoping concentration.

The strong magnetic Dy ions can introduce magnetic dis-order that promotes the formation of magnetic polarons. Asto slightly Dy doped LaMnO3 solid solutions, the Dy ionscan be treated as magnetic impurities located between Mn–Mn spin pairs. For the large Mn–Dy distances the interac-tion between rare earth ions and Mn ions through 3d and4f electrons is smaller than or just comparable with that be-tween nearby Mn–Mn host ions [21–23]. Both the calcu-lations with linear spin wave theory [24] and Monte Carlomethod [25] have confirmed that when the interactions be-tween magnetic impurities and their nearest neighbor host

ions are weak, the local AFM order will be enhanced in thehost spin system. Thus, the Dy ions, in connection with theAFM Mn spin sublattices can form the spin frustrated clus-ters. When the Dy concentration is small, the distances be-tween Dy ions are large and the interactions between Dy im-purities are determined by host Mn spins. A simple picture isthat spin frustrated clusters formed by Dy ions and their sur-rounding Mn ions are randomly scattered in the parent Mnspin sea. When Dy concentration increases, the distances be-tween Dy impurities decrease, and the interactions betweenDy ions increase; at the same time, the lattices become muchmore disordered. The distorted (symmetry-broken) latticeswill lead to a variation of scale exponent γ .

5 Conclusions

The La1−xDyxMnO3 solid solution with x = 0.05,0.10 and0.15 were synthesized by sol–gel method. The doping of Dyions makes the system transit from R-3c gradually to Pnma.A little amount of hexagonal structure was also found inthe x = 0.15 samples. The doping of Dy ions promotes theformation of polarons in LaMnO3, but also blocks the longrange order of polarons. The coupling between spin and dis-torted lattices has led to the variation of scale exponent γ

and the decrease of correlation length of polarons.

Acknowledgements The authors are grateful to Fengchun Hu andBo He for their help during the EXAFS experiment. This work wassupported by united fund of Large scientific apparatus of NSFC andCAS under Grant No. 10979016, Doctoral program 20070286092 forhigher education, and the Graduate Student Innovation Fund of NSRLunder Grant No. 20090621S.

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