Short range magnetic correlations induced by La substitution in Ho1 - xLaxMn2O5

Short range magnetic correlations induced by La substitution in Ho1 − xLaxMn2O5

This article has been downloaded from IOPscience. Please scroll down to see the full text article.

2010 J. Phys.: Condens. Matter 22 246002

(http://iopscience.iop.org/0953-8984/22/24/246002)

Download details:

IP Address: 140.115.30.178

The article was downloaded on 08/06/2010 at 07:29

Please note that terms and conditions apply.

View the table of contents for this issue, or go to the journal homepage for more

Home Search Collections Journals About Contact us My IOPscience

IOP PUBLISHING JOURNAL OF PHYSICS: CONDENSED MATTER

J. Phys.: Condens. Matter 22 (2010) 246002 (8pp) doi:10.1088/0953-8984/22/24/246002

Short range magnetic correlations inducedby La substitution in Ho1−xLaxMn2O5

Chin-Wei Wang1, Chun-Ming Wu1, Chi-Yen Li1, Sunil K Karna1,Chien-Kang Hsu1, Carissa H C Li1, Wen-Hsien Li1,4,Chun-Chen Yu2, Chun-Pin Wu2, Hsiung Chou2

and Jeffrey W Lynn3

1 Department of Physics and Center for Neutron Beam Applications, National CentralUniversity, Jhongli, 32001, Taiwan2 Department of Physics, National Sun Yet-Sen University, Kaohsiung 80424, Taiwan3 NIST Center for Neutron Research, National Institute of Standards and Technology,Gaithersburg, MD 20899, USA

E-mail: [email protected]

Received 1 April 2010, in final form 20 May 2010Published 2 June 2010Online at stacks.iop.org/JPhysCM/22/246002

AbstractMagnetic susceptibility, x-ray diffraction, neutron diffraction and Raman scatteringmeasurements are employed to study the effects of La substitution on the magnetic properties ofmultiferroic HoMn2O5. 9% and 18% La-substituted compounds crystallize into the sameorthorhombic Pbam symmetry as the parent compound. The magnetic responses to an acdriving magnetic field between 40 and 140 K are greatly enhanced by 18% La substitution. Theneutron magnetic diffraction patterns reveal the development of short range magneticcorrelations below 140 K. In addition, two Raman peaks and a series of new x-ray diffractionpeaks suddenly develop below this temperature. Incommensurate long range antiferromagneticorder appears below 38 K. Magnetic frustration could be the main mechanism governing thepresent observations.

(Some figures in this article are in colour only in the electronic version)

1. Introduction

It is known that the interplay between the electric dipoles andmagnetic spins in the multiferroic manganites can generatefruitful physical properties, which provide potential deviceapplications for this class of materials [1–3]. Geometricfrustration, resulting from competitions among the multipleground states, has been recognized to be a key factor governingthe physical properties of such a system [4–6]. Numerousstudies have shown that all sites in RMn2O5 (R125), whereR is a rare-earth element, can effectively alter the physicalbehavior of the compounds. In particular, ferroelectricity candevelop when rare-earths with ionic sizes smaller than thatof Sm (included) are incorporated onto the R sites [5–16].For HoMn2O5 in particular, three magnetic phases have beenidentified. On cooling, the Mn spins order first at 43 K intoan incommensurate configuration. They then lock-in to a

4 Author to whom any correspondence should be addressed.

commensurate arrangement below 39 K as ferroelectricity isinduced. The spin structure transfers finally into a secondincommensurate phase below 21 K [4, 8, 17]. The magneticphase of LaMn2O5, on the other hand, is relatively simple,where the Mn spins become ordered below 31 K, witha propagation vector of (0 0 1/2) [18]. In addition, aspin-glass-like behavior appears at 80 K, which has beensuggested to be an indication of the existence of short rangecorrelations between the Mn3+ sublattices well above theordering temperature [18]. The more complicated behaviorof HoMn2O5 is believed to be associated with both the ionicsize of Ho3+ being 15% smaller than that of La3+ as wellas the magnetic properties of Ho. It is clear that locallattice distortions are created when a noticeable amount of theHo3+ ions are replaced by the significantly larger La3+ ions.Alternations in the magnetic behaviors can then be anticipated.In this paper, we report on the results of studies made on La-substituted HoMn2O5, but focus on the short range magneticcorrelations induced by the La substitution.

0953-8984/10/246002+08$30.00 © 2010 IOP Publishing Ltd Printed in the UK & the USA1

J. Phys.: Condens. Matter 22 (2010) 246002 C-W Wang et al

2. Experimental details

In searching for magnetic responses, the temperature profilesof the ac magnetic susceptibility were measured with andwithout the presence of an applied magnetic field Ha.These measurements were performed on a Physical PropertyMeasurement System, manufactured by Quantum Design,employing the standard setup. Both the in-phase componentχ ′ and the out-of-phase component χ ′′ were measuredsimultaneously, employing various strengths and frequenciesfor the driving magnetic field. During these measurements∼1 g of the powder sample was loaded into a nonmagneticcylindrical container. A pumped 4He cryostat was used to coolthe sample. The lowest temperature achieved was 1.8 K. Wenote that χ ′ measures the response of the system to the drivingfield, while χ ′′ reflects the losses of the driving field to thesystem.

High intensity neutron powder diffraction patterns,covering certain temperature regimes where the susceptibilityshows anomalies, were collected in the search for magneticcorrelations. These measurements were conducted at the NISTCenter for Neutron Research, using the BT-9 (BT-7) triple-axis spectrometer operated in the diffraction mode. Pyrolyticgraphite PG(002) monochromator crystals were used to selectan incident wavelength of λ = 2.359 A (2.444 A), withPG filters to suppress higher-order wavelength contamination.For the measurements performed at BT-9 (BT-7), angularcollimations with horizontal divergences of 40′, 48′ and48′ (50′, 50′ and 50′) full-width-at-half-maximum (FWHM)acceptance were used for the in-pile, monochromatic anddiffracted beams, respectively. For these measurements ∼10 gof the sample was loaded into a cylindrical vanadium-can. Thesample temperature was controlled using a He-gas closed-cyclerefrigerator system.

The Raman spectra were collected on a triple monochro-mator Jobin-Yvon T64000 spectrometer, operated in thebackward scattering configuration, equipped with a focusingmicroscope, a notch filter, and a liquid nitrogen cooled CCDdetector. The 514.5 nm green light from an Ar+ laser was usedas the excitation line, with a low input power of 0.1 mW beingdirected onto the sample to avoid local heating. The sampletemperature was controlled using a pumped nitrogen cryostat.

3. Sample fabrication and crystalline structure

Three polycrystalline samples of nominal compositionsHo1−xLaxMn2O5, with x = 0, 0.09 and 0.18, were preparedby the standard sol–gel method [19]. The precursors werefirst synthesized by a wet-chemistry technique. Stoichiometricamounts of high purity powders of La2O3, Ho2O3 andMn(NO3)2·4H2O were dissolved into a solvent of citric acidand deionized water. The solution was then dried using anevaporator, operated with stepwise temperature increments of10 ◦C h−1 until the temperature reached 180 ◦C, at which pointthe mixture was cured for 2 h. This was followed by dryingin a Pt crucible at 200 ◦C for another 6 h. The precursorsthus obtained were then ground and calcined at 600 ◦C for 6 hand at 800 ◦C for another 8 h to remove organic substances.

Figure 1. Observed (crosses) and fitted (solid lines) x-ray powderdiffraction patterns of the x = 0.18 compound collected at 300 K,assuming an orthorhombic symmetry of space group Pbam. Thedifferences between the calculated and observed intensities areplotted at the bottom. The solid vertical lines mark the calculatedpositions of Bragg reflections for the proposed crystalline structure.

Finally, the sample was ground again, compressed into pellets,and sintered at 1000 ◦C for 12 h in a continuous oxygen flowenvironment. The highest substitution level obtainable for thepresent series was x = 0.18.

The samples thus fabricated were characterized usingx-ray powder diffraction. The diffraction patterns wereanalyzed by the Rietveld method [20], employing the generalstructure analysis system (GSAS) program [21]. All of thediffraction patterns obtained from the three samples couldbe described very well using orthorhombic symmetry withspace group Pbam. The final refinement was performedassuming an orthorhombic symmetry with space group Pbamand taking the pseudo-Voigt function to generate the lineshape of the diffraction peaks. Figure 1 displays the observed(crosses) and fitted (solid lines) patterns of the x = 0.18compound taken at 300 K, with their differences plotted atthe bottom. The results indicate that they agree very well.The chemical compositions obtained from the refinements forthe three samples are HoMn2O4.99, Ho0.91La0.09Mn2O4.98 andHo0.82La0.18Mn2O4.98. No significant oxygen deficiency wasfound in any of the three samples, and we simply quote fulloxygen occupancy. Table 1 summarizes the refined structuralparameters of Ho0.82La0.18Mn2O5 at 300 K. No traces ofimpurity phases, such as MnO, MnO2 or Mn2O3, could beidentified. We estimated the impurity phases in the samples tobe less than 2%. The crystalline structure of Ho1−x Lax Mn2O5

can be viewed as consisting of infinite chains of Mn4+O6

octahedra linked together, along the c axis direction, throughedge- and corner-sharing with slightly distorted tetragonalMn3+O5 pyramids and (Ho/La)O8 units [22]. La substitutionresults in a noticeable expansion of the basal plane of thecrystallographic unit cell. The a and b lattice parameters ofthe 18% La-doped compound increased by 0.97% and 0.40%,respectively, while the axial c lattice parameter expandedby only 0.07%. In addition, the Mn3+–O(1)–Mn3+ angle

2

J. Phys.: Condens. Matter 22 (2010) 246002 C-W Wang et al

Table 1. List of the refined crystalline parameters of Ho0.82La0.18Mn2O4.98 at 300 K, where Biso represents the isotropic temperatureparameter and M represents the multiplicity.

Ho0.82La0.18Mn2O4.98 at 300 KOrthorhombic Pbam (No. 55, Z = 4)

a = 7.3369(6) A, b = 8.5089(7) A, c = 5.6725(3) A, Volume = 354.13(5) A3

X Y z M Biso (A 2) Occupancy

Ho 0.1401(8) 0.1710(4) 0 4g 0.916(2) 0.82La 0.1401(8) 0.1710(4) 0 4g 0.916(2) 0.18Mn1 0 0.5 0.2569(4) 4f 0.142(2) 1Mn2 0.4116(4) 0.3468(7) 0.5 4h 0.142(2) 1O1 0 0 0.2585(2) 4e 0.071(6) 0.99O2 0.1565(4) 0.4457(0) 0 4g 0.458(5) 1.00O3 0.1568(7) 0.4450(4) 0.5 4h 0.087(5) 0.99O4 0.3875(9) 0.2054(6) 0.2398(4) 8i 0.268(5) 1.00

χ2 = 1.451, Rp = 2.24%, Rwp = 2.87%

increased by 1.3% while the Mn3+–O(1) and Mn3+–Mn3+separations enlarged by 1.97% and 2.98%, respectively, inthe 18% La-substituted compound. These alternations willcertainly affect the magnetic interactions within the Mn3+O(1)

layers.

4. Magnetic properties

Figure 2 illustrates the effects of La substitution on thetemperature profiles of (a) χ ′ and (b) χ ′′, measured using aweak driving magnetic field with a root-mean-square strengthof 1 Oe and a frequency of 100 Hz. The χ ′(T ) curve ofthe x = 0 compound departs slightly from the Curie–Weissbehavior (solid line) below 140 K (marked TSR), as shownin figure 3(a). It signals the existence of weak magneticcorrelations below 140 K, which is well above the orderingtemperature of TN = 43 K, as found in a separate study [4].With the presence of an Ha, the amount of departure of χ ′from the Curie–Weiss behavior is noticeably reduced and TSR

shifts to a lower temperature, as shown in figure 3, where thetemperature dependence of 1/χ ′ taken at Ha = 0 and 2000 Oeare displayed. It reveals antiferromagnetic characteristics forthese correlations.

The most pronounced features seen in figure 2 are theenhanced responses and absorptions that appear below 140 K,which result in the appearance of broad peaks centered ataround 60 K in both the response χ ′(T ) curve and in theabsorption χ ′′(T ) curve of the 18% La-substituted compound.These enhancements are nevertheless not clearly revealed inthe x = 0.09 compound. The enhancements seen in thex = 0.18 compound can, however, be suppressed by amodest applied magnetic field Ha, as shown in figure 4.Upon increasing Ha, TSR shifts to a lower temperature thatreaches 100 K at Ha = 1000 Oe, as shown in figure 5.TSR begins to shift to a higher temperature when Ha isfurther increased, and reaches 135 K at Ha = 1 T. Thesebehaviors can be understood as due to the suppression ofthe antiferromagnetic correlations by Ha. An Ha strongerthan 1000 Oe can indeed induce ferromagnetic correlations.No classic spin glass behavior, where χ ′ and χ ′′ responsesignificantly better to a weak or steady driving field, is

Figure 2. Effects of La substitution on (a) χ ′(T ) and (b) χ ′′(T ).Broad peaks develop below 140 K in both the χ ′(T ) and (b) χ ′′(T )curves of the x = 0.18 compound.

observed in the present compounds. Nevertheless, χ ′ and χ ′′

do exhibit a weak but noticeable dependency on the drivingfrequency, as shown in figure 6, reflecting that the relaxationtime of these magnetic correlations is relatively long, whichmay be characterized as spin-glass-like. It is surprising that an18% replacement of magnetic Ho3+ ions by nonmagnetic La3+

3

J. Phys.: Condens. Matter 22 (2010) 246002 C-W Wang et al

Figure 3. Temperature dependence of 1/χ ′ of the x = 0 compoundtaken at (a) Ha = 0 and (b) Ha = 2000 Oe. χ ′ departs slightly fromthe Curie–Weiss behavior when the temperature is below 140 K forthe χ ′(T ) taken without an Ha. The amount of departure of χ ′ fromthe Curie–Weiss behavior is noticeably reduced and TSR shifts to alower temperature for the χ ′(T ) taken at Ha = 2000 Oe.

Figure 4. Effects of the applied magnetic field on χ ′(T ) of thex = 0.18 compound. The response peak at 60 K can be suppressedby an applied magnetic field. The response peak becomes invisible atHa = 1000 Oe.

ions can induce antiferromagnetic correlations between the Mnions at temperatures well above their ordering temperature. Itappears that random lattice distortions associated with the site

Figure 5. Variations of TSR with the applied magnetic field, showinga dip at Ha = 1000 Oe at which TSR shifts to 100 K. TSR begins toshift to a higher temperature when Ha is further increased, andreaches 135 K at Ha = 1 T.

Figure 6. Effects of the driving frequency on χ ′(T ) of the x = 0.18compound, showing a weak but noticeable dependency of χ ′ on thedriving frequency.

substation can indeed induce noticeable magnetic correlationsin this class of materials.

5. Magnetic correlations

The existence of magnetic correlations can be revealed bythe magnetic diffraction of neutrons. Figure 7 shows thedifference pattern between the neutron diffraction patternstaken at 60 and 190 K on BT-7, revealing a broad peak(solid line) covering the whole scattering angles range studied.The appearance of this broad peak signals the existence ofshort range magnetic correlations, presumably between theMn spins. Note that there are peak-like structures around2θ = 20◦, 25◦, 32◦, and 47◦, but the resolution is not superiorenough to distinguish it consists of a single broad peak frommultiple narrower peaks. The magnetic correlation lengthcan be calculated according to ξ = w−1, where w is the

4

J. Phys.: Condens. Matter 22 (2010) 246002 C-W Wang et al

Figure 7. Difference pattern between the neutron diffraction patternstaken at 60 and 190 K of the x = 0.18 compound. A broad peak(solid curve) that signals the existence of short range magneticcorrelations is revealed.

Figure 8. Magnetic diffraction pattern at 5 K, where the patterntaken at 50 K has been subtracted, revealing a series ofresolution-limited magnetic reflections. All the diffraction peaks canbe indexed using propagation vectors �K ± = (h k l) ± �q , with amodulation vector of �q = (0.48 0 0.29), showing an incommensuratestructure of the Mn moments.

HWHM of the peak plotted in A−1

. A magnetic correlationlength of ξ = 0.85 A at 60 K is obtained, when assumingthe magnetic correlations result in a single peak. These shortrange correlations were not observed in the parent compoundand thus originate from La substitution. At low temperatures,these broad peaks develop into resolution-limited peaks, asshown in figure 8, where the difference pattern of the 5 and50 K reflections taken on BT-9 is displayed. These magneticreflections can be indexed using the propagation vectors �K ± =(h k l) ± �q , with a modulation vector �q = (0.48 0 0.29),revealing an incommensurate structure for the Mn moments.This modulation vector obtained for the 18% La-substitutedcompound is similar to but not exactly the same as that ofHoMn2O5 [5].

Figure 9. (a) Thermal evolution of the diffraction pattern around the{100} reflection, showing that the peak position of the magnetic{010}± and {011}− reflections change with temperature.(b) Temperature dependence of the integrated intensity and peakposition of the {010}± reflection.

Figure 9(a) displays the thermal evolutions of the {010}±and {011}− magnetic reflections between 7 and 44 K. Clearly,the peak positions of the magnetic {010}± and {011}−reflections do not remain the same but shift noticeably throughthe transition. The variations in the peak position and theintegrated intensity of the representative {010}± reflection areshown in figure 9(b). A significant increase of 0.32◦ in thescattering angle of the {010}± reflection is seen upon coolingfrom 38 to 18 K, reflecting that the modulation vector �qchanges with temperature. It, however, stabilizes at 18 K,below which no further changes in �q are detected. Thethermal evolution of the magnetic modulation vector of the Mnmoments has been observed in HoMn2O5, where the magneticstructure evolves from an incommensurate phase into a so-called lock-in commensurate one, then stabilizes into anotherincommensurate phase at low temperatures [4, 5]. An 18%La substituted not only alters the thermal evolution profileof the modulation vector, but also disturbs the formation ofa commensurate phase, reflecting that the magnetic structureis associated with the delicate balance of the magneticinteractions in the compound. We believe that the alternationsin the magnetic structure that result from La substitutionare mainly due to perturbations in the next nearest neighborinteractions between the Mn ions, created by the distortions

5

J. Phys.: Condens. Matter 22 (2010) 246002 C-W Wang et al

Figure 10. Temperature dependence of the {010}± peak intensity ofthe x = 0.18 compound, showing an ordering temperature ofTN = 38 K, where an abrupt change in the scattering occurs.Additional magnetic intensity persists up to 140 K.

Figure 11. Raman spectra of the x = 0.18 compound taken atvarious temperatures, revealing the sudden development of two newRaman peaks at 873 and 907 cm−1 when the temperature is below130 K.

wherever the Ho3+ ions are replaced by the significantlylarger La3+ ions. Note that the maximum substitution inthe crystalline structure of the HoMn2O5 lattice is an 18%replacement of Ho by La.

The temperature profile of the {010}± integrated intensityshown in figure 9(b) indicates that the long range orderdisappears at above ∼38 K. Above this temperature short rangecorrelations persist up to much higher temperatures of ∼140 K,as can be seen in figure 10. These are the same temperatureswhere peaks appear in χ ′(T ) and χ ′′(T ). In addition, twonew Raman modes suddenly develop below ∼140 K (alongwith the short range magnetic correlations), suggesting that astructural distortion has occurred. Figure 11 shows portionsof the Raman spectra taken in this temperature regime. In

Figure 12. Direct comparison of the x-ray diffraction patterns of thex = 0.18 compound take between 150 and 90 K. The arrows indicatethe diffraction peaks developed in this temperature regime.

Figure 13. Difference pattern between the x-ray diffraction patternstaken at 110 and 90 K of the x = 0.18 compound, revealing the seriesof diffraction peaks developed in this temperature regime. Thesediffraction peaks may be described using three spatial periodicities of9.48, 8.57 and 7.07 A along mutually perpendicular axis directions.

addition to the Ag and B1g phonon modes at 622 and 693 cm−1

Raman shifts [23], two Raman modes at 873 and 907 cm−1

are also revealed in the patterns taken below 140 K. Moreover,a series of new x-ray diffraction peaks develop below 140 Kas well. Two representative peaks of this additional series areshown in figure 12, where the two peaks indicated by arrowsappear in the 130 and 110 K patterns but not in the 150 and90 K patterns. This series can be more clearly revealed in thedifference pattern between the x-ray diffraction patterns takenat 90 and 110 K shown in figure 13. These additional peaks canbe associated to three spatial periodicities of 9.48, 8.57 and7.07 A along mutually perpendicular axis directions. Thesespatial periodicities cannot be associated to the chemical unitcell, indicating that they are not directly connected to structural

6

J. Phys.: Condens. Matter 22 (2010) 246002 C-W Wang et al

Figure 14. Temperature dependence of the additional Raman andx-ray peaks of the x = 0.18 compound, showing these peaks begin todevelop at 150 K and disappear below 70 K.

change or distortion of the chemical unit cell. Neutrondiffraction patterns taken in this temperature regime reveal nosuch changes but a thermal contraction of the chemical unitcell upon cooling. Figure 14 illustrates the thermal evolutionof the Raman and x-ray diffraction peaks associated withthis transition, showing that they develop below 140 K anddisappear below 70 K. Unfortunately, no enough informationis available to determine the specific nature of this behavior.Further investigations, likely on single crystals, will be neededto clarify the nature of the 140 K phase transition.

6. Discussion and conclusions

Both the 9% and 18% La-substituted compounds, Ho0.91La0.09

Mn2O4.98 and Ho0.83La0.18Mn2O4.98, crystallize into the sameorthorhombic Pbam symmetry as the parent compound.An incommensurate long range antiferromagnetically orderedphase is found below 38 K, while short range magneticcorrelations are observed in the 18% La-substituted compoundup to 140 K. Spin glass behavior associated with the shortrange correlations between the Mn3+ sublattices, with afreezing temperature of 80 K, has been reported to appearin LaMn2O5 [24]. No such behavior has been observed inthe isostructural compounds of the other rare-earths. Thepresent results show that nonmagnetic La3+ ions can indeedinitiate correlations between their neighboring Mn3+ ions. Themagnetic interactions between the Mn ions are mainly dueto the Mn3+–O(1)–Mn3+ superexchange interaction throughthe bridging oxygen ion. The maximum 18% La dopingresults in 1.97% and 2.98% increases in the in-plane Mn3+–O(1) and Mn3+–Mn3+ separations, respectively. Neitherincrease is favorable to strengthening the magnetic correlationsbetween the Mn3+ ions. It is then not the nearest neighboringinteractions that preserve the magnetic correlations up to atemperature that is a factor of 3.7 higher than the orderingtemperature.

Magnetic frustration is known to play a key role in themagnetic behavior of HoMn2O5 [5]. We believe that it stillgoverns the magnetic behavior in the 18% La-substituted

compound. Magnetic frustration can also trigger localspin glass type correlations [25–27]. Incorporation of thesignificantly larger La3+ ions onto the rare-earth sites of aHoMn2O5 lattice creates local distortions, which altering localmagnetic interaction and providing another source of magneticfrustration. Apparently, these alternations become significantenough to disrupt the lock-in commensurate structure duringmagnetic transition and to trigger local magnetic correlationspersist up to much higher temperatures, when 18% of the rare-earth sites are randomly occupied by La ions. It is possible thatthe interactions between these nano-scale magnetic clusterscontribute another component to the magnetic frustrations,as has been suggested in the cases of oxygen excessiveLaMnO3+δ system [28, 29].

Acknowledgments

Identification of commercial equipment in the text is notintended to imply recommendation or endorsement of saidequipments by the National Institute of Standards andTechnology. This work is supported by a grant from theNational Science Council of Taiwan, under Grant No. NSC 98-2112-M-008-016-MY3.

References

[1] Schmid H 1994 Ferroelectrics 162 317[2] Fiebig M, Lottermoser Th, Frohlich D, Goltsev A V and

Pisarev R V 2002 Nature 419 818[3] Hur N, Park S, Sharma P A, Guha S and Cheong S-W 2004

Phys. Rev. Lett. 93 107207[4] dela Cruz C R, Yen F, Lorenz B, Gospodinov M M, Chu C W,

Ratcliff W, Lynn J W, Park S and Cheong S-W 2006 Phys.Rev. B 73 100406(R)

[5] Blake G R, Chapon L C, Radaelli P G, Park S, Hur N,Cheong S-W and Rodrıguez-Carvajal J 2005 Phys. Rev. B71 214402

[6] Chapon L C, Blake G R, Gutmann M J, Park S, Hur N,Radaelli P G and Cheong S-W 2004 Phys. Rev. Lett.93 177402

[7] Kobayashi S, Osawa T, Kimura H, Noda Y, Kagomiya I andKohn K 2004 J. Phys. Soc. Japan 73 1593–6

[8] Vecchini C, Chapon L C, Brown P J, Chatterji T, Park S,Cheong S-W and Radaelli P G 2008 Phys. Rev. B 77 134434

[9] Ma C, Yan J-Q, Dennis K W, Llobet A, McCallum R W andTan X 2009 J. Phys.: Condens. Matter 21 346002

[10] Munoz A, Alonso J A, Casais M T, Martınez-Lope M J,Martınez J L and Fernandez-Dıaz M T 2002 Phys. Rev. B65 144423

[11] Chen Y, Yuan H, Tian G, Zhang G and Feng S 2007 J. SolidState Chem. 180 1340

[12] Golovenchits E and Sanina V 2004 J. Phys.: Condens. Matter16 4325–34

[13] Kobayashi S, Osawa T, Kimura H, Noda Y, Kagomiya I andKohn K 2004 J. Phys. Soc. Japan 73 1031–5

[14] Fukunaga M, Nishihata K, Kimura H, Noda Y andKohn K 2007 J. Phys. Soc. Japan 76 074710

[15] Kobayashi S, Kimura H, Noda Y and Kohn K 2005 J. Phys.Soc. Japan 74 468

[16] Fukunaga M, Nishihata K, Kimura H, Noda Y andKohn K 2008 J. Phys. Soc. Japan 77 094711

7

J. Phys.: Condens. Matter 22 (2010) 246002 C-W Wang et al

[17] Radulova I, Lovchinov V and Daszkiewicz M 2008 J. Magn.Magn. Mater. 320 43–6

[18] Munoz A, Alonso J A, Casais M T, Martınez-Lope M J,Martınez J L and Fernandez-Dıaz M T 2005 Eur. J. Inorg.Chem. 2005 685–91

[19] Yu C C, Huang S Y, Yeh C J, Lee C W, Wu C B andChou H 2008 J. Appl. Phys. 103 07E310

[20] Rietveld H M 1969 J. Appl. Crystallogr. 2 65[21] Larson A and von Dreele R B 1994 General structure analysis

system (GSAS) Los Alamos National Laboratory ReportLAUR 86-748

[22] Alonso J A, Casais M T, Martınez-Lope M J andRasines I 1997 J. Solid State Chem. 129 105

[23] Mihailova B, Gospodinov M M, Guttler B, Yen F,Litvinchuk A P and Iliev M N 2005 Phys. Rev. B 71 172301

[24] Alonso J A, Casais M T, Martınez-Lope M J, Martınez J L andFernandez-Dıaz M T 1997 J. Phys.: Condens. Matter9 8515–26

[25] Huang C Y 1985 J. Magn. Magn. Mater. 51 1–74[26] Awschalom D D, McCord M A and Grinstein G 1990 Phys.

Rev. Lett. 65 783[27] Luo W, Nagel S R, Rosenbaum T F and Rosensweig R E 1991

Phys. Rev. Lett. 67 2721[28] Sankar C R and Joy P A 2005 Phys. Rev. B 72 132407[29] Kim M S, Park J G, Kim K H, Noh T W and Ri H C 2000

J. Korean Phys. Soc. 37 561–4

8