The structural and magnetic phase transitions have been studied on NdFeAsO
single crystals by neutron and x-ray diffraction complemented by resistivity
and specific heat measurements. Two low-temperature phase transitions have been
observed in addition to the tetragonal-to-orthorhombic transition at T_S = 142
K and the onset of antiferromagnetic (AFM) Fe order below T_N = 137 K. The Fe
moments order AFM in the well-known stripe-like structure in the (ab) plane,
but change from AFM to ferromagnetic (FM) arrangement along the c direction
below T* = 15 K accompanied by the onset of Nd AFM order below T_Nd = 6 K with
this same AFM configuration. The iron magnetic order-order transition in
NdFeAsO accentuates the Nd-Fe interaction and the delicate balance of c-axis
exchange couplings that results in AFM in LaFeAsO and FM in CeFeAsO and
PrFeAsO.Comment: revised; 4 pages, 3 figures; accepted for publication in Phys. Rev. 

Dennis, Kevin

Goldman, Alan

Huang, Q.

Jensen, Brandt

Kim, M. G.

Klenzle, P.

Kreyssig, Andreas

Lograsso, Thomas

Lynn, J.

McCallum, R.

McQueeney, Robert

Ratcliff, W.

Tian, W.

Yan, J.-Q.

English

arXiv

W. Tian

W. Ratcliff

M. G. Kim

J.-Q. Yan

P. A. Kienzle

Q. Huang

B. Jensen

K. W. Dennis

R. W. McCallum

T. A. Lograsso

R. J. McQueeney

A. I. Goldman

J. W. Lynn

A. Kreyssig

Crossref

Physical Review B

Interplay of Fe and Nd magnetism in NdFeAsO single crystals

The structural and magnetic phase transitions have been studied on NdFeAsO single crystals by neutron and x-ray diffraction complemented by resistivity and specific-heat measurements. Two low-temperature phase transitions have been observed in addition to the tetragonal-to-orthorhombic transition at TS∼142 K and the onset of antiferromagnetic (AFM) Fe order belowTN∼137 K. The Fe moments order AFM in the well-known stripelike structure in the (ab) plane but change from AFM to ferromagnetic (FM) arrangement along the c direction below T∗∼15 K accompanied by the onset of Nd AFM order below TNd∼6 K with this same AFM configuration. The iron magnetic order-order transition in NdFeAsO accentuates the Nd-Fe interaction and the delicate balance of c-axis exchange couplings that results in AFM in LaFeAsO and FM in CeFeAsO and PrFeAsO.This article is from Physical Review B 82 (2010): 060514(R), doi:10.1103/PhysRevB.82.060514.</p

Digital Repository @ Iowa State University (ISU)

Interplay of Fe and Nd magnetism in NdFeAsO single crystalsW. Tian,1 W. Ratcliff II,2 M. G. Kim,1,3 J.-Q. Yan,1 P. A. Kienzle,2 Q. Huang,2 B. Jensen,1 K. W. Dennis,1 R. W. McCallum,1,4T. A. Lograsso,1 R. J. McQueeney,1,3 A. I. Goldman,1,3 J. W. Lynn,2 and A. Kreyssig1,31Ames Laboratory, Ames, Iowa 50011, USA2NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA3Department of Physics and Astronomy, Iowa State University, Ames, Iowa 50011, USA4Department of Materials Science and Engineering, Iowa State University, Ames, Iowa 50011, USAReceived 6 June 2010; revised manuscript received 9 August 2010; published 25 August 2010The structural and magnetic phase transitions have been studied on NdFeAsO single crystals by neutron andx-ray diffraction complemented by resistivity and specific-heat measurements. Two low-temperature phasetransitions have been observed in addition to the tetragonal-to-orthorhombic transition at TS142 K and theonset of antiferromagnetic AFM Fe order below TN137 K. The Fe moments order AFM in the well-knownstripelike structure in the ab plane but change from AFM to ferromagnetic FM arrangement along the cdirection below T15 K accompanied by the onset of Nd AFM order below TNd6 K with this same AFMconfiguration. The iron magnetic order-order transition in NdFeAsO accentuates the Nd-Fe interaction and thedelicate balance of c-axis exchange couplings that results in AFM in LaFeAsO and FM in CeFeAsO andPrFeAsO.DOI: 10.1103/PhysRevB.82.060514 PACS numbers: 74.70.Xa, 75.25.j, 61.50.Ks, 75.30.KzSince the discovery of high-temperature superconductiv-ity in the fluorine-doped compound LaFeAsO1−xFx with asuperconducting transition temperature of Tc=26 K,1 enor-mous efforts have been made to understand the crystallo-graphic, magnetic, and superconducting properties of theRFeAsO compounds R=rare-earth elements and theirdoped superconducting derivatives with Tc’s up to55 K.2–18 The parent RFeAsO compounds undergo atetragonal-to-orthorhombic phase transition and show anti-ferromagnetic AFM order of Fe and R moments.2,12–18 Al-though doping suppresses both the lattice distortion and Femagnetic order, magnetism is believed to play a major role inthe superconducting pairing mechanism in pnictide super-conductors demonstrated, e.g., by the observation of a mag-netic resonance in the superconducting state.19 Detailedknowledge of the magnetic order and crystallographic struc-ture is a key to gain insight into the magnetic and magneto-elastic coupling but has been hampered, so far, by the ab-sence of sizeable RFeAsO single crystals.Here we report on a neutron and x-ray diffraction study ofrecently available20 NdFeAsO single crystals complementedby resistivity and specific-heat measurements. In prior neu-tron powder studies, the tetragonal-to-orthorhombic latticedistortion has been observed below TS150 K Ref. 13followed by the onset of the well-known stripelike AFM or-der of Fe below TN141 K.12 The onset of the Nd AFMorder was reported for temperatures below TNd2 K in neu-tron powder-diffraction experiments.13,15,18 However,resistivity21 and muon-spin relaxation22 studies indicate alow-temperature phase transition at 5–6 K. Our NdFeAsOsingle crystals show two low-temperature transitions in ad-dition to the high-temperature transitions at TS142 K andTN137 K. Below T15 K, the Fe moments undergo astrongly hysteretic magnetic transition from AFM arrange-ment to a ferromagnetic FM arrangement along the c direc-tion but unchanged AFM stripelike order in the ab plane.The Nd moments order AFM below TNd6 K, coupled tothe Fe magnetic order by a common magnetic unit cell, in-dicating a complex interplay between Nd and Fe magnetism.NdFeAsO single crystals were grown out of NaAs flux asdescribed in Ref. 20. Batches with millimeter-sized singlecrystals and masses up to 7 mg were carefully examined bywavelength dispersive spectroscopy WDS in a JEOL JXA-8200 Superprobe electron probe microanalyzer and x-raypowder diffraction. As in Ref. 20, WDS measurements ondifferent samples from the same batch show no noticeablecompositional variation and confirmed, within the limits ofthe measurement, the atomic ratio of 1:1:1:1. No impurityphases were detected beyond the NaAs flux.Specific heat and resistivity were measured in a QuantumDesign physical property measurement system23 by the re-laxation method and the four-probe technique using EpotekH20E silver epoxy contacts on platinum leads. The homoge-neity of the batches was examined by resistivity measure-ments of several single crystals showing similar behavior.The reproducibility and excellent homogeneity of the grownmaterial were additionally confirmed by neutron-diffractionmeasurements on polycrystalline samples consisting of morethan 300 crystals. They showed well-defined transition tem-peratures and Bragg peaks in excellent agreement with thesingle-crystal measurements reported here. To check poten-tial ferromagnetism, the magnetization, M, was measuredwith increasing temperature from 5 to 20 K for 11 fields, H,between 0.1 and 5 T and the differential susceptibility wascalculated at each temperature. Parallel to the c axis, thezero-field intercept of the M vs H line characterizing a FMcomponent was essentially temperature independent givingan upper limit of 0.012B / f.u. Perpendicular to the c axis thevalue was an order of magnitude smaller.The lattice distortion was characterized by high-resolutionx-ray diffraction measurements using a four-circle diffracto-meter and Cu K1 radiation from a rotating anode x-raysource, selected by a Ge1 1 1 monochromator. The plate-like single crystal was attached to a flat copper sample holderPHYSICAL REVIEW B 82, 060514R 2010RAPID COMMUNICATIONS1098-0121/2010/826/0605144 ©2010 The American Physical Society060514-1on the cold finger of a closed-cycle displex refrigerator. Amosaicity of 0.03° was determined from full width at halfmaximum of the rocking curve of the 0 0 6 Bragg peak.The orthorhombic distortion, , was determined as describedin Ref. 24 using   0 scans through the tetragonal 1 1 7Bragg peak which splits into the orthorhombic 2 0 7 and0 2 7 Bragg peaks below TS.The neutron-diffraction experiments were carried out us-ing the BT9 thermal-neutron triple-axis spectrometer atNCNR, NIST. The crystal with a mass of 3 mg was mountedon a thin aluminum post, oriented in the orthorhombich 0 l scattering plane, and placed in an ILL cryostat.Rocking scans were resolution limited, indicating the highquality of the sample. Neutrons with a fixed incident energyof 14.7 meV and a collimation of 40-48-sample-40-openwere used. Additional measurements were taken on a numberof crystals heat treated in various ways to check the results,and the observed transitions were all consistent within uncer-tainties. Descriptions are given in terms of the orthorhombiccrystallographic nuclear unit cell present below TS.In Fig. 1, we compare the four measurements as afunction of temperature. The anomaly at TS142 K inspecific-heat and resistivity measurements correlates with thephase transition from tetragonal P4 /nmm to orthorhombicCmma. The orthorhombic lattice distortion, , increases be-low TS for decreasing temperature without further anomalies.Figure 2a shows the splitting of the tetragonal 2 2 0Bragg peak into the orthorhombic 4 0 0 and 0 4 0 Braggpeaks from our high-resolution neutron-diffraction measure-ments and demonstrates the bulk nature of the lattice distor-tion and the homogeneity of the sample, through resolution-limited peak shapes and a uniform transition temperature.The second anomaly found in specific-heat and resistivitydata at TN137 K corresponds to the onset of the well-known AFM stripelike Fe order.12 The intensity of the mag-netic 1 0 3/2 Bragg peak increases smoothly below TN seeFig. 1d. The inset illustrates that this transition shows nohysteresis upon cooling and warming through TN consistentwith a second-order transition.At low temperature, a weak and broad anomaly in theresistivity can be associated with the disappearance of themagnetic 1 0 3/2 Bragg peak at T15 K signaling achange in the Fe magnetic order. This behavior is demon-strated in Fig. 2c by rocking scans through the 1 0 3/2peak at selected temperatures. At the same temperature, T,new magnetic Bragg peaks occur as illustrated in Fig. 2bby the 1 0 1 peak and in Fig. 1d by the 1 0 0 peak. Theintensity of these peaks increases with decreasing tempera-ture as the Fe order changes while below TNd6 K the in-crease is much more pronounced. At TNd, a sharp peak is alsoobserved in the specific heat see inset of Fig. 1a and thetemperature derivative of the resistivity indicating the onsetof the Nd magnetic order.Details of the temperature dependence for these low-temperature transitions are shown in Fig. 3. The transition atT15 K shows hysteretic behavior illustrated in Fig. 3b,through the temperature dependence of the magnetic 1 03/2 and 1 0 1 Bragg peaks. The strong hysteresis of ap-prox. 3 K points to the first-order character of this transitionat T. As illustrated in Fig. 3, the 1 0 0 and 1 0 1 Braggpeaks follow the same trend with i weak intensity betweenT and TNd, and ii a strong but smooth and monotonicintensity increase below TNd without reaching saturationdown to the lowest measured temperature of 1.5 K. Bothlow-temperature transitions are reminiscent of Nd2CuO4,where the Nd-Cu coupling leads to abrupt changes in the Cuspin structure along with a significant induced Nd momentbefore the Nd spontaneously orders.25–27 It will be interestingto determine theoretically if a similar type of Fe-Nd couplingis the origin of this behavior in NdFeAsO.For the determination of the magnetic structures, a seriesof five crystallographic and 7, 5, and 15 magnetic Bragg-TsTNTNdT*TNdTFIG. 1. Color online a Temperature dependence of the spe-cific heat. b Resistivity open symbol, left axis and its derivativeline, right axis. c Orthorhombic distortion, , determined byx-ray diffraction as described in the text. d Neutron peak intensityfor the magnetic Bragg peaks at 1 0 0 left axis and at 1 0 3/2right axis. The insets show expanded scales.TIAN et al. PHYSICAL REVIEW B 82, 060514R 2010RAPID COMMUNICATIONS060514-2peak intensities were measured by rocking scans at T=30 K, 10 K, and 1.5 K, respectively, and corrected forinstrumental resolution. The crystallographic Bragg peakswere fit using the published crystal structure13 to determinethe scale factor as the only refinable parameter. The value ofthe Fe magnetic moment was then determined from the in-tensity of the magnetic Bragg peaks considering equal do-main averaging and using this scale factor and the magneticstructure models given below.At TT=30 KTN we find the same stripelike AFMstructure previously reported.12 The Fe moments are col-linearly aligned along the a axis with AFM arrangement inthe a and c directions and FM in the b direction. The mag-netic unit cell for this magnetic structure is doubled along thec direction with respect to the orthorhombic crystallographicnuclear unit cell and yields the observed magnetic Braggpeaks at positions indexed by a propagation vector of 1 01/2. The determined value of 0.543B for the Fe momentis larger than the previously reported value of 0.257B.12The reason for this difference is unknown and points to dif-ferences between polycrystalline and single-crystal samplespotential small deviations in stoichiometry or strain. How-ever, we note that the magnetic peaks associated with ironordering were not reported in several other neutron powderscattering experiments.13,15,18The magnetic diffraction pattern at TNdT=10 KTcan be indexed using a magnetic unit-cell identical to theorthorhombic crystallographic nuclear unit cell. In particu-lar, no doubling along the c direction is now required, sig-naling a change from AFM ordering in this direction aboveT to FM ordering below T. Together with the unchangedstripelike order in the ab plane, this magnetic structure issimply described by a propagation vector of 1 0 0. Theintensity of all measured magnetic Bragg peaks can still befit by Fe moments alone with a reduced moment value of0.323B collinearly aligned along the orthorhombic a axis.At TTNd, the strongly enhanced intensity of the mag-netic Bragg peaks indicates the onset of the Nd magneticorder. The description of the magnetic diffraction pattern re-quires both Fe and Nd moments to be ordered. The positionsof the magnetic Bragg peaks are consistent with the reportedlow-temperature magnetic structure.13,15 However, furthermeasurements are necessary for a detailed analysis of themagnetic structure with its extensive number of parameters.The potential overlapping of Bragg peaks from twin domainswith different orientations together with the small orthor-hombicity require experiments with higher resolution or theusage of detwinned samples.Summarizing, below TN137 K, the Fe moments orderAFM with the ubiquitous stripelike arrangement in the ortho-rhombic ab plane. For TT15 K the c-axis configura-tion changes from AFM as observed in LaFeAsO Ref. 2 toFM as found in CeFeAsO Ref. 28 and PrFeAsO.29,30 Thistemperature-dependent change in the Fe magnetic order inNdFeAsO is unique in the RFeAsO series and demonstratesthat the magnetic coupling in the c direction is at the borderbetween AFM and FM. Further studies of RFeAsO com-pounds with other R elements are necessary to determinewhether this delicate balance in the magnetic coupling iscontrolled by steric effects or the magnetocrystalline aniso-tropy of the R element caused by crystal-electric field effectsFIG. 2. Color online a Neutron-diffraction -2 scansthrough the tetragonal 2 2 0 nuclear Bragg peak at T=158 K andits splitting into two orthorhombic Bragg peaks at T=110 K mea-sured with reduced collimation of 10-10-sample-10-10. Neu-tron diffraction rocking scans through the magnetic Bragg peaks atb 1 0 1 and c 1 0 3/2 at selected temperatures.TNdT*FIG. 3. Color online Neutron peak intensity for the magneticBragg peaks at a 1 0 0 and b at 1 0 1 left axis and 1 0 3/2right axis. The inset in a shows an expanded scale between TNdand T.INTERPLAY OF Fe AND Nd MAGNETISM IN NdFeAsO… PHYSICAL REVIEW B 82, 060514R 2010RAPID COMMUNICATIONS060514-3transferred through a complex coupling of the Nd and Femagnetism. The proximity between the characteristic tem-peratures for the order-order transition of the Fe magneticorder at T to the onset of the Nd magnetic order atTNd6 K together with a common magnetic unit cell at lowtemperatures suggests the second case as the more likelyscenario.Research at Ames Laboratory and Oak Ridge NationalLaboratory was supported by the U.S. Department of Energy,Office of Basic Energy Science, Division of Materials Sci-ences and Engineering and Scientific User Facilities Divi-sion, respectively. Ames Laboratory is operated for the U.S.Department of Energy by Iowa State University under Con-tract No. DE-AC02-07CH11358.1 Y. Kamihara, T. Watanabe, M. Hirano, and H. Hosono, J. Am.Chem. Soc. 130, 3296 2008.2 C. de la Cruz, Q. Huang, J. W. Lynn, J. Li, W. Ratcliff II, J. L.Zarestky, H. A. Mook, G. F. Chen, J. L. Luo, N. L. Wang, and P.Dai, Nature London 453, 899 2008.3 A. S. Sefat, M. A. McGuire, B. C. Sales, R. Jin, J. Y. Howe, andD. Mandrus, Phys. Rev. B 77, 174503 2008.4 Z. A. Ren, J. Yang, W. Lu, W. Yi, G. C. Che, X. L. Dong, L. Sun,and Z. Zhao, Mater. Res. Innovations 12, 105 2008.5 Z.-A. Ren, J. Yang, W. Lu, W. Yi, X.-L. Shen, Z.-C. Li, G.-C.Che, X.-L. Dong, L.-L. Sun, F. Zhou, and Z.-X. Zhao, EPL 82,57002 2008.6 G. F. Chen, Z. Li, D. Wu, G. Li, W. Z. Hu, J. Dong, P. Zheng, J.L. Luo, and N. L. Wang, Phys. Rev. Lett. 100, 247002 2008.7 Z.-A. Ren, G.-C. Che, X.-L. Dong, J. Yang, W. Lu, W. Yi, X.-L.Shen, Z.-C. Li, L.-L. Sun, F. Zhou, and Z.-X. Zhao, EPL 83,17002 2008.8 J. Yang, Z.-C. Li, W. Lu, W. Yi, X.-L. Shen, Z.-A. Ren, G.-C.Che, X.-L. Dong, L.-L. Sun, F. Zhou, and Z.-X. Zhao, Super-cond. Sci. Technol. 21, 082001 2008.9 J. W. G. Bos, G. B. S. Penny, J. A. Rodgers, D. A. Sokolov, A.D. Huxley, and J. P. Attfield, Chem. Commun. Cambridge2008, 3634.10 R. H. Liu, G. Wu, T. Wu, D. F. Fang, H. Chen, S. Y. Li, K. Liu,Y. L. Xie, X. F. Wang, R. L. Yang, L. Ding, C. He, D. L. Feng,and X. H. Chen, Phys. Rev. Lett. 101, 087001 2008.11 C. Wang, L. Li, S. Chi, Z. Zhu, Z. Ren, Y. Li, Y. Wang, X. Lin,Y. Luo, S. Jiang, X. Xu, G. Cao, and Z. Xu, EPL 83, 670062008.12 Y. Chen, J. W. Lynn, J. Li, G. Li, G. F. Chen, J. L. Luo, N. L.Wang, P. Dai, C. de la Cruz, and H. A. Mook, Phys. Rev. B 78,064515 2008.13 Y. Qiu, W. Bao, Q. Huang, T. Yildirim, J. M. Simmons, M. A.Green, J. W. Lynn, Y. C. Gasparovic, J. Li, T. Wu, G. Wu, and X.H. Chen, Phys. Rev. Lett. 101, 257002 2008.14 Q. Huang, J. Zhao, J. W. Lynn, G. F. Chen, J. L. Luo, N. L.Wang, and P. Dai, Phys. Rev. B 78, 054529 2008.15 A. Marcinkova, E. Suard, A. N. Fitch, S. Margadonna, and J. W.G. Bos, Chem. Mater. 21, 2967 2009.16 D. H. Ryan, J. M. Cadogan, C. Ritter, F. Canepa, A. Palenzona,and M. Putti, Phys. Rev. B 80, 220503R 2009.17 A. Marcinkova, D. A. M. Grist, I. Margiolaki, T. C. Hansen, S.Margadonna, and Jan-Willem G. Bos, Phys. Rev. B 81, 0645112010.18 L. Malavasi, G. A. Artioli, C. Ritter, M. C. Mozzati, B. Maroni,B. Pahari, and A. Caneschi, J. Am. Chem. Soc. 132, 24172010.19 A. D. Christianson, E. A. Goremychkin, R. Osborn, S. Rosen-kranz, M. D. Lumsden, C. D. Malliakas, I. S. Todorov, H. Claus,D. Y. Chung, M. G. Kanatzidis, R. I. Bewley, and T. Guidi,Nature London 456, 930 2008.20 J.-Q. Yan, S. Nandi, J. L. Zarestky, W. Tian, A. Kreyssig, B.Jensen, A. Kracher, K. W. Dennis, R. J. McQueeney, A. I. Gold-man, R. W. McCallum, and T. A. Lograsso, Appl. Phys. Lett.95, 222504 2009.21 P. Cheng, H. Yang, Y. Jia, L. Fang, X. Zhu, G. Mu, and H.-H.Wen, Phys. Rev. B 78, 134508 2008.22 A. A. Aczel, E. Baggio-Saitovitch, S. L. Bud’ko, P. C. Canfield,J. P. Carlo, G. F. Chen, P. Dai, T. Goko, W. Z. Hu, G. M. Luke,J. L. Luo, N. Ni, D. R. Sanchez-Candela, F. F. Tafti, N. L. Wang,T. J. Williams, W. Yu, and Y. J. Uemura, Phys. Rev. B 78,214503 2008.23 Any mention of commercial products within this Rapid Commu-nication is for information only; it does not imply recommenda-tion or endorsement by NIST.24 S. Nandi, M. G. Kim, A. Kreyssig, R. M. Fernandes, D. K. Pratt,A. Thaler, N. Ni, S. L. Bud’ko, P. C. Canfield, J. Schmalian, R.J. McQueeney, and A. I. Goldman, Phys. Rev. Lett. 104,057006 2010.25 S. Skanthakumar, H. Zhang, T. W. Clinton, W.-H. Li, J. W. Lynn,Z. Fisk, and S.-W. Cheong, Physica C 160, 124 1989.26 J. W. Lynn, I. W. Sumarlin, S. Skanthakumar, W.-H. Li, R. N.Shelton, J. L. Peng, Z. Fisk, and S.-W. Cheong, Phys. Rev. B41, 2569 1990.27 R. Sachidanandam, T. Yildirim, A. B. Harris, A. Aharony, and O.Entin-Wohlman, Phys. Rev. B 56, 260 1997.28 J. Zhao, Q. Huang, C. de la Cruz, S. Li, J. W. Lynn, Y. Chen, M.A. Green, G. F. Chen, G. Li, Z. Li, J. L. Luo, N. L. Wang, andP. Dai, Nature Mater. 7, 953 2008.29 J. Zhao, Q. Huang, C. de la Cruz, J. W. Lynn, M. D. Lumsden, Z.A. Ren, J. Yang, X. Shen, X. Dong, Z. Zhao, and P. Dai, Phys.Rev. B 78, 132504 2008.30 S. A. J. Kimber, D. N. Argyriou, F. Yokaichiya, K. Habicht, S.Gerischer, T. Hansen, T. Chatterji, R. Klingeler, C. Hess, G.Behr, A. Kondrat, and B. Büchner, Phys. Rev. B 78, 140503R2008.TIAN et al. PHYSICAL REVIEW B 82, 060514R 2010RAPID COMMUNICATIONS060514-4

Interplay between Fe and Nd magnetism in NdFeAsO single crystals

Interplay between Fe and Nd magnetism in NdFeAsO single crystals

Abstract

Similar works

Full text

Available Versions

Crossref

Digital Repository @ Iowa State University (ISU)