Interplay between Co-3d and Ce-4f magnetism in CeCoAsO
Rajib Sarkar, Anton Jesche, Cornelius Krellner, Chandan Mazumdar, Asok Poddar, Michael Baenitz, Cristoph Geibel
aa r X i v : . [ c ond - m a t . s t r- e l ] A ug APS/123-QED
Interplay between Co-3d and Ce-4f magnetism in CeCoAsO
Rajib Sarkar, Anton Jesche, Cornelius Krellner, Michael Baenitz, and Cristoph Geibel
Max-Planck Institute for Chemical Physics of Solids, 01187 Dresden, Germany
Chandan Mazumdar and Asok Poddar
ECMP Division, Saha Institute of Nuclear Physics, 1/AF Bidhannagar, Kolkata-700064, India (Dated: May 1, 2018)We have investigated the ground state properties of polycrystalline CeCoAsO by means of mag-netization, specific heat and solid state NMR. Susceptibility and specific-heat measurements suggesta ferromagnetic order at about, T C =75 K. No further transitions are found down to 2 K. At 6.5 Ka complex Schottky type of anomaly shows up in the specific heat results. The interplay betweenCe-4f and Co-3d magnetism being responsible for that anomaly is discussed. Furthermore AsNMR investigations have been performed to probe the magnetism on a microscopic scale. As-NMRspectra are analysed in terms of first and second order quadrupolar interaction. The anisotropicshift component K ab and K c could be derived from the As powder spectra. Towards lower tem-perature a strong shift anisotropy was found. Nonetheless K iso tracks the bulk susceptibility downto T =50 K very well. Furthermore the presence of weak correlations among the Ce ions in theferromagnetic state is discussed. The observed increase of C/T towards lower temperatures supportsthis interpretation.
PACS numbers: Valid PACS appear here
I. INTRODUCTION:
The rare earth transition metal pnictides RTPnO (R:rare earth, T: transition metal, Pn: P or As) attractedconsiderable attention because of the recent discovery ofsuperconductivity with a transition temperatures T C upto 50 K in the RFeAsO − x F x series of compounds be-ing the highest T C ‘s except for cuprate systems [1–6].While the main studies on these materials are devotedto the superconducting materials, the non superconduct-ing members of these family stays mostly unexplored.Nevertheless studying these compounds may provide in-formation to understand also the superconducting state.Recent studies of CeRuPO and CeOsPO [7–9] indicatedissimilar types of magnetic ordering. CeRuPO is a rareexample for ferromagnetic (FM) Kondo system showinga FM order at T C =15 K and a Kondo energy scale ofabout T K ≃
10 K. CeOsPO exhibits an antiferromag-netic (AFM) order at T N =4.5 K. The recent studies ofthe relative CeFePO suggest that this is a heavy Fermionmetal with strong correlation of the 4f-electrons close toa magnetic instability [10]. On the other hand in Ce-FeAsO, a complex interplay of Ce-4f and Fe-3d mag-netism is found. Here, Ce orders antiferromagneticallyat T N ≃ ≃
151 K followed by SDW type AFM order of Fe at ∼
145 K. Moreover Ce magnetism is not effected strongly bythe presence of the Fe moments. Furthermore, neutronscattering, muon spin relaxation experiments and recentanalysis of χ ( T ), C ( T ) suggest that there is a sizeableinter-layer coupling in CeFeAsO [12–15]. Therefore, thepure CeFeAsO system have already proven to be a richreservior of exotic phenomena. Apart from isoelectronic substitution on CeTPnO withFe, Ru, Os chemically also the T= Co series form. Re-sults on LaCoAsO and LaCoPO were reported to exhibitferromagnetic order of Co moment with Curie tempera-tures T C of about T C =50 K and T C =60 K, respectively.In contrast to Fe, where the 3d magnetism depends onthe Pnictide(P,As), Co stays magnetic in both the P andAs series. In LaCoAsO, Co saturation moments of 0.3-0.5 µ B per Co [16–18] are found. It has been proposed thatspin fluctuations play an important role in the magneticbehavior of LaCoAsO [16][18], as well as the magneticand superconducting properties of the iron-based super-conductors. Last year we reported the detailed physicalproperties of CeCoPO and discussed about the interplaybetween 3d magnetic moments of Co and 4f electrons ofCe [19]. This system, similar to LaCoPO, the Co-3d elec-trons order ferromagnetically. However, here the Ce-ionsare on the border to magnetic order and an enhancedSommerfeld coefficient, γ ∼
200 mJ/mol K was found.In CeTPnO the substitution of P by As change the mag-netism drastically. It is already evident in the case of Ce-FeAsO [20–22]. Therefore it is natural to investigate thephysical and microscopic properties of CeCoAsO com-pound.In this report we present the physical properties of poly-crystalline CeCoAsO using susceptibilty χ ( T ) and spe-cific heat C ( T ) measurements. Additionally, we discussthe preliminary microscopic results as seen by As NMRstudy.
II. EXPERIMENTAL
Samples are prepared by solid state reaction technique.The starting materials that are taken for the preparationof parent CeCoAsO are Ce and As chips and Co, Co O powders. First, CeAs was prepared by taking stoichio-metric amounts of Ce and As in 1:1 ratio, pressed intopellets and sealed in evacuated quartz tube. With re-peated heat treatment, attaining a maximum tempera-ture of 900 ◦ C, and grinding inside a glove box filled withinert Ar gas, single phase CeAs were obtained. CeAswere then mixed thoroughly with Co O and Co pow-der in stoichiometry and pressed into pellets. The pelletswere wrapped with Ta foil and sealed in an evacuatedquartz tube. They were then annealed at 1100-1150 ◦ Cfor 40-45 hours to obtain the final CeCoAsO samples.X-ray powder diffraction revealed a single phase sam-ple with no foreign phases. Susceptibility χ ( T ) measure-ments were performed in a commercial Quantum Design(QD) magnetic property measurement system (MPMS).Specific heat C ( T ), measurements were performed ina QD physical property measurement system (PPMS).For the NMR measurements, polycrystalline powder wasfixed in the paraffin to ensure a random orientation. AsNMR measurements were performed with a standardpulsed NMR spectrometer (Tecmag) at the frequency 48MHz as a function of temperature. The field sweep NMRspectra were obtained by integrating the echo in the timedomain and plotting the resulting intensity as a functionof the field. Shift values are calculated from the reso-nance field H ∗ by K ( T ) = ( H L − H ∗ ) /H ∗ whereas theLarmor field, H L , is given by using GaAs ( As-NMR)as reference compound with K ≃ III. RESULTSA. Magnetisation and specific heat study
Fig. 1 shows the susceptibility of CeCoAsO as a func-tion of temperature at different fields as indicated. Above200 K, the susceptibility follows the Curie Weiss behav-ior with an effective moment µ eff = 2.74 µ B . This valueis higher than the value of µ Ce eff =2.54 µ B , expected for afree Ce ion. This is because of the contribution of Coto the effective moment of µ eff = p µ (Co)2 + µ (Ce)2 . Forthe Co ions we calculate an effective moment of µ Co eff =1.03 µ B . Our results are in good agreement with findings fromOhta and Yoshimura et. al. [18]. A sharp increase of thesusceptibility is observed at around 75 K. Here, a strongfield dependence of the susceptibility typical for a FMsystem is evidenced. In the inset of Fig. 1 the magneti-sation M(H) of CeCoAsO up to 5 T at 2 K is shown.A large hysteresis typical for hard ferromagnets is ob-served. A large hysteresis was also found for CeCoPOwhereas for other RCoAsO no such large hysteresis wasfound [18]. In the left inset of Fig. 1 we compare thesusceptibility results of CeCoAsO with that of LaCoAsOand CeCoPO at 1 T. It should be mentioned that for Ce-CoAsO at smaller fields no peak could be resolved whichis in contrast to CeCoPO [19]. The temperature depen-dence of the susceptibilty is quiet similar for CeCoAsO -6 -4 -2 0 2 4 6-0.4-0.20.00.20.4 ( - m / m o l ) T (K) M ( B / m o l ) H (T) LaCoAsO
CeCoPO CeCoAsO
T(K) ( - m / m o l ) FIG. 1: Temperature dependence of the susceptibility of Ce-CoAsO at different fields. The right inset shows the magneti-sation at 2 K and the left inset shows a comparison of thesusceptibility for CeCoPO (data taken from [19]), CeCoAsO,and LaCoAsO (data taken from [ ? ]) at 1 T. and CeCoPO, whereas there is a pronounced differenceto that of LaCoAsO. In LaCoAsO there is no influence of4f magnetism. Therefore the magnetisation curve lookslike a simple textbook ferromagnet. It has to be men-tioned that we took the data for LaCoAsO from Ref. [ ? ]. For this set of data χ (LaCoAsO) > χ (CeCoAsO) isfound in the FM state but surprisingly the data by Ohtaet al. [18] shows smaller values (at same field) suggest-ing χ (LaCoAsO) < χ (CeCoAsO). Nonetheless our resultsfor CeCoAsO are in perfect agreement with findings in[18]. The different behavior for CeCoPO and CeCoAsOindicate that the Ce-4f creates a significant change inthe overall magnetic behavior. This unusual behavior ofthe susceptibility indicates an intricate magnetic struc-ture with strong polarisation of the itinerant Co mo-ments on the more localised Ce moments. Such effectsare also known from 4f-ion Fe Sb skutterudites. HereEuFe Sb shows similar magnetisation curves[24]. Oneapproach could be to describe CeCoAsO in the frame-work of a classical ferrimagnet like RCo with R=Y, Ce,Pr.... [25]. Magnetisation in these system is goverenedby the two subsystems of the 4f and 3d moments andtheir inter and intra molecular interactions. Because ofthe antiferromagnetic coupling of the rare earth spinswith the Co spin for light rare earth ions like Ce, usu-ally an ferromagnetic alignment of the Ce-4f momentand the Co-3d moments is expected [25] in the orderedstate. This would imply χ (CeCoAsO) < χ (LaCoAsO).Unfortunately because of inconsistent literature for La-CoAsO data we have no prove for that. To summarizethe magnetisation section it should be note that besidesthe high temperature ordering at T C =75 K for CeCoPOand T C =75 K for CeCoAsO no further transitions areevident from magnetisation measurements. This is incontrast to other RCoAsO system with R=Gd, Sm or
65 70 75 800.00.40.81.2 C / T ( - J / m o l K ) T (K) T(K) C / T ( J / m o l K ) C ( J / m o l - K ) T (K)75 K T(K) H (T) C / T ( J / m o l K ) FIG. 2: (Left panel)Temperature dependence of the specificheat of CeCoAsO. The right inset shows the specific heatplotted as
C/T on a logarithmic temperature scale. The leftinset shows the behavior of transition after subtracting thebackground. Right panel shows the specific heat plotted as
C/T vs. T at different field in the temperature range 1.8 -20K (note that 0.1 J/mol-K was added as an offset in the C/Tplot on the right). Nd, where the rare earth moments orders antiferromag-netically at low temperature[18][38]. The temperatureand field dependence of the specific heat C ( T ) of Ce-CoAsO is shown in Fig. 2. Towards high temperatures C ( T ) converges nicely to the classical Dulong Petit limitof ∼
100 J/mol-K. By lowering the temperature a broadanomaly at T C ∼
75 K is visible on the top of the phonondominated specific heat. We have estimated the back-ground using the third order polynomial from 65 to 85K, excluding the temperature range near the peak at 75K. Then the background was subtracted from the data toobtain specific heat. In the left inset we have shown the C ( T ) /T vs. T plot after subtracting the background. Itis worthwhile to mention that at the same temperature(75 K) the susceptibility increases sharply. Therefore,this anomaly is being due to the ferromagnetic orderingof Co. At around 6.5 K an additional broad anomaly inthe C ( T ) shows up. The right inset shows the C ( T ) /T vs. T plots. In this inset both anomalies are rather pro-nounced. To understand the origin of the low temper-ature anomaly we have investigated the field dependentspecific heat in the temperature range 1.8 - 15 K and inthe field range 0-9 T. The right panel of Fig. 2 shows the C/T vs. T plot at different fields (curves are shifted ony axis by 0.1 J/mol-K ). It seems that the effect of fieldon C ( T ) is very small. Nonetheless, the broad maxima isshifted insignificantly towards higher temperatures withincreasing field. The preliminary analysis of the specificheat reveal that this broad anomaly at low temperatureis not due to the ordering of Ce. Rather this is reminis-cent of Schottky type anomaly. This might be attributedto the level splitting CEF ground state of Ce by the po- s p i n ec ho i n t e n s i t y ( a r b . un i t s )
50 K150 K200 K
60 K70 K80 K90 K100 K110 K125 K H (T)
48 MHz .
200 K H (T) As NMR H (T)
100 K H (T) FIG. 3: Temperature dependence of the As field sweepNMR spectra at 48 MHz down to 50 K (left panel). Dottedline indicates the Larmor field estimated from the referencecompound GaAs with K ≃ As spectra below 50 K(right, bottom). As spectra at 200 K and 100 K togetherwith the theoretical simulation (solid line)(right, top). larisation field of Co. For Ce ion is, in a tetragonalenvironment, J = 5 / C/T increases logarithmically and tendsto saturate at further low temperature at γ ∼ . This enhancement might indicate the pres-ence of strong correlations between the Ce ions in the Codominated ferromagnetic ground state.Furthermore, at zero field we have estimated the en-tropy gain by integrating the 4f part to the specific heat C /T in the temperature range 1.8-15 K. For the estima-tion of the entropy gain we have subtracted the phononcontribution by using reference compound data of La-CoAsO in the temperature range 1.8-15 K after Sefat et.al. [ ? ]. However, the contribution of LaCoAsO to thespecific heat is small below 15 K. The estimated entropygain for CeCoAsO at 15 K is 75% of R ln2. This supportsthe scenario based on the splitting of the CEF doubletground state. B. As NMR
Fig.3 shows the field sweep As NMR spectra at dif-ferent temperatures. Because As is a I=3/2 nuclei, thequadrupole interaction should be taken into account forthe interpretation of the spectra. The main effects are I.)first order interaction, occurrence of pronounced satellitepeaks (3/2 ↔ ↔ -3/2), II.) second order interac-tions, splitting of the central -1/2 ↔ Co spectra and the As NMR spectra. We alreadymeasured some Co NMR spectra. The estimated ν Q value from As NMR spectra at high temperature is 3.6MHz. This value is similar to that of LaCoAsO system[36] and somewhat smaller than what was found for theCeFeAsO system [22]. While lowering the temperature ν Q monotonically increases and down to 20 K no dras-tic changes could be detected. This rules out a suddenstructural change in this compound. From the simulationof the spectra, we have estimated the shift components K ab and K c corresponding to H ⊥ c and H k c direc-tion, respectively.Fig. 4 shows the variation of K ab , K c and K iso as a function of temperature. K iso was estimated us-ing the equation K iso =
23 75 K ab +
13 75 K c . From Fig.4 it is evident that K ab and K c increases with de-creasing temperature presenting a strong anisotropy. Athigh temperature the anisotropy is small where as withdecreasing the temperature the anisotropy is enhancedconsiderably. If we compare K ab , K c at 50 K, it isseen that anisotropy of the transferred field is really im-portant here and K ab is 2.5 times larger than K c .From Fig. 4 it is seen that K ab , K c and K iso in-creases with decreasing temperature following the bulksusceptibility down to the temperature 50 K. However,with further lowering the temperature down to 30 Kshift is decreasing with temperature leaving a maximaat around 50 K. This maxima traced back the results ofsusceptibility and specific heat study. Which indicatesthe ferromagnetic Co ordering take place at T c
75 K. Itis worthwhile to mention that the bulk susceptibility isconstantly increasing with decreasing the temperature.It is well established that NMR shift probes the localsusceptibilty, therefore it is normally not influenced bythe small amount of the impurity. There are two pos-
50 100 150 2000246810121416 K ( % ) K ( % ) T (K ) K ab K C K iso
50 K (10 -6 m /mol) K iso FIG. 4: Temperature dependence of the K ab , K c and K iso . Inset shows the plots of K iso against the bulk sus-ceptibility χ measured at 5 T. sibilities for the decrease of the shift. First, the systemhave small amount of impurities which is not tracked bythe XRD measurement and makes the increase of sus-ceptibility. Second, there are polarisation effects on theCe ions by the internal magnetic field of the Co mag-netism. Which in turn changes the transferred hyperfinefield below 50 K. The former one is unlikely because ofthe well-matched magnetisation result of CeCoPO andLaCoAsO [18][19]. Therefore, the later scenario is morelikely. Below 50 K the decrement of the shift reveals thatthe ferromagnetically ordered Co moment polarises theCe moment. Which eventually makes the magnetic struc-ture complicated and changes the hyperfine field in thissystem. NMR probes the magnetism on a microscopicscale. As NMR gives the local hyperfine field arising fromthe 4f-Ce and the 3d-Co ions. Usually for the itinerant3d ions the negative core polarisation is the dominantexchange mechanism whereas for localised Ce momentsthe strong conduction electron polarization contribution(Fermi contact interaction) becomes important. Some-times both fields cancel out each other leading to K = 0condition but often the Fermi contact interaction is morethan one order of magnitude larger [29]. Therefore thetotal shift could be composed as K = K f + K d . Fur-thermore K f couples strongly on the effective Ce-4f-moment. The effective moment is reduced because ofCEF splitting which results in a reduction of K f . Thismight explain the shift maximum observed.For an estimation of the hyperfine coupling constant K iso is plotted as a function of the bulk susceptibility χ in the inset of Fig. 4. For this we have used the sus-ceptibility measured at 5T. We assume that χ = χ iso ,meaning that there is no alignment or the texture in theCeCoAsO sample. From the inset it is seen that K iso is nicely following the bulk susceptibility in the temper-ature range 200-50 K. From the linear curve we haveestimated the hyperfine coupling constants at the Assite, A iso ≈
18 kOe/ µ B . The estimated A iso for theCeCoPO and LaCoAsO is around 14 kOe/ µ B and 24.8kOe/ µ B , respectively [36][37]. Therefore for CeCoAsOthe value of hyperfine coupling constants is higher thanthat of CeCoPO system which could be interpreted asbeing due to a weaker 3d-4f polarisation in CeCoAsO.Below 15 K the additional line broadening shows up inthe spectra. Such a broadening can not be explained bythe impurities or disorder. As a first approach this linebroadening traced back the specific heat anomaly at 6.5K and increase of C/T below 1.8 K. This broadening isalso typical for the onset of correlation. However for thissystem at present with the available data it is not settledwhether this is the result of complicated magnetic struc-ture or it is due to the correlation. For another 4f-3dpnictide NdCoAsO very recently McGurie et. al. pro-posed from neutron scattering multiple phase transitionsat low temperature[38].
IV. DISCUSSION AND SUMMARY
Our findings on CeCoAsO yield several interesting phe-nomena which shall now be discussed. The presented re-sults point to a FM ordering of Co ions at T C =75 Ksimilar to FM order in the P-homolouge[19]. Similar toCeCoPO unusual, but more S-like shaped χ ( T ) curvesbelow T C are observed. This is in contrast to LaCoPOand LaCoAsO where χ ( T ) behave like textbook ferro-magnets. For CeCoPO and CeCoAsO a complex inter-play of Co-3d moments with more localised Ce-4f mo-ments has to be considered. More into detail: Co 3d ionsorder ferromagnetically at T C =75 K and the resulting in-ternal field transferred to the Ce site partially polarizesthe Ce-4f ions. Due to thermal excitation the polarizationis getting stronger towards lower temperatures leading toan S-like shape of χ ( T ). Moreover, in the specific heatan additional broad anomaly at around 6.5 K shows up,which is different to CeCoPO system. This low tempera-ture Schottky anomaly indicates a complex level splittingof the ground state of the rare earth moment by the in-ternal field.In order to get a deeper microscopic insight of this sys-tem we have performed As NMR investigations. TheNMR shift is increasing with decreasing the temperaturefollowing the susceptibility down to 50 K. The shift isdecreased with lowering the temperature further result-ing in a broad maxima at 50 K. This indicates that thereis a change of hyperfine field below 50 K. Furthermore,in the As NMR spectra the additional line broaden-ing below 15 K, traced back the specific heat anomaly at6.5 K. Such broadening effect are typical for the onset ofcorrelations. But also a reconstruction of hyperfine fieldsbecause of CEF splitting could be a possible explanation. To fully understand the low temperature magnetism itdeserves further investigation specially with microscopictool, for instance neutron scattering and/or µ SR.Furthermore based on the presented results dopingstudies on CeCoAsO might be fruitful in the contextof superconductivity. The superconducting state of thedoped RFeAsO systems are suggested to be unconven-tional nature. One key point to get the superconduc-tuvity in the CeFeAsO system is to suppress the Fe mag-netism by changing the carrier concentration. One ap-proach would be to substitute F in place of O, or sub-stitute As by P or Fe by small concentration of Co.In all cases for the specific doping concentration onewould get superconductivity [33][39][4]. Therefore stillit is not settled the nature of the carrier concentra-tion required for this CeFeAsO based superconductor.Apart from the Fe-based pnictides, superconductivitywas also found in LaNiPO and LaNiAsO. Furthermorein the 122 relative Ba(Fe, Co) As T C ‘c up to 22 K arefound. Co NMR investigations clearly reveals that Cois nonmagnetic[34][35]. The absence of superconductiv-ity in CeCo(As/P)O is not surprising considering the factthat here Co carries a moment and long range order is ob-served. Furthermore in these system there is a complexinterplay between the Ce-4f and 3d magnetism playinga crucial role to control the magnetism. It is worthy tomention, as far as 3d magnetism is concern, there is amajor difference between the CeCoAsO to that of Ce-FeAsO. For CeFeAsO, Fe magnetism can be tuned easilyreplacing As by P and SDW type transition diminished,whereas for CeCoAsO, Co magnetism stays rigid withthat and SDW transition is absent. However still thereis a possibility to suppress the Co magnetism either bydoping or pressure, because an ordering temperature ofaround 70 K is rather low for Co ordering. Thereforefurther research has to answer the question whether thesuperconductivity appears in this system after the sup-pression of the Co- magnetism. Which will eventuallyopens up the opportunity to understand the nature ofcoupling between the 4f and itinerant electrons in theRTPnO systems. And NMR/NQR would be the valu-able tool to probe the magnetism and superconductivity.In summary we presented magnetisation, specific heatand As NMR investigations on polycrystalline Ce-CoAsO. The magnetisation and specific heat data revealthat in this system Co orders ferromagnetically at 75 K.Moreover specific heat study shows an Schottky anomalyat low temperature at around 6.5 K, which is likely dueto the level splitting of the ground state of rare earthmoment by the internal field. Furthermore the analysisof the As NMR spectra clearly demonstrate the strongshift anisotropy towards lower temperature. Moreoverthe breakdown of the K vs. χ linearity below 50 K mightbe the signature of CEF interaction. [1] Y. Kamihara, T. Watanabe, M. Hirano, and H. Hosono.J. Am. Chem. Soc. 130(11), 3296(2008). [2] X. H. Chen, T. Wu, G. Wu, R. H. Liu, H. Chen, D. F. Fang, Nature 453, 1224 - 1227(2008) .[3] Z. A. Ren, J. Yang, W. Lu, W. Yi, G.-C. Che, X.-L.Dong, L.-L. Sun, and Z.-X. Zhao, Mater. Res. Innova-tions 12,(2008)105.[4] G. F. Chen, Z. Li, D. Wu, G. Li, W. Z. Hu, J. Dong, P.Zheng, J. L. Luo, N. L. Wang, Phys. Rev. Lett. 100 (24),247002(2008).[5] 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, Supercond.Sci. Technol. 21, 082001(2008).[6] J. G. Bos, G. B. S. Penny, J. A. Rodgers, D. A. Sokolov,A. D. Huxley, and J. P. Attfield, Chem. Commun. (Cam-bridge), 3634(2008).[7] C. Krellner, N. S. Kini, E. M. Br¨uning, K. Koch, H.Rosner, M. Nicklas, M. Baenitz, C. Geibel, Phys. Rev. B76(10) (24), 104418(2007).[8] C. Krellner, C. Geibel, J. Cryst. Growth 310 (7-9), 1875-1880(2007).[9] C. Krellner, T. Forster, H. Jeevan, C. Geibel, J.Sichelschmidt, Phys. Rev. Lett. 100 (6), 066401(2008).[10] E. M. Br¨uning, C. Krellner, M. Baenitz, A. Jesche, F.Steglich, C. Geibel, Phys. Rev. Lett. 101, 117206(2008).[11] Jun Zhao, Q. Huang, Clarina de la Cruz, Shiliang Li, J.W. Lynn, Y. Chen, M. A. Green, G. F. Chen, G. Li, Z.Li, J. L. Luo, N. L. Wang and Pengcheng Dai, NatureMaterials 7, 953 - 959(2008).[12] A. Jesche, C. Krellner, M. de Souza, M. Lang and C.Geibel, New Journal of Physics 11 (2009)103050 (13pp).[13] A. Jesche. et. al. arXiv:1001.4349v1.[14] S. Chi, D. T. Adroja, T. Guidi, R. Bewley, Shiliang Li,Jun Zhao, J.W. Lynn, C. M. Brown, Y. Qiu, G. F. Chen,J. L. Lou, N. L. Wang, and Pengcheng Dai, Phys. Rev.Lett. 101, 217002(2008).[15] H. Maeter, H. Luetkens, Yu. G. Pashkevich, A.Kwadrin,1 R. Khasanov, A. Amato, A. A. Gusev, K. V.Lamonova, D. A. Chervinskii, R. Klingeler, C. Hess, G.Behr, B. B¨uhchner, and H..H. Klauss, Phys. Rev. B 80,094524 (2009).[16] H. Yanagi, R. Kawamura, T. Kamiya, Y. Kamihara, M.Hirano, T. Nakamura, H. Osawa, and H. Hosono, Phys.Rev. B 77, 224431 (2008).[17] A. S. Sefat, A. Huq, M. A. McGuire, R. Jin, B. C. Sales,and D. Mandrus, Phys. Rev. B 78, 104505 (2008).[18] H. Ohta and K. Yoshimura, Phys. Rev. B 77,184407(2009).[19] C. Krellner, U. Burkhardt, and C. Geibel, Physica B 404,3206 (2009).[20] Y. Luo, Y. Li, S. Jiang, J. Dai, G. Cao, and Z. Xu, arXiv:0907.2961v1.[21] Clarina de la Cruz, W. Z. Hu, Shiliang Li, Q. Huang, J.W. Lynn, M. A. Green, G. F. Chen, N. L. Wang, H. A.Mook, Qimiao Si, Pengcheng Dai, arXiv:0907.2853v1.[22] R. Sarkar et. al.
To be published .[23] T. J. Bastow J. Phys.: Condens. Matter 11 (1999)569574.[24] E. Bauer, St. Berger, A. Galatanu, M. Galli, H. Michor,G. Hilscher, Ch. Paul, B. Ni, M. M. Abd-Elmeguid, V. H.Tran, A. Grytsiv, and P. Rogl, Phys. Rev. B. 63, 224414(2001).[25] J.J.M. France and R. J. Radwanski
Handbook of Mag-netic Materials, vol 7, North Holland
Chapter 5.[26] J.G. Cheng, Y. Sui, Z.N. Qian, Z.G. Liu, J.P. Miao, X.Q.Huang, Z. Lu, Y. Li, X.J. Wang, W.H. Su, Solid StateCommunications 134 (2005) 381-384.[27] J. L´ o pez and O. F. de Lima Phys. Rev. B 66, 214402(2002).[28] M. El-Hagary Eur. Phys. J. Appl. Phys. 42, 287-291(2008).[29] G.C. Carter, L.H. Bennett and D.J. Kahan, Metallic shiftin NMR, Pergamon, Oxford, 1977.[30] A. Aeby, F. Hulliger and B. Natterer, Solid State Com-munications 13, 13651368 (1973).[31] V. S. Zapf, N. A. Frederick, K. L. Rogers, K. D. Hof,P.-C. Ho, E. D. Bauer, and M. B. Maple, Phys. Rev. B67, 064405 (2003).[32] Athena S. Sefat, Ashfia Huq, Michael A. McGuire,Rongying Jin, Brian C. Sales, David Mandrus, Lach-lan M. D. Cranswick, Peter W. Stephens, and Kevin H.Stone, Phys. Rev. B 78, 104505 (2008).[33] L. Zhao, D. Berardan, C.Byl, L. Pinsard-Gaudart, andN. Dragoe, J. Phys.: Condens. Matter 22 (2010) 115701(5pp).[34] A. S. Sefat, R. Jin, M. A. McGuire, B. C. Sales, D. J.Singh, and D. Mandrus, Phys. Rev. Lett. 101, 117004(2008).[35] F. L. Ning, K. Ahilan, T. Imai, A. S. Sefat, R. Jin, M.A. McGuire, B. C. Sales, and D. Mandrus Phys. Rev. B79, 140506(R) (2009)[36] H. Ohta, C. Michioka, K. Yoshimura, J. Phys. Soc. Jpn.79, 054703(2010).[37] Baenitz et al. To be published .[38] Michael A. McGuire, Delphine J. Gout, V. Ovidiu Gar-lea, Athena S. Sefat, Brian C. Sales, and David Mandrus,Phys. Rev. B 81, 104405 (2010) .[39] A. Jesche.