Mössbauer spectroscopy of spin dynamics in Mn x Fe 1 x Se 0.85 superconductors: Evidence for an incommensurate-spin-density-wave state

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1 March 2010 EPL, 89 (2010) doi: / /89/ Mössbauer spectroscopy of spin dynamics in Mn x Fe 1 x superconductors: Evidence for an incommensurate-spin-density-wave state H. H. Hamdeh 1(a),M.M.El-Tabey 1(b),R.Asmatulu 2,J.C.Ho 1, T. W. Huang 3,K.W.Yeh 3 and M. K. Wu 3 1 Department of Physics, Wichita State University - Wichita, KS 67260, USA 2 Mechanical Engineering, Wichita State University - Wichita, KS 67260, USA 3 Institute of Physics, Academic Sinica - Taipei, Taiwan received 23 October 2009; accepted in final form 18 March 2010 published online 19 April Ha Magnetic properties including vortex structures and related phenomena Fv Spin-density waves Wx Spin crossover Abstract In the tetragonal crystalline structure of Mn xfe 1 x, the magnetic state contains low- and high-spin Fe 2+, with high-spin numbers equal to that of the combined Mn substitute and Se deficiency atoms. The state is pinned by spin-hopping around substitution centers via highspin low-spin conversions. During the structural distortion from tetragonal to orthorhombic, from 90 K to 70 K, the rate of spin conversions increases and the iterant character of the magnetic state is enhanced. In the orthorhombic structure, the spin dynamics evolve into an incommensurate spin-density wave (ISDW). Excitations of the ISDW decrease with temperature and level out across the superconducting phase. The ISDW appears to have more than one oscillation mode and contributions from high-order harmonics. Copyright c EPLA, 2010 The sighting of superconductivity in the Se-deficient Fe-Se has re-energized efforts to better understand the role of magnetism in all magnetism-mediated superconductivity systems. Theoretical calculations [1 3] have made substantial progress in modeling this role in the newly discovered iron oxypnictide (LaOFe-P/As-F) [4,5] and iron chalcogenides (Fe-Se/Te) [6,7] classes of superconductors. Both classes show the planner feature of the crystalline structure and similar calculated Fermi surfaces. Therefore, identifying the underlining mechanism of superconductivity in one may clarify the nature of superconductivity in the other. The great challenge is to assess the theoretical predictions of spin dynamics by experimental measurements on a time scale comparable to those of the spin excitations. Fortunately, 57 Fe Mössbauer spectroscopy has been shown to be an extremely effective tool in following spin dynamics down to a s time scale (τ M ). Generally, two possible dynamics could become visible to Mössbauer spectroscopy in iron (3d 4 3d 7 ) complexes. The first is a free-spin (HS; high spin) to spin-paired (LS; low (a) hussein.hamdeh@wichita.edu (b) On leave from: Menoufia University - Egypt. spin) conversion, depending on the relative strength of crystalline-field energy to spin-pairing energy. At a critical crystalline field, the difference between the two energies is on the order of the thermal energy (k B T ), and then thermal HS LS is induced. A Mössbauer spectrum shows resonance lines of the individual spin state separately, provided that the lifetime of the particular spin states is longer than τ M. The second dynamic is the fluctuation of the spin vector among the easy directions of magnetization set by the crystalline structure. Typically, Mössbauer spectra display six (sextet) resonance lines for fluctuation time longer than τ M, one (singlet) or two (doublet) resonance lines for fluctuation time shorter than τ M, or a collapsed spectrum for intermediate fluctuation. The valence and spin states of Fe ions are determined from the isomer shift (IS; shift of centroid of spectrum from zero velocity) values. Quadrupole splitting (QS; width of doublet spectrum) values refer also to valence and spin states, and the Fe site symmetry. Magnetic splitting (HMF; width of sextet spectrum) values give information on the magnetic state. At room temperature, α-fese crystallizes to the tetragonal PbO-type structure. Its geometry is described as p1

2 H. H. Hamdeh et al. layers of Fe atoms, with Se atoms forming distorted FeSe 4 tetrahedra. Upon cooling, the structure undergoes phase transition to orthorhombic or monoclinic (nearly orthorhombic) near 90 K [6,8]. For Fe ions, the spin state is usually analyzed by examining the crystalline-electric field splitting of the valence state. In a tetrahedral symmetry, the crystalline field partially lifts the degeneracy of the Fe five d-orbitals (d z2, d x2 y2, d xz, d yz, d xy ), but with structural distortions, or magnetic fields, degeneracy would be completely lifted. Here we report the 57 Fe Mössbauer spectroscopy observation of the interplay between Fe spin, crystalline field and structural-phase transition in Mn x Fe 1 x (x =0, 0.1 and 0.2). The tetragonal and orthorhombic structures of the samples were confirmed by XRD diffraction above and below distortion, respectively. Their superconducting transition temperature T c was found to be 12 K for x = 0, and 11 K for x = 0.1 and 0.2 samples. Whether substoichiometry results in excess Fe occupying interstitial sites [9] or in anion vacancies [2], a HS-Fe 2+ was created by every Se deficiency or Mn substitute atom. After distortion, similar ISDWs evolve in the three samples. The observed similarities in the features of the ISDWs point to the same dynamics, regardless of the level of substitutions. However, the large number of HS-Fe 2+ formed by substitutions provides the crucial probe to track the evolution of the spin dynamics in the normal and superconducting phases, which was not possible to do with low and intermediate spins. Mössbauer spectra in fig. 1 show that the three samples exhibit no spontaneous magnetization at room temperature. In the absence of any magnetic order, the spectra were fitted to a set of two paramagnetic doublets. Calculated hyperfine parameters listed in table 1 are identical for all three samples, but the intensity of doublets (area fraction) differs from one sample to another. The IS values of mm/s and the QS values of mm/s are unambiguously the trade-mark of HS-Fe 2+ (S 2) in a tetrahedral coordination. But IS = 0.47 mm/s and QS = are in the typical range of intermediate to low spin (0 S 1) Fe 2+. Notwithstanding, these parameters are attributed to LS-Fe 2+. The fraction of HS-Fe 2+ and LS-Fe 2+ in each sample, as determined from the area fraction, are also listed in table 1. The consistent match of the number of HS-Fe 2+ with the number of Mn substitute and Se deficiency atoms is very significant for revealing the spin dynamics. It is very likely that the increase of HS-Fe 2+ with Mn substitution is due to the ionic radius of Mn 2+ (0.80 Å), which is larger than that of HS-Fe 2+ (0.74 Å). This increases the length of Mn-Fe bonds as compared to Fe-Fe bonds in the Fe layer, thus decreasing the crystalline electric field at the Fe sites adjacent to Mn and stabilizing the HS state. From a symmetry consideration, the single HS state created by a Mn atom should alternate via HS LS Fe Mn 0.1 Fe 0.9 Mn 0.2 Fe Fig. 1: (Colour on-line) 57 Fe Mössbauer spectra at 300 K resolved into red HS-Fe 2+ and blue LS-Fe 2+ doublets. Table 1: Mössbauer-generated IS, QS, and area fraction of Fe 2+ in Mn xfe 1 xs 0.85 samples at 300 K. Sample Spin IS QS Area state (mm/s) (a) (mm/s) fraction (±0.01) (±0.04) (±0.03) Fe HS LS Mn 0.1 Fe 0.9 HS LS Mn 0.2 Fe 0.8 HS LS (a) Relative to pure Fe p2

3 Mössbauer spectroscopy of spin dynamics in Mn x Fe 1 x superconductors etc. 90 K 80 K 70 K 20 K Fig. 2: (Colour on-line) 57 Fe Mössbauer spectra at indicated temperatures: (90 K) resolved into red HS-Fe 2+ and blue LS-Fe 2+ doublets, (80 K) resolved into a collapsed red spectrum due to increased spin fluctuations and blue LS-Fe 2+ doublet, and (70 K and 20 K) resolved into red magnetic splitting and blue LS-Fe 2+ doublet. conversions among the four Fe ions bonding with Mn. Indeed, there is no significant energy barrier for the spin not to be transmitted, and transmission could be activated by phonons. It is known that HS-Fe 2+ bonds are weaker than LS-Fe 2+ bonds in Fe complexes and HS LS conversions are strongly coupled to vibration modes [10]. For Mössbauer measurements to detect the individual LS and HS states, the transmission time is definitely longer than 2τ M. Consequently, this local spin hopping frustrates the long-range magnetic order and pins the magnetic state around the Mn centers. Similarly, a weak crystalline electric field at excess Fe on an interstitial site, or Fe adjacent to Se vacancy, creates the HS states in the substoichiometric sample x = 0. Developments in the three samples with temperature appear to be parallel down to 4.2 K. The first sign of change comes as broadening to the lines of both doublets. For the x = 0 sample, the weak HS-Fe 2+ signal vanishes into the base of the LS-Fe 2+ at roughly the onset of lattice distortion temperature. The line broadening is quite visible at 90 K for the x =0.2 sample, as shown in fig. 2. This figure also shows Mössbauer spectra at lower temperatures. Starting with the spectrum at 80 K, which is the midway temperature during the structural transition to orthorhombic [8], the HS-Fe 2+ doublet has collapsed, the lines of the LS-Fe 2+ doublet have narrowed, and area fraction of the collapsed doublet has increased to match that of the LS-Fe 2+ doublet. These developments indicate a definite increase in the rate of spin conversions. In fact, spin conversions now are on a time scale shorter than or comparable to τ M.At 70 K, the structural distortion is complete, and the spin dynamics have changed into the ISDW. This is evident from the doublet and the broad hyperfine magneticfield distribution [11] exhibited in the 70 K spectrum. The width of the magnetic splitting, however, continues to expand down to 20 K, signifying a decrease in spin fluctuations in either magnitude or direction. Mössbauer spectra at 4.2 K from the three samples in the superconducting phase are shown in fig. 3, along with their calculated HMF distributions. Qualitatively, the spectra are alike and split about equally between a non-magnetic doublet and a broad magnetic splitting. As mentioned above, HS-Fe 2+ bonds are softer than LS-Fe 2+ bonds, but initially they are localized. Apparently, these local-soft vibration modes, HS LS conversions, and magnetic exchange couplings have played an important role in affecting the structural distortion in order to eliminate the frustration of the magnetic state. The ISDW feature s lack of sensitivity to the initial local parameters manifests the extended nature of the prevailing magnon and phonon states. Here, it is important to emphasize that the spins configurations and dynamics before the magnetic transition are completely different from those after the transition, despite the presence of a central doublet in all spectra. After magnetic transition, the nonmagnetic doublet (S 0) and the convoluted magnetic component (S >0) are integral part of a periodic order in spin magnitude (SDW) and a long-range order in spin direction. In this case, the central doublet arises from diamagnetic-fe and not paramagnetic-fe. According to calculations [11], the intensity of the non-magnetic doublet and the width of the associated HMF distribution increase with contributions from high-order harmonics to the SDW. To quantitatively analyze the data, experimental spectra were processed using the method of Le Caër and p3

4 H. H. Hamdeh et al. 0 Fe Mn 0.1 Fe Mn 0.2 Fe Hyperfine Magnetic Field (kg) Fig. 3: (Colour on-line) 57 Fe Mössbauer spectra at 4.2 K with their HMF distributions as obtained from the method of Le Caër and Dubois. Blue peak labeled 0 depicts the LS-Fe 2+ doublet. Dark green and red peaks from 1 to 6 correspond to the HMF distribution. Dubois [12]. Spectra were fitted to a single doublet and a hyperfine magnetic field distribution. In this method, we assumed two independent linear relationships between IS and HMF (H), on the one hand, and between QS and HMF, on the other. Here, it is worth noting that the assumed linearity to a complex QS-HMF relationship is an oversimplification, which may result in a relatively high uncertainty in the calculated parameters where QS and HMF values are comparable. The two relationships IS (relative to pure Fe)= AH+B and QS = CH+D were initially constructed from measured hyperfine parameters of LS-Fe 2+ and HS-Fe 2+, but they were refined by the fitting routine. The constants A = mm s 1 kg 1, B =0.444 mm s 1, C = mm s 1 kg 1 and D = p4

5 Mössbauer spectroscopy of spin dynamics in Mn x Fe 1 x superconductors etc mm s 1 were obtained from the best fit of the calculated spectrum to the experimental one. These calculated relationships and the HMF distribution mirror the spin nesting of the SDW. In fig. 3, distinct peaks of the HMF distribution are labeled from 1 to 6 and the non-magnetic doublet is depicted by the peak at HMF = 0. The relative intensity of the non-magnetic peak to the peaks of the HMF distribution obtained in the course of the fitting is 1 : 0.75 for all three spectra. This ratio overestimates the non-magnetic fraction since the magnetic signal is shallow and widely spread. We believe that the 1 : 1 ratio obtained from the 80 K spectrum is more accurate. The mean HMF, which is proportional to the amplitude of the ISDW, is found to be at 205, 239, and 240 kg for x = 0, 0.1, and 0.2 samples, respectively. Using the 1 : 1 ratio and the HMF-to-magnetic moment conversion factor of µ B /KG [13], the mean magnetic moment per Fe is estimated at 0.53 µ B for x =0 and 0.62 µ B for both x =0.1 andx =0.2 samples. This mean is over all Fe including diamagnetic-fe. It is easy to notice that the separation between peaks 1 and 2 is about double the approximately equal separation between the adjacent peaks from 2 to 6. The large number of peaks and types of separation suggest that there is more than one oscillation mode and that high-order harmonics is a contributing factor. In conclusion, the structure of this Fe-based superconducting class appears to provide the critical crystalline electric field in which different 3d-spin states of layered Fe 2+ are in virtual thermal equilibrium. The readily induced spin conversions in the Fe layer frustrate the long-range magnetic order. As spins cross over back and forth, they deform the lattice vibration modes, and the two dynamics become strongly coupled. At the onset of structure transformation, spin conversions grow faster just before the transition from frustration to harmony. REFERENCES [1] Subedi A., Zhang L., Singh D. J. and Du M. H., Phys. Rev. B, 78 (2008) [2] Lee K.-W., Pardo V. and Pickett W. E., Phys. Rev. B, 78 (2008) [3] Yildirim T., Phys. Rev. Lett., 101 (2008) [4] Kamihara Y., Hiramatsu H., Hirano M., Kawamura R., Yanagi H., Kamiya T. and Hosono H., J. Am. Chem. Soc., 128 (2006) [5] Kamihara Y., Watanabe T., Hirano M. and Hosono H., J. Am. Chem. Soc., 130 (2008) [6] Hsu F. C., Luo J. Y., Yeh K. W., Chen T. K., Huang T. W., Wu P. M., Lee Y. C., Huang Y. L., Chu Y. Y., Yan D. C. and Wu M. K., Proc. Nat. Acad. Sci. U.S.A., 105 (2008) [7] Yeh K. W., Huang T. W., Huang Y. L., Chen T. K., Hsu F. C., Wu P. M., Lee Y. C., Chu Y. Y., Chen C. L., Luo J. Y., Yan D. C. and Wu M. K., EPL, 84 (2008) [8] Margadonna S., Takabayashi Y., McDonald M. T., Kasperkiewicz K., Mizuguchi Y., Takano Y., Fitch A. N., Suard E. and Prassides K., arxiv: (2008). [9] Bao W., Qiu Y., Huang Q., Green M. A., Zajdel P., Firzsimmoons M. R., Zhernenkov M., Fang M., Qian B., Vehstedt E. K., Yang J., Pham H. M., Spinu L. and Mao Z. Q., arxiv: (2008). [10] Nasser J. A., Topçu S., Chassange L., Bousseksou A., Guillon T. and Alyali Y., Chem. Phys. Lett., 446 (2007) 385. [11] Dubiel S. M., J. Magn. & Magn. Mater., 124 (1993) 31. [12] Le Caër G. and Dubois J. M., J. Phys. E, 12 (1979) [13] Klauss H.-H., Luetkens H., Klingeler R., Hess C., Litterst F. J., Kraken M., Korshunov M. M., Eremin I., Drechsler S.-L., Khasanov R., Amato A., Hamann-Borrero J., Leps N., Kondrat A., Behr G., Werner J. and Büchner B., Phys. Rev. Lett., 101 (2008) p5