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Research motivation

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Research motivation

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  1. Nanosized magnetic clusters and their relation to magnetoresistance in FeCr2S4 spinel Z. Klencsár1, E. Kuzmann1, Z. Homonnay2, A. Vértes1, A. Simopoulos3, E. Devlin3, G. Kallias31 Laboratory of Nuclear Chemistry, Hungarian Academy of SciencesChemical Research Center, Pázmány P. s. 1/A, Budapest, Hungary2 Laboratory of Nuclear Chemistry, Institute of ChemistryEötvös Loránd University, Pázmány P. s. 1/A, Budapest, Hungary3 Institute of Materials Science, NCSR DemokritosAghia Paraskevi 153 10, Athens, GreeceE-mail: z.klencsar@somogy.hu

  2. Research motivation The discovery of colossal magnetoresistance (CMR) in manganese-based perovskites has stimulated intense research on the physical bases of the CMR effect [1,2,3]. The puzzling existence of a considerable intrinsic magnetoresistance reported [4] in FeCr2S4 chalcogenide spinels - that do not possess manganese, oxygen, perovskite structure, or even a metal-to-insulator transition – indicates that phenomena other than the double exchange effect [2] should also be considered in the explanation of magnetoresistance observed in these materials. FeCr2S4 has long been known as a ferrimagnet with semiconducting-like resistivity characteristics [5-15]. It has a crystal structure of a cubic normal spinel where Fe2+ and Cr3+ cations occupy tetrahedrally and octahedrally coordinated positions, respectively [16]. The paramagnetic to ferrimagnetic transition occurs in the range TC = 170..180 K [4,5]. Magnetization of FeCr2S4 measured in 0.1 T magnetic field The structure of FeCr2S4 [4] A.P. Ramirez, R.J. Cava, J. Krajewski: Nature386 (1997) 156. [5] G. Shirane, D.E. Cox, S.J. Pickart: J. Appl. Phys.35 (1964) 954. [6] G. Haacke, L.C. Beegle: Phys. Rev. Lett.17 (1966) 427. [7] M. Eibschutz, S. Shtrikman, Y. Tenenbaum: Physics Letters 24A (1967) 563. [8] G. Haacke, L.C. Beegle:J. Phys. Chem. Solids28 (1967) 1699. [9] C.M. Yagnik, H.B. Mathur: Solid State. Comm.5 (1967) 841. [10] G.R. Hoy, K.P. Singh:Phys. Rev. 172 (1968) 514. [11] F.K. Lotgering, R.P. Van Stapele, G. Van der Steen, J.S. Van Wieringen:J. Phys. Chem. Solids30 (1969) 799. [12] P. Gibart, J.-L. Dormann, Y. Pellerin: Phys. Stat. Sol. 36 (1969) 187. [13] P. Gibart, L. Goldstein, L. Brossard: J. Magn. Magnet. Mat.3 (1976) 109. [14] A.M. Van Diepen, F.K. Lotgering, J.F. Olijhoek:J. Magn. Magnet. Mat. 3 (1976) 117. [15] L. Brossard, J.L. Dormann, L. Goldstein, P. Gibart, P. Renaudin: Phys. Rev. B 20 (1979) 2933. [16] Anthony R. West, Basic Solid State Chemistry, 2nd edn., John Wiley & Sons, New York, 1999, p. 60. [1] R. Helmolt, J. Wecker, B. Holzapfel, L. Schultz, K. Samwer: Phys. Rev. Lett.71 (1993) 2331. [2] C. Zener: Phys. Rev.82 (1951) 403. [3] A.J. Millis: Nature392 (1998) 147. [4] A.P. Ramirez, R.J. Cava, J. Krajewski: Nature386 (1997) 156. TCrange The red and blue arrows indicate the preferred orientation of magnetic moments in the magnetically ordered state. D.G. Wickham, J.B. Goodenough: Phys. Rev. 115 (1959) 1156. A.P. Ramirez, R.J. Cava, J. Krajewski: Nature386 (1997) 156.

  3. Research motivation The recent observation of negative magneto-resistance in FeCr2S4 [4] initiated a reinvestigation of this compound by means of various methods [17‑24]. By the help of 57Fe Mössbauer spectroscopy we investigated the local magnetic and electronic state of Fe2+ in FeCr2S4 as a function of temperature in the range 75 K..290 K. Here we focus our attention on the relation between the magnetic state of iron in FeCr2S4 and the magnetoresistivity as well as the resistivity anomaly observed in this compound at temperatures in the vicinity of the Curie temperature [4]. B= 0 T B= 3 T B= 5 T Z. Yang, S. Tan, Z. Chen, Y. Zhang: Phys. Rev. B62 (2000) 13872. B= 3 T Magnetoresistivity is defined as B= 5 T where r (B) is the resistivity measured in magnetic field B.

  4. T What is happening around the Curie temperature? TCrange A comparison of the temperature dependence of magnetization, resistivity and magnetoresistivity of FeCr2S4 reveals that it is plausible to suppose that the resistivity anomaly as well as the magnetoresistivity are connected to the break down of long-range magnetic order at around the Curie temperature. (Here we prefer to consider the evolution of the physical state as a function of increasing temperature.) In order to find out what is specialabout the break down of magnetic order in FeCr2S4, we recorded 57Fe Mössbauer spectra of this compound as a function of — increasing—temperature. The FeCr2S4 sample that we investigated is the same material as that for which macroscopic magnetization, thermopower, resistivity and CMR measurements have been reported by Ramirez et al. in ref. [4] which article may be consulted for details of the sample preparation. 57Fe Mössbauer spectroscopy measurements were carried out on powdered samples in transmission geometry. During measurements the temperature of the sample was kept constant with a precision of DT0.5 K. The material was first cooled down to 75 K, then measurements were performed by raising the temperature by  5 K between subsequent measurements. 155 K [4] A.P. Ramirez, R.J. Cava, J. Krajewski: Nature386 (1997) 156. 57Fe Mössbauer spectra of FeCr2S4

  5. T 57Fe Mössbauer spectra of FeCr2S4 What is happening below the Curie temperature? Although the Curie temperature, determined by magnetization measurements, was reported to be TC 170 K for this particular FeCr2S4 sample [4], 57Fe Mössbauer spectra indicate that long-range magnetic order starts to break down already at TR 155 K, and the Mössbauer spectrum taken at T = 160 K consists mainly of a singlet. The existence of a magnetically unsplit singlet component in the Mössbauer spectrum of FeCr2S4 below the Curie temperature is due to the magnetic relaxation effect [26], and indicates that in FeCr2S4magnetic order breaks down by the gradual fragmentation of magnetic domains into nanosized, only weakly interacting magnetic clusters of atoms.

  6. Why does long-range magnetic order break down already below the Curie temperature? A hint to the answer to the above questionmay be found in the result ofYang et al., who found that the coercivity of FeCr2S4 decreases with increasing temperature, and goes to zero at around TR. This indicates that magneto-crystalline anisotropy— originating mainly from the tetrahedrally coordinated Fe2+ ions— goes to zero with increasing temperature just about when the break down of long-range magnetic order— and the associated relaxation phenomenon— starts. The spin-orbit coupling characteristic of the Fe2+ ions’ 3d electrons— being at the root of the magnetocrystalline anisotropy in this compound — becomes revealed also by Mössbauer spectroscopy: in the magnetically ordered state it results in a nonvanishing electric field gradient at the iron nuclei, which in turn presents itself as a nonzero quadrupole splitting in the corresponding Mössbauer spectra.The quadrupole splitting, however, also vanishes at T TR and not at T TC . Z.R. Yang, S. Tan, Y.H. Zhang: Appl. Phys. Lett.79 (2001) 3645. TCrange TR TR TCrange

  7. Conclusions On the basis of evidence presented so far, we can already gain a qualitative understanding of the resistivity anomaly and negative magnetoresistance displayed by FeCr2S4: With increasing temperature (1) at TR 155 K spin-orbit coupling at Fe2+ ions becomes ineffective in maintaining strong magnetocrystalline anisotropy: the material enters a soft ferrimagnetic state with near zero coercivity; (2) this new magnetic state evolves under the influence of frustrated (e.g. AFM Cr-Cr and AFM Fe-Cr) magnetic exchange interactions, further magnetic anisotropies (e.g. shape anisotropy) and thermal agitation; (3) in the range TR … TC the minimum-energy state of FeCr2S4 is achieved by a gradual break-down into nanometer-sized magnetic domains; (4) the relaxation frequency of the magneticdomains increases, and enters the frequency window of 57Fe Mössbauer spectroscopy: in the Mössbauer spectra this results in a collapse of the sextet component and the appearance of superparamagnetic-like relaxation; (5) in the range TR … TC electrical resistivity increases due to increasing magnetic disorder and spin-disorder scattering; (6) an external magnetic field can achieve the coalescence of magnetic domains, thereby lowering spin-disorder scattering and electrical resistivity: a negative magnetoresistance is realized. On the basis of evidence presented so far, we can already gain a qualitative understanding of the resistivity anomaly and negative magnetoresistance displayed by FeCr2S4: With increasing temperature (1) at TR 155 K spin-orbit coupling at Fe2+ ions becomes ineffective in maintaining strong magnetocrystalline anisotropy: the material enters a soft ferrimagnetic state with near zero coercivity; (2) this new magnetic state evolves under the influence of frustrated (e.g. AFM Cr-Cr and AFM Fe-Cr) magnetic exchange interactions, further magnetic anisotropies (e.g. shape anisotropy) and thermal agitation; (3) in the range TR … TC the minimum-energy state of FeCr2S4 is achieved by a gradual break-down into nanometer-sized magnetic domains; (4) the relaxation frequency of the magneticdomains increases, and enters the frequency window of 57Fe Mössbauer spectroscopy: in the Mössbauer spectra this results in a collapse of the sextet component and the appearance of superparamagnetic-like relaxation; (5) in the range TR … TC electrical resistivity increases due to increasing magnetic disorder and spin-disorder scattering; (6) an external magnetic field can achieve the coalescence of magnetic domains, thereby lowering spin-disorder scattering and electrical resistivity: a negative magnetoresistance is realized.

  8. What is happening above the Curie temperature? In order to find out what becomes of the nanometer-sized magnetic clusters above the Curie temperature, we performed 57Fe Mössbauer measurements at T = 186.5 K with and without an external magnetic field oriented perpendicular to the direction of the g-radiation. Surprisingly, the 57Fe Mössbauer spectrum measured at T = 186.5 K without external magnetic field displays — apart from the majority singlet component — a minor magnetic component characterized by  4.0(1)T hyperfine magnetic field. Even more striking is that the applied 6 T external magnetic field is able to recover long-range magnetic order, and to fully orient the sample’s magnetization, as reflected by the 3:4:1:1:4:3 relative intensities of the sextet’s peaks. Furthermore, the hyperfine magnetic fields reflected by the sextet components obtained with and without the external magnetic field differ by the magnitude of the externally applied field inside the experimental error. This indicates, that iron magnetic moments become aligned opposite to the externally applied field, thereby proving that at T = 186.5 K the Fe-Cr AFM interaction is still strong enough to bind Fe and Cr magnetic moments together. Obviously, the studied FeCr2S4 sample is not in a paramagnetic state at T = 186.5 K, but it still contains magnetically ordered clusters of atoms. 4.0(1) T 9.89(1) T

  9. References Z. Klencsár, E. Kuzmann, Z. Homonnay, A. Vértes, A. Simopoulos, E. Devlin, G. Kallias: Magnetic relaxation and its relation to magnetoresistance in FeCr2S4 spinel Hyperfine Interactions144/145 (2002) 261. Z. Klencsár, E. Kuzmann, Z. Homonnay, A. Vértes, A. Simopoulos, E. Devlin, G. Kallias: Interplay between magnetic order and the vibrational state of Fe in FeCr2S4 Journal of Physics and Chemistry of Solids64 (2003) 325. [17] Z. Chen, S. Tan, Z. Yang, Y. Zhang: Phys. Rev. B 59 (1999) 11172. [18] Z. Yang, S. Tan, Y. Zhang: Solid State. Comm.115 (2000) 679. [19] Z. Yang, S. Tan, Z. Chen, Y. Zhang: Phys. Rev. B 62 (2000) 13872. [20] V. Tsurkan, M. Lohmann, H.-A. Krug von Nidda, A. Loidl, S. Horn, R. Tidecks: Phys. Rev. B 63 (2001) 125209-1. [21] M.S. Park, S.K. Kwon, S.J. Youn, B.I. Min: Phys. Rev. B 59 (1999) 10018. [22] Z.R. Yang, S. Tan, Y.H. Zhang: Appl. Phys. Lett.79 (2001) 3645. [23] V. Tsurkan, J. Hemberger, M. Klemm, S. Klimm, A. Loidl, S. Horn, R. Tidecks: J. Appl. Phys.90 (2001) 4639. [24] S. Wang, K. Li, Z. Chen, Y. Zhang: Phys. Rev. B 61 (2000) 575.

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