3046 IEEE TRANSACTIONS ON MAGNETICS, VOL. 43, NO. 6, JUNE 2007
Valence States of Transition-Metal Ions and ElectronicStructures of Spinel Fe1 Cu Cr2S4
J.-S. Kang1, S. W. Han2, S. S. Lee1, G. Kim1, C. Hwang2, S. J. Kim3, C. S. Kim3, J.-Y. Kim4, H. J. Shin4, andB. I. Min5
Department of Physics, The Catholic University of Korea, Bucheon 420-743, KoreaKorea Research Institute of Standards and Science, Daejon 305-340, Korea
Department of Physics, Kookmin University, Seoul 136-702, KoreaPohang Accelerator Laboratory (PAL), POSTECH, Pohang 790-784, Korea
Department of Physics, POSTECH, Pohang 790-784, Korea
The valence states and electronic structures of transition-metal ions in spinel Fe1 Cu Cr2S4(0 1 0 5) have been inves-tigated by using scanning photoelectron microscopy (SPEM), photoemission spectroscopy (PES), soft-X-ray absorption spectroscopy(XAS), and soft X-ray magnetic circular dichroism (XMCD). The experimental data have been compared to the calculated density ofstates (DOS). It is found that the valence states of Cr and Cu ions are nearly trivalent (Cr3+) and monovalent (Cu+), respectively. TheFe 2 XAS spectra of Fe1 Cu Cr2S4 are very similar to that of Fe metal, indicating that Fe 3 states are strongly hybridized to S 3states. The XMCD measurements for Fe, Cr, and Cu 2 states show evidence that the magnetic moments of Cr ions are antiparallel tothose of Fe ions and that Cu ions are weakly polarized parallel to Fe ions. Valence-band PES reveals that Cr 3 states are located at
1 5 eV, while Fe 3 states are very broad, in agreement with the calculated DOS. This study indicates that the minority-spinstates of Fe 3 electrons are located very close to EF, suggesting that the hybridized Fe -S 3 states near EF play an important rolein determining the transport properties of Fe1 Cu Cr2S4 for 0 5.
SPINEL compounds of Fe Cu Cr S exhibitthe large negative magnetoresistance (MR) effect [1], [2].
Upon cooling, the resistivity shows a crossover transitionfrom insulator to metal near the magnetic transition tempera-ture , and then again the insulating feature far below [2],[3]. With increasing in Fe Cu Cr S increases mono-tonically, whereas the room temperature resistivity and the MRdecrease first and then increase to exhibit local minima near
and local maxima at [4]–[7]. It is consideredthat each of the Fe and Cr sublattices orders ferromagnetically,while the two sublattices are coupled antiferromagnetically toeach other, resulting in the ferrimagnetic ground states [8]. Inorder to explain the physical properties of Fe Cu Cr S
, two competing models have been proposed, with differentvalence states of the constituent elements. For CuCr S , Lot-gering and van Stapele [9] developed a model considering themonovalent Cu ions. If the Cu ion is monovalent in CuCr S ,the valence configuration of Cu Cr Cr S is expected,implying the formally mixed-valent Cr ions. The ferromagneticmetallic ground state of CuCr S was attributed to the doubleexchange (DE) interaction between Cr and Crions. On the other hand, Goodenough [10] postulated divalentCu ions and trivalent Cr ions for .
Despite extensive studies on the valence states of transition-metal elements in spinel systems, this issue has not been settleddown yet. Therefore, it is necessary to perform the element-spe-
Digital Object Identifier 10.1109/TMAG.2007.892567
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cific experiment that provides the direct information on the elec-tronic and magnetic structure of Fe Cu Cr S
. Photoemission spectroscopy (PES), soft X-ray absorptionspectroscopy (XAS), and X-ray magnetic circular dichroism(XMCD) are good experimental tools for studying the electronicstructures [11], the valence states of transition-metal (T) ions insolids [12], [13], and the element-specific local magnetic mo-ments of both spin and orbital components [14], [15],respectively. The line shapes of XAS and XMCD spectra arestrongly dependent on the occupied electron configurations,the crystal field, and the hybridization of electrons to othervalence electrons. Hence, the peak positions and the line shapeof the XAS spectrum depend on the local electronic struc-ture of the T ion, providing the information about the valencestate and the ground state symmetry of the T ion. The magni-tudes of and can be estimated quantitatively by applyingthe sum rules to the measured XMCD spectrum [15].
We have carried out PES, soft X-ray XAS, and XMCD exper-iments for polycrystalline samples of Fe Cu Cr S
. Polycrystalline samples were prepared by the stan-dard solid-state reaction method [3]. Valence-band PES, XAS,and XMCD measurements were performed at the 8A1 and 2Aundulator beamlines of the PAL. The experimental conditionsare the same as those described in [16]. Scanning photoelectronmicroscopy (SPEM) measurements were performed at the 8A1beamline of the PAL, with the spatial resolution of 0.5 m.Topographic SPEM images were constructed by employing thetotal electron yield method (sample current) so as to representthe bulk features of the measured samples [17]. SPEM is knownto be a powerful method for studying the chemical distributionof specific elements in the sub- m scale.
Fig. 1 shows the measured SPEM image of Fe Cu Cr S ,obtained with the photon energy being set at the Fe
KANG et al.: VALENCE STATES OF TRANSITION-METAL IONS AND ELECTRONIC STRUCTURES OF SPINEL FE CU CR S 3047
Fig. 1. (Left) SPEM image of Fe Cu Cr S and (right) its survey PESspectra , obtained at a (B) bright spot and a (D) dark spot, respectively.
absorption peak ( 706.5 eV) [see Fig. 2(b)]. Therefore,the brightness of the SPEM image in Fig. 1 is proportional tothe concentration distribution of Fe ions [17]. However, the ob-served irregular SPEM image for Fe Cu Cr S in Fig. 1 isinterpreted to reflect the irregular surface morphology and sur-face roughness, caused by scraping, but not to arise from theinhomogeneous Fe concentration distribution. This argumentis supported by the observation that the survey PES spectra,obtained at the bright spot (B) and the dark spot (D), respec-tively, are essentially the same. If the bright and dark spots inthe SPEM image correspond to the Fe-rich and Fe-poor regions,respectively, then their survey PES spectra are expected to bevery different, because the Fe-rich spots should reveal muchlarger Fe-derived peaks. In particular, the resonantly enhancedFe Auger peaks should be observed for the Fe-rich region [17].Therefore, Fig. 1 provides evidence that Cu ions are substitutedfor Fe ions homogeneously at least in the m scale.
Fig. 2 shows the Cr XMCD spectra of Fe Cu Cr S .The top panel of Fig. 2(a) shows the XAS spectra ofFe Cu Cr S , obtained with the photon helicity par-allel to and antiparallel to the magnetization,respectively. The Cr XMCD spectrum (blue)was obtained by taking the difference between and . Thered line represents the integrated value of the XMCD spectrum
, which can be used to estimate and byapplying the sum rule [15].
The Cr XAS spectrum of Fe Cu Cr S is qualitativelysimilar to that of Cr O [18], indicating that Cr ions are inthe formally trivalent states with the configurationfor . Both the Fe XAS and XMCD spectraof Fe Cu Cr S in Fig. 2(b) are very similar to those ofFe metal [15], indicating that the bonding nature of Feelectrons in Fe Cu Cr S is metallic-like, and that thereare the large spin magnetic moments on the Fe sites inFe Cu Cr S . In addition, neither the Fe norpeaks in the XAS and XMCD spectra of Fe Cu Cr Sexhibit multiplet features, which again supports that the Fe
S bonding is very far from the ionic bonding, butrather close to the metallic-like bonding. This finding makes acontrast to those for Cr and Cu XAS spectra that indicate
Fig. 2. (a) Cr 2p XAS spectra of Fe Cu Cr S , obtained with the photonhelicity parallel to (� ) and antiparallel to (� ) to the magnetization, respec-tively. (Blue) The XMCD spectrum corresponds to the difference between �
and � (MCD� � � � ). The red line represents the integrated value of theXMCD spectrum. Similarly for (b) Fe 2p XMCD and (c) Cu 2p XMCD spectrafor Fe Cu Cr S .
the covalent and/or ionic bonding for Cr and Cu electrons.This point will be discussed further in Fig. 3. The Cu XASspectrum of Fe Cu Cr S in Fig. 2(c) is very similar tothat of formally monovalent (Cu ) Cu FeS [19], but quitedifferent from that of CuO [20], providing evidence that Cuions are formally monovalent . These findings of triva-lent Cr ions and monovalent Cu ions in Fe Cu Cr Sseem to support the model by Lotgering and van Stapele [9]rather than that by Goodenough [10]. Note, however, that weare looking at the sample of Fe Cu Cr S for , butthe controversy between two groups is for .
The polarity of the Fe XMCD signals is opposite to thatof the Cr XMCD signals, indicating the antiparallel align-ment of the spin moments between Fe and Cr ions. In addition,
3048 IEEE TRANSACTIONS ON MAGNETICS, VOL. 43, NO. 6, JUNE 2007
Fig. 3. (Left) Comparison of the valence-band PES spectra forFe Cu Cr S (x = 0:1; 0:3; 0:5), obtained with h� = 634 eV, and theCr 3d and Fe 3d PSW’s. (Right) The calculated PDOS and total DOS forFe Cu Cr S .
the Cu XMCD spectrum also shows the very weak polariza-tion, which is parallel to that of Fe ions. The very weak CuXMCD signals, however, suggest that the magnetic moments atthe Cu sites should be very small. As to the integrated XMCD,
is very small for the Cr XMCD, indi-cating that the orbital moment on the Cr site is almost quenched.In contrast, the integrated XMCD of Fe over the whole range
, with respect to that over the edge alone,, is larger than that of Cr, indicating that the or-
bital moment on the Fe site is not completely quenched [15].The left panel of Fig. 3 compares the valence-band PES
spectra for Fe Cu Cr S , obtained at 634 eV, wherethe Cu and S electron emissions are much stronger thanFe and Cr emissions, and the extracted partial spectralweight (PSW) distributions of Fe and Cr states (bottom). Theextraction procedure for the Fe and Cr states is described in[16]. The sharp Cr states are located at 1.5 eV below ,while the Fe states are broad with the center at 4 eV,suggesting that the Fe electrons have the metallic-likebonding character. The intensity of the peak at 2.5 eV(peak A) increases with increasing Cu concentration ,reflecting that this peak has mainly the Cu character. Themeasured PES spectra reveal that all the Cr , Fe , CuPSW’s in Fe Cu Cr S have the very small spectralintensity near .1 The expected metallic Fermi edge is notobserved for , which is probably due to the very smallDOS near . Nevertheless, the findings of the metallic-likebonding of the Fe states with the S states and the broadFe PSW reveal that the top-most valence-band states closestto have mainly the Fe S hybridized character. Thisconclusion is supported by the calculated Fe PDOS and SPDOS, shown in the right panel of Fig. 3. This finding impliesthat the S band is not fully occupied, and that the chargetransfer may occur from S ion to other sites in Fe Cu Cr .
1For studying the electronic structure very close toE , a high-resolution PESstudy on single crystalline samples is required.
Differently from the case of oxide spinels, such charge transfermight be possible because of the smaller electron negativityof a S ion than an O ion. This study suggests that the narrowhybridized Fe S states near play an important rolein determining the transport properties in Fe Cu Cr S for
.The right panel of Fig. 3 provides the calculated total density
of states (DOS) and partial DOS (PDOS) of Fe Cu Cr S[8]. The ferrimagnetic and insulating ground state is revealedwith a small gap at in the total DOS. The valence band ex-tends from to approximately 7 eV below , and the Cubands are nearly occupied, in agreement with the measured PESspectra. The Cr peaks below correspond to the spin-up
band, while the unoccupied peaks above correspondto the spin-up and spin-down and bands. In con-trast, the widely spread Fe states below correspond to themajority spin-down and bands, and the unoccupiedpeaks above correspond to the minority spin-up and
bands. Fe ions are located in the tetrahedral sites, so thatthe energy levels of are lower than the energy levels of .Since Cr ions are located in the octahedral sites, the situation isreversed from the case of Fe. Most of the S states are locatedwell below , and they overlap mainly with the Fe states,but not with the Cu states, indicating the large hybridizationto the Fe states, but a weak hybridization to the Cu states.These findings in the calculated electronic structures are con-sistent with those in the measured PES spectra, as well as withthose of Fig. 2(b).
ACKNOWLEDGMENT
This work was supported in part by the KRF (KRF-2006-311-C00277), by the KOSEF through the CSCMR at SNU, and bythe eSSC at POSTECH. The PAL is supported by the MOSTand POSCO in Korea.
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