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X-ray photoelectron spectroscopy and magnetic properties of CeCo7Mn5 and CeCo8Mn4 isostructural ThMn12 type compounds
Intermetallics 38 (2013) 150e155
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X-ray photoelectron spectroscopy and magnetic properties of CeCo7Mn5 and CeCo8Mn4 isostructural ThMn12 type compounds R. Dudric a, *, A. Popescu a, O. Isnard b, M. Coldea a a b
Babes-Bolyai University, Faculty of Physics, Kogalniceanu Str. 1, 400084 Cluj-Napoca, Romania Institut Néel du CNRS, Université J. Fourier, 25 rue des martyrs, 38042 Grenoble Cedex 9, France
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 January 2013 Received in revised form 7 March 2013 Accepted 9 March 2013 Available online 29 March 2013
The magnetic properties of CeCo7Mn5 and CeCo8Mn4 compounds have been investigated by combining Xray photoelectron spectroscopy (XPS) and magnetic measurements in a wide temperature range (4e550) K and magnetic field up to 12 T. X-ray powder diffraction (XRD) measurements showed that CeCo7Mn5 and CeCo8Mn4 compounds are isostructural and crystallize in the ThMn12 structure type. XPS spectra pointed out the intermediate valence state of Ce atoms and that both Co and Mn atoms carry magnetic moments. The complex magnetic structure of CeCo7Mn5 and CeCo8Mn4 is determined by the competition between the ferromagnetic (CoeCo pairs) and antiferromagnetic (CoeMn and MneMn pairs) interactions. Two different ordering temperatures TN and TC correlated to antiferromagnetic and ferromagnetic coupling of 3d magnetic moments, respectively, are evidenced. Magnetic moments of about 1.6 mB/Co and 3.2 mB/Mn atoms were determined by correlating the magnetic data of the two compounds, in good agreement with the exchange splitting of XPS Co 3s and Mn 3s core levels. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: A. Rare-earth intermetallics B. Magnetic properties B. Electronic structure of metals and alloys
1. Introduction In the ternary metallic system CeeCoeMn, many intermetallic compounds with different crystallographic structures are reported [1]. The nearest-neighbourhood of Co and Mn atoms, as well as, the CoeCo, CoeMn and MneMn distances are different in each compound. The magnetic properties of Ce2Co17xMnx (x ¼ 0e4) [2] compounds with Th2Ni17 structure type and of CeMn4.16Co7.84 [3] compound with ThMn12 structure type were investigated by measuring the magnetization as a function of magnetic field and temperature. It is worth mentioning here that for all these compounds only the overall magnetization per unit formula was determined, without making an estimation of the individual contribution of the constituent atoms to the measured total magnetic moments. Furthermore, no explanation was given about the nature of magnetic interactions between the local magnetic moments for the description of their magnetic behaviour. However, it was suggested that the complex magnetic behaviour of RCo12xMnx (R ¼ CeeSm) compounds might be explained taking into account the possibility that not only Co atoms, but also Mn atoms could have a magnetic moment [3]. It is well known that in many Mn-alloys and
* Corresponding author. Tel.: þ40 264 405 300; fax: þ40 264 591 906. E-mail address: [email protected] (R. Dudric). 0966-9795/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.intermet.2013.03.005
intermetallic compounds, the Mn atoms carry a magnetic moment of about (3e4) mB [4e8]. The electronic 3d band of the transition metals (T) is determined by the overlap between the d orbitals of adjacent atoms and depends on the number of nearest-neighbours and on the hopping integral, which is very sensitive to the TeT distances [9]. The strength and the sign of the interaction between the neighbouring local moments are determined by the occupation fraction of d-orbitals and the orientation of these orbitals in the lattice [10]. The Mn-based alloys and compounds are assumed to exhibit a strong dependence of MneMn exchange interaction as a function of distance between the nearest-neighbour Mn atoms. It is well known that the MneMn interaction is antiferromagnetic when the disA [11]. Typical examples are the tance dMneMn is smaller than 2.9 equiatomic XMn (X ¼ Ni, Pd, Pt) compounds of CuAu(I) e structure type [4,11,12], the Laves phase compounds RMn2 (R ¼ Y, Pr, Nd, Sm, Gd, Tb) [13,14] and ThMn2 [15], and the RMn12 (R ¼ Y and heavy rare earths) of ThMn12 e structure type [16]. Neutron diffraction studies have proved the antiferromagnetic ordering of Mn moments in YMn2 and YMn12 compounds below the Néel temperatures of 100 K [17] and 120 K [18], respectively. Although the magnetic structure of YMn12 compound is not collinear, the antiferromagnetic arrangements of magnetic moments between first neighbouring atoms in 8f, 8i and 8j sites are nearly collinear. These antiferromagnetic couplings have been explained by large negative exchange interactions associated with the short distances of about
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(2.4e2.5) A in RMn12 and around 2.7 A in RMn2 and ThMn2 compounds. On the other hand, in a large number of intermetallic compounds based on Ce and 3d-transition elements a variety of anomalous behaviours in the lattice constants and magnetic properties due to the intermediate valence state of Ce atoms was observed [19e24]. It was also shown that the Ce 4f states in these compounds are not far below the Fermi energy and hybridize with the 3d states, leading to changes in the occupation fraction of the 3d bands and implicit to the decrease of the 3d local moments [25e27]. X-ray photoelectron spectroscopy (XPS) has been proved to be of considerable importance in the understanding of the magnetic properties and electronic structure of the rare earth-transition metal compounds [28]. The exchange splitting of the 3s core level spectra is a direct evidence of the local magnetic moments on T sites [28,29]. By combining XPS and magnetic measurements, we have previously shown that in Ce2Co15Mn3 intermetallic compound with Th2Zn17 structure type both Co and Mn atoms carry magnetic moments and Ce atoms are in the intermediate valence state [30]. A complex magnetic behaviour is expected to occur in CeCo7Mn5 and CeCo8Mn4 compounds, as a result of competing ferro- and antiferromagnetic interactions. The aim of this paper is to study the intrinsic magnetic properties of the CeCo7Mn5 and CeCo8Mn4 isostructural compounds and to elucidate the magnetic state of Ce, Co and Mn elements by correlating the results obtained from XRD data, magnetic measurements and XPS spectra. 2. Experimental The compounds CeCo7Mn5 and CeCo8Mn4 were prepared in a cold crucible induction furnace under a purified argon atmosphere. The samples were melted repeatedly in the same atmosphere to ensure homogeneity. The purity of the starting materials was 99.9% for all the constituent elements. The crystalline structure of the samples was analysed at room temperature by using a D 5000 Brücker BraggeBrentano diffractometer with Cu Ka radiation. The XPS spectra were recorded using a PHI 5600ci ESCA spectrometer with monochromatized Al Ka radiation at room temperature. The pressure in the ultrahigh vacuum chamber was in the 1010 mbar range during the measurements. The samples were cleaved in situ and the surface cleanness was checked by monitoring the O 1s and C 1s core levels in the survey spectra. Their intensity was very low, indicating a negligible contamination. The magnetic measurements were performed in the temperature range 4e550 K and magnetic field up to 12 T using a vibrating sample magnetometer VSM from Cryogenics as well as the extraction method described elsewhere [31]. 3. Results and discussions 3.1. Crystallographic data X-Ray diffraction (XRD) measurements showed that CeCo7Mn5 and CeCo8Mn4 are single phases and crystallize in the ThMn12tetragonal structure type. Fig. 1 shows the XRD patterns for the two compounds together with the calculated profiles using Rietveld method. At room temperature, the lattice parameters are: a ¼ b ¼ 8.496(4) A and c ¼ 4.748(0) A for CeCo7Mn5 and a ¼ b ¼ 8.464(8) A and c ¼ 4.743(4) A for CeCo8Mn4. In this structure type, the Ce atoms are located at 2a sites and Co and Mn atoms may occupy the 8f, 8i and 8j sites. The XRD refinements have been performed assuming a full occupancy of the metallic site and allowing only substitution of Mn by Co atoms. The
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Fig. 1. X-ray diffraction pattern of CeCo7Mn5 and CeCo8Mn4 recorded using a wavelength of 1.540598 A. The experimental data points are shown as dots, the calculated profile using Rietveld method and difference curve are shown as solid lines.
stoichiometry has been constrained according to the initial chemical composition and the absence of secondary phase in the diffraction pattern. Since the atomic radius of Mn atoms is larger than that of Co atoms (1.307 A and 1.252 A, respectively), the 8i sites with a larger Wigner Seitz volume [32] are expected to be preferentially occupied by Mn atoms. In the assumption that Mn atoms occupy entirely the 8i sites, the calculated WignereSeitz cell volumes for CeCo8Mn4 compound are as follows: 12.30 Å3 for Mn in 8i sites, 12.01 Å3 for Co in 8j sites and 10.72 Å3 for Co in 8f sites. This assumption was confirmed by the Rietveld analysis of the XRD data, which yielded the best results for occupancy factors close to zero for Co atoms on the 8i sites. However due to the relatively small scattering length contrast, neutron diffraction investigation would be desirable in order to be more precise on the site occupancy of the Co and Mn and confirm the XRD analysis. The fifth Mn atom in CeCo7Mn5 is supposed to be located at the largest remaining site 8j. Indeed, since this site has similarities with the 8f one including number of neighbours but has larger WignereSeitz volume 12.01 Å3 against 10.72 Å3. This is also in good agreement with the scheme of substitution of Co atoms to Fe in RFe12xCox compounds [33]. It was shown that Co atoms first substitute to the 8f sites, then occupy progressively the 8j and finally the 8i sites. Since the lattice constants of the two studied compounds are very close, the distances between the atoms in different crystallographic sites are almost the same. The nearest-neighbour Mn-Mn distance between the Mn atoms in 8i sites is about 2.36 A, very close to the similar distance in YMn12 isostructural compound (2.39 A). The distances between the Co atoms in both compounds are close to those in pure Co metal, namely w2.5 A. It is consequently reasonable to expect similar exchange interactions between the MneMn or the CoeCo atoms than in the YMn12 and metallic Co, respectively.
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3.2. X-ray photoelectron spectra The XPS spectra of the Ce 4d core levels reflect the mixed valence state of Ce in CeCo7Mn5 and CeCo8Mn4 compounds. The Ce 4d spectra of both compounds show multiplets arising from the interaction of the 4d hole with the 4f electrons, as presented in Fig. 2 for CeCo7Mn5, which is characteristic for all compounds with Ce in the trivalent states. The peaks corresponding to trivalent Ce are superimposed on the Co 3s peaks, while the peaks at higher binding energies correspond to the fraction of tetravalent Ce in the compound. The spin-orbit splittings are 3.11 eV for Ce3þ and 3.30 eV for Ce4þ in CeCo7Mn5 and 3.08 eV for Ce3þ and 3.41 eV for Ce4þ in CeCo8Mn4, in good agreement with the values measured in CeNi5 [19], CeF4 [20], CeMn4Al8 and CeMn6Al6 [34], Ce2Co15Mn3 [30] mixed valence compounds. The energy shift between the barycenters of the structures corresponding to Ce3þ and Ce4þ is about 10 eV, in agreement with the observation made on other Ce mixed valence compounds [20]. The exchange interaction Jdc between the core hole spin s and the 3d electron spin S gives rise to a satellite on the high binding energy side of the main line of the Co 3s spectrum [28]. The two peaks at 101.36 and 105.79 eV in Fig. 2 correspond to the high spin final sate (S þ ½) and the low spin final state (S ½), respectively. The Co 3s core level spectrum shows an exchange splitting of about 4.43 eV for CeCo7Mn5, which is a direct evidence of the local magnetic moments on Co sites. Approximately the same value was obtained for the Co 3s exchange splitting in the isostructural compound CeCo8Mn4. This value is close to that found in the pure Co metal, namely 4.5 eV [29]. The exchange splitting DEex ¼ Jdc(2S þ 1) is proportional with the Co local moment mCo ¼ 2S [35]. This result suggests that the Co local moment decreases only slightly when going from Co metal to CeCo7Mn5 and CeCo8Mn4 compounds. The Ce 3d spectrum of CeCo8Mn4 shows a superposition of spinorbit-split peaks corresponding to the two initial configurations Ce3þ and Ce4þ, as shown in Fig. 3. The spin-orbit splitting is dESO ¼ 18.3 eV. Multiplet effects due to the coupling of the 3d hole with the open 4f shell are visible in both 3d3/2 and 3d5/2 peaks. There is clear evidence of a 3d94f0 component at 914.2 eV which is not present in compounds with all Ce ions in the trivalent state. The intensity of the 3d94f2 final-state component is also higher than for Ce trivalent compounds. These findings confirm the mixed valence state of Ce ions in CeCo8Mn4.
Fig. 4a presents the curve fitting results of Mn 3s spectra in CeCo7Mn5 and CeCo8Mn4 compounds, after background subtraction. The Mn 3s core level spectra show an exchange splitting around 4.9 eV arising from the exchange interactions between the core hole and the open 3d shell. This is a direct evidence of the local magnetic moments on Mn sites. Such splittings of about (4e5.5) eV were observed in the 3s levels of Mn atoms in various intermetallic compounds and alloys, like Mn1xNixAl [36], Mn1xAlxNi3 [37], CeMn4Al8 and CeMn6Al6 [34], Ce2Co15Mn3 [30], MnNi1xSbx [38], MnPd1xSbx [7], etc. For example, the exchange splitting in MnNi is about 5.2 eV and the Mn magnetic moment is 4 mB/Mn [4]. In Fig. 4b is shown the main line of the Mn 2p core level for CeCo7Mn5. The main Mn 2p3/2 line situated at about 639 eV binding energy shows a splitting, as it would consist of two features that are separated by DEex z 1 eV, which is very close to those observed in MnPd3xSbx [39] and Heusler alloys [5]. The line-shape analysis shows that the Mn 2p3/2 spectrum of CeCo7Mn5 can be interpreted as consisting of exchange-split components. The two components of the Mn 2p3/2 line arise from the exchange interactions between the core hole and the Mn 3d valence electrons. In this picture, the high- and lowbinding energy components correspond to the majority- and
Fig. 2. XPS spectrum of the Ce 4d and Co 3s core levels in CeCo7Mn5 (open circles correspond to the experimental spectrum and the continuous curves to the fitting results, after background subtraction).
Fig. 4. XPS spectra of (a) Mn 3s in CeCo7Mn5 and CeCo8Mn4 (open circles correspond to the experimental spectrum and the continuous curves to the fitting results, after background subtraction) and (b) Mn 2p3/2 core levels in CeCo7Mn5.
Fig. 3. XPS spectrum of the Ce 3d level in CeCo8Mn4.
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minority- spin core-hole states respectively and the energy separation between them corresponds to the exchange spitting of the Mn 2p core-holes states. This gives another direct evidence of the existence of local moments confined on Mn sites in CeCo7Mn5 compound. The values obtained for Mn 3s and Mn 2p3/2 exchange splittings in CeCo7Mn5 and also in CeCo8Mn4 indicate a local magnetic moment per Mn atom of about 3 mB. The valence band spectrum of CeCo7Mn5 compound shown in Fig. 5 is mainly the result of the Co 3d, Mn 3d, Ce 5d and Ce 4f states superposition. At Al Ka radiation the Co 3d cross section is about three times larger than the Mn 3d cross section [40] and taking into account the number of the different atoms in the unit formula, the valence band is dominated by the Co 3d states. It is interesting to note that the valence band is centred at about 2 eV (estimated from the half band width at half height), a value close to that observed in pure Co metal (see the inset in Fig. 5). Experimental data and band structure calculations for Ce mixed valence compounds have shown that the 4f state are not far below EF [19,20]. In CeCo5 and Ce2Co17 compounds both the Ce 5d and 4f states hybridize with the Co 3d states [25,41], leading to a decrease of the Co magnetic moment. In addition the spin dependence of the hybridization strength induces antiparallel magnetic moments on the cerium atoms (z0.2 mB/Ce), as evidenced in ferromagnetic compounds like CeCo5 and Ce2Co17 compared with the reference compounds YCo5 and LaCo5, and respectively Y2Co17 [2,25,41,42]. It is therefore expected that the valence band of CeCo7Mn5 and CeCo8Mn4 compounds is influenced by the same 3de5d and 3de4f hybridizations.
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Fig. 6. Magnetization curves of CeCo7Mn5 for temperatures between 4 K and 300 K in magnetic field up to 12 T. The inset shows the temperature dependence of the spontaneous magnetization Msp.
Although the XPS spectra indicate similar electronic structure of Co and Mn atoms in the two compounds, which suggests close values of the local magnetic moments per Co and respectively Mn atoms, the difference in the number of Co and Mn atoms per unit formula leads to very different values of magnetization as a function of magnetic field and temperature. The magnetic field dependences of magnetization recorded for temperatures between 5 K and 300 K for CeCo7Mn5 are shown in Fig. 6. The values and the variations of magnetization with field suggest an antiferromagnetic coupling between the transition metal moments located on different sublattices. The spontaneous magnetizations Msp, determined by extrapolating the
magnetization from high field linear regime to zero field, are shown in the inset of Fig. 6. The magnetization curves show hysteresis for temperatures smaller than 25 K, with a coercive field of HC ¼ 0.392 T at 5 K. The thermal variation of magnetization for CeCo7Mn5 measured in zero-field cooling (ZFC) and field cooling (FC) in applied fields of 0.025 T, 1 T and 2 T is shown in Fig. 7. The ZFC curves present a cusp at low temperatures (Tcs), whose value decreases with the applied magnetic field Happ. Furthermore, the normalized difference between the FC and ZFC magnetization at 5 K also decreases with the increase of the applied magnetic field. The irreversibility of the magnetization of CeCo7Mn5, as measured in ZFC and FC states, is attributed to the movement of domain walls. The spin-glass-like behaviour of magnetization originates from the magnetocrystalline anisotropy [43] and may also originate from the existence of competing magnetic interactions like in frustrated systems. Such later behaviour is not expected to take place here. Since coercivity is related to magnetic anisotropy, the irreversible magnetic behaviour reflects the role of
Fig. 5. XPS spectrum of the valence band region of CeCo7Mn5. In the inset is shown the valence band of pure Co metal for comparison.
Fig. 7. Temperature dependence of the magnetization for CeCo7Mn5 measured in the ZFC and FC states with applied fields of 0.025 T (right axis) and 1 T and 2 T (left axis). The inset shows the dM/dT curve for B ¼ 2 T.
3.3. Magnetization
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anisotropy in determining the shapes of ZFC and FC curves below the ordering temperature. This interpretation is supported by the relationship between the ZFC and FC susceptibilities [44]:
cZFC ¼ MZFC =Happ ; c0FC ¼ MFC = Happ þ HC
(1)
The notation c0FC is used to distinguish this susceptibility from the normal FC susceptibility cFC ¼ MFC =Happ. At 5 K, measurements for CeCo7Mn5 lead to the following results cZFC ¼ c0FC ¼ 1:3mB =f:u:T for Happ ¼ 1 T and HC ¼ 0.392 T. In the inset of Fig. 7, the temperature dependence of the differential magnetization dM/dT for CeCo7Mn5 is displayed for B ¼ 2T. The dM/dT curve exhibits one local minimum near TC z 45 K, ascribed to the ferromagnetic CoeCo coupling and a local maximum near TN z 150 K, which can be ascribed to the antiferromagnetic MneMn coupling. The temperature dependence of the spontaneous magnetization (inset of Fig. 6) may be explained taking into account the residual interaction of Co moments with Mn moments above TC. That is, the Co moments are not in a completely disordered state for TC < T < TN due to the strong MneMn and Mne Co interactions. The magnetic field dependences of magnetization recorded for temperatures between 3.9 K and 300 K for CeCo8Mn4 compound are shown in Fig. 8. Like for CeCo7Mn5 compound, the magnetization does not reach saturation even at high fields, but the values of spontaneous magnetization are much higher than those of CeCo7Mn5. As an example, at 4 K, Msp ¼ 2.6 mB/f.u for CeCo8Mn4, in comparison with Msp ¼ 0.7 mB/f.u for CeCo7Mn5. The M(H) curve at 3.9 K for CeCo8Mn4 shows a hysteresis loop (not shown in Fig. 8) like in the isostructural compound CeMn4.16Co7.84 [3] and originates, as was mentioned for CeCo7Mn5, from the domain wall pinning occurring at low temperatures. The difficulty to reach saturation may be taken as a confirmation of the existence of large magnetocrystalline anisotropy in such system. In both compounds, the ferromagnetic components are pinned by the higher anisotropy of the antiferromagnetic component. In CeCo8Mn4 the magnetization shows a more pronounced tendency towards saturation due to weaker antiferromagnetic interactions in comparison to CeCo7Mn5. The thermal variation of magnetization for CeCo8Mn4 measured in an applied field of 2 T is shown in Fig. 9. The dM/dT curve, shown in the inset of Fig. 9, exhibits one local maximum near TN z 100 K and one local minimum near TC z 173 K, ascribed like for CeCo7Mn5 to the antiferromagnetic and ferromagnetic ordering of the 3d magnetic moments.
Fig. 8. Magnetization curves of CeCo8Mn4 between 3.9 and 300 K in magnetic field up to 10 T.
Fig. 9. Temperature dependence of magnetization for CeCo8Mn4 in 2 T magnetic field. The inset shows the dM/dT curve.
It is worth mentioning that the magnetization curves M(T) given in Figs. 7 and 9 reveal an antiferromagnetic-like magnetic transition at about 155 K for CeCo7Mn5 and 100 K for CeCo8Mn4 (more visible for CeCo7Mn5 compound), which are close to the determined Néel temperatures. A similar anomaly at about 200 K was also observed in the isostructural compounds SmCo6Mn6 and SmCo5Mn7 [45]. The thermal variation of the reciprocal susceptibility for the investigated compounds in the high temperature range is shown in Fig. 10. The experimental data fit a CurieeWeiss law plus a small additional temperature e independent term c0 with the paramagnetic Curie temperatures q ¼ 175 K for CeCo7Mn5 and q ¼ 240 K for CeCo8Mn4.The effective magnetic moments, determined from the Curie constants, have the values meff ¼ 11.2 mB/f.u. for CeCo7Mn5 and meff ¼ 10.6 mB/f.u. for CeCo8Mn4. The complex magnetic behaviour of CeCo7Mn5 and CeCo8Mn4 compounds may be explained taking into account the interactions between Co and Mn magnetic moments located in different crystallographic sites. The coupling between the Co moments from different sites is ferromagnetic as usual for Co containing intermetallic compounds, while the interaction between the Co and Mn atoms can be expected to be antiferromagnetic. Also, like in the YMn12 isostructural
Fig. 10. Reciprocal magnetic susceptibility versus temperature for CeCo7Mn5 and CeCo8Mn4 compounds.
R. Dudric et al. / Intermetallics 38 (2013) 150e155
compound, the interaction between the Mn atoms in 8i sites is expected to be antiferromagnetic. Consequently, the competition between the antiferromagnetic CoeMn and MneMn interactions leads to a complex ferromagnetic þ antiferromagnetic configuration, like in RMn12xFex for 6 < x < 9 [46,47], probably consisting of a canted arrangement of Mn moments relative to the magnetization of the Co sublattice. In order to estimate the values of the Co and Mn moments in CeCo7Mn5 and CeCo8Mn4, we correlated the magnetic data from paramagnetic states for the two compounds. The relations between the effective magnetic moments and the spins of Co and Mn atoms in the paramagnetic state, obtained from the total Curie constant C ¼ CCo þ CMn, are:
m2eff =4 ¼ 7 SCo ðSCo þ 1Þ þ 5SMn ðSMn þ 1Þ for CeCo7 Mn5
(2)
m2eff =4 ¼ 8 SCo ðSCo þ 1Þ þ 4SMn ðSMn þ 1Þ for CeCo8 Mn4
(3)
The above equations are well verified for SCo z 0.8 and SMn z 1.6, yielding for the local magnetic moments confined on Co and Mn sites in the ordered state the values: mCo ¼ 1.6 mB and mMn ¼ 3.2 mB, in good agreement with the above estimation from XPS data. 4. Conclusions The CeCo7Mn5 and CeCo8Mn4 compounds crystallize in the tetragonal ThMn12 structure type and present complex magnetic behaviour. XPS spectra have shown the intermediate valence state of Ce atoms and that both Co and Mn atoms carry magnetic moments. The analysis of the magnetic properties, in the light of crystal structure and of distances between the atoms from different crystallographic sites, suggests the presence of ferromagnetic interactions between CoeCo and antiferromagnetic interactions between CoeMn and MneMn magnetic moments. This competition leads to a complex ferromagnetic þ antiferromagnetic configuration, probably consisting of a canted arrangement of Mn moments relative to the magnetization of the Co sublattice. Typical local magnetic moments of 1.6 mB/Co and 3.2 mB/Mn atoms were estimated by correlating the data from paramagnetic states, in good agreement with the exchange splitting of the XPS Co 3s and Mn 3s core levels. The differences in the magnetic parameters MSP, TC, TN, and q for the two compounds are due to the enhancement of ferromagnetic interactions together with a decrease of antiferromagnetic interactions in CeCo8Mn4, whereas an inverted situation prevails in CeCo7Mn5. Detailed neutron diffraction studies would be necessary to confirm the proposed magnetic structure and the values obtained for Co and Mn magnetic moments. Acknowledgements R.D. acknowledges the financial support from the Romanian UEFISCDI Project No. PN-II-ID-PCE-2011-3-0583 (85/2011). References [1] Vilars P, Calvert LD. Pearson’s handbook of crystallographic data for intermetallic phases. 2nd ed. Materials Park, OH: ASM International; 1991. [2] Sun Zhi-gang, Zhang Shao-ying, Zhang Hong-wei, Wang Jing-yun, Shen Baogen. J Phys Condens Matter 2000;12:2495e504.
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[3] Suski W, Wochowski K, Gilewski A, Mydlarz T, Belan B. Physica B 2001;294e 295:177e81. [4] Pál L, Krén E, Kádár G, Szabó P, Tarnóczi T. J Appl Phys 1968;39:538e44. [5] Plogmann S, Schlathölter T, Braun J, Neumann M, Yarmoshenko YuM, Yablonskikh MV, et al. Phys Rev B 1999;60:6428e38. [6] Coldea M, Neumann M, Chiuzbaian SG, Pop V, Pascut LG, Isnard O, et al. J Alloys Compd 2006;417:7e12. [7] Pacurariu R, Coldea M, Neumann M, Pop V, Isnard O, Räkers M. Phys Status Solidi B 2007;244:3190e7. [8] Wijngaard JH, Hass C, de Groot RA. Phys Rev B 1992;45:5395e405. [9] Mathon J. Contemp Phys 1991;32:143e56. [10] Moriya T. Spin fluctuations in itinerant electron magnetism. New York: Springer; 1985. [11] Kjekshus A, Mollerud R, Andresen AF, Pearson WB. Phil Mag 1967;16: 1063e83. [12] Andresen AF, Kjekshus A, Mollerud R, Pearson WB. Acta Chem Scand 1966;20: 2529e34. [13] Wada H, Nakamura H, Yoshimura K, Shiga M, Nakamura Y. J Magn Magn Mater 1987;70:134e6. [14] Shiga M. Physica B 1988;149:293e305. [15] Deportes J, Lemaires R, Ouladdiaf B, Roudaut E, Sayetat F. J Magn Magn Mater 1987;70:191e3. [16] Deportes J, Givord D, Lemaire R, Nagai H. Physica 1977;86-88B:69e70. [17] Nakamura Y, Shiga M, Kawano S. Physica BþC 1983;120:212e5. [18] Deportes J, Givord D. Solid State Commun 1976;19:845e51. [19] Fuggle JC, Hillebrecht FU, Zolnierek Z, Läser R, Freiburg Ch, Gunnarsson O, et al. Phys Rev B 1983;27:7330e41. [20] Baer Y, Zürcher Ch. Phys Rev Lett 1977;39:956e9. [21] Röhler J. X-ray absorption and emission spectra. In: Gschneidner KA, Eyring JR, Hüfner S, editors. Handbook on the physics and chemistry of rare earths, North-Holland, Amsterdam 1987453e545.. [22] Isnard O, Miraglia S, Fruchart D, Georgetti C, Pizzini S, Dartyge E, et al. Phys Rev B 1994;49:15692e701. [23] Isnard O, Miraglia S, Fruchart D, Giorgetti C, Dartyge E, Krill G. J Phys Condens Matter 1996;8:2437e46. [24] Parks RD, Raaen S, denBoer ML, Murgai V, Mihalisin T. Phys Rev B 1983;2: 3556e8. [25] Nordström Lars, Eriksson Olle, Brooks MSS, Johansson Börje. Phys Rev B 1990;41:9111e20. [26] Duo L, Vavassori P, Braicovich L, Grioni M, Malterre D, Baer Y, et al. Phys Rev B 1996;53:7030e4. [27] Durivault L, Bourrée F, Chevalier B, André G, Etourneau J, Isnard O. J Magn Magn Mater 2001;232:139e46. [28] Hüfner S. Photoelectron spectroscopy principles and applications. Berlin: Springer-Verlag; 1995. [29] Johnson PD, Liu Y, Xu Z, Huang DJ. J Electron Spectrosc Relat Phenom 1995;75: 245e52. [30] Popescu A, Isnard O, Dudric R, Coldea M. J Alloys Compd 2012;535:10e4. [31] Barlet A, Genna JC, Lethuillier P. Cryogenic 1991;31:801e5. [32] Isnard O, Fruchart D. J Alloys Compd 1994;205:1e15. [33] Burriel R, Palacios E, Garcia LM, Soubeyroux JL, Tomey E, Fruchart D. J Magn Magn Mater 1995;140-144:1091e2. [34] Coldea M, Neumann M, Lütkehoff St, Mähl S, Coldea R. J Alloys Compd 1998;278:72e9. [35] Garreau G, Schorsch V, Beaurepaire E, Parlebas JC, Speder O, Rennert P. J Phys IV Colloque C9 1994;4:127. [36] Rednic V, Coldea M, Mendiratta SK, Valente M, Pop V, Neumann M, et al. J Magn Magn Mater 2009;321:3415e21. [37] Rednic V, Rednic L, Coldea M, Pop V, Neumann M, Pacurariu R, et al. Cent Eur J Phys 2008;6:434e9. [38] Pacurariu R, Rednic V, Coldea M, Benea D, Pop V, Isnard O, et al. Phys Status Solidi B 2009;246:50e5. [39] Coldea M, Pop V, Neumann M, Isnard O, Pacurariu R, Takacs AF, et al. Phys Status Solidi B 2006;243:1914e21. [40] Yeh JJ, Lindau I, Data Nucl At. Data Tables 1985;32(1). [41] Zhang L, Tegusi O, Liu ZY, Klaasse JCP, Bruck E, de Boer FR, et al. J Alloys Compd 2000;307:51e5. [42] Sun Zhi-gang, Zhang Hong-wei, Liang Bing, Wang Jing-yun, Shen Bao-gen. J Appl Phys 2000;87:5311e3. [43] Chang Hong, Guo Yong-quan, Liang Jing-kui, Rao Guang-hui. J Magn Magn Mater 2004;278:306e10. [44] Joy PA, Anil Kumar PS, Date SK. J Phys Condens Matter 1998;10:11049e54. [45] Suski W, Wochowski K, Mydlarz T, Belan B. Physica B 2002;319:45e51. [46] Morales M, Bacmann M, Wolfers P, Fruchart D, Ouladdiaf B. Phys Rev B 2001;64:144426. [47] Morales M, Bacmann M, Wolfers P, Fruchart D, Ouladdiaf B. J Magn Magn Mater 2001;236:83e92.
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Intermetallics journal homepage: www.elsevier.com/locate/intermet
X-ray photoelectron spectroscopy and magnetic properties of CeCo7Mn5 and CeCo8Mn4 isostructural ThMn12 type compounds R. Dudric a, *, A. Popescu a, O. Isnard b, M. Coldea a a b
Babes-Bolyai University, Faculty of Physics, Kogalniceanu Str. 1, 400084 Cluj-Napoca, Romania Institut Néel du CNRS, Université J. Fourier, 25 rue des martyrs, 38042 Grenoble Cedex 9, France
a r t i c l e i n f o
a b s t r a c t
Article history: Received 21 January 2013 Received in revised form 7 March 2013 Accepted 9 March 2013 Available online 29 March 2013
The magnetic properties of CeCo7Mn5 and CeCo8Mn4 compounds have been investigated by combining Xray photoelectron spectroscopy (XPS) and magnetic measurements in a wide temperature range (4e550) K and magnetic field up to 12 T. X-ray powder diffraction (XRD) measurements showed that CeCo7Mn5 and CeCo8Mn4 compounds are isostructural and crystallize in the ThMn12 structure type. XPS spectra pointed out the intermediate valence state of Ce atoms and that both Co and Mn atoms carry magnetic moments. The complex magnetic structure of CeCo7Mn5 and CeCo8Mn4 is determined by the competition between the ferromagnetic (CoeCo pairs) and antiferromagnetic (CoeMn and MneMn pairs) interactions. Two different ordering temperatures TN and TC correlated to antiferromagnetic and ferromagnetic coupling of 3d magnetic moments, respectively, are evidenced. Magnetic moments of about 1.6 mB/Co and 3.2 mB/Mn atoms were determined by correlating the magnetic data of the two compounds, in good agreement with the exchange splitting of XPS Co 3s and Mn 3s core levels. Ó 2013 Elsevier Ltd. All rights reserved.
Keywords: A. Rare-earth intermetallics B. Magnetic properties B. Electronic structure of metals and alloys
1. Introduction In the ternary metallic system CeeCoeMn, many intermetallic compounds with different crystallographic structures are reported [1]. The nearest-neighbourhood of Co and Mn atoms, as well as, the CoeCo, CoeMn and MneMn distances are different in each compound. The magnetic properties of Ce2Co17xMnx (x ¼ 0e4) [2] compounds with Th2Ni17 structure type and of CeMn4.16Co7.84 [3] compound with ThMn12 structure type were investigated by measuring the magnetization as a function of magnetic field and temperature. It is worth mentioning here that for all these compounds only the overall magnetization per unit formula was determined, without making an estimation of the individual contribution of the constituent atoms to the measured total magnetic moments. Furthermore, no explanation was given about the nature of magnetic interactions between the local magnetic moments for the description of their magnetic behaviour. However, it was suggested that the complex magnetic behaviour of RCo12xMnx (R ¼ CeeSm) compounds might be explained taking into account the possibility that not only Co atoms, but also Mn atoms could have a magnetic moment [3]. It is well known that in many Mn-alloys and
* Corresponding author. Tel.: þ40 264 405 300; fax: þ40 264 591 906. E-mail address: [email protected] (R. Dudric). 0966-9795/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.intermet.2013.03.005
intermetallic compounds, the Mn atoms carry a magnetic moment of about (3e4) mB [4e8]. The electronic 3d band of the transition metals (T) is determined by the overlap between the d orbitals of adjacent atoms and depends on the number of nearest-neighbours and on the hopping integral, which is very sensitive to the TeT distances [9]. The strength and the sign of the interaction between the neighbouring local moments are determined by the occupation fraction of d-orbitals and the orientation of these orbitals in the lattice [10]. The Mn-based alloys and compounds are assumed to exhibit a strong dependence of MneMn exchange interaction as a function of distance between the nearest-neighbour Mn atoms. It is well known that the MneMn interaction is antiferromagnetic when the disA [11]. Typical examples are the tance dMneMn is smaller than 2.9 equiatomic XMn (X ¼ Ni, Pd, Pt) compounds of CuAu(I) e structure type [4,11,12], the Laves phase compounds RMn2 (R ¼ Y, Pr, Nd, Sm, Gd, Tb) [13,14] and ThMn2 [15], and the RMn12 (R ¼ Y and heavy rare earths) of ThMn12 e structure type [16]. Neutron diffraction studies have proved the antiferromagnetic ordering of Mn moments in YMn2 and YMn12 compounds below the Néel temperatures of 100 K [17] and 120 K [18], respectively. Although the magnetic structure of YMn12 compound is not collinear, the antiferromagnetic arrangements of magnetic moments between first neighbouring atoms in 8f, 8i and 8j sites are nearly collinear. These antiferromagnetic couplings have been explained by large negative exchange interactions associated with the short distances of about
R. Dudric et al. / Intermetallics 38 (2013) 150e155
(2.4e2.5) A in RMn12 and around 2.7 A in RMn2 and ThMn2 compounds. On the other hand, in a large number of intermetallic compounds based on Ce and 3d-transition elements a variety of anomalous behaviours in the lattice constants and magnetic properties due to the intermediate valence state of Ce atoms was observed [19e24]. It was also shown that the Ce 4f states in these compounds are not far below the Fermi energy and hybridize with the 3d states, leading to changes in the occupation fraction of the 3d bands and implicit to the decrease of the 3d local moments [25e27]. X-ray photoelectron spectroscopy (XPS) has been proved to be of considerable importance in the understanding of the magnetic properties and electronic structure of the rare earth-transition metal compounds [28]. The exchange splitting of the 3s core level spectra is a direct evidence of the local magnetic moments on T sites [28,29]. By combining XPS and magnetic measurements, we have previously shown that in Ce2Co15Mn3 intermetallic compound with Th2Zn17 structure type both Co and Mn atoms carry magnetic moments and Ce atoms are in the intermediate valence state [30]. A complex magnetic behaviour is expected to occur in CeCo7Mn5 and CeCo8Mn4 compounds, as a result of competing ferro- and antiferromagnetic interactions. The aim of this paper is to study the intrinsic magnetic properties of the CeCo7Mn5 and CeCo8Mn4 isostructural compounds and to elucidate the magnetic state of Ce, Co and Mn elements by correlating the results obtained from XRD data, magnetic measurements and XPS spectra. 2. Experimental The compounds CeCo7Mn5 and CeCo8Mn4 were prepared in a cold crucible induction furnace under a purified argon atmosphere. The samples were melted repeatedly in the same atmosphere to ensure homogeneity. The purity of the starting materials was 99.9% for all the constituent elements. The crystalline structure of the samples was analysed at room temperature by using a D 5000 Brücker BraggeBrentano diffractometer with Cu Ka radiation. The XPS spectra were recorded using a PHI 5600ci ESCA spectrometer with monochromatized Al Ka radiation at room temperature. The pressure in the ultrahigh vacuum chamber was in the 1010 mbar range during the measurements. The samples were cleaved in situ and the surface cleanness was checked by monitoring the O 1s and C 1s core levels in the survey spectra. Their intensity was very low, indicating a negligible contamination. The magnetic measurements were performed in the temperature range 4e550 K and magnetic field up to 12 T using a vibrating sample magnetometer VSM from Cryogenics as well as the extraction method described elsewhere [31]. 3. Results and discussions 3.1. Crystallographic data X-Ray diffraction (XRD) measurements showed that CeCo7Mn5 and CeCo8Mn4 are single phases and crystallize in the ThMn12tetragonal structure type. Fig. 1 shows the XRD patterns for the two compounds together with the calculated profiles using Rietveld method. At room temperature, the lattice parameters are: a ¼ b ¼ 8.496(4) A and c ¼ 4.748(0) A for CeCo7Mn5 and a ¼ b ¼ 8.464(8) A and c ¼ 4.743(4) A for CeCo8Mn4. In this structure type, the Ce atoms are located at 2a sites and Co and Mn atoms may occupy the 8f, 8i and 8j sites. The XRD refinements have been performed assuming a full occupancy of the metallic site and allowing only substitution of Mn by Co atoms. The
151
Fig. 1. X-ray diffraction pattern of CeCo7Mn5 and CeCo8Mn4 recorded using a wavelength of 1.540598 A. The experimental data points are shown as dots, the calculated profile using Rietveld method and difference curve are shown as solid lines.
stoichiometry has been constrained according to the initial chemical composition and the absence of secondary phase in the diffraction pattern. Since the atomic radius of Mn atoms is larger than that of Co atoms (1.307 A and 1.252 A, respectively), the 8i sites with a larger Wigner Seitz volume [32] are expected to be preferentially occupied by Mn atoms. In the assumption that Mn atoms occupy entirely the 8i sites, the calculated WignereSeitz cell volumes for CeCo8Mn4 compound are as follows: 12.30 Å3 for Mn in 8i sites, 12.01 Å3 for Co in 8j sites and 10.72 Å3 for Co in 8f sites. This assumption was confirmed by the Rietveld analysis of the XRD data, which yielded the best results for occupancy factors close to zero for Co atoms on the 8i sites. However due to the relatively small scattering length contrast, neutron diffraction investigation would be desirable in order to be more precise on the site occupancy of the Co and Mn and confirm the XRD analysis. The fifth Mn atom in CeCo7Mn5 is supposed to be located at the largest remaining site 8j. Indeed, since this site has similarities with the 8f one including number of neighbours but has larger WignereSeitz volume 12.01 Å3 against 10.72 Å3. This is also in good agreement with the scheme of substitution of Co atoms to Fe in RFe12xCox compounds [33]. It was shown that Co atoms first substitute to the 8f sites, then occupy progressively the 8j and finally the 8i sites. Since the lattice constants of the two studied compounds are very close, the distances between the atoms in different crystallographic sites are almost the same. The nearest-neighbour Mn-Mn distance between the Mn atoms in 8i sites is about 2.36 A, very close to the similar distance in YMn12 isostructural compound (2.39 A). The distances between the Co atoms in both compounds are close to those in pure Co metal, namely w2.5 A. It is consequently reasonable to expect similar exchange interactions between the MneMn or the CoeCo atoms than in the YMn12 and metallic Co, respectively.
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3.2. X-ray photoelectron spectra The XPS spectra of the Ce 4d core levels reflect the mixed valence state of Ce in CeCo7Mn5 and CeCo8Mn4 compounds. The Ce 4d spectra of both compounds show multiplets arising from the interaction of the 4d hole with the 4f electrons, as presented in Fig. 2 for CeCo7Mn5, which is characteristic for all compounds with Ce in the trivalent states. The peaks corresponding to trivalent Ce are superimposed on the Co 3s peaks, while the peaks at higher binding energies correspond to the fraction of tetravalent Ce in the compound. The spin-orbit splittings are 3.11 eV for Ce3þ and 3.30 eV for Ce4þ in CeCo7Mn5 and 3.08 eV for Ce3þ and 3.41 eV for Ce4þ in CeCo8Mn4, in good agreement with the values measured in CeNi5 [19], CeF4 [20], CeMn4Al8 and CeMn6Al6 [34], Ce2Co15Mn3 [30] mixed valence compounds. The energy shift between the barycenters of the structures corresponding to Ce3þ and Ce4þ is about 10 eV, in agreement with the observation made on other Ce mixed valence compounds [20]. The exchange interaction Jdc between the core hole spin s and the 3d electron spin S gives rise to a satellite on the high binding energy side of the main line of the Co 3s spectrum [28]. The two peaks at 101.36 and 105.79 eV in Fig. 2 correspond to the high spin final sate (S þ ½) and the low spin final state (S ½), respectively. The Co 3s core level spectrum shows an exchange splitting of about 4.43 eV for CeCo7Mn5, which is a direct evidence of the local magnetic moments on Co sites. Approximately the same value was obtained for the Co 3s exchange splitting in the isostructural compound CeCo8Mn4. This value is close to that found in the pure Co metal, namely 4.5 eV [29]. The exchange splitting DEex ¼ Jdc(2S þ 1) is proportional with the Co local moment mCo ¼ 2S [35]. This result suggests that the Co local moment decreases only slightly when going from Co metal to CeCo7Mn5 and CeCo8Mn4 compounds. The Ce 3d spectrum of CeCo8Mn4 shows a superposition of spinorbit-split peaks corresponding to the two initial configurations Ce3þ and Ce4þ, as shown in Fig. 3. The spin-orbit splitting is dESO ¼ 18.3 eV. Multiplet effects due to the coupling of the 3d hole with the open 4f shell are visible in both 3d3/2 and 3d5/2 peaks. There is clear evidence of a 3d94f0 component at 914.2 eV which is not present in compounds with all Ce ions in the trivalent state. The intensity of the 3d94f2 final-state component is also higher than for Ce trivalent compounds. These findings confirm the mixed valence state of Ce ions in CeCo8Mn4.
Fig. 4a presents the curve fitting results of Mn 3s spectra in CeCo7Mn5 and CeCo8Mn4 compounds, after background subtraction. The Mn 3s core level spectra show an exchange splitting around 4.9 eV arising from the exchange interactions between the core hole and the open 3d shell. This is a direct evidence of the local magnetic moments on Mn sites. Such splittings of about (4e5.5) eV were observed in the 3s levels of Mn atoms in various intermetallic compounds and alloys, like Mn1xNixAl [36], Mn1xAlxNi3 [37], CeMn4Al8 and CeMn6Al6 [34], Ce2Co15Mn3 [30], MnNi1xSbx [38], MnPd1xSbx [7], etc. For example, the exchange splitting in MnNi is about 5.2 eV and the Mn magnetic moment is 4 mB/Mn [4]. In Fig. 4b is shown the main line of the Mn 2p core level for CeCo7Mn5. The main Mn 2p3/2 line situated at about 639 eV binding energy shows a splitting, as it would consist of two features that are separated by DEex z 1 eV, which is very close to those observed in MnPd3xSbx [39] and Heusler alloys [5]. The line-shape analysis shows that the Mn 2p3/2 spectrum of CeCo7Mn5 can be interpreted as consisting of exchange-split components. The two components of the Mn 2p3/2 line arise from the exchange interactions between the core hole and the Mn 3d valence electrons. In this picture, the high- and lowbinding energy components correspond to the majority- and
Fig. 2. XPS spectrum of the Ce 4d and Co 3s core levels in CeCo7Mn5 (open circles correspond to the experimental spectrum and the continuous curves to the fitting results, after background subtraction).
Fig. 4. XPS spectra of (a) Mn 3s in CeCo7Mn5 and CeCo8Mn4 (open circles correspond to the experimental spectrum and the continuous curves to the fitting results, after background subtraction) and (b) Mn 2p3/2 core levels in CeCo7Mn5.
Fig. 3. XPS spectrum of the Ce 3d level in CeCo8Mn4.
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minority- spin core-hole states respectively and the energy separation between them corresponds to the exchange spitting of the Mn 2p core-holes states. This gives another direct evidence of the existence of local moments confined on Mn sites in CeCo7Mn5 compound. The values obtained for Mn 3s and Mn 2p3/2 exchange splittings in CeCo7Mn5 and also in CeCo8Mn4 indicate a local magnetic moment per Mn atom of about 3 mB. The valence band spectrum of CeCo7Mn5 compound shown in Fig. 5 is mainly the result of the Co 3d, Mn 3d, Ce 5d and Ce 4f states superposition. At Al Ka radiation the Co 3d cross section is about three times larger than the Mn 3d cross section [40] and taking into account the number of the different atoms in the unit formula, the valence band is dominated by the Co 3d states. It is interesting to note that the valence band is centred at about 2 eV (estimated from the half band width at half height), a value close to that observed in pure Co metal (see the inset in Fig. 5). Experimental data and band structure calculations for Ce mixed valence compounds have shown that the 4f state are not far below EF [19,20]. In CeCo5 and Ce2Co17 compounds both the Ce 5d and 4f states hybridize with the Co 3d states [25,41], leading to a decrease of the Co magnetic moment. In addition the spin dependence of the hybridization strength induces antiparallel magnetic moments on the cerium atoms (z0.2 mB/Ce), as evidenced in ferromagnetic compounds like CeCo5 and Ce2Co17 compared with the reference compounds YCo5 and LaCo5, and respectively Y2Co17 [2,25,41,42]. It is therefore expected that the valence band of CeCo7Mn5 and CeCo8Mn4 compounds is influenced by the same 3de5d and 3de4f hybridizations.
153
Fig. 6. Magnetization curves of CeCo7Mn5 for temperatures between 4 K and 300 K in magnetic field up to 12 T. The inset shows the temperature dependence of the spontaneous magnetization Msp.
Although the XPS spectra indicate similar electronic structure of Co and Mn atoms in the two compounds, which suggests close values of the local magnetic moments per Co and respectively Mn atoms, the difference in the number of Co and Mn atoms per unit formula leads to very different values of magnetization as a function of magnetic field and temperature. The magnetic field dependences of magnetization recorded for temperatures between 5 K and 300 K for CeCo7Mn5 are shown in Fig. 6. The values and the variations of magnetization with field suggest an antiferromagnetic coupling between the transition metal moments located on different sublattices. The spontaneous magnetizations Msp, determined by extrapolating the
magnetization from high field linear regime to zero field, are shown in the inset of Fig. 6. The magnetization curves show hysteresis for temperatures smaller than 25 K, with a coercive field of HC ¼ 0.392 T at 5 K. The thermal variation of magnetization for CeCo7Mn5 measured in zero-field cooling (ZFC) and field cooling (FC) in applied fields of 0.025 T, 1 T and 2 T is shown in Fig. 7. The ZFC curves present a cusp at low temperatures (Tcs), whose value decreases with the applied magnetic field Happ. Furthermore, the normalized difference between the FC and ZFC magnetization at 5 K also decreases with the increase of the applied magnetic field. The irreversibility of the magnetization of CeCo7Mn5, as measured in ZFC and FC states, is attributed to the movement of domain walls. The spin-glass-like behaviour of magnetization originates from the magnetocrystalline anisotropy [43] and may also originate from the existence of competing magnetic interactions like in frustrated systems. Such later behaviour is not expected to take place here. Since coercivity is related to magnetic anisotropy, the irreversible magnetic behaviour reflects the role of
Fig. 5. XPS spectrum of the valence band region of CeCo7Mn5. In the inset is shown the valence band of pure Co metal for comparison.
Fig. 7. Temperature dependence of the magnetization for CeCo7Mn5 measured in the ZFC and FC states with applied fields of 0.025 T (right axis) and 1 T and 2 T (left axis). The inset shows the dM/dT curve for B ¼ 2 T.
3.3. Magnetization
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anisotropy in determining the shapes of ZFC and FC curves below the ordering temperature. This interpretation is supported by the relationship between the ZFC and FC susceptibilities [44]:
cZFC ¼ MZFC =Happ ; c0FC ¼ MFC = Happ þ HC
(1)
The notation c0FC is used to distinguish this susceptibility from the normal FC susceptibility cFC ¼ MFC =Happ. At 5 K, measurements for CeCo7Mn5 lead to the following results cZFC ¼ c0FC ¼ 1:3mB =f:u:T for Happ ¼ 1 T and HC ¼ 0.392 T. In the inset of Fig. 7, the temperature dependence of the differential magnetization dM/dT for CeCo7Mn5 is displayed for B ¼ 2T. The dM/dT curve exhibits one local minimum near TC z 45 K, ascribed to the ferromagnetic CoeCo coupling and a local maximum near TN z 150 K, which can be ascribed to the antiferromagnetic MneMn coupling. The temperature dependence of the spontaneous magnetization (inset of Fig. 6) may be explained taking into account the residual interaction of Co moments with Mn moments above TC. That is, the Co moments are not in a completely disordered state for TC < T < TN due to the strong MneMn and Mne Co interactions. The magnetic field dependences of magnetization recorded for temperatures between 3.9 K and 300 K for CeCo8Mn4 compound are shown in Fig. 8. Like for CeCo7Mn5 compound, the magnetization does not reach saturation even at high fields, but the values of spontaneous magnetization are much higher than those of CeCo7Mn5. As an example, at 4 K, Msp ¼ 2.6 mB/f.u for CeCo8Mn4, in comparison with Msp ¼ 0.7 mB/f.u for CeCo7Mn5. The M(H) curve at 3.9 K for CeCo8Mn4 shows a hysteresis loop (not shown in Fig. 8) like in the isostructural compound CeMn4.16Co7.84 [3] and originates, as was mentioned for CeCo7Mn5, from the domain wall pinning occurring at low temperatures. The difficulty to reach saturation may be taken as a confirmation of the existence of large magnetocrystalline anisotropy in such system. In both compounds, the ferromagnetic components are pinned by the higher anisotropy of the antiferromagnetic component. In CeCo8Mn4 the magnetization shows a more pronounced tendency towards saturation due to weaker antiferromagnetic interactions in comparison to CeCo7Mn5. The thermal variation of magnetization for CeCo8Mn4 measured in an applied field of 2 T is shown in Fig. 9. The dM/dT curve, shown in the inset of Fig. 9, exhibits one local maximum near TN z 100 K and one local minimum near TC z 173 K, ascribed like for CeCo7Mn5 to the antiferromagnetic and ferromagnetic ordering of the 3d magnetic moments.
Fig. 8. Magnetization curves of CeCo8Mn4 between 3.9 and 300 K in magnetic field up to 10 T.
Fig. 9. Temperature dependence of magnetization for CeCo8Mn4 in 2 T magnetic field. The inset shows the dM/dT curve.
It is worth mentioning that the magnetization curves M(T) given in Figs. 7 and 9 reveal an antiferromagnetic-like magnetic transition at about 155 K for CeCo7Mn5 and 100 K for CeCo8Mn4 (more visible for CeCo7Mn5 compound), which are close to the determined Néel temperatures. A similar anomaly at about 200 K was also observed in the isostructural compounds SmCo6Mn6 and SmCo5Mn7 [45]. The thermal variation of the reciprocal susceptibility for the investigated compounds in the high temperature range is shown in Fig. 10. The experimental data fit a CurieeWeiss law plus a small additional temperature e independent term c0 with the paramagnetic Curie temperatures q ¼ 175 K for CeCo7Mn5 and q ¼ 240 K for CeCo8Mn4.The effective magnetic moments, determined from the Curie constants, have the values meff ¼ 11.2 mB/f.u. for CeCo7Mn5 and meff ¼ 10.6 mB/f.u. for CeCo8Mn4. The complex magnetic behaviour of CeCo7Mn5 and CeCo8Mn4 compounds may be explained taking into account the interactions between Co and Mn magnetic moments located in different crystallographic sites. The coupling between the Co moments from different sites is ferromagnetic as usual for Co containing intermetallic compounds, while the interaction between the Co and Mn atoms can be expected to be antiferromagnetic. Also, like in the YMn12 isostructural
Fig. 10. Reciprocal magnetic susceptibility versus temperature for CeCo7Mn5 and CeCo8Mn4 compounds.
R. Dudric et al. / Intermetallics 38 (2013) 150e155
compound, the interaction between the Mn atoms in 8i sites is expected to be antiferromagnetic. Consequently, the competition between the antiferromagnetic CoeMn and MneMn interactions leads to a complex ferromagnetic þ antiferromagnetic configuration, like in RMn12xFex for 6 < x < 9 [46,47], probably consisting of a canted arrangement of Mn moments relative to the magnetization of the Co sublattice. In order to estimate the values of the Co and Mn moments in CeCo7Mn5 and CeCo8Mn4, we correlated the magnetic data from paramagnetic states for the two compounds. The relations between the effective magnetic moments and the spins of Co and Mn atoms in the paramagnetic state, obtained from the total Curie constant C ¼ CCo þ CMn, are:
m2eff =4 ¼ 7 SCo ðSCo þ 1Þ þ 5SMn ðSMn þ 1Þ for CeCo7 Mn5
(2)
m2eff =4 ¼ 8 SCo ðSCo þ 1Þ þ 4SMn ðSMn þ 1Þ for CeCo8 Mn4
(3)
The above equations are well verified for SCo z 0.8 and SMn z 1.6, yielding for the local magnetic moments confined on Co and Mn sites in the ordered state the values: mCo ¼ 1.6 mB and mMn ¼ 3.2 mB, in good agreement with the above estimation from XPS data. 4. Conclusions The CeCo7Mn5 and CeCo8Mn4 compounds crystallize in the tetragonal ThMn12 structure type and present complex magnetic behaviour. XPS spectra have shown the intermediate valence state of Ce atoms and that both Co and Mn atoms carry magnetic moments. The analysis of the magnetic properties, in the light of crystal structure and of distances between the atoms from different crystallographic sites, suggests the presence of ferromagnetic interactions between CoeCo and antiferromagnetic interactions between CoeMn and MneMn magnetic moments. This competition leads to a complex ferromagnetic þ antiferromagnetic configuration, probably consisting of a canted arrangement of Mn moments relative to the magnetization of the Co sublattice. Typical local magnetic moments of 1.6 mB/Co and 3.2 mB/Mn atoms were estimated by correlating the data from paramagnetic states, in good agreement with the exchange splitting of the XPS Co 3s and Mn 3s core levels. The differences in the magnetic parameters MSP, TC, TN, and q for the two compounds are due to the enhancement of ferromagnetic interactions together with a decrease of antiferromagnetic interactions in CeCo8Mn4, whereas an inverted situation prevails in CeCo7Mn5. Detailed neutron diffraction studies would be necessary to confirm the proposed magnetic structure and the values obtained for Co and Mn magnetic moments. Acknowledgements R.D. acknowledges the financial support from the Romanian UEFISCDI Project No. PN-II-ID-PCE-2011-3-0583 (85/2011). References [1] Vilars P, Calvert LD. Pearson’s handbook of crystallographic data for intermetallic phases. 2nd ed. Materials Park, OH: ASM International; 1991. [2] Sun Zhi-gang, Zhang Shao-ying, Zhang Hong-wei, Wang Jing-yun, Shen Baogen. J Phys Condens Matter 2000;12:2495e504.
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