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Characteristics of the iron moment in Dy–Fe and Dy–FeCo amorphous alloys studied by X-ray magnetic circular dichroism
Journal of Magnetism and Magnetic Materials 220 (2000) 45}51
Characteristics of the iron moment in Dy}Fe and Dy}FeCo amorphous alloys studied by X-ray magnetic circular dichroism K. Fleury-Frenette , S.S. Dhesi, G. van der Laan, D. Strivay, G. Weber, J. Delwiche * Universite& de Lie% ge, Institut de Chimie B6c, Thermodynamique et Spectroscopie Sart Tilman, B-4000 Liege 1, Belgique CLRC, Daresbury Laboratory, Daresbury, Warrington, WA4 4AD, UK Universite& de Lie% ge, Physique Nucle& aire Expe& rimentale B15, Sart Tilman, B-4000 Belgique Received 18 January 2000; received in revised form 17 May 2000
Abstract The local magnetic moment of Fe in Dy}Fe and Dy}FeCo amorphous alloys has been studied using X-ray absorption spectroscopy and X-ray magnetic circular dichroism (XMCD). The Fe orbital and spin magnetic moments have been obtained for a range of alloy compositions by applying the sum rules to the XMCD spectra. The room temperature variations of the average components of the Fe moments as a function of Dy concentration and with the substitution of Fe by Co have been determined. A sharp reversal of the total magnetic moment was found at 28$1 at% Dy for both alloys. 2000 Elsevier Science B.V. All rights reserved. PACS: 75.25.#z; 75.50.Bb; 78.70.Dm Keywords: Thin "lms; Amorphous alloys; XMCD; Synchrotron radiation; Magnetic properties
1. Introduction Amorphous rare-earth}transition-metal (RE}TM) alloys are commonly used as storage layers in magneto-optical disks [1,2]. In order to adjust the key magnetic and magneto-optical properties, such as the perpendicular magnetic anisotropy, Curie temperature, and Kerr rotation, one varies the relative ratios of the di!erent metallic components in these materials [3]. In particular, some Dy}FeCo alloys display characteristics appropriate for magneto* Corresponding author. Tel.: #32-4-366-3435; fax: #32-4366-2941. E-mail address: [email protected] (J. Delwiche).
optical recording [4}6] and, consequently have been used as the writing layer in multilayered light intensity direct overwrite disks [7]. This paper presents some aspects of the magnetism in Dy}Fe binary and Dy}FeCo ternary amorphous alloys, where both the RE and 3d TM can contribute to the magnetization. Our emphasis will be on the magnetic moments of the Fe atoms in thin "lms of these alloys. These systems have been reported to display perpendicular magnetic anisotropy over a wide range of compositions [3,8,9]. The type of magnetic order proposed for these systems is sperimagnetic [2,10,11]. It consists of two oppositely polarized subnetworks of more or less noncollinear magnetic moments * one for the RE and
0304-8853/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 0 ) 0 0 4 6 0 - 1
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K. Fleury-Frenette et al. / Journal of Magnetism and Magnetic Materials 220 (2000) 45}51
the other for the 3d TM. The distribution of the magnetic moments around the magnetization direction is mainly determined by the interplay of the interatomic exchange interactions and the local single-ion anisotropies [12]. The strength of these interactions is expected to vary with composition and, therefore, in#uence the orientation of the magnetic moments. Moreover, the magnitude of the TM moments is considered to be sensitive to compositional variations that modify the d band structure and d orbital occupation. As a result, each magnetic subnetwork will behave di!erently as a function of composition. X-ray magnetic circular dichroism allows one to determine the orbital and spin magnetic moments in a site and element speci"c way [13,14]. It provides, therefore, a particularly powerful technique for the investigation of the magnetic contribution of a speci"c component in complex magnetic materials. Recently, this technique has been applied to TM atoms in RE}TM compounds and alloys [15}19]. Herein, we report on the sum rule analysis of X-ray magnetic circular dichroism (XMCD) spectra at the Fe L edge of several Dy}Fe and Dy}FeCo alloys in order to assess the room temperature variations in the average components of the Fe orbital and spin moments, per d hole and perpendicular to the "lm plane, as a function of Dy concentration and with substitution of Fe by Co. 2. Experimental 2.1. Sample preparation Amorphous thin "lms of Dy}Fe and Dy}FeCo, of ca. 40 nm thickness, were deposited on Si, SiO , or polyimide substrates by radio frequency magnetron sputtering in a 5;10\ mbar Ar atmosphere at a constant power of 0.6 W/cm. The substrates were pretreated using the same deposition technique by covering them with ca. 50 nm thick Al bu!er layer. This layer prevents the di!usion of bulk contaminants into the RE}TM layer as well as possible texturing induced by the substrate. Finally, an ca. 5 nm thick capping layer was added to protect the alloys from oxidation, a layer which was su$ciently thin to allow electron yield measurements. The substrates were rotated at approximately 1 Hz dur-
ing the magnetic layer deposition to ensure composition homogeneity over the entire surface of the "lm. This was necessary because mosaic targets comprising a Dy disk covered by di!erent sized sectors of Fe and Co foils were used. 2.2. Sample characterization A combined Rutherford back scattering (RBS) and particle-induced X-rays emission (PIXE) analysis was carried out at the Experimental Nuclear Physics Institute of the University of Lie`ge, Belgium, in order to calibrate the deposition process and to determine both the composition and the thickness of the "lms. Details of this analysis method are described elsewhere [20]. The perpendicular magnetic anisotropy of the thin "lms was checked by vibrating sample magnetometer measurements. X-ray di!raction with Cr K radiation was used to verify a the amorphous character of the layer. 2.3. XMCD measurements X-ray absorption spectra (XAS) at the Fe L edge were measured using a high-energy spherical grating monochromator at beamline 1.1 of the Synchrotron Radiation Source at Daresbury, England [21]. Measurements at the beam line were carried out using split coils superconducting magnet which encloses a UHV six-way cross [22]. This allows the incident beam to be either parallel or antiparallel to the applied magnetic "eld. Samples are mounted on a manipulator which translates perpendicular to the photon beam. A schematic diagram of the set-up is shown in Fig. 1. The X-rays were &80% circularly polarized with an energy resolution of better than 1 eV. The degree of polarization was calibrated by measuring Fe, Co, and Ni multilayer samples with previously determined orbital magnetic moments. The spectra were collected in the total electron yield (TEY) mode by measuring the drain current from the sample. Repeated spectra were obtained in an external 2 T magnetic "eld applied parallel (I>) A complete description of the beam line and the experimental set-up can be found at the address: http://srs/dl.ac.uk/msg/MSG-Equipment-Mag-Man.html
K. Fleury-Frenette et al. / Journal of Magnetism and Magnetic Materials 220 (2000) 45}51
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Fig. 1. Schematic diagram of the 5 T superconducting magnet at the SRS. The split coil encloses a UHV six-way cross, where the samples can be changed using a translatable manipulator.
and antiparallel (I\) to the helicity vector of the incident X-rays. Fig. 2 shows an example of a pair of XAS spectra together with the resulting di!erence spectrum, which is the XMCD spectrum. A XMCD measurement of orbital and spin moments consists of taking the di!erence in adsorption of circularly polarized X-rays at the 3d metal L and L edges with the photon helicity aligned parallel and antiparallel to the components of the moments. The projections of the ground state moments onto the direction of the incident light are obtained from the intensities of di!erence and sum spectra integrated over the L an L edges according to the sum rules [13,14]. To obtain the moments per d hole, these values have to be normalized to the integrated isotropic L signal. Because for the 3d transition metals the X-ray magnetic linear dichroism is an order of magnitude smaller than the XMCD [23], the isotropic spectrum can be approximated by (I>#I\)/2. To remove contributions due to the continuum states a doublestep background was subtracted from the XAS. The step positions were taken at energies of the L and L maxima and their intensity ratio was assumed to be equal to the statistical ratio of 2 : 1 [24].
Fig. 2. The XAS and XMCD spectra at the Fe L edge in amorphous Dy Fe Co obtained at 300 K. The XAS were collected at room temperature with a 2 T magnetic "eld applied parallel (I>) and anti-parallel (I\) to the helicity vector of the incident X-rays. The lower XMCD spectrum corresponds to the di!erence, I>!I\, between both XAS spectra corrected for the degree of circular polarization.
Because the incident X-rays and magnetization direction were perpendicular to the surface, the measurements gave the values of the orbital and spin magnetic moments normal to the "lm surface, values which we denote as L and 2S , respectively. X X As the number of 3d holes per Fe atom in Dy}Fe and Dy}FeCo amorphous alloys is not precisely known, we will use the orbital and spin moments per hole, respectively, rather than the total moments. In principle, electronic band structure calculations should be able to provide the number of d holes, however, as far as we know such calculations have not yet been carried out for these alloys. To be more precise, the spin sum rule yields 2S #7T , where T is the magnetic dipole term. X X X Although T is expected to be small for 3d X transition metals in cubic symmetry, it may contribute to the dichroic signal in lower symmetry environments such as at the surface. For instance,
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K. Fleury-Frenette et al. / Journal of Magnetism and Magnetic Materials 220 (2000) 45}51
Wu and Freeman [25] calculated for the Fe (0 0 1) surface atoms a T contribution of 8.5% of 2S . For X X amorphous alloy we expect that the atoms are randomly distributed and, therefore, that the local contributions to T will cancel out. This might be X incorrect if some small-scale atomic arrangements predominate. If so, then in the following discussion 2S should be understood to stand for 2S #7T . X X X 3. Results and discussions The measured values of L and 2S per hole of Fe X X in Dy}Fe and Dy}FeCo amorphous alloys are given in Tables 1 and 2, respectively. Fig. 3 shows the variation of the total moment L #2S per hole X X as a function of the Dy concentration. 3.1. Dy}Fe alloys As shown in Table 1, L per hole is always an X order of magnitude smaller than 2S per hole. This X
indicates that the Fe orbital moment is e$ciently quenched by the crystal "eld and band broadening when alloyed with the RE, a quenching which is similar to that observed for a-Fe. For all samples, L and 2S per hole conserve their parallel alignX X ment as expected from Hund's third rule for a more than half-"lled shell. The magnetic moment of Fe is parallel to the applied "eld in the Dy-poor samples and antiparallel in the Dy-rich alloys. The orientation reversal occurs between 27 and 28 at% Dy in our Dy}Fe alloys. The direct observation by magneto-optical Kerr e!ect spectroscopy of the reversal of the TM subnetwork magnetization has been reported in several other RE}TM amorphous alloys [1]. The room temperature compensation composition, x , the composition at which the magnetization of the 3d-TM subnetwork has the same magnitude but the opposite sign from the RE magnetization, yielding a zero net magnetization, lies in the 27}28 at% range of Dy concentration. Mimura et al. [8] reported, however, the room temperature x to be at 23 at% Dy. This
Table 1 Fe orbital and spin moments, per hole, and their ratios in amorphous Dy}Fe thin "lms Composition (at%)
L (l ) X
2S (l ) X
L /2S X X
Dy Fe Dy Fe Dy Fe Dy Fe Dy Fe Dy Fe Dy Fe Dy Fe
0.043$0.005 0.036$0.004 0.031$0.004 0.029$0.004 0.012$0.001 !0.029$0.003 !0.019$0.002 !0.014$0.002
0.29$0.04 0.23$0.03 0.23$0.03 0.28$0.04 0.15$0.02 !0.24$0.03 !0.22$0.03 !0.12$0.02
0.146$0.008 0.156$0.007 0.135$0.008 0.107$0.007 0.077$0.007 0.121$0.007 0.084$0.005 0.116$0.007
Table 2 Fe orbital and spin moments, per hole, and their ratios in amorphous Dy}FeCo thin "lms Composition (at%)
L (l ) X
2S (l ) X
L /2S X X
Dy Fe Co Dy Fe Co Dy Fe Co Dy Fe Co Dy Fe Co Dy Fe Co Dy Fe Co
0.043$0.005 0.053$0.006 0.049$0.005 0.043$0.005 !0.020$0.002 !0.015$0.002 !0.020$0.002
0.55$0.05 0.44$0.05 0.42$0.05 0.28$0.04 !0.17$0.02 !0.17$0.02 !0.15$0.02
0.079$0.004 0.118$0.006 0.116$0.008 0.156$0.008 0.115$0.009 0.089$0.004 0.133$0.008
K. Fleury-Frenette et al. / Journal of Magnetism and Magnetic Materials 220 (2000) 45}51
Fig. 3. The total Fe moment per hole, L #2S , as a function of X X Dy content in amorphous Dy}Fe and Dy}FeCo thin "lms as obtained by applying the sum rules to the XMCD spectra. The lines are a guide to the eye.
discrepancy in x value may originate from dif ferent deposition conditions and/or contamination [26]. Regardless of their alignment, the values of the Fe L and 2S per hole in Dy Fe are considerX X ably smaller than in alloys whose composition is within 1 at% of the room temperature x . This could be explained by local variations of the composition, variations which generate a distribution of compensation temperatures within the amorphous alloy [27]. When this distribution overlaps with the actual temperature, antiparallel magnetic domains may coexist. Their contributions to the average projected moments cancel partially or completely depending on their relative numbers and this leads to the observed reduction. Accordingly, large coercive "elds are measured in the vi-
49
cinity of x for various amorphous RE}TM alloys [8]. This may arise because the Dy Fe sample was not fully saturated by the 2 T "eld applied during the XMCD measurements. The value of the coercive "eld decreases dramatically as one goes away from the compensation composition and domain saturation is, therefore, expected in the remaining samples. As shown in Table 1, 2S per X hole does not vary signi"cantly in Fe-rich Dy}Fe alloys whereas it starts to decrease at higher Dy concentrations beyond x . Here, the TM}TM and RE}TM exchange interactions are the main interactions involving the Fe spin moment [28]. We consider 2S per hole to remain constant, to X a "rst approximation, due to the counterbalanced #uctuations of both interactions. The RE}TM indirect exchange depends partly on the spin polarization of the 3d band [29] such that a consequent weakening of the TM}TM exchange will eventually a!ect it in the same way [30]. The observed decrease of 2S per hole at higher Dy concentrations X may therefore result from a weakening of both interactions due to the dilution of the Fe atoms by the RE. Upon dilution, the Fe}Fe interactions are not only expected to decrease in number but also to be altered by the RE atoms; an outcome of such a perturbation could be the modi"cation of the Fe}Fe interatomic distances leading to weaker exchange constants. The orbital moment decreases with Dy concentration in Dy}Fe alloys on both sides of the compensation composition. At constant spin moment, stronger electrostatic "elds and/or smaller spinorbit couplings are responsible for the reduction of the orbital moment [31,32]. Changes in magnetic anisotropy are also perceived through its projection along the "eld axis. de Castro et al. [19] have shown recently that the iron L /2S ratio roughly X X scales up to the anisotropy energy in crystalline Dy Fe and a-Fe BCC. Our values for this ratio, given in Table 1, are considerably larger than that observed for Fe, in accordance with the perpendicular magnetic anisotropy observed in Dy}Fe alloys at room temperature [8]. Furthermore, several of the investigated samples, mostly the TMrich alloys, show enhanced L per hole again with X respect to the value of 0.025 l per hole for pure Fe [24].
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K. Fleury-Frenette et al. / Journal of Magnetism and Magnetic Materials 220 (2000) 45}51
By considering the total Fe magnetic moment in the Dy}Fe amorphous alloys, even with unlikely hole counts as high as four, the values determined from L #2S per hole would always X X be smaller than 2.0}2.2 l /atom magnetic moment of crystalline Dy}Fe compounds at 4.2 K [33]. An asperomagnetic alignment [10] of reduced moments within the Fe subnetwork could account for the discrepancy. For the comparison to be relevant, temperature considerations might also be required because the Curie temperatures reported by Mimura et al. [8] for Dy}Fe alloys are below 375 K, i.e., close to room temperature. 3.2. Dy}FeCo Neutron di!raction measurements indicate that the local magnetic moment of iron is enhanced upon alloying with Co [34]. This is also apparent from our XMCD measurements, cf. Fig. 3. Within the rigid band model, this is considered to arise from the improved exchange splitting of the dband, a splitting which redistributes electrons among the spin-polarized subbands. The Fe 3d band is more than half-"lled and, prior to alloying, the Fermi level crosses both subbands such that, if their energy splitting increases, electron transfer will occur from minority to majority states, thus resulting in a higher spin moment. The MoK ssbauer Fe hyper"ne "eld, which is related to the Fe magnetic moment, was also found to increase following some substitutions of Fe by Co in Dy}(FeCo) and Dy}(FeCo) pseudo-binary crys talline alloys [35]. For the amorphous alloys investigated herein, the substitutions lead to the observed enhancement of 2S per hole at the correX sponding Dy concentration in the Dy-poor amorphous alloys, whereas no signi"cant in#uence is perceived from our measurements above 25 at% Dy. The e!ect apparently becomes weaker between 16 and 32 at% Dy for alloy with 14$1 at% Co. This may be related to the behaviour of the Curie temperature: at constant Dy content, ¹ increases ! with the Co/Fe composition ratio in amorphous TM-rich alloys, whereas they converge to some extent in Dy-rich alloys [5]. Moreover, Hwang et al. [30] made direct measurements of the
RE}TM exchange constant for some Dy}FeCo amorphous alloys from which they concluded that the interaction between subnetworks weakens between 21 and 27 at% Dy at a constant Co/Fe ratio. The dilution e!ect discussed above, would hinder the action of cobalt in the range of composition for which the e$cient TM}TM interactions are less abundant, and the RE}TM interactions would be weakened, accordingly. One might eventually observe the promoting e!ect in some of the Dy-rich samples by increasing their Co/Fe ratio provided the TM atoms were not too isolated to interact signi"cantly with each other. To some extent the Co-induced changes in Fe L per hole are di!erent from those experienced by X 2S per hole. They share the absence of a signi"cant X e!ect in the Dy-rich alloys, but the enhancement in TM-rich alloys is such that the L /2S observed X X maximum ratio lies at higher Dy concentration than in the Dy}Fe alloys. Fig. 3 and Table 2 show that for both the Dy}FeCo and the Dy}Fe alloys the reversal of the Fe magnetic moment occurs within the same range of Dy concentrations. This is in agreement with the study of several Dy}FeCo amorphous thin "lms by Raasch et al. [5], who detected no signi"cant dependence of the compensation temperature on the Co/Fe ratio. The compensation temperature was found to increase rather linearly with the RE content and the x at room temperature, interpolated from the data of Raasch et al. [5], is approximately 27% Dy.
4. Conclusions We have used the element and site speci"city of the Fe L XMCD to obtain the Fe spin and orbital magnetic moments in Dy}Fe and Dy}FeCo thin "lms with perpendicular magnetic anisotropy. For both types of alloys, we observe a sharp reversal of the Fe spin and orbital moment directions at a Fe concentration of &72$1 at%. The e!ect of the added Co is to increase considerably the Fe total magnetic moment in Fe-rich alloys.
K. Fleury-Frenette et al. / Journal of Magnetism and Magnetic Materials 220 (2000) 45}51
Acknowledgements We gratefully acknowledge the CommunauteH franc7 aise de Belgique (ARC Contract No. 94/99175) and the Institut des Sciences NucleH aires de Belgique for "nancial support. K. Fleury Frenette is also indebted to the CommunauteH franc7 aise de Belgique for a research fellowship. Dr. E. Dudzik is thanked for her assistance during the measurements at Daresbury and to Prof. J.A.D. Matthew at the University of York for support under ESPRC grant GR/L68568. Prof. F. Grandjean, Dr. M.-J. Hubin-Franskin, and Prof. G.J. Long are to be thanked for their numerous advices and support. Mr. J. Heinesch and Dr. D. Vandormael are thanked for their assistance in setting up the sputtering equipment and Mr. A. Joassin for carrying out the X-ray di!raction measurements. K. Fleury Frenette thanks Prof. D. Roy for his continuous interest in his work.
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[11] J.P. Rebouillat, A. Lienard, J.M.D. Coey, R. Arrese-Boggiano, J. Chappert. Physica B 86}88 (1977) 773. [12] R. Harris, M. Plischke, M.J. Zuckermann, Phys. Rev. B 31 (8) (1973) 160. [13] B.T. Thole, P. Carra, F. Sette, G. van der Laan, Phys. Rev. Lett. 68 (1992) 1943. [14] P. Carra, B.T. Thole, M. Altarelli, X. Wang, Phys. Rev. Lett. 70 (1993) 694. [15] J. Vogel, M. Sacchi, R.J.H. Kappert, J.C. Fuggle, J.B. Goedkoop, N.B. Brookes, G. van der Laan, E.E. Marinero, J. Magn. Magn. Mater. 150 (1995) 293. [16] S. Imada, T. Muro, S. Suga, H. Maruyama, H. Yamazaki, J. Electron Spectrosc. Relat. Phenom. 78 (1996) 279. [17] S. Pizzini, L.M. Garcia, A. Fontaine, J.P. Rue!, J. Vogel, R.M. GaleH ra, J.B. Goedkoop, N.B. Brookes, G. Krill, J.P. Kappler, J. Electron Spectrosc. Relat. Phenom. 86 (1997) 165. [18] C. Bordel, S. Pizzini, J. Vogel, K. Mackay, J. Voirin, R.M. GaleH ra, A. Fontaine, P. Auric, J.B. Goedkoop, N.B. Brookes, Phys. Rev. B 56 (1997) 8149. [19] A.R.B. de Castro, G.B. Fraguas, P.T. Fonsesca, R.N. Suave, S. Gama, A.A. Coelho, I.A. Santos, J. Electron Spectrosc. Relat. Phenom. 101}103 (1999) 725. [20] D. Strivay, G. Weber, K. Fleury Frenette, J. Delwiche, in: J.L. Duggan, I.L. Morgan (Eds.), AIP CP475, Applications of Accelerators in Research and Industry, 1999, p. 476. [21] G. van der Laan, Physics of Atoms and Molecules, Plenum Press, New York, 1995, p. 295. [22] G. van der Laan, L.A. Welbourne, Endeavour 20 (1996) 37. [23] G. van der Laan, Phys. Rev. Lett. 82 (1999) 640. [24] C.T. Chen, Y.U. Idzerda, H.-J. Lin, N.V. Smith, G. Meigs, E. Chaban, G.H. Ho, E. Pellegrin, F. Sette, Phys. Rev. Lett. 75 (1995) 152. [25] R. Wu, A.J. Freeman, Phys. Rev. Lett. 73 (1994) 1994. [26] Y. Hoshi, M. Naoe, S. Yamanaka, J. Appl. Phys. 53 (1982) 2344. [27] J.P. Rue!, R.M. GaleH ra, S. Pizini, A. Fontaine, L.M. Garcia, Ch. Giorgetti, E. Dartyge, F. Baudelet, Phys. Rev. B 55 (1997) 3063. [28] K.H.J. Bushow, Rep. Prog. Phys. 40 (1977) 1179. [29] M.S.S. Brooks, L. NordstroK m, B. Johanson, Physica B 172 (1991) 95. [30] W.-K. Hwang, T.-H. Wu, H.-P.D. Shieh, J. Appl. Phys. 81 (1997) 7437. [31] P. Bruno, Phys. Rev. B 39 (1989) 865. [32] G. van der Laan, J. Phys.: Condens. Matter 10 (1998) 3239. [33] A.S. van der Goot, K.H.J. Buschow, J. Less-Comm. Met. 21 (1970) 151. [34] M.F. Collins, J.B. Forsyth, Phil. Mag. 8 (1963) 401. [35] B. Gicala, J. Pszczola, Z. Kucharski, J. Suwalski, Solid State Commun. 96 (1995) 511.
Characteristics of the iron moment in Dy}Fe and Dy}FeCo amorphous alloys studied by X-ray magnetic circular dichroism K. Fleury-Frenette , S.S. Dhesi, G. van der Laan, D. Strivay, G. Weber, J. Delwiche * Universite& de Lie% ge, Institut de Chimie B6c, Thermodynamique et Spectroscopie Sart Tilman, B-4000 Liege 1, Belgique CLRC, Daresbury Laboratory, Daresbury, Warrington, WA4 4AD, UK Universite& de Lie% ge, Physique Nucle& aire Expe& rimentale B15, Sart Tilman, B-4000 Belgique Received 18 January 2000; received in revised form 17 May 2000
Abstract The local magnetic moment of Fe in Dy}Fe and Dy}FeCo amorphous alloys has been studied using X-ray absorption spectroscopy and X-ray magnetic circular dichroism (XMCD). The Fe orbital and spin magnetic moments have been obtained for a range of alloy compositions by applying the sum rules to the XMCD spectra. The room temperature variations of the average components of the Fe moments as a function of Dy concentration and with the substitution of Fe by Co have been determined. A sharp reversal of the total magnetic moment was found at 28$1 at% Dy for both alloys. 2000 Elsevier Science B.V. All rights reserved. PACS: 75.25.#z; 75.50.Bb; 78.70.Dm Keywords: Thin "lms; Amorphous alloys; XMCD; Synchrotron radiation; Magnetic properties
1. Introduction Amorphous rare-earth}transition-metal (RE}TM) alloys are commonly used as storage layers in magneto-optical disks [1,2]. In order to adjust the key magnetic and magneto-optical properties, such as the perpendicular magnetic anisotropy, Curie temperature, and Kerr rotation, one varies the relative ratios of the di!erent metallic components in these materials [3]. In particular, some Dy}FeCo alloys display characteristics appropriate for magneto* Corresponding author. Tel.: #32-4-366-3435; fax: #32-4366-2941. E-mail address: [email protected] (J. Delwiche).
optical recording [4}6] and, consequently have been used as the writing layer in multilayered light intensity direct overwrite disks [7]. This paper presents some aspects of the magnetism in Dy}Fe binary and Dy}FeCo ternary amorphous alloys, where both the RE and 3d TM can contribute to the magnetization. Our emphasis will be on the magnetic moments of the Fe atoms in thin "lms of these alloys. These systems have been reported to display perpendicular magnetic anisotropy over a wide range of compositions [3,8,9]. The type of magnetic order proposed for these systems is sperimagnetic [2,10,11]. It consists of two oppositely polarized subnetworks of more or less noncollinear magnetic moments * one for the RE and
0304-8853/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 0 ) 0 0 4 6 0 - 1
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K. Fleury-Frenette et al. / Journal of Magnetism and Magnetic Materials 220 (2000) 45}51
the other for the 3d TM. The distribution of the magnetic moments around the magnetization direction is mainly determined by the interplay of the interatomic exchange interactions and the local single-ion anisotropies [12]. The strength of these interactions is expected to vary with composition and, therefore, in#uence the orientation of the magnetic moments. Moreover, the magnitude of the TM moments is considered to be sensitive to compositional variations that modify the d band structure and d orbital occupation. As a result, each magnetic subnetwork will behave di!erently as a function of composition. X-ray magnetic circular dichroism allows one to determine the orbital and spin magnetic moments in a site and element speci"c way [13,14]. It provides, therefore, a particularly powerful technique for the investigation of the magnetic contribution of a speci"c component in complex magnetic materials. Recently, this technique has been applied to TM atoms in RE}TM compounds and alloys [15}19]. Herein, we report on the sum rule analysis of X-ray magnetic circular dichroism (XMCD) spectra at the Fe L edge of several Dy}Fe and Dy}FeCo alloys in order to assess the room temperature variations in the average components of the Fe orbital and spin moments, per d hole and perpendicular to the "lm plane, as a function of Dy concentration and with substitution of Fe by Co. 2. Experimental 2.1. Sample preparation Amorphous thin "lms of Dy}Fe and Dy}FeCo, of ca. 40 nm thickness, were deposited on Si, SiO , or polyimide substrates by radio frequency magnetron sputtering in a 5;10\ mbar Ar atmosphere at a constant power of 0.6 W/cm. The substrates were pretreated using the same deposition technique by covering them with ca. 50 nm thick Al bu!er layer. This layer prevents the di!usion of bulk contaminants into the RE}TM layer as well as possible texturing induced by the substrate. Finally, an ca. 5 nm thick capping layer was added to protect the alloys from oxidation, a layer which was su$ciently thin to allow electron yield measurements. The substrates were rotated at approximately 1 Hz dur-
ing the magnetic layer deposition to ensure composition homogeneity over the entire surface of the "lm. This was necessary because mosaic targets comprising a Dy disk covered by di!erent sized sectors of Fe and Co foils were used. 2.2. Sample characterization A combined Rutherford back scattering (RBS) and particle-induced X-rays emission (PIXE) analysis was carried out at the Experimental Nuclear Physics Institute of the University of Lie`ge, Belgium, in order to calibrate the deposition process and to determine both the composition and the thickness of the "lms. Details of this analysis method are described elsewhere [20]. The perpendicular magnetic anisotropy of the thin "lms was checked by vibrating sample magnetometer measurements. X-ray di!raction with Cr K radiation was used to verify a the amorphous character of the layer. 2.3. XMCD measurements X-ray absorption spectra (XAS) at the Fe L edge were measured using a high-energy spherical grating monochromator at beamline 1.1 of the Synchrotron Radiation Source at Daresbury, England [21]. Measurements at the beam line were carried out using split coils superconducting magnet which encloses a UHV six-way cross [22]. This allows the incident beam to be either parallel or antiparallel to the applied magnetic "eld. Samples are mounted on a manipulator which translates perpendicular to the photon beam. A schematic diagram of the set-up is shown in Fig. 1. The X-rays were &80% circularly polarized with an energy resolution of better than 1 eV. The degree of polarization was calibrated by measuring Fe, Co, and Ni multilayer samples with previously determined orbital magnetic moments. The spectra were collected in the total electron yield (TEY) mode by measuring the drain current from the sample. Repeated spectra were obtained in an external 2 T magnetic "eld applied parallel (I>) A complete description of the beam line and the experimental set-up can be found at the address: http://srs/dl.ac.uk/msg/MSG-Equipment-Mag-Man.html
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47
Fig. 1. Schematic diagram of the 5 T superconducting magnet at the SRS. The split coil encloses a UHV six-way cross, where the samples can be changed using a translatable manipulator.
and antiparallel (I\) to the helicity vector of the incident X-rays. Fig. 2 shows an example of a pair of XAS spectra together with the resulting di!erence spectrum, which is the XMCD spectrum. A XMCD measurement of orbital and spin moments consists of taking the di!erence in adsorption of circularly polarized X-rays at the 3d metal L and L edges with the photon helicity aligned parallel and antiparallel to the components of the moments. The projections of the ground state moments onto the direction of the incident light are obtained from the intensities of di!erence and sum spectra integrated over the L an L edges according to the sum rules [13,14]. To obtain the moments per d hole, these values have to be normalized to the integrated isotropic L signal. Because for the 3d transition metals the X-ray magnetic linear dichroism is an order of magnitude smaller than the XMCD [23], the isotropic spectrum can be approximated by (I>#I\)/2. To remove contributions due to the continuum states a doublestep background was subtracted from the XAS. The step positions were taken at energies of the L and L maxima and their intensity ratio was assumed to be equal to the statistical ratio of 2 : 1 [24].
Fig. 2. The XAS and XMCD spectra at the Fe L edge in amorphous Dy Fe Co obtained at 300 K. The XAS were collected at room temperature with a 2 T magnetic "eld applied parallel (I>) and anti-parallel (I\) to the helicity vector of the incident X-rays. The lower XMCD spectrum corresponds to the di!erence, I>!I\, between both XAS spectra corrected for the degree of circular polarization.
Because the incident X-rays and magnetization direction were perpendicular to the surface, the measurements gave the values of the orbital and spin magnetic moments normal to the "lm surface, values which we denote as L and 2S , respectively. X X As the number of 3d holes per Fe atom in Dy}Fe and Dy}FeCo amorphous alloys is not precisely known, we will use the orbital and spin moments per hole, respectively, rather than the total moments. In principle, electronic band structure calculations should be able to provide the number of d holes, however, as far as we know such calculations have not yet been carried out for these alloys. To be more precise, the spin sum rule yields 2S #7T , where T is the magnetic dipole term. X X X Although T is expected to be small for 3d X transition metals in cubic symmetry, it may contribute to the dichroic signal in lower symmetry environments such as at the surface. For instance,
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K. Fleury-Frenette et al. / Journal of Magnetism and Magnetic Materials 220 (2000) 45}51
Wu and Freeman [25] calculated for the Fe (0 0 1) surface atoms a T contribution of 8.5% of 2S . For X X amorphous alloy we expect that the atoms are randomly distributed and, therefore, that the local contributions to T will cancel out. This might be X incorrect if some small-scale atomic arrangements predominate. If so, then in the following discussion 2S should be understood to stand for 2S #7T . X X X 3. Results and discussions The measured values of L and 2S per hole of Fe X X in Dy}Fe and Dy}FeCo amorphous alloys are given in Tables 1 and 2, respectively. Fig. 3 shows the variation of the total moment L #2S per hole X X as a function of the Dy concentration. 3.1. Dy}Fe alloys As shown in Table 1, L per hole is always an X order of magnitude smaller than 2S per hole. This X
indicates that the Fe orbital moment is e$ciently quenched by the crystal "eld and band broadening when alloyed with the RE, a quenching which is similar to that observed for a-Fe. For all samples, L and 2S per hole conserve their parallel alignX X ment as expected from Hund's third rule for a more than half-"lled shell. The magnetic moment of Fe is parallel to the applied "eld in the Dy-poor samples and antiparallel in the Dy-rich alloys. The orientation reversal occurs between 27 and 28 at% Dy in our Dy}Fe alloys. The direct observation by magneto-optical Kerr e!ect spectroscopy of the reversal of the TM subnetwork magnetization has been reported in several other RE}TM amorphous alloys [1]. The room temperature compensation composition, x , the composition at which the magnetization of the 3d-TM subnetwork has the same magnitude but the opposite sign from the RE magnetization, yielding a zero net magnetization, lies in the 27}28 at% range of Dy concentration. Mimura et al. [8] reported, however, the room temperature x to be at 23 at% Dy. This
Table 1 Fe orbital and spin moments, per hole, and their ratios in amorphous Dy}Fe thin "lms Composition (at%)
L (l ) X
2S (l ) X
L /2S X X
Dy Fe Dy Fe Dy Fe Dy Fe Dy Fe Dy Fe Dy Fe Dy Fe
0.043$0.005 0.036$0.004 0.031$0.004 0.029$0.004 0.012$0.001 !0.029$0.003 !0.019$0.002 !0.014$0.002
0.29$0.04 0.23$0.03 0.23$0.03 0.28$0.04 0.15$0.02 !0.24$0.03 !0.22$0.03 !0.12$0.02
0.146$0.008 0.156$0.007 0.135$0.008 0.107$0.007 0.077$0.007 0.121$0.007 0.084$0.005 0.116$0.007
Table 2 Fe orbital and spin moments, per hole, and their ratios in amorphous Dy}FeCo thin "lms Composition (at%)
L (l ) X
2S (l ) X
L /2S X X
Dy Fe Co Dy Fe Co Dy Fe Co Dy Fe Co Dy Fe Co Dy Fe Co Dy Fe Co
0.043$0.005 0.053$0.006 0.049$0.005 0.043$0.005 !0.020$0.002 !0.015$0.002 !0.020$0.002
0.55$0.05 0.44$0.05 0.42$0.05 0.28$0.04 !0.17$0.02 !0.17$0.02 !0.15$0.02
0.079$0.004 0.118$0.006 0.116$0.008 0.156$0.008 0.115$0.009 0.089$0.004 0.133$0.008
K. Fleury-Frenette et al. / Journal of Magnetism and Magnetic Materials 220 (2000) 45}51
Fig. 3. The total Fe moment per hole, L #2S , as a function of X X Dy content in amorphous Dy}Fe and Dy}FeCo thin "lms as obtained by applying the sum rules to the XMCD spectra. The lines are a guide to the eye.
discrepancy in x value may originate from dif ferent deposition conditions and/or contamination [26]. Regardless of their alignment, the values of the Fe L and 2S per hole in Dy Fe are considerX X ably smaller than in alloys whose composition is within 1 at% of the room temperature x . This could be explained by local variations of the composition, variations which generate a distribution of compensation temperatures within the amorphous alloy [27]. When this distribution overlaps with the actual temperature, antiparallel magnetic domains may coexist. Their contributions to the average projected moments cancel partially or completely depending on their relative numbers and this leads to the observed reduction. Accordingly, large coercive "elds are measured in the vi-
49
cinity of x for various amorphous RE}TM alloys [8]. This may arise because the Dy Fe sample was not fully saturated by the 2 T "eld applied during the XMCD measurements. The value of the coercive "eld decreases dramatically as one goes away from the compensation composition and domain saturation is, therefore, expected in the remaining samples. As shown in Table 1, 2S per X hole does not vary signi"cantly in Fe-rich Dy}Fe alloys whereas it starts to decrease at higher Dy concentrations beyond x . Here, the TM}TM and RE}TM exchange interactions are the main interactions involving the Fe spin moment [28]. We consider 2S per hole to remain constant, to X a "rst approximation, due to the counterbalanced #uctuations of both interactions. The RE}TM indirect exchange depends partly on the spin polarization of the 3d band [29] such that a consequent weakening of the TM}TM exchange will eventually a!ect it in the same way [30]. The observed decrease of 2S per hole at higher Dy concentrations X may therefore result from a weakening of both interactions due to the dilution of the Fe atoms by the RE. Upon dilution, the Fe}Fe interactions are not only expected to decrease in number but also to be altered by the RE atoms; an outcome of such a perturbation could be the modi"cation of the Fe}Fe interatomic distances leading to weaker exchange constants. The orbital moment decreases with Dy concentration in Dy}Fe alloys on both sides of the compensation composition. At constant spin moment, stronger electrostatic "elds and/or smaller spinorbit couplings are responsible for the reduction of the orbital moment [31,32]. Changes in magnetic anisotropy are also perceived through its projection along the "eld axis. de Castro et al. [19] have shown recently that the iron L /2S ratio roughly X X scales up to the anisotropy energy in crystalline Dy Fe and a-Fe BCC. Our values for this ratio, given in Table 1, are considerably larger than that observed for Fe, in accordance with the perpendicular magnetic anisotropy observed in Dy}Fe alloys at room temperature [8]. Furthermore, several of the investigated samples, mostly the TMrich alloys, show enhanced L per hole again with X respect to the value of 0.025 l per hole for pure Fe [24].
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K. Fleury-Frenette et al. / Journal of Magnetism and Magnetic Materials 220 (2000) 45}51
By considering the total Fe magnetic moment in the Dy}Fe amorphous alloys, even with unlikely hole counts as high as four, the values determined from L #2S per hole would always X X be smaller than 2.0}2.2 l /atom magnetic moment of crystalline Dy}Fe compounds at 4.2 K [33]. An asperomagnetic alignment [10] of reduced moments within the Fe subnetwork could account for the discrepancy. For the comparison to be relevant, temperature considerations might also be required because the Curie temperatures reported by Mimura et al. [8] for Dy}Fe alloys are below 375 K, i.e., close to room temperature. 3.2. Dy}FeCo Neutron di!raction measurements indicate that the local magnetic moment of iron is enhanced upon alloying with Co [34]. This is also apparent from our XMCD measurements, cf. Fig. 3. Within the rigid band model, this is considered to arise from the improved exchange splitting of the dband, a splitting which redistributes electrons among the spin-polarized subbands. The Fe 3d band is more than half-"lled and, prior to alloying, the Fermi level crosses both subbands such that, if their energy splitting increases, electron transfer will occur from minority to majority states, thus resulting in a higher spin moment. The MoK ssbauer Fe hyper"ne "eld, which is related to the Fe magnetic moment, was also found to increase following some substitutions of Fe by Co in Dy}(FeCo) and Dy}(FeCo) pseudo-binary crys talline alloys [35]. For the amorphous alloys investigated herein, the substitutions lead to the observed enhancement of 2S per hole at the correX sponding Dy concentration in the Dy-poor amorphous alloys, whereas no signi"cant in#uence is perceived from our measurements above 25 at% Dy. The e!ect apparently becomes weaker between 16 and 32 at% Dy for alloy with 14$1 at% Co. This may be related to the behaviour of the Curie temperature: at constant Dy content, ¹ increases ! with the Co/Fe composition ratio in amorphous TM-rich alloys, whereas they converge to some extent in Dy-rich alloys [5]. Moreover, Hwang et al. [30] made direct measurements of the
RE}TM exchange constant for some Dy}FeCo amorphous alloys from which they concluded that the interaction between subnetworks weakens between 21 and 27 at% Dy at a constant Co/Fe ratio. The dilution e!ect discussed above, would hinder the action of cobalt in the range of composition for which the e$cient TM}TM interactions are less abundant, and the RE}TM interactions would be weakened, accordingly. One might eventually observe the promoting e!ect in some of the Dy-rich samples by increasing their Co/Fe ratio provided the TM atoms were not too isolated to interact signi"cantly with each other. To some extent the Co-induced changes in Fe L per hole are di!erent from those experienced by X 2S per hole. They share the absence of a signi"cant X e!ect in the Dy-rich alloys, but the enhancement in TM-rich alloys is such that the L /2S observed X X maximum ratio lies at higher Dy concentration than in the Dy}Fe alloys. Fig. 3 and Table 2 show that for both the Dy}FeCo and the Dy}Fe alloys the reversal of the Fe magnetic moment occurs within the same range of Dy concentrations. This is in agreement with the study of several Dy}FeCo amorphous thin "lms by Raasch et al. [5], who detected no signi"cant dependence of the compensation temperature on the Co/Fe ratio. The compensation temperature was found to increase rather linearly with the RE content and the x at room temperature, interpolated from the data of Raasch et al. [5], is approximately 27% Dy.
4. Conclusions We have used the element and site speci"city of the Fe L XMCD to obtain the Fe spin and orbital magnetic moments in Dy}Fe and Dy}FeCo thin "lms with perpendicular magnetic anisotropy. For both types of alloys, we observe a sharp reversal of the Fe spin and orbital moment directions at a Fe concentration of &72$1 at%. The e!ect of the added Co is to increase considerably the Fe total magnetic moment in Fe-rich alloys.
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Acknowledgements We gratefully acknowledge the CommunauteH franc7 aise de Belgique (ARC Contract No. 94/99175) and the Institut des Sciences NucleH aires de Belgique for "nancial support. K. Fleury Frenette is also indebted to the CommunauteH franc7 aise de Belgique for a research fellowship. Dr. E. Dudzik is thanked for her assistance during the measurements at Daresbury and to Prof. J.A.D. Matthew at the University of York for support under ESPRC grant GR/L68568. Prof. F. Grandjean, Dr. M.-J. Hubin-Franskin, and Prof. G.J. Long are to be thanked for their numerous advices and support. Mr. J. Heinesch and Dr. D. Vandormael are thanked for their assistance in setting up the sputtering equipment and Mr. A. Joassin for carrying out the X-ray di!raction measurements. K. Fleury Frenette thanks Prof. D. Roy for his continuous interest in his work.
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