Realization of a Large Magnetic Moment in the Ferromagnetic CoPt ...

VOLUME 87, NUMBER 6

PHYSICAL REVIEW LETTERS

6 AUGUST 2001

Realization of a Large Magnetic Moment in the Ferromagnetic CoPt Bulk Phase G. S. Chang,1, * Y. P. Lee,2 J. Y. Rhee,3 J. Lee,1 K. Jeong,1 and C. N. Whang1 1 ASSRC and IPAP, Yonsei University, Seoul 120-749, Korea Department of Physics, Hanyang University, Seoul 133-791, Korea 3 Department of Physics, Hoseo University, Asan, Choongnam 336-795, Korea (Received 24 July 2000; published 24 July 2001) 2

Changes in the magnetic moment and other physical properties of a CoPt alloy induced by a new type of ion-beam mixing in an external magnetic field were investigated. This process induces the formation of a metastable phase through extremely rapid quenching from well above the ordering temperature. The measured magnetic moment per Co atom was 2.63mB , larger by 55% and 35% than that of the bulk Co and stable CoPt film, respectively, which is one of the highest values ever observed in the ferromagnetic bulk phase. DOI: 10.1103/PhysRevLett.87.067208

PACS numbers: 75.70. –i, 61.80.Jh, 87.64.Ni, 87.64.Lg

The phenomenological Stoner band theory of ferromagnetism has given useful physical insights into the different magnetic behavior between the single magnetic atom and the magnetic solid, even though the theoretical efforts are still being developed [1]. When atoms are brought together to form a solid, each energy level of the free atom splits and forms a band. As a consequence, the magnetic moment (MM) of magnetic 3d metals is reduced by 4s electrons entering the 3d band owing to the hybridization [2]. This basic concept of a reduced MM due to the formation of a solid has driven extensive studies of an increased MM by controlling the interatomic distance and by localizing the 3d electrons. Moruzzi [3] showed that MM can be enhanced by the volume expansion in a metastable structure from his spin-polarized energy-band calculations. More recently, giant magnetic moments of 0.01 monolayer of Fe and Co atoms on Cs films were achieved through the localization of Fe ions [4]. However, in the case of bulk phases, such metastable ferromagnetic (FM) solids with an expanded lattice spacing and the localized magnetic atoms are not prepared satisfactorily by the conventional equilibrium methods such as physical vapor deposition (e.g., Ref. [5]). In this Letter, we report the newly observed properties of a magnetic Co atom in a metastable CoPt film, whose MM is 2.63mB and about 55% and 35% larger than those of the bulk Co and the stable CoPt film, respectively. The superior properties of this metastable FM thin film are achieved by employing a new ion-beam-mixing method in an external magnetic field, named magnetic ion-beam mixing (MIBM). 共Co 35 Å兾Pt 45 Å兲8 multilayered films (MLF), which match the equiatomic composition, were deposited on Si (001) substrates by e-beam evaporation. A metastable FM phase was obtained by MIBM with 80 keV Ar1 . The ion dose and beam current were fixed at 1.0 3 1016 ions兾cm2 and 1.5 mA, respectively. During the process, an external magnetic field of 500 Oe, normal to the sample surface, was applied for the Lorentz force not to alter the beam

trajectory. To understand the magnetic-field effect, the ion-beam-mixing process without external field was also carried out. For a comparison, we prepared a stable CoPt film with a Pt buffer layer onto the Si (001) by the cosputtering method. The magnetic moments of the Co atom in the samples were measured by magnetic circular dichroism (MCD) at the vacuum ultraviolet beam line of Pohang Light Source, Korea. The MCD data were taken by alternating directions of a magnetic field 共65000 Oe兲 which is larger than the saturation field of investigated samples of 100 Oe confirmed by a vibrating sample magnetometer (VSM). The electronic structures and film textures were investigated by synchrotron-radiation photoemission spectroscopy (PES) and x-ray diffraction (XRD), respectively. Incident photon energies for the PES measurement were chosen to be 50 and 100 eV in order to obtain the relative partial density of states (DOS) of Pt 5d and Co 3d, respectively, by considering the photoionization cross sections [6]. The samples were refreshed by a mild sputtering with 500 eV Ne1 and checked in situ in between repeated measurement sessions to prevent surface contamination. The structural change by MIBM was confirmed by high-resolution transmission electron microscopy (TEM) [Figs. 1(a) and 1(b)]. The micrographs show an interlayer mixing between Co (shown as bright) and Pt (dark), induced by the collision cascades during the irradiation. When energetic ions with several tens to hundreds of keV irradiate a thin film, the thermally activated target atoms move independently in a small volume [7]. Last, the freely moving atoms are quenched to room temperature in a very short period of time 共⬃10211 s兲 [8]. This ultrafast quenching process can freeze the atoms from the excited to the metastable state with an expanded lattice spacing, since the time scale is insufficient for recrystallization to the equilibrium state [9]. In addition, the spins of freely moving atoms at a high effective temperature can be easily affected by an external magnetic field and their spin states can be conserved after the ion-beam mixing because of the ultrafast quenching process. This spin-aligned interatomic interaction results in a localization of the 3d electrons and

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© 2001 The American Physical Society

0031-9007兾01兾 87(6)兾067208(4)$15.00

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XAS (% )

2.0

(a )

Parallel Antiparallel

L3

1.6 1.2

L2

0.8 0.4 0.0

MC D ( % )

0.2

(b )

0.0 -0. 2 MIBM CoPt IBM CoPt Stable CoPt Co/Pt multilayer

-0. 4 -0. 6

760

770 780 790 800 Binding Energy (eV)

FIG. 2. (a) XAS spectra of MIBM CoPt taken at the Co L2,3 edges in the spin-parallel and -antiparallel configurations and (b) corresponding MCD spectra. XAS-normalized MCD spectra for the corresponding MLF and ion-beam-mixed (without Hext ) film, and for a stable alloy film, are also included in (b) for comparison. FIG. 1. Cross-sectional high-resolution TEM micrographs of a Co兾Pt MLF (a) before and (b) after MIBM.

an increase in the interatomic spacing so as not to violate the Pauli principle. In order to clarify this implication, we measured MM of the Co atom in the MIBM film. Figures 2(a) and 2(b) present the L2,3 x-ray absorption spectra (XAS) and the MCD spectra, the areal difference in XAS, of the film, respectively. The measured spectrum is compared with the spectra of the corresponding MLF and ion-beam-mixed film, and a stable alloy film. To evaluate the MM of the Co atom we employed the x-ray MCD sum rules (XMCD-SR) which can provide the spin 共mspin兲 and the orbital 共morbit 兲 moment per Co atom distinguished from those of Pt [10]. The 3d occupation number of the Co atom 共n3d 兲 was chosen to be 7.51, as reported in a recent theoretical calculation [11]. This value offers the minimum increase in MM, because the number is expected to decrease with the localization of the 3d band, as discussed below, and MM / 共10 2 n3d 兲. We checked the validity of these parameters by comparing mCo of the Co sublayer in the MLF with that of the bulk hcp Co. As seen in Table I, the similarity in mCo of both samples justifies the above estimation. However, mCo is dramatically increased after 067208-2

the ion-beam-mixing process, and, especially, mCo of the MIBM sample 共2.63mB 兲 is larger than those of the other samples, being comparable to the maximum MM allowed by the Pauli principle (3mB for Co) [1]. This noble behavior was further supported by VSM combined with a calibration of the film thickness by Rutherford backscattering spectroscopy. The saturated magnetization before MIBM was 1410 emu兾cm3 which is similar to that of bulk Co TABLE I. mspin , morbit, and mtotal per Co atom in mB . The experimental values were evaluated by the XMCD-SR.

Co atom MIBM CoPt Ion-beam-mixed CoPt Stable CoPt Co兾Pt multilayer Bulk Cob Stable CoPt (cal.)c Ion-beam-mixed CoPt (cal.)c MIBM CoPt (cal.)c

mspin

morbit

mtotal

3.00 2.37 2.02 1.79 1.56 1.55 1.80 1.86 2.27

–a 0.26 0.20 0.15 0.13 0.15 0.10 0.11 0.20

3.00 2.63 2.22 1.94 1.69 1.70 1.90 1.97 2.47

a

morbit is assumed to be quenched as it is in the solid. Ref. [5]. c (cal.) stands for “calculated.” b

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共1420 emu兾cm3 兲 and increased to 1800 emu兾cm3 after MIBM, which shows the same trend as the results obtained by MCD. The larger magnetic moments of both ion-beam-mixed and MIBM films with respect to the corresponding stable alloy film are presumably correlated with the aforementioned volume expansion by fast quenching. Figure 3 shows XRD patterns of the investigated samples. Pt (111) peaks at 39.8± are from both the Pt buffer layer of a stable film and the Pt sublayer of the corresponding MLF, while it is not seen after the formation of either ion-beam-mixed or MIBM phases. The (111) peak position of stable alloy is in a good agreement with that for the L10 alloy in other works, even though the (200) peak is invisible compared to the dominant (111) peak as a result of the preferred orientation due to a highly ordered texture [12]. In addition, the lattice constant of the (100)pplane 共d100 苷 3.76 Å兲 of the MIBM film corresponds to 3 3 d111 (2.17 Å), indicating another L10 phase, because the conventional unit cell of the L10 phase has an fcc structure (see inset of Fig. 3) [13]. However, the MIBM CoPt (111) peak is located at 0.4± lower than that of the stable film, which means that the (111) planar spacing of the metastable state is slightly increased with respect to the stable state. This lattice expansion corresponds to a volume expansion of the L10 unit cell by 3%. If the enhancement of MM during MIBM is solely caused by the volume expansion, we have to explain why the increase in magnetic moments (DmCo 苷 0.69mB for MIBM) is appreciably larger than the known enhancement of Co 共DmCo # 1mB 兲 due to a volume expansion by 3% [3]. Additional investigation of the electronic structure can provide a direct way to confirm this excessive enhancement

FIG. 3. XRD patterns of MLF and ion-beam-mixed and MIBM films, together with that of a stable alloy film. (Inset: see text.)

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in mCo of the MIBM sample. Figure 4 presents the valence-band PES near EF of the MIBM film (filled circles) at photon energies of 100 eV [Fig. 4(a)] and 50 eV [Fig. 4(b)]. The spectra of stable alloy film (open circles) are also included for comparison. In the stable alloy, the Co majority-spin states mix strongly with the Pt d states (peak 1), while the position of Co minority-spin states is separated in energy by 2 eV due to an exchange splitting in the Co 3d states (peak 2) [14]. However, the electronic structure of MIBM film is significantly changed. The disappearance of the interatomic spinpolarized interactions in the MIBM film suggests that the aforementioned large mCo of a MIBM film is not the outcome from those interactions for the MM of a stable film [13]. Both valence-band spectra of MIBM film in Fig. 4 show that the Co and Pt atoms are in isolated states of greatly reduced electronic interaction between atoms. Additional striking points are the narrowed valence bands (arrow 3) and the increased DOS near EF 关N共EF 兲兴 for Co (arrow 4). This is direct evidence for less Co coordination in the MIBM film with respect to the stable alloy, which is first realized in the FM bulk phase, because the reduced coordination narrows the d band and generally enhances N共EF 兲 [1]. A local-density approximation [15] shows that a single 3d atom has the maximum MM permitted by the Pauli principle under the intra-atomic exchange. On the other hand, an increase in the MM of a solid

FIG. 4. PES spectra of MIBM CoPt (filled circles) near EF at photon energies of (a) 100 eV and (b) 50 eV. The spectra are predominated by (a) Co 3d and (b) Pt 5d.

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requires that a certain number of electrons be transferred from the minority-spin to the majority-spin band. The cost in energy for this spin redistribution is inversely proportional to N共EF 兲. Thus, a reduced coordination, in general, causes the MM to grow up owing to the atomlike configuration. As mentioned earlier, the relatively free Co atoms in the MIBM state can be less overlapped between Co 3d and Pt 5d at a high effective temperature during ion bombardment than those of the stable film, and this excited state is quenched to a metastable state at an extremely large cooling rate of 1014 K兾s which is impossible to be realized by the conventional film-preparation and annealing methods. Furthermore, the spin alignment induced by an external magnetic field during MIBM plays an additional role to localize the Co 3d state, because this alignment prevents the Co atoms with the same spin from approaching each other to satisfy the Pauli principle. The combined effect conserves the reduced interatomic interaction after MIBM. Therefore, the MIBM film comes to possess the observed novel feature in the electronic structure which is strongly correlated with the large MM. In order to quantify further the large mCo of MIBM film, we also calculated the electronic structures of the CoPt alloy using a full-potential linear muffin-tin-orbital method within the local-spin-density approximation (LSDA). 108 k points of the irreducible Brillouin zone were employed in the self-consistent calculations, and the spin-orbit interaction was included. The enhancement due to the volume expansion of 3% is only 0.07mB which is significantly smaller than the measured values. This implies that the ordinary LSDA could not properly realize the enhancement induced by MIBM, and the on-site Coulomb potential is supposed to be strong for the localized Co 3d electrons in the MIBM phase. Therefore, we employed the “LDA 1 U” method [16] for the metastable MIBM CoPt. The resultant mspin and morbit are 2.27mB and 0.20mB , respectively. These values are so close to the experimental ones that the aforementioned scheme supports satisfactorily the correlation between the localized Co 3d band and the large MM. We also observed an increase by ⬃43% in the calculated N共EF 兲, which is consistent with the results of PES experiments.

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In conclusion, a metastable film with a large MM was achieved by applying the MIBM method. This gigantic MM realized even in a bulk phase is due to the effects of ultrafast quenching and external magnetic field during the process, which leads to a phase with an expanded volume and a narrowed 3d band. This FM metastable bulk phase with a large MM might meet the requirements for further miniaturization of the nanosized magnetic devices and of the high-density storage media, where a serious degradation in the magnetic properties accompanied by the extremely reduced size of magnetic bit size is easily expected. This work was supported by the BK21 Project, the KOSEF through the ASSRC and Project No. 9950200-003-2, and also KRF Grants (No. 99-D00048 and No. 2001-015-DS0015).

*Electronic address: [email protected] [1] J. G. Gay and J. R. Smith, in Ultrathin Magnetic Structures I, edited by J. A. C. Bland and B. Heinrich (SpringerVerlag, Heidelberg, 1994), pp. 21–40. [2] B. D. Cullity, in Introduction to Magnetic Materials, edited by B. D. Cullity (Addison-Wesley, Reading, MA, 1972). [3] V. L. Moruzzi, Phys. Rev. Lett. 57, 2211 (1986). [4] H. Beckmann et al., Phys. Rev. Lett. 83, 2417 (1999). [5] S. Hashimoto et al., J. Magn. Magn. Mater. 88, 211 (1990). [6] J. J. Yeh and I. Lindau, At. Data Nucl. Data Tables 32, 1 (1985). [7] Y. T. Cheng, Mater. Sci. Rep. 5, 45 (1990). [8] W. L. Johnson et al., Nucl. Instrum. Methods Phys. Res., Sect. B 7/8, 657 (1985). [9] B. X. Liu, Phys. Status Solidi 94, 11 (1986). [10] C. T. Chen et al., Phys. Rev. Lett. 75, 152 (1995). [11] R. Wu et al., Phys. Rev. Lett. 73, 1994 (1994). [12] S. Shiomi et al., Jpn. J. Appl. Phys. 32, L315 (1993). [13] D. Weller et al., J. Magn. Magn. Mater. 121, 461 (1993). [14] J. F. van Acker et al., Phys. Rev. B 43, 8903 (1991). [15] J. C. Slater, The Self-Consistent Field for Molecules and Solids: Quantum Theory of Molecules and Solids (McGraw-Hill, New York, 1974), Vol. 4, Chap. 8. [16] V. I. Anisimov et al., J. Phys. Condens. Matter 9, 767 (1997).

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