Correlation between static and dynamic magnetic properties of highly

PHYSICAL REVIEW B 92, 144431 (2015)

Correlation between static and dynamic magnetic properties of highly perpendicular magnetized Co49 Pt51 thin films P. Saravanan,1,2 Jen-Hwa Hsu,1,* Salim Mourad Ch´erif,3,† Yves Roussign´e,3 Mohamed Belmeguenai,3 Andrey Stashkevich,3,4 Nicolas Vernier,5 Akhilesh Kr. Singh,1 and Ching-Ray Chang1 1

Department of Physics, National Taiwan University, Taipei 10617, Taiwan 2 Defence Metallurgical Research Laboratory, Hyderabad, 500058, India 3 Laboratoire des Sciences des Proc´ed´es et des Mat´eriaux, Centre National de la Recherche Scientifique (3407), Universit´e Paris 13-Nord, Sorbonne Paris Cit´e, 99, Av. J.B Cl´ement, 93430, Villetaneuse, France 4 International Laboratory “MultiferrLab”, Information Technologies, Mechanics and Optics University, St. Petersburg 197101, Russia 5 Institut d’Electronique Fondamentale, bˆat 220, Universit´e Paris-Sud, 91405 Orsay, France (Received 6 August 2015; revised manuscript received 21 September 2015; published 30 October 2015) The static and dynamic magnetic behavior of 5-nm-thick Co49 Pt51 films with strong perpendicular magnetic anisotropy (PMA) grown at different deposition temperatures (Td,CoPt ) was investigated using complementary techniques such as vibrating sample magnetometry (VSM), magneto-optical Kerr effect (MOKE) imaging, and Brillouin light scattering (BLS). Our previous study on these films demonstrated the evolution of phase transformation from an A3-disordered (hexagonal) to an L11 -ordered (rhombohedral) structure against increasing Td,CoPt from room temperature (RT) to 350°C. Along these lines, the changes in the domain configuration, magnetization reversal, and spin wave behavior of the 5-nm-thick CoPt films due to varying Td,CoPt are emphasized in this study. The VSM out-of-plane hysteresis loops confirmed the existence of strong PMA for all the CoPt films, irrespective of Td,CoPt . MOKE studies revealed that the films deposited at RT and at 150 ◦ C containing hard and soft magnetic areas, while the films grown at higher Td,CoPt , 250 and 350 ◦ C, are more uniform and homogeneous. The MOKE findings are validated by the BLS spectra in terms of high and low frequency lines corresponding to the hard and soft magnetic areas, respectively. A suitable model is hypothesized to interpret the frequency variation of BLS modes corresponding to the easy saturated regions of the CoPt films. By this means, a good correlation between both static and dynamic behavior of the 5-nm-thick CoPt films has been established in this study. DOI: 10.1103/PhysRevB.92.144431

PACS number(s): 75.60.Ej, 75.70.Kw, 75.30.Gw, 78.35.+c

I. INTRODUCTION

“Perpendicular all the way,” as narrated by Kent [1], thin multilayered films that show large out-of-plane (OOP) magnetic anisotropy have been recognized as candidate materials for applications such as high-density magnetic recording, magnetic random access memory (MRAM) [2], and spin torque nano-oscillators (STNO) [3]. Along these lines, the quest for identifying magnetic materials with high uniaxial magnetic anisotropy (Ku ) – an important factor that imposes perpendicular magnetization, permits stable magnetization states by overcoming thermal fluctuations at room temperature (RT). Over the past several years, L10 -ordered Fe(Co)-Pt(Pd) [4–6], CoFeB [7,8], and Fe(Co)/Pt(Pd) [9,10] multilayered films were extensively investigated for such applications. Though the origin of perpendicular magnetic anisotropy (PMA) in each system is different, these films tend to demonstrate strong PMA with reasonably high Ku values. Despite their strong PMA, the applicability of these films in most of the spintronic devices seems to be not convenient as they suffer from several drawbacks. For instance, the L10 -ordered Fe(Co)-Pt(Pd) films are often processed at higher temperature (>500 ◦ C), which results in islandlike morphology and rough surfaces for the Fe(Co)-Pt(Pd) films [11]. In the case of Co-Fe-B films, a stringent control of Co-Fe-B thickness and * †

Corresponding author: [email protected] Corresponding author: [email protected]

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a smooth MgO/Co-Fe-B interface are mandatory for attaining the strong PMA [12]. In a similar way, the Fe(Co)/Pt(Pd) multilayered structure finds a serious drawback of controlling ˚ All the thickness of interleaving layers precisely to a few A. these issues disqualify the above three categories of films to envisage their applications in the device point of view. In this context, as an alternative, L11 -ordered CoPt films have been proposed by Iwata et al. [13]. Through several experimental studies, it has been demonstrated that these films can impart strong PMA with high Ku at moderate processing temperatures (∼250 ◦ C). By this means, issues such as surface roughness and islandlike morphology can be overcome. Further, the theoretical calculations predicted that the Ku of L11 -CoPt films may exceed 108 erg/cm3 with increasing degree of long-range order [14]. Along these lines, we have recently demonstrated that the Co49 Pt51 films with a disordered A3 hexagonal structure can simply be grown at RT through maneuvering the under layers [15,16]. A noteworthy aspect of the RT-grown A3-CoPt films is that they demonstrate strong PMA (≈ 107 erg/cm3 ) and good thermal stability over a wide range of film thickness: 5−20 nm. In contrast, the ordered L10 or L11 -CoPt films reveal good PMA with a minimum film thickness of about 20 nm [17,18]. As a continuation of our previous efforts, we herein investigate the spin dynamics of 5-nm-thick Co49 Pt51 films as a function of processing temperature using Brillouin light scattering (BLS) toward exploring their applicability in advanced spintronic devices.

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A number of BLS studies dealing with spin waves have significantly grown during the last decade, proving BLS to be a powerful method for investigating the magnetic properties of variety of films, multilayers, and nanostructures [19–22]. In particular, the potential of BLS in understanding the PMA behavior has been well-demonstrated in various systems, such as ultrathin Fe-film grown on GaAs [23], perpendicularlymagnetized Pt/Co/AlOx ultrathin films [24], Co/Ni(111) [25], and Co/Pt multilayers [26] or Fe80 Co20 /MgO junctions [27]. Nevertheless, a very few BLS studies have been focused on the CoPt films. A spin wave study on the sputtered Co1−x Ptx films revealed that the magnetic inhomogeneity due to the grains in the films causes a remarkable broadening in the Brillouin spectra as the Pt content increases and impact the exchange interactions [28]. In another study, a strong decrease in the spin wave stiffness constant was observed in the Co1−x Crx films with x ≈ 12%, which was attributed to the Cr segregation at the grain boundaries [29,30]. In a similar context, it can be expected that the processing temperature of CoPt layers may lead to possible changes in the structural and magnetization behavior, which of course can have significant impact on the spin wave behavior of the CoPt films. Since strong PMA in the CoPt films are mostly observed at elevated temperatures, studies on the spin wave behavior of CoPt films as a function of deposition temperature may provide enormous scope for their utilization in spintronic applications. Accordingly, such an effort has been made in this study. II. EXPERIMENTAL METHODS

Films of CoPt with nominal thickness of 5 nm were grown on glass substrates by co-sputtering under a base pressure of  1 × 10−7 torr. The atomic composition of CoPt films was adjusted close to Co49 Pt51 by controlling the sputtering powers of Co and Pt targets. A 20-nm-thick Ta was initially deposited as a seed layer followed by 20-nm-thick Pt-buffer layer at RT in order to promote strong PMA in the CoPt layer. The CoPt films were fabricated by varying the deposition temperature of the CoPt layer, Td,CoPt from RT to 350 ◦ C. The phase composition of the CoPt films was investigated by x-ray diffraction (XRD) using Cu-Kα radiation. The magnetic hysteresis loops were measured by vibrating sample magnetometer (VSM) as well as by polar Kerr-effect magnetometer at RT. A Veeco atomic force microscope (AFM) with a magnetic tip was used to observe the static magnetic configuration. A first scan in the tapping mode was performed to acquire the film topography. During the second scan, the tip was lifted to 20 nm in order to measure the magnetic force induced by the CoPt film. Magnetic Kerr imaging was accomplished to fully understand both magnetization configuration and reversal behavior of the CoPt films. The dynamic magnetic properties were studied by means of the BLS technique. In the BLS experiment, a beam of a solid laser light operating on a single mode wavelength (532 nm) was used as a probe to reveal the spin waves, which are naturally present in the medium at a spectral frequency range of 3−300 GHz. A 200 mW p-polarized light was focused on the surface of the sample, and the scattered light was analyzed by means of a Sandercock-type 3 + 3 pass tandem Fabry-P´erot interferometer. The phonon lines can be suppressed by introducing a polarizer selecting only

the s-polarized light at the entrance of interferometer. In the present work, a backscattering geometry was used so that the value of the wave vector of the spin waves probed can be fixed to a value, Q = (4π/) sin(θ ), where  and θ denotes the wavelength of laser beam in air and its angle of incidence, respectively. The excited spin waves were studied as a function of applied magnetic field (up to 12 kOe). III. RESULTS AND DISCUSSION A. Structural and static magnetic properties

The XRD studies on the equiatomic CoPt films reveal that the face-centered-cubic (fcc) (A1 phase) stacking sequence with soft magnetism essentially exists for the RT-grown films [31]. However, by properly inserting a 5-nm-thick Ta layer between the buffer layer and glass substrate, a disordered hexagonal (A3) phase with strong PMA could be crystallized (figures not shown) [15,16]. Upon increasing the Td,CoPt from RT to 350 ◦ C, a structural transformation from the disordered hexagonal (A3) to ordered rhombohedral (L11 ) was discerned in the CoPt films [32,33]. Compared to the L11 phase, A3 phase has similar value of Ku and smaller coercive field. In Figs. 1(a) and 1(b), we show the RT magnetization hysteresis curves of 5-nm-thick CoPt films at different Td,CoPt measured by VSM, both under magnetic field applied perpendicular and parallel to the film plane, respectively. It can be noticed that for the field applied perpendicular to the film plane [Fig. 1(a)], the loops show almost a rectangular shape, while the in-plane (IP) loops exhibit S shapes [Fig. 1(b)]. This clearly indicates the existence of strong PMA in these films. In Table I, various magnetic parameters obtained for the 5-nm-thick CoPt films as a function of Td,CoPt are summarized. Upon increasing Td,CoPt from RT to 150 ◦ C, the perpendicular coercive field, Hc⊥ values are increased substantially from 0.62 to 1.060 kOe (62 to 106 mT). When Td,CoPt > 150 ◦ C, there is no significant increase in the Hc⊥ values (1.06−1.21 kOe), while the saturation magnetization Ms values tend to remain constant (780−790 emu/cm3 ). The increased Hc⊥ values are probably due to the enhancement in the ordering parameter (Sord ) with increasing Td,CoPt . However, in this study, the Sord values for the L11 phase could not be estimated from the XRD patterns due to the lower film thicknesses. For the field applied parallel to the film plane, the hysteresis loops demonstrated a distinct S shape-suggesting the existence of IP hard axes in the CoPt films. Further, in these IP loops, a moderate increase in the perpendicular anisotropy field HK (= 2Ku /Ms −4π Ms ) values from 20.4 to 25.4 kOe is evidenced, which is further accompanied by increase in Hsat values as the Td,CoPt increases. The magnetic anisotropy constant (Ku ) was calculated by considering the shape anisotropy term, 2π Ms2 as a correction factor. The estimated Ku values are found to increase from 1.20 to 1.43 × 107 erg/cm3 with an increase of Td,CoPt . B. Magnetic force microscopy

10 × 10 μm MFM images of the CoPt films grown at RT, 150, 250, and 350 ◦ C are depicted in Figs. 2(a)–2(d), respectively, and the magnetic structures are corresponding to their remnant states after OOP saturation. As can be noticed, the magnetization direction is not uniform on the sample surface. Magnetic domains with antiparallel magnetization

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FIG. 1. (Color online) Magnetic hysteresis loops of 5-nm-thick ordered-Co49 Pt51 films at different Td,CoPt measured by VSM under magnetic field applied perpendicular (a) and parallel (b) to the film plane, respectively.

oriented preferentially in the OOP direction are observed, which is a typical of strong PMA films. This behavior is more pronounced in the CoPt films upon increasing the Td,CoPt from RT to 350 ◦ C. In the absence of PMA, the magnetization direction is uniform in the ultrathin films. A periodic configuration called weak-stripe domains appears in a magnetic film, having film thickness higher than the critical value (tc ) and PMA inferior to the demagnetizing energy. At lower film thickness, t < tc , the magnetization lies along the plane of thin film, while beyond tc , the magnetization starts to oscillate OOP in a periodic manner, with a half period being close to the film thickness [34]. The critical thickness depends on the two ratios, Ku /Ms2 ,t/(A/Ku )1/2 , where A is the exchange constant. Muller provided a diagram, which allows deriving the saturation field versus Ku /Ms2 ,t/(A/Ku )1/2 [35]. When the PMA surpasses the demagnetizing energy, the magnetic domains exhibit antiparallel magnetization oriented in the OOP direction, as observed in the case of ordered-Co49 Pt51 films. In fact, for our samples, the Ku value surpasses 2π Ms2 (= 3.92 × 106 erg/cm3 ), and hence, an equilibrium state corresponding to the domains with OOP magnetization is quite obvious. This can be evidenced by the high values of remnant magnetization in the OOP hysteresis loops [Fig. 1(a)].

useful information on the magnetization reversal behavior under an applied field. A polar Kerr microscope with a K¨ohler-reflection configuration, sensitive only to the OOP component of magnetization, was employed for Kerr imaging. A light source of blue LED with wavelength of 455 nm and bandwidth of 25 nm was used. To detect the changes in polarization, an angle of 87 ◦ was maintained between the analyzer and polarizer. The Kerr images were acquired using a charge-coupled device (CCD)-camera cooled at −30 ◦ C in order to minimize the electrical interference. All the Kerr images depicted in Fig. 3 were acquired according to the following procedure. At first, the film was saturated at a magnetic field of 1.6 kOe (160 mT) applied perpendicular to the film plane, i.e., parallel to the easy axis of magnetization, and the Kerr imaging was acquired at the saturated state. The

C. MOKE observations

Kerr imaging is not only considered as a tool for imaging the shape of micrometer-sized domains, but also it provides TABLE I. The calculated magnetic parameters for the 5-nm-thick Co49 Pt51 films grown at different Td,CoPt . Td,CoPt (◦ C) Ms (emu/cm3 ) Hc (kOe) Hk (kOe) Ku (×107 erg/cm3 ) RT 150 250 350

790 790 790 780

0.62 1.06 1.19 1.21

20.4 22.3 24.4 25.4

1.20 1.31 1.35 1.43

FIG. 2. (Color online) MFM images of 5-nm-thick Co49 Pt51 films grown at different Td,CoPt . The image sizes are 10μm × 10μm. 144431-3

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FIG. 3. Successive magnetic states of the 5-nm-thick CoPt film grown at RT after applying a set of magnetic field pulse of duration 20 s. Kerr images were acquired after being subjected to each pulse with different amplitudes. (a) First pulse of 35 mT, (b) second pulse of 40 mT, (c) third pulse of 45 mT, (d) and (e) fourth and fifth pulses of 50 mT, respectively.

Kerr image thus obtained at the saturated condition was used as a reference. Afterward, alternating pulses of magnetic field in an increasing manner were applied in the direction opposite to the first saturating field. After each pulse, the magnetic field was set back to 0, and the Kerr images were then acquired. Subsequently, the magnetic state was obtained by subtracting the reference image. As it is possible to check the domain movement in real time, one can check the blocking of all the domain movements against increasing magnetic field until they attain a critical value. Thus, the remnant state stated here is the state at the end of each field pulse. A set of Kerr images acquired for the RT-grown CoPt films is shown in Fig. 3, which apparently shows the magnetization reversal against the OOP applied magnetic field. Reversed domains appeared at a very low field of 35 mT [Fig. 3(a)]. Between the applied fields of 35 and 50 mT, reversal occurs mainly through the nucleation of small domains [Figs. 3(b) and 3(c)], resulting in a dendritic structure for the domains. For the fields above 55 mT, almost all of the domains are

reversed [Figs. 3(d) and 3(e)]; however, there are some hard spots, which could not be reversed with the fields as high as 150 mT. Perhaps, the reversal in the field range of 35−55 mT explains the rounding of the hysteresis loop [Fig. 1(a)]. Since the hard spots are quite small, a small portion containing one of the hard spots was cropped from the acquired Kerr image, and the same is shown in Fig. 3 for better viewing. Nevertheless, it should be noted that for an applied field of 55 mT, 95% of the area was already reversed in the whole image. As the Td,CoPt increases, we could see nucleation density becoming more and more homogeneous, and the very hard spots disappear. Figure 4 shows the magnetization reversal behavior for the CoPt films grown at 350 ◦ C. It can be noticed that the hard spots are completely vanished, and the sample could be reversed with a field of 150 mT. The main mechanism is still the nucleation. At 95 mT, it can be seen in Fig. 4 that the density of reversed spots is homogeneous on the whole area presented, contrary to what

FIG. 4. Successive magnetic states of the 5-nm-thick CoPt film grown at 350 ◦ C after applying a set of magnetic field pulses. Kerr images were acquired after being subjected to each pulse with different amplitudes and duration. (a) First pulse of 95 mT with 10 s, (b) second pulse of 95 mT with 10 s, (c) third pulse of 95 mT with 70 s, and (d) fourth pulse of 150 mT with 5 s. 144431-4

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FIG. 5. Observed demagnetized state for the 5-nm-thick CoPt films grown at 150 (a) and 350 ◦ C (b).

is seen in the RT-grown sample (Fig. 3). Because of this nucleation process, the shape of the magnetic domains is still dendritic. This can be explained by assuming that the low field nucleations are due to some fluctuations (inhomogeneity) in the sample, which gives rise to the soft spots. With increasing Td,CoPt , improvement in the film homogeneity is therefore obvious, which nullifies the occurrence of soft spots; hence, sufficiently higher applied fields are necessary to induce some nucleation. Finally, it becomes possible to see the demagnetized state created by applying an oscillating decreasing field. The Kerr images in Fig. 5 show the demagnetized state obtained for the CoPt samples grown at 150 and 350 °C. Typical size of the domains after demagnetization is around 4 μm in both the cases. This value is quite small, and it is most probably extrinsic, as suggested by the dendritic nature of the domains during reversal, which implies many pinning centers. The only difference between the two samples is that there is a very hard spot in the 150 ◦ C sample, which does not exhibit any magnetic structure, contrary to the 350 ◦ C sample. For the RT-grown sample, a signature of the hard and soft spots cannot be discerned, as the size of the domain may be probably less than the optical resolution of the system (2 μm). A summary of the observations perceived from the Kerr microscopy is given in Table II; these results are in good agreement with the magnetization loops (Fig. 1). The critical fields are qualitatively similar for all the CoPt samples (Table I). As can be expected, the critical fields obtained from the Kerr microscopy are slightly lower. These lower values are probably attributed to the waiting time required for setting the magnetic field during imaging, which is usually longer for the Kerr microscopy. The rounding of the hysteresis loop is explained by the reversal of some soft parts of the samples when the other parts have not moved yet.

For the BLS studies, the 5-nm-thick CoPt samples were studied under IP Damon-Eshbach (D-E) configuration, i.e., the magnetic field is applied along the film plane. In our case, the field along the film plane becomes the hard-axis direction; hence, the spin waves propagating along the IP direction perpendicular to the applied field were probed. For this reason, the frequency variations did not follow the usual trends of D-E mode. It should be noted that under D-E configuration with uniform magnetization, a variation in the BLS frequencies can be expected in the following manner: Prior to the saturation magnetization, the frequency decreases with the applied field and forms a kink around the saturation field, and then the frequency increases against applied field after the saturation. In contrast, in our CoPt films, since the magnetization state is not uniform, as shown by MFM and MOKE observations, the dynamic magnetization behavior is more complicated. The IP saturation field is higher than 20 kOe for all samples, as shown in Table I. Nevertheless, the samples grown at RT and 150 ◦ C are not homogeneous: The OOP magnetization reversal occurs at high field in small areas of these samples, while the main part of them is reversed at low field, as evidenced by Kerr imaging. All of these features can be related to the BLS spectra. For all the CoPt samples, at lower applied fields, a high frequency line (HFL) in the BLS spectra can be noticed in Fig. 6 (for this analysis, a polarizer was not used in order to increase the BLS intensity). This line vanishes when the applied field increases. Assuming that the HFL is associated to the Kittel mode at 0 field, its frequency can be expressed as follows [36], F = (γ /2π ) (Hu − 4π Ms ) = (γ /2π )Hk ,

(1)

where γ is the gyromagnetic factor and Hu = Ku /Ms . Using the measured values of Hk and F , obtained respectively from VSM and BLS measurements, we obtain (γ /2π ) = 2.9 GHz/kOe. This is consistent with the value, expected for the gyromagnetic factor in the case of ferromagnetic films. Thus, we can infer that the assumption relating the HFL line to the Kittel mode is a good approach for assigning the nature of these spin waves. In order to clearly analyze the low frequency (0−30 GHz) spectra, a polarizer was introduced at the entrance of the spectrometer to suppress the phonon lines. For the CoPt films grown at RT and 150 ◦ C, a low frequency line (LFL) is observed. The observation of such LFL in the case of RT-grown CoPt film at two different applied fields (2 and 7 kOe) is illustrated in Fig. 7. Further, it can be noticed that the corresponding frequency increases with the applied field for the films grown at RT and 150 ◦ C [Figs. 8(a) and 8(b), respectively]. The occurrence of LFLs can be related to the presence of easy saturated regions in the samples. Assuming uniform magnetization, the frequency of the Kittel mode at fields higher than the saturation field can be written as F = (γ /2π )[H (H + 4π Ms − Hu )] 2

1

= (γ /2π ) [H (H − Hk )] 2 , 1

(2)

where H is the applied field and H u is the perpendicular anisotropy field of the easy saturating areas. The adjustment

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TABLE II. Summary of the main results obtained on the 5-nm-thick CoPt films grown at different Td,CoPt using Kerr microscopy. Td,CoPt (◦ C)

RT

150

250

350

Reversal beginning Main reversal Full reversal, except hard spots Hard spots

≈ 35 mT (0.35 kOe) ≈ 50 mT (0.50 kOe) ≈ 60 mT (0.60 kOe) Yes

≈ 70 mT (0.70 kOe) ≈ 95 mT (0.95 kOe) ≈ 120 mT (1.20 kOe) Yes

≈ 85 mT (0.85 kOe) ≈ 105 mT (1.05 kOe) ≈ 135 mT (1.35 kOe) Very few and small

≈ 90 mT (0.90 kOe) ≈ 100 mT (1.00 kOe) ≈ 150 mT (1.50 kOe) May be some

of the experimental frequencies using Eq. (2) is obtained with H k of 170 mT (1.7 kOe) and 500 mT (5 kOe) for the sample grown at RT and at 150 ◦ C. The frequency variation is satisfactorily described when the easy areas are saturated. Furthermore, the low saturation field increases with the Td,CoPt . Finally, at higher Td,CoPt , the LFL disappears in relation with the increase of the low saturating field, as experimentally observed from the BLS spectra and MOKE images. In order to support the above fact, the influence of wave number Q has been studied for the RT-grown CoPt film. At very low film thickness (t  1/Q), the frequency of the mode propagating in the easy saturating region is derived, and the same is described as follows. Consider that the x axis is parallel to the wave vector, the y axis is perpendicular to the film, and the z axis is parallel to the applied field. Assuming that the applied field is strong enough to saturate the film, the magnetization dynamics are ruled by the linearized Landau-

Lifshitz equation of motion: i(ω/γ ) (mx ,my ,0) = (0,0,Ms ) × (hx ,hy + (H u /Ms ) my ,0) + (mx ,my ,0) × (0,0,H ),

(3)

where mx , my are the oscillating components of the magnetization and hx , hy are the components of the associated demagnetizing field. Using the method described in Ref. [37], the equation obtained by averaging Eq. (3) across the thickness reads i(ω/γ ) (mx ,my ,0) = (0,0,Ms ) × (χx mx , χy my + (H u /Ms ) my ,0) + (mx ,my ,0) × (0,0,H ),

(4)

with χx = −4π (Qt/2), χy = −4π (1−(Qt/2)), where Q is the wave number and t is the film thickness. Consequently, the

FIG. 6. (Color online) Brillouin spectra of 5-nm-thick CoPt film grown at different Td,CoPt obtained with an angle of incidence of 20 ◦ and applied magnetic field of 1 kOe. 144431-6

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FIG. 7. (Color online) Brillouin spectra of RT-grown CoPt film obtained with an angle of incidence of 20° and applied magnetic field of 2 and 7 kOe.

frequency is obtained using the relation 

(2π F /γ )2 = (H -χx Ms )(H -Hu -χy Ms ).

FIG. 8. (Color online) Frequency variation as function of IP applied magnetic field for the 5-nm-thick CoPt film grown at (a) RT and (b) at 150 ◦ C. Symbols refer to the experimental values, while the dashed (solid) lines correspond to the theoretical variation below (above) the IP saturation field of magnetically soft regions.

(5)

Keeping only the first order terms, one can obtain 



(2π F /γ )2 = H (H -Hu + 4π Ms ) + 4π Ms (4π Ms -Hu ) (Qt/2). (6) Experimentally, Q varies in the range of 0−20 μm−1 . For our film thickness (t = 5 nm), the calculated difference in the frequencies (F (0)−F (20)) is 0.4 GHz. Figure 9 presents the BLS spectra obtained with angle of incidences 20 and 50 ◦ , corresponding to the wave numbers 8 and 18 μm−1 , respectively. To identify the small negative frequency shift, smoothed Stokes lines are presented in the inset of Fig. 9. The observed difference is in good agreement with the calculation expectations. It is noticeable that for H u > 4π Ms , the frequency decreases with the wave number for fields higher than the saturation field. This behavior is quite different from the usual increase in the frequency with the wave number under the D-E configuration for the IP magnetized films (spontaneously). This qualitative argument about the frequency clearly supports our assumption on the existence of easy saturating regions.

FIG. 9. (Color online) BLS spectra of RT-grown CoPt film obtained with applied field of 5 kOe at two angles of incidence 20 and 50 ◦ corresponding to the wave numbers: 8 (blue line) and 18 μm−1 (red line), respectively. The inset presents smoothed Stokes lines in order to evidence the small negative frequency shift.

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The dynamic magnetic behavior thus revealed by the BLS studies is consistent with the existence of two structural phases, as identified previously by the XRD analysis, and these observations are further complemented with the two kinds of magnetic regions ascertained by the MOKE experiments.

IV. CONCLUSIONS

A detailed investigation on the static and dynamic magnetic behavior of 5-nm-thick Co49 Pt51 films as a function of deposition temperature was carried out using MOKE and BLS. The VSM OOP hysteresis loops demonstrated the occurrence of strong PMA even in the case of RT-grown A3-CoPt films. Also, the most pertinent magnetic parameters extracted from the VSM measurements are substantiated with the spin wave behavior of 5-nm-thick CoPt films grown at different Td,CoPt . Kerr imaging, revealing the existence of magnetically hard regions that gradually expand throughout the film upon increasing the Td,CoPt from RT to 350 ◦ C; these observations are in accordance with the BLS studies. The CoPt films deposited

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at RT and 150 ◦ C showed both HFLs and LFLs in the BLS spectra, corresponding to the hard and soft magnetic regions noticed in the MOKE analysis. The unusual variation in the LFLs with respect to the wave number observed in the case of RT-grown A3-CoPt film was successfully explained by means of an analytical model. The spin wave behavior of 5-nm-thick CoPt films such as those investigated in this study may pave a path to explore the prevailing high Ku values in such disordered structures. ACKNOWLEDGMENTS

This work was supported in part by the National Science Council of Taiwan under Grant No. NSC 102-2112-M002-007-MY2 and by the Ministry of Economic Affairs of Taiwan under Grant No. 103-EC-17-A-01-S1-219 and in part by the Region Ile-de-France in the framework of Domaine d’Int´erˆet Majeur (DIM) C’nano IdF and Agence Nationale de la Recherche (ANR) and Commissariat a` l’Investissementd’Avenir (CGI) through Labex Science and Engineering for Advanced Materials and devices (SEAM).

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