Evolution of microstructure, residual stress, and texture in FePt films during rapid thermal annealing

Accepted Manuscript Evolution of microstructure, residual stress, and texture in FePt films during rapid thermal annealing S.N. Hsiao, L.H. Chen, S.H. Liu, J.L. Tsai, H.Y. Lee PII:

S0925-8388(15)31289-5

DOI:

10.1016/j.jallcom.2015.10.033

Reference:

JALCOM 35588

To appear in:

Journal of Alloys and Compounds

Received Date: 20 March 2015 Revised Date:

17 August 2015

Accepted Date: 4 October 2015

Please cite this article as: S.N. Hsiao, L.H. Chen, S.H. Liu, J.L. Tsai, H.Y. Lee, Evolution of microstructure, residual stress, and texture in FePt films during rapid thermal annealing, Journal of Alloys and Compounds (2015), doi: 10.1016/j.jallcom.2015.10.033. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Evolution of microstructure, residual stress, and texture in FePt films during

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rapid thermal annealing

S. N. Hsiaoa,d,*, L. H. Chenb, S. H. Liua, J. L. Tsaib,*, and H. Y. Leec,e a

Department of Materials Science and Engineering, Feng Chia University, Taichung 407, Taiwan

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Scientific research division, National Synchrotron Radiation Research Center, Hsinchu 300, Taiwan

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Department of Materials Science and Engineering, National Chung Hsing University, Taichung 402, Taiwan

Department of Power Mechanical Engineering, National Tsing Hua University, Hsinchu, 300, Taiwan

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Department of Applied science, National Hsinchu University of Education, Hsinchu, 300, Taiwan

*Corresponding authors: S. N. Hsiao([email protected]); J. L. Tsai

Abstract

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([email protected])

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Dependences of the evolution of microstructure, in-plane tensile stress, and

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crystallographic orientation on rapid thermal annealing in the single-layered FePt films were investigated. By manipulating annealing temperature (450–800 °C), a texture transition from (111) to nearly perfect (001) was induced by a measured huge tensile stress of 2.4 GPa. Based on the microstructural observation and in-plane residual stress measurement, the tensile stress originated from the annihilation of grain boundaries and probably the unexpected surface oxidation of the L10 FePt films during annealing. 1

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Conversely, L10 ordering caused the relaxation of the accumulated tensile stress.

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Keywords: microstructure, residual stress, FePt, (001) preferred orientation

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1. Introduction

Rapid thermal annealing (RTA) techniques, which heat substrate to high temperatures on a timescale within several seconds, have been widely used in semiconductor manufacturing. In the recent years, RTA techniques have also been applied to tailor the

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microstructure, phase transition, and crystallographic orientation in various magnetic materials for a variety of applications, such as high-density recording media, permanent

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magnets, spintronic devices, etc. For development of high-density recording media, L10 ordered FePt is the most promising material due to its remarkable magnetization and

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magnetocrystalline anisotropy energy (~7 × 107 erg/cm3) [1]. To obtain perpendicular magnetic anisotropy (PMA) for the practical applications, an (001) textured L10 FePt is vital owing to its [001] magnetic easy axis. RTA has also been found as an efficient method to induce (001) texture of the FePt films with third element additions and/or multilayered structures on amorphous glass substrate [2-10]. Among these studies, an evidence-based 2

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consensus has been emerged that the accumulation of in-plane tensile stress during RTA not only enhances the [001] orientation but also reduces the surface roughness of the L10 FePt

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films [11-14]. However, the origin and mechanism of the accumulated in-plane tensile stress is still not clear yet. In the previous literatures, it was suggested that the densification

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reactions including rapid recrystallization and abnormal grain growth in thin film material

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were likely to induce huge in-plane tensile stress [6, 8, 13, 15]. Yet the connection between microstructure and in-plane tensile of the (001) L10 FePt induced by RTA is still puzzling.

In this paper, the evolution of microstructure, in-plane residual stress, and

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crystallographic orientation of the FePt films rapid-thermal annealed at different temperatures were investigated. We also examined and discuss the reorientation of crystallographic and magnetic anisotropies during RTA-ordering. The connection of

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in-plane tensile stress to microstructural evolution was addressed for understanding the

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origin of the induced tensile stress in the single-layered FePt films during RTA.

2. Experimental

10 nm-thick Fe52Pt48 single-layered films were deposited onto amorphous glass substrate (Corning 1737) using magnetron sputtering with a background pressure better

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than 2 × 10-7 Torr. An Fe50Pt50 alloy target was used for film preparation. The samples were deposited at room temperature (RT), and then immediately transferred to a RTA chamber

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under vacuum for annealing. The heating rate was fixed at 40 °C/s and the annealing temperature (Ta) was varied from 450–800 °C for 0 second. In-plane residual stress of FePt

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films was examined by asymmetric sin2ψ method, which facilitates residual stress

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measurement in thin film with strong texture, based on x-ray diffractometry (XRD) technique [16]. The grazing incidence angle of x-ray beam was fixed at 3°. Synchrotron radiation x-ray beam with energy of 8 keV was adopted for examination of residual stress at beamline 17B at Taiwan Light Source and beamline 12BM at SPring-8 in Japan. The

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details regarding to stress measurement have been reported elsewhere [17]. Chemical composition of the FePt films was analyzed by inductive coupled plasma. Crystallographic

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structure and texture of the samples was examined by XRD using a Cu Kα radiation.

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Magnetic properties of the films were recorded at RT by a vibrating sample magnetometer (VSM). Prior to VSM measurement, a pulse magnetic field of ~70 kOe was applied on samples to approach their magnetically saturated states. Microstructure of the samples was observed by transmission electron microscopy (TEM). Average grain size was determined by measuring individual grain size in several dark-field images. Surface chemical property was analyzed by x-ray photoelectron spectroscopy (XPS) using synchrotron radiation at 4

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beamline 24A, Taiwan Light Source.

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3. Results and discussion

Fig. 1 (a) shows the x-ray diffraction spectra of the as-deposited films and annealed

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films at various Ta (450 to 800 °C). Note that intensity axis for each pattern is plotted on a logarithmic scale and shifted for clarity. For the as-deposited FePt film, it exhibited a

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disordered fcc structure with (111) texture. After an annealing at Ta = 500 °C, a very weak (001) diffraction peak was found, indicating an emergence of L10 phase. The intensities of (001) and (002) reflections enhanced as increasing RTA temperature, which were even

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stronger than that of (111) reflection in the samples with Ta ≥ 600 °C. This evidences a crystallographic orientation alteration from (111) to (001) plane. To further investigate the

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dependence of Ta on evolution of preferred orientation, a semi-quantitative parameter,

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Lotgering orientation factor (LOF) was adopted[18]. In case of (001) preferred orientation in FePt, LOF is defined as LOF = (p – p0)/(1 – p0), where p and p0 refer to Σ(00l)film/Σ(hkl)film and Σ(00l)powder/Σ(hkl)powder, respectively. Accordingly, LOF ranges from a certain negative value to 1. The negative value represents a preferred orientation other than (001), 0 means a random orientated state, and 1 refers to a prefect (001) texture. Dependence of LOF on Ta is shown in Fig. 1 (b). For the as-deposited film and annealed 5

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film at 450 °C, LOF values were –0.138, indicating a (111) preferred orientation. With increasing Ta from 450 to 700 °C, the LOF value altered from negative value to unity,

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evidencing a significant improvement of the (111) preferred orientation. The integrated intensity ratio of the superlattice (001) and fundamental (002) peaks was used to determine

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the ordering parameter of L10 phase (S) according to Warren’s theory[19]. As illustrated in

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Fig 1 (c), S value increased from 0 to greater than 0.9 as Ta was raised from 450 to 600 °C. No significant variation of S with increasing Ta further was observed. It is worthy to note that the onset temperature (500 °C) for development of S and (001) texture was likely to be the same; however, the progress of S was more rapid than that of (001) orientation. This

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result suggests that the L10 phase transformation is one of key factors for (001) texture formation[11], especially for determination of the onset annealing condition. Furthermore,

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we also noticed that interference fringes in the vicinity of the Bragg peaks for the sample

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annealed at Ta = 700 and 800 °C, indicating the high coherence of lattice and surface flatness.

Out-of-plane and in-plane M-H loops of the rapid-annealed FePt films at various

temperatures are presented in Fig. 2. For the sample annealed at 500 °C (Fig. 2 (a)), a longitudinal magnetic anisotropy with out-of-plane coercivity (Hc) of 2.5 kOe was found

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owing to a LOF value of –0.138. Although the LOF dramatically enhances from –0.138 to 0.88 with increasing Ta from 450 to 650 °C (Fig. 2 (b)-(c)), the magnetic anisotropy barely

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evolves from longitudinal to isotropic state. A significant evolution of magnetic anisotropy from isotropic to perpendicular behavior was observed as annealing temperature was raised

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up to 700 °C (Fig. 2 (d)). The FePt film annealed at 700 °C shows excellent PMA with

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out-of-plane Hc of 9.1 kOe and in-plane Hc of 1.6 kOe. The in-plane MH loop exhibits a slight hysteresis at low applied field owing to the c-axis distribution in the perpendicular direction. For the highest Ta of 800 °C, an excellent PMA with out-of-plane Hc of 10.0 kOe and in-plane Hc of 0.1 kOe was achieved. The (001) textured L10-FePt film with Ts = 800

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°C exhibited a high perpendicular magnetic anisotropy of 2.7±1.3 × 107 erg/cm3, based on the results published elsewhere [20]. One may notice that the significant change in LOF

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and S occurs at 500−550 °C, which seems to be inconsistent with the change in magnetic

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anisotropy (around 700 °C). To fully analyze the variation in texture quantitatively, the pole figure is required to obtain the orientation distribution of crystalline lattice from all directions of a material. However, this is a time-consuming and difficult technique for thin film materials with thicknesses less than 10 nm. The semi-quantitative LOF provide the texture distribution only along the normal direction of the FePt films. One the other hand, the magnetic anisotropy of FePt strongly depends on the easy axis distribution. In some 7

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cases, the FePt thin films with high LOF values barely exhibited a magnetically isotropic state due to the wide distribution of easy axis. [12, 21, 22] Because the magnetic anisotropy

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is much sensitive to the c-axis distribution than the LOF, the discrepancy between magnetic properties and LOF on Ts occurs.

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Figs 3 (a)-(f) show plane-view TEM images of the as-deposited film and samples

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annealed at various temperatures. In Fig. (a), the as-deposited FePt with fcc phase had an average grain size of about 11.5 nm. After an annealing at 450 °C, no distinct microstructural variation was observed, as shown in Fig 3 (b). With increasing Ta to 500 °C,

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the film started L10 ordered, but exhibited a reduced grain size of about 5 nm (Fig. 3 (c)). The reason for this grain size reduction might be due to the nucleation of L10 phases, as indicated by Fig 3(d) where the weak spots represents the (001) L10 ordering domains with

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an average size of about 3 nm using dark field technique. With increasing Ta to 650 °C, the

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sample was in the fully ordered state with a coarsening average grain size of about 8 nm (Fig. 3(e) and (f)). When Ta was raised to 700 °C, the average grain of the film with PMA barely increases to about 9 nm (Fig. 3(g)). For the films with highest Ta, an extensive grain growth with an average grain size of about 25 nm was observed for the FePt with perfect PMA, as shown in Fig. 3(h).

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The dependence of in-plane residual stress (σ) on Ta is illustrated in Fig. 4. For the as-deposited film, σ was in tensile direction with a magnitude of 0.93 GPa and almost

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remained for the sample after a annealing of Ta = 450 °C, since no significant microstructural and grain size alterations were observed. A minimum value of σ was found

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for the sample annealed at 500 °C, which presumably results from the nucleation of L10

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ordering. The L10 ordering leads to a volumetric expansion and atomic rearrangement[23, 24], which releases the preformed tensile stress. A similar stress behavior was also found in the FePt annealed using a conventional vacuum furnace [17]. With increasing Ta from 500 to 650 °C, the σ increases from 0.73 to 1.65 GPa, while the average grain size increased

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from about 5 to 8 nm, as shown in the inset of Fig. 4. Theoretically, this grain growth can induce the tensile stress of FePt film by more than 3 GPa by annihilation of grain

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boundaries [25]. Our samples show a mitigated value of about 1 GPa probably owing to the dynamic stress relaxation during annealing [15, 26]. This interpretation also can be applied

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to the stress behavior of the samples as Ta was increased from 700 to 800 °C, where a measured tensile stress of 0.8 GPa is induced by a significant grain growth from about 9.1 to 25 nm. However, rather interestingly, the findings appear to be inconsistent with that of the sample with increasing Ta from 650 to 700 °C. The fact shows that a huge tensile stress of 0.9 GPa is induced without extensive grain growth. The origin for this tensile stress 9

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generation is not fully understood yet. This might be correlated to the surface oxidation between L10 FePt phase and residual atmosphere in the RTA chamber during annealing

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[10]. In addition, it was suggested that the oxidation of capping layer might result in the in-plane tensile stress on the underlying FePt layer [27]. Surface oxidation causes a

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volumetric expansion of FePt lattice, which in turn induces a compressive surface stress

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since the oxide FePt layer is bonded by the unoxided FePt film. As a counterpart, the unoxided FePt part is expected to bear a tensile stress induced by surface oxidation layer.

Fig. 5 (a) plots the Fe-2p XPS spectrum of the as-deposited and annealed FePt films

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with Ta = 650−800 °C. The vertical dash lines represent the metallic Fe and Fe2O3 energies as references. Note that the samples were sputtered for 1 min to remove the oxygen layers which were physically absorbed on the FePt surfaces in the XPS chamber

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before measurement. Besides, the XPS detectable depth (d) is determined by the formula

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d = 3Msinα, where the M and α are the inelastic mean-free paths (IMFP) and the take off angles for photoelectrons (in this study, α = 45°). For the x-ray beam with energy of 1 the IMFP of the electrons of FePt is about 1.3 nm [28]. Thus, the detectable depth is determined to be around 2.8 nm. For the as-deposited FePt film, the peaks were located the metallic Fe core-level. After an annealing of 650 °C, the peaks remained unchanged,

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indicating that the FePt film was not surface-oxidized during RTA. As Ts was further increased to 700 °C, the peaks started to shift toward Fe2O3 state, which confirms that the

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formation of FeOx on the surface of the L10-FePt film. For the FePt film RTA at 800 °C, a further shift toward Fe2O3 state was observed, owing to the deeper oxygen diffusion and

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formation of thicker oxide layer. The XPS spectra of the FePt films annealed ≥ 700 °C

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show convolution curves from Fe2O3 component and pure FePt component, implying an off-stoichiometric Fe−O compound. The convolution curves indicate that the thicknesses of the oxidation layer should be much less than 2.8 nm.

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To further understand the role of surface oxidation on (001) preferred orientation, the FePt films was capped with an MgO film (1 nm-thick) and then was rapid-annealed at 800 °C. The MgO film was used to prevent exposure to oxygen atmosphere during

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annealing. As shown in Fig. 5 (b), the sample exhibits an L10 structure with S value of

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~0.9. However, a poor perpendicular anisotropy with a LOF value of 0.72 was obtained in the bilayer film. Furthermore, we also examined the σ of the bilayer film which exhibits a much smaller value (1.5 GPa) than that of the samples with free surface (3.3 GPa). Since the capping oxide layer was expected to suppress the surface oxidation of the L10 FePt films during RTA, it can be implied that surface oxidation of the L10 FePt films which

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induces strong tensile stress probably play a very important role in enhancing the perpendicular anisotropy. Further investigations are required to fully understand

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surface-oxidation phenomena and resultant surface stress of the FePt films during RTA.

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4. Conclusion

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In summary, we demonstrate the dependence of annealing temperature on the evolution of microstructure, in-plane residual stress, and crystallographic orientation of the rapid-annealed FePt films. The stress-driven PMA and (001) texture of the L10 FePt films were achieved at Ta ≥ 700 °C. By microstructural observation and in-plan stress

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measurement, we provide an unambiguous evidence for the tensile stress which partly originates from the annihilation of grain boundaries of L10 FePt grains. The progress of

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L10 ordering causes the relaxation of the in-plane tensile stress of the FePt. On the other

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hand, the surface oxidation of the L10 FePt was proposed, for the first time, to be the unexpected second source of the tensile stress during RTA. This fundamental study provides useful knowledge for understanding the stress behavior of FePt during RTA by characterizing the microstructure and crystallographic structure.

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Figure captions

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Figure 1 (a) XRD spectra of the as-deposited and RTA-treated FePt films at different Ta. Dependence of (b) LOF and (c) ordering parameter on Ta in the annealed FePt films.

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Figure 2 Out-of-plane and in-plane MH loops for the FePt films annealed at (a) 500, (b)

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550, (c) 650, (d) 700 and (e) 800 °C.

Figure 3 Plane-view TEM images of the as-deposited (a) and annealed FePt films with (b) Ta = 450 °C, (c) Ta = 500 °C and (d) corresponding dark-field image, (e) Ta = 650 °C and

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(f) corresponding dark-field image, (g) Ta = 700 °C, and (f) Ta = 800 °C.

Figure 4 (a) Dependence of in-plan tensile stress on annealing temperature of the FePt

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of Ta.

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films. The inset shows the evolution of average grain size of the FePt films as a function

Figure 5 (a) Fe-2d XPS spectra for the as-deposited film and the FePt films annealed at 800 °C. (b) XRD pattern for the MgO (1 nm)/FePt (10 nm) bilayer films annealed at 800 °C.

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ACCEPTED MANUSCRIPT Highlights A huge tensile stress (2.5 GPa) induces a texture transition from (111) to (001). Grain boundary annihilation induces part of the accumulation of tensile stress. Progress of L10 ordering causes the relaxation of the in-plane tensile stress. Surface oxidation was proposed to be second source of the tensile stress.

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1. 2. 3. 4.