- Email: [email protected]
Magnetic properties and microstructure of FePtB, FePt(B–Ag) granular films
Journal of Magnetism and Magnetic Materials 329 (2013) 6–13
Contents lists available at SciVerse ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Magnetic properties and microstructure of FePtB, FePt(B–Ag) granular films Jai-Lin Tsai n, Jian-Chiang Huang, Hsueh-Wei Tai, Wen-Chieh Tsai, Yi-Cheng Lin Department of Materials Science and Engineering, National Chung Hsing University, 250 Kuo Kuang Rd., Taichung 402, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 May 2012 Received in revised form 14 July 2012 Available online 23 September 2012
Multilayers [FePt(1 nm)/B(t nm)]10 (t ¼0.05–0.6) were alternately deposited on a glass substrate and subsequently annealed by the rapid thermal process (RTP) at 800 1C for 3 min. After RTP, FePt and B layers intermix to form the FePtB film with (0 0 1) texture. The ordering degree of FePt was slightly increased with doped B. The (Fe–Pt)100 xBx (x ¼0, 5, 10) films show perpendicular magnetization and the minor FeB phase was indexed in isotropic (Fe–Pt)100 xBx (x ¼30, 40, 60) films. By adding Ag into (Fe–Pt)95B5 film, the ordering degree was slightly increased in (Fe–Pt)95(B0.9Ag0.1)5 film. In (Fe– Pt)100 xBx (x ¼5, 10) and (Fe–Pt)95(B0.9Ag0.1)5 granular films, the intermixed B or Ag atoms were diffused among FePt grain boundaries to isolate and refine FePt grains uniformly with average grain sizes of 20, 15, and 6.7 nm, respectively. & 2012 Elsevier B.V. All rights reserved.
Keywords: Granular film Texture Perpendicular magnetization
1. Introduction To improve the thermal stability of perpendicular magnetic recording media, (001) textured L10 FePt film with high magnetocrystalline anisotropy was considered [1–6]. The minimum thermally stable FePt grains size was down to 2.8–3.3 nm in theory [1]. For the writable issue of recording media, high coercivity of FePt film can be tuned by exchange coupled soft/ hard layer [7–12] or reduced by the energy assisted writing process [13,14]. The Curie temperature of FePt is around 500 1C and should be reduced when applied on the heat assisted magnetic recording (HAMR) media [14]. To have high signal to noise ratio (SNR) in FePt media, high degree of ordering, less caxis dispersion and granular structure with uniform grains size are required. High ordered (001) textured FePt granular films have been fabricated and discussed extensively by many researchers. To form the granular FePt film, third element doping with elements such as Ag, B, N, C, Au, and Cu was used in early works [15,16]. Many studies use oxide such as SiO2, MgO, B2O3, Al2O3, TiO2, and Ta2O5 or compounds, for example, BN, B4C, TiN, AlN etc. [17–21]. Recently, lower melting element such as Ag which shows high mobility and diffusivity when combined with interstitial element, for example, B and C was used to prepare FePtB–Ag and FePtAg–C granular films [15]. B and C are low temperature diffusion elements during annealing. In this study, we discuss the attainment, magnetic properties and microstructure of granular FePtB and FePt(B–Ag) films. B and immiscible (B, Ag) doping effects on ordering degree (S), (001) texturing, out-of-
n
Corresponding author. Tel.: þ886 4 22875741; fax: þ886 4 22857017. E-mail address: [email protected] (J.-L. Tsai).
0304-8853/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmmm.2012.09.010
plane coercivity, and grain-separation of L10 FePt film were elucidated.
2. Experiments The sputtering system was designed for ultrahigh vacuum and the load-lock system was used to transfer the substrate via small pre-chamber with base pressure of 5 10 7 Torr. There are four sputtering sources (AJA, US guns) with 2 in. diameter and the targets were FePt, Ag, B and Fe. FePt and Ag were sputtered by direct current (DC) power and B was sputtered by radio frequency (RF) power. The shutters were half-automatic which were controlled via valves. The sample holder can be rotated from one to another target. Multilayers [FePt(1 nm)/B(t nm)]10 (t ¼0, 0.05, 0.1, 0.3, 0.4, 0.6) were alternately deposited on a glass substrate at room temperature by magnetron sputtering. The base pressure of the sputtering system was 5 10 8 Torr with a working pressure of 1.5 10 3 Torr under high purity argon gas. B was deposited by RF sputtering with argon pressure of 1.5 10 3 Torr and the sputtering rate was 0.0143 nm/s at 150 W. The B layer with a thickness of 0.05 nm takes around 3.5 s to deposit. The total thickness of FePt film is 10 nm and the Ag layer with a thickness of 1 nm was capped on the multilayer. After deposition, the films were annealed by using a rapid thermal annealing process (RTP) with a heating rate of 10 1C/s at 800 1C for 3 min thereby forming the granular FePtB thin films. The film chemical composition is Fe48Pt52 measured by energy dispersive spectrometers (EDS). The volume fractions of granular films were represented as (Fe0.48Pt0.52)100 xBx (x ¼0, 5, 10, 30, 40, 60). The (Fe–Pt)100 xBx film was used to present different samples. The composition of
J.-L. Tsai et al. / Journal of Magnetism and Magnetic Materials 329 (2013) 6–13
(Fe0.48Pt0.52)95B5 was our target to add Ag into FePtB film. Multilayers of [FePt (1 nm)/(ByAg1 y)(0.05 nm)]10 (y ¼0, 0.3, 0.4, 0.7, 0.9, 1) were alternately deposited on glass substrates at room temperature. The (B, Ag) layer was deposited by co-sputtering and the total thickness of the FePt layer and the ByAg1 y layer was 10 nm and 0.5 nm, respectively. The volume fractions of B (y) and Ag (1 y) were adjusted based on the pre-calibrated sputtering rates of B and Ag. After deposition, the films were also annealed by using a rapid thermal process (RTP) system at 800 1C for 3 min, forming the granular FePt(B, Ag) thin films. The B, Ag contents in granular FePt(B, Ag) films were estimated
7
from the total deposition time using the calibrated deposition rates for a given sputtering power. The volume fraction of granular films was represented as (Fe0.48Pt0.52)95(ByAg1 y)5 (y ¼0, 0.3, 0.4, 0.7, 0.9, 1). The (Fe–Pt)95B5, (Fe–Pt)95Ag5 and (Fe–Pt)95(ByAg1 y)5 films were used to present different samples. The crystal structure of the samples was identified by the standard x-ray diffraction (XRD) technique (BRUKER, D8 Discover). Magnetic hysteresis loops were measured at room temperature with a vibration sample magnetometer (VSM, Lakeshore 7400) with a maximum magnetic field of 2 T. The film microstructure was observed by transmission electron microscopy (TEM, JEOL JEM-2010).
3. Results and discussion
Fig. 1. XRD patterns of the as-deposited [FePt(1 nm)/B(t nm)]10 t¼(a) 0, (b) 0.05, (c) 0.1, (d) 0.3, (e) 0.4, (f) 0.6 films.
Fig. 2. XRD patterns of (Fe–Pt)100 xBx x ¼(a) 0, (b) 5, (c) 10, (d) 30, (e) 40, (f) 60 annealed films at 800 1C.
Fig. 1 shows the XRD patterns of the as-deposited [FePt(1 nm)/ B(t nm)]10 t¼(a) 0, (b) 0.05, (c) 0.1, (d) 0.3, (e) 0.4, (f) 0.6 films. In Fig. 1(a), the disordered structure (A1 phase) with (111) peak was observed in as-deposited FePt film. In Fig. 1(b–f), when the B layer’s thickness increased, the relative intensity of (111) peak was reduced and became rough. The grains size of disordered FePt was refined when the thickness of B was increased. Fig. 2 shows standard XRD patterns of (Fe–Pt)100 xBx (x¼ 0, 5, 10, 30, 40, 60) annealed films, respectively. The (001) superlattice diffraction peak and the (002) fundamental reflection of the L10 FePt are observed. The XRD profiles suggest that the L10 FePt crystal has [001] texture. The weak (111) diffraction peak was indexed in Fig. 2(d–f) and the soft magnetic FeB phase was found in Fig. 2(f). The ordering parameter, S can be estimated from (In(002)/In(001))1/2(I(001)/I(002))1/2 or proportional to I(001)/I(002) ratio [22]. The (In(002)/In(001))1/2 value, for example, 0.4915 in L10 FePt film was obtained after considering all the corrected factors in the XRD data. The values of corrected factors were listed in Table 1. Fully ordered L10 FePt x-ray diffraction 2 peak intensity is given by In002 =In001 ¼ 9F9 LPDA = 002 2 9F9 LPDA [22]. For L10 FePt, the structure factor F is 001 p iðh þ kÞ þf Pt epiðk þ 1Þ þ epiðh þ 1Þ and f is atomic scatF hld ¼ f Fe 1 þ e tering factor [22]. The Lorentz factor (L) is [1/sin2(y)cos(y)] and the polarization factor (P) is (1þcos2(2y)). The temperature factor (D) is e 2M and M¼(sin(y)/l)2 and absorption factor (A) is 1-exp(2mt/ (siny)). The average mass absorption coefficient was estimated as m ¼[mFe wt%Feþ mPt x wt%Pt]x[XFerFe þXPtrPt] [22]. Here, XFe,XPt and rFe, rPt are the atomic fraction and density of Fe and Pt, respectively. The values for mFe and mPt are tabulated [22]. The I(001) and I(0 0 2) are the integrated peak intensity from the experimental results for a partially ordered film. In Fig. 2(a-f), the ordering parameter S estimated from 0.4915 (I(0 0 1)/I(002))1/2 is 0.80, 0.83, 0.82, 0.83, 0.83 and 0.81, respectively. When B was added, the ordering degree of FePt film was slightly increased. Fig. 3(a-f) shows standard XRD patterns of granular films (Fe– Pt)95(ByAg1 y)5 (y¼1, 0.9, 0.7, 0.4, 0.3, 0) annealed at 800 1C. In Figs. 3(a-e) (y¼1, 0.9, 0.7, 0.4, 0.3, respectively), the (0 0 1)
Table 1 Calculations for (I*(002)/I*(001)), theoretical integrated peak intensity for L10- FePt film. Peak
2h
Cu Ka
sinh/k
jFj2
D
P
L
(0 0 1) (0 0 2)
23.96 48.92
1.542 1.542
0.134 0.268
8798 22440
0.964 0.865
1.835 1.430
23.802 6.3964
Peak
Thickness
l
A
In
(0 0 1) (0 0 2)
10 10
3323.832 3323.832
0.03157 0.015911
11705.18 2828.238
8
J.-L. Tsai et al. / Journal of Magnetism and Magnetic Materials 329 (2013) 6–13
superlattice diffraction peak and the (0 0 2) fundamental reflection of the L10 FePt are clearly observed. The XRD profiles suggest that the L10 FePt crystal has a [001] texture. Fig. 3(f) shows the XRD pattern of the (Fe–Pt)95Ag5 film without B addition. Compared to the FePt film with the B addition shown in Fig. 2(a-e), (0 0 1) and (0 0 2) have a relative weak intensity. A weak (111)
Fig. 3. XRD patterns of (FePt)95(ByAg1 y)5 y ¼ (a) 1.0, (b) 0.9, (c) 0.7, (d) 0.4, (e) 0.3, (f) 0 annealed films at 800 1C.
diffraction peak is slightly visible in Fig. 3(d-f). In Fig. 3(a-f), the ordering parameter S estimated from 0.4915(I(0 0 1)/I(0 0 2))1/2 is 0.83, 0.84, 0.82, 0.76, 0.74 and 0.59, respectively. In (Fe– Pt)95(ByAg1 y)5 films, Ag was added to replace some of B and the ordering degree of FePt film was decreased with added Ag excluding (Fe–Pt)95(B0.9Ag0.1)5 film. Fig. 4(a-f) shows in-plane and out-of-plane magnetic hysteresis loops of the as-deposited [FePt(1 nm)/B(t nm)]10 t¼ (a) 0, (b) 0.05, (c) 0.1, (d) 0.3, (e) 0.4, (f) 0.6 films. Before annealing, disordered FePt film shows soft magnetic properties. The easy magnetization was in the in-plane direction and the saturation magnetization was decreased when B was added. Fig. 5(a-f) shows in-plane and out-of-plane magnetic hysteresis loops of (Fe–Pt)100 xBx (x¼0, 5, 10, 30, 40, 60) films. In Fig. 5(a-c), the (Fe– Pt)100 xBx (x ¼0, 5, 10) films shows perpendicular magnetization with skewed linear like in-plane loops. The out-of-plane coercivity (Hc) is 11 kOe, 7.5 kOe and 9.9 kOe, respectively. With increasing B content in (Fe–Pt)100 xBx (x ¼0, 5, 10), the out-of-plane Hc decreased and the remanence (Mr) increased. In Fig. 5(d), the perpendicular anisotropy of (Fe–Pt)70B30 film was deteriorated and there is a shoulder in in-plane hysteresis loops. The shoulder comes from the magnetization reversal of soft magnetic FeB phase. With further increasing B content in (Fe–Pt)100 xBx (x ¼40, 60) films that become soft with small values of out-ofplane Hc 1.4 kOe, 1.0 kOe shown in Fig. 5(e-f). With increasing B thickness in original multilayer, the FePtB films were changed from perpendicular magnetic anisotropy to isotropy.
Fig. 4. In-plane and out-of-plane magnetic hysteresis loops of the as-deposited [FePt(1 nm)/B(t nm)]10 t¼ (a) 0, (b) 0.05, (c) 0.1, (d) 0.3, (e) 0.4, (f) 0.6 films.
J.-L. Tsai et al. / Journal of Magnetism and Magnetic Materials 329 (2013) 6–13
9
Fig. 5. In-plane and out-of-plane magnetic hysteresis loops of (a) FePt single layer, and (Fe–Pt)100 xBx x ¼ (b) 5, (c) 10, (d) 30, (e) 40, (f) 60 films annealed at 800 1C.
Fig. 6 shows in-plane and out-of-plane magnetic hysteresis loops of (Fe–Pt)95(ByAg1 y)5 (y¼1, 0.9, 0.7, 0.4, 0.3, 0) films. Fig. 6(a) shows the loop of (Fe–Pt)95B5 film, and the out-of-plane Hc is 7.5 kOe. In Fig. 6(b-e), Ag is added to dilute B content, and the out-of-plane Hc increases up to 13 kOe in (Fe–Pt)95 (B0.3Ag0.7)5 film. Fig. 6(f) shows magnetic loops of (Fe–Pt)95Ag5 film, the outof-plane Hc is 12 kOe and in-plane coercivity is also large. In short, the (Fe–Pt)95(ByAg1 y)5 (y¼1, 0.9, 0.7, 0.4, 0.3) shows perpendicular magnetization with linear like in-plane loops. Fig. 7 shows plane-view TEM images and selected area diffraction patterns (SAD) of the as-deposited multilayers [FePt(1 nm)/B(t nm)]10 t ¼ (a) 0, (b) 0.05, (c) 0.1, (d) 0.6. The Ag layer with thickness of 1 nm was capped on the multilayer and may precipitate on top of the FCC FePt grains. From SAD patterns, disordered face center cubic (FCC) FePt (111) and (200) diffraction rings were indexed. The FCC FePt grains were distributed randomly and the grains size of FCC FePt was nearly not changed from Fig. 7(a) to (b). The grains size of FCC FePt was refined with more B as shown in Fig. 7(c-d). Fig. 8 shows plane-view TEM images, grains size distribution and selected area diffraction patterns (SAD) of FePt single layer and FePtB films. In Fig. 8(a), the FePt grains were randomly distributed with average grains size around 22 nm. Fig. 8(b) shows bright field image of (Fe– Pt)95B5 film, the FePt grains were isolated uniformly by the amorphous B and the average grains size was 20 nm. In
Fig. 8(c), the grains size was further refined to 15 nm in (Fe– Pt)90B10 film. In Fig. 8(d), the FePt grains were aggregated. To identify the FeB grains distribution in FePt film, the dark field images of (Fe–Pt)100 xBx (x ¼ 30, 60) films were shown in Fig. 9. In Fig. 9(a) and (c), the very weak FeB (002) diffraction rings were selected and the FeB grains may appear in bright areas (marked by arrows) in Fig. 9(b) and (d). Due to the minor FeB phase, the FePt spots may also include in formation the dark field images. In (Fe–Pt)100 xBx (x¼0, 5, 10) films, the grain size of FePt refined with more B was reduced from 22 nm to 15 nm. In (FePt)40B60 films, appeared soft magnetic FeB phase was evidenced in FeB (002) peak in XRD and soft magnetic hysteresis loops. The FePt and FeB grains were aggregated and the grains size was not defined. Fig. 10 shows plane-view TEM images of (Fe–Pt)95(ByAg1 y)5 (y¼1, 0.9, 0.4, 0) films. In Fig. 10(a), the FePt grains with an average grains size of 20 nm are manifest and the amorphous B resides in the grain boundaries. Fig. 10(b) shows TEM image of (Fe–Pt)95(B0.9Ag0.1)5. The FePt grains are isolated uniformly with thin (B, Ag) grain-boundary layer and the average grains size is calculated as 6.7 nm. Fig. 10(c) shows TEM image of (Fe– Pt)95(B0.4Ag0.6)5. The FePt grains are not separated uniformly and some grains are interconnected. The average grains size is around 12 nm. Fig. 10(d) shows TEM image of (Fe–Pt)95Ag5 film. The over-grown FePt grains become interconnecting due to highly
10
J.-L. Tsai et al. / Journal of Magnetism and Magnetic Materials 329 (2013) 6–13
Fig. 6. In-plane and out-of-plane magnetic hysteresis loops of (FePt)95(ByAg1 y)5 y ¼(a) 1.0, (b) 0.9, (c) 0.7, (d) 0.4, (e) 0.3, (f) 0 films annealed at 800 1C.
diffusive Ag which is abundant in this case. In short, uniform and small FePt grains can be achievable by (B, Ag) co-doping. In Fig. 10(a-c), the grain size distributions estimated by s/Davg were 26%, 20% and 22%, respectively. The finest grains size and narrow grains size distribution was observed in (Fe– Pt)95(B0.9Ag0.1)5 film. The FePt grains were segregated by B that was used to prevent grain growth and grain coarsening at high annealing temperature. In (Fe–Pt)95(B0.9Ag0.1)5 film, the minor Ag doping in multilayer was used to promote chemical ordering. In Fig. 10(c-d), more Ag was added and segregated B was diluted. Finally, the FePt grains were over growth and interconnected. In Fig. 10(c-d), the Ag volume content that comes from capping layer and multilayer was too high and precipitate out of the FePtB on top of the grains [15]. The FePtB microstructure was not influenced significantly by minor Ag doping. Magnetostatic coupling is essentially determined by plane magnetization (Mrt) [23]. The values of plane magnetization (Mrt) were listed in Table 2. The Mrt values in Fe–Pt, (Fe– Pt)95B5 and (Fe–Pt)90B10 films are much higher than (Fe– Pt)100 xBx (x¼30, 40, 60) films. The high Mrt value means large magnetostatic coupling and small grains exchange coupling. From TEM images shown in Fig. 8, the FePt grains in (Fe–Pt)100 xBx (x¼5, 10) films are well separated and aggregated in (Fe– Pt)100 xBx (x¼30, 40, 60) films due to the amount of soft magnetic FeB. The Mrt values and magnetostatic coupling were reduced when Ag were increased in (Fe–Pt)95 (ByAg1 y)5 (y ¼1, 0.9, 0.7, 0.4, 0.3, 0) films. In Fig. 10, the FePt grains were isolated
in (Fe–Pt)95 (ByAg1 y)5 (y¼1, 0.9, 0.4) films but aggregated in (FePt)95Ag5 film. The Wohlfarth relation and Kelly–Henkel plot (dM plot) were used to characterize and classify the hysteresis phenomena, especially the inter-grain magnetic interaction. The dM plot can be determined from the equation [24] dM¼Md(H) [1 2Mr(H)]. Where Md(H) is the normalized dc-demagnetization(DCD) remanence as a function of the reversal field, and Mr(H) is the normalized isothermal remanence(IRM) curve. From the Kelly– Henkel plot, a positive dM indicates that intergrain interactions are ferromagnetic exchange interactions. A negative dM value suggests that intergrain interactions are dipolar interactions (magnetostatic coupling). Fig. 11 shows the dM plots of a FePt single layer, and (Fe–Pt)100 xBx x¼5, 10, 30 films. The Ag layer with a thickness of 1 nm was capped on FePt single layer and FePt/B multilayer before annealing. The FePt single layer and (Fe– Pt)95B5 film show negative dM value at all applied fields. The magnetostatic coupling was dominated between FePt grains. In (Fe–Pt)90B10 film, the dM value was changed from negative to positive and back to negative from low- to high-applied fields. The (Fe–Pt)70B30 film show positive dM values which means a strong intergrain exchange interaction between FePt and soft magnetic FeB grains. The reduction of exchange interaction between FePt grains was achieved by capped Ag layer and B segregant with certain volume fraction. For FePtB film, the doping of B did not reduce the kinetic ordering temperature [25]. But other researchers use B which
J.-L. Tsai et al. / Journal of Magnetism and Magnetic Materials 329 (2013) 6–13
Fig. 7. TEM images and SAD patterns of the as-deposited [FePt(1 nm)/B(t nm)]10 t¼ (a) 0, (b) 0.05, (c) 0.1, (d) 0.6 films.
Fig. 8. TEM images, grains size distribution and SAD patterns of (a) FePt single layer and (Fe–Pt)100 xBx x ¼ (b) 5, (c) 10, and (d) 60 films.
11
12
J.-L. Tsai et al. / Journal of Magnetism and Magnetic Materials 329 (2013) 6–13
Fig. 9. TEM bright (B) and dark (D) field images of (Fe–Pt)100 xBx films (a) B: x ¼ 30, (b) D: x¼ 30, (c) B: x ¼60, (d) D: x¼ 60.
Fig. 10. TEM images, grains size distribution and SAD patterns of (FePt)95(ByAg1 y)5 y ¼(a) 1.0, (b) 0.9, (c) 0.4, (d) 0 films.
J.-L. Tsai et al. / Journal of Magnetism and Magnetic Materials 329 (2013) 6–13
Table 2 The Mrt values of Fe–Pt, (Fe–Pt)100 xBx and (Fe–Pt)95(ByAg1 y)5 films. (Fe–Pt)100 xBx
Mrt (memu/cm2)
(Fe–Pt)95(ByAg1 y)5
Mrt (memu/cm2)
(Fe–Pt) (Fe–Pt)95B5 (Fe–Pt)90B10 (Fe–Pt)70B30 (Fe–Pt)60B40 (Fe–Pt)40B60
0.52 0.63 0.58 0.38 0.15 0.04
(Fe–Pt)95B5 (Fe–Pt)95(B0.9Ag0.1)5 (Fe–Pt)95(B0.7Ag0.3)5 (Fe–Pt)95(B0.4Ag0.6)5 (Fe–Pt)95(B0.3Ag0.7)5 (Fe–Pt)95Ag5
0.63 0.61 0.46 0.53 0.41 0.21
13
4. Conclusions Multilayers [FePt(1 nm)/B(t nm)]10 (t¼0.05–0.6) and [FePt(1 nm)/ (ByAg1 y) (0.1 nm)]10 (y¼0–1) were prepared by controlled magnetron-sputtering. Granular films were obtained after a rapid thermal process at 800 1C for 3 min. By adding B into FePt film, the ordering was increased slightly and amorphous B was segregated at the boundaries of FePt grains and limited the grain coarsening in (Fe– Pt)100 xBx (x¼5, 10) granular films. Furthermore, by adding Ag into (Fe–Pt)95B5 film, the ordering degree and (001) texture was slightly improved in (Fe–Pt)95(B0.9Ag0.1)5 film. Granular (Fe0.48Pt0.52)95 (B0.9Ag0.1)5 film shows high perpendicular magnetization and small grains size (6.7 nm).
Acknowledgment The authors would like to thank the NSC for financial support under grant number NSC100-2221-E-005–044-MY2. We also thank the Center of Nanoscience and Nanotechnology in NCHU for the TEM investigation. References
Fig. 11. dM plots of FePt single layer, and (Fe–Pt)100 xBx (x ¼5, 10, 30) films.
increases coercivity due to reduced ordering temperature [26,27]. However, the coercivity of film depends not only on the film composition but also on its microstructure such as grains size and texture [26]. In this study, by adding B into FePt film, ordering degree just increased slightly. Ternary (Fe–Pt)95B5 and (Fe– Pt)90B10 films were proved to have high perpendicular magnetization with granular FePt structures. The (Fe–Pt)95B5 composition was used in the study with partial B replaced by Ag. An interstitial element combined with a relatively low melting element Ag which has a high mobility and diffusivity, such as co-doped (B, Ag) elements, can well separate FePt grains uniformly even at high annealed temperatures. This is analogous to a recent study using (C, Ag) [15]. In our previous work, Ag capped- and underlayer demonstrated to improve perpendicular magnetization but tend to facilitate aggregation of FePt grains leading to interconnect film structure [28]. In our previous work, we claim that Ag capping layer was used to promote ordering or reduced the ordering temperature [28]. The ordering promotion of FePt film can be explained more detail. Rapid ordering transformation and phase transformation strain are key factors to obtain (0 0 1) textured FePt films [29]. The rapid thermal annealing (RTA) process was used to have high heating rate. In this study, the heating rate was 10 1C/s and the Ag cap layer with thickness of 1 nm was required to enhance the ordering transformation strain. When the heating rate was increased to 40 1C/s, the (001) textured FePt film can be formed without Ag capping layer. This work, the minor addition of Ag in FePt film has been proved to increase the ordering degree slightly and improve perpendicular anisotropy of FePt films, for example, in (FePt)95(B0.9Ag0.1)5 film.
[1] D. Weller, A. Moser, L. Folks, M.E. Best, W. Lee, M.F. Toney, et al., IEEE Transactions on Magnetics 36 (2000) 10. [2] T. Suzuki, Z. Zhang, A.K. Singh, J. Yin, A. Perumal, H. Osawa, IEEE Transactions on Magnetics 41 (2005) 555. [3] J.S. Chen, J.F. Hu, B.C. Lim, Y.K. Lim, B. Liu, G.M. Chow, G. Ju, Journal of Applied Physics 103 (2008) 07F517. [4] M.H. Hong, K. Hono, M. Watanabe, Journal of Applied Physics 84 (1998) 4403. [5] M.L. Yan, N. Powers, D.J. Sellmyer, Journal of Applied Physics 93 (2003) 8292. [6] Y. Xu, J.S. Chen, J.P. Wang, Applied Physics Letters 80 (2002) 3325. [7] A. Yu, Dobin, H.J. Richter, Applied Physics Letters 89 (2006) 062512. [8] D. Suess, Applied Physics Letters 89 (2006) 113105. [9] R.H. Victora, X. Shen, IEEE Transactions on Magnetics 41 (2005) 537. [10] F. Casoli, F. Albertini, L. Nasi, S. Fabbrici, R. Cabassi, F. Bolzoni, C. Bocchi, Applied Physics Letters 92 (2008) 142506. [11] J.L. Tsai, H.T. Tzeng, G.B. Lin, Applied Physics Letters 96 (2010) 032505. [12] D. Goll, A. Breitling, Applied Physics Letters 94 (2009) 052502. [13] J.W. Kim, H.S. Song, J.W. Jeong, K.D. Lee, J.W. Sohn, T. Shima, S.C. Shin, Applied Physics Letters 98 (2011) 092509. [14] M.H. Kryder, E.C. Gage, T.W. McDaniel, W.A. Challener, R.E. Rottmayer, G. Ju, Y.T. Hsia, M.F. Erden, IEEE Transactions on Magnetics 96 (2008) 1810. [15] L. Zhang, Y.K. Takahashi, A. Perumal, K. Hono, Journal of Magnetism and Magnetic Materials 322 (2010) 2658. [16] D.C. Berry, K. Barmak, Journal of Applied Physics 102 (2007) 024912. [17] T. Ichitsubo, S. Tojo, T. Uchihara, E. Matsubara, A. Fujita, K. Takahashi, K. Watanabe, Physical Review B 77 (2008) 094114. [18] G.R. Trichy, D. Chakraborti, J. Narayan, J.T. Prater, Applied Physics Letters 92 (2008) 102504. [19] B.H. Li, C. Feng, X. Gao, J. Teng, G.H. Yu, X. Xing, Z.Y. Liu, Applied Physics Letters 91 (2007) 152502. [20] F.J. Yang, H. Wang, H.B. Wang, X. Cao, C.P. Yang, Q. Li, M.J. Zhou, Y.M. Chong, W.J. Zhang, Journal of Applied Physics 102 (2007) 106101. [21] N. Kaushik, P. Sharma, H. Kimura, A. Inoue, A. Makino, Journal of Applied Physics 103 (2008) 07E121. [22] S.D. Granz, M.H. Kryder, Journal of Magnetism and Magnetic Materials 324 (2012) 287. [23] S.N. Piramanayagam, Journal of Applied Physics 102 (2007) 011301. [24] G. Bertotti (Ed.), Academic Press, 1998, pp. 249–250. [25] B. Wang, K. Barmak, T.J. Klemmer, Journal of Applied Physics 109 (2011) 07B739. [26] K. Nishimura, K. Takahashi, H. Uchida, M. Inoue, Journal of Magnetism and Magnetic Materials 272 (2004) 2189. [27] Y.M. Lee, B.S. Lee, C.G. Lee, B.H. Koo, Y. Shimada, Journal of Magnetism and Magnetic Materials 310 (2004) e918. [28] J.L. Tsai, H.T. Tzeng, G.B. Lin, B.F. Liu, Journal of Alloys and Compounds 487 (2009) 18. [29] J.S. Kim, Y.M. Koo, B.J. Lee, S.R. Lee, Journal of Applied Physics 99 (2006) 053906.
Contents lists available at SciVerse ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Magnetic properties and microstructure of FePtB, FePt(B–Ag) granular films Jai-Lin Tsai n, Jian-Chiang Huang, Hsueh-Wei Tai, Wen-Chieh Tsai, Yi-Cheng Lin Department of Materials Science and Engineering, National Chung Hsing University, 250 Kuo Kuang Rd., Taichung 402, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 May 2012 Received in revised form 14 July 2012 Available online 23 September 2012
Multilayers [FePt(1 nm)/B(t nm)]10 (t ¼0.05–0.6) were alternately deposited on a glass substrate and subsequently annealed by the rapid thermal process (RTP) at 800 1C for 3 min. After RTP, FePt and B layers intermix to form the FePtB film with (0 0 1) texture. The ordering degree of FePt was slightly increased with doped B. The (Fe–Pt)100 xBx (x ¼0, 5, 10) films show perpendicular magnetization and the minor FeB phase was indexed in isotropic (Fe–Pt)100 xBx (x ¼30, 40, 60) films. By adding Ag into (Fe–Pt)95B5 film, the ordering degree was slightly increased in (Fe–Pt)95(B0.9Ag0.1)5 film. In (Fe– Pt)100 xBx (x ¼5, 10) and (Fe–Pt)95(B0.9Ag0.1)5 granular films, the intermixed B or Ag atoms were diffused among FePt grain boundaries to isolate and refine FePt grains uniformly with average grain sizes of 20, 15, and 6.7 nm, respectively. & 2012 Elsevier B.V. All rights reserved.
Keywords: Granular film Texture Perpendicular magnetization
1. Introduction To improve the thermal stability of perpendicular magnetic recording media, (001) textured L10 FePt film with high magnetocrystalline anisotropy was considered [1–6]. The minimum thermally stable FePt grains size was down to 2.8–3.3 nm in theory [1]. For the writable issue of recording media, high coercivity of FePt film can be tuned by exchange coupled soft/ hard layer [7–12] or reduced by the energy assisted writing process [13,14]. The Curie temperature of FePt is around 500 1C and should be reduced when applied on the heat assisted magnetic recording (HAMR) media [14]. To have high signal to noise ratio (SNR) in FePt media, high degree of ordering, less caxis dispersion and granular structure with uniform grains size are required. High ordered (001) textured FePt granular films have been fabricated and discussed extensively by many researchers. To form the granular FePt film, third element doping with elements such as Ag, B, N, C, Au, and Cu was used in early works [15,16]. Many studies use oxide such as SiO2, MgO, B2O3, Al2O3, TiO2, and Ta2O5 or compounds, for example, BN, B4C, TiN, AlN etc. [17–21]. Recently, lower melting element such as Ag which shows high mobility and diffusivity when combined with interstitial element, for example, B and C was used to prepare FePtB–Ag and FePtAg–C granular films [15]. B and C are low temperature diffusion elements during annealing. In this study, we discuss the attainment, magnetic properties and microstructure of granular FePtB and FePt(B–Ag) films. B and immiscible (B, Ag) doping effects on ordering degree (S), (001) texturing, out-of-
n
Corresponding author. Tel.: þ886 4 22875741; fax: þ886 4 22857017. E-mail address: [email protected] (J.-L. Tsai).
0304-8853/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jmmm.2012.09.010
plane coercivity, and grain-separation of L10 FePt film were elucidated.
2. Experiments The sputtering system was designed for ultrahigh vacuum and the load-lock system was used to transfer the substrate via small pre-chamber with base pressure of 5 10 7 Torr. There are four sputtering sources (AJA, US guns) with 2 in. diameter and the targets were FePt, Ag, B and Fe. FePt and Ag were sputtered by direct current (DC) power and B was sputtered by radio frequency (RF) power. The shutters were half-automatic which were controlled via valves. The sample holder can be rotated from one to another target. Multilayers [FePt(1 nm)/B(t nm)]10 (t ¼0, 0.05, 0.1, 0.3, 0.4, 0.6) were alternately deposited on a glass substrate at room temperature by magnetron sputtering. The base pressure of the sputtering system was 5 10 8 Torr with a working pressure of 1.5 10 3 Torr under high purity argon gas. B was deposited by RF sputtering with argon pressure of 1.5 10 3 Torr and the sputtering rate was 0.0143 nm/s at 150 W. The B layer with a thickness of 0.05 nm takes around 3.5 s to deposit. The total thickness of FePt film is 10 nm and the Ag layer with a thickness of 1 nm was capped on the multilayer. After deposition, the films were annealed by using a rapid thermal annealing process (RTP) with a heating rate of 10 1C/s at 800 1C for 3 min thereby forming the granular FePtB thin films. The film chemical composition is Fe48Pt52 measured by energy dispersive spectrometers (EDS). The volume fractions of granular films were represented as (Fe0.48Pt0.52)100 xBx (x ¼0, 5, 10, 30, 40, 60). The (Fe–Pt)100 xBx film was used to present different samples. The composition of
J.-L. Tsai et al. / Journal of Magnetism and Magnetic Materials 329 (2013) 6–13
(Fe0.48Pt0.52)95B5 was our target to add Ag into FePtB film. Multilayers of [FePt (1 nm)/(ByAg1 y)(0.05 nm)]10 (y ¼0, 0.3, 0.4, 0.7, 0.9, 1) were alternately deposited on glass substrates at room temperature. The (B, Ag) layer was deposited by co-sputtering and the total thickness of the FePt layer and the ByAg1 y layer was 10 nm and 0.5 nm, respectively. The volume fractions of B (y) and Ag (1 y) were adjusted based on the pre-calibrated sputtering rates of B and Ag. After deposition, the films were also annealed by using a rapid thermal process (RTP) system at 800 1C for 3 min, forming the granular FePt(B, Ag) thin films. The B, Ag contents in granular FePt(B, Ag) films were estimated
7
from the total deposition time using the calibrated deposition rates for a given sputtering power. The volume fraction of granular films was represented as (Fe0.48Pt0.52)95(ByAg1 y)5 (y ¼0, 0.3, 0.4, 0.7, 0.9, 1). The (Fe–Pt)95B5, (Fe–Pt)95Ag5 and (Fe–Pt)95(ByAg1 y)5 films were used to present different samples. The crystal structure of the samples was identified by the standard x-ray diffraction (XRD) technique (BRUKER, D8 Discover). Magnetic hysteresis loops were measured at room temperature with a vibration sample magnetometer (VSM, Lakeshore 7400) with a maximum magnetic field of 2 T. The film microstructure was observed by transmission electron microscopy (TEM, JEOL JEM-2010).
3. Results and discussion
Fig. 1. XRD patterns of the as-deposited [FePt(1 nm)/B(t nm)]10 t¼(a) 0, (b) 0.05, (c) 0.1, (d) 0.3, (e) 0.4, (f) 0.6 films.
Fig. 2. XRD patterns of (Fe–Pt)100 xBx x ¼(a) 0, (b) 5, (c) 10, (d) 30, (e) 40, (f) 60 annealed films at 800 1C.
Fig. 1 shows the XRD patterns of the as-deposited [FePt(1 nm)/ B(t nm)]10 t¼(a) 0, (b) 0.05, (c) 0.1, (d) 0.3, (e) 0.4, (f) 0.6 films. In Fig. 1(a), the disordered structure (A1 phase) with (111) peak was observed in as-deposited FePt film. In Fig. 1(b–f), when the B layer’s thickness increased, the relative intensity of (111) peak was reduced and became rough. The grains size of disordered FePt was refined when the thickness of B was increased. Fig. 2 shows standard XRD patterns of (Fe–Pt)100 xBx (x¼ 0, 5, 10, 30, 40, 60) annealed films, respectively. The (001) superlattice diffraction peak and the (002) fundamental reflection of the L10 FePt are observed. The XRD profiles suggest that the L10 FePt crystal has [001] texture. The weak (111) diffraction peak was indexed in Fig. 2(d–f) and the soft magnetic FeB phase was found in Fig. 2(f). The ordering parameter, S can be estimated from (In(002)/In(001))1/2(I(001)/I(002))1/2 or proportional to I(001)/I(002) ratio [22]. The (In(002)/In(001))1/2 value, for example, 0.4915 in L10 FePt film was obtained after considering all the corrected factors in the XRD data. The values of corrected factors were listed in Table 1. Fully ordered L10 FePt x-ray diffraction 2 peak intensity is given by In002 =In001 ¼ 9F9 LPDA = 002 2 9F9 LPDA [22]. For L10 FePt, the structure factor F is 001 p iðh þ kÞ þf Pt epiðk þ 1Þ þ epiðh þ 1Þ and f is atomic scatF hld ¼ f Fe 1 þ e tering factor [22]. The Lorentz factor (L) is [1/sin2(y)cos(y)] and the polarization factor (P) is (1þcos2(2y)). The temperature factor (D) is e 2M and M¼(sin(y)/l)2 and absorption factor (A) is 1-exp(2mt/ (siny)). The average mass absorption coefficient was estimated as m ¼[mFe wt%Feþ mPt x wt%Pt]x[XFerFe þXPtrPt] [22]. Here, XFe,XPt and rFe, rPt are the atomic fraction and density of Fe and Pt, respectively. The values for mFe and mPt are tabulated [22]. The I(001) and I(0 0 2) are the integrated peak intensity from the experimental results for a partially ordered film. In Fig. 2(a-f), the ordering parameter S estimated from 0.4915 (I(0 0 1)/I(002))1/2 is 0.80, 0.83, 0.82, 0.83, 0.83 and 0.81, respectively. When B was added, the ordering degree of FePt film was slightly increased. Fig. 3(a-f) shows standard XRD patterns of granular films (Fe– Pt)95(ByAg1 y)5 (y¼1, 0.9, 0.7, 0.4, 0.3, 0) annealed at 800 1C. In Figs. 3(a-e) (y¼1, 0.9, 0.7, 0.4, 0.3, respectively), the (0 0 1)
Table 1 Calculations for (I*(002)/I*(001)), theoretical integrated peak intensity for L10- FePt film. Peak
2h
Cu Ka
sinh/k
jFj2
D
P
L
(0 0 1) (0 0 2)
23.96 48.92
1.542 1.542
0.134 0.268
8798 22440
0.964 0.865
1.835 1.430
23.802 6.3964
Peak
Thickness
l
A
In
(0 0 1) (0 0 2)
10 10
3323.832 3323.832
0.03157 0.015911
11705.18 2828.238
8
J.-L. Tsai et al. / Journal of Magnetism and Magnetic Materials 329 (2013) 6–13
superlattice diffraction peak and the (0 0 2) fundamental reflection of the L10 FePt are clearly observed. The XRD profiles suggest that the L10 FePt crystal has a [001] texture. Fig. 3(f) shows the XRD pattern of the (Fe–Pt)95Ag5 film without B addition. Compared to the FePt film with the B addition shown in Fig. 2(a-e), (0 0 1) and (0 0 2) have a relative weak intensity. A weak (111)
Fig. 3. XRD patterns of (FePt)95(ByAg1 y)5 y ¼ (a) 1.0, (b) 0.9, (c) 0.7, (d) 0.4, (e) 0.3, (f) 0 annealed films at 800 1C.
diffraction peak is slightly visible in Fig. 3(d-f). In Fig. 3(a-f), the ordering parameter S estimated from 0.4915(I(0 0 1)/I(0 0 2))1/2 is 0.83, 0.84, 0.82, 0.76, 0.74 and 0.59, respectively. In (Fe– Pt)95(ByAg1 y)5 films, Ag was added to replace some of B and the ordering degree of FePt film was decreased with added Ag excluding (Fe–Pt)95(B0.9Ag0.1)5 film. Fig. 4(a-f) shows in-plane and out-of-plane magnetic hysteresis loops of the as-deposited [FePt(1 nm)/B(t nm)]10 t¼ (a) 0, (b) 0.05, (c) 0.1, (d) 0.3, (e) 0.4, (f) 0.6 films. Before annealing, disordered FePt film shows soft magnetic properties. The easy magnetization was in the in-plane direction and the saturation magnetization was decreased when B was added. Fig. 5(a-f) shows in-plane and out-of-plane magnetic hysteresis loops of (Fe–Pt)100 xBx (x¼0, 5, 10, 30, 40, 60) films. In Fig. 5(a-c), the (Fe– Pt)100 xBx (x ¼0, 5, 10) films shows perpendicular magnetization with skewed linear like in-plane loops. The out-of-plane coercivity (Hc) is 11 kOe, 7.5 kOe and 9.9 kOe, respectively. With increasing B content in (Fe–Pt)100 xBx (x ¼0, 5, 10), the out-of-plane Hc decreased and the remanence (Mr) increased. In Fig. 5(d), the perpendicular anisotropy of (Fe–Pt)70B30 film was deteriorated and there is a shoulder in in-plane hysteresis loops. The shoulder comes from the magnetization reversal of soft magnetic FeB phase. With further increasing B content in (Fe–Pt)100 xBx (x ¼40, 60) films that become soft with small values of out-ofplane Hc 1.4 kOe, 1.0 kOe shown in Fig. 5(e-f). With increasing B thickness in original multilayer, the FePtB films were changed from perpendicular magnetic anisotropy to isotropy.
Fig. 4. In-plane and out-of-plane magnetic hysteresis loops of the as-deposited [FePt(1 nm)/B(t nm)]10 t¼ (a) 0, (b) 0.05, (c) 0.1, (d) 0.3, (e) 0.4, (f) 0.6 films.
J.-L. Tsai et al. / Journal of Magnetism and Magnetic Materials 329 (2013) 6–13
9
Fig. 5. In-plane and out-of-plane magnetic hysteresis loops of (a) FePt single layer, and (Fe–Pt)100 xBx x ¼ (b) 5, (c) 10, (d) 30, (e) 40, (f) 60 films annealed at 800 1C.
Fig. 6 shows in-plane and out-of-plane magnetic hysteresis loops of (Fe–Pt)95(ByAg1 y)5 (y¼1, 0.9, 0.7, 0.4, 0.3, 0) films. Fig. 6(a) shows the loop of (Fe–Pt)95B5 film, and the out-of-plane Hc is 7.5 kOe. In Fig. 6(b-e), Ag is added to dilute B content, and the out-of-plane Hc increases up to 13 kOe in (Fe–Pt)95 (B0.3Ag0.7)5 film. Fig. 6(f) shows magnetic loops of (Fe–Pt)95Ag5 film, the outof-plane Hc is 12 kOe and in-plane coercivity is also large. In short, the (Fe–Pt)95(ByAg1 y)5 (y¼1, 0.9, 0.7, 0.4, 0.3) shows perpendicular magnetization with linear like in-plane loops. Fig. 7 shows plane-view TEM images and selected area diffraction patterns (SAD) of the as-deposited multilayers [FePt(1 nm)/B(t nm)]10 t ¼ (a) 0, (b) 0.05, (c) 0.1, (d) 0.6. The Ag layer with thickness of 1 nm was capped on the multilayer and may precipitate on top of the FCC FePt grains. From SAD patterns, disordered face center cubic (FCC) FePt (111) and (200) diffraction rings were indexed. The FCC FePt grains were distributed randomly and the grains size of FCC FePt was nearly not changed from Fig. 7(a) to (b). The grains size of FCC FePt was refined with more B as shown in Fig. 7(c-d). Fig. 8 shows plane-view TEM images, grains size distribution and selected area diffraction patterns (SAD) of FePt single layer and FePtB films. In Fig. 8(a), the FePt grains were randomly distributed with average grains size around 22 nm. Fig. 8(b) shows bright field image of (Fe– Pt)95B5 film, the FePt grains were isolated uniformly by the amorphous B and the average grains size was 20 nm. In
Fig. 8(c), the grains size was further refined to 15 nm in (Fe– Pt)90B10 film. In Fig. 8(d), the FePt grains were aggregated. To identify the FeB grains distribution in FePt film, the dark field images of (Fe–Pt)100 xBx (x ¼ 30, 60) films were shown in Fig. 9. In Fig. 9(a) and (c), the very weak FeB (002) diffraction rings were selected and the FeB grains may appear in bright areas (marked by arrows) in Fig. 9(b) and (d). Due to the minor FeB phase, the FePt spots may also include in formation the dark field images. In (Fe–Pt)100 xBx (x¼0, 5, 10) films, the grain size of FePt refined with more B was reduced from 22 nm to 15 nm. In (FePt)40B60 films, appeared soft magnetic FeB phase was evidenced in FeB (002) peak in XRD and soft magnetic hysteresis loops. The FePt and FeB grains were aggregated and the grains size was not defined. Fig. 10 shows plane-view TEM images of (Fe–Pt)95(ByAg1 y)5 (y¼1, 0.9, 0.4, 0) films. In Fig. 10(a), the FePt grains with an average grains size of 20 nm are manifest and the amorphous B resides in the grain boundaries. Fig. 10(b) shows TEM image of (Fe–Pt)95(B0.9Ag0.1)5. The FePt grains are isolated uniformly with thin (B, Ag) grain-boundary layer and the average grains size is calculated as 6.7 nm. Fig. 10(c) shows TEM image of (Fe– Pt)95(B0.4Ag0.6)5. The FePt grains are not separated uniformly and some grains are interconnected. The average grains size is around 12 nm. Fig. 10(d) shows TEM image of (Fe–Pt)95Ag5 film. The over-grown FePt grains become interconnecting due to highly
10
J.-L. Tsai et al. / Journal of Magnetism and Magnetic Materials 329 (2013) 6–13
Fig. 6. In-plane and out-of-plane magnetic hysteresis loops of (FePt)95(ByAg1 y)5 y ¼(a) 1.0, (b) 0.9, (c) 0.7, (d) 0.4, (e) 0.3, (f) 0 films annealed at 800 1C.
diffusive Ag which is abundant in this case. In short, uniform and small FePt grains can be achievable by (B, Ag) co-doping. In Fig. 10(a-c), the grain size distributions estimated by s/Davg were 26%, 20% and 22%, respectively. The finest grains size and narrow grains size distribution was observed in (Fe– Pt)95(B0.9Ag0.1)5 film. The FePt grains were segregated by B that was used to prevent grain growth and grain coarsening at high annealing temperature. In (Fe–Pt)95(B0.9Ag0.1)5 film, the minor Ag doping in multilayer was used to promote chemical ordering. In Fig. 10(c-d), more Ag was added and segregated B was diluted. Finally, the FePt grains were over growth and interconnected. In Fig. 10(c-d), the Ag volume content that comes from capping layer and multilayer was too high and precipitate out of the FePtB on top of the grains [15]. The FePtB microstructure was not influenced significantly by minor Ag doping. Magnetostatic coupling is essentially determined by plane magnetization (Mrt) [23]. The values of plane magnetization (Mrt) were listed in Table 2. The Mrt values in Fe–Pt, (Fe– Pt)95B5 and (Fe–Pt)90B10 films are much higher than (Fe– Pt)100 xBx (x¼30, 40, 60) films. The high Mrt value means large magnetostatic coupling and small grains exchange coupling. From TEM images shown in Fig. 8, the FePt grains in (Fe–Pt)100 xBx (x¼5, 10) films are well separated and aggregated in (Fe– Pt)100 xBx (x¼30, 40, 60) films due to the amount of soft magnetic FeB. The Mrt values and magnetostatic coupling were reduced when Ag were increased in (Fe–Pt)95 (ByAg1 y)5 (y ¼1, 0.9, 0.7, 0.4, 0.3, 0) films. In Fig. 10, the FePt grains were isolated
in (Fe–Pt)95 (ByAg1 y)5 (y¼1, 0.9, 0.4) films but aggregated in (FePt)95Ag5 film. The Wohlfarth relation and Kelly–Henkel plot (dM plot) were used to characterize and classify the hysteresis phenomena, especially the inter-grain magnetic interaction. The dM plot can be determined from the equation [24] dM¼Md(H) [1 2Mr(H)]. Where Md(H) is the normalized dc-demagnetization(DCD) remanence as a function of the reversal field, and Mr(H) is the normalized isothermal remanence(IRM) curve. From the Kelly– Henkel plot, a positive dM indicates that intergrain interactions are ferromagnetic exchange interactions. A negative dM value suggests that intergrain interactions are dipolar interactions (magnetostatic coupling). Fig. 11 shows the dM plots of a FePt single layer, and (Fe–Pt)100 xBx x¼5, 10, 30 films. The Ag layer with a thickness of 1 nm was capped on FePt single layer and FePt/B multilayer before annealing. The FePt single layer and (Fe– Pt)95B5 film show negative dM value at all applied fields. The magnetostatic coupling was dominated between FePt grains. In (Fe–Pt)90B10 film, the dM value was changed from negative to positive and back to negative from low- to high-applied fields. The (Fe–Pt)70B30 film show positive dM values which means a strong intergrain exchange interaction between FePt and soft magnetic FeB grains. The reduction of exchange interaction between FePt grains was achieved by capped Ag layer and B segregant with certain volume fraction. For FePtB film, the doping of B did not reduce the kinetic ordering temperature [25]. But other researchers use B which
J.-L. Tsai et al. / Journal of Magnetism and Magnetic Materials 329 (2013) 6–13
Fig. 7. TEM images and SAD patterns of the as-deposited [FePt(1 nm)/B(t nm)]10 t¼ (a) 0, (b) 0.05, (c) 0.1, (d) 0.6 films.
Fig. 8. TEM images, grains size distribution and SAD patterns of (a) FePt single layer and (Fe–Pt)100 xBx x ¼ (b) 5, (c) 10, and (d) 60 films.
11
12
J.-L. Tsai et al. / Journal of Magnetism and Magnetic Materials 329 (2013) 6–13
Fig. 9. TEM bright (B) and dark (D) field images of (Fe–Pt)100 xBx films (a) B: x ¼ 30, (b) D: x¼ 30, (c) B: x ¼60, (d) D: x¼ 60.
Fig. 10. TEM images, grains size distribution and SAD patterns of (FePt)95(ByAg1 y)5 y ¼(a) 1.0, (b) 0.9, (c) 0.4, (d) 0 films.
J.-L. Tsai et al. / Journal of Magnetism and Magnetic Materials 329 (2013) 6–13
Table 2 The Mrt values of Fe–Pt, (Fe–Pt)100 xBx and (Fe–Pt)95(ByAg1 y)5 films. (Fe–Pt)100 xBx
Mrt (memu/cm2)
(Fe–Pt)95(ByAg1 y)5
Mrt (memu/cm2)
(Fe–Pt) (Fe–Pt)95B5 (Fe–Pt)90B10 (Fe–Pt)70B30 (Fe–Pt)60B40 (Fe–Pt)40B60
0.52 0.63 0.58 0.38 0.15 0.04
(Fe–Pt)95B5 (Fe–Pt)95(B0.9Ag0.1)5 (Fe–Pt)95(B0.7Ag0.3)5 (Fe–Pt)95(B0.4Ag0.6)5 (Fe–Pt)95(B0.3Ag0.7)5 (Fe–Pt)95Ag5
0.63 0.61 0.46 0.53 0.41 0.21
13
4. Conclusions Multilayers [FePt(1 nm)/B(t nm)]10 (t¼0.05–0.6) and [FePt(1 nm)/ (ByAg1 y) (0.1 nm)]10 (y¼0–1) were prepared by controlled magnetron-sputtering. Granular films were obtained after a rapid thermal process at 800 1C for 3 min. By adding B into FePt film, the ordering was increased slightly and amorphous B was segregated at the boundaries of FePt grains and limited the grain coarsening in (Fe– Pt)100 xBx (x¼5, 10) granular films. Furthermore, by adding Ag into (Fe–Pt)95B5 film, the ordering degree and (001) texture was slightly improved in (Fe–Pt)95(B0.9Ag0.1)5 film. Granular (Fe0.48Pt0.52)95 (B0.9Ag0.1)5 film shows high perpendicular magnetization and small grains size (6.7 nm).
Acknowledgment The authors would like to thank the NSC for financial support under grant number NSC100-2221-E-005–044-MY2. We also thank the Center of Nanoscience and Nanotechnology in NCHU for the TEM investigation. References
Fig. 11. dM plots of FePt single layer, and (Fe–Pt)100 xBx (x ¼5, 10, 30) films.
increases coercivity due to reduced ordering temperature [26,27]. However, the coercivity of film depends not only on the film composition but also on its microstructure such as grains size and texture [26]. In this study, by adding B into FePt film, ordering degree just increased slightly. Ternary (Fe–Pt)95B5 and (Fe– Pt)90B10 films were proved to have high perpendicular magnetization with granular FePt structures. The (Fe–Pt)95B5 composition was used in the study with partial B replaced by Ag. An interstitial element combined with a relatively low melting element Ag which has a high mobility and diffusivity, such as co-doped (B, Ag) elements, can well separate FePt grains uniformly even at high annealed temperatures. This is analogous to a recent study using (C, Ag) [15]. In our previous work, Ag capped- and underlayer demonstrated to improve perpendicular magnetization but tend to facilitate aggregation of FePt grains leading to interconnect film structure [28]. In our previous work, we claim that Ag capping layer was used to promote ordering or reduced the ordering temperature [28]. The ordering promotion of FePt film can be explained more detail. Rapid ordering transformation and phase transformation strain are key factors to obtain (0 0 1) textured FePt films [29]. The rapid thermal annealing (RTA) process was used to have high heating rate. In this study, the heating rate was 10 1C/s and the Ag cap layer with thickness of 1 nm was required to enhance the ordering transformation strain. When the heating rate was increased to 40 1C/s, the (001) textured FePt film can be formed without Ag capping layer. This work, the minor addition of Ag in FePt film has been proved to increase the ordering degree slightly and improve perpendicular anisotropy of FePt films, for example, in (FePt)95(B0.9Ag0.1)5 film.
[1] D. Weller, A. Moser, L. Folks, M.E. Best, W. Lee, M.F. Toney, et al., IEEE Transactions on Magnetics 36 (2000) 10. [2] T. Suzuki, Z. Zhang, A.K. Singh, J. Yin, A. Perumal, H. Osawa, IEEE Transactions on Magnetics 41 (2005) 555. [3] J.S. Chen, J.F. Hu, B.C. Lim, Y.K. Lim, B. Liu, G.M. Chow, G. Ju, Journal of Applied Physics 103 (2008) 07F517. [4] M.H. Hong, K. Hono, M. Watanabe, Journal of Applied Physics 84 (1998) 4403. [5] M.L. Yan, N. Powers, D.J. Sellmyer, Journal of Applied Physics 93 (2003) 8292. [6] Y. Xu, J.S. Chen, J.P. Wang, Applied Physics Letters 80 (2002) 3325. [7] A. Yu, Dobin, H.J. Richter, Applied Physics Letters 89 (2006) 062512. [8] D. Suess, Applied Physics Letters 89 (2006) 113105. [9] R.H. Victora, X. Shen, IEEE Transactions on Magnetics 41 (2005) 537. [10] F. Casoli, F. Albertini, L. Nasi, S. Fabbrici, R. Cabassi, F. Bolzoni, C. Bocchi, Applied Physics Letters 92 (2008) 142506. [11] J.L. Tsai, H.T. Tzeng, G.B. Lin, Applied Physics Letters 96 (2010) 032505. [12] D. Goll, A. Breitling, Applied Physics Letters 94 (2009) 052502. [13] J.W. Kim, H.S. Song, J.W. Jeong, K.D. Lee, J.W. Sohn, T. Shima, S.C. Shin, Applied Physics Letters 98 (2011) 092509. [14] M.H. Kryder, E.C. Gage, T.W. McDaniel, W.A. Challener, R.E. Rottmayer, G. Ju, Y.T. Hsia, M.F. Erden, IEEE Transactions on Magnetics 96 (2008) 1810. [15] L. Zhang, Y.K. Takahashi, A. Perumal, K. Hono, Journal of Magnetism and Magnetic Materials 322 (2010) 2658. [16] D.C. Berry, K. Barmak, Journal of Applied Physics 102 (2007) 024912. [17] T. Ichitsubo, S. Tojo, T. Uchihara, E. Matsubara, A. Fujita, K. Takahashi, K. Watanabe, Physical Review B 77 (2008) 094114. [18] G.R. Trichy, D. Chakraborti, J. Narayan, J.T. Prater, Applied Physics Letters 92 (2008) 102504. [19] B.H. Li, C. Feng, X. Gao, J. Teng, G.H. Yu, X. Xing, Z.Y. Liu, Applied Physics Letters 91 (2007) 152502. [20] F.J. Yang, H. Wang, H.B. Wang, X. Cao, C.P. Yang, Q. Li, M.J. Zhou, Y.M. Chong, W.J. Zhang, Journal of Applied Physics 102 (2007) 106101. [21] N. Kaushik, P. Sharma, H. Kimura, A. Inoue, A. Makino, Journal of Applied Physics 103 (2008) 07E121. [22] S.D. Granz, M.H. Kryder, Journal of Magnetism and Magnetic Materials 324 (2012) 287. [23] S.N. Piramanayagam, Journal of Applied Physics 102 (2007) 011301. [24] G. Bertotti (Ed.), Academic Press, 1998, pp. 249–250. [25] B. Wang, K. Barmak, T.J. Klemmer, Journal of Applied Physics 109 (2011) 07B739. [26] K. Nishimura, K. Takahashi, H. Uchida, M. Inoue, Journal of Magnetism and Magnetic Materials 272 (2004) 2189. [27] Y.M. Lee, B.S. Lee, C.G. Lee, B.H. Koo, Y. Shimada, Journal of Magnetism and Magnetic Materials 310 (2004) e918. [28] J.L. Tsai, H.T. Tzeng, G.B. Lin, B.F. Liu, Journal of Alloys and Compounds 487 (2009) 18. [29] J.S. Kim, Y.M. Koo, B.J. Lee, S.R. Lee, Journal of Applied Physics 99 (2006) 053906.