High Anisotropy L1 0 FePt Media for Perpendicular Magnetic Recording Applications

Transcription

1 DISKCON 2008 High Anisotropy L1 0 FePt Media for Perpendicular Magnetic Recording Applications Boon Chow LIM Agency for Science Technology & Research (A*STAR) DATA STORAGE INSTITUTE 5 Engineering Drive 1, (off Kent Ridge Crescent, NUS), Singapore

2 Acknowledgements Dr. Jingsheng CHEN, Dept of Materials Science & Engineering, National University of Singapore Team Members: Dr. Yinfeng DING Dr. Jiangfeng HU Dr. Wei Chuan GOH Mr. Wai Lwin PHYOE Mr. Yuan Kwang LIM 11

3 Areal Density Growth Chart (Adapted from Roger Wood, Intermag 06, CA-01) 22

4 Bit Cell Dimensions at Demonstrated Densities 33

5 Superparamagnetic Limits Higher thermal stability high K u materials 44

6 Why choose FePt? Alloy System Material K u (10 7 erg/cc) M s (emu/cc) H k (koe) T c (K) D p (nm) CoCr 20 Pt Co-alloys Co Co 3 Pt FePd L1 0 Phases FePt CoPt MnAl Rare-earth transition metals (RE-TM) Fe 14 Nd 2 B SmCo K u (10 7 erg/cc) 10 1 Co 5 Sm FePt Fe 14 Nd 2 B CoPt Co/Pt FePd MnAl Co/Pd Co 3 Pt CoPt 3 CoCrPt 1Tbits/in Tbits/in 2 CoCr 20 Pt D P (nm) 55

7 FePt Phase Diagram 2200 K K 1792K (δ-fe) 1667K (γ-fe,pt) or γ) fcc 1573K 1623K 2042K K 1043K fct (FePt) (FePt 3 ) T T c 823K (α-fe) (Fe 3 Pt) 973K T c (ordered) T c (γ) (Fe 3 Pt) (α-fe) Fe at% Pt 66

8 FePt Atomic Structure Face-centered cubic (fcc) a average Fe or Pt atom a a Face-centered tetragonal (fct) Body-centered tetragonal (bct) Bravais Lattice [001] c c c [010] [100] c < a a Fe a Pt a 2 a 2 77

9 Lattice Mismatch Strain y film x substrate a f c f a f < a f c > f c f a f c f a s a s a s a s Before deposition After deposition 88

10 Heteroepitaxial relationship of FePt magnetic layer with Cr underlayer easy axis fcc a c nm fct a nm [001] a nm a nm c/a = bcc Cr [010] [100] a a nm a [110] fct-fept (001)[100] CrX (002)[110] Misfit of lattice constant between FePt (001)[100] and Cr (002)[110] is 5.8% Strain from misfit helps expand a-axis and shrink c-axis, and thus L1 0 FePt (001) texture was obtained at lower deposition temperature According to Vegard s Law, the lattice constant is related to the atomic radius of element X 99

11 Challenges for FePt films as recording media for 1 Tbits/in 2 and beyond Controlling the texture of FePt (001) for perpendicular recording since FePt (111) is preferably formed as it is the closest packed plane Narrow easy axis distribution (< 5 ) Reducing the phase transformation temperature of FePt from fcc to L1 0 FePt Control of microstructure to grain size of < 5 nm grain size with 15% standard deviation. To develop a suitable soft underlayer for FePt media 10

12 Research teams on the control of FePt texture and phase transformation temperature Prof. D.J. Sellmyer, University of Nebraska, USA, high temperature postdeposition annealing of Fe/Pt multilayers (above 550 C to break the multilayers by diffusion to form L1 0 FePt (001) films) Dr. T. Maeda, Toshiba Corporation, doped with Cu followed by postdeposition anneal to reduce the temperature (300 C), but the texture is FePt (111) oriented Dr. Toshio Suzuki, Akita Research Institute of Advanced Technology, MgO underlayer on glass, FePt(001) texture at 375 C. Prof. D.E. Laughlin, Carnegie Mellon University, USA, Ag underlayer on Si, FePt (001) texture is not good and temperature is 375 C Prof.Takao Suzuki, Toyota Technology University, FePt/MgO multilayer by high temperature annealing (600 C) Others mainly focus on magnetic properties of FePt (001) on MgO (001) single crystal substrate or post-deposition anneal FePt on glass substrate without too concerned with the texture (fundamental research) 11

13 Experimental Details All samples deposited using custom built dc/rf magnetron sputtering system and deposition conditions as follows: Base pressure: 5x10-8 Torr Characterisation tools: X-ray diffractometer, vibrating sample magnetometer, FEG-HRTEM 300kV FePt mag. layer Buffer layer CrX underlayer Corning glass substrate Typical film structure Custom designed dc/rf sputter system 12

14 Growth of FePt on MgO single crystal substrate with different intermediate layer Sample Intermediate layer a 1 (Å) (on MgO) a 2 (Å) (on glass) Epitaxial relationship ε (%) A Pt MgO(100)<001> Pt(100)<001> FePt(001)<100> 2.23 B Cr MgO(100)<001> Cr(100)<110> FePt(001)<100> 5.88 C Cr 95 Mo MgO (100)<001> Cr(100)<110> FePt(001)<100> 6.33 D Cr 90 Mo MgO(100)<001> Cr(100)<110> FePt(001)<100> 6.89 E MgO MgO(100)<001> FePt(001)<100> 8.86 Notes: a 1 : Lattice constant of intermediate layer (grown on MgO single crystal substrate) a 2 : Lattice constant of intermediate layer (grown on glass substrate) ε: Lattice mismatch 13

15 Lattice Mismatch Effect A critical lattice mismatch near to 6.33% was believed to be most suitable to improve the chemical ordering of FePt films (I 001 /I 002 ) 1/2 was used to represent the chemical ordering parameter in this study. I 001 and I 002 are the integrated peak intensities of the FePt (001) and (002) peaks respectively 14

16 Intensity (arb. units) FePt (001) Low Temperature Growth of FePt on CrRu FePt (002) + (200) CrRu (200) FePt (003) (e) (d) (c) (b) Momentum transfer, q (Å -1 ) (a) Magnetisation (emu/cc) Magnetisation (emu/cc) out-of-plane in-plane (a) Applied Field (koe) out-of-plane in-plane 150 C 350 C (c) Applied Field (koe) Magnetisation (emu/cc) K u (x10 7 erg/cc) out-of-plane in-plane (b) Applied Field (koe) H c K u 200 C Deposition temperature ( C) Hc (Oe) FePt (20 nm) Pt (2 nm) CrRu (30nm) Perpendicular anisotropic FePt with high K u and was successfully fabricated at temperature below 350 C, far below the 600 C phase transformation temperature to obtain L1 0 phase of FePt by other methods Glass substrate 15

17 Initial Layer Growth in FePt Perpendicular Media In perpendicular recording media, it has long been discovered that an initial growth layer or also known as nucleation layer or dead layer of low M s and low K u Formed in the magnetic layer during the early film growth stage regardless of whether on top of an underlayer or on substrates. The formation of the initial growth layer was attributed to the poor easy axis orientation in the initial stage of the film growth The initial growth layer resulted in the decrease of measured H c and S of the magnetic film Solution: Introduce buffer layer The purpose of the buffer layer (immiscible with underlayer and recording layer) is to block diffusion from the underlayer and promotes heteroepitaxial growth of the L1 0 phase 2 nm Pt buffer was introduced between CrRu underlayer and FePt magnetic layer 16

18 Effect of Pt Buffer Layer on Crystallographic Properties FePt (001) Pt (200) FePt (002) + (200) 20 nm CrRu (200) (d) FePt (001) FePt (002) + (200) 20 nm CrRu (200) (d) Intensity (arb. units) 10 nm 5 nm 3 nm (c) (b) (a) Intensity (arb. units) 10 nm 5 nm 3 nm (c) (b) (a) Momentum transfer, q (Å -1 ) FePt on 2 nm Pt buffer layer Momentum transfer, q (Å -1 ) FePt on CrRu underlayer 17

19 Hysteresis Loops of FePt on Pt buffer layer and CrRu underlayer Magnetisation (emu/cc) Magnetisation (emu/cc) out-of-plane in-plane Applied Field (koe) out-of-plane in-plane Applied Field (koe) (a) (c) Magnetisation (emu/cc) Magnetisation (emu/cc) out-of-plane in-plane Applied Field (koe) out-of-plane in-plane (b) (d) Applied Field (koe) Magnetisation (emu/cc) Magnetisation (emu/cc) out-of-plane in-plane (a) Applied Field (koe) out-of-plane in-plane Applied Field (koe) (c) Magnetisation (emu/cc) Magnetisation (emu/cc) out-of-plane in-plane Applied Field (koe) out-of-plane in-plane (b) Applied Field (koe) (d) FePt on 2 nm Pt buffer layer FePt on CrRu underlayer 18

20 Evaluation of t initial by zero M s Method Segment, n FePt film thickness from interface (nm), bottom up M s(n), on CrRu underlayer M s(n), on Pt buffer layer emu/cc 732 emu/cc emu/cc 602 emu/cc emu/cc 599 emu/cc emu/cc 481 emu/cc M s x t FePt (x10 3 emu/cc x nm) without Pt buffer layer t initial = 1.4 nm (a) t FePt (nm) M s x t FePt (x10 3 emu/cc x nm) 16 with Pt buffer layer t initial = 0.8 nm (b) t FePt (nm) 19

21 Evaluation of t initial by zero K u Method x t FePt (10 8 erg/cc x nm) K u effective N d =0 N d =4π K grain u = 2.75 x 10 7 erg/cc t initial = 3.16nm N d = 4π K grain = 2.36 x 10 7 erg/cc u t initial = 3.20nm N d = t FePt (nm) It is very difficult to accurately determine the demagnetisation factor of the film 20

22 Change of M s and K u over FePt film thickness Bulk FePt region Low M s region Pt buffer layer CrRu underlayer Low K u region Magnetisation (emu/cc) out-of-plane in-plane 2 nm FePt on Pt buffer layer is soft magnetic Applied Field (koe) 21

23 FePt 5 nm Cr 90 Ru 10 22

24 FePt Pt 23

25 New intermediate layer? 1. The interface between FePt and Pt is shaper than that between FePt and CrRu. 2. The initial growth layer thickness estimated by K u model is larger than that estimated by M s, suggesting that there were structural defects existing at the interface which resulted in small anisotropy. But M s does not change due to the structure defect 3. In order to further reduce the initial growth layer thickness, new intermediate layer with suitable crystallographic structure, more difficult to diffuse, is required Solution: MgO was proposed as the buffer layer since sputter deposited MgO formed the (200) texture due to the lowest surface energy plane, and the MgO (200) texture can heteroepitaxially induce the fct-fept (001) texture. 24

26 Advantages of using MgO as buffer layer: Immiscible with FePt recording layer and CrRu underlayer Lattice relationship: MgO (100)<001> FePt (001)<100>, mismatch of 8.66%. Lattice strain maintained Motivation: Advantages of MgO Buffer Layer To study interfacial effects of MgO layer on texture and magnetic properties of FePt as FePt fabrication is carried out at elevated temperature. Interface roughness is expected to change with temperature. a a = nm a Mg 2+ O 2- a 25

27 HRTEM image of 2 nm MgO deposited at 50 C MgO layer appeared continuous MgO-FePt interface also well-defined MgO (200) and FePt (001) planes are parallel to the film plane 26

28 HRTEM image of 2 nm MgO deposited at 350 C Island-like growth of MgO layer, rough interface FePt (111) and (001) oriented grains identified, defect 27

29 M a g n e t ic p r o p e r t ie s ( M g O d e p o s it e d a t 5 0 C ) (a ) (a ) s h o w s o u t-o f-p la n e a n d in -p la n e lo o p s o f F e P t film w h e n M g O is 4 n m S h o w s p e rp e n d ic u la r a n is o tro p y a n d in c re a s e s w ith in c re a s in g M g O th ic k n e s s 28

30 M a g n e t ic p r o p e r t ie s ( M g O d e p o s it e d a t C ) Is o tro p ic m a g n e tiz a tio n w h e n M g O is 1 n m S h o w s p e rp e n d ic u la r a n is o tro p y w ith in c re a s in g M g O th ic k n e s s, fro m 4 n m H c h ig h e r th a n w h e n M g O d e p o s ite d a t 5 0 C, d u e to d e fe c ts 29

31 Summary of effect of MgO deposition temperature and thickness on film properties MgO layer 50 C <4nm 4nm Continuous Continuous 350 C <4nm 4nm Island-like Continuous MgO-FePt Interface Mostly smooth Smooth Rough Rough and Smooth Dominant FePt texture (001) (001) (111) (001) Anisotropy Perpendicular Perpendicular Isotropic Perpendicular 30

32 Reduction of Initial Growth Layer Thickness by MgO Buffer Layer 7.0 M s x t FePt (x10 3 emu/cc x nm) t initial = 0.6 nm t FePt (nm) 31

33 XRD spectra of 12 nm FePt film deposited at various temperatures 400 C 350 C 300 C 280 C Dominant FePt (001) orientation even at 280 C FePt (001) integrated peak intensity increased with deposition temperature Slight shift of broad peak (45-50 ) to higher angle increase in chemical ordering 32

34 Magnetic properties of 12 nm FePt film deposited at various temperatures 280 C 300 C 350 C 400 C Out-of-plane anisotropic even at 280 C Significant increase in H c with increasing temperature to 400 C 33

35 Plan View TEM Images of FePt Doped with Carbon 1.0 out-of-plane Nomalized Kerr Applied Field (koe) 10 vol.% C Normalized Kerr Applied field (koe) 20 vol.% C 34 4

36 5 nm 7.5 nm 10 nm 15 nm 5 nm 20 nm 30 nm 20 nm TEM images of 5 to 30 nm thick FePt:C films (15 vol.% C) FePt:C grain growth observation: It confirms the proposed two-step growth model. 10 nm 35 35

37 Microstructure of FePt+C on Pt buffer layer C C C C C Continuous FePt 5nm FePt Pt layer fcc-fept C fct-fept C fcc-fept fcc-fept fct-fept CrRu glass Continuous FePt layer Pt CrRu Model proposed 15nm FePt C FePt C FePt Glass/CrRu/Pt Glass/CrRu/Pt 36

38 Thin film growth modes Layer by layer growth due to almost similar surface energy of Pt layer and FePt layer (2.9 J/m 2 ) How about oxide underlayer? MgO (1.1 J/m 2 ) Interface interaction between oxide and metal should be higher than between that of metal and metal 37

39 Microstructure of FePt+ x vol%c on MgO buffer layer pure 20% 15% 25% 38

40 Columnar structure of FePt+ x vol%c on MgO buffer layer 2-layer microstructure not favorable for recording application. Adatom mobility of Fe and Pt on substrate were reduced by increasing deposition rate and formed small grain size. Well-isolated FePt-C grains of size about 7.5 nm formed on MgO buffer layer. 1 J.S. Chen et al., Appl. Phys. Lett. 91, (2007) 2 J.S. Chen et al., Appl. Phys. Lett. 90, , (2007) 39

41 Summary Lattice strain induced ordering aided in the reduction of FePt ordering temperature to 350 C and below Buffer layer based on oxide material (MgO) played an important role in lowering the deposition temperature while still keeping chemical ordering high Columnar structure of C doped FePt successfully fabricated 40