VNU JOURNAL OF SCIENCE. Mathem atics - Physics. T.xx, NC3AP, 2004
IN F L U E N C E O F A N N EA LIN G C O N D ITIO N ON T H E S T R U C T U R E AND M A G N ETIC P R O P E R T IE S O F NdFeB T H IN F IL M S
L u u T u a n T a i 1,2, T h a n D uc H i e n 1, T a V an K h o a 1, L e T u a n T ù ' ' International T raining Institute for M aterials Science (ITIM S)- H U T
1 D epartm ent o f Physics, College o f Science, V N U
A b s tra c t: In flu e n ce o f a n n e a lin g con ditio n on th e s tru c tu re a n d m ag ne tic pro p e rtie s o f N d F e B m a g n e tic th in film s ha s been inve stiga ted . T h e re su lts have sho w n th a t N d F e B film s, w h ich w e re pre pa re d at coo ling su b s tra te (T s = 30°C), ha ve s o ft m a g n e tic pro p e rtie s (He = 5 0 0 e ). A fte r c rysta lliza tio n by an n e a lin g at 7 0 0 °c fo r 2 0 m in in A r g a s w ith d iffe re n t rate o f te m p e ra tu re (3 0 0 °c /m in an d 6 0 °c /m in ), ha rd m a g n e tic pro p e rtie s o f th e film s h ave b e en ob se rve d . H ow e ver, s tru c tu re an d m a g n e tic p ro p e rtie s o f th e film s w ith h ig h e r ra te o f a n ne alin g con ditio n (3 0 0 °c /m in ) w e re better. O n th e o th e r hand, p e rp e n d ic u la r an iso tro p y pro p e rtie s o f th e film s h a v e b e en ob serve d.
1. I n t r o d u c t i o n
Application of h ard m agnetic film s in M icroElectroM echnical System (MEMS) has been stu d ie d recently. T hese devices are prepared from relatively thick film s (1 * 5jim) and high energy product of the ra re e a rth m agnets prom ises sm aller device size. The largest m a rk e t lb)- h ard m agnetic film is in th e m agnetic d ata storage on com puter h ard disk. In o rder to exceed densities of 100Gbit/iir, m aterials with a h ig h er unaxial m agnetic anisotropy fire in tere ste d . The high perpendicular m agnetic anisotropy of Nd,Fe,.,B thin films ((4.6 10"erg/cma) pre sen ts th e additional possibility of park in g b its closer together using perpendicular recording [1,2].
There a re m any technical p aram eters (for exam ple elem ental com ponents, s p u tte re d power, s u b s tra te tem p eratu re, annealing condition ...) strongly influence on the s tru c tu re and m agnetic properties of NdFeB thin films. In th is paper, influence of a n n e alin g condition w as studied.
2. E x p e r i m e n t
N dFeB films w ere sp u tte re d onto Si (100, 111) wafer a t nom inal room tem perature.
The s p u tte r gas p re ssu re w as 5 10:! m bar for all layers in th e film stru ctu re . A 20 nm Mo buffer layer an d protective layer w ere used for all NdFeB th in films. A ta rg e t was ingot N dF eB alloy with composition of N dwFe61Bfl. The film composition w as determ ined by EDX.
The fillm thickness w as set to approxim ately 200 nm. After depositing, the sam ples were annealled a t 700"c for 20 rnin u n d er hea tin g ra te of 60"C/min an d 300°c/m in in th e applied field o f 0,7T. M agnetic properties of the film s w ere ev a lu ated by vibrating sam ple m agnetom eter. T he surface im age of th e films w as observed by SEM an d AFM.
1 1 6 L u u T u a n T ai, T h a n D u c H ie n , T a V a n K h o a , L e T u a n T u
3. R e s u lts a n d d is c u s s io n
Influence of h ea tin g ra te on m icrostructure an d m agnetic p ro perties of th e film s has been studied. Fig. 1 show s SEM im ages of NdFeB th in film s a s deposited (a) a n d afte r annealed (b, c). I t show s th a t th e surface of deposited film w ere sm oothly. However, afte r annealed a t 700°c for 20 m in w ith different hea tin g ra te s, th e cry sta l p artic le s w ere clearly observed. F or th e film s w ith h ig h er heating ra te (300°c/m in), m ost of th e crystal particles a re th e sam e size of 100 nm (fig.l b). C ontrarily, for th e lower h e a tin g ra te films (60°c/m in,), th e grain size is different each other, some o f them w ere over 300 nm d iam ete r (fig- 1c)
a) As deposit b, Hi’atinfi rate o f c) H eating rate o f
.W O O C'm m f i o n c .m w
F ig .l. SEM images of NdFeB films as deposit (a), after annealed at 700uc for 20 min (b, c)
AFM im age of th e film after annealed at 700°c for 20 m in w ith h ea tin g ra te of 300°c/min w as showed in fig 2. It corresponds to th e SEM im age in fig lb.
M agnetic properties of NdFeB film s have been investigated by vib ratio n sam ple m agnetom eter.
Fig. 3 shows h y steric loop of deposited film (a), after annealed a t 700°c for 20 m in w ith different hea tin g
ra te 0>, c). I t shows th a t a s deposited film w ere soft FigÆ AFM image oi thc rJm after m agnetic film s (Hc=50 Oe). A fter annealed, h a rd annealed at 7000C for 20 min with m agnetic property h a s been observed, coercivity of heating rate of 3000C/min th e film is th e sam e in both directions
(perpendicular an d p arallel to th e film): He// = H c l
= 1650 Oe an d He// = H c l = 9 0 0 0 e for the sam ple of 300°c/m in an d 60°c/m in h e a tin g rate, respectively.
In th is study, sam ples w ere first annealed a t 700°c for 20 m in w ith h ea tin g ra te of 300°c/m in an d second annealed a t 500°c for 10 m in th en cooling to room tem p e ra tu re in applied field (Ha) of 0.7T. The applied fields w ere both p erp endicular (Hax) and parallel (H;V/) directions to th e films. T he re su lts of He an d M r w ere showed in tab. 1
In flu e n c e o f a n n e a l i n g c o n d itio n on... 1 1 7
a )
Fig.3. Hysteric loops of deposited film (a), after annealed at 700nc for 20 min
with different heating rates
(b,300°c/min; C,60°c/min).
\ r ~
c )
T ab 1. V alues of He and M r of the films with and w ithout field cooling
ha=0 t H,U=0,7T H V/=0,7T
He II 1650 Oe 1320 Oe 1690 Oe
H c l 1650 Oe 1760 Oe 1580 Oe
Mr// 6200 G auss 5800 G auss 6600 G auss
M r l 5300 G auss 5700 G auss 4900 G auss
Tab. 1 show s th a t th e sam ple of perpendicular applied field (Hai); perpendicular coercivity H c l w as increased an d parallel coercivity HeII was decreased. For th e sam ple of parallel applied field H^/J H c lw a s decreased an d He// w as increased. R em anent m agnetization (Mr) w as th e sam e effective a s coercivity.
4. C o n c lu s io n
For th e N dFeB film s w ith high hea tin g ra te (300°c/min), the grains are absolutely the sam e size of lOOnm. T h at cause good m agnetic properties H c=16500e and M r = 6200Gauss.
The coercivity an d re m a n en t m agnetization of th e films, which w ere cooled in applied field, show th a t a r e increased following th e direction of the applied field.
R e fe re n c e s
1. N. N akanishi, M. Ueda, T. Okada, N. Mizutani, Jour. Magn. Magn. Mater., 197(1999), 295- 296.
2. S.N. Piram anayahgam . M. Masumoto, A. Morisaco, s . Takei, Journal o f Alloys and Compounds, 281(1998), 27-31.
