6/nwe)

o d

o

o

~

r ~ ,... i o

NANOSCALE R/T MULTILAYERS 103

o l

E

b 240.

2 0 0 ' 1 6 0

120,

8 0 '

40' 0 - 4 0 '

- 8 0

0

I I I I

5 1 0 1 5 2 0

L a y e r t h i c k n e s s o f g e (A) 2

0 E

(,1

- 2

q) - - 4 . tO

0 --6 ""/ . m - 8

- 1 0 - 1 2

2 5

Fig. 20. Fe layer-thickness dependence of magnetization and measured anisotropy for 4.5 .~ Tb/X .~ Fe at 300 K (after Shan and Sellmyer 1990b).

7/k) samples are summarized in figs. 21, 22 and 23, respectively (Shen 1994). Since the individual layer thicknesses of both Tb and Fe layers are only about 2 atomic layers, it is reasonable to claim that the temperature dependent behavior exhibited in figs. 21-23 originates mainly from the strongly alloyed or interfaeial regions: (i) We have pointed out before that Tb magnetization is dominant at room temperature for this

5 0 0

0

4 0 0

& E

3 0 0

8 0 0 . . j , t . ~ •

,o0

"f:: ill ...

ooo ...

• . . . . o ' ~ . %%

. . ,~ %

. . . , 5.5 ".. ',. ",,

~ ~ - . , °%.%%

5 " ' - . . "':~.

2 0 0

0 i I i I t i i

0 50 100 150 2 0 0 2 5 0 300

T (K)

100

Fig. 21. Temperature dependence of magne- tization for Y/kTb/5 A Fe (Y=5, 5.5, 6, 6.5, and 7) (after Shen 1994).

104 5

Z.S. SHAN and D.J. SELLMYER

4

"0"3

%

1

0 0

I I

• • Y = 5

• 6 . 5 •

i I l I i I i I i I i

5o ~oo ~5o 2oo 2so ~oo

T(K)

Fig. 22. Temperature dependence of intrinsic anisotropy for YATb/5AFe (Y=5, 5.5, 6, 6.5, and 7) (after Shen 1994).

series of samples. As temperature decreases, the Tb magnetization increases because of the exchange interactions between Tb-Fe atoms and Tb-Tb atoms which implies that Tb atoms are getting further ordered magnetically with decreasing temperature. This

O

"1'- O 60

5O

4 0 , t

30 ,',

20

I = I i I ~ I = I

50 100 150 200 250

T (K)

, I

300 Fig. 23. Temperature dependence of coerciv- ity for Y]~Tb/5,~Fe (Y=5, 5.5, 6, 6.5, and 7) (after Shen 1994).

NANOSCALE R/T MULTILAYERS 105

behavior can be well interpreted with a mean-field model (Shan et al. 1990). (ii) The anisotropy Ku (Ku =K'u + 2JrMs 2) increases with decreasing temperature and the sample with thinner Tb layer (Y = 5 A) shows stronger temperature dependence. This behavior is also attributed to the enhancement of the Tb magnetization at lower temperature. (iii) As temperature decreases, the coercivity first increases slowly and then rapidly when the temperature is below -100 K. It is found from the magnetization reversal study that the dominant mechanism of coercivity is domain wall pinning rather than nucleation; because the thermal activation energy decreases with decreasing temperature a larger applied field is required to cause magnetization reversal (Kirby et al. 1994). In sect. 5.1.3 this behavior will be discussed in more detail.

3.2.4.

Interface anisotropy

In this subsection we discuss the experimental determination of interface anisotropy and show some typical results. For R/T multilayers, the R region is ordered magnetically at low temperature and from the energy viewpoint, the anisotropy energy per unit area can be written as

)~K' u = 2Ki(T) + 2Ki(R)

+ [Kv(T) + Kst(T) + Kde(T)] X + [Kv(R) + Kst(R) + Kde(R)] Y, (2) where ~. = X + Y is the bilayer thickness, X and Y are the T and R layer thicknesses, respectively. Ki, Kv, Kst, Kde, represent the interface, volume, stress and demagnetization anisotropy for R and T, respectively. This is a rather complicated expression; however, at room temperature the R interior region is disordered magnetically which causes the fourth term in square brackets in eq. (2) to vanish, and then this equation is often simplified

as

2,K' u = 2Ki + [Kv -

2~rMZlX, (3)

where Ki =Ki(T)+Ki(R) is the sum of the T interface anisotropy and induced interface anisotropy of R layers;

Kv=Kv(T)+Kst(T)

and K d e = 2 ~ M 2. Figure 24 shows an example of 3.K' u vs X for 8 A Dy/8 A Co at room temperature. This plot is helpful to discuss and understand the origin of PMA as follows: (i) It is often the case, for large X (X ~> 15 i, in this figure), that the

~K~(X)

data fall on a straight line with the slope and intercept equal to [ K v - 2 ~ M s 2] and 2Ki, respectively. (ii) The positive intercept implies that the interface anisotropy favors PMA which is the essential feature in multilayers, and the negative slope means that the demagnetization energy term dominates the Kv term, which may be positive at times. (iii) For small X (X ~< 12,~

in this case), there is a general tendency for a maximum (X_~ 6,~ in this case) to be seen in ).K' u and an approach to zero as X--~ 0. This is understandable because this region corresponds to thicknesses less than a few monolayers, where it makes no sense to consider a well-defined volume and interface anisotropy. Moreover, the demagnetization energy will be ill defined. That

).K'u(X )

goes to zero as X ~ 0 is

106 Z.S. SHAN and D.J. SELLMYER

reasonable because the structure approaches that of a homogeneous, isotropic disordered alloy or glass. (iv) The reason that ~,K~u maximum exists in the small X region (X'~ 6.~ in this case) is that the microstructure of such CMF is similar to the interfaces in the CMF with large X, which shows positive interface anisotropy as pointed out in (iii).

I I I 1 I

-".\,

o

I < -2 a~, oy/xA co

-3

- 4 ,,

0 I0 20 50 40 50

Loyer thickness of Co (A) 60

Fig. 24. The measured anisotropy K' multiplied by the bilayer-thickness 3. vs the Co layer-thickness for 8/k Dy/X A Co (after Shah and Sellmyer 1990b).

Figures 25 and 26 summarize the

3,K~u(X)

for Dy/Co and Nd/Fe CMFs. In both of these figures, the slopes of

3,K~u(X)

are negative and the intercepts are positive. Perera, O'Shea and Fert have prepared DyNi/Mo (Perera et al. 1991) and R/Mo (R = Dy, Er) (Perera and O'Shea 1991) multilayers and found similar ~,K~ curves with negative slopes and positive intercepts as shown in fig. 27 for Er/Mo. This means that their PMA originates from the interface anisotropy. It is the single-ion anisotropy of R atoms and the anisotropic distribution of R and T atoms in the interface region which create the PMA.

Equation (3) can be rewritten as

K' u = [Kv - 2arM 2 ] X~_ + ---~-'2Ki (4)

Ertl et al. (1992) employed this formula for Tb/Co multilayers with equal Tb and Co layer thickness (i.e. 2X = Z) and eq. (4) becomes

KIv Ki

X'u = 5 - + /cv = [ x v - (5)

Therefore the curve

of K'(1/X)

should be a straight line for small (I/X) (i.e. for large X) and bent for large (l/X) (i.e. small X). The result is shown in fig. 28. It is seen clearly

NANOSCALE R/T MULTILAYERS 107

2.0

1.O E"

E -. O

•

Y

-< -1.O

-2.0

-5.0

0

1 [

D y / C o