ELSEVIER Journal of Magnetism and Magnetic Materials 168 (1997) 35-42

~ Journal of mm~tlneUsm magnetic .~H materials

Giant magnetoresistance and microstructure of Co-Cu and FeCo-Cu granular films

S h i - H u i G e a'*, Y i n g - Y a n g Li.i a, Z o n g - Z h i Z h a n g a, C h e n g - X i a n L ? , T a o X u b,

J i a - Z h e n g Z h a o b

a Magnetics Laboratory Lanzhou University, State Education Commission, Lanzhou 730 001, China b Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730 000, China

Received 6 September 1996

Abstract

Co-Cu and FeCo-Cu granular films were prepared by DC (or RF) magnetron sputtering and post-deposition annealing. The influence of annealing conditions (temperature, duration) on GMR has been investigated systematically. It is found that the cumulative isothermal annealing at an optimal temperature (moderately high) promotes the precipitation of magnetic grains but limits their excessive growth, and therefore, leads to more homogenous and smaller particles, which is in favor of the GMR improvement. By a combined study of TEM, magnetization and GMR, it is believed that the GMR in these granular films mainly originates from the spin-dependent scattering of conducting electrons in the surfaces between magnetic particles and Cu matrix; especially, the very fine particles with a diameter of a few nm may be most effective for spin-dependent scattering.

Keywords." Giant magnetoresistance; Microstructure; Thin films - granular; Annealing

1. Introduction

The giant magnetoresistance (GMR) in multi- layer systems [1] and in granular films [2, 3] has stimulated a great deal of research activities due to both its fundamental scientific interest and its po- tential application for magnetic recording and magnetic sensors. Granular films, being much easier to prepare, are especially attractive from the

*Corresponding author. E-mail: gesh@spin.lzu.edu.cn; fax: + 86 931 841 5941.

point of view of application. It is evident [4-6] that spin-dependent scattering of conducting electrons at the interfaces between magnetic grains and non- magnetic matrix plays a dominant role in GMR of granular systems. Therefore, G MR is closely re- lated to the features of magnetic granules such as the size, shape and distribution which, to a great extent, depend on the post-deposition annealing procedure. Thus, the choice of post-deposition an- nealing conditions becomes quite crucial to obtain the optimal granular feature and therefore the GMR. But up to now, this problem has not been investigated in detail and is far from completely

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36 Shi-Hui Ge et al. / Journal of Magnetism and Magnetic Materials" 168 (1997) 35-42

understood. So far, most of the experimental stud- ies on post-deposition annealing have focused on the influence of annealing temperature, little atten- tion has been paid to the effects of annealing dura- tion. For example, in the annealing procedure adopted by most studies, samples are annealed in incremental temperatures for a same relatively lon- ger duration such as 10, 30 min, and so on. This annealing is believed to promote grain growth or phase segregation, but at higher temperature it may lead to excessive grain growth which is not in favor of the improvement of GMR. In the present study, we adopted another annealing procedure - cumu- lative annealing at a constant temperature (moder- ately high) and studied how this procedure affects the microstructure and GMR in a different way from the usual procedure. The study is also moti- vated by the fact that the correlation between GMR and annealing condition via microstructure is quite important not only in application but also in the understanding of the underlying mechanism of GMR.

2. Experimental procedure

ature and some at 4.2 K. The annealing for as- deposited films was carried out in a vacuum fur- nace with pressure of 1 x 10-3 Pa. Two kinds of annealing procedures were adopted: (1) isochro- nous annealing (ICA): the sample was annealed in consequently step-increasing temperatures (Ta), staying at each temperature for the same duration, e.g. 10 min in this study. (2) cumulative isothermal annealing (ITA): the sample was annealed at a con- stant temperature (moderately high), e.g. 753 K in this study, for different durations (ta). Unlike usual ITA, the annealing duration in this study is an accumulated duration of consequent annealing steps; in each step, the sample was annealed at 753 K for a very short duration, for example, 3 min, and the total annealing duration is determined by the number of repeated annealing steps. The opti- mal temperature is chosen here by referring to the maximum of GMR versus Ta curve obtained from ICA. In practice, the sample was put into the fur- nace which had been preheated up to the designed temperature and was annealed for a designed dura- tion, then quickly cooled to room temperature for measurement, followed by quick heating again for the next step of annealing.

Co Cu and FeCo-Cu granular films were pre- pared using magnetron sputtering from a com- posite target onto water-cooled glass substrates at an Ar pressure of 0.8 Pa. An automatic pressure controller is used to keep the argon pressure con- stant during the sputtering process for insuring an invariable deposition rate. Both DC and RF powers were used. The composition was adjusted by the number of Co or FeCo chips placed on the Cu disk and was determined finally by energy dis- persive analysis after deposition. X-ray diffraction (XRD) and transmission electron microscope (TEM) were employed to determine the structure of the samples. Magnetic properties were measured by vibrating sample magnetometer (VSM) with ap- plied field up to 2.0 kOe. Magnetoresistance was measured using the conventional four-terminal technique with magnetic field up to 1.2 T both parallel and perpendicular to the film plane. The current of around 1 mA is in the film plane and perpendicular to the magnetic field. Most of the measurements were performed at room temper-

3. Results and discussion

3.1. FeCo-Cu granular films

Fig. 1 shows the X-ray diffraction pattern of as- deposited (Feo.2Coo.8)lsfu82 sample; five diffrac- tion peaks are clearly observed which are identified with the (1 1 1), (2 0 0), (2 2 0), (3 1 1), (2 2 2) diffrac- tions o f F C C Cu. The intensity of Cu(1 1 1) diffrac- tion is much higher than that of grain random orientation, indicating the (1 1 1) texture for this film. The absence of CoFe diffractions was at- tributed to the very small size of CoFe granules, which can be observed only when high-resolution TEM was used.

The dependence of G MR on magnetic field H at room temperature is shown in Fig. 2 for (Feo.2Co0.8)lsCu82 sample in two kinds of anneal- ing conditions. The GMR reported here is refer- enced to the resistance at zero field and is defined as GMR = AR/R(O) = JR(H) - R(O)]/R(O), where

Shi-Hui Ge et al. / Journal o f Magnetism and Magnetic Materials 168 (1997) 35-42 37

r , o

100

80

60

40

20

0

' Cu(I'I 1) '

_J

I

4O

cu(2oo) k ~

"---Jk,__ Cu(220) Cu(311) Cu(222)

i I i I i I i I i I i I

50 60 70 80 90 100

2 Theta (deg.)

Fig. 1. X-ray diffraction pattern using Cu radiation (2 = 1.5407A) for as-deposited (Feo.2Coo.8h8Cus2 film.

1

0

-1

-2

-3

-4

-5 -15

I I I I I

• after ICA

, ,~ , . , * after ITA

r 4 , - , r J ' - J " ~/~ " ~ "--....= .~" n

a D n

n "u n = .,m"

o "n.

I I I I I -10 -5 0 5 10

H (KOe)

15

Fig. 2. GMR ratio versus H at room temperature for (Feo.zCoo.sh 8Cu82 samples under two kinds of annealing condi- tions.

R(H) and R(0) denote the resistances at zero field and the field H, respectively, and AR represents the neat change in resistance. It is seen that even in the highest magnetic field available (1.2 T), the two G M R - H curves are far from saturated; therefore, the G M R values given here are underestimated considerably, but are still big enough for a relative comparison. For the sample after ICA, the GMR is - 2 . 4 % at 1.2 T, while for the sample after |TA,

GMR reaches - 4.1% at the same field, and the

3.0

2.5

~" 2.0

1.5

1.0

0.5

(a)

0.7

0.6

0.5

0.4

0.3

0.2

0.1

Co)

I I I I I I

300 400 500 600 700 800

T A (K)

i i ! i i i

R(O) R

i . i • i , | . i . J .

300 400 500 600 700 800

T A (K)

900

8O

7O

6O

5 0 ~

40 ,--

30

20

10

0 900

Fig. 3. Dependence of GMR (a), AR and R(0) (bj on annealing temperature T, for (Feo.2Coo.s)lsCu82 sample in ICA treat- ment.

saturation field of the latter is higher than that of the former as judged by the trend of G M R - H curve in higher field. But in the same magnetic field, ITA sample exhibits larger G MR field sensitivity which is defined as G M R / H and more useful for applica- tions. The behavior is also observed on other sam- ples with different Cu composition. This result implies that the G MR behaviors under the two annealing conditions are different. The G MR ver- sus H curves measured in the magnetic field per- pendicular to the film plane are the same as those in parallel field, indicating the isotropic characteristic of GMR.

The dependence of G MR on annealing temper- ature Ta in ICA is shown in Fig. 3a for the sample (Feo.2Coo.shsCu82. With the increase of Ta, G MR

38 Shi-Hui Ge et al. / Journal of Magnetism and Magnetic Materials 168 (1997) 35-42

increases very slowly in the lower Ta region then rapidly when T a > 550K, and decreases after reaching a maximum of - 2.4% at Ta = 773 K. Fig. 3b presents the variations of R(0) and AR with Ta. Both AR and R(0) decrease with T~ but R(0) drops more quickly than AR, and this gives rise to the trend of GMR versus T, in Fig. 3a. This behav- ior is similar to the result reported by Wang et al. [73. It is widely accepted [4, 8] that in granular films, spin-dependent scattering of conducting elec- trons in grain surfaces (which can also be under- stood as the interfaces between magnetic grains and nonmagnetic Cu matrix) is the main origin of GMR; therefore, the ratio of the grain surface area to its volume (called specific surface) is closely re- lated to GMR. Thus, the Ta dependence of GMR in Fig. 3 can be explained as follows: on the one hand, with increase in Ta, the FeCo grains in Cu matrix grew and decreased the specific surface of grains and therefore AR, on the other hand, the increasing annealing temperature relaxed the mismatch stress between film and substrate, alleviated disorder, re- duced defects, and as a result, the resistivity R(0) decreases more quickly than AR has, which leads to the increase of GMR. This structure of evolution has been confirmed by TEM, as will be seen below.

In contrast, the dependence of GMR on anneal- ing duration tA in ITA shows different behavior from that in ICA. As can be seen in Fig. 4b that with the increase of tA, AR increases abruptly at the first step then rises very slowly, while R(0) decreases moderately after dropping rapidly at the first step; this gives the GMR versus tA curve in Fig. 4a in which GMR increases quickly at the first step by following a moderate increase and reaches - 4.1% at tA = 30 min. An obvious feature different from that in ICA is that at the first annealing step (753 K × 3 min) both AR and R(0) vary abruptly, beyond this step, both change slowly. Based on the discussion of the paragraph above, it is reasonable to suggest that the precipitation of FeCo phase from Cu matrix occurs mainly at the first step of annealing; the large number of new fine grains increase the specific surface area of grains, which leads to the increase of AR, while the relief of defects, stress and structure disorder brings about the abrupt dropping of R(0), both are in favor of GMR. The consequent annealing only serves as the

4

~ 3

1

i i i i

0 -10 (a)

I I I I

0 10 20 30 t A ( m i l l )

40

0.8

0.7

0.6

0.5

0.4

0.3

0.2 -10 (b)

; ~ " "n i(0) '

I I I I

0 l0 20 30 t^ (min)

8O

70

60

50

40 "-" ,¢

3

20

10 40

Fig. 4. Dependence of GMR (a), AR and R(0) (b) on annealing duration tA for (Feo.2Coo.s)~8Cu82 sample in ITA treatment.

continuous improvement of G MR by following the same mechanism but with much smaller rate. There is no great possibility of making the grains grow excessively in this annealing condition because of the very short annealing duration for each step. Of course, it is possible that some grains existing in the film may grow in the consequent annealing steps, which will decrease the specific surface, but it is also possible that some new small particles may precipi- tate out, which compensates and even exceeds this decrease of specific surface as evidenced by the slowly increasing trend of AR-tA curve. As a result, the grain size in ITA is smaller than that in ICA, which may be responsible for the larger GMR.

The structure of evolution mentioned above has been confirmed by TEM. Fig. 5 shows the bright- field images for (Feo.2Coo.s)lsCus2 sample after ICA and ITA treatments. It is seen that for the ICA

Shi-Hui Ge et al. / Journal of Magnetism and Magnetic Materials 168 (1997) 35-42 39

Fig. 5. TEM micrographs for (Feo.zCo0.s)lsCu82 sample in ICA (a) and ITA (b) states.

film, the grain size distribution covers a wide range from 10 to 30 nm, and the average grain size is of the order of 18 nm. For the ITA sample, however, the grain size decreases to the order of 8 nm or less and shows a more homogenous feature. This result demonstrates that ITA leads to the existence of plenty of fine FeCo particles which would contrib- ute to the improvements of GMR; the fine particles also give rise to the difficulty in saturation, which leads to a larger saturation field. These results are consistent with the experimental observation given in Fig. 2.

3.2. Co-Cu 9ranular films

Based on the investigation on FeCo-Cu system, we adopted ITA treatment in the study of Co-Cu system, concentrating on the relations among the microstructure, magnetic properties and GMR.

The Co composition dependence of GMR at room temperature under a magnetic field up to 1.2 T is shown in Fig. 6 which exhibits a maximum of - 1.7% at x = 20 for as-deposited samples, and the maximum shifts to x = 25 with an increased value of - 4 . 4 % after ITA. Fig. 7 displays the typical G M R versus H curve at 4.2 K under a mag- netic field up to 6 T for the sample with x = 25 in ITA. It can be seen that G M R approaches satura- tion in a magnetic field of about 3.5 T and reaches - 7 . 8 % . In comparison to the GMR value of

FeCo-Cu sample with the same Cu composition, it

5

4

3

2

1

0

-1 0

i i , i i ! i

o as-deposited • after ITA ~

I I I I I I I

5 10 15 20 25 30 35

X (at % Co)

40

Fig. 6. Dependence of GMR on Co composition at RT for CoxCu,0o-x films in as-deposited and ITA states.

is clear that the Fe substitution for Co in Co-Cu granular films could not improve the G MR obvi- ously.

The dependence of G MR and corresponding AR and R(0) on annealing duration tA is shown in Fig. 8 for RF-sputtering film which exhibits behav- ior similar to that of FeCo-Cu film in ITA. The only difference from that of FeCo-Cu sample is that at the first step of annealing, AR increases only little for RF-sputtering Co-Cu sample, indicating that some grains may grow excessively, but the G MR still increases significantly due to the rapid

40 Shi-Hui Ge et al. / Journal o f Magnetism and Magnetic Materials 168 (1997) 35 42

d r o p p i n g of R(0). Beyond this step, both R(0) and AR change modera te ly , and the increased t rend of AR implies the occurrence of new fine par t ic les in this sample which is responsible for the increased G M R .

The above cons ide ra t ion has been conf i rmed by T E M observat ion . Fig. 9 shows the B F micro- g raphs of Co25Cuv5 samples made by RF-spu t t e r - ing at as -depos i ted (a) and in ITA (b) states. I t is seen that even in as -depos i ted state there are some grains with size of 1 0 - 5 0 n m , a m o n g them, the larger grains with m u l t i d o m a i n s t ructure and small

3.5

3.0

2.5

~ 2.0

1.5

1.0

0.5

(a)

i i i

I I I

0 10 20 I I

30 40 50 t A (min)

0 , •

-2

-4

-6

- 8 i I I I I I I [

-40 -20 0 20 40 H (KOe)

0.20 , 20

~" 0.15

• R ( 0 )

o A R

0.10 I t I I I 0

0 10 20 30 40 50 ( b ) t A ( m i l l )

15

10 ~"

5

Fig. 8. Dependence of GMR (a), AR and R(0) (b) on annealing duration tA for RF-sputtered Co25Cu75 sample.

Fig. 7. GMR versus H at 4.2 K for Co25Cu75 film in ITA.

Fig. 9. BF micrographs of Co2~Cu75 sample made by RF-sputtering at as-deposited (a) and ITA (b) states.

Shi-Hui Ge et al. / Journal of Magnetism and Magnetic Materials 168 (1997) 35-42 41

specific surface may be the source of the small G MR in this state. After ITA treatment, besides these grains existing in the as-deposited film which 0.0 grew up a little, there are a large number of new -0.5 small particles with size of 4-10 nm. From the cor- responding electronic diffraction spectra, these ~ - 1 . 0 grains in Cu matrix are confirmed to have FCC Co structure. A number of theoretical studies have ~ -1.5 pointed out that the size of single-domain Co par- ticles lies in the 11.4-32.0 nm range [9, 10] while the critical size of superparamagnetism for spheri- cal particles at room temperature is 14.0 nm for FCC Co and 4 .0nm for HCP Co [11]. Thus, the new fine particles that occur after ITA are believed to have single-domain or superparamagnetism fea- tures, which contribute to GMR.

Magnetization study results support the micro- structure and G M R behavior mentioned above. The correlation between G M R and magnetization M has been deduced by Zhang et al. [83 as

GMR - R(Hc) - R(H) _ A ( M ~ 2. R(Hc) \Ms}

It can be seen that G M R increases with M, ap- proaches saturation at H = Hs due to M = Ms and reaches zero at H = He as M = 0. Fig. 10 presents the G M R - H curve in ITA (a) and M-H curves in as-deposited (b) and ITA (c) states for Co25Cu75 sample. It is immediately found that: (1) The an- nealed sample has larger Hc than the as-deposited one, which can be attributed to the existence of single-domain particles in annealed samples. (2) G M R = 0 at H = 350 Oe, which is much larger than the Hc = 140 Oe given by the M-H curve; considering that the grain size has a wide distribu- :~ tion, and G M R is mainly from the smaller particles ~ _ (single-domain) which have larger He, while M mainly from the larger grains (multi-domain) which possess smaller H¢, this difference can be understood. (3) At H = 1.8 kOe the hysteresis loop has closed, namely, M approaches saturation, but G M R - H curve still varies steeply with H and is far from saturated. Such disagreement between the saturation fields for G M R - H and M-H curves has also been observed in other granular systems [12-14] and explained by spin-glass or super- paramagnetism characteristics of the magnetic enti- ties. In the present study, as obtained from TEM,

0.5 • , . , •

-2.0

-2.5

-3.0

-3.5 -15

(a)

80

60

40

20

0

-20

-40

-60

-80

/ i I s I i I i

-10 -5 0

H (KOe)

\ \ a,

' o

I i I i

5 10

a s - deposited

J -2.0 -1.5 -1.0 -0.5 0.0 0.5

(b) H (KOe)

50 II I I I I 4° l l I I / < after I rA 30

20

10

0

-10

-20

-30

-40

-50 -2.0

(c)

i i m l l

n U B B i n

n// i

-1.5 -1.0 -0.5 0.0

H(KOe)

1.0 1.5 2.0

15

0.5 1.0 1.5 2.0

Fig. 10. G M R - H curve in ITA (a) and M-H curves in as- deposited (b) and ITA (c) states for Co25Cu~5 sample.

42 Shi-Hui Ge et al. / Journal of Magnetism and Magnetic Materials 168 (1997) 35 42

the grain size distributes in a wide range 4-50 nm. Among them, small amount means larger grains which mainly contribute to the M and smaller H~, most of the particles are single-domain-like or superparamagnetic, which are the main origin of GMR. As well known, superparamagnetic particles only have very small contribution to M under not very high field, and therefore, have no big influence on the saturation field of M, but these fine particles may have a big contribution to GMR as pointed by Berkowitz [15] and Maeda [16] et al. Since the magnetic moments of superparamagnetic particles and the single domain particle surfaces align with the external field only in very high magnetic field, this leads to the difficulty of saturation for GMR. Thus, the discrepancy between the saturation fields for M H and GMR-H curves can be attributed to these very fine particles.

4. Conclusions

The GMR of granular CoxCulo0-:, and (FeCo)xCuloo-x films has been confirmed to de- pend on the composition of magnetic entities and the annealing procedure. The latter, to a great ex- tent, determines the microstructure of the films. The optimal Co composition is around 25% at. The cumulative isothermal annealing at a modest high- er temperature is confirmed to be the optimal an- nealing condition which produces a great number of fine and homogenous particles, and therefore, leads to the larger GMR due to the larger specific areas. This study evidences again that the GMR in granular films is mainly to be attributed to the spin-dependent scattering of conducting electrons in the interfaces between magnetic grains and non- magnetic matrix; especially, the very fine particles with a diameter of a few nanometer may be most effective for spin-dependent scattering.

Acknowledgements

This work was supported in part by the National Natural Science Foundation of China, the Natural Science Foundation of Gansu Province and the Magnetics Laboratory, Lanzhou University, State Education Commission.

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