Ag multilayers

~

Journalof magnetism

jR

and

magnetic 44PI~ materials ELSEVIER

Journal of Magnetismand Magnetic Materials 152 (1996) 27-32

Magnetic anisotropy of sputteredNi/Ag multilayers J . W . F e n g *, S.S. K a n g , F . M . P a n , G.J. Jin, M . L u , A. H u , S.S. J i a n g , D. F e n g National Laboratory of Solid State Microstructures and Center for Advanced Studies in Science and Technology of Microstructures, Nanjing University, Nanjing 210093, China

Received 31 January 1995; revised 25 May 1995

Abstract We have studied the magnetic properties and structure of Ni/Ag multilayers, fabricated by the dc magnetron sputtering method. The in-plane uniaxial anisotropy, which is closely related to the sample preparation, has been detected by both vibrating magnetometer (VSM) and ferromagnetic resonance (FMR) measurements where the FMR spectra were obtained as a function of the applied magnetic field orientation in the film plane. The variation of the resonance field with the angle of the applied magnetic field can be well explained by a theoretical model including the in-plane uniaxial anisotropy up to the second-order term. Moreover, both in-plane and perpendicular anisotropy parameters have been deduced by a data-fitting analysis and a negative interface anisotropy constant has been determined.

1. Introduction In the last few years, interest in the magnetic anisotropy of ultrathin metallic films and multilayers has increased rapidly since the discovery of the large uniaxial magnetic anisotropy with the easy axis perpendicular to the film plane in some of these thin films [1]. This property makes them potential candidates for magnetic and magneto-optic recording media. Several possible mechanisms, such as magnetocrystalline anisotropy, shape anisotropy, magnetostriction, and the reduced symmetry of the structure at the interfaces, have been proposed to explain the perpendicular anisotropy, but its exact origin is not yet clear. In previous studies of anisotropy, interest was focused on the out-of-plane anisotropy, and only little effort was devoted to the investigation of the in-plane behaviour. It is well known that ferromagnetic resonance

* Corresponding author. Fax: + 86-25-3300535.

(FMR) is one of the most powerful techniques for providing information on the interlayer coupling [2] and the magnetic anisotropy of thin films [3] (especially the interface-induced and magnetocrystalline anisotropies). The interface anisotropy of Ni (111) thin films in contact with various metals, such as Cu, Pd and Mo [4,5], was investigated by FMR and magnetic measurements, and the interface anisotropy energy constant K S was found to be negative, in contrast with Nrel's prediction. In this paper, the in-plane uniaxial and perpendicular anisotropies for sputtered N i / A g multilayers have been studied using the FMR and VSM methods.

2. Experimental All the multilayers studied here were deposited on glass substrates by the dc magnetron sputtering technique. High-purity Ni and Ag target materials were fixed onto two vertical targets, and the substrates

0304-8853/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0304-8853(95)00445-9

28

J.W. Feng et al. / Journal of Magnetism and Magnetic Materials 152 (1996) 27 32

were placed on a rotating sample holder. The samples with different modulation wavelengths could be prepared by controlling the target power and the speed of rotation of the sample holder. The background pressure was 1.5 × 10 - 6 Tort, and the deposition was carried out at an Ar pressure of 7.5 × 10 -3 Tort with the substrate temperature about 350 K. Before growing the multilayer, a 600 A Ag buffer layer was sputtered. The sputtering rates of Ni and Ag for all N i / A g multilayers were kept constant, at 8 and 10 A/s, respectively. The thicknesses of the Ni and Ag layers were controlled by the exposure time. Finally, a 200 A Ag protective layer was deposited. The multilayers under investigation were designated [Ni( x ) / A g ( y)] X 60, where the Ni layer thickness x = 20-50 ,~, and the Ag layer thickness y = 14-40 X. Just after preparation the N i / A g samples were studied using an X-ray diffractometer with a 12 kW Rigaku rotating anode X-ray source (a Cu anode in the high brilliance 0.2 × 2 mm 2 spot mode) and a symmetric graphite (002) monochromator. The magnetic measurements were performed using the VSM at room temperature. The FMR measurement was carried out with an electronic paramagnetic resonance spectrometer (ER-200D SRC model) operating at a frequency of 9.78 GHz. The FMR spectra were obtained as a function of the orientation of the applied dc field in the film plane at room temperature. All the specimens used in the above measurements had the same area, 2 × 3 mm 2. Assuming that the multilayers have in-plane uniaxial anisotropy and perpendicular anisotropy, then the total free energy density is given by the expression

azimuthal angles of the magnetization M with respect to the film normal and to the in-plane easy axis, respectively. 05H is the azimuthal angle between the applied magnetic field H and in-plane easy axis. The resonance field n r e s c a n be calculated by the general equation derived by Smit et al. [6] O)

M 2sin20[00 2 0052

~ ]

,

(2)

where y is the gyromagnetic ratio and o) is the microwave frequency. Assuming that both the applied field H and the magnetization M are in the film plane, the following equation is obtained

:

[H cos ( 05n -

05) + A + HI I cos205+ H~I cos405]

× [/-/cos (05. - 05)

cos (205)

+ H~L(cos405- 3 cos205 sin205)],

(3)

where A = 4-rrM- 2 K ' / M ; HI I = 2KIJ/M and = 4K / 4. For a given 05H, by minimizing the free energy (1), the static equilibrium of magnetization M is determined by the expression H sin( 05n - 05) = HI hcos 05 sin 05 + H~ cos305sin 05.

(4) From the angle-dependent FMR measurement in the film plane, information on both out-of-plane and in-plane anisotropies can be obtained.

3. Results and discussions

3.1. Structure

E = - H M sin 0 cos (05r! - qS) - 2'rrM 2 sin20 + K ± sin20 - KII sin20 cos205 - K~ sin40 cos405.

(1) The first three terms are the Zeeman energy, the demagnetization energy and the perpendicular uniaxial anisotropy energy, respectively. The last two terms are the in-plane uniaxial anisotropy energy. K " is the perpendicular anisotropy constant. KII and K~ are the first- and second-order in-plane anisotropy constants. 0 and 05 are the polar and

The X-ray diffraction (XRD) pattern of the 0-2 0 scan on the Ni(20 ~,)/Ag(14 A) multilayer is shown in Fig. 1. In the high-angle region, there are three main peaks at 20 = 38.16 °, 39.20 ° and 44.22 °, resulting from the reflections of the buffer layer fcc Ag (111), fcc Ag (111) and fcc Ni (111) planes in the multilayer, respectively. On each side of the Ag (1 1 1) reflection from the multilayer there is a satellite peak, which is evidence that coherent stacking exists in the multilayer. If coherent stacking did not

J.W. Feng et al./Journal of Magnetism and Magnetic Materials 152 (1996) 27-32

4

35 20 ( deg. )

40

Fig. 1. X-ray diffraction pattern of the Ni(20 ~.)/Ag(14 ,~) multilayer in the low- and high-angle regions, z~, v and e represent the reflections of buffer Ag (111), Ag (11 l) and Ni(111) within the multilayer.

exist, these high-angle satellite reflections would be absent [7]. However, these high-angle satellite peaks can not be clearly observed with increasing Ni or Ag layer thickness. This result indicates that the coherent stacking can not be maintained for the larger modulation wavelengths of Ni/Ag multilayers because of the large lattice mismatch (r/= 0.15) between the Ni and Ag. Since no other diffraction peaks were found in the high-angle region, it can be deduced that the N i / A g sample was preferentially grown with the Ag (111) and Ni (111) textures along the growth direction. In the low-angle region, two peaks can be observed. Together with the satellite peaks in the high-angle region, this fact shows that the multilayer is of periodic structure. The period deduced from the low-angle Bragg peaks and the high-angle satellite peak is in good agreement with the designed one. The (111) peak position of the buffer Ag is similar to that of bulk Ag, but the (111) peak positions of Ni and Ag within the Ni(20 ,&)/Ag(14 ,~) multilayer shift slightly towards the low- and highangle directions compared with those expected for bulk samples. The (111) interplanar spacing of Ag within the multilayer is 2.7% smaller than that of bulk Ag. This is due to the difference between the lattice constants of Ni and Ag. The interplanar spacing of Ni (111) is 1.4% larger than that of bulk Ni. This value decreases slightly with increasing Ni layer thickness with a fixed Ag layer thickness (14 ,~). So

29

the stress of Ni layers along the film normal direction is tensile. Due to the rotation of the substrates, on average the adatoms had an in-plane component and a perpendicular one of the relative velocity when they were incident on the substrates. This case is somewhat similar to that of obliquely deposited films [8], and one might expect that the mean [111] texture axis would incline from the film normal towards the film plane. In order to investigate this effect, 0-scans were performed around two different axes parallel and perpendicular to the in-plane component of the relative velocity, while 2 0 was fixed at 39.20 ° (corresponding to the (111) reflection of Ag within the multilayer). However, in these two scans there was no significant difference between the diffraction positions and the full widths at half maximum (FWHM). This indicates that the rotation of the substrates does not obviously affect the quality of (111) texture and the mean [111] axis is along the film normal. 3.2. VSM measurements

Fig. 2(a,b) show typical hysteresis loops for Ni(50 A)/Ag(14 ,~), with the in-plane magnetic field applied parallel and perpendicular to the easy axis, respectively. It is clear that the hysteresis loops in Fig. 2(a,b) express the character of the in-plane uniaxial anisotropy. However, this in-plane uniaxial anisotropy is not revealed only by the loop in Fig. 2(a), where the saturation is reached gradually. This behaviour was also observed by Heinrich et al. [2] in C o / C u / C o and can be explained by the presence of inhomogeneous interlayer antiferromagnetic exchange coupling. For a multilayer with thicker Ag layers, the interlayer exchange coupling is negligible, so that a square loop along easy axis, characteristic of uniaxial anisotropy, may be expected. Fig. 2(c,d) show the hysteresis loops of Ni(30 ~,)/Ag(40 A) along the in-plane easy and hard axes, respectively. As expected, a square loop along the easy axis is obtained (see Fig. 2c). The magnetization of Ni was determined by the VSM with the applied magnetic field in the film plane. The result illustrates that the magnetization increases from 410 to 460 G with increasing Ni layer thickness. These values are about 5-15% smaller than the bulk values.

30

J.W. Feng et al. / Journal of Magnetism and Magnetic Materials 152 (1996) 27-32

f

~0

-800

0

ers, this anisotropy is about two times stronger than that in the N i / A g multilayers. The in-plane uniaxial anisotropy may be induced by intrinsic stress via magnetostriction. Due to the sample rotation during deposition, the stress distributions in the directions parallel and perpendicular to the rotational axis may be different since the incident adatoms had an inplane component of the relative velocity which is perpendicular to the rotational axis. Moreover, bulk Co has a larger magnetostriction constant than bulk Ni, and thus more intense in-plane anisotropy is expected.

800

~0

3.3. Ferromagnetic resonance 1

I

I

-800

0 H(G)

800

--- 0

(c)

-1

-4~0 -260

2&

4~0

200

400

(d) -400

-200

0 H(G)

Fig. 2. (a,b) In-plane hysteresis loops for the Ni(50 A)/Ag(14 ~k) multilayer with the dc field applied parallel and perpendicular to the easy axis, respectively. (c,d) In-plane hysteresis loops for the Ni(30 /~)/Ag(40 ,~) multilayer with the dc field applied parallel and perpendicular to the easy axis, respectively.

Magnetic hysteresis loop measurements carried out in the film plane reveal that all the sputtered N i / A g mulfilayers possess in-plane uniaxial anisotropy, with the hard and easy axes perpendicular and parallel to the rotational axis of the substrates during deposition, respectively. The in-plane uniaxial anisotropy has also been observed in our sputtered C o / A g multilayers [9]. But in the C o / A g multilay-

Fig. 3 shows the two typical FMR spectra of the Ni(50 .~)/Ag(14 ,~) multilayer, with the external magnetic field along the in-plane easy and hard axes. The differences between the resonance fields are obvious. Fig. 4 shows the dependence of the resonance field on the angle of the applied magnetic field in the film plane for Ni(50 A)/Ag(14 ,~) multilayer. In Fig. 4, a twofold symmetric distribution of the resonance with the angle can clearly be seen. This implies the existence of the in-plane uniaxial anisotropy. From Eqs. (3) and (4), the parameters A = 2187 Oe, HII = 158 Oe and H~L= 50 Oe are obtained by matching the data of resonance field versus angle, similar to A u / C o / A u [10] and F e / C u [11]. The calculated result (solid curve) is shown in Fig. 4. The experimental result is in good agreement with the calculation, indicating that the present model is reasonable and the second-order term in anisotropy

-g

i 1000

2000

3000

H(G)

Fig. 3. FMR signal for the Ni(50 ,~.)/Ag(14 ~ ) multilayer with the de field applied (a) along the easy axis (~bH = 0°), and (b) along the in-plane hard axis (~bH = 90°).

J. W. Feng et al. / Journal of Magnetism and Magnetic Materials 152 (1996) 27-32 2400

,

,

6000

,

31

[

~

(.9

4000

Q) I1

0 2200 E0

r~

200C

-gb

6

0.00

9'0

0.04

0.08

1/x ( X )

Angle ( deg. ) Fig. 4. Resonance field versus angle in the film plane for the Ni(50 A)/Ag(14 ,~) multilayer. The solid line is a fit obtained with A = 2187 Oe, HII = 158 Oe and HiI = 50 Oe.

Fig. 5. Variation of (4-rrM, - A) versus 1 / x for Ni/Ag multilayers with a fixed Ag layer thickness of 14 ,~.

i.e.

must be considered. Some of the results of this investigation are summarized in Table 1. It can be seen from the table that for the multilayers with a fixed A g layer thickness ( y = 14 ,~), the parameter A, characteristic of internal field along the film normal, decreases with increasing x. This behaviour can be explained in terms of the negative interface anisotropy. According to previous studies [12,13], a surface (or interface) magnetic anisotropy of multilayers can be deduced through the dependence of the perpendicular anisotropy on the thickness of the magnetic layer, if the interface anisotropy essentially enhances the first-order anisotropy. Then the perpendicular anisotropy field (excluding the demagnetization term) can be written as (5)

H " = I4 v + 2 H J x ,

Table 1 Values of the parameter A and the in-plane anisotropy fields HII and H~I (in Oe) for the Ni/Ag multilayers Samples

A

HII

H~I

Ni(50/~)/Ag(14 A) Ni(40 A)/Ag(14 ,~) Ni(30 ,~,)/Ag(14 ,~) Ni(20 ,~)/Ag(14 ,~)

2187 2295 2753 3679

158 127 162 75

50 0 0 - 30

4'rrM s - A = H v + 2 H s / X .

(6)

The perpendicular anisotropy field H J- includes two terms, a homogeneous volume anisotropy H v = 2 K v / M s, and a surface-induced one H s = 2 K J M s. Fig. 5 shows a plot of (4-rrM s - A) as a function of 1 / x for the multilayers with different Ni layer thicknesses ( x = 20, 30, 40 and 50 .~) and a fixed Ag layer thickness ( y = 14 A). A linear variation of (4-rrM s - A) versus 1 / x is shown. From the slope of the straight line, the value of the interface anisotropy constant K s is deduced to be K s = 0.06 e r g / c m 2, where M s = 460 G is used. Also, by extrapolating the straight line to 1 / x = 0, we obtained H u = 4700 Oe ( K u = 1.1 X 106 e r g / c m 3 ) . Here, K s is a negative value, which means that the interface anisotropy confines the magnetization to the film plane. This value corresponds to an interface anisotropy that is lower in magnitude than the one ( K s = - 0 . 2 e r g / c m z) determined experimentally for Ni films coated with metal Cu, Pd or Re [14]. The negative K s value for N i / A g multilayers may arise from the cancellation between the N6el interface anisotropy and the misfit dislocation anisotropy at the interface through magnetostriction [1]. Furthermore, the anisotropy energy Ku (excluding the demagnetization energy) is much larger than the magnetocrystalline anisotropy energy of cubic Ni. Assuming that a possible contribution to K , arises from the effect of stress through magnetostriction, and taking a typical

32

J.W. Feng et al. /Journal of Magnetism and Magnetic Materials 152 (1996) 27-32

magnetostriction c o n s t a n t All 1 = -- 24 × 10 - 6 , K u = 1.I X 106 e r g / c m 3 would correspond to a 1.5% strain. The estimated strain is of the same order of magnitude as the strain of 1.4% determined by XRD. For N i / A g multilayers with Ag layers of different thicknesses and the same Ni thickness (x = 30 ,~), the FMR peak-to-peak linewidths A H, which arise mainly from both the spatial fluctuations of [111] axes and the spread of the internal fields, were measured with the applied magnetostatic field parallel to the in-plane easy axis. The experimental results show that A H increases with increasing Ag thickness y: A H = 275 G for y = 14.~ and A H = 415 G for y = 40 A. The variation of A H with the Ag layer thickness is caused by the effect of interlayer magnetic exchange coupling across the Ag layer. Since the adjacent Ni layers are coupled together through the above interaction, and the intensity of this coupling decreases rapidly with increasing Ag layer thickness, the spatial fluctuations of the magnetization become larger, which makes A H increase.

4. Conclusions The in-plane and perpendicular anisotropies of sputtered N i / A g (111) multilayers have been studied by in-plane FMR and VSM measurements. In the FMR measurement, resonance field data as a function of the field orientation in the film plane demonstrate that the multilayers possess in-plane uniaxial anisotropy which can not be fully described by using only the first-order term; the second-order term must

also be considered. Moreover, the interface-induced anisotropy has also been revealed by FMR.

Acknowledgements This work is supported by the National Natural Science Foundation of China and the Provincial Natural Science Foundation of Jiangsu.

References [1] F.J.A. den Broeder, W. Hoving and P.J.H. Bloemen, J. Magn. Magn. Mater. 93 (1991) 562. [2] B. Heinrich, J.F. Nichran, M. Kowalewski, J. Kirschner, Z. Celinski, A.S. Arrott and K. Myrtle, Phys. Rev. B 44 (1991) 9348. [3] U. Gradmann, J. Magn. Magn. Mater. 6 (1977) 173. [4] U. Gradmann, J. Magn. Magn. Mater. 54-57 (1986) 733. [5] M.J. Pechan, J. Appl. Phys. 64 (1988) 5754. [6] J. Smit and H.C. Beliers, Philips Res. Rep. 10 (1955) 113. [7] S.S. Jiang, J. Zou, D.J.H. Cockayne, A. Sikorski, A. Hu and R.W. Peng, Phys. Stat. Solidi (a) 130 (1992) 373. [8] K. Hara, K. Itoh and M. Kamiya, J. Magn. Magn. Mater. 102 (1991) 247. [9] J.W. Feng, S.S. Kang, F.M. Pan, G.J. Jin, M. Lu, A. Hu, S.S. Jiang and D. Feng, J. Appl. Phys. 78 (4) (1995). [10] C. Chappert, K. Le Dang and P. Beauvillain, Phys. Rev. B 44 (1992) 3193. [11] S.M. Rezende, J.A.S. Moura, F.M. de Aguiar and W.H. Schreiner, Phys. Rev. B 49 (1994) 15105. [12] G.T. Rado, Phys. Rev. B 26 (1982) 295. [13] U. Gradmann, J. Magn. Magn. Mater. 54-57 (1986) 733. [14] U. Gradmann, R. Bergholz and E. Bergter, IEEE Trans. Magn. 20 (1984) 1940.