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Evidence for Antiferromagnetic Coupling in Ag/Ni Superlattices: a Neutron Scattering Study

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1991 Europhys. Lett. 15 503

(http://iopscience.iop.org/0295-5075/15/5/006)

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EUROPHYSICS LETTERS

Europhys. Lett., 15 (5), pp. 503-507 (1991)

1 July 1991

Evidence for Antiferromagnetic Coupling in Ag/Ni Superlattices: a Neutron Scattering Study.

B. RODMACQ (*), PH. MANGIN (**) and CHR. VETTIER (***) (***) (*) Laboratoire de Mbtallurgie Physique, DFRMC-SPh Centre d’Etudes Nucldaires de Grenoble - B P 85X, 38041 Genoble Cbdex, France (**) Laboratoire de Physique d u Solide, Universitd de Nancy I B P 239, 54506 Vandoeuvre Cbdex, France (***I European Synchrotron Research Facility B P 220, 38043 Grenoble Cddex, France (***) Institut Laue Langevin - B P 156X, 38042 Grenoble Cbdex, France

(received 28 February 1991; accepted in final form 16 April 1991)

PACS. 61.12E - Neutron scattering techniques (inc. small-angle scattering). PACS. 68.65 - Layer structures, intercalation compounds and superlattices, growth, struc-

PACS. 75.50R - Magnetism in interface structures (inc. layer and superlattice structures). ture and nonelectronic properties.

Abstract. - Low-angle neutron scattering experiments in reflection geometry have been performed on Ag/Ni superlattices prepared by sputtering. The low-temperature diffraction diagram shows the appearance of a supplementary low-angle peak attributed to the doubling of the chemical period, as a consequence of the antiparallel alignment of the magnetic moments in successive nickel layers. The variation of the intensity of this antiferromagnetic peak with applied magnetic field is in accordance with a classical model of antiferromagnetism in which the magnetic moments, at first perpendicular to the field, rotate progressively to the direction of the field. A good agreement is found with magnetization and magnetoresistance results obtained on the same samples.

Magnetic coupling in metallic multilayers and superlattices is a subject of current interest, and an increasing number of systems has been investigated in the last few years [l-61. In several cases, it appears that ferromagnetic layers couple antiferromagnetically through the nonmagnetic spacer. As one of the first systems in which such a coupling has been observed was Fe/Cr[2], it was supposed that the antiferromagnetism of chromium played an important role. In fact the same magnetic behaviour manifests itself in systems where ferromagnetic layers are separated by nonmagnetic layers like Cu or Ag. In addition, detailed investigations of some systems have given evidence for the oscillatory character of such a coupling [4,5], with a characteristic period of 15 to 20 A. At this stage, the origin of the antiferromagnetic coupling is not established and experimental data are needed to ascertain the kind of antiferromagnetism which is developed.

In fact this study of antiferromagnetism in multilayers and of its structure is more than a

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simple magnetic study, because it is closely related to the giant magnetoresistance effect recently observed in multilayered samples [4,5,7]. The ((two-current. model of Fert et aZ. (spin up and spin down) [7] clearly shows the relation between the resistivity and the relative orientations of the magnetic moments on both sides of the nonmagnetic material. A study of such effects has been undertaken in the silver-nickel system, and oscillatory magnetic coupling has been inferred from magnetization measurements on samples with Ni layers 8, 10 and 16 A thick. It appears that the saturation magnetic field extracted from the magnetization curves at 5 K goes through a maximum for an Ag thickness of about 11 A, with a concomitant maximum of the magnetoresistance effect of 25% [8].

In this letter we report on a low-angle neutron scattering study of an Ag(l1 A)/Ni(7 A) sample. It reveals the existence of a sharp antiferromagnetic peak at low temperature, corresponding to the doubling of the chemical period. We show that its intensity decreases with increasing applied magnetic field, as expected for a classical antiferromagnet where the magnetizations of the two sublattices are at first perpendicular to the external field and then rotate progressively to the direction of the field. This antiferromagnetic peak disappears for a value of the applied field close to the field determined from magnetization or magneto- resistance measurements.

Ag/Ni superlattices were prepared by d.c.-sputtering onto glass substrates kept at 100 K during the deposition process. The sample consisted of alternating silver and nickel layers 11 and 7 A thick, respectively, with a total thickness of about 4000A. A detailed X-ray study [9] has shown that high-quality, polycrystalline superlattices with (111) texture could be prepared in this way, with elementary layers as thin as a few atomic planes. Moreover, a good agreement between experimental and simulated spectra was obtained by supposing a rectangular composition profile with interplanar (1 11) distances slightly larger than the bulk Ag and Ni values.

Figure 1 shows the magnetization curve MIM, recorded at 5 K with a SQUID magnetometer, the magnetic field being applied parallel to the sample plane. The curve can be decomposed in two parts, a low-field one (below H = 2.2 kOe) where the magnetization increases linearly with the field (except perhaps for very low fields), and a high-field one

Fig. 1. Fig. 2.

Fig. 1. - Magnetization curve at 5 K of a Ag(l1 &lNi(7 A) superlattice, the field being applied parallel to the layers.

Fig. 2. - Schematic representation of the directions of the magnetic moments in Ni layers. The Ni moments lie in the plane of the layers at an angle a from the direction of the external magnetic field. Scattering length densities are given in the case of nonspin flip and spin flip (~,5’)~,~ scattering.

B. RODMACQ et al.: EVIDENCE FOR ANTIFERROMAGNETIC COUPLING ETC. 505

(beyond H = 3 kOe) where saturation is reached. The corresponding saturation field is estimated at H,=2.4 kOe. A much higher saturation field is obtained when the field is applied perpendicular to the sample, showing that the magnetic moments lie in the plane of the layers [8]. As discussed by Fert et al. [2], the behaviour shown in fig. 1 is consistent with a classical antiferromagnet model in which the antiparallel moments are at first perpendicular to the applied field (a = x12, see fig. 2) and then rotate towards the direction of the field, cosa being proportional to H. Below the saturation field H, (corresponding to a = 0), the magnetic susceptibility is given by Mtt /4J , where J is the coupling energy per unit surface and t is the thickness of the magnetic layer. The application to our results provides a value of J = 2 -

Low-angle neutron scattering experiments were performed at Institut Laue Langevin on IN20 instrument with nonpolarized neutrons. The wavelength of the neutron beam was A = 2.36 A. 10' collimation slits were placed on both sides of the sample, with the instrument in the triple axis configuration with the analyser in elastic position, in order to lower the back ound and improve the resolution. Under these conditions the resolution (FWHM) was 0.01 f?' at low angle. The sample was in a cryostat with long tail placed in an electromagnet with hollow polar pieces, the magnetic field being thus applied in the plane of the layers and perpendicular to the scattering vector. The homogeneity of the field was measured as about 5%.

From X-ray experiments, the modulation period of the sample was measured as A = 17 A. A corresponding first peak was observed in the room-temperature neutron experiments at q1 = 2xIA = 0.37 A-'. However, cooling the sample down to 4.2 K led to the appearance of a sharp supplementary peak, as shown in fig. 3, at a scattering vector qli2 = 0.185 A-', that is half the value of the preceding one. The position of this peak, along with its variation with the applied field (see below), is a clear indication of the doubling of the chemical periodicity resulting from an antiferromagnetic coupling of the Ni layers. The absence of such a peak in the neutron experiments at room temperature is explained by the fact that the Curie temperature is around 300 K for such a Ni thickness [lo].

Two points have to be mentioned. Firstly, the width of the antiferromagnetic peak

erglcm2.

Fig. 3. Fig. 4.

Fig. 3. - Low-angle neutron diffraction diagram of a Ag(l1 -k)/Ni(7 A) sample at 4.2 K.

Fig. 4. - Variation with applied magnetic field of the intensity of the antiferromagnetic peak (circles). The dashed line gives the variation of the quantity l - (M/M,) ' , where MIM, is the reduced magnetization curve of fig. 1.

506 EUROPHYSICS LETTERS

(FWHMII2 = 0.011 A-') is close to the one of the <<nuclear)) peak (FWHM, = 0.014 A-'), and of the same order of magnitude as the instrumental resolution. The corresponding correlation lengths amount to more than 1000 A, that is of the order of the thickness of the sample. Secondly, the ratio Z,,z/Il of the intensities of the diffraction peaks evolves as expected from the spin structure depicted in fig. 2.

This theoretical ratio was calculated in an optic model of stratified media [ l l , 121 with refraction indices n = 1 - 6, with 6 = (A2/27c) p/3, where p is the atomic density of the elements and /3 is a combination of the nuclear b (bAg = 0.5922. A [131) and magnetic p (pNi = 0 . 2 7 ~ x A) scattering lengths. ,U is the magnetic moment of nickel (in Bohr magnetons) estimated from our magnetization measurements at ,u = 0 . 5 ~ ~ [lo]. This model is valid when hlsin0 is very large compared to the interatomic distances. In our experimental conditions this requirement is reasonably fulfilled (Ahin 0 > 30 A).

The total diffracted intensity is the sum of two contributions corresponding to nonspin flip and to spin flip scattering of the neutrons. With the quantization axis along H and with the orientation hypotheses of fig. 2 [14], they give rise, respectively, to the intensities Z, and I,,,. According to the kinematic theory, these intensities are proportional to the square of the scattering densities difference, that is

A and bNi = 1.03

where C1 and Cliz are geometrical coefficients. These formulae are simple because the scattering vector is always perpendicular to the magnetic moments.

For H = 0, that is cos a = 0, the above-mentioned optic model leads to a theoretical ratio 11,2/11 of the (1/2) and (1) diffraction peaks equal to 0.11, smaller than the experimentally measured one of 0.24. It can be increased by introducing some interface roughness, and a roughness of 2 A leads to a good agreement with the experimental ratio.

When H increases from 0 to H , ( i . e . cos a increases from 0 to l), ZI is expected to increase by about 3%. The limited available beam time did not allow us to observe this rather small increase. More importantly, IliZ is expected to decrease as sin' a, that is as 1 - (HIH,)' or as 1 - (M/M,),. Figure 4 clearly shows such an expected parabolic behaviour of the diffracted intensity (circles) 'us. applied field. In addition, the dashed line gives the variation of 1 - (M/M,)' as a function of applied field (from the data of fig. l), the vertical scale being chosen so as to fit with the neutron scattering data. Such a parallel variation of both curves is a strong indication of the validity of the model depicted in fig. 2, thus confirming the initial antiparallel alignment of the magnetic moments in successive nickel layers.

In conclusion, the neutron scattering results presented in this letter unambiguously confirm the existence of an antiferromagnetic coupling between successive ultrathin nickel layers in Ag(l1 A)/Ni(7 A) superlattices. The supplementary low-angle peak observed at low temperature demonstrates that the magnetic period is twice the chemical one. The sharpness of the peak indicates an antiferromagnetic correlation length of the order of the thickness of the sample. Both the ratio of the nuclear and antiferromagnetic peaks at zero magnetic field and the parabolic decrease of the intensity of the antiferromagnetic peak with applied field show that the system behaves as an isotropic classical antiferromagnet. This is supported by the small ( S 200 Oe) anisotropy of nickel. These results confirm that the previously reported magnetoresistive effect is related to the antiferromagnetism of the sample. Complementary studies (including thermal behaviour and polarized neutron experiments) are planned in order to get a detailed picture of the magnetic order in these Ag/Ni superlattices.

B. RODMACQ et al.: EVIDENCE FOR ANTIFERROMAGNETIC COUPLING ETC. 507

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