Journal of Magnetism and Magnetic Materials 240 (2002) 229­231 4f­3d exchange coupling in Gd/X/Co (X=Pt, Cr) multilayers G. Suciu, J.C. Toussaint, J. Voiron* Laboratoire Louis N!eel, CNRS, BP 166, 38042-Grenoble Cedex 9, France Abstract We have studied the interlayer exchange coupling between Gd and Co through Pt and Cr spacers in a series of multilayers. Magnetization curves have been analyzed with a model, taking into account bilinear and biquadratic interlayer exchange couplings between Gd and Co. The interlayer exchange coupling constants have been determined as a function of the Pt or Cr spacer layer thickness. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Multilayers; Interlayer exchange coupling; 4f­3d exchange coupling; Magnetization processes; Biquadratic exchange coupling Interlayer exchange coupling (for a review see [1]) in coupling between Gd and Co layers in multilayer magnetic multilayers is widely studied both for the large systems, as a function of the nature and the thickness potentiality of commercial applications and for its of the spacer layer. fundamental interest. If magnetic films are separated We have undertaken studies on the coupling between by a metallic non-magnetic layer, the interlayer coupling rare earth and transition metal through a metallic spacer has been observed to oscillate as a function of the in series of multilayers of type [Gd/X/Co/X]N with spacer-layer thickness. The layers are coupled to each X=Pt, Cr. Several series of samples of different other by an exchange interaction via the electrons of the thicknesses were prepared by sputtering with Pt and spacer layer, which has the same physical origin as the Cr as spacer between cobalt and gadolinium layers. The RKKY interaction between magnetic impurities. Inter- samples were deposited at room temperature on silicon layer coupling has been extensively investigated both in (1 0 0) substrates covered with a tungsten buffer layer transition metal multilayers and in rare-earth multi- (200 (A) from facing targets of pure metals (99.9%). The layers and many theoretical models have been proposed: base pressure was 5 10 8 mbar and the deposit was RKKY-based models [2] are more valid to describe done under an argon pressure of 3.5 10 3 mbar. A magnetism in rare-earth multilayers, while other ap- 200 (A-thick protective layer of tungsten was also proaches such as free electrons models with exchange deposited. The samples are polycrystalline. The whole split bands [3] in ferromagnetic materials are more thickness of each element was determined by Rutherford appropriate for transition metal multilayers. Most of the backscattering spectroscopy. We present in this paper studies have been limited to the coupling between metals some results on series of samples for which thicknesses of the same type, either rare earths or transition metals. are fixed around 16 (A for the Gd layers, 17 (A for the Co Only very few results [4­6] have been presented on layers while the thickness of the spacer layer varies exchange coupling between rare earth and transition between 2 and 30 (A for Cr and Pt. The repetition metals. In intermetallic bulk materials it is well known number N was chosen to 14 to get a large enough that Gd (like other heavy rare earths) and Co are magnetic moment to be measurable. antiferromagnetically coupled. The aim of this study is Magnetization measurements were performed in a to investigate the possible change of the sign of the vibrating sample magnetometer with an in-plane mag- netic field up to 6 T at temperatures ranging from 10 and *Corresponding author. Tel.: +33-4-76-88-79-06; fax: +33- 300 K. Figs. 1­3 show typical examples of these mea- 4-76-88-11-91. surements. Magnetization curves, such as those pre- E-mail address: voiron@polycnrs-gre.fr (J. Voiron). sented in Figs. 1 and 3 are obtained for thin Cr spacer 0304-8853/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 1 ) 0 0 7 7 5 - 2 230 G. Suciu et al. / Journal of Magnetism and Magnetic Materials 240 (2002) 229­231 1.5 0.6 [Gd 16 Å / Cr 4 Å / Co 17 Å / Cr 4 Å] [Gd 16 Å / Pt 9.5 Å / Co 18 Å / Pt 9.5 Å] 14 14 1 T = 10 K 0.4 T = 10 K 0.5 0.2 0 0 -0.5 -0.2 Experiment Magnetization (T) Experiment Magnetization (T) Model Model -1 -0.4 -1.5 -0.6 -4 -2 0 2 4 -6 -4 -2 0 2 4 6 Magnetic field (T) Magnetic Field (T) Fig. 1. Experimental and simulated magnetization curve, typi- Fig. 3. Experimental and simulated magnetization curve, typi- cal of a [Gd/Cr/Co/Cr]14 multilayer with a small Cr thickness cal of a [Gd/Pt/Co/Pt]14 multilayer with Pt spacer layer. (4 (A). with the Cr layer thickness. Samples with Pt also present 1.5 the spin flop transition but the magnetization increases slowly as a function of the applied field before the 1 transition begins; and in high field the magnetization never seems to reach the saturation. 0.5 Fig. 2 shows a typical curve for a sample with large Cr thickness of (>10 (A). No spin-flop transition occurs. 0 This behavior is more like ferromagnetic with the T = 10 K existence of hysteresis. In zero applied field, the Gd -0.5 and Co moments are not aligned and a field (1.3 T in Experiment Magnetization (T) Fig. 2) is necessary to align the moments. This means Model -1 that the indirect interlayer coupling does not tend to align the moments ferromagnetically or antiferro- [Gd 16 Å / Cr 20 Å / Co 17 Å / Cr 20 Å]14 magnetically. -1.5 -4 -2 0 2 4 The determination of the strength of the coupling Magnetic Field (T) requires a model for the magnetization processes. We have developed a magnetic model taking into account all Fig. 2. Experimental and simulated magnetization curve, typi- the energies of the real multilayer system and providing cal of a [Gd/Cr/Co/Cr]14 multilayer with a large Cr thickness the magnetization curves and the magnetization profiles (20 (A). throughout the sample. The numerical simulations take into account the anisotropy of cobalt supposed to be uniaxial, the Zeeman energy, the demagnetizing field layers up to B8 (A and for samples with Pt spacer in the energy and the exchange energy. Considering the small range of thicknesses presented in this paper. These thicknesses of the Gd and Co layers, we suppose that curves present a transition as a function of applied each layer of Gd(Co) is represented by a macrospin M1 magnetic field, which is the signature of an antiparallel (M2). The indirect exchange coupling between these coupling between Gd and Co. The value of the macro-spins has the phenomenological form proposed transition field Ht gives a measure of the interlayer by many authors [7,8] coupling between the Co and Gd layers. The samples with Cr and Pt have different behaviors. For samples Eij ¼ J1ðm1:m2Þ J2ðm1:m2Þ2; with chromium, the magnetization is constant in low field, which indicates a well collinear and antiferromag- where m1 and m2 are the unit vectors of the macro-spins netically coupled moments of Gd and Co. At a certain M1 and M2: The coefficient J1 associated to the bilinear field, (B1 T for the sample of Fig. 1), a spin-flop term represents the usual exchange term between two transition occurs up to the field where saturation is magnetic layers. A negative value of J1 favors a collinear clearly attained. The transition field decreases rapidly antiferromagnetic arrangement of M1 and M2; while a G. Suciu et al. / Journal of Magnetism and Magnetic Materials 240 (2002) 229­231 231 10 [Gd 16 Å / Cr / Co 17 Å / Cr ] ) 14 2 10 2 ) [Gd (16 Å) / Pt / Co (18 Å) / Pt ] T=10K 14 J/m T = 10 K J/m -4 -J -4 1 -J1 5 -J 5 2 -J2 Coupling constants (10 Coupling constants (10 0 0 5 10 15 20 5 10 15 20 25 30 Cr layer thickness (Å) Pt layer thickness (Å) Fig. 4. Variation of the bilinear J1 and biquadratic J2 exchange Fig. 5. Variation of the bilinear J1 and biquadratic J2 exchange coupling constants as a function of the Cr layer thickness, in a coupling constants as a function of the Pt thickness, in a series series of multilayers of type [Gd/Cr/Co/Cr]14. of multilayers of type [Gd/Cr/Co/Cr]14. negative biquadratic term J2 favors a non-collinear thickness for Pt and Cr spacers. Different behaviors arrangement with 901 angles between the magnetic have been evidenced due to the different magnetic states moments M1 and M2 of Gd and Co layers. of Cr and Pt. Measurements of the polarization of From the fit of the magnetization curves with this platinum and chromium in trilayers Gd/X/Co with model it is possible to deduce the values of the exchange X=Cr or Pt by polarized neutron reflectometry are in coupling J1 and J2 as a function of the spacer thickness. progress. Figs. 4 and 5 show results for chromium and platinum. For low thicknesses of Cr, the bilinear term of coupling is sufficient to describe the magnetization curve while for larger thicknesses a biquadratic term is necessary. This References means that for large Cr thicknesses the interlayer coupling yields a canted phase with an intermediate [1] M.D. Stiles, J. Magn. Magn. Mater. 200 (1999) 322. angle between the magnetic moments of Co and Gd [2] P. Bruno, C. Chappert, Phys. Rev. Lett. 67 (1991) layers. Accordingly, Fig. 4 shows a rapid decrease of the 1602. J [3] J. Barnas, J. Magn. Magn. Mater. 111 (1992) L215. 1 coefficient and a strong increase of the biquadratic term J [4] K. Takanashi, H. Kurokawa, H. Fujimori, Appl. Phys. 2 as a function of the Cr thickness. For the platinum spacer, the bilinear term J Lett. 63 (1993) 1585. 1 is always much stronger than J [5] K. Takanashi, H. Kurokawa, H. Fujimori, J. Magn. Magn. 2 and decreases slightly with the Pt Mater. 126 (1993) 242. thickness as shown in the Fig. 5. [6] K. Takanashi, M. Ohba, H. Kurokawa, H. Fujimori, IEEE In summary, the interlayer coupling constants J1 and Trans. J. Magn. Jpn. 9 (1994) 16. J2 between a gadolinium layer and a cobalt layer have [7] S.O. Demokritos, J. Phys. D: Appl. Phys. 31 (1998) 925. been determined as a function of the spacer layer [8] J.C. Slonczewski, Phys. Rev. Lett. 67 (1991) 3172.