Observation of antiparallel magnetic order in weakly coupled Co/Cu multilayers

Polarized neutron reflectivity and scanning electron microscopy with polarization analysis are combined to determine the magnetic structure of Co(6 nm)/Cu(6 nm) multilayers. These data resolve a controversy regarding the low-field state of giant-magnetoresistive (GMR) multilayers with weak coupling. As-prepared samples show a strong antiparallel correlation of in-plane ferromagnetic Co domains across the Cu. At the coercive field, the Co domains are uncorrelated. This irreversible transition explains the decrease in magnetoresistance from the as-prepared to the coercive state and indicates that the as-prepared state best represents the antiparallel configuration assumed in GMR analysis. For both states, the Co moments reside in domains with inplane sizes of The combination of polarized neutron reflectivity (PNR) and scanning electron microscopy with polarization analysis (SEMPA) represents a powerful tool for studying magnetic order in materials with buried magnetic layers, such as multilayers composed of alternating layers of ferromagnetic and nonmagnetic metals. PNR probes the order of the entire sample, while SEMPA produces a direct image of the magnetic domain structure within one magnetic layer at a time. In this letter, we report the successful use of PNR and SEMPA to resolve a controversy in giant magnetoresistance (GMR) in Co/Cu multilayers. The resistance (R) of a GMR multilayer greatly decreases when an external field (H) reorients the inplane magnetization of the ferromagnetic layers parallel (P) to each other [1]. The magnetoresistance, MR(H) = [R(H) R(P)]/R(P), is largest for systems in which R(H) at a low field is associated with antiparallel alignment of adjacent ferromagnetic layers. Theoretical analysis has focused upon this maximum MR [2]. Increasing the thickness of the non-magnetic layer, tn, can lead to an oscillation between antiparallel and parallel states with respectively large and small MR [3]. The strength of the exchange coupling between the ferromagnetic layers decreases with increasing tn. For weak interlayer coupling (tn > 4 5 nm), the magnetoresistance MR(0) for the asprepared multilayer is often larger than the maximum value at the coercive field MR(HC) after saturation [4]. MR(0) usually cannot be restored by field cycling or by demagnetization [5,6]. Because MR(HC) reproduces upon cycling, most investigators have assumed that it approximates the antiparallel state [7]. We have performed PNR and SEMPA measurements on Co/Cu multilayers with Cu layers thick enough (tCu = 6 nm) that the exchange coupling between the Co layers is weak. We find that MR(0) originates from strong antiparallel correlations among the Co domain magnetizations across the Cu layers. In contrast, MR(HC) and MR after demagnetization are both associated with uncorrelated domains in adjoining Co layers. In the as-prepared state, the antiparallel correlation occurs within small comumnar Co domains with an average in-plane size of 0.5 1.5 m. The domain size is essentially unchanged at HC and after the sample is demagnetized. We focus on a multilayer of nominal composition [Co(6 nm)|Cu(6 nm)]20, but supporting results were obtained on additional samples. The sample was sputtered onto a 1 x 1 cm Si substrate as described elsewhere [8]. Specular x-ray reflectivity confirms that the Co and Cu layers are well modulated. The field dependence of the magnetization and magnetoresistance was measured at room temperature for a ‘‘twin’’ sample grown at the same time. SQUID (Superconducting Quantum Interference Device) magnetometer measurements indicate that the Co moments preferentially lie in the layer plane. The magnetization saturates in an inplane field < 200 Oe. As shown in Fig.1, the room temperature current-in-plane MR(0) is 6.6%, whereas MR(HC) is only 4.0%. This ratio of MR(0)/MR(HC) typifies those of sputtered Co/Cu multilayers with similar Co and Cu thicknesses both at room temperature and 4.2 K [4]. We performed PNR studies at room temperature on the NG-1 reflectometer at the NIST Center for Neutron Research. These data are sensitive to the size, in-plane orientation and relative interlayer alignment of magnetic domains in buried layers [912]. For specular and diffuse (i.e., off-specular) experiments, we measured all four cross sections, (), (+ +), (+ -) and (+). (The + and signs indicate polarizations of the incident and scattered neutrons parallel or antiparallel to the external field.) The (-) and (+ +) non-spin-flip (NSF) data depend on the chemical structure, as well as the projection of the inplane magnetization parallel to the applied field. The (+ -) and (+) spin-flip (SF) cross sections arise solely from the projection of the in-plane magnetization perpendicular to this field [9]. We note that the efficiencies of the NG-1 neutron polarizers were > 95% in external fields as small as 1.5 Oe. Figure 2 shows total reflectivity scans along the Qz direction relative to the diffuse scattering for the [Co(6 nm)|Cu(6 nm)]20 sample in the as-prepared state (a) and at the coercive field, HC (b). In both cases, the NSF total reflectivity data have a firstorder structural superlattice peak at Qz = 0.057 Å -1 § , where d = 11.4nm is the bilayer repeat distance. Figure 2 (a) also shows a pronounced magnetic peak in all four cross-sections at the half-order position (Qz = 0.031 Å § ). The magnetic repeat distance in the as-prepared state is twice the bilayer thickness d; i.e., a large fraction of the Co layer moments are oriented antiparallel along the growth axis. The narrow Qz width of the half-order reflection reveals that this antiparallel order is coherent through the entire multilayer thickness. The half-order peak has a substantial diffuse component (open symbols), which FIG. 2 Total PNR (shaded symbols) relative to the diffuse scattering (open symbols) as a function of Qz = 4 for [Co(6 nm)|Cu(6 nm)]20 in the (a) asprepared and (b) coercive state at HC = 54 Oe. The diffuse scattering was measured by offsetting the angle and then scanning Qz. The circles and squares correspond to (-) and (+ +) NSF data respectively. The up and down triangles mark the (+ -) and (+) SF data. No corrections have been made for the polarization efficiencies or sample footprint. The arrows designate the vertical axis for each cross section. The insets show the idealized magnetic structure suggested by the scattering in each state. 2π