Large positive magnetoresistance ͑LPMR͒ effects have been measured at temperatures TϽ100 K in epitaxial Cr/Ag/Cr trilayers grown by molecular beam epitaxy. Compared to single Ag films, the magnetoresistance at 4.2 K is enhanced by nearly two orders of magnitude reaching values up to 120% in a field of 8 Tesla. This behavior is related to a drastic modification of the electron scattering at the Ag interfaces due to the presence of the buffer and cap Cr layers. The LPMR curves measured at

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... t temperatures demonstrate a scaling behavior typical for electron transport in two-dimensional systems. The magnitude of the LPMR is a function of temperature and residual resistance ratio and is influenced by the direction of the applied magnetic field. © 1997 American Institute of Physics. ͓S0003-6951͑97͒00911-X͔ The discovery of the giant negative magnetoresistance effect in magnetic multilayers 1 revived considerably the interest in studies 2 of the charge carrier scattering by surfaces and interfaces. Such studies revealed that the amount of interface roughness plays an essential role in the giant magnetoresistance. On the other hand, the classical positive magnetoresistance ͑PMR͒ effect observed in high quality metallic single crystals 3 receives nowadays much less attention, though the amplitude of these effects may be very large. An essential ingredient to observe large positive magnetoresistance ͑LPMR͒ is a very low resistivity and consequently a long mean free path. 4 Using molecular beam epitaxy ͑MBE͒ techniques, it is now possible to prepare very clean epitaxial films with extremely long mean free paths. Contrary to bulk samples, the electron scattering in these films will be dominated by surface scattering, which may influence substantially the PMR. The surface scattering can be affected by magnetic layers. Recently Tsui et al. 5 reported LPMR effects in Dy/Sc multilayers, which they tentatively explained as arising from a decrease of interfacial reflectivity caused by magnetization of the DySc alloy present at the interfaces. In this letter, we report on the observation of large positive magnetoresistance in high quality Cr/Ag/Cr trilayers and compare the results with data obtained on Cr/Ag bilayers and single Ag films. We will demonstrate that an important contribution to the LPMR arises from scattering of the conduction electrons at the Ag interfaces. The LPMR is also influenced by the direction of the magnetic field, the residual resistance ratio, and the temperature. It should be noted that, contrary to Ref. 5, we did not use magnetic layers to induce LPMR. The Ag-based structures were prepared in a Riber MBE system ͑base pressure 2ϫ10 Ϫ11 mbar͒ using a Knudsen cell for the Ag ͑purity 99.9999%͒ and an electron beam gun for the Cr ͑purity 99.996%͒. The Ag thickness t Ag varies between 20 and 110 nm, the Cr thickness t Cr between 2 and 8 nm. The typical evaporation rate was 1 Å/s. A feedback control system using quadrupole mass spectrometers was used to stabilize the rate of the e-beam gun to within 1%; the temperature stability of the Ag Knudsen cell was better than 1°C. During evaporation, the MgO͑001͒ substrates were kept at room temperature. Reflective high energy electron diffraction ͑RHEED͒ measurements were used to monitor the growth in situ. Ex situ low angle x-ray diffraction confirmed excellent layering quality while high angle measurements have been used to determine the crystalline structure. The films with the Cr buffer layer, i.e., Cr/Ag and Cr/Ag/Cr, grow epitaxially on MgO͑001͒ along the ͓001͔ direction perpendicular to the film plane. The epitaxial relationship in plane is: MgO͓110͔// Cr͓100͔//Ag͓110͔ ͑//Cr͓100͔͒. The Ag films grown directly onto the MgO͑001͒ substrate are mainly ͑100͒ epitaxial, with a small ͑Ͻ5%͒ fraction of ͑111͒. Atomic force microscopy ͑AFM͒ measurements demonstrate that the surface roughness for Ag and Cr/Ag is identical when using the same growth conditions and the same t Ag . RHEED as well as AFM data reveal that the Ag becomes smoother as its thickness increases. For a trilayer structure with t Cr ϭ2 nm and t Ag ϭ40 nm, the AFM rms roughness ϭ2.0 nm on an area of 400 ϫ400 m 2 , for t Ag ϭ80 nm, ϭ0.5 nm. The magnetoresistance was measured by the standard four-point probe method in a temperature controlled cryostat ͑1.5-300 K͒ equipped with a 10 T superconducting magnet and a rotatable sample holder. Figure 1 shows the large positive magnetoresistance of a Cr͑2 nm͒/Ag͑60 nm͒/Cr͑2 nm͒ trilayer measured at 4.2 K for different orientations of the field. The LPMR amplitude is defined as ⌬/ 0 ϭ͓(H)Ϫ 0 ͔/ 0 , where 0 is the zero field resistivity and (H) the resistivity in field H. The largest effect is observed when the field is applied in the plane (H ʈ ) of the film. For a single Ag film of the same thickness, the PMR is negligible (⌬/ 0 Ϸ3%in a field of 8 Tesla͒. The difference in PMR amplitude is mainly due to a difference in ⌬, since 0 for the single Ag film was only 3 times higher. In contrast to the single Ag film, the trilayer does not show the classical parabolic field dependence of the resistance. 6 The amplitude of the LPMR decreases with increasing temperature in a similar way for the three field orientations, and is negligible for TϾ100 K. This decrease closely follows the change for the mean free path l of the a͒ Electronic