Nonlinear optical investigations of magnetic heterostructures (invited)

We report second-harmonic magneto-optic Kerr effect investigations of magnetic heterostructures. The heterostructures studied are multilayers of Si/Al₂O₃/Cu(50 nm)/Ni₈₁,Fe₁₉(x), Si/Al₂O₃/Cu(50 nm)/Co(2 nm)/Ni₈₁Fe₁₉(x), and Si/Al₂O₃Cu(50 nm)/Ni₈₁,Fe₁₉(x)/Cu(5 nm), where x ranges from 1 to 30 nm. For films thinner than the optical penetration depth, the transverse second-harmonic Kerr effect experimentally separates and distinctly identifies each of these multilayered structures. We find that the addition of the 2 nm Co layer between Cu and Ni₈₁Fe₁₉ increases the magnetic signature in the second-harmonic intensity by more than a factor of two. This increase correlates with the observed magnetoresistance increase in spin valves with the addition of Co. For films thicker than the penetration depth, the structures with air-exposed Ni₈₁Fe₁₉ are identical, but they differ significantly from Ni₈₁Fe₁₉ films capped with Cu. We used spectroscopic ellipsometry to determine the linear optical properties of each film in the structure. Modeling of both Cu(50 nm)/Ni₈₁Fe₁₉ and Cu(50 nm)/Ni₈₁Fe₁₉(x)/Cu(5 nm) suggests that the buried magnetic interfaces are better generators of second-harmonic than the air-exposed Ni₈₁Fe₁₉ surface. There is a distinct decay in the magnetic component of the second-harmonic intensity for film thicknesses below 2 nm.