Readout of Magnetic Film-Based Memories by Nonlinear-Optical Magnetic Kerr Effect
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magnetic materials for high density memories. Apart from the traditional magneto-optical materials,
e.g.,
magnetic rare-earth iron garnet
films, a number
of new
magnetic
nanostructures and composite materials have been developed, such as magnetic multilayers and granular films with magnetic nanocrystals. At the same time, the potential use of new memories leads to the search for effective nondestructive readout techniques. In the present
paper magneto-induced second harmonic generation (MSHG) is suggested as a readout for magnetic memories. The choice of the MSHG probe as a readout is motivated by the high sensitivity of the quadratic response to the magnetic properties of nanostructures and low-
dimensional systems [1]. Therefore, the wavelength of readout fundamental radiation can be chosen far from electronic resonances of the film, which makes this probe nondestructive, while the wavelength of the SHG radiation can be resonant and thus provide an effective SHG output. In the present paper the potential of the SHG readout of magnetic memories is demonstrated with the examples of (i) Bi-containing iron garnet films, (ii) thin Co-Cu granular films and (iii) organo-metallic Langmuir-Blodgett (LB) films containing monolayers of Gd ions. The last two systems are of special interest, as the size of the storage unit can be less than 60 nm for the case of Co nanocrystals in granular films and is about 5 nm for a single period of Gd-containing structure in LB films The SHG readout discussed is based on the nonlinear-optical magnetic Kerr effect (NOMOKE), i.e., magneto-induced changes in the SHG intensity and rotation of polarization of the second harmonic (SH) wave. This is a consequence of the general property of changing the nonlinear susceptibility of a material as a result of information recording. The NOMOKE effect has been studied for thin metallic films and multilayers both experimentally [1,2] and theoretically [3,4]. As compared with the magneto-optical Kerr effect (MOKE), a large magneto-induced alteration of the SHG intensity, magneto-induced phase shift and polarization rotation of the SH wave were obtained. For the case of the (210)-oriented Bi-containing irongarnet films, sufficient magnetization induced changes of the SHG intensity were demonstrated for off-resonant and near-resonant fundamental radiation [2,5]. The SHG readout has already been suggested for the photochromic films and iron garnet films in [6]. 605 Mat. Res. Soc. Symp. Proc. Vol. 517 c 1998 Materials Research Society
EXPERIMENT AND RESULTS 3 The output of a Q-switched YAG:Nd + laser at 1064 nm, a pulse duration of 15 ns and a repetition rate of 12.5 Hz is used as the fundamental radiation in the nonlinear-optical experiments. The SHG signal from the films is selected by bandpass filters and detected by a photomultiplier tube (PMT) and gated electronics. To improve the signal-to-noise ratio in nonlinear-optical measurements, the reference SHG channel is used for normalizing the SHG signal reflected from the sample to the square of
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