Photomagnetic Storage in a Ferrimagnetic Material

Photomagnetic storage and retrieval of information in the megahertz frequency range utilizes a ferrimagnetic material as the storage medium in conjunction with parametric excitation of infrared exchange resonance spinwave modes, by means of signal-modulated polarized coherent radiation, for example, from a COz laser at 10.6 micrometer length, focused to the storage medium. Parametric excitation of the spin systems occurs in a high-speed optical scanning process at half the pump frequency. Photomagnetic storage and retrieval of information in the megahertz frequency range comprises the parametric excitation of infrared exchange resonance spinwave modes in thin ferrimagnetic single crystals. Excitation is provided by signal-modulated linearly polarized coherent radiation at 10.6 micrometer wavelength from a CO, laser beam. The beam is focused on the storage medium which is moving orthogonally to the beam direction in a high-speed scanning process (Fig. 1). Excitation of the spin systems utilizes parallel parametric pumping from the laser beam at twice the exchange resonance frequency of the spinwave modes. Excitation occurs with the storage medium at ambient temperatures. The medium is a thin Yttrium Iron Garnet (YIG) single crystal. Photomagnetic effect occurs when the domains of the ferrimagnetic medium have the form of NCel domains, characterizing domains and walls with their magnetization parallel to the surface of the thin single crystal. The crystal surface extends perpendicular to the linearly polarized beam axis with the plane of polarization parallel to the NCel walls. Photomagnetic storage by means of parallel parametric pumping is characterized as the flipping of Nee1 domains (with the intrinsic magnetization parallel to the plane of polarization) to Bloch domains (with the intrinsic magnetization orthogonal to the medium surface and the plane of polarization). Note that the photomagnetic storage process utilizes the ferrimagnetic medium in the demagnetized state ; i. e., no steady magnetic field is applied before, during or after parametric excitation. In order to avoid excessive thermal heating of the lattice structure of the storage medium during the parametric excitation process, high-speed opti~al scanning is provided which separates spin temperatures and lattice temperatures of the medium during the excitation period. In other words, due to the high scanning velocity, the skin depth of the incident thermal laser radiation is negligible compared to the skin depth of parametric excitation. After the parametric excitation of the ferrimagnetic storage medium the spin systems relax to states of minimum energy, which deviate from the magnetic states before excitation. Assuming the medium is in the state of NCel domains before photomagnetization, and providing laser irradiation is within the diffraction limits (focus), the result of the spin relaxations are Bloch domains of the size of the diffraction limits oriented orthogonally to the NCel domains. Due to parallel pumping, the parametrically excited spin waves are extending orthogonally to the direction of magnetization. One might say that the parametric gain yields an increase in magnetization energy in the storage medium. Of course, the temporary heating of the storage medium during the excitation period determines the final state of parametric excitation, the medium being extremely sensitive to photomagnetization close to the Curie temperature. Parallel-pumped parametric excitation of the spin systems occurs at a certain critical power level of the incident laser radiation. When this intensity is reached, the NCel domains flip to Bloch domains, thus drastically changing the original domain structure. There is a characteristic increase in laser power density when approaching the laser focus that is inversely proportional to the beam cross-section. Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1971137 C 1 118 C. H. BECKER Conversion of larger areas of Bloch domains to N&l domains takes place during irradiation of the storage medium with a much wider cross-section of the laser beam, as compared to the diffraction-limited region of the laser focus. Apparently because of magneto-strictive effects the energy of the Bloch domains is lowered to that of the Ntel domains, yielding large areas of Nee1 domains within the YIG single crystal, provided this area previously contained Nee1 domains. The previously known instability between Bloch and Nee1 domain energies, as a function of the thickness of the YIG single crystal, appears to have a close relationship to the observed instability of Bloch to Ntel domain conversion, and vice-versa. Experimental verification of photomagnetic storage and retrieval was performed as follows : A Q-switched, linearly polarized single mode (TEM,,) CO, laser (Coherent Radiation, Type 42 L) was utilized as the infrared coherent radiation source. The beam was linearly polarized. Pulsed power was 1 kW, at a pulse rate of 7.2 kH. Focusing of the incident laser beam was provided by a Germanium double objective of 2.5 centimeter focal length. The thin single crystal YIG was mounted on the rotating quartz disc of an optical scanner. Orientation of the YIG for parallel pumping was so arranged that the Nee1 wall occurred in the exposure region parallel to the plane of polarization of the incident laser beam. After photomagnetization was produced, the rotating quartz disc was removed from the optical scanner and effects observed utilizing a Leitz polarization microscope. The ferrimagnetic background of this work is comprehensively described in Reference [I]. Particular reference is given to : Bloch walls [2] ; ferrimagnetic exchange resonance [3] ; Ntel walls [4] ; domain structure of YIG [5] ; parallel parametric pumping [6] ; and theory of parallel-pumped magnetic instabilities in a two-sublattice ferrimagnetic crystal [7]. The author wishes to acknowledge the invaluable cooperation of L. Herte, W. Kaspari, V. Lieskovsky, N. Schenberger and H. Wong.