Optical Faraday Effect in Ferromagnetic and Ferrite Films R.L
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Optical Faraday effect in ferromagnetic and ferrite films R.L. Coren, M.H. Francombe To cite this version: R.L. Coren, M.H. Francombe. Optical Faraday effect in ferromagnetic and ferrite films. Journal de Physique, 1964, 25 (1-2), pp.233-237. 10.1051/jphys:01964002501-2023300. jpa-00205745 HAL Id: jpa-00205745 https://hal.archives-ouvertes.fr/jpa-00205745 Submitted on 1 Jan 1964 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. LE JOURNAL DE PHYSIQUE TOME 25, JANVIER-FÉVRIER 1964, : 233. OPTICAL FARADAY EFFECT IN FERROMAGNETIC AND FERRITE FILMS By R. L. COREN and 34. H. FRANCOMBE, Philco Scientific Laboratory, Blue Bell, Pennsylvania, U. S. A. Résumé. 2014 On a étudié la dispersion de l’effet Faraday dans les couches de métaux ferroma- gnétiques et de ferrites oxydées dans le domaine de nombres d’onde s’étendant de 1,4 x 104 a 3,7 104. Dans les couches métalliques de Ni, Fe, NiFe2 et CoFe2, la rotation est positive et décroît lorsque le nombre d’onde augmente. Dans les couches de ferrites, la rotation de Faraday est positive pour de grands nombres d’onde et négative pour des nombres d’onde plus petits avec des pics positifs et négatifs. On discute l’absorption optique dans les ferrites. Abstract. 2014 We have studied the Faraday rotation of the plane of polarization of linearly polarized light by thin films of the ferromagnetic metals, Ni, Fe, NiFe2, CoFe2, and of the ferri- magnetic oxides, NiFe2O4 and CoFe2O4. Hysteresis loops of the ferrite films are displayed using a Faraday effect hysteresigraph. Curves are given for the dispersion of Faraday effect in the metal and oxide films, from 1.4 104 to 3.7 x 104 wave numbers. In the metal films the rotation is positive and decreases as wave number increases. Superimposed on this general trend are several dispersive anomalies. Faraday rotation in the ferrite films is positive at large wave numbers and negative at lower wave numbers, displaying positive and negative peaks. Optical absorption in the ferrites is discussed. Introduction. - The Faraday effect in magnetic determination of its frequency dependence, it is materials has attracted interest as a means of worthwhile to reexamine these materials. This studying their technical magnetization processes report describes studies we have made of the and electronic energy structures. The effect has Faraday effect in thin films of several ferro- been employed to examine their hysteretic prop- magnetic metals and alloys and in nickel and erties [1, 2] and domain structures [1, 3]. Meas- cobalt ferrite, over the wavelength range from urements of the magnitude of the Faraday effect 2 700 A to 7 000 Å. .otten shows a strong frequency dependence [4-6], sometimes with pronounced resonance type behav- Experimental. - The saturation rotation of the ior [7]. Since these studies require sample trans- plane of polarization of visible radiation is of the parency they can only be carried out in certain order of 105 degrees/cm for ferromagnetic material frequency bands, or with thin single-crystal sec- and 104 degrees/cm for ferrimagnetic material. tions of sufficient purity and perfection, or on Consequently, with a ferromagnetic film 1 000 Å materials fabricated in film form. The extreme thick or a ferrimagnetic film 10 000 A thick (the opacity of ferrites of the first row of transition approximate limits of transparency to visible radia- elements e.g. Ni, Co, Cu has, thus far, made it tion for metals and for Ni and Co ferrites), one impossible to study thin bulk samples with visible expects a rotation of, at most, a few degrees. radiation. It is desireable to do so since crystal Because of the smallness of the effect, a config- field splitting of the 3d electronic energy levels of uration with the polarizing and analyzing prisms these divalent metal ions gives rise to energy sepa- rotated relatively by 45° produces changes of rations which fall in this range [8]. Transitions intensity of the transmitted radiation which are between the split states have been correlated [9] proportional to the Faraday rotation of a sample with the strong Faraday effect in these materials, placed between them. In our experiments the making the Faraday rotation a likely means of light beam falls on a photomultiplier tube whose examining their electronic structure. With the output is balanced against a constant d. c. source, recent development of techniques for producing amplified, and recorded on the ordinate of an XY films of ferrite materials [2, 10] it is now possible Recorder. By turning the analyzer through a to obtain transparent samples and, therefore, to high ratio, precision speed reducer this axis can be examine their detailed optical properties. calibrated directly. The abscissa of the recorder As a result of improved vacuum facilities and is controlled by the output of a calibrated gauss- techniques, films produced today are more repro- meter whose probe is adjacent to the film, so that ducible and reliable than those studied in the past. the system produces a display of the Faraday rota- For this reason, and because previous studies of tion as a function of applied field. Faraday effect in magnetic metals have often Two separate optical systems were used : a lacked sufficient precision and detail for a complete Faraday effect hysteresigraph, with which to study Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:01964002501-2023300 234 the B - H loop properties of the films, and a Faraday,effect " spectrometer " for studying the spectral dispersion of the Faraday effect. In the hysteresigraph white light passes through the film at an angle of 450 to its plane while the magnetic field is parallel to the film in the plane of incidence. Since the Faraday effect is proportional to the component of magnetization along the light path [11], magnetic changes in the film plane change that component proportionally, so that as the external field is varied the recorder traces the planar hysteresis loop of the sample. To inves- tigate planar magnetic anisotropy the film can be rotated about its normal. The Faraday effect " spectrometer " is con- FIG. 1. - made means structed so that the field and the mono- Typical planar hysteresis loops by magnetic of the effect. The have been nor- chromatic beam optical Faraday loops light both pass normally through malized to the appropriate value uf 47tM (see text). the film. The field is supplied by an electro- with 4 inch magnet cylindrical poles, separated the of the source and 0.5 and which have inch axial holes range used, intensity light by inches, 1/8 the thickness. the curves for the As the field increases sample Consequently, light path. applied in 1 have been normalized to a scale of bulk the to the figure magnetization component perpendicular saturation Discussion of the coer- film increases so that the magnetization. proportionally [12], opti- cive force values obtained from such has been cal rotation with field. loops changes linearly applied included in a Both the When the field H = 47r AJ satu- previous report [2]. loop applied magnetic and coercive forces are ration so that the shapes very reproducible occurs, ferromagnetic Faraday and show little variation with effect reaches its maximum value and remains preparation temper- it is above constant as H is further increased. ature, providing high enough (e.g. Superimposed 700°C for but do film this behaviour is the rotation due to NiFe2O4), depend upon upon Faraday thickness, a constant value above the substrate. Since the substrate is non-ferro- approaching 5 000 A. its effect is to the magnetic Faraday proportional The results of rotation measurements field and can be subtracted. Faraday applied on several metals and are The metal films studied were cathod- ferromagnetic alloys prepared by shown in figure 2. Of the four metallic ferro- ic sputtering from the appropriate element or alloy source, using substrates in the form of micro- scope cover slips or vitreous silica discs, 12 mm in diameter. NiFe2 and CoFe2 alloy films on silica were subsequently oxidized in air at 1000 OC to produce the ferrites. Details of the sputtering and oxidation processes, as well as of the magnetic and structural properties of these films have been described before [2,13]. Results. - Previously reported [13] meas- urements on sputtered films of Ni-Fe alloys have shown that, provided suitable vacuum techniques are used, their magnetic properties are highly reproducible and correspond well with those of bulk material at thicknesses down to 100 A. Magnetic tests on the metal films described here indicate essentially similar behavior. Analysis by electron and X-ray diffraction show the metal and oxide films to be structurally uniform, polycrys- talline and single phase. Magneto-optic hysteresis data for NiFe2O4 and films are shown in figure 1. In obtaining FIG. 2. - Frequency dependence of the initial slope of the CoFe204 curve in elements and these curves the of the measured ordi- Faraday ferromagnetic alloys. amplitude These curves are for films about 300-350 A thick. The nate signal depends upon the absorption and spe- marked points for nickel and iron are taken from Sier- cific rotation of the material over the wavelengths, tsema [5]; the dashed curve is from Skinner and Tool [4 1.