MAGNETIC PROPERTIES OF AMORPHOUS IRON (III) OXIDE THIN FILMS

Abstract

Amorphous iron (III) oxide films were prepared by deposition of iron metal in an oxygen atmosphere of SxlO* torr. Mossbauer spectra show a paramagnetic doublet at 300 K and 77 K. The isomer shift is 0.3 mm/s and a quadrupole splitting AE is 1.06 mm/s at 300 K for the sample obtained at the substrate temperature of 20°C. At 4.2 K a magnetic hyperfine field was observed to be H = 460 kOe. Magnetic susceptibility measurements showed a Neel temperature TN of 48 K. In amorphous iron (III) oxide thin films, H and TJJ become small and AE becomes large as compared with those of crystalline Fe203. Recently we have studied the preparation of iron oxide films by reactive evaporation method /1, 2/. We obtained the films of iron oxide such as FejO^ and a-Pe 0, by evaporation of iron metal in an oxygen atmosphere of pressures of 10~ to 10 torr. We have succeeded in preparing amorphous iron oxide films with a thickness of hundreds angstrom. In the present study, we have investigated the magnetic properties of amorphous iron oxide films through Mossbauer effect and magnetic susceptibility measurements. Experimental procedures were, for the most part, the same as those in the previous works /1,2/. An iron oxide film was obtained by evaporation of iron (purity 99.9 %) in an oxygen atmosphere of pressure of 5x10 * torr. As a substrate an acetylcellulose film was used. Iron (III) oxide films were obtained under the deposition conditions of substrate temperature Ts below 100°C and deposition rate less than 2A/s. A film thickness was roughly 500A. The samples prepared were examined by electron diffraction. The electron diffraction pattern was made up of so called halos, and showed no crystalline phase. Mossbauer spectra were measured with a conventional spectrometer at 4.2 K, 77 K and 300 K. Figure 1 shows the Mossbauer spectra of the film obtained under the deposition condition; Ts = 20°C o and deposition rate of 0.2A/s (referred to as Tg-20). Mossbauer spectra show a paramagnetic doublet at 77 K and 300 K. At 300 K an isomer shift Is is 0.3 mm/s relative to a-Fe and a quadrupole splitting AE is 1.06 mm/s. At 4.2 K a magnetic hyperfine structure was found. From this spectrum we derive a hyperfine field H = 460 kOe. Similar spectra were found on films obtained under the deposition condition; Ts = 60°C o and deposition rate of 0.lA/s (referred to as T -60). The observed Mossbauer parameters are I = 0.3 mm/s and AE = 0.96 mm/s at 300 K, and H = 490 kOe at 4.2 K. Fig. 1 : Mossbauer spectra of amorphous iron (III) oxide film, T -20 (obtained at 20°C at the deposition rate 0.2 A/s). JOURNAL DE PHYSIQUE Colloque Cl, supplement au n° 3, Tome 40, mars 1979, page C2-153 Resume.Des films d'oxyde amorphe de Fe (III) ont ete prepares par depot atomique de fer sous une atmosphere d'oxygene de 5x10'' torr. Les spectres Mossbauer sont un doublet paramagnetique a 300 K et a 77 K. Les valeurs du deplacement isomerique o et de l'effet quadrupolaire AE sont respectivement de 0,3 mm/s et 1,06 mm/s a 300 K et pour un depot effectue a cette temperature. Un champ hyperfin H de 460 kOe peut etre mesure a 4,2 K. Des valeurs de la susceptibilite magnetique, on peut deduire une temperature de Neel TJJ de 48 K. On constate que H et T., sont petits et que AE est grand, compares aux valeurs correspondantes de Fe203 cristallise. Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979254 c2154 JOURNAL DE PHYSIQUE Similar Mzssbauer parameters have been observed in amorphous iron (111) oxides prepared by thermal decomposition of an aerosol on a heated substrate 131 and a gel of iron oxide 141. Magnetic susceptibility measurements were made between 4.2 K and 300 K with a magnetic torsion balance. The sample for magnetic susceptibility measurement was prepared by completely dissolving the acetylcellulose substrate in methyl acetate. Figure 2 shows the variation of the reciprocal susceptibility with temperature for sample Ts-20. Fig. 2 : Reciprocal susceptibility X-l versus temperature T of amorphous iron (111) oxide film, Ts-20. From the maximum of the susceptibility the magnetic ordering temperature TN was estimated to be 48 K. Above TN the susceptibility can be described by the Curie-Weiss law. The paramagnetic moment per iron ion ueff as calculated from the slope of the reciprocal susceptibility versus temperature amounts 5.4 vB, which is close to that expected from ~e+' ions. An asymptotic Curie temperature 0 of -95 K is derived. For the sample Ts-60, Tn was 62 K. The values of ueff and 0 of Ts-60 were the same as those of Ts-20, respectively. The M'6ssbauer spectra and the temperature dependence of magnetic susceptibility suggest that the amorphous iron (111) oxide films are antiferromagnetic. The Mzssbauer parameters and magnetic properties obtained for amorphous iron (111) oxide films are summarized in table I and compared with those for the crystalline a-Fe,03 /5/. In the amorphous iron (111) oxide films, H and T become small and AE beN comes large as compared with those of crystalline a-Fe,O,. These results suggest that in the amorphous iron (111) oxide, the number of the nearest neighbour atoms decreases, and superexchange interaction was weakened due to the change of Fe-0-Fe angles and Fe-0 distances. The difference in the values H, AE and T between the sample Ts-20 and Ts-60 show that N the structure in Ts-60 is closer to the crystalline a-Fe,03 than that in Ts-20.