Optical and magnetic properties of MnBi formed directly by ion beam mixing
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Multilayers of Mn/Bi were irrradiated with 400 keV Xe+ at temperatures from 80 to 550 K. Ferromagnetic MnBi layers oriented perpendicular to the substrate surface were formed directly at mixing temperatures from room temperature (RT) to 500 K. The measurements of Curie point and optical Faraday rotation revealed that 60-70 at. % of MnBi ion-mixed at RT is the metastable phase (QHTP: quenched high temperature phase). The fraction of QHTP is reduced with increase in mixing temperature. It was also found that ion beam mixing yields a pronounced protection effect against the deterioration of MnBi in a moist atmosphere. A detailed discussion is given on the kinetics of ion beam mixing and the growth mechanism of the metastable MnBi grains.
I. INTRODUCTION
II. RESULTS AND DISCUSSION
Much attention has been paid to thin films of MnBi as a magneto-optic recording medium because of its perpendicular magnetic anisotropy and large Faraday rotation.1"3 The films are usually prepared by annealing double layers of successively evaporated Bi and Mn films on a mica or glass substrate. MnBi has two crystallographic phases4'5; upon heating above 360 °C there occurs a first order transition from a low temperature phase (LTP) into a high temperature phase (HTP), which can be retained at room temperature (RT) by quenching. The Curie temperatures of LTP and the quenched HTP (QHTP) are 480 and 180 °C, respectively. The use of QHTP MnBi films for magnetooptic recording would be advantageous because their lower Curie temperature would lead to more writing capability. However, its application faces two major problems: the QHTP films are slowly converted into LTP, and rapid deterioration occurs in the presence of moisture. Several attempts have been made to overcome these problems, for example by partial substitution of the Mn atoms by Ti or Cu.6 In the present work, we try to form directly the QHTP MnBi layers with perpendicular magnetic orientation by ion beam mixing at low temperatures. The study of the kinetics of ion beam mixing of Mn/Bi multilayers reveals the growth mechanism of the ion-mixed MnBi layers oriented perpendicular to the film surface. Perpendicular magnetic orientation is confirmed by measuring the magnetic hysteresis loops and by x-ray diffraction analysis (XRD). The phases formed by ion beam mixing are identified and characterized by measuring the temperature dependence of magnetization and specific Faraday rotation. The film quality of the ion-mixed MnBi is compared with that obtained by the usual heat treatment in connection with surface morphology.
Depth profiles of Mn and Bi before and after 400 keV Xe + irradiation or heat treatment were measured by Rutherford backscattering spectrometry (RBS) followed by computer-simulated spectrum analysis.7 Figure l(a) shows the observed (dotted) and simulated (solid curve) RBS spectra from Al (7 nm)/Mn (31 nm)/Bi (133 nm)/Si irradiated with 400 keV Xe + with a dose of 5 x 1015 Xe + /cm 2 at 400 K. The average projected range and maximum concentration of Xe are calculated to
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