Rdsumd.-Des mesures d'effet KGssbauer ("Fe) et d'aimantation ont 6tb rdalis6e.s sur des poudtes de microcristaux entre 4,2 et 300 K. Les spectres de relaxation magndtique de F~~+ N~$ + B O~ et Fe3+~i2+~0t, ont dt6 observds au-dessous de 102 et 26 K respectivement; ces tempdratures sont 6levbes en comparaison de la tempdrature N6el antiferromagn6tique (54 et 13 K) di5terminde par mesures ma-gnEtiques sur ces mSmes composds. ces particularitss ont pu Stre attribuges B la faible port6e de

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... tion magndtique dans ces compos6s. Abstract.-Mgssbauer ("Fe) and magnetic measurements were performed on powdered single crystals at 4.2 % 300 K. Magnetic relaxation spectra were observed for FeNi2BOs and FeNiBOt, below 102 and 26 K respectively, which are much higher than the antiferromagnetic Ndel temperatures (i.e. 54 and 13 K) determined by the magnetic measurement for the respective compounds. These features may be ascribed to the low dimensional nature of the magnetic interaction in the compounds. The preparation and X-ray structural analysis of the two boron ferrites F ' ' ' M : B õ (or Fez03 .B203. &NO, where M Fe, CO, Ni, Cu) and F~IIIM'IBo~, (or FezOs.B203. 2M0, where M = Fe, CO, Ni, Mg) were reported by Bertaut et al. in 1950 / l , 2 / . Both have orthorhombic crystal cells in which the Fe3+ and M ' + ions are located on the distorted octahedral interstices of the oxygen ions. The octahedra are stacked along the c axis sharing edges., while they I share edges and corners in the ab plane in which the nonmagnetic boron ions make B03 triangles. It is then expected that the magnetic interaction in these compounds are strong along the c axis and weak in the ab plane having low dimensional nature. Therefore the magnetic properties of the boron ferrites are of interest, which have, however, not been studied in detail at present /3,4,5/. It may be partly because of the difficulty in obtaining the sample in single phase without ferromagnetic impurity. In this paper, we describe the gssbauer and magnetic measurements on FeNizBOsand FeNiBOt, crystals which were successfully grown in single phase within the limitation of X-ray analysis by means of CVT method using HC1 carrier gas. Both measurements were done on powdered single crystals below room temperature, using a constant acceleration Mgssbauer spectrometer and a vibrating sample magnetometer. It was found that the sample of FeNizBOs was free from terromzgnetic impurity, since the magnetization showed neither the remnant nor saturation against the external field, while that of FeNiBOt, exhibited a trace of ferromagnetic impurity which was, however, so small that the correction was not made in the calculation of the magnetic susceptibility. As shown in figure 1 , the inverse susceptibility makes a broad minimum at about 54 K for FeNi2BOS and 13 K for FeNiBOt" which may correspond to the antiferromagnetic Nbel temperatures. Fig. 1 : Inverse magnetic susceptibility vs.temperature for FeNizBOs and FeNiBOt,. Figure 2 shows the Mijssbauer spectra of FeNizBOs at various temperatures. The spectrum at the room temperature (285 K) is composed of a sharp quadrupole doubletwithQ.S.=1.17 m / s and I.S.nO.44 m / s (relative to stainless steel), which can be ascribed to the ~e~+ ion located on one site with a strongly distorted environment, as expected from the crystal Article published online by EDP Sciences and available at http://dx.