Modulated Spin Structure Research Articles

The antiferromagnetic structure of Mn5Si3

The antiferromagnetic structure of Mn5Si3 at low temperatures (Neel point 68 °K) has been determined by neutron diffraction using single crystals. The magnetic structure is complicated, and the model proposed consists of a modulated non-collinear spin structure propagated through the crystal in three hexagonally related directions. This model gives good agreement with the observed intensities of 50 independent magnetic reflections. The mean moments at the two manganese sites are MnI 0 4 ± 0 1 μB, MnII 1 2 ± 0 1 μB. In addition, the spin densities of the two mutually perpendicular spin components have been calculated separately. A considerable deviation from spherical symmetry at the MnII site is shown by the larger component.

Magnetic Structure of MnSn2

An abrupt magnetic transition in MnSn2 was reported by Kouvel and Hartelius1 about one year ago. The salient features of the magnetic behavior are a smooth susceptibility maximum at 86°K and an almost discontinuous decrease in susceptibility at 73°K. At the suggestion of Kouvel, a neutron diffraction study was made to investigate possible changes in magnetic structure accompanying the transition. MnSn2 has the body-centered tetragonal C16 structure, consisting of alternating layers of Mn and Sn perpendicular to the c axis. Each Mn atom is surrounded by 8 Sn atoms which connect it to its second and third nearest neighbors through angles of approximately 111° and 146°, respectively. Nearest-neighbor Mn atoms have no intervening Sn atoms and are separated by a distance of 2.72 Å in the c direction. Neutron diffraction patterns show that above the abrupt transition at 73°K, MnSn2 is antiferromagnetic with a Néel temperature in the neighborhood of 330°K, in agreement with recent susceptibility and resistivity measurements of Kouvel2 on the same preparation, as well as with the earlier magnetic measurements of Yasukōchi et al.3 The structure consists of ferromagnetic (110) planes which alternate along the direction perpendicular to the sheets. Individual moments are oriented parallel to the direction of alternation. The superstructure lines, characteristic of the antiferromagnetic state, appear to saturate near nitrogen temperatures and show no anomaly in passing through the susceptibility maximum at 86°K. The value of the Mn moment at 77°K is 2.36 μB. The abrupt transition at 73°K is marked by the disappearance of the high-temperature antiferromagnetic diffraction pattern and the appearance of a new pattern characteristic of a modulated spin structure. The low-temperature structure is a simple modification of the structure above the transition in which the magnitude of the moment is no longer constant but varies sinusoidally along the direction perpendicular to the ferromagnetically aligned sheets. Thus, the short-range structure and spin direction are essentially the same above and below the transition. The observed modulation has a wavelength of approximately 113 Å or twelve times the face diagonal of the chemical unit cell; the amplitude of the modulated moment is 3.11 μB at 4°K.