Multi spin-reorientation process in HrFe6Ge6-type HoMn6Ge6-xGax compounds (x=0.2, 0.4, 1.0)

The HoMn6Ge6-xGax compounds (x=0.2, 0.4, 1.0) have been studied by magnetization measurements and neutron diffraction in the temperature range 2-300 K. The HoMn6Ge5.8Ga0.2 compound orders antiferromagnetically at 415 K and displays a Curie point at 72 Ei;. Neutron diffraction indicates a helimagnetic structure from 300 to 72 Ii and a cone structure at lower temperature (mu (Mn)=2.23(6) mu (B) and mu (Ho)=9.16(12) mu (B) at 2 K). The HoMn6Ge5.8Ga0.4 compound orders antiferromagnetically at 395 Ii. Neutron diffraction study indicates a helimagnetic structure between 250 and 300 K, a ferrimagnetic structure with moments almost aligned along the c-asis between 225 and 180 K, a cone structure in the temperature range 180-70 K and finally a ferrimagetic structure below 70 Ii. At Ii, the moment direction is at 43(1)degrees from the c-axis (mu (Mn)=2.18(6) mu (B) and mu (Ho)=9.59(9) mu (B). The HoMn6Ge5Ga compound is ferrimagnetic in its whole ordered range (T-C=378 K). At room temperature, the moment direction is at 60(4)degrees from the c-axis and rotates continuously towards the c-asis on cooling. In the 250-150 Ii temperature range, the moments are almost aligned along the c-axis and rotate again towards the (001) plane at lower temperature. At 2 K, the moment direction is at 46(1)degrees from the c-asis (mu (Mn)=2.28(5) mu (B) and mu (Ho)=9.81(9) mu (B) at 2 Ii). The anomalous variation of the moment direction is discussed and related to the successive effect of the second and fourth order crystalline field parameters. The evolution of the magnetic order as a function of the Ga content is examined and comparisons are made with the RMn6Ge6-xGax compounds (R=Lu, Tm). (C) 2001 Elsevier Science B.V. All rights reserved.