Neutron diffraction study of HfFe6Ge6-type TmMn6Ge6-xGax compounds (x=0.2, 0.4, 0.6, 0.8 and 1.0)

The TmMn6Ge6-xGax compounds (x=0.2, 0.4, 0.6, 0.8 and 1.0) have been investigated by magnetization measurements and neutron diffraction. The TmMn6Ge5.8Ca0.2 compound orders antiferromagnetically at 471 K in a colinear magnetic structure characterized by a doubling of the c axis. Below T-t = 220 B a helimagnetic structure takes place. Ar 2 K, the spiral plane lies in the (101) plane with mu (Mn) = 2.02(5) mu (B) and mu (Tm) = 6.27(9) mu (B). The TmMn6Ge5.6Ga0.4 compound orders antiferromagnetically at 402 K. From 300 to 42 K, it is characterized by a helimagnetic arrangement with a helical plane lying in the (101) plane. Below 42 K, a conical structure takes place and the direction of the ferromagnetic component is almost aligned along the c axis. The Mn and Tm moments refined at 2 K are mu (Mn) = 2.21(5) mu (B) and mu (TM) = 6-82(8) mu (B). The TmMn6Ge5.4Ga0.4 compound orders antiferromagnetically at 379 K. From 300 to 124 K, it is characterized by a helimagnetic arrangement with a helical plane close to the (001) plane. Below 124 K, this compound becomes ferrimagnetic with moments almost aligned along the c axis. The Mn and Tm moments refined at 2 K are mu (Mn) =2.19(4) mu (B) and mu (Tm) = 6.89(7) mu (B) The TmMn6Ge5.2Ga0.8 and TmMn6Ge5Ga compounds order ferrimagnetically at 360 and 359 K, respectively, and a moment reorientation takes place at 73 and 56 K, respectively At high temperature the moments are close to the (001) plane whereas at low temperature they almost align along the c axis in good agreement with the large coercive field reported at 4.2 K for this compound (H(c)approximate to 20 kOe). The Mn and Tm moments refined at 2 K are mu (Mn) = 2.20(5)mu (B) [2.18(4)] and mu (Tm) = 7.01(8) mu (B) [7.01(8)], respectively. The evolutions of the moment magnitudes, magnetocrystalline anisotropy and interlayer couplings, as a function of the Ga content, are discussed. (C) 2001 Elsevier Science B.V. All rights reserved.