Magnetic resonance in low doped Cu1-xMxGeO3 with different type of 3D AFM ordering

We report a symmetry analysis and a calculation of the frequency - field dependencies of different 3D AFM states for the underdoped Cu1-xMxGeO3. Our consideration based on the supposition about existence or absence of structural distortions, which result into an alternation of the AFM exchange along magnetic chains. Two types of wave vectors k(AF) = (0,1,1/2) and k(SP) =(1/2, 0, 1/2) were considered. The existence or absence of displacements of the copper ions does not result in appearance of DzyaloshinskiiMoriya antisymmetric interaction along chains. However, such an interaction between chains presents in all AFM phases. It is shown that spin wave spectrum contains two acoustical and two exchange branches. Surprisingly, that frequency of exchange mode possesses the same magnitude similar to20 cm(-1) as the gap energy of the triplet state. The role of exchange modes for coexisting dimerized and 3D-AFM states has been discussed.