Cycloid spirals and cycloid cone transition in the HoMn6−xCrxGe6 (T, x) magnetic phase diagramm by neutron diffraction

Abstract

The structures and magnetic properties of the antiferromagnetic hexagonal pseudo ternary compounds HoMn6−xCrxGe6 (x=1, 1.5 and 2) are studied by neutron diffraction in the temperature range 1.5–300K. The substitution of nonmagnetic Cr for Mn greatly affects the magnetic properties of HoMn6Ge6 by reducing the ordering temperature from 466K to 278K, 205K and 130K for (x=0, 1, 1.5 and 2) respectively, increasing the c/a ratio, suppressing the high temperature (HT) commensurate phase with q2=(0, 0, 1/2) and changing the high temperature (LT) q1=(0, 0, q1z) skew spiral rotation plane.HoMn5CrGe6 and HoMn4Cr2Ge6 display in the entire magnetically ordered regime cycloid spiral structures with the wave vector: q=(0, 0, qz), qz≈0.18(3) r.l.u. and Φs≈64.8° turn angle. The Ho and Mn/Cr (001) layers have ferromagnetic arrangements coupled antiferromagnetically. The Ho moments in the z=0 layer are oriented in a direction opposite to the line bisecting the angle 2φMn≈2×28° of the Mn layers at z=±∼0.25. This triple unit changes orientation collectively in the direction of q within the (b, c) plane containing the wave vector. Above 50K, the wave vector length increases linearly from 0.24 to 0.28(1) r.l.u. below TN.The HT HoMn4.5Cr1.5Ge6 cycloid spiral, is stable in the range Tt<T<TN (q1=(0, 0, q1z), q1z=0.1711(3) r.l.u., Φs =61.6° at 40K). At variance to the other compounds, crossing Tt≈36K from above, it undergoes a first order magnetoelastic transition from a cycloid HT q1 to a LT longitudinal cone structure: q2=(0, 0, q2z), q2z=0.149r.l.u., Φs=53.6° and cone angle ΦC=47°. The q1 and q2 phases coexist in the LT range down to 1.5K≤T≤Tt in varying amounts with T. This transition is directly manifested by the splitting of the HT magnetic satellites and the increase of characteristic nuclear intensities. The results are summarised in a (T, x) magnetic phase diagram.