P. Schobinger-Papamantellos

Magnetic properties of DyMn6Ge6 studied by neutron diffraction and magnetic measurements

Abstract The compound DyMn 6 Ge 6 crystallizes in the hexagonal HfFe 6 Ge 6 -type structure ( P 6/ mmm ). It orders antiferromagnetically at T N =420 K but there is a second magnetic phase transition near 100 K. Neutron diffraction has shown that the high temperature magnetic structure is a triple flat spiral consisting of ferromagnetic Dy layers and ferromagnetic Mn layers coupled antiparallelly in a three-layer sequence Mn(+), Dy(−), Mn(+). The moment direction is perpendicular to [001] but the direction in the basal plane changes by a constant angle φ s = πq z on going from one unit cell to another. The wavevector length of the incommensurate structure is temperature dependent and equal to q z =0.184 at 293 K, which corresponds to φ s =66°. Below T t =100 K the wavevector length remains constant ( q z =0.163) and the magnetic structure is an incommensurate triple-cone structure in which both the Dy and Mn sublattices have a ferromagnetic component along the c axis. These components are antiparallel and lead to a net moment in the c direction of 2.0(5) μ B per formula unit. The magnetic isotherm at 4.2 K, studied in fields up to 35 T, suggests that below 22 T the cone angle (55° forB=0) gradually decreases. Above 22 T the magnetic isotherm shows a linear behaviour characteristic of bending of the antiparallel rare earth and 3d sublattice moments towards each other. The intersublattice coupling constant derived from the high field slope of the isotherm equals J Dy  Mn / k = −9.1 K.

A neutron diffraction study of the magnetic ordering of TmMn6Ge6

Abstract The compound TmMn6Ge6 of HfFe6Ge6-type of structure ( P6 mmm ) orders antiferromagnetically below TN = 482 K and undergoes two magnetic transitions at lower temperatures. Neutron powder diffraction has shown that the high temperature region, T > 85 K, consists exclusively of the collinear antiferromagnetic ordering, q 2 = (0,0, 1 2 ), of the Mn sublattice along the c axis. In the low temperature region, T q z = 1 4 . The intersublattice moment angle Mn-Tm-Mn, ± (π - ϕ{Mn}) = ± 142°, and and the angle of the spiral axis with the c axis θs = 69° are slightly temperature dependent. For T > Tt = 35 K the tilted spiral becomes destabilised and the two sublattices become weakly coupled. In the intermediate region 35–85 K the Tm spiral magnetic moment drops progressively to zero; the ordering of the Mn sublattice corresponds to a distorted skewed spiral with fluctuating Mn moment values and directions.

A neutron diffraction study of the magnetic ordering of HoMn6Ge6

Abstract The hexagonal compound HoMn 6 Ge 6 of HfFe 6 Ge 6 -type of structure (P6/mmm) orders antiferromagnetically below T N =466 K and undergoes a second magnetic transition at T t =200 K. Neutron powder diffraction has shown that the low-temperature phase is a triple skewed spiral with wave vector q 1 =(0, 0, q z ) , consisting of ferromagnetic Ho and Mn layers coupled almost antiparallel in a three-layer sequence: Mn (+)- Ho (-)- Mn (+). The plane of the moments is rotated about an axis in the hexagonal plane so that its normal makes a nonzero angle with q 1 . This structure may be regarded as a combination of an elliptic helix with a longitudinal wave with the same wave vector. The wave vector length is incommensurate with the crystal lattice and is temperature dependent, at 9 K q z =0.1979(2) r.l.u., which corresponds to an interplanar turn angle of o S =71.24° in the direction of q 1 . Above T t =200 K a spin reorientation transition, associated with a decoupling of the Mn and Ho sublattices sets in, and the skewed spiral structure gets destabilised. In the transition region 200–260 K the magnetic ordering is described as a superposition of two Fourier coefficients/atom, associated with the wave vectors q 1 =(0, 0, q z ) and q 2 = (0, 0, 1 2 ) corresponding to a distorted spiral with fluctuating Mn moment and directions. The H ± q 1 satellites (skewed spiral) comprise Ho as well as Mn contributions, while the H + q 2 compr only the Mn antiferromagnetic ordering along c . Above 260 K the ordering consists exclusively of the H + q 2 Mn intensity contributions.

Re-entrant ferrimagnetism in TbMn6Ge6

Abstract The magnetic ordering of the hexagonal compound TbMn 6 Ge 6 has been studied by neutron diffraction at various temperatures between 1.8 and 500 K. In almost the whole magnetically ordered regime the magnetic structures are incommensurate with the crystal lattice. The corresponding wave vector q 1 = (0,0, q z ) is strongly temperature dependent and decreases from q z = 0.1307 r.l.u. at 1.8 K to q z = 0.091 r.l.u. at 410 K. The low-temperature range ( T c -direction. At T t = 85 K the net moment disappears and the magnetic ordering changes into a flat spiral structure that probably persists up to the magnetic ordering temperature, T N = 450 K. Before reaching the magnetic ordering temperature, however, an additional ferrimagnetic component perpendicular to the c -direction develops, marking the onset of re-entrant ferrimagnetism. The resulting structure described by two propagation vectors corresponds to a distorted spiral. Various models are discussed. The observed re-entrant ferrimagnetism is in agreement with results of previously reported magnetic measurements.

Crystal structure and magnetic ordering in ErFe6Ge6 studied by X-ray, neutron diffraction and magnetic measurements

Abstract The crystal structure and magnetic ordering of ErFe 6 Ge 6 were studied by means of X-ray and neutron powder diffraction and on a single crystal. Our results suggest that ErFe 6 Ge 6 , when annealed after casting, adopts a crystal structure intermediate between the perfectly ordered HoFe 6 Sn 6 structure type, and the disordered structure of the YCo 6 Ge 6 type. The 293 K (high resolution) and the 4.5 K neutron data show that the antiferromagnetic reflections can be explained by a 2× c cell enlargement of the YCo 6 Ge 6 -type unit cell. The collinear antiferromagnetic magnetic structure is composed of ferromagnetic Fe-sheets coupled antiferromagnetically along the c h -axis. The Er moments become ordered only below 3 K. This ordering is ferromagnetic with the Er-moments directed along the a h -axis. The fact that the ferromagnetic and the antiferromagnetic ordering types are described by two different magnetic space groups indicates the presence of two order parameters, which explains the independent behaviour of the Fe and Er sublattices.

The magnetic ordering and incommensurate phase transition of Tb5Ge3 — A neutron diffraction study☆

Abstract Neutron powder diffraction studies have shown that upon cooling Tb 5 Ge 3 (hexagonal, Mn 5 Si 3 type) orders antiferromagnetically at T N = 85 K, and undergoes an incommensurate magnetic transition in the range of 75-50 K. The moment alignment for the two end members of the transition is within each chemical cell collinear and ferromagnetic, changing the direction of magnetization with the temperature as a spiral along the hexagonal axis. At 77 K the spiral angle measures 180° and the structure can be described by the magnetic space group C c 2/c. The moment distribution for both Tb positions is sine modulated and the moment angles with the hexagonal axis are 67.7° for the 4(d) and 60.2° for the 6(g) atoms. The 4.2 K structure is described as a flat spiral with a helical angle of 166.14° between second nearest neighbours, nearest neighbours being paired. The ordered saturation moment for both Tb positions measures 8.9(1)μ B .

CRYSTALLOGRAPHIC AND MAGNETIC STRUCTURE OF Tb3Ge5

Abstract The compound Tb3Ge5 has an orthorhombic structure related to the tetragonal α-ThSi2 structure. Results of a detailed structure determination based on neutron diffraction are reported. Magnetic measurements and neutron diffraction showed Tb3Ge5 to order antiferromagnetically below TN = 17 K. There is a close relationship between the magnetic structure and the orthorhombic deformation if the crystallographic structure of Tb3Ge5 is viewed as a derivative of the (defect) α-ThSi2 structure.

The antiferromagnetic structures of TbCu2Ge2 and HoCu2Ge2 by neutron diffraction

Abstract The magnetic structures of TbCu 2 Ge 2 and HoCu 2 Ge 2 were studied by neutron diffraction. At 293 K the chemical structure is tetragonal body centered, space group I 4/ mmm . The magnetic cell at 4.2 K is four times larger than the chemical one with a wave vector k = 1 2 0 1 2 . The magnetic space group is triclinic P a 1 (Sh 2 7 ) for both compounds. The moment values and directions are μ Tb = 8.48(6) [ μ B ] along [110] tetr. and μ HO = 6.5(1)[ μ B ] making an angle of 81.4(°) with c and 80(°) with a 1 . The structure consists of ferromagnetic (101) layers stacked antiferromagnetically.

Structure and magnetic ordering in the defect compounds ErGe1.83

Abstract The crystal structure and the magnetic ordering of the novel orthorhombic compound ErGe 2− x has been studied by neutron powder diffraction and magnetic measurements. The crystal structure belongs to the DyGe 1.85 -type (space group Cmc2 1 ). ErGe 2− x ( x =0.17(2)) orders antiferromagnetically below T N =6 K with a uniaxial magnetic moment arrangement. The magnetic cell has the same size as the chemical unit cell ( q =0). The magnetic space group is Cmc2 1 ( Sh 36 173 ). At T =1.5 K the magnetic moments of the two erbium sites have the same ordered magnetic moment value of 7.63(6) μ B /Er and are coupled antiferromagnetically along the a direction.

Crystal structure of the compound DyGe3

Abstract Earlier investigations had indicated the presence of rare earth germanium compounds of a composition RGe x with x close to 3 of unknown structure. Neutron diffraction data obtained on the corresponding dysprosium compound show that the stoichiometry is DyGe 3 . This compound crystallizes in a novel type of structure related to the CrB structure type. The structure is orthorhombic and comprises one dysprosium site and three germanium sites. It can be thought to be built up from DyGe by inserting slabs of trigonal prisms consisting of germanium atoms into the CrB type unit cell of DyGe.