O. Oleksyn

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.

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.

Atomic disorder, magnetic order and phase transitions of TbFe6Ge6 studied by X-ray, neutron diffraction and magnetic measurements (I),

Abstract The crystal structure and the magnetic ordering of TbFe6Ge6 were studied by X-ray and neutron powder diffraction and by magnetic measurements on a single crystal. It has been found that TbFe6Ge6 forms a superstructure described by the wave vector q = ( 1 2, 1 2, 0 ) of the orthohexagonal YCo6Ge6 type of structure ( c , √ 3 a, a ) h . High resolution neutron data of an as cast material annealed at 800°C indicate that the major factor leading to the superstructure formation is associated with a density modulation of the Tb/Ge ordering that can be described by a density modulation function with the wave vector q . The Fe ordering is antiferromagnetic between 1.4 and 293 K with the wave vector q 1 = (0 1 0) . The three Fe sites have a collinear moment arrangement along the x axis with the same moment value. The Tb sublattice, as a result of the density modulation, splits into two sites ( Tb (0, y, 1 4 )/ Tb ∗( 1 2 , y, 1 4)) with (0.89(1)/0.11(1)) occupancy. Both Tb sites order magnetically below T1 = 9 K. Their ordering can be described by two wave vectors q 1 = (0 1 0) and q 2 = (0 0 0) , the former giving rise to a collinear antiferromagnetic ordering along z (perpendicular to the Fe moments), the latter to ferromagnetic ordering along x (parallel with the Fe moments). At 1.4 K the Fe moment values are 2.02(4)μB/Fe atom. Assuming that the two wave vectors correspond to a single magnetic phase, this would lead to the moment values μ Tb = 7.34(8)μ B , μ Tb∗ = 5.1(7)μ B with an angle of ±45° with the z direction in the plane (x 0 z) corresponding to a flopside structure. The nuclear and the magnetic patterns display an anisotropic peak broadening depending on the h k l direction and the annealing temperature. This effect is discussed in terms of microdomain structure associated with fluctuations of density modulation.

Canted antiferromagnetic structure of the novel compound Er3Ge4 by neutron diffraction and magnetic measurements

Abstract The magnetic ordering of the novel orthorhombic compound Er 3 Ge 4 (Cmcm) with two Er sites has been studied by neutron diffraction and magnetic measurements.Er 3 Ge 4 orders antiferromagnetically below T N = 7.3 K. The magnetic moments of the Er 1 atoms (at 8(f)) make an angle of 35.5(3)° with the y -axis while the moments of the Er 2 atoms (at 4(a)) are perpendicular to it within the (O, y, z ) plane. This triangular antifferomagnetic momentsarrangement of the two sublattices in the plane (O, y, z ) relates to the stacking of the rare earth triangular prism slabs along the a axis. Adjacent prisms along b or c are shifted by x = 1 2 and display different chiralities. The 1.5 K ordered magnetic moment values of Er 1 and Er 2 are 7.32(5) μ B /Er and 6.37(6) μ B /Er, respectively. The two Er sublattices order simultaneously but display a completely different thermal behaviour indicating the existence of competing interactions. The magnetic space group C p ( 2 2′2′ m ′ c m ) (Sh 57 391 ) has a primitive lattice associated with the wave vector q = (0 1 0).

Antiferromagnetic ordering in the novel Dy3Ge4 and DyGe1.3 compounds studied by neutron diffraction and magnetic measurements

Abstract The magnetic ordering of the novel binary compounds Dy 3 Ge 4 and DyGe 1.3(1) , has been studied by neutron diffraction and magnetic measurements. Dy 3 Ge 4 with the Er 3 Ge 4 structure type (space group Cmem ) displays a two step antiferromagnetic ordering below T N = 19 K. The two Dy sites (Dy 1 :8(f) and Dy 2 :4(c)) order independently with two distinct order parameters associated with the same wave vector q = (0, 1, 0). Both sublattices have a uniaxial antiferromagnetic moment arrangement but with mutually perpendicular orientations. Dy 1 orders below T N with the moments along the a -axis, Dy 2 orders only below T N = 6 K along the c -axis. At 1.4 K the moment values of Dy 1 and Dy 2 are 7.31(8) μ B and 5.6(3) μ B , respectively. Their arrangement leads to a symmetry reduction described by the monoclinic magnetic space group Cp11 2 1 ′ m 1′(Sh 11 55 ) . The two-dimensional canted antiferromagnetic structure is the result of a strong magnetocrystalline anisotropy and the presence of various types of magnetic interactions. The phase of DyGe 1.3(1) , composition with the AIB 2 structure type (space group P 6/ mmm ) was detected as an impurity in the studied sample. It orders antiferromagnetically below T N = 16 K with collinear moment arrangement associated with a 2 c -cell enlargement ( q = 0,0, 1 2 ) described by the magnetic space group P 2 11 2′ m′ (Sh 11 56 ) . At 1.4 K μ Dy = 7.2(2) μ B and is confined to the (001) plane.

Antiferromagnetic ordering in the novel (two-step) and -type) compounds studied by neutron diffraction and magnetic measurements

The magnetic ordering of the novel binary compound with two Tb sites ( structure type, space group Cmcm) has been studied by neutron diffraction and magnetic measurements. orders antiferromagnetically in two steps. In a first step the sublattice orders below with a collinear moment arrangement along the shortest axis a which persists down to . Below a reorientation of the moments takes place. Simultaneously the moments order with a collinear arrangement along c. The ordering of the and sites is associated with the wavevector q = (010) and is described by the monoclinic magnetic space group . At 1.4 K the ordered moment values of and are and , respectively. The moment is tilted away from the x-direction by an angle of and by from the y-direction within the (0yz) plane. The resulting structure corresponds to a three-dimensional canting with six sublattices. The presence of diffuse scattering in the 16.5 - 53 K temperature interval is attributed to short-range ordered magnetic domains of the mean correlation length A. Fourier analysis of the data reveals that the first- (up to 4.5 A) and third- (6.5 - 7.5 A) neighbour correlations are mainly ferromagnetic; the second (4.5 - 6.5 A) and fourth (7.5 - 8.5 A) ones are antiferromagnetic. The magnetic ordering of the coexisting novel phase with the structure type (space group P6/mmm) has been also studied. The Tb moments order antiferromagnetically below . This collinear moment arrangement is associated with the wavevector and is described by the magnetic space group . At and is confined to the (001) plane.

Crystallographic and magnetic structure of the novel compound ErGe1.83

Abstract The crystal structure and the magnetic ordering of the novel orthorhombic compound ErGe2−x has been studied by neutron powder diffraction and magnetic measurements. The crystal structure belongs to the DyGe1.85-type (space group Cmc21)·ErGe2−x (x = 0.17 (2)) orders antiferromagnetically below TN = 6 K and displays a metamagnetic behaviour. The magnetic cell has the same size as the chemical unit cell ( q = 0 ). The magnetic space group is Cmc21 (Sh36173). At T = 1.5 K the magnetic moments of the two erbium sites have the same ordered magnetic moment values of 7.63 (6)μB/Er and are antiferromagnetically coupled leading to an uniaxial structure along the a direction.

Low-temperature magnetisation and specific heat in antiferromagnetic rare-earth germanides of the type R3Ge4

We have studied the magnetic properties of R3Ge4 compounds with R = Dy and Er by means of specific heat measurements. Both compounds show specific heat anomalies indicative of a two-step magnetic ordering upon cooling. The results are discussed together with neutron diffraction data.