J.H.V.J. Brabers

Magnetic properties of rare-earth manganese compounds of the RMn6Ge6 type

Abstract We determined the magnetic properties of several compounds of the type RMn 6 Ge 6 (R is a heavy rare-earth element). Several of these compounds give rise to two ordering temperatures associated with antiferromagnetic ordering of the R and Mn sublattices. For GdMn 6 Sn 6 and to some extent also for TbMn 6 Sn 6 we observed magnetic behaviour which can be described as “bootstrap ferrimagnetism”: the antiferromagnetic configurations within both the R and Mn sublattices are broken simultaneously by the R-Mn intersublattice interaction, leading to ferrimagnetic alignment of the R and Mn sublattices. This bootstrap mechanism is sufficiently strong only if the intrasublattice Mn-Mn interaction is sufficiently weak. This condition is met only when the unit cell volume is sufficiently large. The interplay between intersublattice coupling and unit cell volume leads to interesting temperature dependences of the magnetization. In the magnetic isotherms the bootstrap mechanism leads to first-order phase transitions.

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.

A neutron diffraction study of the magnetic ordering of LuMn6Ge6

The compound LuMn6Ge6 of the HfFe6Ge6-type of structure (P6/mmm) orders antiferromagnetically below TN = 509 K. Neutron powder diffraction has shown that over the temperature region 1.8–293 K the magnetic ordering consists exclusively of the collinear antiferromagnetic moment arrangement of the Mn sublattice along the c-axis associated with a doubling of the c-axis (q = 0, 0, 12). The Mn ordered moment value of about 1.1 μB found in the low-temperature region, 1.8 K < T < 75 K is inferior to the value found in the other isomorphic RMn6Ge6 compounds.

Giant magnetoresistance in polycrystalline SmMn2Ge2

Abstract The magnetic properties and the magnetoresistance of polycrystalline SmMn 2 Ge 2 were studied. Magnetic phase transitions associated with different types of magnetic ordering were observed near 100 K and 150 K. Between these two temperatures the zero-field magnetization vanishes, but there is a field-induced transition to a basically ferromagnetic state. Measurements made at 125 K suggest that this transition is accompanied by a large magneto-resistance effect ( Δ R/R=16%).

Magnetic properties of rare earth compounds of the type RCo11Ti

Abstract We have investigated the magnetic properties of rare earth compounds of the type RCo 11 Ti (R = Sm, Gd, Tb, Dy, Ho, Er, Tm and Y) by means of high-field and ac susceptibility measurements. From the experimental values of the saturation magnetisation at 4.2 K we have derived the magnitude of the Co moments in these compounds, which were found to be almost invariant across the series ( μ Co = 1.41 μ B ). We have determined the easy magnetisation direction in the various RCo 11 Ti compounds and found that the Co sublattice anisotropy favours an easy moment direction along the c -axis, whereas the rare earth sublattice anisotropy is governed by a positive second-order crystal field parameter ( A 2 0 > 0). Spin reorientation phenomena were observed for compounds with R = Dy, Er and Tm.

Magnetic properties of non-stoichiometric UNiGa

Abstract Magnetization measurements of alloys based on the intermetallic compound UNiGa with deviation from the exact 1:1:1 stoichiometry, namely Ux(Ni0.5Ga0.5)3−x with 0.8 ⩽ x ⩽ 1.2 and UNi1.1Ga, have been performed. The obtained results suggest that the antiferromagnetic ground state of UNiGa can be easily transformed into a ferromagnetic one not only by external magnetic fields but also by changes of the composition.

Magnetic properties of RCr6Ge6 compounds

Abstract Compounds of the composition RCr6Ge6 have been prepared and investigated by X-ray diffraction. The crystal structure was identified as the MgFe6Ge6 type. The magnetic properties of the compounds have been investigated through measurements of the magnetization as a function of applied field and temperature. In the compounds with R  Dy, Ho, Er, no indication of magnetic ordering was found for temperatures down to 5 K. A modest ordering was found in TbCr6Ge6. Magnetization measurements on YCr6Ge6 revealed a very small saturation moment of about 0.2 μB on the Cr ions. In high magnetic fields there is a tendency towards an anti-parallel arrangement of the Tb and Cr moments.

High-field study of the magnetization process in RMn/sub 6-x/Cr/sub x/Ge/sub 6/-compounds

The magnetization of powdered RMn/sub 6-x/Cr/sub x/Ge/sub 6/-compounds (with R=Y, Gd, Tb, Dy, Ho, Er, Lu) was studied at 4.2 K in high magnetic fields up to 38 T. From the high field data values for the Mn-moments were obtained for compounds with K=Y, Gd, Tb, Dy whereas the R-Mn coupling constants were estimated for the compounds with R=Gd, Tb, Dy. The Mn-moments were found to be approximately 2/spl musub B/. Metamagnetism was observed in compounds with R=Y, Lu. The Mn-Mn distance strongly affects the magnetization curves in these latter eases. >