P. J. Brown

Magnetic moments and magnetic site susceptibilities in Mn5Si3

Polarized neutron diffraction has been used to determine the manganese magnetic moments aligned by magnetic fields applied parallel to the [001] axis in the antiferromagnetic phases of Mn5Si3. It has been found that in the two antiferromagnetic phases which exist below TN the magnetic moments on the manganese sites which can be aligned by a magnetic field all have the same order of magnitude, although in the antiferromagnetic structures the ordered moments are very different. The transition, on cooling, from the AF2 to the AF1 phase is accompanied by a sharp decrease in the susceptibilities of all the sites. This uniform decrease is not consistent with disordered local moments on the sites which do not have ordered moments, but can be understood using the same criteria for stability of Mn moments as have been invoked for manganese rare-earth compounds (Ballou R, Lacroix C and Nunez Regueiro M D 1991 Phys. Rev. Lett. 66 1910).

Unusual magnetic interactions in compounds with the ThMn12 structure

Abstract Neutron diffraction experiments have been performed on single crystals of MFe4Al8 where MY, Lu, Dy and U. All compounds have the ThMn12 bct crystal structure with Fe and Al atoms on separate sites. The Fe sublattice order antiferromagnetically above 100 K with a wave vector (ϵ, ϵ, 0), where ϵ = 0.229 for Y and Lu and 0.133 for Dy. For UFe4Al8 ϵ = 0 and the U atom is in position of zero-molecular field from the surrounding Fe atoms. In an applied magnetic field parallel to the easy axis, the U atoms align with the field, but the Fe sublattice has a + − + − antiferromagnetic configuration with moments almost perpendicular to the applied field.

Neutron scattering and magnetization studies of plutonium monochalcogenides

Magnetic susceptibility χ(T) measurements have been performed on single crystals of PuS, PuSe, and PuTe. For T > 50K χ(T) is independent of T with a small value of ≈300 × 10-6 emu/mol. For T<50 K increases are seen in χ. We believe these arise because of defects in the lattice. Polarised neutron diffraction has been used to determine the form factor of the 5f electrons around the Pu ion. The largest induced moment is ≈5 × 10-3μB so the form factor is not accurately determined, but is clearly different from that found in the localised system PuSb. The results are discussed in terms of our present understanding of the Pu monochalcogenides. A theory consistent with all the present measurements is not yet available.

Experiment and theory of actinide intermetallic magnetism: a test case of NpCo2

A detailed neutron study using a single crystal has measured the individual magnetic moments on the Np and Co sites in the compound NpCo2. Furthermore, by studying the angular dependence of the magnetic amplitude, we are able to assign individual values to the spin and orbital moments. These are both large, and oppositely directed. A first-principle LMTO calculation including relativistic effects and both spin and orbital polarization is in excellent agreement with experiment, as far as the partial cancellation of the spin and orbital moments is concerned. This agreement has general implications for our understanding of the complex magnetic behaviour of actinide intermetallic compounds.

Magnetic phase diagram of EuAs3

Abstract The magnetic phase diagram of monoclinic, semimetallic EuAs 3 has been investigated by specific heat, magnetocaloric, electrical resistivity and neutron diffraction measurements. An external magnetic field applied at 4 K parallel to the crystallographic b -axis induces several magnetic phase transitions from commensurate to incommensurate structures and vice versa.

Examination of the magnetic properties of NpCo2

The authors report a series of measurements on both polycrystalline and single-crystal samples of NpCo2 in an effort to understand the low-temperature magnetic properties. Magnetization and Mossbauer experiments strongly suggest a transition to an ordered state. A variety of neutron experiments, including small-angle scattering and searches along the principal crystallographic directions with a single crystal, have failed to reveal any direct evidence for long-range ordering of the Np moments. These measurements do not exclude short-range order or a spin-glass state, although the latter appears to be excluded by the magnetization experiments. An account is given of the polarized-neutron experiments in an applied field that give direct evidence of hybridization between Np 5f and Co 3d electrons.

Magnetic structures of field-induced magnetic phases of EuAs3

Abstract The magnetic field induced magnetic phases of EuAs 3 have been studied with neutron diffraction. Five different magnetic structures have been identified by their moment directions and propagation vectors, only one of them being commensurate with the crystallographic lattice. The change of the magnetic structures with increasing field gives some information on the magnetic exchange interactions.

Magnetic structure of MnS2: single-k or multiple-k, collinear or helical spin density wave ?

The magnetic structure of the cubic type-III antiferromagnet MnS2 has been investigated by unpolarized and polarized single-crystal neutron diffraction measurements. By applying a magnetic field up to 8 T parallel to (0,0,1) and (1,-1,0) the authors have established that the magnetic structure of MnS2 is actually of the single-k type. Polarized neutron diffraction investigations using zero-field neutron polarimeter (CRYOPAD) have further established that the magnetic structure of MnS2 is collinear and is not of the helical spin density wave (HSDW) type.

Crystal structure of aluminium containing superconducting oxides: single crystal study of GdBa2Cu3−xAL x O6.88 (x=0.28) and ErBa2Cu3−yAl y O6.6 (y=0.14)

Following the discovery of the high temperature super-conductivity in YBa2Cu3O7−δ (Tc≃93 K) [1] it has been found that Y atoms can be substituted entirely by almost all of the rare-earth elements, except for Ce, Pr, and Tb, without changing the superconducting properties appreciably [2, 3]. The magnetic moments carried by the rare-earth atoms have apparently no influence on the superconducting properties. In Nd-, Sm-, Gd-, Dy- and Er-based compounds the rare earth moments order at low temperatures (0.025–2.2 K) [4–8] and the ordered antiferromagnetic state coexists with the superconducting state. We have investigated the antiferromagnetic ordering of these compounds by neutron diffraction both on powder and single crystal samples obtained from several laboratories [8–10]. Magnetic structures of all these compounds consist of antiferromagnetic (001) planes stacked ferro- or antiferromagnetically. There have been some controversies as to the stacking of antiferromagnetic (001) planes in GdBa2Cu3O7−δ.

Magnetic site susceptibilities in UPdSn

A study of the magnetization distribution in UPdSn using polarized-neutron scattering is reported. The measurements probed the induced magnetization on a single crystal under an applied magnetic field of 4.6 T at 45 K (paramagnetic phase), and the moment aligned with the field at 4 K (ordered state, canted antiferromagnetic phase I). It was found that the distribution of the magnetic moment in the unit cell is rather localized on the actinide atom. The magnetic moment of the U atom aligned by the field is at 45 K and at 4 K. Any moment on the palladium atom is very small and does not exceed 3% of the aligned moment of the uranium atom at either temperature. The magnetic form factor of the actinide is found to follow closely that of an electronic configuration, with a small reduction of the orbital moment in the ordered state compared to the free-ion value.