Oran Allan Pringle

Neutron-scattering studies of magnetic superconductors

After nearly twenty years of theoretical and experimental attempts to understand the mutual effects of superconductivity and magnetism on each other, the existence of long-range order of both kinds in the same material was finally established recently with the advent of the families of ternary compounds which have come to be known as “magnetic superconductors” [1,2]. The general use of this term does not necessarily imply a simultaneous coexistence (e.g., at a given temperature) of long-range magnetic order and superconductivity, although the very existence of superconductivity in crystals which contain ordered lattices of magnetic ions is itself initially very surprising and is believed to be due to the relative weakness of the exchange spin-flip (and therefore pair-breaking) interaction between the closed shell (necessarily f-like) of electrons on the magnetic ions and the conduction-band electrons. Nevertheless, in many of these compounds, magnetic long-range order (LRO) can simultaneously coexist with superconductivity over certain temperature ranges. This is particularly true of the case in which the magnetic LRO is of the antiferromagnetic type. The case of ferromagnetic LRO leads to the phenomenon of “re-entrance” and will be discussed in detail later. The best studied families of these compounds are the ternary borides with the chemical formula ReRh4B4 (where RE is a rare earth ion), and the Chevrel phase compounds with the formula REMo6S8 or REMo6Se8. There exist by now several review articles on these extremely interesting classes of materials, which the reader may consult for more detailed experimental information [3,4]. In this article, we shall concern ourselves with neutron scattering studies of these compounds. Reviews of the recent theoretical developments of the subject may be found in many other papers of this symposium.

Structural and Magnetic Properties of

Magnetic and structural properties of La0.7Sr0.3Mn1-xCrxO3 (x = 0.05, 0.10, 0.20, 0.30, 0.40 and 0.50) have been studied using neutron and x-ray diffraction and magnetic measurements. All samples are found to be of single phase. At 12 K, we find only ferromagnetic ordering for x < 0.30 and coexisting ferromagnetic and antiferromagnetic domains for x ⩾ 0.30. Our magnetic data indicate temperature dependent ferromagnetic to paramagnetic, and likely antiferromagnetic to paramagnetic transitions. We find no evidence of spin glass or cluster glass behavior, only long-range magnetic order with a transition to antiferromagnetic behavior beginning with x = 0.30 and almost completely antiferromagnetic behavior at x = 0.50.

Small‐angle neutron scattering of (Er0.8Ho0.2)Rh4B4

The (Er1−xHox)Rh4B4 pseudoternary alloy system has a minimum in the phase boundary between the superconducting and ferromagnetic phases near x=0.3. This minimum has been identified as due to the competing magnetic anisotropies of Er and Ho. It has also been suggested that there could be a Lifschitz point near the minimum. Using the 30‐m SANS camera at the National Center for Small‐Angle Scattering Research at ORNL, we have observed a peak in the SANS pattern for (Er0.8Ho0.2)Rh4B4 at Q=0.065 A−1. This peak appears for temperatures between Tc2, measured upon cooling, and Tm, and corresponds to a modulation of the magnetic moment with a wavelength of about 100 A, demonstrating that the modulated moment phase exists away from the ErRh4B4 end of the phase diagram. The wavelength of the modulation is the same as was previously observed in ErRh4B4. The fact that the wavelength of the modulation remains finite near x=0.3 appears to rule out the possibility of Lifschitz behavior near this point.

Structural and magnetic properties of La0.7Sr0.3Mn1−xNixO3 (x ≤ 0.4)

We have studied the structural and magnetic properties of La0.7Sr0.3Mn1−xNixO3 (xu2009=u20090.05, 0.1, 0.20, 0.30, and 0.40) perovskites using x-ray and neutron diffraction and magnetic measurements. Our data consist of neutron (λu2009=u20091.479u2009A) and x-ray (λu2009=u20091.5481u2009A; Cu Kα) powder diffraction and magnetization measurements. We previously suggested these systems transition from ferromagnetic to antiferromagnetic ordering with the intermediate concentrations containing coexisting domains of ferromagnetically and antiferromagnetically ordered states. Upon further detailed examination, we find that the samples are homogeneous and that neutron data can be fitted to a single long-range magnetically ordered state. The compositional dependent changes are driven by a shift in the dominant near neighbor interaction from ferromagnetic to antiferromagnetic. In the intermediate compositions, peaks previously identified as due to antiferromagnetic ordering, in fact arise from charge ordering; the system remains in a ferromagnet...

Magnetism and superconductivity in (Er/sub 0. 16/Ho/sub 0. 84/)Rh/sub 4/B/sub 4/

The superconducting and ferromagnetic phase boundaries in the (Er/sub 1-//sub x/Ho/sub x/)Rh/sub 4/B/sub 4/ mixed ternary alloy system meet in a multicritical point at x/sub cr/approx. =0.9. For x

Magnetism and superconductivity in (Er0.16Ho0.84)Rh4B4

The superconducting and ferromagnetic phase boundaries in the (Er/sub 1-//sub x/Ho/sub x/)Rh/sub 4/B/sub 4/ mixed ternary alloy system meet in a multicritical point at x/sub cr/approx. =0.9. For x

Structural and Magnetic Properties of La₀.₇Sr₀.₃Mn₁₋ₓCrₓO₃(X≤0.5)

Magnetic and structural properties of La0.7Sr0.3Mn1-xCrxO3 (x = 0.05, 0.10, 0.20, 0.30, 0.40 and 0.50) have been studied using neutron and x-ray diffraction and magnetic measurements. All samples are found to be of single phase. At 12 K, we find only ferromagnetic ordering for x < 0.30 and coexisting ferromagnetic and antiferromagnetic domains for x ⩾ 0.30. Our magnetic data indicate temperature dependent ferromagnetic to paramagnetic, and likely antiferromagnetic to paramagnetic transitions. We find no evidence of spin glass or cluster glass behavior, only long-range magnetic order with a transition to antiferromagnetic behavior beginning with x = 0.30 and almost completely antiferromagnetic behavior at x = 0.50.

Electronic Structures and Magnetism of LaNi₅₋ₓFeₓ Compounds

A systematic study of the magnetic moments and hyperfine interactions of the LaNi5–xFex compounds was performed using the spin-polarized tight-binding-linear-muffin-tin-orbital methods (TBLMTO). The calculated results were compared with neutron diffraction and Mossbauer experimental data and found to be in good agreement. Fe atoms exhibit more localized magnetic moments in LaNi5–xFex when the Fe content x is lower than 1.0. The calculated magnetic moments of the Fe atoms approach values as large as 2.5μB due to the dilution effect. The strong s–d hybridization between the Ni and La atoms and the neighboring Fe in the Fe-substituted samples results in a large transferred valence contribution to the total hyperfine field. It is found that the Fe and Ni magnetic moments decrease abruptly at a unit cell volume which is 5% smaller than the observed volume in LaNi4Fe due to the instability of the Fe magnetic moment. LaNi5 becomes ferromagnetic when the unit cell volume is larger than its original volume.

A Magnetic, Neutron Diffraction, and Mössbauer Spectral Study of the Tb 2 Fe 17-x Ga x Solid Solutions, with 7 ⤠x ⤠9

Abstract The compositional dependence of the Curie temperature of the Tb 2 Fe 17− x Ga x solid solutions shows a primary maximum for x equal to three and a secondary maximum for x equal to eight. The Mossbauer spectra and powder neutron diffraction patterns of Tb 2 Fe 9 Ga 8 indicate that gallium avoids the 9d site, completely fills the 6c site, and occupies preferentially the 18f site over the 18h site. This gallium preferential substitution leads to a nearly ordered two-dimensional antiferromagnetic arrangement of the terbium and iron magnetic moments, an increase in Curie temperature, and a surprisingly sharp Mossbauer spectrum.

A Magnetic, Neutron Diffraction, and Mössbauer Spectral Study of Nd₂Fe₁₅Ga₂ and the Tb₂Fe₁₇₋ₓGaₓ Solid Solutions

An x‐ray diffraction study of the substitution of gallium in Tb2Fe17 to form the Tb2Fe17−xGax solid solutions indicates that the compounds adopt the rhombohedral Th2Zn17 structure. The unit cell volume and the a‐axis lattice parameter increase linearly with increasing gallium content. The c‐axis lattice parameter increases linearly from x=0 to 6 and then decreases between x=7 and 8. Magnetic studies show the Curie temperature increases by ∼150° above that of Tb2Fe17 to reach a maximum between x=3 and 4, and then decreases with further increases in x. Neutron diffraction studies of Nd2Fe15Ga2 and Tb2Fe17−xGax, with x equal to 5, 6, and 8, indicate that the gallium completely avoids the 9d site, occupies the 6c ‘‘dumbell’’ site only at high values of x and strongly prefers the 18f site at high values of x. The magnetic neutron scattering indicates both that the terbium sublattice magnetization couples antiferromagnetically with the iron sublattice and that there is a change in easy magnetization direction f...