Arnold R. Moodenbaugh

Lattice parameter, particle size, and thermal expansion results obtained from high‐temperature synchrotron transmission X‐ray diffraction are reported for magnetostructual NiAs‐type MnBi nanorods embedded in a Bi matrix. The structural data are consistent with elevated‐temperature magnetic measurements that indicate a first‐order nanorod Curie transition at 520 K, significantly depressed from the bulk MnBi Curie temperature of 633 K. The data suggest that the unit cell volume dependence of the magnetic behavior—also known as the volume exchange striction—of the MnBi compound is the determining factor underlying this phenomenon. The results imply that materials with magnetostructural transitions of technological interest may be altered by strain effects to tailor the interatomic distances towards the critical transition values.

The perovskites Sr ₂ MnRuO ₆ and LaSrMnRuO ₆ have been investigated and their properties compared with those of the broader family of A ₂ MRuO ₆ double perovskites (A=Sr, La; M=Cr, Mn, Fe, Co, Ni). Neutron powder diffraction shows that both phases lack long‐range Mn/Ru order. Sr ₂ MnRuO ₆ exhibits a cooperative Jahn–Teller distortion, conductivity that proceeds via variable‐range hopping, and antiferromagnetic (AFM) order (C‐type structure, T _N ≅200K, 2.05(1) μ _B per transition metal). LaSrMnRuO ₆ exhibits nearly undistorted octahedra, conductivity that proceeds via diffusion‐assisted small‐polaron hopping, and ferromagnetic (FM) order ( T _C ≅220K, 1.60(4) μ _B transition metal). Band structure calculations show that AFM Mn–O–Ru coupling optimizes polarization of the Ru t _{2 g} orbitals, while FM coupling favors delocalization of the Ru t _{2 g} electrons. The transition from antiferromagnetism to ferromagnetism is linked to a loss of orbital order and stabilized over competing glassy states by delocalization of Ru t _{2 g} electrons.

Rapidly solidified MnxBi(1−x) (x=0.05 Mn5, x=0.10 Mn10) alloys form as NiAs-type low-temperature phase MnBi nanoparticles in a Bi matrix. Microstructural differences underlie diverse magnetic behavior: Mn5 has well-separated nanorods along the basal plane axial directions of Bi, while Mn10 consists of MnBi nanorods and submicron-sized MnBi grains. The Curie transition of Mn10 is TC=630K with a second-order thermodynamic character, while Mn5 nanorods exhibit a reduced and hysteretic TC=520K. Magnetic field annealing fosters (00n) alignment of the MnBi phase. Mn10 also shows alignment of the Bi matrix by a proposed templating mechanism.

As-prepared, single-crystalline bismuth ferrite nanoparticles show strong size-dependent magnetic properties that correlate with: (a) increased suppression of the known spiral spin structure (period length of ∼62 nm) with decreasing nanoparticle size and (b) uncompensated spins and strain anisotropies at the surface. Zero-field-cooled and field-cooled magnetization curves exhibit spin-glass freezing behavior due to a complex interplay between finite size effects, interparticle interactions, and a random distribution of anisotropy axes in our nanoparticle assemblies.

Soft X-ray absorption spectroscopy (XAS), using fluorescence yield, was used to study the boron K near edge in MgB ₂ superconductor. The sample consists of MgB ₂ crystallites randomly oriented in a magnesium matrix. Abrasion of the sample surface in vacuum provides a surface relatively free of boron-containing impurities. The intrinsic boron K near edge spectrum of the sample at a temperature of 295 K is identified. This spectrum is then compared in detail with a spectrum calculated using the full potential linearized augmented plane wave method. Features predicted by the theory appear near the expected energies, with qualitative agreement regarding shape and intensity.