Limits on Superconductivity-Related Magnetization in Sr2_2RuO4_4 and PrOs4_4Sb12_{12} from Scanning SQUID Microscopy

We present scanning SQUID microscopy data on the superconductors Sr2RuO4 (Tc = 1.5 K) and PrOs$_4$Sb$_{12}$ (Tc = 1.8 K). In both of these materials, superconductivity-related time-reversal symmetry-breaking fields have been observed by muon spin rotation; our aim was to visualize the structure of these fields. However in neither Sr$_2$RuO$_4$ nor PrOs$_4$Sb$_{12}$ do we observe spontaneous superconductivity-related magnetization. In Sr$_2$RuO$_4$, many experimental results have been interpreted on the basis of a $px \pm ipy$ superconducting order parameter. This order parameter is expected to give spontaneous magnetic induction at sample edges and order parameter domain walls. Supposing large domains, our data restrict domain wall and edge fields to no more than ~0.1% and ~0.2% of the expected magnitude, respectively. Alternatively, if the magnetization is of the expected order, the typical domain size is limited to ~30 nm for random domains, or ~500 nm for periodic domains.Comment: 8 pages, 7 figures. Submitted to Phys. Rev.

Magnetoelastic coupling in Fe100−xGex single crystals with 4

In this paper we examine the elastic (c′ and c44) and magnetostrictive (λ100 and λ111) behaviors of Fe100−xGex for 4\u3cx\u3c18, quantities used further to find the fundamental magnetoelastic coupling constants b1 and b2 at room temperature. The x dependence ofb1 and b2 for Fe100−xGex is contrasted to those of Fe100−xGax and Fe100−xAlx. While the rhombohedral shear elastic constant c44 is almost insensitive to the type and amount of solute, the tetragonal shear constant c′ shows a pronounced and rapid softening with increasing x for all three alloys but with different decreasing slopes. Similarly, while the rhombohedral magnetostriction λ111 behavior is analogous for all three alloy systems, showing a sign change from negative to positive at the onset of chemical order, the tetragonal magnetostriction λ100 behavior differs. For the Ga and Al alloys, λ100 maintains positive values over the entire x range, both curves showing large peak values, whereasλ100 of Fe100−xGex exhibits a moderate positive peak followed by a negative dip, both of comparable magnitude. Finally the tetragonal coupling constant −b1 of Fe–Ge shows a marked, sharp decrease as chemical order occurs at x ∼ 12 at. % Ge. The decline continues until the ordered D03 phase is fully established at x ∼ 18 at. % Ge. The peak value of |b1| for Fe–Ge is approximately half of those for Fe–Ga and Fe–Al. This smaller value of |b1|, obtained for the higher electron concentration Ge alloy, is consistent with predictions based on band structure calculations. The rhombohedral coupling constant−b2 shows a consistent sign change at the occurrence of chemical ordering in both Fe–Ga and Fe–Ge

The effect of partial substitution of Ge for Ga on the elastic and magnetoelastic properties of Fe–Ga alloys

Both components of the tetragonal magnetoelastic constant b1: the saturation magnetostriction, λγ,2 = (3/2)λ100, and the magnetic-field saturated shear elasticity, c′ = (c11−c12)/2, were investigated over a wide temperature range for the magnetostrictiveFe1−x−yGaxGey alloys, (x+y ≅ 0.125, 0.185, and 0.245; x/y ≅ 1 and 3). The magnetostriction was measured from 77 to 425 K using standard strain gage techniques. Both shear elastic constants (c′ and c44) were measured from 5 to 300 K using resonant ultrasound spectroscopy. Six alloy compositions were prepared to cover three important regions: (I) the disordered solute α-Fe region, (II) a richer solute region containing both disordered and ordered phases, and (III) a rich solute region containing ordered multiphases. Our observations reveal that, when the data is presented versus the total electron/atom (e/a) ratio, the above regions for both the ternary and binary alloys are in almost perfect alignment. Following this analysis, we find that the magnetoelastic coupling, b1, peaks for both the binary and the ternary alloys at e/a ∼ 1.35. The values of c′ as well as of λγ,2 in region I of the ternary alloys, when plotted versus e/a, fall appropriately between the binary limits

A study of strongly correlated electron behavior in the filled skutterudites

In the search for new and interesting strongly correlated electron phenomena, a detailed study has been underway of the rare earth-based filled skutterudite compounds. This work has been driven by the rich variety of strongly correlated electron phenomena exhibited by the filled skutterudites such as metal insulator transitions, heavy fermion behavior, quadrupole ordering, non-Fermi liquid behavior, and heavy fermion superconductivity. The filled skutterudite compounds have the chemical formula MT4X12 where M = alkali metal, alkaline-earth, lanthanide, or actinide; T = Fe, Ru, or Os; and X = P, As, or Sb. To improve the general understanding of these systems and to look for other strongly correlated electron behavior, single crystals of Pr0.87Fe4Sb12, PrOs4As12, \PrOs4P12, SmOs4Sb12, and NdOs4Sb12 were grown and characterized with X-ray diffraction, magnetization, electrical resistivity and specific heat measurements. Measurements of the filled skutterudite compound Pr0.87Fe4Sb12 reveal long rang magnetic ordering below 4.1 K with indications of ferrimagnetic ordering. Features in the magnetization of the filled skutterudite PrOs4As12 are consistent with antiferromagnetic ordering below 2.3 K. The specific heat, electrical resistivity, and magnetization measurements on PrOs4As12 also show features consistent with at least 2 to 3 ordered phases. Strong crystalline electric field effects were observed in PrOs4P12 and SmOs4Sb12, with both systems exhibiting Schottky anomalies in specific heat measurements. Magnetic ordering was observed in SmOs4Sb12 below 2.6 K with a ferromagnetic component as indicated by hysteresis in the magnetization as a function of field at 2 K. Magnetic ordering was also observed in the filled skutterudite compound NdOs4Sb12 below 0.9 K. Analysis of the specific heat measurements on Pr0.87Fe4Sb12, PrOs4As12, SmOs4Sb12, and NdOs4Sb12 reveal an enhanced electron effective mass, with SmOs4Sb12 showing the largest enhancemen

Magnetoelastic coupling in Fe100−xGex single crystals with 4

In this paper we examine the elastic (c′ and c44) and magnetostrictive (λ100 and λ111) behaviors of Fe100−xGex for 4xb1 and b2 at room temperature. The x dependence ofb1 and b2 for Fe100−xGex is contrasted to those of Fe100−xGax and Fe100−xAlx. While the rhombohedral shear elastic constant c44 is almost insensitive to the type and amount of solute, the tetragonal shear constant c′ shows a pronounced and rapid softening with increasing x for all three alloys but with different decreasing slopes. Similarly, while the rhombohedral magnetostriction λ111 behavior is analogous for all three alloy systems, showing a sign change from negative to positive at the onset of chemical order, the tetragonal magnetostriction λ100 behavior differs. For the Ga and Al alloys, λ100 maintains positive values over the entire x range, both curves showing large peak values, whereasλ100 of Fe100−xGex exhibits a moderate positive peak followed by a negative dip, both of comparable magnitude. Finally the tetragonal coupling constant −b1 of Fe–Ge shows a marked, sharp decrease as chemical order occurs at x ∼ 12 at. % Ge. The decline continues until the ordered D03 phase is fully established at x ∼ 18 at. % Ge. The peak value of |b1| for Fe–Ge is approximately half of those for Fe–Ga and Fe–Al. This smaller value of |b1|, obtained for the higher electron concentration Ge alloy, is consistent with predictions based on band structure calculations. The rhombohedral coupling constant−b2 shows a consistent sign change at the occurrence of chemical ordering in both Fe–Ga and Fe–Ge.Copyright 2009 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. The following article appeared in Journal of Applied Physics 105 (2009): 07A932 and may be found at http://dx.doi.org/10.1063/1.3061864.</p

The effect of partial substitution of Ge for Ga on the elastic and magnetoelastic properties of Fe–Ga alloys

Both components of the tetragonal magnetoelastic constant b1: the saturation magnetostriction, λγ,2 = (3/2)λ100, and the magnetic-field saturated shear elasticity, c′ = (c11−c12)/2, were investigated over a wide temperature range for the magnetostrictiveFe1−x−yGaxGey alloys, (x+y ≅ 0.125, 0.185, and 0.245; x/y ≅ 1 and 3). The magnetostriction was measured from 77 to 425 K using standard strain gage techniques. Both shear elastic constants (c′ and c44) were measured from 5 to 300 K using resonant ultrasound spectroscopy. Six alloy compositions were prepared to cover three important regions: (I) the disordered solute α-Fe region, (II) a richer solute region containing both disordered and ordered phases, and (III) a rich solute region containing ordered multiphases. Our observations reveal that, when the data is presented versus the total electron/atom (e/a) ratio, the above regions for both the ternary and binary alloys are in almost perfect alignment. Following this analysis, we find that the magnetoelastic coupling, b1, peaks for both the binary and the ternary alloys at e/a ∼ 1.35. The values of c′ as well as of λγ,2 in region I of the ternary alloys, when plotted versus e/a, fall appropriately between the binary limits.Copyright 2010 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics. The following article appeared in Journal of Applied Physics 107 (2010): 09A926 and may be found at http://dx.doi.org/10.1063/1.3368108.</p