arXiv subjects

R. W. McCallum

Publications and source records attributed to R. W. McCallum.

24 records · Page 2Linked to original sources

arXiv2008

Exotic (anti)ferromagnetism in single crystals of Pr6Ni2Si3

The ternary intermetallic compound Pr6Ni2Si3, is a member of a structure series of compounds based on a triangular structure where the number of Pr atoms in the prism cross section can be systematically varied. Pr6Ni2Si3 contains two distinct Pr lattice sites which result in complex interactions between the magnetic ions. Extensive measurements of specific heat and magnetization on single crystal samples indicate that Pr6Ni2Si3 orders with both a ferromagnet and an antiferromagnet component, with ordering temperatures of 39.6 K and ~ 32 K, respectively. The ferromagnetic component // c-axis is accompanied by a large hysteresis, and the antiferromagnetic component,_|_ c-axis is accompanied by a spin-flop-type transition. More detailed measurements, of the vector magnetization, indicate that the ferromagnetic and the antiferromagnetic order appear independent of each other. These results not only clarify the behavior of Pr6Ni2Si3 itself, but also of the other members of the structure series, Pr5Ni2Si3 and Pr15Ni7Si10.

cond-mat.mtrl-sci↗

arXiv2007

Magnetic and thermal properties of the S = 1/2 zig-zag spin-chain compound In2VO5

Static magnetic susceptibility χ, ac susceptibility χ_{ac} and specific heat C versus temperature T measurements on polycrystalline samples of In2VO5 and χand C versus T measurements on the isostructural, nonmagnetic compound In2TiO5 are reported. A Curie-Wiess fit to the χ(T) data above 175 K for In2VO5 indicates ferromagnetic exchange between V^{4+} (S = 1/2) moments. Below 150 K the χ(T) data deviate from the Curie-Weiss behavior but there is no signature of any long range magnetic order down to 1.8 K. There is a cusp at 2.8 K in the zero field cooled (ZFC) χ(T) data measured in a magnetic field of 100 Oe and the ZFC and field cooled (FC) data show a bifurcation below this temperature. The frequency dependence of the χ_{ac}(T) data indicate that below 3 K the system is in a spin-glass state. The difference ΔC between the heat capacity of In2VO5 and In2TiO5 shows a broad anomaly peaked at 130 K. The entropy upto 300 K is more than what is expected for S = 1/2 moments. The anomaly in ΔC and the extra entropy suggests that there may be a structural change below 130 K in In2VO5.

cond-mat.str-el↗

arXiv2007

Low temperature heat capacity of Fe_{1-x}Ga_{x} alloys with large magneostriction

The low temperature heat capacity C_{p} of Fe_{1-x}Ga_{x} alloys with large magnetostriction has been investigated. The data were analyzed in the standard way using electron ($γT$) and phonon ($βT^{3}$) contributions. The Debye temperature $Θ_{D}$ decreases approximately linearly with increasing Ga concentration, consistent with previous resonant ultrasound measurements and measured phonon dispersion curves. Calculations of $Θ_{D}$ from lattice dynamical models and from measured elastic constants C_{11}, C_{12} and C_{44} are in agreement with the measured data. The linear coefficient of electronic specific heat $γ$ remains relatively constant as the Ga concentration increases, despite the fact that the magnetoelastic coupling increases. Band structure calculations show that this is due to the compensation of majority and minority spin states at the Fermi level.

cond-mat.mtrl-sci↗

arXiv2005

A small sealed Ta crucible for thermal analysis of volatile metallic samples

Differential thermal analysis on metallic alloys containing volatile elements can be highly problematic. Here we show how measurements can be performed in commercial, small-sample, equipment without modification. This is achieved by using a sealed Ta crucible, easily fabricated from Ta tubing and sealed in a standard arc furnace. The crucible performance is demonstrated by measurements on a mixture of Mg and MgB$_2$, after heating up to 1470$^{\circ}{\rm C}$. We also show data, measured on an alloy with composition Gd$_{40}$Mg$_{60}$, that clearly shows both the liquidus and a peritectic, and is consistent with published phase diagram data.

physics.ins-det↗

arXiv2005

Differential thermal analysis and solution growth of intermetallic compounds

To obtain single crystals by solution growth, an exposed primary solidification surface in the appropriate, but often unknown, equilibrium alloy phase diagram is required. Furthermore, an appropriate crucible material is needed, necessary to hold the molten alloy during growth, without being attacked by it. Recently, we have used the comparison of realistic simulations with experimental differential thermal analysis (DTA) curves to address both these problems. We have found: 1) complex DTA curves can be interpreted to determine an appropriate heat treatment and starting composition for solution growth, without having to determine the underlying phase diagrams in detail. 2) DTA can facilitate identification of appropriate crucible materials. DTA can thus be used to make the procedure to obtain single crystals of a desired phase by solution growth more efficient. We will use some of the systems for which we have recently obtained single-crystalline samples using the combination of DTA and solution growth as examples. These systems are TbAl, Pr$_7$Ni$_2$Si$_5$, and YMn$_4$Al$_8$.

cond-mat.mtrl-sci↗

arXiv2004

YbGaGe: normal thermal expansion

We report evidence of the absence of zero thermal expansion in well-characterized high-quality polycrystalline samples of YbGaGe. High-quality samples of YbGaGe were produced from high-purity starting elements and were extensively characterized using x-ray powder diffraction, differential thermal analysis, atomic emission spectroscopy, magnetization, and neutron powder diffraction at various temperatures. Our sample melts congruently at 920 C. A small amount of Yb2O3 was found in our sample, which explains the behavior of the magnetic susceptibility. These observations rule out the scenario of electronic valence driven thermal expansion in YbGaGe. Our studies indicate that the thermal expansion of YbGaGe is comparable to that of Cu.

cond-mat.mtrl-sci↗