B. R. Coles

The role of finite magnetic clusters in Au-Fe alloys near the percolation concentration

Abstract Au-Fe alloys have been studied by magnetic measurements of various types in the composition range where ferromagnetism evolves from spin-glass character. The percolation limit identified from the results and other data is 15.5 + 0.3 at.% Fe. Between this composition and ∼28% Fe, finite clusters co-exist in significant concentrations with the infinite cluster, and a fairly well-defined temperature observed in the magnetic properties is ascribed to a freezing of these clusters that parallels that of finite clusters in the pre-percolation alloys. Magnetic resonance is observed in alloys with more than about 13% Fe (i.e. in alloys containing large nearest-neighbour clusters) and the temperature dependence of the linewidth correlates with the interpretation given of the magnetization data.

Magnetic and electrical properties of UCu2Ge2

Abstract Of the many interesting U and Ce compounds in the ThCr2,Si2, structure UCu2,Ge2, is unusual in showing the onset of ferromagnetism at a fairly high temperature (105 K) but its apparent replacement by antiferrornagnetism at low temperatures. We have made detailed studies of the magnetic behaviour and electrical resistance in this compound and of the effects of Ce substitution for U and some examination of the effects of Si substitution for Ge. Ce lowers the Curie temperature and fairly rapidly raises the temperature at which antiferromagnetic components appear, and by 50% Ce substitution only antiferromagnetic order is found. Si substitutions for Ge have much less effect on UCu2,Ge2.

Studies of the correlated electron system SmB6

We have prepared high-quality, single crystals of SmB 6 under various conditions to improve sample quality. We have measured the resistivity and magnetic susceptibility from room to liquid-helium temperatures to sort samples. We have applied pulsed magnetic fields as high as 50 T at temperatures as low as 40 mK while measuring resistivity. Our samples are of higher quality than previously known. All solvent-grown, single-crystal samples should be etched to remove a surface conductivity.

The loss of ferromagnetism in Gd(X)Al2, where X is U, Th or La

Abstract The results of magnetization, susceptibility and a.c. susceptibility measurements on alloys of GdAl2, in which Gd has been replaced by U, Th or La, are presented. The loss of ferromagnetism in the first two systems is very rapid and is followed by various types of spin-glass behaviour, but they differ in the maintenance of high paramagnetic Curie temperatures in the U system, the sustaining of which is associated with the spin-ftuctuation character of UAl2. The magnetic phase diagram of the LaAl2-GdAl2 system, which is ferromagnetic above about 14% GdAl2, shows a general similarity to that of the f.c.c. Au-Fe system including a re-entrant spin-glass regime.

Observation of a secondary structural phase in the heavy-fermion superconductor UPt3

Abstract Atomic-resolution transmission electron microscopy is used to investigate the microstructure of polycrystalline UPt3. While the host material is the previously reported hexagonal Ni3Sn structure, a second phase having the double-hexagonal Ni3Ti structure is also identified.

The influence of non-stoichiometry on the magnetic properties of UCo2

Abstract Resistance minima and temperature dependent susceptibilities at low temperatures in UCox (where x is close to 2.0) suggest that in all samples some Co atoms occupy wrong sites in the cubic Laves phase structure and give more magnetic character. The smallest fraction of such atoms is found at x = 1.978.

Magnetic ordering in f.c.c. alloys

Abstract A variety of properties is surveyed to consider the character of long range magnetic order in alloys like AuFe and CuMn, and the similarities between their behaviour and that of NiFe alloys.

EPR studies of Gd as a probe of uranium intermetallic compounds and YbCuAl

Abstract g-shifts and linewidths in the X band are given for Gd impurities substituted for U in UAl2, UCo2 and URh3, and for Yb in YbCuAl. No resonance was observed for the first of these alloys, an effect ascribed to 5f spin-fluctuation character. For the other U compounds Δ g and the temperature dependence of the linewidth were fairly small. For YbCuAl a very large positive g-shift and a strong temperature dependence of linewidth were observed. These results are compared with those for other materials in which spin-fluctuation or intermediate-valence character have been suggested.

A preliminary study of magnetic phase transitions in the praseodymium-neodymium alloys

Abstract The results are presented of susceptibility and resistivity studies for alloys across the whole system. The upper ordering temperature increases rapidly with initial additions of Nd to Pr but varies smoothly and has a composition-independent character. Alloys with 85–100%, Nd have transitions at lower temperatures, similar to those in pure Nd, the temperatures of which fall slowly with Pr content. In alloys of 4–75% Nd a susceptibility peak is found, the temperature of which rises up to 35% Nd, remains almost constant at about 9 K up to 53% Nd and then falls slowly. This is associated with the onset of a ferromagnetic component to the magnetization. Some of the alloys in this composition range show a shoulder or second maximum in the susceptibility at lower temperatures.

La(Gd)Al2 - a simple spin glass?

Abstract We compare this system with Au Fe where nearest neighbour interactions of d-d overlap character might give complications. Alloys of up to 9% Gd substitution show simple spin-glass behaviour, those with 10–16% show the “cluster glass” character of Au 15% Fe, and more concentrated alloys show the magnetization character of pure GdAl2 which is less simple than that of Au 30% Fe.