B.D. Rainford

Fundamental magnetic parameters from pure synthetic greigite (Fe3S4)

Pure ferrimagnetic greigite (Fe3S4) has been synthesized by reacting ferric chloride with thiourea and formic acid at 170°C. Sample purity was confirmed by X-ray diffraction, neutron diffraction and Mossbauer spectroscopy, coupled with magnetic measurements. Scanning electron microscope observations indicate clear cubo-octahedral and polyhedral crystal morphologies. The grain sizes are as large as 44 ?m. Detailed low- and high-temperature magnetic measurements document the previously poorly known magnetic properties of greigite. The synthetic greigite samples are dominated by pseudo-single-domain and multi-domain behavior. The saturation magnetization (M s ) at room temperature is ?59 Am2kg?1 (3.13 ? B per formula unit), which is higher than any value previously reported for greigite in the literature largely because of the high purity of this sample compared to others. No low-temperature magnetic transition has been detected; however, a local coercivity minimum is observed at around 130 K, which is probably associated with domain walls present in the studied samples. The high-temperature magnetic properties of greigite are dominated by chemical decomposition above around 250°C, which precludes determination of the Curie temperature, but our evidence indicates that it must exceed 350°C. On the basis of the Bloch spin wave expansion, the spin wave stiffness of greigite was determined for the first time as ?193 meV·A2 from low-temperature M s measurements, with the corresponding exchange constant J AB of ?1.03 meV.

The evolution of magnetic order in CrFe alloys. II. Onset of ferromagnetism

For pt.I see ibid., vol.13, p.441 (1983). The magnetic phase diagram for CrFe alloys was determined by neutron scattering and low-field magnetisation. Two critical concentrations were found as the state of magnetic order evolved from itinerant antiferromagnetism for Fe concentrations less than cA=16.0+or-0.5% to ferromagnetism for Fe concentrations greater than cF=19.0+or-0.5%. Spin-glass behaviour was observed between the two critical concentrations. The onset of ferromagnetism was studied by neutron small-angle scattering. Analysis of the concentration and temperature dependence of the magnetic correlation range below cF suggested that a geometrical element is involved in the development of ferromagnetic long-range order; these results were in semi-quantitative agreement with models for the percolation multi-critical point. The bulk magnetic properties of alloys between the two critical concentrations were consistent with a simple fine magnetic particle model of percolation clusters of Fe moments fluctuating against barriers arising from magnetostatic shape anisotropy.

Evidence for a generic quantum transition in high-T c cuprates

We study the low-energy spin fluctuations and superfluid density of a series of pure and Zn-substituted high-T(c) superconductors (HTS) using the muon spin relaxation and ac-susceptibility techniques. At a critical doping state, p(c), we find (i) simultaneous abrupt changes in the magnetic spectrum and in the superconducting ground state and (ii) that the slowing down of spin fluctuations becomes singular at T=0. These results provide experimental evidence for a quantum transition that separates the superconducting phase diagram of HTS into two distinct ground states.

Superparamagnetism in melt-spun CuCo granular samples

Abstract We have studied the magnetic properties of a melt-spun granular sample of Cu 87 Co 13 . The Cu matrix contains very fine particles of Co, having a bimodal distribution of sizes: ‘larger’ particles ( ∼ 300 A ) and very much smaller particles ( A ) with the latter exhibiting superparamagnetism. The magnetisation data indicate a spread of sizes for the smaller superparamagnetic particles, ranging from individual Co atoms up to clusters of more than 100 atoms of Co. Assuming a simple distribution of particle sizes, we calculated the magnetic field dependence of the magnetisation of the superparamagnetic particles and find excellent agreement with experiment.

Giant magnetoresistance by exchange springs in DyFe2/YFe2 superlattices

Magnetization and magnetoresistance measurements are reported for antiferromagnetically coupled DyFe2/YFe2 multilayers in fields up to 23 T. It is demonstrated that the formation of short exchange springs ( ~20 A) in the magnetically soft YFe2 layers results in a giant magnetoresistance as high as 32% in the spring region. It is shown that both the magnitude of the effect and its dependence on magnetic field are in good agreement with the theory of Levy and Zhang for domain wall induced giant magnetoresistance.

Study of the valence transition in YbInCu4 by inelastic neutron scattering

Abstract In YbInCu 4 a valence transition occurs from an almost stable trivalent phase for T >45K to an intermediate-valence state below 45K. We have investigated the imaginary part of the dynamic susceptibility X ″ of YbInCu 4 above and below the transition and compared it with spectra of the stable valent compound YbInNi 4 . YbInNi 4 and YbInCu 4 in the trivalent phase exhibit crystal-field (CF) excitations, centered at about 4meV. Below the valence transition in YbInCu 4 the CF excitations disappear and instead we observe a broad magnetic signal centered at about 40meV, i.e., energy transfers ten times larger than the position of the CF lines of the stable valent phase.

Ferrofluid particle size distributions from magnetisation and small angle neutron scattering data

Abstract The particle size distributions of three ferrofluids have been determined at room temperature from magnetisation and small angle neutron scattering measurements. The distributions were calculated using a data analysis technique based upon the concept of maximum entropy, and their significance estimated using an adaptation of the Backus-Gilbert technique.

Negative coercivity in epitaxially grown (110) DyFe2/YFe2 superlattices

Molecular beam epitaxial methods have been used to grow single crystal Laves phase DyFe2/YFe2 superlattice samples with a (110) growth direction. Detailed magnetization curves have been obtained for YFe2 dominated multilayer samples [wDyFe2/4wYFe2]×16 with w=45, 50, and 55 A. In particular, it is shown that the formation of magnetic exchange springs in the magnetically soft YFe2 layers, can be used to engineer multilayer samples with a negative coercivity. Further, by using asymmetric field cycling procedures, we have investigated the irreversible parts of the M–B loop, associated with the switching of the DyFe2 multilayers.

Neutron Scattering Study of the Intermediate-Valent Ground State in SmB6

Inelastic-neutron-scattering spectra were measured on the polycrystalline double isotopic sample 154Sm 11B6 in a wide range of the energy transfer. The intermultiplet transitions corresponding to f6 and f5 configurations of the intermediate-valent state of the Sm ion, as well as a low-energy excitation with unusual momentum transfer and temperature dependences have been observed in the magnetic part of the scattering function.

Structural magnetic and transport properties of a new valence fluctuating compound: CeRhSi2

Abstract The compound CeRhSi 2 , crystallizing in the orthorhombic CeNiSi 2 -type structure, has been synthesized and studied by magnetic susceptibility and electrical resistivity between 1.7 and 300 K. The unit cell volume of CeRhSi 2 shows a deviation from the lanthanide contraction. The temperature dependent magnetic susceptibility of CeRhSi 2 exhibits a broad maximum at 80 K, which is characteristic of Ce based valence fluctuating compounds. The electric resistivity also shows a broad maximum at about 100 K, and exhibits a T 2 dependence at low temperatures. These measurements confirm the valence fluctuating nature of CeRhSi 2 . An analysis is presented of the susceptibility data on the basis of the Coqblin-Schrieffer model, and of the resistivity data on the basis of the Freimuth model.