In this chapter, we will restrict our attention to the ferrites and a few other closely related materials. The great interest in ferrites stems from their unique combination of a spontaneous magnetization and a high electrical resistivity. The observed magnetization results from the difference in the magnetizations of two non-equivalent sub-lattices of the magnetic ions in the crystal structure. Materials of this type should strictly be designated as “ferrimagnetic” and in some respects are more closely related to anti-ferromagnetic substances than they are to ferromagnetics in which the magnetization results from the parallel alignment of all the magnetic moments present. We shall not adhere to this special nomenclature except to emphasize effects, which are due to the existence of the sub-lattices.

Ferromagnetic materials

▪ Abstract Metal-hydrogen (M-H) systems are interesting from both a theoretical and a practical point of view. M-H systems are utilized for energy-storage systems, in sensor applications, and in catalysis. These systems are often exploited as models for studying basic material properties, especially when the size of these systems is small and nonbulk-like contributions become dominant. Surfaces, nanocrystals, vacancy- and dislocation-rich materials, thin films, multilayers, and clusters as systems of major interest are addressed in this review. We show that the hydrogen solubility of M-H systems is strongly affected by the morphology and microstructure of and the stress between regions of different hydrogen concentration. For small-sized systems, surface- or interface-related sites become important and change the overall solubility as well as the phase boundaries of M-H systems. In thin films deposited on stiff substrates, compressive stresses evolve during hydrogen loading because the films are effective...

HYDROGEN IN METALS: Microstructural Aspects

The ability to cool and slow atoms with light for subsequent trapping allows investigations of the properties and interactions of the trapped atoms in unprecedented detail. By contrast, the complex structure of molecules prohibits this type of manipulation, but magnetic trapping of calcium hydride molecules thermalized in ultra-cold buffer gas and optical trapping of caesium dimers generated from ultra-cold caesium atoms have been reported. However, these methods depend on the target molecules being paramagnetic or able to form through the association of atoms amenable to laser cooling, respectively, thus restricting the range of species that can be studied. Here we describe the slowing of an adiabatically cooled beam of deuterated ammonia molecules by time-varying inhomogeneous electric fields and subsequent loading into an electrostatic trap. We are able to trap state-selected ammonia molecules with a density of 10(6) cm(-3) in a volume of 0.25 cm3 at temperatures below 0.35 K. We observe pronounced density oscillations caused by the rapid switching of the electric fields during loading of the trap. Our findings illustrate that polar molecules can be efficiently cooled and trapped, thus providing an opportunity to study collisions and collective quantum effects in a wide range of ultra-cold molecular systems.

Electrostatic trapping of ammonia molecules

Experimental and theoretical studies of the linear and nonlinear optical susceptibilities for single crystals of potassium titanyl phosphate KTiOPO4 are reported The state-of-the-art full potentia

Investigation of the linear and nonlinear optical susceptibilities of KTiOPO4 single crystals: theory and experiment.

Hydrogen detection near surfaces and shallow interfaces with resonant nuclear reaction analysis

Laves phase DyFe2/YFe2 multilayers have been grown epitaxially on a YFe2 seed layer, with a (110) growth direction. Magnetic measurements taken in applied fields of up to 12 T, and from 5 K to room temperature, show that short period multilayers (∼100 A) behave, collectively, as a single magnetic entity. As a result, it is possible to engineer magnetic compensation points, in a digital manner, by adjusting the thicknesses of the alternate DyFe2 and YFe2 layers. Nevertheless, the magnetic response of the DyFe2/YFe2 structure and that of the YFe2 seed layer are not completely independent of one another. Because of a mismatch in the Fe–Fe magnetic exchange at the multilayer/seed interface, a 180° magnetic soliton-like domain (“magnetic twister”) is set up in the top of the YFe2 seed layer. A semiquantitative model describing the properties of the magnetic twister is presented and discussed.

Magnetic properties of epitaxial (110) multilayer films of DyFe2 and YFe2

HoHy exhibits dramatic changes in both structural and optical properties as y is varied from zero to three by hydrogen loading This work reports on the effect of such loading upon epitaxial single-crystal Ho films grown by molecular beam epitaxy on (110) Nb (11 0) Al2 O3 substrates Upon loading, the film undergoes a structural transition from hcp metal ( -phase, H in solid solution) to the fcc dihydride ( -phase) There is a further transformation between the dihydride phase and the hexagonal trihydride ( -phase) Three films of HoHy , where, nominally, y = 0,2,3, were studied by XRD, AFM, UFM, SEM and TEM Triangular networks of features of width ~300 nm and height ~20-30 nm that align with the [0 1], [ 01] and [1 0] directions of the dihydride sample are seen on the surface of both the dihydride and the trihydride samples, but have a much greater density on the surface of the latter Such features are not present on the as-grown metal layer In situ controlled-environment transmission electron microscopy (CETEM) studied the effects of hydrogen loading within the dihydride phase, in cross sectional geometry During loading, slip within the dihydride phase was observed The hypothesis that the triangular features are due to the precipitation of the di- and trihydride compounds, with expanded lattices, on the slip planes within the - or -phase host crystal is discussed

https://iopscience.iop.org/article/10.1088/0022-3727/33/8/302/pdf

Structural changes to epitaxial (0001) holmium layers during hydrogen loading

Molecular beam epitaxial methods have been used to grow single crystal Laves phase DyFe2–YFe2 superlattice samples with a (110) growth direction. It is shown that it is possible, in principle, to engineer a desired coercivity between the limits KDyFe2⩽K⩽∞. This can be achieved by adjusting the relative thickness of the individual DyFe2 and YFe2 layers, in multilayer films This novel feature is illustrated, using the superlattice films [x A DyFe2/(100-x) A YFe2]×40, with x=80, 60, 50, and 45. It is found that the measured coercivity is in semiquantitative agreement with a simple theoretical expression, for the nucleation fields in both bilayer and multilayer compounds. However, in practice, exchange spring penetration into the DyFe2 layers can set a limit to the maximum coercivity that can be achieved.

Engineering coercivity in epitaxially grown (110) films of DyFe2–YFe2 superlattices

This paper reports the conductive and ferroelectric properties of RTP crystals that are important for the fabrication of quasi-phase-matched RTP devices. The frequency dependence of the conductivity is shown to be represented by a Debye-function in the lower frequency range ({<}10 kHz) and to obey a power law in the high frequency range. The spontaneous polarization and coercive field are found to be 0.198±0.005 C m-2 and 2.65±0.2 kV mm-1, respectively, by measuring the switching current of RTP under domain reversal. Ferroelectric hysteresis loops are obtained in which the effects of a previously unreported dependence of the ac conductivity on the amplitude of the applied electric field are clearly visible. The domain structure of as-grown crystals and the change in the domain structure under a square waveform of 3 kV mm-1 are revealed through the use of optimized etching conditions.

http://iopscience.iop.org/article/10.1088/0022-3727/33/21/324/pdf

Ferroelectricity, conductivity, domain structure and poling conditions of rubidium titanyl phosphate

Single crystal films of Gd and Nd were grown by MBE techniques using the LaMBE facility in Oxford. The thickness of the layers was 5000 Angstrom and the mosaic spread about 0.2-0.3 degrees. Some of the layers were hydrogenated by KTH in Stockholm to obtain the dihydride. The magnetic structures were determined using the TAS1 triple axis spectrometer at the Rise National Laboratory in Denmark. The magnetic structure of the Gd film was a ferromagnet, similar to that of bulk Gd. In detail, however, there were differences and the spin direction differed from that of the bulk material while the phase with the moments aligned along the c-axis was absent in the him. The magnetic structure of GdH2 was antiferromagnetic with a wavevector of (1/2 1/2 1/2) and a transition temperature of 22 +/- 0.5 K, much lower than that of bulk Gd, 290 K. In contrast, the magnetic structure of NdH2 is incommensurate with a wavevector of (0.532 0.532 1.532) and a magnetic ordering temperature of 34 +/- 1 K.

https://iopscience.iop.org/article/10.1088/0953-8984/12/23/309/pdf

R.C.C Ward

Papers

Magnetic properties of epitaxial (110) multilayer films of DyFe2 and YFe2

Structural changes to epitaxial (0001) holmium layers during hydrogen loading

Engineering coercivity in epitaxially grown (110) films of DyFe2–YFe2 superlattices

Ferroelectricity, conductivity, domain structure and poling conditions of rubidium titanyl phosphate

Magnetic structure of Gd, GdH2 and NdH2 single crystal films