T. Dumelow

Nonreciprocity in the Goos-Hänchen shift on oblique incidence reflection off antiferromagnets

We investigate a lateral shift of the reflected beam on reflection of far-infrared radiation, at oblique incidence, off an antiferromagnet in an external magnetic field, at frequencies close to the magnon resonances. This shift is nonreciprocal and depends on the direction of the applied field. It occurs both at bulk and reststrahlen frequencies, with or without damping. We illustrate this effect using simulations of reflection off MnF2 at low temperature.

Lateral shift of far infrared radiation on normal incidence reflection off an antiferromagnet

A lateral shift, similar to a Goos-Hanchen shift, of a normally incident electromagnetic beam reflected off an antiferromagnet in the presence of an external magnetic field is predicted. This shift is interpreted in terms of nonreciprocity of the reflected phase, and is confirmed using numerical simulation. There is also a lateral displacement of the field within the antiferromagnet, but not of the beam transmitted through an antiferromagnetic slab.

Ionic nitriding in cathodic cage of AISI 420 martensitic stainless steel

Abstract Cylindrical shape samples of AISI 420 martensitic stainless steel were nitrided by cathodic cage plasma nitriding technique at temperatures of 623, 673 and 773 K for 5 h. In this technique the samples are placed on an insulating plate inside a cage that shields the cathodic potential. A systematic study was made to test the efficiency of this technique, when compared with conventional ionic nitriding, related to the elimination of defects such as edge effect. The process present a bigger nitriding rate and the samples nitrided using this new technique displayed crystalline phases and hardness comparable with those obtained using conventional ionic nitriding. Moreover, it was possible to eliminate completely the erosion rings often present in conventionally ionic nitrided samples due to edger effects.

Reentrant spin glass behavior in polycrystalline La0.7Sr0.3Mn1-XFeXO 3

The magnetic and transport properties of the compound La0.7Sr0.3Mn1-xFexO 3 (0.1 < x <0.4) have been studied by means of electrical resistivity, AC magnetic susceptibility, and DC magnetization. At low concentrations (x <0.1), the system displays essentially para-to-ferromagnetic transitions as the temperature is decreased, although a decrease in the magnetic moment has been observed in previous studies at temperatures a little below TC. This ferromagnetism is explained by double exchange theory in terms of the formation of Mn+3/Mn+4 ions pairs in the system. At concentrations in the range 0.1 < x <0.4 the system is more complex. Increased Fe doping not only weakens the ferromagnetic (FM) order and augments the resistivity of the samples, but also induces the appearance of a reentrant spin glass phase at low temperatures (T < 60 K). Irreversibility of the magnetization measured with zero field cooling and with field cooling has been observed. In addition, the AC susceptibility peak position varies with frequency. All these effects are characteristic of spin glass behavior. The results have been interpreted based in an increase of frustration due to increasing competition between FM Mn+3/Mn+4 interactions and antiferromagnetic interactions between ions at the boundaries of Fe clusters.

Far Infrared Slab Lensing and Subwavelength Imaging in Crystal Quartz

Departamento de F´isica, Universidade Federal do Ceara´, Campus Pici, 60455-900 Fortaleza CE, Brazil(Dated: November 20, 2012)We examine the possibility of using negative refraction stemming from the phonon polaritonresponse in an anisotropic crystal to create a simple slab lens with plane parallel sides, and show thatimaging from such a lens should be possible at room temperature despite the effects of absorptionthat are inevitably present due to phonon damping. In particular, we consider the case of crystalquartz, a system for which experimental measurements consistent with all-angle negative refractionassociated with the phonon polariton response have already been demonstrated. Furthermore, weinvestigate the possibility of subwavelength imaging from such materials, and show that it shouldbe possible for certain configurations.

Giant magnetoimpedance in FeAg granular alloys

We have observed giant magnetoimpedance (GMI) in an FeAg granular alloy. Impedance and transverse susceptibility (TS) measurements as a function of the external dc magnetic field were made at room temperature. The variation of the magnetoimpedance ratio with magnetic field has been related to a corresponding TS ratio change as a function of the magnetic field. The peaks in the GMI and TS curves are in the same position and have been associated in both cases to effective anisotropy fields.

Beam shifts on reflection of electromagnetic radiation off anisotropic crystals at optic phonon frequencies

We discuss possible beam shifts on external reflection of electromagnetic radiation off an anisotropic crystal around its optic phonon frequencies. In certain situations, these displacements may oppose the energy flow within the crystal. We show examples for reflection off crystal quartz in various configurations. Displacements of the order of a wavelength should occur with significant reflected intensity.

Nonreciprocal Phenomena on Reflection of Terahertz Radiation off Antiferromagnets

There are a number of ways that reciprocity principles in opt ics may be affected by the presence of a static magnetic field (Potton, 2004). A classic example is Faraday rotation in which a plane polarised electromagnetic beam propagating through a suitable medium is rotated in the presence of a static magnetic field along the direction of propagation. The handedness of this rotation depends on the propagation direction, a nonreciprocal effect usefully applied to the construction of optical isolators (Dotsch et al., 2005). Nonreciprocal effects of this type are closely related to the idea that magnetic fields break time reversal symmetry. Similar nonreciprocal phenomena can occur, in various guises, on reflection off a semi-infinite sample. We discuss such behaviour in the present chapter, in the context of reflection off antiferromagnetic materials. In contrast to nonreciprocal phenomena based on the Faraday effect, our interest is in the Voigt geometry, in which the static magnetic field is perpendicular to the direction of propagation. We consider the well established phenomena of nonreciprocity in the intensity and phase of oblique incidence radiation, but concentrate mainly on recent developments on nonreciprocal power flow and finite beam effects. We restrict discussion to the simple two dimensional geometry shown in Figure 1. Radiation is reflected, in the xy plane, off a semi-infinite sample, isotropic in this plane, in the presence of a static magnetic field B0 along z (into the page). Note that, in this configuration, we do not have to worry about polarisation effects, since there is no mixing between s-polarised (electromagnetic E field component along z) and p-polarised (electromagnetic H field component along z) radiation.

Magnetic properties of the granular alloy Fe10Ag90 as a function of annealing temperature

Fe10Ag90 granular alloys have been prepared using a sol-gel process, sintered at 300 °C and annealed at temperatures between 400 °C and 700 °C. The mean size of the iron particles, obtained from X-ray diffraction, is 30.0 ± 0.7 nm. Due to the existence of a distribution of particle sizes in these samples, both blocked (BL) and superparamagnetic (SPM) particles are present simultaneously, as confirmed by magnetization measurements at room temperature. AC susceptibility measurements as a function of temperature reveal a magnetic phase transition at about 770 °C, indicating the presence of particles exhibiting bulk behavior, in the samples annealed above 550 °C. The presence of these particles has been attributed to an atomic diffusion process between the grains, forming bulk-like multiple-domain Fe particles having Curie temperatures near that of bulk a-Fe phase (TC = 770 °C).

Beam shifts of Far-Infrared Radiation on Reflection off the Anisotropic Crystal LiYF 4

We investigate the Goos-Hanchen shifts associated with phonons in the anisotropic crystal LiYF4. In p-polarization the shifts can be either positive or negative, and various mechanism are discussed.