R. S. Edwards

Exchange bias in Ni4 single-molecule magnets

Abstract The syntheses and physical properties are reported for three single-molecule magnets (SMMs) with the composition [Ni(hmp)(ROH)Cl]4, where R is CH3 (complex 1), CH2CH3 (complex 2) or CH2CH2C(CH3)3 (complex 3) and hmp− is the monoanion of 2-hydroxymethylpyridine. The core of each complex is a distorted cube formed by four NiII ions and four alkoxide hmp− oxygen atoms at alternating corners. Ferromagnetic exchange interactions give a S=4 ground state. Single crystal high-frequency EPR spectra clearly indicate that each of the complexes has a S=4 ground state and that there is negative magnetoanisotropy, where D is negative for the axial zero-field splitting DŜz2. Magnetization versus magnetic field measurements made on single crystals with a micro-SQUID magnetometer indicate these Ni4 complexes are SMMs. Exchange bias is seen in the magnetization hysteresis loops for complexes 1 and 2.

Definitive spectroscopic determination of the transverse interactions responsible for the magnetic quantum tunneling in Mn(12)-acetate.

We present detailed angle-dependent single crystal electron paramagnetic resonance (EPR) data for field rotations in the hard plane of the S=10 single molecule magnet Mn(12)-acetate. A clear fourfold variation in the resonance positions may be attributed to an intrinsic fourth-order transverse anisotropy (O(4)/(4)). Meanwhile, a fourfold variation of the EPR line shapes confirms a recently proposed model wherein disorder associated with the acetic acid of crystallization induces a locally varying quadratic (rhombic) transverse anisotropy [O (2)/(2) identical with E(S (2)/(x)-S(2)/(y))]. These findings explain most aspects of the magnetic quantum tunneling observed in Mn(12)-acetate.

Rayleigh wave interaction with surface-breaking cracks

This paper investigates Rayleigh wave interaction with simulated, surface breaking cracks using a finite element method, in which the scattered wave modes giving rise to the in-plane and out-of-plane displacements are presented. By looking at the contribution from all of the transmitted, reflected, and mode-converted signals at the crack, the magnitude of signal enhancement in the near field and the mechanism by which this occurs can be fully explained. Furthermore, oscillations in the Rayleigh wave reflection and transmission coefficients with crack depth in the far field can be explained by means of multiple reflected and transmitted wave modes at the crack, whose relative amplitudes are dependent on the crack depth. Results agree with previously published experimental measurements.

Surface-breaking crack gauging with the use of laser-generated Rayleigh waves

This paper studies crack depth gauging using wideband Rayleigh waves generated by laser in the thermoelastic regime. A finite element method (FEM) is used to calculate the scattering of the Rayleigh waves. The reflected and transmitted Rayleigh waves have different propagation paths and have very different frequency spectra. The arrival times of these scattered waves are crack depth related and can be used for crack depth gauging. Experimental measurements agree well with FEM predictions and validate the usefulness of the crack depth gauging method.

Coupling mechanism of electromagnetic acoustical transducers for ultrasonic generation

Electromagnetic acoustic transducers (EMATs) generate ultrasonic waves in metals through an electromagnetic coupling mechanism. A concept for EMAT generation, using a coil alone without a permanent magnet, but with a pulse generator and a sample, is introduced. A simplified equivalent coil circuit is given and has been validated by experimental measurements. Such an equivalent circuit can be used for variations in excitation current calculations, which have often been neglected in previous publications in this area but have proved to be of great importance in considering the efficiency and frequency characteristics of ultrasonic generation. The equivalent coil inductance is dependent on the distance between the coil and the sample, due to coil interactions with eddy currents and the Ampere current of the magnetization mechanism. Analytical solution for an annular coil above metal samples is given, and the influences of the lift-off, coil radius, material magnetic permeability and electrical conductivity o...

High-frequency electron paramagnetic resonance investigations of tetranuclear nickel-based single-molecule magnets

We report preliminary high-frequency electron paramagnetic resonance (EPR) investigations for several tetranuclear nickel complexes which exhibit single-molecule magnetism, including low-temperature (below ∼1 K) hysteresis loops and resonant magnetic quantum tunneling. The combination of a cavity perturbation technique and a split-coil magnet facilitates high-sensitivity, multifrequency (40 to 200+ GHz), angle dependent single-crystal EPR measurements. The data confirm the expected S=4 ground states, and a negative magnetocrystalline anisotropy for each member in the series. An unusual splitting of the easy-axis EPR peaks is observed, which may be interpreted in terms of distinct Ni4 species within the crystals. Overall, however, the trends associated with the splitting, as well as the EPR linewidths and shapes, suggest that intermolecular exchange interactions are important. Indeed, differences between the EPR spectra obtained for different complexes correlate nicely with the expected strength of exchang...

Signal enhancement of the in-plane and out-of-plane Rayleigh wave components

Several groups have reported an enhancement of the ultrasonic Rayleigh wave when scanning close to a surface-breaking defect in a metal sample. This enhancement may be explained as an interference effect where the waves passing directly between source and receiver interfere with those waves reflected back from the defect. We present finite element models of the predicted enhancement when approaching a defect, along with experiments performed using electromagnetic acoustic transducers sensitive to either in-plane or out-of-plane motion. A larger enhancement of the in-plane motion than the out-of-plane motion is observed and can be explained by considering ultrasonic reflections and mode conversion at the defect.

Non-linear enhancement of laser generated ultrasonic Rayleigh waves by cracks

Laser generated ultrasound has been widely used for detecting cracks, surface and sub-surface defects in many different materials. It provides a non-contact wideband excitation source, which can be focused into different geometries. Previous workers have reported enhancement of the laser generated Rayleigh wave, when a crack is illuminated by pulsed laser beam irradiation. We demonstrate that the enhancement observed is due to a combination of source truncation, the free boundary condition at the edge of the crack and interference effects. Generating a Rayleigh wave over a crack can lead to enhancement of the amplitude of the Rayleigh wave signal, a shift in the dominant frequency of the wideband Rayleigh wave and strong enhancement of the high frequency components of the Rayleigh wave.

Enhancement of the Rayleigh wave signal at surface defects

We present details of a non-contacting technique that uses ultrasound to detect reliably surface defects with depths that are significantly smaller than the average ultrasonic wavelength to many times this depth. Two electro-magnetic acoustic transducers (EMATs) are used to generate and detect a broadband Rayleigh wave signal with a central frequency close to 200 kHz and extending to approximately 400 kHz. These transducers are moved with a fixed separation between them along a sample towards a defect. Enhancement of the ultrasonic signal is observed when the detector is very close to the defect, with the enhanced signal about 1.6 times the signal away from the defect. A similar enhancement is observed when using an EMAT for generation and a laser interferometer to measure the absolute surface displacement of the sample. We attribute the enhancement of the signal to straightforward interference between the ultrasonic signal that passes directly from the generating coil to the detector and that reflected from the defect. The enhancement can be used to pinpoint accurately a surface or subsurface defect, with the wide-band nature of the signal allowing us to gauge the depth of a wide range of surface-breaking defects.

Temperature contour maps at the strain-induced martensitic transition of a Cu–Zn–Al shape-memory single crystal

We study temperature changes at the reverse strain-induced martensitic transformation in a Cu–Zn–Al single crystal. Infrared thermal imaging reveals a markedly inhomogeneous temperature distribution. The evolution of the contour temperature maps enables information to be extracted on the kinetics of the interface motion.