Foldover Effect Research Articles

Ferromagnetic resonance and nonlinear absorption in p-NPNN

Ferromagnetic Resonance (FMR) study has been reported in β-phase p-NPNN (nitrophenyl nitronyl nitroxide) with Tc=0.6K. We have observed non-linear line shapes at the lowest temperatures. The line shape is related to the effect known as the “foldover effect” [1]. We confirmed that the temperature dependent g-value so far reported can be understood by the demagnetization effect using spherically shaped samples. An intrinsically anisotropic g-values are observed depending on the applied field directions. This is due to the anisotropy field along the crystal b-axis.

Photothermal investigation of magnetostatic modes in yttrium iron garnet at high microwave power levels

Laterally resolved magnetostatic mode spectra in an yttrium-iron-garnet single-crystal (0.98 mm*8.40 mm*4.40 mm) were obtained by microwave spectroscopy using the photothermal laser beam deflection technique. For a microwave frequency of 9.33 GHz and a power of 2.2 W, both magnetostatic surface and magnetostatic backward volume wave modes were observed. The lateral variation of these modes can be explained by a modified WKB approximation for the magnetostatic potential and by taking into account the presence of an inhomogeneous demagnetization field inside the sample. At high incident microwave power, a foldover effect in the range of the surface wave modes and one distinctly larger volume wave mode peak occur. The photothermal signal phase provides information on the penetration depth of the surface wave modes.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

The second-order spin-wave instability threshold in single-crystal yttrium-iron-garnet films under perpendicular pumping

The second-order spin-wave instability threshold in yttrium-iron-garnet single-crystal thin films has been investigated under conditions of ferromagnetic resonance (FMR). The samples were (111) etched circular films of 500 μm diam and 5.1 μm thickness, grown on gadolinium gallium garnet substrates by liquid-phase epitaxy. The pulsed microwave power FMR absorption curves were measured by standard signal averaging techniques at 9.18 GHz and room temperature for three different perpendicular pumping configurations: (1) in-plane static field and out-of-plane microwave field, (2) mutually perpendicular in-plane static and microwave fields, and (3) out-of-plane static field and in-plane microwave field. The experimental results show that the upsweep and downsweep FMR curves for lower pulsed microwave powers at low duty cycle of 1% are identical, but there are foldover effects in the FMR profiles at higher powers. Theoretical expressions for the instability threshold for these three configurations were obtained by extending previous theories. The calculated thresholds are in good agreement with the experimental results for configurations (1) and (2). The results for configuration (3) indicate the presence of a true foldover effect, also consistent with instability theory.

Spin-wave instability and ‘‘true’’ foldover in single-crystal YIG films (abstract)

High-power pulsed microwave measurements of ferromagnetic resonance (FMR) in yttrium iron garnet (YIG) films have been extended to include all three in-plane and out-of-plane microwave and static field combinations for FMR. The 5.1-μm-thick films were prepared by liquid phase epitaxy on single-crystal gadolinium gallium garnet substrates and subsequently etched to form 500-μm circles. The FMR spectra were obtained with a 9.18-GHz pulse spectrometer as described in Ref. 1. The results confirm the conclusion of Ref. 1. The pulsed microwave FMR at high-power levels yields line broadening due to spin-wave instability processes, rather than the shifts and distorted profiles which occur due to heating under cw excitation conditions. The key new result is the explicit correlation between the microwave field amplitude for the onset of the line broadening and the FMR pump configuration. In addition to these line broadening effects at high power, a ‘‘true’’ foldover effect is observed for the perpendicular-to-plane static field case: the FMR shoulder shifts down in field at high powers, due to the nonlinear response and the limitations on that response from the spin wave instability processes.

Analysis of low-frequency noise reduction by autozero technique

This letter analyses the low-frequency noise reduction produced by the autozero technique. A very simple model is established, and the output average power spectrum in the case of lowpass filtered white noise and 1/f noise is calculated. This analysis clearly shows the limitation to low-frequency noise reduction introduced by the foldover effect.

Double foldover effect in ferromagnetic resonance

It is theoretically shown that a new kind of uniform precession instability is possible.

Nonlinear Behavior of Ferromagnetic Thin Films

The authors report a study of the behavior of Permalloy thin films at high microwave signals. The cases where films are polarized either parallel or perpendicular to the film plane are successively investigated at X and V bands. With a parallel polarization, the absorption via a first-order process can be present at X band either as a subsidiary absorption or as a premature decline of the main resonance, depending on the film thickness. At V band, only a subsidiary absorption was expected; it was effectively observed in some films with a large critical field (≃45 Oe). With perpendicular polarization, new experimental observations (such as an important main resonance shifting, a large variation of the linewidth at low level, a saturation of the uniform mode via a second order process, and a foldover effect leading to relaxation oscillations) at X and V bands were made and explained. Finally, with a strong interaction between films and electromagnetic waves having thus been shown, possible applications such as power limiters are indicated.

Excitation of spin waves during ferromagnetic resonance

A non-linear equation is solved which includes the excitation of spin waves by Suhl non-linear coupling and by the scattering of homogeneous precession on inhomogeneities. The solution of this equation takes into account the dependence of the spin-wave frequencies on the degree of excitation of the homogeneous precession and on the degree of excitation of the spin waves. The result of the dependence just quoted is the finality of the amplitudes of spin waves for arbitrary values of the angle of homogeneous precession and the “foldover effect”, connected with hysteresis and discontinuities in the excitation of the spin waves. From the solution it is also apparent that for values of the homogeneous precession amplitude higher than the Suhl critical value the resonance curve splits into two curves, i.e. two groups of spin waves are excited. These two groups differ in frequency by a value of the same order as their relaxation frequency.