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Nowadays microfabrication techniques originating from micro-electronics enable to create mechanical objects of micron-size. The field of Micro-Electro-Mechanical devices (MEMS) is continuously expanding, with an amazingly broad range of applications at room temperature. Vibrating objects (torsional oscillators, vibrating wires) widely used at low temperatures to study quantum fluids, can be replaced advantageously by Silicon MEMS. In this letter we report on the study of Silicon vibrating wire devices. A goal-post structure covered with a metal layer is driven at resonance by the Laplace force acting on a current in a magnetic field, while the induced voltage arising from the cut magnetic flux allows to detect the motion. The characteristics of the resonance have been studied from 10 mK to 30 K, in vacuum and in $^4$He gas. In this article, we focus on the results obtained above 1.5 K, in vacuum and gas, and introduce some features observed at lower temperatures. The resonant properties can be quantitatively understood by means of simple models, from the linear regime to a highly non-linear response at strong drives. We demonstrate that the non-linearity is mostly due to the geometry of the vibrators. We also show that in our device the friction mechanisms originate in the metallic layers, and can be fully characterized. The interaction with $^4$He gas is fit to theory without adjustable parameters., Comment: Long paper. Both experiment and theory

In the weak ferromagnetic MnCO$_3$ system, a low-frequency collective spin excitation (magnon) is the hybridized oscillation of nuclear and electron spins coupled through the hyperfine interaction. By using a split-ring resonator, we performed transmission spectroscopy measurements of MnCO$_3$ system and observed, for the first time, avoiding crossing between the hybridized nuclear-electron magnon mode and the resonator mode in the NMR-frequency range. The splitting strength is quite large due to the large spin density of $^{55}$Mn, and the cooperativity value $C=0.2$ (magnon-photon coupling parameter) is close to the conditions of strong coupling. The results reveal a new class of spin systems, in which the coupling between nuclear spins and photons is mediated by electron spins via the hyperfine interaction, and in which the similar normal-mode splitting of the hybridized nuclear magnon mode and the resonator mode can be observed., Comment: 5 pages, 3 figures

We present mechanical measurements performed at low temperatures on cantilever-based microelectro-mechanicalstructures(MEMS) coated with a metallic layer. Two very different coatings are presented in order to illustrate the capabilities of the present approach, namely (soft) aluminum and (hard) niobium oxide. The temperature is used as a control parameter to access materials properties. We benefit from low temperature techniques to extract a phase-resolved measurement of the first mechanical resonance mode in cryogenic vacuum. By repeating the experiment on the same samples, after multiple metallic depositions, we can determine accurately the contribution of the coating layers to the mechanical properties in terms of surface stress, additional mass, additional elasticity and damping. Analytic theoretical expressions are derived and used to fit the data. Taking advantage of the extremely broad dynamic range provided by the technique, we can measure the anelasticity of the thin metallic film. The key parameters describing the metals' dynamics are analyzed in an original way in order to provide new experimental grounds for future theoretical modelings of the underlying mechanisms.

3He confined in aerogel in the millikelvin temperature domain exemplifies a Fermi liquid in the presence of disorder. In confined 3He systems, a solid layer of 3He atoms forms on the confining medium. This system can then be viewed as a model system for the study of the (strongly interacting) Fermi liquid in contact with a (ferromagnetic) "2D-like" adsorbed solid. This interaction, studied experimentally through NMR T2 experiments, is described in the framework of the "fast exchange" model. A complete analytical descripion of the model is given, explaining our measurements as well as related normal-state confined 3He NMR literature.

We report on low temperature measurements performed on micro-electro-mechanical systems (MEMS) driven deeply into the non-linear regime. The materials are kept in their elastic domain, while the observed non-linearity is purely of geometrical origin. Two techniques are used, harmonic drive and free decay. For each case, we present an analytic theory fitting the data. The harmonic drive is fit with a Lorentz-like lineshape obtained from an extended version of Landau and Lifshitz's non-linear theory. The evolution in the time domain is fit with an amplitude-dependent frequency decaying function derived from the Lindstedt-Poincar'e theory of non-linear differential equations. The technique is perfectly generic and can be straightforwardly adapted to any mechanical device made of ideally elastic constituents, and which can be reduced to a single degree of freedom, for an experimental definition of its non-linear dynamics equation.

In this Letter we discussed the parametric instability of texture of homogeneous (in time) spin precession, explaining how spatial inhomogeneity of the texture may change the threshold of the instability in comparison with idealized spatial homogeneous case, considered in our JETP Letter \textbf{83}, 530 (2006), cond-mat/0605386. This discussion is inspired by critical Comment of I.A. Fomin (cond-mat/0606760) related to the above questions. In addition we considered here results of direct numerical simulations of the full Leggett-Takagi equation of motion for magnetization in superfluid 3He-B and experimental data for magnetic field dependence of the catastrophic relaxation, that provide solid support of the theory of this phenomenon, presented in our 2006 JETP Letter., Comment: 5 pages, 1 fig. included, JETP Lett. style, submitted to JETP Lett. as response to Comment cond-mat/0606760