We have measured and calculated the magnetocaloric effect in macroscopic samples of oriented high-spin molecular clusters like Mn 12 and Fe 8 as a function of the temperature and both the intensity and the sweeping rate of the applied magnetic field.We have observed a high magnetic entropy variation around the blocking temperature of the magnetic moment of molecules and calculated the shift of the entropy variation and cooling temperature, with the sweeping rate of the magnetic field.

No abstract

Monolithic mass sensors for ultrasensitive mass detection in air conditions have been fabricated using a conventional 0.35 m complementary metal-oxide-semiconductor ͑CMOS͒ process. The mass sensors are based on electrostatically excited submicrometer scale cantilevers integrated with CMOS electronics. The devices have been calibrated obtaining an experimental sensitivity of 6 ϫ 10 −11 g/cm 2 Hz equivalent to 0.9 ag/ Hz for locally deposited mass. Results from time-resolved mass measurements are also presented. An evaluation of the mass resolution have been performed obtaining a value of 2.4ϫ 10 −17 g in air conditions, resulting in an improvement of these devices from previous works in terms of sensitivity, resolution, and fabrication process complexity. © 2007 American Institute of Physics. ͓DOI: 10.1063/1.2753120͔ Cantilever-based sensors are very attractive transducers for physical, chemical, and biological sensors based on micro-/nanoelectromechanical systems 1-11 ͑NEMSs͒ due to its simplicity, wide range of sensing domains, and extremely high sensitivity when cantilever is scaled down into submicrometer scale dimensions. [6][7][8][9][10] One approach is the use of these cantilevers in dynamic mode for mass sensing applications where the mass is measured as a change of the resonance frequency. Monolithic integration of the cantileverbased transducer with readout electronics provides interesting advantages in terms of size, portability, and cost 10,11 in front of the use of optical readout techniques commonly used. [2][3][4][5][6][7][8][9] In previous works, 10 a post-complementary metal-oxidesemiconductor ͑CMOS͒ fabrication process based on a combination of electron beam lithography and direct write laser lithography has been used in order to define and release the cantilever of fully integrated mass sensors. In this letter, the mechanical structures are defined directly during the standard CMOS process used to fabricate the overall sensor ͑cantilever-based transducer plus readout circuitry ͓Fig. 1͑a͔͒, without the need of any additional lithographic process. 12 The reported results corresponding to calibration, real time mass measurements, and resolution analysis indicate the benefits in terms of resolution and operation stability of a fully integrated mass sensor operating in air conditions, which has been fabricated using a conventional CMOS technology.Cantilever structures 10 m long ͑l͒, 600 nm wide ͑w͒, and 750 nm thick ͑t͒ ͓Fig. 1͑b͔͒ have been fabricated by using the top metal layer of a commercial 0.35 m CMOS technology. 12,13 We use a three-electrode configuration constituted by the cantilever that is biased at a dc voltage ͑V dc ͒ and two electrodes for electrostatic excitation ͑V ac ͒ and capacitive readout purposes ͑V sense ͒, respectively. The cantilever displacement is detected with a full-custom designed readout CMOS circuit with a high-sensitivity and low-noise front-end stage. 14 The electrical scheme used presents an input capacitance as low as 11 fF and an input refereed noise of 21 nV/ Hz...

This letter presents the design, fabrication, and demonstration of a CMOS-MEMS filter based on two high-Q submicrometer-scale clamped-clamped beam resonators with resonance frequency around 22 MHz. The MEMS resonators are fabricated with a 0.35-μm CMOS process and monolithically integrated with an on-chip differential amplifier. The CMOS-MEMS resonator shows high-quality factors of 227 in air conditions and 4400 in a vacuum for a bias voltage of 5 V. In air conditions, the CMOS-MEMS parallel filter presents a programmable bandwidth from 100 to 200 kHz with a < 1-dB ripple. In a vacuum, the filter presents a stop-band attenuation of 37 dB and a shape factor as low as 2.5 for a CMOS-compatible bias voltage of 5 V, demonstrating competitive performance compared with the state of the art of not fully integrated MEMS filters. Index Terms-Bandpass filter, CMOS-MEMS, micromechanical filter, system-on-chip.

No abstract

The question addressed in this paper is that of the influence of the density of dislocations on the spin tunneling in Mn 12 clusters. We have determined the variation in the mosaicity of fresh and thermally treated single crystals of Mn 12 by analyzing the widening of low angle x-ray diffraction peaks. It has also been well established from both isothermal magnetization and relaxation experiments that there is a broad distribution of tunneling rates which is shifted to higher rates when the density of dislocations increases. DOI: 10.1103/PhysRevB.66.161407 PACS number͑s͒: 75.45.ϩj, 75.50.Xx Within the last few years molecular clusters have emerged as truly interdisciplinary objects. This is so because these materials allow us to test the border between quantum and classical mechanics, 1 they may also be used as a hardware for quantum computers 2,3 and for low temperature magnetic cooling. 4 The magnetic hysteresis in molecular clusters results from the existence of 2Sϩ1 spin levels in the two wells of the magnetic anisotropy barrier. Mn 12 molecular clusters have Sϭ10 at low temperature and are equivalent to a single domain particle with magnetic moment 20 B . The occurrence of magnetic relaxation at temperatures at which the thermal fluctuations die out is due to spin resonant tunneling between degenerate S Z states in the two wells of the anisotropy potential wells. [5][6][7][8][9][10][11][12][13][14][15] To the first approximation, the spin Hamiltonian used previously [5][6][7][8][9][10][11][12][13][14][15] to fit the magnetic data obtained for the different Mn 12 Ac molecular clusters is written aswhere Dϭ0. 65 K 16 -18 and HЈ contains anisotropy terms of fourth order of the spin operator. 16 -19 The last two terms correspond to the contribution of both dipolar and hyperfine fields to the transverse magnetic field. The first term of Eq. ͑1͒ generates spin levels S Z inside each well, while the symmetry violating terms inducing tunneling are those associated with the transverse component of the magnetic anisotropy and the transverse dipolar and hyperfine fields. Very recently, however, Chudnovsky and Garanin 20 have suggested that tunneling due to the magnetoelastic coupling H me may be even larger than that due to the terms written in Eq. ͑1͒. This may be so, as a consequence of the local transverse anisotropy and magnetic fields associated with dislocations. Also recently, Mertes et al. 21 have performed Landau-Zener relaxation studies in Mn 12 clusters using different sweeping rates of the applied magnetic field. Their data clearly show that ͑i͒ there is a distribution of tunneling splittings associated with the second-order anisotropy term; ͑ii͒ the fraction of Mn 12 molecules remaining in the metastable state obey the scaling law suggested theoretically for the case when the tunneling is driven by dislocations. In a series of resonant experiments at frequencies between 37 and 110 GHz, using both fresh and distorted single crystals, 22 we have noticed that in order to fit the data for these last cr...

No abstract

CoFe 2 O 4 magnetic nanoparticles were prepared by in situ precipitation and oxidation of Co 2ϩ and Fe 2ϩ within a sulfonated polystyrene resin. The nanometric particles were characterized by X-ray diffraction. A ferrofluid was prepared from the CoFe 2 O 4 mineralized polymer resin and water. Poly(vinyl alcohol) (PVA)-based nanocomposite materials were obtained by mixing different amounts of ferrofluid (compositions ranging within 0 -51 wt % of mineralized resin) with an aqueous solution of the polymer. The PVA composite materials were characterized by TGA, DSC, and stressstrain testing. The thermal and mechanical properties of PVA change with filler content, exhibiting an initial increase in these properties due to polymer-filler interactions. After a maximum value, at about 15 wt % of mineralized resin, the mechanical properties decrease probably due to particle aggregation which causes phase separation. The results obtained show that the nanoparticles are dispersed in the amorphous regions of the polymer, the crystalline zones remaining unaltered up to compositions as high as 30 wt %.