The activated torsion oscillation magnetometer

The activated torsion oscillation magnetometer exploits the mechanical resonance of a cantilever beam, driven by the torque exerted on the sample by an ac field applied perpendicularly to the film plane. We describe a model for the cantilever dynamics which leads to the calculation of the cantilever dynamic profile and allows the mechanical sensitivity of the instrument to be expressed in terms of the minimum electronically detectable displacement. We have developed a capacitance detector of small oscillations which is able to detect displacements of the order of 0.1 nm. We show that sensitivities of the order of 0.5(10-11 Am2 can be in principle achieved. We will subsequently describe the main features of the ATOM prototype which we have built and tested, with particular attention to the design solutions which have been adopted in order to reduce the effects of parasitic vibrations due either to acoustic noise, originating from the ac field coil, or to eddy currents in the capacitor electrodes. The instrument is mounted in a continuous flow cryostat and can work in the 4.2-300 K temperature range. Finally, we will show that our experimental set-up has a second mode of operation, named Torsion Induction Magnetometer (TIM).Comment: Invited Talk at the Moscow International Symposium on Magnetism, 2002 to appear in the J. Mag. Mag. Mat Revised versio

Giant entropy change at the co-occurrence of structural and magnetic transitions in the Ni2.19Mn0.81Ga Heusler alloy

In this paper we report the existence of a giant magnetocaloric effect (MCE) in a intermetallic compound non-containing rare-earth. This effect is associated with the concomitant occurrence of a structural and a magnetic transition. The result has been compared with that obtained in a parent compound in which magnetic and structural transition occur separately.Comment: PDF file from MS-Word 2000 document, 13 pages (text) plus 6 figures; corrected typo

Lattice strain accommodation and absence of pre-transition phases in Ni50_{50}Mn25+x_{25+x}In25−x_{25-x}

The stoichiometric Ni$_{50}$Mn$_{25}$In$_{25}$ Heusler alloy transforms from a stable ferromagnetic austenitic ground state to an incommensurate modulated martensitic ground state with a progressive replacement of In with Mn without any pre-transition phases. The absence of pre-transition phases like strain glass in Ni$_{50}$Mn$_{25+x}$In$_{25-x}$ alloys is explained to be the ability of the ferromagnetic cubic structure to accommodate the lattice strain caused by atomic size differences of In and Mn atoms. Beyond the critical value of $x$ = 8.75, the alloys undergo martensitic transformation despite the formation of ferromagnetic and antiferromagnetic clusters and the appearance of a super spin glass state.Comment: Appearing in Journal of Physics: Condensed Matte

Influence of thermal conductivity on the dynamic response of magnetocaloric materials

We compare the magnetocaloric effect of samples prepared with different thermal conductivities to investigate the potential of composite materials. By applying the magnetic field under operating conditions we test the material’s response and compare this to heat transfer simulations in order to check the reliability of the adiabatic temperature change probe used. As a result of this study we highlight how the material’s thermal conductivity influences τ , the time constant of temperature change. This parameter ultimately limits the maximum frequency of a refrigerant cycle and offers fundamental information about the correlation between thermal conductivity and the magnetocaloric effect

Negative magnetic relaxation in superconductors

It was observed that the trapped magnetic moment of HTS tablets or annuli increases in time (negative relaxation) if they are not completely magnetized by a pulsed magnetic field. It is shown, in the framework of the Bean critical-state model, that the radial temperature gradient appearing in tablets or annuli during a pulsed field magnetization can explain the negative magnetic relaxation in the superconductor

preliminary investigation on a rotary magnetocaloric refrigerator prototype

Abstract Environmental legislations are currently imposing important restrictions to regulate the use of refrigerant fluids in order to reduce the greenhouse gases emissions and global warming potential. To overcome these issues, a valid alternative to replace conventional refrigeration systems can be represented by magnetic refrigeration. Since magnetic refrigeration is based on the magnetocaloric effect it represents an environmental friendly technology that avoids the use of Chlorinated refrigerants. In this paper a preliminary analysis of a novel magnetocaloric refrigerator is presented. The magnetocaloric refrigeration prototype uses Gadolinium as refrigerant and water as heat exchange medium, and relies on permanent magnets as magnetic field source. The device operates according to the active regenerative principle with a rotary movement. A detailed description of the main components included in the design of the prototype device is presented along with a schematic representation of the hydraulic circuit. Focusing on the regenerators beds, some simulations have been carried out to quantify the heat energy fluxes between water and gadolinium. The results of the simulations show a decrease on gadolinium temperature distribution cycle by cycle highlighting the actual effect of the regeneration

Pr 0.5 Ca 0.5 MnO 3 thin films deposited on LiNbO 3 substrates

Thin films of Pr 0.5 Ca 0.5 MnO 3 have been deposited on z-cut LiNbO 3 by pulsed laser ablation. The X-ray diffraction measurements showed that the films have grown highly oriented on LiNbO 3 , with a pseudocubic (111) preferred growth direction. The thicknesses of the films, measured by low angle X-ray reflectivity, are between 13 and 140 nm. Their electrical resistivity present a semiconducting-like behaviour with an anomaly around 240 K, that corresponds to the charge ordering transition. The temperature of the transition (T_CO) was estimated from ln(r) vs. (1/T) plots. The charge ordering temperature was found to be dependent on the strain induced by the lattice mismatch on the films.Fundação para a Ciência e a Tecnologia (FCT

Field-induced segregation of ferromagnetic nano-domains in Pr0.5_{0.5}Sr0.5_{0.5}MnO3_3, detected by 55^{55}Mn NMR

The antiferromagnetic manganite Pr$_{0.5}$Sr$_{0.5}$MnO$_3$ was investigated at low temperature by means of magnetometry and $^{55}$Mn NMR. A field-induced transition to a ferromagnetic state is detected by magnetization measurements at a threshold field of a few tesla. NMR shows that the ferromagnetic phase develops from zero field by the nucleation of microscopic ferromagnetic domains, consisting of an inhomogeneous mixture of tilted and fully aligned parts. At the threshold the NMR spectrum changes discontinuously into that of a homogeneous, fully aligned, ferromagnetic state, suggesting a percolative origin for the ferromagnetic transition.Comment: Latex 2.09 language. 4 pages, 3 figures, 23 references. Submitted to physical Review