327 research outputs found
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
Non-contact direct measurement of the magnetocaloric effect in thin samples
An experimental setup, based on a non-contact temperature sensor, is proposed to directly measure the magnetocaloric effect of samples few micrometers thick. The measurement of the adiabatic temperature change of foils and ribbons is fundamental to design innovative devices based on magnetocaloric thin materials or micro-structuring bulk samples. The reliability of the proposed setup is demonstrated by comparing the measurements performed on a bulk gadolinium sample with the results obtained by an experimental setup based on a Cernox bare chip thermoresistance and by in-field differential scanning calorimetry. We show that this technique can measure the adiabatic temperature variation on gadolinium sheets as thin as 27 µm. Heat transfer simulations are added to describe the capability of the presented technique
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
Thermally activated magnetization reversal in bulk BiFe0.5Mn0.5O3
We report on the synthesis and characterization of BiFe0.5Mn0.5O3, a
potential type-I multiferroic compound displaying temperature induced
magnetization reversal. Bulk samples were obtained by means of solid state
reaction carried out under the application of hydrostatic pressure at 6 GPa and
1100{\deg}C. The crystal structure is an highly distorted perovskite with no
cation order on the B site, where, besides a complex scheme of tilt and
rotations of the TM-O6 octahedra, large off-centering of the bismuth ions is
detected. Below T1 = 420 K the compound undergoes a first weak ferromagnetic
transition related to the ordering of iron rich clusters. At lower temperatures
(just below RT) two distinct thermally activated mechanisms are superimposed,
inducing at first an enhancement of the magnetization at T2 = 288 K, then a
spontaneous reversal process centered at T3 = 250 K, finally giving rise to a
negative response. The application of fields higher than 1500 Oe suppresses the
process, yielding a ferromagnetic like behaviour. The complementary use of
SQuID magnetometry and M\"ossbauer spectroscopy allowed the interpretation of
the overall magnetic behaviour in terms of an uncompensated weak competitive
coupling between non-equivalent clusters of interactions characterized by
different critical temperatures and resultant magnetizations. PACS numbers:
75.85.+t, 75.60.Jk, 76.80.+y, 75.30.Et, 75.30.KzComment: 30 pages, 13 figure
Scale-Up of Magnetocaloric NiCoMnIn Heuslers by Powder Metallurgy for Room Temperature Magnetic Refrigeration
We present a new approach for a large-scale production of the rare-earth free NiCoMnIn Heusler alloy for room temperature magnetic refrigeration applications. This class of compounds has recently attracted attention, thanks to the large reversible isothermal entropy change (ΔSiso) and adiabatic temperature change (ΔTad) associated to a first-order magnetostructural phase transition. A large-scale production method, however, has not yet been proposed. For giant magnetocaloric materials and especially for Heusler compounds, the synthesis has a predominant role in tailoring the physical–chemical properties, due to the high sensitivity of the first-order transition characteristics on chemical composition and microstructure. Up to 250 g of the nominal composition Ni45.7Co4.2Mn36.6In13.3 alloy was prepared in a unique sample starting from industrial-grade powdered elements. The phase transition temperatures and magnetocaloric properties were investigated by magnetic and direct adiabatic temperature measurements and were found to be homogeneous in the whole sample. The mechanical stability of the produced alloy and its workability were investigated. A low-temperature thermal treatment was identified and showed promising results by reducing hysteresis and transition width
Conditions for the growth of smooth La0.7Sr0.3MnO3 thin films by pulsed electron ablation
We report on the optimisation of the growth conditions of manganite
La0.7Sr0.3MnO3 (LSMO) thin films prepared by Channel Spark Ablation (CSA). CSA
belongs to pulsed electron deposition methods and its energetic and deposition
parameters are quite similar to those of pulsed laser deposition. The method
has been already proven to provide manganite films with good magnetic
properties, but the films were generally relatively rough (a few nm
coarseness). Here we show that increasing the oxygen deposition pressure with
respect to previously used regimes, reduces the surface roughness down to unit
cell size while maintaining a robust magnetism. We analyse in detail the effect
of other deposition parameters, like accelerating voltage, discharging energy,
and temperature and provide on this basis a set of optimal conditions for the
growth of atomically flat films. The thicknesses for which atomically flat
surface was achieved is as high as about 10-20 nm, corresponding to films with
room temperature magnetism. We believe such magnetic layers represent appealing
and suitable electrodes for various spintronic devices.Comment: original paper, thin film optimization, 25 pages, 9 figure
Waste of batteries management: Synthesis of magnetocaloric manganite compound from the REEs mixture generated during hydrometallurgical processing of NiMH batteries
In the present study, rare earth elements (REEs, i.e., La, Ce, Nd, and Pr) were hydrometallurgically recovered in oxalate form with presence of very low concentration of Co, Al, Zn and Ni from solution after processing of spent Nickel metal hydride (Ni-MH) batteries. The recovered mixture was used as alternative source in the synthesis of magnetocaloric materials. In this study, a manganite sample with general formula ABO3 was selected to be prepared since it is relatively easy to synthesize and is tuneable by adjustment of the doping concentration. The conventional solid-state reaction method was used to prepare an orthorhombic structure of manganite with presence of REE2O3 and MnO2 as secondary phases reported from x-ray pattern at room temperature. The thermomagnetic measurements showed a PM to FM transition at 184 K in a 0.01 T magnetic field that shifts to 194 K by increasing the magnetic field to 1.8 T. The magnetocaloric properties were determined by calculating the isothermal entropy change and directly measuring the adiabatic temperature change. A reversible magnetocaloric effect was observed
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