Magnetocaloric effect in GdGeSi compounds measured by the acoustic detection technique: Influence of composition and sample treatment

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)In this paper we explore the acoustic detection method applied to the investigation of the magnetocaloric effect in Gd and Gd(5)(Ge(1-x)Si(x))(4) compounds, in the temperature range from 230 to 360 K and for magnetic fields up to 20 kOe. Measurements were performed in as-cast materials, both for powder and pellet samples, and in tree samples with compositions around Gd(5)Ge(2)Si(2) that underwent different thermal treatments. Small differences were observed when comparing powder and pellet samples of Gd and Gd(5)(Ge(1-x)Si(x))(4) compounds with 0.500<x <= 1.00. For the alloys with composition around Gd(5)Ge(2)Si(2), which exhibit giant magnetostriction and coexistence of distinct phases, expressive changes were observed when comparing powder and pellet samples. Based on these cases, it is easy to see that the acoustic method can distinguish a second-order phase transition from a first-order magnetic-crystallographic one, and that it presents good sensitivity to detect spurious material phase in small quantities. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3357375]1077Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)FAEPEX-UnicampFundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES

Cytotaxonomy of Lychnophoriopsis Sch.Bip. And Paralychnophora MacLeish species (Asteraceae : Vernonieae : Lychnophorinae)

The genera Lychnophoriopsis and Paralychnophora are endemic to Brazilian 'campo rupestre' in the States of Minas Gerais and Bahia. To analyse the status of the cytotaxonomy of these genera, we carried out chromosome counts on several species. Two chromosome numbers were obtained for four species of Paralychnophora: 2n = 36 and 2n = 38. For three species of Lychnophoriopsis, only 2n = 36 was observed. The chromosome size (1.0-2.58 mu m) and morphology (mainly metacentric, with some submetacentric) were analysed in four species of Paralychnophora and showed relative constancy. (C) 2007 The Linnean Society of London.154110911

Chromosome numbers in the genus Lychnophora Mart. (Lychnophorinae, Vernonieae, Asteraceae)

The genus Lychnophora Mart. comprises about 68 endemic species in "campos rupestres" in the States of Minas Gerais, Bahia and Goias. Ecological conditions of this environment possibly favored an intense speciation, and could be one factor for this endemism. The highest diversity is found in Minas Gerais, a state also considered as the major center of dispersal of this genus. To contribute to the biosystematic study of the genus, we carried out a chromosomic study in 18 species of Lychnophora, belonging to 3 sections, collected at the mountain ranges of Cipo and of the regions of Carrancas and Diamantina, all in the State of Minas Gerais. Three chromosome numbers, n=17, 18 and 19 were found in different species and a polyploid population with n=34 was detected in L. staavioides. These different chromosome numbers are distributed among species of the 3 sections and can not be used as distinctive characters at an infrageneric level. On the other hand, they are very important to differentiate some species whose limits sometimes are questioned.55436737

Chromosomal studies of three species of Bidens (L.) (Asteraceae)

The chromosomal numbers of three species of Bidens were determined based on an analysis of somatic metaphases. The species studied were Bidens pilosa L., Bidens subalternans DC and Bidens alba (L.) DC, all of which belong to the Bidens pilosa species complex. The three species have a similar morphology: Bidens pilosa and B. subalternans are widely distributed in agricultural areas, in disturbed habitats and along roadsides, whereas B. alba occurs only along the seacoast. Bidens pilosa has 2n=72 and B. subalternans and B. alba has 2n=48. These numbers agreed with the basic chromosomal number of n=12 reported for the tribe Heliantheae and show that the number of metaphase chromosomes can be used to distinguish Bidens pilosa from the other two species of the B. pilosa complex in southeastern Brazil.591141

Angular dependence of the photothermally modulated magnetic resonance in Gd thin films

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)In this paper we use electron spin resonance and photothermally modulated magnetic resonance techniques to investigate gadolinium thin films as a function of the orientation of the film surface with respect to the external magnetic field and of the temperature, around the magnetic phase transition temperature. We observe that, in the ferromagnetic phase, the resonance line is shifted up to higher external magnetic fields when the angle between the film surface and the field increases, revealing the magnetic anisotropy of the sample. At the same time, when the temperature is augmented to values higher than the phase transition temperature, the external field of the resonance collapses back to the expected value in the paramagnetic phase for all orientations. We also demonstrated that, even for the perpendicular orientation (magnetic field perpendicular to the sample surface), the photothermally modulated magnetic resonance signal is maximized near the magnetic phase transition temperature. Furthermore, in the ferromagnetic phase the photothermally modulated magnetic resonance intensity is very sensitive to the orientation, showing a significant enhancement in the perpendicular direction.1122403409Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)FAEPEX-UnicampFundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq

Large magnetocaloric effect and refrigerant capacity near room temperature in as-cast Gd5Ge2Si2-xSnx compounds

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Large values of isothermal entropy change (Delta S-T) and refrigerant capacity have been found in Gd5Ge2Si2-xSnx compounds. Values of the order of 20 J kg(-1) K-1 for -Delta S-T were obtained in as-cast samples when submitted to a magnetic field variation of 2 T. First-order-magneto-structural transition is induced by the substitution of silicon by tin and it is shifted to lower temperatures with the tin content. It means that the magnetocaloric effect on this series can be properly tuned to a specific practical thermodynamic cycle, including near room temperature range. (C) 2013 AIP Publishing LLC.10219Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq

Acoustic detection of the magnetocaloric effect: Application to Gd and Gd5.09Ge2.03Si1.88

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)In this paper we present a simple method for the determination of the total magnetocaloric effect based on the acoustic detection of the adiabatic temperature rise caused by the application of an ac magnetic field of small amplitude. The continuous scanning of a superimposed dc magnetic field allows, by numerical integration, the determination of large temperature variations caused by magnetic field steps from zero to tens of kOe. Absolute values of temperature rise are easily acquired after the calibration of the microphone signal using an appropriate reference sample. Once the calibration is done, no further information about the sample's thermal properties is necessary since the measured signal is directly proportional to the temperature variation. Measurements were made in Gd and Gd5.09Ge2.03Si1.88 samples in the temperature range from 240 to 320 K. The technique shows to be suitable for the investigation of materials undergoing both purely magnetic phase transitions, as in the case of Gd, and magnetic-crystallographic first-order ones, as observed for Gd5.09Ge2.03Si1.88. Besides the ability to determine the temperature variation due to a large magnetic field step through the continuous scanning of the magnetic field, the technique is also very suitable for measuring the magnetocaloric effect under very small magnetic field steps since it has sensitivity below millikelvin. Moreover, it is able to detect temperature variations in very small amount of sample, leading to its potential application in magnetocaloric thin films.8013Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)FAEPEX-UnicampFundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES

Determination of the entropy change using the acoustic detection technique in the investigation of the magnetocaloric effect

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)In this paper we demonstrate the use of the acoustic detection as an alternative way to determine the entropy variation, Delta S(T), a parameter normally used to characterize the magnetocaloric effect. The measurements were performed for a Gd sample in the 252-316 K temperature range for magnetic fields from zero up to 50 kOe. The reversible adiabatic curves were built in a T versus H diagram, and specific heat data obtained at zero-magnetic field were employed to assign the entropy values of each curve. Subsequently, the entropy was plotted as a function of temperature for fixed magnetic fields, and therefore the isothermal entropy variation, Delta S(T), was found as a function of the temperature for several magnetic field steps.4344Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)FAEPEX-UnicampFundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES

Photothermally modulated magnetic resonance applied to the study of the magnetic phase transition in gadolinium thin films

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)We explore the photothermally modulated magnetic resonance technique to investigate gadolinium thin films deposited on fused quartz substrate, as a function of thickness and thermal treatment, around the magnetic phase transition temperature. It has been observed that the maximum amplitude of the photothermally modulated magnetic resonance (PM-MR) signal takes place near the phase transition temperature, similarly to the magnetocaloric effect, for which Gd has been the prototype material. The reason is that both depend on the temperature derivative of the magnetization, which maximizes at the phase transition. Besides, there is a narrowing of transition with thermal treatment, confirming that thermal treatment stabilizes the film structure. For frequency scan measurements, the heat diffusion in a two-layer system was considered, and a depth profile study was carried out in order to investigate heterogeneities along the film thickness. From the PM-MR response as a function of the modulation frequency it was possible to estimate the thermal properties of the Gd film. Magnetization, X-ray and electron spin resonance measurements were used to complement the analysis and support the conclusions. (c) 2011 Elsevier B.V. All rights reserved.520936343640Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES)FAEPEX-UnicampFundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES