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38 results on '"Oliveira, G. N. P."'

1. Probing $Ca_3Ti_2O_7$ crystal structure at the atomic level: Insights from $^{111m}Cd/^{111}Cd$ PAC spectroscopy and ab-initio studies

Author

Rocha-Rodrigues, P., Miranda, I. P., Santos, S. S. M., Oliveira, G. N. P., Leal, T., Marcondes, M. L., Correia, J. G., Assali, L. V. C., Petrilli, H. M., Araújo, J. P., and Lopes, A. M. L.

Subjects
Condensed Matter - Materials Science
Abstract

Perturbed angular correlation spectroscopy combined with $ab-initio$ electronic structure calculations is used to unravel the structural phase transition path from the low-temperature polar structure to the high-temperature structural phase in $Ca_3Ti_2O_7$, a hybrid improper ferroelectric. This procedure explores the unique features of a local probe environment approach by monitoring the evolution of the electric field gradient tensor at the calcium sites. The local environments, observed above 1057 K, confirm a structural phase transition from the $A2_1am$ symmetry to an orthorhombic $Acaa$ symmetry in the $Ca_3Ti_2O_7$ crystal lattice, disagreeing with the frequently reported avalanche structural transition from the polar $A2_1am$ phase to the aristotype $I4/mmm$ phase. Moreover, the EFG temperature dependency, within the $A2_1am$ temperature stability, is shown to be sensitive to the recently proposed $Ca_3Ti_2O_7$ ferroelectric polarization decrease within the 500-800~K temperature range.

Published
2024

2. Probing Ca 3 Ti 2 O 7 crystal structure at the atomic level : Insights from perturbed angular correlation spectroscopy and ab initio studies

Author

Rocha-Rodrigues, P., Miranda, Ivan P., Santos, S. S. M., Oliveira, G. N. P., Leal, T., Marcondes, M. L., Correia, J. G., Assali, L. V. C., Petrilli, H. M., Lopes, A. M. L., Araujo, J. P., Rocha-Rodrigues, P., Miranda, Ivan P., Santos, S. S. M., Oliveira, G. N. P., Leal, T., Marcondes, M. L., Correia, J. G., Assali, L. V. C., Petrilli, H. M., Lopes, A. M. L., and Araujo, J. P.

Abstract

Perturbed angular correlation spectroscopy combined with ab initio electronic structure calculations is used to unravel the structural phase transition path from the low -temperature polar structure to the high -temperature structural phase in Ca 3 Ti 2 O 7 , a hybrid improper ferroelectric. The experimental procedure, conducted at ISOLDE, explores the unique features of a local probe environment approach by monitoring the evolution of the electric field gradient tensor at the calcium sites. The local environments, observed above 1057 K, confirm a structural phase transition from the A 2 1 am symmetry to an orthorhombic Acaa symmetry in the Ca 3 Ti 2 O 7 crystal lattice, disagreeing with the frequently reported avalanche structural transition from the polar A 2 1 am phase to the aristotype I 4 / mmm phase. Moreover, the EFG temperature dependency, within the A 2 1 am temperature stability, is shown to be sensitive to the recently proposed Ca 3 Ti 2 O 7 ferroelectric polarization decrease within the 500-800 K temperature range.

Published
2024
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3. Influence of short time milling in R5(Si,Ge)4, R =Gd and Tb, magnetocaloric materials

Author

Pires, A. L., Belo, J. H., Turcaud, J., Oliveira, G. N. P., Araújo, J. P., Berenov, A., Cohen, L. F., Lopes, A. M. L., and Pereira, A. M.

Subjects
Condensed Matter - Materials Science
Abstract

The effect of the short milling times on R5(Si,Ge)4 R =Gd, Tb magnetocaloric material properties was investigated. In particular, the effect of milling on atomic structure, particles size and morphology, magnetic, and magnetocaloric effect was studied. With short milling times (< 2.5h), a reduction of the Gd5Si1.3Ge2.7 and Tb5Si2Ge2 particles size was achieved down to approximately 3.5 DSm. For both compositions the main differences are a consequence of the milling effect on the coupling of the structural and magnetic transitions. In the Gd5Si1.3Ge2.7 case, a second-order phase transition emerges at high temperatures as a result of ball milling. Consequently, there is a decrease in the magnetocaloric effect of 35% after 150 minutes of milling. Interestingly, an opposite effect is observed in Tb5Si2Ge2 where a 23% increase of the magnetocaloric effect was achieved, driven by the enhancement of the coupling between magnetic and structural transitions arising from internal strain promoted by the milling process., Comment: 15 pages, 6 figures, Submitted to Materials & Design

Published
2015

7. Ca$_3$Mn$_2$O$_7$ structural path unraveled by atomic-scale properties: A combined experimental and ab initio study

Author

Rocha-Rodrigues, P, Santos, S S M, Miranda, I P, Oliveira, G N P, Correia, J G, Assali, L V C, Petrilli, H M, Araújo, J P, and Lopes, A M L

Subjects
Nuclear Physics - Experiment
Abstract

The structural phase transition path from the low-temperature polar structure up to the highest symmetric phase in the hybrid improper ferroelectric Ca$_3$Mn$_2$O$_7$ compound is here investigated at atomic scale. Measurements using the perturbed angular correlation local probe technique are combined with ab initio electronic structure calculations to observe the evolution of the electric field gradient parameters at the Ca site within the 10–1200 K temperature range. The results show that polar-phase clusters persist at temperatures as high as 500 K. In addition, evidence is given for a structural phase transition occurring above 1150 K. The high-temperature symmetry is here confirmed to be $I4/mmm$.

Published
2020

8. Poster: Memristive Systems

Author

Ho Yoon, Jung, primary, Jung, Hyung-Suk, additional, Hwan Lee, Min, additional, Hwan Kim, Gun, additional, Ji Song, Seul, additional, Yeong Seok, Jun, additional, Jean Yoon, Kyung, additional, Seong Hwang, Cheol, additional, Besland, M.-P., additional, Tranchant, J., additional, Souchier, E., additional, Moreau, P., additional, Salmon, S., additional, Corraze, B., additional, Janod, E., additional, Cario, L., additional, Zazpe, Raúl, additional, Ungureanu, Mariana, additional, Llopis, Roger, additional, Golmar, Federico, additional, Stoliar, Pablo, additional, Casanova, Félix, additional, Eduardo Hueso, Luis, additional, Hermes, C., additional, Wimmer, M., additional, Menzel, S., additional, Fleck, K., additional, Rana, V., additional, Salinga, M., additional, Böttger, U., additional, Bruchhaus, R., additional, Wuttig, M., additional, Waser, R., additional, Lentz, F., additional, Rösgen, B., additional, Selle, T., additional, Marchewka, Astrid, additional, Menzel, Stephan, additional, Böttger, Ulrich, additional, Waser, Rainer, additional, Hoskins, Brian, additional, Alibart, Fabien, additional, Strukov, Dmitri, additional, Pellegrino, Luca, additional, Manca, Nicola, additional, Kanki, Teruo, additional, Tanaka, Hidekazu, additional, Biasotti, Michele, additional, Bellingeri, Emilio, additional, Sergio Siri, Antonio, additional, Marré, Daniele, additional, M. Padilha, Antonio Claudio, additional, Martini Dalpian, Gustavo, additional, Reily Rocha, Alexandre, additional, Prodromakis, Themistoklis, additional, Salaoru, Iulia, additional, Khiat, Ali, additional, Toumazou, Christopher, additional, Gale, Ella M., additional, Madhavan, A., additional, Adam, G., additional, Alibart, F., additional, Gao, L., additional, Strukov, D. B., additional, Wamwangi, D., additional, Welnic, W., additional, Gholipour, Behrad, additional, Huang, Chung-Che, additional, Anastasopoulos, Alexandros, additional, Al-Saab, Feras, additional, Hayden, Brian E., additional, Hewak, Daniel W., additional, Lan, Rui, additional, Endo, Rie, additional, Kuwahara, Masashi, additional, Kobayashi, Yoshinao, additional, Susa, Masahiro, additional, Baumeister, Paul, additional, Wortmann, Daniel, additional, Blügel, Stefan, additional, Mazzarello, Riccardo, additional, Li, Yan, additional, Zhang, Wei, additional, Ronneberger, Ider, additional, Simon, Ronnie, additional, Gallus, Jens, additional, Bessas, Dimitrios, additional, Sergueev, Ilya, additional, Wille, Hans-Christian, additional, Pierre Hermann, Raphaël, additional, Luckas, Jennifer, additional, Rausch, Pascal, additional, Krebs, Daniel, additional, Zalden, Peter, additional, Boltz, Janika, additional, Raty, Jean-Yves, additional, Salinga, Martin, additional, Longeaud, Christophe, additional, Wuttig, Matthias, additional, Kim, Haeri, additional, Kim, Dong-Wook, additional, Phark, Soo-Hyon, additional, Hong, Seungbum, additional, Park, C., additional, Herpers, A., additional, Verbeeck, J., additional, Egoavil, R., additional, Borgatti, F., additional, Panaccione, G., additional, Offi, F., additional, Dittmann, R., additional, Clima, Sergiu, additional, Sankaran, Kiroubanand, additional, Mees, Maarten, additional, Yin Chen, Yang, additional, Goux, Ludovic, additional, Govoreanu, Bogdan, additional, Wouters, Dirk J., additional, Kittl, Jorge, additional, Jurczak, Malgorzata, additional, Pourtois, Geoffrey, additional, Calka, P., additional, Martinez, E., additional, Delaye, V., additional, Lafond, D., additional, Audoit, G., additional, Mariolle, D., additional, Chevalier, N., additional, Grampeix, H., additional, Cagli, C., additional, Jousseaume, V., additional, Guedj, C., additional, Shrestha, Pragya, additional, Ochia, Adaku, additional, Cheung, Kin. P., additional, Campbell, Jason, additional, Baumgart, Helmut, additional, Harris, Gary, additional, Scherff, Malte, additional, Meyer, Bjoern, additional, Scholz, Julius, additional, Hoffmann, Joerg, additional, Jooss, Christian, additional, Xiao, Bo, additional, Tada, Tomofumi, additional, Gu, Tingkun, additional, Tawara, Arihiro, additional, Watanabe, Satoshi, additional, Young, Tai-Fa, additional, Yang, Ya-Liang, additional, Chang, Ting-Chang, additional, Hsu, Kuang-Ting, additional, Chen, Chao-Yu, additional, Burkert, A, additional, Valov, I., additional, Staikov, G., additional, van den Hurk, Jan, additional, Valov, Ilia, additional, Tappertzhofen, Stefan, additional, van der Hurk, Jan, additional, Hoskins, B., additional, Kim, Gun Hwan, additional, Seok, Jun Yeong, additional, Ho Yoon, Jeong, additional, Yoon, Jung Ho, additional, Yoon, Kyung Jin, additional, Shuai, Yao, additional, Wu, Chuangui, additional, Zhang, Wanli, additional, Zhou, Shengqiang, additional, Bürger, Danilo, additional, Slesazeck, Stefan, additional, Mikolajick, Thomas, additional, Helm, Manfred, additional, Schmidt, Heidemarie, additional, Gale, Ella, additional, Pearson, David, additional, Kitson, Stephen, additional, Adamatzky, Andrew, additional, Costello, Ben de Lacy, additional, Lehtonen, Eero, additional, Poikonen, Jussi, additional, Laiho, Mika, additional, Kanerva, Pentti, additional, Lim, Hyungkwang, additional, Jang, Ho-won, additional, Jeong, Doo Seok, additional, Cao, Xun, additional, Jiang, Meng, additional, Zhang, Feng, additional, Liu, Xinjun, additional, Jin, Ping, additional, Zhang, Kai, additional, Tangirala, Madhavi, additional, Kittiwatanakul, Salinporn, additional, Lu, Jiwei, additional, Wolf, Stuart, additional, Pallem, Venkateswara, additional, Dussarrat, Christian, additional, Pinto, S., additional, Krishna, R., additional, Dias, C., additional, Pimentel, G., additional, Oliveira, G. N. P., additional, Teixeira, J. M., additional, Aguiar, P., additional, Titus, E., additional, Gracio, J., additional, Ventura, J., additional, and Araujo, J. P., additional

Published
2013
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11. Local symmetry lowering in CdMn2O4 spinel.

Author

Oliveira, G. N. P., Teixeira, R., Mendonça, T. M., Silva, M. R., Correia, J. G., Lopes, A. M. L., and Araújo, J. P.

Subjects
*SPINEL, *ELECTRIC fields, *SYMMETRY, *ANGULAR correlations (Nuclear physics), *JAHN-Teller transitions, *PHASE transitions
Abstract

This work presents an atomic scale study of the electric field gradient (EFG) in the tetragonally distorted CdMn2O4 spinel manganite. The EFG temperature dependence at the Cd and Mn sites was followed via perturbed angular correlation measurements with the 111In and 111mCd probes, from 873 down to 12K at Isolde-CERN. The results show that in the 12–600K temperature range, a single Jahn-Teller distorted local phase exists. However above 100 K, a dynamic lattice distortion, evidenced by time dependent EFG fluctuations, sets in suggesting a structural instability. Above 600 K, a local MnO6 octahedra with relaxed Jahn-Teller distortions emerge and grow in the low temperature matrix, although no macroscopic tetragonal to cubic phase transition was observed. [ABSTRACT FROM AUTHOR]

Published
2014
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14. Local symmetry lowering in CdMn2O4 spinel

Author

Oliveira, G. N. P., primary, Teixeira, R., additional, Mendonça, T. M., additional, Silva, M. R., additional, Correia, J. G., additional, Lopes, A. M. L., additional, and Araújo, J. P., additional

Published
2014
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22. Dynamic off-centering of Cr3+ ions and short-range magneto-electric clusters in CdCr2S4.

Author

Oliveira, G. N. P., Pereira, A. M., Lopes, A. M. L., Amaral, J. S., dos Santos, A. M., Ren, Y., Mendonça, T. M., Sousa, C. T., Amaral, V. S., Correia, J. G., and Araújo, J. P.

Subjects
*CHROMIUM ions, *SPINTRONICS, *DIELECTRICS, *ANGULAR correlations (Nuclear physics), *DEGREES of freedom
Abstract

The cubic spinel CdCr2S4 gained recently a vivid interest, given the relevance of relaxor-like dielectric behavior in its paramagnetic phase. By a singular combination of local probe techniques, namely, pair distribution function and perturbed angular correlation, we firmly establish that the Cr ion plays the central key role on this exotic phenomenon, namely, through a dynamic off-centering displacement of its coordination sphere. We further show that this off-centering of the magnetic Cr ion gives rise to a peculiar entanglement between the polar and magnetic degrees of freedom, stabilizing, in the paramagnetic phase, short-range magnetic clusters, clearly seen in ultralow-field susceptibility measurements. Moreover, the Landau theory is here used to demonstrate that a linear coupling between the magnetic and polar order parameters is sufficient to justify the appearance of magnetic cluster in the paramagnetic phase of this compound. These results open insights on the hotly debated magnetic and polar interaction, setting a step forward in the reinterpretation of the coupling of different physical degrees of freedom. [ABSTRACT FROM AUTHOR]

Published
2012
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23. Local distortions in multiferroic AgCrO2 triangular spin lattice.

Author

Lopes, A. M. L., Oliveira, G. N. P., Mendonça, T. M., Moreira, J. Agostinho, Almeida, A., Araüjo, J. P., Amaral, V. S., and Correia, J. G.

Subjects
*ELECTRIC fields, *SILVER compounds, *LATTICE dynamics, *MAGNETISM, *ELECTRIC distortion
Abstract

In this work an electric-field gradient and magnetic hyperfine field study of the multiferroic AgCrO2 triangular spin lattice is presented. Perturbed angular correlation measurements at different temperatures, revealed the coexistence of two electric-field gradients, i.e., two distinct local environments at temperatures below 100 K. The emerging second local environment appears as a distortion of the Cr surrounding resulting in a local symmetry lowering. This local distortion emerges much above TN and concomitantly with the onset of short-range magnetic correlations. We claim that, through a magnetoelastic instability, distinct Cr-Cr exchange interaction pathways appear providing a channel for magnetic frustration release. [ABSTRACT FROM AUTHOR]

Published
2011
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24. Analysis of cantilever beam test using digital holography

Author

de Oliveira, G. N., Nunes, L. C. S., and dos Santos, P. A. M.

Abstract

In the present work an interferometric approach based on the implementation of a digital holography setup is discussed. It is used to determine interferograms of materials under small deformations. In order to do this, a fixed-free cantilever beam subjected to a concentrated force applied at the free end is taken into account. The experimental procedure to estimate displacements is carried out using digital Fourier transform holography (DFTH) technique. This is obtained making a very simple modification in the classical setup architecture of the DFTH setup. It is also demonstrated the easy and practical viability of the setup in an interferometric application for mechanical parameters determination.

Published
2011
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25. Dynamic moir� patterns and Michelson fringe patterns for profilometry: a results comparative analysis

Author

de Oliveira, G. N., de Oliveira, M. E., and dos Santos, P. A. M.

Abstract

In the present work we present a simple comparative analysis of the results in the optical Fourier transform profilometry which fringe projection is produced by two different methods, dynamic moir�-like patterns and Michelson interferometer. First, the Fourier transform profilometry technique is applied in the projected moir� fringe pattern onto an object surface. Then, this result is compared to the Michelson fringe pattern projection in the same method of profilometry that is applied using the same test object. We have conclude that dynamic moir�-like fringe patterns could be used with some advantages, comparatively to the results obtained by the classical Michelson fringe patterns.

Published
2011
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28. Vectors with a prescribed minimal polynomial

Author

Goncalves, A. D. S., Machado, J. F. C., and De Oliveira, G. N.

Abstract

Let V be a finite dimensional vector space over the field Fand φ (x)∈F[x].LetxV → V be a linear operator. Let Sφbe the set consisting of the vectors whose minimal polynomial φ(x)together with the zero vector We give necessary and sufficieni condition for S φ to be a subspace.

Published
1985
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32. Matrices with prescribed entries and eigenvalues. I

Author

Oliveira, G. N. de

Abstract

It is proved that there exists a matrix (over an appropriate field) with prescribed eigenvalues and a prescribed nonprincipal diagonal. It is also proved that there exists an $ n \times n$ prescribed arbitrary entries.

Published
1973

38. Influence of short time milling in R5(Si,Ge)4, R = Gd and Tb, magnetocaloric materials.

Author

Pires, A. L., Belo, J. H., Turcaud, J., Oliveira, G. N. P., Araújo, J. P., Berenov, A., Cohen, L. F., Lopes, A. M. L., and Pereira, A. M.

Subjects
*MILLING (Metalwork), *MAGNETOCALORIC effects, *ATOMIC structure, *PARTICLE size distribution, *CRYSTAL morphology, *MAGNETIC properties
Abstract

The effect of the shortmilling times on R5(Si,Ge)4 R=Gd, Tbmagnetocaloricmaterial propertieswas investigated. In particular, the effect of milling on atomic structure, particles size and morphology, magnetic, and magnetocaloric effect was studied. With short milling times (b2.5 h), a reduction of the Gd5Si1.3Ge2.7 and Tb5Si2Ge2 particles sizewas achieved down to approximately 3.5 μm. For both compositions the main differences are a consequence of the milling effect on the coupling of the structural and magnetic transitions. In the Gd5Si1.3Ge2.7 case, a second-order phase transition emerges at high temperatures as a result of ballmilling. Consequently, there is a decrease in themagnetocaloric effect of 35% after 150 min of milling. Interestingly, an opposite effect is observed in Tb5Si2Ge2 where a 23% increase of the magnetocaloric effectwas achieved, driven by the enhancement of the coupling betweenmagnetic and structural transitions arising from internal strain promoted by the milling process. [ABSTRACT FROM AUTHOR]

Published
2015
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