Efeitos de campos magnéticos aleatórios em sistemas de vidro de spin com clusters
Autor(a) principal: | |
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Data de Publicação: | 2019 |
Tipo de documento: | Tese |
Idioma: | por |
Título da fonte: | Manancial - Repositório Digital da UFSM |
Texto Completo: | http://repositorio.ufsm.br/handle/1/17547 |
Resumo: | This work investigates the effects of the presence of random magnetic fields (RFs) in thermodynamic quantities that characterize the spin glass state (VS). We adopt a cluster spin model that considers long-range disordered interactions between magnetic clusters and short-range interactions (ferromagnetic, FE, or antiferromagnetic, AF) between spins belonging to the same cluster, as well as a local random magnetic field acting in each spin, which can follow a bimodal or Gaussian probability distribution. A replica approach is adopted to obtain an effective single cluster problem that is solved analytically. For the system with intracluster FE interactions, we discuss the behavior of order parameters, specific heat, nonlinear susceptibility and phase diagrams for different configurations of disorder. In the absence of a random field, the nonlinear susceptibility ( 3) exhibits a divergence in the freezing temperature (Tf ), while the specific heat shows a broad ma- ximum at a temperature T , which depends on the FE interaction. In particular, the intensity of the FE interaction is set to find T around 30% above the Tf , as expected for conventional VS systems. The presence of RFs alters this scenario, Cm still shows the maximum in T that is weakly dependent on RF intensity. However, the Tf is shifted to lower temperatures, considerably increasing the ratio T =Tf . In addition, the divergence in 3 is replaced by a maximum rounded at a temperature T , which becomes higher than Tf with increasing the RF intensity. As a consequence, we suggest that the paramagne- tic (PM) phase is unfolded in three regions: (i) a conventional paramagnetism of high temperatures (T > T ); (ii) a region with short-range order formation with spins frozen (T < T < T ) e (iii) a region with slow growth of free energy barriers that retard the spin dynamics before the transition to VS (Tf < T < T ). These results qualitatively reproduce some findings for the compound LiHoxY1�����xF4 such as the maximum rounded at 3, which is triggered by RFs and the deviation of the conventional relation between Tf and T . For the case with intracluster AF interactions, the results show that the VS region is gradually reduced with increasing the intensity of the AF couplings until only a PM phase is found. Thus, we highlight two distinct regions in relation to the disorder: (i) strong disorder one, where we have a PM/VS second-order phase transition with the RF decreasing Tf similarly to the case FE; (ii) weak disorder, where only the PM state with fully compensated clusters and no long range order is found. In this region (ii), when the RF intensity increase, we observe a VS phase induced by the random field. This result suggests that the presence of RFs can introduce a VS state in weakly disordered AF cluster system. |
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Efeitos de campos magnéticos aleatórios em sistemas de vidro de spin com clustersEffects of random magnetic fields on spin glass systems with clustersCampos aleatóriosVidro de spinClustersDesordemRandom fieldsSpin glassClustersDisorderCNPQ::CIENCIAS EXATAS E DA TERRA::FISICAThis work investigates the effects of the presence of random magnetic fields (RFs) in thermodynamic quantities that characterize the spin glass state (VS). We adopt a cluster spin model that considers long-range disordered interactions between magnetic clusters and short-range interactions (ferromagnetic, FE, or antiferromagnetic, AF) between spins belonging to the same cluster, as well as a local random magnetic field acting in each spin, which can follow a bimodal or Gaussian probability distribution. A replica approach is adopted to obtain an effective single cluster problem that is solved analytically. For the system with intracluster FE interactions, we discuss the behavior of order parameters, specific heat, nonlinear susceptibility and phase diagrams for different configurations of disorder. In the absence of a random field, the nonlinear susceptibility ( 3) exhibits a divergence in the freezing temperature (Tf ), while the specific heat shows a broad ma- ximum at a temperature T , which depends on the FE interaction. In particular, the intensity of the FE interaction is set to find T around 30% above the Tf , as expected for conventional VS systems. The presence of RFs alters this scenario, Cm still shows the maximum in T that is weakly dependent on RF intensity. However, the Tf is shifted to lower temperatures, considerably increasing the ratio T =Tf . In addition, the divergence in 3 is replaced by a maximum rounded at a temperature T , which becomes higher than Tf with increasing the RF intensity. As a consequence, we suggest that the paramagne- tic (PM) phase is unfolded in three regions: (i) a conventional paramagnetism of high temperatures (T > T ); (ii) a region with short-range order formation with spins frozen (T < T < T ) e (iii) a region with slow growth of free energy barriers that retard the spin dynamics before the transition to VS (Tf < T < T ). These results qualitatively reproduce some findings for the compound LiHoxY1�����xF4 such as the maximum rounded at 3, which is triggered by RFs and the deviation of the conventional relation between Tf and T . For the case with intracluster AF interactions, the results show that the VS region is gradually reduced with increasing the intensity of the AF couplings until only a PM phase is found. Thus, we highlight two distinct regions in relation to the disorder: (i) strong disorder one, where we have a PM/VS second-order phase transition with the RF decreasing Tf similarly to the case FE; (ii) weak disorder, where only the PM state with fully compensated clusters and no long range order is found. In this region (ii), when the RF intensity increase, we observe a VS phase induced by the random field. This result suggests that the presence of RFs can introduce a VS state in weakly disordered AF cluster system.Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPESOs efeitos da presença de campos magnéticos aleatórios (RFs) em quantidades termodi- nâmicas que caracterizam um estado vidro de spin (VS) são investigadas neste trabalho. Adotamos um modelo de spin com clusters que considera interações desordenadas de longo alcance entre momentos magnéticos de clusters e interações de curto alcance (fer- romagnética, FE, ou antiferromagnética, AF) entre spins pertencentes ao mesmo cluster, além de um campo magnético aleatório local atuando em cada spin, o qual pode seguir uma distribuição de probabilidade bimodal ou gaussiana. Uma abordagem com réplicas é adotada para obter um problema efetivo de cluster único que é resolvido analiticamente. Para o sistema com interações intracluster FE, discutimos o comportamento dos parâ- metros de ordem, calor específico, susceptibilidade não linear e diagramas de fases para diferentes configurações de desordem. Na ausência de campo aleatório, a susceptibilidade não linear ( 3) exibe uma divergência na temperatura de congelamento (Tf ), enquanto o calor específico (Cm) mostra um máximo amplo a uma temperatura T , que depende da interação FE. Em particular, a intensidade da interação FE é ajustada para localizar T em torno de 30% acima da Tf , como esperado para sistemas VS convencionais. A pre- sença de RFs altera este cenário, o Cm ainda mostra o máximo em T que é fracamente dependente da intensidade do RF. No entanto, a Tf é deslocada para temperaturas mais baixas, aumentando consideravelmente a relação T =Tf . Além disso, a divergência na 3 é substituída por um máximo arredondado a uma temperatura T , que se torna cada vez maior que a Tf com o aumento da intensidade do RF. Como consequência, sugerimos que a fase paramagnética (PM) é desdobrada em três regiões: (i) um paramagnetismo conven- cional de altas temperaturas (T > T ); (ii) uma região com formação de ordem de curto alcance com spins congelados (T < T < T ) e (iii) uma região com lento crescimento de barreiras de energia livre que retardam a dinâmica do spin antes da transição ao VS (Tf < T < T ). Esses resultados reproduzem qualitativamente algumas descobertas para o composto LiHoxY1xF4 como o máximo arredondado na 3, comportamento desencade- ado por RFs e o desvio da relação convencional entre Tf e T . Para o caso com interações intracluster AF, os resultados mostram que a região VS é gradativamente reduzida com aumento da intensidade dos acoplamentos AF, até que somente uma fase paramagnética é encontrada. Assim, destacamos duas regiões distintas em relação à desordem: (i) de- sordem forte, onde temos uma transição de fase de segunda ordem PM/VS com o RF decrescendo a Tf semelhante ao caso FE; (ii) desordem fraca, onde somente a fase PM com clusters totalmente compensados e sem ordem de longo alcance é encontrada. Nessa região (ii), quando a intensidade do RF é aumentada, observamos o surgimento da fase VS induzida pelo campo aleatório. Esse resultado sugere que a presença de RFs pode introduzir um estado VS em sistemas com cluster AF fracamente desordenados.Universidade Federal de Santa MariaBrasilFísicaUFSMPrograma de Pós-Graduação em FísicaCentro de Ciências Naturais e ExatasZimmer, Fábio Mallmannhttp://lattes.cnpq.br/6328420212181284Calegari, Eleonir Joãohttp://lattes.cnpq.br/6030572534512173Dorneles, Lucio Strazzaboscohttp://lattes.cnpq.br/7244173039310066Lazo, Matheus Jatkoskehttp://lattes.cnpq.br/2253520712024445Erichsen Junior, Rubemhttp://lattes.cnpq.br/2461173825552717Romitti, Marcos Vinicios2019-07-24T15:36:54Z2019-07-24T15:36:54Z2019-03-08info:eu-repo/semantics/publishedVersioninfo:eu-repo/semantics/doctoralThesisapplication/pdfhttp://repositorio.ufsm.br/handle/1/17547porAttribution-NonCommercial-NoDerivatives 4.0 Internationalhttp://creativecommons.org/licenses/by-nc-nd/4.0/info:eu-repo/semantics/openAccessreponame:Manancial - Repositório Digital da UFSMinstname:Universidade Federal de Santa Maria (UFSM)instacron:UFSM2019-07-25T06:02:35Zoai:repositorio.ufsm.br:1/17547Biblioteca Digital de Teses e Dissertaçõeshttps://repositorio.ufsm.br/ONGhttps://repositorio.ufsm.br/oai/requestatendimento.sib@ufsm.br||tedebc@gmail.comopendoar:2019-07-25T06:02:35Manancial - Repositório Digital da UFSM - Universidade Federal de Santa Maria (UFSM)false |
dc.title.none.fl_str_mv |
Efeitos de campos magnéticos aleatórios em sistemas de vidro de spin com clusters Effects of random magnetic fields on spin glass systems with clusters |
title |
Efeitos de campos magnéticos aleatórios em sistemas de vidro de spin com clusters |
spellingShingle |
Efeitos de campos magnéticos aleatórios em sistemas de vidro de spin com clusters Romitti, Marcos Vinicios Campos aleatórios Vidro de spin Clusters Desordem Random fields Spin glass Clusters Disorder CNPQ::CIENCIAS EXATAS E DA TERRA::FISICA |
title_short |
Efeitos de campos magnéticos aleatórios em sistemas de vidro de spin com clusters |
title_full |
Efeitos de campos magnéticos aleatórios em sistemas de vidro de spin com clusters |
title_fullStr |
Efeitos de campos magnéticos aleatórios em sistemas de vidro de spin com clusters |
title_full_unstemmed |
Efeitos de campos magnéticos aleatórios em sistemas de vidro de spin com clusters |
title_sort |
Efeitos de campos magnéticos aleatórios em sistemas de vidro de spin com clusters |
author |
Romitti, Marcos Vinicios |
author_facet |
Romitti, Marcos Vinicios |
author_role |
author |
dc.contributor.none.fl_str_mv |
Zimmer, Fábio Mallmann http://lattes.cnpq.br/6328420212181284 Calegari, Eleonir João http://lattes.cnpq.br/6030572534512173 Dorneles, Lucio Strazzabosco http://lattes.cnpq.br/7244173039310066 Lazo, Matheus Jatkoske http://lattes.cnpq.br/2253520712024445 Erichsen Junior, Rubem http://lattes.cnpq.br/2461173825552717 |
dc.contributor.author.fl_str_mv |
Romitti, Marcos Vinicios |
dc.subject.por.fl_str_mv |
Campos aleatórios Vidro de spin Clusters Desordem Random fields Spin glass Clusters Disorder CNPQ::CIENCIAS EXATAS E DA TERRA::FISICA |
topic |
Campos aleatórios Vidro de spin Clusters Desordem Random fields Spin glass Clusters Disorder CNPQ::CIENCIAS EXATAS E DA TERRA::FISICA |
description |
This work investigates the effects of the presence of random magnetic fields (RFs) in thermodynamic quantities that characterize the spin glass state (VS). We adopt a cluster spin model that considers long-range disordered interactions between magnetic clusters and short-range interactions (ferromagnetic, FE, or antiferromagnetic, AF) between spins belonging to the same cluster, as well as a local random magnetic field acting in each spin, which can follow a bimodal or Gaussian probability distribution. A replica approach is adopted to obtain an effective single cluster problem that is solved analytically. For the system with intracluster FE interactions, we discuss the behavior of order parameters, specific heat, nonlinear susceptibility and phase diagrams for different configurations of disorder. In the absence of a random field, the nonlinear susceptibility ( 3) exhibits a divergence in the freezing temperature (Tf ), while the specific heat shows a broad ma- ximum at a temperature T , which depends on the FE interaction. In particular, the intensity of the FE interaction is set to find T around 30% above the Tf , as expected for conventional VS systems. The presence of RFs alters this scenario, Cm still shows the maximum in T that is weakly dependent on RF intensity. However, the Tf is shifted to lower temperatures, considerably increasing the ratio T =Tf . In addition, the divergence in 3 is replaced by a maximum rounded at a temperature T , which becomes higher than Tf with increasing the RF intensity. As a consequence, we suggest that the paramagne- tic (PM) phase is unfolded in three regions: (i) a conventional paramagnetism of high temperatures (T > T ); (ii) a region with short-range order formation with spins frozen (T < T < T ) e (iii) a region with slow growth of free energy barriers that retard the spin dynamics before the transition to VS (Tf < T < T ). These results qualitatively reproduce some findings for the compound LiHoxY1�����xF4 such as the maximum rounded at 3, which is triggered by RFs and the deviation of the conventional relation between Tf and T . For the case with intracluster AF interactions, the results show that the VS region is gradually reduced with increasing the intensity of the AF couplings until only a PM phase is found. Thus, we highlight two distinct regions in relation to the disorder: (i) strong disorder one, where we have a PM/VS second-order phase transition with the RF decreasing Tf similarly to the case FE; (ii) weak disorder, where only the PM state with fully compensated clusters and no long range order is found. In this region (ii), when the RF intensity increase, we observe a VS phase induced by the random field. This result suggests that the presence of RFs can introduce a VS state in weakly disordered AF cluster system. |
publishDate |
2019 |
dc.date.none.fl_str_mv |
2019-07-24T15:36:54Z 2019-07-24T15:36:54Z 2019-03-08 |
dc.type.status.fl_str_mv |
info:eu-repo/semantics/publishedVersion |
dc.type.driver.fl_str_mv |
info:eu-repo/semantics/doctoralThesis |
format |
doctoralThesis |
status_str |
publishedVersion |
dc.identifier.uri.fl_str_mv |
http://repositorio.ufsm.br/handle/1/17547 |
url |
http://repositorio.ufsm.br/handle/1/17547 |
dc.language.iso.fl_str_mv |
por |
language |
por |
dc.rights.driver.fl_str_mv |
Attribution-NonCommercial-NoDerivatives 4.0 International http://creativecommons.org/licenses/by-nc-nd/4.0/ info:eu-repo/semantics/openAccess |
rights_invalid_str_mv |
Attribution-NonCommercial-NoDerivatives 4.0 International http://creativecommons.org/licenses/by-nc-nd/4.0/ |
eu_rights_str_mv |
openAccess |
dc.format.none.fl_str_mv |
application/pdf |
dc.publisher.none.fl_str_mv |
Universidade Federal de Santa Maria Brasil Física UFSM Programa de Pós-Graduação em Física Centro de Ciências Naturais e Exatas |
publisher.none.fl_str_mv |
Universidade Federal de Santa Maria Brasil Física UFSM Programa de Pós-Graduação em Física Centro de Ciências Naturais e Exatas |
dc.source.none.fl_str_mv |
reponame:Manancial - Repositório Digital da UFSM instname:Universidade Federal de Santa Maria (UFSM) instacron:UFSM |
instname_str |
Universidade Federal de Santa Maria (UFSM) |
instacron_str |
UFSM |
institution |
UFSM |
reponame_str |
Manancial - Repositório Digital da UFSM |
collection |
Manancial - Repositório Digital da UFSM |
repository.name.fl_str_mv |
Manancial - Repositório Digital da UFSM - Universidade Federal de Santa Maria (UFSM) |
repository.mail.fl_str_mv |
atendimento.sib@ufsm.br||tedebc@gmail.com |
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1805922124609617920 |