15747

1620 دكتري

دكتري

ماده چگال

اصفهان : دانشگاه صنعتي اصفهان

1399

بيست، 144ص.: مصور، جدول، نمودار

مجتبي اعلايي

فرهاد شهبازي

عبدالله لنگري، سعيد جلالي اسدآبادي، فرهاد فضيله

1399/06/19

كتابنامه

فيزيك

فيزيك

1399/06/25

2630405

First principle investigation of magnetic properties and terms of spin Hamiltonian of 3d pyrochlores Mohammad Amirabbasi August 11 2020 Department of Physics Isfahan University of Technology Isfahan 84156 83111 Iran University Code IUT 77142 Degree Doctor of Philosophy Language Farsi supervisor Dr Mojtaba Alaei AbstractIn this thesis magnetic and critical properties of 3d pyrochlores such as FeF3 Y2 Mn2 O7and NaCaNi2 F7 are investigated For each of these materials spin Hamiltonian has obtainedthrough ab initio calculations in the framework of Density Functional Theory Using thisHamiltonian the critical and Curie Weiss temperature as well as magnetic order are inves tigated by Monte Carlo simulations We obtain spin Hamiltonian of FeF3 under the relativevolume change with respect to the experimental volume V from 0 2 to 0 2 We show V0that the spin system undergoes a weakly first order transition to AIAO at small volumesthat turn to a second order transition close to the experimental structure However increas ing V to 0 2 system shows a transition to a modular spin structure In the following V0we have investigated Y2 Mn2 O7 magnetic properties There is a discrepancy between theexperimental results for the magnetic ground state of Y2 Mn2 O7 We resolve this issue byemploying density functional theory calculations and Monte Carlo simulations We derivedifferent spin models by varying the Hubbard U parameter in ab initio GGA U calcula tions By comparing Monte Carlo simulations of these models with the experiments we finda ferromagnetic ground state for Y2 Mn2 O7 as the most compatible with experiments Wealso consider Y2 Mo2 O7 as a prototype of the defect free pyrochlore system with spin glassbehavior and compare it with Y2 Mn2 O7 Our GGA U calculations reveal d orbital degreesof freedom for Y2 Mo2 O7 and a fixed orbital orientation in Y2 Mn2 O7 Finally the groundstate magnetic properties of NaCaNi2 F7 pyrochlore has been determined It is found thatdifferent Na Ca distributions can change magnetic symmetry of Ni ions in such a way thatthere are different exchange parameters for each Na Ca distributions Dzyaloshinskii Moriyacoupling constant is indirect which means that the ground state has global degeneracy Wehave shown that the elastic neutron scattering has different patterns at low temperatureswhich indicate the ground state is spin glass Keywords DFT Geometrical frustration magnetic pyrochlores spin glass pyrochlores

مجتبي اعلايي

فرهاد شهبازي

عبدالله لنگري، سعيد جلالي اسدآبادي، فرهاد فضيله