Dynamical phase transitions study in a Co(1−x)Rux magnetic system of two-dimensional Ising type

ABSTRACT: The study of the magnetic properties in new materials has allowed great advances from the technological and industrial point of view. Therefore, the understanding of the physical phenomena that involve magnetism is a challenge that is still in force and that increasingly includes more research. This continuous search has been focused on the development and study of new experimental techniques, analytical and/or theoretical models that can provide the necessary tools to transcend current technological barriers and demands. Hence, the manufacture of nano-structured materials has led to the discovery of new and exciting magnetic properties and devices. That is why, in the first part of this thesis, we will try to elaborate and develop a methodological procedure that allows the use and adaptation of advanced materials manufacturing tools, which will enable the synthesis of nano-structured magnetic thin films. This thin films have a high reproducibility and also are ferromagnetic at room temperature, a relevant aspect for a technological perspective. Likewise, these ultrathin systems can be structurally manipulated to modify the critical magnetic transition temperature or Curie Tc temperature, an intrinsic property of all ferromagnetic materials. Furthermore, for the study of these systems it is necessary to develop an understanding of how the magnetization process occurs and its magnetization reversal (change of sign) in systems with a high anisotropy, that is, systems that have an easy axis of magnetization. These materials are currently widely used in magnetic recording processes. Combined with this last fact and the current demands for more information and speed capability of electronic devices by the final consumer, it is essential to understand the dynamic processes of magnetization under the effect of time-dependent magnetic fields. Therefore, in the second part of our work we will find some important peculiarities of the study of magnetic transitions in low dimensional systems using external excitations with temporal dependence. For this, we will take advantage of the Kerr magneto-optical effect or MOKE, as our main study tool. This phenomenon consists of the change of polarization in an electromagnetic wave when reflected by a magnetized surface, allowing more information about some fundamental magnetic observables even at the limit of monolayers (a few atoms). Therefore, by employing the Kerr effect in conjunction with a suitable series of test samples, we are able to study the dynamic magnetic behavior in the vicinity of the dynamic phase transition (DPT). Likewise, by changing the intrinsic Curie temperature in our thin film systems, i.e. the reduced temperature T/Tc we can access experimentally to a wider portion of the dynamic phase diagram which has not been addressed before either theoretically or experimentally for dynamic magnetic systems. For this purpose, we have fabricated Co1−xRux (1010) epitaxial thin films with uniaxial in-plane anisotropy by means of sputter deposition in the concentration range 0.0 ≤ x ≤ 0.26. All samples are ferromagnetic at room temperature, exhibit an abrupt magnetization reversal along their easy axis, and represent a unique Tc and thus T/Tc-ratio according to their Ru concentration. The dynamic magnetic behavior was measured by using an ultra-sensitive transverse magneto-optical detection method, and the resulting dynamic states were explored as a function of the applied magnetic field amplitude H0 and period P, as well as an additional bias field Hb, which is the conjugate field of the dynamic order parameter Q. Our experimental results demonstrate that the qualitative behavior of the dynamic phase diagram is independent from the T/Tc-ratio, and that for all T/Tc values, we observe metamagnetic anomalies in the dynamically paramagnetic state. These anomalies are characterized by a rapid rise in the order parameter in a small region of bias field, which are not present in the corresponding thermodynamic phase diagram. However, quantitatively these metamagnetic anomalies are very strongly dependent on the T/Tc-ratio, leading to an about 20-fold increase in magnitude for the metamagnetic fluctuations in the paramagnetic regime as the T/Tc-ratio increases from 0.37 to 0.68. Also, the phase space range, in which these anomalous metamagnetic fluctuations occur, extends closer and closer to the critical point as T/Tc increases.

• Autores: Marín Ramírez, Juan Marcos
• Tipo de recurso: Doctoral thesis
• Fecha de publicación: 2020
• Institución: Universidad de Antioquia
• Repositorio: Repositorio UdeA
• Idioma: eng
• OAI Identifier: oai:bibliotecadigital.udea.edu.co:10495/19522
• Acceso en línea: http://hdl.handle.net/10495/19522
• Palabra clave: Ising model; Phase transformations (Statistical physics); Mean field theory; Kerr effect; Magnetismo; Magnetism; Magnetización; Magnetization; Sistemas dinámicos; http://id.loc.gov/authorities/subjects/sh85068376; http://id.loc.gov/authorities/subjects/sh85100646; http://id.loc.gov/authorities/subjects/sh96010226; http://id.loc.gov/authorities/subjects/sh85072072
• Rights: openAccess
• License: Atribución-NoComercial-CompartirIgual 2.5 Colombia (CC BY-NC-SA 2.5 CO)