Ph.D. Thesis Defense: Phase Transitions in Nanoscale Designed Magnetic Thin Films

CIC nanoGUNE Seminars

Mikel Quintana
Pre-doctoral Researcher, Nanomagnetism
CFM Auditorium
Andreas Berger
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Ph.D. Thesis Defense: Phase Transitions in Nanoscale Designed Magnetic Thin Films

The general occurrence of thermodynamic phase transitions (TPT) in matter, associated with abrupt changes of certain physical quantities in such systems, is of upmost relevance, both in their pure fundamental understanding, but also in terms of technological applications. Examples of technologically relevant TPT include the superconducting phase transition, used, for example, to reduce the energy consumption in magnetic resonance imaging techniques, or phase changing materials, used in computer cooling systems or thermal energy storage.

Ferromagnetic (FM) materials are another type of systems undergoing TPTs. In these materials, a TPT is known to occur in a specific temperature called the Curie temperature TC, bellow which the system exhibits an ordered FM phase. At temperatures below TC, two equivalent FM states can be found in absence of an external magnetic field, also separated by a different type of TPT. The occurrence of such TPTs in ferromagnets and the associated FM states are also relevant in widespread technological applications involving magneto-caloric refrigeration or magnetic recording media.

In general, thin film research and technologies involving FM materials have historically considered abrupt interfaces mainly. Such abrupt interfaces induce and/or enhance effects required, for example, in the electronic read-out of such FM states. However, gradual changes in the physical properties of thin films are also known to improve their performance in certain conditions.

In this regard, gradual changes in the exchange coupling strength are known to influence the occurrence of the TPT in FM thin films. Here, separate Quasi- PM/FM phases can coexist in the same ferromagnetic thin film. Such formation of separate phases in the same film can furthermore be controlled by means of temperature upon suitable design of the exchange-graded profiles. Thus, exchange- graded FM thin films are a novel tool that can lead to potentially relevant technological applications upon suitable design of such exchange coupling strength profile.

Parallel, the occurrence of TPT is associated to thermodynamic equilibrium conditions in which all thermodynamic quantities remain constant in time. However, phase transitions are also known to happen in systems that are far from equilibrium in presence of a time-dependent driving force that initiates the dynamic trajectory or pattern. Indeed, these so-called dynamic phase transitions (DPT) have been known to exist in FM materials as well. However, their experimental verification has only been possible recently, by means of specifically designed experiments. The understanding of DPTs is crucial in non-equilibrium physics because of their similarities with respect to the TPT. Such similarities allow for the use of methodologies for dynamical states and systems that were originally devised for TPTs only.

In this thesis, a series of investigations related to the occurrence of phase transitions in nanoscale-designed ferromagnetic materials are explained in a comprehensive manner, and associated scientific conclusions are drawn. More specifically, relevant aspects related to the two aforementioned phenomena are explored, namely, the occurrence of phase transitions in exchange-graded FM materials, and the occurrence of dynamic phase transitions in FM thin films.

In Chapter 1, the main theoretical aspects of this thesis are discussed. These include the key ideas of ferromagnetism, together with a review of the most important aspects of TPTs in FM materials. In Chapter 2, the main experimental techniques employed in this work are explained.

Chapters 3-6 represent the main results of this thesis. Chapters 3 and 4, explain the fabrication and structural and magnetic characterization of thin films exhibiting exchange-graded ferromagnetic properties along the film depth. More specifically, Chapter 3 explores the thermodynamic equilibrium behavior of exchange profiles, exhibiting a single FM domain that expands with decreasing temperature. In this work, such type of exchange-graded profiles is shown to bypass universality and, thus, modify the thermodynamic equilibrium critical exponents; a fact that is otherwise basically impossible to control.

In Chapter 4, a more complex exchange-graded profile is designed and investigated. Such profile allows one to create separate FM regions at different depths in the thin film that can interact with each other by means of proximity- induced coupling. Accordingly, such particular sample design is shown to induce strongly temperature dependent coupling or bias fields over extended temperature ranges. The experimental results presented in this chapter demonstrate that such an exchange coupling profile can be used to obtain temperature independent coercivities over large temperature ranges by using otherwise conventional magnetic materials, making them particularly relevant for applications that are relying on stable operation points.

Chapters 5 and 6 explain the here obtained results related to DPTs in FM thin films. In Chapter 5, the first ever experimental quantification of the scaling behavior in the vicinity of the DPT and associated critical exponents is achieved. This is a major achievement, given that such dynamic critical exponents have been theorized to be of the same universality class of the TPT, but no experimental work had verified such universality. For this work, the dynamic behavior of magnetization in ultrathin FM films with in-plane uniaxial anisotropy has been investigated by means of transverse magneto-optical Kerr effect magnetometry. The here obtained results are compatible with the 2D Ising model, as expected. Interestingly, they are shown to differ from the critical exponents of the TPT, that agree with the 3D Ising model. Such observation points towards fundamentally different length-scales for the dimensional crossover between the dynamic and equilibrium phase transition, an entirely novel fact that has not been explored in the literature so far.

In Chapter 6, the existence of a generalized conjugate field for the dynamic order parameter of the DPT is explored, both theoretically and experimentally. For this purpose, time-dependent magnetic field sequences without half-wave asymmetry are considered in the immediate vicinity of the critical point of the DPT. The here obtained results show that one can construct a generalized conjugate field definition and associated values that allow for a renormalization of the entire dynamic phase space. Furthermore, within the mean-field approximation, the dynamic critical exponents are preserved if one employs the proper renormalized phase space for driving forces that do not have half-wave asymmetry.

In Chapter 7, several concluding remarks and an outlook to the present work are discussed.

Supervisor: Andreas Berger