Macroscopic Quantum Phenomena in Spintronics

Herbert P. Simanjuntak
Departemen Fisika, Universitas Indonesia, Depok, Jawa Barat, Indonesia

Series: Physics Research and Technology
BISAC: SCI055000

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Although the discussion is general, this book focuses on the problem of macroscopic quantum phenomena using systems of spintronics. The spintronics considered are ferromagnetic and antiferromagnetic spintronics. To represent the macroscopic quantum phenomena in spintronics, transitions from one state to another of the magnetization of ferromagnetic spintronics are considered, and of the Néel vector of antiferromagnetic spintronics. The authors have studied transitions from a metastable state to a more stable one, as well as quantum coherence between two degenerate stable states.

Quantum and classical rates of transitions are presented as functions of temperature, magnetic field and the spin-polarized current flowing through the spintronics. With this method, one can immediately observe the effect of the spin-polarized current on the transitions of the magnetization and the Néel vector when comparing the results to those of the earlier ones on magnetic systems that did not have spin-polarized current. Specifically, while dissipations in magnetic system are intrinsic, the book shows how the total dissipation in spintronics can be controlled and eliminated by varying the spin-polarized current appropriately that depends on the temperature.

The study of transitions from a metastable state to a more stable one in ferromagnetic spintronics shows that the rate of transitions of the magnetization at low temperatures is low and vanishes at zero temperature, so that the magnetization is relatively more stable than that in ferromagnetic materials without existence of spin-polarized currents. In the case of antiferromagnetic spintronics, the behavior and characteristics of transitions of the Néel vector is in contrast to those of ferromagnetic spintronics, where the low-temperature rate of transitions in antiferromagnetic spintronics varies exponentially small in temperature, and is finite and non-vanishing at zero temperature.
In addition to the theoretical aspects, the book also discusses experimental and technological aspects that one may obtain.

Measurements of the rate of transitions can be used to provide an independent method to determine certain parameters being involved, such as the anisotropy parameter Kc of tetragonal crystals, which is an important parameter but usually difficult to obtain. Eliminating dissipation in ferromagnetic and antiferromagnetic spintronics would be desired so as not to have unnecessary loss of energy. Low rate of transitions corresponds to the initial state that is relatively stable. Technologically, the stability of the states of the magnetization and Néel vector in spintronics are important, for example, for memory storage.
(Imprint: Nova)

Chapter 1. Introduction

Chapter 2. Formulation and the Method

Chapter 3. Ferromagnetic Spintronics

Chapter 4. Antiferromagnetic Spintronics

Chapter 5. Ferromagnetic Quantum Coherence

Chapter 6. Antiferromagnetic Quantum Coherence

Chapter 7. Summary and Discussion

Appendices

References

Index

“Spintronics” is the field wherein the net spin of an ensemble of electrons is non-zero. One can monitor the degree of spin polarization in order to study the source of the spin polarization and/or use the polarized electrons to control the behavior of another system. The degree of polarization can be controlled by varying the electric current passing through a layer of metal whose electrons are spin polarized.

This monograph deals with the use of spin polarized electrons to affect the behavior of a magnetic layer, either ferromagnetic or antiferromagnetic. The magnetic layer is macroscopic in that it contains many electrons, perhaps as many as one hundred thousand, yet small enough as to be able to exhibit the phenomenon of “macroscopic quantum tunneling” - referred to as “MQT”. MQT is a phenomenon that occurs at the crossover between the quantum level and the microscopic level and has been richly studied for the past few decades in a number of systems. To observe MQT of magnetization, the magnetization is oriented, typically by an external magnetic field, so that its energy is in a potential well with a barrier of extremely small height so that the magnetization can spontaneously make a transition into a neighboring well at a reasonably large rate.

The transition rate is affected by dissipation. For an isolated sample of a magnetic layer, the dissipation is fixed. However, since spin polarization contributes to the dissipation, our ability to control the degree of spin polarization allows us to control the degree of dissipation in the magnetic layer. That control is an extremely useful tool in a study of the effect of dissipation on MQT.

The ultimate goal of this monograph is to provide the reader with the transition rates of a ferromagnetic or antiferromagnetic layer that has various crystalline structures. The author has been a leader in the field of making these complex calculations so that the results should be of great use to researchers in this field.

The reader should certainly have studied quantum theory at an advanced level. To follow the calculations, the reader should have a background in the use of path integrals to calculate transitions rates for MQT." - Leon Gunther, Emeritus Professor of Physics, Tufts University

Researchers and Graduate students in Physics, Materials science and Materials Engineering, in general, and especially those working in the problem of macroscopic quantum phenomena. People such as those working with spintronics, memory storage, etc., should find the book useful especially in regards to the idea of reducing power loss, and increasing efficiency in systems of spintronics.

Quantum Physics, macroscopic quantum coherence, metastable states, quantum and classical transitions, magnetization and Ne’el vector transitions, dissipative systems, ferromagnetic and antiferromagnetic spintronics

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