Studying Of Microscopic Magnetic Ordered Systems With Spin Resonance Technology

When magnetic coupling inside a magnetic sample is larger than thermal fluctuation,the sample can be magnetic ordered.Because of multiple forms of magnetic coupling,with the help of multi-parameter coupling including magnetoelectric,magnetoelastic coupling,the physics of magnetic ordered systems is very rich.At the same time,magnetic ordered systems have promising applications in magnetic storage,spintronics and magnonics.Hence the study of magnetic ordered systems is valuable in both theory and application.There are two ways to study magnetic ordered system:one is the top-down way,i.e.to detect a magnetic ordered system from the outside,then using the results to infer the details of the interaction within the system.The other one is the bottom-up way,i.e.to construct the Hamiltonian of the system,then obtaining the general properties of the system by means of simulation.The spin resonance technique is an important tool for studying the magnetic properties of materials.According to the types of spin resonances,there are electron spin resonance and nuclear spin resonance.We can use spin resonance to study the microscopic magnetic ordered system.There are two approaches to achieve this:one is to develop a real-space scanning imaging technique based on the NV electron spin resonance in diamond following the top-down way,and analysis the microscopic magnetic order from the point of view of real space imaging;the other one is to construct the Hamiltonian of a magnetic ordered system in a nuclear spin resonance system following the bottom-up way,and obtaining the relevant information of the microscopic magnetic order with the method of quantum simulation.I used spin resonance techniques to study microscopic magnetic ordering systems during my PhD,my research includes:1.I built a self-developed scanning imaging microscope based on the NV color center electron spin resonance.The microscope has a sensitivity at the scale of～μT/Hz1/2 and a spatial resolution at the scale of～10 nm;it works at room temperature in the atmosphere,the temperature stability is～±5 mK/day.The innovations in this part are:(a)Reducing the temperature drift by increasing the temperature stability and compact mechanical design.(b)A new magnetic applying method which has the capability to realize vector magnetic field adjustment in a scanning microscope with a single NV.(c)A new microwave antenna which could realize a flexible microwave control.The relevant theoretical and technical issues are published in"Nanoscale magnetic imaging based on quantum sensing with diamond and its applications to condensed matter physics",in which the theory part is in the second and third chapter,and the technical part is in the sixth chapter of this thesis.2.I used the self-developed NV scanning microscope to image the freestanding bismuth ferrite films and adjust its stress in-situ.Bismuth ferrite is an antiferromagnetic material,a weak local magnetization of～0.07μB/u.c.is generated by DM(Dzyaloshinskii-Moriya)interaction.We engaged the NV color center electron spin close to the sample at～50 nm and obtained the effective magnetic imaging signal.During the imaging to bismuth ferrite,the in-plane uniaxial stress in films was controlled by in-situ stretching,by this operation,the orientation of the cycloidal magnetic sequence was adjusted.The innovations in this part are:(a)A continuous,in-situ tuning of uniaxial strains over freestanding BFO films,which is integrated into a scanning NV microscope.(b)The first observation of the continuous rotation of the BFO film’s magnetic order.(c)The relation between the magnetic order’s rotation and the structural distortion induced by the strain is explained with a first-principle calculation.The paper discussing this work is in preparation and planned to be submitted as"Observation of the relation between antiferromagnetic order and mechanical strain in a freestanding BFO film".This part is included in the fourth chapter of this thesis.3.The coherence local distribution of three-spin system is studied by quantum simulation based on nuclear spin resonance.And quantum coherence was used to characterize the changes in the magnetic order during the phase transition of the quantum simulation.In this work,we improved the quantum coherence measure based on quantum Jensen-Shannon divergence,constructed the coherent components related to the internal structure of the spin system and gave the relevant triangle inequalites.Adiabatic quantum simulations of three-spin systems were carried out using nuclear spin resonance,the evolution of each component in adiabatic simulation was measured.And the phase diagram of the Ising model was analyzed with quantum coherence.(a)A novel coherence measure,which could provide inequality relations for an adiabatic evolution of a many-body system.(b)Utilizing the coherence measure to observe magnetic phase transition of a spin system.This part was published on NPJ Quantum Information,7:145(2021).In the future I will continue to use NV scanning imaging microscopy to study novel systems in condensed matter like two dimensional ferromagnetism in CrI3 and FGT and topological edge states in graphene.Besides,I will keep developing NV scanning imaging microscopy,including:improving the spatial resolution to the scale of a few nanometers;extending the working temperature to 4 K;improving magnetic sensitivity and imaging speed by combining advanced quantum sensing technology.This will greatly expand the microscopic magnetic ordering that we can observe.In addition,it is expected to combine our imaging technology with nuclear spin resonance based quantum simulation to realize the simulation of any magnetic Hamiltonian and the readout of any spin site.