Cr2O3中反铁磁自旋波的低频拉曼光谱研究.pdf
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1、第 43 卷第 5 期2023 年 10 月物理学进展PROGRESS IN PHYSICSVol.43 No.5Oct.2023Low-Frequency Raman Detection of Antiferromagnetic Spin Waves inCr2O3Dong Biao,CUI Jun,TIAN Yuan-zhe,WU Di,ZHANG QiNational Laboratory of Solid State Microstructures and Department of Physics,Nanjing University,Nanjing 210093,ChinaAbst
2、ract:The antiferromagnetic(AFM)spin waves are promising for being utilized in high-speed and energy-efficient information processing.However,the excitation and detection ofterahertz spin waves in AFM systems is challenging.Here,we demonstrate low-frequencyRaman spectroscopy as a powerful tool for sp
3、in-wave detection in AFM systems.We presenta systematic study of AFM magnons in Cr2O3,a prototypical uniaxial antiferromagnet,viaRaman measurements down to 2.3 cm1(69 GHz).We resolved the magnon Zeeman splittingand the spin-flop transition.We further determined the sign of angular momentum of themag
4、non branches via polarization-resolved Raman processes.We also obtained the anisotropyenergy,the g-factor,and the spin-flop field of Cr2O3as a function of temperatures and magneticfields.A spin-wave renormalization theory accounts for all experimental observations.Key words:antiferromagnetism;spin w
5、ave;low-frequency Raman spectroscopy;spin-flopCLC number:O469Document code:ADOI:10.13725/ki.pip.2023.05.002CONTENTSI.Introduction142II.Results and discussion143A.LFRS set-up and the magnetic structure ofCr2O3143B.Temperature dependence of the AFM magnon ofCr2O3144C.Temperature dependence of the magn
6、on g-factorand the spin-flop field145D.Polarization-resolved magneto-Raman spectra ofmagnons146III.Conclusions148Acknowledgments148References148I.INTRODUCTIONAntiferromagnets with fast spin dynamics andsuperior robustness in external fields are promisingReceived date:2023-7-29 E-mail:for facilitatin
7、g high-speed spin-based information pro-cesses and long-term data storage1-4.Antiferromag-netic(AFM)spin waves,which predominately locatein the terahertz regime,have the potential for beingused as information carriers and play a central rolein AFM-based spintronics and magnonics.However,unlike their
8、 ferromagnetic counterparts,it is challeng-ing to effectively excite and detect AFM magnons dueto their terahertz frequency5-7,especially in thin filmsand microstructures.Ferromagnetic spin waves are commonly launchedand probed with microwave resonance through mag-netic dipole transitions8-11,or wit
9、h Brillouin lightscattering spectroscopy via inelastic scattering pro-cesses between photons and magnons12-16.Themajority of AFM spin waves are beyond the frequencyrange of those conventional approaches for ferromag-nets.On one hand,advances in terahertz opticsoffer new possibilities to address AFM
10、magnons4,17-18,where the magnetic components of terahertz waves cou-ple with AFM spin waves,similar to the microwaveresonance.On the other hand,regarding the inelasticArticle ID:1000-0542(2023)05-0142-9142Dong Biao et al.:Low-Frequency Raman Detection of Antiferromagnetic Spin Waves in Cr2O3143light
11、 scattering approach of magnon detection,conven-tional Raman spectroscopy is hard to resolve magnonsbelow 100 cm1(3 THz).Hence,there exists adetection gap from 100 GHz to 3 THz between Bril-louin light scattering and regular Raman spectroscopy,which unfortunately coincides with the essential fre-que
12、ncy range for most practical AFM magnonic sys-tems.Although a triple-stage spectrometer can resolveRaman shift down to 1 cm1level,it suffers fromcomplicated equipment settings and low efficiency dueto the photon loss from passing multiple gratings.The recent development of low-frequency Raman spec-t
13、roscopy(LFRS)based on Bragg notch filters and aregular single-stage spectrometer fills the gap betweenBrillouin light scattering and conventional Ramanmethods.In this work,we show that LFRS can be a power-ful tool for AFM spin wave detection down to 2.3 cm1(69 GHz).We choose the classic AFM insulato
14、r Cr2O3as a model system and perform a comprehensive inves-tigation of the sub-terahertz magnons in this system.II.RESULTS AND DISCUSSIONA.LFRS set-up and the magnetic structure ofCr2O3Chromium oxide(Cr2O3)is one of the classic AFMinsulators that has been extensively investigated overthe past decade
15、s19-22,and is the first material wheremagneto-electric coupling was discovered.Cr2O3isan easy-axis AFM with a corundum(hexagonal)crys-tal structure and a magnetic point group of3m.ItsNel temperature(TN)is 307 K.The magnetic struc-ture of Cr2O3is shown in Fig.1(b),and spins fromthe two sublattices ar
16、e along with the trigonal c-axis(0001)23-24.The AFM spin wave of Cr2O3locates at0.165 THz at 0 K20,25,measured by high-frequencymicrowave resonance.Recently,terahertz spin pump-ing of AFM magnon was achieved in Cr2O326,andsignatures of magnon-phonon coupling was also re-ported27.The-cut single cryst
17、al Cr2O3fromPrMat.Inc.is used in this study.The experimental set-up of LFRS is schematicallyshown in Fig.1(a).Magneto-Raman measurementsare conducted in the back-scattering geometry.Themagnetic field is applied perpendicular to the samplesurface(along the c-axis)and parallel to the directionof light
18、 propagation.A HeNe laser of 632.8 nm is usedas the excitation light source.The laser beam goesthrough a Bragg bandpass filter and gets focused ontothe Cr2O3single crystal by an objective,with a spotsize of 1 micron.Inelastically scattered photons arecollected and further filtered by multiple Bragg
19、notchfilters,and finally sent to a single-stage spectrometerwith a liquid-nitrogen-cooled CCD array.The schematic Stokes and anti-Stokes processes forcreating and annihilating a magnon in Cr2O3,respec-tively,are illustrated in Fig.1(c).Cr2O3possessesa three-fold rotational symmetry along the c-axis.
20、Itenables a quasi-angular momentum transfer of 3 hfrom the crystal lattice,which assists in the conser-vation of total angular momentum during the Ramanprocess.It can be viewed as the rotational analog ofthe umklapp process28.The total angular momentum(Jtot)consists of three parts,the magnons(Jmag),
21、pho-ton(Jph),and crystal(Jcry),and needs to be conserved,which givesJtot=Jmag+Jph+Jcry=0.(1)As shown in the left panel of Fig.1(c),for a Stokesprocess under left-handed circular excitation and right-handed detection(+)configuration,Jphequals2 h,and the creation of a magnon with h angu-lar momentum g
22、ives Jmag=h,then the crystalcan offer 3 h quasi-angular momentum,Jcry=3 h,which conserves the total angular momentum.For acorresponding anti-Stokes process with the same+polarization configuration(the right panel of Fig.1(c),Jphequals 2 h,and the annihilation of a h magnongives Jmag=h,and Jcry=3 h,t
23、hen Jtotis con-served.It indicates the polarization selection rules areopposite for magnons with opposite angular momentain Stokes and anti-Stokes processes.The selection rulesare also opposite between the two cross-circular config-urations(+and+),which allow the experimen-tal identification of the
24、sign of magnon angular momen-tum via circular polarization-resolved Raman measure-ments.144Dong Biao et al.:Low-Frequency Raman Detection of Antiferromagnetic Spin Waves in Cr2O3Fig.1.(a)Schematic diagram of the low-frequency magneto-Raman spectroscopy set-up.A laser beam passes through thebandpass
25、filter(BPF),then gets focused onto the sample by an objective,scattered by magnons.The scattered photonsare filtered by multiple Bragg notch filters(BNF)then collected by a spectrometer.(b)The magnetic structure of Cr2O3,only Cr atoms are shown.(c)Illustration of Raman processes and selection rules
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