Diamond Annual Review 2019/20

42 43 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 1 9 / 2 0 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 1 9 / 2 0 MagneticMaterials Group Beamline I10 We performed temperature-dependent XMCD measurements at the Cr L 2,3 absorption edges of the Bi 2 - x Cr x Se 3 thin films on I10. Circularly polarized X-rays with ~100% polarization were used in normal incidence with respect to the sample plane and parallel to the applied magnetic field. The XMCD was obtainedbytakingthedifferenceoftheabsorptionspectrabyflippingtheX-ray helicity at a fixedmagnetic field. Atomic multiplet theory was used to calculate the electric-dipole transitions, which reveals two deconvoluted Cr spectra uniquely representing the surface and the bulk properties of the Bi 2 - x Cr x Se 3 , respectively. In line with the electrical magneto-transport measurements, the XMCD-derived spin moment exhibits a Curie-like behaviour, pointing to a ferromagnetic phase of the Bi 2 Se 3 thin films at low temperatures. Remarkably, the fact that the spin moment of the surface and the bulk electrons show distinct temperature dependencies points to the presence of dual magnetic states simultaneously existing within one sample. With careful comparison of the respective magnetisation modes of the bulk and the surface, a‘three-steps’ transition model was concluded for the magnetic TIs against temperature (Fig. 2). During phase I both the surface and bulk are magnetically ordered belowT c ; between T c and T c ’(phase II) the surface retains magnetisation whilst the bulk does not any longer; eventually above T c ’(phase III), both the surface and the bulk lose their magnetic ordering. To conclude, an experiential approach to determine the magnetic ground stateina‘surface-specific’mannerusingthesynchrotron-basedX-raytechnique has been defined and validated. An enhanced surface magnetic ordering of the Bi 2 - x Cr x Se 3 systems with a significantly large surface magnetic moment and high ordering temperature has been unambiguously observed. A‘three-steps- transition’model has been demonstrated, in which a temperature ‘window’of ~15 K exists where the surface of the TI is magnetically ordered but the bulk is not. Future work to explore the tuning of this ‘window’ and understand the dual magnetization process will provide important information to refine the physical model of magneticTIs and lays the foundation for making use of them in the emerging spintronic technologies. References: 1. Liu C. X. et al. Quantum anomalous Hall effect in Hg 1−y Mn y Te quantum wells. Phys. Rev. Lett. 101 , 146802 (2008). DOI: 10.1103/PhysRevLett.101.146802 2. Hsieh D. et al. A tunable topological insulator in the spin helical Dirac transport regime. Nature 460 , 1101 (2009). DOI: 10.1038/nature08234 3. Chen Y. L. et al. Massive Dirac fermion on the surface of a magnetically doped topological insulator. Science 329 , 659 (2010). DOI: 10.1126/science.1189924 Funding acknowledgement: UK EPSRC (EP/S010246/1), Royal Society (IEC\NSFC\181680), and Leverhulme Trust (LTSRF1819\15\12). Corresponding author: Dr Wenqing Liu, Royal Holloway University of London, wenqing.liu@rhul. ac.uk 1 QL 1 nm Global-doped Surf-doped Mid-doped Bulk Dopants Bi 2 Se 3 Matrices Surface Dopants ρ (x) ρ (x) ρ (x) Gapless Gap a b Figure 1: (a) Conceptual illustration of the Dirac fermion states of Bi 2 Se 3 . (b) Sample configuration of the global-, surf-, and mid-doped Bi 2 Se 3 . Figure 2: M-T relationships. Upper row: The XMCD-derived m spin and m orb versus temperature. Lower rows: schematic illustration of the ‘three-steps’ transition. The extraordinarymagnetic transition in topological matters Related publication: LiuW., XuY., He L., van der Laan G., Zhang R. &Wang K. Experimental observation of dual magnetic states in topological insulators. Sci. Adv. 5 , eaav2088 (2019). DOI: 10.1126/sciadv.aav2088 Publication keywords: X-raymagnetic circular dichroism (XMCD);Topological insulator; Surfacemagnetism T he recently discovered topological phase offers new possibilities for condensed matter and spintronics. Three-dimensional (3D) topological insulators (TIs) exhibit novel phases of quantum matter, and sharp transitions, in the electronic structure near their surfaces. Because these can give rise to an anomalous quantum Hall effect and other coherent spin transport phenomena that minimise heat dissipation, 3D TIs have potential uses for next-generation energy-efficient electronics. Although themetallic surface states of TIs have been extensively studied, there has been less direct comparison of their surface and bulk magnetic properties. Researchers used the X-ray Magnetic Circular Dichroism (XMCD) technique at BLADE (Beamline for Advanced Dichroism Experiments), also known as I10, to investigate prototype magnetic TIs. XMCD is one of the most powerful tools for the study of surface phenomena, offering unique elemental selectivity and atomic-scale high sensitivity. They were able to quantitatively address the different magnetic moments and transition temperatures, respectively, for the surface and the bulk of the TI. The researchers demonstrated a ‘three-steps’ transition model, with a temperature window of around 15 K where the surface of the TI is magnetically ordered while the bulk is not. These surface states of magnetic TIs are immune to material disorders. As a result, they have substantial implications for emerging technologies such as dissipationless transport and quantum computing. Understanding the dual magnetisation process will aid in defining the physical model of magnetic TIs and lays the foundation for making use of them in information technology. Three-dimensional (3D) TIs feature novel phases of quantum matter with sharp transitions in the electronic structure near their surfaces. It is well known the different electronic properties of the surface and the bulk universally exist in all solids owing to the termination of the periodic lattice structure. In addition, however, TIs present a new class of nontrivial surface states arising from the intrinsic strong spin-orbit coupling and characterised by a Rashban spin texture 1-3 . These low-dimensional surface states are immune to localisation caused by disorders as long as the disorder potential is time- reversal invariant and therefore have strong implications for the emerging technologies such as dissipationless transport and quantum computation. Breaking the time-reversal invariance by introducing a magnetic perturbation (Fig. 1a), on the other hand, reveals a complex phenomenology associated with a tuneable excitation band gap of the surface spectrum. Such a magnetic TI system resembles that of a massive Dirac fermion, which represents an ideal laboratory to study the interplay between magnetism and topology. While the presence of the metallic surface state has been well studied, experimental evidence on the magnetic properties of the TI surface is far from conclusive. It is possible to distinguish the surface moment of a magnetic TI from that of the bulk using the synchrotron-based X-ray absorption technique, X-ray Magnetic Circular Dichroism (XMCD). This is on the basis of the modified surface band structure, i.e. surface-atom core-level shift, which is reflected in a different surface valence state of metallic elements and can be experimentally observed in their characteristic X-ray absorption spectra. Tuning the absorption to the magnetic resonant edges, XMCD can be obtained for the surface and bulk dopants, respectively, revealing the magnetic ground state and temperature dependence in an unambiguous surface-bulk-resolved manner. Using the absorption end station of I10 at Diamond Light Source, this was successfully demonstrated with a set of modulation-doped Bi 2 Se 3 , a prototype magnetic TI. This has been achieved by accurately controlling the dopant distribution profiles along the growth direction using the slow- deposition molecular beam epitaxy technique (Fig. 1b).

RkJQdWJsaXNoZXIy OTk3MjMx