Ultrafast and Energy-Efficient Quenching of Spin Order: Antiferromagnetism Beats Ferromagnetism

Sun

et al. 2018

J. Phys. Chem. Lett.

To date only a few two-dimensional (2D) magnetic crystals were experimentally confirmed, such as CrI 3 and CrGeTe 3 , all with very low Curie temperatures (T C ). High-throughput first-principles screening over a large set of materials yields 89 magnetic monolayers including 56 ferromagnetic (FM) and 33 antiferromagnetic compounds.Among them, 24 FM monolayers are promising candidates possessing T C higher than that of CrI 3 . High T C monolayers with fascinating electronic phases are identified: (i) quantum anomalous and valley Hall effects coexist in a single material RuCl 3 or VCl 3 , leading to a valley-polarized quantum anomalous Hall state; (ii) TiBr 3 , Co 2 NiO 6 and V 2 H 3 O 5 are revealed to be half-metals. More importantly, a new type of fermion dubbed type-II Weyl ring is discovered in ScCl. Our work provides a database of 2D magnetic materials, which could guide experimental realization of high-temperature magnetic monolayers with exotic electronic states for future spintronics and quantum computing applications. KEYWORDS: Magnetic two-dimensional crystals, high throughput calculations, quantum anomalous Hall effect, valley Hall effect. 2 / 12The discovery of two-dimensional (2D) materials opens a new avenue with rich physics promising for applications in a variety of subjects including optoelectronics, valleytronics, and spintronics, many of which benefit from the emergence of Dirac/Weyl fermions. One example is transition-metal dichalcogenide (TMDC) monolayers, whose finite direct bandgap enable novel optoelectronic controls as well as valley-dependent phenomena, such as circular dichroism and valley Hall effect (VHE) 1-2 . Due to the equivalent occupation of K and K' valleys in TMDC materials, the VHE does not produce observable macroscopic Hall voltage. This is overcome in magnetic counterparts, resulted from time reversal symmetry breaking 3-8 .In contrast to fermions in nonmagnetic systems, such as massless Dirac fermions in borophene, black phosphorus, Cu 2 Si 9-13 , and massive Dirac fermions in graphene and bilayer bismuth 14-15 , few types of fermions in 2D magnetic systems are proposed, implying the scarcity of intrinsic magnetic 2D crystals. In particular, 2D magnets with massive Dirac fermions can support quantum anomalous Hall state leading to topologically protected spin current, promising for low-power-consumption devices. Unfortunately, the state is only realized in 2D materials with non-intrinsic magnetism at extremely low temperature (e.g. 30 mK for Cr-doped (Bi,Sb) 2 Te 3 thin film) [16][17][18][19][20][21][22] , hampering its extensive applications. Therefore, discovery of 2D magnets at an elevated temperature is of great importance, providing optimal platforms to enable realistic spintronic and quantum devices, as well as to realize new electronic states.To date 2D magnetic materials are limited to marginal modifications to existing compounds, for instance by: (i) adsorbing hydrogen on graphene 23 ; (ii) reconstructing surface/edge [24][25] ; or (iii) creating defects in MoS 2 n...

Section: / 12mentioning

confidence: 99%

Screening Magnetic Two-Dimensional Atomic Crystals with Nontrivial Electronic Topology

Sun

et al. 2018

J. Phys. Chem. Lett.

“…ltrafast magnetism belongs to one of the most active fields in solid-state physics with frequent discoveries of new fundamental microscopic phenomena. Prominent examples include all-optical magnetization switching mediated by a transient ferromagnetic state 1,2 , control of antiferromagnetic order [3][4][5][6][7] and transfer of angular momentum via terahertz spin currents [8][9][10][11][12][13] . Very recently it was shown theoretically, that in multi-component magnetic systems laser-driven optical transitions can induce a spin-selective charge flow between sub-lattices causing significant magnetization changes, including switching from antiferromagnetic to ferromagnetic order [14][15][16][17][18] .…”

mentioning

confidence: 99%

Optical inter-site spin transfer probed by energy and spin-resolved transient absorption spectroscopy

et al. 2020

Self Cite

Optically driven spin transport is the fastest and most efficient process to manipulate macroscopic magnetization as it does not rely on secondary mechanisms to dissipate angular momentum. In the present work, we show that such an optical inter-site spin transfer (OISTR) from Pt to Co emerges as a dominant mechanism governing the ultrafast magnetization dynamics of a CoPt alloy. To demonstrate this, we perform a joint theoretical and experimental investigation to determine the transient changes of the helicity dependent absorption in the extreme ultraviolet spectral range. We show that the helicity dependent absorption is directly related to changes of the transient spin-split density of states, allowing us to link the origin of OISTR to the available minority states above the Fermi level. This makes OISTR a general phenomenon in optical manipulation of multi-component magnetic systems.

“…[1,2,[8][9][10][11][12] Specialized techniques allow for assigning timescales to specific electronic processes and orbitals or bands. [3,[13][14][15][16]] This is particularly relevant in the magnetic rare earths, where the exchange interaction between the localized 4f spin and orbital magnetic moments is mediated by the itinerant 5d6s conduction electrons via the RKKY interaction. [17,18] Ultrafast electron diffraction (UED) or ultrafast x-ray diffraction (UXRD) experiments that directly observe the transient lattice strain induced by ultrafast demagnetization have been discussed only sporadically.…”

mentioning

confidence: 99%

Ultrafast negative thermal expansion driven by spin disorder

et al. 2019

We measure the transient strain profile in a nanoscale multilayer system composed of Yttrium, Holmium and Niobium after laser excitation using ultrafast X-ray diffraction. The strain propagation through each layer is determined by transient changes of the material-specific Bragg angles. We experimentally derive the exponentially decreasing stress profile driving the strain wave and show that it closely matches the optical penetration depth. Below the Neel temperature of Ho, the optical excitation triggers negative thermal expansion, which is induced by a quasi-instantaneous contractive stress, and a second contractive stress contribution rising on a 12 ps timescale. These two timescales have recently been measured for the spin-disordering in Ho [Rettig et al, PRL 116, 257202 (2016)]. As a consequence we observe an unconventional bipolar strain pulse with an inverted sign travelling through the heterostructure.In most of the research on ultrafast magnetism the lattice was only considered as an angular momentum sink.[1-3] Ultrafast effects on the lattice induced by demagnetization have been discussed surprisingly rarely.[4-7] Time-resolved magneto-optical Kerr measurements and optical picosecond ultrasonics are the workhorse for many researchers. [1,2,[8][9][10][11][12] Specialized techniques allow for assigning timescales to specific electronic processes and orbitals or bands. [3,[13][14][15][16]] This is particularly relevant in the magnetic rare earths, where the exchange interaction between the localized 4f spin and orbital magnetic moments is mediated by the itinerant 5d6s conduction electrons via the RKKY interaction. [17,18] Ultrafast electron diffraction (UED) or ultrafast x-ray diffraction (UXRD) experiments that directly observe the transient lattice strain induced by ultrafast demagnetization have been discussed only sporadically. [4,5,[19][20][21] Several ultrafast diffraction studies on the transition metals Ni and Fe [22][23][24] discuss the strain waves excited by electron and phonon stresses σ e and σ ph , and theory predicts relevant electron-phonon (e-ph) coupling constants [25] even with mode-specificity.[26] Very recently granular FePt films were studied by UED. The rapid out-of-plane lattice contraction could be convincingly ascribed to changes of the free energy of the spin system.[5] The macroscopic Grüneisen coefficients (Gc) Γ e,ph describe the efficiency for generating stress σ e,ph = Γ e,ph ρ Q e,ph by a heat energy density ρ Q e,ph .[27] If Γ e = Γ ph , ultrafast diffraction allows inferring the time-dependent σ(t) from the observed transient strain ε(t).[22, 23, 28] Hooke's law relates ε linearly to σ and hence to the energy densities ρ Q e,ph deposited in each subsystem. This concept was extended to stress resulting from spin-excitations in Ni and Fe [29,30] but the experimental verification remained ambiguous. [22][23][24] Thermodynamic analysis affirms that the Gcs generally measure how entropy S depends on strain ε.[31] The phenomenon of negative thermal expansion (NTE) generally oc...