Session AAA02: V: Spin-Dependent Phenomena in Semiconductors, Including 2D Materials and Topological Systems

Dual nature of the magnetic excitations and Kondo effect in a van der Waals Metallic Ferromagnet Fe3-xGeTe2

In the local or itinerant extreme, magnetic excitations can be described by the Heisenberg model which treats electron spins as localized moments, or by the itinerant-electron model where the exchange interaction between electrons leads to unequal numbers of electrons with up and down spins. However, the nature of the magnetic excitations has been elusive when both local moments and itinerant electrons are present in the intermediate range. In this talk, I will show direct spectroscopic evidence obtained from inelastic neutron scattering for the coexistence of and interplay between local moments and itinerant electrons in a van der Waals metallic ferromagnet Fe2.72GeTe2, which can sustain tunable room-temperature ferromagnetism down to the monolayer limit. We find that there exist ferromagnetic spin-wave excitations dispersing from the zone center at low energies resulting from local moments and a columnlike broad continuum at the zone boundary at high energies up to over 100 meV resulting from itinerant electrons. Our neutron spectroscopic data reveal that the low-energy spin waves at 100 K are more coherent than those at 4 K, which is evidence of the weakening of the Kondo screening at high temperatures. These results unambiguously demonstrate the coexistence of local moments and itinerant electrons and the Kondo effect between these two components in Fe2.72GeTe2. Such behaviors are generally expected in heavyfermion systems with heavy f electrons but are rarely clearly observed in materials with light d electrons. These findings shed light on the understanding of magnetism in transition-metal compounds.   

Hydroxide Semiconductors: New Series of van der Waals Magnets

Electronic memory storage devices utilize different magnetic configurations for data storage. Among potential magnetic systems, antiferromagnetically (AFM) oriented systems possess timescales of magnetization faster than ferromagnetic (FM) ones, leading to more efficient operations. Vast efforts have been devoted to explore the 2D FM and AFM semiconducting materials in recent years for their usage in MRAM devices. With the help of first-principles calculations using GGA-PBE functional with vdW correction, we have predicted a new series of 2D magnetic transition metal hydroxide semiconductors X(OH)2 (X= Mn, Fe, Co Ni). We have explored the layer-dependent magnetic properties and the formation as well as exfoliation energies of the corresponding 2D monolayers. The exfoliation energies for monolayers of Mn, Fe, Co and Ni hydroxides are 9.51, 2.97, 8.62, and 7.19 meV/Å2 respectively.  The spin-polarized electronic structures of bulk and monolayer have been calculated to investigate their in-plane and out-of-plane lowest-energy magnetic configurations. All of these systems are observed to have a wide bandgap in their bulk state and stabilize in the collinear AFM state having A-type, G-type, C-type and A-type configurations for Mn, Fe, Co and Ni hydroxides respectively. There are band gap crossover transitions when thickness of the systems is reduced from bulk to monolayer. The magnetic phonon dispersion shows that the bulk systems are devoid of any structural instabilities.

    

Stimuli-assisted magnetism in two-dimensional (2D) magnets

Magnetic phase control and room temperature magnetic stability in two-dimensional (2D) materials are indispensable for realising advanced spintronic and magneto-electronic functions. Our current work employs first-principles calculations to comprehensively study the magnetic behaviour in several 2D intrinsically magnetic systems, uncovering the impact of strain and electric field on the material. Our studies have revealed that uniaxial strain leads to the feasibility of room temperature ferromagnetism in the 2D-oxychloride system and also detected the occurrence of a ferromagnetic - antiferromagnetic phase transition in the system, which is anisotropic along the armchair and zigzag directions. Beyond such a strain effect, the coupling of strain and electric field leads to a remarkable enhancement of the Curie temperature (Tc) ~ 450 K in CrOCl [1, 2]. Furthermore, the current-voltage (I–V) response showed spin-resolved conductance with 100% spin filtering, and conductance fluctuations, characterised by peak-to-valley ratio and switching efficiency offering high strain-assisted tunability [3]. These predictions based on our detailed simulations show the prospect of multi-stimuli magnetic phase control, which could have great significance for realizing magneto-mechanical sensors.

Ref:

(1)  Nair, A. K., Shivani Rani, M. Venkata Kamalakar, and Soumya Jyoti Ray. "Bi-stimuli assisted engineering and control of magnetic phase in monolayer CrOCl." Physical  Chemistry Chemical Physics 22, no. 22 (2020): 12806-12813.

(2)Kar, S., S. Rani, and S. J. Ray. "Stimuli assisted electronic, magnetic and optical phase control in CrOBr monolayer." Physica E: Low-dimensional Systems and Nanostructures 143 (2022): 115332.

(3)Rani, Shivani, A. K. Nair, M. Venkata Kamalakar, and Soumya Jyoti Ray. "Spin-selective response tunability in two-dimensional nanomagnet." Journal of Physics: Condensed Matter 32, no. 41 (2020): 415301.

    

Strongly temperature-dependent spin-orbit torque in sputtered WTex

Type-II Wely semimetals have attracted considerable interest due to the strong spin-orbit coupling and Fermi arc surface states. Recently, large charge-to-spin conversion was also observed in amorphous Wely semimetals. Here, we report the strong temperature dependence of the SOT efficiency of sputtered WTe_x. The SOT efficiency is 0.35 under 295 K, dramatically increasing to a sizeable SOT efficiency above 3.5 under 12 K. We further perform current-induced switching of perpendicular magnetization in sputtered WTe_x devices from 12 K to room temperature. Our results show the potential of the industrial application based on sputtered Wely semimetals.

    

Symmetry enriched criticality in a spin triangular ladder model

The phase diagram and universality class of a triangular cluster Ising ladder are studied by infinite time-evolving block decimation algorithm. There is a critical line hosting a phase transition between a spontaneous symmetry breaking phase and a non-trivial symmetry protected topological phase. Except an exceptional point, critical physics along the critical line always falls into the topological Ising universality class. By adding an irrelevant boundary perturbation, we further systematically study the topological degeneracy of the critical line and decaying behavior of finite-size energy splitting of edge mode. After introducing a localized correlation length, we propose that the physical quantity draws a hyperfine structure of the topological Ising universality class, which goes beyond both the Landau paradigm and the conformal field theory.   

Chiral Spin Textures in Amorphous Rare-earth Transition-metal Ferrimagnets

Chiral spin textures including magnetic skyrmions have drawn extensive interest for their potential in serving as the building blocks in future memory and logic devices. These chiral spin textures are stabilized through the Dzyaloshinskii-Moriya interactions (DMI). While skyrmions have been observed experimentally in amorphous rare-earth transition-metal ferrimagnets, challenges remain in identifying other possible chiral spin textures. In this work, we report calculated chiral spin textures beside skyrmions in 10 nm thick CoGd thin films. Using the atomistic Landau–Lifshitz–Gilbert (LLG) equation, we evolve various initial spin textures to uncover metastable spin configuration states in magnetic energy landscape. This study can help the design of next-generation spintronics using chiral spin textures.   

Silicon passivation of zigzag graphene edge enabling robust spin-polarized nanogap quantum transport

Zigzag graphene edges (ZGEs) can ideally host spin-polarized edge states, providing significant potential for spintronics applications. However, because the high chemical reactivity of a pure sp² termination easily destabilizes zigzag edges, preparing well-defined ZGEs at ambient conditions remains a formidable practical challenge. Performing first-principles calculations, we herein demonstrate that the silicon passivation of ZGEs is a reliable method to preserve the spin-polarized ZGE states and furthermore to improve their electronic connectivity with neighboring nanostructures. We find that the desired structural stabilization is driven by the formation of polysilene-like quasi-one-dimensional puckered silicon chain configurations along the ZGE, which satisfies both the strong propensity of silicon atoms toward the sp³-type bonding and the preservation of the sp²-type graphene C edge structure. Calculating the quantum tunneling across DNA nucleobase located in a nanogap between two Si-passivated ZGEs, we finally demonstrate that silicon passivation expands the ZGE electron transport channels both spatially and energetically and significantly enhances spin-polarized sensing currents. We find that 8-oxo-guanine accommodates very large spin-polarized currents due to the hybridization of the spin-polarized reactive oxygen atom and ZGE orbitals, providing an intriguing possibility of the quantum-mechanical sensing of oxidatively damaged DNA.   

Spin parity effects in monoaxial chiral ferromagnetic chain

We describe a fully quantum mechanical treatment of solitons in monoaxial chiral ferromagnets in 1d, which contrasts with the majority of theoretical studies to date  on chiral magnetic solitons (in 1d)/skyrmions (2d) that rely on a classical or at best semiclassical framework.  We begin with a reasonably generic model for this class of magnets, where we find numerically that  the magnetization curve behaves differently depending on  whether the spin quantum number is half-odd integer or integer: a clear indication that quantum effects are at play.  As a leverage for unraveling how this comes about, we then construct a model where the number of solitons of heights 1 through 2S are all seperately  conserved quantities. This leads us to an exact formula relating the set of soliton numbers to the crystal momentum of the ground state, which in turn reproduces the observed behavior. Finally, we establish numerically that the constructed Hamiltonian is a good description of the generic model we started out with. We think the present approach, which replaces the semiclassical "winding-spin picture" of solitons with spin configurations involving quantized steps,  can help gain a better understanding of the quantum nature of solitons, skyrmions and domain walls, in chiral and non-chiral magnets alike. This contribution is based on preprint arXiv:2209.04227, authored by Sohei Kodama,  Yusuke Kato, and the presenter.