Magnetization and Curie Temperature of Ferromagnetic Ultrathin Films: The Influence of Magnetic Anisotropy and Dipolar Interactions (invited)

Monolayer Magnetic Metal with Scalable Conductivity

2D magnets have emerged as a class of materials highly promising for studies of quantum phenomena and applications in ultra-compact spintronics. Current research aims at design of 2D magnets with particular functional properties. A formidable challenge is to produce metallic monolayers: the material landscape of layered magnetic systems is strongly dominated by insulators; rare metallic magnets, such as Fe3GeTe2, become insulating as they approach the monolayer limit. Here, electron transport measurements demonstrate that the recently discovered 2D magnet GdAlSi - graphene-like AlSi layers coupled to layers of Gd atoms - remains metallic down to a single monolayer. Band structure analysis indicates the material to be an electride, which may stabilize the metallic state. Remarkably, the sheet conductance of 2D GdAlSi is proportional to the number of monolayers - a manifestation of scalable conductivity. The GdAlSi layers are epitaxially integrated with silicon, facilitating applications in electronics.

Emerging 2D Ferromagnetism in Graphenized GdAlSi

2D magnets are expected to give new insights into the fundamentals of magnetism, host novel quantum phases, and foster development of ultra-compact spintronics. However, the scarcity of 2D magnets often makes a bottleneck in the research efforts, prompting the search for new magnetic systems and synthetic routes. Here, an unconventional approach is adopted to the problem, graphenization - stabilization of layered honeycomb materials in the 2D limit. Tetragonal GdAlSi, stable in the bulk, in ultrathin films gives way to its layered counterpart - graphene-like anionic AlSi layers coupled to Gd cations. A series of inch-scale films of layered GdAlSi on silicon is synthesized, down to a single monolayer, by molecular beam epitaxy. Graphenization induces an easy-plane ferromagnetic order in GdAlSi. The magnetism is controlled by low magnetic fields, revealing its 2D nature. Remarkably, it exhibits a non-monotonic evolution with the number of monolayers. The results provide a fresh platform for research on 2D magnets by design.

Exchange Bias State at the Crossover to 2D Ferromagnetism

The inherent malleability of 2D magnetism provides access to unconventional quantum phases, in particular those with coexisting magnetic orders. Incidentally, in a number of materials, the magnetic state in the bulk undergoes a fundamental change when the system is pushed to the monolayer limit. Therefore, a competition of magnetic states can be expected in the crossover region. Here, an exchange bias state is observed at the crossover from 3D antiferromagnetism to 2D ferromagnetism driven by the number of monolayers in the metalloxene GdSi₂. The material constitutes a stack of alternating monolayers of Gd and silicene, the Si analogue of graphene. The exchange bias manifests itself as a shift of the hysteresis loop signifying coupling of magnetic systems, as evidenced by magnetization studies. Two features distinguish the phenomenon: (i) it is intrinsic, i.e. it is detected in an individual compound; (ii) the exchange bias field, 1.5 kOe, is unusually high, which is conducive to applications. The results suggest magnetic derivatives of 2D-Xenes to be prospective materials for ultracompact spintronics.

2D magnetic phases of Eu on Ge(110)

2D magnetic materials are at the forefront of research on fundamentals of magnetism; they exhibit unconventional phases and properties controlled by external stimuli. 2D magnets offer a solution to the problem of miniaturization of spintronic devices. A technological target of materials science is to find suitable magnetic materials and scale their thickness down as much as possible, a single monolayer being a natural limit. However, magnetism does not halt at one monolayer - it may persist beyond this boundary, to sparse but regular lattices of magnetic atoms. Here, we report 2D magnetic phases of Eu on the Ge(110) surface. We synthesized two submonolayer structures Eu/Ge(110) employing molecular beam epitaxy. The phases, identified by electron diffraction, differ in the surface density of Eu atoms. At low temperature, they exhibit magnetic ordering with magnetic moments lying in-plane. Strong dependence of the effective magnetic transition temperature on weak magnetic fields points at the 2D nature of the observed magnetism. The results are set against those on the Eu/Si system. The study of Eu/Ge(110) magnets demonstrates that a variety of substrates of different structure and symmetry can host submonolayer 2D magnetic phases, suggesting the phenomenon to be rather general.

Two-Dimensional Magnets beyond the Monolayer Limit

Intrinsic two-dimensional (2D) magnetism has been demonstrated in various materials scaled down to a single monolayer. However, the question is whether 2D magnetism extends beyond the monolayer limit, to chemical species formed by sparse but regular 2D arrays of magnetic atoms. Here we show that sub-monolayer superstructures of Eu atoms self-assembled on the silicon surface exhibit strong magnetic signals. Robust easy-plane magnetism is discovered in both one- and two-dimensionally ordered structures with Eu coverage of half monolayer and above. The emergence of 2D magnetism manifests itself by a strong dependence of the effective transition temperature on weak magnetic fields. The results constitute a versatile platform for miniaturization of 2D magnetic systems and seed an expandable class of atomically thin magnets for applications in information technologies.

Near Degeneracy of Magnetic Phases in Two-Dimensional Chromium Telluride with Enhanced Perpendicular Magnetic Anisotropy

The discovery of atomically thin van der Waals magnets (e.g., CrI₃ and Cr₂Ge₂Te₆) has triggered a renaissance in the study of two-dimensional (2D) magnetism. Most of the 2D magnetic compounds discovered so far host only one single magnetic phase unless the system is at a phase boundary. In this work, we report the near degeneracy of magnetic phases in ultrathin chromium telluride (Cr₂Te₃) layers with strong perpendicular magnetic anisotropy highly desired for stabilizing 2D magnetic order. Single-crystalline Cr₂Te₃ nanoplates with a trigonal structure (space group P3̅1c) were grown by chemical vapor deposition. The bulk magnetization measurements suggest a ferromagnetic (FM) order with an enhanced perpendicular magnetic anisotropy, as evidenced by a coercive field as large as ∼14 kOe when the field is applied perpendicular to the basal plane of the thin nanoplates. Magneto-optical Kerr effect studies confirm the intrinsic ferromagnetism and characterize the magnetic ordering temperature of individual nanoplates. First-principles density functional theory calculations suggest the near degeneracy of magnetic orderings with a continuously varying canting from the c-axis FM due to their comparable energy scales, explaining the zero-field kink observed in the magnetic hysteresis loops. Our work highlights Cr₂Te₃ as a promising 2D Ising system to study magnetic phase coexistence and switches for ultracompact information storage and processing.

Multiferroicity in atomic van der Waals heterostructures

Materials that are simultaneously ferromagnetic and ferroelectric - multiferroics - promise the control of disparate ferroic orders, leading to technological advances in microwave magnetoelectric applications and next generation of spintronics. Single-phase multiferroics are challenged by the opposite d-orbital occupations imposed by the two ferroics, and heterogeneous nanocomposite multiferroics demand ingredients' structural compatibility with the resultant multiferroicity exclusively at inter-materials boundaries. Here we propose the two-dimensional heterostructure multiferroics by stacking up atomic layers of ferromagnetic Cr2Ge2Te6 and ferroelectric In2Se3, thereby leading to all-atomic multiferroicity. Through first-principles density functional theory calculations, we find as In2Se3 reverses its polarization, the magnetism of Cr2Ge2Te6 is switched, and correspondingly In2Se3 becomes a switchable magnetic semiconductor due to proximity effect. This unprecedented multiferroic duality (i.e., switchable ferromagnet and switchable magnetic semiconductor) enables both layers for logic applications. Van der Waals heterostructure multiferroics open the door for exploring the low-dimensional magnetoelectric physics and spintronic applications based on artificial superlattices.

Two-dimensional magnetic crystals and emergent heterostructure devices

Magnetism, originating from the moving charges and spin of elementary particles, has revolutionized important technologies such as data storage and biomedical imaging, and continues to bring forth new phenomena in emergent materials and reduced dimensions. The recently discovered two-dimensional (2D) magnetic van der Waals crystals provide ideal platforms for understanding 2D magnetism, the control of which has been fueling opportunities for atomically thin, flexible magneto-optic and magnetoelectric devices (such as magnetoresistive memories and spin field-effect transistors). The seamless integration of 2D magnets with dissimilar electronic and photonic materials opens up exciting possibilities for unprecedented properties and functionalities. We review the progress in this area and identify the possible directions for device applications, which may lead to advances in spintronics, sensors, and computing.

Emerging two-dimensional ferromagnetism in silicene materials

The appeal of ultra-compact spintronics drives intense research on magnetism in low-dimensional materials. Recent years have witnessed remarkable progress in engineering two-dimensional (2D) magnetism via defects, edges, adatoms, and magnetic proximity. However, intrinsic 2D ferromagnetism remained elusive until recent discovery of out-of-plane magneto-optical response in Cr-based layers, stimulating the search for 2D magnets with tunable and diverse properties. Here we employ a bottom-up approach to produce layered structures of silicene (a Si counterpart of graphene) functionalized by rare-earth atoms, ranging from the bulk down to one monolayer. We track the evolution from the antiferromagnetism of the bulk to intrinsic 2D in-plane ferromagnetism of ultrathin layers, with its characteristic dependence of the transition temperature on low magnetic fields. The emerging ferromagnetism manifests itself in the electron transport. The discovery of a class of robust 2D magnets, compatible with the mature Si technology, is instrumental for engineering new devices and understanding spin phenomena.

Creep and flow regimes of magnetic domain-wall motion in ultrathin Pt/Co/Pt films with perpendicular anisotropy

We report on magnetic domain-wall velocity measurements in ultrathin Pt/Co(0.5-0.8 nm)/Pt films with perpendicular anisotropy over a large range of applied magnetic fields. The complete velocity-field characteristics are obtained, enabling an examination of the transition between thermally activated creep and viscous flow: motion regimes predicted from general theories for driven elastic interfaces in weakly disordered media. The dissipation limited flow regime is found to be consistent with precessional domain-wall motion, analysis of which yields values for the damping parameter, alpha.