Magnetic insulators are a key resource for next-generation spintronic and topological devices. The family of layered metal halides promises varied magnetic states, including ultrathin insulating multiferroics, spin liquids, and ferromagnets, but device-oriented characterization methods are needed to unlock their potential. Here, we report tunneling through the layered magnetic insulator CrI as a function of temperature and applied magnetic field. We electrically detect the magnetic ground state and interlayer coupling and observe a field-induced metamagnetic transition. The metamagnetic transition results in magnetoresistances of 95, 300, and 550% for bilayer, trilayer, and tetralayer CrI barriers, respectively. We further measure inelastic tunneling spectra for our junctions, unveiling a rich spectrum consistent with collective magnetic excitations (magnons) in CrI.

Abstract.The observation of ferromagnetic order in a monolayer of CrI 3 has been recently reported, with a Curie temperature of 45 Kelvin and off-plane easy axis. Here we study the origin of magnetic anisotropy, a necessary ingredient to have magnetic order in two dimensions, combining two levels of modeling, density functional calculations and spin model Hamiltonians. We find two different contributions to the magnetic anisotropy of the material, favoring off-plane magnetization and opening a gap in the spin wave spectrum. First, ferromagnetic super-exchange across the ≃ 90 degree Cr-I-Cr bonds, are anisotropic, due to the spin orbit interaction of the ligand I atoms. Second, a much smaller contribution that comes from the single ion anisotropy of the S = 3/2 Cr atom. Our results permit to establish the XXZ Hamiltonian, with a very small single ion easy axis anisotropy, as the adequate spin model for this system. Using spin wave theory we estimate the Curie temperature and we highlight the essential role played by the gap that magnetic anisotropy induces on the magnon spectrum.

Quantum spin networks having engineered geometries and interactions are eagerly pursued for quantum simulation and access to emergent quantum phenomena such as spin liquids. Spin-1/2 centers are particularly desirable because they readily manifest coherent quantum fluctuations. Here we introduce a controllable spin-1/2 architecture consisting of titanium atoms on a magnesium oxide surface. We tailor the spin interactions by atomic-precision positioning using a scanning tunneling microscope (STM), and subsequently perform electron spin resonance (ESR) on individual atoms to drive transitions into and out of quantum eigenstates of the coupled-spin system. Interactions between the atoms are mapped over a range of distances extending from highly anisotropic dipole coupling, to strong exchange coupling. The local magnetic field of the magnetic STM tip serves to precisely tune the superposition states of a pair of spins. The precise control of the spin-spin interactions and ability to probe the states of the coupled-spin network by addressing individual spins will enable exploration of quantum many-body systems based on networks of spin-1/2 atoms on surfaces.Building networks of spin-1/2 objects with adjustable interactions represents a versatile approach for quantum simulation of model Hamiltonians [1, 2] because it provides direct experimental access to quantum emergent phenomena, such as topologically generated gapped excitations [3], spin liquids [4] and anyon excitations [5]. However, the precise control of spin interactions and integration beyond a few spins, while maintaining the ability to address individual spins, remains notoriously challenging [6]. Atomically engineered spin networks on surfaces, such as coupled atomic dimers, chains [7,8], ladders [9] and arrays [10], provide a bottom-up realization of tailored spin systems, by using STM to position and address individual atoms [9,11]. Atoms with large spin S generally exhibit strong magnetocrystalline anisotropy that results in Ising-like interactions [4,12]. In contrast, quantum fluctuations scale in proportion to 1/S, so they are maximal for the smallest possible spin, S = 1/2 [4].Spins interact via exchange and dipolar interactions. At the scale of a few coupled spins, shortrange exchange coupling can give rise to magnetic ordering such as magnetic bistability [9,13] and

The advent of devices based on single dopants, such as the single atom transistor 1 , the single spin magnetometer 2,3 and the single atom memory 4 , motivates the quest for strategies that permit to control matter with atomic precision. Manipulation of individual atoms by means of low-temperature scanning tunnelling microscopy 5 provides ways to store data in atoms, encoded either into their charge state 6,7 , magnetization state 8-10 or lattice position 11 . A defining challenge at this stage is the controlled integration of these individual functional atoms into extended, scalable atomic circuits. Here we present a robust digital atomic scale memory of up to 1 kilobyte (8,000 bits) using an array of individual surface vacancies in a chlorine terminated Cu(100) surface. The memory can be read and rewritten automatically by means of atomic scale markers, and offers an areal density of 502 Terabits per square inch, outperforming state-of-the-art hard disk drives by three orders of magnitude. Furthermore, the chlorine vacancies are found to be stable at temperatures up to 77 K, offering prospects for expanding large-scale atomic assembly towards ambient conditions.Since the first demonstration of atom manipulation, 25 years ago 5 , the preferred approach for assembling atomic arrangements has been the lateral positioning of atoms or molecules evaporated onto a flat metal surface, most notably the (111) crystal surface of copper [12][13][14][15] . While ideal for experiments comprising up to several hundreds of constituents, the absence of a large-scale defect-free detectable grid on this surface prohibits the construction of architectures involving correlated lattice-placement of atoms separated by more than a few nanometres. Moreover, thermal motion of the adatoms restricts the technique to temperatures below 10 K. As we demonstrate below, we find that manipulation of missing atoms in a surface (vacancies) 16 , as opposed to additional atoms atop, permits a dramatic leap forward in our capability to build functional devices on the atomic scale.To this purpose, we take advantage of the self-assembly of chlorine atoms on the Cu(100) surface [17][18][19][20] , forming a flat two-dimensional lattice with several convenient properties. First, it provides large areas of a perfect template grid, with a controllable coverage of vacancies. Second, the chlorine lattice remains stable up to a large density of vacancies and up to relatively high temperature (77 K). And third, critical for our purpose, the precise location of the vacancies can be manipulated by STM with a very high level of control (and without the need to pick-up atoms with the tip, i.e. vertical atom manipulation). As we show below, these properties allow us to position thousands of vacancies at predefined atomic sites in a reasonable timeframe.The chlorinated copper surface is prepared in ultra-high vacuum through the evaporation of anhydrous CuCl 2 powder heated to 300 °C onto a clean Cu(100) crystal surface. The crystal is pre-heated to 100-150 °C prior...

Taking advantage of nuclear spins for electronic structure analysis, magnetic resonance imaging, and quantum devices hinges on knowledge and control of the surrounding atomic-scale environment. We measured and manipulated the hyperfine interaction of individual iron and titanium atoms placed on a magnesium oxide surface by using spin-polarized scanning tunneling microscopy in combination with single-atom electron spin resonance. Using atom manipulation to move single atoms, we found that the hyperfine interaction strongly depended on the binding configuration of the atom. We could extract atom- and position-dependent information about the electronic ground state, the state mixing with neighboring atoms, and properties of the nuclear spin. Thus, the hyperfine spectrum becomes a powerful probe of the chemical environment of individual atoms and nanostructures.

Twisted graphene bilayers develop highly localized states around AA-stacked regions for small twist angles. We show that interaction effects may induce either an antiferromagnetic or a ferromagnetic (FM) polarization of said regions, depending on the electrical bias between layers. Remarkably, FM-polarized AA regions under bias develop spiral magnetic ordering, with a relative 120°misalignment between neighboring regions due to a frustrated antiferromagnetic exchange. This remarkable spiral magnetism emerges naturally without the need of spin-orbit coupling, and competes with the more conventional latticeantiferromagnetic instability, which interestingly develops at smaller bias under weaker interactions than in monolayer graphene, due to Fermi velocity suppression. This rich and electrically controllable magnetism could turn twisted bilayer graphene into an ideal system to study frustrated magnetism in two dimensions. DOI: 10.1103/PhysRevLett.119.107201 Magnetism in 2D electronic systems is known to present a very different phenomenology from its three-dimensional counterpart due to the reduced dimensionality and the increased importance of fluctuations. Striking examples are the impossibility of establishing long range magnetic order in a 2D system without magnetic anisotropy [1] or the emergence of unique finite-temperature phase transitions that are controlled by the proliferation of topological magnetic defects [2]. In the presence of magnetic frustration, in, e.g., Kagome [3,4] or triangular lattices [5][6][7][8], 2D magnetism may also lead to the formation of remarkable quantum spin-liquid phases [3,9,10]. The properties of these states remain under active investigation, and have recently been shown to develop exotic properties, such as fractionalized excitations [11], long-range quantum entanglement of their ground state [12,13], topologically protected transport channels [14], or even high-T C superconductivity upon doping [4,15,16].The importance of 2D magnetism extends also beyond fundamental physics into applied fields. One notable example is data storage technologies. Recent advances in this field are putting great pressure on the magnetic memory industry to develop solutions that may remain competitive in speed and data densities against new emerging platforms. Magnetic 2D materials are thus in demand as a possible way forward [17]. Of particular interest for applications in general are 2D crystals and van der Waals heterostructures. These materials have already demonstrated a great potential for a wide variety of applications, most notably nanoelectronics and optoelectronics [18][19][20]. Some of them have been shown to exhibit considerable tunability through doping, gating, stacking, and strain. Unfortunately, very few 2D crystals have been found to exhibit intrinsic magnetism [21,22], let alone magnetic frustration and potential spin-liquid phases.In this work we predict that twisted graphene bilayers could be a notable exception, realizing a peculiar magnetism on an effective triangular supe...

Shrinking spintronic devices to the nanoscale ultimately requires localized control of individual atomic magnetic moments. At these length scales, the exchange interaction plays important roles, such as in the stabilization of spin-quantization axes, the production of spin frustration, and creation of magnetic ordering. Here, we demonstrate the precise control of the exchange bias experienced by a single atom on a surface, covering an energy range of four orders of magnitude. The exchange interaction is continuously tunable from milli-eV to micro-eV by adjusting the separation between a spin-1/2 atom on a surface and the magnetic tip of a scanning tunneling microscope (STM). We seamlessly combine inelastic electron tunneling spectroscopy (IETS) and electron spin resonance (ESR) to map out the different energy scales. This control of exchange bias over a wide span of energies provides versatile control of spin states, with applications ranging from precise tuning of quantum state properties, to strong exchange bias for local spin doping. In addition we show that a time-varying exchange interaction generates a localized AC magnetic field that resonantly drives the surface spin. The static and dynamic control of the exchange interaction at the atomic-scale provides a new tool to tune the quantum states of coupled-spin systems.Exchange interaction between magnetic atoms gives rise to exotic forms of quantum magnetism such as quantum spin liquids [1], and spin transport in magnetic insulators [2,3]. It is also of great technological importance in tailoring magnetic devices [4][5][6][7][8]. For instance, in magnetic read heads, a ferromagnetic layer is "biased" to a specific magnetization direction by strong exchange coupling to an antiferromagnetic layer [4]. Weaker exchange interaction also plays an important role, in the spin dynamics of quantum magnets [9], in giant magnetoresistance devices [10], and in magnetic phases of coupled spins that depend on next-nearest-neighbor interactions such as spin chains [11] and spin glasses [12].

A clear demonstration of topological superconductivity (TS) and Majorana zero modes remains one of the major pending goal in the field of topological materials. One common strategy to generate TS is through the coupling of an s-wave superconductor to a helical half-metallic system. Numerous proposals for the latter have been put forward in the literature, most of them based on semiconductors or topological insulators with strong spin-orbit coupling. Here we demonstrate an alternative approach for the creation of TS in graphene/superconductor junctions without the need of spin-orbit coupling. Our prediction stems from the helicity of graphene's zero Landau level edge states in the presence of interactions, and on the possibility, experimentally demonstrated, to tune their magnetic properties with in-plane magnetic fields. We show how canted antiferromagnetic ordering in the graphene bulk close to neutrality induces TS along the junction, and gives rise to isolated, topologically protected Majorana bound states at either end. We also discuss possible strategies to detect their presence in graphene Josephson junctions through Fraunhofer pattern anomalies and Andreev spectroscopy. The latter in particular exhibits strong unambiguous signatures of the presence of the Majorana states in the form of universal zero bias anomalies. Remarkable progress has recently been reported in the fabrication of the proposed type of junctions, which offers a promising outlook for Majorana physics in graphene systems.Comment: 14 pages, 8 figures. Included simulations of Andreev spectroscopy and mor