Session G02: 2D Ferroelectrics and Multiferroics

Strain-mediated magnetoelectric coupling of LSMO twisted bilayer membranes on PMN-xPT

Strain-mediated magnetoelectric (ME) coupling in ferroelectric (FE) and ferromagnetic (FM) heterostructures provides an exciting path towards the development of next-generation memory storage and sensing technologies [1-2], particularly for low-power applications [3-4]. Theory predicts that strain can control magnetic orientation in heterostructures with noncollinear easy axes [5]. By stacking free-standing membrane thin films, which are released from their original substrates through various methods [6-8], we can vary the twist-angle between individual membrane layers which has paved the way for a new category of devices termed “twistronics.” [9] We want to utilize this twist angle degree of freedom to achieve parallel to antiparallel magnetic switching in twisted bilayers of La_(1-x)Sr_(x)MnO₃ (LSMO) transferred onto a giant piezoelectric (1-x)Pb(Mg_1/3Nb_2/3)O₃-(x)PbTiO₃ (PMN-xPT). We will discuss experiments involving in-operando Polarized Neutron Reflectometry (PNR) to measure the depth dependence of magnetism which helps us study how individual layers of LSMO respond to the strain provided by PMN-xPT.

[1] Adv. Mater. 28, 15-39 (2016). [2] J. Phys. D. Appl. Phys. 51, 243001 (2018). [3] Sci. Adv. 7, eabh2294 (2021). [4] Nat. Comm. 2, 553 (2011). [5] Adv. Theory 4, 2000215 (2021). [6] Nat. Mater. 15, 1255-1260 (2016). [7] Nature 544, 340- 343 (2017). [8] Appl. Phys. Lett. 119, 202903 (2021). [9] Phys. Rev. B 95, 075420 (2017).   

Fabrication and Characterization of SrMnO3 Membranes

Multiferroic materials exhibiting both ferromagnetism and ferroelectricity show promise as future memory, sensor, and spintronic devices due to the coupling of the electric and magnetic orders. One such material, SrMnO₃ (SMO), achieves a multiferroic ferroelectric-antiferromagnetic state at high strains (>1%).  At even higher strains (>4.5%), however, SMO is predicted to undergo a transition into a ferroelectric-ferromagnetic state. This would enable direct manipulation of magnetic polarization through an electric signal, which is critical for low-energy memory devices. Traditional epitaxial methods have so far only achieved strains of 3.8%, well below the predicted ferromagnetic transition, and is itself on the limits of strains possible by that method. As such, observation of the ferromagnetic-ferroelectric state will have to be achieved through other sources of strain. Herein, we report fabrication of high quality SMO membranes, as a first step to achieving the strain required to achieve the ferromagnetic-ferroelectric state. Pulsed laser deposition is used to deposit SMO on top of a template consisting of a sacrificial layer grown on a SrTiO₃ (001) substrate. The sacrificial layer is subsequently dissolved in water, and the membrane transferred onto a polyimide film.  XRD, XRR, AFM, and transport measurements were performed before and after membrane fabrication to ensure membrane quality.   

Enhanced magnetoelectric coupling in two-dimensional hybrid multiferroic heterostructures

Magnetoelectric coupling in insulating multiferroic materials holds significant value in both fundamental research and multifunctional device applications. However, material realization remains highly challenging. We predict enhanced magnetoelectric coupling in a multiferroic van der Waals heterostructure composed of CrCl₃ and CuCrP₂S₆. In this heterostructure, the intralayer orders exhibit characteristics of a type-I multiferroic, while the interlayer orders are more akin to a type-II multiferroic, forming a hybrid-type multiferroic. Notably, the interlayer magnetic configuration can be switched upon electric polarization reversal, while maintaining the insulating property without any nontrivial phase transitions. Our analysis reveals substantial band alignment variations and a strongly asymmetric spin local-field effect under electric polarization. Particularly, these two factors synergistically work together in deciding the interlayer exchange interactions, achieving enhanced magnetoelectric coupling. Finally, we demonstrate that this magnetoelectric effect is robust in multi-layer structures and can even control the Néel vector, which is known difficult to be manipulated. This study indicates promising prospects to search for enhanced magnetoelectric couplings in hybrid multiferroic heterostructures.   

Exploring the Magnetic Ground State and Electric Control in Two-Dimensional Multiferroics

Two-dimensional magnets have recently gained significant attention due to their unique properties and high tunability compared to their bulk counterparts. Among these materials, type-II 2D multiferroics have emerged as intriguing systems that exhibit rich physics arising from the interaction between a chiral helical magnetic order and a magnetism-induced ferroelectric order. Motivated by the experimental discovery of a single-layer multiferroic in the NiI2 system, our study focuses on investigating the magnetic ground state of NiI2 under the combined influence of electrostatic doping and an electric field in a single-gate device structure. To elucidate the dynamics of electric and magnetic orders, we employ photocurrent and linear dichroism measurements. Remarkably, we observe a pronounced photocurrent upon applying gate voltage, displaying asymmetric voltage dependence. Furthermore, we observe the emergence of a possibly novel state at intermediate temperature, which has not been previously observed. These findings not only provide valuable insights into the behavior of 2D multiferroics, chiral magnetic textures, and magnetoelectric coupling but also hold promise for potential applications in magneto-electric devices. We will present a comprehensive analysis of the electric control over the magnetic ground state in NiI2, shedding light on the underlying mechanisms and offering new avenues for exploring 2D multiferroics and their technological potential.   

Designing unconventional ferroelectrics in van der Waals heterostructures

Achieving atomically thin ferroelectric materials for the use in ferroelectric non-volatile memory remains a significant challenge in materials science, primarily due to the depolarization effects in ultra-thin scales. To address this challenge, we present a novel approach to engineer atomically thin ferroelectrics using van der Waals heterostructures. Our method involves artificially inducing ferroelectricity by manipulating the stacking angle of non-ferroelectric materials such as bilayer boron nitride and bilayer transition metal dichalcogenides. This technique enables us to produce one of the thinnest out-of-plane ferroelectrics that operates as a non-volatile memory at room temperature.

 

We specifically highlight its device performance as a ferroelectric field effect transistor. The artificial ferroelectrics offers atomically-thin devices that enables ultrafast and high-endurance switching, which outperforms some of the traditional constraints. This exceptional performance stems from the unique ferroelectric mechanism, where the polarization is switched by the interlayer sliding motion between the van der Waals layers.

 

Additionally, we introduce the novel concept of moiré ferroelectrics achieved by twisting the two layers. This results in a unique ferroelectric state characterized by an alternating out-of-plane polarization network. We further discuss the utility of moiré ferroelectrics as substrates to modulate the band structure of 2D materials in momentum space.   

Title:Oral: Room temperature multiferroicity in alloys of 2D van der Waals transition metal dichalcogenide materials

Multiferroics are materials that simultaneously exhibit more than one type of ordering such as magnetic, electric, or elastic. This coexistence of multiple ferroic orders is a scarce property to be found in materials. Historically, this state has been found mainly in 3-dimensional complex oxides, but so far this state has still been elusive for the most widely studied and characterized family of 2-dimensional van der Waals compounds, the transition metal dichalcogenides (TMD). TMDs are composites of the type MX₂, with M a transition-metal atom (Mo, W, etc) and X a chalcogen atom (S, Se or Te). In this talk I will present evidence of multiferroic states in alloys of this family of materials, at room temperature, in bulk single crystals of Te-doped WSe₂. We observe the coexistence of ferromagnetism and ferroelectricity through the presence of magnetization and piezoresponse force microscopy hysteresis loops. Our results prefigure a design strategy of 2D multiferroics based on alloying these compounds by means of chemical  substitutions in the chalcogen site (in this case, W[Te_x Se_1-x]_2-δ, where δ representa vacancies of the chalcogen atoms). These findings open the possibility of widening the use and study of van der Waals-based multifunctional devices for new nanoelectronics and spintronics applications.

[1] Cardenas-Chirivi, G., Vega-Bustos, K., Rojas-Páez, H. et al. Room temperature multiferroicity in a transition metal dichalcogenide. npj 2D Mater Appl 7, 54 (2023). https://doi.org/10.1038/s41699-023-00416-x   

Strain Engineering of Ferroelectricity in a Monolayer CuInP2S6

This study explores the strain effect on the ferroelectric properties of a monolayer CuInP2S6 , which can greatly impact the performance of the emerging van der Waals ferroelectric devices such as the ferroelectric tunnel junctions, ferroelectric random-access memories, and ferroelectric field-effect transistors. By employing density-functional theory calculations, we elucidate the effects of strain on the electronic structures and ferroelectric properties of the material. The underlying mechanism of such strain engineering is revealed through an analysis of strain-dependent formation energy, charge distribution, density of states, and more. These findings highlight the potential of strain as a tool for lowering transition barriers and facilitating ferroelectricity switching via an electric field, thus enhancing the performance of monolayer CIPS in energy-efficient devices.   

Electronic and ferroelectric properties of Niobium and Titanium-based 2D Janus monolayers

Two-dimensional (2D) ferroelectric materials have emerged as a compelling class of substances distinguished by their distinctive electronic and structural properties. One such class of material is the Janus material, characterized by two distinct faces or sides, each possessing unique properties. Janus materials have been investigated for their piezoelectric characteristics; their potential ferroelectric properties have received comparatively limited attention. Our study aims to investigate ferroelectric behavior in Janus materials, focusing on Niobium and Titanium-based Janus monolayers. Employing density functional theory (DFT), our research unveils in-plane spontaneous polarization in these Nb/Ti oxide dihalides. Furthermore, our findings demonstrate the spontaneous polarization in Janus materials with varying halogen atom arrangements. The 2D Janus TiOClI material exhibits a bandgap of 2.862 eV, a value consistent with the existing literature, and displays a notable spontaneous polarization of 1.988 x 10^-10 C/m, which is close to SnS monolayer. Thus, Janus 2D materials can be useful for non-volatile memories, sensors, and nonlinear optoelectronics. The modeling aspect and the results will be discussed in our presentation.    

Effects of structure dimensionality and chemical identity on persistent spin textures

Persistent spin textures, materials with a unidirectional spin configuration in momentum space, show promise for spintronics applications owing to their theoretically infinite spin lifetimes. For noncentrosymmetric bulk materials, it has been shown that the PST can be enforced by the symmetry of the material [1]. Bulk ternary oxides that exhibit both symmetry-protected PSTs (type I) and ferroelectric polarizations are ideal as the spin-orientation may be switched by an electric field; however, few examples have been found. Group-IV monochalcogenide MX monolayers (M=Sn, Ge and X = S, Se, Te) also host both an in-plane ferroelectric polarization and a PST [2]. Through comparing these two groups of materials we discuss the effect material dimensionality, chemistry, and local coordination has on the effective spin-orbit coupling (SOC) parameters and what the implications are for the PST quality and the persistent helix mode. This discussion will be oriented around assessing PSTs feasibility for use in spintronic devices and clarify future directions for research in this field.

[1] X. Lu and J.M. Rondinelli. Discovery Principles and Materials for Symmetry-Protected Persistent Spin Textures with Long Spin Lifetimes. Matter 3, 1211 (2020).

[2] M.A Absor and F. Ishii. Intrinsic persistent spin helix state in two-dimensional group-IV monochalcogenide MX monolayers (M = Sn or Ge and X = S, Se, Te). Phys. Rev. B, 100, 115104 (2019).   

Electronic and optical properties of monochalcogenides van der Waals heterostructure from atomistic simulations

Layering two-dimensional (2D) ferroelectric monochalcogenides in heterostructures offers a powerful approach to tailor the properties of materials and devices for a wide range of applications, including field-effect transistors, nanoelectronics, and sensors. Here, density functional theory (DFT) is used to investigate the electronic and optical properties of monochalcogenides van der Waals heterostructure GeS/SnS. Formation energy and dynamical dispersions confirm the chemical and dynamical stability of the heterostructure. The heterostructure shows the indirect band gap of 1.14 eV by using PBE, and a band gap of 2.54 eV by using HSE06 hybrid functional. Exotic properties can be achieved by computationally engineering 2D van der Waals (vdW) materials and their heterostructures with a suitable choice of stacking order, thickness, and interlayer interactions. Biaxial strain tunes the electronic properties, and indirect to direct band gap conversion of heterostructure is found under biaxial strain. We further investigate the optical properties of heterostructure and show their potential use in ferroelectric memory devices.   

Creating a three-dimensional intrinsic electric dipole on rotated CrI3 bilayers

New multiferroic platforms by sliding or rotation are being explored in two-dimensional (2D) materials [1,2,3]. Antiferromagnetically-coupled CrI₃ bilayers are one of the most studied magneto-electric multiferroic 2D materials [4]. Without considering magnetism, those bilayers possess a crystalline point of inversion, which can only be removed by antiparallel spin configuration between their two monolayers [4]. However, relative rotations between layers break the crystalline point of inversion, resulting in an inherent electric dipole moment P. Magnetoelectric couplings can be enhanced this way on a two-dimensional bilayer that is experimentally accessible [5].

References: 

[1] Wu et al. ACS Nano, 11, 6382-6388, 2017.

[2] Marmolejo-Tejada et al. Nano Lett. 22, 19, 2022.

[3] Junyi Ji et al. Phys. Rev. Lett. 130, 146801, 2023.

[4] Sun et al. Nature 572, 497, 2019.

[5] Poudel et al. Phys. Rev. B 107, 195128, 2023.