PCSI2024 Session PCSI-TuM1: Magnetic Materials (2D, Monolayers, & Heterostructures)

The emergent two-dimensional (2D) layered magnets provide ideal platforms to enable the atomically thin magneto-optical and magnetoelectric devices. Though many have envisioned that 2D magnets should allow efficient control of magnetism by a variety of external stimuli, true breakthroughs are still lacking, with limited proof-of-concept demonstrations reported thus far. There appear to be fundamental obstacles for efficient control, e.g., through electrical and optical means. In this talk I will analyze the challenges and present our theoretical and experimental progress on efficient electrical and optical control of 2D magnets. Specifically, the results show that the voltage of a few volts can effectively change the magnetic anisotropy of 2D magnets and the laser shinning of tens of uW/um² can effectively affect the domain behaviors of 2D magnets. These efficient controls of 2D magnets potentially open up new avenues towards low-power spintronics and photonics.

The successful isolation of monolayer to few-layer magnetic atomic crystals from van der Waals (vdW) magnets have opened a new pathway of researching two-dimensional (2D) magnetism [1,2,3]. Over the past half a decade, the vdW and 2D magnet library has been greatly expanded, and new magnetic phenomena have been discovered in the 2D limit. Yet, one key question has been brought up: what is the distinction amongst bulk, surface and 2D magnetism for a vdW magnet? This question is well motivated by the observations of 2D behaviors in 3D vdW magnets, as well as the contrasts between 2D layers and 3D bulk, for systems such as CrI₃, CrSBr, NiPS₃, etc.

In this talk, we will show the surface-bulk difference in two archetype vdW magnets, CrI₃ [4] and CrSBr [5]. In CrI₃, it has been thought that the 3D bulk hosts the ferromagnetic (FM) state below T_c = 61K whereas the 2D films realizes the layered antiferromagnetic (AFM) order below T_N = 45K. We will show from our optical magneto-Raman spectroscopy measurements that even in a 3D bulk CrI₃, we capture clear signatures of layered AFM, in addition to the known bulk FM. We attribute the layered AFM signature here to the surface magnetism, which is the same as that of the 2D layers but distinct from that deep in the 3D bulk (Figure 1a). In CrSBr, it has the same layered AFM order in both 3D bulk and 2D layers, but surprisingly with a higher critical temperature in the 2D case. We will show with our nonlinear optical measurements that multiple characteristic temperature scales appear in the 3D bulk CrSBr, including a surface (T_surface) and a bulk (T_bulk) onset temperature between which the surface one is unexpectedly higher than the bulk one (Figure 1b). Our results on these two systems demonstrate that the surface of vdW magnets can well be distinct from their bulk.

[1] Cheng et al Nature 546, 265 (2017)

[2] Huang et al Nature 546, 270 (2017)

[3] Wang et al ACS Nano, 16, 6960 (2022)

[4] Li et al Phys. Rev. X, 10, 011075 (2020)

[5] Guo et al arXiv 2309.01047 (2023)

In recent years, Mn₃Ga has garnered significant attention due to its exotic physical properties and potential applications in spintronic devices [1,2]. One of the most intriguing, yet less explored, phases is the hexagonal antiferromagnetic phase of Mn₃Ga (D0₁₉), which exhibits anomalous Hall effect and topological Hall effect in distinct temperature ranges [2]. In this presentation, we will delve into the growth and surface studies of a thin film of D0₁₉-Mn₃Ga on a Ga polar- GaN (0001) substrate.

The experiments are carried out in an ultra-high vacuum chamber equipped with a molecular beam epitaxy system and a room-temperature scanning tunneling microscope. Initially, the GaN epilayer is deposited on a GaN (0001) substrate at 700 ⁰C under gallium-rich conditions, followed by the growth of D0₁₉-Mn₃Ga at 250 ⁰C under manganese-rich conditions. Reflection high-energy electron diffraction is used during growth to monitor the sample, and the in-plane lattice constant is evaluated. Various in-situ techniques confirm that the grown sample exhibits epitaxial growth. Furthermore, scanning tunneling microscopy image shows the hexagonal atomic arrangements with an average in-plane atomic spacing of 5.37 ± 0.05 Å. However, the atomic spacing varies in the local region. The 1 x 1 surface structure of hexagonal D0₁₉-Mn₃Ga (a = 5.40 Å [2]) is shown in Fig. 1. Moreover, multiple flat terraces and steps with height of 2.20 Å are observed. The measured step height corresponds to the c/2 value of D0₁₉-Mn₃Ga (c = 4.39 Å [2]). The ex-situ­ X-ray diffraction clearly shows the Mn₃Ga 0002 peak, and the calculated d-spacing matched well with the step heights measured by scanning tunneling microscope. These measurements are consistent with the theoretically reported c-value of D0₁₉-Mn₃Ga. The concentration of manganese and gallium in the sample is confirmed to be 3.2:1.0 by Rutherford backscattering. Various in-situ and ex-situ measurements confirm the D0₁₉-Mn₃Ga growth. Further work is planned to investigate the non-collinear antiferromagnetism using spin polarized scanning tunneling microscope.

This work is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Award No. DE-FG02-06ER46317.

[1] L. Song, B. Ding, Appl. Phys. Lett. 119, 152405 (2021).

[2] Z. H. Liu, Scientific Reports 7, 515 (2017).

Because of the ability to manipulate their structure and properties, metallic 2D van der Waals materials that exhibit ferromagnetism (FM) are of considerable potential interest for spintronics applications. Cr₅Te₈ is such a system whose structure consists of layers of CrTe₂ having additional Cr intercalated between the layers.CrTe₂ itself is known to be a strong ferromagnet up to room temperature [1].Cr₅Te₈ is FM below T_c1=155K with perpendicular magnetic anisotropy and it exhibits a large (10%) negative magneto-resistance effect above T_c1 over a narrow temperature range [2].

We have performed neutron diffraction measurements to explore the magnetic behavior in a temperature range above T_c1 and as a function of applied magnetic field.A modulated antiferromagnetic phase is observed, which has a wavevector perpendicular to the van der Waals layers and a period that is triple the unit cell length.The modulated spin structure is canted with a significant component in the van der Waals layers.The modulation is robust with field applied in-plane but it is quickly destroyed with a field applied perpendicular to the layers.Our magnetic phase diagram shows that the transition from FM to the modulated phase at T_c1 is strongly first-order with a true FM transition occurring at a higher temperature, T_c=180K.We show that the large magnetoresistance observed in transport arises from the in-plane components of the magnetic moments.Since the spin modulation is controlled at relatively low magnetic field and the intercalated Cr can be tuned, 2D systems such as these have potential for spintronic applications.

Support: NSF-DMR; the University of Missouri Research Reactor.Spallation Neutron Source at Oak Ridge National Lab is supported by the US Department of Energy.

[1] Room-temperature intrinsic ferromagnetism in epitaxial CrTe2 ultrathin films X. Zhang et al., Nature Communications12:2492 (2021) [https://www.nature.com/articles/s41467-021-22777-x]

[2] Self-Intercalation Tunable Interlayer Exchange Coupling in a Synthetic Van der Waals Antiferromagnet X. Zhang et al., Advanced Functional Materials 2202977 (2022) [https://onlinelibrary.wiley.com/doi/abs/10.1002/adfm.202202977]