Session S38: Controlling Magnetic States in Thin Films and on Surfaces

Live

We experimentally observe both U- and S-shaped zero-field magnetic configurations in single square permalloy dots, with a given dot’s configuration dependent on field history and temperature. Magnetization of 170 nm x 170 nm x 7 nm dots is probed using 4-terminal resistance measurements via the anisotropic magnetoresistance (AMR). RT field sweeps parallel to the square’s edge produce AMR curves characteristic of both the known U-shaped state [1], as well as curves characteristic of a previously unreported S-shaped state for identical field sweep parameters. Measurements at 4K do not generate the S-state pattern, suggesting thermal activation, a result confirmed by micromagnetic simulation. The S-state is also realized by sweeping an off-axis field, consistent with minimizing Zeeman interaction energy. Existence of multiple zero-field configurations complicates the modeling of dots on this scale, precluding use of the Stoner-Wolfarth model and necessitating a more complicated configurational picture.
[1] Endean, et. al, Appl Phys Lett 103, 042409 (2013)   

Live

Many-body phenomena are paramount in physics. In condensed matter, their hallmark is considerable on a wide range of material characteristics spanning electronic, magnetic, thermodynamic and transport properties. They potentially imprint non-trivial signatures in spectroscopic measurements, such as those assigned to Kondo, excitonic and polaronic features, whose emergence depends on the involved degrees of freedom. In this talk, we address systematically zero-bias anomalies detected by scanning tunneling spectroscopy on Co atoms deposited on Cu, Ag and Au(111) substrates, which remarkably are almost identical to those obtained from first-principles [1]. These features originate from gaped spin-excitations induced by a finite magnetic anisotropy energy, in contrast to the usual widespread interpretation relating them to Kondo resonances. Resting on relativistic time-dependent density functional and many-body perturbation theories, we furthermore unveil a new many-body feature, the spinaron, resulting from the interaction of electrons and spin-excitations localizing electronic states in a well defined energy.
[1] Bouaziz, Guimaraes, Lounis, Nat. Commun. (accepted) 2020 -- arXiv:2003.01746   

Live

Combining electron paramagnetic resonance (EPR) with scanning tunneling microscopy (STM) enables detailed insight into the interactions and magnetic properties of single atoms on surfaces [1]. A requirement for EPR-STM is the efficient coupling of a microwave excitation to the tunnel junction. Here, we present a coupling efficiency of the order of unity by using a radiofrequency (RF) antenna placed parallel to the STM tip [2]. This highly efficient coupling allows us to observe the EPR of individual atoms on an MgO surface routinely at 4 K. Using this technique, we perform a systematic study of the EPR of Fe and hydrogenated Ti atoms on MgO, comparing different tunneling parameters, frequency and magnetic field sweeps as well as amplitude and frequency modulation in order to maximize the EPR signal. We interpret the data based on density functional theory and charge transfer multiplet calculations, revealing the important role of the tip magnetic field in EPR-STM [3].

[1] S. Baumann, W. Paul, T. Choi, et al., Science 350, 417 (2015).
[2] T. S. Seifert, S. Kovarik, C. Nistor, et al., Phys. Rev. Research 2, 013032 (2020).
[3] T. S. Seifert, S. Kovarik, D. M. Juraschek, et al., Sci. Adv. 6, eabc5511 (2020).   

Live

Combining the sub-atomic resolution of scanning tunneling microscopy (STM) with the spectral resolution of electron-paramagnetic resonance (EPR) allows for sensitively probing magnetic interactions of single atoms on a surface [1]. However, the experimental requirements for driving the EPR transitions are still under debate. In-depth understanding of the EPR-STM driving is mandatory to explore novel material systems and optimize the sensitivity of this technique. Here, we acquire and model EPR spectra of single Fe and hydrogenated Ti atoms on bilayer MgO on Ag [2]. We systematically investigate the impact of RF excitation strength and tunneling parameters on the EPR signal and find strong evidence for a piezoelectric coupling mechanism [3]. In this mechanism, the surface atom oscillates at RF frequencies in the inhomogeneous tip magnetic field. Based on density functional theory and atomic-multiplet calculations, we reveal different driving mechanisms for single Fe and hydrogenated Ti atoms on the surface. Specifically, transverse magnetic field gradients drive the spin-1/2 hydrogenated Ti, whereas longitudinal magnetic field gradients drive the spin-2 Fe.
[1] S. Baumann et al., Science 350 (2015); [2] T.S. Seifert et al., PRR 2 (2020); [3] T.S. Seifert et al., Sci. Adv. 6 (2020)   

Live

A class of porous metal-organic framework systems, called "magnetic sponge" is attracting interest as gas-sensing material with high designability. One of the systems, [{Ru₂(3,5-F₂PhCO₂)₄}₂{TCNQ(MeO)₂}]3(DCM)1.5(DCE), is a magnetically active compound with a high capability of gas adsorption. While it shows uniform magnetic ordering below around 80 K at normal pressures, its magnetization profile strongly depends on the species of adsorbed gas. In particular, oxygen adsorption sensitively changes the magnetization process according to the gas pressure and alters the low-temperature state to antiferromagnetic. In this contribution, we focus on the antiferromagnetic interaction between the adsorbed oxygen (O) molecules and TCNQ complex and construct an effective spin model with temperature-dependent magnetic interactions by integrating out the O degrees of freedom. The resultant model well explains the magnetization process and the adsorption isotherm at the same time. In particular, we clarify why magnetism can be controlled by oxygen, despite the considerable difference in energy scale between the magnetic interaction and adsorption energy.   

Live

We report the controlled chemical vapor deposition method of an organic-based magnet, vanadium tetracyanoethylene (V[TCNE]_x, x≈2) [1,2]. V[TCNE]_x is a promising candidate for applications in the fields of spintronics and magnonics and has been applied in photovoltaics, light-emitting diodes, and spin valves [3,4]. V[TCNE]_x magnetically orders above room temperature, T_c ~ 400 K. We discuss in detail the method of thin film deposition of V[TCNE]_x via tight control of the parameters in the low temperature (< 350 K) deposition process to achieve high purity films with good reproducibility. The properties of V[TCNE]_x films are confirmed by broad-band ferrimagnetic resonance (FMR) and spin pumping measurement by inverse spin-Hall effect (ISHE). [1] Harberts et al., JoVE. 10.3791/52891 (2015; [2] Froning et al., Applied Physics Letters 106, 122403 (2015). [3] Liu et al., Nature Mater 17, 308–312 (2018); [4] Liu et al., Adv. Mater., 32, 2002663 (2020).   

Live

We report on the structural and magnetic properties of manganese phthalocyanine (MnC₃₂N₈H₁₆) thin films. Several 3d transition metal phthalocyanine thin films have been studied with the aim to tune their magnetic properties based on the metal-ion's strong inter-chain interactions. One promising candidate includes manganese phthalocyanine thin films, which were thermally deposited on substrates covered with both insulating and metallic surfaces at different deposition temperatures ranging from room temperature to 260^oC. The diffusion-driven induced structural changes are observed both with x-ray diffraction and atomic force microscopy. While samples deposited at low temperatures with small crystallites tend to follow diamagnetic behavior, thin films of the same material deposited near 230^oC show saturation magnetization and strong deviations from a diamagnetic behavior below 20 K suggesting a structure-induced magnetic transition.   

Live

Magnetocrystalline anisotropy (MCA) is one of the key parameters investigated in spin-based electronics (spintronics), e.g. for memory applications. Here, we employ first-principles calculations to study MCA in thin film full Heusler alloy Co₂CrAl. This material was studied in the past, and has been reported to exhibit half-metallic electronic structure in bulk geometry. In our recent work, we showed that it retains a 100% spin-polarization in thin-film geometry, at CrAl atomic surface termination. Here, we show that the same termination results in a robust perpendicular magnetic anisotropy, while Co surface termination not only destroys the half-metallicity, but also results in in-plane magnetization orientation. To the best of our knowledge, this is one of the first reports of half-metallic thin-film surface with perpendicular magnetic anisotropy. This result may be of interest for potential nano-device applications, and may stimulate a further experimental study of this and similar materials.   

Live

The realization of magnetic order at the two-dimensional limit is currently a priority for Materials Science. In this arena, self-intercalated 2D transition metal chalcogenides have emerged as candidate magnetic materials with unprecedented robust chemical stability, which could enable their integration in durable, flexible magnetic devices [1]. Here we perform combined atomic-scale structural and electronic characterization of few layer Cr₅Se₈ with its mesoscopic magnetic characterization. We have studied the atomic, electronic and magnetic structure of MBE-grown few-layer Cr₅Se₈ on graphene substrates (BLG/SiC(0001) and HOPG) by means of 4.2K-STM/STS and XMCD measurements. STM imaging reveals that Cr₅Se₈ present both Se- and Cr-terminations, the latter showing a 2x2 periodicity in the Cr plane stable up to room temperature. Both terminations exhibit a semiconducting behavior with an accused layer-dependent gap value maximized at 1.2 eV for ML. Lastly, our XMCD measurements are compatible with a weak ferromagnetic ground state down to 2K.   

Live

Quasi-2D magnetic materials Cr₅Te₈ has attract intensive research for its new discovered fascinating properties, such as topological Hall effect and anomalous Hall effect. Besides, the magnetic anisotropy and relative high Curie temperature (~ 231K) among quasi-2D magnetic materials make it become a candidate for the development of next generation spintronics. Here we report angle-resolved photoemission spectroscopy study of the electronic structure of Cr₅Te₈. We perform systematic experiments on band component resonance behavior and temperature dependence. The results are helpful to understand the fertile magnetic phenomenon in this material and shed light on the understanding for magnetism origin in correlated materials.   

Live

Recently, several two-dimensional (2D) materials of magnetic order, including CrI₃ and Cr₂Ge₂Te₆, have been reported, which opens up opportunities for device applications integrating these magnets with other van der Waals (vdW) crystals. However, most of these materials are vdW structures which rely on mechanical exfoliation techniques to obtain ultra-thin flakes. Furthermore, most 2D magnets have poor stability and low magnetic transition temperature, which limit their practical applications. To search for 2D materials with improved magnetic properties, one needs to look beyond vdW crystals. In this work, the non-vdW 2D magnetic Cr₂Te₃ with a thickness down to one-unit-cell was synthesized by chemical vapor deposition. The Cr₂Te₃ 2D crystals display robust ferromagnetism with a relatively high Curie temperature of 180 K, a large perpendicular magnetic anisotropy of 7×10⁵ J m^-3, and a high coercivity of ~ 4.6 kG at 20 K. First principles calculations further show a transition from canted to collinear ferromagnetism, and emergent half-metallic behavior in atomically-thin Cr₂Te₃, paving the way for its potential application such as injecting carriers with high spin polarization into spintronic devices.   

Live

Band structure engineering has become a crucial way to explore ferromagnetism in two-dimensional materials in the past decade. However, if band structures could also be used to guide the magnetic anisotropy, it will accelerate and simplify the study of magnetic properties in general two-dimensional magnetic systems. Here, through first-principles calculations, we report the connection between magnetic anisotropy and band structures in Γ-specialized In₂Se₃/Fe₃GeTe₂ heterostructure, where band distortion occurs in the surrounding of Γ point with a formed valley. As the band-distortion-caused valley moves relative to the quasi valence band maximum, the magnetic anisotropy switches between in-plane and out-of-plane magnetic anisotropy. Such rule applies to either the strained or polarization-switched heterostructure. This is effectively a solution of the “inverse design” problem applied to the highly demanding realm of nano-engineered magnetic materials   

Live

Until now, the two-dimensional ferromagnets remain mostly limited to exfoliated micron-sized samples. Large-scale thin films are desirable for the fabrication of integrated devices for spintronic and memory storage applications. Among this group of materials, Fe₃GeTe₂ (FGT) can host ferromagnetic states at room temperature upon optimized gating. We achieve, by molecular beam epitaxy, the synthesis of high-quality monolayer and multilayer FGT films. Surface x-ray diffraction confirms its quintuple layer substructures with hexagonal symmetry. Thickness-dependent transport measurements are used to probe and characterize magnetic order. Ferromagnetic states exist in 1-10 layer thick Fe₃GeTe₂, with Curie temperatures ranging from ~75 K in one layer samples to above 175 K in ten layer samples. A ferromagnetic phase with significant magnetic anisotropy is revealed for all layer numbers.   

Live

Palladium is a versatile transition metal, from a fundamental and applied physics perspective. Being a nearly-ferromagnetic metal, it shows magnetic spin glass behavior and giant magnet moments when doped with small amounts of magnetic elements such as iron. Due to its considerable spin-orbit coupling, it has recently become of interest in the field of spin-orbitronics. Applications of Pd nanomaterial include, among others, hydrogen storage and purification, catalysis in the context of metal-organic frameworks, magnetoresistance spin valves and phase coherence superconducting junctions.
Understanding Pd finite-size effects is therefore relevant for many fields of research.
We performed depth-dependent low-energy muon spin spectroscopy (MuSR) studies on three undoped and iron-doped Pd 100 nm thin films. Muons implanted in the surface and substrate interface of the samples probe an increased local magnetic field. The field increase is temperature-independent, stronger for iron-doped samples, accompanied by an increase in local field inhomogeneity and it extends over a few nanometers. We discuss various potential origins for this magnetic surface state and conclude that orbital moments induced by the surface/interface and by localized spins/charges are the most likely explanation.   

On Demand

Ferromagnetism in two-dimensional (2D) van der Waals materials has attracted increasing attention due to its potential for developing a new generation of spintronic devices. In this work, we propose an alternative approach that consists in decorating 2D TMD materials with nanoparticles of manganese oxide. The nanoparticles were deposited on the surface of monolayer MoSe2 using a chemical vapor deposition technique. Micro-Raman and PL spectroscopy were used to study their optical properties; while the morphology, chemical composition and thickness of the samples were characterized via SEM, STEM and AFM, respectively. Magnetic measurements reveal a weak ferromagnetic behavior in individual MoSe2 films. Similarly, weak ferromagnetism is observed for MnOx nanoparticles deposited on a SiO2 substrate. However, when the MnOx nanoparticles are deposited on 2D MoSe2, the hybrid MnOx/MoSe2 system displays enhanced room-temperature ferromagnetism, suggesting a strong magnetic exchange coupling between the two material components. A detailed analysis on the temperature dependence of saturation magnetization and coercive field indicates stronger magnetic coupling at lower temperatures. Our results provide an alternative route to induce room-temperature ferromagnetism in 2D systems.