Session M54: Nanomagnetic Structures: From Patterned Arrays to Molecular Magnets

Anomalous Hall Effect due to Topological Magnetic Charge Correlation in Permalloy Honeycomb Lattice.

Two-dimensional artificial honeycomb lattice of connected ferromagnetic elements undergoes temperature dependent evolution of magnetic phases and provides a facile platform for exploring novel electric and magnetic properties. Hall probe measurements on a honeycomb lattice of connecting permalloy elements, with a typical length of 12 nm, suggest that as temperature decreases, the system undergoes a magnetic phase transition that renders the Hall resistance from the anomalous Hall effect (AHE) type to an unusual oscillatory type at low temperature. Magnetic measurements in a perpendicular magnetic field of the same system also reveal quantized magnetization, characterized by two field-induced jumps in the magnetization. The oscillatory behavior in Hall resistance and the quantized magnetization provide evidence to the emergence of topologically nontrivial chiral magnetic vortex loops in the novel spin solid state at low temperature. The experimental observation is supplemented by first-principle theoretical calculations. Finally, we will discuss the implication of the Hall probe results in a broader perspective of condensed matter physics research on 2D materials.   

Using X-ray beams with orbital angular momentum to probe an antiferromagnetic ground state

Artificial spin ice systems present an interesting platform to study magnetic interactions and dynamics. A square array of permalloy nanomagnets can be used to create an artificial antiferromagnetic lattice. Furthermore, topological defects such as edge dislocations can be introduced into the lattice. Resonant X-ray scattering from such an artificial spin ice with a defect of topological charge Z = 2 imparts an orbital angular momentum onto the scattered beam [1]. Here, we show how circular dichroism in the scattered diffraction pattern can be used to determine the ground state of the antiferromagnet and the magnetization configuration around the defect. By thermal cycling, two different degenerate ground states can be reached, and these can be differentiated using circular dichroism.

[1] J. S. Woods, X. M. Chen, et al. Phys. Rev. Lett. 126, 117201 (2021).   

Quantum disordered state of magnetic charges in nanoengineered honeycomb lattice

A quantum magnetic state due to magnetic charges is never observed, even though they are treated as quantum mechanical variables in theoretical calculations. Here, the occurrence of a novel quantum disordered state of magnetic charges in a nanoengineered magnetic honeycomb lattice of ultra-small connecting elements is demonstrated. A magnetic honeycomb lattice with competing exchange interactions between Ising moments is theoretically predicted to exhibit disordered magnetic states with macroscopic degeneracy. The experimental research, performed using spin-resolved neutron scattering, reveals a robust massively degenerate ground state, comprised of low integer and energetically forbidden high integer magnetic charges, that manifests cooperative paramagnetism at low temperature. We explored the competing physics of energy vs. entropy in a thermally tuned magnetic phase transition. The system tends to preserve the degenerate configuration even under large magnetic field applications. It exemplifies the robustness of the disordered correlation of magnetic charges in a 2D honeycomb lattice. The realization of a quantum disordered ground state elucidates the dominance of exchange energy, which is enabled due to the nanoscopic magnetic element size in nanoengineered honeycomb. Consequently, an archetypal platform is envisaged to study quantum mechanical phenomena due to emergent magnetic charges.   

Magnetic charge's relaxation propelled electricity in a two-dimensional magnetic honeycomb lattice

Emerging new concepts, such as magnetic charge dynamics in two-dimensional magnetic material, can provide a novel mechanism for spin-based electrical transport at macroscopic length. In artificial spin ice of single domain elements, magnetic charge's relaxation can create an efficient electrical pathway for conduction by generating fluctuations in the local magnetic field that couple with conduction electron spins. In a first demonstration, we show that the electrical conductivity is propelled by more than an order of magnitude at room temperature due to magnetic charge defects sub-nanosecond relaxation in an artificial magnetic honeycomb lattice. For the first time, the neutron spin-echo (NSE) measurement technique is utilized to extract sub-nanosecond relaxation of magnetic charge defect dynamics in artificial spin ice. More importantly, we will also show that magnetic charges remain highly dynamic to the lowest measurement temperature in the artificial permalloy honeycomb lattice of single domain elements. The direct evidence of the proposed electrical conduction mechanism in a two-dimensional frustrated magnet points to the untapped potential for spintronic applications in this system.   

Magnetoresistance study of the effects of Fibonacci Distortions on Kagome Artificial Spin Ice Systems

Nanofabrication techniques allow magnetic thin films to be lithographically patterned into arrays of interacting macro-spins designed to exhibit emergent physical properties.  We study the effects of continuous symmetry breaking on the magnetoresistive behavior of frustrated Kagome (alt. honeycomb) ASI whose periodic lattice is aperiodically distorted by repeated application of a substitution algorithm:  A Fibonacci sequence of binary digits is mapped into short (d1) and long (d2) primitive lattice translations, which alters the magnetic moments and angular coordination of the three-fold Kagome vertices.  Kagome arrays with variable distortions are patterned in series to permit simultaneous longitudinal and transverse magnetoresistances measurements in external magnetic fields.  The direction and magnitude of applied field was varied to produce distinct, interesting differences in the magnetoresistance response of the distorted Kagome arrays, compared to the undistorted arrays.   

Detection of Spin Interactions with Single Magnetic Molecules

Magnetic single molecules are receiving intensifying research focus because they are promising candidates for realizing the spatial limit of memory storage and qbits. Scanning tunneling microscopy (STM) offers particular experimental advantages given its ability to image and characterize single atoms and molecules. With an ultrahigh vacuum sub-Kelvin high magnetic field STM, inelastic electron tunneling spectroscopy (IETS) can be used to study the spin-flip dynamics of single atoms and molecules as well as to characterize Zeeman splitting with respect to the external magnetic field. Here, we successfully prepared Al₂O₃ film by partial oxidation the atomic clean NiAl(110) surface and deposited NiCp₂  (short for Ni(cyclopentadienyl)₂) molecules on both NiAl(110) surface as well as the Al₂O₃ films. The magnetic anisotropy of energy (MAE) of NiCp₂ molecules adsorbed on different sites are measured. We showed that the MAE of NiCp₂ molecules varies with adsorption geometry. The spin-vibration coupled transitions of NiCp₂ molecules and the spin exchange interaction between two NiCp₂ molecules are also observed. With NiCp₂ Adsorbed at the apex of the STM tip, exchange interactions with another molecule adsorbed on NiAl(110) surface are dectected.    

Spatial Impact Range of Single Molecule Magnet on Magnetic Tunnel Junction-Based Molecular Spintronic Devices (MTJMSDs)

Advanced and energy efficient logic and memory devices are needed to produce next generation computers for the adaptation of artificial intelligence like technologies in day-to-day life. Magnetic Tunnel junction based Molecular Spintronic Devices (MTJMSD) can effectively combine ferromagnetic electrode (FME) with magnetic molecules for producing spintronic devices for futuristic computers. This work investigates the effect of FME's length and thickness variation on MTJMSD's molecule-induced correlated magnetic phases and spatial range of molecule impact. Our experimental transport studies show that in a strong FME-molecule coupling regime 10 to 20 nm change in thickness changed MTJMSD conductivity by>1000 fold. Magnetic Force Microscopy (MFM) showed the FME extending beyond the cross-junction area developed multiple room temperature stable magnetic phases. To explore other possible effects, we also conducted Monte Carlo Simulation (MCS) using Heisenberg atomic modeling. Our computational results agree with our experimental observations. According to the MCS study, increasing FME length produced multiple magnetic phases around MTJMSD. On the other hand, increasing FME thickness from 5 to 25 atoms reduced the penetration of molecule coupling impact along the thickness.   

Dipole Switching by Intramolecular Electron Transfer in Single-Molecule Magnetic Complex [Mn12O12(O2CR)16(H2O)4]

Intramolecular electron transfer in single-molecule magnetic complex [Mn₁₂O₁₂(O₂CR)₁₆ (H₂O)₄] for R = -H, -CH_3, -CHCl₂, -CH₂Cl, -C₆H₄F ligands as a mechanism for switching of the dipole moment of the molecule is studied. We use the density functional theory with onsite Coulomb energy correction (DFT+U) to calculate localized states. We find that the extra electron can only localize on alternating Mn sites of the outer ring and all sites in the core. The lowest energy path for an electron going across the molecule is through the localized states in the core Mn atoms. In this talk we will discuss energetics, geometry, ligands and water isomer configuration, and effect of electric field due to counterion.   

Majorana Zero Modes Emulated in a Magnetic Molecule Chain

We theoretically predict the presence of Majorana zero modes (MZMs) in a Co trimer molecular magnet. Using parameters extracted from ab initio calculations, we find a low-energy subspace of this trimer which realizes an effective anisotropic spin-1/2 chain. We show the presence of MZMs in this system and find their response to electronic paramagnetic resonance. This response gives a fingerprint for the MZMs in this system and, moreover, can be used to extract the occupation of the MZMs in the fermionic representation.   

Perpendicular edge magnetization fluctuations in a double mesospin system

Elongated nanomagnets, ‘mesospins’, are used as building blocks in a range of mesoscopic spin systems in order to study, for example, frustrated physics and phase transitions in magnetic metamaterials. These mesospins are designed in such a way that shape anisotropy determines their effective spin dimesionality. Previously, we have performed a micromagnetic study of the effect of temperature on the internal texture of Ising-like mesospins, and found the excitation of thermal magnons along with the occurrence of stochastic switching of the magnetization at the edge of the nanomagnet. In this work, we explore in a micromagnetic framework the effects on the edge fluctuations when a second, oppositely magnetized Ising-like mesospin is added. We map out the energy landscape of this system and identify two different energy barriers, which are modified as the spacing between the mesospins is varied. We gather switching statistics for this system and obtain energy barriers and attempt frequencies via Arrhenius law. We find inverse scaling of the low energy barrier compared to the associated attempt frequency. Finally, we calculate the mode spectrum for this system and investigate the relevance of these magnon modes for the switching behavior.   

Efficient Monte Carlo Simulation of Ferromagnetic Spin Configuration

Due to the long-range dipole-dipole interactions (DDI) in ferromagnets (FM), the time complexity of Monte Carlo (MC) simulation is order O(N²), in a FM system with N spins. Tradition trivial MC algorithm is unbearably computationally expensive, as the size of system N increases. To solve this dilemma, we introduce an efficient algorithm named stochastic cutoff (SCO) algorithm and reduce the time complexity of O(N*log N). In this talk, I will show our simulation results of spin configuration in FM with different shapes.

Considering the anisotropy of DDI in FM, shape anisotropy will affect the spin configuration. As a result in long thin ferromagnets, spins prefer to align parallel to the long side of ferromagnets. In addition, magnetic domains of flux-closure shape can be observed in ferromagnets. These two kinds of spin configuration is related to not only the competition of the nearest-neighbouring exchange interaction J and dipole-dipole interaction D, but also the shape of the ferromagnts. We simulate the spin configuration from a first-principle perspective and study the phase diagram of spin configuration with different system sizes and interaction constants.   

Underlying mechanism for exchange bias in single molecule magnetic junctions

Magnetic proximity has been observed in variety of solid-state magnetic devices but less discussed at the molecular scale. In this study [1], the magnetotransport calculation is carried out using the generalized Landau-Lifshitz-Gilbert (LLG) equation combined with our self-developed  DFT+JunPy calculated spin torque effect [2,3]. Except the current driven spin torque, which is a promising approach for magnetization switch in magnetic random access memory, the equilibrium field-like spin torque also plays a crucial role in the strain-controlled exchange bias with current-controlled magnetic coercivity in single molecule magnetic junctions. The tight-binding model is further employed to clarify the critical role of interfacial spin filter effect arising from the hybridization between linker and Co apex. These multidisciplinary DFT+JunPy+LLG results may provide important and practical implications in dual control of magnetic proximity and magnetization switching in molecular spintronics at low temperature, either by tensile strain or via smaller applied current density in the order of MA/cm².

Refereces

[1] Y. -H. Tang and B. -H. Huang, Phys. Rev. Research, 3, 033264 (2021).

[2] Y. –H. Tang and B. –H. Huang, J. Phys. Chem. C 122, 20500 (2018).

[3] B. -H. Huang et al., AIP Advacnes 11, 015036 (2021).