Session B23: Fabrication and Characterization of Magnetic Nanostructures

Unconventional magnetoresistance and its implications in spin-torque characterization

Magnetoresistance (MR) is essential in characterizing spin torques, especially in spin-torque ferromagnetic resonance (ST-FMR) and harmonic Hall analysis. Some experiments utilize a ferromagnetic (FM) metal that hosts anisotropic MR, where current in an adjacent material generates spin torques exerting on the magnetization and changes the MR. The variation of MR can in turn be used to quantify the spin torques, even when the spin-torque generation is attributed to unconventional mechanisms such as the out-of-plane anti-damping torque recently discovered in WTe₂. A different type of experimental setup involves an insulating FM driven and detected by a heavy metal. In such a system, there is no shunting current in the FM insulator, so the spin Hall effect in the heavy metal generates the spin torque and at the same time detects the FM dynamics through the change of the spin Hall MR. However, when it comes to an FM insulator driven by a non-spin-Hall system, such as WTe₂/Cr₂Ge₂Te₆ bilayer, there has been no clear understanding of the MR, let alone using the MR to characterize the spin torques. In this work, we first investigate the unconventional form of MR in a non-spin-Hall geometry arising from the spin diffusion effect in the presence of spin backflow. Next, by solving the Landau-Lifshitz-Gilbert equation in combination with the harmonic expansion of the unconventional MR, we derive a series of expressions for measurable ST-FMR and harmonic Hall signals, which are compared with conventional theories based on the spin Hall MR. Verified by detailed numerical results, our finding lays the theoretical foundation for characterizing spin torques originating from unconventional mechanisms beyond the spin Hall effect, which can not only guide ongoing experimental endeavors but also stimulate future experimental designs.   

Polaron-induced intrinsic ferromagnetic ordering in Cu-doped ZnO thin films

We prepared high-quality Cu-doped ZnO films by the spray pyrolysis technique and explored the correlations between their magnetic ordering and the existing native defect states in the ZnO lattice. The magnetization curves showed a paramagnetic phase, which coexists with the ferromagnetic phase at room temperature in all doped films. The results indicated that the detected ferromagnetic ordering is intrinsic and induced by the Cu dopants and correlated with the existing defect states and vacancies in the lattice. By increasing the Cu dopant in the films, we detected a clear improvement in the ferromagnetic ordering. This is correlated with an obvious enhancement in the oxygen vacancy (V_O) and zinc interstitial (Zn_i) concentrations and a slight reduction in the concentration of zinc vacancies and oxygen interstitial levels. The magnetic ordering mechanism in these films is associated with the long-range ferromagnetic coupling between Cu ions mediated by the intrinsic defects of V_O and Zn_i, through the reaction defect complexes networks of Cu⁺²-V_O-Cu⁺ and Cu⁺²-Zn_i-Cu⁺. The bound-magnetic polarons (BMPs) are formed through these indirect exchange interactions. The magnetization curves are closely fitted with the BMPs model. The concentration of these BMPs is found to be above the reported percolation threshold of BMPs to construct long-range ferromagnetic coupling in Cu-doped ZnO.   

Ac-Susceptibility Studies of the Energy Barrier to Magnetization Reversal in Frozen Magnetic Nanofluids of Different Concentrations

We used ac-susceptibility to measure the blocking temperature, T_B, and energy barrier to magnetization reversal, E_B, of nanomagnetic fluids of different concentrations, c. We collected data on five samples synthesized by dispersing Fe₃O₄ nanoparticles of average diameter áDñ=8 nm in different volumes of carrier fluid (hexane). We found that T_B increases with the increase of c, a behavior predicted by the Dormann-Bessais-Fiorani (DBF) theory. In addition, our observed T_B vs. c dependence is excellently described by a power law T_B=A∙c ^g, with A=64 K and g=0.056. Our data also show that a Néel-Brown activation law  describes the superspin dynamics in the most diluted sample, whereas an additional energy barrier term, E_ad, is needed at higher concentrations, according the DBF model: We found E_B/k_B=366K and additional energy barriers E_ad/k_B that increase linearly with the common logarithm of the volume concentration, from 138 K at c= 8.3×10⁻⁴ % to 745 K at c = 4×10⁻² %. These results add to our understanding of the contributions by different factors to the superspin dynamics. In addition, the quantitative relations that we established between the T_B, E_ad, and c support the current efforts towards the rational design of functional nanomaterials.   

Magnetic reversal time of a cobalt nanoparticle with defects

Transition metal magnetic nanoparticles can exhibit a wide range of magnetic reversal times which cannot be explained by crystalline, shape and surface anisotropies. In this talk we consider how the presence of planar defects can affect the thermal switching properties of such Co particles. Specifically, we develop a formulation for the magnetic switching rate of a model defected Co nanoparticle based on the harmonic transition state theory which uses knowledge of the underlying magnetic energy landscape. We find that both the Arrhenius energy barrier and the prefactor depend on nanoparticle size and defect density. This result is able to rationalize the experimental findings.   

Superlattices of Co2MnAl and Fe2MnAl Heusler Alloys

MBE growth and properties of short period superlattices of Co2MnAl and Fe2MnAl have been studied. Superlattices with periods between one and two unit-cell of the Heusler compound were grown on Ge (111) and (001) substrates by sequentially depositing atomic layers of the constituent elements, instead of co-deposition of each ternary alloy. Multiple superlattices with different elemental stacking sequences and the same average composition were grown on the same substrate under the same conditions by the use of precision shadow masks, in order to explore the potential for artificially sequenced complex materials and functionalities. Growing surface was probed in situ by realtime scanning reflection high energy electron diffraction. Sample structure, composition, and magnetic properties were characterized by x-ray diffraction, energy dispersive x-ray spectroscopy, and magneto-optical Kerr effect, respectively.   

Temperature-dependence of interlayer antiferromagnetic interactions in even and odd-layered synthetic antiferromagnets

Synthetic antiferromagnets (SAFM) are multilayered materials made up of successive ferromagnetic layers separated by non-magnetic metallic spacers of a suitable thickness which promotes an antiferromagnetic interaction between the ferromagnetic layers. In this work, a systematic and comparative study of the dependence of the interlayer exchange field constant on temperature in even and odd-layered SAFMs is made.  Specifically, we select permalloy as a ferromagnetic material and ruthenium as a non-magnetic spacer.  All samples were characterized using standard magnetometry methods.  By comparing magnetometry data to a macrospin model, the interlayer exchange field constant field can be extracted.  In general, we find that the interlayer exchange interaction increases as temperature is decreased. More importantly, the rate of change of the exchange field with respect to temperature is found to depend on the thickness of the ferromagnetic layers.  This study motivates a systematic study of the temperature dependence of exchange field constant in even and odd-layered synthetic antiferromagnets of the higher number of layers, and, ultimately, this information is helpful in providing freedom while choosing suitable magnetic heterostructure in spintronic applications.     

Nanocolumnar Heterostructure Metamaterial Platforms for Advanced Magnetic and Optical Applications

In this study, we employed a custom-built ultra-high vacuum electron-beam glancing angle deposition technique [1] to fabricate spatially-coherent nanocolumnar heterostructure metamaterials from both hard (cobalt) and soft (permalloy) magnetic materials. Furthermore, we have utilized the atomic layer deposition technique to successfully incorporate a conformal ultrathin interface layer of Al₂O₃ between the magnetic columnar subsegments, thereby enhancing the tunability of magnetic and optical properties. Hence, the anisotropic Bruggeman effective medium model based on the spectroscopic ellipsometry data allowed to extract the dielectric properties of the fabricated structures [2]. Our analysis of the materials also involved the use of generalized vector magneto-optic ellipsometry and vibrating sample magnetometer measurements to determine the magnetic properties. In addition to the experimental work, we conducted a series of systematic micromagnetic and finite element modeling simulations to delve into the fundamental driving mechanisms behind the tunable magneto-optic responses exhibited by the proposed metamaterial platforms. We anticipate that these innovative structural designs may underpin the development of next-generation sensing devices, magnetic recording technologies, and on-chip nanophotonic applications.

[1] Kilic, U., et al. Sci Rep 9, 71 (2019).

[2] Schmidt, D., and Schubert, M., J. Appl. Phys. 114.8 (2013).   

Growth and magnetic properties of covalent 2D magnet FexSey

Recent advances in 2D magnetism have reignited interest in this field, driven by the realization of atomically thin van der Waals (vdW) magnets through exfoliation and the characterization of 2D magnets down to the monolayer limit. These materials have become fertile ground for exploring fundamental physics, including the Mermin-Wagner theorem, the Heisenberg and Ising models, as well as dimensional crossover and scaling behaviors. Moreover, their 2D nature endows them with a high sensitivity to external stimuli, such as electric gating, light, and strain, which allows for the manipulation of their magnetic properties. There are certain limitations in exfoliated 2D magnets in terms of their instability, relatively low Curite temperature, and limited choice of materials. Thus, an effective approach to realize controlled synthesis of two-dimensional magnetic materials beyond vdW systems is required. Herein, we report the synthesis of covalent 2D magnet Fe x Se y using chemical vapor deposition. With employment of space confined strategy, two different phases (tetragonal and hexagonal) can be realized with proper adjustment of growth temperature and amount of precursor. Magnetic characterization of our Fe x Se y compounds showed ordering temperature above 250K. Our method provides a controllable way to grow ultrathin Fe x Se y crystals and platform for further exploration of new phenomena and applications.   

Electrodeposited FeCoNiCu High Entropy Alloy Thin Films and Nanowires

High-entropy alloys (HEAs) have emerged as an exciting platform for explorations of new materials phases and novel functionalities. These alloys are typically formed using bulk synthesis or physical vapor deposition techniques. We have investigated the synthesis of FeCoNiCu high-entropy alloys by electrochemical deposition under ambient conditions. One major challenge is that it is nontrivial to find a common set of electrodeposition conditions to allow simultaneous deposition of the alloys with designed compositions. We have succeeded in realizing near equiatomic ratio FeCoNiCu thin films. Electrodeposition conditions, including potential, electrolyte composition, electrolyte pH, etc., have been optimized to yield continuous films with a metallic finish. X-ray diffraction has yielded a single-phase BCC structure. Magnetic properties of such films are comparable to those made by magnetron sputtering. Additionally, the synthesis approach has allowed the fabrication of HEA nanowires, which have been used as building blocks to construct nanoporous metallic foams. Such electrodeposited HEA nanostructures not only offer a new arena to explore the vast parameter space of HEAs, but also present opportunities for studies of their functionalities.   

Magnetic anisotropy in Fe-based ferromagnetic materials

Fe-based magnetic materials are widely applied in technologies and industries, with most of the applications for soft magnetic materials, because of the low magnetocrystalline anisotropy (MCA) of bcc Fe. However, it is possible to realize magnetic hardening in Fe-based materials as we have learned from the early carbon steel permanent magnets, although the coercivity of the magnets was modest. Recent efforts to search for rare-earth-free hard magnetic materials have shown more promising evidences for achieving high MCA in Fe-based materials. In this presentation, we discuss recent developments of Fe-based hard and semi-hard magnetic materials with a focus on mechanisms of high MCA in alloys and intermetallic systems with Fe as the major content. We have identified the structures and the magnetic properties of the Fe-based binary or ternary systems containing p-block and d-block elements with considerable MCA. Furthermore, it is also important to know and to understand that the MCA in Fe-based magnetic materials can be tailored/enhanced through chemical and/or structural modifications that will lead to “artificially engineered” hard and semi-hard magnetic materials for advanced permanent magnets in the future. Experimental results on the Fe-rich carbide nanoparticles and nanorods with different aspect ratio will be discussed as an example of the materials investigated.    

Novel method for synthesis of magnetic Graphene Oxide and GO-Ferrocene/Nickleocene hybrids at room temperature using mechanical defects and insertion.

In this study, we report on the synthesis and magnetic properties of hybrid materials consisting of graphene oxide (GO) and ferrocene/nickleocene (Fc/Nc). The samples were prepared by a facile ball-milling process and characterized using various analytical techniques, including scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), electron paramagnetic ressonance (EPR), and vibrating sample magnetometry (VSM). Our results show that the GO-Fc/Nc hybrids exhibit ferromagnetic behavior at room temperature, with magnetization values reaching as high as 3.0 emu/g. This is a significant improvement over previous attempts at ferromagnetic graphene, which have reported values of 0.2 emu/g or less at around 2K, and 5.5 emu/g at 62K [1]. The observed magnetism in our samples can be attributed to the formation of a spin-coupled network between the ferrocene or nickleocene molecules and graphene oxide sheets. The increase in magnetization with increasing milling time can be correlated with the enhanced homogenization and decreased particle size of the ferrocene/nickleocene molecules in the graphene oxide matrix. Furthermore, our findings suggest that the GO-Fc/Nc hybrids could potentially serve as a new class of molecular magnets with improved magnetic properties compared to metallocene sandwich compounds, which are currently the most popular candidates for molecular magnets [2][3]. Overall, our study provides new insights into the magnetic properties of graphene oxide hybrids and offers a promising route for the development of high-performance magnetic materials.