Magnetic Simulations of Core–Shell Ferromagnetic Bi-Magnetic Nanoparticles: The Influence of Antiferromagnetic Interfacial Exchange

Magnetic properties of ferromagnetic nanostructures were studied by atomistic simulations following Monte Carlo and Landau–Lifshitz–Gilbert approaches. First, we investigated the influence of particle size and shape on the temperature dependence of magnetization for single cobalt and gadolinium nanoparticles and also in bi-magnetic Co@Gd core–shell nanoparticles with different sizes. The Landau–Lifshitz–Gilbert approach was subsequently applied for inspecting the magnetic hysteresis behavior of 2 and 4 nm Co@Gd core–shell nanoparticles with negative, positive, and zero values of interfacial magnetic exchange. We were able to demonstrate the influence of finite-size effect on the dependence of the Curie temperature of Co and Gd nanoparticles. In the Co@Gd core–shell framework, it was possible to handle the critical temperature of the hybrid system by adjusting the Co core size. In addition, we found an improvement in the coercive field values for a negative interfacial exchange energy and for a different core size, suggesting an exchange spring behavior, while positive and zero values of interfacial exchange constant showed no strong influence on the hysteresis behavior.

Determination of the Dzyaloshinskii-Moriya interaction using pattern recognition and machine learning

Machine learning is applied to a large number of modern devices that are essential in building an energy-efficient smart society. Audio and face recognition are among the most well-known technologies that make use of such artificial intelligence. In materials research, machine learning is adapted to predict materials with certain functionalities, an approach often referred to as materials informatics. Here, we show that machine learning can be used to extract material parameters from a single image obtained in experiments. The Dzyaloshinskii–Moriya (DM) interaction and the magnetic anisotropy distribution of thin-film heterostructures, parameters that are critical in developing next-generation storage class magnetic memory technologies, are estimated from a magnetic domain image. Micromagnetic simulation is used to generate thousands of random images for training and model validation. A convolutional neural network system is employed as the learning tool. The DM exchange constant of typical Co-based thin-film heterostructures is studied using the trained system: the estimated values are in good agreement with experiments. Moreover, we show that the system can independently determine the magnetic anisotropy distribution, demonstrating the potential of pattern recognition. This approach can considerably simplify experimental processes and broaden the scope of materials research.

Altered calcium handling in cardiomyocytes from arginine:glycine amidinotransferase knockout mice is rescued by creatine

The creatine kinase system facilitates energy transfer between mitochondria and the major ATPases in the heart. Creatine-deficient mice, which lack arginine:glycine amidinotransferase (AGAT) to synthesize creatine and homoarginine, exhibit reduced cardiac contractility. We studied how the absence of a functional CK system influences calcium handling in isolated cardiomyocytes from AGAT knockouts and wild-type littermates as well as in AGAT knockout mice receiving lifelong creatine supplementation via the food. Using a combination of whole-cell patch clamp and fluorescence microscopy, we demonstrate that the L-type calcium channel (LTCC) current amplitude and voltage range of activation was significantly lower in AGAT knockout compared to wild-type littermates. Additionally, the inactivation of LTCC and the calcium transient decay were significantly slower. According to our modeling results, these changes can be reproduced by reducing three parameters in knockout mice when compared to wild-type: LTCC conductance, the exchange constant of calcium transfer between subspace and cytosol, and SERCA activity. Since tissue expression of LTCC and SERCA protein were not significantly different between genotypes, this suggests the involvement of post-translational regulatory mechanisms or structural reorganization. The AGAT knockout phenotype of calcium handling was fully reversed by dietary creatine supplementation throughout life. Our results indicate reduced calcium cycling in cardiomyocytes from AGAT knockouts and suggest that the creatine kinase system is important for the development of calcium handling in the heart.

Effects of molecular adsorption on the spin-wave spectrum and magnon relaxation in two-dimensional Cr2Ge2Te6

Molecular adsorption has remarkable impacts on the exchange constant and magnon–phonon scattering of magnetic materials.

Complementary atomic force microscopy and magnetic force microscopy study for the magnetic transition of cobalt clusters onto gold substrates

Atomic and magnetic force microscopy were employed in order to investigate the local magnetic state of individual cobalt clusters electrodeposited onto gold electrodes. The analysis indicated that the diameter of the clusters was similar regardless of the potential formation, or growth stage, but the height and surface coverage changed as the potential augmented. Individual examination of the clusters showed that the height determined the magnetic transition from the single to the multi domain state. By using a theoretical single-domain ferromagnetic model in terms of the clusters dimensions, the magnetic exchange constant at different potential voltages was obtained, which was higher than the cobalt bulk value. In addition, a micromagnetic simulation study was employed to validate the experimental results, and it correctly confirmed the experimental magnetic transition. In addition, a fcc crystalline structure and some insights of the clusters growth mechanism on the gold surface were inferred from the results.

Local Magnetic Anisotropy of the Co-Based Amorphous Alloy

The magnetization, initial magnetic susceptibility, and magnetostriction of a multicomponent Co-based amorphous alloy have been studied. The exchange constant a and the Curie temperature TC of the alloy are determined. On the basis of a method based on the theory of stochastic magnetic structure for amorphous ferromagnets and using the magnetization curves, the correlation field Hℓ, the field Ha, the effective constant of local magnetic anisotropy Keff, and the stochastic characteristics of local anisotropy – the mean square field fluctuations and the correlation radius – have been calculated. The temperature behavior of the examined magnetic characteristics is analyzed. The results of magnetostriction research allow a conclusion to be drawn that the local magnetic anisotropy of the alloy has a single-ion origin.

Tuning of the exchange interaction and the Curie temperature by mixed crystal formation of the bridging anionic ligands

A strategy to continuously tune the exchange constant J and the critical temperature Tc in the 1D coordination polymers [Co(NCS)x(NCSe)2−x(pyridine)2]n is reported. For such mixed crystals, prepared by annealing and from solution, an increase of J and Tc with increasing selenocyanate content is observed.

Эффект спинового насоса в системе ферромагнитный изолятор/нормальный металл: простая квантово-механическая модель

We suggest a simple quantum-mechanical model of the spin pumping effect that appears when the ferromagnetic resonance is excited in a magnetic insulator that has a flat contact with a nonmagnetic metal. The model is based on the solution of Schrodinger equation for conductance electrons in the metal. We show that electron reflection with spin flip at the boundary with the insulator leads to the dc and ac (oscillating with the frequency of microwave field exciting the resonance) spin flow from ferromagnet to insulator. The dc effect is small compared to the ac effect; the small parameter here is the ratio of exchange constant in the ferromagnet to the energy of potential barrier between the metal and insulator which is greater than the Fermi energy. The obtained result agrees to known experimental data. The developed model provides a simple and spectacular insight into the phenomenon roots and allows to make analysis of the effect dependence on the parameters of considered system.

Kinetics and Mechanism of Alkaline Hydrolysis of N-(o-Aminophenyl)Phthalimide in the Presence and Absence of Cationic Micelles and Sodium Salts of Aliphatic Acids

Pseudo first-order rate constants, k obs, for the alkaline hydrolysis of N-( o-aminophenyl) phthalimide, obtained at constant [CTABr]T (total concentration of cetyltrimethylammonium bromide) and varying concentrations of inert salt MX (MX = sodium formate, sodium acetate, sodium propanoate, sodium butyrate, sodium valerate and sodium hexanoate) follow the relationship: [Formula: see text], where θ and KX/S are empirical constants and S = HO-. The values of KX/S for each aliphatic acid decrease with an increase in [CTABr]T. This relationship gives an empirical constant whose magnitude is the measure of the ability of an ion, X-, to expel another counterion S-, from the cationic micellar surface to the bulk aqueous phase. The values of the empirical constant, KX/S, are used to derive the ion exchange constant, KBrX.