Exchange-spring Magnets Research Articles

Possible New Approach for Exchange-Spring Magnet and Continuous Spin State Transition in Half-Doped High-Entropy Cobaltite Perovskite

Possible New Approach for Exchange-Spring Magnet and Continuous Spin State Transition in Half-Doped High-Entropy Cobaltite Perovskite

Understanding magnetic interactions in SrFe12O19/CoFe2O4 nanocomposites: role of particle structure on exchange spring magnet behaviour

This study explores the impact of particle structure on the magnetic properties of SrFe12O19/CoFe2O4 nanocomposites. The investigation focuses on variations in particle sizes of SrFe12O19 (SHF) and CoFe2O4 (COF) with a mass ratio of 80:20, considering micro-scale poly-crystallite particles (Pp) and nano-scale mono-crystallite particles (Mp). Four sample structures (COM1, COM2, COM3, COM4) were prepared, each with different combinations of poly-crystallite and mono-crystallite particles. Morphological analysis through scanning electron microscopy and energy-dispersive x-ray spectroscopy revealed consistent grain shape and size post-sintering. Magnetic properties analysis using Permagraph indicated that grain fineness and uniformity influence the resulting magnetic properties, with composite samples featuring nano-sized hard or soft phases showing improved M r , M s , and (BH) max values compared to individual phases. The nanocomposite sample COM4, characterized by fine and uniform particle size, exhibited the highest remanence magnetization, M r (34.55 emu g−1), saturation magnetization, M s (66.44 emu g−1), and the maximum energy product, (BH) max (1.17 MGOe).

A Comparison of the magnetic characteristics and exchange spring effect of Y-substituted Co2Mn1-xYxO4 composite spinel oxides

Magnetic spinel oxides are materials that exhibit unique magnetic properties due to their crystal structure and composition. Herein, we report the sol-gel synthesis of Y-substituted Co2Mn1-xYxO4 (0 ≤ x ≤ 0.1) inverse spinel oxides. X-ray diffraction pattern confirms the single-phase cubic structure with space group Fd3m. Magnetic susceptibility measurement results exhibited the para-ferrimagnetic phase transition near TC = 174 K. Also, another phase transition near a specific temperature T* = 76 K, below TC of Y-doped samples. Furthermore, the magnetic entropy change analysis also confirmed the magnetic phase transition near the temperature of T*. Moreover, with the phase transition near T*, Y-doped samples show magnetic anomalies at low temperatures. Similarly, the analysis of magnetic entropy change and the magnetic hysteresis loops show the coexistence of “soft " and “hard” ferromagnetic phases at low temperatures in Y-substituent Co2Mn1-xYxO4 samples. Using the coupling of the “soft/hard” ferromagnetic phase boundary, a noticeable exchange spring effect can be observed at low temperatures. Due to the exchange spring effect, the magnetic energy product (BH)max up to 3.1 MJ m−3 can be obtained at 2 K. Such insights can help to optimize the behavior of magnetic materials and structures, leading to improved performance and functionality in applications such as magnetic data storage, magnetic sensors, spintronics, and magnetic nanoparticles.

Tuning in-plane magnetic anisotropy and temperature stability in amorphous trilayers

Better understanding of the nature of magnetic coupling in soft/hard nanocomposites paves the way for tailored exchange-spring magnets. We have investigated a series of amorphous magnetic thin films and trilayers of magnetically soft Co85(Al70Zr30)15 and magnetically hard Sm12Co81Ti7, produced with DC magnetron sputtering. The overall magnetic properties of the trilayers were investigated with focus on the effects of layer configuration and thicknesses on coercivity, originating from the Sm12Co81Ti7 phase, and uniaxial in-plane anisotropy induced from the Co85(Al70Zr30)15 phase. In addition, we found that the thermal stability of a 20 nm Sm12Co81Ti7 layer was significantly increased if surrounded by two 10 nm Co85(Al70Zr30)15 layers in a trilayer structure.

Probing strongly exchange coupled magnetic behaviors in soft/hard Ni/CoFe2O4 core/shell nanoparticles.

Exchange coupling in a model core-shell system is demonstrated as a step on the path to 3d exchange spring magnets. Employing a model system of Ni@CoFe2O4, high quality core-shell nanoparticles were fabricated using a simple two-step method. The microstructural quality was validated using TEM, confirming a well-defined interface between the core and the shell. A strongly temperature dependent two-phase magnetic hysteresis loop was measured, wherein an analysis of step heights indicates coupling of roughly 50% between the core and the shell. Element-specific XMCD hysteresis confirms the presence of exchange coupling, suppressing the superparamagnetism of the Ni core at room temperature, and reaching a coercivity of >6 kOe at 80 K. These results provide a pathway to the development of heterostructured metal-oxide exchange coupled nanoparticles with improved maximum energy product.

Aspects of long range spin–triplet correlations in superconductor/ferromagnet hetero-structures

The notion of competing ferromagnetic (F) and superconducting (S) orders in F/S hybrid structures was transformed by the first realization of ferromagnetic Josephson π-junctions and the almost simultaneous prediction of a possibility of spin–triplet correlations in such structures, almost two decades back. Such hybrid structures in various configurations are now studied as rich sources of emergent states and new effects. Unlike the spin singlet Cooper pairs, the spin triplet Cooper pairs are much less affected by the exchange field of a ferromagnet and, therefore, immediately finds interest in the field of spintronics. Theoretically, it has been shown that the basic protocol for spin–singlet to spin–triplet supercurrent conversion is the presence of magnetic non-collinearity at the superconductor–ferromagnet interface. Therefore, almost all experiments in this direction have utilized transport measurements on F/S systems with artificial magnetic non-collinearity formed by combination of several ferromagnetic layers next to the superconducting layer. Here we highlight two aspects of studying these heterostructures. Firstly we show that natural magnetic inhomogeneities, found in domain walls of ferromagnets, can also be used to achieve singlet–triplet conversion, instead of artificial magnetic inhomoheneities. This possibility was explored via transport measurements in nano-scale planar Nb–Ni–Nb junctions and nano-SQUIDs, where a domain wall was pinned at the Josephson junction barrier. By this method we were able to show Josephson coupling across about 70 nm of strong ferromagnetic planar barrier. Secondly we show that spin–triplet correlations at the F/S interface are robust enough to be probed by the diamagnetic screening currents at the interface. This was probed by studying the change in sperconducting transition temperature of Nb/Co/Py/Nb multilayers in presence of small in-plane magnetic field. The Co/Py combination, which is a soft-hard type magnetic exchange spring, worked as magnetic inhomogeneity for triplet generation at the interface of the superconducting Nb. These observations may promote new experiments in the field of superconducting-spintronics.

Combined characterization approaches to investigate magnetostructural effects in exchange-spring ferrite nanocomposite magnets

Combining multiple characterization probes (neutrons, X-rays, electrons) illustrated the hierarchical structure of exchange-spring nanocomposites & revealed unreported degradation of parent phase.

Magnetoelastic coupling enabled tunability of magnon spin current generation in two-dimensional antiferromagnets

We theoretically investigate the magnetoelastic coupling (MEC) and its effect on magnon transport in two-dimensional antiferromagnets with a honeycomb lattice. MEC coeffcient along with magnetic exchange parameters and spring constants are computed for monolayers of transition metal trichalcogenides with N\'eel order ($\text{MnPS}_3$ and $\text{VPS}_3$) and zigzag order ($\text{CrSiTe}_3$, $\text{NiPS}_3$ and $\text{NiPSe}_3$) by $ab$ $initio$ calculations. Using these parameters, we predict that the spin-Nernst coefficient is significantly enhanced due to magnetoelastic coupling. Our study shows that although Dzyaloshinskii-Moriya interaction can produce spin Nernst effect in these materials, other mechanisms such as magnon-phonon coupling should be taken into account. We also demonstrate that the magnetic anisotropy is an important factor for control of magnon-phonon hybridization and enhancement of the Berry curvature and thus the spin-Nernst coefficient. Our results pave the way towards gate tunable spin current generation in 2D magnets by SNE via electric field modulation of MEC and anisotropy.

Tuneable exchange-spring stiffness in amorphous magnetic trilayer structures

We investigate the magnetic properties of amorphous Sm10Co90/Co60(Al70Zr30)40/Co85(Al70Zr30)15 exchange-spring magnet trilayers. The magnetically soft Co85(Al70Zr30)15 layer is coupled to the magnetically hard Sm10Co90 layer through the weakly magnetic low-T c Co60(Al70Zr30)40 spacer layer. The strength of the coupling can be controlled with temperature and the coupling persists above the intrinsic T c of the spacer layer due to a long-range magnetic proximity effect. Polarized neutron reflectivity is used to examine the magnetic profile of the trilayers during magnetization reversal. A two-step switching occurs, with the switching angle of the soft layer strongly dependent on the strength of the coupling. In the strong coupling regime a magnetic state can be achieved where the soft layer magnetization is perpendicular to the hard layer whereas in the weak coupling regime the soft layer reverses fully.

Self-organized exchange-spring magnet in epitaxial β-Fe(Ni)Si2/Si system

Self-organized formation of exchange-spring magnets in Permalloy-derived epitaxial silicide nanostructures, fabricated by deposition of Ni80Fe20 onto a vicinal Si(111) substrate, is reported. The crystal structure of bar-shaped silicide nanostructures decorating Si(111) surface steps, following thermally activated reaction between Ni, Fe, and Si surface atoms, was identified as ternary β-Fe(Ni)Si2 phase, in well-defined (1¯11¯) Si || (2¯2¯0) β and [011] Si || [001] β orientation relations with the substrate. At the same time, chemical composition within the islands was Fe-rich and Fe-deficient in comparison with the original stoichiometric Permalloy (Ni80Fe20) and not uniform, with higher concentration of Ni at the island bottoms, close to the interface with Si(111), creating de facto “compositional interfaces” within the islands, though no physical interfaces could be detected in a high-resolution structural-crystallographic analysis by transmission electron microscopy. Analysis of temperature-dependent magnetization reversal loops revealed, that the thicker and magnetically soft top part of the islands and yet magnetically softer and thinner bottom part, were magnetically exchange-coupled via the above “virtual” interfaces. Micromagnetic simulations, consistent with exchange-spring magnet, had further corroborated this conclusion.

Co/Ni]-NiFe 교환스프링 자석의 자성 특성에 관한 연구

Co/Ni]-NiFe 교환스프링 자석의 자성 특성에 관한 연구

Reconstructing phase-resolved hysteresis loops from first-order reversal curves

The first order reversal curve (FORC) method is a magnetometry based technique used to capture nanoscale magnetic phase separation and interactions with macroscopic measurements using minor hysteresis loop analysis. This makes the FORC technique a powerful tool in the analysis of complex systems which cannot be effectively probed using localized techniques. However, recovering quantitative details about the identified phases which can be compared to traditionally measured metrics remains an enigmatic challenge. We demonstrate a technique to reconstruct phase-resolved magnetic hysteresis loops by selectively integrating the measured FORC distribution. From these minor loops, the traditional metrics—including the coercivity and saturation field, and the remanent and saturation magnetization—can be determined. In order to perform this analysis, special consideration must be paid to the accurate quantitative management of the so-called reversible features. This technique is demonstrated on three representative materials systems, high anisotropy FeCuPt thin-films, Fe nanodots, and SmCo/Fe exchange spring magnet films, and shows excellent agreement with the direct measured major loop, as well as the phase separated loops.

Canted standing spin-wave modes of permalloy thin films observed by ferromagnetic resonance

Non-collinear spin structures in materials that combine perpendicular and in-plane magnetic anisotropies are of great technological interest for microwave and spin wave-assisted magnetization switching. [Co/Pt] multilayers are well-known perpendicular anisotropy materials that have the potential to pin the magnetization of a soft magnetic layer, such as permalloy (Py), that has in-plane anisotropy, thereby forming a magnetic exchange spring. Here we report on multilayered [Co/Pt]/Pt/Py films, where an additional ultrathin Pt spacer has been included to control the coupling between the sub-units with in-plane and perpendicular magnetic anisotropy. Vector network analyser (VNA)-ferromagnetic resonance (FMR) measurements were made to obtain a complete picture of the resonant conditions, while the dynamical response of the sub-units was probed by synchrotron-based element- and phase selective x-ray detected FMR (XFMR). For all samples, only slight pinning of the dynamic magnetization of the Py by the [Co/Pt] was noted, and the FMR results were dominated by the 50 nm thick Py layer. Out-of-plane VNA-FMR maps reveal the presence of additional modes, e.g. a perpendicular standing spin-wave (PSSW) state. However, as the magnetic field is reduced below the saturation field, the PSSW state morphs continuously through a series of canted standing spin-wave (CSSW) states into a horizontal standing spin-wave (HSSW) state. The PSSW, CSSW and HSSW states are well described using a multilayer model of the Py film. The observation of CSSW modes is of particular relevance to microwave assisted magnetic recording, where microwave excitation stimulates precession of a soft layer canted out of plane by a pulsed magnetic field.

Amorphous exchange-spring magnets with crossed perpendicular and in-plane anisotropies

We study the magnetic coupling in a thin film bilayer exchange-spring magnet composed of an amorphous ${\mathrm{Tb}}_{10}{\mathrm{Co}}_{90}$ layer with a large perpendicular anisotropy and an amorphous ${\mathrm{Co}}_{85}{({\mathrm{Al}}_{70}{\mathrm{Zr}}_{30})}_{15}$ layer with uniaxial in-plane anisotropy. The CoAlZr is directly exchange coupled to the TbCo with an interface region of at least 7.5 nm within which the CoAlZr magnetization is perpendicular to the plane and switches with the underlying TbCo. The influence of the coupling extends up to 30 nm into the CoAlZr resulting in an effective tilted anisotropy. The coercivity of the CoAlZr is greatly enhanced due to the coupling and is tuneable over a large range by varying its thickness.

The role of FeCo and FeCoCr nanowires array on the formation of single phase bi-magnets

Nanowire (NWs) arrays of FeCo and FeCoCr were prepared by AC pulsed electrodeposition embedded into an alumina template. They were then employed as an underlayer to provide the dot array of cobalt ferrite thin films. Highly ordered arrays of NWs with the diameter of 30 nm and length of 10 µm were fabricated and then annealed in various environments. The cobalt ferrite thin films were deposited by the sputtering approach with the almost constant thickness of 70 nm as the hard magnetic phase. The combination of FeCo and cobalt ferrite could provide an exchange spring magnet with the magnetic coupling feature. X-ray diffraction (XRD) patterns of the NW arrays indicated the formation of the body centered cubic (bcc) FeCo phase with [110] texture. Field emission electron microscopy (FESM) equipped by EDS also characterized the morphological features and elemental composition. Magnetization and coercivity of NWs were examined using the vibrating sample magnetometer. The maximum coercivity of NWs array is 2950 Oe which is related to H2 annealing FeCo NWs array. The magnetization reversal process of the as-deposited and annealed FeCo and FeCoCr NWs arrays was performed by angular dependence coercivity and squareness. Further, FORC diagrams indicated an interacting single domain behavior for as-deposited FeCo NWs and a weakly-interacting single domain state for the annealed samples. The morphological and structural features of cobalt ferrite were also studied by XRD, FE-SEM and AFM techniques. It was found that the cobalt ferrite dot array with a uniform chemical composition and structure was formed. The inter-diffusion of atoms between underlayer and thin films was confirmed by EDS. The maximum coercivity of the nanocompsite was 5800 Oe that is obtained for FeCoCr underlayer with argon annealing. The VSM analysis showed that sufficient exchange coupling was formed by using the FeCoCr underlayer in hydrogen annealing, while argon annealing provided a kink in the hysteresis loops. The obtained nanocomposites could be, therefore, considered as the potential candidate for use in the exchange spring magnet.

Direct observation of magnetic proximity effects in amorphous exchange-spring magnets by neutron reflectometry

In this letter we report a direct observation of a magnetic proximity effect in an amorphous thin film exchange-spring magnet by the use of neutron reflectometry. The exchange-spring magnet is a trilayer consisting of two ferromagnetic layers with high $T_c$'s separated by a ferromagnetic layer, which is engineered to have a significantly lower $T_c$ than the embedding layers. This enables us to measure magnetization depth profiles at which the low $T_c$ material is in a ferromagnetic or paramagnetic state, while the embedding layers are ferromagnetic. A clear proximity effect is observed 7 K above the $T_c$ of the embedded layer, with a range extending 50 $\unicode{xC5}$.

Magnetic properties improvement through exchange-coupling in hard/soft SrFe12O19/Co nanocomposite

In this paper, an exchange spring magnet with a new composition and improved magnetic performance was prepared and investigated. Accordingly, SrFe12O19/Co nanocomposite with various hard/soft magnetic phase ratios were prepared and inspected. Due to importance of particle size control in establishing full exchange coupling behavior and avoiding oxidation of cobalt nanoparticles, non-aqueous polyol method was exerted to prepare the nanocomposite. In order to investigate structural and morphological properties, X-ray diffraction and field emission scanning electron microscopy were employed and it was confirmed that polyol method performed well in size control and avoiding formation of impurities. Vibrating sample magnetometer was utilized for analyzing magnetic properties and single-phase magnetization behavior implied that nanocomposite components were appropriately exchange coupled. Investigations proved that nanocomposite with 15% mass ratio of soft phase demonstrated highest exchange coupling interactions.

Effect of Silver Nanoparticles on Properties of Cobalt Ferrites

Ferrite nanomaterials in the form of CoAgxFe2−xO4 (x = 0.0, 0.03, 0.07, 0.1, and 0.2) have been prepared by the citrate autocombustion method. Structural analysis was carried out by x-ray diffraction. Fourier-transform infrared spectroscopy confirmed the formation of a spinel crystal structure. High-resolution transmission electron microscopy revealed the formation of particles with mixed morphologies. The average particle size ranged from 22 nm to 31 nm. Vibrating-sample magnetometry revealed that the saturation magnetization Ms and coercivity Hc were affected by the grain size variation. The electrical properties of the samples were investigated over the frequency range from 102 Hz to 105 Hz. Electric modulus analysis showed that the conduction was due to short-range mobility of charge carriers. The variation of the dielectric loss with frequency at different temperatures indicated two types of conduction mechanism. All these results suggest that the synthesized materials can be recommended for fabrication of exchange spring magnets and electrode materials.

Phase- and Composition-Tunable Hard/Soft Magnetic Nanofibers for High-Performance Permanent Magnet

An exchange-spring magnet is a next-generation permanent magnetic model that possesses a synergistic effect of single-phased hard and soft magnets, thereby giving rise to enhanced magnetic performa...

Effect of the soft layer thickness on magnetization reversal process of exchange-spring nanomagnet patterns

A systematic study of the magnetization reversal behavior in the regular arrangement of L10-FePt based exchange-spring nanomagnets with different thicknesses of the Co soft magnetic layer is presented. The magnetic property of the hard magnet is compared to two tuned exchange-spring magnets: its systems of 20 nm L10-FePt/3 nm, and 7 nm Co. In particular, we focus on the switching field distribution. The exchange coupling showed narrower SFD, in spite of the decoupled part switches earlier. The magnetization switching mechanism of exchange-spring nanomagnets patterns has been revealed with a first-order reversal curves technique and the switching field distribution. Further, the microscopic results using magnetic force microscopy show that the spin rotation of the non-interacting part in the thicker soft layered exchange-spring magnet. The part influences the magnetization reversal process. According to the experimental results, exchange coupling strength can be tuned by the thickness of the soft magnetic layer.