The exchange interaction in ferromagnetic ultra-thin films is a critical parameter in magnetization-based storage and logic devices, yet the accurate measurement of it remains a challenge. While a variety of approaches are currently used to determine the exchange parameter, each has its limitations, and good agreement among them has not been achieved. To date, neutron scattering, magnetometry, Brillouin light scattering, spin-torque ferromagnetic resonance spectroscopy, and Kerr microscopy have all been used to determine the exchange parameter. Here we present a novel method that exploits the wavevector selectivity of Brillouin light scattering to measure the spin wave dispersion in both the backward volume and Damon-Eshbach orientations. The exchange, saturation magnetization, and magnetic thickness are then determined by a simultaneous fit of both dispersion branches with general spin wave theory without any prior knowledge of the thickness of a magnetic "dead layer". In this work, we demonstrate the strength of this technique for ultrathin metallic films, typical of those commonly used in industrial applications for magnetic random-access memory.
Herein, some magnetic nanoparticles (MNP)/clay/polymer nanocomposites have been prepared, whose saturation magnetization is higher than that of pure oleic acid coated MNP component. The existence of unique 'nano-network' structure and tight three-phase nano-interface in the nanocomposites contribute to the surprising saturation magnetization.
The mystery of giant saturation magnetization of Fe16N2 has remained for 37 years. In this letter, X-ray absorption spectroscopy (XAS) and magnetic circular dichroism (XMCD) are used to study the electron states of Fe atoms in Fe16N2 and Fe8N films. Localized Fe 3d electron states, which are not expected from current theories (models), are observed. The XMCD spectra and elemental hysteresis loops reveal the polarization of N atoms and the hybridization of N and Fe atoms. This discovery points to the origin of giant saturation magnetization in Fe16N2.
Whether αdouble prime-Fe16N2 possesses a giant saturation magnetization (Ms) has been a daunting problem among magnetic researchers for almost 40 years, mainly due to the unshakable faith of famous Slater-Pauling (SP) curve and poor consistency on evaluating its Ms. Here we demonstrate that, using epitaxy and mis-fit strain imposed by an underlying substrate, the in-plane lattice constant of Fe16N2 thin films can be fine tuned to create favorable conditions for exceptionally large saturation magnetization. Combined study using polarized neutron reflectometry and X-ray diffraction shows that wi…
A new model is proposed for the strong ferromagnetism associated with partially localized orbitals in the Fe16N2 metallic system which draws substantially from models of heavy fermion metals. We demonstrated that an unusual correlation effect is brought up within the Fe-N octahedral cluster region and the effective on-site 3d-3d Coulomb interaction increases due to a substantial 3d electrons charge density difference between the clusters and its surroundings, which leads to a partially localized high spin electron configuration with a long range ferromagnetic order. First principle calculation…
The effect of the saturation field on the magnetization reversal of FePt(L10)/FeCo/CoO/FeCo multilayer (ML) has been investigated to understand the origin of asymmetric magnetization reversal and its correlation with exchange bias (EB). In the ML structure, the bottom FeCo layer is coupled to the hard FePt(L10) layer, and the top FeCo layer is comparatively free due to the relatively more distance from it. The ML has been deposited under UHV conditions and characterized at each stage of growth using magneto-optical Kerr effect and x-ray reflectivity techniques. Magnetization reversal is furthe…
Magnetic materials with giant saturation magnetization have been a holy grail for magnetic researchers and condensed matter physicists for decades because of its great scientific and technological impacts. As described by the famous Slater-Pauling curve the material with highest Ms is the Fe65Co35 alloy. This was challenged in 1972 by a report on the compound Fe16N2 with Ms much higher than that of Fe65Co35. Following this claim, there have been enormous efforts to reproduce this result and to understand the magnetism of this compound. However, the reported Ms by different groups cover a broad…
We report a synthesis route to grow iron nitride thin films with giant saturation magnetization (Ms) through an N inter-diffusion process. By post annealing Fe/Fe-N structured films grown on GaAs(001) substrates, nitrogen diffuses from the over-doped amorphous-like Fe-N layer into strained crystalline Fe layer and facilitates the development of metastable Fe16N2 phase. As explored by polarized neutron reflectometry, the depth-dependent Ms profile can be well described by a model with the presence of a giant Ms up to 2360 emu/cm3 at near-substrate interface, corresponding to the strained region…
The half-metallic half-Heusler alloy NiMnSb is a promising candidate for applications in spintronic devices due to its low magnetic damping and its rich anisotropies. Here we use ferromagnetic resonance (FMR) measurements and calculations from first principles to investigate how the composition of the epitaxially grown NiMnSb influences the magnetodynamic properties of saturation magnetization $M_S$, Gilbert damping $α$, and exchange stiffness $A$. $M_S$ and $A$ are shown to have a maximum for stoichiometric composition, while the Gilbert damping is minimum. We find excellent quantitative agre…
Applicability of dilute magnetic semiconductors (DMS) in electronic devices relies upon the understanding and control of their magnetic anisotropy. This paper explores one of the ways in engineering magnetic anisotropy in epitaxial layers of DMS by forming them into magnonic structures. For this purpose the canonical ferromagnetic DMS, namely (Ga,Mn)As, is employed. The anti-dot systems based on (Ga,Mn)As layers of various thicknesses are fabricated with focused ion beam apparatus and studied by means of microscopy as well as magnetometry. The magnetometric data - collected along two nonequiva…
We present a micromagnetic analysis of spin-wave propagation in a magnonic crystal realized as a permalloy spin-wave waveguide with a spatial periodical variation of its saturation magnetization. Frequency band gaps were clearly observed in the spin-wave transmission spectra and their origin is traced back to an overlap of individual band gaps of the fundamental and the higher order spin-wave width modes. The control of the depth, width and the position in frequency and space of the rejection band gaps by the width areas with a reduced magnetization and by the modulation level, are discussed i…
Nanoparticles with their specific properties newly have drawn a great deal of attention of researchers [1-3]Yttrium iron Garnet magnetic nanoparticles (YIG-NPs) are promising materials with novel applications in microwave, spintronics, magnonics, and magneto-optical devices. However, achieving stable and remarkable magnetic YIG-NPs has been remaining as a great challenge. In this paper, synthesized YIG-NPs by modifying co-precipitation (MCP) method is reported. Structural and magnetic properties of final products are compared to those of the materials prepared by citrate-nitrate (CN) method. S…
Single crystals of Fe5B2P were grown by self-flux growth technique. Structural and magnetic properties are studied. The Curie temperature of Fe5B2P is determined to be 655$pm$2K. The saturation magnetization is determined to be 1.72 MuB/Fe at 2K. The temperature variation of the anisotropy constant K1 is determined for the first time, reaching ~0.50 MJ/m3 at 2K, and it is comparable to that of hard ferrites. The saturation magnetization is found to be larger than the hard ferrites. The first principle calculations of saturation magnetization and anisotropy constant are found to be consistent w…
The magnetization process of the S=1/2 Heisenberg antiferromagnet on the kagome lattice is studied by the numerical-diagonalization method. We successfully obtain a new result of the magnetization process of a 42-site cluster in the entire range. Our analysis clarifies that the critical behavior around one-third of the height of the saturation is different from the typical behavior of the well-known magnetization plateau in two-dimensional systems. We also examine the effect of the $\sqrt{3}\times\sqrt{3}$-type distortion added to the kagome lattice. We find at one-third of the height of the s…
We have used time-resolved scanning Kerr microscopy (TRSKM) and micromagnetic simulations to demonstrate that, when driven by spatially uniform microwave field, the edges of patterned magnetic samples represent both efficient and highly tunable sources of propagating spin waves. The excitation is due to the local enhancement of the resonance frequency induced by the non-uniform dynamic demagnetizing field generated by precessing magnetization aligned with the edges. Our findings represent a crucial step forward in the design of nanoscale spin-wave sources for magnonic architectures, and are al…
Spin-Hall oscillators are promising sources of spin-wave signals for magnonics applications, and can serve as building blocks for magnonic logic in ultralow power computation devices. Here, we analytically and micromagnetically study magnetization dynamics excited in a Spin-Hall oscillator with oblique magnetization when the spin-Hall effect and interfacial Dzyaloshinskii-Moriya interaction act simultaneously. Our key results are (i) excitation of nonreciprocal spin-waves propagating perpendicularly to the in-plane projection of the static magnetization, (ii) skyrmions generation by pure spin-…
We present a study on the magnetic behavior of dextran-coated magnetite nanoparticles (DM NPs) with sizes between 3 and 19 nm, synthesized by hydrothermal-assisted co-precipitation method. The decrease of saturation magnetization ($M_s$) with decreasing particle size has been modeled by assuming the existence of a spin-disordered layer at the particle surface, which is magnetically dead. Based on this core-shell model and taking into account the weight contribution of the non-magnetic coating layer (dextran) to the whole magnetization, the dead layer thickness ($t$) and saturation magnetizatio…
Reversible control of magnetization by electric fields without assistance from a subsidiary magnetic field or electric current could help reduce the power consumption in spintronic devices. When increasing temperature above room temperature, FeRh displays an uncommon antiferromagnetic to ferromagnetic phase transition linked to a unit cell volume expansion. Thus, using the strain exerted by an adjacent piezoelectric layer, the relative amount of antiferromagnetic and ferromagnetic regions can be tuned by an electric field applied to the piezoelectric material. Indeed, large variations in the s…
Electrodynamic theory applied to the analysis of TEn0p mode resonances in ferromagnetic spheres placed either in metallic cavities or in the free space is compared with Walker-Fletcher's theory of so-called magnetostatic modes. The influence of the diameter of the sample, its permittivity and the permittivity of the surrounding media on the resonance frequencies of a few modes is analyzed. It is shown that the dominant resonances are essentially related either to negative values of the diagonal component of the permeability tensor or, for clockwise circularly polarized magnetic fields, to nega…
The anisotropic magnetoresistance (AMR) in thin permalloy strips was calculated at each steps during magnetization by the finite element method. The magnetization at equilibrium under different external fields was obtained by micromagnetic simulations, while the resistance with different magnetization was obtained by solving the Poisson equations iteratively until self-consistence. We find that the relation between magnetization and AMR deviates from the Stoner-Wohlfarth prediction when the magnetization is reduced from saturation. The reason is that the demagnetization is not necessarily from…
Homogeneous freestanding films have been obtained by the direct current (DC) magnetron sputtering technique using a sacrificial layer. After annealing, the films are crystallized with a strong out-of-plane texture along the (022) direction. The stoichiometry of the annealed films is close to the target composition and leads to a martensitic transformation around 255K. The annealed films demonstrate ferromagnetic behavior with a Curie temperature of about 362K. The magnetization process has been studied on the both states and during the martensitic transition. The saturation magnetizations have…
6H-SiC (silicon carbide) single crystals containing VSi-VC divacancies are investigated with respect to magnetic and structural properties. We found that an initial increase of structural disorder leads to pronounced ferromagnetic properties at room temperature. Further introduction of disorder lowers the saturation magnetization and is accompanied with the onset of lattice amorphization. Close to the threshold of full amorphization, also divacancy clusters are formed and the saturation magnetization nearly drops to zero.
The structure of domain walls delimiting magnetic bubbles in L10 FePd thin layers is described on the basis of Lorentz transmission electron microscopy (LTEM) and multiscale magnetic simulations. Images obtained by high resolution LTEM show the existence of magnetization reversal areas inside domain walls, called vertical Bloch lines (VBLs). Combining these observations and multiscale simulations on various geometries, we can identify the structure of these VBLs, notably the presence or not of magnetic singularities.
Co3Sn2S2, a quasi-two-dimensional system with kagome lattice, has been found as a magnetic Weyl semimetal recently. In this work, the anisotropies of magnetization and transport properties of Co3Sn2S2 were investigated. The high field measurements reveal a giant magnetocrystalline anisotropy with an out-of-plane saturation field of 0.9 kOe and an in-plane saturation field of 230 kOe at 2 K, showing a magnetocrystalline anisotropy coefficient Ku up to 8.3 * 10^5 J m-3, which indicates that it is extremely difficult to align the small moment of 0.29 μB/Co on the kagome lattice from c axis to ab …
We investigate the ground state of a uniaxial ferromagnetic plate with perpendicular easy axis and subject to an applied magnetic field normal to the plate. Our interest is the asymptotic behavior of the energy in macroscopically large samples near the saturation field. We establish the scaling of the critical value of the applied field strength below saturation at which the ground state changes from the uniform to a branched domain magnetization pattern and the leading order scaling behavior of the minimal energy. Furthermore, we derive a reduced sharp-interface energy giving the precise asym…
We report the use of Lorentz microscopy to observe the domain wall structure during the magnetization process in FePd thin foils. We have focused on the magnetic structure of domain walls of bubble-shaped magnetic domains near saturation. Regions are found along the domain walls where the magnetization abruptly reverses. Multiscale magnetic simulations shown that these regions are vertical Bloch lines (VBL) and the different bubble shapes observed are then related to the inner structure of the VBLs. We were thus able to probe the presence of magnetic singularities as small as Bloch points in t…
We use femtosecond optical pulses to induce, control and monitor magnetization precession in ferromagnetic Ga0.965Mn0.035As. At temperatures below ~40 K we observe coherent oscillations of the local Mn spins, triggered by an ultrafast photoinduced reorientation of the in-plane easy axis. The amplitude saturation of the oscillations above a certain pump intensity indicates that the easy axis remains unchanged above ~TC/2. We find that the observed magnetization precession damping (Gilbert damping) is strongly dependent on pump laser intensity, but largely independent on ambient temperature. We …
Single phase Mn doped (2 at %) ZnO samples have been synthesized by solid-state reaction technique. Before the final sintering at 500 C, the mixed powders have been milled for different milling periods (6, 24, 48 and 96 hours). The grain sizes of the samples are very close to each other (~ 32 \pm 4 nm). However, the defective state of the samples is different from each other as manifested from the variation of magnetic properties and electrical resistivity with milling time. All the samples have been found to be ferromagnetic with clear hysteresis loops at room temperature. The maximum value f…
The asymmetric magnetization reversal is studied in a single exchange-biased microbar of 1.5 x 13 micrometer with anisotropic magnetoresistance and magnetic force microscopy. The particle has a moment of less than 10^-9 emu and is not accessible with standard magnetometry. The asymmetric hysteresis loop of CoFe/CrMnPt shows a repeatable rotation process, followed by an irreversible nucleation process that is marked by jumps in the magnetoresistance. The induced unidirectional anisotropy enhances the rotation process in one branch of the hysteresis loop, followed by a sped up nucleation process…
The magnetic properties of the alloy system Fe3-xMnxSi have been studied by measuring magnetization for samples with x = 0, 0.1, 0.25, 0.5, and by thermal scanning techniques for samples with x = 0, 0.1. The results reveal that the system is ferromagnetic in this composition range. Zero field cooling and field cooling magnetization measurements indicate a similar magnetic ordering and magnetic anisotropy in all samples. The saturation magnetization for the annealed samples was higher than that for as prepared samples. This is attributed to the reduction of magnetic domain boundaries rather tha…
We calculate magnetization in magnetically doped semiconductors assuming a local exchange model of carrier-mediated ferromagnetic mechanism and using a number of complementary theoretical approaches. In general, we find that the results of our mean-field calculations, particularly the dynamical mean field theory results, give excellent qualitative agreement with the experimentally observed magnetization in systems with itinerant charge carriers, such as Ga_{1-x}Mn_xAs with 0.03 < x < 0.07, whereas our percolation-theory-based calculations agree well with the existing data in strongly insulatin…
Three amorphous Co$_x$Nb$_{100-x}$ alloys, Co$_{25}$Nb$_{75}$, Co$_{57}$Nb$_{43}$ and Co$_{80}$Nb$_{20}$, were produced by Mechanical Alloying starting from the elemental powders. Their magnetic properties were determined using an alternating gradient force magnetometer (AGFM), and the remanent magnetizations, saturation fields and coercive fields were obtained from the hysteresis loop. The alloys have a relatively high saturation field, which decreases as the composition becomes richer in Co. The coercivity and remanent magnetization reach an optimal value around 57% at.Co, making {\em a}-Co$…
Co2FeAl (CFA) nanoparticles (NPs) of different sizes were synthesized by chemical route. The effect of the size of NPs upon the structure and magnetization compared to its bulk counterpart was investigated. The structure and composition were determined from X-ray diffraction (XRD) and electron microscopy. XRD analysis shows that the samples are having single (A2-type) disordered phase. Magnetization measurements suggest that the samples are soft ferromagnetic in nature with very low coercivity. Enhanced magnetic properties like saturation magnetization, coercive force, retentivity, and Curie-t…
We investigate the $S=1/2$ quantum spin antiferromagnets on the triangular and Kagome lattices in magnetic field, using the numerical exact diagonalization. Particularly we focus on an anomalous magnetization behavior of each system at 1/3 of the saturation magnetization. The critical exponent analyses suggest that it is a conventional magnetization plateau on the triangular lattice, while an unconventional phenomenon, called the magnetization ramp, on the Kagome lattice.
In this paper we study magnetization excitations induced in a thin extended film by a spin-polarized dc-current injected through a point contact in the current-perpendicular-to-plane (CPP) geometry. Using full-scale micromagnetic simulations, we demonstrate that in addition to the oscillations of the propagating wave type, there exist also two localized oscillation modes. The first localized mode has a relatively homogeneous magnetization structure of its kernel and corresponds to the so called 'bullet' predicted analytically by Slavin and Tiberkevich (Phys. Rev. Lett., 95 (2005) 237201). Magn…
Due to a ferromagnetic in-chain coupling between Co$^{3+}$ ions at trigonal sites, chains Co$_2$O$_6$ are considered as large rigid spin moments. The antiferromagnetic Ising model on the triangular lattice is applied to describe an interchain ordering. An evolution of metastable states in a sweeping magnetic field is investigated by the single-flip technique. At the first approximation two steps in the magnetization curve and a plateau at 1/3 of the saturation magnetization are found. Four steps in magnetization are determined in high-order approximations in agreement with experimental results…
We present a method to map the saturation magnetization of soft ultrathin films with perpendicular anisotropy, and we illustrate it to assess the compositional dependence of the magnetization of CoFeB(1 nm)/MgO films. The method relies on the measurement of the dipolar repulsion of parallel domain walls that define a linear domain. The film magnetization is linked to the field compressibility of the domain. The method also yields the minimal distance between two walls before their merging, which sets a practical limit to the storage density in spintronic devices using domain walls as storage e…
The magnetization process of the spin-S Heisenberg antiferromagnet on the kagome lattice is studied by the numerical-diagonalization method. Our numerical-diagonalization data for small finite-size clusters with S=1, 3/2, 2, and 5/2 suggest that a magnetization plateau appears at one-third of the height of the saturation in the magnetization process irrespective of S. We discuss the S dependences of the edge fields and the width of the plateau in comparison with recent results obtained by real-space perturbation theory.
We theoretically investigate the spin Seebeck effect (SSE) in antiferromagnets and ferrimagnets, and show that the SSE vanishes in antiferromagnets but survives in ferrimagnets even at the magnetization compensation point despite the absence of its saturation magnetization. The non-vanishing SSE in ferrimagnets stems from two non-degenerate magnons. We demonstrate that the magnitude of the SSE in ferrimagnets is unchanged across the magnetization compensation point.
We report a study of the magnetization reversals and skyrmion configurations in two systems - Pt/Co/MgO and Ir/Fe/Co/Pt multilayers, where magnetic skyrmions are stabilized by a combination of dipolar and Dzyaloshinskii-Moriya interactions (DMI). First Order Reversal Curve (FORC) diagrams of low-DMI Pt/Co/MgO and high-DMI Ir/Fe/Co/Pt exhibit stark differences, which are identified by micromagnetic simulations to be indicative of hybrid and pure Néel skyrmions, respectively. Tracking the evolution of FORC features in multilayers with dipolar interactions and DMI, we find that the negative FORC …
Using femtosecond optical pulses, we have investigated the ultrafast magnetization dynamics induced in a dielectric film of bismuth-substituted yttrium iron garnet (Bi-YIG) buried below a thick Cu/Pt metallic bilayer. We show that exciting the sample from Pt surface launches an acoustic strain pulse propagating into the garnet film. We discovered that this strain pulse induces a coherent magnetization precession in the Bi-YIG at the frequency of the ferromagnetic resonance. The observed phenomena can be explain by strain-induced changes of magnetocristalline anisotropy via the inverse magnetos…
We study the magnetization process of the spin-$1/2$ Heisenberg antiferromagnet on a distorted square-kagome lattice by the numerical-diagonalization method. The magnetization jump at one-third of the height of the saturation is examined in detail; we find that the jump becomes larger when a small distortion is switched on and that it is accompanied by an abrupt change in lines along microscopic spin directions. Our finite-size results successfully confirm that the magnetization jump in a spin-isotropic system is a macroscopic jump that survives in the thermodynamic limit and that the changes …
The structural and magnetic properties of a series of superlattices consisting of two ferromagnetic metals La$_{0.7}$Sr$_{0.3}$MnO$_3$ (LSMO) and SrRuO$_3$ (SRO) grown on (001) oriented SrTiO$_3$ are studied. Superlattices with a fixed LSMO layer thickness of 20 unit cells (u.c.) and varying SRO layer thickness show a sudden drop in magnetization on cooling through temperature where both LSMO and SRO layers are ferromagnetic. This behavior suggests an antiferromagnetic coupling between the layers. In addition, the samples having thinner SRO layers (n \TEXTsymbol{<} 6) exhibit enhanced saturati…
Direct observation of propagating spin waves in a spin-Hall nano-oscillator
University of Cambridge（剑桥大学）；University of Gothenburg（哥德堡大学）；Max-Planck-Institut für Festkörperforschung（马克斯·普朗克固体研究所）；Indian Institute of Technology Bhubaneswar（布巴内斯瓦尔印度理工学院）；Tohoku University（东北大学）；Max-Planck-Institut für Intelligente Systeme（马克斯·普朗克智能系统研究所）；Helmholtz-Zentrum Berlin für Materialien und Energie（柏林亥姆霍兹材料与能源中心）
We have investigated the room temperature ferromagnetic order that develops in Al-substituted magnesium oxide, Mg(Al)O, nanoparticles with Al fractions of up to 5 at.%. All samples, including undoped MgO nanoparticles, exhibit room temperature ferromagnetism, with the saturation magnetization reaching a maximum of 0.023 emu/g at 2 at.% of Al. X-ray photoelectron spectroscopy identifies the presence of oxygen vacancies in both doped and undoped MgO nanoparticles, with the vacancy concentration increasing upon vacuum annealing of Mg(Al)O, resulting in two-fold enhancement of the saturation magne…
We establish a solvable Heisenberg-Ising model on a spin-1 Ni-containing polymer chain, $[Ni (NN'-dmen) (μ-N_3)_2]$, with $NN'-dmen$ being $NN'-dimethylethylenediamine$, that fully covers the interaction characteristics of the material and by which, we can characterize all the peculiar magnetic features of the polymer, which has been partly studied in experiment. By purely analytical calculations, we can see that the magnetization exhibits three plateaus at zero, mid, and 3/4 of the saturation value at low temperatures below 2 K. The corresponding featuring peaks of magnetic susceptibility are…
