Surface Magnetic Anisotropy Research Articles

Phase transitions in rare-earth ferrimagnets with surface anisotropy near the magnetization compensation point

Theoretical model is proposed for calculating the phase H-T diagrams of a rare-earth ferrimagnet, considering the effects of each of the magnetic sublattices and surface anisotropy. Magnetic phase diagrams are numerically calculated. The presence of surface anisotropy leads to blurring of the second-order phase transition lines between the collinear and angular phases, displacement of the tricritical point, as well as the possibility of the formation of new phase transition lines.

Contributions of interfacial spin–orbit coupling to magnetic and spintronic properties in AuPt/ferromagnet bilayers

We investigate the relationships between various magnetic and spintronic properties within AuPt/ferromagnet (FM) bilayers (FM = CoFe, CoFeB, Py, and Co). A linear correlation between the volume and surface magnetic anisotropies is identified, potentially influenced by the magnetoelastic effect. The FM thickness dependence of the magnetic damping indicates that spin-memory loss due to the interfacial spin–orbit coupling (ISOC) and spin pumping to the heavy-metal layer contribute little to the damping. Instead, a notable contribution from two magnon scattering to the damping is recognized in AuPt/(Co, CoFe, CoFeB) bilayers, possibly originating from a magnetic inhomogeneity due to the ISOC. In addition, in contrast to the magnetic damping, spin–orbit-torque efficiencies are unlikely related to the ISOC in AuPt/FM systems. This work offers valuable insights into the correlations among magnetic and spintronic parameters arising from the interfaces, ultimately aiding in the advancement of magnetic memory and information processing systems.

Harnessing ion beam erosion engineering for controlled self-assembly and tunable magnetic anisotropy in epitaxial films

The engineering of the surface morphology and the structure of the thin film is one of the essential technological assets for regulating the physical properties and functionalities of thin film-based devices. This study presents an easy and handy approach to tailor the surface structure of epitaxial thin films utilizing low-energy ion beam. Here, we investigate the evolution of the surface structure and magnetic anisotropy (MA) in epitaxial Fe/MgO (001) model systems subjected to multiple cycles of ion beam erosion (IBE) after thin film growth. The growth of Fe film occurs in the form of three–dimensional islands and exhibits intrinsic biaxial MA. Following a few cycles of IBE, an induced uniaxial magnetic anisotropy leads to a split in the hysteresis loop, and the film displays almost uniaxial magnetic switching behavior. More distinctly, we present a clear and conclusive evidence of (2 × 2) reconstruction of the Fe surface due to the atomic rearrangement by IBE. Furthermore, 57Fe isotope sensitive nuclear resonance scattering measurement provides insight into the depth-resolved magnetic information due to the modified surface topography. We also demonstrate that thermal annealing can reversibly tune the surface reconstruction and induced UMA. The feasibility of the IBE technique by adequately selecting IBE parameters for surface structure modification has been highlighted apart from conventional tailoring of the morphology for the tuning of UMA and introduces a new dimension to our understanding of self-assembled surface morphology evolution by IBE.

Tailoring magnetic softness of Fe-based amorphous alloys with superior magnetization by magnetic field annealing

The inverse relationship between the saturation magnetic flux density (Bs) and coercivity (Hc) of Fe-based amorphous alloys is a very active research topic that has been extensively debated. In this work, we conducted a detailed investigation on the magnetic softness of Fe83.2-xCoxB10C6Cu0.8 (x = 0 and 6 at.%) amorphous alloys based on analysis of the surface morphology, microstructure, magnetic anisotropy, and magnetic domain structure. Enhanced magnetic softness-magnetization synergy was realized in the present alloys by magnetic field annealing (MFA) during the de-stressing process. A dramatic 84 % reduction of Hc to 2.2 A/m was achieved for the Co-doped alloy under MFA, exhibiting excellent magnetic performance with a superb Bs of 1.86 T. The consistency between the experimental results and theoretical analysis revealed that the MFA process can mitigate the trade-off between stress-induced anisotropy and induced uniaxial anisotropy owing to the homogenized structure formed by field annealing. Thus, the process favored a low Hc due to the significant continuous decline in the total magnetic anisotropy, which coincided well with the results of Magneto-optical Kerr microscopy. The study elucidates a mechanism for tuning Hc in Co-doped alloy systems and affords a possible pathway for softening amorphous alloys with high Bs.

Significant modulation of Gilbert damping in ultrathin ferromagnetic films by altering the surface magnetic anisotropy

Significant modulation of Gilbert damping in ultrathin ferromagnetic films by altering the surface magnetic anisotropy

3D Magnetization Textures: Toroidal Magnetic Hopfion Stability in Cylindrical Samples.

Topologically non-trivial magnetization configurations in ferromagnetic materials on the nanoscale, such as hopfions, skyrmions, and vortices, have attracted considerable attention of researchers during the last few years. In this article, by applying the theory of micromagnetism, I demonstrate that the toroidal hopfion magnetization configuration is a metastable state of a thick cylindrical ferromagnetic nanodot or a nanowire of a finite radius. The existence of this state is a result of the competition among exchange, magnetostatic, and magnetic anisotropy energies. The Dzyaloshinskii-Moriya exchange interaction and surface magnetic anisotropy are of second importance for the hopfion stabilization. The toroidal hopfion metastable magnetization configuration may be reached in the process of remagnetizing the sample by applying an external magnetic field along the cylindrical axis.

Influence of Free Layer Surface Roughness on Magnetic and Electrical Properties of 300 mm CMOS-Compatible MTJ Stacks

The magnetic tunnel junction (MTJ) is a highly versatile device widely used in today’s spintronic applications such as magnetoresistive random-access memory (MRAM), magnetic sensors and prospectively as a read device in racetrack memory. Tuning the perpendicular (p-)MTJ stack to match the desired properties, such as tunnel magnetoresistance (TMR), magnetic anisotropies or coercive field of the free layer, requires careful optimization of the deposition parameters as well as precise layer thickness control. Here, the deposition of individual layers in a wedged manner across 300 mm wafers is proposed to engineer the thicknesses within the stack more efficiently. Furthermore, this technique provides detailed insights into effects related to surface roughness, magnetic anisotropy and TMR.

Manipulations of Spin Wave by Photoelectrons in Ferromagnetic/Non-Ferromagnetic Alloyed Film.

Spin wave is considered an alternative carrier with great promise for information sensing. The feasible excitation and low-power manipulation of spin waves still remain a challenge. In this regard, natural light enables spin wave tunability is profoundly investigated in Co60 Al40 alloyed film. A reversible manner with 2° of the critical angle of the body spin wave is achieved successfully. Meanwhile, an eye-catching shift (817Oe) of the ferromagnetic resonance (FMR) field is obtained optically, leading to changes in magnetic anisotropy. Based on the modified Puszkarski's surface inhomogeneity model, the sunlight control of spin-wave resonance (SWR) can be comprehended by a photoelectron-doping-induced effective surface magnetic anisotropy change. Furthermore, the body spin wave is modulated stably with natural light illumination, confirming a non-volatile, reversible switching behavior. This work has both practical and theoretical importance for developing future sunlight-tunable magnonics/spintronics devices. This article is protected by copyright. All rights reserved.

Out-of-plane Ferromagnetic Resonance and Surface Spin Wave Modes in Ni80Fe20 Film on Ripple-Patterned Sapphire Substrate

The study of surface magnetic anisotropy of magnetic films is beneficial to understanding the behaviors of films at high frequencies and further promoting their high-frequency magnetic applications. We investigated the out-of-plane ferromagnetic resonance (FMR) and surface spin wave modes in 40-nm Ni80Fe20 (NiFe) films deposited on ripple-patterned sapphire substrate with periodicity and a flat sapphire substrate by broad-band FMR technique. When measuring the FMR of the film on ripple-patterned substrate, the in-plane angle (\U0001d711 H of the external applied magnetic field was along the direction of the ripples (\U0001d711 H = 90°) and perpendicular to the ripple direction (\U0001d711 H = 0°) respectively. The spin-wave resonance spectra consisting of up to two surface spin wave (SSW) modes were observed as the external magnetic field polar angle θH varied from “in-plane configuration” (θH = 90° ) to “out-of-plane configuration” (θH = 0°). As the decrease of θH , the two surface spin excitations disappeared in sequence and further only FMR mode was observed in the region θH which was smaller than the “critical angle” θc . The θc of \U0001d711 H = 90° is found to be different from that for \U0001d711 H = 0°. The dependences of field shift vs. out-of-plane angles θH for each SSW mode were analyzed by the surface inhomogeneity model. From the model, the surface anisotropy constants Ks of films were obtained. The results show that the values of surface anisotropy constants for the two SSW modes are different. Compared with the film sputtered on flat sapphire substrate, after introducing ripple-patterned substrate, the variation of Ks is only on the order of 0.01 erg/cm2 for the film of this thickness.

The manifestation of surface and size effects in the magnetic properties of ε-Fe-=SUB=-2-=/SUB=-O-=SUB=-3-=/SUB=- nanoparticles. (Brief overview)

Polymorphic modification of iron oxide, known as ε-Fe2O3, exists only in the form of nanoparticles with characteristic sizes up to several tens of nanometers. Particles of these sizes exhibit a large coercive force, about 20 kOe at room temperature. In the temperature range of 80-150 K, a magnetic transition occurs in ε-Fe2O3, accompanied by a sharp decrease in the coercive force. At the same time, there are significant differences in the magnetic behavior of "large" (~20 nm) particles and ultra-small particles (up to 6 nm). A number of experimental facts indicate the manifestation of size effects that lead to a change in the magnetic structure in particles of these sizes. In addition, a surface effect is also manifested for such particles --- a significant contribution to the magnetic behavior is governed by surface magnetic anisotropy. In this paper, a brief review of the manifestation of these size and surface effects in the magnetic properties of ε-Fe2O3 nanoparticles is carried out. Keywords: iron oxide ε-Fe2O3, nanoparticles, size effect, surface magnetic anisotropy.

Size dependence of the surface spin disorder and surface anisotropy constant in ferrite nanoparticles.

The magnetic properties of nanoscale magnets are greatly influenced by surface anisotropy. So far, its quantification is based on the examination of the blocking temperature shift within a series of nanoparticles of varying sizes. In this scenario, the surface anisotropy is assumed to be a particle size-independent quantity. However, there is no solid experimental proof to support this simplified picture. On the contrary, our work unravels the size-dependent magnetic morphology and surface anisotropy in highly uniform magnetic nanoparticles using small-angle polarized neutron scattering. We observed that the surface anisotropy constant does not depend on the nanoparticle's size in the range of 3-9 nm. Furthermore, our results demonstrate that the surface spins are less prone to polarization with increasing nanoparticle size. Our study thus proves the size dependence of the surface spin disorder and the surface anisotropy constant in fine nanomagnets. These findings open new routes in materials based on a controlled surface spin disorder, which is essential for future applications of nanomagnets in biomedicine and magnonics.

Significant suppression of two-magnon scattering in ultrathin Co by controlling the surface magnetic anisotropy at the Co/nonmagnet interfaces

To enable suppression of two-magnon scattering (TMS) in nanometer-thick Co (ultrathin Co) layers and realize low-magnon damping in such layers, the magnon damping in ultrathin Co layers grown on various nonmagnetic seed layers with different surface magnetic anisotropy (SMA) energies are investigated. We verify the significantly weak magnon damping realized by varying the seeding layer species used. Although TMS is enhanced in ultrathin Co on Cu and Al seeding layers, the insertion of a Ti seeding layer below the ultrathin Co greatly suppresses the TMS, which is attributed to suppression of the SMA at the interface between Co and Ti. The Gilbert damping constant of the ultrathin Co layer on Ti (3 nm), 0.020, is comparable to the value for bulk Co, although the Co layer thickness here is only 2 nm. Realization of such weak magnon damping can open the door to tunable magnon excitation, thus enabling coupling of magnons with other quanta such as photons, given that the magnetization of ultrathin ferromagnets can be tuned using an external electric field.

From Termination Dependent Chemical Sensitivity of Spin Orientation in All-bcc Fe/Co Magnetic Superlattices toward the Concept of an Artificial Surface of a Ferromagnet

Adsorption of gases on the surface of all-bcc (Fe/Co)N superlattices drives the in-plane, 90° magnetization rotation of the bulk-like Fe(110) supporting ferromagnet. Both experimental and theoretical results prove that terminating the surface of (Fe/Co)N superlattices either by Co or by Fe switches “ON” or “OFF” the spin orientation sensitivity to adsorption. Results indicate that purely surface limited adsorption processes strongly modify the magnetic anisotropy of the entire (Fe/Co)N superlattice, which acts as a kind of “artificial” surface of the bulky Fe(110) ferromagnet. Such an artificial magnetic surface anisotropy concept not only enhances the surface contribution in classical surface–bulk competition but also provides its additional chemical sensitivity.

Suppressing Magnetic Damping Related to Two-Magnon Scattering in Ultrathin NiFe Films by Interface Engineering

Designing the ferromagnetic/nonmagnetic (FM/NM) heterostructure with desirable low magnetic damping is important for spintronic devices to reduce the critical current density of magnetization switching. For ultrathin FM/NM films, magnetic damping from the two-magnon scattering (TMS) effect is a critical component, and suppressing TMS damping becomes one of the crucial ways to achieve low damping. In this paper, we have demonstrated the depressed linewidth in Ta (1 nm)/NiFe (3 nm)/NM (4 nm)/Ta (1.5 nm) heterostructure with Cu instead of Pt as the NM layer, where the maximal linewidth of ferromagnetic resonance (FMR) decreases from 440 to 190 Oe. And the decrease of TMS damping from 55 to 0 Oe is one of the main reasons for the lower FMR linewidth. The nonzero surface magnetic anisotropy implies that the suppression of interfacial defect scattering causes reduced TMS damping. Meanwhile, a noticeable enhancement of TMS damping from the lack of seed layer may be due to the lattice structure change. Thereby, our findings provide a way toward the design of low-damping spintronic devices.

Thickness-dependence of magnetic damping related to two-magnon scattering in ultrathin Ni0.81Fe0.19 films

Magnetic damping is of critical importance for devices due to its influences on magnetization dynamics. When the ferromagnetic layer goes into the nanometer scale, the magnetic damping related to the two-magnon scattering (TMS) effect becomes the main damping in nonmagnetic/ferromagnetic films. Here, we experimentally demonstrated the thickness-dependence of TMS damping, one part dependent on ferromagnetic thickness (), another part independent, in both Cu/NiFe and Pt/NiFe films. The scale of TMS damping can be explained based on the ‘Arias–Mills’ mode, and its origin is the distribution of surface magnetic anisotropy due to interface defects. The rest of TMS damping is independent on is attributed to the more generic defects creating inhomogeneity, and its origin can be sorted into the ‘McMichael–Krivosik’ mechanism. The quantitative results reveal that the -dependent and -independent TMS damping are of equal importance in ultrathin NiFe films. Meanwhile, the approximately equal intrinsic damping was displayed for Cu/NiFe and Pt/NiFe thin films. Our results add a clearer understanding of TMS damping in ultrathin ferromagnetic films.

Surface magnetic anisotropy-mediated spin Hall magnetoresistance and spin Seebeck effects in a YIG/Pt heterostructure

The role of magnon-phonon coupling in the low-temperature behavior of the spin Seebeck effect (SSE) in YIG/Pt has been puzzling for more than a decade. To elucidate the origin of the anomalous peak around 80 K, we investigate the temperature evolution of SSE, spin Hall magnetoresistance (SMR), and magnetic anisotropy in the same YIG/Pt heterostructure. We find that these effects, along with magnetic damping, show the peaks at the same temperature (∼80 K). This simultaneous occurrence, where no heat is applied in the case of SMR, rules out the phonon-magnon drag related origin of SSE in the YIG/Pt system. We further show that the intrinsic surface anisotropy behavior in YIG is responsible for controlling the SSE, SMR, and magnetic damping in the YIG/Pt structure. Our findings not only help to understand these effects fundamentally but also provide an effective way for improving them by manipulating the surface magnetic anisotropy for spin caloritronic applications.

First-principles Calculations of Magnetic Properties for Analysis of Magnetization Processes in Rare-earth Permanent Magnets

It has been empirically known that the coercivity of rare-earth permanent magnets depend on the size and shape of fine particles of the main phase in the system. Also, recent experimental observations have suggested that the atomic scale structures around the grain-boundaries of the fine particles play a crucial role to determine their switching fields. In this article, we review a theoretical attempt to describe the finite temperature magnetic properties and to evaluate the reduction of the switching fields of fine particles of several rare-earth permanent magnet materials based on an atomistic spin model that is constructed using first-principles calculations. It is shown that, over a wide temperature range, the spin model gives a good description of the magnetization curves of rare-earth intermetallic compounds such as R2Fe14B (R = Dy, Ho, Pr, Nd, Sm) and SmFe12. The atomistic spin model approach is also used to describe the local magnetic anisotropy around the surfaces of the fine particles, and predicts that the rare-earth ions may exhibit planar magnetic anisotropy when they are on the crystalline-structure surfaces of the particles. The dynamical simulation of the atomistic spin model and the corresponding micromagnetic simulation show that the planar surface magnetic anisotropy causes a reduction in the switching field of fine particles by approximately 20-30%, which may be relevant to the atomic scale surface effects found in the experimental studies.

Ferromagnetic resonance and spin-wave exchange stiffness of FeGaB/Al2O3 multilayer thin film stack for microwave applications

In this article, we have illustrated the deposition of multilayer film stack of Ta (5 nm)/[FeGaB (15 nm)/Al2O3 (3 nm)] n/Ta (3 nm)/Al2O3 (2 nm); n = 1,5, and 10, onto Si substrates, and studied magnetodynamic properties with multilayer thickness and temperature dependence. The results of surface morphology of thin film stacks suggest dipolar coupling generates for n = 5 and 10 thin film stack, which influence the ferromagnetic absorption results. From results of ferromagnetic resonance spectroscopy (FMR), the resonance peak for each excitation frequency provides the effective magnetization, the gyromagnetic ratio and damping factor. The damping factor for multilayer stack n = 1 shows a lower value compared to the other two samples. The multilayer stack provides perpendicular surface magnetic anisotropy (ks) from thickness dependence of effective magnetization. The analysis of spin-wave resonance spectra at multiple frequencies allows manipulate the exchange stiffness (D) of multilayer film stack n = 5 and n = 10 and temperature dependence the exchange stiffness follows the itinerant-electron model (T2 law) and suggests electron - magnon scattering exists in the thin film stack.

Локальная магнитная анизотропия в наноструктурированных покрытиях FeCo-C, синтезированных методами зеленой химии

Fe-Co alloys attracting interest due to their high magnetic induction and Curie temperature, were synthesized by an eco friendly electroless deposition with carbohydrates as reducing agents. It was shown that Fe1-xCox-С composite coatings retain high induction, while demonstrating an unusual behavior of magnetization at low temperatures. It was found that for each coating measured at different temperatures, there is a correlation between the local magnetic anisotropy constant K at a given temperature and the correlation radius of the local easy magnetization axis Rc. The functional type of this correlation is typical for nanoparticles or nanogranules in a composite, which made it possible to estimate the volume and surface magnetic anisotropy constants of metal granules of Fe-Co-C coatings.

Local magnetic anisotropy in nanostructured FeCo-C coatings synthesized by green chemistry methods

Fe-Co alloys attracting interest due to their high magnetic induction and Curie temperature, were synthesized by an eco friendly electroless deposition with carbohydrates as reducing agents. It was shown that Fe1-xCox-C composite coatings retain high induction, while demonstrating an unusual behavior of magnetization at low temperatures. It was found that for each coating measured at different temperatures, there is a correlation between the local magnetic anisotropy constant K at a given temperature and the correlation radius of the local easy magnetization axis Rc. The functional type of this correlation is typical for nanoparticles or nanogranules in a composite, which made it possible to estimate the volume and surface magnetic anisotropy constants of metal granules of FeCo-C coatings. Keywords: magnetic anisotropy, FeCo-C coatings, electroless deposition, approach to magnetic saturation law.