Perpendicular Anisotropy Research Articles

Traps for pinning and scattering of antiferromagnetic skyrmions via magnetic properties engineering

Micromagnetic simulations have been performed to investigate the controllability of the skyrmion position in antiferromagnetic nanotracks with their magnetic properties modified spatially. In this study, we have modeled magnetic defects as local variations on the material parameters, such as the exchange stiffness, saturation magnetization, perpendicular magnetocrystalline anisotropy, and Dzyaloshinskii–Moriya constant. Thus, we have observed not only pinning (potential well) but also scattering (potential barrier) of antiferromagnetic skyrmions, when adjusting either a local increase or a local reduction for each material parameter. In order to control the skyrmion motion, it is very important to impose certain positions along the nanotrack where the skyrmion can stop. Magnetic defects incorporated intentionally in antiferromagnetic racetracks can be useful for such a purpose. In order to provide guidelines for experimental studies, we vary both material parameters and the size of the modified region. The results obtained show that the efficiency of skyrmion traps depends on a suitable combination of magnetic defect parameters. Furthermore, we discuss the reason why skyrmions are either attracted or repelled by a region magnetically modified.

Features of the AMR Effect in Magnetic Strips with Perpendicular Anisotropy

A theoretical model is proposed for structures of collective micromagnetic spin in magnetoresistive FeNiCo nanostrips with perpendicular anisotropy. Atypical dependences of the magnetoresistance on the magnitude of the external field obtained experimentally indicate there is a complex domain structure in each sample. The curves of the AMR effect, calculated in accordance with this model, are in quantitative agreement with the experimental results for the above samples, while calculations using the specialized OOMMF package show only qualitative similarities.

Improved Magnetic and Microwave Properties of La-Substituted Barium Hexaferrite Screen-Printed Thick Films

This paper investigates the structural, morphological, magnetic, and microwave absorption properties of screen-printed Ba1−xLaxFe12O19 (x = 0.0, 0.1, 0.15, 0.20) thick films. Single phase P63/mmc was confirmed by X-ray diffraction patterns in fabricated thick films. SEM revealed dense packed microstructure with an average grain size of 2–3 μm for all films. La-substituted films showed increase in magnetization, coercivity, magnetocrystalline anisotropy with enhancement in perpendicular anisotropy. Overall microwave absorbance was increased for La-substituted BaM in Ku-band.

Switching of skyrmioniums induced by oscillating magnetic field pulses

Skyrmioniums are exotic magnetic configurations that have several potential applications, as devices for magnetic recording, in Spintronics. In this work, using micromagnetic simulation, we show that it is possible to switch a skyrmionium in an isolated nanodisk. First, we have obtained the ground state of the nanodisk for a fixed value of perpendicular anisotropy constant Kz and for several values of Dzyaloshinskii-Moriya exchange constant Dint. Next, in order to obtain the frequencies of the spin wave mode, we have calculated the power spectrum of the fluctuations of the spatial z-component of the magnetization for values of Dint where the ground state of nanodisk is a skyrmionium. Our results show that applying oscillating perpendicular magnetic field pulses with a frequency equal to that of the spin wave mode, and tuning the values of the intensity of the applied magnetic field and its duration in the sub-nanosecond range, one can switch the polarity of the skyrmionium in the nanodisk. We have also obtained, from the micromagnetic simulation of the process of inversion of polarity of the skyrmioniums, the several intermediate magnetic configurations of the nanodisk, revealing the full complexity of this phenomenon.

Criterion of non monotonic magnetic relaxation in Pt/Co/Ir/Co/Pt synthetic ferrimagnet with perpendicular anisotropy

Abstract In Pt/Co/Ir/Co/Pt synthetic ferrimagnet, non-monotonic magnetic relaxation (NMMR) caused by switching of external magnetic dependends on Co layer thicknesse and temperature. In this paper, we have varied thickness of one of the Co layers in the Pt/Co/Ir/Co/Pt synthetic ferrimagnet in the 0.6–1.0 nm range at fixed Co layer thickness of another layer 1.1 nm in the 50–300 K range to find experimental conditions for NMMR. We found interdependence between Co layer thickness tCo and temperature T providing NMMR. Energy balance between magnetic anisotropies and exchange interaction of the two Co layers stipulates linear dependence of nessecary T on tCo. Exsact expression limitating magnetic anisotropies of the thick and thin Co layers and predicting NMMR conditions in tCo - T space is proposed. Altough all mentioned results relates to experiments in permanent magnetic field, the contribution of the NMMR to the magnetic hysteresis loops recorded in sweeping magetic field was experimentally found. Obtained results can be used for wide family of synthetic ferrimagnets with perpendicular anisotropy.

Tuning magnetic properties of single-layer MnTe2 via strain engineering

We systematically study the strain-tuning effect on the electronic and magnetic properties of single-layer MnTe2 using first-principles calculations. The calculations show that the unstrained single-layer MnTe2 is an intrinsic ferromagnetic metal with a perpendicular magnetocrystalline anisotropy. The in-plane biaxial tensile strain greatly enhances the perpendicular magnetocrystalline anisotropy. The orbital-projected spin-orbit coupling energy indicates that the magnetocrystalline anisotropy energy mainly comes from the contribution of the Te atom although its magnetic moment is small. The ferromagnetic interaction between the nearest two Mn atoms is also strengthened significantly by the tensile strain. The Curie temperature increases from 88 K without strain to 440 K at 10% strain. Moreover, the single-layer MnTe2 becomes a half metal when the tensile strain is larger than 6%. If compressive strain is applied, the single-layer MnTe2 undergoes a spin reorientation transition from out-of-plane to in-plane magnetization. The compressive strain also induces the transition of the Mn–Mn exchange coupling from ferromagnetic to antiferromagnetic. The intrinsic ferromagnetic properties and the strong strain-tuning effect make the single-layer MnTe2 a promising candidate for spintronics applications.

Reduction of the switching current in perpendicularly magnetized nanomagnets using an antiferromagnetic coupling structure

This paper reports a current-induced magnetization switching with a nanosecond-scale pulse current in a nanomagnet using a perpendicularly magnetized synthetic antiferromagnetic coupling (p-AFC) structure. The results indicate that the magnetization switching current in the p-AFC structure is less than that in the single-nanomagnet structure with perpendicular anisotropy when the differences in thickness and saturation magnetization between the upper and lower layers of the p-AFC structure are small and the Gilbert damping constant is also small. The results also show that the p-AFC structure can reduce the switching current when the pulse duration is short and its structure is effective for a high-speed switching. The results of this study shall be useful in the design of spin-transfer torque random access memory.

Creating skyrmions and skyrmioniums using oscillating perpendicular magnetic fields

Magnetic skyrmions and skyrmioniums are exotic magnetic configurations that have several potential applications in Spintronics. In this work, using micromagnetic simulations, we show that it is possible to create metastable skyrmions and skyrmioniums in an isolated cobalt nanodisk, that has as its minimum energy configuration a single domain with perpendicular magnetization. First, we have determined the ground state of the nanodisk and the frequencies of the spin wave modes of skyrmions and skyrmioniums for different values of perpendicular uniaxial anisotropy constant Kz and Dzyaloshinskii-Moriya exchange constant Dint. Next, in order to create a skyrmion or skyrmionium, we have applied an oscillating perpendicular magnetic field with a frequency equal to that of the spin wave modes. Our results show that it is possible to switch from single domain to skyrmion or skyrmionium, tuning parameters such as the intensity of the applied perpendicular magnetic field, and its duration.

Fabrication and magnetic properties of Co based thin films synthesized by physical vapor deposition under vacuum

We used thermal heating process, under a pressure of 10−7 mbar, to fabricate thin films of cobalt on monocrystalline silicon Si (100) substrates. The incident beam strikes the substrates under normal and oblique incidences within a home-made evaporation chamber. The thickness of the deposited films ranges from 18 to 400 nm. Microscopic characterizations of the films are performed with X-ray diffraction (XRD) measurements and infer that all the samples are polycrystalline, with an hexagonal close packed (h.c.p.) structure and exhibit a <0001> preferred orientation, for the perpendicularly evaporated films. The grain size is found to increase with the thickness of the magnetic layer. The values of the computed parameters allow concluding to a compressive stress for the thinnest films. The static magnetic properties have been performed by means of magnetic force microscopy (M.F.M.) and Alternating Gradient Field Magnetometer (A.G.F.M.) tools. The dynamic magnetic properties have been investigated using Brillouin Light Scattering (B.L.S.) measurements. The M.F.M. observations were performed after in-plane ac demagnetization, and stripe patterns are only observed for the thickest films, showing the weaker perpendicular anisotropies. We used the experimental results provided by these tools to compute the effective magnetic anisotropy factors Ku . Values of Ku anisotropy constants, higher than 13 × 106 erg.cm−3, have been found.

Multistate Magnetization Switching Driven by Spin Current From a Ferromagnetic Layer

Spin-charge interconversion in ferromagnets has recently drawn great attention due to its ability to generate spin current via the anomalous Hall, planar Hall, or intrinsic spin Hall effect. In this work, we demonstrate the magnetization switching of perpendicular anisotropy ferromagnets by the transversely polarized spin current generated from an easy-plane ferromagnet such as (Fe, Mn)Pt and (Co, Fe)B. The spin-torque efficiency is characterized to be about 21% of that of the same structure with the easy-plane ferromagnet replaced by Ta. In addition, we find that the magnetization switching consists of multiple intermediate states, which can be robustly set by the applied current pulse amplitude and repetition number. This behavior can be used to mimic synaptic devices that is potentially useful for neuromorphic computing.

Suppression of the skyrmion Hall effect in planar nanomagnets by the magnetic properties engineering: Skyrmion transport on nanotracks with magnetic strips

Micromagnetic simulations have been performed to investigate the suppression of the skyrmion Hall effect in nanotracks with their magnetic properties strategically modified. In particular, we study two categories of magnetically modified nanotracks. One of them, repulsive edges have been inserted in the nanotrack and, in the other, an attractive strip has been placed exactly on the longest axis of the nanotrack. Attractive and repulsive interactions can be generated from the engineering of magnetic properties. For instance, it is known that the skyrmion can be attracted to a region where the exchange stiffness constant is decreased. On the other hand, the skyrmion can be repelled from a region characterized by a local increase in the exchange stiffness constant. In order to provide a background for experimental studies, we vary not only the magnetic material parameters (exchange stiffness, perpendicular magnetocrystalline anisotropy and the Dzyaloshinskii-Moriya constant) but also the width of the region magnetically modified, containing either a local reduction or a local increase for each one of these magnetic properties. In the numerical simulations, the skyrmion motion was induced by a spin-polarized current and the found results indicate that it is possible to transport skyrmions around the longest axis of the nanotrack. In practice, the skyrmion Hall effect can be completely suppressed in magnetic nanotracks with strategically modified magnetic properties. Furthermore, we discuss in detail 6 ways to suppress the skyrmion Hall effect by the usage of nanotracks with repulsive edges and nanotracks with an attractive strip.

Manipulating the Energy Balance of Perpendicular-Anisotropy Synthetic Antiferromagnets by He + -Ion Irradiation

${\mathrm{He}}^{+}$-ion irradiation enables controlled postdeposition modification of layered magnetic thin-film systems. The degree of modification and its depth profile can be tuned by the irradiation dose and energy. Here, we use magnetometry and magnetic force microscopy to explore the impact of gentle ${\mathrm{He}}^{+}$-ion irradiation on synthetic antiferromagnets, consisting of ferromagnetic $\mathrm{Co}/\mathrm{Pt}$ multilayers with perpendicular magnetic anisotropy, which are antiferromagnetically (AF) coupled via $\mathrm{Ru}$ interlayers. This system shows a rich variety of magnetic domain patterns due to the strong competition between different magnetic energies. We show that AF interlayer exchange and perpendicular interface anisotropy energy are gradually reduced by the ion irradiation while the demagnetization energy is mainly preserved, which thus results in multiple successive magnetic-phase transitions.

Bidirectional switching assisted by interlayer exchange coupling in asymmetric magnetic tunnel junctions.

We study the combined effects of spin transfer torque, voltage modulation of interlayer exchange coupling and magnetic anisotropy on the switching behavior of perpendicular magnetic tunnel junctions (p-MTJs). In asymmetric p-MTJs, a linear-in-voltage dependence of interlayer exchange coupling enables the effective perpendicular anisotropy barrier to be lowered for both voltage polarities. This mechanism is shown to reduce the critical switching current and effective activation energy. Finally, we analyze the possibility of having switching via interlayer exchange coupling only.

Epitaxially grown Cu2Sb-type MnGaGe films with large perpendicular magnetic anisotropy

A perpendicularly magnetized film is essentially important for spintronics applications, such as spin-transfer-torque type magnetoresistive random access memories, STT-MRAMs, and large perpendicular anisotropy energy, Ku, with small saturation magnetization, Ms, is required for reducing the critical current for the STT induced magnetization switching phenomenon. In this study, an equiatomic intermetallic compound, MnGaGe, was investigated in epitaxially grown film samples. Perpendicularly magnetized films with an Ms value of about 260 emu/cm3 at 300 K were fabricated onto MgO (100) single crystal substrates. The maximum value of Ku was 8.1 × 106 erg/cm3 at room temperature. Microstructure analysis using a scanning transmission electron microscope atomically revealed the Cu2Sb type crystal structure in the film sample. First principles calculations were also carried out to discuss the electronic structure. Using the analysis of the projected density of states for 3d-orbitals, the hybridization of the 3d-orbitals was suggested as a possible factor for promoting magnetocrystalline anisotropy. The MnGaGe film with the large Ku and relatively small Ms would be potentially used for spintronics applications.

Pitch Angle Dependence of Electron and Ion Flux Changes During Local Magnetic Dipolarization Inside Geosynchronous Orbit

AbstractIn this study we have statistically examined flux changes of 1–1,000‐keV electrons, and hydrogen, helium, and oxygen ions during local magnetic dipolarization inside geosynchronous orbit, with a focus on their pitch angle dependence. Using 144 dipolarization events that were selected in a previous study with Van Allen Probes observations in 2012–2016, we have performed a superposed epoch analysis of differential flux changes after the dipolarization onset for each species. On average the electron flux increases primarily around pitch angle (α) = 90° at &gt;80 keV and almost isotropically at 10–50 keV. The electron flux at &lt;5 keV increases at α = 0° and 180° but slightly decreases (or remains unchanged) at α = 90°. On the other hand, the ion flux at &gt;80 keV increases around α = 90°, while at &lt;30 keV it decreases nearly independent of pitch angle. Only the low‐energy (&lt;5 keV) helium and oxygen ion flux changes indicate a strong field‐aligned enhancement, which could be attributed to their outflows from the topside ionosphere. After the dipolarization onset, &gt;5‐keV electron and &gt;30‐keV ion fluxes exhibit a perpendicular anisotropy, which is most pronounced for 50–200‐keV electrons. Both distributions become more isotropic with decreasing energy. A noticeable field‐aligned anisotropy is seen for &lt;5‐keV ion fluxes. These statistical pitch angle‐dependent electron and ion properties during dipolarizations may be explained by combining various processes, including adiabatic acceleration and/or transport, wave‐particle interactions, and ion outflow.

Thermal nucleation and high-resolution imaging of submicrometer magnetic bubbles in thin thulium iron garnet films with perpendicular anisotropy

Nanometer-thin rare-earth iron garnet films with perpendicular magnetic anisotropy are among the most promising materials for fast and low-energy spintronics applications. Here, the authors demonstrate ultrafast control of intrinsically stabilized submicrometer bubble domains in these materials. By employing powerful x-ray imaging techniques both in transmission and with surface sensitivity, and by combining this with nanosecond electrical pulses and femtosecond laser excitation, they deterministically create, and image, these bubble states $i\phantom{\rule{0}{0ex}}n$ $s\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}u$. The topology, the chirality, and the switching phase diagram of these twisted spin structures are all resolved by direct imaging.

A New Substorm Onset Mechanism: Increasingly Parallel Pressure Anisotropic Ballooning

AbstractRecent observations demonstrating that auroral beads are observed prior to the majority of substorm onsets (Kalmoni et al., 2017, https://doi.org/10.1002/2016GL071826) have reinforced the potential importance of ballooning instabilities for near‐Earth magnetospheric substorm onset. Here we examine pressure anisotropic ballooning instabilities in stretched magnetotail geometries. Our results show that transition from an initially perpendicular anisotropy toward parallel anisotropy reduces the plasma β threshold for triggering a ballooning instability. Such increasingly parallel anisotropies can form as a direct consequence of tail stretching that occurs during the late substorm growth phase through the well‐known effects of drift shell splitting and the competition between betatron and Fermi processes in the tail. We propose such increasingly parallel ballooning triggers auroral substorm onset on field lines in the transition region between dipolar and tail‐like fields, consistent with observational constraints on the location of the onset arc with respect to the ion isotropy boundary in the magnetotail.

Temperature dependence of magnetic anisotropy in heavily Fe-doped ferromagnetic semiconductor (Ga,Fe)Sb

We study the temperature dependence of magnetic anisotropy (MA) of ferromagnetic semiconductor (Ga0.7,Fe0.3)Sb thin films with thicknesses of 20 nm and 40 nm using ferromagnetic resonance. With decreasing temperature from 300 to 10 K, the easy magnetization axis of the 40-nm-thick film changes from in-plane to perpendicular at 250 K, while that of the 20-nm-thick film lies in the film plane at all measurement temperatures (10–300 K). In the 40-nm-thick film, the uniaxial perpendicular anisotropy (K2⊥) significantly increases with decreasing temperature and surpasses the thin-film shape anisotropy, leading to perpendicular magnetic anisotropy (PMA). We present a possible scenario that the increase in K2⊥ originates from the formation of the Fe-rich nanocolumns with the perpendicular shape anisotropy, which induces the PMA in the whole film at low temperatures due to carrier-mediated exchange interactions between Fe spin in the Fe-rich nanocolumns and Fe spin in the (Ga,Fe)Sb matrix. Our findings on the temperature dependence of MA of heavily Fe-doped (Ga,Fe)Sb provide insights into the mechanism of high-Curie-temperature ferromagnetism.

Contributions of magnetic structure and nitrogen to perpendicular magnetocrystalline anisotropy in antiperovskite ɛ−Mn4N

To study how nitrogen contributes to perpendicular magnetocrystalline anisotropy (PMA) in the ferrimagnetic antiperovskite $\mathrm{M}{\mathrm{n}}_{4}\mathrm{N}$, we examined both the fabrication of epitaxial $\mathrm{M}{\mathrm{n}}_{4}\mathrm{N}$ films with various nitrogen contents and first-principles density-functional calculations. Saturation magnetization $({M}_{\mathrm{s}})$ peaks of 110 mT and uniaxial PMA energy densities $({K}_{\mathrm{u}})$ of $0.1\phantom{\rule{0.16em}{0ex}}\mathrm{MJ}/{\mathrm{m}}^{3}$ were obtained for a ${\mathrm{N}}_{2}$ gas flow ratio ($Q$) of \ensuremath{\sim}10% during sputtering deposition, suggesting nearly single-phase crystalline $\ensuremath{\varepsilon}\text{\ensuremath{-}}\mathrm{M}{\mathrm{n}}_{4}\mathrm{N}$. Segregation of \ensuremath{\alpha}-Mn and nitrogen-deficient $\mathrm{M}{\mathrm{n}}_{4}\mathrm{N}$ grains were observed for $Q\ensuremath{\approx}6%$, and were responsible for a decrease in the ${M}_{\mathrm{s}}$ and ${K}_{\mathrm{u}}$. The first-principles calculations revealed that the magnetic structure of $\mathrm{M}{\mathrm{n}}_{4}\mathrm{N}$ showing PMA was ``type B,'' having a collinear structure, whose magnetic moments couple parallel within the $c$ plane and alternating along the $c$ direction. In addition, the ${K}_{\mathrm{u}}$ calculated using $\mathrm{M}{\mathrm{n}}_{32}{\mathrm{N}}_{x}$ supercells showed a strong dependence on nitrogen deficiency, in qualitative agreement with the experimental results. The second-order perturbation analysis of ${K}_{\mathrm{u}}$ with respect to the spin-orbit interaction revealed that not only spin-conserving but also spin-flip processes contribute significantly to the PMA in $\mathrm{M}{\mathrm{n}}_{4}\mathrm{N}$. We also found that both contributions decreased with increasing nitrogen deficiency, resulting in the reduction of ${K}_{\mathrm{u}}$. It was noted that the decrease in the spin-flip contribution occurred at the Mn atoms in face-centered sites. This is one of the specific PMA characteristics we found for antiperovskite-type $\mathrm{M}{\mathrm{n}}_{4}\mathrm{N}$.

High temperature spin cluster glass behavior in Co- and Si-substituted garnet ferrite thin films

In this study, lutetium iron garnet (Lu3Fe5O12; LuIG) thin films have been grown on Y3Al5O12 (1 1 1) substrates by a pulsed laser deposition and are co-substituted with strong magnetically anisotropic ions of Co2+ and nonmagnetic ones of Si4+. LuIG is a garnet-structured ferrimagnet with an exceptionally low Gilbert damping constant as 10−5. The substitution of Co2+ and Si4+ induces a chemical disorder and competing antiferromagnetic and ferromagnetic interactions, which induce a spin glass behavior. When the Co–Si substitution ratio is increased, a spin-freezing behavior is observed below the spin-freezing temperature TG = 190–220 K of the Lu3Fe4Co0.5Si0.5O12 film; this indicates the presence of a cluster glass phase. The film demonstrates a characteristic memory effects in the temperature range 120–180 K, which are below TG; this reveals the presence of a metastable magnetic state in the multi-valley energy landscape with shallow activation energy barriers. Ferromagnetic resonance measurements confirm increased perpendicular anisotropy energy induced by Co substitution and spin-wave characteristics including the inverse spin Hall effect voltage (VISHE). Lu3Fe5-2xCoxSixO12 films show an increase in damping and a decrease in VISHE at the cryogenic temperature, which are attributed to the impurity relaxation mechanism. Therefore, the Co- and Si-substituted films exhibit strong spin-wave damping at a low temperature, which reflects the spin-freezing dynamics of cluster glass behaviors.