Intrinsic Magnetocrystalline Anisotropy Research Articles

Exceptional magnetic and magnetoelastic behavior of rare-earth non-centrosymmetric Sm7Pd3

Magnetic compounds possessing an intrinsic combination of near-zero magnetization with high magnetic anisotropy are highly desirable for spinronic applications and memory recording. A comprehensive study of Sm7Pd3 binary compound uncovered a unique combination of strong magnetoelastic behavior, very low net magnetization, and exceptionally high magnetic coercivity. The temperature-dependent X-ray synchrotron powder diffraction study indicates the abrupt changes in the compound's lattice parameters at the magnetic ordering temperature of TC=169 K, although the crystal structure remains non-centrosymmetric hexagonal Th7Fe3-type down to 6 K. Density functional theory calculations confirm high intrinsic magnetocrystalline anisotropy of Sm7Pd3, which explains the extremely large coercivity of the polycrystalline sample, up to Hcr = 130 kOe at 2 K. This discovery brings to life a novel class of highly anisotropic materials that are distinctly different from known spintronic materials, making them interesting future systems for magnetic memory research.

Epitaxial growth and electric control of magnetic behaviors in exchange biased IrMn/FeGa/MgO/PMN-PT heterostructures

We grew exchange biased (001)-textured IrMn/epitaxial FeGa bilayers on ferroelectric PMN-PT(001) substrate with assistance of an MgO buffer layer and investigated the electric control of magnetic behaviors. The as-grown sample exhibits square, asymmetrically shaped, and two-step loops at different magnetic field orientations due to the combined effect of the intrinsic magnetocrystalline anisotropy and the collinear uniaxial and unidirectional magnetic anisotropies, which can be interpreted by the magnetization reversal of domain wall nucleation. After polarized by an electric field, another kind of asymmetrically shape loops with three steps in the descending and two steps in the ascending branch is additionally observed because the uniaxial magnetic anisotropy is greatly enhanced and becomes perpendicular to the exchange bias. The angular dependent magnetic behavior suggests the exchange bias is slightly rotated by the electrically induced strain. The electric field dependence of coercive field and uniaxial magnetic anisotropy exhibit an asymmetrical butterfly-like shape, indicating a strain-mediated magnetoelectric coupling with a net 109° polarization switching of PMN-PT.

Fourfold magnetic anisotropy varied by interfacial exchange coupling in epitaxial Fe/IrMn bilayers

We report variable in-plane fourfold magnetic anisotropy in epitaxial exchange-biased Fe/IrMn bilayers with different layer thicknesses. The fourfold magnetic anisotropy of the bilayers continuously decreases and even appears negative as the thickness of the Fe layer decreases, indicating a change in the easy axes from Fe 100 to Fe 110, which can be interpreted by the competition between the intrinsic magnetocrystalline anisotropy along Fe 100 and the extrinsic fourfold magnetic anisotropy along Fe 110 induced by the interfacial exchange coupling to IrMn. As the IrMn thickness increases, the fourfold magnetic anisotropy of the bilayers decreases rapidly from a value close to the intrinsic magnetocrystalline anisotropy of Fe and then gradually reaches saturation. The extrinsic fourfold magnetic anisotropy depends on the amount of the induced uncompensated IrMn spins near the edge of antiferromagnetic domains. In contrast, the exchange bias is determined by the net difference of the opposite uncompensated IrMn spins.

Fourfold magnetic anisotropy induced in CoFeB/IrMn bilayers by interfacial exchange coupling

Exchange bias (EB) occurring in ferromagnetic (FM)/antiferromagnetic (AFM) bilayers conventionally can lead to a unidirectional magnetic anisotropy () as well as an accompanied uniaxial magnetic anisotropy (). We observed an additional fourfold magnetic anisotropy () induced by interfacial exchange coupling in amorphous CoFeB/epitaxial IrMn bilayers with an EB. Because of the combined effect of the three kinds of magnetic anisotropies, one- and two-step magnetic switching processes were observed at different magnetic field orientations, which usually appear in single-crystal FM layer with an intrinsic magnetocrystalline anisotropy but not in amorphous FM layer. The angular dependent magnetic switching fields can be nicely fitted by a phenomenological model based on domain wall nucleation and propagation with the in-plane along <100>. The ferromagnetic resonance measurements indicate that the specific strength of for EB along [100] is larger than that for EB along [110]. The induced can be understood by considering two types of AFM domains caused by both monatomic steps and defects and their induced net uncompensated spins along the in-plane <100> axes. The different dependence of on the EB direction are because of the different effects of growth magnetic field on the presence of AFM domains.

Influence of ferromagnetic layer thickness on the Gilbert damping and magnetocrystalline anisotropy in PLD grown epitaxial Co2FeSi Heusler alloy thin films

In the present work, full Heusler alloy (FHA) Co2FeSi (CFS) epitaxial thin films with thickness (tCFS) ranging from 5–25 nm were grown by Pulse laser deposition (PLD) on MgO (001) substrates at an optimized deposition temperature of 600°C. Dynamic magnetization response of PLD-grown CFS thin films was systematically investigated as a function of tCFS. Thickness-dependent structural and magnetic correlation exhibit a non-monotonic variation in Gilbert damping parameter, α. The ferromagnetic resonance (FMR) investigation revealed that the presence of lattice strain and a lower degree of atomic ordering in the thinner FHA CFS films, results in additional enhancement of α. The lowest α value of 4×10−3 is obtained from the theoretical fit of experimental data in a 20 nm thick film. This low value of α is suitable in magnonic devices, such as spin-transfer torque magnetic random-access memories (STT-MRAMs) where an efficient spin injecting source from a FHA ferromagnet to a non-magnetic interface is required. Further, in-plane angle-dependent (φ-variation) FMR measurements of the sample yield an intrinsic four-fold magnetocrystalline anisotropy (MCA) along with the presence of uniaxial anisotropy that has non-monotonicity with tCFS. A significant value of cubic anisotropy constant of 5.16×105erg/cm3 has been obtained in the film with a thickness of around 15nm. Further investigation suggests that uniaxial anisotropy in the CFS/MgO film is a combination of strain-induced magneto-elastic and interface-related contribution that is intrinsic in nature. The experimental observations have been verified by using micromagnetic simulations run on the epitaxial crystal structure. FMR mode position and linewidth of varied thickness CFS films are generated by micromagnetic simulations utilizing the Landau–Lifshitz–Gilbert formalism.

Magnetic Nanoparticles: Synthesis, Anisotropy, and Applications.

Anisotropy is an important and widely present characteristic of materials that provides desired direction-dependent properties. In particular, the introduction of anisotropy into magnetic nanoparticles (MNPs) has become an effective method to obtain new characteristics and functions that are critical for many applications. In this review, we first discuss anisotropy-dependent ferromagnetic properties, ranging from intrinsic magnetocrystalline anisotropy to extrinsic shape and surface anisotropy, and their effects on the magnetic properties. We further summarize the syntheses of monodisperse MNPs with the desired control over the NP dimensions, shapes, compositions, and structures. These controlled syntheses of MNPs allow their magnetism to be finely tuned for many applications. We discuss the potential applications of these MNPs in biomedicine, magnetic recording, magnetotransport, permanent magnets, and catalysis.

Ferromagnetic resonance of superparamagnetic nanoparticles: The effect of dipole–dipole interactions

Superparamagnetic iron-oxide nanoparticles play an important role in a wide range of applications and determining their magnetic state is crucial. Typically, it is assumed that dipole–dipole interactions are not relevant in the superparamagnetic state due to thermal fluctuations. Here, we show evidence of how dipole–dipole interactions modify the collective magnetic state. Ferromagnetic resonance spectroscopy of iron-oxide nanoparticles with a diameter of 10 nm reveals that the configuration of the particles has a strong effect on their effective magnetic anisotropy in short time scales. Fits of the experimental spectra with a theoretical model enable the quantification of these anisotropy fields. Particles in suspension exhibit an easy-axis anisotropy due to the field-induced alignment, whereas condensed dry particles exhibit easy-plane anisotropy due to clustering, and the difference between uniaxial anisotropy in suspension and in the condensed state is on the order of 0.1 T. These findings highlight that dipole–dipole interactions have a strong effect on the collective magnetic state despite thermal fluctuations and should be taken into account in any high-frequency application because in short time scales, the configuration of the particles exhibits effective anisotropy that is an order of magnitude larger than the intrinsic magnetocrystalline anisotropy.

Electric Field Control of Magnetic Permeability in Co-Fired Laminate Magnetoelectric Composites: A Phase-Field Study for Voltage Tunable Inductor Applications.

Control of magnetic permeability through electric field in magnetoelectric materials promises to create novel voltage tunable inductors (VTIs). VTIs synthesized using co-fired ceramic processing exhibit many advantages over traditional epoxy bonding method, but the internal residual stress in co-fired VTIs resulting from thermal expansion mismatch hinders a full exploitation of the tunability of permeability. To find the optimal condition for high tunability of co-fired VTIs, domain-level phase field modeling and computer simulation are employed to study co-fired magnetoelectric composites comprising NiZn ferrite and PZT. Two key factors important toward increasing the inductor tunability are systematically investigated: intrinsic magnetocrystalline anisotropy of the ferrite material and internal residual stress caused by the co-firing process. The simulations indicate that in order to achieve a large tunability, the tuned permeability should be confined within the linear region of the reciprocal of susceptibility and stress. Additionally, both magnetocrystalline anisotropy and residual stress should be as small as possible. These results provide a design strategy for realizing high-tunability co-fired VTIs.

Formation of Fe(III)-phosphonate Coatings on Barium Hexaferrite Nanoplatelets for Porous Nanomagnets.

Amorphous coatings formed with mono-, di-, and tetra-phosphonic acids on barium hexaferrite (BHF) nanoplatelets using various synthesis conditions. The coatings, synthesized in water with di- or tetra-phosphonic acids, were thicker than that could be expected from the ligand size and the surface coverage, as determined by thermogravimetric analysis. Here, we propose a mechanism for coating formation based on direct evidence of the surface dissolution/precipitation of the BHF nanoplatelets. The partial dissolution of the nanoplatelets was observed with atomic-resolution scanning transmission electron microscopy, and the released Fe(III) ions were detected with energy-dispersive X-ray spectroscopy and electron energy loss spectroscopy in amorphous coating. The strong chemical interaction between the surface Fe(III) ions with phosphonic ligands induces the dissolution of BHF nanoplatelets and the consequent precipitation of the Fe(III)-phosphonates that assemble into a porous coating. The so-obtained porous nanomagnets are highly responsive to a very weak magnetic field (in the order of Earth’s magnetic field) at room temperature, which is a major advantage over the classic mesoporous nanomaterials and metal–organo-phosphonic frameworks with only a weak magnetic response at a few kelvins. The combination of porosity with the intrinsic magneto-crystalline anisotropy of BHF can be exploited, for example, as sorbents for heavy metals from contaminated water.

Microstructure, magnetic and optical properties of Nb3+ and Y3+ ions co-substituted Sr hexaferrites

Series of SrNbxYxFe12-2xO19 (0.00 ≤ x ≤ 0.05) hexaferrites (HFs) were fabricated via citrate sol-gel approach. Structural and magneto-optical properties of ensembles were investigated in detail. The structural and morphological analyses revealed the formation of M-type Sr hexaferrite nanoparticles. Diffuse reflectance data were registered to estimate the direct optical energy band gaps (Eg) in a range of 1.77 eV-1.87 eV. Room temperature (RT, 300 K) and low temperature (10 K) magnetic hysteresis curves were recorded by enforcing applied dc magnetic field up to ±70 kOe. Magnetic parameters were significantly tuned due to coordination of Nb3+ and Y3+ rare earth ions. Specified magnetic data reveal the strong ferromagnetic characteristics of pristine SrFe12O19 and co-doped HFs with Nb3+ and Y3+ ions at both temperatures. RT squareness ratio (SQR) has an exception only for pristine sample as 0.506, which is in the margin of theoretical limit assigning the single-domain nature with uniaxial anisotropy. However, all co-doped samples have SQR = 0.288–0.485 values that are smaller than theoretical limit of 0.50, implying multi-domain nature at RT and at 10 K. Co-doped ions cause lowering in super-exchange interactions between different sites and resulting the decrements of intrinsic magneto-crystalline anisotropy and coercivity fields. The specified magnetic characteristics make our fabricated SrNbxYxFe12-2xO19 (0.00 ≤ x ≤ 0.05) HFs good candidates as permanent magnets applications and high-density recording media.

Anisotropic magnetocaloric effect in Fe3\u2212xGeTe2

We present a comprehensive study on anisotropic magnetocaloric porperties of the van der Waals weak-itinerant ferromagnet Fe3−xGeTe2 that features gate-tunable room-temperature ferromagnetism in few-layer device. Intrinsic magnetocrystalline anisotropy is observed to be temperature-dependent and most likely favors the long-range magnetic order in thin Fe3−xGeTe2 crsytal. The magnetic entropy change ΔSM also reveals an anisotropic characteristic between H//ab and H//c, which could be well scaled into a universal curve. The peak value -Delta {S}_{M}^{{max }} of 1.20 J kg−1 K−1 and the corresponding adiabatic temperature change ΔTad of 0.66 K are deduced from heat capacity with out-of-plane field change of 5 T. By fitting of the field-dependent parameters of -Delta {S}_{M}^{{max }} and the relative cooling power RCP, it gives -{rm{triangle }}{S}_{M}^{{max }} ∝ Hn with n = 0.603(6) and RCP ∝ Hm with m = 1.20(1) when H//c. Given the high and tunable Tc, Fe3−xGeTe2 crystals are of interest for fabricating the heterostructure-based spintronics device.

Computational study of cobalt-modified nickel-ferrite/PZT magnetoelectric composites for voltage tunable inductor applications

Control of magnetic permeability through voltage in magnetoelectric materials promises to create novel voltage tunable inductors. Domain-level phase field modeling and computer simulation are employed to study magnetoelectric composites of Co-modified NiCuZn ferrite and PZT, where ferrite outperforms metallic magnetostrictive materials at high frequencies important for inductor applications. The simulations focus on the interplay between magnetocrystalline anisotropy and stress-induced anisotropy and reveal different regimes of permeability and tunability behaviors. It is shown that the permeability and its tunability can be significantly increased by reducing the intrinsic magnetocrystalline anisotropy through doping NiCuZn-ferrite with Co2+ to form solid solution (1-x)(Ni0.6Cu0.2Zn0.2)Fe2O4-xCoFe2O4, and the optimal performance is achieved at vanishing magnetocrystalline anisotropy constant K1 = 0 corresponding to composition x∼0.02. The findings are confirmed by complementary experiments with good agreements. The effect of internal bias stress is further investigated, which is shown to shift the permeability regimes and thus provides an effective engineering technique to optimize the voltage tunable inductor performance. A tunability as high as ∼500% is obtained under the optimal condition, which is much higher than that obtained in conventional high-frequency voltage tunable inductors.

A Comparative Study of Magnetic Properties of Large Diameter Co Nanowires and Nanotubes.

A comparative study of the magnetic properties of the arrays of Co nanowires and nanotubes with large external diameters (180 nm) has been carried out. The nanowires/nanotubes were grown by electrodeposition into the self-assembled pores of anodic alumina membranes. The experimental study of their magnetic behavior was focused on the angular dependence of hysteresis loops and their parameters. In both nanowire and nanotube arrays, from the analysis of experimental data, effective longitudinal magnetic anisotropy is concluded, which is stronger in the case of the nanotube array. In addition, the extremely small remanence observed for all loops indicates the important role played by magnetostatic interactions. Micromagnetic simulations were first performed considering intrinsic shape and magnetocrystalline anisotropy terms, together with an effective easy-plane anisotropy to account for those magnetostatic interactions. A qualitative agreement between experiments and simulations is found despite the complexity introduced by the intrinsic and extrinsic array properties (i.e., large diameters, grain structure, and array configuration). In addition, simulations were also carried out for individual nanowire/nanotube with a particular emphasis to understand their differences at the remanence, due to pure geometry contribution.

Lanthanum ion substituted cobalt ferrite nanoparticles and their hyperthermia efficiency

We investigated the structural, compositional, and magnetic properties as well as the AC magnetic hyperthermic response of CoFe2-xLaxO4(x=0.0,0.2,0.5) nanoparticles. We found that the La3+ ions substituted into the Fe3+ ion sites, and resulted in an increased magnetocrystalline anisotropy with x, and altered the time-dependent magnetism. To provide a better understanding of the AC magnetic hyperthermia response, a series of temperature versus time measurements were done by varying the magnetic field amplitude, the carrier medium viscosity and the concentration of the nanoparticles as parameters that governed the heating efficiency. A decrease of specific loss power was observed with an increase of the viscosity of the carrier medium for x=0 and x=0.25 substituted Co-ferrite nanoparticles, while a small increase was observed with the x=0.1 La3+ substituted Co-ferrite nanoparticles (due to their higher intrinsic magnetocrystalline anisotropy).

Strain tuned magnetocrystalline anisotropy in ferromagnetic H-FeCl2 monolayer

For the utilization of two-dimensional materials with ferromagnetism in high density storage, it is very important to find an effective method to enhance their perpendicular magnetocrystalline anisotropy. In this paper, we investigated the impact of strain on structure and magnetism of the H-FeCl2 monolayer by employing first-principles calculations. Our results show that stressless H-FeCl2 monolayer not only is dynamically stable, but also has intrinsic ferromagnetism and perpendicular magnetocrystalline anisotropy. Under the strains ranging from −3% to 3%, the structure of the H-FeCl2 monolayer is always dynamically stable and its ground state is always ferromagnetic (FM) configuration. Interestingly, applying compressive strain can enhance the FM. More importantly, we found the compressive strain can enhance the perpendicular magnetocrystalline anisotropy of H-FeCl2 monolayer by 20.9%. Contrarily, tensile strian can make the perpendicular magnetocrystalline anisotropy decrease by 14.5%. The analysis of density of state (DOS) and the dedications to magnetocrystalline anisotropy energy (MAE) of 3d orbitals of Fe atom demonstrate that the comepressive (tensile) strain influence depends on the decrease (increase) of the negative part to MAE from the hybridization between 3dyz and 3dz2 states through spin-orbit coupling interaction. Our study indicates that applying compressive strain can effectively enhance the ferromagnetism and perpendicular magnetocrystalline anisotropy of H-FeCl2 monolayer for its application in high density data storage.

Achievement of Diverse Domain Structures in Soft Magnetic Thin Film through Adjusting Intrinsic Magnetocrystalline Anisotropy

Oriented soft magnetic hcp-Co1 − xIrx films with a fixed thickness of 120 nm were fabricated. All prepared films exhibit soft magnetic properties but various magnetocrystalline anisotropies with the variation of Ir content. The measured data shows that diverse domain structures including the Néel wall, Bloch wall, and stripe domains present in a fixed film thickness. It is singular for the single-layer soft magnetic film to possess diverse domains in a fixed thickness. This phenomenon was explained by introducing intrinsic magnetocrystalline anisotropy energy into soft magnetic films rather than the structural parameters of the film, inner stress, and microstructure effect.

Effect of Ar+ beam sputtering on the magnetic anisotropy of Fe thin films deposited on the MgO(0 0 1) substrate

The Fe thin films with different initial thickness were prepared on MgO(001) substrate. Ar+ ion beam having energy of 3keV was used to sputter the film at incident angle of 80° with respect to the film normal. Low energy electron diffraction (LEED) was employed to study the morphology and crystal quality of the film. Moreover surface magneto-optical Kerr effect (SMOKE) and anisotropic magnetoresistance (AMR) setups were used to investigate the magnetic properties of the Fe film. The values of magnetocrystalline anisotropy K1 and induced in-plane uniaxial magnetic anisotropy (UMA) Ku were derived from torque curves on the base of AMR results and found that the value of Ku was increasing with increasing the sputtering time while the intrinsic magnetocrystalline anisotropy K1 was not affected by sputtering. Similarly it is observed that the value of K1 increases with the thickness. We noted that ion beam sputtering can be used as an effective technique of manipulating the magnetic anisotropy of ultrathin magnetic films.

Influence of thickness-dependent structural evolution on ultrafast magnetization dynamics in Co2Fe0.4Mn0.6Si Heusler alloy thin films

We experimentally investigate thickness $(t)$-dependent evolution of structural and magnetic properties in $\mathrm{C}{\mathrm{o}}_{2}\mathrm{F}{\mathrm{e}}_{0.4}\mathrm{M}{\mathrm{n}}_{0.6}\mathrm{Si}$ (CFMS) thin films and correlate them with ultrafast demagnetization time $({\ensuremath{\tau}}_{\mathrm{d}})$ and relaxation time $({\ensuremath{\tau}}_{1})$ as well as the Gilbert damping coefficient ($\ensuremath{\alpha}$). Structural ordering and magnetic parameters, including $\ensuremath{\alpha}$, exhibit a nonmonotonic variation with increasing $t$. A remarkably low value of $\ensuremath{\alpha}$ of 0.009 is obtained for the CFMS film with $t=20\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$ without any buffer layers, which helps to avoid possible diffusion of the buffer layer into CFMS. Highest saturation magnetization, lowest coercivity, and the $\ensuremath{\alpha}$ value imply CFMS film with $t=20\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$ is most suitable for integrated spintronics devices, viz. low-current switched spin transfer torque, and magnetic tunnel junction with a high tunnel magnetoresistance ratio at room temperature. Despite the presence of strain, a lower degree of chemical ordering in the low-$t$ regime, and increased defect density in the high-$t$ regime, we obtained a reasonably low value of damping. In addition to the intrinsic fourfold magnetocrystalline anisotropy, an induced uniaxial anisotropy is found, which also varies nonmonotonically with $t$. Finally, unique band structure controlled demagnetization and fast relaxation in half-metallic CFMS is correlated to $\ensuremath{\alpha}$.

Enhanced magnetocrystalline anisotropy in an ultra-dense array of air-exposed crystalline cobalt nanowires

The magnetic anisotropy of an ultradense array of crystalline cobalt nanowires is investigated by means of broadband ferromagnetic resonance and magnetic torque measurements. The array is grown epitaxially in solution on a Pt(111) film and consists of single crystalline metallic wires with a diameter of 6.2 nm and a center-to-center interwire distance of 9.6 nm. The shape anisotropy and the Co hexagonal compact structure with the c-axis along the wire axis combine with each other to impose a perpendicular magnetic anisotropy despite the high density of 8 × 1012 wires/in.2. The intrinsic uniaxial magnetocrystalline anisotropy constants K1 and K2 are extracted from the ferromagnetic resonance and torque measurements using a mean field approach accounting for the interwire dipolar interactions. At room temperature, and despite air exposure, an unexpected increase of K1 and K2 of more than 40% with respect to the bulk is evidenced.

Controlled growth of large-area arrays of gadolinium-substituted cobalt ferrite nanorods by hydrothermal processing without use of any template

CoFe2−xGdxO4 nanorods were synthesized by a hydrothermal process without use of any template and surfactant. X-ray diffraction, field-emission scanning electron microscope, energy dispersive X-ray microanalyzer, and vibrating sample magnetometer were employed to evaluate structural and magnetic properties of synthesized nanorods. The X-ray diffraction analysis indicated that single phase spinel ferrites were obtained. The XRD data were processed for Rietveld refinement of structure by Reflex program. The FE-SEM micrographs of the synthesized samples showed the presence of large-area arrays of ferrite nanorods . The possible formation mechanism for the synthesis of ferrites nanorods has been preliminarily explained. Based on the EDS analysis, it was suggested that the applied process for preparation of CoFe2−xGdxO4 nanorods is a suitable method for the synthesis of spinel ferrites with homogeneity in composition. It was observed from the magnetic hysteresis loop at a room temperature that with substitutions of gadolinium cations, the coercive field increased from 590.35Oe for x=0–826.10Oe for x=0.05. It was also found that with an increase in gadolinium content, the values of saturation magnetization decreased from 72.36emu/g for x=0–53.61emu/g for x=0.05. Likewise, the demagnetizing factors of ferrite nanorods is calculated. Based on the FE-SEM micrographs, it was observed that with an increase in gadolinium content, the axial ratio c/a of nanorods decrease from 21 for x=0–13 for x=0.05. Therefore, with the substitutions of gadolinium cations, the values of demagnetizing factor along c increased from 0.006 for x=0–0.013 for x=0.05. We can say that at a maximum axial ratio of about 20, the shape-anisotropy constant Ks is lower than the first crystal-anisotropy constant K1 (neglecting K2). Consequently, we proposed that with an increase in gadolinium content, the intrinsic magnetocrystalline anisotropy dominates over the shape anisotropy and thus dictates the magnetic behavior of the system in the present work.