Uniaxial Anisotropy Field Research Articles

Widen-dynamic-range surface acoustic wave magnetic sensors with high sensitivity

The ferromagnetic resonance (FMR) driven by surface acoustic wave (SAW) has advantages in magnetic sensors due to its high sensitivity and wireless characteristics. However, the magnetic field remains limited to a small range due to the narrow field range for FMR at a fixed resonance frequency. This paper demonstrates that introducing of a small uniaxial anisotropy field (3 mT) in a Ni film can produce a wide dynamic range with high sensitivity along different magnetic field directions. This effectively widens the field range without sacrificing sensitivity, paving the path for developing the SAW magnetic sensor with both a high sensitivity and wide dynamic range.

Magnetic Stirling Cycle for Qubits with Anisotropy near the Quantum Critical Point

We studied the performance of a quantum magnetic Stirling cycle that uses a working substance composed of two entangled antiferromagnetic qubits (J) under the influence of an external magnetic field (Bz) and an uniaxial anisotropy field (K) along the total spin in the y-direction. The efficiency and work were calculated as a function of Bz and for different values of the anisotropy constant K given hot and cold reservoir temperatures. The anisotropy has been shown to extend the region of the external magnetic field in which the Stirling cycle is more efficient compared to the ideal case.

Analytical solutions to the precession relaxation of magnetization with uniaxial anisotropy

Based on the Landau–Lifshitz–Gilbert (LLG) equation, the precession relaxation of magnetization is studied when the external field H is parallel to the uniaxial anisotropic field H k. The evolution of three-component magnetization is solved analytically under the condition of H = nH k (n = 3, 1 and 0). It is found that with an increase of H or a decrease of the initial polar angle of magnetization, the relaxation time decreases and the angular frequency of magnetization increases. For comparison, the analytical solution for H k = 0 is also given. When the magnetization becomes stable, the angular frequency is proportional to the total effective field acting on the magnetization. The analytical solutions are not only conducive to the understanding of the precession relaxation of magnetization, but also can be used as a standard model to test the numerical calculation of LLG equation.

Ultrahigh in-plane uniaxial magnetic anisotropy of amorphous FeCoHf composition gradient films

Amorphous FeCoHf films with a Hf composition gradient were prepared at room temperature. The composition gradient sputtering technique was used to control the composition gradient of the doped element Hf by changing the deposition power of Hf. Interestingly, in the as-deposited FeCoHf films, an in-plane uniaxial magnetic anisotropy field of up to 640 Oe was achieved, and the resonance frequency was 6.5 GHz. The large anisotropy field can be attributed to the high sensitivity of the amorphous FeCoHf films to the composition gradient. The ultrahigh anisotropy exhibited by FeCoHf films at room temperature holds significant technological potential for various applications.

The effect of anisotropic field on magnon dispersion of antiferromagnetic metal fluoride materials

This article investigates the dynamics of spin waves in two sublattice antiferromagnetic (AFM) system spins’ interaction and magnon dispersion relation. The analysis is done starting with a standard model that includes a uniaxial magnetic anisotropy field. Quantum field theory is employed in formulating the problem, and double-time temperature-dependent Green function technique is used to obtain magnon dispersion, whereby random phase approximation is considered to decouple and diagonalize the higher order terms. At low temperatures and long wavelength approximation, dispersion of uniaxial symmetric AFM crystal lattice of metallic fluoride materials, such as MnF2, FeF2, and RbMnF3, is analyzed for illustration. Our findings indicate that as the anisotropic field increases, the magnon dispersion vs wave vector k gradually varies, attempts linearity from being curved, and attains a sinusoidal structure when extended further in the first Brillouin zone. Perhaps, these results are useful to understand the feature of magnon dispersion of stable similar AFM materials for practical application.

Магнитные свойства тонких пленок La-=SUB=-0.7-=/SUB=-Sr-=SUB=-0.3-=/SUB=-MnO-=SUB=-3-=/SUB=-, полученных магнетронным распылением

The results of studying the influence of the thickness of La0.7Sr0.3MnO3 films obtained by magnetron sputtering on (110) NdGaO3 substrates on the magnetic and crystallographic properties using ferromagnetic resonance and X-ray spectroscopy are presented. The dependences of the uniaxial and cubic anisotropy fields on the sample thickness are established. Furthermore, it is shown that the magnetic and crystallographic properties of a film obtained by magnetron sputtering strongly depend on the target region from which it is made. The results obtained will be useful for interpreting the experimental data and creating a series of samples.

Effect of the laser fluence on the microstructure and the relating magnetic properties of BaFe12O19 films grown on YSZ(111) by PLD for optimized perpendicular recording

High-quality BaFe12O19 (BaM) films with high uniaxial anisotropy fields of HA = 17.5 and 18.5 kOe were obtained by pulsed laser deposition (PLD) at two fluences of 1.5 and 5.1 J/cm2 on YSZ(111) substrate, using a platinum interlayer for reducing lattice mismatch. We demonstrated that the microstructure, morphology, and stoichiometry of the hexaferrite BaFe12O19 films can be affected by raising the corresponding energy per pulse from 25 to 75 mJ. However, we also concluded that the increase of fluence leads to the formation of a non-stoichiometric BaM film through two nucleation steps and an output growth of small grains in addition to the increase of the defect density. In turn, this has contributed to the enhancement of the coercive field from Hc = 1769 Oe to Hc = 2166 Oe as it is required for the improvement of perpendicular recording resolution. We found that both the lateral coherent block size and misorientation of mosaic blocks are remarkably affected by the growth kinetics, which itself depends on the energy per pulse. For a deep understanding of the effect of laser fluence on the microstructure, chemical composition, and on the magnetic properties of thin BaM films, the results of complementary methods are combined. These methods comprise high-resolution X-ray diffraction, atomic force microscopy, high-resolution transmission electron microscopy (TEM), scanning TEM combined with energy-dispersive X-ray spectroscopy, and vibrating sample magnetometer.Graphical abstract

Effect of external stress anisotropy on the ferromagnetic resonance properties in thin film

Effects of in-plane and out-of-plane external stress anisotropy of ferromagnetic thin film with in-plane and out-of-plane uniaxial anisotropy on the ferromagnetic resonance frequencies have been theoretically investigated by using ferromagnetic resonance method. It is shown that resonance frequency can be enhanced by increasing in-plane external stress anisotropy field and uniaxial anisotropy field, decreasing out-of-plane uniaxial anisotropy field. For out-of-plane external stress case, however, the resonance frequency and two critical fields have no significant change. Also, compared with the case of the in-plane external stress anisotropy, the FMR frequencies are smaller. Additionally, interesting features are obtained for the stress case and compared with the case of without stress. The obtained results provide further insights on the effect of external stress on the properties of ferromagnetic thin film.

Magnetocrystalline anisotropy study of Co-substituted M-type strontium hexaferrite single crystals

The study on the magnetocrystalline anisotropy (MA) of La–Co co-substituted strontium hexaferrite (La–Co SrM) shows a joint effect of Fe2+ and Co2+ ions in the enhancement of MA. Since the role of Fe2+ single ion has been studied with La-substituted strontium ferrite, in this work, Co-substituted strontium hexaferrite SrFe12-xCoxO19 (Co-SrM) single crystals were successfully grown for 0 ≤ x ≤ 0.31 by the Na2CO3 flux method to elucidate the anisotropy of Co2+ single ions. Co-substitution in this preparation condition has a limit solubility of 0.31 and enhances uniaxial magnetic anisotropy field HA by 19% for 0.03 = x ≤ 0.11, with a mere loss of 7% of saturation magnetization MS at 5 K. The enhanced HA of Co-SrM is reported for the first time, even higher than that of La–Co SrM, which is suitable to be used as permanent magnets in this concentration range. But with the further substitution of Co, the planar anisotropy of x = 0.31 was observed at 5 K. The potential nonlinear magnetic structure of Co-SrM remains to be discovered for magnetoelectric effects. This work is also of great significance as a complement to the magnetocrystalline anisotropy study of La–Co SrM.

Study of growth and broadband FMR spectroscopy in PLD grown Y3Fe5O12/Gd3Ga5O12 thin films

We report the structural and magnetization dynamics of pulsed laser ablated Y3Fe5O12 (YIG) thin films grown on gadolinium gallium garnet (GGG[110]) substrate. The co-planar waveguide ferromagnetic resonance measurement was performed to study the thickness and inhomogeneity effect on damping constant. We also estimated the effective demagnetization () value which was lower than that of single-crystalline bulk YIG. A detailed line width analysis revealed the significant two-magnon scattering and inhomogeneity contribution to damping which varied for different thickness films. The Gilbert damping constant (α) of the grown film was found to be lowest in the case of 100 nm film (≈3 × 10−4), and the corresponding inhomogeneous broadening (ΔH O) was recorded to be 77 Oe. Further angular measurements on 100 nm thin film revealed the presence of strong uniaxial magnetic anisotropy field of 735 Oe at 10 GHz RF frequency.

Strain induced anisotropy in liquid phase epitaxy grown nickel ferrite on magnesium gallate substrates

This work focuses on the nature of magnetic anisotropy in 2.5–16 micron thick films of nickel ferrite (NFO) grown by liquid phase epitaxy (LPE). The technique, ideal for rapid growth of epitaxial oxide films, was utilized for films on (100) and (110) substrates of magnesium gallate (MGO). The motivation was to investigate the dependence of the growth induced anisotropy field on film thickness since submicron films of NFO were reported to show a very high anisotropy. The films grown at 850–875 C and subsequently annealed at 1000 C were found to be epitaxial, with the out-of-plane lattice constant showing unanticipated decrease with increasing film thickness and the estimated in-plane lattice constant increasing with the film thickness. The uniaxial anisotropy field Hσ, estimated from X-ray diffraction data, ranged from 2.8–7.7 kOe with the films on (100) MGO having a higher Hσ value than for the films on (110) MGO. Ferromagnetic resonance (FMR) measurements for in-plane and out-of-plane static magnetic field were utilized to determine both the magnetocrystalline the anisotropy field H4 and the uniaxial anisotropy field Ha. Values of H4 range from −0.24 to −0.86 kOe. The uniaxial anisotropy field Ha was an order of magnitude smaller than Hσ and it decreased with increasing film thickness for NFO films on (100) MGO, but Ha increased with film thickness for films on (110) MGO substrates. These observations indicate that the origin of the induced anisotropy could be attributed to several factors including (i) strain due to mismatch in the film-substrate lattice constants, (ii) possible variations in the bond lengths and bond angles in NFO during the growth process, and (iii) the strain arising from mismatch in the thermal expansion coefficients of the film and the substrate due to the high growth and annealing temperatures involved in the LPE technique. The LPE films of NFO on MGO substrates studied in this work are of interest for use in high frequency devices.

In-plane current induced nonlinear magnetoelectric effects in single crystal films of barium hexaferrite

This report is on the observation and analysis of nonlinear magnetoelectric effects (NLME) for in-plane currents perpendicularly to the hexagonal axis in single crystals and liquid phase epitaxy grown thin films of barium hexaferrite. Measurements involved tuning of ferromagnetic resonance (FMR) at 56–58 GHz in the multidomain and single domain states in the ferrite by applying a current. Data on the shift in the resonance frequency with input electric power was utilized to estimate the variations in the magnetic parameter that showed a linear dependence on the input electric power. The NLME tensor coefficients were determined form the estimated changes in the magnetization and uniaxial anisotropy field. The estimated NLME coefficients for in-plane currents are shown to be much higher than for currents flowing along the hexagonal axis. Although the frequency shift of FMR was higher for the single domain resonance, the multi-domain configuration is preferable for device applications since it eliminates the need for a large bias magnetic field. Thus, multidomain resonance with current in the basal plane is favorable for use in electrically tunable miniature, ferrite microwave signal processing devices requiring low operating power.

Twofold and fourfold anisotropies in zinc ferrite thin films investigated by ferromagnetic resonance

By means of the ferromagnetic resonance (FMR) technique (at the $X$ band) we investigated the magnetic properties of ${\mathrm{ZnFe}}_{2}{\mathrm{O}}_{4}$ (ZFO) films grown on different substrates. Angular dependence of the FMR field showed the existence of twofold and fourfold anisotropies for the same material, grown on Si(111), MgO(100), and STO(100), respectively. By using a model that takes into consideration the relevant contributions to the energy, we were able to extract the following parameters: effective magnetization ${M}_{\mathrm{eff}}$, cubic magnetocrystalline anisotropy field, in-plane and out of plane uniaxial anisotropy fields, and the $g$ factor. The samples deposited on Si showed a twofold in-plane anisotropy while a fourfold magnetic anisotropy was observed for the ZFO/MgO and ZFO/${\mathrm{SrTiO}}_{3}$ samples. All samples showed a strong perpendicular uniaxial anisotropy above 1 kOe and low in-plane uniaxial and cubic anisotropy field, on the order of tens of oersteds. The FMR linewidth ($\mathrm{\ensuremath{\Delta}}H$) fitted from FMR data shows $\mathrm{\ensuremath{\Delta}}H$ values above 190 Oe for all samples, indicating high damping of this material to be $\ensuremath{\sim}{10}^{\ensuremath{-}2}$ for the ZFO/MgO. The present analysis compared the FMR data obtained for the in-plane and out of plane measurements. Likewise, the effective magnetization and perpendicular uniaxial anisotropy field by FMR were compared with the magnetic response obtained by a vibrating sample magnetometer.

Theoretical investigation of ferromagnetic resonance in a ferromagnetic thin film with external stress anisotropy

We use the ferromagnetic resonance (FMR) method to study the properties of ferromagnetic thin film, in which external stress anisotropy, fourfold anisotropy and uniaxial anisotropy are considered. The analytical expressions of FMR frequency, linewidth and the imaginary part of magnetic susceptibility are obtained. Our results reveal that the FMR frequency and the imaginary part of magnetic susceptibility are distinctly enhanced, and the frequency linewidth or field linewidth are broadened due to a strong external stress anisotropy field. The hard-axis and easy-axis components of magnetization can be tuned significantly by controlling the intensity and direction of stress and the in-plane uniaxial anisotropy field.

Anisotropic Magnon–Magnon Coupling in Synthetic AntiferromagnetsSupported by the National Natural Science Foundation of China (Grant Nos. 51871235, 51671212, 52031014, 51771198, and 51801212), the National Key Research and Development Program of China (Grant Nos. 2016YFA0300701, 2017YFB0702702, and 2017YA0206302), and the Key Research Program of Frontier Sciences, CAS (Grant Nos. QYZDJ-SSW-JSC023, KJZD-SW-M01, and ZDYZ2012-2). J.L. acknowledges support from the Natural Science Foundation

Magnon–magnon coupling in synthetic antiferromagnets advances it as hybrid magnonic systems to explore the quantum information technologies. To induce magnon–magnon coupling, the parity symmetry between two magnetization needs to be broken. Here we experimentally demonstrate a convenient method to break the parity symmetry by the asymmetric structure. We successfully introduce a magnon–magnon coupling in Ir-based synthetic antiferromagnets CoFeB(10 nm)/Ir(t Ir = 0.6 nm, 1.2 nm)/CoFeB(13 nm). Remarkably, we find that the weakly uniaxial anisotropy field (∼ 20 Oe) makes the magnon–magnon coupling anisotropic. The coupling strength presented by a characteristic anticrossing gap varies in the range between 0.54 GHz and 0.90 GHz for t Ir = 0.6 nm, and between 0.09 GHz and 1.4 GHz for t Ir = 1.2 nm. Our results demonstrate a feasible way to induce magnon–magnon coupling by an asymmetric structure and tune the coupling strength by varying the direction of in-plane magnetic field. The magnon–magnon coupling in this highly tunable material system could open exciting perspectives for exploring quantum-mechanical coupling phenomena.

Nonlinear magnetoelectric effects in Al-substituted strontium hexaferrite

This report is on the observation and theory of electric field E induced non-linear magnetoelectric (NLME) effects in single crystal platelets of ferrimagnetic M-type strontium aluminum hexagonal ferrite. Using microwave measurement techniques, it was found that a DC electric field along the hexagonal c-axis results in significant changes in the saturation magnetization and uniaxial magneto-crystalline anisotropy field and these changes are proportional to the square of the applied static electric field. The NLME effects were present with or without an external bias magnetic field. The E-induced variation in magnetic order parameters is attributed to weakening of magnetic exchange and spin–orbit interactions since conduction electrons in the ferrite are effectively excluded from both interactions while being in transit from one Fe ion to another. We present a phenomenological theory which considers magneto-bielectric effects characterized by a quadratic term in electric field E in the free energy density. The coefficients for the NLME coupling terms have been calculated from experimental data and they do show variations with the Al substitution level and the largest rates of change of the saturation magnetization and anisotropy constant change with the applied power were observed for x = 0.4. It was also clear from the study that strength of the NLME effect does not depend on the amount Al substitution, but critically depends on the electrical conductivity of the sample with the highest NLME coefficients estimated for the sample with the highest conductivity. Results of this work are of importance for a new family of electric field tunable, miniature, high frequency ferrite devices.

Ferromagnetic Resonance and Elastic Vibrations in Epitaxial Yttrium Iron Garnet Films

The ferromagnetic resonance (FMR) spectra of epitaxial yttrium iron garnet films are experimentally studied. The field dependences of the FR frequency are used to determine the effective fields induced by the first and second anisotropy constants and the effective uniaxial anisotropy field. The FMR frequency and linewidth are found to increase in the fields lower than 8 Oe applied in the bismuth-containing film plane. This increase is related to the formation of a nonuniform magnetization distribution, which is supported by a direct magnetooptical observation. The FMR spectra are found to be modulated by a set of narrow lines, the amplitude of which is proportional to the FMR signal intensity. The frequency position of the lines is independent of the magnitude and direction of a dc magnetic field. This modulation is shown to be associated with the resonance excitation of transverse elastic vibration modes. The effective magnetoelastic coupling parameter and the linewidth of the elastic vibrations excited by resonance pumping are estimated.

Evidence for strain control of magnetic anisotropy in epitaxial nickel ferrite thin films grown on strontium titanate substrates

This report is on strain control of magnetic order parameters in 50 nm epitaxial NiFe2O4 (NFO) films grown on (001), (110), and (111) SrTiO3 (STO) single-crystal substrates by pulsed laser deposition. The related in-plane strains for NFO films on (001), (110), and (111) STO substrates are found to be ―1.36 %, 1.35 %, and 1.52 %, respectively, where the lattice mismatch between film and substrate is as high as 6.7 %. Our analysis of magnetization and ferromagnetic resonance on all the NFO films reveals the presence of a uniaxial anisotropy field, which is perpendicular to the film plane. The anisotropy field decreases with the increase of strain in NFO films. Specifically, NFO films on STO (111) and STO (001) have the lowest and highest uniaxial anisotropy fileds of 1.4 kOe and 6.1 kOe, respectively. These experimental values of anisotropy field are a factor of 5–10 times smaller than those of the calculated ones due to the film defects induced by the high lattice mismatch. The studies of magnetic anisotropy of spinel ferrite films are of interest for spintronics and microwave devices.

Effects of substrate annealing on uniaxial magnetic anisotropy and ferromagnetic resonance frequency of Ni80Fe20 films deposited on self-organized periodically rippled sapphire substrates

Ni80Fe20 (NiFe) films have been deposited on self-organized sapphire substrates with periodical surface ripples formed by 2–10 h thermal annealing at 1400°C. By extracting static and dynamic magnetic parameters from magneto-optical Kerr effect (MOKE) loops and ferromagnetic resonance (FMR) spectra, we quantified and analyzed the substrate annealing effects on the induced in-plane uniaxial magnetic anisotropy and ferromagnetic resonance frequency in these films. The study demonstrates that the uniaxial anisotropy of these NiFe films is strongly related to the annealing-time dependent substrate faceting and coarsening mechanisms. The uniaxial anisotropy field Hu changes from ~170 Oe to ~250Oe, and the maximum value of it is obtained in film on 8-h annealed substrate. The ferromagnetic resonance response and high frequency characteristics of films can be modulated by self-organized substrate is also confirmed. When external magnetic field is parallel to the direction of ripple, the ferromagnetic resonance frequency of films grown on the self-organized substrates is increased by ~0.3−1 GHz compared with that of reference film on plane substrate. This work would be helpful to design the high frequency properties of spintronic devices and micro-wave magnetic devices by utilizing self-organized substrates.

Crystal field model simulations of magnetic response of pairs, triplets and quartets of Mn3+ ions in GaN

A ferromagnetic coupling between localized Mn spins was predicted in a series of ab initio and tight binding calculations and experimentally verified for the dilute magnetic semiconductor Ga1−x Mn x N. In the limit of small Mn concentrations, x ≲ 0.01, the paramagnetic properties of this material were successfully described using a single ion crystal field model approach. In order to obtain the description of magnetization in (Ga,Mn)N in the presence of interacting magnetic centers, we extend the previous model of a single substitutional Mn3+ ion in GaN by considering pairs, triplets and quartets of Mn3+ ions coupled by a ferromagnetic superexchange interaction. Using this approach we investigate how the magnetic properties, particularly the magnitude of the uniaxial anisotropy field, change as the number of magnetic Mn3+ ions in a given cluster increases from 1 to 4. Our simulations are then exploited in explaining experimental magnetic properties of Ga1−x Mn x N with x ≅ 0.03, where the presence of small magnetic clusters gains in significance. As a result the approximate lower and upper limits for the values of exchange couplings between Mn3+ ions in GaN, being in nearest neighbors (nns) J nn and next nns J nnn positions, respectively, are established.