Ferrimagnetic Resonance Linewidth Research Articles

An exotic uniaxial hexagonal ferrite with narrow self-biased ferrimagnetic resonance linewidths: Cu 18H hexaferrite

Traditional circulators utilize ferrites with low magnetocrystalline anisotropy driven by external magnetic bias fields. Hexagonal ferrites with a strong uniaxial anisotropy enable self-biased circulators, significantly reducing their volume, weight, and cost. Here, a unique uniaxial hexaferrite, Cu 18H hexaferrite, with narrow ferrimagnetic resonance (FMR) linewidth at zero magnetic bias field is reported. The temperature-dependent magnetic hysteresis loops show a significant decrease in coercivity from cryogenic to room temperature, indicating a transition from the hard to soft magnetic state. The crystallographic texture ensures a squareness ratio greater than 0.65 and leads to a strong internal bias field induced by the aligned easy-magnetization axes. Owing to the highly dense and oriented crystallites, the zero-field FMR exhibits excellent linewidths of ∼270–320 Oe, which are lower than those of most hexaferrites reported at the Ka-band. These results reveal the great potential of Cu 18H hexaferrite as a low-loss microwave ferrite used for self-biased circulators.

Synergetic effect of site-controlled two-step Ca doping on magnetic and electrical properties of M-type strontium hexaferrites

M-type strontium (SrM) hexaferrites exhibiting large magnetocrystalline anisotropy, high saturation magnetization and coercivity, and narrow ferrimagnetic resonance (FMR) linewidth have a great potential for applications in high-performance motors and self-biased circulators. Ca doping in SrM hexaferrites is an effective method for tailoring the intrinsic electromagnetic parameters and modifying polycrystalline microstructure to meet the application requirements. However, the regulation mechanism of Ca doping is still obscure, and its regulation effect is not optimized. Here, the magnetic and electrical properties of SrM hexaferrites with site-controlled two-step Ca doping are reported. The spatial concentration distribution and diffusion dynamics of Ca2+ ions are investigated when introduced by CaCO3 as the reactant and sintering additive, respectively. The existence of the Ca2+ ions in the crystallites added as the reactant reduces the diffusion of the Ca2+ ions in the grain boundaries added as the additive, which synergistically tailors the magnetocrystalline anisotropy field, crystallite orientation, and polycrystalline morphology, and further optimizes the coercivity, remanence, and maximum energy product. Moreover, the impact of the Ca2+ ions on the resistivities of the grain and grain boundary is also investigated, revealing the synergetic effect of Ca2+ ions at different sites on lowering the loss. These results prove the effectiveness of the site-controlled two-step Ca doping in synergistically enhancing the magnetic and electrical properties of SrM hexaferrites.

Vacancy-engineered Gd3+-substituted yttrium iron garnet with narrow ferrimagnetic linewidth and high Curie temperature

With the urgent need for high-power low-loss microwave systems, next-generation wireless communications call for high-performance yttrium iron garnet with narrow ferrimagnetic resonance (FMR) linewidth, large spin-wave linewidth, and high Curie temperature, which can be tailored by cation substitution and vacancy engineering. Here, the electromagnetic properties of vacancy-engineered Gd3+-substituted garnet ferrites are reported. Cation vacancies on the dodecahedral sites of the garnet crystal structure are introduced by deficient Gd3+ substitution for the Y3+ ions. The polycrystalline microstructure, magnetic properties, and electrical properties of both the Gd3+-sufficient and Gd3+-deficient samples are comprehensively examined and compared. The deficient Gd3+ substitution promotes the densification process and grain growth during the sintering stage. Without detriment to the spin-wave linewidth and electrical resistivity, the vacancies are beneficial to reducing the FMR linewidth and increasing the Curie temperature. With appropriate vacancy concentrations, the garnet ferrite samples exhibit a 58% reduction in the FMR linewidth and a 13% increase in the Curie temperature. These results reveal the great potential of the vacancy-engineered Gd3+-substituted garnet ferrites for high-power low-loss microwave communication systems.

Barium hexaferrites with narrow ferrimagnetic resonance linewidth tailored by site‐controlled Cu doping

AbstractBarium hexagonal ferrites exhibiting unique self‐biasing characteristics have great potential for application in planar microwave devices, such as self‐biased circulators. Cu doping is an effective method to tailor their anisotropy field and ferrimagnetic resonance (FMR) linewidth to meet the requirements for low‐frequency low‐loss microwave devices. However, the regulation mechanism of Cu doping is still obscure, and its regulation effect is not optimized. Here, the magnetic and microwave properties of two groups of barium hexaferrites with site‐controlled Cu doping are reported. The diffusion dynamics of Cu2+ ions are comprehensively investigated, revealing significant differences in the concentration distribution and polycrystalline morphology, when comparing CuO as a reactant and sintering additive. The accumulation of Cu2+ ions at grain boundaries contributes to the increase in coercivity, whereas the dispersion of Cu2+ ions in crystallites leads to the decrease in the anisotropy field. Moreover, by introducing Cu2+ ions into the interstitial positions of the lattice, barium hexaferrites with a narrow FMR linewidth of 303 Oe and a high remanence ratio of 0.82 are achieved. These results represent the lowest FMR linewidth reported in polycrystalline hexagonal ferrites and prove the great technological value and commercial potential of Cu‐doped barium hexaferrites for next‐generation planar microwave devices.

Iron‐depleted Bi‐YIG having enhanced gyromagnetic properties suitable for LTCC processing

AbstractThe traditional method of ferrite synthesis is based upon solid‐state reaction. The advantages of this approach include simplicity, cost effectiveness, and suitability for commercial process scaling. However, there exist shortcomings in that impurities and defects adversely affect magnetic properties. Here, iron‐depleted compositions of bismuth‐substituted yttrium iron garnet (ie Bi:YIG) were studied with emphasis placed on the impact of synthesis processing parameters upon the structure and gyromagnetic properties. It was shown that ferrimagnetic resonance linewidth experienced a minimum of 180.3 Oe (X‐band) when the sample stoichiometry was 4% depleted in iron (x = 0.2), a reduction of 29.3% from the parent compound. Concomitantly, iron depletion caused a systematic change in lattice parameter that resulted in a dilation of Goodenough‐Kanamori‐Anderson bond angles increasing exchange energy and magnetization. The substitution of Bi for Y on the dodecahedron sites altered the reaction path and decreased the activation energy effectively reducing sintering temperature. To establish a functional processing protocol, an ion diffusion model is presented to explain the demonstrated processing, structure, and performance paradigm.

Significant reduction in spin pumping efficiency in a platinum/yttrium iron garnet bilayer at low temperature

The temperature evolution of a direct-current electromotive force (EMF) generated by spin pumping and the inverse-spin Hall effect in a platinum (Pt)/yttrium iron garnet (YIG) bilayer was investigated down to 80 K. The magnitude of the EMF decreased significantly with decreasing temperature and disappeared at approximately 80 K. 40-nm-thick YIG films fabricated by a metal organic decomposition method exhibited single-peak ferrimagnetic resonance (FMR) spectra without any spin wave resonance, which allowed us to precisely analyze the FMR spectra. We determined that the temperature evolution of the Gilbert damping constant is the dominant factor in the temperature dependence of the EMF. The comparison of the FMR linewidth between the X- and Q-bands revealed that an increase in Gilbert damping constant at low temperatures is not due to the enhancement of the spin pumping efficiency but due to an additional spin relaxation in the YIG film itself, which reduces the precession angle of the magnetization under the FMR conditions.

Large magneto-optic enhancement in ultra-thin liquid-phase-epitaxy iron garnet films

Significant departures from bulk-like magneto-optic behavior are found in ultra-thin bismuth-substituted iron-garnet films grown by liquid-phase-epitaxy. These changes are due, at least in part, to geometrical factors and not to departures from bulk-composition in the transient layer at the film-substrate interface. A monotonic increase in specific Faraday rotation with reduced thickness is the signature feature of the observed phenomena. These are traced to size-dependent modifications in the diamagnetic transition processes responsible for the Faraday rotation. These processes correspond to the electronic transitions from singlet 6S ground states to spin-orbit split excited states of the Fe3+ ions in the garnet. A measurable reduction in the corresponding ferrimagnetic resonance linewidths is found, thus pointing to an increase in electronic relaxation times and longer lived excitations at reduced thicknesses. These changes together with a shift in vibrational frequency of the Bi-O bonds in the garnet at reduced thicknesses result in greatly enhanced magneto-optical performance. These studies were conducted on epitaxial monocrystalline Bi0.8Gd0.2Lu2Fe5O12 films.

The Magnetoelastic Ion: Friend and Foe to Microwaves

From the earliest years of ferrite development, spin-lattice interactions from select transition-metal (3dn) and rare-earth (4fn) ions have influenced key parameters of microwave device performance. Hysteresis loop squareness and coercivity, and magnetic loss that is related to the ferrimagnetic resonance linewidth will continue to depend on cautious inclusion of ions of the 3dn and 4fn series. As the promise of films and layers for planar devices are made more realizable by rapidly advancing deposition technologies, new challenges appear from the presence of stress imparted by mismatches at film/substrate interfaces. To offset unwanted effects of bias strains, increased use of magnetoelastic ions will become necessary. An additional feature made possible by deposition processing at temperatures lower than those of bulk ceramic sintering or melt-grown single crystals is the stabilization of cation valence states that do not normally occur in the thermodynamic equilibrium of conventional bulk ceramic processing. In the future that will undoubtedly involve structures of steadily decreasing dimensions, ferrite applications will have to account for surface compositional variations and interface stress effects. To this end, this article examines the physics that underlies the possible advantages and potential hazards to microwave technologies of the future, where magnetoelastic 3dn ions Mn3+, Fe4+, Fe2+, Co4+, Co2+, and Cu2+, as well as other ions such as Ho3+ or Dy3+ of the 4fn shell can influence the microwave performance.

Growth and characterization of 144 μm thick barium ferrite single crystalline film for microwave device application

Liquid phase epitaxy technique was used to grow 144 μm thick barium ferrite (BaFe12O19; BaM) single crystalline films on (111) Gd3Ga5O12 substrate. The growth rate of 72 μm/h was achieved with a flux system of Fe2O3–BaCO3–Na2CO3. The grown BaM films show single crystalline (000l) orientation that was confirmed by x-ray diffraction and magnetic torque curves. The saturation magnetization (4πMs) and the anisotropy field (Hk) were found to be 4.2 kG and 16.0 kOe, respectively. The ferrimagnetic resonance linewidth (ΔH) at 35 GHz was measured to be 0.1 kOe.

Ferrite-Coupled Line Circulator Simulations For Application at X-Band Frequency

We have designed and simulated a circulator circuit in which the magnetization aligns along the plane of an yttrium iron garnet (YIG) film. For an X-band frequency (8-12 GHz) circulator we have utilized a YIG slab (~200 mum thick) with saturation magnetization (4piMS ) and ferrimagnetic resonance (FMR) linewidth (DeltaH) of ~139.26 kA/m and 10 Oe, respectively. Broadband circulator operation was realized for frequencies above FMR, f=5 GHz. The applied FMR field was 79.58 kA/m. The Ansoft HFSS software suite was used to simulate the circulator response. The insertion loss S21 and the isolation S12 were calculated to be ~0.9 dB and ~-52 dB, respectively, with ~15% bandwidth at the center frequency of 10.1 GHz. We believe that this in-plane circulator design may enable high performance with significant volume and weight reduction

The effect of SrTiO3 addition on the magnetic and microwave properties of yttrium iron garnet

The effect of SrTiO3 addition on the microstructure and magnetic properties of yttrium iron garnet prepared by a conventional ceramic method was investigated using scanning electron microscopy, X-ray diffractometry, vibrating sample magnetometry, and ferrimagnetic resonance experiments. A YIG specimen sintered at 1693 K with 0.2 mol% SrTiO3 addition showed above 98% densification of theoretical density without changing in magnetic properties of YIG. A YIG specimen sintered at 1693 K with 0.2 mol% SrTiO3 showed ferrimagnetic resonance linewidth ΔH of about 80 Oe.

Microwave and magnetic properties of self-biased barium hexaferrite screen printed thick films

The interest in barium hexaferrite thick films, particularly with high remanent magnetization, is driven by the development of small planar ferrite microwave devices. We report here processing and microwave characterization of BaFe12O19 ferrite thick films (100–400μm). The films were deposited on silicon and alumina substrates by screen printing, oriented under a magnetic field of 8kOe, then annealed at 250°C and sintered at temperatures ranging from 850to1300°C. Scanning electron microscopy and x-ray diffraction exhibited strong crystallographic alignment of c-axis crystals perpendicular to the film plane. The magnetization measurement indicated that a typical dense film with 270μm thickness yielded a high squareness (Mr∕Ms) of 0.93. Ferrimagnetic resonance (FMR) measurements were performed in the frequency range of 40–55GHz. From the linear dependence of FMR frequency on the external field, a g factor of 2.03±0.08 was deduced, while the smallest linewidth was obtained to be 1.2kOe at 40GHz. The broadening of the FMR linewidth arises predominantly from magnetic inhomogeneities in the polycrystalline films. The thick films have great potential for use in future microwave and millimeter wave monolithic integrated circuits.

Thick M-type barium hexaferrite films grown on garnet substrates

The deposition and characterization of thick M-type barium hexaferrite (BaFe12O19) films produced by a modified liquid phase epitaxy deposition technique are reported. The films are deposited on (111) oriented single crystal garnet (Gd3Ga5O12) substrates. The thickness of the films ranged between 45 and 80μm with growth rates of up to 40μm∕h. This growth rate is about five times greater than films grown on substrates of (111) magnesium oxide. Although the films are relatively thick, the ferrimagnetic resonance linewidth is remarkably narrow (∼50Oe) for external fields applied along the c axis.

Preparation of high-quality hexaferrite thick films by an improved liquid phase epitaxy deposition technique

In this paper, we report on the deposition of thick and high-quality films of BaM on (111) gadolinium gallium garnet (GGG) and m-plane, (11_00) or (101_0), sapphire (Al/sub 2/O/sub 3/) substrates. The deposition rate is sufficiently high to allow for large batch production. The total thickness ranged from 50 to 200 /spl mu/m for 2 h of the liquid phase epitaxy deposition. Therefore, the growth rate of the films ranged from /spl sim/25 to /spl sim/100 /spl mu/m/h. In previous work, growth rates in BaM films were quite small. Equally important, the ferrimagnetic resonance (FMR) linewidth (/spl Delta/H) was /spl sim/0.068 kOe at 58 GHz for the BaM films on (111) GGG and /spl sim/0.08 kOe at 59.9 GHz for the BaM film on m-plane sapphire substrates. The FMR linewidth films of thick BaM films deposited by pulsed laser deposition in earlier work ranged from 0.5 to 1.2 kOe.

Microwave and magnetic properties of barium hexaferrite films having the c-axis in the film plane by liquid phase epitaxy technique

We report the growth of thick barium hexaferrite (BaFe12O19,BaM) films on m-planes ([11_00] or [101_0]) sapphire (Al2O3) substrates by the liquid phase epitaxy deposition technique. The procedure entailed the deposition of seed layers of BaFe12O19 onto the substrate by the pulsed laser ablation deposition and then dipping the substrate into a molten flux for 2 h. The total thickness ranged from 60 μm to 200 μm. The vibrating sample magnetometer (VSM) measurement data showed that the film exhibited magnetic uniaxial anisotropy axis in the film plane. The coercive field was relatively small, range of Hc was from 0.007 kOe to 0.08 kOe, and the saturation magnetization (4πMs)≅4.42 kG. The ferrimagnetic resonance (FMR) linewidth (ΔH) at 59.9 GHz was ∼0.08 kOe. This value of ΔH should be contrasted with previous FMR linewidth measurement of films deposited by the PLD technique which showed a ΔH of ∼1.20 kOe.

Magnetization behavior of scandium-substituted barium hexaferrite films having uniaxial axis in the film plane

Highly oriented films of ( 1 0 1 ̄ 0 ) BaSc 0.6Fe 11.4O 19 having the magnetic easy axis (crystallographic [0 0 0 1] axis) in the film plane were deposited by pulsed laser deposition onto a-plane ( 1 1 2 ̄ 0 ) sapphire (Al 2O 3) substrates. Magnetometry and torque magnetometry measurements showed in-plane permanent magnet (magnetic dipole) behavior below a coercive field value of 450 Oe, while high field measurements yielded a uniaxial anisotropy field value of 10 kOe. Measurements at temperatures from 123 to 650 K showed the uniaxial anisotropy field was insensitive to temperature for T<400 K, while the coercive field had a maximum value near 525 K before declining to zero at T C=638 K. This temperature dependence was modeled by an empirical Brown's-type equation for T⩽525 K, where the fitting parameters indicated the presence of surprisingly large effective local demagnetizing fields. The film microstructure is inferred to significantly affect the magnetization behavior, and is also presumed to cause significant ferrimagnetic resonance linewidth broadening compared to BaFe 12O 19 (0 0 0 1) films.

Microwave and magnetic properties of double-sided hexaferrite films on (111) magnesium oxide substrates

We have deposited hexaferrite films of BaFe12O19 on both sides of a magnesium oxide, (111) MgO, substrate by the liquid phase epitaxy (LPE) deposition technique. The procedure entailed the deposition of seed layers of BaFe12O19 onto the substrate by laser ablation deposition and then dipping of the substrate into a molten flux. Whereas the seed layers were the order of 0.5 μm, the total thickness after LPE deposition was ∼45 μm. The ferrimagnetic resonance linewidth at 60 GHz, 27 Oe, is as low as it has ever been measured before in BaFe12O19 to the best of our knowledge. The static field measurements of the saturation magnetization and anisotropy field were in agreement with previous measured bulk values.

Variation in material parameters for hexagonal barium ferrite films on (111) magnesium oxide as a function of oxygen pressure

This work examines the material characteristics and magnetic properties of hexagonal M-type barium ferrite (BaFe12O19) films deposited by pulsed laser ablation deposition onto (111) magnesium oxide substrates at different oxygen growth pressures. X-ray diffraction data showed the lattice constant of the film grown at 300 mTorr c=23.23 Å increased to c=23.30 Å for the film grown at 20 mTorr. The saturation magnetization ranged from 4.03 to 4.33 kG and the magnetic uniaxial anisotropy field ranged from 16.73 to 19.49 kOe as the oxygen pressure was varied from 10 to 300 mTorr. The narrowest ferrimagnetic resonance (FMR) linewidths (ΔH) for as-produced films were obtained for the 10 mTorr films, where ΔH=0.39 kOe. Films grown with oxygen pressure at 200 mTorr or higher showed two distinct resonance modes, which exhibited ΔH1⩾0.90 kOe and ΔH2⩾1.10 kOe at 59 GHz for a film deposited at 925 °C. The FMR linewidths for this film decreased to ΔH1=0.52 kOe and ΔH2=0.45 kOe after annealing the film at 1000 °C for 1 h. These results indicate that thick films grown at higher oxygen pressures have magnetic parameters closer to bulk values, but also show larger magnetic losses than films grown at lower oxygen pressures.

Oriented Y-type hexaferrites for ferrite device

We have developed a technique to orient particles of Y-type hexaferrite in which the c axis of the particles is oriented perpendicular to a plane. Disks of oriented Ba2MnZnFe12O22 particles were characterized in static and dynamic field excitations and exhibited the following properties: 4πMs=2300 G, Ha=9500 G, 4πMeff=4πMs+Ha=11800 G, and Hc=60 Oe. The ferrimagnetic resonance linewidth at 27 GHz measured to be 350 Oe and the g factor approximately equal to 2. The magnetic parameters of the oriented particles were very similar to the bulk parameters previously published.

Ferrimagnetic resonance of manganese ferrites with iron excess

Ferrimagnetic resonance linewidths of polycrystalline and single crystal manganese ferrites, MnxFe3-xO4 with x<1.0, were measured at 8.9 GHz in the range 77–320 K. Conductivity measurements were carried out from 100 Hz to 40 MHz. The behavior of the ferrimagnetic resonance linewidth and of the resonance field was different from that obtained for manganese ferrites with x=1. These differences are attributed to the presence of a resonance mode in addition to the uniform resonance mode. The variation of the skin depth as a function of the temperature and the high conductivity of the samples is responsible for the appearance of this extra mode. This assumption is corroborated by the analysis of the asymmetry of the absorption curve.