Ferromagnetic Resonance Frequency Shift Research Articles

Magnetic anisotropy and GGG substrate stray field in YIG films down to millikelvin temperatures

Quantum magnonics investigates the quantum-mechanical properties of magnons, such as quantum coherence or entanglement for solid-state quantum information technologies at the nanoscale. The most promising material for quantum magnonics is the ferrimagnetic yttrium iron garnet (YIG), which hosts magnons with the longest lifetimes. YIG films of the highest quality are grown on a paramagnetic gadolinium gallium garnet (GGG) substrate. The literature has reported that ferromagnetic resonance (FMR) frequencies of YIG/GGG decrease at temperatures below 50 K despite the increase in YIG magnetization. We investigated a 97 nm-thick YIG film grown on 500 μm-thick GGG substrate through a series of experiments conducted at temperatures as low as 30 mK, and using both analytical and numerical methods. Our findings suggest that the primary factor contributing to the FMR frequency shift is the stray magnetic field created by the partially magnetized GGG substrate. This stray field is antiparallel to the applied external field and is highly inhomogeneous, reaching up to 40 mT in the center of the sample. At temperatures below 500 mK, the GGG field exhibits a saturation that cannot be described by the standard Brillouin function for a paramagnet. Including the calculated GGG field in the analysis of the FMR frequency versus temperature dependence allowed the determination of the cubic and uniaxial anisotropies. We find that the total crystallographic anisotropy increases more than three times with the decrease in temperature down to 2 K. Our findings enable accurate predictions of the YIG/GGG magnetic systems behavior at low and ultralow millikelvin temperatures, crucial for developing quantum magnonic devices.

Giant demagnetization effects induced by superconducting films

We show that a ferromagnetic (F) slab with the in-plane magnetization sandwiched between two superconducting (S) films experiences strong demagnetization effect due to the Meissner screening of the stray magnetic field by superconductors. In the extreme case, the transition of the S film from normal to the superconducting state can switch the demagnetization factor from 0 to 1, which is in a sharp contrast with the S/F bilayers where such transition affects the magnetic field inside the F film only slightly. The giant demagnetization effect is shown to be qualitatively robust against the decreasing superconducting film thickness and may provide a hint toward the explanation of the anomalously large ferromagnetic resonance frequency shift recently observed for the S/F/S structures [Golovchanskiy et al., Phys. Rev. Appl. 14, 024086 (2020)].

Tuning of electromagnetic wave absorbing properties in Fe-deficient SrFe9.6-xCo1.2Ti1.2O19 hexaferrite-epoxy composites

We report the tunable electromagnetic (EM) wave absorption properties of Fe-deficient SrFe9.6-xCo1.2Ti1.2O19 hexaferrite–epoxy composites. SrFe9.6-xCo1.2Ti1.2O19 hexaferrite powders were prepared via solid-state reaction routes. It was observed that Sr–Ti-rich second phases were formed as x increased, i.e., the Fe content decreased. The ferromagnetic resonance (FMR) frequency of the composites gradually decreased from 8.8 GHz to 4.8 GHz with increasing x, and accordingly, the EM absorption frequency range also gradually changed. The gradual FMR frequency shift was attributed to the compositional shift in the mother phase. It is predicted that the Fe deficiency caused a decrease in the magnetocrystalline anisotropy, and in turn, it shifted the FMR frequency and modified the corresponding EM absorbing properties. All the samples demonstrated a high EM absorption performance with the lowest reflection loss of < −40 dB at the optimized frequency and thickness.

Tunable ferromagnetic resonance in La-Co substituted barium hexaferrites at millimeter wave frequencies

Transmittance measurements have been performed on La-Co substituted barium hexaferrites in millimeter waves. Broadband millimeter-wave measurements have been carried out using the free space quasi-optical spectrometer, equipped with a set of high power backward wave oscillators covering the frequency range of 30 – 120 GHz. Strong absorption zones have been observed in the millimeter-wave transmittance spectra of all La-Co substituted barium hexaferrites due to the ferromagnetic resonance. Linear shift of ferromagnetic resonance frequency as functions of La-Co substitutions have been found. Real and imaginary parts of dielectric permittivity of La-Co substituted barium hexaferrites have been calculated using the analysis of recorded high precision transmittance spectra. Frequency dependences of magnetic permeability of La-Co substituted barium hexaferrites, as well as saturation magnetization and anisotropy field have been determined based on Schlömann’s theory for partially magnetized ferrites. La-Co substituted barium hexaferrites have been further investigated by DC magnetization to assess magnetic behavior and compare with millimeter wave data. Consistency of saturation magnetization determined independently by both millimeter wave absorption and DC magnetization have been found for all La-Co substituted barium hexaferrites. These materials seem to be quite promising as tunable millimeter wave absorbers, filters, circulators, based on the adjusting of their substitution parameters.

Ferromagnetic resonance frequency shift model of laminated magnetoelectric structure tuned by electric field

Based on Smith–Beljers theory and classical laminate theory, an explicit model is proposed for the ferromagnetic resonance (FMR) frequency shift of a stress-mediumed laminated magnetoelectric structure tuned by an electric field. This model can effectively predict the experimental phenomenon that the FMR frequency increases under a parallel magnetic field and decreases under a perpendicular magnetic field when the electric field ranges from −10 kV/m to 10 kV/m. Besides, this theory further shows that the FMR frequency increases monotonically as the angle between the direction of the external magnetic field and the outside normal direction of the laminated structure increases, and the frequency will increase as great as 7 GHz. In addition, when the angle reaches a certain critical value, the external electric field fails to tune the FMR frequency. When the angle is above the critical value, the increase of the electric field induces the FMR frequency to increase, and the opposite scenario happens when it is below the critical value. When the angle is 90° (parallel magnetic field), the FMR frequency is the most sensitive to the change of the electric field.

Theoretical Model of Electric Field Tunable FMR Frequency of Magnetoelectric Tri-Layered Structure

To study the magnetic-electrical-mechanical coupling mechanism of microwave ME (magnetoelectric) tri-layered structures, we proposed a theoretical model of electric tunable FMR (Ferromagnetic Resonance) frequency shift for bias magnetic field in different directions through the theory of Smith-Beljers and free energy density of ferrite. A deformation produced by the applied electric field called strain could be obtained through the theory of classical laminated plate. This model effectively predicts the stress of laminated structure increases when the piezoelectric coefficient increases, the shift of electric field tunable FMR frequency is more obvious when saturation magnetization and magnetostriction coefficient of ferrite increase. Moreover, it qualitatively explains the experimental phenomena that the directions of FMR frequency shift are opposite when apply the in-plane and out-of-plane magnetic field respectively, and provides a theoretical basis for electric field and magnetic field dual tunable microwave devices.

Strain-mediated magnetoelectric coupling in magnetostrictive/piezoelectric heterostructures and resulting high-frequency effects

Magnetoelectric coupling terms are derived in piezoelectric/magnetostrictive (multiferroic) thin film heterostructures using Landau-Ginzburg free energy expansions in terms of strain and by considering strain boundary conditions between the two materials. Then, a general effective medium method for solving for the complete electromagnetic susceptibility tensor of such heterostructures is used to calculate the ferromagnetic resonance frequency in a BaTiO$_3$/NiFe$_2$O$_4$ superlattice. This method differs from existing methods for treating magnetoelectric heterostructures since the magnetic and electric dipolar fields are not assumed constant but vary from one film to another. The ferromagnetic resonance frequency shift is calculated as a function of applied electric field and is compared to some experimental results.

Modified Composition of Cobalt Ferrite as Microwave Absorber in X-Band Frequencies

The magnetic and electromagnetic wave absorbing properties were investigated in the Ni substituted cobalt ferrites, Co1-xNixFe1.9Mn0.1O4 (0≤ x ≤ 1.0), prepared by solid state technique with the increase in concentration of Ni doping uniformly in grain size and densification have improved, while permeability increased remarkably. Saturation magnetization and coercivity both decreased with Ni concentration in cobalt ferrite. Electromagnetic parameters of ferrite samples were measured in X-band frequency range (8–12 GHz) by vector network analyzer (VNA) and their absorbing properties were calculated according to transmission-line theory. Phase formation of all samples has been confirmed by XRD. Ferromagnetic resonance frequency shift has been observed with concentration of Ni concentration which is an important phenomenon of cobalt ferrite to absorb microwave at higher frequencies in X-band. Based on magnetic and microwave measurements Co1-xNixFe1.9Mn0.1O4 (x = 0.8) may be a good potential candidate for electromagnetic compatibility and for other practical applications at high frequencies.

Microwave magnetoelectric effects in bilayers of piezoelectrics and ferrites with cubic magnetocrystalline anisotropy

The strain mediated microwave magnetoelectric coupling is studied in bilayers of ferrites with cubic anisotropy and piezoelectrics. The strength of magnetoelectric coupling is determined from data on ferromagnetic resonance frequency shift Δf versus applied electric field E. Studies at 6–10 GHz on samples of nickel zinc ferrite and polycrystalline lead zirconium titanate, lead magnesium niobate–lead titanate or lead zinc niobate-lead titanate (PZN–PT) show shifts Δf on the order of 30–150 MHz for E=10 kV/cm. Bilayers of nickel zinc ferrite and PZN-PT demonstrated the highest magnetoelectric coefficient of 5.3 Oe cm/kV. Theoretical estimates of the frequency shifts are in very good agreement with the data.

Modified composition of barium ferrite to act as a microwave absorber in X-band frequencies

The magnetic and electromagnetic wave absorbing properties of the Co–Mn–Ti substituted barium ferrites, BaCo x Mn x Ti 2 x Fe 12−4 x O 4 (0≤ x≥0.5), prepared by a solid-state technique, were investigated. The grain size uniformity and densification improved with increasing concentration of Co–Mn–Ti, while the permeability increased and the coercivity decreased remarkably. The reflection coefficients ( S 11 , S 12 ) of the ferrite samples were measured in the X-band frequency range (8–12 GHz) and their absorbing properties were calculated according to transmission line theory. The maximum reflection loss (−14.7 dB) is observed for the x = 0.4 sample. A ferromagnetic resonance frequency shift has been observed to depend on the Co–Mn–Ti concentration, which is an important phenomenon of barium ferrite for absorbing microwaves at high frequencies. Based on the magnetic and microwave measurements, BaCo x Mn x Ti 2 x Fe 12−4 x O 4 may be a good potential candidate for electromagnetic compatibility and for other practical applications at high frequencies.

Electric field tunable 60 GHz ferromagnetic resonance response in barium ferrite-barium strontium titanate multiferroic heterostructures

A magnetic-ferroelectric film heterostructure with a large electric field tuning of the ferromagnetic resonance (FMR) mode was fabricated. Pulse laser deposited 30 nm thick Pt electrodes and 3 μm thick barium strontium titanate films on Nb-doped strontium titanate substrates were capped with an unbonded 200 μm thick single crystal in-plane c-axis barium hexaferrite slab. The structure gives a 60 GHz FMR frequency shift of 16 MHz at a bias of 29 V, for an average response of 0.55 MHz/V. The maximum incremental tuning response at 29 V was 1.3 MHz/V. This is a hundredfold improvement over previous results.