FMR Frequency Research Articles

Static and dynamical behaviour of magnetically coupled Co/Cu/CoFeB trilayers

Engineering the magnetic behaviour of stacks of ferromagnetic and non-magnetic thin films is mainly controlled by the anisotropy of individual layers plus the interlayer exchange coupling constant. Then, Co/Cu/CoFeB trilayers with different Cu layer thicknesses, tCu, were fabricated by sputtering. The effects of tCu on the magnetization reversal processes, the interlayer coupling, FMR frequencies and damping parameters were investigated. The analysis of the hysteresis loops have demonstrated how the two ferromagnetic films can be strongly coupled and behave like a single system for tCu ≤ 3 nm, or like two almost independent or uncoupled layers for tCu ≥ 3 nm. It is shown that the interlayer coupling is at the origin of the observed two FMR resonance modes: the acoustic and the optic modes. Particularly, the resonance frequency of the optic mode is very sensitive to the interlayer coupling constant and high resonance frequencies can be achieved (over 25 GHz) for ferromagnetically coupled Co/CoFeB bilayers with interlayer exchange coupling Jeff = (2.7 ± 0.8) erg/cm2. However, the technological application of such high resonance frequencies of the optic mode can be limited by the increase in the damping parameter mainly due to the spin-pumping effect.

Method for measuring magnetic susceptibility of magnetic nanoparticles up to millimeter-wave frequency range

A novel method for measuring the complex magnetic susceptibility of magnetic nanoparticles (MNPs) has been developed by a broadband microstrip line-type probe with a frequency range of up to 50 GHz. Two external orthogonal DC fields, Hhard and Heasy, were applied to change the susceptibility of MNPs (Streptavidin coated, 200 nmϕ, number of MNPs about 1010) based on magnetization switching theory. The susceptibility of MNPs in the liquid, solid state, and after biotin-avidin reaction with polymer beads (2 μmϕ, number of polymer beads about 108) was evaluated. A clear FMR frequency of MNPs and the same FMR frequency shift trend as Hhard changed was observed around 2–10 GHz in both liquid and solid states. The susceptibility of MNPs reached maximum when Hhard was 0.1 T which is approximately equal to Heasy (0.135 T). The maximum values of real and imaginary parts increased by a factor of 3.39 and 1.66 compared to when Hhard was 0. The obtained results indicate that the magnetic anisotropy was successfully controlled in the liquid and solid states of MNPs aggregate. Furthermore, after the biotin-avidin reaction of MNPs with polymer beads, the imaginary part of susceptibility showed a decreasing trend regardless of the increase in Hhard, and its half-width expanded by 35.1% at maximum when Hhard was 0.15 T.

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.

Interlayer Coupling and High-Frequency Performance in Magnetic Anisotropic FeCoB/Hf/FeCoB Trilayers with Various Hf Thicknesses

FeCoB (25 nm)/Hf(tHf)/FeCoB (25 nm) sandwich films with different hafnium thicknesses tHf were fabricated using a modified compositional gradient sputtering method to obtain self-biased high-frequency performances. The effects of tHf on the interlayer coupling and FMR frequency were investigated. It is revealed that interlayer coupling enhanced the resonance frequency by 48%, and a ferromagnetic coupling between the FeCoB films occurred for the trilayers with tHf < 3.0 nm, likely due to the interface roughness and pinhole effect. In this case, only acoustic mode resonance was observed with the same high-frequency performance as the corresponding FeCoB single layer. In contrast, a tHf-dependent antiferromagnetic interlayer coupling appeared at tHf > 3.0 nm. The coupling coefficient J1 was antiferromagnetic, and a biquadratic coupling J2 appeared at tHf > 3.5 nm. The coupling mechanism was simulated and verified by Layadi’s rigid model, and the simulation was consistent with the experimental results.

Isotropic FMR frequency enhancement in thin Py/FeMn bilayers under strong magnetic proximity effect

Exchange biasing in ferromagnet/antiferromagnet bilayers is known to enhance the material’s ferromagnetic resonance frequency and make it strongly angle dependent due to the unidirectional anisotropy induced at the interface. We observe a ten-fold enhancement in frequency and angle-independent ferromagnetic resonance in bilayers of Py/FeMn with ultrathin FeMn, accompanied by a significantly enhanced magnetic moment. The observed isotropic frequency enhancement is consistent with rotatable rather than unidirectional magnetic anisotropy and the induced magnetic moment links this anisotropy with the ferromagnet-proximity effect. The estimated effective anisotropy field acting on the proximity-induced moment in ultrathin FeMn can be as high as 0.5 T at room temperature. Our results show the potential of the ferromagnetic proximity effect combined with the inherent exchange anisotropy in antiferromagnets for high-speed spintronic applications.

Inductive Decoupling between Two Parallel MSLs using Co-Zr-Nb Film

The effect of the negative permeability of the magnetic film on the inductive coupling was quantitatively investigated using a magnetic circuit analysis of two parallel microstrip line (MSLs) with a Co-Zr-Nb film. First, an electromagnetic field simulation was performed to obtain the magnetic flux distribution around the MSLs with the Co-Zr-Nb film. A simple magnetic circuit model considering the negative permeability, based on the simulated magnetic flux distribution, was constructed. The self-inductance of the MSL and the mutual inductance between the MSLs were estimated. Finally, the coupling coefficient of magnetic flux was obtained using the self and the mutual inductances. As a result, it was shown that the inductive coupling could be reduced to almost zero in the certain frequency range above the FMR frequency caused by the negative permeability of the magnetic film. This result indicates the possibility of developing the design guideline of the magnetic films as the crosstalk suppressors.

Tuning the particle size, natural ferromagnetic resonance frequency and magnetic properties of ε-Fe2O3nanoparticles prepared by a rapid sol–gel method

A fast method for the synthesis of ε-Fe2O3, yielding 100% pure material with a variable FMR frequency, is proposed.

Ferromagnetic resonance of a two-dimensional array of nanomagnets: Effects of surface anisotropy and dipolar interactions

We develop an analytical approach for studying the FMR frequency shift due to dipolar interactions and surface effects in two-dimensional arrays of nanomagnets with (effective) uniaxial anisotropy along the magnetic field. For this we build a general formalism on the basis of perturbation theory that applies to dilute assemblies but which goes beyond the point-dipole approximation as it takes account of the size and shape of the nano-elements, in addition to their separation and spatial arrangement. The contribution to the frequency shift due to the shape and size of the nano-elements has been obtained in terms of their aspect ratio, their separation and the lattice geometry. We have also varied the size of the array itself and compared the results with a semi-analytical model and reached an agreement that improves as the size of the array increases. We find that the red-shift of the ferromagnetic resonance due to dipolar interactions decreases for smaller arrays. Surface effects may induce either a blue-shift or a red-shift of the FMR frequency, depending on the crystal and magnetic properties of the nano-elements themselves. In particular, some configurations of the nano-elements assemblies may lead to a full compensation between surface effects and dipole interactions.

Electric-field tuning of ferromagnetic resonance in CoFeB/MgO magnetic tunnel junction on a piezoelectric PMN-PT substrate

We investigated the strain-induced modulation of in-plane magnetic anisotropy in single crystal Pb(Mg1/3Nb2/3)0.7Ti0.3O3 (PMN-PT) with a (011) orientation/seed layer/IrMn/CoFeB/MgO/CoFeB/capping layer structure using spin-torque ferromagnetic resonance (ST-FMR) at room temperature. An in-plane biaxial strain is produced by an electric field applied across the substrate, which results in the tuning of the magnetic anisotropy of the CoFeB layer. As the change in the electric field value progresses, the MR loop becomes slanted, indicating the change in the easy axis direction. A strain-induced FMR frequency shift dependence of 145 MHz⋅cm⋅kV−1 is obtained by analyzing the experimental FMR spectra. An analytical model of the potential dynamic mechanism based on the strain induced reorientation of magnetization was discussed. Our results suggest that PMN-PT based magnetic tunnel junctions can have applications in electric-field driven spintronic devices.

Epitaxially grown BaM hexaferrite films having uniaxial axis in the film plane for self-biased devices

Barium hexaferrite (BaM) films with in-plane c-axis orientation are promising and technically important materials for self-biased magnetic microwave devices. In this work, highly oriented BaM films with different thickness and an in-plane easy axis (c-axis) of magnetization were grown on a-plane single-crystal sapphire substrates by direct current magnetron sputtering. A procedure involving seed layers, layer-by-layer annealing was adopted to reduce the substrate-induced strains and allow for the growth of thick (~3.44 μm) films. The epitaxial growth of the BaM film on sapphire was revealed by high-resolution transmission electron microscopy with dislocations being observed at the film-substrate interface. The orientation was also verified by X-ray diffraction and more notably, polarized Raman scattering. The magnetic properties and ferromagnetic resonant frequencies were experimentally characterized by a vibrating sample magnetometry and a frequency-swept ferromagnetic resonant flip-chip technique, respectively. The micron-thick BaM films exhibited a large remanence ratio of 0.92 along in-plane easy axis and a small one of 0.09 for the in-plane hard axis loop measurement. The FMR frequency was 50.3 GHz at zero field and reached 57.9 GHz under a magnetic field of 3 kOe, indicating that the epitaxial BaM films with strong self-biased behaviors have good electromagnetic properties in millimeter-wave range.

Free Layer Characterization of TMR Head with Multi-Stripe Height by Using Ferro Magnetic Resonance Analyzer (FMRA)

As continued reducing the track width (TW) of magnetic read sensor for gaining areal density in hard disk drive (HDD) industry, stripe height (SH) is needed to be reduced as keeping aspect ratio(TW/SH) for maintain resistance and signal constant. This paper is proposed to characterize the free layer behavior of tunneling magneto resistive (TMR) head with multiple SH by using ferro magnetic resonance analyzer (FMRA) that is able to analyze by layer-level state without applying external field. The results are found that, at shorter SH, effective stiffness field of free layer is increased which is based on decreasing FMR amplitude and increasing FMR frequency. As a result, magnetic read sensor is shown less sensitivity and signal to noise ratio (SNR) with shorter SH. The SH is a correlated function of the effective stiffness field in free layer that is referred to changed shape anisotropy field which has an effect to read ability.

Application of sub-micrometer patterned permalloy thin film in tunable radio frequency inductors

Electrical tunable meander line inductor using coplanar waveguide structures with patterned permalloy (Py) thin film has been designed and implemented in this paper. High resistivity Si substrate is used to reduce the dielectric loss from the substrate. Inductor is implemented with a 60 nm thick Py deposited and patterned on top of the gold meander line, and Py film is patterned with dimension of 440 nm × 10 μm to create the shape anisotropy field, which in turn increases the FMR frequency. Compared to a regular meanderline inductor without the application of sub-micrometer patterned Py thin film, the inductance density has been increased to 20% for the implemented inductor with patterned Py. Measured FMR frequency of the patterned Py is 4.51 GHz without the application of any external magnetic field. This has enabled the inductor application in the practical circuit boards, where the large external magnet is unavailable. Inductance tunability of the implemented inductor is demonstrated by applying a DC current. Applied DC current creates a magnetic field along the hard axis of the patterned Py thin film, which changes the magnetic moment of the thin film and thus, decreases the inductance of the line. Measured results show that the inductance density of the inductor can be varied 5% by applying 300 mA DC current, larger inductance tunability is achievable by increasing the thickness of Py film.

A compact frequency tunable radio frequency phase shifter with patterned Py enabled transmission line

A well designed frequency tunable phase shifter using patterned Py with different thickness has been demonstrated. Phase shifter is implemented with a slow wave coplanar wave guide (CPW) transmission line, where the signal line has alternate short narrow and wide sections. Py is patterned on the top of narrow section for high inductance density, and inter-digital capacitor is implemented in wide section for high capacitance density. Compared with phase shifter using regular CPW, the dimension of the developed phase shifter has been reduced from 14.86 mm to 4.70 mm at 2 GHz. Phase shifter based on 100 nm and 200 nm thick patterned Py with the same dimensions (14 μm × 10 μm) are implemented and investigated comprehensively. FMR frequency of 3.2 GHz and 3.6 GHz without any external magnetic field has been achieved for 100 nm and 200 nm thick Py film, respectively. Thicker Py has increased inductance density from 1067.2 nH/m to 1193.2 nH/m while the center frequency of the phase shifter has been shifted to 1.80 GHz. Frequency tunability of the phase shifter has been also demonstrated with DC current. The phase shifter can provide 90° phase shift continuously from 2 GHz to 1.80 GHz with DC current from 0 mA to 150 mA. The design concept has great potential in design arbitrary tunable RF components such as filters and couplers.

Compositional dependence of magnetic and high frequency properties of nanogranular FeCo-TiO2 films

A series of soft magnetic nanogranular FeCo-TiO2 films with different compositions of FeCo were fabricated on Si substrates by co-sputtering FeCo and TiO2, and the magnetic and high frequency properties were studied in details. X-ray diffraction analysis confirmed that the films were nanocrystalline/amorphous composites. A high saturation magnetization of 14 kGs, a low coercivity of 5.3 Oe, a FMR frequency of 3.2 GHz, and a high resistivity of 1068 μΩ cm were obtained when the volume fraction of FeCo was 0.68. The excellence electromagnetic properties make the films an excellent candidate for microwave applications.

Mapping microwave field distributions via the spin Hall effect

The spatial variation of the spin pumping–inverse spin Hall effect was studied in a palladium/permalloy bilayer via a coplanar waveguide ferromagnetic resonance (CPW-FMR) broadband technique. The inverse spin Hall signal is both inhomogeneous and asymmetric with respect to both the position along the CPW and the excitation port. Based on this observation, we show how the inverse spin Hall effect can be used as a sensitive probe for mapping the microwave magnetic field distribution in the FMR frequency range.

교환 바이어스 인위적 준강자성 기준층을 포함한 자기 터널 접합의 강자성 공명

Spin dynamics of magnetic tunnel junctions with free and fixed reference layers is investigated by ferromagnetic resonance micromagnetic simulations. First, in magnetic tunnel junctions with an exchange biased synthetic ferrimagnetic reference layer, a magnetization direction of each layer and the tunneling magnetoresistance are calculated for a DC magnetic field. To investigate the spin exciting modes in magnetic tunnel junctions, we simulate the ferromagnetic resonance frequency spectra with small RF magnetic fields. Exciting modes of the tunneling magnetoresistance calculated by an included angle between free and reference layers is interpreted from those of each layer. Spin exciting modes are different according to a signs of the DC magnetic field. In a negative magnetic field, FMR frequency spectra of free and reference layers are well elucidated by the modified Kittel`s equation. However, in a positive magnetic field, there is no simple analytic solution related to FMR frequency spectra due to the coupled modes. Since ferromagnetic layers in magnetic tunnel junctions are interactive each other, careful considerations of the reference and fixed layer as well as the free layer are required for understanding on the spin dynamics of magnetic tunnel junctions with an exchange biased synthetic ferrimagnetic reference layer.

High frequency characteristics of NiO/(FeCo/NiO)10 multilayers with exchange anisotropy

Abstract [NiO/Fe 65 Co 35 ] 10 exchange-coupled multilayer films for high frequency applications are fabricated, and their static magnetic property and microwave permeability are studied systematically. Both exchange bias field and ferromagnetic resonance frequency of the multilayers increase with decreasing Fe 65 Co 35 thickness, which means that the microwave properties such as permeability and FMR frequency can be controlled by changing Fe 65 Co 35 thickness in the exchange-coupled films. Ferromagnetic resonance frequencies beyond 7 GHz of the films are measured and reported for the first time.

Broadband RF Noise Suppression by Magnetic Nanowire-Filled Composite Films

The characteristics of the electromagnetic noise suppression have evaluated using magnetic nanowire-filled composite film on a microstrip line in the broadband radio-frequency range from 0.5 to 20 GHz. The ferromagnetic resonance frequency and the relative complex permeability with the change of aspect ratio were calculated by Kittel's and Landau-Lifshitz-Gilbert equations. The FMR frequency is gradually increased with the increment of aspect ratio (up to 50) of magnetic nanowire. When the magnetic nanowires were mixed with various aspect ratio in composite, the FMR frequency exhibited the broadband frequency regions with 10-28 GHz, 5-14 GHz, 2.5-7 GHz, and 1-2.7 GHz for the change of magnetizations (4piMs: 20, 10, 5, and 2 kG), respectively. Using the calculated FMR frequency profile, the power losses were obtained in broadband frequency region. The transmission power absorption of the composite film on microstrip line was simulated using a 3-D FEM HFSS simulation program. The microstrip line model was composed of Cu conductor line and grounded dielectric substrates, which was designed by IEC standard (IEC 62333-2). The conduction electromagnetic noise was suppressed by the various aspect ratios of the magnetic nanowire-filled composite films in broadband frequency region.

Annealing Temperature Dependence of the Saturation Field and High Frequency Magnetic Property of Slit-Patterned Mn-Ir/Fe-Si Exchange-Coupled Film

Slit-patterned Mn-Ir/Fe-Si exchange-coupled film was annealed under an applied dc magnetic field in vacuum, and its magnetic properties were then evaluated. When a film without slits was annealed, the saturation field Hs in the hard axis direction decreased with increasing annealing temperature, and the ferromagnetic resonance frequency after annealing was lower than that before. On the other hand, when the slit-patterned film was annealed, the saturation field Hs after annealing was larger than that of the film without slits, due to the effect of a shape anisotropy field. In addition, the ferromagnetic resonance frequency changed very little with the annealing temperature. Hence, on account of the shape anisotropy resulting from the introduction of slits, Mn-Ir/Fe-Si exchange-coupled film has a higher FMR frequency. It will be applicable in device fabrication that includes thermal processes because of its uniaxial anisotropy, which includes shape anisotropy that is not influenced by thermal processes.

Fabrication, magnetism and high frequency application of exchange-coupled Fe 65Co 35±2–SiO 1.7±0.2 granular films

A series of (Fe 65Co 35±2) x –(SiO 1.7±0.2) 1− x nano-granular films with various metal volume fractions ( x) were fabricated by rf sputtering. In a wide range, excellent soft magnetic properties have been achieved. In the x range from 0.7 to 0.48, the films exhibit small coercivity H c not exceeding 4 Oe and high electrical resistivity ρ up to 1.15 × 10 4 μΩ cm. And a minimum H c value of 1.65 Oe was obtained for the sample of x = 0.57 with ρ = 2.86 × 10 3 μΩ cm. At a frequency lower than 2.0 GHz, the real part μ′ of complex permeability of this sample is more than 170 and the FMR frequency is as high as 2.6 GHz, implying a high cut-off frequency for high frequency applications. With decreasing Fe 65Co 35±2 volume fraction, the resistivity of films increases remarkably and the grain size decreases obviously. At the same time, the coercivity H c decreases with grain size decreasing, which is consistent with the conclusion resulted from random anisotropy model quoted by Herzer. Study on Henkel plots shows that intergranular ferromagnetic exchange coupling exists among grains and is important for realizing soft magnetic properties.