Vector Network Analyzer Ferromagnetic Resonance Research Articles

Influence of an ultrathin Mn ‘spy layer’ on the static and dynamic magnetic coupling within FePt/NiFe bilayers

Abstract We explore whether insertion of an ultrathin Mn ‘spy layer’ within a magnetic hard/soft bilayer can enable depth-sensitive element-specific measurements of the static and dynamic magnetization, while avoiding significant disruption of the original magnetic state. MgO(110)/FePt(100 Å)/NiFe(200 Å)/Mn(tMn Å)/NiFe(200 Å) samples with Mn thicknesses of tMn=0, 5, and 10Å were fabricated by magnetron sputtering and studied by element-selective x-ray magnetic circular dichroism (XMCD), vector network analyzer ferromagnetic resonance (VNA-FMR), and x-ray detected ferromagnetic resonance (XFMR). For tMn= 5 Å, the magnetic reversal properties remain broadly similar to tMn= 0 Å. For tMn=10 Å, the two NiFe layers decouple with XMCD hysteresis loops at the Mn edge showing two switching events that suggest the presence of two distinct Mn-containing regions. While the Mn moments within each region have ferromagnetic order, their relative alignment is antiparallel at high field. Analysis of the magnetic data and additional scanning transmission electron microscopy (STEM) measurements point to the presence of a Mn layer at the lower NiFe/Mn interface, and the formation of a NiFeMn alloy at the upper Mn/NiFe interface. The Mn moments of the former region lie antiparallel to those of the underlying NiFe layer. The VNA-FMR data suggests that for tMn= 5 and 10 Å, the interfacial exchange coupling at the FePt/NiFe is suppressed and the in-plane uniaxial magnetic anisotropy of the NiFe is increased, perhaps due to migration of Mn towards the buried interface. The above findings show that Mn is a problematic magnetic spy, and that a Mn thickness of less than 5Å would be required.

Material Characterization Study of Magnetite Nanocrystals for RF Sensing.

Magnetite nanocrystals show promise for electrically small gigahertz frequency applications, which could lead to miniaturizing transformer cores and new sensing technologies. This work presents a rigorous radiofrequency characterization of these nanocrystals using vector network analyzer (VNA) ferromagnetic resonance (FMR) measurements. For the first time, two different average diameters of Fe3O4 nanocrystals are investigated (7.3 and 20.2 nm). When the VNA-FMR results were compared to micromagnetic simulations, the magnetic anisotropy (K 1) deviated from the ideal mean orientation value of K 1/2 to K 1/80 for the 7.3 nm nanocrystal. In contrast, the obtained magnetic anisotropy in 20.2 nm nanocrystals slightly deviated to K 1/11 due to less structural deformations. These findings resulted in a newly proposed methodology for an approximate simulation based on VNA-FMR measurements. In addition, this work estimates the approximate amount of nanocrystals needed to measure a useful VNA-FMR spectrum.

Temperature dependent intrinsic Gilbert damping in magnetostrictive FeCoSiB thin film

A low Gilbert damping factor is crucial for emerging magneto-acoustic devices. In current work, angle-dependent and temperature-dependent Gilbert damping of magnetostrictive FeCoSiB thin films have been investigated using electron spin resonance (ESR) and vector network analyzer ferromagnetic resonance (VNA-FMR) techniques, respectively. A very low Gilbert damping factor ∼0.0038 was measured with in-plane magnetic fields at room temperature. Temperature-dependent VNA-FMR results between 10 K and 300 K exhibit a conductivity-like damping feature, which can be attributed to the spin-orbital coupling dominantly controlled by the intraband scattering. Our results clearly indicate that highly magnetostrictive or piezomagnetic film does not necessarily have a high intrinsic damping factor.

A short-circuited coplanar waveguide for low-temperature single-port ferromagnetic resonance spectroscopy setup to probe the magnetic properties of ferromagnetic thin films

A coplanar waveguide shorted in one end is proposed, designed, and implemented successfully to measure the properties of magnetic thin films as a part of the vector network analyzer ferromagnetic resonance (FMR) spectroscopy setup. Its simple structure, potential applications, and easy installation inside the cryostat chamber made it advantageous especially for low-temperature measurements. It provides a wide band of frequencies in the gigahertz range essential for FMR measurements. Our spectroscopy setup with a short-circuited coplanar waveguide has been used to extract the Gilbert damping coefficient and effective magnetization values for standard ferromagnetic thin films, such as Permalloy (Py) and Cobalt (Co). The thickness and temperature-dependent studies of those magnetic parameters have also been done here for the afore-mentioned magnetic samples.

High frequency dynamics and magnetic anisotropy of bcc Co films grown on Si (0 0 1) substrate

Dynamic magnetization and magnetic anisotropy of Cu/bcc-Co/CoSi2 /Si(001) system were investigated by Magneto-Optical Kerr Effect (MOKE) analysis and vector network analyzer ferromagnetic resonance (VNA-FMR). Typical fourfold magnetocrystalline anisotropy of bcc Co film and superposed uniaxial magnetic anisotropy were observed. The uniaxial magnetic anisotropy originated from shape anisotropy as confirmed by micromagnetic simulation. The large magnetocrystalline anisotropy results in high resonance frequency even in the zero applied field. The Gilbert damping parameter α obtained by analysis of the dynamic process is close to the theoretically estimated value of bcc phase Co. The Gilbert damping parameter of the bcc-Co film is larger than those of fcc-Co and hcp-Co films. This work indicates the Co damping parameter can be enhanced by using bcc structure, which will help create high-speed magnetoelectronic devices.

Development of a high‐sensitivity VNA‐FMR spectrometer with field modulation detection and its application to magnetic characterization

AbstractWe have developed a vector network analyzer ferromagnetic resonance (VNA‐FMR) spectrometer with field modulation detection. This system provides broadband coverage and very high sensitivity to resolve the FMR signal on a ultra‐thin (∼ 1 nm thick) magnetic film. This paper first presents the detail of the VNA‐FMR spectrometer built in this work, including its operation principle and design guideline, and second various magnetic properties obtained by the VNA‐FMR measurement results.

Erratum: "Vector network analyzer ferromagnetic resonance spectrometer with field differential detection" [Rev. Sci. Instrum. 89, 053901 (2018)

Erratum: "Vector network analyzer ferromagnetic resonance spectrometer with field differential detection" [Rev. Sci. Instrum. 89, 053901 (2018)

Dynamic interlayer exchange coupling in magnetic anisotropic FeCoB/Ru/FeCoB sandwich trilayers

Transverse and longitudinal vector network analyzer ferromagnetic resonance (VNA-FMR) spectroscopy, which can easily distinguish the optical and acoustic modes of FMR, is used to study the dynamic interlayer exchange coupling (IEC) in the oblique sputtered Fe56Co24B20(26.5 nm)/Ru(tRu)/Fe56Co24B20(26.5 nm) trilayers with various Ru thicknesses tRu. Comparing with the static IEC research method, the dynamic method not only analyzes the frequency response characteristics caused by IEC, but also identifies the nature of IEC. In the studied system, an oscillatory interlayer coupling shows up in the VNA-FMR dynamic measurement. An antiferromagnetic (AFM) coupling appears at 0.35 nm < tRu≤0.9 nm, while a strong ferromagnetic coupling dominates at tRu≤0.3 nm and a weak ferromagnetic coupling dominates at tRu≥1 nm. The strongest AFM coupling with coupling coefficient J1 of −0.87 erg/cm2 occurs at the intermediate thickness oftRu ~0.5 nm, while a strong FM coupling with J1 as large as 1.1 erg/cm2 dominates in the relatively thin 0.23 nm Ru spacers. The coupling mechanism is simulated with Layadi’s rigid model and the oscillatory behavior of the bilinear IEC coefficient J1 versus tRu is obtained.

Effects of Thermal Annealing and Ni Addition on the Magnetic Properties of Co–CoO Multilayers

The influence of mild thermal annealing and nickel addition on the magnetic properties of Co–CoO multilayers is investigated. Co–CoO and CoNi–CoNiO multilayers were fabricated by radio-frequency magnetron sputtering and natural oxidation. Their magnetic properties have been studied via the magneto-optical Kerr effect and a vector network analyzer ferromagnetic resonance setup. The Co–CoO multilayers were annealed up to 90[Formula: see text]min at [Formula: see text]C and they were found to be always in-plane magnetized. The value of Gilbert damping was evaluated. The multilayers after annealing or Ni addition show a small increase in coercivity without considerable changes in their anisotropy. The phenomenon of inverted hysteresis with negative remanence, with the field being applied perpendicular to the film plane, has been recorded. It was modeled with two coexisting magnetic moments with in-plane and perpendicular anisotropies, respectively.

Giant anisotropy of magnetic damping and significant in-plane uniaxial magnetic anisotropy in amorphous Co40Fe40B20 films on GaAs(001)**Project supported by the National Key Research and Development Program of China (Grant No. 2016YFA0300803), the National Natural Science Foundation of China (Grant Nos. 51971109, 51771053, and 51471085), and Scientific Research Foundation of Nanjing Institute of Technology (Grant Nos. ZKJ201708 and CKJB201708).

Tuning magnetic damping constant in dedicated spintronic devices has important scientific and technological implications. Here we report on anisotropic damping in various compositional amorphous CoFeB films grown on GaAs(001) substrates. Measured by a vector network analyzer-ferromagnetic resonance (VNA-FMR) equipment, a giant magnetic damping anisotropy of 385%, i.e., the damping constant increases by about four times, is observed in a 10-nm-thick Co40Fe40B20 film when its magnetization rotates from easy axis to hard axis, accompanied by a large and pure in-plane uniaxial magnetic anisotropy (UMA) with its anisotropic field of about 450 Oe. The distinct damping anisotropy is mainly resulted from anisotropic two-magnon-scattering induced by the interface between the ferromagnetic layer and the substrate, which also generates a significant UMA in the film plane.

Large anisotropy of magnetic damping in amorphous CoFeB films on GaAs(001)

Amorphous CoFeB films grown on GaAs(001) substrates demonstrating significant in-plane uniaxial magnetic anisotropy were investigated by vector network analyzer ferromagnetic resonance. Distinct in-plane anisotropy of magnetic damping, with a largest maximum–minimum damping ratio of about 109%, was observed via analyzing the frequency dependence of linewidth in a linear manner. As the CoFeB film thickness increases from 3.5 nm to 30 nm, the amorphous structure for all the CoFeB films is maintained while the magnetic damping anisotropy decreases significantly. In order to reveal the inherent mechanism responsible for the anisotropic magnetic damping, studies on time-resolved magneto-optical Kerr effect and high resolution transmission electron microscopy were performed. Those results indicate that the in-plane angular dependent anisotropic damping mainly originates from two-magnon scattering, while the Gilbert damping keeps almost unchanged.

Tunable ferromagnetic resonance in coupled trilayers with crossed in-plane and perpendicular magnetic anisotropies

An original approach to tune the ferromagnetic resonance frequency of a soft magnetic Ni80Fe20 (Permalloy = Py) film with in-plane magnetic anisotropy (IMA) based on the controlled coupling to a hard magnetic NdCox film with perpendicular magnetic anisotropy (PMA) through a nonmagnetic Al spacer is studied. Using the transverse magneto-optical Kerr effect (T-MOKE), alternating gradient magnetometry (AGM), and vector network analyzer ferromagnetic resonance (VNA-FMR) spectroscopy, the influence of both the Co concentration and the Al spacer thickness on the static and dynamic magnetic properties of the coupled IMA/PMA system is investigated. Compared to a single Py film, two striking effects of the coupling between IMA and PMA layers can be observed in their FMR spectra. First, there is a significant increase in the zero-field resonance frequency from 2.0 GHz up to 6.4 GHz, and second, an additional frequency hysteresis occurs at low magnetic fields applied along the hard axis. The maximum frequency difference between the frequency branches for increasing and decreasing magnetic fields is as high as 1 GHz, corresponding to a tunability of about 20% at external fields of typically less than ±70 mT. The origin of the observed features in the FMR spectra is discussed by means of magnetization reversal curves.

Microwave-Assisted Magnetization Reversal in Dispersed Nanosized Barium Ferrite Particles for High-Density Magnetic Recording Tape

This paper investigates microwave (MW)-assisted magnetization reversal (MAMR) in dispersed nanosized barium ferrite (BaFe) particles. MAMR in dispersed nanosized BaFe particles was observed using vector network analyzer ferromagnetic resonance spectroscopy with coplanar waveguides, in this investigation. The average size of BaFe particles was 1750 nm3. A frequency-dependent reduction in the switching field was observed in response to an MW impulse of duration 50 ns and with a frequency of approximately 22 GHz. This result suggests that MAMR was observed in dispersed nanosized BaFe particles.

Thickness-dependent magnetic anisotropy in obliquely deposited Fe(001)/Pd thin film bilayers probed by VNA-FMR**Project supported by the National Basic Research Program of China (Grant Nos. 2015CB921403 and 2016YFA0300701), the National Natural Science Foundation of China (Grant Nos. 51427801, 11374350, and 51671212), and the Chinese Government Scholarship (Grant No. 2015GXYG37).

The thickness-dependent magnetic anisotropy of obliquely deposited Fe(001)/Pd thin films on Mg(001) is investigated by fitting the field-dependent resonant field curve using the Kittel equation. In this study, three Fe film samples with thicknesses of 50 monolayers (ML), 45 ML, and 32 ML deposited at 0°, 45°, and 55°, respectively, are used. The magnetic anisotropy constant obtained from ferromagnetic resonance (FMR) spectra exhibits a dominant fourfold magnetocrystalline anisotropy (MCA) at the normal deposition angle with larger Fe thickness. However, the in-plane uniaxial magnetic anisotropy (UMA) is induced by a higher oblique deposition angle and a smaller thickness. Its hard axis lies between the [100] and [010] directions. The FMR data-fitting analysis yields a precise measurement of smaller contributions to the magnetic anisotropy, such as in-plane UMA. Due to MCA, when the magnetic field is weaker than the saturated field, the magnetization direction does not always align with the external field. The squared frequency-dependent resonant field measurement gives an isotropic Landé g-factor of 2.07. Our results are consistent with previous experiments conducted on the magneto-optical Kerr effect (MOKE) and anisotropic magnetoresistance (AMR) systems. Thus, a vector network analyzer ferromagnetic resonance (VNA-FMR) test-method for finding UMA in obliquely deposited Fe(001)/Pd bilayer ferromagnetic thin films, and determining the magnetic anisotropy constants with respect to the film normal deposition, is proposed.

Variation of Magnetization Dynamics of Co/Ni Multilayer by Capturing Magnetic Nanoparticles

[Co (0.29 nm)/Ni (0.71 nm)]10 multilayers were deposited on Ta (30 nm) and Pt (30 nm) buffer layers, and the variation of the magnetization dynamics by the adsorption of Fe3O4 nanoparticles was studied by vector network analyzer—ferromagnetic resonance method. The surface roughness of the Co/Ni multilayer was 0.52 nm, and significantly increased after the adsorption of the Fe3O4 nanoparticles which was captured by large field gradient on the demagnetized Co/Ni multilayer. The anisotropy field $H_{k}$ , damping constant $\alpha $ , and anisotropy distribution $\Delta H_{k}$ of the Co/Ni on Ta buffer layer were 2.6 kOe, 0.012, and 0.32 kOe, respectively. By adsorbing of the Fe3O4 nanoparticles, the stray field from the nanoparticles $H_{{\text {np}}}$ was applied to the Co/Ni, and $H_{k} + {H}_{{\text {np}}}$ was estimated to increase to 3.7 kOe. Moreover, $\alpha $ and $\Delta H_{k}$ also increased to 0.020 and 0.42 kOe by the adsorption, respectively. Similarly, $H_{k} + H_{\mathrm{np}}$ , $\alpha $ , $\Delta H_{k}$ of Pt buffered Co/Ni also increased from 2.4 kOe, 0.033, 0.43 kOe to 4.2 kOe, 0.055, 0.53 kOe, respectively. These results suggest that the adsorption of Fe3O4 nanoparticles significantly modifies the magnetization dynamics of Co/Ni multilayer.

Determination of the spin Hall effect and the spin diffusion length of Pt from self-consistent fitting of damping enhancement and inverse spin-orbit torque measurements

Understanding the evolution of spin-orbit torque (SOT) with increasing heavy-metal thickness in ferromagnet/normal metal (FM/NM) bilayers is critical for the development of magnetic memory based on SOT. However, several experiments have revealed an apparent discrepancy between damping enhancement and damping-like SOT regarding their dependence on NM thickness. Here, using linewidth and phase-resolved amplitude analysis of vector network analyzer ferromagnetic resonance (VNA-FMR) measurements, we simultaneously extract damping enhancement and both field-like and damping-like inverse SOT in Ni$_{80}$Fe$_{20}$/Pt bilayers as a function of Pt thickness. By enforcing an interpretation of the data which satisfies Onsager reciprocity, we find that both the damping enhancement and damping-like inverse SOT can be described by a single spin diffusion length ($\approx$ 4 nm), and that we can separate the spin pumping and spin memory loss (SML) contributions to the total damping. This analysis indicates that less than 40% of the angular momentum pumped by FMR through the Ni$_{80}$Fe$_{20}$/Pt interface is transported as spin current into the Pt. On account of the SML and corresponding reduction in total spin current available for spin-charge transduction in the Pt, we determine the Pt spin Hall conductivity ($\sigma_\mathrm{SH} = (2.36 \pm 0.04)\times10^6 \Omega^{-1} \mathrm{m}^{-1}$) and bulk spin Hall angle ($\theta_\mathrm{SH}=0.387 \pm0.008$) to be larger than commonly-cited values. These results suggest that Pt can be an extremely useful source of SOT if the FM/NM interface can be engineered to minimize SML. Lastly, we find that self-consistent fitting of the damping and SOT data is best achieved by a model with Elliott-Yafet spin relaxation and extrinsic inverse spin Hall effect, such that both the spin diffusion length and spin Hall conductivity are proportional to the Pt charge conductivity.

Vector network analyzer ferromagnetic resonance spectrometer with field differential detection.

This work presents a vector network analyzer ferromagnetic resonance (VNA-FMR) spectrometer with field differential detection. This technique differentiates the S-parameter by applying a small binary modulation field in addition to the DC bias field to the sample. By setting the modulation frequency sufficiently high, slow sensitivity fluctuations of the VNA, i.e., low-frequency components of the trace noise, which limit the signal-to-noise ratio of the conventional VNA-FMR spectrometer, can be effectively removed, resulting in a very clean FMR signal. This paper presents the details of the hardware implementation and measurement sequence as well as the data processing and analysis algorithms tailored for the FMR spectrum obtained with this technique. Because the VNA measures a complex S-parameter, it is possible to estimate the Gilbert damping parameter from the slope of the phase variation of the S-parameter with respect to the bias field. We show that this algorithm is more robust against noise than the conventional algorithm based on the linewidth.

Determination of the spin Hall angle in single-crystalline Pt films from spin pumping experiments

We report on the determination of the spin Hall angle in ultra-clean, defect-reduced epitaxial Pt films. By applying vector network analyzer ferromagnetic resonance spectroscopy to a series of single crystalline Fe (12 nm) /Pt (tPt) bilayers we determine the real part of the spin mixing conductance (4.4 ± 0.2) × 1019 m−2 and reveal a very small spin diffusion length in the epitaxial Pt (1.1 ± 0.1) nm film. We investigate the spin pumping and ISHE in a stripe microstucture excited by a microwave coplanar waveguide antenna. By using their different angular dependencies, we distinguish between spin rectification effects and the inverse spin Hall effect. The relatively large value of the spin Hall angle (5.7 ± 1.4)% shows that ultra-clean e-beam evaporated non-magnetic materials can also have a comparable spin-to-charge current conversion efficiency as sputtered high resistivity layers.

Inductive detection of fieldlike and dampinglike ac inverse spin-orbit torques in ferromagnet/normal-metal bilayers

Functional spintronic devices rely on spin-charge interconversion effects, such as the reciprocal processes of electric field-driven spin torque and magnetization dynamics-driven spin and charge flow. Both dampinglike and fieldlike spin-orbit torques have been observed in the forward process of current-driven spin torque and dampinglike inverse spin-orbit torque has been well studied via spin pumping into heavy metal layers. Here, we demonstrate that established microwave transmission spectroscopy of ferromagnet/normal metal bilayers under ferromagnetic resonance can be used to inductively detect the ac charge currents driven by the inverse spin-charge conversion processes. This technique relies on vector network analyzer ferromagnetic resonance (VNA-FMR) measurements. We show that in addition to the commonly extracted spectroscopic information, VNA-FMR measurements can be used to quantify the magnitude and phase of all ac charge currents in the sample, including those due to spin pumping and spin-charge conversion. Our findings reveal that ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}\text{/Pt}$ bilayers exhibit both dampinglike and fieldlike inverse spin-orbit torques. While the magnitudes of both the dampinglike and fieldlike inverse spin-orbit torque are of comparable scale to prior reported values for similar material systems, we observed a significant dependence of the dampinglike magnitude on the order of deposition. This suggests interface quality plays an important role in the overall strength of the dampinglike spin-to-charge conversion.

Static and dynamic magnetic properties of stripe-patterned Fe20Ni80 soft magnetic films

Stripe-patterned soft magnetic Fe20Ni80 films were fabricated on silicon substrate via radio frequency magnetron sputtering technology. The static and dynamic magnetic properties of samples were measured by a vibrating sample magnetometer and vector network analyzer. The vector network analyzer ferromagnetic resonance technique was used to analyze the experimental results, which showed that damping and in-plane uniaxial anisotropy can be tuned significantly for the samples with various stripe widths from 5 to 20 µm. A stripe-shaped anisotropy model was used to analyze the experimental results, which were in accord with the theoretical predictions. Moreover, the variation of damping was investigated in detail.