Pulsed Inductive Microwave Magnetometer Research Articles

Magnetic properties of chromium-doped Ni80Fe20 thin films

This paper investigates the properties of thin films of chromium-doped Ni80Fe20 (Permalloy) that could potentially be useful in future low-power magnetic memory technologies. The addition of chromium reduces the saturation magnetization, Ms, which is useful for low-energy switching, but does not significantly degrade the excellent switching properties of the host material even down to 10 K, the lowest temperature measured, in films as thin as 2.5 nm. As an example, an alloy film composed of 15% chromium and 85% Ni80Fe20 has an Ms just over half that of pure Ni80Fe20, with a coercivity Hc less than 4 Oe, an anisotropy field Hk less than 1 Oe, and an easy-axis remanent squareness Mr/Ms of 0.9 (where Mr is the remanent magnetization). Magnetodynamical measurements using a pulsed inductive microwave magnetometer showed that the average Landau Lifshitz damping λ was relatively constant with changing Cr content, but increased significantly for thinner films (λ ≈150 MHz for 11 nm, λ ≈250 MHz for 2.5 nm), and at low bias fields likely due to increased magnetic dispersion. Density functional theory calculations show that chromium reduces Ms by entering the lattice antiferromagnetically; it also increases scattering in the majority spin channel, while adding almost insignificant scattering to the minority channel.

High frequency magnetization dynamics metrology using a pulsed field inductive microwave magnetometer

We report the development of a pulsed inductive microwave magnetometer for measurements of ultra-fast magnetization dynamics of soft magnetic thin films. This paper details the instrument construction and the measurement procedure. We show experimentally that our instrument detects nanosecond and sub-nanosecond magnetization precession/relaxation processes, in real time, without the need for data averaging, interpolation or any other post-measurement data processing. This enhanced detection resolution has been achieved by careful design and optimization of the coplanar waveguide, which will be described in detail in this paper. Moreover, we show that, unlike other similar instruments, our apparatus is significantly simplified, reducing the cost as well as the measurement procedure.

Frequency- and time-domain investigation of the dynamic properties of interlayer-exchange-coupledNi81Fe19∕Ru∕Ni81Fe19thin films

Pulsed inductive microwave magnetometer (PIMM), conventional ferromagnetic resonance (FMR), and vector network analyzer FMR (VNA-FMR) have been used for complementary studies of the various excited modes in exchange-coupled $\mathrm{Ni}\mathrm{Fe}(30\phantom{\rule{0.3em}{0ex}}\mathrm{nm})∕\mathrm{Ru}({d}_{\mathrm{Ru}})∕\mathrm{Ni}\mathrm{Fe}(30\phantom{\rule{0.3em}{0ex}}\mathrm{nm})$ films with variable Ru thicknesses ${d}_{\mathrm{Ru}}$. For antiferromagnetically coupled layers, two modes, which vary in their relative intensity as a function of the bias field, are detected. These two modes, which are observable simultaneously over a limited range of the bias field with PIMM, are identified as optic and acoustic modes. The mode frequencies and the interlayer exchange coupling are found to oscillate as a function of the Ru layer thickness with a period of $8.5\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$. The frequency oscillations of the optic mode are coupling dependent, while those of the acoustic mode are indirectly related to coupling via the canting angle of the layer magnetizations below the saturation. Comparison between PIMM and VNA-FMR in terms of frequency of modes shows good agreement, but the optic mode is observed over a wider field range with VNA-FMR. Furthermore, we clearly observed different behaviors of the FMR linewidths as a function of the spacer thickness for the optic and acoustic modes. In addition, perpendicular standing spin waves have been studied as a function of coupling. The FMR linewidth of the different modes increases with the microwave frequency and typical damping constants of $\ensuremath{\alpha}=0.0073$ have been measured. The effect of the pulse field amplitudes on the properties of the various excited modes has been simulated and studied experimentally.

Experimental determination of the inhomogeneous contribution to linewidth in Permalloy films using a time-resolved magneto-optic Kerr effect microprobe

We adapted a time-resolved magneto-optic microprobe for use with a pulsed inductive microwave magnetometer apparatus, to measure the magnetization dynamics of a thin Permalloy film at micrometer and millimeter length scales under exactly the same experimental conditions. The optical microprobe has a spatial resolution of 1 μm. We compare the data obtained over these different length scales to quantitatively determine the localized inhomogeneous contribution to magnetic damping. When measured directly with the magneto-optic technique, the inhomogeneous contributions are in agreement with the value 88±16 A/m (1.1±0.2 Oe) extrapolated from PIMM measurements of linewidth versus frequency.

Interpretation of magnetisation dynamics using inductive magnetometry in thin films

We present ferromagnetic resonance data from a Py film using both a pulsed inductive microwave magnetometer (PIMM) and conventional FMR. An increase in the damping is seen at low field resonances in the PIMM data from what is expected using conventional FMR. This is explained by the influence of the PIMM’s spatially inhomogeneous excitation field and quantified using an intuitive argument. We demonstrate from this derivation how excitation of non-uniform wavevectors can explain the measured increase in damping at low fields observed by the PIMM. We also present results from a coupled Py and Cobalt system, demonstrating that inductive magnetometry can be a sensitive technique for measuring exchange coupling over interfaces and surfaces.

Pulsed inductive measurement of ultrafast magnetization dynamics in interlayer exchange coupled NiFe∕Ru∕NiFe films

Pulsed inductive microwave magnetometer (PIMM), conventional ferromagnetic resonance (FMR), and vector network analyzer FMR (VNA-FMR), as well as quasistatic methods such as a magneto-optic Kerr effect magnetometer and a vibrating sample magnetometer, have been used for complementary studies of interlayer exchange coupled Si∕Ta∕NiFe(30nm)∕Ru(t)∕NiFe(30nm)∕Ta films of variable Ru thickness t. The coupling constants were determined from the measured magnetization curves by fitting them to simulated ones. The dynamics measurements revealed two spin wave modes, which could be identified as optical and acoustical ones. The optical mode changes significantly with the coupling, whereas the slight change of the acoustical mode is due to different relative orientations of the layer magnetizations. Numerical simulations helped to identify optical and acoustical modes. The intensity of these two modes changes with the bias field and coupling. Using VNA-FMR the optical mode could be observed up to higher bias fields than with the PIMM at small excitation angles. While varying the pulse field up to 150Oe using the PIMM, the intensity ratio of optical and acoustical modes changed and additional modes occurred.

Ferromagnetic resonance linewidth in metallic thin films: Comparison of measurement methods

Stripline (SL), vector network analyzer (VNA), and pulsed inductive microwave magnetometer (PIMM) techniques were used to measure the ferromagnetic resonance (FMR) linewidth for a series of Permalloy films with thicknesses of 50 and 100nm. The SL-FMR measurements were made for fixed frequencies from 1.5to5.5GHz. The VNA-FMR and PIMM measurements were made for fixed in-plane fields from 1.6to8kA∕m (20–100Oe). The results provide a confirmation, lacking until now, that the linewidths measured by these three methods are consistent and compatible. In the field format, the linewidths are a linear function of frequency, with a slope that corresponds to a nominal Landau-Lifshitz phenomenological damping parameter α value of 0.007 and zero frequency intercepts in the 160–320A∕m (2–4Oe) range. In the frequency format, the corresponding linewidth versus frequency response shows a weak upward curvature at the lowest measurement frequencies and a leveling off at high frequencies.

Enhanced ferromagnetic damping in Permalloy∕Cu bilayers

We have investigated the enhancement of ferromagnetic damping for thin Permalloy (Ni80Fe20) films grown with Cu capping layers of variable thickness (5–1000 nm). The measurements were performed with a pulsed inductive microwave magnetometer in the frequency range between 2.3 and 2.7 GHz. The damping was enhanced if the Cu layers were thicker than the spin-diffusion length of ls≈250nm. For example, the damping was enhanced by 30% for a Permalloy (3nm)∕Cu (1000 nm) bilayer relative to the damping for Permalloy with a 5-nm-thick Cu capping layer. Existing theory for spin pumping from the Permalloy layer into the Cu layer was used to model the additional contribution to damping for these bilayer systems. Additional experiments on Permalloy (5nm)∕Cu(xnm)∕Ta (5 nm) provided indirect evidence for spin accumulation inside the Cu layer.

Gyromagnetic damping and the role of spin-wave generation in pulsed inductive microwave magnetometry

The dependence of the magnetodynamic response of thin permalloy films was measured with a pulsed inductive microwave magnetometer as a function of varying width of the coplanar waveguide center conductor (220to990μm), frequency (0.6to2GHz) and film thickness (25to93nm) to ascertain the role of magnetostatic spin-wave generation in the low-frequency enhancement of the measured decay rate. A component of the decay rate depends on δw, the ratio of film thickness to center conductor width as theoretically predicted. However, there is an anomalous contribution to the frequency dependence of the decay rate exists that cannot be attributed to the generation of spin-waves.

Pulsed inductive microwave magnetometer response calculated for IrMn/FeNi bilayers

Micromagnetic simulations of a pulsed inductive microwave magnetometer (PIMM) experiment are performed using a well established model for exchange bias. The model (Interacting Grain Model) consists of ferromagnetic grains and antiferromagnetic grains with randomly distributed easy axes. A perfectly compensated interface between the ferromagnet and the antiferromagnet is assumed which leads to spin flop coupling. The antiferromagnetic layer is modelled as two totally antiparallel sublattices with a small intergrain exchange between each antiferromagnetic sublattice. Simulations of an experimental PIMM setup provide a shift of the minimum of the resonance frequency which is also observered experimentally.

Inductive time-domain measurement of magnetization dynamics in epitaxial Fe1−xCox single and double layers

A pulsed inductive microwave magnetometer (PIMM) is used to examine magnetization dynamics. The thin film sample is brought into close proximity to the coplanar waveguide, which allows for simple changing of samples. The angle between the easy axis (e.a.) and field direction can easily be varied by rotation of the sample on the waveguide. The magnetization dynamics, i.e., precessional frequency, decay time, and precessional amplitude, are determined with respect to this angle or the bias field for epitaxial Fe1−xCox films with different anisotropies. The two precessional motions of a magnetic double layer (FeCo∕Au∕Ni80Fe20) where resolved with the PIMM, which is promising for future investigations on exchange coupled layers.

Optic and acoustic modes measured in a cobalt/Permalloy exchange spring bilayer using inductive magnetometry

The resonance frequencies of a cobalt (100nm)/Permalloy (50nm) exchange spring bilayer have been measured using a pulsed inductive microwave magnetometer. Two field dependent frequencies are detected which vary in relative intensity as a function of applied magnetic field. The two frequencies are identified as the optic and acoustic spin wave modes of coupled ferromagnetic films. Only over a limited range of fields are both frequencies observable. In low fields the acoustic mode is dominant, and in higher fields the optic mode gives the largest intensity. These intensity variations are explained in terms of off-resonant driving of cobalt modes by the uniform mode in Permalloy and a field dependent interfacial pinning.

Dynamic and static magnetic anisotropy in thin-film cobalt zirconium tantalum

The magnetic anisotropy values of thin amorphous cobalt zirconium tantalum (CZT) films were determined from static and dynamic measurements. Dynamic techniques show a rotatable component of anisotropy that decreases with increasing longitudinal bias field from 200 to 0±48A/m (2.5 to 0±0.6Oe). The dynamic value of the anisotropy is important when using CZT in high-frequency magnetic applications. Static values were obtained with an induction-field magnetometer while dynamic values were obtained using a pulsed inductive microwave magnetometer.

Dynamic anisotropy in amorphous CoZrTa films

The high-frequency response of amorphous CoZrTa thin films was measured by using a pulsed inductive microwave magnetometer. The anisotropy of the magnetic films was varied by magnetic field annealing. Static anisotropy field values ranging from Hk=100 to 1920 A/m were obtained. The dynamically determined anisotropy field is shifted to higher values compared to the static anisotropy by an additional isotropic internal field Hadd. This internal field is independent of the strength of the static anisotropy field. We determined a value of about Hadd=510 A/m.

Variation of magnetization and the Landé g factor with thickness in Ni–Fe films

We have measured the Landé g factor, the effective magnetization Meff, the uniaxial anisotropy Hk, and the Gilbert damping parameter α, as a function of Permalloy film thickness from 2.5 to 50 nm. We used a pulsed inductive microwave magnetometer capable of generating dc bias fields of 35.2 kA/m (440 Oe). A significant decrease in g is observed with decreasing thickness below 10 nm. Also, Meff decreases with decreasing thickness consistent with a surface anisotropy constant of 0.196±0.025 mJ/m2. The decrease in g can arise from the orbital motion of the electrons at the interface not being quenched by the crystal field. We also compare our data to a model of an effective g factor suggesting that the decrease in g factor might also stem from the Ni–Fe interface with a Ta underlayer.

Different dynamic and static magnetic anisotropy in thin Permalloy™ films

The values of uniaxial anisotropy Hk in thin polycrystalline Permalloy™ films measured by static and dynamic methods differ by as much as a factor of 1.5. The anisotropy obtained with a pulsed inductive microwave magnetometer in 2.5 to 100 nm thick Permalloy films exhibits an additional isotropic component of 120 to 240 A/m not observed in static measurements. The static value of anisotropy was obtained with an inductive magnetic hysteresis loop tracer. The time-resolved precessional response was measured as a function of in-plane applied magnetic bias field and the angle between the easy axis and that of the applied bias field. We interpret the constant-offset field as a transient component of the magnetic susceptibility that affects only dynamical response at time scales below 10 ns.

Cryogenic pulsed inductive microwave magnetometer

A cryogenic pulsed inductive microwave magnetometer is used to characterize the switching dynamics in thin-film magnetic materials at low temperatures and microwave frequencies. The system is contained inside a 20-cm-diam ultrahigh vacuum chamber and cooled by a cryopump that allows measurements between 20 and 350 K. A temperature controller regulates the sample temperature using two silicon diodes as sensors. Applied magnetic fields of up to 36 kA/m (450 Oe) are generated by a four-pole, water-cooled electromagnet with independent control of each axis. Magnetic switching in the sample is driven by high-speed current step pulses in a coplanar waveguide structure with the sample placed in a flip-chip configuration. A 20 GHz sampling oscilloscope is used to record the dynamics of the magnetic reorientation. The switching dynamics are given for a 10-nm-thick Ni–Fe film at 30 K in response to a 1 kA/m field step.

Damping as a function of pulsed field amplitude and bias field in thin film Permalloy

We have measured the step response in thin film Permalloy as a function of both a hard-axis pulsed field amplitude and an easy-axis longitudinal magnetic bias field using a pulsed inductive microwave magnetometer. The bias field ranged from 0 to 8000 A/m (0 to 100 Oe) and the pulsed field varied from 0.32 to 320 A/m (0.004 to 4 Oe). The rotation angle of the equilibrium magnetization direction varied from 0.002° to 40° for this range of field values. Data were analyzed to extract the Gilbert damping parameter, α. The damping parameter decreased monotonically with an increase in longitudinal bias field. However, there is no observed dependence of α on the pulse amplitude, indicating that the damping is independent of the angle of rotation. We conclude that there is no significant nonlinear generation of spin waves that affects the damping in the case of free induction decay for the range of field pulses employed.

Pulsed inductive microwave magnetometer

We describe the apparatus, software, and measurement procedures for a pulsed inductive microwave magnetometer (PIMM). PIMM can measure the dynamical properties of materials used in recording heads for magnetic storage applications, and it can be used as a general magnetodynamics diagnostic tool. PIMM uses a coplanar waveguide as both a source of fast pulsed magnetic fields and as an inductive flux sensor. Magnetic field pulses are provided by a 10 V, 55 ps risetime pulse generator; a 20 GHz digital sampling oscilloscope is used to acquire the fast pulse data; and orthogonal Helmholtz pairs provide the bias and saturating fields required for the measurement. The system can measure dynamical behavior as a function of several variables, including applied magnetic bias field, magnetic pulsed field amplitude and width, and sample orientation. Using a fast Fourier transform, PIMM can determine the frequency dependence of the complex magnetic permeability, as well as the step and impulse responses of the magnetic system. Data from 50 nm Ni–Fe and rare-earth-doped Ni–Fe thin films are presented.