Pulsed Inductive Microwave Magnetometry Research Articles

Coherent control of magnetization precession in electrically detected time domain ferromagnetic resonance

We demonstrate coherent control of time domain ferromagnetic resonance by all electrical excitation and detection. Using two ultrashort magnetic field steps with variable time delay we control the induction decay in yttrium iron garnet (YIG). By setting suitable delay times between the two steps the precession of the magnetization can either be enhanced or completely stopped. The method allows for a determination of the precession frequency within a few precession periods and with an accuracy much higher than can be achieved using fast fourier transformation. Moreover it holds the promise to massively increase precession amplitudes in pulsed inductive microwave magnetometry (PIMM) using low amplitude finite pulse trains. Our experiments are supported by micromagnetic simulations which nicely confirm the experimental results.

Spin precession by pulsed inductive magnetometry in thin amorphous plates

Broadband magnetic loss and damping behavior of Co-based amorphous ribbons and thin films have been investigated. The permeability and loss response of the transverse anisotropy ribbon samples in the frequency range DC to 1 GHz is interpreted in terms of combined and distinguishable contributions to the magnetization process by domain wall displacements and magnetization rotations. The latter alone are shown to survive at the highest frequencies, where the losses are calculated via coupled Maxwell and Landau–Lifshitz–Gilbert (LLG) equations. Remarkably high values of the LLG damping coefficient α = 0.1–0.2 are invoked in this theoretical prediction. Direct measurements of α by pulsed inductive microwave magnetometry are thus performed, both in these laminae and in amorphous films of identical composition, obtaining about one order of magnitude increase of the α value upon the 100 nm÷10 μm thickness range. This confirms that dissipation by eddy currents enters the LLG equation via large increase of the damping coefficient.

Inductive determination of the optimum tunnel barrier thickness in magnetic tunneling junction stacks for spin torque memory applications

We use pulsed inductive microwave magnetometry to study the precessional magnetization dynamics of the free layer in CoFeB/MgO/CoFeB based magnetic tunneling junction stacks with varying MgO barrier thickness. From the field dependence of the precession frequency we are able to derive the uniaxial anisotropy energy of the free layer and the exchange coupling between the free and the pinned layer. Furthermore the field dependence of the effective damping parameter is derived. Below a certain threshold barrier thickness we observe an increased effective damping for antiparallel orientation of free and pinned layer which would inhibit reversible low current density spin torque magnetization reversal. Such inductive measurements, in combination with wafer probe station based magneto transport experiments, allow a fast determination of the optimum tunnel barrier thickness range for spin torque memory applications in a lithography free process.

Inductive and magneto-resistive measurements of Gilbert damping in Ni81Fe19 thin films and microstructures

We study time resolved precessional magnetization dynamics of permalloy (Ni81Fe19) thin films by time resolved detection of the anisotropic magneto-resistance (AMR) and by pulsed inductive microwave magnetometry (PIMM). AMR experiments are performed on microstructured permalloy discs of 4 µm diameter. PIMM experiments are performed on unpatterned macroscopic thin films of the same material. The magnetization dynamics observed by AMR can be well described by mainly single domain precession neglecting spin wave excitations. In contrast PIMM experiments on macroscopic samples show evidence of multiple precessional modes which cannot be described by a single Gilbert damping parameter. Consequences of the measurement of the intrinsic Gilbert damping parameter by the two methods are discussed.

Comment on “Dynamic anisotropy of thin Permalloy films measured by use of angle-resolved pulsed inductive microwave magnetometry” [Appl. Phys. Lett. 86, 202503 (2005)

Comment on “Dynamic anisotropy of thin Permalloy films measured by use of angle-resolved pulsed inductive microwave magnetometry” [Appl. Phys. Lett. 86, 202503 (2005)

Comparison of frequency, field, and time domain ferromagnetic resonance methods

We present vector network analyzer ferromagnetic resonance measurements of epitaxial Fe films having a thickness of 16 monolayers. Our objective is to test the reliability of this novel frequency domain technique with respect to frequency and damping. For this purpose we compare vector network analyzer ferromagnetic resonance to pulsed inductive microwave magnetometry, time resolved magneto-optic Kerr effect (both methods in the time domain), and conventional ferromagnetic resonance (measured in the field domain) in terms of position and width of the ferromagnetic resonance. In addition, we compare the various techniques with respect to the signal to noise ratio of the raw data. All data is obtained using the same well characterized ultrathin magnetic Fe/GaAs (0 0 1) film. Finally, we demonstrate the potential of the vector network analyzer ferromagnetic resonance technique for the investigation of nano-structured magnetic elements having nonuniform magnetization configuration. The absorption spectrum of Permalloy disks with a diameter of 200 nm and a thickness of 15 nm shows up to eight distinct resonance peaks. The spatial structure of the corresponding modes was derived from numerical calculations and reveals that azimuthal modes up to the fifth order have been observed inductively.

Large-angle magnetization dynamics measured by time-resolved ferromagnetic resonance

A time-resolved ferromagnetic resonance technique was used to investigate the magnetization dynamics of a 10 nm thin Permalloy film. The experiment consisted of a sequence of magnetic field pulses at a repetition rate equal to the magnetic systems resonance frequency. We compared data obtained by this technique with conventional pulsed inductive microwave magnetometry. The results for damping and frequency response obtained by these two different methods coincide in the limit of a small angle excitation. However, when applying large amplitude field pulses, the magnetization had a non-linear response. We speculate that one possible cause of the nonlinearity is related to self-amplification of incoherence, known as the Suhl instabilities.

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.

Dynamic anisotropy of thin Permalloy films measured by use of angle-resolved pulsed inductive microwave magnetometry

In this study, angle-resolved pulsed inductive microwave magnetometry is used to investigate the symmetry of the dynamic anisotropy of thin Permalloy films. We measured the dynamic anisotropy field as a function of angle between the easy axis and the applied bias field. We found that, in addition to the expected uniaxial anisotropy, there is a rotatable component of anisotropy. This component of the anisotropy is present only during the dynamics measurements and is attributed to surface effects in the thin films. However, the native oxide layer is not the cause of the rotatable anisotropy components in these films.

Finite coplanar waveguide width effects in pulsed inductivemicrowave magnetometry

The effect of finite coplanar waveguide (CPW) width on the measurement of the resonance frequency in thin ferromagnetic films has been characterized for pulsed inductive microwave magnetometry. A shift in resonant frequency is a linear function of the ratio of sample thickness to CPW width. The proportionality constant is experimentally determined to be 0.74±0.1 times the saturation magnetization of the film. The frequency shift may be modeled as arising from an effective magnetic-anisotropy field.

High-Frequency Properties of FeCoSiB Thin Films With Crossed Anisotropy

Investigations on amorphous FeCoSiB/SiO/sub 2//FeCoSiB thin-film samples with crossed anisotropies are presented in this paper. The samples consist of two magnetic layers, 40 nm thick each, where the second magnetic layer's uniaxial anisotropy is rotated by an angle of 90/spl deg/ with respect to the anisotropy of the first layer. The individual layers were separated by a 20 nm SiO/sub 2/ thin film. The angular dependent magnetic properties of the samples have been characterized by vibrating sample and inductive magnetometry. The high-frequency (HF) response was determined by HF-permeameter measurements and pulsed inductive microwave magnetometry (PIMM). Good HF-response independent of the magnetic field excitation angle is achieved. The results are compared to an 80 nm FeCoSiB reference film with uniaxial anisotropy. Our results provide new insights important for the design of HF magnetic devices.

Structure and Magnetic Switching of Thin-Film a-HITPERM/<tex>$hboxSiO_2$</tex>Soft Magnetic Multilayers

Laminated (Fe/sub 0.7/Co/sub 0.3/)/sub 88/Zr/sub 7/B/sub 4/Cu/sub 1/ amorphous HITPERM/SiO/sub 2/ multilayer thin films have been studied by room-temperature (RT) pulsed inductive microwave magnetometry (PIMM) in single, bilayer, and trilayer films. Switching has been measured in a multilayer film with six (50 nm) HITPERM layers, separated by five (2 nm) SiO/sub 2/ layers. Films were investigated by conventional transmission electron microscopy (TEM), high-resolution TEM (HREM), and superconducting quantum interference device (SQUID) magnetometry. HREM and TEM show BCC FeCo nanocrystals to nucleate on top of SiO/sub 2/ layers. Plan view TEM on the top layer reveals FCC nanocrystals that align in chains with spacing of /spl sim/50-100 nm. SQUID magnetometry shows reversal to begin by rotation in a single layer for H<14.4 kA/m (180 Oe) followed by nearly simultaneous reversal of several layers. A final switching event is thermally activated, requiring fields in excess of 8 kA/m (100 Oe) to switch at 2 K, but switching at the same field as other layers for elevated temperatures (RT).