Measurements Of Magnetization Dynamics Research Articles

Magnetic domain-dependent ultrafast optical demagnetization in stripe domain films

We investigated femto- and picosecond-time magnetization dynamics in a ferromagnetic Ni80Fe20 film with varying thicknesses (wedge-shaped film). We observed that the thickness gradient strongly affects the magnetic moment distribution, causing a magnetization reorientation from in-plane to out-of-plane, and formatting a stripe domain (SD) at the thicker end of the wedge. The magnetization dynamics measurements reveal that the part of the film displaying SDs follows a substantially faster demagnetization and magnetization recovery and smaller magnetization quenching compared to the in-plane domain film. The experiments and micromagnetic simulations support that the decrease in relaxation time is caused by a magnetic anisotropy of the films introduced by SD formation. Our results point out that the micromagnetic structure plays an important role in the magnetization dynamics in ferromagnetic films after optically triggered demagnetization.

Direct spectroscopic observation of Berry-phase interference in the Ni4 single-molecule magnet

Berry phase effects in spin systems lead to the suppression of tunneling effects when different tunneling paths interfere destructively. Such effects have been seen in several single-molecule magnets (SMMs) through measurements of magnetization dynamics, where the experimental signal may arise from the contributions of numerous energy levels. Here we present experimental measurements of Berry phase interference effects that are determined through electron-spin resonance on a four-fold symmetric SMM. Specifically, we measure transitions between tunnel-split excited states in the Ni$_4$ SMM in the presence of a transverse field in the hard plane of the crystalline sample. By using a home-built rotation apparatus, the direction of the sample can be changed \textit{in situ} so that that the field direction can be swept through the entire hard plane of the sample. When the field is in certain directions in the plane, we observe a splitting of the transition, a hallmark of Berry phase interference. The experimental results are well reproduced by theoretical predictions, and fitting of the data provides information about the effects of dipolar interactions and sample misalignment.

A novel method for time-resolved measurement of magnetization dynamics induced by femtosecond laser pulse in highly absorbing and metallic layer coated thin films based on a magnetic loop antenna

Femtosecond laser-induced magnetization dynamics is a popular research topic, due to its potential applications in fast magnetic storage devices and the field of spintronics. In the article we present a novel technique for time-resolved measurement of femtosecond laser-induced magnetization dynamics, based on inductive sensing by a loop antenna. It allows performing investigation of magnetization dynamics in highly absorbing samples, samples with patterned surfaces and samples coated by a thin metallic layer, which are difficult to study using optical techniques. To test the technique, both thick and thin films made of highly absorbing and diffusing materials, were used. Results of tests for several ferromagnetic materials including Gadolinium are discussed in the article. The test results prove that the method provides a simple tool to observe and study magnetization dynamics phenomena in the vicinity of the materials’ Curie temperature.

Quantitative analysis and optimization of magnetization precession initiated by ultrafast optical pulses

Magnetic storage and magnetic memory have recently shifted towards the use of magnetic thin films with large perpendicular magnetic anisotropy (PMA) to simultaneously satisfy the requirements in storage density and thermal stability. Understanding the magnetic switching process and its dependence on the Gilbert damping (α) of materials with large PMA is crucial for developing low-power consumption, fast-switching, and high-thermal stability devices. The need to quantify α of materials with large PMA has resulted in the development of the all-optical ultrafast Time-Resolved Magneto-optical Kerr Effect (TR-MOKE) technique. While TR-MOKE has demonstrated its capability of capturing magnetization dynamics of materials with large PMA, a quantitative analysis regarding the operational optimization of this emerging technique is still lacking. In this paper, we discuss the dependence of the TR-MOKE signal on the magnitude and angle of the applied field, by utilizing a numerical algorithm based on the Landau-Lifshitz-Gilbert equation. The optimized operational conditions that produce the largest TR-MOKE signals are predicted. As an experimental verification, we conduct TR-MOKE measurements on a representative sample of a tungsten-seeded CoFeB PMA thin film to show the excellent agreement of the model prediction with measurements. Our analysis results in a better understanding of the external field influence on the magnetization precession processes. The results of this work can also provide guidance on selecting operational conditions of the TR-MOKE technique to achieve optimal signal-to-noise ratios and thus more accurate measurements of magnetization dynamics.

Optical-helicity-driven magnetization dynamics in metallic ferromagnets

Recent observations of switching of magnetic domains in ferromagnetic metals by circularly polarized light, so-called all-optical helicity dependent switching, has renewed interest in the physics that governs the interactions between the angular momentum of photons and the magnetic order parameter of materials. Here we use time-resolved-vectorial measurements of magnetization dynamics of thin layers of Fe, Ni and Co driven by picosecond duration pulses of circularly polarized light. We decompose the torques that drive the magnetization into field-like and spin-transfer components that we attribute to the inverse Faraday effect and optical spin-transfer torque, respectively. The inverse Faraday effect is approximately the same in Fe, Ni and Co, but the optical spin-transfer torque is strongly enhanced by adding a Pt capping layer. Our work provides quantitative data for testing theories of light–material interactions in metallic ferromagnets and multilayers.

Amorphous <inline-formula> <tex-math notation="TeX">${\rm CoFe}_{2}{\rm O}_{4}$ </tex-math></inline-formula> Nanoparticles: Synthesis, Crystallization Process, and Magnetic Properties

Amorphous ${\rm CoFe}_{2}{\rm O}_{4}$ nanoparticles have been successfully synthesized by the chemical microwave-assisted method. The crystallization process and the hard magnetic properties of prepared nanoparticles were systematically investigated in terms of thermal analysis and in situ measurement of magnetization dynamics. The as-prepared sample was amorphous, which was confirmed by various techniques, such as high resolution transmission electron microscopy, with electron diffraction and X-ray diffraction. The thermal analysis using differential scanning calorimetry showed that crystallization from ${\rm CoFe}_{2}{\rm O}_{4}$ amorphous state to ${\rm CoFe}_{2}{\rm O}_{4}$ crystalline started at 290 $^{\circ}{\rm C}$ . Crystallization activation energy was determined using the Kissinger model with a value of 1.09 eV. A study of the temperature dependence of magnetization supported the crystallization process of cobalt ferrite nanoparticles. Magnetic measurement indicated that the as-prepared amorphous particles were superparamagnetic. Compared with the bulk material, the observed value of saturation magnetization of the annealed sample was significantly low (27 emu/g) due to its nanocrystalline nature.

Ferromagnetic resonance: Electrical detection vs conventional absorption measurements

Ferromagnetic resonance (FMR) in point contacts to a soft CoSiBFeNb magnet is examined by three different techniques: electrical detection via rectification, power-absorption measurements, and time-resolved measurements of magnetization dynamics. The electrical detection is realized by taking advantage of the point contact as a nano-scale bolometer which monitors the microwave current absorbed by the sample. The power-absorption measurements exploit an external sensor to monitor the microwave absorption as a function of applied magnetic field. Finally, a high-bandwidth sampling oscilloscope allows for the time-resolved measurement of magnetization dynamics during FMR and provides the phase- and amplitude-characterization of FMR at 4-8 GHz. Simultaneous detection of the rectification signal, microwave absorption, and time-resolved microwaves allows for a direct comparison of the electrical detection and conventional FMR schemes. A good agreement between different FMR methods was found and the expected phase shift and amplitude variation for a driven resonance were confirmed.

Ultrafast element-specific magnetization dynamics of complex magnetic materials on a table-top

We review recent progress in femtosecond magnetization dynamics probed by extreme ultraviolet pulses from high-harmonic generation. In a transverse magneto-optical Kerr geometry, we established an ultrafast, element-specific experimental capability – on a table-top – for the measurement of magnetization dynamics in complex multi-sublattice magnets and multilayer magnetic structures. We show that this newly introduced technique is an artifact-free magnetic sensor, with only negligible non-magnetic (optical) contributions from the transient variation of the refractive index due to the presence of a non equilibrium hot-electron distribution. We then use these new experimental capabilities of ultrahigh time-resolution, combined with element-specific simultaneous probing, to disentangle important microscopic processes that drive magnetization dynamics on femtosecond timescales. We elucidate the role of exchange interaction on magnetization dynamics in strongly exchange-coupled alloys, and the role of photo-induced superdiffusive spin currents in magnetic multilayer stacks.

Time- and vector-resolved Kerr microscopy of hard disk writers

Time-resolved scanning Kerr microscopy has been used to make wafer level measurements of magnetization dynamics within the yoke and pole piece of partially built hard disk writer structures. Three Cartesian components of the vector magnetization were recorded simultaneously using a quadrant photodiode polarization bridge detector. The rise time, relaxation time, and amplitude of each component has been related to the magnetic ground state, the initial torque, and flux propagation through the yoke and pole piece. Dynamic images reveal “flux-beaming” in which the magnetization component parallel to the symmetry axis of the yoke is largest along that axis.

Determination of the spin-flip time in ferromagnetic SrRuO3from time-resolved Kerr measurements

We report time-resolved Kerr effect measurements of magnetization dynamics in ferromagnetic SrRuO${}_{3}$. We observe that the demagnetization time slows substantially at temperatures within 15 K of the Curie temperature, which is $~$150 K. We analyze the data with a phenomenological model that relates the demagnetization time to the spin-flip time. In agreement with our observations, the model yields a demagnetization time that is inversely proportional to $T\ensuremath{-}{T}_{c}$. We also make a direct comparison of the spin-flip rate and the Gilbert damping coefficient, showing that their ratio is very close to ${k}_{B}{T}_{c}$, indicating a common origin for these phenomena.

Comparison of electrical techniques for magnetization dynamics measurements in micro/nanoscale structures

Three all-electrical detection techniques—differential resistance (dV/dI), rf/microwave transmission, and spin rectification measurements—were used to measure the magnetic response of individual patterned Permalloy structures. The investigated techniques enable the identification of distinctive magnetization states, such as vortex nucleation and excitation, domain wall motion, ferromagnetic resonance, and spin wave excitations both in the quasistatic and dynamic regimes. The techniques differ in signal-to-noise ratio and sensitivity to the different types of magnetization responses studied here, suggesting that the use of multiple techniques can significantly improve the understanding of magnetically complex structures.

Optically induced magnetization dynamics and variation of damping parameter in epitaxialCo2MnSiHeusler alloy films

All-optical pump-probe measurements of magnetization dynamics have been performed upon epitaxial ${\text{Co}}_{2}\text{MnSi}(001)$ Heusler alloy thin films annealed at temperatures of 300, 400, and $450\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$. An ultrafast laser-induced modification of the magnetocrystalline anisotropy triggers precession which is detected by time-resolved magneto-optical Kerr effect measurements. From the damped oscillatory Kerr rotation, the frequency and relaxation rate of the precession is determined. Using a macrospin solution of the Landau-Lifshitz-Gilbert equation the effective fields acting upon the sample magnetization are deduced. This reveals that the magnetization is virtually independent of the annealing temperature while the fourfold magnetocrystalline anisotropy decreases dramatically with increasing annealing temperature as the film structure changes between the B2 and $\text{L}{2}_{1}$ phases. From the measured relaxation rates, the value of the apparent Gilbert damping parameter is found to depend strongly upon the static field strength and in-plane static field orientation. The variation of the apparent damping parameter is generally well reproduced by an inhomogeneous broadening model in which the presence of B2 and $\text{L}{2}_{1}$ phases leads to a large dispersion of the magnetocrystalline anisotropy. However, for the sample annealed at a temperature of $300\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C}$, the lack of a detailed fit to the data suggests that the apparent anisotropy of the apparent damping parameter might alternatively arise due to a network of dislocations with fourfold symmetry.

Phase and amplitude of element-specific moment precession in [formula omitted

We present a time and element-resolved measurement of magnetization dynamics during ferromagnetic resonance (FMR) in a single layer of Ni 81 Fe 19 . Small-angle ( < 1 ∘ ) precession of elemental Ni, Fe moments could be measured directly and quantitatively using time-resolved X-ray magnetic circular dichroism (XMCD) in transmission. The high temporal and rotational sensitivity of this technique has allowed quantification of the phase and amplitude of driven FMR motion at 2.3 GHz, verifying basic expectations for a driven resonance.

Measurement of magnetization dynamics in single-molecule magnets induced by pulsed millimeter-wave radiation

We describe an experiment aimed at measuring the spin dynamics of the Fe8 single-molecule magnet in the presence of pulsed microwave radiation. In earlier work, heating was observed after a 0.2ms pulse of intense radiation, indicating that the spin system and the lattice were out of thermal equilibrium at millisecond time scales [Bal et al., Europhys. Lett. 71, 110 (2005)]. In the current work, an inductive pickup loop is used to probe the photon-induced magnetization dynamics between only two levels of the spin system at much shorter time scales (from nanoseconds to microseconds). The relaxation time for the magnetization, induced by a pulse of radiation, is found to be on the order of 10μs.

A crossed coplanar waveguide design for ultrafast magnetization switching utilizing polymer insulation layers

A crossed coplanar waveguide design using polymer insulation layers is developed as a tool to experimentally investigate fast, precessional switching phenomena in view of applications in magnetic random access memory. The photosensitive polymer Cyclotene 4024™ (benzocyclobutene) is used as an electric insulation layer to realize a multilayer sample consisting of several different conducting and insulating layers. We report on the processing of Cyclotene 4024™ multilayers with optimized surface and electric insulating properties after hard cure. The micropatterned impedance matched coplanar waveguides have a bandwidth of 8.5GHz according to −2dB criterion, and for the −3dB criterion the bandwidth exceeds the measurement range of up to 15GHz. These values allow for short rise and fall times of the pulse for ultrafast magnetization dynamics measurements.

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.

Retrieving pulse profiles from pump-probe measurements on magnetization dynamics

A method for back-tracing magnetic field pulses in pump-probe-type magnetization dynamics measurements is presented. Solving vectorially the Landau–Lifshitz–Gilbert equation for our field-induced measurement geometry yields field pulse profiles fulfilling the theoretical expectations on a 100 ps timescale. Applying the method to our earlier, all-optical-type pump-probe measurements, the claim that the optical pulse triggers an ultrafast anisotropy field pulse gets a direct proof: we derive a pulse consisting of a delta-peak type, full width at half maximum &amp;lt;5 ps impulse on top of a very fast rising step-like profile. Application of the method to other, less trivial pump-probe schemes can contribute to the development of novel type magnetic recording technologies.

Inductive measurement of the high frequency permeability of a Permalloy thin film

An improved broadband inductive technique for the measurement of magnetization dynamics in thin ferromagnetic films is presented. The technique is demonstrated using 9.3 nm thick Permalloy films, and allows for time and frequency domain measurements to be made in parallel. Free oscillations after short duration pulses (200 ps) and broadband response to radio frequency fields up to 6 GHz have been observed, and can be precisely matched to existing theories. This opens the way for detailed studies of the relaxation mechanisms above 1 GHz .

Interface coupling and magnetic properties of exchange-coupled Ni81Fe19/Ir22Mn78 bilayers

Hysteresis loop measurements using magnetooptic Kerr effect magnetometry in the low frequency region and magnetization dynamics measurements using high frequency permeametry are carried out to study the exchange anisotropy in Ni81Fe19/Ir22Mn78 bilayers. These two measurement techniques provide different exchange anisotropies for bilayers with thin Ir22Mn78 films. The observations can be understood by assuming that the AF grains break into domains due to the interface random field. By analysing the results from these two techniques, the interface coupling strength and the magnetic properties of the bilayers can be quantitatively determined.

Advanced Soft Magnetic Materials for Magnetic Recording Heads and Integrated Inductors

AbstractFor Abstract see ChemInform Abstract in Full Text.