Ferromagnetic Resonance Experiments Research Articles

Transport Properties of Pure Spin Currents in a Polycrystalline Gd Wire

We have investigated the magneto- and spin-current-transport properties in patterned Gd wires in order to understand the interplay between the spin current and the spin entropy. From the magneto-transport measurements, the electrical resistance was found to decrease with decreasing the spin entropy because of the suppression of the spin fluctuation. We also found that the Gd acts as a strong absorber both for the ac and dc pure spin currents from the ferromagnetic resonance and nonlocal spin injection experiments.

Dynamic and rotatable exchange anisotropy in Fe/KNiF3/FeF2 trilayers

Results from ferromagnetic resonance experiments carried out on epitaxially grown Fe/KNiF3/FeF2 trilayers are presented. Exchange coupling between the KNiF3, a weak anisotropy antiferromagnet, and the Fe leads to shifts in the resonance field of the ferromagnet. The field shifts can be described by a temperature-dependent exchange anisotropy . depends on the orientation direction of the applied field relative to the magnetic anisotropy axis, and a non-monotonic dependence on KNiF3 thickness. Three thickness regimes appear that correspond to different values of exchange bias in each region. A qualitative understanding of the basis for these three thickness regimes due to spin canting at the interfaces is presented. Our results illustrate a method to tune the value of exchange anisotropy using a combination of different antiferromagnets.

Ferromagnetic resonance behavior of spark plasma sintered Ni–Zn ferrite nanoparticles produced by a chemical route

Ferrite nanoparticles of composition Zn0.5Ni0.5Fe2O4 were prepared by forced hydrolysis in a polyol (polyol process) from the corresponding iron, nickel, and zinc acetates. Synthesis conditions allowed for obtaining polycrystalline epitaxial clusters of about 22 nm in size with an average crystal size of about 5 nm. These powders were subsequently consolidated by spark plasma sintering (SPS) technique under a pressure of 80 MPa and temperatures in the 350 to 500 °C range for short periods (5 to 10 min). Densities reached 92% to 94% of the theoretical density. Particle size remained smaller than 61 nm, even for the highest temperature. Ferromagnetic resonance experiments at 77 K showed broadened resonance lines. In contrast with powdered nanoparticles, these high-density bodies can be used in many high-frequency applications.

Annealing effects on the magnetic dead layer and saturation magnetization in unit structures relevant to a synthetic ferrimagnetic free structure

The changes in the magnetic dead layer (MDL) and saturation magnetization of the CoFeB layers are investigated as a function of the annealing temperature for four different unit structures, that are relevant to the synthetic ferrimagnetic free structure in MgO-based magnetic tunnel junctions. The MDL results for these unit structures are then converted into those for the constituent interfaces of the free structure. Most of the changes in the MDL thickness occur during annealing at a low temperature of 150 °C while those in the saturation magnetization occur at a high annealing temperature of 350 °C. These results for the MDL and saturation magnetization are critically tested by using the synthetic ferrimagnetic free structures with various thickness asymmetries. The observed switching properties of these tested structures are in good agreement with those expected from the results for the MDL and saturation magnetization, confirming the accuracy of the present results. The accuracy of the saturation magnetization is further confirmed by ferromagnetic resonance experiments.

Temperature dependence of exchange bias in NiFe/FeMn bilayers

Ferromagnetic resonance (FMR) and dc-magnetometry measurements were used to investigate the temperature dependence of the anisotropies responsible for the exchange-bias coupling in NiFe/FeMn bilayer. At room temperature, both measurements showed the same exchange-bias angular distributions due to a low coupling strength, ${H}_{\text{E}}$, with respect to the antiferromagnet domain-wall formation anisotropy. The anisotropies derived from both techniques displayed identical ${H}_{\text{E}}$ values but different uniaxial anisotropies. This disagreement may be ascribed to different stabilities of antiferromagnetic grains in both techniques. At low temperature, strong disagreements were found on the exchange-bias values. They are explained by the occurrence of the slow relaxer mechanism in FMR experiments.

High Frequency Magnetic Properties and Intergranule Interaction of Nano-Granular Films with High Resistivity and High Permeability

In order to get magnetic films with high permeability and high resistivity for applications in high frequency devices, (Fe65Co35)x(SiO2)1-x nano-granular films with low metal volume fraction were fabricated by radio frequency magnetron sputtering, and excellent soft magnetic properties have been acquired by annealing the sample with x = 0.38 at 260 degrees C in a magnetic field. The real part micro' of complex permeability is larger than 100 when frequency f is lower than 0.7 GHz, and the ferromagnetic resonance frequency f(r) is as high as 1.8 GHz. More interestedly, the resistivity of this sample reaches 13.4 momega x cm, 4 orders higher than that of pure FeCo alloy. Ferromagnetic resonance experiment and deltam-H curve study were employed to elucidate the mechanism of soft magnetic properties. The results reveal that the intergranule exchange coupling plays an important role in realizing good soft magnetic properties for metal-insulator granular films.

Temperature evolution of self-organized stripe domains in ultrathin Fe films on MnAs/GaAs(001)

A detailed investigation of the magnetic behavior of an ultrathin Fe film epitaxially grown onto MnAs/GaAs(001) is performed by means of both ferromagnetic resonance (FMR) and magneto-optical Kerr-effect (MOKE) experiments, as a function of temperature. When the MnAs underlayer is fully in the hexagonal ferromagnetic $\ensuremath{\alpha}$ phase (at low temperature) or in the orthorhombic non magnetic $\ensuremath{\beta}$ one (at high temperature), the magnetic behavior of the Fe film is found to be nearly the same and independent of the substrate. In contrast, at intermediate temperatures, when the MnAs underlayer presents a self-organized phase consisting of $\ensuremath{\alpha}\text{-MnAs}$ and $\ensuremath{\beta}\text{-MnAs}$ striped domains, the Fe film is strongly influenced by the dipolar (stray) fields arising from the substrate. As a consequence, the Fe film breaks in two distinct families of striped domains. Theoretical analysis of the FMR data in the whole investigated temperature range is performed using a free-energy density model which provides an accurate determination of the temperature-dependent magnetic parameters. Furthermore, the mechanism of magnetization reversal has been analyzed as a function of temperature by MOKE. The evolution of the magnetization curves can be satisfactory interpreted starting from the main achievements of the theoretical analysis of FMR data.

Magnetoresistance effect in (La, Sr)MnO3 bicrystalline films

The angular dependence of the magnetoresistance effect has been measured on bicrystallineLa0.75Sr0.25MnO3 films. The measurements have been performed on an electronically lithographedWheatstone bridge. The study of the angular dependence of both the magnetoresistanceand the resistance of single-crystalline and grain-boundary regions of the samples allowedus to isolate two contributions of low-field magnetoresistance in manganites. One of them isassociated with the spin–orbit effect, i.e. the anisotropic magnetoresistance of ferromagneticcompounds, and the other one is related to spin-disorder regions at the grainboundary. Complementary x-ray diffraction, ferromagnetic resonance and lowtemperature magnetization experiments contribute to the characterization of themagnetic anisotropy of the samples and the general comprehension of the problem.

Ferromagnetic resonance studies of exchange coupled ultrathin Py/Cr/Py trilayers

Magnetic properties of ultrathin Py/Cr/Py trilayers have been investigated as a function of Cr spacer layer thickness by using ferromagnetic resonance (FMR) and vibrating sample magnetometer (VSM) techniques. The Cr spacer layer thickness was increased from 4 to 40 Å with 1 Å steps to determine the dependence of interlayer exchange coupling between ferromagnetic layers on the spacer layer thickness. Two strong and well resolved peaks were observed which correspond to a strong (acoustic) and weak (optic) modes of magnetization precession in the effective dc field due to the exciting external microwave field as the external dc field orientation comes close to the film normal. The separation of the two modes in the field axis depends on the thickness of Cr spacer layer. An interchange in the relative positions of the acoustic and optic modes has been observed for a particular thickness of Cr spacer layer as well. A computer program for magnetically exchange coupled N magnetic layers was written to simulate the experimental FMR spectra and to obtain the magnetic parameters of ultrathin Py/Cr/Py trilayers. FMR data have been analyzed from every aspect by using this program and interlayer exchange coupling constant was calculated for the prepared structures. It was found that the relative position of the peaks depends on the nature (sign) of the interlayer exchange coupling between ferromagnetic layers through Cr spacer layer. In Py/Cr/Py trilayers, strength of the interlayer exchange coupling constant oscillates and changes its sign with Cr spacer layer thickness with a period of about 11 Å.

Competing interparticle interactions and surface anisotropy in NiO nanoparticles

We report unconventional magnetic properties on NiO nanoparticles of an average diameter ∼5.8(7) nm obtained by coprecipitation method. To investigate the effect of the intra and interparticles interactions in the magnetic properties nanoparticles were dispersed in a polyvinyl-pyrrodone matrix at two different concentrations. X-ray, ac, and dc magnetization and ferromagnetic resonance experiments were carried out on powder and dispersed NiO systems. Our results show that dispersed and concentrated samples exhibit following two different magnetic behaviors: (i) a high temperature peak related to the blocking of the particle core and (ii) a low temperature maximum likely related to the freezing of the frustrated spins on surface particle. Besides, we have observed that the low temperature maximum is not field-dependent and depend strongly on the distance among particles. This result can be understood taking account the decreasing of the dipolar interaction to more dispersed samples.

Temperature dependence of the FMR absorption lines in viscoelastic magnetic materials

The magnetic properties of the viscoelastic materials filled with magnetic nanoparticles strongly depend on the viscosity of these materials. The time–temperature changes of the viscosity can affect the orientation ordering of the anisotropy axes of the magnetic nanoparticles after an external magnetic field is applied. In consequence, the absorption lines obtained in the ferromagnetic resonance experiment (FMR) will possess an additional temperature dependence through the viscosity of the materials under consideration. The particular case of the temperature dependence of the FMR signal detected from the γ-Fe 2O 3 (maghemite) magnetic nanoparticles dispersed at low concentration (0.1 wt.%) in a poly(ether-ester) multiblock copolymer (PEN-block-PTMO)) matrix has been investigated. A strong increase of the resonance line amplitude is observed with increasing temperature. We note also two qualitatively different temperature dependences of the magnetic resonance field at low and high temperatures. A simplified theoretical model of a single cluster consisting of N magnetic nanoparticles in an elastic polymer matrix is introduced to explain these temperature dependences of the absorption lines. The model is based on the stochastic version of the Lifshitz–Landau equation for nanoparticle magnetization.

Magnetic properties of ferromagnetic nanowire arrays: Theory and experiment

Magnetic nanowires are good candidates for microwave filters, sensors and data storage applications. An investigation of magnetic properties of single-component nanowires as a function of diameter and aspect ratio is performed in this work. Nickel nanowire (with 15 and 100 nm diameter and 6000 nm length) are grown with electrodeposition in polycarbonates templates. Two reversal modes (coherent and curling) are studied versus nanowire diameter. Magnetostatic interaction among wires and its effect on nanowire magnetic properties is also studied. Using vibrating magnetometer (VSM) and X-band ferromagnetic resonance (FMR) experiments at room temperature we infer that the interaction field Hc value may vary significantly and may cause a change of magnetic easy axis orientation along geometrical wire axis (for large diameter) to an easy magnetic plane perpendicular to the nanowire axis (for small diameter).

Microwave Magnetic Properties of NiFe Nanostrips

The dynamic excitations in the nano-sized Permalloy arrays exhibit a strong resonance mode in ferromagnetic resonance experiments (conventional as well as network analyzer based) for the magnetic field applied in the plane of the array and along the short axis and long axis of the arrays. In network-analyzer experiments, when the dc field was applied along the short axis of the arrays, the resonance frequency goes down with an increasing interstrip distance up to 1 mum and then saturates for higher values. This behavior clearly demonstrates the presence of dipolar interactions between the individual elements, resulting in the increased resonance frequency for the smaller spacing whereas when the field is applied along the long axis, the resonance frequencies go up with increasing interstrip distance.

Magnetoelectricity in Piezoelectric/Magnetostrictive Nanocomposites at Microwave Frequencies

We present a comprehensive study of the magnetic and microwave properties of piezoelectric BaTiO3 /magnetostrictive Ni nanocomposites (NCs), fabricated under uniaxial compression, at room temperature. In the current work, we investigated samples in function of magnetic phase content in a piezoelectric matrix and from 6 to 28 GHz using broadband ferromagnetic resonance (FMR-stripline) experiments in the microwave regime in combination with atomic (AFM) and magnetic force microscopy (MFM), X-ray diffraction (XRD), electron transport, and broadband (0.1-6 GHz) microwave spectroscopy experiments to correlate magnetization dynamics properties, electromagnetic materials parameters, and microstructural information to the magnetoelectric (ME) coupling coefficient. We provide the experimental evidence for ME effect, i.e., the effective permittivity at microwave frequencies can be controlled by an external magnetic field, which makes these nanostructures ready for microwave tunable devices, sensors, and transducers.

Finite-size scaling in band ferromagnets with non-universal critical behavior

The ultra-high sensitivity of the ferromagnetic resonance (FMR) techniquehas been fully exploited to study the finite-size effects in the criticalregion near the ferromagnetic to paramagnetic phase transition inCr75−xFe25+x (x = 0, 5) thin films of high structural and magnetic quality. Conclusiveexperimental evidence is provided for the validity of finite-size scaling.Irrespective of the film thickness and composition, the critical exponentsβ,γ andν for spontaneous magnetization, initial magnetic susceptibility and spin–spin correlationlength retain their bulk values so that no dimensionality crossover occurs within the filmthickness range covered in the FMR experiments. The present results indicate that (i) likeCr75−xFe25+x, the previously studiedFe, Co, Ni, and CoNi3 thin films behave as itinerant-electron (band) ferromagnets inwhich the isotropic long-range interactions between spins decay asJ(r)∼r−(d+σ) (σ>0), and (ii) the latticedimensionality d,spin dimensionality m, and range of spin–spin interactions (via the material-specific parameterσ) decide the (non-universal) values of the critical exponents.

Magnetoelectric effect in BaTiO3/Ni particulate nanocomposites at microwave frequencies

We present a comprehensive study of the magnetic and microwave properties of piezoelectric BaTiO3/magnetostrictive Ni nanocomposites (NCs), fabricated under uniaxial compression, at room temperature. In the current work, we investigated samples in the compositional range between 0≤fNi≤33.5 vol % and from 0.1 to 6 GHz using broadband microwave spectroscopy in combination with atomic and magnetic force microscopy (MFM), x-ray diffraction (XRD), electron transport, and broadband (6–28 GHz) ferromagnetic resonance (FMR) experiments in the microwave regime to correlate magnetization dynamics, electromagnetic materials parameters, and microstructural information. The static magnetic response is consistent with a model of a composite medium with an unmodified Ni phase in a nonmagnetic matrix. We provide the experimental evidence for a magnetoelectric (ME) effect, i.e., the effective permittivity at microwave frequencies can be controlled by an external magnetic field, which makes these nanostructures ready for microwave tunable devices, sensors, and transducers. We show in the analysis that this magnetic field dependence is inconsistent with expectations from magnetoresistance and magnetocapacitance effects, and propose as an alternative an explanation based on the striction across the interfaces between the magnetic and piezoelectric phases. By varying the Ni content and frequency, room temperature broadband FMR was performed in order to investigate the different contributions, e.g., inhomogeneous broadening, to the effective linewidth and microwave damping. The line broadening and asymmetry of the FMR features are not intrinsic properties of the metallic nanophase but reflects the local nonmagnetic environment in which they are embedded. The increase in the effective Gilbert damping coefficient as function of the Ni content is related to the strong increase in the damping experienced by the precessing magnetization in the Ni phase. One of the characteristic features of the present results is the significant correlation between the internal field probed by FMR and the ME coupling coefficient evaluated by microwave spectroscopy which was not observed in our previous study of ZnO/Ni NCs. The present results highlight the strong influence of interfaces of the composite constituent play a crucial role in the analysis of the ME coupling. In addition MFM has been successfully used to detect the strong magnetic contrast between the phases of these nanostructures which indicates local changes in composition and structure.

Magnetization relaxation and structure of CoFeGe alloys

The magnetic relaxation of 10 and 50 nm thin films of (CoFe)100−xGex (0 at. %≤x≤35 at. %) alloys was investigated by broadband ferromagnetic resonance (FMR) experiments. 10 nm thin films exhibit a significant two magnon contribution to the FMR linewidth. The 50 nm films exhibit very low damping constants of α≈0.0025 and relaxation rates as low as 33 MHz in the composition range of 20 at. %≤x≤30 at. % Ge after annealing. Structural characterization revealed B2 order for these compositions. First principles calculations confirm a pseudogap in the minority channel for B2 ordered (CoFe)75Ge25 which may cause the low damping parameters and high ΔRA in CoFeGe based current perpendicular to the plane giant magnetoresistance spin valves.

Charge pumping and the colored thermal voltage noise in spin valves

Spin pumping by a moving magnetization gives rise to an electric voltage over a spin valve. Thermal fluctuations of the magnetization manifest themselves as increased thermal voltage noise with absorption lines at the ferromagnetic resonance frequency and/or zero frequency. The effect depends on the magnetization configuration and can be of the same order of magnitude as the Johnson-Nyquist thermal noise. Measuring colored voltage noise is an alternative to ferromagnetic resonance experiments for nanoscale ferromagnetic circuits.

Microwave studies of magnetic anisotropy of Co nanowire arrays

The effect of magnetocrystalline anisotropy and dipolar interactions in Co nanowire arrays is studied by ferromagnetic resonance (FMR). Microwave measurements performed by the microstripline method are reported for two series of crystalline hcp Co (with the c axis nominally perpendicular [Co(c⊥)] and parallel to the wires [Co(c∥)]) and an amorphous alloy with Co as the main component—Co94Fe5B1. Extrapolation of the high field linear part of the resonance curve (frequency versus dc field) permitted an evaluation of the effective anisotropy fields for saturated samples, as well as of the intrinsic fields HK, showing that the great differences between the three series are due to the magnetocrystalline anisotropy. The HK values for the two series of Co are discussed in terms of a model which accounts for the effect of the distributions of the c axis orientation in systems of uniaxial ferromagnets. The observed dependence of the effective anisotropy fields on the array geometry (wire length and diameter) is interpreted in terms of the interwire dipolar interactions and found to be in agreement with theoretical predictions based on a micromagnetic model. The fact that the resonance frequencies at Hdc=0 are geometry dependent shows that magnetostatic interactions are still significant at remanence. A second series of FMR experiments was performed at constant frequency (38 GHz), with the purpose of obtaining the angle dependence of the resonance field. These experiments provided an alternative method for the evaluation of the effective anisotropy field. The angle dependence of the resonance field for Co(c⊥) fitted the simplest equation for magnets with uniaxial anisotropy, obtained considering only the first order term in the expression of the magnetocrystalline energy as a function of the magnetization orientation. The same is not true for Co(c∥), which required inclusion of a second order term.

Voltage controlled inversion of magnetic anisotropy in a ferromagnetic thin film at room temperature

The control of magnetic properties by means of an electric field is an important aspect in magnetism and magnetoelectronics. We here utilize magnetoelastic coupling in ferromagnetic/piezoelectric hybrids to realize a voltage control of magnetization orientation at room temperature. The samples consist of polycrystalline nickel thin films evaporated onto piezoelectric actuators. The magnetic properties of these multifunctional hybrids are investigated at room temperature as a function of the voltage controlled stress exerted by the actuator on the Ni film. Ferromagnetic resonance spectroscopy shows that the magnetic easy axis in the Ni film plane is rotated by 90° upon changing the polarity of the voltage Vp applied to the actuator. In other words, the in-plane uniaxial magnetic anisotropy of the Ni film can be inverted via the application of an appropriate voltage Vp. Using superconducting quantum interference device (SQUID) magnetometry, the evolution of the magnetization vector is recorded as a function of Vp and of the external magnetic field. Changing Vp allows to reversibly adjust the magnetization orientation in the Ni film plane within a range of approximately 70°. All magnetometry data can be quantitatively understood in terms of the magnetic free energy determined from the ferromagnetic resonance experiments. These results demonstrate that magnetoelastic coupling in hybrid structures is indeed a viable option to control magnetization orientation in technologically relevant ferromagnetic thin films at room temperature.