Perpendicular Ferromagnetic Resonance Research Articles

Engineering spin wave spectra in thick Ni80Fe20 rings by using competition between exchange and dipolar fields

Control of the spin wave dynamics in nanomagnetic elements is very important for the realization of a broad range of novel magnonic devices. Here we study experimentally the spin wave resonance in thick ferromagnetic rings (100 nm) using perpendicular ferromagnetic resonance spectroscopy. Different from what was observed for the continuous film of the same thickness, or from rings with similar lateral dimensions but with lower thicknesses, the spectra of thick patterned rings show a nonmonotonic dependence of the mode intensity on the resonance field for a fixed frequency. To explain this effect, the theoretical approach by considering the dependence of the mode profiles on both the radial and axial coordinates was developed. It was demonstrated that such unusual behavior is a result of the competition between exchange and dipolar fields acting at the spin excitations in the structure under study. The calculations are in a good agreement with the experimental results.

Nonlinear domain-mode ferromagnetic resonances in garnet films with perpendicular anisotropy

The zero-field domain-mode ferromagnetic resonances (DM-FMRs) in the weakly nonlinear regime have been investigated for garnet films with a perpendicular anisotropy supporting parallel stripe domains by means of broadband microwave measurements, and analytical and numerical approaches. First, the perpendicular ferromagnetic resonance (FMR) in the saturated state is considered as a reference case. A redshift of the resonance frequency and a broadening of the resonance line are experimentally observed for increasing the input microwave power. These power-dependent characteristics are nicely reproduced by an analytical model taking into account the linear polarization of the pumping field and a nonlinear Gilbert-type damping parameter. Then, the effect of the stripe domain pattern is analyzed. A redshift and a line broadening are also experimentally revealed both for the acoustic and the optical modes with an anisotropic character between the two modes. Numerical simulations based on the DM-FMR model extended in the nonlinear regime and including a nonlinear Gilbert-type damping parameter account quantitatively for the power dependences of the DM resonance frequency shifts and the DM resonance linewidths. These results highlight the requirement of a nonlinear damping term for explaining the FMR and DM-FMR spectra in the weakly nonlinear regime within the framework of the phenomenological approach of magnetization dynamics.

Axially and radially quantized spin waves in thick permalloy nanodots

We have investigated the spin wave modes in perpendicularly magnetized permalloy nanodots as a function of film thickness using field modulated perpendicular ferromagnetic resonance spectroscopy. We observed mode intensity inversion with an increase in dot thickness. In thicker dots, one ellusive antisymmetric spin wave mode was also detected due to the asymmetric nature of the excitation signal. In the case of the thickest nanodot, a circulating chiral mode near the dot surface was also observed. Our experimental results are in good qualitative agreement with the dynamic micromagnetic simulations.

Spin reorientation transition in Co/Au multilayers

We report a study about the spin reorientation transition (SRT) from perpendicular easy axis to in-plane easy axis of magnetization in Co/Au multilayers. A series of multilayers of Si/Au(100Å)/{[Co(tCo)/Au(20Å)]20}/Au(50Å) family were studied, with Co layer thickness varying between 6Å to 30Å. The thickness of the Au layer was chosen large enough in order to minimize the interlayer exchange coupling between Co layers. In such thick Au-layer samples the magnetic properties are mainly the result of competition between interlayer magnetostatic coupling due to stray field, perpendicular magnetic anisotropy and shape anisotropy. The effective anisotropy constant Keff and the second order anisotropy K2 were deduced from the fit of the resonant magnetic field obtained from out of plane dependence Ferromagnetic Resonance (FMR) experiments. To study the SRT, we have plotted the phase diagram between Keff and K2. The results show that SRT occurs through the metastable region with K2≤−½Keff, (Keff>0). It is interesting to note that FMR shows the coexistence of two modes with different anisotropy for small Co thickness, while for thick Co layers the modes have the same anisotropy. Moreover, in thick Co layer samples, volume and surface spin wave resonance (SWR) modes were also excited by the microwave field, around the perpendicular FMR geometry, giving a clear evidence of a magnetic coupling between the Co layers.

FMR and Magnetic Studies on Polycrystalline YIG Thin Films Deposited Using Pulsed Laser

Ferromagnetic resonance (FMR) and magnetic studies have been carried out on polycrystalline YIG films deposited by pulsed laser deposition (at TS:750 °C, O2:5×10-2 mbar) using a range of laser energies (240 to 350 mJ). The films were ex situ annealed (Ta:700 °C, in air for 2 h). The film thickness increases from 100 to 290 nm for an hour of deposition with the increase in laser energy from 240 to 350 mJ. The FMR linewidth was found to reduce from 340 to 70 Oe for the same incremental variation in laser energy. Multiple resonance modes were observed in the perpendicular FMR spectra of our samples which are related to intrinsic as well as extrinsic mechanisms such as in homogeneities. Saturation magnetization measurement (Ms) is found to be dependent on the laser energy. The value of Ms is found to increase with laser energy and is close to 90% of the bulk value for the film with highest power. An attempt has been made to correlate the magnetization results with the microstructure.

Perpendicular ferromagnetic resonance in soft cylindrical elements: Vortex and saturated states

Linear magnetic excitations in perpendicularly magnetized micrometer-sized disks have been investigated in detail both in the saturated and the vortex states using ferromagnetic resonance spectroscopy and micromagnetic simulations. Broadband ferromagnetic resonance spectra measured in disk arrays reveal a set of discrete resonance lines associated with the dipole-exchange spin-wave modes quantized by the disk edge in the saturated state and several new resonance lines (up to four) with negative slopes for the frequency-field dispersion relation $\ensuremath{\omega}({H}_{z})$ in the vortex state at intermediate magnetic fields. The micromagnetic simulations performed for a Py disk array (regime of negligible coupling between the disks) allow us to identify the four excitations occurring in the deformed vortex state as vortex core, disk edge, and coupled vortex core/disk edge modes, and to reproduce very satisfactorily their experimental $\ensuremath{\omega}({H}_{z})$ curves. In addition, the nonlinear frequency dependence of the resonance linewidth for the predominant coupled vortex core/edge mode experimentally observed is in agreement with the numerical prediction. These findings are finally confirmed by magnetic resonance force microscopy measurements conducted on an isolated NiMnSb disk. The remarkable similarity between the experimental results coming from two magnetic systems and using two different microwave probes demonstrates the robustness of the physical phenomenon.

Damping phenomena in Co90Fe10/Ni multilayers and alloys

We used perpendicular ferromagnetic resonance to measure the damping parameter in Co90Fe10/Ni multilayers over a wide range of layer thicknesses. The magnetic anisotropy within this range varied from in-plane to out-of-plane. We measured (Co90Fe10)xNi1−x alloys of identical thicknesses over the same compositional range of Co90Fe10 and Ni in order to isolate the influence of the multilayer structure. The damping parameter varied from 0.004 to 0.030 and depended only on the relative amounts of Co90Fe10 and Ni and was independent of the magnetic anisotropy and layer structure.

Ferromagnetic resonance study of magnetic phases in FeNi∕Al∕FeMn∕Al and FeMn∕Al∕FeNi∕Al multilayers

A comparative ferromagnetic resonance (FMR) study of the magnetic properties of Si(100)[FeNi∕Al∕FeMn∕Al]×10 and Si(100)[FeMn∕Al(6Å)∕FeNi(30Å)∕Al]×20 multilayers is reported. Besides the uniform mode, the parallel and perpendicular FMR spectra of both multilayers also give evidence of additional magnetic phases in the films. The perpendicular FMR spectrum of the first multilayer also shows the presence of a volume spin-wave resonance (SWR) mode, while the spectrum of the second shows the presence of a surface SWR mode. This is attributed to the structural differences between the two multilayers in terms of the thicknesses of the magnetic (FeNi) and nonmagnetic (FeMn and Al) layers.

Imaging of ferromagnetic-resonance excitations in Permalloy nanostructures on Si using scanning near-field thermal microscopy

The investigated structures were Permalloy (Py) stripes of 3×0.3μm2 and a thickness of 24nm. Two samples with lattices of stripes of 2μm distance between the stripes and a lattice constant of 5μm are reported in this paper. One of the samples had crossed perpendicular lattices. The samples were prepared using standard lithography lift-off processes and comprised large arrays of at least 1000 stripes, such that enough material was available to perform conventional ferromagnetic-resonance (FMR) measurements. The overall anisotropy and resonance line behavior of the samples were determined by conventional FMR measurements. The origin of the FMR modes was then deduced locally resolved in single stripes using a scanning thermal microscope (SThM), which features a lateral resolution of 100nm and a temperature resolution of the order of a few millikelvins, mounted on a conventional FMR setup. The technique is based on the detection of the dissipated heat due to microwave absorption while in FMR. This setup provides an exact correlation of the SThM-FMR image and the simultaneously taken topography. In the crossed lattice sample the two resonance lines in the conventional spectra were identified as the parallel and perpendicular FMR excitations in each sublattice, respectively. A homogenous heating of a single stripe is observed while in FMR. In the single Py stripe lattice with external field parallel to the long axis of the stripe an additional resonance was found with slightly higher resonance field than the uniform mode. SThM-FMR measurements prove that this additional excitation is located at the polar edges of one stripe and is due to the round edges of each stripe and stray field effects.

Structural and magnetic properties of NiMnSb/InGaAs/InP(001)

The structural and magnetic properties of NiMnSb films, 5–120 nm thick, grown on InGaAs/InP(001) substrates by molecular-beam epitaxy, were studied by x-ray diffraction, transmission electron microscopy (TEM), and ferromagnetic resonance (FMR) techniques. X-ray diffraction and TEM studies show that the NiMnSb films had the expected half-Heusler structure, and films up to 120 nm were pseudomorphically strained at the interface, greater than the critical thickness for this system, about 70 nm (0.6% mismatch to InP). No interfacial misfit dislocations were detected up to 85 nm, however, relaxation in the surface regions of films thicker than 40 nm was evident in x-ray reciprocal space maps. TEM investigations show that bulk, planar defects are present beginning in the thinnest film (10 nm). Their density remains constant but they gradually increase in size with increasing film thickness. By 40 nm these defects have overlapped to form a quasicontinuous network aligned closely with ⟨100⟩ in-plane directions. The associated strain fields and or compositional ordering from these defects introduced a reduction in crystal symmetry that influenced the magnetic properties. The in-plane and perpendicular FMR anisotropies are not well described by bulk and interface contributions. In thick films, the in-plane uniaxial and fourfold anisotropies increased with increasing film thickness. The lattice defects resulted in a large extrinsic magnetic damping caused by two-magnon scattering, an increase in the coersive field with increasing film thickness, and a lower magnetic moment (3.6 Bohr magnetons) compared to the expected value for the bulk crystals (4 Bohr magnetons).

Ferromagnetic resonance spectra in Co/Nb multilayers with large Co thickness

The perpendicular ferromagnetic resonance spectra for four Co/ Nb multilayered films have been studied. Three of these multilayers had the same thickness of Co (210 Å) with Nb layer thicknesses varying between 18 and 135 Å. The fourth film was a trilayer sandwich film. A multimode spectrum was observed for all the films. The results indicated that the Co films are magnetically coupled even when the Nb film thickness was 135 Å. A new approach to understand the multimode ferromagnetic resonance in magnetic/nonmagnetic multilayer thin film structures was worked out. In this approach, the equation of motion is solved in the magnetic layers, which are assumed to be coupled through a boundary condition. The theoretical calculations gave a good agreement with the experimentally observed field positions when it was assumed that the magnetic properties of the top and the bottom Co layers are slightly different from the intermediate ones.

Comparison of parallel and perpendicular ferromagnetic resonance in an exchange-biased bilayer

We have investigated the ferromagnetic resonance spectra of an exchange-biased Ni 80Fe 20/CoO bilayer between room temperature and 4 K. Primary attention has been paid to the effect of the antiferromagnetic CoO film on the temperature-dependent resonance field shift of the ferromagnetic Ni 80Fe 20 film with respect to that of an unbiased film. At low temperatures, the field shift with the magnetic field applied perpendicular to the plane was determined to be more than twice the magnitude of the parallel field shift, and of the same sign, while an unoxidized single ferromagnetic film has much smaller parallel and perpendicular low-temperature shifts (here defined with respect to room temperature) of opposite sign. This observation implies that the anisotropy axis can rotate with the applied field, provided that the primary cause of the anisotropy is the interaction between the adjacent ferromagnetic and antiferromagnetic films. Since the perpendicular shift is more than a factor of two larger than the parallel field shift, the rotatable anisotropy is, in fact, anisotropic in this bilayer.

Angle dependence of the ferromagnetic resonance linewidth and two magnon losses in pulsed laser deposited films of yttrium iron garnet, MnZn ferrite, and NiZn ferrite

Shorted waveguide ferromagnetic resonance (FMR) measurements were made at 9.5 GHz for pulsed laser deposited yttrium iron garnet (YIG), MnZn ferrite, and NiZn ferrite films with thicknesses of 1.8, 0.5, and 1.5 μm, respectively. The FMR field versus the field angle confirmed the operational assumption of a uniform mode response. For the YIG, the linewidth was 10 Oe when the external field and magnetization vectors were perpendicular to the disk (perpendicular FMR) and increased smoothly to a maximum value of 27 Oe when the field and magnetization were in the film plane (parallel FMR). For the MnZn, the linewidth was 49 Oe at perpendicular FMR, increased with the angle between the film normal and the external field to a broad maximum of 80 Oe at 35°, and then dropped to 65 Oe at parallel FMR. For the NiZn, the linewidth was 310 Oe at perpendicular FMR, increased with the field angle to a broad maximum of 1530 Oe at 45°, and then dropped to 960 Oe at parallel FMR. The linewidths were larger than predicted for reasonable values of the Landau–Lifshitz damping and showed angle dependences which indicated nonintrinsic contributions to the loss. Two magnon scattering was used to model these linewidth differences. The angle dependences of the excess linewidths show qualitative agreement with the two magnon predictions, with inhomogeneity sizes in the submicron range and volume fractions below 1%.

A study of interlayer coupling of Co/Pb multilayers

Interlayer coupling of Co/Pb multilayers was studied by magnetization and Magnetoresistance (MR) measurements and ferromagnetic-resonance (FMR). The Pb layer thickness dependence of coupling strength derived from the separation between the resonance fields of the `optical mode’ and uniform mode in perpendicular FMR spectra is consistent qualitatively with the variation of the in-plane remanence ratio and MR ratio with spacer thickness.

Characterization of epitaxial La0.7Ba0.3MnO3 structures using ferromagnetic resonance

We have compared a single layer of La0.7Ba0.3MnO3 and a trilayer structure of SrTiO3/La0.7Ba0.3MnO3/SrTiO3, both grown epitaxially on a LaAlO3 substrate, using information obtained by ferromagnetic resonance (FMR). The trilayer samples have a more uniform magnetization and are not susceptible to environmental degradation. This may be due to the strain relief that the buffer SrTiO3 layer provides for the La0.7Ba0.3MnO3 layer. We have also studied the magnetic homogeneity of the trilayer structure as a function of the deposition temperature. The perpendicular FMR linewidth, Γ⊥, shows a clear window in the deposition temperature where the linewidth is <50 Oe. However, the parallel linewidth, Γ∥, is nearly ten times larger than Γ⊥ with only a weak dependence on the deposition temperature. This broadening of the parallel linewidth compared to the perpendicular linewidth can be explained by invoking a local unidirectional anisotropy in the plane of the film.

Magnetic and crystallographic properties of molecular beam epitaxially grown Fe3O4/NiO superlattices and Fe3O4 films

Ferromagnetic resonance, SQUID magnetometry, and x-ray diffraction have been used to characterize a set of [Fe3O4(68 Å)/NiO(17 Å)]N superlattices (SL) with N=3, 10, 30, and 100, as well as a 1.5-μm-thick Fe3O4 film. For this NiO thickness, Fe3O4 layers are strongly coupled and the in-plane anisotropy is much less than the 330-Oe ferromagnetic resonance (FMR) linewidth at 35 GHz. Both in-plane and perpendicular FMR at 9.5 and 35 GHz have been used, with the 9.5-GHz data showing significant hysteresis associated with the sample magnetization. X-ray diffraction indicates that both the film and SL’s are nearly cubic single-crystalline structures with long-range coherence. The 300 K magnetization data indicate the presence of small cubic anisotropy in the SL’s, although bulklike Fe3O4 magnetic ordering in the thick single film. When the Fe3O4 film is cooled below the Verwey transition in a 10 kOe field (aligned along 〈100〉), the FMR shows that the sample develops a large uniaxial (Ku=1.8 kOe) in-plane anisotropy with its easy axis along the field direction. Decreased microwave loss also occurs abruptly at the film TV.

Structural and magnetic characterization of amorphous Gd2Fe14B thin films

Thin amorphous films of nominal composition Gd2Fe14B were fabricated on Si (100) substrates using ion-beam sputtering for the purpose of establishing their structural and magnetic properties at room temperature. X-ray-diffraction scans performed on as-deposited and annealed films revealed an amorphous structure. The as-deposited saturation magnetization was found to be 8101 G at room temperature suggesting ferrimagnetic ordering with anti-parallel coupling similar to other heavy rare earth-transition metal systems. Results from in-plane and perpendicular ferromagnetic resonance experiments conducted at 9.108 GHz showed the films to be inhomogeneous. This was evident from the presence of two in-plane resonance lines and spin-wave spectra that were not quadratic. Annealing at 300 °C for 30 min significantly reduced the absorption of the second in-plane resonance and restored classical n2 law spin-wave behavior. It is speculated that the origin of the inhomogeneities is due to the kinetics of the deposition process where the increased mobility of the condensing Fe atoms leads to interspersed amorphous regions which are rich in Fe and Gd, respectively.

INTERLAYER EXCHANGE COUPLING IN (Co/Pt) MULTILAYER FILMS BY FERROMAGNETIC RESONANCE

A model assuming exchange coupled layers is used to evaluate the interlayer exchange coupling strength, A12(tPt), and the effective uniaxial anisotropy field, Heff(tPt), within each Co layers as a function of the Pt thickness, tPt, from the perpendicular ferromagnetic resonance spectra for a series of Co/Pt multilayer films. It was found that A12(tPt) decreases exponentially with tPt having a characteristic length, t0 ∼ 7 A, indicating the importance of the polarization of Pt to mediate the interlayer exchange coupling between the Co layers. The decrease of Heff(tPt) with the increase of tPt also follows an exponential function of tPt with a decay length, τ ∼ 13 A, suggesting that the surface interaction on each Co/Pt interface is not fully developed unless the Pt layer thickness is greater than τ.

Out-of-plane ferromagnetic resonance study of magnetically coupled Fe/Cr/Fe films (abstract)

Magnetically coupled single-crystal Fe/Cr/Fe (001) sandwiches have been studied by ferromagnetic resonance (FMR) techniques. Such films have been previously shown to be antiferromagnetically coupled for a chromium layer thickness range 12 Å<tCr<25 Å by variable frequency ferromagnetic resonance measurements along principal crystallographic axes, and other techniques.1 These measurements have been extended to include oblique angle application of the applied magnetic field (H) between principal crystallographic axes in the film plane and the film normal. Measured films had tCr=13, 16, 17, and 28 Å. Ferromagnetic resonance measurements were taken using a fixed frequency 9.5-GHz cavity. Resonances corresponding to the in-phase and out-of-phase precession of film moments M1 and M2 were observed for the three thinnest films with H lying in the film plane. These resonances could be tracked to high (∼85°) oblique angles before disappearing, with the in-phase resonance strengthening near perpendicular. Perpendicular FMR measurements on the tCr=13-Å-thick sample show a weaker resonance ∼680 Oe above the in-phase resonance, corresponding to the out-of-phase precessional mode. This value can be directly compared to 4J/Ms∼540 Oe result from earlier work,1 perhaps indicating some anisotropic exchange. An extension of the model presented in Ref. 1 is presented to explain the resonance behavior as a function of angle.<ks>

Ferromagnetic resonance measurements on epitaxial grown Fe/ZnSe and Co/ZnSe films on GaAs substrates

Ferromagnetic resonance (FMR) measurements have been performed over a broad frequency range on epitaxial films of Co and bcc α-Fe grown onto epilayers of ZnSe by molecular-beam-epitaxy techniques on (001) and (110) GaAs substrates. In-plane and perpendicular FMR results were used in deducing the magnetic parameters and spin-wave resonance (SWR) dispersion of the films. The magnetic parameters deduced for Fe films agree with previous results obtained on the (001) Fe/ZnSe film. Well-resolved SWR modes were observed in the Fe/ZnSe films, allowing extraction of the exchange stiffness constant. Few spin-wave excitations were observed for the Co/ZnSe films. Both (110) Fe/ZnSe and (110) Co/ZnSe show evidence for surface spin-wave excitations. These excitations are attributed to variations in the internal field near the interface.