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Spintronics is a recently emerged research field in both fundamental science and technology, a successful development which promises a breakthrough in energy efficient information processing. The ever increasing demand for faster information processing raises a fundamental question about the feasibility to realize the concepts of spintronics at THz frequencies.

Two ferromagnetic layers magnetically decoupled by a thick normal metal spacer layer can be, nevertheless, dynamically coupled via spin currents emitted by the spin-pump and absorbed through the spin-torque effects at the neighboring interfaces. A decrease of damping in both layers due to a partial compensation of the angular momentum leakage in each layer was previously observed at the coincidence of the two ferromagnetic resonances. In case of non-zero magnetic coupling, such a dynamic exchange will depend on the mutual precession of the magnetic moments in the layers. A difference in the linewidth of the resonance peaks is expected for the acoustic and optical regimes of precession. However, the interlayer coupling hybridizes the resonance responses of the layers and therefore can also change their linewidths. The interplay between the two mechanisms has never been considered before. In the present work, the joint influence of the hybridization and non-local damping on the linewidth has been studied in weakly coupled NiFe/CoFe/Cu/CoFe/MnIr spin-valve multilayers. It has been found that the dynamic exchange by spin currents is different in the optical and acoustic modes, and this difference is dependent on the interlayer coupling strength. In contrast to the acoustic precession mode, the dynamic exchange in the optical mode works as an additional damping source. A simulation in the framework of the Landau-Lifshitz-Gilbert formalism for two ferromagnetic layers coupled magnetically and by spin currents has been done to separate the effects of the non-local damping from the resonance modes hybridization. In our samples both mechanisms bring about linewidth changes of the same order of magnitude, but lead to a distinctly different angular behavior. The obtained results are relevant for a broad class of coupled magnetic multilayers with ballistic regime of the spin transport.

Magnetic vortex that consists of an in-plane curling magnetization configuration and a needle-like core region with out-of-plane magnetization is known to be the ground state of geometrically confined submicron soft magnetic elements. Here magnetodynamics of relatively thick (50-100 nm) circular Ni80Fe20 dots were probed by broadband ferromagnetic resonance in the absence of external magnetic field. Spin excitation modes related to the thickness dependent vortex core gyrotropic dynamics were detected experimentally in the gigahertz frequency range. Both analytical theory and micromagnetic simulations revealed that these exchange dominated modes are flexure oscillations of the vortex core string with n = 0,1,2 nodes along the dot thickness. The intensity of the mode with n = 1 depends significantly on both dot thickness and diameter and in some cases is higher than the one of the uniform mode with n = 0. This opens promising perspectives in the area of spin transfer torque oscillators. This work was supported by the SMF-NUS New Horizon Awards, National Research Foundation, Prime Minister's Office, Singapore under its Competitive Research Programme (CRP Award No. NRF-CRP 10-2012-03) and Ministry of Education of Singapore. K.Y.G. acknowledges support by IKERBASQUE (the Basque Science Foundation) and the Spanish MINECO grants PIB2010US-00153, FIS2010-20979-C02-01. The authors would like to thank Dr. N. Singh from A*Star Institute of Microelectronics, Singapore for his help with template fabrication.

When the in-plane bias magnetic field acting on a flat circular magnetic dot is smaller than the saturation field, there are two stable competing magnetization configurations of the dot: the vortex and the quasi-uniform (C-state). We measured microwave absorption properties in an array of non-interacting permalloy dots in the frequency range 1-8 GHz when the in-plane bias magnetic field was varied in the region of the dot magnetization state bi-stability. We found that the microwave absorption properties in the vortex and quasi-uniform stable states are substantially different, so that switching between these states in a fixed bias field can be used for the development of reconfigurable microwave magnetic materials. G M and Y K are grateful to the Rohde & Schwarz Oestereich GmbH Representative Office for the opportunity to use the R&S® ZVA8 Vector Network Analyzer. This work was supported in part by grant No. UU34/008 from the State Fund for Fundamental Research of Ukraine, by the National Science Foundation of USA (grant DMR-1015175), by DARPA, by the contract from the US Army TARDEC, by the US DOE Office of Science (contract DEAC02-06CH11357), and by the Spanish MINECO grants PIB2010US-00153 and FIS201020979- C02-01. K G acknowledges support by IKERBASQUE (the Basque Science Foundation). G N K is supported by Foundation of Science & Technology of Portugal through the program 'Investigador FCT (grant IF/00981/2013)'.

The granular layered magnetic system Co/sub 80/Fe/sub 20/(t)/Al/sub 2/O/sub 3/ (3 nm), where the Co/sub 80/Fe/sub 20/ layers of nominal thickness t form separate, almost spherical magnetic granules of typical diameter 2-3 nm between the Al/sub 2/O/sub 3/ spacers, was studied. We discuss measurements of the dc and ac magnetic susceptibility /spl chi/ for 1 nm

Polycrystalline samples Bi 4 − x Gd x Ti 3 O 12 ( x = 1, 1.5, 2) were investigated by X-ray diffraction, piezoresponse force microscopy and SQUID-magnetometry techniques. Increasing the gadolinium content was shown to suppress the spontaneous polarization in Bi 4 − x Gd x Ti 3 O 12 , resulting in a polar-to-nonpolar phase transition near x = 1.5. In contrast to previous expectations, all these samples were found to be paramagnets. It was thus proven that introducing magnetically-active Gd ions into the lattice of the ferroelectric Aurivillius-type compound should not be considered as an effective way to achieve multiferroic behavior.

Ordered hexagonal arrays of Co (x)Ni (1-x) nanowires (NWs) with different content of cobalt (0.35 < x < 0.75), were successfully fabricated by a potentiostatic electrodeposition technique inside anodic aluminum oxide (AAO) templates. The CoNi NWs were electrodeposited using only one electrolyte bath and tuning the Co% by the applied deposition potential. The systematic study of the morphology, chemical composition and magnetic properties of the CoNi NW arrays was performed. The obtained results unambiguously demonstrated the influence of the Co content on the magnetic properties of CoNi NWs. The Co% was found to decrease when increasing the applied cathodic potential and the coercivity of the CoNi NWs linearly increased with the Co%. The magnetic texture of the fabricated NW arrays was found lower for smaller Co concentrations, which was in good agreement with the squareness results.

Dynamic microwave properties of arrays of circular Ni and Ni81Fe19 dots were studied by X-band ferromagnetic resonance (FMR) technique. All of the dots had the same radius 0.5μm, thickness 50–70nm, and were arranged into rectangular or square array with different interdot separations. In the case of perpendicular magnetization multiple (up to 8) sharp resonance peaks were observed below the main FMR peak, and the relative positions of these peaks were independent of the interdot separations. Quantitative description of the observed multiresonance FMR spectra is given using the dipole-exchange spin wave dispersion equation for a perpendicularly magnetized film where in-plane wave vector is quantized due to the finite dot radius, and the inhomogenetiy of the intradot static demagnetization field in the nonellipsoidal dot is taken into account.

Magnetization of ErFeO3 single crystal was measured by a super conducting quantum interference device magnetometer with greatly improved precision. A dramatic (70%) increase of magnetization was observed in the spin-reorientation interval (88–97 K). This and the temperature dependence of the spin-rotation angle are accurately described by the modified mean field theory, which uses the key fact of the magnetic anisotropy of erbium subsystem. Calculated temperature dependence of the rotation angle shows a remarkable correspondence with the experiment with no fitting parameters. Proposed model is general and can be used to analyze orientation transitions in many orthoferrites.

Ferromagnetic resonance at 9.2 GHz (X band) was used to characterize the uniaxial magnetic anisotropies in rectangular arrays of submicron circular Ni dots. The in-plane anisotropy, originated from interdot interactions in the rectangular lattice, and the perpendicular anisotropy, due to individual dot shape and magnetostriction, were explored. For in-plane dependencies of the resonance field (Hr), the main resonance mode angular dependence was well described by the standard Kittel formula. As the interdot distances decreased from 800 to 50 nm, the in-plane uniaxial anisotropy field changed from 5 to 130 Oe, in reasonable agreement with calculations. Simultaneously, the position of perpendicular Hr increased from 6.38 to 6.83 kOe, also following Kittel’s formula.

Magnetization reversal processes have been analyzed by Magnetic Force Microscopy in dense arrays of Co bars with well defined shape anisotropy and strong magnetostatic interactions. Two different geometries have been used: rectangular and rhombic so that the sign of dipolar interactions between adjacent chains of bars is changed from antiferromagnetic (rectangular array) to ferromagnetic (rhombic array), having a profound influence on the shape of a nucleus of inversion at the magnetization reversal., Spanis MICINN: FIS2008-06249, HP 2008-0032; Asturias FICYT: IB 08-106; CRUP of Portugal: E41/09 and FCT of Portugal: SFRH/BD/24012/2005.

Nanoparticles of Co10Cu90 alloy have been prepared by sonochemical wet method. According to transmission electron microscopy, bimetallic particles with typical diameter of 50-100 nm consisting of nanocrystallites with average diameter of 15-20 nm were obtained. The samples were annealed at 300 degrees C and 450 degrees C. Zero field cooled and field cooled temperature dependences of magnetization in the temperature range of 5-400 K at 50 Oe, as well as magnetization hysteresis loops at 15, 100 and 305 K were measured by vibrating sample magnetometry. Presence of antiferromagnetic phase, most probably of the oxide Co3O4, was observed in as-prepared sample. The lowest coercivity was found for the CoCu sample annealed at-300 degrees C, whereas for as prepared sample and the one annealed at 450 degrees C it was significantly higher. The samples were additionally probed by continuous wave ferromagnetic resonance at room, temperature using a standard X-band electron spin resonance spectrometer. A good correspondence between evolution of the coercivity and the microwave resonance fields with annealing temperature was observed.

We performed ferromagnetic resonance measurements of square arrays of noninteracting Permalloy circular dots for different orientations of external magnetic field with respect to the patterned film plane (\ensuremath{\theta}). Out-of-plane angular dependence of the main resonance peak was measured in the whole range of the field angles 0\ifmmode^\circ\else\textdegree\fi{} \ensuremath{\leqslant} \ensuremath{\theta} \ensuremath{\leqslant} 90\ifmmode^\circ\else\textdegree\fi{}. The main eigenmodespatial distribution is strongly nonuniform due to the dot nonellipsoidal shape. Nevertheless, for dots with small aspect ratio $b=L/R\ensuremath{\leqslant}0.1$ (where $R$ is dot radius and $L$ is dot thickness) Kittel's equation, assuming uniform dynamic magnetization (no pinning at the dot lateral edges), describes the peak position with high accuracy. Analytical calculations and micromagnetic simulations confirmed the gradual evolution of the main mode profile and a smooth transition from the strong to relatively weak pinning conditions with the change of external magnetic field angle.

We studied NMR spectra at 1.5 K in a series of [Co 80 Fe 20 ( t )/Al 2 O 3 (3 nm)] 10 multilayers. The spectra are interpreted by mixed contributions from Co atoms inside CoFe granules and at their interface with Al 2 O 3 . They display three characteristic shapes when t varies from 0.8 to 2 nm. These shapes have been interpreted as resulting from granular CoFe layers for the thinnest ones ( t t >1.6 nm), with an intermediate regime—pancake grains of different sizes—in between. These structural data correlate with three magnetic states deduced from the FMR and SQUID measurements in this system.

Discontinuous metal–insulator multilayers [Co 80 Fe 20 ( t )/Al 2 O 3 (3 nm)] n were studied by SQUID magnetometry and AC susceptometry. CoFe forms ferromagnetic particles in the Al 2 O 3 matrix. In this paper, we focus on the field dependence of the AC susceptibility of samples with t =1.3 nm. We find strong evidence for transitions from a superpara- to a superferromagnetic and, finally, to a reentrant superspin glass state.

3 páginas, 3 figuras.-- et al., Epitaxial discontinuous Fe/MgO multilayers have been grown by pulsed laser deposition on MgO(001) single-crystal substrates. The multilayers with 0.6 nm nominal Fe layers thickness are superparamagnetic and demonstrate tunneling magnetoresistance (TMR) in the current-in-plane configuration. Increasing deposition temperature causes an improvement in crystal quality and is accompanied by higher TMR ratios. The maximum value (9.2% TMR at room temperature and 18 kOe magnetic field) trebles that of polycrystalline samples deposited simultaneously on glass substrates. A model formula for TMR ratio that includes both spin-dependent tunneling and spin-filtering effect is proposed to explain these results., Financial support by Spanish Ministry of Science (through project No. MAT2008-06567-C02 including FEDER funding) and by the Aragón Regional Government (through projects E26 and PI059/08) is acknowledged. Work of A.V. and G.N.K. was supported by Portuguese FCT through “Ciencia 2008” and “Ciencia 2007” programs, respectively.

We report on the discovery of an isothermal structural transition observed in Bi${}_{1\ensuremath{-}x}$La${}_{x}$FeO${}_{3}$ ($0.17\ensuremath{\leqslant}x\ensuremath{\leqslant}0.19$) ceramics. At room temperature, an initially pure polar rhombohedral phase gradually transforms into a pure antipolar orthorhombic one. The polar phase can be recovered by annealing at $Tg300 {}^{\ifmmode^\circ\else\textdegree\fi{}}$C. In accordance with neutron powder diffraction data, an inverse isothermal antipolar-polar transition takes place at $Tg300$ ${}^{\ifmmode^\circ\else\textdegree\fi{}}$C, where the polar phase becomes more stable. The antipolar phase is characterized by a weak ferromagnetic state, whereas the polar phase has been obtained in a mixed antiferromagnet--weak ferromagnet state. The relatively low external pressure induces polar-antipolar transition, but there is no evidence of electric-field-driven antipolar-polar transition. The observed large local piezoelectric response is associated with structural instability of the polar phase, whereas local multistate piezoelectric loops can be related to the domain wall pinning effect.

4 páginas, 5 figuras.-- PACS number(s): 75.60.Ej, 75.60.Jk, 75.70.Kw, 75.75.Cd.-- et al., Rectangular and rhombic patterned superlattices of magnetic bar elements have been experimentally studied and theoretically modeled in order to analyze the role of the array geometry in their magnetization reversal and coercivity. The results show that a dominating part of the coercive field (≈250 Oe) is due to the reversal processes within a single bar element (independent of the array geometry) which is well described by the standard micromagnetic calculation. Otherwise, a smaller (≈60 Oe) but significant difference between the magnetization loops in the two geometries is related to the magnetostatic coupling effects between the bars and it is reasonably accounted for with a simple model of Coulomb-like interaction between terminal magnetic charges. The possibility to use this geometry effect to control the performance of artificial magnetic media is discussed., The support of this work by Spanish and Portuguese Grants Nos. HP2008-0032, FIS2008-6249, PCTI IB08-106, FCT SFRH/BPD/72329/2010 (J.M.T.) is gratefully acknowledged. G.N.K. acknowledges support from Portuguese FCT through the “Ciencia 2007” program. Work in Ukraine was supported by STCU through the Project No. 5210.

Ferromagnetic resonance (FMR) has been studied in CoFe(t)/Al 2 O 3 (30 A) multilayers. With growing nominal thickness t of CoFe layers, transitions from superparamagnetic (SP) to ferromagnetic (FM) state at t 1 13 A and from granular to continuous structural state at t 2 18 A, previously revealed by magnetotransport studies on this system, are confirmed by the FMR data. The effective anisotropy field H eff (t) clearly displays three distinct regions: (i) SP at t < t 1 , (ii) mixed SP + FM at t 1 < t < t 2 , and (iii) FM at t 2 < t. The FMR linewidth AH(t) increases when passing from the continuous to discontinuous regime, mainly due to static fluctuations of internal field on granules. At room temperature AH(t) peaks near t 1 , the magnetic percolation point, while at 77 K it still grows below this point, probably due to getting below the AC blocking temperature.

Two sets of [Co0.74Si0.26(5nm)∕Si(s)]n amorphous films were prepared by magnetron sputtering: one in the form of multilayers with the Si spacer thickness s fixed at 3nm, and the number n of periods varying from 1 to 10 and the other with only two periods and s varying from 3to24nm (trilayers). In both sets, the Co0.74Si0.26 layer thickness t was fixed at 5nm. All the samples except the one with s=24nm manifest antiferromagnetic coupling. Their magnetic properties at room temperature were probed using the magnetooptical transverse Kerr effect (MOTKE) and ferromagnetic resonance (FMR). The relative increase in the saturation magnetization Ms (for trilayers, relative to a structure with s=24nm; for multilayers, relative to the single-layer structure) determined from the FMR measurements was compared with the exchange coupling strength HAFJ obtained from the MOTKE studies. The dependences of HAFJ and Ms on n and s were found to be very similar to each other. Possible mechanisms of this similarity are discussed., This work was supported by MICINN of Spain (grants Nos. HP2008-0032, NAN2004-09087 and FIS2008-06249) and CRUP of Portugal (grant No. E41/09) in the framework of the Spanish-Portuguese Integrated Action. G.N.K acknowledges support from FCT of Portugal through the “Ciencia 2007” program. C.Q. acknowledges support from the Spanish Government and European Social Fund under the “Ramón y Cajal” program.

We report on detailed broadband ferromagnetic resonance measurements of azimuthal and radial spin wave excitations in circular Permalloy dots in the vortex ground state. Dots with aspect ratio (Beta =height over radius) varied from 0.03 to 0.1 were explored. We found that for Beta exceeding approximately 0.05, variation of the spin wave eigenfrequencies with Beta deviates from the predicted dependence. The frequency splitting of two lowest azimuthal modes was observed. The experimentally observed dependence of the frequency splitting on beta was reasonably well described by dynamic splitting model accounting the spin-waves and, vortex gyrotropic mode interaction.

Magnetization and magnetoresistance in function of the magnitude and orientation of applied magnetic field were studied in Co-Fe discontinuous multilayers close to their structural percolation. The high pulsed magnetic fields up to 33 T were used in the 120--310 K temperature range. Comparison between longitudinal and transverse (with respect to the film plane) field configurations was made in the low-field and high-field regimes in order to clarify the nature of the measured negative magnetoresistance. Coexistence of two distinct magnetic fractions, superparamagnetic (SPM, consisting of small spherical Co-Fe granules) and superferromagnetic (SFM, by bigger Co-Fe clusters), was established in this system. These fractions were shown to have different relevance for the system magnetization and magnetotransport. While the magnetization is almost completely (up to $\ensuremath{\sim}97\mathrm{%}$) defined by the SFM contribution and practically independent of temperature (in this range), the magnetoresistance experiences a crossover from a regime dominated by Langevin correlations (suppressed with temperature) between neighbor SPM and SFM moments at low fields, to that dominated by spin scattering (enhanced with temperature) of charge carriers within SFM clusters at high fields. Also, the demagnetizing effects, sensitive to the field orientation, were found to essentially define the low-field behavior and characteristic crossover field.

3 páginas, 2 figuras.-- et al., Broadband ferromagnetic resonance in square arrays of Permalloy circular dots with different interdot separations was measured in the vortex ground state. The detected spin excitations show a complicated dependence of their frequencies on the interdot coupling strength. A considerable influence of the interdot separation on the gyrotropic vortex frequency and splitting of the azimuthal spin wave frequencies was detected. The gyrotropic frequency and the first azimuthal doublet frequency splitting depend nonmonotonously on the interdot spacing, whereas the dependence of the second doublet frequency splitting on this parameter is monotonous. The observed effects are explained by the influence of both the dipolar and quadrupolar contributions to the dynamic magnetostatic interactions., The work in Madrid was supported by Spanish MICINN (Grant Nos. MAT2009-10139, MAT2006-28183-E, and CSD2007-00010) and CAM (Grant No. S-505/MAT0194). K.Y.G. and G.R.A. acknowledge support by IKERBASQUE (the Basque Science Foundation), JAE-doc grant and the SAIOTEK Grant No. S-PC09UN03. G.N.K. acknowledges support from Portuguese FCT through the “Ciencia 2007” program. The work in Kiev was supported by the STCU Grant No. 5210 and the Grant No. 220-10 from NAS of Ukraine.

Low temperature (400°C) deposition of ferromagnetic Ni-Mn-Ga thin films is successfully performed via rf magnetron sputtering technique using co-deposition of two targets Ni50Mn50 and Ni50Ga50 on sapphire (0001) and Si (100) substrates. The films are in part amorphous with significant degree of crystallinity. The obtained crystallographic structure is shown to be substrate-dependent. Films on both substrates are ferromagnetic at room temperature (Curie temperature ∼ 332.5K) with well-defined hysteresis loops, low coercivity (∼ 100 Oe) and a saturation magnetization of ∼ 200 emu/cc. At low temperature (5 K), both films show increased magnetization value with wider hysteresis loops having higher coercivity and remanent magnetization. The process is therefore effective in achieving the appropriate thermodynamic conditions to deposit thin films of the Ni-Mn-Ga austenitic phase (highly magnetic at room temperature) at relatively low substrate temperature without the need for post-deposition annealing or further thermal treatment, which is prerequisite for the device fabrication.

We report on linear spin dynamics in the vortex state of Permalloy cylindrical dots subjected to an in-plane bias magnetic field. We demonstrate experimentally by a broadband ferromagnetic resonance technique and by simulations that breaking the cylindrical symmetry of the magnetic vortex gradually changes and suppresses the azimuthal spin eigenmodes below the vortex nucleation field and leads further to the appearance of new eigenmodes. The parallel microwave field pumping is shown to be a unique tool to observe spin excitation modes localized near the strongly shifted vortex core for the bias field between the vortex nucleation and annihilation fields. Meanwhile, the perpendicular field pumping, which excites the spin waves throughout the entire dot, reveals a crossover between two dynamic vortex regimes near the nucleation field.

Hysteresis loops in a $3.9\ifmmode\times\else\texttimes\fi{}3.9\ifmmode\times\else\texttimes\fi{}3.1\text{ }{\text{mm}}^{3}$ single-crystal ${\text{ErFeO}}_{3}$ sample were studied in the $4lTl70\text{ }\text{K}$ temperature interval. Above and near the compensation point ${T}_{\text{comp}}=46\text{ }\text{K}$ the hysteresis loops are rectangular, with the coercive force diverging at ${T}_{\text{comp}}$. As the temperature is lowered toward the erbium ordering transition at ${T}_{N2}=4.1\text{ }\text{K}$, the shape of the loops experiences a dramatic change. First, the loops develop triangular ``tails.'' Then the triangles become prominent while the central rectangular part collapses. A double-loop hysteresis pattern with two triangular loops emerges. This behavior is explained by a reversible motion of a single magnetic domain wall in the sample. The simplicity of the magnetic state is related to the small magnetization and correspondingly large domain sizes in orthoferrites. Our model reproduces the correlation of the loops' shape with the temperature dependence of the total magnetic moment of ${\text{ErFeO}}_{3}$.

We present an analysis of the effect of intergranular interactions on magnetic susceptibility of granular magnetic systems. The RKKY-type interaction between magnetic granules, via the conduction electrons, is found non-oscillatory antiferromagnetic, unlike in multilayer systems. The demagnetizing effect of dipolar interaction is shown to produce deviations from simple superparamagnetic behavior predicted by the Langevin model.

Ion beam sputtered Co/sub 80/Fe/sub 20/(t)/Al/sub 2/O/sub 3/ (30 /spl Aring/) multilayers were obtained. The Co/sub 80/Fe/sub 20/ layers become discontinuous for nominal thickness t/spl les/18 /spl Aring/. Tunnel magnetoresistance was measured in CIP and CPP geometries, reaching up to 6.5% at room temperature and 11% at 15 K, for as-deposited films in CIP geometry. The temperature dependence of MR was found quite different for the two geometries: fairly strong in the CIP case and almost absent in the CPP geometry. A model is proposed to explain these large differences in behavior.

Nanogranular layered magnetic films are a new class of magnetoresistive (MR) systems, consisting of well separated layers of closely spaced ferromagnetic nanoparticles within an insulating matrix. Most commonly they are realized in the form of discontinuous metal-insulator magnetic multilayers (DMIM), representing a general similarity with the formerly known magnetic multilayered giant magnetoresistance (GMR) systems (Baibich et al., 1988). Compared to the continuous multilayers, the DMIM systems possess the same advantages (easy preparation, high chemical/electrical stability, insensitivity to pinholes) as the cermet films, that is the arrays of magnetic nanogranules isotropically distributed in the insulating host (Gittleman et al., 1972; Mitani et al., 1998). However, but they display a much higher sensitivity of MR to low and moderate magnetic fields than cermets. An example of a DMIM system is given in Fig. 6.1, showing a cross-section micrograph of nanogranular CoFe layers in the insulating AI2O3matrix.

2-D modulated structures consisting of square arrays of 60 nm thick Ni80Fe20 circular dots underneath a continuous Ni80Fe20 film were fabricated using deep ultraviolet lithography. The array pitch for all the samples was fixed at 620 nm, while the dot diameter varied from 300 nm to 550 nm. Four-fold in-plane magnetic anisotropy (FFA) was detected in the samples using ferromagnetic resonance technique. It was found that the deposition of continuous film on top of the array drastically increases the FFA of the system, with a maximum anisotropy field of 120 Oe being observed for the sample with intermediate diameter. This increase is due to the appearance of a field well in the film underneath the dots with a shape that is dependent on magnetic field orientation and to the coupling of magnetization oscillations in dots and film.

The dynamic magnetic properties of micron-sized permalloy disks have been investigated using magnetic resonance force microscopy. From local spectroscopy data, information about the global sample properties is obtained. The experimental results are in good agreement with theoretical predictions, which account for magnetostatic modes with quantized in-plane wave vector. Ferromagnetic resonance spectral features due to the permalloy dots in close proximity to the micromagnetic probe tip provide two-dimensional images of the sample.

We discuss the four-fold anisotropy of in-plane ferromagnetic resonance (FMR) field $H_r$, found in a square lattice of circular Permalloy dots when the interdot distance $a$ gets comparable to the dot diameter $d$. The minimum $H_r$, along the lattice $$ axes, and the maximum, along the $$ axes, differ by $\sim$ 50 Oe at $a/d$ = 1.1. This anisotropy, not expected in uniformly magnetized dots, is explained by a non-uniform magnetization $\bm(\br)$ in a dot in response to dipolar forces in the patterned magnetic structure. It is well described by an iterative solution of a continuous variational procedure., 4 pages, 3 figures, revtex, details of analytic calculation and new references are added

The magnetization behavior in the vicinity of the reorientation phase transition was studied experimentally for a number of orthoferrites. The proposed model convincingly describes the experimental data with no fitting parameters.

Precise measurements of YbFeO_3 magnetization in the spin-reoirentation temperature interval are performed. It is shown that ytterbium orthoferrite is well described by a recently developed modified mean field theory developed for ErFeO_3. This validates the conjecture about the essential influence of the rare earth ion's anisotropic paramagnetism on the magnetization behavior in the reorientation regions of all orthoferrites with Gamma{4} -> Gamma{24} -> Gamma{2} phase transitions.

Spontaneous spin-reorientation transitions in ${\mathrm{ErFeO}}_{3}$ are studied using ultrasound, magnetic susceptibility, and magnetic moment measurements. The properties of the first-order transition happening in magnetic field sweeps through zero are studied and a three-dimensional $(H,T)$ magnetic phase diagram of the material is constructed. Direct measurements of the magnetic moment allow for a precise determination of the temperature dependence of the magnetization direction in the reorientation region. The mean-field theory of orientation transitions is modified to account for the magnetism of the erbium subsystem, and an excellent agreement with experiment is achieved.

Summary form only given. In a series of ion-beam deposited metal-insulator CoFe(t)/Al/sub 2/O/sub 3/(30 /spl Aring/) multilayers, ranging from discontinuous insulating (t = 9 /spl Aring/) to continuous metallic (t = 25 /spl Aring/) behavior, SQUID magnetization measurements, both zero-field-cooled (ZFC) and field-cooled (FC), were done on a total of 9 samples, over temperatures T from 8 to 300 K and in applied magnetic fields H from 10 to 100 Oe. The data permit an extraction of the basic characteristics of different magnetic phases present, in some cases coexisting, in this system.

We present micromagnetic simulations of ferromagnetic resonance in patterned permalloy films and isolated stripes. Films of the total thickness 20 nm or 40 nm are patterned in the form of 1D periodic structures with rectangular profile (rectangular grooves of depth varying from zero up to the film thickness) and in-plane period of 500 nm. The direction of the applied dc magnetic field is varied in the film plane from the direction parallel to the stripes to perpendicular one. The thickness of the patterned elements and direction of the bias field affect essentially the resonance peaks (changing their position, amplitude and number) and the corresponding dynamical magnetization profiles. We simulated from one up to three ferromagnetic resonance peaks and found the areas of microwave magnetization localization for them.

Vortex gyrotropic modes in ferromagnetic nanostructures can be described as flexure oscillations of the vortex core line with different number of nodes n along the dot thickness. By conducting broadband ferromagnetic resonance measurements in the absence of external magnetic field on Ni80Fe20 circular nanodots with radius R = 150 nm and thickness 50 ≤ L ≤ 100 nm, we established that above L = 70 nm the intensity of more complicated n = 1 vortex mode is unexpectedly higher than the one of n = 0 mode. The observed behavior is explained on the basis of the inhomogeneous vortex mode phase profiles extracted from micromagnetic simulations. The phase difference of vortex core gyrations at the top and bottom dot faces is essentially different from 0 and π. The difference is increasing with increase in the dot aspect ratio L/R for the 0th order mode, whereas an inverse relationship is observed for the 1st order mode. The analytical theory indicates that this phase difference has magnetostatic origin.

International audience; The aim of the present work is to show a simple method that combines conventional laser interferometry and standard thin film deposition techniques to fabricate modulated magnetic nanostructures with lateral periodicity, and to tailor the magnetic properties by varying geometrical parameters. Well defined Ni80Fe20 magnetic thin films with sinusoidal grating profiles were obtained with a periodicity of 1.2 μm and different grating depths. Magnetic studies via ferromagnetic resonance and magneto optical Kerr effect demonstrate the tunability of the induced in-plane magnetic anisotropy with depth profile.

Magnonic crystals with periodically modulated thickness were fabricated using deep ultraviolet lithography by covering Ni80Fe20 nanowire arrays with continuous Ni80Fe20 film. It is shown that the magnetization reversal and ferromagnetic resonance fields can be tuned either varying the film thickness (t) or the stripe width (w). For t ≥ 20 nm, two different stable magnetic configurations in zero external field were obtained. These configurations are characterized by noticeably different frequencies of ferromagnetic resonance up to 1 GHz and can be switched by applying magnetic pulses of proper amplitude and polarity.

Dipolar superferromagnetism with reentrant low-temperature superspin glass behavior is observed on a randomly distributed ferromagnetic nanoparticle systems in discontinuous metal-insulator multilayers $[{\mathrm{Co}}_{80}{\mathrm{Fe}}_{20}(t)/{\mathrm{Al}}_{2}{\mathrm{O}}_{3}(3\mathrm{}\mathrm{n}\mathrm{m}){]}_{10}$ with nominal thickness $1.1l~tl~1.3\mathrm{nm}$ by use of ac susceptometry and dc magnetometry. At $t=1.0\mathrm{nm},$ superspin glass-like freezing is evidenced by the criticality of dynamic and nonlinear susceptibilities.

The effective fields in magnetic thin films, their origins and peculiarities are described by Heinrich, Cochran and Kowalewski in this chapter. A particular attention is focused on FMR and Brillouin light scattering (BLS) techniques, which are used for the study of effective fields in films and multilayers. In the theoretical description the effective field is believed being an admixture of the interface and bulk effective fields. Dipole (demagnetizing) field, in-plane and perpendicular uniaxial anisotropies fields as well as magnetocrystalline anisotropy field has been considered as main contributions. The advantages of their investigation and measurements by means of FMR and BLS were also shown in [1].

Modulated structures consisting of square arrays of 60 nm thick Ni80Fe20 circular dots underneath a continuous Ni80Fe20 film were fabricated using multi-level process based on deep ultraviolet lithography at 248 nm exposure wavelength. We observed a drastic change in both the static and dynamic properties of the modulated structures by varying the Ni80Fe20 film thickness t in the range from 5 to 60 nm. It was revealed via comparison between experimental results and micromagnetic simulations that the dots create perturbations of internal fields in the neighbor regions of the film which can be controlled by magnetic field and film thickness.

We use the resonant and non-resonant microwave absorption to probe the dynamic and static magnetic parameters of weakly coupled spin valves. The sample series include spin valve structures with varying thickness of the non-magnetic metallic spacer and reference samples comprised only a free or fixed magnetic layer. Beside the common resonance absorption peaks, the observed microwave spectra present step-like features with hysteretic behavior. The latter effect is a direct manifestation of the interlayer coupling between the ferromagnetic layers and provides two static magnetic parameters, the switching field and coercivity of the fixed layer. The analysis of the microwave absorption spectra under in-plane rotation of the applied magnetic field at different spacer thicknesses permits a deeper insight in the magnetic interactions in this system as compared to the conventional magnetometry. We combine the standard Smit-Beljers formalism for the angular dependence of the resonance fields with a Landau-Lifshitz-Gilbert dynamics extended to describe in detail the intensity of microwave absorption in the spin valves. In this way, we extract a set of parameters for each layer including the effective magnetization and anisotropy, exchange bias and interlayer coupling, as well as Gilbert damping. The model reproduces well the experimental findings, both qualitatively and quantitatively, and the estimated parameters are in a reasonable agreement with the values known from the literature. The proposed theoretical treatment can be adopted for other multilayered dynamic systems as, e.g., spin-torque oscillators.