Ferromagnetic resonance in thin cobalt films

The paper gives the results of measuring ferromagnetic resonance on thin cobalt films, vacuum deposited on unheated glass slides. The values of the (g-factor, the width of the curve, the effective stress and uniaxial induced anisotropy were determined as a function of the thickness of the film from measurements of the ferromagnetic absorption in a magnetic field normal and parallel to the surface of the film. Measurements were carried out on a frequency of 9200 MHz and on film thicknesses of 180 to 1800 Å. A qualitative explanation of the observed dependences is given.

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The author thanks V. Kamberský and Z. Málek, C. Sc., for providing some of the cobalt films and for help in depositing and measuring the thicknesses, S. Kadeková and M. Polcarová for valuable advice in determining the structure of the films, J. Míová for carefully plotting the results of measurements and Z. Málek and O. tirand for carefully reading the paper and for valuable discussions.     

Ferromagnetic resonance in systems with competing uniaxial and cubic anisotropies

We develop a model for ferromagnetic resonance in systems with competing uniaxial and cubic anisotropies. This model applies to (i) magnetic materials with both uniaxial and cubic anisotropies, and (ii) magnetic nanoparticles with effective core and surface anisotropies; We numerically compute the resonance frequency as a function of the field and the resonance field as a function of the direction of the applied field for an arbitrary ratio of cubic-to-uniaxial anisotropy. We also provide some approximate analytical expressions in the case of weak cubic anisotropy. We propose a method that uses these expressions for estimating the uniaxial and cubic anisotropy constants, and for determining the relative orientation of the cubic anisotropy axes with respect to the crystal principle axes. This method is applicable to the analysis of experimental data of resonance type measurements for which we give a worked example of an iron thin film with mixed anisotropy.     

Ferromagnetic resonance and bistability field in a uniaxial magnetic film

Peculiarities of ferromagnetic resonance (FMR) corresponding to bias along the “hard” magnetic axis of a film with 2D uniaxial anisotropy are studied based on numerical solution of magnetic moment dynamics equations. It is shown that an additional resonance peak is formed in the FMR spectrum in the vicinity of “bistability field” H _b . The dependence of this field on the amplitude of the microwave field and damping parameters is analyzed.     

Geometrical effects in nanodimensional epitaxial films of barium strontium titanate

The effect of the thickness of 6- to 950-nm-thick Ba_0.8Sr_0.2TiO₃ films epitaxially grown on (001)MgO on their ferroelectric properties is investigated. Raman scattering and X-ray diffraction data indicate a structural phase transition taking place at a film thickness of ≈70 nm, which changes drastically the lattice parameters of the film. Raman spectra taken of the films confirm that they are in the ferroelectric state and that their symmetry changes in going over a critical thickness, as revealed by a sharp displacement of the peaks corresponding to the A ₁(TO) and E(TO) components of the soft mode. At film thicknesses of <100 nm, the permittivity versus thickness dependence exhibits two peaks at thicknesses of ～18 and ～36 nm. Near the first peak, the dielectric nonlinearity is considerably higher than near the second one.     

Ferromagnetic resonance studies in compositionally modulated cobalt-niobium films

We have prepared multilayered compositionally modulated films by alternately depositing Co and Nb layers of 50 and 24 Å nominal thickness, respectively. Two e-beam evaporation systems in ultra high vacuum were used. Magnetization and B-H loop measurements were done at 293 K. Ferromagnetic resonance was observed at 9.79 GHz using cavity techniques and as a function of frequency in the range 4–14 GHz using a planar microwave structure. The magnetization 4πM=13 kG is found to be lower than the Co bulk value. The observed frequency dependence of the linewidth in the parallel orientation can be interpreted. Possible explanations for the reduction of 4πM are discussed.     

Ferromagnetic resonance studies in granular CoCu codeposited films

Experimental ferromagnetic resonance (FMR) data are presented for CoCu granular films obtained by electron beam codeposition. Co_xCu_100−x films were prepared for x=10 and 30 and were subsequently submitted to a thermal treatment. For the 10% Co samples one found three main resonance modes, one of them constituting of non-interacting Co particles. External field angular variation and temperature-dependent FMR experiments were proceeded and straight calculations of the involved resonance intensities as well. A strict correlation of the presented FMR data with the previous X-Ray diffraction, magnetization, and particle size investigations was possible. The link between structural changes and magnetic properties of the samples could be verified. For the 30% Co samples a broad resonance line for the in-plane configuration was found, which is characteristic for polycrystalline systems. In the perpendicular configuration a spin wave spectra were present on all samples and one could identify some of the modes as surface modes. These samples with 30% Co become more heterogeneous with increasing thermal treatment.     

Magnetic and structural properties of M-type barium hexaferrites films with transition metal substitutions

We have investigated the correlation between structural and magnetic properties of M-type BaFe₁₂O₁₉ thin films (～1.4 $\mu m$) with Co-Ti (magnetic/non-magnetic) and Co-Ni(magnetic/magnetic) substitution, as BaFe_12-2xCo_xTi_xO₁₉ and BaFe_12-2xCo_xNi_xO₁₉ (0?≤?x?≤?1). With structural properties sensitively related to the magnetic properties, where ferro-ferri phase transition is involved, it has been found that magnetic properties can be substantially controlled by substitution concentration.     

Ferromagnetic order and critical behavior at surfaces of ultrathin epitaxial films

The critical behavior, ferromagnetic order and magnetic anisotropies of ultrathin, epitaxial, magnetic films is studied using electron capture spectroscopy (ECS), which is capable of probing the long-ranged and short-ranged electron spin polarization (ESP) at the topmost surface layer of uncoated and coated magnetic structures. For all systems [Ni(100)/Cu(100), Ni(100)/NaCl(100), fcc Fe(111)/Cu(111), Fe(100)/Ag(100), Tb/Fe(100)/Ag(100), Fe(100)/Au(100), hcp Tb(0001)/W(110), Fe(110)/W(110), V(100)/Ag(100), Pd(100)/Ag(100), Pd/W(110)] investigated so far, ferromagnetic order is detected. It is found that the surface Curie temperatureT _Cs depends on film thicknessd. ECS data obtained at the surface of various systems reveal the existence ofT- andd-dependent magnetic anisotropies. Although for V(100)/Ag(100) the measured critical exponent=0.128 agrees very well with=1/8 predicted for the two-dimensional Ising model, for other systems, such as Fe(100)/Au(100), the measured value (0.25) is in disagreement with theoretical predictions. The experimental results are discussed within the framework of presently available experimental and theoretical data.     

Domain structures in barium and strontium hexaferrites

Powder figures have been used to study the domain structure of monocrystals of barium hexaferrite; the domain boundaries on a side face respond to a magnetic field. The domain structure of strontium hexaferrite is reported; the star structure on the end face has a definite order, and the pattern changes to wavy bands as the thickness of the crystal is reduced. The domain structure of both ferrites corresponds generally to that of cobalt monocrystals.     

Ferromagnetic resonance and relaxation in single crystal metal films

We review the results of oblique-angle ferromagnetic resonance experiments in single-crystal metal films, and compare them with recent calculations for oblique-angle resonance based on isotropic metals and on anisotropic insulators with magnetoelastic interactions.Supported in part by the National Science Foundation.We are please to acknowledge the contributions of our colleagues J. L. Bleustein, T. Kobayashi, and C. Vittoria.     

Ferromagnetic resonance in Ni-Mn-Ga thin films and thin-film tubes

We report on FMR experiments performed for the first time on thin Ni-Mn-Ga films clamped to the mica substrates and then fully released from them. The aim is to evaluate the role of magnetoelastic coupling in stressed Ni-Mn-Ga Heusler alloy films that undergo martensitic transformation. The experimental results show that the difference in the effective magnetization 4π(M_eff ^tubes-M_eff ^films) is negligible in the austenite phase and it increases to about 1–1.5 kG at temperatures well below the martensitic transformation. The data suggests that magnetoelastic coupling in the martensite phase of Ni-Mn-Ga thin films is typical of normal thin magnetic films with magnetostriction of about 50 ppm.     

Ferromagnetic resonance ripple effects in single crystal nickel films

An explanation of ferromagnetic resonance ripple effects in single crystal nickel films is given in terms of the magnetic effects associated with dislocations.     

Hybrid electromagnetic-spin oscillations in layered structures with uniaxial hexaferrites

Spectra of magnetostatic oscillations in layered dielectric-ferrite-metal structures are studied theoretically and experimentally. Hybridization of the magnetostatic oscillations and dielectric resonant modes is experimentally investigated both under saturation and in the domain region in the presence of a cylindrical magnetic domain. The experiments are performed on the platelets of magnetoplumbite, as well as barium and strontium hexaferrites. In composite structures, quartz platelets of various thicknesses are used as dielectrics.