Magnetic anisotropy in thin ferromagnetic films

The magnetic properties of thin ferromagnetic films are studied taking into account the magnetic anisotropy term in the Hamiltonian. In the second approximation equations are obtained for the magnetization of the monatomic layers parallel to the surface of the thin film. From these equations one obtains the Curie temperature, which depends on the thickness of the thin film and the ratio a between the anisotropy constant and the exchange energy between two neighbours. A value can be chosen for such that the thin film becomes ferromagnetic only for a thickness greater than a definite value and in this manner the theoretical results can be fitted to the experimental data. The situation in cobalt thin films is dealt with in particular.

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The author extends his thanks to the research workers of CIFA 1 as well as to Dr. L. Valenta for information on the same subject.     

Co anisotropy in amorphous Y1-xCox films

Transverse biased initial susceptibility measurements have been performed in amorphous Y_1-xCo_x films. In the framework of the random anisotropy model for amorphous magnetism, a local anisotropy constant for Co of the order 10⁷ erg/cm³ may be inferred from the results. This agrees with the low symmetry of environments in the amorphous state.     

Selective-resputtering-induced perpendicular magnetic anisotropy in amorphous TbFe films

Perpendicular magnetic anisotropy energy in rf magnetron sputtered amorphous TbFe films is measured to increase exponentially with pair-order anisotropy induced by the selective resputtering of surface adatoms during film growth.     

Magnetic anisotropy of strained epitaxial manganite films

The in-plane magnetic anisotropy of epitaxial La_0.7Sr_0.3MnO₃ (LSMO) films is studied at room temperature by the following three independent techniques: magnetooptical Kerr effect, ferromagnetic resonance at a frequency of 9.61 GHz, and recording of absorption spectra of electromagnetic radiation at a frequency of 290.6 MHz. The films are deposited onto NdGaO₃ (NGO) substrates in which the (110)NGO plane is tilted at an angle of 0–25.7° to the substrate plane. The uniaxial magnetic anisotropy induced by the strain of the film is found to increase with the tilt angle of the (110)NGO plane. A model is proposed to describe the change in the magnetic anisotropy energy with the tilt angle. A sharp increase in the radio-frequency absorption in a narrow angular range of a dc magnetic field near a hard magnetization axis is detected The anisotropy parameters of the LSMO films grown on (110)NGO, (001)SrTiO₃, and (001)[(LaAlO₃)_0.3 + (Sr₂AlTaO₆)_0.7] substrates are compared.     

Magnetic properties of amorphous heisenberg ferromagnet with random anisotropy

Magnetization, susceptibility, and the critical temperature for a spin-one amorphous Heisenberg ferromagnet with random local anisotropy are calculated in the molecular field approximation for the exchange interactions. The effects of random fluctuations of the magnitude of the local anisotropy about its crystalline value are studied for various probability distributions. It is found that for a given randomness, measured in terms of the root mean square fluctuation of the magnitude of the local anisotropy, the magnetization and the critical temperature depend more on the range, denned as the magnitude of the maximum value of the fluctuation from its mean value, than the shape of the probability distribution function. The effect of randomness decreases as the crystalline value of the anisotropy increases. Calculation of the susceptibility shows that above the critical temperature, randomness has no effect.     

Magnetic behavior of amorphous Sm-Co alloy films

Amorphous films of Sm_100–x Co _x with 70x90 were made by vapor deposition on flat glass substrates kept at 300 K. The films crystallize above 700 K. The film plane is the easy plane with the planar anisotropy of the order of 10⁷ erg cm^–3, indicating a local anisotropy of the order 10⁸ erg cm^–3. With regard to local anisotropy and magnetization, the films appear to correspond to the random anisotropy concept of amorphous magnets. Films evaporated in a magnetic field parallel to the film plane have an induced uniaxial anisotropy of the order of 10⁶ erg cm^–3. Hysteresis loops in fields parallel to the easy axis are perfectly rectangular with coercive fieldsH _c in the range 30 Oe<H _c<3000 Oe, depending on composition, temperature and heat treatment. The magnetization reverses by a thermally activated domain nucleation and growth process having a narrow distribution of time constants. Aging at temperatures below the crystallization temperature reduces the anisotropies andH _c.     

Magnetic surface anisotropy of transition metal ultrathin films

In order to study the magnetic anisotropy of transition metal ultrathin films, we have performed tight-binding calculations including spin-orbit coupling. Beside the anisotropy energy these calculations also yield the orbital moment, which turns out to be much more anisotropic than in bulk materials. The effects of interfacial mismatch and roughness are discussed within phenomenological models. We also briefly review experimental results on the magnetic surface anisotropy (MSA) in transition metal ultrathin films. In some cases such as Au/Co/Au(111) sandwiches the MSA wins the competition with the shape anisotropy arising from the magnetostatic energy: below a critical thickness this leads to aperpendicular spontaneous magnetization. We show the effects of this crossover on the hysteresis loops and on the magnetoresistance, and the effects of interface roughness on the critical thickness.     

Analyzing in-plane magnetic anisotropy from surface morphology for amorphous films

A method is developed to analyze the in-plane magnetic anisotropy from surface morphology for amorphous films. The lateral sizes along radial direction (R_RD) and tangent direction (R_TD) of rotational substrate, which are extracted from the surface morphology of Co_66.3Zr_33.7 amorphous films, are used to calculate stress anisotropy energy E_σ. It is found that E_σ is consistent with the magnetic anisotropy energy K_μ for the samples deposited on Si (1 0 0) substrate and then a relationship K_μ ∝ 1/R_RD − 1/R_TD can be obtained. This method is sensitive to the initial state of substrate so its application range is discussed.     

Dynamic self-organization of magnetic domains in amorphous films with perpendicular anisotropy

Dynamic self-organization of magnetic domains is observed in amorphous gadolinium-cobalt films in narrow temperature intervals on both sides of the magnetic compensation point. Spiral dynamic domains form in a limited range of ac magnetic field amplitudes and frequencies. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 11, 688–692 (10 December 1997)     

Magnetic anisotropy determined by differential magnetization measurements in twisted amorphous ribbons

A new method of measurement, which enables a determination of the in-plane anisotropy in magnetic metallic glass ribbons, has been developed. It can be used for materials with the easy axis of magnetization parallel as well as perpendicular to the ribbon axis. The method is based on measurements of the stress dependence of the inhomogeneous magnetization obtained by twisting the ribbon around the ribbon axis. The required experimental equipment is very simple, and the value of the anisotropy constant comes out directly as an equivalent internal stress field.     

Magnetic properties of amorphous Ge–Fe thin films

Thin films of Ge_100−xFe_x (x in at%) alloys, fabricated by thermal co-evaporation, have an amorphous structure at compositions x<∼40, although an unidentified crystalline phase with an FCC symmetry also exists at low Fe content. Magnetization versus temperature curves show that saturation magnetization is non-zero (1 to 2.5 emu/cm³) and remains nearly unchanged up to the highest measured temperature of 350 K. Magnetic hysteresis loops at room temperature show a typical ferromagnetic shape, complete saturation occurring by 1–2 kOe. These results may indicate ferromagnetic ordering at room temperature. No definite tendency is observed in the compositional dependence of saturation magnetization.