M. From

Exchange stiffness in thin film Co alloys

The exchange stiffness (A(ex)) is one of the key parameters controlling magnetization reversal in magnetic materials. We used a method based on the spin spiral formation in two ferromagnetic films ...

MBE growth and FMR, BLS and MOKE studies of exchange coupling in Fe whisker/Cr/Fe(001) and in Fe/Cu/Fe(001) ‘loose spin’ structures

Abstract The exchange coupling has been studied in structures which consist of two ferromagnetic layers separated by non-ferromagnetic spacers (trilayers). The exchange coupling was measured using FMR and BLS techniques in the temperature range 77–400 K. Two systems were investigated: (a) Fe whisker/Cr/Fe(001) and (b) Fe/Cr/Fe(001). The oscillatory thickness dependence of the exchange coupling through a spin-density wave Cr spacer will be discussed and compared with recent data obtained by other groups. Cu interlayers were deposited either in a pure form, or a single monolayer of Cu Fe alloy (‘loose spins’) was inserted between two pure bcc Cu(001) layers. Several such ‘loose spin’ structures were engineered to test the behavior of ‘loose spin’ structures. It was found that the presence of Fe impurity atoms has a strong tendency to decrease the direct bilinear exchange coupling. The contribution of ‘loose spins’ to the exchange coupling can be made significant, and even dominant, by a suitable choice of the RKKY coupling energy between the ‘loose spins’ and the surrounding ferromagnetic layers.

Temperature dependence of the exchange coupling in the Fe(001) whisker/11 ML Cr/20 ML Fe structure

The exchange coupling between iron layers separated by 11 monolayers (ML) of Cr(001) has been investigated using a structure in which the Cr(001) was grown on a bulk iron whisker Fe(001) surface at a temperature of approximately 300 °C. This temperature was selected to produce near optimum smoothness of the Cr layer. The Cr(001) deposition was followed by the deposition of 20 ML of Fe(001) at room temperature, and by the deposition of a 20 ML Au(001) protective layer. The frequencies corresponding to the magnetic excitations in this structure were measured by means of Brillouin light scattering (BLS). One of the observed frequencies corresponds to a surface mode in the bulk iron whisker. Another observed frequency corresponds to the lowest lying precessional mode of the magnetization in the 20 ML thick Fe(001) thin film. Typically, the thin film frequency exhibits a dependence on applied magnetic field that displays two cusps. The positions of the cusps are dependent on the exchange coupling between the 2...

‘‘Loose spins’’ in Fe/Cu/Fe(001) structures

Slonczewski recently proposed a model for the exchange coupling between ferromagnetic layers separated by a nonferromagnetic spacer based on the concept of ‘‘loose spins.’’ ‘‘Loose spins’’ contribute to the total exchange energy. We have studied the role of ‘‘loose spins’’ in bcc Fe/Cu/Fe(001) structures. bcc Fe/Cu/Fe(001) trilayers deposited at room temperature were investigated extensively in our previous studies. In our ‘‘loose spin’’ studies, the Fe was added inside the Cu interlayer. Several structures were atomically engineered in order to test the behavior of ‘‘loose spins:’’ One additional atomic layer of an (Fe+Cu) alloy were located in appropriate positions in a Cu spacer. The bilinear and biquadratic exchange coupling in the above structures was quantitatively studied with FMR in the temperature range 77–370 K and with MOKE at RT.

Measuring the speed of sound in a solid

The speed of sound in a solid is measured using an oscilloscope, a square-wave oscillator and a piezo-electric pick-up. A study of the relationship between the distance traveled and the time of arrival of the sound pulse allows a graphical determination of the speed of the pulse in the lucite rod.

Thermopower of Fe-Zr metallic glasses

Abstract The thermopower of Fe-Zr metallic glasses for Fe concentrations in the range 20 to 43 at.% has been measured from 4K to 500K. The form of the thermopower changes dramatically from a nearly linear temperature dependence when the glasses are non magnetic to strongly non-linear dependence when the glasses become magnetic. The data for the non magnetic glasses can be explained using the usual Faber-Ziman approach. For the magnetic glasses inelastic magnetic scattering must be specifically included.

Bilinear and Biquadratic Exchange Coupling in BCC Fe/Cu/Fe Trilayers. Exchange Coupling in Fe Whisker/Cr/Fe(001) Structures

Surface Magneto-Optical Effect (SMOKE) studies of the exchange coupling in bcc Fe/Cu/Fe(001) structures and Brillouin Light Scattering (BLS) results on Fe(OOl) whisker/Cr/Fe(001) structures are presented. It is shown that the interfaces in bcc Fe/Cu/Fe(001) trilayers can be significantly improved by choosing an appropriate growth procedure. The exchange coupling in bcc Fe/Cu/Fe trilayers was studied as a function of the interlayer thickness. The interpretation of magnetization loops for Fe/Cu/Fe trilayers requires the simultaneous presence of bilinear and biquadratic exchange coupling between the magnetic layers. Computer calculations were used to determine the strength of the bilinear and biquadratic exchange couplings. It is shown that the strength of the biquadratic exchange coupling increases with increasing terrace width. The measured values of the bilinear and biquadratic exchange coupling were compared with a model recently proposed by Slonczewski, which treats the exchange coupling in trilayers with imperfect interfaces. Slonczewski’s model was used to deconvolute the data to obtain the intrinsic behavior of the bilinear exchange coupling in bcc Fe/Cu/Fe(001) trilayers. It is shown that the exchange coupling unobscured by interface roughness exhibits a strong short wavelength oscillatory behavior which is in agreement with recent first principles band calculations.

Magnetic and transport properties of sputtered Gd‐Y multilayers

Gd‐Y‐Gd multilayers were prepared that show a magnetoresistance enhancement for an Y layer separation of 30 A. This magnetoresistance enhancement is an interface effect and occurs in samples where some degree of antiferromagnetic coupling is present.

Dependence of anti-Stokes/Stokes intensity ratios on substrate optical properties for Brillouin light scattering from ultrathin iron films

Brillouin light scattering experiments have been used to investigate the intensity of 5145 A laser light backscattered from spin waves in 20 monolayer thick Fe(001) films. The experiments have shown that the ratio of frequency upshifted light intensity to frequency downshifted light intensity depends upon the material of the substrate used to support the iron films. For a fixed magnetic field and for a fixed angle of incidence of the laser light this intensity ratio is much larger for an iron film deposited on a sulphur passivated GaAs(001) substrate than for an iron film deposited on a Ag(001) substrate. The data have been compared with a calculation that takes into account multiple scattering of the optical waves in the iron film and in a protective gold overlayer. The observations are in qualitative agreement with the theory, except for angles of incidence greater than 60°.

Investigation of the dependence of BLS frequencies on angle of incidence for thin iron films

Brillouin light‐scattering experiments have been done at various angles of incidence, θ, for four specimens prepared by molecular‐beam epitaxy. The specimens were single ultrathin films of Fe deposited on single‐crystal Ag substrates. Dependence of magnon frequency on θ is easily resolvable in all specimens. We find that the magnitude of this dependence is in good agreement with a theoretical calculation that takes into account magnetic anisotropies, dipole‐dipole, and exchange interactions. Our results imply that magnetic excitations in these specimens are correlated over distances of at least 5000 A.