A. B. Drovosekov

Spin-glass state of Fe/Cr multilayer structures with ultrathin iron layers

The evolution of the magnetic properties of Fe/Cr superlattices with a decrease in the nominal thickness of the iron layers down to atomic dimensions at which these layers are not continuous has been analyzed. Investigations have been carried out with multilayer samples with Fe-layer thicknesses in a range of 2–6 Å and Cr-layer thicknesses of 10 and 20 Å. It has been found that the system with various Fe-layer thicknesses and at various temperatures exhibits various magnetic phases—superparamagnetic, magnetically ordered, and nonergodic—characterized by the dependence of the magnetization of the sample on its magnetic prehistory. It has been shown that the observed nonergodic phase has the properties of a spin glass. A qualitative phase diagram of the magnetic states of the system has been obtained.

Interlayer interaction in a Fe/Cr/Fe system: Dependence on the thickness of the chrome interlayer and on temperature

A three-layer sample of Fe(100 Å)/Cr(0–20 Å)/Fe(100 Å) is used to study the dependence of inter-layer exchange on the thickness of the chrome interlayer and on temperature. The method of Kerr magnetometry in the temperature range from 77 to 473 K and the method of Brillouin scattering of light by spin waves at room temperature are used. The data for magnetization curves and spin wave spectra are treated in the model of biquadratic exchange. The range of validity of this model is established, which is apparently determined by interlayer exchange. The resultant dependence of the constant of bilinear interaction on the interlayer thickness demonstrates an oscillating behavior with two oscillation periods of about 3 and 18 Å. Within the experimental error, the magnitude of this constant, the amplitude, and the period and phase of its oscillation are independent of temperature. It is found that the constant of biquadratic exchange decreases in inverse proportion to the chrome thickness, the proportionality factor decreasing linearly as the temperature rises. In order to interpret the observed singularities in the behavior of the biquadratic exchange constant, a theoretical model is suggested which includes the nonideality of the interface and the presence of magnetic hardness in the chrome interlayer. This rigidity exceeds in magnitude the interaction on the interface between iron and chrome. The suggested model gives an adequate qualitative description of the experimental results.

Interlayer coupling in Fe/Cr/Gd multilayer structures

The effect of the chromium layer thickness on the magnetic state of an [Fe/Cr/Gd/Cr]n multilayer structure is studied. A series of Fe/Cr/Gd structures with Cr spacer thicknesses of 4–30 Å is studied by SQUID magnetometry and ferromagnetic resonance in the temperature range 4.2–300 K. The obtained experimental results are described in terms of an effective field model, which takes into account a biquadratic contribution to the interlayer coupling energy and a nonuniform magnetization distribution inside the gadolinium layer (which was detected earlier). Depending on the magnetic field and temperature, the following types of magnetic ordering are identified at various chromium layer thicknesses: ferromagnetic, antiferromagnetic, and canted ordering. A comparison of the experimental and calculated curves allowed us to determine the dependence of the bilinear (J1) and biquadratic (J2) exchange constants on chromium layer thickness tCr. Weak oscillations at a period of about 18 Å are detected in the J1(tCr) dependence in the range 8–30 Å. The interlayer coupling oscillations in the system under study are assumed to be related to the RKKY exchange interaction mechanism via the conduction electrons of Cr.

Temperature dependence of interlayer coupling in Fe/Cr superlattices — FMR studies

Abstract The ferromagnetic resonance in a [Fe(30xa0A)/Cr(10xa0A)]10 superlattice with a relatively large value of biquadratic coupling constant was studied in a temperature range from 400xa0K down to liquid-nitrogen temperature. The monocrystalline sample was grown by means of the MBE technique on a MgO [1xa00xa00] substrate. Measurements were performed in magnetic fields up to 10xa0kOe at frequencies ranging from 17 to 37xa0GHz under both transversal and longitudinal FMR excitations. Resonance spectra with several modes including the acoustic and the optical branches were observed in the whole temperature range. Temperature dependence of both bilinear and biquadratic coupling constants was derived from the experimental spectra.

The observation of non-homogeneous FMR modes in multilayer Fe/Cr structures

Abstract The spectrum of excitations in [Fe/Cr]n structures with a non-collinear magnetic ordering was studied by means of the FMR technique. The measurements were carried out at room temperature in the frequency range of 9.5–37xa0GHz with both static and microwave magnetic fields lying in the film plane. Along with the acoustic FMR mode several additional modes were observed in the longitudinal pumping configuration. The resonance spectrum of an infinite magnetic superlattice was calculated analytically on the basis of the biquadratic exchange coupling model. Both cases were considered: external magnetic field H parallel and perpendicular to the film plane. It was shown that the observed additional FMR modes correspond to the excitation of standing spin waves with wave vectors perpendicular to the superlattice plane.

Inhomogeneous ferromagnetic resonance modes in [Fe/Cr]n superlattices with a high biquadratic exchange constant

In a set of [Fe/Cr]n superlattices, magnetization curves and spectra of ferromagnetic resonance under an in-plane magnetic field have been studied at room temperature. Along with the acoustic branch, several additional branches have been observed in resonance spectra. Resonance spectra have been calculated analytically for a structure with an infinite number of layers and numerically for finite numbers of layers in real samples using a model of biquadratic exchange taking account of the fourth-order magnetic anisotropy. A possibility of describing both static and resonance properties of the system in terms of this model has been demonstrated.

Ferromagnetic resonance in multilayer [Fe/Cr]n structures with noncollinear magnetic ordering

The excitation spectrum in an [Fe/Cr]n multilayer structure with non-collinear magnetic ordering was studied by the ferromagnetic resonance (FMR) method in the frequency interval 9.5–37 GHz at room temperature. Besides an acoustic branch, several additional modes were observed under parallel excitation of resonance. The FMR spectrum was calculated analytically in a biquadratic exchange model, neglecting in-plane anisotropy, for an infinite number of layers in the structure and numerically for a finite number of layers contained in real samples. It was shown that the observed modes correspond to excitation of standing spin waves with wave vectors perpendicular to the film plane.

Temperature dependence of interlayer coupling in a Fe/Cr/Fe wedge sample. MOKE and MBLS studies

Abstract The interlayer coupling in a Fe(100xa0A)/Cr(0–22.5xa0A)/Fe(100xa0A) sample with a wedge-type chromium spacer was investigated as a function of chromium thickness and temperature. Two experimental methods were used for the interlayer coupling determination: the static magnetization curves of the sample were measured by means of the magneto-optical Kerr effect (MOKE) technique in the 77–473xa0K temperature range, and the Mandelstam–Brillouin light scattering (MBLS) experiments were performed to measure the spin-wave spectra of the sample at room temperature. The data obtained via two methods can be well described in the frame of the biquadratic coupling model and are in a good agreement with each other. The bilinear coupling oscillates with the chromium spacer thickness having a period of two monolayers (in agreement with the known experimental results). The biquadratic coupling constant J 2 decays with the increase of the spacer thickness monotonically, showing a 1/ t Cr behaviour at all temperatures. The bilinear coupling temperature dependence is very weak and could not be distinguished within our experimental accuracy, while the biquadratic coupling demonstrates a relatively strong temperature dependence, which can be considered as linear.

Magnetization and ferromagnetic resonance in a Fe/Gd multilayer: experiment and modelling

Static and dynamic magnetic properties of a [Fe(35 Å)/Gd(50 Å)]12 superlattice are investigated experimentally in the temperature range 5-295u2009K using SQUID magnetometery and the ferromagnetic resonance (FMR) technique at frequencies 7-38 GHz. The obtained magnetization curves and FMR spectra are analysed theoretically using numerical simulation on the basis of the effective field model. At every given temperature, both static and resonance experimental data can be approximated well within the proposed model. However, a considerable temperature dependence of the effective field parameter in gadolinium layers has to be taken into account to achieve reasonable agreement with the experimental data in the entire temperature range studied. To describe the peculiarities of experimental FMR spectra, a non-local diffusion-type absorption term in Landau-Lifshitz equations is considered in addition to the Gilbert damping term. Possible reasons for the observed effects are discussed.

Magnetic states of multilayer Fe∕Cr structures with ultrathin iron layers

The evolution of the magnetic properties of Fe∕Cr superlattices is studied as the nominal thickness of the iron layers is reduced to atomic values, when these layers are no longer continuous. The studies were done on multilayer samples with Fe thicknesses of 2–6A and chromium spacer thicknesses of 10 and 20A. The samples were prepared by molecular beam epitaxy. The static magnetization and complex magnetic susceptibility were measured and FMR spectra taken. It was found that, depending on the thickness of the Fe layers and temperature, different magnetic phases are realized in the system: supermagnetism, magnetic ordering, and a nonergodic state characterized by a dependence of the magnetization of a sample on its magnetic prehistory. The observed nonergodic phase is found to exhibit spin glass properties. A qualitative phase diagram of the magnetic states of this system is constructed.