Magnetic properties of layered structures based on transition and rare-earth metals (TMs and REMs) such as Fe and Gd have attracted attention of researchers since 1990s. These materials are artificial ferrimagnets with reach magnetic phase diagrams, which make it possible to realize a wide spectrum of predefined properties. In recent years, a new surge of the interest in such systems was evoked by observations of new peculiar dynamic effects in these materials, including optical magnetization reversal and ultrafast motion of domain walls, as well as the possibility of realization of skyrmion magnetic states. In this article, a brief review of the most interesting features of magnetism and magnetic dynamics of layered ferromagnetic TM/REM structures is presented. The results of our investigation concerning Fe/Gd superlattices and the effects of Cr interlayers on their magnetic properties are reported. The surface cant of the magnetization in these structures has been observed directly using the magnetooptical Kerr effect, and the magnetic phase diagram of the system has been obtained. We have analyzed peculiarities of nonuniform modes of magnetic oscillations excited in the Fe/Gd superlattice by the ferromagnetic resonance method. The possibility of sign reversal of the exchange interaction of Fe and Gd layers from antiferromagnetic to ferromagnetic type upon the introduction of Cr interlayers between them has been demonstrated.

Static and dynamic magnetic properties of a ferrimagnetic [Fe(35 angstrom)/Gd(50 angstrom)](12) superlattice were investigated in a wide 4-300 K temperature range using magneto-optical Kerr effect (MOKE) and ferromagnetic resonance (FMR) techniques. The multilayer structure was sputtered on a transparent glass substrate which made it possible to perform MOKE measurements on both Fe and Gd terminated sides of the superlattice. These experiments allowed us to detect a transition between field-aligned and canted magnetic states on both sides of the film and to distinguish between the bulk and surface twisted phases of the superlattice. As a result, the experimental H - T magnetic phase diagram of the system was obtained. FMR studies at frequencies 7-36 GHz demonstrated a complex evolution of absorption spectra as temperature decreased from room down to 4 K. Two spectral branches were detected in the sample. Theoretical simulations show that the observed spectral branches correspond to different types of inhomogeneous resonance modes in the multilayer with non-uniform magnetization precession inside Gd layers.

In this work, we analyze the role of a thin Cr spacer between Fe and Gd layers on the structure and magnetic properties of a [Fe(35 Å)/Cr(tCr)/Gd(50 Å)/Cr(tCr)]12 superlattice. Samples without the Cr spacer (tCr = 0) and with a thin spacer (tCr = 4 Å) are investigated using X-ray diffraction, polarized neutron and resonance X-ray magnetic reflectometry, static magnetometry, magneto-optical Kerr effect, and ferromagnetic resonance techniques. Magnetic properties are studied experimentally in a wide temperature range 4–300 K and analyzed theoretically using numerical simulation on the basis of the mean-field model. We show that a reasonable agreement with the experimental data can be obtained considering temperature dependence of the effective field parameter in gadolinium layers. The analysis of the experimental data shows that besides a strong reduction of the antiferromagnetic coupling between Fe and Gd, the introduction of Cr spacers into Fe/Gd superlattice leads to modification of both structural and magnetic characteristics of the ferromagnetic layers.

We studied the magnetoresistance (MR) of twisted bilayer graphene (tBLG) on electron transparent substrate. Samples of tBLG were assembled on free-standing silicon nitride (SiNx) membranes (<100 nm thick) by transferring chemical vapor deposition (CVD)-grown single layer graphene (SLG) twice; this allowed the measurement of the angle of rotation between the two layers, the twist angle, by electron diffraction using a transmission electron microscope (TEM). To compare with the previous reports on tBLG, we performed Raman spectroscopy on our samples. We measured the MR of tBLG for two different twist angles: 2 degrees (small) and 18 degrees (large). The MR showed superposition of two Shubnikov de Haas (SdH) oscillations for both angles. An analysis of the oscillation peaks by Landau fan diagrams showed difference as twist angle. While the large twist angle (18 degrees) sample had two anomalous pi Berry's phases, the small twist angle (2 degrees) sample had conventional 2 pi and anomalous pi Berry's phase depending on carrier density. (C) 2016 Elsevier B.V. All rights reserved.

Upsurge of interest in the study of magnetic multilayer structures began in the mid-80s of the twentieth century and was caused by significant progress in the technology of ultrathin metallic films. For a few years, phenomena extremely interesting from both fundamental and applied point of view have been found in these structures: the antiparallel arrangement of the ferromagnetic layers, giant magnetoresistance, alternating oscillations of the interlayer exchange, noncollinear interlayer ordering of magnetic moments. All this and the closeness of technologies, used for the sample preparation to those traditionally applied in microelectronics, promised great potentials for developing compact magnetic field sensors, non-volatile memory for electronic devices, etc. To date, some of these features are already implemented in the mass commercial products (e.g., read heads of hard drives), and others still wait in the wings. The Nobel Prize, awarded in 2007 to A. Fert and P. Grünberg with the words “For the discovery of the giant magnetoresistance effect” can be considered as recognition of the importance of the discoveries made in this area. A special place in these works was given to the Fe/Cr/Fe system. It is there for the first time almost all of the most interesting phenomena were observed that caused a boom in the physics of magnetic films. However, until recently the nature of the exchange interaction in this system aroused great debates. First of all, it was due to a complex phase diagram of chromium spacer and strong influence on the sample properties of technological perfection of its structure. Here we made a brief review of the main experimental and theoretical studies on the multilayer systems Fe/Cr/Fe, as well as recounted the results of our own studies (performed with a group of co-authors), which greatly clarify the mechanism of the interlayer interaction in this system.

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.

The interlayer magnetic coupling of iron layers as a function of the chromium spacer thickness and temperature has been studied for three-layer epitaxial Fe/Cr/Fe films by the methods of Kerr magnetometry and Mandelstam-Brillouin scattering. The results obtained indicate that the short-period component of the interlayer exchange is related to the spin density wave in the chromium spacer.

The interlayer coupling in an Fe/Cr/Fe layer system with wedge-type chromium spacers was investigated as a function of the chromium-layer thickness and temperature using Kerr magnetometry. The system with a moderate roughness demonstrates an interlayer coupling that can be well described in the framework of the biquadratic coupling model. The smoother sample shows interlayer coupling that can be described using both the biquadratic coupling and the proximity magnetism model proposed by Slonczewski.

The interlayer coupling in Fe/Cr/Fe layer system with wedge-type chromium spacers was investigated as a function of chromium thickness and temperature using Kerr magnetometry and Mandelstam–Brillouin light scattering techniques. The system with a moderate roughness demonstrates an interlayer coupling which can be well described in the frame of the biquadratic coupling model. On the contrary, the smoother sample shows the interlayer coupling, which can be perfectly described by the proximity magnetism model, proposed by Slonczewski.

The local modification of antiferromagnetic (AF) interlayer exchange coupling by focused ion-beam irradiation has been studied experimentally in the epitaxial Fe/Cr/Fe(001) trilayer systems. Square ferromagnetic (FM) areas of 200×200 μm2 were created in the initially AF trilayer by ion irradiation with a fluence of 1015 ions/cm2. It was found, that in the range of the external magnetic field of about ±200 Oe, the change of magnetic properties at the boundaries separating FM and AF areas occurs within distances of less than 200 nm. This fact allows the use of the technique for magnetic patterning of antiferromagnetically coupled trilayers on the submicrometer scale.

The manipulation of the antiferromagnetic interlayer coupling in epitaxial Fe/Cr/Fe(001) trilayers by 5 keV He ion beam irradiation has been investigated. It is shown that even for irradiation with low fluences a drastic change in strength of the coupling appears. For thin Cr spacers (below 0.6-0.7 nm) it decreases with fluence, becoming ferromagnetic for fluences above 2x10(14) ions/cm(2). The effect is connected with the creation of magnetic bridges in the layered system due to atomic exchange events caused by the bombardment. For thicker Cr spacers an enhancement of the antiferromagnetic coupling strength is found. A possible explanation of the enhancement effect is given.

The interlayer coupling in a Fe(100Å)/Cr(0–22.5Å)/Fe(100Å) 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 77–473K 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 J2 decays with the increase of the spacer thickness monotonically, showing a 1/tCr 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.

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.

The ferromagnetic resonance in two Fe/Cr superlattices with a relatively large value of biquadratic coupling constant was studied in a temperature range from 400 K down to liquid helium temperature. Monocrystalline samples [Fe(30 A)/Cr(10 A)] 10 and [Fe(20 A)/Cr(11 A)] 8 were grown by means of the MBE technique on MgO [100] and Al 2 O 3 [101] substrates, respectively. Measurements were performed in magnetic fields up to 10 kOe at frequencies ranging from 17 to 37 GHz under both transversal and longitudinal FMR excitation. Resonance spectra including the acoustic and the optical branches show significant temperature dependence. The interlayer coupling parameters were numerically calculated from the experimental spectra on the basis of the biquadratic coupling model. The data obtained for temperature dependence of both bilinear and biquadratic coupling constants were discussed in the framework of existing theories.

In a set of [Fe/Cr](n) superlattices with strong biquadratic coupling, FMR spectra and magnetization curves have been studied at room temperature, A number of resonance modes were detected including the acoustic and optical branches. Resonance spectra have been calculated in a biquadratic exchange model analytically for an infinite number of layers in the structure and numerically for a finite number of layers contained in real samples.

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.