Results are presented from an experimental and theoretical analysis of the quasi-stationary magnetization reversal of ferrite–garnet films with complex anisotropy. A magneto-optical setup based on the Faraday effect is used to determine conditions that allow reduction of the area of a domain structure. Destruction of the homogeneous magnetization of the magnetic film in four narrow regions relative to one another at right angles is also established. A theory in good agreement with the experimental results is proposed.

Shungite is considered as a filler of composite materials. It is a natural composite of carbon nanoparticles with a variety of micro- and nano-sized mineral impurities that give it high conductivity and electromagnetic radar-shielding properties. The radar-absorbing and shielding characteristics of a composite material based on fine shungite and urea–formaldehyde resin are studied in the 500 MHz to 4 GHz range of frequencies. The effect the thickness of a sample has on the electromagnetic properties of the studied composite is determined.

An experimental study is performed using an advanced magneto-optical setup that simplifies the preparation and observation of domain structures which arise upon magnetizing ferrite-garnet films with in-plane anisotropy.

Results from magneto-optical studies that provide a greater understanding of the stationary domain structure in ferrite–garnet films with both biaxial and in-plane anisotropy are discussed. Two mechanisms of films magnetization by weak magnetic fields on the order of 1 Oe are identified: from a change in the 90° and 180° domain boundaries and the sprouting of wedge-shaped domains from closing domains.

Разработана теория изменения сопротивления сплошной среды между двумя электродами при наличии в среде движущегося тела. Правильность представленных теоретических расчетов подтверждается результатами эксперимента, выполненного в рамках данной работы.

The technical devices which allow to significantly extend the time range of the studied pulsed magnetization and magnetization reversal processes of magnets that are, e.g., plates, flat films (ferrite-garnet films with easy-plane anisotropy, iron borate monocrystals), along with ensuring measurements of maximum accuracy are considered.

The regime of 180° pulsed magnetization reversal of ferrite-garnet films with planar anisotropy in the region of external fields, in which the mechanism of uniform rotation of the magnetization operates, is investigated for the first time. An analysis of numerical solutions of the Landau–Lifshitz equation and our experimental studies show that, as in the case of the 90° pulsed magnetization, the presence of biaxial anisotropy in real ferrite–garnet films also leads to the so-called “effect of delayed acceleration of the transient process.” In addition, it is found that under certain conditions it is possible to achieve two stable final positions of the magnetization vector that correspond to 180° and 90°.

Впервые исследован режим 180o импульсного перемагничивания плeнок ферритов-гранатов с плоскостной анизотропией в области внешних полей, в которой действует механизм однородного вращения намагниченности. Анализ численных решений уравнения Ландау-Лифшица, рассчитанных и экспериментальных сигналов показал, что наличие в реальных плeнках ферритов-гранатов двухосной анизотропии приводит к тому, что при конечной длительности фронта перемагничивающего импульса начальное медленное вращение в определeнный момент времени резко ускоряется --- так, что за интервал ~0.7 ns азимутальный угол изменяется от 45o до равновесного значения (160-170o). В результате становится возможным возникновение нелинейных затухающих колебаний намагниченности с периодом основной гармоники ~1.5 ns. DOI: 10.21883/FTT.2017.10.44955.299

The regime of 180° pulsed magnetization reversal of ferrite–garnet films with planar anisotropy in the region of external fields, in which the mechanism of uniform rotation of the magnetization operates, is investigated for the first time. An analysis of the numerical solutions of the Landau–Lifshitz equation and the calculated and experimentally obtained signals showed that the presence of biaxial anisotropy in real ferrite–garnet films leads to the fact that at finite duration of the remagnetizing pulse front the initial slow rotation at definite moment of time is sharply accelerated so that over an interval of ~0.7 ns the azimuthal angle changes from 45° to the equilibrium value (160°–170°). As a result, appearence of the nonlinear damped oscillations of magnetization with a fundamental harmonic period of ~1.5 ns become possible.

Time dependences of the azimuthal component of the torque T φ(t) acting on magnetization are calculated to understand the nature of the delayed magnetization acceleration effect observed during the 90° pulsed magnetization of real ferrite–garnet films, in which biaxial anisotropy exists alongside with in-plane anisotropy. A calculation technique based on analyzing an operating point trajectory is used. Calculations show that if the effective anisotropy field H K2 is comparable to the magnetizing pulse amplitude H ma, abruptly ascending regions at characteristic times t* in curves T φ(t) arise, in the limit of which nonlinear magnetization oscillations formed. The shape of these regions depends weakly on the magnetizing pulse front duration τf. This explains the reason of the weak dependence of the nonlinear magnetization oscillations on duration of the magnetizing pulse front. Calculations also show that the main features of the delayed acceleration effect are less clear upon an increase of the pulse amplitude: the behavior of curves T φ(t) becomes smoother near times t*, and an increase in the pulse front duration is accompanied by a stronger drop in the intensity of magnetization oscillations.

The torque acting on the magnetization vector in the course of 90° pulsed magnetization of real garnet ferrite films with in-plane and biaxial anisotropy is calculated by a method in which the operating point trajectory is analyzed. The position of the operating point is described by azimuthal angle φ and torque component T m produced by pulsed magnetizing field H m . The time dependence of resultant torque T φ has a sharply ascending portion, within which the nonlinear magnetization oscillations are excited. Additionally, the shape of the curve T φ ( t ) within this portion depends on pulse rise time τ f only slightly. These results explain the weak dependence of the magnetization oscillation strength on τ f , which was experimentally found previously. It is shown analytically that when τ f decreases to 2.5–3.0 ns within the initial portion of the curve T φ ( t ) at φ ≤ 10°, there arises an extra maximum of torque T φ . Simultaneously, an additional voltage peak appears in the initial part of the longitudinal magnetization signal. The appearance of the additional voltage peak is confirmed experimentally.

The possibility of suppression of gyroscopic magnetization oscillations by increasing an effective in-plane anisotropy field is discussed. It is shown that the presence of biaxial anisotropy in real films with the effective field ranging from 10 to 70 Oe should not cause any difficulties at suppressing the oscillations.