A direct experimental study of the temperature dependence of the field HJ proportional to the energy of the interlayer exchange interaction J is carried out in heterophase samples with perpendicular magnetic anisotropy consisting of ferromagnetic Co layers separated by a nonmagnetic Pd interlayer. Using Kerr microscopy, the fields of domain nucleation HN in samples magnetized to saturation and the fields of interlayer exchange interaction are measured at different thicknesses of the nonmagnetic interlayer in the temperature range from 15 to 300 K. It is shown that the nucleation field in both ferromagnetic layers decreases steadily with increasing temperature. The threshold temperatures Ti are found to be dependent on the thickness of Pd. At T < Ti, domains of the new phase are generated simultaneously in both layers. At T > Ti, the nucleation of domains in layers occurs in different fields and regions of the samples. In the specified temperature range, field dependences of the velocity of the domain boundaries v(H) in one of the layers are obtained with the other layer uniformly magnetized. It is established that the fields for the v(H) dependence differ by 2HJ when the boundaries move in opposite directions (with magnetic-field strengths of +H and –H). As the temperature decreases, the value of this field increases. This increase is greater in samples with a thinner interlayer, which is consistent with the theoretical dependences obtained earlier. However, a number of important unpredicted features of the behavior of the domain structure with changes in temperature are revealed. It is shown for the first time that at low temperatures the value of HJ is proportional to T–1.

Using Kerr microscopy, the effect of temperature on the displacement of domain walls in ultrathin exchange-coupled ferromagnetic layers in Pt/Co/Pt/Co/Pt heterostructures with perpendicular magnetic anisotropy and a nonmagnetic wedge-shaped spacer is experimentally studied. The exchange interaction between Co layers was studied for spacer thicknesses from 5 to 6 nm in the temperature range from 200 to 300 K. Independent displacement of domain walls in Co layers under a perpendicular magnetic field occurs in the thickness range d0 d dCR. In the temperature range, when the domain walls are moved along the Pt wedge, they are stabilized in the equilibrium positions. These positions depend on the field strength, the thickness of the nonmagnetic interlayer, and temperature and is determined by the balance of forces caused by the external field acting on the boundary, the effective field of the exchange coupling between the layers Co and the coercivity field. After the external field is removed, under the influence of the exchange field, the domain walls relax to the initial state with d = d0. The characteristics of the relaxation process depend on temperature. The mechanism of domain wall stabilization near dCR is considered. It is shown that the critical thickness of the nonmagnetic spacer dCR and the coercivity field have opposite temperature dependences.

Using Kerr microscopy, the effect of temperature on the displacement of domain boundaries in ultrathin exchange-coupled ferromagnetic layers in heterophase Pt/Co/Pt/Co/Pt films with perpendicular magnetic anisotropy and a nonmagnetic wedge-shaped spacer layer is experimentally studied. The exchange interaction between the Co layers is investigated for spacer-layer thicknesses ranging from 5 to 6 nm within the temperature range of 200 to 300 K. The independent displacement of domain boundaries in the Co layers under the action of a perpendicular magnetic field applied to the sample surface occurs within the range of thicknesses d0 < d < dCR. Throughout the entire temperature range, the displacement of domain boundaries along the Pt wedge results in their stabilization in an equilibrium position. This position depends on the magnitude of the applied field, the thickness of the nonmagnetic spacer layer, and temperature. It is determined by the balance of forces acting on the boundary, including the external field, the effective exchange field between the Co layers, and the coercivity field. Upon the removal of the external field, the domain boundaries relax to the initial state with d = d0 due to the effect of the exchange field. The characteristics of this relaxation depend on the temperature. The study of the mechanism of domain-boundary stabilization near dCR reveal that the critical thickness of the nonmagnetic spacer layer dCR and the coercivity field exhibit oppositely directed dependences on temperature.

Using Kerr microscopy, the magnetization reversal of the Co(0.4 nm)/Pd(6 nm)/Co(0.4 nm) heterostructure was studied in the temperature range 15–300 K. The temperature dependence of the domain nucleation field in a sample magnetized to saturation was obtained. The nucleation field in both ferromagnetic layers was shown to decrease monotonically with increasing temperature. A region of unstable temperatures of 160–174 K was found, below which the through domains of the new phase nucleated simultaneously in both layers, while in this region domains also nucleated simultaneously in different layers, but in different sites of the sample. The temperature dependence of the effective field HJ of the interlayer exchange interaction was obtained, which increased or decreased the pressure on the domain wall depending on whether this field was added to or subtracted from the external field.

Domain wall mobility as a function of nonmagnetic interlayer thickness and temperature was studied in ultrathin exchange-coupled ferromagnetic layers using magneto-optic Kerr microscopy. The system under study is a Pt/Co/Pt/Co/Pt heterostructure having perpendicular magnetic anisotropy and a middle Pt layer with spatially variable thickness. The ferromagnetic interaction between the Co layers is observed when the Pt interlayer thickness varies from 5 to 6 nm in a temperature range of 200–300 K. There is a certain interval of Pt layer thickness where domain walls in both ferromagnetic layers move independently. Nonlinear dependence of the domain wall displacement on the applied field was measured. It is shown that an equilibrium position of the relaxed domain wall depends on field, temperature, and the nonmagnetic interlayer thickness. This position is determined by the energy balance: (i) domain wall displacement provided by the applied field, (ii) interlayer exchange interaction in the area swept by the domain wall, and (iii) domain wall coercivity. The mechanism of domain wall stabilization in terms of independent wall motion near critical thickness was considered. It is found that both the coercivity of the Co layer and the critical thickness decrease at higher temperature, while the interlayer exchange constant J is changed weakly.

Ta|Pt|GdFeCo|IrMn|Pt multilayer structure with perpendicular magnetic anisotropy in vicinity of the ferrimagnet compensation temperature TK has been carried out. It has been established that the distribution of the magnetic moment in the GdFeCo ferrimagnetic film exchange-coupled to the IrMn antiferromagnet is largely determined by the magnitude and orientation of both the field applied during cooling from room temperature to T = 2 K and the testing field over the entire temperature range. It is shown that the direction of the domain wall moving changes to the opposite one at a fixed value of the amplitude of the magnetic field pulse when the temperature passes through the TK.

Ta|Pt|GdFeCo|IrMn|Pt multilayer structure with perpendicular magnetic anisotropy in vicinity of the ferrimagnet compensation temperature T K has been carried out. It has been established that the distribution of the magnetic moment in the GdFeCo ferrimagnetic film exchange-coupled to the IrMn antiferromagnet is largely determined by the magnitude and orientation of both the field applied during cooling from room temperature to T=2 K and the testing field over the entire temperature range. It is shown that the direction of the domain wall moving changes to the opposite one at a fixed value of the amplitude of the magnetic field pulse when the temperature passes through the T K . Keywords: ferrimagnets, heterostructures, perpendicular magnetic anisotropy, magnetic moment, compensation temperature, domain wall.

We studied the in-field evolution of the domain structure in ultrathin Co(0.6 nm)/Pt(t)/Co(0.6) nm trilayers with perpendicular magnetic anisotropy for 5 nm < t < 6 nm using polar Kerr microscopy. The critical interlayer thickness tcr = 5.3 nm was found to separate two principal patterns of domain behavior including interlayer correlations and motility of the domain walls. It is shown that magnetization in both Co layers is coupled with strong ferromagnetic interaction for small Pt thickness (t < tcr), while this coupling is weak for thicker (t > tcr) Pt layers. Nonlinear dependence of the wall displacement on the field value is observed. The established final position of domain walls after relaxation depends on the Pt layer thickness. It is determined by balance of the interlayer exchange and energy gain due to the field. The mechanism of wall stabilization is considered in the case of independent wall motion. In the region with weak coupling, dependence of the interlayer interaction energy on Pt thickness was measured.

The dynamics of domain walls in ultrathin Pt/Co(0.6 nm)/Pt(t)/Co(0.6 nm)/Pt heterostructures with perpendicular magnetic anisotropy, where 0 nm < t < 10 nm, is studied. A transition from correlated to independent magnetization reversal is revealed at a Pt interlayer thickness of t = 5.5 nm. In the transition region, the domain wall is found to slowdown and stop in the “soft” layer at a constant magnetic field, which indicates a change in the nature of interlayer coupling. It is established that the energy of exchange coupling between Co layers in the transition region is equal to ~17 µJ/m2. Enhancement of the exchange coupling between the layers leads to the relaxation of magnetization in the “soft” layer and to magnetization in the “hard” layer when the external field is removed; in this case, the movement of the wall is described by an exponential function. The dynamics of domain walls with independent and coupled magnetization reversal of layers is considered; it corresponds to a creep mode in all cases at low fields. It is shown that the velocities of the walls for different layers are noticeably different and are due to the different roughness of interfaces.

The uniaxial anisotropy and kinetics of the transformation of the domain structure in a ferrimagnetic GdFeCo film exchange-coupled to the IrMn antiferromagnet have been studied in a wide temperature range. Ta/Pt/GdFeCo/IrMn/Pt multilayer structures with perpendicular magnetic anisotropy demonstrate the compensation of the sublattice magnetization at K and K. Nonmonotonic temperature dependences of the exchange bias field H EX and coercivity H C are revealed. It is established that the slope of the dependence and the quantity H EX change sign at T 1 . An orientational phase transition is detected at a temperature of T CR = 35 K, below which uniaxial anisotropy, as well as the mechanism of coercive force formation, changes.

—The domain structure and magnetization reversal mechanisms are studied in quasi-two-dimensional exchange-biased NiFe/FeMn and NiFe/NiO nanomagnetics, using a complex method of magnetooptical indicator films and acoustic emission. The presence of the axial dispersion of unidirectional anisotropy in grains of the antiferromagnetic layer is shown to determine the statistic distribution and chirality of spin springs near the interface. The acoustic emission signals caused by the excitation of elastic Lamb waves upon the magnetization reversal of NiFe/NiO heterostructure are found. The coercive force of these systems is due to irreversible processes to overcome potential barriers that are induced by the formation of spin springs with different chirality, localized in the antiferromagnetic near the ferromagnetic–antiferromagnetic boundary.

Abstract —The domain structure and magnetization reversal mechanisms are studied in quasi-two-dimensional exchange-biased NiFe/FeMn and NiFe/NiO nanomagnetics, using a complex method of magnetooptical indicator films and acoustic emission. The presence of the axial dispersion of unidirectional anisotropy in grains of the antiferromagnetic layer is shown to determine the statistic distribution and chirality of spin springs near the interface. The acoustic emission signals caused by the excitation of elastic Lamb waves upon the magnetization reversal of NiFe/NiO heterostructure are found. The coercive force of these systems is due to irreversible processes to overcome potential barriers that are induced by the formation of spin springs with different chirality, localized in the antiferromagnetic near the ferromagnetic–antiferromagnetic boundary.

Magnetization reversal of polycrystalline NiFe/NiO bilayers was investigated using magneto-optical indicator film imaging and acoustic emission techniques. Sporadic acoustic signals were detected in a constant magnetic field after the magnetization reversal. It is suggested that they are related to elastic waves excited by sharp shocks in the NiO layer with strong magnetostriction. Their probability depends on the history and number of repetitions of the field cycling, thus testifying the thermal-activation nature of the long-time relaxation of an antiferromagnetic order. These results provide evidence of spontaneous thermally activated switching of the antiferromagnetic order in NiO grains during magnetization reversal in ferromagnet/antiferromagnet (FM/AFM) heterostructures. The respective deformation modes are discussed in terms of the thermal fluctuation aftereffect in the Fulcomer and Charap model which predicts that irreversible breakdown of the original spin orientation can take place in some antiferromagnetic grains with disordered anisotropy axes during magnetization reversal of exchange-coupled FM/AFM structures. The spin reorientation in the saturated state may induce abrupt distortion of isolated metastable grains because of the NiO magnetostriction, leading to excitation of shock waves and formation of plate (or Lamb) waves.

Based on the experimental data of the magnetic hysteresis and magneto-optical images of the magnetic structure during magnetization process, in this work, we have carried out estimation of the value of radial mechanical stresses in the surface region of the metal core of microwire. The composition of the sample microwire was Fe73.9B13.2Si10.9C2 with a positive magnetostriction constant. The magneto-optical images of the domain structure during magnetization and the hysteresis loops have been used to define the magnetic characteristics of the sample (the saturation magnetization Ms and the saturation field Hs). We have determined that there is a relationship between the orientation of the magnetic moments of the ring-shaped surface domains and the internal cylindrical domains. On the basis of the experimental data, we have estimated the value of radial mechanical stress of the microwire in the surface region of the metal core where the ring-shaped domains are located.

Avalanchelike behavior reflected in power-law statistics is a ubiquitous property of extended systems addressed in a number of generic models. The paper presents an experimental investigation of the effect of thresholding on the statistics of durations and waiting times between avalanches using acoustic emission accompanying unstable plastic deformation. It is found that durations of acoustic events obey power-law statistical distributions robust against thresholding. The quiescent time distributions follow the Poisson law for low threshold values. Both these results corroborate the hypothesis that plastic deformation is akin to the phenomena associated with self-organized criticality (SOC), often advanced on the basis of power-law amplitude statistics. Increasing the threshold height enforces deviation from the Poisson distributions toward apparent power-law behavior. Such a thresholding effect may hinder the experimental determination of SOC-like dynamics because of the inevitable noise.

The effects of shape and edges in magnetic elements with reduced dimensions on the magnetization reversal of cross- and framed cross- shaped Ni79Fe21 (30nm) films were studied. Remagnetization details in the strips of the patterned structures, which had 3 to 30µm widths and ~100µm lengths, were visualized by the magneto-optical indicator film technique. The magneto-optic images revealed three different types of the domain structure formation and evolution in the samples during their magnetization reversal: (i) spin rotation with growth and annihilation of a cross-tie structure in the strips perpendicular to the applied field, (ii) nucleation and fast motion of special boundaries, which consist of a number of coupled vortices located along both edges of the strips parallel to the applied field, and (iii) switching by ripple structure formation with macrodomain nucleation and domain wall motion in the large unpatterned part of the films. It was experimentally revealed that there exists a dependence of the critical field for nucleation and motion of domain walls in the parallel-to-field strips on their width and frame width. In particular, an inverse proportionality between this nucleation field and strip width was found in these strips having micrometer sized widths. Both experimental and simulation results show that, in cases (i) and (ii), the magnetostatic fields, which are formed on the edges of the strips and at their intersections, play a crucial role in the formation of spin inhomogeneities and switching of the samples.