A. B. Chizhik

Magneto-optical investigation of magnetization reversal in nearly zero magnetostrictive Co-rich wire and microwire

Abstract Investigations of the magnetic reversal in amorphous wire of nominal composition (Co 94 Fe 6 ) 72.5 Si 12.5 B 15 and microwire of nominal composition (Co 1− x Mn x ) 75 Si 10 B 15 ( X =0.07; 0.11) using transverse and longitudinal magneto-optical Kerr effect have been performed. Changes of the surface hysteresis loops of the amorphous wire after annealing (without and under torsion stress) have been studied. The analysis of the obtained results allows to establish that the outer shell of the as-quenched wire consists of circular domains, in contrast to the annealed wire displaying the domain structure with magnetization directed along the wire axis. Such a modification of the domain structure with thermal treatment could be related to the change of the sign of magnetostriction from negative to positive. The effect of a thermal treatment under torsion stress on the shape of the hysteresis loops of the amorphous wire has also been investigated. The results obtained for this case can be understood by considering the formation of the helical magnetic structure in the wire. The Kerr effect measurements of Co-rich microwires with different Mn content demonstrate the variety of the shapes of magnetization reversal loop, which can be attributed to the change of sign of the magnetostriction. The rectangular shape of the hysteresis loop in circular magnetic field of the microwire with X =0.07 can be interpreted by considering that the magnetization process takes place by large Barkhausen jumps of circular domain structure, while the rectangular shape of the hysteresis loop in axial magnetic field of the microwire with X =0.11 could be attributed to the large Barkhausen jumps within the axial domain structure.

Formation of noncollinear spin configurations during magnetization reversal in multilayered Fe/Si films

Abstract Magnetization reversal processes in Fe/Si multilayered films have been studied by means of the longitudinal magneto-optical Kerr effect at room temperature. The multilayers have been grown by DC sputtering on the single-crystal GaAs substrate. The Kerr effect curves had areas with an invariable or slightly varying rotation and had different remanent rotation value in the zero magnetic field for different directions of the magnetic field in the film plane. The observed features are related to the formation of stable and metastable noncollinear spin configurations. The experimental results are explained in the framework of the theory, which takes into account the competition between the bilinear exchange term on the one hand and the biquadratic exchange or cubic magnetic anisotropy terms on the other hand.

Magnetooptical studies of the H–T phase diagram of an Fe/Si multilayered film

The magnetization reversal of a multilayered Fe/Si film having cubic magnetocrystalline anisotropy is investigated in the temperature interval 25–300 K by magnetooptical methods. It is found that the growth of the biquadratic exchange interaction as the temperature is lowered causes a spontaneous second-order phase transition from a collinear antiferromagnetic state to a noncollinear state. The presence of cubic anisotropy in the film gives rise to spontaneous and magnetic-field-induced first-order phase transitions between noncollinear states. Magnetooptical studies permit constructing the H–T magnetic phase diagram of the multilayered Fe/Si film for an orientation of the external field along the hard magnetization axis [110]. A calculation of the H–T phase diagram in the framework of a model taking into account the bilinear exchange and cubic anisotropy, with constants I1 and K that are assumed to be independent of temperature, and also the biquadratic exchange with a linearly temperature-dependent cons...

Visualization of the antiferromagnetic insulator–ferromagnetic metal phase transition in manganite Nd0.5Sr0.5MnO3

It is found that the first-order phase transition from a nonmetallic antiferromagnetic (AFM) state to a metallic ferromagnetic (FM) state in manganite Nd0.5Sr0.5MnO3 is accompanied by a significant change in the reflectivity of the visible light. This effect is used for visualizing the AFM–FM phase transition in Nd0.5Sr0.5MnO3. The coexistence of AFM and FM phases was observed visually during spontaneous and field-induced AFM–FM transitions. In both cases, the transition occurs through nucleation and expansion of domains of the phase that is favorable from the energy point of view. However, the periodic domain structure of the intermediate magnetic state was not formed during the phase transition. A striped domain structure was formed in the AFM state while the FM phase had a uniform structure.

Noncollinear magnetic structures in an Fe/Si/Fe film with a ferromagnetic interlayer exchange interaction

Results are presented from magnetic and magnetooptical studies of the magnetic configurations in a three-layer film Fe(30 A)/Si(14 A)/Fe(30 A) in which ferromagnetic and biquadratic exchange between iron layers is observed and which also possesses cubic and uniaxial anisotropy. These studies were done at temperatures of 300 and 10 K. It was found that in the absence of magnetic field a noncollinear magnetic structure, the form of which changes with temperature, is observed in this system. A calculation of the stable noncollinear configurations is done on the basis of a model in which it is assumed that both the exchange interaction constant and the cubic and uniaxial anisotropy constants in the iron layer change with temperature. The values of the angles characterizing the magnetization direction in the iron layers in the stable noncollinear configurations are determined on the basis of a comparative analysis of the experimental data and the results of the calculation. It is shown that both the angle betw...

H–T phase diagram of a multilayered Gd/Si/Co film with ferrimagnetic ordering of the layers

The magnetization process of multilayered films [Co(30 A)/Si(5 A)/Gd(75 A)/Si(5 A)]20 having a ferrimagnetic ordering of the magnetic moments of the cobalt and gadolinium layers and a compensation temperature Tcomp≈118 K is studied by magnetometric and magnetooptical methods. A magnetic-field-induced spin-orientation transition from the collinear to the noncollinear state is observed. The transition field is determined and the H–T magnetic phase diagram is constructed for the multilayered ferrimagnet Gd/Si/Co in the temperature interval 5–140 K. The phase diagram is also calculated in the framework of molecular field theory, and satisfactory agreement with the experimental results is obtained. The constant of the antiferromagnetic exchange interaction between the cobalt and gadolinium layers is established.

Coercive properties of a Gd/Si/Co multilayer film with a compensation point

The temperature dependence of the coercive field Hc is determined for a multilayer [Gd75 A/Si5 A/Co30 A/Si5 A]20 film with ferrimagnetic ordering of the magnetic moments of the cobalt and gadolinium layers. The maximum value of Hc is observed at a temperature of around 118 K, which corresponds to the compensation point of the film. It is established that in the entire interval of temperatures studied, the variation of the coercive field is due to the variation of the spontaneous magnetization.

Noncollinear Structure in Surface Gadolinium Layer of Multilayer Gd/Fe Films Induced by Magnetic Field

In the Gd/Fe multilayers, the magnetic moments of the ferromagnetically ordered layers of Gd and Fe lie in the film plane and are oriented antiparallelly due to antiferromagnetic exchange interaction at the interface. It is known that a magnetic field applied in the plane of such film induces the phase transition to the twisted state [1–4]. The collinearity of the magnetic moments of Gd and Fe layers is destroyed at the transition. The spin reorientation to the twisted phase originates from the surface of the Gd/Fe multilayers if the magnetic moment of the outermost layer is opposite to the field direction [3,4]. In the present paper, the other type of the surface spin reorientations induced by a magnetic field has been found in the Gd/Fe multilayers terminated with the Gd layer. A magnetic field induced a noncollinear spin configuration within the outermost Gd layer of multilayer.