Magnetoresistance and noncollinear structures of multilayer ferromagnetic nanoparticles

The distribution of the magnetization over multilayer particles, including three ferromagnetic layers separated by insulating spacers, is studied experimentally and theoretically. Experimental data on the magnetic state of these particles are obtained by measuring their magnetoresistance. For the case of zero applied field, it is shown that a multilayer particle with easy-plane magnetic anisotropy is a noncollinear helical state.     

Tunnel barrier and noncollinear magnetization effects on shot noise in ferromagnetic/semiconductor/ferromagnetic heterojunctions

Based on the scattering approach, we investigate transport properties of electrons in a one-dimensional waveguide that contains a ferromagnetic/semiconductor/ferromagnetic heterojunction and tunnel barriers in the presence of Rashba and Dresselhaus spin-orbit interactions. We simultaneously consider significant quantum size effects, quantum coherence, Rashba and Dresselhaus spin-orbit interactions and noncollinear magnetizations. It is found that the tunnel barrier plays a decisive role in the transmission coefficient and shot noise of the ballistic spin electron transport through the heterojunction. When the small tunnel barriers are considered, the transport properties of electrons are quite different from those without tunnel barriers.     

Two-stage magnetization reversal of ferromagnetic films

The two-stage magnetization relaxation of thin ferromagnetic films during the magnetization reversal process is considered in the context of the generalized Kolmogorov three-phasecrystallization model. The experimental data agree qualitatively with the considered model.     

Domain morphology in ultrathin ferromagnetic films with perpendicular magnetization

We determine the minimal domain structure for the equilibrium thickness of stripes as well as for the minimal energy of the domain configuration in ultrathin films of ferromagnetically coupled spins, where the easy direction of magnetization is perpendicular to the film. It is found that the equilibrium thickness of stripes and walls depend on the exchange energy. The normalized anisotropy, f, depends on interplay between the magnetic and anisotropy energies and is almost independent of the exchange energy inside the wall. The results are compared with the experimental data for thin Ag/Fe/Ag (0 0 1) films and a good coincidence is obtained between both results.     

Stable multidomain structures formed in the process of magnetization reversal in GaMnAs ferromagnetic semiconductor thin films

The process of magnetization reversal in ferromagnetic Ga(1-x)Mn(x)As epilayers has been systematically investigated using the planar Hall effect (PHE). Interestingly, we have observed a pronounced asymmetry in the PHE hysteresis when the range of the field scan is restricted to fields below the final magnetization transition. The observed behavior indicates that (a) multidomain structures are formed as M undergoes a reorientation, (b) the domain landscape formed in this way remains stable even after the magnetic field is switched off, and (c) the reorientation of magnetization directions corresponding to the transition points in PHE takes place separately within each domain.     

Inhomogeneous configurations of magnetization of ferromagnetic films with biaxial anisotropy

The system of the Landau-Lifshitz equations and magnetostatic equations for a ferromagnetic film with biaxial anisotropy and a Q-factor smaller than unity is reduced to a single scalar equation for the magnetostatic potential. Such a procedure is possible if the magnetization modulation scale in the sample considerably exceeds the characteristic magnetic length. The solutions to this equation describing inhomogeneous periodic magnetic configurations are obtained. The energy analysis of these configurations is carried out.     

Reflection and transmission of light by a ferromagnetic structure with a noncollinear orientation of the magnetization of layers

The transmission and reflection of light by a ferromagnetic structure with a noncollinear orientation of the magnetization vectors of layers lying in the plane of the film is considered. The characteristic matrix of the structure that relates the wave amplitudes at the entrance to the system and at the exit from it and that determines the magnetooptical properties of the structure is calculated. The magnetooptical characteristics of this structure are shown to significantly depend on the angle between the magnetization vectors of layers and on the number of layers. Both the magnitude and the character of these dependences are analyzed in relation to the angle of incidence and to the polarization of the incident wave. Such an analysis makes it possible to optimize the magnetooptical properties of this structure.     

Critical current in planar SNS Josephson junctions

Specific features of the proximity effect and Josephson behavior of submicron planar SNS junctions fabricated by electron beam lithography and shadow evaporation have been studied experimentally and theoretically. The critical current of the junctions has been found to drastically increase with a decrease in temperature, which is associated with a change in the effective size of the weak link owing to the additional SN interface.     

Josephson current versus potential strength of the interface in ferromagnetic superconductors

Based on the scattering theory, we calculate the Josephson current in a junction between two ferromagnetic superconductors as a function of the interface potential z. We consider the ferromagnetic superconductor(FS) in three different Cooper pairing states: spin singlet s-wave pairing(SWP) state, spin triplet opposite spin pairing(OSP) state, and spin triplet equal spin pairing(ESP) state. We find that the critical Josephson current as a function of z shows clear differences among the SWP, OSP, and ESP states. The obtained results can be used as a useful tool for determining the pair symmetry of the ferromagnetic superconductors.     

Josephson current through a ferromagnetic semiconductor/semiconductor/ferromagnetic semiconductor structure

We theoretically calculate the Josephson current for two superconductor/ferromagnetic semiconductor (SC/FS) bilayers separated by a semiconductor (SM) layer. It is found that the critical Josephson current I_C in the junction is strongly determined by not only the relative orientations of the effective exchange field of the two bilayers and scattering potential strengths at the interfaces but also the kinds of holes (the heavy or light) in the two FS layers. Furthermore, a robust approach to measuring the spin polarization P for the heavy and light holes is presented.     

Critical current and order parameter in Josephson arrays

We give a definition of the order parameter for the paracoherent-ferrocoherent phase transition in a Josephson array and we show that the amplitude of the critical current in the array is proportional to this order parameter. We then discuss some qualitative properties of a Josephson array.     

Domain structure and magnetization reversal in thin ferromagnetic elinvar films

A study was made of single-layer Fe-Ni-Cr Elinvar films ranging in thickness from 400 to 10,000 Å and prepared by the thermal sputtering process. For the first time the domain structure of these alloys was examined and found to vary depending on the technological process conditions as well as on the film thickness. The coercive force H is shown here as a function of the specimen thickness, with a maximum value H = 32 Oe in a 640 Å film. The hysteresis loop of Elinvar films has uniaxial anisotropy.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii Fizika, No. 11, pp. 114–117, November, 1971.     

Asymmetric kinetics of magnetization reversal of thin exchange-coupled ferromagnetic films

This paper reports on the results of the investigation of the kinetics of magnetization reversal in FeNi-FeMn ferromagnet-antiferromagnet thin hybrid films grown by magnetron sputtering on silicon substrates in the presence of in-plane magnetic field, which provided unidirectional in-plane magnetic anisotropy in the ferromagnetic layer and a single-domain structure of the ferromagnet in the absence of an external magnetic field. The constructed hysteresis loops and magnetization loci have made it possible to reveal the specific features of the magnetization reversal process of an exchange-coupled ferromagnet, to establish new types of asymmetry, and to obtain new proofs for the existence of a spin spring at the ferromagnet-antiferromagnet interface. The visualization of the magnetization reversal process has allowed one to establish a one-to-one correspondence between the macrocharacteristics of the material and the real processes occurring in ferromagnet-antiferromagnet hybrid structures.     

Josephson current in ferromagnetic d-wave superconductor/ferromagnetic d-wave superconductor junction

We solve a self-consistent equation for the d-wave superconducting gap and the effective exchange field in the mean-field approximation, study the Zeeman effects on the d-wave superconducting gap and thermodynamic potential. The Josephson currents in the d-wave superconductor (S)/insulating layer (I)/d-wave S junction are calculated as a function of the temperature, exchange field, and insulating barrier strength under a Zeeman magnetic field on the two d-wave Ss. It is found that the Josephson critical currents in d-wave S/d-wave S junction depend to a great extent on the relative orientation of the effective exchange field of the two S electrodes, and the crystal orientation of the d-wave S. The exchange field can under certain conditions enhance the Josephson critical current in a d-wave S/I/d-wave S junction.     

Critical magnetization and hysteresis of nanogranular films with perpendicular anisotropy

The magnetic-field dependences of the stability boundaries of the nonequilibrium magnetic states that exist in a nanogranular film with perpendicular anisotropy in tilted magnetic fields are theoretically described, and the corresponding critical magnetization is calculated. The field dependences of the critical magnetization of the film are analyzed at various ratios of the anisotropy field of particles to the maximum possible demagnetizing field of the film. In a tilted magnetic field, the magnetization reversal curves, which include hysteresis loops, are shown to consist of segments of the following three types: equilibrium stable magnetization, nonequilibrium stable magnetization, and critical type of magnetization.     

Domain and wall structures in films with helical magnetization profile

We study soft magnetic bilayers having orthogonal, in-plane easy axes. The layers are thicker than the Bloch wall width linked to the anisotropy, so that a helical magnetization with a large angle exists across the sample thickness. The magnetic domains structure has been investigated at both sample surfaces, using magneto-optical microscopy. The domain structure is found to be similar to that of double films with biquadratic coupling. Two kinds of domain walls are identified, namely with a 90° and 180° rotation of the average magnetization. The detailed structure and energy of these walls are studied by micromagnetic calculations.     

Critical current oscillations in strong ferromagnetic pi junctions

We report magnetic and electrical measurements of Nb Josephson junctions with strongly ferromagnetic barriers of Co, Ni, and Ni80Fe20 (Py). All these materials show multiple oscillations of critical current with a barrier thickness implying repeated 0-pi phase transitions in the superconducting order parameter. We show, in particular, that the Co barrier devices can be accurately modeled using existing clean limit theories and that, despite the high exchange energy (309 meV), the large IcR(N) value in the pi state means Co barriers are ideally suited to the practical development of superconducting pi-shift devices.     

Ferromagnetic Josephson junction with precessing magnetization

The Josephson current in a diffusive superconductor-ferromagnet-superconductor junction with precessing magnetization is calculated within the quasiclassical theory of superconductivity. When the junction is phase biased, a stationary current (without ac component) can flow through it despite the nonequilibrium condition. A large critical current is predicted due to a dynamically induced long range triplet proximity effect. Such an effect could be observed in a conventional hybrid device close to the ferromagnetic resonance.