Ferromagnetic Bilayer System Research Articles

Time-resolved x-ray imaging of nanoscale spin-wave dynamics at multi-GHz frequencies using low-alpha synchrotron operation

Time-resolved x-ray microscopy is used in a low-alpha synchrotron operation mode to image spin dynamics at an unprecedented combination of temporal and spatial resolution. Thereby, nanoscale spin waves with wavelengths down to 70 nm and frequencies up to 30 GHz are directly observed in ferromagnetic thin film microelements with spin vortex ground states. In an antiparallel ferromagnetic bilayer system, we detect the propagation of both optic and acoustic modes, the latter exhibiting even a strong non-reciprocity. In single-layer systems, quasi-uniform spin waves are observed together with modes of higher order (up to the 4th order), bearing precessional nodes over the thickness of the film. Furthermore, the effects of magnetic material properties, film thickness, and magnetic fields on the spin-wave spectrum are determined experimentally. Our experimental results are consistent with numerical calculations from a micromagnetic theory even on these so-far unexplored time- and length scales.

Slow-Wave-Based Nanomagnonic Diode

Spin waves, the collective excitations of the magnetic order parameter, and magnons, the associated quasiparticles, are envisioned as possible data carriers in future wave-based computing architectures. On the road towards spin-wave computing, the development of a diode-like device capable of transmitting spin waves in only one direction, thus allowing controlled signal routing, is an essential step. Here, we report on the design and experimental realization of a microstructured magnonic diode in a ferromagnetic bilayer system. Effective unidirectional propagation of spin waves is achieved by taking advantage of nonreciprocities produced by dynamic dipolar interactions in transversally magnetized media, which lack symmetry about their horizontal midplane. More specifically, dipolar-induced nonreciprocities are used to engineer the spin-wave dispersion relation of the bilayer system so that the group velocity is reduced to very low values for one direction of propagation, and not for the other, thus producing unidirectional slow spin waves. Brillouin light scattering and propagating spin-wave spectroscopy are used to demonstrate the diode-like behavior of the device, the composition of which was previously optimized through micromagnetic simulations. simulations.

Multiorbital physics in Fermi liquids prone to magnetic order

The interplay of spin-orbit-coupling and strong electronic correlations is studied for the single-layer and the bilayer compound of the strontium ruthenate Ruddlesden-Popper series by a combination of first-principles band-structure theory with mean-field rotationally invariant slave bosons. At equilibrium strongly renormalized (spin-orbit-split) quasiparticle bands are traced and a thorough description of the low-energy regime for the nearly ferromagnetic bilayer system in accordance with experimental data is presented. The metamagnetic response of Sr$_3$Ru$_2$O$_7$ in finite magnetic field $H$ is verified and a detailed analysis of the underlying correlated electronic structure provided. Intriguing multi-orbital physics on both local and itinerant level, such as e.g. competing paramagnetic and diamagnetic contributions, is observed with important differences depending on the magnetic-field angle $\theta$ with the crystallographic c axis.

Spin-wave excitation in the antiferromagnetic bilayer ruthenateCa3Ru2O7

We study the spin-wave excitation spectrum of the antiferromagnetic bilayer ruthenate Ca${}_{3}$Ru${}_{2}$O${}_{7}$ through inelastic neutron scattering measurements. The material behaves as a quasi-two-dimensional ferromagnetic bilayer system with very weak interbilayer antiferromagnetic coupling, and its magnetic dispersion can be well described with a nearest-neighbor Heisenberg model. We find that the intrabilayer exchange interaction along the out-of-plane $c$-axis is much stronger than the in-plane intralayer interaction, in sharp contrast to the previously reported bilayer manganite system. And our study reveals a finite lifetime effect of the magnetic excitation, presumably attributable to the existence of a small number of itinerant charge carriers within the planes.

The eigenvalues of quantized spin waves and theeffect of the uniaxial anisotropy

The eigenproblems of quantized spin waves in a (100) ferromagnetic bilayer system have been investigated theoretically by using the interface rescaling approach. The energy-band structure of the system has been obtained and the effect of the uniaxial bulk anisotropy field of easy-axis type on the energy bands were explored thoroughly. It is found that there are bulk modes, perfect confined modes and interface modes in the biferromagnetic system, the existing condition of which is determined not only by the bulk exchange coupling constant, the interface exchange coupling constant, the spin of lattice, the number of atomic layers, but also by the uniaxial bulk anisotropy.

The energy bands of quantized spin waves and the uniaxial anisotropy in a biferromagnetic system

The eigenproblems of quantized spin waves in a (100) ferromagnetic bilayer system have been investigated theoretically using the interface rescaling approach. The energy-band structure of the system has been obtained and the effect of the uniaxial bulk anisotropy field of easy-axis type on the energy bands are explored thoroughly. It is found that all kinds of eigenmodes of the spin waves in the biferromagnetic system are determined by the bulk exchange coupling constant, the interface exchange coupling constant, the spin of lattice, the number of atomic layers and the uniaxial bulk anisotropy. Especially, we explore the effect of the uniaxial bulk anisotropy field of easy-axis type on the energy bands of first Brillouin zone and the existence condition of all kinds of the eigenmodes.

Spin waves modulated in a ferromagnetic bilayer system calculated using the interface rescaling approach

The eigenproblems of spin waves in a ferromagnetic bilayer system with periodic boundary conditions are solved using the interface-rescaling approach. The Brillouin zone mapping of the system is investigated. Besides three types of known eigenmodes, the bulk mode, the interface mode, and the perfect confined mode, we also find a type of eigenmode, the critical mode, on the boundary of those known eigenmodes, which is composed of the spin wave modulated. This spin wave possesses both the feature of the bulk spin wave and that of the interface spin wave.

Brillouin-zone mapping of the spin waves in a ferromagnetic bilayer system

The eigenproblems of spin waves in a heterogeneous ferromagnetic bilayer system with periodic boundary conditions are solved using the interface-rescaling approach. Brillouin zone mapping and the eigenmodes of the system are investigated. We find three types of spin waves may exist in the system: the bulk mode, the interface mode, and the perfect confined mode. The fine structure of the energy band in the heterogeneous bilayer system is first given for the whole two-dimensional Brillouin zone. Conditions for the existence of the interface mode are discussed. Finally, we analyze the resonant-confined spin waves in bulk modes and their oscillating behavior.

Numerical Analysis of Thermally Assisted Magnetization Reversal in Rectangular MRAM Cells Consisted of Exchange Coupled Bilayer

Time evolution of magnetization during a cooling process in thermally assisted magnetization reversal of rectangular MRAM cell has been numerically investigated by taking account of not only temperature variations of material parameters but also a thermal fluctuation effect. The rectangular MRAM cell with 100×150×20 nm3 in size is assumed, where the shape anisotropy dominates the switching property. The threshold amplitude of magnetic field required for a successful thermally assisted switching is decreased as the exchange stiffness constant is changed from 1×10-6 to 1×10-7 erg/cm. The time for the magnetization to relax into the bias field direction can be reduced by increasing the bias field amplitude. To switch the magnetization within 1 ns, it is found that the bias field should be larger than 165 Oe. An exchange coupling field in a ferromagnetic bilayer system is considered to be available for producing such a large bias field required for a faster switching in thermally assisted MRAMs.

Spin dynamics of bilayer manganites

Crystal and magnetic structures of bilayer (n = 2) Ruddlesden-Popper phase La1-2x Sr1+2x Mn2O7 have been reviewed and their relationship with the colossal magnetoresistive behavior discussed. Experimental results of inelastic and quasielastic neutron scattering on the 40% hole-doped (x = 0.4) bilayer man-ganite La1.2Sr1.8Mn2O7 have been described. A fully quantum theory of the spin dynamics of ferromagnetic manganites is developed, starting from the minimal double exchange model. This is compared with experimental results for a ferromagnetic bilayer system. A mean-field type theory of doping dependence of the exchange interactions in bilayer manganites has also been developed and compared is compared with the experimental results.

The energy bands and the dispersion relations of the spin waves in a ferromagnetic bilayer system

The eigenproblems of spin waves in a ferromagnetic bilayer system with periodic boundary conditions are solved by using the interface-rescaling approach. The energy bands and the dispersion relations of the system are investigated. In addition, the effect of the uniaxial bulk anisotropy field of easy-axis type on the energy bands and the dispersion relations is also discussed. Furthermore, we conjecture that there may be the resonant-confined spin waves in the system.

Influence of the spin structure on the exchange bias of antiferromagnetic and ferromagnetic bilayer

The exchange coupling bias field in the antiferromagnetic (AFM) and ferromagnetic bilayer system is investigated within the framework of the classical Heisenberg model. The spin structures in L1/sub 0/-type ordered and disordered /spl gamma/ phase of AFM alloys including magnetic and nonmagnetic atoms are calculated, and the magnetization loops are obtained by using the Landau-Lifshitz motion equation. The AFM layer having the 3Q structure can generate an exchange bias field. However, no magnetization loop shift can be obtained in cases of the 1Q, 2Q, and AF-I structures of AFM alloys.

Magnetic properties of the spin- [formula omitted] transverse ferromagnetic bilayer system with disordered interfaces

The effects of an applied transverse magnetic field on the magnetic properties in a ferromagnetic bilayer system with disordered interfaces consisting of spin- 1 2 atoms are investigated using the effective field theory. The magnetization profiles and the phase diagrams, namely, T c versus the concentration p, T c versus the interlayer exchange interaction and T c versus the transverse field plots are discussed. It is shown that the existence of the disordered interfaces may severely affect the phase diagrams of T c. When the transverse field is applied and the interlayer exchange interaction is selected as the large value, some characteristic effects due to them can be obtained in the critical properties of the system, such as the reentrant phenomenon observed for special cases.

反強磁性Ｍｎ‐Ｐｔ合金／強磁性積層界面の磁気構造

Monte Carlo calculation for the classical Heisenberg model was performed to investigate the magnetic structure of the antiferromagnetic (AFM) and ferromagnetic (FM) bilayer system. In the case of MnPt with Co, the spin structure obtained at the interface was a “spin flop structure” as given by Koon, in which the direction of spins is relaxed from the original collinear structure. A similar relaxation of spins was observed in the case of Mn3Pt with Co, where the triangular spin structure appears in the Mn3Pt layer. This relaxation of spins is strongly related to the exchange bias in an AFM and FM bilayer system, and the canting angle of spins corresponds to the strength of the exchange bias.

Amorphization and anisotropy effects on a ferromagnetic bilayer system

A ferromagnetic amorphous bilayer system, consisting of two magnetic monolayers (A and B) with spins ( S A = 1 2 and S B = 3 2 ) and different interaction constants coupled together with interlayer coupling, is studied by the use of effective field theory. In addition to the influence of crystal field interaction the effects of the amorphization in the monolayer B and in the interlayer coupling give some interesting phenomena in the system. Phase diagrams and magnetization are investigated in detail.

Anisotropy, exchange coupling and phase transition in bilayer systems

Phase transitions in magnetic multilayer structures are studied with mean-field theory. Transitions between different magnetization configurations in bilayer systems with different anisotropies are discussed by means of variational method. The responsible mechanism is the competition between the anisotropy energy and the exchange coupling. The para-ferro magnetic phase transition in a ferromagnetic bilayer system is investigated with the Ginzburg-Landau theory. The effect of finiteness of the layer thickness has been revealed. Critical phenomena according to the variance of the layer thickness are clarified.

Properties of perfect confined modes and interface modes of spin-waves in a ferromagnetic bilayer system

The exact solutions of spin-wave modes in a ferromagnetic bilayer system with a periodic boundary condition are investigated by using the interface-rescaling approach. The dispersion relations of the modes in the sublayers are obtained. The perfect confined modes and the interface spin-wave modes are given and discussed.

Interface-rescaling approach to the exact solutions of quantum low-energy eigenproblems for a ferromagnetic bilayer system.

Exact solutions of the low-energy quantum eigenstates and eigenvalues of spin waves in a ferromagnetic bilayer system with periodic boundary condition are investigated by use of the interface-rescaling approach and quantum theory. The relations between spin waves and interface coupling are discussed. It is shown that resonant confined modes of the spin waves occur when the interface coupling is comparatively strong and that the spin waves are nearly pinned at the interface.