Tailor-made nano-structured materials for perpendicular recording media and head—precise control of direct/indirect exchange coupling

Tailor-made nano-structured spin materials obtained by precisely controlled nano-scale fabrication technologies for use in ultra-high density hard disk drives (HDDs), as well as an understanding of their nanomagnetics, are essential from the view point of materials, processes, and physics. Artificial control of the exchange coupling among ferromagnetic layers through the RKKY interaction (indirect) and direct exchange coupling represented as the exchange bias at the ferromagnetic (FM)/antiferromagnetic (AFM) interface are of great interest and have received significant attention to induce new modulated spin structures in conventional simple FM materials. In particular, soft magnetic under layer (SUL) with strong synthetic antiferromagnetic (SAF) coupling between two adjacent soft magnetic layers, exchange coupled stacked media introducing exchange coupling between FM layers and giant exchange anisotropy at the FM/AFM interface have attracted significant attention from the view point of applications. Within the framework of the present paper, we discuss future technical trends for SUL, granular media and the spin-valve head from the viewpoint of direct and/or indirect exchange coupling based on our recent results.     

Magnetic and magnetooptical properties of multilayer ferromagnet-semiconductor nanostructures

The magnetic and magnetooptical properties of spin-tunneling multilayer permalloy-silicon carbide nanoheterostructures deposited by rf sputtering have been studied. Magnetometric and magnetooptical methods are used to show that the magnetic-semiconducting nanostructures have a complex magnetic structure and to track the evolution of the magnetic properties of these structures as functions of the magnetizing field and the thickness and sequence order of ferromagnetic and semiconducting layers in them. The induction response and the field and orientation dependences of the transversal Kerr effect are found to have anomalies. The experimental results are interpreted under the assumption that there is exchange interaction between the ferromagnetic and semiconducting layers through a thin magnetically ordered transition layer formed inside the interface.     

Magnetic phase diagram of the spin-valve structure with an antiferromagnetic oxide layer

The magnetic phase diagram of a three-layer ferromagnet-antiferromagnet-ferromagnet structure with an antiferromagnetic oxide layer of variable thickness has been investigated. It has been predicted that there are three ranges of thicknesses in which domain walls created by the frustration of the exchange interaction between the layers have different structures. The structure of these domain walls induced by edges of atomic steps at the interfaces between the layers has been described. The predictions of the theory have been compared with the available experimental results, in particular, with the data indicating the existence of the nanodomain state.     

Roughness of layer interfaces and phase diagram of magnetic multilayer structures

The dependence of the type of magnetic ordering in multilayer structures on the magnitude of the exchange interaction between the magnetic layers and on the degree of roughness of the interfaces between the layers is investigated. In the case of a three-layer system the regions of existence of the domain structure, of noncollinear ordering of the homogeneous magnetic layers, and of the collinear ferromagnetic phase and collinear antiferromagnetic phase are found. Fiz. Tverd. Tela (St. Petersburg) 39, 1244–1247 (July 1997)     

Exchange couplings in magnetic films

Recent advances in the study of exchange couplings in magnetic films are introduced.To provide a comprehensive understanding of exchange coupling,we have designed different bilayers,trilayers and multilayers,such as anisotropic hard/soft-magnetic multilayer films,ferromagnetic/antiferromagnetic/ferromagnetic trilayers,[Pt/Co]/NiFe/NiO heterostructures,Co/NiO and Co/NiO/Fe trilayers on an anodic aluminum oxide(AAO) template.The exchange-coupling interaction between soft-and hard-magnetic phases,interlayer and interfacial exchange couplings and magnetic and magnetotransport properties in these magnetic films have been investigated in detail by adjusting the magnetic anisotropy of ferromagnetic layers and by changing the thickness of the spacer layer,ferromagnetic layer,and antiferromagnetic layer.Some particular physical phenomena have been observed and explained.     

The effect of a nonmagnetic Zr layer on the magnetic and magneto-optical properties of Fe/Zr and Fe/Zr/Fe thin-film systems

The magnetic and magneto-optical properties of nanocrystalline Fe/Zr and Fe/Zr/Fe thin-film systems have been studied using the magneto-optical method. The strong effect of Zr layer thickness t _Zr on the magnetic properties of Fe/Zr samples was discovered. It was found that the value of the saturation field of the Fe/Zr/Fe systems oscillates as a function of t _Zr, which is explained by the oscillating character of the exchange interaction between ferromagnetic layers via a Zr spacer with the change in t _Zr. It was established that the values of the transverse Kerr effect depend on the thicknesses of both magnetic and nonmagnetic layers.     

Brillouin light scattering of CoFe/IrMn-based top and bottom spin-valves

The spin-wave Brillouin light scattering (BLS) was observed from CoFe/IrMn-based bottom and top spin-valve structures. The magnitude of perpendicular magnetic anisotropy of these structures was investigated by means of BLS. The experimental spin-wave frequency was simulated using a simple model which includes crystallographic, exchange, in-plane uniaxial, and in- and out-of-plane anisotropies and takes the exchange interaction between the two ferromagnetic layers into account. This enabled us to determine, in addition to the other magnetic parameters, both in- and out-of-plane anisotropy constants near the surface. It was found that the top spin-valve is in a strong out-of-plane anisotropy whereas in-plane anisotropy is dominant for the bottom spin-valve.     

The mechanism of interlayer exchange coupling in iron-chromium type nanostructures

A mechanism of the interlayer exchange coupling in layered structures of the Fe/Cr(001) type with rough interfaces is proposed. The theory is based on a model of the charge-induced spin density wave (SDW) formed in the chromium layer. It is shown that the effective magnetic coupling between thick ferromagnetic layers arises due to variations of the SDW vector orientation in the antiferromagnetic layer over a characteristic length ζ determined by the exchange stiffness of chromium. A general expression for the effective magnetic coupling energy E(ψ) as a function of the angle ψ between magnetic moments of the ferromagnetic layers is obtained and numerically analyzed for an arbitrary value of the parameter ρζ, where ρ is the density of monoatomic steps on the interface. For ρζ?1, the form of E(ψ) is typical of a model with the “ biquadratic” interaction, while in the case of ρζ?1, the dependence obtained differs significantly. The proposed mechanism is used to interpret the results of measurements of the interlayer exchange coupling in Fe/Cr(001) structures.     

Frustrated multilayer ferromagnet-antiferromagnet structures: Beyond the scope of the exchange approximation (a review)

The frustrations of exchange interaction between ferromagnetic and antiferromagnetic layers, which arise at the uncompensated interface between the layers due to the interface roughness, have been described. The distribution of magnetic order parameters in the vicinity of the interface between the layers has been investigated, and the “layer thickness-roughness” magnetic phase diagram has been obtained in the case of the two-layer ferromagnet-antiferromagnet system and the ferromagnet-antiferromagnet-ferromagnet spin-valve system. An analysis has been performed taking into account the single-ion anisotropy energy, i.e., beyond the scope of the exchange approximation. It has been demonstrated that the number of easy axes in the layer plane, in many respects, determines the existence of an exchange shift of the hysteresis loop of the ferromagnet due to its interaction with the antiferromagnetic substrate.     

Slow Oscillations of the Perpendicular Magnetization of the Pt/Co/Ir/Co/Pt Spin Valve

The time dependences of the transfer of the magnetic moment between stable magnetic states of heterostructures with two Pt/Co/Ir/Co/Pt ferromagnetic layers with the perpendicular magnetization have been studied. Spontaneous oscillations of the macroscopic magnetization with a period of several hours are observed after switching the magnetic field to a new value. The phase portrait of the magnetic relaxation corresponds to damped oscillations. The macrospin oscillation may be due to the high nucleation rate of the reverse magnetization phase induced by the exchange and magnetic dipole interaction between the phase nucleation centers, which arise in different layers. The changes in the Zeeman energy of the system under magnetic oscillations are considered.     

Brownian dynamics simulations of a cubic haematite particle suspension with a more effective treatment of steric layer interactions

We have proposed a new repulsive layer model for describing the interaction between steric layers of coated cubic particles. This approach is an effective technique applicable to particle-based simulations such as a Brownian dynamics simulation of a suspension composed of cubic particles. 3D Brownian dynamics simulations employing this repulsive interaction model have been performed in order to investigate the equilibrium aggregate structures of a suspension composed of cubic haematite particles. It has been verified that Brownian dynamics employing the present steric interaction model are in good agreement with Monte Carlo results with respect to particle aggregate structures and particle orientational characteristics. From the viewpoint of developing a surface modification technology, we have also investigated a regime change in the aggregate structure of cubic particle in a quasi-2D system by means of Brownian dynamics simulations. If the magnetic particle–particle interaction strength is relatively strong, in zero applied magnetic field the particles aggregate in an offset face-to-face configuration. As the magnetic field strength is increased, the offset face-to-face structure is transformed into a more direct face-to-face contact configuration that extends throughout the whole simulation region.     

Phase Diagram of Antiferromagnetically Exchange-Coupled Bilayer

Magnetic hysteresis properties of antiferromagnetically exchange-coupled bilayer structures, in which the two magnetic layers have different magnetic parameters and thicknesses, are studied within the framework of the Stoner-Wohlfarth model. Analytical expressions for the switching fields corresponding to the linear magnetic states are obtained. By adjusting the magnetic parameters or thicknesses of layers, nine different types of easy-axis hysteresis loops may exist. The phase diagram of easy-axis hysteresis loops is mapped in the k1 and k2 plane, where k1 and k2 are the ratios of magnetic anisotropy to the interlayer exchange coupling of the two magnetic layers, respectively.     

Compensation behaviors in a tri-layers nano-dicoronylene like-structure with ferrimagnetic mixed spins (3/2,1): Monte Carlo study

This paper outlines the magnetic properties of a tri-layers nano-dicoronylene like-structure, using Monte Carlo simulations under the Metropolis algorithm in the Blume Capel model. The top and bottom layers are composed by S-spins (S = 0, ±1), While the middle layer is composed by σ-spins (σ=±3/2, ±1/2). Each layer contains 48 atoms. We establish, in six various planes, the ground state phase diagrams. Still, we present the thermal variation of partials and total magnetization and susceptibility for several values of exchange coupling interactions and crystal field. Also, the hysteresis cycles have been reported for selected values of temperature and exchange coupling interaction. The compensation and blocking temperatures values increases when increasing the exchange coupling parameter in the extreme layers.     

Exchange bias in the IrMn/Co structures with alternative sequences of antiferromagnetic and ferromagnetic layers

The magnetic properties of Mo/IrMn/Co/Mo/SiO₂/Si structures with alternative sequences of the antiferromagnetic and ferromagnetic layers have been studied by measuring the angular dependence of the high-frequency radiation absorption in the ferromagnetic resonance region. The layers have been prepared by pulsed laser deposition in the absence of a magnetic field. It has been found that thermal annealing and cooling make it possible to create the exchange bias in the structure with the upper antiferromagnetic layer at a temperature much below the Néel temperature. At the same time, the identical heat treatment does not induce the exchange bias in the structure with the upper ferromagnetic layer. The possible mechanisms of the phenomena observed are discussed.     

Ferromagnetic glass on the base of aggregates of Ni amorphous nanogranules

We have studied theoretically (including computer simulations) magnetic properties of aggregates of ferromagnetic amorphous nanogranules in the presence of direct exchange between the neighboring granules and random anisotropy fields. We show that such a system can be considered as ferromagnetic glass. We demonstrate (basing on analytical considerations as well as on the results of numerical simulations) that the system is decomposed to clusters or domains with nearly collinear orientation of magnetization. The size of the domains depends on the ratio of the exchange interaction and random anisotropy. For quasi-2D structures we predict that the dipole-dipole interactions between the granules lead to a formation of magnetic vortices. Moreover, the computer simulations also reproduce the puzzling increase of the thermoremanent magnetization observed experimentally, which is expected to be a result of a temperature-dependent decrease in the anisotropy (or a temperature-dependent increase in the exchange). We also consider the structures with weak intergranular exchange and show that they are characterized by the presence of two critical temperatures.     

Noncollinear interlayer exchange in Fe/Cr/Fe magnetic structures with different interface roughnesses

The interaction of iron layers through a chromium spacer in Fe/Cr/Fe trilayers with different roughnesses of interfaces was studied by the Kerr magnetometry and Mandel’shtam-Brillouin light scattering techniques so as to trace the interlayer exchange coupling of the Fe layers depending on the Cr spacer thickness and the sample temperature. It is established that, in a broad range of these parameters, the interlayer exchange in Fe/Cr/Fe structures with sufficiently smooth interfaces is adequately described using the proximity magnetism and half-angle coupling models taking into account the antiferromagnetic properties of chromium. As the interface roughness increases, the well-known biquadratic exchange model becomes valid. This is evidence for the decisive role of the magnetic stiffness of a Cr spacer and the structure of interfaces on the noncollinear exchange coupling in Fe/Cr/Fe trilayers.     

Effect of anisotropy and dipole interaction on long-range order magnetic structures generated by Dzyaloshinskii–Moriya interaction

Self-organized long-range order structures, such as stripe domains and magnetic skyrmion lattices, are formed by the competition between ferromagnetic exchange interaction and Dzyaloshinskii–Moriya (DM) interaction. We investigated the properties of the magnetic structures generated by a DM interaction under the influence of anisotropy or magnetic dipole interaction, by performing Monte-Carlo simulated annealing. We constructed phase maps in external-field and anisotropy space to study the effect of anisotropy or dipole interaction on the phase boundaries between the magnetic structures. The simulation results show that the phase boundaries are sensitive to perpendicular anisotropy and that the skyrmion lattice region in phase space is extended under easy-plane anisotropy. The effect of the long-range dipole interaction was studied and was found to stabilize the skyrmion lattice phase and reduce the size of the magnetic structures.     

Electronic Structure and Magnetic Properties of Cr-Doped AlN

To understand the electronic and magnetic properties, we have studied Cr-doped zinc-blende AlN system in detail by applying a first-principle plane wave pseudopotential method based on the density functional theory within the local spin density approximation. The analyses of the band structures, density of states, exchange interactions, and magnetic moments show that Al1-xCrxN alloys may exhibit a half-metallic ferromagnetism character, that Cr in the diluted doping limit forms near-midgap deep levels, and that the total magnetization of the cell is 3μB per Cr atom, which does not change with Cr concentration. Moreover, we have succeeded in predicting that Al1-xCrzN alloys in x = 0.0625 has a very high Curie temperature, and lind that ferromagnetic exchange interaction between magnetic dopants is short-ranged.     

The fragile magnetic structures of Fe/CeH2−δ multilayers

Fe/CeH_2−δ multilayers exhibit at room temperature evidence of interlayer exchange coupling. Subsequent Fe layers are either parallel or antiparallel to each other, depending on the Fe and CeH_2−δ layer thickness. However, when both layers have thickness larger than 15 Å, the antiferromagnetic structure becomes fragmented into domains laterally limited to a few microns, and the magnetic structures become very fragile. Small magnetic fields of a few Oersteds acting on the samples during growth induce helimagnetic configurations which coexist with antiferromagnetic coupling. The magnetic structures can be permanently destroyed by applying magnetic fields larger than 150 Oe.