Exchange-coupled Layers Research Articles

Spacer Thickness and Temperature Dependences of the Interlayer Exchange Coupling in a Co/Pt/Co Three-Layer Structure

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.

Effect of Multilayered Structure on the Static and Dynamic Properties of Magnetic Nanospheres.

The development of flexible and lightweight electromagnetic interference (EMI)-shielding materials and microwave absorbers requires precise control and optimization of core–shell constituents within composite materials. Here, a theoretical model is proposed to predict the static and dynamic properties of multilayered core–shell particles comprised of exchange-coupled layers, as in the case of a spherical iron core coupled to an oxide shell across a spacer layer. The theory of exchange resonance in homogeneous spheres is shown to be a limiting special case of this more general theory. Nucleation of magnetization reversal occurs through either quasi-uniform or curling magnetization processes in core–shell particles, where a purely homogeneous magnetization configuration is forbidden by the multilayered morphology. The energy is minimized through mixing of modes for specific interface conditions, leading to many inhomogeneous solutions, which grow as 2n with increasing layers, where n represents the number of magnetic layers. The analytical predictions are confirmed using numerical simulations.

Suppression of Coercivity in Nanoscale Graded Magnetic Materials

We investigate temperature-dependent magnetization reversal of $\mathrm{Co}\mathrm{Ru}$ graded films, in which a predefined depth-dependent exchange-coupling strength J follows a V-shaped profile. Magnetometry reveals an extended temperature range below the Curie temperature ${T}_{C}\phantom{\rule{0.25em}{0ex}}$ where the reversal of the magnetization M is not accompanied by the conventionally occurring hysteresis, in stark contrast with homogeneous $\mathrm{Cr}\mathrm{Ru}$ reference films. This is caused by the temperature-driven paramagnetic (PM)-ferromagnetic (FM) phase transition, which does not occur in the entirety of the graded material but only in well-defined nanoscopic regions at any given temperature, enabling the creation of two internal PM/FM interfaces that assist the external magnetic field in reversing the magnetization of the FM graded sample region. Hysteretic reversal is recovered at sufficiently low sample temperatures or by using graded structures with very steep J gradients, so that no PM/FM interfaces form inside the exchange-coupled layer material that could influence the magnetization reversal process. Our findings open interesting material design options, and we envision wide application potential, since we succeed in engineering a temperature gap between the temperature dependent magnetization and hysteresis onsets, a capability of interest for any technology benefitting from field-free magnetization switching.

Ferromagnetic Resonance Studies in Magnetic Nanosystems

Ferromagnetic resonance is a powerful method for the study of all classes of magnetic materials. The experimental technique has been used for many decades and is based on the excitation of a magnetic spin system via a microwave (or rf) field. While earlier methods were based on the use of a microwave spectrometer, more recent developments have seen the widespread use of the vector network analyzer (VNA), which provides a more versatile measurement system at almost comparable sensitivity. While the former is based on a fixed frequency of excitation, the VNA enables frequency-dependent measurements, allowing more in-depth analysis. We have applied this technique to the study of nanostructured thin films or nanodots and coupled magnetic layer systems comprised of exchange-coupled ferromagnetic layers with in-plane and perpendicular magnetic anisotropies. In the first system, we have investigated the magnetization dynamics in Co/Ag bilayers and nanodots. In the second system, we have studied Permalloy (Ni80Fe20, hereafter Py) thin films coupled via an intervening Al layer of varying thickness to a NdCo film which has perpendicular magnetic anisotropy.

Exchange interactions and sensitivity of the Ni two-hole spin state to Hund's coupling in doped NdNiO2

Using density functional based LDA+U method and linear-response theory, we study the magnetic exchange interactions of the superconductor Nd{1-x}Sr{x}NiO2. Our calculated nearest-neighbor exchange constant J1=82 meV is large, weakly affected by doping and is only slightly smaller than that found in the sister compound CaCuO2. We however find that the hole doping significantly enhances the inter--layer exchange coupling as it affects the magnetic moment of the Ni-3d{3z2-r2} orbital. This can be understood in terms of small hybridization of Ni-3d{3z2-r2} within the NiO2 plane which results in a flat band near the Fermi level, and its large overlap along z direction. We also demonstrate that the Nd-5d states appearing at the Fermi level, do not affect the magnetic exchange interactions, and thus may not participate in the superconductivity of this compound. Whereas many previous works emphasize the importance of the Ni-3d{x2-y2} and Nd-5d orbitals, we instead analyze the solution of Ni-3d{x2-y2}/Ni-3d{3z2-r2} minimal model using Dynamical Mean Field Theory. It reveals an underlying Mott insulating state which, depending on precise values of the intra--atomic Hund's coupling less or larger than 0.83 eV, selects upon doping either S=0 or S=1 two--hole states at low energies leading to very different quasiparticle band structures. We propose that trends upon doping in spin excitational spectrum and quasiparticle density of state can be a way to probe Ni 3d8 configuration.

Ferromagnetism in the Ferromagnetic Yttrium Iron Garnet Film/Ferromagnetic Intermetallic Compound Heterostructure

We report on the investigations of the magnetic properties and ferromagnetic resonance in the heterostructure consisting of an epitaxial yttrium iron garnet (Y3Fe5O12) film and a nanometer rare-earth intermetallic superlattice comprised of the (TbCo2/FeCo)n exchange-coupled layers. The (TbCo2/FeCo)n superlattice exhibits the giant magnetostriction and the controlled magnetic anisotropy induced by a magnetic field or elastic stresses. The magnetic interaction of the films in the (TbCo2/FeCo)n/Y3Fe5O12 heterostructure has been experimentally established and the spin current flowing through their interface has been detected.

Ферромагнетизм в гетероструктуре ферромагнитная пленка железо-иттриевого граната/ферромагнитный интерметаллид

Abstract We report on the investigations of the magnetic properties and ferromagnetic resonance in the heterostructure consisting of an epitaxial yttrium iron garnet (Y_3Fe_5O_12) film and a nanometer rare-earth intermetallic superlattice comprised of the (TbCo_2/FeCo)_ n exchange-coupled layers. The (TbCo_2/FeCo)_ n superlattice exhibits the giant magnetostriction and the controlled magnetic anisotropy induced by a magnetic field or elastic stresses. The magnetic interaction of the films in the (TbCo_2/FeCo)_ n /Y_3Fe_5O_12 heterostructure has been experimentally established and the spin current flowing through their interface has been detected.

Magnetic Interactions on Oxide Ferromagnet/Ferromagnetic Intermetallide Interface

The magnetic properties of heterostructures consisting of two films are studied. The upper layer involves rare-earth intermetallic nanostructured superlattices consisting of exchange-coupled layers (TbCo2/FeCo)n (TCFC), and the lower layer includes either epitaxial manganite La0.7Sr0.3MnO3 (LSMO) with optimum strontium doping or an epitaxial film of an yttrium–iron garnet Y3Fe5O12 (YIG) with a Bi additive. TCFC is a ferromagnet having high Curie temperature and provides controllable induced magnetic anisotropy. Experimental studies showed that the interlayer interaction of the TCFC/LSMO heterostructure is antiferromagnetic. There was an increase in the FMR line width in the structures due to the flow of a spin current through the interface between two films. There was electric voltage in the TCFC/YIG heterostructure induced in the TCFC intermetallide film, due to an inverse spin Hall effect under ferromagnetic resonance conditions.

Effects of Resonant Enhancement of Evanescent Spin Waves in Exchange-Coupled Layered Magnetic Structures with and without an Inversion Center

In the system of planar exchange-coupled antiferromagnetic layers with and without an inversion center, the hybridization of waveguide and antiwaveguide spin wave propagation modes induces an interference mechanism of resonant enhancement of both the amplitudes of evanescent magnons and the accompanying first order effects of nonspecular spin-wave refraction in the course of transmission and (or) reflection processes (including those corresponding to the magnon analog of Fano resonance).

Solid-Phase Epitaxy of BiFeO3 Films with Magnetoelectric Properties on Sapphire

The ferroelectric domain structure has been investigated in bismuth-ferrite rhombohedral phase films prepared on the rhombohedral plane of sapphire by solid-phase epitaxy. The piezoelectric modulus has been estimated to be d33 ~ 2 pm/V. In films of the Bi25FeO39–BiFeO3 and Bi2Fe4O9–BiFeO3 systems on sapphire, the magnetoelectric-switching effect has been observed in bismuth ferrite crystallites without deposition of exchange-coupled layers. The values of the magnetic moment in bismuth ferrite microcrystallites have been estimated.

Твердофазная эпитаксия пленок BiFeO-=SUB=-3-=/SUB=- с магнитоэлектрическими свойствами на сапфире

AbstractThe ferroelectric domain structure has been investigated in bismuth-ferrite rhombohedral phase films prepared on the rhombohedral plane of sapphire by solid-phase epitaxy. The piezoelectric modulus has been estimated to be d _33 ~ 2 pm/V. In films of the Bi_25FeO_39–BiFeO_3 and Bi_2Fe_4O_9–BiFeO_3 systems on sapphire, the magnetoelectric-switching effect has been observed in bismuth ferrite crystallites without deposition of exchange-coupled layers. The values of the magnetic moment in bismuth ferrite microcrystallites have been estimated.

Switching of Exchange-Coupled Perpendicularly Magnetized Layers Under Spin–Orbit Torque

We proposed a multilayer structure to reduce the critical spin current density of perpendicularly magnetized layers switched by spin-orbit torque (SOT) from the spin Hall effect. In the structure, we introduced exchange-coupled perpendicularly magnetized layers composed of soft and hard magnetic layers in lieu of a single magnetic layer. The switching loops and dynamic processes under periodic spin currents have been studied by micromagnetic simulation. We have found that the critical spin current density can be substantially reduced when the saturation magnetization of the soft layer is sufficiently low. Moreover, the critical spin current density is quite insensitive to the change of exchange interaction between the two magnetic layers, which enables the multilayer structure to tolerate the variation of material composition or structure imperfections during fabrication. The introduction of exchange-coupled layers provides a way to design and optimize the SOT-driven devices and paves the path for their applications.

Ferromagnetic resonance linewidth in coupled layers with easy-plane and perpendicular magnetic anisotropies

Magnetic bilayers with different magnetic anisotropy directions are interesting for spintronic applications as they offer the possibility to engineer tilted remnant magnetization states. We investigate the ferromagnetic resonance (FMR) linewidth of modes associated with two interlayer exchange-coupled ferromagnetic layers, the first a CoNi multilayer with a perpendicular magnetic anisotropy, and the second a CoFeB layer with an easy-plane anisotropy. For antiferromagnetic interlayer exchange coupling, a local maximum in the FMR linewidths is observed below a characteristic field. This is in contrast to what is found in uncoupled, ferromagnetically coupled and single ferromagnetic layers in which the FMR linewidth increases monotonically with field. We show that the existence of the local maximum as well as the characteristic field below which there is a dramatic increase in FMR linewidth can be understood using a macrospin model with Heisenberg-type exchange coupling between the layers.

Largely enhanced coercivity of cobalt adjacent to straight-stripe mixed-phase bismuth ferrites

Exchange coupling at the interface of an antiferromagnet and a ferromagnet is the major route toward the future magnetoelectric applications of multiferroic bismuth ferrite at room temperature. By using angle-resolved longitudinal magneto-optic Kerr effect microscopy, we have investigated the exchange anisotropy in a ferromagnetic Co layer adjacent to electrically aligned straight-stripe mixed-phase-boundary (MPB) regions of La-5%-doped $\mathrm{BiFe}{\mathrm{O}}_{3}$. We have found that the magnetic easy axis of the exchange-coupled Co layer becomes parallel to the in-plane crystallographic axis nearest to the adjacent MPB elongation axis. The coercive field of the exchange-coupled Co layer along the magnetic easy axis has prominently increased by $\ensuremath{\sim}66$ Oe, i.e., 30 times larger than that of a control sample (2.3 Oe), wherein a 5-nm-thick nonmagnetic Ta spacer is placed between the antiferromagnetic and ferromagnetic layers and thus it is intended to have only the shape anisotropy arising from the surface nanostructure of the MPB regions. The finding opens a promising avenue for magnetoelectric applications at room temperature.

Current-driven domain wall dynamics in ferromagnetic layers synthetically exchange-coupled by a spacer: A micromagnetic study

The current-driven domain wall motion along two exchange-coupled ferromagnetic layers with perpendicular anisotropy is studied by means of micromagnetic simulations and compared to the conventional case of a single ferromagnetic layer. Our results, where only the lower ferromagnetic layer is subjected to the interfacial Dzyaloshinskii-Moriya interaction and to the spin Hall effect, indicate that the domain walls can be synchronously driven in the presence of a strong interlayer exchange coupling, and that the velocity is significantly enhanced due to the antiferromagnetic exchange coupling as compared with the single-layer case. On the contrary, when the coupling is of ferromagnetic nature, the velocity is reduced. We provide a full micromagnetic characterization of the current-driven motion in these multilayers, both in the absence and in the presence of longitudinal fields, and the results are explained based on a one-dimensional model. The interfacial Dzyaloshinskii-Moriya interaction, only necessary in this lower layer, gives the required chirality to the magnetization textures, while the interlayer exchange coupling favors the synchronous movement of the coupled walls by a dragging mechanism, without significant tilting of the domain wall plane. Finally, the domain wall dynamics along curved strips is also evaluated. These results indicate that the antiferromagnetic coupling between the ferromagnetic layers mitigates the tilting of the walls, which suggest these systems to achieve efficient and highly packed displacement of trains of walls for spintronics devices. A study, taking into account defects and thermal fluctuations, allows to analyze the validity range of these claims.

Exchange coupling of a perpendicular ferromagnet to a half-metallic compensated ferrimagnet via a thin hafnium interlayer

A thin Hafnium film is shown to act both as an effective diffusion barrier for manganese at a thickness of 0.7 nm and as an effective exchange coupling layer in a sandwich structure with perpendicular magnetic anisotropy. The two magnetic layers are Co20Fe60B20 and the low moment ferrimagnet Mn2RuxGa (MRG). The relative orientation of the magnetic layers changes sign at the compensation temperature of MRG. The exchange energy reaches 0.11 mJ m−2 for the thinnest Hf interlayers. Ruthenium, the usual metal of choice for coupling ferromagnetic layers in thin film heterostructures, cannot be used with the zero-moment half metal MRG because of Ru interdiffusion. Due to its large coercivity near compensation, the MRG can potentially act as an effective source of exchange pinning.

(Invited) Towards Topological Antiferromagnetic Spintronics

Magnetic topological insulators in the tetradymite family, such as Cr/V doped Bi2Se3, Bi2Te3, Sb2Te3, and their ternary compounds, provide a new route for realizing the novel time-reversal symmetry-breaking physics [1, 2]. To effectively manipulate these versatile quantum states, various heterostructures are demonstrated by utilizing the unique Néel order in an antiferromagnet CrSb and the topological order in a magnetic topological insulator Cr-doped (Bi,Sb)2Te3 [3-5] grown by molecular beam epitaxy. It is shown that emergent interfacial magnetic interactions can be tailored through deliberate artificial structural engineering. By investigating bilayers, trilayers, and superlattices made of these two components, interfacial exchange coupling and antiferromagnetic exchange coupling were shown to affect the topological magnetism as mediated by the massive Dirac fermions. The relationship of magnetic spin textures of the antiferromagnet to that of the magnetic topological insulator was further explored through polarized neutron reflectometry/diffraction, magneto-electrical transport measurements as well as the control of the mass of Dirac fermions by using different Cr doping concentrations. From these studies, the antiferromagnet is shown to be an efficient interfacial- and interlayer- exchange coupling layer for the massive Dirac fermions of the magnetic topological insulator, which additionally gives rise to a giant enhancement in magnetic ordering. This work provides a new framework for new topological antiferromagnetic spintronics. *The work was in part supported by the SHINES Center, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award # S000686, National Science Foundation, Army Research Office program under contract 15-1-10561, TANMS, FAME Center, and a Semiconductor Research Corporation program sponsored by MARCO and DARPA.

Evidence for asymmetric rotation of spins in antiferromagnetic exchange-spring

We demonstrate an asymmetric rotation of the antiferromagnetic (AFM) spins in the exchange-spring driven by perpendicularly magnetized Co/Pt. The static and dynamic behaviors of the twisted spin structure are directly revealed by a combination of element specific soft-x-ray absorption spectra and magnetoresistance measurements. X-ray magnetic linear dichroism spectra as a function of AFM thickness clarify the features of the whole exchange-spring, while the interfacial uncompensated spins are identified by the x-ray magnetic circular dichroism spectra. Moreover, the observed tunneling anisotropic magnetoresistance (TAMR) in AFM-based junctions based on this asymmetric rotation provides an electrical approach to monitoring the dynamic twist of the AFM spins. These investigations not only provide a deep insight into the spin structure of the exchange coupling layers but would also advance the development of AFM spintronics.

Demonstration of the thermal stability advantage of advanced exchange coupled composite media

We systematically vary the strength of the vertical interlayer exchange coupling (VIEC) between the magnetic layers (MLs) of an advanced exchange coupled composite media and experimentally investigate, through magnetic viscosity and dynamic coercivity measurements, the impact on the thermal stability. The strength of the VIEC is controlled through exchange coupling layers of varied thicknesses, whereas MLs are unchanged. We estimate the thermal energy barrier ΔEo as a function of the VIEC within the standard Arrhenius-Neel relaxation and Sharrock formalisms and characterize the effect of the VIEC on the degree of incoherency in the grain reversal mechanism. The analysis shows that while ΔEo drops for weaker VIEC, the ratio of ΔEo to MsHo energy per activation volume is maintained near 2 and weakly optimizes for intermediate VIEC.

Ferromagnetic resonance study of structure and relaxation of magnetization in NiFe/Ru superlattices

The structural properties and relaxation processes of magnetization in [Ni81Fe19(t1)/Ru(t2)]N superlattices (N=number of bilayers) were analyzed by ferromagnetic resonance (FMR) with a fixed microwave frequency. One series of samples was deposited with constant NiFe layer thickness (t1) and variable Ru layer thickness (t2); the other series, with constant t2 and variable t1. A single FMR mode was observed for t2<15Å and t1>75Å and it has been attributed to the resonance of the exchange-coupled NiFe layers across the Ru interlayers. For the other values of t1 and t2, several FMR modes appeared and they were associated to non-coupled magnetic phases with different effective magnetization formed during the multilayer growth. The FMR linewidths were analyzed as a function of the magnetic layer thickness and a strong dependence on t1−2 was observed. It was attributed to the contribution of the two-magnon scattering mechanism for the linewidth.