Tuning Exchange Anisotropy of Exchange-Biased System

Exchange anisotropy in FM/AFM bilayers has given a lot of static magnetization properties such as enhanced coercivity and magnetization loop shifts. These phenomena are primarily from the effective anisotropies introduced into a ferromagnet by exchange coupling with a strongly anisotropic antiferromagnet. These effective anisotropies can also be used to explain the dynamic consequences of exchange-biased bilayers. In this article, the dynamic consequences such as exchange-induced susceptibility, exchange-induced permeability, and the corresponding domain wall characteristics in the exchange-biased structures of ferromagnet/antiferromagnet1/antiferromagnet2 are studied. The results show that the second antiferromagnetic layer can largely affect the dynamic consequences of exchange-biased bilayers. Especially in the case of critical temperature, the effects become more obvious. Practically, the exchange anisotropy of biased bilayer system can be tuned by exchange coupling with the second antiferromagnetic layer.     

Symmetry driven irreversibilities at ferromagnetic-antiferromagnetic interfaces

The coupling between a ferromagnet and an antiferromagnet can establish a directional anisotropy called exchange bias. In many systems this exchange bias is reduced upon subsequent field cycling, which is referred to as training effects. Numerical simulations of a simple coherent rotation model suggest that the symmetry of the anisotropy in the antiferromagnet is crucial for the understanding of training effects in exchange bias systems. Namely, the existence of multiple antiferromagnetic easy anisotropy axes can initially stabilize a noncollinear arrangement of the antiferromagnetic spins, which relaxes into a collinear arrangement after the first magnetization reversal of the ferromagnet.     

Influence of an interface layer on the effective coupling at a ferromagnet/antiferromagnet interface

We examine the exchange anisotropy induced at a ferromagnetic/antiferromagnetic interface when an antiferromagnetic interface layer exists. We show that competition between exchange couplings in the interface layer can result in a ferrimagnetic-like compensation point. This leads to a reversal of the effective field acting on the ferromagnet, and a consequent sign change of the exchange bias for temperatures near the Néel temperature of the antiferromagnet. A surprising result is the sensitive dependence of the compensation point on exchange interactions. Even minute modifications of the exchange interactions near the interface can result in a reversal of the effective field, provided certain conditions are met.     

“Unusual” domain walls in multilayer systems: Ferromagnet + layered antiferromagnet

The structure and conditions for the onset of a new type of domain wall in multilayer systems comprising a ferromagnet and a layered antiferromagnet is investigated by numerical simulation. Domain walls occur as the result of frustrations produced by interface roughness, i.e., by the existence of atomic steps on them. The domain walls are investigated both in a ferromagnetic film on a layered antiferromagnetic substrate and in multilayer structures. It is shown that a domain wall broadens with increasing distance from the interface; this trend is attributed to the nontrivial dependence of the wall energy on the thickness of the layer. The structure of the domain walls in multilayer ferromagnet-layered antiferromagnet systems varies dramatically as a function of the energies of interlayer and in-layer exchange interactions between adjacent layers. Zh. éksp. Teor. Fiz. 114, 1817–1826 (November 1998)     

Probing antiferromagnetic order with heat capacity in exchange-biased structures

We explore the magnetic heat capacity in exchange-biased ferromagnet/antiferromagnet bilayers theoretically. We show that changes in the antiferromagnetic structure due to the reversal of the ferromagnet layer can be detected by distinct features in the heat capacity. This offers a method for probing antiferromagnetic domains in exchange-biased systems.     

Spin-transfer torques in antiferromagnetic metals from first principles

In spite of the absence of a macroscopic magnetic moment, an antiferromagnet is spin-polarized on an atomic scale. The electric current passing through a conducting antiferromagnet is polarized as well, leading to spin-transfer torques when the order parameter is textured, such as in antiferromagnetic noncollinear spin valves and domain walls. We report a first principles study on the electronic transport properties of antiferromagnetic systems. The current-induced spin torques acting on the magnetic moments are comparable with those in conventional ferromagnetic materials, leading to measurable angular resistances and current-induced magnetization dynamics. In contrast to ferromagnets, spin torques in antiferromagnets are very nonlocal. The torques acting far away from the center of an antiferromagnetic domain wall should facilitate current-induced domain wall motion.     

Anomalous spontaneous reversal in magnetic heterostructures

We observe a thermally induced spontaneous magnetization reversal of epitaxial ferromagnet/antiferromagnet heterostructures under a constant applied magnetic field. Unlike any other magnetic system, the magnetization spontaneously reverses, aligning antiparallel to an applied field with decreasing temperature. We show that this unusual phenomenon is caused by the interfacial antiferromagnetic coupling overcoming the Zeeman energy of the ferromagnet. A significant temperature hysteresis exists, whose height and width can be tuned by the field applied during thermal cycling. The hysteresis originates from the intrinsic magnetic anisotropy in the system. The observation of this phenomenon leads to open questions in the general understanding of magnetic heterostructures. Moreover, this shows that in general heterogeneous nanostructured materials may exhibit unexpected phenomena absent in the bulk.     

Influence of a transport current on a domain wall in an antiferromagnetic metal

We consider the influence of an electric current on the position of a domain wall in an antiferromagnetic metal. We first microscopically derive an equation of motion for the Néel vector in the presence of current by performing, in the transport steady state, a linear-response calculation in the deviation from collinearity of the antiferromagnet. This equation of motion is then solved variationally for an antiferromagnetic domain wall. We find that, in the absence of dissipative or non-adiabatic coupling between magnetization and current, the current displaces the domain wall by a finite amount and that the domain wall is then intrinsically pinned by the exchange interactions. In the presence of dissipative or non-adiabatic current-to-domain-wall coupling, the domain wall velocity is proportional to the current and is no longer pinned.     

Field evolution of tilted vortex cores in exchange-biased ferromagnetic dots

We developed an analytical model for the magnetization reversal via vortex nucleation and annihilation in double-layer ferromagnetic/antiferromagnetic cylindrical dots. The coupling of the ferromagnet to the antiferromagnet is modeled by means of an interfacial exchange field. The nonuniformity of the magnetization reversal mode perpendicular to the layers is explicitly included and results in a tilted vortex core (tilted Bloch line). The vortex core tilt results in an asymmetry of the nucleation and annihilation fields, which are calculated as a function of the dot geometry.     

Exchange field induced magnetoresistance in colossal magnetoresistance manganites.

The effect of an exchange field on the electrical transport in thin films of metallic ferromagnetic manganites has been investigated. The exchange field was induced both by direct exchange coupling in a ferromagnet/antiferromagnet multilayer and by indirect exchange interaction in a ferromagnet/paramagnet metallic superlattice. The electrical resistance of the metallic manganite layers was found to be determined by the magnitude of the vector sum of the effective exchange field and the external magnetic field.     

Transverse magnetic ordering

There is an increasing number of ferromagnets and antiferromagnets which are observed to undergo either a further long range magnetic order or spin glass transition in components of the moment transverse to either the ferromagnetic or antiferromagnetic ordering direction. Necessary conditions include exchange frustration and some atomic disorder. We discuss the observation of transverse antiferromagnetic order in the ferromagnet (Fe, Mn)₃Si and the transverse spin glass phase observed in the ferromagnetic glassy metal a-(Fe, Zr) and the antiferromagnet γ-Mn–Cu.     

Rotation of the pinning direction in the exchange bias training effect in polycrystalline NiFe/FeMn bilayers

For polycrystalline NiFe/FeMn bilayers, we have observed and quantified the rotation of the pinning direction in the exchange bias training and recovery effects. During consecutive hysteresis loops, the rotation of the pinning direction strongly depends on the magnetization reversal mechanism of the ferromagnet layer. The interfacial uncompensated magnetic moment of antiferromagnetic grains may be irreversibly switched and rotated when the magnetization reversal process of the ferromagnet layer is accompanied by domain wall motion and domain rotation, respectively.     

Uncompensated moments in the MnPd/Fe exchange bias system

The element-specific magnetic structure of an epitaxially grown Mn_52Pd_48/Fe bilayer showing exchange bias was investigated with atomic-layer depth sensitivity at the antiferromagnet/ferromagnet interface by soft-x-ray magnetic circular dichroism and magnetic reflectivity. A complex magnetic interfacial configuration, consisting of a 2-monolayer-thick induced ferromagnetic region, and pinned uncompensated Mn moments that reach far deeper (approximately 13 A), both in the antiferromagnet, were found. For the latter, a direct relationship with the magnitude of the exchange bias is verified by similar measurements perpendicular to the field cooling direction.     

Exchange bias on epitaxial Ni films due to ultrathin NiO layer

Exchange anisotropy refers to the effect that an antiferromagnetic (AF) layer grown in contact with a ferromagnetic (FM) layer has on the magnetic response of the FM layer. The most notable changes in the FM hysteresis loop due to the surface exchange coupling are a coercivity enhanced over the value typically observed in films grown on a nonmagnetic substrate, and a shift in the hysteresis loop of the ferromagnet away from the zero field axis. A typical observation is that the thickness of the antiferromagnet needs to exceed a critical value before exchange bias is observed. Here we report on the exchange bias properties observed in an epitaxial Ni/NiO system where a thin NiO layer forms spontaneously and is observed after annealing epitaxial Ni films MBE grown on MgO substrates.     

Magnetization reversal studies of continuous and patterned exchange biased NiFe/FeMn thin films

In this article we present a detailed investigation of the structural and magnetic properties of exchange biased NiFe (ferromagnet)/FeMn (antiferromagnet) thin films. The influence of the shape anisotropy on exchange bias and the magnetization reversal mechanism in a sample with patterned lines is compared with a continuous two-dimensional reference sample. Polarized neutron reflectivity (PNR) is employed to study the magnetization reversal by analyzing the spin-flip and non-spin-flip reflectivities. PNR measurements show that the magnetization reversal in the reference two-dimensional film and patterned lines is by domain wall motion rather than coherent rotation of magnetization.     

Coercivity enhancement in exchange biased systems driven by interfacial magnetic frustration

We report the temperature and cooling field dependence of the coercivity of exchange biased MnF(2)/Fe bilayers. When the antiferromagnetic surface is in a state of maximum magnetic frustration and the net exchange bias is zero, we observe a strong enhancement of the coercivity, which is proportional to the exchange coupling between the layers. Hence, the coercivity can be tuned in a reproducible and repeatable fashion in the same sample. We propose that a frustrated interface provides local energy minima which effectively pin the propagating domain walls in the ferromagnet, leading to an enhanced coercivity.     

Current-induced torques due to compensated antiferromagnets

We analyze the influence of current-induced torques on the magnetization configuration of a ferromagnet in a circuit containing a compensated antiferromagnet. We argue that these torques are generically nonzero and determine their form by considering spin-dependent scattering at a compensated antiferromagnetic interface. Because of symmetry dictated differences in the form of the current-induced torque, the phase diagram which expresses the dependence of the ferromagnetic configuration on the current and external magnetic field differs qualitatively from its ferromagnet-only counterpart.     

Asymmetric reversal in inhomogeneous magnetic heterostructures

Asymmetric magnetization reversal is an unusual phenomenon in antiferromagnet/ferromagnet (AF/FM) exchange biased bilayers. We investigated this phenomenon in a simple model system experimentally and by simulation assuming inhomogeneously distributed interfacial AF moments. The results suggest that the observed asymmetry originates from the intrinsic broken symmetry of the system, which results in local incomplete domain walls parallel to the interface in reversal to negative saturation of the FM. The magneto-optical Kerr effect unambiguously confirms such an asymmetric reversal and a depth-dependent FM domain wall in accord with the magnetometry and simulations.     

Magnetization reversal by electric-field decoupling of magnetic and ferroelectric domain walls in multiferroic-based heterostructures

We demonstrate that the magnetization of a ferromagnet in contact with an antiferromagnetic multiferroic (LuMnO(3)) can be speedily reversed by electric-field pulsing, and the sign of the magnetic exchange bias can switch and recover isothermally. As LuMnO(3) is not ferroelastic, our data conclusively show that this switching is not mediated by strain effects but is a unique electric-field driven decoupling of the ferroelectric and antiferromagnetic domain walls. Their distinct dynamics are essential for the observed magnetic switching.     

Site-diluted antiferromagnet in a uniform field

We study a site-diluted Ising antiferromagnet in a square lattice, in an external uniform field (DAFF). We use a real space renormalization group technique to obtain the phase diagram as a function of temperature (T), concentration of magnetic ions (p) and magnetic field (H). At zero temperature the phase diagram, in the H × T plane, has a steplike structure separating the disordered from the ordered antiferromagnetic phase. We also discuss the connection between this problem and that of a ferromagnet in a random field.