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1.
Hoffmann A 《Physical review letters》2004,93(9):097203
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. 相似文献
2.
Miltenyi P Gierlings M Keller J Beschoten B Guntherodt G Nowak U Usadel KD 《Physical review letters》2000,84(18):4224-4227
The exchange bias coupling at ferromagnetic/antiferromagnetic interfaces in epitaxially grown Co/CoO layers can intentionally be increased by a factor of up to 3 if the antiferromagnetic CoO layer is diluted by nonmagnetic defects in its volume part away from the interface. Monte Carlo simulations of a simple model of a ferromagnetic layer on a diluted antiferromagnet show exchange bias and explain qualitatively its dilution and temperature dependence. These investigations reveal that diluting the antiferromagnet leads to the formation of volume domains, which cause and control exchange bias. 相似文献
3.
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. 相似文献
4.
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. 相似文献
5.
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. 相似文献
6.
Aspects of exchange bias between antiferromagnets and ferromagnets remain unclear despite recent research. An outstanding issue is the relationship between exchange bias and enhanced coercivity in the ferromagnetic layer. This Letter reports the unexpected finding that a substantial exchange bias can be generated between an antiferromagnet (FeMn) with a higher ordering temperature than that of the ferromagnet (CuNi). We interpret the result in terms of a temperature-dependent competition between interfacial exchange and antiferromagnet anisotropy energies. Crossover of these energies during cooling is responsible for the onset of exchange bias at the blocking temperature. 相似文献
7.
M. Holtschneider W. Selke 《The European Physical Journal B - Condensed Matter and Complex Systems》2008,62(2):147-154
Classical uniaxially anisotropic Heisenberg and XY antiferromagnets
in a field along the easy axis on a square lattice
are analysed, applying ground state
considerations and Monte Carlo techniques. The models are known to
display antiferromagnetic and spin-flop phases. In the Heisenberg
case, a single-ion anisotropy is added to the XXZ antiferromagnet,
enhancing or competing with the uniaxial exchange anisotropy. Its
effect on the stability of non-collinear structures of biconical
type is studied. In the case of the anisotropic XY antiferromagnet,
the transition region between the antiferromagnetic and spin-flop
phases is found to be dominated by degenerate bidirectional
fluctuations. The phase diagram is observed to resemble closely
that of the XXZ antiferromagnet without single-ion anisotropy. 相似文献
8.
We present evidence for the creation of an exchange spring in an antiferromagnet due to exchange coupling to a ferromagnet. X-ray magnetic linear dichroism spectroscopy on single crystal Co/NiO(001) shows that a partial domain wall is wound up at the surface of the antiferromagnet when the adjacent ferromagnet is rotated by a magnetic field. We determine the interface exchange stiffness and the antiferromagnetic domain wall energy from the field dependence of the direction of the antiferromagnetic axis, the antiferromagnetic pendant to a ferromagnetic hysteresis loop. The existence of a planar antiferromagnetic domain wall, proven by our measurement, is a key assumption of most exchange bias models. 相似文献
9.
Micromagnetic simulations of a pulsed inductive microwave magnetometer (PIMM) experiment are performed using a well established model for exchange bias. The model (Interacting Grain Model) consists of ferromagnetic grains and antiferromagnetic grains with randomly distributed easy axes. A perfectly compensated interface between the ferromagnet and the antiferromagnet is assumed which leads to spin flop coupling. The antiferromagnetic layer is modelled as two totally antiparallel sublattices with a small intergrain exchange between each antiferromagnetic sublattice. Simulations of an experimental PIMM setup provide a shift of the minimum of the resonance frequency which is also observered experimentally. 相似文献
10.
The magnetic anisotropy in antiferromagnetic 500 A thick NiO films, before and after the establishment of an exchange bias field with Co84Fe16 ferromagnetic layers, was measured using magnetic linear dichroism in soft x-ray absorption. Both <111> textured NiO and untextured NiO films show exchange-bias induced in-plane magnetic anisotropy of nearly equal magnitude and with the Ni moment axis being nearly parallel to the exchange bias field direction. These results represent the first observation of the key step in the exchange biasing process, namely, repopulation of the antiferromagnetic domains whose magnetization axis is closest to the exchange bias field direction. 相似文献
11.
讨论了铁磁-反铁磁双层膜中交换偏置和矫顽场随温度变化的关系。在本模型中,温度的依赖性来源于系统态的热激发以及相关磁学参量的温度依赖性。数值结果显示:低温下,交换偏置和矫顽场随温度的升高而减少,但是随着界面的交换耦合的增强或铁磁层各向异性的减少,其交换偏置变得平坦。随着温度的升高,交换偏置减少直至零;而矫顽场却达到峰值后再减为零。这些结果与实验结果定性一致。根据数值计算结果,可以预见软的铁磁层耦合上硬的反铁磁层,在恰当的交换耦合强度下,可构建具有大的交换偏置、小矫顽场;并在某温度区几乎不随温度变化的磁存贮器件. 相似文献
12.
比较了铁磁单层膜与铁磁/反铁磁双层膜结构中的磁畴演化行为, 发现由于反铁磁层膜对铁磁层膜的耦合作用使得系统的磁畴壁厚度、 磁畴壁等效质量、磁畴壁移动速度等发生了改变, 系统的矫顽场增强, 并出现了交换偏置场. 文章具体研究了反铁磁层耦合作用下其磁畴壁厚度、 等效质量以及磁畴壁移动速度等与反铁磁层的净磁化、 磁各向异性、界面耦合强度以及温度等的关系; 并研究了其对铁磁/反铁磁双层膜中的交换偏置场、矫顽场的影响. 进而 从磁畴结构的形成及其演化上揭示了铁磁/反铁磁双 层膜中出现交换偏置以及矫顽场增加的物理机制. 相似文献
13.
Khomenko E. V. Chechenin N. G. Dzhun’ I. O. Perov N. S. Samsonova V. V. Go?khman A. Yu. Zenkevich A. V. 《Physics of the Solid State》2010,52(8):1701-1708
The preparation conditions and the magnitudes of the uniaxial and unidirectional magnetic anisotropies of IrMn/Co structures
with an alternative sequence of deposition of antiferromagnetic and ferromagnetic layers upon heat treatment and cooling in
an external magnetic field have been investigated. It has been revealed that the unidirectional anisotropy (exchange bias)
arises in the structure with an antiferromagnetic layer deposited on a ferromagnetic layer (TS structure) at an annealing
temperature of higher than 100°C. In structures with a ferromagnetic layer deposited on an antiferromagnetic layer (BS structure),
the exchange bias does not arise in the annealing temperature range under investigation. The possible factors responsible
for this effect and the ratio between the temperature of the appearance of the exchange bias and the Néel temperature have
been discussed. 相似文献
14.
Martí X Park BG Wunderlich J Reichlová H Kurosaki Y Yamada M Yamamoto H Nishide A Hayakawa J Takahashi H Jungwirth T 《Physical review letters》2012,108(1):017201
We employ antiferromagnetic tunneling anisotropic magnetoresistance to study the behavior of antiferromagnetically ordered moments in IrMn exchange coupled to NiFe. Experiments performed by common laboratory tools for magnetization and electrical transport measurements allow us to directly link the broadening of the NiFe hysteresis loop and its shift (exchange bias) to the rotation and pinning of antiferromagnetic moments in IrMn. At higher temperatures, the broadened loops show zero shift, which correlates with the observation of fully rotating antiferromagnetic moments inside the IrMn film. The onset of exchange bias at lower temperatures is linked to a partial rotation between distinct metastable states and pinning of the IrMn antiferromagnetic moments in these states. The observation complements common pictures of exchange bias and reveals an electrically measurable memory effect in an antiferromagnet. 相似文献
15.
Griselda García Miguel Kiwi José Mejía-López Ricardo Ramírez 《Journal of magnetism and magnetic materials》2010,322(21):3329-3332
The magnitude of the exchange bias field of patterned systems exhibits a notable increase in relation to the usual bilayer systems, where a continuous ferromagnetic film is deposited on an antiferromagnet insulator. Here we develop a model, and implement a Monte Carlo calculation, to interpret the experimental observations which is consistent with experimental results, on the basis of assuming a small fraction of spins pinned ferromagnetically in the antiferromagnetic interface layer. 相似文献
16.
Alexandra Mougin Stéphane Mangin Jean-Francois Bobo Alois Loidl 《The European Physical Journal B - Condensed Matter and Complex Systems》2005,45(2):155-155
The study of layered magnetic structures is one of the hottest topics in magnetism due to the growing attraction of
applications in magnetic sensors and magnetic storage media, such as random access memory. For almost half a century,
new discoveries have driven researchers to re-investigate magnetism in thin film structures. Phenomena such as giant
magnetoresistance, tunneling magnetoresistance, exchange bias and interlayer exchange coupling led to new ideas to
construct devices, based not only on semiconductors but on a variety of magnetic materials
Upon cooling fine cobalt particles in a magnetic field through the Néel temperature of their outer antiferromagnetic
oxide layer, Meiklejohn and Bean discovered exchange bias in 1956. The exchange bias effect through which an
antiferromagnetic AF layer can cause an adjacent ferromagnetic F layer to develop a preferred direction of
magnetization, is widely used in magnetoelectronics technology to pin the magnetization of a device reference layer in a
desired direction. However, the origin and effects due to exchange interaction across the interface between
antiferromagneic and ferromagnetic layers are still debated after about fifty years of research, due to the extreme
difficulty associated with the determination of the magnetic interfacial structure in F/AF bilayers. Indeed, in an AF/F
bilayer system, the AF layer acts as “the invisible man” during conventional magnetic measurements and the presence of
the exchange coupling is evidenced indirectly through the unusual behavior of the adjacent F layer. Basically, the
coercive field of the F layer increases in contact with the AF and, in some cases, its hysteresis loop is shifted by an
amount called exchange bias field. Thus, AF/F exchange coupling generates a new source of anisotropy in the F layer. This
induced anisotropy strongly depends on basic features such as the magnetocrystalline anisotropy, crystallographic and spin
structures, defects, domain patterns etc of the constituant layers.
The spirit of this topical issue is, for the first time, to gather and survey recent and original
developments, both experimental and theoretical, which bring new insights into the physics of exchange bias. It has been
planned in relation with an international workshop exclusively devoted to exchange bias, namely IWEBMN’04 (International
Workshop on Exchange Bias in Magnetic Nanostructures) that took place in Anglet, in the south west of France, from
16th to 18th September 2004. The conference gathered worldwide researchers in the area, both
experimentalists and theoreticians. Several research paths are particularly active in the field of magnetic exchange
coupling. The conference, as well as this topical issue, which was also open to contributions from scientists
not participating in the conference, has been organized according to the following principles:
1. Epitaxial systems:
Since the essential behavior of exchange bias critically depends on the atomic-level chemical and spin structure at the
interface
between the ferromagnetic and antiferromagnetic components, epitaxial AF/F systems in which the quality of the interface and
the
crystalline coherence are optimized and well known are ideal candidates for a better understanding of the underlying physics
of
exchange bias. The dependence of exchange bias on the spin configurations at the interfaces can be accomplished by selecting
different crystallographic orientations. The role of interface roughness can also be understood from thin-film systems by
changing the growth parameters, and correlations between the
interface structure and exchange bias can be made, as reported in this issue.
2. Out-of-plane magnetized systems:
While much important work has been devoted to the study of structures with in-plane magnetization, little has been done
on the study of exchange bias and exchange coupling in samples with out-of-plane magnetization. Some systems can exhibit
either in-plane or out-of-plane exchange bias, depending on the field cooling direction. This is of particular interest
since it allows probing of the three-dimensional spin structure of the AF layer. The interface magnetic configuration is
extremely important in the perpendicular geometry, as the short-range exchange coupling competes with a long-range
dipolar interaction; the induced uniaxial anisotropy must overcome the demagnetization energy to establish perpendicular
anisotropy films. Those new studies are of primary importance for the magnetic media industry as perpendicular recording
exhibits potential for strongly increased storage densities.
3. Parameters tuning exchange bias in polycrystalline samples and magnetic configurations:
Different parameters can be used to tune the exchange bias coupling in polycrystalline samples similar to those used in
devices. Particularly fascinating aspects are the questions of the appearance of exchange bias or coercivity in
ferromagnet/antiferromagnet heterostructures, and its relation to magnetic configurations formed on either side of the
interface. Several papers report on either growth choices or post preparation treatments that enable tuning of the exchange
bias in bilayers. The additional complexity and novel features of the exchange coupled interface make the problem
particularly rich.
4. Dynamics and magnetization reversal:
Linear response experiments, such as ferromagnetic resonance, have been used with great success to identify interface,
surface anisotropies and interlayer exchange in multilayer systems. The exchange bias structure is particularly well
suited to study because interface driven changes in the spin wave frequencies in the ferromagnet can be readily related
to interlayer exchange and anisotropy parameters associated with the antiferromagnet. Because the exchange bias is
intimately connected with details of the magnetization process during reversal and the subsequent formation of
hysteresis, considerations of time dependence and irreversible processes are also relevant. Thermal processes like the
training effect manifesting itself in changes in the hysteretic characteristics depending on magnetic history can lead to
changes in the magnetic configurations. This section contains an increasing number of investigations of dynamics in
exchange bias coupled bilayers, and in particular those of the intriguing asymmetric magnetization reversal in both
branches of a hysteresis loop.
The Editors of the topical issue:
Alexandra Mougin
Laboratoire de Physique des Solides, UMR CNRS 8502, Université Paris Sud, F-91405 Orsay, France
Stéphane Mangin
Laboratoire de Physique des Matériaux, UMR CNRS 7556, Université Henri Poincaré, F-54506 Nancy, France
Jean-Francois Bobo
Laboratoire de Physique de la Matière Condensée - NMH, FRE 2686 CNRS ONERA, 2 avenue Edouard Belin,
F-31400 Toulouse, France
Alois Loidl
Experimentalphysik V, EKM, Institut für Physik, Universität Augsburg, Universitätsstrasse 1, D-86135, Augsburg, Germany 相似文献
17.
This work introduces a realistic model for the magnetic behavior of polycrystalline ferromagnet/antiferromagnet (FM/AF) systems with granular interfaces. It considers that, for strong enough interface exchange coupling, the AF layer breaks the adjacent FM into small-sized domains and that at the interface there exist grains with uncompensated spins interacting with the FM magnetizations; the classification of these grains as unstable (rotatable, responsible for a coercivity enhancement) or stable (adding to the bias) depends on both the anisotropy and the magnetic coupling with the adjacent FM. The distinctive characteristic of the model is that the effective rotatable anisotropy changes when the external magnetic field is varied resulting in a non-zero hard-axis coercivity, a feature commonly observed, though little understood and often ignored. The applicability of this model was checked on a typical magnetron-sputtered IrMn/Co bilayer and excellent agreement between experiment and simulation was achieved. 相似文献
18.
Using the principle of minimal energy and S-W model, the exchange bias for ferromagnetic/antiferromagnetic bilayers has been
investigated when the uniaxial anisotropy is misaligned with the exchange anisotropy. According to the relation between the
energy of the bilayer and the orientation of ferromagnetic magnetization, it is found that the bilayer will be in the monostable
state or bistable state when the external field is absent in the initial magnetization state. The monostable state or bistable
state of the bilayer, which determines the angular dependence of exchange bias directly, is controlled by the competition
between the exchange anisotropy and uniaxial anisotropy. When the applied field is parallel to the intrinsic easy axes and
intrinsic hard axes, one of the switching fields of the hysteresis loop shows an abrupt change, while the other keep continuous
by analyzing the magnetization reversal processes. Consequently, the exchange bias field and the coercivity will show a jump
phenomenon. The numerical calculations indicate that both the magnitude and direction of the exchange anisotropy will significantly
affect the angular dependence of exchange bias. The jump phenomenon of exchange bias is an intrinsic property of the bilayer,
which is dependent on the interfacial exchange-coupling constant, the orientation of the exchange anisotropy, the thickness
and uniaxial anisotropy constant of the ferromagnetic layer. 相似文献
19.
The rotational anisotropies in the ferromagnetism/antiferromagnetism 1/ antiferromagnetism 2 exchange bias structures
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The rotational anisotropies in the exchange bias structures of
ferromagnetism/antiferromagnetism 1/antiferromagnetism 2 are studied in this
paper. Based on the model, in which the antiferromagnetism is treated with
an Ising mean field theory and the rotational anisotropy is assumed to be
related to the field created by the moment induced on the antiferromagnetic
layer next to the ferromagnetic layer, we can explain why in experiments for
ferromagnetism (FM)/antiferromagntism 1 (AFM1)/antiferromagnetism 2 (AFM2)
systems the thickness-dependent rotational anisotropy value is
non-monotonic, i.e. it reaches a minimum for this system at a specific
thickness of the first antiferromagnetic layer and exhibits oscillatory
behaviour. In addition, we find that the temperature-dependent rotational
anisotropy value is in good agreement with the experimental result. 相似文献
20.
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. 相似文献