Exchange Bias In Polycrystalline Research Articles

In situ observation of the structural transformations behind the emergence and disappearance of exchange bias in polycrystalline Ni-Mn/Fe20Ni80 films

The antiferromagnetic L10θ-NiMn compound has a Néel temperature of up to 800 °C and is capable of providing the exchange bias effect in multilayered structures with adjacent ferromagnetic layers, such as Fe20Ni80, up to 350 °C. The value of the exchange bias field can be non-decreasing up to 150 °C, making Ni-Mn a compelling choice for applied devices based on the exchange bias effect. In this work we study the structural transformations occurring in magnetron-sputtered Ni-Mn/Fe20Ni80 films that first lead to the emergence of exchange bias in these structures, and then to its disappearance. Employing in situ X-ray diffractometry and vibrating-sample magnetometry we observe how under annealing the phases of Ni-Mn follow Ostwald’s rule of stages on their way to form the equilibrium θ-NiMn. Our findings suggest that at temperatures of around 350 °C diffusion of iron from the ferromagnetic layer causes decomposition of θ-NiMn, ultimately resulting in the irreversible disappearance of exchange bias. Our results reveal the role of adjacent Fe20Ni80 layers in the transformations, and how crystal texture in the Ni-Mn layer affects exchange bias. The results of our work not only clarify the fundamental mechanisms but also provide valuable insights for engineering and utilizing of applied devices with Ni-Mn, making it of interest to material scientists and engineers of different specializations.

Viscous magnetization decrease in first-order reversal curves induced by rotatable magnetic anisotropy in polycrystalline exchange-biased bilayers

A prototypical polycrystalline in-plane exchange bias system exhibited a viscous decrease of the ferromagnetic magnetization upon increasing external magnetic field when measuring first-order reversal curves. The phenomenon is investigated by means of angular-resolved vectorial magneto-optical Kerr magnetometry complemented by fits of model calculations and by Kerr microscopy. It is found that the viscous magnetization decrease is mediated by a rotatable magnetic anisotropy arising from thermally unstable antiferromagnetic grains coupled to the probed ferromagnet. This additionally manifests itself by a creeping domain wall motion in the ferromagnet due to thermally activated processes in the antiferromagnet. The investigations are in agreement with a generalized description of polycrystalline exchange bias systems and emphasize the relevance of understanding minor loop behavior addressing nonsaturated magnetic states for systems susceptible to dynamic changes on the hysteresis loop timescale.

Preferential weakening of rotational magnetic anisotropy by keV-He ion bombardment in polycrystalline exchange bias layer systems

The consequences of keV light-ion bombardment of polycrystalline exchange bias layer systems on individual magnetic anisotropies composing the effective unidirectional magnetic anisotropy are investigated by vectorial magneto-optic Kerr effect measurements. Preferential reduction of rotatable magnetic anisotropy as compared to the other magnetic anisotropies is observable, disproving the intuitive assumption of an equable weakening of all magnetic anisotropies due to the bombardment. It is concluded that light-ion bombardment has a stronger impact on the magnetic characteristics of smaller grains in a polycrystalline antiferromagnet as compared to those of larger ones.

Time-dependent rotatable magnetic anisotropy in polycrystalline exchange-bias systems: Dependence on grain-size distribution

Angular resolved measurements of the exchange bias field and the coercive field are a powerful tool to distinguish between different competing magnetic anisotropies in polycrystalline exchange bias layer systems. No simple analytical model is as yet available, which considers time dependent effects like enhanced coercivity arising from the grain size distribution of the antiferromagnet. In this work we expand an existing model class describing polycrystalline exchange bias systems by a rotatable magnetic anisotropy term to describe grain size correlated effects. Additionally, we performed angular resolved magnetization curve measurements using vectorial magnetooptic Kerr magnetometry. Comparison of the experimental data with the proposed model shows excellent agreement and reveals the ferromagnetic anisotropy and properties connected to the grain size distribution of the antiferromagnet. Therefore, a distinction between the different influences on coercivity and anisotropy becomes available.

Large exchange bias in polycrystalline MnN/CoFe bilayers at room temperature

We report on the new polycrystalline exchange bias system MnN/CoFe, which shows exchange bias of up to 1800Oe at room temperature with a coercive field around 600Oe. The room temperature values of the interfacial exchange energy and the effective uniaxial anisotropy are estimated to be $J_\mathrm{eff} = 0.41\,\mathrm{mJ}/\mathrm{m}^2$ and $K_\mathrm{eff} = 37\,\mathrm{kJ}\,/\,\mathrm{m}^3$. The thermal stability was found to be tunable by controlling the nitrogen content of the MnN. The maximum blocking temperature exceeds $325^\circ$C, however the median blocking temperature in the limit of thick MnN is $160^\circ$C. Good oxidation stability through self-passivation was observed, enabling the use of MnN in lithographically defined microstructures. As a proof-of-principle we demonstrate a simple GMR stack exchange biased with MnN, which shows clear separation between parallel and antiparallel magnetic states. These properties come along with a surprisingly simple manufacturing process for the MnN films.

Lateral grain size effect on exchange bias in polycrystalline NiFe/FeMn bilayer films

This study explores the effect of lateral grain size on the exchange bias in the magnetron sputtered NiFe(5nm)/FeMn(20nm) bilayer films. The thicknesses of ferromagnetic (NiFe) and antiferromagntic (FeMn) layers were kept constant in the study. The lateral grain size variation was induced by increasing Ta buffer layer thickness. The cross-sectional transmission electron microscopy revealed that bilayers deposited on thicker Ta buffer layers have larger lateral grain diameter. Grain-to-grain epitaxy from buffer layer controls lateral grain size at NiFe/FeMn interface. A large increase in exchange bias field was observed with increasing thickness of Ta buffer layer, which is attributed to the enhanced ferromagnetic/antiferromagnetic coupling originated from larger interface lateral grain area.

Effect of Mn Interface Doping in Polycrystalline Exchange Bias Thin Films

We report on the effect of 0-5 monoatomic layers of Mn deposited at the interface of a CoFe/IrMn polycrystalline thin film. All samples were produced via sputtering. Thermal activation measurements were carried out using the well-established York Protocols. In conjunction with grain size analysis performed on each sample the value of the antiferromagnet (AF) anisotropy KAF was found. An increase in exchange bias Hex was found in the case of 1-2 atomic layers, however a sharp decrease was seen with the addition of further layers. There was no change in the median grain diameter DM , or in the median blocking temperature (TB ), and consequently KAF, with the addition of Mn. However in the samples with the Mn interfacial layers Hex was found to increase by up to 100 Oe when the setting field Hset, was varied from 5 to 20 kOe.

Thermal exchange bias field drifts after 10 keV He ion bombardment: Storage temperature dependence and initial number of coupling sites

Sputter deposited Mn83Ir17(30 nm)/Co70Fe30(10 nm)/Ta thin films have been investigated for their thermal exchange bias field drift at different storage temperatures after 10 keV He+ ion bombardment in an externally applied in-plane magnetic field. It is experimentally shown that the drift coefficient in an intermediate time interval, as given in a recently developed model, is proportional to T and proportional to the initial number of coupling sites in the polycrystalline exchange bias layer system used.

Influence of magnetocrystalline anisotropy on the magnetization reversal mechanism in exchange bias Co/CoO bilayers

We investigated the reversal mechanism in a Co/CoO exchange bias bilayer with a pronounced magnetocrystalline anisotropy in the ferromagnet. The anisotropy, which is induced by the growth of a highly textured Co layer, imposes a distinct reversal mechanism along the magnetically easy and hard direction. It is shown that exchange bias can be induced along both directions, despite the magnetocrystalline anisotropy. The interplay between the magnetocrystalline anisotropy and exchange bias induces a different reversal mechanism for the subsequent reversals in the two crystallographic directions. Along the hard axis, the magnetization reverses according to the reversal mechanism observed before in polycrystalline exchange bias bilayers, i.e. domain wall nucleation and motion for the first reversal and coherent rotation for the subsequent ones. Along the easy axis, domain wall motion remains the dominant reversal mechanism and magnetization rotation has only a minor contribution.

Experimental evidence for exchange bias in polycrystalline BiFeO3/Ni81Fe19 thin films

We report on experimental evidence of exchange bias between a polycrystalline antiferromagnetic BiFeO3 and a ferromagnetic Ni81Fe19 film at room temperature. The measured 17 Oe hysteresis loop shift corresponds to an exchange energy of 11×10-3 erg/cm2 per unit area of the interface coupling the two films, which is comparable with those observed for similar epitaxially-grown systems. The azimuthal behavior of the longitudinal and transverse magnetization components revealed the presence of induced unidirectional and biquadratic anisotropies. A misalignment between unidirectional and biquadratic anisotropy axes was also observed.

Interfacial and bulk order in polycrystalline exchange bias systems

A study of the effect of the setting conditions on the two coercive fields HC1 and HC2 in an exchange bias system is presented. The study was performed on a polycrystalline thin film sample of composition Si/Cu(5 nm)/IrMn(10 nm)/CoFe(2 nm)/Ta(5 nm) with a median blocking temperature ⟨TB⟩=240 K. An experiment analogous to a thermoremanence experiment, where order is quenched in by field cooling, is compared to a setting field experiment at constant temperature. These processes can be used to probe the order of the interfacial spin clusters and the bulk of the antiferromagnet. HC1 and HC2 follow different setting temperature dependences, indicating that different mechanisms are dominant in the two branches of the loop.

Asymmetric recovery effect of exchange bias in polycrystalline NiFe/FeMn bilayers

For exchange bias in polycrystalline NiFe/FeMn bilayers, the hysteretic behavior of the angular dependence and the recovery effect has been studied. In particular, the pinning direction (PD) at the ending remanent state of each hysteresis loop is identified. In the hysteretic behavior, in addition to the coercivity, the PD also demonstrates different angular dependence between clockwise and counterclockwise rotations of the external magnetic field. Measurements of the recovery effect consist of two major steps. In the first step, the PD is deviated from the initial one by using its hysteretic effect and training effect. For polycrystalline NiFe/FeMn bilayers, the rotated PD is located at the maximal angle θPD0 of ±22° with respect to the initial ones. As for the second step, an external magnetic field is applied at a specific orientation θH−RE and then switched off at the same orientation. For the negative θPD0, the recovery effect only occurs for 0<θH−RE<180° with the maximal effect at θH−RE=90° and vanishes for 180°<θH−RE<360°, and vice versa for the positive θPD0. Therefore, the recovery effect shows an asymmetric angular dependence on θH−RE. The recovery effect of the PD also depends on the magnitude and the application time of the recovery magnetic field. For the exchange field and the coercivity, similar recovery behaviors are observed and attributed to the recovery effect of the PD. These phenomena clearly show that the motion of antiferromagnet spins not only obeys the thermally activated transition but also strongly depends on the magnetization reversal mechanism of the ferromagnet layer.

Probing the exchange bias in Co/CoO nanoscale antidot arrays using anisotropic magnetoresistance

The dependence of exchange bias in polycrystalline Co/CoO nanoscale antidot arrays on temperature and Co layer thickness ${t}_{\text{Co}}$ has been systematically probed using the anisotropic magnetoresistance technique. Our experimental results reveal a relatively small degree of asymmetry in the magnetization reversal process of the antidot arrays as compared to a continuous film of identical composition, attributable to the configurational anisotropy of the antidot arrays and the competition between interfacial ferromagnetic-antiferromagnetic (FM-AFM) exchange anisotropy and FM uniaxial anisotropy. The strong interplay between thermal activation effects and AFM domain size confinement in the antidot arrays results in the exchange bias field ${H}_{E}$ being either smaller or larger than the continuous film depending on the temperature. Furthermore, with increasing ${t}_{\text{Co}}$, the asymmetry in the magnetization reversal of the antidot arrays increases monotonously due to enhancement in the FM anisotropy. This enhancement is accompanied by a reduction in the magnitudes of ${H}_{E}$ and coercive field ${H}_{C}$ with increasing ${t}_{\text{Co}}$ at all temperatures.

Magnetic force microscopy study of the training effect in polycrystalline Co/CoO bilayers

The training effect in polycrystalline exchange bias Co/CoO bilayers has been investigated with low-temperature magnetic force microscopy (MFM). After field cooling the bilayer to 13 K, no MFM contrast was detected but a clear MFM contrast related to the domain structure appears after the first magnetization reversal of the ferromagnetic Co layer. Once formed, the ferromagnetic domains survive even at very large fields and cannot be erased by the application of a magnetic field along the cooling field direction. On the other hand, it is possible to partially remove the magnetic domains by performing a hysteresis loop with a magnetic field perpendicular to the cooling field direction.

Origin of the Training Effect and Asymmetry of the Magnetization in Polycrystalline Exchange Bias Systems

The training effect and asymmetry in exchange-coupled polycrystalline CoO/Co bilayers with in-plane magnetization has been investigated. This system is selected for its large training effect and initial asymmetry of the magnetic hysteresis after field cooling, which is removed after training. Applying an in-plane magnetic field perpendicular to the cooling field largely restores the untrained state with its pronounced asymmetry. The possibility to reinduce the asymmetry strongly depends on the magnitude of the perpendicular field, providing the key to identify the physical origin of training and removal of the asymmetry. These effects result from misalignment between the ferromagnetic magnetization and the uncompensated magnetization of the granular antiferromagnet.

Multiple antiferromagnet/ferromagnet interfaces as a probe of grain-size-dependent exchange bias in polycrystalline Co/Fe 50Mn 50

We have used ferromagnet/antiferromagnet/ferromagnet trilayers and ferromagnet/antiferromagnet multilayers to probe the grain size dependence of exchange bias in polycrystalline Co/Fe 50Mn 50. X-ray diffraction and transmission electron microscopy show that the Fe 50Mn 50 (FeMn) grain size increases with increasing FeMn thickness in the Co (30 Å)/FeMn system. Hence, in Co(30 Å)/FeMn( t AF Å)/Co(30 Å) trilayers the two Co layers sample different FeMn grain sizes at the two antiferromagnet/ferromagnet interfaces. For FeMn thicknesses above 100 Å, where simple bilayers have a thickness-independent exchange bias, we are therefore able to deduce the influence of FeMn grain size on the exchange bias and coercivity (and their temperature dependence) simply by measuring trilayer and multilayer samples with varying FeMn thicknesses. This can be done while maintaining the (1 1 1) orientation, and with little variation in interface roughness. Increasing the average grain size from 90 to 135 Å results in a fourfold decrease in exchange bias, following an inverse grain size dependence. We interpret the results as being due to a decrease in uncompensated spin density with increasing antiferromagnet grain size, further evidence for the importance of defect-generated uncompensated spins.

All-optical probe of magnetization dynamics in exchange biased bilayers on the picosecond timescale

All-optical control of the magnetization of polycrystalline exchange bias bilayer systems is achieved using short picosecond laser pulses. Due to the photoexcitation, the spin temperature across the interface between the ferromagnetic and antiferromagnetic layer is elevated, resulting in a collapse of the interfacial exchange coupling. Thus, within the first 10 ps, a fast reduction of both the exchange bias field and the coercive field is observed for three different exchange bias systems comprising both different ferromagnets and antiferromagnets. The fast thermal unpinning is followed by a slower heat diffusion dominated relaxation process, which strongly depends on the thermal conductivity of the used buffer layers and substrates. The fast optical unpinning can be understood in terms of an internal anisotropy pulse field capable of triggering ultrafast precessional magnetization dynamics of the ferromagnetic layer, which makes heat-assisted coherent magnetization rotation feasible.

Micromagnetic calculations of bias field and coercivity of compensated ferromagnetic antiferromagnetic bilayers

Exchange bias in polycrystalline IrMn/NiFe was found at perfectly compensated interfaces. The energy associated with unidirectional anisotropy is stored in lateral domain walls in the antiferromagnet. In addition to exchange bias, this mechanism leads to a training effect. The bias field shows a maximum of μ0Hb=4 mT at an antiferromagnetic layer thickness of 22 nm. The coercivities are on the order of μ0Hc=10 mT. The coercive field increases with decreasing intergrain exchange interactions within the ferromagnet.