Ferromagnetic/antiferromagnetic exchange coupling in SrFe<Subscript>12</Subscript>O<Subscript>19</Subscript>/CoO composites

The surface of SrFe₁₂O₁₉ coated with a CoO layer reveals a strong exchange bias characterized by magnetic hysteresis loops. The low-temperature coercivity, H_C, and the squareness, M_R/M_S, of a permanent magnet of SrFe₁₂O₁₉/CoO powder prepared by the sol–gel method are enhanced after field cooling through the Néel temperature (T_N=290 K) when compared to those after zero-field cooling. The existence of loop shifts and the enhancement of H_C indicate that exchange-bias effects, which are induced by the ferromagnetic/antiferromagnetic (FM/AFM) exchange-coupling interactions, are responsible for these behaviors. According to our experimental results, some of the factors controlling the exchange bias, such as FM/AFM interfaces and the CoO amount of the antiferromagnetic layer, are discussed. PACS 75.30.Et; 75.50.Ee; 75.60.Ej; 75.60.Gm; 75.70.Cn     

Shape-induced oscillation of exchange bias in the Co nanopillar with lateral oxidation

A modified Monte Carlo Metropolis method is performed to simulate the hysteresis loops of a laterally oxidized Co nanopillar with an excised arc or an inclined circular plane. At low temperature after field cooling, an oscillatory exchange bias is observed with excised length or angle of inclination. However, the behaviors of coercivity exhibit a monotonous decrease with excision but an oscillation with tilt. The phenomena are intriguing and corroborated by means of the microscopic spin configurations and the magnetization reversal mechanisms. The occurrence of oscillatory exchange bias in such a Co/CoO composite nanopillar with direct exchange couplings is mainly because of the shape inducement in the nanopillar, which results in the energy fluctuations of interfacial coupling and the nonuniformity of domain distribution.     

The synthesis and exchange bias effect of monodisperse NiO nanocrystals

Monodisperse NiO nanocrystals with an average particle size of 3 ± 0.4 nm are successfully synthesized by the thermal decomposition of Ni-oleylamine complex in an organic solvent under a continuous O2 flux. The crystalline structure and the morphology of the product are investigated by X-ray diffraction, X-ray photoelectron spectroscopy, and transmission electron microscopy. Magnetization and alternating-current (ac) susceptibility measurements indicate that the structure of the particles can be considered as consisting of an antiferromagnetically ordered core and a spin-glass-like surface shell. In addition, both the exchange bias field and the vertical magnetization shift can be observed in this system at 10 K after field cooling. This observed exchange bias effect is explained in terms of the exchange interaction between the antiferromagnetic core and the spin-glass-like shell.     

Revealing the origin of the vertical hysteresis loop shifts in an exchange biased Co/YMnO(3) bilayer

We have investigated exchange bias effects in bilayers composed of the antiferromagnetic o-YMnO(3) and ferromagnetic Co thin film by means of SQUID magnetometry, magnetoresistance, anisotropic magnetoresistance and the planar Hall effect. The magnetization and magneto-transport properties show pronounced asymmetries in the field and magnetization axes of the field hysteresis loops. Both exchange bias parameters, the exchange bias field H(E)(T) as well as the magnetization shift M(E)(T), vanish around the Néel temperature T(N)???45?K. We show that the magnetization shift M(E)(T) is also measured by a shift in the anisotropic magnetoresistance and planar Hall resistance having a similar temperature dependence as the one obtained from magnetization measurements. Because the o-YMnO(3) film is highly insulating, our results demonstrate that the M(E)(T) shift originates at the interface within the ferromagnetic Co layer. To show that the main results obtained are general and not because of some special characteristics of the o-YMO(3) layer, similar measurements were done in Co/CoO micro-wires. The transport and magnetization characterization of the micro-wires supports the main conclusion that these effects are related to the response of the ferromagnetic Co layer at the interface.     

Reversing the training effect in exchange biased CoO/Co bilayers

We performed a detailed study of the training effect in exchange biased CoO/Co bilayers. High-resolution measurements of the anisotropic magnetoresistance (AMR) display an asymmetry in the first magnetization reversal process and training in the subsequent reversal processes. Surprisingly, the AMR measurements as well as magnetization measurements reveal that it is possible to partially reinduce the untrained state by performing a hysteresis measurement with an in-plane external field perpendicular to the cooling field. Indeed, the next hysteresis loop obtained in a field parallel to the cooling field resembles the initial asymmetric hysteresis loop, but with a reduced amount of spin rotation occurring at the first coercive field. This implies that the antiferromagnetic domains, which are created during the first reversal after cooling, can be partially erased.     

Temperature dependence of exchange bias and coercivity in ferromagnetic/antiferromagnetic bilayers

A model for the temperature dependence of exchange bias and coercivity in epitaxial ferromagnetic (FM)/ antiferromagnetic (AFM) bilayers is developed. In this model, the interface coupling includes two contributions, the direct coupling and the spin-flop coupling. The temperature dependence arises from the thermal disturbance to the system, involved in the thermal fluctuations of magnetization of AFM grains and the temperature modulation of the relevant magnetic parameters. In addition, the randomness of original orientations of easy axes of AFM grains after field cooling is taken into account. A self-consistent calculation scheme is proposed and numerical treatment is carried out. The results show that the temperature dependence of exchange bias and coercivity is closely related to the sizes of AFM grains and the interface exchange coupling constants. Especially, the exchange bias will have a peak and the blocking temperature will increase if the spin-flop coupling plays a role. On the other hand, the original orientation distribution of easy axes of AFM grains will affect exchange bias and coercivity prominently. The prediction has been well supported by experiments.Received: 12 May 2004, Published online: 31 August 2004PACS: 75.30.Et Exchange and superexchange interactions - 75.50.Ee Antiferromagnetics - 75.30.Gw Magnetic anisotropy     

Exchange bias with Fe substitution in LaMnO<Subscript>3</Subscript>

We observe the negative shift of the magnetic hysteresis loop at 5 K, while the sample is cooled in external magnetic field in case of 30% of Fe substitution in LaMnO₃. The negative shift and training effect of the hysteresis loops indicate the phenomenon of exchange bias. The cooling field dependence of the negative shift increases with the cooling field below 7.0 kOe and then, decreases with further increase of cooling field. The temperature dependence of the negative shift of the hysteresis loops exhibits that the negative shift decreases sharply with increasing temperature and vanishes above 20 K. Temperature dependence of dc magnetization and ac susceptibility measurements show a sharp peak (T_p) at 51 K and a shoulder (T_f) around 20 K. The relaxation of magnetization shows the ferromagnetic and glassy magnetic components in the relaxation process, which is in consistent with the cluster-glass compound.     

Enhanced asymmetric magnetization reversal in nanoscale Co/CoO arrays: competition between exchange bias and magnetostatic coupling

Magnetization reversal was studied in square arrays of square Co/CoO dots with lateral size varying between 200 and 900 nm. While reference nonpatterned Co/CoO films show the typical shift and increased width of the hysteresis loop due to exchange bias, the patterned samples reveal a pronounced size dependence. In particular, an anomaly appears in the upper branch of the magnetization cycle and becomes stronger as the dot size decreases. This anomaly, which is absent at room temperature in the patterned samples, can be understood in terms of a competition between magnetostatic interdot interaction and exchange anisotropy during the magnetic switching process.     

Asymmetric magnetization reversal on exchange biased CoO/Co bilayers

We study magnetic hysteresis loops after field cooling of a CoO/Co bilayer by MOKE and polarized neutron reflectivity. The neutron scattering reveals that the first magnetization reversal after field cooling is dominated by domain wall movement, whereas all subsequent reversals proceed essentially by rotation of the magnetization. In addition, off-specular diffuse scattering indicates that the first magnetization reversal induces an irreversible change of the domain state in the antiferromagnet.     

The influence of finite size and shape anisotropy on exchange bias: A study of patterned Co/CoO nanostructures

The influence of finite dimensions on the exchange bias effect in patterned polycrystalline Co/CoO ferromagnet/antiferromagnet exchange bias systems was studied. Magnetization measurements on the smallest structures reveal that the exchange bias shift increases as the structure size becomes smaller. Off-specular neutron scattering experiments were used to study the asymmetric magnetization reversal behaviour.     

Magnetization reversal in exchange biased Co/CoO patterns

The influence of patterning on exchange bias has been investigated using arrays of micron-sized Co/CoO dots with different lateral confinement and length-to-width ratio. The patterned samples show higher coercive and exchange bias fields than a continuous Co/CoO bilayer. As in unpatterned film, magnetization reversal mechanisms on opposite sides of the hysteresis loops of the microstructured samples are different. However, with the increase of lateral confinement and shape anisotropy of the dots, the asymmetry in the magnetization reversal starts to differ from that observed in continuous Co/CoO films.     

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.     

Unified nonequilibrium dynamical theory for exchange bias and training effects

We have investigated the exchange bias and training effect in the ferromagnetic/antiferromagnetic (FM/AF) heterostructures using a unified Monte Carlo dynamical approach. The magnetization of the uncompensated AF layer is still open after the first field cycling is finished. Our simulated results show obvious shift of hysteresis loops (exchange bias) and cycling dependence of exchange bias (training effect) when the temperature is below 45~K. The exchange bias field decreases with decreasing cooling rate or increasing temperature and the number of the field cycling. Essentially, these two effects can be explained on the basis of the microscopical coexistence of both reversible and irreversible moment reversals of the AF domains. Our simulations are useful to understand the real magnetization dynamics of such magnetic heterostructures.     

Asymmetric magnetization reversal in exchange-biased hysteresis loops

Polarized neutron reflectometry is used to probe the in-plane projection of the net-magnetization vector M--> of polycrystalline Fe films exchange coupled to twinned (110) MnF (2) or FeF (2) antiferromagnetic (AF) layers. The magnetization reversal mechanism depends upon the orientation of the cooling field with respect to the twinned microstructure of the AF, and whether the applied field is increased to (or decreased from) a positive saturating field; i.e. , the magnetization reversal is asymmetric. The reversal of the sample magnetization from one saturated state to the other occurs via either domain wall motion or magnetization rotation on opposite sides of the same hysteresis loop.     

Growth of CuPd nanoalloys encapsulated in carbon-shell

Phase characteristics of the partially oxidized cobalt particles were investigated. It is shown that the studied system consists of Co, CoO, and Co₃O₄ phases. The paper presents results for the magnetic characteristics of Co particles covered by a CoO shell. Magnetic measurements were performed at 77 K. It is shown that the shapes of the hysteresis loop of the investigated system are different for different cases of cooling, with and without applications of external magnetic field. Structural investigations were performed on single particles.     

Finite-sized Heisenberg chains and magnetism of one-dimensional metal systems

We present a combined experimental and theoretical study of the magnetization of one-dimensional atomic cobalt chains deposited on a platinum surface. We discuss the intrinsic magnetization parameters derived by X-ray magnetic circular dichroism measurements and the observation of ferromagnetic order in one dimension in connection with the presence of strong, dimensionality-dependent anisotropy energy barriers of magnetocrystalline origin. An explicit transfer matrix formalism is developed to treat atomic chains of finite length within the anisotropic Heisenberg model. This model allows us to fit the experimental magnetization curves of cobalt monatomic chains, measured parallel to the easy and hard axes, and provides values of the exchange coupling parameter and the magnetic anisotropy energy consistent with those reported in the literature. The analysis of the spin–spin correlation as a function of temperature provides further insight into the tendency to magnetic order in finite-sized one-dimensional systems. PACS 57.10.Pq; 75.30.-m; 75.30.Gw; 78.70.Dm     

Field cooling-induced magnetic anisotropy in exchange biased CoO/Fe bilayer studied by ferromagneticresonance

Exchange-biased CoO/Fe bilayer grown on MgO (0 0 1) substrate by sputtering, studied by variable angle and temperature ferromagnetic resonance. Room temperature in-plane measurements reveal that the Fe layer was epitaxially grown on MgO substrate with a fourfold cubic symmetry. The data also show that the easy axis of magnetization is in the film plane and makes an angle of 45° with the [1 0 0] crystallographic direction of MgO substrate. The low temperature data exhibit a sudden onset of a field cooling-induced and shifted cubic anisotropy below the Néel temperature of CoO. This results in a twofold uniaxial or fourfold cubic symmetry for in-plane magnetic anisotropy depending on a field cooling direction. Low temperature measurements also present a reduction in the resonance fields due to the antiferromagnetic/ferromagnetic coupling. The developed theoretical model perfectly simulates the experimental data of coupled CoO/Fe bilayer.     

Interface induced exchange bias effect in La0.67Sr0.33MnO3/SrIrO3 multilayer

Epitaxial superlattices of ferromagnetic/paramagnetic La_0.67Sr_0.33MnO₃/SrIrO₃ materials have been prepared on SrTiO₃ (100) substrate using pulse laser deposition technique. An unexpected onset of interface magnetic interaction has been observed around 40 K. Interestingly, magnetic exchange bias effect has been observed in both field cooled and zero field cooled magnetization loops, however, the shifting of loop is opposite in both measurements. Exchange bias field vanishes as temperature increases to interface magnetic ordering temperature. Moreover, exchange bias field is found to decrease with increasing cooling field. We believe that tuning of magnetic exchange at interface during field cooling induces this evolution in nature of exchange bias field.     

Structural and magnetic properties of ion-beam deposited NiFe/Co-oxide bilayers

The structural and magnetic properties of NiFe/Co-oxide bilayers were studied. XRD investigation indicates the NiFe (Permalloy) layers (a= 3.53 Å) grow with a (111) preferred orientation. The Co-oxide layers were fabricated with oxygen content in the deposition assist beam ranging from 8% (rock-salt CoO, a= 4.27 Å) to 34% (spinel Co3O4, a= 8.21 Å). Both the coercivity Hc and exchange bias field Hex closely follow an inverse NiFe thickness relationship. A strong temperature dependence of Hc and Hex is found in these NiFe/Co-oxide bilayers. At T= 289 K, the NiFe/CoO film exhibits an enhanced Hc relative to pure NiFe/Co or NiFe/Co3O4 bilayers, an indication of exchange coupling between the NiFe and CoO phases. At T= 10 K, the NiFe/Co3O4 film exhibits an exchange-bias loop shift of Hex∼- 200 Oe that persists to temperatures greater than 30 K (the Néel temperature of bulk Co3O4). The transition temperature of the Co3O4 film component has increased above the bulk value via exchange coupling with the permalloy. Results indicate that the exchange coupling interaction between FM and AFM layers are responsible for both enhanced coercivity and cross-tie domains.     

Size-dependent exchange bias in single phase Mn_3O_4 nanoparticles

Glassy magnetic behavior and exchange bias phenomena are observed in single phase Mn_3O_4 nanoparticles.Dynamics scaling analysis of the ac susceptibility and the Henkel plot indicate that the observed glassy behavior at low temperature can be understood by taking into account the intrinsic behavior of the individual particles consisting of a ferrimagnetic(FIM) core and a spin-glass surface layer.Field-cooled magnetization hysteresis loops display both horizontal and vertical shifts.Dependence of the exchange bias field(H_E) on the cooling field shows an almost undamped feature up to 70 kOe,indicating the stable exchange bias state in Mn_3O_4.H_E increases as the particle size decreases due to the higher surface/volume ratio.The occurrence of the exchange bias can be attributed to the pinning effect of the frozen spin-glass surface layer upon the FIM core.