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Register a new userPropagation of Exchange Bias in CoFe/FeMn/CoFe Trilayers
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CoFe/FeMn, FeMn/CoFe bilayers and CoFe/FeMn/CoFe trilayers were grown in magnetic field and at room temperature. The exchange bias field $H_{eb}$ depends strongly on the order of depositions and is much higher at CoFe/FeMn than at FeMn/CoFe interfaces. By combining the two bilayer structures into symmetric CoFe/FeMn($t_mathrm{FeMn}$)/CoFe trilayers, $H_{eb}^t$ and $H_{eb}^b$ of the top and bottom CoFe layers, respectively, are both enhanced. Reducing $t_mathrm{FeMn}$ of the trilayers also results in enhancements of both $H_{eb}^b$ and $H_{eb}^t$. These results evidence the propagation of exchange bias between the two CoFe/FeMn and FeMn/CoFe interfaces mediated by the FeMn antiferromagnetic order.
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Antiferromagnetic spintronic devices have the potential to outperform conventional ferromagnetic devices due to their ultrafast dynamics and high data density. A challenge in designing these devices is the control and detection of the orientation of the anti-ferromagnet. One of the most promising ways to achieve this is through the exchange bias effect. This is of particular importance in large scale multigranular devices. Due to the large system sizes, previously, only micromagnetic simulations have been possible, these have an assumed distribution of antiferromagnetic anisotropy directions. Here, we use an atomistic model where the distribution of antiferromagnetic anisotropy directions occurs naturally and the exchange bias occurs due to the intrinsic disorder in the antiferromagnet. We perform large scale simulations, generating realistic values of exchange bias. We find a strong temperature dependance of the exchange bias, which approaches zero at the blocking temperature while the coercivity has a peak at the blocking temeprature due to the superparamagnetic flipping of the antiferromagnet during the hysteresis loop. We find a large discrepancy between the exchange bias predicted from a geometric model of the antiferromagnetic interface indicating the importance of grain edge effects in multigranular exchange biased systems. The grain size dependence shows the expected peak due to a competition between the superparamagnetic nature of small grains and reduction in the statistical imbalance in the number of interfacial spins for larger grain sizes. Our simulations confirm the existence of single antiferromagnetic domains within each grain. The model gives insights into the physical origin of exchange bias and provides a route to developing optimised nanoscale antiferromagnetic spintronic devices.
Using an atomistic spin model, we have simulated spin wave injection and propagation into antiferromagnetic IrMn from an exchange coupled CoFe layer. The spectral characteristics of the exited spin waves have a complex beating behavior arising from the non-collinear nature of the antiferromagnetic order. We find that the frequency response of the system depends strongly on the strength and frequency of oscillating field excitations. We also find that the strength of excited spin waves strongly decays away from the interfacial layer with a frequency dependent attenuation. Our findings suggest that spin waves generated by coupled ferromagnets are too weak to reverse IrMn in their entirety even with resonant excitation of a coupled ferromagnet. However, efficient spin wave injection into the antiferromagnet is possible due to the non-collinear nature of the IrMn spin ordering.
Naturally occurring spin-valve-type magnetoresistance (SVMR), recently observed in Sr2FeMoO6 samples, suggests the possibility of decoupling the maximal resistance from the coercivity of the sample. Here we present the evidence that SVMR can be engineered in specifically designed and fabricated core-shell nanoparticle systems, realized here in terms of soft magnetic Fe3O4 as the core and hard magnetic insulator CoFe2O4 as the shell materials. We show that this provides a magnetically switchable tunnel barrier that controls the magnetoresistance of the system, instead of the magnetic properties of the magnetic grain material, Fe3O4, and thus establishing the feasibility of engineered SVMR structures.
Tailoring Gilbert damping of metallic ferromagnetic thin films is one of the central interests in spintronics applications. Here we report a giant Gilbert damping anisotropy in epitaxial Co$_{50}$Fe$_{50}$ thin film with a maximum-minimum damping ratio of 400 %, determined by broadband spin-torque as well as inductive ferromagnetic resonance. We conclude that the origin of this damping anisotropy is the variation of the spin orbit coupling for different magnetization orientations in the cubic lattice, which is further corroborate from the magnitude of the anisotropic magnetoresistance in Co$_{50}$Fe$_{50}$.
Multiferroic BaMnF$_4$ powder were prepared by hydrothermal method. Hysteretic field dependent magnetization curve at 5 K confirms the weak ferromagnetism aroused from the canted antiferromagnetic spins by magnetoelectric coupling. The blocking temperature of 65 K for exchange bias coincides well with the peak at 65 K in the zero-field cooled temperature-dependent magnetization curve, which has been assigned to the onset temperature of two-dimensional antiferromagnetism. An upturn kink of exchange field and coercivity with decreasing temperature was observed from 40 K to 20 K, which is consistent with the two-dimensional to three-dimensional antiferromagnetic transition at Neel temperature (~26 K). In contrast to the conventional mechanism of magnetization pinned by interfacial exchange coupling in multiphases, the exchange bias in BaMnF$_4$ is argued to be a bulk effect in single phase, due to the magnetization pinned by the polarization through magnetoelectric coupling.
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