Strain Control of Phase Transition and Exchange Bias in Flexible Heusler Alloy Thin Films

Full electrical manipulation of perpendicular exchange bias in ultrathin antiferromagnetic film with epitaxial strain

Achieving effective manipulation of perpendicular exchange bias effect remains an intricate endeavor, yet it stands a significance for the evolution of ultra-high capacity and energy-efficient magnetic memory and logic devices. A persistent impediment to its practical applications is the reliance on external magnetic fields during the current-induced switching of exchange bias in perpendicularly magnetized structures. This study elucidates the achievement of a full electrical manipulation of the perpendicular exchange bias in the multilayers with an ultrathin antiferromagnetic layer. Owing to the anisotropic epitaxial strain in the 2-nm-thick IrMn3 layer, the considerable exchange bias effect is clearly achieved at room temperature. Concomitantly, a specific global uncompensated magnetization manifests in the IrMn3 layer, facilitating the switching of the irreversible portion of the uncompensated magnetization. Consequently, the perpendicular exchange bias can be manipulated by only applying pulsed current, notably independent of the presence of any external magnetic fields.

Flexomagnetic Effect Enhanced Ferromagnetism and Magnetoelectrochemistry in Freestanding High-Entropy Alloy Films

Freestanding thin films of functional materials enable the tuning of properties via strain and strain gradients, broadening their applications. Here, a systematic approach is proposed to fabricate freestanding CrMnFeCoNi high-entropy alloy (HEA) thin films by pulsed laser deposition using expansion-contraction of NaCl substrates and weak van der Waals interaction of the interface, which form wrinkles with inhomogeneous strain gradients when transferred to a viscous handle. We demonstrate that the nonuniform gradients of external strain (flexomagnetic effect) can induce the partial structural phase transition from FCC to BCC in the wrinkled HEA film, resulting in a 10-fold increase in its room-temperature saturation magnetization compared with the unstrained flat HEA film. Furthermore, after applying an external magnetic field, due to the different electron transfer behavior caused by the electron scattering in wrinkled and flat HEA films, their electrocatalytic magnetic responses showed a diametrically opposite picture. Our work provides a promising strategy for tuning physical and chemical properties via complex strained geometries.

Ni-Mn-Sn-Cu Alloys after Thermal Cycling: Thermal and Magnetic Response

Heusler Ni-Mn-Sn-based alloys are good candidates for magnetic refrigeration. This application is based on cycling processes. In this work, thermal cycles (100) have been performed in three ribbons produced by melt-spinning to check the thermal stability and the magnetic response. After cycling, the temperatures were slowly shifted and the thermodynamic properties were reduced, the entropy changed at about 3–5%. Likewise, the thermomagnetic response remains similar. Thus, these candidates maintain enough thermal stability and magnetic response after cycling. Likewise, Cu addition shifts the structural transformation to higher temperatures, whereas the Curie temperature is always near 310 K. Regarding magnetic shape memory applications, the best candidate is the Ni₄₉Mn₃₆ Sn₁₄Cu₁ alloy.

Optimization of spontaneous exchange bias in Mn-rich Heusler alloys

At low temperature, spontaneous (zero-field-cooled, SEB) and traditional (field-cooled) exchange bias effects may be induced in a series of NiMn-based Heusler alloys, and the exchange bias is commonly sensitive to alloying elements and compositions, while the mechanisms especially for SEB are still elusive. Therefore, the SEB in Mn-rich Heusler alloys with coexistence of ferromagnetic and antiferromagnetic exchange interactions is numerically studied by performing a modified Monte Carlo simulation. The intrinsic magnetocrystalline anisotropies (K_AF), exchange interactions (J_FM-AF and J_AF-AF), and occupation probabilities (x_FM) are directly tuned to establish their dependencies of zero-field-cooled/field-cooled thermomagnetic curves and zero-field-cooled magnetization hysteresis loops. The results indicate that the freezing temperature is monotonically enhanced with increasing K_AF and varies nonmonotonically with other parameters, and at 5 K, the irreversibility arising from antiferromagnetic components becomes high enough to trigger SEB even though no spin glass state exists. The SEB is nonmonotonic with K_AF, J_FM-AF, J_AF-AF, and x_FM, and its maximum value will be obtained at K_AF = 4.5 × 10⁶ J m^-3, J_FM-AF = 5 meV, J_AF-AF = -5 meV, or x_FM = 0.3. On the contrary, the coercivity is also nonmonotonic with K_AF and J_FM-AF while monotonic with J_AF-AF and x_FM. The values of the SEB field are nearly one order of magnitude smaller than those of coercivity, consistent with experimental data. The magnetic relaxation properties are calculated to propose two factors, i.e., ferromagnetic-like domain between ferromagnetic and antiferromagnetic components and decay rate, to determine the final SEB. This work demonstrates the mechanisms to optimize SEB in Mn-rich Heusler alloys, and physically the results obtained are also suitable for other material systems with spontaneous ferromagnet/antiferromagnet phase separations.