The volatile and nonvolatile magnetization switching of CoNi thin films manipulated by electric-field

Multiferroic and magnetoelectric materials have attracted significant scientific interest due to their potential application in magnetic information storage devices and magnetic sensors . In those materials, magnetization state could be controlled by using electric-field (E-field), which paves a new way for novel memory devices . In this work, we observe a giant reversible magnetization and magnetic anisotropy reorientation in a magnetoelectric heterostructure, consisting of polycrystalline CoNi thin film and (011)-oriented Pb(Mg_1/3Nb_2/3)O₃-xPbTiO₃(PMN-xPT) single crystal . Taking the different piezoelectric response of PMN-PT with different PbTiO₃ content, volatile and nonvolatile magnetization switching have been demonstrated in CoNi/PMN-PT heterostructures . The CoNi films were deposited on single crystal PMN-PT substrate by a magnetron sputtering system at room temperature . The composition of CoNi layer was optimized to Co₂₃Ni₇₇, which shows a large magnetostriction coefficient and good soft magnetic properties . In this work, two kind of PMN-PT substrate with 30% PbTiO₃ (here after denoted PMN-30%PT) and 32% PbTiO₃ (PMN-32%PT) were used . The magnetic properties of the sample under different E-fields were measured by using a vibrating sample magnetometer (VSM) . For CoNi/PMN-30%PT heterostructure, the as-deposited film shows almost in-plane isotropic magnetic properties, as shown in figure 1(a) . With increasing E-field, due to the E-field induced compressive strain along x[100] direction and negative magnetostriction coefficient of CoNi, the y[0-11] direction becomes more and more harder to magnetize . Under a 10kV/cm E-field, the saturation field along y[0-11] direction reaches up to 350Oe and the normalized remanence (M_r/M_s) decreases to 1 .7% . The continuous Mr/Ms vs E-fields loop (figure 1(c)) shows a typical butterfly-shape behavior,- indicating that the anisotropy change is almost volatile, which is consistent with the strain vs E-field (figure 1(d)) . For PMN-32%PT substrate, as shown in figure 2(a), the strain response along [0-11] direction is neglectable, while along [100] direction it shows large compressive strain under large E-field . When the E-field returns from 10kV/cm to zero, a large remnant strain (-2200ppm) is obtained, suggesting the E-filed induced strain is nonvolatile . As a result, the corresponding magnetic anisotropy change and magnetization switching in CoNi/PMN-32%PT structure, as shown in figure 2(b,c,d), is also nonvolatile . A minor loop shown in figure 2(d) suggests that when the E-field return from different polarization states (5kV/cm and -2kV/cm) back to zero, two different M_r/M_s values (0 .35 and 0 .73) are obtained . Based on the nonvolatile magnetization switching in CoNi/ PMN32%PT structure, we demonstrate two stable magnetization states manipulated by alternately E-field pulse . In summary, the giant E-field induced volatile and nonvolatile magnetization switching has been observed in CoNi/PMN-PT heterostructure with 30% and 32% PbTiO₃, respectively . The nonvolatile magnetization switching opens a new way for designing novel memory devices .