Optically Readable Multilevel Magnetic Memory States in Perpendicularly Exchange-Biased Ferromagnetic Multilayers

Abstract

The construction of multilevel magnetic states using materials with perpendicular magnetic anisotropy (PMA) offers a novel approach to enhancing the storage density and read/write efficiency of nonvolatile magnetic memory devices. In this study, optically readable multilevel magnetic domain states are achieved by inducing asymmetric interlayer interactions and decoupling the magnetic reversal behavior of individual ferromagnetic (FM) layers in exchange-biased FM multilayers with PMA. Hepta-level magnetic domain states are formed in [Co/Pt]_n FM multilayers grown on an antiferromagnetic Fe₂O₃ layer within a relatively low magnetic field range of ∼±400 Oe. Raising and lowering operations between states are demonstrated to be achievable, enabling the writing of new information without the need for initialization in multilevel magnetic memory applications. This design concept, leveraging multilevel magnetic domain states and facilitating noncontact optical reading of stored information, demonstrates the potential to enhance the storage density of nonvolatile magnetic memory devices as it eliminates the need for electrical circuits typically required in other resistive memory technologies.

Keywords: exchange‐biased ferromagnetic multilayers; magnetic states; memory devices; perpendicular magnetic anisotropy materials.

Figure 1

a) Schematics comparing the AHE and MOKE magnetic hysteresis loops of two SAF‐like FM layers with symmetric interlayer interaction and two exchange‐biased FM layers with asymmetric interlayer interaction. b) MOKE magnetic hysteresis loops of a single FM [Co/Pt]₁ layer and two FM [Co/Pt]₂ layers on Al₂O₃ and Si substrates. c) MOKE magnetic hysteresis loops of a single FM [Co/Pt]₁ layer and two FM [Co/Pt]₂ layers on AF Fe₂O₃ layers. d) MOKE magnetic domain images of a single FM [Co/Pt]₁ layer and two FM [Co/Pt]₂ layers on Al₂O₃ and Si substrates. e) MOKE magnetic domain images of a single FM [Co/Pt]₁ layer and g) two FM [Co/Pt]₂ layers on AF Fe₂O₃ layers. The size of the magnetic domain images is 405 × 340 µm².

Figure 2

a) MOKE magnetic hysteresis loops of the exchange‐biased single FM layer configured as Fe₂O₃/Pt(t_i )/[Co(0.7)/Pt(2.8)]₁ with varying insertion layer thicknesses(t_i ). b) H_ex and H_C values versus t_i in Fe₂O₃/Pt(t_i )/[Co(0.7)/Pt(2.8)]₁. c) MOKE magnetic hysteresis loops of the exchange‐biased double FM layers configured as Fe₂O₃/Pt(0.7)/[Co(0.7)/Pt(t_s )]₂ with varying spacer layer thicknesses(t_s ). d) H_ex and H_C values versus t_i in Fe₂O₃/Pt(0.7)/[Co(0.7)/Pt(t_s )]₂.

Figure 3

a–d) MOKE magnetic hysteresis loops of exchange‐biased FM multilayers configured as Fe₂O₃/Pt(0.7)/[Co(0.7)/Pt(2.8)] _n , with n values ranging from three to six. e) Schematic illustrations depicting magnetic reversal sequences in exchange‐biased FM multilayers with n values of up to six. Here, FM layers (F1 to F6) are numbered sequentially from the bottom layer, closest to the AF layer.

Figure 4

Anomalous Hall resistance and MOKE magnetic hysteresis loops of exchange‐biased FM multilayers configured as Fe₂O₃/Pt(0.7)/[Co(0.7)/Pt(2.8)] _n with n = 1−4. The inset displays a Hall‐bar‐patterned device fabricated by depositing FM layers through a metal shadow mask with a line width of 100 µm.

Figure 5

a) Full MOKE hysteresis loop and minor hysteresis loops of the exchange‐biased FM multilayers configured as Fe₂O₃/Pt(0.7)/[Co(0.7)/Pt(2.8)] _n with n = 2 and 3. b) Full MOKE hysteresis loop and minor hysteresis loops of SAF‐structured FM multilayers configured as Al₂O₃/Pt(0.7)/[Co(0.7)/Pt(2.8)] _n with n = 2 and 3. Solid lines represent the full hysteresis loop, while open data points indicate the minor hysteresis loops.

Figure 6

a) Full MOKE hysteresis loop and magnetic domain images of exchange‐biased FM multilayers configured as Fe₂O₃/Pt(0.7)/[Co(0.7)/Pt(3.5)]₄. b–d) Minor hysteresis loops correspond to ±1 level changes, ±2 level changes, and ±3 level changes, respectively, within specific field ranges. Solid lines represent the full hysteresis loop, while open data points indicate the minor hysteresis loops.