Local Writing of Exchange Biased Domains in a Heterostructure of Co/Pd Pinned by Magnetoelectric Chromia

Abstract

wileyonlinelibrary.comto an understanding of EB. In reality, the formation of antiferromagnetic domains, either lateral [9] or through the depth of the film, [10] will reduce the EB field. In addition, atomic level roughness and defects at the interface can have a significant impact on the density of uncompensated surface spins, also reducing the net exchange bias.Here, we demonstrate laser written domains of EB in a heterostructure consisting of a 300 nm film of Cr 2 O 3 (0001) and perpendicular anisotropic Co/Pd multilayers. The spatial and temperature dependence of the EB will constrain the resolution of the domains that can be written. Careful measurements of the temperature dependence reveal a narrow distribution of blocking temperatures and spatially resolved measurements indicate that the spatial variations in EB are minimal. Both are facilitated by the robust roughness insensitive boundary magnetization [11,12] of Cr 2 O 3 , a well-known magnetoelectric (ME) AFM insulator with a bulk Néel temperature of 307 K. The boundary magnetization (BM) is present at the (0001) surface below the Néel temperature, enabled by the symmetry properties of magnetoelectric antiferromagnets [11][12][13] and is coupled to the AFM order parameter. Below the Néel temperature, two equivalent AFM domain configurations (and corresponding boundary magnetizations) [11,12] are possible. The roughness insensitive boundary magnetization fundamentally alters the EB mechanism-the statistical fluctuations seen in the Malozemoff model [14] and the influence of defects do not play a role. Note that in our sample, the very large exchange bias that would be expected in the presence of this robust boundary magnetization is dramatically reduced due to the presence of the 5 A Pd seed layer. We demonstrate local control of the EB using a focused laser beam and show that the ultimate size of these local regions may be manipulated by extrinsic controls, including the focusing optics, the power of the laser and the speed of writing. The process of domain writing involves the systematic control of sample temperature, the magnetization state of the FM layer, and the application of controlled localized heating provided by the focused laser beam. At a fundamental materials level, the unique boundary magnetization of Cr 2 O 3 provides excellent intrinsic control over the exchange bias. We will show that the narrow distribution of blocking temperatures facilitates the ability to write well-defined patterns of exchange bias. Technologically, our work demonstrates the possibility of using heat-assisted magnetic recording (HAMR) to write a state variable by altering the EB, rather than the coercivity, of magnetic memory cells. The increased stability of EB coupledThe writing of micrometerscaled exchange bias domains by local, laser heating of a thinfilm heterostructure consisting of a perpendicular aniso tropic ferromagnetic Co/Pd multilayer and a (0001) oriented film of the mag netoelectric antiferromagnet Cr 2 O 3 (chromia) is reported. Exchange coupling b...

Magnetoelectric Switching Energy of Antiferromagnetic Cr2O3 Used for Spintronic Logic Devices and Memory

“…Notably, the curve in Figure 3 is similar to a magnetic hysteresis loop of an antiferromagnet/ferromagnet exchange‐coupling system with H ex . [ 9–29 ] It should be noted that the hysteresis loop in Figure 1b assumes that the coercivity field is reduced to less than the H ex by inserting a metal layer between Cr 2 O 3 and the ferromagnet. [ 30 ] As previously introduced, the top spin direction of Cr 2 O 3 decides the AFM domain statuses and the H ex sign.…”

Spin Reversal Mechanism of a Perpendicular Exchange Bias System with an Antiferromagnet Incorporating a Parasitic Ferromagnetism

“…Reading can be realized through PEB‐coupled ferromagnetic materials (Co, Co/Pt, Co/Pd, etc. ) or a spin polarized paramagnet . Both theoretical and experimental demonstrations of T N enhancement up to ≈400 K and some recent new discoveries such as colossal magnetoresistance make Cr 2 O 3 ‐based devices suitable for practical applications in the near future.…”

Magnetoelectric manipulation and enhanced operating temperature in antiferromagnetic Cr2O3 thin film

“…Recently, research studies motivated by device applications have also become active. For example, some studies related to the local writing of exchange bias have been exploited 65,71) for storage application; 32) further applications of this field, including the use of the spin Seebeck effect, 112) spin transport, 113) and graphene transistors, [114][115][116][117] were also reported. Further progress is expected for electrically controllable AFM Cr 2 O 3 thin films.…”