Emergence of Multispinterface and Antiferromagnetic Molecular Exchange Bias via Molecular Stacking on a Ferromagnetic Film

Abstract

Heterointerfaces may exhibit unexpected physical properties distinct from intrinsic properties of component materials. In particular, metal–organic interfaces can drive unique interfacial spin moments, which are often called molecular spinterface. Here, van der Waals stacking of molecular layers may lead to variations in the intra/interlayer exchange coupling resulting in multiple ground states, which is highly desired for multifunctional magnetic devices. In this report, the emergence of molecular multispinterface of paramagnetic cobalt‐octaethyl‐porphyrin (CoOEP) layers in a Fe/CoOEP heterostructure is demonstrated through the interfacial layer and a successive antiferromagnetic molecular spin chain. The disentangled interfacial ferromagnetic spins lead to multiple magnetic ground states and behave as additional spin‐dependent scattering centers, as evidenced through the magnetotransport study. In addition, the antiferromagnetic molecule spin chain derives tunable exchange bias, which signifies the dominance of the antiferromagnetic interfacial interaction. Theoretical calculations demonstrate spin configurations of the molecular chain and the antiferromagnetic interfacial coupling through oxygen intermediaries. The development of the molecular multispinterface and controllable exchange bias therein will provide a promising route for the active control of multivalued data processing at the nanoscale.

Encoding Information on the Excited State of a Molecular Spin Chain

“…To encode information in a technologically relevant way, the excited quantum state of a molecular spin chain (MSC) should thus exhibit a magnetic steady state that can be manipulated independently. Noting that a FM can set the spin referential of a MSC, [ 15,30 ] we begin by sandwiching a thin film of antiferromagnetic spin chains formed by Co phthalocyanine (Pc) molecules with S = 1/2 [ 7,29 ] between FM metallic thin films acting as injector/analyzer of the current's spin polarization. Here, the antiferromagnetic molecular layer is the device's active spintronic spacer, rather than an adjunct layer to magnetically pin a FM electrode through the exchange bias effect as is commonplace in spintronic devices.…”

“…This hard unit comprises the bottom FM layer that is coupled across the spinterface to a molecular spin chain through exchange bias. [ 15,30,40–45 ] In the spin chain's ground state, they represent one magnetic unit (thick green arrow in Figure 1d). However, in the experimental conditions of the data of Figure 1c, the AF spin chain is in the electrically excited state (Figure 1b) and can act as a distinct steady‐state magnetic unit (yellow/green arrow and box of Figure 1d).…”

“…It has a very low magnetic anisotropy and is exchange‐coupled with energy J 12 MSC to the bottom FM layer (Figure 1d). Due to exchange bias, [ 15,30,40–45 ] the excited spin chain can switch its magnetic orientation at H ≈ 1.1 T (yellow/green arrow in Figure 1c). The bottom magnetic unit, comprising the FM layer and spinterface, eventually switches at higher H (bottom green arrow in Figure 1c).…”

“…[ 46 ] To the best of our knowledge, this is the first observation of a specific MR signal that is driven to appear due to bias voltage, and whose amplitude tracks a conductance increase. An electrically driven [ 47,48 ] generation of interfacial MR at a spinterface due to so‐called “magnetic hardening” [ 30,40,43–45 ] past an electric field threshold is not expected to yield a MR term that scales with d I/ d V . Instead, given the above interpretation of d I/ d V , we conclude that MR ES arises from the opening of spin‐flip channels of transport across MSCs.…”

Evidence of Spin-Glass state in Molecular Exchange-Bias System

“…Molecular exchange-bias effect [1][2][3][4][5] is a particular manifestation of ferromagnet/molecule spin-interface study [6] in a widely investigated topic of interfaceassisted molecular spintronics [7,8]. Here, complex charge transfer and hybridization effects between the molecular orbitals and the spin-polarised bands of the ferromagnet (FM) surface create new hybridized interface states that provide novel electronic and magnetic properties [7][8][9].…”