Interfacial Frustration Research Articles

Realization of exchange bias control with manipulation of interfacial frustration in magnetic complex oxide heterostructures

Rich exchange bias (EB) behaviors were previously observed when ferromagnetic (FM) materials contacted a spin glass, demonstrating magnetic degrees of freedom of the coupling between the glass and FM spins. However, the correlation between the degree of magnetic spin frustration and the strength of the resulting EB is far from being understood. Here, we systematically investigate the dependency of EB on interfacial spin frustration in magnetic complex oxide heterostructures including ${\mathrm{La}}_{0.7}{\mathrm{Ca}}_{0.3}\mathrm{Mn}{\mathrm{O}}_{3}\text{/}\mathrm{CaMn}{\mathrm{O}}_{3}$ (LCMO/CMO) systems. The experimental analysis revealed that the extent of interfacial spin frustration is determined by the subtle competition between different types of magnetic orders related to the glassy spin behaviors at the interface. Such spin frustration can be manipulated through strain engineering through changes in the Mn ${e}_{g}$ orbital by alternating the stacking sequence of the heterostructures. A highly tunable EB field with 95% change of strength between the highly and weakly frustrated heterostructures has been achieved. Magnetic depth profiles of the heterostructures provide convincing evidence that a magnetically depressed region always occurs in the LCMO layer at the LCMO/CMO interfaces irrespective of the stacking sequence. Finally, EB is established at the magnetic interface in the LCMO layer.

Interfacial Lewis Acid-Base Adduct Formation Probed by Vibrational Spectroscopy.

Understanding Lewis pair (LP) interactions at heterogeneous environments is important for controlling surface reactions. We report the formation of interfacial Lewis adducts with tris(pentafluorophenyl)borane as the Lewis acid and 4-mercaptobenzonitrile attached to gold as the Lewis base. We use the nitrile vibrational frequency as a probe of adduct strength, with stronger adducts leading to larger frequencies. The vibrational frequency shifts of the surface adducts were measured via sum frequency generation spectroscopy and compared to the frequency shifts of bulk adducts. Our results show a distinctly smaller frequency shift for the surface adducts compared to the bulk, indicating a weaker Lewis acid-base interaction near the surface. We explore three possible origins of this difference: interfacial frustration, surface electric fields, and electronic energy level alignment. We highlight the relevance of each and note that likely more than one of them affect the observed surface LP interactions.

Numerical study of the influence of interfacial roughness on the exchange bias properties of ferromagnetic/antiferromagnetic bilayers

Exchange bias and coercivity are both studied numerically in antiferromagnetic/ferromagnetic (AFM/FM) bilayers in the presence of a rough interface. The roughness is modeled by an AFM atomic mesa of variable width, in a periodic bidimensional system. Unlike the flat interface case, roughness can favor the presence of magnetic interfacial frustration or the formation of sharp magnetic domain walls pinned within the first AFM planes, inside the AFM mesa, in a Peierls potential well. We demonstrate by using athermal steepest-descent calculations that irreversible processes can occur during the hysteresis loops, when the AFM mesa width is less than half of the system period. In this case, the depinning of the domain wall from the Peierls potential well during the descending branch is not followed by its rewinding in a certain range of the AFM anisotropy. This leads to a large increase of both exchange bias and coercivity at low temperature and to an athermal training effect. When the thermal activation is taken into account by using Monte Carlo simulations, we show that a random walk of the domain wall occurs within the AFM layer. These processes induce changes in the AFM spin configuration when the system is cycled several times and produce a thermally activated training effect. Our simulations, interpreted in the context of periodic Peierls potential, provide an explanation for two important features of the exchange bias phenomenon, i.e., the thermal variation of its characteristic fields and the different contributions giving rise to the training effect (AFM bulk vs interface). More generally, the presence of interfacial atomic roughness reduces both exchange bias and coercivity with respect to the perfect interface case.

Spin-glass–like ground state and observation of exchange bias in Mn0.8Fe0.2NiGe alloy

The ground-state magnetic properties of a hexagonal equiatomic alloy of nominal composition Mn0.8Fe0.2NiGe were investigated through dc magnetization and heat capacity measurements. The alloy undergoes a first-order martensitic transition below 140 K with simultaneous development of long-range ferromagnetic ordering from the high-temperature paramagnetic phase. The undoped compound MnNiGe has an antiferromagnetic ground state and it shows martensitic-like structural instability well above room temperature. Fe doping at the Mn site not only brings down the martensitic transition temperature, but it also induces ferromagnetism in the sample. Our study brings out two important aspects regarding the sample, namley i) the observation of exchange bias at low temperature, and ii) spin-glass–like ground state which prevails below the martensitic and magnetic transition points. In addition to the observed usual relaxation behavior, the spin glass state is confirmed by the zero-field–cooled memory experiment, thereby indicating cooperative freezing of spin and/or spin clusters rather than uncorrelated dynamics of superparamagnetic-like spin clusters. We believe that doping disorder can give rise to some islands of antiferromagnetic clusters in the otherwise ferromagnetic background which can produce interfacial frustration and exchange pinning responsible for spin glass and exchange bias effect. A comparison is made with doped rare-earth manganites where a similar phase separation can lead to a glassy ground state.

Spin-glass-like state in GdCu: Role of phase separation and magnetic frustration

We report investigations on the ground state magnetic properties of intermetallic compound GdCu through dc magnetization measurements. GdCu undergoes first order martensitic type structural transition over a wide temperature window of coexisting phases. The high temperature cubic and the low temperature orthorhombic phases have different magnetic character and they show antiferromagnetic and helimagnetic orderings below 145 K and 45 K respectively. We observe clear signature of a glassy magnetic phase below the helimagnetic ordering temperature, which is marked by thermomagnetic irreversibility, aging and memory effects. The glassy magnetic phase in GdCu is found to be rather intriguing with its origin lies in the interfacial frustration due to distinct magnetic character of the coexisting phases.

Emergence of noncollinear anisotropies from interfacial magnetic frustration in exchange-bias systems

Exchange bias, referred to the interaction between a ferromagnet (FM) and an antiferromagnet (AFM), is a fundamental interfacial magnetic phenomenon, which is key to current and future applications. The effect was discovered half a century ago, and it is well established that the spin structures at the FM/AFM interface play an essential role. However, currently, ad hoc phenomenological anisotropies are often postulated without microscopic justification or sufficient experimental evidence to address magnetization-reversal behavior in exchange-bias systems. We advance toward a detailed microscopic understanding of the magnetic anisotropies in exchange-bias FM/AFM systems by showing that symmetry-breaking anisotropies leave a distinct fingerprint in the asymmetry of the magnetization reversal and we demonstrate how these emerging anisotropies are correlated with the intrinsic anisotropy. Angular and vectorial resolved Kerr hysteresis loops from FM/AFM bilayers with varying degree of ferromagnetic anisotropy reveal a noncollinear anisotropy, which becomes important for ferromagnets with vanishing intrinsic anisotropy. Numerical simulations show that this anisotropy naturally arises from the inevitable spin frustration at an atomically rough FM/AFM interface. As a consequence, we show in detail how the differences observed for different materials during magnetization reversal can be understood in general terms as originating from the interplay between interfacial frustration and intrinsic anisotropies. This understanding will certainly open additional avenues to tailor future advanced magnetic materials.

Magnetic phases of thin Fe films grown on stepped Cr(001)

Magnetic phases of Fe films grown on curved Cr(001) with steps parallel to [100] are studied using the surface magneto-optic Kerr effect (SMOKE). We found that the atomic steps (1) induce an in-plane uniaxial magnetic anisotropy with the easy magnetization axis parallel to the step edges, and (2) generate magnetic frustration either inside the Fe film or at the Fe-Cr interface, depending on the Fe film thickness and the vicinal angle. For thickness greater than 35 {Angstrom}, the Fe film forms a single magnetic domain and undergoes an in-plane magnetization switching due to the competition of the step-induced anisotropy and the Fe-Cr interfacial frustration. For thickness less than 35 {Angstrom}, the Fe film forms multiple magnetic domains at low vicinal angle, and transforms into a single domain at high vicinal angle. A magnetic phase diagram in the 30{endash}45 {Angstrom} thickness range was obtained using a wedge-shaped Fe film. {copyright} {ital 1999} {ital The American Physical Society}