Stabilization of Antiferromagnetic Order in FeO Nanolayers

Abstract

We have studied the evolution of the magnetic state of a nanometer thick antiferromagnetic (AFM) FeO layer during its formation using nuclear resonant scattering of synchrotron radiation. In contact to ferromagnetic Fe, the FeO layer does not show magnetic order at room temperature (RT). Once embedded between two Fe layers, magnetic coupling to the adjacent ferromagnets leads to a drastic increase of the Néel temperature far above RT, while the blocking temperature remains below 30 K. The presented results evidence the role that the ferromagnetic surrounding plays in modifying the magnetic state of ultrathin AFM layers. DOI: 10.1103/PhysRevLett.103.097201 PACS numbers: 75.70.Cn, 75.25.+z, 75.75.+a, 76.80.+y Magnetic data storage technology is reaching the bottom of the nanoscale. The stabilization of magnetic order in low-dimensional structures becomes a key issue. Below a critical thickness, ferromagnets undergo a superparamagnetic relaxation where spins are thermally excited, and the system does not show magnetic hysteresis anymore. Exchange bias bilayer systems composed of a ferromagnet (FM) and an antiferromagnet (AFM) have been recently reported to substantially reduce the critical thickness under which the ferromagnet undergoes superparamagnetic relaxation [1,2]. Layered FM/AFM systems, where an AFM layer (with high anisotropy) is used to pin the magnetization of the FM, are used to create reference magnetic layers in devices. Below a certain temperature (the blocking temperature T B ) the spins in the antiferromagnet are frozen and exchange coupling at the FM/AFM interface leads to a shift of the hysteresis loop and an increase of the coercivity, this phenomenon being known as the exchange bias effect [3,4].In the ultrathin film limit, the thermal stability of these FM/AFM bilayers is strongly modified [5,6]. In exchange bias systems, one usually distinguishes the Néel (ordering) temperature T N of the antiferromagnet and the blocking temperature T B below which exchange bias occurs [7]. For thick AFM layers (> 50 nm), T N and T B are usually found to be equivalent. Field cooling the system below T N directly leads to the freezing of spins (and the appearance of exchange bias). Early studies showed that T B was usually decreasing with decreasing thickness of the AFM layer, and it was assumed that T N was decreasing accordingly due to a finite size effect [5]. Recently, Vallejo-Fernandez et al. found out that the evolution of the exchange bias field H e and T B for polycrystalline films was in fact linked to the average grain volume of the AFM [6], which typically decreases with the layer thickness. This results in a decreased stability against thermal excitation and a decrease of T B . It should be noted that there is still an underlying physics problem as data on epitaxially grown AFM/FM bilayers also show a reduction of T B with a decreasing AFM layer thickness [8,9]; this is still the subject of different theoretical considerations [5,10]. The evolution of T N relative to T B was investigated by Va...

Ion irradiation of Fe-Fe oxide core-shell nanocluster films: Effect of interface on stability of magnetic properties

“…Note that for bulk FeO the Néel temperature (T N ) is 198 K, but it has been shown that thin FeO layers with Fe on either side show T N well above room temperature due to exchange interactions with FM spins. 60 Given the large fractional decrease in ferromagnetic components, it is unexpected that the saturation magnetization did not decrease more, since FeO fraction is 64 wt%. It may be that Fe/FeO interfaces are ferrimagnetic, resulting in more saturation than would be expected given a mixture of the pure phases.…”

Positive exchange bias in epitaxial permalloy/MgO integrated with Si (100)

“…The authors attributed this to the formation (unwanted) of an Fe-O reaction product at the interface introducing an AFM spin coupling. In our sample, the possible formation of Fe-related AFM oxides such as Fe-O [30], c-Fe 2 O 3 [31] and Fe 3 O 4 [32] at the Py/MgO interface cannot be entirely ruled out. But, the T b 's associated with these oxides have been found to be $30 K, 100 K and 120 K, respectively, well below the observed T b > 400 K in our work.…”

Nuclear resonant scattering of synchrotron radiation: Applications in magnetism of layered structures

“…The origin for this coupling is the reduction of the multi-valent (2 + , 3 + ) native Fe-oxide to single valent (2 + ) FeO upon coverage with metallic Fe [33]. The ultrathin buried FeO layers exhibit an enhanced Neel temperature well above room temperature [34]. The exchange coupling between the Fe layers and the laterally inhomogeneous (rough) FeO layers leads to a spin frustration that results in canted moments rather than parallel or antiparallel moments between adjacent Fe layers [24].…”