Electrically Controlled Surface Magnetism

Xi He, Yi Wang, N. Wu, K. Belashchenko, P. Dowben,
A. Gruverman, and Ch. Binek
Nebraska MRSEC

Manipulation of magnetically ordered states by electrical means is among the most promising approaches towards novel spintronic applications. Modern devices of the information age are expected to be highly integrated and ultra fast which in turn demands for lowest power consumption to avoid overheating. Electric control of magnetism provides an almost powerless approach to manipulate magnetic states for data storage and processing purposes. Electric control of exchange bias, a specific control of the magnetic hysteresis of a ferromagnetic thin film, will play an important role in a large class of future spintronic devices. We work on its realization with the help of the magnetoelectric antiferromagnet Cr₂O₃. The latter replaces today’s passive pinning layers in exchange bias heterostructures by an active magnetoelectric material. Recently, a very unusual specific surface magnetic order which enables magnetoelectric control of a net magnetic moment of the Cr₂O₃ (111) surface has been predicted by our first principle calculations and experimental evidence has been found by magnetometry and spin polarized photoemission. The very unusual robustness of this highly spinpolarized surface state which exists close to room temperature together with the ability to control it by electrical means makes this finding very attractive for further investigations and the implementation of Cr₂O₃ as unique material in prototypical spintronic devices.
This research is supported by the National Science Foundation, Division of Materials Research, Materials Research Sciences and Engineering Program, Grant 0820521.

Picture: A net surface magnetization evolving in the antiferromagnetic single domain state of Cr₂O₃ (111).

Magnetic Doping of Golden-Cage Clusters?

X.C. Zeng, J. Bai
Nebraska MRSEC

L.M. Wang, L.S. Wang, W. Huang
Washington State University/PNNL

D. Schooss, M. Kappes
Karlsruhe, Germany

Understanding local magnetic properties of dilute magnetic impurities in nonmagnetic hosts is of both fundamental and practical importance. Atomic metal clusters provide a unique medium for exploring local magnetism, as the cluster size, the number of valence electrons, and the local structures can be readily controlled and varied. In particular, a single magnetic atom trapped in a metallic cage (i.e., core/shell cluster) can be an ideal molecular model for dilute magnetic alloys. We have found the golden-cage Au16– cluster, which has a sufficiently large internal volume to encapsulate a foreign atom. We showed that the most stable structure of bimetallic MAu16– (M = Fe, Co, Ni) clusters is the core/shell MAu16– structure, but with considerable distortions to the parent Au16– shell. Fe@Au16– and Co@Au16– are found to have similar structures with C2 symmetry, while a C1 structure is found for Au16Ni–. The 4s electrons are observed to transfer to the Au16 cage, whereas atomic-like magnetism due to the unpaired 3d electrons is retained for all the doped clusters. Fe@Au16– and Co@Au16– have high spins (5µB and 4µB), while Ni@Au16– has a lower spin (1µB), consistent with the stronger Ni-cage interactions [Phys. Rev. B 79, 033413 (2009)].
This research is supported by the National Science Foundation, Division of Materials Research, Materials Research Sciences and Engineering Program, Grant 0820521.

Picture: Fe, Co, and Ni atom in the gold-cage Au₁₆^–

Stripes are Stars! Pt Helps Fe to Stay Magnetized

A. Enders, R. Skomski
Nebraska MRSEC

J. Honolka, K. Kern, K. Fauth, G. Schuetz
MPI Stuttgart, Germany

P. Varga
TU Vienna, Austria

H. Ebert
LMU München, Germany

The magnetic anisotropy energy is among the most important functional properties of magnetic elements. It determines the orientation and stability of the magnetization as well as the mechanisms and the dynamics of the magnetization reversal. New materials with extremely large anisotropy values are currently sought for the development of ultrahigh density magnetic recording media. As one example, FePt or CoPt alloys exhibiting L1₀ structure are currently in the focus of intensive research, as they were found to exhibit unprecedented anisotropy values. Recently Nebraska MRSEC researchers in collaboration with their international partners at Stuttgart, Vienna and München have discovered that atomically small nanostructures of Fe and Pt can also, under certain conditions, exhibit magnetic anisotropy values similar to those of their L1₀ bulk counterparts (Phys. Rev. Lett. 102, 067201, (2007)). This discovery was possible after advances in nanostructure synthesis have been made to embed Fe atoms as monatomic Fe stripes or in other configurations into the surface of Pt single crystals. It was found that the Pt plays a critical role in the magnetism of these structures, as it dictates the structure’s magnetic anisotropy and thus helps the Fe to stay magnetized. Intriguingly, in the system studied, there is an interesting interplay between ordinary magnetization states and non-collinear spin structures at the nanoscale, leading to rather complex magnetic properties of the FePt nanostructures.
This research is supported by the National Science Foundation, Division of Materials Research, Materials Research Sciences and Engineering Program, Grant 0820521.

Picture: STM image of FePt surface alloy with atomic chemical contrast. The Fe atoms (yellow) are embedded in the Pt surface (bluish, not resolved). Fe chains separated by Pt is the key to large magnetic anisotropy.

Sculptured Cobalt Nanomagnets

E. Schubert, A. C. Kjerstad, D. Schmidt, M. Schubert, S.-H. Liou, and R. Skomski
Nebraska MRSEC

Nanomagnets establish a promising field of research with a broad variety of applications in magnetic memory, bioengineering or medical diagnosis and treatment. However, exploitation requires understanding, control and manipulation of magnetic properties, which are mainly influenced by the nanomagnet’s size and shape as well as by interactions between ensembles of nanomagnets arranged in large scale arrays. We fabricate chiral and non-chiral three-dimensional thin-film nanomagnets by means of oblique particle flux deposition. The material exhibits unique magnetic properties associated with the shape of the structures and with magnetic interactions between adjacent features. An example is the preferential magnetization direction (easy axis) in non-chiral cobalt films from slanted nanoneedles. Measurements reveal that the easy axis is rotated towards the film rather than being parallel to the needle direction, as expected from the needles’ shape anisotropy. Model calculations suggest that magnetostatic interactions between the needles are responsible for the observed behavior. The calculations predict that the easy axis depends on both the tilting angle and the packing fraction of the nanoneedles. Accordingly, the easy axis varies from parallel to the needles for low packing fractions to almost in the film plane for close-packed needle films, which was indeed detected in our experiments.
This research is supported by the National Science Foundation, Division of Materials Research, Materials Research Sciences and Engineering Program, Grant 0820521.

Picture: Sculptured Thin Films from Cobalt a) non-chiral (inclined needles), b) chiral (spirals)

How Strong is It?

Roger D. Kirby
Nebraska MRSEC

University of Nebraska MRSEC faculty worked with first-grade students at Morley Elementary School for seven science lessons to study many properties of magnetism and magnetic materials. These enthusiastic students used hands-on activities to learn how to tell whether a material is magnetic, how to make magnets in a variety of ways, how the earth behaves as a giant magnet, and how like poles repel, and unlike poles attract. They were especially intrigued by the question of how “strong” a magnet could be, and delighted in testing to find out.
These programs are supported by the National Science Foundation, Division of Materials Research, Materials Research Science and Engineering Program, Grant 0820521.