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It is demonstrated that a charge-trapping layer placed in proximity to a ferromagnetic metal enables efficient electrical and optical control of the metal's magnetic properties. Retention of charge trapped inside the chargetrapping layer provides nonvolatility to the magnetoelectric effect and enhances its efficiency by an order of magnitude. As such, an engineered charge-trapping layer can be used to realize the magnetoelectric equivalent to today's pervasive charge trap flash memory technology. Moreover, by supplying trapped charges optically instead of electrically, a focused laser beam can be used to imprint the magnetic state into a continuous metal film. KEYWORDS: Magnetoelectric effect, electric field control of magnetic properties, charge trap flash memory, magneto-optical writing, magnetic anisotropy E lectric field control of the magnetic state of a material could enable new low-power logic devices and nonvolatile memory cells.1−17 Magneto-electric coupling has typically been achieved using complex materials such as multiferroic oxides, 5 dilute magnetic semiconductors, 1,2 or strain-coupled magnetostrictive/piezoelectric composites.3 Only recently, direct electrical control of magnetic anisotropy 4,6,7,9−12 and Curie temperature 8 has been demonstrated in transition metal ferromagnets at room temperature. These effects open the door to electrically gated spintronic devices based on materials amenable to integration with conventional semiconductor electronics.Although magnetoelectric effects are precluded from bulk ferromagnetic metals due to the very short screening length, in films thinner than a few nanometers, spin-dependent charge screening and band level shifting can lead to pronounced electric field-driven changes to magnetic properties. 14−17Unfortunately, magnetoelectric effects in metals require a relatively strong electric field and are inherently volatile. In this work, we show that a charge-trapping layer integrated into the gate dielectric can provide the missing nonvolatility to the magnetoelectric effect and enhances its efficiency by an order of magnitude. We report the largest voltage-induced change to surface magnetic anisotropy yet demonstrated for a metallic thin film, and directly correlate this change with the density of trapped charge in an adjacent charge storage layer. This magnetoelectric charge-trap heterostructure provides a natural interface between conventional electronic and magnetic storage and logic devices, and the possibility to exploit nonvolatility in both the electronic and magnetic order offers the potential for multibit storage. 18 The functionality is similar to recently proposed ferroelectric/ferromagnetic stacks 19,20 but avoids the associated materials processing challenges by replacing the ferroelectric material with a well-established charge-trapping dielectric layer. Moreover, in the present implementation, charge trapping is optically assisted, providing a novel mechanism to optically manipulate the magnetic state. We use a simple capacitor de...

We grew tetragonally distorted FexCo1-x alloy films on Pd(001). Theoretical first-principles calculations for such films predicted a high saturation magnetization and a high uniaxial magnetic anisotropy energy for specific values of the lattice distortion c/a and the alloy composition x. The magnetic anisotropy was investigated using the magneto-optical Kerr effect. An out-of-plane easy axis of magnetization was observed for Fe0.5Co0.5 films in the thickness range of 4 to 14 monolayers. The magnetic anisotropy energy induced by the tetragonal distortion is estimated to be almost 2 orders of magnitude larger than the value for bulk FeCo alloys. Using LEED Kikuchi patterns, a change of the easy axis of magnetization can be related to a decrease of the tetragonal distortion with thickness.

The magnetic anisotropy of Fe film grown on vicinal Ag(1,1,10) surfaces was studied with the in situ magneto-optic Kerr effect. Below 200 K, strong oscillations of the uniaxial magnetic anisotropy as a function of Fe thickness with a period of 5.7 monolayers are found, which can even cause the easy magnetization axis to oscillate between perpendicular and parallel to the steps. Such novel oscillation of the anisotropy is attributed to the quantum well states of d-band electrons at the Fermi level in the Fe film. This is unlike the previously observed oscillatory behaviors of ferromagnetic films caused by the quantum well states in nonmagnetic interfacing layers.

Using soft-x-ray resonant magnetic scattering in combination with first-principles calculations for noncollinear magnetic configurations we present a new model of the magnetism in ultrathin fcc Fe films on Cu(001). We find the presence of blocks with robust magnetic structure, while the relative directions of the moments of different blocks are sensitive to the detailed atomic structure and temperature. The magnetic noncollinearity is directly demonstrated, which has not been possible so far.

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We studied tetragonally distorted Fe(1-x)Co(x) alloy films on Rh(001), which show a strong perpendicular anisotropy in a wide thickness and composition range. Analyzing x-ray magnetic circular dichroism spectra at the L_(3,2) edges we found a dependence of the Co magnetic orbital moment on the chemical composition of the Fe(1-x)Co(x) alloy films, with a maximum at x=0.6. For this composition, we observed an out-of-plane easy axis of magnetization at room temperature for film thickness up to 15 monolayers. Since both the magnetic orbital moment and the anisotropy energy show similar composition dependence, it confirms that both quantities are directly related. Our experiments show that the adjustment of the Fermi level by a proper choice of the alloy composition is decisive for the large magnetic orbital moment and for a large magnetic anisotropy in a tetragonally distorted lattice.

Tetragonally distorted FexCo1−x alloy films are grown on Rh (001) showing a strong perpendicular magnetic anisotropy in a wide thickness and composition range. This large perpendicular magnetic anisotropy is chemical composition dependent and reaches a maximum at x=0.4. For this composition, we observed an out-of-plane easy axis of magnetization at room temperature with film thickness up to 15 ML. Our experiments show that the proper adjustment of the Fermi level (EF) by the variation of the FexCo1−x alloy composition and the corresponding tetragonal distortion results in a large perpendicular magnetic anisotropy.