Voltage-controlled spin electronics is crucial for continued progress in information technology. It aims at reduced power consumption, increased integration density and enhanced functionality where non-volatile memory is combined with high-speed logical processing. Promising spintronic device concepts use the electric control of interface and surface magnetization. From the combination of magnetometry, spin-polarized photoemission spectroscopy, symmetry arguments and first-principles calculations, we show that the (0001) surface of magnetoelectric Cr(2)O(3) has a roughness-insensitive, electrically switchable magnetization. Using a ferromagnetic Pd/Co multilayer deposited on the (0001) surface of a Cr(2)O(3) single crystal, we achieve reversible, room-temperature isothermal switching of the exchange-bias field between positive and negative values by reversing the electric field while maintaining a permanent magnetic field. This effect reflects the switching of the bulk antiferromagnetic domain state and the interface magnetization coupled to it. The switchable exchange bias sets in exactly at the bulk Neel temperature.

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Robust isothermal electric control of exchange bias at room temperature

Roughness-insensitive and electrically controllable magnetization at the (0001) surface of antiferromagnetic chromia is observed using magnetometry and spin-resolved photoemission measurements and explained by the interplay of surface termination and magnetic ordering. Further, this surface in placed in proximity with a ferromagnetic Co/Pd multilayer film. Exchange coupling across the interface between chromia and Co/Pd induces an electrically controllable exchange bias in the Co/Pd film, which enables a reversible isothermal (at room temperature) shift of the global magnetic hysteresis loop of the Co/Pd film along the magnetic field axis between negative and positive values. These results reveal the potential of magnetoelectric chromia for spintronic applications requiring non-volatile electric control of magnetization.

/pdf/robust-isothermal-electric-switching-of-interface-4m66t0dqv7.pdf

Robust isothermal electric switching of interface magnetization: A route to voltage-controlled spintronics

Voltage-controlled spin electronics is crucial for continued progress in information technology. It aims at reduced power consumption, increased integration density and enhanced functionality where non-volatile memory is combined with high-speed logical processing. Promising spintronic device concepts use the electric control of interface and surface magnetization. From the combination of magnetometry, spin-polarized photoemission spectroscopy, symmetry arguments and first-principles calculations, we show that the (0001) surface of magnetoelectric Cr(2)O(3) has a roughness-insensitive, electrically switchable magnetization. Using a ferromagnetic Pd/Co multilayer deposited on the (0001) surface of a Cr(2)O(3) single crystal, we achieve reversible, room-temperature isothermal switching of the exchange-bias field between positive and negative values by reversing the electric field while maintaining a permanent magnetic field. This effect reflects the switching of the bulk antiferromagnetic domain state and the interface magnetization coupled to it. The switchable exchange bias sets in exactly at the bulk Néel temperature.

http://absimage.aps.org/image/MAR11/MWS_MAR11-2010-000594.pdf

Angle-resolved photoemission spectroscopy (ARPES) has emerged as a leading experimental probe for studying the complex phenomena in quantum materials, a subject of increasing importance The power of this technique stems from the directness and the richness of the momentum-resolved information it can provide, such as band dispersion, Fermi surface topology, and electron self-energy Over the past decade, the significantly improved energy and momentum resolution and carefully matched experiments have turned this technique into a sophisticated tool in characterizing the electronic structure of complex materials This revolution is mostly evident in the study of cuprate high-temperature superconductors More recently, this technique has played a critical role in advancing our understanding of the newly discovered iron-based superconductors and topological insulators In this paper we review some of the recent ARPES results and discuss the future perspective in this rapidly developing field

Angle-Resolved Photoemission Studies of Quantum Materials

Rational material design can accelerate the discovery of materials with improved functionalities. This approach can be implemented in Heusler compounds with tunable magnetic sublattices to demonstrate unprecedented magnetic properties. Here, we have designed a family of Heusler alloys with a compensated ferrimagnetic state. In the vicinity of the compensation composition in Mn-Pt-Ga, a giant exchange bias (EB) of more than 3 T and a large coercivity are established. The large exchange anisotropy originates from the exchange interaction between the compensated host and ferrimagnetic clusters that arise from intrinsic anti-site disorder. Our design approach is also demonstrated on a second material with a magnetic transition above room temperature, Mn-Fe-Ga, exemplifying the universality of the concept and the feasibility of room-temperature applications. These findings may lead to the development of magneto-electronic devices and rare-earth-free exchange-biased hard magnets, where the second quadrant magnetization can be stabilized by the exchange bias.

Design of compensated ferrimagnetic Heusler alloys for giant tunable exchange bias

Characterization and control of the interface structure and morphology at the atomic level is an important issue in understanding the magnetic interaction between an antiferromagnetic material and an adjacent ferromagnet in detail, because the atomic spins in an antiferromagnet change direction on the length scale of nearest atomic distances. Despite its technological importance for the development of advanced magnetic data-storage devices and extensive studies, the details of the magnetic interface coupling between antiferromagnets and ferromagnets have remained concealed. Here we present the results of magneto-optical Kerr-effect measurements and layer-resolved spectro-microscopic magnetic domain imaging of single-crystalline ferromagnet-antiferromagnet- ferromagnet trilayers. Atomic-level control of the interface morphology is achieved by systematically varying the thicknesses of the bottom ferromagnetic and the antiferromagnetic layer. We find that the magnetic coupling across the interface is mediated by step edges of single-atom height, whereas atomically flat areas do not contribute.

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Tuning the magnetic coupling across ultrathin antiferromagnetic films by controlling atomic-scale roughness

The magnetic coupling between epitaxial single-crystalline Co and FeMn layers on Cu(001) was investigated by element-resolved magnetic circular dichroism domain imaging using a photoelectron emission microscope. As-grown Co domain patterns reveal the presence of many small domains in the antiferromagnet. The coupling of the Co layer is found to be along $〈100〉$ crystallographic directions. This is discussed in terms of a $45\ifmmode^\circ\else\textdegree\fi{}$ coupling due to frustrations at topological $90\ifmmode^\circ\else\textdegree\fi{}$ domains in the FeMn layer. Coercivity oscillations as a function of FeMn thickness with atomic monolayer period support the importance of such step-induced domains in the coupling.

Magnetic interface coupling in single-crystalline Co/FeMn bilayers

We demonstrate the use of a photoelectron emission microscope in connection with a retarding field electron energy analyzer for the fast acquisition of two-dimensional momentum resolved photoelectron angular distribution patterns. This opens the possibility to combine spatial, momentum, and energy resolution of photoelectrons within the same instrument. We have applied this to observe the Cu(001) Fermi surface from a selected region of the sample. A well defined bulk Fermi surface is quickly mapped in this way.

/pdf/microspectroscopic-two-dimensional-fermi-surface-mapping-4qeebi2ij0.pdf

Microspectroscopic two-dimensional Fermi surface mapping using a photoelectron emission microscope

Using photoemission electron microscopy in combination with x-ray magnetic circular dichroism, element selective magnetic domain images have been obtained from single-crystalline Co/FeMn and FeMn/Co bilayers epitaxially grown on a Cu(001) single crystal. The contact with ferromagnetic Co leads to the observation of a net magnetic moment in Fe and Mn, independently of the paramagnetic or antiferromagnetic state of the FeMn thin films. Only a small fraction of this moment might mediate the magnetic interaction at the interface, and thus be responsible for the exchange bias effect.

Induced Fe and Mn magnetic moments in Co-FeMn bilayers on Cu(001)

Layer-resolved magnetic domain images of epitaxially grown Co/Cu/Ni trilayers on Cu(001) have been studied, taken by photoelectron emission microscopy using x-ray magnetic circular dichroism as a magnetic contrast mechanism. In these trilayers the Ni layers are magnetized perpendicularly to the film plane, whereas the Co magnetization is in the film plane. Comparison of the as-grown magnetic domain images of the Co and Ni layers reveals the influence of the magnetostatic stray fields from Ni domain walls on the Co domain pattern as a lateral displacement of the Co domain wall position compared to the Ni domain walls. The effect is quantified by comparing to the effect of external magnetic fields, and is found to be equivalent to about 250 Oe. Micromagnetic simulations using the Landau-Lifshitz-Gilbert equation confirm that size of the Ni domain wall stray field interaction.

Li Chelaru

Papers

Tuning the magnetic coupling across ultrathin antiferromagnetic films by controlling atomic-scale roughness

Magnetic interface coupling in single-crystalline Co/FeMn bilayers

Microspectroscopic two-dimensional Fermi surface mapping using a photoelectron emission microscope

Induced Fe and Mn magnetic moments in Co-FeMn bilayers on Cu(001)

Layer-resolved imaging of magnetic interlayer coupling by domain-wall stray fields