Session B34: Focus Session: Nanomagnetism -- Exchange & Multiferroics

Strain-induced magnetic domain re-configuration in magnetostrictive Fe$_{70}$Ga$_{30}$ thin films

It has been long understood that the magnetic order parameter in many metals carries with it a symmetry-breaking distortion of the lattice that manifests as a macroscopic strain. In the inverse effect, a strain can give a magnetic anisotropy, restructuring the magnetic domains of a ferromagnet under applied force. We use Lorentz--force transmission electron microscopy to observe magnetic domain structure dynamics induced via direct application of strain on magnetostrictive iron gallium (Fe$_{70}$Ga$_{30})$ thin films. Iron-gallium films are deposited on flexible free-standing membranes, and using a mechanically manipulated tip, a strain is applied to the sample. The varied hysteretic behaviors under applied magnetic and strain fields have been modeled and will be presented. Strain-mediated magnetic domain switching has previously been investigated with multiferroic devices, where the applied strain is induced from an underlying piezoelectric film introducing clamping effects, and we will extend our observations to this technologically-relevant multi-ferroic system.   

Piezoelectric tuning of exchange bias from negative to positive bias fields

Tuning of the exchange bias has been attempted using magnetoelectric and multiferroic systems. Alternatively, we propose tuning of the exchange bias via the piezoelectric property of ferroelectric material. A ferromagnetic Co thin film is deposited on top of a ferroelectric tetragonal BaTiO$_{3}$ (001) by using MBE at a base pressure of 1.5$\times $10$^{10 }$m bar. An ex-situ antiferromagnetic CoO film is naturally formed on top of the Co Hereby, the piezoelectric BaTiO$_{3}$ induced electrically tunable stress in the adjacent Co film. The stress induced strain alters the magnetic anisotropy of the Co film and by that the magnetization at the Co/CoO-interface modifying the exchange bias field. This includes sign change of the exchange bias from negative to positive bias fields by increasing electric field applied on BaTiO$_{3}$. The observed complex electric field dependence of the exchange bias is interpreted through competition between ferromagnetic and antiferromagnetic exchange at the rough Co/CoO interface. The competition involves weakening of negative exchange bias through deviations from collineraity of the Co and CoO interface magnetization and simultaneous activation of antiferromagnetic exchange giving rise to a crossover into positive exchange bias.   

Magnetization reversal controlled by an applied voltage in La$_{0.7}$Ca$_{0.3}$MnO$_{3}$/(Ba, Sr)TiO$_{3}$/La$_{0.7}$Ca$_{0.3}$MnO$_{3 }$ multiferroic tunneling junctions

We report the magnetization reversal in La$_{0.7}$Ca$_{0.3}$MnO$_{3}$/(Ba, Sr)TiO$_{3}$/La$_{0.7}$Ca$_{0.3}$MnO$_{3}$ magnetic tunneling junctions by measuring the resistance switch between the two magnetic states after applying a voltage pulse. The junction size is 10 x 10 $\mu $m$^{2}$ with the barrier thickness in the range of 1-1.5 nm. The ferroelectric barrier allows us to apply a large voltage ($\sim $ 3V) with very low current. We have found that magnetic state of the junction can be switched both from the antiparallel to parallel state and from the parallel to antiparallel state in certain field ranges. The switching does not depend on the polarity of the electrical field direction. The critical voltage for the switching depends on the magnetic field with higher voltage needed for lower magnetic field. The switching current density is in the order of 10$^{3}$ - 10$^{4}$A/cm$^{2}$, which is much smaller than the usually observed $\sim $10$^{7}$/cm$^{2}$ in metallic tunneling junctions by spin transfer torques. The details of the switching voltage as a function of magnetic field and the manipulation of the magnetic states using a voltage will be discussed.   

A New Synthesis Strategy For High-Quality Fe / BaTiO$_{3}$ Multiferroics

Ultrathin film BaTiO$_{3}$/Nb-SrTiO$_{3}$ (BTO/STO) and Fe/BTO nanostructures were investigated with variable temperature scanning tunneling microscopy (STM) and low energy electron diffraction (LEED) under ultrahigh vacuum. BTO films of 8 -- 13 unit cells thickness were grown by ozone-assisted molecular beam epitaxy on STO, and transferred through air into the STM chamber. Atomically flat, clean and unreconstructed films were recovered after annealing them in 1$\times $10$^{-4}$ mBar oxygen at 970K. Fe nanoclusters were deposited on the BTO by noble gas buffer layer assisted growth, and further studied by LEED and STM. The sharp 1x1 LEED images after cluster deposition show that the clusters are crystalline, suggesting that the interface oxidation is minimal. This synthesis route has thus the potential to fabricate ordered, atomically flat interfaces by suppressing interface mixing and Fe oxidation, which is a significant advantage over MBE deposition of Fe on BTO. The samples obtained are currently used as model system for the study of interface contributions to the magnetoelectric effect in multiferroics.   

Nature of the magnetoelectric coupling in PZT/LSMO multiferroic structures

By using a combination of electric, transport, magnetic and spectroscopic techniques, we shed light on the origin of the magnetoelectric coupling recently observed in La$_{0.8}$Sr$_{0.2}$MnO$_3$/Pb(Zr$_{0.2}$Ti$_{0.8}$)O$_3$ (LSMO/PZT) multiferroic heterostructures [1]. Hall-bar device structures are grown by molecular beam epitaxy (LSMO) and off-axis rf magnetron sputtering (PZT). From x-ray absorption near edge spectroscopy measurements as a function of the PZT polarization, we find a modification in the Mn valence state, in good agreement with the expected charge modulation, demonstrating the electronic origin of the large magnetoelectric coupling effect found in these multiferroic heterostructures. We also show that the large change in magnetic moment observed in this system cannot be fully accounted for by the measured change in the Mn valency. This result indicates the occurrence of a change in the spin configuration of the interfacial LSMO layer, from a ferromagnetic spin configuration to an antiferromagnetic spin arrangement with respect to the underlying LSMO layers. This effect is attributed to the balance between charge and competing super-exchange, along with double exchange interactions that are a hallmark of these strongly correlated oxides. [1] Molegraaf et al, , Advanced Materials 21 (2009) 3470.   

Influence of ferroelectric polarization on magnetic anisotropy

Thin film heterostructures of transition metal ferromagnets (FM) and polymer ferroelectrics (FE) are investigated to look for changes in the magnetic anisotropy of the FM layer that occur on switching the FE polarization (with an ensuing change in the electric field direction).[1] Samples of [Glass/ Pd (50 nm)/Co wedge (0.9-2.6nm)/ferroelectric P(VDF-TrFE) (53 nm)/Al (30nm)] are deposited via sputtering or evaporation for the metallic layers and via Langmuir-Schaefer deposition for the polymer ferroelectric. [2] Magnetic and FE properties have been characterized using the Magneto-Optical Kerr Effect (MOKE) and the pyroelectric effect. Polar and longitudinal MOKE loops are measured across the Co wedge for both positive and negative FE polarization and the difference in the two MOKE loops is ascribed to the changes in the magnetic anisotropy of the FM layer. [3] These changes are most apparent in the region where the Co undergoes a transition from in-plane to out-of-plane anisotropy. This research is supported by the NSF MRSEC through Grant No. DMR- 0820521 1. Chun-Gang Duan et al, Appl. Phys. Lett. \textbf{92}, 122905 (2008) 2. A. V. Bune, et al, Nature \textit{(London)} \textbf{391}, 874 (1998) 3. P. F. Carcia, J.Appl. Phys. 63, 5066 (1988)   

First-Principles Study of Enhanced Magnetoelectric Effects at the Fe/MgO(001) Interface

The magnetoelectric effect allows affecting magnetic properties of materials by electric fields with potential for technological applications such as electrically controlled magnetic data storage. In this study we explore, using first-principles methods, the magnetoelectric effect at the Fe/MgO(001) interface$^{,1}$. By explicitly introducing an electric field in our density-functional calculations we demonstrate that the magnetic moment of Fe atoms at the interface changes linearly as a function of the applied electric field with the surface magnetoelectric coefficient being strongly enhanced as compared to that for the clean Fe(001) surface.$^{1}$ The effect originates from the increased screening charge associated with a large dielectric constant of MgO. The influence of electric field on relative occupancy of the Fe-3$d$ orbitals leads to significant change in the surface magnetocrystalline anisotropy. These results are compared with the available experimental work.$^{2}$ Our results indicate that using high-k dielectrics at the interface with ferromagnetic metals may be very effective in controlling the magnetic properties by electric fields thereby leading to interesting device applications. $^{1 }$C.-G. Duan et al., Phys. Rev. Lett. \textbf{101}, 137201 (2008). $^{2}$ T. Maruyama et al., Nat. Nanotech., \textbf{4}, 158 (2009).   

Mechanism for normal chirality at hexagonal interfaces

We study the net chirality created by the Dzyaloshinkii-Moriya interaction (DMI) at the boundary between hexagonal layers of magnetic and non-magnetic materials. It is determined that another mechanism besides elastic torsion is required to understand the change in chirality observed in Dy/Y multilayers during field cooling. We show that due to the overlap between magnetic and non-magnetic atoms, interfacial steps may produce a DMI normal to the interface in magnetic heterostructures.   

Aging in Magnetic Superlattices

Aging phenomena can be observed in non-equilibrium systems with slow relaxation dynamics. Magnetic specimens with well defined interactions and dimensions can serve as model systems for universal aspects of aging. Magnetic thin films provide access to a wide range of microscopic parameters. Superlattice structures allow tuning the intra and inter-plane exchange and enable geometrical confinement of the spin fluctuations. We use Co/Cr thin film superlattices to study magnetic aging. The static and dynamic magnetic properties are affected via the Co and Cr film thicknesses. The Curie temperature of the Co films is reduced from the bulk value by geometrical confinement. Cr provides antiferromagnetic coupling between the Co films. In-plane magnetic set fields of some 10-100 mT are applied and the sample is exposed to the latter for various waiting times. After removing the field, relaxation of the magnetization is recorded via longitudinal Kerr-magnetometry and SQUID. The relaxation data are analyzed by scaling plots revealing universal aspects of aging. Financial support by NRI, and NSF through EPSCoR, Career DMR-0547887, DMR-0904999, and MRSEC.   

Finite size effects with variable range exchange coupling in thin-film Pd$/$Fe$/$Pd trilayers

The magnetic properties of thin-film Pd$/$Fe$/$Pd trilayers in which an embedded $\sim$1.5$\AA$-thick ultrathin layer of Fe induces ferromagnetism in the surrounding Pd have been investigated. The thickness of the ferromagnetic trilayer is controlled by varying the thickness of the top Pd layer over a range from 8~\AA~to 56~\AA. As the thickness of the top Pd layer decreases, or equivalently as the embedded Fe layer moves closer to the top surface, the saturated magnetization normalized to area and the Curie temperature decrease whereas the coercivity increases. These thickness-dependent observations for proximity-polarized thin-film Pd are qualitatively consistent with finite size effects that are well known for regular thin-film ferromagnets. The functional forms for the thickness dependences, which are strongly modified by the nonuniform exchange interaction in the polarized Pd, provide important new insights to understanding nanomagnetism in two-dimensions.   

Persistent interlayer coupling by an antiferromagnetic spacer above its Neel temperature (a Monte Carlo study)

It has been demonstrated by neutron diffraction experiments\footnote{J.A. Borchers et al., Phys. Rev. Lett. {\bf 70}, 1878 (1993).} that if a thin film of antiferromagnetic (AFM) material of bulk Neel temperature $T_N$ is placed between two AFM layers or between two ferromagnetic (FM) layers with much higher transition temperatures, then a short-range AFM ordering in the ``sandwiched'' layer may persist well above $T_N$, and it may maintain magnetic coupling between the two adjacent layers which are still in their ordered phase. We report MC simulations of exchange-coupled FM/AFM/FM trilayers with an even number (4, 6, or 8) of AFM monolayers. In these systems the magnetization vectors of the FM blocks are antiparallel, but an external magnetic field $B$ tends to incline them toward its direction. By varying $B$, we investigated the strength of the interlayer coupling between the FM films. In a system with a spacer consisting of 4 AFM layers the FM blocks remain coupled even at temperatures 50\% higher than $T_N$. We believe that such trilayers may be used for making new types of TMR sensors with ``temperature-tunable'' sensitivity to the magnetic field.   

Exploring the Interlayer Exchange Coupling of Ferromagnetic Films and Nanoclusters Across MgO

We investigate the interlayer exchange coupling (IEC) in Fe/MgO/Fe and Fe/MgO/Co systems. Samples are fabricated through molecular beam epitaxy (MBE) synthesis and magnetic properties are examined through magneto-optic Kerr effect (MOKE) measurements. First, comparing the bilinear coupling (J1) observed in the Fe/MgO/Fe and Fe/MgO/Co systems, we find an enhanced coupling in Fe/MgO/Co showing that J1 depends on magnetic material composition. Next, we embed Fe nanoclusters (NC) within the MgO spacer and find that we can tune the J1 by varying the position the NC. Lastly, examining J1 between ferromagnetic thin-film and NC in Fe/MgO/NC and Co/MgO/NC systems, we find a large enhancement in J1 for Co/MgO/NC with differences larger then what is seen in the analogous thin-film systems. This amplification provides evidence for enhanced coupling due to nano-scaling effects.   

Electric field-driven magnetocrystalline anisotropy switching of Fe/MgO: Towards full understanding from first principles

Controlling magnetic properties by an external electric field ($E$-field) is a key challenge in modern magnetic physics. Here, we investigate the magnetocrystalline anisotropy (MCA) modification by an $E$-field for thin films of Fe on a MgO substrate from first principles. Calculations were carried out by using the film FLAPW method\footnote{Wimmer, Krakauer, Weinert, and Freeman, PRB {\bf 24}, 864 (1981).} with full optimization by atomic force calculations in which an $E$-field effect is incorporated.\footnote{Nakamura et al., PRL {\bf 102}, 18702 (2009); Weinert et al., J. Phys.: Condens. Matter {\bf 21}, 084201 (2009).} Results predict that the Fe/MgO interface gives rise to a large out-of-plane MCA due to an Fe-O hybridization at the interface and a MCA modification is induced by a change in the $d$-band structures at the Fermi level when an $E$-field is introduced. Importantly, however, the existence of an interfacial iron- oxide layer between the Fe layer and the MgO substrate is found to play a key role in demonstrating an electric field-driven MCA switching, {i.e.,} from out-of-plane MCA to in-plane MCA --- as observed in experiments.\footnote{Shiota et al., Appl. Phys. Express {\bf 2}, 063001 (2009).}   

Uncompensated magnetization and structural phase transitions at Cr$_{2}$O$_{3}$ (0001) surface: Theory

Active research is currently aimed at achieving voltage-controlled magnetic functionality in nanostructures. While multiferroics attract most attention in this respect, magnetoelectric Cr$_{2}$O$_{3}$ offers an alternative route. Here we show that the Cr$_{2}$O$_{3}$ (0001) surface (in the single antiferromagnetic domain state) has an unusual feature of having an uncompensated magnetization, which persists even if the surface is rough. This magnetization persists up to the bulk N\'{e}el temperature. Reversible isothermal electrical switching of exchange bias field utilizing this feature was demonstrated experimentally [1]. We further investigate the termination of the Cr$_{2}$O$_{3}$ (0001) surface using total energy calculations and identify two competing sites for surface Cr atoms. We build a surface configurational Hamiltonian using supercell calculations and study its thermodynamics using the Monte Carlo method. An ordering phase transition from $1\times 1$ to $\sqrt 3 \times \sqrt 3 $ structure is found in agreement with LEED measurements [2]. \\[4pt] [1] X. He \textit{et al.}, contributed abstract MAR10-2009-006536. \\[0pt] [2] M. Bender \textit{et al.}, JPCM \textbf{7}, 5289 (1995).