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J C Yang,
Q He,
S J Suresha,
C Y Kuo,
C Y Peng,
R C Haislmaier,
M A Motyka,
G Sheng,
C Adamo,
H J Lin, [......], E Arenholz,
N J Podraza,
M Bernhagen,
R Uecker,
D G Schlom,
V Gopalan,
L Q Chen,
C T Chen,
R Ramesh,
Y H Chu
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ABSTRACT: A new orthorhombic phase of the multiferroic BiFeO_{3} has been created via strain engineering by growing it on a NdScO_{3}(110)_{o} substrate. The tensile-strained orthorhombic BiFeO_{3} phase is ferroelectric and antiferromagnetic at room temperature. A combination of nonlinear optical second harmonic generation and piezoresponse force microscopy revealed that the ferroelectric polarization in the orthorhombic phase is along the in-plane ⟨110⟩_{pc} directions. In addition, the corresponding rotation of the antiferromagnetic axis in this new phase was observed using x-ray linear dichroism.
Physical Review Letters 12/2012; 109(24):247606. · 7.37 Impact Factor
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J. Li, E. Arenholz,
Y. Meng,
A. Tan,
J. Park,
E. Jin,
H. Son,
J. Wu,
C. A. Jenkins,
A. Scholl,
H. W. Zhao,
Chanyong Hwang,
Z. Q. Qiu
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ABSTRACT: Fe/NiO/MgO/Ag(001) films were grown epitaxially, and the Fe and NiO spin orientations were determined using x-ray magnetic dichroism. We find that the NiO spins are aligned perpendicularly to the in-plane Fe spins. Analyzing both the in-plane and out-of-plane spin components of the NiO layer, we demonstrate unambiguously that the antiferromagnetic NiO spins undergo a continuous spin reorientation transition from the in-plane to out-of-plane directions with increasing of the MgO thickness.
Phys. Rev. B. 07/2011; 84(1).
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ABSTRACT: Single crystalline FeMn/Ni bilayer was epitaxially grown on Cu(001) substrate and investigated by photoemission electron microscopy (PEEM). The FeMn and Ni films were grown into two cross wedges to facilitate an independent control of the FeMn (0-20 ML) and Ni (0-20 ML) film thicknesses. The Ni magnetic phases were determined by Ni domain images as a function of the Ni thickness (d<sub>Ni</sub>) and the FeMn thickness (d<sub>FeMn</sub>). The result shows that as the Ni thickness increases, the Ni film undergoes a paramagnetic-to-ferromagnetic state transition at a critical thickness of d<sub>FM</sub> and an in-plane to out-of-plane spin reorientation transition at a thicker thickness d<sub>SRT</sub>. The phase diagram shows that both d<sub>FM</sub> and d<sub>SRT</sub> increase as the FeMn film establishes its antiferromagnetic order.
IEEE Transactions on Magnetics 07/2011; · 1.36 Impact Factor
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Y. Meng,
J. Li,
A. Tan,
E. Jin,
J. Son,
J. S. Park,
A. Doran,
A. T. Young,
A. Scholl, E. Arenholz,
J. Wu,
C. Hwang,
H. W. Zhao,
Z. Q. Qiu
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ABSTRACT: NiO/Ag/CoO/Fe single crystalline films are grown epitaxially on a vicinal Ag(001) substrate using molecular beam epitaxy and investigated by photoemission electron microscopy. We find that after zero-field cooling, the in-plane Fe magnetization switches from parallel to perpendicular direction of the atomic steps of the vicinal surface at thinner CoO thickness but remains in its original direction parallel to the steps at thicker CoO thickness. CoO and NiO domain imaging result shows that both CoO/Fe and NiO/CoO spins are perpendicularly coupled, suggesting that the Fe magnetization switching may be associated with the rotatable-frozen spin transition of the CoO film.
Applied Physics Letters 05/2011; 98(21):212508-212508-3. · 3.84 Impact Factor
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Q He,
Y-H Chu,
J T Heron,
S Y Yang,
W I Liang,
C Y Kuo,
H J Lin,
P Yu,
C W Liang,
R J Zeches,
W C Kuo,
J Y Juang,
C T Chen, E Arenholz,
A Scholl,
R Ramesh
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ABSTRACT: Magnetoelectrics and multiferroics present exciting opportunities for electric-field control of magnetism. However, there are few room-temperature ferromagnetic-ferroelectrics. Among the various types of multiferroics the bismuth ferrite system has received much attention primarily because both the ferroelectric and the antiferromagnetic orders are quite robust at room temperature. Here we demonstrate the emergence of an enhanced spontaneous magnetization in a strain-driven rhombohedral and super-tetragonal mixed phase of BiFeO₃. Using X-ray magnetic circular dichroism-based photoemission electron microscopy coupled with macroscopic magnetic measurements, we find that the spontaneous magnetization of the rhombohedral phase is significantly enhanced above the canted antiferromagnetic moment in the bulk phase, as a consequence of a piezomagnetic coupling to the adjacent tetragonal-like phase and the epitaxial constraint. Reversible electric-field control and manipulation of this magnetic moment at room temperature is also shown.
Nature Communications 03/2011; 2:225. · 7.40 Impact Factor
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ABSTRACT: We studied the magnetic behavior of bilayers of multiferroic and nominally
antiferromagnetic o-YMnO$_3$ (375~nm thick) and ferromagnetic
La$_{0.7}$Sr$_{0.3}$MnO$_3$ and La$_{0.67}$Ca$_{0.33}$MnO$_3$ ($8 \ldots
225~$nm), in particular the vertical magnetization shift $M_E$ and exchange
bias field $H_E$ for different thickness and magnetic dilution of the
ferromagnetic layer at different temperatures and cooling fields. We have found
very large $M_E$ shifts equivalent to up to 100\% of the saturation value of
the o-YMO layer alone. The overall behavior indicates that the properties of
the ferromagnetic layer contribute substantially to the $M_E$ shift and that
this does not correlate straightforwardly with the measured exchange bias field
$H_E$.
09/2010;
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ABSTRACT: Epitaxial Py/Cu/Co/Cu(001) trilayers were patterned into micron sized disks and imaged using element-specific photoemission electron microscopy. By varying the Cu spacer layer thickness, we study how the coupling between the two magnetic layers influences the formation of magnetic vortex states. We find that while the Py and Co disks form magnetic vortex domains when the interlayer coupling is ferromagnetic, the magnetic vortex domains of the Py and Co disks break into anti-parallel aligned multidomains when the interlayer coupling is antiferromagnetic. We explain this result in terms of magnetic flux closure between the Py and Co layers for the antiferromagnetic coupling case.
Journal of Physics Condensed Matter 09/2010; 22(34):342001. · 2.55 Impact Factor
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ABSTRACT: CoO/Fe thin films were grown epitaxially onto vicinal Ag(001) and investigated using magneto-optic Kerr effect, x-ray magnetic circular dichroism (XMCD), and x-ray magnetic linear dichroism (XMLD) techniques. We show that the CoO film in the ultrathin regime does not induce a uniaxial magnetic anisotropy but a coercivity enhancement. This result provides a mechanism for the microscopic origin of the rotatable magnetic anisotropy. XMLD measurement further reveals that the underlying mechanism is that the CoO spins are totally rotatable in the ultrathin regime to follow the Fe magnetization.
Applied Physics Letters 07/2010; 97(4):042505-042505-3. · 3.84 Impact Factor
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ABSTRACT: The exchange bias of epitaxially grown CoO/Fe/Ag(001) was investigated using x-ray magnetic circular dichroism and x-ray magnetic linear dichroism (XMLD) techniques. A direct XMLD measurement on the CoO layer during the Fe magnetization reversal shows that the CoO compensated spins are rotatable at thinner thickness and frozen at larger thickness. By a quantitative determination of the rotatable and frozen CoO spins as a function of the CoO film thickness, we find the remarkable result that the exchange bias is well established before frozen spins are detectable in the CoO film. We further show that the rotatable and frozen CoO spins are uniformly distributed in the CoO film.
Physical Review Letters 05/2010; 104(21):217204. · 7.37 Impact Factor
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ABSTRACT: Co/NiO/Fe trilayers are grown on Ag(001) substrate using molecular-beam epitaxy and investigated by element-specific magnetic domain images using x-ray magnetic circular dichroism and x-ray magnetic linear dichroism techniques. By comparing the Co, Fe, and NiO magnetic domain images, we identify that the anomalous Co-Fe interlayer coupling from a 90° coupling to a collinear coupling with increasing the NiO film thickness is due to a transition from a collinear to 90° coupling at the NiO/Fe interface while retaining a 90° coupling at the Co/NiO interface. Uncompensated Ni spins are found at the Co/NiO interface but are absent at the NiO/Fe interface. No evidence of spiral NiO spin structure is found in this Co/NiO/Fe sandwich.
Phys. Rev. B. 07/2009; 80(1).
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ABSTRACT: FeMn/Ni/Cu(001) bilayer films are grown epitaxially and investigated by photoemission electron microscopy and magneto-optic Kerr effect. We find that as the FeMn overlayer changes from paramagnetic to antiferromagnetic state, it switches the ferromagnetic Ni spin direction from the out-of-plane to an in-plane direction of the film. This phenomenon reveals the mechanism of creating magnetic anisotropy by the out-of-plane spin frustration at the FeMn-Ni interface.
Phys. Rev. B. 06/2009; 79(21).
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ABSTRACT: We present a x-ray dichroism study of graphite surfaces that addresses the origin and magnitude of ferromagnetism in metal-free carbon. We find that, in addition to carbon $\pi$ states, also hydrogen-mediated electronic states exhibit a net spin polarization with significant magnetic remanence at room temperature. The observed magnetism is restricted to the top $\approx$10 nm of the irradiated sample where the actual magnetization reaches $ \simeq 15$ emu/g at room temperature. We prove that the ferromagnetism found in metal-free untreated graphite is intrinsic and has a similar origin as the one found in proton bombarded graphite. Comment: 10 pages, 5 figures, 1 table, submitted to New Journal of Physics
05/2009;
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ABSTRACT: We present work done on EuO films with thicknesses varying from 10 to 60 Å grown as a stepped wedge on Si / SiO <sub>2</sub>/ Cr (20 Å )/ Cu (90 Å ) and capped with Y(20 Å)/Al(80 Å). The films were characterized by x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) at the europium M<sub>5</sub> and copper L<sub>3</sub> edges. The films' high quality and consistent magnetic properties were confirmed by superconducting quantum interference device magnetometry, which revealed a constant saturation moment independent of film thickness. XAS at the Cu L<sub>3</sub> edge showed that the bottom Cu electrode is metallic (oxidation free). We report an XMCD intensity of 52% (±4.3), in excellent agreement with theoretical calculations.
Journal of Applied Physics 05/2009; · 2.17 Impact Factor
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ABSTRACT: The x-ray absorption spectra of the Fe and Zn L edges for 6.7 nm Fe <sub>3</sub> O <sub>4</sub> nanoparticles grown inside 12 nm ferritin protein cages with 10%, 15%, 20%, and 33% zinc doping show that Zn is substitutional as Zn <sup>2+</sup> within the iron oxide host structure. A Neel–Arrhenius plot of the blocking temperature in frequency dependent ac-susceptibility measurements shows that the particles are noninteracting and that the anisotropy energy barrier is reduced with Zn loading. X-ray magnetic circular dichroism of the Fe displays a linear decrease with Zn doping in sharp contrast to the initial increase present in the bulk system. The most plausible explanation for the decrease in moment is that Zn substitutes preferentially into the tetrahedral A site as a Zn <sup>2+</sup> cation, generating a mixed spinel.
Journal of Applied Physics 05/2009; · 2.17 Impact Factor
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ABSTRACT: Using soft x-ray spectromicroscopy we show that NiO(001) exhibits a crystallographic and magnetic domain structure near the surface identical to that of the bulk. Upon Co deposition a perpendicular coupling between the Ni and Co moments is observed that persists even after formation of uncompensated Ni spins at the interface through annealing. The chemical composition at the interface alters its crystallographic structure and leads to a reorientation of the Ni moments from the ⟨112⟩ to the ⟨110⟩ direction. We show that this reorientation is driven by changes in the magnetocrystalline anisotropy rather than exchange coupling mediated by residual uncompensated spins.
Phys. Rev. B. 02/2009; 79(5).
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ABSTRACT: This paper deals with the growth and characterization of ferromagnetic cobalt doped TiO <sub>2</sub> thin films deposited by liquid precursor metal organic chemical vapor deposition using a combination of the source materials Co ( TMHD )<sub>3</sub> , tetrahydrofuran, and titanium isopropoxide. An array of experiments reveals the intrinsic ferromagnetic nature of the grown films and suggests that the magnetism is not generated by oxygen vacancies.
Journal of Applied Physics 11/2008; · 2.17 Impact Factor
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ABSTRACT: A large exchange splitting of the conduction band in ultrathin films of the ferromagnetic semiconductor EuO was determined quantitatively, by using EuO as a tunnel barrier and fitting the current-voltage characteristics and temperature dependence to tunneling theory. This exchange splitting leads to different tunnel barrier heights for spin-up and spin-down electrons and is large enough to produce a near-fully spin-polarized current. Moreover, the magnetic properties of these ultrathin films (<6 nm) show a reduction in Curie temperature with decreasing thickness, in agreement with theoretical calculation [R. Schiller, Phys. Rev. Lett. 86, 3847 (2001)10.1103/Phys. Rev. Lett.86.3847].
Physical Review Letters 10/2008; 101(14):147201. · 7.37 Impact Factor
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ABSTRACT: Antiferromagnetic (AFM) NiO thin films are grown epitaxially on vicinal Ag(118) substrate and investigated by x-ray linear dichroism (XLD). We find that the NiO AFM spins exhibit an in-plane spin-reorientation transition from parallel to perpendicular to the step edge direction with increasing the NiO film thickness. In addition to the conventional L2 absorption edge, XLD effect at the Ni L3 absorption edge is also measured and analyzed. The results identify a small energy shift of the L3 peak. Temperature-dependent measurement confirms that the observed XLD effect in this system at the normal incidence of the x rays originates entirely from the NiO magnetic ordering.
Phys. Rev. B. 08/2008; 78(6).
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ABSTRACT: Ferromagnetic Ni-Cu-Co and Ni-Fe-Co sandwiches were grown epitaxially onto a vicinal Cu(001) substrate and investigated using magneto-optical Kerr effect and x-ray magnetic circular dichroism techniques. We find that the atomic steps of the vicinal surface break the magnetic reversal symmetry to induce an exchange bias in the Ni perpendicular magnetic hysteresis loop. The Ni exchange bias direction can be switched by changing the direction of the in-plane Co magnetization. In addition, the exchange bias can be tailored by changing the Cu or Fe spacer layer thickness.
Physical Review Letters 09/2007; 99(7):077203. · 7.37 Impact Factor
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ABSTRACT: The magnetic properties of antiferromagnetic NiO thin films grown on curved Ag(001) substrate were investigated using x-ray magnetic linear dichroism. We show that atomic steps on a vicinal Ag(001) surface induce an in-plane uniaxial magnetic anisotropy, which favors the Ni spins to be perpendicular to the steps, which are parallel to a [110] axis and parallel to the steps for steps that are along a [100] axis. Temperature- and thickness-dependent measurements show that [110] and [100] steps produce different in-plane crystal field, which may be responsible for the different NiO in-plane magnetic anisotropy.
Phys. Rev. B. 12/2006; 74(21).
