Magnetoelectric (ME) coupling is the coupling of magnetic and electric properties within a material. It allows the possibility of dual control of the material through the manipulation of either electric or magnetic fields and therefore... more
Magnetoelectric (ME) coupling is the coupling of magnetic and electric properties within a material. It allows the possibility of dual control of the material through the manipulation of either electric or magnetic fields and therefore could potentially revolutionize the current technology. However, little is known about the factors that influence the strength of this magnetoelectric coupling. In the presented research,
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ABSTRACT The ferroelectric and antiferromagnetic properties of epitaxial, hexagonal (0001) YMnO3 thin films grown on GaN/Al2O3 substrates were studied using second harmonic generation. A Ti:sapphire laser with a 15 W Nd:YVO4 pump was used... more
ABSTRACT The ferroelectric and antiferromagnetic properties of epitaxial, hexagonal (0001) YMnO3 thin films grown on GaN/Al2O3 substrates were studied using second harmonic generation. A Ti:sapphire laser with a 15 W Nd:YVO4 pump was used to generate the second harmonic signal. Above the N'eel temperature, ferroelectric ordering was clearly observed as deduced from angular plots of the incoming and outgoing polarization of the second harmonic generation (SHG) signals. Additional antiferromagnetic order was identified below the N'eel temperature. The ferroelectric-magnetic coupling studied via SHG will be discussed.
ABSTRACT Fairly weak magnetoelectric coupling observed in the only single phase material (BeFiO3) exhibiting magnetism and ferroelectricity at room temperature has pushed scientists to consider alternative systems. Multilayers prove... more
ABSTRACT Fairly weak magnetoelectric coupling observed in the only single phase material (BeFiO3) exhibiting magnetism and ferroelectricity at room temperature has pushed scientists to consider alternative systems. Multilayers prove promising theoretically and experimentally, however, most modern techniques are blind to the interfacial mechanisms causing the coupling. Without a full understanding of the physical mechanism for these effects, significant improvements in the design and multiple potential applications of magnetoelectric coupling will be difficult to achieve. Optical measurements including second harmonic generation are crucial tools to solve this problem, as they provide complementary insight into the magnetic/ferroelectric properties and resulting carrier dynamics. For example, angular dependence SHG of magnetic LaSrMnO3 and ferroelectric PbZrTiO3 bilayers indicates the symmetry and magnetization as we vary thicknesses of the magnetic and ferroelectric layers and its implication to magnetoelectric coupling.
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Theory and experiments support that magnetoelectric coupling (electrical control of magnetism and vice versa) can be enhanced by taking advantage of interfacial coupling between magnetic and ferroelectric films. The interface between... more
Theory and experiments support that magnetoelectric coupling (electrical control of magnetism and vice versa) can be enhanced by taking advantage of interfacial coupling between magnetic and ferroelectric films. The interface between ferroelectric PZT and ferromagnetic LSMO ...
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We study the scaling behavior of dipolar interacting nanoparticles in 3D samples of various sizes but constant particle density. Ferromagnetic gamma-Fe2O3 clusters embedded in a polystyrene matrix are fabricated by thermal decomposition... more
We study the scaling behavior of dipolar interacting nanoparticles in 3D samples of various sizes but constant particle density. Ferromagnetic gamma-Fe2O3 clusters embedded in a polystyrene matrix are fabricated by thermal decomposition of metal carbonyls. Transmission electron microscopy reveals a narrow size distribution of 12 nm clusters. They are randomly dispersed in the matrix with an average separation of 80
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ABSTRACT The temperature dependence of the coercivity is studied in (Co/CoO)n periodic multilayer thin film superstructures below and above the exchange bias blocking temperature. The ferromagnetic Co thin films are grown with the help of... more
ABSTRACT The temperature dependence of the coercivity is studied in (Co/CoO)n periodic multilayer thin film superstructures below and above the exchange bias blocking temperature. The ferromagnetic Co thin films are grown with the help of MBE at a base pressure of 10E-10 m.bar whereas antiferromagnetic CoO thin films are grown from in-situ oxidized Co. The thicknesses of these films are monitored by reflection high energy electron diffraction (RHEED). A mean-field theory^1 is outlined which provides an analytic and intuitive expression for the enhancement of the coercivity of the ferromagnet which experiences the exchange coupling with a neighboring antiferromagnet. An experimental approach is developed allowing to determine the interface susceptibility of an individual antiferromagnetic pinning layer by systematic change in the thickness of the antiferromagnet thin films in various sets of superlattice samples measured at different temperatures, respectively. The experiment enables us to separate out the intrinsic coercivity from the contribution induced by exchange coupling at the interface. It is the goal of our study to evidence or disprove if it is simply this susceptibility of the reversible interface magnetization creating the spin drag effect and by that the coercivity enhancement. Financial support by NSF through CAREER DMR-0547887, NRI and Nebraska MRSEC. ^1G. Scholten, K. D. Usadel, and U. Nowak, Phys. Rev B. 71, 064413 (2005).
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ABSTRACT Fairly weak magnetoelectric coupling observed in the only single phase material (BeFiO3) exhibiting magnetism and ferroelectricity at room temperature has pushed scientists to consider alternative systems. Multilayers prove... more
ABSTRACT Fairly weak magnetoelectric coupling observed in the only single phase material (BeFiO3) exhibiting magnetism and ferroelectricity at room temperature has pushed scientists to consider alternative systems. Multilayers prove promising theoretically and experimentally, however, most modern techniques are blind to the interfacial mechanisms causing the coupling. Without a full understanding of the physical mechanism for these effects, significant improvements in the design and multiple potential applications of magnetoelectric coupling will be difficult to achieve. Optical measurements including second harmonic generation are crucial tools to solve this problem, as they provide complementary insight into the magnetic/ferroelectric properties and resulting carrier dynamics. For example, angular dependence SHG of magnetic LaSrMnO3 and ferroelectric PbZrTiO3 bilayers indicates the symmetry and magnetization as we vary thicknesses of the magnetic and ferroelectric layers and its implication to magnetoelectric coupling.
Low-temperature atomic layer deposition of conformal ZnO on a self-assembled block polymer lithographic template comprising well-ordered, vertically aligned cylindrical pores within a poly(styrene) (PS) matrix was used to produce... more
Low-temperature atomic layer deposition of conformal ZnO on a self-assembled block polymer lithographic template comprising well-ordered, vertically aligned cylindrical pores within a poly(styrene) (PS) matrix was used to produce nanocrucible templates with pore diameters tunable via ZnO thickness. Starting from a PS template with a hexagonal array of 30 nm diameter pores on a 45 nm pitch, the ZnO thickness was progressively increased to narrow the pore diameter to as low as 14 nm. Upon removal of the PS by heat treatment in air at 500 °C to form an array of size-tunable ZnO nanocrucibles, permalloy (Ni80Fe20) was evaporated at normal incidence, filling the pores and creating an overlayer. Argon ion beam milling was then used to etch back the overlayer (a Damascene-type process), leaving a well-ordered array of isolated ZnO nanocrucibles filled with permalloy nanodots. Microscopy and temperature-dependent magnetometry verified the diameter reduction with increasing ZnO thickness. The largest diameter (30 nm) dots exhibit a ferromagnetic multidomain/vortex state at 300 K, with relatively weakly temperature-dependent coercivity. Reducing the diameter leads to a crossover to a single-domain state and eventually superparamagnetism at sufficiently high temperature, in quantitative agreement with expectations. We argue that this approach could render this form of block polymer lithography compatible with high-temperature processing (as required for technologically important high perpendicular anisotropy ordered alloys, for instance), in addition to enabling separation-dependent studies to probe interdot magnetostatic interactions.
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Magnetoelectric (ME) coupling is the coupling of magnetic and electric properties within a material. It allows the possibility of dual control of the material through the manipulation of either electric or magnetic fields and therefore... more
Magnetoelectric (ME) coupling is the coupling of magnetic and electric properties within a material. It allows the possibility of dual control of the material through the manipulation of either electric or magnetic fields and therefore could potentially revolutionize the current technology. However, little is known about the factors that influence the strength of this magnetoelectric coupling. In the presented research,
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X-ray absorption spectroscopy (XAS) and photoemission electron microscopy (PEEM) have been used to determine the magnetic properties of magnetoelectrics, possessing both ferroelectric and magnetic order; however, the additional... more
X-ray absorption spectroscopy (XAS) and photoemission electron microscopy (PEEM) have been used to determine the magnetic properties of magnetoelectrics, possessing both ferroelectric and magnetic order; however, the additional sensitivity to the ferroelectricity in ...
... 11:15 AM–2:15 PM, Wednesday, February 29, 2012. Room: 252B Sponsoring Unit: DMP Chair:Kyle Shen, Department of Physics, Cornell University. ... Evan Wolfe (West Virginia University). Charles Frye (West Virginia University). Disheng... more
... 11:15 AM–2:15 PM, Wednesday, February 29, 2012. Room: 252B Sponsoring Unit: DMP Chair:Kyle Shen, Department of Physics, Cornell University. ... Evan Wolfe (West Virginia University). Charles Frye (West Virginia University). Disheng Chen (West Virginia University). ...
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ABSTRACT Piezoelectrically controlled strain is used for electric tuning of exchange-bias fields. A generic exchange-bias Co/CoO bilayer is deposited on the surface of a ferroelectric and thus piezoelectric BaTiO3 substrate which allows... more
ABSTRACT Piezoelectrically controlled strain is used for electric tuning of exchange-bias fields. A generic exchange-bias Co/CoO bilayer is deposited on the surface of a ferroelectric and thus piezoelectric BaTiO3 substrate which allows to apply electrically and thermally tunable stress in the adjacent ferromagnetic Co thin film. The stress-induced strain alters foremost the magnetic anisotropy of the Co film and by that the magnetization orientation at the Co/CoO interface modifying the exchange-bias field. This results in a pronounced electrically induced weakening of the exchange bias but also includes the possibility of tuning the exchange-bias field through a subtle sign change from regular negative to positive values. The electrically controlled crossover from negative to positive exchange bias is consistently observed at various temperatures in the rhombohedral phase of BaTiO3. This complex electric field dependence of the exchange-bias field is the result of the long-range nature of strain and interpreted through competition between ferromagnetic and antiferromagnetic exchange at the Co/CoO interface. Our data suggest competition between regular negative and positive exchange bias. Weakening of negative exchange bias originates from noncollinear alignment of the Co and CoO interface magnetizations. Positive exchange bias is activated when stress induces antiferromagnetic exchange through atomic displacements changing the exchange paths at the Co/CoO interface.
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Magnetoresistance (MR) measurements are carried out on a Co(8 nm)/CoO(3.5 nm) bilayer in the exchange bias (EB) state prepared by molecular beam epitaxy. With the applied magnetic field parallel to the current, the EB MR curves show an... more
Magnetoresistance (MR) measurements are carried out on a Co(8 nm)/CoO(3.5 nm) bilayer in the exchange bias (EB) state prepared by molecular beam epitaxy. With the applied magnetic field parallel to the current, the EB MR curves show an asymmetric behavior about the minimum, in contrast to the symmetric one for non-EB systems. We generalize a well-known analytical expression used for the field dependence of the MR of paramagnets. Our generalization incorporates coercivity and EB in a new phenomenological MR expression. Excellent fits of the latter to the experimental MR data are achieved, showing the way to use MR techniques for the quantitative characterization of EB systems. Furthermore, the temperature dependence of the EB field obtained from MR loops can be described with a power law, which yields a value of 96.6 K for the EB blocking temperature, which is significantly below the Néel temperature of 293 K for bulk CoO.
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Theory and experiments support that magnetoelectric coupling (electrical control of magnetism and vice versa) can be enhanced by taking advantage of interfacial coupling between magnetic and ferroelectric films. The interface between... more
Theory and experiments support that magnetoelectric coupling (electrical control of magnetism and vice versa) can be enhanced by taking advantage of interfacial coupling between magnetic and ferroelectric films. The interface between ferroelectric PZT and ferromagnetic LSMO ...
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We report on the optimization of Magneto Optical Kerr setups. Photoelastic modulation and phase sensitive detector methodology have been used to measure the first and second harmonics of the reflected light intensity related to the... more
We report on the optimization of Magneto Optical Kerr setups. Photoelastic modulation and phase sensitive detector methodology have been used to measure the first and second harmonics of the reflected light intensity related to the magnetization-dependent off-diagonal reflection ...
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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... more
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 ...
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We measured the anisotropic magnetoresistance of a Co(11nm)/CoO bilayer in exchange biased and unbiased states. The bilayer was fabricated on a-Al2O3 substrate maintained at 300^oC by molecular beam epitaxy at a base pressure of... more
We measured the anisotropic magnetoresistance of a Co(11nm)/CoO bilayer in exchange biased and unbiased states. The bilayer was fabricated on a-Al2O3 substrate maintained at 300^oC by molecular beam epitaxy at a base pressure of 10-11mbar. theta-2theta X-ray diffraction scans reveal hcp (001) texture of the Co film. No peak associated with the naturally formed CoO top layer was identified. Small
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ABSTRACT The ferroelectric and antiferromagnetic properties of epitaxial, hexagonal (0001) YMnO3 thin films grown on GaN/Al2O3 substrates were studied using second harmonic generation. A Ti:sapphire laser with a 15 W Nd:YVO4 pump was used... more
ABSTRACT The ferroelectric and antiferromagnetic properties of epitaxial, hexagonal (0001) YMnO3 thin films grown on GaN/Al2O3 substrates were studied using second harmonic generation. A Ti:sapphire laser with a 15 W Nd:YVO4 pump was used to generate the second harmonic signal. Above the N'eel temperature, ferroelectric ordering was clearly observed as deduced from angular plots of the incoming and outgoing polarization of the second harmonic generation (SHG) signals. Additional antiferromagnetic order was identified below the N'eel temperature. The ferroelectric-magnetic coupling studied via SHG will be discussed.
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The exchange bias (EB) training effect is studied in a Co/CoO heterostructure using low temperature longitudinal Kerr rotation. After field cooling the sample to below the N\'{e}el temperature of CoO the EB training effect manifests... more
The exchange bias (EB) training effect is studied in a Co/CoO heterostructure using low temperature longitudinal Kerr rotation. After field cooling the sample to below the N\'{e}el temperature of CoO the EB training effect manifests itself by a decrease of the EB field upon cycling the Co ...
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Epitaxial (111) oriented thin films of magnetoelectric (ME) Cr2O3 are grown by MBE. These films are the key component of Cr2O3(111)/(Co/Pt)3 heterostructures allowing for electrically controlled exchange bias (EB) and novel spintronic... more
Epitaxial (111) oriented thin films of magnetoelectric (ME) Cr2O3 are grown by MBE. These films are the key component of Cr2O3(111)/(Co/Pt)3 heterostructures allowing for electrically controlled exchange bias (EB) and novel spintronic applications [1]. Pure voltage control of ...
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We study the scaling behavior of dipolar interacting nanoparticles in 3D samples of various sizes but constant particle density. Ferromagnetic $\gamma $-Fe$_{2}$O$_{3}$ clusters embedded in a polystyrene matrix are fabricated by thermal... more
We study the scaling behavior of dipolar interacting nanoparticles in 3D samples of various sizes but constant particle density. Ferromagnetic $\gamma $-Fe$_{2}$O$_{3}$ clusters embedded in a polystyrene matrix are fabricated by thermal decomposition of metal carbonyls. ...