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Construction of the Magnetic Phase Diagram of FeMn/Ni/Cu(001) Using Photoemission Electron Microscopy

Description: 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}) and the FeMn thickness (d{sub FeMn}). 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} and an in-plane to out-of-plane spin reorientation transition at a thicker thickness d{sub SRT}. The phase diagram shows that both d{sub FM} and d{sub SRT} increase as the FeMn film establishes its antiferromagnetic order.
Date: January 4, 2011
Creator: Wu, J.; Scholl, A.; Arenholz, E.; Hwang, C. & Qiu, Z. Q.
Partner: UNT Libraries Government Documents Department

Stripe-to-bubble transition of magnetic domains at the spin reorientation of (Fe/Ni)/Cu/Ni/Cu(001)

Description: Magnetic domain evolution at the spin reorientation transition (SRT) of (Fe/Ni)/Cu/Ni/Cu(001) is investigated using photoemission electron microscopy. While the (Fe/Ni) layer exhibits the SRT, the interlayer coupling of the perpendicularly magnetized Ni layer to the (Fe/Ni) layer serves as a virtual perpendicular magnetic field exerted on the (Fe/Ni) layer. We find that the perpendicular virtual magnetic field breaks the up-down symmetry of the (Fe/Ni) stripe domains to induce a net magnetization in the normal direction of the film. Moreover, as the virtual magnetic field increases to exceed a critical field, the stripe domain phase evolves into a bubble domain phase. Although the critical field depends on the Fe film thickness, we show that the area fraction of the minority domain exhibits a universal value that determines the stripe-to-bubble phase transition.
Date: June 9, 2010
Creator: Wu, J.; Choi, J.; Won, C.; Wu, Y. Z.; Scholl, A.; Doran, A. et al.
Partner: UNT Libraries Government Documents Department

Correlation between exchange bias and pinned interfacialspins

Description: Using x-ray magnetic circular dichroism, we have detectedthe very interfacial spins that are responsible for the horizontal loopshift in three different exchange bias sandwiches, chosen because oftheir potential for device applications. The "pinned" uncompensatedinterfacial spins constitute only a fraction of a monolayer and do notrotate in an external magnetic field since they are tightly locked to theantiferromagnetic lattice. A simple extension of the Meiklejohn and Beanmodel is proposed to account quantitatively for the exchange bias fieldsin the three studied systems from the experimentally determined number ofpinned moments and their sizes.
Date: July 1, 2003
Creator: Ohldag, H.; Scholl, A.; Nolting, F.; Arenholz, E.; Maat, S.; Young, A.T. et al.
Partner: UNT Libraries Government Documents Department

Effect of NiO spin orientation on the magnetic anisotropy of the Fe film in epitaxially grown Fe/NiO/Ag(001) and Fe/NiO/MgO(001)

Description: Single crystalline Fe/NiO bilayers were epitaxially grown on Ag(001) and on MgO(001), and investigated by Low Energy Electron Diffraction (LEED), Magneto-Optic Kerr Effect (MOKE), and X-ray Magnetic Linear Dichroism (XMLD). We find that while the Fe film has an in-plane magnetization in both Fe/NiO/Ag(001) and Fe/NiO/MgO(001) systems, the NiO spin orientation changes from in-plane direction in Fe/NiO/Ag(001) to out-of-plane direction in Fe/NiO/MgO(001). These two different NiO spin orientations generate remarkable different effects that the NiO induced magnetic anisotropy in the Fe film is much greater in Fe/NiO/Ag(001) than in Fe/NiO/MgO(001). XMLD measurement shows that the much greater magnetic anisotropy in Fe/NiO/Ag(001) is due to a 90{sup o}-coupling between the in-plane NiO spins and the in-plane Fe spins.
Date: February 10, 2010
Creator: Kim, W.; Jin, E.; Wu, J.; Park, J.; Arenholz, E.; Scholl, A. et al.
Partner: UNT Libraries Government Documents Department

Determination of rotatable and frozen CoO spins and their relationship to exchange bias in CoO/Fe/Ag(001)

Description: The exchange bias of epitaxially grown CoO/Fe/Ag(001) was investigated using X-ray Magnetic Circular Dichroism (XMCD) 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, i.e. fixed in direction to the lattice, 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, contrary to the common assumption that the majority of antiferromagnetic spins need to be frozen to generate the exchange bias. We further show that the rotatable/frozen CoO spins are uniformly distributed in the CoO film.
Date: February 10, 2010
Creator: Wu, J.; Park, J.; Kim, W.; Arenholz, E.; Liberati, M.; Scholl, A. et al.
Partner: UNT Libraries Government Documents Department

A direct measurement of rotatable and frozen CoO spins in exchange bias system of CoO/Fe/Ag(001)

Description: 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.
Date: March 10, 2010
Creator: Wu, J.; Park, J. S.; Kim, W.; Arenholz, E.; Liberati, M.; Scholl, A. et al.
Partner: UNT Libraries Government Documents Department

Writable graphene: Breaking sp2 bonds with soft X-rays

Description: We study the stability of various kinds of graphene samples under soft x-ray irradiation. Our results show that in single-layer exfoliated graphene (a closer analog to two-dimensional material), the in-plane carbon-carbon bonds are unstable under x-ray irradiation, resulting in nanocrystalline structures. As the interaction along the third dimension increases by increasing the number of graphene layers or through the interaction with the substrate (epitaxial graphene), the effect of x-ray irradiation decreases and eventually becomes negligible for graphite and epitaxial graphene. Our results demonstrate the importance of the interaction along the third dimension in stabilizing the long range in-plane carbon-carbon bonding, and suggest the possibility of using x-ray to pattern graphene nanostructures in exfoliated graphene.
Date: June 9, 2010
Creator: Zhou, S.; Girit, C.; Scholl, A.; Jozwiak, C.; Siegel, D.; Yu, P. et al.
Partner: UNT Libraries Government Documents Department

Ultrafast magnetization dynamics studies using an x-ray streakcamera

Description: The spin dynamics of ferromagnetic thin films following an excitation by ultrashort 100-fs near-infrared laser pulses has recently received much attention. Here, a new approach is described using x-ray magnetic circular dichroism to investigated emagnetization and magnetization switching processes. In contrast to magneto-optical measurements, x-ray dichroism has the advantage of determining separately the spin and orbital components of the magnetic moment. The relatively low time resolution of the synchrotron x-ray probe pulses (80 ps FWHM) is overcome by employing an ultrafast x-ray streak camera with a time resolution of <1 ps. A description of the experimental setup including the x-ray/IR laser pulse synchronization and the streak camera is given.
Date: July 13, 2005
Creator: Bartelt, A.F.; Comin, A.; Feng, J.; Nasiatka, J.; Padmore, H.A.; Scholl, A. et al.
Partner: UNT Libraries Government Documents Department

An ultrafast x-ray detector system at an elliptically polarizingundulator beamline

Description: An ultrafast x-ray detector system is under development atLawrence Berkeley National Laboratory (LBNL) for application primarily tostudyies of ultrafast magnetization dynamics. The system consists of a fslaser, an x-ray streak camera and an ellipitically polarization undulator(EPU) beamline. Polarized x-rays from an EPU can be used to measure x-raymagnetic circular dichroism (XMCD) of a sample. XMCD has the uniqueability to independently measure orbit and spin magnetization withsub-monolayer sensitivity and element specificity. The streak camera hassimultaneously a sub-picosecond temporal resolution and a high spatialresolution. The combination of the streak camera and EPU allows us tostudy the transfer of angular momentum from spin to orbit to the latticein the sample on an ultrafast time scale. We describe here theperformance of the ultrafast detector, the laser and the x-raysynchronization system. The observation of the demagnetization process ofdifferent samples demonstrates the ability of the apparatus.
Date: May 1, 2007
Creator: Feng, J.; Comin, A.; Bartelt, A.F.; Shin, H.J.; Nasiatka, J.R.; Padmore, H.A. et al.
Partner: UNT Libraries Government Documents Department

Domain-size-dependent exchange bias in Co/LaFeO3

Description: X-ray microscopy using magnetic linear dichroism of a zero-field-grown, multi-domain Co/LaFeO{sub 3} ferromagnet/antiferromagnet sample shows a local exchange bias of random direction and magnitude. A statistical analysis of the local bias of individual, micron-size magnetic domains demonstrates an increasing bias field with decreasing domain size as expected for a random distribution of pinned, uncompensated spins, which are believed to mediate the interface coupling. A linear dependence with the inverse domain diameter is found.
Date: September 22, 2004
Creator: Scholl, A.; Nolting, F.; Seo, J.W.; Ohldag, H.; Stohr, J.; Raoux,S. et al.
Partner: UNT Libraries Government Documents Department

Creation of an antiferromagnetic exchange spring

Description: We present evidence for the creation of an exchange spring in an antiferromagnet due to exchange coupling to a ferromagnet. X-ray magnetic linear dichroism spectroscopy on single crystal Co/NiO(001) shows that a partial domain wall is wound up at the surface of the antiferromagnet when the adjacent ferromagnet is rotated by a magnetic field. We determine the interface exchange stiffness and the antiferromagnetic domain wall energy from the field dependence of the direction of the antiferromagnetic axis, the antiferromagnetic pendant to a ferromagnetic hysteresis loop. The existence of a planar antiferromagnetic domain wall, proven by our measurement, is a key assumption of most exchange bias models.
Date: April 6, 2004
Creator: Scholl, A.; Liberati, M.; Arenholz, E.; Ohldag, H. & Stohr, J.
Partner: UNT Libraries Government Documents Department

Continuous spin reorientation transition in epitaxially grown antiferromagnetic NiO thin films

Description: 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.
Date: March 1, 2011
Creator: Li, J.; Arenholz, E.; Meng, Y.; Tan, A.; Park, J.; Jin, E. et al.
Partner: UNT Libraries Government Documents Department

Direct observation of imprinted antiferromagnetic vortex state in CoO/Fe/Ag(001) disks

Description: In magnetic thin films, a magnetic vortex is a state in which the magnetization vector curls around the center of a confined structure. A vortex state in a thin film disk, for example, is a topological object characterized by the vortex polarity and the winding number. In ferromagnetic (FM) disks, these parameters govern many fundamental properties of the vortex such as its gyroscopic rotation, polarity reversal, core motion, and vortex pair excitation. However, in antiferromagnetic (AFM) disks, though there has been indirect evidence of the vortex state through observations of the induced FM-ordered spins in the AFM disk, they have never been observed directly in experiment. By fabricating single crystalline NiO/Fe/Ag(001) and CoO/Fe/Ag(001) disks and using X-ray Magnetic Linear Dichroism (XMLD), we show direct observation of the vortex state in an AFM disk of AFM/FM bilayer system. We observe that there are two types of AFM vortices, one of which has no analog in FM structures. Finally, we show that a frozen AFM vortex can bias a FM vortex at low temperature.
Date: December 21, 2010
Creator: Wu, J.; Carlton, D.; Park, J. S.; Meng, Y.; Arenholz, E.; Doran, A. et al.
Partner: UNT Libraries Government Documents Department

FeMn/Fe/Co/Cu(1,1,10) films studied using the magneto-optic Kerr effect and photoemission electron microscopy

Description: FeMn/Fe/Co/Cu(1,1,10) films were grown epitaxially and investigated using the magneto-optic Kerr effect and photoemission electron microscopy. We found that FeMn/Fe/Co/Cu(1,1,10) exhibits the same properties as FeMn/Co/Cu(1,1,10) for the ferromagnetic phase of the face centered cubic (fcc) Fe film but a different property for the non-ferromagnetic phase of the fcc Fe film. This result indicates that the characteristic property reported in the literature for FeMn/Co/Cu(001) comes from the FeMn spin structure and is independent of the ferromagnetic layer.
Date: July 31, 2011
Creator: Meng, Y.; Li, J.; Tan, A.; Park, J.; Jin, E.; Son, H. et al.
Partner: UNT Libraries Government Documents Department

Determination of the Fe magnetic anisotropies and the CoO frozen spins in epitaxial CoO/Fe/Ag(001)

Description: CoO/Fe/Ag(001) films were grown epitaxially and studied by X-ray Magnetic Circular Dichroism (XMCD) and X-ray Magnetic Linear Dichroism (XMLD). After field cooling along the Fe[100] axis to 80 K, exchange bias, uniaxial anisotropy, and 4-fold anisotropy of the films were determined by hysteresis loop and XMCD measurements by rotating the Fe magnetization within the film plane. The CoO frozen spins were determined by XMLD measurement as a function of CoO thickness.We find that among the exchange bias, uniaxial anisotropy, and 4-fold anisotropy, only the uniaxial magnetic anisotropy follows thickness dependence of the CoO frozen spins.
Date: April 28, 2011
Creator: Meng, J. Li, Y.; Park, J. S.; Jenkins, C. A.; Arenholz, E.; Scholl, A.; Tan, A. et al.
Partner: UNT Libraries Government Documents Department

Element-specific spin and orbital momentum dynamics of Fe/Gdmultilayers

Description: The role of orbital magnetism in the laser-induced demagnetization of Fe/Gd multilayers was investigated using time-resolved X-ray magnetic circular dichroism at 2-ps time resolution given by an x-ray streak camera. An ultrafast transfer of angular momentum from the spin via the orbital momentum to the lattice was observed which was characterized by rapidly thermalizing spin and orbital momenta. Strong interlayer exchange coupling between Fe and Gd led to a simultaneous demagnetization of both layers.
Date: March 23, 2007
Creator: Bartelt, A.F.; Comin, A.; Feng, J.; Nasiatka, J.R.; Eimuller, T.; Ludescher, B. et al.
Partner: UNT Libraries Government Documents Department

Ni spin switching induced by magnetic frustration in FeMn/Ni/Cu(001)

Description: Epitaxially grown FeMn/Ni/Cu(001) films are investigated by Photoemission Electron Microscopy and Magneto-Optic Kerr Effect. We find that as the FeMn overlayer changes from paramagnetic to antiferromagnetic state, it could switch the ferromagnetic Ni spin direction from out-of-plane to in-plane direction of the film. This phenomenon reveals a new mechanism of creating magnetic anisotropy and is attributed to the out-of-plane spin frustration at the FeMn-Ni interface.
Date: March 8, 2009
Creator: Wu, J.; Choi, J.; Scholl, A.; Doran, A.; Arenholz, E.; Hwang, Chanyong et al.
Partner: UNT Libraries Government Documents Department

Probing the evolution of antiferromagnetism in multiferroics

Description: This study delineates the evolution of magnetic order in epitaxial films of the room-temperature multiferroic BiFeO3 system. Using angle- and temperature-dependent dichroic measurements and spectromicroscopy, we have observed that the antiferromagnetic order in the model multiferroic BiFeO3 evolves systematically as a function of thickness and strain. Lattice-mismatch-induced strain is found to break the easy-plane magnetic symmetry of the bulk and leads to an easy axis of magnetization which can be controlled through strain. Understanding the evolution of magnetic structure and how to manipulate the magnetism in this model multiferroic has significant implications for utilization of such magnetoelectric materials in future applications.
Date: June 9, 2010
Creator: Holcomb, M.; Martin, L.; Scholl, A.; He, Q.; Yu, P.; Yang, C.-H. et al.
Partner: UNT Libraries Government Documents Department

Magnetism at spinel thin film interfaces probed through soft x-ray spectroscopy techniques

Description: Magnetic order and coupling at the interfaces of highly spin polarized Fe{sub 3}O{sub 4} heterostructures have been determined by surface sensitive and element specific soft x-ray spectroscopy and spectromicroscopy techniques. At ambient temperature, the interface between paramagnetic CoCr{sub 2}O{sub 4} or MnCr{sub 2}O{sub 4} and ferrimagnetic Fe{sub 3}O{sub 4} isostructural bilayers exhibits long range magnetic order of Co, Mn and Cr cations which cannot be explained in terms of the formation of interfacial MnFe{sub 2}O{sub 4} or CoFe{sub 2}O{sub 4}. Instead, the ferrimagnetism is induced by the adjacent Fe{sub 3}O{sub 4} layer and is the result of the stabilization of a spinel phase not achievable in bulk form. Magnetism at the interface region is observable up to 500 K, far beyond the chromite bulk Curie temperature of 50-95 K.
Date: December 16, 2009
Creator: Chopdekar, R.V.; Liberati, M.; Takamura, Y.; Kourkoutis, L. Fitting; Bettinger, J. S.; Nelson-Cheeseman, B. B. et al.
Partner: UNT Libraries Government Documents Department

Switching a magnetic vortex by interlayer coupling in epitaxially grown Co/Cu/Py/Cu(001) trilayer disks

Description: 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.
Date: July 16, 2010
Creator: Wu, J.; Carlton, D.; Oelker, E.; Park, J. S.; Jin, E.; Arenholz, E. et al.
Partner: UNT Libraries Government Documents Department

Exploring the microscopic origin of exchange bias with photo-electron emission microscopy (PEEM)

Description: It is well known that magnetic exchange coupling across the ferromagnet--antiferromagnet interface results in a unidirectional magnetic anisotropy of the ferromagnetic layer, called exchange bias. Despite large experimental and theoretical efforts, the origin of exchange bias is still controversial, mainly because detection of the interfacial magnetic structure is difficult. We have applied photoelectron emission microscopy (PEEM) on several ferromagnet - antiferromagnet thin-film structures and microscopically imaged the ferromagnetic and the antiferromagnetic structure with high spatial resolution. Taking advantage of the surface sensitivity and elemental specificity of PEEM, the magnetic configuration and critical properties such as the Neel temperature were determined on LaFeO{sub 3} and NiO thin films and single crystals. On samples coated with a ferromagnetic layer, we microscopically observe exchange coupling across the interface, causing a clear correspondence of the domain structures in the adjacent ferromagnet and antiferromagnet. Field dependent measurements reveal a strong uniaxial anisotropy in individual ferromagnetic domains. A local exchange bias was observed even in not explicitly field-annealed samples, caused by interfacial uncompensated magnetic spins. These experiments provide highly desired information on the relative orientation of electron spins at the interface between ferromagnets and antiferromagnets.
Date: January 22, 2001
Creator: Scholl, A.; Nolting, F.; Stohr, J.; Regan, T.; Luning, J.; Seo, J.W. et al.
Partner: UNT Libraries Government Documents Department

Studies of the magnetic structure at the ferromagnet - antiferromagnet interface

Description: Antiferromagnetic layers are a scientifically challenging component in magneto-electronic devices such as magnetic sensors in hard disk heads, or magnetic RAM elements. In this paper we show that photo-electron emission microscopy (PEEM) is capable of determining the magnetic structure at the interface of ferromagnets and antiferromagnets with high spatial resolution (down to 20 nm). Dichroism effects at the L edges of the magnetic 3d transition metals, using circularly or linearly polarized soft x-rays from a synchrotron source, give rise to a magnetic image contrast. Images, acquired with the PEEM2 experiment at the Advanced Light Source, show magnetic contrast for antiferromagnetic LaFeO{sub 3}, microscopically resolving the magnetic domain structure in an antiferromagnetically ordered thin film for the first time. Magnetic coupling between LaFeO{sub 3} and an adjacent Co layer results in a complete correlation of their magnetic domain structures. From field dependent measurements a unidirectional anisotropy resulting in a local exchange bias of up to 30 Oe in single domains could be deduced. The elemental specificity and the quantitative magnetic sensitivity render PEEM a perfect tool to study magnetic coupling effects in multi-layered thin film samples.
Date: January 2, 2001
Creator: Scholl, A.; Nolting, F.; Stohr, J.; Luning, J.; Seo, J.W.; Locquet, J.-P. et al.
Partner: UNT Libraries Government Documents Department