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ABSTRACT: We studied how the application of an in-plane field affects the asymmetries of the domain nucleation activity and domain wall velocity revealed previously in ultrathin Co films with perpendicular anisotropy. It is established that the asymmetries survive and new effects arise under application of an additional field parallel to the film surface. It is found that the mobility of different parts of a circular domain wall vary drastically under application of additional planar field. The domain wall velocity in ultrathin Co films, unlike that in low-damping garnet films, slows down dramatically upon application of a planar field. The domain wall part that is moving perpendic-ular to the in-plane field direction decelerates most relative to the other parts of the domain wall. Moreover, domain wall parts moving in the same direction as the in-plane field and in the opposite direction have considerably different velocities. This anisotropy of domain wall velocity rotates with the field as the in-plane field direction is varied. A new asymmetry in the domain wall velocity in the presence of both in-plane and perpendicular fields is observed when the perpendicular field is reversed. Index Terms—Cobalt, domain wall, in-plane magnetic field, perpendicular anisotropy, ultrathin film.
IEEE Transactions on Magnetics 01/2220; 46. · 1.36 Impact Factor
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ABSTRACT: The effect of varying the interlayer coupling between two 0.6 nm thick Co layers with perpendicular anisotropy on their magnetic ground state and the domain nucleation processes during a field reversal was studied. A transition from out-of-plane to in-plane anisotropy was revealed as the Pt spacer thickness decreased below 1.6 nm. For Pt thicknesses in the range of 1.6 nm – 5.4 nm, domain nucleation occurred in both Co layers in a correlated manner, and subsequent motion of the domain walls proceeded as though bound together. A transition to uncorrelated domain nucleation and independent wall motion was observed at Pt thicknesses above 5.4 nm. Both conventional and "asymmetrical" domain nucleation centers were observed in the whole range of platinum spacer thicknesses.
Journal of Applied Physics 01/2013; 113(17):17C101. · 2.17 Impact Factor
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ABSTRACT: We studied how the application of an in-plane field affects the asymmetries of the domain nucleation activity and domain wall velocity revealed previously in ultrathin Co films with perpendicular anisotropy. It is established that the asymmetries survive and new effects arise under application of an additional field parallel to the film surface. It is found that the mobility of different parts of a circular domain wall vary drastically under application of additional planar field. The domain wall velocity in ultrathin Co films, unlike that in low-damping garnet films, slows down dramatically upon application of a planar field. The domain wall part that is moving perpendicular to the in-plane field direction decelerates most relative to the other parts of the domain wall. Moreover, domain wall parts moving in the same direction as the in-plane field and in the opposite direction have considerably different velocities. This anisotropy of domain wall velocity rotates with the field as the in-plane field direction is varied. A new asymmetry in the domain wall velocity in the presence of both in-plane and perpendicular fields is observed when the perpendicular field is reversed.
IEEE Transactions on Magnetics 07/2010; · 1.36 Impact Factor
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ABSTRACT: We report on a very interesting magnetic behavior in the intermetallic compound Zr<sub>9</sub>N<sub>11</sub>, namely, (i) spin-glass behavior and (ii) fluctuations in the magnetization versus temperature curves. The magnetic properties are consistent with structural results obtained by neutron diffraction and by scanning and transmission electron microscopies. The fluctuations are believed to arise from the Dzyaloshinskii-Moriya interactions occurring in the Zr<sub>9</sub>N<sub>11</sub> compound. Between 7.86 kA/m and 31.85 kA/m, the temperature (Tp) of the zero-field cooled (Mzfc ) peak shifted to lower temperatures with increasing field. Tp is found to be proportional to H<sup>b</sup>, where H is the applied field and the exponent b < 1. The value of b is found to be 0.68, which is close to the de Almeida-Thouless 2/3 power law that applies to spin glasses. Published neutron diffraction data showed that a Zr<sub>9</sub>N<sub>11</sub> compound possessed Zr<sub>9</sub>Pt<sub>11</sub>-type tetragonal structure with columnar atomic chains parallel to the c axis and consisting of alternating Zr and Ni atoms. Small fluctuations present in the Mfc versus T plot at 31.85 kA/m are believed to be the result of atomic displacements along these chains over short distances. Another interesting featured observed in the Zr<sub>9</sub>N<sub>11</sub> compound is the fact that at 3.98 kA/m the Mzfc values reside mostly above the corresponding Mfc values.
IEEE Transactions on Magnetics 03/2010; · 1.36 Impact Factor
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ABSTRACT: Exchange bias in bilayer magnetic systems is usually ascribed to the existence of transient magnetic structures at the interfaces between adjacent layers on top of each other. Here, we report the observation of a similar phenomenon in the lateral direction, indicated by an asymmetry in the magnetization reversal of a ferromagnetic film covered with a square grid of an antiferromagnetic layer. We show the antiferromagnet not only changes the properties of the ferromagnet regions immediately underneath it, but despite the large pattern period, also drastically affects the remagnetization behavior in the adjacent uncovered parts of the ferromagnet. Specifically, we demonstrate pattern-controlled nucleation of domain walls and an overall asymmetry in the reversal behavior of the uncovered ferromagnet when the polarity of the in-plane field is reversed. A canted orientation of the intrinsic anisotropy of the ferromagnet with respect to an induced exchange anisotropy and the presence of artificial topologically stable domain walls are discussed as likely origins of the effects.
Phys. Rev. B. 04/2009; 79(14).
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ABSTRACT: Intact liposome-based targeted nanoparticle delivery systems (NDS) are immobilized by non-selective binding and characterized by scanning probe microscopy (SPM) in a fluid imaging environment. The size, size distribution, functionality, and stability of an NDS with a payload consisting of a super-paramagnetic iron oxide contrast agent for magnetic resonance imaging are determined. SPM results are combined with information obtained by more familiar techniques such as superconducting quantum interference device (SQUID) magnetometry, dynamic light scattering, and electron microscopy. By integrating the methods presented in this work into the NDS formulation and manufacturing process, size-dependent statistical properties of the complex can be obtained and the structure-function relationship of individual, multi-component nanoscale entities can be assessed in a reliable and reproducible manner.
Nanotechnology 07/2008; 19(30):305101. · 3.98 Impact Factor
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Functional Materials. 01/2008; 15(1):51-55.
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ABSTRACT: We studied the magnetization reversal in ultrathin [Co/Pt] n films (n ¼ 1, 2, and 4) using magneto-optical Kerr microscopy. These materials demonstrate unusual asymmetries in the activity of nucleation centers and domain wall motion. It was found that application of very high holding magnetic field prior to magnetization reversal, exceeding some critical value much larger than the apparent saturation field, suppresses the subsequent 'asymmetric' nucleation centers, activity. We revealed that the 'asymmetric' nucleation centers become active again after subsequent reversal cycles coming from a smaller holding field and studied how the asymmetry returns with the decrease of applied holding field. It was found that in low-coercivity ultrathin Co films, the asymmetry in domain wall velocity decreased sharply with the applied field increase and disappeared when the reversal field is greater than m 0 H ¼ 1.5 mT.
Journal of Magnetism and Magnetic Materials 01/2008; 320:2044-2048. · 1.78 Impact Factor
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ABSTRACT: We studied the magnetization reversal in ultrathin [Co/Pt](n) films (n = 1, 2, and 4) using magneto-optical Kerr microscopy. These materials demonstrate unusual asymmetries in the activity of nucleation centers and domain wall motion. It was found that application of very high holding magnetic field prior to magnetization reversal, exceeding some critical value much larger than the apparent saturation field, suppresses the subsequent 'asymmetric' nucleation centers, activity. We revealed that the 'asymmetric' nucleation centers become active again after subsequent reversal cycles coming from a smaller holding field and studied how the asymmetry returns with the decrease of applied holding field. It was found that in low-coercivity ultrathin Co films, the asymmetry in domain wall velocity decreased sharply with the applied field increase and disappeared when the reversal field is greater than mu H-0 = 1.5 mT. (c) 2008 Elsevier B.V. All rights reserved.
J. Magn. Magn. Mater. 01/2008; 320(15):2044-2048.
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ABSTRACT: Applications of magnetic nanoparticles, including hyperthermia for cancer treatments, require knowledge of how the colloidal environment affects the magnetic properties of the nanoparticles. Here, 10 nm diameter cobalt nanoparticles synthesized by thermodecomposition in 1,2-dichlorobenzene (DCB) are used to study the effect of the colloidal environment on the magnetic behavior of such materials. The magnetic properties are investigated by magnetization (M) versus temperature (T) measurements and vector magnetometry performed on the samples under zero-field-cooled conditions. Of particular interest in the M versus T data is a continuous rise in the magnetization observed around the DCB melting point during sample heating and a discontinuous drop around the DCB supercooling point during sample cooling. Vector magnetometer measurements quantify the portion of the sample that does not respond to the applied field. The magnitude of this unreversed component doubles with decreasing temperature as the temperature cools through the supercooling point in DCB. There is also an increase in the uniaxial anisotropy of the sample from 61.1(7)times10<sup>-7</sup> J to 104.2(9)times10<sup>-7</sup> J as the liquid-to-solid transition is traversed.
IEEE Transactions on Magnetics 07/2007; · 1.36 Impact Factor
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ABSTRACT: We report unexpected phenomena during magnetization reversal in ultrathin Co films and Co/Pt multilayers with perpendicular anisotropy. Using magneto-optical Kerr microscopy and magnetic force microscopy we have observed asymmetrical nucleation centers where the reversal begins for one direction of the field only and is characterized by an acute asymmetry of domain-wall mobility. We have also observed magnetic domains with a continuously varying average magnetization, which can be explained in terms of the coexistence of three magnetic phases: up, down, and striped.
Physical Review Letters 04/2007; 98(11):117204. · 7.37 Impact Factor
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ABSTRACT: Remagnetization of the FeNi∕FeMn bilayer was investigated using the magneto-optical indicator film imaging technique. We show the formation and breakdown of the homogeneous exchange spring into exchange springs of opposite chiralities during reversal in a rotating magnetic field. In reversal with a linear field, contrary to theoretical predictions, the winding of the exchange spring occurs without net magnetization rotation. It initiates by the formation of local spin spirals with opposite chirality and terminates with the formation of a single chiral state through the propagation of a specific kind of boundary separating regions with this single chirality from those with the mixed chiral state.
Phys. Rev. B. 05/2006; 73(18).
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ABSTRACT: Unusual magnetization reversal of [Co(4 Å)/Pt(10 Å)]4 multilayers with perpendicular magnetic anisotropy has been revealed macroscopically by magnetometry measurements and microscopically by magneto-optical Kerr effect microscopy and magnetic force microscopy (MFM) imaging. During the first-order reversal process, the magnetization first decreases, then reaches a plateau, and finally rises back to saturation, corresponding to expanding bubble domains, stationary domains, and fading contrast but unchanged boundary domains, respectively. MFM imaging reveals the existence of many submicron-scaled unreversed channels within the boundary of the “bubble” domains. The magnetization reversal behavior can be accounted for by the evolution of the unusual domain structures in different field regimes.
Journal of Applied Physics 04/2006; 99(8):08C905-08C905-3. · 2.17 Impact Factor
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ABSTRACT: We report on the first experimental demonstration of the details of the transient response in the four sequential processes of active magnetic regenerative refrigeration: magnetization, warm blow, demagnetization, and cold blow. The experimental results display the details of the gradual temperature divergence due to regeneration. The temperature change of the stationary solid bed is due to the magnetocaloric effect in a periodic field. The theory for the active magnetic regenerative refrigeration is applied to the transient processes in magnetocaloric regeneration. A time and spatial dependent model of temperature profile of the magnetization and demagnetization with thermal wave regenerative processes is developed.
IEEE Transactions on Magnetics 07/2005; · 1.36 Impact Factor
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ABSTRACT: While magnetoresistance (MR) has generally been found to be symmetric in applied field in nonmagnetic or magnetic metals, we have observed antisymmetric MR in Co/Pt multilayers. Simultaneous domain imaging and transport measurements show that the antisymmetric MR is due to the appearance of domain walls that run perpendicular to both the magnetization and the current, a geometry existing only in materials with perpendicular magnetic anisotropy. As a result, the extraordinary Hall effect gives rise to circulating currents in the vicinity of the domain walls that contributes to the MR. The antisymmetric MR and extraordinary Hall effect have been quantitatively accounted for by a theoretical model.
Physical Review Letters 02/2005; 94(1):017203. · 7.37 Impact Factor
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ABSTRACT: A time-dependent model of temperature in magnetization and demagnetization processes is developed based on thermodynamic and heat transfer laws, material properties, and variable-field magnet parameters. Test results of a magnetic refrigeration test bed are compared with the model calculation. The model can be used to simulate optimum operating conditions and duty cycle of magnetic regenerative refrigeration. The effects of the thermal conductivity between the magnetocaloric material and the thermocouple on temperature measurements and thermal lag are presented.
IEEE Transactions on Magnetics 08/2004; · 1.36 Impact Factor
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ABSTRACT: Distribution of a magnetic moment in an exchange-coupled bilayer Fe/SmCo epitaxial structure grown on a (110) MgO substrate
is visualized by the magnetooptic indicator film technique. The direction and the magnitude of the effective magnetization
in this structure are determined both under external magnetic fields of variable magnitude and direction and after the removal
of these fields. It is shown that such a heterostructure is remagnetized by a nonuniform rotation of a magnetic moment both
along the thickness of a sample and in its plane. A field antiparallel to the axis of unidirectional anisotropy gives rise
to spin springs with opposite chiralities in different regions of the magnetically soft ferromagnetic layer. The contributions
of these springs to the net magnetization cancel out, thus decreasing the averaged magnetic moment and the remanent magnetization
without their rotation. When the external field deviates from the easy axis, the balance is violated and the sample exhibits
a quasi-uniform rotation of the magnetic moment. Asymmetry in the rotation of the magnetic moment is observed under the reversal
of the field as well as under repeated remagnetization cycles. It is established that a monochiral spin spring is also formed
in a rotating in-plane magnetic field when the magnitude of the field exceeds the critical value. Possible mechanisms of remagnetization
in this system are discussed with regard to the original disordered orientation of magnetization of the magnetically soft
layer with respect to the easy axis, which is defined by the variance of unidirectional anisotropy axes of this layer on the
interface.
Journal of Experimental and Theoretical Physics 01/2004; 99(3):602-612. · 1.03 Impact Factor
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ABSTRACT: Domain processes were observed at 300 K using the magneto-optic indicator film technique (MOIF) in an exchange-coupled ferromagnet (FM)/antiferromagnet (AF) bilayer Fe76Mn6C18 (150 Å)/FeMn (100 Å) deposited under the presence of a 0.4 mT magnetic field (H) applied in the plane of the sample. The hysteresis loop for this sample was comprised of two half-loops symmetrically shifted in opposite directions from the origin. At H=0, MOIF observations showed the presence of domains in the FM with magnetization (M) vectors along the axis of the preparation field separated by 180° walls. Upon field application along that axis, saturation of the FM was achieved by the nucleation and growth of domains. In this state, at not very high fields, it was possible to observe an unusual MOIF contrast at the location of the original FM domain walls in the as-prepared ground state, associated with the intersection of domain walls in the AF with the FM. Upon field reduction M reversed only in regions which had reversed during the prior field application, so that at H=0 the domain structure was the same as that in the original ground state. If H was off-axis, during the original field application domain growth occurred in that same direction; upon field reduction, the preferred domain growth direction was at an angle with reversed sign. These results prove the AF domain walls do not move during the motion of FM domain walls, and that an exchange spring is created parallel to the AF/FM interface as the ferromagnetic layer reverses. © 2003 American Institute of Physics.
Journal of Applied Physics 05/2003; 93(10):8603-8605. · 2.17 Impact Factor
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ABSTRACT: As revealed by the observation of memory effects and domain imaging, the antiferromagnetic (AFM) spin structure in exchange bias is not static. During reversal, the AFM spins form an exchange spring connected with the ferromagnet (FM). We have observed hybrid domain walls consisting of FM and AFM sections and their evolution using the magnetooptical indicator film technique. The external magnetic field moves only the FM section of the hybrid domain walls, leading to the formation of an exchange spring parallel to the interface. The nucleation and unwinding of the exchange spring occur at different locations, and the propagation depends strongly on the chirality of the FM domain walls.
IEEE Transactions on Magnetics 10/2002; · 1.36 Impact Factor
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J S Jiang,
S D Bader,
H Kaper,
G K Leaf, R D Shull,
A J Shapiro,
V S Gornakov,
V I Nikitenko,
C L Platt,
A E Berkowitz,
S David,
E E Fullerton
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ABSTRACT: We highlight our experimental studies and micromagnetic simulations of the rotational hysteresis in exchange-spring magnets. Magneto-optical imaging and torque magnetometry measurements for Sm–Co/Fe exchange-spring films with uniaxial in-plane anisotropy show that the magnetization rotation created in the magnetically soft Fe layer by a rotating magnetic field is hysteretic. The rotational hysteresis is due to the reversal of the chirality of the spin spiral structure. Micromagnetic simulations reveal two reversal modes of the chirality, one at low fields due to an in-plane untwisting of the spiral, and the other, at high fields, due to an out-of-plane fanning of the spiral.
Journal of Physics D Applied Physics 09/2002; 35(19):2339. · 2.54 Impact Factor
