Magnetic dissipative droplets are localized, strongly nonlinear dynamical modes excited in nano-c... more Magnetic dissipative droplets are localized, strongly nonlinear dynamical modes excited in nano-contact spin valves with perpendicular magnetic anisotropy. These modes find potential application in nanoscale structures for magnetic storage and computation, but dissipative droplet studies have so far been limited to extended thin films. Here, numerical and asymptotic analyses are used to demonstrate the existence and properties of novel solitons in confined structures. As a nanowire's width is decreased with a nano-contact of fixed size at its center, the observed modes undergo transitions from a fully localized two-dimensional droplet into a two-dimensional droplet edge mode and then a pulsating one-dimensional droplet. These solitons are interpreted as dissipative versions of classical, conservative solitons, allowing for an analytical description of the modes and the mechanisms of bifurcation. The presented results open up new possibilities for the study of low-dimensional solitons and droplet applications in nanostructures.
... Phys. Lett. 79 3848 [11] Liu K, Chien CL, Searson PC and Yu-Zhang K 1998 Appl. Phys. Lett. ..... more ... Phys. Lett. 79 3848 [11] Liu K, Chien CL, Searson PC and Yu-Zhang K 1998 Appl. Phys. Lett. ... Phys. Lett. 77 3815 [41] Liu K, Baker SM, Tuominen M, Russell TP and Schuller IK 2001 Phys. Rev. B 63 060403 [42] Sun L, Ding Y, Chien CL and Searson PC 2001 Phys. Rev. ...
ABSTRACT Using ac susceptibility, we determine the critical current density Jc and the flux creep... more ABSTRACT Using ac susceptibility, we determine the critical current density Jc and the flux creep activation energy U of an a-axis-oriented HgBa2CaCu2O6+delta thin film. The critical current density at helium temperatures is found to be 4.6×104 A/cm2, i.e., about two orders of magnitude smaller than for corresponding films with c-axis orientation. The temperature and ac field dependent activation energy is consistent with dislocation-mediated flux creep and well described by U(T,Hac)=U0(1-t4)H-1/2ac with t=T/Tc, Tc=120 K, and U0=0.77 eV Oe1/2 for temperatures T>45 K and in the field range studied. The activation energy is of the same order as that found in c-axis-oriented films. Below T=45 K the activation energy is observed to decrease as thermally assisted quantum creep becomes increasingly important.
ABSTRACT The temperature, ac and dc field, and current dependent activation energy U(T,H)[(Jc0/Jc... more ABSTRACT The temperature, ac and dc field, and current dependent activation energy U(T,H)[(Jc0/Jc)mu-1]/mu governing low-field flux creep in epitaxial c-axis-oriented HgBa2CaCu2O6+delta thin films has been determined from measurements of the frequency-dependent in-phase ac susceptibility. Above 35 K three different thermally activated flux creep regimes can be identified: (i) dislocation-mediated plastic flux creep, described by U(T,H)=U0(1-t4)H-1/2 and mu=0, (ii) elastic collective flux creep which decreases with temperature and has a weaker field dependence of H0.22 above a field-dependent temperature Tcm(H) where mu acquires finite values, and (iii) reappearance of dislocation-mediated plastic flux creep which rapidly increases as Tc is approached. It is argued that the re-entrant plastic-elastic-plastic vortex creep behavior is driven by the underlying temperature and field dependence of the shear modulus c66. Tcm(H) marks a line in the H-T plane where the increasing c66 promotes long-range correlations in the dilute vortex phase and creep becomes collective. At high H and T, c66 again decreases and plastic creep reappears as the ordered phase starts to melt. Evidence for thermally assisted quantum creep is observed up to temperatures as high as T0=35 K.
... Casey W. Miller, Ivan K. Schuller, RW Dave, JM Slaughter, Yan Zhou, Johan Åkerman. Abstract. ... more ... Casey W. Miller, Ivan K. Schuller, RW Dave, JM Slaughter, Yan Zhou, Johan Åkerman. Abstract. ... BNEngel, J.Åkerman, B.Butcher, RWDave, M.DeHerrera, M.Durlam, G.Grynkewich, J.Janesky, SVPietambaram, NDRizzo, JMSlaughter, K.Smith, JJSun, and S.Tehrani, IEEE Trans. ...
ABSTRACT Magnetization reversal of antiferromagnetically coupled (AFC) soft and hard (Co/Pd) mult... more ABSTRACT Magnetization reversal of antiferromagnetically coupled (AFC) soft and hard (Co/Pd) multilayers was studied as a function of temperature. While the hard [Co(0.3 nm)/Pd(0.8 nm)]�10 was kept unchanged, the softness of the [Co(t)/Pd(0.8 nm)]�3 was controlled by varying the thickness t of the Co sublayer. Clear two-step hysteresis loops were observed for all the investigated multilayers with t ranging between 0.4 and 1 nm. The spin reorientation of the soft layer magnetization from in-plane direction to out-of-plane direction was investigated from 50 to 300 K. The antiferromagnetic field HAFC measured from the shift of the minor hysteresis loop reveals a good agreement to the quantum-well model. From the out-of-plane hysteresis loop of the uncoupled soft layer, its magnetization shows an in-plane orientation for t�0.6 nm. The strong HAFC helps to induce an out-of plane orientation of the soft layer with a linear decrease of its coercivity with temperature. These investigated structures show the possibility to reduce the unwanted stray field and improving the out-of-plane anisotropy even for relatively thicker soft layer. VC
Although magnetic ``tunnel" junctions (MTJs) have been reported with magnetoresistance (MR) ... more Although magnetic ``tunnel" junctions (MTJs) have been reported with magnetoresistance (MR) as high as 50%, it was recently found that very large MR (300%) also can be achieved in magnetic nanocontacts. Since most of the so-called ``Rowell" criteria only apply to superconducting structures, no solid criteria exist at present to unequivocally establish whether a magnetic trilayer is or is not
The magnetodynamical properties of nanometer-thick yttrium iron garnet films are studied using fe... more The magnetodynamical properties of nanometer-thick yttrium iron garnet films are studied using ferromagnetic resonance as a function of temperature. The films were grown on gadolinium gallium garnet substrates by pulsed laser deposition. First, we found that the damping coefficient increases as the temperature increases for different film thicknesses. Second, we found two different dependencies of the damping on film thickness: at room temperature, the damping coefficient increases as the film thickness decreases, while at T = 8 K, we find the damping to depend only weakly on the thickness. We attribute this behavior to an enhancement of the relaxation of the magnetization by impurities or defects at the surfaces.
We have studied exchange bias in Ta(5nm)/Au(10)/[Ni(t)/Co(0.4)]x5/IrMn(8)/Ta(5) multilayers for N... more We have studied exchange bias in Ta(5nm)/Au(10)/[Ni(t)/Co(0.4)]x5/IrMn(8)/Ta(5) multilayers for Ni thicknesses of 0.8-1.2 nm. The samples were deposited via sputtering and no deposition field was used. The samples were annealed at 200 C in an applied field of 1500 Oe for different durations, then measured by polar Magneto-Optical Kerr Effect at room temperature. We find that the field annealing significantly alters the hysteresis loop shape, giving it more single domain character while simultaneously inducing exchange bias in the direction of the annealing field. After 34 (2 and 32) hrs annealing, the exchange bias of the samples each reach a maximum value ranging from 100 Oe for the thickest Ni to 35 Oe for the thinnest Ni. We find that the samples with the thinner Ni layers approach their exchange bias maximum values for shorter annealing times.
ABSTRACT We describe the possibility of using nano-contact spin-torque oscillators (NC-STOs) as f... more ABSTRACT We describe the possibility of using nano-contact spin-torque oscillators (NC-STOs) as fundamental magnonic building blocks. NC-STOs can act as spin wave generators, manipulators, and detectors, and can hence realize all the fundamental functions necessary for fully integrated magnonic devices, which can be fabricated using available CMOS compatible large-scale spin-torque device production processes. We show in particular how a 200 nm sized nano-contact located on an out-of-plane magnetized permalloy “free” magnetic layer can generate spin waves at f≈15 GHz that propagate up to 4 μm away from the nano-contact with wavelength λ=200–300 nm, decay length λ r ≈2 μm and group velocities v g ≈3 μm/ns. We propose that the same type of NC-STOs can be used as spin wave manipulators, via control of the local Gilbert damping, and as spin wave detector using the spin torque diode effect.
Several experiments report the presence of finite jumps in the properties of spin–torque oscillat... more Several experiments report the presence of finite jumps in the properties of spin–torque oscillators at room temperature, such as oscillation frequency and power as functions of current or field. On the basis of micromagnetic simulations, this paper links those experimental discontinuities to the changes in the curve slope numerically observed in the absence of thermal effects. Our numerical results show the key ingredient triggering this behavior is the presence of abrupt changes in the oscillation axis of the magnetization precession. We also predict that by fixing the bias point of the oscillator near those critical regions, it is possible to observe hysteretic synchronization. This result should be a key point in the design of nanoscale on-chip phase-locked loop receivers with improved sensitivity.
Magnetic dissipative droplets are localized, strongly nonlinear dynamical modes excited in nano-c... more Magnetic dissipative droplets are localized, strongly nonlinear dynamical modes excited in nano-contact spin valves with perpendicular magnetic anisotropy. These modes find potential application in nanoscale structures for magnetic storage and computation, but dissipative droplet studies have so far been limited to extended thin films. Here, numerical and asymptotic analyses are used to demonstrate the existence and properties of novel solitons in confined structures. As a nanowire's width is decreased with a nano-contact of fixed size at its center, the observed modes undergo transitions from a fully localized two-dimensional droplet into a two-dimensional droplet edge mode and then a pulsating one-dimensional droplet. These solitons are interpreted as dissipative versions of classical, conservative solitons, allowing for an analytical description of the modes and the mechanisms of bifurcation. The presented results open up new possibilities for the study of low-dimensional solitons and droplet applications in nanostructures.
... Phys. Lett. 79 3848 [11] Liu K, Chien CL, Searson PC and Yu-Zhang K 1998 Appl. Phys. Lett. ..... more ... Phys. Lett. 79 3848 [11] Liu K, Chien CL, Searson PC and Yu-Zhang K 1998 Appl. Phys. Lett. ... Phys. Lett. 77 3815 [41] Liu K, Baker SM, Tuominen M, Russell TP and Schuller IK 2001 Phys. Rev. B 63 060403 [42] Sun L, Ding Y, Chien CL and Searson PC 2001 Phys. Rev. ...
ABSTRACT Using ac susceptibility, we determine the critical current density Jc and the flux creep... more ABSTRACT Using ac susceptibility, we determine the critical current density Jc and the flux creep activation energy U of an a-axis-oriented HgBa2CaCu2O6+delta thin film. The critical current density at helium temperatures is found to be 4.6×104 A/cm2, i.e., about two orders of magnitude smaller than for corresponding films with c-axis orientation. The temperature and ac field dependent activation energy is consistent with dislocation-mediated flux creep and well described by U(T,Hac)=U0(1-t4)H-1/2ac with t=T/Tc, Tc=120 K, and U0=0.77 eV Oe1/2 for temperatures T>45 K and in the field range studied. The activation energy is of the same order as that found in c-axis-oriented films. Below T=45 K the activation energy is observed to decrease as thermally assisted quantum creep becomes increasingly important.
ABSTRACT The temperature, ac and dc field, and current dependent activation energy U(T,H)[(Jc0/Jc... more ABSTRACT The temperature, ac and dc field, and current dependent activation energy U(T,H)[(Jc0/Jc)mu-1]/mu governing low-field flux creep in epitaxial c-axis-oriented HgBa2CaCu2O6+delta thin films has been determined from measurements of the frequency-dependent in-phase ac susceptibility. Above 35 K three different thermally activated flux creep regimes can be identified: (i) dislocation-mediated plastic flux creep, described by U(T,H)=U0(1-t4)H-1/2 and mu=0, (ii) elastic collective flux creep which decreases with temperature and has a weaker field dependence of H0.22 above a field-dependent temperature Tcm(H) where mu acquires finite values, and (iii) reappearance of dislocation-mediated plastic flux creep which rapidly increases as Tc is approached. It is argued that the re-entrant plastic-elastic-plastic vortex creep behavior is driven by the underlying temperature and field dependence of the shear modulus c66. Tcm(H) marks a line in the H-T plane where the increasing c66 promotes long-range correlations in the dilute vortex phase and creep becomes collective. At high H and T, c66 again decreases and plastic creep reappears as the ordered phase starts to melt. Evidence for thermally assisted quantum creep is observed up to temperatures as high as T0=35 K.
... Casey W. Miller, Ivan K. Schuller, RW Dave, JM Slaughter, Yan Zhou, Johan Åkerman. Abstract. ... more ... Casey W. Miller, Ivan K. Schuller, RW Dave, JM Slaughter, Yan Zhou, Johan Åkerman. Abstract. ... BNEngel, J.Åkerman, B.Butcher, RWDave, M.DeHerrera, M.Durlam, G.Grynkewich, J.Janesky, SVPietambaram, NDRizzo, JMSlaughter, K.Smith, JJSun, and S.Tehrani, IEEE Trans. ...
ABSTRACT Magnetization reversal of antiferromagnetically coupled (AFC) soft and hard (Co/Pd) mult... more ABSTRACT Magnetization reversal of antiferromagnetically coupled (AFC) soft and hard (Co/Pd) multilayers was studied as a function of temperature. While the hard [Co(0.3 nm)/Pd(0.8 nm)]�10 was kept unchanged, the softness of the [Co(t)/Pd(0.8 nm)]�3 was controlled by varying the thickness t of the Co sublayer. Clear two-step hysteresis loops were observed for all the investigated multilayers with t ranging between 0.4 and 1 nm. The spin reorientation of the soft layer magnetization from in-plane direction to out-of-plane direction was investigated from 50 to 300 K. The antiferromagnetic field HAFC measured from the shift of the minor hysteresis loop reveals a good agreement to the quantum-well model. From the out-of-plane hysteresis loop of the uncoupled soft layer, its magnetization shows an in-plane orientation for t�0.6 nm. The strong HAFC helps to induce an out-of plane orientation of the soft layer with a linear decrease of its coercivity with temperature. These investigated structures show the possibility to reduce the unwanted stray field and improving the out-of-plane anisotropy even for relatively thicker soft layer. VC
Although magnetic ``tunnel" junctions (MTJs) have been reported with magnetoresistance (MR) ... more Although magnetic ``tunnel" junctions (MTJs) have been reported with magnetoresistance (MR) as high as 50%, it was recently found that very large MR (300%) also can be achieved in magnetic nanocontacts. Since most of the so-called ``Rowell" criteria only apply to superconducting structures, no solid criteria exist at present to unequivocally establish whether a magnetic trilayer is or is not
The magnetodynamical properties of nanometer-thick yttrium iron garnet films are studied using fe... more The magnetodynamical properties of nanometer-thick yttrium iron garnet films are studied using ferromagnetic resonance as a function of temperature. The films were grown on gadolinium gallium garnet substrates by pulsed laser deposition. First, we found that the damping coefficient increases as the temperature increases for different film thicknesses. Second, we found two different dependencies of the damping on film thickness: at room temperature, the damping coefficient increases as the film thickness decreases, while at T = 8 K, we find the damping to depend only weakly on the thickness. We attribute this behavior to an enhancement of the relaxation of the magnetization by impurities or defects at the surfaces.
We have studied exchange bias in Ta(5nm)/Au(10)/[Ni(t)/Co(0.4)]x5/IrMn(8)/Ta(5) multilayers for N... more We have studied exchange bias in Ta(5nm)/Au(10)/[Ni(t)/Co(0.4)]x5/IrMn(8)/Ta(5) multilayers for Ni thicknesses of 0.8-1.2 nm. The samples were deposited via sputtering and no deposition field was used. The samples were annealed at 200 C in an applied field of 1500 Oe for different durations, then measured by polar Magneto-Optical Kerr Effect at room temperature. We find that the field annealing significantly alters the hysteresis loop shape, giving it more single domain character while simultaneously inducing exchange bias in the direction of the annealing field. After 34 (2 and 32) hrs annealing, the exchange bias of the samples each reach a maximum value ranging from 100 Oe for the thickest Ni to 35 Oe for the thinnest Ni. We find that the samples with the thinner Ni layers approach their exchange bias maximum values for shorter annealing times.
ABSTRACT We describe the possibility of using nano-contact spin-torque oscillators (NC-STOs) as f... more ABSTRACT We describe the possibility of using nano-contact spin-torque oscillators (NC-STOs) as fundamental magnonic building blocks. NC-STOs can act as spin wave generators, manipulators, and detectors, and can hence realize all the fundamental functions necessary for fully integrated magnonic devices, which can be fabricated using available CMOS compatible large-scale spin-torque device production processes. We show in particular how a 200 nm sized nano-contact located on an out-of-plane magnetized permalloy “free” magnetic layer can generate spin waves at f≈15 GHz that propagate up to 4 μm away from the nano-contact with wavelength λ=200–300 nm, decay length λ r ≈2 μm and group velocities v g ≈3 μm/ns. We propose that the same type of NC-STOs can be used as spin wave manipulators, via control of the local Gilbert damping, and as spin wave detector using the spin torque diode effect.
Several experiments report the presence of finite jumps in the properties of spin–torque oscillat... more Several experiments report the presence of finite jumps in the properties of spin–torque oscillators at room temperature, such as oscillation frequency and power as functions of current or field. On the basis of micromagnetic simulations, this paper links those experimental discontinuities to the changes in the curve slope numerically observed in the absence of thermal effects. Our numerical results show the key ingredient triggering this behavior is the presence of abrupt changes in the oscillation axis of the magnetization precession. We also predict that by fixing the bias point of the oscillator near those critical regions, it is possible to observe hysteretic synchronization. This result should be a key point in the design of nanoscale on-chip phase-locked loop receivers with improved sensitivity.
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