Here we study the effect of La doping in EuO thin films using SQUID magnetometry, muon spin rotat... more Here we study the effect of La doping in EuO thin films using SQUID magnetometry, muon spin rotation ($\mu$SR), polarized neutron reflectivity (PNR), and density functional theory (DFT). The $\mu$SR data shows that the La$_{0.15}$Eu$_{0.85}$O is homogeneously magnetically ordered up to its elevated $T_{\rm C}$. It is concluded that bound magnetic polaron behavior does not explain the increase in $T_{\rm C}$ and an RKKY-like interaction is consistent with the $\mu$SR data. The estimation of the magnetic moment by DFT simulations concurs with the results obtained by PNR, showing a reduction of the magnetic moment per La$_{x}$Eu$_{1-x}$O for increasing lanthanum doping. This reduction of the magnetic moment is explained by the reduction of the number of Eu-4$f$ electrons present in all the magnetic interactions in EuO films. Finally, we show that an upwards shift of the Fermi energy with La or Gd doping gives rise to half-metallicity for doping levels as high as 3.2 %.
We have used low-energy implanted muons as a volume sensitive probe of the magnetic properties of... more We have used low-energy implanted muons as a volume sensitive probe of the magnetic properties of EuO1-x thin films. We find that static and homogeneous magnetic order persists up to the elevated TC in the doped samples, and the muon signal displays the double dome feature also observed in the sample magnetization. Our results appear incompatible with either the magnetic phase separation or bound magnetic polaron descriptions previously suggested to explain the elevated TC, but are compatible with an RKKY-like interaction mediating magnetic interactions above 69 K.
We have studied how the magnetic properties of oxygen-deficient EuO sputtered thin films vary as ... more We have studied how the magnetic properties of oxygen-deficient EuO sputtered thin films vary as a function of thickness. The magnetic moment, measured by polarized neutron reflectometry, and the Curie temperature are found to decrease with reducing thickness. Our results indicate that these surface-induced effects are caused by the reduced number of nearest neighbors, band bending, and the partial depopulation of the 4f states of Eu.
We investigate equilibrium properties of an exchange-spring magnetic system constituted of a soft... more We investigate equilibrium properties of an exchange-spring magnetic system constituted of a soft layer (e.g., Fe) of a given thickness on top of a hard magnetic layer (e.g., FePt). The magnetization profile M(z) as a function of the atomic position ranging from the bottom of the hard layer to the top of the soft layer is obtained in two cases with regard to the hard layer: (i) in the case of a rigid interface (the FePt layer is a single layer), the profile is obtained analytically as the exact solution of a sine-Gordon equation with Cauchy’s boundary conditions. Additional numerical simulations also confirm this result. Asymptotic expressions of M(z) show a linear behavior near the bottom and the top of the soft layer. In addition, a critical value of the number of atomic planes in the soft layer, that is necessary for the onset of spin deviations, is obtained in terms of the anisotropy and exchange coupling between the adjacent plane in the soft layer. (ii) In the case of a relaxed interface (the FePt layer is a multilayer), the magnetization profile is obtained numerically for various Fe and FePt films thicknesses and applied field.
Magnetization dynamics in exchange spring magnets have been studied using simulations of the FePt... more Magnetization dynamics in exchange spring magnets have been studied using simulations of the FePt/Fe bilayer system. The FePt hard layer exhibits strong perpendicular magnetocrystalline anisotropy while the soft (Fe) layer has negligible magnetocrystalline anisotropy. The variation in the local spin orientation in the Fe layer is determined by the competition of the exchange coupling interaction with the hard layer and the magnetostatic energy which favors in-plane magnetization. Dynamics were studied by monitoring the response of the Fe layer magnetization after the abrupt application of a magnetic field which causes the systems to realign via precessional motion. This precessional motion allows us to obtain the frequency spectrum and hence examine the dynamical magnetization motion. Since the rotation of the spins in the soft layer does not have a well-defined magnetic anisotropy, the system does not present the usual frequency field characteristics for a thin film. Additionally we obtain multipeaked resonance spectra for the application of magnetic fields perpendicular to the film plane, though we discount the existence of spin-wave modes and propose that this arises due to variations in the local effective field across the Fe layer. The dynamic response is only considered in the Fe layer, with the FePt layer held fixed in the perpendicular orientation.
Here we study the effect of La doping in EuO thin films using SQUID magnetometry, muon spin rotat... more Here we study the effect of La doping in EuO thin films using SQUID magnetometry, muon spin rotation ($\mu$SR), polarized neutron reflectivity (PNR), and density functional theory (DFT). The $\mu$SR data shows that the La$_{0.15}$Eu$_{0.85}$O is homogeneously magnetically ordered up to its elevated $T_{\rm C}$. It is concluded that bound magnetic polaron behavior does not explain the increase in $T_{\rm C}$ and an RKKY-like interaction is consistent with the $\mu$SR data. The estimation of the magnetic moment by DFT simulations concurs with the results obtained by PNR, showing a reduction of the magnetic moment per La$_{x}$Eu$_{1-x}$O for increasing lanthanum doping. This reduction of the magnetic moment is explained by the reduction of the number of Eu-4$f$ electrons present in all the magnetic interactions in EuO films. Finally, we show that an upwards shift of the Fermi energy with La or Gd doping gives rise to half-metallicity for doping levels as high as 3.2 %.
We have used low-energy implanted muons as a volume sensitive probe of the magnetic properties of... more We have used low-energy implanted muons as a volume sensitive probe of the magnetic properties of EuO1-x thin films. We find that static and homogeneous magnetic order persists up to the elevated TC in the doped samples, and the muon signal displays the double dome feature also observed in the sample magnetization. Our results appear incompatible with either the magnetic phase separation or bound magnetic polaron descriptions previously suggested to explain the elevated TC, but are compatible with an RKKY-like interaction mediating magnetic interactions above 69 K.
We have studied how the magnetic properties of oxygen-deficient EuO sputtered thin films vary as ... more We have studied how the magnetic properties of oxygen-deficient EuO sputtered thin films vary as a function of thickness. The magnetic moment, measured by polarized neutron reflectometry, and the Curie temperature are found to decrease with reducing thickness. Our results indicate that these surface-induced effects are caused by the reduced number of nearest neighbors, band bending, and the partial depopulation of the 4f states of Eu.
We investigate equilibrium properties of an exchange-spring magnetic system constituted of a soft... more We investigate equilibrium properties of an exchange-spring magnetic system constituted of a soft layer (e.g., Fe) of a given thickness on top of a hard magnetic layer (e.g., FePt). The magnetization profile M(z) as a function of the atomic position ranging from the bottom of the hard layer to the top of the soft layer is obtained in two cases with regard to the hard layer: (i) in the case of a rigid interface (the FePt layer is a single layer), the profile is obtained analytically as the exact solution of a sine-Gordon equation with Cauchy’s boundary conditions. Additional numerical simulations also confirm this result. Asymptotic expressions of M(z) show a linear behavior near the bottom and the top of the soft layer. In addition, a critical value of the number of atomic planes in the soft layer, that is necessary for the onset of spin deviations, is obtained in terms of the anisotropy and exchange coupling between the adjacent plane in the soft layer. (ii) In the case of a relaxed interface (the FePt layer is a multilayer), the magnetization profile is obtained numerically for various Fe and FePt films thicknesses and applied field.
Magnetization dynamics in exchange spring magnets have been studied using simulations of the FePt... more Magnetization dynamics in exchange spring magnets have been studied using simulations of the FePt/Fe bilayer system. The FePt hard layer exhibits strong perpendicular magnetocrystalline anisotropy while the soft (Fe) layer has negligible magnetocrystalline anisotropy. The variation in the local spin orientation in the Fe layer is determined by the competition of the exchange coupling interaction with the hard layer and the magnetostatic energy which favors in-plane magnetization. Dynamics were studied by monitoring the response of the Fe layer magnetization after the abrupt application of a magnetic field which causes the systems to realign via precessional motion. This precessional motion allows us to obtain the frequency spectrum and hence examine the dynamical magnetization motion. Since the rotation of the spins in the soft layer does not have a well-defined magnetic anisotropy, the system does not present the usual frequency field characteristics for a thin film. Additionally we obtain multipeaked resonance spectra for the application of magnetic fields perpendicular to the film plane, though we discount the existence of spin-wave modes and propose that this arises due to variations in the local effective field across the Fe layer. The dynamic response is only considered in the Fe layer, with the FePt layer held fixed in the perpendicular orientation.
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Papers by Pedro Monteiro