Murielle Villeret

Exchange coupling in magnetic multilayers: effect of partial confinement of carriers

The exchange coupling J(l) between magnetic layers across a non-magnetic spacer is observed to oscillate as a function of the spacer thickness l. In an earlier work a theory of the oscillatory exchange was proposed which shows that the oscillation periods are characteristic of the spacer. The theory relied on the assumption of an infinitely large exchange splitting in the magnetic layers, which leads to complete confinement of magnetic carriers of one spin in the ferromagnetic configuration of the sandwich. While this may be valid for strong ferromagnets such as Co or Ni, the complete confinement model is not a realistic approximation for iron which has holes in its majority-spin band. The theory is now generalized to the case of partial confinement of carriers in the spacer appropriate to a sandwich with weakly ferromagnetic layers. An exactly solvable hole-gas model of the coupling as well as numerical tight-binding results are presented which demonstrate that the oscillation period is unaffected but the amplitude and phase depend critically on the degree of confinement. Asymptotic expansions for J(l) valid at finite temperature and for an arbitrary single tight-binding band are also obtained. They show that the period and temperature dependence of the oscillations are directly related to the properties of the spacer Fermi surface but the amplitude and phase depend also on the exchange splitting in the ferromagnetic layers.

Theory of oscillatory exchange in magnetic multilayers: effect of partial confinement and band mismatch

Abstract In an earlier work a theory of the oscillatory exchange coupling between two ferromagnetic layers across a nonmagnetic spacer was proposed. It relied on size-quantization of the energy of electrons of one spin confined in the spacer (complete confinement model). Detailed tight-binding calculations of the coupling between Fe layers across Cr(001) using five d orbitals and the complete confinement model are reported. The coupling exhibits oscillations with several different periods but the amplitude and phase do not agree with experiment. An exactly solvable hole gas model and new tight-binding results are presented which demonstrate that the oscillation period is unaffected but the amplitude and phase depend critically on the degree of confinement. In particular, the amplitude is reduced dramatically for a weak confinement appropriate to iron. The dependence of the coupling on the occupancy of the spacer layer d band is also investigated. It is shown that a mismatch between the ferromagnet and spacer layer d bands, which increases with increasing number of holes in the spacer, leads to a systematic variation of the coupling strength across the transition metal series in agreement with experiment.

Enhanced magnetoresistance effect in layered systems

Magnetoresistance ratios several orders of magnitude higher than those of conventional multilayers may be obtained with much smaller saturation fields in magnetic layers separated by a periodically modulated structure. Conditions for the occurrence of such effect, as well as the possible use of these systems as spin-filter devices and magnetic logical gates, are discussed.

Optical and magnetic properties of Fe2+ and Cr2+ in II–VI semiconductors: the Jahn-Teller effect

The 5 D terms of Fe 2+ and Cr 2+ in the tetrahedral potential at cation sites in II-VI compounds split into orbital doublet and triplet states. While in Cr 2+ the orbital triplet has lower energy than the doublet, the opposite is the case in Fe 2+ . Both ions have singlet ground states after the spin-orbit interaction is taken into account and, hence, both are Van Vleck paramagnets. The optical absorption spectra of Fe and Cr based materials differ and are explained on the basis of a dynamic Jahn-Teller effect in the former and a static one in the latter. These considerations permit us to explain the optical as well as the magnetic properties observed in these materials.

Enhanced magnetoresistance and spin-filter effects in magnetic heterostructures

We show that ballistic transport through a pair of superlatticed magnetic junctions connected by a low-density-of-carriers conducting medium can provide an efficient spin filter and cause an enhanced magnetoresistive response.

Spin filtering and an enhanced regime of giant magnetoresistance

A new regime of magnetoresistance (MR) in systems composed of magnetic layers separated by non-magnetic modulated structures is studied. The ability to tailor the electronic structure of superlatticed systems enables one to engineer contrasting spin-dependent transport properties, enhancing the magnetoresistance ratios. When the system acts as a spin filter the MR ratios reach values many orders of magnitude larger than those of conventional giant magnetoresistance. Rather than a mere enhancement of the magnetoresistance ratios, this new regime involves ingenious combinations of spin-polarized currents. Moreover, it results from a magnetic-field-induced metal-insulator transition along the modulation direction, characterizing a highly anisotropic transport behaviour. The existence of an insulating phase circumvents the experimental challenge of probing excessively small resistances in the current-perpendicular-to-plane (CPP) geometry of magnetoresistance. A picture in terms of the bulk Fermi surfaces of the constituent materials emerges and provides general guidelines on how to achieve this regime.

Theory of the systematic variation of the exchange coupling strength in magnetic multilayers

Abstract The amplitude of the exchange coupling between Co or Fe layers separated by different nonmagnetic transition metal spacers is observed to decrease exponentially with decreasing number of electrons per atom in the spacer. An earlier theory based on size quantization of magnetic carriers in the spacer layer is generalized to show that the systematic variation of the coupling strength across the transition metal series is due to a misalignment between the ferromagnet and spacer bands which increases with increasing number of holes in the spacer. A general physical picture of the effect based on conservation laws and phase space availability is presented. Tight-binding calculations confirm the validity of the band misalignment theory. It is also shown that the systematic variation of the coupling strength cannot be explained by simple RKKY type theories.

The effect of electron-phonon interaction in iron-doped III-V cubic semiconductors

A theoretical study of optical absorption and emission measurements of Fe2+ as a substitutional impurity in InP and GaP is presented. A new interpretation of the low-temperature absorption spectrum is proposed based on a weak Jahn-Teller interaction between the electronic excited states and a local gap mode of 5 symmetry. The model also includes the crystal potential, hybridization with the orbitals of the ligands of the host crystal, spin-orbit interaction and a weak dynamic Jahn-Teller coupling of the orbital ground state of Fe2+ with transverse acoustic phonons of 3 symmetry. The theoretical model describes with good accuracy the measured positions and relative intensities of the spectral lines. In addition, the mass dependence of the local gap mode of 5 symmetry reproduces the general features of the fine structures associated with the isotopic shifts of the zero-phonon line and the contribution to the isotopic shifts arising from the difference in zero-point energy between the initial and final states of the transition is evaluated.

Spin current and exchange coupling in magnetic multilayers

Abstract The role of quantum wells in exchange coupling between magnetic layers is discussed. Applications of the Greens function torque method of determining exchange coupling, which involves the calculation of the spin current flowing between the layers, are summarized. New results are reported on the effect of interface roughness.

Isotope splitting of the zero-phonon line of Fe2+ in cubic III–V semiconductors

Abstract A theoretical study of the isotopic-mass dependence of the internal transitions of Fe2+ at a cation site in a cubic zinc-blende semiconductor is presented. The model used is based on crystal-field theory and includes the spin-orbit interaction and a weak dynamic Jahn-Teller coupling between the 5Φ5 excited manifold of Fe2+ and a local vibrational mode (LVM) of Φ5 symmetry. The mass dependence of the LVM frequency is described, in the harmonic approximation, within two different limits: the rigid-cage model and a molecular model. In the rigidcage model, the Fe2+ ion undergoes a displacement but the rest of the lattice is fixed. In this case, a simple M −1 2 dependence of the frequency is obtained and the Jahn-Teller energy, EJT, is independent of the mass. In the molecular model, the four nearest neighbors of the magnetic ion are allowed to move and the LVM then behaves as the Φ5 mode of a MX4 tetrahedral molecule leading to a more complicated dependence of the frequency on the isotopic mass and to a mass-dependence of EJT. The theoretical results obtained with these two models are compared with the observed isotopic shifts of the zero-phonon lines in InP:Fe and GaP:Fe corresponding to an optical transition between the vibronic Φ1 ground state and the lowest Φ5 state originating from the 5Φ5 excited orbital multiplet. A prediction of the isotopic shifts of the zero-phonon line in GaAs:Fe is also presented.