Tuning ferromagnetism at room temperature by visible light

Most digital information today is encoded in the magnetization of ferromagnetic domains. The demand for ever-increasing storage space fuels continuous research for energy-efficient manipulation of magnetism at smaller and smaller length scales. Writing a bit is usually achieved by rotating the magnetization of domains of the magnetic medium, which relies on effective magnetic fields. An alternative approach is to change the magnetic state directly by acting on the interaction between magnetic moments. Correlated oxides are ideal materials for this because the effects of a small external control parameter are amplified by the electronic correlations. Here, we present a radical method for reversible, light-induced tuning of ferromagnetism at room temperature using a halide perovskite/oxide perovskite heterostructure. We demonstrate that photoinduced charge carriers from the CH3NH3PbI3 photovoltaic perovskite efficiently dope the thin La0.7Sr0.3MnO3 film and decrease the magnetization of the ferromagnetic state, allowing rapid rewriting of the magnetic bit. This manipulation could be accomplished at room temperature; hence this opens avenues for magnetooptical memory devices.

Magnetization Reversal Dynamics in Ferromagnetic Semiconductors

Liza Herrera Diez

This thesis is dedicated to the study of the magnetization reversal dynamics in compressively strained (Ga,Mn)As and differently functionalized (Ga,Mn)As materials. In the first part the domain wall dynamics of pure (Ga,Mn)As/GaAs materials is in the focus. The changes in the magnetic anisotropy energy landscape occurring as a function of temperature are monitored in detail by different experimental techniques. These measurements provide the necessary information for the identification of the temperature ranges corresponding to different magnetic anisotropy regimes. Knowing the biaxial anisotropy and uniaxial anisotropy dominated regimes the dependence of the domain wall dynamics on the magnetic anisotropy landscape is studied. Critical changes in domain wall alignment observed by Kerr microscopy are found upon changing from biaxial to uniaxial anisotropy. These changes could be partially attributed to the tendency of the system to minimize magnetic free poles at the domain boundaries. To complement the space resolved studies of the magnetization reversal, magnetic time relaxation effects are addressed. In particular the magnetic aftereffect in (Ga,Mn)As/GaAs is studied in detail. The irreversible aftereffect is evidenced as a critical reduction of the domain wall velocity with time under a constant applied magnetic field below coercivity. This time relaxation is tracked as a function of both time and magnetic field. The measurements show that the overall relaxation is composed of two relaxation processes acting in parallel at short and long time scales. By fitting these experimental results the values of the relaxation times of both relaxation processes were obtained. By this modeling also the values of the activation volumes for two independent (Ga,Mn)As materials are estimated. In the second part of this thesis the possibilities of tuning the electrical and magnetic properties of (Ga,Mn)As by volume and surface treatments are explored. As for the volume modification, oxygen species were incorporated into the (Ga,Mn)As films by means of exposure to an oxygen plasma. The incorporation of oxygen was evidenced by depth profile X-ray photoelectron spectroscopy. This treatment is found to weaken the ferromagnetism, visible as a reduction in the Curie temperature, and to hinder the electrical transport evidenced as an increase in the electrical resistance. In agreement with theories accounting for the hole mediated ferromagnetism in (Ga,Mn)As this behaviour was related to a hole compensation mechanism arising from the presence of oxygen impurities sitting in interstitial positions of the (Ga,Mn)As Zinc blende structure. In the last part, the modulation of the electrical and magnetic properties via surface functionalization is presented. The effects produced by the adsorption of molecular species on (Ga,Mn)As are similar to those found after oxygenation, namely a strong weakening of the magnetic properties and an increase in the values of the electrical resistance. However, in this case the distinctive feature is provided by the possibility of regulating the hole quenching effect by exploiting the additional degree of freedom provided by the chemical properties of the adsorbates. The adsorbate chosen for these experiments are dye molecules that absorb light in the visible range, in this way allowing for a light modulated interaction with the substrate. Using this concept is was possible to observe a modulation of the ferromagnetic transition temperature, the coercive field and the electrical resistance upon illumination. These observations provide a proof of principle for the realization of photo-sensitized (Ga,Mn)As devices where the magnetic properties can be regulated by light.

EPFL2011

Three-dimensionality of field-induced magnetism in a high-temperature superconductor

Henrik Moodysson Rønnow

Many physical properties of high-temperature superconductors are two-dimensional phenomena derived from their square-planar CuO2 building blocks. This is especially true of the magnetism from the copper ions. As mobile charge carriers enter the CuO2 layers, the antiferromagnetism of the parent insulators, where each copper spin is antiparallel to its nearest neighbours(1), evolves into a fluctuating state where the spins show tendencies towards magnetic order of a longer periodicity. For certain charge-carrier densities, quantum fluctuations are sufficiently suppressed to yield static long-period order(2-6), and external magnetic fields also induce such order(7-12). Here we show that, in contrast to the chemically controlled order in superconducting samples, the field-induced order in these same samples is actually three-dimensional, implying significant magnetic linkage between the CuO2 planes. The results are important because they show that there are three-dimensional magnetic couplings that survive into the superconducting state, and coexist with the crucial inter-layer couplings responsible for three-dimensional superconductivity. Both types of coupling will straighten the vortex lines, implying that we have finally established a direct link between technical superconductivity, which requires zero electrical resistance in an applied magnetic field and depends on vortex dynamics and the underlying antiferromagnetism of the cuprates.

2005

Search for nuclear magnetic resonance evidence of spin accumulation

Aurore Rudolf

Developments towards an understanding of the nature of conductance at the interface between two different metallic layers – ferromagnetic and non magnetic – as well as the discovery of giant magnetoresistance have stirred attention from both the scientific community as well as the electronics industry, with the prospect of using the spin of electrons in fast, nano-sized electronic devices. The spin polarization obtained by driving a current through a magnetic nanostructure is referred to as "spin accumulation". In this thesis work, we use 59Co zero-field nuclear magnetic resonance (NMR) under an additional current in order to probe spin accumulation. To the best of our knowledge, no local measurement of spin accumulation has yet been reported. NMR was chosen as the most appropriate technique for this study, as it is a local probe of the electronic structure and its dynamics. In the first part of this thesis, we present the main characteristics of NMR in ferromagnets. We also discuss the effect expected in the NMR due to spin accumulation. In the second part, we describe and characterize the different samples that we investigated during this study: granular samples, multilayered nanowires and micro-pillars. The third part is devoted to the description of 59Co NMR lineshapes obtained for different samples. In addition, we measured the spin-spin and spin-lattice relaxation times in the Co/Cu granular samples. This study is of tremendous importance to choosing the best candidate sample for NMR experiments under additional current. Finally, we used a method of electrically excited and electrically detected ferromagnetic nuclear resonance (EDFNR) developed during the course of this work. This technique presents several advantages over standard FNR: neither coil nor resonant circuit is needed and, overall, EDFNR opens the possibility of studying samples with much fewer nuclei than is achievable with standard nuclear resonance methods. This could provide an important development to decrease the current needed for the same current density in the samples, which is of great interest due to the smaller magnetic field created by the reduced current. EDFNR was applied to measure the nuclear resonance in the samples under an additional applied current. The current densities achieved were high enough to induce measurable effects in the EDFNR spectra, which are interpreted in the final part.