Thesis

Important!

The manuscript of my PhD dissertation is hosted on the TEL (“Thèses En Ligne”) server  but you can also get it from here (pdf, 19 Mo).

GeMn nanocolumns : magnetic and structural properties in light of synchrotron radiation

Preamble

I give here a short account of the work I did during my PhD thesis (2007-2010) under the supervision of Joël Cibert and Salia Cherifi at the Institut Néel and Vincent Favre-Nicolin at the CEA-Grenoble in Grenoble, France. Experts might want to scroll down directly to the last three paragraphs or go take a look directly at the manuscript linked above.

Probing matter with X-rays

In order to probe the properties of a material, it can be useful to throw a bunch of particles at it and look at what happens. They are many types of particles that can be used as probes (electrons, photons, neutrons, muons, alpha particles …) and they all interact differently with matter. In the studies I conduct, I use X-ray photons. These X-ray photons, or X-rays for short, interact with the electrons of the atoms that make up the material (as opposed, for example, to neutrons that would interact with the nuclei). Additionally, the usual distinction is made between hard and soft X-rays. Hard X-rays have a larger energy (103-105 eV) and conversely the soft X-ray a lower energy (10-104 eV) (yes, there is a bit of overlap, it’s not a very precise definition). I use both types of X-rays, but for different purposes.

Hard X-rays to study the position of atoms

The wavelength of hard X-rays (0.1 to 10 Angström) is well matched with the interatomic spacing in condensed matter, therefore they can give rise to interferences between the different interaction sites. The position of the electronic clouds (and indirectly that of the corresponding atom) can then be computed and the crystal structure of the material can be studied.

Soft X-rays to study their magnetic moments

The energy of soft X-ray is in the range of the L absorption edge in 3d transition metals (i.e. the usually magnetic ones: Ni, Co, Fe, Mn …). This means that if the X-ray energy is exactly matched to one of the electronic transition, there will be a resonance effect and a large enhancement of the associated interaction. This allows for the study of the electronic properties with element selectivity. Furthermore, if the electronic transition is carefully chosen, one can access the magnetic properties of the element.

The (big) X-ray source: the synchrotron

How about the X-ray source ? Where do all the X-ray photons come from ? There are several sources of X-ray out there, typically in hospitals or at airport security checks. Such machines are routinely used in labs as well, but there are several limitations for the use I would have: among others, the two main ones are that the energy cannot be continuously changed (to be tuned to the resonance) and the photon flux is not very high (to study small effects). In order to circumvent these, I use synchrotron radiation. Synchrotron radiation is produced in large scale facilities (called “synchrotrons”) scattered over the globe. Ranging in few hundreds of meters in circumference, such facilities are much bigger that the average lab X-ray source. However they produce X-ray beams of high intensity – the correct word is “brilliance”, i.e. they produce a large number of X-ray photons per second in a small spot, all going in the same direction and with the same energy – which allows me to study small effects in a reasonable time (the average experiment still lasts a few day). High level of polarization or polarization control (e.g. right- or left-handed circular polarization), as well as the pulsed nature of the X-ray beam, are additional beneficial properties of synchrotron light. Synchrotron X-rays are therefore a very valuable tool that can be used to probe the structural, electronic and magnetic properties of the material of interest.

(Ge,Mn) nanocolumns for spintronics

The material I studied consists in a compound of manganese and germanium. The prospect of combining a magnetic element  and a semiconductor is very promising for spintronics applications. In order to make this new material, Molecular Beam Epitaxy (MBE) was used by my collaborators at the CEA-Grenoble. MBE consists in depositing atoms of both species in a controlled ratio (typically few percent of Mn) on substrate, atomic layer after atomic layer. The result is a thin film, a few nanometers thick. Under the right MBE growth conditions, a self-assembly of Mn-rich GeMn nanocolumns has been observed, oriented along the film thickness. Those nanocolumns have a diameter of a few nanometers and are embedded in a Mn-poor germanium matrix. Depending on the growth parameters, coherent GeMn nanocolumns, amorphous GeMn nanocolumns and/or Ge3Mn5 nanoclusters can be present in the sample. In my manuscript, I report on the investigation on the electronic, magnetic and structural properties of the GeMn nanocolumns using synchrotron techniques.

Structural properties of the (Ge,Mn) thin film and strain correlation

X-ray scattering measurements were done using a grazing incidence geometry to maximize the contribution from the thin film: below the critical angle, the X-rays are totally reflected and only the surface layer is probed over a depth of typically a few nanometers. In samples containing coherent nanocolumns, free from Ge3Mn5 precipitates, we observed that some disorder was present in the nanocolumns and that the surrounding Ge matrix was strained. This strain field showed a correlation length equal to the inter-column correlation distance and by measuring the diffuse scattering perpendicular to the plane it was unambiguously attributed to the strain of the matrix around each nanocolumn. Detailed reciprocal space maps around in-plane reflection (e.g. (220), (400), (620)) were measured and quantitatively explained by considering the scattering of the Ge matrix strained by the inclusion of the nanocolumns in the matrix and their correlations in position, without requiring the consideration of different additional phases.
Bragg-GISAXS_L

(click to enlarge) Grazing Incidence Small Angle X-ray Scattering (GISAXS) around the in-plane (220) Bragg reflection measured in a single angular scan around the reflection with a linear position sensitive detector (E = 6.5 keV, incidence angle = 0.4 degree). Qang and Qz are respectively parallel and perpendicular to the sample surface. Three finite distances can be seen: “T” along Qz is due to the finite length of the columns (60 nm), “D” along Qang shows the in-plane correlations between the columns (9 to 15 nm) and “R” along Qang shows a longer range surface roughness.

Magnetic moments in the nanocolumns

XAS-XMCD_L

(click to enlarge) X-ray Absorption Spectra (XAS) at the Mn L2,3 absorption edges for a circular polarization parallel (I+) or antiparallel (I-) to the applied magnetic field (5 T) (top). The difference between the two spectra is the X-ray Magnetic Circular Dichroism (XMCD) and provides information on the ratio of the spin to orbital magnetic moment (bottom). Under additional assumptions (number of holes, symmetry), quantitative information on the amplitude of the spin and orbital magnetic moment can be separately obtained.

X-ray absorption spectroscopy and x-ray magnetic circular dichroism allow for the specific probing of the Mn magnetic properties in samples free of Ge3Mn5clusters. It is very important to consider samples containing only nanocolumns since this technique is sensitive to all the Mn atoms present in the sample. The lineshapes of the XAS-XMCD spectra in the nanocolumns were found to be very similar to those in the well-known metal Ge3Mn5.The local magnetic moment on the Mn  atom possess a small but non-zero orbital component and its total magnitude is much smaller (0.8±0.1 µB) than that in Ge3Mn5 (∼ 2.6 µB) or than that expected for fully substitutional Mn atoms (∼ 3 µB). This points to a different nature of the nanocolumns. The XAS-XMCD spectra have been calculated for several structural models, including simple defects and new crystalline phases, and critical parameters for the calculations have been identified. In particular, we suggested a new approach to calculate the XAS/XMCD spectra more accurately. The best agreement was found for a new Ge2Mn crystalline phase.