Preserving Metamagnetism in Self-Assembled FeRh Nanomagnets.

Preparing and exploiting phase-change materials in the nanoscale form is an ongoing challenge for advanced material research. A common lasting obstacle is preserving the desired functionality present in the bulk form. Here, we present self-assembly routes of metamagnetic FeRh nanoislands with tunable sizes and shapes. While the phase transition between antiferromagnetic and ferromagnetic orders is largely suppressed in nanoislands formed on oxide substrates via thermodynamic nucleation, we find that nanomagnet arrays formed through solid-state dewetting keep their metamagnetic character. This behavior is strongly dependent on the resulting crystal faceting of the nanoislands, which is characteristic of each assembly route. Comparing the calculated surface energies for each magnetic phase of the nanoislands reveals that metamagnetism can be suppressed or allowed by specific geometrical configurations of the facets. Furthermore, we find that spatial confinement leads to very pronounced supercooling and the absence of phase separation in the nanoislands. Finally, the supported nanomagnets are chemically etched away from the substrates to inspect the phase transition properties of self-standing nanoparticles. We demonstrate that solid-state dewetting is a feasible and scalable way to obtain supported and free-standing FeRh nanomagnets with preserved metamagnetism.

Controlling the Metamagnetic Phase Transition in FeRh/MnRh Superlattices and Thin-Film Fe50-xMnxRh50 Alloys.

Equiatomic and chemically ordered FeRh and MnRh compounds feature a first-order metamagnetic phase transition between antiferromagnetic and ferromagnetic order in the vicinity of room temperature, exhibiting interconnected structural, magnetic, and electronic order parameters. We show that these two alloys can be combined to form hybrid metamagnets in the form of sputter-deposited superlattices and alloys on single-crystalline MgO substrates. Despite being structurally different, the magnetic behavior of the alloys with substantial Mn content resembles that of the FeRh/MnRh superlattices in the ultrathin individual layer limit. For FeRh/MnRh superlattices, dissimilar lattice distortions of the constituent FeRh and MnRh layers at the antiferromagnetic-ferromagnetic transition cause double-step transitions during cooling, while the magnetization during the heating branch shows a smooth, continuous trend. For Fe50-xMnxRh50 alloy films, the substitution of Mn at the Fe sites introduces an effective tensile in-plane strain and magnetic frustration in the highly ordered epitaxial films, largely influencing the phase transition temperature TM (by more than 150 K). In addition, Mn acts as a surfactant, enabling the growth of continuous thin films at higher temperatures. Thus, the introduction of hybrid FeRh-MnRh systems with adjustable parameters provides a pathway for the realization of tunable spintronic devices based on magnetic phase transitions.

Acetone and the precursor ligand acetylacetone: distinctly different electron beam induced decomposition?

In focused electron beam induced deposition (FEBID) acetylacetone plays a role as a ligand in metal acetylacetonate complexes. As part of a larger effort to understand the chemical processes in FEBID, the electron-induced reactions of acetylacetone were studied both in condensed layers and in the gas phase and compared to those of acetone. X-ray photoelectron spectroscopy (XPS) shows that the electron-induced decomposition of condensed acetone layers yields a non-volatile hydrocarbon residue while electron irradiation of acetylacetone films produces a non-volatile residue that contains not only much larger amounts of carbon but also significant amounts of oxygen. Electron-stimulated desorption (ESD) and thermal desorption spectrometry (TDS) measurements reveal striking differences in the decay kinetics of the layers. In particular, intact acetylacetone suppresses the desorption of volatile products. Gas-phase studies of dissociative electron attachment and electron impact ionization suggest that this effect cannot be traced back to differences in the initial fragmentation reactions of the isolated molecules but is due to subsequent dissociation processes and to an efficient reaction of released methyl radicals with adjacent acetylacetone molecules. These results could explain the incorporation of large amounts of ligand material in deposits fabricated by FEBID processes using acetylacetonate complexes.

Dissociative electron attachment to hexafluoroacetylacetone and its bidentate metal complexes M(hfac)2; M = Cu, Pd.

Beta-diketones are a versatile class of compounds that can complex almost any metal in the periodic table of elements. Their metal complexes are found to be fairly stable and generally have sufficient vapor pressure for deposition techniques requiring volatile metal sources. Motivated by the potential role of low energy electrons in focused electron beam induced deposition, we have carried out a crossed electron∕molecular beam study on the dissociative electron attachment and non-dissociative electron attachment (NDEA) to hexafluoroacetylacetone (HFAc) and its bidentate metal complexes: bis-hexafluoroacetylacetonate copper(II), Cu(hfac)2 and bis-hexafluoroacetylacetonate palladium(II), Pd(hfac)2. The relative ion yield curves for the native precursor to the ligand as well as its stable, 16 valence electron Pd(II) complex and open shell, 17 valence electron Cu(II) complex, are presented and compared. For HFAc, the loss of HF leads to the dominant anion observed, and while NDEA is only weakly pronounced for Pd(hfac)2 and loss of hfac(-) is the main dissociation channel, [Cu(hfac)2](-) formation from Cu(hfac)2 dominates. A comparison of the ion yield curves and the associated resonances gives insight into the role of the ligand in the attachment process and highlights the influence of the central metal atom.

Absolute cross sections for dissociative electron attachment and dissociative ionization of cobalt tricarbonyl nitrosyl in the energy range from 0 eV to 140 eV.

We report absolute dissociative electron attachment (DEA) and dissociative ionization (DI) cross sections for electron scattering from the focused electron beam induced deposition (FEBID) precursor Co(CO)(3)NO in the incident electron energy range from 0 to 140 eV. We find that DEA leads mainly to single carbonyl loss with a maximum cross section of 4.1 × 10(-16) cm(2), while fragmentation through DI results mainly in the formation of the bare metal cation Co(+) with a maximum cross section close to 4.6 × 10(-16) cm(2) at 70 eV. Though DEA proceeds in a narrow incident electron energy range, this energy range is found to overlap significantly with the expected energy distribution of secondary electrons (SEs) produced in FEBID. The DI process, on the other hand, is operative over a much wider energy range, but the overlap with the expected SE energy distribution, though significant, is found to be mainly in the threshold region of the individual DI processes.

Gas phase low energy electron induced decomposition of the focused electron beam induced deposition (FEBID) precursor trimethyl (methylcyclopentadienyl) platinum(IV) (MeCpPtMe3).

Relative cross sections for dissociative electron attachment (DEA) and dissociative ionization (DI) of the FEBID precursor, trimethyl (methylcyclopentadienyl) platinum(iv), MeCpPtMe(3), are presented. The most pronounced DEA process is the loss of one methyl radical, while the loss of two or three methyl groups along with hydrogen is the main pathway in DI. Further fragments are formed in DEA and through DI by more complex rearrangement reactions but complete dissociation to bare Pt(-) in DEA or Pt(+) in DI is minor. The transient negative ion (TNI) formation in DEA is discussed and fragmentation mechanisms are proposed for individual processes. From the thermodynamics of the DEA processes we derive a lower limit for the electron affinity of the MeCpPtMe(2) radical (1.7 eV). Appearance energies (AE) of MeCpPtMe(3)(+) (7.7 eV) and Pt(+) (18.6 eV) formation through electron impact ionisation (EI) and through DI, respectively, are determined. Finally, the current DEA and DI results are compared and brought into context with earlier surface science studies on electron-induced decomposition of adsorbed MeCpPtMe(3) as well as gas phase and surface science studies on the FEBID precursors [Co(CO)(3)NO] and [Pt(PF(3))(4)]. These comparisons strongly indicate that DEA is an important process in the electron-induced decomposition of these molecules in FEBID.

Sodium controlled selective reactivity of protonated deoxy-oligonucleotides in the gas phase.

Metastable fragmentation of the positively charged, hexameric oligonucleotides 5'-d(TTXYTT) (X and Y are dC, dG, or dA) and 5'-d(CTCGTT), 5'-d(TTCGTC) and 5'-d(CTCGTC) is studied after matrix assisted laser desorption/ionization (MALDI). The influence of the degree of sodiation, i.e., when the acidic protons are one by one exchanged against sodium ions, is systematically studied for the exchange of up to seven protons against sodium ions. Exchanging the acidic protons against sodium gradually quenches the backbone cleavage through the w and a-B channels, and quantitative quenching of these channels is generally achieved with the exchange of four protons against sodium ions. At the same time, the exchange of protons against sodium ions promotes the loss of a neutral, high proton affinity base. The formation of the w and a-B fragments is found to be highly dependent on the sequence of the central bases. A single mechanism consistent with these observations is proposed. In addition to the quenching of the classical w and a-B reaction channels, a drastic and abrupt on/off-switching of new reaction channels is observed as the degree of sodiation successively increases. These channels involve selective loss of the two central bases and the excision of a phosphodiester group and a sugar unit from the center of the oligonucleotides. Synchronously, the two terminal fragments recombine to form a tetramer containing the two terminal nucleosides from each end of the hexamer. Possible mechanism explaining these remarkable channels are discussed.

Dissociative electron attachment to gas phase valine: a combined experimental and theoretical study.

Using a crossed electron/molecule beam technique the dissociative electron attachment (DEA) to gas phase L-valine, (CH(3))(2)CHCH(NH(2))COOH, is studied by means of mass spectrometric detection of the product anions. Additionally, ab initio calculations of the structures and energies of the anions and neutral fragments have been carried out at G2MP2 and B3LYP levels. Valine and the previously studied aliphatic amino acids glycine and alanine exhibit several common features due to the fact that at low electron energies the formation of the precursor ion can be characterized by occupation of the pi* orbital of the carboxyl group. The dominant negative ion (M-H)(-) (m/Z=116) is observed at electron energies of 1.12 eV. This ion is the dominant reaction product at electron energies below 5 eV. Additional fragment ions with m/Z=100, 72, 56, 45, 26, and 17 are observed both through the low lying pi* and through higher lying resonances at about 5.5 and 8.0-9.0 eV, which are characterized as core excited resonances. According to the threshold energies calculated here, rearrangements play a significant role in the formation of DEA fragments observed from valine at subexcitation energies.

Effective quenching of fragment formation in negative ion oligonucleotide matrix-assisted laser desorption/ionization mass spectrometry through sodium adduct formation.

Metastable decay of the sodium-free and the sodium adducts of the negatively charged hexameric nucleotides 5'-d(TTXYTT)-3' (X and Y are dC, dG or dA) in matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-ToF-MS) is reported. We show that the sodium adduct formation may be used to quantitatively quench dissociation channels in oligonucleotide MALDI-MS, and we argue, that by means of effective steps in the sample preparation that lead to per-sodiation of the sample, the sample preparation and the mass resolution of oligonucleotide mass spectra in MALDI may be improved substantially. The dependency of the fragment formation on the degree of sodiation is also discussed in context with the underlying fragmentation mechanisms.

Electron attachment to higher fullerenes and to Sc3N@C80.

We report on attachment of free electrons to fullerenes C(n) (n = 60, 70, 76, 78, 80, 82, 84, 86) and to Sc(3)N@C(80). The attachment cross sections exhibit a strong resonance at 0 eV for all species. The overall shape of the anion yield versus electron energy is quite similar for the higher fullerenes, with a minimum around 1 eV and a maximum which gradually shifts from 6 eV for C(60) to approximately 4 eV for large n. The endohedral Sc(3)N@C(80) exhibits a particularly shallow minimum and a maximum below 4 eV. We model autoionization of the anions with due consideration of the internal energy distributions. The relatively low electron affinity of Sc(3)N@C(80) is reflected in its reduced ion yield at higher attachment energies.

Reactions in condensed formic acid (HCOOH) induced by low energy (< 20 eV) electrons.

The interaction of low energy (< 20 eV) electrons with a five monolayer (ML) film of formic acid (HCOOH) deposited on a cryogenically cooled monocrystalline Au substrate is studied by electron stimulated desorption (ESD) of negatively charged fragment ions. A comparison with results from gas phase experiments demonstrates the strong effect of the environment for negative ion formation via dissociative electron attachment (DEA). From condensed phase formic acid (FA) a strong H desorption signal from a resonant feature peaking at 9 eV is observed. In the gas phase, the dominant reaction is neutral hydrogen abstraction generating HCOO- within a low energy resonance, peaking at 1.25 eV. ESD studies on the isotopomers HCOOD and DCOOH indicate effective H/D exchange in the precursor ion at 9 eV prior to dissociation. The evolution of the desorption signals in the course of electron irradiation and the features in the thermal desorption spectra (TDS) of the electron irradiated film suggest the formation of CO2 at electron energies above 8 eV.

Low energy electron driven reactions in single formic acid molecules (HCOOH) and their homogeneous clusters.

Low energy (0-3 eV) electron attachment to single formic acid (FA) and FA clusters is studied in crossed electron/molecular beam experiments. Single FA molecules undergo hydrogen abstraction via dissociative electron attachment (DEA) thereby forming HCOO(-) within a low energy resonance peaking at 1.25 eV. Experiments on the isotopomers HCOOD and DCOOH demonstrate that H/D abstraction occurs at the O-H/O-D site. In clusters, electron attachment is strongly enhanced leading to a variety of negatively charged complexes with the dimer M2(-) (M[triple bond]HCOOH) and its dehydrogenated form M (M-H)(-) as the most abundant ones. Apart from the homologous series containing the non-dissociated (Mn(-)) and dehydrogenated complexes (M(n-1) (M-H)(-), n > or = 1) further products are observed indicating that electron attachment at sub-excitation energies (approximately 1 eV) can trigger a variety of chemical reactions. Among these we detect the complex H2O (M-H)(-) which is interpreted to arise from a reaction initiated in the cyclic hydrogen bonded dimer target. In competition to hydrogen abstraction yielding the dehydrogenated complex M (M-H)(-) the abstracted hydrogen atom can react with the opposite FA molecule forming H2O and HCO with the polar water molecule attached to the closed shell HCOO(-) ion. The FA dimer can thus be used as a model system to study the response of a hydrogen bridge towards dehydrogenation in DEA.