Magnetic Circular Dichroism Experiments Research Articles

A Joint Experimental and Theoretical Study on the Structural and Spectroscopic Properties of the Piv-Pro-d-Ser-NHMe Peptide.

In this paper, we investigate the secondary structure of the Piv-Pro-d-Ser-NHMe peptide by means of nuclear magnetic resonance (NMR) and electronic circular dichroism (ECD) experiments, in conjunction with theoretical simulations based on molecular dynamics and time-dependent density functional theory calculations including polarizable embedding to account for solvent effects. The various experimental and theoretical protocols are assessed and validated, and are shown to provide a consistent description of the turn structure adopted by this peptide in solution. In addition, a simple fitting procedure is proposed to make the simulated and experimental ECD almost perfectly match. This full methodology is finally tested on another small peptide, enlightening its efficiency and robustness.

Single scan STEM-EMCD in 3-beam orientation using a quadruple aperture

The need to acquire multiple angle-resolved electron energy loss spectra (EELS) is one of the several critical challenges associated with electron magnetic circular dichroism (EMCD) experiments. If the experiments are performed by scanning a nanometer to atomic-sized electron probe on a specific region of a sample, the precision of the local magnetic information extracted from such data highly depends on the accuracy of the spatial registration between multiple scans. For an EMCD experiment in a 3-beam orientation, this means that the same specimen area must be scanned four times while keeping all the experimental conditions same. This is a non-trivial task as there is a high chance of morphological and chemical modification as well as non-systematic local orientation variations of the crystal between the different scans due to beam damage, contamination and spatial drift. In this work, we employ a custom-made quadruple aperture to acquire the four EELS spectra needed for the EMCD analysis in a single electron beam scan, thus removing the above-mentioned complexities. We demonstrate a quantitative EMCD result for a beam convergence angle corresponding to sub-nm probe size and compare the EMCD results for different detector geometries.

Investigating the electronic states of UTe2 using X-ray spectroscopy

The recent discovery of superconductivity in paramagnetic UTe2 turns spotlight on a serious candidate for spin-triplet state. To draw a complete picture of the superconducting state in UTe2 precise knowledge of the electronic properties of the 5 f states of Uranium is missing. We report on x-ray absorption and magnetic circular dichroism experiments performed at the U M4,5 edges at 2.7 K. At ambient pressure the 5 f electron count is found to be in-between 2.6 and 2.8. Partial delocalization of the 5 f electrons is further confirmed by the reduced value of the U orbital to spin magnetic moment ratio. The 5 f count is reduced by as large as 7 percent at the transition to a magnetically ordered state at Pc ≈ 1.5 GPa. At pressures above 4 GPa, the 5 f count increases back towards U3+ in the tetragonal phase. The observed “valence instabilities” and their interplay with magnetism seem to be important ingredients to understand the electronic structure in UTe2 in different phases.

Nonmonotonous temperature fluctuations of the orbital moment and spin-orbit coupling in multiferroic gallium ferrite thin films

Temperature dependent x-ray absorption spectroscopy and x-ray magnetic circular dichroism experiments were performed at the Fe ${L}_{2,3}$ edges to unveil the magnetic orbital moment and spin-orbit coupling (SOC) behavior in thin films of the multiferroic ${\mathrm{Ga}}_{0.6}{\mathrm{Fe}}_{1.4}{\mathrm{O}}_{3}$ (GFO) compound. A thorough analysis of the data evidenced a nonmonotonous behavior for both magnetic orbital moment and SOC with a minimum at approximately 120 K, accompanied with indications of disturbances of the magnetic exchange and electric field dipole. Resonant photoemission spectroscopy experiments performed at the Fe ${L}_{2,3}$ edges rendered an important modification of the crystal field configuration around this temperature value, pointing to a temperature-driven structural trigonal distortion. These instabilities can be compared to those observed in other iron oxides, in particular, during transitions such as the Verwey transition, with the difference that on GFO they do not have any impact on the macroscopic magnetic properties, such as the coercive field and the global magnetization. This work demonstrates that the absence of instabilities in magnetism at the macroscopic scale does not guarantee the absence of fluctuations at the atomic scale. Such atomic-scale fluctuations of the SOC are undoubtedly shown here and have an impact on the spin Hall magnetoresistance response of the films. This point is of importance for prospective applications of GFO in spin-orbitronics.

Experimental confirmation of the delayed Ni demagnetization in FeNi alloy

Element-selective techniques are central for the understanding of ultrafast spin dynamics in multi-element materials, such as magnetic alloys. Recently, however, it turned out that the commonly used technique of the transverse magneto-optical Kerr effect (T-MOKE) in the extreme ultraviolet range may have issues with unwanted crosstalk between different elemental signals and energy-dependent non-linear response. This problem can be sizeable, which puts recent observations of ultrafast spin transfer from Fe to Ni sites in FeNi alloys into question. In this study, we investigate the Fe-to-Ni spin transfer in a crosstalk-free time-resolved x-ray magnetic circular dichroism (XMCD) experiment with a reliable time reference. With XMCD near the absorption maxima, we find a very similar Fe and Ni dynamics as with T-MOKE from identical samples. Considering the potential non-linearities of the T-MOKE response, such a good agreement in our findings is remarkable. Our data provide the ongoing discussion about ultrafast spin-transfer mechanisms in FeNi systems with a broader experimental basis.

Pre-miRNA-149 G-quadruplex as a molecular agent to capture nucleolin

One of the most significant challenges in capturing and detecting biomarkers is the choice of an appropriate biomolecular receptor. Recently, RNA G-quadruplexes emerged as plausible receptors due to their ability to recognize with high-affinity proteins. Herein, we have unveiled and characterized the capability of the precursor microRNA 149 to form a G-quadruplex structure and determined the role that some ligands may have in its folding and binding capacity to nucleolin. The G-quadruplex formation was induced by K+ ions and stabilized by ligands, as demonstrated by nuclear magnetic resonance and circular dichroism experiments. Surface plasmon resonance measurements showed a binding affinity of precursor microRNA 149 towards ligands in the micromolar range (10−5–10−6 M) and a strong binding affinity to nucleolin RNA-binding domains 1 and 2 (8.38 × 10−10 M). Even in the presence of the ligand PhenDC3, the binding remains almost identical and in the same order of magnitude (4.46 × 10−10 M). The molecular interactions of the RNA G-quadruplex motif found in precursor miRNA 149 (5′-GGGAGGGAGGGACGGG- 3′) and nucleolin RNA-binding domains 1 and 2 were explored by means of molecular docking and molecular dynamics studies. The results showed that RNA G-quadruplex binds to a cavity between domains 1 and 2 of the protein. Then, complex formation was also evaluated through polyacrylamide gel electrophoresis. The results suggest that precursor microRNA 149/ligands and precursor microRNA 149/nucleolin RNA-binding domains 1 and 2 form stable molecular complexes. The in vitro co-localization of precursor microRNA 149 and nucleolin in PC3 cells was demonstrated using confocal microscopy. Finally, a rapid and straightforward microfluidic strategy was employed to check the ability of precursor microRNA 149 to capture nucleolin RNA-binding domains 1 and 2. The results revealed that precursor microRNA 149 can capture nucleolin RNA-binding domains 1 and 2 labeled with Fluorescein 5-isothiocyanate in a concentration-dependent manner, but PhenDC3 complexation seems to decrease the ability of precursor microRNA 149 to capture the protein. Overall, our results proved the formation of the G-quadruplex structure in the precursor microRNA 149 and the ability to recognize and detect nucleolin. This proof-of-concept study could open up a new framework for developing new strategies to design improved molecular receptors for capture and detection of nucleolin in complex biological samples.

5f states in UGa2 probed by x-ray spectroscopies

The $5f$-based ferromagnet $\mathrm{U}{\mathrm{Ga}}_{2}$ with the Curie temperature ${T}_{\mathrm{C}}=125\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ was investigated by x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) experiments at the $\mathrm{U}--{M}_{4,5}$ and Ga--K edges. The position of the $\mathrm{U}--{M}_{4}$ white line, determined in the high-energy resolution fluorescence detection XAS, suggests that $\mathrm{U}{\mathrm{Ga}}_{2}$ is neither a localized $5{f}^{2}$ nor an itinerant system with $5f$ occupancy close to ${n}_{5f}=3$. The analysis of the acquired ${M}_{4,5}$ XANES and XMCD spectra indicates the $5f$ occupancy close to 2.5 and a large orbital magnetic moment of the uranium $5f$ states (3.18 ${\ensuremath{\mu}}_{\mathrm{B}}$) that is partly compensated by the antiparallel spin moment (1.31 ${\ensuremath{\mu}}_{\mathrm{B}}$). Thus, the total $5f$ magnetic moment of 1.87 ${\ensuremath{\mu}}_{\mathrm{B}}$ is obtained, which is smaller than the known bulk magnetization of 3.0 ${\ensuremath{\mu}}_{\mathrm{B}}$ per formula unit, while the magnetic moments of the Ga atoms are negligible. Several methods based on density-functional theory were applied and the obtained results were compared with XAS spectral features, the Sommerfeld coefficient of the electronic specific heat, and the size of the U moments and $5f$ occupancies. A clear correlation is revealed between the $\mathrm{U}--{M}_{4}$ white-line position of three metallic uranium compounds and the calculated uranium ionicity. It is demonstrated that only electronic structure methods taking appropriate care of orbital magnetism and related atomic multiplet effects can successfully describe all considered properties.

Induced magnetization in Cu atoms at the Fe-Co/Cu3Au(001) interface: X-ray magnetic circular dichroism experiments and theoretical results

The induced magnetization in Cu atoms at the interface between ferromagnetic (FM) Fe/Co ultrathin films and a nonmagnetic (NM) Cu3Au(001) substrate was explored by the investigation of two structures of five alternated Fe and Co monoatomic layers with different stacking orders with a Fe or a Co layer in direct contact with the Cu3Au(001) surface. First principles calculations were applied to disentangle the origin of magnetic proximity effects at these FM/NM interfaces. The hybridization between the electronic states of the FM layers, resulting in a pronounced widening of the d-bands of the local density of states of Fe-Co interface atoms, has a fundamental effect in the spin-polarization of the NM substrate. X-ray magnetic circular dichroism measurements at the L2,3-edges of Fe, Co and Cu allowed to extract the spin and orbital magnetic moments of Fe and Co, and to measure extremely low magnetic moments induced in Cu atoms. It is shown that the magnetism is induced only in the Cu and Au atoms at the first CuAu monolayer at the very surface of the substrate, i.e., the Cu XMCD signal is due to only 0.5 ML magnetic Cu atoms.

Direct observation of the magnetic ordering process in the ferromagnetic semiconductor Ga1−xMnxAs via soft x-ray magnetic circular dichroism

In order to understand the mechanism of the ferromagnetism in the ferromagnetic semiconductor Ga1−xMnxAs [(Ga,Mn)As], we have investigated the magnetic behavior on a microscopic level through systematic temperature (T)- and magnetic field (H)-dependent soft x-ray magnetic circular dichroism (XMCD) experiments at the Mn L2,3 absorption edges. The T and H dependences of XMCD intensities have been analyzed using a model consisting of the ferromagnetic (FM), paramagnetic, and superparamagnetic (SPM) components. Intriguingly, we have found a common behavior for the ferromagnetic ordering process in (Ga,Mn)As samples with different Mn concentrations (4% and 10.8%) and different Curie temperature (TC) values (65, 120, and 164 K). In particular, the SPM component develops well above TC, indicating that local FM regions are formed well above TC. The present findings indicate that the onset of ferromagnetic ordering is triggered by local electronic states around the substitutional Mn ions rather than uniform electronic states considered by mean-field theories. Insight into the most representative ferromagnetic semiconductor, (Ga,Mn)As, provided by the present study will be an important step in understanding the mechanism of ferromagnetic ordering in various ferromagnetic semiconductor families.

Two Spectroscopies in One: Interference of Circular Dichroism and Raman Optical Activity

AbstractPreviously, we and other laboratories have reported an unusual and strong Raman optical activity (ROA) induced in solvents by chiral dyes. Various theories of the phenomenon appeared, but they were not capable of explaining fully the observed ROA band signs and intensities. In this work, an analysis based both on the light scattering theory and dedicated experiments provides a more complete understanding. For example, double‐cell magnetic circular dichroism and magnetic ROA experiments with copper‐porphyrin complex show that the induced chirality is observed without any contact of the solvents with the complex. The results thus indicate that a combination of electronic circular dichroism (ECD) with the polarized Raman scattering is responsible for the effect. The degree of circularity of solvent vibrational bands is a principal molecular property participating in the event. The insight and the possibility to predict the chirality transfer promise future applications in spectroscopy, chemical analysis and polarized imaging.

Two Spectroscopies in One: Interference of Circular Dichroism and Raman Optical Activity.

Previously, we and other laboratories have reported an unusual and strong Raman optical activity (ROA) induced in solvents by chiral dyes. Various theories of the phenomenon appeared, but they were not capable of explaining fully the observed ROA band signs and intensities. In this work, an analysis based both on the light scattering theory and dedicated experiments provides a more complete understanding. For example, double‐cell magnetic circular dichroism and magnetic ROA experiments with copper‐porphyrin complex show that the induced chirality is observed without any contact of the solvents with the complex. The results thus indicate that a combination of electronic circular dichroism (ECD) with the polarized Raman scattering is responsible for the effect. The degree of circularity of solvent vibrational bands is a principal molecular property participating in the event. The insight and the possibility to predict the chirality transfer promise future applications in spectroscopy, chemical analysis and polarized imaging.

Advanced Magnetic Resonance Studies of Tetraphenylporphyrinatoiron(III) Halides

High-Frequency and -Field EPR (HFEPR) studies of Fe(TPP)X (X = F, Cl, Br; I, TPP2−= meso-tetraphenylporphyrinate dianion) and far-IR magnetic spectroscopic (FIRMS) studies of Fe(TPP)Br and Fe(TPP)I have been conducted to probe magnetic intra- and inter-Kramers doublet transitions in these S = 5/2 metalloporphyrin complexes, yielding zero-field splitting (ZFS) and g parameters for the complexes: Fe(TPP)F, D = +4.67(1) cm−1, E = 0.00(1) cm−1, g⊥ = 1.97(1), g|| = 2.000(5) by HFEPR; Fe(TPP)Cl, D = +6.458(2) cm−1, E = +0.015(5) cm−1, E/D = 0.002, g⊥ = 2.004(3), g|| = 2.02(1) by HFEPR; Fe(TPP)Br, D = +9.03(5) cm−1, E = +0.047(5) cm−1, E/D = 0.005, giso = 1.99(1) by HFEPR and D = +9.05 cm−1, giso = 2.0 by FIRMS; Fe(TPP)I, D = +13.84 cm−1, E = +0.07 cm−1, E/D = 0.005, giso = 2.0 by HFEPR and D = +13.95 cm−1, giso = 2.0 by FIRMS (the sign of E was in each case arbitrarily assigned as that of D). These results demonstrate the complementary nature of field- and frequency-domain magnetic resonance experiments in extracting with high accuracy and precision spin Hamiltonian parameters of metal complexes with S > 1/2. The spin Hamiltonian parameters obtained from these experiments have been compared with those obtained from other physical methods such as magnetic susceptibility, magnetic Mossbauer spectroscopy, inelastic neutron scattering (INS), and variable-temperature and -field magnetic circular dichroism (VT-VH MCD) experiments. INS, Mossbauer and MCD give good agreement with the results of HFEPR/FIRMS; the others not as much. The electronic structure of Fe(TPP)X (X = F, Cl, Br, I) was studied earlier by multi-reference ab initio methods to explore the origin of the large and positive D-values, reproducing the trends of D from the experiments. In the current work, a simpler model based on Ligand Field Theory (LFT) is used to explain qualitatively the trend of increasing ZFS from X = F to Cl to Br and to I as the axial ligand. Tetragonally elongated high-spin d5 systems such as Fe(TPP)X exhibit D > 0, but X plays a key role. Spin delocalization onto X means that there is a spin–orbit coupling (SOC) contribution to D from X•, as opposed to none from closed-shell X−. Over the range X = F, Cl, Br, I, X• character increases as does the intrinsic SOC of X• so that D increases correspondingly over this range.

High-Throughput Screening at the Membrane Interface Reveals Inhibitors of Amyloid-β.

Aggregation and the formation of oligomeric intermediates of amyloid-β (Aβ) at the membrane interface of neuronal cells are implicated in the cellular toxicity and pathology of Alzheimer's disease. Small molecule compounds have been shown to suppress amyloid aggregation and cellular toxicity, but often the presence of a lipid membrane negates their activity. A high-throughput screen of 1800 small molecules was performed to search for membrane active inhibitors, and 21 primary hits were discovered. Through the use of fluorescence-based assays, transmission electron microscopy, and dot blot assays, the initial 21 primary hits were narrowed down to five lead compounds. Nuclear magnetic resonance and circular dichroism experiments were used for further confirmation of amyloid inhibition at the membrane interface and to obtain insights into the secondary structure of amyloid-β, while size exclusion chromatography was used to characterize the size of Aβ species. Lastly, dye-leakage assays allowed us to understand how the addition of the five lead compounds affected amyloid-β's ability to permeate the lipid bilayer. These results provide insights into small molecules that stabilize small amyloid species in the presence of membranes for the development of tool compounds for deeper investigations of these transient species.

Strong ferromagnetic coupling and tunable easy magnetization directions of FexCo1−x layer(s) intercalated under graphene

We report the magnetic response of FexCo1−x layer(s) intercalated under graphene, compared with the bulk equiatomic FeCo(1 0 0) specimen. Graphene grown on Ir(1 1 1) can act as a protective membrane, allowing confined epitaxial growth of homogeneous FexCo1−x layer(s). Few intercalated FexCo1−x layers exhibit a significant enhancement of the spin (Seff) and orbital (L) moments with respect to both the single elements and the bulk equiatomic FeCo(1 0 0) configuration, as demonstrated by means of magnetic circular dichroism experiments. Reducing the intercalant thickness down to a single layer of FexCo1−x, the easy magnetization direction switches from parallel to perpendicular to the surface plane, with a magnetic response being influenced by the underlying metal. Element-sensitive hysteresis loops disclose identical magnetization response at the Fe and Co sites, thus constituting a single magnetic unit with enhanced ferromagnetic response, with Fe acting as a strong ferromagnet, differently from its bulk bcc configuration. This is a significant step towards the control at the atomic scale of the magnetic response in low dimensional 3d transition metal alloys, with tunable easy-magnetization direction, enhanced spin moments and magnetic anisotropy, in a inert and protected configuration thanks to the 2D graphene cover.

The Synthesis of a Quasi-One-Dimensional Iron-Based Telluride with Antiferromagnetic Chains and a Spin Glass State.

The report on the superconductivity of the two-legged spin ladders BaFe2S3 and BaFe2Se3 has established 123-type iron chalcogenides as a novel subgroup in the iron-based superconductor family and has stimulated the continuous exploration of other iron-based materials with new structures and potentially novel properties. In this paper, we report the systematic study of a new quasi-one-dimensional (1D) iron-based compound, Ba9Fe3Te15, including its synthesis and magnetic properties. The high-pressure synthesized Ba9Fe3Te15 crystallized in a hexagonal structure that mainly consisted of face-sharing FeTe6 octahedral chains running along the c axis, with a lattice constant of a = 10.23668 Å; this led to weak interchain coupling and an enhanced one-dimensionality. The systematic static and dynamic magnetic properties were comprehensively studied experimentally. The dc magnetic susceptibility showed typical 1D antiferromagnetic characteristics, with a Tmax at 190 K followed by a spin glass (SG) state with freezing at Tf ≈ 6.0 K, which were also unambiguously proved by ac susceptibility measurements. Additionally, X-ray magnetic circular dichroism (XMCD) experiments revealed an unexpected orbital moment for Fe2+, i.e., 0.84 μB per Fe in Ba9Fe3Te15. The transport property is electrically insulating, with a thermal activation gap of 0.32 eV. These features mark Ba9Fe3Te15 as an alternative type of iron-based compound, providing a diverse candidate for high-pressure studies in order to pursue some emerging physics.

Anomalous and topological Hall effects in epitaxial thin films of the noncollinear antiferromagnet Mn3Sn

Noncollinear antiferromagnets with a D0$_{19}$ (space group = 194, P6$_{3}$/mmc) hexagonal structure have garnered much attention for their potential applications in topological spintronics. Here, we report the deposition of continuous epitaxial thin films of such a material, Mn$_{3}$Sn, and characterize their crystal structure using a combination of x-ray diffraction and transmission electron microscopy. Growth of Mn$_{3}$Sn films with both (0001) c-axis orientation and (40$\bar{4}$3) texture is achieved. In the latter case, the thin films exhibit a small uncompensated Mn moment in the basal plane, quantified via magnetometry and x-ray magnetic circular dichroism experiments. This cannot account for the large anomalous Hall effect simultaneously observed in these films, even at room temperature, with magnitude $\sigma_{\mathrm{xy}}$ ($\mu_{0}H$ = 0 T) = 21 $\mathrm{\Omega}^{-1}\mathrm{cm}^{-1}$ and coercive field $\mu_{0}H_{\mathrm{C}}$ = 1.3 T. We attribute the origin of this anomalous Hall effect to momentum-space Berry curvature arising from the symmetry-breaking inverse triangular spin structure of Mn$_{3}$Sn. Upon cooling through the transition to a glassy ferromagnetic state at around 50 K, a peak in the Hall resistivity close to the coercive field indicates the onset of a topological Hall effect contribution, due to the emergence of a scalar spin chirality generating a real-space Berry phase. We demonstrate that the polarity of this topological Hall effect, and hence the chiral-nature of the noncoplanar magnetic structure driving it, can be controlled using different field cooling conditions.

CfbA promotes insertion of cobalt and nickel into ruffled tetrapyrroles in vitro.

The nickel chelatase CfbA is the smallest member of the chelatase family, but the mechanism by which this enzyme inserts nickel into sirohydrochlorin is unknown. In order to gain mechanistic insight, metal binding, tetrapyrrole binding, and enzyme activity were characterized for a variety of substrates using several spectroscopic and computational approaches. Mass spectrometery and magnetic circular dichroism experiments revealed that CfbA binds an octahedral, high-spin metal substrate. UV/Vis absorption spectroscopy demonstrated that the enzyme binds a wide range of tetrapyrrole substrates and perturbs their electronic structures. Based upon activity assays, CfbA promotes insertion of cobalt and nickel into several tetrapyrroles, including cobalt insertion into protopophyrin IX. Finally, density functional theory models were developed which strongly suggest that observed spectral changes upon binding to the enzyme can be explained by tetrapyrrole ruffling, but not deprotonation or saddling. The observation of an octahedral, high-spin metal bound to CfbA leads to a generalization for all class II chelatases: these enzymes bind labile metal substrates and metal desolvation is not a rate-limiting step. The conclusion that CfbA ruffles its tetrapyrrole substrate reveals that the CfbA mechanism is different from that currently proposed for ferrochelatase, and identifies an intriguing correlation between metal substrate specificity and tetrapyrrole distortion mode in chelatases.

An ab Initio Linear Response Method for Computing Magnetic Circular Dichroism Spectra with Nonperturbative Treatment of Magnetic Field.

Magnetic circular dichroism (MCD) experiments provide a sensitive tool for exploring geometric, magnetic, and electronic properties of chemical complexes and condensed matter systems. They are also challenging to simulate because of the need to simultaneously treat the perturbations of a finite magnetic field as well as an optical field. In this work, we introduce an ab initio approach that treats the external magnetic field nonperturbatively with London orbitals for simulating the MCD spectra of closed-shell systems. Effects of a magnetic field are included variationally in the spin-free nonrelativistic Hamiltonian, followed by a linear response formalism to directly calculate the difference in absorption between the left and right circularly polarized light. In addition to the presentation of underlying mathematical formalism and implementation, the method developed in this paper has been applied to simulations of MCD spectra of the sodium anion, 2,2,6,6-tetramethylcyclohexanone, and 3-methyl-2-hexanone. Results are discussed and compared to experiments.

Soft x-ray spectroscopic endstation at beamline 08U1A of Shanghai Synchrotron Radiation Facility.

A spectroscopic endstation with magnetic field, voltage, and low temperature control has been installed and commissioned at the soft X-ray beamline 08U1A of Shanghai Synchrotron Radiation Facility, which can obtain a magnetic field up to ±0.53 T, applied current and bias voltage, and cryogenic temperatures down to 14 K with a Gifford-McMahon cycle cryocooler. The endstation can perform soft X-ray absorption spectroscopy methods including total electron yield, fluorescence yield, and X-ray excited optical luminance. Combined with an elliptically polarized undulator and the in situ conditions, the endstation can effectively perform X-ray magnetic circular and linear dichroism experiments in the soft X-ray range between photon energies of 250 and 2000 eV.

Microscopic Nature of the First-Order Field-Induced Phase Transition in the Strongly Anisotropic Ferrimagnet HoFe_{5}Al_{7}.

We report on x-ray magnetic circular dichroism experiments in pulsed fields up to 30T to follow the rotations of individual magnetic moments through the field-induced phase transition in the ferrimagnet HoFe_{5}Al_{7}. Near the ground state, we observe simultaneous stepwise rotations of the Ho and Fe moments and explain them using a two-sublattice model for an anisotropic ferrimagnet with weak intersublattice exchange interactions. Near the compensation point, we find two phase transitions. The additional magnetization jump reflects the fact that the Ho moment is no longer rigid as the applied field acts against the intersublattice exchange field.