Frank O. Schumann

MAGNETIC INSTABILITY OF ULTRATHIN FCC FEXNI1-X FILMS

The {open_quotes}invar effect{close_quotes} in Fe{sub x}Ni {sub 1{minus}x} alloys occurs when the Fe content approaches 65{percent}. At this point, the magnetization falls to zero, and a martensitic structural transformation from a fcc to a bcc lattice occurs. This paper addresses the question: {open_quotes}What happens if the structural transformation is suppressed in an ultrathin alloy film?{close_quotes} We present results to this effect, showing the variation of the magnetization with changing composition in ultrathin films grown on Cu(100). We find a new low-spin, ferromagnetic phase of matter, which is a sensitive function of the atomic volume. {copyright} {ital 1997} {ital The American Physical Society}

Magnetic properties of pseudomorphic ferromagnetic alloy films on Cu(100)

We report the variation of the thickness dependence of the Curie temperature for a range of ultrathin ferromagnetic alloy films. By simultaneously depositing Fe or Co and Ni on Cu(100) we produce pseudomorphic alloys with controlled stoichiometry. An analysis of the Curie temperature versus thickness curves with an empirical finite size scaling formula reveals that the Curie temperature in the thick film limit increases monotonically with Co or Fe concentration. We compare the bulk alloy Curie temperatures to those of the alloy films extrapolated from the few monolayers limit.

Magnetic dichroism effect of binary alloys using a circularly polarized x ray

We have studied the magnetic properties of CoNi binary alloy films with various atomic compositions using the soft x-ray magnetic circular dichroism technique. The alloy films were deposited on a single Cu(100) crystal in situ using our well established epitaxial growth technique to achieve a layer-by-layer growth and a metastable face-centered-cubic structure, with all the films exhibiting an in-plane magnetic anisotropy. The high density, circularly polarized x-ray beam was supplied by the elliptically polarizing undulator at the Stanford Synchrotron Radiation Laboratory. Utilizing the element-specific ability and nanostructure magnetization sensitivity of this technique, we were able to perform the absorption measurements at L2 and L3 edges of Co and Ni atoms and we observed large dichroism signals. The extraction of spin moment and orbital moment for varying elemental stoichiometry using magneo-optical sum rules is discussed.

Growth and magnetic properties of FexNi1−x ultrathin films on Cu(100)

We have investigated ultrathin FexNi1−x films grown epitaxially on Cu(100) with different stochiometry. With the surface magneto‐optic Kerr effect (SMOKE) we measured the variation of the Curie temperature TC as a function of the film thickness n in monolayers (ML). Using the results of our previous investigations on finite‐size scaling (Huang et al.), we are able to extrapolate the value TC(∞) for samples with different Fe content. In particular, alloy films with Fe concentrations close to 65% remain ferromagnetic. This is in contrast to bulk Fe65Ni35, which shows a collapse of long range order, which is the so‐called invar effect associated with a fcc to bcc structural transition. Growing these alloy films on a Cu(100) substrate forces them to adapt the Cu lattice spacing, thereby suppressing the structural relaxation.

Direct extraction of exchange splittings from magnetic x-ray dichroism in photoelectron spectroscopy

It will be demonstrated that core-level exchange splitting can be extracted directly from normalized difference curves in magnetic X-ray circular dichroism (MXCD) in angle-resolved photoelectron spectroscopy (PES). Although high resolution is a requirement for the method, this determination can be performed without resorting to time-consuming and difficult spectral simulations. For well-defined cases, it will be shown empirically that this method may also work for the analysis of magnetic X-ray linear dichroism (MXLD). Applying this approach, it will be possible to use MXCD and MXLD in PES for direct surface magnetometry with full elemental specificity.

Comparison of magnetic linear dichroism in 4f photoemission and 4d–4f photoemission from Gd on Y(0001)

Magnetic linear dichroism (MLD) in 4d–4f resonant and 4f nonresonant photoemission (PE) is studied from thin epitaxial gadolinium films. In an angle resolved and high-energy resolution mode, experiments were conducted with the electric-field vector of the incident light perpendicular to the sample magnetization. Our results show a significant difference in behavior of MLD in resonant PE as compared to that in nonresonant PE. Off-resonance, the MLD signal is dominated by a negative feature at the low binding energy side of the peak. Near the 4d–4f resonance maximum, the MLD displays a plus–minus shape, with a negative signal at the low binding energy side of the 4f peak and a positive signal at the high binding energy side. Analysis of MLD in 4d–4f resonant PE may provide insight into interactions of the 4d core hole with the 4f core level in the intermediate state.

Generalized description of magnetic x-ray circular dichroism in Fe 3p photoelectron emission

Extending a single electron picture previously developed by Venus, it will be shown that a generalized prediction of the magnetic x-ray circular dichroism in the angle-resolved photoelectron spectroscopy of the Fe 3p can be obtained. This determination can be performed without resorting to time-consuming and difficult spectral simulations.

On the magnetic instability of ultrathin FCC FexNi1-x films

Abstract The `invar effect’ in Fe x Ni 1− x alloys occurs when the Fe content approaches 65%. At this point, the magnetization falls to zero, and a martensitic structural transformation from an FCC to BCC lattice occurs. This paper addresses the question: what happens if this structural transformation is suppressed in an ultrathin alloy film?

Effects of symmetry on circular and linear magnetic dichroism in angle-resolved photoemission spectra of Gd/Y (0001) and Fe-Ni//Cu (001)

We have observed circular and linear magnetic dichroism in angle- resolved photoemission spectra of 50-monolayer Gd film grown on Y(0001) and 6-monolayer Fe-Ni alloy films grown on Cu(001). The 4f level of Gd and the Fe 3p level of the Fe-Ni alloy were measured. A different geometry was used for the magnetic circular dichroism than was used to measure the magnetic linear dichroism. The geometries were chosen so that the shape of the magnetic circular dichroism is predicted to be equal to the shape of the magnetic linear dichroism for four-fold symmetric Fe-Ni/Cu(001) but not for three-fold symmetric Gd/Y(0001). Experimental results are presented. In this paper we examine the effect of symmetry (experimental geometry and sample geometry) on magnetic linear and circular dichroism in angle- resolved photoemission. In particular we chose separate geometries for measuring magnetic circular and magnetic linear dichroism. The geometries were chosen such that samples with four-fold symmetry about the sample normal may have magnetic circular and magnetic linear dichroism of the same shape. But samples with three-fold symmetry should not exhibit circular and magnetic linear dichroism of the same shape. The samples studied are three-fold symmetric Gd films grown on Y(0001) and four-fold symmetric Fe-Ni alloy grown on Cu(001). After presenting the methods of the experiment, we briefly review parts of a model of magnetic dichroism developed by Venus and coworkers and our specialization and extension of it, particularly for FeNi/Cu(001). We then show the results of our measurements.

Surface-sensitive, element-specific magnetometry with x-ray linear dichroism

It is shown that the x-ray magnetic linear dichroism (XMLD) in x-ray photoemission signal can be used to monitor the element specific magnetic moments in ultra thin alloy films. Comparison with recent superconducting quantum interference device data provides a quantitative check that demonstrates that the total magnetization derived from summing the constituent elemental moments changes with the composition of the alloy. This is illustrated by the contrasting behavior of FeNi and CoNi alloys.