G. Meigs

Soft X-ray magnetic circular dichroism and magnetic films

Abstract Soft X-ray magnetic circular dichroism is the difference in the total absorption cross-section of positive and negative helicity circularly-polarized soft X-rays at the magnetically interesting L 2 and L 3 absorption edges of 3d transition metals or the M 4 and M 5 absorption edges of the rare earths. The absorption edges can be measured using a variety of techniques including transmission, partial electron yield, total electron yield, partial fluorescence yield, sample current, and reflection. The different measurement schemes represent different probing depths, allowing depth dependent studies, and are complementary to each other. A description of the relative merits of the various measurement schemes is presented. In addition, complications such as beam instability, saturation effect, and incident beam Bragg scattering into the detectors are discussed.

Magnetic circular dichroism of FexCo1−x single-crystal thin films

Abstract Soft X-ray magnetic circular dichroism (SXMCD) of the L 2 and L 3 edges of Fe and Co is used to independently determine magnetic information for single-crystal Fe x Co 1− x alloy thin films (0≤ x ≤1) grown on ZnSe(001). The SXMCD L 3 to L 2 relative intensity of the Fe-Co alloy system is found to rise monotonically with reducing atomic concentration for both the Co and Fe, indicating an increase in the orbital to spin moment contribution at the dilute limits. This suggests that the orbital moment may play a significant role in the magnetism of transition metal alloy systems.

Electron spectroscopic studies of colossal magnetoresistance material La1−xCaxMnO3

High resolution photoemission measurements have been performed for the ferromagnetictransition of La0.67Ca0.33MnO3 and for the order–disorder transition of La0.4Ca0.6MnO3. The band gap collapsed below the Curie temperature and the density of states at the Fermi level increases with cooling. Upon cooling through the order–disorder transition temperatures, T CO, the band gap increases by ∼50 meV. This change in the gap is consistent with the change in the activation energy above and below T CO estimated from conductivity data. We have also performed soft x‐ray magnetic circular dichroism measurements at Mn L 2,3 edges for ferromagnetic La0.67Ca0.33MnO3 and La0.8Ca0.2MnO3.

Multidomain and incomplete alignment effects in giant magnetoresistance trilayers

The element specific magnetic hysteresis curves of Fe/Cu/Co trilayer structures can be used in conjunction with the measured magnetoresistance curves to extract the coefficient of the giant magnetoresistance (GMR) independent of magnetic domain effects and incomplete alignment effects, allowing for a measure of the maximum attainable GMR for that trilayer. Information concerning the details of the trilayer switching can be extracted showing that sputtered polycrystalline films of Fe/Cu/Co deposited on Si switch their magnetization directions like multidomain Ising magnets.

EXTRACTION OF THE COEFFICIENT OF GIANT MAGNETORESISTANCE

Element specific magnetic hysteresis (ESMH) loops have been used in conjunction with measured magnetoresistance curves to extract the coefficient of giant magnetoresistance (GMR), which is the maximum achievable value of the GMR independent of magnetic domain effects and incomplete moment alignment effects, for sputtered Fe/Cu/Co sandwiches on silicon substrates. Using the ESMH loops to calculate GMR curves, comparison with the measured GMR curves shows that the magnetization reversal process is best described by multidomain Ising behavior.

Magnetic structure of Fe/Cr/Fe trilayers

The magnetic structure of Cr films deposited on single crystal Fe(001)/GaAs(001) substrates is investigated by soft x‐ray magnetic circular dichroism (SXMCD) of the L2 and L3 absorption edges. We find that 0.25 ML Cr coverages result in Cr moments aligned with each other and antialigned to the underlying Fe moment direction. The Cr moment is measured to be 0.6±0.2 μB. Additional Cr deposition results in a monotonically reducing averaged moment consistent with the island growth of ferromagnetic Cr sheets of a single layer, antialigned with adjoining sheets. SXMCD measurements of the trilayer structures clearly show the antiferromagnetic coupling of the two Fe films through the interlying Cr film.

Magnetic circular dichroism studies with soft x-rays

Soft-x-ray magnetic circular dichroism (SXMCD) is a powerful tool to study the element and site specific magnetic moments in multi-component systems. This is demonstrated for a thin metal film overlayer system and a rare-earth/transition-metal compound. The magnetic properties of ultrathin Ni films on a Cu(100) substrate are investigated at the Ni L2,3 absorption edges. Temperature dependent SXMCD data of various film thickness yield Curie temperatures and the critical exponents used in scaling theories which describe two dimensional magnetic phase transitions. The magnetic moments of Gd and Fe in gadolinium iron garnet (Gd3Fe5O12) were probed at 77 K and 300 K at the Gd M4,5 and at the Fe L2,3 absorption edges. The SXMCD signal at each edge allows the magnetic ordering for each specific ion to be independently determined, and the temperature dependence confirms that the reversal of the macroscopic magnetization is due to the reversal of each local magnetic moment of the Fe and the Gd atoms when the compensation point (Tcomp equals 288 K) is crossed.

Application of magnetic circular dichroism to magnetic thin films

Abstract The L 2,3 edge magnetic circular dichroism (MCD) spectra of 3d transition metal magnetic thin films and overlayers is used to identify the presence and alignment of the magnetic moment of overlayers on ferromagnetic and non-magnetic substrates. A controversial application of MCD is the determination of the magnitude of magnetic moments and the extraction of orbital and spin contributions to the moment. To study this, we have measured, in transmission, the helicity-dependent, total absorption cross-section at the L 2 and L 3 absorption edge of Fe and Co ultrathin films deposited on 1 μm parylene foil. Using no artificial background subtraction or curve renormalization, we obtain clean spectra for comparison with theoretical methods of the extraction of the orbital and spin contributions to the total moment.

Element-specific vector magnetometry of buried layers

Abstract Magnetic circular dichroism is used as an element-specific vector magnetometer to determine the behavior of a buried 4.3 monolayer Mn film in a Fe 25 Co 75 /Mn/Fe 25 Co 75 trilayer, which exhibits a 90° coupling between the FeCo layers. The data indicate a net ferromagnetic Mn moment canted by 23° from the net Fe (and Co) moments.

Giant magnetic effects in the L‐edge extended x‐ray absorption fine structure of 3d transition metals

A systematic extended x‐ray absorption fine structure (EXAFS) study at the L edges of ferromagnetic Fe, Co, and Ni thin films has been conducted using circular polarized soft x rays. A very large helicity dependence has been observed in the L‐edge EXAFS spectra. Surprisingly, in contradiction with prior works, these magnetic EXAFS results indicate a nonlinear dependence of this effect on the magnetic polarization of the scattering atom. The data further indicate that the dominant scattering paths responsible for the effect are different from those responsible for EXAFS. A discussion of the results in relation to the present understanding of the underlying mechanism and its ramifications to the existing theoretical frameworks is also presented.