Element-specific Hysteresis Loops Research Articles

Element-specific magnetic hysteresis loops observed in hexagonal ErFeO3 thin films

We investigated the magnetic properties of multiferroic hexagonal ErFeO3 thin films by using x-ray magnetic circular dichroism (XMCD) and synchrotron Mössbauer spectroscopy. In order to reveal the element-specific magnetic properties, temperature-dependent XMCD experiments were conducted at the Fe L 2,3 and Er M 4,5-edges. Apparent magnetic hysteresis loops appear for both ions below the Néel temperature, which suggests the existence of ferromagnetism. The temperature evolutions of the coercive field and spontaneous magnetization for both ions show similar behavior. These results indicate that Fe ions influence the magnetism of Er ions. Our results deepen the understanding of the physical properties of hexagonal rare-earth ferrite system.

Microstructures and Interface Magnetic Moments in Mn2VAl/Fe Layered Films Showing Exchange Bias.

Heusler alloys are a material class exhibiting various magnetic properties, including antiferromagnetism. A typical application of antiferromagnets is exchange bias that is a shift of the magnetization curve observed in a layered structure consisting of antiferromagnetic and ferromagnetic films. In this study, a layered sample consisting of a Heusler alloy, MnVAl and a ferromagnet, Fe, is selected as a material system exhibiting exchange bias. Although the fully ordered MnVAl is known as a ferrimagnet, with an optimum fabrication condition for the MnVAl layer, the MnVAl/Fe layered structure exhibits exchange bias. The appearance of the antiferromagnetic property in the MnVAl is remarkable; however, the details have been unclear. To clarify the microscopic aspects on the crystal structures and magnetic moments around the MnVAl/Fe interface, cross-sectional scanning transmission electron microscope (STEM) observation, and synchrotron soft X-ray magnetic circular dichroism (XMCD) measurements were employed. The high-angle annular dark-field STEM images demonstrated clusters of MnVAl with the L2 phase distributed only around the interface to the Fe layer in the sample showing the exchange bias. Furthermore, antiferromagnetic coupling between the Mn- and Fe-moments were observed in element-specific hysteresis loops measured using the XMCD. The locally ordered L2 phase and antiferromagnetic Mn-moments in the MnVAl were suggested as important factors for the exchange bias.

Thermomagnetic switching in strong/weak/strong ferromagnet stack detected with resonant X-ray magnetic reflectometry

It is of interest to explore magnetic configurations and phase transitions at the interfaces between different magnetic materials by employing nondestructive and element-specific methods such as resonant X-ray magnetic reflectometry (RXMR). Using RXMR, we have measured element-specific hysteresis loops of a layered stack of TaO(15 nm)/Ir20Mn80(25 nm)/Co90Fe10(3 nm)/Ni67Cu33(10 nm)/Ni80Fe20(10 nm)/quartz substrate at the Fe и Ni absorption edges. The goal here is to highlight the contribution of the weakly ferromagnetic Ni67Cu33 spacer in this system where the spacer is sandwiched between strong ferromagnets, Ni80Fe20 and Co90Fe10. We find two kinds of the spacer effects on magnetization switching in the system. One of them is that switching of the Co90Fe10 magnetization displays at the Ni absorption edge. Such a behavior of the magnetic reflectivity results from strong exchange coupling of the spacer to the Co90Fe10 surrounding across the interface between them. Another one is that temperature variation around the Curie point of the spacer (TC ≈ 240 K) provides a stronger change of the magnetic contrast at the Ni80Fe20/Ni67Cu33 interface than that at the Co90Fe10/Ni67Cu33 one. This feature indicates weaker exchange coupling across the Ni80Fe20/Ni67Cu33 interface.

FromFe3O4/NiO bilayers toNiFe2O4-like thin films through Ni interdiffusion

Ferrites with (inverse) spinel structure display a large variety of electronic and magnetic properties making some of them interesting for potential applications in spintronics. We investigate the thermally induced interdiffusion of Ni$^{2+}$ ions out of NiO into Fe$_3$O$_4$ ultrathin films resulting in off-stoichiometric nickelferrite-like thin layers. We synthesized epitaxial Fe$_3$O$_4$/NiO bilayers on Nb-doped SrTiO$_3$(001) substrates by means of reactive molecular beam epitaxy. Subsequently, we performed an annealing cycle comprising three steps at temperatures of 400\,$^{\circ}$C, 600\,$^{\circ}$C, and 800\,$^{\circ}$C under an oxygen background atmosphere. We studied the changes of the chemical and electronic properties as result of each annealing step with help of hard x-ray photoelectron spectroscopy and found a rather homogenous distribution of Ni and Fe cations throughout the entire film after the overall annealing cycle. For one sample we observed a cationic distribution close to that of the spinel ferrite NiFe$_2$O$_4$. Further evidence comes from low energy electron diffraction patterns indicating a spinel type structure at the surface after annealing. Site and element specific hysteresis loops performed by x-ray magnetic circular dichroism uncovered the antiferrimagnetic alignment between the octahedral coordinated Ni$^{2+}$ and Fe$^{3+}$ ions and the Fe$^{3+}$ in tetrahedral coordination. We find a quite low coercive field of 0.02\,T, indicating a rather low defect concentration within the thin ferrite films.

Temperature dependent studies of capped magnetic nanowires using XMCD

Aligned cobalt nanowires, 1, 2, and 3 atoms wide, were grown on platinum (997) surfaces under UHV conditions and capped with five monolayers of gold. X‐ray magnetic circular dichroism (XMCD) measurements were performed at the L2,3 edges of Co. Element specific magnetic hysteresis loops were measured as a function of temperature and coercivities were extracted. The easy axis of magnetization was confirmed to be perpendicular to the (111) terrace for all samples. The temperature dependence of the coercivity, which varies by an order‐of‐magnitude over a 150 K temperature range for the 1‐atom‐wide wire, was found to be well described by the Gaunt “strong” domain wall pinning model for all three wires. The observation that capping increases the Curie temperature of the 1‐atom‐wide nanowire by at least 270 K, together with the strong temperature dependence of the coercivity and the perpendicular magnetic anisotropy, may have important implications for the technological application of low dimensional, nanoscale magnetic materials.

The manipulation of magnetization damping in FeNi1–xNdx/Cu/FeCo1–yGdy sandwich structured multilayers

The magnetic properties and magnetization damping in a series of Ta(5 nm)/Fe20Ni80Nd0.075(3 nm)/Cu(tCunm)/Fe50Co50Gd0.07(3 nm)/Cu(2 nm) multilayers with varying nonmagnetic Cu layer thickness are investigated by vibrating sample magnetometer and ferromagnetic resonance (FMR). Two-step hysteresis loops are observed, indicative of individual switching of the two magnetic layers. The saturation magnetization of the multilayers possesses a very close magnitude to the sum of those for the Fe20Ni80Nd0.075 and Fe50Co50Gd0.07 single layer films, and the difference between each other is within 3% only. Element-specific hysteresis loops of each magnetic elements involved the multilayers are further probed by x-ray magnetic circular dichroism, which reveals that Fe, Ni, and Co are ferromagnetically coupled with Nd and anti-ferromagnetically with Gd. The FMR spectra for the multilayers exhibit two-resonance peaks originated from the two magnetic layers. Compared with the NiFe-Nd and FeCo-Gd single magnetic films, the linewidth of NiFe-Nd layer in the multilayers shows a large enhancement, while that of the FeCo-Gd layer decreases. Theoretical fitting of the FMR data indicates that the contribution of two-magnon scattering has played an important role in the observed linewidth of single FeCo-Gd films, which, in turn, causes the larger FMR linewidth than that in the multilayers. The intrinsic Gilbert damping constant of both FeNi-Nd and FeCo-Gd layers is found to increase with increasing thickness of nonmagnetic Cu layer from 0.8 nm to 2 nm.

The influence of Nd dopants on spin and orbital moments in Nd-doped permalloy thin films

Magnetic properties of NdX-Ni80Fe20(1−X) thin films have been investigated using x-ray absorption spectroscopy and x-ray magnetic circular dichroism (XMCD) at room temperature. With the Nd concentration increasing, the ratio of orbital-to-spin moment of Ni and Fe increases significantly, indicating that the spin-orbit coupling in permalloy thin films is enhanced due to the Nd impurities. The spin and orbital moments have been obtained by the sum rules analysis, which shows that the Nd impurities lead to a strong dispersion of spin moments of Fe and Ni while have no effect on orbital moments in Nd-doped permalloy thin films. Element-specific XMCD hysteresis loops suggest an antiferromagnetic coupling between the magnetic moments of Nd and permalloy at room temperature. The static magnetic properties have been studied by vibrating sample magnetometer for comparison, which shows a nice agreement with the XMCD results.

X-ray absorption spectroscopy and magnetic circular dichroism studies of L1-Mn-Ga thin films

Tetragonally distorted Mn3−xGax thin films with 0.1<x<2 show a strong perpendicular magnetic anisotropy and low magnetization and thus have the potential to serve as electrodes in spin transfer torque magnetic random access memory. Because a direct capping of these films with MgO is problematic due to oxide formation, we examined the influence of a CoFeB interlayer and of two different deposition methods for the MgO barrier on the formation of interfacial Mn-O for Mn62Ga38 by element specific X-ray absorption spectroscopy (XAS) and magnetic circular dichroism (XMCD). A highly textured L10 crystal structure of the Mn-Ga films was verified by X-ray diffraction measurements. For samples with e-beam evaporated MgO barrier no evidence for Mn-O was found whereas in samples with magnetron sputtered MgO, Mn-O was detected, even for the thickest interlayer thickness. Both XAS and XMCD measurements showed an increasing interfacial Mn-O amount with decreasing CoFeB interlayer thickness. Additional element specific full hysteresis loops determined an out-of-plane magnetization axis for the Mn and Co, respectively.

Direct observation of frozen moments in the NiFe/FeMn exchange bias system

We detect the presence of frozen magnetic moments in an exchange biased NiFe ferromagnet at the NiFe/FeMn ferromagnet/antiferromagnet interface by magnetic circular dichroism in x-ray absorption and resonant reflectivity experiments. Frozen moments are detected by means of the element-specific hysteresis loops. A weak dichroic absorption with unidirectional anisotropy can be linked to frozen magnetic moments in the ferromagnet. A more pronounced exchange bias for increasing the thickness of the FeMn layer correlates with an increase in orbital moment for interface Ni atoms carrying a frozen moment. These atoms compose about a single monolayer, but only a fraction of the atoms contributes by means of a strongly enhanced orbital moment to the macroscopic exchange bias phenomenon. The microscopic spin–orbit energy associated with these few interface frozen moment atoms appears to be sufficient to account for the macroscopic exchange bias energy.

Element-Specific Hysteresis Loop Measurements on Individual 35 nm Islands with Scanning Transmission X-Ray Microscopy

Using scanning transmission X-ray microscopy combined with X-ray magnetic circular dichroism, element-specific hysteresis loops with a 25 nm X-ray probe are obtained on 35 nm Fe/Gd multilayer nanoislands fabricated by extreme ultra-violet interference lithography. Local hysteresis loops measured for the individual islands and the antidot film between the islands display similar behavior resulting from the lateral confinement. Line scan measurements confirm ferrimagnetic coupling between Fe and Gd in the patterned region. The ability to measure magnetization reversal with X-rays at high spatial resolution will provide an important tool for future characterization of sub-50 nm nanostructures.

Non-collinear magnetization configuration in interlayer exchange coupled magnetic thin films

Element specific magnetic hysteresis loops of the interlayer exchange coupled CoFeB/Ru/[Co/Ni]4 structure were measured utilizing x-ray magnetic circular dichroism. It was found that the Co/Ni multilayer and the CoFeB layer have Ru thickness dependent oscillatory interlayer coupling. Due to its interlayer coupling with the perpendicularly magnetized Co/Ni multilayer, the CoFeB magnetization direction is slightly tilted out-of-plane from its in-plane magnetic easy axis. Quantitative measurements show that the tilting angle is small (<12°) and that a small in-plane magnetic field (∼50 Oe) applied to this structure will result in a completely in-plane CoFeB magnetization.

Magnetocrystalline anisotropy of magnetic grains in Co80Pt20:oxide thin films probed by x-ray magnetic circular dichroism

Using angle-dependent x-ray magnetic circular dichroism, we have measured magnetic hysteresis loops at the Co L2,3 edges of oxide-doped Co80Pt20 thin films. The magnetocrystalline anisotropy energy (MAE) of the Co atoms, which is the main source of the magnetocrystalline anisotropy of the CoPt magnetic grains, has been determined directly from these element-specific hysteresis loops. When the oxide volume fraction (OVF) is increased from 16.6% to 20.7%, the Co MAE has been found to decrease from 0.117 meV/atom to 0.076 meV/atom. While a larger OVF helps one to achieve a smaller grain size, it reduces the magnetocrystalline anisotropy, as demonstrated unambiguously from the direct Co MAE measurements. Our results suggest that those Co80Pt20:oxide films with an OVF between 19.1% and 20.7% are suitable candidates for high-density magnetic recording.

Tuning the coercive field of Ni and CuNi thin films with the embedding of Co nanoparticles: An element-specific study

Cobalt nanoparticles have been embedded either within magnetic thin films of CuNi or Ni, or at their interface with an Al capping layer. Soft X-ray magnetic circular dichroism (XMCD) was used to measure element-specific hysteresis loops, allowing discrimination between the nanoparticles and the host film. The hysteresis shows coupling between the nanoparticles and the films and a reduction in coercivity is observed when positioning the nanoparticles within the film, as opposed to at its interface with the cap. Sum rule analysis of the XMCD spectra makes it possible to determine the orbital to spin moment ratios for Co and Ni in the samples. The difference in the calculated ratios is consistent with the differences in the coercivities, the films appearing more homogenous due to hybridization of the 3d bands of Co and Ni when the nanoparticles are embedded deeper within the films, creating less pinning sites and hence a lower coercivity.

Interfacial structure and magnetic properties of Co2FeAl0.5Si0.5/MgO heterostructures

The interfacial properties of the Co2FeAl0.5Si0.5/MgO based magnetic tunnel junction have been investigated using x ray absorption spectroscopy (XAS), angle resolved x ray photoelectron spectroscopy (ARXPS), x ray magnetic circular dichroism (XMCD), and element-specific hysteresis loops. The XAS demonstrates a multiplet structure at the Co L3 edge which could be attributed to the formation of CoO at the interface due to the high annealing temperature. The XMCD sum-rule analysis and the element-specific hysteresis loops show a higher magnetic moment, a change in the loop shape, and an increase in the Co coercive field when probing more close to the interface layer. The chemical and structural disorder at the interface has been further revealed by the ARXPS measurements.

Ultrafast Demagnetization Dynamics at theMEdges of Magnetic Elements Observed Using a Tabletop High-Harmonic Soft X-Ray Source

We use few-femtosecond soft x-ray pulses from high-harmonic generation to extract element-specific demagnetization dynamics and hysteresis loops of a compound material for the first time. Using a geometry where high-harmonic beams are reflected from a magnetized Permalloy grating, large changes in the reflected intensity of up to 6% at the M absorption edges of Fe and Ni are observed when the magnetization is reversed. A short pump pulse is used to destroy the magnetic alignment, which allows us to measure the fastest, elementally specific demagnetization dynamics, with 55 fs time resolution. The use of high harmonics for probing magnetic materials promises to combine nanometer spatial resolution, elemental specificity, and femtosecond-to-attosecond time resolution, making it possible to address important fundamental questions in magnetism.

Temperature-Induced Reversal of Magnetic Interlayer Exchange Coupling

For epitaxial trilayers of the magnetic rare-earth metals Gd and Tb, exchange coupled through a nonmagnetic Y spacer layer, element-specific hysteresis loops were recorded by the x-ray magneto-optical Kerr effect at the rare-earth M5 thresholds. This allowed us to quantitatively determine the strength of interlayer exchange coupling (IEC). In addition to the expected oscillatory behavior as a function of spacer-layer thickness dY, a temperature-induced sign reversal of IEC was observed for constant dY, arising from magnetization-dependent electron reflectivities at the magnetic interfaces.

Microscopic Nature of Ferro- and Antiferromagnetic Interface Coupling of Uncompensated Magnetic Moments in Exchange Bias Systems

Exchange bias in layered CoO/Fe structures is investigated by x-ray resonant magnetic reflectivity (XRMR) measurements. Element-specific hysteresis loops are obtained from x-ray magnetic circular dichroism effects in the XRMR spectra. Evidence is provided for the existence of different types of uncompensated moments in the antiferromagnetic material. Explanations are given for the microscopic nature of these moments and the complex exchange interactions that determine the magnetization reversal in exchange bias systems.

Interface effects, magnetic, and magneto-optical properties ofAl∕Co∕V∕MgO(100)structures

The magnetic and magneto-optical properties of Al/ Co/ V / MgO100 structures and the influence of the Co/ V interface for different thicknesses of the Co and V layers have been studied experimentally. From the element-specific hysteresis loops obtained by x-ray magnetic circular dichroism XMCD at the Co and V L3 edges we find that the V layers are magnetically polarized antiparallel to the Co layers and that magnetization reversal occurs at the same magnetic field in both layers. The effect of the V atomic volume and the relative thicknesses of V and Co layers on the magnetic moments of the V and Co atoms are also studied. Contrary to what might be expected, the atomic magnetic moments of the V and Co atoms do not depend on the V atomic volume, however they depend strongly on the thicknesses of the V and Co layers. From the trend in the Co and V atomic magnetic moments as a function of Co and V thicknesses and the absence of any spectroscopic evidence for the formation of a VCo alloy, a long-range spin polarization of the V atoms is proposed. The extracted magneto-optical MO constants of V and Co as a function of photon energy clearly evidence a large amount of polarization in the V films in agreement with the XMCD results, which is indicative of the magnetic coupling between Co and V. The experimental MO results and simulations also show the strong influence of these polarized V layers in the MO properties of the complete system. These results, given the absence of spectroscopic evidence for the formation of a VCo alloy, support a long-range magnetization of the V atoms and a reduction of the Co atomic magnetic moments.

Rotation of a Gd Moment Vector in an Fe/FeGd/Fe Trilayer Observed by Hysteresis Measurements of the Resonant X-ray Magnetic Reflectivity

We have developed an experimental technique to measure element-specific hysteresis loops of films consisting of a few layers on the thick substrate by resonant X-ray magnetic scattering. The hysteresis loops of Gd in the Fe/FeGd/Fe trilayer are obtained by measuring magnetic field dependence of the magnetic specular scattering intensity at a fixed scattering angle. The Gd magnetization is obtained as a vector from the measurements in two experimental geometries with applied field parallel and perpendicular to the X-ray propagation. This technique can be applied more widely than the usual hysteresis measurements by X-ray magnetic circular dichroism because it requires no X-ray transmission through the sample.

Observation of nitrogen polarization in Fe–N using soft x-ray magnetic circular dichroism

X-ray magnetic circular dichroism (XMCD) measurements were performed on epitaxial iron nitride thin films at the nitrogen K and at the Fe LIII and LII absorption edges. The iron nitride samples measured with XMCD consist primarily of γ′ phase with no observed enhancement on their magnetic moment as compared to that of pure Fe. The XMCD difference signal between left and right elliptically polarized x rays shows a small effect in the 10−5 range at the N edge, indicative of the possible polarization of nitrogen in the γ′ Fe–N samples. XMCD spectra collected with higher energy resolution reveal the presence of multiple components in the Fe LIII absorption edge that are related to different environments for the Fe atoms, i.e., near neighbors or next near neighbors to the N sites. Finally element specific hysteresis loops were also recorded at the N and Fe absorption edges using the difference signal in various incident beam angle geometries.