Element-specific Magnetic Properties Research Articles

Isotropic orbital magnetic moments in magnetically anisotropic SrRuO3 films

Epitaxially strained SrRuO3 films have been a model system for understanding the magnetic anisotropy in metallic oxides. In this paper, we investigate the anisotropy of the Ru 4d and O 2p electronic structure and magnetic properties using high-quality epitaxially strained (compressive and tensile) SrRuO3 films grown by machine-learning-assisted molecular beam epitaxy. The element-specific magnetic properties and the hybridization between the Ru 4d and O 2p orbitals were characterized by Ru M2,3-edge and O K-edge soft X-ray absorption spectroscopy and X-ray magnetic circular dichroism measurements. The magnetization curves for the Ru 4d and O 2p magnetic moments are identical, irrespective of the strain type, indicating the strong magnetic coupling between the Ru and O ions. The electronic structure and the orbital magnetic moment relative to the spin magnetic moment are isotropic despite the perpendicular and in-plane magnetic anisotropy in the compressive-strained and tensile-strained SrRuO3 films; i.e., the orbital magnetic moments have a negligibly small contribution to the magnetic anisotropy. This result contradicts Bruno model, where magnetic anisotropy arises from the difference in the orbital magnetic moment between the perpendicular and in-plane directions. Contributions of strain-induced electric quadrupole moments to the magnetic anisotropy are discussed, too.

Cr doping-induced ferromagnetism in the spin-glass Cd1−xMnxTe studied by x-ray magnetic circular dichroism

The prototypical diluted magnetic semiconductor Cd1−xMnxTe is a spin glass (x < 0.6) or an antiferromagnet (x > 0.6) but becomes ferromagnetic upon doping with a small amount of Cr atoms. To investigate the origin of the ferromagnetism in Cd1−x−yMnxCryTe, we have studied its element specific magnetic properties by x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD). Measured XAS and XMCD spectra indicate that both Cr and Mn atoms are divalent and that the magnetic moments of Cr and Mn are aligned parallel. The magnetization of Mn increases with increasing Cr content, suggesting that ferromagnetic interaction exists between neighboring Mn and Cr ions despite the largely antiferromagnetic interaction between Mn atoms. By analyzing the element-specific magnetization curves, we conclude that Cr doping leads to the formation of ferromagnetic or superparamagnetic clusters consisting of several Cr ions surrounded by a much larger number of Mn ions.

Comprehensive investigation of crystallographic, spin-electronic and magnetic structure of [formula omitted]: Unraveling the suppression of configuration entropy in high entropy oxides

High entropy oxides (HEOs) are a rapidly emerging class of functional materials consisting of multiple principal cations. The original paradigm of HEOs assumes cationic occupations with the highest possible configurational entropy allowed by the composition and crystallographic structure. However, the fundamental question remains on the actual degree of configurational disorder in HEOs, especially, in systems with low enthalpy barriers for cation anti-site mixing. Considering the experimental limitations due to the presence of multiple principal cations in HEOs, here we utilize a robust and cross-referenced characterization approach using soft X-ray magnetic circular dichroism, hard X-ray absorption spectroscopy, Mössbauer spectroscopy, neutron powder diffraction and SQUID magnetometry to study the competition between crystal field stabilization energy and configurational entropy governing the cation occupation in a spinel HEO (S-HEO), (Co0.2Cr0.2Fe0.2Mn0.2Ni0.2)3O4. In contrast to the previous studies, the derived complete structural and spin-electronic model, (Co0.6Fe0.4)(Cr0.3Fe0.1Mn0.3Ni0.3)2O4, highlights a significant deviation from the hitherto assumed paradigm of entropy-driven non-preferential distribution of cations in HEOs. An immediate correlation of this result can be drawn with bulk as well as the local element specific magnetic properties, which are intrinsically dictated by cationic occupations in spinels. The real local lattice picture presented here provides an alternate viewpoint on ionic arrangement in HEOs, which is of fundamental interest for predicting and designing their structure-dependent functionalities.

Single-domain perpendicular magnetization induced by the coherent O 2p -Ru 4d hybridized state in an ultra-high-quality SrRuO3 film

We investigated the Ru $4d$ and O $2p$ electronic structure and magnetic properties of an ultrahigh-quality $\mathrm{SrRu}{\mathrm{O}}_{3}$ film on $\mathrm{SrTi}{\mathrm{O}}_{3}$ grown by machine-learning assisted molecular-beam epitaxy. The high itinerancy and long quantum lifetimes of the quasiparticles in the Ru $4d$ ${t}_{2g}$-O $2p$ hybridized valence band are confirmed by observing the prominent well-screened peak in the Ru $3d$ core-level photoemission spectrum, the coherent peak near the Fermi energy in the valence-band spectrum, and quantum oscillations in the resistivity. The element-specific magnetic properties and the hybridization between the Ru $4d$ and O $2p$ orbitals were characterized by Ru ${M}_{2,3}$-edge and O $K$-edge soft x-ray absorption spectroscopy and x-ray magnetic circular dichroism measurements. The ultrahigh-quality $\mathrm{SrRu}{\mathrm{O}}_{3}$ film with the residual resistivity ratio of 86 shows the large orbital magnetic moment of oxygen ions induced by the strong orbital hybridization of the O $2p$ states with the spin-polarized Ru $4d$ ${t}_{2g}$ states. The film also shows single-domain perpendicular magnetization with an almost ideal remanent magnetization ratio of 0.97. These results provide detailed insights into the relevance between orbital hybridization and the perpendicular magnetic anisotropy in $\mathrm{SrRu}{\mathrm{O}}_{3}/\mathrm{SrTi}{\mathrm{O}}_{3}$ systems.

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.

Realizing large out-of-plane magnetic anisotropy in L10-FeNi films grown by nitrogen-surfactant epitaxy on Cu(001)

Rare-metal free ferromagnetic material $L{1}_{0}$ FeNi with large uniaxial magnetic anisotropy has attracted much attention for broad use of magnetic materials in various applications. While single-crystal films of $L{1}_{0}$ FeNi can be made by layer-by-layer epitaxial growth, magnitude of the uniaxial anisotropy has been lower than expected because of the imperfection of the crystal atomic structure. Here, a nitrogen (N) -surfactant epitaxial growth below 150 K is employed for overcoming interlayer mixing of the atoms at the interface. Ordered $L{1}_{0}$ FeNi films are successfully prepared on the Cu(001) substrate precovered by a ${\mathrm{Ni}}_{2}\mathrm{N}$ monolayer. The role of the N surfactant and the absence of the intermixing among Fe and Ni during the deposition and annealing processes were investigated by scanning tunneling microscopy and x-ray photoemission spectroscopy. Element-specific magnetic properties were in situ studied by soft-x-ray magnetic circular dichroism (XMCD). The observed Fe hysteresis curve and XMCD of the nitrogen-covered Ni/Fe/Ni trilayer indicate an out-of-plane magnetocrystalline anisotropy large enough for dominating the film shape anisotropy. The N-surfactant epitaxy provides a useful method for fabricating well-ordered magnetic materials without rare metals.

Interfacial exchange coupling between transition metals and Mn1.5Ga studied by X-ray magnetic circular dichroism

We investigated the element-specific magnetic properties at interfaces of Fe (1 nm)/Mn1.5Ga and Co (1 nm)/Mn1.5Ga using X-ray magnetic circular dichroism (XMCD). We found that the spins in the Fe and Co layers were coupled with and parallel to those in the Mn1.5Ga under the as-grown conditions, while the post-annealing at 350 °C changed the interface magnetic coupling to antiferromagnetic in Co. The element-specific hysteresis curves at each XMCD absorption edge revealed large coercive fields in Fe and Co through the exchange coupling with Mn1.5Ga. After the annealing process, XMCD spectral line shapes indicate that the interfacial reactions promote the formation of Mn2CoGa-like secondary alloys, which influences the performance of tunnel magnetoresistance using the interfaces between transition metals and MnGa.

Electron vortex beams prepared by a spiral aperture with the goal to measure EMCD on ferromagnetic films via STEM.

X-ray magnetic circular dichroism is a well established method to study element specific magnetic properties of a material, while electron magnetic circular dichroism (EMCD), which is the electron wave analogue to XMCD, is scarcely used today. Recently discovered electron vortex beams, that carry a discrete orbital angular momentum (OAM) L, are also predicted to reveal dichroic signals. Since electron beams can be easily focused down to sub-nanometer diameters, this novel technique promises the possibility to quantitatively determine local magnetic properties with unrivalled lateral resolution. As the spiralling wave front of the electron vortex beam has an azimutally growing phase shift of up to 2π and a phase singularity in its axial center, specially designed apertures are needed to generate such non-planar electron waves. We report on the preparation and successful implementation of spiral apertures into the condenser lens system of an aberration-corrected FEI Titan(3) 80-300 transmission electron microscope (TEM). This setup allows to perform scanning TEM (STEM) with vortex beams carrying user-selected OAM. First experiments on the interaction of the vortex beam with a poly-crystalline sample are presented. Within the achieved signal to noise ratio no EMCD signal has been detected. This finding is supported by simulations of inelastic scattering of a beam generated by spiral aperture.

Spectroscopic investigation of an intrinsic room temperature ferromagnetism in Co doped ZnO nanoparticles

Pure and Co substituted ZnO nano crystalline particles were prepared by solution combustion technique using l-Valine as a fuel. As synthesized powder samples were characterized by X-ray diffractometer and SQUID magnetometer to confirm the formation of single phase wurtzite structure and to study the bulk magnetic response of the sample, respectively. Magnetic studies show that Co doped ZnO nanoparticles exhibit ferromagnetism (FM) at room temperature (RT). Furthermore, the electronic structure and element specific magnetic properties were investigated by near-edge X-ray absorption fine structure (NEXAFS) and X-ray magnetic circular dichroism (XMCD) measurements, respectively. The effect of Co substitution on the spectral features of Co–ZnO at O K-edge, Co L3,2 edge, Zn L3,2 edge have been investigated. The spectral features of NEXAFS at Co L3,2 edge is entirely different from the spectral features of metallic clusters and other impurity phases, which rules out the presence of impurity phases. The valence state of ‘Co’ ion is found to be in +2 state. The FM nature of the sample was confirmed through XMCD spectra, which is due to the incorporation of divalent ‘Co’ ions. Hence the presented results confirm the substitution of ‘Co’ ions at ‘Zn’ site in the host lattice, which is responsible for the RTFM.

Scattering of electron vortex beams on a magnetic crystal: Towards atomic-resolution magnetic measurements

Use of electron vortex beams (EVB), that is convergent electron beams carrying an orbital angular momentum (OAM), is a novel development in the field of transmission electron microscopy. They should allow measurement of element-specific magnetic properties of thin crystals using electron magnetic circular dichroism (EMCD)---a phenomenon similar to the x-ray magnetic circular dichroism. Recently it has been shown computationally that EVBs can detect magnetic signal in a scanning mode only at atomic resolution. In this follow-up work we explore in detail the elastic and inelastic scattering properties of EVBs on crystals, as a function of beam diameter, initial OAM, acceleration voltage and beam displacement from an atomic column. We suggest that for a 10 nm layer of bcc iron oriented along (001) zone axis an optimal configuration for a detection of EMCD is an EVB with OAM of $1\hbar$ and a diameter of 1.6 \AA, acceleration voltage 200 keV and an annular detector with inner and outer diameters of $G$ and $5G$, respectively, where $\mathbf{G}=(100)$.

Separation of magnetic properties at uranium and cobalt sites in UCoAl using soft x-ray magnetic circular dichroism

Temperature ($T$) and magnetic field ($H$) dependence of the magnetic properties in metamagnetic UCoAl have been investigated using a soft x-ray magnetic circular dichroism (XMCD). In order to extract element-specific magnetic properties at the U and Co sites, the XMCD experiment has been performed at the U 4$d$-5$f$ (${N}_{4},5$) and Co 2$p$-3$d$ (${L}_{2},3$) absorption edges, respectively. Directions of magnetic moments at the U and Co sites have been determined from shapes of the XMCD spectra. The directions of the total magnetic moments at the U and Co sites are parallel to the $H$ direction ($c$ axis), but the direction of the spin magnetic moment at the U site is opposite to that at the Co site. The XMCD intensities at both the U and Co sites at $T=5.5$ K increase steeply at $H=0.77$ T (${H}_{\mathrm{m}}$), corresponding to the metamagnetic transition. The XMCD intensities do not saturate, even in the field-induced ferromagnetic state above ${H}_{\mathrm{m}}$. In addition, the ratio of the increase of the XMCD intensity at the Co site is smaller than that at the U site. From comparison of the $H$ dependence of the XMCD intensities at $T=25$ and 5.5 K, we found that the magnetic behavior of the Co atom has a stronger $T$ dependence than that of the U atom.

Element-specific magnetic properties of Co2(Mn1−xFex)Si films probed by x-ray magnetic circular/linear dichroism

Element-specific magnetic properties of epitaxial Co${}_{2}$Fe${}_{x}$Mn${}_{1\ensuremath{-}x}$Si/MgO(100) Heusler films prepared by laser ablation are measured by circular dichroism in x-ray absorption spectroscopy (XMCD). Surface and bulk magnetization of the 100-nm-thick films are equal as tested by comparing the total electron yield and transmission measurements and follow the generalized Slater-Pauling rule. For Co${}_{2}$FeSi, the large magnetization of 6 ${\ensuremath{\mu}}_{B}$ per formula unit indicates half-metallic behavior. Calculations using the local spin density approximation (LDA) result in a half-metallic band structure for this compound when an additional electron-electron-correlation correction ($\text{LDA}+U$) is considered. Simulated XMCD spectra using the LDA and $\text{LDA}+U$ approaches are compared to the experimental spectra. While the electron-electron Coulomb correlation has only a small effect on the observable XMCD spectra, the calculations predict that it will cause a significant change in the x-ray magnetic linear dichroism (XMLD).

X-선 자기 원형 이색성

X-ray magnetic circular dichroism (XMCD) has been used as an important tool of magnetics due to its unique abilities to measure element-specific magnetic properties and to separate the orbital and the spin magnetic moments. These abilities allow researchers to access the microscopic origin of the magnetic properties of transition metal and rare earth compounds. In this report, I explain the principle of XMCD and the experimental set-up. Recent a few research examples using XMCD will be also introduced.

X-ray magnetic circular dichroism of pulsed laser deposited Co2MnSn and Co2MnSb thin films grown on GaAs (001)

We present the structural and element specific magnetic properties of Co2MnSn and Co2MnSb thin films grown on GaAs (100) substrates using pulsed laser deposition. X-ray magnetic circular dichroism (XMCD) spectra were measured for 400Å thick films at the L2,3 edges of Co and Mn. Element specific moments for Co and Mn in Co2MnSn were calculated from the x-ray absorption and XMCD spectra using the XMCD sum rules. The ratios of orbital to spin magnetic moments for Co and Mn were calculated for Co2MnSn and Co2MnSb.

Element-specific study of the temperature dependent magnetization of Co–Mn–Sb thin films

Magnetron sputtered thin Co–Mn–Sb films were investigated with respect to their element-specific magnetic properties. Stochiometric Co1Mn1Sb1 crystallized in the C1b structure has been predicted to be half-metallic and is therefore of interest for spintronic applications. It should show a characteristic antiferromagnetic coupling of the Mn and Co magnetic moments and a transition temperature TC of about 480 K. Although the observed transition temperature of our 20 nm thick Co32.4Mn33.7Sb33.8, Co37.7Mn34.1Sb28.2, and Co43.2Mn32.6Sb24.2 films is in quite good agreement with the expected value, we found a ferromagnetic coupling of the Mn and Co magnetic moments which indicates that the films do not crystallize in the C1b structure and are probably not fully spin polarized. The ratio of the Co and Mn moments does not change up to the transition temperature and the temperature dependence of the magnetic moments can be well described by the mean-field theory.

Element specific magnetic properties of Gd-doped GaN: Very small polarization of Ga and paramagnetism of Gd

Element specific x-ray magnetic dichroism measurements have been carried out at the $\text{Ga}\text{ }K$ and Ga and $\text{Gd}\text{ }{L}_{3}$ edges of the dilute magnetic semiconductor (DMS) Gd:GaN grown by molecular beam epitaxy. This DMS material has previously been reported to exhibit room temperature magnetic order accompanied by colossal effective magnetic moments. We detect only a very small magnetic polarization of the order of at most ${10}^{\ensuremath{-}5}{\ensuremath{\mu}}_{B}/\text{Ga}$ atom in Gd:GaN which cannot account for the colossal effective magnetic moments. Further, the element specific magnetic properties at the Gd sites do not reveal any ferromagneticlike signatures but rather behave paramagnetic. Thus, the ferromagnetism in Gd:GaN is caused by polarization of the N site or by extrinsic mechanisms such as magnetic polarization of defects or residual oxygen in these samples.

Sum rules for electron energy loss near edge spectra

We derive four sum-rule expressions for spectra measured in electron energy loss near edge structure experiments. These sum rules permit the determination of spin and orbital magnetic moments, spin-orbit interaction, and number of states, analogously to the sum rules of x-ray magnetic circular dichroism. The derivation of the sum rules is based on dynamical electron diffraction theory and the properties of the mixed dynamic form factor. The accuracy of the sum rules is tested by a complete evaluation of the thickness-dependent electron energy loss spectra for iron, cobalt, and nickel crystals. We find that the sum rules reproduce both spin and orbital moments with very good accuracy. Our results provide a foundation for the use of the energy loss magnetic chiral dichroism technique as a quantitative probe of element-specific magnetic properties.

Element specific investigation of ultrathin Co2MnGa∕GaAs heterostructures

We have used x-ray magnetic circular dichroism to study the element specific magnetic properties of ultrathin films of the Heusler alloy Co2MnGa at room temperature. Nine films were grown by molecular beam epitaxy on GaAs substrates and engineered to vary in stoichiometry as Co1.86Mn0.99Ga1, Co1.95Mn0.98Ga1, and Co1.97Mn0.96Ga1, with thicknesses of 5.3, 7.6, and 9.7nm, and were capped with Al to prevent oxidization. Sum rule analysis revealed magnetic moments significantly lower than the values predicted theoretically, especially in the case of the Mn total magnetic moment. The results do not support the hypothesis that a large magnetic moment is transferred from Co to Mn, and may suggest that diffusion and reacted layers at ultrathin thicknesses can effect the establishment of a full Heusler structure.

Interface magnetization of ultrathin epitaxial Co2FeSi(110)/Al2O3 films

Element-specific magnetic properties of ultrathin epitaxial Co2FeSi(110) films were measured using x-ray magnetic circular dichroism (XMCD). The epitaxial Heusler films were grown by RF magnetron sputtering on substrates. The magnetization of thicker films as determined by XMCD is smaller than expected for a half-metallic material. In addition, the magnetization decreases considerably for films thinner than 10 nm. The thickness dependence of the magnetic moment can be described by introducing a certain number of dead layers representing a deficiency of magnetization at the interfaces. Quantitative evaluation results in a dead layer thickness of 0.8 nm at room temperature, consisting of a temperature induced size effect of 0.1 nm and a surface effect of 0.15 nm at the top and 0.55 nm at the bottom interface.

Cr adsorption effect of magnetic property of Fe/Cu(001)

Epitaxial growth of iron on Cu(001) below 11 ML shows a perpendicular magnetic anisotropy. The sub-monolayer Cr adsorbed fct Fe/Cu(001) have been investigated by x-ray absorption spectroscopy and x-ray magnetic circular dichroism (XMCD) to understand the element specific magnetic properties and the electronic states. It is found that the XMCD signal has been clearly observed in the Cr L2,3 edge, which indicates the anti-parallel magnetic moment in the adsorbed Cr atom to that of the Fe layer. The smaller remnant magnetization for the larger amount of Cr has been observed for both 2.5 and 7.4 ML Fe films, where their magnetizations disappear upon the 0.4 and 0.2 ML Cr adsorption at 185 K, respectively. This behavior is turned out to be caused by the suppression of the Curie temperature. It is also found that the modification of the magnetic property for 7.4 ML Fe is similar to that for 2.5 ML. [DOI: 10.1380/ejssnt.2006.345]