Experimental realization of linearly polarized X-ray detected ferromagnetic resonance

We present the first theoretical and experimental evidence of time-resolved dynamic X-ray magnetic linear dichroism (XMLD) measurements of GHz magnetic precessions driven by ferromagnetic resonance in both metallic and insulating thin films. Our findings show a dynamic XMLD in both ferromagnetic Ni80Fe20 and ferrimagnetic Ni0.65Zn0.35Al0.8Fe1.2O4 for different measurement geometries and linear polarizations. A detailed analysis of the observed signals reveals the importance of separating different harmonic components in the dynamic signal in order to identify the XMLD response without the influence of competing contributions. In particular, RF magnetic resonance elicits a large dynamic XMLD response at the fundamental frequency under experimental geometries with oblique x-ray polarization. The geometric range and experimental sensitivity can be improved by isolating the 2ω Fourier component of the dynamic response.These results illustrate the potential of dynamic XMLD and represent a milestone accomplishment towards the study of GHz spin dynamics in systems beyond ferromagnetic order.

Tensor description of X-ray magnetic dichroism at the Fe L 2,3-edges of Fe3O4

A procedure to build the optical conductivity tensor that describes the full magneto-optical response of the system from experimental measurements is presented. Applied to the Fe L 2,3-edge of a 38.85 nm Fe3O4/SrTiO3 (001) thin-film, it is shown that the computed polarization dependence using the conductivity tensor is in excellent agreement with that experimentally measured. Furthermore, the magnetic field angular dependence is discussed using a set of fundamental spectra expanded on spherical harmonics. It is shown that the convergence of this expansion depends on the details of the ground state of the system in question and in particular on the valence-state spin–orbit coupling. While a cubic expansion up to the third order explains the angular-dependent X-ray magnetic linear dichroism of Fe3+ well, higher-order terms are required for Fe2+ when the orbital moment is not quenched.

Magnetocrystalline anisotropy imprinting of an antiferromagnet on an amorphous ferromagnet in FeRh/CoFeB heterostructures

Magnetic anisotropy is a fundamental key parameter of magnetic materials that determines their applications. For ferromagnetic materials, the magnetic anisotropy can be easily detected by using conventional magnetic characterization techniques. However, due to the magnetic compensated structure in antiferromagnetic materials, synchrotron measurements, such as X-ray magnetic linear dichroism, are often needed to probe their magnetic properties. In this work, we observed an imprinted fourfold magnetic anisotropy in the amorphous ferromagnetic layer of FeRh/CoFeB heterostructures. The MOKE and ferromagnetic resonance measurements show that the easy magnetization axes of the CoFeB layer are along the FeRh〈110〉 and FeRh〈100〉 directions for the epitaxially grown FeRh layer in the antiferromagnetic and ferromagnetic states, respectively. The combined Monte Carlo simulation and first-principles calculation indicate that the fourfold magnetic anisotropy of the amorphous CoFeB layer is imprinted due to the interfacial exchange coupling between the CoFeB and FeRh moments from the magnetocrystalline anisotropy of the epitaxial FeRh layer. This observation of imprinting the magnetocrystalline anisotropy of antiferromagnetic materials on easily detected ferromagnetic materials may be applied to probe the magnetic structures of antiferromagnetic materials without using synchrotron methods.

X-ray magnetic linear dichroism as a probe for non-collinear magnetic state in ferrimagnetic single layer exchange bias systems

Ferrimagnetic alloys are extensively studied for their unique magnetic properties leading to possible applications in perpendicular magnetic recording, due to their deterministic ultrafast switching and heat assisted magnetic recording capabilities. On a prototype ferrimagnetic alloy we demonstrate fascinating properties that occur close to a critical temperature where the magnetization is vanishing, just as in an antiferromagnet. From the X-ray magnetic circular dichroism measurements, an anomalous ‘wing shape’ hysteresis loop is observed slightly above the compensation temperature. This bears the characteristics of an intrinsic exchange bias effect, referred to as atomic exchange bias. We further exploit the X-ray magnetic linear dichroism (XMLD) contrast for probing non-collinear states which allows us to discriminate between two main reversal mechanisms, namely perpendicular domain wall formation versus spin-flop transition. Ultimately, we analyze the elemental magnetic moments for the surface and the bulk parts, separately, which allows to identify in the phase diagram the temperature window where this effect takes place. Moreover, we suggests that this effect is a general phenomenon in ferrimagnetic thin films which may also contribute to the understanding of the mechanism behind the all optical switching effect.

Extraction of magnetic circular dichroism effects from blended mixture of magnetic linear dichroism signals in the cobalt/Scotch tape system

Circular dichroism (CD) signals revealed in some materials may arise from different origins during measurements. Magnetic field dependent CD (MCD) emanating from the spin-polarized band provides direct insight into the spin–spin interband transitions in magnetic materials. On the contrary, natural CD effects which are artefactual signals resulting from the linear polarization (LP) components during the polarization modulation with a photo-elastic modulator in anisotropic polymer systems were usually observed. There is no simple method to reliably distinguish MCD effect due to spin polarized band structures from natural CD effect, which limits our understanding of the magnetic material/polymer hybrid structures. This paper aims to introduce a general strategy of averaging out the magnetic linear dichroism (MLD) contributions due to the anisotropic structure and disentangling MCD signal(s) from natural MCD signal(s). We demonstrate the effectiveness of separating MCD from natural MCD using rotational MCD measurement and presented the results of a sample with Co thin film on polymer Scotch tape (unplasticized polyvinyl chloride) glued on a quartz substrate. We demonstrate that the proposed method can be used as an effective tool in disentangling MCD and natural MCD effects, and it opens prospects to study the magnetic material /polymer hybrid systems.

Tuning the Néel temperature in an antiferromagnet: the case of NixCo1−xO microstructures

We show that it is possible to tune the Néel temperature of nickel(II)-cobalt(II) oxide films by changing the Ni to Co ratio. We grow single crystalline micrometric triangular islands with tens of nanometers thickness on a Ru(0001) substrate using high temperature oxygen-assisted molecular beam epitaxy. Composition is controlled by adjusting the deposition rates of Co and Ni. The morphology, shape, crystal structure and composition are determined by low-energy electron microscopy and diffraction, and synchrotron-based x-ray absorption spectromicroscopy. The antiferromagnetic order is observed by x-ray magnetic linear dichroism. Antiferromagnetic domains up to micrometer width are observed.