Noncollinear magnetization structure at the thickness-driven spin-reorientation transition in epitaxial Fe films on W(110)

Transfer of magnetic anisotropy in epitaxial Co/NiO/Fe trilayers

The magnetic properties of Co(10 Å)/NiO(40 Å)/Fe trilayer epitaxially grown on W(110) substrate were investigated with use of x-ray magnetic linear dichroism (XMLD) and x-ray magnetic circular dichroism (XMCD). We showed that magnetic anisotropy of Fe film that can be controlled by a thickness-driven spin reorientation transition is transferred via interfacial exchange coupling not only to NiO layer but further to ferromagnetic Co overlayer as well. Similarly, a temperature driven spin reorientation of Fe sublayer induces a reorientation of NiO spin orientation and simultaneous switching of the Co magnetization direction. Finally, by element specific XMCD and XMLD magnetic hysteresis loop measurements we proved that external magnetic field driven reorientation of Fe and Co magnetizations as well as NiO Néel vector are strictly correlated and magnetic anisotropy fields of Fe and Co sublayers are identical despite the different crystal structures.

Fine tuning of ferromagnet/antiferromagnet interface magnetic anisotropy for field-free switching of antiferromagnetic spins

We show that in a uniform thickness NiO(111)/Fe(110) epitaxial bilayer system, at given temperature near 300 K, two magnetic states with orthogonal spin orientations can be stabilized in antiferromagnetic NiO. Field-free, reversible switching between these two antiferromagnetic states is demonstrated. The observed phenomena arise from the unique combination of precisely tuned interface magnetic anisotropy, thermal hysteresis of spin reorientation transition and interfacial ferromagnet/antiferromagnet exchange coupling. The possibility of field-free switching between two magnetic states in an antiferromagnet is fundamentally interesting and can lead to new ideas in heat assisted magnetic recording technology.

Driving the polar spin reorientation transition of ultrathin ferromagnets with antiferromagnetic-ferromagnetic phase transition of nearby FeRh alloy film

We show that in-plane to out-of-plane magnetization switching of a ferromagnetic layer can be driven by antiferromagnetic-ferromagnetic phase transition in a nearby FeRh system. For FeRh/Au/FeAu trilayers, the impact of the magnetic phase transition of FeRh onto the perpendicular magnetization of monoatomic FeAu superlattices is transferred across the Au spacer layer via interlayer magnetic coupling. The polar spin reorientation process of the FeAu spins driven by the magnetic phase transition in the FeRh reveals its major features; namely it is reversible and displays hysteresis.

How a ferromagnet drives an antiferromagnet in exchange biased CoO/Fe(110) bilayers

Antiferromagnet/ferromagnet (AFM/FM) bilayers that display the exchange bias (EB) effect have been subjected to intensive material research, being the key elements of novel spintronics systems. In a commonly accepted picture, the antiferromagnet, considered as a rigid material due to its high anisotropy and magnetic hardness, controls the magnetic properties of the ferromagnet, such as a shift of the hysteresis loop or coercivity. We show that this AFM-FM master-slave hierarchy is not generally valid and that the influence of the ferromagnet on the magnetic anisotropy (MA) of the neighbouring antiferromagnet must be considered. Our computer simulation and experimental studies of EB in an epitaxial CoO/Fe(110) bilayer show that the ferromagnetic layer with strong uniaxial magnetic anisotropy determines the interfacial spin orientations of the neighbouring AFM layer and rotates its easy axis. This effect has a strong feedback on the EB effect experienced by the FM layer. Our results show new physics behind the EB effect, providing a route for grafting a desired anisotropy onto the AFM and for precise tailoring of EB in AFM/FM systems.

Nuclear resonance reflectivity from a [57Fe/Cr]30 multilayer with the Synchrotron Mössbauer Source

Mössbauer reflectivity spectra and nuclear resonance reflectivity (NRR) curves have been measured using the Synchrotron Mössbauer Source (SMS) for a [⁵⁷Fe/Cr]₃₀ periodic multilayer, characterized by the antiferromagnetic interlayer coupling between adjacent ⁵⁷Fe layers. Specific features of the Mössbauer reflectivity spectra measured with π-polarized radiation of the SMS near the critical angle and at the `magnetic' maximum on the NRR curve are analyzed. The variation of the ratio of lines in the Mössbauer reflectivity spectra and the change of the intensity of the `magnetic' maximum under an applied external field has been used to reveal the transformation of the magnetic alignment in the investigated multilayer.

Switching of Dirac-Fermion Mass at the Interface of Ultrathin Ferromagnet and Rashba Metal

We have performed spin- and angle-resolved photoemission spectroscopy on tungsten (110) interfaced with an ultrathin iron (Fe) layer to study an influence of ferromagnetism on the Dirac-cone-like surface-interface states. We found an unexpectedly large energy gap of 340 meV at the Dirac point, and have succeeded in switching the Dirac-fermion mass by controlling the direction of Fe spins (in plane or out of plane) through tuning the thickness of the Fe overlayer or adsorbing oxygen on it. Such a manipulation of Dirac-fermion mass via the magnetic proximity effect opens a promising platform for realizing new spintronic devices utilizing a combination of exchange and Rashba-spin-orbit interactions.

Effect of substrate interface on the magnetism of supported iron nanoparticles

In situ X-ray photo-emission electron microscopy is used to investigate the magnetic properties of iron nanoparticles deposited on different single crystalline substrates, including Si(001), Cu(001), W(110), and NiO(001). We find that, in our room temperature experiments, Fe nanoparticles deposited on Si(001) and Cu(001) show both superparamagnetic and magnetically stable (blocked) ferromagnetic states, while Fe nanoparticles deposited on W(110) and NiO(001) show only superparamagnetic behaviour. The dependence of the magnetic behaviour of the Fe nanoparticles on the contact surface is ascribed to the different interfacial bonding energies, higher for W and NiO, and to a possible relaxation of point defects within the core of the nanoparticles on these substrates, that have been suggested to stabilise the ferromagnetic state at room temperature when deposited on more inert surfaces such as Si and Cu.

Noncollinear spin reorientation transition in S = 1 ferromagnetic thin films

An in-plane spin reorientation transition in thin ferromagnetic films is discussed in terms of the thermodynamics of inhomogeneous low-dimensional systems based on a Néel sublattices concept while using a spin 1 Heisenberg Hamiltonian. The model allows us to investigate in a straightforward manner the layer-dependent phenomena. In this context, we propose a model of noncollinear magnetization structure based on the appropriate distribution of the anisotropy parameters inside the Fe films on W(110). The spin reorientation transition originates at the Fe/W(110) interface and proceeds via noncollinear magnetization structure toward the surface with increasing film thickness in accordance with the experimental findings. The temperature-driven spin reorientation transition in freestanding Fe films and in Fe/W(110) systems is also discussed in detail.

Grazing-incidence synchrotron-radiation ⁵⁷Fe-Mössbauer spectroscopy using a nuclear Bragg monochromator and its application to the study of magnetic thin films

Energy-domain grazing-incidence (57)Fe-Mössbauer spectroscopy (E-GIMS) with synchrotron radiation (SR) has been developed to study surface and interface structures of thin films. Highly brilliant (57)Fe-Mössbauer radiation, filtered from SR by a (57)FeBO(3) single-crystal nuclear Bragg monochromator, allows conventional Mössbauer spectroscopy to be performed for dilute (57)Fe in a mirror-like film in any bunch-mode operation of SR. A theoretical and experimental study of the specular reflections from isotope-enriched ((57)Fe: 95%) and natural-abundance ((57)Fe: ∼2%) iron thin films has been carried out to clarify the basic features of the coherent interference between electronic and nuclear resonant scattering of (57)Fe-Mössbauer radiation in thin films. Moreover, a new surface- and interface-sensitive method has been developed by the combination of SR-based E-GIMS and the (57)Fe-probe layer technique, which enables us to probe interfacial complex magnetic structures in thin films with atomic-scale depth resolution.

Structure, morphology, and magnetic properties of Fe nanoparticles deposited onto single-crystalline surfaces

BACKGROUND

Magnetic nanostructures and nanoparticles often show novel magnetic phenomena not known from the respective bulk materials. In the past, several methods to prepare such structures have been developed - ranging from wet chemistry-based to physical-based methods such as self-organization or cluster growth. The preparation method has a significant influence on the resulting properties of the generated nanostructures. Taking chemical approaches, this influence may arise from the chemical environment, reaction kinetics and the preparation route. Taking physical approaches, the thermodynamics and the kinetics of the growth mode or - when depositing preformed clusters/nanoparticles on a surface - the landing kinetics and subsequent relaxation processes have a strong impact and thus need to be considered when attempting to control magnetic and structural properties of supported clusters or nanoparticles.

RESULTS

In this contribution we focus on mass-filtered Fe nanoparticles in a size range from 4 nm to 10 nm that are generated in a cluster source and subsequently deposited onto two single crystalline substrates: fcc Ni(111)/W(110) and bcc W(110). We use a combined approach of X-ray magnetic circular dichroism (XMCD), reflection high energy electron diffraction (RHEED) and scanning tunneling microscopy (STM) to shed light on the complex and size-dependent relation between magnetic properties, crystallographic structure, orientation and morphology. In particular XMCD reveals that Fe particles on Ni(111)/W(110) have a significantly lower (higher) magnetic spin (orbital) moment compared to bulk iron. The reduced spin moments are attributed to the random particle orientation being confirmed by RHEED together with a competition of magnetic exchange energy at the interface and magnetic anisotropy energy in the particles. The RHEED data also show that the Fe particles on W(110) - despite of the large lattice mismatch between iron and tungsten - are not strained. Thus, strain is most likely not the origin of the enhanced orbital moments as supposed before. Moreover, RHEED uncovers the existence of a spontaneous process for epitaxial alignment of particles below a critical size of about 4 nm. STM basically confirms the shape conservation of the larger particles but shows first indications for an unexpected reshaping occurring at the onset of self-alignment.

CONCLUSION

The magnetic and structural properties of nanoparticles are strongly affected by the deposition kinetics even when soft landing conditions are provided. The orientation of the deposited particles and thus their interface with the substrate strongly depend on the particle size with consequences regarding particularly the magnetic behavior. Spontaneous and epitaxial self-alignment can occur below a certain critical size. This may enable the obtainment of samples with controlled, uniform interfaces and crystallographic orientations even in a random deposition process. However, such a reorientation process might be accompanied by a complex reshaping of the particles.