Paramagnetic Spacer Research Articles

Dipole-induced exchange bias.

The discovery of dipole-induced exchange bias (EB), switching from negative to positive sign, is reported in systems where the antiferromagnet and the ferromagnet are separated by a paramagnetic spacer (AFM-PM-FM). The magnitude and sign of the EB is determined by the cooling field strength and the PM thickness. The same cooling field yields negative EB for thin spacers, and positive EB for thicker ones. The EB decay profile as a function of the spacer thickness, and the change of sign, are attributed to long-ranged dipole coupling. Our model, which accounts quantitatively for the experimental results, ignores the short range interfacial exchange interactions of the usual EB theories. Instead, it retains solely the long range dipole field that allows for the coupling of the FM and AFM across the PM spacer. The experiments allow for novel switching capabilities of long range EB systems, while the theory allows description of the structures where the FM and AFM are not in atomic contact. The results provide a new approach to design novel interacting heterostructures.

Direct surface charging and alkali-metal doping for tuning the interlayer magnetic order in planar nanostructures

The continuous reduction of magnetic units to ultrasmall length scales inspires efforts to look for a suitable means of controlling magnetic states. In this study, we show two surface charge alteration techniques for tuning the interlayer exchange coupling of ferromagnetic layers separated by paramagnetic spacers. Our ab initio study reveals that already a modest amount of extra charge can switch the mutual alignment of the magnetization from antiferromagnetic to ferromagnetic or vice versa. We also propose adsorption of alkali metals as an alternative way of varying the electronic and chemical properties of magnetic surfaces. Clear evidence is found that the interlayer magnetic order can be reversed by adsorbing alkali metals on the magnetic layer. Moreover, alkali-metal overlayers strongly enhance the perpendicular magnetic anisotropy in FePt thin films. These findings combined with atomistic spin model calculations suggest that the electronic or ionic way of surface charging can have a crucial role for magnetic hardening and spin state control.

A Quantum Exchange Bias Model for Coupling Across a Nonmagnetic Interlayer

Exchange bias (EB) has been observed in systems where a paramagnetic (PM) spacer separates an antiferromagnet (AFM) from a ferromagnet. This coupling extends to PM spacer thicknesses of up to 100A in Py/(Ag, Au, Cu)/CoO systems. We develop a quantum fluctuation model in which a dipole field is created at the AFM surface due to the breaking of translational invariance symmetry. The uncompensated spins of the semiinfinite AFM induce a field that extends all the way to the CoO/(Ag, Au, Cu) interface and thus generate EB. The magnitude of the EB field that the present model yields is of the same order of magnitude as the one measured in recent experiments.

Elastic and inelastic neutron scattering for the study of magnetic thin films and heterostructures

Neutron-based techniques have proved for decades to be essential tools in the investigation of magnetic materials. The strong experience gathered in neutron centers together with high performance instruments have also largely contributed to the researches on magnetic nanosystems. We will present here a few examples of the use of elastic and inelastic neutron scattering for the study of Dysprosium-based thin films and heterostructures. Elastic neutron scattering in rare earth-based nanosystems provides crucial information both on the crystal structure and the modulated magnetic order. This has especially permitted to connect the drastic change in Curie temperature observed in Dy epitaxial films to modifications of both magnetoelastic and exchange contributions. In superlattices, elastic neutron scattering has been successfully used to analyze the magnetic coupling phenomena, in particular the long range coherent propagation of the Dy helical order. The example of Dy/Er superlattices highlights this fascinating coupling mechanism through either a paramagnetic or magnetic spacer. Inelastic Neutron Scattering is the technique of choice to investigate magnetic excitations over the entire Brillouin Zone. The very low inelastic scattering cross section can however make any investigation of nanosystems very difficult and very few studies have indeed been reported so far on nanosystems. We will show that relevant information could be obtained from high quality single crystalline Dy films and superlattices. These promising results are paving the way to new spin wave explorations in a wider range of magnetic nanosystems.

Deposition rate enhancement in HiPIMS without compromising the ionized fraction of the deposition flux

We systematically investigate and quantify different physical phenomena influencing the deposition rate, aD, of Nb coatings prepared by high power impulse magnetron sputtering (HiPIMS), and propose a straightforward approach for deposition rate enhancement through the control of the magnetron's magnetic field. The magnetic field strength at the target surface, B, of a 50 mm diameter magnetron was controlled by the application of paramagnetic spacers with different thicknesses in between the magnetron surface and the target. We found that lowering B achieved by the application of a 2.8 mm thick spacer led to an increase in aD by a factor of ∼4.5 (from 10.6 to 45.2 nm min−1) when the discharge was operated at a fixed average pulse target power density (2.5 kW cm−2). However, the ionized fraction of the deposition flux onto the substrate was found to be comparable, despite a large difference in B-dependent discharge characteristics (magnetron voltage and discharge current). We show that the decrease in aD commonly observed in HiPIMS (ranging from 33% to 84% in comparison with dc magnetron sputtering in the presented experiments) is governed by different physical processes, depending on the value of B: for high B, the back-attraction of the target ions towards the target is the dominant effect, while for low B the ion back-attraction, the sub-linear dependence of the sputtering yield on the ion energy, and the variation in material transport effects are all important. Finally, we offer a theoretical background for the observed results, demonstrating that the here-presented conclusions may be applicable to HiPIMS discharges using different metal targets and different inert gases.

Controlled Synthesis of Heterotrimetallic Single‐Chain Magnets from Anisotropic High‐Spin 3 d–4 f Nodes and Paramagnetic Spacers

By using the node-and-spacer approach in suitable solvents, four new heterotrimetallic 1D chain-like compounds (that is, containing 3d-3d'-4f metal ions), {[Ni(L)Ln(NO(3))(2)(H(2)O)Fe(Tp*)(CN)(3)]⋅2 CH(3)CN⋅CH(3)OH}(n) (H(2)L = N,N'-bis(3-methoxysalicylidene)-1,3-diaminopropane, Tp* = hydridotris(3,5-dimethylpyrazol-1-yl)borate; Ln = Gd (1), Dy (2), Tb (3), Nd (4)), have been synthesized and structurally characterized. All of these compounds are made up of a neutral cyanide- and phenolate-bridged heterotrimetallic chain, with a {-Fe-C≡N-Ni(-O-Ln)-N≡C-}(n) repeat unit. Within these chains, each [(Tp*)Fe(CN)(3)](-) entity binds to the Ni(II) ion of the [Ni(L)Ln(NO(3))(2)(H(2)O)](+) motif through two of its three cyanide groups in a cis mode, whereas each [Ni(L)Ln(NO(3))(2)(H(2)O)](+) unit is linked to two [(Tp*)Fe(CN)(3)](-) ions through the Ni(II) ion in a trans mode. In the [Ni(L)Ln(NO(3))(2)(H(2)O)](+) unit, the Ni(II) and Ln(III) ions are bridged to one other through two phenolic oxygen atoms of the ligand (L). Compounds 1-4 are rare examples of 1D cyanide- and phenolate-bridged 3d-3d'-4f helical chain compounds. As expected, strong ferromagnetic interactions are observed between neighboring Fe(III) and Ni(II) ions through a cyanide bridge and between neighboring Ni(II) and Ln(III) (except for Nd(III) ) ions through two phenolate bridges. Further magnetic studies show that all of these compounds exhibit single-chain magnetic behavior. Compound 2 exhibits the highest effective energy barrier (58.2 K) for the reversal of magnetization in 3d/4d/5d-4f heterotrimetallic single-chain magnets.

Steady state discharge optimization in high-power impulse magnetron sputtering through the control of the magnetic field

High-power impulse magnetron sputtering (HiPIMS) is a pulsed DC sputtering technique utilizing high power density peaks of typically more than 100 W cm−2. The discharge operation at such elevated powers can be hindered by the magnetron configuration (size and magnetic field) and/or the target conditions (e.g., material and thickness). In addition, target erosion is an important issue significantly affecting process reproducibility. In the present work, we propose a simple approach for the stabilization of the HiPIMS discharge by controlling the target magnetic field using paramagnetic spacers with different thicknesses in between the magnetron surface and the target. We demonstrate a straightforward discharge optimization, while using various target materials, such as Nb, Ta, Cr, Al, Ti, Si, and even C (graphite). The existence of a steady state high density discharge above the graphite target and the other targets in general is discussed in terms of the magnetic field configuration and the gas rarefaction effect.

Probing the magnetism of nanostructures buried in metallic surfaces and their possible utilization

AbstractWe discuss the possibility to probe magnetic properties of nanostructures buried beneath a metallic surface by means of local probe techniques. We prove, that those properties can be deduced from the spin‐resolved local density of states (LDOS) above the surface. In‐plane polarization maps in vacuum above the surface are shown to allow one to simultaneously detect electronic, magnetic, and even geometric properties of subsurface structures. It is argued, that the coupling of buried nanostructures to each other can be deduced from the symmetry of the polarization map. To underline the importance of studying buried nanostructures several possible applications of buried magnetic impurities are pointed out. The exchange coupling of an adatom to a nanostructure or a monolayer (ML) across a paramagnetic spacer is shown to oscillate with the thickness of the latter. This could provide one with reliable means to stabilize the spin of a magnetic adatom in either a ferromagnetic or an antiferromagnetic configuration with respect to the magnetic orientation of the monolayer or nanostructure. The possibility to tailor the exchange coupling between single magnetic impurities on a surface through the adjustment of the overlayer thickness and the interatomic separation in a dimer, is discussed.

A heterotrimetallic 3d–3d′–4f single chain magnet constructed from anisotropic high-spin 3d–4f nodes and paramagnetic spacers

By connecting [LCuTb](3+) nodes with [Fe(CN)₆]³⁻ spacers a 1-D coordination polymer with slow relaxation of magnetization is obtained (L²⁻ is the N,N'-propylene-bis(3-methoxysalicylideneiminato) anion).

First Heterotrimetallic {3 d‐4 d‐4 f} Single Chain Magnet, Constructed from Anisotropic High‐Spin Heterometallic Nodes and Paramagnetic Spacers

Three in one: Heterotrimetallic 1D coordination polymers are designed by assembling heteronuclear nodes (see figure), {Cu2Ln}, with paramagnetic octacyanido-metallates, [M(CN)8]3− (M=Mo, W), acting as spacers. By choosing a strongly anisotropic lanthanide (DyIII), a new single chain magnet is obtained.

Enhancement of the Josephson current by magnetic field in superconducting tunnel structures with a paramagnetic spacer

The dc Josephson critical current of a (S/M)IS tunnel structure in a parallel magnetic field is investigated (here S is a superconductor, S/M is a proximity-coupled S and paramagnetic metal M bilayer, and I is an insulating barrier). We consider the case when, due to Hund’s rule, in the metal M the effective molecular interaction aligns the spins of the conduction electrons antiparallel to the localized spins of magnetic ions. It is predicted that for the tunnel structures under consideration there are conditions such that the destructive action of the internal and the applied magnetic fields on Cooper pairs is weakened, and increase of the applied magnetic field causes field-induced enhancement of the critical tunnel current. The experimental realization of this interesting effect of the interplay between superconductivity and magnetism is also discussed.

Size effects in the giant magnetoresistance of segmented nanowires

We calculate the resistivity and giant magnetoresistance (GMR) of a segmented nanowire consisting of two ferromagnetic segments separated by a thin paramagnetic spacer. Spin-dependent surface electron scattering is taken into account. The quantization of the electron motion due to the small nanowire cross section leads to oscillations of the resistivity and the GMR. The interplay between spin-dependent electron scattering in the bulk and the surface results in a complex behavior of the GMR as a function of nanowire radius and surface-potential strength. Both increase and decrease of the GMR can be obtained as the spin-dependent surface scattering grows.

ChemInform Abstract: Magnetic Heterostructures: Examples and Perspectives

Abstract Artificial magnetic heterostructures and superlattices have received much attention in recent years because of their scientific and technological relevance. Heterostructures consist of different material layers promising jointly to display physical properties different from any of their single layers. Paramagnetic layers sandwiched between ferromagnetic films are an excellent example of a magnetic heterostructure, since together they display an oscillatory exchange coupling as a function of the paramagnetic spacer thickness not present in any single layer. The strength of the exchange coupling and the oscillation period depend on the details of the Fermi surfaces involved, whereas the overall features of the exchange coupling appear universal. More complex couplings are observed for magnetic superlattices with Cr spacer layers. This is due to the intrinsic spin-density wave state of Cr. Extensive experiments with synchrotron and neutron radiation have recently unravelled the Neel state of thin Cr...

Magnetic heterostructures: examples and perspectives

Abstract Artificial magnetic heterostructures and superlattices have received much attention in recent years because of their scientific and technological relevance. Heterostructures consist of different material layers promising jointly to display physical properties different from any of their single layers. Paramagnetic layers sandwiched between ferromagnetic films are an excellent example of a magnetic heterostructure, since together they display an oscillatory exchange coupling as a function of the paramagnetic spacer thickness not present in any single layer. The strength of the exchange coupling and the oscillation period depend on the details of the Fermi surfaces involved, whereas the overall features of the exchange coupling appear universal. More complex couplings are observed for magnetic superlattices with Cr spacer layers. This is due to the intrinsic spin-density wave state of Cr. Extensive experiments with synchrotron and neutron radiation have recently unravelled the Neel state of thin Cr layers and proximity effects between Fe and Cr, elucidating the mutual interdependence of the Cr spin structure and the Fe exchange coupling. In Co/Cr superlattices the structural mismatch between hep Co and bcc Cr adds another complexity, which affects strongly the magnetic anisotropy. Utilizing the different coercivities of Co and Fe layers, spin valve systems can be constructed from Co/Cr/Fe heterostructures. Both the current status and some future perspectives are briefly reviewed here.

Magnetic heterostructures: examples and perspectives

Abstract Artificial magnetic heterostructures and superlattices have received much attention in recent years because of their scientific and technological relevance. Heterostructures consist of different material layers promising jointly to display physical properties different from any of their single layers. Paramagnetic layers sandwiched between ferromagnetic films are an excellent example of a magnetic heterostructure, since together they display an oscillatory exchange coupling as a function of the paramagnetic spacer thickness not present in any single layer. The strength of the exchange coupling and the oscillation period depend on the details of the Fermi surfaces involved, whereas the overall features of the exchange coupling appear universal. More complex couplings are observed for magnetic superlattices with Cr spacer layers. This is due to the intrinsic spin-density wave state of Cr. Extensive experiments with synchrotron and neutron radiation have recently unravelled the Neel state of thin Cr...

Effect of spin-flip scattering on current-in-plane giant magnetoresistance

We use Boltzmann's equation to study the effect of spin-flip scattering on the current-in-plane giant magnetoresistance (GMR). When the spin-flip scattering is in the paramagnetic spacer layer, the GMR is suppressed. When the spin-flip scattering is only in the ferromagnetic layers, the GMR due to bulk-spin-scattering anisotropy is suppressed, while the GMR due to surface-spin-scattering anisotropy is selectively enhanced and can actually lead to an increase in the GMR. For spin diffusion lengths comparable to the mean free path in the ferromagnetic layers, the GMR is due almost exclusively to surface scattering.

Interlayer exchange coupling: The role of electron confinement in the overlayer due to the vacuum barrier

The phenomenon of interlayer exchange coupling between two ferromagnetic layers separated by a paramagnetic spacer is usually attributed to quantum interferences due to electron confinement in the spacer. This paper considers the effect of electron confinement in the paramagnetic overlayer due to the vacuum barrier. It is shown that this leads to a novel oscillatory behavior of the interlayer exchange coupling: the latter is predicted to oscillate versus overlayer thickness. This is confirmed by several experimental investigations.

Quantum-well states and induced magnetism in Fe/CuN/Fe bcc (001) trilayers

We have used a first-principles Green's function technique to investigate the formation of magnetic moments in Fe/Cu-N/Fe bcc (001) trilayers. We show that the magnetic moment in the paramagnetic spacer material to a first approximation may be described a

Magnetic fingerprints of quantum well states in bcc (001) Fe/CuN/Fe trilayers

A first-principles Green's function technique has been used to investigate the formation of magnetic moments in Fe/CuN/Fe bcc (001) trilayers (N = integer number of atomic interlayers). We show that the magnetic moment in the paramagnetic spacer material may be accurately described by a superposition of the magnetic profiles of two single Fe/Cu interfaces. We identify the deviation between the superposed and the self-consistently calculated moments with the contribution from magnetic quantum well states.

Exchange coupling between ferromagnetic layers: Effect of quantum well states (abstract)

It is a well-established experimental result that the exchange coupling between ferromagnetic metals Fe, Co, or Ni is an oscillatory function of the thickness of the paramagnetic spacer. Theories of this coupling are almost in complete unanimity in attributing the observed periodicity to the properties of the Fermi surface of the paramagnetic spacer. Little has been done, and no consensus reached, as to the origin of the strength of the coupling. A realistic assessment of this quantity requires the use of an itinerant picture of the ferromagnetic constituent. From a recent study based on this picture, it became clear that the ratio of the spin-split band gap to the Fermi energy is important in establishing not only the strength of the coupling, but the phase of the oscillations in the nonasymptotic spacer thickness regime. Here we extend the work, which was limited to cases in which the minority-spin band of the ferromagnet was energy matched to the paramagnetic band, to cases in which the match is to that of the majority-spin band. In the latter arrangement, electrons of one spin are trapped in a well whose width equals the thickness of the spacer. We calculated the spin-flip current, and hence, the torque, exerted by one ferromagnet on the other as the magnetization direction of the former is rotated with respect to that of the latter. As in the earlier case, the torque leads to higher-order (in the cosine of the angle between the magnetizations of the ferromagnets) coupling as well as the usual linear coupling. The bound states produced by the quantum well structure are most important when the band splitting in the ferromagnet is large. The new features produced by these states will be discussed; in particular, their effects on the coupling strength and the phase shift will be discussed.