The remagnetization process in spin-valve structures

The process of magnetization reversal in ultrathin magnetic trilayer is analyzed. It is shown that the shape of magnetization hysteresis loops and the giant magnetoresistance essentially depend on the relative magnitudes of magnetic parameters of the top and the bottom layers. New types of hysteresis loops are found for characteristic relative magnitudes of the parameters. Analysis of the dependence of the shape of hysteresis loops on the magnitude of interlayer exchange is performed. The phase diagram which determines the regions of existence of characteristic hysteresis loops for different relative magnitudes of the uniaxial anisotropy constant and exchange constant J₁ is constructed.     

Domain state model for exchange bias: thickness dependence of diluted antiferromagnetic Co1−yO on exchange bias in Co/CoO

The thickness dependence of different diluted antiferromagnetic Co_1−yO layers on the exchange bias (EB) in ferro/antiferromagnetic Co/Co_1−yO bilayers is investigated. For undiluted samples the EB decreases above a layer thickness of 5 nm whereas it increases and saturates for AFM layers thicker than 20 nm for diluted samples. These findings support the domain state model for EB.     

Interlayer exchange coupling: an in situ investigation via ferromagnetic resonance

A new absolute way to deduce the interlayer exchange coupling via in situ ferromagnetic resonance measurements in Cu/Ni/Cu/Ni/Cu(0 0 1) films is presented. The signal of the first magnetic layer before and after the deposition of the second one is compared and the exchange coupling is extracted from the shift of the resonance position in the coupled system with respect to the Cu/Ni bilayer.     

Hard magnetic CoCr layer in ferromagnetic tunnel junctions

In magnetic tunnel junctions a highly spin-polarizing layer is usually exchange biased by an antiferromagnetic layer, an artificial antiferromagnetic layer system or a combination of both, while the magnetically soft layer is free to rotate. The use of a single layer of a hard magnetic material is rarely investigated up to now. In this paper, we present the electric and magnetic properties of tunnel junctions with a hard magnetic Co₈₃Cr₁₇ layer. The soft magnetic electrode consists of either a single Co layer or a Co/Ni₈₀Fe₂₀ bilayer. The magnetic anisotropy and coercive field H_C of the CoCr layer depend on its thickness and the kind of the bottom layer (Cu or Ta) and can vary from H_C=50–700 Oe. It is found that a thin Co cap layer also influences the hysteretic behavior. Furthermore, only small changes after annealing up to 450°C promise a high thermal stability for the application in magnetic tunnel junctions. Measurements of the tunnel magnetoresistance on large area junctions, however, show a strong magnetic coupling of the hard and soft electrodes.     

Further evidence against mean-free-path effects in the CPP-MR

Mean-free-paths were recently claimed to be scaling lengths in the current-perpendicular-to-plane magnetoresistance when they are longer than the layer thicknesses of magnetic multilayers. We provide two new sets of evidence against such a claim; one involves reducing mean-free-paths by alloying, the other inserting a source of strong spin flipping.     

Stability of magnetic tunnel junctions

When magnetic tunnel junctions (MTJ) are built into a memory device they will be arranged in a matrix; therefore some of the not addressed elements will be exposed to a significant field during the switching of one element. It has to be avoided that the state of not selected MTJ is changed during this process. Here we present data on the stability of MTJ against small fields which occur during a writing process.     

Effect of Co doping on the magnetic properties of La0.85Ag0.15(Mn1−yCoy)O3

The mixed valent manganites (La_0.85Ag_0.15)MnO₃ with perovskite structure has been prepared by doping up to 50% of Co at the Mn site. Paramagnetic (PM) to ferromagnetic (FM) transitions have been observed in all the prepared materials. However, the long-range magnetic ordering observed in (La_0.85Ag_0.15)MnO₃ is systemically reduced to cluster glass-type (short-range) of FM ordering due to the introduction of Co. The FM transition temperature was found to decrease with increase in Co doping up to 20% and for further increase in Co doping, the T_c was found to increase. They are explained on the basis of competition between FM double exchange interactions in Mn–O–Mn and Co–O–Co networks. In addition to PM–FM transition, evidences of FM to antiferromagnetic (AFM), and AFM to reentrant spin-glass transitions have been observed. The shift in spin-glass freezing temperature, T_f has been observed from the frequency variation of ac susceptibility measurements. The observed magnetic transitions are explained on the basis of magnetic interactions in different Mn–O–Mn, Mn–O–Co and Co–O–Co networks and such transitions are also observed from the measurement of third harmonic susceptibility. Metal–insulator transition and colossal magneto-resistivity have been observed up to 10% of Co doping.     

Appearance of inverse giant magnetoresistance in noncollinear magnetic systems

We show for a simple d-band TB Hamiltonian that noncollinear magnetic configurations can contribute to large inverse giant magnetoresistance (IGMR) ratios. We make a systematic study as a function of band filling, magnetic moment and canting angle for some simple model examples and use the outcome of this study to interpret the experimentally observed IGMR ratios on LaMn₂Ge₂.     

Brillouin light scattering investigation of FeSm/FeTaN trilayer

The static and dynamic magnetic properties of a FeTaN/FeSm/FeTaN trilayer are investigated by alternating gradient field magnetometry and Brillouin light scattering. Evidence is given for a strong in-plane uniaxial anisotropy induced by a magnetic field applied during deposition. The spin-wave spectrum consists of different modes, with either surface or bulk character. A nonreciprocal behavior of the spin-wave frequency due to the exchange field exerted on the FeTaN layer at the FeSm interface is observed.     

Coupled perpendicular magnetization in Fe/Cu/Fe trilayers

Ultrathin epitaxial Fe films on Cu(1 0 0) with perpendicular magnetization have been used as templates for the preparation of FCC Fe/Cu/Fe trilayers. The magnetic anisotropy and the coupling of these films have been studied by in-situ magneto optical Kerr effect measurements and Kerr microscopy. The magnetic coupling of both Fe layers is found to be dominated by magnetostatic interaction. Adsorbate-induced spin reorientation in the top layer also causes spin reorientation in the bottom layer. The governing role of the Fe-vacuum interface for the magnetism of the whole trilayer is demonstrated.     

Mini-gaps and novel giant negative magnetoresistance in InAs/GaSb semimetallic superlattices

We have studied the temperature dependence of the magnetoresistance of semimetallic InAs/GaSb structures with magnetic field applied parallel to the layers. We present the first unambiguous evidence for the presence of a mini-gap at the crossing point between the electron and hole dispersion relations. The resistivity is found to change from semiconductor-like behaviour with a strong temperature dependence at low parallel magnetic fields to that of a semimetal with a weak temperature dependence at high field. Furthermore, the magnetoresistance, for intrinsic samples, is found to decrease with field by as much as 70% at low temperatures. As the parallel magnetic field is increased the centres of the electron and hole dispersions are shifted apart in k-space and at sufficiently high field the mini-gap is destroyed and the bands overlap fully. Finally, a theoretical model allows us to estimate that the mini-gap is of order 7 meV.     

Probing antiferromagnetic order with heat capacity in exchange-biased structures

We explore the magnetic heat capacity in exchange-biased ferromagnet/antiferromagnet bilayers theoretically. We show that changes in the antiferromagnetic structure due to the reversal of the ferromagnet layer can be detected by distinct features in the heat capacity. This offers a method for probing antiferromagnetic domains in exchange-biased systems.     

Structural study and magnetoresistance effect of epitaxial La0.67Sr0.33MnO3–δ and Pr0.7Ca0.3MnO3−δ multilayer films

La_0.67Sr_0.33MnO_3−δ (LSMO) and Pr_0.7Ca_0.3MnO_3−δ (PCMO) multilayer epitaxial films, which were fabricated with different LSMO and PCMO layer thickness on LaAlO₃ single crystal substrates of (0 0 1) orientation by a direct current magnetron sputtering technique, were studied further, after the structure, magnetoresistance effect and magnetic properties of LSMO/PCMO/LSMO (LPL) trilayer epitaxial films were systemically studied. The superlattice structures of multilayer films were observed according to the diffraction peaks of X-ray diffraction patterns at small angles. The metal–insulator transition temperature (T_P) and peak resistivity (ρ_max) obviously changed when we altered the thickness of PCMO middle layer and the intra-field related with the thickness of those layers and their interaction. Considering the effect of the distribution of electrical field and current, and the interaction among the layers of LSMO and PCMO, an effective fact n* was introduced to replace n (the number of layer). All the calculated values of ρ (the resistivity of multilayer films) accorded with the experimental values.     

Potassium doping effect on the lattice softening and electronic structure of Ba(1-x)K(x)Fe(2)As(2) probed by X-ray absorption spectroscopy

Ba(1-x)K(x)Fe(2)As(2) superconducting samples (x = 0, 0.2, 0.4, 0.5) were synthesized by the solid-state reaction method. In this contribution the doping effect of potassium on the lattice dynamics in this newly discovered Ba(1-x)K(x)Fe(2)As(2) superconductor has been investigated by extended X-ray absorption fine-structure spectroscopy. The analysis shows that with potassium doping an increased disorder in the iron layers is mainly related to the softening of the Fe-Fe bond. Information about the electronic structure of these materials has also been obtained by looking at the X-ray absorption near-edge structure spectra that point out the presence of holes in the Fe-3d/As-4p hybridized orbital of the BaFe(2)As(2)-based system.     

Theory of the angular magnetoresistance in CPP spin valves

The resistance of CPP spin valve is a continuous function of the angle θ between the magnetizations of both ferromagnets. We use the circuit theory for non-collinear magnetoelectronics to compute the angular magnetoresistance of CPP spin valves taking the spin accumulation in the ferromagnetic layers into account.     

Exchange bias in Fe0.6Zn0.4F2/Fe heterostructures

The exchange bias field, H_e, is measured in Fe_0.6Zn_0.4F₂/Fe heterosystems prepared from Fe layers of 14 and 5 nm thickness which are deposited on top of the compensated (1 1 0) surface of the antiferromagnet. Deviations from a linear dependence of H_e on the magnetization of the Fe layer are attributed to ferromagnetic domains. Moreover, piezomagnetism and its influence on H_e are evidenced.     

Tunnel magnetoresistance of magnetic junctions with cubic symmetry of the layers

A tunnel magnetic junction is considered with magnetic hard and magnetic soft layers of cubic symmetry. The magnetic switching of the layers is analyzed for a magnetic field perpendicular to the initial magnetizations. In such a situation, an additional peak in the tunnel magnetoresistance ratio appears at the magnetic field value that is substantially lower than the anisotropy field of the soft layer.     

Local density of states and order parameter configurations in layered ferromagnet-superconductor structures

We analyze the local density of states (LDOS) of heterostructures consisting of alternating ferromagnet, F, and superconductor, S, layers. We consider structures of the SFS and SFSFSFS type, with thin nanometer scale F and S layers, within the ballistic regime. The spin-splitting effects of the ferromagnet and the mutual coupling between the S regions, yield several nontrivial stable and metastable pair amplitude configurations, and we find that the details of the spatial behavior of the pair amplitude govern the calculated electronic spectra. These are reflected in discernible signatures of the LDOS. The roles that the magnetic exchange energy, interface scattering strength, and the Fermi wavevector mismatch each have on the LDOS for the different allowed junction configurations, are systematically investigated.     

Layer-resolved microscopy of magnetic domains in multi-layered systems

Photoelectron emission microscopy in connection with magnetic circular dichroism in soft X-ray absorption can be used for the microscopic imaging of magnetic domains in layered thin film structures consisting of several magnetic layers. Due to the element-selectivity of the method, the different magnetic layers in such a structure can be imaged separately, provided that they contain different elements. This has been applied for the investigation of Co/Cu/Ni trilayers, epitaxially grown on Cu (001). The magnetic coupling between the Co and Ni layers can be directly visualized from comparing layer-resolved magnetic domain images of both layers. As a consequence of the competition between the anisotropy energies of the two magnetic layers and the magnetic coupling energy, spin-reorientation transitions between collinear and non-collinear magnetic configurations are observed. Apart from this globally observable magnetic interlayer coupling a micromagnetic coupling mechanism is also evident from the layer-resolved domain images. It is caused by magnetostatic interaction of local stray fields from domain walls. Received: 22 August 2002 / Accepted: 2 October 2002 / Published online: 5 February 2003 RID="*" ID="*"Corresponding author. Fax: +49-345/5511-223, E-mail: kuch@mpi-halle.de     

Waveguide modes of 1D photonic crystals in a transverse magnetic field

We analyze waveguide modes in 1D photonic crystals containing layers magnetized in the plane. It is shown that the magnetooptical nonreciprocity effect emerges in such structures during the propagation of waveguide modes along the layers and perpendicularly to the magnetization. This effect involves a change in the phase velocity of the mode upon reversal of the direction of magnetization. Comparison of the effects in a nonmagnetic photonic crystal with an additional magnetic layer and in a photonic crystal with magnetic layers shows that the magnitude of this effect is several times larger in the former case in spite of the fact that the electromagnetic field of the modes in the latter case is localized in magnetic regions more strongly. This is associated with asymmetry of the dielectric layers contacting with the magnetic layer in the former case. This effect is important for controlling waveguide structure modes with the help of an external magnetic field.