arXiv subjects

J. Aarts

Publications and source records attributed to J. Aarts.

At least 37 records · Page 2Linked to original sources

arXiv2009

In search for the superconducting spin-switch: Magnetization induced resistance switching effects in La$_{0.67}$Sr$_{0.33}$MnO$_3$/YBa$_2$Cu$_3$O$_{7-δ}$ bi- and trilayers

We have studied the influence of the magnetization on the superconducting transition temperature ($T_c$) in bi- and trilayers consisting of the half-metallic ferromagnet La$_{0.67}$Sr$_{0.33}$MnO$_3$ (LSMO) and the high-temperature superconductor YBa$_2$Cu$_3$O$_{7-δ}$ (YBCO). We have made use of tilted epitaxial growth in order to achieve contacts between the two materials that are partly in the crystallographic $ab$-plane of the YBCO. As a result of uniaxial magnetic anisotropy in the tilted structures, we observe sharp magnetization switching behavior. At temperatures close to $T_c$, the magnetization switching induces resistance jumps in trilayers, resulting in a magnetization dependence of $T_c$. In bilayers, this switching effect can be observed as well, provided that the interface to the ferromagnetic layer is considerably rough. Our results indicate that the switching behavior arises from magnetic stray fields from the ferromagnetic layers that penetrate into the superconductor. A simple model describes the observed behavior well. We find no evidence that the switching behavior is caused by a so-called superconducting spin-switch, nor by accumulation of spin-polarized electrons. Observation of magnetic coupling of the ferromagnetic layers, through the superconductor, supports the idea of field induced resistance switching.

cond-mat.supr-con↗

arXiv2009

Correlations between the morphology and the electronic structure at the surface of thin film manganites, investigated with STM

Thin-film colossal magnetoresistance manganites such as La$_{0.67}$Ca$_{0.33}$MnO$_{3}$ (LCMO) have now been intensely studied for more than a decade, but the issue of possible nanoscale electronic phase separation is not fully solved. Scanning Tunneling Microscopy / Spectroscopy (STS) has been pivotal in studying phase separation, but is hindered by being surface- rather than bulk-sensitive. For our sputtered LCMO films the data indicates a strong correlation between surface morphology and signatures of phase separation; rough films show phase separation while atomically flat films are electronically homogeneous but have a more or less inactive surface layer. Regardless of surface morphology, the film bulk is electronically and magnetically active. Many of the reported conclusions about electronic inhomogeneities measured by STS have been confused by this issue. We study both strained and unstrained films and find no correlation between substrate-induced strain and either electronic phase separation or dead layers.

cond-mat.str-el↗

arXiv2007

Transport properties of microstructured ultrathin films of La0.67Ca0.33MnO3 on SrTiO3

We have investigated the electrical transport properties of 8 nm thick La0.67Ca0.33MnO3 films, sputter-deposited on SrTiO3 (STO), and etched into 5 micrometer-wide bridges by Ar-ion etching. We find that even slight overetching of the film leads to conductance of the STO substrate, and asymmetric and non-linear current-voltage (I-V) characteristics. However, a brief oxygen plasma etch allows full recovery of the insulating character of the substrate. The I-V characteristics of the bridges are then fully linear over a large range of current densities. We find colossal magnetoresistance properties typical for strained LCMO on STO but no signature of non-linear effects (so-called electroresistance) connected to electronic inhomogeneites. In the metallic state below 150 K, the highest current densities lead to heating effects and non-linear I-V characteristics.

cond-mat.mtrl-sci↗

arXiv2007

Spin dynamics in a superconductor / ferromagnet proximity system

The ferromagnetic resonance of thin sputtered Ni80Fe20 films grown on Nb is measured. By varying the temperature and thickness of the Nb the role of the superconductivity on the whole ferromagnetic layer in these heterostructures is explored. The change in the spin transport properties below the superconducting transition of the Nb is found to manifest itself in the Ni80Fe20 layer by a sharpening in the resonance of the ferromagnet, or a decrease in the effective Gilbert damping co-efficient. This dynamic proximity effect is in contrast to low frequency studies in these systems, where the effect of the superconductor is confined to a small region in the ferromagnet. We interpret this in terms of the spin pumping model.

cond-mat.supr-con↗

arXiv2007

Vortex relaxation and coupling in superconducting heterostructures studied by STM

In a sandwich consisting of two superconducting films, one weakly pinning and one strongly pinning, the vortex positions in both films are determined by the strongly pinning material and the vortex lattice is disordered in both films. We used (strongly pinning) NbN and (weakly pinning) MoGe and studied, by directly imaging the vortex core positions with a scanning tunnelling microscope, how this disorder is restored with increasing thickness of MoGe layer or when the interface is made insulating. For clean interfaces we find that the first reordering of the vortex lattice is found at a layer thickness wich is compatible with the first bending mode of the vortex lines. Making the interface insulating we find that order is restored quickly. We argue that this is can be understood from the competition between the Josephson force working on the vortex segments on the one hand, and the elastic restoring forces inside the weakly pinning layer on the other hand.

cond-mat.supr-con↗

arXiv2006

Flux-flow induced giant magnetoresistance in all-amorphous superconductor-ferromagnet hybrids

We present magnetoresistance measurements on all-amorphous ferromagnet (F) / superconductor (S) heterostructures. The F/S/F trilayers show large magnetoresistance peaks in a small field range around the coercive field of the F layers, at temperatures within and below the superconducting transition. This is interpreted as flux flow of weakly pinned vortices induced by the stray field of Bloch magnetic domains in the F layers. Bilayers show much smaller effects, implying that the Bloch walls of the F-layers in the trilayer line up and focus the stray fields. The data are used to discuss the expected minimum F-layer thickness needed to nucleate vortices.

cond-mat.supr-con↗

arXiv2006

Charge Localization due to RKKY Interaction in the Spin Glass AuFe

Measurements of electrodynamic response of of spin glass AuFe films in comparison with pure gold films are performed at frequencies from 0.3 THz (10 cm-1) up to 1000 THz (33000 cm-1) using different spectroscopic methods. At room temperatures the spectra of pure gold and of AuFe are typically metallic with the scattering rate of carriers in AuFe being significantly enlarged due to scattering on localized magnetic moments of Fe ions. In the spin-glass phase of AuFe at T = 5 K a pseudogap in the conductivity spectrum is detected with the magnitude close to the Ruderman-Kittel-Kasuya-Yosida (RKKY) energy for AuFe: Delta(RKKY) = 2.2 meV. The origin of the pseudogap is associated with partial localization of electrons which mediate the RKKY interaction between localized magnetic Fe centers.

cond-mat.str-el↗

arXiv2005

Inverse spin switch effects in Ferromagnet / Superconductor hybrids with strong ferromagnets

In F/S/F trilayers where the magnetization directions of the F layers can be controlled separately, it has theoretically been predicted that the antiparallel (AP) configuration can have a higher superconducting transition temperature T_c than the parallel configuration. This is the so-called spin switch, which also experimentally has been found for the case of weak ferromagnets. Here we show that strong ferromagnets yield the opposite effect. We study the transport properties of F/S/F trilayers with F = Ni_0.80Fe_0.20 (Permalloy, Py) and S = Nb, structured in strips of different sizes. Using two different thicknesses for the Py layers, we can switch in a well-defined way between the AP- and P-configurations. In the superconducting transition we find a clear increase of the resistance in the AP-state. We ascribe this to enhanced reflection of spin-polarized quasiparticles at the S/F interfaces which leads to a stronger suppression of superconductivity on the S-side.

cond-mat.supr-con↗

arXiv2005

Critical voltage of a mesoscopic superconductor

We study the role of the quasiparticle distribution function f on the properties of a superconducting nanowire. We employ a numerical calculation based upon the Usadel equation. Going beyond linear response, we find a non-thermal distribution for f caused by an applied bias voltage. We demonstrate that the even part of f (the energy mode f_L) drives a first order transition from the superconducting state to the normal state irrespective of the current.

cond-mat.supr-con↗

arXiv2004

Enhancement of the superconducting transition temperature in Nb/Permalloy bilayers by controlling the domain state of the ferromagnet

In (S/F) hybrids the suppression of superconductivity by the exchange field h_ex of the ferromagnet can be partially lifted when different directions of h_ex are sampled simultaneously by the Cooper pair. In F/S/F trilayer geometries where the magnetization directions of the two F-layers can be controlled separately, this leads to the so-called spin switch. Here we show that domain walls in a single F-layer yield a similar effect. We study the transport properties of Ni_0.8Fe_0.2/Nb bilayers structured in strips of different sizes. For large samples a clear enhancement of superconductivity takes place in the resistive transition, in the very narrow field range (order of 0.5 mT) where the magnetization of the Py layer switches and many domains are present. This effect is absent in microstructured samples. Comparison of domain wall width δ_w to the temperature dependent superconductor coherence length ξ_S(T) shows that δ_w ~ ξ_S(T), which means that the Cooper pairs sample a large range of different magnetization directions.

cond-mat.supr-con↗

arXiv2003

Depairing currents in superconducting films of Nb and amorphous MoGe

We report on measuring the depairing current J_{dp} in thin superconducting films as a function of temperature. The main difficulties in such measurements are that heating has to be avoided, either due to contacts, or to vortex flow. The latter is almost unavoidable since the sample cross-section is usually larger than the superconducting coherence length ξ_s and the magnetic field penetration depth λ_s. On the other hand, vortex flow is helpful since it homogenizes the distribution of the current across the sample. We used a pulsed current method, which allows to overcome the difficulties caused by dissipation and measured the depairing current in films of thin polycrystalline Nb (low λ_s, low specific resistance ρ) and amorphous Mo_{0.7}Ge_{0.3} (high λ_s, high ρ), structured in the shape of bridges of various width. The experimental values of J_{dp} for different bridge dimensions are compared with theoretical predictions by Kupriyanov and Lukichev for dirty limit superconductors. For the smallest samples we find a very good agreement with theory, over essentially the whole temperature interval below the superconducting critical temperature.

cond-mat.supr-con↗

arXiv2003

Depairing currents in superconductor / ferromagnet Nb/CuNi trilayers close to T$_c$

In superconductor/ferromagnet (S/F) heterostructures, the exchange field $h_{ex}$ of the F-layer suppresses the superconducting order parameter in the S-layer via the proximity effect. One issue in current research is the effect of a domain state, or, more generally, different directions of $h_{ex}$, on the superconductivity. We used a pulsed-current technique in order to measure the superconducting transport properties of Cu$_{1-x}$Ni$_x$/Nb/Cu$_{1-x}$Ni$_x$ ($x$ = 0.54) F/S/F trilayers structured in strips of about 2 $μ$m wide and 20 $μ$m long as function of a small in-plane magnetic field. We find that the depairing current is tied to the magnetization behavior. In particular, we show that the suppression of superconductivity in the S-layer is smallest when the external magnetic field equals the coercive field H$_{c}$ of the F-layers.

cond-mat.supr-con↗

arXiv2003

Proximity effects in the superconductor / heavy fermion bilayer system Nb / CeCu_6

We have investigated the proximity effect between a superconductor (Nb) and a 'Heavy Fermion' system (CeCu_6) by measuring critical temperatures $T_c$ and parallel critical fields H_{c2}^{\parallel}(T) of Nb films with varying thickness deposited on 75 nm thick films of CeCu_6, and comparing the results with the behavior of similar films deposited on the normal metal Cu. For Nb on CeCu_6 we find a strong decrease of T_c with decreasing Nb thickness and a finite critical thickness of the order of 10 nm. Also, dimensional crossovers in H_{c2}^{\parallel}(T) are completely absent, in strong contrast with Nb/Cu. Analysis of the data by a proximity effect model based on the Takahashi-Tachiki theory shows that the data can be explained by taking into account both the high effective mass (or low electronic diffusion constant), {\it and} the large density of states at the Fermi energy which characterize the Heavy Fermion metal.

cond-mat.str-el↗

arXiv2002

STM Imaging of Flux Line Arrangements in the Peak Effect Regime

We present the results of a study of vortex arrangements in the peak-effect regime of 2H-NbSe_2 by scanning tunneling microscopy. By slowly increasing the temperature in a constant magnetic field, we observed a sharp transition from collective vortex motion to positional fluctuations of individual vortices at the temperature which coincides with the onset of the peak effect in ac-susceptibility. We conclude that the peak effect is a disorder driven transition, with the pinning energy winning from the elastic energy.

cond-mat.supr-con↗

arXiv2002

Disorder-induced melting of the charge order in thin films of Pr0.5Ca0.5MnO3

We have studied the magnetic-field-induced melting of the charge order in thin films of Pr0.5Ca0.5MnO3 (PCMO) films on SrTiO3 (STO) by X-ray diffraction, magnetization and transport measurement. At small thickness (25 nm) the films are under tensile strain and the low-temperature melting fields are of the order of 20 T or more, comparable to the bulk value. With increasing film thickness the strain relaxes, which leads to a strong decrease of the melting fields. For a film of 150 nm, with in-plane and out-of-plane lattice parameters closer to the bulk value, the melting field has reduced to 4 T at 50 K, with a strong increase in the hysteretic behavior and also an increasing fraction of ferromagnetic material. Strain relaxation by growth on a template of YBa2Cu3O(7-delta) or by post-annealing yields similar results with an even stronger reduction of the melting field. Apparently, strained films behave bulk-like. Relaxation leads to increasing suppression of the CO state, presumably due to atomic scale disorder produced by the relaxation process.

cond-mat.mtrl-sci↗

arXiv2001

Inhomogeneous superconductivity induced in a weak ferromagnet

Under certain conditions, the order parameter induced by a superconductor (S) in a ferromagnet (F) can be inhomogeneous and oscillating, which results e.g. in the so-called pi-coupling in S/F/S junctions. In principle, the inhomogeneous state can be induced at T_c as function of the F-layer thickness d_F in S/F bilayers and multilayers, which should result in a dip-like characteristic of T_c(d_F). We show the results of measurements on the S/F system Nb/Cu_{1-x}Ni_x, for Ni-concentrations in the range x = 0.5-0.7, where such effects might be expected. We find that the critical thickness for the occurrence of superconductivity is still relatively high, even for these weak ferromagnets. The resulting dip then is intrinsically shallow and difficult to observe, which explains the lack of a clear signature in the T_c(d_F) data.

cond-mat.supr-con↗

arXiv2001

Thickness dependence of the ground-state properties in thin films of the heavy-fermion compound CeCu_6

High-quality thin polycrystalline films of the heavy-fermion compound CeCu_6 were prepared by sputter deposition. The thicker of these films (with thickness up to around 200 nm) reproduce the properties of the bulk compound CeCu_6. As the thickness of the films is decreased, our measurements display strong deviations from the bulk properties, namely, a suppression of the heavy-fermion state. We show that possible `external' effects, like disorder, oxidation and morphology can be excluded and that this size effect is therefore an intrinsic property of CeCu_6. In addition, we investigate possible scenarios explaining the size effect, and find that the proximity of CeCu_6 to a quantum phase transition can account for this striking result.

cond-mat.str-el↗

arXiv2001

Strain release of (La,Ca)MnO3 thin films by YBa2Cu3O7-d

La1-xCaxMnO3 (x = 0.3; LCMO) films of different thickness were sputter-deposited on SrTiO3(100) (STO) single crystal substrates with and without YBa2Cu3O7-d (YBCO) as template layer. The electric and magnetic properties and the microstructure of the films were studied. Clear differences in the out-of-plane lattice parameter and the temperature of the metal-insulator transition show that the YBCO buffer layer is very effective in relaxing the strain of the LCMO films, which is quite difficult to release in LCMO films directly deposited on STO. Use of such buffer layers for strain release may prove to be a quite general tool.

cond-mat.mtrl-sci↗