arXiv subjects

Ludwig Schultz

Publications and source records attributed to Ludwig Schultz.

At least 19 recordsLinked to original sources

arXiv2017

Reversible tuning of magnetocaloric Ni-Mn-Ga-Co films on ferroelectric PMN-PT substrates

Tuning functional properties of thin caloric films by mechanical stress is currently of high interest. In particular, a controllable magnetisation or transition temperature is desired for improved usability in magnetocaloric devices. Here, we present results of epitaxial magnetocaloric Ni-Mn-Ga-Co thin films on ferroelectric Pb(Mg$_{1/3}$Nb$_{2/3}$)$_{0.72}$Ti$_{0.28}$O$_3$ (PMN-PT) substrates. Utilizing X-ray diffraction measurements, we demonstrate that the strain induced in the substrate by application of an electric field can be transferred to the thin film, resulting in a change of the lattice parameters. We examined the consequences of this strain on the magnetic properties of the thin film by temperature and electric field dependent measurements. We did not observe a change of martensitic transformation temperature but a reversible change of magnetisation within the austenitic state, which we attribute to the intrinsic magnetic instability of this metamagnetic Heusler alloy.

cond-mat.mtrl-sci↗

arXiv2017

Role of twin and anti-phase defects in MnAl permanent magnets

We quantify and explain the effects of both anti-phase boundaries and twin defects in as-transformed $τ$-MnAl by carrying out micromagnetic simulations based closely on the results of microstructural characterization. We demonstrate that magnetic domain walls nucleate readily at anti-phase boundaries and are strongly pinned by them, due to anti-ferromagnetic coupling. Likewise, twin boundaries reduce the external field required to nucleate domain walls and provide strong pinning potentials, with the pinning strength dependent on the twinning angle. The relative strengths of the known twin defect types are quantified based on the anisotropy angles across their boundaries. Samples that have undergone heat treatment are imaged using electron back-scatter diffraction. The precise crystallographic orientation is mapped spatially and converted into a number of realistic finite element models, which are used to compute the effects of large concentrations of twin domains in a realistic morphology. This is shown to have a negative effect on the remanence coercivity and squareness. The maximum energy product $(BH)_{\mathrm{max}}$ is therefore significantly lower than the theoretical limit of the material and much lower than MnAl permanent magnets that have been further processed to remove twin defects. The knowledge gained in this study will allow the optimization of processing routes in order to develop permanent magnets with enhanced magnetic properties.

cond-mat.mtrl-sci↗

arXiv2017

Nucleation and growth of hierarchical martensite in epitaxial shape memory films

Shape memory alloys often show a complex hierarchical morphology in the martensitic state. To understand the formation of this twin-within-twins microstructure, we examine epitaxial Ni-Mn-Ga films as a model system. In-situ scanning electron microscopy experiments show beautiful complex twinning patterns with a number of different mesoscopic twin boundaries and macroscopic twin boundaries between already twinned regions. We explain the appearance and geometry of these patterns by constructing an internally twinned martensitic nucleus, which can take the shape of a diamond or a parallelogram, within the basic phenomenological theory of martensite. These nucleus contains already the seeds of different possible mesoscopic twin boundaries. Nucleation and growth of these nuclei determines the creation of the hierarchical space-filling martensitic microstructure. This is in contrast to previous approaches to explain a hierarchical martensitic microstructure. This new picture of creation and anisotropic, well-oriented growth of twinned martensitic nuclei explains the morphology and exact geometrical features of our experimentally observed twins-within-twins microstructure on the meso- and macroscopic scale.

cond-mat.mtrl-sci↗

arXiv2016

Growth, microstructure and thermal transformation behaviour of epitaxial Ni-Ti films

Epitaxial films have the potential to be used as model systems for fundamental investigations on the martensitic transformation in binary NiTi. In this paper, we discuss growth of binary NiTi thin films on single crystalline MgO substrates. Sputter deposition is used to grow NiTi films. Films prepared by complementary preparation routes (with different deposition temperatures and subsequent heat treatments) are investigated by X-ray diffraction, electron microscopy, atomic force microscopy, and electrical resistivity measurements, with the aim of optimizing film properties, particularly to obtain a well defined orientation of the austenitic unit cell and smooth surfaces. Our results show that deposition at elevated temperatures and carefully controlled subsequent heat treatments allow to produce epitaxially grown and smooth NiTi films that exhibit reversible one- or two-step martensitic transformations.

cond-mat.mtrl-sci↗

arXiv2016

On the origin of perpendicular magnetic anisotropy in strained Fe-Co(-X) films

Very high magnetic anisotropies have been theoretically predicted for strained Fe-Co(-X) and indeed several experiments on epitaxial thin films seemed to confirm strain induced anisotropy enhancement. This study presents a critical analysis of the different contributions to perpendicular anisotropy: volume, interface and surface anisotropies. Tracing these contributions, thickness series of single layer films as well as multilayers with Au-Cu buffers/interlayers of different lattice parameters have been prepared. The analysis of their magnetic anisotropy reveals a negligible influence of the lattice parameter of the buffer. Electronic effects, originating from both, the Au-Cu interface and the film surface, outrange the elastic effects. Surface anisotropy, however, exceeds the interface anisotropy by more than a factor of three. A comparison with results from Density Functional Theory suggests, that the experimentally observed strong perpendicular surface anisotropy originates from a deviation from an ideal oxide-free surface. Accordingly, tailored Fe-Co-X/oxide interfaces may open a route towards high anisotropy in rare-earth free materials.

cond-mat.mtrl-sci↗

arXiv2016

Magnetic properties of single nanomagnets: EMCD on FePt nanoparticles

Energy-loss magnetic chiral dichroism (EMCD) allows for the quantification of magnetic properties of materials at the nanometer scale. It is shown that with the support of simulations that help to identify the optimal conditions for a successful experiment and upon implementing measurement routines that effectively reduce the noise floor, EMCD measurements can be pushed towards quantitative magnetic measurements even on individual nanoparticles. With this approach, the ratio of orbital to spin magnetic moments for the Fe atoms in a single L$1_0$ ordered FePt nanoparticle is determined to be ${m_l}/{m_s} = 0.08 \pm 0.02$. This finding is in good quantitative agreement with the results of XMCD ensemble measurements.

cond-mat.mtrl-sci↗

arXiv2015

Enhanced and Tunable Spin-Orbit Coupling in Tetragonally Strained Fe-Co-B Films

We have synthesized 20 nm thick films of tetragonally strained interstitial Fe-Co-B alloys epitaxially grown on Au55Cu45 buffer layer. The strained axis is perpendicular to the film plane and the corresponding lattice constant c is enlarged with respect to the in-plane lattice parameter a. By adding the interstitial boron with different concentrations 0, 4, and 10 at.% we were able to stabilize the tetragonal strain in 20 nm Fe-Co films with different c/a ratios of 1.013, 1.034 and 1.02, respectively. Using ferromagnetic resonance (FMR) and x-ray magnetic circular dichroism (XMCD) we found that the orbital magnetic moment increases with increasing the c/a ratio, pointing towards the enhancement of spin-orbit coupling (SOC) at larger strain. Our results show that careful doping of ferromagnetic films allows to control the SOC by stabilizing anisotropic strain states. These findings are applicable in material design for spintronics applications. We also discuss the influence of B doping on the Fe-Co film microstructure, its magnetic properties and magnetic relaxation.

cond-mat.mtrl-sci↗

arXiv2015

From soft to hard magnetic Fe-Co-B by spontaneous strain: A combined first principle and thin film study

In order to convert the well-known Fe-Co-B alloy from a soft to a hard magnet, we propose tetragonal strain by interstitial boron. Density functional theory reveals that when B atoms occupy octahedral interstitial sites, the bcc Fe-Co lattice is strained spontaneously. Such highly distorted Fe-Co is predicted to reach a strong magnetocrystalline anisotropy which may compete with shape anisotropy. Probing this theoretical suggestion experimentally, epitaxial films are examined. A spontaneous strain up to 5 % lattice distortion is obtained for B contents up to 4 at%, which leads to uniaxial anisotropy constants exceeding 0.5 MJ/m^3. However, a further addition of B results in a partial amorphization, which degrades both anisotropy and magnetization.

cond-mat.mtrl-sci↗

arXiv2015

Influence of substrate type on transport properties of superconducting FeSe0.5Te0.5 thin films

FeSe0.5Te0.5 thin films were grown by pulsed laser deposition on CaF2, LaAlO3 and MgO substrates and structurally and electro-magnetically characterized in order to study the influence of the substrate on their transport properties. The in-plane lattice mismatch between FeSe0.5Te0.5 bulk and the substrates shows no influence on the lattice parameters of the films, whereas the type of substrates affects the crystalline quality of the films and, therefore, the superconducting properties. The film on MgO showed an extra peak in the angular dependence of critical current density Jc(θ) at θ = 180° (H || c), which arises from c-axis defects as confirmed by transmission electron microscopy. In contrast, no Jc(θ) peaks for H || c were observed in films on CaF2 and LaAlO3. Jc(θ) can be scaled successfully for both films without c-axis correlated defects by the anisotropic Ginzburg-Landau (AGL) approach with appropriate anisotropy ratio γJ. The scaling parameter γJ is decreasing with decreasing temperature, which is different from what we observed in FeSe0.5Te0.5 films on Fe-buffered MgO substrates.

cond-mat.supr-con↗

arXiv2014

Increased magnetocrystalline anisotropy in epitaxial Fe-Co-C thin films with spontaneous strain

Rare earth free alloys are in focus of permanent magnet research since the accessibility of the elements needed for nowadays conventional magnets is limited. Tetragonally strained iron-cobalt (Fe-Co) has attracted large interest as promising candidate due to theoretical calculations. In experiments, however, the applied strain quickly relaxes with increasing film thickness and hampers stabilization of a strong magnetocrystalline anisotropy. In our study we show that already 2 at% of carbon substantially reduce the lattice relaxation leading to the formation of a spontaneously strained phase with 3 % tetragonal distortion. In these strained (Fe$_{0.4}$Co$_{0.6}$)$_{0.98}$C$_{0.02}$ films, a magnetocrystalline anisotropy above 0.4 MJ/m$^3$ is observed while the large polarization of 2.1 T is maintained. Compared to binary Fe-Co this is a remarkable improvement of the intrinsic magnetic properties. In this paper, we relate our experimental work to theoretical studies of strained Fe-Co-C and find a very good agreement.

cond-mat.mtrl-sci↗

arXiv2013

Mesoscopic twin boundaries in epitaxial Ni-Mn-Ga films

Twin boundaries play an essential role in the use of magnetic shape memory alloy Ni-Mn-Ga as active material. Only if twin boundaries can be moved by an external magnetic field, high strain values of up to 10 % can be obtained. Therefore, understanding the observed twin microstructure of thin films is crucial for future application. We exemplarily present two different microstructural pattern, type X and type Y microstructure, using the example of two Ni-Mn-Ga films with similar film thickness and composition. The analysis of microstructure and structure of the thin films shows that both, type X and type Y pattern, are formed by mesoscopic 14M twin boundaries. The mesoscopic 14M twin boundaries are either tilted from the substrate surface in case of type X pattern or they are perpendicular to the surface for type Y pattern. Based on a recently proposed scenario for the nucleation of martensite in austenite, we can trace back the difference in twin boundary orientation to differently oriented martensitic nuclei.

cond-mat.mtrl-sci↗

arXiv2013

Oxypnictide SmFeAs(O,F) superconductor: a candidate for high-field magnet applications

The recently discovered oxypnictide superconductor SmFeAs(O,F) is the most attractive material among the Fe-based superconductors due to its highest transition temperature of 56 K and potential for high-field performance. In order to exploit this new material for superconducting applications, the knowledge and understanding of its electro-magnetic properties are needed. Recent success in fabricating epitaxial SmFeAs(O,F) thin films opens a great opportunity to explore their transport properties. Here we report on a high critical current density of over 10^5 A/cm^2 at 45 T and 4.2 K for both main field orientations, feature favourable for high-field magnet applications. Additionally, by investigating the pinning properties, we observed a dimensional crossover between the superconducting coherence length and the FeAs interlayer distance at 30-40 K, indicative of a possible intrinsic Josephson junction in SmFeAs(O,F) at low temperatures that can be employed in electronics applications such as a terahertz radiation source and a superconducting Qubit.

cond-mat.supr-con↗

arXiv2013

Electric field control of magnetic exchange coupling

Electric control of magnetism is a vision which drives intense research on magnetic semiconductors and multiferroics. Recently, also ultrathin metallic films were reported to show magnetoelectric effects at room temperature. Here we demonstrate much stronger effects by exploiting reduction/oxidation reactions in a naturally grown oxide layer exchange coupled to an underlying ferromagnet. For the exemplarily studied FePt/iron oxide composite in an electrolyte, a large and reversible change of magnetization and anisotropy is obtained. The principle can be transferred to various metal/oxide combinations. It represents a novel approach towards multifunctionality.

cond-mat.mtrl-sci↗

arXiv2013

Intrinsic pinning and the critical current scaling of clean epitaxial Fe(Se,Te) thin films

We report on the transport properties of clean, epitaxial Fe(Se,Te) thin films prepared on Fe-buffered MgO (001) single crystalline substrates by pulsed laser deposition. Near Tc a steep slope of the upper critical field for H||ab was observed (74.1 T/K), leading to a very short out-of-plane coherence length, ξc, of 0.2 nm, yielding 2ξc(0) approximately 0.4 nm. This value is shorter than the interlayer distance (0.605 nm) between Fe-Se(Te) planes, indicative of modulation of the superconducting order parameter along the c-axis. An inverse correlation between the power law exponent N of the electric field-current density (E-J) curve and the critical current density, Jc, has been observed at 4 K, when the orientation of H was close to the ab-plane. These results prove the presence of intrinsic pinning in Fe(Se,Te). A successful scaling of the angular dependent Jc and the corresponding exponent N can be realized by the anisotropic Ginzburg Landau approach with appropriate Γvalues 2~3.5. The temperature dependence of Γbehaves almost identically to that of the penetration depth anisotropy.

cond-mat.supr-con↗

arXiv2013

Versatile fluoride substrates for Fe-based superconducting thin films

We demonstrate the growth of Co-doped BaFe2As2 (Ba-122) thin films on AEF2 (001) (AE: Ca, Sr, Ba) single crystal substrates using pulsed laser deposition. All films are grown epitaxially despite of a large misfit of -10.6% for BaF2 substrate. For all films a reaction layer is formed at the interface confirmed by X-ray diffraction and by transmission electron microscopy. The superconducting transition temperature of the film on CaF2 is around 27 K, whereas the corresponding values of the other films are around 21 K. The Ba-122 on CaF2 shows identical crystalline quality and superconducting properties as films on Fe-buffered MgO.

cond-mat.supr-con↗

arXiv2013

Magnetic domain pattern in hierarchically twinned epitaxial Ni-Mn-Ga films

Magnetic shape memory alloys exhibit a hierarchically twinned microstructure, which has been well examined in epitaxial Ni-Mn-Ga films. Here we analyze consequences of this twin within twins microstructure on the magnetic domain pattern. Atomic and magnetic force microscopy are used to probe the correlation between the martensitic microstructure and magnetic domains. We examine consequences of different twin boundary orientations with respect to the substrate normal as well as variant boundaries between differently aligned twinned laminates. A detailed micromagnetic analysis is given which describes the influence of the finite film thickness on the formation of magnetic band domains in these multiferroic materials.

cond-mat.mtrl-sci↗

arXiv2012

Enhancing magnetocrystalline anisotropy of the Fe70Pd30 magnetic shape memory alloy by adding Cu

Strained epitaxial growth provides the opportunity to understand the dependence of intrinsic and extrinsic properties of functional materials at frozen intermediate stages of a phase transformation. In this study, a combination of thin film experiments and first-principles calculations yields the binding energy and magnetic properties of tetragonal Fe70Pd30-xCux ferromagnetic shape memory thin films with x = 0, 3, 7 and structures ranging from bcc to beyond fcc (1.07 1.41 the samples undergo structural relaxations through adaptive nanotwinning. For all tetragonal structures, we observe a significant increase of the magnetocrystalline anisotropy constant K1, which reaches a maximum of K1=-2.4*10^5 Jm^-3 at room temperature around c/a_bct=1.33 and is thus even larger than for binary Fe70Pd30 and the prototype Ni-Mn-Ga magnetic shape memory system. Since K1 represents the driving force for variant reorientation in magnetic shape memory systems, we conclude that Fe-Pd-Cu alloys offer a promising route towards microactuators applications with significantly improved work output.

cond-mat.mtrl-sci↗

arXiv2012

Magnetic domain structure of epitaxial Ni-Mn-Ga films

For the magnetic shape memory effect, knowledge about the interaction between martensitic and magnetic domain structure is essential. In the case of Ni-Mn-Ga bulk material and foils, a staircase-like magnetic domain structure with 90{\deg}- and 180{\deg}-domain walls is known for modulated martensite. In the present paper we show that the magnetic domain pattern of thin epitaxial films is fundamentally different. Here we analyze epitaxial Ni-Mn-Ga films by atomic and magnetic force microscopy to investigate the correlation between the twinned martensitic variants and the magnetic stripe domains. The observed band-like domains with partially perpendicular outof-plane magnetization run perpendicular to the microstructure domains defined by twinning variants. These features can be explained by the finite film thickness, resulting in an equilibrium twinning period much smaller than the domain period. This does not allow the formation of a staircase domain patter. Instead the energies of the magnetic and martensitic microstructures are minimized independently by aligning both patterns perpendicularly to each other. By analyzing a thickness series we can show that the observed magnetic domain pattern can be quantitatively described by an adapted band domain model of Kittel.

cond-mat.mtrl-sci↗