2016 Publications

Torque magnetometry of perpendicular anisotropy exchange-spring heterostructures
P. Vallobra, T. Hauet, F. Montaigne, E.G Shipton, E.E. Fullerton, S. Mangin (2016)
J. Appl. Phys. 120, 013903

Up to now, torque magnetometry has been mostly used for studying the magnetic properties of single system. Here we used it to characterize the magnetic configurations under field of a multilayer thin film, namely [Co/Pd]15 /TbFeCo exchange-spring system. The experimental results are compared to a 1D micromagnetic simulation. The good agreement between experiments and simulations allows us to deduce the evolution of the in-depth magnetic configuration as a function of the applied field orientation and amplitude. The chirality transition of the interfacial domain wall developing in the structure can also be determined with this technique.

Reproducible formation of single magnetic bubbles in an array of patterned dots
T. Liu, V. Puliafito, F. Montaigne, S. Petit, C. Deranlot, S. Andrieu, O. Ozatay, G. Finocchio and T. Hauet (2016)
J. Phys. D: Appl. Phys. 49, 245002

Here we study the nucleation of magnetic bubble in arrays of nano- and micro-patterned dots The formation conditions of single magnetic bubbles through in-plane field demagnetization are investigated in an array of Co/Ni circular dots by magnetic force microscopy and compared to micromagnetic calculations. We demonstrate high success rates in nucleating stable bubbles.

Magnetic moment and local magnetic induction of superconducting/ferromagnetic structures subjected to crossed fields: Experiments on GdBCO and modelling.
Fagnard, J.-F., Morita, M., Nariki, S., Teshima, H., Caps, H., Vanderheyden, B., & Vanderbemden, P. (2016)
Superconductor Science and Technology, 29(12), 125004

Recent studies have shown that ferromagnetic materials can be used together with bulk high temperature superconductors in order to improve their magnetic trapped field. Remarkably, it has also been pointed out that ferromagnets can help in reducing the crossed field effect, namely the magnetization decay that is observed under the application of AC transverse magnetic fields. In this work, we pursue a detailed study of the influence of the geometry of the ferromagnetic part on both trapped fields and crossed field effects. The magnetic properties of the hybrid superconducting/soft ferromagnetic structures are characterized by measuring the magnetic moment with a bespoke magnetometer and the local magnetic field density with Hall probes. The results are interpreted by means of 2D and 3D numerical models yielding the distribution of the superconducting currents as a function of the ferromagnet geometry. We examine in details the distortion of the shielding superconducting currents distribution in hybrid structures subjected to crossed magnetic fields. These results confirm the existence of an optimum thickness of the ferromagnet, which depends on the saturation magnetization of the ferromagnetic material and the current density of the superconductor. A hybrid structure providing an efficient protection against the crossed magnetic field while maintaining the magnetic induction along the axis of the structure is suggested. The limitations of the 2D modelling in this configuration are discussed

Synthesis, structure, and physical properties of new rare earth ferrocenoylacetonates
P.S. Koroteev, Z.V. Dobrokhotova, A.B. Ilyukhin, N.N. Efimov, M. Rouzières, M.A. Kiskin, R. Clérac and V.M. Novotortsev (2016)
Dalton Transactions, 45(15), 6405-6417

New ferrocenoylacetonate complexes of several rare earth elements, [Ln(fca)3(bpy)]·MeC6H5 (Ln = Pr (1), Eu (2), Gd (3), Tb (4), Dy (5), Ho (6), Y (7); bpy – 2,2′-bipyridine; Hfca – FcCOCH2COMe) as well as scandium ferrocenoylacetonate [Sc(fca)3]·0.5MeC6H5 (8), were synthesized and characterized by single crystal X-ray diffraction analysis. In the crystal lattice of the isostructural complexes 1–7, two [Ln(fca)3(bpy)] molecules form a pair due to stacking interactions between the bpy ligands. The Ln3+ ions are coordinated in a square antiprism geometry with a coordination number of 8. The Sc3+ ions in complex 8 are coordinated in an octahedral geometry. Thermolysis of complexes 1–7 was studied under air and argon atmospheres; in the first case, it affords perovskites LnFeO3 as one of the products. Complexes 4–6 display single-molecule magnet properties, and the effective relaxation barrier for the Dy complex 5, was found to be Δeff/kB = 241 K, which is one of the highest values obtained for a mononuclear β-diketonate lanthanide complex.

A low spin manganese(IV) nitride single molecule magnet
M. Ding, G.E. Cutsail, D. Aravena, M. Amoza, M. Rouzières, P. Dechambenoit, Y. Losovyj, M. Pink, E. Ruiz, R. Clérac and J.M. Smith (2016)
Chemical science, 7(9), 6132-6140

Structural, spectroscopic and magnetic methods have been used to characterize the tris(carbene)borate compound PhB(MesIm)3Mn[triple bond, length as m-dash]N as a four-coordinate manganese(IV) complex with a low spin (S = 1/2) configuration. The slow relaxation of the magnetization in this complex, i.e. its single-molecule magnet (SMM) properties, is revealed under an applied dc field. Multireference quantum mechanical calculations indicate that this SMM behavior originates from an anisotropic ground doublet stabilized by spin–orbit coupling. Consistent theoretical and experiment data show that the resulting magnetization dynamics in this system is dominated by ground state quantum tunneling, while its temperature dependence is influenced by Raman relaxation.

Magneto-optical micromechanical systems for magnetic field mapping
A. Truong, G. Ortiz, M. Morcrette, T. Dietsch, P. Sabon, I. Joumard, A. Marty, H. Joisten, B. Diény (2016)
Scientific Reports, 6, 31634

A new method for magnetic field mapping based on the optical response of organized dense arrays of flexible magnetic cantilevers is explored. When subjected to the stray field of a magnetized material, the mobile parts of the cantilevers deviate from their initial positions, which locally changes the light reflectivity on the magneto-optical surface, thus allowing to visualize the field lines. While the final goal is to be able to map and quantify non-uniform fields, calibrating and testing the device can be done with uniform fields. Under a uniform field, the device can be assimilated to a magnetic-field-sensitive diffraction grating, and therefore, can be analyzed by coherent light diffraction. A theoretical model for the diffraction patterns, which accounts for both magnetic and mechanical interactions within each cantilever, is proposed and confronted to the experimental data.

Chiral damping of magnetic domain walls
E. Jué, C. K. Safeer, M. Drouard, A. Lopez, P. Balint, L. Buda-Prejbeanu, O. Boulle, S. Auffret, A. Schuhl, A. Manchon, I. Mihai Miron, G. Gaudin (2016)
Nature materials, 15(3), 272

Structural symmetry breaking in magnetic materials is responsible for the existence of multiferroics1, current-induced spin–orbit torques and some topological magnetic structures. In this Letter we report that the structural inversion asymmetry (SIA) gives rise to a chiral damping mechanism, which is evidenced by measuring the field-driven domain-wall (DW) motion in perpendicularly magnetized asymmetric Pt/Co/Pt trilayers. The DW dynamics associated with the chiral damping and those with Dzyaloshinskii–Moriya interaction (DMI) exhibit identical spatial symmetry. However, both scenarios are differentiated by their time reversal properties: whereas DMI is a conservative effect that can be modelled by an effective field, the chiral damping is purely dissipative and has no influence on the equilibrium magnetic texture. When the DW motion is modulated by an in-plane magnetic field, it reveals the structure of the internal fields experienced by the DWs, allowing one to distinguish the physical mechanism. The chiral damping enriches the spectrum of physical phenomena engendered by the SIA, and is essential for conceiving DW and skyrmion devices owing to its coexistence with DMI.

Spin–orbit torque magnetization switching controlled by geometry
C. K. Safeer, E. Jué, A. Lopez, L. Buda-Prejbeanu, S. Auffret, S. Pizzini, O. Boulle, I. Mihai Miron, G. Gaudin (2016)
Nature nanotechnology, 11(2), 143

Magnetization reversal by an electric current is essential for future magnetic data storage technology1, such as magnetic random access memories. Typically, an electric current is injected into a pillar-shaped magnetic element, and switching relies on the transfer of spin momentum from a ferromagnetic reference layer (an approach known as spin–transfer torque). Recently, an alternative technique has emerged that uses spin–orbit torque (SOT) and allows the magnetization to be reversed without a polarizing layer by transferring angular momentum directly from the crystal lattice. With spin–orbit torque, the current is no longer applied perpendicularly, but is in the plane of the magnetic thin film. Therefore, the current flow is no longer restricted to a single direction and can have any orientation within the film plane. Here, we use Kerr microscopy to examine spin–orbit torque-driven domain wall motion in Co/AlOx wires with different shapes and orientations on top of a current-carrying Pt layer. The displacement of the domain walls is found to be highly dependent on the angle between the direction of the current and domain wall motion, and asymmetric and nonlinear with respect to the current polarity. Using these insights, devices are fabricated in which magnetization switching is determined entirely by the geometry of the device.

Second order anisotropy contribution in perpendicular magnetic tunnel junctions
A. Timopheev, R. Sousa, M. Chshiev, H. T. Nguyen, B. Dieny (2016)
Scientific reports, 6, 26877

Hard-axis magnetoresistance loops were measured on perpendicular magnetic tunnel junction pillars of diameter ranging from 50 to 150 nm. By fitting these loops to an analytical model, the effective anisotropy fields in both free and reference layers were derived and their variations in temperature range between 340 K and 5 K were determined. It is found that a second-order anisotropy term of the form −K2cos4θ must be added to the conventional uniaxial –K1cos2θ term to explain the experimental data. This higher order contribution exists both in the free and reference layers. At T = 300 K, the estimated −K2/K1 ratios are 0.1 and 0.24 for the free and reference layers, respectively. The ratio is more than doubled at low temperatures changing the ground state of the reference layer from “easy-axis” to “easy-cone” regime. The easy-cone regime has clear signatures in the shape of the hard-axis magnetoresistance loops. The existence of this higher order anisotropy was also confirmed by ferromagnetic resonance experiments on FeCoB/MgO sheet films. It is of interfacial nature and is believed to be due to spatial fluctuations at the nanoscale of the first order anisotropy parameter at the FeCoB/MgO interface.

Tunable spin-wave frequency gap in anisotropy-graded FePt films obtained by ion irradiation
S. Tacchi, M.G. Pini, A. Rettori, G. Varvaro, A. Di Bona, S. Valeri, F. Albertini, P. Lupo, F. Casoli (2016)
Physical Review B, 94(2), 024432

The effect of graded anisotropy on static and dynamic magnetic properties of Ar+-irradiated FePt films has been investigated by static magnetometry, magnetic force microscopy, and Brillouin light scattering from thermally excited spin waves. A gradual variation of magnetic anisotropy with film thickness was obtained by Ar+-irradiation. The irradiation incidence angle influences the anisotropy profile: on decreasing α, a decreasing thickness of the hard L10 phase and an increasing thickness of the soft A1 phase were obtained. Accordingly, the zero-field spin-wave frequency gap was found to decrease. In the sample with the highest soft-phase thickness the spin-wave frequency gap takes a substantial value (ν0 ≈ 6 GHz), which could be reproduced assuming the presence of a nonzero “rotatable” anisotropy (i.e., any direction in the film plane can be established as the easy axis by the application of a saturating magnetic field along this direction). The hypothesis is supported by both magnetometry and magnetic force microscopy data.

Much More Than a Glass: The Complex Magnetic and Microstructural Properties of Obsidian
V. Mameli, A. Musinu, D. Niznansky, D. Peddis, G. Ennas, A. Ardu, C. Lugliè, C. Cannas (2016)
The Journal of Physical Chemistry C, 120(48), 27635-27645

Obsidian is a natural volcanic glass in which nanocrystalline and microcrystalline phases can coexist with the glassy one. In this paper, magnetic properties of Monte Arci obsidians are investigated by an experimental approach commonly applied to synthetic nanostructured materials and rarely to natural ones, and correlated with the mineralogical composition and microstructure. Among the different crystalline phases, the iron-containing components are found to be responsible for a great variety of magnetic behaviors, including paramagnetism, antiferromagnetism, ferromagnetism, and superparamagnetism. The combined use of powder X-ray diffraction (PXRD), 57Fe Mössbauer spectroscopy, DC magnetometry, and transmission electron microscopy (TEM/HRTEM) provides new insights in the Monte Arci obsidian: (i) the presence of magnetite nanoparticles spread into the glassy matrix; (ii) the presence of an antiferromagnetic phase responsible for a discontinuity at about 45 K; (iii) exchange bias phenomena, for the first time revealed in obsidians, due to the coupling between the nanostructured ferrimagnetic phase and the antiferromagnetic one; (iv) Goldanskii-Karyagin effect (GKE) associated with biotite.

Studying the effect of Zn-substitution on the magnetic and hyperthermic properties of cobalt ferrite nanoparticles
V. Mameli, A. Musinua, A. Ardu, G. Ennas, D. Peddis, D. Niznansky, C. Sangregorio, C. Innocenti, N.T. K. Thanh, C. Cannas (2016)
Nanoscale, 8(19), 10124-10137

The possibility to finely control nanostructured cubic ferrites (MIIFe2O4) paves the way to design materials with the desired magnetic properties for specific applications. However, the strict and complex interrelation among the chemical composition, size, polydispersity, shape and surface coating renders their correlation with the magnetic properties not trivial to predict. In this context, this work aims to discuss the magnetic properties and the heating abilities of Zn-substituted cobalt ferrite nanoparticles with different zinc contents (ZnxCo1−xFe2O4 with 0 < x < 0.6), specifically prepared with similar particle sizes (∼7 nm) and size distributions having the crystallite size (∼6 nm) and capping agent amount of 15%. All samples have high saturation magnetisation (Ms) values at 5 K (>100 emu g−1). The increase in the zinc content up to x = 0.46 in the structure has resulted in an increase of the saturation magnetisation (Ms) at 5 K. High Ms values have also been revealed at room temperature (∼90 emu g−1) for both CoFe2O4 and Zn0.30Co0.70Fe2O4 samples and their heating ability has been tested. Despite a similar saturation magnetisation, the specific absorption rate value for the cobalt ferrite is three times higher than the Zn-substituted one. DC magnetometry results were not sufficient to justify these data, the experimental conditions of SAR and static measurements being quite different. The synergic combination of DC with AC magnetometry and 57Fe Mössbauer spectroscopy represents a powerful tool to get new insights into the design of suitable heat mediators for magnetic fluid hyperthermia.

Characterization of superconducting thin films and nanoSQUIDs for nanoparticle investigation at high magnetic field
R. Russo, E. Di Gennaro, E. Esposito, A. Crescitelli, D. Fiorani, C. Granata, A. Vettoliere, R. Cristiano, M. Lisitskiy and D. Peddis (2016)
IEEE Transactions on Applied Superconductivity, 26(3), 1-5

We present an experimental investigation aimed to fabricate a nanoSQUID for high-magnetic-field applications. In particular, we produced niobium films having different thicknesses (5, 10, and 15 nm) with passivation layers to investigate the transport properties of very thin niobium film in high magnetic field parallel to the film plane. The results indicate that niobium thin films are superconducting at high field; however, the passivation layers are not sufficient to avoid a degradation of superconducting properties when niobium films are cycled between room and cryogenic temperatures. We also fabricated and measured a nanosensor based on niobium nitride. Niobium nitride films have much higher magnetic critical field, and they are more stable compared with niobium ones. The characterization of the nanodevice in the temperature range from 2 to 7 K includes measurements of current-voltage and critical current versus magnetic flux characteristic. Albeit the coherence length of niobium nitride is very short compared with the niobium one, and much smaller than the Dayem bridge length, the nanoSQUIDs present modulations of critical current larger than 5% at 2 K. The modulation can be observed up to 8 T, making NbN nanoSQUIDs good candidates to measure magnetic properties of magnetic nanoparticles and nanostructures.

L‐DOPA‐Coated Manganese Oxide Nanoparticles as Dual MRI Contrast Agents and Drug‐Delivery Vehicles
B. H. McDonagh, G. Singh, S. Hak, S. Bandyopadhyay, I. Lovise Augestad, D. Peddis, I. Sandvig, A. Sandvig, W. R. Glomm (2016)
Small, 12(3), 301-306

Manganese oxide nanoparticles (MONPs) are capable of time‐dependent magnetic resonance imaging contrast switching as well as releasing a surface‐bound drug. MONPs give T2/T2* contrast, but dissolve and release T1‐active Mn2+ and L‐3,4‐dihydroxyphenylalanine. Complementary images are acquired with a single contrast agent, and applications toward Parkinson’s disease are suggested.

Engineering perpendicular magnetic anisotropy in Fe via interstitial nitrogenation: N choose K
H. Zhang, I. Dirba, T. Helbig, L. Alff, and O. Gutfleisch (2016)
APL Materials 4, 116104

In this work, combining experimental results and first principles calculations, we show that interstitial nitrogen not only serves for inducing tetragonality in α′-Fe8Nx but is also essential for achieving a high degree of perpendicular magneto-crystalline anisotropy, K. Our results demonstrate that the orbital magnetic moments of the iron atoms above and below N in the direction of magnetization are much more susceptible to the applied magnetic field than their in-plane counterparts, leading to a giant value of K as compared to a hypothetical distorted material without N.

Magnetic properties of the Laves-type phases Ti2Co3Si and Ti2Fe3Si and their solid solution
C. M. Hamm, D. Gölden, E. Hildebrandt, J. Weischenberg, H. Zhang, L. Alff and C. S. Birkel (2016)
Journal of Materials Chemistry C, 4(31), 7430-7435

Interest in Fe and Co containing Laves-type phases arises from their promising magnetic properties, especially in the context of magnetostrictive and magnetocaloric materials. Most compounds that are used for these applications consist of rare earth elements that are economically and environmentally problematic. Ti2Co3Si and Ti2Fe3Si are therefore attractive rare earth-free candidates, however their magnetic behavior has not been studied in full detail yet. Consequentially, we have prepared the full solid solution between both phases by arc melting and spark plasma sintering. The samples were subject to comprehensive investigation by means of synchrotron X-ray diffraction, EDX analysis and SQUID measurements. The magnetic properties of the mostly single phase materials generally follow a clear trend with an increase in the magnetic moment with increasing Fe content. Interestingly, we found a sudden drop in magnetic moment between samples Ti2Fe3Si and Ti2(Co0.2Fe0.8)3Si. This phase transition from the ferromagnetic to the antiferromagnetic configuration for these compositions can indeed be confirmed by theoretical calculations.

Micromagnetic simulations on the grain shape effect in Nd-Fe-B magnets
M. Yi, O. Gutfleisch, and B.-X. Xu (2016)
Journal of Applied Physics 120, 033903

Micromagnetic simulations were performed to study the effect of grain shape and defect layer in Nd-Fe-B magnets. It was found that the coercivity can be increased by a factor of ∼2 by changing the grain shape from the triangular prism to the spheroid. Both the anisotropy field contribution and the shape contribution to the coercivity, and thus also the final coercivity, were found to decrease in the order: spheroid > circular prism > hexagonal prism > square prism > triangular prism. Sputtered columnar grains and hot-deformed platelet grains with a constant volume were also considered. Results show that the coercivity initially increases with the aspect ratio and then nearly saturates above the ratio of ∼4. Simulations of multigrain ensembles showed that depending on the grain shape, compared to the case of single grain, a further decrease of ∼10%–45% in the coercivity is induced by magnetostatic coupling.

Direct measurement of the magnetocaloric effect in cementite
B. Kaeswurm, K. Friemert, M. Gürsoy, K. P. Skokov, and O. Gutfleisch (2016)
Journal of Magnetism and Magnetic Materials, 410, 105-108

Measurements of the magnetocaloric effect of cementite at its Curie temperature of 475 K are presented. An adiabatic temperature change of 1.76±0.01 K was measured using a direct measurement technique. The isothermal entropy change was determined from measurements of magnetisation isotherms and was shown to be 3.07 J K−1 kg−1 in a field change of 2 T. The field dependencies of both magnetocaloric properties follow the H2/3 dependence typical for ferromagnetic materials with a second order phase transition. The material may be of interest in magnetocaloric applications such as magnetic refrigeration or thermomagnetic power generation.

Mastering hysteresis in magnetocaloric materials
O. Gutfleisch, T. Gottschall, M. Fries, D. Benke, I. Radulov, K. P. Skokov, H. Wende, M. Gruner, M. Acet, P. Entel, M. Farle (2016)
Phil. Trans. R. Soc. A, 374(2074), 20150308

Hysteresis is more than just an interesting oddity that occurs in materials with a first-order transition. It is a real obstacle on the path from existing laboratory-scale prototypes of magnetic refrigerators towards commercialization of this potentially disruptive cooling technology. Indeed, the reversibility of the magnetocaloric effect, being essential for magnetic heat pumps, strongly depends on the width of the thermal hysteresis and, therefore, it is necessary to understand the mechanisms causing hysteresis and to find solutions to minimize losses associated with thermal hysteresis in order to maximize the efficiency of magnetic cooling devices. In this work, we discuss the fundamental aspects that can contribute to thermal hysteresis and the strategies that we are developing to at least partially overcome the hysteresis problem in some selected classes of magnetocaloric materials with large application potential. In doing so, we refer to the most relevant classes of magnetic refrigerants La–Fe–Si-, Heusler- and Fe2P-type compounds.

Multiferroic Clusters: A New Perspective for Relaxor‐Type Room‐Temperature Multiferroics
L. F. Henrichs, O. Cespedes, J. Bennett, J. Landers, S. Salamon, C. Heuser, T. Hansen, T. Helbig, O. Gutfleisch, D. C. Lupascu, H. Wende, W. Kleemann and A. J. Bell (2016)
Advanced Functional Materials, 26(13), 2111-2121

Multiferroics are promising for sensor and memory applications, but despite all efforts invested in their research no single‐phase material displaying both ferroelectricity and large magnetization at room‐temperature has hitherto been reported. This situation has substantially been improved in the novel relaxor ferroelectric single‐phase (BiFe0.9Co0.1O3)0.4–(Bi1/2K1/2TiO3)0.6, where polar nanoregions (PNR) transform into static‐PNR as evidenced by piezoresponse force microscopy (PFM) and simultaneously enable congruent multiferroic clusters (MFC) to emerge from inherent strongly magnetic Bi(Fe,Co)O3 rich regions as verified by magnetic force microscopy (MFM) and secondary ion mass spectrometry. The material’s exceptionally large Néel temperature TN = 670 ± 10 K, as found by neutron diffraction, is proposed to be a consequence of ferrimagnetic order in MFC. On these MFC, exceptionally large direct and converse magnetoelectric (ME) coupling coefficients, α ≈ 1.0 × 10−5 s m−1 at room‐temperature, are measured by PFM and MFM, respectively. It is expected that the non‐ergodic relaxor properties which are governed by the Bi1/2K1/2TiO3 component to play a vital role in the strong ME coupling, by providing an electrically and mechanically flexible environment to MFC. This new class of non‐ergodic relaxor multiferroics bears great potential for applications. Especially the prospect of a ME nanodot storage device seems appealing.

First-Order Reversal Curve (FORC) Analysis of Magnetocaloric Heusler-Type Alloys
V. Franco, T. Gottschall, K. P. Skokov, and O. Gutfleisch (2016)
IEEE Magnetics Letters, PP (99) p. 1.

The thermomagnetic hysteresis loops of a Ni45.7Mn36.6In13.5Co4.2Heusler-type alloy exhibiting inverse magnetocaloric effect were studied with the help of first-order reversal curves (FORC). These have been measured using two different protocols (either upon heating or cooling the sample) and using different applied magnetic fields. For proper comparison, FORC distributions were shifted according to the field dependent center of the M(T) hysteresis loop, which follows a linear trend. The qualitative behavior of FORC distributions remains the same, allowing their use for fingerprinting the transition, while there is a shift of their maxima along the hysteretic temperature axis and their distributions also get broader along the interaction temperature axis with increasing magnetic field. This was evidence that FORC distributions are dependent on the intensive variables temperature and field. As a consequence, it is necessary to obtain them for different temperatures and fields in order to accurately model the transition.