2017 Publications

The 2017 Magnetism Roadmap
D Sander et al (2017)
Journal of Physics D: Applied Physics, Volume 50, Number 36

The 2017 Magnetism Roadmap edition consists of 14 sections, each written by an expert in the field and addressing a specific subject on two pages. The first material focused pillar of the 2017 Magnetism Roadmap contains five articles, which address the questions of atomic scale confinement, 2D, curved and topological magnetic materials, as well as materials exhibiting unconventional magnetic phase transitions. The second pillar also has five contributions, which are devoted to advances in magnetic characterization, magneto-optics and magneto-plasmonics, ultrafast magnetization dynamics and magnonic transport. The final and application focused pillar has four contributions, which present non-volatile memory technology, antiferromagnetic spintronics, as well as magnet technology for energy and bio-related applications. As a whole, the 2017 Magnetism Roadmap article, just as with its 2014 predecessor, is intended to act as a reference point and guideline for emerging research directions in modern magnetism.

Generalization of magnetostatic method of moments for thin layers with regular rectangular grids
R. Szewczyk (2017)
Acta Physica Polonica A 131, 845.

Possibilities of the modeling of the flux density distribution in thin films are significantly limited using the finite elements method due to the fast increase of the number of tetrahedral elementary cells with reduction of the thickness. For this reason, method of the moments is very important alternative for finite elements method in the case of thin layers, where layer’s thickness should be considered. Method of the moments overcomes this barrier, due to the possibility of operation on uniform grids with limited number of cells. Moreover, in opposite to the finite elements method, the method of the moments requires solving of the well defined linear equations, instead of the set of ill-posed differential equations. Paper presents the generalization of the method of the moments for thin layers with given thickness. Layers are defined as the 2D rectangular grids. Within the generalization, four key equations describing the influence of rectangular cell’s border on the magnetization of cells are stated. On the base of these dependences, the set of 2NM linear equations was determined, where N and M are the numbers of rectangular cells in the rows and columns of regular grid. Finally, the set of linear equations is solved and magnetic flux density distribution in the thin layer is calculated.

Coexistence of long-range antiferromagnetic order and slow relaxation of the magnetization in the first lanthanide complex of a 1,2,4-benzotriazinyl radical
I.S. Morgan, A. Mansikkamäki, M. Rouzières, R. Clérac and H.M. Tuononen (2017)
Dalton Transactions, 46(38), 12790-12793

The first lanthanide complex of a 1,2,4-benzotriazinyl radical (1), Dy(1)(tbacac)3 (2, tbacac = 2,2,6,6-tetramethyl-3,5-heptane-dionato), was synthesised and found to have an antiferromagnetically ordered ground state with a metamagnetic phase diagram and a critical field of 0.91 T at 1.85 K. The application of a small dc field revealed the single-molecule magnet behaviour of 2, illustrating the coexistence of long-range antiferromagnetic order and slow relaxation of the magnetization.

A symmetric miniature diamond anvil cell for magnetic measurements on dense hydrides in a SQUID magnetometer
Adrien Marizy, Bastien Guigue, Florent Occelli, Brigitte Leridon and Paul Loubeyre (2017)
High Pressure Research, 37(4), 465-474

A new miniature diamond anvil cell was specifically designed to detect superconductivity using a SQUID (Superconducting QUantum Interference Device) magnetometer in dense hydrides directly synthesized by the reaction of hydrogen with a chemical element. The cell, made of a CuTi alloy, is fully symmetric with a very low magnetic background allowing the detection of the superconductivity of a sample as small as 3.4 × 104 µm3 without background subtraction. DC measurements or AC measurements in a Magnetic Property Measurement System 3 SQUID magnetometer from Quantum Design could be performed at temperatures as low as 3 K. This high pressure cell is inserted in a modified conventional membrane diamond anvil cell to be driven for hydrogen gas loading and for fine pressure increase before magnetic measurements are performed. To synthetize and structurally characterize the superconducting sample, a 21° optical and 8.6° X-ray acceptance angle allows one to perform laser heating and X-ray diffraction at the same time. A first measurement is shown on the PdH system.

Enhanced annealing stability and perpendicular magnetic anisotropy in perpendicular magnetic tunnel junctions using W layer
J. Chatterjee, R.C. Sousa, N. Perrissin, S. Auffret, C. Ducruet, B. Diény (2017)
Applied Physics Letters, 110(20), 202401

The magnetic properties of the perpendicular storage electrode (buffer/MgO/FeCoB/Cap) were studied as a function of annealing temperature by replacing Ta with W and W/Ta cap layers with variable thicknesses. W in the cap boosts up the annealing stability and increases the effective perpendicular anisotropy by 30% compared to the Ta cap. Correspondingly, an increase in the FeCoB critical thickness characterizing the transition from perpendicular to in-plane anisotropy was observed. Thicker W layer in the W(t)/Ta 1 nm cap layer makes the storage electrode highly robust against annealing up to 570 °C. The stiffening of the overall stack resulting from the W insertion due to its very high melting temperature seems to be the key mechanism behind the extremely high thermal robustness. The Gilbert damping constant of FeCoB with the W/Ta cap was found to be lower when compared with the Ta cap and stable with annealing. The evolution of the magnetic properties of bottom pinned perpendicular magnetic tunnel junctions (p-MTJ) stack with the W2/Ta1 nm cap layer shows back-end-of-line compatibility with increasing tunnel magnetoresistance up to the annealing temperature of 425 °C. The pMTJ thermal budget is limited by the synthetic antiferromagnetic hard layer which is stable up to 425 °C annealing temperature while the storage layer is stable up to 455 °C.

Investigation of magnetic coupling in FePt/spacer/FePt trilayers
A. Kaidatzis, G. Giannopoulos, G. Varvaro, G. Dimitrakopulos, V. Psycharis, J.M. Garcia-Martin, A.M Testa, G. Barucca, T. Karakostas, P. Komninou, D. Niarchos (2017)
Journal of Physics D: Applied Physics, 50(44), 445002

The effect of different spacer materials (MgO, W, and Pt) on the magnetic coupling in FePt/spacer/FePt trilayers has been carefully investigated. MgO results in magnetically coupled FePt layers with perpendicular magnetic anisotropy (PMA); W gives rise to a magnetically coupled system consisting of layers with PMA and in-plane magnetic anisotropy whereas Pt results in magnetically decoupled FePt layers with PMA. The trilayer microstructure is essential for explaining the obtained results. The growth mode of the top FePt layer is strongly affected by the underlying non-magnetic spacer, with occurrence of different morphologies; in particular, L10 FePt islands grow on MgO, a continuous FePt layer with fcc crystal structure is obtained on W, whereas a continuous layer with L10 structure is observed when the top layer is deposited on Pt.

Single nanoparticles magnetization curves by controlled tip magnetization magnetic force microscopy
L. Angeloni, D. Passeri, D.Peddis, D. Mantovani and Marco Rossi (2017)
Nanoscale, 9(45), 18000-18011

The development of high spatial resolution and element sensitive magnetic characterization techniques to quantitatively measure magnetic parameters of individual nanoparticles (NPs) and deeply understand and tune their magnetic properties is a hot topic in nanomagnetism. Magnetic force microscopy (MFM), thanks to its high lateral resolution, appears as a promising technique for the magnetic characterization of single nano-sized materials although it is still limited by some drawbacks, especially by the presence of electrostatic artifacts. Recently, these limitations have been overcome by the development of a particular MFM based technique called controlled magnetization – MFM (CM-MFM) allowing, in principle, a quantifiable correlation between the measured magnetic signal and the magnetization of the object under investigation. Here we propose an experimental procedure, based on the use of CM-MFM technique, to measure the magnetization curve of single magnetic NPs individuating their saturation magnetization, magnetic field, and coercivity. We measured, for the first time, the magnetization curves of individual Fe3O4 nanoparticles with diameters in the range of 18–32 nm by using a MFM instrument. Results are in very good agreement with the quantitative data obtained by SQUID analysis on a macroscopic sample, showing the high potential of the technique in the field of nanomagnetometry.

Remanence plots as a probe of spin disorder in magnetic nanoparticles
J.A. De Toro, M.Vasilakaki, Su S. Lee, M. S. Andersson, P. S. Normile, N. Yaacoub, P. Murray, P. Muñiz, D. Peddis, Mathieu, K.Liu, J. Geshev, K. N. Trohidou, J.Nogués (2017)
Chemistry of Materials, 29(19), 8258-8268

Remanence magnetization plots (e.g., Henkel or δM plots) have been extensively used as a straightforward way to determine the presence and intensity of dipolar and exchange interactions in assemblies of magnetic nanoparticles or single domain grains. Their evaluation is particularly important in functional materials whose performance is strongly affected by the intensity of interparticle interactions, such as patterned recording media and nanostructured permanent magnets, as well as in applications such as hyperthermia and magnetic resonance imaging. Here, we demonstrate that δM plots may be misleading when the nanoparticles do not have a homogeneous internal magnetic configuration. Substantial dips in the δM plots of γ-Fe2O3 nanoparticles isolated by thick SiO2 shells indicate the presence of demagnetizing interactions, usually identified as dipolar interactions. Our results, however, demonstrate that it is the inhomogeneous spin structure of the nanoparticles, as most clearly evidenced by Mössbauer measurements, that has a pronounced effect on the δM plots, leading to features remarkably similar to those produced by dipolar interactions. X-ray diffraction results combined with magnetic characterization indicate that this inhomogeneity is due to the presence of surface structural (and spin) disorder. Monte Carlo simulations unambiguously corroborate the critical role of the internal magnetic structure in the δM plots. Our findings constitute a cautionary tale on the widespread use of remanence plots to assess interparticle interactions as well as offer new perspectives in the use of Henkel and δM plots to quantify the rather elusive inhomogeneous magnetization states in nanoparticles.

Determination of Blocking Temperature in Magnetization and Mössbauer Time Scale: A Functional Form Approach
G. Concas, F. Congiu , G. Muscas , D. Peddis (2017)
The Journal of Physical Chemistry C, 121(30), 16541-16548

We studied the temperature dependence of the magnetization in an ensemble of monodomain nanoparticles both with dc magnetometry and Mössbauer spectroscopy. The analytical form of the temperature dependence is given by the complementary cumulative distribution function. This allows to determine the magnetization blocking temperatures of the sample by a fitting procedure. It is possible to calculate the Mössbauer blocking temperature by a single spectrum and the dc magnetization blocking temperature by two points of the thermoremanent magnetization curve, thus with a large reduction of the experimental work. The method may be used for particles with not too strong interactions, such happens in the Fe28 sample and not for samples with strong interactions as N30; it may be used for interparticle interaction energies up to 2 yJ and not for energies larger than 60 yJ. This method of analysis of the data should be used in the future work concerning the thermoremanent magnetization and Mössbauer spectra of magnetic nanoparticles.

Folate targeted coated SPIONs as efficient tool for MRI
C. Scialabba, R. Puleio, D. Peddis, G. Varvaro, P. Calandra, G. Cassata, L. Cicero, M. Licciardi, G. Giammona (2017)
Nano Research, 10(9), 3212-3227

The development of more sensitive diagnostic tools allowing an early-stage and highly efficient medical imaging of tumors remains a challenge. Magnetic nanoparticles seem to be the contrast agents with the highest potential, if properly constructed. Therefore, in this study, hybrid magnetic nanoarchitectures were developed using a new amphiphilic inulin-based graft copolymer (INU-LAPEG-FA) as coating material for 10-nm spinel iron oxide (magnetite, Fe3O4) superparamagnetic nanoparticles (SPION). Folic acid (FA) covalently linked to the coating copolymer in order to be exposed onto the nanoparticle surface was chosen as the targeting agent because folate receptors are upregulated in many cancer types. Physicochemical characterization and in vitro biocompatibility study was then performed on the prepared magnetic nanoparticles. The improved targeting and imaging properties of the prepared FA-SPIONs were further evaluated in nude mice using 7-Tesla magnetic resonance imaging (MRI). FA-SPIONs exhibited the ability to act as efficient contrast agents in conventional MRI, providing a potential nanoplatform not only for tumor diagnosis but also for cancer treatment, through the delivery of anticancer drug or locoregional magnetic hyperthermia.

Robust Ferromagnetism of Chromium Nanoparticles Formed in Superfluid Helium
S. Yang, C. Feng, E. Latimer, D. Spence, A. Ellis, C. Binns, D. Peddis, S. S. Dhesi, L. Zhang, Y. Zhang, K. N. Trohidou, M. Vasilakaki, I. MacLaren (2017)
Advanced Materials, 29(1)

Chromium nanoparticles are formed using superfluid helium droplets as the nanoreactors, which are strongly ferromagnetic. The transition from antiferromagentism to ferromagnetism is attributed to atomic‐scale disorder in chromium nanoparticles, leading to abundant unbalanced surface spins. Theoretical modeling confirms a frustrated aggregation process in superfluid helium due to the antiferromagnetic nature of chromium.

Evidence of tetragonal distortion as the origin of the ferromagnetic ground state in gamma-Fe nanoparticles
V. Augustyns, K. van Stiphout, V. Joly, T. A. L. Lima, G. Lippertz, M. Trekels, E. Menéndez, F. Kremer, U. Wahl, A. R. G. Costa, J. G. Correia, D. Banerjee, H. P. Gunnlaugsson, J. von Bardeleben, I. Vickridge, M. J. Van Bael, J. Hadermann, J. P. Araújo, K. Temst, A. Vantomme, and L. M. C. Pereira (2017)
Phys. Rev. B 96, 174410

γ−Fe and related alloys are model systems of the coupling between structure and magnetism in solids. Since different electronic states (with different volumes and magnetic ordering states) are closely spaced in energy, small perturbations can alter which one is the actual ground state. Here, we demonstrate that the ferromagnetic state of γ−Fe nanoparticles is associated with a tetragonal distortion of the fcc structure. Combining a wide range of complementary experimental techniques, including low-temperature Mössbauer spectroscopy, advanced transmission electron microscopy, and synchrotron radiation techniques, we unambiguously identify the tetragonally distorted ferromagnetic ground state, with lattice parameters a = 3.76(2) Å and c = 3.50(2) Å, and a magnetic moment of 2.45(5) μB per Fe atom. Our findings indicate that the ferromagnetic order in nanostructured γ−Fe is generally associated with a tetragonal distortion. This observation motivates a theoretical reassessment of the electronic structure of γ−Fe taking tetragonal distortion into account.

Interface induced out-of-plane magnetic anisotropy in magnetoelectric BiFeO3-BaTiO3 superlattices
V. Lazenka, J. K. Jochum, M. Lorenz, H. Modarresi, H. P. Gunnlaugsson, M. Grundmann, M. J. Van Bael, K. Temst, and A. Vantomme (2017)
Appl. Phys. Lett. 110, 092902

Room temperature magnetoelectric BiFeO3-BaTiO3 superlattices with strong out-of-plane magnetic anisotropy have been prepared by pulsed laser deposition. We show that the out-of-plane magnetization component increases with the increasing number of double layers. Moreover, the magnetoelectric voltage coefficient can be tuned by varying the number of interfaces, reaching a maximum value of 29 V/cm Oe for the 20×BiFeO3-BaTiO3 superlattice. This enhancement is accompanied by a high degree of perpendicular magnetic anisotropy, making the latter an ideal candidate for the next generation of data storage devices.

Synthesis and magnetic properties of the thin film exchange spring system of MnBi/FeCo
S. Sabet, E. Hildebrandt and L. Alff (2017)
Journal of Physics: Conference Series (Vol. 903, No. 1, p. 012032). IOP Publishing

Manganese bismuth thin films with a nominal thickness of ~40 nm were grown at room temperature onto quartz glass substrate in a DC magnetron sputtering unit. In contrast to the usual multilayer approach, the MnBi films were deposited using a single sputtering target with a stoichiometry of Mn55Bi45 (at. %). A subsequent in-situ annealing step was performed in vacuum in order to form the ferromagnetic LTP of MnBi. X-ray diffraction confirmed the formation of a textured LTP MnBi hard phase after annealing at 330 °C. This film shows a maximum saturation magnetization of 530 emu/cm3, high out-of-plane coercivity of 15 kOe induced by unreacted bismuth. The exchange coupling effect was investigated by deposition of a second layer of FeCo with 1 nm and 2 nm thickness onto the LTP MnBi films. The MnBi/FeCo double layer showed as expected higher saturation magnetization with increasing thickness of the FeCo layer while the coercive field remained constant. The fabrication of the MnBi/FeCo double layer for an exchange spring magnet was facilitated by deposition from a single stoichiometric target.

Low-Temperature Phase c-axis Oriented Manganese Bismuth Thin Films With High Anisotropy Grown From an Alloy Mn55Bi45 Target
S. Sabet, E. Hildebrandt, F. M. Römer, I. Radulov, H. Zhang, M. Farle, and L. Alff (2017)
IEEE Transactions on Magnetics, 53(4), 7776882

Manganese bismuth thin films were deposited from a Mn55Bi45 (at.%) alloy target onto glass substrates at room temperature using dc magnetron sputtering. The ferromagnetic low-temperature phase (LTP) of MnBi was formed through a subsequent vacuum annealing step. The resulting thin films were highly c-axis textured. Magnetic measurement shows a maximum saturation magnetization of 600 emu/cm3 (0.60 MA/m). A magnetic uniaxial anisotropy energy density of ~1.86·107 erg/cm3 (~1.86 MJ/m3) was measured by torque magnetometry. The coercive field has a positive temperature coefficient and reaches 12 kOe (1.2 T) and 14 kOe (1.4 T) at 300 K for the out-of-plane and in-plane direction, respectively. Density functional theory calculations have confirmed that the magnetocrystalline anisotropy energy increases with increasing temperature as a result of a spin-reorientation occurring around 100 K. Growing LTP MnBi thin films directly from an alloy Mn55Bi45 target is an important step toward facilitating the synthesis of multilayers for spintronics or in an exchange spring magnet configuration.

YCo5±x thin films with perpendicular anisotropy grown by molecular beam epitaxy
S. Sharma, E. Hildebrandt, S.U. Sharath, I. Radulov, and L. Alff (2017)
Journal of Magnetism and Magnetic Materials, 432, 382-386

The synthesis conditions of buffer-free (00l) oriented YCo5 and Y2Co17 thin films onto Al2O3 (0001) substrates have been explored by molecular beam epitaxy (MBE). The manipulation of the ratio of individual atomic beams of Yttrium, Y and Cobalt, Co, as well as growth rate variations allows establishing a thin film phase diagram. Highly textured thin films were stabilized with saturation magnetization of 517 emu/cm3 (0.517 MA/m), coercivity of 4 kOe (0.4 T), and anisotropy constant, K1, equal to erg/cm3 (0.53 MJ/m3). These magnetic parameters and the perpendicular anisotropy obtained without additional underlayers make the material system interesting for application in magnetic recording devices.

Multiscale Examination of Strain Effects in Nd-Fe-B Permanent Magnets
M. Yi, H. Zhang, O. Gutfleisch, and B.-X. Xu (2017)
Phys. Rev. Applied 8, 014011

We perform a combined first-principles and micromagnetic study on the strain effects in Nd-Fe-B permanent magnets. First-principles calculations on Nd2Fe14B reveal that magnetocrystalline anisotropy (K) is insensitive to deformation along the c axis, and that a−b in-plane shrinkage is responsible for K reduction. The predicted K is more sensitive to lattice deformation than the previous phenomenological model suggests. The biaxial and triaxial stress states have a greater impact on K. Negative K occurs in a much wider strain range in the a−b biaxial stress state. Micromagnetic simulations of Nd-Fe-B magnets using first-principles results show that a 3% to 4% local strain in a 2-nm-wide region near the interface around the grain boundaries and triple junctions leads to a negative local K and thus, remarkably, decreases the coercivity by about 60%, or 3 to 4 T. The local a−b biaxial stress state is more likely to induce a large loss of coercivity. In addition to the local stress states and the strain levels themselves, the shape of the interfaces and the intergranular phases also makes a difference. Smoothing the edge and reducing the sharp angle of the triple regions in Nd-Fe-B magnets would be favorable for a coercivity enhancement.

Atomic structure and domain wall pinning in samarium-cobalt-based permanent magnets
M. Duerrschnabel, M. Yi, K. Uestuener, M. Liesegang, M. Katter, H.-J. Kleebe, B. Xu, O. Gutfleisch and L. Molina-Luna (2017)
Nature Communicationsvolume 8, Article number: 54

A higher saturation magnetization obtained by an increased iron content is essential for yielding larger energy products in rare-earth Sm2Co17-type pinning-controlled permanent magnets. These are of importance for high-temperature industrial applications due to their intrinsic corrosion resistance and temperature stability. Here we present model magnets with an increased iron content based on a unique nanostructure and -chemical modification route using Fe, Cu, and Zr as dopants. The iron content controls the formation of a diamond-shaped cellular structure that dominates the density and strength of the domain wall pinning sites and thus the coercivity. Using ultra-high-resolution experimental and theoretical methods, we revealed the atomic structure of the single phases present and established a direct correlation to the macroscopic magnetic properties. With further development, this knowledge can be applied to produce samarium cobalt permanent magnets with improved magnetic performance.

Grain boundary diffusion of different rare earth elements in Nd-Fe-B sintered magnets by experiment and FEM simulation
K. Loewe, D. Benke, C. Kübel, T. Lienig, K. P. Skokov, O. Gutfleisch (2017)
Acta Materialia, 124, 421-429.

In the present work, we explore the influence of a surface-bulk coercivity gradient in Nd-Fe-B magnets produced by the Grain Boundary Diffusion Process (GBDP) on the overall coercivity. In our systematic and comprehensive study we diffused four different rare earth elements (Dy, Tb, Ce and Gd) in two different kinds of commercial Nd-Fe-B magnets, one very Dy-lean and one Dy-rich. By means of cutting the magnets into thin slices we obtain lateral coercivity profiles, from which diffusion constants are extracted. We find that in both magnets Tb diffuses significantly faster than Dy. The diffusion is generally slower in the Dy-lean magnet, which is attributed to the different chemistry and a smaller grain size. Ce diffuses slightly slower than Dy and the overall coercivity decrease is similar for Ce and Gd. With scanning electron microscopy it is revealed that, contrary to the magnets diffused with the heavy rare earths, the microstructure in the magnets treated with Ce show no (Nd,Ce)-Fe-B shells in the surface regions. While not of practical importance this allows some interesting insights into the metallurgy of (Nd,Ce)-Fe-B system. High-resolution scanning transmission electron microscopy coupled with electron probe microanalysis show the nano-scale distribution of Tb around the grain boundaries located in the bulk of the magnet. Finally, a simple model for the magnetization reversal in grain boundary diffusion processed gradient Nd-Fe-B magnets was developed and implemented into a FEM software. Our calculated demagnetization curves correspond very well for the Dy and Tb samples, but deviate significantly for Ce and Gd.