Department of Engineering Sciences

Magnetic materials

Within the Magnetic materials research group we are studying a wide range of magnetic materials with properties that make them interesting for applications in biotechnology, energy technology and spintronics. We combine magnetic nanoparticles with molecular tools to develop simple yet sensitive diagnostic methods that provide rapid patient responses; we study new permanent magnet materials without content of rare earth metals which can be used in renewable energy sources and magnetocaloric materials that can be used to build energy-smart refrigerators and heat pumps; and we are examining microwave properties of magnetic films and micro / nanostructured magnetic surfaces with the intent to tailor material properties for spintronic applications. We also have a more basic research-oriented activity in which we study mesocrystals composed of magnetic nanoparticles, frustrated magnetic systems and strongly correlated electron systems.

Ongoing research areas

Read our publications

Biomagnetism

There is an increasing interest within society, industry and academia to develop new user-friendly, fast and low-cost biosensor platforms which can be used for various applications such as veterinary and human disease diagnostics and detection of pathogenic bacteria and virus relating to food safety, food production and drinking water quality assessment.

Biosensor principles relying on the use of magnetic nanoparticles (beads) have a considerable potential to be fundament of such platforms. Furthermore, it is desirable to integrate and automate all steps in the bioassay (target recognition, labelling and read-out) in a single chip-based unit with integrated microfluidics; this in denoted lab-on-a-chip concept.

For molecular recognition of the pathogens within this project advanced molecular tools such as padlock probes, rolling circle amplification and circle-to-circle amplification are used. In this assay protocol macromolecular coils of single-stranded DNA having a repeating sequence are formed. Magnetic nanobeads with Brownian relaxation behaviour are conjugated with detection probes complementary to a part of the repeating sequence in the DNA coils. Thereby, the nanoparticles will hybridize to the DNA coils and experience a huge increase in hydrodynamic size. We are also evaluating immuno-magnetic detection strategies (antibodies attached to the nanoparticles).

Within this project an optomagnetic setup consisting of a Blu-ray laser, AC magnetic excitation coils and a photodetector (see illustration) is mainly used as read-out unit. The excitation field gives rise to a frequency-dependent modulation of the transmitted light intensity. Magnetic nanoparticles with an increased hydrodynamic size will have a different optomagnetic response than free nanoparticles which can be used for target quantification.

An alternative read-out platform under evaluation is based on magnetophoresis. Micron-sized magnetic beads functionalized with probes are mixed together with the sample containing the target and the mixture is injected to a cell containing a chip with a gold detection area having transport lines made of ellipsoid-shaped magnetic elements (see illustration). By applying a rotating magnetic field the beads will move along the lines. Probes are attached on the detection area so that beads are immobilized on the area in presence of target (sandwich assay). Target quantification is then accomplished by counting the number of beads immobilized on the detection area in a standard optical microscope.

magnetophoresis sensor system

At the end of the project we intend to achieve a prototype of a complete detection platform with integrated and automated sample preparation.

The biosensor platform developed within this project has a great potential to be used in resource-poor regions of the world for cost-efficient, rapid and user-friendly detection of pathogens relating to food safety and drinking water quality screening. Pathogens of particular relevance for the projects are e.g. Salmonella, Campylobacter and E. coli.

Contanct person and principal investigator: Dr. Mattias Strömberg, Division of Solid State Physics, Uppsala University

Participating/collaborating research groups:

  • Uppsala university, Division of Solid State Physics, Experimental Magnetism Group: Mattias Strömberg (PI), Tian Bo (PhD student) and Changgang Xu (postdoc). Mainly responsible for running the project.

  • Stockholm University/Science for Life Laboratory:Mats Nilsson and Annika Ahlford. Contribute by knowledge around advanced molecular tools and microfluidic sample preparation.

  • Technical University of Denmark: Mikkel Fought Hansen and Marco Donolato. Development of chip-based magnetic microsensor systems (planar Hall effect, optomagnetic) with integrated microfluidics.

  • Swedish National Veterinary Institute: Mikael Leijon and others. Knowledge on pathogens relating to veterinary medicine and food industry.

  • Uppsala University, Division of Nanotechnology and Functional Materials: Maria Strømme och Teresa Zardàn Gòmez de la Torre.

  • Uppsala University, Department of Medical Sciences, Clinical Microbiology and Infectious Medicine.

Financier: Formas, young researcher grant (project number 221-2012-444)

Project period: 2013-06-01 to 2017-12-31 (end date preliminary)

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Frustrated magnetic systems

Interatomic forces strive to condense matter to states of perfect order. However, randomness and defects, quenched disorder, necessarily accompany the atomic structural order of condensed matter. Magnetic materials mirror and illustrate consequences of these counteracting influences. Strong direct exchange interaction between the atoms in iron causes ferromagnetic order well above room temperature but the degree of atomic disorder in the crystal structure governs the magnetic functionality of iron (and iron based alloys) – allowing soft (transformer sheet) or hard (permanent magnet) magnetic behaviour. Many alloys and compounds possess more complex interaction patterns between the magnetic atoms or entities; in some materials competing ferro- and antiferromagnetic interactions occur which give rise to frustration (see figure) conveying conflicting information to the atomic magnetic moments on the direction to point in and low temperature states with disordered order: Spin Glasses, Superspin Glasses, frustrated ferro- or antferromagnets. These systems have complex magnetic properties including non-equilibrium glassy magnetic dynamics which state several yet unsolved physical questions. We address such questions experimentally by studies of atomic, magnetic particle or nanostructured model systems of frustrated and disordered magnets.

Frustrated magnetic systems
Satisfied (A,B) and frustrated (C,D) interaction patterns between
three spins ((+) denotes ferromagnetic and (–) denotes
antiferromagnetic interaction).

Contact: Professor Per Nordblad or senior lecturer Roland Mathieu

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Magnetic materials for energy and raw material economization

Rare Earth Metal-Free Permanent Magnetic Materials

The development of environmental friendly energy sources largely relies upon the access of high quality permanent magnets (PM). A wind turbine, e.g., uses 300 kg of magnets. Today, the most powerful PMs are based on rare-earth (RE) metals. However, export quotas posed by the main producer China, have resulted in a dramatic prize increase of RE materials. This has stimulated intense effort towards the search for new non-RE containing PM.

Figures of merit for PMs are the maximum energy product (BH)max and the coercivity, Hc. (BH)max is the highest available energy that can be stored in the PM and it is dependent on the remanent magnetic induction, Br. Hc gives the robustness of the PM against demagnetization due to external magnetic fields. Currently, the best values are (BH)max = 400 kJ/m3 and Hc = 2 MA/m, obtained for RE based PMs. Equally important is the critical temperature, Tc, above which the material ceases to be magnetic. There exist a number of families of PMs as shown in the figure below. The Nd2Fe14B magnets are by far the best while ferrites are the most widely used due to their low price. However, since ferrites have low Br, much more material is needed in applications compared to RE magnets.

Remanent magnetic induction and coercivity for different permanent magnet materials

One objective in this project is to find a PM with (BH)max above 100 kJ/m3, which is much higher than ferrite PMs but lower than the best RE based PM. This will require Br close to 0.7 T and Hc close to 1 MA/m. The targeted Tc is > 600 K. These objectives suggest that we should begin our search for new PM materials by looking for low symmetry, iron rich intermetallic compounds. The low symmetry materials are needed since they are known to exhibit large magnetic anisotropy (which is a prerequisite for a large Hc). Our quest is conducted in a cross-disciplinary effort, involving experimental chemistry and physics, materials theory and industry.

One material investigated is (Fe1-xCox)2B, having uniaxial magnetic anisotropy, Tc > 900 K and a potential Br = 1.3T. We will also study tetragonally distorted iron carbides. Theoretically, it has recently been shown that alloying additions have marked impact on the crystal lattice, which possibly would increase the magnetic anisotropy. But for the moment it is MnAl compounds that attract our main interest. Mn has a large magnetic moment per atom, but the moments are antiferromagnetically coupled. Forming an alloy with Al, the distance  between the Mn atoms increases and the interaction between the Mn moments turns ferromagnetic. Adding e.g. carbon to the alloy stabilizes the structure. The research project has already produced Mnal-based materials with highly promising magnetic properties.

The outcome of this project will be a novel RE-free PM material with the capability of being formed using metallurgical production routes.

Contact: Senior lecturer Klas Gunnarsson

Magnetocaloric Materials

Cooling and heating systems using vapor-compression techniques totally dominate the market for refrigeration, heating, ventilation and air-conditioning (RHVAC). Alternative techniques that are more energy efficient and do not use greenhouse gases (hydrofluorocarbon (HFC) refrigerants) are requested. Magnetocaloric thermodynamic processes are more efficient than vapor-compression processes and do not need HFC refrigerants. Energy savings of more than 20% would be gained if magnetocaloric systems were to substitute current RHVAC systems.  

In an inter-institutional research project supported by the Swedish Research Council (VR) we are investigating and searching for suitable magnetic materials for magnetocaloric applications. The system (Fe1-yMny)2P1-xSix experiences a composition dependent 1st order ferro- to paramagnetic transition near room temperature (cf. figure for y=0.5). Extensive research on this material system in Uppsala and elsewhere indicates that certain compositions of (Fe1-yMny)2P1-xSix will have applicable magnetocaloric properties.

Magnetocaolric properties
Phase diagram of FeMn(P1-xSix) showing e.g. the  Curie temperature Tc and magnetization M as a function of  x. Figure from V. Höglin et al. RSC Adv. 2015, 5, 8278.

Contact: Professor Per Nordblad

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Magnetic nanoparticles

A magnetic fluid is a dispersion of superparamagnetic particles that remain dispersed even in very strong magnetic fields. Magnetic fluids, or ferrofluids, were originally developed by NASA in the 1960s as an attempt to manipulate fluids in space, but later found use in many everyday technical applications, including shaft seals, lubricators, dampeners, and coolants.

A magnetic nanoparticle could either contain a single magnetic nanocrystal in a magnetic or non-magnetic shell, or several nanocrystals bound together in a non-magnetic matrix - so-called multi-core - as illustrated in Figure 1 A and B respectively. The colloidal stability of the nanoparticle dispersion is very important when using nanoparticles for biotechnological applications. Also, when used in biotechnological applications the surface of the nanoparticles is usually functionalized with e.g. detection probes. In order to prevent agglomeration and facilitate surface functionalization, the nanoparticles are coated, illustrated in Figure 1 C. This coating also protects the nanocrystals from oxidation and/or erosion.

Magnetic nanoparticle
Figure 1 A) Single-core magnetic nanoparticle. B) Multi-core magnetic nanoparticle. C) Multi-core magnetic nanoparticle with surface functionalization
Different nanoparticles
Figure 2 Six different types/sizes of single-core iron-oxide nanoparticles.

NanoMag

The objectives of the EU financed NanoMag project are to standardize, improve and redefine analyzing methods of magnetic nanoparticles. Using improved manufacturing technologies, synthesized magnetic nanoparticles with specific properties will be analyzed with a multitude of characterization techniques (focusing on both structural as well as magnetic properties).

NanoMag brings together leading experts in; manufacturing of magnetic single- and multi-core nanoparticles, analyzing and characterization of magnetic nanostructures, and national metrology institutes. In the NanoMag consortium we have gathered partners within research institutes, universities and metrology institutes, all carrying out front end research and developing applications in the field of magnetic nanoparticles.

Magnetic nanorods

This postdoctoral project will be a part of a larger Formas young researcher project aiming at developing a magnetic biosensor platform for veterinary medicine applications where the readout relies on changes in frequency-dependent optomagnetic properties of magnetic nanoparticles suspended in liquid. In order to optimize the detection sensitivity of the platform the properties of the nanoparticles, such as their morphology and type of magnetic material, should be optimized for obtaining maximal optomagnetic response, which will be the main focus of the postdoctoral work. The magnetic material could for instance be iron oxide (magnetite/maghemite) or a ferromagnetic element such as cobalt. In the project, rod shaped and/or oval shaped nanoparticles with tunable dimensions will be solvothermally synthesized and characterized with respect to static and dynamic magnetic and optomagnetic (anisotropic light scattering) properties. Later on in the project, the nanoparticles found to have the optimal properties for the biosensor application will be surface-modified to allow for attachment of different types of biomolecular probes, e.g. DNA or antibodies.

Mesocrystals

Mesocrystal made of nanoparticles
Figure 3 Mesocrystal with iron-oxide nanoparticles as building block

Mesocrystals constitute a special class of crystals. In conventional crystalline materials, the subunits correspond to individual atoms in perfect 3D order. In the mesocrystals, however, the subunits are still ordered but could instead be magnetic nanoparticles.  Mesocrystals composed of polydisperse nanocrystals, with little or no spatial correlation at the nm scale, are found in numerous biominerals e.g., sea urchin spines and plankton shells.

In this work, monodisperse iron oxide nanoparticles shaped as cubes with average sizes of 9.6 and 12.6 nm are synthesized using thermal decomposition, after which they are assembled on a solid surface in a lateral magnetic field using an evaporation process. The nanoparticles spontaneously form mesocrystals in the form of needle-shaped arrays (diameter 10 µm) with very large aspect ratios. The temperature dependent AC-magnetic properties are measured parallel and transverse to the long axes of the needle shaped arrays. The measured data deviate from those of obtained from measurements performed on non-interacting particles in dilute dispersions indicating that the properties of the needle-shaped arrays are collective in origin.

Contact: Professor Peter Svedlindh

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Transition metal oxides and strongly correlated electrons

The order or disorder of the spin and orbital degrees of freedom of electrons in transition metal oxides is responsible for a wide variety of phenomena [1]. For example, the colossal magnetoresistance (CMR) effect observed in manganese oxide compounds with an ABO3 perovskite structure (e.g. (La,Sr)MnO3 or (Pr,Ca)MnO3) results of the complex spin-orbital coupled state of the manganese lattice [2].

In materials with complex orbital order and/or spin structures, an electric polarization may be induced, making the material (in a large sense) multiferroic [3]. Such materials display magnetoelectric effects, permitting e.g. the control of the magnetization of a material by means of an electric field or reciprocally the control of its polarization by means of a magnetic field, and are thus very attractive for application in many devices [4].

There are however few spintronic magnetoresistive oxides or magnetoelectric multiferroics which can be used at room temperature. We are thus investigating new materials, to be able to explore further, and extract new fundamental information on the mechanisms bringing forth those properties. We are interested in novel bulk materials, and well as nanoparticles systems and nanocomposites, that we study with colleagues from Uppsala and abroad; a list of collaborators and articles can be found on this page as well as here. Recent results are illustrated below.

Nanoscale homogeneity in the ferromagnetic insulator (La,Ca)MnO3.
Nanoscale homogeneity in the ferromagnetic insulator (La,Ca)MnO3. Adapted from P. Anil Kumar et al., Phys. Rev. X 4, 011037 (2014).
New M3TeO6 family of magnetoelectric multiferroics: (Ni,A)3BO6 (A=Ni, B=Te or A=In,Sc, B=Sb).
New M3TeO6 family of magnetoelectric multiferroics: (Ni,A)3BO6 (A=Ni, B=Te or A=In,Sc, B=Sb). Adapted from S. A. Ivanov et al., Chem. Mater. 25, 935 (2013).
Ferrimagnetic ilmenite and antiferromagnetic perovskite states of Mn2FeSbO6.
Ferrimagnetic ilmenite and antiferromagnetic perovskite states of Mn2FeSbO6. Adapted from R. Mathieu et al., Phys. Rev. B 87, 014408 (2013).
Multiglass state and room-temperature magnetodielectric effects in La2NiMnO6.
Multiglass state and room-temperature magnetodielectric effects in La2NiMnO6. Adapted from D. Choudhury et al., Phys. Rev. Lett. 108, 127201 (2012).

References:
[1] “Complexity in Strongly Correlated Systems”, E. Dagotto, Science 309, 257 (2005).
[2] “Orbital Physics in Transition-Metal Oxides”, Y. Tokura and N. Nagaosa, Science 288, 462 (2000).
[3] “Classifying multiferroics: Mechanisms and effects ”, D. Khomskii, Physics 2, 20 (2009).
[4] “Multifunctional Magnetoelectric Materials for Device Applications”, N. Ortega, A. Kumar, J. F. Scott, and R. S. Katiyar, preprint; available at http://arxiv.org/abs/1403.1838

Contact: senior lecturer Roland Mathieu

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Magnetic materials with tailored properties for applications in spintronics and magnonics

This activity, which is supported by Knut and Alice Wallenberg foundation, involves collaboration between the departments of Engineering Sciences and Physics and Astronomy at the Ångström laboratory, as well as with the group headed by Prof. Johan Åkerman at the University of Gothenburg. The activity is a cross-disciplinary project to study the spin dynamics of magnetic materials, using experimental and theoretical tools.

We are studying low damping magnetic thin films and multilayers with either in-plane or out-of-plane magnetization. The present focus is on three different systems; i) half-metallic Co-based Heusler alloys, ii) half-metallic Fe3O4 and iii) binary Mn- Al alloys. The objectives for all systems is to tailor properties being of importance for spintronic and magnonic applications, in particular the saturation magnetization, the effective magnetic anisotropy and the damping of spin waves. As a result of this work, we will be able to by choice of material tune the intrinsic resonance frequencies of the spin dynamics from a few GHz to frequencies well above 100 GHz. Moreover, by nanostructuring of films and multilayers into 2D magnonic crystals (MCs) we will be able to study propagation of different spin wave modes. The objective here is to design MCs with waveguiding capabilities and to achieve group velocities of propagating spin waves spanning the range from a few to several tens of km/s. By using optimized half-metallic magnetic materials in combination with nonmagnetic metallic and semiconducting materials with large spin-orbit coupling, we are searching for material combinations exhibiting enhanced Spin Hall Effect (SHE). The inverse SHE (ISHE) also exists, where e.g. spin pumping from a FM layer generates spin currents in the nonmagnetic layer that due to spin-orbit coupling is converted to a charge current. We will use the ISHE to determine the spin Hall angle for materials of interest.

Lastly, we will use spin pumping from ferromagnetic strips to inject pure spin currents into graphene, which will be used for transportation of spin currents.

Scheme for a material with strong spinn-electron movement correlation

Contac: Professor Peter Svedlindh

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