MnxGa1−x nanodots with high coercivity and perpendicular magnetic anisotropy

Significance Statement

Metastable tetragonal D022 phase of manganese-gallium exhibits unique attributes that can be exploited for a number of spintronic device applications. The material shows low saturation magnetization owing to the ferrimagnetic alignment of manganese on two inequivalent sub-lattices in the structure; the magnetization can be changed by altering the manganese content. Above all, it has low Gilbert damping, huge uniaxial magnetic anisotropy, spin polarization, and high Curie temperature. These material properties make it important for spin torque driven memories and devices including spin transfer torque MRAMs as well as spin transfer oscillators.

Both spin transfer torque MRAMs and spin transfer oscillators necessitate nano-structuring of the material used. Therefore, several fabrication studies have been done concerning many magnetic materials including D022 manganese-gallium. However, there are problems still outstanding. Studies of D022 manganese-gallium nanostructures where the nanostructures resulted from a 3D growth mode of the material led to multiphase particles or irregularly shaped particles were obtained.

Typical optical and sequential nano-lithographic methods have been used to generate a high degree of patterning regularity in a low area. However, these methods incur high costs when implemented in an industrial environment. Self-assembly of polymeric nano-spheres have been identified recently as an alternative to typical nano-lithographic processes. The method allows for easy patterning of a wide region. In order to obtain a high degree of hexagonal close packing in the course of self-assembly, a functionalization of the surface of the nano-spheres is an important fabrication parameter.

In a recent paper published in Applied Surface Science Julie Karel and co-workers presented nano-lithographic patterning of the manganese-gallium D022 phase by self-assembly of polystyrene nano-spheres. The method was inexpensive with the capacity to pattern large areas; therefore, it is attractive for prospective device fabrication.

The authors prepared manganese-gallium thin films through RF sputtering. The film was grown on a single crystal strontium titanate substrate at 300°C. They then deposited a 5nm platinum capping layer to curtail oxidation. Nano-structuring was then done implementing a self-assembled nanolithography procedure.

After the process of nano-structuring, annealing temperatures below 200°C were used to enhance the properties of the nanostructured samples. The authors performed characterization using X-ray diffraction and scanning electron microscopy.

The authors observed nanodots, which had similar features as the parent film, i.e. large coercivity and perpendicular magnetic anisotropy. They observed a kink in the magnetic hysteresis loop in the nanostructured material and referenced it to chemical disorder of the D022 phase induced in the course of the lithographic process.

The chemical order was recovered in the nanostructures after low temperature annealing. The nanodots posted several magnetic domains owing to their size, thus future research will include reducing the size of the nanostructures.

The results presented in their paper demonstrate that the nano-sphere lithography is a potential method for effectively preparing magnetic nanostructures with the magnetization oriented perpendicular to the substrate surface. Future research will constitute reducing the size of the nanodot and obtaining single domain nanodots, which will make this approach relevant for the fabrication of spintronic devices such as spin transfer oscillators and spin transfer torque MRAMs.

MnxGa1−x nanodots with high coercivity and perpendicular magnetic anisotropy- Advances in Engineering

About The Author

Benedikt Ernst did his studies at the University of Göttingen, Germany and wrote his Diploma thesis about optical properties on PLD deposited oxide layers. Now he is doing his PhD at the max Planck institute of chemical physics of Solid in Dresden, Germany. He is working on thin films of topological Heusler materials.

About The Author

Claudia Felser studied chemistry and physics at the University of Cologne and completed her doctorate in physical chemistry there in 1994. After postdoctoral fellowships at the MPI in Stuttgart and the CNRS in Nantes (France), she joined the University of Mainz. She was a visiting scientist at Princeton University (USA) in 1999 and at Stanford University in 2009/2010 and a visiting professor at the University of Caen (France).
She became a full professor at the University of Mainz in 2003. In Dec, 2011 she became director of the Max Planck Institute for Chemical Physics of Solids. She is the chair of the DFG research group “New Materials with High Spin Polarization” and is the director of the Graduate School of Excellence “Materials Science in Mainz” of the German Science Foundation (DFG). She was honored with the order of merit “Landesverdienstorden” of the state Rhineland-Palatinate for the foundation of a lab for school students at the University of Mainz. The materials under investigation are Heusler compounds and compounds with related structure types. In 2010, she was the distinguished lecturer of the IEEE Magnetic Society, she received the Nakamura lecture award of the UC Santa Barbara and the SUR-grant award of IBM.

In July 2014, Prof. Felser received the GRC-Alexander-M-Cruickshank-Lecturer Award at the “Gordon Research Conference” in New London, NH, USA, as well as the Tsungmin Tu research prize by the Ministry of Science and Technology of Taiwan. This is the highest academic honor granted to foreign researchers in Taiwan. She has written more than 400 articles and been granted several patents. Her recent research focuses on the rational design of new materials for spintronics and energy technologies such as solar cells, thermoelectric materials, topological insulators and superconductors.

About The Author

Franca Albertini is the leader of the Magnetic Materials Group at the Institute of Materials for Electronics and Magnetism (IMEM) of the Italian National Research Council (CNR). She is Member of the General Council of the European Magnetism Association (EMA) and AdCom elected member of IEEE Magnetics Society.

Her research interests are focused on nanostructured and bulk magnetic materials for applications in energy efficient technologies, magnetic memories and sensors, and nanomedicine.

She is author of 125 publications and more than 200 presentations at international conferences.

About The Author

Francesca Casoli is a research scientist at the Institute of Materials for Electronics and Magnetism of the Italian National Research Council (IMEM – CNR). She obtained her PhD in Physics from the University of Parma in 2005, investigating magnetic thin films and multilayers with perpendicular anisotropy and exchange-spring properties.

Her research is currently focused on magnetic thin films, nanostructures and nanocomposites with new functionalities or multi-functionalities. She has published more than 60 peer-reviewed papers on magnetic materials for data storage, sensors/actuators and biomedicine.

About The Author

Federica Celegato obtained her degree in Materials Science at the University of Turin in 2003. She is currently technical collaborator at INRIM in the Nanoscience and Materials Division. Her currently research activity is mainly focused on: a) growth of  metallic/magnetic thin films by vacuum techniques;  b) bottom up/top down nanolithography process to design nanostructures; c) study of magnetization reversal process in magnetic nanostructures by MFM techniques.

About The Author

Dr. Julie Karel is currently a research fellow in the Department of Materials Science and Engineering at Monash University.  Her research focuses on utilizing either thin film growth techniques or application of external electric fields to control and create new, metastable magnetic or electronic states in materials.  For instance, she is particularly interested in the electronic modifications in materials resulting from very large electric fields applied during liquid electrolyte gating and how to utilize these changes to make novel computing devices.  Dr. Karel also studies magnetic thin films for future magnetoelectronic applications, where she uses disorder and nanostructuring to control the properties.

About The Author

Pierpaolo Lupo received his PhD degree in Materials Science and Engineering from the University of Parma in 2012  with a thesis on “Heterostructures based on L10-FePt for spintronics and magnetic recording.” He spent two years as PostDoc in IMEM-CNR, Parma, Italy, working on materials for magneto-plasmonic applications, before moving to the Information Storage Materials Laboratory (ISML) at National University of Singapore, Singapore, where he worked as research fellow in the field of magnonics. He is currently working in the industrial R&D sector.

About The Author

Paola Tiberto got her Ph.D in experimental physics at the Physics Department of Torino Politecnico in 1993. Since 1994 she is working at the IEN Materials Dept (presently INRIM) as a researcher. She focused her scientific activity basically on the following topics: phase transformation in metastable ferromagnetic alloys; magnetic characterization of rapidly solidified alloys in ribbon and wire form, in amorphous and nanocrystalline alloys; magnetotransport properties of thin films and multilayer; study of magnetization process in materials obtained by means of non-equilibrium technique (powder and thin films); magnetic nanoparticles synthesized via chemical route.

Recently, she focused her activity on the study of nanostructure in magnetic thin films by top-down and bottom up lithographic techniques. She is author of more than 200 scientific papers published in International Peer-reviewed Journals in the field of magnetism.

About The Author

Dr. Sahoo is currently working as a post doctoral research fellow in the Max Planck Institute for Chemical Physics of Solids, Dresden, Germany. She is mainly involved in growing, characterizing and studying physical properties (magnetic and transport) of Heusler thin films that can be integrated into spintronic devices.

Reference

Karel, F. Casoli, P. Lupo, F. Celegato, R. Sahoo, B. Ernst, P. Tiberto, F. Albertini, C. Felser. MnxGa1−x nanodots with high coercivity and perpendicular magnetic anisotropy. Applied Surface Science, volume 387 (2016), pages 1169–1173.

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