Tailoring magnetic properties of Mn-Cr chalcogenide alloys and heterostructures

Lead Research Organisation: University of Bath
Department Name: Physics

Abstract

We shall develop new, thin-layer materials composed of transition metal elements (Cr, Mn) combined with group VI elements (S, Se,Te); these little-known materials offer the prospect of satisfying the requirements for a wide range of spin-dependent electronic ("spintronic") devices. This proposal aims to take the first steps in making and investigating single- and multi-layer materials from this family with the necessary compositions and crystal quality, but we shall also manufacture and study selected demonstrator devices within the timescale of the project.

The term "spintronics" encompasses many proposed devices (for example, but not exclusively, sensors, memory elements, diodes and transistors) in which it is the spin of the electrons that is manipulated in order to sense, store, carry or process information. In other words, these devices exploit the intrinsic magnetic properties of electrons as well as their charge and, as a result, may offer improvements in energy efficiency, speed or size. The prizes for the realization of such devices are enormous; for example, the phenomenal commercial success of the device at the heart of hard disk read heads (which can be classed as a spintronic device) has stimulated great excitement and intense research efforts aiming at wider applications of spin. Despite this, it has not proved possible to produce industrially-useful magnetic semiconductors from the first few candidate materials identified by early theoretical predictions. As a result, many groups in the worldwide spintronics community are now engaged in widening both the scope of the materials considered, and the types of magnetic behaviour that can be exploited; this search has revitalised the whole field of spintronics. This proposal addresses a candidate material family that has been proposed in the theoretical literature but (apart from our trials) has not yet been produced in the laboratory.

The (Cr, Mn)(S,Se,Te) material family satisfies several crucial requirements. Firstly, thin layers of these compounds will be grown on industry standard GaAs substrates and will adopt the same crystal structure as the substrate. This makes the resulting structures highly compatible with existing semiconductor technologies. Secondly, preliminary studies of ours and theoretical studies of several groups imply that we will be able to produce all the potentially useful types of magnetic behaviour by tuning the composition. These include ferromagnets and half-metals (where the transition metal magnetic moments align parallel to each other and add), antiferromagnets (where they align oppositely and cancel) and ferrimagnets (where dissimilar transition metals align oppositely but are not equivalent and so cannot exactly cancel). Layers of any of the above magnetic types can form the active layer in different types of spintronic devices. Thirdly, these materials are chemically and structurally compatible with non-magnetic semiconductors (e.g, ZnSe, MgS) that we can grow as parts of multi-layer structures; these allow the necessary electrical contacts and electrical barriers to be formed.

Our programme will involve a substantial effort in growing these new materials to obtain layers of high crystal quality; preliminary work indicates that there will be no fundamental obstacles to success. The structural, magnetic and electrical properties of the materials will be investigated to identify the most promising compositions and most appropriate target device designs based on them. Demonstrator devices will then be produced to test how the materials perform in realistic device contexts and to promote interest in the work. This work requires a broad base of experience and so we have formed a team having expertise in MBE growth, electrical device fabrication and measurement, magneto-optical spectroscopy and magnetometry.

Planned Impact

At the earliest stages in the fabrication of these materials and heterostructures, which do not exist in nature and which are known only through theoretical predictions, we expect the immediate impact of this project (during its 42 month period) to be amongst academic groups, as discussed in the Academic Beneficiaries section above. This impact is likely to be significant independent of whether the most ambitious objectives of this project are achieved, since the new insights into material growth and into the modelling of magnetic behaviour of these systems that will be generated will be of fundamental interest to those respective communities.

In the medium term (most likely, towards the conclusion of the project and onwards), a successful demonstration of high temperature ferromagnetic behaviour, or half-metallicity, and successful spin injection would impact also on industrial groups interested in exploitation of spin effects for novel semiconductor devices. Significant research and development work would then be needed to optimise device design and fabrication with a much more restricted set of the best candidate materials and that would be likely to require a programme of academic/industrial collaboration. We can anticipate significant continuing involvement of the present project partners, especially since expertise in the growth of these materials will be unique to HWU, but a much wider network with suitable device engineering skills would become necessary. How we will reach out to such industrial groups is discussed in the Pathways to Impact section of the main proposal. Part of the appeal of our proposed materials is their relatively easy integration with existing III-V technologies, and this is why we take the ambitious step of including demonstrator devices already within this project.

The ultimate economic and societal impact of novel semiconductor technologies is potentially enormous; past breakthroughs in, for example, blue light emitting diodes and lasers, or GMR hard disk read heads, were commercialised on the scale of 10 to 15 years following the initial materials science discoveries and these are now the bases of large-scale industrial activity and huge international markets. Furthermore, the resulting transformations in information technology have (in both cases) had a wide societal impact. Some of the possible device applications of magnetic semiconductors (which are not discussed in detail in the proposal, note, since our project is mainly focused on a much earlier stage in the overall programme, of fundamental materials science) are based on providing similar functionality to existing types of semiconductor device but at much reduced power consumption and/or smaller physical size. As is widely recognised, such devices, whilst not breaking new grounds in terms of function, could have significant positive implications for sustainability and for wider access to information technology. Beyond this, more disruptive concepts such as quantum information technology and quantum computing require the manipulation of quantum bits. Many possible strategies for this exist, one of which is based on the use of electron spins within nanoscale ferro- and paramagnetic materials. Here again, the materials we propose to study could have a role to play and the fabrication of quantum dot structures from them will be feasible by well-established means.
 
Description Within the computation modelling part of the project, we have started the first ever studies of hypothetical zincblende rhenium monoselenide, showing that it should be possible to manufacture it via molecular beam epitaxy and that it is expected to be a paramagnetic material with some resemblances to the recently-discovered superconductor FeSe. The aim of the project is to search for ferromagnetic semiconductors and so we have eliminated ReSe as a suitable candidate; however, the link to FeSe still makes it a very interesting material and we are seeking research partners able to attempt to make it for the first time. One publication and a conference talk have resulted from this work. At teh same time, the search for new magnetic semiconductors has led us to carry out extensive investigations of ReS2 and ReSe2 and this work has attracted a great deal of academic interest worldwide, because these materials have several features that make them unique amongst layered semiconductors.
Exploitation Route ReSe: we hope to collaborate with a group equipped to make this material for the first time. We are also building up a substantial level of activity on ReS2 and ReS2 materials, where theoretical predictions suggest magnetism can be introduced in the same way as in the materials originally listed in the proposal. These new materials are even more topical since they are examples of layered 2D semiconductors and our study of these has lead to several successful applications for synchrotron beam time, not originally proposed as part of this work, but an extremely valuable extension of it. We have attracted considerable academic attention to these new materials since we first publicised them and the rate of publication concerning them has increased dramatically.
Sectors Chemicals,Electronics,Energy,Manufacturing, including Industrial Biotechology

 
Description DIAMOND beam time (I05 beam line)
Amount £0 (GBP)
Funding ID SI15905 
Organisation Diamond Light Source 
Sector Private
Country United Kingdom
Start 02/2017 
End 07/2017
 
Description Nano-ARPES studies of novel transition metal dichalcogenides
Amount £5,396 (GBP)
Funding ID EP/P004830/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 05/2016 
End 11/2016
 
Description SOLEIL beamtime allocation
Amount € 1 (EUR)
Funding ID 20151237 
Organisation SOLEIL Synchrotron 
Sector Academic/University
Country France
Start 02/2016 
End 06/2016
 
Title Lattice dynamics of the rhenium and technetium dichalcogenides 
Description The rhenium and technetium dichalcogenides are layered van der Waals semiconductors which show a large number of Raman-active zone centre phonon modes as a result of their unusually large unit cells and deviation from hexagonal symmetry. They thus offer the possibility of introducing in-plane anisotropy into composite heterostructures based on van der Waals materials, and Raman spectroscopy is generally used to determine their in-plane orientation. We show that first principles calculations give a good description of the lattice dynamics of this family of materials and thus predict the zone-center phonon frequencies and Raman activities of TcS2. We consider the distribution of the phonon modes in frequency and their atomic displacements, and give a unified understanding of the phonon frequencies and Raman spectra of ReS2, TcS2 and ReSe2 in terms of the scaling of Raman frequency with the chalcogen mass. 
Type Of Material Database/Collection of data 
Year Produced 2016 
Provided To Others? Yes  
 
Title Rhenium Dichalcogenides: Layered Semiconductors with Two Vertical Orientations 
Description Raw data for Nano Letters DOI: 10.1021/acs.nanolett.5b04838 
Type Of Material Database/Collection of data 
Year Produced 2016 
Provided To Others? Yes  
 
Title Rhenium monoselenide: an investigation by density functional theory 
Description Rhenium monoselenide: an investigation by density functional theory: supporting data for publication 
Type Of Material Database/Collection of data 
Year Produced 2016 
Provided To Others? Yes  
 
Title Sulfur and oxygen impurities in ReSe2: symmetry breaking and local vibrational modes 
Description This dataset supports the publication "Identifying light impurities in transition metal dichalcogenides: the local vibrational modes of S and O in ReSe2 and MoSe2". It contains shell scripts for running Quantum Espresso code and the original spectroscopic data in Origin Project (.opj) file format. 
Type Of Material Database/Collection of data 
Year Produced 2017 
Provided To Others? Yes  
 
Title Supporting information for "Valence band structure of ReS2 by Angle Resolved Photoemission Spectroscopy" 
Description Datasets contributing to the experimental figures (Figs. 2, 3, 4, 5) of the associated paper are contained here. Also included (code section) are scripts and associated C codes for running the band structure calculations (Figs 5 and 6); within these are the atomic coordinates which lead to the reciprocal space Brillouin Zone of Fig. 1. 
Type Of Material Database/Collection of data 
Year Produced 2017 
Provided To Others? Yes  
 
Title Supporting information for "Valence band structure of ReSe2 investigated by angle-resolved photoemission spectroscopy" 
Description Data here represents raw computational and experimental data for the following work: ReSe2 and ReS2 are unusual compounds amongst the layered transition metal dichalcogenides as a result of their low symmetry, with a characteristic in-plane anisotropy due to in-plane rhenium 'chains'. They preserve inversion symmetry independent of the number of layers and, in contrast to more well-known transition metal dichalcogenides, bulk and few-monolayer Re-TMD compounds have been proposed to behave as electronically and vibrational decoupled layers. Here, we probe for the first time the electronic band structure of bulk ReSe2 by direct nanoscale angle-resolved photoemission spectroscopy. We find a highly anisotropic in- and out-of-plane electronic structure, with the valence band maxima located away from any particular high-symmetry direction. The effective mass doubles its value perpendicular to the Re chains and the interlayer van der Waals coupling generates significant electronic dispersion normal to the layers. Our density functional theory calculations, including spin-orbit effects, are in excellent agreement with these experimental findings. 
Type Of Material Database/Collection of data 
Year Produced 2017 
Provided To Others? Yes  
 
Description ARPES at DIAMOND 
Organisation Diamond Light Source
Country United Kingdom 
Sector Private 
PI Contribution We proposed project and were granted beam time at DIAMOND
Collaborator Contribution DIAMOND staff assisted in proof of principle experiments, advised on experimental details, and are training PG and PDRA
Impact Collaboration at very early stage - we have initial data but no outputs
Start Year 2017
 
Description Magnetic semiconductors: SQUID access 
Organisation Philipp University of Marburg
Country Germany 
Sector Academic/University 
PI Contribution We provide materials for study and characterisation of them. We also fabricate electronic devices from these materials.
Collaborator Contribution The Marburg group will carry out SQUID measurements and optical spectroscopy on these materials.
Impact None so far
Start Year 2015
 
Description Polish Academy of Sciences 
Organisation Polish Academy of Sciences
Country Poland 
Sector Public 
PI Contribution We propose experiments and design suitable samples.
Collaborator Contribution The Warsaw MBE group manufacture samples.
Impact Many over several EPSRC and Leverhulme grants pre-Researchfish reporting period.
 
Description XPS and ARPES 
Organisation University of St Andrews
Country United Kingdom 
Sector Academic/University 
PI Contribution We provide novel semiconductor materials and characterise them by a variety of means, as well as fabricating devices based on these materials. We also model them theoreticallly.
Collaborator Contribution St Andrews will perform XPS and ARPES measurements to reveal fundamentals of the electronic band structure of these materials to test our theoretical and experimental work.
Impact None so far
Start Year 2015
 
Description nano-ARPES 
Organisation SOLEIL Synchrotron
Country France 
Sector Academic/University 
PI Contribution We proposed the experiments, shall provide the samples, and shall be responsible for the data analysis.
Collaborator Contribution SOLEIL provides the beamline (unique world-wide at present) and the support in running the experiments.
Impact None yet.
Start Year 2015
 
Description International advisory committee: International Conference on II-VI Compounds and Related Materials 2017 
Form Of Engagement Activity A formal working group, expert panel or dialogue
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Member of international advisory committee: International Conference on II-VI Compounds and Related Materials 2017
Year(s) Of Engagement Activity 2017
 
Description Invited talk: International Conference on Physics of 2D Crystals 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Invited talk: International Conference on Physics of 2D Crystals at Malta, May 2018
Year(s) Of Engagement Activity 2018
 
Description Invited talk: International Conference on Physics of 2D Crystals, 2017 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact invited talk at International Conference on Physics of 2D Crystals conference, Ha Long, Viet Nam, April 2017
Year(s) Of Engagement Activity 2017
 
Description Invited talk: Rhenium dichalcogenides: 2D layered semiconductors in a class of their own 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Invited talk: "Rhenium dichalcogenides: 2D layered semiconductors in a class of their own" at seminar in LMU Munich
Year(s) Of Engagement Activity 2018
 
Description Invited talk: Smart Nanomaterials 2018 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Invited talk at Smart Nanomaterials 2018 conference, Paris, December 2018
Year(s) Of Engagement Activity 2018
 
Description Public lecture 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Local
Primary Audience Public/other audiences
Results and Impact Public lecture organised by the IOP at the Bath Royal Scientific and Literary Institution, Queen Square, Bath,attended by IOP members and the general public.
Year(s) Of Engagement Activity 2016
 
Description Speaker at Photovoltaic Science, Application and Technology Conference 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Postgraduate students
Results and Impact Invited speaker at Photovoltaic Science, Application and Technology Conference, Bangor, April 2018.
Year(s) Of Engagement Activity 2018