Spin electronics is expected to displace volatile silicon memory technology within the next decade and is already in existence in the read head of hard disk drives (HDDs). The demand/supply disruption of the rare metal Iridium is already under way and, as spin electronics becomes more ubiquitous, the level of disruption and higher cost will only increase. The price of Iridium has risen by a factor of 4 in the last five years and by more than a factor of 10 in the last decade. It is expected to soar perhaps by a factor of 100 due to its wider applications.
Our research programme impacts this key material directly by providing an improved understanding of a wide ranging class of ternary alloys and we will seek to find new materials and new compositions of Heusler Alloys (HAs) to resolve this issue. Specifically, we intend to develop antiferromagnetic (AF) HA films to replace the AF alloy Iridium Manganese (IrMn). HAs are alloys of the transition metals Iron (Fe), Cobalt (Co) or Manganese (Mn) with materials such as Silicon (Si) or Aluminium (Al). All these materials are abundant on Earth and hence success in our project would eliminate the problem of the soaring cost of Iridium and perhaps its availability in the future. As shown in the attached chart, Iridium is one of the world’s most scarce elements – three times as rare as Gold and Platinum, for example – with an average occurrence in igneous rock of only 0.001 parts per million.
We combine our expertise in ab initio calculations and HA film growth techniques to seek highly anisotropic AF HA films. These films will be characterised both structurally and magnetically using synchrotron beamlines, high-resolution (scanning) transmission electron microscopy and highly sensitive electrical and magnetic measurement facilities available within the consortium. We will demonstrate a device concept with the developed AF HA films at the end of this project, fabricating both tunnelling (TMR) and giant magnetoresistive (GMR) junctions showing an exchange bias (EB) greater than 1 kOe in sheet form and a blocking temperature (TB) greater than 300K. The developed AF HA films will be patented with the royalties shared equally among the partners in the consortium. To our knowledge we are the first group to realise the criticality of the position with regard to the supply of Ir. The innovation within HARFIR is therefore extremely high.
We undertook a wide ranging survey of the properties of HAs to determine AF or compensated ferrimagnetic (CF) compositions. Four HA compounds – Fe2VAl, Ni2MnAl, Ru2MnAl and Cr2MnSb – were selected for the initial studies, based on theoretical calculations and crystallisation temperature measurements. We found that crystallising Fe2VAl requires 500°C – very close to the limit for practical applications – whereas Ni2MnAl crystallises readily below 500°C and Ru2MnAl crystallises into the B2 structure at 400°C. We selected the following X2YZ compositions for further studies: Fe2VAl (having the greatest potential for AF ordering); Mn2VAl (replaces the Fe2 with the very high moment Mn2); (Co,Mn)2VAl (analogous to the previous two except that X2 uses 2 high moment atoms); Ni2MnAl (has a high moment with Mn replacing V); Ru2MnAl (has the potential advantage of both X and Y being high moment atoms and is known to exhibit AF ordering).
We undertook ab initio calculations and computer simulations of the structure and magnetic properties of AF HAs. We developed an atomistic model to calculate the inter-atomic spin interactions successfully and determined the origin of their AF behaviour. We also calculated EB using modelling and found the calculations agree with experimental results.
We grew thin films of AF and CF HAs using ultrahigh- and high-vacuum sputtering techniques. We employ sputtering from a single compound target with the optimised atomic compositions and 3 targets with a combinatorial method. We mainly focus on the AF phases appearing in the L2_1-ordered Ru2MnSi, B2-ordered Ni2MnAl and A2-ordered Mn2VAl. By combining our atomistic modelling, we anticipate revealing the origin of the AF phases in these alloys with different crystalline ordering.
We structurally analysed the AF HA films using various tools – especially XRD and high resolution TEM. We found that the interfacial atomic sharpness of the AF films controls the magnetic properties, namely EB, and we fed back the interfacial structures for the optimisation of the AF film growth processes.
We developed a resistivity measurement to determine the AF/CF properties of AF/CF HAs – in particular the Néel temperature (AF), the compensation temperature (CF) and the Curie temperature (CF). We measured the temperature dependence of resistivity and found the gradient changes at these characteristic temperatures related to the changes in the magnetic ordering.
We have been continuing to develop a material which in sheet form has an EB > 1 kOe and to find an AF HA film with a TB >300K with standard deviation of <0.3. Based on our analysis for the first objective, the origin of the AF phases with different crystalline ordering is anticipated to assist the development of an AF HA film to satisfy these properties.
We have started to establish the corrosion resistance of AF/CF HAs and demonstrate the stability of prototype devices based on these alloys. We fabricated both TMR and GMR junctions and found that our HAs are corrosion resistive.
We characterised the AF behaviour of the HA films using synchrotron light sources and HR-(S)TEM. Neutron-, muon-, soft X-ray- and electron-beams were also used for these characterisations. An XMCD analysis of epitaxially grown Ru2MnGe covered with 2 nm Fe showed an AF coupling of the Manganese moments in this compound – i.e. for the correct phase, the XMCD signal vanishes.
A Press Release about the project was distributed to relevant news outlets. Open workshops in the EU and Japan have attracted many external researchers. We have participated in a number of Critical Raw Materials meetings, given presentations at public scientific forums, written articles about our work for several journals and have exchanged staff between the EU and Japanese collaborators.
We expect to develop inexpensive HAs capable of replacing expensive (and rare) IrMn in spin electronic devices, and we will create GMR and TMR devices which use optimised AF or CF HA films to demonstrate their performance.
It is widely recognised that spin electronic technologies will displace semiconductor technology within the next decade, yielding many areas where the project’s results can be applied. These range from existing technologies such as read heads in HDDs, to solid-state storage such as MRAM and thermally assisted MRAM (TA-MRAM) – a system proposed as a replacement for the common silicon-based dynamic random access memory (DRAM). The start-up time when switching on a computer or similar device would then be reduced dramatically because the information storage would be permanent rather than charge-based or capacitive – where recharging the system is required and data has to be reloaded from a permanent storage system such as a HDD.
The other major advantage of proposed MRAM technologies is that they are faster and capable of storage at a higher density than, for example, flash memory. They are also much less complex than flash and have the advantage of increased reliability, with no limit to the read-write cycles that can be performed. This is thanks to the infinite reversibility of a magnetic material’s orientation without damage to its structure – something not true of the charge-based systems that compose flash memory. It is also the case that flash memory has to be rewritten as an entire block when its data is modified. For large-scale storage or archiving, this power cost, with consequences for CO2 emissions, becomes considerable.
In addition to the storage capabilities of MRAM, there is also the distinct potential for magnetic logic systems based on similar technologies. These are known to have the capability to be faster and have higher aerial densities than is possible with current Silicon technologies. The decrease in complexity and the increase in reliability mean that magnetic logic systems would have major cost advantages over current Silicon techniques.
Direct or near term exploitation of HARFIR is unlikely to occur due to the thermal stability issues associated with the low anisotropy of the AF phases of the HAs that have been studied. However, following the initiation of HARFIR, there has been a very significant upsurge not only in the study of HAs for technological applications themselves but also in the search for alternative AFs to IrMn. To our knowledge, there are now about seven significant research groups around the world involved in this activity, whereas at the outset there were none. The level of interest in HAs for technological applications is manifest by the fact that Seagate are now partially supporting a PhD student at the University of York who is looking at ferromagnetic phases of these alloys for potential application in HDD read heads.
Indirect exploitation of the HARFIR work may well occur though due to our recent development of what we believe is the highest resolution magnetoresistive measurement system in the world. This system was developed as part of our structural studies to enable us to determine the AF/paramagnetic transition temperature in the HAs (known as the “Néel temperature”). The design of this device is not patentable as, in essence, it is no different to many other similar devices in operation around the world. However its resolution is much greater than any other system of which we are aware and it exceeds the specification on the leading commercial system by >100. When a suitable opportunity arises (e.g. at the forthcoming MMM conference) we will discuss with potential interested parties – such as companies specialising in magnetic instrumentation – whether or not they would have an interest in collaborating with us on commercialising such a system.