PHYS01 - A New Paradigm for Exchange Bias in Polycrystalline Films
Submitting Institution
University of YorkUnit of Assessment
PhysicsSummary Impact Type
TechnologicalResearch Subject Area(s)
Chemical Sciences: Analytical Chemistry
Engineering: Materials Engineering
Medical and Health Sciences: Cardiorespiratory Medicine and Haematology
Summary of the impact
A new procedure for the measurement and characterisation of
polycrystalline exchange bias
systems has been developed which has impacted significantly the
manufacture of computer hard
drive read-heads by companies such as Seagate Inc and Western Digital
Corp. The new
measurement procedure has enabled a typical 40% increase in the thermal
stability of the
antiferromagnetic materials used in computer hard drive read heads. The
procedure has also
improved the manufacturing process of the read-heads giving increased
material performance and
has resulted in a ~25% improvement in the resolution of detecting a bit.
Underpinning research
Since 1996 read heads based on the Giant Magneto Resistive effect have
used exchange bias
technology. Exchange bias systems are a ferromagnetic (F) film grown on an
antiferromagnetic
(AF) film pinning the magnetic orientation of the ferromagnet. Control of
the behaviour of an
antiferromagnetic material used in exchange bias systems is vital. Thin
film antiferromagnetic
materials do not give a measureable signal related to their degree of
order and thermal stability by
any direct technique making the study of these properties difficult.
Prof O'Grady and Dr Vallejo-Fernandez who at the time was a PhD student
funded by an EU
Training Network known as NEXBIAS, have developed a new measurement
protocol which allows
the thermal stability within AF layers to be determined. These layers
consist of polycrystalline
grains with sizes 5-15 nm produced by sputtering. To initiate the exchange
bias effect the sample
is heated and field cooled with the F layer saturated in one direction.
Reversing the direction of the
F layer then reverses the direction of the exchange field on the AF and
allows for thermal activation
to be used progressively to alter the orientation of the AF grains. The
measurement protocols
developed at York based on this concept enables the so-called distribution
of blocking
temperatures of the AF grains to be determined. Such exchange bias
characterisation takes 15
hours per sample for a statistically significant measurement.
Using the developed exchange bias characterisation procedure, it was
found that the thermal
stability of the AF materials is a grain volume dependent effect. The
measurement of a grain
volume distribution is challenging. Typically 800 grains are required to
characterise the median
volume and the standard deviation. It was shown that the initial alignment
process used by
manufacturers was incomplete and a significant fraction of the grains were
not stable at room
temperature. The exchange bias effect is proportional to the integral over
the volume distribution.
The developed analysis procedure produced a remarkable fit to experimental
data for the film
thickness and the grain size within a series of films. It was possible to
predict the rate of alignment
of AF grains during the setting process. This has an impact on the design
of the setting
procedures used in manufacturing. The grain size work, whilst not
undertaken in collaboration with
industry, was rapidly adopted first by Western Digital Corporation and
subsequently by Seagate
Technology. These two companies manufacture 90% of the world's read heads
with a value of
~$10Billion. Dr A Johnson (Vice President, Transducer Development, Seagate
Technology,
Northern Ireland in a letter of support states: "(this work)...constituted
the only known mechanism
for assessing the degree of order and thermal stability of
antiferromagnetic grains."
The exchange bias work has shown that the interfaces between the layers
consist of spin clusters
whose alignment can occur spontaneously at low temperatures, be increased
by high setting fields
and behave independently of the AF grains. Subsequently O'Grady reported
on factors which
control the anisotropy of the AF grains including texture, composition and
impurities. The model
has been extended to explain exchange bias in single crystal and large
grain systems based on a
strong domain wall pinning concept. A patent for a new design for advanced
solid state magnetic
memory has been filed and is being developed in collaboration with Hitachi
Europe Ltd.
References to the research
[1] Measurement of the Anisotropy Constant of Antiferromagnets in
Metallic Polycrystalline
exchange biased systems (37 citations)
G Vallejo-Fernandez, L E Fernandez-Outon and K O'Grady
Appl. Phys. Lett. 91, (2007) p. 212503
[2] Interfacial Spin Order in Exchange Biased Systems (10 citations)
L E Fernandez-Outon, G Vallejo-Fernandez, Sadia Manzoor, B Hillebrands and
K O'Grady
J. Appl. Phys. 104, (2008) 093907
[3] Antiferromagnetic Grain Volume Effects in Metallic Polycrystalline
Exchange Bias Systems
(28 citations)
G Vallejo-Fernandez, L E Fernandez-Outon and K O'Grady
J. Phys. D: Appl. Phys. 41, (2008) 112001 (5pp)
[4] Tuning of Anisotropy in IrMn/CoFe Exchange Bias Systems (7
citations)
N.P. Aley, R. Kroeger, B. Lafferty, J. Agnew, Y. Lu and K. O'Grady
IEEE TransMag, 45 (10) (2009), p.3869 - 3872
[5] A New Paradigm for Exchange Bias in Polycrystalline Thin Films
(invited) 97 citations)
K. O'Grady, L.E.Fernandez-Outon and G.Vallejo-Fernandez
J. Magn. Magn. Mater. 322 (2010) p883 - 899
All citations taken from Scopus, 14/11/2013
Details of the impact
The impact of the York Model of Exchange Bias has been significant in
that every magnetic
recording read head in the world is now designed based on the underpinning
physical insight that
was developed. In 2011 alone this involved about one billion components,
400 million of which
were manufactured by Seagate Technology in Northern Ireland. The total
revenue from these
products is approximately $6 billion based on an output of 2.5 million
heads/day each with a unit
value of circa $6. Following a series of mergers Seagate and Western
Digital now command over
90% of the world-wide hard drive industry with a turnover of $34 billion
per annum. . In a letter of
support from Dr Mahendra Pakela (former Director Western Digital Fremont)
to Professor O'Grady,
the commercial value of this impact is confirmed: "I am happy to confirm
that your work, which is
now broadly accepted as the correct interpretation of exchange bias in
granular antiferromagnetic
layers, has enabled my company to design and develop improved layers which
we incorporate in
all the read heads we currently manufacture. This amounts to hundreds of
millions of components
each year." Using the work it was possible to design an antiferromagnetic
layer in terms of its
crystal orientation, grain size, size distribution and layer thickness,
tailored to the
equipment/process used to achieve exchange bias. This in turn allowed for
the development of
layers with up to two times greater alignment and thermal stability. In
consequence thinner layers
could be used for increasing resolution.
The close relationship between O'Grady's group and the major magnetic
recording companies
ensured that the work was disseminated, not only via publications, but via
regular visits to the three
major hard drive manufacturers, i.e. Seagate, Western Digital and Hitachi
GST. Hitachi GST and
Western Digital are in the process of merging which means that all hard
drive manufacturers are
involved in utilising this work. Additionally Seagate Northern Ireland,
have sponsored students in
O'Grady's group continuously for the last six years and Seagate Media
Research in Fremont
sponsored students prior to that. Subsequently Western Digital provided
financial support for
contract measurements which supported post docs in York and Seagate
Technology, Northern
Ireland, are now doing the same. Further support for another PhD student
for 3 years has been
secured from Seagate from January 2012. A further contract with Seagate
with a value of $400K
has been secured which includes 50% support for a new lectureship (taken
up Vallejo-Fernandez).
All the major manufacturers of read heads in the world have modified both
the growth processes
for antiferromagnetic layers and their setting processes based on the new
physical insights into
exchange bias that we developed.
All work associated with the development of the York Model of Exchange
Bias was undertaken
here in York. The York Model has now displaced all other models of
exchange bias based on
computer simulations of complex AF domain structures. Working in
collaboration with Western
Digital in 2009, an increase in the maximum reported value of exchange
bias of more than a factor
2 to 3.6 kOe was reported. This system was designed using the York Model
and remains the
world record value [4].
The new insight into exchange bias materials that has been established
has improved the thermal
stability of the AF layers in certain designs of read heads by up to a
factor 2. The new
understanding has also enabled AF materials to be designed for purpose.
For example, for
perpendicular recording the overall thickness of the read head stack
determines the linear data
density along the track. In collaboration with Seagate in Northern
Ireland, studies have been
undertaken that allowed for the stack thickness to be reduced from 12nm to
6nm for certain
applications. This allows for a potential doubling of the linear density
along track although in
practice the increased resolution was used to improve signal to noise.
Sources to corroborate the impact
Relevant references have been cited but in particular reference to the
article from 2010, `A New
Paradigm for Exchange Bias in Polycrystalline Thin Films' [4], describes
the bulk of the original
work. A more recent major invited work `Exchange Bias in Polycrystalline
Metallic Thin Films'
shows subsequent results obtained using the York Model.
There are confidential reports to both Seagate and Western Digital but
these cannot be made
available for commercial reasons.
The University's financial records show that 8 short contracts were
undertaken between 2007 and
2013 with a total value of $100k.
Letters are available confirming the use of the York Model of Exchange
Bias from a former
Director from Western Digital in Fremont, and the VP R&D of Seagate
Technology in Northern
Ireland/Minneapolis MN.