Talks - Orbitronics 2022 - Mathias KLÄUI, University of Mainz
Summary
TLDRThe speaker discusses experimental results in the field of spintronics, focusing on the challenges faced by experimentalists in advancing beyond current technology. They delve into the potential of spin-orbit torques and the role of orbital angular momentum, highlighting recent discoveries that suggest a significant increase in efficiency compared to traditional spin transfer torques. The talk emphasizes the importance of collaboration between theorists and experimentalists to understand the origins of observed effects and the potential for new materials to enhance performance in memory and logic devices.
Takeaways
- 😀 The speaker expresses excitement about seeing attendees in person after a period of virtual meetings.
- 🔬 The presentation focuses on experimental results and the practical challenges faced by experimentalists in the field of spintronics.
- 🌟 The speaker is from Mines and has a co-affiliation at NTNU, emphasizing the importance of understanding complex theories for practical applications.
- 🛠 There is a discussion on the limitations of spin transfer torque devices in the market, highlighting the efficiency limit of one Bohr magneton per electron.
- 📈 The potential of spin-orbit torques (SOTs) to overcome these limitations by allowing the transfer of more than one Bohr magneton per electron is introduced.
- 🔍 The experimental challenge of distinguishing between different types of torques (damping-like and field-like) is highlighted, as only the effective field's direction and strength can be measured.
- 🌀 The concept of orbital angular momentum and its potential to enhance torque efficiency beyond spin angular momentum alone is explored.
- 💡 The importance of collaboration between theoreticians and experimentalists to interpret experimental results and understand the origins of different torques is emphasized.
- 📚 The speaker references previous work and theoretical calculations that motivate the experimental approach to identifying and quantifying orbital effects in materials.
- 🛑 The experimental results show a significant increase in torques when certain materials like copper oxide are used, suggesting a strong orbital current generation.
- 🔧 The use of different ferromagnets in experiments reveals a unique dependence on the material, suggesting that the conversion of orbital currents to spin currents is material-specific.
Q & A
What is the main focus of the speaker's presentation?
-The speaker's presentation focuses on experimental results related to spin and orbital angular momentum in materials, particularly the effects of spin-orbit torques and the potential for increasing efficiency beyond current limitations.
What is the significance of seeing 80 people 'alive in 3D' compared to 'tiles of Zoom'?
-The speaker is expressing the value of in-person interactions over virtual meetings, highlighting the importance of live attendance for a more engaging and dynamic experience.
Why is the efficiency of spin transfer torque limited?
-The efficiency of spin transfer torque is limited because it is fundamentally constrained by the efficiency of one Bohr magneton (h bar) per electron for spin transfer torque, which necessitates a certain amount of current to reverse the magnetization.
What is the potential advantage of orbital angular momentum over spin angular momentum in this context?
-Orbital angular momentum offers the potential to transfer more than one Bohr magneton per electron, which can lead to increased efficiency in torques compared to traditional spin transfer torques.
What are the two fundamental mechanisms the speaker refers to in relation to spin-orbit torques?
-The two fundamental mechanisms are the spin Hall effect, where spin-polarized electrons are scattered into a ferromagnet, and the inverse spin galvanic effect or Edelstein effect, where a non-equilibrium spin density forms and exerts a torque by exchange interaction on the magnetization.
Why is it challenging for experimentalists to identify the origin of a torque?
-It is challenging because the only measurable quantities are the direction and strength of the effective field, which can have the symmetry of either a damping-like or field-like torque. The actual origin of the torque requires indirect interpretation and input from theoreticians.
What is the significance of the 16-fold increase in torques when copper oxide is added on top of platinum?
-The 16-fold increase indicates a highly efficient generation of orbital currents that are converted into spin currents in the platinum layer, which then generate a significant spin-orbit torque acting on the underlying ferromagnet.
How does the speaker's research group address the experimental challenge of identifying orbital effects?
-The group uses a combination of torque measurements and magneto-resistance effects, varying the thickness of different layers in their samples, and comparing results across multiple materials to infer the origins of the observed effects.
What is the importance of being able to manipulate iron cobalt boron instead of nickel in MRAM devices?
-The ability to manipulate iron cobalt boron is important because it is preferred by companies for MRAM devices due to its compatibility with MgO tunnel junctions, which nickel does not offer.
What is the role of theoreticians in this field of research?
-Theoreticians play a crucial role in interpreting experimental results, providing calculations and models that help experimentalists understand the underlying physics of the observed phenomena, and guiding the design of new experiments.
Outlines
🧲 Spin Transfer and Spin-Orbit Torques
The speaker begins by expressing excitement about the live audience and then dives into the topic of spin transfer torque devices available in the market. These devices operate by polarizing electrons and using their spin to change the magnetization of a free layer. However, they are limited by an efficiency of one Bohr magneton (h bar) per electron. The speaker introduces the concept of spin-orbit torques, which have shown potential for higher efficiency, and discusses the fundamental mechanisms behind them, including the spin Hall effect and the inverse spin galvanic effect. The talk also touches on the experimental challenges faced by researchers in distinguishing between different types of torques and the importance of collaboration with theoreticians to interpret experimental results.
🔬 Exploring the Origins of Spin-Orbit Torques
The speaker discusses the complexities of identifying the origins of different types of torques in experiments. Initially, it was believed that the spin Hall effect and the Edelstein effect produced distinct types of torques, but it has been established that both mechanisms can generate both types of torques. The speaker emphasizes the need for careful sample variation and measurement to understand the origins of these torques. The introduction of orbital effects in the context of spin-orbit torques is highlighted, with the potential for significantly enhancing the efficiency of torques, which is an exciting prospect for students and researchers in the field.
🌐 Orbital Hall Effect and Its Impact on Torques
The speaker explores the concept of the orbital Hall effect, which is analogous to the spin Hall effect but involves the generation of a transverse orbital current from a longitudinal charge current. This effect, along with the orbital Edelstein effect, can lead to the generation of orbital angular momentum flow, which is a promising area of research. The speaker also discusses the motivation for studying these effects, including the potential for large orbital Hall conductivities in certain materials and the possibility of avoiding the use of heavy, rare, and environmentally harmful materials in device fabrication.
📈 Measuring Orbital Effects Through Torques and Magnetoresistance
The speaker describes the experimental approaches to measuring orbital effects, focusing on torque measurements and magnetoresistance effects. The use of magnetic insulators to eliminate complications from charge currents within the material is highlighted. The speaker presents unpublished results showing a significant increase in torques when copper oxide is added to platinum, suggesting the generation of an orbital current that is converted into a spin current, which then acts on the magnetization. The talk also covers the measurement of spin Hall magnetoresistance effects and the potential for identifying orbital effects through these measurements.
🔍 Investigating the Mechanisms of Orbital Torques and Magnetoresistance
The speaker delves deeper into the investigation of orbital torques and magnetoresistance, discussing the use of different materials and the impact of their properties on the observed effects. The talk presents results from experiments with permalloy and copper, highlighting the differences in the length scales of the effects observed. The speaker also discusses the importance of understanding the physical origins of these effects, comparing them to standard spin Hall magnetoresistance and suggesting that the observed effects may be due to orbital Hall inverse or orbital Rashba Edelstein magnetoresistance.
🛠️ Enhancing MRAM Performance with Orbital Torques
The speaker discusses the application of orbital torques in improving the performance of Magnetic Random Access Memory (MRAM) devices. The talk presents results from experiments with ruthenium and niobium, which have large orbital Hall conductivities, and the use of platinum layers to enhance the conversion of orbital currents into spin currents. The speaker also addresses the challenges of anomalous torques and self-talks, and how they can be mitigated by using specific material combinations. The talk concludes with the significance of these findings for the development of MRAM devices and the potential for further research in this area.
🏆 Conclusions and Acknowledgments
In the concluding part of the talk, the speaker summarizes the key findings and contributions of the research, emphasizing the potential of orbital effects in enhancing the efficiency of torques and the importance of collaboration with theoreticians. The speaker acknowledges the work of the research team, including specific individuals who contributed significantly to the studies. The talk also credits external collaborators and funding sources that supported the research. The speaker invites the audience to a poster session for further details and expresses gratitude for the attention of the audience, looking forward to discussions.
Mindmap
Keywords
💡Spin-transfer torque
💡Spin Hall effect
💡Orbital angular momentum
💡Damping-like torque
💡Field-like torque
💡Orbital Hall effect
💡Rashba-Edelstein effect
💡Magnetoresistance
💡Magnetic insulator
💡Skermion
💡Niobium
Highlights
The transition from theory to addressing experimental challenges in spin transfer torques.
The limitations of spin transfer torque efficiency and the fundamental limit of one Bohr magneton per electron.
Introduction of spin-orbit torques as a next step beyond spin transfer torques.
The role of spin Hall effect and the inverse spin galvanic effect in generating spin-orbit torques.
The difficulty in experimentally distinguishing between different types of torques due to measurement limitations.
The potential of orbital angular momentum in enhancing torque efficiency beyond the spin transfer torque limit.
Experimental results showing a 16-fold increase in torques with the addition of copper oxide.
The significance of the non-monotonic dependence of torques on the thickness of platinum.
The discovery of orbital Rashba-Edelstein magnetoresistance effects in certain material systems.
The importance of selecting appropriate ferromagnetic materials for efficient orbital to spin current conversion.
Demonstration of large orbital-induced torques in niobium and ruthenium systems.
The ability to manipulate iron cobalt boron using platinum layers to enable large torques.
The experimental validation of the absence of intrinsic large effects in nickel itself.
The theoretical and experimental collaboration that led to the discovery of enhanced orbital torques.
The practical implications for MRAM devices in utilizing iron cobalt boron instead of nickel.
The demonstration of orbital to spin conversion in permalloy itself without the need for additional layers.
The strong dependence on the ferromagnet in the orbital effects, highlighting a unique characteristic of orbital-induced phenomena.
Transcripts
okay yeah thanks a lot so it's great to
be here it's also great to see so many
faces live
in the last three years we had some
spice workshops where i saw 80 tiles of
zoom so this is a lot better seeing 80
people alive
in 3d that someone wrote not just 2d
yeah so so today i'm going to show some
experimental results and i'm kind of
going to kind of change gears a little
bit from a lot of really exciting theory
talks which you have seen to some of the
banal problems that experimentalists
actually face
um yeah so i'm local from mines and i'm
also have a co-affiliation at ntnu
so firstly also for the students why are
we doing all this complex stuff
i mean obviously it's exciting signs but
if you look at what is already in the
market you can see that actually there
are already been transfer talk devices
which you can buy
and they work by polarizing a current in
a polarizing layer and the current these
electrons which are spin polarized then
go to a free layer and change the
magnetization of the free layer and this
is really good it works you can buy it
it's actually a product out there but it
is also limited by an efficiency of one
h bar per electron for spin transfer
torque and that is of course something
that fundamentally
means that you need a certain amount of
current a certain amount of spins in
order to reverse the magnetization and
we heard from
the horn this morning if you go to low
magnetization materials this can
actually improve things but
fundamentally we are still limited for
every electron that has a spin of h bar
over two it can flip from plus h b over
two to minus h b over two so transfer
only one h bar
and the same also holds when you look at
things like domain bar motion or
skermion motion that when the electron
goes across a domain wall then in the
adiabatic limit it just switches its
spin from up to down so by one h bar so
you transfer one h bar onto the
magnetization of the domain wall and
that then limits the domain wall
velocity now of course we all know
because we hear that there is something
more than spin angular momentum and that
is orbital angular momentum
and so with this it was shown that you
can transfer more than one h bar per
electron if you have multiple
interactions often electrons say with
the domain wall as the electron passes
across it
so this is fundamentally a really
exciting opportunity that you can
increase and it's been shown
experimentally affected 10
in terms of the efficiency of the
torques compared to the previously used
spin transfer talks
so the spinomid torques which is kind of
the next step beyond spin transfer
torques this is something which you
already heard also again from a few
speakers before there are two
fundamental mechanisms which is the spin
hall effect there's a nice review from
hiro
and this is essentially
spin right electrons which are scattered
or with intrinsic uh moving to the top
interface and then they diffuse into a
ferromagnet and then they act on the
magnetization and now comes again the
experimental banality and that is what
we can measure is only the direction of
the exerted torques or even worse we
only measure effective fields which
either have the symmetry of a so-called
damping-like or field-like torque and
that's all we can provide you with so so
please theoreticians believe us we do
not measure
a spin hall effect torque or rush by
edelstein talk what we measure is an
effective field
and we can tell you the symmetry of the
field that's it and all the rest is
indirect interpretation
now in addition to this bulky effect
which is the spin hall effect you also
have this inverse spin galvanic effect
or rush by either shine effect where we
have a non-equilibrium spin density that
forms f4 current flowing at the
interface which then also exerts a
torque by exchange interaction on the
magnetization
so this type of torque that is exerted
can also have the two symmetries of
damping like oslanjeski like and now
there is a lot of literature and now for
the students some people call this
damping like talks lonzewski talker
anti-damping or in-plane or anti-dumping
which i thought was really nice um
i don't need to find out the paper i
said published paper where it's an
anti-dumping talk
um
okay and there's a field like which some
people also call the effective here the
rush ba
like perpendicular out of plane and so
on now what we are always interested in
is of course if we are fundamental
physicists to understand which talk
originates from which origin
and again we also have these orbital
effects that i'm going to talk about
next but again please remember that the
only thing we can provide you with is
the strength of the effective field
which is mini tesla current density so
melee tesla per amps per square meter
and the direction so the symmetry if
it's damping like or a field like torque
and now we need the input from the
theoreticians to actually see what do we
have to vary in our samples in order
then to understand where one talk
originates from and that is actually
quite tough when i started the business
with spin orbit torx in my group i don't
remember eight years ago or something
like that everyone was saying
spin hall effect generates a damping
like torque rushed by edelstrine effect
generates a feed like torque and that is
absolutely not correct so i think we
have meanwhile established that both
mechanisms can generate both torques
also theoretically but also that a lot
of simple experiments like varying a
thickness of a layer are not sufficient
because when you vary a layer you change
interfaces you change strain a lot of
things that can happen so it is a
difficult task but it's not to put you
off so for the students this is super
exciting because we suddenly have big
effects
i wasted my youth on permaloyd permaloy
has nice domain walls and you can move
them to spin transfer talk but it's a
very small effect the fields are
a few earths at 10 to the 12 m's per
square meter in spin orbit talks we
already have 10 times more we have milli
tesla per 10 to the 12 amps per square
meter and now with orbital talks i'm
going to show you another factor 10. so
we are 100 times better than where we
were at uh 20 years ago when i did my
phd
okay so that's kind of the introduction
now comes the interesting and difficult
signs
um you already heard all of this so in
an analogy to the spin-hole effect that
generates a transfer spin current from a
longitudinal charge current we also have
an orbital hall effect that generates a
transverse orbital current from a loot
longitudinal charge current and we have
an analogy to the inverse being galvanic
effect or the spin rust by edelstein
effect that generates a non-equilibrium
spin density at the interface we can
generate by the orbital rushed by either
side effect and non-equilibrium orbital
density at the interface
so okay
now again remember we measure two
torques and now we already have four
origins so actually there is a lot of
scope for you know getting things wrong
um but of course before we actually go
into the details to see what we can
learn let me just uh
you know my summary of what we heard
from hyun woo
yesterday so
in the 3d metals typically we have in a
ground state orbital quenching but when
we actually have an electric field or
also some other excitation we can get
orbital angular momentum flow or
accumulation and that makes it really
exciting so even though
in most typical 3d metals and apparently
what i tell my students in my magnetism
lecture course is not wrong in the
ground state the orbital moment is
largely quenched when you excite it you
can generate a strong orbital uh angular
momentum flow so i should also say that
i'm absolutely new to this field so
there's already tens of years of
experience in this there's some original
work here of course don't work work from
a couple of years ago so we came to this
experimentally via a lot of motivation
from dongbok and jerome kruzov to
actually understand some of our results
so that's also really nice to be
motivated by theoreticians to look for a
new effect
now one very motivating slide which i
took from drunk work is this one here
where we compare the conductivity the
spin whole conductivity in the orbital
hole conductivity and then we saw that
if you compare theoretical calculation
of platinum and manganese we can get
like an order of magnitude more orbital
hole conductivity that obviously is
extremely motivating because i can tell
you after a lot of frustration in
permaloya very small effects measuring
large effects is simply better fun also
for the students
so
yeah that motivated us um and there's
this this work here where they
calculated the orbital conductivity and
found that for some materials which you
know for us were not very exciting
people for
very very large or potentially large
orbital conductivities
now in addition to the bulk orbital hall
effect we have also the orbital rush by
edelstein effect where you have a chiral
double angular momentum texture in k
space that induces the orbital angular
momentum this an accumulation of orbital
angular momentum which is analogous to
the spin rush beta stein effect and what
is exciting is that it can exist in
light metals and acidic structures like
copper copper peroxide without the need
for strong spinal coupling
and one has to also point out again also
for the students that
if you talk to companies like intel
they're not super excited about using
tons of iridium ruthenium or platinum
because it's rare it's heavy
it's also
sometimes toxic to the environment so
cadmium is also really nice material for
some experiments but i can tell you no
no if you talk to the european
commission they're not very happy to
have cadmium in any of the devices that
you develop i think if you run write a
grant proposal we're going to do mercury
and cadmium
might have some issues but
so that is another motivation that i
think we should not underestimate that
there is not necessarily now a need for
heavy environmentally detrimental and
rare and also expensive atom species so
i'll show you later also some results
that we have on niobium which is a
relatively abundant and relatively light
material
again there's some original work
here more than 10 years ago
and if you're interested have a look at
that
now
this is all great i mean this you
probably all heard about it just with a
little spin so now comes the
experimental problem how can we identify
what can we measure
and there are three things and we
already heard about a really nice
measurement which is the curve effect
measurement where you just have the
single material i think in this case
titanium i think this is a really nice
approach of course as we saw it's also
tough i mean nanowrite i mean maybe
theoreticians don't realize but
measuring nanowright rotation is not a
lot so it's not so easy
um so it's really a nice feat um but we
are actually interested in
measuring the uh orbital effects by
measuring torques and measuring magneto
resistance
um and i think these are the two things
that i'd like to walk you through and
also show you some unpublished hot
results
that we just got in the last couple of
months
so let's start with the torques so the
first
thing that i would like to show is that
there are of course ways to quantify
torques that we use for spin orbit
torques and then there are ways to
indirectly infer where they come from
so if you look at
these two pictures here
then in this case here we have a
light metal let's assume it's niobium or
whatever where we generate by a
longitudinal charge current a transverse
orbital current and this orbital current
then enters a ferromagnet and now you
know what should it do there are two
ways that you can actually act it can
directly interact with the orbit of the
ferromagnet or as we heard also
yesterday we can convert the orbital car
into a spin current and the spin current
then acts on the magnetization
so in the end what we're measuring is a
change is an effective field acting on
the magnetization you can also have the
or the opposite which is orbital pumping
so this i think was first shown in this
paper here again i think we should give
a lot of credit to the people that
actually did this a little bit
underestimated i think for four or five
years no one gave a about that pair
very few people cared about that paper
until people actually understood how
groundbreaking this was and then you
know as you can see we were like four
years later
um
so the experiment that we wanted to do
is this one here we want to measure the
torques acting on a magnet
and the first experiment we did was to
work on a magnetic insulator
now if you have a magnetic metal you
have a lot of things happening you have
current flowing in the metal you get
self torques anomalous torques
so i i really like magnetic insulators
not because they're easy to make they're
actually not easy to make but because
you don't have the problem that you have
any charge kind of flowing in them so
anything that happened has to happen by
the charge current at the interface
there's no electrons entering into the
magnetic material so what we chose is
sodium ion garnet platinum of course it
also has germion so you know it's an
amazing material and skeletons are very
hot and everyone loves skermions but i'm
not going to talk about it but just
partly we had this material at hand
because we were studying in this paper
here actually skermions in this thalium
iron garnet
then we put platinum on top
to start us studies without torques and
yes so this is a measurement of the
spinova talks and there are different
ways to measure that if you're
interested here's the paper but what is
important is if we vary the thickness of
a platinum
we see that it increases and levels off
and that's very typical for all spin
orbit torque measurements
so you have initially an increase up to
the thickness of the spin diffusion
length in the platinum and then it
levels off because essentially all the
spin current that is generated in the
platinum then
a further away from the interface
doesn't get to the interface and you
maximize the efficiency of the torque
here
so this is actually pretty much what we
get in yik platinum thorium iron garnet
platinum lots of garnets platinum with
something like a spin diffusion length
of 1.8 nanometers in the platinum that's
all standard
now my very smart student chile ding he
put copper copper oxide on top partly
motivated by this existing material
and i was very skeptical because firstly
copper is you know light secondly copper
oxide is insulating so somehow you know
i don't see why it should do anything
useful
and thirdly because you know he just let
it rot at air which i thought was not a
very well defined process but then this
is what happened
he saw that if you put copper oxide on
top you get a 16 fold increase of the
acting torques so sometimes for the
students do something crazy and you know
even if you're a supervisor or don't
tell your supervisor i only tell him
when you get the curve
um so uh i think that this you know he
came back with that curve like that you
know something is wrong so we did a lot
of tests we checked you know what is the
conductivity of the copper oxide we
removed the platinum see what happened
but actually it's robust
and we saw that for
a certain thickness of the platinum you
actually have a strong increase in the
torques and you know not a strong but a
16-fold increase that's gigantic i mean
you know we were always fighting for a
factor two here just you get for free
effective 16.
so what is happening
interpretation was that
in the copper copper peroxide you
generate an orbital current at the
interface and now this orbital current
in the platinum is actually converted
into a spin current and the spin current
then generates a spin-orbit torque that
acts on the thorium iron garnet
and what is interesting is of course if
you look at here there's non-monotonic
dependence and that can be described
quite nicely because of course when you
have very thick platinum then eventually
the orbital current that is converted in
the first one point uh something
nanometers of the platinum
then simply doesn't get any more spin
current to the fullium iron garnet so if
you make a lot of platinum
of course nothing will happen if you put
one meter of platinum and you put some
copper oxide on top nothing will exit at
the bottom interface but for thin
platinum layers you have an extremely
efficient torque generation and that
means you must have a huge orbital
current because you now have two times
things that need to happen you generate
the orbital current it needs to be
converted to a spin current and the spin
current then acts on the magnetization
so fundamentally one would say should be
less efficient just means there's a huge
orbital current that is being generated
yeah and then we could fit this and get
the length scales of the orbital to char
orbital to spin current conversion in
about one nanometer
now what is also nice about these iron
garnets is they have typically nice
spin-hole magnet resistance effects and
then we measure the spin hole magnitude
resistance in the system and we also
found this peak at something like one
point something nanometers of platinum
thickness so here again this is the
series a is a series without the copper
copper oxide
and then we see there is a typical
increase and then decrease just because
you get shunting
and
if you put the copper sorry in the
series b the copper copper oxide on top
you have this huge increase the 16 fold
16 fold increase and then the reduction
as you get more shunting and also we put
some actually all the credit to chile he
did it
put also mgo to protect the copper from
oxidation so not let it rot in air and
then you see there is no increase at all
so it's really the copper copper oxide
that makes a difference not the copper
in this case
and we should say that you know a lot of
the explanation was supported by donbook
and euron
okay so this is the first thing we can
do so we can generate huge torques by
putting some really crazy stuff on top
of platinum
the second thing we did is we wanted to
look at magnetoresistance effects to see
if we can actually also obtain magnesium
resistance effects in these type of
systems and here we remove the platinum
so what we did is we just did permalink
copper you know this is from my youth is
like permaloy and so i said how about we
just put parapromelo and then copper
copper oxide and see if we actually need
this conversion layer
and then for those of you that are into
magnetic resistance effects there are
three scans that we can do we can rotate
the field in plane we can rotate the
field out of plane perpendicular to the
current or out of plane in the current
direction and then if we do this beta
scan then we get the smr type spin hall
magnetoresistance type of resistance in
this case is orbital rashba edenstein
magneto resistance and we do this now
for different thicknesses but this time
we vary the copper thickness so the
permeable is fixed but we vary the
copper thickness and what we see is that
actually we get an increase with
increasing copper
and again we have nothing on top so we
get copper copper oxide
and then as we increase the copper
thickness eventually the copper oxide
doesn't oxidize through and we get
shunting and the effect goes down again
now what is interesting about this is
if we now compare
the different thickness of permaloy and
the different thickness of the copper
we can see that if we just put permeable
platinum we again get an increase of the
spin hall magneto resistance for the
first two nanometers or so and then a
decrease
but if we put copper oxide on top we get
a different length scale we get a peak
at something like seven nanometers
compared to three nanometers here and
the length scale of 5 nanometers
compared to 1.83 nanometers here and
that shows the actual magneto-resistance
effect must have a different mechanism
must have a different physical origin
so
here it's standard um inverse spin hall
effect so spin hole magnitude resistance
and here we have orbital hall inverse or
orbital rush by edenstein
and inverse orbital rochelstein effect
so this i think is really exciting
because now we have two means to
determine
the possible orbital effects that can
occur one is measuring torques one is
measuring many resistance effects
now
copper copper oxide is great
effects are big but it's also badly
defined you know theoreticians don't
like it you know if you talk to don't
work he doesn't like calculating copper
oxide because we cannot tell him what we
really have you know we don't have good
copper copper oxide what we have is
copper that rots in air we also do it in
plasma but then anyway so
you know then we have peter peter comes
up with all these amazing calculations
so this is from this archive paper here
and then you know he compares and it's
not only him obviously there's this work
here there's don rook's work
and and many others and then he
calculates the spin hole conductivity
and the orbital hall conductivity and
then you just go through this and you
just pick something where you see a peak
like athenium and you select let's do
that ruthenium is great we have
ruthenium in the chamber and therefore
we can just deposit it and you know we
don't need this oxide which is badly
defined it's also a little bit
frustrating if a paper like this comes
out because this is like you know 10
years of work for an experimentalist to
go through this
and maybe three months of calculation
anyway so what we did is we picked
ruthenium and niobium because they have
large orbital hole conductivity but they
have very small spin hole conductivity
so that's what we did and also don't
work did the calculation uh in a bit
more detail for niobium ruthenium and we
see huge
orbital hall conductivity and in red
very little spin hall conductivity and
then we did measurements and now all the
credit to arnhem ana bose he's a postdoc
working with me and the paper that we
just finished is from him and he will
show details tomorrow in uh the poster
so what we measured was niobium
nic iron cobalt boron and a niobium
nickel and we measured the acting
torques
and we first measured with iron cobalt
boron which is our standard system for
tmr junctions and the torque is super
small we see virtually no torque
and then we measure on nickel and we
have a huge increase in the torque and
now this is something important again
for the students this does not happen
for spinova torques for spin orbit
torques the effect is big on all
ferromagnets or small on all
ferromagnets here there is a huge
difference in that we have a major
difference whether we act on a different
ferromagnet or nickel or iron cobalt
boron
so the same holds for ruthenium
again withinium and iron cobalt boron
very little effect and ruthenium nickel
huge effect and the interpretation is
that you need to convert the orbital
current into a spin current and that
works well in nickel
but it does not work well in iron cobalt
boron so i think here is a huge field
also for students to invest yourself
experimentally trying out different
ferromagnets to see what ferromagnet
makes the best conversion from an
orbital current to a spin current by the
way this was measured by sdf homogeneous
in case someone's interested ask arnob
about details tomorrow
and now i think we had yesterday the
question from kung jin li how about
anomalous talks and and um
self-talks so what we did is we actually
then
sandwiched the nickel between ruthiem on
both sides so that we have equal
interfaces and then the effect goes away
so it goes away from niobium and
ruthenium so there is no large intrinsic
effect in the nickel itself so this is i
think also a very important check now i
said okay
there's no
strong talk on iron cobalt boron but
there's a strong talk on nickel can we
make actually iron cobalt boron to be
manipulated
and well it turns out we can because
what we can do is we can again inject
our platinum layer
so now we have a more complex system
where we have ruthenium not directly on
iron cobalt boron but with a platinum
layer and we again put a second platinum
layer on the other side to cancel out
the spin hole conductivity and if you
just do platinum iron cobalt boron
platinum then the effect is virtually
zero but now we put ruthenium on one
side and we're going to get a large
torque
so now we are able to generate a large
orbital carbon in ruthenium
convert it to a large spin current and
platinum and add spin current works
effectively on the iron cold boron and
that is important because companies
don't like nickel nickel is not a very
nice material for an amram device they
want cobaltine boron because they want a
good mgo tunnel junction afterwards and
so the fact that we can use just one
nanometer of platinum to get again this
very large torque allowing us to act on
the iron cobalt boron or coal dye and
boron is super important because then
this allows to actually switch iron
cobalt boron which is what we want to do
for an mram device
now all the credit to anup and his
poster tomorrow so please do see his
poster and now i see my time's already
up so this is the people that did all
the work i just get to talk
and travel around the world or travel
not so far around the world and give
talks so these are the people that were
involved in particular arnab fabian and
ross saoin yen chile ding sven becker
kyung jin lee
klaus rabb and lucia and the others as
well in particular three staff
scientists martin jordan gerhard jacob
and our visiting professor hartmut zabal
this is what we looked like last year
and oh sorry
so this is what we looked like last year
now this is what we look like this year
things are going uphill also a lot of
credit go to our colleagues from picking
university who worked with us
together with chile on these first two
prls and also a lot of credit go to the
theoreticians that motivated us to get
into that direction so don vuk and frank
from eulek and yura also from euleg and
mainz and then colleagues in sendai
sydney and also the group of hiro helen
gomonae karen averso city has now moved
to duisburg and people wear measurements
as well as some of the funding and some
work on the iron garnets with caroline
ross and yeah most of the work was
funded by the germ research foundation
as well as the eu and also some from
companies
and uh yeah with that i summarize so i
mean this is the banal
thing analogous to the spin hall effect
in the rush beta shiny effect are the
orbital versions but they are not easy
to identify as an experimentalist it's
not straightforward to say what we
measure is due to one or the other
however there are ways to indirectly
infer it by looking at multiple samples
with different stacks and understanding
how orbital currents can be converted
into spin currents in heavy metal layer
or layers that have large spin-over
coupling and this was shown in this
first pll here
we find this very very strongly enhanced
orbital talks which i found extremely
surprising and then exciting because it
means we can go beyond what is the state
of the art
we measured the orbital rush by eighteen
strike magnetoresistance where we saw
that we can even get away from the
platinum and use a stack just comprising
a ferromagnet and a light metal layer so
we have orbital to spin conversion in
the permaline itself so not necessary to
have another layer
however if you do want to have iron
cobalt boron switch you need this
additional layer and there we find that
in well-defined niubium ruthenium where
we know the crystallite size we know the
crystalline structure we can tell don't
work what we actually have so you can
calculate something reasonable uh we get
a strong dependence on the ferromagnet
which is unique to at least as far as i
can see to this orbital effect is not
present in spin-orbit effects
and yeah please see on us poster
tomorrow and there's a nice review that
don book wrote and i contributed a
little bit to it and with that i thank
you for your attention and i'm looking
forward to the discussions
Посмотреть больше похожих видео
5.0 / 5 (0 votes)