Leonard Spinu: I think I had the list number
of FORCs in my presentation,
and I wish I had
this presentation before Kai's because I have
a presentation based on
some results you obtain and you can
present FORC results only on two systems,
magnetic nanowires and exchange bias system.
The difference between the system is
that magnetic nanowires is homogeneous and I
say uncoupled system and
exchange bias is an example of couple system.
I will try before arriving to the full diagram,
I'm going to present the reason and try
to explain some of the keywords appearing in my title.
First of all, now structures,
and this was the reason I
was thinking that probably this was good to
have presentation before Kai because I know we
have several students in our audience
and I'll try to get this progression.
In terms of nanostructure, this is
a scale of things chart,
I think it is from DOE,
and actually have on the left hand side
features nano scales that are nature
made and actually go progression from
larger scale to small scale nanometer scale here,
up to the DNA.
Also we have man made things that are nanometer scales,
and here we have the regular scale that we know.
The idea is that we try to do
these nanostructures and the idea
is to obtain new phenomena,
not necessarily miniaturization,
like in these mum's examples.
The motivation is that using this structures, actually,
we can address very important problem
we have nowadays is information,
which actually we assume that we like to
have it anywhere anytime and for everyone.
In this case, basically,
we face this production,
transmission, storage, and retrieving
information of a large amounts of data.
That's why we like to do this
very quickly and accurately.
That's why you need to have each stage of
this process of information processing.
We need to have device available materials
able to handle this type of information.
Here I show in chart showing, for example,
in terms of recording of
information using magnetic materials,
where you see that how during the different years,
we have different ways to
store the information and different technology
appearing and here,
actually we are talking about
the heat-assisted magnetic recording
or patterned magnetic recording,
and we see that actually we are
basically approaching the one the abit per square inch.
Now, this is a huge progression actually
to see this in other terms,
I have these pictures which showing a hard drive,
which is forklifted in airplane.
Actually this is IBM 305 RAMAC.
Actually to have a better perspective here,
basically showing what's inside,
you have huge wheels of 24" platters,
and this has only 4.4 megabyte,
and we have 50 of those and compare for example with
a regular hard drive and you see
that it's amazing advancement.
For this, actually, for
precession information you need to have high speed,
non volatile, endurance and
high density and small and low voltages.
That's why I go to nanostructured materials,
which can be in different flavors,
for example, three dimensional.
All three dimension are smaller than 100 nanometers
or you can have only one dimension, smaller.
We have this multi layer,
therefore the vertical distance is a scale of
nanometers or you have
two dimensions like in the case of nanowires.
In this case, you are able to arrive to a technology like
magnetic resistive random access memory
or actually compared with
other type of memory technology has
the potential to be universal memory that it's fast,
also non volatile and also amazing properties.
Now, talking about the MRAM actually
here show the evolution of the concept of MRAM.
Actually, the way we store the information,
you have actually a fixed layer shown here
in blue and also a red layer, which is a free layer,
and depending on the reciprocal orientation of
the fix and free layer,
you are able to store
the information and as the time passes, actually,
it was showing that actually instead of using
a simple homogeneous layer like uncoupled system,
you need to use
a exchange bias material
to pin the layer for the fixed layer.
Also later, we even use a synthetic antiferromagnet with
another couple magnetic system that actually you're
used to do this memory possible.
That's why you have not only simple magnetic systems,
but also have couple system.
That's why when you do and start the switching,
you need to have these two perspective take into account.
Now what we understand by switching because this
was another keyword I have in my title.
Actually here showing two ends of this process.
In the sense that you can understand that we have
a magnet here show and this particle with the spin up,
you like to orient that particle spin down and this case,
you need to overcome this energy barrier
go from up to down.
This can be understood in
a very classic or simple way
through Stonner-Wohlfarth model.
I have actually a chart showing all this.
Now, if you like to do a fast memory,
you need to take into account how much time it
takes from up to down and
that's why you need to take into
account the time dependence of the switches and
that's why you need to treat it with
the time domain approach
like Landa Lisa Gilbert equation.
Now, for switching, we talk about
static switching because here we talk
about FORC and actually FORC is
a static measurement method because
the measurement time is much smaller than
let's say relaxation time for a magnetic system.
Now, here's an example from a paper that I
hope that any graduate student working in
magnets find time to look at and probably already did.
But this paper of total evolve actually is I think one of
the most important model
to be understood and very simple to
understood in the case of magnetic switching.
The idea that Wohlfarth
was a student of Stonner and actually as a PhD,
I think this was his work.
It was this simple problem, having a particle,
was a question how it switches and
what we can understand the switching
from one orientation to another one.
In this case, you need to take
the energy that we can take
into account in this case
for switching and you have the field energy,
the isotropy energy,
and finally to arrive to
this well known plot where we see this.
This loop depends on
the orientation of magnetic field respect
to easy access in this case.
Now, this is a very interesting and very important result
and it was only later
understood better when Slonczewski
basically came with this correlation
between the regis loop and actually the critical curve.
Critical curve is something that actually
separates the region inside where we
have just stable equilibrium position.
From outside, we have a single equilibrium position,
and that's why it's very important to understand
for people working in the memories.
Here it's a correlation,
I have here a short animation for showing the correlation
between hysteresis loop here and
the energy barrier and here, the critical curve.
You see that actually as you go from
positive saturation and we
have the switch here when
the energy barrier disappears right now,
you have a switch where
the magnetization irreversibly switches.
Therefore, I can understand
the hysteresis loop by thinking energy barrier,
but also having also another image
of this switching using this critical curve.
Now, this is not only a theoretical concept.
Usually are people working on this area
they're able to compare
this theoretical with experiment
and also another example,
therefore, you're able to do that.
I show this because this is a static and
also is a way to switch when you have a uncoupled system.
Now, we have a couple system like the one
present in a memory.
This show a memory stack where you
actually have a base electrode,
we have a pinning electrode,
and we have a synthetic antiferromagnet and
also we have a tunnel barrier and again,
a synthetic antiferromagnet,
you have multiple layers here.
Therefore you see we don't have isolated layers.
Therefore switching could be more complicated and
indeed it is because if I'm
looking of the critical curve here,
from that simple uncouple,
you're getting this weird shape and actually
it's more complicated the switches for uncoupled system.
Actually, this is a parallel between
uncoupled versus couple system where
the switches can be described by
a single variable orientation of magnetization.
Here, if I have let's say,
a synthetic antiferromagnet or you
have two magnetization to take care of,
we have a more complex picture
and just to show the same example,
we had with uncoupled system,
I show here the same plot where we
have here the hysteresis loop
or synthetic antiferromagnet,
here the energy barrier.
In this case, we deal with
two variables and also the critical curve
and we have two magnetizations which is and so on.
For couple versus uncouple is very important.
I said I'm going to present some FORC results
on one for uncoupled system,
namely my internal wires and
another one for exchange bias systems.
Now, magnetic nanowires,
this is the way they look like
after I prepared and actually,
we are able to do this work in our lab.
The idea behind using nanowire is that are
many application can be
achieved using this type of structures.
In order to prepare them,
you can use the template based method,
which is an essence using a mold,
and then you put in that mold,
melt the material in such a way that you basically
cloned the material to
have all cells or
all elements have the size and the same characteristics.
Actually using this template based method,
you can do a variety of materials and here
show some of the materials you're able to do in our lab.
In terms of nanowires,
one of the first work on growing nanowires using
template bed method is in this paper in 1990s.
Actually, you have a template and you put electrode
and you put an electrolyte and actually
you have electric deposition,
the metal deposited in this is the
porous of the membrane and you can obtain
different wires like simple wires or
more complicated wires depending on
the complexity of your template.
Now, when you have a collection nanowires, of course,
you have a certain distance between
them and we heard from
previous speakers that interaction
between them can affect their properties.
That's why there are several studies
before showing that actually it's very
important to take into account the interaction.
That's why we had this idea how we
can try to understand
better interaction in magnetic samples.
That's why we're focused on magnetic nanowires,
and our goal was to understand the interaction.
First of all, by preparing very good samples.
We wanted to have samples
with very well known characteristics.
That's why if you
like to do samples using template bait method,
you need to prepare the template with a high accuracy.
That's why in our institute actually,
we use this anodized aluminum oxide template
and actually have a pretty standard fabrication process.
Actually, you start from aluminum foil and actually have
a first anodization to create these channels.
Then you have etching and then you increase the height
of the ports and you eliminate the bottom and then
you put an electrode which we use for it to deposition.
In this case, depending on
the anodization technique because you use acids,
so the time, therefore the strength of the acid,
therefore the concentration, also the time
of acid using anodization,
you can obtain different templates.
Here are shown example of the templates
obtained using different anodization time.
You see that actually are pretty much
the same center to center distance,
but we have actually larger diameter.
Therefore you are able to prepare these templates.
We try to get the distance between nanowires also
varied and that's why we use
so called double anodization and
actually have a mild and hard
anodization on the same template.
This is, for example, the set of wires that can be
grown in a simple template with a single anodization.
Let's call it mild and by
mild means that concentration is
not very strong and actually in this case,
you can have very small diameter.
Now if I take this samplate and I make it
at the top with
a hard anodization using higher concentration,
I'm going to promote these larger channels,
but when the larger channels are going to grow,
they're going to operate some of
the smaller channel on the other end.
If I use this mild hard anodized membrane,
I can grow wires only on
the places where the metal can get in.
Therefore, I can compare this set of wires with
this set of wires which are
now larger space between them.
Therefore, I have the same type of
size distribution because I started from
the same type of template,
but I have different distance between them
by this mild hard anodization.
Also, I can go the other way around to grow
the wires on the hard side and get this larger wires,
but the same center to center distance,
or I can hit the combination of those.
Here show some real examples of
this mild and hard anodized membrane
and here of a series of
the sample we consider in our study.
We have so called mild anodized samples,
therefore grown only in the simple wire.
Then we have mild hard
therefore they have those which some wires are
operated and also we have the
hard which are larger wires or a combination of both.
Now this first task
having good sample was being achieved.
Now the question is how we
can actually measure them and how to quantify them.
There are several methods for study in
magnetic systems and the point for
using one of the other is
have this simple aspect in mind.
First of all, if you have a method
for measured or quantified interaction,
it need to be simple to
implement because it's very complex,
algorithm is very difficult to
implement and it's not useful.
Then you need to start from
a magnetic state which is very reproducible.
It's very important that all the time,
when you want to repeat
that protocol for identified interaction, the sample,
you need to make sure that all the time you
tied the same state because as you know,
magnetic materials are affected by hysteresis
and that's why we want to make
sure that we eliminate any.
Then we need to make sure that
the protocol has a reasonable number of data,
therefore not too large,
because in this case you
may take too long and actually you're not going
to be very convenient to implement.
That's why there are
several methods we take into account.
For example, one of them was major hysteresis loop.
By just looking at the shape
and as some of the previous speaker mentioned,
if a shape is a little bit tilted,
therefore it's indication having
mean field interaction, and so on.
For DeltaM plots, another protocol that was
previously proposed, or Henkel plots.
Now in terms of hysteresis loop, it's simple.
Therefore, they're required to satisfy,
and we have, let's say, a loop like this,
and we have main parameters like coercivity
and remanence and squareness,
which is the ratio of two,
and you can try to look at the interaction.
But problem is too simple because you may
have features that are not present in hysteresis loop,
but you can have different samples.
This was actually highlighted by
previous speakers showing that actually samples
with more or less the same hysteresis loop
they present different FORC diagrams.
In terms of DeltaM plots and Henkel plots,
this protocol is based on
a paper of Wohlfarth where actually relates to
different magnetizations like remanent magnetization
, and DC magnetization.
This is a relation that relates this two.
The problem for this protocol is
that the demagnetizing state
was very well defined because
we're showing that actually if you use
a DC demagnetizing state or use
a thermal demagnetizing state
you basically have the same zero magnetization,
but actually the structure inside is
different and actually you obtain
different Henkel plots.
Actually, in this paper, actually we're showing
that actually we have this problem.
That's why I arrived to take into
consideration the higher order magnetization curves.
By higher order, I
mean that actually I'm going to reverse several time.
I'm going to come here let's say,
then reverse one second, third, and so on,
and that's why we
talk about first order reversal curve
and I'm going to insist on this slide,
and as the previous speaker that we have
this magnetization depending on
the reversal field and
the field I stop and you can build this FORC diagram.
As a matter of fact we consider this possibility.
I think we are the first one
showing that actually we can use
FORC diagram to describe the interaction in nanowire.
This is a paper in 2004 and soon after,
actually this possibility was recognized by
other researcher and working on the field of nanowires.
We took our samples and actually
we got the FORC and then we got
the FORC diagram and we did all the
systematic for the samples I just
mentioned and these are the results.
On the left hand side I have the nanowires,
which have the same diameter left and right.
The difference between them is
the distance between nanowires.
This is showing wires
with 100 nanometers center to center here, 250.
You see clearly that
actually the difference between them is
just this distribution of interaction,
which is clearly visible here.
We use the same type of template,
the difference that we operated
by hadronization, some of them.
Therefore it's clearly we're able to
identify and make this comparison
between strongly interacted and
weakly interacted nanowire.
That's why you could easily get
this coercivity and also the interaction distribution.
Now we choose wires which have
the same center to center distance between them,
but they have different diameter.
Again, the fact that we have wires with larger diameter,
therefore are closer edge to edge,
we have stronger interaction and here
show exactly here on this diagram.
Actually we have this comparison
between these two samples.
We discussed previously about
the presence of this ridge here.
Actually, was mentioned by Alexander, actually.
Some of the researcher try to identify the press of
these two distribution with more coercivity and
higher coercivity by the press
of two entities in the system.
Actually, it was I think Baron group in Brazil,
they took nanowires and they polish them,
and they claim that actually by polishing them,
actually they made all the wires with
the same length and actually this ridge disappear.
They claim that actually they saw
that ridge and actually
as evidence for the existence
of two different population wires.
Now, in our case we rule out this possibility
because if we look of different set of wires,
we see that we have the ridge when we have,
let's say, this simple wires.
We do not have it when we have these large wires and
the separation between mild anodized wires,
this one here and the hard one is very well defined.
What I'm trying to say is that actually the length
of the small wires which
present this ridge actually is very well defined.
Therefore, they are not two different population.
Therefore, here we may think
that when the wire were grown,
you may have different heights.
But here, the component that presents this ridge,
they have the same length because if you look at
this image of hard and mild,
you see that actually the hard side
and mild is very well defined,
therefore, all the wires they're going to
have pretty much the same height.
Therefore, we rule out that.
Actually, we actually assign this feature existence
of highly symmetric [inaudible] similar that was
observed in the previous work on the system,
Edward Henderson Ising spin glass.
Actually, we gave
explanation that this is intrinsic feature
of the material itself,
not necessarily of the presence of two entities.
Now, we are able by this analysis of
FORC now we have all wires,
we have simple, and
then we have this complex and you see
how the interaction distribution changes and you see
that actually we have here
very pointed interaction distribution.
Here for hard part we have this very large.
When you combine the two, we have
some combination to see that actually we can by
this fabrication technique to tune
the way we have the interaction in our system therefore
by careful fabrication of the sample.
Now we're able to do a statistic analysis
and for all the samples and we're able
to do a fit of all these features we observe.
Actually we are able to get this graphs of
bias and interaction and
also the interaction the way is varying on the system.
Yeah, this is
actually a conclusion that basically we have
the dipolar interaction affected the behavior
of switching in Ni nanowires and
we have the FORC that can give
insight in the way this interaction occurs.
The second system that I'd like to mention is
this coupled system that we have, exchange bias,
and this is a very old effect
observed in 1950s and actually this is a paper by Bean.
They observe that we have
cobalt ferrite and also it's oxidized.
We have cobalt oxide and when you cool in zero field,
we have this hysteresis loop which is shifted.
It can be explained by
this interface between
a ferromagnet and also antiferromagnet.
Actually, we have here
a simple explanation how this phenomenon occurs.
As Kay mentioned,
still this exchange bias, how you measure,
how you quantify is
very debated question because we have static techniques,
we have dynamic techniques,
and we are trying to see,
using different techniques how we can
quantify exchange biases and different samples.
We have a series of iridium manganese and also coupled
and we have three samples
and we have the repetition number of
these multilayers and the thickness
of iridium manganese was kept constant,
only the ferromagnet thickness
was modified in such a way that
overall sample after all repetition
being done is kept constant.
We consider static but also dynamic type measurements.
I will refer just the first order reversal curve and
also the major hysteresis loop method
how we are able to identify the exchange bias.
In terms of major hysteresis loop it is very simple.
You have hysteresis loop and
actually the displacement of
hysteresis loop with respect to origin give
you an average exchange bias.
Here show the different loop of
different angles and different orientation
and we have 180 and zero degrees, therefore orientation,
but also we have zero and you see
the displacement for zero and 180
is opposite while the 90 degrees
actually don't have this displacement.
For three different samples,
you observe that S1 have
a larger displacement, the largest displacement,
therefore it's easy to have
this analysis and look at the exchange bias value.
We have 70 Oersted for the first sample,
37 and 25 for the sample S3.
Now, we wanted to get
more details in terms of the exchange bias.
Exchange bias also is more
complicated to be captured by a single number,
therefore that's why we wanted to get more details.
That's why we look on the FORC,
and actually we have these samples
and we perform a FORC analysis.
Actually, at this point we see that yeah,
we observe the displacement of the curves,
which is displaced by exchange bias,
but also we have other features that can be observed in
the four diagrams by
the present additional satellite distribution,
which give information about
the homogeneity of the samples,
but also the way these are coupled and
the other information can be captured is also
the width of the distribution.
By comparing and analyzing the four diagrams,
you're able to get this exchange bias values
and we have value
but also the width of distribution.
Now, just for the sake of showing this,
we have also all different methods.
You have static and dynamics and actually
using ferromagnetic resonance we were
able to get about the same values,
but there are some variation and actually
this paper explain why you can get
this variation in terms of
exchange by it's complex phenomenon and actually
FORC maybe one of the methods that
can give some more insight in this phenomenon.
Conclusion for this part of exchange bias.
Actually, we observe
this increase of exchange
bias increase with the number of layers,
and actually the FORC can provide
more insight in exchange bias.
We observe some difference between
the values you obtained by static and dynamic.
With this I would like to thank the sponsor
of research in Embry, National Science Foundation,
and also Board of Regions of Louisiana
that also partially
supported my work and also I would like to thank you for
being here and listening to my presentation.
MALE_1: Hi. I really like your samples.
I'd love to measure them myself.
I'm wondering when I try and simulate
strongly interacting systems like
you have in your nanowires,
I make the simplest possible approximation,
which is I treat everything as a point dipole
and a stray field from a point dipole.
I'm just wondering if you have a sense from your work
how bad an approximation
or good an approximation that is.
Leonard Spinu: Well, it's a starting point.
But the things are getting complex because the diameter
is nanometers or you have it
larger for the hard anodized,
and you have nickel, which is pretty soft.
Therefore, that's why the assumption that it's
just macrospin approximation, a good starting point.
I did not do fully mathematic simulation
on that just to compare.
We had some work on
treating and explaining by
modeling the interaction nanowires.
We did as you mentioned,
using a macrospin approach.
MALE_1: Did you get close to the
experimental forms with the approach?
Leonard Spinu: Yeah. But we did we
did some other assumption,
working with different amortizing factors and try
to change and to amorate this model.
Therefore, not just simple macrospin model.
Yeah. But right now we are
doing 40 micromagnetic simulation
using a GPU accelerated micromagnetic code,
mumax is one of them that's available.
We ported this on a supercomputer and
actually right now we are able to
get quite nice results in a reasonable amount of time.
MALE_1: Define reasonable. How long?
How long do the simulations take?
Leonard Spinu: It depends on
the size of the system, I would say,
but for example we are using a supercomputer which
has 1,000 K20 GPUs.
With this, we have like say in two days you can get,
let's say, a pretty large system.
They have called a macro
geometry periodic boundary condition,
which allows you to simulate real size systems.
Because regular predicate boundary condition,
you just simple infinitely repeat the system left,
right, top, down.
But actually with macro geometry you can say,
this is the size of my system,
and this way, I'm going to rate this.
MALE_2: Thanks for the talk.
You characterize the FORC measurement as
a static measurement versus the dynamic measurement.
Is FORC dependent on the rate of field change?
Is there a frequency effect involved,
and has that been well studied?
Leonard Spinu: Yes. I think that the best person to ask
is I think the group
from Yash because they are doing some studies.
I don't know if they have a poster related to this.
Actually the poster, they
maybe have something related to that
where they have this dynamic effects.
For the question,
yes, it's depending.
MALE_2: What do you suppose the highest frequency
or rate of field change
would be for those types of measurement?
Leonard Spinu: I don't know. I don't know the experiment.
I don't know. Dorine, what is the frequency?
Professor Sanko in your case,
what are the frequency?
I think it's kilohertz. Yeah.
MALE_2: Thank you.
Leonard Spinu: But still from my point of view,
it's still static from my point of view.
Everything is below, let's say,
one gigahertz static. Yeah.
MALE_2: I agree.
Leonard Spinu: Yeah.