Tunable thermal hysteresis in CoGd alloys

Article abstract of DOI:10.1063/1.2132530

We show that tunable magnetic thermal hysteresis can be realized for a simple ferrimagnetic CoGd alloy. The magnetization of a Co0.8 Gd0.2 alloy shows a bow-tie curve as a function of temperature, which indicates supercooled and superheated phase transitions. The width of the magnetic thermal hysteresis can be tuned by an external magnetic field, with values of 190 K at fields near 100 Oe and values of 20 K near 400 Oe. Experimental results are in reasonable agreement with theoretical calculations.

Full text of DOI:10.1063/1.2132530

Tunable thermal hysteresis in CoGd alloys
S. Demirtas, R. E. Camley, and A. R. Koymen
Citation: Applied Physics Letters 87, 202111 (2005); doi: 10.1063/1.2132530
View online: http://dx.doi.org/10.1063/1.2132530
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APPLIED PHYSICS LETTERS 87, 202111 ͑2005͒
Tunable thermal hysteresis in CoGd alloys
S. Demirtas
Department of Physics, The University of Texas at Arlington, Arlington, Texas 76019
R. E. Camley
Department of Physics, University of Colorado at Colorado Springs, Colorado Springs, Colorado 80933
a͒
A. R. Koymen
Department of Physics, The University of Texas at Arlington, Arlington, Texas 76019
͑
Received 1 August 2005; accepted 26 September 2005; published online 11 November 2005͒
We show that tunable magnetic thermal hysteresis can be realized for a simple ferrimagnetic CoGd
alloy. The magnetization of a Co Gd alloy shows a bow-tie curve as a function of temperature,
0
.8
0.2
which indicates supercooled and superheated phase transitions. The width of the magnetic thermal
hysteresis can be tuned by an external magnetic field, with values of 190 K at fields near 100 Oe and
values of 20 K near 400 Oe. Experimental results are in reasonable agreement with theoretical
calculations. © 2005 American Institute of Physics. ͓DOI: 10.1063/1.2132530͔
There have been a number of recent investigations into
tunable thermal hysteresis in magnetic multilayer systems.
holds the spins in place. Eventually, however, this configu-
1
–3
ration becomes unstable and the structure reverses with the
Gd moments along the field and the Fe moments antiparallel
to the applied field. A similar situation occurs on heating the
system from low temperatures.
The thermal hysteresis occurs between two stable magnetic
states and can be exceptionally large, spanning up to 200 K.
In addition, the thermal hysteresis can be dramatically tuned
or eliminated by changing a small external magnetic field.
These studies have concentrated on multilayers made of al-
ternating films of transition metals and the rare-earth metal
Gd. Such a two-state tunable system could be employed in a
nonvolatile magnetic memory, however the operational tem-
perature in the previous studies centered around 200 K.
In this letter we report on tunable thermal hysteresis in a
Samples were prepared in a dc magnetron sputtering sys-
tem at room temperature. They were deposited on Corning
glass substrates and 20-nm-thick Ag layers were used as
buffer and cap layers. Deposition thicknesses were moni-
tored in situ using a quartz thickness monitor calibrated by a
stylus profilometer. 20 nm Co Gd alloy was created by
0
.8
0.2
codeposition from pure Co and Gd targets. Magnetization
͑thermal hysteresis͒ measurements in the temperature inter-
val 10–390 K were taken using a superconducting quantum
interference device magnetometer starting from 390 K under
a constant in-plane external magnetic field ͑100–800 Oe͒.
The temperature is first reduced to 10 K and then increased
back to 390 K. The temperature sweep rate was 10 K/min.
In Fig. 1, magnetization versus temperature is shown for
a 2-nm-thick Co Gd alloy for different values of the ex-
4
–10
CoGd alloy.
Alloys can have a number of technological
advantages over multilayer systems. First, multilayers sys-
tems often depend sensitively on the properties of the inter-
faces, for example the degree of intermixing or the smooth-
11
ness of the interface. Second, multilayer systems can be
nonuniform since one film may be slightly thicker or thinner
than an equivalent film in a different portion of the structure.
Third, multilayer systems may have large surface
0
.8
0.2
1
2–14
effects,
since films at the surfaces do not have the same
ternal magnetic field. In Fig. 1͑a͒ the system at 390 K is
originally in a Co-aligned state at 100 Oe and remains in this
state as the temperature is reduced to 160 K. Magnetic an-
exchange coupling as those in the central portion. Finally, we
note that the operational temperature for thermal hysteresis
in the CoGd alloy is centered around room temperature, well
above that found in the multilayer system. In fact, small
changes in composition could easily move the operational
temperature to 400 K.
isotropy stabilizes the structure so that
a
negative
2
5,26
magnetization
is observed. Below 160 K, the structure
switches from the Co-aligned to the Gd-aligned configura-
tion. Positive total moment is attained around 120 K and
becomes more positive as the temperature is reduced. On
heating from 10 K, the Gd-aligned state is stable up to 340 K
exhibiting a negative total moment above 300 K. Around 340
K, the system switches back to its initial Co-aligned configu-
ration. The resulting “bow-tie” shape is characteristic for the
thermal hysteresis. The thermal hysteresis width, defined as
the temperature interval between the minima of the bow-tie
curve, spans over 190 K for a 100 Oe external magnetic
field. For 200 Oe and higher external magnetic fields the
width decreases as the field increases, but retains the bow-tie
shape as shown in Figs. 1͑b͒ and 1͑c͒. At 800 Oe, as shown
in Fig. 1͑d͒, the bow-tie vanishes.
The thermal hysteresis depends on the existence of two
stable states at the same temperature. In the alloy, Co and Gd
moments have strong antiferromagnetic exchange coupling
1
5,16
and are arranged antiparallel to each other.
The first state
is a Co-aligned state where the Co magnetization is aligned
with the external field and the Gd is opposite. There is also a
1
7–24
Gd-aligned state
where the Gd magnetization is aligned
with the external field and Co is opposite. At high tempera-
tures the system is in the Co-aligned state; the Gd moments,
which are small, are oppositely directed to the field. As the
temperature decreases the Gd moments increase in their ther-
mal averaged magnitude and the net magnetic moment can
actually be opposite to the external field if some anisotropy
The theoretical calculations are based on a mean field
2
7,28
approach.
An individual Co spin feels a net effective
a͒Electronic mail: koymen@uta.edu
magnetic field, H , which is produced by the external field,
ជ
Co
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003-6951/2005/87͑20͒/202111/3/$22.50 87, 202111-1 © 2005 American Institute of Physics
29.12.233.190 On: Sun, 30 Nov 2014 14:18:58
0
1

202111-2
Demirtas, Camley, and Koymen
Appl. Phys. Lett. 87, 202111 ͑2005͒
FIG. 2. Theoretical calculations for magnetization as a function of tempera-
ture for 2-nm-thick Co0.772Gd0.228 alloy. The theory is in good agreement
with experimental results of Fig. 1.
fraction of Gd is 22.8% and Co is 77.2%, close to experi-
mental values. We note that changes of 1% in the composi-
tion may vary the compensation temperature by over 50 K in
the theoretical calculations.
As seen in Fig. 2, theoretical calculations are in reason-
able agreement with experimental results. The compensation
temperature, the temperature at which the net moment of the
Gd spins is equal and opposite to that of the Co spins, is at
FIG. 1. Thermal hysteresis with increasing external magnetic fields for
-nm-thick Co0.8Gd0.2 alloy. The Co and Gd magnetization directions are
shown as black and white arrows, respectively. The external magnetic field,
2
H, points to the right as shown.
H, the anisotropy field ͑assumed to be uniaxial͒, H , and the
a
3
00 K in agreement with the experiment. In addition, the
exchange field from neighboring Co and Gd spins.
bow-tie shape, and the decrease in the width of the thermal
hysteresis as the applied field is increased are all reproduced
by the theoretical calculations. The differences between ex-
periment and theory may be due to changes in anisotropy
fields with temperature which are not included in the theory.
We note that we have also performed calculations where
ជ
z
H
= H zˆ + H ͗m Co͘/mCo + J ͗ mជ Gd͘nGd + J ͗ mជ ͘n .
Co
a
I
Co
Co Co
͑
1͒
Here J is the effective exchange coupling between a Gd and
I
a Co spin, JCo is the exchange coupling between Co spins,
and nGd and nCo given the number of nearest neighbor Gd
and Co spins, assuming a random distribution. A similar
equation is used to find the effective field acting on the Gd
spins. The direction and thermal averaged Gd and Co mo-
ments, ͗ mជ Gd͘ and ͗ mជ Co͘, are then found by an iterative pro-
cess. An initial direction is chosen for Co and Gd moments,
the effective fields for each spin is calculated and then used
to find the new directions of the magnetic moments and the
thermal averaged moments through the Brillouin function,
e.g., ͗m ͘=g␮ S B ͑g␮ S H /kT͒. These new thermal
SCo has its bulk value and J was about four times larger. In
I
this case, the overall fit for M vs T does not match the ex-
periment quite as well, but the general trends seen in Fig. 2
could be reproduced with this alternate parameter set. Thus
we conclude that the appearance and behavior of the bow-tie
is not an oddity of a particular parameter choice.
In Fig. 3, the width of the bow-tie curves for 2 nm
Co Gd alloy is shown as a function of external magnetic
0
.8
0.2
field. The width of the bow-tie decays exponentionally with
increasing external magnetic field. In addition the theoretical
results for Co0.772Gd0.228 alloy are also shown. Good agree-
ment is seen between experiment and theory.
Co
B
Co
S
B Co Co
averaged moments are then used, in turn, to find new effec-
tive fields. The process continues until a self-consistent state
evolves.
As shown in Fig. 4, the measured and the theoretical
results for the coercive field, H , of 2 nm Co Gd alloy
C
0.8
0.2
Exchange parameters for the calculation are chosen to
match the bulk behavior of the individual materials. We take
show an asymptotic maximum as a function of temperature
near Tcomp. Coercive field measures how easy it is to reverse
the magnetization. The field at which this reversal takes
place depends on the competition between the anisotropy
energy, which would like to hold the moments in their cur-
rent positions, and the Zeeman energy of the net moment in
the applied field. So as the net moment decreases ͑near the
compensation point͒ the anisotropy energy wins out and it
takes a larger field to reverse the spins. The agreement be-
4
g=2, S =0.63, SGd=3.5, ␮BJGd=3.56ϫ10 Oe, ␮ J
Co
B
Co
6
5
=
1.53ϫ10 Oe and ␮ J =−1.51ϫ10 Oe. The ratio of the
B I
exchange values is close to the values used by Mansuripur in
2
7
earlier mean-field calculations for CoGd alloys. Further-
more, the magnetic moment on the Co atoms is reduced to
about 73% of its usual bulk moment, again in agreement
2
7
with Mansuripur. The anisotropy field is 19 Oe in the Co, a
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value close to numbers found in multilayer systems. The
tween experiment and theory is reasonable, especially con-
129.12.233.190 On: Sun, 30 Nov 2014 14:18:58

202111-3
Demirtas, Camley, and Koymen
Appl. Phys. Lett. 87, 202111 ͑2005͒
reasonable agreement with theoretical calculations. The mid-
point of the thermal hysteresis is at 300 K, but calculations
and previous studies have shown that this can easily be ad-
justed by small changes in the composition of the alloy. The
temperature dependence of the coercive field is also ex-
plained using the same theoretical model as is used for the
thermal hysteresis.
The work at The University of Texas at Arlington is
supported by a grant from The Welch Foundation. The work
by R.E.C. was supported by a grant from the US ARO
No. DAAD19-02-1-0174.
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FIG. 4. Coercive field as a function of temperature for 2 nm Co0.8Gd0.2
alloy, experiment and theory ͑Co0.772Gd0.228͒ are both shown.
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1