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- Abstract:Anisotropic and giant magneto-resistance (AMR and GMR)
measurements for a molecular beam epitaxy grown (110) multilayer film
[50 DyFe2/50 YFe2] x 40 at 100 K are reported.
The film possesses a bending field BB similar to 5 T and an irreversible switching
field BIS similar to 5 T. These features allow the study of both AMR,
and GMR caused by the creation not only of longitudinal but also transverse magnetic exchange springs,
in the magnetically soft YFe2 layers. Finally, rotation experiments in applied fields show that like DyFe2 films,
there is a metastable state aligned along a [110] axis at 100 K.
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- Abstract:Spin-polarised neutron reflectometry has been used to investigate
a magnetic exchange spring, formed in a (75 DyFe2/150 YFe2) x 18 multilayer
thin film. The existence of an exchange spring is found to enhance the level of neutron spin
flipping during reflection, a strong indicator of the formation of transverse magnetic moments
within the YFe2 layers.
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- Abstract: The direction of easy magnetisation in molecular beam
epitaxy grown (110) films of DyFe2 is known to change from [0 0 1] at low
temperatures to close to [3 5 1] at room temperature. In this paper we
present compelling anisotropic magneto-resistance measurements for the
presence of a meta-stable magnetic state aligned along the [I 10] axis at
100K.
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- Abstract: It has been known for many years that the
rare-earth intermetallic compound terfenol (Tb0.3Dy0.7)Fe2
is characterized by "giant magnetostriction," which has found practical
applications. In this article we report the magnetic properties of a
series of molecular beam epitaxially (MBE) grown (110) Tb1-xDyxFe2
alloy films, on sapphire substrates. All the measurements were performed
at room temperature with the field applied along an in-plane [-110] axis.
However, unlike bulk Tb1-xDyxFe2,
it is shown that the coercivity does not fall to zero at the magic ratio
of x similar to 0.7. Instead,
the coercivities fall almost on a straight line, with the maximum
coercivity for pure TbFe2 (0.64 T) and the smallest for DyFe2
(0.22 T). The difference between the bulk and MBE films is attributed to
the presence of a magnetoelastic strain term, induced during crystal
growth by the sapphire substrate. In practice, the measured coercivities
can be described, approximately, with a modified Stoner-Wohlfarth model.
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- Abstract: A vector vibrating sample magnetometer has
been used to determine the direction of easy magnetisation in a thin film
of DyFe2. The 4000 Angstrom thick film, with a [110] growth
direction, was prepared using molecular beam epitaxy. The results show
that the direction of easy magnetisation is temperature dependent. At 4 K
this direction corresponds to the [0 0 1] type axes. But at 290 K, the
easy axis is close to a [-110] direction, but pointing out of the plane of
the film at angle of similar to (14 +/- 3)degrees. On the basis of the
vector magnetometer results at 290 K we find two easy magnetisation axes
close to [-350] or [-530] (and opposite directions). Finally, it is shown
that DyFe2 films are unique in that the remanent magnetisation
can be manipulated to point either in or out of the plane of the film.
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- Abstract: The Stoner-Wohlfarth model has proved
reasonably successful in describing the coercivities of
antiferromagnetically coupled DyFe2/YFe2 hard/soft
superlattices in the absence of magnetic exchange springs. In particular,
the coercivity rises sharply as the net magnetic moment of the
superlattice approaches zero. However the situation becomes more
complicated as the thickness of the YFe2 layers is increased.
Two distinct "instability fields" can be identified: the bending field BB,
signifying the onset of a magnetic exchange spring, and the irreversible
switching field B-IS associated with magnetic reversal. We have developed
a computational model to address this problem. In particular, it is shown
that the two instability fields in question are characterized by vanishing
eigenvalues in the matrix formed by the double energy derivatives partial
d2E/dqidqj
, where E is the total energy and qi
the angle of each individual monolayer. It is shown that the model
provides a very good description of the M-B-app loops of DyFe2/YFe2
multilayer films. In particular, the coercivity of a nearly magnetically
compensated multilayer (75 Angstrom DyFe2/150 Angstrom YFe2)
is much reduced below the prediction of the Stoner-Wolhfarth model, in
accord with experiment.
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- Abstract: We present some universal results for exchange
springs in multilayer systems with alternating hard and soft layers. In
particular, the reduced magnetisation <cos q>,
for both symmetric exchange springs (0-->f-->0)
and 180degrees exchange springs (0-->p), is
given in terms of elliptic functions. The result for the symmetric
exchange spring is found to be in good accord with experimental results on
MBE grown DyFe2/YFe2 multilayers.
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- Abstract: The coercivity of MBE-grown Laves-phase DyFe2-YFe2
thin-layered ( less than or equal to 50 Angstrom) superlattice samples,
with a (110) growth direction, can be varied by adjusting the relative
thickness of the YFe2 and DyFe2 layers. However, for
sufficiently thick layers, the coercivity is found to be dependent on the
thickness of the YFe2, layers, even when the ratio of DyFe2/YFe2,
is fixed. This feature is illustrated using the superlattice films [wDyFe2/wYFe2]x40,
with w = 75, 100 and 150 Angstrom. The underlying reason for the decrease
in coercivity with increasing width w, is traced to the action of the
magnetic exchange springs in the YFe2 layers.
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- Abstract: MBE methods have been used to grow
single-crystal Laves-phase DyFe2-YFe2 superlattice
samples with a (1 1 0) growth direction. In this paper, magnetisation
curves for the YFe2 dominated multilayer samples [wDyFe2/4wYFe2]
x N, where w = 20, 30, 40, and
50 Angstrom, and N = 40, 27, 20, and 16, respectively, are presented and
discussed. It will be shown that the measured M-H loops are dominated by
magnetic exchange springs, which reduce the magnetic coercivity,
considerably. In particular, it is possible to engineer samples with a
negative coercivity.
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- Abstract: Molecular beam epitaxial methods have been
used to grow single crystal Laves phase DyFe2/YFe2
superlattice samples with a (110) growth direction. Detailed magnetization
curves have been obtained for YFe2 dominated multilayer samples
[wDyFe2/4wYFe(2)]x16
with w=45, 50, and 55 Angstrom.
In particular, it is shown that the formation of magnetic exchange springs
in the magnetically soft YFe2 layers, can be used to engineer
multilayer samples with a negative coercivity. Further, by using
asymmetric field cycling procedures, we have investigated the irreversible
parts of the M-B loop, associated with the switching of the DyFe2
multilayers.
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- Abstract: Magnetization and magnetoresistance
measurements are reported for antiferromagnetically coupled DyFe2/YFe2
multilayers in fields up to 23 T It is demonstrated that the formation of
short exchange springs (similar to 20 Angstrom) in the magnetically soft
YFe2 layers results in a giant magnetoresistance as high as 32%
in the spring region. It is shown that both the magnitude of the effect
and its dependence on magnetic field are in good agreement with the theory
of Levy and Zhang for domain wall induced giant magnetoresistance.
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- Abstract: Molecular beam epitaxial methods have been used to grow
single crystal Laves phase DyFe2/YFe2 superlattice
samples with a (110) growth direction. In particular, the magnetic
properties of the YFe2 dominated multilayer samples [wDyFe2/4wYFe2]
x N with w = 20, 30, 40, 45,
50, and 55 Angstrom are presented and discussed. In principle, the
multilayer films should possess similar magnetic properties because they
are all nominally Dy0.2Y0.8Fe2. However,
it is shown that their magnetic properties depend strongly on the
thickness of the DyFe2 layers w.
Those films with w greater than
or equal to 45 Angstrom possess negative coercivities, while those with
w less than or equal to 40
Angstrom are positive. It is argued that this behavior can be understood
in terms of a strong increase of the intrinsic coercivity of the DyFe2/YFe2
superlattice, taken as a whole, as w
is reduced. For w less than or equal to 40 Angstrom almost none of the
DyFe2 moments can be flipped over in the available field range
(12 T).
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- Abstract: The properties of soft magnetic
exchange-springs in both bilayer and multilayer samples are investigated,
with particular emphasis on the discrete nature of the spring. it is shown
that, in a mean-field model, a very simple relationship exists between the
bending field BB, the exchange field BEX and the
number of monolayers N in the soft magnetic layer. For bilayers BB/BEX
= (p /2N)2, whereas for multilayers
BB/BEX = (p /N)2
. In addition, it is shown that Jacobi elliptic functions, originally used
by Goto at al for continuous bilayer springs, provide a
surprisingly robust description of discrete bilayer and symmetric
multilayer exchange-springs. Finally, the problem of soft exchange-spring
penetration into neighboring hard magnetic layers is discussed.
Calculations show that this is an important effect, which leads to a
reduction in the bending field Bs.
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- Abstract: Molecular beam epitaxy methods have been used
to grow single-crystal Laves phase DyFe2-YFe2,
superlattice samples with a (110) growth direction. In this paper,
magnetization curves for the multilayer samples [wDyFe2
/ wYFe2] x 40, with
w = 50, 75, 100, 150, and 300
Angstrom, are presented and discussed. Below TC, the magnetic
moments of the DyFe2 layers are coupled antiferromagnetically
to those of the YFe2 layers. However, in applied fields, it is
possible to generate magnetic exchange springs in the YFe2
layers, with well-defined bending (onset) fields BB. In
particular, it is demonstrated, both experimentally and theoretically,
that the bending field BB is closely proportional to 1/w2,
where w is the thickness of the
magnetically soft YFe2 layer.
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- Abstract: Molecular beam epitaxial methods have been
used to grow single crystal Laves phase DyFe2-YFe2
superlattice samples with a (110) growth direction. It is shown that it is
possible, in principle, to engineer a desired coercivity between the
limits K(DyFe2) less than or equal to K less than or equal to
infinity. This can be achieved by adjusting the relative thickness of the
individual DyFe2 and YFe2 layers, in multilayer
films This novel feature is illustrated, using the superlattice films [x
Angstrom DyFe2 / (100-x)
Angstrom YFe2]x40, with x=80,
60, 50, and 45. It is found that the measured coercivity is in
semiquantitative agreement with a simple theoretical expression, for the
nucleation fields in both bilayer and multilayer compounds. However, in
practice, exchange spring penetration into the DyFe2 layers can
set a limit to the maximum coercivity that can be achieved.
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