ORIGIN OF HIGH COERCIVITIES IN AS-CAST
Dy-Fe-C MAGNETS
G. Hadjipanayis, N. Venkateswaran, J. Strzeszewski
To cite this version:
G. Hadjipanayis, N. Venkateswaran, J. Strzeszewski. ORIGIN OF HIGH COERCIVITIES IN
AS-CAST Dy-Fe-C MAGNETS. Journal de Physique Colloques, 1988, 49 (C8), pp.C8-639-C8640. <10.1051/jphyscol:19888289>. <jpa-00228460>
HAL Id: jpa-00228460
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Submitted on 1 Jan 1988
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JOURNAL DE PHYSIQUE
Colloque C8, SupplCment au no 12, Tome 49, dQcembre1988
ORIGIN OF HIGH COERCIVITIES IN AS-CAST Dy-Fe-C MAGNETS
G . C. Hadjipanayis, N. Venkateswaran and J. Strzeszewskil
Department of Physics, Carwell Hall, Kansas State University, Manhattan, K S 66506, U.S.A.
Abstract. - As-cast Dy15Fe77Cs alloys develop high coercivities after a heat treatment at 900 OC. Magnetic and
microstructure measurements show the presence of two phases: the anisotropic Dy2Fe14Cphase and a DyFeC phase with
Tc around 40 K. The high coercivities are attributed to localized domain wall pinning at the boundaries between the
DyzFe14C and DyFeC phases.
Introduction
The high coercivities observed in R-Fe-B alloys are
only obtained in microcrystalline melt-spun samples
[I, 21 and in permanent magnets made by sintering [3,
41 die-upsetting [5] and liquid dynamic compaction [6].
-4s-cast alloys do not have a high coercivity. On the
contrary, as-cast R-Fe-C(B) alloys do show a high coercivity after a heat-treatment [7] at around 900 OC.
The high coercivity has been attributed to a fine cellular microstructure which is produced with carbon
substitution.
In the present study we have extended the magnetic measurements over a wider temperature range
to search for any phases present in this system that
can possibly help us understand better the origin of
high coercivity.
enough t o saturate the magnetization resulting in a
minor loop. The steep increase of magnetization at
low fields is similar to that observed in anisotropic
sintered magnets [8] indicating the presence of rather
large grains with a preferred orientation.
The field dependence of coercivity is shown in figure 2. The coercivity increases almost linearly with applied field and is not saturated even at fields of 55 kOe.
This kind of behavior is characteristic of nucleation
type or localized domain wall pinning magnets 181.
Experimental
As-cast Dy ,,Fe17Cs alloys were prepared by arcH, kOe1
melting. Samples were heat-treated at 900 OC for
24 hours to produce the high coercivity. The mag- Fig. 2. - Field dependence of coercivity in a heat-treated
netic properties were measured with a vibrating sam- Dy-Fe-C sample
ple magnetometer and a SQUID magnetometer. The
microstructure of the samples was determined with a
Thermomagnetic data MH (T) (Fig. 3) show that
JEOL lOOC scanning transmission electron microscope. the majority phase in the as-cast samples is Dy,Fe17,
consistent with the results of Liu et al. [7]. This phase
Results a n d discussion
has an easy plane resulting in low coercivity. SimThe as-cast samples are magnetically soft. A coercivity greater than 12 kOe is obtained in the heat- ilar data on heat-treated samples show the presence
of a phase with a Curie temperature around 300 OC.
treated samples (Fig. 1). In the latter samples, the
maximum magnetic field used (17 kOe) was not large This phase is believed to be the tetragonal Dy,Fel*C
with the c-axis as the easy axis of magnetization. The
large decrease of magnetization and coercivity below
room temperature observed in zero field-cooled (ZFC)
Fig. 1. - Hysteresis loops for Dy-Fe-C samples.
Fig. 3. - Thermomignetic data in Dy-Fe-C samples.
' o n leave from the Institute of Physics, Warsaw Technical University, Warsaw, Poland.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19888289
C8 - 640
JOURNAL DE PHYSIQUE
samples is related t o hysteresis effects (Fig. 4). The
misotropy of the samples increases substantially below room temperature resulting in minor loops with
a small coercivity. Field cooled (FC) (55 kOe) samples, on the other hand show giant coercivities and a
smaller magnetization
decrease at low temperatures as
expected for strong anisotropic samples. The increase
of magnetization observed a t cryogenic temperatures
is related to the magnetic ordering of another phase
with a Curie temperature about 40 K. This phase is
believed to be carbon rich, possibly the FeDyC phase
suggested by Liu et al. [7].
scope studies verified the existence of the phases mentioned previously. Figure 6 shows the microstructure
of the C-rich phase which is also heavily faulted like
the B-rich phase [lo]. The lattice parameters of the
C-rich phase as found by electron diffraction are approximately a = 8 A and c = 24 A.
Fig. 6.
- (a) Microstructure.
This type of microstructure can explain the magnetic hysteresis behavior of the Dy-Fe-C magnets. The
domain walls move easily inside the matrix phase resulting in the steep increase of magnetization at low
fields. Localized domain wall pinning possibly takes
place at the boundary region between the Dy2Fe14C
and DyFeC phase resulting in the higher coercivities.
Lorentz microscope studies are required to verify this
hypothesis.
Fig. 4. - Magnetization and coercivity data below roomtemperature.
Scanning electron microscope data (Fig. 5) show a
microstructure consisting of a continuous matrix phase
with two other phases randomly distributed in the matrix phase. The majority phase has an Fe/Dy ratio
corresponding to Dy,l?e14C. The first minority phase
is richer in Dy showing a ratio of Fe/Dy N 1. This
is probably the phase with Tcaround 40 K. A similar B-rich phase has been reported [9] in R-Fe-B magnets but with different composition RFe4B4. The other
phase which is sparsely found in the sample is believed t o be Dy,Fel7. Transmission electron micro-
Fig. 5. - Grain structure of heat-treated Dy-De-C samples,
A
Dy2FelsC, B m DyFeC.
Acknowledgments
Work supported by both DOEFG02-86ER45263
and the CEAM Program of the European Community.
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