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L.Y. Ma et al. / Journal of Alloys and Compounds 723 (2017) 197e200
we fabricated two amorphous hybrids using a combination of
Gd50Co50, Gd50Co48Fe2, Gd50Co48Mn2 and Gd50Co45Mn5 amor-
phous ribbons with Tc ranging from 245 K to 278 K, according to a
particular fraction (wt%), and obtained table-like MCE profiles near
the freezing point of water, with a width of 20e40 K. The amor-
phous hybrids are expected to be applied as magnetic refrigerants
in house-hold refrigerators.
2. Experimental methods
Gd50Co50, Gd50Co48Fe2, Gd50Co48Mn2 and Gd50Co45Mn5 ingots
were prepared separately by arc-melting Gd, Co, Fe and Mn -
elements with a purity of at least 99.9% (at%) under a titanium-
gettered argon atmosphere. As-spun ribbons of each ingot
were prepared under a pure argon atmosphere by a single copper
wheel with a surface speed of about 30 m/s. The amorphous
structures of the as-spun ribbon were checked by X-ray diffraction
(XRD) on
radiation. Gd50Co50, Gd50Co48Fe2, Gd50Co48Mn2 and Gd50Co45Mn5
amorphous ribbons with approximate dimensions mm
(long) ꢁ 0.6 mm (wide) ꢁ 40 m (thick) were glued together ac-
a Rigaku D\max-2550 diffractometer using Cu Ka
8
m
cording to specific fractions (wt%) for magnetic measurements. The
first amorphous hybrid sample (sample 1) was fabricated by gluing
together with 70% (wt%) Gd50Co48Fe2 and 30% (wt%) Gd50Co48Mn2
amorphous ribbons; while the second amorphous hybrid sample
(sample 2) was glued together with 65% (wt%) Gd50Co48Fe2, 25%
(wt%) Gd50Co45Mn5 and 10% (wt%) Gd50Co50 amorphous ribbons.
The magnetic properties of the amorphous ribbons and the hybrid
samples were measured by a Quantum Design Physical Properties
Measurement System (PPMS 6000). The applied field was parallel
to the longitudinal direction along the length of the sample in order
to minimize the demagnetization factor.
3. Results and discussion
Fig. 1 shows the XRD patterns of the Gd50Co50, Gd50Co48Fe2,
Gd50Co48Mn2 and Gd50Co45Mn5 as-spun ribbons. The ribbons are
predominantly amorphous without any obvious crystalline peaks
on the XRD patterns. The temperature dependence of the magne-
tization (M-T) curves for the Gd50Co50, Gd50Co48Fe2, Gd50Co48Mn2
and Gd50Co45Mn5 amorphous ribbons measured under a field of
0.03 T are shown in Fig. 2 (a). Tc values of the amorphous alloys are
obtained from the derivative of their M-T curves. As marked clearly
Fig. 2. (a) M-T curves of the Gd50Co50, Gd50Co48Fe2, Gd50Co48Mn2 and Gd50Co45Mn5
amorphous ribbons under a field of 0.03 T, (b) the (ꢀ
DSm)-T curves of the amorphous
ribbons under a field of 5 T.
on the M-T curves, the Tc of the Gd50Co50 amorphous alloy is
enhanced to about 277 K by minor Fe addition, but decreases with
Mn addition to about 258 K for the Gd50Co48Mn2 amorphous rib-
bon and to about 245 K for the Gd50Co45Mn5 amorphous ribbon.
There are three kinds of exchange interactions in Gd-Co amorphous
alloys: the indirect Gd-Gd, Gd-Co interactions and the direct Co-Co
interaction [17e19]. The direct Co-Co interaction plays an impor-
tant role in determining the Tc of the amorphous alloys, while the
magnetic behavior of these alloys is mainly determined by the in-
direct interactions. Therefore, the improved Tc of the Gd50Co48Fe2
amorphous alloy is closely related to the enhanced Co-Co interac-
tion by the minor Fe addition, and the reduced Co-Co interaction in
the Gd50Co48Mn2 and Gd50Co45Mn5 amorphous alloys leads to the
decrease of Tc with Mn addition.
The temperature dependence of the magnetic entropy change
((-DSm)-T) curves under a field of 5 T for the Gd50Co50, Gd50Co48Fe2,
Gd50Co48Mn2 and Gd50Co45Mn5 amorphous ribbons derived from
their isothermal magnetization (M-H) curves according to the
thermodynamic Maxwell equation are shown in Fig.
The ꢀ
Spmeak of the amorphous alloys obviously increases with the
decrease of their Curie temperatures, which follows the relation-
2 (b).
D
ship between ꢀ
D
Spmeak and Tc proposed by Belo et al. from mean field
Fig. 1. XRD patterns of the Gd50Co50, Gd50Co48Fe2, Gd50Co48Mn2 and Gd50Co45Mn5 as-
spun ribbons obtained at a wheel surface speed of 30 m/s.
theory [23].