Effect of Carbon Doping on the Structure and Magnetic Phase Transition in (Mn,Fe2(P,Si))
Effect of Carbon Doping on the Structure and Magnetic Phase Transition in (Mn,Fe2(P,Si))
0 1.-Fundamental Aspects of Materials and Energy, Department of Radiation Science and Technology, Delft University of Technology , Mekelweg 15, 2629 JB Delft , The Netherlands. 2.-Neutron and Positron Methods in Materials, Department of Radiation Science and Technology, Delft University of Technology , Mekelweg 15, 2629 JB Delft , The Netherlands. 3.-
Given the potential applications of (Mn,Fe2(P,Si))-based materials for roomtemperature magnetic refrigeration, several research groups have carried out fundamental studies aimed at understanding the role of the magneto-elastic coupling in the first-order magnetic transition and further optimizing this system. Inspired by the beneficial effect of the addition of boron on the magnetocaloric effect of (Mn,Fe2(P,Si))-based materials, we have investigated the effect of carbon (C) addition on the structural properties and the magnetic phase transition of Mn1:25Fe0:70P0:50Si0:50Cz and Mn1:25Fe0:70P0:55Si0:45Cz compounds by x-ray diffraction, neutron diffraction and magnetic measurements in order to find an additional control parameter to further optimize the performance of these materials. All samples crystallize in the hexagonal Fe2Ptype structure (space group P-62m), suggesting that C doping does not affect the phase formation. It is found that the Curie temperature increases, while the thermal hysteresis and the isothermal magnetic entropy change decrease by adding carbon. Room-temperature neutron diffraction experiments on Mn1:25Fe0:70P0:55Si0:45Cz compounds reveal that the added C substitutes P/Si on the 2c site and/or occupies the 6k interstitial site of the hexagonal Fe2Ptype structure.
Room-temperature magnetic refrigeration
exploiting the magnetocaloric effect (MCE) of
magnetic materials has the potential to address the
disadvantages of conventional vapor-compression
refrigeration when it comes to the environmental
impact, energy efficiency and device volume.1–3
Magnetic marterials showing large low-field
magnetocaloric effect have been attracting increasing
attention over the past few decades due to their
potential applications for magnetic refrigeration.
During the past decades, a large MCE in the
roomtemperature range has been observed in several
classes of materials including Gd5(Si,Ge)4;4 MnAs
and Mn(As,Sb);5,6 (Mn,Fe)2(P,X) with X = As, Ge,
Si;7–9 (Mn,Fe)2(P,Si,B);10 MnCoGeBx;11 MnCoGe1 x
hydridesM;14n,1C5o1Lax(FMexnS,Fi;1e3,Si)13Hz;16 Fe49Rh5117 and
Gax;12 La(Fe,Si)13 and their
Heusler alloys.18,19 A combination of a large MCE,
tuneable Curie temperature, limited thermal
hysteresis, non-toxic and abundant ingredients makes
(Mn,Fe)2(P,Si)-based compounds one of the most
attractive candidate materials for commercial
roomtemperature magnetic refrigeration.
In order to cover a wide range of temperatures,
different magnetocaloric materials with the desired
variation in TC are required, while having both a
large MCE and a small thermal hysteresis. With the
aim to tune the Curie temperature and reduce the
thermal hysteresis, while improving the mechanical
stability and maintaining an acceptable MCE in the
(Mn,Fe)2(P,Si) system, much work has recently been
done by balancing the Mn:Fe ratio and P:Si
ratios,20,21 by the introduction of nitrogen,22,23 by
varying the duration and temperature of the heat
treatment24 and by Co-B and Ni-B co-doping.25 Miao
et al. (Ref. 23) have recently shown that the magnetic
transition of (Mn,Fe)2(P,Si) can be tailored by adding
C. The C atoms were found to occupy the interstitial
6k and 6j sites in the hexagonal structure. The aim of
the present study is to obtain the complementary
information on the influence of C additions on the
magnetocaloric properties, which is key information
that needs to be taken into account for practical
applications. Based on the earlier studies by Miao
et al. (Ref. 23) the C atoms were expected to be
introduced interstitially, and; therefore, the C was
added to the composition (rather than substituted for
To study the influence of C on the structural and
magnetocaloric properties of (Mn,Fe)2(P,Si)-based
materials, in this work, C was added to the
Mn1:25Fe0:70P0:50Si0:50 and Mn1:25Fe0:70P0:55Si0:45
compounds. These two compounds have been chosen
for this work due to their different magnitude of
latent heat. In fact, an increase in P/Si ratio leads to a
stronger first-order magnetic transition. The
influence of carbon addition on the structural, magnetic
and magnetocaloric properties of the compounds
obtained was systematically investigated by x-ray
diffraction and magnetic measurements. In order to
determine the occupancy of C added in the crystal
structure, room-temperature neutron diffraction was
employed for Mn1:25Fe0:70P0:55Si0:45Cz compounds.
This may allow understanding the relation between
the changes in crystal structure and in the magnetic
To investigate the influence of carbon addition on
the structural properties and magnetic phase
transition, two series of samples, Mn1:25Fe0:70P0:50Si0:50Cz
and Mn1:25Fe0:70P0:55Si0:45Cz, were prepared by
highenergy ball milling followed by a double-step
annealing process.26 The mixtures of 15 g starting
materials, namely Fe, Mn, red-P, Si and C (graphite), were
ball milled for 16.5 h (having a break for 10 min every
15-min milling) with a constant rotation speed of
380 rpm in tungsten-carbide jars with seven
tungsten-carbide balls under argon atmosphere. The fine
powders obtained were compacted into small tablets
and were then sealed into quartz ampoules with
200 mbar argon before the heat treatment was
Magnetic properties were characterized using a
commercial superconducting quantum interference
device (SQUID) magnetometer (Quantum Design
MPMS XL) in the reciprocating sample option (RSO)
mode. X-ray powder diffraction experiments using a
PANalytical X-pert Pro diffractometer with Cu-Ka
radiation were carried out at room temperature. The
room temperature neutron diffraction data were
collected on the neutron powder diffraction
instrument PEARL27 at the research reactor of Delft
University of Technology. For neutron
measurements, 8–10 g powder samples were put into a
vanadium can with a diameter of 6 mm and a height of
50 mm. Structure refinement of the x-ray and neutron
diffraction data was done by using the Rietveld
method implemented in the Fullprof program.28
RESULTS AND DISCUSSION
The room temperature XRD patterns of the
Mn1:25Fe0:70P0:50Si0:50Cz (z ¼ 0:00, 0.05, 0.10 and
0.15) compounds indicate that all samples exhibit
the hexagonal Fe2P-type main phase. The
temperature dependence of the magnetization for the
Mn1:25Fe0:70P0:50Si0:50Cz compounds was measured
during cooling and heating after removing the
‘virgin effect’29 under an applied magnetic field of
1 T and is shown in Fig. 1. All samples show sharp
ferro-to-paramagnetic phase transitions
accompanied by a small thermal hysteresis. The Curie
temperature (TC) increases while the thermal
hysteresis (D Thys) decreases as carbon is added.
However, the change in TC is not linear as a
function of the carbon content. Compared to B
doping,30 the influence of C doping on both TC and
D Thys is less pronounced.
The isothermal entropy change (D Sm) of the
Mn1:25Fe0:70P0:50Si0:50Cz compounds in a field
change of 0.5 T, 1.0 T, 1.5 T and 2.0 T derived from
the isofield magnetization curves for cooling using
the Maxwell relation is shown in Fig. 2 and
summarized in Table I. It is noticeable that for magnetic
field changes of between 0.5 T and 2.0 T, DSm
decreases as a function of carbon concentration
although TC does not show a systematic change for
increasing carbon concentration. Moreover, the
Mn1:25Fe0:70P0:50Si0:50C0:05 compound shows nice
magnetocaloric properties in low field (0.5 T)
accompanied by a very small (negligible) thermal
hysteresis. An acceptable magnetocaloric effect at lower
magnetic field strength would be a significant
advantage for practical applications, since it allows
reducing the mass of permanent magnets needed to
generate the magnetic field. Thus, it is highly
desirable to verify the effect of carbon doping on
Fig. 4. Isothermal magnetic entropy change of the Mn1:25Fe0:70P0:55Si0:45Cz compounds as a function of temperature for a field change of 0.5 (a),
1.0 (b), 1.5 (c) and 2 T (d).
the thermal hysteresis, magnetic phase transition
and magnetocaloric properties of
To verify the influence of carbon added on the
magnetic phase transition and the thermal
hysteresis of (Mn,Fe)2(P,Si)-based compounds, another
series of samples with the parent compound was
prepared. Room-temperature XRD patterns of
Mn1:25Fe0:70P0:55Si0:45Cz compounds indicate that
the hexagonal Fe2P-type structure remains
unchanged by adding C. This confirms that the
carbon addition preserved the crystal structure of
Figure 3 shows the temperature dependence of
the magnetization for the Mn1:25Fe0:70P0:55Si0:45Cz
compounds. A remarkable thermal hysteresis
confirms that the nature of the phase transitions in the
parent and doped compounds is of the first order. It
is noticeable that the Curie temperature can be
tuned between 202 K and 226 K, while maintaining
the sharp magnetic phase transition and reducing
the thermal hysteresis by the introduction of carbon
in the parent Mn1:25Fe0:70P0:55Si0:45 compound. The
Curie temperature of all the carbon-doped
compounds is higher than that of the parent compound.
Similar to the Mn1:25Fe0:70P0:55Si0:45Cz series, the
change in the Curie temperature of the
Mn1:25Fe0:70P0:55Si0:45Cz compounds does not
linearly increase as a function of carbon doping
concentration. It is worth mentioning that the
introduction of interstitial carbon atoms in other
well-known MCE materials such as
LaFe11:5Si1:5Cx31 leads to an increase in the Curie
temperature, while the Curie temperature
decreases with increasing the carbon concentration
for Ni43Mn46Sn11Cx,33 and
Mn38Fe2M2AnlA40sCCxx.,3342 However, no further
investigation has been done on these compounds to resolve
the occupancy of C in the crystal structure.
The DSm of the Mn1:25Fe0:70P0:55Si0:45Cz
compounds in a field change of 0.5 T, 1.0 T, 1.5 T and
2.0 T derived from the isofield magnetization data is
shown in Fig. 4 and summarized in Table II. As
shown in Fig. 4, the DSm for a field change of both
0.5 T and 1.0 T hardly changes as C is added.
However, there is a slight decrease in the DSm for a
field change of 1.5 T and 2.0 T with carbon addition.
Hence, a certain amount of C can be added to
(Mn,Fe)2(P,Si) compounds in order to tune the
magnetic phase transition and reduce the thermal
hysteresis, while preserving an acceptable
magnetocaloric effect for practical applications.
To quantify the concentration of C in the obtained
samples, the combustion method using a LECO
element analyzer was employed. The results
obtained from the elemental analysis are in good
agreement with the nominal compositions and are
summarized in Table III. However, it is necessary to
investigate how much and where the C atoms have
entered the structure. This is not possible with
xrays as C is hardly visible for x-rays. Hence, neutron
diffraction experiments were performed at room
temperature to resolve the occupancy of C atoms in
the crystal structure of the doped compounds.
In Fig. 5, the room-temperature neutron
diffraction patterns for the Mn1:25Fe0:70P0:55Si0:45Cz
compounds in the paramagnetic state are shown as an
example. The Rietveld refinement using the
FullProf package for all samples confirms the Fe2P-type
hexagonal structure (space group P-62m) with two
specific metallic and non-metallic sites. It is worth
mentioning that <2 wt.% of the (Mn,Fe)3Si
impurity phase is detected in these samples. The unit-cell
volume is expected to increase if C atoms enter the
structure as an interstitial element. However, the
initial reduction in the unit-cell volume when
carbon is added suggests that in this case C atoms
substitute non-metal atoms on the 2c/1b sites, since
C has a smaller atomic radius than both P and Si.
Moreover, the unit-cell volume hardly changes after
further C doping, indicating that part of the C added
Space group: P
62m. Atomic positions: 3f (x1, 0, 1/2); 3g (x2, 0, 1/2); 2c (1/3,2/3,0) and 1b (0,0,1/2).
may also enter the interstitial sites. Hence, two
different atomic models with C substituting P/Si on
the 2c site and/or occupies the 6k interstitial sites
have been used to resolve the occupancy of C atoms
in the crystal structure. The structural parameters
derived from the Rietveld refinement for the
Mn1:25Fe0:70P0:55Si0:45Cz compounds are
summarized in Tables IV and V. It is found that in both
cases the total C occupation is not strongly
influenced by the amount of C added, and the Rietveld
refinements are not sensitive enough to distinguish
the C atom occupancy at the substitutional and/or
interstitial sites. However, the unit-cell volume
decreases as C is added and hardly changes after
further C doping, indicating that C atoms may enter
the crystal structure both as an interstitial and a
substitutional element rather than only occupy the
substitutional sites. Note that Miao et al. (Ref. 23)
observed an increase in the unit-cell volume as a
function of the C concentration instead and pointed
out that C occupies the 6k and 6j interstitial sites.
This difference may come from different preparation
methods since the samples of Miao and coworkers
are prepared by melt spinning.
z ¼ 0:050
The influence of C addition on the structure and the
magnetic phase transition of Mn1:25Fe0:70P0:50Si0:50Cz
and Mn1:25Fe0:70P0:55Si0:45Cz compounds fabricated
by high-energy ball milling and a solid-state
reaction has been investigated. The experimental
results indicate that C doping allows to tune the
Curie temperature of the parent alloys and to
reduce the thermal hysteresis. The magnetic
softness of the C doped compounds results in large MCE
even in lower magnetic fields compared to the
parent compounds. The refinements based on the
room-temperature neutron diffraction data indicate
that C substitutes P/Si on the 2c site and/or occupies
the 6k interstitial site of the hexagonal Fe2P-type
The authors acknowledge A.J.E Lefering, Bert
Zwart and David van Asten for their technical
assistance. This work is a part of an Industrial
Partnership Program IPP I28 of the Dutch
Foundation for Fundamental Research on Matter (FOM),
co-financed by BASF New Business.
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