In the 3d transition metals, the 3d electrons are not localized but itinerant
and form the energy band. The ferromagnetic state is characterized by exchange
splitting of the energy band. There is no exchange splittingt in the paramagnetic
state. This is a Pauli paramagnet. It has been clarified that, in some
itinerant electron systems, the ferromagnetic state is abruptly induced
by applying high magnetic field to a Pauli paramagnetic state. This is
called itinerant electron metamagnetism (IEM). The typical example is shown
in the left panel, which is the magnetization curves of Co(S0.88Se0.12)2 at various temperatures. The origin of the IEM is the peculiar structure
of the density of states (DOS) near the Fermi level. Similar behavior is
also observed for the system showing a first-order magnetic transition
from a ferromagnetic state to a paramagnetic state just above TC. The right panel illustrates the temperature dependence of magnetization
of a first-order magnetic transition system, Co(S0.92Se0.08)2. These properties are well described by the theory of itinerant electron
metamagnetism, in which the effects of spin fluctuations are taken into
consideration.We focus on the magnetocaloric effect ant transport properties of IEM and
first-order magnetic transition systems.
When a magnetic solid is exposed to a magnetic field at a certain temperature,
its entropy is reduced. By removing magnetic field in an adiabatic condition,
the temperature of the magnetic solid is decreased. These properties are
called the magnetocaloric effect (MCE). Magnetic refrigeration is a cooling
technology based on the MCE. This technology has a number of advantages
compared with conventional gas refrigeration, namely, energy efficiency
and environmental safety. To realize the magnetic refrigeration,
it is strongly required to develop magnetic refrigerant materials with large
2001, we have reported that MnAs1-xSbx compounds exhibit
giant MCEs near room temperature. The origin of giant MCEs in the compounds is
the FOMT. At almost the same time, Bruck
and his co-workers reported the ginat MCEs of MnFeP1-xAsx. These facts have demonstrated that (i) giant
MCEs can be obtained in the FOMT systems, and (ii) the 3d transition metal
compounds can be potential candidates for magnetic refrigerant materials. These
studies have inspired subsequent research of the MCE of 3d transition metal
As-free MnFe P1-xSix compounds are found to show giant MCEs near room temperature. In order
to improve the magnetocaloric properties of the compounds at desired temperatures,
we are studying the alloying effects on the MCEs of MnFe
P1-xSix. The right panel displays the temperature dependence of magnetic entropy
change of Ru substituted MnFe
P1-xSixcompounds. The peak values are in the range of 13-15 J/K kg, which are
about three times as large as that of metallic Gd.
Significant changes in transport properties, such as magnetoresistance
and Hall effect, are expected for the IEM. Magnetoresistance (MR) means
the change of electrical resistivity under magnetic fields. In general,
magnetic materials show negative MR, because magnetic moments are aligned
by applying magnetic field, which reduces scattering of conduction electrons.
However, we observed positive magnetoresistance for Co(S1-xSex)2 associated with the IEM. The MR curves of Co(S0.86Se0.14)2 at various temperatures are shown in the left panel. In the light panel,
the MR curves of Co(S1-xSex)2 at 4.2 K are displayed. In both figures, the residual resistivity was
subtracted. The resistivity shows a distinct jump at the metamagnetic transition
field. The magnetoresistance ratio at 4.2 K is about 180 %. Such a giant
positive MR is quite rare. This giant positive MR can be understood in
terms of a significant change of spin polarization during the IEM. Early
electronic structure calculations have reported that CoS2 is a highly or completely spin polarized ferromagnet. We note that the
saturation magnetization of Co(S1-xSex)2 in the ferromagnetic state is independent of x up to x = 0.20. This suggests that the induced ferromagnetic state is also highly
spin polarized. Based on this picture, we have a following scenario for
a giant positive MR. In the paramagnetic ground state, both spin bands
contribute to the electrical conductivity. When the ferromagnetic state
is induced by a magnetic field, the minority spin DOS at Fermi level is
considerably reduced. As a result, the total DOS is drastically decreased,
which leads to giant positive MR.
The fied dependence of Hall resistivity of Co(S1-xSex)2 with x=0.10 and 0.13at various temperatures are illstrated in the figures
(a) and (b), respectively. In these figures, negative Hall resistivity
is shown as a function of magnetic field. The Hall resistiviy shows sinificant
change associated with the IEM, which is attributable to the anomalous
Hall effect by the onset of ferromagnetic state. For both compounds, the
fied dependence of Hall resistivity has negative slopes below the transition
field and postive slopes above the transition field. Our careful analyses
have revealed that the normal Hall coefficient, R0 is positive for both the paramagnetic and ferromagnetic states and R0 of the ferromagnetic state is much larger than that of the paramgentic
state. The increase in R0 by magnetic field suggests the shrinkage of the Fermi surface due to IEM.
This is consistent with the scenario above mentioned in which a highly
polarazed state is realized in the induced ferromagnetic state Co(S1-xSex)2.
1. Giant magnetocaloric effect of
MnAs1-xSbx, H. Wada and Y. Tanabe, Appl.
Phys. Lett. vol. 79 (2001)
2. Giant magnetocaloric effect of
MnAs1-xSbx in the vicinity of first-order magnetic
transition, H. Wada, T. Morikawa, K. Taniguchi, T. Shibata, Y. Yamada and Y.
Akishige, Physica B vol. 328, (2003) 114-116.
properties and magnetic refrigerant capacity of MnFeP1-xSix , K. Katagiri, K. Nakamura and H. Wada,J. Alloys Compd. vol. 553 (2013) 286-290.
4. Tuning the Curie Temperature
and Thermal Hysteresis of Giant Magnetocaloric (MnFe)2PX (X = Ge and Si) Compounds by the Ru Substitution, Hirofumi Wada, Koshi
Nakamura, Kodai Katagiri, Takayuki Ohnishi, Keiichiro Yamashita, Akiyuki
Matsushita, Jpn. J. Appl. Phys. vol. 53 (2014) 063001 (4 pages).
1. Magnetocaloric Effect and Magnetoresistance due to Itinerant Electron
Metamagnetic Transition in Co(S1-xSex)2, Hirofumi Wada, Daichi Kawasaki and Yoshiro Maekawa, IEEE Trans. Magn.
vol. 50 (2014) 2501806 (6 pages).
2. High-field transport properties of iItinerant electron metamagnetic Co(S1-xSex)2, Hirofumi Wada, Yoshiro Maekawa and Daichi Kawasaki, Journal of Science:
Advanced Materials and Devices (JSAMD) vol. 1 (2016) 179-184.
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