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Itinerant electron metamagnetism (IEM)

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.

Magnetocaloric effect (MCE)

 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 MCEs.  In 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 compounds. Recently, 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.

Transport propeties under magnetic field
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.


References

MCE

1. Giant magnetocaloric effect of MnAs1-xSbx, H. Wada and Y. Tanabe, Appl. Phys. Lett. vol. 79 (2001) 3302-3304.

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.

3. Magnetocaloric 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).

Transport properties

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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