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Physics of Magnetism

  • Hirofumi Wada, Professor
  • Akihiro Mitsuda, Associate Professor
Our group studies magnetic properties of metallic materials (alloys and compounds) from both fundamental and applied aspects of physics. Current research topics are as follows:
  1. Magnetocaloric effect of first-order magnetic transition systems.
  2. High-field transport properties of itinerant electron metamagnetism.
  3. Valence instability of 4f electron systems.
  4. Exotic phase transitions of superconducting systems.

1. 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.

Figure 1. Temperature dependence of the magnetic entropy change, ΔSM, of Mn1.2Fe0.8-yRuyP0.5Si0.5.

To realize the magnetic refrigeration, it is strongly required to develop magnetic refrigerant materials with large MCEs. In 2001, we have pointed out that the compounds undergoing a first-order magnetic transition (FOMT) exhibits giant MCEs. Figure 1 displays the temperature dependence of magnetic entropy change of Ru substituted (MnFe)2PSi compounds [1]. 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. Moreover, the peak temperature can be tuned by changing the Ru content. These results suggest that Ru substituted (MnFe)2PSi compounds are promising candidates for the magnetic refrigerant materials near room temperature.

2. Itinerant electron metamagnetism (IEM)

Figure 2. Magnetoresistance of Co(S0.86Se0.14)2 at various temperatures.

In some itinerant electron systems, the ferromagnetic state is abruptly induced by applying high magnetic field to a paramagnetic state. This is called itinerant electron metamagnetism (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 [2]. The MR curves of Co(S0.86Se0.14)2 at various temperatures are shown in Figure2.

The resistivity shows a distinct jump at the metamagnetic transition field. The magnetoresistance ratio at 4.2 K is about 180 %. Such 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. 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 density of states (DOS) at Fermi level is considerably reduced. As a result, the total DOS is drastically decreased, which leads to giant positive MR. To confirm this scenario, hall effect measurements are on progress.

3. Valence instability of 4f electron systems.

In some rare-earth compounds, 4f electrons can be delocalized due to strong mixing with conduction electrons. When the configuration 4fn of a rare earth ion is nearly degenerate with a 4fn-1 state and a surplus conduction electron, the 4f state will fluctuate between two valence states. This behavior is called valence fluctuation. The valence fluctuating state of Eu is realized between Eu3+(4f6) and Eu2+(4f7) configurations. In some cases, valence changes abruptly in its temperature dependence, which is the valence transition. Recently, we have observed that EuRh2Si2 exhibits a valence transition under pressure [3]. The temperature-pressure diagram is depicted in Figure3. The antiferromagnetic Eu2+ state is replaced by a nonmagnetic nearly Eu3+ state at low temperatures above 1GPa.

In other cases, the two valence states are ordered periodically below the characteristic temperature due to strong Coulomb repulsion between electrons. This is the valence ordering. The ternary Eu pnictide, EuPtP has been believed to be a valence ordering system. The structure contains layers of Eu and Pt-P atoms alternating in the c-direction. This compound undergoes two first-order phase transitions at T1=235 K and T2= 190 K. Early reports suggested that the valence ordering takes place at T1 and another valence order state is realized below T2. We prepared single crystals and performed resonant X-ray diffraction measurements as collaboration work with JASRI [4]. The results are shown in Figure 4. We have observed (1 1 1) reflection below T2 and superlattice reflections (1 1 2/3) and (1 1 4/3) between T1 and T2. These results have revealed that the valence order patterns are Eu2+Eu3+Eu2+Eu3+ at T < T2 and Eu2+Eu2+Eu3+Eu2+Eu2+Eu3+ at T2 < T < T1.

Figure 3. Temperature-pressure digram of EuRh2Si2.
Figure 4. Resonant X-ray diffraction spectra and proposed valence ordering of EuPtP.

References

[1] H. Wada et al., Jpn. J. Appl. Phys. vol. 53 (2014) 063001.
[2] H. Wada et al.,IEEE Trans. Magn. vol. 50 (2014) 2501806.
[3] A. Mitsuda et al., J. Phys. Soc. Jpn. vol. 81 (2012) 023709.
[4] T. Inami et al. Phys. Rev. B vol. 82 (2010) 195133.