Discovery of paramagnetism with anomalously large magnetic susceptibility in high-pressure phase β (fcc)-cobalt Materials Science : Electronic & Magnetic Properties 70 Pressure-induced magnetic transitions have received much attention because a new phase at high pressure gives considerable insight into the correlation between the crystal structure and ferromagnetism. In 2000, Yoo et al . discovered the ε → β (hcp → fcc) martensitic structural transition of Co at ~100 GPa by means of X-ray powder diffraction [1]. Because this transition is regarded to be analogous to the pressure-induced martensitic transition of Fe (ferromagnetic bcc → nonmagnetic hcp at ~14 GPa), they proposed that β -Co is nonmagnetic. Theoretical calculations have also predicted that β -Co is nonmagnetic [2]. However, the transition pressure is so high that the direct observation of the magnetic states has so far not been achieved. X-ray magnetic circular dichroism (XMCD) is a spectroscopic technique that enables us to probe magnetically polarized electronic states even under high pressure. To investigate the magnetic state of β -Co, XMCD measurement at the Co K -edge was carried out at beamline BL39XU using a diamond anvil cell. XMCD spectra were measured by the helicity-modulation method. A magnetic field H of 0.6 T was applied parallel to the incident X-ray beam and in the direction of surface normal of the sample. In this study, we succeeded in the direct observation of the magnetic state up to 170 GPa [3]. The measurement was done at room temperature. The incident X-ray beam was focused using Kirkpatrick and Baez (K-B) mirrors; a circularly polarized beam with a size of 7 (v) × 6 (h) μ m 2 was realized at the sample position, which is a suitable condition for a very small sample in the diamond anvil cell at P > 100 GPa. Figure 1 shows the pressure dependence of XANES spectra. The marked changes in the XANES profile can be recognized in the absorption maximum referred to as b ( b ') and c ( c '), which is caused by the ε - β structural transition. The pressure variation in the XANES can be divided into three regions, I, II, and III. The ε -Co single phase is stable in region I, the ε - and β -Co phases coexist in region II, and the β single phase is formed in region III. The boundary pressures were determined to be P t = 81(8) GPa and P t ' = 134(6) GPa. P t represents the onset of the structural transition. In region III ( P > P t '), the residual ε phase disappears, and Co completely transforms to the β -phase. The magnetic states in ε - and β -Co are investigated by the pressure variation in the XMCD spectrum shown in Fig. 2. As the pressure increases, the K -edge XMCD amplitude shows a gradual reduction. Figure 2 demonstrates that the small amplitude is maintained up to 170 GPa without changes in the spectral shape. The integrated intensity of XMCD, I XMCD , plotted in the inset of Fig. 2, shows that a sharp drop of I XMCD occurs at the onset of the ε - β transition ( P t ). As the pressure increases, I XMCD gradually decreases again at region II, and it reaches a constant value at region III above P t '. The nonzero I XMCD in region III suggests that β -Co possesses finite magnetization due to the magnetic polarization of 3 d electrons. Hence, β -Co is not nonmagnetic, in contrast to the theoretical predictions. 7.72 7.76 7.80 0.0 1.0 Photon Energy (keV) 170 GPa 153 140 72 56 48 34 21 AP a b' c' a c E 0 b 128 114 102 89 P t ' P t I: I: ε ε - - ph pha as se e I: ε - phase I II I: : ε+β ε+β ph pha as se e II: ε+β phase I II II: I: β β p ph ha as se e III: β phase XANES (arb. units) Fig. 1. XANES profiles at the K -edge of Co up to 170 GPa. For clarity, each profile is shifted upward. The pressure dependence is divided into three regions, I, II, and III. 71 In region III, the XMCD amplitude is nearly proportional to the magnetic field ( H ) and approaches zero at H = 0 T without hysteresis, suggesting that β -Co is paramagnetic. The marked H dependence indicates that measurable magnetization is easily induced by an external magnetic field in the paramagnetic β -Co. Under the assumption that I XMCD is proportional to the magnetization, the mass susceptibility of β -Co is estimated to be χ = 1.5(4) × 10 -3 cm 3 /g. The evaluated χ is three orders of magnitude larger than the typical value for nonmagnetic 4 d and 5 d transition metals and is nearly the same as the reported value of γ (fcc)-Co above T C at ambient pressure (AP). The anomalously large susceptibility indicates that the total energies of the states with and without a finite magnetic moment are comparable. Our result also reveals that the magnetic states of the high-pressure phases in Co and Fe are clearly different. In the case of ε (hcp)-Fe, the zero intensity of XMCD demonstrates the nonmagnetic state of ε (hcp)-Fe [4]. Because this experiment was carried out at room temperature, the magnetic ground state of β -Co is still an open question. There are two possible magnetic states of β -Co: (i) a ferromagnetic ground state with the Curie temperature T C located below room temperature or (ii) a paramagnetic ground state accompanied with large magnetic susceptibility. The present results encourage theoretical researchers to reexamine the magnetic states of β -Co. XMCD measurements at low temperature should be carried out to investigate the magnetic ground state. 7.72 7.76 7.80 –0.002 –0.001 Photon Energy (keV) 170 GPa 153 128 102 89 72 34 AP 0 50 100 150 Pressure (GPa) I II I XMCD (arb. units) E 0 P t ' P t P t P t ' 0.000 III XMCD (arb. units) Fig. 2. XMCD spectra at the K -edge of Co at selected pressures. The inset shows integrated intensity of XMCD, I XMCD , as a function of pressure (filled circles). Naoki Ishimatsu Graduate School of Science, Hiroshima University E-mail: naoki@sci.hiroshima-u.ac.jp References [1] C.S. Yoo et al .: Phys. Rev. Lett. 84 (2000) 4132. [2] J.E. Saal et al .: J. Phys.: Condens. Matter 22 (2010) 096006. [3] N. Ishimatsu, N. Kawamura, H. Maruyama, M. Mizumaki, T. Matsuoka, H. Yumoto, H. Ohashi and M. Suzuki: Phys. Rev. B 83 (2011) 180409(R) . [4] N. Ishimatsu et al .: J. Phys. Soc. Jpn. 76 (2007) 064703.
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