Ordering Phenomena in Transition Metal Oxides

New phases of transition metal compounds   

Recent advances in the characterization of new magnetic phases in transition metal compounds with focus on Mn-Si alloys and compounds will be reviewed. (1) Mn₅Si₃ is a well-known antiferromagnet (T_N = 90 K) which by doping with interstitial C can be turned into a ferromagnet with a maximum Curie temperature T_c = 152 K for Mn₅Si₃C_x with x = 0.22 [1]. We recently succeeded in driving T_c to 350 K by magnetron sputtering at elevated substrate temperatures [2]. This is accomplished by an enhanced non-equilibrium C concentration x = 0.75 where the average moment per Mn atom amounts to ≈ 0.9 μ_B. X-ray absorption spectroscopy does not show a significant valence change of the inequivalent-site Mn_I or Mn_II atoms, ruling out a double-exchange mechanism as origin of the stabilization of magnetic order. For Mn₅Ge₃C_x, even a maximum of T_c = 445 K has been reached. (2) Mn₃Si has long been known as an incommensurate antiferromagnet (T_N = 25 K) with a sequence of alternating Mn_I and Mn_II ferromagnetic planes, separated by a Si plane, and ordered moments &mu_I = 1.7 &mu_B and &mu_II = 0.19 &mu_B oriented in plane [3]. Here a surprising stability of the magnetically ordered state in magnetic ordered fields up to 14 T was found, as evidenced by the invariance of specific heat, magnetization, and electrical resistivity measured on polycrystalline Mn₃Si samples [4]. This invariance under magnetic field is in marked contrast to the rather low T_N and suggests the presence of a yet unknown high-energy scale which might be related to the most unusual magnetic fluctuation spectrum in this compound [3]. (3) MnSi is a particularly clean system where the magnetic ordering temperature can be tuned to T = 0 by hydrostatic pressure. The long-wavelength spiral structure (wavelength 180 Å) retains its periodicity when approaching the critical pressure where T_c → 0 but loses its orientation, as observed via neutron scattering under pressure [5]. This “partial melting’’ is reminiscent of orientational order in liquid crystals and presents a truly novel magnetic phase. How this partial order is related to the non-Fermi-liquid behavior observed in electronic transport over an extended pressure and field range [6] has yet to be established.

[1] J. P. Senateur et al., Bull. Soc. Fr. Min. Crist. 90, 537 (1967)
[2] C. Sürgers et al., Phys. Rev. B 68, 174423 (2003) and refs. therein
[3] S. Tomiyoshi et al., J. Phys. Soc. Jpn. 39, 295 (1975); Phys. Rev. B 36, 2181 (1987)
[4] C. Pfleiderer et al., Phys. Rev. B 65, 172404 (2002)
[5] C. Pfleiderer et al., Nature 427, 227 (2004)
[6] C. Pfleiderer et al., Nature 414, 427 (2001)