Itinerant-electron metamagnetism and magneto-volume effects in Lu(Co_1-xAl_x)₂Laves phase compounds

Abstract

The itinerant-electron metamagnetic transition (MT) and the effects of hydrostatic pressure on the critical transition field BC of the MT, on the spontaneous magnetization MS and on the Curie temperature TC have been investigated for well homogenized Lu(Co1-xAlx)2 Laves phase compounds. The critical field BC decreases with increasing x, maintaining a linear relationship with the inverse susceptibility at the temperature where the susceptibility exhibits a maximum value, χ-1(Tmax). On applying pressure, the magnetization M of the ferromagnetic compound with x = 0.100 is drastically decreased at a critical pressure, resulting in a paramagnetic state. In addition, the metamagnetic transition from the paramagnetic to the ferromagnetic state is induced by applying an external magnetic field. The effect of pressure on the Curie temperature TC is extremely large and negative in the vicinity of the critical concentration for the onset of ferromagnetism. The pressure coefficient of the Curie temperature, ∂ln TC/∂P, is much larger than that of the spontaneous magnetization, ∂ln MS/∂P, below x = 0.150. These results can be explained by the theory for itinerant ferromagnets having a negative coefficient b of the fourth-order term in the Landau expansion. The Landau expansion coefficients estimated from the experimental results are in accord with the theories. From these estimated values, it is concluded that the magneto-volume effect decreases the critical transition field BC. It has been confirmed that the results for Lu(Co1-xAlx)2 are very much analogous to those for Lu(Co1-xGax)2.

Ferromagnetism of (ScCa)Co2 Laves phase compound synthesized under high pressure

“…It has been pointed out that the smoothing of DOS peaks occurs due to alloying effect of Co-band when Co is partially substituted by Al in YCo 2 and a slight increase of DOS at the Fermi level E F brings about appearance of ferromagnetism [12]. In Lu(Co 1 − x Al x ) 2 systems, the onset of ferromagnetism Table 1 The Curie temperature T C , quantities of pressure effect dln T C /dP, dln M/dP at 4.2 K and dln T C /dln M, spin wave stiffness constant D s and the ratio of D s /T C for (Sc 0.85 Ca 0.15 )Co 2 , Lu(Co 0.85 Al 0.15 ) 2 and Lu(Co 0.80 Al 0.20 ) 2 Laves compounds should be similar to Y(Co 1 − x Al x ) 2 , although the critical Al concentration for the onset of ferromagnetism in the former is lower than that in the latter [9]. On the other hand, the substitution of Ca for Sc in ScCo 2 , the position of E F shifts towards the main peak [13].…”

“…On the other hand, the value of dln T C /dP of (Sc 0.85 Ca 0.15 )Co 2 is smaller than those of Lu(Co 1−x Al x ) 2 Laves phase compounds, namely, the ratio of dln T C /dln M is small. It has been reported that the thermal variation of M 2 loc due to thermal spin fluctuations, which is significant in Lu(Co 1 − x Al x ) 2 [9]. In the system, which exhibits a significant increase of thermal spin fluctuations, T C decreases drastically on applying pressure [16], because T C is strongly related with the renormalization effect of spin fluctuations in the magnetic free energy in the finite temperatures [16].…”

“…The values of T C , dln T C /dP, dln M/dP at 4.2 K and (dln T C /dP)/(dln M/dP) = dln T C /dln M of (Sc 0.85 Ca 0.15 )Co 2 are given in Table 1, together with those of Lu(Co 0.85 Al 0.15 ) 2 and Lu(Co 0.80 Al 0.20 ) 2 Laves compounds which show the second-order magnetic phase transition from the ferromagnetic to the paramagnetic state [9]. In addition, the compounds with x = 0.15-0.16 exhibit the highest T C 130-150 K in Lu(Co 1 − x Al x ) 2 system [9,15]. It should be pointed out that the present compound exhibits the highest T C among the ScCo 2 , YCo 2 and LuCo 2 based quasi-binary compound systems.…”

“…In the system, which exhibits a significant increase of thermal spin fluctuations, T C decreases drastically on applying pressure [16], because T C is strongly related with the renormalization effect of spin fluctuations in the magnetic free energy in the finite temperatures [16]. It has been pointed out that the positive curvature of the DOS at the Fermi level E F remains up to around x = 0.15 in Lu(Co 1 − x Al x ) 2 , and hence a negative mode-mode coupling among spin fluctuations leads to large values of dln T C /dP and dln T C /dln M [9,16]. Therefore, it is expected that the difference between the shape of DOS around E F brings about the difference in the stiffness of M 2 loc against pressure in (Sc 0.85 Ca 0.15 )Co 2 and in Lu(Co 0.85 Al 0.15 ) 2 .…”

“…By applying high magnetic field, a metamagnetic transition from the paramagnetic to the ferromagnetic state occurs at 69 T for YCo 2 and 74 T for LuCo 2 [6,7]. Moreover, a ferromagnetic state is induced by a slight change of the electronic structure, that is, a ferromagnetic state is established by partial substitution of Co for Al or Ga in LuCo 2 [8,9]. A weak ferromagnetic state with a low Curie temperature T C ≈ 20 K also appears by partial substitution of Co for Al or Ga in YCo 2 [10].…”