MAGNETIC PHASE-DIAGRAM OF ERGE1-XSIX (0-LESS-THAN-X-LESS-THAN-1)

The composition-temperature magnetic phase diagram of ErGe1-xSix (0 < x < 1) has been deduced from the powder neutron diffraction investigat ion of the magnetic structure of several samples in the 1.5-15 K range . These compounds present an antiferromagnetic behaviour with 7.2 < T( N) < 11.5 K. Four magnetic phases are present: two that are commensura te with the crystal lattice (wavevectors (1/2, 0, 1/2) and (0, 0, 1/2) ) and two incommensurate (wavevectors (0, 0, k(z)) and (k(x)', 0, k(z) ') with a slight deviation of k(x), k(x)' and k(z)' from 1/2). Whateve r x, an incommensurate phase appears below T(N), the wavevector being (0, 0, k(z)) for x < 0.40 and (k(x)', 0, k(z)') for x > 0.40. For 0.17 greater-than-or-equal-to x less-than-or-equal-to 0.55, a first-order transition occurs as function of the temperature between these two pha ses. For x greater-than-or-equal-to 0.65, a lock-in transition takes p lace at T(IC), leading from the wavevector (k(x)', 0, k(z)') to (1/2, 0, 1/2), as was already observed in ErSi. Finally, for x < 0.17 or 0.5 5 < x < 0.65, the wavevectors of the incommensurate phases characteriz ed by (0, 0, k(z)) or (k(x)', 0, k(z)') respectively remain unchanged in the whole temperature range below T(N). For x greater-than-or-equal -to 0.65, a small amount of a magnetic phase characterized by the wave vector (0, 0, 1/2) coexists with the main phases, below a Neel tempera ture T(N)' slightly lower than T(N). In all cases, the erbium magnetic moments are colinear along the orthorhombic a-axis; the arrangement o f the moments in the commensurate phases is the same as in ErSi and th e incommensurate orderings correspond to sine-wave amplitude modulatio ns. A brief account on the theoretical interpretation of this phase di agram is finally given.