Abstract:
The magnetic properties of the low temperature phase (LTP) MnBi thin films of islands structure are discussed. The LTP MnBi islands are formed onto silica substrates after the multilayers Bi(3.2nm)/Mn(2nm)x N are deposited and then annealed at 450C for 0.5hr, where N is the number of the repetition of a pair of Mn and Bi layer. Those islands are found to be of the LTP MnBi, with the c-axis orientation along the normal to the sample plane for N=10 ∼ 40. Their size vary from place to place, but are averagely of about a few hundred nm in height and a few μm in width for N from 10 to 40. For N=200, the elongated islands are formed densely, with the length of about a few tens of μm. The coverage of those islands increases with N. The temperature dependence of saturation magnetization M_s is qualitatively similar to that for bulk, though the absolute values for M_s are smaller by 20%. The magnetic anisotropy constants of K_u1 and K_u2 are evaluated for the samples with N=10 ∼ 40, where K_u1 and K_u2 are the magnetic anisotropy constants corresponding to the second and fourth power term in the uniaxial magnetic anisotropy energy expression. It is found that the K_u1 increases with T monotonously, reaching to about 1x10⁷ erg/cc at 400K. On the other hand, the K_u2 remains nearly zero for temperatures below 300K, and then becomes negative, reaching to about 7 x 10⁶ erg/cc at 400K. This is the first to report of the temperature dependence of K_u1 and K_u2 in the LTP MnBi of an island structure. It is also noted that the decrease of K_u for a temperature range beyond around 450 K is possibly due to the decrease of the K_u2 component, as demonstrated in the present study.
