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 Ms is qualitatively similar to that for bulk, though the absolute values for Ms are smaller by 20%. The magnetic anisotropy constants of Ku1 and Ku2 are evaluated for the samples with N=10 ∼ 40, where Ku1 and Ku2 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 Ku1 increases with T monotonously, reaching to about 1x107 erg/cc at 400K. On the other hand, the Ku2 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 Ku1 and Ku2 in the LTP MnBi of an island structure. It is also noted that the decrease of Ku for a temperature range beyond around 450 K is possibly due to the decrease of the Ku2 component, as demonstrated in the present study.
Highlights
Voluminous works have been carried out on the magnetic properties of the low-temperature phase (LTP) MnBi in the forms of bulk, thin films and powders in conjunction with their structures and morphologies.[1,2,3,4] In particular, the unique temperature dependence of magnetocrystalline anisotropy constant Ku, that increases with temperature T in the order of 107erg/cc for temperatures above about 200K, is the subject for many years.[5,6,7,8] Yet, despite many works, there is still a lack of understanding of the magnetic anisotropy mechanism in conjunction with the structure and morphology of the samples investigated
The low temperature phase (LTP) MnBi islands are formed onto silica substrates after the multilayers {Bi(3.2nm)/Mn(2nm)}x N are deposited and annealed at 450◦C 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
Summary
The theory has failed to account for the experimental result of the relation between Ku and T if one used the experimentally obtained lattice constant values of a and c to explain the change of Ku with a and c Another recent work[10] on LTP MnBi of island structure showed the discrepancy between the theory and experiment. It was pointed out that the measured dependence of Ku on Ms over a wide temperature range from 5 to 400K showed that the Ku is inversely proportional to the nth power of Ms, where n is 5 ∼ 11.10,11 In this present work, in order to elucidate further the temperature dependence of magnetic anisotropy of the LTP MnBi, a systematic experimental study has been performed for various total thicknesses of the samples. The temperature dependence of both the first and second magnetic anisotropy constants, Ku1 and Ku2 are discussed
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