Suppression of the increase of high-temperature coercivity in MnBi thin films by Al interlayers

Abstract

By tailoring the microcrystalline structure of MnBi films, using Al interlayers, a reduction of the high-temperature coercivity by a factor of 3 is achieved. The separation of Bi/Mn bilayers by Al interlayers acts as a diffusion barrier perpendicular to the surface. After annealing, the MnBi layers contain single-domain particles surrounded by an Al matrix exhibiting no significant increase of the coercive field with increasing temperature. ͓S0003-6951͑97͒01619-7͔The large negative Kerr rotation of nearly ⌰ K ϭϪ1.2°i n the UV spectral region and the large perpendicular magnetic anisotropy make MnBi a promising material for highdensity, short-wavelength magneto-optic recording. 1 A laser beam is commonly used for reading and writing. As a consequence, the smallest diffraction-limited spot-size diameter is 0.56 /NA. The spot size can be reduced not only by decreasing the wavelength but also by increasing the numerical aperture NA. The recently developed blue semiconductor lasers with ϭ417 nm reduce the theoretical spot-size minimum to nearly 240 nm. Therefore, in order to get a low-noise readout signal, the diameter of the crystallites has to be minimized to sizes smaller than 25 nm. A MnBi grain size of more than Ϸ1 m in diameter gives rise to a small signal-to-noise ratio of the readout signal. 1 The grain size reduces by adding a small content of Al, Sb or Pt to the MnBi films. 2,3 For example, MnBi films with Al interlayers exhibit grain sizes smaller than about 100 nm in diameter. 2 An unusual increase of the coercive field, H c , reaching 2.0 T at 550 K is detrimental to the thermomagnetic writing process. 4 Several years ago, a hybrid domain-wall-pinning model was proposed to explain the temperature dependence of the coercive field. 5 The main feature of this model is the thermal activation of domain walls pinned by nonmagnetic defects or impurities during the magnetization-reversal process. In the case of single-domain particles, a temperaturedependent pinning mechanism of domain walls is not important. By comparing the magnetic energy of a single-domain MnBi sphere with that of a sphere showing internal flux closure, the upper limit in size for a single-domain state is estimated to be nearly 100 nm. For calculating this critical diameter a Bloch-wall energy of S B ϭ0.016 J/m 2 , a saturation magnetization of M S ϭ660 kA/m, and a uniaxial anisotropy constant K u ϭ1160 kJ/m 3 were used. 5 In this letter, we show that adding Al interlayers to MnBi in a suitable way will reduce MnBi grain size below 100 nm and, therefore, effectively suppress the domain-wall pinning mechanism. This leads to a vanishing of the strong increase of H c between room temperature and 550 K.A sequence of three Bi ͑18 nm͒/Mn ͑12 nm͒ bilayers has been deposited on quartz substrates in a vacuum of ϳ10 Ϫ6 mbar. The deposition rates of the Bi and Mn layers were fixed to 0.4 and 2.0 Å/s, respectively. 6 The Bi/Mn bilayers are separated from each other by 2.0-nm-thick Al interlayers and covered by a 2.0-nm-thick Al capping layer. In order t...

Structure, magnetic, and magneto-optical properties of MnBi films grown on quartz and (001)GaAs substrates

“…The unusual large coercive force at RT is comparable to the results of Ref. 23. For single domain particles the coercivity is limited by the intrinsic anisotropy energy only and, therefore, it will decrease linearly as the diameter of the single domain particles increases.…”

“…For single domain particles the coercivity is limited by the intrinsic anisotropy energy only and, therefore, it will decrease linearly as the diameter of the single domain particles increases. 24 By comparing the magnetic energy of a single domain MnBi sphere with that of a sphere showing internal flux closure, Rüdiger et al 23 estimated the upper limit of the size for a single domain state to be around 100 nm. Assuming that the MnBi particles are in single domain state we can evaluate the grain size to be 100 nm or less, i.e., smaller than estimated from the SEM images.…”

Micromagnetism and high temperature coercivity of MnBi/Al multilayers

“…14 As reported before, multilayered MnBi/Al thin films or, in general, MnBi thin films with a small Al content exhibit a clear reduction of the MnBi grain size 4,22,[31][32][33] in comparison to pure MnBi films of similar thickness without a significant change of the MO performance. 4,22,31,32 A cosputtering technique has been employed, which leads to a MnBi grain size of only 20 nm without using any dopants. 34 In the MnBi/Al multilayered films reported the average MnBi grain size becomes smaller than approximately 50 nm and a dramatic change in the magnetization reversal process has been observed from one driven by domain wall motion toward coherent rotation.…”

“…As shown in previous articles 14,22 creases from H c ϭ0.25 T at room temperature to H c ϭ2.0 T at Tϭ550 K.…”

Optical and magneto-optical properties of thin films for high density information recording

“…Thin films of MnBi or MnBiX have been studied extensively for development as magneto-optical (MO) storage media because they show a large magneto-optical Kerr effect (MOKE) signal. However, there are problems related to a phase transformation, chemical stability of MnBi films and the grain boundary noise [3]. In order to solve these problems we have proposed to substitute Bi for Sb in MnSb to obtain the compound Mn(Sb 1-x Bi x ), which possesses better physical and chemical properties compared to MnBi or MnSb.…”