MnBi Magnetic Material: A Critical Review

Manganese Bismuth (MnBi) - the ferromagnetic material attracting a great interest of the world magnetic society last years. The absence of rare-earth elements in compositions, the sizable magnetocrystalline anisotropy, the room-temperature moderate but high-temperature reasonable magnetization plus the positive temperature coefficient of coercivity and the moderate Curie temperature make MnBi bulk magnets very potential for high-temperature magnet application. This bright future is a little gray because the research results in the past were not as expected. The paper summarizes the results concerning the MnBi alloys, powders and bulk magnets investigated during past 67 years. The look on the difficulties inhibiting the development of this material is given and the proposals that might allow overcoming the difficulties and push again the efforts of research towards the goal of 12 MGOe for the energy product (BH)max of MnBi bulk magnets are discussed.

Layered Manganese Bismuth Tellurides with GeBi4Te7– and GeBi6Te10–type Structures: Towards Multifunctional Materials

<p>The crystal structures of new layered manganese bismuth tellurides with the compositions Mn0.85(3)Bi4.10(2)Te7 and Mn0.73(4)Bi6.18(2)Te10 were determined by single-crystal X-ray diffraction, including the use of microfocused synchrotron radiation. These analyses reveal that the layered structures deviate from the idealized stoichiometry of the 12<i>P</i>-GeBi4Te7 (space group <i>P</i>3<i>m</i>1) and 51<i>R</i>-GeBi6Te10 (space group <i>R</i>3<i>m</i>) structure types they adopt. Modified compositions Mn1–<i>x</i>Bi4+2<i>x</i>/3Te7 (<i>x </i>= 0.15 – 0.2) and Mn1–<i>x</i>Bi6+2<i>x</i>/3Te10 (<i>x </i>= 0.19 – 0.26) assume cation vacancies and lead to homogenous bulk samples as confirmed by Rietveld refinements. Electron diffraction patterns exhibit no diffuse streaks that would indicate stacking disorder. The alternating quintuple-layer [M2Te3] and septuple-layer [M3Te4] slabs (M = mixed occupied by Bi and Mn) with 1:1 sequence (12<i>P </i>stacking) in Mn0.85Bi4.10Te7 and 2:1 sequence (51<i>R </i>stacking) in Mn0.81Bi6.13Te10 were also observed in HRTEM images. Temperature-dependent powder diffraction and differential scanning calorimetry show that the compounds are high temperature phases, which are metastable at ambient temperature. Magnetization measurements are in accordance with a MnII oxidation state and point at predominantly ferromagnetic coupling in both compounds. The thermoelectric figures of merit of n-type conducting Mn0.85Bi4.10Te7 and Mn0.81Bi6.13Te10 reach <i>zT </i>= 0.25 at 375 °C and <i>zT </i>= 0.28 at 325 °C, respectively. Although the compounds are metastable, compact ingots exhibit still up to 80% of the main phases after thermoelectric measurements up to 400 °C.</p>

Layered Manganese Bismuth Tellurides with GeBi4Te7– and GeBi6Te10–type Structures: Towards Multifunctional Materials

<p>The crystal structures of new layered manganese bismuth tellurides with the compositions Mn0.85(3)Bi4.10(2)Te7 and Mn0.73(4)Bi6.18(2)Te10 were determined by single-crystal X-ray diffraction, including the use of microfocused synchrotron radiation. These analyses reveal that the layered structures deviate from the idealized stoichiometry of the 12<i>P</i>-GeBi4Te7 (space group <i>P</i>3<i>m</i>1) and 51<i>R</i>-GeBi6Te10 (space group <i>R</i>3<i>m</i>) structure types they adopt. Modified compositions Mn1–<i>x</i>Bi4+2<i>x</i>/3Te7 (<i>x </i>= 0.15 – 0.2) and Mn1–<i>x</i>Bi6+2<i>x</i>/3Te10 (<i>x </i>= 0.19 – 0.26) assume cation vacancies and lead to homogenous bulk samples as confirmed by Rietveld refinements. Electron diffraction patterns exhibit no diffuse streaks that would indicate stacking disorder. The alternating quintuple-layer [M2Te3] and septuple-layer [M3Te4] slabs (M = mixed occupied by Bi and Mn) with 1:1 sequence (12<i>P </i>stacking) in Mn0.85Bi4.10Te7 and 2:1 sequence (51<i>R </i>stacking) in Mn0.81Bi6.13Te10 were also observed in HRTEM images. Temperature-dependent powder diffraction and differential scanning calorimetry show that the compounds are high temperature phases, which are metastable at ambient temperature. Magnetization measurements are in accordance with a MnII oxidation state and point at predominantly ferromagnetic coupling in both compounds. The thermoelectric figures of merit of n-type conducting Mn0.85Bi4.10Te7 and Mn0.81Bi6.13Te10 reach <i>zT </i>= 0.25 at 375 °C and <i>zT </i>= 0.28 at 325 °C, respectively. Although the compounds are metastable, compact ingots exhibit still up to 80% of the main phases after thermoelectric measurements up to 400 °C.</p>

Layered manganese bismuth tellurides with GeBi4Te7- and GeBi6Te10-type structures: towards multifunctional materials

Non-stoichiometry is the key to single-phase layered compounds in the system Mn/Bi/Te, which is essential to evaluate their multifunctional properties.