Evolution of the magnetic phase transition in MnO confined to channel type matrices: Neutron diffraction study

Abstract

Neutron diffraction studies of antiferromagnetic MnO confined to MCM-41 type matrices with channel diameters 24÷87Å demonstrate a continuous magnetic phase transition in contrast to a discontinuous first order transition in the bulk. The character of the magnetic transition transforms with decreasing channel diameter, showing the decreasing critical exponent and transition temperature, however the latter turns out to be above the Néel temperature for the bulk. This enhancement is explained within the framework of Landau theory taking into consideration the ternary interaction of the magnetic and associated structural order parameters.

Effect of size reduction on the ferromagnetism of the manganite La1−xCaxMnO3(x=0.33)

“…This crossover of the first-order transition to the second-order transition on size reduction has not been reported before for manganites. There are, however, reports of a change in the nature of transition from first order to second order in the AFM transition in MnO nanoparticles supported in a porous matrix with sizes down to 14 nm [29].…”

Antiferromagnetic order in MnO spherical nanoparticles

“…16 On the other hand, in ionic oxide compounds, due to the relatively localized electronic distribution, the moment is less affected by the surface and surface disorder results in a reduced average moment. 17 A reduced moment and enhanced transition temperature have been observed in MnO particles [11][12][13][14][15] and in the spinel ferrites Ni(Mn)Fe 2 O 4 . 18 In either case, the origin of the moment and ordering temperature enhancement or reduction is unclear and, consequently, further study of magnetic nanoparticles is important.…”

“…9 In contrast, an increase has been observed in the magnetic ordering temperature and magnetic moment in several nanoscale materials. [10][11][12][13][14][15] For example, in some nanoscale 3d metals, the moment is enhanced as a result of band narrowing at the surface resulting from the reduced coordination number. This behavior has been observed experimentally in iron, cobalt, and nickel clusters 10 and has been supported by theoretical calculations.…”

Синтез Кластеров Оксидов Железа В Мезопорах Монодисперсных Сферических Частиц Кремнезема

“…Затем рас-плав вновь заполняет поры и т. д. Расплавный метод заполнения мезопор исключает присутствие массивного вещества на внешней поверхности частиц [21,25]. При полном заполнении пор оксидом происходит формиро-вание нанонитей, диаметр которых равен диаметру пор в темплате [26,27]. В настоящей работе время контакта частиц с кристаллогидратом (2 h) было достаточным для " подпитки" незаполненного объема пор расплавом, поэтому заполнение пор было больше расчетного зна-чения.…”