Magnetic transitions in α-Fe2O3 nanowires

Abstract

Magnetic transitions in single-crystal ␣-Fe 2 O 3 ͑hematite͒ nanowires, grown by thermal oxidation of iron powder, have been studied in the range of 5 -1023 K with a superconducting quantum interference device below room temperature and with a vibrating sample magnetometer at higher temperatures. The broad temperature range covered enables us to compare magnetic transitions in the nanowires with the transitions reported for bulk hematite. Morin temperatures ͑T M ͒ of the nanowires and of hematite bulk reference powder were found to be 123 and 263 K, respectively. Also the Néel temperature ͑T N ͒ of the nanowires, 852 K, was lower than the bulk T N value. Measurements of the magnetization as a function of temperature show an enhanced signal in the nanowires, which suggests a decrease in the antiferromagnetic coupling. A coercive field observed below T M in the hysteresis loops of the nanowires is tentatively explained by the presence of a magnetic phase.

Suppression of the Néel temperature in hydrothermally synthesized α-Fe2O3 nanoparticles

“…The measured exponent is in better agreement with the theoretical prediction for the Ising systems [15], which is consistent with the fact that Co behaves more like an Ising than a Heisenberg system [17]. On the other hand, a T N reduction of about 100 K was reported for single-crystal α-Fe 2 O 3 nanowire arrays with d = 150 nm [10]. This reduction seems too large to be compatible with Eq.…”

“…Such a T N reduction in these nanowires (d = 150 nm) is too large to be compatible with the finite-size scaling relations inferred from both ferromagnetic thin films [2] and antiferromagnetic thin films [9]. Moreover, the assigned T N value [10] [1]. Our current results also show that coating nanoparticles with SiO 2 can effectively protect nanoparticles from oxidation or reduction, which is important to technological applications of nanoparticles.…”

“…Recently, finite-size effects have been investigated in antiferromagnetic hematite (α-Fe 2 O 3 ) nanowire arrays with a mean diameter d of about 150 nm [10]. A magnetic transition at 852 K has been assigned to the Néel temperature T N of the α-Fe 2 O 3 nanowire arrays [10].…”

“…A magnetic transition at 852 K has been assigned to the Néel temperature T N of the α-Fe 2 O 3 nanowire arrays [10]. Then the speculated T N of 852 K for the nanowires is over 100 K below the bulk value (960 K [10]).…”

“…A magnetic transition at 852 K has been assigned to the Néel temperature T N of the α-Fe 2 O 3 nanowire arrays [10]. Then the speculated T N of 852 K for the nanowires is over 100 K below the bulk value (960 K [10]). Such a T N reduction in these nanowires (d = 150 nm) is too large to be compatible with the finite-size scaling relations inferred from both ferromagnetic thin films [2] and antiferromagnetic thin films [9].…”