The Verwey Transition in Polycrystalline Cobalt-Iron Ferrites

Abellán, J.; Ortuño, M.

doi:10.1002/pssa.2210960226

Cited by 8 publications

(3 citation statements)

References 18 publications

(3 reference statements)

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“…5͒ indicates two transitions in both the FC and NFC measurements. The first dramatic intensity reduction at 120 K may be attributed to the Verwey transition as observed by others 5,23,32,[64][65][66] with the higher temperature transition indicating the blocking temperature ͑ϳ315 K͒. The T B values observed here are within a 50 K range of those reported for cobalt ferrites and suggest larger relative particle size and increased relaxation time, although the exact mechanism responsible for blocking temperature value is controversial.…”

Section: Temperature Dependence Of the Magnetizationsupporting

confidence: 79%

Cobalt ferrite nanoparticles: Achieving the superparamagnetic limit by chemical reduction

Jeppson

Sailer

Jarabek

et al. 2006

Journal of Applied Physics

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An unanticipated superparamagnetic response has been observed in cobalt ferrite materials after thermal treatment under inert atmosphere. Cobalt ferrite particles were prepared via normal micelle precipitation that typically yields Co x Fe 3−x O 4 nanoparticles ͑x = 0.6− 1.0͒. While samples thermally treated under oxygen show majority spinel phase formation, annealing in nitrogen gas yields materials consisting of Co-Fe alloy, FeS, and CoFe 2 O 4 spinel. After thermal treatment, thermomagnetic studies reveal composition-insensitive, but highly treatment-sensitive, saturation magnetization, coercivity, blocking temperature, and Verwey transition temperature dependence. Extremely high saturation magnetization ͑159 emu/g͒ with low coercivity ͑31 Oe͒ was observed for one of the treated compositions, which drastically deviates from prototypical cobalt ferrite with large magnetocrystalline anisotropy. We attribute such unique magnetic response to Co-Fe alloy coexisting with FeS and CoFe 2 O 4 spinel where the diameter of the metallic phase is below the superparamagnetic limit. While thermal treatment in nitrogen was not anticipated to yield Co-Fe alloy, chemisorbed surfactant molecules ͑i.e., sodium dodecylsulfate͒ are postulated to act as reducing agents in the present scenario.

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Section: Temperature Dependence Of the Magnetizationsupporting

confidence: 79%

Cobalt ferrite nanoparticles: Achieving the superparamagnetic limit by chemical reduction

Jeppson

Sailer

Jarabek

et al. 2006

Journal of Applied Physics

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“…Indeed, as reported by Abellan et al, the Verwey transition in Co y Fe 3−y O 4 disappears when y is larger than 0.04. 36 In our NHSs, the presence of metal phases strongly increases M S (30% more at room temperature) and the coercive field (90% more) with respect to those of magnetite NPs (for example, 42 nm magnetite NPs exhibit M S = 89 A m 2 kg −1 and μ 0 H C = 0.021 T, at room temperature). 37 It is interesting to note that when the hysteresis loop was measured at low temperature (5 K) after a 5 T FC procedure, no evidence of exchange bias occurred.…”

Section: Resultsmentioning

confidence: 64%

Unraveling the mechanism of the one-pot synthesis of exchange coupled Co-based nano-heterostructures with a high energy product

et al. 2020

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“…With decreasing temperature, the structure changes from cubic to monoclinic associated with an abrupt decrease of conductivity, lower magnetic susceptibility, and increased magnetocrystalline anisotropy while its magnetic anisotropy easy axis changes from <111> to <100> [31,32]. Cation substitution (in particular by Co), oxygen vacancies, and stress are known to influence the transition temperature [33]. Another type of phase transition, known as Morin transition [34], is found in hematite (Fe 2 O 3 ) at T M ∼ 250 K. Above T M , hematite is a weak ferromagnet [35,36] with spins lying in the basal c-plane of the rhombohedral lattice [35][36][37].…”

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confidence: 99%

Magnetic phase transitions in Ta/CoFeB/MgO multilayers

Barsukov

Safranski

et al. 2015

Applied Physics Letters

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We study thin films and magnetic tunnel junction nanopillars based on Ta/Co20Fe60B20/MgO multilayers by electrical transport and magnetometry measurements. These measurements suggest that an ultrathin magnetic oxide layer forms at the Co20Fe60B20/MgO interface. At approximately 160 K, the oxide undergoes a phase transition from an insulating antiferromagnet at low temperatures to a conductive weak ferromagnet at high temperatures. This interfacial magnetic oxide is expected to have significant impact on the magnetic properties of CoFeB-based multilayers used in spin torque memories.The ferromagnet/oxide (FM/Ox) interfaces play an important role in modern spintronics. Insulating oxide layers sandwiched between two metallic ferromagnets form magnetic tunnel junctions (MTJs) [1][2][3] which find applications in magnetic sensors [4][5][6][7], spin torque oscillators [8][9][10][11], and non-volatile spin torque memory (STT-RAM) [12]. A number of FM/Ox interfaces exhibit large perpendicular magnetic anisotropy (PMA) [13][14][15][16] needed for enhancing thermal stability of . Furthermore, some FM/Ox interfaces exhibit magneto-electric coupling that allows control of the interfacial PMA with an electric field applied perpendicular to the interface. This effect, known as voltagecontrolled magnetic anisotropy (VCMA) [20][21][22][23], can be used for energy-efficient voltage-driven switching of magnetization at low current densities [15,24,25].The interface between Co x Fe y B z (CoFeB) ferromagnet and MgO insulator is one of the most important interfaces in spintronics because the STT-RAM technology is based on CoFeB/MgO/CoFeB MTJs [26]. Comprehensive understanding of structural, magnetic and electronic properties of CoFeB/MgO-based multilayers is at the forefront of applied spintronics research. In this Letter, we report magnetometry and electrical transport studies of (i) CoFeB/MgO/CoFeB nanoscale magnetic tunnel junctions and (ii) CoFeB/MgO based multilayer films. These studies reveal surprising anomalies in the temperature dependence of resistance and magnetization of the CoFeB films interfaced with MgO. Our data suggest that ultrathin magnetic oxide layers are formed at the CoFeB/MgO interfaces prepared under typical deposition and annealing conditions employed in fabrication of CoFeB/MgO/CoFeB MTJs with high tunneling magnetoresistance (TMR) [1,2]. Interestingly, the high TMR observed in these MTJs seems to be largely unaffected by the interfacial oxide formation.We Prior to patterning, the multilayers are annealed for 2 hours at 300• C in a 1 Tesla in-plane magnetic field that sets the exchange bias direction for the SAF bottom layer parallel to the long (easy) axis of the nanopillar.The CoFeB/MgO based multilayer film composition is Si/ SiOx/ Ta(5 nm)/ Co 20 Fe 60 B 20 (d)/ MgO(1.1 nm) /Ta(1 nm)/ Ru(2 nm), where the CoFeB layer thickness d ranges from 0.9 nm to 2.5 nm. In order to minimize the sample-to-sample variations, the films were grown in a single run without changing the deposition parameters. The films w...

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The Verwey Transition in Polycrystalline Cobalt-Iron Ferrites

Cited by 8 publications

References 18 publications

Cobalt ferrite nanoparticles: Achieving the superparamagnetic limit by chemical reduction

Cobalt ferrite nanoparticles: Achieving the superparamagnetic limit by chemical reduction

Unraveling the mechanism of the one-pot synthesis of exchange coupled Co-based nano-heterostructures with a high energy product

Magnetic phase transitions in Ta/CoFeB/MgO multilayers

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