Exchange bias beyond the superparamagnetic blocking temperature of the antiferromagnet in a Ni-NiO nanoparticulate system

Roy, Aparna; Toro, J. A. De; Amaral, V. S.; Muniz, Pablo Rodrigues; Riveiro, José Manuel Suárez; Ferreira, J.M.F.

doi:10.1063/1.4866196

Cited by 24 publications

(18 citation statements)

References 20 publications

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“…As a result, the average coordination number for Ni 2+ ions at the surface would be less than that in the bulk, which can result in a distribution of exchange energy barriers for the surface spins. 40 Furthermore, as the superexchange interaction is sensitive to bond angles and bond lengths, they are likely to be modified at the surface as compared to that in the bulk. This is supported by the absence of 2M mode in Raman spectra for nanoparticles.…”

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confidence: 99%

Enhanced room temperature ferromagnetism in antiferromagnetic NiO nanoparticles

2015

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We report systematic investigations of structural, vibrational, resonance and magnetic properties of nanoscale NiO powders prepared by ball milling process under different milling speeds for 30 hours of milling. Structural properties revealed that both pure NiO and as-milled NiO powders exhibit face centered cubic structure, but average crystallite size decreases to around 11 nm along with significant increase in strain with increasing milling speed. Vibrational properties show the enhancement in the intensity of one-phonon longitudinal optical (LO) band and disappearance of two-magnon band due to size reduction. In addition, two-phonon LO band exhibits red shift due to size-induced phonon confinement effect and surface relaxation. Pure NiO powder exhibit antiferromagnetic nature, which transforms into induced ferromagnetic after size reduction. The average magnetization at room temperature increases with decreasing the crystallite size and a maximum moment of 0.016 μB/f.u. at 12 kOe applied field and coercivity of 170 Oe were obtained for 30 hours milled NiO powders at 600 rotation per minute milling speed. The change in the magnetic properties is also supported by the vibrational properties. Thermomagnetization measurements at high temperature reveal a well-defined magnetic phase transition at high temperature (TC) around 780 K due to induced ferromagnetic phase. Electron paramagnetic resonance (EPR) studies reveal a good agreement between the EPR results and magnetic properties. The observed results are described on the basis of crystallite size variation, defect density, large strain, oxidation/reduction of Ni and interaction between uncompensated surfaces and particle core with lattice expansion. The obtained results suggest that nanoscale NiO powders with high TC and moderate magnetic moment at room temperature with cubic structure would be useful to expedite for spintronic devices.

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confidence: 99%

Enhanced room temperature ferromagnetism in antiferromagnetic NiO nanoparticles

2015

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“…In contrast, the reference samples have vanishing M R and H C at T = 300 K, revealing their superparamagnetic state ( Figure S1 [34]). [25,27,[43][44][45][46]. This highlights that using a high-T N material is not sufficient in itself to reach high-temperature stability.…”

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confidence: 96%

“…The origin of the enhanced magnetic stability must reside in some coupling existing between the Co nanoparticles and the high-T N AFM matrix, NiO (T N =520 K), since using CoO alone as matrix limits the T B enhancement to 290 K [T N (CoO)] [4]. However, NiO is known to have a low anisotropy [42], leading to small H E and low T B [H E ] (often below RT) [25,27,[43][44][45][46]. This highlights that using a high-T N material is not sufficient in itself to reach high-temperature stability.…”

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confidence: 99%

High Temperature Magnetic Stabilization of Cobalt Nanoparticles by an Antiferromagnetic Proximity Effect

et al. 2015

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Thermal activation tends to destroy the magnetic stability of small magnetic nanoparticles, with crucial implications in ultra-high density recording among other applications. Here we demonstrate that low blocking temperature ferromagnetic (FM) Co nanoparticles (T B <70 K) become magnetically stable above 400 K when embedded in a high Néel temperature antiferromagnetic (AFM) NiO matrix. The origin of this remarkable T B enhancement is due to a magnetic proximity effect between a thin CoO shell (with low Néel temperature, T N ; and high anisotropy, K AFM ) surrounding the Co nanoparticles and the NiO matrix (with high T N but low K AFM ). This proximity effect yields an effective AFM with an apparent T N beyond that of bulk CoO, and an enhanced anisotropy compared to NiO. In turn, the Co core FM moment is stabilized against thermal fluctuations via core-shell exchange-bias coupling, leading to the observed T B increase. Mean-field calculations provide a semi-quantitative understanding of this magnetic-proximity stabilization mechanism. 2The current miniaturization trend in magnetic applications has led to a quest to suppress spontaneous thermal fluctuations (superparamagnetism) in ever-smaller nanostructures [1][2][3][4][5], which is a clear example of the fundamental efforts of condensed matter physics to meet technological challenges [6] (e.g., the continued growth of recording density [7]). Despite the foreseeable change of recording paradigm from continuous to patterned media, where each bit is recorded in an individual nanostructure [7], the key for sustained storage density increase will remain the introduction of progressively more anisotropic (high K) materials [8], which allow for magnetic stability at very small volumes, V (i.e., blocking temperature, T B ∝ KV, above room temperature, RT). Two main strategies are largely investigated to achieve high K (both of them with implications in other active technologies beyond information storage, such as permanent magnets, magnetic hyperthermia or even sensors [5,[9][10][11]): (i) the use of compounds with intrinsically high magnetocrystalline anisotropy (such as FePt [3,8]) and (ii) the design of exchange-coupled nanocomposites [4,[12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29].Unfortunately, most high-K materials require high-temperature annealing processes to obtain the desired phase, which could hamper their implementation in certain structures. Thus, FM-AFM exchange coupling alternatives may be an appealing option. In fact, it has been demonstrated [4] that ferromagnetic-antiferromagnetic (FM-AFM) interfacial exchangecoupling is an effective method, later patented by Seagate [12], to increase the effective K of FM nanoparticles. However, a T B enhancement beyond RT using this approach has been rarely reported [22][23][24][25][26] (where often broad particle size distribution can partly account for the "apparent" T B increase [22][23][24][25]). The reason for this scarcity is that high Néel temperature (T N ) AFMs tend to h...

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“…an enhancement of the coercivitytypically a shi of hysteresis loop, 9,10 and combating the superparamagnetic limit in magnetic recording media. 11,12 The Co/ CoO core-shell structure is one of the most thoroughly studied systems whose EB behavior was observed in the coreshell structure at the ferromagnetic/antiferromagnetic (FM/ AFM) interface for the rst time. 13 Here we report a comprehensive study of the Ni/NiO particle system with distinguishable characteristics from Co/CoO and other relevant structures: compared with the Fe and Co nanoparticles that ignite spontaneously in air, Ni nanoparticles have better stability and resistance to oxidization 14 and its oxide, NiO, has a higher Néel temperature T N (523 K) than typical 3d transition oxides like CoO (293 K) and FeO (198 K), 15 which enables the Ni/NiO system to generate a robust EB effect even at room temperature.…”

Section: Introductionmentioning

confidence: 99%