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

Abstract

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...

Size effects on the Néel temperature of antiferromagnetic NiO nanoparticles

“…As a consequence, NiO can play a crucial role for addressing the challenge of storage density increase in magnetic recording media by allowing magnetic stability at very low volumes far above room temperature. 11 The current work presents a systematic study on how size reduction affects the Néel temperature of NiO NPs. In order to address this issue, neutron diffraction experiments have been carried out on NiO NPs with average sizes in the sub-10 nm range.…”

Vacancy-Induced Ferromagnetic Behavior in Antiferromagnetic NiO Nanoparticles: A Positron Annihilation Study

“…As a consequence, it can cause high storage density of magnetic recording media due to allowing magnetic stability at very low volumes far above room temperature. 17 In the recent few decades, great efforts have been devoted to exploring the unexpected magnetic properties in NiO nanomaterials, such as large magnetic moments, enhanced coercivity, spin glass freezing and hysteretic loop shifts. However, up to now the underlying mechanism for the FM is still unclear.…”

Tunable High-Field Magnetization in Strongly Exchange-Coupled Freestanding Co/CoO Core/Shell Coaxial Nanowires

“…12,50,58,59 It is precisely this lack of indepth studies, particularly for measuring fields applied perpendicular to the wire axis, which has so far precluded the discovery of previously not envisaged exchange bias effects in this particular geometry.…”

“…[15][16][17]36,38,[42][43][44][45][46][47][48][49][50][51][52][53][54][55][56][57][58][59] However, new effects could emerge if the exchange coupling is induced in such a way that it directly competes with the shape anisotropy.…”

“…Actually, the study of one dimensional (1D) structures (e.g., nanowires, nanorods, or nanotubes), has attracted ample attention during the last few years, not only from the basic [15][16][17]36,38,[42][43][44][45][46][47][48][49][50][51][52][53][54][55][56][57][58][59] The exchange coupling between the FM core and the AFM or FiM shell gives rise to phenomena similar to the ones observed in thin films and nanoparticles, e.g., loop shifts and coercivity enhancement. [15][16][17]36,38,[42][43][44][45][46][47][48][49][50][51][52][53][54][55][56][57][58][59] However, new effects could emerge if the exchange coupling is induced in such a way that it directly competes with the shape anisotropy.…”