Onset of long-range order in superlattices: Mean-field theory

Wang, R. W.; Mills, D. L.

doi:10.1103/physrevb.46.11681

Cited by 69 publications

(49 citation statements)

References 2 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…where m AFM is the magnetization of the Fe in the FeO layer, yields a T N of 800 AE 30 K, a value much larger than the bulk value of 198 K. This large increase of T N is in accordance with the prediction of the mean field theory used to describe the evolution of the magnetic order in ultrathin FM/AFM superlattices [9,16,17]. In the case of a FM/AFM system, the Curie temperature T C of the FM is only weakly dependent on the relative thickness of the FM and AFM layers [17].…”

Section: Prl 103 097201 (2009) P H Y S I C a L R E V I E W L E T T Esupporting

confidence: 75%

Stabilization of Antiferromagnetic Order in FeO Nanolayers

et al. 2009

View full text Add to dashboard Cite

We have studied the evolution of the magnetic state of a nanometer thick antiferromagnetic (AFM) FeO layer during its formation using nuclear resonant scattering of synchrotron radiation. In contact to ferromagnetic Fe, the FeO layer does not show magnetic order at room temperature (RT). Once embedded between two Fe layers, magnetic coupling to the adjacent ferromagnets leads to a drastic increase of the Néel temperature far above RT, while the blocking temperature remains below 30 K. The presented results evidence the role that the ferromagnetic surrounding plays in modifying the magnetic state of ultrathin AFM layers. DOI: 10.1103/PhysRevLett.103.097201 PACS numbers: 75.70.Cn, 75.25.+z, 75.75.+a, 76.80.+y Magnetic data storage technology is reaching the bottom of the nanoscale. The stabilization of magnetic order in low-dimensional structures becomes a key issue. Below a critical thickness, ferromagnets undergo a superparamagnetic relaxation where spins are thermally excited, and the system does not show magnetic hysteresis anymore. Exchange bias bilayer systems composed of a ferromagnet (FM) and an antiferromagnet (AFM) have been recently reported to substantially reduce the critical thickness under which the ferromagnet undergoes superparamagnetic relaxation [1,2]. Layered FM/AFM systems, where an AFM layer (with high anisotropy) is used to pin the magnetization of the FM, are used to create reference magnetic layers in devices. Below a certain temperature (the blocking temperature T B ) the spins in the antiferromagnet are frozen and exchange coupling at the FM/AFM interface leads to a shift of the hysteresis loop and an increase of the coercivity, this phenomenon being known as the exchange bias effect [3,4].In the ultrathin film limit, the thermal stability of these FM/AFM bilayers is strongly modified [5,6]. In exchange bias systems, one usually distinguishes the Néel (ordering) temperature T N of the antiferromagnet and the blocking temperature T B below which exchange bias occurs [7]. For thick AFM layers (> 50 nm), T N and T B are usually found to be equivalent. Field cooling the system below T N directly leads to the freezing of spins (and the appearance of exchange bias). Early studies showed that T B was usually decreasing with decreasing thickness of the AFM layer, and it was assumed that T N was decreasing accordingly due to a finite size effect [5]. Recently, Vallejo-Fernandez et al. found out that the evolution of the exchange bias field H e and T B for polycrystalline films was in fact linked to the average grain volume of the AFM [6], which typically decreases with the layer thickness. This results in a decreased stability against thermal excitation and a decrease of T B . It should be noted that there is still an underlying physics problem as data on epitaxially grown AFM/FM bilayers also show a reduction of T B with a decreasing AFM layer thickness [8,9]; this is still the subject of different theoretical considerations [5,10]. The evolution of T N relative to T B was investigated by Va...

show abstract

Section: Prl 103 097201 (2009) P H Y S I C a L R E V I E W L E T T Esupporting

confidence: 75%

Stabilization of Antiferromagnetic Order in FeO Nanolayers

et al. 2009

View full text Add to dashboard Cite

show abstract

“…We do not have a quantitative model relating the coupling strength and susceptibility; however, previous calculations of ferromagnetic/paramagnetic multilayers showed that the induced magnetic order in the paramagnetic layer falls off quickly from the interface and is strongly temperature dependent, in agreement with our findings. 21 As discussed above, one application for an amorphous coupling layer is to couple a highanisotropy magnetic layer to a CoFeB layer to act as a reference layer for PMA STT-MRAM. Figure 3 shows the magnetic moment of CoFeB in CoFeB/spacer (Ta or Fe-Ta alloy) multilayers as a function of the nominal CoFeB layer thicknesses.…”

mentioning

confidence: 99%

Paramagnetic FexTa1-x alloys for engineering of perpendicularly magnetized tunnel junctions

et al. 2013

View full text Add to dashboard Cite

Exchange coupling between two magnetic layers through an interlayer is of broad interest for numerous recent applications of nano-magnetic systems. In this Letter we study ferromagnetic exchange coupling through amorphous paramagnetic Fe-Ta alloys. We show that the exchange coupling depends exponentially on spacer thickness and scales with the Fe-Ta susceptibility, which can be tuned via the alloy composition and/or temperature. Such materials are of high interest for the engineering of perpendicularly magnetized CoFeB-MgO based tunnel junctions as it enables ferromagnetic coupling of magnetic layers with differing crystalline lattices, suppresses dead layers and can act as an inter-diffusion barrier during annealing.

show abstract

“…Since the first third of the 20th century, when research focussed on the basic relation between quantum mechanics and magnetic order, 1,2 interest has shifted towards critical phenomena 3,4 and the behavior of particular magnetic features such as surfaces 5,6 and artificial structures. [7][8][9][10][11][12][13] In the case of multilayers, mean-field Curie temperatures are obtained by diagonalizing tridiagonal exchange-interaction matrices, [7][8][9][10] whereas Wang and Mills 11 used a Landau-Ginzburg type analysis to elaborate that the spontaneous magnetization varies continuously across phase boundaries.…”

Section: Introductionmentioning

confidence: 99%

Curie temperature of multiphase nanostructures

Skomski

Sellmyer

2000

Journal of Applied Physics

View full text Add to dashboard Cite

The Curie temperature and the local spontaneous magnetization of ferromagnetic nanocomposites are investigated. The macroscopic character of the critical fluctuations responsible for the onset of ferromagnetic order means that there is only one Curie temperature, independent of the number of magnetic phases present. The Curie temperature increases with the grain size and is, in general, larger than predicted from the volume averages of the exchange constants. However, the Curie-temperature enhancement is accompanied by a relative reduction of the spontaneous magnetization. Due to the quadratic dependence of the permanent-magnet energy product on the spontaneous magnetization, this amounts to a deterioration of the magnets performance. The length scale on which an effective intergranular exchange coupling is realized ͑coupling length͒ depends on the Curie-temperature difference between the phases and on the spacial distribution of the local interatomic exchange. As a rule, it is of the order of a few interatomic distances; for much bigger grain sizes the structures mimic an interaction-free ensemble of different ferromagnetic materials. This must be compared to the magnetic-anisotropy coupling length, which is of the order of 10 nm. The difference is explained by the nonrelativistic character of the Curie-temperature problem.

show abstract

Onset of long-range order in superlattices: Mean-field theory

Cited by 69 publications

References 2 publications

Stabilization of Antiferromagnetic Order in FeO Nanolayers

Stabilization of Antiferromagnetic Order in FeO Nanolayers

Paramagnetic FexTa1-x alloys for engineering of perpendicularly magnetized tunnel junctions

Curie temperature of multiphase nanostructures

Contact Info

Product

Resources

About