Evidence for suppression of collective magnetism in Fe-Ag granular multilayers

Evidence for the suppression of collective magnetic behavior of dipolarly interacting Fe nanoparticles is found in Fe-Ag granular multilayers. Interaction of Fe particles located in neighboring Fe layers is studied as a function of the nominal thickness of the Ag layer in between only two Fe layers. The surprisingly increasing interaction with increasing Ag-layer thickness, verified by memory-effect measurements, is explained by the formation of pinholes in the Ag layer at small Ag thicknesses, allowing direct ferromagnetic coupling between Fe particles in neighboring Fe layers which may hinder the frustration of superspins favored by dipolar interactions. At larger Ag thicknesses, the Ag layer is continuous without pinholes and frustration leads to the appearance of the superspin-glass state. The effect of increasing interactions correlates well with the growing deviation at low temperatures of the measured field-cooled (FC) magnetization from the interaction-free FC curve calculated by a model based on the relaxation of two-level systems. Similar phenomenon is reported in a recently published paper (S\'anchez et al., Small 2022, 18, 2106762) where a dense nanoparticle system is studied. The collective magnetic behavior of the particles due to dipolar interactions is suppressed when the anisotropy energy of the individual particles exceeds a certain threshold.Comment: 13 pages, 3 figure

Szuperferromágnesség nanoszerkezetekben = Superferromagnetism in nanostructures

Izolált, nem-kölcsönható mágneses részecskék szuperparamágneses viselkedést mutatnak akkor, amikor a termikus energia meghaladja a részecske mágneses anizotrópia energiáját, azaz az un. blocking hőmérséklet felett. A részecskék koncentrációjának, azaz kölcsönhatásának növelésével komplex mágneses állapotok jönnek létre, amelyek a spinüveg állapottól az un. ""szuperferromágneses"" állapotig terjednek. Utóbbi a részecskék közötti erős kölcsönhatás okozta mágneses momentum rendeződés. Külső mágneses tér jelentősen megváltoztatja a minta mágneses állapotát és kutatásaink célja a mágnesesen korrelált állapotok tanulmányozása a mágneses szemcsék méretének és pakolási sűrűségének függvényében. Igen vékony (1 nm körüli illetve vékonyabb) Fe/Al és Fe/Ag multirétegeket, nem-stöichimetrikus Fe0.5+xAl0.5-x (x=0-0.1) ötvözetek mechanikai őrlésével előállított porokat és részben kristályosított, különböző elemekkel (Cr, Co, stb.) hígitott FeZrCuB amorf ötvözetek Fe-Co szemcséinek mágneses viselkedését tanulmányoztuk. Mössbauer méréseket végeztünk a szobahőmérsékletű 6 kG elektromágneses mérések mellett 4.2 - 300 K között 7 T külső mágneses térig, a mágneses mérésekre pedig egy 5 T-ás SQUID magnetométert használtunk. | The superparamagnetic behaviour of isolated (non-interacting) magnetic particles is well known in the case when the thermal energy exceeds the magnetic anisotropy energy (i.e. when the temperature is higher than the so-called blocking temperature). As the volume fraction of the particles increases the interaction among the particles is enhanced which results in complex magnetic structures spanning from the spin glass behaviour to superferromagnetism. The latter is the ordering of the particle magnetic moments caused by the strong interaction of the particles. The aim of the present research is to study the correlated magnetic states as a function of the size and packing fraction of the magnetic particles. Ultra-thin Fe/Al and Fe/Ag multilayers, ball-milled off-stoichiometric Fe0.5+xAl0.5-x (x=0-0.1) alloys and partially crystallized amorphous FeZrBCu alloys diluted by Cr, Co, etc. were the investigated systems. Mössbauer measurements between 4.2 and 300 K and up to 7 T external magnetic fields, further measurements in a 6 kG electromagnet at 300 K and SQUID magnetic measurement were performed

Multiréteg struktúrák mágneses tulajdonságai = Magnetic properties of multilayer structures

A nanométeres skálájú szerkezet, a határfelületi keveredés és a mágneses tulajdonságok kapcsolatát tanulmányoztuk Fe-Al és Fe-Ag multirétegekben és nano-kompozitokban. Az ultravékony rétegvastagságú Fe-Al multirétegekben tapasztalt szokatlanul nagy mágneses anizotrópiát a két monoréteg vastagságú Fe tartományoknak tulajdonítottuk, miután ezek megjelenését az alacsony hőmérsékleten mért Fe hiperfinom terek alapján kimutattuk. A szuperparamágneses tulajdonságok alapján meghatároztuk a nem-folytonos Fe rétegű Fe-Ag multirétegekben található Fe szemcsék méretét. A tömbi mágnesség mérések és a lokális jellegű Mössbauer spektroszkópia hasonló eredményeket adott és ezekből arra következtettünk, hogy a Fe szemcsemérete csökken az Ag elválasztó réteg vastagságának növekedésével. Bizonyos szemcseméret alatt egy közel merőleges mágneses anizotrópiát tapasztaltunk, amiről megmutattuk, hogy független az olyan mechanikai feszültségektől, amely a hőtágulásból vagy fedőréteg alkalmazásából származik. A komponensek együttes párologtatásával készített Fe-Ag granuláris ötvözetek esetén azt találtuk, hogy a Fe szemcsék mérete ugyancsak a nanométeres tartományba esik, de merőleges mágneses anizotrópiát nem tapasztaltunk. Több mint két komponensű multirétegek esetén bevezettük a szekvencia permutált multirétegek gondolatát és alkalmaztuk arra, hogy a Fe réteg felső (Fe/B) és alsó (B/Fe) határrétegét vizsgáljuk Ag/Fe/B és Ag/B/Fe szekvencia permutált multiréteg párokban. | The connection between the nanoscale structure, the interface mixing and the magnetic properties were studied in Fe-Al and Fe-Ag multilayers and nanoscale composites. The unusually large magnetic anisotropy of ultrathin Fe-Al multilayers has been related to two monolayer thick Fe regions deduced from the low temperature Fe hyperfine fields. The size of Fe grains in Fe-Ag multilayers with discontinuous Fe layers could be determined from the superparamagnetic properties. Bulk magnetization and local Mössbauer spectroscopy measurements gave similar results and the decrease of the grain size with increasing Ag spacer thickness was deduced. Below certain grain-size a close to perpendicular magnetic anisotropy was observed, which was shown to be independent of mechanical stresses arising from heat dilatation or application of cover layer. Fe-Ag granular alloys prepared by co-evaporation of the components were found to contain Fe grains also in the nano-size range, but no perpendicular magnetic anisotropy could be observed. The notion of sequence permutated multilayers with more than two components was introduced and applied to investigate the top (Fe/B) and bottom (B/Fe) interfaces of Fe with B in Ag/Fe/B- and Ag/B/Fe sequence permutated multilayer pairs. The results show that both interfaces are amorphous, but B concentration of the top interface is higher

Study of the unusual increase in the Curie temperature of the residual amorphous phase in nanocrystalline Fe90Zr7B2Cu1

Nanocrystalline Fe90Zr7B2Cu1 with ferromagnetic BCC nanocrystals of about 10-20 nm size embedded in a residual amorphous matrix was produced from amorphous precursor by partial crystallization. A significant increase in the Curie temperature of the residual amorphous phase (T-C) as compared to that of the amorphous precursor was found by combined bulk magnetic and Mossbauer measurements. The unusual increase of T-C for alloys with different nanocrystalline fractions correlates with the quantity of the BCC phase

Mössbauer study of the interface of iron nanocrystallites

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Superparamagnetic behaviour of Fe80B12Zr7Cu alloys with different fractions of primary nanocrystalline phase

The superparamagnetic behaviour of nanocrystalline (nc) Fe80B12Zr7Cu alloys, having different amounts of body centred cubic (bcc) fraction controlled by DSC heat treatments was studied by high temperature SQUID magnetometry and Mössbauer spectroscopy. The granule diameter calculated from magnetic measurements is significantly higher than the X-ray determined grain size but is proportional to that value. The superparamagnetic behaviour is observed at substantially lower temperatures by the Mössbauer spectroscopy than by the macroscopic magnetisation studies, which means that the applied magnetic field significantly influences the superparamagnetic behaviour

Magnetic properties of ball-milled FeAl nanograins

Ball milled alloys were investigated by X-ray diffraction, magnetic measurements and Mössbauer spectroscopy. No magnetic saturation is observed up to 5 T and neither the magnetization nor the Mössbauer measurements show a well-defined phase transition but a gradual disappearance of the magnetism with increasing temperature. The hyperfine field distributions and the X-ray grain size data enable the identification of Fe atoms with magnetic moments as some atomic layers' thick surface of the nonmagnetic nanometric Fe–Al grains. The unusual magnetic behaviour is attributed to the large magnetic anisotropy of the low dimensional magnetic surfaces