Size Dependent Exchange Bias Research Articles

Size dependent exchange bias in single-phase Zn0.3Ni0.7Fe2O4 ferrite nanoparticles

We report the microstructural and magnetic characterization of single phase nanocrystalline partially inverted Zn0.3Ni0.7Fe2O4 mixed spinel ferrite. The samples were annealed at 200 °C, 400 °C, 600 °C, 800 °C and 1000 °C. X-ray diffraction results indicate phase purity of all the samples and application of Debye- Scherrer yielded a crystallite size variation from 5 nm to 33 nm for the different samples. Magnetic measurements have revealed the freezing of interfacial spins which were the cause of the large horizontal M-H loop shift causing large exchange bias with high anisotropy. The magnetic measurements show a hysteresis loop with high effective anisotropy constant due to highly magnetically disordered surface spin at 5 K.

Size-dependent exchange bias in single phase Mn3O4 nanoparticles**Project supported by the National Natural Science Foundation of China (Grant No. 11464007), the Natural Science Foundation of Guangxi, China (Grant Nos. 2012GXNSFGA060002 and 2014GXNSFBA118241), the Guangxi Key Laboratory of Information Material Foundation, China (Grant No. 131021-Z), and the Guangxi Department of Education Foundation, China (Grant Nos. YB2014120 and KY2015YB104).

Glassy magnetic behavior and exchange bias phenomena are observed in single phase Mn3O4 nanoparticles. Dynamics scaling analysis of the ac susceptibility and the Henkel plot indicate that the observed glassy behavior at low temperature can be understood by taking into account the intrinsic behavior of the individual particles consisting of a ferrimagnetic (FIM) core and a spin-glass surface layer. Field-cooled magnetization hysteresis loops display both horizontal and vertical shifts. Dependence of the exchange bias field (HE) on the cooling field shows an almost undamped feature up to 70 kOe, indicating the stable exchange bias state in Mn3O4. HE increases as the particle size decreases due to the higher surface/volume ratio. The occurrence of the exchange bias can be attributed to the pinning effect of the frozen spin-glass surface layer upon the FIM core.

Particle Size Effects on Charge Ordering and Exchange Bias in Nanosized Sm0.43Ca0.57MnO3

Size dependence of magnetic properties of Sm0.43Ca0.57MnO3 nanoparticles with average size of 15–60 nm has been investigated. Charge ordering, characteristic for the bulk form of this material, was found to be gradually suppressed with decreasing particle size and fully disappearing in 15 nm nanoparticles. Onsets of ferromagnetic contributions to the magnetization appear below 90 K, independently on the particle size, while spontaneous magnetization at 10 K increases with decreasing particle size. Magnetic hysteresis loops exhibit size dependent exchange bias effect manifested by horizontal HEB and vertical MEB shifts, appearing after field cooling. Variation of HEB and MEB shows opposite size dependences; HEB decreases while MEB increases with decreasing particle size. For large 60 nm particles, HEB decreases monotonously with increasing temperature, while for the smallest 15 nm particles it shows surprising nonmonotonic temperature dependence. Size and temperature dependences are discussed in terms of m...

Enhanced exchange bias effect in size modulated Sm0.5Ca0.5MnO3 phase separated manganite

The effect of grain size modulation on exchange bias effect in CE-type antiferromagnetic Sm0.5Ca0.5MnO3 phase separated manganite is reported here. With the reduction of particle size, ferromagnetic clusters are found to form in the charge ordered antiferromagnetic matrix and gradually become larger. The horizontal and vertical shifts of the magnetic hysteresis loops in the field cooled magnetization process clearly indicate the size dependent exchange bias effect and it can be tuned with the reduction of particle sizes. The values of exchange bias parameter, i.e., exchange bias field (HE), coercivity (HC), remanence asymmetry (ME), and magnetic coercivity (MC) are found to depend strongly on the particle size. The variations of HE follow non-monotonic dependencies with reduction in particle size and show maximum (1205 Oe) at particle size of 150 nm at T = 5 K, which can be ascribed due to the changes in uncompensated surface spins. The values of HE and ME are found to decrease exponentially with increasing temperature below the spin- or cluster-glass like freezing temperature. The spin relaxation model has been employed for analysis of large magnetic training effect. The linear relationship between HE and ME further confirms the role of uncompensated surface spins. In view of spintronics application of manganites, the present observation of large exchange bias shift in this half-doped manganite may have great technological importance.

Size-dependent magnetism and exchange bias effect in Sm0.27Ca0.73MnO3 nanoparticles

Magnetic properties of compacted Sm0.27Ca0.73MnO3 nanoparticles with average particle size ranging from 20 to 80 nm have been investigated in wide temperature and magnetic field range. It has been found that charge ordering transition gradually shifts to lower temperatures with decreasing particle size and almost disappears for 20 nm particles. At the same time, the relative volume of the ferromagnetic phase increases monotonously. Field-induced transition from antiferromagnetic to ferromagnetic state in 80 nm particles appears at the same magnetic field as in the bulk. In small 20 nm particles, the transition is strongly suppressed by increasing surface spins disorder. Magnetic hysteresis loops show size-dependent exchange bias effect with exchange field, remanence asymmetry, and magnetic coercivity that depend on cooling magnetic field and temperature. Magnetic training effect has been observed in 20 nm particles and analyzed using a spin relaxation model. The thermoremanence and isothermoremanence curves have provided fingerprints of irreversible magnetization originating from the glassy component. Analysis of remanence curves has showed that the inner core of small 20 nm particles behaves as a two-dimensional diluted antiferromagnet.

Synthesis and size dependent exchange bias effect in CoCr2O4/Cr2O3 nanogranular systems

Structure and magnetic properties have been investigated on the nanogranular systems of ferrimagnetic (Ferri) CoCr2O4 nanoparticles embedded in antiferromagnetic (AFM) Cr2O3 matrix, synthesized by a phase segregation route from diluted Cr2(1−x)Co2xO3 (x=0.1) oxides. The CoCr2O4 nanoparticles with average particle size (DCCO) from 8 to 60nm can be produced by varying synthesis temperatures. Magnetization measurement show that, both exchange bias fields (HEB) and vertical magnetization shift (MShift) can be found at 10K after field-cooled from 350K. With increasing particle size (DCCO), both HEB and MShift decrease monotonically and finally disappear as particle size is above 60nm. The linear relationship between HEB and MShift indicates that the interfacial frozen uncompensated spins play a critical role in inducing exchange bias effect.

Nanometer Size Effect on Structural and Magnetic Properties of La0.2Ca0.8MnO3

Structural and magnetic properties of La0.2Ca0.8MnO3 nanoparticles with average size of 15-37 nm, prepared by the glycine-nitrate method, have been studied. Synchrotron experiments performed in the temperature range of 80-300 K have shown that a structural transition from room temperature orthorhombic Pnma to monoclinic P2(1)/m space group, associated with orbital ordering, occurs below 200 K in the studied particles. This transition observed for the largest particles of 37 nm is very close to that of bulk, as seen by similar temperature variation of lattice parameters and orthorhombic strain. The transition is highly suppressed for smaller 15 nm particles. Horizontal and vertical shifts of magnetic hysteresis loops (M(Shift) and H(EB)), displayed in a field cooled process, indicate size dependent exchange bias effect. It is also shown that M(shift) and H(EB), as well as the remanent magnetization M(r) and coercive field H(c) at low temperatures, exhibit a non-monotonic size dependence for particles around 23 nm. These effects may be attributed to the changes in uncompensated spins at the surface, anisotropy or alternatively to a transition from a multi-domain to the single domain state.

Magnetic properties of Sm0.1Ca0.9MnO3 nanoparticles

Magnetic properties of compacted Sm0.1Ca0.9MnO3 nanoparticles with average particle size of 25 and 60 nm have been investigated. It was found that the relative volume of the ferromagnetic phase decreases with decreasing particle size. Magnetization curves measured in field cooled and zero field cooled mode separate near the transition temperature TC and remain different even in magnetic field of 15 kOe. AC-susceptibility is strongly frequency dependent below TC, although the temperature of the maximum depends on frequency only slightly. Magnetization hysteresis loops exhibit horizontal and vertical shifts, relatively small in 60 nm and much larger in 25 nm particles, due to size-dependent exchange bias effect. The exchange bias field and the coercive field depend in a non-monotonic way on cooling magnetic field, while the asymmetry of remanence magnetization and magnetic coercivity increase monotonously with the increase of cooling field. Applied pressure enhances Curie temperature TC of nanoparticles with a pressure coefficient dTC/dP ≈ 0.6 K kbar−1, close to that of the bulk, suggesting that magnetic state of the core is similar to the bulk state. The thermoremanance and isothermoremanance curves provide fingerprints of irreversible magnetization originating from the presence of glassy component. We have ascribed the magnetic behavior of the nanoparticles to a core-shell scenario with phase separated core containing ferromagnetic clusters embedded in an antiferromagnetic matrix and partially disordered antiferromagnetic or paramagnetic shell. The suppression of the ferromagnetic phase in the core with decreasing particle size may account for the enhancement of the exchange bias effect seen in smaller particles.

Unusual magnetic properties in nanosized La 0.4Ca 0.6MnO 3

The magnetic properties of nanosized La 0.4Ca 0.6MnO 3 samples with different particle size were investigated. Reducing the particle size can significantly suppress antiferromagnetism and charge ordering. Unlike other charge ordering manganites, in which the exchange bias (EB) field shows a maximum for a certain particle size, the EB field shows a minimum value at ∼180 nm in the present samples. The size-dependent EB can be understood in terms of the antiferromagnetic (AFM) domain state model in small particles and the decrease of the volume of ferromagnetic (FM) component in large particles.

Size effect on the magnetic properties of antiferromagneticLa0.2Ca0.8MnO3nanoparticles

Magnetic properties of electron-doped ${\text{La}}_{0.2}{\text{Ca}}_{0.8}{\text{MnO}}_{3}$ manganite nanoparticles with average particle size ranging from 15 to 37 nm, prepared by the glycine-nitrate method, have been investigated in temperature range 5--300 K and in magnetic fields up to 90 kOe. A monotonous enhancement of weak ferromagnetism linked to the reduction in the particle size was observed for all nanoparticles. Magnetic hysteresis loops also indicate size-dependent exchange bias effect displayed by horizontal and vertical shifts in field-cooled processes. The magnetization data reveal two ferromagnetic components: first one appears at $T\ensuremath{\sim}200\text{ }\text{K}$ and may be attributed to surface magnetization and second one appears as a result of spin canting of antiferromagnetic core or is developed at some interfaces inside nanoparticles. Time evolution of magnetization recorded in magnetic fields after the field cooling to low temperatures exhibits a very noisy behavior that may be caused by formation of collective state of nanoparticles with no clear tendency to reach equilibrium state. Magnetic properties of the nanoparticle samples are compared with those of the bulk ${\text{La}}_{0.2}{\text{Ca}}_{0.8}{\text{MnO}}_{3}$.

Comment on ‘Particle size dependent exchange bias and cluster-glass states inLaMn0.7Fe0.3O3 ’

Thakur et al (2008 J. Phys.: Condens. Matter 20 195215) have recently reportedmagnetization hysteresis loops shifted along the field axis of the cluster-glass compoundLaMn0.7Fe0.3O3, attributed there to exchange bias induced at ferromagnetic/spin-glass-like interfaces. Thepresent comment affirms that their results are insufficient for assigning the phenomenonsolely to exchange bias since the corresponding field shift, if any, cannot be separated fromthat of a minor hysteresis loop of a ferromagnet, naturally displaced from theorigin.

Reply to comment on ‘Particle size dependent exchange bias and cluster-glass states inLaMn0.7Fe0.3O3 ’

In reply to the comment by Geshev we emphasize that loop shift in the compounds is not asimplified phenomenon of minor loop effect of a ferromagnet rather, it is a genuinesignature of exchange bias effect. The estimate of anisotropy field and the plot ofexchange bias field at 5 K with the maximum field used for the measurement ofhysteresis loop, in addition to the previously reported results such as temperaturedependence of exchange bias field, training effect, etc, confirm the exchange bias effect.

Size-dependent exchange bias in half-doped manganite nanoparticles

Magnetic properties of the nanosized half-doped manganite of Sm0.5Ca0.5MnO3 with different particle sizes prepared by a sol-gel method are investigated. Exchange-bias phenomena are observed in the field-cooled magnetic hysteresis loops for these nanoparticles. The values of the exchange field, coercivity, remanence asymmetry, and remanent magnetization are found to depend strongly on the particle size. Particularly, as the particle size decreases, the exchange field shows a nonmonotonic variation with a maximum at ∼120 nm. These magnetic behaviors have been explained in terms of size effects on the charge ordered and antiferromagnetic manganite nanoparticles.

Particle size dependent exchange bias and cluster-glass states inLaMn0.7Fe0.3O3

The cluster-glass compound LaMn0.7Fe0.3O3 was synthesized with average particle sizes∼20,∼90,and ∼300 nm. We observed a shift of the magnetic hysteresis loop in the field axis while the samplewas cooled in an external magnetic field. The systematic shift of the hysteresis loops andthe cooling field dependence of the shift indicated the phenomenon of exchange bias.The exchange bias field was found to be strongly dependent on the particle size,where the exchange bias field decreased considerably with an increase of particlesize, and the weak effect of exchange bias was observed for particles with size∼300 nm. A cluster-glass state with short range ferromagnetic clusters embedded in thespin-glass like host has been proposed, where the average size of the ferromagnetic clusterincreases with particle size.