Field-cooled Hysteresis Loops Research Articles

Thermosensitivity through Exchange Coupling in Ferrimagnetic/Antiferromagnetic Nano-Objects for Magnetic-Based Thermometry.

Temperature is a fundamental physical quantity important to the physical and biological sciences. Measurement of temperature within an optically inaccessible three-dimensional (3D) volume at microscale resolution is currently limited. Thermal magnetic particle imaging (T-MPI), a temperature variant of magnetic particle imaging (MPI), hopes to solve this deficiency. For this thermometry technique, magnetic nano-objects (MNOs) with strong temperature-dependent magnetization (thermosensitivity) around the temperature of interest are required; here, we focus between 200 K and 310 K. We demonstrate that thermosensitivity can be amplified in MNOs consisting of ferrimagnetic (FiM) iron oxide (ferrite) and antiferromagnetic (AFM) cobalt oxide (CoO) through interface effects. The FiM/AFM MNOs are characterized by X-ray diffraction (XRD), (scanning) transmission electron microscopy (STEM/TEM), dynamic light scattering (DLS), and Raman spectroscopy. Thermosensitivity is evaluated and quantified by temperature-dependent magnetic measurements. The FiM/AFM exchange coupling is confirmed by field-cooled (FC) hysteresis loops measured at 100 K. Magnetic particle spectroscopy (MPS) measurements were performed at room temperature to evaluate the MNOs MPI response. This initial study shows that FiM/AFM interfacial magnetic coupling is a viable method to increase thermosensitivity in MNOs for T-MPI.

Origin of spin-glass-like magnetic anomaly in the superconducting and multiferroic spin ladder BaFe2Se3

The spin ladder system ${\mathrm{BaFe}}_{2}{\mathrm{Se}}_{3}$ presents puzzling magnetic behaviors, such as a spin-glass-like phase below $\ensuremath{\sim}50$ K. In this paper, an exchange bias effect with a large vertical shift in the field-cooled hysteresis loop is observed below 50 K. We also evidence the existence of uncompensated spins by susceptibility, magnetic remanence, and hysteresis loop measurements. The thermoremanent and isothermoremanent magnetization curves evidence a two-dimensional diluted antiferromagnet (DAFF) nature. Moreover, a nanometer-sized layered structure is observed by scanning electron microscope (SEM) and confirmed by scanning transmission electron microscope (STEM) coupled with electron energy loss spectroscopy (EELS). The discovery of exchange bias in antiferromagnetic ${\mathrm{BaFe}}_{2}{\mathrm{Se}}_{3}$ crystals adds a new dimension to the research of its superconducting and multiferroic properties.

Insight into the negative magnetization and anomalous exchange-bias in DyFe5Al7 through neutron depolarization and neutron diffraction studies

We have provided the mesoscopic and microscopic understandings of polarity reversal of the magnetization or negative magnetization (NM) below T COMP = 93 K in an exotic magnetic material containing three magnetic sublattices, viz., DyFe5Al7 crystallizing in ThMn12 structure, using neutron depolarization and neutron diffraction techniques. A full recovery of the neutron beam polarization at the T COMP in a neutron depolarization experiment reveals a total compensation of magnetization inside the magnetic domains in the sample. The temperature-dependent neutron diffraction study under zero magnetic field has provided temperature dependencies of antiparallelly coupled Dy (M Dy(2a)) and Fe (M Fe(8f) and M Fe(8j)) sublattice magnetic moments along [100] direction. The dominance of |M Dy(2a)| over total Fe moment, , below T COMP leads to the NM in the compound. The magnetization versus magnetic field curves below the T COMP indicate the presence of field-induced spin reorientation in the compound. The magnetic field required for spin reorientation (H SR) is maximum at the lowest temperature and it decreases to zero as the temperature is increased to T COMP. Interestingly, the compound shows a finite exchange-bias (H EB) below the T COMP only, as evident from the field-cooled hysteresis loops, while at T > T COMP, H EB is almost zero. The cooling-field (H COOL) dependent study of H EB shows a slope change at H COOL ∼ H SR indicating a correlation of exchange-bias with spin-reorientation in the compound. This study, apart from revealing microscopic understanding of magnetic behavior of an exotic three magnetic sublattice system, provides a correlation among exchange-bias, magnetic compensation, and spin-reorientation phenomena.

Spin Glass State in Strained La2/3Ca1/3MnO3 Thin Films.

Epitaxial strain modifies the physical properties of thin films deposited on single-crystal substrates. In a previous work, we demonstrated that in the case of La2/3Ca1/3MnO3 thin films the strain induced by the substrate can produce the segregation of a non-ferromagnetic layer (NFL) at the top surface of ferromagnetic epitaxial La2/3Ca1/3MnO3 for a critical value of the tetragonality τ, defined as τ = |c − a|a, of τC ≈ 0.024. Although preliminary analysis suggested its antiferromagnetic nature, to date a complete characterization of the magnetic state of such an NFL has not been performed. Here, we present a comprehensive magnetic characterization of the strain-induced segregated NFL. The field-cooled magnetic hysteresis loops exhibit an exchange bias mechanism below T ≈ 80 K, which is well below the Curie temperature of the ferromagnetic La2/3Ca1/3MnO3 layer. The exchange bias and coercive fields decay exponentially with temperature, which is commonly accepted to describe spin-glass (SG) behavior. The signatures of slow dynamics were confirmed by slow spin relaxation over a wide temperature regime. Low-energy muon spectroscopy experiments directly evidence the slowing down of the magnetic moments below ~100 K in the NFL. The experimental results indicate the SG nature of the NFL. This SG state can be understood within the context of the competing ferromagnetic and antiferromagnetic interactions of similar energies.

Large exchange bias due to spin-glass behavior in hexagonal-YMn0.8Fe0.2O3 thin film

We report spin-glass (SG) behavior and large exchange bias effect at low temperature (∼2 K) in Fe-doped hexagonal-YMnO3 (YMn0.8Fe0.2O3) thin film. The thin films of YMnO3 and YMn0.8Fe0.2O3 are deposited on Pt(111)/Al2O3(0001) substrate by pulsed laser deposition method. The temperature dependent magnetization measurements show a characteristic peak for YMn0.8Fe0.2O3 film indicating the presence of SG state as confirmed by memory effect measurements. The hysteresis loops reveal high coercivity in YMn0.8Fe0.2O3 as compared to YMnO3 thin film. Field cooled hysteresis loop measurements at 2 K show the presence of exchange bias in YMn0.8Fe0.2O3 thin film. Maximum exchange bias of ∼909 Oe has been observed for hysteresis loop with cooling field of 20 kOe at 2 K. The exchange bias is further confirmed by training effect as well as negative and positive field cooled hysteresis loops. Exchange bias effect in YMn0.8Fe0.2O3 thin film can be attributed to the interaction between spin-glass/Anti-ferromagnetic phases.

Coercivity and exchange bias in size reduced iron obtained through chemical reduction

Size dependent coercivity of Fe particles synthesized through polyol process in a vast average size range from 210 nm to 19 nm has been reported. The large particles exhibit multidomain behaviour whereas the smaller particles show single domain nature and exchange bias. Saturation magnetization upto 211 emu/g, close to bulk Fe, has been obtained for particles with the largest average size. The major contribution to magnetization reversal is influenced by the magnetocrystalline anisotropy of Fe particles. The room temperature coercivity of multidomain particles with an average size of 210 nm is less than 100 Oe while that of 50 nm single domain particles reaches a maximum of 328 Oe. The TEM analysis of 50 nm and 19 nm particles reveal core shell morphology, with an iron oxide shell of thickness 3–4 nm. The increase in exchange anisotropy contribution due to size reduction has been studied from field cooled hysteresis loop measurements at 15 K. The interfacial exchange coupling between the core and shell gives rise to an exchange bias field of 16 Oe and 38 Oe for the 50 nm and 19 nm particles, respectively. Exchange bias blocking temperature of 50 K is observed for particles of average size 19 nm. The increase in exchange bias field for the particles of average size 19 nm has been correlated to interfacial effects using the Malozemhoff’s random field model with a roughness factor of 2.51.

Interfacial spin coupling and exchange anisotropy in core–shell MnFe2O4/α-Fe2O3 nanocomposites

Core–shell MnFe2O4/α-Fe2O3 nanoparticles having core diameters from 5 nm to 28 nm are synthesized by chemical co-precipitation route and investigated structurally and magnetically. The existence of the cubic spinel phase of MnFe2O4 (FiM) core together with the rhombohedral phase of α-Fe2O3 (AFM) shell was confirmed by X-ray diffraction. The field-cooled hysteresis loop at 5 K reveals a strong exchange bias of 1.1 kOe along with a high coercivity of 4.81 kOe and vertical shift of 1.83% for the particles with the smallest core diameters. The exchange-bias (HE) and coercive (HC) fields decrease with increasing core diameter. Exchange-spring behavior was also observed in the hysteresis loops in all nanocomposite systems at 5 K due to the presence of the magnetically hard (α-Fe2O3) and soft (MnFe2O4) phases. The superparamagnetic limit shifted above room temperature even for the sample having core diameter of 5 nm due to the presence of strong interfacial spins coupling, and this coupling was also seen in the hysteresis loops obtained at room temperature.

Magnetic field enhanced coercivity of Fe nanoparticles embedded in antiferromagnetic MnN films

The exchange coupling effect in nanocomposite samples with ferromagnetic (FM) body-centred tetragonal (bct) Fe nanoparticles (NPs) (~20 nm) embedded in an MnN antiferromagnetic (AFM) matrix is investigated. Both the bct Fe NPs control sample and the MnN (×nm)\\bct Fe NPs\\MnN (20–35 nm)\\Ta (5 nm) nanocomposite samples are synthesized by a gas-phase condensation method. Both the coercivity and the remanence ratio of the nanocomposite samples are significantly enhanced compared with the bct Fe NPs sample. The coercivity and the remanence ratio increase with MnN thickness up to 30 nm and then decrease. Additionally, the exchange coupling strength between the bct Fe NPs and AFM MnN matrix is enhanced by magnetic field training. The magnetic field training leads to a higher coercivity and remanence ratio after one cycle of field-cooled hysteresis loop measurement. The coercivity of the composite sample increases by 80%, while the remanence ratio increases by around 30% compared with the bct Fe NPs sample. This work may provide an alternative approach to the design and manufacturing of FM-AFM exchange-coupled permanent magnets.

Observation of spin glass behavior in chiral Mn48Fe34Si18 with a β-Mn related structure

The ternary silicide system Fe-Mn-Si exhibits several complex magnetic phases and the chemical similarity of Fe and Mn enables these two elements to be mutually substituted over a wide compositional range. Because this substitution results in chemical disorder and often causes competing magnetic exchange interactions, frustrated spin glass ordering is expected to occur for some Fe-Mn-Si phases. However, the observation of a spin glass state within these alloys has been elusive to date. This paper reports magnetization and ac susceptibility measurements on Mn48Fe34Si18. The compound crystallizes in a Mn3IrSi-type structure, which is closely related to that of β-Mn. Typical spin glass behavior is observed at low temperatures. This comprises a bifurcation of field-cooled and zero-field-cooled magnetization, a displacement of the field-cooled hysteresis loop, magnetic relaxation, a memory effect, and a sharp, frequency-dependent cusp in the ac susceptibility at the freezing temperature. Mn48Fe34Si18 is demonstrated to be a canonical spin glass.

Exchange bias behaviors up to room temperature in NiCo2O4/NiO nanoparticle system

Exchange bias behaviors up to room temperature have been observed in NiCo2O4/NiO nanoparticle system. The field-cooled magnetic hysteresis loops exhibit shifts both in the field and magnetization axes simultaneously. The variation of exchange bias field and magnetization shift depends strongly on the measuring temperature. In addition, the dependence of exchange bias effect on measuring temperature is not monotonous, which is reflected in the different tendency during the decrease process of magnetization shift and exchange bias field. The exchange bias field is decreased firstly at the range of 10–50 K followed by an increase of 50–200 K and decreased again of 200–320 K. By analyzing various magnetic measurements, the special exchange bias effect in NiCo2O4/NiO can be owing to the spin glass state and the ferrimagnetic/antiferromagnetic coupling during different temperature ranges.

Magnetic properties and exchange bias effect of Y2−xSrxCoMnO6 (0 ≤ x ≤ 0.5) double perovskites

The structural and magnetic properties of Y2−xSrxCoMnO6 (0 ≤ x ≤ 0.5) compounds are investigated in this report. X-ray photoelectron spectroscopy reveals that Co and Mn ions are presented in the form of Co2+/Co3+ and Mn3+/Mn4+, respectively. The broadening of Raman spectra suggests the existence of antisite disorders in these double perovskite oxides. The ferromagnetic transition temperature slightly increases with Sr doping, while the saturated magnetic moment shows a substantial decrease, further confirming the presence of Co/Mn antisite disorders. The signature of exchange bias effect in Y2−xSrxCoMnO6 (0 ≤ x ≤ 0.5) double perovskites has been observed, which is demonstrated by the displacement of field-cooled magnetic hysteresis loops. Moreover, the exchange bias field is strongly correlated with the degree of antisite disorders and reaches a maximum of 3.4 kOe for x = 0.25. The origin of exchange bias effect can be explained as the interfacial exchange coupling between ferromagnetic regions and antiferromagnetic antiphase boundaries created by antisite disorders in these specimens.

Electron doping induced exchange bias and cluster glass magnetism in multiferroic Sc0.8Zr0.2MnO3

Since few decades, several studies on hole-doping in rare-earth manganites (RMnO3) have been reported exploring various novel phenomena. Comparatively electron-doping studies in RMnO3 are scarcely found and the intrinsic properties are still unidentified. We report on the occurrence of exchange bias phenomenon and glassy magnetic interaction in multiferroic compound Sc0.8Zr0.2MnO3 induced by electron doping. The compound possesses long-range antiferromagnetic ordering, (TN = 90 K) and electron doping was found to induce additional ferromagnetic interaction (TC = 60 K) in it. The study provides compelling experimental evidences that the system behaves more like a cluster glass compound than a canonical spin glass system with a freezing temperature (Tf) of 13 K. Field-cooled magnetic hysteresis loops are found to shift in the field axis (HE) and show asymmetry in remanent magnetization (ME). The direction of loop shift depends on the sign of the cooling field used. We present details of field cooling and temperature dependence of HE and ME and training effect in Sc0.8Zr0.2MnO3. A comparison with the commonly observed exchange bias phenomena in typical phase separated systems is also presented. The present study assumes significance as it helps the understanding of new phenomena consequent to electron doping.

Reversed exchange-bias effect associated with magnetization reversal in the weak ferrimagnet LuFe0.5Cr0.5O3

The exchange-bias (EB) effect with sign reversal was found in $\mathrm{LuF}{\mathrm{e}}_{0.5}\mathrm{C}{\mathrm{r}}_{0.5}{\mathrm{O}}_{3}$ ferrite-chromite, which is a weak ferrimagnet below ${T}_{\mathrm{N}}=265\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, exhibiting antiparallel orientation of the ferromagnetic (FM) moments of the Fe and Cr sublattices due to opposite sign of the Fe-Cr Dzyaloshinskii vector, as compared to that of the Fe-Fe and Cr-Cr. The weak FM moments of the studied compound compensate each other at temperature ${T}_{\mathrm{comp}}=230\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, leading to the net magnetic moment reversal and to observed negative magnetization, at moderate applied fields, below ${T}_{\mathrm{comp}}$. Both vertical and horizontal shifts from the origin were gotten in the field-cooled magnetization hysteresis loops. The EB sign was found to be positive below ${T}_{\mathrm{comp}}$ and negative above ${T}_{\mathrm{comp}}$, with nonmonotonic dependence on cooling field ${H}_{\mathrm{cool}}$. It sharply increases at small values of magnetic fields up to ${H}_{\mathrm{cool}}\ensuremath{\sim}1\phantom{\rule{0.16em}{0ex}}\mathrm{kOe}$, then remains almost unchanged in the range 1--30 kOe and strongly decreases with further increase of ${H}_{\mathrm{cool}}$. This unusual behavior results from the competition of various Dzyaloshinskii-Moriya interactions between $\mathrm{F}{\mathrm{e}}^{3+}$ and $\mathrm{C}{\mathrm{r}}^{3+}$ ions.

Phase coexistence and exchange-bias effect in LiMn2O4 nanorods

In this paper, the magnetic properties of $\mathrm{LiM}{\mathrm{n}}_{2}{\mathrm{O}}_{4}$ nanorods with an average diameter of $\ensuremath{\sim}100\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$ and length of $\ensuremath{\sim}1\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{m}$ are investigated. The temperature dependences of dc and ac susceptibility measurements show that $\mathrm{LiM}{\mathrm{n}}_{2}{\mathrm{O}}_{4}$ nanorods experience multiple magnetic phase transitions upon cooling, i.e., paramagnetic (PM), antiferromagnetic (AFM), canted antiferromagnetic (CAFM), and cluster spin glass (SG). The coexistence between a long-range ordered AFM phase due to a $\mathrm{M}{\mathrm{n}}^{4+}\text{\ensuremath{-}}\mathrm{M}{\mathrm{n}}^{4+}$ interaction and a cluster SG phase originating from frozen AFM clusters at low temperature in $\mathrm{LiM}{\mathrm{n}}_{2}{\mathrm{O}}_{4}$ nanorods is elucidated. Field-cooled hysteresis loops (FC loops) and magnetic training effect (TE) measurements confirm the presence of an exchange-bias (EB) effect in $\mathrm{LiM}{\mathrm{n}}_{2}{\mathrm{O}}_{4}$ nanorods below the N\'eel temperature $({T}_{\mathrm{N}}\phantom{\rule{4pt}{0ex}}\ensuremath{\sim}\phantom{\rule{4pt}{0ex}}60\phantom{\rule{4pt}{0ex}}\mathrm{K})$. Furthermore, by analyzing the TE, we conclude that the observed EB effect originates completely from an exchange coupling interaction at the interface between the AFM and cluster SG states. A phenomenological model based on phase coexistence is proposed to interpret the origin of the EB effect below 60 K in the present compound. In turn, the appearance of the EB effect further supports the coexistence of AFM order along with a cluster SG state in $\mathrm{LiM}{\mathrm{n}}_{2}{\mathrm{O}}_{4}$ nanorods.

Investigation of novel inverted NiO@NixCo1-xO core-shell nanoparticles

Inverse core-shell nanoparticles, comprised of an antiferromagnetic (AFM) core covered by a ferromagnetic (FM) or ferrimagnetic (FiM) shell, are of current interest due to their different potential application and due to the tunability of their magnetic properties. The antiferromagnetic nature of NiO and high Néel temperature (523 K) makes this material well suited for inverse core-shell nanoparticle applications. Our primary objective in this project has been to synthesize and characterize inverted core-shell nanoparticles (CSNs) comprised of a NiO (AFM) core and a shell consisting of a NixCo1-xO (FiM) compound. The synthesis of the CSNs was made using a two-step process. The NiO nanoparticles were synthesized using a chemical reaction method. Subsequently, the NiO nanoparticles were used to grow the NiO@NixCo1-xO CSNs using our hydrothermal nano-phase epitaxy method. XRD structural characterization shows that the NiO@NixCo1-xO CSNs have the rock salt cubic crystal structure. SEM-EDS data indicates the presence of Co in the CSNs. Magnetic measurements show that the CSNs exhibit AFM/FiM characteristics with a small coercivity field of 30 Oe at 5 K. The field cooled vs zero field cooled hysteresis loop measurements show a magnetization axis shift which is attributed to the exchange bias effect between the AFM NiO core and an FiM NixCo1-xO shell of the CSNs. Our ab initio based calculations of the NixCo1-xO rock salt structure confirm a weak FiM character and a charge transfer insulator property of the compound.

Synthesis and Characterization of Novel Inverted NiO@NixMn1-xO Core-Shell Nanoparticles

ABSTRACTMagnetic core-shell nanoparticles have the potential for numerous applications, such as in magnetic recording media, magnetic resonance imaging, drug delivery or hyperthermia, and spin valves. Inverse core-shell nanoparticles, comprised of an antiferromagnetic (AFM) core covered by a ferromagnetic (FM) or ferrimagnetic (FiM) shell, are of current interest due to the tunability of their magnetic properties. NiO is typically antiferromagnetic in nature and has a Néel temperature of 523 K. Our primary objective in this project is to synthesize and characterize inverted core-shell nanoparticles (CSNs) comprised of a NiO (AFM) core and a shell consisting of a NixMn1-xO (FM/FiM) compound. The synthesis of the CSNs was made using a two-step process. The NiO nanoparticles were synthesized using a chemical reaction method. Subsequently, the NiO nanoparticles were used to grow the NiO@NixMn1-xO CSNs using our hydrothermal nano-phase epitaxy method. XRD structural characterization shows that the NiO@NixMn1-xO CSNs have the rock salt cubic crystal structure throughout. SEM-EDS data indicates the presence of Mn in the CSNs. SQUID magnetic measurements show that the CSNs exhibit AFM/FM or AFM/FiM characteristics with a coercivity field of 425 Oe at 5 K. The field cooled vs zero field cooled hysteresis loop measurements show a significant exchange bias effect between the AFM NiO core and FM/FiM NixMn1-xO shell of the CSNs. The results of additional TEM and magnetic characterization are discussed.

Thickness dependent interfacial magnetic coupling in [La2NiMnO6/LaMnO3] multilayers

In the present work, interfacial magnetic exchange coupling at FM/AFM interface has been studied by varying the thickness of AFM layer (LaMnO3) in (La2NiMnO6/LaMnO3)15 multilayer thin film based system. In multilayer thin films, the thickness of LMO was varied from 30 to 50Å, while the thickness of LNMO was kept constant at 100Å. Thin films of LNMO, LMO and LNMO/LMO multilayers have been deposited by pulsed laser deposition technique on (001) LaAlO3 substrate. The thin films have been studied for their structural and magnetic properties. XRD analysis reveals the c-axis epitaxial growth of LNMO, LMO and their multilayer thin films. Exchange bias (EB) effect has been observed in the field cooled hysteresis loops of multilayer thin films and the interaction between FM and AFM spins at the interface is responsible for the observed effect. EB measurements reveal that the thickness variation influences the interfacial interaction between two layers. The EB increases with increasing AFM film thickness in multilayer thin film samples and maximum EB (∼740Oe) is observed for sample with 50Å thickness of AFM layer. Temperature dependence of EB and training effect measurements have also been performed to confirm the EB in the sample.

Robust Interfacial Exchange Bias and Metal-Insulator Transition Influenced by the LaNiO3 Layer Thickness in La0.7Sr0.3MnO3/LaNiO3 Superlattices.

Artificial heterostructures based on LaNiO3 (LNO) have been widely investigated with the aim to realize the insulating antiferromagnetic state of LNO. In this work, we grew [(La0.7Sr0.3MnO3)5-(LaNiO3)n]12 superlattices on (001)-oriented SrTiO3 substrates by pulsed laser deposition and observed an unexpected exchange bias effect in field-cooled hysteresis loops. Through X-ray absorption spectroscopy and magnetic circular dichroism experiments, we found that the charge transfer at the interfacial Mn and Ni ions can induce a localized magnetic moment. A remarkable increase of exchange bias field and a transition from metal to insulator were simultaneously observed upon decreasing the thickness of the LNO layer, indicating the antiferromagnetic insulator state in 2 unit cells LNO ultrathin layers. The robust exchange bias of 745 Oe in the superlattice is caused by an interfacial localized magnetic moment and an antiferromagnetic state in the ultrathin LNO layer, pinning the ferromagnetic La0.7Sr0.3MnO3 layers together. Our results demonstrate that artificial interface engineering is a useful method to realize novel magnetic and transport properties.

Probing core and shell contributions to exchange bias inCo/Co3O4nanoparticles of controlled size

Coupling at the interface of core/shell magnetic nanoparticles is known to be responsible for the exchange bias (EB) and the relative sizes of core and shell components are supposed to influence the associated phenomenology. In this work, we have prepared core/shell structured nanoparticles with the total averaged diameter around $\sim$ 27 nm and a wide range of shell thicknesses through the controlled oxidation of Co nanoparticles well dispersed in an amorphous silica host. Structural characterizations give compelling evidence of the formation of Co$_3$O$_4$ crystallite phase at the shells surrounding the Co core. Field cooled hysteresis loops display nonmonotonous dependence of the exchange bias $H_E$ and coercive $H_C$ fields, that become maximum for a sample with an intermediate shell thickness, at which lattice strain is also maximum for both the phases. Results of our atomistic Monte Carlo simulations of the particles with the same size and compositions as in experiments are in agreement with the experimental observations and have allowed us to identify a change in the contribution of the interfacial surface spins to the magnetization reversal giving rise to the maximum in $H_E$ and $H_C$.

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.