Effective Magnetic Anisotropy Field Research Articles

Temperature Evolution of Magnon Propagation Length in Tm3Fe5O12 Thin Films: Roles of Magnetic Anisotropy and Gilbert Damping.

The magnon propagation length, ⟨ξ⟩, of a ferro-/ferrimagnet (FM) is one of the key factors that controls the generation and propagation of thermally driven magnonic spin current in FM/heavy metal (HM) bilayer based spincaloritronic devices. For the development of a complete physical picture of thermally driven magnon transport in FM/HM bilayers over a wide temperature range, it is of utmost importance to understand the respective roles of temperature-dependent Gilbert damping (α) and effective magnetic anisotropy (Keff) in controlling the temperature evolution of ⟨ξ⟩. Here, we report a comprehensive investigation of the temperature-dependent longitudinal spin Seebeck effect (LSSE), radio frequency transverse susceptibility, and broad-band ferromagnetic resonance measurements on Tm3Fe5O12 (TmIG)/Pt bilayers grown on different substrates. We observe a significant drop in the LSSE voltage below 200 K independent of TmIG film thickness and substrate choice. This is attributed to the noticeable increases in effective magnetic anisotropy field, HKeff (∝Keff) and α that occur within the same temperature range. From the TmIG thickness dependence of the LSSE voltage, we determined the temperature dependence of ⟨ξ⟩ and highlighted its correlation with the temperature-dependent HKeff and α in TmIG/Pt bilayers, which will be beneficial for the development of rare-earth iron garnet based efficient spincaloritronic nanodevices.

Experimental investigation of the effect of topological insulator on the magnetization dynamics of ferromagnetic metal: and heterostructure

We have studied the spin-pumping phenomenon in ferromagnetic metal (FM) ()/topological insulator (TI) () bilayer system to understand magnetization dynamics of FM in contact with a TI. TIs embody a spin–momentum-locked surface state that spans the bulk band gap. Due to this special spin texture of the topological surface state, the spin-charge interconversion efficiency of TI is even higher than that of heavy metals. We evaluated the parameters like effective damping coefficient (), spin-mixing conductance () and spin current density () to demonstrate an efficient spin transfer in heterostructure. To probe the effect of the topological surface state, a systematic low-temperature study is crucial as the surface state of TI dominates at lower temperatures. The exponential increase of for all different thickness combinations of FM/TI bilayers and the enhancement of effective damping coefficient () with lowering temperature confirms that the spin chemical potential bias generated from spin-pumping induces spin current into the TI surface state. Furthermore, low-temperature measurements of effective magnetization () and magnetic anisotropy field (Hk ) showed anomaly around the same temperature region where the resistivity of TI starts showing metallic behavior due to the dominance of conducting TI surface state. The anomaly in Hk can result from the emerging exchange coupling between the TI surface state and the local moments of the FM layer at the interface without any long-range ferromagnetic order in TI at the interface.

Effective anisotropy in Fe-Ni nanowire arrays with strong dipolar interaction

The effective magnetic anisotropy field Heff of Fex Ni(100-x) (x = 15, 25, 38) nanowires of 65 nm diameter forming ordered arrays within dense alumina templates of ∼35% porosity, was determined. Different values for Heff were obtained depending on the determination method, which were: magnetization evolution along major hysteresis loops; ferromagnetic resonance at 34 GHz; and magnetization switching by curling nucleation (through angular variation of the coercive field). Heff values determined from magnetization measurements differ from those arising from FMR measurements, being these latter quite smaller in every case. No evidence of an easy plane behavior was found, as theoretically predicted for high porosity arrays as ours.These results suggest that new internal degrees of freedom need to be considered for the effective anisotropy description in quasistatic hysteresis loops, during polarization reversal and in ferromagnetic resonance dynamic processes, to understand and model these experimental findings. We discuss possible causes.

Magnetization dynamics in layered systems with coexisting bilinear and biquadratic interlayer exchange coupling

Important aspects of exchange-coupled magnetic layered structures are related to the noncollinear arrangement of sublayer magnetizations which can arise from competition between bilinear (BL) and biquadratic (BQ) interlayer exchange coupling (IEC). In this work, the influence of coexisting BL and BQ IEC of different strengths on magnetization precession in layered systems is investigated both experimentally and theoretically. Laser-induced magnetization precession has been studied in the $\text{Fe/Si}({d}_{\text{Si}}$) multilayers (MLS) as a function of the amplitude ($H$) and orientation angle (${\ensuremath{\theta}}_{H}$) of external magnetic field using time-resolved magneto-optical Kerr (TRMOKE) effect. Strongly changing characters of precession frequency dependencies $\ensuremath{\omega}(H,{\ensuremath{\theta}}_{H})$ for Fe sublayer thickness ${d}_{\text{Fe}}=3$ nm and Si spacer-layer thicknesses (${d}_{\text{Si}}$) varying in the range of 0.9--2.4 nm have been observed. Analytical formulas for acoustic and optic mode dispersion relations with coexisting BL and BQ IEC, scaled by ${J}_{1}$ and ${J}_{2}$ parameters, respectively, for the in-plane effective magnetic anisotropy and arbitrary magnetic field direction were derived, and very good agreement with the experimentally observed frequency dependencies has been obtained. It is shown that BQ coexisting with BL IEC significantly influences on the magnitude and form of dispersion relations. From analytical formula derived, it follows that zero-field optical mode frequency tends to zero as $|{J}_{1}|$ approaches $2{J}_{2}$. The acoustic and optic mode-crossing effect has been observed and it is found that values of crossing fields and frequency gaps strongly increase as ${\ensuremath{\theta}}_{H}$ angles decrease and depend on relative BL and BQ IEC strengths. The BL IEC is of ferromagnetic type with ${J}_{1}\ensuremath{\approx}1.6$ mJ/${\mathrm{m}}^{2}$ for the MLS with ${d}_{\text{Si}}=0.9$ nm, and changes to antiferromagnetic one with ${J}_{1}\ensuremath{\approx}\ensuremath{-}0.9$ mJ/${\mathrm{m}}^{2}$ for the MLS with ${d}_{\text{Si}}=1.4$ nm, while the ${J}_{2}$ parameter of BQ IEC decreases from 1.8 to 1.0 mJ/${\mathrm{m}}^{2}$. The coupling strengths decrease by one to two orders of magnitude for the sample with ${d}_{\text{Si}}=2.4$ nm, but both mode frequencies are still observed and well reproduced by the theory. It is shown that ${J}_{1}$ and ${J}_{2}$ parameters obtained in the TRMOKE experiment coincide within the estimated error bars with the determined from independent measurements of magnetization processes in the static magneto-optical Kerr effect and interpreted with the use of analytical formulas derived. Numerical solutions of coupled Landau-Lifshitz-Gilbert (LLG) equations for acoustic and optic modes, with inclusion of BL IEC, intrinsic Gilbert damping, and spin-pumping damping terms, and extended to include BQ IEC, were performed and fitted to experimental data. It is shown that determined effective damping coefficients on $H$ and ${\ensuremath{\theta}}_{H}$ dependencies for acoustic and optic modes are very well simulated with the use of LLG equation solutions with Gilbert damping, spin-pumping-damping--related effective spin-mixing conductance, and spin-diffusion length parameters included. The dependencies of the parameters on ${d}_{\text{Si}}$ spacer-layer thickness are discussed and compared with available data for other systems.

Constituent contribution to the magnetocrystalline anisotropy in Mn(Al1−xGax)

The phase stability and magnetocrystalline anisotropy (MCA) of tetragonal Mn(Al1−xGax) with the L10-type structure (P4/mmm) has been studied using first-principles density functional calculations. The calculated decomposition energy indicates that partial replacement of Al by Ga suppresses the formation of Mn5(Al,Ga)8 and enhances the thermal stability of the L10 phase while the total magnetic moment per formula unit (f.u.) remains almost unchanged. The site- and atomic-resolved MCA calculations show that the MCA energy (MAE) comes mainly from the Mn atoms, and the total MAE increases from 0.25 meV/f.u. (x = 0) to 0.34 meV/f.u (x = 1). Spin resolved MCA and band structure calculations indicate that the high MCA is mainly due to spin flipping behavior near Fermi level. The derived effective magnetic anisotropy field increases from 37 kOe (x = 0) to 46 kOe (x = 1), in agreement with experiments. Doping with Ga improves the thermal stability of the L10 structure and enhances the magnetic anisotropy field, which facilitates developing high coercivity Mn-Al magnets.

Validity of DFT-based spin-orbit torque calculation for perpendicular magnetic anisotropy in iron thin films

In this study, the validity of our DFT-based self-developed JunPy+SOT calculation method has been rigorously confirmed by the perpendicular magnetic anisotropy (PMA) induced effective magnetic anisotropy field of 5.6 kOe in a 1.5 nm-thick iron thin film. The angular dependence of the magnetic anisotropy energy (MAE) and the total spin-orbit torque (SOT) indicate the uniaxial PMA corresponding to the out-of-plane rotations of the magnetization. Our results agree with the conventional MAE calculation but provide deeper insights into atomistic spin dynamics of local magnetic moments. The main advantage of this method is that the well-decomposed layer-resolved SOT and SOC-dominated spin current accumulation, which exhibit maximum magnitude but opposite sign at the surface, provide comprehensive physical understanding in angular momentum transfer between spin and orbital, competition between SOT and spin current accumulation, and precise spin torque acting on each local magnetic moment that is crucial for the atomistic spin dynamics especially in magnetic heterostructures.

Controlling magnetic anisotropy in nickel films on LiNbO3

Nickel thin films, deposited on 128° Y-cut LiNbO3, and subsequently annealed at low temperature (≈ 300 °C), exhibit a number of interesting and unusual magnetic properties. These include: hysteresis branch crossings, negative Barkhausen jumps, square hysteresis loops (squareness > 0.99), Stoner-Wohlfarth behavior, and a well-defined, uniaxial, magnetic anisotropy. Previous work suggests the magnetic anisotropy is created by magneto-elastic coupling between the film and the underlying substrate. The difference of thermal coefficient of expansion between film and substrate was thought to generate a well-defined strain upon cooling from the annealing temperature. Here we show that the changes in the effective magnetic anisotropy field are directly correlated with thermal strain. This mechanism provides a technique for creating and modifying magnetic anisotropy in a controlled fashion.

Enhanced spin-orbit torque efficiency in Pt100–x Ni x alloy based magnetic bilayer* *Project supported by the National Natural Science Foundation of China (Grant Nos. 51901025 and 51625101), the Fundamental Research Funds for the Central Universities, China (Grant No. 310421101), and the Beijing Natural Science Foundation, China (Grant No. Z190007).

The binary alloy/ferromagnetic metal heterostructure has drawn extensive attention in the research field of spin–orbit torque (SOT) due to the potential enhancement of SOT efficiency via composition engineering. In this work, the magnetic properties and SOT efficiency in the Pt100 – x Ni x /Ni78Fe22 bilayers were investigated via the spin-torque ferromagnetic resonance (ST-FMR) technique. The effective magnetic anisotropy field and effective damping constant extracted by analyzing the ST-FMR spectra show a weak dependence on the Ni concentration. The effective spin-mixing conductance of 8.40 × 1014 Ω−1 ⋅ m−2 and the interfacial spin transparency T in of 0.59 were obtained for the sample of Pt70Ni30/NiFe bilayer. More interestingly, the SOT efficiency that is carefully extracted from the angular dependence of ST-FMR spectra shows a nonmonotonic dependence on the Ni concentration, which reaches the maximum at x = 18. The enhancement of the SOT efficiency by alloying the Ni with Pt shows potential in lowering the critical switching current. Moreover, alloying relatively cheaper Ni with Pt may promote to reduce the cost of SOT devices.

Study on relatively large response of rectifying voltage in Ni wires fabricated on a LiNbO3 substrate

The rectifying electric ferromagnetic resonance spectroscopy was performed to examine the magnetization dynamics in the stripe magnetic domain structure induced by the competition between shape and heterojunction-induced magnetic anisotropy. The observed angle dependence is in good agreement with the analytical calculations based on the macro-spin model when the applied magnetic field is larger than the effective magnetic anisotropy field. When the external magnetic field is smaller than the threshold field at which destroys the stripe domain structure is destroyed, a large rectifying amplitude is generated in a Ni wire aligned perpendicular to the x-axis of the substrate. The relatively large response is attributed to that the coupled oscillations of the stripe domains. In addition, the Gilbert damping factor is found to be enhanced in the system because of stripe domain structure formation and strain-induced modulation.

Features of magnetic and magnetoelectric properties, H-T phase diagram of GdCr3(BO3)4

Comprehensive studies of magnetic properties of GdCr3(BO3)4 single crystal have been carried out. The integrals of intrachain (J/k = 4 ± 1 K) and interchain (J΄/k ≈ −0.7 K) exchange interactions in the Cr subsystem have been determined and the strength of Cr-Gd exchange interaction (JCr-Gd/k ~ −0.2 K) has been estimated. The values of the exchange field 2HE ≈ 17 T and the effective magnetic anisotropy field HA ≈ 0.05 T of GdCr3(BO3)4 have been estimated. The electric polarization ΔPa along the a axis in the longitudinal geometry of the experiment has been detected. Correlations between the electric polarization and the magnetization of the studied compound have been found. The spin-reorientation phase transition in the magnetically ordered state has been found. This transition exists for the external magnetic field applied along any crystallographic direction and the transition field depends weakly on the direction of the field. Nature of the spin-reorientation phase transition has been discussed. Magnetic phase diagram has been constructed and spin configurations for the low-field and high-field phases have been proposed.

Temperature Dependence of the Effective Magnetic Anisotropy Field and Ferromagnetic Resonance Linewidth in Polycrystalline Complex Substituted Magnetically Uniaxial Hexagonal Ferrites in the Frequency Range of 25–67 GHz

Temperature dependences of the effective magnetic anisotropy field and ferromagnetic resonance linewidth in samples of isotropic and anisotropic hexaferrite SrFe11.2Al0.1Si0.15Ca0.15O19 and anisotropic hexaferrite BaFe10.4Al1.4Si0.15Mn0.1O19, obtained by ceramic technology with wet blanks pressed in the magnetic field of 10 kOe, were studied in the frequency range from 25 to 67 GHz. The studies were carried out at temperatures from 25 to 85°C. The change in the magnetic anisotropy in this temperature range was found to be 9.8 Oe/°C for barium hexaferrite and 4.2 Oe/°C for strontium hexaferrite; the change in the ferromagnetic resonance linewidth was 12.2 Oe/°C for barium hexaferrite and 10–12.3 Oe/°C for strontium hexaferrite.

Magnetic irreversibility and saturation criteria in ultrasmall bi-magnetic nanoparticles

Ultrasmall magnetic particles are notorious for exhibiting a magnetization increase even at quite intense applied fields, behavior which can be interpreted as a non-saturation of the magnetic disordered shell even if the nanoparticle magnetization is reversible. In this work we study two kinds of ultrasmall core@shell nanoparticles (3 nm) with contrasting core anisotropy, composed of MnFe2O4 and CoFe2O4 cores covered by a thin maghemite layer. In order to investigate the saturation criterion associated to the closure of major loops (in contrast to minor loops), we use several procedures to determine, at moderate fields, if the effective anisotropy energy barrier is overcome or not. Firstly, we carefully evaluate the closure field of the hysteresis loop, interpreted as the effective anisotropy field, correspondent to two different contributions. One, arising from the nanoparticle core, is related to the coercivity and the other one is associated to pinned spins of the nanoparticle shell. Secondly, the ZFC-FC magnetization measurements taken at different fields give us information about both the anisotropy energy barrier distribution and the thermal dependence of effective anisotropy field. Thirdly, forced minor loops are performed, measured after either ZFC or FC processes, to counter-check the effective magnetic anisotropy at low temperature of both samples. Finally, we perform major hysteresis loop calculations in order to better understand the magnetization processes involved. To go further, we propose in this study a procedure to extract information about the two contributions which composes the effective magnetic anisotropy field.

Температурная зависимость эффективного поля магнитной анизотропии и ширины линии ферромагнитного резонанса поликристаллических сложнозамещенных гексагональных магнитноодноосных ферритов в диапазоне частот 25-67 GHz

In the work in the frequency range 25 – 67 GHz the temperature changes of the effective magnetic anisotropy field and ferromagnetic resonance linewidth of the samples isotropic and anisotropic hexaferrite SrFe11.2Al0.1Si0.15Ca0.15O19 and anisotropic hexaferrite BaFe10.4Al1.4Si0.15Mn0.1O19 were studied. The samples obtained by ceramic technology with the pressing of the raw blanks in a magnetic field of 10 kOe. The studies were carried out in the temperature range+25 – +85 ºC. It was found that in the specified temperature range, the change in magnetic anisotropy is 9.8 Oe/ºC for barium hexaferrite and 4.2 Oe/ºC for strontium hexaferrite, and the change in ferromagnetic resonance linewidth is 12.2 Oe/ºC for barium hexaferrite and 10 – 12.3 Oe/ºC for strontium hexaferrite.

Effect of second-order magnetic anisotropy on nonlinearity of conductance in CoFeB/MgO/CoFeB magnetic tunnel junction for magnetic sensor devices

We studied the effect of second-order magnetic anisotropy on the linear conductance output of magnetic tunnel junctions (MTJs) for magnetic-field-sensor applications. Experimentally, CoFeB/MgO/CoFeB-based MTJs were fabricated, and the nonlinearity, NL was evaluated for different thicknesses, t of the CoFeB free layer from the conductance. As increasing t from 1.5 to 2.0 nm, maximum NL, NLmax was found to decrease from 1.86 to 0.17% within the dynamic range, Hd = 1.0 kOe. For understanding the origin of such NL behavior, a theoretical model based on the Slonczewski model was constructed, wherein the NL was demonstrated to be dependent on both the normalized second-order magnetic anisotropy field of Hk2/|Hkeff| and the normalized dynamic range of Hd/|Hkeff|. Here, Hkeff, Hk2, are the effective and second-order magnetic anisotropy field of the free layer in MTJ. Remarkably, experimental NLmax plotted as a function of Hk2/|Hkeff| and Hd/|Hkeff|, which were measured from FMR technique coincided with the predictions of our model. Based on these experiment and calculation, we conclude that Hk2 is the origin of NL and strongly influences its magnitude. This finding gives us a guideline for understanding NL and pioneers a new prospective for linear-output MTJ sensors to control sensing properties by Hk2.

Influence of aluminum substitution on the fi eld of effective magnetic anisotropy and the degree of magnetic texture of anisotropic polycrystalline hexagonal ferrites of barium and strontium for substrates of microstrip devices of microwave electronics

Abstract. In this paper, the effect of Al3+ ions substitutions on the value of the effective magnetic anisotropy field НАeff and the degree of magnetic texture f of the anisotropic polycrystalline hexagonal barium and strontium ferrites were studied. The samples were obtained by the ceramic technology method and the texture was formed by pressing in a magnetic field. The sample preparation technology presented in detail. The batches of barium hexaferrites were synthesized with the concentration of Al3+ ions: 0.9; 1.4; 2.5 and 2.6 formula units while strontium hexaferrites had Al3+ concentration of 0.1 formula units. It has been shown that by this technology barium and strontium hexaferrites with high value of (in range of 19—35 kOe) and with f = 80—83% could be obtained. The achieved values of НАeff and f could be sufficient for the production of substrates for microstrip microwave devices in millimeter−wave region.For the first time a raise in the degree of magnetic texture of polycrystalline barium hexaferrites with an increase of concentration of Al3+ ions were detected; a slight (5.5—5.8%) magnetic texture of isotropic strontium hexaferrites was also detected. The achieved results discussed in detail. For studied hexaferrites the mechanism of magnetic texture formation during the process of synthesis is proposed.

Ultrafast magnetization dynamics in an epitaxialNi54.3Mn31.9Sn13.8Heusler-alloy film close to the Curie temperature

The influence of the amplitude of an external magnetic field ($H$) and femtosecond laser pulse fluence ($F$) on ultrafast magnetization dynamics has been investigated in a ferromagnetic ${\mathrm{Ni}}_{54.3}{\mathrm{Mn}}_{31.9}{\mathrm{Sn}}_{13.8}$ Heusler-alloy film using the time-resolved magneto-optical Kerr effect. A large slowing down of the demagnetization process was observed and characteristic parameters of magnetization precession were determined for a wide range of $H$ and $F$ values. Long demagnetization times of the order of hundreds of picoseconds have been found and explained as a result of the Curie temperature (${T}_{\mathrm{C}}$) proximity in the alloy film studied. Effective magnetic anisotropy field (${H}_{\text{k}}^{\text{eff}}$) and Gilbert damping parameter dependencies were determined. A significant reduction of the precession frequency versus $F$ of the uniform Kittel mode was found. A strong decrease of ${H}_{\text{k}}^{\text{eff}}$ with $F$ was well simulated in the frame of an extended version of the microscopic three-temperature model (eM3TM), and explained by the ${T}_{\mathrm{C}}$ proximity effect. The estimated low values of the eM3TM model parameters, the demagnetization rate and electron-lattice coupling constant, appeared essential to explain the slowing down effect of demagnetization. Precession amplitude dependencies were explained by a phenomenological approach taking into account ${H}_{\text{k}}^{\text{eff}}$ and changes of the equilibrium magnetization angles induced by pump-pulse excitation.

Femtosecond laser-heating effect on the magnetization dynamics in perpendicularly magnetized Ta/CoFeB/MgO film

We have studied the effect of ultrafast laser-heating on the magnetization dynamics of perpendicularly magnetized CoFeB film by means of the time-resolved magneto-optical Kerr rotation effect. The effective perpendicular magnetic anisotropy field is significantly decreased with enhancing the pump laser-fluence in a moderate range of 5–12 mJ cm−2. The Gilbert damping, however, is found to be independent of the pump fluence. These findings provide a new method of separately manipulating the Gilbert damping and perpendicular magnetic anisotropy.

Measurement of effective magnetic anisotropy field and ferromagnetic resonance bandwidth at ferromagnetic resonance frequency in magnetically uniaxial hexagonal ferrites

In this work we have considered metrological problems and measurement of magnetic parameters and presented methods of measuring effective magnetic anisotropy field HAeff and ferromagnetic resonance bandwidth ∆H in magnetically uniaxial hexagonal ferrites in the electromagnetic microwave working frequency range. The methods allow measuring HAeff in the 10–23 and 28–40 kE ranges and ∆H in the 0.5–5.0 range. One method (suitable for wavelength measurements in free space in the 3-mm wavelength range) has been implemented for the 78.33–118.1 GHz range. The other method (based on the use of microstrip transmission lines) has been implemented for the 25–67 GHz range. The methods have been tested for polycrystalline specimens of hexagonal barium and strontium ferrites with nominal composition or complex substituted and having high magnetic texture. The measurement results have been compared with those obtained using conventional measurement methods and spherical specimens. Our methods prove to be highly accurate and reliable.

A Strain‐Mediated Magnetoelectric‐Spin‐Torque Hybrid Structure

AbstractMagnetization dynamics induced by spin–orbit torques in a heavy‐metal/ferromagnet can potentially be used to design low‐power spintronics and logic devices. Recent computations have suggested that a strain‐mediated spin–orbit torque (SOT) switching in magnetoelectric (ME) heterostructures is fast, energy‐efficient, and permits a deterministic 180° magnetization switching. However, its experimental realization has remained elusive. Here, the coexistence of the strain‐mediated ME coupling and the SOT in a CoFeB/Pt/ferroelectric hybrid structure is shown experimentally. The voltage‐induced strain only slightly modifies the efficiency of SOT generation, but it gives rise to an effective magnetic anisotropy and rotates the magnetic easy axis which eliminates the incubation delay in current‐induced magnetization switching. The phase field simulations show that the electric‐field‐induced effective magnetic anisotropy field can reduce the switching time approximately by a factor of three for SOT in‐plane magnetization switching. It is anticipated that such strain‐mediated ME‐SOT hybrid structures may enable field‐free, ultrafast magnetization switching.

Magnetic Anisotropy and Switching Behavior of Fe3O4/CoFe2O4 Core/Shell Nanoparticles

A uniform core/shell nanoparticle system composed of a soft magnetic core (Fe3O4) and a hard magnetic shell (CoFe2O4) was synthesized and characterized to understand how the shell influences the magnetism and exchange coupling of the system. In the case of Fe3O4(8 nm)/CoFe2O4(2 nm) core/shell nanoparticles, DC and AC susceptibility measurements revealed three features associated with the blocking temperatures of the core/shell system (TB-cs ∼ 300 K), the CoFe2O4 shell (TB-s ∼ 200 K), and the Fe3O4 core (TB-c ∼ 50 K). Radio-frequency transverse susceptibility gave a direct probe of the effective magnetic anisotropy field (HK) and switching field (HS), as well as their temperature evolutions. Interestingly, we found that HK of the core/shell structure increased with decreasing temperature. HS was observed only below TB-s, which first decreased drastically with lowering temperature and then increased sharply below TB-c. This is attributed to the effect of a coercive field of CoFe2O4 on the spin flipping of Fe3O4 in the superparamagnetic state (TB-c < T < TB-s) and the blocked state (T < TB-c), respectively. Our study sheds light on the magnetic exchange coupling mechanism in core/shell nanoparticle systems and demonstrates the possibility of controlling the nanomagnetism of a soft magnetic core to which the hard magnetic shell is coupled in such systems.