Radio-frequency Transverse Susceptibility 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.

Tunable competing magnetic anisotropies and spin reconfigurations in ferrimagnetic Fe100−xGdx alloy films

We report a comprehensive study of the temperature evolution of in-plane (IP) and out-of-plane (OOP) effective magnetic anisotropies in compensated ferrimagnetic ${\mathrm{Fe}}_{100\ensuremath{-}x}{\mathrm{Gd}}_{x}$ alloy films by employing direct current magnetometry and radiofrequency (RF) transverse susceptibility (TS) measurements. We suggest that our ${\mathrm{Fe}}_{100\ensuremath{-}x}{\mathrm{Gd}}_{x}$ system is chemically inhomogeneous and phase segregates into Fe- and Gd-enriched regions. Our IP and OOP magnetometry results indicate that the system undergoes a temperature-driven transformation from an IP-spin-configuration-dominated state to an OOP-spin-configuration-dominated state below a certain temperature (spin reorientation temperature). A two-step reversal behavior emerges in the OOP $M$($H$) loop near compensation, which we attribute to the sequential magnetization reversals of Fe- and Gd-enriched domains. Field-induced spin-flop transitions were also observed near the compensation. Our RF TS measurements indicate that the effective magnetic anisotropy for the OOP configuration dominates over that for the IP configuration below a certain spin reorientation temperature. Both IP and OOP anisotropy fields determined from our TS measurement exhibit a minimum around the compensation temperature, which has been explained in the framework of the Stoner-Wohlfarth model.

Strain-modulated helimagnetism and emergent magnetic phase diagrams in highly crystalline MnP nanorod films

We explore strain-modulated helimagnetism in highly crystalline MnP nanorod films grown on Si(100) substrates using molecular beam epitaxy. The strained MnP film exhibits a paramagnetic to ferromagnetic (FM) phase transition at ${T}_{C}\ensuremath{\sim}279\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, and the FM to helical phase transition at ${T}_{N}\ensuremath{\sim}110\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. The value of ${T}_{N}$ is greater than ${T}_{N}\ensuremath{\sim}47\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ for the MnP single crystal, indicating strong strain-modulated helimagnetic states in the MnP nanorod film. The presence of significant thermal hysteresis in the helical phase indicates the coexistence of competing magnetic interactions, leading to the first-order metamagnetic transition. Similar to its single-crystal counterpart, an anisotropic magnetic effect is observed in the MnP film, which is independently confirmed by magnetic hysteresis loop and radio-frequency transverse susceptibility (TS) measurements. The evolution of the screw to the cone and fan phases is precisely tracked from magnetization vs magnetic field/temperature measurements. The temperature dependence of the anisotropy fields, extracted from the TS spectra, yields further insight into the competing nature of the magnetic phases. Unfolding of the different helical phases at $T<120\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ $(\ensuremath{\sim}{T}_{\mathrm{N}})$ is analyzed by the temperature- and field-dependent magnetic entropy change. Based on these findings, the comprehensive magnetic phase diagrams of the MnP nanorod film are constructed for both the in-plane and out of plane magnetic field directions, revealing emergent strain/dimensionality-driven helical magnetic features that are absent in the magnetic phase diagram of the MnP single crystal.

Magnetic anomalies associated with domain wall freezing and coupled electron hopping in magnetite nanorods

Magnetite has fascinated researchers for decades, due to its wide range of applications from spintronics to biomedicine. Despite a large body of works aimed at its magnetic properties, no consensus has been reached on the physical origin of the low temperature magnetic anomalies observed in magnetite. Although, a lot of work has been done in studying magnetite nanoparticles, but studies on the low temperature anomalies in those nanoparticles still remains unresearched. We report on the observation of the low temperature magnetic anomalies in highly crystalline, stoichiometric Fe3O4 nanorods and relate them to the coupled electron hopping relaxation process and domain wall motion. Both DC and AC susceptibility show the presence of a hump around 35 K, which is associated with the relaxation of Fe+2 extra electrons. Radio-frequency transverse susceptibility (TS) measurements indicated a noticeable increase in anisotropy field below ~ 25 K, which is attributed to the rearrangement of Fe+2 electrons in the octahedral sites. TS experiments also revealed the domain wall freezing below ~ 35 K. Our combined DC, AC and TS susceptibility studies shed light on the complex nature of the low-temperature magnetic behavior in nanostructured magnetite.

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.

Enhanced room-temperature spin Seebeck effect in a YIG/C60/Pt layered heterostructure

We report on large enhancement of the longitudinal spin Seebeck effect (LSSE) in the Y3Fe5O12 (YIG)/Pt system at room temperature due to the addition of a thin layer of organic semiconductor (C60) in between the YIG and the Pt. LSSE measurements show that the LSSE voltage increases significantly, from the initial value of 150 nV for the YIG/Pt structure to 240 nV for the YIG/C60(5nm)/Pt structure. Radio-frequency transverse susceptibility experiments reveal a significant decrease in the surface perpendicular magnetic anisotropy (PMA) of the YIG film when C60 is deposited on it. These results suggest that the LSSE enhancement may be attributed to increased spin mixing conductance, the decreased PMA, and the large spin diffusion length of C60.

Probing the temperature-dependent magnetic anisotropy and longitudinal spin Seebeck effect in Y3Fe5O12

Y3Fe5O12 (YIG) has attracted growing interest since a large Longitudinal Spin Seebeck Effect (LSSE) was discovered in this material. However, the origin of the LSSE and its temperature dependence are not well understood. We report here, the temperature dependence of the effective magnetic anisotropy field (HK) and LSSE voltage (VLSSE) of single crystal YIG, measured using the radio-frequency transverse susceptibility (TS) and LSSE techniques, respectively. VLSSE is found to vary from 40 nV to 97 nV from 100 to 300 K, which is consistent with the previously reported experimental results. Interestingly, we find the temperature dependence of HK resembles that of VLSSE in the high temperature regime (100 – 300 K), with the sudden changes in both VLSSE and HK at ∼175 K. Our results indicate the possible role of magnetic anisotropy in the LSSE in YIG and provide important insights into improving LSSE in magnetic materials through manipulating their magnetic anisotropy.

Transverse Susceptibility as a Biosensor for Detection of Au-Fe3O4 Nanoparticle-Embedded Human Embryonic Kidney Cells

We demonstrate the possibility of using a radio-frequency transverse susceptibility (TS) technique based on a sensitive self-resonant tunnel-diode oscillator as a biosensor for detection of cancer cells that have taken up magnetic nanoparticles. This technique can detect changes in frequency on the order of 10 Hz in 10 MHz. Therefore, a small sample of cells that have taken up nanoparticles when placed inside the sample space of the TS probe can yield a signal characteristic of the magnetic nanoparticles. As a proof of the concept, Fe3O4 nanoparticles coated with Au (mean size ∼60 nm) were synthesized using a micellar method and these nanoparticles were introduced to the medium at different concentrations of 0.05, 0.1, 0.5, and 1 mg/mL buffer, where they were taken up by human embryonic kidney (HEK) cells via phagocytosis. While the highest concentration of Au-Fe3O4 nanoparticles (1 mg/mL) was found to give the strongest TS signal, it is notable that the TS signal of the nanoparticles could still be detected at concentrations as low as 0.1 mg/mL.

Asymmetric hysteresis loops and its dependence on magnetic anisotropy in exchange biased Co/CoO core-shell nanoparticles

The origin of asymmetry in field cooled (FC) hysteresis loops exhibiting exchange bias (EB) is investigated by studying the static and dynamic magnetic properties of core-shell Co/CoO nanoparticles. Two distinct freezing temperatures coresponding to the core (Tf-cr ∼ 190 K) and the shell moments (Tf-sh ∼ 95 K) are obtained from the energy barrier distribution. The FC loops are symmetric in the temperature range Tf-sh ≤ T ≤ Tf-cr, however, asymmetry in hysteresis is observed immediately below Tf-sh. These intriguing features are also probed by radio frequency transverse susceptibility (TS) experiments. We show that the first anisotropy fields obtained from the demagnetization and return curves of field cooled TS measurement, shift along the negative field axis and strikingly resemble the temperature dependence of EB. Field cooled TS measurements reveal the effect of competing Zeeman and anisotropy energy above and below Tf-sh to account for the development of asymmetry. Our study indicates that asymmetry in FC hysteresis loops is intrinsic to core-shell nanoparticles and develops only below the freezing temperature of the shell due to enhanced magnetic anisotropy.

Signatures of spin-glass freezing in Co/CoO nanospheres and nanodiscs

We present a study of the magnetic properties of Co nanoparticles having a combination of both spherical and disk shapes. The hcp Co nanospheres with an average diameter of 11nm and nanodiscs of dimensions ∼2.5×15nm2 were prepared by thermal decomposition of di-cobalt octacarbonyl in the presence of an amine surfactant. The as-synthesized nanoparticles were oxidized to grow an antiferromagnetic layer. High resolution transmission electron microscopy showed the presence of a ferromagnet/antiferromagnet (Co/CoO) interface with a 2.2-nm thick CoO shell on the spherical nanoparticles and 0.5nm thick on nanodiscs. We report the temperature and field dependent DC magnetization, frequency, field, and temperature dependent AC susceptibility, and the radio frequency transverse susceptibility. A low temperature paramagnetic behavior was observed in the DC magnetization at high fields and is assigned to defects in the CoO shell that are not coupled to the antiferromagnetic lattice. Our results support the existence of a low temperature frozen, disordered magnetic state, characterized by a strong exchange coupling between the structurally disordered, spin-glass CoO shell and Co core.

Fabrication and magnetic response probed by RF transverse susceptibility in La 0.67Ca 0.33MnO 3 nanowires

The temperature and magnetic field dependence of the radio-frequency (RF) transverse susceptibility ( χ T ) of La 0.67Ca 0.33MnO 3 crystalline nanowires has been studied using a very sensitive self-resonant tunnel-diode oscillator (TDO) technique. The nanowires were synthesized using porous templates of anodized alumina by chemical solution deposition technique, and the crystalline nature of the nanowires with the average diameter of 70 nm was confirmed by TEM, SAED, and HREM. RF transverse susceptibility experiments reveal the presence of a double-peak structure at T≤245 K (the Curie temperature) but a single peak at T>245 K. This distinguishes the low temperature ferromagnetic state from the high temperature paramagnetic state. The effective magnetic anisotropy field ( H K ), which corresponds to the peak location of χ T , has been found to increase with decrease in temperature from the Curie temperature.

Phase coexistence and magnetic anisotropy in polycrystalline and nanocrystalline LaMnO3+δ

We report on the phase coexistence and magnetic anisotropy in polycrystalline (bulk) and nanocrystalline (∼15 nm) LaMnO3+δ materials, which were prepared by solid state reaction and sol-gel methods, respectively. In addition to standard magnetization measurements, radio-frequency transverse susceptibility (TS) based on a very sensitive, self-resonant tunnel diode oscillator method was used to probe magnetic anisotropy and switching fields in the samples. The results revealed a coexistence of the ferromagnetic (FM) and antiferromagnetic (AFM) phases in both samples. For the bulk sample, the AFM phase significantly changed in volume fraction at ∼30 K and completely vanished around 120 K. Size reduction to the nanometer scale (∼15 nm) significantly suppressed the AFM phase while inducing surface spin disorder in the material. The large magnetic anisotropies were probed by TS experiments in both samples. Our studies showed that the magnetic properties of bulk LaMnO3+δ were strongly modified by size reduction.

Magnetic switching and magnetic transitions in ErCo2 probed by radio-frequency transverse susceptibility

ErCo2 represents a typical example of magnetism of itinerant electron systems and metamagnetic processes and has been the subject of extensive research. We present here the first study of radio-frequency transverse susceptibility (TS) of bulk ErCo2 using a self-resonant tunnel-diode oscillator technique. TS measurements reveal the collective magnetic switching of the Er moments at temperatures below the ferrimagnetic transition temperature, Tc∼32 K, and the existence of Co nanoclusters with short-range correlations at Tc&amp;lt;T&amp;lt;Tf (Tf denoted as the flipping temperature). The difference in the magnetic configuration between the ferrimagnetic, parimagnetic, and paramagnetic states, as well as the change from the paramagnetic to parimagnetic regime upon varying dc magnetic fields are also probed by TS experiments. These findings are discussed in the context of our previous investigations using other different techniques which provide further insights into the magnetism and the so-called parimagnetism phenomenon in ErCo2.

Interparticle interactions in coupled Au–Fe3O4 nanoparticles

Complex ac susceptibility measurements are reported on composite Au–Fe3O4 nanoparticles of two different configurations—the so-called “dumbbell” and “flower” configurations. The frequency-dependent blocking temperature was fitted to two separate models in an attempt to understand the relaxation and the role of interactions present in the nanoparticle arrays. While the Néel–Arrhenius model failed to accurately describe the blocking behavior of both types of particles, the Vogel–Fulcher model was shown to fit the dumbbell particles indicating the importance of weak interparticle interactions in this system. The flower nanoparticles, however, failed to yield physical fit parameters for both models, indicating that the interactions present in these particles are not solely dipolar but likely associated with competing intraparticle interactions. Radio-frequency transverse susceptibility measurements also confirm these features.

Magnetic anisotropy in epitaxialCrO2andCrO2∕Cr2O3bilayer thin films

We have investigated the effective magnetic anisotropy in CVD-grown epitaxial $\mathrm{Cr}{\mathrm{O}}_{2}$ thin films and ${\mathrm{Cr}}_{2}{\mathrm{O}}_{3}∕\mathrm{Cr}{\mathrm{O}}_{2}$ bilayers using resonant radio-frequency transverse susceptibility (TS). While $\mathrm{Cr}{\mathrm{O}}_{2}$ is a highly spin polarized ferromagnet, ${\mathrm{Cr}}_{2}{\mathrm{O}}_{3}$ is known to exhibit magnetoelectric effect and orders antiferromagnetically just above room temperature. In $\mathrm{Cr}{\mathrm{O}}_{2}$, the measured values for the room temperature anisotropy constant scaled with the film thickness and the TS data is influenced by magnetoelastic contributions at low temperature due to interfacial strain caused by lattice mismatch with the substrate. In $\mathrm{Cr}{\mathrm{O}}_{2}∕{\mathrm{Cr}}_{2}{\mathrm{O}}_{3}$ bilayers $M\text{\ensuremath{-}}H$ loops indicated an enhanced coercivity without appreciable loop shift and the transverse susceptibility revealed features associated with both the ferromagnetic and antiferromagnetic phases. In addition, a considerable broadening of the anisotropy fields and large ${K}_{\mathrm{eff}}$ values were observed depending on the fraction of ${\mathrm{Cr}}_{2}{\mathrm{O}}_{3}$ present. This anomalous behavior, observed for the first time, cannot be accounted for by the variable thickness of $\mathrm{Cr}{\mathrm{O}}_{2}$ alone and is indicative of possible exchange coupling between $\mathrm{Cr}{\mathrm{O}}_{2}$ and ${\mathrm{Cr}}_{2}{\mathrm{O}}_{3}$ phases that significantly affects the effective magnetic anisotropy.

Inter-particle interactions and magnetism in manganese–zinc ferrite nanoparticles

Manganese–zinc ferrite (Mn x Zn 1− x Fe 2O 4) nanoparticles were synthesized by reverse micelle technique using two different surfactant media—(1) bis-(2-ethylhexl) sodium sulfosuccinate (AOT) and (2) mix of nonylphenol poly(oxyethylene) 5 and nonylphenol poly(oxyethylene) 9 (NP) followed by annealing of precursors to remove the surfactant coating and to obtain better crystalline phase. A comparison of the magnetic properties showed distinct differences in blocking temperature, coercivity and saturation magnetization. Radio-frequency (RF) transverse susceptibility (TS) measurements were in agreement with the static magnetization data. Our precise TS measurements further revealed features associated with anisotropy fields that were dependent on the grain size, crystallinity and inter-particle interactions. Overall, we have demonstrated that RF TS is an excellent probe of the dynamic magnetization and influence of effects such as crystallinity and inter-particle interactions in soft ferrite nanoparticles.