Magnetic Compensation Temperature Research Articles

Temperature dependence of magnetoresistance in GdFeCo/Pt heterostructure

The temperature dependence of magnetoresistance is investigated in ferrimagnetic GdFeCo/Pt heterostructures. An anomalous Hall effect (AHE) shows a monotonic behavior in temperature even across the magnetization compensation temperature TM, implying that the FeCo moment is responsible for the magnetotransport properties. An anisotropic magnetoresistance (AMR) exhibits a steep increase at low temperatures, which we ascribe to the contribution of a weak antilocalization in an amorphous GdFeCo layer. A spin Hall magnetoresistance (SMR) is found to exist in ferrimagnet/Pt systems and shows a moderate temperature dependence, in contrast to the SMR in YIG/Pt where a significant temperature dependence was observed. These results provide a basic understanding of the magnetotransport in amorphous ferrimagnets/heavy metal heterostructures.

Crystallization and magnetic characterizations of DyIG and HoIG nanopowders fabricated using citrate sol-gel

Abstract Dy and Ho iron garnets in form of nanoparticles were synthesized by citrate sol-gel method. Phase formation, lattice constant and average crystallite sizes of the samples were determined via XRD measurements. Morphology and particle size distribution were studied by TEM and chemical composition was checked by EDX. Magnetic measurements in temperature range 5–600 K and in the maximum applied field of 50 kOe were carried out by using SQUID and VSM. Their magnetic parameters, including Curie temperature, magnetization compensation temperature, spontaneous magnetization, high-field susceptibility, magnetic coercivity were discussed in the framework of three interacting magnetic sublattices, magnetocrystalline anisotropy, core-shell model and compared to those of the bulk materials. Based on these analyses further evaluation on the crystallinity and homogeneity of the samples has been made.

Deterministic character of all-optical magnetization switching in GdFe-based ferrimagnetic alloys

Using photo-emission electron microscopy with X-ray magnetic circular dichroism as a contrast mechanism, new insights into the all-optical magnetization switching (AOS) phenomenon in GdFe based rare-earth transition metal ferrimagnetic alloys are provided. From a sequence of static images taken after single linearly polarized laser pulse excitation, the repeatability of AOS can be measured with a correlation coefficient. It is found that low coercivity enables thermally activated domain wall motion, limiting in turn the repeatability of the switching. Time-resolved measurement of the magnetization dynamics reveal that while AOS occurs below and above the magnetization compensation temperature $T_\text{M}$, it is not observed in GdFe samples where $T_\text{M}$ is absent. Finally, AOS is experimentally demonstrated against an applied magnetic field of up to 180 mT.

Femtosecond X-ray magnetic circular dichroism absorption spectroscopy at an X-ray free electron laser.

X-ray magnetic circular dichroism spectroscopy using an X-ray free electron laser is demonstrated with spectra over the Fe L(3,2)-edges. The high brightness of the X-ray free electron laser combined with high accuracy detection of incident and transmitted X-rays enables ultrafast X-ray magnetic circular dichroism studies of unprecedented sensitivity. This new capability is applied to a study of all-optical magnetic switching dynamics of Fe and Gd magnetic sublattices in a GdFeCo thin film above its magnetization compensation temperature.

Modeling of Temperature Dependence of Magnetization in TbFe Films — An Atomistic Spin Simulation Study

In this paper, we performed spin simulations at atomistic level to study the temperature dependent properties of perpendicularly magnetized TbFe thin films. The crystallographically amorphous feature of TbFe ferrimagnetic alloys is modeled by using a lattice system with disordered site occupation of rare earth (RE) and transition metal (TM) spins. The simulated Curie temperature ([Formula: see text]) is consistent well with the mean-field approximation theory. With the increase of Tb concentration, the [Formula: see text] decreases almost linearly, whereas the magnetization compensation temperature ([Formula: see text]) increases gradually until the [Formula: see text] value is reached. The inter-sublattice exchange coupling strength [Formula: see text] between the RE and TM atoms can significantly affect [Formula: see text], but has less impact on [Formula: see text]. With the increase of Tb concentration, the TbFe sample of high [Formula: see text] exhibits a much faster increase in [Formula: see text] than the sample with low [Formula: see text]. Moreover, we have tested the simulation code to model the laser pulse induced ultrafast nonequilibrium spin dynamics. As an example, the femto-second pulse laser induced demagnetization and recovery process is clearly reproduced. These features are in a good agreement with the experiments, indicating that the simulation model can capture the basic physics in describing the high temperature dependent magnetic property as well as the ultrafast spin dynamics.

Exchange bias and magnetization reversal in Ni(Cr1−xFex)2O4 (x=0–0.20)

Exchange bias and magnetization reversal in single phase samples of Ni(Cr1−xFex)2O4 (x=0–0.20) were studied through magnetic measurements. Substitution of Fe for Cr changes the crystal structure at room temperature from tetragonal (space group: I41/amd) to cubic (space group: Fd3¯m) form. Temperature variation of magnetization measurements show that these samples undergo ferrimagnetic transitions and the transition temperature (TC) increases from 73K for x=0.0 to 314K for x=0.20. An interesting magnetization reversal phenomenon was observed for x=0.06 sample with a magnetic compensation temperature of 49K. M–H loop measurements at different temperature show the signature of presence of strong antiferromagnetic interactions especially at low temperature (T<50K). Tunable exchange bias behavior with a maximum exchange bias field of 5670Oe is observed for x=0.06 sample and it is explained by considering the exchange anisotropy between the ferrimagnetic and the antiferromagnetic components. The exchange bias field and the vertical shift in magnetization decrease exponentially with increase in temperature.

Reentrant spin-glass behavior and bipolar exchange-bias effect in “Sn” substituted cobalt-orthotitanate

We report the co-existence of longitudinal ferrimagnetic behavior with Néel temperature TN ∼ 46.1 K and reentrant transverse spin-glass state at 44.05 K in Tin (Sn) doped cobalt-orthotitanate (Co2TiO4). The ferrimagnetic ordering is resulting from different magnetic moments of Co2+ on the A-sites (3.87 μB) and B-sites (5.069 μB). The magnetic compensation temperature (TCOMP) shifts from 31.74 K to 27.1 K when 40 at. % of “Sn4+” substitutes “Ti4+” at B-sites where the bulk-magnetization of two-sublattices balance each other. For T &amp;gt; TN, the dc-magnetic susceptibility (χdc = M/Hdc) fits well with the Néel's expression for the two-sublattice model with antiferromagnetic molecular field constants NBB ∼ 15.44, NAB ∼ 32.01, and NAA ∼ 20.88. The frequency dependence of ac-magnetic susceptibility χac data follows the Vogel-Fulcher law, and the power-law of critical slowing-down with “zν” = 6.01 suggests the existence of spin-clusters (where “z” and “ν” being dynamic critical-exponent and correlation length of critical-exponent, respectively). This system exhibits unusual hysteresis loops with large bipolar exchange-bias effect (HEB ∼ 13.6 kOe at 7 K) after zero-field cooling process from an un-magnetized state, and a dramatic collapse of remanence (MR) and coercive field (HC) across TCOMP. The possible origins of such anomalous characteristics were discussed.

Sign reversal of magnetization in Mn substituted SmCrO3

Single phase samples of orthorhombic SmCr1−xMnxO3 compounds were prepared for x=0 to 0.50. Analysis of X-ray diffraction patterns shows a systematic increase in lattice parameters with increase in Mn concentration. The phenomenon of magnetization reversal is observed for x=0.10–0.30 samples with a maximum magnetic compensation temperature of 126K. The mechanism of magnetization reversal is explained by considering the competition between the paramagnetic moments of Mn3+ and Sm3+ ions under the influence of negative internal field due to antiferromagnetically ordered Cr3+ ions and the canted ferromagnetic component of Cr3+ ions. For x≥0.40, the samples exhibit ferromagnetic like behavior.

Magnetooptics of single and microresonator iron-garnet films at low temperatures

We have investigated the low-temperature behavior of the optical and magneto-optical properties of (Bi, Gd, Al)-substituted yttrium iron-garnet films that are either single or microresonator, i.e. sandwiched between two dielectric Bragg mirrors. It was shown that the magneto-optical properties of the microresonators with a magnetic film core are mainly determined by the properties of the constituent magnetic films. Special attention was paid to the compositions possessing magnetic compensation temperatures. The phenomenon of the temperature hysteresis was found and discussed for several samples. This testifies the fact that the magnetic moment reorientation in a magnetic field occurs by the full cycle of the first-order phase transitions “collinear phase – non-collinear phase – collinear phase”. The Faraday hysteresis curves at around magnetic compensation temperatures are demonstrated to be very informative concerning composition of a sample. In particular, the hysteresis curves measured for the magnetic films on the garnet substrates showed bursts that indicates formation of a transition layer.

Temperature Dependence of Faraday Rotation in Microcavity 1D-MPCs Containing Magneto-Optical Layers with a Compensation Temperature

The low-temperature behavior of optical and magneto-optical properties of microcavity 1D-MPCs with the Bi, Gd, Al-substituted YIG cavity layers of different compositions including compositions with a magnetic compensation temperatures were investigated. The optical transmittance and position of the resonant peaks in optical transmittance and the Faraday rotation spectra were found to be independent of temperature in all types of 1D-MPCs. The temperature dependence of changing the sign of Faraday rotation experienced the hysteresis with a value 8 K in one of two investigated MPCs with magnetic compensation temperature, while in the another MPC the hysteresis did not exceed 1 K. The possible origins of these properties are discussed.

Sign reversal of magnetization and tunable exchange bias field in NdCr1−xFexO3 (x=0.05–0.2)

Abstract Magnetization reversal and tunable exchange bias behavior are observed in NdCr 1− x Fe x O 3 compounds for x =0.05–0.20. The magnetic compensation temperature ( T comp ) is found to increase with increase in Fe concentration and its maximum value is 198 K for x =0.15 sample. The observed magnetization reversal is explained by considering the competition between the weak ferromagnetic component of Cr 3+ ions and the paramagnetic moments of Nd 3+ and Fe 3+ ions under the influence of negative internal magnetic field. The exchange anisotropy between the above two components of magnetic moments give rise to tunable positive and negative exchange bias fields. The sign reversal of exchange bias field also coincides with T comp . Bipolar switching of magnetization is demonstrated at T comp by just varying the positive magnetic field.

Cylindrical Ising nanowire in an oscillating magnetic field and dynamic compensation temperature

The nonequilibrium magnetic properties of a spin-1/2 cylindrical Ising nanowire system with core/shell in an oscillating magnetic field are studied by using a mean-field approach based on the Glauber-type stochastic dynamics (DMFT). We employ the Glauber-type stochastic dynamics to construct set of the coupled mean-field dynamic equations. First, we study the temperature dependence of the dynamic order parameters to characterize the nature of the phase transitions and to obtain the dynamic phase transition points. Then, we investigate the temperature dependence of the total magnetization to find the dynamic compensation points as well as to determine the type of behavior. The phase diagrams in which contain the paramagnetic, ferromagnetic, ferrimagnetic, partially nonmagnetic, surface fundamental phases and tree mixed phases as well as reentrant behavior are presented in the reduced magnetic field amplitude and reduced temperature plane. According to values of Hamiltonian parameters, the compensation temperatures, or the N-, Q-, P-, R-, S-type behaviors.

Bipolar switching of magnetization and tunable exchange bias in NdCr1−xMnxO3 (x = 0.0–0.30)

Polycrystalline samples of NdCr1−xMnxO3 (x = 0.0–0.30) were prepared by sol-gel method. Samples in the composition range, x = 0.10 to 0.20, exhibit magnetization reversal, and the magnetization compensation temperature (Tcomp) is found to increase from 110 K to 157 K with increase in Mn concentration. The magnetization reversal behavior is explained by considering paramagnetic moments of Nd3+ and Mn3+ ions under the influence of negative internal magnetic field and the canted ferromagnetic component of Cr3+ ions. Bipolar switching of magnetization by varying either the applied field H = 200 Oe to 3000 Oe or temperature through Tcomp is shown. Tunable exchange bias behavior was observed for x = 0.10 to 0.20 samples, and the value of exchange bias field, HEB, could be tuned from −1.6 kOe to +0.39 kOe for x = 0.10 sample. The origin of exchange bias is explained by considering the antiferromagnetic interaction between ferromagnetic component of Cr3+ ions and the paramagnetic components of Nd3+ and Mn3+ ions. The x = 0.30 sample exhibits ferromagnetic like behavior due to super exchange interaction in Cr3+–O2−–Mn3+ networks.

Magneto-optical properties of the iron garnet Tb3Fe5O12 near the magnetic compensation temperature

The magneto-optical susceptibility and magnetic hysteresis loops of the Faraday effect, which accompany the technical magnetization of the iron garnet Tb3Fe5O12, have been investigated experimentally in the temperature region near the magnetic compensation point of this ferrimagnet T c = 249 K. It has been found that, during the technical magnetization as the temperature approaches the magnetic compensation point T c , the velocity of domain walls increases, whereas the magneto-optical susceptibility has local maxima to the right and to the left from T c . Mentioned features of magneto-optical properties of the iron garnet Tb3Fe5O12 are associated with the resonance natural frequency of the domain wall oscillations in the crystal with the frequency of the alternating magnetic field. It has been shown that the used theoretical model of the magnetic resonance of domain walls makes it possible to consistently describe (at the qualitative level) the revealed regularities of the variation of the Faraday effect in the iron garnet Tb3Fe5O12 near T c .

Negative magnetization and the tunable exchange bias field in LaCr0.8Mn0.2O3

Manganese substituted Lanthanum chromite LaCr0.8Mn0.2O3 exhibits negative magnetization with decrease in temperature under the field cooled (FC) condition for the applied field H≤2000Oe. The maximum magnetic compensation temperature, (Tcomp) was 147K. A reentrant positive magnetization was observed at T≤50K due to low temperature transition. The negative magnetization is explained by considering the paramagnetic moment of Mn ions under the influence of negative internal field. Measurement of magnetic hysteresis loops under FC condition shows the presence of exchange bias field at T<Tcomp and its magnitude can be tuned by varying the temperature and cooling field.

Sign reversal of magnetization and exchange bias field in LaCr0.85Mn0.15O3

The sign reversal of magnetization is observed in LaCr0.85Mn0.15O3 compound and the maximum magnetic compensation temperature (Tcomp) was at around 100 K. We have also observed negative and positive values of exchange bias field (HEB) having sign reversal at around Tcomp. The value of HEB could be tuned from −2.1 kOe to + 2.6 kOe with change in temperature. The competition between the magnetic moment due to the canted ferromagnetic component of Cr3+ ions and the paramagnetic component of doped Mn3+ ions under the negative internal field gives rise to magnetization reversal. The origin of sign reversal of HEB is discussed in detail.

The Influence of Domain Structure on the Faraday Effect in Terbium Garnet Ferrite in the Vicinity of the Magnetic-Compensation Temperature

The temperature dependence of the Faraday effect in terbium garnet ferrite, Tb3Fe5O12, is investigated near its magnetic-compensation temperature, Т с = 249 K. A non-monotonous variation in the value of the Faraday rotation angle Ф is observed in a weak magnetic field as the temperature approaches Т с : the temperature plot of the Faraday rotation angle has two local maxima observed left and right of the magnetic compensation point. A theoretical model is proposed, which follows from the phenomenological theory of domain-boundary displacement under the action of a magnetic field, offering an unambiguous description of the principles of domain-structure influence on the Faraday effect in Tb3Fe5O12 near Т с .

The study of ultrafast magnetization reversal across magnetization compensation temperature in GdFeCo film induced by femtosecond laser pulses

The subpicosecond magnetization reversal is observed by a femtosecond (fs) laser ultrafast heating of the GdFeCo film across magnetization compensation temperature, under the saturated magnetic field. Then the film is induced by fs laser without external magnetic field, and the curve of magnetization dynamics also shows magnetization reversal crossing zero magnetization. Such result shows that the ultrafast magnetization reversal in GdFeCo system induced by ultrafast heating the film across magnetization compensation temperature does not originate from the extrinsic magnetic field. To study the origin of the magnetization reversal and reveal the role of the extrinsic magnetic field and the fs-laser pump, the external field dependence and the pump fluence dependence of magnetization dynamics are measured. The external field dependence result shows that the curves of ultrafast magnetization reversal almost overlap in the initial several picoseconds (ps) and then the nucleation and growth of the reversed domains are accelerated with increasing external fields. And the pump fluence dependence result shows that the magnetization reversal occurs only when the pump fluence is intense enough and the magnetization reversal increases with the pump fluence. Therefore the ultrafast magnetization reversal during the initial several picoseconds may originate from the intense-enough ultrafast heating of fs laser, and the extrinsic magnetic field is responsible for the later domain formation and domain-wall motion.

Magnetization and coercivity of nanocrystalline gadolinium iron garnet

Gadolinium iron garnet (GdIG) nanoparticles with mean particle size of about 37nm have been synthesized by citrate precursor gel formation followed by annealing at 800°C for 2 hours. Magnetic behavior of clustered GdIG nanoparticles was studied in temperature range from 5K to above Curie temperature. The sample shows a magnetization compensation temperature Tcomp∼286.5K and a Curie temperature TC∼560K. In comparison with the bulk saturation magnetization, the sample exhibits lower spontaneous magnetization in the temperature region from 5K to Tcomp whereas higher spontaneous magnetization is observed at higher temperatures up to near the Curie point. The magnetization curves show a differential susceptibility in high fields which increases sharply below 50K. At very low temperatures, irreversibility was observed in the magnetization loops, enduring in the fields up to ∼12.5kOe. The spontaneous magnetization, high-field susceptibility and low-temperature irreversible effect were discussed based on a model for the interacting particles consisting of ferrimagnetically aligned core spins and disordered spins in surface layer which become frozen at low temperatures. We proposed a mechanism for the enhancement of the spontaneous magnetization above Tcomp in which the Gd and Fe spins in the surface layer are largely decoupled at high temperatures and the surface Fe spins realign to the magnetic moment of the core. The magnetic coercivity Hc at low temperatures is governed by the effective anisotropy whereas in the vicinity of the compensation point a peak in the coercive force shows up as a result of the so-called paraprocess with the maximum value of 1.2kOe at Tcomp and by further increasing temperature the coercivity decreases and eventually vanishes at about 500K. The interparticle interactions were found to play an important role in the hysteresis behavior of the sample.

Structure and property changes and femtosecond laser-induced magnetization dynamics of two-year-old high coercive TbFeCo films

An uncoated TbFeCo film on glass substrate exposed to dry air for 2 years is studied using magneto-optical polar Kerr spectroscopy and vibrating sample magnetometry (VSM). The out-of-plane hysteresis loop by VSM is found to become slanted and meanwhile, an in-plane loop is also observable. The Kerr loop measured from the exposed surface is also slanted compared to that measured from the substrate-contacted surface, which indicates that the exposed surface is oxidized. So a bilayer structure of the film is proposed. When a pump laser is switched on, the Kerr loops measured from both surfaces at the delay time of −5ps become anomalous, showing the occurrence of magnetization reversal across magnetization compensation temperature. Unlike that measured from the substrate-contacted surface, femtosecond laser-induced magnetization dynamics measured from the exposed surface does not show magnetization reversal crossing zero magnetization. This can be explained by the bilayer structure as the compensation effect of demagnetization recovery of the oxidized layer on magnetization reversal of perpendicularly anisotropic TbFeCo layer across magnetization compensation temperature. Above experimental results show that the uncoated TbFeCo film cannot resist oxidation in dry air at room temperature for 2 years while the SiO2-coated surface can do so for over 2 years.