Magnetic Field Sweep Research Articles

Defect-Assisted Domain Nucleation Drives Unique Exchange-Bias Phenomena in MnBi2Te4

The study of the mechanism of exchange-bias phenomena and the achievement of its efficient control are of great importance, as it promotes the revelation of unique exchange interactions and the development of exotic applications. However, it is challenging due to the elusive interface between magnetic phases. In this study, we report an unprecedented exchange-bias phenomenon observed in ultrathin, uncompensated, antiferromagnetic MnBi2Te4. The magnitude and direction of the exchange field can be intentionally controlled by designing a magnetic-field sweep protocol without a field cooling process. The combined experimental and theoretical simulation results indicate that the spin-flip process assisted by the ubiquitous defect-induced pinning domain sites with varying inner exchange interactions might give rise to the emergence and robustness of this peculiar exchange bias. The temperature and thickness dependence of the exchange-bias phenomena are systematically investigated for further study and exploitation of its unique properties. This mechanism allows for highly tunable exchange bias in prevalent magnetic systems by engineering the properties of domain structures and also offers promising avenues for the design of spintronic devices combing its topology based on MnBi2Te4. Published by the American Physical Society 2024

Precise Calorimetry of Small Metal Samples Using Noise Thermometry

AbstractWe describe a compact calorimeter that opens ultra-low-temperature heat capacity studies of small metal crystals in moderate magnetic fields. The performance is demonstrated on the canonical heavy fermion metal YbRh$$_{2}$$ 2 Si$$_{2}$$ 2 . Thermometry is provided by a fast current sensing noise thermometer. This single thermometer enables us to cover a wide temperature range of interest from 175 µK to 90 mK with temperature-independent relative precision. Temperatures are tied to the international temperature scale with a single-point calibration. A superconducting solenoid surrounding the cell provides the sample field for tuning its properties and operates a superconducting heat switch. Both adiabatic and relaxation calorimetry techniques, as well as magnetic field sweeps, are employed. The design of the calorimeter results in an addendum heat capacity which is negligible for the study reported. The keys to sample and thermometer thermalisation are the lack of dissipation in the temperature measurement and the steps taken to reduce the parasitic heat leak into the cell to the tens of fW level.

Parahydrogen-Induced 13C Hyperpolarizer Using a Flow Guide for Magnetic Field Cycling to Evoke 1H-13C Spin Order Transfer Toward Metabolic MRI.

The pair-wise addition of parahydrogen, the singlet form of molecular hydrogen, to unsaturated precursors evokes the hyperpolarization of two parahydrogen-derived 1H nuclear spins through a process known as parahydrogen-induced polarization (PHIP). Subsequent spin order transfer (SOT) from the 1H to the surrounding 13C nuclear spins via magnetic field cycling (MFC) results in substantial signal enhancement in 13C magnetic resonance imaging (MRI). Here, we report the development of a unique PHIP 13C hyperpolarizer system using a flow guide for MFC. The optimal MFC scheme for 1H to 13C spin order transfer was quantum-chemically simulated using the J-coupling values of 13C-labeled metabolic tracers. The flow guide system was three-dimensionally designed based on the simulated MFC scheme and pre-measured magnetic field distribution in a zero-field chamber. The system efficiently transfers the spin order of hyperpolarized 1H to a particular 13C spin when the parahydrogenated tracer passes through the flow guide at a designated flow rate. The 13C MRI signal is enhanced more than 40,000 times in 13C-labeled pyruvate and fumarate, compared to the thermal equilibrium level at 1.5 T, was achieved for conducting in vivo metabolic MRI of mice. A fully automated PHIP-based 13C polarizer was developed using a unique flow guide to conduct the MFC for 1H to 13C SOT. The PHIP hyperpolarizer with a flow guide can conduct efficient 1H-13C SOT without a MFC magnetic field sweep system and offers a cost-effective alternative to conventional dynamic nuclear polarization.

Room temperature chirality switching and detection in a helimagnetic MnAu2 thin film

Helimagnetic structures, in which the magnetic moments are spirally ordered, host an internal degree of freedom called chirality corresponding to the handedness of the helix. The chirality seems quite robust against disturbances and is therefore promising for next-generation magnetic memory. While the chirality control was recently achieved by the magnetic field sweep with the application of an electric current at low temperature in a conducting helimagnet, problems such as low working temperature and cumbersome control and detection methods have to be solved in practical applications. Here we show chirality switching by electric current pulses at room temperature in a thin-film MnAu2 helimagnetic conductor. Moreover, we have succeeded in detecting the chirality at zero magnetic fields by means of simple transverse resistance measurement utilizing the spin Berry phase in a bilayer device composed of MnAu2 and a spin Hall material Pt. These results may pave the way to helimagnet-based spintronics.

Shear stiffening and magneto-induced properties of magnetorheological elastomer based on self-healing poly(urethane-urea) matrix

A novel multifunctional composite material, defined as self-healing magnetorheological elastomer (SH-MRE), which exhibits both shear stiffening and magnetorheological effects as well as self-healing ability, is obtained by dispersing carbonyl iron particles in a poly(urethane-urea) matrix. The dynamic shear properties are tested through oscillation frequency and magnetic field sweeps. When shear stress is applied with an excitation frequency of 0.1 Hz to 100 Hz, the storage modulus of the poly(urethane-urea) matrix increases sharply, showing a good shear stiffening effect. In addition to the magnetically dependent mechanical properties with a magnetic flux density of 0–0.894 T, the self-healing efficiency of the isotropic SH-MREs with destructive shear damage after healing can be influenced by the mass fraction of the carbonyl iron particles, the applied magnetic field strength and the support of trace solvents. Based on the experimental results, it is believed that the hydrogen bonds in the dynamic hard domains and the magnetic interaction forces between the carbonyl iron particles induced by the magnetic field are the cause of the good multifunctional properties.

Unveiling the Anomalous Hall Response of the Magnetic Structure Changes in the Epitaxial MnBi2Te4 Films.

Recently discovered as an intrinsic antiferromagnetic topological insulator, MnBi2Te4 has attracted tremendous research interest, as it provides an ideal platform to explore the interplay between topological and magnetic orders. MnBi2Te4 displays distinct exotic topological phases that are inextricably linked to the different magnetic structures of the material. In this study, we conducted electrical transport measurements and systematically investigated the anomalous Hall response of epitaxial MnBi2Te4 films when subjected to an external magnetic field sweep, revealing the different magnetic structures stemming from the interplay of applied fields and the material's intrinsic antiferromagnetic (AFM) ordering. Our results demonstrate that the nonsquare anomalous Hall loop is a consequence of the distinct reversal processes within individual septuple layers. These findings shed light on the intricate magnetic structures in MnBi2Te4 and related materials, offering insights into understanding their transport properties and facilitating the implementation of AFM topological electronics.

Atomic resolution imaging using a novel, compact and stiff scanning tunnelling microscope in cryogen-free superconducting magnet.

We present the design and performance of a novel scanning tunnelling microscope (STM) operating in a cryogen-free superconducting magnet. Our home-built STM head is compact (51.5mm long and 20mm in diameter) and has a single arm that provides complete openness in the scanning area between the tip and sample. The STM head consists of two piezoelectric tubes (PTs), a piezoelectric scanning tube (PST) mounted on a well-polished zirconia shaft, and a large PT housed in a sapphire tube called the motor tube. The main body of the STM head is made of tantalum. In this design, we fixed the sapphire tube to the frame with screws so that the tube's position can be changed quickly. To analyse the stiffness of the STM head unit, we identified the lowest eigenfrequencies with 3 and 4kHz in the bending modes, 8kHz in a torsional mode, and 9kHz in a longitudinal mode by finite element analysis, and also measured the low drift rates in the X-Y plane and in the Z direction. The high performance of the home-built STM was demonstrated by images of the hexagonal graphite lattice at 300 K and in a sweeping magnetic field from 0 T to 9 T. Our results confirm the high stability, vibration resistance, insensitivity to high magnetic fields and the application potential of our newly developed STM for the investigation of low-frequency systems with high static support stiffness in physics, chemistry, material and biological sciences.

Bi2Se3 interlayer treatments affecting the Y3Fe5O12 (YIG) platinum spin Seebeck effect

In this work, we present a method to enhance the longitudinal spin Seebeck effect at platinum/yttrium iron garnet (Pt/YIG) interfaces. The introduction of a partial interlayer of bismuth selenide (Bi2Se3, 2.5% surface coverage) interfaces significantly increases (by ∼380%–690%) the spin Seebeck coefficient over equivalent Pt/YIG control devices. Optimal devices are prepared by transferring Bi2Se3 nanoribbons, prepared under anaerobic conditions, onto the YIG (111) chips followed by rapid over-coating with Pt. The deposited Pt/Bi2Se3 nanoribbon/YIG assembly is characterized by scanning electron microscope. The expected elemental compositions of Bi2Se3 and YIG are confirmed by energy dispersive x-ray analysis. A spin Seebeck coefficient of 0.34–0.62 μV/K for Pt/Bi2Se3/YIG is attained for our devices, compared to just 0.09 μV/K for Pt/YIG controls at a 12 K thermal gradient and a magnetic field swept from −50 to +50 mT. Superconducting quantum interference device magnetometer studies indicate that the magnetic moment of Pt/Bi2Se3/YIG treated chips is increased by ∼4% vs control Pt/YIG chips (i.e., a significant increase vs the ±0.06% chip mass reproducibility). Increased surface magnetization is also detected in magnetic force microscope studies of Pt/Bi2Se3/YIG, suggesting that the enhancement of spin injection is associated with the presence of Bi2Se3 nanoribbons.

Effect of light ellipticity and polarization angle on the transient dynamics of the ground-state Hanle effect

We report theoretical investigations on the transient dynamics of the ground-state Hanle effect in the Voigt geometry, where a time-varying scanning magnetic field B (t) is applied perpendicular to the light propagation direction. Transient Hanle signals are calculated for different light polarizations using a theoretical model based on the density matrix approach. First, we study the ellipticity dependence of transient Hanle signals in two configurations in which the major axis of the light polarization ellipse () is aligned either perpendicular or parallel to the field B . For B , elliptical polarization of the light produces oscillations of frequencies 2ΩL(t) and ΩL(t) simultaneously, where ΩL(t) is the Larmor frequency of the corresponding field B (t). On the other hand, in the case of || B , only ΩL(t) oscillation is observed using elliptically polarized light. Second, we vary the tilt angle between the polarization vector of a linearly polarized light and the orientation of the field B and study its effect on the transient response of Hanle signals. Both the amplitude and the decay rate of the oscillations show a strong dependence on the light ellipticity and tilt angle of the polarization vector. In addition, the influence of the magnetic field sweep rate on the transient signals is demonstrated for given values of light ellipticity and polarization angle.

Landau Diamagnetism and de Haas-van Alphen Oscillations, Formed in Single Crystals of Y3Fe5O12, in Local Nanodimensional-Sized 2D Phase Separation Regions, Located inside Layered Domain Walls at Room Temperature and T = 77 K.

This paper presents results of the magnetic dynamics study (the microwave power absorptions at the fixed frequencies during magnetic field sweeping) in samples of Y3Fe5O12 single crystals in the form of plates and spheres of various sizes, at frequencies exceeding 30 GHz, in magnetic fields up to 18 kOe, at room temperature, and T = 77 K. It was found that in this case, the inhomogeneity's of the magnetic state manifested itself in the Y3Fe5O12 samples as 2D local phase separation regions. Such 2D phase separation regions formed inside layered domain walls representing superlattices with sizes of 700-900 Å. Depending on the shape and size of the studied plates and spheres, Landau diamagnetism or de Haas-van Alphen oscillations were observed in the 2D phase separation regions at room temperature and T = 77 K.

Hysteretic magnetoresistance in superconducting SrTiO3/LaAlO3/SrTiO3 trilayer interface system

We investigated the electrical transport properties of the SrTiO3/LaAlO3/SrTiO3 (STO/LAO/STO) trilayer interface system. We found that the trilayer exhibits superconductivity at temperatures below 0.2 K. In the superconducting regime, the magnetoresistance (MR) of the system shows pronounced hysteresis, possibly due to the interplay of ferromagnetism and superconductivity. The magnitude of MR hysteresis strongly depends on the magnetic field sweep rate, and we observed a threshold field-sweep rate below which no MR is detected. At high sweep rates, the MR exhibits superconducting-normal-superconducting transition behavior. To explain these observations, we propose a model based on the ohmic heating from superconducting phase slip centers beneath Bloch-type magnetic domain walls in the ferromagnetic layer. Furthermore, we observed complex features in the MR curves that are likely due to domain wall motion in the system.

Nonreciprocal transmission of magnetoacoustic waves in compensated synthetic antiferromagnets

We investigate the interaction between surface acoustic waves (SAWs) and spin waves (SWs) in a Pt/Co($2\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$)/Ru($0.85\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$)/Co($2\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$)/Pt compensated synthetic antiferromagnet (SAF) composed of two ferromagnetic layers with equal thicknesses separated by a thin nonmagnetic Ru spacer layer. Because of the combined presence of interlayer dipolar coupling fields and interfacial Dzyaloshinskii--Moriya interaction (iDMI), the optical SW mode shows a large nondegenerate dispersion relation for counter-propagating SWs. Due to resonant SAW-SW interaction, we observe a nonreciprocal SAW transmission in the prepared piezoelectric/SAF hybrid device. We demonstrate that the nonreciprocity of the SAW transmission in symmetric SAFs with equal thicknesses of the magnetic layers can show a substantially different characteristic behavior in comparison to asymmetric SAFs or magnetic single layers with iDMI. For the prepared SAF, the nonreciprocal shift of the magnetoacoustic resonance fields and the magnetoacoustic SW excitation efficiency depend on the external magnetic field sweep direction. For one magnetic field sweep direction and angle of the magnetic field, the resonance fields of the waves propagating in one direction are larger than for the waves propagating in the opposite direction. In addition, the magnitude of the nonreciprocal field shift is at minimum if the external magnetic field is aligned perpendicular to the SW propagation direction. The experimental results are in agreement with a phenomenological SAW-SW interaction model.

Room temperature giant magnetocaloric effect in Pd doped FeRh and the effect of martensitic transition

FeRh 0.8 Pd 0.2 is known to exhibit near room-temperature first-order antiferromagnetic (AF)–ferromagnetic (FM) transition, typical of chemically ordered FeRh system. In addition, it is also reported to show martensitic transition at a lower temperature. In this work, the effect of sample history on transition temperature (Tt) and the magnetocaloric effect (MCE) have been studied. The experimentally determined MCE parameters across FM to AF transition induced either by isothermal magnetic field sweep or by temperature sweep showed a large isothermal change in entropy around room temperature, i.e., 14 J/kg K for 50 kOe magnetic field change, whereas MCE corresponding to AF–FM transition depends on cooling history. Our study shows that in the presence of martensite phase, the peak value of MCE is shifted to higher temperatures but with significantly reduced magnitude.

Time and temperature dependent magnetic viscosity experiments on Sr/Co nanoferrite particles

Magnetic viscosity experiments have been performed in order to investigate the magnetization reversal in Sr nanoferrite particles (nanoscale SrFe12O19) and interacting Sr/Co nanoferrite particles (SrFe12O19–CoFe2O4 nanocomposites). The magnetic viscosity S = d M ( t ) / d l n ( t ), where M ( t ) is the magnetization as a function of time, has been collected. For Sr nanoferrite S shows a maximum close to the coercive field, reflecting the relation between S and the energy barrier distribution. We evidence that magnetic viscosity experiments on Sr nanoferrite and interacting Sr/Co nanoferrite particles provide reliable qualitative results for the different magnetic field sweep rate and saturating field H s a t considered. In addition, the activation volumes extracted from the magnetic viscosity experiments performed at different temperatures on Sr nanoferrite are quantitatively correlated to anisotropy changes.

Landau theory-based relaxational modeling of first-order magnetic transition dynamics in magnetocaloric materials

The magnetocaloric effect is often largest within the neighborhood of a first-order phase transition. This effect can be utilized in magnetocaloric refrigeration, which completely eliminates the need for the greenhouse gases utilized in conventional refrigeration. However, such transitions present unique dynamical effects and are accompanied by hysteresis, which can be detrimental for such refrigeration applications. In this work, a Landau theory-based relaxational model is used to study the magnetic hysteresis and dynamics of the first-order magnetic transition of LaFe13−x Si x . Fitting the experimental magnetization data as a function of applied magnetic field under different field sweep rates with this model provided the Landau parameters (A, B, and C) and the kinetic coefficient of the studied material. We demonstrate the tendency of the magnetic hysteresis to increase with the magnetic field sweep rate, underlining the importance of studying and minimizing the magnetic hysteresis in magnetic refrigerants at practical field sweep rates. While evaluating the temperature dependence of the time required for a complete transition to occur, a nonmonotonic behavior and a sharp peak were found for temperatures near the transition temperature. Such peaks occur at the same temperature as the peak of the magnetic entropy change for low fields, whereas for higher fields the two peaks decouple. This information is critical for technological applications (such as refrigerators/heat pumps) as it provides guidelines for the optimization of the magnetic field amplitude in order to reduce the transition timescale and consequently maximize the machine operational frequency and amount of heat that is pumped in/out per second.

A Novel Atomically Resolved Scanning Tunneling Microscope Capable of Working in Cryogen-Free Superconducting Magnet.

We present a novel homebuilt scanning tunneling microscope (STM) with atomic resolution integrated into a cryogen-free superconducting magnet system with a variable temperature insert. The STM head is designed as a nested structure of double piezoelectric tubes (PTs), which are connected coaxially through a sapphire frame whose top has a sample stage. A single shaft made of tantalum, with the STM tip on top, is held firmly by a spring strip inside the internal PT. The external PT drives the shaft to the tip-sample junction based on the SpiderDrive principle, and the internal PT completes the subsequent scanning and imaging work. The STM head is simple, compact, and easy to assemble. The excellent performance of the device was demonstrated by obtaining atomic-resolution images of graphite and low drift rates of 30.2 pm/min and 41.4 pm/min in the X-Y plane and Z direction, respectively, at 300K. In addition, we cooled the sample to 1.6 K and took atomic-resolution images of graphite and NbSe2. Finally, we performed a magnetic field sweep test from 0 T to 9 T at 70 K, obtaining distinct graphite images with atomic resolution under varying magnetic fields. These experiments show our newly developed STM's high stability, vibration resistance, and immunity to high magnetic fields.

Thermomagnetic instabilities in 3D network of superconducting lead nanofilaments in porous glass

Thermomagnetic instabilities in the lead-porous glass (Pb-PG) nanocomposite in superconducting state were studied, and a model was proposed that relates magnetization jumps and heat release spikes with average critical current density. The samples were created by filling porous glass (characteristic pore size d = 7 nm) with lead from a melt under pressure, which created a 3D multiply connected system of lead filaments. The critical temperature of superconducting transition in Pb-PG T c = 7.2 K is the same as for bulk lead and the critical magnetic field H c (0) ≈ 40 kOe is much higher than H c bulk (0) ≈ 800 Oe for bulk lead. Hysteresis and magnetic flux jumps were observed in the magnetization curves m(H) of the Pb-PG nanocomposite, which are connected with the formation of magnetic flux gradients in a system of interconnected contours. Magnetic flux jumps are thermomagnetic processes accompanied by heat release which was directly observed by measuring adiabatic temperature change in the sample during magnetic field sweep. The flux jumps in the magnetization and heat release events were almost periodic in the magnetic field. The period dependences on the magnetic field are similar to the calculated average critical current density dependence j c (H).

Correlated insulating states in carbon nanotubes controlled by magnetic field

Recent experiments have shown that nearly metallic nanotubes remain insulating throughout magnetic field sweeps, in which the magnetic flux through the nanotube is expected to pass through a critical point where the single-particle band gap closes in one valley. Here, the authors investigate correlated insulating states of zigzag carbon nanotubes, which emerge from the interplay of electron-electron interactions, magnetic flux, strain, and spin-orbit coupling. They find that the gap for charged excitations generically remains open near the critical flux value, and predict a novel mirror symmetry breaking phase that may arise in the experimentally relevant regime of spin-orbit coupling.

Polarizing insensitive nuclei at ultralow magnetic fields using parahydrogen: A facile route to optimize adiabatic magnetic field sweeps.

Parahydrogen induced polarization (PHIP) provides a powerful tool to enhance inherently weak nuclear magnetic resonance signals, particularly in biologically relevant compounds. The initial source of PHIP is the non-equilibrium spin order of parahydrogen, i.e., dihydrogen, where the two protons make up a singlet spin state. Conversion of this spin order into net magnetization of magnetic heteronuclei, e.g., 13C, provides one of the most efficient ways to exploit PHIP. We propose a facile route to increase the performance of PHIP transfer in experiments with adiabatic sweeps of the ultralow magnetic field. To date, this technique yields the highest efficiency of PHIP transfer, yet, it has been mostly utilized with linear field sweeps, which does not consider the underlying spin dynamics, resulting in sub-optimal polarization. This issue was previously addressed by using the "constant" adiabaticity method, which, however, requires extensive calculations for large spin systems. In this work, the field sweep is optimized by utilizing the field dependence of the average 13C polarization. Both the experimental detection and the numerical simulation of this dependence are straightforward, even for complex multi-spin systems. This work provides a comprehensive survey of PHIP transfer dynamics at ultralow fields for two molecular systems that are relevant for PHIP, namely, maleic acid and allyl pyruvate. The proposed optimization allowed us to increase the resulting 13C polarization in 13C-allyl pyruvate from 6.8% with a linear profile to 8.7% with an "optimal" profile. Such facile optimization routines are valuable for adiabatic experiments in complex spin systems undergoing rapid relaxation or chemical exchange.

The Ising critical quantum Otto engine

We study a four-stroke Otto engine whose working fluid is a quantum Ising chain. The thermodynamic cycle consists in sweeps of the transverse magnetic field occurring in thermal isolation, alternated by thermalisation strokes with reservoirs at different temperatures. The system–environment coupling is modelled in a thermodynamically consistent way by means of a nonlocal Lindblad master equation. We show that the engine may operate in four different operation modes, depending on the various parameters, in particular it can act as a heat engine and as a refrigerator. We detect an enhancement of the thermodynamic performance as the critical point is crossed, and investigate it in detail.