Inverted Hysteresis Loops Research Articles

Susceptibility and Inverted Hysteresis Loop of Prussian Blue Analogs with Orthorhombic Structure

The magnetic susceptibility of ternary metal Prussian blue analogues with orthorhombic structure is studied using Ising model. Within the frame work of effective-field theory with correlations, the roles of the mole fraction y, uniaxial magnetic anisotropy, transverse and longitudinal magnetic field are discussed in details. The temperature dependence of the magnetic susceptibility is also investigated. The interesting phenomenon of the inverted magnetic hysteresis loop has been found. The results can help to understand the experimental work of the molecule-based ferri-ferrimagnet.

Magnetic properties of Pr0.7Ca0.3MnO3/SrRuO3 superlattices

High-quality Pr0.7Ca0.3MnO3/SrRuO3 superlattices were fabricated by pulsed laser deposition and were investigated by high-resolution transmission electron microscopy and superconducting quantum interference device magnetometry. Superlattices with orthorhombic and tetragonal SrRuO3 layers were investigated. The superlattices grew coherently; in the growth direction Pr0.7Ca0.3MnO3 layers were terminated by MnO2 and SrRuO3 layers by RuO2-planes. All superlattices showed antiferromagnetic interlayer coupling in low magnetic fields. The coupling strength was significantly higher for orthorhombic than for tetragonal symmetry of the SrRuO3 layers. The strong interlayer exchange coupling in the superlattice with orthorhombic SrRuO3 layers led to a magnetization reversal mechanism with a partially inverted hysteresis loop.

Inverted hysteresis and giant exchange bias in La0.7Sr0.3MnO3/SrRuO3 superlattices

The magnetization reversal mechanisms in a La0.7Sr0.3MnO3/SrRuO3 superlattice with ultrathin individual layers were studied. Due to the strong exchange bias between La0.7Sr0.3MnO3 and SrRuO3 layers inverted hysteresis loops were observed at temperatures below 62 K; at higher temperatures the superlattice showed an unconventional reversal mechanism with the magnetically hard SrRuO3 layers switching first on reducing the magnetic field from saturation. These observations were corroborated by micromagnetic simulations and were interpreted as arising from interfacial Bloch walls.

Size-Effect Induced Short-Range Magnetic Ordering in Germanium Nanostructures

Formation of ordered magnetic states in germanium nanostructures embedded in SiO2 has been investigated. Samples with the nanostructures were prepared by sputtering deposition on Si(100) substrates, followed by thermal annealing in vacuum. Transmission electron microscopy, energy dispersive X-ray spectrometry, and Raman spectroscopy have been used to characterize the samples. Magnetic measurements were performed using a superconducting quantum interference device. Size-effect induced magnetic orderings in the germanium nanostructures were found to be present at room temperatures and below. Superparamagnetic behavior was observed at temperatures above 230 K, whereas thermal excitation of spin reorientation and magnetic coupling has been revealed at temperatures below 60 K. Inverted hysteresis loops with negative remanences and multiple plateaus revealed the ferri- or antiferromagnetic nature of the coupling. Inter-domain coupling and effect of magnetic anisotropy will be discussed based on the experimental results and simulations with a spin reorientation model.

Exchange bias in a single phase ferrimagnet

We report on peculiar magnetic hysteresis behavior of Ru0.25Cr0.75O2 thin films. We describe three unusual magnetic phenomena inherent to this material; (i) intrinsic positive exchange bias in a single phase epitaxial thin film ferrimagnet with the absence of a clear and distinct magnetic-magnetic interface, (ii) field modulation of the shifted M(H) loop along the field axis direction, and (iii) inverted hysteresis loops in which the overall area of the loop is nearly zero over the entire field cycle. These results can be understood within the context of a very unusual interface of the epitaxial film with the TiO2 (001) substrate.

Structure and magnetism of Co:CoO core–shell nanoclusters

Transmission electron microscopy (TEM) and electron diffraction (ED) are used to investigate the nanostructures of two ensembles of Co:CoO core–shell particles. TEM images show that particles of size about 12 nm are almost fully oxidized, while particles with size about 18 nm have a core–shell structure where a Co core is surrounded by a shell of CoO. ED simulation confirms that the larger particles have an fcc-structured Co core and a rock-salt CoO shell structure, while the smaller particles mostly have the rock-salt CoO structure. The core–shell structure is responsible for the unusual magnetic properties of the Co:CoO nanoclusters, especially the occurrence of inverted hysteresis loops (proteresis), but previous research has been indirect, largely based on magnetic measurements and on a cross-comparison with granular materials. Our measurements show that the structures have ferromagnetic fcc Co cores of varying sizes down to 1 nm which are surrounded by antiferromagnetic rock-salt CoO shells. The core radii obtained from the TEM pictures are used to estimate the exchange interactions responsible for proteresis and to pinpoint the core-size window in which proteresis occurs.

Magnetisation inverted hysteresis loops in the molecular magnets [M(Cp*)2][Ni(α-tpdt)2] (M = Fe, Mn)

The low temperature magnetic properties of the compounds belonging to the family [M(Cp)(2)][Ni(alpha-tpdt)(2)], where M = Fe (), Mn () and Cr () (alpha-tpdt = 2,3-thiophenedithiolate) are studied, with emphasis on the magnetisation curves. These compounds share a common structure, composed of alternating layers, each consisting of mixed parallel D(+)A(-)D(+)A(-)D(+)A(-) chains; the chains of adjacent layers being perpendicular. At low temperatures, a rather unusual inverted hysteresis loop with negative remanence is observed in 1, and a similar reversed hysteresis is also detected in the magnetic field induced transition of 2. This effect is associated with the layered structure of these compounds, together with the considerable magnetic anisotropy of the [Fe(Cp)(2)](+) and [Mn(Cp)(2)](+) donors, being to the best of our knowledge, the first time such inverted hysteresis loops are observed in a bulk molecule based material.

Inverted hysteresis loops observed in a randomly distributed cobalt nanoparticle system

We observed abnormal magnetic behaviors (so-called inverted hysteresis loops or negative remanence) in a randomly distributed Co nanoparticle (NP) system, where the Co NPs are prepared using laser irradiation. The inverted hysteresis loops are observed when ferromagnetic and superparamagnetic phases coexist within the NP system. Normal hysteresis loops appear in samples having only one phase, either ferromagnetic or superparamagnetic. The ferromagnetic and superparamagnetic Co NPs, with average diameters of 25 and 4 nm, respectively, showed negative remanence over a temperature range of 92--300 K. We have proposed that these abnormal features are directly related to the dipolar interaction between the superparamagnetic Co NPs and a ferromagnetic Co NP.

Magnetic Properties of Fe/Ho Multilayers Prepared by Electron-Beam Evaporation

Fe/Ho multilayers were prepared by electron-beam evaporation. A vector vibrating sample magnetometer is adopted to investigate the multilayers in-plane and out-plane magnetic properties, and an inverted hysteresis loop is obtained in the direction perpendicular to the applied magnetic field. The loop shape is sensitive to the angle between the film plane and the direction of magnetic field. A proposal module is used to explain this mechanism. The results of magnetoresistance measurement show that the multilayers have a positive magnetoresistance. The Fe layer thickness has a marked effect on the electronic resistance and magnetoresistance properties of the Fe/Ho multilayers.

Inverted hysteresis loops: Experimental artifacts arising from inappropriate or asymmetric sample positioning and the misinterpretation of experimental data

Inverted hysteresis loops (IHL) of negative net area have been measured for some films using vibrating sample magnetometer (VSM), SQUID magnetometer or magneto-optic Kerr effect magnetometer. All of the IHL were observed in the field direction along the hard axis. An IHL violates the thermo-mechanical second law. This work points out that an IHL arises when | M x | ≪ | M z | ( M x and M z : the parallel and normal to the field direction components of magnetization vector) if the sample is set at certain inappropriate positions in VSM and SQUID magnetometer experiments, and the sense voltage is solely attributed to M x while it also relates to M z .

Negative coercivity characteristics in antiferromagnetic coupled hard/soft multilayers

We investigated the negative coercivity ( H c) by using the Stoner–Wohlfarth model in antiferromagnetic-coupled hard/soft bilayers. We found out the conditions for negative H c in terms of thickness ( d H) and anisotropy energy( K H) of hard layer such as d H<0.5 and d H K H>2 J in antiferromagnetic-coupled bilayers. In order to test our predictions experimentally, we prepared a series of synthetic exchange-coupled multilayer structure of MnIr/CoFe/Ru( x)/CoFe/MnIr thin film with x = 0.9 , 1.0 and 1.1 nm. The inverted hysteresis loop with negative H c was observed in antiferromagnetic-coupled multilayers, and not in ferromagnetic-coupled sample. The negative H c behavior was well agreed with the theoretical predictions.

Electrical and magnetic properties of CoFeAlO thin films

Influences of oxygen-partial pressure and annealing on the electrical and magnetic properties of CoFeAlO thin films were systematically investigated by means of resistivity, permeability, magnetization and ferromagnetic resonance (FMR) measurements. It was found that, with increasing oxygen-partial pressure or under annealing, the electrical resistivity of the film increased and the magnetic softness decreased, which is attributed to the microstructural change of the film. Interestingly, an as-deposited Co 45.30Fe 20.65Al 19.34O 14.71 film was found to exhibit an inverted hysteresis loop with negative coercivity, and this peculiar phenomenon disappeared upon effects of oxygen-partial pressure and annealing. It was also found that the as-deposited films owned a narrow FMR line width that increased with increasing oxygen-partial pressure or under annealing.

Inverted hysteresis in exchange biased Cr2O3 coated CrO2 particles

Exchange bias and inverted hysteresis loops were observed in the Cr2O3 coated CrO2 particles with a wide distribution of the blocking temperature. The ferromagnetic coupling between the CrO2 cores and Cr2O3 shell is responsible for exchange bias observed below 292K. In the temperature range of 261K–∼340K, the particles show inverted hysteresis loops, i.e., the negative remanence and coercivity. The most interesting observation is that the particles show an exchange-biased (or shifted) inverted hysteresis loops in between 261K and 292K; and that the maximum negative remanence appears at 292K where exchange bias disappears. An antiferromagnetic type of interaction between the blocked particles and the superparamagnetic particles is believed to be responsible to the inverted hysteresis loops.

Inverted near-surface hysteresis loops in heterogeneous (nanocrystalline/amorphous) Fe81Nb7B12 alloys

Inverted hysteresis loops were observed for the first time in the near-surface layers of heterogeneous (nanocrystalline/amorphous) Fe81Nb7B12 alloys. In particular, a negative residual magnetization is retained when a positive magnetic field applied in the sample plane is decreased to zero. The inverted hysteresis is qualitatively explained within the framework of a two-phase model, according to which the heterogeneous alloys contain two dissimilar phases exhibiting uniaxial magnetic anisotropy and featuring antiferromagnetic exchange interaction.

Inverted surface hysteresis loops in heterogeneous (nanocrystalline/amorphous) Fe81Nb7B12 alloys

Inverted hysteresis loops were observed for the first time in the near-surface layers of heterogeneous (nanocrystalline/amorphous) Fe81Nb7B12 alloys. In particular, a negative residual magnetization is retained when a positive magnetic field applied in the sample plane is decreased to zero. The inverted hysteresis is qualitatively explained within the framework of a two-phase model, according to which the heterogeneous alloys contain two dissimilar phases exhibiting uniaxial magnetic anisotropy and featuring antiferromagnetic exchange interaction.

Negative Remanence in Synthetic Antiferromagnetically Coupled FeCo/Ru/FeCo Films

Inverted hysteresis loops in a sputtered FeCo/Ru/FeCo bilayer have been investigated. A Stoner-Wohlfarth energy minimization calculation reproduced the negative remanence assuming a small misalignment of the easy axes of the ferromagnets. During the initial stage of the growth of the top layer, the external field fails to overcome the strong antiferromagnetic exchange coupling between the layers. The magnetizations, as a result, are no longer collinear and the easy axis of the top layer will vary with its thickness. The negative remanence disappeared following an annealing of the sample in a strong field confirming the theory.

Inverted hysteresis loops in annealed Co–Nb–Zr and Co–Fe–Mo–Si–B amorphous thin films

Amorphous CoNbZr and CoFeMoSiB films with uniaxial anisotropy were obtained onto Mylar and Kapton substrates, and magnetically annealed under a field transverse to the as-prepared easy axis. Annealed samples exhibited a remarkable behaviour: inverted loops with M r/ M s<0 were found for directions within an interval of 65°–90° to that of a maximum M r/ M s, the latter being found between the as-prepared EA and annealing field directions. Results are explained by considering non-collinear effective uniaxial and biaxial anisotropies acting on the mean magnetization.

A NEW TYPE HYSTERESIS LOOP IN SiN/GdFeCoSi/SiN SANDWICH STRUCTURE

A new type of inverted hysteresis loop was observed in an amorphous SiN/GdFeCoSi/SiN film, in which a magnetic GdFeCoSi layer with a thickness of 40 nm was sandwiched by SiN capping and buffer layers 5 nm thick. An inverted hysteresis loop with negative remanence appeared, when an applied field was perpendicular to the film plane, with remanance ratio of 0.4 and HC=120 Oe. An FMR experiment study showed that two magnetic phases existed in the system. A major magnetic phase had an easy-plane anisotropy and the other minor magnetic phase had an easy-normal anisotropy. By assuming an antiferromagnetic exchange coupling between them, the inverted loops can be explained.

Inverted hysteresis loops in magnetically coupled bilayers with uniaxial competing anisotropies: Theory and experiments

The magnetization reversal processes in magnetic bilayers with individual uniaxial anisotropies have been studied, both theoretically and experimentally, to analyze the possible existence of inverted hysteresis loops, that is, with negative remanent magnetization ${(M}_{r}).$ Kerr effect measurements in amorphous ${\mathrm{YCo}}_{2}/{\mathrm{YCo}}_{2}$ bilayers and alternating gradient magnetometry in polycrystalline FeNi/FeNi samples reveal that ${M}_{r}&lt;0$ can be observed for certain directions of the applied magnetic field in the sample plane. This property has also been found in CoNbZr films annealed under an applied field. Our theoretical approach shows that the behavior of these magnetic heterogeneous systems with two coupled uniaxial anisotropies can be understood in terms of two competing effective anisotropies, one biaxial (with ${K}_{\mathrm{biax}})$ and one uniaxial (with ${K}_{\mathrm{uniax}}).$ In particular, a phase diagram has been deduced for the conditions on ${K}_{\mathrm{biax}}$ and ${K}_{\mathrm{uniax}}$ that can produce negative remanence. This description indicates that, under those anisotropy conditions, inverted hysteresis loops can be observed for an applied field close to the hard axis of the effective uniaxial anisotropy, when magnetization reversal is driven by rotations and not by domain nucleation and wall movement. To consider the real situation in a ${\mathrm{YCo}}_{2}/{\mathrm{YCo}}_{2}$ bilayer sample, the predictions of this phenomenological model have been further improved by micromagnetic calculations, which are in very good agreement with the magneto-optical measurements.

Design and preparation of a bulk magnet exhibiting an inverted hysteresis loop

We report on a magnet exhibiting an inverted magnetic hysteresis loop, i.e., the magnetization becomes negative in the decreasing part even when the applied field is still positive, while the magnetization becomes positive in the increasing part when the applied field is still negative. The key to obtaining this unusual magnet is to utilize a competing effect between the spin-flip transition and the uniaxial magnetic anisotropy. On the basis of the model calculations considering this effect, we have succeeded in synthesizing a bulk magnet showing the inverted magnetic hysteresis loop with the system of samarium(III) gadolinium(III) hexacyanochromate(III).