Temperature-dependent Magnetization Reversal Research Articles

Suppression of Coercivity in Nanoscale Graded Magnetic Materials

We investigate temperature-dependent magnetization reversal of $\mathrm{Co}\mathrm{Ru}$ graded films, in which a predefined depth-dependent exchange-coupling strength J follows a V-shaped profile. Magnetometry reveals an extended temperature range below the Curie temperature ${T}_{C}\phantom{\rule{0.25em}{0ex}}$ where the reversal of the magnetization M is not accompanied by the conventionally occurring hysteresis, in stark contrast with homogeneous $\mathrm{Cr}\mathrm{Ru}$ reference films. This is caused by the temperature-driven paramagnetic (PM)-ferromagnetic (FM) phase transition, which does not occur in the entirety of the graded material but only in well-defined nanoscopic regions at any given temperature, enabling the creation of two internal PM/FM interfaces that assist the external magnetic field in reversing the magnetization of the FM graded sample region. Hysteretic reversal is recovered at sufficiently low sample temperatures or by using graded structures with very steep J gradients, so that no PM/FM interfaces form inside the exchange-coupled layer material that could influence the magnetization reversal process. Our findings open interesting material design options, and we envision wide application potential, since we succeed in engineering a temperature gap between the temperature dependent magnetization and hysteresis onsets, a capability of interest for any technology benefitting from field-free magnetization switching.

Self-organized exchange-spring magnet in epitaxial β-Fe(Ni)Si2/Si system

Self-organized formation of exchange-spring magnets in Permalloy-derived epitaxial silicide nanostructures, fabricated by deposition of Ni80Fe20 onto a vicinal Si(111) substrate, is reported. The crystal structure of bar-shaped silicide nanostructures decorating Si(111) surface steps, following thermally activated reaction between Ni, Fe, and Si surface atoms, was identified as ternary β-Fe(Ni)Si2 phase, in well-defined (1¯11¯) Si || (2¯2¯0) β and [011] Si || [001] β orientation relations with the substrate. At the same time, chemical composition within the islands was Fe-rich and Fe-deficient in comparison with the original stoichiometric Permalloy (Ni80Fe20) and not uniform, with higher concentration of Ni at the island bottoms, close to the interface with Si(111), creating de facto “compositional interfaces” within the islands, though no physical interfaces could be detected in a high-resolution structural-crystallographic analysis by transmission electron microscopy. Analysis of temperature-dependent magnetization reversal loops revealed, that the thicker and magnetically soft top part of the islands and yet magnetically softer and thinner bottom part, were magnetically exchange-coupled via the above “virtual” interfaces. Micromagnetic simulations, consistent with exchange-spring magnet, had further corroborated this conclusion.

Temperature dependent magnetization reversal process of a Ga-doped Nd-Fe-B sintered magnet based on first-order reversal curve analysis

A Ga-doped Nd-Fe-B sintered magnet has attracted significant attention as a heavy-rare-earth-free high-performance magnet. We have studied the temperature dependent magnetization reversal process of a Ga-doped Nd-Fe-B sintered magnet based on the first-order reversal curve (FORC) analysis. The FORC diagram pattern of the Ga-doped Nd-Fe-B sintered magnet changes from single spot in the high field region at room temperature to double spots in the low and high field regions at 200 °C, indicating that the dominant magnetization reversal process changes from single domain type to multidomain type. The single domain magnetization reversal at room temperature is well confirmed by using the soft X-ray magnetic circular dichroism microscopy observation. This change in the magnetization reversal process is well discussed by the temperature dependent local demagnetization field and the saturation field of multidomain state. Moreover, we have demonstrated the quantitative analysis of the FORC diagram pattern, which makes a deeper understanding of the magnetization reversal process of the Ga-doped Nd-Fe-B sintered magnet.

Temperature Dependent Magnetization Reversal Process of a Ga-Doped Nd-Fe-B Sintered Magnet Based on First-Order Reversal Curve Analysis

A Ga-doped Nd-Fe-B sintered magnet has attracted significant attention as a heavy-rare-earth-free high-performance magnet. We have studied the temperature dependent magnetization reversal process of a Ga-doped Nd-Fe-B sintered magnet based on the first-order reversal curve (FORC) analysis. The FORC diagram pattern of the Ga-doped Nd-Fe-B sintered magnet changes from single spot in the high field region at room temperature to double spots in the low and high field regions at 200 C, indicating that the dominant magnetization reversal process changes from single domain type to multidomain type. The single domain magnetization reversal at room temperature is well verified using soft X-ray magnetic circular dichroism microscopy observation. This change in the magnetization reversal process is well explained by the temperature dependent saturation field of multidomain grains. Moreover, we have demonstrated the quantitative analysis of the FORC diagram pattern, which makes a deeper understanding of the magnetization reversal process of the Ga-doped Nd-Fe-B sintered magnet.

Temperature-dependent magnetization reversal in exchange bias NiFe/IrMn/NiFe structures

We demonstrate the magnetization reversal features in NiFe/IrMn/NiFe thin-film structures with 40% and 75% relative content of Ni in Permalloy in the temperature range from 80 K to 300 K. At the descending branches of the hysteresis loops, the magnetization reversal sequence of the two ferromagnetic layers is found to depend on the type of NiFe alloy. In the samples with 75% relative content of Ni, the bottom ferromagnetic layer reverses prior to the top one. On the contrary, in the samples with 40% of Ni, the top ferromagnetic layer reverses prior to the bottom one. These tendencies of magnetization reversal are preserved in the entire range of temperatures. These distinctions can be explained by the morphological and structural differences of interfaces in the samples based on two types of Permalloy.

Reversal of spontaneous magnetization and spontaneous exchange bias for Sm1−xYxCrO3: The effect of Y doping

We report the crystal and electronic structures and magnetic properties of non-magnetic Y3+ ion doped SmCrO3 crystals. Structural distortion and electronic structure variation are caused by cation disorder due to Y doping. Although the spin moment of Sm3+ is diluted by nonmagnetic Y ions, spin reorientation continues to exist, and the temperature-dependent magnetization reversal effect and the spontaneous exchange bias effect under zero field cooling are simultaneously induced below Neel temperature. Significantly, the method of doping promotes the achievement of temperature dependent tunable switching of magnetization and sign of a spontaneous exchange bias from positive to negative. Our work provides more tunable ways to the sign reversal of magnetization and exchange bias, which have potential application in designing magnetic random access memory devices, thermomagnetic switches and spin-valve devices.

The reversal of the spontaneous exchange bias effect and zero-field-cooling magnetization in La1.5Sr0.5Co1-xFexMnO6: the effect of Fe doping.

The crystal structure, electronic structure and magnetic properties were systematically studied in a series of Fe-doped La1.5Sr0.5CoMnO6 double perovskites. The X-ray diffraction patterns of the samples are all refined with a rhombohedral (R3[combining macron]c) structure. The parameters a and c continuously increase with increasing Fe doping concentration x. X-ray photoelectron spectroscopy (XPS) spectra of the Mn, Co, and Fe 2p core levels, consistent with the soft X-ray absorption spectroscopy (XAS) spectra of Mn, Co, and Fe L2,3 edges, indicate that their valence states are Mn3+ and Mn4+, Co2+ and Co3+, and Fe3+, respectively. However, relative to samples with x ≤ 0.1, there is an abrupt change of photon energy in the Co- and Fe-2p XAS spectra for x ≥ 0.2, implying the spin state transition is from high to low. In addition, this is further confirmed by a comparison between the calculated effective spin moment from the paramagnetic data and the theoretical value. Interestingly, we demonstrate the reversal of both zero-field-cooling magnetization and the sign switching of the spontaneous exchange bias (SEB) with the doping concentration from magnetic measurements. The magnetization reverses from positive to negative with the temperature decreasing across the compensation temperature at the critical concentration x = 0.2. Meanwhile, the exchange bias field of the SEB reverses from large negative values to positive ones. Our findings allow us to propose that the spin state transition caused by inhomogeneity is considered to play an important role in the reversal of the magnetization and the SEB effect.

Understanding temperature and magnetic-field actuated magnetization polarity reversal in the Prussian blue analogue Cu0.73Mn0.77[Fe(CN)6].zH2O, using XMCD

We have investigated the microscopic origin of temperature and magnetic-field actuated magnetization reversal in Cu0.73Mn0.77[Fe(CN)6].zH2O, using XMCD. Our results show a fair deviation from the mean-field-theory in the form of different ordering temperatures of Fe and Mn sublattices. A preferential sign reversal of Mn spin under magnetic field and different spin cant angles for the two sublattices have also been observed. An antiferromagnetic coupling between the Fe and Mn sublattices along with different ordering temperatures (sublattice decoupling) for these sublattices explain the temperature-dependent magnetization reversal. Whereas, Mn spin reversal alone (under external magnetic field) is responsible for the observed field-dependent magnetization reversal. The dissimilar magnetic behavior of Fe and Mn sublattices in this cubic 3d-orbital system has been understood by invoking disparity and competition among inter-sublattice magnetic control parameters, viz. magnetic Zeeman energy, exchange coupling constant and magnetic anisotropy constant. Our results have significant design implications for future magnetic switches, by optimizing the competition among these magnetic control parameters.

Temperature-dependent magnetization reversal process and coercivity mechanism in Nd-Fe-B hot-deformed magnets

Low coercivity and its large temperature dependence of a Nd2Fe14B magnet with respect to its magnetic anisotropy field have been addressed as the coercivity problem. To elucidate the physical origin of this problem, we have investigated the temperature dependence of the magnetization reversal behavior in the Nd-Fe-B hot-deformed magnet. Based on the analysis of the energy barrier evaluated from magnetic viscosity measurements, the coercivity problem is discussed in terms of the following three aspects: magnetization reversal process, intrinsic coercivity without thermal demagnetization effect, and energy barrier height. The analyses lead us to conclude that domain wall pinning is dominant in the magnetization reversal in the Nd-Fe-B hot-deformed magnet. The temperature dependences of the intrinsic coercivity and the energy barrier height are explained by the grain boundary model with an intermediate layer. These analyses would be utilized to discuss the detailed structure and magnetic properties of the grain boundary, which gives a new insight to overcome the coercivity problem.

Magnetic and thermodynamic properties of the lightly electron-doped manganite compound (Ca,Sr)Mn 0.95 Sb 0.05 O 3

Abstract We report on the dc magnetization and ac magnetic susceptibility of the lightly electron-doped manganite compound ( Ca 1 − x 2 + Sr x 2 + ) Mn 0.9 4 + Mn 0.05 3 + Sb 0.05 5 + O 3 2 − (x=0.0, 0.05, 0.10, 0.15, 0.16, 0.17, and 0.2) with a fixed carrier content, to examine the effect of chemical pressure on magnetization reversal of this system. In a weak magnetic-field-cooled measurement, diamagnetic magnetization is observed for x ≤ 15 % , which changes to positive values for x > 15 % . We present a magnetic phase diagram as a function of Sr concentration in several magnetic fields. The ac magnetic susceptibility measurement indicates the existence of magnetic frustration for the Sr substituted samples exhibiting diamagnetic behavior. To better understand the thermodynamic properties of this system, we have measured the specific heat as a function of temperature over a field cooling of 100 Oe. Our data show no anomalies associated with the temperature-dependent magnetization reversal, indicating the absence of long-range magnetic ordering.

Temperature dependent magnetization reversal of exchange biased magnetic vortices in IrMn/Fe microcaps

We have investigated the magnetization reversal of vortex structures in Fe as well as in IrMn/Fe magnetic caps at elevated temperatures up to 450 K. The caps were formed by film deposition onto dense arrays of self-assembled silica particles of 900 nm diameter. In the investigated temperature range, the magnetization reversal in Fe caps evolves via nucleation and annihilation of magnetic vortices. However, in exchange coupled IrMn/Fe caps, the magnetic vortex at zero magnetic field vanishes as the temperature approaches the blocking temperature of IrMn accompanied by an increase in coercivity. Further increase in temperature, approaching the Néel temperature of IrMn, results in a reduction of coercivity as well as remanence indicating the re-stabilization of a vortex state.

Reversal mode instability and magnetoresistance in perpendicular (Co/Pd)/Cu/(Co/Ni) pseudo-spin-valves

We have observed distinct temperature-dependent magnetization reversal modes in a perpendicular (Co/Pd)4/Co/Cu/(Co/Ni)4/Co pseudo-spin-valve, which are correlated with spin-transport properties. At 300 K, magnetization reversal occurs by vertically correlated domains. Below 200 K the hysteresis loop becomes bifurcated due to laterally correlated reversal of the individual stacks. The magnetic configuration change also leads to higher spin disorders and a significant increase in the giant magnetoresistance effect. First order reversal curve measurements reveal that the coupled state can be re-established through field cycling and allow direct determination of the interlayer coupling strength as a function of temperature.

Magnetization reversal and chemical pressure effect in the electron-doped manganite CaMn0.95Sb0.05O3

We have demonstrated the effect of Sr substitution on the temperature-dependent magnetization reversal in the electron-doped manganite CaMn0.95Sb0.05O3. X-ray photoemission spectroscopy reveals the substitution of the Sb ion with its valence of 5+ at a Mn4+ site, resulting in one eg-electron doping, which is consistent with the negative Seebeck coefficient previously reported. For the (Ca1−ySry)Mn0.95Sb0.05O3 system, anomalously-diamagnetic behaviors are observed for y ≤ 15% in the weak-field-cooled magnetization. For Sr contents beyond 15%, the behavior of the negative magnetization disappears. We believe that the local lattice distortion due to Sb substitution causes a tilting of the Jahn-Teller active Mn3+O6 octahedron and stabilizes the canted spin state in a direction opposite that of applied field through the Dzyaloshinsky-Moriya interaction, thus contributing to the diamagnetic response.

Bipolar magnetization switching and its control in a Prussian blue type molecular magnetic compound

Prussian blue analogue (PBA) molecular magnetic materials have opened up the possibility of combining the molecular functionalities with their magnetic properties. A tremendous effort is on for finding appropriate PBAs that are suitable for memory and sensor applications. We have observed the magnetic pole reversal in multi-metal Prussian blue type compounds {CuxMn1-x}1.5[Fe(CN)6].zH2O. Our molecular field theory calculations agree very well with the observed temperature dependent magnetization reversal behaviour. As the applied magnetic field is increased, the negative magnetization keeps changing its character and finally becomes positive with typical ferrimagnetic/ferromagnetic type behaviour. A highly reversible-bipolar switching of magnetization using low applied magnetic fields is demonstrated. Possible applications of the observed magnetic pole reversal phenomenon in novel magneto-electronic devices such as volatile magnetic memory and thermo-magnetic switches have been revealed.

Temperature- and magnetic-field-controlled magnetic pole reversal in a molecular magnetic compound

A highly reversible (bipolar) switching of magnetization in a Prussian blue type molecular magnet Cu0.73Mn0.77[Fe(CN)6]⋅zH2O using low magnetic fields is demonstrated. The studied molecular compound also shows both positive and negative magnetocaloric effects below its magnetic ordering temperature. A molecular field theory calculation has also been done to explain the observed temperature dependent magnetization reversal behavior. Possible applications of the magnetic pole reversal phenomenon in magnetoelectronic and magnetocaloric devices such as magnetic memory and magnetic cooling/heating based constant temperature bath have been revealed.

Switching-field characteristics of exchange-coupled double layers (ECDLs)

Exchange-coupled double layers were prepared by RE sputtering and their temperature-dependent magnetization reversal behaviour was investigated by Kerr and VSM measurements. The switching field data were used to deduce the general magnetization reversal behaviour of antiparallel and parallel coupled double layers. With respect to application, a formalism based on the switching-field diagram was developed, which allows testing of the direct overwrite (DOW) or the magnetically induced superresolution (MSR) capability of MO media by using data obtained from Kerr measurements only. As an example, the formalism is applied to a double layer structure to test the writing performance of the DOW process.