Anisotropic Ferromagnet Research Articles

Intrinsic Properties of Fe-Substituted $L1_{0}$ Magnets

First-principle supercell calculations are used to determine how 3d elemental additions, especially Fe additions, modify the magnetization, exchange and anisotropy of L1 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> -ordered ferromagnets. Calculations are performed using the VASP code and partially involve configurational averaging over site disorder. Three isostructural systems are investigated: Fe-Co-Pt, Mn-Al-Fe, and transition metal-doped Fe-Ni. In all three systems the iron strongly influences the magnetic properties of these compounds, but the specific effect depends on the host. In CoPt(Fe) iron enhances the magnetization, with subtle changes in the magnetic moments that depend on the distribution of the Fe and Co atoms. The addition of Fe to MnAl is detrimental to the magnetization, because it creates antiferromagnetic exchange interactions, but it enhances the magnetic anisotropy. The replacement of 50% of Mn by Fe in MnFeAl <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> enhances the anisotropy from 1.77 to 2.5 MJ/m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> . Further, the substitution of light 3d elements such as Ti, V, Cr into L1 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> -ordered FeNi is shown to substantially reduce the magnetization.

Remagnetization of permanent magnets in closed and open magnetic circuits

The homogeneity of the strength of the magnetic field on the surface of a magnet placed in open and closed magnetic circuits is investigated using a program for modeling the remagnetization processes in highly anisotropic uniaxial ferromagnets. It is shown that the inhomogeneity of the magnetic field on the surface of a magnet is a consequence of magnetostatic interaction between the magnet’s microvolumes. A comparison of the calculation results and experimental data shows their close resemblance.

Aligned CuO nanorod arrays: fabrication and anisotropic ferromagnetism

Copper oxide (CuO) is a p-type semiconductor with a band gap of 1.2 eV, which is well known in high-temperature superconductor and antiferromagnetic (AFM) materials through Cu–O–Cu super-exchange interaction. In this paper, we report the strong anisotropic ferromagnetism (FM) in aligned CuO nanorod arrays synthesized by a microwave-assisted hydrothermal method. The transmission electron microscopy (TEM) image shows that the CuO nanorod consists of a large number of smaller nanorods with almost the same growth direction. The X-ray diffraction (XRD) pattern indicates that the CuO nanorods are well crystallized with highly preferred orientation of the [020] direction. These CuO nanorod arrays show room-temperature ferromagnetism, with strong magnetic anisotropy when the magnetic field is applied perpendicular or parallel to the rod axis. This phenomenon of room-temperature ferromagnetism in those aligned CuO nanorods might originate from uncompensated surface spins and shape anisotropy of the nanorods.

Epitaxial strain effect on transport properties in Ca2−xSrxRuO4thin films

We have grown Ca${}_{2\ensuremath{-}x}$Sr${}_{x}$RuO${}_{4}$ ($x$ $=$ 0, 0.1, 0.5, and 2) epitaxial thin films using a pulsed laser deposition method and characterized their structures and magnetotransport properties. We find that the $x$ $=$ 0, 0.1, and 0.5 films grown on LaAlO${}_{3}$ substrates exhibit coherent strain with tetragonal structure. The nature of strain is dependent on Sr content: the Ca${}_{2}$RuO${}_{4}$ ($x$ $=$ 0) film features biaxial compressive strain, while the $x$ $=$ 0.5 film shows biaxial tensile strain. The strain in the $x$ $=$ 0.1 film is relatively weak and strongly anisotropic, with compressive strain along the $a$ axis and tensile strain along the $b$ axis. In contrast, the Sr${}_{2}$RuO${}_{4}$ films show strain relaxation. The epitaxial strain effect leads the properties of the $x=0$, 0.1, and 0.5 films to be distinct from those of bulk materials. The bulk material shows antiferromagnetic Mott-insulating properties for $x$ 0.2 and a nearly ferromagnetic state for $x$ \ensuremath{\sim} 0.5 [Nakatsuji and Maeno, Phys. Rev. Lett. 84, 2666 (2000)], whereas the film displays itinerant ferromagnetism for $x$ $=$ 0 and 0.1 and paramagnetic metal for $x$ $=$ 0.5. Furthermore, in the $x$ $=$ 0 and 0.1 films, we observed distinct fourfold ferromagnetic anisotropy, with the minimum magnetoresistivity along the diagonal directions for $x$ $=$ 0 and $a$ and $b$ directions for $x$ $=$ 0.1. Such evolution of magnetic anisotropy may be associated with the tuning of the spin-orbit coupling by the epitaxial strain.

Orbital magnetism and magnetic anisotropy in thin-film ferromagnets disturbed from the ground state

We report first-principles study of the orbital magnetism and magnetic anisotropy energy (MAE) in ultrathin films of Fe, Co, and FePt disturbed from the ferromagnetic ground state. The deviation of ferromagnetic spin configurations from the easy axis leads to the noncollinearity of the spin and orbital moments. In Fe and Co, this noncollinearity correlates with the magnitude of MAE. In contrast, in FePt a complex interplay of competing processes leads to unusual phenomenon of the orthogonality of the inducing spin and induced orbital moments. Considering the influence of noncollinearity of spin moments we obtain opposite trends in the MAE variation for Fe and Co films. This finding correlates with the absence of universal temperature behavior of MAE in itinerant magnets and is shown to originate in the variation of the electronic structure with the change of spin configuration.

Ferromagnetic Exchange Anisotropy from Antiferromagnetic Superexchange in the Mixed3d−5dTransition-Metal CompoundSr3CuIrO6

We report a combined experimental and theoretical study of the unusual ferromagnetism in the one-dimensional copper-iridium oxide Sr(3)CuIrO(6). Utilizing Ir L(3) edge resonant inelastic x-ray scattering, we reveal a large gap magnetic excitation spectrum. We find that it is caused by an unusual exchange anisotropy generating mechanism, namely, strong ferromagnetic anisotropy arising from antiferromagnetic superexchange, driven by the alternating strong and weak spin-orbit coupling on the 5d Ir and 3d Cu magnetic ions, respectively. From symmetry consideration, this novel mechanism is generally present in systems with edge-sharing Cu(2+)O(4) plaquettes and Ir(4+)O(6) octahedra. Our results point to unusual magnetic behavior to be expected in mixed 3d-5d transition-metal compounds via exchange pathways that are absent in pure 3d or 5d compounds.

Antiferromagnetic layer thickness dependence of the coercivity of the ferromagnetic/antiferromagnetic two‐phase system

The antiferromagnetic layer thickness dependence of the coercivity of the ferromagnetic/antiferromagnetic two‐phase system was studied. With mathematical transformations, equivalent anisotropy field and equivalent external field were obtained analytically. In this way, the two‐phase system was transformed into a single‐phase magnet and its hysteresis loops and coercivities were obtained by using a three‐dimensional hybrid hysteresis model. It is shown that the emergence of a peak in the coercivity curve may be related with the competing between the ferromagnetic and antiferromagnetic anisotropy field, or the offset of the equivalent easy axis. The results are discussed and compared with previous work.

Effect of mechanical boundary conditions on the dynamic and static properties of a strongly anisotropic ferromagnet

The spectra of coupled magnetoelastic waves in a semi-infinite strongly anisotropic easy-plane ferromagnet with a rigidly fixed face are analyzed for two variants of fixation (in the basal plane and perpendicularly to it). The phase states of the system are determined. Differences in the phase diagrams and elementary excitation spectra depending on the choice of the sample fixation plane are considered. When rotational invariance is taken into account, the nonreciprocity effect for the velocities of sound in a crystal appears. It is shown that the velocity of sound in the sample considerably depends on the symmetry of the imposed mechanical boundary conditions. The phase diagrams of the system under investigation are presented.

Spin-state responses to light impurity substitution in low-spin perovskite LaCoO3

We studied the spin-state responses to light impurity substitution in low-spin perovskite LaCoO${}_{3}$ (Co${}^{3+}$: ${d}^{6}$) through magnetization, x-ray fluorescence, and electrical resistivity measurements of single-crystal LaCo${}_{0.99}{M}_{0.01}$O${}_{3}$ ($M$ $=$ Cr, Mn, Fe, Ni). In the magnetization curves measured at 1.8 K, a change in the spin-state was not observed for Cr, Mn, or Fe substitution but was observed for Ni substitution. Strong magnetic anisotropy was also found in the Ni-substituted sample. The fluorescence measurements revealed that the valences were roughly estimated to be Cr${}^{3+}$, Mn${}^{(4\ensuremath{-}\ensuremath{\delta})+}$, Fe${}^{(3+{\ensuremath{\delta}}^{\ensuremath{'}})+}$, and Ni${}^{3+}$. From the observed chemical trends, we propose that the chemical potential is a key factor in inducing the change of the low-spin state. By expanding a model of the ferromagnetic spin-state heptamer generated by hole doping [Podlesnyak et al., Phys. Rev. Lett. 101, 247603 (2008)], the emergence of highly anisotropic spin-state molecular ferromagnets induced by low-spin Ni${}^{3+}$ with Jahn-Teller activity is suggested. We also discuss applicability of the present results to other materials with Fe (${d}^{6}$).

Critical behavior of the 3D-Ising ferromagnets Cd[CrxTiy]Se4

The critical behavior of the single-crystalline Cd[CrxTiy]Se4 spinels (where y=0.07, 0.42 and 0.59) investigated around the paramagnetic–ferromagnetic phase transition revealed the values of the critical exponents close to those predicted by the calculations for a three-dimensional (3D) anisotropic short-range Ising ferromagnet. The dc magnetic susceptibility and magnetization measurements showed the Curie temperature of 130K and the saturation magnetization above 5.19μB/f.u. slightly depend on the Ti content. An increase of the positive value of the temperature independent magnetic susceptibility as the Ti-content increases suggests a significant Van Vleck contribution.

Size dependent ferromagnetic resonance and magnetic anisotropy of hexagonal barium and strontium ferrite powders

Ferrite powders with different particle diameters were characterized in millimeter wave frequency range. Quasi-optical spectrometer in transmission mode powered by high power backward wave oscillator was employed to measure the transmittance of the powder samples. Strong ferromagnetic absorptions were acquired in the millimeter wave frequency range. The absorption frequencies exhibit distinct shift related to the size of ferrite particles. The complex dielectric permittivity and magnetic permeability are determined from the transmittance spectra. The size dependence of ferromagnetic resonance is clearly observed.

Propagation of an electromagnetic soliton in an anisotropic biquadratic ferromagnetic medium

Information storage technology based on anisotropic ferromagnets with sufficiently high magneto-optical effects has received much attention in recent years. Magneto-optical recording combines the merits of magnetic and optical techniques. We investigate the magneto-optical effects on a biquadratic ferromagnet and show that the dynamics of the system are governed by a perturbed nonlinear Schrödinger equation. The evolutions of amplitude and velocity of the soliton are found to be time independent, thereby admitting the lossless propagation of the electromagnetic soliton in the medium, which may have potential applications in soliton based optical communication systems. We also exploit the role of perturbation, which has a significant impact on the propagation of an electromagnetic soliton.

Study of the magnetic and electronic properties of nanocrystalline PrCo3 by neutron powder diffraction and density functional theory

Nanocrystalline PrCo3 powder has been synthesized by high-energy milling and was subsequently annealed from 873 to 1273 K for 30 min to optimize the extrinsic properties. The structure and magnetic properties of the nanocrystalline PrCo3 have been investigated by means of x-ray and neutron diffraction as well as magnetization measurements. All compounds crystallize in the same PuNi3 type structure, with grain sizes between 28 and 47 nm. As the annealing temperature increases, a maximum coercive field of 12 kOe at 300 K (55 kOe at 10 K) was obtained by annealing at 1023 K for a grain size of 35 nm. The refinement of the neutron powder diffraction patterns (NPD) of PrCo3 from 1.8 to 300 K shows an expansion of the parameter a and a contraction of the parameter c, leading to a decrease of the ratio c/a. The evolution of the Co and Pr magnetic sublattices measured by NPD indicates that this compound is a highly anisotropic uniaxial ferromagnet with the easy magnetization axis parallel to . This experimental study has been completed by a theoretical investigation of the electronic structure of the PrCox (x = 2, 3 and 5) compounds. Band structure calculations with collinear spin polarization were performed by using the local approximation of the density functional theory scheme implemented in the projector-augmented wave method. The electronic structure of PrCo3 compound in both directions of spin shows that the majority of occupied states are dominated by the 3d states of Co, with a strong electronic charge transfer from Pr to Co. The PrCo3 electronic structure can be explained by a superimposition of those of PrCo2 and PrCo5, as expected from its crystal structure. The magnetic anisotropy has been confirmed for PrCo3, as a non-collinear spin calculation with the polarization along the c axis is shown to be more stable than with the polarization in the () plane.

Influence of strong anisotropy of CoFe layer on the reversal asymmetry and training effect in Ir22Mn78/Co60Fe40 bilayers

In this work, we report a study of the influence of large anisotropy of CoFe layer on the reversal asymmetry and training effect in exchange biased IrMn/CoFe bilayers. The existence of a strong single cycle training effect and an accompanying distinct reversal asymmetry at 15 K have been demonstrated using magnetoresistance as a probe. The temperature dependent change in the symmetry (uniaxial to biaxial) of IrMn anisotropy has been proposed as the primary cause of the occurrence of pronounced reversal asymmetry and large training effect below 50 K. In particular, the present findings indicate that the observed differences between IrMn/CoFe and IrMn/NiFe [Fulara et al., Appl. Phys. Lett. 101, 142408 (2012)] systems in their asymmetric behaviour of magnetization reversal and training effect are linked with the interplay between the interfacial spin frustration and the intrinsic ferromagnetic anisotropy.

Effect of mechanical strain on magnetic properties of flexible exchange biased FeGa/IrMn heterostructures

We have fabricated flexible exchange biased heterostructures with magnetostrictive Fe81Ga19 alloy as the ferromagnetic layer and Ir20Mn80 as the antiferromagnetic layer on polyethylene terephthalate substrates. The mechanical strain can modify both the strength and the orientation of the uniaxial anisotropy, giving rise to the switching between the easy and hard magnetization directions. Different from the previously reported works on rigid exchange biased systems, a drastic decrease in exchange bias field was observed under a compressive strain with magnetic field parallel to the pinning direction, but only a slightly decrease was shown under a tensile strain. Based on a Stoner-Wohlfarth model calculation, we suggested that the distributions of both ferromagnetic and antiferromagnetic anisotropies be the key to induce the mechanically tunable exchange bias.

Tuning the formation and functionalities of ultrafine CoFe2O4 nanocrystals via interfacial coherent strain

Complex oxide nanocrystals with a spinel structure show their remarkable optical, electronic, mechanical, thermal, and magnetic properties. In this study, we present a simple yet versatile strategy to grow self-assembled epitaxial CoFe2O4 nanocrystals with well-controlled size (less than 10 nm) and single orientation. CoFe2O4 nanocrystals were fabricated via phase separation in a BiFeO3-CoF2O4 ultrathin film by pulsed laser deposition. The coherent strain at the BiFeO3-CoF2O4 interface suppressed the growth of the nanocrystals regardless of substrate temperatures. This strain also resulted in the ferromagnetic anisotropy and interesting conducting behaviors of ultrafine CFO nanocrystals.

Post-Annealing Treatments and Interface Effects on Anomalous Magnetic Characteristics of HfOx Film

Interface microstructures and anomalous magnetic characteristics of HfOx film annealed under different conditions have been investigated using X-ray diffraction, X-ray photoelectron spectroscope, high-resolution transmission electron microscopy and superconducting quantum interference device. It is demonstrated that HfOx samples have monoclinic crystal structure and an interface layer of Hf-silicate mixed with Hf-silicide. Remarkable ferromagnetic signals and obvious magnetic anisotropy are observed, and they are greatly affected by the post-annealing temperature and ambient. It is believed that oxygen vacancies and interface defects are responsible for the anomalous magnetic characteristics of HfOx films.

Structural and Magnetic Studies of CoCr Thin Films Prepared by Physical Vapor Deposition

We use physical vapor deposition (PVD) to produce, under vacuum, CoCr thin films onto Si (100) and glass substrates. The thicknesses of the samples were measured with a DEKTAK 150 profilometer and the micro structural properties were studied using a Siemens D500 X-ray diffractometer. The static magnetic characterization was done by means of an Alternating Gradient Field Magnetometer (A.G.F.M.) 2900 MicroMag. The films deposited on Corning glass are amorphous. Among all the samples deposited on Si(100) with thickness ranging from 100 nm to 340 nm, only those with thickness 163 nm, 178 nm and 340 nm are polycrystalline and present a hexagonal close-packed (h.c.p.) structure with 〈0001〉 texture. The magnetization curve study infers that all the samples present a planar ferromagnetic anisotropy. The maximum saturation magnetization M s value being 400 emu/cm3, the coercive field decreases vs. thickness beyond a maximum value (178 nm). All these results will be presented and discussed.

Phase diagram of UGe2.: Whether there are quantum phase transitions ?

The phase diagram of several itinerant ferromagnets reveals the common feature. The phase transition temperature decreases with pressure increase and reaches zero value at some critical pressure Pc such that at low enough temperatures one can expect critical behavior specific for quantum phase transition. It is not the case, however. Being the second order at ambient pressure the transition from paramagnetic to ferromagnetic state at high pressures -low temperatures is transformed to the discontinuous jump.We discuss the magneto-elastic mechanism of development of the first order type instability at the phase transition to the ferromagnet state in strongly anisotropic ferromagnet UGe2. Using the parameters characterizing the properties of UGe2 we argue the effectiveness of this mechanism transforming the very weak first order type transition to the really observable one.

Fabrication of single-dot planar nano-devices and the application to the exchange bias characterization in nano-pillar devices

Single dot [Co/Pd]5/FeMn nano-pillar devices with split nano-contacts are fabricated down to 150 × 150 nm2 dimensions, to understand the effects of nano-patterning on perpendicular exchange bias (PEB) characteristics. Using extraordinary Hall effect measurements, magnetic force microscopy, and numerical calculations, it is shown that the exchange bias field initially increases from the thin-film value, with decreasing dimensions down to a critical dimension below, which it again reduces. The PEB characteristics of the nano-pillar devices are found to be influenced by changes to the ferromagnetic (FM) layer anisotropy, exchange coupling between ferromagnetic and antiferromagnetic layers, in addition to edge effects caused by the fabrication process.