Spin-flop State Research Articles

Selective damping modulation in a synthetic antiferromagnet induced by spin–orbit torque

Magnetization dynamics in a synthetic antiferromagnet with an adjacent Pt layer are investigated. Using dc bias, magnetic damping can be effectively controlled by the spin–orbit torque from the Pt layer. In the spin-flop state, the acoustic mode is modulated by the dc bias, but the optical mode is not sufficiently modulated. In the saturation state, the optical mode is effectively modulated. By appropriately selecting acoustic and optical modes and magnetization states such as the spin-flop and saturation states, modulation of the mutual phases of the ac spin currents driven by the ac damping torques can be realized.

Antiferromagnetic resonance and magnetic anisotropy in PrxY1−xFe3(BO3)4 crystals in the region of the magnetic structure transformation "easy axis – easy plane"

The spin dynamics, magnetic structures and magnetic anisotropy of single crystals Pr x Y 1−x Fe 3 (BO 3 ) 4 have been studied using antiferromagnetic resonance (AFMR) in a wide range of frequencies, magnetic fields, and temperatures. The frequency-field dependences of AFMR for the crystals with x = 0.25 and 0.45 are characteristic of antiferromagnets with the easy plane (EP) anisotropy. The crystals with x = 0.75 and 1.0 exhibit frequency-field dependences that are typical for antiferromagnets with the easy axis (EA) anisotropy. In these crystals, a significant decrease in the effective anisotropy fields of praseodymium upon the transition to the spin-flop state has been found. It is shown that this is the main reason for the large lability intervals, within which the regions of coexistence of the collinear and spin-flop states overlap. In the crystal with x = 0.67, the magnetic field applied along the trigonal axis of the crystal leads to the spin reorientation transition from the EA to the EP state. A magnetic phase diagram of the states on the plane "magnetic field - temperature" is built. In this crystal, the effective anisotropy field of praseodymium also decreases upon the transition to the field-induced EP state. Diamagnetic dilution of the praseodymium subsystem leads to the contribution of this subsystem to the total anisotropy field depending almost linearly on the praseodymium concentration. • In crystal with х = 0.67 magnetic field H||c causes spin reorientation transition EA→EP. • For х = 0.67÷1.0 a decrease in praseodymium anisotropy upon transition to field-induced EP or spin-flop state is found. • Large interval of coexistence of collinear and spin-flop states in crystals with х = 0.75÷1.0

Pressured-induced superconducting phase with large upper critical field and concomitant enhancement of antiferromagnetic transition in EuTe2

We report an unusual pressure-induced superconducting state that coexists with an antiferromagnetic ordering of Eu2+ moments and shows a large upper critical field comparable to the Pauli paramagnetic limit in EuTe2. In concomitant with the emergence of superconductivity with Tc ≈ 3–5 K above Pc ≈ 6 GPa, the antiferromagnetic transition temperature TN(P) experiences a quicker rise with the slope increased dramatically from dTN/dP = 0.85(14) K/GPa for P ≤ Pc to 3.7(2) K/GPa for P ≥ Pc. Moreover, the superconducting state can survive in the spin-flop state with a net ferromagnetic component of the Eu2+ sublattice under moderate magnetic fields μ0H ≥ 2 T. Our findings establish the pressurized EuTe2 as a rare magnetic superconductor possessing an intimated interplay between magnetism and superconductivity.

Noncollinear spin state and unusual magnetoresistance in ferrimagnet Co-Gd

Rare-earth transition-metal ferrimagnetic materials are promising candidates for next-generation spintronic devices owing to their small magnetic moments and fast dynamics. However, only a few studies have been conducted thus far on the transport properties of noncollinear ferrimagnetic spin textures. In this study, we investigate the transverse and longitudinal resistances in a ferrimagnet/heavy-metal heterostructure composed of Co-Gd and Pt. The transition-metal sublattice arrangement and magnetic field--temperature phase diagram are obtained based on unconventional hysteresis loops observed near the compensation point. In the collinear regime, the spin Hall magnetoresistance (SMR) and anisotropic magnetoresistance (AMR) exhibit normal oscillation curves. In the noncollinear regime, the SMR and AMR signals are inverted for the existence of a 90\ifmmode^\circ\else\textdegree\fi{} phase shift between magnetization and magnetic fields. Owing to the conical spin textures in the spin-flop state, sharp peaks emerge when the field is perpendicular to the film plane. The abnormal magnetoresistance in our results can be used to analyze complex magnetic structures, which plays an important role in the all-electrical readout of noncollinear spintronics devices.

High-field spin-flop state in green dioptase

The high-field magnetic properties and magnetic order of the gem mineral green dioptase Cu$_6[$Si$_6$O$_{18}]\cdot 6$H$_2$O have been studied by means of single-crystal neutron diffraction in magnetic fields up to $21~$T and magnetization measurements up to $30~$T. In zero field, the Cu$^{2+}$-moments in the antiferromagnetic chains are oriented along the $c$-axis with a small off-axis tilt. For a field applied parallel to the $c$-axis, the magnetization shows a spin-flop-like transition at $B^*=12.2~$T at $1.5~$K. Neutron diffraction experiments show a smooth behavior in the intensities of the magnetic reflections without any change in the periodicity of the magnetic structure. Bulk and microscopic observations are well described by a model of ferromagnetically coupled antiferromagnetic $XXZ$ spin-$\frac{1}{2}$ chains, taking into account a change of the local easy-axis direction. We demonstrate that the magnetic structure evolves smoothly from a deformed N\'eel state at low fields to a deformed spin-flop state in a high field via a strong crossover around $B^*$. The results are generalized for different values of interchain coupling and spin anisotropy.

Use of a Spin-Flop State for the Creation of Spin-Valve Elements for a Full Wheatstone Bridge

A method of universal thermomagnetic treatment forming a pairwise opposite exchange bias in spin valves, which are sensor elements of a Wheatstone bridge, has been developed. The method is based on the formation of two magnetic phases in a spin valve upon the transition of a synthetic antiferromagnet into a spin-flop state. Thermomagnetic treatment in a two-phase state leads to the formation of a pairwise mutually opposite exchange bias in different elements of a Wheatstone bridge. The direction of a formed exchange bias is governed by the uniaxial anisotropy of every element.

Spin valve based sensor elements for full Wheatstone bridge

Present work deals with methods for creating opposite pinning directions in micro-objects based on a spin valve by one thermomagnetic treatment. The methods are based on the thermomagnetic treatment in spin-flop state of synthetic antiferromagnet. We use splitting the magnetic structure in spin-flop state of synthetic antiferromagnet to form opposite pinning directions in different micro-objects by one thermomagnetic treatment. The positive characteristic (dR/dH) was obtained in the two sensor elements of the full Wheatstone bridge, and the negative characteristic (dR/dH) in the other two elements.

Antiferromagnetic domain wall control via surface spin flop in fully tunable synthetic antiferromagnets with perpendicular magnetic anisotropy

Antiferromagnetic (AF) domain walls have recently attracted revived attention, not only in the emerging field of AF spintronics, but also more specifically for offering fast domain wall velocities and dynamic excitations up to the terahertz frequency regime. Here we introduce an approach to nucleate and stabilize an AF domain wall in a synthetic antiferromagnet (SAF). We present experimental and micromagnetic studies of the magnetization reversal in [(Co/Pt)$_{X-1}$/Co/Ir]$_{N-1}$(Co/Pt)$_X$ SAFs, where interface induced perpendicular magnetic anisotropy (PMA) and AF interlayer exchange coupling (IEC) are completely controlled via the individual layer thicknesses within the multilayer stack. By combining strong PMA with even stronger AF IEC, the SAF reveals a collective response to an external magnetic field applied normal to the surface, and we stabilize the characteristic surface spin flop (SSF) state for an even number N of AF-coupled (Co/Pt)$_{X-1}$/Co multilayer blocks. In the SSF state our system provides a well-controlled and fully tunable vertical AF domain wall, easy to integrate as no single crystal substrates are required and with uniform 2D-magnetization in the film plane for further functionalization options, such as for example lateral patterning via lithography.

High-field magnetization and magnetic phase diagram of α−Cu2V2O7

High-field magnetization of the spin-$1/2$ antiferromagnet $\alpha$-Cu$_2$V$_2$O$_7$ was measured in pulsed magnetic fields of up to 56 T in order to study its magnetic phase diagram. When the field was applied along the easy axis (the $a$-axis), two distinct transitions were observed at $H_{c1}=6.5$~T and $H_{c2}=18.0$~T. The former is a spin-flop transition typical for a collinear antiferromagnet and the latter is believed to be a spin-flip transition of canted moments. The canted moments, which are induced by the Dzyaloshinskii-Moriya interactions, anti-align for $H_{c1}<H<H_{c2}$ due to the anisotropic exchange interaction that favors the antiferromagnetic arrangement along the $a$-axis. Above $H_{c2}$, the Zeeman energy of the applied field overcomes the antiferromagnetic anisotropic interaction and the canted moments are aligned along the field direction. Density functional theory was employed to compute the exchange interactions, which were used as inputs for quantum Monte Carlo calculations and then further refined by fitting to the magnetic susceptibility data. Contrary to our previous report in Phys. Rev. B {\bf 92}, 024423, the dominant exchange interaction is between the third nearest-neighbor spins, which form zigzag spin-chains that are coupled with one another through an intertwining network of the nonnegligible nearest and second nearest-neighbor interactions. In addition, elastic neutron scattering under the applied magnetic fields of up to 10 T reveals the incommensurate helical spin structure in the spin-flop state.

Ferromagnetic resonance study of interface coupling for spin waves in narrow NiFe/Ru/NiFe multilayer nanowires

A systematic investigation is presented for the magnetization dynamics in trilayer nanowires, consisting of two permalloy (${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$) layers separated by a nonmagnetic Ru spacer layer. The width of the wires ranges from 90 to 190 nm. By varying the Ru thickness between 0.7 and 2.0 nm, the interlayer coupling can be effectively controlled, modifying the corresponding magnetic ground state and the spin-wave dynamics. By contrast with previous work on coupled trilayer nanowires with larger widths (270 nm and more), the focus here is on nanowire arrays where the strong shape anisotropy competes with the Ruderman-Kittel-Kasuya-Yosida interactions and biquadratic exchange interactions across the Ru interface, as well as dipolar interactions and Zeeman energy. As a result, the spin-wave spectrum is found to be drastically modified. Ferromagnetic resonance and hysteresis loop measurements are reported over a wide range of applied magnetic fields, showing that the overall magnetization alignment between the permalloy layers may be parallel, antiparallel, or in a spin-flop state, depending on the overall interlayer coupling. The experimental results for different stripe widths are successfully analyzed using a microscopic dipole-dipole theory and micromagnetic simulations.

Spin-flop states in a synthetic antiferromagnet and variations of unidirectional anisotropy in FeMn-based spin valves

Spin valves with a synthetic antiferromagnet have been prepared by magnetron sputtering. Regularities of the formation of single- and two-phase spin-flop states in the synthetic antiferromagnet have been studied using magnetoresistance measurements and imaging the magnetic structure. A thermomagnetic treatment of spin valve in a field that corresponds to the single-phase spin-flop state of synthetic antiferromagnet was shown to allow us to obtain a magnetically sensitive material characterized by hysteresis-free field dependence of the magnetoresistance.

Field-induced spin reorientation in[Fe/Cr]nmultilayers studied by nuclear resonance reflectivity

We present depth-resolved nuclear resonance reflectivity studies of the magnetization evolution in ${[^{57}\mathrm{Fe}(3\phantom{\rule{0.16em}{0ex}}\mathrm{nm})/\mathrm{Cr}\phantom{\rule{0.16em}{0ex}}(1.2\phantom{\rule{0.16em}{0ex}}\mathrm{nm})]}_{10}$ multilayer under applied external field. The measurements have been performed at the station BL09XU of SPring-8 at different values of the external field (0--1500 Oe). We apply the joint fit of the delayed reflectivity curves and the time spectra of the nuclear resonance reflectivity measured at different grazing angles for enhancement of the depth resolution and reliability of results. We show that the azimuth angle, which is used in all papers devoted to the magnetization profile determination, has a more complicated physical sense due to the partially coherent averaging of the scattering amplitudes from magnetic lateral domains. We describe how to select the true azimuth angle from the determined ``effective azimuth angle.'' Finally we obtain the noncollinear twisted magnetization depth profiles where the spin-flop state appears sequentially in different $^{57}\mathrm{Fe}$ layers at increasing applied field.

Reentrant spin-flop transition in nanomagnets

Antiferromagnetic chains with an odd number of spins are known to undergo a transition from an antiparallel to a spin-flop configuration when subjected to an increasing magnetic field. We show that in the presence of an anisotropy favoring alignment perpendicular to the field, the spin-flop state appears for both weak and strong field, the antiparallel state appearing for intermediate fields. Both transitions are second order, the configuration varying continuously with the field intensity. Such re-entrant transition is robust with respect to quantum fluctuations and it might be observed in different types of nanomagnets.

Modulated quasi-two-dimensional antiferromagnetic structures in La1 − y Nd y MnO3 + δ manganites

The structural, electronic, and magnetic phase transitions induced by the isovalent substitution of the rare-earth Nd3+ ion for the La3+ ion with a larger radius have been investigated in the system of self-doped manganites La1 − yNdyMnO3 + δ (0 ≤ y ≤ 1; δ ∼ 0.1). For the average radius of the ions in A-sites of the lattice 〈rA〉 〈 1.19 A (y 〉 0.5), the phenomena revealed in the manganites are as follows: the ordering of Mn eg orbitals, the transition from the pseudocubic O* phase to the orthorhombic O’ phase, the opening of the dielectric Jahn-Teller gap, the frustration of the collinear ferromagnetic (FM) state, and the transition from the lowtemperature canted FM to canted antiferromagnetic (AFM) state of Mn spins. It is assumed that, in samples with neodymium concentrations y = 0.9 and 1.0 (〈rA〉 ≈ 1.16 A) at temperatures T < 12 K, there coexist A- and E-type modulated AFM states similar to the sinusoidal and helical structures of Mn spins, which were previously studied in RMnO3 multiferroics. The magnetic T-H phase diagrams of these samples are characteristic of quasi-two-dimensional antiferromagnets with a very low (zero) magnetic anisotropy in the ab planes. Under these conditions, the phase transition from the A-type AFM phase to the spin-flop state occurs in a relatively weak magnetic field. The AFM ordering of the Nd magnetic moments with a critical phase transition temperature TNd ≌ 6 K is induced in magnetic fields with a strength H ≥ 3.5 kOe. For the NdMnO3 + δ manganite in a magnetic field H = 10.7 kOe, the curves M(T) are characterized by additional very narrow peaks near temperatures T1 ≌ 4.5 K and T2 ≌ 5 K. The additional features revealed for the first time in the magnetization near T = 0 are assumed to be caused by the quantization of the spectrum of free holes in the ab planes by a strong magnetic field.

Magnetism and thermodynamic properties of a spin-1/2 ferrimagnetic diamond XY chain in magnetic fields at finite temperatures

The elementary excitation spectra of a one-dimensional ferrimagnetic diamond chain in the spin-1/2 XY model at low temperatures have been calculated by using an invariant eigen-operator (IEO) method, the energies of elementary excitations in different specific cases are discussed, and the analytic solutions of three critical magnetic field intensities (HC1, HC2, and Hpeak) are given. The magnetization versus external magnetic field curve displays a 1/3 magnetization plateau at low temperatures, in which HC1 is the critical magnetic field intensity from the disappearance of the 1/3 magnetization plateau to spin-flop states, HC2 is the critical magnetic field intensity from spin-flop states to the saturation magnetization, and Hpeak is the critical magnetic field intensity when the temperature magnetization shows a peak in the external magnetic field. The temperature dependences of the magnetic susceptibility and the specific heat show a double peak structure. The entropy and the magnetic susceptibility versus external magnetic field curves also exhibit a double peak structure, and the positions of the two peaks correspond to HC1 and HC2, respectively. This derives from the competition among different types of energies: the temperature-dependent thermal disorder energy, the potential energy of the spin magnetic moment, the ferromagnetic exchange interaction energy, and the anti-ferromagnetic exchange interaction energy. However at low temperatures, the specific heat as a function of external magnetic field curve exhibits minima at the above two critical points (HC1 and HC2). The origins of the above phenomena are discussed in detail.

Singular Field Response and Singular Screening of Vacancies in Antiferromagnets

For isolated vacancies in ordered local-moment antiferromagnets we show that the magnetic-field linear-response limit is generically singular: The magnetic moment associated with a vacancy in zero field is different from that in a finite field h in the limit h→0(+). The origin is a universal and singular screening cloud, which moreover leads to perfect screening as h→0(+) for magnets which display spin-flop bulk states in the weak-field limit.

Low Temperature Magnetization of Yb2Pt2Pb with the Shastry–Sutherland Type Lattice and a High-Rank Multipole Interaction

We measured DC magnetization of the antiferromagnet Yb 2 Pt 2 Pb ( T N ∼2.1 K) with the Shastry–Sutherland lattice for two in-plane directions H ∥[100] and [110] down to 0.08 K. Multiple metamagnetic transitions are found to develop in the field dependence of the magnetization M ( H ) near the I–II phase boundary. Substantial anisotropy observed in M ( H ) is well explained on the basis of a two-sublattice model with strong Ising anisotropy along the orthogonal [110] and [1\bar10] directions arising from the wave function of the ground state Kramers doublet predominantly comprised of |±7/2>, as recently proposed by Ochiai et al. In the high-field region of phase II, however, M ( H ) increases nearly linear to H and the full saturation is reached by a second-order transition even at the base temperature of 0.08 K, just like a spin flop state of an ordinary antiferromagnet. The results are interpreted by a pseudospin-flop state, which can be stabilized by a high-rank multipole, possibly a rank-7 octacosahec...

Exchange-spring-driven spin flop in an ErFe2/YFe2multilayer studied by x-ray magnetic circular dichroism

X-ray magnetic circular dichroism at the Er ${M}_{4,5}$ edge is used to study the switching behavior of the hard ErFe${}_{2}$ layers in an epitaxial [ErFe${}_{2}$(70 \AA{})/YFe${}_{2}$(150 \AA{})] \ifmmode\times\else\texttimes\fi{} 25 exchange-spring superlattice. Magnetic hysteresis loops for the Er magnetization, at temperatures $T$ 200 K, reveal a single irreversible switch between a vertical exchange spring and its reversed state. Experiments at $T$ &gt; 200 K reveal a crossover to a regime with two irreversible switching processes. Computational modeling for this system gives good agreement with the experiment, revealing that the observed high-temperature switching behavior is due to an exchange-spring-driven spin-flop-like transition. In contrast to the conventional spin-flop transition in an antiferromagnet, the increase in anisotropy energy of the hard magnetic ErFe${}_{2}$ layers and Fe-Fe exchange energy is overcome by a decrease in overall Zeeman energy. Computational studies also reveal two types of transitions between vertical exchange-spring and spin-flop states with first-order and second-order character.

Magnetization processes in a dot of ferrimagnetic garnet near the compensation temperature

At the compensation temperature of ferrimagnetic materials, magnetization vanishes but domain wall energy does not. Magnetic domains and coercive field become then very large. We are interested in small objects of such materials, in order to study the effect of size, shape, edges… on the domain structure and on the magnetization processes. A magnetic garnet film was patterned by chemical etching, leading to dots 7 μm thick and 10 to 100 μm wide. Domain observations were performed close to compensation (213 K) under magnetic field up to 22 mT. In the case where the dots are connected by a thin continuous magnetic film (partial etching), magnetization reversal occurs via an intermediate spin-flop state, coexisting with perpendicular domains. In order to analyze this process, we performed a mean field computation to characterize the spin-flop state itself, by the amplitude and orientation of the magnetization of each sublattice.

On the control of spin flop in synthetic antiferromagnetic films

The paper presents a systematic study of anneal induced anisotropies in a CoFe/Ru/CoFe synthetic antiferromagnet (SAF) exchange coupled with an IrMn film. When the annealing is done with the SAF in a spin flop state, the magnetic layers can be pinned perpendicular to the annealing field direction. The main parameters controlling this process are identified and analyzed: the value and the direction of the annealing field along with the Ruderman-Kittel-Kasuya-Yosida coupling energy between the two ferromagnetic layers. The induced anisotropy is predicted within a theoretical model taking into account the thermal variation of the coupling constants. Finally, the spin flop annealing is used to orthogonally pin the reference and the detection electrodes in an IrMn/CoFe/Ru/CoFe/Cu/CoFe/IrMn spin valve structure. The magnetoresistance variation in this structure is analyzed as a function of the pinning direction of the SAF acquired during the annealing in the spin flop state. A very good agreement is observed between the experimental and theoretically predicted responses.