Thickness dependence of positive exchange bias in ferromagnetic/antiferromagnetic bilayers

For the ferromagnetic (FM)/antiferromagnetic (AFM) bilayers, both negative and positive exchange bias H_E have been observed for low and high cooling field H_CF, respectively. The thickness dependence of H_E and coercivity H_C have been investigated for the cases of negative and positive H_E. It is found that the negative H_E and the positive one have similar FM thickness dependence that is attributed to the interfacial nature of exchange bias. However, the AFM thickness dependence of positive H_E is completely contrary to that of the negative one, which clearly demonstrates that the AFM spins play different roles for the cases of positive and negative H_E. In particular, the AFM thickness of positive H_E was first highlighted by an AFM spin canting model. These results should be attributed to the interfacial spin configuration after field cooling procedure.     

Temperature dependent exchange bias effect in polycrystalline BiFeO3/FM (FM = NiFe, Co) bilayers

Extensive studies on the temperature (T) dependent exchange bias effect were carried out in polycrystalline BiFeO₃(BFO)/NiFe and BFO/Co bilayers. In contrast to single-crystalline BFO/ferromagnet (FM) bilayers, sharp increase of the exchange bias field (H _E ) below 50 K were clearly observed in both of these two bilayers. However, when T is higher than 50 K, H _E increases with T and decreases further when T is larger than 230 K (for BFO/NiFe) or 200 K (for BFO/Co), which is similar to those reported in single-crystalline BFO/FM bilayers. After the exploration of magnetic field cooling, the temperature dependent exchange bias can be explained considering two contributions from both the interfacial spin-glass-like frustrated spins and the polycrystalline grains in the BFO layer. Moreover, obvious exchange bias training effect can be observed at both 5 K and room temperature and the corresponding results can be well fitted based on a recently proposed theoretical model taking into account the energy dissipation of the AFM layer.     

Theoretical investigation of exchange bias

The exchange bias of bilayer magnetic films consisting of ferromagnetic (FM) and antiferromagnetic (AFM) layers in an uncompensated case is studied by use of the many-body Green's function method of quantum statistical theory. The effects of the layer thickness and temperature and the interfacial coupling strength on the exchange bias H_E are investigated. The dependence of the exchange bias H_E on the FM layer thickness and temperature is qualitatively in agreement with experimental results. When temperature varies, both the coercivity H_C and H_E decrease with the temperature increasing. For each FM thickness, there exists a least AFM thickness in which the exchange bias occurs, which is called pinning thickness.     

Effects of component ratio and easy axes distribution on the exchange bias in ferromagnetic-antiferromagnetic random alloys

For a ferromagnetic (FM)-antiferromagnetic (AFM) system with composition x(FM)+(AFM)_1−x, a modified Monte Carlo Metropolis method is performed to study the effects of x and easy axes distribution at the FM/AFM nearest neighbors on exchange bias field H_E, coercivity H_C, and vertical magnetization shift M_E after cooling under different magnetic fields h_CF. When the orientations of easy axes are uniform, the x dependence of H_E and M_E undergo a non-monotonous to monotonous process with the increase of h_CF, whereas H_C shows a more complex behavior. On the other hand, for the case of the random orientation, H_C has a peak around x=0.5, while M_E decreases with the increase of x. H_E exhibits negative extrema at small x and disappears for larger x. However, abnormal positive H_E observed depends on the frustration and the distinct trends of two coercive fields with x in such a special model.     

Peculiarities of the magnetic,galvanomagnetic, elastic,and magnetoelastic properties of Sm<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Sr<Subscript>x</Subscript>MnO<Subscript>3</Subscript> manganites

Manganites of the Sm1?xSr_xMnO₃ system (x=0.33, 0.4, and 0.45) possess giant negative values of the magnetoresistance Δρ/ρ and the volume magnetostriction ω near the Curie temperature T_C. In the compound with x=0.33, the isotherms of Δρ/ρ, ω, and magnetization σ exhibit smooth variation and do not reach saturation up to maximum magnetic field strengths (120 kOe) studied (according to the neutron diffraction data, this substance comprises a ferromagnetic (FM) matrix with distributed clusters of a layered antiferromagnetic (AFM) structure of the A type). In the compounds with x=0.4 and 0.45 containing, besides the FM matrix and A-type AFM phase, a charge-ordered AFM phase of the CE type (thermally stable to higher temperatures as compared to the A-type AFM and the FM phases), the same isotherms measured at T ≥ T_C show a jumplike increase in the interval of field strengths between H_c1 and H_c2 and then reach saturation. In the interval H_c1 > H > H_c2, the σ, ω, and Δρ/ρ values exhibit a metastable behavior. At temperatures above T_C, the anisotropic magnetostriction changes sign, which is indicative of rearrangements in the crystal structure. The giant values of ω and Δρ/ρ observed at T ≥ T_C for all compounds, together with excess (relative to the linear) thermal expansion and a maximum on the ρ(T) curve, are explained by the phenomenon of electron phase separation caused by a strong s-d exchange. The giant values of magnetoresistance and volume magnetostriction (with ω reaching ～10^?3) are attributed to an increase in the volume of the FM phase induced by the applied magnetic field. In the compound with x=0.33, this increase proceeds smoothly as the FM phase grows through the FM layers in the A-type AFM phase. In the compounds with x=0.4 and 0.45, the FM phase volume increases at the expense of the charge-ordered CE-type AFM structure (in which spins of the neighboring manganese ions possess an AFM order). The jumps observed on the σ(H) curves, whereby the magnetization σ reaches ～70% of the value at T=1.5 K, are indicative of a threshold character of the charge-ordered phase transition to the FM state. Thus, the giant values of ω and Δρ/ρ are inherent in the FM state, appearing as a result of the magnetic-field-induced transition of the charge-ordered phase to the FM state, rather than being caused by melting of this phase.     

Peculiarities of magnetic,galvanomagnetic, elastic,and magnetoelastic properties of Eu<Subscript>0.55</Subscript>Sr<Subscript>0.45</Subscript>MnO<Subscript>3</Subscript>

Magnetic, elastic, magnetoelastic, transport, and magnetotransport properties of the Eu_0.55Sr_0.45MnO₃ ceramics have been studied. A break was detected in the temperature dependence of electrical resistivity ρ(T) near the temperature of the magnetic phase transformation (41 K), with the material remaining an insulator down to the lowest measurement temperature reached (ρ=10⁶ Ω cm at 4.2 K). In the interval 4.2≤T≤50 K, the isotherms of the magnetization, volume magnetostriction, and ρ were observed to undergo jumps at the critical field H_C1, which decreases with increasing T. For 50≤T≤120 K, the jumps in the above curves persist, but the pattern of the curves changes and H_C1 grows with increasing T. The magnetoresistance Δρ/ρ = (ρ _H ?ρ_H=0)/ρ _H is positive for H<H_C1 and passes through a maximum at 41 K, where Δρ/ρ = 6%. For H>H_C1, the magnetoresistance is negative, passes through a minimum near 41 K, and reaches a colossal value of 3×10⁵ % at H=45 kOe. The volume magnetostriction is negative and attains a giant value of 4.5×10^?4atH=45 kOe. The observed properties are assigned to the existence of three phases in Eu_0.55Sr_0.45MnO₃, namely, a ferromagnetic (FM) phase, in which carriers are concentrated because of the gain in s-d exchange energy, and two antiferromagnetic (AFM) phases of the A and CE types. Their fractional volumes at low temperatures were estimated to be as follows: ～3% of the sample volume is occupied by the FM phase; ～67%, by the CE-type AFM phase; and ～30%, by the A-type AFM phase.     

Mictomagnetic State in an EuBaCo<Subscript>2–<Emphasis Type="Italic">x</Emphasis></Subscript>O<Subscript>5.5–δ</Subscript> Single Crystal

The magnetic properties of an EuBaCo_1.9O_5.36 single crystal are studied in the temperature range T = 2–300 K and the magnetic field range H ≤ 90 kOe. This binary layered cobaltite single crystal has vacancies in the cobalt and oxygen sublattices, in contrast to the stoichiometric EuBaCo₂O_5.5 composition. All cobalt ions in EuBaCo1.9O5.36 are in a trivalent state. The single crystal has an orthorhombic structure with space group Pmmm, and its unit cell parameters are a = 3.883 Å, b = 7.833 Å, and c = 7.551 Å. The field and temperature dependences of the magnetization of the single crystal demonstrate that it is ferrimagnet below T_C = 242 K. At T < 300 K, all three spin states of the Co³⁺ ions are present. The nearest-neighbor interactions give antiferromagnetic (AFM) and ferromagnetic (FM) contributions to the exchange energy. The ratio of the AFM to the FM contributions changes when temperature decreases because of a change in the spin state of the Co³⁺ ions. The single crystal exhibits signs of mictomagnetism at low temperatures in high magnetic fields. At T = 2 K and H = 90 kOe, the zero-field and nonzero-field magnetizations are strongly different because of a uniaxial magnetic anisotropy, which tends to set magnetization along the magnetic field applied in cooling throughout the crystal volume. As a result, a complex ferrimagnetic structure with a noncollinear direction of Co³⁺ spins appears. The following phenomena characteristic of mictomagnets are also observed in the EuBaCo_1.9O_5.36 single crystal: a shift in a magnetization hysteresis loop when temperature decreases, retained hysteretic phenomena and no magnetization saturation in high magnetic fields, and an orientation transition. The mictomagnetic state in EuBaCo_1.9O_5.36 is shown to be caused by the structural distortions induced by vacancies in the cobalt and oxygen sublattices and by the frustration of AFM and FM exchange interactions.     

Ferromagnetic/antiferromagnetic exchange coupling in SrFe<Subscript>12</Subscript>O<Subscript>19</Subscript>/CoO composites

The surface of SrFe₁₂O₁₉ coated with a CoO layer reveals a strong exchange bias characterized by magnetic hysteresis loops. The low-temperature coercivity, H_C, and the squareness, M_R/M_S, of a permanent magnet of SrFe₁₂O₁₉/CoO powder prepared by the sol–gel method are enhanced after field cooling through the Néel temperature (T_N=290 K) when compared to those after zero-field cooling. The existence of loop shifts and the enhancement of H_C indicate that exchange-bias effects, which are induced by the ferromagnetic/antiferromagnetic (FM/AFM) exchange-coupling interactions, are responsible for these behaviors. According to our experimental results, some of the factors controlling the exchange bias, such as FM/AFM interfaces and the CoO amount of the antiferromagnetic layer, are discussed. PACS 75.30.Et; 75.50.Ee; 75.60.Ej; 75.60.Gm; 75.70.Cn     

Effects of divalent alkaline earth ions on the magnetic and transport features of La0.65A0.35Mn0.95Fe0.05O3 (A = Ca, Sr, Pb, Ba) compounds

We have studied the magnetic and transport properties of Fe doped La_0.65A_0.35Mn_0.95Fe_0.05O₃ (A = Ca, Sr, Pb, Ba) manganites. All the compositions show ferromagnetic/metal to paramagnetic/insulator transition (T_C) except the Pb doped sample which is insulating and ferromagnetic (FM) in the entire temperature range. The magnetization and T_C are decreased by decreasing the cation size on La site. The transition temperature and magnetic moment at 77 K is a maximum for Sr doped sample and is decreasing if we increase or decrease the cation size from Sr size. The maximum value of T_C and magnetic moment for Sr based sample is most likely due to the closer ionic sizes of La and Sr as compared to the other dopants (Ca, Pb, and Ba). We observed a spin freezing type effect in the Pb doped sample below 120 K in resistivity, ac susceptibility and in magnetization. This suggests that the AFM interactions introduced by the Fe are most effective in the Pb doped composition leading to increased competition between the FM and AFM interactions. This FM and AFM interaction generates some degree of frustration leading to the appearance of spin glass like phase whose typical magnetic behavior is studied for small ion when the metallic like behavior is lost.     

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 La_{1 ? y} Nd _y MnO_{3 + δ} (0 ≤ y ≤ 1; δ ～ 0.1). For the average radius of the ions in A-sites of the lattice 〈r _A 〉 〈 1.19 Å (y 〉 0.5), the phenomena revealed in the manganites are as follows: the ordering of Mn e _g 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 (〈r _A 〉 ≈ 1.16 Å) 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 T _Nd ≌ 6 K is induced in magnetic fields with a strength H ≥ 3.5 kOe. For the NdMnO_{3 + δ} manganite in a magnetic field H = 10.7 kOe, the curves M(T) are characterized by additional very narrow peaks near temperatures T ₁ ≌ 4.5 K and T ₂ ≌ 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.     

Magnetoresistance of lanthanum manganites with activation-type conductivity

The temperature dependence of the resistivity and magnetic moment of La_0.85Ba_0.15MnO₃ and La_0.85Sr_0.15MnO₃ manganite single crystals in magnetic fields up to 90 kOe is investigated. Analysis of the experimental results shows that the magnetoresistance of lanthanum manganites far from the Curie temperature T _C can be described quantitatively by the s-d model normally used for ferromagnets and taking into account only the exchange interaction between the spins of charge carriers and magnetic moments. These data also show that the features of lanthanum manganites responsible for colossal magnetoresistance (CMR) are manifested in a narrow temperature interval δT ≈ 20 K near T _C. Our results suggest a CMR mechanism analogous to the mechanism of giant magnetoresistance (GMR) observed in Fe/Cr-type multilayers with nanometer layer thickness. The nanostratification observed in lanthanum manganites and required for GMR can be described taking into account the spread in T _C in the CMR range δT.     

Magnetocaloric effect in manganites

The magnetocaloric effect (MCE) in La_{1 ? x} Sr _x MnO₃, Sm_0.55Sr_0.45MnO₃, and PrBaMn₂O₆ compounds is studied. The maximum values of MCE (??T _max) determined by a direct method in the second and third compositions and in La_0.9Sr_0.1MnO₃ are found to be much lower than those calculated from the change of the magnetic part of entropy in the Curie temperature (T _C) and the Néel temperature (T _N) range. The negative contribution of the antiferromagnetic (AFM) part of a sample in the La_{1 ? x} Sr _x MnO₃ system at 0.1 ?? x ?? 0.3 decreases ??T _max and changes the ??T(T) curve shape, shifting its maximum 20?C40 K above T _C. Lower values of ??T _max are detected in the range T _C = 130?142 K in polycrystalline and single-crystal Sm_0.55Sr_0.45MnO₃ samples cooled in air. If such samples were cooled in an oxygen atmosphere (which restores broken Mn-O-Mn bonds and, thus, increases the volume of CE-type AFM clusters), the maximum in the temperature dependence of MCE is located at T _N (243 K) for CE-type AFM clusters. A magnetic field applied to a sample during the MCE measurements transforms these clusters into a ferromagnetic (FM) state, and both types of clusters decompose at T = T _N. The PrBaMn₂O₆ composition undergoes an AFM-FM transition at 231 K, and the temperature dependence of its MCE has a sharp minimum at T = 234 K, where MCE is negative, and a broad maximum covering T _C. The absolute values of MCE at both extrema are several times lower than those calculated from the change in the magnetic entropy. These phenomena are explained by the presence of a magnetically heterogeneous FM-AFM state in these manganites.     

Magnetic phase transitions of antiperovskite Mn3−xFexSnC (0.5≤x≤1.3)

The magnetization and electrical resistivity of Mn_3−xFe_xSnC (0.5≤x≤1.3) were measured to investigate the behavior of the complicated magnetic phase transitions and electronic transport properties from 5 to 300 K. The results obtained demonstrate that Fe doping at the Mn sites of Mn₃SnC induces a more complicated magnetic phase transition than that in its parent phase Mn₃SnC from a paramagnetic (PM) state to a ferrimagnetic (FI) state consisting of antiferromagnetic (AFM) and ferromagnetic (FM) components, while, with the change of Fe-doped content and magnetic field, there is a competition between the AFM component and FM component in the FI state. Both the Curie temperature (T_C) and the saturated magnetization M_s increase with increasing x. The FM component region becomes broader with further increasing Fe-doped content x. The external magnetic field easily creates a saturated FM state (and increased T_C) when . Fe doping quenches the negative thermal expansion (NTE) behavior from 200 to 250 K reported in Mn₃SnC.     

The instability point H s of the Meissner state for large-κ superconductors

The critical field H _s corresponding to the emergence of vortices in a superconductor without a threshold is found near the transition temperature and in the limit as T→0 for an arbitrary value of the depairing factor Γ. In superconductors of the second kind, this field value coincides with the absolute instability point of the Meissner state. In large-κ superconductors, the order parameter tends to zero on the surface of the super-conductor if the external magnetic field reaches the value H _s. We obtain that H _s=H _cm (where H _cm is the thermodynamic critical field) for an arbitrary value of the depairing factor Γin the temperature region near T _c and at T=0.     

On the field dependence of the interface energy in AF/FM bilayers

In the investigations of antiferromagnetic (AF)/ferromagnetic (FM) bilayer samples, often distinct experimental techniques yield different values for the measured exchange anisotropy field (H_E). We propose that the observed discrepancy may be accounted in part by the dependence of the unidirectional anisotropy with the value of the externally applied cooling field (h). Using a simple microscopic model for representing the AF/FM interface, which incorporates the effect of interface roughness, we show that the interface energy between the AF and FM layer indeed varies with h, as recently observed in anisotropic magnetoresistance measurements, lending support to our proposal.     

Fine structure of the field-induced magnetic transition in Nd0.1La0.9Fe11.5Al1.5

For the Nd_0.1La_0.9Fe_11.5Al_1.5 compound, the fine structure of the magnetic transition from the ferromagnetic (FM) to the antiferromagnetic (AFM) states has been studied carefully by means of magnetization (M) and heat capacity (C_p) measurements. Although a single phase with the cubic NaZn₁₃-type structure (Fm3c) has been proved by the room temperature X-ray diffraction pattern, the phase transition has been clearly found to be a stepwise process in M(T) and C_p(T) curves under proper fields. Due to the strong competition between the FM order and AFM order, the characteristic is especially evident under low fields, weakens gradually with the increasing applied field and finally vanishes when the field is higher than 2 T. This multi-step magnetic transition results from the inhomogeneity of the sample, probably due to the inhomogeneous distribution of Nd atoms.     

Exchange Bias in Layered GdBaCo<Subscript>2</Subscript>O<Subscript>5.5</Subscript> Cobaltite

It is established that excess oxygen content δ influences the exchange bias (EB) in layered GdBa-Co₂O_{5 + δ} cobaltite. The EB effect arises in p-type (δ > 0.5) cobaltite and disappears in n-type (δ < 0.5) cobaltite. The main parameters of EB in GdBaCo₂O_5.52(2) polycrystals are determined, including the field and temperature dependences of EB field H _EB , blocking temperature T _B , exchange coupling energy J _i of antiferromagnet–ferromagnet (AFM–FM) interface, and dimensions of FM clusters. The training effect inherent in systems with EB has been studied. The results are explained in terms of exchange interaction between the FM and AFM phases. It is assumed that the EB originates from the coexistence of Co³⁺ and Co⁴⁺ ions that leads to the formation of monodomain FM clusters in the AFM matrix of cobaltite.     

Enhanced magnetoresistance in La0.82Sr0.18MnO3-π-conjugated semiconducting polymer heterostructure

We report a large enhancement (∼90%) in magnetoresistance in La_0.82Sr_0.18MnO₃ (LSMO) layers by incorporating a π-conjugated semiconducting polymer layer in between them. The epitaxial LSMO layers were deposited by DC magnetron sputtering on SrTiO₃ single crystal substrates and have FM transition temperature (T_C)∼310 K. A semiconducting polymer poly(3-octylthiophene) (P3OT) layer was deposited over the epitaxial LSMO layer by solution dip coating technique and with subsequent deposition of another epitaxial LSMO layer, forming a LSMO-P3OT-LSMO heterostructure. The effect of P3OT incorporation on magnetotransport properties of this heterostructure has been examined in the temperature range 77-350 K. Large MR enhancement observed near room temperature in the FM regime is explained in terms of efficient magnetic field dependent carrier injection at LSMO/P3OT interface.     

Ni81Fe19/Cr82Al18双层膜中的交换偏置

研究了直流磁控溅射法制备的Ni₈₁Fe₁₉/Cr₈₂Al₁₈双层膜中的交换耦合.样品的室温矫顽力与1/t^{3/2_FM(t_FM为铁磁层厚度）近似成正比例关系,从而表明在Ni₈₁Fe₁₉/Cr₈₂Al₁₈中交换耦合为铁磁/反铁磁界面的随机相互作用.另外还讨论了反铁磁层厚度对交换偏置的影响. 关键词：}     

Observation of exchange bias and spin-glass-like ordering in ɛ-Fe2.8Cr0.2N nanoparticles

Nanoparticles of ɛ-Fe_2.8Cr_0.2N system exhibit the exchange bias phenomenon due to the exchange coupling of the spins of the antiferromagnetic (AF) oxide/oxynitride surface layer and the ferromagnetic (FM) nitride core. Exchange bias is observed at 10 K both in the absence and presence of cooling field. Due to the interface disorder, a mixture of parallel and anti-parallel/perpendicular coupling of the AF and FM spins is observed. The roughness of AF-FM interface induces disorder due to the random exchange anisotropy. The saturation magnetization is also found to be drastically lowered as compared to parent ɛ-Fe₃N. Below 58 K, the broad peak (T _E ≅ T _f ) in zero-field cooled (ZFC) magnetization curves indicates the presence of unidirectional anisotropy and spinglass-like ordering, that arises from the freezing of localized frustrated spins.