Magnetic field cycling effect on the non-linear current-voltage characteristics and magnetic field induced negative differential resistance in α− Fe1.64Ga0.36O3 oxide R... The magnetic
Trang 1Magnetic field cycling effect on the non-linear current-voltage characteristics and magnetic field induced negative differential resistance in α− Fe1.64Ga0.36O3 oxide
R N Bhowmik and G Vijayasri
Citation: AIP Advances 5, 067126 (2015); doi: 10.1063/1.4922511
View online: http://dx.doi.org/10.1063/1.4922511
View Table of Contents: http://aip.scitation.org/toc/adv/5/6
Published by the American Institute of Physics
Trang 2Magnetic field cycling effect on the non-linear
current-voltage characteristics and magnetic field induced negative differential resistance in α-Fe1.64Ga0.36O3 oxide
R N Bhowmikaand G Vijayasri
Department of Physics, Pondicherry University, R.Venkataraman Nagar, Kalapet,
Puducherry - 605 014, India
(Received 13 April 2015; accepted 31 May 2015; published online 10 June 2015)
We have studied current-voltage (I-V) characteristics of α-Fe1.64Ga0.36O3, a typical canted ferromagnetic semiconductor The sample showed a transformation of the I-V curves from linear to non-linear character with the increase of bias voltage The I-V curves showed irreversible features with hysteresis loop and bi-stable electronic states for up and down modes of voltage sweep We report positive magnetoresistance and magnetic field induced negative differential resistance as the first time observed phenomena in metal doped hematite system The magnitudes of critical voltage at which I-V curve showed peak and corresponding peak current are affected by magnetic field cycling The shift of the peak voltage with magnetic field showed a step-wise jump between two discrete voltage levels with least gap (∆VP) 0.345(± 0.001) V The magnetic spin dependent electronic charge transport in this new class of magnetic semiconductor opens a wide scope for tuning large electrore-sistance (∼500-700%), magnetoreelectrore-sistance (70-135 %) and charge-spin dependent conductivity under suitable control of electric and magnetic fields The electric and magnetic field controlled charge-spin transport is interesting for applications
of the magnetic materials in spintronics, e.g., magnetic sensor, memory devices and digital switching C 2015 Author(s) All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License [http://dx.doi.org/10.1063/1.4922511]
I INTRODUCTION
Development of a new class of ferromagnetic semiconductor, based on metal (Al, Ti, Ga) doped hematite (α-Fe2O3) system, is matter of recent interest for understanding novel magneto-electric phenomena and their potential applications in spintronic devices.1 5The hematite system stabilizes
in rhombohedral structure with space group R¯3C The spins of Fe3+ ions in each rhombohedral planes (say, two alternating planes A and B) from ferromagnetic (FM) order, but spins of Fe3+ ions between A and B planes are coupled by antiferromagnetic (AFM) superexchange interac-tions (Fe3+
A–O2−–Fe3+
B).3 A weak (canted) ferromagnetic state appears in hematite system due
to canting of spins between Fe3+ ions in A and B planes, where spin canting is controlled by anisotropic Dzyaloshinsky-Morya (DM) interactions [∼⇀−D.(−→Sn×−S−−n+1→)] The interesting aspect is that doping of non-magnetic Ga atoms in α-Fe2O3system enhances ferromagnetic properties.5 7
The ferromagnetic enhancement in Ga doped α-Fe2O3 occurs due to modified spin structure and superexchange interactions in rhombohedral planes of the crystal structure Electrically, hematite
is a charge-transfer type semiconductor with band gap ∼ 2.2 eV and non-suitable for spintronics applications due to its low electrical conductivity.8Recent reports3 , 4 , 8 , 9predicted enhancement of electrical conductivity in metal doped hematite There is not much progress to understand either the mechanism of enhanced conductivity or the hidden magneto-transport properties in metal doped
a Corresponding author: Tel.: +91-9944064547; Fax: +91-413-2655734 E-mail: rnbhowmik.phy@pondiuni.edu.in
Trang 3067126-2 R N Bhowmik and G Vijayasri AIP Advances 5, 067126 (2015)
hematite Many exciting magneto-transport properties can be expected due to coupling between ferromagnetism and conductivity in metal doped hematite system C Papaioannoua et al.10 sug-gested a possible charge-spin coupling in hematite Our recent work11 has highlighted the effect
of magnetic spins flipping on magneto-electric coupling in α-Fe1.6Ga0.4O3system The ferromag-netic semiconductor properties of Ga doped α-Fe2O3system are not like conventional III-V ions based diluted ferromagnetic semiconductor.1More experimental evidences are needed to establish charge-spin coupling in this new class of ferromagnetic semiconductor The coupling between electronic charge and spin is essential for the spintronics applications of magnetic semiconductors.2 , 12
The current-voltage (I-V) curve correlates the charge flow through a ferromagnetic semicon-ductor under test (FMSUT) with applied voltages and it is an important tool to determine suitability
of the material in electronic devices The I-V characteristics can be used to estimate electrical conductivity, charge accumulation and charge injection efficiency at the interfaces of FMSUT and electrodes.13 – 15The I-V curves of a metal/FMSUT/metal junction device are controlled by the injec-tion rate of charge carriers from electrode into FMSUT, and transport of the charge carriers through FMSUT The transport of charge carriers is either injection limited due to a mismatch between the electrode work function and the electronic energy level of the FMSUT or bulk transport limited due to intrinsic charge mobility of the FMSUT or space charge limited at the interfaces and grain boundaries, respectively.16 The control of space charge limited current (SCLC) and charge-spin coupling in metal electrode-semiconductor junction has become a key issue for device applications and understanding the electronic charge transport in FMSUT.14 , 17 The magnetic field controlled I-V curve has emerged as an effective route for testing SCLC and charge-spin coupling in a FM-SUT.13 – 15 , 18The I-V characteristics under magnetic field are also useful to extract the information
of the effects of magnetic domain wall motion and magnetic domain rotation on the current flow mechanism in FMSUT.1 , 19To our knowledge, there is no work which dealt magnetic field controlled electronic properties for metal doped hematite system, despite the fact that such system has a bright future for the applications in multifunctional devices.20 Our earlier works5 , 6 have discussed the canted ferromagnetic properties in Ga doped hematite system In this work, we report a detailed study of I-V characteristics of α-Fe1.64Ga0.36O3 sample and demonstrate the magnetic field cycl-ing effect on its I-V characteristics to explore hidden magneto-transport properties in Ga doped hematite system
II EXPERIMENTAL
The α-Fe1.64Ga0.36O3 compound in rhombohedral structure (space group R¯3C) has been pre-pared by the coprecipitation of Fe and Ga hydroxides in alkaline medium (pH ∼ 11) at 80 oC The precipitated powder after washing has been annealed under vacuum (10−5mbar) at 800 OC and formation of single phased structure has been confirmed from synchrotron X-ray diffraction pattern Details of the material preparation and characterization have been discussed elsewhere.6In this work, we describe the I-V characteristics in the absence and presence of dc magnetic field A disc shaped (Ø13 mm and thickness ∼ 1 mm) sample has been sandwiched between two platinum (Pt) electrodes for I-V curve measurement The contact between Pt electrodes and the sample has been made by adjusting pressure from both sides of the Pt plates using homemade sample holder The sample holder has been placed in pole gap of an electromagnet (MicroSense, USA) The bias voltage (within ± 10 V) across the sample has been applied by connecting the Pt electrodes to Keithley Meter (Model: 2410-C) The current flow and magnetic field are directed perpendicular
to the surface area of the disc The current through the sample has been measured by sweeping the dc voltage across the sample in up (0 → ± 10 V) and down (± 10 V → 0) modes We have extracted the static resistance (R = V/I) at each point and dynamic resistance (rd= ∆V/∆I) over a range of I-V curves The magnetic field (H) cycling effect on the I-V curves has been recorded in the presence of selected magnetic fields in five segments, i.e., first segment (S1): increase of H from
0 to + 15 kOe, second segment (S2): reducing the field from + 15 kOe to 0, third segment (S3): reduction of field from 0 to -15 kOe, fourth segment (S4): increase of field from -15 kOe to 0, and fifth segment (S5): increase of field from 0 to +15 kOe These five segments have been decided based on the field dependent magnetization [M(H)] loop of the sample However, I-V curves for
Trang 4all the applied magnetic fields are not shown in figures for better representation of the data The measurement has been designed for the delay time 100 ms between two consecutive bias voltage points The sample has been waited (tw) for 0 s and 60 s, respectively between the application of magnetic field and starting of I-V curve measurement In order to confirm the intrinsic nature of the non-linear I-V curves of the sample, we have repeated some of the measurements in the presence and absence of magnetic field by silver coating on both sides of the sample disc and sandwiching the silvered disc between two pressure-controlled Pt electrodes We have noted the reproducibility
of some important features, leaving aside small differences due to silver coating For example, magnetic field induced negative differential resistance (NDR) and positive magnetoresistance (MR) have been confirmed for the sample disc with and without silver coating However, I-V curves exhibited nearly two order increase of measured current when the sample surface has been silver coated We noted similar increase of current after silver coating on hematite sample, but hematite does not show NDR effect We believe that current flow in silver coated sample is increased due
to diffusion of some silver atoms at the sample surface We focus on the following aspects, (1) magnetic field cycling effect on I-V curves, (2) magnetic field induced NDR effect, (3) space charge controlled current flow, and (4) charge-spin controlled electronic properties by sandwiching the disc-shaped sample without silver coating between two Pt electrodes
III EXPERIMENTAL RESULTS
A I-V curves measurement in the presence of magnetic field with zero waiting time
The current has been measured in positive (0 to +10 V) and negative (0 to -10 V) bias of the sweeping voltage in the absence and presence of set magnetic field The measurement started immediately without giving any waiting time (tw=0) for applied magnetic field before recording the I-V curve Fig.1 shows the I-V curves The absolute current values on positive and negative bias voltages are nearly identical, suggesting good electrical contact on both sides of the sample The I-V curves showed nearly linear character with increase of voltage up to ± 4 V The rate of current increment decreases above ± 5 V, implying a non-linear behavior at higher voltage The vertical arrows in Fig.1represent the directions of magnetic field (H) and current (I) during S1-S5 segments The nature of I-V curves during different segments of magnetic field cycling has been discussed later in terms of the R(V) curves The linear behavior of I-V curves follows a power law: I(V) ∼ Vm, where the power factor (m) has been obtained by m = ∂lnI/∂lnV As shown in Fig.1(f), the power factor (m) in the absence of magnetic field is ∼ 1.4 and stabilized in the range 1.0 ± 0.2 under magnetic field Fig.2shows the I-V curves during down modes of the sweeping voltage (±10
to 0 V) under constant magnetic fields The I-V curves in the down modes do not follow the paths
of up modes (Fig.1) This shows irreversible character (hysteresis loop) and two distinct resistance states, i.e., low resistive state (LRS) during voltage up mode and high resistive state (HRS) during voltage down mode in I-V curves The ferromagnetic-semiconductor with bi-stable electronic states
is interesting for applications in magnetic memory devices.21In the lower voltage regime of I-V curves during down modes of voltage sweep, the power factor (m) has stabilized in the range 1.3-1.7 under magnetic field The non-linear I-V curves suggest a good amount of change of electrical resistance with bias voltage, a phenomenon known as electroresistance (ER).22Fig.3(a)-3(h)shows the variation of static resistance (R) with bias voltage (V), derived from up and down modes of I-V curves Some important features can be noted from R(V) curves First, R(V) curves depend
on magnetic fields Second, the nature of R(V) curves for down modes (sweeping voltage ± 10 V
to 0) is different from the nature of R(V) curves for up modes (sweeping voltage 0 to ± 10 V) The R(V) curves during up modes are in LR state and weakly dependent on voltage sweep up to ± 4 V, followed by an electric field induced increment above ± 5 V On the other hand, R(V) curves for the down modes are in HR state that increases rapidly for voltage swept from ± 10 V to 0 We estimated the change of ER using the formula ER (%) = [R (V H )−R(V L )
R (V L ) ] x100 Considering an uncertainty in the accuracy of measured current for sweeping voltage approaching to zero volt, we have taken R(VH) and R(VL) as the resistances at 10 V and 1V, respectively Fig 4 shows the variation of ER(%) during all five segments of the magnetic field cycling in the up and down modes of voltage
Trang 5067126-4 R N Bhowmik and G Vijayasri AIP Advances 5, 067126 (2015)
FIG 1 (a-e) I-V curves (voltage increasing mode) at selected magnetic fields and without wating time after application of fields (f) the power factor obtained from fitting of I-V curves at low voltage regime Vertical up arrows represent positive direction and vertical down arrows represent negative directions for magnetic field and current flow through the disc shaped sample.
sweep We observed that ER (%) is positive for the up mode and negative for the down mode of the voltage sweep The ER values in down mode (- 90 (±10) % for positive and - 92 (±8) % for negative bias voltage) are larger in comparison to the values in up mode (∼ + 50 (±20) % for positive and +30 (±15) % for negative bias voltage) It is not good to draw any decreasing or increasing pattern
of the variation of ER (%) in different segments of magnetic fields, because the ER(%) data are
a bit scattered due to low accuracy of R(VL) data at 1 V or due to insufficient waiting time for magnetic field to achieve a quasi-equilibrium magnetic spin ordered state.23 The magnetic field effect on R(V) curves has been confirmed from the magnetoresistance (MR), a phenomenon where resistance at constant bias voltage changes with magnetic field variation We used the I-V curves
at selected voltages to extract M R (%) = (R(H )−R(0)
R (0) ) x100 R(H) is the resistance at magnetic field
H and R(0) is the resistance at magnetic field zero Fig.5(a)-5(e)shows the variation of MR (%) with magnetic field cycling at positive bias voltages The behavior of MR (%) with magnetic field
at negative voltages is identical to that found for positive bias voltages (not shown) We have also directly measured the MR(%) at selected magnetic fields and the data are shown in Fig.5(f)-5(j)
Trang 6FIG 2 (a-e) I-V curves (return paths) shown at selected magnetic fields and without wating time after application of fields (f) power factor obtained by fitting the I-V curves at low voltage regime.
In direct measurement, the current has been measured at selected magnetic fields under constant voltage The measurement of current at each magnetic field has been repeated 20 times The average current at each point of magnetic field cycling has been used to calculate resistance (R = V/I) and MR(%) Fig.5shows the MR(%) vs H plots The MR(%) data with H from direct measurement and extracted from I-V curves are well consistent to each other For example, MR(H) curves followed different paths during increase and decreasing modes of magnetic field variation The plots showed
a butterfly wings shaped loop, which is associated with bi-stable electronic states and controlled by magnetic field cycling The variation of ER(%) with H and the variation of MR(%) with V indicate
a strong magneto-electric coupling in the sample The increasing MR(%) at lower magnetic field regime to achieve a peak and subsequent decrease of MR(%) at higher magnetic field has been found in different ferromagnetic systems, where spin polarization of electrons controls the transport properties.24 , 25We estimated the MR(%) maximum (∼ 105 %, 135 %, 95 %, 90 %, and 70 % for the bias voltage at 1 V, 3 V, 5 V, 8 V, and 10 V, respectively) using the MR(%) curves from direct measurement The maximum MR(%) initially increases with V up to 3 Volts and then decreases
on further increase of V This feature can be corroborated with the nature of I-V curves, where current has rapidly increased for bias voltage up to ∼ 4 V and then slowed down at higher voltage This shows a critical value of bias voltage for achieving maximum MR(%) The positive MR(%)
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FIG 3 Resistance vs sweeping voltage at selected magnetic fields at different segments for the initial paths (0 to ± 10 V)
in (a-d) and for return paths (± 10V to 0) in (e-h), extracted from Fig 1 and Fig 2 , respectively.
indicates that resistance of the sample increases under magnetic field Such increase of resistance can be attributed to spins scattering effect at the interfaces or grain-boundaries of the magnetic material.16 , 19Space charge also exhibited a great impact on the large positive MR in non-magnetic material.17However, the nature of the MR(%) curves in our sample with H at selected voltages is different from that observed due to space charge effect We have discussed later that space charge effect is not the main mechanism in our sample for controlling magneto-transport properties It may be noted by comparing the MR(%) vs H data from I-V curves and direct measurement that the data extracted from I-V curves showed a bit fluctuation Although current measurement started immediately after setting the magnet field, the repetition of the current measurement for 20 times
at each magnetic field takes nearly 20-30 s We understand that the waiting time of magnetic field before current measurement plays a significant role to suppress thermal induced noise and increase
of electron spin ordering at the grain boundaries.25We elaborate the magnetic spin controlled charge transport process by increasing the waiting time of magnetic field so that spins structure can achieve
a quasi-equilibrium state before I-V curve measurement
Trang 8FIG 4 Variation of electroresistance (%) with applied magnetic fields for the initial (a-b) and return (c-d) paths of the positive and negative biasing of voltage.
B I-V curves measurement in the presence of magnetic field with 60 s waiting time
In this measurement, the sample has been waited for nearly 60 s under set magnetic field before recording the I-V characteristics Fig 6 shows the I-V curves recorded during up modes
of voltage sweep at constant magnetic fields The I-V curve in the absence of magnetic field is obviously identical to that shown in Fig 1 However, the I-V curves measured after 60 s waiting time at set magnetic field have brought remarkable changes We found three distinct regimes, as indicated in Fig.6(d) In low voltage ohmic type regime (LOR), current increases almost linearly
up to a critical voltage (VP) The most remarkable feature is the negative differential resistance (NDR: ∂V/∂I < 0) effect above VP In NDR regime, the I-V curve showed non-ohmic character where current decreases with increase of voltage to achieve a current valley at voltage Vm In the high voltage ohmic regime (HOR) at V > Vm, the current once again increases with the increase
of voltage up to 10 V The NDR effect is useful for applying materials in spintronics devices,26 – 29
especially in memory and switching devices.30 , 31We have shown in Fig.6(f)that the power factor (m = ∂lnI/∂lnV) in the LOR regime varied in the range 0.90-1.20 These values of m matched with the values obtained from I-V curves measured at zero waiting time for magnetic field We found the critical voltage (VP) in the range ∼ 2- 4 V and ∼ - (2.5 - 4.5 V) for positive and negative
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FIG 5 Variation of the change of magnetoresistance obtained from I-V curves (a-e) and direct measurements (f-j), respectively.
bias voltages, respectively We plotted the values of VP and corresponding peak current (IP) at each applied magnetic field in Fig 7(a)-7(b) and Fig 7(c)-7(d), respectively We noted that VP corresponding to the peak current shifts with applied magnetic field The shift of VPwith magnetic field showed a step-wise jump between two discrete voltage levels, e.g., + 2.06912 V, + 3.44870 V, +3.79284 V for positive bias and -2.75925 V, -3.79297 V, -4.48322 V for negative bias The least gap (∆VP) between two discrete VPlevels has been found 0.345(± 0.001) V The discrete levels have been found as the integer multiple of ∆VP Fig.7(c)-7(d)shows that the variation of peak current (IP) with magnetic field cycling is nearly identical for both positive and negative bias voltages, except the direction of current is opposite The IPpath with magnetic field cycling is irreversible The IPvaried within a limited range (2.5±1.5 µA) in the S1-S3 segments of magnetic field cycling
It increases rapidly in S4 segment to reach the highest value ∼ 8.5±0.5 µA as the magnetic field increases from -15 kOe to +3 kOe On further increase of magnetic field up to +15 kOe in S5
Trang 10FIG 6 I-V curves measured at selected points of magnetic field cycling (a-e) and power factor from I-V curves at different points of magnetic field cycling (f) Three distinct regimes in I-V curves under magnetic field have been shown in (d).
segment, the IPdecreases down to the level that has been achieved at +15 kOe during S1 segment, and completes the IP loop If we compare the IP(H) loop with ferromagnetic M(H) loop,6 it is realized that there is no remarkable change of peak current during domain wall motion in the S1 segment (magnetization is orienting parallel to magnetic field) and also during magnetic domains rotation from up (parallel to magnetization) to down (opposite to magnetization) directions How-ever, a spectacular increase of IP is started as the magnetic domains started rotating from down
to up directions when magnetic field was increasing from -15 kOe to positive field direction An estimation of the negative differential resistance (rd= ∆V/∆I) from NDR regime (Fig 7(e)-7(f)) shows relatively large value at higher magnetic fields This means the rate of decrease of current
in the NDR regime is affected by the magnetic field cycling effect We have also estimated static resistance (R) from the I-V curves (Fig 6) and plotted the data in Fig.8(a)-8(e) In the absence
of magnetic field (S1 segment), the resistance initially decreases to reach a minimum value at bias voltage ± 1.72 V, followed by an increasing trend with further increase of voltage in both positive and negative bias The features of R(V) curves drastically changed when magnetic field is applied The resistance is nearly independent or weakly dependent up to the bias voltage ∼ ± 3.79 V, which
is consistent to ohmic character of the sample at lower voltage Then, static resistance increases